Neotectonics of the Hengchun Area, Southern Taiwan

The collision process around Taiwan can be distinguished as subduction stage and mature arc-continent collision stage in the south and north of Hengchun area respectively (Fig.1: Reed et al., 1992; Huang et al., 2000). The Hengchun area, therefore, is regarded as a transition zone since the collision migrates southward as previous study proposed (Suppe, 1981; Teng, 1990). The major goal of this study is to understand the transient process through the neoteconic study. We combine both onshore/offshore geology and geomorphology to establish a spatial and temporal framework. Forty-meter-DEM and aerial photos are used to identify the onshore geomorphic features, and then confirmed by field investigation. We also analyze seismic profiles and bathymetry to look into the details of the submarine structures and landforms. We basically divide our study area into onshore Hengchun Peninsula and offshore Hengchun Ridge, and then integrate our discoveries to form the entire picture. The most dominant structure in the Hengchun Peninsula is the NW-trending Hengchun Fault (HF) passing through the middle of the Hengchun Peninsula (Fig.1). It is an east-dipping thrust fault evidenced by the topographic high and pervasive marine terraces on the hanging wall. It also has left-lateral component, which is supported by the triangular facets, pressure ridges and offset ridges along the fault trace. To the southern offshore of the Hengchun Peninsula, a clear linear scarp is probably the southern extension of the Hengchun Fault (Fig.1). We analyze two seismic profiles normal to the scarp, and confirm the tectonic origin and recent activity of the offshore Hengchun Fault. To the west of Hengchun Fault, the east-facing Western Hengchun Tableland is composed of four-step marine terraces, which were dated from Holocene to late Pleistocene (Chen, 1993; Shyu, 1986). From bathymetry, the Western Hengchun Tableland is a NW-trending anticline, which we tentatively named as Western Hengchun Tableland Anticline (WHTA). The seismic profiles reveal a NW-trending fault, named A Fault (AF) tentatively, being developed in the front of the western lim