Study On Tectonic Deformation Characteristics And Chronology Of The Southern Qiangtang Thrust Belt

The Cenozoic thrust nappe structure is one of the most important structural patterns in the Southern Qiangtang basin,which runs through the whole basin.As a product of extrusion,these thrusts play an important role in the shortening of the crust and the uplift of the Tibet.Here,we have studied the tectonic deformation in its footwall strata and hanging wall strata.In the western part of the SQTT,the Sewa formation（J1-2s）and Sahqiaomu formation（J2sq）overlay the footwall Niubao formation（E1-2nb）,thrust faults are imbricated,alternatively,it might be the duplex structure.In the eastern part,there are no big faults,as the klippe,Riganpeicuo formation（T₃r）covers the Jurassic strata in it’s footwall.The nappe pushed at least 10 km from north to south.In the hanging and footwall strata,roughly three stages of tectonic deformation are divided:Phase 1（D1）is mainly seen in the Sewa formation(J_1-2s)and Shaqiaomu formation（J₂sq）,the deformation in this period is mainly manifested as closed syncline-recumbent folds,the axial plane cleavages of folds have strong permeability,they lean at high angles in the direction of N,NE,and NW.The tectonic deformation at this stage is related to the subduction and closure of the middle Tethys ocean.The second phase（D2）is the vertical fold in east-west direction on the axial plane formed under the north-south compressive stress,with the penetrative cleavage S2.In the Saqiaomu formation（J₂sq）,this phase of fold deformation is also manifested as large-scale folds,it has a connection with the India-Eurasia collision.The third phase（D3）refers to thrust structure and associated fold deformation and fault-related cleavages S3,thrust faults usually cut the previous cleavage S1 and S2.In the footwall,the Niubao formation(E_1-2nb)developed a broad gentle fold with a wavelength overs 2 km.This deformation（D3）should be related to the continuous compression of India-Eurasia.In this study,we reported for the first time the dating results of fault gouge size grading method on SQTT.The activity time of the main fault was between 30.88 Ma and 30.12Ma.This proves that the Southern Qiangtang basin was subjected to a strong thrust during the Oligocene.In addition,the gouge age of 36.5 Ma was obtained from the fault zone in the east of the study area.In addition,we also got the detrital zircon ages come from the footwall strata-the Niubao formation（E1-2nb）,the most young ages are 48.6Ma and 55.4 Ma,respectively.Combined with previous research results,we consider the sedimentary age of Niubao formation to be the Paleogene,this information also provides a constraint for the time of the SQTT.K-Ar dating of fault gouge gives the direct active age of SQTT,the study on the deformation characteristics of the thrust nappe structure shows that the Qiangtang has undergone multiple tectonic deformation,indicating that its uplift began in the Mesozoic.Since the Cenozoic era,India-Eurasia-continuous extrusion has formed a series of thrust nappe structures in the Southern Qiangtang,which has made an important contribution to the uplift of the Southern Qiangtang since the Cenozoic.