Geomorphic evidence and kinematic model for quaternary transfer faulting of the Pakuashan anticline, Central Taiwan

摘要

The Pakuashan-Tatushan anticline and the related Changhua thrust are characterized by a regional curvature of the deformation front, which deviates from the general trend of the thrust system in the Western Foothills of Taiwan. A Quaternary transfer fault zone, which accommodates the motion of connected thrust sheets over an oblique feature of the preexisting margin basement, is proposed to explain this regional phenomenon. There are distinct morphological features in the Pakuashan ridge related to the transfer faulting. The stream valley orientations responding to the regional slope change and left-laterally offset valleys concur to support the existence of the tear faults. These observations lead us to propose a 2-D kinematic model in order to illustrate the geometry of the Pakuashan ridge adjusted by the tear faulting. Based on this kinematic model, we infer that the Changhua thrust system moves over an oblique ramp which bounds the Peikang Basement High. The oblique ramp is composed of three segments corresponding to the northern, central and southern Pakuashan. The gentle northern and southern ramps are connected by the steep central ramp. This ramp model can illustrate the kinematics of the Pakuashan transfer fault zone and has the potential to predict the local deformation (extension and compression) with respect to the geometry of the intersection of oblique ramps. The Pakuashan-Tatushan anticline and the related Changhua thrust system is one of typical examples of thrust sheet deformation associated with oblique ramps.