Thrust belt architecture of the central and southern Western Foothills of Taiwan

A structural model of the central and southern Western Foothills Fold and Thrust Belt (WFFTB) was constructed from serial balanced cross sections. The cross sections are constrained by published surface and subsurface geologic data and thermochronological data. The regional detachment zone is constrained by thrust focal mechanisms to a depth of 10-15 km. Because the current geometries observed within the WFFTB are influenced by the presence of pre-existing normal faults, the WFFTB overall is an inverted basin. Most of the faults of the Western Foothills started their activity before the deposition of the Cholan Fm (~3.5 Ma). There is not a strict forward-breaking thrust sequence in Taiwan. Out-of-sequence faulting may be due to localized erosion and fault inversion. A new regional structure, the sub-Yuching anticline, is identified in the southern WFFTB. 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 Tingpinglin 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 on the central and southern segments. 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 hypotheses favoring a deep rooted Central Range.