*Article* **Timing of Contractional Tectonics in the Miocene Foreland Basin System of the Umbria Pre-Apennines (Italy): An Updated Overview**

**Francesco Brozzetti 1,2, Daniele Cirillo 1,2,\* and Lucina Luchetti <sup>3</sup>**


**Abstract:** A large dataset of lithostratigraphic and biostratigraphic data, concerning the Early-Late Miocene turbidite succession of the Umbria pre-Apennines, is presented and analyzed. The data come from the study of 24 sections that are representative of all the main tectonic units cropping out between the front of the Tuscan allochthon and the Umbria-Marche calcareous chain. The sections have been dated using quantitative calcareous nannofossil biostratigraphy and, wherever possible, they were correlated through key-beds recognition. Such a multidisciplinary approach allowed us to reconstruct the evolution of the Umbria foredeep over time and to unveil the chronology of compressive deformations by defining: (i) the onset of the foredeep stage in each structural unit, (ii) the age of depocenter-shifting from a unit to the adjacent one, (iii) the progressive deactivation of the western sector of the foredeep due to the emplacement of allochthon units, and (iv) the internal subdivisions of the basin due to the presence of foreland ramp faults or thrust-related structures. A further original outcome of our study is having brought to light the Late Burdigalian "out-of-sequence" reactivation of the Tuscan allochthon which bounded westward the foredeep, and the subsequent protracted period of tectonic stasis that preceded the deformations of the Umbrian parautochthon.

**Keywords:** Umbria pre-Apennines; foreland basin systems evolution; timing of contractional tectonics; biostratigraphic constraints to foredeep deposition

#### **1. Introduction**

It is widely recognized that, in a foreland basin system, the spatial-temporal distribution of sedimentary processes reflects the tectonic deformations which drove its structuring [1,2]. Such a close link between the evolution of the chain and the depositional events that occurred in the foredeep and the associated satellite basins is well documented in any stage of the Apennines orogenic system [3–7].

During their eastward migration, the foreland basin systems of the Apennines included the following tectono-sedimentary zones [1,2,8]: (i) piggyback or thrust-top basins, characterized by continental and shallow marine deposition occurred unconformably above older foredeep successions, (ii) foredeep basin, a high-subsidence basin hosting deep-water turbidites lying in slight unconformity on pre-orogenic successions, and (iii) forebulge sector, where sedimentation of hemipelagic ramp-mud [9] occurred, in substantial conformity, on the foreland monocline.

We tried to reconstruct the nature and timing of the tectonic events which affected the Umbria pre-Apennines based on a detailed litho- and bio-stratigraphic analysis of the sin-tectonic successions outcropping in the sections shown in Figure 1, which have been selected taking into account also their present structural setting.

**Citation:** Brozzetti, F.; Cirillo, D.; Luchetti, L. Timing of Contractional Tectonics in the Miocene Foreland Basin System of the Umbria Pre-Apennines (Italy): An Updated Overview. *Geosciences* **2021**, *11*, 97. https://doi.org/10.3390/ geosciences11020097

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

Received: 5 January 2021 Accepted: 8 February 2021 Published: 19 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/).

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**Figure 1.** Structural Geological scheme of the study area showing the main tectono-stratigraphic units in which the Early-Middle Miocene Marnoso Arenacea basin is presently split according to the data reported in [10–16]. Asterisks and acronyms locate the studied sections as explained in the following: REN Unit: Ml = I Molini, Ms = Monte Sperello, Cv = Castelvieto, Im = Il Molino; Mt Nero Unit: Ba = Balconcelli, Co = Corciano, Mn = Monestevole, Fb = Fosso della Badia, Sp = Case Spertaglia, Mc = Monte Casale, Ve = Vesina; M. S. Maria Tiberina area: Sl = San Lorenzo, Mr = Monte Cedrone; Pietralunga Unit: Pz = Piazze, Sc = Santa Cecilia, Po = Portole, Vm = Valmarcolone, Ss = Sassarone, Pt = Pietralata, Mo = Moravola; Gubbio Unit: Cs = Contessa, Be = Bavelle; Mf = Madonna dei 5 Faggi, Bf = Belfiore.

The Umbria pre-Apennine is a ~40 km-wide belt delimited to the west by the front of the Tuscan Falterona nappe and to the east by the inner border of the Umbria-Marche calcareous chain (Figure 1) [10–14].

Its shallow structure consists mainly of an east-verging imbricate fan formed by the stacking of Early to Late Miocene successions of turbidites and hemipelagites. Within the pre-Apennine, the Meso-Cenozoic Umbria-Marche multilayer crops out only in correspondence of narrow culminations located in the Perugia Mts ridge (west of the Tiber river) and in the Gubbio and Mt Subasio doubly plunging anticlines (eastern Umbria, Figure 1).

The major thrust-sheets correspond to regional tectono-stratigraphic units referred to, in the literature, as Mt Rentella, Mt Nero, Pietralunga, Gubbio, and Mt Vicino units, from west to east (Figure 1) [11–15]. The turbidite succession characterizing each one of these units differs from the neighboring ones as regards the age of the top of the basal ramp-mud, the time interval during which the turbidite sedimentation took place, the feeding areas of the gravity flows, and their dispersal pattern [15–20].

A clear eastward-younging trend is documented by the ages through which each unit evolved, from the foreland to the foredeep stage, and was progressively incorporated into the chain.

In fact, as highlighted by previous biostratigraphic data, thrusting rejuvenated from the Late Burdigalian, in the Mt Rentella Unit, to the Early-Late Tortonian, in the easterner Gubbio ad Mt Vicino units [15–17,21–23].

The various tectonic stages which involved the basin were marked by major changes in local and regional stratigraphy, in the sedimentary rates, in turbidite facies, and arenite composition.

An additional factor that had a significant influence on foredeep sedimentation and its duration over time was the emplacement, on the western border of the basin, of allochthon terrains of Tuscan and Ligurian pertinence. The latter became, for a long period, the source areas of transversal sedimentary inputs and of regional-scale olistostromic bodies that were embedded at various levels within the Marnoso Arenacea Fm [17,19,24,25].

In such a general picture, an overall and updated synthesis of the depositional architecture and the structural setting of the pre-Apennine foreland basin is still lacking, and the timing of deformation has been defined only in broad terms, except for restricted sectors.

The first objective of our work is therefore to revise the stratigraphy of the Miocene successions at the scale of the entire basin, defining the vertical and lateral relationships among the different units.

Once the stratigraphic framework has been reconstructed, we highlight those depositional events that were driven by tectonics and are useful to define a high-resolution timing of the deformational events that affected the foredeep, from its onset to the complete accretion into the chain.

In particular, an issue that was not previously explored in-depth, and deserves to be addressed, is the influence exerted on sedimentation by the concurrent activity of contractional structures and foreland ramp faults, both displacing, during the Middle-Late Miocene, the Meso-Cenozoic carbonate substratum [7,21,22].

We have pursued the aforementioned goals by analyzing, reviewing, and correlating a total of 24 stratigraphic sections representative of the entire foreland basin system of the Umbria pre-Apennine, most of which unpublished. The sections, which have been studied through quantitative nannofossil biostratigraphy, are located in the six following key-areas; from west to east, these are (Figure 1): (1) Mt Rentella, (2) Perugia Mts ridge, (3) Alpe della Luna, (4) M. S. Maria Tiberina, (5) Pietralunga-Mt Urbino ridge and (6) Gubbio-Serra Maggio.

This paper aims to provide an overview of all turbidite successions referred, in the literature, to the Umbrian domain, giving useful chronological constraints to future research on the tectonics and geodynamics of the Tyrrhenian-Apennines system, focusing on the Miocene deformational stages.

We are also confident that our work can be explanatory of a methodological approach, based on the intimate integration of stratigraphy and tectonics, that we consider highly effective in the study of foreland basin systems.

#### **2. Materials and Methods**

#### *2.1. Physical Stratigraphy*

We started from an accurate revision of a huge bulk of unpublished lithostratigraphic data, acquired during original research surveys and the CARG project 289-Città di Castello Sheet of the Carta Geologica d'Italia [20].

These data, which had already been calibrated by nannofossils biostratigraphy [26], were subsequently homogenized with the datasets reported in previous works dealing with the tectono-stratigraphic setting of the Mt Rentella [15], M. S. Maria Tiberina [7,21,27], and Mt Nero [18] units. Further stratigraphic sections have been recently studied in sectors of the pre-Apennine where, in the course of this study, the need for further integrative data emerged.

An accurate physical-stratigraphic analysis was carried out on all the sections, to distinguish the stratigraphic units of various ranks within the Miocene successions, such as formations, members, facies associations, and key beds.

Logging was carried out at a 1:50 scale factor, which allowed us to collect and describe all the major sedimentary features characterizing the different types of strata and their composition and provenance, and ultimately formulate hypotheses on the depositional mechanisms and the types of causative gravity flows.

An expeditious compositional determination was also performed by comparing hand specimens with the petrofacies described in several type-sections of the high Tiber Valley by [28–31]. These determinations, integrated with facies analysis (based on the classification scheme defined by [32]) and paleocurrent measurements, allowed us to distinguish the arenites of alpine provenance, generally characterized by fine grain size, distal facies, and low thickness, from the deposits originated by transversal supply [25], which were fed from piggyback basins lying above the Apennines front.

#### *2.2. Biostratigraphy*

During stratigraphic logging, the sections were sampled for micropaleontological purposes. The marly samples collected for the biostratigraphic analysis were prepared as smear slides according to the procedures indicated by [33–35] and subsequently studied under a polarizing optical microscope at 1000× with both parallel and crossed nicols.

The microscopic analyses of nannofossil assemblages were referred to the biozonations schemes proposed by [35–37] for the Late Oligocene and Early-Middle Miocene of the Mediterranean domain (Figure 2).

The adopted schemes still show a better resolution either with respect to the "standard" ones [38,39] or to the recent upgrades of the tertiary nannofossil biostratigraphy [40,41], which have been mainly defined based on ocean bio-events recognized on a global scale. Other updated schemes of the Miocene Mediterranean nannofossils biostratigraphy are available but, having reviewed only some certain time-intervals (the Burdigalian stage in [42] and the Langhian stage in [43]), their use would lead to some problems of chronostratigraphic correlations with the biozonation proposed by [35–37].

The latter works, including the entire Miocene, are therefore still to be preferred for the study of stratigraphic successions that cover a large part of this epoch.

In each sample, the identification of marker species has been integrated by quantitative analysis, carried out following the methodologies developed by several authors [35,44].

It consists of two kinds of counting: the first counts the species percentages within a population of 300 specimens of the whole assemblage; the second is carried out only for the most significant species whose percentage is determined in comparison with a fixed number of coccoliths pertaining to the same genus (f.i. counting of *S. heteromorphus* within 100 sphenoliths).

The application of quantitative methodologies has also allowed the aforesaid authors to establish new bio-horizons based on variations in the abundance of some marker species, such as the First Common Occurrence (FCO), the Beginning of the Paracme (PB), and the End of the Paracme (PE) of *Sphenolithus heteromorphus.*

Finally, in the case of the individuals belonging to the genus *Reticulofenestra*—which, although having been studied in depth [34,45–49], still present classification problems—a biometric approach has been applied. The latter led to referring to *Reticulofenestra pseudoumbilicus* individuals with size > 7 [35,37,44,50] and to distinguish them from those


characterized by a smaller size (which have been, in turn, shared in two classes, respectively characterized by size <6 μm and >6 μm).

**Figure 2.** Biostratigraphic scheme for the Upper Oligocene-Middle Miocene interval. The scheme correlates the nannofossils biozonations proposed by [35,37], used in this work, with the Standard ones by [38,39], recently updated by [41]. The scheme is also integrated (bold rectangle) with the main bio-event characterizing the Mediterranean area.

The variation in abundance of the marker species, within each succession, was highlighted by plotting the relative distribution diagrams alongside the corresponding lithostratigraphic Log.

#### *2.3. Field Geology*

A large part of the fieldwork was carried out in the past years using the traditional geological mapping techniques. These former surveys, however, were recently revised and integrated with digital surveys and new data collection, aided by an Apple iPad-Pro in which a dedicated mapping software, whose details are given in the following section, was installed.

Due to the considerable extension of the study area, most of the structural survey was performed in selected key outcrops to characterize the major tectonic contacts and the spatial relationships between the Tectono-Stratigraphic units, synthesized in Figure 1.

Two commercial seismic lines provided by [51], hereinafter referred to as L1 and L2 (traces in Figure 1), have been interpreted to assess the present thickness of Miocene foredeep deposits and detect the geometry of the contractional deformations. The seismic interpretation through Move software was performed on both the aforesaid lines.

#### **3. An Overview on the Tectono-Stratigraphic Units of the Umbria Pre-Apennines** *3.1. Mt Rentella Unit*

The Mt Rentella unit (REN) [15] crops out in a narrow ~NS striking belt, tectonically sandwiched between the Tuscan Falterona Nappe, to the west, and the Marnoso Arenacea Fm of the Mt Nero Unit, to the east. Its peculiar stratigraphy justifies its classification as an independent tectono-stratigraphic unit derived from a paleogeographic domain located in an intermediate position between the two aforementioned units.

In the type locality (sections Im, Ms, Ml, and Cv, Figure 1), the REN includes, from the bottom, the polychromic marls of Mt Rentella, the cherty marls of Mt Sperello, and the Castelvieto turbiditic sandstones (also referred to as La Montagnaccia Fm by [13,52–55]). The nannofossil assemblages [15] show that the polychromic marls are referable to the MNP25b-MNN1d biozones (Late Oligocene-Aquitanian), the cherty-marly interval to the MNN1d-MNN2a biozones (approximating the Aquitanian-Burdigalian boundary), and the turbidite succession deposited during the MNN2a-MNN2b biozones (Early Burdigalian) [15,53,54]. A bio-chronostratigraphic correlation table, showing the overall stratigraphy, reconstructed "mending" the portions of the REN succession logged in four reference sections (I Molini = Ml, Monte Sperello = Ms, Castelvieto = Cv, and Il Molino = Im, location in Figure 1) is provided in Figure S1 (Supplementary Materials). Based on these results, the Mt Rentella sequence cannot be biostratigraphically correlated either to the topmost part of the Falterona Nappe, which is older [52–57], or to the outcropping turbiditic sequence of the Umbria domain, which is younger [7,17,18].

As regards the compositional data concerning the fine-grained lithic fragments, the arenites characterizing the turbidite succession of the REN differ sensibly from the Macigno sandstones, whereas they are comparable with some Alpine-fed beds of the Marnoso Arenacea Fm [31–51,58,59].

The reported litho- and bio-stratigraphic constraints suggest that the REN represents a tectonic slice originated by the innermost marginal sector of the Umbria domain which, during the Chattian and most of the Aquitanian, was located in the peripheral bulge of the Tuscan foredeep, which will never be reached later on, by the deposition of the Macigno turbidites. Such a hypothesis would account for the remarkable facies affinity between the polychromic marls of Mt Rentella and the "Scisti Policromi" Auctt. occurring at the top of the Tuscan Scaglia, the former being characterized by a larger content of calcium carbonate.

In the study area, the succession of the REN is rootless, being detached on the polychromic marls and tectonically superimposed to the Marnoso Arenacea Fm of the Mt Nero unit. For this reason, its stratigraphic passage to the original carbonate substrate was nowhere observed.

According to [15], this substrate would correspond to the succession cropping out in the Mt Peglia-Mt Piatto ridge (south-western Umbria), characterized by an "Umbria type" facies but showing some affinities with the Tuscan terms, during the Oligocene-Early Miocene [60].

#### *3.2. Units Derived from the Marnoso Arenacea Foredeep*

The Marnoso Arenacea foredeep basin [24,61,62] developed, since Early-Middle Burdigalian times, in front of a tectonic pile made of the REN unit and the overlying Falterona Nappe. The turbidite deposition lasted, within this basin, until the Late Miocene, when it was tectonized and completely incorporated within the Umbria pre-Apennines.

