TECTONIC CHARACTERISTICS OF THE MIDDLE SECTION OF THE AILAO MOUNTAINS

Through 1: 50000-scale regional surveys,a gigantic tectonic detachment zone has been found in the Ailaoshan Group,and with the detachment zone as the boundary the metamorphic series of the Ailaoshan Group may be divided into the upper and lower parts. Both parts show notably different features in respect to metamorphism,tectonic deformation,protolith formationand magmatism. This area has undergone superimposition and reworking of different complextectonic processes such as extension,compression,torsion,shear and uplift and subsidence.According to the tectonic deformation stages and compounding relationships,three phases of tectonism may be distinguished:（1）the early phase （prior to the Triassic）was a stage of formationof the "metamorphic core complex"under the conditions of continental extension;（2）the middlephase（related to the Triassic Paleo-Tethys closure）witnessed overthrusting under the conditions of continental collision;（3）the late phase（terminal Yanshanian-Himalayan）again sawstrike-slip shear and strong uplift,accompanied by subsidence due to mormal-faulting on theeastern side of the Honghe River fault zone.