대만 중동부의 3차원 지하구조와 지체역학적 중요성

Abstract

The active collision zone suture, the Longitudinal Valley, in central eastern Taiwan is sandwiched between two NNE-SSW trending mountain ranges, the Central Range and the Coastal Range. Beneath the ~160 km long and ~10 km wide suture zone, crust thickness is relatively thin (~23 km) that is imaged by a 3-D tomographic inversion of local earthquake data and is validated by the anomalous Pn waves observed at seismic stations located along the axial direction of the suture zone where ~4 seconds earlier Pn arrivals at stations at distance as close as 60 km are consistently observed as comparing to other stations at the same distance but outside of the suture zone region. After all earthquakes are relocated using a 3-D Vp and Vs crust model, seismicity associated with known active faults or previously unknown active faults becomes apparent. A westerly dipping high-angle thrust fault in northern suture zone extends from near surface to ~30 km depth that marks a boundary between the oceanic and continental crust and a transition from active collision to active subduction where 2/3 of the high converging velocity between the Eurasian and Philippine Sea plates has been accommodated. An easterly dipping fault in southern suture zone extends from near surface to ~25 km that marks a zone of initial collision, i.e. a zone of high Vp/Vs due to high concentration of fractures and/or rich in water during collision. The central suture zone is, however, characterized by aseismic, high geothermal activity, and creeping dominant deformation. Therefore, crustal structural variations from northern to southern suture zone reveal that the elevated high velocity and hot oceanic upper mantle beneath the suture zone and its interactions with the surrounding continental and oceanic crust play very important roles on the tectonic evolution and mountain building process in the Taiwan region via thermal deformation, crustal thickening, and uplifting.