THE ARCHITECTURE OF THE LUZON ARC IN THE TAIWAN ARC-CONTINENT COLLISION (Invited Presentation)

The obliquity of the Taiwan arc-continent collision allows us to constrain both the pre-collisional architecture of the Luzon Arc and its collisional deformation, based on an integration of surface geology, stratigraphy, bathymetry, marine seismic reflection and refraction profiles, local tomography and abundant seismicity. We present evidence that the Miocene and younger northern Luzon arc is built on early Cretaceous oceanic lithosphere of the Huatung Basin of the Philippine Sea plate, which had undergone ~100Ma of pelagic sedimentation prior to the onset of arc volcanism at ~17-18 Ma. This 1-2km thick pelagic sequence overlying oceanic crust provides thin-skinned detachments that were activated in Pleistocene to recent collisional deformation, producing a pair of thin-skinned thrust belts on the two sides of the detached arc. In addition, seismic reflection data show that this pelagic sequence has been uplifted in the arc by 1-3 km in what appear to be large laccolithic intrusions above oceanic crust. Early Cretaceous radiolarian cherts are found as zenoliths in Lanyu Island volcano, which is consistent with a Huatung Basin substrate to the northern Luzon arc. In addition, Miocene pelagic red shales deposited close to the CCD with interbedded arc volcanics on Lanyu island indicate ~3km uplift of the earliest arc materials. However, in contrast with this evidence for a deep oceanic substrate to the northern Luzon arc, widespread evidence exists for contamination by Eurasian crustal signatures. In particular Luzon arc volcanics contain abundant detrital zircons of distinctive Eurasian affinity, leading to proposals for a hidden allochthonous Eurasian crustal fragment under the northern Luzon arc. The most extreme case on Lanyu island shows >80% of zircons being detrital. However, the detrital zircon age spectra suggest they are derived from subducted Eurasian Miocene continental rise sediments of Yangze River origin rather than older Eurasian continental crust. Forearc lithosphere has been lost to subduction. Taiwan derived Plio-Pleistocene orogenic strata record the onset of arc-continent collision, with forearc basin strata onlapping the arc and spilling over the arc through submarine canyons to form deep submarine fans overlying the active retro-arc thrust belt, extending far into the Huatung Basin.