Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya

Journal Pre-proofs Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya Vinee Srivastava, Malay Mukul PII: DOI: Reference:

S1367-9120(19)30444-4 https://doi.org/10.1016/j.jseaes.2019.104092 JAES 104092

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Please cite this article as: Srivastava, V., Mukul, M., Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya, Journal of Asian Earth Sciences (2019), doi: https://doi.org/ 10.1016/j.jseaes.2019.104092

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Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya

Vinee Srivastava, Malay Mukul Continental Deformation Laboratory, Department of Earth Sciences, IIT Bombay, Powai, Mumbai 400076

Abstract Pure strain is an important component of the total displacement vector in fold-thrust belts like the Himalayan orogenic belt. We describe a methodology to compute cataclastic strain from external thrust sheets using the Bootstrapped Modified Normalized Fry Method and the results of its use in two different structural settings in the frontal Himalaya. In the frontal imbricate zone of the Dharan salient in Darjiling Himalaya we measure the highest strain ratios from a central imbricate thrust (T3) and also find that strain decreases away from the fault zones within individual thrusts. Modelling of our results indicates that there was significant vertical flattening in the fault zones while layer parallel shortening related strain dominated the thrust sheets. Also, fault parallel shear decreased away from the fault zones. In contrast, vertical flattening was largely absent in the fault zone associated with the Main Frontal thrust and the MFT sheet in the Mohand Range of the Dehradun recess. Here the strain distribution pattern was consistent with a trishear fault propagation monocline which is our preferred model for the structure of the MFT sheet in the recess. Modelling of our results suggests that fault parallel shear decreased away from the MFT fault zone like in the thrust sheets in the Dharan salient and that the fault propagation folding was accomplished by flexural slip folding. We contend that our methodology can be used effectively to quantify and study the pure strain part of the total displacement vector in external thrust sheets from fold-thrust belts worldwide.

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1. Introduction Crustal evolution in fold-thrust belt (FTBs) settings like the Himalayan orogenic belt involves kinematic analysis of deformation. This requires the quantification of the total displacement vector in individual thrust sheets and their step-wise restoration in balanced cross sections in a foreland-to-hinterland sequence. The total displacement vector consists of rigidbody translation and rotation. In addition, pure strain, which is shape change without any rigidbody deformation, is also recognized as an important component of the total displacement vector in internal and transitional thrust sheets (Mitra, 1994). FTBs typically consist of strongly deformed quasi-plastic (ductile) internal, less deformed elastico-frictional (brittle) external thrust sheets and transitional sheets deformed by both elastico-frictional and quasi-plastic (brittleductile) deformation mechanisms (Bally et al., 1966; Dahlstrom, 1970; Mitra, 1978, 1979; Boyer and Elliott, 1982; Woodward, 1985; Boyer and Mitra, 1988; Yonkee, 1992; Yonkee and Mitra, 1993; Mitra, 1994; McNaught and Mitra, 1999; Mukul and Mitra, 1998; Mukul, 1999; Yonkee and Weil, 2010; Long et al., 2011; Parsons et al., 2016; Srivastava et al., 2018). Computation of pure strain (referred to as strain henceforth), therefore, is an important component of kinematic studies in FTBs as it contributes to more accurate retro-deformation of balanced cross sections by removing internal deformation in addition to translation and rotation in thrust sheets; this also results in a more accurate estimation of wedge taper (Schwerdtner, 1977; Hossack, 1978, 1979; Woodward et al., 1986; Protzman and Mitra, 1990; Mitra, 1994; Mukul, 1998) at different scales (Wu, 1993). Critical wedge theory (Davis et al., 1983; Dahlen, 1990; DeCelles and Mitra, 1995; Mitra, 1997) applications to FTB kinematics suggests that low finite strain in a thrust sheet points to high initial wedge taper. Also, high finite strain indicates internal shortening to attain critical taper during emplacement of thrust sheets (Boyer, 1995; Mitra, 1997).

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1.1. Strain in Fold-thrust belts Strain can be developed in thrust sheets in a FTB in different ways. In FTBs, strain can develop during early stage layer parallel shortening (LPS) prior to thrusting and emplacement of thrust sheets (e.g. Geisser and Engelder, 1983; Gray and Mitra, 1993; Mitra, 1994; McNaught and Mitra, 1996; Ong et al., 2007; Yonkee and Weil, 2009; Long et al., 2011). LPS strain is characterized by near vertical strain ellipse/ellipsoid (Gray and Mitra, 1993) and affects internal, external and transitional thrust sheets. LPS is typically followed by layer or thrust-parallel simple shear, thrust-normal pure shear or a combination of both i.e. sub-simple shear after thrusts develop in the FTB in response to shear failure in the deforming wedge (Coward and Kim, 1981; Sanderson, 1982; Gray and Mitra, 1993; Mitra, 1994; Seno et al., 1998; Mukul and Mitra, 1998; Mukul, 1999; Yonkee, 2005). The resultant strain ellipses/ellipsoids are inclined depending on the amount of shearing in the rocks; strain ellipses/ellipsoids associated with very high shearing are sub-parallel with the shearing direction. 1.2. Strain and Grain Shape Preferred Orientation (GSPO) Deformation of rocks in a fault zone results in an overall grain-size reduction by brittle, brittle-ductile or ductile deformation processes (e.g. Sibson, 1977; Passchier and Trouw, 2005; Blenkinsop, 2000). Strain is typically a characteristic feature of brittle-ductile to ductile shear zones of internal and transitional thrust sheets in FTBs, where deformation mechanism and grainsize reduction is predominantly governed by crystal-plastic or diffusion processes (Mitra, 1984; Mitra, 1994; McNaught and Mitra, 1996; Mukul and Mitra, 1998; Mukul, 1999; Yonkee and Weil, 2010; Long et al., 2011). Unlike brittle fault zones, there is no continuity loss across the ductile shear zone, and shear strain magnitude varies smoothly across the zone (e.g. Marshak and Mitra, 1988). In brittle-ductile to ductile regime, grain supported-preferred orientation (GSPO) form a fabric due to deformation by crystal-plastic processes such as dislocation creep (Passchier and Trouw, 2005). The strength and orientation of the GSPO developed depends on

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finite strain that can be quantified by the strain ellipsoid (Passchier and Trouw, 2005). Elasticofrictional brittle fault zones are largely thought to be strain free. However, cataclastic strain has been reported in the brittle regime (Wojtal, 1989; Ismat and Mitra, 2001; Long et al., 2011; Srivastava et al., 2018). Moreover, GSPO can also develop in the brittle regime during cataclastic flow as a result of disintegration of grains when they slide past each other and rotate during the comminution process. The GSPO can form due to (i) matrix supported cataclastic flow where the clasts embedded in matrix slide past and rotate against each other or (ii) block supported cataclastic flow where movement along network of fractures can make blocks slide past each other (Ismat, 2013). Therefore, GSPO can develop under brittle, brittle-ductile and ductile deformation conditions. GSPO development, irrespective of the deformation mechanism and conditions, systematically changes the spacing between grain centers during deformation (e.g., Mair et al., 2002; Collettini et al., 2009; Ismat, 2013). This can be quantified using a center-to-center nearest neighbor strain determination technique like the Fry method if the grain centers are anti-clustered (Fry, 1979). Strain in elastico-frictional regime or cataclastic strain (Ismat, 2013) could also result during early stage deformation in layer parallel shortening (LPS, Long et al., 2011) as well as subsequent fault related simple/sub-simple shear. 1.3. Strain in the Himalayan Fold-thrust belt In the Himalayan FTB most of the strain work has been carried out in the higher Himalayan high grade metamorphic rocks from internal thrust sheets. Strain has been quantified from the Tethyan, Greater, Lesser and Sub-Himalayan thrust sheets from Garhwal and Bhutan Himalaya (Jain and Anand, 1988; Patel and Jain, 1997; Srivastava et al., 2000; Long et al., 2011). In the western Himalaya Panjal traps from Tethyan sedimentary zone, strain axial ratios ranged from ~1.5 to 5 at the contact of Panjal Traps and Karsha Formation (Patel and Jain, 1997). In the Garhwal Himalaya, strain was computed from Higher Himalayan MCT thrust sheet and central

