Structural and rheological features of the northeastern Sanyang shear zone in the eastern Jiangnan orogen: Indications for early Paleozoic orogeny in the South China Block

Abstract

The N55°E−trending Sanyang shear zone is located in the eastern Jiangnan orogen, and the northeastern junction between the Cathaysia and Yangtze blocks is the key to understanding the evolution of the South China Block (SCB) for early Paleozoic crustal reworking. Most shear-sense indicators (e.g., asymmetric folds, porphyroclasts, S–C fabrics, and mica fish) indicate that the Sanyang shear zone is characterized mainly by oblique sinistral shear and locally by dextral shear. Mineral deformation behavior, quartz C-axis fabrics, and the Kruhl thermometer demonstrate that shear deformation developed under high greenschist-to low amphibolite facies conditions with temperatures ranging from 450 to 550 °C. Differential stresses deduced from recrystallized quartz grain sizes were approximately 22.4–46.5 MPa, and strain rates were on the order of 10−9 to 10−10 S−1. Three dimensional (3D) strain ellipsoid measurements and the Flinn diagram of the Sanyang shear zone indicate that all samples yield oblate and flattened strain ellipsoids, which imply a contractional regime. The kinematic vorticity (Wk) 0.716–0.999, estimated by using 3D strain ellipsoid-based methods, indicates a high simple shear contribution in the shear zone. All structural, strain, and vorticity analyses suggest that the Sanyang shear zone experienced general shear with transpression. The numerical estimates allow kinematic modeling of the Sanyang shear zone, which provides important information about the structural framework for the SCB. The development of the Sanyang shear zone was a response to the oblique convergence of the Yangtze and Cathaysia blocks during the early Paleozoic orogeny.