TECTONICS OF SIWALIK HIMALAYA IN DEHRADUN-ROORKEE SECTION, INDIA

The collision between the Indian- and the Eurasian plate produced the Himalayan orogen ~ 55 Ma back. Fieldwork in the Siwalik Himalaya in the Dehradun-Roorkee section revealed ~ top-to-SW/SSW (up), top-to-SW (down), top-to-NE/NNE (up), top-to-NE/NNE (down), top-to-NW (down), top-to-SE/SSE (up), top-to-SE/SSE (down), top-to-NW/NNW (up) sense of brittle shear, (h0l) and (hkl) joints, vertical to sub-vertical joints and faults from conglomerates and sandstones.

Palaeostress analyses suggest two compressional and two extensional deformation phases. Correlation amongst deformation phases are obscure from field observation since those were merely spot observations, and one brittle plane was not found to cut across the other. A NE-SW compression by Main Frontal Thrust (MFT). A simultaneous syn- to post- extensional phase developed due to gravitational adjustment in the active orogenic wedge. A compression along NNW-SSE co-occurred. Vertical joints and faults were produced probably under isostatic adjustment.