Abstract:

The strain characteristics and tectonic significance of the Sanjiang ductile shear zone are studied by analyzing the macro-micro structures, magnetic fabrics and quartz EBSD fabrics to constrain the Early Paleozoic orogenic processes of the Yangtze and Cathaysia block. The Sanjiang ductile shear zone has typical macro-micro scopic ductile deformation fabrics, including S-C fabric, tensile lineation, A-type fold, domino structure, mechanical twin, permeable foliation, core-mantle  structure and quartz dynamic recrystallization. The magnetic fabrics and macro-micro structure show that the Sanjiang ductile shear zone extends more than 40 km in NNE direction with a width of 3.7-7.5 km. The mylonitic C-foliation strikes 262°-321° with the dip at 31°-87° and the overall average is 295 °∠ 69°. The mean of 36 groups of magnetic foliations with susceptibil ity anisotropy (Pvalue)≥1.05 is 304°∠ 83°. The clockwise rotation of the magnetic foliation relative to the C-plane foliation of mylonite indicates a sinistral sense of the Sanjiang ductile shear zone. Macro-micro structures show that the Sanjiang ductile shear zone is dominated by sinistral thrust shear and dextral normal slip shear. The rock strain of the Sanjiang ductile shear zone is mainly flattening and tensile strain. At the same time, it shows both pure shear and simple shear. Quartz EBSD fabric analysis reveals that the Sanjiang ductile shear zone is characterized by the late mid-low temperature deformation (300-500 ℃ ) superimposed on the early midhigh temperature deformation (480-600 ℃ ). Based on the research of macro-micro structures, magnetic fabrics and quartz EBSD fabrics, combined with regional geological data, it is considered that the Sanjiang ductile shear zone was formed in the tectonic background of compression and extrusion of the Cathaysia block from SE toward the Yangtze block. In the early extrusion and collision stage, the Sanjiang ductile shear zone underwent mid-high temperature sinistral thrust shear. After the late orogenic extension, the Sanjiang ductile shear zone experienced dextral normal slip shear at mid-low temperature. The above analysis reveals the strain characteristics, deformation temperature and dynamic background of the Sanjiang ductile shear zone and provides new constraints for better understanding of the Early Paleozoic tectonic events in South China.

Key words: magnetic fabric, quartz EBSD fabric, strain characteristic, ductile shear zone, western Hunan-northern Guangxi