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crystalline zone using Rf and φ method (Ramsay, 1967; Lisle, 1977; Jain and Anand, 1988; Patel and Jain, 1997; Srivastava et al., 2000). Progressive increase in strain was found from the boundary to the center of ductile shear zone of crystalline rocks of MCT sheet in Bhagirathi valley, Garhwal Himalaya (~1.5-4.5; Srivastava et al., 2000). Strain axial ratios of ~1.5-2.0 were reported from the central crystalline zone and quartzite of the Lesser Himalaya in Tons-SupinRupin Valley and from the Garhwal Himalaya near the base of the MCT and Jutogh thrusts using Rf and φ method (Jain and Anand, 1988). In the Bhutan Himalaya, moderate to high strain were reported from the Lesser to Greater Himalaya (~1.5 to 4.0) in the transport direction due to layer normal shortening (Long et al., 2011). Low magnitude strain (~1.1-1.2) was found from the foreland in the Siwaliks rocks in the Bhutan Himalaya rocks (Long et al., 2011). The strain was computed using the Rf and φ and the Normalized Fry method (Erslev, 1988) and attributed to low-temperature compaction, dissolution of matrix and micro-fracturing during early stage LPS in samples showing feeble preferred orientation. However, 3 out of their 6 Siwalik samples were not oriented and could only be used as minima for X direction (Long et al., 2011). Strain was also reported in the Middle Siwalik sandstones from the Main Frontal thrust (MFT) sheet in the Mohand Range, Dehradun recess which is the frontal most sheet in the NW Indian Himalaya characterized by brittle deformation (Srivastava et al., 2018). We, therefore, concluded that the Main Frontal thrust sheet was the ideal location to systematically study the development of cataclastic strain in an external thrust sheet in more detail. Long et al., (2011) and Srivastava et al., (2018) successfully used the Normalized Fry method to compute strain from Siwalik rocks in the Bhutan and NW Indian Himalaya. This suggests that the Fry method can be used to quantify cataclastic strain in the brittle regime of external thrust sheets like ductile strain from internal and transitional sheets. We present the results of our systematic attempt to study cataclastic strain from Middle Siwaliks sandstone using

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the Bootstrapped Modified Normalized Fry method (McNaught, 1994; 2002) from two different settings in the Indian Himalaya. The aim of this study is (a) to establish a systematic methodology for quantification of cataclastic strain from external thrust sheets that have undergone cataclastic flow in FTBs worldwide and (b) to study cataclastic strain variation in an external schuppen or imbricate zone and the Main Frontal thrust (MFT) sheet in the Indian Himalaya and (c) use the computed finite strain to improve balanced cross sections from the imbricate zone and the MFT sheet. We used Middle Siwalik sandstone samples from the frontal schuppen zone (Mukul, 2000; Kundu et al., 2011; 2012) in the Dharan salient in the Darjiling Himalaya and the MFT sheet in the Dehradun recess in the frontal Garhwal Himalaya (Srivastava et al., 2016; 2018) for our study. 2. Geologic setting The Himalayan boundary is sinuous in plan forming salients and recesses from west to east (e. g. Powers et al., 1998; Singh and Tandon, 2010; Mukul, 2010; Srivastava et al., 2016). In Darjiling Himalaya this sinuosity is marked by Dharan salient-Gorubathan recess pair (Fig. 1) separated by the Gish Transverse Zone (GTZ) (Mukul et al., 2009; Mukul, 2010; Srivastava et al., 2017; Mukul et al., 2018). The Dharan salient lies between Kosi River, eastern Nepal in the west and Gish River, West Bengal in the east (Fig. 1). The Dharan salient and the Gorubathan recess in the Darjiling Sikkim Himalaya contain E-W trending South Tibetan Detachment (STD), Main Central thrust (MCT) and the Ramgarh thrust (RT). However, the Main Boundary thrust (MBT) and Main Frontal thrust (MFT) are blind in the recess (Srivastava et al., 2017 and references therein). The structural mountain front in the Gorubathan recess is marked by RT (Matin and Mukul, 2010 and Srivastava et al., 2017) and the MFT in the Dharan salient (Mukul et al., 2007). The Nahan salient-Dehradun recess pair is separated by the Yamuna Tear fault in the frontal Garhwal Himalaya (Kaushal et al., 2017; Srivastava et al., 2018 and references therein). The MFT fault 6

core and the hanging wall damage zone is exposed locally in the Dehradun recess (Srivastava et al., 2016) and a fault propagation monocline with a frontal sub-horizontal limb and a northeastdipping backlimb is observed in the MFT sheet (Srivastava et al., 2018).

This

work

mainly focuses on the Middle Siwaliks sandstones from both the frontal Dharan salient and the Dehradun recess in the Darjiling and Garhwal Indian Himalaya, respectively. In the Dharan salient, the RT thrusts Precambrian Daling rocks over Permian Gondwana rocks. South of RT, MBT carries Gondwana rocks over Tertiary Siwalik rocks (Heim and Gansser, 1939; Yin 2006; Kundu et al., 2011, 2012; Srivastava et al., 2017). A schuppen/imbricate zone lies in the footwall of the MBT which consists of several imbricates. The South Kalijhora thrust (SKT), which carries Lower Siwalik in its hanging wall over Middle Siwalik in its footwall (Table 1), is the first imbricate south of the MBT (Basak and Mukul, 2000; Mukul, 2000). South of SKT eleven imbricates (T1-T11) were reported (Kundu et al., 2011, 2012) in the Tista River section and five imbricates (T1-T5) were reported in the Lish River section (Kundu et al., 2015). All these imbricates repeated the Middle Siwaliks section so that both the hanging wall and footwall of these faults consisted of Middle Siwaliks rocks (Mukul, 2000). The southernmost exposed imbricate thrust carrying Middle Siwaliks rocks in its hanging wall was referred to as the MFT (Mukul et al., 2007). We collected a total of 13 Middle Siwaliks samples from the frontal schuppen zone for our study (Table 1 and 2). In the Dehradun recess, the MFT thrusts Middle and Upper Siwaliks rocks over the younger Indo-Gangetic alluvium. The Middle Siwaliks sandstone is deformed in the MFT fault core and damage zone and is also present in the frontal part of the MFT sheet. The Middle Siwaliks sandstones also exhibit a gradational contact with the conglomerate dominated Upper Siwaliks. The Middle Siwaliks sandstone is present in the Dehradun recess and we quantified cataclastic strain in this unit from 21 sandstone samples (Table 3). 3. Methodology 7

We carried out geological fieldwork along the Tista and Lish River sections in the Dharan salient, Darjiling Sikkim Himalaya (Fig. 2) to collect bedding orientation data and oriented samples of Siwalik rocks for cataclastic strain analysis. Thirteen oriented samples of Middle Siwaliks sandstones were collected along Tista (TV1-TV7) and Lish (LV1-LV6) River sections in the frontal imbricate zone of the Dharan salient to study the deformation mechanisms and quantify finite cataclastic strain. We sampled the imbricate thrusts at two different elevations (ellipsoidal heights) between 170-239 m and 442-623 m in the Tista and Lish River sections, respectively south of the MBT (Table 1). We collected a single representative sample from each of the SKT, T2, T4, T6 and T8 thrust sheets, 2 samples from T5 sheet and 3 samples from the T1 and T3 sheets depending on the quality of exposure in the study area (Fig. 2; Tables 1 and 2). Similarly, we collected 21 oriented, Middle Siwaliks sandstone samples from two regional transects (Fig. 3; Table 3) in the MFT sheet in the Dehradun recess; this is an expanded sample set that includes 11 samples from Srivastava et al., (2018). These include 6 samples MR1-MR4, MR7 and MR10 (CM1-CM6, respectively in Srivastava et al., 2018) in the central Mohand Range. In the western Mohand Range, we used the 5 samples BB1-BB4 and BB8 as WM1WM5, respectively in Srivastava et al., (2018). Samples KJ1-KJ3 from Khajnawar Rao section were collected from islands in the MFT damage zone (Srivastava et al., 2016) . Samples MR1MR3 and BB1 were collected from the horizontal frontal limb, MR3 and BB2 from the monoclinal hinge and MR4-MR10 and BB3-BB8 from the NE-dipping backlimb of the faultpropagation monocline (Srivastava et al., 2018). Regional transport-parallel (N-S and NE-SW for Dharan salient and Dehradun recess, respectively) thin sections were prepared for our study (Sibson, 1977; Mukul, 1998; McNaught, 1994, 2002; Blenkinsop, 2000; Mukul et al., 2004; Passchier and Trouw, 2005). Assuming plane strain, our thin-sections represented the XZ section (Mitra, 1994) as maximum shortening occurs in Z-direction (Ramsay and Huber, 1989) and best

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deformation features are developed in the XZ during shearing or thrusting (e.g. Fossen, 2010). We then quantified the centroid coordinates and the area (X, Y, A) of each grain from the photomicrographs (e.g. Mukul, 1998; Mukul et al., 2004) using Sigma Scan Pro 4.0 software. We used the centroid coordinates and grain area computed from each of the thin-sections to compute cataclastic strain using the nearest neighbor, center-to-center Bootstrapped Modified Normalized Fry method to compute Rf (axial ratio) and φ (orientation of long axis of ellipse from east axis) with uncertainties (McNaught, 2002). We chose this method as it is more robust than conventional and Normalized Fry methods (Fry, 1979; Erslev 1988. Ersleve and Ge, 1990, McMaught, 1994) and not only computes the strain but also its uncertainty using Bootstrap resampling and eliminates the effect of outlier grains (Tables 2 and 3). Computation of uncertainty is essential to determine the sensitivity of the method and to determine if the differences in the values are statistically significant or not. Rf and φ values were computed successfully for all 13 samples in the Dharan salient and 21 samples in the Dehradun recess (Table 2). Attitude of long axis of ellipses (φ) were computed using Allmendinger’s Stereonet 9.0 software (Allmendinger et al., 2012). We also computed the angle () between the restored strain ellipse long axis and restored bedding pole (vertical) using Stereonet 9.0 (Allmendinger et al., 2012; Cardozo and Allmendinger, 2013) (Tables 2 and 3). Finally, we used StrainSim v.3.6.1 (McEachran et al., 1986, Allmendinger et al., 2012) to model our cataclastic strain results using combinations of horizontal and vertical pure shear and horizontal simple shear (right-lateral or dextral angular shear is treated as positive) in the 34 Siwaliks rocks samples from Dehradun recess and Dharan salient. We have taken the N-to-S or top-to-the-S sense of shear, which is the direction of regional transport in the Dharan salient, as dextral/+ve. Similarly, we have taken NE-to-SW or top-to-the-SW sense of shear as dextral/+ve in the Dehradun recess. We modelled the axial ratio (Rfm) and the angle between restored long axis of the ellipse and pole to bedding plane (θm) for each of the sampled imbricate thrust sheet. 9

