*Article* **Constraining the Passive to Active Margin Tectonics of the Internal Central Apennines: Insights from Biostratigraphy, Structural, and Seismic Analysis**

**Giovanni Luca Cardello 1,2,\*, Giuseppe Vico 3, Lorenzo Consorti 4,5, Monia Sabbatino 6, Eugenio Carminati <sup>1</sup> and Carlo Doglioni 1,7**


**Abstract:** The polyphase structural evolution of a sector of the internal Central Apennines, where the significance of pelagic deposits atop neritic carbonate platform and active margin sediments has been long debated, is here documented. The results of a new geological survey in the Volsci Range, supported by new stratigraphic constraints from the syn-orogenic deposits, are integrated with the analysis of 2D seismic reflection lines and available wells in the adjacent Latin Valley. Late Cretaceous syn-sedimentary faults are documented and interpreted as steps linking a carbonate platform to the adjacent pelagic basin, located to the west. During Tortonian time, the pelagic deposits were squeezed off and juxtaposed as mélange units on top of the carbonate platform. Subsurface data highlighted stacked thrust sheets that were first involved into an initial in-sequence propagation with top-to-the-ENE, synchronous to late Tortonian foredeep to wedge-top sedimentation. We distinguish up to four groups of thrust faults that occurred during in-sequence shortening (thrusts 1–3; about 55–60 km) and backthrusting (thrust 4). During Pliocene to recent times, the area has been uplifted and subsequently extended by normal faults cross-cutting the accretionary wedge. Beside regional interest, our findings bear implications on the kinematic evolution of an orogenic wedge affected by far-traveled units.

**Keywords:** Central Apennines; passive margin inversion; mélange; pelagic deposits; thrust sheets; backthrust; cretaceous; Miocene; nannoplankton

#### **1. Introduction**

Carbonate platforms are a type of passive margin sedimentary succession that can be commonly involved in the thrust-sheet imbrication of an orogenic wedge [1–3]. During in-sequence ongoing deformation, the wedge propagates by incorporating new portions of the foreland, which is commonly made up of crystalline basement, clastic and/or carbonatic successions, and overriding foredeep/foreland clastics with variable thickness and composition [4–6]. The so formed fold-and-thrust belt, incorporating distinctive tectonostratigraphic units, is the combined product of inherited syn-sedimentary structures and orogenic dynamics [7,8]. Thus, the wedge-related deformation style may strongly depend on the stratigraphic architecture and in particular on the presence and depth of décollement

**Citation:** Cardello, G.L.; Vico, G.; Consorti, L.; Sabbatino, M.; Carminati, E.; Doglioni, C. Constraining the Passive to Active Margin Tectonics of the Internal Central Apennines: Insights from Biostratigraphy, Structural, and Seismic Analysis. *Geosciences* **2021**, *11*, 160. https://doi.org/10.3390/ geosciences11040160

Academic Editors: Domenico Liotta, Giancarlo Molli and Angelo Cipriani

Received: 1 February 2021 Accepted: 28 March 2021 Published: 1 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

layers within the stratigraphic successions (i.e., salt [9]). In this sense, thick-skinned deformation (see, e.g., in [10]) can dominate when there is no suitable detachment horizon. On the contrary, when preferred slip-levels occur, thin-skinned tectonics develop, generating flat-ramp-flat geometries and disharmonic folding, which, for example, can occur within base-of-slope to pelagic successions [11,12]. At the transition between such structural domains, strain localization can occur, nucleating thrusts by inverting previous listric boundary extensional faults (see, e.g., in [13]).

During inversion of hyperextended passive margins, orogenesis forms far-traveled units that can reach a high-degree of internal deformation [14–16]. The chaotic structure of these so-formed mélange units is the result of the superposition of tectonic, sedimentary, and mud-diapiric processes [17], to which gravitative processes add, by incorporating both allochthonous and autochthonous blocks [18]. Despite the subsequent orogenic deformation overprint, occurring within far-traveled thrust-sheets, the structural heritage may be preserved and studied (see, e.g., in [19–22]).

The Apennines are a fold-and-thrust belt involving basinal and platform-derived thrust sheets and mélange units (Figure 1) that offer well-outcropping structures representative of inverted hyperextended passive margins. The present-day deep structure of the Apennines has been a long matter of debate, as the amount of thrust allochthony and the involvement of the crystalline basement are widely discussed (see, e.g., in [23–29]. In this frame, the recognition of inherited structures also bears implications on the reconstructions of the pre- to syn-orogenic evolution [30–33]. For the Central Apennines, timing of deformation and shortening rates through time were reconstructed by coupling kinematic reconstructions with dating of the deposits overlying the forebulge unconformity [34] or, more classically, by dating the siliciclastic syn-orogenic deposits of the foredeep and wedgetop basins by using biostratigraphy (see, e.g., in [35–38]). However, controversial age interpretations may be derived due to the occurrence of few index fossils or reworked specimens from cannibalized foredeep and wedge-top deposits (see, e.g., in [34–39]). Recently, thermo-chronological studies have provided absolute dating of calcite and fault-gouge that have supported the reconstruction of regional thrust evolution [40–43].

Considering that the central Apennines represent an orogen that involves large volumes of the Adriatic plate, identification and description of the most internal thrust sheets are fundamental to highlight the role of inherited structures in determining the dynamics of far-traveled thrusting. In particular, one of the most crucial problems is deciphering the degree of distance covered by the units after detachment within foreland, foredeep, and wedge-top basins during shortening. In this paper, we provide (i) a review of the existing literature of the Volsci Range (VR; Figure 1) and of the adjacent Latin Valley; (ii) a comprehensive stratigraphic and structural analysis based on new age determinations of the syn-orogenic deposits; and (iii) a reinterpretation of a composite dataset of public well data and seismic lines, integrated by unpublished data provided by Pentex Italia Limited. We recognize a polyphase structural evolution based on the documentation of the characteristic mélange structures in the Chaotic complex and the distinction of foreland-directed thrusts cross-cut by younger hinterland-directed reverse faults. As a brand-new outcome, the reconstruction of the pre-orogenic heritage and the syn-orogenic Miocene structures allows us to constrain a previously unpublished regional inversion tectonic process and its peculiar evolution of thrusting. In this frame, the internal Central Apennines represent an example of the kinematic evolution of platform and basin-derived thrust sheets. Our study can help unravel the evolution of similar belts worldwide, and more specifically contributes to the understanding of far-traveled thrust sheets.

**Figure 1.** (**a**) Simplified Tectonic map of Central Italy (modified from the works in [30,38,44], showing the active margin units and the Meso-Cenozoic passive margin units. The shortening time is in italic. (**b**) Crustal cross-section (modified after the work in [45]). Deep well location is taken from in [23].

#### **2. Geological Setting**

#### *2.1. The Central Apennines*

The Apennines (Figure 1) are a ~1500 km long accretionary wedge made of different pre-orogenic and syn-orogenic units accreted together during the progressive E/NE-ward migration of leading-edge frontal thrusts and associated active margin units deposited within foredeep and wedge-top basins (see, e.g., in [46–50]. From Miocene time, the Apennine foreland became progressively involved in pre-thrusting bulging, uplift, and erosion resulting from the wedge migration [51–56]. Since Tortonian time (~11 Ma), the west-directed subduction of the Adriatic slab drove the development of the accretionary wedge now exposed in the central sector of the Apennine belt [49,54]. Subsequently, the fold-and-thrust belt underwent severe crustal stretching, related to back-arc extension that progressively migrated from the Sardinian margin to the axial part of the central Apennines [49,57]. The chain is now uplifted and cross-cut by Quaternary normal faults and also affected by several volcanic centers along the Tyrrhenian margin [45,47,49,58,59].

The central Apennines constitute a mountain chain sector bounded by two major NNEtrending tectonic lines (Figure 1), comprised between two arcs with polyphase activity: the Ortona–Roccamonfina and the Olevano–Antrodoco–Sibillini lines [60,61]. The latter can be considered as the positive transpressive reactivation (see in [7] and the references therein) of a Mesozoic extensional fault system associated with continental rifting, the Ancona–Anzio line [62] (Figure 1).

The Mesozoic paleogeography was characterized by different domains defined by peculiar stratigraphic successions. West of the Olevano–Antrodoco–Sibillini line, Meso-Cenozoic pelagic sequences occur in the northern Apennines. East of the Ancona–Anzio line, the central Apennines are mainly formed by neritic carbonate platform units that are bounded by base-ofslope to basinal domains (e.g., Gran Sasso [30]). According to the works in [63,64], drowning of the Mesozoic carbonate platform of the VR occurred during the latest Cretaceous or Cenozoic times and is testified by basinal deposits lying on top of platform carbonates. More internal basinal/oceanic units, referred to the Sicilide and Ligurian Accretionary Complex, crop out both in the southern Apennines [44] and along the coast west of Rome (i.e., Tolfa region [65,66]; Figure 1). These units are traditionally recognized as allochthonous units that were involved into the wedge in Miocene time. The occurrence of similar internal allochthonous units in the central Apennines is still debated. A stratigraphic correlation between the deposits atop the neritic carbonates of the VR and the Ligurian-Sicilian basinal units of Sicily and southern Apennines was first made by [67]. A different interpretation was proposed by the authors of [65,66], who recognized the marly–terrigenous terrains atop the VR carbonates as the remobilization of the Cenozoic basinal succession.

The terrigenous units cropping out in the central Apennines mostly occur in NW-striking valleys (e.g., Latin Valley [68]; Figure 2). These units are representative of foreland basin deposits, whose formation nomenclature varies from region to region, i.e., the Frosinone Formation [64] shares similar timing and facies with the Termini and Pietraroja formations of the southern Apennines [69,70]. To harmonize their occurrence throughout the central and southern Apennines, we have grouped them in four different units, representative of progressively more external and younger stages of the wedge accretion towards the east (Figure 1a). To the south, as shown by well logs and outcrops in the Pontian islands and at Circeo Mt., Mesozoic basinal units overthrust Oligocene to early Miocene flysch units [42,71]. South of Naples (Figure 1), Serravallian to lower Tortonian flysch represent internal terrigenous foredeep units [44]. Serravallian syn-orogenic units, indicative of plate flexuration, were recognized as well in more internal positions within the Volsci Range [72]. Such flexural deposits rejuvenate towards the east suggesting a progressive shift of the wedge towards the outer portions of the arc. Intermediate terrigenous units of late Tortonian–earliest Messinian age occur in the Latin Valley and underneath the overthrusted platform carbonates of Campanian age.

North of the Latin Valley, the Simbruini-Ernici Mts are built up of NW-striking imbricate carbonate thrust sheets that overthrusted onto the outer terrigenous units of Messinian age (e.g., within the Latin valley, Figure 1 [73,74]). This is well evidenced by the Trevi well that shows the juxtaposition, at considerable depths (3000 m), of Triassic terrains onto Cretaceous and Miocene carbonates, testifying for the doubling of the Mesozoic succession [75]. A horizontal displacement in the order of 30 km and vertical offset of about 5 km has been proposed for this thrust [76,77], although field evidence from the Simbruini thrust front is at odds with this interpretation [49]. These ridges constitute the backbone of the internal sectors of the Central Apennines (internal Central Apennines), which first overthrust onto the outer active margin deposits and, during late Messinian time, were involved into renewed shortening [43]. Differently from the Internal Apennines, the axial and external parts of the chain, that occur more to the northeast, were involved into the wedge respectively during Messinian (Abruzzi) and Pliocene (Majella Mountain deformed Apulian terrains; Figure 1; see in [78]) times. During middle Pliocene time, the outermost terrigenous units experienced compression, while back-arc extension was affecting the internal part of the chain.

**Figure 2.** Simplified stratigraphic columns of the Volsci Range and the adjacent Latin Valley correlated, on the left, with the official cartography [64]. On the right, tectonic context and stratigraphy of basin deposits is reported from the literature (see Appendix A) and original data at representative localities. Localities from the Ernici unit are highlighted by vertical gray stripes. Below, the geological map of the study area with the studied locations and their respective numbers.

#### *2.2. The Volsci Range and the Latin Valley*

The VR is traditionally subdivided into major mountain groups, i.e., West Lepini, East Lepini, Ausoni, and West and East Aurunci Mts (Figure 2), that are separated by major valleys or mountain passes. More to the SW, the Mount Massico structural high occurs. These groups share a similar tectonic and stratigraphic evolution. The VR is mostly composed of passive margin Mesozoic neritic carbonates belonging to the Latium and Abruzzi platform or Apennine carbonate platform (see, e.g., in [79–81]). The Mesozoic dominant facies are representative of inner to rim carbonate platform environments (see, e.g., in [63,64,82,83]).

A compilation of the Mesozoic lithostratigraphic units cropping out in the Lepini sectors is presented in Figure 2. The Upper and Lower Volsci thrust sheets differ from the Upper Ernici unit on the basis of the Cenozoic stratigraphy. Of note, the VR succession generally bears a thin and incomplete succession of Paleocene to Miocene deposits [84] atop late Cretaceous formations of different ages, possibly due to progressive drowning of some sectors of the platform during Late Cretaceous time [63]. On the other hand, in the Latin Valley, the Ernici unit is thicker and also contains Eocene to early Tortonian foreland units and late Tortonian to earliest Pliocene active margin siliciclastic formations (see in [63] and the references therein).

Seismic interpretation studies in the Latin Valley, carried out by AGIP and other companies (www.videpi.com) (accessed on 20 January 2021), trace top-platform seismic horizons that allowed us to locally outline a fold-and-thrust structure [85]. According to the authors of [64,86], the VR front propagation affected the Latin Valley foredeep deposits that were doubled or even triplicated [45]. Upper and lower units in the Volsci Range and in the Ernici units of the Latin Valley were thus distinguished. As also shown in the cross sections in [64], thrusting involved the Cretaceous carbonates of the Ernici unit together with upper Tortonian foredeep sediments of the Frosinone Formation [63,64]. Finally, out-of-sequence thrusting during and after the Messinian salinity crisis was documented in [77,87], possibly related to backthrusting, like at Carpineto Romano [88]. The thrust front does not crop out, but according to the most recent reconstructions, it is offset by normal faults [45,86]. At least from Middle Pliocene time, the study area experienced regional uplift, accompanied by subaerial exposure and consequent diffuse erosional processes that generated erosional surfaces, now found at different elevations [63].

According to the authors of [89], just north of VR the uplift rate increased during the last 2.4 Myr. In the VR, no such detail was reached yet. However, early to late Pleistocene slope, river, and lacustrine paralic and continental deposits were mapped within depressions bounded by high-angle NW- and NE-striking normal faults that dissected the fold-and-thrust fabric. Further, E-striking transtensional faults contribute to generate middle Pleistocene wrench zones and basins between the Latin Valley and the Pontina Plain. Syn- to post-tectonic upper Pliocene–middle Pleistocene continental successions are preserved in the Middle Latin Valley, the Pontina Plain, and locally in the VR intermontane depressions [64]. Further, during late Pliocene to possibly Holocene times, the fold-and-thrust belt was progressively cross-cut by a system of conjugate synthetic and antithetic normal faults determining the formation of the coastal plain and intra-mountain depressions [64,90,91]. The VR hosts volcanic terrains of Pleistocene age from both nearby volcanic districts and local eruptive centers belonging to the Volsci Volcanic Field (VVF; Figure 1 [64,92]).

#### **3. Materials and Methods**

#### *3.1. Stratigraphic Review and New Paleontological Determinations*

The lithostratigraphic architecture of the Meso-Cenozoic carbonate platform succession has been reviewed, following the scheme in [45], and it has been integrated with a stratigraphic chart that compares eighteen different key localities representative of preorogenic passive margin to syn-orogenic foreland basin lithostratigraphic units throughout the study area (Figure 2). Erosive submarine and karst-related unconformities are reported to support the regional review of the syn-orogenic evolution, also constrained by the absolute ages provided in [43] for the Massico Mt ridge. The overall stratigraphic setting allowed us to correlate diachronous events among different structural units from the Volsci Range and Latin Valley. Lithologies not constrained by biomarkers are traced by a question mark, whereas lithologic and biostratigraphic information coming from the review of the existing literature is resumed in the table of Appendix A. We have harmonized the stratigraphic information published in the 1:100,000 maps (i.e., Latina, Frosinone, and Alatri; https://www.isprambiente.gov.it/) (accessed on 20 January 2021), and in the more recent and detailed 1:50,000 maps (i.e., Anagni, Ceccano, and Velletri; https://www.isprambiente.gov.it/) (accessed on 20 January 2021) as well as and in other papers (i.e., in [72,84,86–88,93,94] and, using the stratigraphic nomenclature after that in [64], then grouped the deposits into the broader informal lithostratigraphic subdivision of Figure 2.

New biostratigraphic information was acquired by studying Upper Cretaceous– Miocene to early Pliocene samples collected from fifteen localities at Colle Cantocchio, Gorga, Gavignano, Carpineto Romano, Caccume Mt., and Siserno Mt. (Figure 2). Further sampling through the Latin Valley at Morolo, Ferentino, and Frosinone localities was performed in order to determine facies and fossil content of syn-orogenic deposits. Hard rock samples have been prepared for thin sections analysis, which provided thirty-three new age determinations. Further, we collected seventeen samples for nannoplankton using samples prepared under smear slide technique, and following the procedures described in [95]. We observed the nannoplankton content through the polarized light microscope Zeiss Axioscop equipped with an ×100 oil immersion objective lens. We performed a qualitative evaluation of the assemblages on all the samples, but only twelve of them proved to be fossiliferous, while five other ones are barren or poorly fossiliferous. Important time maker nannoplankton taxa were identified up to species level, as presented in Supplementary Material. We base our time determination on the micro-biostratigraphic frames in [82,96–98] for the shallow-water carbonate assemblage and the biostratigraphic scale in [99–101] for the nannoplankton.

#### *3.2. Structural Analysis*

A new structural-geological survey of the carbonate and siliciclastic succession integrates previous work of the Geological Survey of Italy (ISPRA) (i.e., in [64,102,103] and the references therein). The resulting new geological map is built also considering a specific review of the 1:50,000 geological sheets "Anagni" and "Ceccano" in order to avoid lithostratigraphic synonymy (see Appendix A) [64,103].

Bedding attitude was retrieved from existing map sheet tables at the scale 1:25,000 on a stripe of about one kilometer to each side of the main cross section (Figure 3). In order to constrain fault kinematics, field measurements of faults, fractures, and slicken-fibers were collected at key localities and plotted by means of TectonicsFP software [104] with lowerhemisphere projections and rose diagrams. In particular, at each locality eigen vectors are calculated from the bedding and are indicative of the orientation of the axes of deformation, where the gray circles are representative of the plane between the principal and minimal eigenvector. In general, an eigenvector is a vector which gets stretched, but not rotated, when operated on by the matrix. Considering that eigenvectors have corresponding eigenvalues, the amount of squeezing or stretching (the strain) is called the eigenvalue. Eigenvectors from key localities are reported in Table S1 (Supplementary material).

**Figure 3.** (**a**) Comparison among lithostratigraphic data from wells in the Latin Valley. (**b**) Sketched geological map with the location of the studied wells and seismic lines. Wells 1, 2, and 4 are from a public dataset (www.videpi.com) (accessed on 20 January 2021). Wells, 3, 5, 8, 11, 12, and 13 are provided by Pentex Limited Italia (see Acknowledgments). Wells 6, 7, 9, and 10 are reported in [105]. Full lines are stratigraphic correlations within the same structural unItal. Black dashed lines are uncertain stratigraphic and tectonic correlations (blue lines). Regional cross section AB (Figure 6) and the detailed structural maps of the Figures 6 and 9 are also shown.

#### *3.3. Borehole Data from the Latin Valley*

Composite well log data from the exploration and production of hydrocarbon activity were used to calibrate the seismic lines (Figure 3). Fifteen wells were drilled through the syn-orogenic lithologies, and they provide insights on late Miocene siliciclastic deposits. Four wells are from a public database (www.videpi.com) (accessed on 20 January 2021), the others were extrapolated from the literature [64,106,107] or confidential reports provided by Pentex Italia Ltd. The stratigraphic calibration of the seismic profiles was performed by using (i) the Frosinone 1 well, which is located within a relatively dense network of seismic lines and drilled at total depth of 684 m, reaching the Orbulina Marl Fm at 526 m and the CBZ at 551 m, while the Cretaceous carbonate platform top was encountered at 620 m, and (ii) the Anagni 1 well, which encountered mesozoic platform carbonates between 47 and 162 m and reached again the carbonate top at 862 m after having crossed a thick siliciclastic succession (Figures 2 and 3). Three wells were characterized by velocity data that allowed us to calibrate seismic data and/or calculate average and interval velocity for the identified macro-units. Where velocity logs were not available, an average interval velocity based on our calculations was applied to fit with the correspondent lithology and reflector detected on seismic profile. In few cases, velocity logs were available for a direct local time-depth chart; in the other cases, average velocity obtained by the analysis of the available logs and from literature was used. These two velocity laws were used to depth-convert the two-way-time interpretation on seismic dataset, in order to define thickness and depth of the main top interpreted horizons to set the geological cross section (Figure 14). Biostratigraphic data are available only for a few key wells (i.e., Paliano 1, Gavignano 1, Anagni 1, Frosinone 1, Liri 1, and Farnese 1) and have been anchored using the regional scale in [96].

