Reconstructing Taiwan. A New View of a Classic Orogen

A compilation of magnetic, geologic, surface velocity (GPS) and seismic data suggest that the colliding Chinese passive margin is more heterogeneous than previously thought and that these heterogeneities have played a significant role in the kinematics and dynamics of the Taiwan orogen. Magnetic data show a prominent, but irregularly shaped high that in the pre-collision zone southwest of Taiwan separates continental crust of normal thickness (i.e., ~ 30 km) from thinner and more heterogeneous transitional crust southeast of the high. In the collision zone, the high ends abruptly and appears to be offset to the northwest, suggesting a rift-related, left-lateral fracture zone in the continental margin. The offset high forms a triangular-shaped continental margin promontory that includes the Peikang High and the Puli Basin. The northeastern edge of the promontory correlates with the Sanyi-Puli seismic zone and marks the southern topographic front of the Hsuehshan Range. The eastern tip of the promontory correlates with a second, crustal-scale cluster of seismicity. The promontory, however, is relatively aseismic – the 1999 Chi-Chi earthquake is a notable exception. We propose that the early history of the collision (e.g., ~ 5 Ma to ~1 Ma) involved the subduction and collision of transitional crust rather than true oceanic or continental crust and that later stages of the collision (~ 1 Ma to present) involved continental crust of more normal thickness (~30 km). This recent involvement of continental crust of full thickness may explain the proposed increase in the rate of exhumation cooling. GPS data also indicate divergent flow around the promontory, suggesting that it acts as a relatively rigid indenter. These observations show how lower plate heterogeneities alter the collisional process, creating along strike-variations in deformation, erosion, and underplating in the interior of the orogen and making it difficult to trade space for time along strike.