Tectonic Evolution Of The Pre-Cenozoic Basement Of The Southern South China Sea Basin

The Southren South China Sea?SSCS?,as an important strategic resource area,is rich in oil,natural gas and other mineral resources,which attract much attention to geologists on the globe.With the improvement of China's deepwater exploration technology,Pre-Cenozoic in SSCS will become a new field and new formations of exploration.Therefore,it is necessary to do further research in the basic geology of the Pre-Cenozoic strata in SSCS.The data about the Pre-Cenozoic formation?South Vietnam,Natuna Islands,Borneo,the Philippines,and so on?,regional faults and suture zones of SSCS and adjacent areas are collected and summarized,and then,the field investigation of the Pre-Cenozoic strata of the continental margin of SSCS is done.Based on these data and investigation,we analysis statistically the whole-rock major and trace element data,zircon isotope data and the drilling data in SSCS.On the basis of the above,we divide the basement of SSCS and adjacent areas into 9 tectonic units,and conduct a comprehensive study on the Mesozoic lithofacies paleogeography,paleontology and tectonic features in SSCS.At last,this paper establishes Pre-Cenozoic tectonic evolution sequences of SSCS to provide the necessary support for the exploration of oil and gas in the Mesozoic strata of SCS.Through the above comprehensive study,some main achievements obtained are as follows:?1?Under the constraints of the geological features of the land surrounding SCS,taking ocean basin edge faults,suture zone?ophiolite and melange belts?,regional strike-slip faults,and trenches as the boundary of two adjacent tectonic units,the basement of SSCS and adjacent areas is divided into nine tectonic units,that is Indochina,Sunda block,Sibu accretionary wedge,East Kalimantan,Nansha block,Sulu block,Central Basin,Zhongsha-Xisha block and Philippine island arc.Taking strike-slip faults as the boundary of three adjacent tectonic units,the basement of Nansha block is divided into five tectonic units and the basement of Sulu block is divided into three tectonic units.?2?The metamorphic basements of SSCS adjacent areas are different.From Indochina to Kuching Zone to Sibu accretionary wedge to Miri accretionary wedge,the time of crystal-metamorphic basements is from old to new,the metamorphic grade is middle-high class to low-middle class,and the deposition process is different.The age of metamorphic basement is pre-paleozoic in Indochina,where three sets of Marine sediments are superimposed with one set of sea-continental interactive sedimentary formation.The age of metamorphic basement is devonian in Kuching Zone,where two sets of marine sediments are superimposed with one set of sea-continental interactive sedimentary formation.The age of metamorphic basement is carboniferous in North Palawan,where two sets of Marine sediments are superimposed with one set of sea-continental interactive sedimentary formation.From Natuna to Sarawak to South Palawan,the age of ophiolite suite has changed from old to new.?3?It is found that the age of basalt is 248.1±4.6 Ma,and the geochemical characteristic is oceanic basalt,by field investigation,geochemistry and isotope dating in Sabah area.PSCS was an inheritance development of the Paleo-Tethys remnants in Triassic,the invasion Meso-Tethys and Paleo-Pacific superimposed impact.In the end of the Late Cretaceous,PSCS subducted southward under the Sunda block.Its ruins is the line of ophiolite belt?the ophiolite belts of Natuna Island and Sabah,and Lubok Antu melange?near Lupar Line,and the accretionary wedge?Sibu accretionary wedge?in Northwest Borneo.?4?According to the research of the SSCS blocks strata,regional structure,tectonic atrributes and continental collages/slivers of terranes by the opening and closing of the successive oceans,Pre-Cenozoic tectonic evolution in SSCS is divided into five stages:?1?The stage of the effect of Paleo-Tethys?Pz₂?.In Late Paleozoic,the northern SCS blocks,Indochina block and Nansha block all were free micro-blocks in the Paleo-Tethy archipelagic ocean.?2?The stage of the combined effect of Paleo-Tethys and Paleo-Pacific?T₁-T₂?.Paleo-Tethys?mainly flowing through SongMa Ocean into SSCS?and Paleo-Pacific together affected sedimentary and paleontological features of SSCS.?3?The stage of Paleo-Tethys subduction and Paleo-Pacific tectonic domain?T₃-J₁?.In the Late Triassic,the effect Paleo-Tethys had onSSCS was over,because Song Ma ocean closed.So the SSCS sedimentary was only affected by Paleo-Pacific tectonic domain.?4?Superimposed influence stage by Meso-Tethy and Paleo-Pacific?J₂-K₁?.In the Late Jurassic,Nansha block subducted along the Mesozoic subduction zone beneath Cathysia.In the Early Cretaceous,Meso-Tethys gradually closed,and Nansha block collaged together with Cathysia.And East Kalimantan simultaneously drifted to West Kalimantan.?5?The PSCS extinction and the new South China Sea expansion stage?K₂-E?.In the Late Cretaceous,Meso-Tethys completely closed,and because of the margin expansion between Cathysia and Nansha block.Then proto-South China Sea began to subduct beneath Kalimantan.