Subsidence, Migration, and Destruction of a Plio-Pleistocene Retrowedge Foredeep Basin in Eastern Taiwan: Evidence for Un-Steady Growth of the Collisional Orogen

The island of Taiwan is a well studied arc-continent collision where topography is shaped by interactions between tectonically-driven rock uplift and climate-driven erosion. Many studies conclude or assume that: (1) the colliding Luzon arc creates a sub-vertical backstop in the east that guides advection of crust accreted in the west through the orogen; (2) there is no transfer of crust from the Philippine Sea plate into the metamorphic core of the orogen; and (3) mountain building and exhumation are steady-state processes that propagate south through time. New data from the geologic record, however, contradict these interpretations. For example, high-pressure mafic and ultramafic rocks of the Yuli Belt that were previously thought to be Cretaceous fragments of the Eurasian margin are now recognized as Miocene oceanic crust and mantle with possible Luzon-arc affinity. The Coastal Range in eastern Taiwan preserves a thick sedimentary record of deposition in a Plio-Pleistocene retrowedge foredeep basin that provides an ideal natural laboratory for testing models of orogen-scale dynamics. Miocene volcanic rocks are overlain by a 5-7 km thick sequence of marine mudstone, turbidites, conglomerates and olistostromes derived from the orogen. The base of this succession is an unconformity that records 1-3 Myr of uplift followed by rapid subsidence and basin filling. Subsidence rates increased from 1 to 6 mm/yr between 4.0 and 0.8 Ma in a flexural response to tectonic loading. Convergence between the orogen and foredeep basin took place by shortening on the west-dipping, east-vergent Central Range fault that consumed a wide region of forearc crust. The Lichi Melange is a sedimentary olistostrome facies that prograded rapidly to the east over marine turbidites at 1.7-1.5 Ma during eastward propagation of the thrust front into the arc. Detrital thermochronology records increased rate of exhumation in the orogen at 2-1 Ma. Eastern Taiwan underwent a major tectonic reorganization at 0.8-0.6 Ma when thrust faults stepped offshore to the east, initiating modern uplift and erosion of foredeep deposits and Luzon-arc crust in the Coastal Range. This event may be related to break-off of the subducting Eurasian slab, and suggests a need to re-evaluate prevailing models for steady-state southward-propagating collision in Taiwan.