Tectonic Characteristics And Rheological Parameter Estimation Of Yuanbaoshan Ductile Shear Zone

Located at the southernwest junction points between Yangtze and Huaxia block,the Yuanbaoshan area in the northern Guangxi is crucial to the research about the tectonic history of those two blocks.Previous studies concentrated on sedimentology,geochronology and isotope geochemistry of the strata or magmatite.However,not enough attentions have been given to the study of ductile shear zone.To get a better understanding of the tectonic evolution between Yangtze and Huaxia block,through the field geological investigation?microstructural observation?rock finite strain measurement?deformation temperature analysis and rheological parameter estimation,we summarized the main results as follows:As a gigantic dxtral strike-slip thrust ductile shear zone,Yuanbaoshan ductile shear zone consisted of two ductile shear belts-the west belt and the east balt which exceeds 40km length in NNE direction with a thrusting direction from NWW to SEE.The west belt is about 3.5-5km width,dipping toward in 270°-320°direction with dip angles from 15°to 30°while the east belt is about 1.5-5 km width dipping toward in 250°-300°with dip angles from 45°to70°.Sibao Group?Danzhou Group and Nanhuan System were involved into the ductile shear zone as well as Yuanbaoshan granite and some mafic-ultramafic rocks.Rocks deformed pervasively,there are mylonitic foliation?A folds?elongation lineation?schistosity?S-C fabric and Asymmetric augen.In plane those two belts shows dextral shearing consistently which reflects the same tectonic settings.Systematic measurement of the finite strains of the mylonites indicates that 8 out of 10measurement points are compressed strain type in the west belt,8 out of 13 measurement points are compressed strain type in the east belt;The strain degrees of the west belt are between 1.91 to 2.84 with a average 2.26 while the strain degrees of the east belt are between1.52 to 8.34 with a average 2.59;The rock compressibility of the west belt is 39.2%and the east belt is 34.9%;Both strain degree and rock compressibility suggest that the deformation of the west belt is stronger than the east belt.The kinematic vorticties show that the west belt is mainly pure shear while the east belt is mainly simple shear.By quantitative estimation,the west belt compression ratio is 10.79%while the east belt is 4.92%with a thickness reduction about 7-22 km and 20-90 km respectively;Quartz C-axis fabrics,quartz recrystallization deformation behaviors,quartz grain size-frequency diagram and Kruhl themometer demonstrate that deformation temperature evolved from high temperature?650°C?to middle temperature?500°C?,then to low temperature?400°C?;The paleo-stress is about 15.38-43.83 MPa which probably represents the lower limit of the paleo-stress during mylonization.The strain rates range from 10^-15 s^-1 to10^-10 s^-1.Considering the situation of South China continent surrounded by the others plates chronically?nonequilibrium conditions between Huaxia and Yangtze blocks in which the dynamic of tectonic evolution of Yuanbaoshan ductile shear zone was provided.We suggest that Yuanbaoshan ductile shear zone might resulted from the thrusting of Yangtze block to Huaxia block at northern Guangxi during the later Caledonian.