Role of initial basin width in partitioning total shortening in the Lesser Himalayan fold-thrust belt: Insights from regional balanced cross-sections

Journal of Asian Earth Sciences 116 (2016) 122–131

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Role of initial basin width in partitioning total shortening in the Lesser Himalayan fold-thrust belt: Insights from regional balanced cross-sections Kathakali Bhattacharyya ⇑, Farzan Ahmed Department of Earth Sciences, Indian Institute of Science Education and Research – Kolkata (IISER K), Mohanpur 741246, West Bengal, India

a r t i c l e

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Article history: Received 22 June 2015 Received in revised form 15 October 2015 Accepted 13 November 2015 Available online 14 November 2015 Keywords: Himalayan fold thrust belt Balanced cross-sections Lateral variation in shortening Lesser Himalayan sequence (LHS) Initial basin width

a b s t r a c t Published, regional, balanced cross-sections constructed across various transects along the Himalayan fold-thrust belt (FTB) suggest significant lateral variations in the magnitude and partitioning of total minimum shortening among various lithotectonic units. The variation in shortening is greatest in the Lesser Himalayan sequence (LHS). Western Nepal Himalaya records the highest shortening. Shortening variation shows non-uniform spatial distribution along the length of the FTB, with regions of lower shortening estimates lying in between regions of higher shortening estimates. We measured the initial and final lengths of the Lesser Himalayan sequence (LHS) from existing published balanced cross-sections. There is a direct correlation between the initial width of the LHS and the total minimum shortening distribution accommodated in all the lithotectonic units of the FTB. This indicates that the initial width of the LHS controlled the lateral variation in the total minimum shortening, and provides a new interpretation for minimum shortening variation in the FTB. The initial width of the LHS also controlled the lateral variation in structural architecture of the LHS by affecting the geometry and total number of LHS imbricates and horses along the FTB. The variation in structural geometry of the LHS along the Himalayan FTB resulted in non-uniform distribution of lateral variation in initial and current LHS outcrop widths. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction At convergent plate boundaries, crustal rocks deform at various scales to accommodate the contraction or shortening associated with the convergence. This shortening is manifested by formation of fold-thrust belts (FTB) along such plate boundaries (Dahlstrom, 1970; Boyer and Elliott, 1982). The minimum shortening accommodated along specific transects of these FTB can be estimated from construction of restorable, kinematically viable, transportparallel, regional balanced cross-sections along such transects (e.g., Price and Mountjoy, 1970; Hossack, 1979; Elliott, 1983; Stockmal and Waldron, 1993; Srivastava and Mitra, 1994; McQuarrie and DeCelles, 2001; McQuarrie, 2004). Comparing these estimates from the Himalayan FTB to address how and why the minimum shortening distribution vary laterally have been the focus of some recent studies (DeCelles et al., 2002; Mitra et al., 2010; Long et al., 2011; Bhattacharyya, 2010; Bhattacharyya et al., 2013, 2015a). ⇑ Corresponding author. E-mail address: [email protected] (K. Bhattacharyya). http://dx.doi.org/10.1016/j.jseaes.2015.11.012 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

In the Himalayan FTB, 580–900 km of convergence–related. This shortening is manifested (Le Pichon et al., 1992; DeCelles et al., 2002) has been accommodated primarily by a folded thrust system (Medlicott, 1864; Heim and Gansser, 1939; Gansser, 1964; Valdiya, 1980; Srivastava and Mitra, 1994; Pearson and DeCelles, 2005). From north to south these are the Main Central thrust system (MCT), the Pelling–Munsiari thrust (PT), the Ramgarh thrust, the Lesser Himalayan duplex, the Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) (Fig. 1). A compilation of minimum shortening estimates from the published, regional balanced cross-sections constructed along the FTB reveals a lateral variation in the total minimum shortening, and its partitioning among the different thrust sheets (Fig. 2; DeCelles et al., 2002; Mitra et al., 2010; Long et al., 2011; Bhattacharyya et al., 2015a). The lithotectonic unit structurally bounded above by the MCT and below by the MBT is the Lesser Himalayan sequence (LHS) that lies in the middle of the FTB. The LHS preserves the highest number of imbricates in the FTB with greater connectivity than the major Himalayan thrust faults forming the Lesser Himalayan duplex. The lateral variation in the geometry of these imbricates

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Fig. 1. Regional map of the Himalayan FTB showing the major longitudinal lithotectonic subdivisions separated by major faults. The Lesser Himalayan sequence (LHS) is bounded by the Main Central thrust (MCT) in the north and the Main Boundary thrust (MBT) in the south. Rectangular boxes show the locations of the studied 12 published balanced cross-sections. 1. Webb, 2013; 2. Srivastava and Mitra, 1994; 3. Robinson et al., 2006; 4. Khanal and Robinson 2013; Robinson and Martin, 2014; 5. Bhattacharyya et al., 2015a; 6. McQuarrie et al., 2013; 7. Long et al., 2011; 8. Yin et al., 2011.