Measured finite strain for all 34 Siwalik samples were modelled to gain insight into the possible origin (tectonic or diagenetic) of the finite strain ellipses using combinations of fault parallel simple shear and pure shear involving layer parallel shortening (LPS) and/or vertical flattening (Tables 4 and 5). 4. Results We first discuss our results from the microstructural and cataclastic strain analysis from the Middle Siwalik sandstones in the Lish and Tista River sections from the frontal schuppen zone in the Dharan salient, Darjiling Himalaya. We then describe our results from the MFT thrust sheet in the Dehradun recess, Garhwal Himalaya. 4.1. Dharan salient, Darjiling Himalaya Microstructural analysis of Middle Siwalik samples in the frontal Dharan salient in the footwall of the MBT was mainly done to identify deformation features and their mechanisms. Sub-Himalayan thrust sheets associated with frontal thrusts (e.g. MFT) predominantly exhibited elastico-frictional (brittle) deformation features. Subsequently, we studied the cataclastic strain distribution from the two sections in the frontal schuppen/imbricate zone. 4.1.1. Deformation microstructures in Siwaliks sandstones from the Tista River section Twelve imbricate thrusts (SKT, T1-T11) were reported from the schuppen zone of the Tista River section south of the MBT (Kundu et al., 2011, 2012). The SKT carried the Lower Siwalik rocks over Middle Siwalik sandstones and rest of the 11 imbricates repeated the Middle Siwalik rocks. The Middle Siwalik sandstones in the Tista River section were mainly arkosic ranging from fine to coarse grains with some pebble horizons and consisted of predominantly quartz and some orthoclase, plagioclase, litho-fragments and biotite grains (Kundu et al., 2011, 2012). We studied 6 Middle Siwalik (TV1-TV6) and 1 Lower Siwalik (TV7) sandstone samples (Fig. 2) in detail for microscopic deformation features. The southernmost sample TV1 from the 10

T8 thrust sheet (Table 1) was fractured and had more matrix content than others. Sample TV2 from the T6 fault zone was intensely fractured (Fig. 4). Farther north, TV3 and TV4 were located close to the T5 and T3 fault zones, respectively. Sample TV5 was located in the T3 sheet (Fig. 2; Table 1) and was highly fractured and fine grained (Fig. 5). However, all the three samples (TV3-TV5) exhibited weak to moderate GSPO (Fig. 5). TV6 located south of SKT in the T1 fault zone (Fig. 2) was also fractured (Fig. 4). Northernmost TV7 sample from the Lower Siwaliks sandstones in the SKT sheet (Fig. 2; Table 1) was less deformed and did not show intense fracturing or GSPO (Fig. 5). The Tista River section Middle Siwalik sandstone samples also showed quasi-plastic (ductile) deformation features such as kinked mica (KnM), undulose extinction (Un), re-crystallized grains (RG) and deformation bands (DfB) (Fig. 4). However, these ductile deformation features were randomly distributed. 4.1.2. Deformation microstructures in the Siwalik sandstones from the Lish River section The Lish River section in the Dharan salient consists of five imbricates (BT1-BT5, Kundu et al., 2016) south of MBT. The SKT in the Tista River section was absent in the Lish River section (Kundu et al. 2011, 2012). The Middle Siwalik sandstones in the Lish River section were mainly arkosic and similar to the Tista River section (Kundu et al., 2015, 2016). We first correlated the thrusts in the Tista and Lish River sections and identified Middle Siwalik rocks and renamed the thrusts in the Lish section as T1-T5. Our 6 Middle Siwalik samples (LV1-LV6) along the Lish River section were sampled south to north from the frontal imbricate system that repeated the Middle Siwalik rocks (T1-T5 in Table 1; Fig. 2). The southernmost sample (LV1) in the fault zone of the frontal most imbricate (T5) in Middle Siwalik was matrix dominated and contained fractured clasts. Farther north, sample (LV2) was located in the T4 sheet (Fig. 2; Table 1) and had less matrix (Fig. 5) compared to LV1. Sample LV3 from the T3 thrust sheet (Fig. 2; Table 1) was fractured (Fig. 4). The grains in LV3 were marginally coarser than both LV1 and LV2 and exhibited a weak GSPO (Fig. 5). The LV4 sample from the T2 thrust sheet (Fig. 2;

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Table 1) was fractured (Fig. 4) with a weak GSPO (Fig. 5). The northern most LV5 and LV6 samples from the T1 sheet were deformed with fractures (Fig. 4) and showed a weak GSPO (Fig. 5). Elastico-frictional (brittle) deformation features, such as intragranular fractures (IF) within the grain and intergranular fractures (INF) that cut-across several grains, were found in all Middle Siwalik samples (LV1-LV6) with varying intensity (Fig. 4). Apart from the brittle features, some randomly distributed ductile features i.e. re-crystallized grains (RG), undulose extinction (Un), kinked-mica (KnM), deformation bands (DfB) and sub-grain rotation (SGR) were also found in the Middle Siwalik samples (Fig. 4). 4.2. Finite strain in imbricate thrust sheets from frontal Dharan salient Microstructural analysis results confirmed brittle deformation in the Middle Siwalik sandstones from the frontal schuppen zone and the presence of GSPO. We quantified finite cataclastic strain (Rf, φ) associated with the GSPO from the sandstone thin-sections using the Bootstrapped Modified Normalized Fry Method (McNaught, 2002). 4.2.1. Tista River section Finite strain values (Rf) in the Middle Siwalik rocks from the Tista River section ranged from 1.19 to 1.87 with uncertainties in the second decimal place. Rf gradually increased and then decreased from south to north (Tables 1 & 2; Fig. 6). The frontal most sample TV1, located half a kilometer from the mountain front near Sevok, in the T8 thrust sheet had a Rf of 1.32±0.03. Farther north, Rf increased to 1.52±0.05 and 1.78±0.06 in samples TV2 and TV3 in T6 and T5 fault zones, respectively (Tables 1 & 2). Highest Rf value of 1.87±0.04 was obtained from TV4 in the T3 fault zone. Farther north in the T3 sheet, Rf decreased to 1.76±0.06 in TV5. In the T1 sheet, Rf for sample TV6 was 1.49±0.05 near the T1 fault zone. The northernmost sample TV7 collected from the Lower Siwalik sandstones in the SKT thrust sheet measured the lowest Rf of 1.19±0.01 in the Tista River section (Tables 1 & 2; Figs. 2 & 6). In the Tista section, φ varied

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from 58º - 113º and the angle between pole to restored bedding and the long axis of the strain ellipse () varied from 38º - 89º in different directions (Table 2). 4.2.2. Lish River section Finite cataclastic strain values (Rf, φ) vary in the Middle Siwalik samples from the Lish River section south of the MBT in the frontal schuppen zone (Tables 1 and 2; Fig. 5). Axial ratios (Rf) varied from 1.26 to 1.59 with uncertainties in the second decimal place (Fig. 6, Table 2). Rf values for the southernmost sample, LV1 was 1.33±0.05 near the T5 fault zone (Fig. 2). Farther north, LV2 had a Rf of 1.26±0.02 in the T4 sheet near the T3 imbricate thrust ~3 km from the mountain front (Figs. 2 and 6; Tables 1 and 2). High Rf of 1.47±0.05 was measured in the T3 sheet from LV3. Rf in the T2 sheet proximal to the fault zone was 1.26±0.03 in LV4 (Fig. 6; Table 2). The highest Rf of 1.59±0.05 was seen in LV5 near the T1 fault zone. A marginally lower Rf of 1.46±0.05 was measured in LV6 in the T1 sheet (Figs. 2 and 6; Table 2). In the Lish section, φ varied from 63º - 96º and the angle between pole to restored bedding and long axis of the strain ellipse () varied from 7º - 89º in different directions (Table 2). 4.3. Dehradun recess, Garhwal Himalaya We collected 21 oriented, Middle Siwaliks sandstone samples from two regional NE-SW transport parallel transects (in areas BB, KJ and MR in Fig. 3) in the MFT sheet in the Dehradun recess. Microstructural analysis of the Middle Siwalik samples in the MFT sheet was carried out to identify deformation features and their mechanisms. This was followed by cataclastic strain study on the 21 samples we collected from the two sections in the MFT sheet. 4.3.1. Deformation microstructures in the MFT sheet Mohand Range We observed predominantly brittle deformation microstructures such as intragranular fractures (IF) within individual grains and intergranular fractures (InF) that cut across more than one grain (Fig. 7). We also found that sandstone samples exhibit a weak grain shape-preferred orientation (GSPO) of quartz grains in the NE-SW transport plane (Fig. 7). Furthermore, we also 13

