Arc-parallel shears in collisional orogens: Global review and paleostress analyses from the NW Lesser Himalayan Sequence (Garhwal region, Uttarakhand, India)

Interest in hydrocarbon exploration from the the Lesser Himalayan Sequence (LHS) has recently been revived amongst petroleum geoscientists. Understanding the paleostress regime and the deformation processes are the two important steps to understand the structural geology of any (petroliferous) terrane. Arc-parallel shear is an integral deformation process in orogeny. The scale of the consequent deformation features can range from micro-mm up to regional scale. Unlike orogenperpendicular shear, different driving forces can produce orogen-parallel shears. We review these mechanisms/theories from several orogens including the Himalaya and compile 44 locations worldwide with reported orogen-parallel shear. Due to continuous crustal shortening by the IndiaEurasia collision, the squeezed rock mass at the plate interface has produced the Himalayan Mountain chain. In addition, the rock mass also escapes laterally along the orogenic trend. Tectonic stress-field governs this mass flow. Field study and microstructural analysis in the northwest LHS (India) reveals orogen-parallel brittle and ductile shear movement. Yand Pbrittle shear planes, and the Sand Cductile shear planes reveal the following shears documented on the ~ NW-SE trending natural rock selections: (i) top-to-NW up, (ii) top-to-SE up, (iii) top-to-NW down, and (iv) top-to-SE down. Our paleostress analysis indicates top-to-SE down and top-to-NW down shears occurred due to stretching along ~ 131°-311° (Dext), whereas top-to-SE up and top-to-NW up shear fabric originated due to shortening along ~133.5°-313.5° (Dcompr). Previous authors considered that the orogen-parallel extension generated ~ 15-5 Ma due to vertical thinning of the Himalaya. The NEtrending Delhi-Haridwar Ridge below the LHS plausibly acted as a barrier to the flowing mass and piled up the rock mass in the form of NW-SE/orogen-parallel compression. The NW-SE compression can be correlated with the D3 of Hintersberger et al. (2011) during ~ 4-7 Ma. (Words: 290) Manuscript accepted for publication in Marine and Petroleum Geology


Introduction
"…although many of the key concepts in the structural geology of fold and thrust belts have earlier origins in other orogens, the impetus has come from the desire to exploit geological resources that reside in the subsurface." - Hammerstein et al. (2020) Structural geology and tectonic models of fold and thrust belts/ collisional orogens primarily control the spatial distribution of the hydrocarbon reserves (Cooper, 2007). Small and complicated structures in such terrains hold the possibility of hydrocarbon (review in Hammerstein et al., 2019). In 1960s the Oil and Natural Gas Commission (ONGC) tried to explore hydrocarbon in the Himalaya and got a limited success. Independent and infrequent geoscientific studies in the Himalaya continued in this direction (e.g., Rao, 1986;Mukherjee and Chakrabarti, 1996). Palaeontological studies on Ediacara led Tewari (2012) to speculate hydrocarbon source rock from the Neo-Proterozoic terrain of the Lesser Himalayan Sequence (LHS), Garhwal Indian Himalaya. In fact, Neoproterozoic-Cambrian hydrocarbon reserves have been gaining attention worldwide. For example, Proterozoic rocks in the Jammu region (NW Himalaya, India) are presently under study for hydrocarbon (Hakhoo et al., 2016).
Despite pessimist scientific views that fold and thrust belts/collisional orogens such as the Himalaya in general has a poor chance of hydrocarbon exploration (Goffey et al., 2010), or the postulation that some specific part of the LHS can be non-productive (Mishra and Mukhopadhyay 2012), ONGC has been targeting the Krol unit of rocks from the LHS, NW Himalaya, India (Bhattacharya, Internet Reference; Bose and Mukherjee 2020). The Krol and Tal units of the LHS are potential reservoir of moderate quality (reviewed in table 1 of Mishra and Mukhopadhyay, 2012). Besides, the Blaini unit in the LHS have a good hydrocarbon potential (review in Craig et al., 2018). LHS rocks in Pakistan (Riaz et al., 2019) and in Nepal (Neupane et al. 2020) also have quite good potential and active hydrocarbon (shale gas, liquid H-C etc.) producing fields (e.g., Pakistan: Jhelum riverine system; Nepal: oil and gas seepages in Dailekh).
Several research papers on paleostress analysis elucidate the role of such a study in deciphering the petroleum geology of (sedimentary) terrains. To refer a few, Sippel et al. (2010) reconstructed the paleostress field in the petroliferous Oslo graben (Norway) and linked them with the rift-related local igneous activities. Kulikowski and Amrouch (2018) used 3D seismic data to analyze paleostress from the Cooper-Eromanga Basin (Australia) that has implication in understanding the hydrocarbon migration pathways (also see Zeng et al., 2010 for a similar study). Scheiber and Viola (2018) analyzed paleostress from fractures in the country rocks at the northern Bømlo islands (Norway). They separated deformation phases and linked those with the Caledonian orogeny and rifting. A similar approach of separating out deformation phases by performing paleostress studies was conducted by Ju et al. (2017) from the Sichuan basin (China). Kleinspehn et al. (1989) utilized paleostress study in deciphering the subsidence history of the petroliferous Central Basin of Spitsbergen, which is an important input in the basin's petroleum geology.
Around 55 Ma back, the northward advancing Indian plate made a continent-continent collision with the Eurasian plate (Klootwijk et al., 1992;Yin, 2006;Copley et al., 2010) and started initiating the classic Cenozoic Himalayan Mountain chain. The orogen commonly comprises of compressional (Yin, 2006) and intriguing extensional structures (Kellet et al., 2019). These can be well understood at orogen-parallel natural (sub-)vertical rock sections (Godin et al., 2006). Not only the shortened rock materials at the interface of Indian and the Eurasian plate led to orogenesis, but rock materials also flowed/sheared laterally (Molnar and Tapponier, 1975). This is well understood for the Tibetan region as well where materials extended E-W in addition to significant N-S shortening (GPS velocity study, Zhang et al., 2004). However, in the Himalaya ~ N-S shortening also dominates in terms of insequence and out-of-sequence thrusting, that verges both towards foreland (towards ~ S) and hinterland (towards ~ N) (e.g., Ni and Barazangi, 1984;Mukherjee, 2013;Bose and Mukherjee, 2019a,b;Ghosh and Mukherjee 2021).
Orogen-parallel extension and compression have been reported from worldwide (review in Fig. 1), including the Himalaya (review in Fig. 2). Twelve possibilities of geneses are compiled for such deformation (Table 1).
Orogensis has been deduced mostly based on the study of regional thrusts [e.g., Gansser, 1964;Burchfiel et al., 1992; in case of the Himalaya] in which the smallscale brittle deformation signatures have sometimes been missed. However, these (sub)meter-scale faults and ductile shear zones have great potential to provide detail deformation phases of different segments of the mountain. For example, the Karakorum fault situated between the Tibetan Plateau and the Himalaya slipped > 1000 km to accommodate the crustal shortening (Peltzer and Tapponnier, 1988). Interestingly, study of small-scale brittle structures from the southern part of the Karakorum Fault tightly constrained the slip magnitude of this fault to be only 65 km (e.g., Murphy et al., 2002) to < 400 km (e.g., Lacassin et al., 2004).
In this article, we document shear senses parallel to the regional orogenic trend in the NW Lesser Himalaya Sequence (Uttarakhand state, India) to understand the deformation behaviour parallel to the orogen. We attempt to correlate orogen-parallel rock movement with the existing models. We present meso-and micro-scale evidence of orogen-parallel shears and perform paleostress analysis of brittle shears. Various models explaining origin of extensional and compressional deformation parallel to the arc are compared. However, lack of cross-cut relation restricts our understanding of their relative timing. This work is important since orogen-parallel shears can trigger landslides and earthquakes.
2. Orogen-parallel rock movement 2.1 Orogen-parallel extension, the concept Extensional structures parallel to the orogen trend has been recorded profusely ( Fig. 1) but lacks unified theory for their genesis. From over-thickening of the crust to extension within the orogen marks a remarkable shift in the stress regime. Shape/geometry of an orogen, gravity driven mass flow, change in mantle flow along the collisional margin, angular convergence between two colliding plates, dome extrusion, approaching plate acting as rigid component, basement cover relationship etc. can govern the orogen-parallel extension. To explain these, the following models have been postulated with different driving forces.
(a) Gravitational collapse (mechanism: Fig. 3.i; global occurrence: Fig. 1) Collisional margin induces a significant change in gravitational potential energy during lithospheric deformation. A gravity-driven ductile flow compensates the generated anomalous potential energy producing extensional structures parallel to the ductile flow direction (Rey et al., 2001). The mechanism depends on horizontal compressional stresses, basal shear stress between the two plate boundaries, change in potential energy, strength of the adjoining materials and the crustal strength (Rey et al., 2001 and references therein).
(b) Radial thrusting and expansion (mechanism: Fig. 3.ii; global occurrence: Fig. 1) Converging plate acting as an indentor after collision gives rise to radially outward thrust system dying out at the orogen margin. To preserve such an expanding shape of the collisional margin, local extensional and strikeslip features Armbuster, 1984, Murphy andCopeland, 2005) develop synchronous with the radial listric thrusts. Extension is presumably maximum at the central part of the collisional margin and dies out towards the flanks. Platt and Vissers (1989) additionally suggested that due to additional emplacement of asthenosphere mantle under the collisional margin, surface elevates and eventually the orogen grows.
(c) Oroclinal bending (mechanism: Fig. 3.iii; global occurrence: Fig. 1) Orogens bent around a vertical axis of rotation are called oroclines (Carey, 1955). Progressive oroclines (Johnston et al., 2013) are characterized by small-scale structures, thinskinned tectonics, a single stress-field generating the thrust belt as well as the orogenic bending. Secondary oroclines (Johnston et al., 2013) are plate-scale. Here orogeny involves lithospheric mantle along with the crust and orogenic bending. A compression near-perpendicular to the original compressive stress-field develops. In secondary oroclines, lithospheric mantle plays a vital role in modifying surface features. The theory is popular in many orogens and explains their extensional structures.
(d) Removal of Mantle (mechanism: Fig. 3.iv; global occurrence: Fig. 1) England and Houseman (1989) numerically modelled collision-induced overthickened lithosphere where the hot asthenosphere replaces the colder and denser mantle lithosphere. This involves "convectional thinning" of the lithosphere that elevates the topography. A differential stress between the elevated topography and its low-lying surroundings may lead to extensions locally in a regionally compressive tectonic setting giving rise to orogen-parallel extension.
(e) Oblique convergence (mechanism: Fig. 3.v; global occurrence: Fig. 1) When a plate approaches another plate obliquely, the line of action of stress normal to the plate boundary results in compression within the plate, whereas the orogen-parallel component results in strike-slip faults, extensional faults/shear zones within the forearc region (Avé Lallemant and Guth, 1990). Inter-plate earthquake data along the convergent margin can confirm this oblique convergence based on the angle between the shortening slip directions and the direction of convergence . Oblique convergence is characterized by an orogenparallel extension with meso-scale cylindrical isoclinal folds and stretching lineations subparallel to the orogenic trend (Ellis and Watkinson, 1987).
(g) Rigid plate indenters (mechanism: Fig. 3.vii; global occurrence: Fig. 1) A rigid subducting basement, upon collision with another plate, results in initially gently dipping thrusts followed by orogen-parallel extension in the form of a "continental escape" in response to crustal thickening . The collisional features are governed by the geometry and the dip of the approaching rigid plate. Bonini et al. (1999) studied the progressively changing geometry of a rigid indentor after collision and coined the term "effective indentor". In this model, the normal and the thrust faults predate the orogen-parallel strike slip faulting (Martinod et al., 2000).
(h) Basement Influence on cover (mechanism: Fig. 3.vii; global occurrence: Fig. 1) Reactivation of pre-existing structures in the basement is common in collisional margins and plays a crucial role in orogenesis (Brown et al., 1999 and references therein). Lacombe and Bellahsen (2016) pointed out the requirement of thick-skinned tectonics to understand crustal deformation on a collisional boundary. Basement highs in the present-day foreland regions placed at a high-angle to the collisional margin shortens the crust leading to orogenparallel extensions on the cover rocks.

