Structural Characteristics And Geological Implications Of The Middle To Northern Segment Of Ailao Shan Metamorphic Belt

Ailao Shan metamorphic belt can be subdivided into two metamorphic belts from their characteristics of metamorphism and deformation, a high-grade metamorphic belt in the east and a low-grade metamorphic belt in the west. Three boundary faults are the Red River fault, the Ailao Shan fault and the Jiujia-Anding fault distributed from NE to SW. On the basis of detailed studies of literature, we finished systematic geological survey of three geological sections (i.e. Shuitang-Heping, Mosha-Malu and Yuanjiang-Anding sections). Lineations and foliations are measured and a group of samples were taken and analyzed. Based on these studies, we found that the high-grade belt includes a sequence of high-temperature mylonite (L tectonites and L-S tectonites) formed in lower amphibolite facies-upper greenschist facies and the low-grade belt contains a series of low-temperature phyllonite (S tectonites) formed in lower greenschist facies.Three types of tectonites are distributed at different tectonic settings. L-S tectonites are mainly observed near fault zones in high-grade belt and some are away from the fault zones in high-grade belt. S tectonites are located around the Ailao Shan fault in low-grade belt. L tectonites are distributed in high-grade belt. It is concluded in combination of regional chronological data, the different types of tectonites are formed in different stages of Cenozoic evolution of the Ailao Shan shear zone. L tectonites are formed before 56~58Ma in a transtensional regime, whereas L-S tectonites between 22~24Ma by simple shearing. The S tectonites, however, are formed in a later transpressional event at around 12~13Ma, that is accompanied with thrust faulting.Regional structural analysis reveals that the L tectonites or high-temperature mylonite are formed in the deep crustal level. They are exhumed to the surface in a later event. The tectonites experienced shearing at lower amphibolite facies to upper greenschist facies forming L-S tectonites. Both of the tectonites undergo transformation by low-temperature shearing at lower greenschist facies, forming S tectonites.