DEFORMATION OF THE OVERRIDING PLATE IN TAIWAN ARC-CONTINENT COLLISION

In currently active arc-continent collision belts, such as Taiwan, Timor, and Papua New Guinea, we have perhaps a clearer understanding of deformation in accretionary wedges derived from the subducting plate. In contrast, deformation of the upper crust and lithosphere in forearc-to-backarc regions of the overriding plate where significant plate motion can be consumed, is still obscure. In this study, we present new constraints on the structures of the overriding Philippine Sea plate where the Luzon arc collides with Asian continental margin during Taiwan arc-continent collision since 2 Ma, based on surface geological data, seismic reflection data, bathymetry data, 3-D tomography and earthquake locations. Important constraints include the following: (1) The pre-stack depth migrated seismic reflection images that show thin-skin east-vergent thrusts in the Philippine Sea plate east of the Luzon Arc with respect to west-vergent thrusts in the forearc and arc regions. The east-vergent thrusts are detached along the top of the Huatung Basin oceanic crust whereas the west-vergent thrusts are detached along the base of Miocene arc volcanic rocks deposited on older forearc crust. The combined east- and west-vergent thrusts form a 5-6 km thick doubly vergent wedge on the overriding Philippine Sea plate. Both thrust belts contain a thick cover of Taiwan-derived orogenic strata that record the history of upper plate deformation. The minimum shortening of the combined thrust belts is ~50-60 km in the last ~1Ma, which is consistent with an observed geodetic rate of ~60 mm/y. (2) The total upper plate shortening in the forearc-to-backarc collisional zone is around 120 km in the last 2 Ma. (3) A >90 km long subducted slab beneath the eastern edge of Taiwan in 3-D regional tomography is identified as the missing subducted lower crust and mantle lithosphere of the deformed edge of the over-riding Philippine Sea plate during arc-continent collision. This subducted lithosphere is expected as the original basement of the bivergent thin-skin imaged in the upper 5-6 km of the over-riding plate.