Orogen-parallel compression-The concept:
Orogen-parallel compression has been recorded only from a few orogens globally (Fig.1). Schellart and Lister (2004) doubted compression parallel to the orogen based on the large-scale elastic behaviour of the lithosphere and the retaliate behaviour of the lithospheres surrounding the orogen. Orogenparallel compression appears to be areaspecific and can be due to rotation of the colliding plates, oblique convergence, and existence of barrier to prevent lateral massflow. Models for orogen-parallel compression are presented below.
(a) Oblique Convergence: (mechanism: Fig. 4.i; global occurrence: Fig. 1) When a low-dipping subducting plate attains ahigh-angle of obliquity at collisional margin in between two plates, friction between the colliding plates increases temporally to effectively produce orogen-parallel compression (Boutelier and Oncken, 2010). Plates may curve as a response to stress, and this plays a vital role to develop such compression (Boutelier and Oncken, 2010).
(b) Plate rotation (mechanism: Fig. 4.ii; global occurrence: Fig. 1) If a microplate is involved in a collisional margin, rotation and reorientation of such plate accompanied by continuous collision produces orogen-parallel compression (Boccaletti and Sani, 1998;. A small plate rotating under compression must have high mechanical strength and remain in no contact with surrounding plates   (Fig.  3iii). Wide-scale bend, strike-slip fault and small-scale lateral escape of materials are characteristics of orogen-parallel compression (Mantovani et al., 1997a(Mantovani et al., , 2002Gelabert et al., 2002;.
(c) Abrupt termination of lateral mass flow (mechanism: Fig. 4.iii; global occurrence: Fig. 1) Collisional boundaries are characterized by overthickening followed by lateral escape of such mass to dissipate the accumulated energy. If these laterally escaping mass encounters a sudden dead end/stress buttress [(regional structural boundary, e.g., Nanga Parbat syntaxis in Himalayan orogen, ], the mass tends to accumulate against such termini. If the termini are ~ perpendicular to orogen, orogen-parallel compression will be the outcome .

Orogen-parallel shear in Himalaya:
Orogen-parallel shear from the arcuate Himalayan chain have been recorded by several workers from Tethyan-, Higher-, Lesser-and the Siwalik Himalaya (Fig. 2).

