USING ELECTRON BACKSCATTER DIFFRACTION (EBSD) TO CHARACTERIZE STRESS AND CRUSTAL RHEOLOGY IN TAIWAN

The extent and style of deformation that occurs during orogenesis is a function of the rheology of the crust and the applied tectonic stress. The crust is recognized as having depth-dependent rheology where deformation is brittle near the surface and ductile at depth. Ductile deformation may be accommodated by a variety of deformation mechanisms, depending on factors like temperature, strain rate, and fluids. The grain size and orientation of quartz crystals, which evolve during deformation as a result of variably activated deformation mechanisms, can be used to determine the conditions of deformation (e.g. temperature, stress, strain geometry), and provide a way to understand the large-scale mechanical behavior of the crust, particularly in complex tectonic regions such as collisional orogenies. Here, we undertake a microstructural study of quartz via electron backscatter diffraction (EBSD) in order to constrain the crustal rheology of the rapidly eroding collisional orogen of Taiwan. The Taiwan Central Range is thought to have exhumation pathways that are deeper to the north and east and shallower to the south and west, as determined by kinematics and thermochronometry. This setting serves as a natural laboratory to study the stress and rheology of the crust along a gradient of exhumation depth from north to south. We present new EBSD results from thirty samples collected along the length of the Eastern Central Range. Our results indicate that dislocation creep is an active deformation mechanism throughout the orogen. Analysis of crystallographic preferred orientations (CPOs) indicate that the highest temperature dislocation creep deformation mechanisms (400°C to >500°C) are active in the northern and central portions of the orogen, while the southern part of the orogen is dominated by low-temperature (~300 to 400°C) deformation mechanisms, consistent with independent estimates of exhumation. Further, by using an EBSD-calibrated paleopiezometer, we find the stress of deformation increases from north to south along the orogen, from 32 MPa to 65 MPa. These results serve to quantify the deformation conditions of the Taiwanese orogeny, potentially highlighting the unroofing of the brittle-ductile transition as a result of variable exhumation pathways in the orogen.