# Kinematic segmentation of a DSGSD on the flank of the exhumed Leo Pargil Dome (northwestern Himalaya) using InSAR

Gökhan Aslan, Marcello de Michele, Tim Redfield, Reginald Hermanns, John Dehls

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Gökhan Aslan, Marcello de Michele, Tim Redfield, Reginald Hermanns, John Dehls. Kinematic segmentation of a DSGSD on the flank of the exhumed Leo Pargil Dome (northwestern Himalaya) using InSAR. Geomorphology, 2026, 501, pp.110248. 10.1016/j geomorph.2026.110248. hal-05535240

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# Kinematic segmentation of a DSGSD on the flank of the exhumed Leo Pargil Dome (northwestern Himalaya) using InSAR

Gökhan Aslan<sup>a,∗</sup>, Marcello de Michele<sup>b</sup>, Tim Redfield<sup>a</sup>, Reginald Hermanns<sup>a,c</sup>, John Dehls<sup>a</sup>

## ABSTRACT

Deep-seated landslides in high mountain terrain typically deform slowly over years to decades, making their internal kinematics difficult to characterise from field observations alone. Here we investigate the Leo Pargil landslide on the southwestern flank of the exhumed Leo Pargil gneiss dome in the northwestern Indian Himalaya, a large active deep-seated gravitational slope deformation (DSGSD) in the region. Multi-temporal persistent scatterer interferometric synthetic aperture radar (PS-InSAR) analysis of Sentinel-1 data is combined with geological and structural information to characterise the landslide's present-day kinematics and internal segmentation. Ascending and descending Sentinel-1 data from 2018 to 2023 are combined to decompose InSAR-derived displacement into vertical and horizontal components, allowing spatial variations in displacement magnitude and direction to be resolved across the landslide body. The displacement field is strongly segmented, with faster-moving sections separated by sharp displacement gradients and relatively stable zones, indicating the presence of distinct internal sectors within the landslide. The upper part of the slope shows displacement patterns consistent with a rotational component near the headscarp, whereas the lower part is characterised by predominantly slope-parallel motion. Spatial variations in motion broadly follow mapped structural and lithological boundaries, indicating that inherited structures and slope geometry may influence the present-day deformation pattern. The results show that the Leo Pargil landslide is a segmented deep-seated gravitational slope deformation on the flank of an exhumed gneiss dome, with its present-day kinematics consistent with inherited structural heterogeneity and ongoing geomorphic processes.

## 1. Introduction

Deep-seated gravitational slope deformations (DSGSDs), also referred to as sackungen, are large, deep-seated slope instabilities that typically deform slowly over timescales of years to decades. However, under certain conditions, deformation may localize, leading to faster movement or partial failure of the slope. In high-relief mountain belts, DSGSDs contribute to landscape evolution by progressively modifying hillslope geometry over long timescales (Densmore et al., 1997; Densmore and Hovius, 2000; Korup, 2006). By mobilizing large volumes of material, they play an important role in denudation and long-term topographic adjustment in response to tectonic uplift and climate-driven surface processes (Hovius et al., 2000; Montgomery and Brandon, 2002; Korup et al., 2007; Weidinger and Korup, 2009). In rapidly uplifting mountain belts, giant DSGSDs may therefore contribute to the coupling between tectonic uplift and surface erosion (Agliardi et al., 2013). In addition to their geomorphic importance, DSGSDs can also represent a natural hazard for communities in tectonically active, high-relief mountain regions (Hermanns et al., 2001). Understanding the spatial variability and kinematic behaviour of DSGSDs remains

challenging due to the combined effects of geological structure, slope morphology, and external forcing such as climate variability and earthquakes (Korup et al., 2007; Schulz et al., 2009; Roering et al., 2015; Kargel et al., 2016; Aslan et al., 2021).

DSGSDs in such settings are often bounded on their flanks and segmented internally by structures that are morphologically similar to tectonic features (Fleming and Johnson, 1989; Jaboyedoff et al., 2011, 2013; Schleier et al., 2016; Guerriero et al., 2021). This morphological similarity has led to frequent misinterpretations, where DSGSD-related features have been attributed to tectonic faulting or folding (Hippolyte et al., 2006; Hart et al., 2012; Krieger et al., 2013). Conversely, tectonic faults and scarps have also occasionally been misinterpreted as gravitational slope deformations, especially where active tectonics, slope processes, and post-glacial landscape adjustment overlap (Jaboyedoff et al., 2011; Krieger et al., 2013). At the same time, many of these morphological characteristics are also diagnostic of DSGSDs, which develop through progressive deformation of slopes and, in some cases, entire ridges driven by deep-seated creep (Agliardi et al., 2001). This ambiguity makes it difficult to infer controlling mechanisms from morphology alone and highlights the need for spatially resolved

<footnote><sup>a</sup> Geological Survey of Norway (NGU), Trondheim, Norway</footnote>

<footnote><sup>b</sup> French Geological Survey (BRGM), Orléans, France</footnote>

<footnote><sup>c</sup> Norwegian University of Science and Technology (NTNU), Trondheim, Norway</footnote>

<footnote>* Corresponding author.
E-mail address: gokhan.aslan@ngu.no (G. Aslan).</footnote>
kinematic observations that can identify coherent moving sectors and their boundaries within large deep-seated slope systems.