Orogen-parallel extension in the Himalaya-Tibet system
(a) Gravitational collapse (Fig. 3.i for mechanism; Himalayan occurrence: Fig. 2) Higher elevations are prone to gravitational collapse, and as compression continues material escapes laterally in form of east-west extension in Himalaya-Tibet region (e.g., Tapponier, 1975, 1978;Tapponnier and Molnar, 1976;Tapponnier et al., 1982;Valli et al., 2007. (b) Radial thrusting and expansion (Fig. 3.ii for mechanism; Himalayan occurrence: Fig. 2) The radial expansion theory is a step further from the radial thrusting (Section 2.1.b) in case of the Himalaya, where the sequential development of the thrust faults viz., the Main Central Thrust (MCT, ~ 25 -0.7 Ma; Godin et al., 2006, Larson et al., 2015review in Martin, 2017b, the Main Boundary Thrust (MBT, ~ 11 -9 Ma; review in Godin et al., 2018) and the Main Frontal Thrust (MFT, < 2.5 Ma; review in , is accounted for. Due to its arcuate geometry, arc-parallel stretching in the Himalaya increases towards the foreland. However, given that the MCT is the oldest amongst the three major discontinuities, Murphy et al. (2009) identified that the Tethyan Himalaya and the Trans-Himalaya are the areas of maximum orogen-parallel stretching followed by Higher Himalayan Crystallines (HHC), LHS and then the Siwalik Himalaya.
(c) Oroclinal bending (Fig. 3.iii for mechanism; Himalayan occurrence: Fig. 2) The Karakorum Fault and Yadong-Gulu rift cover a large part of Tibetan N-S rift system (e.g., Klootwijket al., 1985;Ratschbacheret al., 1994) The central part of the Himalaya and southern Tibet are the most affected regions due to the arcuate shape of the Himalayan arc. Opposite sense of shear (Dutta and Mukherjee, 2019) at the syntaxes indicates rotation of crustal blocks due to the orogenic bending.
(d) Removal of Mantle (Fig. 3.iv for mechanism; Himalayan occurrence: Fig. 2) With the ongoing collision between the Indian and the Eurasian plate, an instability within mantle convection led to up-arching of the crustal surface (England and Houseman, 1989). Stress difference between the elevated surface and the surroundings produced orogen-parallel extension in the Himalaya.
(e) Oblique convergence (Fig. 3.v for mechanism; Himalayan occurrence: Fig. 2) No extensional structures developed in the central part of the Himalaya as the orientation of the interface between the Indian plate and Eurasian plate is ~ orthogonal to the direction of convergence. As per this model, Western Himalaya experiences westward material flow and the eastern Himalaya towards east due to the regional resultant stress orientation (Styron et al., 2011).
(g) Rigid plate indenters (Fig. 3.vii for mechanism; Himalayan occurrence: Fig. 2) The Indian plate as the rigid indenter collided with Eurasian plate and as a result orogenparallel extensional rift system developed. As per this theory, extension was restricted in the Central Himalaya (Kapp and Guynn, 2004).
(h) Basement Influence on cover (Fig. 3.viii for mechanism;occurrence in the Himalaya: Fig. 2) Reactivation of pre-existing basement fault system within the Indian continent created lateral deformation zones within the Himalayan cover by affecting the geometry of the Main Himalayan Thrust (Godin et al., 2018). Bouguer gravity anomaly data reveals linear structural features along the Himalayan Mountain range are deeply connected with the underplated basement faults .
(i) Mass accumulation and stored energy dissipation (Fig. 3.ix for mechanism;occurrence in the Himalaya: Fig. 2) After Indo-Eurasian plate collision, the Himalaya and the Tibetan plateau continuously stored mass and therefore gravitational potential energy. A full grown orogen starts squandering the energy. At this time, domination in between accumulated energy and dissipated energy takes place. The stored energy is dissipated by the Himalayan front by re-organizing the structural and the erosion patterns. Such reorganization can be accomoapned by seismicity. The reorganization can produce orogen-parallel extension. In a full grown orogen squandering of energy dominates as the accumulation of energy fades out (Hodges et al., 2001). Orogen-parallel compression in the Himalaya is deciphered from the syntaxial boundaries. In east, the Namcha Barwa syntaxis (Wadia, 1931) and in the west the Nanga Parbat syntaxis (Madin et al., 1989) in form of antiforms  and are coeval with the Himalayan regional compression (Burg et al., 1998).

Orogen-parallel compression in Himalaya
In the NW Himalaya, orogen-parallel compression has been well documented within the LHS (e.g., in form of fold, doubly plunging folds, faults, klippes, cross-cut relations; Agarwal and Kumar, 1973;Misra and Bhattacharya, 1973;Jain 1987;Chakroborty and Mavaliya, 1995;Dubey and Jayangondaperumal, 2005) and within the Siwalik range in the Dehradun-Roorkee transect in terms of brittle deformed clasts (Dutta et al., 2019). To the authors' knowledge, orogenparallel compressional structures are absent in the Central and in the Eastern Himalaya (Fig. 2).
Major deformation within Himalaya pre-dating the Tertiary collision is not universally accepted. In the context of thick-skinned tectonics, pre-Himalayan deformation and its possible influence on Himalayan orogeny are to be considered profoundly. See Repository 1 for detail pre-Himalayan deformation (and metamorphism).

Timings of orogen-parallel shear
After the Indian plate collided with the Eurasian plate, massive crustal thickening perpendicular to the orogen triggered lateral movement of mass manifested as orogen-parallel extension between 15-5 Ma. For example, Nagy et al. (2015) reported extension ~15-13 Ma from the upper Karnali valley (Nepal) based on 40 Ar/ 39 Ar dating of white micas from the HHC. Zircon fission track dating reveals 12-11 Ma activation of orogen-parallel extension from the HHC in the Sutlej valley, NW Indian Himalaya . Inception of orogen-parallel rock movement is presumed to be ~ 10 Ma within the HHC (cross-cut relation between the 12.5 Ma old Khula Kangri Granite and the South Tibetian Detachment: Edward and Harrison, 1997). Repository 2 presents orogen-parallel shear from the Tibetan region.

Geology of the study area
The LHS is bounded by two active thrusts, the MBT to the south, and the MCT to the north. The MCT and the MBT separate the LHS from the HHC and the Siwalik Himalaya, respectively. The LHS is sub-divided by various major and minor longitudinal thrust sheets (Fig. 2, Table  2). Lack of ubiquitous fossil evidence, low-grade metamorphism (Miller et al., 2000) and brittle to brittle-ductile deformation render it difficult to compile a detailed and exhaustive stratigraphy. Nevertheless, previous workers (Valdiya, 1980;Srivastava and Mitra, 1994;Ahmad et al., 2000;DeCelles et al., 2001;Célérier et al., 2009;Richards 2005) broadly divided the LHS into: (i) Outer Lesser Himalayan Sequence (OLHS), and (ii) Inner Lesser Himalayan Sequence (ILHS), based on the degree of deformation, age difference, strain rate and temperature. The north-dipping Tons Thrust (Célérier et al., 2009, Ahmad et al., 2000, Richards et al., 2005, separates the OLHS at south from the ILHS at north.

