Evolution And Exhumation Of The Mesozoic Qiangtang And Maqu-Linzhou Basins,Tibet

Reconstructing the subsidence,burial and uplift histories of sedimentary basins provides evidence to directly interrogate the filling,hydrocarbon accumulation and tectonic evolution of a basin.The Mesozoic Qiangtang basin and the Gangdese retro-arc basin in Tibet contain a lot of key information about the early evolution of the plateau,climate change and hydrocarbon accumulation.Despite the scientific significance,there is still considerable controversy regarding the type of the basin and the sedimentary environment.This thesis focused on the Mesozoic marine strata in the Qiangtang basin and the Shexing Formation in the Gangdese retro-arc basin.The stratigraphy,sedimentology,and geochronology methods are used to reconstruct the subsidence,burial and uplift histories of the basins.A new understanding has been obtained about the sedimentary environment,the early-stage development of the Tibetan Plateau,and the relationship between the evolution of the basins and oil and gas accumulation.According to the subsidence history and low-temperature thermochronology data,the North Qiangtang Basin is a retro-foreland basin.The subsidence history of the South Qiangtang Basin can be divided into a pro-foreland basin stage and a retro-foreland basin stage.The former phase shows acceleration on the subsidence curve,and the latter phase is similar to that of the North Qiangtang Basin.The evolution of the Qiangtang Basin proves that it began to show thickening of the crust at about 148 Ma.Thermochronological analysis show that the earliest rapid exhumation in the central part of the plateau occurred between 140 and 80 Ma.The burial and hydrocarbon generation histories show that the Qiangtang basin has experienced two peaks of oil production.The oil and gas traps formed in Mesozoic were damaged by thrusts in Cenozoic times,which is not conducive to oil and gas exploration.The Miocene oil and gas generation is the main period of oil generation in the basin,and the Cenozoic structures are dominated by extensional structures,which are small in scale and short-lived.They have limited impacts on the oil and gas accumulation.The lithofacies of the Shexing Formation in the Gangdese retro-arc basin is obviously three-stage.The sedimentary environment is sandy meandering river and floodplain.The time span of the Shexing Formation is from 100 to 80 Ma.The sandstone compositions,paleocurrents,and detrital zircon ages indicate that the Gangdese magmatic arc and the Carboniferous-Triassic sedimentary strata in the Lhasa terrane are the main provenance of the Shexing Formation.Thermochronology data have shown that the deformation of the Shexing Formation occurred at 75-60 Ma before the India-Asia collision.The Andean-type Gangdese mountain arc began to rise,and the southern margin of the Lhasa terrane began to acquire high-altitude terrain since then.Combined with the uplift histories in the central part,we concluded that a proto-plateau was formed in the North Qiangtang during 80-70 Ma,and then the growth of the Tibetan plateau has expanded southwards.The southern edge of the Lhasa terrane began to uplift at 75-40 Ma.Finally,the prototype of the Tibetan plateau had been formed across the Lhasa and Qiangtang terranes by 45-40 Ma.