Gravity and tectonic model across the Sulaiman fold belt and the Chaman fault zone in western Pakistan and eastern Afghanistan

Abstract

Gravity data from the western boundary of the Indian subcontinent have been analysed to infer the gross crustal structure across the Indian/Afghan collision zone. Seismic reflection profiles reveal the gross structural (duplex) geometry of the Sulaiman fold belt. These data show that the wedge, which is 10 km thick at the deformation front, thickens northwestward to attain a tectonic thickness (thickness due to deformation) of about 20 km in the hinterland. Gravity modeling depicts the depth of the Moho between 33 and 35 km at the deformation front of the Sulaiman fold belt. The Moho depth shows an upward convexity along an E-W profile. It decreases northward with a gentle gradient of 1.1° (20 m/km) below the Sulaiman fold belt, and then deepens abruptly with a gradient of about 7.8° (136 m/km) across the Chaman fault zone, attaining a depth of about 57 km in eastern Afghanistan. The model suggests that the Sulaiman fold belt is underlain by transitional crust (15–27 km thick), in contrast to the continental crust (38 km thick) underneath the fold belt of the Himalayan collision zone in northern Pakistan. The about 57-km-thick crystalline crust in eastern Afghanistan may be due to: (1) underplating by crust of the Indian subcontinent; and (2) structural thickening within the Afghan block.