In this context, the Himalaya represents a region where DSGSDs occur under conditions of active tectonics, high relief, and strong climatic variability. The region is characterised by frequent earthquakes, rapid mountain building, and intense erosional processes, which together are conducive to the development of large-scale slope deformation (Dortch et al., 2009; Fig. 1a). Many of the largest terrestrial landslides (>10⁸ m³) have occurred in specific sectors of major active mountain belts where relief and erosion rates are highest (Ponomareva et al., 2006; Korup et al., 2007; Hewitt, 2009). Within the Himalaya, the northwestern sector contains numerous giant DSGSDs, highlighting the combined influence of geological structure, tectonic deformation, and surface processes on deep-seated slope instability (Hewitt, 1988, 1998, 2009; Owen et al., 2008; Schroder, 1998; Korup et al., 2007; Dortch et al., 2009).

Building on this regional context, we focus on the Leo Pargil landslide, a large active DSGSD on the southwestern flank of the exhumed Leo Pargil gneiss dome in the northwestern Himalaya (Fig. 1b). Here we examine the landslide as a deep-seated slope deformation in a tectonically active, high-relief setting, with the aim of characterising its internal kinematic segmentation and assessing how the observed deformation patterns relate to the local structural and geomorphic setting. Multi-temporal Sentinel-1 InSAR time series are used to estimate surface displacement rates and identify internally coherent moving sectors across the landslide. Using both ascending and descending acquisitions, the InSAR measurements are further decomposed into vertical and horizontal (east-west) displacement components to resolve spatial variations in motion and to distinguish rotational from translational behaviour within the segmented slope. Geological and geomorphological mapping, together with published low-temperature thermochronology for the Leo Pargil gneiss dome, are then used to place the observed deformation patterns in a broader geological context and to examine how exhumation history, inherited structural weaknesses, and surface processes may have conditioned the present-day deformation.

## 2. Study area and setting

### 2.1. Study area

The Leo Pargil landslide is located in the Kinnaur District of Himachal Pradesh in northwestern India, on the southwestern flank of the exhumed Leo Pargil gneiss dome near the India-China border (Fig. 1b). It covers an area of approximately 55 km² and extends for more than 15 km downslope. The landslide toe lies along the Spiti River (a tributary of the Sutlej River) at ~2800 m a.s.l., and the main backscarp reaches ~5800 m a.s.l., giving a local relief of ~3000 m across the slope system, corresponding to an approximate mean slope angle of ~11° based on the overall elevation difference and landslide length. The morphology of the slope is characteristic of a deep-seated gravitational slope deformation, with translated rock blocks forming back-tilted surfaces and a prominent steepened toe that reflects ongoing fluvial incision by the Spiti River (Fig. 2). The landslide surface is structured by multiple scarps and internally displaced blocks that create step-like topography across the slope (Fig. 2). In addition to these large-scale morphostructural elements, the slope contains talus accumulations, active rock glaciers, and glacial meltwater channels, reflecting a landscape shaped by combined fluvial and glacial processes. Denudation dominates the present-day geomorphic regime, while local depositional activity occurs where retreating glaciers release sediment onto the slope system (Gupta and Sah, 2008).

### 2.2. Geologic and structural setting

The study area lies within the northwestern Himalaya, where convergence between the Indian and Eurasian plates has driven long-term crustal shortening and thickening since the Eocene (Searle et al., 1987). During the Eocene to early Miocene, north-south-oriented shortening led to the development of high topography in the region (Searle et al., 1987; Ratschbacher et al., 1994). Since the middle Miocene, deformation has been dominated by orogen-parallel translation and extension, accommodated by strike-slip and normal fault

