LATERAL VARIATIONS IN THE CRUSTAL STRUCTURE OF THE PAKISTAN REGION

The tectonic environment of Pakistan is complex and involves the interaction of three tectonic plates -- the Indian Plate, the Arabian Plate and the Eurasian Plate -- with deformation on three active plate boundaries -- the convergent boundaries characterized by continent-continent collision and thrusting in the northern region of the Himalayas and by oceanic crust subduction with a volcanic arc and a wide accretionary wedge in the southern region of the Makran. These plate boundaries are connected by the Charman transform zone, characterized by very large left-lateral strike-slip and lesser thrusting. As a result of the complex tectonic environment, the Pakistan crust is probably complex with large lateral variations in structure, but almost nothing is known of the crustal structure of the region. In this study, we derive a broad-scale crustal model of the region from the analysis of receiver functions from teleseismic data recorded at 57 sites and surface wave dispersion measured from regional earthquakes. The joint inversion of the receiver functions and surface wave dispersion provides strong constraints on the shear wavespeed and thickness of the crust. Seismograph coverage is sparse in the south, dense across the transition from the Potwar Plateau to the Himalayas and sparse in the high Himalaya. The analyses of recordings in the Makran shows the thinnest (33-34 km thick) and slowest (3.35 km/s) crust overlying the subducting Arabian Seafloor. Farther to the north the crust beneath the Indus Plains varies from 40-44 km thick with a wavespeed of 3.75 km/s in the south to 63-65 km thick with a wavespeed of 3.84 km/s in the north. Along the western border with Afghanistan, the crustal structure varies from 60-63 km thick and an average wavespeed of 3.60 km/s beneath the Sulaiman Range in the south to the Safed Koh Range in the north but significantly thinner (42-43 km thick) and slower (3.41 km/s) below the Salt Range. The crust across the Potwar Plateau -- Himalayan transition varies from 43-44 km thick beneath the Potwar Plateau to 52-56 km thick to the north of the Main Boundary Thrust to 70-72 km thick beneath the high Himalayas; however, the average crustal wave speed across the transition does not show a significant variation in wavespeed (3.48 km/s beneath the Potwar Plateau to 3.46 km/s beneath the high Himalayas).