A high-resolution study of clay mineralogy and major element geochemistry has been conducted on high sedimentation rate cores (MD01-2393 and MD97-2150) collected off the Mekong River mouth in the southwestern South China Sea in order to reconstruct the erosional and weathering history of the Mekong Basin. The chronology is based upon planktonic foraminiferal oxygen isotope records combined with carbonate stratigraphy. Clay minerals and major element results suggest that the Mekong River is the major sedimentary source over the past 190 kyr for both cores. Illite and chlorite were derived mainly from the eastern Tibetan Plateau. Kaolinite was derived mainly from active erosion of inherited clays from reworked sediments in the middle part of the Mekong Basin. Smectites originated mainly through chemical weathering of parent aluminosilicate and ferromagnesian silicate under warm and humid conditions in the middle to lower parts of the Mekong Basin. Smectites/(illite+chlorite) and smectites/kaolinite ratios coupled to K2O/SiO2 and Al2O3/SiO2 ratios allow us to reconstruct a history of chemical weathering versus physical erosion. Good correlations between those ratios suggest monsoon-controlled weathering and erosion over the eastern Tibetan Plateau and the Mekong Basin. Variations with time scales of 0.3-0.6kyr for the last 70kyr and 1-2kyr for 70-190kyr ago in the chemical weathering/erosional history are strongly related to the East Asian monsoon evolution, which is itself related to the summer solar radiation calculated for the Northern Hemisphere.

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: 87Sr/86Sr=0.7206-0.7240 and εNd(0)=-11.1 to -12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite+chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5-6.5Am/15-55Am siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial-interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.

This work reconstructs the depositional history and estimates the mass of sediments stored in the Hoh Xil basin, northern Tibet, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau. The mass estimate is derived from over 75 geologic sites and 21 measured stratigraphic sections, with a total thickness of 13,478m. The results show that the Hoh Xil basin underwent six stages of development from the Early Eocene to the Early Miocene, with a hiatus in sedimentation in the Late Oligocene. The Fenghuoshan Group was deposited during the first four stages 56.0-52.0, 52.0-43.0, 43.0-38.2, and 38.2-31.5Myr; the Yaxicuo Group comprises stage 5, 31.5-30.0Myr; the Wudaoliang Group covered the entire basin during the last stage,～23.0 to～16.0Myr. The strata of the Fenghuoshan and Yaxicuo groups were strongly deformed, mainly during the Late Oligocene, whereas only minor tilting has occurred since then in the Wudaoliang Group. The depositional history indicates that the Hoh Xil basin could have been formed as a piggyback basin and that the onset of northeastward growth of the central Tibet was from the Early Eocene (about 56 Myr) to the Late Oligocene. The analyses of subsidence history and mass accumulation indicate that both accelerated subsidence and sudden increases of accumulation rate occurred at the four periods of about 52.0, 40.5, 34.5, and 31.5Myr in three sub-basins and over the entire basin from the Early Eocene to the Early Oligocene. During the four periods, the deposits were either lacustrine turbidite sandstone or fan-delta conglomerate, which resulted from the tectonic movement. On the basis of the mechanism of northeastward growth of the piggyback basins and the consistency of accelerated subsidence, depositional systems, and mass accumulation, we suggest the continental collision and early uplift of the Tibetan Plateau controlled the formation and evolution of the Hoh Xil basin. The event that occurred at 52.0Myr could represent the continental collision between India and Asia, whereas the other three events that happened during about 40–30 Myr could show the early uplift of the Tibetan Plateau. This study on sedimentary records in the Hoh Xil basin, along with widespread magmatic activity in eastern and western Tibet, suggests a diachronous uplift history for the Tibetan Plateau from east to west.

A sedimentary succession more than 5800m thick, including the Lower Eocene to Lower Oligocene Fenghuoshan Group, the Lower Oligocene Yaxicuo Group, and the Lower Miocene Wudaoliang Group, is widely distributed in the Hoh Xil piggyback basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan plateau. The strata of the Fenghuoshan and Yaxicuo groups have undergone strong deformation, whereas only minor tilting has occurred in the Wudaoliang Group. We analyze their sedimentary facies and depositional systems to help characterize continental collision and early uplift of the Tibetan plateau. The results indicate fluvial, lacustrine, and fan-delta facies for the Fenghuoshan Group, fluvial and lacustrine facies for the Yaxicuo Group, and lacustrine facies for the Wudaoliang Group. Development of the Hoh Xil basin underwent three stages: (1) the Fenghuoshan Group was deposited mainly in the Fenghuoshan-Hantaishan sub-basin between 56.0 and 31.8Ma ago; (2) the Yaxicuo Group was deposited mainly in the Wudaoliang and Zhuolai Lake sub-basins between 31.8 and 30.0Ma ago; and (3) the Wudaoliang Group was deposited throughout the entire Hoh Xil basin during the Early Miocene. The Fenghuoshan and Yaxicuo groups were deposited in piggyback basins during the Early Eocene to Early Oligocene, whereas the Wudaoliang Group was deposited in a relatively stable large lake. The Hoh Xil basin underwent two periods of strong north-south shortening, which could have been produced by the collision between India and Asia and the early uplift of the Tibetan plateau. The study suggests the Hoh Xil region could reach a high elevation during the Late Oligocene and the diachronous uplift history for the Tibetan plateau from east to west.

Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau, provide abundant information of paleoclimate changes. A 350-m thick section in the middle-lower Yaxicuo Group was analyzed to reveal the climatic history that occurred in the Hoh Xil region during the early Oligocene interval 31.30-30.35Ma, dated with the paleomagnetic chronostratigraphy. The results indicate that arid and cold climate dominated the Hoh Xil region during the early Oligocene in general, being related to the global cooling and drying events that occurred in the earliest Oligocene. Within this period, relatively warm and wet climate accompanied by strong tectonic activity occurred in the 31.05-30.75Ma interval; while arid and cold climate and relatively inactive tectonics occurred in the 31.30-31.05 and 30.75-30.35Ma intervals. Furthermore, spectral analyses of high-temporal resolution paleoclimatic records show orbital periods including eccentricity, obliquity, and precession. It is concluded that paleoclimate changes during the early Oligocene in the Hoh Xil region were forced by both tectonic activity and orbital periods.