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Tectonic setting and location of the Leo Pargil landslide in the upper Sutlej basin. (a) Major mapped tectonic structures and faults superimposed on shaded topography derived from ALOS data (12.5 m resolution), with major active faults highlighted in black (Zelenin et al., 2022). Blue lines show the Sutlej River and tributaries; the red outline marks the study area. (b) Topographic overview of the Leo Pargil landslide. The red dashed line indicates the tentative landslide boundary (50 m contour interval shown). Arrows show satellite flight and line-of-sight (LOS) directions for the Sentinel-1 geometries used in this study (ascending Track 129 and descending Track 63). Glacier outlines are from the Global Land Ice Measurements from Space database (GLIMS, 2018). For reference and subsequent analysis, the landslide is divided into a northern and a southern sector, as used throughout this paper.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Geomorphic anatomy of the Leo Pargil landslide. Shaded topography with slope gradients emphasised in red to highlight steep areas. Lobate talus-derived active rock glaciers are shown in orange. Landslide scarps and translated rock blocks are mapped, together with five profile lines (a–a' to e–e') used for kinematic interpretation. Profile a–a' is a longitudinal profile spanning the southern sector, while profiles b–b' to e–e' are transverse profiles aligned with the orientation of translated blocks and headscarps within the southern sector.</figcaption></figure>

systems linked to metamorphic dome exhumation (Vannay et al., 2004; Hintersberger et al., 2011).

The Leo Pargil Dome (LPD) is one such metamorphic dome located
near the border between northwest India and western Tibet (Fig. 1a). It
forms a prominent topographic high, with Leo Pargil peak reaching
~6790 m a.s.l. (Thiede et al., 2006). Its southwest margin is defined by
the Leo Pargil Shear Zone (LPSZ), a ductile extensional shear zone that
progressively evolved toward brittle normal faulting during exhumation
and is regionally linked to the broader fault architecture of the north-
west Himalaya (Fig. 3a) (Ni and Barazangi, 1985; Thiede et al., 2006;
Langille et al., 2012). On the southeastern side of the dome, the Qusum
detachment defines a major boundary structure that records a similar
transition from ductile to brittle deformation (Zhang et al., 2000).

Near Nako and Leo villages (Fig. 1b), the LPSZ includes a several-
hundred-metre-thick deformed sequence within the Haimanta Group,
comprising sheared quartzite, marble, and leucogranite; kinematic in-
dicators consistently record a top-down-to-the-west shear sense
(Langille et al., 2012). The western margin of the dome is further
bounded by the Kaurik–Chango normal fault zone, which trends NNE
and dips steeply westward and is expressed by high-angle brittle normal
faults that cut both hanging wall and footwall units (Fig. 3a, b) (Thiede
et al., 2006). Seismological observations, including analyses of the 1975
Kinnaur earthquake and its aftershock sequence, indicate ongoing ac-
tivity of this extensional fault system in the wider region (Molnar and
Chen, 1983; Thiede et al., 2006), although the direct relationship be-
tween this fault activity and the present-day landslide kinematics cannot
be resolved from the available data.

**3. Data and methods**

**3.1. Sentinel-1 InSAR data**

This study uses C-band Sentinel-1A/B synthetic aperture radar (SAR)
data acquired in interferometric wide swath TOPS mode. The analysis is
based on two acquisition geometries covering the Leo Pargil landslide:
an ascending orbit (Track 129) and a descending orbit (Track 63). In
total, 204 Sentinel-1 scenes were processed, comprising 100 ascending
acquisitions (Track 129) and 104 descending acquisitions (Track 63),
acquired between January 2018 and June 2023. All Sentinel-1 data were
obtained from the Copernicus Open Access Hub.

The study area lies in a high-elevation, semi-arid sector of the
Himalayan–Tibetan orogen, where sparse vegetation and widespread
exposed rock surfaces promote high interferometric phase stability,
enabling persistent scatterer interferometry in steep alpine terrain.

**3.2. PS-InSAR processing**

We processed Sentinel-1 data using the GSAR-GTSI processing sys-
tem developed by NORCE for the Norwegian National Ground Motion
Service (InSAR.no) (Dehls et al. 2019). The workflow follows the
principles of persistent scatterer interferometry (PSI) (Ferretti et al.,
2001) and is specifically adapted for large-area Sentinel-1 TOPS acqui-
sitions. Coregistration was performed using enhanced spectral diversity
to ensure accurate burst-to-burst alignment, which is critical for TOPS
mode processing (Prats-Iraola et al., 2012). Interferograms were
generated using a single-reference strategy, where all acquisitions were
connected to a primary reference scene selected to minimize both spatial
and temporal baselines.

Topographic phase was removed using the Shuttle Radar
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Geological and structural framework of the western flank of the Leo Pargil Dome. (a) Simplified geological map of the study area showing the main lithological units and major structures, including the Kaurik-Chango normal fault and the Leo Pargil Shear Zone (LPSZ). (b) Structural cross-section A–A' at the latitude of Nako village (location in Fig. 1b), illustrating the geometric relationship between the Kaurik-Chango normal fault, the LPSZ, and the principal lithological units underlying the Leo Pargil landslide. Lithological boundaries and structural interpretations are modified after Thiede et al. (2006) and Langille et al. (2012).</figcaption></figure>

Topography Mission (SRTM) 3-arcsecond digital elevation model. Atmospheric phase delays were mitigated by estimating and correcting an atmospheric phase screen (APS), thereby reducing spatially correlated tropospheric noise across the interferometric time series. Persistent scatterer candidates were identified based on temporal phase stability after removal of modeled deformation and atmospheric contributions. Pixels affected by water bodies, geometric distortions (layover and

shadow), or low mean amplitude were excluded prior to time-series analysis. The final set of persistent scatterers was selected using a model-fit criterion applied to the displacement time series, allowing for both long-term trends and seasonal variability, and the resulting LOS velocity fields were referenced to the median velocity of the retained persistent scatterers within each acquisition geometry.

