Fast-eroding Taiwan and transfer of orogenic sediment to forearc basins and trenches in the Philippine and South China seas

Abstract

This article reviews what is known about sediment composition and transport within and offshore Taiwan Island, a place on Earth where ultrarapid erosion is induced by strong tectonic activity, powerful earthquakes, high relief, and intense precipitation during the passage of typhoons. Literature information is here integrated with new petrographic, heavy-mineral, clay-mineral, elemental-geochemistry, and Sr and Nd isotope-geochemistry data on river sands and fluvial, shelf, and deep-sea muds to obtain a general overview of sediment generation and offshore dispersal pathways.

The Taiwan thrust belt is bracketed by two opposite-verging subduction zones, and orogenic detritus is transferred across rugged relief to surrounding forearc basins and trenches in the deep sea. Despite humid tropical climate, physical erosion is so fast that both sand petrography and clay mineralogy faithfully reflect the lithology of source rocks. In the Coastal Range, Miocene Luzon Arc andesites shed feldspatho-lithic volcaniclastic sediment rich in pyroxenes and smectite, whereas quartzo-lithic sand is recycled from the overlying Pliocene-Pleistocene sandstones. Basement rocks of the Tananao Complex supply quartzo-lithic metamorphiclastic sand with epidote and amphibole and mud rich in chlorite and illite. Lower Cenozoic mudrocks of the Slate Belt produce lithic to quartzo-lithic metasedimentaclastic sand with durable ZTR minerals and mostly illite, whereas upper Cenozoic strata of the Western Foothills shed recycled quartzo-lithic to litho-quartzose sedimentaclastic sand with ZTR minerals, garnet, and mixed clay-mineral assemblages. Scarce kaolinite, inconsistent behavior of mobile elements in sediment derived from opposite sides of Taiwan Island, and lack of correlation between geochemical and climatic parameters consistently indicate a weathering-limited regime. High weathering indices in sediment of western Taiwan rivers thus cannot be ascribed to present conditions but were largely inherited from recycling of Paleogene sedimentary rocks, generated in humid mainland China at a time of global greenhouse climate and subsequently accreted to the frontal part of the Taiwan thrust belt.

Offshore dispersal pathways are traced by the spatial distribution of clay minerals and geochemical fingerprints. Abundant illite and high ⁸⁷Sr/⁸⁶Sr ratio indicate the Lanyang River as a major sediment contributor to the Ryukyu forearc basin, whereas smectite-bearing sediments of the Hualian, Xiuguluan, and Beinan rivers are conveyed to the Ryukyu Trench via the Hualian, Chimei, and Taitung canyons. Sediment from southern Taiwan feeds the Luzon forearc basin and reaches the Manila Trench via the Gaoping River and submarine canyon. Smectite transported long-distance from the Luzon Arc by the Kuroshio Current reaches as far north as the Ryukyu accretionary wedge, the Manila accretionary wedge and the Tainan Shelf, where only a little kaolinite-bearing sediment is supplied by South China rivers.