The tectonic history of Kohistan and its implications for Himalayan structure

Abstract

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.