For quality control, each displacement time series was fitted with a
third-order polynomial plus a seasonal term, which allows non-linear
deformation behaviour to be represented while limiting overfitting.
Persistent scatterers were retained only if the root-mean-square error of
the fit was ≤5.0 mm. This threshold provides a consistent criterion for
rejecting noisy measurements and ensures that retained time series
represent coherent deformation signals.

3.3. *LOS velocity decomposition*

The InSAR technique captures ground displacement only as a pro-
jection along the satellite-to-ground line-of-sight (LOS) vector, which
limits direct interpretation of true three-dimensional motion. This lim-
itation can be reduced by combining observations from multiple viewing
geometries over the same area. Combining LOS velocities from both
ascending and descending Sentinel-1 acquisitions allows the vertical and
horizontal components of surface deformation to be estimated, provided
that appropriate kinematic assumptions are applied. In this study, we
assume negligible north–south (N–S) motion, which is a reasonable
approximation for the Leo Pargil landslide given its predominantly west-
facing slope orientation and the absence of evidence for significant N–S
displacement. Under this assumption, LOS velocities were decomposed
into vertical and east–west displacement components.

Our initial step involved resampling the mean line-of-sight (LOS)
velocities from both ascending and descending tracks. This resampling
was conducted to ensure alignment onto a consistent grid, featuring 50
m pixel spacing and covering the entire spatial extent of our tracks. In
our approach, we apply the nearest neighbor resampling technique,
which selectively incorporates persistent scatterer pixels that have a
nearest neighbor within a 50 m radius from the center of each grid nodal
point. In the second step, we retained only grid nodes where both the
ascending and descending resampled velocity fields were available,
ensuring that the decomposition is based on paired observations. Before
decomposition, we referenced both tracks' InSAR mean velocity fields by
aligning them to a common reference frame. This transformation was
achieved by utilizing the average velocity within the selected area (point
P in Fig. 4), considered a stable area.

4. Results

4.1. Spatial displacement pattern

Mean line-of-sight (LOS) velocity fields derived from Sentinel-1A/B
data for 2018–2023 reveal a coherent deformation pattern across the
Leo Pargil landslide (Fig. 4). The ascending Track 129 results are shown
in Fig. 4a (map view) and Fig. 4c (3D perspective), while the descending
Track 63 results are shown in Fig. 4b (map view) and Fig. 4d (3D
perspective). In both geometries, the landslide body is characterised by a
spatially continuous area of measurable LOS velocities that contrasts
with the surrounding slopes, where velocities are generally close to zero.

The spatial extent of the deforming area is similar between the two
viewing geometries, but the LOS velocity sign differs across large parts
of the landslide, reflecting the different acquisition directions. In the
ascending geometry (Track 129), the main deforming area is charac-
terised by positive LOS velocities (cold colours; motion toward the sat-
ellite), whereas in the descending geometry (Track 63) the same area
shows predominantly negative LOS velocities (warm colours; motion
away from the satellite) (Fig. 4a–b). Within the deforming zone, spatial
variability is present, with patches of higher-magnitude LOS velocities
occurring within broader areas of lower-magnitude deformation. The 3D
views (Fig. 4c–d), generated from viewpoint P (Fig. 4a–b), show that
higher LOS velocity magnitudes are concentrated on the mapped land-
slide body, while adjacent valley floors and surrounding slopes exhibit
comparatively low velocities.

Persistent scatterer coverage varies across the study area. In partic-
ular, PS density decreases in glacier- and snow-covered areas at higher
elevations and is locally reduced in very steep terrain, where geometric
distortions and decorrelation limit phase stability (Fig. 4). These varia-
tions in PS density are reflected in local gaps within the velocity field but
do not affect the overall spatial coherence of the deformation pattern.

