**6. Results**

*6.1. InSAR, PS, and Time Series Analysis*

The results of the large-scale InSAR analysis showed that most PSs were located in stable areas, while high deformation rates were recorded in the slopes of Pardu Valley, where slope-failure processes—in particular, rockfalls and DGSDs—were widespread. All four focus areas were analyzed in detail with the Sentinel 1 data (from 2014 to 2020). For the left flank of Rio Pardu, the ERS data (from 1992 to 2000) were also used in descending order of acquisition. The data from the periods 1992–2000 and 2014–2020 indicated areas with large slopes that were identified as DGSDs that were active in Pardu Valley. We used

only PSs with high coherence (0.6–1) that were located in the rocky outcrops and in the urban structures, while low-coherence points located in rockfall deposits and in vegetated areas were not considered. The PS analysis allowed the recognition of active DGSDs and the measurement of their movement rates, which turned out to be extremely slow, ranging from 6 to 20 mm/year (Figures 5 and 6). We identified a downslope movement of up to 1 cm/y in the right slope of the Pardu Valley and a movement of up to 2 cm/y in the left slope. Continuous movements that did not change over years with both linear and seasonal trends were observed (Figure 6). The InSAR analysis showed no perceptible movements on the slopes of Rio Quirra.

In the Ulassai area, the PS analyses showed a stable surface in the urban area and on the west slope of the main extensional trench of Pranedda Canyon (Figures 5a and 6a). However, in accordance with the geomorphological evidence, downstream from the main trench, the speeds of the PSs showed LOS displacements of up to 1 mm/y. In this sector, the PSs were located in rocky dolomitic outcrops on the top edge of the plateau, in the total absence of vegetation and in excellent exposure conditions. No movements were detected in the DGSD downstream from Bruncu Pranedda due to the low PS coherence due to dense vegetation. Using Sentinel data from 2014 to 2020, we measured a total of 5 cm (orange star in Figure 5). It was possible to observe seasonal deformation trends with an excellent correlation among all of the PSs analyzed. Generally, no movement was observed during the winter and spring, but an acceleration was observed during the summer and autumn.

In Osini, a cluster of PSs were well defined within the inhabited center, particularly in the northwest and southeast sectors, where there was a speed of between 4 and 6 mm/y, with a maximum of 8 mm/y (Figures 5c and 6b) with a seasonal trend. Spotlights were located on the roofs of the buildings. In the surroundings of the inhabited center, the dense vegetation resulted in a low coherence of the PSs; therefore, they were not considered.

In the Gairo sector, the InSAR data showed a large area that was greater than 1 km<sup>2</sup> with a high diffusion of PSs. Based on the high-resolution field surveys, the PSs are located on rocky metamorphic outcrops. The speeds were, on average, greater than 8 mm/y, with a maximum of 2 cm/y. The cluster identified a well-defined area with a circular shape that was delimited by PSs with zero or negligible speed (Figures 5b and 6d1,d2). The higher speeds were located in the central and basal part of the DGSD, while towards the top and lateral flanks, the speeds decreased. In the lateral and top parts, the DGSD was delimited by stable PSs (speeds of 0–2 mm/y), which allowed the deformed area to be circumscribed in detail. in the PSs on the foot slope with a low coherence due to the continuous movement of slope deposits and the vegetation were not considered. The deformation's progression was continuous and linear, and an excellent correlation was found between the Sentinel 1 and ERS data. In the southern part of the DGSD, a high concentration of PSs were located in the abandoned village of Old Gairo with speeds that were sometimes greater than 1 cm/y. The town of New Gairo, which was built after the 1951 catastrophe, showed displacements limited to 2–4 mm/y.