After the seminal work of [10], several studies contributed to identify, within the deformed Marnoso Arenacea of the Umbria area (MAR hereinafter), a number NW-SE trending sectors characterized by significant stratigraphic differences and diachronic tectonic evolutions [7,11–18,21,28,29,61–66].

These sectors, delimited by regional-scale thrust surfaces, can be considered as tectonostratigraphic units, namely thrust sheets whose internal stratigraphy is substantially homogeneous but showing significant differences compared to the adjacent ones (Figure 3). In the present work, for reasons of conciseness, the term "Unit" is used with the meaning above.

**Figure 3.** Comprehensive stratigraphic scheme of the Umbria pre-Apennine Marnoso Arenacea succession cropping out East of the Falterona Nappe and Mt Rentella frontal thrusts to the inner side of the carbonate chain. The columns represent, to all effects, composite Logs obtained by correlating the studied sections, located in Figure 1.

> Within each Unit, sedimentation was mainly controlled by the following tectonic processes: (a) flexural retreat of the regional monocline, which caused the system depocenterramp-peripheral bulge of the foredeep to shift eastward [9], (b) progressive emplacement, along the western border of the foredeep, of allochthon units of Tuscan and Ligurian pertinence [7,15], (c) activity of foreland ramp synsedimentary normal faults, and (d) nucleation and progressive growth of compressional structures in the carbonatic substratum of the basin.

> Ultimately, the MAR Units derive from pre-contractional ~ chain-parallel isopic zones which differed from each other in what concerns the age of onset of turbiditic sedimentation, its duration over time, and, partly, the sedimentological and compositional characteristics of the beds.

> Based on the aforesaid criteria, in the Umbria pre-Apennines, the Mt Nero, Pietralunga, Gubbio, and Mt Vicino Units have been recognized, from west to east (Figures 1, 3 and 4). Their sedimentary and tectonic evolution is synthetically described below.

**Figure 4.** Interpreted seismic reflection profiles crossing the deformed Marnoso Arenacea, cropping out between the Tiber valley Quaternary graben and the Apennine calcareous chain; (**a**) = line L1, (**b**) = line L2 whose traces are shown in Figure 1. Both the "clean" sections are taken from [51] and here re-interpreted in light of the different aims of our work.

#### 3.2.1. Mt Nero-Unit

The Mt Nero Unit includes the more internal successions of the MAR, which originated from the Burdigalian-Langhian depocenter of the Umbria foredeep.

From a tectonic point of view, it belongs to the so-called "internal Umbria Romagna parautochthon" [16], which overrides, along a NW-SE trending thrust fault (Bocca Trabaria thrust), the Pietralunga Unit (Figure 1). Clear exposures of the Mt Nero succession can be observed in the Perugia Mts ridge and on the east sides of the northern Valtiberina, where these successions are also referred to as "Alpe della Luna sequence" (Figure 1).

In previous studies, the lithostratigraphy of the Mt Nero succession had been pointed out for both these areas [17,28,64,65,67], but an updated biostratigraphic calibration was defined only during the recent survey of the 289-Città di Castello Sheet of the Carta Geologica d'Italia [20].

In the Alpe della Luna (Figure 1), the stratigraphic sections are quite continuous and, despite some minor gaps, allow for an investigation of the entire succession lying over the Schlier Fm (Figure 3 and Figure S2). In this area, the MAR, from the bottom, is composed by:


In the Perugia Mts ridge, where only the two lower mrs of the MAR crop out, the exposures are discontinuous (Figure 1) and separated by a thrust fault which splits the Mt Nero Unit in two distinct thrust sheets, the western or "Mt Acuto" and the eastern or "Mt Corona" tectonic elements. Consequently, the stratigraphic continuity between the two members cannot be directly observed.

The Mt Acuto element consists only of the pelitic-arenaceous lower facies of the MAR1 mr, referable to the MNN3a-MNN4a biozones (sections Balconcelli = Ba, Corciano = Co and Monestevole = Mn, location in Figure 1, stratigraphy in Figure 3 and Figure S2) [18].

The Mt Corona element is instead characterized, at the surface, by the higher interval of the MAR1 mr and part of the MAR2 mr (Section Fosso della Badia = Fb in Figure 1), which, consistently with the section of Mt Casale, have been dated to the Middle-Late Langhian, MNN4b-MNN5a zones [18].

Synthesizing, within the entire Mt Nero Unit, the beginning of the MAR sedimentation (i.e., the onset of the foredeep stage s.s.) can be referred to the upper part of the Early Burdigalian, being the lowest passage to the Schlier Fm identifiable in the MNN3a biozone.

Conversely, the top of the Fm is markedly diachronic because it corresponds, in the western sector (Mt Acuto) to the intermediate portion of the MAR1 (MNN4a subzone, Late Burdigalian) and in the eastern sector (Alpe della Luna) to the uppermost MAR3 (MNN5b, Late Langhian; Figure S2).

The Mt Acuto succession does not include the higher part of the MAR1 because, in the internal sector of the foredeep, the MAR sedimentation was interrupted, during the MNN4a biozone, by the emplacement of the Falterona Nappe.

East of the leading edge of the Falterona Nappe, in the easternmost portion of the Mt Nero Unit, the topmost MAR1 mr and the following MAR2 (Late Burdigalian-Early Langhian) and MAR3 (Early-Late Langhian) members continued regularly to deposit, at least up to the top of the MNN5a biozone.

#### 3.2.2. The Monte Santa Maria Tiberina Succession

The M. S. Maria Tiberina area has long been considered a crucial area to investigate the connection between the stratigraphic evolution of the Marnoso Arenacea Fm and the progressive emplacement of the Falterona Nappe along the western boundary of the foredeep.

Conflicting interpretations were formulated in the past on the outcropping Miocene succession, which has recently been re-named as M. S. Maria Tiberina Fm (SMT) in the 239-Città di Castello Sheet of the Carta Geologica d'Italia [30].

The stratigraphic reconstructions formulated by [7,21,30], based on robust biostratigraphic constraints, solved the debate between "autochthonist" [28,68] and "allochtonist" [10,11] interpretations, showing the SMT to have sedimented upon two different substrates, that are the Falterona Nappe and its semi-allochthon cover, in the internal sector, and the lower part of the Marnoso Arenacea Fm (MAR1 mr), in the external one (Figure S3).

More precisely: (i) to the West, the SMT Fm deposited unconformably over the Vicchio Marls of the Late Burdigalian age (MMN4a nannofossil subzone) and, after that, the Falterona Nappe was thrusted upon the inner side of the Umbria foredeep; (ii) in the intermediate zone (Gioiello syncline), the SMT covered the Nappe front and sealed it; and (iii) in the external sector (Mt Cedrone-Uppiano, Section Mr, in Figure 1), the SMT Fm deposited conformably on the Marnoso Arenacea MAR1 mr, whose topmost beds also show a Late Burdigalian age (MNN4a subzone) (Figure S3).

The integrated stratigraphic analysis of the SMT Fm (Section Sl in Figure 1) provides additional information about the timing of the former contractional phases affecting the succession of the Umbria domain.

In fact, the distribution of benthic foraminiferal taxa in the intermediate-to-upper part of this Fm clearly points to a shallowing upward trend of the paleodepth [27] during the Early Serravallian (the boundary between the MNN5b-MNN6a biozones).

This marked change, preluding the end of SMT sedimentation and also the emplacement of a minor olistostromic body, topping the formation [30], can be related to the growth of incipient anticlines involving the underlying carbonate multilayer of the Mt Nero Unit.

According to this interpretation, the contractional tectonics affected the western Umbria domain from the Middle-Late Serravallian, leading to the thrusting and folding of the MAR and the re-folding of the overlying Falterona Nappe front.

#### 3.2.3. Pietralunga Unit

The Pietralunga Unit consists of a succession of turbidites and hemipelagites of Middle Langhian-Late Serravallian age [12,16] deformed by east-verging folds and minor thrusts (Figures 1 and 3).

Its structural arrangement and the thickness of its succession can be well appreciated in two SW-NE striking commercial seismic lines, published by [51], that cross nearly the entire Umbria-pre-Apennines, from the internal contact with the Mt Nero Unit as far as the Gubbio Normal Fault to the East (Figure 4, traces in Figure 1).

Although these profiles have been acquired for deeper targets, deformations affecting the Pietralunga Marnoso Arenacea MAR with the adjacent Mt Nero (to the west) and Gubbio (to the East) Units.

The proposed interpretations allow us to refer the Pietralunga Unit to an imbricate fan, detached on the top of the Meso-Cenozoic carbonates, in which each splay thrust branches upward, in-sequence, from the basal sole thrust.

Nevertheless, some faults seem to penetrate the carbonates, in correspondence with the major structures of Perugia Mts and Gubbio, refolding the overhanging shallow thrust.

In the intermediate part of the L2 line, in a sector unaffected by significant thrust and tectonic doubling, the top of the Meso-Cenozoic carbonates is located at a pseudo-depth of ~1 s TWT, which, assuming a v = 4.0 km/s seismic velocity [51], provides, for the Miocene succession including the Bisciaro, Schlier, and MAR Fms, a maximum thickness of ~2000 m.

Within the Pietralunga Unit, the age of the basal passage of the turbidites to the Schlier Fm is undetermined, as well as the latero-vertical stratigraphic relationships with the Mt Nero succession, because the lower part of the Unit is buried, and the occurrence, below it, of the MAR1-MAR3 members is uncertain.

The presence of Early Langhian terms, possibly with reduced thickness compared to the western Umbria succession, is suggested by some limited outcrops of a peliticarenaceous succession containing a 7 m-thick calcarenite, sampled in the Piazza area (5 km SE of Pietralunga). This section (Pz in Figure 1) has been tentatively related, in age and composition, with the topmost MAR3 of the Mt Nero Unit including the Poggio La Rocca marker bed (Figures 1 and 3).

The Pietralunga succession shows an outcropping thickness > 1000 m and, based on the facies analysis, can be divided into four members, hereinafter referred to as MAR4–MAR7.

The ~750 m-thick MAR 4 mr spans in age from the highest part of the MNN5b to the topmost MNN6b nannofossil biozones (Late Langhian-Middle Serravallian) according to [26] and our unpublished data (Figure S4), whereas all the remaining MAR5-7 mrs fall within the MNN7 biozone of the Late Serravallian age.

The overall stratigraphic reconstruction of this member was carried out by correlating eight sections (Vm, Sc, Po, Pz, Ss, Pi, Mo, and Mf, full names and location in Figure 1), that have been logged and analyzed from the litho- and bio-stratigraphic points of view.

The MAR4 mr is mainly characterized by the typical association of marls, arenite layers, and scattered media to thick-bedded calcarenites and hybrid arenites, with a predominant A/P ratio varying from <1 to <<1.

Despite an apparent monotonous facies, this member embodies several outstanding layers, including the well-known Contessa mega-bed, the "Colombine" calcarenites (occurring above the Contessa bed [25,66]) and some very thick and laterally continuous arenite and hybrid arenite beds, useful as key-layers for both geological mapping and basin analysis purposes (Figures S4 and S5 and detailed stratigraphy in [26]).

As aforesaid, the upper part of the Pietralunga succession is characterized by three further members, all of them falling in the MNN7 biozone, Late Serravallian in age.

The MAR5 mr corresponds to a quite thin (nearly 150 m) pelitic-arenaceous interval, characterized by thin-bedded fine-grained alpine-supplied arenites including some slump episodes.

The passage to the MAR6 mr is marked by a sharp increase in bed thickness (thick to very thick beds with an arenitic portion up to 8 m) and by the complete absence of calcarenite layers.

Finally, the MAR7 mr consists mainly of a pelitic-arenaceous facies association, characterized by a thinning-upward trend and a gradually decreasing A/P ratio, until the complete disappearance of the arenite beds.

The upper part of the MAR4 mr contains at least two large episodes of submarine sliding [12,24,25,66], the lower of which, referred to as "Lame-Castiglione olistostrome", occurs nearly 600 m above the Contessa bed [66], whereas the uppermost one, or "San Faustino-Scritto olistostrome", is located at least another 400 m above the former (Figure 3 and Figure S4).

The two olistostromes consist of wide lenses of chaotic materials within which disrupted rock fragments of different origin and size (up to decametric strata-fragments) are scattered in a polychromic sandy-clayey, and locally scaly, matrix. These lithotypes are derived from the Tuscan succession (Scaglia Toscana, jaspers, Triassic gypsum, and anhydrites) and from the Liguride units (Alberese and Palombini shales).

Generally, a thick interval, with a few meters, characterized by slumped intraformational materials, occurs at the base of both the olistostromes.

In the southern part of the study area the upper olistostrome consists of a quite continuous and undamaged stack of strata ("Scritto Limestones", according to [17,24]) which should be correlated with the sub-Ligurian Canetolo succession and the Ligurian units [17]. This attribution implies that, during the Middle Serravallian, the Apennine allochthon had reached the frontal part of the Falterona Nappe.

The detailed lithostratigraphy of the sections studied within the Pietralunga Unit exceeds the objectives of this work.

Anyhow, a composite Log, synthesizing the reconstructed stratigraphy of the entire Unit and showing the occurrence and position of the significant key layers, used for horizontal correlations, is provided in Figure S4.

For a more detailed description of all the sections mapped in Figure 1, the reader is sent back to the original work by [26].

#### 3.2.4. Gubbio Unit

The Marnoso Arenacea of the Gubbio Unit overlies the Meso-Cenozoic multilayer of the homonym anticline in eastern Umbria [12,42,69].

In the reference sections of the Contessa Road and Bevelle (Cs and Be, in Figure 1), the Late Langhian—Serravallian succession, from the Contessa mega-bed (MNN5b biozone) upward [26], is well correlated with the Pietralunga one, in what concerns the dominant facies and the occurrence of the major key-beds (compare Figures S4 and S5).

Conversely, in the aforesaid sections, the age of the basal stratigraphic passage to the Schlier Fm differs from that found in all the western outcrops, being localized in the Early Langhian, highest MNN5a subzone.

The absence of the MAR1-MAR3 members (Figure 2) implies that, during the entire Burdigalian-Early Langhian time span, the Marnoso Arenacea Fm did not deposit in this outer sector of the foredeep.

Actually, in the "Contessa" section (Figure 1), the homonym key-layer is placed nearly 105 m above the base of the MAR (Figure 2 and Figure S5), whereas it was drilled in the Mt Civitello well (location in Figure 1, Log available at: https://www.videpi.com/videpi/ pozzi/dettaglio.asp?cod=3896; accessed on 10 February 2021) at least ~700 m above.

The significant reduction of thickness of the pre-Contessa turbidite succession passing from the Pietralunga to the Gubbio Units occurs quite abruptly, straddling the Gubbio normal fault, and can be explained by its synsedimentary activity during Burdigalian-Langhian times.

The interpretation of seismic reflection data by [70] confirms this hypothesis, showing that the Langhian terms of the Pietralunga Unit display an eastward-thickening wedgeshape geometry that is compatible with a growth on the Gubbio fault.

The hypothesis of an early structuring of the Gubbio anticline suggested in [12] could also explain the absence of the lower Langhian Marnoso Arenacea in this area. However, even if it cannot be ruled out a priori, we note that it does not explain the eastwardthickening of the succession highlighted by [70], west of the Gubbio fault.

A further significant feature, which differentiates the Gubbio succession from the more internal ones, is the absence of olistostromic bodies at all stratigraphic levels.

The lateral continuity of the Serravallian terms from the Pietralunga to the Gubbio units (including the related marker beds) leads us to exclude that the lack of olistostromes was due, in that time, to a subdivision of the foredeep into several smaller basins. More reasonably, it was caused by the emplacement mechanism of the olistostromes that, moving by sliding processes on a tectonically unstable slope, could not reach the outermost areas of the basin.

Finally, our biostratigraphic data do not confirm the occurrence of the Tortonian in the highest part of the succession, as already suggested in [12,69].

