Out of sequence faulting in the backbone range, Taiwan: Implications for thickening and exhumation processes

The Taiwan orogenic belt results from convergence between the Philippine Sea plate and Eurasia plate since the late Cenozoic. An extremely high exhumation rate has been observed in the Backbone Range, which has motivated interpretive models that show underplating as the primary process in driving uplift and thickening. Here we integrate new (U-Th)/He and fission-track dates of detrital zircons with previously published thermochronology to document a significant out-of-sequence thrust in the core of Taiwan orogen. The thrust, informally named the Tayulin fault system, is identified by structural kinematics and offset metamorphic temperature trends, low-temperature thermochronometric dates, and seismic tomography. The OOST appears to be composed of three segments that crop out along the western flank of the Backbone Range. The apparent age-elevation profiles suggest a relatively slow exhumation rate, <1.0 mm/yr, in the early stages from 8 Ma to 2 Ma followed by a stage of significantly higher exhumation rates, ranging from 2.3 to 6.5 mm/yr after 2 Ma. The early slow exhumation stage is related to regional folding and foliation development. In contrast, we suggest the younger rapid exhumation stage is related to slip on the out-of-sequence thrust.