On the competition of interplate shear and surface erosion at convergent plate boundaries

Abstract

Convergent plate boundary tectonics is characterised by a variety of deformation patterns from subduction orogenesis to continental collision zones. These tectonic features are associated with the long-term deformation of the lithospheric wedge overriding the subducting lithospheric slab. Two sets of forces appear to be predominant in the process of tectonic structure formation: the shear forces corresponding to relative plate motion along the tilted bottom of the lithospheric wedge and the changing load pattern along the free surface of the lithosphere caused by erosion and redeposition. According to the large scales involved in space and time one might argue in terms of an induced mass flow in the lithosphere driven by interplate shear, topographic erosion and isostatic readjustment.

We study the cooperation of these well established phenomena by means of a scale consistent numerical finite element approach. In order to represent the overall mechanical behaviour in various scales appropriately, we adopted an elasto-visco-plastic constitutive equation for the strain rate. It incorporates a yield criterion as well as power law creep depending on thermal energy. The long-term mass flow is traced by a regridding technique of the numerical scheme. The cooperation of subduction-driven tectonic erosion and topography-driven surface erosion leads to a rather stable structural pattern of deformation. Broad topographic uplift is reflected by a dynamically formed root-zone. This orogenic kernel is coincidently the location of exhumation of deep seated matter at the surface while in the neighbourhood on both sides fore- and back-arc or fore- and hinterland basins are formed. The hinterland basin is a result of thinning by lateral mass flow beyond the orogenic section into a hinterland plateau. Our numerical results suggest a competitive influence between both tectonic and topographic erosion. The influence of the tilt angle of the shear interface is of comparable importance for the formation of the dominant features during the long-term process. The relative strength of the lower crustal layer compared to the upper mantle structure controls the lateral distance of the induced mass flow pattern within the lithospheric wedge. Altogether, the extremely simplified scale consistent approach provides a new insight into the complex nonlinear tectonics of convergent plate boundaries.