Structural development and major unconformities of the NAM Conson basin, offshore Vietnam

Abstract

Two major unconformities, Middle Miocene (15.5 Ma) and Top of Basement (66.5 Ma), were identified and mapped within the Nam Conson basin using 2700 km of migrated, multifold, seismic reflection data. The results show that the Nam Conson basin is an asymmetric, extensional basin. It formed as a result of Oligocene and then Middle Miocene, NW-SE extension. The extrusion of the Indochina microplate and synchronous, left lateral motion at the terminus of the Mekong fault caused the Oligocene extension that formed the initial structure of the Nam Conson basin. In the Early Miocene, NW-SE seafloor spreading in the South China Sea southwest subbasin rapidly propagated southwestward reaching the Nam Conson basin in the Middle Miocene. This rapid southwest propagation was probably due in part to a NE-SW zone of weakness caused by the Oligocene extension. The resulting, Middle Miocene extension produced additional extensive NE-SW striking normal faulting, formation of half grabens, and the emplacement of an intrusive, axial zone on strike with the southwest subbasin spreading center.These results confirm the Miocene age given to the magnetic anomalies of the southwest subbasin by Taylor and Hayes (1980, 1983) and more recently by Briais et al. (1993). This study throws doubt on the model of Ru and Pigott (1986), who concluded that the South China Sea basin initially opened in the southwest during the Cretaceous. The two periods of NW-SE extension found in the Nam Conson basin created by the Oligocene left-lateral fault movement and the Middle Miocene seanoor spreading of the southwest subbasin support some components of the collision extrusion model proposed by Tapponnier et al. (1982, 1986). Identification of several additional unconformities in the seismic data show how the Nam Conson basin developed after the Middle Miocene, through the subsequent infilling of accommodation space overlying the tilted fault blocks; compression: deposition of a transparent sequence during a period of quiescence and thermal subsidence; and channelization of the transparent sequence followed by the deposition of another sedimentary sequence. Apparent Miocene to Recent volcanoes were also identified along 1090 E.