Tuhin Biswas, Narayan Bose , Dripta Dutta , Soumyajit Mukherjee

Interest in hydrocarbon exploration from the the Lesser Himalayan Sequence (LHS) has recently been revived amongst petroleum geoscientists. Understanding the paleostress regime and the deformation processes are the two important steps to understand the structural geology of any (petroliferous) terrane. Arc-parallel shear is an integral deformation process in orogeny. The scale of the consequent deformation features can range from micro-mm up to regional scale. Unlike orogen-perpendicular shear, different driving forces can produce orogen-parallel shears. We review these mechanisms/theories from several orogens including the Himalaya and compile 44 locations worldwide with reported orogen-parallel shear. Due to continuous crustal shortening by the India-Eurasia collision, the squeezed rock mass at the plate interface has produced the Himalayan Mountain chain. In addition, the rock mass also escapes laterally along the orogenic trend. Tectonic stress-field governs this mass flow. Field study and microstructural analysis in the northwest LHS (India) reveals orogen-parallel brittle and ductile shear movement. Y- and P- brittle shear planes, and the S- and C- ductile shear planes reveal the following shears documented on the ~ NW-SE trending natural rock selections: (i) top-to-NW up, (ii) top-to-SE up, (iii) top-to-NW down, and (iv) top-to-SE down. Our paleostress analysis indicates top-to-SE down and top-to-NW down shears occurred due to stretching along ~ 131°-311° (Dext), whereas top-to-SE up and top-to-NW up shear fabric originated due to shortening along ~133.5°-313.5° (Dcompr). Previous authors considered that the orogen-parallel extension generated ~ 15-5 Ma due to vertical thinning of the Himalaya. The NE-trending Delhi-Haridwar Ridge below the LHS plausibly acted as a barrier to the flowing mass and piled up the rock mass in the form of NW-SE/orogen-parallel compression. The NW-SE compression can be correlated with the D3 of Hintersberger et al. (2011) during ~ 4-7 Ma.