#### *3.4. Seismic Dataset*

The structural setting of the Latin Valley presented in this study largely relies on thirty-eight 2D seismic reflection profiles irregularly arranged (map view Figure 3b). In the north, some seismic lines gather around the Gavignano 1 and the Anagni 1 wells, while in the south they occur together with different wells (Figure 3). The seismic sections originate from different acquisition campaigns carried out in the 1980s and 1990s for the exploration of hydrocarbons by AGIP and recently by Sovereign and Pentex. Most seismic lines are part of a public dataset (ViDEPI Project. Available online: https://www.videpi.com accessed on: 20 January 2021. This public network has been integrated by a few other seismic lines from different surveys, to better constrain the structural setting of the Latin Valley. The interpreted seismic dataset was a stack version. Public data were in raster format, so we produced segy files for each raster seismic line in order to be able to import all the dataset into the interpretation software (OpendTect). This was achieved using Kogeo© 2.7, a free and open software for 2D/3D seismic data analysis that allows to create a geo referenced seg-y file from a scanned seismic image (http://www.kogeo.de/index.htm) (accessed on 20 January 2021). Seismic quality is good to poor, probably due to a lack of reprocessing and therefore interpretation may be inaccurate in some points. In those cases, we have integrated the outcropping geological information to reconstruct a geological model along the seismic profile, identifying when possible the main reflectors.

The most evident reflectors are the unconformities at the top of the upper Cretaceous carbonates (Figure 4), and of the Orbulina Marl Fm. (UAM; Figure 2). To calibrate and detect the main reflectors/markers in the Latin Valley, a synthetic seismogram was created for the Anagni-1 well (Figure 4) by focusing on the following formation discontinuities (from the bottom to the top): at the top of the Cretaceous limestones (UK), at the top of the Bryozoa and Lithothamnium limestone (CBZ), and at the base of the Frosinone Formation (FFS). For the interpretation of the seismic profiles, we identified the top-CBZ as the key reflector with the strongest acoustic impedance contrast observed over the entire Latin Valley. This often corresponds to the UAM lithostratigraphic unit (Figure 2), which at the basin scale corresponds with one of the most used reflectors that tie wells with seismic lines [108–110]. Miocene and Cretaceous near-top reflectors are well recognizable because of the characteristic geometry and energy picks that are stronger than the adjacent reflectors. In particular, the marly layers reflect most of the down-going seismic energy, obscuring the siliciclastic sequence or the underthrusted carbonate units. Despite the limited thickness of UAM, this reflector was followed also on the poorer quality seismic lines.

**Figure 4.** Simplified stratigraphic column of the Anagni 1 borehole with velocity log, synthetic seismogram, and an extract of a seismic section passing by the well (location in Figure 3). The seismic marker horizons and additional stratigraphic horizons interpreted in this study are also shown.

#### **4. Results**

*4.1. Stratigraphic Constraints*

4.1.1. Stratigraphic Review

The stratigraphy of the study area is schematically reported from the literature in Figure 2, where the lithostratigraphic units are anchored to the exposed sections at each of the eighteen localities presented in the map. The basics of the different tectonic units are exposed in Section 2.2. A new set of ages is proposed for the succession cropping out at the northern Volsci Range, as shown in the next section.

The Upper Campanian to Eocene carbonate platform succession that rest on the Hippuritid and Radiolitid limestone is generally missing [64], possibly due to a widespread depositional hiatus, although it locally crops out (e.g., at Gorga [103,111]). Note that the shallow-water Spirolina limestone (lower to mid-Eocene [112]), which crops out only in rare patches comprised between two unconformities—probably related to emersion events—was found at Gorga [112], Ferentino, and Castelforte (Figure 2; see also in [96,113], while it was recognized in well logs of Paliano 1 DIR and Farnese 001dir (Figure 3). In the Volsci Range, the Bryozoa and Lithothamnium limestone (CBZ) was dated as middle Miocene (see, e.g., in [64,87]). However, our data from the Volsci Range show that at least the CBZ base is early Miocene in age (see Section 4.1.2). Locally in the Volsci Range (e.g., Carpineto Romano, Figure 2), the CBZ lithotype is reported to occur within and beneath the allochthonous sub-Ligurian units [72], that can be compared with the Falvaterra Chaotic complex in [63].

Overall, the Falvaterra Chaotic complex is an ensemble of Paleocene to middle Miocene lithoclasts (from dm to decametric) wrapped within a matrix, whose best age constraints were provided mostly from the outcrops of Colle Cavallaro [114]. The basal contact of the Chaotic complex, although tectonically overprinted [63], is often marked by a ferruginous-limonitic veneer that occurs as a calcareous-detrital iron-oxide cruston. Differently from the classical carbonate hardgrounds, that are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor under an extremely low sedimentation rate, the crustons of the Volsci Range could be either of karstic origin and/or the product of fluids involved into thrust faulting. Near Formia these crustons occur on top of peritidal limestones with benthic foraminifera (redetermined after the work in [63]) including *Spirolina* sp. [115], which can be possibly attributed to the early Eocene [111]. In particular, the foraminifera shown in [63] (their Figure 4) appear closer to some shallow-water discorbidae rather than planktonic forams. However, this need to be verified with new determinations. Our data constrain the top platform units providing new insights on the correlation, envisaged in [63], between these crustons and the Upper Cretaceous–lower Miocene succession preserved in the Chaotic complex (cf. Section 5.1 on the basis of the new stratigraphic constraints presented in Section 4.1.2.). Concerning the stratigraphic evidence from the Paleogene-early Aquitanian pelagic terms atop (Figure 2), they are mostly represented by Scaglia lithotypes (e.g., Formia and Spigno Saturnia, Figure 2). These lithotypes also crop out beneath the thrust south of Carpineto Romano, and beneath the Caccume Mt. and Colle Cavallaro klippen (Figure 2). Further, Scaglia *sensu latu* lithotypes were found as blocks of various dimensions wrapped in clayey matrix together with: early-middle Miocene lithoclasts (Figure 2; Appendix A), upper Serravallian cherty marl, and massive to laminated arcosic greywackes with mica [103]. The latter resulted sterile at the Caccume Mt. [84]. Lithologies of clasts involved into the Chaotic complex belong to a wide chronostratigraphic interval (i.e., Paleogene-Serravallian pro parte; Figure 2). More to the south, beneath the Vele Mt. thrust, siliciclastic marly deposits, mapped as Chaotic complex equivalent units, occur. Our data provide age constraints for the northern Volsci Range, see Section 4.1.2, and provide insights on the stratigraphic development of the sedimentary succession later deformed as Chaotic complex.

In the Latin Valley, the Frosinone Fm. was homogeneously attributed to late Tortonian time, while on the northeastern edge of the valley the base seems to be younger (i.e., uppermost Tortonian [87]). The upper part of the Frosinone Fm. unit bears olistoliths and

olistostromes [115], from Mesozoic platform and Chaotic complex equivalent lithologies. They are reported at Sgurgola [35] and in the Torre Ausente Valley [64,116], although not as nicely cropping out as at the Massico Mt. [37].

Well data show a highly variable facies pattern of the siliciclastic units that include carbonate intercalations and thick marly successions with minor to rare sandstone horizons (Gavignano 1; Anagni 1; Frosinone 01; Farnese 001 wells; Figure 3). Due to tectonic juxtaposition, these successions may appear repeated at least twice and thus also reaching a total thickness of about 1.8 to 2.5 km at Gavignano and Liri and Farnese wells. Single thrust-bounded siliciclastic units are up to some 0.7 km thick.

In particular, the Gavignano 1 well hits four repeated siliciclastic-marly sequences bounded by thrust faults juxtaposing older terrains above younger ones. The uppermost unit is constituted of Upper Cretaceous (UK) limestones (cf. Anagni 1 well). The deeper fault-bounded units are about 600–900 m thick. Their siliciclastic sequence is defined by different lithofacies associations including alternations of sandstone, marl, and limestone. By correlating the wells providing detailed biostratigraphic information (e.g., Paliano, Gavignano, and Frosinone), we have correlated similar lithostratigraphic units, thus providing a formation identification. Biostratigraphic data from wells do not report Messinian taxa. Thus, we consider the Messinian Monte San Giovanni Campano unit (MVP) following the work in [63] and composed of wedge-top clastics [87], including other formally defined lithostratigraphic units (i.e., Torrice Sandstone Fm, Figure 2). Despite this lack of subsurface biostratigraphic information, its occurrence at depth cannot be excluded. Further, the correlation among conglomerates bearing exotic clasts of granitoids (SBG) is not clear as not supported by resolutive available stratigraphic information. However, their occurrence is of regional relevance as they could be representative of the transition from late orogenic [117] to backarc settings (i.e., Formia; Figure 2).

#### 4.1.2. New Stratigraphic Constraints

New stratigraphic data from the northern Volsci Range and Latin Valley constrain the age of sedimentary units (Appendix B). The uppermost Cretaceous carbonate units were studied at different localities to reconstruct the tectono-stratigraphic setting of the top of the platform before thrusting. This information is provided by the variable thickness and facies distribution of the carbonate units between the Hippuritid and Radiolitid limestone and the ferruginous cruston on top, which usually marks the top of the platform. East of Gorga (Figure 2), the Hippuritid and Radiolitid limestone is overlain by some decameters of Maastrichtian bioclastic limestone and dolostone. This unit is truncated at the top by breccias, indicating an unconformity on the Upper Cretaceous succession. Those breccias are intercalated with a middle Burdigalian shallow-water marly level (Lep 12c, Appendix B) passing upward to typical CBZ limestone.

The Mesozoic platform top was found on top of the Lower Volsci Unit at the Caccume Mt., where it occurs as an encrusted breccia. At Carpineto Romano (Figure 5), atop of the platform succession of the Lower Volsci Unit, when preserved, discontinuous thin patches of proximal early Miocene CBZ limestone and middle Miocene Orbulina Marl formations occur (cf. Cosentino et al., 2002). At Colle Cantocchio (Figure 2; Appendix B), the early Miocene CBZ limestone was found disconformable on the Jurassic-Cretaceous limestone, which is marked by a hardground (structural details in Section 4.2.1).

Atop the Meso-Cenozoic carbonate units, the Chaotic complex occurs as a mélange that contains both native and exotic blocks, the latter being Cretaceous to Miocene basinal to distal ramp deposits that are coeval with the in situ formerly described proximal succession (Figure 5). Both block types are internally folded. South of Carpineto Romano (Figure 3), the deformed platform blocks involved within the Chaotic complex are stratigraphically comparable with the encrusted carbonates that are preserved at the top of the Lower Volsci Unit (cf. Figure 2). In particular, within the Chaotic complex, we have mapped several lenses of Cenomanian to early Campanian limestones covered by middle Campanian karstic breccias and ferruginous to limonitic cruston (Figure 5; structural details in Section 4.2.2).

**Figure 5.** Sampled lithologies of the top of the platform and Chaotic complex. (**a**) Carpineto Romano (Pian della Faggeta; cf. Figures 6–8), encrusted top of platform crossed by E-trending thrust grooves and later veins having growth-fiber lineations plunging towards the NE (corresponding to plot 3 in Figure 8; 41◦34 51" N/13◦6 30" E); (**b**–**c**) encrusted native block within the Chaotic complex; (**d**) Campanian breccia beneath cruston; (**e**) Sampling site of the top of the Lower Volsci Unit north of Caccume Mt. and inherited paleo-topographic reconstruction; (**f**–**g**) outcrop detail of the cruston and underneath discordant units. (**h**) example of discordant Santonian-Campanian breccia beneath Chaotic complex. (**i**) Small-scale dykes of the grooved top platform cruston (41◦34 32" N/13◦14 5" E); (**j**,**k**) lower Miocene blocks 41◦34 33" N/13◦13 20" E; (**l**) Tortonian turbidites from Caccume Mt. north.

Differently from the native blocks, the Scaglia-type pelagic to hemipelagic limestones (with rare planktonic foraminifera and iron oxides) occur as exotic inclusions. In this

category, at Carpineto Romano and Caccume Mt. (Figures 2 and 5; Appendix B), we have found CBZ blocks of early Miocene age represented by red dots (iron oxide spherules) glauconitic calcarenite associated with micaceous intercalations and chert (Figure 5). Minor lenses of hemipelagic middle Miocene marl and sandstone occur as well. Overall, the blocks are wrapped within a sandy-clayey matrix that is alternated with shales, foliated brownish marl, greenish arenaceous beds with exotic lithic, and coarse-grained micro-conglomerate with carbonatic and crystalline elements.

The matrix of the Chaotic complex at the base of the Caccume and Siserno mounts, includes Paleocene-Eocene, Oligocene-early Miocene, middle Miocene, and perhaps also late Tortonian-Messinian nannofossil assemblages (Appendix B). A similar wide span of ages was obtained from the shaly units of Colle Cantocchio (Figure 2), where Mesozoic to Tortonian nannoplankton reworked specimens were found beneath a major thrust (Appendix B; see also Section 4.2.1).

In the Latin Valley, the nannoplankton from the Frosinone Fm. can be referred, although rare or hardly diagnostic, to late Tortonian time. Wedge-top conglomerate deposits were studied at two key localities. At Gavignano (Figure 3), folded calcareous conglomerate occurs atop karstified Cenomanian limestones that according to the well data are juxtaposed on arenaceous deposits (cf. Figure 3). The clasts of mixed origin are from the Upper Cretaceous carbonates (i.e., Coniacian-Campanian and Albian-Cenomanian; see also Farinacci, 1965) and from the Tortonian Orbulina Marl Fm. The embedding matrix is made of abundant quartz grains along with reworked *Amphistegina* and *Elphidium* that make it possible to refer the whole Gavignano clastic deposit to the MVP unItal. In particular, the fining upward series with rare sandy matrix at the base (LEP10L) are dated to the latest Tortonian-earliest Zanclean and the clay marl at the top (LEP10M) to the Messinian. Thus, we consider this topmost constrain as indicative of the Messinian age of the MVP unit in the Latin Valley.

Within the eastern Lepini backbone, the conglomerates of Gorga are composed of pebbles and rounded blocks of reworked conglomerates whose clayey matrix and a bioturbated marly pebble were investigated. The age of these samples is late Tortonian for the marly pebble due to the presence of the coccolithophore *Discoaster surculus*, and top Tortonian–earliest Zanclean for the clay matrix bearing the marker *Amaurolithus primus*.

#### *4.2. Structural Analysis of the Volsci Range*

In this section, we document the field data used to reconstruct a geological cross section across the northern Volsci Range. The Western Lepini Mounts essentially consist of a 3 km thick Jurassic to Cretaceous carbonates dipping to (E)NE, whose local variations are shown in the stereoplots from 1 to 6 in Figure 6. The Neogene lithostratigraphic units atop are locally preserved beneath a few klippen structures that we document in detail in the next paragraphs. In the map and in the cross section of Figure 6, two areas are highlighted and described in detail as they preserve novel insights about pre-orogenic and syn-orogenic tectonics, which are presented from the oldest to the youngest event.

Near the western edge of the Western Lepini Mounts, a detailed survey performed at Colle Cantocchio allowed us to update the previous work by providing details on the stratigraphic contacts and fault kinematics (Figure 7). In particular, we integrate the data from in [93] by describing the pre-orogenic contacts and the low-angle fault juxtaposing Cretaceous rocks onto the Orbulina Marl Fm. As we can see from the panoramic view and cross section (Figure 7), lower Cretaceous calcareous dolostones (LK) are juxtaposed to a thick Jurassic-Cretaceous succession. The LK unit is downthrown towards the WSW and it overall consists of a striated proto-cataclasite of a normal fault (in orange). The fault has a cut off angle of about 40◦ with the footwall bedding. On top of this fault (paleofault, orange line in Figure 7), patches of lower Miocene CBZ occur sealing the contact (see Section 4.1.2). At the contact, an oxidized bluish rim of Mesozoic limestones marks the paleoescarpment (yellow dotted line in Figure 7), which is surrounded by altered shales (late Serravallian-Tortonian pp. Orbulina Marl Fm).

Such an inherited tectono-stratigraphic setting is preserved at the footwall of a thrust, whose hanging wall consists of a one-hundred-meter-thick pile of Upper Cretaceous (earlymid Campanian) limestone, and whose base constitutes the roof of a cave. The cave is defined by an iron oxide-rich striated principal slip surface. In the hanging wall, cataclastic bands are crosscut by minor mirror-like faults.

**Figure 6.** (**a**) Geological map of the western Lepini sector. (**b**) Stereoplots (lower hemisphere projection, equal area) summarizing orientation data for the structural elements representative of the subdivided areas in panel (**a**). Eigen vectors are indicative of the orientation of the axes of deformation calculated from the bedding, where the gray circles represent planes that contain the intermediate and maximum eigenvectors, as shown also by the data reported in the supplementary material. (**c**) Cross-section of the Volsci Range limited to the Malaina Mount to the northeast.

**Figure 7.** (**a**) Geological map of Colle Cantocchio modified after Cocozza and Praturlon (location in Figure 6 [93]). (**b**) Structural overview looking eastward. Blue line: thrusts and transpressive faults; yellow dotted line: paleoescarpment unconformity below Middle Miocene terrains (T); orange line (paleofault). (**c**) Larger geological cross section from Figure 6 and detailed (**d**) cross section (bold line traced in panel (**a**)) with stereoplots (lower hemisphere projection, equal area) of faults with slickenlines measured at the paleofault and in the roof of the cave. (**e**) Detail of the paleoescarpment contact of the pebbly calcarenite (**f**) over the hardground composed of oxidized Upper Jurassic peritidal limestones (41◦34 29" N/13◦0 9" E); (**g**) Polygenic breccia composed by Miocene and Cretaceous calcareous clasts with a reddish cement and calcareous matrix. (**h**) Cave details, grooved-base thrust fault zone constituted by foliated cataclasite bands (**i**,**j**). Sampling sites are referred to Appendix B.

**Figure 8.** (**a**) Geological map of the Lower and Upper Volsci Unit deformation preserved between Pian della Faggeta and Occhio di bue localities (Figure 6) modified after [72] and the related geological cross section D-E in panel (**b**). (**c**) Stereoplots summarizing orientation data for the structural elements representative of the different key outcrops (from 1 to 6 in panel a). (**d**,**e**) Hanging wall and footwall of a (E)NE-directed thrust occurring in a cave near the top of the platform RTDb limestone (corresponding to plots 1–2; 41◦34 48" N/13◦6 21" E). (**f**) S/C top-to-the NE structures affecting lower–middle Miocene limestone and marl lithotypes (41◦35 18.18" N/13◦6 16.40" E). (**g**) Detail of the (E)NE verging fold (41◦35 18.96" N/13◦6 19.28" E) and striated bedding (41◦36 11" N/13◦5 36" E) of the Upper Volsci Unit (corresponding to plot 6). The sketch on top left shows the geometry of the outcrop that consists of a fault-propagation fold later tilted towards the foreland to the NE.

As constrained by nannoplankton analysis on samples from the fault core, both clasts and matrix (see Appendix B) are representative of different levels of a basinal sedimentary succession. The cataclasite also includes fragments of calcite mineralizations. The internal fabric is marked by the occurrence of slip surfaces associated with transpressive S/C structures indicating top-to-the-NE thrusting. Overall, the thrust seems to cut up-section although bounded and possibly tilted by later normal faults. The NW edge of the cave is bounded by a NE-striking normal fault with a displacement in the order of 20–40 of meters (red line in Figure 7h). At the top of the hill, the overall structure is topped by transgressive polygenic marine breccia composed by Miocene and Cretaceous calcareous clasts with a reddish cement and calcareous matrix, possibly crosscut by a SW-dipping normal fault with a displacement in the order of 150 m.

#### 4.2.1. Thrusting at the top of Lower Volsci Unit

Figure 8 summarizes the kinematic indicators affecting the top of the Mesozoic platform and the Chaotic complex in six localities at the top of the Mesozoic succession of the Lower Volsci Unit in the Western Lepini Mounts.

Starting from the base of this deformed area, the Hippuritid and Radiolitid limestone (Campanian RDTb; Appendix B) of the Lower Volsci Unit is affected by bedding-parallel proto-cataclasite bands crossed at low-angle by striated curvy fault mirrors with dm<sup>2</sup> to m2 dimensions (Figure 8). Across the most evident fault mirror (Figure 8), both footwall (plot-1) and hanging wall (plot-2) are characterized by top-to-the-NE slicken fibers, measured also on smaller fault mirrors. Crustons are disconformably topped by veined and laminated beige sandy calcarenites (plot-3). The thin carbonate blocks embedded in the Chaotic complex at Pian della Faggeta (plot-4) have variable thickness (up a few meters thick) and limited lateral extent (up to some dozens of meters). The native carbonate lithons are internally deformed and in places, display a sharp contact at their base with the siliciclastic units, and can be internally affected by top-to-the-(E)NE asymmetric folding. On the top of some of these slices, E-trending thrust grooves are cross-cut by NE-stretching mode-I veins. Beside the dominant NE-stretching, provided by the fiber direction of veins, more to the south (plot-4, Figure 8), veins crossing carbonate slices in similar structural positions also show NW-directed stretching.

At Occhio di bue locality (plot-5), a block of middle Miocene limestones and marls with chert topped by light green clay of late Serravallian age (c.f., Cosentino et al., 2003) is affected by S/C structures indicating top-to-the NE shear. Coherently, at the contact with the Cenomanian limestone on top, 1–2 m of foliated proto-cataclasite bands are topped by (E)NE verging folds (plot-6; Figure 8). In the same plot, top-to-the-NE striated bedding is reported as it crops out more to the north at the top of the same lithon. While bedding is folded around N- to NNW-striking axes (cf. stereoplots 7–8; Figure 6), northeast of a major backthrust it is folded around NW-striking axes of folds (stereoplots 9–10).

As the Chaotic complex is concerned, field data from the Eastern Lepini Mounts highlight the top-to-the-ENE juxtaposition of the Upper Volsci unit above the Chaotic complex (i.e., Caccume Mt., Siserno Mt.), which in the Volsci Range is preserved in a few klippen atop the Lower Volsci Unit, whereas in the Latin Valley it is found on top of the Frosinone Formation (Figures 9a and 10a). At the Caccume Mt., we report structural information from the juxtaposition of folded Cenomanian Lower Cretaceous limestone on the Chaotic complex. The regional folding affecting the Lower Volsci Unit defines a well-marked NW-striking open fold while the Upper Volsci unit of the Caccume Mt. displays rather dipping beds folded around an NNW-striking axis. The basal contact of the Chaotic complex is marked by thrust grooves and ferruginous faint slicken lines along the crustons, while at the top of the Chaotic complex, S/C and C' structures display topto-(E)NE shearing. Cross-cutting field relationships show that thrust grooves are further cross-cut by high-angle en-échelon shear zones and normal faults.