In the San Giorgio sector, scattered PSs were identified with speeds greater than 10 mm/y on the large blocks of the rock avalanche on the slope (Figures 5d and 6c). These blocks, with dimensions of up to 30 m per side, were collateral landslides related to the collapsed DGSD located at the edge of the plateau above. All of the PSs showed a linear trend with a slowdown in the winter and spring between 2016 and 2017. This slowdown, which was observed in all of the PSs, indicates that the causes of the movement are to be found in processes that involve a greater portion of the slope, and not only in the large blocks. The surrounding area did not allow a PS analysis due to the importance of the wooded vegetation, but evidence of deformation was visible in the road infrastructure.

#### *6.2. Deep-Seated Gravitational Slope Deformation*

Various areas affected by DGSDs and landslides that were located of the slopes of Pardu Valley and on the slope of Monte Arbu of Tertenia were identified (Table 2).

**Figure 5.** Analysis of the focus areas with InSAR data. The points represent high-coherence permanent scatters located on buildings, rocky outcrops, and blocks of large rock avalanches. The stars represent the PSs used to analyze the time series shown in Figure 6. (**a**) Bruncu Pranedda lateral spread. (**b**) Gairo DGSD. (**c**) Osini landslide. (**d**) San Giorgio paleo-rock avalanche.

On the east side of Tacco di Ulassai and Tisiddu Mountain, three DGSDs were analyzed (Figure 7). The main structures that indicated deep gravitational phenomena were large and deep extensional trenches that were evident in the dolomitic lithotypes. The extensional trenches had lengths of several hundreds of meters and a decametric opening and depth. This slope was characterized by the Mesozoic marine deposits resting on the Paleozoic metamorphites.

The Bruncu Pranedda DGSD (Figure 7b2,c1) is constituted by two regions with different settings located on the top and middle slopes. On the top slope, toward the east of the largest extensional trenches in the area called the Pranedda Canyon, the rock mass fracturing increased, and the attitude of the Dorgali Formation was toward the east, with a dip of up to 40◦. In this area, both facies of the Dorgali Formation were visible, with the summit comprising dolomitic banks and the lower part being characterized by an alternation of well-stratified dolomites and marls. This subdivision was not observed in the middle slope, where basal marly levels did not appear on the surface. This indicates that the basal facies (approximately 30 m) were partially covered by slope deposits; however, they also sank a few meters inside the fractured and altered Paleozoic metamorphic basement. This could be correlated with the field observations at the same altitude, as well as with the basement and the massive facies of the Dorgali Formation [6].

**Figure 6.** Time series extracted with the representative permanent scatters. The vertical axes represent the cumulative LOS displacement; the horizontal axes represent the time. (**a**) Bruncu Pranedda lateral spread—seasonal displacement trend, maximum displacement of 5 cm from 2014 to 2020; (**b**) Osini landslide—seasonal displacement trend, maximum displacement of 6 cm from 2014 to 2020; (**c**) San Giorgio paleo-rock avalanche—constant movement trend of the large blocks, maximum displacement of 6 cm from 2014 to 2020; (**d**) Gairo DGSD; (**d1**) the ERS data show a constant deformation trend, with a maximum displacement o f23 cm from 1992 to 2000; (**d2**) the Sentinel 1 data show a constant deformation trend that is correlated with the ERS data, with a maximum displacement of 10 cm from 2014 to 2020. The colors of the points agree with the colors of the stars that identifies the location of the PS in Figure 5.



The Scala San Giorgio DGSD (Figure 7b1,c2,d1) is located north of Osini Village and is characterized by two major extensional trenches that are parallel to the slope affecting the Dorgali Formation with a dip amount of up to 20◦. All of the sequences of the Dorgali Formation are exposed; however, the Genna Selole Formation is covered by rockfall deposits.

The Tisiddu Mountain DGSD (Figure 7b3,d2) to the south of Ulassai Village is characterized by a highly fractured segmen<sup>t</sup> of the Dorgali Formation located tens of meters downstream. Only the tops of the massive banks of dolostones are visible. The basal level partially sank into the metamorphic basement.

