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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 fault zone. These three belts of mylonitic rocks are interpreted to have been originally a single through-going zone of left-lateral shear that was disrupted by younger events in an evolving shear zone. The Day Nui Con Voi belt terminates to the NW in Yunnan in a NW-plunging antiform, where it is surrounded by Triassic rocks that belong to the Yangtze platform of South China. Within Yunnan, a carapace of metasedimentary Triassic rocks locally forms the cover for the Day Nui Con Voi mylonite, but the cover is interpreted to be a tectonic cover with an original low-angle normal fault at its base. Ordovician to Triassic strata near the Vietnamese border can be inferred to be the cover of the Ailao Shan based on geological relations between the sedimentary rocks and the metamorphic and mylonitic rocks, which appear to have been joined in Mesozoic time by plutons that intruded into both sequences. The nature of the cover, whether the sedimentary rocks are the normal stratigraphic cover or a tectonic cover, is unknown. Disruption of the mylonitic belts by late Cenozoic right-lateral faulting occurred along the Red River fault zone.

The preliminary tectonic conclusions suggest: (1) Triassic metamorphism, plutonism, and tectonism may have been important in the Vietnamese part of the mylonitic belts, but the extent of this event into Yunnan cannot be determined at present. (2) The early Cenozoic SE extrusion of Indochina was bounded on the NE side by a narrow mylonitic belt formed from Precambrian rocks and Paleozoic to Cenozoic igneous rocks. (3) During early Cenozoic transpressional extrusion, rocks in the evolving NE-dipping mylonitic belt were decoupled from rocks in the footwall and hanging wall by a thrust below and normal fault above. (4) While left-lateral shear was the dominant mode of deformation, upward extrusion of the ductile lower- and middle-crustal mylonitic rocks occurred early in the deformational process. (5) Decoupling of the mylonitic belt from adjacent rocks in the crust and also from rocks in the lower crust and mantle indicates that all rock units have moved relative to one another and makes determination of the magnitude of displacement difficult. (6) The Paleozoic to Triassic cover of the Ailao Shan correlates best with rocks near Erhai Lake, suggesting ~250–300 km of displacement relative to South China rocks in the hanging wall of the normal fault at the top of the mylonite belt, but they are still separated by an unknown amount of displacement from the Lanping-Simao tectonic unit farther to the SW. (7) The magnitude of displacement during disruption of the original continuous mylonite zone later in evolving transpressional left-lateral shear is limited by the broad continuity of the South China Triassic cover north of the Day Nui Con Voi and may be no more than 100 km. (8) The magnitude of late Cenozoic disruption by right-lateral brittle faults of the Red River fault zone is likewise limited by the distribution of the South China rocks and is probably ~50 km.

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