3차원 속도구조, 지진위치, Pn 속도를 이용한 대만동부 판충돌부의 지구조연구

Abstract

Tectonic structures associated with the active collision between the Eurasian and Philippine Sea plates in eastern Taiwan can be manifested by metamorphic deformation, crustal thickening and uplifting observed on two NNE-SSW trending mountain ranges, the Central Range and the Coastal Range, separated by a 160 km long collision suture, the Longitudinal Valley. Early stage of plate collision initializes from the southeastern Taiwan region constituting a well-defined eastward dipping seismic zone to depth ~ 25 km along a high Vp/Vs ratio or water rich and highly fractured zone. To the north of the aseismic and predominantly creeping deformation central collision zone, the most advanced collision occurs at the northern collision zone near Hualien area where 2/3 of the high converging velocity between the Eurasian and Philippine Sea plates has been accommodated by a steeply westward dipping seismic zone extending from near surface to ~30 km. This high-angle thrust fault defines not only the eastern boundary of the Central Range separating the oceanic and continental crusts but also serves as a transition from an active plate collision to the south to an active plate subduction immediately to the north. An elongated NNE-SSW trending and narrowly defined high velocity oceanic upper mantle can be consistently seen to elevate to ~20 km depth beneath the collision suture from recent 3-D structural images. The elevated oceanic upper mantle and relatively thin crust beneath the collision suture has further been supported by the recent observations of anomalous Pn waves from shallow eastern Taiwan earthquakes observed at seismic stations along the collision suture with epicenter distances as small as 60 km. Therefore, the contact between the high-velocity and hot oceanic upper mantle and the mid-to-lower continental crust across the collision suture play very important roles on the tectonic evolution and mountain building process in Taiwan region via thermal deformation, c