**Figure 9.** (**a**) Geological map of the Eastern Lepini sector and part of the Latin Valley. (**b**) Stereoplots (lower hemisphere projection, equal area for locations 11–20; numbering following after Figure 6) summarizing orientation data for the structural elements representative of the areas in panel a). Plot-13 shows E-striking folds interposed in the frontal thrust zone near Morolo, while plot-18 represents the N-S trending flank of a salient associated with transpressive S/C structures of Plot-19. (**c**) Sketched geological cross section and structural overview of the Volsci Range front (Caccume Mt. lower and upper unit, respectively, correspond to plots 16 and 17). Normal faults dip towards the NE, crosscut the Upper thrust. Sampling sites are reported in Appendix B. (**d**) Caccume Mt. front, detail of the encrusted top of the platform affected by E-trending D1 grooves and later crossed by oxides-rich (D2+3) en-echelon fractures and later NW-striking oxides-free and cemented veins; 41◦34 46" N/13◦13 60" E). (**e**) Upper thrust juxtaposing the Cenomanian neritic limestone over the Chaotic complex (41◦34 15.00" N/13◦13 55.13" E), which, as shown as the sampling site of LEP67 on a lithotype that in panel (**f**), is affected by top-to-the-(E)NE S/C structures.

**Figure 10.** (**a**) Structural overview over two frontal klippen of the Latin Valley cropping out at Siserno Mt. where the Chaotic complex is juxtaposed to the Frosinone Fm. (**b**) Near Frosinone, an unconformity subdivides folded FFS units from the channelized facies on top**.** (**c**) Detail of the unconformity. (**d**) Vertical pelitic-arenaceous succession with (**e**) bioturbated levels. (**f**) Structural overview of the Gavignano area with stereoplot (lower hemisphere projection, equal area) of bedding and eigenvectors, that are indicative of the orientation of the axes of deformation related to the MVP thrust top conglomerates of Gavignano with (**g**–**i**) location of sampling localities. Conglomerates at the base are affected by pressure solutions and in the most calcareous beds also by veins. Sampling sites are referred to the Table in Appendix B.

4.2.2. The Volsci Range Thrust Front and the Latin Valley Structures

The geometries of the frontal part of the Volsci Range and Latin Valley are shown from the SW to the NE (stereoplots 11–15, Figure 9). The thrust front between the Ernici and Lower Volsci units occurs as a series of imbricates of overturned Cretaceous to CBZ layers (i.e., NW of Morolo; Figure 9). New data allowed us to recognize a salient at the front of the Eastern Lepini Mounts. This structure is accompanied by a change in the fold trend from NW to W (plots 12 and 13; Figure 9) and by transpressive top-to-the-NE kinematics. The frontal part is defined by a large-scale anticline in the west and a syncline in the east (Figure 9). The two folds are separated by a series of NNW-striking tear faults with inferred right-lateral kinematics (Figure 9). More to the east (plot-18), the N-S trending flank of the salient is associated with transpressive S/C structures in Cretaceous limestones (plot 19). Overall, the fold-and-thrust fabric is cross-cut by NE-dipping normal faults at the northeastern VR edge. As it is downfaulted, the thrust front does not outcrop further north. In the VR, a salient has been mapped between Morolo and Patrica (Figure 9), its most external point being characterized by the outcrop of Jurassic limestones. Upper Cretaceous units occur as klippe above the imbricated Chaotic complex juxtaposed to the foredeep deposits of the Frosinone Fm.

At the southern edge of the studied area of the Latin Valley (Figure 10a), the Chaotic complex was mapped as juxtaposed on the Frosinone Fm., and it reaches its maximum thickness west of the Siserno Mt. (about 250 m).

There (Figure 10a), we identify two thrusts: one juxtaposing the Upper Volsci Unit on the Chaotic complex (white dashed line) and the other juxtaposing the Chaotic complex onto the Frosinone Formation (black thrust). At Frosinone, a new road cut exposes a major intraformational unconformity within the Frosinone Fm. (yellow dotted line, Figure 10b, c) between folded layers beneath and sub-horizontal channelized deposits atop.

The channelized facies is made of arenaceous-pelitic associations with sets of thin pelitic-arenaceous and marly beds intercalated in thick massive arenaceous-pelitic layers. Southwest of Ferentino, paleocurrents are marked by a NW–SE direction, whereas the Frosinone formation is internally deformed and displays verticalized to overturned successions (Figures 9 and 10). There, the facies consists of an arenaceous association of amalgamated massive beds with arenaceous-pelitic and pelitic-arenaceous sets. As shown on the map (Figure 3), north of Sgurgola and north of the Siserno Mt., an anticline with upper Cretaceous and CBZ limestone belonging to the Ernici Unit emerges from the Latin Valley siliciclastics, which are locally bioturbated. In the syncline between this ridge and the Volsci Range, pelitic facies of the Frosinone Fm. occur.

At Gavignano (Figure 10f), the MVP Messinian calcareous conglomerate occurring on top of the Upper Volsci Unit overthrusting the Frosinone Formation is folded along an NNW-striking axis and is near vertical in places. In the most calcareous layers, pressure solution seams and veins crosscut the pebbles as typical of load-driven compaction.

#### 4.2.3. Backthrusts and Normal Faults

Backthrusts best crop out in the northwestern part of the VR, where their presence is highlighted by some pockets of Messinian-earliest Pliocene heterogeneous conglomerate (Figure 11). Transpressive kinematics associated with a general top-to-the-(E)SE sense of thrusting was observed on the reverse faults along the Montelanico-Carpineto Backthrust. As typical of cannibalized wedge-top basins, blocks of conglomerates occur within a marly-conglomeratic matrix near Gorga (Figure 11).

In Figure 11, we sketch the structural setting related to the backthrusts, which cross-cut and preserve the top-to-the-(E)NE Chaotic complex at the footwall of the Montelanico-Carpineto Backthrust. This major backthrust (i.e., Montelanico-Carpineto Backthrust) bounds the East Lepini structure, a large-scale anticline with its culmination at the Malaina Mt. (Figures 6 and 11). The backthrust is accompanied by recumbent folds and minor high-angle reverse faults. In the southwestern sectors of the VR (Figure 11), normal faults cross-cut older contractional structures. More to the SW, another high-angle backthrust

was mapped west of Bassiano (Figure 6). This structure allows the juxtaposition of the Jurassic and Early Cretaceous carbonate onto the upper Cretaceous and it is defined by transpressive kinematics (stereoplots in Figure 6).

**Figure 11.** (**a**,**b**) Structural overview of the backthrusts in the northern Volsci Range with sampling sites (see Appendix B) and stereoplot (lower hemisphere projection, equal area) of bedding and backthrusts. (**c**) Thrust zone detail. (**d**) Block of conglomerate within conglomerate with clayish marly matrix. (e) Pebble of bioturbated marl with chondrites. (**f**,**g**) Structural overview of the Lepini sector and the Montelanico-Carpineto backthrust continuation towards the south beneath the Eastern Lepini Pop-up.

Along the southern slope of Semprevisa Mt. (i.e., the Semprevisa Fault), a major normal fault dissects the whole Jurassic-Upper Cretaceous succession, while along the northern slope, the top of the Mesozoic succession is overthrust by Upper Cretaceous units (documented in depth in the following sections). To the southwest, stepwise segments of normal faults bound the Pontina Plain (Figure 2). Further to the northeast, domino-like blocks are bounded by 2–3 km spaced faults, each with about 0.5 km downdip offset. More details on the Quaternary fault system are in [45].

#### *4.3. Seismic Interpretation of the Latin Valley*

By tracing the reflectors of the unconformable contact between the Meso-Cenozoic carbonates and the upper Miocene siliciclastic deposits on top (cf. Section 3.4), two major seismic units were recognized in the subsurface of the Latin Valley: (i) the Upper Ernici unit and (ii) the Lower Ernici unItal.

The Upper Ernici unit crops out at Ceccano (Figure 2), and northwest of Morolo (Figure 9), where it constitutes a carbonate ridge in the middle of the Latin Valley. Coupled seismic and field geological evidence shows that the ridge is represented by detached Upper Cretaceous carbonates topped by a thick CBZ succession sealed by UAM and FFS units. The Upper Ernici Unit was drilled by the Frosinone 1, Ripi I, Ripi II, Pofi 1, and Ceprano 1 wells (Figure 3). This thrust-bounded unit is composed of a stratigraphic succession that can be correlated with the upper units of the Gavignano-1 well.

The Lower Ernici unit, apart from the distinctive near-top reflections, displays a variable amplitude and frequency with a discontinuous and chaotic pattern of reflectors that generally is characterized by noisy seismic facies. We exclude that this reflector is a coherent noise (multiple) as it can be followed over the entire study area and it displaces geometries that roughly differ from the above reflectors. Due to the scarce penetration of the seismic signal, this unit can be considered as the acoustic substratum of the area. No boreholes reached this unItal. By comparison due to our reconstruction of the thrust geometry, the top of the Lower Ernici seismic unit is possibly represented by the Meso-Cenozoic carbonates that crop out northeast of the Latin Valley (Figure 2). Due to the above reported uncertainty, marks indicate the less-constrained portions of the interpreted cross sections.

Within the Latin Valley, minor thickness changes of the carbonate tectonic units occur. Due to the repetition of the top-CBZ reflector accompanied by underlying top-UK reflectors, we have recognized multiple repetitions of the Upper Ernici unit due the occurrence of several thrust faults. The Ripi I well [106]), although crossing a major thrust zone, shows no siliciclastic deposits under the Mesozoic carbonates, but rocks of the Orbulina Marl and CBZ formations.

To show the general structural trend of the research area, we present three representative seismic lines (Figure 12), constrained by field and borehole data, showing thrust sheets characterized by a general top-to-the-NE sense of shear. Major thrusts, although occurring in all of the seismic lines, are well evident but discontinuous in number and distribution from line to line. Four major groups of thrusts form before the occurrence of normal faulting (Figure 13). From the most internal to the outermost we describe them as (1) the first group (thrust-1) marks the juxtaposition of the Chaotic complex on top of the FFS units and it can be correlated with the Upper Volsci thrust. (2) Thrust-2 marks the translation and doubling of the Upper Ernici unit within the Frosinone foredeep domain. No clear indication of the front could be recognized in the study area, possibly due to subsequent erosion. This structure is also represented by a series of thrust splays that cross-cut the formerly formed the fold-and-thrust fabric. Carbonate thrust-sheet units as thick as 0.6–0.8 s intervals have undergone significant translation in the order of 20–25 km. Considering that no thrust ramp could be observed toward the SE, this is a minimum estimate calculated on the hanging wall flat. (3) Thrust-3 is a group of reverse faults with flat-ramp-flat geometries that involve both the Upper and Lower Ernici units. The thrust-3 records a minimum offset in the range of 5 to 8.5 km. (4) The latest reverse faults belonging to the thrust-4 include the backthrusts at the northern edge of the Latin Valley. Such backthrusts cross-cut the previous 1–3 thrust faults and allow the formation of a triangle structure, during the deposition of the MVP deposits in the structural lows. In the southernmost section (Figure 12; Section 3), the cut-off relationships provided by the latest thrusts may have allowed the exposure of Thrust-2.

**Figure 12.** Two-times travel (TWT) seismic lines (from www.videpi.com (accessed on 20 January 2021); below, interpreted), also showing the projection of the wells. The location of both wells and seismic line traces of Section 1 (line label FR-309-80), Section 2 (FR-306-82), and Section 3 (FR-302-80) are in Figure 3. The vertical gray stripes highlight the Lower Ernici UnItal.

**Figure 13.** (**a**) Top-platform unconformities related to the upper (orange arrows) and lower Ernici units (green arrows) (Dip seismic line); (**b**) detail (right, interpreted) showing the angular unconformity between the Lower Frosinone seismic subunit (FFS1) and the Upper Frosinone seismic subunit (FFS2); (**c**) W-E view (Strike seismic line), showing the lateral variability of seismic facies FF1 and FFS2. (**d**) FR-314-82. Strike view of the Gavignano klippe, the purple dashed line marks the thrust onto the Frosinone Formation (transparent facies FFS), while the yellow dotted line highlights the top reflectors of the carbonates with the MVP conglomerate atop. Seismic line traces and well location in Figure 3.

> The most prominent of this group of thrusts generates the outcrop of basal platform at the foothill of the VR Front. A few backthrusts were recognized at depth, with vertical displacement up to 1–2 km. In Figure 12, normal faults with appreciable offset were identified (labeled with number 5). NE-striking faults concentrate at the Latin Valley edges and do not clearly show in seismic lines. NW-striking faults bound Quaternary graben, where travertine, continental, and volcaniclastic deposits were cumulated. The normal fault trace in seismic lines was drawn when it is anchored to the outcrop evidence. In

these cases, we have extended the minimum offset recognized at surface to the deeper structural levels.

The most distinctive unconformities occur at the top of the Mesozoic carbonate succession and above the Middle Miocene CBZ Fm., onlapped by late Serravallian-early Tortonian UAM horizons (Section 3 in Figures 12 and 13). At the borehole scale this contact may appear as a paraconformity but the discontinuous and variable thickness of both CBZ and UAM suggest that this is actually an unconformity with an irregular erosional surface. Three subunits, divided by two major unconformities, can be observed within the siliciclastics deposits and labeled as Lower Frosinone seismic subunit (FFS1), Upper Frosinone seismic subunit (FFS2), and Monte San Giovanni Campano seismic unit (MVP); the first two are made by the late Tortonian Frosinone Fm., while MVP is formed by the Messinian piggyback deposits (Monte San Giovanni Campano unit; see MVP in Figures 2 and 12).

The thickness of the syn-orogenic units varies depending on the fold-and-thrust belt structure, being the siliciclastic deposits thicker to the south and to the north (up to 0.600 sec) and thinner in the central part (usually limited to 0.180 sec). As shown in Figure 13, Subunit FFS1 is folded together with the underlying carbonates, showing a transparent seismic facies, while Subunit FFS2 is thicker in the syncline and thinner towards the anticline and it is possibly related to Thrust-2. In FFS2, minor internal unconformities, typical of syn-depositional antiforms in foredeep basins, are here expressed by lobatetype seismic facies. In detail, the antiformal-growth geometries are crestal erosional truncations and diverging/converging reflection patterns around the hinge of the anticlines. In the piggyback basins, the FFS2 is defined by well-reflecting horizons and is marked by an erosive unconformity that at Ceprano cross-cuts both FFS1 at anticline culminations (Figure 13). This anticline is sealed by FFS2 and is formed on top of Thrust-3. As shown by the strike section in Figure 13, the thrust-and-fold geometry changes laterally as also reported for the Gavignano klippe more to the north.

#### **5. Discussion**

The tectono-stratigraphic analysis of field and subsurface data enabled us to define different thrust units, providing insights for a time-deformation analysis of one of the innermost portions of the Central Apennines. Hereby, we present a geological cross section, interpretative of the deep structures produced after the integration of field and subsurface structures (Figure 14), that includes pre-orogenic passive margin deposits, mélange units, foredeep, and wedge-top deposits. In the following, we discuss the main novel features of the geologic history that led to the development of the geological setting of Figure 14. In the cross section, we correlate the Upper Volsci Unit remnants of the Colle Cantocchio, Carpineto Romano, and Caccume Mt klippen. Based on the mixed exotic-native composition of the blocks of the Chaotic complex, we recognize that they were overthrusted together with the Upper Volsci Unit on top of the Lower Volsci UnItal. As shown in the cross section, the Lower Volsci Unit of the Western Lepini Mounts is a monocline essentially composed of Jurassic to Cretaceous carbonates dipping to (E)NE, that together with the remnants of the upper units was further crossed by high-angle faults. In detail, the Montelanico-Carpineto backthrust, bounds the Eastern Lepini pop-up that is affected by small-scale folds and reverse faults, whose geometry suggests positive reactivation of pre-orogenic normal faults during shortening. The wedge-top pockets preserved by the backthrusts are infilled by MVP Messinian conglomerate that was deposited directly on the Lower Volsci Unit, when the Upper Volsci unit was already dismantled. Thrusts and folds are mostly evident in the Latin Valley (Figure 12), whose substrate has been reconstructed by applying a depth conversion on a structural model published in [45].

By studying the top of the Mesozoic carbonate platform both in the Lower Volsci Unit and in the blocks embedded in the Chaotic complex (Appendix B; Figure S1), we have reported the occurrence of an irregular surface at the top of the platform. Such a paleotopography was likely the result of Late Cretaceous syn-sedimentary tectonics. In such scenario, the most elevated structures might have been affected by karstism (possibly with

the formation of ferruginous crustons) during the latest Cretaceous (see Section 4.1). The occurrence of a Late Cretaceous tectonics is supported by the lithostratigraphic unit we refer to the "Gorga bioclastic limestone and dolostone" upper Campanian to Maastrichtian in age, whose lateral change and abrupt facies shift points to syn-depositional tectonics (Figure 2). At Gorga (Figure 3), this unit is represented by about 250 m thick rock volume [112], that thins rapidly towards the west, whereas it lacks in the rest of the Volsci Range. In particular, as recognized at Caccume Mt. and near Carpineto Romano (Figure 5), the unconformity occurring at the top of the platform is marked by a very thin younger breccia partially overprinted by a dolomitic and ferrougeneous cruston (cf. Figures 5 and 9), whose age and origin need to be further constrained.

**Figure 14.** Geological cross section AB (trace in Figure 3) interpretative of both field and subsurface data converted to depth (see methods). Numbers related to the group of faults are disposed as in Figure 12. In the Volsci Range, the Upper Volsci Unit experiences about 25 km of thrusting (Thrust-1) towards the ENE. Thrust-2 accommodated the overthrust of the Volsci Range and Upper Ernici Unit on top of the Frosinone Formation. In Latin Valley, the Upper Ernici unit is doubled by the breaching of Thrust-3. Late reverse faults (Thrust-3 and -4) contribute to forming a triangle zone in the Latin Valley and backthrusts in the rear. Normal faults generate a graben in the Latin valley and SW dipping faults towards the Pontina Plain.

> In the Apennine platform, the transition from the Upper Cretaceous carbonates to Paleocene–Eocene margin, slope, and Scaglia-type basin deposits was guided by a synchronous regional extension during Maastrichtian–Eocene time that affected both the Jurassic base-of-slope domains [30] and the demised sectors of the neritic platforms [118]. We recognize that the discordant stratigraphic contacts of Colle Cantocchio are due to the development of a submarine paleoescarpment, guided by normal faults down-stepping towards the WSW. The bluish hardground (highlighted by yellow dots Figure 15) can be interpreted as a submarine unconformity marking the onlap (escarpment contact) of the lower Miocene intraformational pebbly calcarenite on the Mesozoic carbonates. Similar facies have been reported elsewhere by the authors of [119] and are here interpreted as a diagenetic effect on the articulated inherited physiography of the previously unedited fault escarpment described in Figure 6. A simplified back-restoration of section C-D (Figure 7c) is attempted in Figure 15, where a fault step occurred to the south with an offset in the order of 700–1000 m due to the exposure of the Jurassic terrains and the downthrowing of the Cretaceous units in the hanging wall. The Semprevisa Fault can be still recognized laterally for over 10 km, although overprinted by later Pliocene-Quaternary tectonics, and possibly remarks at least part of this inherited structure. In our interpretation, as shown by the stratigraphic contacts, the Jurassic units of the southwestern slope of the Semprevisa Mt. were already exposed in early Miocene time (Figure 15). As suggested by the clasts within the Chaotic complex, coeval basinal sedimentation occurred more to the WSW [120]. In particular, the recognition of Cretaceous-Paleogene Scaglia lithotypes and of distal early Miocene CBZ limestones in the exotic blocks of the Chaotic complex (see Figures 7, 9 and 10) suggest that sedimentation occurred in a bypass slope setting during Paleogene-Neogene time. In particular, the Paleogene is recorded by a condensed to hemipelagic sedimentation, evolving during the Miocene to mixed calcareous-siliciclastic turbidites with chert. The Orbulina Marl Fm. (Serravallian pp.) sealed the pre-orogenic early Miocene topography.

The Colle Cantocchio pre-orogenic fault is a part of the normal fault system that produced the steps from the exposed Jurassic carbonates to the basin and is here proposed to be at least Eocene in age, although older ages cannot be excluded. Synthetizing, according to the new data, we propose a provenance of the Chaotic complex (i.e., including the exotic blocks) from a hemipelagic paleogeographic domain with slow depositional rates placed to the WSW of the present-day Volsci Range.

**Figure 15.** Reconstruction of the ramp-flat geometry of the upper thrust after restoration of section C-D by removing late backthrusts and normal faults. During Tortonian time, inversion tectonics of inherited structures occurred on a ramp by the overthrusting of the Upper Volsci UnItal. At the transition from ramp to flat, native blocks were scraped off from the Lower Volsci UnItal. On the right, peculiar settings inspired by field examples are contextualized to understand the mélange formation. During Serravallian time, the inherited structure was sealed by Orbulina Marl hemipelagic deposits. To the southwest, base-of-slope to basinal Cretaceous-Miocene deposits occurred on a fault-controlled step of the platform. On the right, a detail of the paleoscarpment setting prior to thrusting.

