COMPLEXITIES FO SHEARING BETWEEN SOUTH CHINA AND INDOCHINA DURING EXTRUSION FROM TRIASSIC TO RECENT TIME

The parallel Ailao Shan shear zone (ASSZ) and the Red River fault zone (RRFZ) are often considered parts of the same tectonic system, but should be separated as distinct entities, because they were active at two different times, have different exposed structural expressions and have different senses of displacement. The ASSZ is An early Cenozoic left-lateral mylonitic shear zone active until about 17 Ma, but its time of initiation remains controversial. The RRFZ is an active right-lateral brittle fault zone, but its beginning is at least 2 Ma but probably older. Nowhere is this separation more clear than near China-Vietnam border where within SE Yunnan the ASSZ becomes a narrow (~1km wide) mylonite zone and the metamorphic rocks in the Ailao Shan are intruded by abundant plutons of both Mesozoic and Cenozoic age that are largely unmylontized and becomes a broader terrane in Vietnam. Faults of the RRFZ separate two additional mylonitic belts of rocks along at the border and in Vietnam NE of the ASSZ. These two belts are also partially separated by fault-bounded narrow zones of Triassic and older (?) sedimentary and metasedimentary strata and by Neogene conglomerates. The middle belt of mylonitic rocks widens to the SE into Vietnam and the northeastern belt continues into Vietnam as the Day Nui Con Voi (DNCV) that is commonly connected with the ASSZ. None of mylonitic belts can be followed directly into one another other. Each of the mylonitic belts have cover rocks of Paleozoic (ASSZ, middle mylonitic belt) or Triassic age (DNCV). Most importantly the DNCV terminates in Yunnan in a NW-plunging antiform covered by South China Triassic rocks. The DNCV consists of both early HT and younger LT mylonites. The LT mylonite of the DNCV cannot mark a zone of significant lateral displacement and DNCV must have a South China basement protolith. These relations suggest a complex history of protracted shear zone development beginning possibly in the Triassic extending into the Neogene modified by the brittle Neogene to recent RRFZ. The margin of South China is truncated by the shear zones, but a reconstruction remains uncertain.