Evidence for late Quaternary brittle deformation and back thrusting within the Indus Suture Zone, Ladakh Himalaya

Highlights

• The most hinterland fault in the suture zone is neotectonically active.

• Evidence from deformed fluvial, lacustrine and fault gouges.

• Chronology established using optically stimulated luminescence dating.

Abstract

Although the tectonic evolution of the southern Himalaya has been extensively studied, relatively few studies have focused on potential neotectonic activity in the most hinterland part of Himalaya, north of the region that separates the southern edge of the Tibetan plateau from the peaks of the Higher Himalayan Range. Here we use a combination of new structural and geomorphic field data, Optically Stimulated Luminescence (OSL) chronology, as well as previously published seismicity and denudation rate dataset, to suggest that the Trans-Himalaya within the region of the Indus Suture Zone (ISZ) is neotectonically active. Based on fault gouge, tilted paleolake deposits, and the chronology of deformed fluvial gravels constrained by OSL dating, the present study provides evidence for the tectonic activity along the ISZ between 78 and 58 ka. The occurrence of two earthquakes of Mw ≥ 4.0 and the focal mechanism solution of a shallow focus earthquake (Mw: 3.0, depth ~ 8 km) near the town of Upshi, shows a pure thrust mechanism. We suggest that this earthquake was likely associated with the Upshi-Bazgo thrust, a major structure which separates the mid-Miocene Indus Molasse from the Cretaceous Ladakh Batholith. It is envisaged that the fault and fold systems in the Ladakh region such as the Upshi-Bazgo thrust originated due to (1) acceleration in the northward movement of Indian plate relative to the Eurasian plate, and (2) post-collisional shortening parallel to plate convergence. On account of this, the Eocene to Miocene post-collisional sedimentary sequences, such as the Indus Molasse, rode over the now-extinct Ladakh Arc, as it acted as a rigid body, and formed a system of backthrusts, which are neotectonically active.

Introduction

Neotectonic activity along the major structures of the Himalaya i.e. from the southernmost, Himalayan Frontal Thrust (HFT), the Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the South Tibet Detachment system (STDs) have been extensively explored (Lavé and Avouac, 2000; Thiede et al., 2004; Bookhagen et al., 2005; Dutta et al., 2012; Devrani and Singh, 2014; Morell et al., 2015, Morell et al., 2017) and the regional evolution of the Himalaya is well described by several kinematic models such as the critical taper wedge and channel flow models (Molnar and England, 1990; Royden, 1993; Nelson et al., 1996; Beaumont et al., 2001). The critical taper wedge theory suggests that the Himalayan orogenic front progresses foreland-ward via younger in-sequence frontal thrusts (Molnar and England, 1990; Royden, 1993), while the channel flow model explains the exhumation of Greater Himalaya through the extrusion of partially molten crust (Nelson et al., 1996; Beaumont et al., 2001). However, much less work has focused on the possibility of neotectonically-active thrusts in the northern Himalaya of northwest India, where the mountain ranges of the High Himalaya changes northwards to rocks that represent the ancient collision zone between the Indian and Eurasian plates in a region termed the Indus Suture Zone (ISZ).

This study is focused on the ISZ region near Leh, Ladakh, India, which represents the boundary between syn-collisional sediments of the Indus Molasse (mid-Miocene) from intrusive rocks of the Ladakh Batholith, within a region that is ~70–100 km north of the Zanskar Himalaya in the northwestern Himalaya. Early studies described the northwestern part of the Indus Molasse to be in a depositional, onlapping contact with the Ladakh Batholith (Searle et al., 1990, Searle et al., 1997) and, recent cosmogenic radionuclide (CRN) based studies have suggested that denudation rates along the ISZ are overwhelmed by glaciogenic surface processes and do not reflect tectonic processes to a first order (Munack et al., 2014). However, there are evidence that imply formation of backthrust between Indus Molasses and Ladakh Batholith (i.e. Upshi-Bazgo Thrust and Stok thrust; Tripathy-Lang et al., 2013) and that being neotectonically active at least since 45 ka (Sinclair et al., 2017). Neotectonic activity across this boundary is further supported by a study of strath terraces along the Indus River that crosses the ISZ. Strath terraces point to average incision rates ranging between 0.8 and 3.0 mm/a (Kumar and Srivastava, 2017). These large incision rates, caused by rapid uplift of the northwestern syntaxial region and the uplift along the Upshi-Bazgo thrust, led to the formation of ~62 ± 16 ka-aged strath terraces at ~134 ± 24 m above the current river level (Kumar and Srivastava, 2017). The individual terrace height, along the Indus river ranges from 81.2 to 158.8 m above the river level and the average strath elevation of 134 ± 24 m incorporates the propagation of error from individual terrace (discussed in Kumar and Srivastava, 2017). In this study, we bracket the timing of recent fault slip on the Upshi-Bazgo thrust, which separates the Indus Molasse from the Ladakh Batholith (also termed the Stok Thrust). We use new luminescence chronology of fault gouge, combined with field, structural and seismicity datasets as well as analysis of published denudation rate data. These data collectively argue for neotectonic deformation along the ISZ and provide evidence for active backthrusting within this region during the late Quaternary.