The ongoing research in the southern Volsci Range, is providing constraints for the determination of the age of the encrusted normal faults bounding the Formia plain and Spigno Saturnia areas, whose data from the literature are reinterpreted above (cf. Figure 2). A comparable syn-sedimentary setting, leading to the deposition of Scaglia deposits has been recorded nearby the VR [8,121] and documented at the western tip of the Volsci Range [122]. Of note, at Colle Cantocchio (Figure 5), the early Miocene transgression over the Jurassic-Lower Cretaceous rocks occurred on a step of the escarpment, where there was no record of Paleogene basinal sedimentation. In alternative, this sector could be associated with renewed normal faulting activity along a pre-existing Cretaceous-Paleogenic normal

fault, which may have further exposed the Mesozoic rocks with its reactivation and allowed the CBZ-UAM units to settle on top prior to the Tortonian onset of thrusting.

#### *5.1. Chaotic Complex Emplacement and Thrust Propagation*

To define the overthrusting towards the (E)NE of the Upper Volsci Unit and to understand the evolution of the Chaotic complex, we correlated the carbonate klippen by documenting the stratigraphic and structural elements of the syn-orogenic deposits. This correlation was initially proposed by Accordi [71], but inherited structures, thrust kinematics, and age of the syn-orogenic deposits needed to be better constrained. With the degree of allochthony and origin of the Chaotic complex being long debated [45,64,67,86,123], in this section we discuss the Chaotic complex origin and the role of the thrust propagation towards the foreland into the late Miocene wedge growth.

Starting from the southwest, the Colle Cantocchio cataclasite and shale preserved underneath the Upper Volsci Thrust can be interpreted as a thin Chaotic complex unit juxtaposed on the paleo escarpment setting (c.f. Section 5.1). In this frame, the inherited topography produced a ramp in the upper thrust during shortening. A comparable setting occurs more to the south at the Vele Mt. (Figure 2), where the siliciclastic deposits underneath the thrust could be correlated with the Chaotic complex sliver of Colle Cantocchio (Figure 15). As commonly occurring in mélange complexes [124,125], the Chaotic complex formed at the expenses of the Lower Volsci Unit, whose inherited and articulated top was scraped off and grooved (see Figures 7–9). The Chaotic complex is a combination of (i) autochthonous "native" and (ii) allochthonous "exotic" blocks (Figure 15). The latter derive form a discontinuous series of Paleogene-Burdigalian pelagic deposits deposited more to the south and progressively mixed with lower Serravallian to upper Tortonian siliciclastic units bearing also crystalline clasts.

In particular, the matrix of the Chaotic complex shows the same composition of the embedded blocks, but it also shows the occurrence of late Tortonian-Messinian nannofossil assemblages, which may have deposited during the final stage of thrusting related to the Upper Volsci Thrust. Further, we are able to further narrow this time range to the late Tortonian, considering also the absence of *Amaurolithus* sp., typical marker of top Tortonian-Messinian. Provided that the overthrust of the pelagic elements of the Chaotic complex is due to the juxtaposition of the Upper Volsci Unit, which squeezed them out towards the foredeep, they must have originated from about the same distance reached by the Upper Volsci Thrust front (Thrust-1).

In this frame, the SE-ward termination of the Chaotic complex and the lens-like shape of the outcrop at Carpineto Romano (Figure 6) provide an example of interaction between inherited top-platform physiography and thrust geometry. In our interpretation, this structure is an inherited depression at the top of the platform that was later crosscut by the Upper Volsci Thrust. At its southern tip, as demonstrated by Accordi [71], this thrust still occurs as it doubles of the upper Cretaceous units although not involving anymore the Chaotic complex, whereas, as shown on the map (Figure 6), at the northern of the Upper Volsci Thrust, the younger Montelanico-Carpineto backthrust cross-cut it (Figure 11).

The Upper Volsci Unit is mainly composed by Upper Cretaceous neritic carbonates (e.g., Carpineto Romano, Figure 8), implying that this unit detached essentially above the uppermost Lower Cretaceous Orbitolina Marl level during shortening. However, although rare, older Mesozoic rocks can also be found. A second detachment level, highlighted by subsurface data, corresponds with the Orbulina Marl Fm, which allowed the doubling. The chronological relationship between Thrust-1 (marking the overthrust of the Upper Volsci Unit on to the Upper Ernici unit) and Thrust-2 (between the Ernici Units of the Latin Valley) is beneath the resolution of our data. However, provided their geometrical distribution, these thrusts are likely to represent a classical thrust propagation towards more external and lower structural levels through time (i.e., towards the foreland). The minimal shortening associated with Thrust-1 is of about 25–30 km, which corresponds with the approximated present-day distance between Colle Cantocchio and the frontal klippe

along the ENE-directed Thrust-1; while Thrust-2 ranges about 20 to 25 km as shown by the thrust-2 structures in Figure 14. These amounts are comparable with the shortening estimated at the thrust fronts of the Gran Sasso Massif (>20 km [30]) and of the Apennine platform in the southern Apennines (>60 km [126]), while it is significatively lower than the translation that affected the Ligurian Accretionary Complex onto the foredeep units (> 100 km [127]). In this frame, the Tortonian southern Apennine platform thrusting [28] matches our thrust dynamics (Figure 15). As also typical of the far-traveled Sicilian platform units [128], the thrust geometry is characterized by long flats (10–15 km) and thin thrustsheets, that in our case can be as thin as about 0.7 km near the front. This implies that the Orbitolina level and Orbulina Marl Fm preferred slip levels were very efficient in allowing far-traveled thrusting.

As shown by thickness and facies variations of the siliciclastic deposits of the Latin Valley, the Thrust-2 shortening stage was accompanied by syn-sedimentary folding of the deposits of the FFS2 seismic unit (Figure 13). In our interpretation, while the unconformable FFS1 contact with the CBZ limestone marks the flexuration of the foredeep, the unconformable contact associated with wedge shape and channelized FFS2 facies marks the growth of pop-up anticlines, thus being representative of wedge-top settings initially developed during Thrust-2.

The channelized facies may be, respectively, representative of syn-tectonic fringe and lobe deposits and of inner channelized sand bodies, while pelitic facies are rather typical of outer fans [129]. In particular, the observed syn-sedimentary folded channelized structures (Figure 10b), show that, the deposition of the Frosinone Fm. thus encompassed an increasing input (mostly during the FFS1 stage), later followed by a progressive channelization of turbidity flows onto the synclines during the FFS2 stage. As already suggested in [130] for the Latin Valley on the channelization of the foredeep to wedge-top sediments, the active margin possibly followed a comparable evolution similar to what elsewhere envisaged in the southern Apennines by Casciano et al. [131].

At the front of our study area, a transition between the mélange and the flysch units occurs. Based on published maps [64], wells, and seismic lines on the southwestern edge of the Latin Valley, we also confirm that the Chaotic complex is juxtaposed to the Frosinone Fm. of the upper Ernici unit (cf. Gavignano; Figures 10 and 13). For this feature, the authors of [132] proposed an olistostrome origin, while Centamore et al. (2007) proposed gravitational sliding of the Chaotic complex off the Volsci Upper UnItal. Further, this level can correlate with the mélange levels of the Massico Mt. [43,133].

To explain the abrupt thickening of the Chaotic complex east of the Caccume Mt. (Figure 10), we suggest that a growth structure was forming during the initial uplift of the Volsci Range front as testified by fault-propagation fold (Figure 14) at the hanging wall of thicker FFS units with syn-sedimentary folds (Figures 10 and 12). This generated the glide of the Chaotic complex on top of the FFS units. Similar contexts were reconstructed for other mélange units at thrust fronts, where the remobilization of the formerly emplaced thrust sheets, allows the incorporation of the extrabasinal (exotic) lithologies within the foredeep [18,134]. An alternative possible explanation to allow the juxtaposition of the Upper Volsci unit onto the FFS units, would envisage thrusting to occur during the uppermost Tortonian-earliest Messinian.

#### *5.2. The Late Stages of Shortening*

As observed in seismic lines (Figures 12 and 13), thrust-3 produced the doubling of the flat of the far-traveled Thrust-2, by involving deeper carbonates in the thrust ramps. We have also shown that in the area break back thrusting occurred [135] (Figure 16). As shown near Ceprano well (Figure 13), MVP wedge-top deposits that include calcareous pebbles from the CBZ unit [87] were directly deposited on Mesozoic carbonates deformed by an anticline. This contact is representative of a wedge with regional subsidence slower than local antiformal growth [136]. Nannoplankton determination finally allowed constraining the age of the folded conglomerates and atop marls of Gavignano, thus allowing a correlation

with the MVP stratigraphic unit (Figure 10). This unit represents a folded Messinian thrust top deposit and this constraint attributes this late folding stage to late Messinian-earliest Pliocene time. As supported by subsurface data (Figures 3 and 13) the Gavignano klippe was involved into the renewed deformation of the VR front, which would correspond with the latest stage of thrusting and veining dated in [43] at the late Messinian on the Massico Mt. (cf. Figure 2). Those absolute constraints can be used to review the regional thrust kinematics. In this sense, the ages determined along the thrusts in areas more to the south can be compared to what provided in [114]. These authors have attributed a late Miocene-Pliocene age to the clayey matrix beneath the thrust at the front of the Siserno Mt. Similar to what reported for the Chaotic complex in this work (Appendix B), they have also reported that the exotic clasts are representative of a wide range of ages, from Late Cretaceous (including Scaglia Rossa pelagic limestone) to early-middle Miocene. The degree of fragmentation of microfauna embedded within the Chaotic complex [114] suggests active deposition during the late Miocene-Pliocene as well. Therefore, we can envisage a late involvement of Pliocene deposits into the reactivated thrust zones at the VR front. In this interpretation, the Chaotic complex was already exhumed likely after the strong erosion related to the Messinian salinity crisis [137–140], which also affected the Ernici Mts [77], implying reactivation in the rear [49].

**Figure 16.** Sketch of relative timing and geometries of fore- and back-thrust involving different generations of thrusts (1–4) within the Apennine wedge through time. Backthrusts generate at progressively lower depths, moving towards the hinterland (to the left), due to the dip of the basal detachment.

> In this context, the late Messinian shortening event could be correlated with the late orogenic structures in the northern VR that are crossed by a series of SW-directed backthrusts (Figure 11). In our interpretation, the SW-directed Montelanico-Carpineto backthrust cross-cuts the top-to-the (E)NE older Upper Volsci Thrust. Despite the lack of valuable data from the main lineament, minor thrusts show that top to the SW-backthrusting,

could be accounted as partially reactivating the older fabric. Further, the fault strike of the backthrusts diverges about 20◦ from the trend of the upper Volsci Thrust that is underthrusted beneath the Eastern Lepini pop-up (Figures 5 and 11).

So far, scarce constraints of top-to-the-SW shear were found, although backthrusting is possibly localized more to the NE of the studied area of Figure 11. Our stratigraphic constraints (Appendix B) from the MVP conglomerate near Gorga, document Messinain Lago-Mare conglomerates that are produced after iterative cannibalization of older wedgetop deposits. The further occurrence of upper Messinian deposits in the Pian della Faggeta area (Figure 5), is a possible clue indicating depositional activity on top of the Volsci Range during the Messinian salinity crisis (5.96–5.33 Ma). During that time, the area was exposed to linear erosion followed by the deposition of sandy gravels that Centamore et al. (2010) dated at the early Pliocene (south of Castro dei Volsci; Figure 3). This implies that the major valleys were already formed before the latest orogenic compressional events affected both the VR and Latin Valley [117]. Field evidence in the rear (Figure 5), suggests the presence of a major backthrust with transpressive kinematics further south, possibly implying that a deeper backthrust affected the southwestern slope of the VR during the early Pliocene. At that time, the Apennines experienced renewed shortening with frontal thrusting accompanied by backthrusting and tilting toward the foreland to the northeast (Figure 16).

During late orogenic deformation, thrust front migrated towards the outermost active margin units (Figure 1), and the inherited fold-and-thrust belt of the external Apennines was folded together with lower Pliocene syn-orogenic conglomerates (i.e., Rigopiano conglomerate [30,78]). Meanwhile, the previous in-sequence structure of the internal Apennines was truncated by triangle zones (Figure 12) and by more internal backthrusts (e.g., in the Volsci Range, Figures 11 and 14).

In our interpretation (Figure 16), the backthrusting roots at deeper levels, by following the dip of the basal detachment towards the backarc. In this sense, moving to the inner parts of the wedge, the inner wedge is remobilized, affecting a larger volume with respect to the external part. In the case of late orogenic deformation affecting only the sedimentary cover, shortening localizes within the weakest stratigraphic levels, possibly by reactivating the décollement of the older fore-thrusts [136,141–145], while in the rear faulting tends to broaden and possibly involve also deeper structural levels.

Finally, Pleistocene to Holocene NW- and NE-trending normal faults deeply affected the fold-and-thrust belt structure. In particular, the almost constant NE-dip shown by the bedding planes of the studied carbonates might be interpreted as the result of the activity of the major NW-striking and SW-dipping listric normal faults bordering the Pontina Plain, which were also documented at depth [146].

#### **6. Conclusions**

This study contributes to constraining the timing of initiation and progressive development of platform-derived thrust sheets, mélange units, foreland, foredeep, and wedge-top sediments of the internal Central Apennines. The main phases of the evolution of the belt are as follows:


juxtaposed as a mélange unit on top of the carbonate platform together with early to middle Miocene calcareous-cherty-siliciclastics. The Chaotic complex also bears highly deformed basinal exotic and native blocks of neritic carbonates, the latter being scrapped off by the overthrust of the embedding Chaotic complex, whose Paleogene-Miocene matrix includes up to Tortonian nannoplankton. Seismic analysis supported by well logs at the regional scale highlighted repeated carbonate thrust sheets that have first been involved into an initial in-sequence propagation towards the foreland to the ENE occurred during foredeep to wedge-top sedimentation.


Finally, our findings bear implications on platform derived thrust sheets associated with active margin successions and mélange units. The far-traveled thrust sheets, hereby documented both in the field and in the subsurface, constitute a key aspect for the development of the internal Apennines, whose degree of allochthony and role of inherited structures was long debated. Furthermore, at the light of our new interpretation, the deeper platform units could be a new focus for hydrocarbon accumulation and may provide targets for geothermal and/or hydrocarbon research in the area. Beside the regional geological aspects, this work bears implications on the modes of involvement of mélange units at the transition from passive margin to foreland basin systems.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/geosciences11040160/s1.

**Author Contributions:** Conceptualization, G.L.C., G.V., L.C., M.S., and E.C.; methodology, G.L.C., and G.V.; software, G.V.; validation, E.C. and C.D.; formal analysis, G.L.C., G.V., L.C., and M.S.; investigation, G.L.C., G.V., L.C., M.S., and E.C.; resources, E.C., L.C., and C.D.; data curation, G.L.C., G.V., L.C., and M.S.; Writing—Original draft preparation, G.L.C., G.V., and L.C., Writing—Review and editing, G.L.C., G.V., L.C., M.S., and E.C.; visualization, G.L.C., G.V., L.C., and M.S.; supervision, E.C. and C.D.; project administration, G.L.C. and E.C.; funding acquisition, E.C., L.C., and C.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Progetti di Ateneo 2016 (C. Doglioni), 2017 and 2019 (C. Doglioni and E. Carminati) and by the Spanish Ministry of 'Economía y Competitividad' (projects CGL2012-33160 and CGL2015-69805-P).

**Data Availability Statement:** Data used for seismic interpretation and model reconstruction can mainly be found in the public VIDEPI database (www.videpi.com) (accessed on 20 January 2021) and in the Pentex Ltd. database. Data available at the ENI data room were also viewed.

**Acknowledgments:** We gratefully acknowledge Pentex Limited Italia and Luigi Albanesi for the permission to analyze their subsurface data and publish the seismic lines of the Strangolagalli Oil Concession area. We acknowledge ENI for the permission to participate at the data room as requested in San Donato Milanese, Milan, and in particular to analyze some seismic lines on old Permit Areas. We thank the G. Wang and the D. Liotta, G. Molli and A. Cipriani for the opportunity to share our research in the Special Issue "The Apennines: Tectonics, Sedimentation, and Magmatism from the Palaeozoic to the Present". Enrico Tavarnelli, Andrea Artoni and an anonymous reviewer are acknowledged for insightful comments and suggestions. We are grateful to "Gruppo Grotte Castelli

Romani", "Federazione Speleologica del Lazio", Andrea Cesaretti, Piero Ciccaglione, Pio Di Manna, Simone Fabbi, Luca Forti, Angelo Giuliani, Domenico Mannetta and Anne Mérienne for their support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **Appendix A**

In the following, we report the Biostratigraphic and lithostratigraphic data of outcrops and stratigraphic units available from the literature related to syn-orogenic deposits shown in the representative stratigraphic logs of Figure 2 in the main manuscript. The formation labels are also related to Figure 2.








#### **Appendix B**

In the following, we report the new Stratigraphic constraints and age determination of the samples collected from twenty-five different localities in the study area representative stratigraphic logs of Figure 2 in the main manuscript.





#### **References**


### *Review* **The Alps-Apennines Interference Zone: A Perspective from the Maritime and Western Ligurian Alps**

**Fabrizio Piana 1,\*, Luca Barale 1, Carlo Bertok 2, Anna d'Atri 1,2, Andrea Irace <sup>1</sup> and Pietro Mosca <sup>1</sup>**


**\*** Correspondence: fabrizio.piana@cnr.it

**Abstract:** In SW Piemonte the Western Alps arc ends off in a narrow, E-W trending zone, where some geological domains of the Alps converged. Based on a critical review of available data, integrated with new field data, it is concluded that the southern termination of Western Alps recorded the Oligocene-Miocene activity of a regional transfer zone (southwestern Alps Transfer, SWAT) already postulated in the literature, which should have allowed, since early Oligocene, the westward indentation of Adria, while the regional shortening of SW Alps and tectonic transport toward the SSW (Dauphinois foreland) was continuing. This transfer zone corresponds to a system of deformation units and km-scale shear zones (Gardetta-Viozene Zone, GVZ). The GVZ/SWAT developed externally to the Penninic Front (PF), here corresponding to the Internal Briançonnais Front (IBF), which separates the Internal Briançonnais domain, affected by major tectono-metamorphic transformations, from the External Briançonnais, subjected only to anchizonal metamorphic conditions. The postcollisional evolution of the SW Alps axial belt units was recorded by the Oligocene to Miocene inner syn-orogenic basin (Tertiary Piemonte Basin, TPB), which rests also on the Ligurian units stacked within the adjoining Apennines belt in southern Piemonte. The TPB successions were controlled by transpressive faults propagating (to E and NE) from the previously formed Alpine belt, as well as by the Apennine thrusts that were progressively stacking the Ligurian units, resting on the subducting Adriatic continental margin, with the TPB units themselves. This allows correlation between Alps and Apennines kinematics, in terms of age of the main geologic events, interference between the main

structural systems and tectonic control exerted by both tectonic belts on the same syn-orogenic basin.

**Keywords:** tectonics; sedimentation; exhumation; Western Alps; Apennines

#### **1. Introduction**

In the southern part of the Piemonte region (NW Italy) the Western Alps arc ends in a narrow, E-W trending zone (here named "southern termination of Western Alps"), where some of the main geologic domains of the Alps are now strictly juxtaposed. The Alps and the Apennines presently join in southern Piemonte where they have been intergrowing since the Paleogene: see [1–10] with references therein.

The southern termination of the Western Alps consists of three main geomorphologic sectors: a northwestern sector comprised between the Maddalena Pass and the Stura di Demonte valley, a central sector comprehending the Gesso Valley and extended eastward to the Tenda Pass and Vermenagna valley, and an eastern part, the western Ligurian Alps, here considered as the mountain range comprised between the Tenda Pass and the Tanaro valley (Figure 1). This region is very well suited for studying the relations between the Alps and the Apennines orogenic systems in terms of both the age of formation and the way in which the two main tectonic belts developed. This is mainly because: (i) the Maritime and Ligurian Alps formed later than other sectors of Western Alps [11–14]; (ii) they preserve, on top of their polymetamorphic basements, extensive Mesozoic to Oligocene sedimentary

**Citation:** Piana, F.; Barale, L.; Bertok, C.; d'Atri, A.; Irace, A.; Mosca, P. The Alps-Apennines Interference Zone: A Perspective from the Maritime and Western Ligurian Alps. *Geosciences* **2021**, *11*, 185.

https://doi.org/10.3390/geosciences11050185

Academic Editors: Jesus Martinez-Frias, Giancarlo Molli, Domenico Liotta and Angelo Cipriani

Received: 9 February 2021 Accepted: 19 April 2021 Published: 25 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

successions, which provide chronological constraints to the reconstruction of the regional tectono-sedimentary evolution, and (iii) the adjoining syn-orogenic basins recorded the tectonic history of both the two belts.

At the crustal scale, the SW Alps are interpreted as a composite tectonic belt detached at a depth of about 15 km in its north-eastern part and 5 km in the southwestern part, on the European crust. A high-density body (southern prolongation of the Ivrea body Auct.) occurs at a depth of about 40 km below the eastern margin of the SW Alps in southern Piemonte (Figure 2). The Ivrea body could have played an important role in the postcollisional Alpine tectonic evolution [15–17].

The first part of this paper concerns the analysis of the concepts (e.g., the Penninic Front and the Briançonnais Front) used in the literature to subdivide the SW Alps orogenic belts into domains that have different paleogeographic pertinence and/or show different geodynamic evolution, so that their effective relations with the other sectors of the Western Alps can be clearly defined. Once the main Alpine subdivisions are traced within the study area, a further analysis is carried out in the second part of the paper investigating how, when and at which extent the SW Alps were involved in the Apennine dynamics, which started during the late Oligocene when the westward-directed subduction of the Adriatic plate began [11–14,18].

This paper is thus based on the assumption that the evolution of the north-westernmost part of the Apennines can be studied referring to Alpine geodynamics because, although the Alps and the Apennines are two distinct geomorphologic and geophysical entities at the scale of the Western Mediterranean area [14], they share consistent kinematic evolution and common synorogenic basins in their junction zone of NW Italy. The steps of the Alps-Apennines evolution have been clearly recorded by a set of regional scale Oligocene to Pleistocene unconformities that can be continuously traced at surface in the southern part of the Piemonte region and in the subsurface of the western Po plain [19].

