**3. Geodynamic Setting**

The river networks' geometry and gravity processes show a young conformation of the landscape, which is typical of a recent tectonic setting. The geodynamic setting is associated with the collisional dynamics between the African and European plates [2] (Figure 1). The structural setting is associated with the Alpine cycle, which first appeared with a strike-slip fault in the Oligo–Miocene, and then in the Pliocene and Quaternary with an extensional component [1–3,56,69–72].

The major features in the study area are the NW–SE and N–S faults on which, respectively, the Pardu Valley and Quirra River are engraved, and the secondary fault directions include ENE–WSW and NNE–SSW [57].

The Plio-Quaternary tectonic phase is associated with conspicuous N–S faults [73]. These rectilinear and normal faults are also evident in the continental margin and control its morphology (Figure 2). In the continental region, these N–S faults are associated with alkaline basalts with an age of approximately 3.9 Ma—Pleistocene [74]. Especially during the upper Pliocene, a general areal elevation occurred throughout the island, highlighted by the traces of the paleo-surfaces and by the numerous and superimposed paleo-hydrographies; moreover, the Neogenic sediments, which were already affected by Oligo-Miocene Tectonics, are currently also found at altitudes of 700 m, such as on the Tacco di Laconi, and are widely found above 500 m of altitude in various locations on the island. The reasons for these events are related to the more general distensive tectonics that affect the whole Tyrrhenian area [75].

Based on preliminary geodetic data from the Peri-Tyrrhenian Geodetic Array network, Ferranti et al. (2008) [76] revealed the presence of low internal deformation in Sardinia. In Sardinia, seismicity is typically scattered and sporadic, except for the dozen tremors detected following the ML4.7 earthquake of 7 July 2011 in the Corsican Sea, which primarily characterized the edges of the continental lithosphere block. Significant seismic events also occurred in the eastern sector—in particular, three events with a magnitude > 4 (26 April 2000, magnitudes ML 4.2 and 4.7, and 18 December 2004, magnitude ML 4.3)— located in the central Tyrrhenian Sea, approximately 60 km east of Olbia in the Comino depression [77].The most recent low-magnitude earthquake events were ML 1.8 (Escalaplano, 4 April 2019) and ML 1.6 (Perdasdefogu, 14 October 2020) [78].

Along the Ogliastra coast, recent movements have acted by conditioning the trend of the hydrographic network and the morphological evolution. The basaltic plateau of the Teccu in Barisardo can be related to these movements along an N–S line.

The Sardinian continental margin started from around 9 Ma, following the opening of the Tyrrhenian Sea, which caused the thinning of the continental crust and the formation of tectonic depressions, which are now sites of deep intra-slope basins.

The continental margin off the Ogliastra is represented by the continental shelf, the continental slope, and the plain called the Ogliastra basin, which reaches the deepest point of the whole Sardinian margin at 1750 m depth. The continental shelf is very narrow with less than 10 km of width, and it is indented by several submarine canyons [58,70].

## **4. Geomorphological Setting**

The landscape, which is characterized by sub-horizontal carbonate plateaus, represents the result of the paleogeographic evolution of the region. The current dolomitic plateaus represent the extensive carbonate sedimentation due to the Jurassic marine transgression on the peneplanated Paleozoic metamorphites during the Permian and the Triassic. The continental phase following the post-Mesozoic emergence determined the setting of a tectonic control hydrographic network represented by deep rectilinear valleys engraved in the Paleozoic basement for several hundreds of meters [4,5] (Figure 4). Erosion primarily acted on the Oligo-Miocene strike-slip faults with an increase in the erosive rate during the Plio-Pleistocene uplift phases [25]. The presence of major regional faults has influenced the watercourses, which maintain a prevalent N–S direction in the Pardu and Quirra Rivers (set on the main fault).

**Figure 4.** Three-dimensional (3D) model of the Pardu River and Quirra River. Blue lines represent major hydrographic features, and red areas represent the major DGSDs. (a) Fluvial capture elbow; (b) Lequarci waterfall.

The evolution of the Pardu River is closely associated with that of the Quirra River [7,57,79]. The Pardu River flows from the NW toward the SE and then abruptly changes direction toward the NE. At this point, a capture elbow adjacent to the present head of the Quirra River is well developed. The upstream part of the Quirra River flows at an altitude of approximately 200 m higher than the Pardu River. It also presents an over-sized and over-flooded valley with respect to the upstream catchment area. Moreover, there are various orders of river terraces and slope deposits of the Pleistocene. This setting indicates that the Pardu River, previously flowing south along the Quirra River, was captured by the Pelau River [7,79]. Considering the descriptive parameters, longitudinal profile, and the evolutionary conditions, the Pardu Valley is associated with a cycle of underdeveloped fluvial erosion, suggesting a relatively young age of engraving [4,5,25].

DGSDs are present in both river basins and cause collateral landslides. In particular,rockfalls and toppling occur along carbonate cornices, while rotational slide occurs in the metamorphic rocks [6]. We focused on the DGSDs in this study, as they are important in the morphological evolution of the slopes.

