Active Tectonics in the Tibetan Plateau Region as a Consequence of Plate-Scale Forces on the Eurasian Plate: a Model Study

Abstract:

We study the forces acting on the Eurasian plate and the resulting present-day deformation. We use mechanically balanced models based on plate contact forces (continental collision, plate boundary friction at transform faults and subduction contacts, and slab roll-back forces), lithospheric body forces (from lateral variations in topography, density structure) and mantle convective tractions including dynamic topography. These forces drive Eurasia in the direction of absolute motion and fit observed horizontal stress directions to first order. We employ plane stress spherical finite elements and linear visco-elastic rheology to compute the lithosphere-averaged mechanical response. We consider the influence of including the major active faults in these models. Here we focus on intra-plate deformation in the Tibetan Plateau. We assume five different compositions for the upper and lower crust and one for the upper mantle, and we use geotherms and crustal thickness maps to constrain depth-dependent rheology profiles and to estimate vertically averaged viscosities.

Predicted velocities show to be very sensitive to the reference point: Eurasia consists of cratonic regions surrounded by more recently active “mobile belts”. Using the Siberian or the East European craton as a reference gives significantly different deformation solutions. Best-matching velocities are obtained using Eurasia’s “center of deformation”, defined on the basis of force moments and located in the southeastern Siberian craton. Comparison with horizontal GPS velocities shows a good correlation in velocity directions and magnitudes in the Tibetan Plateau, Tarim Basin and Tien Shan. Strain rate and vertical axis rotation rates also provide a good fit.

Velocity field and clockwise rotation pattern in Southeast Asia are highly dependent on the vertically averaged horizontal viscosity distribution contrast and its geometry. Faults do not significantly affect the predicted surface velocity field. Furthermore, deformation turns out to be very sensitive to the edge force distribution. In our best-fitting model, the magnitude of the India-Asia collision force is at least four times larger than any other force in the plate and indicates that the Tibetan Plateau deforms due to the largest line force in the planet (10-12 TN/m).