observed ductile deformation features such as deformation bands (DfB), undulose extinction (Un), and recrystallization (RG) in the quartz grains, and kinked micas (Fig. 7). Interestingly, these ductile microstructures were randomly distributed relative to the MFT fault zone in the individual thin sections. Nevertheless, the observed grain shape-preferred orientation of quartz grains near the MFT suggested fabric strain. 4.3.2 Finite strain in the MFT sheet Mohand Range Finite strain (Rf , φ) measurements in Middle Siwalik sandstones record variable Rf and φ in Mohand Rao (samples MR1-MR10) and Khajnawar Rao (samples KJ1-KJ3) in the central Mohand area and Badhshahibag Rao (samples BB1-BB8; Fig. 3) in the western Mohand Range. Axial ratios of the finite strain ellipses (Rf) varied from 1.2 to 1.7 and 1.3 to 1.6 in the central and western transects, respectively (Table 3; Fig. 8). In Central Mohand, in the Mohand Rao transect, the highest Rf (1.73) occurred ~0.5 to 2.5 km from the range front in the MFT damage zone (MR1) in the preserved horizontal limb of the monocline containing meter-scale folds (MR2) (Srivastava et al., 2018). The Rf value decreased from MR2 (1.69) to MR3 Rf (1.22) over the next ~0.1 km farther northeast near the monoclinal hinge and also farthest from the trishear zone or the MFT (MR3; Table 3; Fig. 8). Beyond 2.6 km distance, the Rf values were intermediate in the monoclinal NE-dipping limb (Srivastava et al., 2018) and ranged from 1.3-1.5 (MR4MR10) (Table 3; Fig. 8). We also sampled Middle Siwalik rocks from islands in the MFT damage zone in the Khajnawar Rao transect where the MFT fault zone was best exposed (Srivastava et al., 2016). The Rf values in the islands from Khajnawar Rao varied between 1.2 and 1.45 (Table 3; Fig. 8). In the western Mohand Range, the frontal part of the structure was eroded. Our southernmost sample (BB1) was collected from a SW-dipping bed in the remnants of the eroded frontal monocline containing meter-scale folds (Table 3; Fig. 8). This sample (BB1) had a Rf of 1.6 followed by 1.3 for the next sample (BB2) near the monoclinal hinge farther to the northeast. The rest of the samples (BB3 to BB8) were collected from the monoclinal

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NE-dipping limb with Rf ranging from 1.4-1.6 (Table 3; Fig. 8) with the exception of BB6 with Rf of 1.3. The angle φ varied from 59º-119º and the angle between pole to restored bedding and long axis of the strain ellipse () ranged from 03º-86º between N and E directions in the central Mohand Range. The angle φ varied from 71º-136º and the angle  ranged from 19º-70º between N and S directions in the western Mohand Range (Table 3). 5. Discussion Microstructural study of all the Siwaliks sandstone samples reveal elastico-frictional (brittle) and quasi-plastic (ductile) microstructural deformation features in both the Dehradun recess and the Dharan salient. However, random distribution of ductile deformation features from the sandstones in both Dharan salient and Dehradun recess suggests that they were inherited from the high grade rocks in the FTB (Figs. 4 and 7; Long et al., 2011; Srivastava et al., 2016). We observed intense fracturing, grain size reduction, inter-granular and intra-granular microfractures cutting across several and within individual grains, respectively due to frictional sliding in the Siwaliks sandstones (Sibson, 1977; Blenkinsop, 2000). These brittle features were more dominant in samples that were in the proximity of fault zones related to thrusts. We interpret, therefore, that the Middle Siwaliks sandstones were deformed under near-surface, elastico-frictional (brittle) conditions. Some of our samples exhibited GSPO under these conditions. The grain shape preferred orientation (GSPO) in the quartz grains from the Middle Siwaliks sandstones was not random like the quasi-plastic features in both the Dharan salient and the Dehradun recess. Presence of GSPO in Siwalik rocks is intriguing and, if not inherited from a higher provenance within the FTB, must be systematically developed due to cataclastic flow under brittle conditions. GSPOs are typically recognized as ductile features formed from alignment of flaky-minerals due to deviatoric stress under ductile deformation conditions (Fossen, 2010). However, the presence of GSPO in rocks that deformed at shallow crust level has been attributed to cataclastic flow (Ismat, 15

2013). Cataclastic flow (Engelder, 1974; Sibson, 1977; Paterson, 1978; Tullis and Yund, 1987; Evans et al., 1990; Babaie et al., 1991; Hirth and Tullis, 1994; Cladouhos, 1999; Blenkinsop, 2000; Rawling and Goodwin, 2003, Ismat, 2013) can form centimeter to kilometer scale folds in upper crust due to frictional sliding along the set of fractures or faults (Marshak et al., 1982; Laubach, 1988; Ismat and Mitra, 2005) and are either block-supported or matrix supported (Ismat, 2013). Block-supported cataclastic flow involves meso-scale fault / fracture bounded blocks sliding past each other along sets of faults/fractures. Matrix-supported cataclastic flow occurs in fault zones where grain size reduction is dominant and facilitates progressive decrease of block or clast size (Ismat, 2013) or in sandstones having a large matrix content to begin with. Therefore, matrix-supported cataclastic flow is likely in matrix-dominated (LV6, TV1) and MFT fault damage zone samples (KJ1). Block-supported cataclastic flow dominated in all the other samples in the Dharan salient and Dehradun recess. We, therefore, interpret that the GSPO in Siwaliks rocks seen in the frontal schuppen belt in the Dharan salient and the MFT sheet in the Dehradun recess formed due to cataclastic flow as they show widespread brittle deformation features. This is further corroborated by the fact that the samples were taken from sheets carried by thrusts that sole to the basal decollément (MHT) at a depth of 550 m (cf. fig. 1 Kundu et al., 2012) and 5-6 km (Srivastava et al., 2018) in the Dharan salient and Dehradun recess, respectively. We further tested our interpretation through systematic measurements of cataclastic strain using the Bootstrapped Normalized Fry method (McNaught, 2002) from the Dehradun recess and the Dharan salient in the Garhwal and Darjiling Indian Himalaya, respectively. 5.1. Cataclastic strain in the frontal schuppen belt in the Dharan salient The Middle Siwaliks sandstone samples from the lower (170 to 239 m) Tista River valley section appears more deformed as the Rf values range from 1.32-1.87 compared to the 1.26-1.59 in the higher (442-623 m), ridge-top, Lish River section (Tables 1 and 2). The restored bedding pole-long axis of cataclastic strain ellipse angle () varies between 38-89º in the Tista section 16

compared to 07-89º in the Lish section. This indicates that, in addition to LPS and fault-parallel shear (which results in variable ), layer-perpendicular flattening (causing  > 50º but low Rf) has also affected the imbricate thrusts (e. g. Mukul, 1999). The Tista samples appear to be sheared more than the Lish samples as they have higher Rf and lower  variability (Tables 1 & 2). TV7 was the northern most sample in the SKT sheet in the Dharan salient. In the hanging wall of the SKT (sample TV7), which was last active around ~40 ka (cf. Table 1 of Mukul et al., 2007; Table 1), we measured the minimum cataclastic finite strain (Rf = 1.19). However, we also measured  = 69ºS in TV7 which indicates top-to-the NW shear. Our preferred StrainSim3.6.1 model for the measured cataclastic finite strain in the SKT sheet was 7º sinistral fault-parallel shear followed by 6.5% vertical (pure shear) flattening; this resulted in a modelled axial ratio Rfm of 1.20 and m of 68.84ºS. As the SKT sheet is folded into upright antiforms and synform (Mukul, 2000; Basak and Mukul, 2000) and the sample was collected from a south easterlydipping bed, we interpret the top-to-the-NW, sinistral shearing to be related to layer-parallel shearing during folding followed by vertical flattening (Table 4). Cataclastic strain analysis in the T1 sheet, south of the SKT (Fig. 2; Table 1), from samples TV6, LV5 & LV6 measured Rf values ranging from 1.46-1.59 (± 0.05) and the angle between the restored bedding pole and long axis of cataclastic strain ellipse () from 37-63º. The T1 sheet accommodated more cataclastic strain (1.5-1.6) than the SKT (1.19) and a marginally greater fault-parallel shear as  = 58º-63ºN in samples (TV6 and LV5) near the T1 fault zone. Our preferred StrainSim3.6.1 model for the measured cataclastic finite strain in the T1 sheet near the fault zone in LV5 was 23.5º dextral, top-to-the-SE, fault-parallel shear followed by 5% vertical (pure shear) flattening; this resulted in a modelled Rfm = 1.59 and m = 57.46ºN. Similarly, in TV6 18º dextral, top-to-the-SW, fault-parallel shear followed by 9% vertical (pure shear) flattening results in a modeled Rfm = 1.48 and m = 63ºN. Away from the T1 fault zone 17

