Late Triassic-Early Cretaceous Sedimentary Record

The South China Block experienced a tectonic regime transformation from the east-west-oriented stress field of the Tethyan tectonic domain to the northeast-southwest-oriented stress field of the Paleo-Pacific tectonic domain during the early Mesozoic,and this tectonic regime transformation must be related to the subduction of the westward subduction of the Paleo-Pacific Plate,which has been widely accepted,however,debate continues regarding the specific timing of this change and there is a debate on when and how the Paleo-Pacific plate subducted beneath Southeast China in the Mesozoic.The East Guangdong Basin and Yong'an Basin,located in the south-eastern South China Block,were influenced by both the Tethyan tectonic domain and the Paleo-Pacific tectonic domain,and the records of basin sedimentation in these areas preserve important geological information about the Mesozoic tectonic setting of the South China Block,and the investigation of sedimentary record in the retro-arc basin may lead to an improved understanding of the connection between plate subduction history and basin sedimentation of overriding plate.In this study,we applied detailed field geological investigation,systematic sampling of the Late Triassic Early Cretaceous sedimentary sequences,sedimentary facies analysis,detrital zircon U-Pb geochronology,petrography,and detrital zircon geochemistry to establish the "source-to-sink" system of the two basins,and to discuss the timing of the tectonic regime transformation and subduction model of the Paleo-Pacific plate.Sedimentary facies analysis shows that the East Guangdong Basin and Yong'an Basin were shallow sea and delta facies in the Late Triassic,and the scope of the shallow sea facies was expanded in the Early Jurassic.And they were lacustrine,fan delta,Delta,and fluvial facies in the Middle Jurassic,while they were volcanic basins from the Late Jurassic to the Earliest Cretaceous.The change of basin sedimentary facies was the response to the transgression and regression of southeast China,and also the response to basin expansion to contraction."Source-to-sink" system analysis suggests that the Late Triassic detritus in the East Guangdong Basin was derived from the south-western or southern margin of the Yangtze Block,Hainan Island,the Yunkai Massif,and the Nanling tectonic belt in the South China Block and that the detritus in the Yong'an Basin was sourced from the southern Wuyi Massif to the north of the basin,Nanling tectonic belt,and Hainan Island.The Jurassic sediments were transported from multiple sources including the intracraton Mountains and tectonic belts including Yunkai Massif,the south-western or southern margin of the Yangtze Block,southern Wuyi Massif,southwest Zhejiang and northeast Fujian region,southwest Japan,and Nanling tectonic belt,and the inferred active continental margin arc.And the clastic composition of the Early Cretaceous tuff suggests it was sourced from north of the basins,China's southeast coast,the eastern magmatic arc,crust material,and deep crust-mantle coupling material.The variations in the main age groups of the detrital zircons in the study areas and the corresponding provenance areas indicate that the tectonic regime of the South China Block experienced a transition from the Tethyan tectonic domain to the Paleo-Pacific tectonic domain in the early Mesozoic and the transition occurred between the Early Jurassic and completed at 165± 5 Ma.Based on all the U-Pb detrital dating,sedimentation,and structure data,we infer that there developed an active continental magmatic arc along the coast of Southeast China,which was migrated inland from the Early Jurassic to Early Cretaceous(200 Ma-135 Ma).The westward subduction of Paleo-Pacific Plate initiated in the Early Jurassic(ca.200 Ma)and continued to the Early Cretaceous(ca.135 Ma),and the angle of subduction kept increasing.The Paleo-Pacific Plate subduction resulted in uplift of southwest Zhejiang and northeast Fujian region and the eastern South China,and controlled the provenance characteristics and structural pattern of the sedimentary basins during the Jurassic to Early Cretaceous(200 Ma-135 Ma)in Southeast China,The East Guangdong Bassin and Yong'an Basin are back-arc foreland basin controlled by the stress field of the Paleo-Pacific tectonic domain during the Jurassic to Early Cretaceous(200 Ma-135 Ma).