Escape Mechanism with Shallow Ramp and Décollements in Southwest Taiwan

Abstract

The escape structure in southwest Taiwan has long been discussed. The counterclockwise rotation in GPS ground motion is argued to be evidence of tectonic escape. However, tectonic escape events worldwide are always bounded by lithosphere-cutting strike-slip faults at the boundary of the lithospheric-scale rotating block, and these have not observed in Taiwan. In this study, we propose that the escape structure in southwest Taiwan is a thin-skinned deformation due to the open boundary in the continental slope, the ramp, and the shallow décollements. We tested this shallow escape hypothesis using a 3D numerical simulation with elastoplastic rheology. We found that a conjugate pair of forethrust and backthrust developed above the ramp. The strike and location of the forethrust mainly follow the ramp. However, the strike of the forethrust rotates perpendicular to the open boundary when in proximity. From north to south, the strike of the forethrust transfers from NNE to NE, and the deformation transfers from thrust to dextral thrust. This leads to a counterclockwise rotation in ground motion, which matches the GPS observation. This research provides a different explanation of the mechanism for the escape structure in southwest Taiwan.