(LV6), the strain (Rf = 1.46) was marginally lower and  = 37ºSW reflected top-to-the-NE shear in northerly-dipping beds (Tables 1 & 2). We modelled this using 8% LPS (Mitra, 1994) and 17.5º horizontal sinistral fault-parallel shear in StrainSim3.6.1. This resulted in modelled Rfm of 1.46 and m of 36.85ºS suggesting that the vertical flattening was dominant closer to the fault zone and LPS away from it in the T1 sheet. Conjugate fracturing and faulting was widely observed in the T1 sheet and we interpret that southerly-dipping faulting with top-to-the-NE shear was probably responsible for the  = 37ºS observed in the T1 sheet. The T1 fault was also found to be active around ~20 ka as an out-of-sequence thrust in the Tista Valley (cf. Table 1 of Mukul et al., 2007; Table 1). Sample LV4 was the only sample collected from the T2 sheet near the T2 fault zone just south of the T1 sheet (Fig. 2). Sample LV4 accommodated lesser cataclastic finite strain than T1 (Rf = 1.26) but  = 89º suggested a significant component of vertical flattening or near pure shear deformation. Our preferred StrainSim3.6.1 model for the measured cataclastic finite strain in sample LV4 located near the T2 fault zone is 0.46º sinistral or top-to-the-NE fault-parallel shear followed by 12% vertical (pure shear) flattening; this resulted in a modelled Rfm of 1.255 and m of 89ºS (Table 4) supporting the interpretation that vertical flattening is present close to the fault zones in the imbricate thrusts. Alternatively, the observed strain may also be mainly diagenetic in origin because of  = 89º. We collected three samples from north-dipping beds in the T3 sheet from the T3 fault zone (TV4), in the T2 footwall proximal to T2 fault zone (TV5) and the T3 sheet (LV3) (Table 1). We measured Rf = 1.87 and  = 72ºS in TV4 (Table 2) and modelled this using 21.5º sinistral or top-to-the-NW fault parallel shear followed by 22.8% vertical flattening in StrainSim3.6.1; this resulted in modelled Rfm of 1.87 and m of 71.93ºS (Table 4). Farther north, in the T3 sheet, LV3 measured Rf = 1.47 and  = 71ºS and was modelled by 13.5º sinistral or top-to-the-NW fault-parallel shear followed by 14.5% vertical flattening in StrainSim3.6.1; this resulted in 18

modelled Rfm of 1.47 and m of 71.21ºS (Table 4). Conjugate fracturing and faulting was also widely observed in the T3 fault zone and the thrust sheet. We, therefore, interpret that southdipping faulting with top-to-the-NW shear was responsible for the  = 71-72ºS observed in T3 fault zone and the sheet and both the shearing and the vertical flattening decreased away from the fault zone. Farther north in the T3 sheet near the T2 footwall (TV5), we also measured Rf = 1.76 and  = 77ºN and modelled this by 14.5º dextral or top-to-the-SE fault-parallel shear followed by 26.3% vertical flattening in StrainSim3.6.1; this resulted in modelled Rfm of 1.76 and m of 77.15ºN (Table 4). We interpret the increase in flattening and fault-parallel shearing in TV5 compared to LV3 to be the result of proximity to the T2 fault zone. Presence of both topto-the-NW and SE fault parallel shear and  = 71-72ºS as well as 77ºN in the T3 sheet supports our field observation of conjugate fracturing and faulting in the T3 sheet (Tables 1, 2 & 4). Sample LV2 from the T4 sheet measured Rf = 1.26 and  = 07ºN in north-dipping bed indicating that the finite cataclastic strain in the Lish River section of the sheet was dominantly LPS. We modelled 10.7% LPS (Mitra, 1994) accompanied by 2.6º dextral or top-to-the-SE fault parallel shear which resulted in Rfm of 1.26 and m of 7ºN. The T4 fault was also found to be active around ~45 ka in the Tista Valley (cf. Table 1 of Mukul et al., 2007; Table 1). As T4 is associated with a significant topographic break and is emergent, T4 was mapped as the MFT in the Dharan salient (Mukul et al., 2007; Tables 1, 2 & 4). The cataclastic strain in the sample TV3 from the T5 fault zone (Table 1) was high Rf = 1.78 but  = 89ºNW in north-dipping beds (Table 2) indicates a significant component of vertical flattening or near pure shear deformation in the Tista section in this sheet. We modelled this by 1.25º dextral or top-to-the-SE fault parallel shear followed by 33.3% vertical flattening in StrainSim3.6.1; this resulted in modelled Rfm of 1.78 and m = 89ºN (Table 4). However, in the Lish section (LV1) Rf = 1.33 and  = 77ºE from south-dipping bed of meter-scale folds suggests comparatively lesser vertical flattening and increased fault parallel shear. Our model for LV1 19

invokes 7.25º top-to-the-W fault parallel shear followed by 13% vertical flattening in StrainSim3.6.1; this resulted in modelled Rfm of 1.33 and m of 76.84ºE (Table 4). This suggests greater flattening (33.3%) in the Tista section (TV3) compared to the 13% flattening in the Lish section (LV1) in the T5 sheet probably due to greater overburden in the Tista section (Tables 1, 2 & 4). In the T6 sheet, sample TV2 measured Rf = 1.52 and  = 54ºNE close to the T6 fault zone from north-dipping beds (Tables 1 & 2). Our preferred StrainSim3.6.1 model for TV2 invokes 22º dextral or top-to-the-SW fault parallel shear followed by 2.5% vertical flattening; this resulted in modelled Rfm of 1.51 and m of 54.25ºN (Table 4). Our southernmost sample TV1 from north-dipping bed in the middle of the T8 sheet measured Rf = 1.32 and  = 38ºS (Tables 1 & 2). We modelled this in StrainSim3.6.1 by 5% LPS accompanied by 14º sinistral or top-to-the-NE sinistral fault parallel shear which resulted in Rfm of 1.32 and m of 38ºS (Table 4). We also interpret that south-dipping conjugate faulting with top-to-the-NE shear was probably responsible for the  = 38ºS observed in the T8 sheet (Table 2). Our strain analysis, and subsequent StrainSim3.6.1 modelling of our results, suggests that the Middle Siwalik rocks were subject to a maximum of 10.7% LPS (LV2 from T4 sheet). We also modelled LPS of 5% and 8% in the frontal T8 (TV1) and T1 (LV6) sheets. An upper bound of 11% LPS in the frontal schuppen zone appears to be reasonable considering that the LPS in LV2 was modified by only 2.6º top-to-the-SE fault parallel shearing related to T4. We, therefore, removed 11% LPS between the mountain front and the SKT from the balanced cross section in the Tista valley section (Fig. 9). This increased the minimum shortening estimate south of the SKT in the Dharan salient from 2.5 km (Kundu et al., 2012) to 2.8 km. Fault-parallel shear models in the imbricate thrust fault zones and sheets varied between 1.25°-23.5° top-to-the-south to 0.5°-21.5° top-to-the-north on north-dipping imbricate thrusts and south-dipping faults conjugate to the thrusts observed in the schuppen zone, respectively. In 20

addition, two samples were also taken from south-dipping limbs of meter-scale folds in the SKT (TV7) and the T5 (LV1) sheets. These were modelled by ~7º top-to-the-north (TV7) and top-tothe-west (LV1) bedding-parallel slip in the folds. Layer-perpendicular flattening ranging from 2.5-33.5% was also modeled in all samples except LV2, LV6 and TV1 where LPS was measured. The flattening percentage was highest close to the fault zones (TV3, TV4 & TV5) of the imbricate thrusts and decreased into the thrust sheets (Tables 1, 2 & 4). Thrust sheets T1, T3 and T5 contained samples from both the Lish and Tista River sections. In T1, samples LV5 and TV6 had almost identical cataclastic strain (Rf =1.5) that was modeled by 18º-24º top-to-the-south fault-parallel shear and 5-9% layer-perpendicular flattening. In T3, the Tista sample TV4 was close to T3 fault zone and was modelled by 21.5º top-to-north fault-parallel shear which was higher than TV5 in the footwall of T2 (top-to-south 14.5º) and LV3 in the middle of the T3 sheet (top-to-north 13.5º). The layer-perpendicular flattening was comparable at 23% and 26% in TV4 and TV5 respectively and higher than LV3 (15%) (Tables 1, 2 & 4). In T5, the Tista sample TV3 had a lower modelled fault-parallel shear (1.25º) than LV1 (7.25º) but much higher layer-perpendicular flattening (33% compared to 13%). Therefore, in general it appears that the Tista samples were more deformed than the Lish samples in the frontal schuppen zone in the Dharan salient of the Darjiling Himalaya. Available dates (cf. Table 1 of Mukul et al., 2007; Table 1) indicate out-of-sequence deformation at 20 ka in the T1 sheet which may also explain the high finite cataclastic strain values (Rf = 1.5-1.6) and modelled fault-parallel shear (17-24º) in the sheet (Tables 1, 2 & 4). Our results in the Dharan salient suggest (i) cataclastic flow related strain exists in the imbricate thrust sheets in the frontal schuppen/imbricate belt (ii) measured strain values within an individual thrust sheet show consistency both in Rf and  and decrease away from the fault zone into the thrust sheet (iii) the strain values in the central thrust sheet T3 in the imbricate zone is highest possibly because of out-of-sequence reactivation of T3. This suggests that systematic 21

