THE STRUCTURAL FRAMEWORK OF PRE DEVONIAN METAMORPHIC ROCKS IN XIMENG COUNTY, WESTERN YUNNAN

The Ximeng area, western Yunnan, is the only area where the basement of the Baoshan Shan State massif is exposed in China. The pre Devonian metamorphic rocks in this area are divided into the Proterozoic structural layer and the overlying Paleozoic structural layer. The former, comprising the Pake Complex experiencing amphibolite facies metamorphism, is characteristized by three phases of N S trending deformation structures; the latter, consisting of the Wangya Formation and Yungou Formation, is characterized by two phases of N S trending deformation structures and low greenschist facies metamorphism. Detailed structural studies demonstrate that the Laojiezi Pake blind thrust antiformal imbricate structure and the Wangya Yungou backthrusting imbricate system dominate the structural framework of the pre Devonian metamorphic rocks in the Ximeng area. The former is the predominant structure and comprised of eastward overridding thrusts propagating in a piggy back sequence; the latter, which is characterized by the development of the westward thrusting faults and westward verging folds and foliation, is a coeval cover deformation system responsive to the former.