#### *3.3. Mt Vicino Unit*

The Mt Vicino Unit, which is the easternmost unit of the Umbria pre-Apennine, is located just at the back of the present carbonate chain. In this area the upper boundary of the Schlier Fm was dated to the Early Serravallian and the MAR Fm to the Early Serravallian-Early Tortonian (section Bf, Figure 1) [26,71].

During the uppermost part of Early Tortonian and the lower part of the Middle Tortonian, the turbidite flows continued to affect only a residual furrow east of the Gubbio structure, leading to the deposition of the Mt Vicino Fm [72,73].

The M. Vicino Fm consists of a 600 to 1400 m thick turbidite succession in which, from the bottom, the following facies associations have been recognized: (i) thin-bedded pelite alternating with subordinate thin to medium bedded arenites, (ii) middle to thickbedded arenaceous-pelitic turbidites, (iii) medium to thick-bedded arenites and bioclastic hybrid arenites.

The Mt Vicino Fm reaches its maximum thickness, of a few hundred meters, in the axial zone of the narrow depression which also corresponds to the core of the Serra Maggio syncline, suggesting that the latter was already in an advanced stage of structuring during sedimentation.

In other words, as proposed by [69], the Mt Vicino Fm deposited when the innermost folds of the Apennines were already growing, marking the late evolution of the MAR foredeep into the wedge-top basin stage.

Such a suggestion is confirmed by the occurrence, at intermediate stratigraphic levels within the succession, of slumped layers and coarse-grained poorly cemented turbiditic sandstones, showing Apennine provenance and both NW and SE paleocurrents [72,73].

#### **4. Reconstructed Timing of Deformations**

In the following, we reconstruct the timing of the main tectono-sedimentary stages and events which can be inferred based on the sedimentary evolution of the Marnoso Arenacea basin, described in the previous section.

Such stages, graphically schematized in the steps of Figures 5 and 6, document the setting, evolution, and progressive tectonization of the Umbria foreland basin system during the Miocene. Their temporal constraint is provided by the high-resolution biostratigraphic

analysis performed on all the 24 studied sections, scattered throughout the entire northern Umbria pre-Apennines.

**Figure 5.** Exemplified tectono-sedimentary evolution of the Umbria pre-Apennines foreland basin system, during the Early and Middle Miocene; the stages, referred to the biozonation scheme proposed by [35–37] synthesized in Figure 2, are: (**A**) Incipient foredeep, (**B**) early Etruscan; (**C**) late Etruscan; (**D**) early Umbria-Romagna, and (**E**) late Umria Romagna stages (detail in the text; n.b.: sections are schematic and not in scale).

#### *4.1. Incipient Umbria Foredeep (Aquitanian-Burdigalian Boundary; MNN2a-MNN3a Nannofossil Biozones)*

The beginning of the regional subsidence related to the embryonic stage of the Umbria foredeep can be identified in the passage from hemipelagic (cherty marls of Mt Sperello) to turbidite deposition (Castelvieto-Montagnaccia Sandstones), within the REN Unit (Figures 5A and 6). At this stage the Apennine tectonic pile bounding the basin to the

west included the Falterona Nappe, passively carrying the Liguride l.s. units and some wedge-top basins in which piggyback deposition took place (Celle Sandstones and Vicchio Marls, [7,74].

During the uppermost MNN2b-MNN3a zones (Middle Burdigalian), the REN Unit was affected by contractional deformations and progressively accreted into the orogen.

#### *4.2. Early Etruscan Stage (Middle-Late Burdigalian; MNN3a-MNN4a Nannofossil Biozones)*

The eastward migration of the orogenic wedge led to the setup of the Etruscan depocenter of the foredeep, in which the Mt Nero succession settled down (Figure 5B).

Up to the earlier Langhian, turbidite sedimentation affected exclusively this westernmost sector of the Umbria domain.

This inference is suggested by the stratigraphy of the Civitello1 borehole (Figures 1 and 4a, https://www.videpi.com/videpi/pozzi/dettaglio.asp?cod=3896; accessed on 10 February 2021), in which the base of the MAR of the Pietralunga Unit was attributed to an unspecified "Langhian"; moreover, the lithological descriptions of these lowermost turbidites show no facies affinity with the MAR1 and MAR2 members of Mt Nero Unit.

During the MNN3b-MNN4a (p.p.) interval, the Falterona Nappe underwent an outof-sequence reactivation (Figures 5B and 6) and, after overtaking the deformed REN unit, arrived at the west side of the foredeep and covered its westernmost sector, interrupting the sedimentation.

At the beginning of Late Burdigalian (lower part of the MNN4a zone), its leadingedge can reasonably be thought to have reached its present location. This latter assertion is corroborated by the reciprocal consistency of the following points: (i) the topmost beds of MAR1, sampled just below the Falterona thrust in the Perugia Mts ridge (Monestevole = Mn section in Figure 1 and Figure S2, [18]), provided a MNN4a age, (ii) this same age was determined in the lowermost strata of the M. S. Maria Tiberina Fm, which has been proven to seal the front of the Tuscan allochthon in the Mt Cedrone area [21,27] (Figure S3), and (iii) the lacking evidence, in the younger and more eastern MAR of the Mt Nero Unit, of any signal of subsequent reactivation of the allochthon front.

#### *4.3. Late Etruscan Stage (Late Burdigalian-Late Langhian; MNN4a-MNN5a/b Nannofossil Biozones)*

This stage corresponds to the time which followed the emplacement of the Tuscan allochthon and pre-dates the nucleation of the major contractional structures within the western Umbria carbonates. During such an interval, most of the turbidite flows sedimented in the outer sector of the basin, giving rise to the upper MAR1 and the following MAR2 and MAR3 mrs.

Further west, in the wedge-top structural position, and also above the slope connecting the chain front with the foredeep depocenter (Figure 5C) Apennine-supplied low- and high-density turbidites, slumped bodies and pelite mud drapes continued to deposit (SMT3 and SMT4 mrs of the M.S. Maria Tiberina Fm).

No reliable data allows us to constrain the transversal extent of the foredeep in this stage beeing the lateral continuity of the Langhian succession not strictly constrained.

Nevertheless, the aforementioned stratigraphy of the Mt Civitello1 well indicates that, during the Langhian, the depocenter of the basin included the whole Mt Nero and part of the Pietralunga Units.

We can exclude with certainty that, during this stage, the MAR deposited east of the area where the Gubbio and Mt Subasio anticlines are presently located. This area was placed, up to the Early Langhian, in the peripheral bulge of the MAR foredeep, and the sedimentation of the Schlier Fm was still going on (Figures 5C and 6).

#### *4.4. Early Umbria-Romagna Stage (Late Langhian-Early Serravallian; MNN5b-MNN6b Nannofossil Biozones)*

We refer to this stage as "Umbria-Romagna" because our stratigraphic data, compared with the literature data collected over the past decades [3,9–13], undoubtedly demonstrates that a single and undivided foredeep developed in front of the Early Miocene northern paleo-Apennines.

The main argument supporting this inference is the lateral continuity of several keylayers over the entire MAR of the Umbria and Romagna area. These layers include the Contessa mega-bed, tens of "Colombina type" calcarenites, and some other noticeable arenite beds (Figure 3, Figures S4 and S5) [3,9,25,66].

During this stage, which was characterized by quite stationary sedimentary conditions, the depocenter of the foredeep was localized in correspondence to the present Pietralunga Unit, whereas its eastern side extended to the Gubbio zone (Figures 5D and 6).

In other words, at least since the MNN5b zone, the MAR Fm was settling, with fairly uniform thickness, into a single wide basin.

The peripheral bulge was shifted to the Mt Vicino and the inner chain areas, where up to the MNN6a-(6b?) zone, the deposition of the Schlier ramp-muds persisted (Figures 3, 5D, 6 and S6).

Orogenic contraction began to affect the older and more internal succession deposited during the Etruscan stage.

The onset of thrusting and, in particular, the growth of the Mt Cedrone anticline (see geological section of Figure S3) has been referred by [21] to the Early Serravallian (MNN6a subzone).

Actually, the sudden fining-upward evolution of the M. S. Maria Tiberina Fm and the concurrent shallowing of the seafloor [27] may be interpreted as the first sign of ongoing compression at shallow crustal levels.

Uplift processes appear to have involved the entire western sector of the basin since the very Early Serravallian, as no turbidites younger than Late Langhian (MNN5b zone) have ever been detected at the top of the Mt Nero Unit.

As better explained in the following Section 5, we hypothesize that this regional-scale uplift is the shallow effect of the activation of the regional thrust fault displacing the Permo-Triassic basement in the subsurface of the Perugia Mts ridge [75–77].

#### *4.5. Late Umbria-Romagna Stage (Middle-Late Serravallian; MNN6b-MNN7)*

We place the beginning of this stage, in correspondence with the earliest involvement of the Serravallian MAR, in the orogenic deformations.

In particular, we interpret the emplacement of the Lame-Castiglione olistostrome (topmost MNN6b biozone) as evidence that the front of the chain, which had now extended to the completely detached Mt Nero Unit, was thrusted over the inner Pietralunga succession (Figure 5E). The inner part of this latter Unit was subsequently detached from the underlying Meso-Cenozoic carbonates and, at least since the earlier MNN7 biozone, was progressively shortened "in-sequence". Due to such progressive deformation, during the upper MNN7 biozone the width of the foredeep underwent a severe reduction and, as a consequence, the higher stratigraphic members of the succession (MAR6–MAR 7 mrs) deposited only in the central-east part of the basin (Figure 5E).

The end of this stage can be located straddling the Serravallian-Tortonian boundary (top of MNN7 biozone), as we have not found Upper Miocene deposits in the Pietralunga and Gubbio successions (Figure 6 and Figure S6).

#### *4.6. Wedge-Top and Accretion Stage (Early-Middle Tortonian; MNN8-MNN9)*

At the beginning of the Late Miocene, the Langhian-Serravallian successions of both the Pietralunga and Gubbio Units were diffusely affected by contractional tectonics, giving rise to structures of variable scale: (i) regional macro-folds affecting the underlying Meso-Cenozoic carbonates (Figure 4), (ii) low-wavelength folds (some hundred meters to 1–2 km wide), detached at the Schlier Fm (Figure 4), and (iii) mesoscopic folds nucleated above local intra-formational decollements.

In the easternmost sector of the pre-Apennine, the growth of the Gubbio and Mt Subasio anticlines further reduced the active part of the foredeep, which, during the earlier Tortonian, was restricted to the Mt Vicino-Serra Maggio trough (Figure 1). This area was the only one located west of the chain, which hosted the MAR sedimentation during the Tortonian.

Above the MAR, the Mt Vicino sandstone (upper part of the Early Tortonian) deposited unconformably.

Reasonably, at the end of Early Tortonian, the further eastward progression of the contractional front caused the Mt Vicino sheet to override the innermost Umbria-Marche anticlines (Scheggia-Mt Maggio anticlines), which, according to [78] were in their initial stages of growth.

#### **5. Discussion and Final Remarks**

The systematic litho- and bio-stratigraphic study of the Umbria turbidite successions allowed for the recognition of the main evolutionary stages of the Umbria pre-Apennines' foredeep and the definition of the tectonic events that have involved the Miocene foreland basin system in this sector of the orogen.

Based on such high-resolution tectono-stratigraphic history, schematized in the steps of Figure 5, we have elaborated a kinematic model which describes the distribution of the structural paleo-domain over time (Figure 7).

**Figure 7.** Distribution of the pre-Apennines structural paleo-domain, as inferred by the high-resolution tectono-stratigraphic timing summarized in Figure 6, during the: (**a**) Late Aquitanian-Early Burdigalian, (**b**) Middle-Late Burdigalian, (**c**) Early-Middle Langhian, (**d**) Early Serravallian, (**e**) Late Serravallian; (**f**) Early Tortonian. Note that the migrating paleo-domains are shown by the brown, orange, and blue colors and that the thrust faults refer to the present configuration, taken from Figure 1.

A first fundamental achievement of our reconstruction was the precise definition of the age of emplacement of the thrust-stack bounding west of the foredeep, which, at the end of the Aquitanian, included the Tuscan Falterona Nappe and the overlying allochthon units of Ligurian (l.s.) pertinence.

The over-thrusting of such tectonic pile above the Umbria domain is bracketed between the very Early Burdigalian and the Late Burdigalian and was articulated in two distinct steps: (a) "in-sequence" thrusting above the REN, during the MNN2b-MNN3a zones [15], and (b) "out-of-sequence" thrusting above the MAR, after overtaking the REN, which was, in turn, tectonically superimposed on the MAR of the Mt Nero Unit (Figure 6—Eastern Tuscany column- and Figure 7).

The out-of-sequence phase of the Falterona Nappe had not been previously recognized in Umbria and deserves to be verified in other sectors of the chain, e.g., in the Tuscan-Emilian Apennines, to understand if it must be considered a significant event on the geodynamic scale.

The eastward advancement of the Nappe, in Middle-Late Burdigalian times, progressively narrowed the Umbria foredeep of the early Etruscan stage and interrupted the sedimentation in its western sector [7,18]. The reduction of the basin-width, resulting in a lower availability of the accommodation space for the incoming gravity flows, might explain the increase in the average thickness of turbidite beds, observed at the passage from the lower member (MAR1-MNN3a-MNN4a biozones) to the intermediate one (MAR2 mr-MNN4b-MNN5a biozones) of the Mt Nero Unit (Figure S2). Such an increase is consistent with a sharp change in the foredeep physiography that, during the late Etruscan stage, evolved from an open submarine plain to a strongly subsiding and transversely confined basin. In this regard, we must also consider that, during the aforementioned stages, the foredeep basin did not reach its maximum W-E extent, being delimited eastward by a set of west-dipping sinsedimentary normal faults that offset the foreland ramp.

Although the influence of foreland extensional faults on the MAR sedimentation is poorly investigated, the Gubbio normal fault seems to have played a significant role [70], producing a sharp uplift of the peripheral bulge that was located, in Early-Middle Langhian times, in its footwall block (Figure 7c).

After the emplacement of the allochthon units, the Umbria domain appears to have experienced an interval with absent or scarce deformation, at least from the Middle Burdigalian to the Early Serravallian (MNN4b-MNN6a biozones)—that is, for most of the Early Umbria-Romagna stage (Figure 7b,c). Such a ~3.5 ma-long standstill can be bracketed between the time at which the front of the Falterona Nappe was sealed by the sedimentation of the M. S. Maria Tiberina Fm (lower MNN4a zone) and the first clear evidence of compression and uplift affecting the western Umbria carbonate multilayer (uppermost MNN6a zone).

We point out that the corresponding time interval, spanning from ~17 to ~13.5 Ma, matches the transition between the collisional and post-collisional stages of the Apennine orogenesis [79–83]. The latter, which is still in progress, is associated with the Tyrrhenian rifting and is characterized by the eastward migration of two coupled-sub-parallel and synchronous tectonic belts causing contraction at the front and extension at the rear of the chain.

The evidence of a break in the shift of compressive deformation toward the foreland indicates that the passage between these two stages did not occur with continuity, at least in the more peripheral zone of the orogen.

In such a zone, the change in the geodynamic regime seems not to have produced significant tectonic manifestations during the time required for the new stress-field to propagate outside the collisional suture.

We are aware that such a hypothesis is preliminary and highly speculative. Anyhow, we want to stress that having highlighted such a protracted stasis in the orogenic deformations is an unexpected result, worthy of being further investigated, to verify its consistency also in other areas of the Middle Miocene Apennine front.

We refer the compression of the western Umbria domain (Figure 7d) to the Early Serravallian (MNN6a-MNN6b biozones), as suggested by the shallowing-upward trend of the topmost M. S. Maria Tiberina Fm and by the subsequent interruption of its sedimentation [21,27].

During this time interval, a major thrust fault nucleated within the Permian-Triassic basement [51,75–77]. It displaced the Meso-Cenozoic Multilayer of the Perugia Mts ridge and, branching at shallower structural levels, gave rise to an imbricate fan and associated folds affecting the MAR.

