Morphotectonic analysis and GNSS observations for assessment of relative tectonic activity in Alaknanda basin of Garhwal Himalaya, India

Highlights

• Alaknanda basin in the Garhwal Himalaya, India, is a tectonically active region owing to ongoing crustal deformation.

• Relative tectonic activity in the basin was evaluated by using eight geomorphic indices and seismicity.

• The integrated product, relative tectonic activity index (TAI) map, was produced with three classes.

• Results were compared with high crustal movement rate computed through Global Navigation Satellite System (GNSS).

Abstract

Alaknanda basin in the Garhwal Himalaya, India, is a tectonically active region owing to ongoing crustal deformation, erosion, and depositional processes active in the region. Active tectonics in this region have greatly affected the drainage system and geomorphic expression of topography and provide an ideal natural set up to investigate the influence of tectonic activity resulting from the India-Eurasia collision. We evaluated active tectonics by using high resolution digital elevation model (DEM) based on eight geomorphic indices (stream length gradient index, valley floor width-to-height ratio, hypsometric integral, drainage basin asymmetry, transverse topography symmetry factor, mountain front sinousity index, bifurcation ratio, and basin shape index) and seismicity in eight subbasins of Alaknanda basin. The integrated product, relative tectonic activity index (TAI) map, was classified into three classes such as: ‘highly active’ with values ranging up to 2.0; ‘moderately active’ with values ranging from 2.0 to 2.25; and ‘less active’ with values > 2.25. Further, the results were compared with relatively high crustal movement rate of 41.10 mm/y computed through high precession Global Navigation Satellite System (GNSS) based continuous operating reference station (CORS) data. Thus, we concluded that this new quantitative approach can be used for better characterization and assessment of active seismotectonic regions of the Himalaya and elsewhere.

Introduction

The highest mountain range of the world, Himalaya is the centre of attraction for tectonic studies for more than a century mainly because of active tectonics and seismicity (Heim and Gansser, 1939, Srivastava and Ahmad, 1979, Hodges and Silverberg, 1988, Valdiya, 1995, Yin, 2006). Morphometric analysis has been widely used and is established as a proven technique to understand development of geomorphic features in the young mountain belt (Keller and Pinter, 1996, Keller and Pinter, 2002, Mesa, 2006, Ozdemir and Bird, 2009, Perez-Pena et al., 2010, Bali et al., 2012). In mountainous terrain, active tectonics can be viewed as the main factor shaping the present-day topography, which is a result of the combined effect of tectonics and erosional/denudational processes (Andermann and Gloaguen, 2009, Harkins et al., 2005, Perez-Pena et al., 2009). The drainage pattern in tectonically active regions is very sensitive to processes such as upliftment, folding, faulting, and tilting - which are responsible for river incision, basin asymmetry, drainage geometry, and river deflections (Cox, 1994). The geomorphic indices are important indicators capable of decoding landform responses to active deformation processes and have been widely used as a reconnaissance tool to differentiate zones influenced by active tectonics (Keller and Pinter, 2002, Chen et al., 2003). The information about geomorphic indicators of active tectonics can be retrieved through the analysis of topographic maps, aerial photographs, satellite images and quantification of morphotectonic features (Horton, 1945, Keller and Pinter, 1996, Keller and Pinter, 2002).

In the present study an attempt was made to develop a quantitative methodology to analyse geomorphic indices derived from the digital elevation model (DEM), seismicity, and ground-based GNSS observation in Alaknanda basin (Fig. 1), a major tributary of the Ganges in Garhwal Himalaya, Uttarakhand, to evaluate relative tectonic activity across the basin. The Alaknanda basin is one of the most tectonically active regions in Uttarakhand as it lies in ‘Zone-5’, the highest seismic hazard zone map of India and has experienced last major seismic event of the region, Chamoli earthquake, Mw 6.8 (29 March 1999) and many tremors in the recent past. The seismic hazard exposure has increased many fold because of higher tourist influx to pilgrimage sites, hydropower and other infrastructure development in the region. The tectonic activity and seismicity in this region is attributed to the India-Eurasia collision resulting in major faults such as the South Tibetan detachment (STD), Main Central Thrust (MCT), Main Boundary Thrust (MBT), and many other faults such as Ramgarh Thrust, North Almora Thrust, etc. observed in the study area (Valdiya, 2010). Present study was attempted to understand relative active tectonics by analysing eight geomorphic indices and seismicity in eight subbasins of Alaknanda basin. These eight geomorphic indices and seismicity index were combined to provide a regional estimator to characterize tectonic activity and compare the same with actual ground movements observed from the GNSS receiver. Similar studies using morphometric parameters were found to be useful in various tectonically active regions such as the Hindu Kush (Mahmood and Gloaguen, 2012), southeast Spain (Perez-Pena et al., 2010), Iran (Dehbozorgi et al., 2010), and southwestern Sierra Nevada of Spain (El Hamdouni et al., 2008). The objective of this study is to quantify and integrate several geomorphic indices and seismicity and to validate with ground observation to produce a single index that can be used to characterize relative influence of tectonic activity.