Tectonics, topography, and river system transition in East Tibet: Insights from the sedimentary record in Taiwan

Abstract

The Cenozoic in East Asia is marked by major changes in tectonics, landscapes, and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th, and eNd values in these rocks show an abrupt change between 31 and 25 Ma. Generally the higher eNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower eNd values suggest that the main sources had evolved to inland southern China by 25 Ma. The SHRIMP U-Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup unconformity to be between 39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before 33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to 33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.

Topographic architecture and drainage reorganization in Southeast China: Zircon U-Pb chronology and Hf isotope evidence from Taiwan

“…7 and 10). Thus, the variation trends of U-Pb ages and Hf isotope of detrital zircon grains from Cenozoic sandstones in Taiwan indicate that their provenance migrated inland greatly during Late Oligocene and Early Miocene, which is consistent with the variation of the Nd isotope values reported from mudstones (Lan et al, 2014). The source region of Neogene formations might have already reached up to the Lower Yangtze area or West Cathaysia, although it is more likely derived from the Yangtze Craton.…”

“…Together with the evolution of drainage systems recorded by the Cenozoic sediments in Taiwan in this study, it is possible that the opening of marginal sea, such as the South China Sea, exerted a major influence on the evolution of topography and drainage system in southeast China. This can also be proved from the diachronism features of breakup unconformity and transition of provenance in basins in northern margin of South China Sea (Lan et al, 2014). The transition time of provenance in rifted basins in northern margins of South China Sea, the same with breakup unconformity, shows obvious diachronism.…”

“…3) Tsukeng Anticline (Fig. 2A, C), the indigenously buried Discocyclina dispansa foraminifera was identified together with interbedded tuff (Pinglin Tuff) that was dated as ca.39 Ma (Huang et al, 2013;Lan et al, 2014), suggesting that the sedimentary history of the Western Foothills is much longer than previously recognized (Ho et al, 1956;Ho, 1961). The Hsuehshan Range is located at the east of Western Foothills and the west of Central Range ( Fig.…”

“…Meanwhile, large-scale rift systems developed in southeast China during Late Mesozoic-Early Cenozoic (Yao et al, 1994), which finally led to the formation of a series of rifted basins and culminated in the opening of marginal seas, such as South China Sea during the Paleogene (Taylor and Hayes, 1980;Briais et al, 1993;Lan et al, 2014). During the transition from rifting to seafloor spreading, a series of breakup unconformities develops in the rifted basins in northern margin of South China Sea (Zhou et al, 1995;Lin et al, 2003;Lan et al, 2014). The diachronous westward younging of these breakup unconformities is consistent with seafloor spreading propagating from east to west (Lan et al, 2014).…”

“…During the transition from rifting to seafloor spreading, a series of breakup unconformities develops in the rifted basins in northern margin of South China Sea (Zhou et al, 1995;Lin et al, 2003;Lan et al, 2014). The diachronous westward younging of these breakup unconformities is consistent with seafloor spreading propagating from east to west (Lan et al, 2014). Although the surface uplift of Tibetan Plateau is believed to have significantly influenced the tectonics, landforms and drainage systems during the Cenozoic in East Asia (Harrison et al, 1992;Burchfiel et al, 1995;Fielding, 1996;Wang, 1998;Li and Fang, 1999;Wang, 2004), the role of marginal sea spreading in shaping the topography of East Asia has yet to be further studied.…”