We identify the main surface expression of this regional thrust in the frontal thrust of the Mt Nero Unit, which is well-exposed in the Alpe della Luna (Afra Valley) and has been recognized to discontinuously crop out also even further south, in the hydrographic left of the Tiber (Figures 1 and 4).

Considering the amount of the associated displacement and the considerable continuity of this structure, all along the entire central Umbria (Figure 1), we hypothesize its possible correlation with the "Mandriacce Line", described in the Marnoso Arenacea Romagnola [84], which causes the systematic superposition of Langhian terms above Serravallian ones.

According to our reconstruction, thrusting in western Umbria went on until the Late Serravallian, as shown by the timing of emplacement of the upper olistostromic slices (earlier MNN7a zones) within the MAR of the Pietralunga Unit (Figures 5D,E and 7e).

The few public subsurface data, available in the area comprised between the Tiber Valley and the Umbria-Marche chain, highlights that the contractional deformations affecting the Pietralunga MAR are arranged in a shallow imbricate whose basal decollement is localized at the top of the Meso-Cenozoic Carbonate multilayer (Figure 4a,b).

The geometry of such a thrust system, joined to the observation that the uppermost members of the succession (MAR5-7) occur only in the easternmost sector of the Pietralunga Unit, supports the inference of an "in-sequence" thrusting, within this unit, during the very Late Serravallian-Early Tortonian (uppermost MNN7b-MNN8 biozones, Figures 5 and 6).

However, it should be noted that such argument is not conclusive, as it cannot be excluded that the complete absence of such members, in the innermost areas, might also be due to widespread erosion, rather than their non-deposition.

Just after the beginning of the Early Tortonian, the Umbria foredeep was entirely undergoing compression, whereas the origin of the Serra Maggio syncline can be confidently placed in the Late Tortonian, taking into account the piggyback sedimentation of the Mt Vicino sandstones.

Subsequently, the so-called "intra-Messinian" phase [22,72] split the previously unitary foredeep in several minor basins interposed to the rising Umbria–Marche anticlines. Inside them, a terrigenous-evaporitic sedimentation occurred up to the earlier Pliocene.

At the end of this latter period, after the uplift and accretion into the chain of such residual elongated furrows, the tectono-sedimentary history of the Umbria foreland basin system was over.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2076-326 3/11/2/97/s1, Figures S1–S6.

**Author Contributions:** F.B. and L.L. conceived the work. F.B. and D.C. performed the fieldwork. L.L. carried out the nannofossil analyses and the biostratigraphic determinations. F.B. wrote the manuscript. D.C. and F.B. interpreted the seismic lines and prepared the figures. D.C. geo-referenced the maps and carried out the final review of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by DiSPUTer Departmental Research grants 2020 (Resp. F. Brozzetti).

**Acknowledgments:** We thank two anonymous referees for the constructive comments, that have been useful for improving the manuscript.

**Software Used in This Work:** Field survey was partly performed through Fieldmove software (Move™, produced by Midland Valley Exploration Ltd. 2018 Glasgow, and Petroleum Experts Edinburgh, Scotland UK—https://www.petex.com/products/move-suite/digital-field-mapping/, accessed on 10 February 2021) installed on an Apple iPad-Pro. The newly acquired data were managed in a GIS database elaborated through ArcGIS v.10.8. Figure 1 was created using ArcGIS v.10.8 (http://desktop.arcgis.com; accessed on 10 February 2021). Figures 2–6 and all the supplementary figures were drawn using CorelDRAW graphics suite 2020 (https://www.coreldraw.com; accessed on 10 February 2021). The interpretation of seismic lines and their conversion to depth (Figure 4) were carried out with the Move suite software 2019.1.8 (https://www.petex.com/products/move-suite/; accessed on 10 February 2021), in particular using the 2D depth conversion tool. Microsoft Excel was used to elaborate the distribution diagrams for the marker species of nannofossils, placed alongside the stratigraphic columns of Figures S2, S4 and S5.

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

#### **References**


### *Article* **Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview**

**Stefano Conti 1, Claudio Argentino 2, Chiara Fioroni 1, Aura Cecilia Salocchi <sup>1</sup> and Daniela Fontana 1,\***


**\*** Correspondence: daniela.fontana@unimore.it

**Abstract:** The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins.

**Keywords:** seep-carbonates; cold seepage; Miocene; northern Apennines; accretionary wedge

#### **1. Introduction**

The seepage of methane-rich fluids at the seafloor, also known as cold seeps, has been frequently observed in accretionary wedges, where active tectonics generate pore-fluid overpressures and induce fluid migration through the sediments [1–5]. Cold seepage along the slope of continental margins is often associated with a large variety of sedimentary processes (e.g., landslides, mud volcanism, diapirism) and fluid escape structures (e.g., pockmarks, carbonate mounds) [6–10].

Due to the fluxes of reduced carbon and sulfur compounds reaching the seafloor, these environments are inhabited by peculiar microbial consortia and chemosymbiotic macrofaunal assemblages [11–16] and marked by specific geochemical imprinting [17,18]. Fossil analogous to modern systems have been recognized in exposed sedimentary successions on all continents (except Antarctica) and have allowed the investigation of the long-term evolution of hydrocarbon seepage in relation to tectonic processes and climate change [12]. Spectacular seep-carbonate examples have been reported from Miocene outer shelf and upper slope deposits at Hikurangi Margin, New Zealand [10,19,20] and in Cretaceous deposits linked to cold seepage in forebulge setting (Tepee Buttes carbonate mounds) cropping

**Citation:** Conti, S.; Argentino, C.; Fioroni, C.; Salocchi, A.C.; Fontana, D. Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview. *Geosciences* **2021**, *11*, 53. https://doi.org/10.3390/ geosciences11020053

Academic Editors: Domenico Liotta and Jesús Martínez-Frías Received: 23 December 2020 Accepted: 25 January 2021 Published: 28 January 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/).

out in Colorado, USA [21]. It is worth mentioning the well-exposed and widely-studied Mesozoic seep deposits in the shelf and fore-arc successions of Japan

Ref [22], and seep carbonates in Oligocene flysch related to foreland basins of the Outer Carpathians, Poland [23]. The global sedimentary record of cold seepage provided statistical evidence for the fact that sea-level forcing and rates of organic carbon burial in ocean basins have been the main factors controlling overall seepage activity since the early Cretaceous [24], resulting in cycles with periodicity in the order of tens of Myr.

Fossil seeps have been widely reported from the Italian Apennine chain, in the form of seep-carbonate outcrops, and mostly hosted in Miocene successions [25]. These deposits are historically known under the informal lithostratigraphic name of Calcari a *Lucina*, as they include densely-packed large lucinid bivalves [26]. These authigenic carbonates are characterized by very negative <sup>δ</sup>13C values (<−30‰ VPDB), peculiar chemosynthetic fauna (vesicomyid and lucinid clams, bathymodiolid mussels) and distinctive carbonate facies related to fluid expulsion processes [27,28], and testify a long history of cold seepage during the Neogene building phase of the Apennine chain. Seep-carbonate precipitation and fluid expulsion processes occurred in different tectonic settings of the Apennine foreland, from wedge-top basins through the outer slope of the accretionary prism, and at the leading edge of the deformational front in the inner foredeep, in correspondence of fault-related anticlines. Outcrop distribution highlights a causal relationship between tectonics and seepage occurrence. Recent studies also showed that some seep deposits could have originated during sea-level low-stands [29–31] and from gas hydrate destabilization events [32,33].

In this paper we provide an overview of the main geochemical, sedimentological and stratigraphical features of the northern Apennine seep-carbonates located in the Emilia to Umbria-Marchean area (Figure 1), included in Burdigalian to Messinian successions. The extraordinary dataset reported here, covers ~10,000 km2 area and the examined carbonate outcrops are representative of different structural settings along the paleo-Apennine wedge, thus serving as a record of the regional-scale spatial and temporal evolution of cold seepage systems which can be used to better understand analogous systems in other modern convergent geo-settings.

**Figure 1.** Simplified geological map of the studied area in the northern Apennines. Numbers refer to the examined and correlatable outcrops. 1. Montebaranzone; 2. Cappella Moma; 3. Traversetolo; 4. Lavino Valley; 5. Moggiona; 6. Fosso Riconi; 7. Deruta; 8. Poggio Campane; 9. S. Sofia-S. Vernicio; 10. S. Sofia-Case Buscarelle; 11. Salsomaggiore-Case Gallo; 12. Salsomaggiore-Case Cagnotti-Zappini; 13. Montepetra; 14. L Lame-Pietralunga; 15. Caresto; 16. Brisighella; 17. Brasimone–Suviana; 18. Poggio Michelino; 19. Telecchio; 20. Castagno d'Andrea-Corella; 21. Acquadalto (Podere Filetta-Monte Citerna); 22. Bibbiana-Le Fogare; 23. Colline-Mondera; 24. Prati Piani; 25. Case Bandirola.

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

The northern Apennine chain results from the convergence and collision between the European and African plates, with the interposition of the Adria and Corsica-Sardinia microplates. During the early stages of the collision (late Oligocene), the internal oceanic units (Ligurides) were placed over the adjacent thinned margin of the continental Adria microplate, represented by Subligurian units [34]. From Miocene to Recent, the thrust system migrated towards the foreland, involving the continental Tuscan and Umbria-Marchean units deposited on the Adria microplate. This collisional stage involved the subduction of the Adria under the Corsica-Sardinia lithosphere coupled with the flexuring of the foreland and the formation of foredeep basins, progressively migrating towards the northeast [35,36].

The progressive migration of the foredeep produced a segmentation of its inner part [37] by the growing of anticlines on top of synsedimentary blind thrust faults, creating intrabasinal highs. Sedimentation on top of thrust-related anticlines consisted of hemipelagites and diluted turbidites (drape mudstones) forming up to a hundred meter thick fine-grained intervals. The structural evolution of these thrust-related anticlines created favorable conditions for gas accumulation and gas hydrate formation at the ridge crest, promoting the development of cold seepage systems and inducing sediment remobilization along the ridge flanks [28]. The progressive closure of the foredeep, with its involvement in the accretionary wedge (closure phase), is marked by the deposition of slope marls characterized by sediment instability due to the strong tectonic activity of the substrate [38]. Slope sedimentation is stopped by the overriding of the Ligurian units. During the overriding of the Ligurian units, from the Burdigalian to the early Tortonian, sedimentation also occurred in wedge-top basins (Epiligurian succession) at the top of the accretionary prism [39], and marked by intense seepage processes. Authigenic seep-carbonates in Upper Miocene sediments of the Tertiary Piedmont Basin are reported in [40,41].

The age of the Apennine successions becomes younger moving toward northeast, following the direction of wedge advancement and foredeep migration. In the Burdigalian, the closure of the Tuscan foredeep (filled by turbidites of the Falterona-Cervarola Fm) is followed by the deposition of fine-grained hemipelagites of the Vicchio Fm. During the Langhian, the newly formed foredeep is filled by the thick turbidite succession of the Marnoso-arenacea Fm (Umbro-Marchean units) partially sealed by the Verghereto Marls (late Serravallian-early Tortonian) and by the Ghioli di letto mudstones (late Tortonian-Messinian).

#### **3. Structural Distribution of Apennine Seep-Carbonates**

Miocene seep-carbonates occur in specific positions of the Apennine wedge-foredeep system, reported below from the inner to the outer sectors (Figure 2; Table 1):


**Figure 2.** (**a**) Block diagram showing the northern Apennine wedge-foredeep system during the Miocene. Seep carbonates occurred in wedge-top basins, on the slope and in the inner foredeep basin. (**b**) Simplified paleogeographic sketch of the Apennine domains during the middle Miocene (modified from [26]).


**Table 1.** Structural distribution and paleogeographic domain of the examined outcrops (described in the text).

It is worth noting that the temporal/structural relationship between the slope and the foredeep is complicated by the migration of the accretionary wedge. This leads to the progressive incorporation of the foredeep units in the slope, and the creation of a new foredeep in a more external position.

In this work we report the location (Figure 1), morphology (Figure 3), facies (Figures 4 and 5), geochemistry (δ13C, δ18O; Figure 6) and stratigraphic distribution (Figure 7) of 13 seep-carbonate outcrops representative of the different geological settings (Table 1).

**Figure 3.** Seep-carbonate outcrops from the three examined geological settings. (**a**) Seep-carbonates hosted in the wedge-top basin (Termina Fm) in the Montebaranzone area (outcrop 1 in Figure 1), dotted line marks the geometry of the seep-carbonate bodies. (**b**) Castagno d'Andrea seep-carbonates enclosed in pelitic interval of the inner foredeep (Marnoso-arenacea Fm), (outcrop 20 in Figure 1). (**c**) Montepetra seep-carbonates hosted in slope fine-grained sediments (Ghioli di letto Fm), (outcrop 13 in Figure 1).

#### *3.1. Wedge-Top Epiligurian Basins* Montebaranzone

*Location*. Emilia Apennines. Hundreds of seep-carbonate bodies crop out extensively between Parma and Bologna Apennines (Figure 1), enclosed within marly sediments of the Termina Fm (late Serravallian−early Messinian), made up of slope deposits up to 500 m thick. In the upper portion, the Termina Fm is represented by the Montardone mélange, a chaotic body made up of polygenic and heterometric blocks dispersed in a fine-grained matrix. Seep-carbonate deposits reach the maximum concentration in the Montebaranzone syncline (Modena area), associated with the Montardone mélange (for geological details see [42]).

*Carbonate size and distribution*. Seep-carbonates consist of stratiform bodies and large pinnacles with lateral extent from a few meters to 100 m and a maximum thickness of 25–30 m, mainly concentrated within a 50 m thick stratigraphic interval of slope marlstones. The lithologies are micritic limestones and calcareous marls, with wide portions of carbonate breccias. The contacts with the host sediment vary from sharp to transitional. Conversely, seep-carbonates occurring on the western side of the mélange consist of small and laterally isolated marly and marly-calcareous bodies, which exhibit a lenticular, domed, columnar to irregular shape. Their dimensions vary from a few tens of centimeters to 4–5 m; the lateral contact to enclosing sediment is gradual.

*Seep-related facies*. Polygenic breccias occur at the base of seep-carbonate bodies close to the contact with the Montardone mélange and form units of variable thickness from some centimeters to a few meters, often interdigitated with fine-grained carbonate cemented sediments. Polygenic breccias contain clasts of different dimensions and provenance, carbonate, arenitic and pelitic, chaotically floating in the micritic matrix. Extraformational clasts largely derive from the basal complex of the Ligurian units. Clasts are heterometric (from some millimeters to ~50 cm), generally angular (Figure 5a). Intraformational clasts are sourced from previously precipitated seep-carbonates and from the Termina marls. Disarticulated and isolated lucinid shells have been observed, as well as fragments of shells forming packstones or grainstones. Pseudo-fluidal textures and soft sediment deformations are observed. Dense localized semi-infaunal chemosynthetic fauna often in living position (mainly lucinid and vesicomyid bivalves) are present in the middle upper portions of the outcrops (Figure 4a). In thin section, authigenic minerals consist of micro- to cryptocrystalline micrite and sparry calcite with minor dolomite. Micrite is the dominant authigenic phase and includes abundant shell fragments, associated with planktonic foraminifera and terrigenous particles in variable amounts.

*Carbon and oxygen isotopic composition*. Carbon isotopic composition differs for various carbonate bodies and inside a single mass (δ13C from −39.1 to −18.2‰ VPDB), with the most negative δ13C values obtained in sparry calcite cement in the brecciated portions. Samples are significantly enriched in δ18O (δ18O between +0.3 and +5.5‰ VPDB); the δ18O composition reaches the maximum values in the brecciated portions, close to the contact with the Montardone mélange.