In all cases, the shear zones are located in different geological units that represent structural weaknesses (Figure 7d1,d2). (I) The top of the metamorphites was affected by sub-horizontal foliation and advanced weathering, which was highlighted by the reddish or whitish color of the rocks. This type of alteration could be linked to the pre-transgressive Mesozoic period [65]. (II) The Genna Selole Formation was characterized by plastic clay layers; (III) basal levels of the Dorgali Formation were characterized by the alternation of marl and dolomite.

A large landslide that affected the town of Osini and the northernmost slope downstream of the San Giorgio DGSD was identified by using InSAR data. The inhabited center of Osini is built over an extensive cemented paleo-rockfall deposit that rests on the Paleozoic basement. Geomorphological evidence is difficult to observe due to the extensive vegetation around the village.

**Figure 7.** DGSD on the right slope of the Pardu River. (**a**) Orthophoto of the area of Ulassai, Osini, and Jerzu. The Jurassic dolostone plateaus on the metamorphic basement are shown in blue. The yellow square represents the analyzed DGSD. (**b**) UAV images of the DGSD showing the major geomorphological and structural features. The white dashed lines represent the major extensional trenches. (**b1**) San Giorgio lateral spread, (**b2**) Bruncu Pranedda lateral spread, (**b3**) Monte Tisiddu sackung. (**c**) Three-dimensional LiDAR model of the DGSDs with a colored elevation scale. The black dashed lines represent the major extensional trenches. The white dashed lines represent the major stratigraphic discontinuity between the marine Mesozoic sequence and the metamorphic basement. (**c1**) Bruncu Pranedda lateral spread. (**c2**) San Giorgio lateral spread. (**d**) Interpretative geological cross-sections passing through the DGSD in the study area. The hypothetical basal shear zone is highlighted with black dotted lines. (**d1**) San Giorgio lateral spread. (**d2**) Monte Tisiddu sackung.

The left side of the Rio Pardu is characterized by a different geological and structural context compared to the opposite side (Figure 8a). There are metamorphic lithologies belonging to the formation of Monte Santa Vittoria and the Filladi del Gennargentu. The slope is affected by a dip-slope Hercynian thrust that brings the two formations belonging to two different tectonic units into contact. This structure plays a fundamental role in the deep gravitational processes, as it is marked by intense fracturing and alteration of the lithotypes. Based on the geomorphological evidence and the analysis of the InSAR data, a large landslide with a DGSD character was identified in the northwest sector with respect to the town of Gairo (Figure 8b–f). The DGSD extends from the top slope to the middle-lower part of the slope and is about 1 km wide. The crown is circular (Figure 8c,d) and joins laterally rectilinear structural flanks (Figure 8e). Analyzing the profile of the slope along the DGSD, the concave upstream part and the convex downstream part are clearly evident. The foot of the landslide is covered by landslide and slope deposits that reach the valley floor, where lateral erosion by the Rio Pardu is affected (Figure 8).

On the right side of the Rio Quirra, in correspondence with the Tacco di Tertenia, complex gravitational morphologies linked to paleo-DGSDs are evident (Figure 9). The morphology of Mount Arbu is also affected by the complex tectonic structure, which is characterized by a sub-horizontal thrust that brings the Pyllades del Gennargentu Formation into contact with the Metavolcanites of the Monte Santa Vittoria Formation (Figure 9a). The morphological analysis of the slope shows convexity and concavity linked to different DGSDs that are distributed at various altitudes of the slope. The DGSDs consist of portions of the Dorgali Formation, which is tilted up to 30–40◦ and is translated along the slope. The most complex and evolved movement was identified in the NE sector (Figure 9b,c1,c2). The area extends for a length of about 1800 m from the top of the plateau to the valley floor. The fan-shaped landslide body has a foot with a length of 2 km. The crown is located in the plateau edge, which is affected by faults and distension trenches. The latter delimit mega-blocks of the Dorgali Formation With a prismatic shape and inclination of up to 40◦. The foot of the DGSD, which is represented by the Dorgali Formation, is marked by dolomitic outcrops with vertical heights of up to 40 m with sometimes sub-horizontal attitudes of the strata. On these walls, terraced alluvial deposits rest in onlap. Paleo-DGSDs are widespread in the upper part of the slope, with greater diffusion in the southern part of Mount Arbu, but they do not evolve until reaching the valley floor (Figure 9b,d).

