Early Paleozoic ductile deformation of the South China Block: the polyphase shear zones in the eastern Jiangnan belt

Three parallel east-west-trending shear zones are components of the polyphase shear zones in the eastern Jiangnan belt. The meso- and microstructural characteristics and the estimates of deformation temperatures, vorticities and geochronology of these shear zones provide significant information regarding the tectonic evolution of the South China Block (SCB) during the early Paleozoic. Most shear sense indicators (e.g. asymmetric folds, S-C fabrics and rotating porphyroclasts) reveal that the kinematics were characterized by east-west dextral shear. Finite strain measurements indicate an oblate strain ellipsoid shape with S-type and SL-type tectonites. The rotated rigid porphyroclasts and Mohr circle methods yield vorticity (Wk) values of 0.58-0.80 and 0.66-0.89, respectively, indicating that the flow was generally shear-dominated with almost equal proportions of simple and pure shear. Both mineral deformation thermometers and quartz c-axis fabrics indicate that the deformation occurred at medium temperature conditions of 400-550 degrees C. Combined with our new age and regional geological data, the east-west-trending shear zones might represent an increment of deformation during the last period of the early Paleozoic orogeny in the SCB.