quantification of cataclastic strain from external thrust sheets that have undergone cataclastic flow in FTBs is possible and this methodology can be used in other fold-thrust belts. 5.2. Cataclastic strain in the Main Frontal thrust in the Mohand Range, Dehradun recess We mapped a fault-related monocline in the MFT sheet (Srivastava et al., 2018) in the Mohand Range, Dehradun recess. Our preferred fault-related fold model for MFT sheet in the Mohand Range is trishear fault-propagation folding (Srivastava et al., 2018; Fig. 10). In this model high Rf values are seen in and near the trishear zone, low values outside the trishear zone, and intermediate values in the dipping backlimb of the fault propagation fold (Allmendinger, 1998). We systematically sampled the fault propagation monocline developed in the MFT sheet (Srivastava et al., 2018) for strain measurements. Extreme grain size reduction in the MFT core or the trishear zone produced incohesive fault gouge (Srivastava et al., 2016) where sample collection for strain measurements was not possible. We, however, collected samples from the preserved islands in the MFT fault damage zone. We observed a systematic trend in Rf values from southwest to northeast along the transport direction (1.2 to 1.7 in Figs. 3 & 8; Table 3) in the MFT sheet. Our samples (KJ1-3, MR1, MR2) were collected from the MFT fault damage zone and were also intensely fractured. Our highest strain values (Rf = 1.6-1.7; MR1-2, BB1) were from the MFT damage zone where meter-scale folds were also observed. The lowest (Rf = 1.2-1.3; MR3, BB2) values were from near the monoclinal hinge. However, we also found intermediate (Rf = 1.3-1.6; MR4-MR10, BB3-BB8) values in samples from the monoclinal NE-dipping limb (Table 3; Fig. 8). Our results are largely consistent with the Rf distribution pattern predicted by the trishear fault propagation model. However, our results also indicate that the trishear zone in the Mohand Range has gradational, rather than discrete boundaries and that strain, along with the meter-scale folding, decreases away from the MFT fault core in the damage zone and the MFT sheet. Our results also imply that monoclinal, back-limb tilting occurred by frictional sliding and block-supported

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cataclastic flow. We observed that the angle between restored bedding poles and the cataclastic strain ellipse long axis () ranged from 3°-86° in the Mohand Range (Table 3). This indicates (a) varying degrees of rotation in the Middle Siwalik sandstones in the MFT sheet possibly due to shear strain and (b) confirms our model of a gradational trishear zone related deformation. In summary, these results confirm that MFT-related shearing in the Mohand Range caused grainsize reduction by elastico-frictional matrix- and block-supported cataclastic flow during fault propagation folding in the MFT sheet. 5.2.1 Finite strain in the MFT damage zone The sample (KJ1) from an island in the gouge dominated part of the MFT fault damage zone in Khajnawar Rao (Srivastava et al., 2016) measured Rf = 1.2 ± 0.02 and  = 3ºE (Figs. 3&8; Table 3) which suggested dominantly LPS strain in the island as the finite strain ellipse was almost perpendicular ( = 3ºE) relative to the restored bedding. StrainSim3.6.1 modelling of KJ1 indicated a LPS of 8.8% and a low (9º) dextral or top-to-the-SW fault-parallel shear (Table 5). The samples (KJ2 and KJ3) were collected from Middle Siwaliks sandstones samples from the rock dominated part of the MFT damage zone (Srivastava et al., 2016). Rf of 1.4 and a variable  (14º-77ºN) points to variable fault parallel shear in the samples. StrainSim3.6.1 modelling of KJ2 indicated a LPS of 15% and a low (7º) dextral or top-to-the-SW fault-parallel shear (Table 5). Our results indicate absence of layer-perpendicular flattening in the damage zone samples. This is consistent with our field observations of the MFT fault zone in which the fault rocks were seen capped only by Quaternary gravels. LPS dominated the strain in KJ1 and KJ2 whereas fault parallel shear was dominant in KJ3. 5.2.2 Finite strain in the frontal horizontal limb of the fault propagation monocline In the frontal horizontal limb of the fault propagation monocline in the central and western Mohand Range, respectively samples MR1, MR2 and BB1 measured Rf = 1.6-1.7 and  variable between 19º-54ºN & S (Table 3; Fig. 8) pointing to variable fault parallel shear in the 23

frontal limb of the fault propagation monocline. StrainSim3.6.1 modelling of MR1 indicated a LPS of 21% and a 12.5º dextral or top-to-the-SW fault-parallel shear (Table 5). BB1 was modelled by a LPS of 12% and a 19º sinistral or top-to-the-NE fault-parallel shear and MR2 by a LPS of 0.1% and a 28º dextral or top-to-the-SW fault-parallel shear (Table 5). Our results also point to absence of layer-perpendicular flattening in the frontal horizontal limb of the fault propagation monocline in the MFT sheet like the MFT fault damage zone. Near the hinge zone of the monocline, samples MR3 and BB2 measured Rf = 1.2-1.3 and  variable between 14º-46ºN (Table 3; Fig. 8). We also noted that the sample MR3 measured the lowest Rf (1.22) and  (14°E) (Table 3; Fig. 8). It is also located farthest (2.5 km) from the MFT fault zone in the horizontal limb of the fault propagation monocline. Therefore, the MR3 sample is least affected by fault parallel shearing and, consequently, is the most representative of the layer parallel shortening (LPS) related strain in the fault propagation structure in the Mohand Range. We modelled the MR3 strain using 8.7% LPS and 4.5º dextral or top-to-the-W simple shear and the BB2 strain using 1.5% LPS and 15.5º dextral or top-to-the-SW simple shear (Table 5) using StrainSim3.6.1. The result from the MR3 sample was also corroborated by sample KJ1 that indicated a LPS of 8.8% and a 9º top-to-the-SW fault parallel shear. Therefore, LPS strain of ~9% was removed from the balanced cross sections constructed in the Mohand Range (Srivastava et al., 2018). This resulted in restoration of an additional ~ 3 km layer parallel shortening in the central Mohand Range and increased the total shortening in the central Mohand Range to 9 km (or 20%) (Srivastava et al., 2018; Fig. 10). 5.2.3 Finite strain in the NE dipping back limb of the fault propagation monocline Measured Rf varied between 1.3-1.6 in the samples (MR4-MR10 and BB3-BB8; Table 3 and Fig. 8) from the NE-dipping back limb of the fault propagation monocline in the Mohand Range. Bedding-parallel shear ranging from 3.7-21.5º dextral or top-to-the-SW was modelled by StrainSim3.6.1 in samples from the north-dipping limb of the monocline (MR4-MR10, BB324

BB7). The only exception to this trend was BB8 in which we modelled the bedding-parallel shear to be 12º sinistral or top-to-the-NE. Layer-parallel slip in the north-dipping limb of the monocline indicated that the fault propagation folding in the MFT occurred by flexural-slip folding. We also modelled layer perpendicular shortening or vertical flattening in samples MR10, BB5 and BB7. This is best explained by the fact that these samples were close to the Upper SiwalikMiddle Siwalik lithocontact. The Upper Siwalik unit consists of conglomerates which contains a high matrix percentage characteristic of the upper part of the Middle Siwalik section. Therefore, the deformation mechanism is likely to be matrix-supported cataclastic flow in the basal Upper Siwalik as opposed to block-supported cataclastic flow in the Middle Siwalik section which is dominated by sandstones. Therefore, we interpret that our StrainSim3.6.1 modelling indicates that samples MR10, BB5 and BB7 have been subjected to 13-16% vertical flattening in addition to bedding-parallel slip. In contrast to the Dharan salient, vertical flattening appears to be absent in rest of the samples (including KJ2 and KJ3 from the MFT fault zone) indicating that the fault propagation folding occurred under near-surface conditions without significant overburden. Our results from both the Dharan salient and the Dehradun recess record systematic variation in finite strain ratios within individual as well as across multiple thrust sheets. This implies that our methodology can be used as a new method to systematically study cataclastic strain from rocks exhibiting GSPO in external thrust sheets like quasi-plastic strain has been traditionally studied from internal sheets in FTBs worldwide. In the Himalayan FTB, our methodology opens up the possibility of quantification of cataclastic strain from the Siwalik sandstones. Our methodology, therefore, can be applied to quantification of cataclastic strain in addition to translation and rotation components of the total displacement vector in the kinematic analysis of FTB deformation from external thrust sheets from FTB worldwide. This also implies

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that GSPO in rocks can be effectively used from internal, transitional as well as external thrust sheets to quantify pure strain from FTBs resulting from dislocation creep or cataclastic flow. 6. Conclusions We describe a new methodology to quantify cataclastic strain from GSPO observed in the Middle Siwalik sandstones from the Himalaya deformed by cataclastic flow. We have validated this methodology using samples from (i) the frontal schuppen zone in the Dharan salient along Tista and Lish River sections in the Darjiling Himalaya, (ii) the fault propagation monocline in the MFT sheet in the Mohand Range of NW Himalaya. Major outcomes of this study are as follows: 

Microstructural analysis shows presence of dominantly brittle or elastico-frictional deformation features and grain shape preferred orientation (GSPO) in the Middle Siwalik sandstones from the frontal Himalaya.