4.2. Decomposed displacement components

The mean LOS velocity fields were decomposed into east–west hor-
izontal and vertical displacement-rate components using paired
ascending and descending observations (Figs. 5 and 6). To highlight
subtle gradients within the landslide body, Fig. 5 uses a narrowed colour
scale (–5 to +5 mm yr–1), such that values outside this range are

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Mean line-of-sight (LOS) velocity of the Leo Pargil landslide derived from Sentinel-1A/B PS-InSAR time series (2018–2023). (a) Ascending Track 129 and (b) descending Track 63 LOS velocity maps. (c–d) Oblique 3D views of the same fields draped over ALOS 12.5 m topography and optical imagery, viewed from point P indicated in (a) and (b). Positive LOS velocities (cold colours) indicate motion toward the satellite, and negative velocities (warm colours) indicate motion away from the satellite.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Decomposition of LOS velocities into 2D displacement rates using a narrow colour scale. (a) East–west horizontal and (b) vertical velocity fields inverted from paired Sentinel-1 ascending and descending LOS velocity fields (2018–2023).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Decomposition of LOS velocities into 2D displacement rates using an extended colour scale. (a) East–west horizontal and (b) vertical velocity fields inverted from paired Sentinel-1 ascending and descending LOS velocity fields (2018–2023).</figcaption></figure>

saturated. In contrast, Fig. 6 uses an expanded scale (−50 to +50 mm yr⁻¹) to show the full range of observed velocities without saturation.

The decomposed fields show that deformation varies spatially across the landslide. In the upper northern sector, displacement is low, with horizontal velocities of about 5 mm yr⁻¹ and vertical motion close to 2 mm yr⁻¹ (Fig. 5). In contrast, the steep toe zone at the base of the northern sector shows higher displacement rates, with vertical velocities reaching up to 50 mm yr⁻¹ and horizontal velocities up to 110 mm yr⁻¹ (Fig. 5).

In the southern sector, the horizontal velocity field indicates predominantly westward horizontal motion (Fig. 6a). Horizontal velocities increase from about 10 mm yr⁻¹ near the headscarp to about 100 mm yr⁻¹ at the toe (Fig. 6a). The vertical velocity field shows a similar gradient, with subsidence rates increasing from about 10 mm yr⁻¹ on the upper slope to about 30 mm yr⁻¹ toward the toe (Fig. 6b).

A glacier-fed stream crosses the southern sector, and a zone of comparatively high displacement rates spatially coincides with this channelized area (Fig. 6). In addition, active rock glaciers located above the southern sector display higher horizontal and vertical motion than the surrounding landslide body and are clearly identifiable in the decomposed fields; their displacement is characterised by a stronger horizontal component relative to the vertical component (Fig. 6).

### 4.3. Displacement profiles (southern sector)

To examine along-slope and across-slope variability in the decomposed displacement field, we extracted cumulative displacement time series and mean displacement rates along one longitudinal profile (a–a') and four transverse profiles (b–b'-e-e') across the southern sector (profile locations in Fig. 2; results in Figs. 7–8).

Along the longitudinal profile (a–a'), the cumulative displacement time series extracted from the ascending and descending datasets show systematic spatial gradients in deformation between 2018 and 2023 (Fig. 7a). The descending track shows a larger LOS displacement magnitude and more pronounced spatial gradients than the ascending track, which is consistent with the acquisition geometry relative to the predominantly west-facing slope. When the decomposed components are sampled at 100 m spacing along the profile, the displacement field is dominated by the horizontal (east–west) component, with westward motion increasing steadily downslope (Fig. 7b).

Several distinct step-like changes in the mean displacement rates occur along the upper and mid-slope portions of the profile, marking sharp kinematic discontinuities that generally coincide with the main breaks in slope and mapped internal scarps visible in the geomorphic inventory (Fig. 2) and along the profile trace (Fig. 7b). In contrast, the
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Displacement patterns along the longitudinal profile aa' across the southern sector (profile location in Fig. 2). (a) Cumulative displacement time series extracted along the profile from Sentinel-1 ascending Track 129 and descending Track 63 (2018–2023). (b) Mean decomposed horizontal (east–west) and vertical displacement rates sampled at 100 m intervals along aa'. (c) Decomposed displacement vectors shown in a common reference frame to illustrate spatial changes in direction and magnitude along the profile. (d) The same vectors plotted along the topographic profile. Elevation is from the ALOS PALSAR 12.5 m DEM. (e) Geological cross-section along aa', adapted from Langille et al. (2012), showing the main lithological units and major structures intersecting the profile.</figcaption></figure>

lower sector is characterised by a more translation-dominated pattern with fewer clearly defined discontinuities. The lowest rates occur in the uppermost part of the profile, while the highest rates are concentrated toward the lower slope, where motion locally reaches the largest observed values along the profile near the toe (Fig. 7b). The vector representation shows that the upper slope is characterised by relatively stronger vertical contributions, whereas the lower slope shows more slope-parallel, translation-dominated motion expressed mainly by the horizontal component (Fig. 7c–d).

Overall, the longitudinal profile indicates a segmented deformation pattern, with discrete kinematic domains separated by narrow transition zones where displacement rates change over short distances (Fig. 7).