result in the greatest lateral variation of minimum shortening within the LHS (Fig. 2; Bhattacharyya et al., 2015a). The published cross-sections do not incorporate penetrative strain from the different thrust sheets, and smaller-scale contractional structures. In all these transects, the hanging wall cut-offs wherever not preserved, are assumed to lie immediately above the current erosional surface. Thus, they provide minimum shortening estimates, which we refer to throughout this paper. The hanging wall cutoffs of the LHS are not preserved in any of the studied balanced cross-sections. In spite of this limitation, there is a significant lateral variation in the LHS minimum shortening in the FTB (Fig. 2) suggesting a possible causative factor that is

independent of the preservation of the LHS. Recent studies have evaluated some of the causative factors that may have led to the observed lateral variation in total minimum shortening along the FTB (DeCelles et al., 2002; Mitra et al., 2010; Long et al., 2011; Bhattacharyya et al., 2013, 2015a). These studies have demonstrated that the minimum shortening variations do not completely mimic the width of the Tibetan plateau, as was suggested by DeCelles et al. (2002) (Mitra et al., 2010; Long et al., 2011). The minimum shortening estimates also do not show a progressive increase toward the eastern Himalaya, as was predicted by an increase in convergence rate (Guillot et al., 1999). Similar eastward increase in shortening estimates was also predicted due to an

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Fig. 2. Plot showing lateral variation of total minimum shortening and its partitioning in the various thrust sheets of the Himalayan FTB (after Bhattacharyya et al., 2015a). The corresponding data and the references are listed in Table 1. The LHS records the maximum lateral variation in minimum shortening in the FTB.

Table 1 Compilation of current outcrop width, initial width of LHS, LHS minimum shortening, minimum shortening in the GHS + LHS + SHS, total minimum shortening (THS + GHS + LHS + SHS), minimum shortening percentages of LHS and total minimum shortening percentage from the 12 studied cross-sections. Transects of studied balanced cross sections

Current outcrop width of LHS (km)

Initial width of LHS (km)

Total shortening (THS + GHS + LHS + SHS) (km)

Shortening in GHS + LHS + SHS (km)

Shortening in LHS (km)

LHS shortening percentage (%)

Total shortening percentage (%)

Himachal (Webb, 2013) Garhwal (Srivastava and Mitra, 1994) Kumaon (Srivastava and Mitra, 1994) Chainpur (Robinson et al., 2006) Simikot (Robinson et al., 2006) Western Central. Nepal (Robinson and Martin, 2014) Eastern Central Nepal (Khanal and Robinson, 2013) Darjeeling–Sikkim (Bhattacharyya et al., 2015a) Western Bhutan (McQuarrie et al., 2013) Mangde Chu (Long et al., 2011) Trashigang (Long et al., 2011) Arunachal (Yin et al., 2010)

111 95 85 98 145 68

495 256 404 444 597 275

773 578 581 664 818 492

518 197 445 471 574 355

335 114 250 349 458 199

72 69 73 79 77 70

73 68 79 76 74 77

85

353

741

400

245

71

69

56 76 28 73 44

427 509 342 401 454

581 693 520 550 654

466 545 387 405 515

331 341 178 266 308

82 70 52 66 73

82 76 70 75 76

increase in erosion rate toward the east (Grujic et al., 2006; Yin et al., 2006; Long et al., 2011). Further, the shortening distribution does not record the greatest shortening in Sikkim–Bhutan Himalaya, as was predicted from an oblique convergence of the Indian plate (Molnar and Lyon-Caen, 1989; McCaffrey and Nabelek, 1998; Mitra et al., 2010; Bhattacharyya et al., 2015a). In this paper, we evaluate the lateral shortening variation in the FTB in the context of an additional factor, i.e., the initial width of the LHS basin in the Himalayan FTB (Mitra et al., 2010; Bhattacharyya et al., 2013). Regional map of the Himalayan FTB shows that the current width of the mountain belt is higher in the western Himalaya than that in the east (Fig. 1). Additionally, the current outcrop width of the LHS shows a significant lateral variation from the west to the east; it is highest in western Nepal and generally decreases farther east in the Sikkim–Bhutan

Himalaya (Fig. 1, Table 1). We evaluate how the initial width of the Lesser Himalayan basin controlled the lateral variation in minimum shortening in the mountain belt, and how the resultant shortening variation affected the current outcrop with of the LHS. We also discuss the implications of the variation in the initial length of the LHS in the overall structural evolution of the Lesser Himalayan FTB. Additionally, this study provides insights into palinspastic reconstruction of the northern extent of the undeformed LHS prior to the initiation of translation on the MCT. 2. Method We have used 12 published, regional, restorable balanced crosssections through the Himalayan FTB as our database (Table 1); their locations are shown on the regional map (Fig. 1). Following