*Biostratigraphy of host sediments.* The interval hosting seep-carbonates has been ascribed to the Serravallian to early Tortonian based on planktonic foraminiferal assemblages (MMi7- MMi9 Biozone). The oldest ages (Serravallian) are recorded in eastern bodies closest to the vertical contact with the mélange.

*Correlatable outcrops*. Cappella Moma (δ13C from −19.0 to −16.0‰ VPDB; <sup>δ</sup>18O from +2.1 to +2.9‰) Traversetolo, Lavino Valley (Figure 1).

#### *3.2. Outer Slope of the Accretionary Prism*

In slope mudstones of the Tuscan and Umbria-Marchean domains (Tuscan-Romagna-Marchean-Umbria Apennines), two different phases of seepage are recognized: the first during the initial part of the foredeep closure stage; the second marks the final part of the closure stage when slope mudstones were topped by Ligurian overthrust (Table 1).

**Figure 4.** Representative seep-carbonate facies. (**a**) Densely packed articulated bivalves from the Sasso delle Streghe carbonate pinnacle (Montebaranzone area, outcrop 1 in Figure 1). (**b**) Large conduit and a complex network of vuggy fabric and doughnut structures from Poggio Campane outcrop (8 in Figure 1). Bar length = 2 cm.

#### 3.2.1. Tuscan Domain

Moggiona

*Location*. Seep-carbonates are present in the basal member of Vicchio Fm made up of marls and silty marls with centimeter thick layers of fine-grained arenites (early phase of Table 1). Seep-carbonates are located in the footwall syncline at the base of the thrust within a stratigraphic interval 20–30 m thick, with a lateral extent of ~200 m. Carbonate bodies have a vertical attitude and are concordant to host sediments (geological detail in [25,28]).

*Carbonate size and distribution.* Seep-carbonate bodies are 5–100 m wide and up to 8 m thick; two smaller meter-sized blocks are also present. Morphologies are mostly stratiform with an irregular profile and strong lateral thickness variations. The contacts between carbonates and host sediments vary from sharp to gradual. Pinch-out lateral terminations, bifurcations, multiple interdigitation of carbonates with enclosing marls, lateral repetitions of rounded concretions and nodules are observed.

*Seep-related facies*. The basal portion of large carbonate bodies is characterized by a dense arrays of conduits, vertical to subhorizontal, with crosscutting relationships. Conduits are generally a few centimeters in diameter, with circular sections, and a few tens of centimeters long; conduit infilling consists of silty particles typically associated with shell debris. The top of the carbonate bodies is commonly characterized by assemblages of chemosynthetic fauna, either articulated and in life position or dismembered shells oriented parallel to bedding.

*Carbon and oxygen isotopic composition.* <sup>δ</sup>13C ranges from −40.2 to −13.6‰; <sup>δ</sup>18O from−9.9 to + 0.7‰.

*Biostratigraphy of host sediments.* Based on nannofossil assemblages of the enclosing marls, the age of the seep carbonates is ascribed to the Burdigalian MNN3b biozone. *Correlatable outcrops*. Poggio Corniolo.

#### Fosso Riconi

*Location.* The outcrop (final phase of the closure) is situated in the Tuscan Units (Vicchio Fm) cropping out in the Mugello area. The outcrop includes one of the most extensive exposures of a fossil seep system in the Apennines. About 80 lenses of authigenic carbonates are hosted in the topmost 30 m of marls with a lateral extent of about 500 m; the attitude is conformable to bedding of the enclosing marls (geological detail in [32,43]).

*Carbonate size and distribution.* Seep-carbonates have various geometries from elongated bed-like to lenticular bodies and pinnacles. The thickness of each body ranges from 1.5 m to 6 m and the lateral extent is from 1.5 to 10 m. The transition to host sediments vary from sharp to gradual. Nodular structures (2–3 cm in diameter), cylindrical to encircling concretions (4–5 cm in diameter) are present in the marginal portion of the seep-carbonates and arranged along stratification. Fossils are irregularly concentrated and consist of densely packed articulated and disarticulated bivalves up to 25 cm long. The most common lithotype is marly limestone.

*Seep-related facies.* Common facies are mottled carbonates, with irregular patches of micrite, pervaded of sinuous pipes, conduits and tubules, (Figure 4b), varying in diameter from 2–3 mm to 1–2 cm. Conduit sections are circular to elliptical, with the central hole filled by authigenic micrite, sparry calcite, coarser sandy sediment and shell fragments. In thin section, calcite is the dominant authigenic phase, associated with minor ankerite and dolomite. A very subordinate detrital fraction is made of illite-muscovite, chlorite, quartz and albite.

*Carbon and oxygen isotopic composition*. <sup>δ</sup>13C values range from −39 to −4.7‰ VPDB and <sup>δ</sup>18O values from −2 to +4‰ VPDB.

*Biostratigraphy of host sediments*. Based on planktonic foraminifera and nannofossil, the seep-carbonate precipitation approximates the Langhian/Serravallian boundary (MNN6a Biozone).

#### 3.2.2. Umbria-Marchean Domain (Serravallian to Early Tortonian)

Seep outcrops are included in the Verghereto Marls and other coeval slope deposits. As previously described for the Tuscan slope, two different seepage phases mark respectively the early phase of the closure stage, and the final part of the closure stage when slope mudstones were topped by Ligurian overthrust.

#### Deruta

*Location.* Seep-carbonates in Deruta (early phase of the closure) are included in the Marnoso-arenacea Fm at different stratigraphic levels associated with coarse-grained pebbly sandstones and conglomerates (delta-slope and large-scale mass-wasting deposits) in the wedge-top area, prograding into the inner Marnoso-arenacea foredeep. The release of abundant methane-rich fluids through thrust faults pervaded coarse-grained sediments, causing the precipitation of authigenic seep-carbonates both along the delta-slope and the adjacent foredeep (geological details in [44]).

*Carbonate size and distribution*. Authigenic seep-carbonates occur as large (50 m wide, 10 m thick) fossiliferous lenses, as concretions and cements in previously reworked coarsegrained deposits, and as reworked blocks (cobble and boulder) in slide/slump horizons.

*Seep-related facies*. Common facies in large lenses are mottled micrite and biomicrite with densely packed seep-bivalves (lucinids, vesicomyids) mainly articulated. Disarticulated shells are scattered in brecciated portions. Vuggy structures are present, with void infilling made up of carbonate cements and/or coarser-grained sediments (Figure 5c). Monogenic breccias consist of angular clasts (a few millimeters to 5–10 cm) made up of previously precipitated micrite. In fine- to coarse-grained calcarenitic limestones, concentric and radial patterns of carbonate veins and micritic patches are frequent; fossils are absent. In deltafront conglomerates and sandstones authigenic micrite precipitated as intergranular cement, dense irregular networks of carbonate-filled veins and extensional fractures are frequent. Veins contain abundant black iron sulfides. The fossil content is scarce, with scattered disarticulated clams or articulated lucinids. Stratified marlstones cemented by authigenic micrite barren of fossils represent a transitional facies to normal marine conditions.

*Carbon and oxygen isotopic composition*. δ13C largely differs in the different lithofacies, ranging from −46.0 to −11.0‰ PDB, and <sup>δ</sup>18O ranges from −4.7 to +2.4‰.

*Biostratigraphy of host sediments*. Nannofossil analyses of the enclosing marls indicate the MNN6b subzone, Serravallian in age.

**Figure 5.** Recurrent seep-carbonate facies. (**a**) Polygenic breccias with clasts sourced from Ligurian units (Montebaranzone area, outcrop 1 in Figure 1). (**b**) Polygenic breccias with disarticulated bivalves; clasts sourced from previously formed seep-carbonates and from the underlying Marnosoarenacea Fm (Colline, outcrop 23 in Figure 1). (**c**) Complex network of veins and conduits (Deruta, outcrop 7 in Figure 1). (**d**) Layered structures associated with disarticulated bivalves and mottled micrite (Montepetra, outcrop 13 in Figure 1). Bar length = 5 cm.

#### Poggio Campane

*Location*. Seep carbonates are included in Verghereto marls capping the inner Marnosoarenacea unit coinciding with a main phase of the Apennine overriding (Figure 1).

*Carbonate size and distribution.* Seep-carbonates consist of three large stratiform bodies and several large pinnacles ranging in extent from a few meters to 20 m and with a maximum thickness of 8–10 m. Several minor irregular blocks and lenses (30 cm to 1 m in extent) are scattered around the main bodies. The lithologies are calcarenites and calcareous marls, with wide portions of carbonate breccias.

*Seep-related facies*. Marly limestones, calcareous marls, fine to very fine calcarenitic limestones, grey to pale brown in color, with abundant fossil content. Fossils are denselypacked articulated and disarticulated lucinid clams, with maximum diameter up to 25 cm. Other facies include mottled micrites with veins and shell debris, vuggy marly limestones with fractures and cavities, complex vein networks and doughnut structures (Figure 4b). Monogenic breccias are common.

*Carbon and oxygen isotopic composition*. <sup>δ</sup>13C = −32.2‰; <sup>δ</sup>18O = +2.2. *Biostratigraphy of host sediments.* Based on nannofossil assemblages, the host sediment indicates the MNN7 biozone.

#### Santa Sofia

*Location*. Seep-carbonate blocks are hosted in slope sediments of the Verghereto Marls consisting of laminated, fine grained sandstones and marly-muddy beds, deposited by low density turbulent flows. Seep-carbonates have a wide extent and mark the closure stage of the foredeep before the overriding of the Ligurian units.

*Carbonate size and distribution*. Several tens of bodies of various dimensions and shapes from large stratiform (up to 50 m in lateral extent and 10 m thick) to irregular metric blocks and lenses. Carbonates consist of lightly colored, micritic limestones rich in mussels and clams [45]. The passage to enclosing marls is gradual.

*Seep-related facies.* Common facies are: polygenic and monogenic breccias with isolated articulated or disarticulated clams, centimetric conduits and doughnut fabric, network of conduits filled by calcite cements, laminated micritic limestones with alternance of whitish and brownish laminae.

*Carbon and oxygen isotopic composition*. <sup>δ</sup>13C from −36.4 to −27.2‰; <sup>δ</sup>18O from −0.3 to +3.6‰.

*Biostratigraphy of host sediments*. The nannofossil biostratigraphy indicates the MNN8 biozone (San Vernicio outcrop) and MNN8–9 (Case Buscarelle outcrop).

*Correlatable outcrops*. Salsomaggiore: Case Gallo, Case Cagnotti and Zappini (δ13C from −41.4 to −8.7‰; <sup>δ</sup>18O from −2.4 to +2.8‰) (Figure 1) [46].

**Figure 6.** Carbon and oxygen isotopic values measured in seep-carbonates from the examined outcrops. The bar length represents the range of variability of the isotopic values; circles indicate the average of the endmember values and numbers specify the outcrop as listed in the caption of Figure 1. Rhombic features represent single measurement values.

#### 3.2.3. Umbria-Romagna Domain (Tortonian to Messinian)

Seep carbonates are hosted in the Ghioli di letto mudstones and coeval slope deposits: during the early phase of the closure stage (Montepetra outcrops) and at the end of the closure stage below the contact with the Gessoso-solfifera Fm. Seep carbonates are also hosted in minor basin formed during the final stages of the Umbria-Marchean foredeep.

#### Montepetra

*Location.* The Montepetra outcrop formed along the outer slope of the accretionary prism, close to the front of the orogenic wedge. Seep-carbonates are hosted in fine-grained sediments (Ghioli di letto Fm, late Tortonian-early Messinian) draping thrust-bounded folds and buried ridges, constituted by the older accreted turbiditic units. The Montepetra outcrop is located in the south-eastern edge of a regional anticline, extending for more than twenty kilometers in a NW−SE direction. Seep-carbonates crop out in the hinge zone of the anticline and at the top of the mass transport deposits (geological details in [47]).

*Carbonate size and distribution.* Seep-carbonates consist of irregular metric lenses (up to 25 m) and blocks with different morphologies: lenticular-amygdaloid, mound-like irregular bodies, pinnacles, concretions of variable thickness (Figure 3c).

**Figure 7.** Nannofossil and foraminifera biostratigraphic framework indicating the distribution of the examined outcrops (numbers refer to Figure 1).

*Seep-related facies*. Monogenic and polygenic breccias, with intraformational and extraformational clasts (from Ligurian units), networks of conduits and veins, with scarce disarticulated and reworked fossils prevail in the basal portion of seep-carbonate bodies, indicating repeated phases of carbonated precipitation followed by fracturing. Moderate

to strong fluid seepages are suggested by laminated and mottled marly limestones with vuggy fabrics chaotically mixed with polygenic microbreccias rich in articulated and/or disarticulated lucinid clams (Figure 5d). Stratified micritic limestones and fine-grained calcarenites with plane-parallel laminations and articulated lucinid-like bivalves occur in the upper portion of the seep bodies during phases of low and diffuse fluid circulation.

*Carbon and oxygen isotopic composition*. Seep-carbonates yielded depleted δ13C values with a large dispersion from −52.7 to −19.1‰, and positive or slightly negative oxygen (δ18O from −0.7 to +6.0‰). Brecciated levels show the most depleted carbon isotope values (δ13C from −52.7 to −36.0‰), associated with heavy <sup>δ</sup>18O values (from +2 to +6.0‰).

*Biostratigraphy of host sediments*. Based on planktonic foraminifera, the age spans from the late Tortonian to the early Messinian MMi12-13 Zones (Figure 7).

*Correlatable outcrops.* Le Lame, Pietralunga [28,48], Caresto (geological description in [49].

#### Brisighella

*Location*. Numerous seep-carbonate bodies are located at the top of the euxinic marls of the Ghioli di letto Fm, and mark the contact with the overlying Gessoso-solfifera Fm. Seep-carbonates and evaporitic levels are involved in several back-thrusts striking parallel to the main NW−SE Apennine structures; the detachment level is located at the base of the euxinic marls bearing carbonate bodies. The attitude of seep-carbonate bodies is concordant with the Gessoso-solfifera Fm.

*Carbonate size and distribution*. Seep-carbonates vary from stratiform to pinnacular (from 1 to 30 m in extent and up 10 m thick), to minor irregular lenses and blocks.

*Seep-related facies*. Monogenic and polygenic breccias are a common facies, with isolated articulated or disarticulated clams and conduits (diameter of a few cm). Other facies are marly limestones rich in densely packed bivalves (giant lucinids and rarely mussels), massive to laminated micritic limestones resembling *Beggiatoa* beds, stratified marly limestones with small lucinids and gastropods. Spongy fabric with a complex network of cavities and veins and doughnut fabric, are common.

*Carbon and oxygen isotopic composition*. <sup>δ</sup>13C from −51.7 to −27.4‰; <sup>δ</sup>18O from −1.6 to +5.0‰ [50].

*Biostratigraphy of host sediments*. Early Messinian. *Correlatable outcrops*. See outcrops reported in [50,51].

#### Minor Basins

*Location*. Several small seep-carbonates in the Borello and Savio valley of the Romagna Apennines (Case Bandirola outcrop of Figure 1) are hosted in late Tortonian proximal turbidites belonging to minor basins of the Romagna-Marchean foredeep, known in literature under various names (Molasse grossolane, Fontanelice Sandstones, Urbania Sandstones),

*Carbonate size and distribution*. Carbonates are present as small blocks (thickness ranging from tens of centimeters to meters) in arenaceous coarse-grained turbidites, as crusts or as arenite cement.

*Seep-related facies.* Common facies are mottled micrite with small articulated bivalves, monogenic breccias with disarticulated bivalves and spongy fabric.

*Carbon and oxygen isotopic composition*. δ13C values are reported in [50]. Biostratigraphy of host sediments. Late Tortonian.

#### *3.3. Intrabasinal Highs of the Inner Foredeep*

3.3.1. Tuscan Foredeep (Cervarola Fm)

Brasimone-Suviana

*Location.* The pelitic interval enclosing seep-carbonates in the M. Cervarola Fm crops out in the northern limb of the Granaglione-Montepiano overturned anticline [52]. The pelitic interval consists of fine-grained marly turbidites up to 40–50 m thick and with a lateral extent of 20 km. Carbonate bodies are located in proximity of the tectonic contact separating the M. Cervarola Fm from the Sestola-Vidiciatico Unit. Geological details in [52].

