Geomorphic characteristics of landscape development and formation of lakes in the zone of Munsiari Thrust, Garhwal Himalaya, Uttarakhand, India
Introduction
Topography of the mountain belts results from tectonic uplift caused by plate boundary forces and the efficiency of erosional processes, mainly determined by lithology together with climatic changes (Strecker et al., 2007) and tectonics as well. However, the geomorphic systems that evolve under constant uplift, the climate and lithology tend towards a steady state because of the establishment of a dynamic equilibrium between erosion and uplift (Willet and Brandon, 2002). Since the tectonics involves both horizontal displacements and vertical motions of the Earth's crust (Hay, 1996), active tectonics, particularly in the mountainous region, is a leading factor contributing to rock uplift and therefore the topographic modification is considered as a result of the coupling of tectonics and erosional processes (Andermann and Gloaguen, 2009; Pérez-Peña et al., 2010). These processes include vertical tectonic motion, erosion, glacial activity and high monsoon precipitation (Montgomery, 2001).
The Himalaya is flanked by intracrustal boundary thrusts (viz., Main Boundary Thrust (MBT), Main Central Thrust (MCT) and associated thrusts/faults, e.g., Himalayan Frontal Thrust (HFT) and South Tibetan Detachment system (STDS)/Trans Himadri Fault (THF), which are demarcated by different litho-tectonic units as Sub-Himalaya, Lesser Himalaya, Higher Himalaya and Tethys Himalaya from south to north respectively (Gansser, 1964; Tapponier and Molnar, 1977; Valdiya, 1980; Nakata, 1989; Godin, 2003; Yin, 2006; Célérier et al., 2009) (see Fig. 1a). The present study area falls in the central part of the Himalayan belt in the Garhwal Himalaya and is critical for studying the typical characteristics of the Himalayan fold-and-thrust belt (Srivastava and Mitra, 1994). The Garhwal Lesser Himalaya is cut by various thrusts/faults, e.g., MBT, MCT, HFT, Munsiari Thrust (MT), Ramgarh Thrust (RT), Tons Thrust (TT)/Berinag Thrust (BT) and STDS. This middle latitudinal domain, tectonically bounded at the top by the MCT and base by the MBT, comprises mainly of Precambrian clastic sediments and meta-sedimentary rocks (Brookfield, 1993; Yin and Harrison, 2000). The MCT zone has largest width of about 80 km in the Kumaun and Garhwal regions (Thakur, 1992). Geologically, the Chail/Ramgarh/Srinagar Thrust, the Budhakedar Thrust (MCT III), the Jutogh/Almora/Munsiari Thrust (MCT II) and the Vaikrita Thrust (VT/MCT I) are principal thrusts in the Garhwal Himalaya (Valdiya, 1980; Saklani and Bahuguna, 1986; Saklani, 1993; Naithani et al., 2002) and the investigated area is located in the zone of the MT (MCT II). Towards south of Vaikrita Thrust, the MT forms a long thrust flat that extends far south into geographically defined Lower Himalaya and splits in its up dip direction into two fault splays, e.g., the RT below and the Almora Thrust above (Valdiya, 1980). The MT is considered to have activated within the Lower Himalaya, approximately 1–2 Ma ago (Mukherjee, 2015) and thus recognized as out of sequence thrust (Célérier et al., 2009; Webb, 2013; Mukherjee, 2015; Yu et al., 2015), and an out-of sequence thrust can also reactivate other thrusts (Mukhopadhyay and Mishra, 1999). A number of earlier studies have also documented active nature of the MCT including MT in the Garhwal and Kumaun regions (Seeber and Gornitz, 1983; Valdiya, 2001; Célérier et al., 2009; Tyagi et al., 2013; Joshi and Kotlia, 2015; Kothyari and Luirei, 2016; Kothyari et al., 2017). Therefore, the lithology of area along the MT and other MCT zones in Garhwal Himalaya is fragile and susceptible for landslide disasters and has also been affected by larger floods in the recent past (Srivastava and Singh, 2017). Increasing impact of landslides and flood events in this part needs extra focus on the geomorphology and tectonics to identify vulnerable and geomorphologically sensitive pockets to help making applicable policies which may be helpful in minimizing the future loss of property and lives.
The tectonically active Himalayan domain has witnessed intense orogenic movements (Nakata, 1989; Valdiya, 1993), which have resulted in the reactivation of thrust/faults and impounding of ancient rivers/streams and formation of lakes during the Quaternary period (Kotlia et al., 1997, 2000; 2010; Valdiya and Kotlia, 2001). As such, the lake formation is coeval with fast vertical movements and disappearance coincides with increased erosion across the outlets (Castellanos, 2006; Kotlia et al., 2010). On the geological time scales, lakes are ephemeral systems that, once the forcing mechanism ceases, disappear by outlet erosion and/or sediment overfill (e.g., Ollier, 1981). Therefore, quantification and discrimination of the authigenic and detrital inputs within the basin and identification of mechanism of the detrital processes are crucial to understand the tectonic variability and in Himalayan region, coupling of the climate and tectonics has largely controlled the regional geomorphology of the Quaternary basins (Sangode et al., 2007).
