A NEW INTERPRETATION OF THE 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. It is constrained by published surface and subsurface geologic data, a regional detachment zone illuminated by focal mechanisms at a depth of 10-15 km and published thermochronologic data. The WFFTB is composed of four main thrust sheets with minor splays. On the east, the Tulungwan fault carries a series of low-grade metamorphosed hinterland imbricates. It evolves from a basement-cored fold in the north (around 24°10' N) with Slate Belt and Foothills rocks are conformable on its western flank. To the south this fault breaks the back limb of the fold and gains displacement. The next thrust sheet to the west includes the Schuantung, Fenghuangchan, Luku, Tatou, Hopiya, and Pingchi faults. This fault system is interpreted as being underlain by a long flat with small ramps along a Miocene detachment, not a series of imbricates as previously interpreted. The next thrust sheet to the west is the Chulungupu-Chukou-Lunhou fault system which is characterized by the presence of a wide monocline in the central Foothills related to the Chulungpu fault and two wide synclines in the south, the Yuching and Tinpligling synclines. Modeling of the latter two structures shows that both have been uplifted with respect to the regional level on a wide and flat feature and that the footwall of the Lunhou fault is a monocline. A geometric solution to uplift of the Lunhou system above regional is a low-ramp angle fault-bend-fold with a detachment at ~13 km. The last thrust system toward the west shows a series of structures which are closely associated with the Peikang high implying that the structures are either inversion structures or new thrust faults whose ramps are located in pre-existing normal faults.