The present-day distributions of mylonitic belts within Yunnan and north Vietnam, along the northern boundary of early Cenozoic extruded crustal fragments around the eastern Himalayan syntaxis, collectively called the Ailao Shan–Day Nui Con Voi shear zone, cannot be traced directly into one another. There are at least three belts of mylonitic rocks, the Ailao Shan, a middle belt, and the Day Nui Con Voi, and, in Yunnan, these are separated by zones of weakly to unmetamorphosed sedimentary rocks of Triassic and possibly early Paleozoic age, narrow fault-bounded deposits of Neogene age, and young or active faults of the Red River...