Geometries And Kinematics Of Structures In Southwest Sichuan Basin: Indications To The Geo-dynamic Settings Of The Southeast Margin Of Qinghai-tibet Plateau

Sichuan Basin belongs to Yangtze Plate, and lies on southeast margin ofQinghai-Tibet Plateau. It is a kind of superposed basin based on the Middle and UpperYangtze Craton and having undergone multi-stage evolution. Its tectono-deposintionalevolution was controlled by evolution of South China Plate, North China Plate andintracontinental subduction caused by collision of the two plates before Cenozoic.During Cenozoic, uplifting of Qinghai-Tibet Plateau had an apparent effect onformation of Sichuan Basin. Particularly, the southwest region of Sichuan Basin wasthe most obvious and the strongest area which was interacted by collision anduplifting. Moreover, this region had a lot of complex structures which were differentfrom other structures in the middle and north basin. These differences reflecte thatthere had been a complex structural changing in the basin. In addition, this region alsodeposited Jurassic System, Cretaceous System, Paleogene System and NeogeneSystem discontinuous in horizon, and this phenomenon reflectes the changing oftectonic environment and strong effect of structural uplifting. However, thesestructures’ uplifting time, rate and stages are not well answered, and the problemshave a key role in constraining structural deformational time, rate and mechanism.Therefore, based on structural chronology, fault related folds, structural geologyand critical taper wedge, and generally taking advantage of geologic maps, fieldgeologic section, seismic reflection profiles and drilling data, this paper wellinterprets and analyzes seismic reflection profiles in order to study distribution andgeometrics of the structures in region of interest and to calculate quantitativelystructures’ deformational time and shortening and to build the regional geologicconfiguration section. The information can provide rational interpretation scheme forgeologic configuration and valid references for the oil and gas exploration in region ofinterest, and also provide some indications to uplifting settings of southeast margin ofQinghai-Tibet Plateau. The results and conclusions are as following:(1)The structures of region of interest distribute in rows in geologic map, and canbe divided into seven belts from northwest to southeast. They are Longmen Mountains frontier belt, Daxing Anticline (buried structure), Xiongpo-DongguachangAnticline belt, Longquanshan Anticline, Weiyuan Anticline, Kongtan-DengjingguanAnticline belt and Qingshanling Anticline, respectively. The geologic section can alsobe divided into three zones in vertical through the Sinian System weakness layer andLower Triassic Jialingjiang Formation-Middle Triassic Leikoupo Formation saltdetachment, which is shallow zone, middle zone and deep zone structural patterns.And different zones have different structural deformational features.(2)The shallow zone, middle zone and deep zone of region of interest have total19.1km,5.8km, and15.4km shortening, respectively, which reflect that there aremulti-detachment layers with different structural deformation.(3)According to the apatite fission track results, the south Longmen Mountains,Xiongpo Anticline and Weiyuan Anticline have some differences in uplifting time,stages and rate. Longmen Mountains begins to form quickly from Cretaceous, andrises more quickly from30Ma at least than ever before, and uplifts strongly from5Ma.This means that effect of Sichuan Basin which Qinghai-Tibet Plateau has had maybegin to be from30Ma, and be more obviously from5Ma.(4)The region of interest has two critical taper wedges. One is a deep critical taperwedge which is constituted by Sinian System weakness layer and surface slope. Andthe other one is a shallow critical taper wedge which is made up of Lower TriassicJialingjiang Formation-Middle Triassic Leikoupo Formation salt detachment andsurface slope. The reliable field of two wedges’ strength is0.5-1.0, and the reliablefields of their basement detachment strength are0.018-0.03and0.025-0.042,respectively, which reflect detachment of two wedges is extremely weak.(5)The fluid pressure ratio of deep wedge is larger than shallow wedge’s, whichmeans that the deep detachment is much weaker than shallow detachment.(6)The deep wedge is still not reaching to critical taper, but the shallow wedge hadreached. Therefore, the unstable, sub-critical deep taper wedge resulted in manyearthquakes in Longmenshan Mountains thrust belts.