#### **2. Geological Setting of the Southern Termination of the Western Alps**

The southern termination of the Western Alps comprehends several tectonic units juxtaposed by NW-SE striking, mainly steeply dipping Alpine tectonic contacts. These units constitute the southern part of a double vergent structure developed at the regional scale [20,21] that involves the Briançonnais Domain in the internal northeastern side, and the Dauphinois-Provençal Domain in the external southwestern side [22,23]. The Briançonnais Domain, referred to as the distal part of the European continental palaeomargin of the Alpine Tethys [24], is subdivided by the Internal Briançonnais Front into an internal sector (Internal Briançonnais, mostly cropping out in the Cottian and Ligurian Alps) affected by HP-LT metamorphism [21], and an external sector affected by very low-grade metamorphism [25,26]. The Dauphinois-Provençal Domain, representing the proximal part of the European continental palaeomargin, was affected only by anchizone metamorphism, and is bounded along its inner side by the External Briançonnais Front. It may be subdivided into a basinal area where a several km-thick and clay-rich Mesozoic succession was deposited (Dauphinois succession), and a shallow water area, which is characterized by a reduced succession with carbonate platform facies (Provençal succession).

The more eastern part of the Western Alps southern termination, i.e., the western Ligurian Alps, shows a tectono-metamorphic and geometric setting [23,25–29] quite similar to that of the southern Cottian and Maritime Alps, although the fan-like, double-vergent structure is less pronounced than in the Maritime and southern Cottian Alps.

Finally, to the south of the External Briançonnais Front, in the investigated area the Western Ligurian Flysch units are present. These units, made up of Helminthoides Flyschtype successions [22,30–32] (also known as "Embrunais–Ubaye nappes" north of the Argentera Massif [33,34] and San Remo-M.Saccarello Unit to the SE of it [29,35,36]) are a stack of tectonic units composed of Lower Cretaceous–Lower Paleocene deep-water sediments referred to as the proximal Ligurian Domain and detached from their original substrate (i.e., the European continental margin). In the study area, the Western Ligurian Flysch, which were detached and emplaced in the early stages of the alpine tectonics [37], were later thrust over the Alpine Foreland Basin and/or the Dauphinois-Provençal succession, and are in turn involved in the Dauphinois-Briançonnais fold and thrust belt [23].

#### *Tectono-Stratigraphic Evolution of the Southern Termination of the Western Alps*

In the southern termination of the Western Alps, the Briançonnais domain represents a part of the more internal, uplifted sector, of the European distal margin, close to the Mesozoic Tethyan ocean (residual H-block [38,39]). The polymetamorphic Briançonnais basement crops out discontinuously in the Acceglio zone where it consists of micaschists, metabasites and granite. The overlying succession starts with Permian volcanic and volcaniclastic deposits and Lower Triassic fluvial to littoral conglomerate, quartzite sandstone and lagoonal pelite, followed by a Middle Triassic peritidal carbonate succession. The top of the Triassic succession is truncated by an unconformity due to a regional uplift and related subaerial exposure during the Tethyan syn-rift stage [40–42]. The succession continues with Middle Jurassic platform carbonates and Upper Jurassic pelagic plateau limestone, followed by mineralised Lower Cretaceous hard ground and Upper Cretaceous hemipelagic sediments.

The Dauphinois-Provençal domain represents the proximal margin of the Mesozoic Tethyan ocean [43,44] developed above the continental crust (i.e., the Argentera Massif in the study area). The succession starts with Carboniferous–Permian continental sediments and Lower Triassic coastal and lagoonal deposits, followed by Middle Triassic peritidal carbonates and Upper Triassic evaporites and lagoonal pelites. Starting from the Late Triassic–Early Jurassic, the Dauphinois-Provençal domain was affected by intracontinental rifting, and partitioned into fault-bounded rift-basins [41]. From the Early Jurassic to Early Cretaceous, the rift basins (Dauphinois domain) progressively subsided, and thick successions of deep-water marl, limestone and shale with interbedded resedimented calcirudite and calcarenite layers were deposited. Toward the south the Dauphinois domain passed laterally to a structural high (Provençal domain) that remained during the Middle Jurassic-earliest Cretaceous in shallow water conditions with the development of carbonate platforms [45]. In the Valanginian, the carbonate platform drowned and was covered by a few metres to a few tens of metres of condensed, open marine deposits locally rich in authigenic minerals (Hauterivian-Albian; [35,46]). The Dauphinois-Provençal succession ends with Upper Cretaceous hemipelagic marly limestone, locally rich in resedimented, mainly siliciclastic, layers [27,47,48].

The present geological setting of the Briançonnais and Dauphinois-Provençal domains of the southern termination of the Western Alps is mainly due to the progressive involvement of the European continental palaeomargin, which these domains belonged to in the Alpine tectonic belt since the middle Eocene (e.g., [34]). The first stages of this tectonic evolution caused the development, on top of the Mesozoic succession, of a regional unconformity corresponding to a hiatus spanning the latest Cretaceous–middle Eocene, overlain by the Alpine Foreland Basin succession [49], middle Eocene discontinuous continental to lagoonal deposits (Microcodium Formation), middle Eocene mixed carbonate–siliciclastic ramp deposits (Nummulitic Limestone), upper Eocene hemipelagic sediments (Globigerina Marl) and upper Eocene–lower Oligocene turbidite succession (Grès d'Annot, [50]).

Both the continental margin and the foreland basin successions have experienced, since the latest Eocene, a multistage tectonic evolution characterized first by southwestward brittle–ductile thrusting and superposed foldings, then by northeastward back-vergent folding and, lastly, by southward brittle thrusting and flexural folding [23]. The regional structural setting resulted from a transpressional regime [23,25,51], documented by a postearly Oligocene NW-SE transcurrent shear zone (Limone Viozene Zone) extending for some tens of kilometres through the study area. This shear zone is probably superimposed on a long-lived shearing corridor, active since the Jurassic–Cretaceous and reactivated during the Cenozoic [23,52].

In the southern termination of the Western Alps, the Alpine Foreland Basin succession is overthrust by the Helminthoides Flysch units (Western Ligurian Flysch units in [23]). The Helminthoides Flysch units [22,30–32] are composed of Lower Cretaceous–lower Paleocene deep-water sediments detached from their original substrate and referred to as the Ligurian Domain. These units consist of carbonate-poor, thin-bedded varicoloured pelites (the basal complex) interpreted as basin plain deposits, thick-bedded, coarse-grained sandstones deposited in internal deep-sea fans and thick-bedded, mainly carbonate turbidite successions deposited in external deep-sea fans [31].

#### **3. The SW Alps Transfer**

On the basis of the paleogeographic reconstructions proposed in the literature (see [53] with references therein), the geological evolution of the southern termination of the Western Alps arc (Figures 1 and 2) was controlled, since at least the Early Cretaceous, by major transcurrent fault zones.

In the latest Eocene (35 Ma ago), the onset of the Europe-Adria continental collision induced the westward indentation of the Adria continental block, together with its highdensity roots (Ivrea geophysical body [17]) and its counterclockwise rotation with respect to Europe, marking a dramatic change in convergence and thrusting direction ("Oligocene revolution" *sensu* [54]).

**Figure 1.** Geological sketch of the southern termination of the Western Alps. The main tectonic domains are represented with different colours. The black dashed rectangle corresponds to the study area of Figure 3. Legend: ARG: Argentera Massif; EBr: External Briançonnais Domain; IBr: Internal Briançonnais Domain; Pm: Piemonte Domain; PPm: Pre-Piemonte Domain; DM: Dora-Maira Domain; DauPro: Dauphinois—Provençal Domain; ExLBr: External Ligurian Briançonnais Domain; ILBr: Internal Ligurian Briançonnais Domain; TPB: Pliocene sediments and Tertiary Piemonte Basin; WestLF: Western Ligurian Flysch Domain; EBF: External Briançonnais Front; IBF/PF: Internal Briançonnais Front/Penninic Front; LiVZ: Limone-Viozene Zone; DAZ: Demonte-Aisone Zone; REZ: Refrey Zone; HFT: Helminthoides Flysch Basal Thrust; TTT: Tenda-tunnel Thrust; COT-A: Cottian Alps; MAR-A: Maritime Alps; WeLI-A: Western Ligurian Alps; CeLi-A: Central Ligurian Alps. The black line within the dashed inset refers to trace of the section of Figure 2. Adapted with permission from ref. [23]. Copyright 2016 Springer-Verlag Berlin Heidelberg.

At the same time, south of the Ligurian Alps, a switch in the subduction polarity occurred from a SE-dipping polarity to a W-dipping polarity, and the eastward retreat of

the Apennine slab began, accompanied by onset of back-arc rifting in the Liguro-Provençal area [13,14,18,55]. Both these processes were quite absent to the north east of the Ligurian Alps, suggesting the presence of a major transfer zone between these two realms, which should have been oriented NNW-SSE in the first (early Oligocene) stages and WNW-ESE in the last (early-middle Miocene) ones ([9,23]. The activity of this major transfer zone, with active steep crustal tectonic features, extended at depth and allowed the west-ward indentation of Adria [34] that in the northern part of the Western Alps was accommodated by dextral shearing along the Periadriatic line [56]. Then, since the Oligocene, the transfer zone should have acted as a strike-slip fault zone along the southern margin of the Adria indenter [54]. This major transfer zone placed at the southern termination of the Western Alps may have developed partly on inherited Mesozoic extensional and transtensional faults active before the onset of the Alpine collisional regime (as discussed in [57]), which may have compensated for the different seafloor spreading rate of the Ligurian ocean to the south and Piemonte ocean to the north [34,53,54,58] Consequently, the geological units presently placed at the southern termination of the Western Alps arc should bear clear and widespread evidence of such distinctive geological evolution in terms of sedimentary evolution, deformational history and related hydrothermal activity.

A large amount of recently published data [23,25–28,46,52,57,59–63] document that since Early Cretaceous up to at least the early Miocene, the geological units presently placed at the southern termination of the Western Alps experienced a tectono-sedimentary evolution mainly related to large scale strike-slip and transpressive faulting, with significative migration of syntectonic fluids.

**Figure 2.** Schematic geological section (location in Figure 1) across the Argentera massif, the western Ligurian Alps and the TPB syn-orogenic basin. Reference data from [17,23,25,26,64,65]. Legend: TPB: Tertiary Piemonte Basin; GVZ: Gardetta-Viozene Zone; LIVZ: Limone-Viozene Zone; REZ: Refrey Zone; WLF: Western Ligurian Flysch (Helminthoides Flysch); IBF/PF: Internal Briançonnais Front/Penninic Front; Late Burd, Late Chat: seismic imaged unconformities of inferred late Burdigalian and late Chattian age.

The field evidence of such an important, often invoked, transfer zone has been reported by [23] who described the southern termination of Western Alps arc as an assemblage of juxtaposed tectonic units, mainly belonging to the Briançonnais and Dauphinois-Provençal sedimentary domains of the palaeo-European margin and to the overlying Alpine Foreland basin, elongated on average ESE-WNW direction [20,47] within a km-scale transpressive deformation zone (SW Alps Transfer, SWAT). A progressive transition from high pressure metamorphic rocks in the internal (NE) part of the transfer zone, to very low grade and nonmetamorphic rocks in the external ones (SW) can be presently observed [21,23,26,66] (Figures 1 and 3). The SWAT is probably rooted within the underlying European continental basement [16,17] (Figure 3) made up of mono and polymetamorphic rocks. These units crop out in the Argentera Massif, an ellipsoid-shaped body exhumed in Miocene times [67] through the Briançonnais, Provençal and Dauphinois sedimentary successions, from which it is presently separated on both sides by Miocene-Pliocene boundary fault systems [68] (Sanchez et al., 2011a) (Figures 1–3).

**Figure 3.** Geological sketch map of the southern termination of the Western Alps adapted from [23]. The Gardetta-Viozene Zone is highlighted (in grey). The main tectonic units are represented referring to their different paleogeographic pertinence (different colors) and geometrical position with respect to the main tectonic boundaries and deformation zones. The location of geological cross-sections of Figure 4 (sections A-A', B-B', C-C', D-D', E-E') is reported. The Internal Briançonnais Front (IBF) here corresponds to the external boundary of the green-schist to eclogite facies metamorphic axial belt of the Alps (*sensu* [69]) and can be thus considered as the surface expression of the Penninic Front.

#### **4. The Penninic Front at the Southern Termination of Western Alps**

To discuss the evolution of the southern termination of Western Alps in the frame of the collisional and postcollisional kinematics of the orogenic belt, it is necessary to unambiguously define the intended meaning of the "Penninic Front concept", in order to understand what are the relations of the SWAT with the ideal main tectonic boundary of the Western Alps belt, i.e., the so-called Penninic Front. Before starting to debate on this matter, a brief examination of the historical intended meaning(s) of the "Penninic Front concept" is given in the following.

The Penninic nappe stack can be seen, in a very simplified view, as the suture zone between the European (including the Briançonnais domain) and Adriatic plates, comprising remnants of the former subducted oceanic crust or exhumed mantle, as well as the extended European continental crust [70]. A slightly different, definition of the Penninic zone is given by Dal Piaz (ets al.) [71], as a stack of generally metamorphic nappes scraped off the subducting oceanic lithosphere and European passive continental margin (distal part), mainly accreted during the Paleogene, whose outer boundary is the Penninic frontal thrust.

In many other papers [16,34,72,73] the Penninic domain has been conceived differently from the above cited papers because the Penninic frontal thrust (Penninic Front) has been placed in the Southwestern Alps at the boundary between the Briançonnais and Dauphinois-Provençal domains, irrespective of the distribution of the metamorphism. Further south, the Penninic Front has been placed along the external boundary of the nonmetamorphic Western Ligurian Flysch Unit (Helminthoides Flysch Units) of the SW Alps, which is detached over the Dauphinois-Provençal domain, and labelled as Penninic Basal Contact or "AlpineThrust Front" [29,37].

In this paper, the Penninic Front is intended (in the sense of [69]) as the frontal thrust which bounds the Alpine Axial Belt, which comprises continental units derived from the Adriatic margin ("Austroalpine units"), as well as parts of the European continental margin together with oceanic units originated from the Mesozoic Piemonte–Liguria Ocean, labelled together as "Penninic Units" (Figure 1). All these units of the Alpine Axial Belt underwent alpine metamorphism ranging from greenschist facies to ultra-high-pressure eclogite facies conditions. The Penninic Front bounds the penninic units from the less metamorphosed external parts of the paleoEuropean margin, which can pertain to the External Briançonnais, Helvetic, and Dauphinois-Provençal domains.

#### **5. Structure of the Southern Termination of the Western Alps: Increasing Deformation, Metamorphism and Relations with the Adjoining Alps-Apennines Syn-Orogenic Basins**

A review of the tectonic setting and evolution of the southern termination of the Western Alps, as well as the dating of its main stages, is necessary to provide the elements to correlate it with the evolution of the northernmost part of the Apennines belt. We specify that the following description of the metamorphic conditions of the tectonic units refers to the "metamorphic degree", defined on the basis of mineralogical indexes (such as the 'crystallinity' degree of phyllosilicate minerals [26]), even for the very poorly transformed rocks of the external Briançonnais and Dauphinois-Provençal domains, which were subjected to anchizonal or very low-grade metamorphic conditions [26]. Conversely, if textural criteria are used to define the metamorphic features in the study area [19,74], the rocks of the studied external Briançonnais and Dauphinois-Provençal domains would be classified as "nonmetamorphic", since their primary features are mostly preserved from the effect of secondary petrogenetic processes and do not show any major internal reorganization or recrystallization.

The overall aspect of the southern termination of the Western Alps is presently a fan-like setting that evokes a flower structure, well known in the Briançonnais domain of the Guillestre, Briançon and Moûtiers transects [75–79], as well as in the Maritime Alps (Stura valley and Tenda Pass area [27]) and western Ligurian Alps [23,25,26,29,80].

This double-vergent tectonic system [20,21] involves the Briançonnais Domain in the internal, northeastern side and the Dauphinois-Provençal Domain in the external, southwestern side [22,23]. This system consists of NE-vergent thrusts and transpressive fault systems developed to the north of the External Briançonnais Front, and SW-vergent ones to the south of it. In map view (Figure 1), the external and internal Briançonnais Fronts, as well as the frontal thrust of the Piemonte Zone, get closer to each other from west to east, while changing their directions from NW–SE to west–east, almost merging together in the Stura valley, east of Aisone (Figures 1 and 2). Due to the deflection of tectonic fronts and the consequent strong reduction in thickness of the main structural domains, the transition from the HP-LT metamorphic units of the internal Brianconnais to the very low-grade units of the external Briançonnais and Dauphinois-Provençal domains occurs over a relatively short distance (about ten kilometers) along a NE-SW oriented geological section (Figures 1–3). The eastward prolongation of the tectonic stack made up of the Piemonte Zone and the Internal Briançonnais tectonic domains, is concealed by the sediments of TPB and overlying Pliocene and Quaternary successions: see [19] with

references therein. At the southern termination of the Western Alps, the transition between the internal Alpine metamorphic tectonic prism and the external Briançonnais-Dauphinois-Provençal fold-and-thrust belt occurs.

This geological transect is suitable for investigation of how the postsubduction Alpine tectonic evolution has been recorded since the early Oligocene by the adjoining synorogenic basins of the Piemonte region, whose tectono-sedimentary setting was controlled, at least since the late Oligocene, by the beginning of the Apennines subduction and related surface tectonics. In this way, a correlation between the Alpine and Apennine main tectonic events can be attempted, as discussed in the following sections. As reported in Section 3, in the southern termination of the Western Alps there is evidence of a major transfer zone (the SWAT) represented by an assemblage of juxtaposed tectonic units, mainly belonging to the Briançonnais, Dauphinois-Provençal and Alpine Foreland Basin sedimentary domains, elongated on average in an ESE-WNW direction [23]. The SWAT represents an ideal macroscale tectonic feature, whose existence can be inferred on the base of kinematic modeling at the regional scale [15,34,81]. A number of effective deformation units (*sensu* GeosciML), and/or strongly deformed tectonic units, exist in the southern termination of the Western Alps, which are defined in Figures 2, 4 and 5 and listed as follows, starting from the more internal units in the footwall of the IBF: (a) the steepened Triassic dolostone succession of the Rocca la Meja unit; (b) the Gardetta Deformation Zone, consisting of several deformation units made up of Carboniferous (?)-Permian (?) volcanics and schists, Triassic quartzarenite, quartzite and km-scale slices of gypsum, intensively folded and steepened along the WNW-ESE faults that bound and dissect the deformation zone; (c) the steepened and strongly sheared external Briançonnais M.Omo Unit and Provençal Giordano-Savi Unit; and (d) the Demonte- Aisone deformation unit of [23]; (e) the tectonic slice system of the Roaschia Unit [27], which passes laterally to the transpressive Limone-Viozene Zone [23,25]. All these units share a succession of geologic events and related mesoscale regional foliations, as well as consistent kinematic features, that formed in response to a succession of deformation phases as described in the following. The assemblage of the above-described tectonic units is here labelled as the Gardetta-Viozene Zone (GVZ), which can be interpreted as the effective representation of the SWAT.

#### *5.1. Rock Deformation and Metamorphism across the IBF in Southern Cottian Alps*

The transition from the green-schist facies metamorphic rocks of the internal Briançonnais to the anchizonal external Briançonnais, which occurs across the IBF, can be observed through the watershed between the Grana and Arma valleys (San Magno-Fauniera Pass area) and along the Preit-Gardetta transect in the right side of the Maira valley (Figure 5). Near San Magno, the Permian (?)-Early Triassic conglomerate and quarzarenite [47] of the internal Briançonnais (site GRA4 in Figure 5) are transformed into greenschist gneisses, while in the Fauniera Pass area (External Briançonnais) they crop out as a very poorly stretched conglomerate with pink quartz and rhyolite pebbles (sample PRV in Figures 5 and 6a,c). Similarly, in the Preit valley, the Permian (?)-Early Triassic quartzite have a marked gneissic structure (sample SRV in Figures 5 and 6b,d), while the quartzarenite and dolostone cropping out SW of the IBF, in the Rocca la Meja tectonic slice do not show evidence of major metamorphic transformations, as also suggested by the presence of preserved microfacies in the Triassic dolostone (sample PRE6, Figure 6e) and very wellpreserved Archosauriform footprints recently discovered [82] a few hundred meters from the IBF. The IBF cuts across the Middle-Upper Triassic evaporites and the Lower Triassic quartzite/quartzarenite layers, which, conversely, in the external (SW) part of the GVZ and in the fold and thrust belt comprised between the IBF and the Argentera northern boundary faults, are not displaced by high angle faults and seem to have played a role of regional detachment horizons.

### *5.2. Field Constraints to the Age of Tectonic Events and Kinematic Interpretation*

#### 5.2.1. Tectonic Phases

The southern termination of the Western Alps shows structural and metamorphic characters acquired through a succession of three "phases of deformation" (D1–D3) with prevalent folding and thrusting, followed by a transpressive, mostly disjunctive phase (D4) and a late (D5) extensional phase.


The D1 was probably characterized by transpressional kinematics, since the sedimentary successions were shortened by duplexes and steepened (Figure 7a) after being detached from the Triassic gypsum and quartzite/quartzarenite levels, while the S1 foliation developed mostly parallel to the steepened bedding [21,25]. With the D1 related strain, a regional scale transpressional deformation unit formed, developed mainly at the expense of the external Briançonnais and inner Dauphinois-Provençal domains, which can be still recognised from the high Stura valley to the Tanaro valley (Figure 2) [23] within the GVZ. Gypsum masses of Triassic age [47] are involved in the GVZ (Figures 7b and 8e,f), probably dragged from the basal detachment level placed at the base of the external Briançonnais and Dauphinois-Provençal succession where these rocks largely occur [23,85,86].