GSPO in Middle Siwalik rocks is best explained by matrix- and block-supported cataclastic flow as all the samples were collected from thrust sheets deformed in the elastico-frictional or brittle deformation regime.



GSPO induced cataclastic strain was quantified successfully using the Bootstrapped Modified Normalized Fry method. Our results from both the Dharan salient and the Dehradun recess record systematic variation in finite strain ratios within individual as well as across multiple thrust sheets.



In the Dharan salient, higher strain values (Rf = 1.19-1.87; φ = 58°-113°) were obtained from samples collected from the Tista valley compared to samples collected along the ridge in the Lish River section (Rf = 1.26-1.59; φ = 63°- 96°) as the imbricate thrust sheets were sampled at different structural and topographic elevations. T3 sheet recorded the highest strain among all the thrust sheets. Modelling of measured cataclastic strain results using StrainSim3.6.1 revealed superposition of LPS, fault-parallel simple shear 26

and bedding-perpendicular flattening in the imbricate thrust sheets repeating the Middle Siwalik section in the Dharan salient. In individual thrust sheets strain and vertical flattening decreased away from the fault zone. 

In the Dehradun recess, maximum finite strain (Rf = 1.6-1.7) was measured close to the MFT fault zone. The minimum strain (Rf = 1.2-1.3) was obtained near the fault propagation monoclinal hinge. Intermediate strain (Rf = 1.4-1.6) was measured in the NE-dipping limb of the monocline. This finite strain pattern was consistent with our preferred trishear fault propagation fold model for the MFT related structure in the Mohand Range.



Modelling of our results in the Dehradun recess indicate that fault parallel shear decreased away from the MFT fault zone. Bedding parallel shear was also modelled in the NE-dipping limb of the monocline suggesting that the fault propagation folding involved flexural-slip folding.



This work effectively allows the quantification of pure strain part of the total displacement vector of deformation in internal, transitional and external thrust sheets from fold-thrust belts worldwide.

Acknowledgments The fieldwork and sample collection for this paper was supported by IIT Bombay Grant No RI/0516-10001077-001 to MM. VS acknowledges IIT Bombay for a Teaching Assistantship in the Department of Earth Sciences. We thank Mark McNaught for his program ANGBOOT and Rick Allmendinger for StrainSim 3.6.1. We also take this opportunity to honor Prof. Dhruba Mukhopadhyay for his contributions in field-based structural geology and for inspiring MM to study strain in rocks.

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Figure captions Figure 1: Generalized map of the Dharan salient and the Gorubathan recess. Our study area is shown by a black rectangle in the Dharan salient in the frontal Darjiling Himalaya, India.

Figure 2: Structural map of the Dharan salient (after Kundu et al., 2012). Sample locations along the Tista (TV1-TV7) and Lish (LV1-LV6) River sections in the along frontal imbricate thrust system. The cross-section along P-Q is shown in Fig. 9.

Figure 3: Sample locations along the two transects in the Mohand Range from the Main Frontal thrust (MFT) sheet. Samples BB1-BB8 were collected in the western Mohand Range along Badshahibag Rao (BB). Samples MR1-MR10 and KJ1-KJ3 were collected in the Central Mohand Range along Mohand Rao (MR) and Khajnawar Rao (KJ), respectively. The Khajnawar Rao (KJ) samples were collected from the islands in the hanging wall damage zone of the MFT. 40

Figure 4: Brittle deformation features from Middle Siwaliks rock in Lish (LV3-LV6) and Tista River (TV2, TV6) sections. Elastico-frictional microstructures include intragranular fractures (IF), and intergranular feature (INF). Quasi-plastic deformation microstructures such as kinked mica (KnM), recrystallized grains (RG), and deformation bands (DfM) are also seen.

Figure 5: Siwaliks rocks from the Tista (TV1-TV7) and Lish (LV1-LV6) River sections with their corresponding Rf and phi() values. Weak to moderate grain shape preferred orientation (GSPO) is observed in these rocks.

Figure 6: Strain axial ratios (Rf) in the Middle Siwaliks rocks from (A) Tista (samples TV1TV6) and (B) Lish (samples LV1-LV6) River sections in the frontal schuppen belt consisting of 11 imbricate thrust faults (T1-T11) in the Dharan salient (after Kundu et al., 2012). TV7 is a sample from the Lower Siwaliks section in the South Kalijhora thrust (SKT) sheet (C) Comparison of strain variation in both the river sections. Each color represents the imbricate thrust sheet identified by the label on its left (or southern) boundary.

Figure 7: Photomicrographs of brittle and ductile deformation microstructures in the Middle Siwaliks sandstones. Intragranular (IF) fractures are seen in samples MR3 and BB3-BB8. Intergranular fractures (InF) cutting across several grains are seen in samples MR3 and BB3. MFT damage zone sample (MR7) exhibits GSPO. Quartz grain showing deformation bands (DfB) with undulose extinction (Un), Recrystallized quartz grains (RG) are observed in samples MR3, MR7, BB3 and BB8. Kinked mica (KnM) in cross-polarized light shows high and variable birefringence in biotite in response to kinking in samples MR7, BB3 and BB8.

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Figure 8: Strain axial ratios (Rf) in Middle Siwalik sandstones in the central and western Mohand transects near Paonta Sahib and Mohand. In the central Mohand Range, Rf has been computed for Mohand Rao (MR) and Khajnawar Rao (KJ) streams. In Mohand Rao, samples were distributed for ~ 6 km from the mountain front. In Mohand Rao, Rf = 1.7 (between X = 0.4 - 2.5 km) in the frontal limb and drops to 1.2 (at X = 2.6 km) at the monoclinal hinge. Rf ranges from 1.4 - 1.5 (between X = 2.6 to 5.7 km) in the backlimb with an exception at MR6 with Rf = 1.33. The Khajnawar Rao samples were within half-a-kilometer of mountain front. Lowest Rf was 1.20 (X = 0.2 km) which gradually increased to 1.45 (X = 0.6 km). Similarly, in the western Mohand Range in the Badshahibag Rao (BB) transect, Rf = 1.6 (at X = 0.1 km) in the frontal sub-horizontal limb of the monocline and 1.33 (at X= 0.3 km) at the monoclinal hinge. Rf ranged from 1.4 to 1.6 (between X = 0.3 - 3.6 km) in the NE-dipping backlimb with an exception at BB6 where Rf = 1.28.

Figure 9: (A) Deformed Tista River section (along PQ of Fig 2; modified after Kundu et al., 2012) showing the imbricate thrust zone that deformed the Middle Siwaliks sandstones between the South Kalijhora thrust (SKT) and the mountain front (MF). (B) Deformed cross section after removing the LPS strain between the MF and the SKT. (C) Restored cross section of the Middle Siwaliks Subgroup in the study area. Total shortening of 2.55 km was accommodated without taking strain into account (Kundu et al., 2012). However, after adding ~11% LPS strain we computed a total of 2.76 km shortening between the MF and the SKT. (D) Variation in strain (Rf) in each thrust sheet corresponding to the balanced cross section.

Figure 10: Transport-parallel balanced cross sections between the Santaurgarh and Main Frontal thrusts for the western (A-A') and central (B-B') Mohand Range transects using the trishear fault propagation fold model at the MFT tip (Modified after Srivastava et al., 2018).

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Top panels are the deformed sections. Finite strain ellipses using the Bootstrapped Normalized Fry method (McNaught, 2002) in Middle Siwalik sandstones are shown in the sections. Long axes of the computed strain ellipses show variable plunges indicating varying degrees of rotation possibly due to fault- or bed-parallel shear strain. The middle panel for central Mohand Range (B-B') shows the deformed section after finite (LPS) strain is removed. SnBT = Santaurgarh thrust splay; MFT = Main Frontal thrust. Horizontal dashed lines track the shortening accommodated in the two transects. Bottom panels for both the sections show the area and line-length balanced, restored cross sections. Strain distribution from southwest to northeast in the MFT sheet in the western and central Mohand Range is shown separately below the Mohand Range sections.