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previous studies (DeCelles et al., 2002; Mitra et al., 2010; Long et al., 2011; Bhattacharyya et al., 2013, 2015a) and incorporating recent datasets from Himachal (Webb, 2013) and central Nepal Himalaya (Khanal and Robinson, 2013; Robinson and Martin, 2014), we have updated the plot of lateral variation in total minimum shortening in the Himalayan FTB. We measured the finite, deformed state lengths of the balanced cross-sections, between the pinlines. Next, we have measured the restored lengths of the Greater Himalayan sequence (GHS) that lies in the hanging wall of the Main Central thrust (MCT). Following the standard Himalayan definition (Medlicott, 1864; Heim and Gansser, 1939), we measured the restored lengths of the LHS, bounded by the MCT in the north and the MBT in the south. Finally, we have measured the restored lengths of the SubHimalayan sequence (SHS) lying in the hanging wall of the Main Frontal thrust (MFT). In all such sections, the measurements were carried out between the trailing and the leading branch lines of the bounding faults, along the crosssections. The restored lengths of the LHS, measured along different transects, is referred to as the initial width of the LH basin throughout the text. We calculated the minimum shortening amount and the shortening percentages by comparing the undeformed and deformed state lengths of the balanced cross-sections. We have also measured the current outcrop width of the LHS along each of the transects from the accompanying published maps and cross-sections. To arrive at the total minimum shortening of the FTB, we have added the published minimum shortening estimates from the Tethyan Himalayan sequence (THS; Ratschbacher et al., 1994; Searle et al., 1997;Corfield and Searle, 2000) to the existing cross-sections, following previous studies (Long et al., 2011; Bhattacharyya et al., 2015a). To the western transects (Srivastava and Mitra, 1994; Webb, 2013), estimates from THS from northwest India (Searle et al., 1997; Corfield and Searle, 2000) are added. For western Nepal (Robinson et al., 2006), THS estimates from Murphy and Yin (2003) are added. The THS estimates from north of eastcentral Nepal (Ratschbacher et al., 1994) were incorporated in the central Nepal Himalaya minimum shortening estimates (Khanal and Robinson, 2013; Robinson and Martin, 2014). For Sikkim–Bhutan and western Arunachal estimates (Bhattacharyya et al., 2015a; Long et al., 2011; McQuarrie et al., 2013; Yin et al., 2010), we have added THS estimates from a section that lies north of Sikkim (Ratschbacher et al., 1994). The results are tabulated in Table 1.

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3. Results 3.1. Lateral variation in the current width of the LHS Based on structural (Heim and Gansser, 1939; Valdiya, 1980; Sinha-Roy, 1982; Gansser, 1983; Le Pichon et al., 1992; Srivastava and Mitra, 1996; Srivastava and Tripathi, 2007; Bhattacharyya and Mitra, 2011, 2014; Imamura et al., 2011) and geochemical characters (DeCelles et al., 1998, 2001; Robinson et al., 2001; Martin et al., 2005; Catlos et al., 2001; Catlos et al., 2004; Long et al., 2011; Bhattacharyya et al., 2015a), we define the LHS as the package of rocks bounded by the MCT in the north and the MBT in the south. Therefore, the LHS comprises two major thrust systems: (a) the Munsiari thrust sheet (Srivastava and Mitra, 1994; Robinson and Pearson, 2013) in the north, and (b) the Lesser Himalayan duplex (Srivastava and Mitra, 1994; Robinson et al., 2006; Bhattacharyya and Mitra, 2009; Mitra and Bhattacharyya, 2011; Long et al., 2011) in the south. In the Sikkim Himalaya, the Munsiari thrust is equivalent to the Pelling thrust (Bhattacharyya et al., 2015b). Therefore, we are defining the northern system as the Pelling–Munsiari thrust. We have measured the current outcrop width of the exposed LHS, along the lines of cross-sections, for all the 12 cross-sections (Table 1), and have plotted them with respect to the arc distance of the Himalayan FTB (Fig. 3). The current width of the LHS in the Himachal Himalaya in the westernmost transect is 111 km and progressively decreases eastward in Garhwal (95 km) and Kumaon (85 km) Himalaya (Table 1, Fig. 3). It increases eastward and becomes widest at the Simikot transect in western Nepal (145 km; Table 1, Fig. 3). However, east of this transect the outcrop widths do not show a gradual variation. In central Nepal it decreases to 68 km and increases to 85 km farther east (Fig. 3). The outcrop width remains lower in eastern Himalaya and varies from 56 km in Sikkim to 76 km in western Bhutan (Fig. 3). Farther east, in Mangde Chu in central Bhutan, the outcrop width is the least (28 km), and increases in eastern Bhutan along Trashigang (73 km) (Fig. 3). The current outcrop width decreases in the far-eastern transect of Arunachal Himalaya where it is 44 km (Fig. 3). Therefore, the current LHS outcrop width is widest in western Nepal along Simikot transect, and east of this transect, the width alternates between decreasing and increasing trends (Fig. 3). Superimposition of the total LHS minimum shortening on these data (Fig. 3) suggests that although the current

Fig. 3. Plot showing variation in current outcrop width of the LHS, as measured along the 12 regional transects, versus the arc-length of the Himalayan FTB. Total LHS minimum shortening, estimated from restored sections, is superimposed on the plot. The current outcrop width of the LHS is higher in the western Himalaya than in the east. See Table 1 for details.

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Fig. 4. Plot showing variation in initial width of the LHS, as estimated from the restored balanced cross-sections, with respect to the arc-length of the FTB. The initial width of LHS was highest along western Nepal, and remained high in western Bhutan; there is no progressive increase or decrease in the initial width from west to east. The current outcrop width, the total LHS minimum shortening, LHS minimum shortening percentage and total minimum shortening percentage are superposed. Details of the data are tabulated in Table 1.

