POLARITY REVERSAL OF ACTIVE PLATE BOUNDARY AND ELEVATED OCEANIC UPPER MANTLE BENEATH THE COLLISION SUTURE IN CENTRAL EASTERN TAIWAN

CHIU, J.M., CERI, The University of Memphis, Memphis, TN 38152, USA, KIM, K.H., KORDI, Anshan, Korea; PUJOL, J., Dept. of Earth Sciences, The University of Memphis, Memphis, TN 38152, USA; CHEN, K.C., Institute of Earth Sciences, Academia Sinica, Taiwan, ROC, jerchiu@memphis.edu.

The active collision between the Eurasia and Philippine Sea plates in eastern Taiwan has been explored from the recently determined 3D velocity images and relocated hypocenters. A NNE-SSW trending high velocity zone corresponding to the oceanic upper mantle is narrowly defined underneath the collision suture from Hualien to Taitung. This elevated and hot oceanic upper mantle must have played an important role in the tectonic evolution/mountain building process of the adjacent continental crust. A NW-dipping seismic zone can be identified extending from the surface to ~30 km depth which correlates with the northern Longitudinal Valley Fault (LVF). This zone marks a transitional plate boundary separating the high Vp and high Vp/Vs oceanic crust to the east and the high Vp and Vs upper crust and low Vp and low Vp/Vs mid-to-lower continental crust to the west. A significant amount of plate convergence along the suture has been consumed by the high-angle thrusting along the northern LVF. In contrast, a SE-dipping seismic zone can be identified extending from the surface to ~25 km depth near Taitung in the southern collision zone. This zone coincides with a region of high Vp and high Vp/Vs, suggesting that earthquakes occurred within a highly fractured or fluid-rich zone which marks the boundary corresponding to the early phase of plate collision. The central collision zone is creeping and aseismic that can be attributed to the high heat flow and geothermal activity as well as due to the inter-seismic period since the 1951 Taitung earthquake.