Structural signatures and 40Ar/39Ar geochronology of the Indosinian Xuefengshan tectonic belt, South China Block

To advance our understanding of the kinematics of the Xuefengshan tectonic belt (XFSTB) and the Mesozoic tectonics of the South China Block (SCB), this paper presents new data and interpretations from our structural studies and thermochronological dating. The XFSTB is characterized by greenschist-facies metamorphism and development of S and S–L tectonic fabrics. An asymmetric positive flower structural pattern is composed of NW–WNW- and SE–ESE-dipping cleavages, faults and shear zones. Kinematic indicators indicate a dominant top-to-WNW-thrusting with a sinistral strike-slip component on the ESE-dipping shear zones and top-to-ESE-back-thrusting on the WNW-dipping shear zones. The quartz c-axis orientations of mylonitic rocks exhibit monoclinic point-maximum asymmetry, indicative of a sinistral shear sense under the 〈a〉 basal gliding condition. The timing of the major deformation event has been constrained to the middle Triassic to early Jurassic (244–195 Ma) on the basis of 40Ar/39Ar geochronology and other geological observations. The structural characteristics of the XFSTB are probably related to an Indosinian oblique convergent zone, in combination with tectonic wedging and associated back thrusting, all above a low-angle, SE-dipping basal detachment. Oblique northwestward and upward movement along the basal detachment was partitioned into the NW–WNW-directed thrusting, SE–ESE-directed back-thrusting and subsidiary strike-slip movement along the NE- and NNE-trending faults and shear zones. The XFSTB may represent part of a huge structural fan between the Yangtze and Cathaysian blocks, and can be interpreted as a product of the Indosinian intracontinental collision involving a weak zone.