*Carbonate size and distribution.* Numerous carbonate lenses (a few decimeters to several meters wide, 20 cm to 4–5 m thick) are made up of micritic and calcarenitic lenses, strongly brecciated and rich in densely packed lucinid bivalves. The micritic groundmass is commonly associated with pyrite, abundant bioclastic debris (planktonic foraminifera and fragments of shells) and locally fine-grained sand grains, made of quartz, feldspars and low-grade metamorphic rock fragments (phyllite, serpentinite, chlorite-schist) similar to those of the M. Cervarola arenites.

*Seep-related facies.* Common facies are mottled micrite, strongly bioturbated, monogenic and polygenic breccias frequently barren of fossils, usually disarticulated, micrite with dense networks of calcite veins and extensive vuggy fabrics. Micritic doughnuts, nodular and cylindrical concretions and pipe-like structures are interpreted as fluid-flow conduits. Chaotic structures and soft sediment deformation are common and consist of small slumps involving marly and carbonate deposits.

*Carbon and oxygen isotopic composition.* <sup>δ</sup>13C = −15.9‰; <sup>δ</sup>18O = +1.4; Poggio Michelino: <sup>δ</sup>13C from −29.5 to −14.7‰; <sup>δ</sup>18O from −8.7 to −2.6‰; Telecchio: <sup>δ</sup>13C = −21.0, <sup>δ</sup>18O = −4.0.

*Biostratigraphy of host sediments*. The nannofossil biostratigraphy indicates the MNN5a Biozone.

*Correlatable outcrops*. Telecchio, Poggio Michelino (Figure 1).

#### 3.3.2. Umbria-Marchean Foredeep (Marnoso-Arenacea Fm)

Thousands of seep-carbonates crop out in pelitic intervals draping intrabasinal highs in the Marnoso-arenacea foredeep. We recognized several pelitic intervals, each containing numerous seep-carbonate outcrops (Figure 1): Castagno-Corella (geological details in [25,28]; Acquadalto (Podere Filetta, Monte Citerna, Capanne di Favaglie and correlate outcrop of Poggio Cavalmagra); Susinello-Romiceto-Casaglia-M.Colonna-Nasseto described in [53]; Bedetta-Archetta (Colline, Mondera), Visignano (Prati Piani, case Termine). Here we describe the most representative outcrops. The more ancient pelitic interval (Castagno d'Andrea-Corella) contains numerous (up to 40) seep carbonate bodies scattered at various stratigraphic intervals; here we describe the two main outcrops, at the base (Castagno d'Andrea) and at the top (Corella) of the Castagno-Corella pelitic interval.

#### Castagno D'Andrea

*Location*. Four large seep-carbonate bodies are vertically stacked and distributed on three stratigraphic horizons concordant to the enclosing sediments and to the main structural trends. Small-scale slumps occur locally in host sediments above the carbonate bodies (geological details in [25,28]).

*Carbonate size and distribution*. Seep-carbonates vary from pinnacle-like to stratiform, 12 to 30 m wide and 5 to 10 m thick. Stratiform bodies are connected by pinnacular structures (Figure 3b). The basal contact is highly irregular marked by <0.5 m sized micritic concretions. Lateral transitions to enclosing sediments are sharp to gradual, with lateral repetitions of small concretions.

*Seep-related facies*. Stratiform bodies are characterized by an irregular framework of fractures and drusy-like cavities, associated with branched veins; in the upper portions, dense arrangements of lucinid-like clams are observed. In pinnacle-like bodies polygenic breccias prevail, with mixing of clasts from older underlying successions and from previously precipitated carbonate crusts and disarticulated clams. Monogenic breccias generated by autoclastic processes are common.

*Biostratigraphy of host sediments*. Calcareous nannofossils of the enclosing marls indicate the Langhian MNN5a subzone [54].

#### Corella

*Location.* The outcrop is located within the same pelitic interval as Castagno d'Andrea, approximately 10 km northwest, in a higher stratigraphic position. Six large carbonate bodies and several minor meter-sized blocks are concentrated in two horizons (geological details in [25,28]).

*Carbonate size and distribution*. Carbonate bodies are stratiform to lenticular, with a lateral extent from 50 m to 230 m and thickness up to 30 m. Basal and upper surfaces are flat. Lateral contacts with the host sediment are usually sharp, with pinch-out terminations. Larger carbonate bodies are vertically connected by irregular minor meter-sized bodies, or by highly cemented sediment.

*Seep-related facies*. The basal portion of the bodies is characterized by an irregular framework of fractures, conduits, drusy-like cavities and polygenic breccias (mm to cm sized) associated with disarticulated bivalves (Figure 5b). Monogenic breccias occur at various levels. A dense concentration of articulated bivalves is observed at the top of the bodies.

*Carbon and oxygen isotopic composition*. <sup>δ</sup>13C ranges from −42.3 to −26.6‰ VPDB. <sup>δ</sup>18O ranges from −5.7 to +1.2‰ VPDB.

*Biostratigraphy of host sediments.* Calcareous nannofossils of the enclosing marls indicate the Langhian MNN5a subzone.

#### Acquadalto

*Location*. The outcrops (Podere Filetta-Monte Citerna) are located within the pelitic interval of Acquadalto (geological detail in [54]). The interval has a thickness from 40 to 75 m and is cut by normal faults with Apenninic direction. Seep-carbonates are numerous at different levels commonly aligned along bedding. Some bodies show evidence of moderate reworking within the slopes of the intrabasinal high.

*Carbonate size and distribution.* Seep-carbonates consist of numerous marly-calcareous lenses, or irregular column-like and stratiform bodies, ranging in size from some decimeters to 3–4 m, and with a thickness from 20–30 cm to 3 m. The lateral contact with host sediments is gradual and interfingering; the transition is marked by carbonate-rich marly nodules.

*Seep-related facies.* The basal portion of bodies is characterized by an irregular framework of fractures, large conduits, drusy-like cavities and polygenic breccias (mm to cm sized) associated with disarticulated bivalves (Figure 5b). Monogenic breccias occur at various levels. Densely packed articulated lucinid-like bivalves occur at the top of the bodies. Mottled micrites pervaded by doughnut-like structures are common. Many marly nodules are connected to seep-carbonates by irregular and intertwined conduits.

*Carbon and oxygen isotopic composition.* <sup>δ</sup>13C ranges from −15.8 to −3.6‰ VPDB. <sup>δ</sup>18O ranges from −4.5 to −0.4‰ VPDB.

*Biostratigraphy of host sediments*. Calcareous nannofossils of the enclosing marls indicate the Langhian MNN5b subzone.

Similar features characterize the outcrops of Colline Mondera (Bedetta pelitic interval) (more details in [28]), and Prati Piani (Visignano pelitic interval) [55], all referable to the nannofossil MNN7 biozone.

The Prati Piani interval has a lateral extent of about 15 km and a thickness from 60 to 120 m, mainly constituted by hemipelagic marls and thin bedded fine-grained turbidites rich in ichnofossils. Extraformational bodies are present mainly in the basal portion, whereas the top is marked by glauconitic arenites. Seep-carbonates occur both at the base and in the upper portion of the interval.

*Correlatable outcrops*. Le Caselle-Pontevecchio [56].

#### **4. Discussion**

We report the spatial and stratigraphic distribution of seep-carbonate outcrops over an area of ~10,000 km<sup>2</sup> in the Emilia and Umbria-Marchean sector of the Apennine chain, northern Italy. The examined 13 outcrops are representative of three different structural

positions along the Miocene Apenninic accretionary system, and document a clear causal relationship between active tectonics and their origin and distribution. The outcrops formed within wedge-top basin, along the outer continental slope close to the orogenic front, and in a more external position in the inner foredeep, in correspondence with fault-related anticlines. The widely diverse outcrop-scale spatial patterns, facies and morphologies of seep-carbonates reported in our study indicate that several factors influenced seepage activity. From the examination of this dataset it follows that:


that could be longer than the seepage activity. It is also evident that the tectonic processes played a primary role in favoring the development of seepage systems, by creating the structural pathways for fluid advection (thrust faults) as well as trapping mechanisms for the accumulation of fluids (fold structures). Nevertheless when comparing seep distributions with third-order eustatic curves [61], they seem matching phases of sea-level low-stands. In particular the detailed study of one of the best exposed Apennine outcrops (Fosso Riconi [43]) indicates that the onset of the seepage approximates to the Mi3b cooling event (13.82 Ma). These results also match with results of previous authors that proposed the correspondence of seep-carbonates with cold climate and sea-level lowering. In this proposed scenario, the reduction in hydrostatic pressure acting on the plumbing system, and related to sea-level falls, would shift the bottom of the gas hydrate stability zone to shallower depths, inducing gas hydrate destabilization.

#### **5. Summary**

The Miocene seep-carbonate outcrops of the northern Apennines (Italy) reflect a long history of methane-rich fluid emissions along the paleo-accretionary wedge. In the last decades, studies on the lithostratigraphic distribution of seep-carbonate deposits in the Emilia to Umbria-Marchean sector of the Apennine chain highlighted the fact that cold seeps developed in three main tectonic settings, corresponding to wedge-top basins, outer slope and intrabasinal highs located at the deformational front. Structural and biostratigraphic analyses conducted over the years provided solid evidence for the causal relationship between tectonic phases related to the building of the Apennine wedge and the evolution of fluid plumbing systems, and critical estimates of the duration of methane emissions on the paleo-seafloor, that in the case of the largest deposits can reach several hundreds of thousands of years of carbonate precipitation. In some cases, a climate forcing has been proposed as a contributing factor to the inception of seepage. Detailed sedimentological (facies, microstructures) and geochemical (δ13C, δ18O) investigations of seep-carbonates revealed the involvement of paleo-gas hydrates, but further studies are required in order to support the hypothesis of a regional-scale hydrate destabilization.

This study provided an overview on seep-carbonate deposits of the northern Apennines (Emilia to Umbria-Marchean sector) and factors controlling the regional-scale spatial and temporal evolution of Miocene cold seepage systems, representing a remarkably complete record that can be used to better understand fluid plumbing systems at modern convergent margins.

**Author Contributions:** Methodology—writing—review & editing, S.C., C.A., C.F., A.C.S. and D.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was founded by University of Modena and Reggio Emilia (FAR 2018-2020).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Our thanks to Luca Martire and anonymous reviewer for constructive criticism that led us to improve the paper.

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

#### **References**


### *Article* **Plio–Quaternary Structural Evolution of the Outer Sector of the Marche Apennines South of the Conero Promontory, Italy**

**Mario Costa 1, Jessica Chicco 2, Chiara Invernizzi 3,\*, Simone Teloni <sup>3</sup> and Pietro Paolo Pierantoni <sup>3</sup>**


**Abstract:** Some new results and preliminary remarks about the Plio–Quaternary structural and evolutionary characteristics of the outer Marche Apennines south in the Conero promontory are presented in this study. The present analysis is based on several subsurface seismic reflection profiles and well data, kindly provided by ENI S.p.A. and available on the VIDEPI list, together with surface geologic–stratigraphic knowledge of Plio–Quaternary evolution from the literature. Examples of negative vs. positive reactivation of inherited structures in fold and thrust belts are highlighted. Here, we present an example from the external domain of the Marche Apennines, which displays interesting reactivation examples from the subsurface geology explored. The study area shows significant evolutionary differences with respect to the northern sector of the Marche region previously investigated by the same research group. The areal distribution of the main structures changes north and south of the ENE–WSW oriented discontinuity close to the Conero promontory. Based on the old tripartite classification of the Pliocene, the results of this work suggest a strong differential subsidence with extension occurring during the Early Pliocene and principal compressive deformation starting from the Middle Pliocene and decreasing or ceasing during the Quaternary. The main structure in this area is the NNW–SSE Coastal Structure, which is composed of E-vergent shallow thrusts and high-angle deep-seated normal faults underneath. An important right-lateral strike–slip component along this feature is also suggested, which is compatible with the principal NNE–SSW shortening direction. As mentioned, the area is largely characterized by tectonic inversion. Starting from Middle Pliocene, most of the Early Pliocene normal faults became E-vergent thrusts.

**Keywords:** Plio–Quaternary evolution; outer Marche Apennines; seismic reflection profiles; tectonic inversion; Coastal Structure; extensional and contractional deformation

#### **1. Introduction**

In the Apennines of Italy, and especially the Adriatic foreland domain, it is possible to infer the foreland deformation process and explore the impacts of inherited faults and basins on the subsequent evolution thanks to the milder deformation in the area and the good geological and geophysical record documenting an interaction between normal, thrust, and strike–slip faults.

Foreland domains are often affected by inherited rift-related or flexure-related synsedimentary normal faults becoming involved in the advancing fold-and-thrust belt. This introduces an element of further complications into the evolution of the foredeep systems subsequently involved in the mountain belts, as evidenced by numerous studies in different contexts, such as the Northern Apennines, Po Plain, and South-Eastern Pyrenean foreland basins ([1–4], among others).

The tectonic and structural features of the Umbria-Marche Apennines (Figure 1) are widely described in the literature, and several models have been proposed. The most

**Citation:** Costa, M.; Chicco, J.; Invernizzi, C.; Teloni, S.; Pierantoni, P.P. Plio–Quaternary Structural Evolution of the Outer Sector of the Marche Apennines South of the Conero Promontory, Italy. *Geosciences* **2021**, *11*, 184. https://doi.org/ 10.3390/geosciences11050184

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

Received: 30 January 2021 Accepted: 20 April 2021 Published: 24 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/).

important model is found in [5], which proposes a thin-skinned imbricate belt detached above the crystalline basement (see also [6]). This model indicates strong shortening (in the order of hundreds of kilometres) and important repetitions of the sedimentary cover. Further studies on the geometries and evolution of the outer Marche sector, as well as their extent, style, and age of deformation, are thoroughly reported in many works. Among others, [7–13] mainly focus on stratigraphic record, geological setting, and sedimentary evolution; [14] on the anatomy of the Apennine orogen; [15–20] on the structural and deformation style; and [14,21] on the role of inherited structures and tectonic inversion.

**Figure 1.** On shore schematic geological map of the Marche region (modified from [22]). The work in [23] was considered for the thrust location. Dashed square: study area; dashed red line: ViDEPI seismic profile B-441 with Elisa 1 well (Figure 3). Inset: geographic location of the study and location of the Transects. Transects: results for 1 to 7 are published in [24,25]. Transects 8 to 12: this work.

The acquisition of new data (such as the CROP project, well stratigraphy and seismic reflection profiles, and sedimentological and paleo-thermometric data) have shed new light on the evolution of the area and introduced models that indicate the crystalline basement's involvement (thick-skinned model) and the reactivation of inherited faults (inversion tectonic model). As a main outcome, the amount of shortening affecting this area was progressively reduced from hundreds of kilometres to tens of kilometres [26].

New observations about onshore and offshore outcropping and buried Neogene– Quaternary structures, as well as their possible implications for deep geothermal fluid circulation, were recently integrated into the tectonic framework of the northern outer Marche Apennines [24,25]. These studies highlighted new findings mainly characterized by the presence of positive flower structures to be considered as common features along the whole outer sector of the Northern Apennine chain [24]. This suggests the more relevant influence of strike–slip kinematics in recent times, with implications for seismic assessment and deep fluid circulation [25].

The southern sector of the outer Marche Apennines has been long investigated by authors who addressed specific features in this area as related to a complex foreland– foredeep geometry. In particular, several works explore the influence of thrust-system propagation on the distribution of sedimentary sequences, their 3D geometric organization, and the burial and exhumation history of these units [27–31]. These features were identified as the link between the inner, uplifted, and Early Miocene Apennine fold-and-thrust belt and the outer and younger belt to the east [31]. The interpretations of integrated structural and stratigraphic studies indicate this to be the result of turbidite deposition in a complex foredeep, strongly affected by tectonic activity and Messinian–Pliocene climate changes ([29,32] and references therein).