A significant karstic process has acted on plateau surfaces, comprising ancient paleoforms and, currently, hypogeal and superficial morphologies [6,7,80]. Karst paleoforms represented by complex cockpit doline types have been characterized, and they belong in a humid and warm paleo-morphoclimatic setting [6,81–83]. These dolines are separated by residual reliefs called Fengcong, which are sorted among the major structural features. The hypogean karst enabled the development of sinkholes, karst springs, cavities, and caves (e.g., Su Marmuri Cave and Is lianas Cave). The combined action of karst, uplift, river erosion, and gravity has led to the formation and evolution of hanging valleys on the plateau surfaces [5]. The geomorphological analysis of the continental margin off the coast shows that the area occupied by the shelf is rather narrow and is engraved with numerous submarine canyons [58,84,85] (Figure 2). The structural lines coincide with those of the continental part that has emerged—mainly N–S, accompanied by normal tectonic lines in

the E–W direction. The shelf break is about −130 m; however, locally, it is at about −60 m due to the erosion of retrogressive canyons. The submerged and emerged morphologies highlight the extremely young landscape conformation, which is associated with the Neogene and Quaternary geodynamic events, implying a series of problems related to the slope process. The control factors of the DGSDs are associated with the geo-structural characteristics and the Neogene and Quaternary geomorphological evolution of the river valley, which is associated with the recent uplift [6].

We can summarize the events that dominated the valleys' evolution [4,6,79]:


#### **5. Materials and Methods**

A morphotectonic analysis of the River Pardu and River Quirra was carried out based on an integrated approach that incorporated a cartographic and morphometric analysis [86–88]. Remote sensing analysis and geological and geomorphological field mapping in slopes and the valley floor of the Rio Quirra and Rio Pardu were performed from the head to the mouth on a scale of 1:10,000. The field surveys were based on the interpretation of data from remote sensing on a large scale. Particular attention was paid to the study of morphologies related to river dynamics (fluvial and orographic terraces) and slope gravitational process (DGSDs and collateral landslides).

Multi-scale field surveys were carried out to analyze the geological and structural setting of the slopes—in particular, the plateaus' edges and the left slope of the Pardu Valley [89–94].

The DGSDs were surveyed in detail by reconstructing the structural setting and analyzing the relationships with the surrounding collateral landslide and alluvial deposits. The study areas were often not accessible due to their steep slopes; therefore, they required remote sensing survey systems to complete the field investigations. Uncrewed aerial vehicle digital photogrammetry (UAV-DP) is a robust methodology for the investigation of DGSDs and large landslides. In particular, it was used for the recognition of large lateral spreads in Malta and Tunisia [95,96]. We used UAV-DF and light detection and ranging (LiDAR) to extract high-resolution topographic 3D DGSD models and perform detailed morphometric analyses.

DGSD displacement and rate were evaluated using space-borne interferometric synthetic aperture radar (InSAR). Over the last 30 years, InSAR techniques have been widely used to investigate geological (e.g., volcano activity, earthquakes' ground effects, etc.) and geomorphological processes—in particular, DGSD. In different geological and climatic contexts, this technique allows one to analyze extremely slow DGSDs and to identify displacements of about 1–2 mm in favorable conditions [95–103].

Based on previous studies on the fluvial deposits of Rio Quirra and Rio Pardu [5,6,79], the geological analysis was implemented by using high-resolution topographies based on UAV-DP and LiDAR. Detail-scale field surveys were carried out in the alluvial quaternary deposits with the aim of the identification and mapping of various terraced orders and the reconstruction of the relative chronology among morphostratigraphy and sedimentological indicators. Stratigraphic profiles relating to the various orders of river terraces and landslide deposits were surveyed in the natural outcrops of the alluvial plains.

#### *5.1. Aerial and Uncrewed Aerial Vehicle Remote Sensing*

LiDAR and aerial photogrammetric data produced by the Autonomous Region of Sardinia were used to perform visual and morphometric analysis of DGSDs and fluvial morphologies. A detailed orthophoto dating from 2016 was used together with LiDAR data with a cell size of 1 × 1 m and vertical resolution of 30 cm.

The aerial surveys were performed using UAVs (DJI Phantom 4 and DJI Matrix 200) flying at altitudes of 50–60 m above ground level. The acquired images were analyzed and processed using the photogrammetric Agisoft MetaShape software and constrained by 10–12 ground control points using GEODETIC LEICA GNSS for each area. The resulting orthorectified mosaic and DEM (WGS 84 datum and UTM 32N projection) had a cell size of 5 cm/pixel and were considered sufficiently precise to be used for the geomorphological analysis.

To analyze the DGSDs at the local scale, we used high-resolution digital elevation models (DEMs) acquired via structure from motion from a UAV-DF [8,103–106].

The 3D high-resolution UAV-DF models were used to develop interpretative superficial models by using geomorphological evidence and stratigraphic and structural data of the DGSDs. Geological interpretative cross-sections of geologic features crossing the major DGSDs were also generated to define the movement kinematics, deformative style, and deep geometries of the DGSDs.

The DTMs were used to analyze the morphometric parameters of the hydrographic basins under analysis. The longitudinal and transversal profiles of the valleys were extracted in such a way as to highlight different erosive structures in relation to river capture and to analyze the different altitudes of the various river terraces.