#### *6.3. River Capture Analysis*

The area has a deep cut made by the Rio Pardu Valley and Rio Quirra Valley, which extend in an NNW–SSE direction, following a major Tertiary fault. For most of the Pardu River's course, the talweg is set on rock, indicating its predominantly erosive nature. Downstream, the river is captured, turning in an eastward direction, and its name changes to Rio Pelau; then, it flows into the Tyrrhenian Sea. South of the capture, the abandoned Rio Pardu Valley continues southward as Rio Quirra. This valley is characterized by a bottom filled with Pleistocene and Holocene terraced alluvial deposits and slope deposits, which are currently undergoing erosion. It is clear that in the past, Rio Pardu was captured by Rio Pelau (Figure 10), causing a rapid incision upstream. Longitudinal profiles were constructed for Rio Pardu, Rio Quirra, and Rio Pelau. Rio Pardu flows up to 750 m below the dolostone near Ulassai, where the main active DGSDs are located. The evolutionary hypotheses are related to the Pliocene and Quaternary uplift, which led to an important erosive phase.

The triggering process can be justified in the following ways:


**Figure 8.** (**a**) Geological map of the Gairo slope with the DGSD localization. (**b**) Orthophoto with the main geomorphological feature of the DGSD. (**c**) Photo of the DGSD head. (1) Crown; (2) right slope of the Pardu River; (**d**) photo showing a 3D view with the DGSD border marked in red; (**e**) linear flank of the DGSD; (**f**) interpretative geological cross-section of the DGSD showing it (in transparent orange) sliding on the highly fractured rock due the underlying dip-slope Paleozoic thrust. Geolithological legend: MSV—Monte Santa Vittoria Formation; GEN—Filladi del Gennargentu Formation; ald—current alluvial deposits; sld—slope deposits.

**Figure 9.** (**a**) Geological map of the eastern slope of Monte Arbu (Tertenia). Geolithological legend: MSV—Monte Santa Vittoria Formation; GEN—Filladi Del Gennargentu Formation; GNS—Genna Selole Formation; DOR—Dorgali Formation; al—terraced and current alluvial deposits; sl—slope deposits. [57]. (**b**) LiDAR hillshade with the main geomorphological feature of the DGSD. (**c1**) Photographic 3D view with the DGSD border marked in red and the terraced alluvial deposit in blue. (**c2**) Three-dimensional LiDAR of the Tertenia paleo-DGSD with the border marked in red and the terraced alluvial deposit in blue. (**d**) Interpretative geological cross-section of the DGSD showing it sliding on highly fractured rock due the underlying Paleozoic thrust.

**Figure 10.** Three-dimensional LiDAR model of the river capture sector. In the north, the Pardu River flows eastwards, taking the name of Rio Pelau. The blue and light blue show the Holocene alluvial deposit of the Pardu River. South of Genna and Crexia is the head of the Rio Quirra. In red is shown the paleo-slope and paleo-alluvial deposits of the Quirra River.

#### *6.4. Fluvial Morphostratigraphic Analysis*

A morphostratigraphic analysis was performed first on Rio Quirra and later on Rio Pardu, which isolated it following the capture (Table 3).


#### **Table 3.** Morphostratigraphic synthesis.

In the valley of the Rio Quirra, above the current riverbed, the following were identified (Figure 11):

T0—Actual flood surface consisting of pebbles and clastosustained gravels with a scarce sandy matrix (Holocene).

T1—Sub-current Holocene terrace with a maximum height on the riverbed of about 20–30 cm up to 1.5–2 m. The dark brown matrix is subordinate to the coarse fraction, which is represented by heterometric and polygenic pebbles. This terrace often forms alluvial islands in the upstream part of the river; they reach a good stability due to the dense vegetation that has settled there (Upper Pleistocene–Holocene).