The D1 phase generated the oldest composite tectonic foliation (S1, [87], locally axialplanar to F1, SW verging recumbent folds and syn-D1 reverse/sinistral shear zones ("charriages" Auctorum; [20,88]) Gidon, 1972; Lefèvre, 1983), within which the S1 foliation (Figure 7c,d,f) formed mostly parallel to the boundary shear planes [23]. Evidence of the D1 phase in Dauphinois-Provençal Domain is poorly documented at the mesoscale, except for the Giordano-Savi unit close to the EBF in the left side of the Stura valley, where D1 folds are preserved (Figure 8b) but rotated and displaced by syn-D2/D3, SW-vergingSW vergent thrusts, with related drag folds (M. Bodoira, Figure 4).

At the end of the D1 phase, local transtension should have occurred in some parts of the GVZ, as inferred by the activity of relatively hot fluids, which circulated along NW-SE fault systems, that led to the formation of hydrothermal marbles at the expense of the Jurassic-Cretaceous Dauphinois succession (Valdieri marble, [63]) and to intense recrystallization of some parts of the Alpine Foreland Basin succession (Aisone Flysch, [23,63,89]).


**Figure 4.** Geological cross-sections across the Gardetta-Viozene Zone (GVZ; location in Figure 3); modified from [20] (sections AA', BB'); [27] (sections CC', DD'); [26] (section EE'). Tectonic units: ENUD: Dauphinois succession of Entracque Unit; ENU, For: Foreland Basin succession of Entracque Unit; ENUP: Provençal succession of Entracque Unit; GAZ: Gardetta Zone; GSU: Giordano–Savi Unit; GSV: Gesso–Stura–Vésubie terrane; IBr: internal Briançonnais units; LiVZ: Limone–Viozene Zone; LiVZFor: tectonic slices of Foreland Basin succession involved in the Limone–Viozene Zone; LiVZHF: tectonic slices of Western Ligurian Flysch Unit involved in the Limone–Viozene Zone; MAU: Monte Marguareis Unit; MEJ: Rocca la Meja Unit; OMO: Monte Omo Unit; SMU: Sambuco Unit; SMUFor: Foreland Basin succession of Sambuco Unit; TES: Cima Test Unit; REZ: Refrey Zone; REZFor: Foreland Basin succession of the Refrey Zone; ROU: Roaschia Unit; RYU: Upper Roya Unit; RYUFor: Foreland Basin succession of Upper Roya Unit; SRU: San Remo–Monte Saccarello Unit. Main boundary faults: ABF: Argentera boundary fault system; BNF: Bersaio–Nebius Fault; EBF: External Briançonnais Front; HFT: Helminthoides Flysch basal Thrust; IBF: Internal Briançonnais Front; LiVZf: external boundary faults; SGT: Serra Garb Thrust; TTT: Tenda-tunnel Thrust.

**Figure 5.** Geological map of the Gardetta highland area (modified from [19]), showing the sampling points of samples PRE6, PRV, and SRV and the position of outcrops pictured in Figure 6a,b. ABF: Argentera boundary fault system; BNF: Bersaio-Nebius Fault; EBF: External Briançonnais Front; IBF: Internal Briançonnais Front; PmF: Liguria-Piemonte units Front; PRS: Preit fault ("Preit Scar").

The folding phase D2 folded the D1 duplexes and reactivated the D1 steep transpressive shear zones (Figure 8c,d). A well-developed spaced crenulation cleavage (S2) is associated with F2 folds (Figure 7c–f). S2 planes are in several places reactivated and reoriented by shear deformation (S2-shear, generated by the D3, Figure 7e,f) kinematically consistent with the D2 phase that often induced the displacement or partial transposition of F2 folds hinge zones [25]. In the major transpressive shear zones (LiVZ), the S1 and S2 surfaces are almost subparallel and often form a composite foliation [25] (Figure 6c).

**Figure 6.** (**a**) Permian (?)-Early Triassic interbedded conglomerates and sandstones made up mainly of quartz and volcanics clasts. External Briançonnais Domain (Viridio Unit), Fauniera Pass area. (**b**) Permian (?)-Early Triassic quartzitic-rhyolitic metaconglomerate and metasandstone, Internal Briançonnais Domain (near Preit village), showing isooriented and stretched clasts that mark a gneissic structure. (**c**) Transmitted-light, crossed polars photomicrograph of sample PRV (external Briançonnais Domain, Fauniera Pass area; location in Figure 5), a coarse lithic sandstone corresponding to the finer levels of Figure 7a. A poorly defined lamination is evidenced by grain size variations. (**d**) Transmitted-light, crossed polars photomicrograph of sample SRV (internal Briançonnais Domain, Preit Valley; location in Figure 5), a metalithic sandstone corresponding to the finer levels of Figure 7b. A well-defined foliation is evidenced by the iso-orientation of the arenitic grains and by the occurrence of sub-millimetre thick levels of neoblastic iso-oriented white mica. (**e**) Transmitted-light photomicrograph of sample PRE 6 (Rocca la Meja tectonic slice, near the GVZ inner boundary, location in Figure 5), a dolomitized grainstone with oolites, echinoderm fragments and other bioclasts. Note the well-preserved morphology and internal structure of the oolites. (**f**) Transmitted-light photomicrograph of bioclastic arenaceous limestone (Eocene Nummulitic Limestone, Alpine Foreland Basin succession) from a tectonic slice within the LIVZ, Lago dei Signori pass, Marguareis area. Although a mm-scale foliation is present, evidenced by pressure dissolution seams and cataclastic levels, no volumetric dissolution or recrystallization occurred, as documented by the very well-preserved macroforaminifera and bryozoan.

The S1/S2 foliation is clearly reoriented and reactivated by the D3 shearing event in all the sectors of the GVZ, from the Gardetta Pass to the Marguareis area. The S2 cleavage is well exposed in the Ligurian Briançonnais units (Marguareis area [25,83]) due to the presence of the Upper Cretaceous marly limestone (Upega Formation) that recorded it with clear evidence. Conversely, this important regional foliation is poorly represented in the external Briançonnais units cropping out to the North of the Stura Valley, where, although present, it can be less frequently observed (Figure 7d).


Among the main evidence for the D4 phase are the boundary faults of the Argentera Massif, with the related huge mass of gypsum-bearing brecciated fault rocks (Carnieules Auct. [47]) aligned all along the boundary faults of the massif.


#### 5.2.2. Age of Tectonic Phases and Metamorphism

The southern termination of Western Alps shows a metamorphic evolution ranging from anchizonal and very low-grade facies in the Dauphinois-Provençal and external Briançonnais domains to low grade, high-pressure greenschist facies and carpholite-quartz (blueschist) facies in the internal Briançonnais domain: see [21] with references therein) and [26]. In the more internal Briançonnais and prePiemontese successions, as well as the Acceglio zone, adjoining to the study area the metamorphic degree reached the eclogite facies; see [91] and [21] with references therein.

The metamorphic conditions and the ages related to the above-described tectonic phases are described in the following.


**Figure 7.** (**a**) The sub-vertical Triassic dolostone succession of Rocca la Meja (Meja unit, GVZ, Gardetta highland). In the foreground (right lower corner) an outcrop of steepened Lower Triassic quartzarenite of the Gardetta Deformation Zone is visible. (**b**) Intensively folded and steepened gypsum within the Gardetta Deformation Zone. (**c**) Upper Cretaceous marly limestones in the external part of the GVZ (Dauphinois-Provençal succession) close to the Tenda pass showing a well-developed spaced crenulation cleavage (S2, black lines), crosscutting the older S1 foliation (black dashed lines). Note that in this area the S2 is NE-dipping. (**d**) Upper Cretaceous marly limestone in the External Briançonnais M. Omo Unit, close to Valcavera pass showing a well-developed spaced crenulation cleavage (S2, black lines), crosscutting the S1 foliation (black dashed lines). (**e**) Strain localization along the lithological contact between Upper Cretaceous marly limestone (Upega Fm.) and Upper Jurassic grey limestone (M. Marguareis, Ligurian Briançonnais domain). The S2 spaced cleavage (black lines) is dragged by S2-shear planes (S2-sh, white dashed lines) developed during the late stages of D2 and probably also during the D3 phase. (**f**) Upper Cretaceous marly limestones of the External Briançonnais succession in the Limone-Viozene Zone, Marguareis area. A well-developed spaced crenulation cleavage (S2, black lines) was generated as axial planar surfaces of F2 folds that folded both bedding and older foliation S1 (black dashed lines). Locally, S2 surfaces evolved from an axial plane foliation to a slip cleavage (S2sh, white dashed lines), giving origin, in some places, to dm-thick shear zones, which reoriented all the pre-existing foliations.

The radiometric age of the late D1 hydrothermal fluids responsible for the formation of the Valdieri marble (see above), defined at 30–31.6 Ma by U-Pb analyses on recrystallised and vein carbonate and neoblastic silicate minerals, provides a useful constraint for dating not only the D1, but also the D2 phase, which clearly postdates the marble [63]. It is remarked that a major gap in the distribution of metamorphism is not recorded across an ideal internal Briançonnais-Dauphinois cross section, i.e., across the front of the alpine tectonic prism, as occurring in other orogens (e.g., across the Himalayan main thrust front [94]), but the decrease of the metamorphic grade from the inner to the outer tectonic units seems to occur gradually (see above, Section 5.1), although a relatively major boundary can be established along the internal Briançonnais Front, corresponding to the "Penninic Front" in the sense of the assumption defined in Section 4, following [69].


Other constraints for dating the D2/D3 phases are provided by the faults of the northern part of the Argentera Massif [96]. The kinematics of these faults have been related to the shifting of the regional shortening to N-S directions, which is here thought as the reason for the shearing evolution of the D2 folding phase, i.e., the D3 phase. This occurred probably between 26 Ma and 20 Ma when the D3 phase induced the main uplifting of the Argentera Massif, controlled by dextral transpression along NW-SE fault systems [68,97]. In this time span, the internal sectors of the uplifting SW Alps crossed the apatite fission track closure temperature of 120 ◦C [9], while in the external sectors (Argentera and Dauphinois-Provençal) this occurred at about 14–12 Ma [68]. These data suggest that after the D3 phase, the tectonic evolution (D4 and D5 phases) should have been characterized by development of purely brittle discrete fault systems and/or individual faults.

The D2 NE-vergent tectonics recorded in the external Ligurian Briançonnais have been ascribed to the late Oligocene also by [29].

During the D2/D3 time span the uplifting was recorded by the sedimentary evolution of the adjoining internal syn-orogenic basin, the so-called Tertiary Piemonte Basin, which in the sector close to the Ligurian Alps is characterized by localized fault bounded basins whose tectono-sedimentary evolution has been interpreted as controlled by transtensional and strike-slip tectonics since the late Rupelian [9,10,65].


**Figure 8.** (**a**) Slickensided strike-slip fault surface bounding the GDZ on its southern side along the EBF (Dogger limestones of M. Bodoira); (**b**) steepened folds affecting the Jurassic-Cretaceous successions close to the EBF in the M. Bodoira tectonic unit; (**c**) subvertical, intensively sheared beds in the hundred-meters scale "Servagno transcurrent zone". This zone is located on the GDZ southern boundary, along the EBF and it affects the Jurassic-Cretaceous succession of M.Bodoira near Servagno pass. (**d**) Detail of Figure 10c, drag folds showing vertical axes and vertical axial planes developed along the individual strike-slip faults of the Servagno transcurrent zone; (**e**) tight to isoclinal folds in the gypsum masses of the GDZ to the west of Valcavera pass; (**f**) major gypsum tectonic slices in the central part of the GDZ, bounded by steeply dipping faults belonging to the EBF fault system (in the background the vertical tectonic slices of Monte Salè, reported in the cross-section of Figure 4).

#### *5.3. Subsurface Stratigraphic Constraints to the Uplifting Stages*

The eastern extension of the SWAT/GVZ system (as described in previous sections) can be traced in the Cuneo-Mondovì area. In this area, outcrop and subsurface data constrain the tectono-depositional evolution of Oligocene to Miocene synorogenic basins developed on the western Alpine basements: see [7] and references therein. The line drawing in Figure 9 (SL1, [65]) refers to a N-S seismic line running from the Ligurian Alps to the adjacent plain. It shows strike-slip faults with flower geometries, deep-seated in the western Ligurian Alps basement. During the Oligocene and early Miocene, these faults (roughly E-W striking in map view) controlled evolution of basins (presently buried below younger successions) filled by continental to marginal marine/slope successions [7] and

laterally equivalent to the cropping out eastern TPB successions (Molare and San Paolo formations [99,100]). Unconformities and onlap terminations point out the progressive Oligocene to Miocene syntectonic uplift of this part of the Ligurian Alps.

**Figure 9.** Seismic line drawing crossing the TPB sediments in the subsurface of the Cuneo-Mondovì area. The unconformities are base Rupelian (B-RU), base Chattian (B-CH), late Chattian (L-CH), late Burdigalian (L-BU) and base Langhian (B-LA). From [7,65].

#### **6. Discussion: The Late Eocene to Miocene Evolution of the Internal SW Alps in the Frame of the Adria Indenter Kinematics and First Stages of the Apennines Orogenesis**

Some points for discussion on the relations between the kinematic evolution of the SW Alps and the Adria indenter in the frame of the ongoing Apennines orogenesis, are here proposed.

(a) The first point concerns the dynamic context in which the exhumation of the alpine units occurred. As reviewed in the above sections, it can be suggested that the exhumation of the tectonic units derived from the Briançonnais, Dauphinois-Provençal and Alpine Foreland basin domains occurred in a transpressional regime. It was stated by [21] that whereas the exhumation of the adjoining Alpine units, such as the Dora-Maira UHP-HP eclogite rocks and the Monviso meta-ophiolites, mostly occurred through extrusion in the subduction channel and then late extensional tectonics, "the Briançonnais nappes were exhumed mostly through transpressional deformation at the bottom of a collapsing and eroded orogenic wedge". This interpretation is consistent with the ideas sustained in this paper, and is supported by: (i) the overall structural setting of the Briançonnais-Dauphinois-Provençal transect in the SW Alps, corresponding to a mega-macroscale fan-like geometry evoking a flower structure; (ii) the transpressional characters of the structural associations developed in the D1–D3 phases, consisting of several juxtaposed tectonic domains dominated, in turn, by steep reverse and strike-slip faults, low-angle thrust surfaces and folded domains with alternating low-angle axial surface folds and vertical axes folds; (iii) the progressive and gradual decrease of the metamorphic grade from the internal Briançonnais to the external Dauphinois-Provençal units, indicating that no major vertical offset, and no related metamorphic gap, occurred across the tectonic unit boundaries; (iv) the tectono-sedimentary setting of the adjoining syn-orogenic basins placed in the internal side of the Western Alps, in southern Piemonte, that recorded the kinematics and uplifting stages of the Alps tectonic belt.

The subpoint (ii) demands further considerations on the geometrical setting of the GVZ, which has been described in detail only for its eastern part (LIVZ, [23,25]). The western part of the GVZ, consists of a number of tectonic slivers made up of different rock types arranged in a km-scale deformation zone (Gardetta Deformation Zone, Figure 10) developed between the internal (IBF) and the external (EBF) Briançonnais fronts. The

tectonic slices are made up of both the rocks of the internal Briançonnais domain (metavolcanics and volcanoclastite, phyllite, quartzite and meta quartzarenite) and of the external Briançonnais (volcanoclastite, quartzarenite and parts of the Triassic-Cretaceous carbonate succession), as well as gypsum masses derived probably from the base of the Briançonnais or the Dauphinois successions. The slices are separated by steeply dipping tectonic contacts, oblique to the GDZ boundaries, and consisting of individual faults showing strike-slip slickensided surfaces (Figure 8a), or transcurrent shear zones. The internal setting of the slices can be represented by contractional structural associations (thrusts with related ramp folds, symmetrical folds in the core of the GDZ, Figure 8b) and/or by transcurrent strained domains showing drag folds with subvertical axes and axial planes (Figure 8c,d), and subhorizontal extension lineations. The huge gypsum masses involved in the GDZ show isoclinal to tight folds with axial planes subparallel to the bedding (Figure 8e) but are always bounded by steeply dipping faults (Figure 8f). Although the detailed description of the GDZ structural setting is beyond the scope of the paper, we believe that a qualitative interpretation of the GDZ as a strain-partitioned transpressive zone should be accountable on the basis of the above reported observations, as well as the overall geometric setting on map view (Figure 10). The macro and mesoscale features of the GDZ seem to fit with the diagnostic features described for the transpressive zones in the basic works of [101–103]. Furthermore, the abundant presence within the GDZ of lithotypes that rarely occur in the external Briançonnais domain (e.g., the basic volcanics of the Becco Nero slices, Figure 10) suggest that the GDZ could have originated on some prealpine lithological inhomogeneity.

(b) The second discussion point refers to the effective relations of the transpressive tectonics, discussed at point (a), with the inferred presence of the regional transfer zone, here named SWAT (see Section 3), that should have contributed to the west-ward indentation of Adria and its counterclockwise rotation with respect to Europe [34]. This transfer kinematics, coeval with the continuing shortening due to the Adria-Europe indentation, seems to be effectively recorded by the structural setting of the southern termination of Western Alps, namely by the Gardetta-Viozene Zone (GVZ), consisting of an assemblage of transpressive deformation units [20,23], which can be followed quite continuously from the NW in the Cottian Alps to the Tanaro valley in the western Ligurian Alps (Figure 3).

**Figure 10.** Geotectonic map of the Gardetta Deformation Zone, developed between the internal Briançonnais front (IBF) and the external Briançonnais front (EBF) (modified from [19]).

The structural setting of the SWAT/GVZ corresponds to a double-vergent tectonic system, mainly developed during the D1/D2/D3 phases, where NW-trending major folds, and both NE dipping and SW dipping reverse and strike slip faults developed. In the NW sectors [75–77], the NE-vergent (back-vergent) branch of the transect is indeed developed mostly inwards (NE) of the GVZ, in the internal Briançonnais units [21] (Figure 4). Conversely, in the SE sectors the NE-vergent fold systems are intensively developed within the GVZ itself [25,80,83], while the SW-vergent folds are less abundant, except near the tip zone of the SW-vergent thrusts (Figure 4). The presence of steepened slices of anhydrite and gypsum in the northern branch of the GVZ (Valcavera-Gardetta highland at the altitude of more than 2000 m), which have been found more to the East, at the northern termination of the LIVZ (not far from the IBF, at an altitude of less than 700 m, in the "Buzzi tunnel" between Roaschia and Robilante [104]) and, more extensively, along the Tenda Tunnel thrust at about 1000–1200 m a.s.l [23,86], suggests that the transpressive GVZ probably merged into the basal detachment of the external Briançonnais-Dauphinois-Provençal thrust belt from where it could have dragged, up to higher geometric positions, the above cited gypsum and anhydrite-bearing tectonic slices. The presence, within the fault core zone (namely in the restraining bends), of duplexes and slices extruded from deeper levels, is indirect evidence of transpressional fault setting [101,105,106]. The development of the GVZ unlikely occurred as the result 6tof an homogeneous transpression, as it shows a marked internal strain partitioning. Further analysis is required to ascertain if the strain partitioning was achieved mostly during the first deformation stage (D1) or continuously during the D1 to D3 stages.

(c) As the SWAT activity was recorded by the tectonic evolution of the basement and covers (Mesozoic succession and Eocene-Oligocene Foreland Basin succession) at the southern termination of Western Alps, a third crucial point, concerning the sedimentary recordings of this tectonics in the syn-orogenic basins is to be discussed. The data reported in the above sections indicate that the exhumation/uplifting of the southern Cottian-Maritime-western Ligurian system occurred during the formation of the Alps-Apennines syn-orogenic basin known as TPB (Tertiary Piemonte Basin [9,19,74,107] with references therein). The TPB succession (Figure 1) was deposited starting from the early Oligocene on the exhuming metamorphic complexes of the Western Alps, as well as on the top of the overthrusting Ligurian units involved in the northwestern Apennines. The TPB successions recorded the Alps-Apennines tectonics through some regional scale unconformities related to main Geologic Events (Figure 11) that divided the succession into a number of unconformity-bounded stratigraphic units (Synthems [108]) continuous at the regional scale [19]. The analysis of a seismic line (Figure 9) available for the TPB sectors adjoining the Western Ligurian Alps [65], evidenced that the uplifting of the basement occurred during the depositions of the Oligocene-early Miocene succession, as evidence by late Chattian and Burdigalian reflectors that onlap distinct tectonic units, sealing the progressively younger activity of the faults branching from the main steep fault systems, interpreted by the authors as roughly E-W strike-slip faults deep-seated in the alpine basement. We suggest that this fault system could be consistent (as close to it and showing similar geometric and kinematic features) with the inferred eastward prolongation of the GVZ system (Figure 2), whose evidence in this sector of the western Ligurian Alps (i.e., the Limone-Viozene Zone [25,28]) has been confirmed by surface data. The activity of the GVZ/SWAT system thus occurred first during the early Oligocene sedimentation stages, when the basement of the TPB underwent a stretching, consistent with the sinistral transcurrent tectonics of the D1/D2 phase, which controlled the deposition of the lower Oligocene continental and coarse-grained marine sediment (Molare Fm. [99]), and induced an intense vertical mobility leading to a main regional denudation episode [109] and the onset of differentially subsiding sub-basins, bounded by high angle transtensional faults ([6,10] with references therein) and flanked by fastly uplifting areas. This stage was concomitant with the rifting phase of the Balearic

basin [110] that in the internal part of the Ligurian Alps was less pronounced and rapidly decreased toward the North, maybe due to the hindering effect of the southern prolongation of the Ivrea high-density body [9]. The gradual decrease of the rifting was probably partitioned by the transpressive faults of the GVZ/SWAT system, as suggested by [10], during the D1/D2 phase.

