Identifying active structures in the Chitwan Dun, Central Nepal, using longitudinal river profiles and SL index analysis

Highlights

• The Chitwan dun located in a seismic gap in the central sub-Himalaya is investigated in this study.

• Active faults are mapped on the basis of longitudinal profile and SL index analysis of rivers flowing in Chitwan dun.

• Nine active faults that are potential sites of rupturing during a future mega earthquake are identified in this study.

Abstract

Previous studies have identified a seismic gap that has not witnessed any surface rupture earthquakes in the last 200–500 years located in the central sub-Himalaya (i.e., area around Chitwan intermontane valley). A higher rate of convergence in these parts poses a great threat of a surface rupturing earthquake in future. Recent analysis of the 2015 Gorkha earthquake to the north of this region suggests that it failed to rupture the surface; moreover, the stress is suggested to have transferred towards south. Therefore, the Chitwan intermontane valley - located in the sub-Himalaya of the central Himalayan seismic gap - is investigated in this study to identify the active structures. The longitudinal profile of a river considered to be one of the important geomorphic markers is used in this study for the identification of the active structures. The Hack profile and Stream-Length gradient index (SL index) is used to identify the anomalous segments of the rivers; these anomalous reaches are verified in the field for the presence of structures. Geomorphology of the region also supports the presence of active reverse faults in the valley. Abrupt break in the continuity of the geomorphic units and its correlation with the knick points and high SL index values indicate presence of around nine active structures in the Chitwan intermontane valley. Six new structures viz., Jharahi Thrust (JHT). Belani Thrust (BT), West Chitwan Thrust (WCT), Danda Thrust (DT), Central Churia Thrust 2 (CCT 2) and Shaktikhor Thrust (ST) have been identified in this study; whereas, three previously mapped structures viz., Main Boundary Thrust (MBT), Barakot Fault (BKF) and Central Churia Thrust (CCT) are also found to be active during the Quaternary/Holocene. Most of the newly identified structures appear to be splays of either the MBT or the CCT. These structures have displaced Quaternary landforms within the valley and are potentially active. The structures can get re-activated during large magnitude earthquake events in future.

Introduction

The Himalayas which form the northern boundary of the Indian plate are one of the most tectonically active regions of the world. Continued convergence between the Indo-Eurasian plates result in an active seismicity along the length of the Himalayas. Since the beginning of the Himalayan orogeny in early Cenozoic, the mountain front has migrated southwards, resulting in three distinct NW-SE trending segments of the Himalayan orogen - the Higher Himalaya, the Lesser Himalaya and the Sub-Himalaya (Gansser, 1964, LeFort, 1975; An Yin, 2006). These segments have originated from north to south as a result of thrusting along three major thrust systems viz., Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT; also called as Himalayan Frontal Thrust or HFT) respectively. These major thrusts represent branches arising from a gently dipping decollement (Main Himalayan Thrust: MHT) which separates the Himalayan sequence from the basement rocks of the Indian shield (Powers et al., 1998, Schelling and Arita, 1991, Schelling, 1992, Khanal and Robinson, 2013). The MFT is the youngest thrust system and forms the southernmost boundary of the Himalaya. Some studies (e.g., Lavé and Avouac, 2000) have shown that along the MFT the majority of convergence has been accommodated between the Indian and the Eurasian plates during the Holocene. Active deformation along the MFT has led to the evolution of the frontal foreland belt/Sub-Himalaya which is composed of Siwalik strata. According to wedge tectonic theory, a new fault develops near the frontal part of the taper and maximum deformation also occurs in the same zone (Davis et al., 1983, Dahlen et al., 1984). Evidences of surface rupture has been shown in several parts of the frontal Himalaya from NW to SE (Kumar et al., 2001, Kumar et al., 2006, Mugnier et al., 2005, Malik et al., 2010). However, not all ruptures occurs along the MFT; there are evidences of surface ruptures to the north of the MFT as well, indicating presence of active structures to the north of the MFT (e.g., Mugnier et al., 1998, Mugnier et al., 1999, Mugnier et al., 2005, Thakur and Pandey, 2004, Wobus et al., 2005, Malik et al., 2007, Singh and Tandon, 2008, Singh and Tandon, 2010; Singh et al., 2008, Thakur et al., 2014, Hossler et al., 2016, Dey et al., 2016).

