Cooking of high-pressure rocks in the subduction zones: An insight from lubrication dynamics

Abstract:

The exhumation mechanism of high/ultra-high pressure (HP/UHP) rocks from depths >100 km still stand out as a subject of strong debate in subduction tectonics. Using lubrication theory we propose a viable mechanical model to explain the extrusion dynamics of HP/UHP rocks. The model suggests that the subducting plate motion produces viscous flow-induced dynamic pressure (P) of large magnitudes (0.1- 2 GPa) in the subduction wedge, which supports the excess pressure (D) of the relatively denser overburden. For a given geometry of the convergent setting, P, which is found to hold a linear relation with the wedge viscosity (µ) and subducting plate velocity (u_s) modulates the exhumation process. For a dynamic condition P > D, there is no sign of exhumation, and the overriding plate undergoes contractional deformation to accommodate the convergence movement. A dramatic tectonic transition occurs when P < D, where the overriding block undergoes rotational collapse under the effects of its own gravity load, and forces the deep-crustal wedge materials to extrude through a narrow channel to the surface. From computational fluid dynamics (CFD) simulations, we demonstrate fast exhumation events of UHP rocks at rates (several cm/yr) comparable to the plate velocity. It is shown the exhumation history can track episodic events, modulated by inversions in the P versus D relation. We finally discuss implications of the lubrication model in interpreting the origin of HP and UHP rocks reported from many subduction zones, including the Himalaya-Tibet system.