Landscape inversion episodes in SE China during the Mesozoic−early Cenozoic: Constrained by trace-element contents, Nd isotope geochemistry, and detrital zircon U-Pb geochronology of sedimentary basins

Abstract

The tectonics and landscape of SE China experienced significant changes throughout the Mesozoic and early Cenozoic, largely in response to variations in the slab dynamics of the paleo-Pacific plate, which was subducting beneath continental Asia. We investigated the Mesozoic Yong’an basin in western Fujian Province of SE China in comparison to the sedimentary records of coeval basins in the region to document how its clastic sediment types and their provenance varied through time during the Mesozoic and what regional geologic processes may have controlled these variations. The average εNd value of samples from the Middle Jurassic Zhangping Formation is −16.6, and its detrital zircons are dominated by 1800 Ma and 2000 Ma grains, sourced from the northern Wuyishan Mountains. These mountains underwent significant rock and surface uplift by the Middle Jurassic and became the main source of clastic sediments in SE China. The Lower Cretaceous Bantou Formation contains pyroclastic rocks and represents fluvial-lacustrine deposits with εNd values of −14.8 to −12.4 and abundant 160−120 Ma detrital zircons, sourced from Late Jurassic granitoid rocks, which were widely exposed at the surface in SE China by this time. The upper Lower and lower Upper Cretaceous Shaxian Formation contains coarse-grained and poorly sorted sandstones-conglomerates with volcanic and granitic rock fragments, and it rests unconformably on the Bantou Formation. The Shaxian Formation represents fluvial- to alluvial-fan deposits, and its formation marks the timing of a rapid uplift of the paleo−Coastal Mountains. The Upper Cretaceous Chong’an Formation (>2000 m thick) contains abundant volcanic and granitic rock clasts and represents alluvial-fan and fluvial deposits. The average εNd values of the Shaxian and Chong’an Formations range between −9.3 and −7.5, and their most abundant detrital zircon ages are between 120 Ma and 80 Ma. By the end of the Late Cretaceous, the paleo−Coastal Mountains constituted a nearly 4-km-high magmatic belt, with much of SE China situated in its rain shadow at a lower elevation to the north. Eocene−Oligocene sedimentary basin rocks in Taiwan have an average εNd value of −10.9 and abundant Phanerozoic detrital zircons. The sediment source for these rocks was the paleo−Coastal Mountains. The Miocene basinal strata in Taiwan have more negative εNd values (−13.0) and contain Jurassic−Cretaceous as well as abundant Paleoproterozoic and Neoproterozoic zircons, indicating that the Wuyishan Mountains were again the main sediment source later in the Cenozoic. Denudation rates in the SE margin of South China were high (0.12−0.10 km/yr) during the Cretaceous (140−60 Ma), while they were very low in SW China and in the interior of South China during the same period. These differences confirm the existence of high coastal mountains in SE China until the Late Cretaceous. Denudation rates in eastern South China, particularly the coastal areas, were very low (0.06−0.02 km/yr) during the late Cenozoic (30−0 Ma), whereas they were the fastest (0.14−0.16 km/yr) in the northern Nanling belt and the Yangtze block farther inland to the north, indicating the surface elevation became higher in the western part of South China but lower in its eastern part in the late Cenozoic. This dynamic landscape evolution of SE China through multiple and major shifts throughout the Mesozoic and early Cenozoic was driven by the subducting slab dynamics and the tectonics of the Tibetan Plateau.