Cenozoic seismic stratigraphy and structure of the continental margin offshore Vietnam, South China Sea

Time-structure maps of the basement in the study area show two different sets of faults. These faults trend NNW-SSE in the northern and middle areas and NE-SW in the southern area. Strain ellipse indicates that the fault patterns in the northern area may have been formed by a left-lateral movement. This left-lateral movement may be related to the southeastward extrusion and clockwise rotation of Sundaland during the collision of India and Asia. Faults in the southern area that trend NE-SW may have been initiated during the Paleocene seafloor spreading in the Southwest sub-basin. The faults may have been reactivated during the middle Miocene.

Cenozoic sediments offshore Vietnam, with thicknesses that range from 0.5 to more than 4 seconds of two-way travel-time, consist of syntectonic and posttectonic sequences. Syntectonic sediments overlie a pronounced rift-onset unconformity. Posttectonic sediments, which overlie a breakup unconformity, contain five sequences that are inferred to be upper Oligocene - lower Miocene, lower Miocene - middle Miocene, middle Miocene - upper Miocene, Pliocene, and upper Pliocene - Holocene sediments. Maximum rates of posttectonic sedimentation are smallest during the early-middle Miocene. The highest increase in sedimentation rates occurred during the upper Pliocene - Holocene in the middle area. This may have been related to the increase of sediment flux in the middle area and to the slight increase in basin subsidence during the upper Pliocene- Holocene.

Thirteen seismic facies within posttectonic sequences were recognized and mapped. These facies indicate shelfal deposits, deltaic sediments, turbidite sediments, channel fill deposits, upper fan deposits, and shale diapirs. Shelfal sediments are commonly found in each sequence. Upper fan deposition occurred in the middle area during the upper Oligocene - lower Miocene. Deltaic sediments are found in the lower Miocene - middle Miocene, middle- upper Miocene, and upper Pliocene- Holocene sequences. Shelf-edge progradation and upper slope slumping, which have occurred since the early Miocene, were intensive during the upper Pliocene - Holocene.