Insights into the dynamics of an orogenic wedge from lubrication theory: Implications for the Himalayan tectonics

Abstract

Using lubrication theory, we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate. The model suggests that the subducting plate motion produces a dynamic pressure in the wedge, which supports the gravity load of the overriding plate lying above it (stable condition). A drop in the dynamic pressure results in a rotational collapse of the overriding plate (unstable condition). We present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges. The lubrication model is also implemented in computational fluid dynamics (CFD) simulations to perform real scale numerical experiments, considering mainly three variables: 1) plate convergence angle (α), 2) subduction rate (us), and 3) viscosity ratio (R) between the overriding plate and wedge. The overriding plates attain mechanical stability when us exceeds a threshold value (us∗); otherwise, they become unstable as us<us∗. A linear increase of us∗ with α as well as R widens the unstable fields. We demonstrate that the stable to unstable transition initiates a crustal flow channel in the orogenic wedge, driven by the collapse of the overriding plate. Such a collapse is associated with a kinematic transformation from contraction to extension in the overriding plate. Based on geological proxies, this article finally explains the formation of the orogenic channel in the Himalaya as a consequence of the transition from stable to unstable state in overriding Tibet, lying above the Himalayan wedge, when the Indo-Asia collision rate reduced to ~5 cm/yr at ~22 Ma.