Lithologic & structural information
The entire LHS was deposited under a marine condition along with the Tethyan Himalayan Sequence (THS) within two separate basins (reviews in Thakur, 1992;Srivastava and Mitra, 1994). Fore-structures are foreland verging and dip towards the hinterland. Back-structures, on the other hand, dip and verge towards the foreland and hinterland, respectively. Fore and back structures are well documented and reported from the study area (Banerjee et al., 2019;Bose and Mukherjee, 2019a). Table 2 compiles the succession.
U-Pb isotopic signatures of detrital zircon suggest deformed LHS sediments' depositional ages, Paleoproterozoic to Neoproterozoic, are comparable with undeformed Vindhyan sediments within the Indian craton (McKenzie et al., 2011). The OLHS (succession in Table 2), underwent fault-propagation folding to create the Mussoorie Syncline and Garhwal Syncline ( fig. 1 in Kumar and Dhaundiyal, 1980) with the Tal Formation at the core (Shanker and Ganesan, 1973;Valdiya, 1978). Parallel to the synclinal axis is the northerly dipping Kathu-ki-Chail Thrust, which displaced the Tal Formation rocks locally creating klippes of the Precambrian Ramgarh Group and the Almora Group Crystallines (Fig. 5;Dubey and Jayangondaperumal, 2005). The Nagthat and the Chandpur Formations are separated by a north-dipping Aglar Thrust to the south, and a south-dipping Basul Thrust to the north (Fig. 5, Table 2; Jain, 1971). The Tons Thrust marks the contact between the OLHS and the ILHS (Célérier et al., 2009). It is a prominent backthrust , which is also named as the Srinagar Thrust (Thakur and Kumar, 1994;Valdiya 2010) and the North Almora Thrust (Agarwal and Kumar, 1973;Kayal et al., 2002), Dharkot Thrust, and the Chail 3 Thrust (Bhatt, 1996). The ILHS is intensely folded and faulted (Valdiya, 1995). The Berinag Thrust (/Dharasu Thrust / Singauni Thrust / Dunda Thrust/ Uttarkashi Thrust, Jain, 1971;Agarwal and Kumar, 1973) is the most prominent discontinuity within the ILHS and is a folded thrust belt, separates the Rautgara-, Mandhaniand Deoban Formations with the Berinag Formation (Fig. 2, Table 2). The north dipping Munsiari Thrust (Lower strand of the MCT: MCTL) marks the upper limit of the LHS (Mukherjee, 2013) located near the Sainj village (also see Catlos et al., 2020;Montemagni et al. 2020). See Repository 3 for local structural detail.
Unlike the much well-studied orogenperpendicular shear in ductile and brittle regimes (e.g., Mukhopadhyay and Mishra, 2005;Célérier et al., 2009;Agarwal et al., 2016;Bose and Mukherjee, 2019a,b), orogen-parallel shear received much less attention from the LHS. In and around our study area, no absolute timing/constrain on orogen-parallel compression and extension has been available.
The Berinag Thrust-fold belt responsible for klippes within ILHS sheared S/SW up . Mandal et al. (2016) predicted Berinag Thrust to be genetically equivalent to the Munsiari Thrust (Ramgarh-Munsiari Thrust). However, Singh et al. (2012) suggested a younger age (apatite fission track date of 0.3 ± 0.1 to 0.9 ± 0.2 Ma) for the Berinag Thrust than that of the Munsiari Thrust (1.7 ± 0.4 Ma, 1.8 ± 0.2 Ma and 2.3 ± 0.3 Ma; apatite fission track) and correlated with reactivation of duplex within the ILHS. 40 Ar/ 39 Ar dating and zircon fission track dating by  suggests Munsiari Thrust developed during 4-7 Ma from the Sutlej valley, Himachal Pradesh, India. Catlos et al. (2002) interpreted that the Munsiari Thrust emplaced ~ 5.9 ± 0.2 Ma based on Th-Pb monazite dating (also see ~ 5-4 Ma age recently deduced by Montemagni et al. 2020). However, an older age of ~ 19.8 ± 2.6 Ma for the Munsiari Thrust was also noted by Metcalfe (1993; Ar/Ar dating on hornblende).
Brittle deformation study reveals that the NE-SW compression (correlated with D1 by Hintersberger et al., 2011) predates the NW-SE orogen-parallel compression. Ductile to brittle deformation transition within the Himalaya as well as the NE-SW compression happened possibly around 15-17.5 Ma (Hintersberger et al. 2011). Hintersberger et al. (2011) reported NE-SW extension (D3) within LHS ~ 4-7 Ma based on 40 Ar/ 39 Ar datingof white mica from the LHS (Sutlej valley). This can be correlated with the NW-SE compression.

Seismicity & subsurface information
The LHS is one of the most seismically active regions in the mountain belt (e.g., Molnar et al., 1973, review in Rajendran et al., 2017Fig. 5 for compiled plots in the study area). The International Seismological Center (ISC) data reveals 44 moderates to large earthquake events (5 ≤ ML ≤ 6.3) in last 60 years (in between 1958-2017) in the Kumaun-Garhwal Himalaya (Pasari and Arora, 2017). Between April 2005 to June 2008, Kanna and Gupta (2020) identified 27 earthquakes of ML ≤ 3.5 in the Garhwal Himalaya. Within the ILHS, most of the seismic activities are recorded in between the Berinag Thrust and the Munsiari Thrust (Kanaujia et al., 2016). Besides, scattered seismicity was also observed in between the Berinag Thrust and the Tons Thrust, however, mostly restricted in the vicinity of the latter. Most of the earthquake focal points are 30-50 km deep below the LHS (Thakur, 1992). The 20-Oct-1991 6.8 Mw earthquake in the Uttarkashi area and the 28-March-1999 6.5 Mw earthquake at the Chamoli area are the two major seismic events in the Garhwal Lesser Himalaya (Khattri et al., 1994;Kayal et al., 2002). Kumar and Mahajan (1994) reported maximum damage in between Bhatwari and Gangori area due to Uttarkashi Earthquake. Low value of fractal dimension of earthquakes in the Garhwal Himalaya indicates that the main faults (MFT, MBT, MCT) are primarily responsible for seismicity (Teotia et al., 1999).
The present-day GPS-derived stress direction broadly trends SSW/SW direction. However. a subordinate/ anomalous SSE trend has also been reported (Gautam et al., 2017 and references therein). The later is worth mentioning in the contenxt of orogen-parallel rock movement (Fig. 5).
The ~ NE-SW trending Delhi-Haridwar Ridge (DHR; Arora et al., 2012, also referred as Delhi-Haridwar-Harsil ridge in Bagri, 2006a,b: yellow dash line in Fig. 5), a northward protrusion of the Aravalli range, lies underneath the LHS from 28 N-30 N to 76-79 E (Verma et al., 1995). Small-scale transverse structures within Lesser Himalaya having similarities with Aravalli trend gives the first indication of extension of Aravalli range underneath the Himalaya (Valdiya, 1976 and references therein). The northernmost end of the ridge has remained indeterminate. Godin and Harris (2014) suggest that the ridge continues up to the Karakorum Fault.As per one view, DHR's northern extension is up to Uttarkashi (Bagri, 2006a). However, Kanujia et al. (2016) proposed that the DHR does not extend beyond the MCT, with an offset at the Tons Thrust. This subsurface ridge has possibly affected the stress field, seismicity, and the exhumation rate in its vicinity, and played a major role in deforming the LHS (e.g., Khattri, 1992;Raval, 1995;Bollinger et al., 2004;Gahalaut and Kundu, 2012;Ravi Kumar et al., 2013, Srivastava andCobbold, 2014;Godin and Harris, 2014).

Landslides
Areas with high slope, area affected by seismic events, unconsolidated soil, barren land, high relative relief (150-200 m), joint sets, slopes facing south, near road, near drainage, near reservoir and areas near geological structures/ weak planes are susceptible for landslides in the study area (compiled in Fig. 5; e.g., Saha et al., 2002;Pareek et al., 2013;Kumar and Anabalagan, 2016). The detail is presented in Repository 4.

Present work 6.1 Field observations
Structural fieldwork was carried out from ~10 km NE of Uttarkashi up to Mussoorie along the National Highway-34, and State Highway-30 for a total of ~ 152 km in the LHS. Exposures of schists, phyllites, slates, quartzites, dolomites, conglomerates and limestones occur, covering a part of the MCT schist zone, and all of the ILHS and OLHS.
Y-plane/boundary faults and P-planes/Riedel shears, sigmoid rock bulges and vein geometries are utilized from the sub-vertical road sections to decipher the shear senses. Shear movements perpendicular those given by the fore and back structures are documented in this study to understand the deformation produced.
We encountered two ubiquitous cases of 3Dsigmoidal bulge of sheared rock mass (Figs. 6,7). In both the cases, the sigmoid geometry is preserved only on the NW-SE sections (Figs. 6b,d;7a,d) and not on the NE-SW sections. In the later rock section, the sigmoids manifest merely as parallel straight lines (Figs. 6c; 7b,e) and thus are unsuitable for shear sense determination. Although the field transect trends ~ NE-SW, local turns along the road sections allow us to study NW-SE sections selectively. To pick up orogen-parallel deformation we documented four categories of brittle shear senses on the NW-SE sections: (a) top-to-NW up (Fig. 8), (b) top-to-NW down ( Fig.  9), (c) top-to-SE up (Fig. 10), and (d) top-to-SE down (Fig. 11). However, lack of cross-cut relations amongst these shears renders their relative temporal relations indeterminate.

