Structural Deformation, Geochronological Characterization And Regional Tectonic Implication Of The Ailaoshan-Red River Shear (Fault) Zone

As the boundary between the Indochina and the Yangtze blocks, the Ailaoshan-Red River shear (fault) zone has been extensively studied, focusing principally on its structural evolution and accommodation to the India-Asia collision. The Ailaoshan-Red River shear (fault) zone underwent an early sinistral strike-slip ductile shearing and a late normal dextral strike-slip brittle faulting. The former is characterized by deformation structures along the Ailaoshan range and the latter occur as the brittle Red River fault.Based on macro- and micro-structural observations and fabric analysis of quartz in the sheared rocks, two stages of Cenozoic tectonic activities of the Ailaoshan-Red River shear zone have been established. During the early stage, the deformation along the Ailaoshan belt is dominated by contractional deformation under pure shearing. Ductile simple shearing characterized the late stage evolution of the shear zone. The dextral strike-slip shearing along the Red River fault zone has important effects on the geomorphology on the river sides. Thus, the displacement of late dextral strike-slip could be estimated up to 3km by offset and deflection of the tributary of Red River.Based on tectono-magmatic relationship, the left-lateral ductile shearing along the Ailao Shan- Red River shear zone started at 28Ma, lasted to 23Ma and ended at 21Ma. According to the previous apatite FT dating results (5.5~1.5Ma) along the Red river fault zone, and the dating result along the Dien Bien Phu fault (5Ma) related to Red river fault in this paper, the late dextral strike-slip shearing initiated at 5Ma.According to Morphotectonics, fault activity at shallow crustal levels and focal mechanisms of earthquakes at mid-shallow levels, the overall stress field in eastern Tibet and Three River region since Pliocene changed regularly from north to south. Taken the eastern Himalayan syntaxis as a turning point, it is shown to be a regional E-W extensional stress field in the southern region while the E-W compressional in the northern region.Contractional deformation under early pure shearing, ductile deformation under late simple shearing, and magmatic activity along the Ailaoshan-Red river shear zone during Oligo-Miocene, and the variation of stress field from the northern to southern part in eastern and southeastern Tibet, are attributed to the Indian-Eurasian collision, and the subsequent rotation of tectonic plates and crustal blocks.