Motion and evolution of the Chaochou Fault, Southern Taiwan

The Chaochou Fault (CCF) is both an important lithologic boundary and a significant topographic feature in the Taiwan orogenic belt. It is the geologic boundary between the Slate Belt to the east, and the Western Foothills to the west. Although the fault is known to be a high angle oblique sinistral thrust fault in places, both its kinematic history and its current role in the development of the orogen are poorly understood. Field fabric data suggest that structural orientations vary along strike, particularly in the middle segment, the suspected location of the intersection of the on-land Eurasian continent-ocean boundary and the Luzon Island Arc. Foliation/solution cleavage is oriented NE-SW and in the northern and southern sections, but ESE-WNW in the middle segment. Slip lineations also reveal a change in fault motion from dip-parallel in the north to a more scattered pattern in the south. This correlates somewhat with recent GPS results, which indicate that the direction of current horizontal surface motion changes along strike from nearly perpendicular to the fault in the northern field area, to oblique and nearly parallel to the fault in the southern field area. The magnitude of vertical surface motion vectors, relative to Lanyu Island, decreases to the south. Surface morphology parameters, including mountain front sinuosity and valley floor width/valley height ratio indicate higher activity and uplift in the north. These observations correlate well with published apatite/zircon fission track data that indicate un-reset ages in the south, and reset ages in the northern segment. Geodetic and geomorphic data indicate that the northern segment of the CCF and Slate Belt are currently undergoing rapid uplift related to oblique arc-continent collision between the Eurasian continent and the Luzon arc. The southern segment is significantly less active perhaps because the orogen is not yet involved in direct arc-continent collision.