outcrop width of the LHS is lower in the eastern Himalaya, the total LHS shortening remained generally high in this region. We discuss the possible cause and implication of this observation in later sections (Sections 4.1 and 4.3). 3.2. Lateral variation in the initial width of the LHS We have measured the restored LHS lengths from each of the 12 available cross-sections. In all these cases, the hanging wall cutoffs, wherever not preserved, are assumed to lie immediately above the current erosional surface to minimize the shortening estimates. The Himachal transect in the western Himalaya (Webb, 2013) records an initial LHS width of 495 km followed by a significant decrease in the Garhwal section (256 km; Srivastava and Mitra, 1994) (Table 1, Fig. 4). It progressively increases eastward in Kumaon (404 km; Srivastava and Mitra, 1994) and Chainpur (444 km; Robinson et al., 2006), and becomes widest along the Simikot transect (597 km; Robinson and Pearson, 2006), in western Nepal (Table 1, Fig. 4). The initial width decreases significantly to 275 km, east of Simikot in central Nepal (Robinson and Martin, 2014). Farther east, it progressively increases in east-central Nepal (353 km; Khanal and Robinson, 2013) to Sikkim (427 km; Bhattacharyya et al., 2015a)–western Bhutan (509 km; McQuarrie et al., 2013) Himalaya (Table 1, Fig. 4). The initial width of the Lesser Himalayan basin decreases eastward in central Bhutan (342 km), followed by an increase in eastern Bhutan (401 km; Long et al., 2011) (Table 1, Fig. 4). In the easternmost transect in western Arunachal Himalaya (Yin et al., 2010), the initial width of LHS increases to 454 km. Thus, the initial width of LHS does not show a progressive increasing or decreasing trend from west to east. The basin was widest in western Nepal, followed by western Bhutan, and remained high in the westernmost and the easternmost sections (Fig. 4). Additionally, although the initial width was high in the eastern Himalaya, the finite deformed lengths of the LHS reduced significantly (28–76 km) in this part (Figs. 3 and 4, Sections 4.2 and 4.3). 3.3. Lateral variation in total minimum shortening estimates in the Himalayan FTB Lateral variation in total minimum shortening of the Himalayan FTB has been examined in detail in some recent studies (DeCelles et al., 2002; Mitra et al., 2010; Long et al., 2011; Bhattacharyya

et al., 2015a). We briefly summarize the results in this section (Fig. 2, Table 1). The total minimum shortening comprises the Tethyan Himalayan sequence (THS), the Greater Himalayan sequence (GHS), the LHS, and the SubHimalayan sequence (SHS). The total minimum shortening is 773 km in the Himachal Himalaya (Webb, 2013). It decreases eastward in Garhwal Himalaya to 578 km (Srivastava and Mitra, 1994;Table 1, Fig. 2). It progressively increases eastward in Kumaon (581 km; Srivastava and Mitra, 1994), Chainpur (664 km; Robinson et al., 2006), till Simikot (818 km; Robinson et al., 2006) where it becomes the highest (Table 1, Fig. 2). East of Simikot, there is a significant decrease in minimum shortening value in west central Nepal (492 km; Robinson and Martin, 2014) that increases in the east central Nepal (741 km; Khanal and Robinson, 2013) (Table 1, Fig. 2). Farther east, the total minimum shortening alternates between decreasing and increasing trends. It decreases through Darjeeling–Sikkim (581 km; Bhattacharyya et al., 2015a), western Bhutan (693 km; McQuarrie et al., 2013) and Mangde Chu of central Bhutan (520 km; Long et al., 2011), followed by an increase in eastern Bhutan (550 km; Long et al., 2011) and western Arunachal Himalaya (654 km; Yin et al., 2010) (Table 1, Fig. 2).

4. Discussion 4.1. Initial basin width of the LHS and the total minimum shortening in the FTB We evaluate the lateral variation of the initial width of the Lesser Himalayan basin with respect to the total minimum shortening recorded within the FTB (Fig. 5). As the THS shortening estimates are not from along the lines of 12 studied cross-sections, but are projected from nearby transects, we did not include the THS shortening in the total minimum shortening estimate of the FTB for this analysis, to minimize error. Thus, the minimum shortening accommodated within the GHS, LHS and SHS form the total shortening discussed in the rest of this section. On superposing the minimum shortening of the wedge (GHS + LHS + SHS) on the initial width of the LHS basin, we generally find a positive correlation between the two datasets (Fig. 5). The highest shortening is recorded in western Nepal (574 km; Simikot transect, Robinson and Pearson, 2006), where the LH basin was also the widest (597 km; Fig. 5). Similarly, the second highest shortening is

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Fig. 5. Plot showing initial width variation of the LHS with respect to the arc length of the Himalayan FTB. Minimum shortening (GHS + LHS + SHS) and the total Himalayan minimum shortening (THS + GHS + LHS + SHS) are superimposed. There is a direct correlation between the minimum shortening and the initial width of the LHS indicating that the latter controlled the former. The details of the data are tabulated in Table 1. Width of Tibetan plateau measured in an arc normal direction is also plotted. See text for discussion.