Microstructural study
Three oriented rock samples -two biotite schists (samples S1 and S2) and a single quartzmica phyllite (sample S3) -are collected from the ILHS and the OLHS, respectively. The purpose is to examine and confirm the presence of orogen-parallel shear at microscale, which otherwise looks ubiquitous repeatedly in the meso-scale. In all other field locations we confirmed the arc-parallel shear based on the well-known shear sense indicators, therefore, we did not go for thinsection studies from rocks from those locations. Samples S-1 and S-2 ( Fig. 5) come from the MCT-Zone i.e., the Berinag Formation and are collected from the footwall of the Munsiari Thrust. The outcrops for S-1 and S-2 exhibit top-to-NW up (Fig. 12a). and top-to-SE down ( Fig. 12b) shear senses, respectively. The Thin-sections parallel to the dip directions [section trends for S-1 (110°-290°), S-2 (165°-345°), S-3 (101°-281°) and perpendicular to the primary shear planes are prepared. In absence of clear-cut lineations in the field (e.g., Piazolo and Passchier 2002, this is the most reasonable way of choosing orientation of thin-sections. Attitude of the main foliations (primary shear planes) are ploted in Fig. 5. The purpose is to confirm the shear sense where we had doubt in shear sense observed in field. In the subsequent paleostress analyses, we incorporated field data, and no inputs were taken from these thin-sections. Significant grain size reduction, alterations of existing minerals, mainly feldspar, recrystallization, shear fabrics etc. indicate that the rocks are significantly deformed in ductile and brittle regimes. S-1 and S-2 show ductile features viz., as S-C fabrics ( Fig.13c), quartz ribbons ( Fig. 14d) and partial recrystallization of feldspar grains (Fig.13d). S-3 consists of typical brittle deformation features e.g., extensional joints (Figs. 13a, 14b) and curved fracture planes and grain-size reduction (Figs. 13a,14b). Curved extensional brittle plane developed within quartz-sericite mylonite (Fig. 13a), and partly altered sigmoid feldspar porphyroclast within biotite-rich matrix (Fig.  13b), biotite fish within mylonitized matrix (Fig.  14c), and asymmetric ribbon of quartz within biotite matrix (Fig. 14d) show a top-to-SE shear. Top-to-NW shear is recorded as curved quartz vein within the quartz-sericite mylonitic matrix (Fig. 14b), and partly recrystallized sigmashaped feldspar within the mylonitic matrix ( Fig. 13d) near the MCT. Sigma-shaped aggregates of fragmented feldspar pophyroclasts bound by steeply ( Fig. 13c) and gently ( Fig. 15b) dipping C-planes near the MCT zone record a top-to-NW down shear. Top-to-SE up shear is represented by sigmoid biotite fish ( Fig. 14a) from the mylonitic schist of the Berinag Formtion. Extensional shear (top-to-SE down) (Fig. 15a) is also observed from the quartz-feldspar-biotite mylonite.

Paleostress Analysis
Repository 5 presents the principals involved in this study.

6.3.1
Steps 84 fault kinematic data viz. brittle Y-, Pplanes/riedel shears; S-, C-planes; sigmoids; vein geometries (assumed to represent the outcomes of the youngest deformation in this region) have been documented from more than 98 locations in the field (locations in Fig. 5). Plane Y-and plane C-attitudes are separately processed using three paleostress analysis software: , and (iii) Win-TENSOR (5.8.8; detailed in Repository 5). Paleostress analysis has been recommended to be run by different means in order to confirm the results (Simón, 2019). Field data exhibits an abundance of gently to moderately dipping fault/shear planes and few steeply dipping ones. Due to lack of striation data, movement along the fault/ shear plane was assumed to be dip-slip in nature. This is a standard assumption adopted by field geologists. Y-Plane/ C-plane are considered to be the fault planes, along with the geographic direction of movement i.e. NW or SE. The analyses are carried out to identify primarily the paleostress principal axes orientations.
The data to be input by the user for each of the software are presented in the Appendix-I.
Slip senses collected from the field are classified into four categories. (i) Top-to-NW up, (ii) top-to-SE up, (iii) top-to-NW down, and (iv) top-to-SE down. A two-step process is followed. In the first run (R-1), paleostress solution is derived for each individual set of slip-sense. Subsequently, the entire dataset is used as input and the software clubbed them into two groups based on their two sets of slipsenses supposedly developed under the same stress regime (R-2). We finally compare the results deduced from R-1 and R-2. (Table 3, Fig. 16) Different software results are tabulated for R-1 as well as R-2. Comparison on the aforementioned three paleostress software reveals a close match in between WinTensor and SG2PS results, however, T-Tecto displays not a very significant deviation (~ 5-11°) in the orientation of principal stress axes with others. This happens because the software has different statistical ways of analyzing the data.

Results
Principal stress axes in SG2PS are oriented like an Andersonian stress regime, i.e., orthogonal to each other with two stress axes lying horizontal. Whereas, for T-Tecto and WinTensor, the principal axes orientations remain ~ orthogonal and at a very low angleto the horizontal. With all the84 data input, SG2PS identifies two deformation events, marked by two different orientations of principal stress axes (Table. 3), however, represents the result in a single stereo-plot superposing the results one on the other. This often forces the user to identify, separate and define the data group beforehand. From Win-Tensor, additionally, stress ratio (R= σ2-σ3/ σ1-σ3: Delvaux and Sperner, 2003) can be derived and then the stress index (R / ) can be obtained. For example, for extensional stress tensor R / =R, strike-slip stress tensor R / = (2-R) and under compressional stress tensor type R / = (2+R). Based on the stress index, stress regimes can be of various types. For example, for top-to-NW up and topto-SE up shears in our study area, R / = 2.73 indicates pure compressive regime. For top-to-NW down and top-to-SE down shears, R / = 0.21 denotes radial extensive stress regime. However, Win-Tensor software has a limited control over dataset and often seen to be using a selected amount of data thus neglecting the remaining data, which may have an influence in the paleostress results. For example, the software considered only 72 fault data as good data amongst 84. By analyzing the epaleostress data using different methods, we checked the the authenticity of the results.
Rarely, T-Tecto neglects any data from dataset. It segregates individual deformation phases and generates two different plots smoothly.

Discussions
The new data set consists of attitudes of meter to cm-scale shear planes (foliation plane in case of ductile shear and fault plane/ Y-plane for brittle shear) documented along the Bhagirathi section from ~10 km NE of Uttarkashi up to Mussoorie.
Arc-parallel extension and compression are observed throughout the area sporadically. Few areas are affected with denser data under the influence of major regional structures. Such locations are the hangingwall and on the footwall blocks of the Tons Thrust, Berinag Thrust and around the Mussoorie synclinal axis. Based on field observations from HHC, orogenparallel extension within NW Himalaya was earlier predicted to be extended in lower elevation such as the LHS (Hintersberger et al., 2011). The prediction is re-iterated by the field evidence cited in the present study.
Pre-Himalayan extensional and compressional events, though reported from various places from the orogen, are scace (Repository 1). NEplunging isoclinal folds have been reported from the Kumaon Himalaya (Patel et al., 2011) parallels the orogenic trend. Abundant and prominent orogen-parallel deformation features within the Garhwal LHS indicate the recorded deformations are much recent events. Overall acceptance of pre-Tertiary deformations within Himalaya is still unpopular and in various cases recorded pre-Himalayan deformations were later linked with Himalayan deformation as the understanding of the orogeny upgraded (Bhargava et al., 2011). However, reactivation of pre-existing weak planes belonging to Indian plate can produce deformation within the cover rock (Godin et al., 2018).
The Dehradun lineament (Fig. 2) underlying the study area trends NW-SE can be linked with orogen-parallel extensional shear fabrics. From Paleostress analyses using three software in this work identified that the orogen-parallel compressional shear direction is ~ 43.5˚NE and that for the orogen-parallel extension is ~ 41˚NE.

