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Left-lateral motion along the Ailao Shan‐Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-floor spreading in the South China Sea. Our new 40 Ar/ 39 Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began 027 Ma and lasted until 022 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y ˇ1 . Such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea. # 2000 Elsevier Science Ltd. All rights reserved.