All the early Oligocene sub-basins of the southern TPB then underwent, in late Rupelian and early Chattian times, a general drowning and subsidence, with the deposition of outer shelf and slope marly sediments (Rocchetta and Rigoroso Fms. [107]). This event occurred in a transtensional regime concomitant with crustal thinning and opening of back-arc basins [55] and was coeval with the initial stages of the Apennines dynamics, i.e., with the beginning of the Adriatic (Apennines) subduction [111].

Then, a marked inversion of southern TPB structures occurred during the Aquitanian-Early Burdigalian in response to an important geologic event induced by the change in the direction of motion of the Adriatic indenter with respect to Europe from NWward to WNW-ward at about 20 My [53,112,113]. This resulted in conditions of oblique convergence and increased collisional tectonics, whose effects are recorded in a large part of the western Mediterranean area and caused the switch from transtensional to contractional and transpressional regime in the southern TPB, inducing inversion of a great number of the formerly active structures [5,7,9,10,114–116]. In this period (early to middle Miocene) the TPB underwent a counterclockwise rotation of ca. 50◦ with respect to Africa [117].

In the southern western Alps, the Aquitanian-Burdigalian tectonic stage induced a marked regional uplift, with subsequent high denudation rates. This caused the eastward migration of fan-delta systems, prograding from the western margin of the TPB (the "Saluzzo-Monregalese belt" of [7]), accompanied by significant change in sediment composition [118]. In the western Ligurian Alps, the uplifting stage can be referred to the D3 phase, coeval with the main uplifting of the Argentera Massif [68,97] induced by dextral transpression along the northern boundary faults of the massif and D2/D3 back-thrusting. These thrusts are clearly sealed by the late Burdigalian reflectors reported in the line drawing of Figure 9.

The D2/D3 phase developed a penetrative regional foliation through large sectors of the Maritime and western Ligurian Alps: this foliation postdates the early Oligocene (see also [63]) and could be ascribed to compressional stages coeval and dynamically consistent with those recorded in the adjoining TPB successions, i.e., the main Apennines related tectonics that occurred at the Aquitanian-Burdigalian boundary, active while the Maritime and western Ligurian Alps continued their uplifting. The propagation of the N- and NEvergent thrust front in the westernmost part of the Apennines was thus hindered by the uplifting Alpine basement and related covers. The E and NE-vergent transpressive faults propagating from the previously formed alpine belt also involved and stacked the Ligurian units resting on the Adria crust, as well as the same syn-orogenic sedimentary successions that were flanking the alpine units while they were uplifting [5,51,65]. It becomes clear now, in our view, that the NE-vergent contractional tectonic systems affecting the TPB can be defined alpine or Apennines-related depending on the substrate they displaced (alpine metamorphic units vs. Ligurian non-metamorphic units), but they developed indeed within the same geodynamic context since the late Oligocene, making it appropriate, for the westernmost Alpine-Padane realm, to refer to a single "Alps-Apennines orogenic system", as in [19,74].


**Figure 11.** List of the Alpine Geologic events (D1–D5) recorded in the southern termination of Western Alps since Early Oligocene. The U1, U2 ... D7 unconformities correspond to the D1, D2 ... D7 regional unconformities of the TPB, as defined in [19,74]. The column "geodynamics" describes the relations with the main stages of the tectono-sedimentary evolution of the southern termination of the Western Alps and of north-western Apennines.

#### **7. Conclusions**

Based on a critical review of surface and subsurface geological data, integrated with new data and interpretations, it is concluded that the southern termination of the Western Alps arc recorded the Oligocene-Miocene activity of a regional transfer zone (the southwestern Alps Transfer, SWAT) whose existence has been often postulated in literature [15,34,81] and that should have allowed, since early Oligocene, the westward indentation of Adria and its counterclockwise rotation with respect to Europe. This "virtual" transfer zone, inferred on the basis of geodynamic constraints and reconstructions, could be partially seen, at shallow crustal level, in an effective system of deformation units and km-scale shear zones, here defined as the Gardetta-Viozene Zone (GVZ). The GVZ is developed externally to the internal Briançonnais Front (IBF), involving the external Briançonnais and Dauphinois-Provençal domains and the overlying Eocene-Oligocene sediments of the Alpine Foreland basin. The IBF, which represents the inner boundary of the SWAT, is thought to correspond to the Penninic Front, here intended as the frontal thrust which bounds the Alpine Axial Belt, i.e., the metamorphic orogenic prism (in the sense of [69]). Thus, in the southern termination of western Alps, the Penninic Front divides the external from the internal Briançonnais domains. Consequently, it can be argued that, in this area, the Briançonnais domain did not experience subduction and exhumation as a whole. The internal Briançonnais underwent major tectono-metamorphic transformations, while the external Briançonnais was subjected only to anchizonal P-T conditions. The relatively gradual transition (although stepwise across distinct tectonic fronts) from HP-LT metamorphism and very low-grade to anchizone metamorphism through the Briançonnais-Dauphinois

transect of SW Alps, suggests a low entity of the thrust vertical offsets, as expected in an overall transpressive or strike-slip regional context.

The southwestern Alps Transfer acted outwardly of the IBF, in a foreland fold and thrust belt, consisting of the external Briançonnais and the Dauphinois-Provençal domains with related Alpine Foreland Basin successions, which was detached above quartzites and anhydrite-gypsum levels of inferred Triassic age, now locally involved in the core of the SWAT shear zones. Conversely, the IBF cut across the Triassic evaporite and quartzite level, bounding the external domains affected by "cover tectonics" from the internal levels, where the stacking involved the Permian metavolcanics and some levels of the underlying polymetamorphic basement [119,120]. The Oligocene to Miocene kinematic evolution of the above-described Alpine units was well recorded by the tectono-sedimentary evolution of the inner syn-orogenic basins, i.e., the so-called Tertiary Piemonte Basin, as evidenced by stratigraphic, sedimentological and geophysical data. This allows correlation with the Apennines kinematics and dynamics, in terms of the age of the main geologic events, the interference between the main structural systems and the tectonic control exerted by both the tectonic belts on the same syn-orogenic basin.

**Author Contributions:** Conceptualization, F.P.; investigation: A.d., L.B. and C.B.; data curation, F.P., A.d., L.B., C.B., A.I. and P.M.; writing—original draft preparation, F.P.; writing—review and editing, F.P., A.d., L.B., C.B., A.I. and P.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The study did not report any organised data sets.

**Acknowledgments:** Two anonymous reviewers are kindly thanked for their useful suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Article* **Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines)**

**Enrico Capezzuoli 1,\*, Amalia Spina 2, Andrea Brogi 3,4, Domenico Liotta 3,4, Gabriella Bagnoli 5, Martina Zucchi 3, Giancarlo Molli <sup>5</sup> and Renzo Regoli <sup>6</sup>**


**Abstract:** The Pre-Mesozoic units exposed in the inner Northern Apennines mostly consist of Pennsylvanian-Permian successions unconformably deposited on a continental crust consolidated at the end of the Variscan orogenic cycle (Silurian-Carboniferous). In the inner Northern Apennines, exposures of this continental crust, Cambrian?-Devonian in age, have been described in Northern Tuscany, Elba Island (Tuscan Archipelago) and, partly, in scattered and isolated outcrops of southern Tuscany. This paper reappraises the most significant succession (i.e., Risanguigno Formation) exposed in southern Tuscany and considered by most authors as part of the Variscan Basement. New stratigraphic and structural studies, coupled with analyses of the organic matter content, allow us to refine the age of the Risanguigno Fm and its geological setting and evolution. Based on the low diversification of palynoflora, the content of sporomorphs, the structural setting and the new field study, this formation is dated as late Tournaisian to Visean (Middle Mississippian) and is not affected by pre-Alpine deformation. This conclusion, together with the already existing data, clearly indicate that no exposures of rocks involved in the Variscan orogenesis occur in southern Tuscany.

**Keywords:** northern Apennines; Risanguigno Formation; Carboniferous; southern Tuscany; Monticiano-Roccastrada Unit; Tuscan Palaeozoic; palynology

#### **1. Introduction**

Stratigraphic reconstructions of the deep successions involved in orogens later affected by post-collisional extensional tectonics are always tempting, since these are normally metamorphosed and involved in polyphase deformation, are laterally segmented and, consequently, are exposed in scattered outcrops. This is even crucial for the metamorphosed, deep successions of the Northern Apennines [1], which experienced the Variscan sedimentary and tectonic evolution (Devonian-Carboniferous), then the Alpine cycle (Triassic-Oligocene), and ultimately the extensional process leading to the opening of the Tyrrhenian Basin (Miocene-Quaternary). Nowadays, the so-called Tuscan Crystalline Basement (Cambrian?-Devonian [2]) is discontinuously exposed, and the scarcity of fossils remains inhibits precise age determination [3–5]. Thus, in absence of fossil records, the Tuscan Basement is traditionally related to the well-known and better exposed Palaeozoic succession of southeastern Sardinia, where the Alpine deformation is relatively minor [6–9].

To strengthen this approach, several studies of the pre-Alpine metamorphic rocks of the Tuscan Archipelago and Apuan Alps have incorporated palaeontological, stratigraphic

**Citation:** Capezzuoli, E.; Spina, A.; Brogi, A.; Liotta, D.; Bagnoli, G.; Zucchi, M.; Molli, G.; Regoli, R. Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines). *Geosciences* **2021**, *11*, 84. https://doi.org/10.3390/ geosciences11020084

Academic Editors: Jesus Martinez-Frias and Rodolfo Carosi

Received: 5 January 2021 Accepted: 7 February 2021 Published: 12 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and tectonic data [2,9–14]. On the other hand, a few studies have focused on southern Tuscany, where contrasting interpretations on datings and palaeogeographic reconstructions have been proposed [15–20]. More recently, radiometric (Ar/Ar, U/Th [21–23]) and palynological studies [24,25] have served to motivate a review of the entire Tuscan Palaeozoic successions based on bio/chronological markers. These studies contribute more precise age datings and provide new evolutionary scenarios in the context of two distinct Palaeozoic cycles (Mississippian-early Permian and middle-late Permian) [5,26]. Accordingly, we re-consider the Risanguigno Formation, which is regarded as part of the Tuscan Crystalline Basement and constitutes the oldest outcrops in southern Tuscany. In this view, this isolated and scarcely studied formation strongly influenced the reconstruction of the entire Pre-Alpine Apenninic succession. Therefore, the aim of this paper is to document and illustrate a newly discovered palynofloral content and, consequently, to provide the precise age of the Risanguigno Fm, together with its structural setting. This will lead to determination of Variscan formations in this key sector of Northern Apennines from a stratigraphic and palaeogeographic perspective.

#### **2. Geological Outline of the Palaeozoic Units of Tuscany**

The inner Northern Apennines (Figure 1) resulted from the convergence (Cretaceous-Eocene) and collision (Oligocene-early Miocene) between the European Corsica-Sardinia massif and the Adria microplate of the Africa pertinence. This process produced the stacking of tectonic units deriving from oceanic and continental palaeogeographic domains [27,28].

**Figure 1.** Structural sketch map of (**a**) the Northern Apennines and (**b**) Northern Tyrrhenian Sea.

In southern Tuscany these are, from top to bottom (Figure 2): (a) the Ligurian and Sub-Ligurian Units, consisting of remnants of Jurassic oceanic and transitional crust and their related Cretaceous–Oligocene sedimentary cover; (b) the Tuscan Units including the Triassic-early Miocene sedimentary (Tuscan Nappe) and Palaeozoic-Triassic metamorphic succession. According to [29,30], this metamorphic succession can be broadly subdivided in (i) a late Cambrian?-Mississippian basement (affected by deformation during the Variscan orogenesis) and (ii) a Late Pennsylvanian to Triassic sedimentary cover, deposited during the Variscan post-collisional events.

**Figure 2.** Tectono-stratigraphic columns illustrating the main features of the paleogeographic domains of the inner Northern Apennines (redrawn from [27,31]).

The Palaeozoic basement is made up of quartzites and phyllite with acidic to intermediate metavolcanic rock (porphyroid). At their top, black shale, radiolarite (lydian stone) and metacarbonate deposits (dolostone, calcschist) have been detected [4,10,32–34]. This succession, attributed to the Cambrian?-Devonian, on the basis of scattered fossils [4] and U/Th radiometric dating [21–23], is classically related to the central-southern Sardinia succession [35] and is considered to be involved in the Variscan orogeny during the early Carboniferous [12]. In the Inner Northern Apennines, Palaeozoic rocks involved in the Variscan deformation extensively crop out in the La Spezia-Apuan Alps-Mt. Pisani area, while smaller exposures are located in the Tuscan Archipelago (Elba island) and southern Tuscany (Figure 3). It is noteworthy that some deep wells in northern Tuscany (Pontremoli) and in the geothermal area of southern Tuscany are believed to have intersected deformed Variscan rocks [2,8,9,12,14,36–40].

The "post-Variscan" Palaeozoic-Triassic sedimentary succession (referable to the Phyllite-Quartzitic Group of [41]) is mostly exposed in the Monticiano-Roccastrada Unit (Figures 3 and 4), along the Middle Tuscan Ridge, in three different main tectonic units, as defined by [19]: Iano Sub-Unit 1; Monte Quoio-Montagnola Senese Sub-Unit 2; Monte Leoni-Farma Sub-Unit 3—Figure 4. Only minor outcrops are present elsewhere, and locally drilled by boreholes [1,9,10,37,42–46].

**Figure 3.** Distribution of metamorphic units in Tuscany, including Variscan deposits (in black).

This succession is formed by phyllite, metasandstone, metaconglomerate with local carbonate levels attributed to Mississippian-late Permian on the basis of the fossil [16,18,44,47–49], palynoflora content [24,25] and radiometric dating [23]. Its evolution is related to rifting [1], transcurrent/transtensive pull-apart basins [5,50] or to late Variscan compressional events [9,19].

The uppermost part of the succession is represented by the typical Triassic continental quartz-dominated clastic sedimentation belonging to the Verrucano Group [51–53].

During the Apennines collisional stages, the above-mentioned Palaeozoic-Triassic successions were involved in duplex structures, up to HP-LT conditions (P ≥ 1.1 GPa and T ~ 350–400 ◦C) and retrograde green schist metamorphic conditions [54–61]. Their exhumation was favoured by the development of Miocene extensional detachments [26,62], which produced extensional horses (i.e., megaboudins [63]) and the lateral segmentation of the previously stacked tectonic units.

**Figure 4.** (**a**) Schematic sketch of the Middle Tuscan Ridge with geographic distribution of the three structural Sub-Units; (**b**) Simplified stratigraphy and tectonic relation among the three Sub-Units (redrawn from [64]).

#### *Variscan Basement in Southern Tuscany: The Risanguigno Formation*

In this framework, the Risanguigno Fm represents the only cropping out unit in southern Tuscany assigned to the Palaeozoic basement. It is part of Sub-Unit 2 (Monte Quoio-Montagnola Senese) of the Monticiano-Roccastrada Unit (Figure 4).

Such a formation was initially defined in the type locality of the Risanguigno Creek by [4]. These main exposures were previously described by [47,65], although interpreted as part of another formation (Boccheggiano Fm). Subsequently, [20,66,67] related other

outcrops exposed in the surroundings (Farma River, Figures 4 and 5) to the Risanguigno Fm, furthermore recognized in a few boreholes [68].

**Figure 5.** New geological maps of the study areas and related geological cross sections: (**a**) Risanguigno Creek; (**b**) Farma River (previous maps from [20,64,65]). See Figure 4 for their location.

The base of the formation is never exposed, although [20] postulated the presence of a basal stratigraphic unconformity separating the Risanguigno Fm from the underlying Variscan deformed units.

At the top, the Risanguigno Fm is in contact with the Poggio al Carpino Fm [64], a middle-late Permian [19,25,69] clast- to matrix-supported polymictic conglomerate, often alternated with grey quartzose sandstone and subordinate dark grey phyllite [70]. The contact between the Risanguigno and Poggio al Carpino formations is described as an angular unconformity by [18], in contrast with [20], indicating that the Poggio al Carpino Fm stratigraphically overlies the lower unit.

From a sedimentary point of view [2,18,19], the Risanguigno Fm is composed by blackgrey graphitic to bituminous phyllite intercalated with cm- to dm-thick alternations of: (i) grey-greenish to black quartzose, granolepidoblastic metasandstone and siltstone with iron-rich carbonate matrix and detritic mica, (ii) cm-thick microcrystalline, granoblastic dolostone rich in detritic quartz and white mica, (iii) silicified grey metalimestone, and (iv) thinly bedded, grey-greenish to black chert and radiolarian lydite. A chert sub-sequence, up to 4.5 m thick and intercalated with fine-grained clastics, was also recognized in close outcrops by [66], and later correlated with the small chert sequence present also in the Risanguigno type locality [20]. Anhydrite in the silicified limestone is reported by [17,65], while this is not described by [68]. Local post-tectonic chloritoid needles are reported in the metasandstones and metasiltstones by [2].

Rocks are strongly deformed, making the stratigraphic reconstruction difficult. By this, and due to the fact that the basal contact is not exposed, the thickness of the Risanguigno Fm is unknown and only inferred in 40 m, at least [20].

Regarding the fossiliferous content, [4,71] reported a conodont fauna, characterized by *Ozarkodina denckmanni*, *Panderodus unicostatus* and *Icriodus* sp. This fauna was recovered from the dolostone levels in the type locality at the altitude of 304 m along the Risanguigno Creek.

Regarding the chert-subsequence, [4,68] accounted for the presence of recrystallized radiolaria, often well preserved although flattened during deformation.

The formation, originally attributed to a generic Carboniferous by [47,65], was ascribed to the Early Devonian on the basis of the conodont fauna [4]. Alternatively, [66] suggested a Tournaisian-Visean age based on the radiolaria observed in the chert sub-sequence, while [17,20] related these siliceous portions to late Devonian-early Carboniferous (late Emsian to Visean?) on the basis of the lithological correlation with similar deposits in the circum-Mediterranean area.

Similarly, the interpretation of the depositional environment is matter of debate. Ref. [4] proposed a shallow marine origin, while [17] favoured a moderately deep water basin origin, owing to the presence of the siliceous portions. In contrast, [71] suggested an epicontinental shelf characterized by recurrent anoxic conditions, while [20] accounted for a highly condensed sequence deposited in a starved, low energetic, distal and relatively deep marine environment.

From a tectonic point of view, the Risanguigno Fm is described as intensely deformed and marked by a metamorphic grade higher than the one affecting the overlying formation, i.e., the Poggio al Carpino Fm [4]. In this view, according to [9,37], the Risanguigno Fm evidences relics of a pre-Alpine deformation, interpreted as a Variscan syn-metamorphic tectonic foliation relatable to the Sudetic event.

#### **3. Materials and Methods**

A detailed field survey was carried out in key areas where the Risanguigno Fm is exposed. The fieldwork was dedicated to field mapping and data collection for describing the deformation affecting the Risanguigno Fm and the overlying units. During the survey, 21 samples of black phyllite, metasiltstone and metacarbonate (16 samples from the Risanguigno Creek and 5 samples from the Farma River (Table 1) were collected for petrographic, microfacies and biostratigraphic studies. Palynological samples (c. 20 g each for phyllite and metasiltstone lithologies and 100 g each for metacarbonate samples) were treated by standard palynological acid maceration (with 37% HCl, 50% HF, boiling HCl 10%), density separation of the organic matter (using a ZnCl2 solution) and filtration of

the organic-rich residue at 10 μm. As a result of the high degree of thermal alteration, the organic residue was treated with Schultz solution and filtered with 10 μm sieve.

**Table 1.** Analysed samples, with related geographical coordinates, lithology and content (quotes in meter above sea level —m a.s.l.).


Light microscope observations were performed on palynological slides using a Leica DM1000 microscope (Leica, Wetzlar, Germany) using the differential interference contrast technique in transmitted light. Images were captured using the camera on the digital microscope and successively corrected for contrast and brightness using the open-source Gimp

software. The palynological slides are stored at the Sedimentary Organic Matter Laboratory of the Department of Physics and Geology, University of Perugia, Italy. Metacarbonate samples were collected from dolostone levels for the analyses of conodont content and processed by standard procedures using 10% acetic acid. The residue was washed through a 71 μm sieve.

#### **4. Results**

The results are summarized in different sections, according to the main issues of lithology, fossil content, and deformation.

#### *4.1. Lithological Characteristics*

The outcrops exposed in the Risanguigno Creek and Farma River were revisited (Figure 5).

In the Risanguigno Creek, the formation crops out in two small windows (quote 304 m and quote 324 m a.s.l.) in correspondence of the riverbed (Figure 5a). A small supplementary outcrop, never described before, was discovered along the riverbed at quote 300 m. The Risanguigno Fm is mostly dominated by black to grey phyllite, locally intercalated by cm-thick level and lenses of metasandstone and metasiltstone (Figure 6a,b). Only in the outcrop of quote 304 m is phyllite intercalated with cm-thick beds and lenses of microcrystalline dolostone and silicified grey metacarbonate (Figure 6c). These are geometrically positioned below a small succession (max 2 m thick) displaying alternation of phyllite and chert beds/lydite in thinly bedded laminae (Figure 6d). The transition from the Risanguigno Fm to the overlying grey quartzose metasandstone and metaconglomerate formation (Poggio al Carpino Fm) is marked by a sharp angular unconformity (Figure 6e).