recorded along western Bhutan transect (545 km; McQuarrie et al., 2013) where the basin was second widest (509 km) (Fig. 5). In contrast, the narrowest part of the basin (256 km; Garhwal transect, Srivastava and Mitra, 1994) records the minimum shortening (197 km; Fig. 5). This correlation holds for all the 12 studied transects (Fig. 5). There are two interesting observations with respect to the initial LH basin width and total minimum shortening that are recorded in central Nepal (Robinson and Martin, 2014) and in central Bhutan (Long et al., 2011) sections. In western central Nepal, the initial LHS basin width is 275 km (Table 1) that accommodated 199 km LHS shortening (Robinson and Martin, 2014), and 355 km shortening in GHS + LHS + SHS (Table 1). Whereas, the width of the initial LH basin was 342 km in central Bhutan that accommodated 178 km LHS shortening (Long et al., 2011), and 387 km shortening in GHS + LHS + SHS (Table 1). Therefore, although the initial LH basin width was narrower in central Nepal than in central Bhutan, the former accommodated more LHS shortening and total shortening than central Bhutan. The geometry of the imbricates and horses of the Lesser Himalayan duplex can explain this counter-intuitive observation. The dip of the horses of the Lesser Himalayan duplex in Bhutan section (Long et al., 2011) is gentler than in Nepal section (Robinson and Martin, 2014), and their longer forelimbs are preserved in Bhutan. The gentler horses possibly caused lower shortening in the central Bhutan LHS by increasing its deformed state length. Additionally, the longer forelimbs of the LHS horses in Bhutan allowed better preservation of the overlying MCT synformal hinges in Bhutan thereby preserving the MCT klippe. This, in turn, increased the total GHS shortening in the Bhutan Himalaya. Availability of new minimum shortening estimates from Himachal (Webb, 2013), central Nepal (Robinson and Martin 2014; Khanal and Robinson, 2013), Sikkim (Bhattacharyya et al., 2015a) and western Bhutan (McQuarrie et al., 2013) Himalaya enables us to evaluate the existing hypotheses on lateral variation in shortening estimates. To examine this variation, we have included the THS minimum shortening in the total minimum shortening plot (Fig. 5). The updated plot reveals that the total minimum shortening in the Himachal Himalaya (773 km) is 277 km greater than the width of Tibetan plateau (496 km) measured along an arcnormal direction (Fig. 5). The total minimum shortening in the Kumaon–Garhwal Himalaya (578–581 km) is similar to width

of Tibetan plateau (591 km), but is 150 km greater than the arc-normal width of the Tibetan Plateau in the western Nepal Himalaya (Fig. 5). Although the total minimum shortening and the Plateau width are similar in western Bhutan and in Arunachal Himalaya, the total shortening is 137 km lower than the Plateau width in Sikkim–Bhutan Himalaya (Fig. 5). Thus, the total minimum shortening does not mimic the width of the Tibetan Plateau (Mitra et al., 2010; Long et al., 2011) as proposed by earlier study (DeCelles et al., 2002). Similarly, the minimum shortening estimates do not progressively increase eastward (Mitra et al., 2010; Long et al., 2011) as predicted by an increase in convergence rate toward east (Guillot et al., 1999) and an increase in erosion rate (Grujic et al., 2006; Yin et al., 2006; Long et al., 2011). Although, the western Bhutan transect (McQuarrie et al., 2013) records a higher magnitude of minimum shortening (693 km, Fig. 5), these estimates in Sikkim (581 km) and eastern Bhutan (550 km) are lower than the western Himalaya (578–773 km). Therefore, the updated data do not support the greatest minimum shortening being recorded in Sikkim–Bhutan Himalaya as predicted by higharc normal convergence in this region (Mitra et al., 2010). Additionally, the highest shortening is also not recorded in the middle of the orogen, i.e., in central Nepal, and neither does it decrease progressively toward east and west. Thus, the bow- and -arrow

Table 2 Table showing the number of imbricates and horses from the LHS along the studied balanced cross-sections. Transects of studied balanced cross sections

Number of imbricates in LHS (Ni)

Number of horses in LHS (Nh)

Himachal (Webb, 2013) Garhwal (Srivastava and Mitra, 1994) Kumaon (Srivastava and Mitra, 1994) Chainpur (Robinson et al., 2006) Simikot (Robinson et al., 2006) C. Nepal (Robinson and Martin, 2014) C. Nepal (Khanal and Robinson, 2013) Darjeeling–Sikkim (Bhattacharyya et al., 2015b) Western Bhutan (McQuarrie et al., 2013) Mangde (Long et al., 2011) Trashigang (Long et al., 2011) Arunachal (Yin et al., 2010)

