Arc-parallel shears in collisional orogens: Global review and paleostress analyses from the NW Lesser Himalayan Sequence (Garhwal region, Uttarakhand, India)

Abstract

Interest in hydrocarbon exploration from the Lesser Himalayan Sequence (LHS) has recently been revived amongst the petroleum geoscientists. Understanding the paleostress regime and the deformation mechanism are the two important steps to understand the structural geology of any (petroliferous) terrains. Arc-parallel shear is an integral deformation process in orogeny. The scale of such deformation features can range from micro-mm up to regional-scale. Unlike arc-orogen-perpendicular shear, different driving forces can produce arc-parallel shears. We review these mechanisms/theories from several orogens including the Himalaya, and compile 44 locations worldwide with arc-parallel shear. Due to continuous crustal shortening by the India-Eurasia collision, the squeezed rock mass at the plate interface has been building 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 arc-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 arc-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/arc-parallel compression. The NW-SE compression can be correlated with the D3 of Hintersberger et al. (2011) during ~ 4–7 Ma.