Possible mechanisms for orogen-parallel deformation (extension and compression) in NW Indian LHS
The arc-parallel extension in the NW Indian LHS could be explained by: (i)"radial thrusting and expansion model" combining arc-geometry and major thrust fronts (e.g. Seeber and Armbuster, 1984;Murphy et al., 2009), (ii) "removal of mantle" (e.g. England and Houseman 1989), (iii) accumulation of mass and energy under collision and dissipation of the stored energy (Hodges et al., 2001), or (iv) basement influence on cover (e.g. Godin et al., 2018;Lacombe and Bellahsen, 2016).The "oblique convergence model" (McCaffrey and Nablek, 1998; can explain both arcparallel compression and extension provided the local stress magnitude exceeds than the regional one (Boutelier and Oncken, 2010). This local stress can either create extension or compression parallel to the arc. Presence of both the arc-perpendicular (NE-SW) and arcparallel (NW-SE) brittle/ductile shear fabricsdocumented in this study from the NW Indian LHS makes the model consistent with either arc-parallel compression or else arc-parallel extension.
Consequently, alternate mechanisms could be required to address the coexistence. "Gravitational collapse", which discusses escape tectonics can explain arcparallel shear, however, those brittle structures are more likely to trend E-W and are unlike observations of this work (NW-SE shear) from the field and also under an optical microscope.
Although models such as "orogen bending" and "rigid plate indentor" justify the occurrences of arc-parallel extensional structures within the LHS (and the HHC), they are incompatible to develop orogen-parallel shears at the eastern and the western portions of the Himalaya. The NW Indian Lesser Himalaya Sequence is devoid of extruded domes, thus we can dismiss "dome extrusion"-induced extension in the region. Table 4 presents the remainder models and their chances of occurring orogen-parallel extension.

Most suitable model(s) for arc-parallel extension in NW Indian LHS
The model "radial thrusting and extension" incorporate the southward migration of the thrust fronts along with their arcuate geometries. It addresses the development of NE-SW striking extensional features in the NW Indian LHS due to arc-parallel stretching, to preserve the arcuate geometry of the orogen. Within NW Higher Himalayan Crystalline both ductile and brittle shear fabrics are correlated with the model (Hintersberger et al., 2011).
Lithosphere underneath the collisional margin experiences "convectional thinning" and thus develops rifts. Through such rifts are found in Tibet, these are rare within the Himalaya.
Several authors (e.g., England and Houseman, 1989) attempted to correlate Himalayan orogen-parallel extension with thick-skinned tectonics. Validity of such a model would require a deeper insight and thus remains as a possibility to explain arc-parallel shear within NW Indian Himalaya. Taloor et al. (2020) identified five levels of river terraces and based on geomorphic studies from the Garhwal LHS. Pandey et al. (2005) recorded five stages of uplift. These point out that the "mass accumulation and stored energy dissipation" could have happened in the LHS. Dharasu and Varunavat (Fig. 5 for location; e.g., Sarkar et al., 2011, Joshi et al., 2003 are localities around which severe landslides have been reported. Besides, frequent landslides along the existing river pathways in the LHS have been common (locations in Fig. 5). Theoretically, landslides are in direct link with geological structures, earthquake epicenters, high relative relief, joint sets, slopes facing towards the regional rock movement (S direction), apart from human activities such as road and reservoir construction. Squandering of accumulated energy in form of landslides, erosional activites within Garhwal Himalaya under tectonic influence is well evident. Orogen-parallel extension towards NW-SE trend is thus can be correlated with such process.
The fourth and the final possibility ("basement influence on cover") for the orogen-parallel extension involves thick-skinned tectonics, in which the subsurface pre-existing surface plays a crucial role in deforming the surface structure. The Delhi-Haridwar ridge (Godin and Harris, 2014) situated below the NW LHS plausibly influenced the surface structure with its additional elevation from the surroundings (Fig. 17a). The DHR has been stated to be the cause of mass movement in Varunavat and a reason or the Uttarkashi earthquake (Bagri 2006a). Over-riding mass experiences an additional stretching (NE-SW trending) and can develop NW-SE extensional structures/ shear fabrics (Fig. 17b).
Several ridge and lineaments (Fig. 2) oriented NE-SW within Indian plate can generate seismic events, modifying river drainage pattern and the Himalayan surface morphology (Godin et a., 2018). As per Godin et al. (2018), based on analogue centrifuge models, the lineaments/ basement faults of the Indian plate are capable of producing orogen-parallel extension at surface level. Our field-evidence, NE-SW trending extensional shear fabrics above the DHR, justifies this model. It is further premised by us that such kind of shear fabrics exist in the Himalayan rocks overlying these sub-surface lineaments/ basement faults ( Fig. 2; e.g., Simla, Dehradun, Lucknow, Pokhara, Kathmandu, 88°E, Kolkata linemants; Delhi-Haridwar ridge, Faizabad ridge and Munger-Saharsa ridge). This observation provokes a new opening for future Himalayan research.
The first three hypotheses are regionally significant, especially at higher elevations of the Himalaya-Tibet region. On the other hand, the fourth hypothesis i.e., presence of the DHR would locally influence the Siwalik and the LHS only. The four possibilities are not mutually exclusive and might act in different degrees to produce the arc-parallel extension. Sub-surface presence of the DHR and its direct correlation to the arc-parallel extension is debated, however, we suspect that the high elevation did influence the surface structures in the region.

Best model for arc-parallel compression in the NW Indian Lesser Himalaya Sequence
As per Section 5.2, the "oblique convergence" model -although a popular one -can either justify arc-parallel extension or compression, but not both. This is a major limitation to the applicability of this model in our study area where both the regional (NE-SW deformation: fore-structures) and the local stress-induced structures occur. Besides, their explanation is conditional i.e., the local stress component should dominate over the regional one.
We eliminated the "plate rotation" theory since neither the Indian nor the Eurasian plates are micro-plates. This left us with the only model proposed by : "abrupt termination of the lateral mass-flow". Arcparallel compression requires an abrupt boundary to prevent the lateral mass-flow. South to our study area in sub-surface the elevated DHR can cause an indirect effect to terminate the flowing mass and allow them to pile up, thereby generating arc-parallel compression (Fig. 17c).
Previous workers (Agarwal and Kumar, 1973;Misra and Bhattacharya 1973;Jain, 1987;Dubey and Jayangondaperumal, 2005) also reported orogen-parallel compression from the Garhwal LHS as byproducts of some other studies. Although earlier studies expressed the existence of orogen-parallel compressive stress-field, for the first time the present work attempts to genetically correlate orogenparallel compression with basement high (Fig.  17c). It is further predicted such kind of deformation can be observed above the other Himalayan sub-surface ridges.

