Originally defined as a thrust fault, the Chinshan Fault of northern Taiwan can be readily recognized by the stratigraphic offset in the Tertiary sedimentary strata exposed in the Chinshan and Kuantu-Peitou areas. However, between Chinshan
and Kuantu, it is mostly covered by the Quaternary Tatun volcanics, in which no obvious fault traces and stratigraphic offsets can be recognized. Seismological and structural geological studies show prominent wrench and extensional tectonisms in the Tatun volcanic area, which is not consistent with the thrust nature of the Chinshan Fault. Its location and character, hence, have remained a subject of conjecture and debate, especially in the Tatun volcanic area. To solve this problem, we examined the geomorphology and structural geology of the Tatun volcanic area in the hope of better understanding this fault.
Based on a comprehensive map of lineaments extracted from the digital terrain model data, we delineated the minor faults in the Tatun volcanic area by aerophotos and field check. All the minor faults are distributed in a NE-SW trending zone
stretching from Chinshan to Peitou. The zone is about 3.5 km wide and 16 km long, and generally parallels the surface trace of the alleged Chinshan Fault. It indicates that the present Chinshan Fault, as shown on the surface, is not a through-going fault but a zone of distributed minor faults. The zone encloses not only all the tectonic fractures but all the hydrothermal activities in the Tatun volcanic area. Most of the minor faults are oriented NE-SW, largely parallel to the enclosing fault zone. According to the detailed fault-slip and paleostress analysis, the Chinshan Fault has gone through three stages of deformation. The first stage involves reverse faulting in response to the NW-SE
