The Role of Pre-Existing Extensional Structure and Stratigraphy on the Geometry, Evolution and Neotectonics of the Western Taiwan Foothills Fold-and-Thrust-Belt

The western foothills of Taiwan are a classic active foreland fold and thrust belt, resulting from the oblique, and hence diachronous, collision between the Eurasian passive margin and the Luzon island arc of the Philippine Sea plate. The regional structural trends are oblique to those of the former passive margin of the South China Sea in two senses. First, the obliquity of the plate shortening direction leads to oblique slip on individual thrust faults as shown by the 1999 Chi-Chi earthquake. Secondly the mountain belt is typically oblique to the fabric of the passive margin, including both the regional stratigraphic facies trends of available detachment horizons and old normal fault structures and compartment boundaries. The complexly structured foreland is affected by both Paleogene and Neogene normal faulting, which take on different trends, giving rise to important segmentation on the scale of individual thrust sheets. Particularly important are the Neogene normal faults, which are typically perpendicular to the thrust faults.

Based on biostratigraphic and sequence stratigraphic correlations we review the stratigraphic architecture of the western foothills thrust belt and its foreland, identifying stratigraphic omissions on structural highs and their control on distribution of activated detachment horizons. In addition, N-S-trending along-strike sections illustrate the role of pre-existing normal faults and half grabens, which affected the passive margin up to the level of the Upper Miocene to Lower Pliocene Kueichulin formation. These faults are largely held responsible for the often highly non-cylindric geometry of many thrust sheets, often characterized by splay faults and oblique ramps. We illustrate these phenomena with examples from the Shuangtung thrust sheet in the area of Puli (central Taiwan) and the densely populated area around Chia-Yi (southwestern Taiwan), where the seismically active Meishan fault represents a highly oblique ramp nucleating along a reactivated normal fault.