This paper represents a continuation of the above-mentioned studies [24,25] and aims at highlighting the significant structural and depositional differences between the northern and southern outer Marche Apennine, as well as discussing the timing and style of deformation in the outermost sector of the belt toward the Adriatic foreland, where milder deformation and mainly buried structures are present.

To this end, a detailed study along the sector south of the Conero promontory to S. Benedetto del Tronto was conducted (Figure 1) using seismic reflection profile interpretations and well data for hydrocarbon purposes, kindly provided by ENI S.p.A. Available online data: https://www.videpi.com/videpi/sismica/sismica.asp (accessed on 30 March 2021), and published studies further contributed to acquiring complete information and enriching the results in our previous works.

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

At the continental scale, the Alps and Apennines orogens are located in the hanging wall of two opposite subduction zones. The Alps resulted from the Cretaceous to present via the European plate being subducted beneath the Adriatic plate to the east, whereas the Apennines resulted from the Eocene to the present via subduction of the Adriatic plate to the west ([33] and references therein). The Adriatic plate itself is also subducted below the Dinarides in its easternmost part [33,34].

The arcuate-shaped, NE-verging Umbria–Marche Apennines form the external part of the Northern Apennines foreland fold-and-thrust belt (see [35] and references therein). This belt resulted from the convergence between a mosaic of minor blocks of the Africa–Eurasia plates, such as the European Corsica–Sardinia plate to the West and the African–Adria plate to the east ([36–39] and references therein).

In the Umbria–Marche area, starting from the Miocene, the previously rifted and telescoped African-bearing continental margin was involved in the compressive phase. Here, different styles and degrees of the positive inversion of pre-orogenic faults controlled the location, geometry, and evolution of compressive structures in several cases [16,28,40–44]. In addition, the inner portion of the chain was involved in the Late Miocene to present day extension [14,23,45,46], with episodes of negative inversion [43,47–49].

The study area belongs to the outer Umbria–Marche Apennines chain. The general tectonic–sedimentary evolution of the Umbria–Marche sequence can be framed in three main stages: pre-, syn-, and post-orogenic sedimentation [23]. The pre-orogenic sedimentation is characterized by basin carbonates and marly lithostratigraphic units (Late Triassic to Paleogene in age; [23,35] and references therein, Figure 1). Both syn- and post-orogenic sedimentation is characterized by prevalent terrigenous deposits from Neogene to Quaternary in age and hosted in a wide foredeep basin (Periadriatic Foredeep; [50]). This basin was generated by the flexure of the Adria plate under the Apennine Chain [51], migrating eastward. The foredeep filling includes siliciclastic turbidites (e.g., the Messinian Laga Basin), Plio–Pleistocene marine deposits [51–53], and wedge-top basin sediments [31]. The foredeep itself was gradually involved into the fold-and-thrust belt during the Late Miocene to present.

In the present study, we investigate an area lying in the outer portion of the southern Marche Apennines between the Conero promontory and S. Benedetto del Tronto (Figure 1). In particular, the double effect of the Sibillini thrust to the west and the Gran Sasso thrust to the south (the Abruzzo area in Figure 1) influences the Messinian foredeep geometry and depth. The foredeep hosts thick, internally deformed, turbiditic fan complexes (the Laga Formation; [30,54]) and some positive structures (Acquasanta, Montagna dei Fiori and Coastal Structure) described in the literature (see [2,4,7,30]). The outcropping succession consists of Messinian turbiditic deposits (Figure 2), including a thick, arenaceous basal member whose source is partially provided by the Eocene–Oligocene westernmost chain [7,54] and shallow water facies (S. Donato and Argille a Colombacci Formations). The Argille a Colombacci Formation is always above S. Donato Formation, while the latter may rest discordantly above different members of the Laga Formation (see [22] and references therein). Messinian deposits are followed by the Pliocene succession, whose base marks the marine transgression that occurred after the "lago-mare" phase (sedimentation breck-off; [55]) and the subsequent filling of the Central Adriatic foredeep [56].

The Plio–Pleistocene foredeep basin is associated with deep marine to alluvial sedimentation that shows progressive infill of the basin and a final vertical regressive trend [41]. The infill mainly consists of hemipelagic mudstones deeply incised by coarse-grained canyon-fill deposits [57,58] indicating slope degradation and sediment supply from the uplifted Apennines [32]. Many authors associate these deposits with the outermost part of the orogenic wedge [28,32]; with the formation of thrust fronts and folded structures in the Early Pliocene [11,52]; followed by intense deep water clayey sedimentation in the deepest areas until the Pleistocene; and a new compressive phase right after, likely linked to the reactivation of Late Pliocene thrusts [10]. Deformation of the foredeep via thrusting likely yielded open piggy-back basins and structural highs filled up by shallow-water deposits, likely due to the tightness and blockage of the system against a stable platform, as hypothesized in [11]. The sedimentation within the basin was also partially controlled by the Pliocene–Pleistocene obliquity/precession cycles of the Earth's orbit driving climatic changes, as suggested in [28].

In its outermost portion, the belt shows compressive to transpressive flower structures, which are NW–SE oriented and generally covered by Plio–Pleistocene deposits or partial outcropping on the seafloor. These structures were identified and described in [25] and are located further north of the study area as well as some NE-SW trending faults, which affects the continuity of the previous structures.

In the considered area, the main structural element is the NNW–SSE trending Coastal Structure ("Struttura Costiera" in [11]) which is located near the coastline. This structure continues southwards in the Abruzzo area with similar characteristics [50].

Two main deformation events in the area were recognized by previous authors: an extensional Messinian–Early Pliocene event due to the Adria plate flexure [50] followed by a compressive phase ascribable to the late Early Pliocene [55] or to the Middle Pliocene [50].

**Figure 2.** Synthetic stratigraphic scheme of the Messinian–Pleistocene of the Central Periadriatic Basin (CPB; slightly modified from [32]). This scheme includes the stratigraphic schemes of previous studies [11,13,27,50,59], but in the right column we list only the units and members of our study area.

#### **3. Dataset and Working Methods**

In the onshore and offshore areas between the Conero promontory and San Benedetto del Tronto locality, numerous ENI S.P.A. seismic profiles have been interpreted in addition to those available from VIDEPI (https://www.videpi.com/videpi/sismica/sismica.asp; accessed on 30 March 2021). The ENI seismic profiles were migrated, while the VIDEPI ones were stacked and already interpreted. Some ENI profiles were of a MERGE type and good quality, resulting from advanced reprocessing. All the wells available in the area corresponding to the interpreted seismic profiles were used for the interpretation. However, the materials supplied by ENI are confidential, and we are thus not able to represent them on the seismic profiles. Only a general well location was reported. For the seismic velocities of the sedimentary sequences, we referred to [20,60–62]. We then compared the seismic stratigraphy of the seismic profile VIDEPI B-441 001 (Figure 1) with the log data of the Elisa 1 well placed on it (Figure 3). This comparison indicates that velocity, Vp, for the Plio–Pleistocene sequence is 2000 m/s in agreement with the literature in the same area [50,63].

Seismic profiles were then homogenized and scaled to 1:100.000 horizontally and 1 s TWT = 2 cm vertically. In this way, the horizontal and vertical scales were harmonized for the Plio–Pleistocene sequence of the seismic profile, and the geometries of the tectonic and seismic–stratigraphic elements were preserved. As velocity increased at depths below the lower Pliocene deposits, the dip angles of these structures became higher.

To determine the Plio–Quaternary's tectono–stratigraphic evolution, specific seismo– stratigraphic horizons were considered, as follows:


Within the interpreted profiles, the seismo–stratigraphic horizons are highlighted with different colours (see Figures 3 and 4 and Plate 1 in Supplementary materials). Unconformities are shown in green dots (see Figures 4, 5, and 7). Some additional reflectors are also highlighted (light blue) because these reflectors allow the main structures to be better marked and identified. The boundary between the Pliocene and Quaternary deposits has been always defined based on the available well stratigraphy, where Calabrian is considered to be the base of the Quaternary, while the new bio–stratigraphic scale from https://stratigraphy.org/ (accessed on 30 March 2021) includes the Gelasian (2.58–1.8 Ma) to Quaternary. This scale could introduce some differences compared to recent cartography [22] but is consistent with [7,62].

Some of the best seismic profiles were selected and organized in 5 almost-parallel Transects with a SW–NE direction within the above-mentioned area. Each Transect is composed of several seismic profiles that are aligned or partially overlapping and aim at realizing a single element.

**Figure 3.** Stratigraphic correlation between the ELISA 1 well and a segment of the ViDEPI B-441 seismic profile where the well is placed (Figure 1).

**Figure 4.** Transect 8. The Transect is composed of several seismic profiles labelled with letters *A*, *B*, *C*, see Figure 1). Quaternary deposits are highlighted in blue, Middle–Upper Pliocene deposits are in yellow, Lower Pliocene deposits are in orange, and top Messinian/Pre-Pliocene deposits are in brown. No colour is used for the pre-Pliocene sequence. Unconformities are shown in green dots. Light blue: undefined seismic reflectors. This legend is valid for all Transects (Figures 5–8).

**Figure 5.** Transect 9. The purple line shows the hypothetical basement.

**Figure 6.** Transect 10.

**Figure 7.** Transect 11.

**Figure 8.** Transect 12.

Figure 1 presents traces of the Transects and each seismic profile within them. These traces complete our previous analyses of the outer Apennine Marche sector north of the Conero promontory, where seven Transects have already been observed [24,25]. For this reason, the new Transects are numbered from 8 to 12.

These Transects are represented individually in Figures 4–8 and are reported using a high-resolution plate in the supplementary material (Plate 1).

#### **4. Results by Wells and Seismic Profiles Interpretation**

#### *4.1. Transects*

The northernmost Transect (number 8 in this work; Figure 1) developed in the onshore and offshore areas just south of the Conero promontory and includes the seismic reflection profiles *A*, *B,* and *C* (Figure 4). Overall, the quality of these seismic reflection profiles is good; among these profiles, profile *B*, which was already interpreted in the VIDEPI catalogue, was further interpreted here using different colours.

In the onshore area, a WSW–ENE seismic profile (*A*) extends from the east of the Appignano locality to the coastline (ENE). Along this profile, some wells (Figure 1) allow good calibration of the top of the Messinian/Pre-Pliocene and near the top of the Early Pliocene seismo–stratigraphic horizons. An unconformity is also present within the Middle– Upper Pliocene deposits. In the offshore area, two seismic reflection profiles are present (*B* and *C*) and are aligned along a WSW–ENE direction. In particular, the *B* profile partly overlaps the *A* profile, and two hydrocarbon wells occur nearby.

Transect 8 is characterized by three structurally well-defined areas. On the western area a wide syncline is present, affecting a large thickness of about 3000 m Pliocene deposits. Lower Pliocene deposits cover the Messinian deposits in transgression. These deposits have an almost constant thickness, while those above the Middle–Upper Pliocene feature have variable thickness (ranging between 1800 m in the syncline core and about 1000 m along the limbs). Quaternary deposits have a thickness of a few hundred meters.

The western limb of the syncline is affected by a high-angle, W-dipping reverse fault system. This system deforms the whole Lower to Middle Pliocene sequence without involving that of the Upper Pliocene. However, in the westernmost part, some faults deform the overlying unconformity placed within the uppermost part of the Middle–Upper Pliocene sequence.

In the central area, a very complex compressive and uplifted structure ("Coastal Structure") is present. This structure is characterized by shallow East-verging thrusts affecting the Lower–Middle Pliocene sequence and, marginally, the Upper Miocene sequence. Below this structure, an E-dipping reverse fault and a slightly W-dipping sub-vertical fault reaching the relevant depths (>4 s TWT) are present. Quaternary deposits were likely

involved in the deformation of the upper and frontal sectors of this Coastal Structure. As highlighted in the *A* seismic profile, Quaternary deposits outcropping on the western side of the Coastal Structure show reduced thickness compared to those on the eastern side. The Lower Pliocene deposits are indeed less than 100 m in thickness in front of the Coastal Structure and about 1000 m within the syncline behind.

On the eastern area (seismic profiles *B* and *C*), numerous reverse high angle W- and E-dipping faults are present. Overall, the vertical displacement of these faults is moderate, rarely exceeding 500 m. Lower Pliocene deposits have an average thickness of about 100 m or can be absent in the proximity of some structural highs (see *C* in Figure 4 and the wells presented here). The Middle–Upper Pliocene deposits show more variable thickness, from about one hundred meters on the structural highs to more than 1000 m in the proximity of faults and in structurally deeper areas. This extreme variability together with the characteristics of unevenness and chaos of the seismic horizons suggest a syn–tectonic origin of these deposits. The middle lower part of this sequence is certainly affected by reverse faults, while the upper part does not appear to be involved in deformation (seismic profiles *B* and *C* in Figure 4). Indeed, in this area the Quaternary deposits show a regular trend—increasing their thickness toward the east—and are not involved in deformation.

Transect 9 (Figure 5), which includes several wells, shows similar structural and stratigraphic characteristics to those of Transect 8. These differences relate to the greater thickness of the Lower Pliocene and Quaternary deposits facing the Coastal Structure and the high angle faults that are more evident below this structure. A Middle-Upper Pliocene unconformity is also clear in this area and was displaced by frontal thrusts. In this Transect, seismic reflection profile *A* overlaps profile *B* moving eastward toward the coastline. This profile exhibits a shallow compressive structure characterized by east- and west-verging thrusts.

In this structure, the Lower Pliocene deposits are concordant with the Messinian ones, featuring a thickness of about 800 m and more than 1000 m. Eastward, the thickness is notably reduced (about 150 m). Moreover, an unconformity present in the Middle-Upper Pliocene deposits separates the upper portion of the sequence, which is characterized by onlap geometry, from the lower one featuring pinch-out geometry. Furthermore, below the surface thrusts of the Coastal Structure, seismic profiles *A* and *B* from Figures 4 and 5 show very evident high angle W- and E-dipping faults. Offshore, seismic reflection profiles *C* and *D* show pre-Pliocene bedrock widely deformed by high angle west- and east-dipping compressive faults forming gentle pop-up structures with reduced vertical displacement. The thickness of the Lower Pliocene deposits is always very low (<100 m, as also reported in well stratigraphy), while the Middle–Upper Pliocene deposits are syn–tectonic with high variable thickness (from a few to several hundreds of meters) close to compressive structures. Quaternary deposits have a relatively constant thickness (about 600–800 m) and are not affected by deformation. All the other Transects (10–12, see Figures 6–8) show similar characteristics to those described above. As already mentioned, due to the different resolutions of seismic profiles and/or local factors, certain features are clearer than others.

In the westernmost sector of Transects 10 and 11, a deeply rooted sub-vertical structure is highlighted. Transect 10 (Figure 6) shows a branched flower structure that separates the Laga Formation units by the Colombacci Formation at the surface (Figure 1; [25]). This is a branched structure with a possible strike–slip component. In these two Transects, the compressive, W-dipping structures observed in Transect 8 are absent. Furthermore, along Transect 11 (profile *A* in Figure 7), an important normal E-dipping fault (more than 3000 m of vertical displacement) defines the Lower Pliocene basin to the west and is covered by transgressive deposits of the Middle–Upper Pliocene. In the same Transect, the abovementioned unconformity within the Middle–Upper Pliocene sequence is clearly visible within the syncline. Transect 11 shows that during the Middle/Upper Pliocene, there was simultaneous subsidence (with transgression) in the current onshore to the west together with compression and uplift to the east (Coastal Structure, profiles *A-B* in Figure 7). In both

Transect 11 and 12 (Figure 8), compressive E-dipping faults under the Coastal Structure thrusts are clearly present, as in Transects 8 and 9.