0 3 3 5 5 2 2 2 3 0 0 0

14 4 3 9 6 7 4 12 11 11 13 6

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rule (Elliott, 1976) also does not explain the updated minimum shortening plot. Hence, the initial Lesser Himalayan basin width variation resembles the lateral shortening variation more closely than any of the proposed hypotheses. Therefore, it is a critical factor that needs to be considered for explaining lateral variation in total minimum shortening in the FTB. 4.2. Initial and deformed widths of the Lesser Himalayan basin Comparison of the deformed and the restored lengths of the LHS from the 12 studied cross-sections reveals that the lateral distribution of the two do not mimic each other, completely (Fig. 4, Table 1). The current outcrop width is highest in western Nepal (145 km), where the basin was also the widest (597 km; Table 1, Fig. 4). The outcrop width becomes narrower (28–85 km) east of this transect than in the western part (85–111 km); however, the initial basin width remains high in the eastern Himalaya (342– 509 km; Table 1, Fig. 4). At a first order, this suggests that the shortening structures in the LHS accommodated more minimum shortening in the eastern part than in the west. In order to examine this aspect, we calculated the total LHS minimum shortening and the LHS minimum shortening percentages from the studied transects (Table 1). The lateral variation in LHS minimum shortening is devoid of any particular trend from west to east. Variation in LHS minimum shortening along the FTB was studied by Mitra et al. (2010). However, availability of newer published sections from Himachal (Webb, 2013), central Nepal (Robinson and Martin, 2014; Khanal and Robinson, 2013), Sikkim Himalaya (Bhattacharyya et al., 2015a), western Bhutan (McQuarrie et al., 2013), together with revising of LHS stratigraphy from the Sikkim Himalaya (Bhattacharyya et al., 2015a,b) require an updated discussion. The LHS minimum shortening (Fig. 4, Table 2) is high at the westernmost Himachal section (335 km; Webb, 2013) followed by a rapid decrease in Garhwal Himalaya (114 km; Srivastava and Mitra, 1994). It progressively increases eastward in Kumaon (250 km; Srivastava and Mitra, 1994), Chainpur (349 km; Robinson et al., 2006) Simikot (458 km; Robinson and Pearson, 2006) where the LHS minimum shortening is highest (Fig. 4). Farther east, it significantly decreases in western central Nepal section (199 km; Robinson and Martin, 2014), followed by a progressive increase in eastern central Nepal (245 km; Khanal and Robinson, 2013), Darjeeling–Sikkim (331 km; Bhattacharyya et al., 2015b) and Western Bhutan (341 km; McQuarrie et al., 2013). The shortening decreases farther east in Mangde (178 km; Long et al., 2011), followed by a progressive increase eastward in eastern Bhutan (266 km; Long et al., 2011) and western Arunachal Himalaya (308 km; Yin et al., 2010). Therefore, there is a direct correlation between the total LHS minimum shortening and the initial LHS width indicating that the latter controlled the total LHS minimum shortening in the FTB (Table 1, Fig. 4). In contrast, the current outcrop width variation does not generally follow the lateral variation of initial width of the LHS and the total LHS minimum shortening (Table 1, Fig. 4, Section 3.1). In order to examine the cause of the lower current outcrop width of the LHS in the eastern Himalaya, we estimated the LHS minimum shortening percentages (Fig. 4). Generally, in the western Himalaya, the LHS shortening percentage varies between 69% (Garhwal) and 79% (Simikot). The highest LHS shortening percentage is recorded in Darjeeling–Sikkim Himalaya (Bhattacharyya et al., 2015a) at 82%. It reduces to 69% in western Bhutan (McQuarrie et al., 2013), and varies between 52% and 66% in eastern Bhutan (Long et al., 2011). The Arunachal section records 73% LHS minimum shortening (Yin et al., 2010). Therefore, the LHS minimum shortening percentage does not show an increase in the Bhutan–Arunachal Himalaya. Therefore, based on these data,

the lower current outcrop width of the LHS in the far-eastern Himalaya does not directly indicate a greater LHS shortening in the Bhutan–Arunachal Himalaya. However, the high LHS shortening percentage provides an explanation for the lower current LHS outcrop width in the Sikkim Himalaya. Next, we examine if the total shortening percentage variation in the FTB had controlled the current outcrop width variation of the LHS (Table 1, Fig. 4). The variation in magnitudes of the LHS minimum shortening and total shortening percentages remain similar in the western Himalaya–Sikkim–Arunachal Himalaya (Table 1). Western central Nepal has a low current LHS outcrop width (66 km; Robinson and Martin, 2014), and records a greater total shortening percentage (77%) than the LHS minimum shortening percentage (70%). Similarly in western Bhutan (McQuarrie et al., 2013) the total minimum shortening percentage is higher (76%) than the LHS minimum shortening (70%). Similar trends are also seen in central and eastern Bhutan where the total minimum shortening percentage (70–75%) is higher than the LHS minimum shortening percentage (52–66%). Therefore, the higher total minimum shortening in Sikkim–Bhutan Himalaya suggests a greater minimum shortening in the overlying MCT sheet that helped carry the MCT sheet farther south in this region than in the west (Mitra et al., 2010). This could have structurally covered the underlying LHS, and can explain the lower current LHS outcrop width in the eastern Himalaya. 4.3. Implications of initial width on the deformation architecture of the mountain belt The role of the Lesser Himalayan duplex in controlling the evolution of the Himalayan wedge has been well studied (e.g., Mitra et al., 2010). In this section, we examine how the initial width of the Lesser Himalayan basin may have controlled the deformation architecture of the LHS rocks by examining the number and geometry of horses and imbricates that are documented in each of the studied transects. The exposed horses and imbricates are generally constrained by field evidence, however, the blind horses or imbricates may not be uniquely constrained in these cross sections. We are aware of this limitation; nevertheless, we attempt to examine how the laterally varying number of imbricates, horses and their geometry may explain the structural architecture of the FTB. In the Himachal transect, the entire shortening in the LHS is accommodated by 14 hinterland-dipping horses of the Lesser Himalayan duplex (Table 2, Fig. 6; Webb, 2013). In the Kumaon– Garhwal transects, the LHS minimum shortening is accommodated by a combination of three thrust imbricates, namely, the Munsiari thrust, the Tong/Kapkot thrust and the Ramgarh thrust, and 3–4 hinterland-dipping horses of the Lesser Himalayan duplex (Table 2, Fig. 6; Srivastava and Mitra, 1994). In the western Nepal transects, the Chainpur cross section has 5 imbricates and 9 horses in the LHS, while the Simikot transect has 5 imbricates and 6 horses (Robinson et al., 2006) (Table 2, Fig. 6). The horses of the Lesser Himalayan duplex are predominantly hinterland-dipping. In central Nepal, the western transect has one imbricate, the Ramgarh thrust, and 7 hinterland-dipping horses (Table 2, Fig. 6; Robinson and Martin, 2014); the eastern transect has 2 imbricates (Ramgarh thrust and Trishuli thrust) and 4 hinterland-dipping horses (Table 2, Fig. 6; Khanal and Robinson, 2013). Farther east, in the Darjeeling–Sikkim transect, the LHS minimum shortening is accommodated by the Pelling thrust and the Ramgarh thrusts along with two duplex systems that have 2 and 10 horses, respectively (Table 2, Fig. 6; Bhattacharyya and Mitra, 2009, 2014; Mitra and Bhattacharyya, 2011) (Table 2). The duplex has 3 forelanddipping horses (Bhattacharyya and Mitra, 2009). The LHS structural geometry in Bhutan remains similar. The minimum shortening is accommodated by 3 imbricates and 11 horses: 4 foreland-