Along the Farma River, outcrops are located close at the Ferriera locality, on the right bank of the riverbed at quote 270 m and 265 m a.s.l. (Figure 5b). Black bituminous phyllite, locally smelly and rich in millimetric-sized crystals of pyrite, is the dominant lithotype. Conversely, metasandstone and metasiltstone, as also dolostone and metacarbonate, are less diffuse. The 4.5-m-thick chert sequence evidenced by [66] constitutes the main lithological variation and morphological prominence (Figure 6f,g). Similarly to the Risanguigno Creek, here, also, the chert beds are positioned at the top of the succession, immediately below the Poggio al Carpino Fm.

In both valleys, the complete stratigraphic reconstruction is prevented by an intense folding (see the next paragraph).

#### *4.2. Fossiliferous Content*

Twenty-one samples were obtained from almost all the analysed outcrops. In the Risanguigno Creek, three samples were from quote 324 (RIS14, 15, 16) and thirteen from quote 304 (RIS1, 2, 3, 4, 5, 6, 7, 11, 12, 13, 22, 23, 24). In the Farma River, two samples were from quote 265 (RIS17,18) and three from quote 270 (RIS19, 20, 21). All of them were analysed for their palynological content. Four samples were analysed for conodont content (Table 1).

#### 4.2.1. Palynological Content

Ten samples out of 21 were productive, even yielding strongly degraded palynofloral assemblages (Table 1). Degradation was mainly due to intense in situ pyritization affecting the exine of miospores. A low metamorphic grade with a temperature of about 350– 400 ◦C [56] was recognized. Nonetheless, different organic microfossils were recognized, adding new data for the age determination of the Risanguigno Fm. The palynological assemblage mainly consists of ornamented forms as *Auroraspora balteola*, *Claytonispora distincta*, *Retialetes radforthii*, *Vallatisporites? hystricosus*, *Perotrilites magnus*, *Spelaeotriletes balteatus*, *S. pretiosus* and *Grandispora* sp. Different tetrads of indeterminate apiculate spores also occur (Figure 7).

**Figure 6.** Lithological characteristics of the Risanguigno Fm: (**a**) view of the dominant black phyllite; (**b**) thin intercalations of metasiltstone lenses; (**c**) examples of metacarbonate beds; (**d**) example of thin laminae and beds of lydite; (**e**) upper contact of the Risanguigno Fm with the Poggio al Carpino Fm with evidence of the angular unconformity; (**f**) chert sequence cropping out along the Farma River; (**g**) detail of thinly laminated chert sequence.

#### 4.2.2. Conodont Content

All processed samples were barren in terms of conodont content.

#### *4.3. Deformation*

The Risanguigno Fm, together with the overlying units (Permian Poggio al Carpino Fm and Triassic Verrucano Group) exposed in the Risanguigno Creek and Farma River (Figure 5), are commonly involved in polyphase folding characterized by superposed F1 and F2 folds, with NS and NS-NNE axial trends, respectively.

**Figure 7.** Miospores from Risanguigno Formation. Scale bar indicates 10 μm. (**1**) *Claytonispora distincta* Playford and Melo 2012 (slide: RIS 3). (**2**) *Pustulatisporites* sp. (slide: RIS 3). (**3**) *Retialetes radforthii* Staplin 1960 (slide: RIS 15). (**4**,**15**) Indeterminate miospore (slide: RIS 15). (**5**) Tetrad of indeterminate apiculate spores (slide: RIS 3). (**6**,**7**). *Vallatisporites? hystricosus* (Winslow) Byvscheva 1985 (slide: RIS 15). (**8**) *Auroraspora balteola* Sullivan 1964 (slide: RIS 3); (**9**) *Spelaeotriletes balteatus* (Playford) Higgs 1975 (slide: RIS 15). (**10**,**11**) Indeterminate spore with a heavily pyritized exine. (slide: RIS 15). (**12**) *Spelaeotriletes pretiosus* (Playford) Neves and Belt 1970 (slide: RIS 18). (**13**) *Grandispora* sp. (slide: RIS 15). (**14**) *Perotrilites magnus* Hughes and Playford 1961 (slide: RIS 15).

Both folding events are referrable to the Alpine evolution. F1 folds are the prominent structures and involve the entire succession, defining the main shape and geometries of the exposures (Figures 8 and 9). F1 folds range from map-scale to outcrop-scale and have hectometre- to decimetre sizes (Figure 8). These consist of tight and isoclinal recumbent folds, with axial planes steeply dipping toward west. F1 hinge lines mostly dip gently toward S-SE (Figure 8), although in some rare cases they dip toward N-NW (Figure 9).

**Figure 8.** N-S trending F1 folds affecting the metasandstone and metapelite succession of the Poggio al Carpino Fm exposed in the Risanguigno Creek. (**a**) F1 sub-isoclinal folds and related S1 axial planar tectonic foliation and related stereographic diagram (lower hemisphere, equiareal diagram); (**b**,**c**) enlarged sector of the F1 hinge zones indicated in (**a**) and showing the S0/S1 angular relationships also shown in the stereographic diagram (lower hemisphere, equiareal diagram); (**d**,**e**) hinge zone of F1 fold with a pervasive axial planar S1 tectonic foliation developed within the metapelite; relationships between S0 and S1 are indicated in the stereographic diagram (lower hemisphere, equiareal diagram).

In the quartz-metasandstone and metaconglomerate, S1 is a rough cleavage, as highlighted by the differentiated domains when alternating quartzitic and micaceous layers are present. The L1 object lineation occurs in the phyllite and metasilstone, defined by elongated quartz and mica lenses tracking the x axis of the finite strain ellipse.

**Figure 9.** Macroscopic scale deformation pattern of the phyllite and metacarbonate belonging to the Risanguigno Fm. (**a**) detail of the contact separating the Risanguigno Fm from the Poggio al Carpino Fm and geometrical relationships between S0/S1 and S2 foliations (see the text for more details) also indicated in the stereographic diagram (lower hemisphere, equiareal diagram). (**b**) Penetrative S1 foliation crossing the metaconglomerate level belonging to the basal part of the Poggio al Carpino Fm. (**c**) Centimetre-scale isoclinal F1 folds and the S1 axial planar tectonic foliation crossed by the S2 tectonic foliation. (**d**,**e**) F2 open folds affecting the S0/S1 foliations affecting phyllite and metacarbonate levels.

At the microscopic scale, S1 relates to a continuous foliation, mainly defined by elongate quartz layers, formed by flattened and dynamically recrystallized grains, alternated with mica-rich domains (Figure 10a,b). Mica domains are mainly composed of fine-grained white mica and biotite (Figure 10a–d) with locally developed chloritoid crystals, grown both along the main foliation and crossing it (Figure 10d).

**Figure 10.** (**a**,**b**) Phyllitic quartzite (Risanguigno Fm) showing the S1 foliation consisting of metamorphic layering made up of quartz and white mica + biotite levels ((**a**) plane polarized light; (**b**) crossed polars). (**c**,**d**) Microscale F1 fold with associated S1 foliation mainly formed by quartz + white mica + biotite + chloritoid. This latter is also represented by post-kinematic bigger crystals (see (**e**)), suggesting syn- and post-S1 development ((**c**) plane polarized light; (**d**) crossed polars). (**f,g**) Mineralogical association of the S1 foliation developed within carbonate rich levels, mainly composed by qtz + cc + white mica + biotite + chloritoid ((**f**) plane polarized light; (**g**) crossed polars). (**h**,**i**) s-c shear zone (top-to-the right) affecting the organic matter-bearing phyllite and developed coevally with the S1 foliation; the latter is affected by a later crenulation cleavage possible related to the F2 folding event ((**h**) plane polarized light; (**i**) crossed polars).

Syn-kinematic calcite locally developed within the polycrystalline quartz-rich layers (Figure 10e,f). Localized mylonitic layers with mica fish structures (Figure 10 g,h) developed mainly at the boundary between quartz- and mica-dominated domains. F2 folds are also recognisable both at the map and outcrop scale and display hectometre to decimetre sizes (Figures 8 and 9). F2 folds deformed F1 isoclinal folds and their related S1 axial planar foliation. F1/F2 fold interferences have been reconstructed in the Farma Creek area (Figure 8), where F1 folds affecting the Triassic and Palaeozoic succession have been deformed by top-to-the-East verging F2 folds. F2 folds consist of gentle to close folds, in some cases overturned. Axial planes are gently to moderately dipping toward West. F2 hinge lines are sub-horizontal or deep gently toward SSW (Figure 9).

An axial-planar foliation (S2) is associated with F2 folds, well developed only in the metapelite levels (Figure 9). It ranges from spaced disjunctive to a crenulation cleavage. At the microscopic scale, the S2 consists of a spaced foliation often producing zonal crenulation cleavage defined by symmetric or asymmetric microfolds.

#### **5. Discussion**

The newly obtained data, especially from the bio-chronological perspective, allow us to frame the Risanguigno Fm in a new scenario with fallouts in the Palaeozoic palaeogeography of Gondwana. We describe this in the following sections.

#### *5.1. New Bio-Chronological Framework of the Risanguigno Fm*

The palynological assemblage shows similar compositional characteristics to those documented in the Mississippian successions of Western Europe, northern Gondwana and other areas.

*Auroraspora balteola* was documented in the mid-Visean within the *Knoxisporites triradiatus-Knoxisporites stephanephorus* (TS) Zone of Kammquartzite Formation in the Rhenohercynian Zone (Germany; [72]) and in the late Visean of England [73] in assemblage with *Spelaeotriletes pretiosus* in the Tournaisian of eastern Scotland [74]. This last taxon marks the base of the *Spelaeotriletes pretiosus-Raistrickia clavata* (PC) Zone attributed to the late Tournaisian and first described from SW Britain [75] and from Ireland [76]. Later, in the latter country, [77] also documented the PC biozone, characterized by the occurrence of *S. balteatus* and *Claytonispora distincta* within a stratigraphic interval attributed to middlelate Tournaisian on the basis of conodont fauna. In Belgium, the base of the PC biozone occurred within the upper *Siphonodella crenulata* conodont Zone (late Tournaisian, [78]). *Spelaeotriletes pretiosus* was also reported in assemblage with *S. balteatus* from other Mississippian sequences of Western Europe [79–82], North America [83–86] and China [87]. The species was also documented from similar-aged rocks in some regions of Northern Gondwana. In particular, in North Africa, [88,89] considered microfloristic assemblage marked by the occurrence of *S. pretiosus*, *S. balteatus* and *Vallatisporites vallatus* of late Tournaisian age, without excluding a younger early Visean age. In Algeria, *S. pretiosus* occurred in the middle Tournasian-lower Visean palynozones [90,91]. In Libya, a similar microflora was found in the late Tournaisian-Visean time interval (palynozones XI and XII [92]; palynozones 13 and 14 [93,94]). Analogous palynoflora also occurs in the Tournaisian-early Visean of Saudi Arabia [95,96] and the Central Iranian Basin [97]. In Southeastern Turkey, [98] tentatively correlated the *Spelaeotriletes pretiosus-Aratrisporites saharensis* assemblage, where *Vallatisporites hystricosus* also occurs, with the PC biozone of Western Europe. In Western Gondwana regions, a similar assemblage also characterizes the late middle to early late Tournaisian *Spelaeotriletes pretiosus-Colatisporites decorus* Biozone documented from Brazil [99–105]. On the other hand, in the northern Gondwana regions, *S. balteatus* was also documented in slightly younger time-intervals (e.g., Visean of Libya [89]; Visean of Morocco [106]; Visean-Bashkirian of Saudi Arabia [95,107]).

Regarding the conodont content previously reported by [4], the new investigation carried out in the same levels was not productive. This negative evidence, coupled with the contemporaneous presence of a younger-aged rich microflora, suggests that the previously reported conodonts were reasonably reworked fossils, deriving from older deposits.

Therefore, based on the stratigraphic range of the recorded microflora, we can confirm the age of Risanguigno Fm as being late Tournaisian-Visean, as already suggested by [66] on the basis of radiolarian fossil content.

#### *5.2. Paleoenvironmental Insights*

The Risanguigno Fm depositional environment was highly debated in previous studies and alternatively attributed to shallow [4], moderate [7,64] or relatively deep marine environments [20]. The presence of Middle Mississippian metacarbonate/dolostone and siliceous portions (lydite beds) seems consistent with carbonate-to-radiolarite platform environment, also recognized in several lower Carboniferous tectofacies (eastern Southern Alps, Karawanken Mountains, external Dinarides, southern margin of the Pannonian Basin, Aegean islands, Calabria and southern Sardinia [7,107]) of the central Mediterranean area. Accordingly, the lydite deposits do not necessarily require a deep-water environment since these can develop in different depositional areas [108,109], especially if associated with a local silica-enrichment related to volcanic activity in nearby zones [4]. In this view, it is worth remembering that the Variscan evolution was associated with a widespread magmatism during the late Carboniferous [110–112], as well as during the Mississippian [112–115]. On the other hand, the organic-rich property of the phyllite supports the deposition in a starved, oxygen-deficient environment. In fact, the finding of spores characterized by pseudosculpture induced by deposition of pyrite crystals in the wall (exine) interstices is indicative of syn-depositional pyrite, suggesting that the water/sediment interface was in a strongly reducing state [116–119]. Regarding the bathymetric definition of this anoxic environment, the interpretation remains difficult. Nonetheless, the type and morphology of the recovered microflora are indicative of a shallow-marine-to-epicontinental depositional environment: the presence of ornamented spores and tetrads suggest a proximal depositional environment since the spores were selected according to their hydrodynamic equivalence, and the tetrads did not maintain their integrity along the distal direction [120].

Consequently, we interpreted the Risanguigno Fm as being deposited during the Middle Mississippian in a shallow-marine-to-epicontinental setting, characterized by starved, anoxic condition in its lower portion and progressively evolving to carbonate-radiolarite platform. Some authors [121] have evidenced that chert sedimentation dominated during the late Devonian and Mississippian in the tropical Palaeotethys strait, and associated their development with sea-level rise.

The organic-rich deposit could also be related to oceanic anoxic events known for the late Frasnian to Late Mississippian age and influenced by global climatic and oceanographic changes. One of these corresponds to the mid-Tournaisian carbon isotope excursion (TICE) [122,123], as indicated by the largest positive δ13C excursion in the Phanerozoic. This is related to the climatic transition between the Devonian greenhouse and the late Paleozoic ice age [124]. Such TICE was interpreted as being the result of either Oxygen Carbon sequestration in foreland basin deposits (tectonic-sedimentation driver [122,125]) or oxygen minimum zone expansion (marine anoxia driver [126–129]).

#### *5.3. Stratigraphic Setting*

The new palynological evidence frames the Risanguigno Fm in the Mississippian, thus implying a reconsideration of the southern Tuscany Palaeozoic setting.

The Risanguigno Fm represented an issue in the lateral juxtaposition with the other southern Tuscany Palaeozoic deposits belonging to the three sub-units of the Monticiano Roccastrada Unit (i.e., Sub-Unit 1: Scisti di Iano Fm—[130,131]; Sub-Unit 3: Calcari di S.Antonio-Scisti a Spirifer formations—[16,18]—Figure 4b). The new attribution of the Risanguigno Fm to the Middle Mississippian implies a stratigraphic correlation with all these Carboniferous deposits as representing different portions of a same marine depositional environment, evolving through time.

In this view, the Risanguigno Fm is interpreted as the older cropping out deposits of the basin. This shallow marine-to-epicontinental setting was progressively evolving, in its upper part, to a Moscovian shale-carbonate deposition (Calcare di S.Antonio Fm— Scisti a Spirifer Fm; [16,18]—Figure 11) and open marine environment during Upper Pennsylvanian (Scisti di Iano Fm. [131]). A similar age (up to lower Permian) is also testified for the continental succession (Scisti di San Lorenzo Fm; [132]) exposed in the northernmost area of Tuscany (Figure 11).

**Figure 11.** Stratigraphic chart relating the different successions of the Monticiano-Roccastrada Unit located in the Middle Tuscan Ridge (for location, see Figure 4a): Pisani Mts: 1a—Scisti di San Lorenzo Fm; 1b—Breccia di Asciano Fm; Iano (Sub-Unit 1): 2a—Scisti di Iano Fm; 2b—Breccia e Conglomerati di Torri Fm; 2c—Scisti Porfirici Fm; 2d—Fosso del Fregione Fm; Mt. Quoio-Mt. Senese (Sub-Unit 2): 3a—Risanguigno Fm; 3b—Poggio al Carpino Fm; Mt. Leoni- Farma River (Sub-Unit 3): 4a—Calcare di Sant'Antonio Fm; 4b—Scisti a Spirifer Fm; 4c—Farma Fm—Falsacqua Fm; 4d—Carpineta Fm—Quarziti di Poggio alle Pigne Fm; 4e—Le Cetine Fm.-Conglomerato di Fosso Pianacce Fm. See the main text for references.

> This Carboniferous-lower Permian deposition was succeeded by a second middlelate Permian sedimentary cycle (see, e.g., [5] for a review) where the older deposits were partially dismantled and accumulated in the new one. This is also testified in the Monticiano-Roccastrada area by the occurrence of numerous clastic fragments relatable to the Risanguigno Fm [4], or by the presence of middle Carboniferous (late Visean-early Namurian: [16,47–49,133,134]) clasts, bioclasts and olistoliths embedded within the middlelate Permian Farma and Carpineta formations [70].

#### *5.4. Deformation Insights*

The deformation evidenced by the structural survey indicates that the Risanguigno Fm shared its tectono-metamorphic evolution with the overlying middle-late Permian Poggio al Carpino Fm and Triassic Verrucano Group, therefore highlighting their involvement in the Alpine deformational history. Noteworthy, neither outcrop-scale nor microscopic-scale evidence suggests the involvement of the Risanguigno Fm in a pre-Alpine deformation. This implies that the depositional environment of the Risanguigno Fm remained reasonably external to the orogenesis of Variscan chain, even during the formation of foreland and/or piggy-back basins. In this view, the presence of a Variscan tectonic phase in explaining the angular unconformity separating the Risanguigno Fm with the overlying Poggio al Carpino Fm and attributed to the Sudetic [15,43] or Bretonian phase [9,18,38] is

denied. Therefore, such an angular unconformity is to be considered as having developed during the Carboniferous-Permian post-collisional tectonic regime [32,45,53], giving rise to short-lived, possibly pull-apart basins, dominated by continental to shallow-marine conditions [5].

#### *5.5. Paleogeographical Implications*

According to several reconstructions [1,5,135–141], during the Variscan evolution the Mississippian foredeep and piggy-back basin facies are always represented by coarsedominated deposits (Culm facies—[141–144]) rapidly involved in the orogenesis and then progressively dismantled during exhumation and uplift. Accordingly, this foredeep basin was considered as possibly having been affected by late Variscan deformation [9,19], thus determining basins and rises, bringing to highly diverse depositional settings [20]. Coupling this latter interpretation with the results of the new structural survey (which rules out the Variscan deformation), we conclude that the Risanguigno Fm is not related to the Culm deposits. Thus, we propose the Risanguigno Fm as the oldest deposits of this sedimentary succession promoted in the "stable" Gondwana foreland that developed within fairly narrow continental or epicontinental domains. These depositional features could have favoured the low-energy, anoxic environments.

These settings evolved during the Late Pennsylvanian-Permian [32,45,64], originating graben/semigraben [1] or transcurrent/transtensive pull-apart basins [5,32,145] dominated by continental (Scisti di San Lorenzo Fm [131]) to shallow-marine conditions (Scisti di Iano Fm [130]), or local development to carbonate platform (Calcare di S. Antonio Fm [7]).

#### **6. Conclusions**

The new palynological-fossiliferous data for the Risanguigno Fm, coupled with its sedimentary and deformational setting, make it possible to assign it to the Middle Mississippian (late Tournasian-Visean) and to exclude its encompassment in the Variscan basement.

For this reason, it is possible now to exclude in southern Tuscany the outcrops of successions deformed during Variscan Orogenesis. Consequently, the Tuscan Crystalline Basement (Cambrian?-Devonian) is only exposed in the northern Tuscany (Apuan Alps, Pisani Mts and La Spezia area) and Tuscan Archipelago (Elba Island).

Sedimentation of Risanguigno Fm occurred in a shallow-marine-to-epicontinental setting, characterized by starved, anoxic conditions. This setting, localized in the Variscan foreland, evolved to open marine during the Pennsylvanian-Permian without any involvement in the Variscan Orogenesis.

On these bases, the Tuscan Palaeozoic-Triassic sedimentary succession (Phyllite-Quartzitic Group of [40]), classically considered as "post-Variscan" and now comprising the Middle Mississippian Risanguigno Fm, is no more to be related to the Variscan Orogenesis.

**Author Contributions:** Conceptualization, E.C., D.L., A.B. and A.S.; methodology, A.S., A.B. and E.C.; software, E.C., A.B., A.S. and M.Z.; formal analysis, A.S., G.B., A.B. and E.C.; investigation, E.C, A.B., A.S. and R.R.; writing—original draft preparation, E.C.; writing—review and editing, A.B., D.L., A.S., G.B., M.Z., G.M. and R.R.; project administration, E.C., A.S.; funding acquisition, E.C. and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Research Project of the Department of Physics and Geology (University of Perugia) 'RED-AB—REDiscovering the Apennine Basement: a multidisciplinary correlation among the Northern Apennine Upper Paleozoic–Triassic successions (CAPEZBASE2017) is acknowledged by E.C. and A.S. E.C. thanks to the Research Project 2021 of the Department of Earth Sciences (University of Firenze) "Circolazione di fluidi nel sottosuolo e travertini: esempi in aree chiave del Settore Tirrenico dell'Appennino Settentrionale (Italia) e dell'Anatolia (Turchia)". A.S. thanks also PRIN 2017RX9XXXY.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The presented data are available on request from the corresponding author.

**Acknowledgments:** Enrico Capezzuoli and AmaliaSpina warmly thanks Mauro Aldinucci (Eni) and Geoff Clayton (University of Sheffield, UK) for the discussions on the treated subjects. The authors thanks to Ausonio Ronchi, and an anonymous reviewer for their constructive comments.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