Some thrusts of the Coastal Structure, as shown in Transect 11, affect the Quaternary deposits, such as in Transect 8. In Transect 12, only the shallowest Quaternary deposits are transgressive and are not involved in the deformation. Instead, in Transect 10 the thrusts affect only the Middle–Upper Pliocene sequence. In all Transects, the Quaternary succession covering the offshore flower structure is undeformed. Furthermore, evidence of fore-set Quaternary sedimentation is present in Transects 10 and 12 (Figures 6 and 8).

#### *4.2. Characteristics and Distribution of the Plio–Quaternary Deformation*

#### 4.2.1. Early Pliocene

Based on well data logs and interpretations of both VIDEPI and ENI seismic profiles, we achieved a reconstruction of the thicknesses and distribution of the Lower Pliocene stratigraphic sequence (Figure 9).

**Figure 9.** Schematic map of the distribution and thickness of Lower Pliocene deposits within the Marche region and the adjacent Adriatic Sea. Marche Adriatic Structural High (MASH) and smaller structural highs and basins are highlighted with the same colour. The study area is located in the dashed square.

This sequence has significant and sudden variations in thickness, and we were able to distinguish between the true sedimentary thicknesses and local tectonic repetitions or highly inclined bedding in proximity of compressive structures. This does not allow us to reconstruct a reliable isopaches map. Afterward, for an immediate view of the Lower Pliocene deposits distribution, we identified two distinct thickness classes (less than 200 m and greater than 500 m). This simple representation makes it possible to easily locate the position, geometry, and kinematics of the faults affecting this sequence. Finally, in Figure 9 the whole outer Marche Apennine sector has been reproduced to show the distribution of this succession.

The thickness distribution of the Lower Pliocene deposits shows evidence of a wide semi-submerged Marche Adriatic Structural High (MASH) area mainly located on the current Adriatic offshore (light green in Figure 9), as well as evidence of a wide basin area located in the northern portion of the same offshore area and in the south onshore area (dark green in Figure 9). This area also includes the northern part of the Marche territory, which is only marginally examined in this work.

Within the MASH area, the thickness of these deposits is very modest (a few tens of meters and, locally, not more than 200 m). In the basin area, the thickness rapidly increases, ranging from more than 500 to 3330 m. The limit between the plateau and basin areas features an NNW–SSE trend south of the Conero promontory lies slightly eastward of the current coastline, which shows instead an NE–SW trend in proximity to the Fano offshore area.

This spatial distribution is likely due to a normal or transtensive fault system that separates the wide and stable MASH area, which appears to be slightly subsident and located in the central–southern Adriatic offshore, from a basin area that is strongly subsident towards its western and northern portions.

Furthermore, the western side of the basin is marked by a normal fault system (Transects 10*A*, 11*A*; Figures 6 and 7). This transtensive fault system was active soon before the onset of the compressive phase highlighted within the Transect.

As underlined in the previous section, this normal fault system consists of syn– sedimentary high angle W- and E-dipping faults characterized by remarkable vertical displacement reaching thousands of meters, which is clearly detectable in the interpreted seismic profiles.

The main faults were likely placed in proximity of the NNW–SSE and NE–SW boundaries of uplifted and subsident areas. Other minor faults further disarticulated both the basin and the MASH areas, defining local thickness variations in the sequence.

#### 4.2.2. Middle-Late Pliocene-Quaternary

Based on our investigation, three structurally well-defined areas along a W–E direction are identified (Figure 10).

The western area is characterized by a wide syncline. In the northern part of this area, the syncline is locally intersected by W-dipping high-angle reverse faults (Figure 4); in the southern portion, Lower Pliocene deposits end against a high-angle E-dipping normal fault to the West, covered by transgressive Middle–Upper Pliocene deposits. The syncline axis is about N–S oriented. W of the syncline, a sub-vertical fault system deeply rooted with a N–S trend can be observed.

The central area is marked by a compressive structure (Coastal Structure). This structure consists of a series of E-verging thrusts within the shallower sequence, mainly affecting the Lower–Middle Pliocene deposits and only marginally affecting the Messinian ones. Thrust displacements are rapidly reduced within the Messinian and Lower Pliocene deposits. Just below this horizon, E-dipping reverse faults and deeper high-angle Wdipping faults are present. The Coastal Structure shows an NNW–SSE, almost continuous, trend, and sometimes crops out close to the coastline. This structure was formed starting during the Middle Pliocene, and its deformation continued until the Upper Pliocene, in some parts up to the Quaternary.

**Figure 10.** Structural sketch map of the outer Marche area south of Conero promontory. The main Plio–Quaternary structures are highlighted.

The eastern area is characterized by W- and E-verging high-angle reverse faults, giving rise to gentle flower structures with an NW–SE trend. These structures were formed from the Middle Pliocene and developed structural highs, some of which were still emerging during the Upper Pliocene/Pleistocene (Transects 10, 11; Figures 6 and 7). Compressive deformation stopped during the Upper Pliocene, and Quaternary deposits were not affected.

#### **5. Discussion**

During the Messinian, this part of the Marche Apennines outer sector emerged or was close to emersion (the "lago-mare" succession in Figure 2), with sedimentary break-off [55]. The top Messinian/pre-Pliocene seismic horizon was always clearly evident in the examined seismic profiles, with frequent characteristics of an erosive surface (Figures 6 and 7). The Lower Pliocene deposits, however, are often transgressive or discordant over the underlying Messinian or pre-Pliocene ones. Furthermore, no important evidence of Messinian active tectonics was found in this area. This part of the sector started to deform during the Early Pliocene when normal or transtensive faults with an NNW–SSE trend were enucleated. These faults separated heavily subsident basin areas from almost-stable structural highs (Figure 9).

The basin area was mainly located along the current onshore, while the Marche Adriatic Structural High (MASH) was located in the current offshore. This feature continued southward in the Abruzzo region with quite similar characteristics, as described in [28]. According to these authors, in Abruzzo, the basin formed due to horst and graben structures starting in the Messinian–Pliocene transition due to flexural extension of the under-thrusting Adria Plate. In our study area, this extensional phase started in the Early

Pliocene, as indicated by the erosive top Messinian and the transgressive and discordant Pliocene deposits above it.

North of the Conero promontory, the area's slightly more complex setting was also due to an important NE–SW trending fault that segmented the MASH, yielding a basin area to the NW (Figure 9). This structure, already identified in [24], continues from the Fano offshore to the SW along the river valley (Figure 1). Other local features with an NE–SW trend segmented both the basin and the MASH, forming lower-ranking depressions and structural highs (Figure 9). The northernmost transverse structures correspond to the Cattolica seismogenic system [24].

Starting from the Middle Pliocene, a compressive regime was established in the sector south of the Conero promontory, growing the structures underlined in the Transects and in Figure 10.

In more detail, in the study area we highlighted a wide syncline with an almost N–S trend to the west, the Coastal Structure with an NNW–SSE trend in the central portion, and the NW–SE-trending gentle-flower structure system to the east. The syncline was thus formed in correspondence with the Lower Pliocene basin, and the Coastal Structure formed in correspondence with the normal faults bordering the same basin to the east (Figures 9 and 10). The Middle Pliocene deposits are continuous with those of the Lower Pliocene at the syncline core while resting on the same deposits with a pinch-out feature and reduced thickness in proximity to the growing Coastal Structure western flank (Transects 9, 11, and 12; Figures 5, 7 and 8). Variable thickness, with greater thickness close to the faults, attests to the syn–tectonic origins of these deposits in the offshore area.

Further to the west of the syncline, the N–S Amandola-positive flower structure (Figure 6) separates different Messinian units [7]. This structure is high-angle and deeply rooted (Transects 10 and 11; Figures 6 and 7), likely extending farther than the representation in Figure 10. The push-up in the western part of Transect 8a (Figure 4) is likely a continuation of the Amandola structure or one of its branches. All these structural elements are slightly divergent from each other and are interrupted along a complex transverse structure approximately ENE–WSW oriented and located immediately south of the Conero Promontory (Figure 10).

The compressive deformation phase ended in the western and eastern areas during the Late Pliocene-Early Pleistocene. The unconformity within the Middle–Upper Pliocene deposits (Transects 9 and 11; Figures 5 and 7) indicates that the syncline has not deepened since the Late Pliocene. Upper Pliocene deposits rest in an on-lap over the underlying ones above the unconformity and reduce their thickness in proximity of the western flank of the Coastal Structure. These features indicate that, within the syncline, the lower parts of the Middle–Upper Pliocene deposits are syn–tectonic, while those of the upper part are post-tectonic.

The flower structures of the Adriatic offshore are sealed by the Quaternary deposits. In the central area, the Coastal Structure continued its activity even during the Quaternary, as shown in several areas (Transects 8,9, and 11 in Figures 4, 5 and 7). Therefore, all these structures were formed during the Middle Pliocene. Most of these were deactivated at the end of the Late Pliocene, while a few others were probably still active during the Early Pleistocene (Transects 10–12 in Figures 6–8).

The Coastal Structure is characterized by low-angle faults close to the surface and high angle faults at depth. Low-angle faults are mainly involved in the Lower Pliocene deposits, making their repetition clearly visible in all Transects. The underlying Messinian deposits were, instead, not significantly involved, likely due to detachment between the two sequences. In [11], however, Messinian deposits were considered to be strongly involved in deformation. At the western edge of the syncline, and underneath the highly deformed close-to-the-surface succession (Transect 11), parts of the original faults bordering the Lower Pliocene basin are still recognizable. Looking at the Lower Pliocene deposits distribution map (Figure 9), it can be seen that the Coastal Structure is nucleated in the same position as the faults bordering the previous Lower Pliocene basin to the east and perfectly follows the trend of the latter. Therefore, this structure was formed by partially inverting or deforming (Figure 11) the previous high-angle normal/transtensive faults (see also Figures 5–8). These faults may have acted initially as a barrier, forcing the involved sequences to climb upwards; in some cases (Figures 5, 7 and 8), the innermost thrusts show a higher angle than the external ones. Subsequently, as the shortening increased, the upper parts of the Early Pliocene faults were decapitated (see [43]) and included within the superficial low-angle, E-verging thrust sheets, which mainly affect the Lower Pliocene succession that is partially detached from the underlying one (Figure 11).

**Figure 11.** Sketch diagram showing the evolution of the Coastal Structure from the Early to Middle Pliocene across two representative cross-sections (not to scale). The size of the grey arrows is proportional to the intensity of vertical movement.

Thus, in the later stage (Late Pliocene–Quaternary), some of the thrust sheets partially covered the westernmost flower structures of the eastern Adriatic area (Transects 8, 10, and 12 in Figures 4, 6 and 8). The compressive Coastal Structure formed due to the inversion of previous extensional features following the "interaction of extensional and contractional deformation" model proposed by [43,64] and in [18,65] for the nearby Montagna dei Fiori and Cingoli structures (Figure 11).

The Coastal Structure continues southward, in the Abruzzo region, with quite similar litho–structural characteristics and ages of deformation [50,65]. Additionally, in that area, E-vergent thrusts are mainly found in the Lower Pliocene deposits, which, in this case, are completely detached from the underlying Messinian ones. However, unlike the process proposed for our study area, these authors suggest that the previous Early Pliocene normal faults bordering the basin were already enucleated during the Messinian. Furthermore, these normal faults were not involved in compressive deformation but were simply covered by the thrust sheets. According to these authors, compressive deformation began in the Early Pliocene.

As a result of the compression that determined the Coastal Structure's development, tilting of the block between this structure and the Amandola structure to the W likely also have occurred. During the Middle Pliocene, there was simultaneous uplift of the eastern front (enucleation and uplift of the Coastal Structure) and subsidence of the western side (transgression of the Middle Pliocene deposits on those previously raised during the pre-Pliocene time; Transect 11). The horizontal rotational axis may correspond to the syncline axis. This mechanism is similar to that described in [66] for the Po Valley. During the Late Pliocene–Quaternary this rotation ceased, and the deposits of the same age became horizontal.

The Amandola structure, the syncline, and the Coastal Structure show a straight and regular trend. As previously mentioned, the trend of these main onshore structures is somewhat divergent from the offshore one, even though they all formed during the same time interval. This can be attributed to the influence of pre-existing features inherited by previous deformation phases such as the faults shown in Figure 9. These structures are compatible with the main local shortening oriented in an NNE–SSW direction during the Middle–Late Pliocene (compression with the P axis about NNE–SSW; Figure 12), which emerged in the northern sector of the Marche region [24,67,68] and, more generally, in the overall Central Adriatic area [69,70].

In this context, right-lateral transpression developed along the Coastal Structure and likely enhanced the gentle flower systems of the Adriatic offshore (Figure 12)

The Coastal Structure schematically represented as continuous and regular in Figure 10 is most likely composed of several structures, some of which were still active during the Quaternary, as shown by fairly significant earthquake sequences (Mw = 5, Porto San Giorgio sequence, [71,72]) that occurred recently (Figure 12).

As previously mentioned, the described structures were somehow interrupted to the north along a transverse ENE–WSW-oriented structure. The existence of transverse faults has been highlighted in literature by various authors, particularly in the Marche-Abruzzo onshore (see [7,24,73] and reference therein). In our study area, several structures underwent sudden changes in characteristics (age of deformation, geometry, and direction) that are observable when compared to those mapped in [24] in the areas west, east, and north of the Conero promontory. Furthermore, the structures present immediately northward of this transverse element and of our study area, e.g., the Early Pliocene transpressive structure of Strada-Moie and S. Andrea di Suasa (see Figure 7 in [23]) are no longer present in the south. Indeed, in this southern area, extension was still occurring during the Early Pliocene.

**Figure 12.** Kinematic sketch map. Red lines indicate fault systems still active during the Quaternary. The black arrow represents the main shortening direction, red arrows describe the right lateral strike–slip component, and the plus symbol (+) is the narrow strongly uplifted area. Focal mechanisms (beach balls) of the main earthquakes of 1987 Porto S. Giorgio and 2013 south of Conero seismic sequences are shown.

The transverse structure south of the Conero promontory, already partially present in [7], interrupts structures with Quaternary activity, i.e., the Coastal Structure to the south and the Conero compressional structure [25] to the north. Therefore, this tectonic element must be Quaternary itself, as also attested by recent earthquakes and seismic sequences in the offshore along the element (Figure 12). These focal mechanisms are predominantly strike–slip, with P-axes oriented around the ENE–WSW and sub-vertical planes [67,68]. Furthermore, the epicentres of the seismic sequences described by these authors are aligned ENE–WSW.

#### **6. Conclusions**

Seismic profile interpretations and well stratigraphic data allowed us to describe the Plio–Quaternary evolution of the outer Marche Apennines south of the Conero promontory. The main results can be summarized as follows:


dextral strike-slip Amandola structure, the NNW–SSE dextral transpressive Coastal Structure, and an NW–SE-striking system of gentle flower structures (offshore).


Based on our results, we conclude that during the Plio–Quaternary times, this portion of the outer Apennine sector is mainly affected by a right-lateral transpressive deformation, and by widespread kinematic inversion of pre-existing structures. Former studies proposed a simple E-vergent compressive deformation for the same area.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/geosciences11050184/s1.

**Author Contributions:** Investigation, M.C.; resources, J.C., M.C., S.T, and P.P.P.; data curation, S.T., P.P.P., and M.C.; writing—original draft preparation, C.I., M.C., and J.C.; writing—review and editing, M.C., C.I., and J.C.; funding acquisition, P.P.P. and C.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by grants from P.P. Pierantoni (FAR—Fondo di Ricerca Ateneo, Università di Camerino) and C. Invernizzi (Interreg Coastenergy project ID: 10045844; FAR—Fondo di Ricerca Ateneo, Università di Camerino).

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** ENI S.p.A. is thanked for supplying seismic profiles, for their facilities, and for permission to publish the data. Stefano Mazzoli and Claudio Di Celma are kindly thanked for their useful discussions. The authors also thank the reviewers for several insightful comments that consistently improved the manuscript and Giancarlo Molli, Angelo Cipriani, and Domenico Liotta for their editorial work.

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

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