**1. Introduction**

The Pliocene and Quaternary geodynamic processes related to the Tyrrhenian basin opening led an uplift in Sardinia [1–3]. This is evidenced by a morphotectonic setting linked to fluvial and gravitative morphologies [4,5]. Therefore, the hydrographic basins of the Rio Quirra and the Rio Pardu have been studied in detail in order to analyze their evolutionary scenarios in relation to a river capture.

Rio Pardu and Rio Quirra are two of the most important rivers in central eastern Sardinia. The two basins are separated by a river capture caused by the Rio Pelau, which isolated Rio Pardu, the catchment area of Rio Quirra. Rio Pardu flows from northwest to southeast and then flows towards the northeast through a river capture elbow with the name of Rio Pelau. Rio Quirra flows from the north to the south parallel to the coast and then abruptly bends towards the Tyrrhenian Sea near the mouth. The flow directions are

**Citation:** Demurtas, V.; Orrù, P.E.; Deiana, G. Evolution of Deep-Seated Gravitational Slope Deformations in Relation with Uplift and Fluvial Capture Processes in Central Eastern Sardinia (Italy). *Land* **2021**, *10*, 1193. https://doi.org/10.3390/land10111193

Academic Editor: Giulio Iovine

Received: 15 October 2021 Accepted: 3 November 2021 Published: 5 November 2021

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**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/).

closely related to the structural conditioning of the main alpine structural setting and are linked to the opening of the Tyrrhenian basin [6,7] (Figure 1).

**Figure 1.** Geographical location and structural features of the study area, modified after [8]; red lines represent thrust fronts; white lines are the Sardinian–Corse Block translation at 30 Ma; the pink line represents the Sardinian–Corse Block translation at 25 Ma; the yellow line represents the Calabrian block translation at 10 Ma; the green line represents the Calabrian block translation at 5 Ma [2].

River drainage systems are very dynamic features of the landscape. Geological changes can cause fluvial captures, leading to abnormal large-scale river networks [7,9–16]. The main geological changes that cause river captures are glaciation and tectonic movements associated with earthquakes and faults [17]. Tectonic movements, especially landscape uplift, are much slower than glacial processes. Therefore, the development of tectonic river capture normally requires hundreds of thousands of years, or even millions of years [18,19].

The particular evolutionary characteristics of the Pardu and Quirra valleys in relation to slope instability dynamics have been the subject of various studies [4–7]. This sector of Sardinia represents one of the most susceptible areas to landslides in the region. This high hazard is closely linked to the particular vulnerability to important weather events, especially rainstorms. Rainfall-induced landslides represent a relevant threat to the population, infrastructure, buildings, and cultural heritage [20–24]. Among the most important catastrophic geological events in Sardinia are those that occurred in the Rio Pardu valley, which involved the inhabited centers of Gairo, Osini, Ulassai, and Jerzu. Between 15 and 17 October, 1951, extreme rainfall of about 1000 mm involved this area, triggering mudflows and landslides. This catastrophic event caused abandonment of the villages of Osini and Gairo [5,7,25]. These settlements have been rebuilt at least in part, sometimes with transfer to another area on the same slope. However, these measures proved useless, as the new sites present the same geo-hydrological risks as the previous ones [26,27]. Landslides affected schistose Paleozoic metamorphites on the left slope, while on the right, there was also widespread rockfall. Recent studies have highlighted the presence of deep-seated gravitational slope deformations with sacking [28–32] and lateral spread [33–35] characteristics that affect the sub-horizontal carbonate succession and the underlying metamorphites [6].

Deep-seated gravitational slope deformation (DGSD, [36]) is a complex type of rock slope failure characterized by large dimensions generated in stone rocks [37]. DGSDs are characterized by slow movements that can suddenly accelerate and cause catastrophic

collapse of sections of the deformed slopes [30,38–41]. Therefore, this phenomenon represents an important geo-hazard in relation to the deformation of large infrastructures and secondary collateral landslides. Although DGSDs play an important role in slope evolution and geo-hydrological risk, knowledge about them was scarce for a long time [42]. They are characterized by very slow deformation rates [34], landform assemblages (such as double-crested ridges, trenches, synthetic and antithetic scarps, tension cracks, and convex bulged toes), and deep basal shear zones [43–47]. Often, shear zones present characteristics of cataclastic breccias with an abundant fine matrix [48] and thicknesses up to tens of meters [41]. DGSD is a common phenomenon in the relief of the Mediterranean Sea in relation to the particular geodynamic context that characterizes the region and to the widespread orogenetic chains. In this context, DGSDs play an important role in slope relief evolution, showing at least geometric analogies with gravity-accommodated structural wedges. Often, DGSD phenomena are influenced by the scale structural context of the slope and use pre-existing tectonic structures (fault and thrust) to guide their evolution, which is also in relation to a reactivation linked to a slope stress field variation [49,50].

