Neoproterozoic Rift Effect Of The Sichuan Basin And Three Dimensional Numerical Simulation Of Extensional Rift

Based on the combination of the tectonic characteristics of the top of the Upper Sinian and the present,the deposition characteristics,the deformation features and the petroleum exploration situation of Sichuan Basin,the Sichuan Basin was divided into five first-order tectonic units and twelve secondary tectonic units.The five first-order tectonic units are:(1)the uplift belt in the middle Sichuan,(2)the western Sichuan depression,(3)the northern Sichuan depression,(4)the eastern Sichuan fold belt and(5)the southern Sichuan fold belt.The structure framework of the Sichuan Basin were established by the interpretation of six large seismic reflection sections across the basin.Based on the the field investigation and literature survey,this paper explores the spatial distribution pattern of the Neoproterozoic mafic dykes in the Upper Yangtze area,and explores the spatial distribution pattern of the Neoproterozoic rift system in the Sichuan Basin based on the seismic interpretation.The results show that the Neoproterozoic mafic dykes developed in the the Upper Yangtze area have different preferred directions at different stages,in the southeastern area of the Yangtze block:the Guibei area(NWW-SEE,828 Ma),the Longsheng area(NNE-SSW,761 Ma);in the western area of the Yangtze block:the Huangtian area(NEE-SWW,792Ma),the Kangding area(main NEE-SWW,sub-NE-SW,779Ma),the Yingxin village(NEE-SSW)and the Qingping village(NNW-SSE).The age of tuff layer samples collected from the Third member of the Dengying formation in the Xianfeng section,E'bian County,Leshan city,Southwest Sichuan by the LA-ICP-MS zircon U-Pb dating shows that the deposition period of this layer is(539.6±1.4)Ma,suggesting a lower placement of the Precambrian-Cambrian boundary in the Sichuan Basin.We eatablished a evolution time frame of late Sinian to early-Cambrian intracratonic rift in the middle Sichuan basin based on four isochronous strata,the bottom of the Dengying Formation(551.1 Ma),the Third Member of the Dengying Formation(539.6 Ma),the Maidiping Formation(535.2 Ma)and the Ni-Mo sulfide enriched layer in the lower Cambrian(521 Ma).We propose that the formation of the late-Sinian(Ediacaran)to early-Cambrian intracratonic rift in central Sichuan Basin was mainly controlled by the boundary fault.Due to the deep-water depositional environment in the rift,the Maidiping Formation began to deposit rapidly after the deposition of the thin Third and Fourth Member of the Dengying Formation,the Maidiping Formation and the Qiongzusi Formation inside the rift and the Fourth Member of the Dengying Formation outside the rift are contemporary heteropical sediments.The tuff layers in the Third member of the Dengying formation are important markers for the correlation of the Dengying formation in the Sichuan basin.Based on stratigraphic correlation of famous sections of the Dengying Formation in Yangtze Block,we find that the tuff layers in the Third Member of the Dengying Formation distributed widely in the Upper Yangtze,and a carbon isotopic negative shift event was recorded by the Third Member of the Dengying Formation and its equivalent strata in the Upper Yangtze area.Zircon ages from three locations constrained the deposition period of this layer from 539.6 Ma to 542.6 Ma.We propose that the tuff beds in the Third Member of the Dengying Formation and its equivalent layer distributed widely and recorded one crabon isotopic neagative shift event which wasn't documented,this layer can be used as a new line for constructing the Sinian(Ediacaran)-Early Cambrian time frame.In this paper,the theory of extensional fault propagation modeling is studied systematically and the evolution process of the Neoproterozoic intracratonic rift in middle Sichuan is modeled.The evolution process is divided into three stages:(1)during the deposition stage of the Nanhua series,the rift system started;(2)the deposition stage of the Doushantuo Formation to the Second Fember of the Dengying Formation,regional geological setting is stable.(3)the deposition stage from the Third Member of the Dengying Formation to the Canglangpu Formation,the boundary fault of the rift system restarted to move,the syn-sedimentary strata was deposited.