Cenozoic Cyclic Brittle-ductile Transitional Deformation Of The Cangshan Complex In Western Yunnan

The Cangshan complex is part of the Ailaoshan-Red River shear zone developed during the Indian-Eurasian convergence in eastern Tibet,and suffered multi-stage Cenozoic deformation and metamorphism.There are anatectic gneiss-mylonitic gneiss-mylonite tectonite series in the Cangshan shear zone,and the corresponding metamorphic facies change from high amphibolite to low green-schist facies,and the C-axis fabric of quartz is in the gradation form of Z-axis maxima-Y-axis maxima-X-axis maxima.By calculating and analyzing the differential stress and strain rate of the deformed rocks,the strain intensity model and minimum strain rate of the rocks in the middle-crustal shear zone of the Cangshan complex are obtained as 4⁵×10^-14s^-1.Besedes,the rheological evolution processes and deformation events sequence of different rock types were reconstructed combining with the 40Ar/39Ar cooling curve of the Cangshan complex.The multi-stage syntectonic granite dykes in the study area record the cyclical brittle-ductile transitions during ductile deformation,which lead to the formation of multi-stage high-temperature fractures.The temperature of the early H-T fractures are>550?,and the host rocks were under pure shear deformation along with weak mylonitization;The temperature of these late H-T fractures formed in amphibolite facies and green-schist facies rocks are 510⁵50?and 450⁵00?,respectively.The rocks were under simple shearing,and strongly foliated during this stage of deformation.In addition,the differential stress of amphibolite facies and green-schist facies rocks was 9¹4MPa and 10²4MPa respectively when the H-T fractures were formed,and the whole rock underwent plastic deformation under the lower differential stress conditions.According to the magmatic sources of syntectonic granite dikes,which are mostly the partial melting of the middle and lower crustal rocks under shearing?Trigger temperature?550??,we argue that the brittle-ductile transition of lower-crustal rocks was dominated by melt.The presence of melt could dramatically reduce the rock strength,and cause melt overpressure.These processes may lead to plastic instability and early H-T fractures;The upward propagation of lower-crustal fractures led to suddenly change in strain rate of the middle-crustal rocks,and the strain hardening processes occurred under the deformation dominated by dislocation creep.Meanwhile,the positioning of the magma chamber increased the differential stress on the rocks to its friction-slip strength,resulting in the formation of late H-T fractures.Finally,we establish the model of brittle fracture during plastic deformation of the Cangshan shear zone.The cyclical brittle-ductile transitions,corresponding to the cyclical activity of the lower-crustal melt during shearing and exhumation,indicate the cyclical fluctuations of melt pressure and strain rate on regional scale.Therefore,the fracture behavior under steady-state plastic deformation of Cangshan complex is fundamentally controlled by the competition between the stress loading and melt pressure bulid up.Crystallization rate of magma in place after the formation of brittle fracture will be the main controlling factor of melt pressure fluctuation.The multi-stage H-T fractures represented by syntectonic granite dikes in the Cangshan complex may be the indicator of slow earthquakes or tectonic tremors.Hence,the mechanismes of brittle failure at different levels in the ductile deformation region presented in this paper are of great significance for understanding the causes of slow earthquakes and tectonic tremors observed in similar tectonic backgrounds.