Oblique convergence between India and Eurasia

[1] Oblique convergence is the general rule along convergent boundaries. Such convergence produces asymmetric deformation (topography) along the collision boundary. To quantify this asymmetry, we develop a method that allows us to calculate the asymmetry in topography with respect to a reference line drawn perpendicular to the collision boundary. The asymmetry in topography is presented as a number, or series of numbers, called here the asymmetry index. A positive asymmetry index indicates that there is more area with topography greater or equal to certain value to the east (right) of the reference line than to the west (left). The asymmetry index is negative for the opposite topography distribution. We use oblique convergent boundary conditions with a thin viscous sheet model to investigate the observed topography along the India-Eurasia collision boundary. The goal of our study is to reproduce the observed asymmetry in the topography (i.e., the high topography in the Tibetan Plateau is shifted to the east). We consider a laterally varying velocity (both magnitude and angle, which we call the angle of obliquity) along this boundary and use a homogeneous lithosphere for Eurasia. With boundary conditions alone we are unable to account for the degree of asymmetry present in the observed topography. The asymmetry in the calculated topography is enhanced by considering a rheologically stronger anomalous zone representing the Tarim basin. The use of a constant velocity (both magnitude and angle) boundary condition, as in earlier studies, produces topographic asymmetry in the opposite direction to the observed topography. This indicates that in the study of obliquely converging boundaries it is important to consider the variation of velocity (magnitude and angle) in the collisional boundary.