India-Asia collision and the Cenozoic slowdown of the Indian plate: Implications for the forces driving plate motions

The plate motion of India changed dramatically between 50 and 35 Ma, with the rate of convergence between India and Asia dropping from ∼15 to∼4 cm/yr. This change is coincident with the onset of the India-Asia collision, and with a rearrangement of plate boundaries in the Indian Ocean. On the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate, we perform force balance calculations for the precollision and postcollision configurations. We show that the observed Euler poles for the Indian plate are well explained in terms of their locations and magnitudes if (1) the resistive force induced by mountain building in the Himalaya-Tibet area is ∼5-6 ×10¹² N/m, (2) the net force exerted upon the Indian plate by subduction zones is similar in magnitude to the ridge-push force (∼2.5 ÷ 10¹² N/m), and (3) basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame. The third point implies an asthenospheric viscosity of ∼2-5 × 10¹⁹ Pa s, assuming a thickness of 100-150 km. Synthetic Euler poles show that crustal thickening in the Tibetan Plateau was the dominant cause of the Cenozoic slowdown of the Indian plate.