Structural Style, Strain And Tectonic Implication Of The Qingchuan-Pingwu Region In The Northeastern Songpan-Ganze Terrane

The geological background of Qingchuan-Pingwu region is mainly based on NEE trending Qingchuan-Pingwu fault. The Qingchuan-Pingwu fault lies at the northern segment of Longmen Shan thrust belt and the south of the Bikou terrane in the SW Qinling-Dabie orogenic belt. This fault is located in the junction of Qinling-Dabie orogenic belt, Songpan-Ganze orogenic belt and Yangtze block, and thus has a implication for the understanding of tectonic evolution because of its special tectonic position. The upper wall、footwall of the Qingchuan-Pingwu fault and the deformation characteristics、difference of the fault itself are one of the key to constraint the Qingchuan-Pingwu fault formation time. The upper wall、footwall of the Qingchuan-Pingwu fault is the main research targets of this paper. On the basis of previous studies, combined with the regional tectonic background of the area, using the macro and micro combined methods of Qingchuan-Pingwu region deformation sequence and metamorphism for more detailed research and uses strain analysis to make quantitatively structural constraints.Two deformational episodes D1 and D2 within the Qingchuan-Pingwu region are identified on the basis of detailed structural analysis. D1 is represented by Indosinian thrust with spaced cleavage(S1), moderately tight fold(f1), mineral lineation(L1) and corresponded kinematics; D2 has characteristics of extensional tectonics with detachment fault of Yanshanian ages. D2 is represented by ductile shear zone with penetrative cleavage(S2), recumbent fold(f2) and mineral lineation(L2) and brittle normal fault. Kinematic deformation of rotational porphyroclast system, pressure shadow and mineral fish are common and consistently indicate an extensional tectonic event. Both in micro and macro scales, the foliation S1 is mostly replaced by well developed and penetrative S2.Due to the general development of the D2 phase structure,EBSD quartz fabric analysis indicates that the D2 generally experienced a medium temperature to low-medium temperature. On the basis of structural analysis, comprehensive strain measurements, draw three strain intensity profile map through Qingchuan-Pingwu fault. According to the distribution of the strain intensity which obtained from the sampling points along the profile. Both in the upper wall of the fault in the north of the study area(near the mianlue) and near each fault the strain intensity are relatively high, And mainly for flattening effect. Use the porphyroclast aspect ratio method to measure the ductile shear zone for kinematic vorticity. Respectively as 0.42、0.6 and 0.68,are less than 0.75. The result shows that the mixed ductile deformation type is dominated by pure shear.