Crustal deformation and block kinematics in transition from collision to subduction: Global positioning system measurements in northern Taiwan, 1995-2005

We present global positioning system (GPS) measurements for the period 1995-2005 at 125 campaign-surveyed sites in northern Taiwan. Based on elastic, rotating block modeling analyses derived from the GPS data, we describe the transitional tectonics from arc continent (Luzon-Chinese) collision to the converging Ryukyu trench subduction and back-arc opening along the Chinese continental margin. Station velocities relative to station S01R, in the Chinese stable continental margin, were estimated from coordinate time series of each station by using the weighted least squares technique. We found two distinct deformation patterns in two geological areas, which are basically separated by the surface projection of the NW-trending boundary of the subducting Philippine Sea plate across northern Taiwan: (1) a waning collision area to the west and (2) a transition zone to the east. In the waning collision area, the horizontal velocity field shows vectors of 0.3-7.3 mm/yr toward the NW in the foothills and the Hsuehshan Range of northwestern Taiwan. The tectonic blocks represent a significant NW-SE internal contraction along with a small block rotation rate (< 3.0 degrees/Myr). The transition zone can be further divided into an outer range and inner range with distinguishing rotation rates and deformation behaviors. In the outer range of the transition zone, velocities of 1.0-7.8 mm/yr from south to north rotating from 008 degrees to 143 degrees is found in the northernmost foothills and the Hsuehshan Range. The tectonic blocks within the outer range are characterized by a coherent rotation (low internal strain rate of < 0.10 mu strain/yr) with an angular velocity of similar to 5.1 degrees/Myr, where the Euler pole is located near its southeastern boundary. In the inner range of transition zone, a larger clockwise rotation from west to east, with horizontal velocities of 9.3-41.2 mm/yr from 053 degrees to 146 degrees, are found in the northernmost Central Range. The tectonic blocks of the inner range reveal a remarkable NW-SE internal extension with an ultrarapid clockwise rotation (similar to 47.3 degrees/Myr) where the Euler pole is near the southern boundary of the range close to the collision corner with the colliding Luzon arc. The trench roll-back together with back-arc opening are interpreted to be substantially superposed on the arc-continent collision-induced rotation in the transition zone with particular regard to the inner range of the northeast Taiwan mountain belt.