In some parts, to the north of the Siwalik Hills (bounded by the MFT to its south) relatively flat terrain called as intermontane valleys (also known as dun in local language) are present. These valleys usually correspond with the sinuosity of the MBT – occurring in the recess (or the part concave towards the foreland); the salient (or the convex part) lack intermontane valleys. As a result the width of the Sub-Himalaya is greater along the recesses in comparison to the salients. The Soan, Pinjaur and Dehra duns are intermontane valleys along the northwestern Himalayan front, and the Dang, Deukhury, Chitwan and Hetauda duns are intermontane valleys along the central Himalayan front (Tandon and Singh, 2014). These intermontane valleys represent longitudinal depressions developed on the hanging wall of the MFT and are characterized by the presence of several landforms and structures that have evolved during the Quaternary period. Several previous workers (e.g., Nakata, 1972, Malik and Nakata, 2003, Mugnier et al., 2005, Philip and Virdi, 2007, Singh et al., 2008, Singh and Tandon, 2008, Singh and Tandon, 2010, Philip et al., 2011) have shown active faults within these intermontane valleys. These active faults displace the quaternary landforms in the valley and hence represent out-of sequence/reactivated thrusts in the MFT hanging wall. They are developed mostly as splays or back thrusts arising from the MBT and MFT, due to later reactivation. However, some intermontane valleys which occupy strategically important locations along the central part of the Himalayan front (e.g. Chitwan dun, Hetauda dun) are not very well studied (Divyadarshini and Singh, 2016). This makes them interesting areas to investigate for active faults.

The recent 2015 Gorkha Nepal earthquakes occurred to the north of the Chitwan dun which is located around 100 km southwest of Kathmandu in the central part of the Himalaya (Fig. 1a). The earthquakes originated in the Lesser Himalaya, north of the MBT. Slip due to the major (M_w 7.8) event which initiated on the MHT (décollement), propagated towards SE and failed to rupture the Himalayan front (Fig. 1b) (Avouac et al., 2015, Bilham, 2015, Luo and Chen, 2016, Kumahara et al., 2016). The failure of southwards rupture propagation of the Gorkha earthquake has resulted in a transfer of large amount of stresses to the southern part of the Central Nepalese Himalaya (Bilham, 2015). Post seismic slip due to the earthquake is reported along a thrust segment (Main Dun Thrust - MDT) present to the north of the MFT, SE of Kathmandu (Elliott et al., 2016). In another study, Bollinger et al. (2016) have suggested that a large earthquake is due in the area to the south of Kathmandu and Pokhara (i.e., area around Chitwan dun). Hence, this area becomes an important location to identify active faults and potential sites of rupture in case of a high magnitude earthquake.

The Chitwan dun is largely unexplored despite the significance of the location in relation to the regional seismo-tectonics of the Central Himalaya. Keeping in view that previous studies (e.g. Mugnier et al., 2005, Philip and Virdi, 2007, Mukul et al., 2007, Singh et al., 2008, Singh and Tandon, 2010) and recent study (Elliott et al., 2016) have identified either reactivation/out-of-sequence thrusting or stress accumulation in the areas to the north of MFT, we aim to investigate the Chitwan Dun area in this study. While, it is very difficult to identify the exact fault that will rupture during an earthquake event, identification of active faults in the area would suggest other potential sites in the central Himalayan region apart from the MFT, which is considered to be the most active in the Himalaya.

There are several techniques such as geophysical, paleo-seismic, and morpho-tectonic studies which are followed worldwide for the investigation of active faults. The existence of faults (whether blind or associated with surface rupturing) largely control the development of topography in a region. Thus, landscapes in actively deforming terrains bear evidences of active faulting in the form of geomorphic markers (Burbank and Anderson., 2012). Fluvial systems are the most sensitive landscape components which respond directly to tectonic perturbations (e.g Schumm, 1986, Holbrook and Schumm, 1999, Snyder et al., 2000, Brocard et al., 2003, Larue, 2008). River profiles have long been used to identify the anomalies and deformation in tectonically active areas (e.g., Demoulin, 1998, Kirby and Whipple, 2001) and even in areas of low tectonic activity (e.g., Carretier et al., 2006, Ambili and Narayana, 2014), due to their high sensitivity. They have been shown to record spatial and temporal deformation pattern (Kirby and Whipple, 2012). Sudden changes in gradient along river profiles appear in the form of prominent breaks known as Knick points (Hack, 1957, Hack and Young, 1959). The generation of knick points along a river profile is a result of either change in lithology or due to presence of active structures (Hack, 1973, Bishop et al., 2005, Larue, 2008). Therefore, in tectonically active regions, such as the Himalaya, knick points are useful indicators of active structures. The stream length (SL) index allows a better detection of knick points along river profiles (Hack, 1973, Seeber and Gornitz, 1983; Keller and Pinter, 2012, Pérez-Peña et al., 2009, Chang et al., 2015). In Himalaya, several studies have successfully used river profile and SL index analysis to identify tectonically active segments (e.g., Seeber and Gornitz, 1983, Delcaillau et al., 2006, Singh et al., 2008, Singh and Tandon, 2008, Singh and Jain, 2009, Singh and Awasthi, 2010, Bhat et al., 2013). In this study, we investigate active faults in Chitwan dun by analysing the longitudinal profile and SL index of rivers flowing within the dun. These are further correlated with the displacement in the landforms to assess activity along them.