Polarity Reversal of Active Plate Boundary and Elevated Oceanic Upper Mantle beneath the Collision Suture in Central Eastern Taiwan

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 north-northeast-south-southwest-trending high- velocity zone corresponding to the oceanic upper mantle is narrowly defined under- neath 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 northwest-dipping seismic zone can be identified in the northern collision zone extending from the surface to 30 km depth, which can be correlated 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 con- vergence along the suture has been accommodated by the high-angle thrusting along the northern LVF. In contrast, a southeast-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. The reverse polarity of active-plate boundary faults marks two distinguished transition boundaries, one from eastward subduction in southern Taiwan to east-west collision in the southern collision zone corresponding to the early phase of plate collision, and the other from east-west collision to northwest subduction in the northern collision zone corre- sponding to the advanced phase of plate collision. The central collision zone is creep- ing and aseismic, which can be attributed to the high heat flow and geothermal activity during an interseismic period since the 1951 Taitung earthquake.