Considering the role of tectonics, presumably added by the climate to modify the landscape in the Himalaya, a part of 55 km long Lastar Gad (southwardly flowing tributary of the Mandakini River in the upper catchment of Bhagirathi/Ganga basin in the Garhwal Himalaya was investigated (Fig. 1b and c). This sector (300 26′ N to 300 29′ N and 780 54′ E to 780 56’ E; elevation 1600–2100 m) is crossed by the RT and MT (Fig. 1b and c). The thrusts lie south of the Vaikrita thrust (MCT-I) zone which is not only seismically active but is also influenced by intense annual rainfall (Hodges et al., 2004). It should be mentioned here that in tectonically active regions, the rivers are mostly controlled by tectonics and climatic variability (Seeber and Gornitz, 1983). Simultaneously, they also play significant role due to their ability to incise, which ultimately sets the rate of lowering of a landscape resulting in mass removal in the uplifted regions (Snyder et al., 2000; Whipple, 2004; Pazzaglia, 2013). The aim of present study was (a) to delineate the neotectonic activity along the Lastar Gad using geomorphic features, added by significant morphometric parameters, and (b) to evaluate relative intensity of the tectonics towards the formation and disappearance of the lakes in the study area.
Section snippets
Geology, tectonics and seismicity
The Kumaun-Garhwal region of the NW Himalaya is divided into Inner (older) and Outer (younger) Lesser Himalaya (Valdiya, 1980; Célérier et al., 2009), the later lies between the MCT and MBT. Structurally, there lie two major thrust sheets in the Lesser Himalaya, viz., RT and MT (Singh and Patel, 2016). Different thrusts in the MCT zone are always associated with parallel and sub-parallel thrusts, branching off from the main thrust (Valdiya, 2014). These crystalline formations are termed as
Methodology and significance of morphometric indices in the study
Our study was based on the detailed field investigations (survey of India toposheets at 1:50,000 scale) including identification of various geomorphic features and distribution of fluvio-lacustrine deposits. The Shuttle Radar Topography Mission/Global Digital Elevation Model (SRTM/GDEM) and GDEM data were obtained to generate slope and aspect maps using Microdem-10 and GIS softwares. The chronology of Badanital lake core (herefater called as BTP) was taken from Kotlia and Joshi (2013) and that
Lithology and chronology of palaeolake deposits and modern lake core
The present day Badanital is a closed lake basin (elevation 2085 m) and is 125 m long and 60 m wide with inner periphery of 490 sq. m. (Fig. 3a and b). There are large grasslands, swamps/marshy areas within the outer periphery covering an area of 1100 sq. m. We identified >1m thick lacustrine deposit over landslide fan within the periphery of the present day Badanital (Fig. 3c). The Gethina palaeolake (GJP) profile, downhill of modern Badanital lake in hanging wall of the MT zone (Fig. 1c), is
Geomorphic observations on neotectonic activity
During the Quaternary period, recurring tectonic movements along the thrusts/faults around the study area are manifested in form of the frequent slope movements, tilted and displaced terraces, sloping of alluvial and colluvial deposits, fault scarps and facets, abrupt formation of deep gorges, river cascading, entrenched meandering and shifting of channels etc. (Nakata, 1989; Valdiya, 1993, 1999; Valdiya and Kotlia, 2001; Kotlia et al.,. 1997, 2010; 2018; Kotlia and Joshi, 2013; Goswami and
Slope instability and landslide activity
Tectonically controlled lithology in the MT and RT zones is sensitive for mass movement during the high rainfall events. This area is hit by several old and active as well as inactive landslides, fans and debris flow deposits (see Fig. 8a–d). The rocks are highly sheared and crushed with four sets of joints and large fractures. Among the four sets of joints (J1 to J4), J1 is principle/bedding plane joint, dipping 300 → 500, J2 as 550 → 2200, J3 as 550 → 750 and J4 dips as 520 → 2550.
Morphometric parameters
To infer the basin shape, drainage pattern, tilting and incision and uplift and erosion, we computed a number of morphometric indices as mentioned below.
Evaluation of tectonic activity along Lastar Gad
A significant role of the tectonic activity was interpreted by collecting together a number of data set including field geomorphic features and morphometric parameters and by showing their relationship in the Lastar Gad basin. The geomorphic characteristics, e.g., uplifted and scattered lake deposits near Badanital, several old/stabilized and active landslide fans, wide and straight course of river with depositional paired or unpaired terraces, waterfalls, deflected stream channels, incision,
Conclusion
The purpose of this study is to analyze the drainage pattern and landscape evolution in order to evaluate the tectonic activity within the actively deformed eastern most sector of the Indian Himalaya. The active tectonics in the MCT II zone is recognized based on the field geomorphic features as discussed above. Our field investigations are further supported by computation of morphometric indices including low values of Vf, elongated shaped basin, tectonic tilting and higher bifurcation ratio.
Acknowledgements
LMJ is thankful to Science and Engineering Research Board and Council of Scientific and Industrial Research New Delhi for financial assistant under Fast Track project (No. SR/FTP/ES-91/2012) and SRA fellowship under scientist pool scheme (Pool no 8885-A). BSK is thankful to the Ministry of Earth Sciences, New Delhi (MoES/PO/Geosci/43/2015) for partial financial assistance. We are gratified to the Head, Department of Geology, Kumaun University, Nainital for providing working facilities.
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Present address: CAS in Geology, Lucknow University, Lucknow, 226,007, India.