Tectonite Series In The Middle-southern Segment Of Ailao Shan-Red River Shear Zone And Their Geological Implication

Much attension has been paid, in the last decades, to the Ailao Shan-Red River (ASRR) shear zone, an important tectonic boundary in eastern Tibet. The shear zone cut across and broke-up the Ailao Shan ophiolite zone formed by opening and closing of Paleozoic ocean. It experienced a long and complex history as a structural accommodation zone during the subsequent India-Asian collision and post-collisional evolution. This paper focuses on structural characterization of the middle-southern segment of the ASRR shear zone. The spatial and temporal sequences of tectonites along the shear zone are studied to reveal the Cenozoic tectonic evolution and deformation regimes along ASRR shear zone.The ASRR shear zone, extending in NW-SE direction, is composed of the Ailao Shan shear zone, Red River fault zone and high grade metamorphic rocks between them. Various types of tectonites are observed along the shaear zone. Based on structural, microstructural and fabric analysis of tectonites from 6 geological sections and outcrops along stirke, four groups of tectonites are recognized along the middle-southern segment of the ASRR shear zone, i.e. high-temperature tectonite series (including high-temperature mylonites), low-temperature mylonite series, cataclasite series and superposed tectonite series.It is shown that the tectonites have very complicated deformation and metamorphism history. A general tendency of evolution is as follows: 1) Rocks of upper greenschist-amphibolite facies in Ailao Shan group experienced left-lateral strike-slip ductile shearing at an early stage, forming high-temperature mylonite series with high-strains in Ailao Shan shear zone and Red River fault zone respectively; 2) The tectonites were superimposed by middle-low greenschist facies (retrograde metamorphic condition) left-lateral strike-slip ductile shearing at a late stage, and resulted in the formation of low-temperature mylonite series, and superposed tectonites. In low-temperature mylonites there are different structural features as a result of progressive deformation. The qualitative analysis of quartz C-axis by EBSD also provides information on superposition of two stages of ductile deformation; 3) Cataclastite series were formed during right-lateral strike-slip activity along Red River fault zone.The tectono-magmatic relationship between Ailao Shan shear zone and Fan Si Pan granite indicates that the onset of left-lateral movement along ASRR shear zone should start after ~35Ma. Combined with previous geochronology data and analysis on relationship between lineation-foliation fabrics and deformation regimes, the structural evolution of ASRR shear zone can be subdivided into three stages. During the main active period of ASRR shear zone at 35~21Ma, L-type tectonites were developed as high-temperature mylonites under a transtension system (but regionally transpression). From 17 to 12Ma, low-temperature mylonitization is attributd to strike-slip deformation regime along ASRR and Red River fault zone with transpression system regionally. From ~5.5Ma, cataclasis characterizes a transtension system along the Red River fault zone.