ANALYSIS OF FIELD BASED KINEMATIC AND STRUCTURAL FABRIC DATA FROM SOUTHWESTERN TAIWAN TO DETERMINE THE STRESS ORIENTATION

We analyzed unpublished field-derived fault kinematic and structural fabric data from southwestern Taiwan compiled by co-author Lee and his collaborators. Preliminary analyses reveal multiple expressions of progressive collision and evidence for ongoing tectonic escape to the southwest accommodated by brittle structures. These observations further support evidence found in the literature from paleostress analyses and geodetic inversions. In particular, existing inversions of fault-slip suggest E-W compression overprinted by WNW to ESE collisions. Furthermore, inversions of GPS data suggest active SW directed extension. We find that the intensity of deformation increases from west to east crossing from the westernmost foothills into the southern portion of the slate belt. To the west, the dominant structure is orthogonal joints that are normal to bedding. The most western exposures of these appear consistently oriented, whereas toward the east they begin to fan about a subvertical axis. The fanning suggests either rotation of the rocks in a fixed stress field or a rotating stress field. In the eastern most features, we find deformation is dominated by strike-slip faults, thrust faults, and mineralized veins. These structures appear to record regional collisional processes with some influence of the local fold-and-thrust structures. In the features south of the Chishan Transfer Fault Zone, we find deformation is dominated by strike-slip faults oriented in sets E to W and WNW to ESE, along with unconstrained thrust and normal faults due to rapid uplift of unconsolidated badland-like material. These features appear to record compression E to W and WNW to ESE and extension to N to S and ENE to WSW, respectively; suggesting the possibility of tectonic escape.