CRUSTAL THICKENING VERSUS LATERAL EXPULSION IN THE INDIAN-ASIAN CONTINENTAL COLLISION

Since the, beginning of the continental collision between India and Asia there has been about 2500 km of convergence, and the northward movement of India has been accommodated by major internal deformation of the Asian lithosphere. The crustal thickening in and around the Tibetan Plateau is clearly a direct consequence of this collision, but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern Asia and southern China. Numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere. We employ a thin viscous sheet model of the lithosphere with a depth-averaged nonlinear viscous rheology described by a stress-strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless Argand number Ar. The eastern boundary for the collision region is described as a lithostatic boundary; the precollision normal stress is determined by static balance, and that constant stress is applied throughout the collision. The experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1/4 of the indentation rate, with only minor variation due to geometry or rheology. Crustal thickening rates in the region of the plateau are reduced by between 10% and 25% of the corresponding rates determined for similar experiments with a rigid eastern boundary. To generalize from the results of these experiments, the total north-south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3:1 and more probably 4:1.