Across the transverse profiles (b–b'-e–e'), the cumulative displacement time series show a distinct contrast between stable ground outside the mapped deformation zone and ongoing motion within the active

landslide body (Fig. 8). In Fig. 8, the east–west component is plotted horizontally for clarity, with westward displacement directed toward the viewer. In all profiles, displacement increases steadily through time within the central part of the landslide, whereas the flanks display near-zero or much lower displacement.

The transitions at the lateral margins are sharp and expressed as pronounced gradients in both horizontal and vertical components and mark the lateral limits of the actively deforming zone along each profile. Within the active zone, displacement magnitudes are generally highest near the centre of the profiles and decrease toward the margins, producing a systematic cross-slope velocity gradient. Although the horizontal (east–west) component dominates the total signal, the vertical component varies locally along the profiles and contributes to the overall kinematic pattern in the most active sections.

The shaded regions in Fig. 8 highlight the portions of each profile
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Cumulative displacement time series along transverse profiles in the southern sector (profile locations in Fig. 2). Panels (a–d) correspond to profiles bb', cc', dd', and ee', respectively. The colored time-series points show cumulative displacement evolution from 2018 to 2023. Decomposed displacement vectors are overlain on the elevation profiles; the vertical axis represents the vertical component, while the horizontal axis represents the east–west component plotted horizontally for visualization (westward directed toward the viewer, as indicated). Shaded regions highlight the portions of each profile corresponding to the active deformation zone.</figcaption></figure>

where deformation is concentrated, showing that the width and internal distribution of the active zone vary between profiles from the lower to the upper parts of the southern sector. Together, the transverse profiles demonstrate that deformation is laterally heterogeneous, with distinct gradients and clearly identifiable lateral limits across the southern sector.

# 5. Discussion

## 5.1. Kinematic characteristics of the southern sector

The decomposed InSAR velocity fields and the displacement profiles (Figs. 5–8) show that the Leo Pargil landslide does not deform uniformly but instead exhibits spatially distinct zones of motion separated by pronounced changes in displacement rate. These observations are used to describe the deformation characteristics of the southern sector and to compare the observed kinematic patterns with mapped surface features and major structural elements on the western flank of the Leo Pargil Dome.

In the southern sector, deformation is primarily characterised by westward horizontal motion, visible as a laterally continuous zone of displacement in the 2D decomposition maps (Figs. 5–6) and along the displacement profiles (Figs. 7–8). Along the longitudinal profile a–a', horizontal displacement rates increase downslope and are marked by abrupt changes that coincide with the positions of the mapped internal scarp and translated blocks at the surface (Figs. 2 and 7b). The displacement vectors along the profile show a dominant downslope component, while also exhibiting local variations in vector orientation and magnitude across the same locations where displacement rates change (Fig. 7c). Vertical motion is generally smaller than the horizontal

component, but it varies across these locations and contributes locally to the overall displacement pattern, particularly in the upper and mid-slope parts of the profile (Fig. 7b).

The spacing and abrupt nature of the changes in displacement rate along a–a', together with the grouping of displacement vectors, indicate that deformation is concentrated along relatively narrow zones where displacement rates change over short distances (Fig. 7c). These zones align with the main scarp and the mapped limits of translated blocks, which may indicate preferential strain localisation along discrete surfaces rather than uniform deformation across the slope (Figs. 2 and 7). The transverse profiles show a similar spatial pattern: displacement is highest near the centre of the deforming area and decreases rapidly toward the margins, where the transition to stable ground occurs over short distances (Fig. 8). This cross-slope pattern indicates that the actively deforming area is laterally confined, with deformation concentrated near the lateral margins and along internal discontinuities.

Additional information on the deformation pattern is provided by the spatial variations in the relative contributions of the vertical and horizontal components from the headward part of the southern sector toward the toe. In the upper and mid-slope sections, the vertical component is locally more pronounced, particularly across the main scarp, where displacement vectors show a stronger vertical contribution (Fig. 7b–d). The increased vertical contribution across the main scarp may reflect localized extension within the deforming mass. These zones can be interpreted as extensional, normal fault-like deformational structures that accommodate internal stretching of the landslide body rather than simple open cracks. Similar structural patterns have been documented in large earth-flow systems and deep-seated slope deformations, where internal deformation is partitioned along discrete extensional surfaces (e.g., Guerriero et al., 2014). In contrast, the lower
slope is characterised mainly by westward, slope-parallel displacement with a smaller vertical component (Figs. 7–8), reflected by displacement vectors that are increasingly horizontal downslope. Together, these observations document a consistent along-slope variation in displacement behaviour across the southern sector.

