Large-Scale Rotational Motion Within the Main Central Thrust Zone in the Darjeeling-Sikkim Himalaya, India

摘要

Much of the shortening associated with the India-Eurasia collision has been accommodated by orogen-spanning shear zones within the fold-and-thrust belt that forms the Himalayan orogen. The kinematics on these shear zones and how they accommodated large-scale displacements remain poorly understood. In general, rock flow has been described as orogen perpendicular towards the south. Here, structural mapping and microstructural analysis are applied to demonstrate that the Darjeeling-Sikkim section of the Main Central Thrust Zone (MCTZ) records an anticlockwise rotation of the resolved stresses within the orogenic wedge, changing from south to north over the period between 20 and at least 9 Ma. The transport direction during thrusting varies systematically defining broad arcs at the scale of 50 to 100 km, rotating by up to 140 degrees from SE- or S-vergence in the north, to SW-vergence in the south. This rotation is independent of the similar to 60-km-wide dome in the footwall of the MCTZ, exposed as a broad half-window. Rotation may have been a response to rheological heterogeneities within the orogeny forming large-scale flow impediments.