Kinematics of active deformation in the Sulaiman Lobe and Range, Pakistan

Abstract

The western margin of the Indian plate is highly oblique to the direction of convergence between India and Asia and represents an excellent example of large‐scale oblique continent‐continent collision. Determining the strain field in western Pakistan and how it relates to the plate motion and plate margin geometry affords an exceptional opportunity for understanding oblique margin processes in general. Through the inversion of regional and teleseismic body waves, we have determined the source parameters of 10 moderate‐sized earthquakes that occurred between 1964 and 1985 in and around the Sulaiman Range, Pakistan. The earthquakes are dominantly thrust events with slip vectors that are approximately perpendicular to the lobate Sulaiman mountain front. Slip vector orientations rotate 60°–70° from a N‐S to a WNW‐ESE direction of compression, consistent with the geometries of the complex, festoon‐shaped mountain belts of this margin. We have estimated the spatial variation of the horizontal strain rate and velocity fields within Sulaiman using vertically averaged models that accommodate plate motion constraints within a deforming layer. The most important factors determining the style of strain rotation in the Sulaiman Lobe and Range are the presence of pure strike‐slip motion along the Chaman Fault, and the relatively rigid and undeformed Katawaz Basin that is therefore allowed to translate obliquely relative to India. This same conclusion is obtained using either a three‐dimensional, frictional, analogue model with significant basal tractions or a thin sheet viscous numerical model without basal tractions. Thrusting in a predominantly NW‐SE direction in the Sulaiman Range accommodates 5–14 mm/yr of N‐S motion between India‐Eurasia and 3–6 mm/yr of E‐W shortening. Seismic moment release this century within the India‐Eurasia plate boundary zone, west of the western Himalayan Syntaxis, constitutes roughly 40% of the expected total seismic moment release for this time period. Particularly significant moment rate deficits exist within the Sulaiman Range and along the Chaman Fault.