In Sardinia, the studies and evidence of DGSDs are quite scarce, but the distensive tectonics and the Plio-Quaternary uplift could justify the favorable conditions for the development of DGSDs, which could also be due to local reactivation of Hercynian and Alpine tectonic structures. In this context, the slope evolutionary characteristics are analyzed—in particular, the DGSDs and the evolution of watercourses in relation to the uplift. The aim is to correlate these different aspects through geomorphological analysis with both field surveys and remote sensing techniques. Furthermore, the choice to analyze these basins takes on a particular characteristic due to the economic and social repercussions that the conditions of instability of the slopes determine in the populations of the towns of Ulassai, Osini, Jerzu, and Gairo. In fact, as is well known, these inhabited centers are continually threatened by disasters. Different types of interventions were carried out to protect inhabited centers and infrastructures, but they were carried out without a global study of the problem and, therefore, without real knowledge of the evolutionary modalities of the valley and the real gravitational dynamics of the slopes. Understanding the kinematics and temporal behavior of DGSDs and landslides is important for designing monitoring systems based on strong process knowledge. In some cases, continuous monitoring is the only way to reduce risk [51–55].

We hypothesize that the Plio-Quaternary tectonics and uplift in the Ogliastra area are the main forcing mechanisms for sustaining the necessary gravitational forces of DGSDs.

Here, we present an innovative approach for analysis of DGSDs and fluvial dynamics by using morphotectonic, morphostratigraphic, and geomorphic data and time-series InSAR data in the Pardu and Quirra rivers. We also integrated stratigraphic and morphotectonic data of the drainage basin scale to support our observations and analyses about the relation between DGSD activity and fluvial capture.

## **2. Geological Setting**

East-central Sardinia (Italy) is characterized by widespread Jurassic dolomitic plateaus— called "Tacchi" in Sardinia—overlying a Paleozoic basement (Figures 2 and 3a,b) [56,57].

The area is characterized by the Pardu River Valley in the north, the Quirra River Valley in the south, and the Rio Pelau toward the east (Figure 2). The geological basement primarily comprises low-grade Paleozoic metamorphites affected by complex plicative structures, while in the coastal sector, there are widespread outcrops of carboniferous granites placed in the terminal phases of the Hercynian orogeny [1,59–61]. The major metamorphic Paleozoic units are the Filladi del Gennargentu Formation and Monte Santa Vittoria Formation, which are constituted by metasandstones, quartzites, phyllites, and metavolcanites (Middle Cambrian–Middle Ordovician) [57,62,63]. The summit of the metamorphic basement has suffered chemical alteration associated with a warm humid climate during the Permian and Triassic periods [64,65].

The marine and transitional Mesozoic sedimentary succession rests on the metamorphic basement in angular unconformity. These Mesozoic deposits are extensive and decipherable from their plateau morphology and are clearly visible along the right slope of the Rio Pardu and the Rio Quirra (Figure 2). The basal layers are primarily fluvial sediments of the Genna Selole Formation (Middle Jurassic) (Figure 3d), which are overlain by dolomitic limestones of the Dorgali Formation (Middle–Upper Jurassic) (Figure 3e). [57,64,66,67].

**Figure 2.** Geolithological sketch map of the study area based on geological data of the Autonomous Region of Sardinia. Continental margin topography by [58]. Black dashed lines show the analyzed rivers.

**Figure 3.** Lithostratigraphic sketch of lithological units: Low-grade metamorphic rocks: (a) Monte Santa Vittoria Fm; (b) Filladi del Gennargentu Formation; (c) granitic plutonic complex and dikes; marine and transitional Mesozoic sedimentary succession; (d) gluvial and deltaic conglomerates, sandstones, and mudstones (Genna SeloleFormation); (e) dolostone (Dorgali Formation); (f) cemented paleo-rockfall deposits; (g) paleo-rockfall deposits; (h) active rockfall; (i) slope deposits; (l) terraced alluvial deposits; (m) alluvial deposits (modified after Demurtas et al. [6]).

The Genna Selole Formation [67,68] represents a mixed succession of siliciclastic to siliciclastic–carbonate deposits. The presence of clay layers is important as a predisposing factor for lateral spread. The Dorgali Formation is represented by dolomitic sequences with thicknesses of up to tens of meters. The lower part, with a thickness of approximately 30 m, is affected by marl intercalations, whereas the upper part is typically massive. The attitude of the strata of the Mesozoic units is sub-horizontal with a dip of approximately N90/0–5◦, while at the plateau edges, it can reach a dip of up to 40◦ and a direction parallel to the slope owing to the DGSDs. Quaternary covers, which are represented by continental deposits, are primarily gravitative and alluvial deposits. The most extensive outcrops are represented by landslide deposits, including rockfalls, toppling, and collapsed DGSDs, and are abundant in the lower part on the right slopes of the Pardu Valley and Quirra Valley (eastern slope of Monte Arbu). Downslope, actual and terraced alluvial deposits have also been identified, and they are well developed and hierarchized in the Rio Quirra [57].

The deposits of the rockfalls and toppling landslides have been characterized by their different sedimentological features based on age (Figure 3f–h). These deposits are associated with rockfalls affecting the plateau edge wall and the collapse of some parts of the DGSDs [6].
