HIGH-TEMPERATURE METAMORPHISM IN A MAJOR STRIKE-SLIP SHEAR ZONE - THEAILAO-SHAN-RED RIVER, PEOPLES-REPUBLIC-OF-CHINA

Petrographic and thermobarometric analysis provides constraints on the PT path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indoc hina during the Tertiary. Two different paragenesis, P1 and P2, are co eval with this deformation and correspond respectively to the amphibol ite and greenschist facies. Microprobe analysis reveals that P1 garnet s bear a chemical zonation from core to rim. This zonation indicates a temperature increase during garnet growth. Conditions of formation of garnet rims (P1b), which are estimated using biotite-garnet and plagi oclase-garnet thermobarometers, are close to the granitic solidus (710 +/- 70-degrees-C and 4.5 +/- 1.5 kbar). P2 conditions are estimated t o be approximately 500-degrees-C and < 3.8 kbar. Both P1b and P2 condi tions correspond to much higher temperatures than expected for their d epths in the continental crust, suggesting a perturbed geothermal grad ient during strike-slip deformation along the Ailao Shan-Red River she ar zone. Thermochronology results suggest that cooling between P1b and P2 was fast (almost-equal-to 100-degrees-C/Ma) and may have been asso ciated with significant uplift. Uplift during the left-lateral shearin g may have resulted from a slight reverse or, more probably normal, co mponent of movement along the strike-slip fault. A simple numerical mo del suggests that the high temperatures in the shear zone at the time of deformation may be explained by shear heating in the more competent upper mantle and by advection of this heat along the shear zone by as cent of magmas and/or fluids. In this hypothesis, the medium-pressure and temperature schists bounding the mylonitic gneisses to the southwe st previously interpreted as resulting from collision-related metamorp hism result instead from 'contact' metamorphism of the shear zone at m id-crustal depths.