KINEMATIC AND MODELING CONSTRAINTS ON THE TRANSITION FROM ACCRETION TO COLLISION IN SOUTHERN TAIWAN

Southern Taiwan marks the transition from accretion above the Manila Trench to the south and Eurasia - Philippine Sea Plate collision to the north. Serial balance sections through the southern Western Foothills Fold and Thrust Belt (WFFTB) show that the structures making up the transition are influenced by the presence of large pre-existing normal faults. Therefore, the WFFTB overall is best understood as an inverted basin detached near the base of the pre-existing normal-fault basins. A new regional structure, the sub-Yuching anticline is a low ramp angle fault-bend-fold with a detachment at ~13 km. The sub-Yuching anticline explains the uplift of the Yuching and Tignpinglin synclines above their regional level and may lie above the continuation of the Manila trench beneath Taiwan. The estimated aggregate shortening for the easternmost fault of the WFFTB is about 40 km. The restored position of the preexisting normal faults places the current trace of the Western Foothills-Slate Belt boundary beneath the Coastal Range. The restoration of WFFTB's rocks to their depositional locations also supports the tectonic model of a crustal-scale thin-skin collisional orogen and rejects a previous hypothesis favoring a deep rooted Central Range. Published GPS and thermochronological data indicate that the depth of involvement of the deformation in the orogen deepens to the east but changes character and perhaps deformation mechanism across the Slate Belt -Western Foothills boundary. Except for the Longitudinal Valley, strain and dilation rates are highest in the frontal fold and thrust belt, as also suggested by modeling results. Specifically, finite element models employing a plastic rheology show that as each new ramp is initiated, plastic strain concentrates in that region and is lessened in previous ramp regions. The rate of accumulation of strain in the hinterland is reduced after the first ramp forms; displacements tend to parallel the basal detachment in the hinterland as well. Areas above and behind ramps revert to the elastic state after a thrust ramp has formed. Taken together these results indicate that, while ‘tectonic escape' is an attractive hypothesis in general, it must be taking place at two crustal levels to explain the observed structures and active deformation.