Deformation Kinematics and Dynamics in the Philippine Sea - Eurasia Plate Collision Zone in the Taiwan Island Determined From Recent GPS Data

The 300 km long segment of the Philippine Sea and Eurasia convergent plate boundary through the Taiwan Island involves collision, subduction, and back-arc extension tectonics. Various dynamic models have been proposed for the collision tectonics in the Taiwan Island based on geological or seismological evidence. We investigate possible dynamic processes that shape the modern tectonics in and around the Taiwan area. We first use recent GPS data to quantify the contemporary deformation kinematics in the region. A self-consistent strain rate tensor field is determined using an inversion procedure that minimizes the misfit between the model and observed GPS velocities in a common reference frame. The strain rate tensor field determined from GPS data predicts deformation styles that are in agreement with earthquake focal mechanisms and local geology, indicating that the interseismic deformation field represents the style of long-term deformation field reasonably well. The Longitudinal Valley fault zone (LVF) has the highest strain rates with magnitude of 1-2.5x10^-6/yr. The strain rates in the deformation front fault zone (DFFZ), west of the Central Range in western Taiwan, are also relatively high, with magnitudes ranging from 1x10^-7 to 2x10^-6/yr, increasing from north to south. The deformation style in both the LVF and DEFZ are primarily pure compression in northwest-southeast direction. In the Central Range, between LVF and DEFZ, extensional strain rates predominate. The extension regime in the Okinawa basin north of the Ryukyu subduction zone extends westward to the Ilan plain in northeast Taiwan. The rate of NS extension in the east, offshore of northeast Taiwan, could be as high as 20-40 mm/yr. West of Ilan plain, the extension rates decrease rapidly. The transition from extension to compression occurs in the northern Central Range, where strain rates are close to zero. In order to understand the dynamics driving the deformation we directly solve the force balance equations, using both a thin elastic layer and a thin viscous sheet formalism, for estimates of the styles and magnitudes of vertically averaged deviatoric stress. The observations that constrain the solutions are gravitational potential energy, inferred from topography, and the styles of strain, inferred from the kinematic solution. The solutions consist of two parts: vertically averaged deviatoric stresses associated with internal body forces (gravitational potential energy), and stress boundary conditions. The stress boundary conditions are sought such that the styles of total deviatoric stress match the styles of deformation inferred from the interseismic deformation field. Vertically averaged stress levels are approximately 100 bars in northern Taiwan, and are smaller in the southern Taiwan with stress levels ranging from 10-50 bars. Both the elastic and viscous solutions yield extensional deviatoric stresses over high topography, consistent with gravitationally driven extension. Although full solutions will require the treatment of basal tractions, the presence of extension over high topography is consistent with crustal scale, thin-skinned, type models where there is significant coupling between the elastic upper crust and a deeper viscous layer.