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Fig. 6. Plot showing the lateral variation of initial width of the LHS along the Himalayan arc together with the number of imbricates (Ni) and horses (Nh) within the LHS from the studied transects.

dipping and 7 hinterland-dipping (Table 2, Fig. 6; McQuarrie et al., 2013). In the central and eastern Bhutan transects, there are 13 and 11 hinterland-dipping horses, respectively (Table 2, Fig. 6; Long et al., 2011). In the western Arunachal cross section, there are 6 hinterland-dipping horses (Table 2, Fig. 6; Yin et al., 2010). Therefore, there is a lateral variation in the structural architecture of the LHS. The number of LHS imbricates and horses in the Lesser Himalayan duplex are generally higher along the transects with high initial Lesser Himalayan basin (Fig. 6); however, this observation is not valid for all the transects. There are two exceptions to this observation: (a) The Mangde Chu transect of central Bhutan has 11 horses with a relatively low initial LHS width (342 km) (Fig. 6; Long et al., 2011), (b) The Arunachal section has a high initial LHS width (454 km) but a low number of horses (6) (Fig. 6; Yin et al., 2010). The first aberration can be explained as due to a lower initial width of the LHS basin in central Bhutan, the rocks accommodated progressive deformation by generating more horses along this transect. Additionally, the variation in lithology may have played a role in resulting in larger number of horses along this transect. The Baxa Group of the LHS is dominantly quartzite rich in eastern Bhutan; it becomes progressively richer in dolomite and phyllite toward western Bhutan (Long et al., 2011). Dolomite not being an efficient glide horizon, may not have accommodated a significant amount of displacement in imbricates passing through them. Hence, the minimum shortening was accommodated by a larger number of horses and imbricates along this transect. For the Arunachal section, the horses have a relatively large displacement on them. Thus, overall, there is a positive correlation between the initial width of the LH basin and the number of imbricates and horses in the LHS. Hence, the initial basin width controlled the structural architecture of the LHS in the Himalayan FTB. The initial width of the Lesser Himalayan basin remained high in the eastern Himalaya (342–509 km), although the deformed state width is lower (56 km) in this region than in the west (Fig. 4). This study reveals that the increasing complexity of the LHS structure, i.e., with a larger number of horses and imbricates, along with presence of foreland-dipping horses (Bhattacharyya and Mitra, 2009; Long et al., 2011), resulted in more shortening in the LHS, and transferred the slip to the overlying MCT in the eastern Himalaya (Mitra et al., 2010). Additionally, the longer forelimbs of the horses may have preserved the synformal hinges of the overlying MCT sheet better in eastern Himalaya thereby

preserving the MCT klippen in Sikkim–Bhutan Himalaya. This, along with large slip transfer on the MCT, can provide an explanation for the lower current outcrop width of the LHS in the eastern Himalaya.

5. Conclusions The most updated compilation of minimum shortening estimates from the published 12 regional balanced cross-sections in the Himalayan FTB reveals that the shortening does not show progressive variation; transects of higher minimum shortening are punctuated by lower minimum shortening. Western Nepal records the highest minimum shortening estimate at 818 km. Among THS, GHS and SHS, the LHS records the greatest lateral variation in minimum shortening along the FTB. Restoration of the LHS from the regional balanced crosssections suggest that the initial width of the LHS basin varied laterally, and it remained high in the Sikkim–Bhutan Himalaya (342–509 km). There is a direct correlation between the initial width of the LHS basin, and the total minimum shortening recorded in the Himalayan FTB that includes minimum shortening estimates from the GHS, LHS and SHS. This indicates that the minimum shortening distribution in the Himalayan FTB was controlled by the initial width of the LHS. The number of horses and imbricates in the LHS are generally higher in regions with higher initial LHS width; there are two exceptions to this observation in central Bhutan and in Arunachal Himalaya. We explain these two aberrations by lithologic variation, and greater displacements on LHS imbricates. Therefore, the width of the initial LHS basin also controlled the deformation architecture of the LHS. The current outcrop width of the LHS in the Himalayan FTB varies laterally; it remains wide in Himachal–Kumaon–Garhwal Himalaya (85–111 km) in the west. The current LHS outcrop width is widest in western Nepal (145 km), and it decreases in the eastern Himalaya. The lateral distribution of initial and final widths of the LHS do not mimic each other. The complexity of the LHS structures increase in the Sikkim–Bhutan Himalaya with greater number of horses (11–12) and imbricates (2) than in the western Himalaya (4–9 horses, and 3–5 imbricates). This resulted in greater minimum shortening of the LHS rocks (178–341 km) and sliptransfer to the overlying MCT in the Sikkim–Bhutan Himalaya than in anywhere else. Additionally, the longer forelimbs of the LHS

