INTRAPLATE EXTENSION PRIOR TO CONTINENTAL EXTRUSION ALONG THE AILAO SHAN RED RIVER SHEAR ZONE

Left-lateral movement of the Ailao Shan-Red River shear zone lends sup port to the hypothesis of continental extrusion resulting from the col lision of India with Asia, Our new observations from northwestern Yunn an, China, and northwestern Vietnam on different sides of the shear zo ne demonstrate that the sinistral offset was similar to 600 km accordi ng to correlations of Permian-Triassic flood basalt successions and la te Paleogene highly potassic mafic magmas. We conclude that the shear was propagating on the South China continental margin and does not cor respond to a suture between South China and Indochina Furthermore, the highly potassic magmas were emplaced from ca. 40 to 30 Ma, before the shear movement, which was caused by the late Oligocene to early Mioce ne (ca, 27-22 Ma) extrusion activity, This suggests that a late Eocene to early Oligocene intraplate extension, possibly induced by delamina tion of thickened continental lithosphere, took place in northwestern Yunnan (or eastern Tibet) as a response to the India-Asia collision, T his extension, and sea-floor spreading of the South China Sea that beg an ca, 30 Ma, could have accounted for the initiation of the Ailao Sha n-Red River shear zone.