The Sedimentary Basin Evolution And Tectonic Significance Of Northwestern Margin In The Tarim Basin During The Early-middle Permian Epoch

Northwestern margin of Tarim Basin and South Tianshan orogenic belt, both of which contain rich mineral resources and reserves significant information about the formation and evolution of paleo-asian ocean and Tarim block. The early-middle Permian epoch is a vital period' during the sedimentary-tectonic evolution history,which is significant to the evolution history of South Tien orogenic belt, as well as the oil-gas geological conditions of northwestern margin of the Tarim basin in upper Paleozoic erathem.The paper is a research about the sedimentary-tectonic evolution in the northwestern margin of the Tarim basin's in early-middle Permian epoch, trying to set up the relationship between the northwestern margin of the Tarim sedimentary basin , South Tien residual sea basin and South Tien orogenic belt by bilateral study of deposition and conformation.Based on field geology investigation,lithologic- lithofacies analysis, granite petrology, geochemistry and Zircon U-Pb chronology, the article reveals the spatiotemporal variation of the northwestern margin of the Tarim basin from Carboniferous to Permian, especially in early-middle Permian. The study holds that early-middle Permian sedimentary basin in this area shows typical sedimentary structure feature of peripheral foreland basin Fold-thrust zone,foredeep belt,fore bulge zone and back-bulge deposit zone can be identified from north to south, possessing their own depositional feature. Fold-thrust belt characterized by turbidite of middle Longlin Age within late Carboniferous-early Permian, has no any new strata upwards. It is the front zone of southward ovethrust nappe of South Tien orogenic belt. Foredeep belt, featured by huge thick turbidite, moved to south continuously from late Longlin age-Qixia age of middle permian, which can be showed by the turbidite sedimentation on the platform facies strata during top part of Upper carboniferous-middle part of Longlin age of Lower permian. It is in accordance with the southward overthrust nappe acting time of South Tianshan orogenic belt. Fore bulge belt, featured by comparatively small thickness of platform facies in lower-middle Permian period, has developed reef limestone of Qixia stage on its north edge in lower-middle Permian period. Back-bulge deposit basin consists of comparatively small thickness fine clastic rock, grouped into lower Permian platform facies and middle-upper Permian meandering river. In northwestern margin of Tarim Basin, the strata from Maokou stage, middle Permian upward was all absent, which means the sedimentrary basin of early-middle Permian became absent either.The research on the spatiotemporal evolution feature of strata and lithofacies shows that during Permo-Carboniferous, this area has distinctive zonal distribution spatially. As sedimentary emvironment changing from marine to terrestrial during middle-late Permian in this region, residual ocean basin of southwestern Tianshan region gradually disappeared during middle Permian, and southwestern Tianshan Orogenic Belt formed during lower Permian.The study on the petrology, geochemistry and isotope chronology of moyite in Halajun shows that the belt changed from collisional-orogenic stage to postcollisional-extensional stage during middle Permian, and then molasse began to deposit in some area within south Tianshan Orogenic Belt during late Permian. The early-middle Permian sedimentation of northwestern margin of Tarim Basin responsed timely to the collisinal orogeny of south Tianshan Orogenic Belt, which offers new evidence for the belt. And the west-to-east oblique collision between Tarim block and the one to the north can be proved by depositional features of the foreland basin from one broadside.