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Table 1: Details of Siwalik samples (Middle & Lower) collected from imbricate thrusts in the Dharan salient in the Tista and Lish River sections and SKT, Splay and MFT fault zone (FZ) samples age from Mukul et al., (2007). Imbricate thrust

Tista River Section

Lish River Section

Mukul et al., 2007 (Age of fault gouge in ka)

Elevation (m)

Sample

Elevation (m)

Sample

Sample

Elevation (m)

Age (ka)

SKT

179

TV7 (SH)

-

-

SKTFZ

164

42 ± 10

T1

200

TV6 (FZ)

574 565

LV6 (SH) LV5 (FZ)

SPLAY FZ

212

20 ± 6

T2

-

-

623

LV4 (FZ)

-

-

-

T3

170 219

TV5 (SH) TV4 (FZ)

621

LV3 (SH)

-

-

-

T4 T5

209

TV3 (FZ)

587 442

LV2 (SH) LV1 (FZ)

MFTFZ -

596 -

45 ± 7 -

T6

212

TV2 (FZ)

-

-

-

-

-

-

-

-

-

-

T8 239 TV1 (SH) SH – Thrust sheet; FZ – Fault zone

Table 2: Finite strain values from Tista and Lish River sections, Dharan salient

44

Sample no. (From south to North)

Axial Ratio (Rf)

Attitude of bedding (Dip, Dip Direction,°)

Phi (φ)

Attitude of Long axis of ellipse (°) (Deformed section)

Attitude of Long axis of ellipse, (°) (Restored section)

Angle () between pole to bedding and long axis of strain ellipse (°) (Restored Section)

52, 201 36, 072 01, 296 18, 149 13, 354 27, 009 21, 153

38S 54N 89N 72S 77N 63N 69S

13, 091 83, 328 19, 110 01, 200 32, 349 53, 259

77E 07N 71S 89S 58N 37S

TISTA RIVER SECTION 1 2 3 4 5 6 7

TV1 TV2 TV3 TV4 TV5 TV6 TV7

1.32±0.03 73.58±2.88 1.52±0.05 71.83±4.27 1.78±0.06 66.35±5.82 1.87±0.04 102.37±1.30 1.76±0.06 92.13±5.83 1.49±0.05 112.91±4.85 1.19±0.01 58.41±6.96

20, 340 30, 006 40, 000 40, 022 45, 350 30, 002 45, 122

36, 190 42, 097 17, 290 07, 332 58, 358 57, 013 55, 180

LISH RIVER SECTION 1 2 3 4 5 6

LV1 LV2 LV3 LV4 LV5 LV6

1.33±0.05 1.26±0.02 1.47±0.05 1.26±0.03 1.59±0.05 1.46±0.05

95.58±5.63 70.77±3.27 87.66±4.82 62.53±5.66 80.19±4.72 92.29±5.09

30, 180 20, 353 32, 350 30, 008 50, 070 27, 357

12, 084 76, 185 02, 115 28, 022 26, 319 42, 231

Table 3: Finite strain values from central and western Mohand Range, MFT sheet.

Samp le no. (Fro m South to North )

Distanc e from mounta in front (km)

Axial Ratio (Rf)

Phi (φ)

Attitude of bedding (Dip, Dip Direction ,°)

Attitude of Long axis of ellipse (°) (Deform ed section)

Attitud e of Long axis of ellipse, (°) (Restor ed section)

Angle (θ) betwee n pole to bedding and long axis of strain ellipse (Restor 45

ed section)

1 2 3 4

Central Mohand

5 6 7 8 9 1 0 1 2 3 1

Western Mohand

2 3 4 5 6 7 8

MR1

0.35

MR2

2.36

MR3

2.45

MR4

2.55

MR5

3.71

MR6

4.20

MR7

4.77

MR8

5.47

MR9

5.51

MR10

5.71

KJ1

0.23

KJ2

0.44

KJ3

0.58

BB1

0.10

BB2

0.32

BB3

0.63

BB4

1.37

BB5

1.82

BB6

2.98

BB7

3.42

BB8

3.59

1.73±0. 05 1.69±0. 05 1.22±0. 03 1.51±0. 05 1.45±0. 03 1.33±0. 04 1.48±0. 04 1.48±0. 03 1.35±0. 04 1.37±0. 03 1.20±0. 02 1.43±0. 02 1.45±0. 03 1.58±0. 03 1.33±0. 03 1.45±0. 03 1.59±0. 04 1.62±0. 06 1.28±0. 03 1.46±0. 04 1.41±0. 04

Mohand Rao 118.67±3. 45, 202 53 92.61±6.5 26, 060 3 73.84±6.0 28, 050 0 59.15±3.6 30, 020 5 91.78±6.5 20, 055 8 118.60±3. 50, 020 17 104.62±6. 35, 030 76 104.40±3. 16, 020 46 102.90±5. 20, 015 59 94.84±4.6 35, 023 6 Khajnawar Rao 95.38±6.0 39, 181 2 96.87±4.4 56, 040 2 64.55±3.9 38, 040 9 Badshahi Bag Rao 71.31±3.7 77, 206 5 136.81±4. 27, 062 88 106.77±5. 50, 043 31 113.88±3. 32, 064 33 103.96+3. 50, 052 08 103.34±5. 15, 178 48 82.36±6.1 32, 075 5 111.58±4. 30, 080 87

28, 008

71, 344

19N

62, 064

36, 062

54N

72, 198

76, 082

14E

72, 350

44, 010

46N

80, 076

60, 051

30N

80, 240

57, 008

33N

80, 058

46, 037

44N

80, 125

72, 055

18N

25, 006

0.5,007

85.5N

26, 102

17, 091

73E

50, 004

87, 092

03E

44, 234

76, 356

14N

25, 210

13, 029

77N

24, 058

56, 135

34S

68, 023

44, 044

46N

58, 216

71, 058

19N

53, 296

59, 344

31N

65, 007

21, 035

69N

50, 049

58, 066

45N

46, 036

20, 047

70N

35, 242

64, 227

26S

46

Table 4: Modelling of measured strain in Siwalik rocks from Tista and Lish River section using StrainSim.v3.6.1. (Dextral/right lateral is +ve showing sense from north to south) Sample no. Layer parallel (From shortening south (LPS) to (%) North) 1 2 3 4 5 6 7

TV1 TV2 TV3 TV4 TV5 TV6 TV7

05 -

1 2 3 4 5 6

LV1* LV2 LV3 LV4 LV5 LV6

10.7 08

Fault/bed Layer parallel parallel shear Shear extension or (FPS)(º) strain Vertical D-Dextral flattening (%) S-Sinistral TISTA RIVER SECTION 14S -0.2493 22D 0.4040 2.5 1.25D 0.0218 33.3 21.5S -0.3939 22.8 14.5D 0.2586 26.3 18D 0.3249 09 07S -0.1228 6.5 LISH RIVER SECTION 7.25D 0.1272 13 02.6D 0.0454 13.5S -0.2401 14.5 0.46S -0.0080 12 23.5D 0.4348 05 17.5S -0.3153 -

Rfm

θm(º)

1.325 1.513 1.778 1.871 1.761 1.479 1.199

37.99 54.25 88.97 71.93 77.15 62.99 68.84

1.327* 1.261 1.468 1.255 1.589 1.461

76.84 07.03 71.21 88.99 57.46 36.85

Rfm – Rf modelled, θm - Modelled θ (Angle between long axis of strain ellipse and pole to bedding plane after restoration), *Sample LV1 was modelled in the E-W plane and top-to-west was taken as dextral.

Table 5: Modelling of measured strain in Middle Siwaliks rocks from central and western

Central Mohand

Mohand using StrainSim.v3.6.1. (Dextral/right lateral is +ve showing sense from NE to SW)

1 2

Sample no. (From South to North)

Layer Parallel Shortenin g (%) (LPS)

MR1 MR2

21.2 0.1

Fault/Bed Parallel Shear Shear (FPS) (°) Strain D- Dextral S-Sinistral Mohand Rao 12.5D 0.222 28D 0.532

Layer Parallel Extension (%) (Flattening )

*Rfm

θm (°)

-

1.733 1.689

19.06 52.34 47

3 4 5 6 7 8 9 10

MR3 MR4 MR5 MR6 MR7 MR8 MR9

8.7 3.7 11.1 7.1 4.3 15.6 16

4.5D 21.5D 14.5D 12.9D 20D 9.6D 3.7D

0.079 0.394 0.259 0.229 0.364 0.169 0.065

-

MR10

-

10D

0.176

13

-

1.222 1.511 1.451 1.332 1.474 1.481 1.346 #1.36 9

13.91 45.60 29.58 32.98 43.86 18.04 85.45 73.27

Khajnawar Rao

Western Mohand

1

KJ1

8.8

0.9D

0.016

-

2 3

KJ2 KJ3

15 -

-

1 2

BB1

11.7

7.0D 0.123 9.5D 0.167 Badshahi Bag Rao 19S -0.344

BB2

1.5

15.5D

0.277

-

3 4 5 6 7 8

BB3 BB4 BB5 BB6 BB7 BB8

14.5 13.5 1.4 12.5

10D 17.5D 18.5D 13.5D 14D 12S

0.176 0.315 0.335 0.196 0.249 -0.213

16 13.5 -

-

#1.20

2 1.425 1.452 1.583 #1.32 6 1.454 1.588 1.623 1.277 1.459 1.405

02.92 13.99 76.98 34.13 45.99 19.51 59.19 69.19 44.83 69.95 25.70

Rfm – Rf modelled, # θm – Modelled θ (Angle between long axis of strain ellipse and pole to bedding plane after restoration; # Sample MR3, MR10 and KJ1 was modelled in the E-W plane and top-to-west was taken as dextral/positive.

48

Highlights 

We establish that strain can be quantified from rocks deformed by cataclastic flow



Strain pattern in the trishear model of fault propagation folding is validated



Balanced cross sections in the frontal Himalaya are improved by strain inclusion



Our methodology can be used in external thrust sheets in all fold-thrust belts

49

Graphical abstract

50

Declaration of interests

☒ 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 paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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