Relation between arc-parallel shear with landslides, seismicity and regional faults in NW LHS
In this section, brittle and ductile deformation ranging from µm to m-scale and its possible relation and effect are viewed in larger scale taking in consideration of geodynamic framework of the LHS.
Natural landslides in the study area are concentrated along the river section where undercutting of stream plays the most important role. South/ south-east/ south-west facing planes are more susceptible to landslides upon receiving of heavy rainfall and the direction of rock-movement is overall southward in the Himalaya. SE-dipping planes being a part of orogen-parallel deformation thus further provides additional weak zones from which the landslides can initiate, given proper geological and geomorphic settings.
The study area consists of several earthquake epicentres (Fig. 5). GPS derived stress pattern in NW Lesser Himalaya reveals present day SSEtrending stress axis, thus orogen-parallel deformations could contribute in generating earthquakes in the LHS. This may further nucleate landslides as observed along the Alakananda valley in the Garhwal Himalaya. After the 28-March-1999 Chamoli earthquake, 56 seismicity-related active landslides occurred in the Garhwal Himalaya (Barnard et al., 2001).
The Bhagirathi River course within the study area is guided by several zone of weaknesses (e.g., strike-slip faults, thrusts, antiformal and synformal hinges). The observed orogenparallel shear although appears ubiquitously within ILHS and OLHS, however, density of their occurrence can roughly be correlated with the regional structures (e.g., Tons Thrust, see Fig.  5).
At present the study lacks direct link between orogen-parallel shears and landslides, earthquake epicenters and regional faults. Nevertheless their close spatial occurrences (Fig. 5) will require more investigations along with GPR studies.
The Ganga basin south to the Himalaya is typically characterized by two depressions (Sarda deep and Gandhak deep) situated in between three established ridges (Delhi-Haridwar ridge, Faizabad ridge and Munger Saharsa ridge; e.g., Sinha et al., 2005;Negi and Thakur, 1989). Based on geomorphic studies, Goswami (2012) observed a direct link amongst sedimentary thickness, basement highs and reactivated basement faults. Basement structures are capable of bending cover rocks and can bend an orogen itself (Goswami, 2012). Such a structure controlling orogen-parallel shear could be possible.

Conclusions
A renewed interest in LHS for hydrocarbon exploration led us to study its paleostress pattern based on hetherto much less described arc-parallel shear from this terrain. The NW LHS in Uttarakhand (India) experienced two distinct phases of Himalayan arc-parallel deformations: (i) orogen-parallel extension (Dext), and (ii) orogen-parallel compression (Dcompr). Paleostress analysis reveals Dext generates an expansion along ~ 131°-311° and Dcompr a shortening along ~ the same direction (133.5°-313.5°). We could not derive temporal relation between these two phases due to the absence of cross-cut evidence in the field, under an optical microscope and inconclusive geochronologic information available in the literature.
Based on previous studies and this work, we understand that the arc-parallel shear continues from the Indus-Tsangpo Suture Zone (ITSZ) up to the Siwalik range. Thus, arc-parallel deformation is much more ubiquotous in the Himalayan belt than previously discussed. We speculate that the Delhi-Haridwar Ridge (DHR) and the Dehradun lineament played a significant role in modifying the strain, seismicity and deformation pattern within the NW Indian LHS at the upper crustal level.
The study proposes collision-induced crustal overthickening escapes rock masses laterallyresulting in arc-parallel extension. The DHR plausibly indirectly acted as a barrier to the lateral flow, may cause the material to pile up against its wall thereby producing arcparallel compression. Onset of orogen-parallel extension ranged from ~ 15-5 Ma (Stockli et al., 2002;Nagy et al., 2015). NW LHS experienced orogen-parallel compression that presumably initiated ~ 4-7 Ma during the NE-SW extension (D3 of Hintersberger et al., 2011). This can mean that the arc-parallel compression is syn to postkinematic to the arc-parallel extension. The new structural geological input obtained from this study will be helpful in tectonic modeling of the terrain, and thefore in petroleum geoscience of this terrain.

Acknowledgements
Part of the Ph.D. thesis of TB. Research Policy Committee members Profs. G.N. Jadhav, P. Naraga and S. Dasgupta (IIT Bombay) provided many critical suggestions. Fieldwork was conducted using IIT Bombay's CPDA grant provided to SM. Associate Editor Tiago Alves and reviewers (Subhobroto Mazumder, Vicor Alania and anonymous) are acknowledged for detail comments in two rounds. Appendix-I: SG2PS: (i) data ID; (ii) group code -'A' to 'I' is accepted for different set of data from the same location (our data are assigned to the same group); (iii) colour code -'0' to '9' can be assigned to plot the groups in different colours; (iv) location details -a minimum of one is required; (v) LOC_X and LOC_Y -coordinates of the locations may or may not be provided; (vi) formation name -not manadatory; (vii) data type -'striae' for slickenside dataset; (viii) DIP_DIR -dip direction of the Y-plane/ C-plane; (viii) DIP -dip amount of the Y-plane/ C-plane; (ix) L_DIP_DIR -rake of the slickenline on the fault/shear plane; (x) L_DIP -geographic direction from which rake is calculated; (xii) slip sense -'up' (reverse slip), 'down' (normal slip); (xi) PALEONORTH -represents previously known north direction (ranging 0°-360°), is left blank; (xii) comments -comments on the data type. We chose Angelier's (1990) inversion technique from the list as the method is able to determine a misfit vector and can correct in between the measured and the calculated shear vector (detail in Sasvári and Baharev, 2014).   , 2005). Isoclinal folds in gneiss-migmatite formation are demarcated as Pre-Himalayan orogen product (Jain et al., 2002 and references therein). In Nepal, rare pre-India-Euresian collisional deformation signatures were found in form of Cambrian-Ordovician granite, which cuts through regional folds and foliation, remain younger than garnet-grade regional metamorphism (Bhargava et al., 2011 and references therein). From the Kumaon HHC Himalaya, Patel et al. (2011) identified pre-Himalayan deformation (D1) along with three Himalayan deformations (D2, D3 and D4). Coaxial reorientation of isoclinal folds (D1) into NE-plunging folds (D2) are rare in nature. However, Moharana et al. (2013) suggested the isoclinal folds are product of progressive Himalayan collisional deformation based on the observations within MCT zone (Madlakia-Munsiari Dhapa section, NE Kumaun). During Eocene Eohimalayan phase NW-SE trending isoclinal folds (D1) presumably resulted due to reactivation of earlier existing structures in THS of western Himalaya (NW India and N Nepal; Aikman et al., 2008). Godin et al. (2018) identified several lineaments trending NW-SE based on Bouguer gravity data belonging to Indian plate extending up to Tibet covered by Himalayan rocks. Analogue centrifuge model predicted the reactivation of such lineaments can produce orogen-parallel extension structures on the cover rock. U-Pb dating of zircon postulates Bhatwari Gneiss (Garhwal Lesser Himalaya, India) was involved in arc-magmatism in the process of forming Columbia Supercontinent (Sen et al., 2018).

Authorship contribution statement
Based on muscovite 40 Ar/ 39 Ar and K-Ar whole rock cooling age, Oliver et al. (1995) implied pre-Himalayan metamorphism reached up to biotite-grade in the Garhwal Himalaya, whereas garnet Sm-Nd dating data from Zanskar Himalaya suggests Pre-Himalayan metamorphism rose to garnet grade (Prince et al., 1998).

