Arc-parallel compression in the NW Himalaya: Evidence from structural and palaeostress studies of brittle deformation from the clasts of the Upper Siwalik, Uttarakhand, India

The sub-Himalayan Upper Siwalik rocks, between the Main Boundary Thrust (MBT) to the north and the Main Frontal Thrust (MFT) to the south, are intensely brittle sheared and jointed. Our field studies around Dehradun (India) furnished at least eight small-scale brittle slip directions, viz., ∼ top-to-SW/SSW (up), top-to-SW/SSW (down), top-to-NE/NNE (up), top-to-NE/ENE (down), top-to-NW (down), top-to-SE/SSE (up), top-to-SE/SSE (down) and top-to-NW/NNW (up). Additionally, we report near-vertical faults, four sets of joints (inclined: J and J ; near-vertical: J and J ). Palaeostress analyses using T-TECTO Studio X5 with all joint sets reveal two compression directions ∼ ENE –WSW and ∼ NNW –SSE. We propose two possible temporal relations between the joint sets: (i) J , J , J and J are coeval (∼ ENE –WSW compression) and (ii) J and J developed coevally (∼ ENE –WSW compression) followed by J and J (∼ NNW –SSE compression), because arc-parallel compression (if any) occurs later than arc-perpendicular compression. The presence of already well-known strike-slip faults, viz., the Yamuna tear fault and the Ganga tear fault, at high angles, ∼ 55 and ∼ 85 to the orogenic trend, implies a possible arc-parallel compression in the Siwalik Himalaya in the study area. This ∼ NNW –SSE compression could also indicate a localised stress reorientation due to the curvature of the Thrust planes, viz., the MFT and the Asan Thrust (as observed in plan view) close to the study area. This study further shows that arc-parallel compression need not be restricted to the inner arc of an orogen, and/or, as in the case of the Himalaya, near the syntaxes.