Sediment compositions in offshore southern Taiwan and their relations to the source rocks in modern arc-continent collision zone

Abstract

Modal analyses of 31 sand samples collected by piston coring document variations in sediment composition along and across the developing collision zone off southern Taiwan and help constrain sediment transport paths and, by inference, of sediment sources. Overall, sand composition from this region is dominated by lithic-fragment populations, with a QtFL average of Qt 27F 21L 52. Three geographic domains are based on morphotectonics and variations in sand composition: (1) South China Sea and the adjacent slope of the accretionary prism; (2) suture zone south of southern Taiwan and north of the Luzon forearc; and (3) the Luzon forearc basin (North Luzon Trough) and immediate adjacent slopes. Sands from the accretionary prism and the suture zone contain subequal amounts of sedimentary and metamorphic lithic fragments (Ls 51Lv 5Lm 45 and Ls 47Lv 5Lm 48), respectively, whereas sands from the forearc basin are dominated by volcanic lithic fragments (Ls 29Lv 49Lm 22). In addition, compositions of individual sand beds vary dramatically in the forearc basin. This heterogeneity indicates that sediments from different sources have been deposited sequentially, but do not typically mix during transport and deposition. Similar, but less dramatic, within-core variations occur over the accretionary prism. The major sediment input for the accretionary prism is from western Taiwan and is dominated by sedimentary and low-grade metamorphic lithic fragments. There appears to be a second sediment source, however, from southeastern China. This implies that a significant amount of sediment is the input from China to this region. The major component of sands in the suture zone is derived from Taiwan, but medium-grade metamorphic lithic fragments are rather sparse in these sands, considering the extensive and high-relief exposures of metamorphic rocks on the island of Taiwan. The major source of sands in the forearc basin is the active volcanoes of the Batan islands. Episodically, minor components appear to be fed to the forearc basin from the arcward slope of the accretionary prism, presumably by submarine mass wasting. This observation provides support for the hypothesized olistostromal origin for the Lichi Mélange of eastern Taiwan.