Repository 3:
Local structural detail: Based on field observations and cross-cut relations between the structural features, an orthogonal switch in the stress fields trending NE-SW and NW-SE was recorded by the previous authors (e.g., Agarwal and Kumar, 1973;Misra and Bhattacharya, 1973;Jain, 1987;Dubey and Jayangondaperumal, 2005). Agarwal and Kumar, (1973) and Misra and Bhattacharya (1973) recorded first phase of deformation resulting NW-SE trending fold later refolded into NE-SW trending doubly plunged folding (second deformation phase). Saklani (1979) and Shekhar et al, (2006) considered folds developed under NW-SE compression are of third deformation phase. Fourth generation of folding trends E-W in Garhwal Lesser Himalaya (Shekhar et al., 2006).
In contrary, Valdiya (1976), Pant et al., (2012 and Sharma and Bhatt (1990) proposed NE-SW trending folds to be of first deformation product. Sharma and Bhatt (1990) proposed an additional arc-parallel folding in ENE-WSW trend as second generation folding. Sharma and Bhatt (1990) noted a third set of folding with axial plane-oriented WNW-ESE in the form of an arc-parallel folding. Saxena (1974) and Sharma and Bhatt (1990) added another set of deformation in the form of NNW-trending folds.
Bagri (2006a) mapped metabasic rocks layer running south of the MCTL. No sharp contact between the LHS and the HHC is found near Sainj since the MCTL is covered by landslide materials, which connotes the active nature of the MCTL (Bagri 2006a). Two sets of joints are found from the Uttarkashi area, with one striking ~40°NE and the other 130°-140°SE .
Three deformation phases have been worked out in field from Purola near Uttrakashi of unknown timing. These are D1: S1 foliation development; D2: S2 foliation development and flexure slip folding; D3: refolding of veins and chevron folding (Pachauri 2005). Bhatt and Saklani (1990) showed in micro-scale that the quartz grains in the Pratapnagar thrust sheet (=Berinag/Dunda thrust), the Pratapnagar quartzite (=Berinag quartzite) are stretched along ~ NE-SW and underwent progressive plane and constrictional strain. Bhatt and Saklani (1994) further show that rocks on the Pratapnagar thrust were dominantly simple sheared and bear lineations plunging 10-25 0 , whereas those away from the thrust were pure sheared. The Pratapnagar Thrust shows both brittle and ductile shear evidence (Bhatt and Saklani, 1994). Dunda Thrust and Singauni Thrust show opposite sense of shears (Bhatt 1996). From Garhwal Himalaya structures locally show significant variations. For example, the thrust sheets have been affected by four phases of deformation. These are D1: coaxial F1 and F2 folding, F3 folding during D2 shear, and D3 and D4 produced F4 folding and transverse faults, respectively (Bhatt 1996). Saklani (1979) reported NW and SE dip direction of schistosity of the Mukhem Schist. Pandey et al. (2005) identified ~ NE-SW trending 1096 lineaments within the Tehri-Uttarkashi District in the LHS. Landsat images reveal several major lineaments within the Garhwal Lesser Himalayaviz., the Nagaon Lineament that trends 130˚-310˚ parallels the Tons Thrust for 110 km, the 170˚-350˚ trending Tehri Lineament, which continues for ~ 180 km from the Indo-Gangetic plane up to the MCT and across both the ILHS and OLHS (Bharktya and Gupta, 1982). Geomorphic studies by Pandey et al. (2005)  In Tehri dam foundation, the main foliation dips ~ S. Here several shear planes were traced that are either longitudinal or diagonal to the foliation plane (Nawani and Sanwal 1996). Several younger steep faults cut across older thrusts e.g., the Ramgarh Thrust cuts the MCT. Such younger thrusts are the out-of-sequence thrusts (review in . Around 6 mm y -1 arc normal extension from Tethyan NW Himalaya was estimated by Jade et al. (2014) and correlated with the E-W Tibetan extension.

Repository 4:
Landslides: Bernard et al. (2001) predicted ~ 66% of the landslides within the Garhwal Himalaya are initiated due to increase in human interference (e.g. road construction). Heavy construction works involving landslides lead earthquakes in the past (e.g., Kashmir earthquake, 2005;Owen et al., 2008). Kumar and Anbalagan (2016) pointed out reservoir water fluctuation area to be one of the major factors for the increased frequency of landslide around Tehri reservoir.  suggested, movement of Delhi-Haridwar Ridge in response with Indian plate movement triggers the rock mass slumping. The 23-Sept-2003 Varunavat landslide, one of the significant landslides within Garhwal Himalaya, was due to heavy rainfall (Sarkar et al., 2011).
Type of rocks and geologically structure contributes 72% of the landslides occurring within Garhwal Himalaya (Naithani 2007). Greywackes, siltstone, phyllites, limestones and slates are more prone to landslides within Garhwal Himalaya (Pareek et al., 2013). Rautgara Formations are more prone to landslides , whereas Kumar and Anabalagan (2016) pointed out that the Blaini and the Rautgara formation are stable in terms of landslide susceptibility, whereas the Chandpur Formation is highly vulnerable. Mithal (1988) considered that the joint planes and foliation planes with less vegetation made Krol limestone and Nagthat quartzite more landslide prone, compared to rocks of higher metamorphic grade or massive in nature (massive Krol limestone). Saha et al. (2005) suggested bivariate statistical method to be useful in marking regional scale landslide zone, dealing with landslide prone area along structural weak zones, dividing smaller areas from very low to very high susceptibility zones. Repositories 6 and 7 present landslides documented in this study.

Repository 5:
Fundamentals and principles involved in the paleostress studies: Paleostress analysis was introduced in 1970s to decode past stress regimes based on brittle plane data (e.g., Carey and Brunier, 1974). The stress inversion method relies on a few basic assumptions (reviewed in Vanik et al., 2018;Shaikh et al., 2020): (i) stress field under which brittle deformation took place should be homogeneous; (ii) all slip data (attitudes of fault planes and that of the slip lineations) are the product of a single deformation, therefore to run paleostress analysis, manual segregation of heterogenous data set into homogenous subset provides a better control (Kounov, 2011); (iii) deformation does not involve block rotation; (iv) slips along separate fault planes are mutually independent; (v) displacement is small compared to the size of the fault; and (vi) resolved maximum shear stress direction parallels the direction of slip.
The software Structural Geology to Postscript (SG2PS) version 2 (Sasvári and Baharev, 2014) compiles seven paleostress inversion algorithms contributed by various authors into a single software. The software offers a range of different paleostress methods e.g., Turner (1953), Spang (1972), Michael (1984, Angelier (1990), Fry (1999), Shan et al. (2004), and Mostafa (2005). Field dataset can be run using any of these methods -the choice lies with the user. Apart from determining the orientation of principal stress axes, the software also runs Bingham statistics, which calculates the arithmetic average of a processed dataset to produce directional distribution of fractures by generating a density ellipsoid in which the principal stress axes directions are modified as maximum, intermediate, and minimum density directions. The distribution geometry is denoted by the axes length of the density ellipsoid.
The T-TECTO version X5 is a paleostress software (Žalohar and Vrabec, 2007) that applies the Gauss numerical method to perform the inversion operation. Apart from the Right Dihedra Method (RDM), it also provides Visualization of the Gauss Function Method (VGF Method: Žalohar and Vrabec, 2007). This yields a statistical averaged solution for the orientation of principle stress axes and stress ratio (a ratio between normal stress and shear stress on the slip-plane). The VGF method can be utilized for different type of fault planes (Vanik et al. 2018), and additionally all types of fracture planes without visible slip (Dutta et al. 2019).

Blaini Formation (Permo-Carboniferous c / Neoproterozoic j )
Greenish-grey boulder conglomerate layers with sedimentary and low grade metamorphic provenance, along with carbonaceous as well as green-purple shale, grey siltstone, grey and purple dolomite, grey limestone, grey quartzites with non-fossiliferous glaciomarine origin.

BASUL THRUST AND AGLAR THRUST Chadpur Formation/ Dharmandal Group/ Saryu
Greyish green phyllite intercalated with grey and very fine metasiltstone and Muddy quartzite (subgreywacke to sublitharenite) altered with greygreenish and purple color phyllite and green, purple, greyish black color slate. Ripple marks and mudcracks indicate delta deposit.

Deoban Formation (Neoproterozoic e,h )
Thick layers of stromatolite-cherty dolomites altered with quartzite and grey slates deposited under shallow marine condition.