Kinematics and Dynamics of the Pamir, Central Asia

Abstract:

The Pamir region of the India-Eurasia collision zone is the site of a rare geological phenomenon: ongoing subduction of continental crust. We investigate the surface expression of continental subduction in the region by modeling the distribution and magnitude of strain rates and quantifying the effects of body forces (gravitational potential energy) and boundary forces in driving surface deformation.

We first construct kinematic strain rate and velocity fields on a 0.25° by 0.25° grid using continuous spline interpolations of observed strain rate data in a Eurasia-fixed reference frame. Strain rate observations include India plate motion, velocities at 506 GPS stations, estimated Quaternary fault slip rates, and fault slip styles. We then calculate gravitational potential energy (GPE) using the Crust1.0 crustal thickness model and assuming Airy isostasy. Following the methods of Flesch et al. (2001), we then solve the depth averaged 3-D force balance equations to determine the vertically averaged deviatoric stress field associated with GPE. We next solve for stress field boundary conditions by minimizing the differences between a) the directions of deviatoric stress principal axes and strain rate principal axes, and b) the style of deformation predicted by the stress field and the style of deformation predicted by the strain rate field. These stress field boundary conditions are assumed to represent effects of plate motions.

Our best-fit strain rate field shows regions of high strain rate along the Chaman Fault, Darvaz Fault, Zebak-Munjab Fault, Main Boundary Thrust, Main Pamir Thrust, boundaries of the western Tarim Basin and across the western Tien Shan. GPS data indicate strain along the Main Frontal Thrust. Our results confirm north-south compression and east-west extension in the central Pamir, as reported in Mohadjer et al. 2010. Despite high GPE/topography in the central Pamir and low GPE/topography in the adjacent Tarim Basin, the Pamir Mountains do not appear to collapse to the east, implying the presence of an additional process bolstering the eastern Pamir.

The strain rate and velocity fields produced in this study provide a detailed investigation into surface kinematics and dynamics in the Pamir and offer a first-order look at regions with increased vulnerability to seismic hazards.