An analysis of subduction related tectonics offshore southern and eastern Taiwan
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Arc-continent collision is associated with vigorous mountain building and terrane accretion on relatively short (<10 Ma) geologic timescales. It is believed to be an important mechanism for the growth of continents. Taiwan represents one of the few active examples of this process. As such, is the perfect natural laboratory to investigate the nature of the continent ocean boundary and the uncertain behavior of the accretionary prism and extended, transitional rifted margin crust during the collision process. Taiwan also provides a unique opportunity to investigate structures in the backarc, yielding key insights into the still controversial tectonic conditions that were responsible for the unique subduction-collision system observed today. The obliquity of the collision between the North Luzon Arc and the Chinese rifted margin allows for examination of different temporal stages of collision at different locations. Recently acquired seismic reflection and wide-angle seismic refraction data, offshore Taiwan, document the crustal structure of the incipient mountain belt and of the Philippine Sea Plate in the backarc domain to the east. Geophysical profiles offshore southern Taiwan show evidence for a transition from the subduction of ocean crust to highly extended, transitional continental crust of the northern South China Sea distal margin. During oceanic subduction, accretion and underplating of thick sedimentary cover sequences create a large 13-15 km thick accretionary prism. Prior to the encroachment of the continental shelf, there is evidence for further underplating of transitional distal margin crust to the base of the prism. These findings support a multi-phase collisional model in which early growth of the mountain belt is driven by structural underplating of the previously sedimentary-only accretionary prism with blocks of transitional crust from the distal rifted margin. Geophysical profiles offshore eastern Taiwan show evidence for asymmetric crustal thickening, from 12-18 km, along the entire length of the Gagua Ridge suggesting the West Philippine Basin oceanic crust is underthust beneath that of the Huatung Basin. In this interpretation, the Gagua Ridge was the result of a failed subduction initiation event during the early Miocene that may have existed simultaneously and, for a short time, competed with the Manila subduction zone in accommodating convergence between the Eurasia and Philippine Sea plates.