Provenance of Miocene Sedimentary Sequences in Hengchun Peninsula, Southern Taiwan, and Implications for the Modern Taiwan Orogen

The Hengchun Peninsula (HCP) of southern Taiwan represents the newest emergent segment of the orogen built by the ongoing oblique collision between the Luzon arc and Eurasia continent. Newly recognized exposures indicate that a unit with the lithology of the Shihtzutou unit underlies the Lilongshan unit with a depositional contact. This discovery shows that Shihtzutou-type sands accumulated at least twice during the deposition of the Mutan Formation, and are in sedimentary contact with the Lilongshan unit. Fission-track analysis of detrital zircons reveals three age components from the samples analyzed from the HCP, which are 15, 34, and 66 Ma. These age components coincide with the thermal events of the opening of the South China Sea.Uplift rates calculated from the pattern of horizontal motions observed by a network of GPS stations, coupled with the equation of conservation, indicate the need for the Fengkang fault to accommodate a significant differential in uplift rates north and south of this fault. To a first degree, predicted uplift rates agree with data available on uplift across Taiwan. The source terranes for the Miocene sedimentary sequences of the HCP likely formed by the following scenario. Rifting of the South China Sea (SCS) began in the Late Cretaceous, which created rift basins off the southeast coast of China. A second rift episode of the SCS during the Eocene then intersected the early rift basins and carried some rifted blocks of the continental margin farther from Asia, while at the same time creating new mafic crust that was later accreted into the forearc of the approaching Luzon arc. Between 10 Ma and 5 Ma the Manila trench encountered and accreted the rifted microcontinental blocks and blocks of young mafic crust, and raised them to a subaerial environment to provide eroded detritus including rounded pebbles to the HCP basin. These sediments were deposited into the HCP basin in different fan systems with different source terranes and paleoflow directions. From 5 Ma to the present, the HCP basin itself was incorporated into the accretioanry wedge and brought to its current position by the fast-moving Philippine Sea plate.