Morphotectonics, slope stability and paleostress studies from the Bhagirathi river section, western Himalaya (Uttarakhand, India)

Abstract

We study parts of Tethyan, Higher and Lesser Himalayan rocks along the Bhagirathi river valley for morphotectonic analysis. The spatial and linear properties of the 21 sub-watersheds (S-WSs) and the Bhagirathi main watershed provide strong evidence of active tectonics mainly in the S-WS 3 (through which the South Tibetan Detachment passes), S-WS 9 (no fault runs), S-WS 12 (Vaikrita, Munsiari and Tons Thrusts cross) and S-WS 17 (Basul and Tons Thrusts occur). The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) has been used to classify the sub-watersheds as per the intensity of their recent tectonic activity. Seven morphometric parameters are used for the TOPSIS analysis. From the Lesser Himalayan section additionally, we perform landslide and paleostress studies. Eleven slopes cuts and 24 landslides were investigated to determine the mode of failure in a portion of the Rishikesh-Gangotri Highway. Landslides in soil strata is caused mainly by the low cohesion and due to the presence of coarse-grained loose materials. In the present study, most landslides (and earthquakes) have occurred in the vicinity of major thrusts. Where there is a high frequency of slickenside related to brittle normal faulting (K2 zone, near Dunda, Singuni and Dharasu Thrusts), a higher earthquake frequency of 3.5–5.2 magnitude is observed from the data set of around last 75 years. Paleostress analysis on data-sets of normal, reverse and strike-slip movements using the WinTensor software (ver. 5.8.8) yields NNE-SSW direction of extension for normal slip, NE-SW compression for reverse movement, and a pure strike-slip tensor with NNE-SSW shortening and WWN-SSE direction of maximum extension. The K2 zone where these deformations were most documented is also the place of slope instability and high present-day tectonic activity.