Collapse of the Luzon Volcanic Arc during onset of arc‐continent collision: Evidence from a Miocene‐Pliocene unconformity, eastern Taiwan

Abstract

A regional Miocene‐Pliocene unconformity in the Coastal Range of eastern Taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc‐continent collision. The unconformity separates precollisional volcanic and volcaniclastic rocks of the Miocene Luzon arc and forearc basin from an overlying thick sequence of syncollisional, Pliocene‐Pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of Taiwan. The duration of time represented by the unconformity averages 1–2 m.y. and locally ranges up to 4 m.y. The basal unit of the younger syncollisional sequence consists of bathyal, mega‐slump‐folded mudstone, pebbly mudstone, and thin‐bedded turbidites. Sharp stratigraphic juxtaposition of these deep‐marine deposits directly above shallow‐marine limestone and epiclastic rocks (upper part of the older arc sequence) requires extremely rapid subsidence during initiation of the collisional basin. The Coastal Range unconformity is interpreted to represent a period of latest Miocene nondeposition and/or erosion in the Luzon arc that was related to initial subduction of the Eurasian continental margin along the Manila Trench. This period was followed in Pliocene time by very rapid subsidence, or collapse, of the arc platform to 1–2 km water depth. There are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin: (1) extension, rifting, and crustal thinning in the Luzon arc massif to form a series of small rift or pull‐apart subbasins or (2) flexural subsidence of oceanic lithosphere in a foreland‐style basin due to crustal loading in the expanding accretionary wedge of proto‐Taiwan. Although data are insufficient to provide a unique solution, the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east, which may record eastward migration of an outer flexural bulge. Notable differences between the unconformity and paleogeography of the early collisional basin and analogous present‐day basins in the offshore region to the south suggest that arc‐continent collision and basin development in eastern Taiwan have experienced secular change since late Pliocene time.