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horses may have preserved the synformal hinges of the overlying MCT sheet better in eastern Himalaya thereby preserving the MCT klippen in Sikkim–Bhutan Himalaya. This, along with large slip transfer on the MCT, can provide a possible explanation for the lower current outcrop width of the LHS in the eastern Himalaya than in the west. The restored lengths of the LHS also provide insights into laterally varying northern extent of the undeformed LHS prior to the initiation of translation on the MCT. Acknowledgements The work was supported by Academic Research Fund (ARF) of IISER Kolkata to K. Bhattacharyya. We acknowledge A. Kumar for his help on compiling some of the preliminary data for this work. We benefitted from discussions with G. Mitra. Detailed and insightful reviews by A. Webb, two anonymous reviewers and the Editor, M. Faure, substantially improved the quality of the paper. References Bhattacharyya, K., Mitra, G., 2009. A new kinematic evolutionary model for the growth of a duplex – an example from the Rangit duplex, Sikkim Himalaya, India. Gondwana Res. 16, 697–715. Bhattacharyya, K., 2010. Geometry and Kinematics of the Fold-Thrust Belt and Structural Evolution of the Major Himalayan Fault zones in the Darjeeling– Sikkim Himalaya, India. PhD thesis, University of Rochester, pp. 1–263. Bhattacharyya, K., Mitra, G., 2011. Strain softening along the MCT zone from the Sikkim Himalaya: relative roles of quartz and micas. J. Struct. Geol. 33, 1105– 1121. Bhattacharyya, K., Kumar, A., Mitra, G., 2013. Role of initial basin width in partitioning total shortening in the lesser Himalayan fold-thrust belt: insights from regional balanced cross sections. Geol. Soc. Am. Abstr. Prog. 45 (7). Bhattacharyya, K., Mitra, G., 2014. Spatial variations in deformation mechanisms along the Main Central thrust zone: implications for the evolution of the MCT in the Darjeeling–Sikkim Himalaya. J. Asian Earth Sci. 96, 132–147. Bhattacharyya, K., Mitra, G., Kwon, S., 2015a. Geometry and kinematics of the Darjeeling–Sikkim Himalaya India. Implicat. Evol. Himalayan Fold-Thrust Belt, http://dx.doi.org/10.1016/j.jseaes.2015.09.008. Bhattacharyya, K., Dwivedi, H.V., Das, J.P., Damania, S., 2015b. Structural geometry, microstructural and strain analyses of L-tectonites from Paleoproterozoic orthogneiss: insights into local transport-parallel constrictional strain in the Sikkim Himalayan fold thrust belt. J. Asian Earth Sci. 107, 212–231. Boyer, S.E., Elliott, D., 1982. Thrust systems. Am. Assoc. Pet. Geol. Bull. 66, 1196– 1230. Catlos, E.J., Dubey, C.S., Harrison, T.M., Edwards, M.A., 2004. Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north-east India. J. Metamorph. Geol. 22, 207–226. Catlos, E.J., Harrison, T.M., Kohn, M.J., Grove, M., Ryerson, F.J., Manning, C.E., Upreti, B.N., 2001. Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya. J. Geophys. Res.: Solid Earth 106, 16177–16204. Corfield, R.I., Searle, M.P., 2000. Crustal shortening estimates across the north Indian continental margin Ladakh NW India. In: Khan, M.A., Treloar, P.J., Searle, M.P., Jan, M.Q. (Eds.), Tectonics off the Nanga Parbat Syntaxis and the Western Himalaya. The Geological Society of London Special Publication 170, pp. 395– 410. DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, P.A., Upreti, B.N., 1998. Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal. Geol. Soc. Am. Bull. 110, 2–21. DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Garzione, C.N., Copeland, P., Upreti, B.N., 2001. Stratigraphy, structure and tectonic evolution of the Himalayan fold-thrust belt in western Nepal. Tectonics 20, 487–509. DeCelles, P.G., Robinson, D.M., Zandt, G., 2002. Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau. Tectonics 21, 1062– 1087. Dahlstrom, C.D.A., 1970. Structural geology of the eastern margin of the Canadian Rocky Mountains. Bull. Canad. Petrol. Geol. 18, 331–406. Elliott, D., 1976. The motion of thrust sheets. J. Geophys. Res. http://dx.doi.org/ 10.1029/JB081i005p00949. Elliott, D., 1983. The construction of balanced cross-sections. J. Struct. Geol. 5, 101. Gansser, A., 1964. Geology of the Himalayas. Wiley-Interscience, New York, pp. 1– 289. Gansser, A., 1983. Geology of the Bhutan Himalaya. Birkhauser Verlag, Boston, pp. 1–181. Grujic, D., Coutand, I., Bookhagen, B., Bonnet, S., Blythe, A., Duncan, C., 2006. Climatic forcing of erosion, landscape and tectonics in the Bhutan Himalaya. Geology 34, 801–804.

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