ELSEVIER Tectonophysics 24 3 (1995) 209-222 TE TONOPHYSI S The Red River fault system in the Tonkin Gulf Vietnam C. Rangin a M. Klein b D. Roques a X. Le Pichon c LeVan Trong d a Ddpartemen t de Gdotectonique, CNRS UR A 1759, T 26 -0 Unit ersit~ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris, Paris Cedex 05, France b Total, la Dffense, Paris, France c Ecole No rmale Supdrieure, Paris, Fran ce Petrocietnam, Hanoi, Viet Na m Received 24 March 1994 ; revised version accepted 27 September 19 94 Abstract The R ed River fault system in the Tonkin G ulf offshore Haip hong wa s studi ed using seismic profi les ca librated by deep wells. Well characterised left- later al strik e-sli p occurr ed contin uousl y withi n a narrow 30-km-wid e zone south west of the Vinh Minh fault between 30 Ma and 5.5 Ma. However, the corresponding amount of offset probably does not excee d a few tens of kilometres . No sign of po st-5. 5 Ma rig ht-lateral motion can be det ected. Prior to 30 Ma, there is a widespread extension in a wider 100-km-wide zone which could be relate d to a significa nt amount of left-lateral motio n. Th e motion of the fault splays to the no rtheast of the Vinh Minh fault prior to 30 Ma was absorbed in the rifting of the Gulf of Beibu. A 15.5-Ma unconfor mity separates the transten siona l regime from a later transpre sional regime. This 15.5-Ma date coincide s with the cessation of sea-floor spreading in the So uth China Sea I Introduction The Red River fault zone (RRFZ) in Asia is a major strike-slip fault zone that may be followed from Tibet to the South China Sea for over 1000 kin. This fault is presently locally active with a right-lateral displacement (Allen et al., 1984). However left-lateral motion resulting in the southeast escape of the Indochina block in re- sponse to the India-Eurasia collision was pro- posed by Tapponnier et al. (1986). Tapponnier et al. (1990) and Sharer et al. (1990) showed the presence of Cenozoic syntectonic metamorphism within this left-lateral shear zone. In the Ailao- Shan Massif, monazite and xenotime U/Pb ages from late syntectonic leucogranitic veins parallel to the foliation and affected by left-lateral shear, cluster at 23 + 0.2 Ma (Sharer et al., 1990). How- ever, other undated metamorphic rocks present along the RRFZ may have a different age. Esti- mates of left-lateral offsets of geological features across the shear zone made by the team of Tap- ponnier range between 300 and 700 km (Leloup, 1991; Lacassin et al., 1993), although others con- sider that the offset may be very small (Dewey et al., 1989). Finally, the chronology of the opening of the South China Sea between 30 and 15 Ma, as deduced from magnetic anomalies (Taylor and Hayes, 1983), appears to be compatible with the timing of strike-slip faulting along the RRFZ as deduced from Tapponnier et al. (1990). A quite 0040-1951/95/ 09.50 © 1995 Elsevier Science B.V. A ll r ights reserved SSDI 0040-1951(94)00207-X
