An Analysis Of Structure Style Of The Mega Thrust Nappe Structure And Tectonic Stress Fields Characteristics Of The Southern Qiangtang Basin,Tibet

Due to the Indian-Eurasian continental sustained collision and the closure of the Neo-tethys in Cenozoic,The Northward Subduction of the Lhasa block,and The Southern Qiangtang terrane(SQT)thrust southward to the above of the Lhasa block.Thus far,nevertheless,the questions about the structure composition,the structure style,the formation age and the structure shorten distribution of the thrusts and the fold deformation that is sandwiched between the mega thrusts are still hotly controversy.Basing on the detailed field investigation,We analysised the Geometry,kinematics feature of the Xiaochaka-Shuanghu thrust fault(XST),the Longeni thrust-fold belt(LT),the Doma-Qixiang co thrust fault(DQT),the Saibu co-Zagya zangbo thrust fault(SZT),and the fold structure which exist between the faluts.The research shows that the structure style of the faults are mainly consists of the forward thrust.the main thrust,the normal fault behind the main fault,and the reverse thrust,The faults are imbricate thrust nappe structure whose striking direction is nearly E-W or NEE-SWW with a main dip angle of 45~75°.The mega thrust nappe faults and secondary thrust faults of SQT are closely linked to the cenozoic Kangto group,Which is at the footwall of the faults with the missing of the bottom conglomerate Stratigraphic sequence,showing that the thrust faults are the same period imbricated thrust napped structure.and the sediment era of the Kangto group is the lower active time limit of the thrust faults.Significantly,The oblique bedding,graded bedding and the secondary structure et al.,reflecting the overturned-stratum of the Kangto group that is at the footwall of the faults.The faults and folds structure with different properties and sizes in the SQT indicating the strong shorten deformation of the basin.The thrusts and fold structure are all cut by the NE-SW or NW-SE striking transtension normal faults,which holds penetrative transtension jionts,the quaternary hot springs,beaded spring sinter and tectonic basins that implicts the strong fault activity,and the activity of transtension normal faults is the upper active time limt of the thrusts.According to the comprehensive analysis of this study,the active time of the main thrust in the SQT is in the late paleogene period,and the late oligogene to the early of Miocene may be the main active period of the thrusts.Moreover,This paper thesis widely collected the tectonic chronology,palaeomagnetism and deep geophysical research data that is related the SQT,established the tectonic framework of the basin in detail and discussed the tetonic evolution of the basin under the regional tectonic evolution.By analysising the fold feature in the field,Combining with the level,joint surface and fault surface sliding vector analsis,We identified four Periods tectonic stress field of the SQT:1)The early N-S trending Compressive tectonic stress in Meso-tethys.2)As the closing of the Bangonghu-Nujiang suture zone,the stress field of the SQT changed to the N-S trending detaching.3)In Cenozoic,the Tibetan platen entered the stage of intra-continental deformation and the SQT performed a NE-SW trending compressing feature,and then the mega thrust Nappe structure of the SQT actived in late paleogene,in which a series of north to south exhibition type imbricated thrusts,which thrusts the SQT to the the above of the Lhasa,formed.4)With the continuous uplift of plateau,the North and south rift valley actived in about 14 Ma,and the tectonic stress turned to SE-NW trending detachment in which the Shuanghu-rifts formed.