| Convergent plate boundary basins are sedimentary reservoirs of plate subduction and collision processes,record the processes and modes of subduction,accretion and orogenesis,which are of great significance for understanding the mechanical processes of the subduction zone.The Sewa Formation and the Shaqiaomu Formation were selected to conduct sedimentology and stratigraphy analysis in the Jurassic South Qiangtang Basin in Rongma area,Tibet.The prototype basin type,tectonic setting and the spatio-temporal evolution process of the basin were discussed based on provenance analysis and tectonic subsidence reconstruction.The Marine strata of Riganpeicuo Formation,Sewa Formation and Shaqiaomu Formation in the southern Qiangtang Basin from Late Triassic to Jurassic are in angular unconformity contact with the underlying Paleozoic strata and the overlying Upper Cretaceous continental strata.Sedimentology analysis indicates that the Sewa Formation was mainly deposited in the abyssal to semi-abyssal submarine fan environment,and the Shaqiaomu Formation was deposited in the coastal to shallow shelf environment.U-Pb dating of detrital zircons indicates that the youngest zircon age is 184.5±6.9 Ma and the YC1-?(2+)age is 190.0±10.0 Ma in sandstones of the Sewa Formation.The age of the youngest zircon in sandstones of Shaqiaomu Formation is 158.3±3.4Ma and the age of YC1-?(2+)is 165.6±3.0 Ma.Combined with the regional stratigraphic correlation,previous biostratigraphic data and isotope chronology data,the sedimentary age of the Sewa Formation is defined as the Early Jurassic to Middle Jurassic Bajocian Period,and the sedimentary age of the Shaqiaomu Formation is defined as the Middle Jurassic Bajocian Period to the Late Jurassic Oxford Period in the study area.Comparison with the contemporaneous deposits in the South Qiangtang Basin reflects the water is shallow in the north and east of the basin,and is deep in the south and west of the basin.Tthe clastic provenance areas include the North Qiangtang Block,the Central Qiangtang Metamorphic Belt,the South Qiangtang Block and the Accretionary wedge of the northward subduction of the Bangonghu-Nujiang Tethyan Ocean.The age distribution of detrital zircons from the Sewa and the Shaqiaomu Formations in the study area is strongly consistent,showing peak age groups of 200-360 Ma,400-510 Ma,720-860 Ma,930-990 Ma,1750-2000 Ma,and2400-2650 Ma.The age peak of young detrital zircons of 158-186 Ma in the Shaqiaomu Formation indicates the addition of debris from the southern Qiangtang epicontinental arc.Sandstones in the Sewa formation and the Shaqiaomu formation contain abundant lithic debris and quartz.The lithofacies characteristics indicate the tectonic setting of the source area of subduction complex belt,arc orogenic belt and collision orogenic belt.The tectonic subsidence reconstruction curve of the basin is mainly linear from the Late Triassic to the Middle Jurassic Bajocian Period,with a slightly concave-up characteristic.After the Middle Jurassic Bajocian period,the tectonic uplift occurred in the basin.The spatial distribution relationship,sedimentary facies and filling sequence,provenance indication and tectonic subsidence curve of the South Qiangtang Basin show the characteristics of forearc basin.Combined with the continental margin arc in the South Qiangtang Basin,eclogite,slab-derived adakite in the Bangonghu-Nujiang Suture Zone,the research suggests that the collision between the Ejiu ocean island and the trench during the Early-Middle Jurassic may have weakened the arc-type magmatism in the study area,resulting in few pyroclastic deposits in the South Qiangtang Basin.After the Middle Jurassic Bajocian Period,the mid-oceanic ridge of the Bangonghu-Nujiang Tethyan Ocean may have subducted northward to somewhere near the trench,which caused the tectonic uplift in the study area and the subsidence deceleration in the eastern part of the basin,and finally caused the sedimentary hiatus in the basin after the Late Jurassic Oxford Period. |
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