The observed segmentation shows that displacement within the southern sector varies spatially, with sharp changes in the vertical-to-horizontal displacement ratio along slope (Figs. 7–8). These observations document a systematic along-slope pattern in the displacement field. While this kinematic configuration shares similarities with translational rock-slide behaviour, its large spatial extent, internal segmentation, and structural association with the Leo Pargil Dome suggest that the deformation cannot be fully explained by a simple, discrete rock-slide mechanism alone. In the following section, we examine whether this pattern can be related to the lithological and structural features of the Leo Pargil Dome (Sect. 5.2).

## 5.2. Dome exhumation as a geomechanical driver of slope dynamics

Deep-seated slope deformation on the flanks of the Leo Pargil Dome is considered here in relation to the dome's long-term tectonic and exhumational history, which is marked by regional extension and normal faulting in the upper crust (Ni and Barazangi, 1985; Thiede et al., 2006). Previous structural and thermochronological studies indicate that dome exhumation involved the development of an extensional structural setting on the southwest flank, including the Leo Pargil Shear Zone and associated normal-fault systems (Ni and Barazangi, 1985; Thiede et al., 2006; Langille et al., 2012). These studies further

document a progressive overprint of earlier ductile shear-zone fabrics by brittle structures during exhumation (Thiede et al., 2006; Langille et al., 2012). Later stages of erosion-driven exhumation and incision have been suggested to increase slope relief and reduce basal support along valley flanks, potentially enhancing the susceptibility of steep slopes to gravitational deformation (Thiede et al., 2006). The geological context outlined above provides a basis for examining why present-day deformation is concentrated in certain parts of the slope and displays spatial segmentation in the InSAR-derived displacement fields.

### 5.2.1. Transition in stress fields

The tectonic record of the Leo Pargil region documents a shift from earlier crustal shortening to Miocene extension across the northwest Himalaya (Thiede et al., 2006 and references therein). Thiede et al. (2006) inferred a regional stress reorganization characterised by a vertical maximum principal stress and approximately east-west-oriented extension. In the Leo Pargil area, this tectonic context is reflected in extensional structures, including the Leo Pargil Shear Zone and associated brittle normal faults (Thiede et al., 2006; Langille et al., 2012), as well as the Kaurik-Chango fault system and related active normal faulting (Ni and Barazangi, 1985; Thiede et al., 2006; Langille et al., 2012). Because the Kaurik-Chango fault system lies close to the land-slide, it may contribute additional brittle faults and fractures on the western dome flank. We do not treat this fault system as the main control on the InSAR pattern, but as part of the brittle fault network that may influence how the slope breaks into blocks.

From a slope-stability perspective, the extensional framework is unlikely to represent a direct triggering mechanism; rather, it provides

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Conceptual and geological framework relevant to slope deformation at the Leo Pargil Dome. (a) Conceptual scheme illustrating gravity-driven zones of extension and compression on a steep ridge, after Savage and Varnes (1987). (b) Simplified geological cross-section and block diagram of the Leo Pargil Dome, showing the regional structural architecture and extensional setting, after Thiede et al. (2006). (c) Northwest-oriented geological cross-section across the western flank of the Leo Pargil Dome at the latitude of Nako village, highlighting major lithological units and extensional structures, after Langille et al. (2012).</figcaption></figure>
inherited discontinuities and mechanical anisotropy that may favour the development of gravitational deformation (Agliardi et al., 2001; Jaboyedoff et al., 2011). Studies of DSGSDs elsewhere suggest that transient tectonic stress or strain changes can modulate ongoing deformation in critically stable slopes, superimposed on gravitational driving stresses (Stemberk et al., 2017; Baron et al., 2019).

Conceptual models of gravitational ridge spreading predict systematic zones of extension and compression, as well as the development of normal-fault-like scarps and internal discontinuities driven primarily by self-weight (Savage and Varnes, 1987; see also Jaboyedoff et al., 2011; Fig. 9a). In tectonically active mountain belts, such gravity-driven stress patterns may exploit inherited tectonic fabrics where their orientation and continuity are favourable with respect to slope geometry, allowing slope-scale fracture networks and shear zones to develop preferentially within mechanically weakened parts of the slope (Agliardi et al., 2001; Jaboyedoff et al., 2011).

In the Leo Pargil area, this conceptual understanding offers a way to examine whether gravitational deformation may preferentially localize along extensional discontinuities associated with the LPSZ and related brittle fault networks mapped along the dome flank (Thiede et al., 2006; Langille et al., 2012; Figs. 9b, c). Consistent with this interpretation, the InSAR results show distinct zones of displacement separated by pronounced gradients and surface scarps (Sections 4.1 and 5.1; Figs. 4–8; see also Fig. 2). Furthermore, the longitudinal cross-section suggests that deformation appears to be concentrated along a deeper-seated surface beneath the landslide body, rather than being distributed uniformly through the slope (Section 5.1; Fig. 7e).

