MIDCRUST KINEMATICS AND VORTICITY IN THE ZHANGBALING TRANSPRESSION BELT, SE CHINA

The Zhangbaling (ZB) belt, SE China, is exposed along the NE trending Tan-Lu fault and represents the transition zone between the Dabie Shan and Sulu ultra-high pressure (UHP) belts. Ar-Ar dating of phengitic white micas shows ages of 225-236 Ma interpreted as fabric-formation ages; these are consistent with ages reported for the formation of the UHP belts. Therefore, the ZB belt likely developed during oblique convergence along the transform zone that joined the Dabie Shan and Sulu belts. The ZB belt displays gently dipping foliation in a dome-and-basin pattern and a lineation that changes gradually from NE in the south to N and NW in the north, with top-to-south sense of shear. This subhorizontal high strain zone developed mainly in the ZB group felsic metavolcanics and in Sinian metatillite and is overlain by a ductile to brittle transition zone exposed beneath a foreland fold-thrust belt with a clear tectonic vergence toward the SE.

Strain/vorticity analyses are based on finite-strain measurements in metatillite and analysis of the shape preferred orientation (SPO) of rigid clasts in the mylonitic felsic metavolcanics. Results suggest dominantly plane to constriction strain with a finite vorticity Wk~0.8 (combined pure and simple shear). Quartz c-axes fabric patterns imply basal slip system under relatively low temperature. Based on these results and on thermochronology, the dominant fabrics were probably developed during burial and partial exhumation of this transpression belt. In order to explain the existence of gently dipping foliation, with lineation curving from belt-parallel to belt-perpendicular, in a transpression zone, several models are explored. A model of attachment between a transpression belt below and the rigid upper crust (the fold-thrust belt) above is successful in predicting the orientation of foliation-lineation, the sense of shear, and the trend toward constriction finite strain. The geometry, kinematics, and vorticity of the ZB belt are consistent with a fabric developed during progressive exhumation by belt-parallel shearing and extension in an overall transpressional system.