Abstract

As part of the 1993 French-Vietnamese PONAGA cruise, profiles comprising swath bathymetric profiling, six-channel reflection seismics, gravity and magnetic data were acquired across the N-S trending margin off Central Vietnam. These data enable us to recognise structural features striking N060 degrees E. The seismic data show progressive crustal thinning from NW to SE: large tilted blocks in shallow water (Triton ridge), smaller blocks under a thin sedimentary layer (terraces), a buried structural high, and a deep basin (Nha Trang basin) with a thick sedimentary fill, a sequence typical of a passive margin. In map view all these structures branch in horsetail fashion onto the N-S striking Vietnam scarp, suggesting that the stretching is accommodated by N-S dextral motion with respect to the Indochina peninsula. Estimated crustal thicknesses of about 19 km for the lower terrace and 11 km for the Nha Trang basin correspond to extension factors of 1.7 and 2.9, respectively, if the crust is of continental origin. The total amount of extension then must have been at least 165 km and must have ended by anomaly 6 (20.5 Ma), age of the oldest oceanic crust adjacent to the basin. Assuming steady state symmetrical rifting, it must have started near anomaly 8 ( approximately 28 Ma), corresponding to approximately 165 km of opening prior to anomaly 6. Alternatively, strongly asymmetrical rifting implies a slightly younger age, near anomaly 7 ( approximately 26 Ma), while additional stretching further north leads to an older age, possibly up to 29 Ma. This result would then imply that the opening of the South China Sea after 29 to 26 Ma occurred along the right-lateral Vietnam scarp and was not primarily controlled by the extrusion of the Indochina.

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