Structural features and Quaternary tectonics of the Chinshan Fault, northern Taiwan

Originally defined as a thrust fualt, 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 Taun 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 Taun 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 3.5km wide and 16km 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 Fualt has gone through three stages of deformation. The first stage involves revdrse faulting in response to the NW-SEe compression, which produced not only the Chinshan Thrust Fault but also the minor faults in thet surrounding Tertiary strata. The second stage is dominated by wrench tectonism, which transformed the Chinshan Fault into a zone of distributed strike-slip faults. The last stage is characterized by the extensional tectonism, which reactivated these strike-slip faults into normal faults. The three-stage tectonic evolution of the Chinsyhan Fault is related to the Late Cenozoic buildup and consecutive collapse of the Taiwan mountain belt. At present, the Chinshan Fualt zone is still undergoing extensional tectonism such that its seismogenic potential should not be overlooked.