Structural evolution of the Kangra recess, Himachal Himalaya: a model based on magnetic and petrofabric strains

摘要

Structural style, supplemented by magnetic and petrofabric strain studies, suggests that the Kangra recess is the result of combined frontal and oblique thrust ramps, which were initiated as a listric normal fault during the pre-Himalayan tensional phase and were later reactivated as a listric thrust fault during the Himalayan orogeny. The displacement out of the tectonic transport plane has resulted in the formation of a pull-apart basin parallel to the oblique ramp. The shear strain associated with the faulting was accommodated within a narrow zone characterized by the presence of salt. The structures in the hanging wall were developed as a result of a combination of simple shear and pure shear. The orientations of the folds were controlled by interference patterns between simultaneously developing folds, and thrust ramp structures. There is no marker horizon across the Main Boundary Thrust (MBT) which could be used to measure the amount of displacement, but a method based on the geometry of the pull-apart basin is proposed to estimate the strike-slip displacement along the oblique ramp. The method indicates a horizontal displacement of ～ 33.5 km along the MBT, which corresponds well to other geological data. A sequence of stages in the structural evolution of the area is proposed.