NORMAL FAULTS COUPLED WITH MAJOR THRUSTS OF LESSER AND OUTER HIMALAYAS: OBSERVATIONS FROM NORTHWESTERN HIMALAYA AND THEIR GEOLOGIC IMPLICATIONS

The Himachal Himalaya, in NW India, southward successively exposes Neo Proterozoic-Palaeozoic Tethyan Basin, Meso to Neo Proterozoic Central crystalline rocks with younger intrusives and sediments, Palaeo to Neo Proterozoic Lesser Himalayan sequence, Unmetamorphosed Palaeogene and Neogene Belts and Indo-Gangetic Alluvial Plain, each separated by thrust contact except for South Tibet Detachment Fault (STD) between Tethyan and Central Crysltalline belt, considered a normal fault. No other orographic parallel normal fault recorded or recorded as local subdued feature. Other normal faults are mostly across the Himalayan orographic.

First time, We report the systematic occurrence of orographic parallel normal faults associated with all the major thrusts of Central Crystallines, Lesser and Outer Himalayas except for the Himalayan Frontal Thrust (HFT).

These occur in the proximity of a thrust, preferably on footwall side as south verging steep fault with top to the norward movement. The presence of major down drag, domal structure on footwall side and moderate sag-drag in thrust block are associated characteristic. These faults post date a thrust and continue regionally but influenced by cross tears. They are also associated with basic intrusive and hydrothermal mineralization, hot springs occasionally in Lesser Himalayas. Major landslides such as Kotrupi near Mandi and those along Satluj and Giri valleys also located on this fault. Associated tilted Quaternary terraces and sediments are also indicative of their neotectonic behavior.

These faults may have developed in response to isostatic sinking of thrust block during weaken or cessation of a compressive tectonic events. The mechanism evolved for evolution of the thrust, is in conformity with Critical Wage Tapering model with a variance that instead a critical litho-static load, the steep fault zone has been considered acting like a wall support for the development of successive thrust towards foreland.