Interpreting low-temperature thermochronometric data in fold-and-thrust belts: An example from the Western Foothills, Taiwan.

摘要

New apatite fission track and (U-Th)/He data document the cooling history and the structural development of the Taiwan Western Foothills fold-and-thrust belt which lies at the edge of one of the worlds most actively growing orogens. This dissertation develops and uses a 2-dimensional thermo-kinematic model of thrust faulting and folding to understand the cooling history recorded by the new low-temperature thermochronometric data. The results of this dissertation provide the previously lacking long-term record of uplift and erosion in the western foothills fold-and-thrust belt.; In western Taiwan, the apatite fission track and (U-Th)/He cooling ages record thrust exhumation over the last 3.5 Ma. Material is rapidly exhumed above a blind thrust ramp buried beneath the eastern part of the thrust belt or western Hsueshan Range. After exhumation above the ramp, material is transported horizontally along a decollement in the Miocene strata of the western foothills. The thrust ramp links the main Taiwan decollement in the Oligocene rocks of the Hsueshan Range to the decollement in the Miocene section and migrates with time.; Slip along the deep buried ramp and the upper decollement is passed to thrust ramps at the deformation front along the western edge of the thrust belt. This deformation explains the distribution of cooling ages, stratigraphy, and the patterns of geodetic shortening across western Taiwan. There is no evidence for out-of-sequence thrusting in the thrust belt and the evolution recorded by the thermochronometric data is a steady southward and westward propagating sequence of thrusts.; The fault structures in southern Taiwan document the transition from subduction to arc-continent collision. The thermo-kinematic model is used to 'retro-deform' thrust faults and thermochronometric samples. This retro-deformation shows that prior to arc-continent collision, stratigraphy and subduction-related faults were buried in a depositional basin. The onset of arc-continent collision is marked by rapid exhumation above a thrust ramp that forms at the rear of the thrust belt beneath the old subduction-related accretionary-prism faults.; The studies in this dissertation highlight the power of using thermo-kinematicthermochronologic modeling to understand detailed cooling histories in fold-and-thrust belts.