Reef-building Mechanism in the Early-Middle Permian of the East Kunlun Mountains TIAN Shugang(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037) and FAN Jiasong(Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029) Reef-building was controlled by the community succession, sea-level elevation and basement changes caused by tectonics. Six kinds of reef-bnilding communities have been sorted out, which possess different compositions and occupy respective ecological sets. Types and thriving of communities were distinctive and replaced one after another in the four stages of reef development. Sedimentary cyclothems are clearly discernible in the reef-facies strata, which could be divided into