Within this conceptual framework, the potential role of secondary shear structures, such as Riedel-type shears, can also be considered. These obliquely oriented shear zones typically develop at acute angles to the principal stress direction in extensional or transtensional settings and have been shown in other studies to act as localized zones of strain accommodation through slip and fracturing (Agliardi et al., 2001). Conceptual models of gravitational spreading predict that such secondary shears may intersect pre-existing or developing normal faults, thereby increasing the structural complexity of the deforming slope (Savage and Varnes, 1987; Fig. 9a).

Although Riedel shears are not directly identified in the present dataset, their potential development provides a plausible conceptual mechanism by which inherited discontinuities and fault segments could become mechanically linked, facilitating localized deformation within mechanically weakened parts of the slope (Agliardi et al., 2001). Similar structural interactions have been documented in deep-seated gravitational slope deformations in the Alps, where gravitational reactivation of tectonic fabrics exerts a strong control on long-term slope kinematics (Agliardi et al., 2001). In this sense, the schematic stress configuration and structural relationships illustrated in Fig. 9 help interpret the observed kinematic segmentation, but do not imply direct identification of specific shear structures in the InSAR data.

### 5.2.2. Exhumation stages and surface-process-driven weakening

Building on the structural template described in Section 5.2.1, thermochronological constraints indicate that exhumation of the Leo Pargil Dome progressed through distinct stages (Thiede et al., 2006; Langille et al., 2012). During exhumation and cooling, earlier ductile fabrics were progressively overprinted and crosscut by brittle faulting as rocks were brought toward shallower crustal levels (Thiede et al., 2006; Langille et al., 2012). Thiede et al. (2006) summarize three cooling-exhumation phases, including rapid cooling at ~16–14 Ma, followed by slower cooling between ~10 and 4 Ma, and renewed accelerated cooling/exhumation after ~4 Ma. For slope stability, the relevance of this staged exhumation is that progressive exhumation is commonly associated with a shift from ductile deformation at depth to brittle fracturing at shallower levels, which can increase the abundance and continuity of slope-scale discontinuities. In this setting, inherited shear-zone fabrics and later brittle faults provide potential planes of weakness

that may be reactivated under gravitational loading, promoting progressive internal partitioning of the rock mass (Agliardi et al., 2001; Jaboyedoff et al., 2011).

Apatite fission-track data presented by Thiede et al. (2006) indicate renewed acceleration in cooling after ca. 4 Ma, which they interpret as potentially reflecting enhanced erosion and incision rather than purely fault-driven exhumation. If incision increased during the late Cenozoic, it would provide a plausible long-term geomorphic context for progressive slope adjustment by toe erosion and relief growth, especially where the valley floor is persistently lowered by river processes. Here, we treat this as a reasonable long-term preconditioning scenario rather than a uniquely demonstrated driver of present-day motion. At the scale of mountain belts, analogous links between long-term exhumation, relief development, and the distribution of large non-catastrophic slope deformations have been discussed for the European Alps (Agliardi et al., 2013). In their orogen-scale analysis, DSGSDs preferentially cluster in areas with intermediate exhumation rates and their abundance correlates with valley incision along major drainage corridors (Agliardi et al., 2013). Although the Alpine setting is not a direct analogue for Leo Pargil, it supports the general point that incision and relief organization can influence where large, persistent slope deformations develop in anisotropic bedrock terrains. This comparison is used to place the Leo Pargil case within a broader geomorphic framework, rather than to suggest that the processes operate in the same way or at the same scale.

## 6. Conclusion

This study provides a detailed kinematic characterization of the Leo Pargil landslide, a large active deep-seated gravitational slope deformations (DSGSDs) in the northwestern Himalaya. By combining InSAR-derived displacement fields with geological and geomorphological data, we show that the landslide's present-day kinematics reflect a strongly segmented deformation pattern. The observed kinematic segmentation is discussed in relation to the regional structural and geomorphic context of the exhumed Leo Pargil Dome.

The kinematic segmentation observed in our InSAR data suggests that gravitational deformation is influenced by pre-existing structural discontinuities. One plausible interpretation is that these discontinuities include fabrics and brittle structures associated with dome exhumation, but their specific role and origin cannot be resolved from InSAR data alone. Fluvial incision at the slope toe by the Spiti River may also contribute to maintaining slope relief and therefore to the persistence of deformation. Overall, our results are consistent with an interpretation in which present-day deformation reflects gravitational driving stresses acting on a slope that exhibits structural anisotropy and has been pre-conditioned by long-term exhumation and valley incision.

### CRediT authorship contribution statement

**Gökhan Aslan:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Marcello de Michele:** Writing – review & editing, Writing – original draft, Conceptualization.
**Tim Redfield:** Writing – review & editing, Writing – original draft, Conceptualization.
**Reginald Hermanns:** Writing – review & editing.
**John Dehls:** Writing – review & editing, Data curation.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

### Data availability

Data will be made available on request.
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