T2—In this terrace, the matrix, which is decidedly prevalent in the coarse fraction, has a dark brown color. There is no evidence of prolonged chemical alterations due

to climatic conditions other than the current ones. The pebbles are less varied: mainly quartz with, subordinately, granite and schistose. On average, the height of T1 with respect to the riverbed is about 2 m, with a maximum of 5–6 m and a minimum of 50 cm. The deposits that form this terrace show forms of erosion linked to secondary climatic pulsations (Upper Pleistocene).

**Figure 11.** (**a**) Morphostratigraphic profiles of the Quirra River. (**b**) UAV photo in the river alluvial plain. (**c**) Outcrop of Terrace T3. Lithological legend: (1) Filladi Grigie del Gennargentu Formation; (2) Monte Santa Vittoria Formation; (3) paleo-DGSD; (C1) paleo-conoid; (4) T3; (5) T2; (6) T1; (7) T0.

T3—This is the oldest terrace, with an average height of 6–7 m and a maximum of 10 m (Figure 10c). The matrix–skeleton relationship is not constant. The depository is made up of alternations of fine and large sediments that testify to the variations in the river's energy. The matrix is red and sometimes whitish. In the first case, the color derives from Fe oxides, indicating a warm, humid climate typical of tropical and sub-tropical regions; in the second case, the oxides have been leached and for an eluviation horizon. The pebbly fraction does not have a varied lithological composition. It is mainly schistose

and, subordinately, quartz. The deposit is well cemented. This terrace rests directly on the slope. The frame of erosion along the riverbed is clear, and the lower terraces rest on it (Middle Pleistocene).

In Rio Pardu, the alluvial deposits cover a valley floor characterized by a well-defined flood bed, which is limited by banks that are intensely affected by landslides. Two orders of alluvial terraces up to 2 m above the current level were detected (Figure 12). The maturity of the flood clasts is very low due to the continuous supply of material from the slopes, while the grain size distribution along the longitudinal profile reflects the trend characterized by the high slope. By analyzing the longitudinal profile of the Rio Pardu, it can be observed that it is divided into two well-defined parts separated by the knickpoint in Ponti Mannu. In the initial part, near the steeply sloping trunk, there is the head of the valley, which continues until an area with a low slope where alluvial deposits appear. Downstream of the Ponte Mannu, after a section of the river in which the waters flow on the rock, the river becomes slightly sloped and establishes an alluvial plain with anastomotic channels and river islands.

Active and quiescent dejection cones are distributed over the Quirra and Pardu Valleys. The active conoids are well highlighted by the morphology, and they have a poorly elaborated clastic component and an uncemented dark brown matrix. In the terminal part of the Quirra, a terraced dejection cone (C1) assumes a certain importance due to its size and evolutionary stage (Inner–Middle Pleistocene). The often large pebbles are very elaborate and have blackish patinas of Mn oxides on their surfaces. Oxides also accumulate inside the matrix, which presents an intense redness. In the upper part of the Rio Pardu, a paleo-conoid (C2) with large pebbles and a brown matrix is currently engraved by the current course of the river (Upper Pleistocene and Holocene).

The paleo-slope deposits are characterized by coarse, elaborate, and sharp-edged components. The matrix is very abundant, strongly cemented, and bright red in color due to the accumulation of pockets of Mn oxides. These deposits are located at the same altitude as that of T3 or sometimes at higher altitudes, and they are connected to the base of the slope (Inner-Middle Pleistocene).

**Figure 12.** (**a**) Morphostratigraphic profiles of the Pardu River. (**b**) UAV photo near the head of the Pardu River. (**c**) The bottom of the Pardu Valley near the capture elbow. Lithological legend: (1) Filladi Grigie del Gennargentu Formation; (2) Monte Santa Vittoria Formation; (3) paleo-rockfall deposits; (C2) paleo-conoid; (4) Terrace T1; (5) Terrace T0.
