Timing and Tectonic implications of basin inversion in the Nam Con Son Basin and adjacent areas, southern South China Sea

Abstract

The Nam Con Son (NCS) Basin, located offshore of SE Vietnam, is one of several Tertiary rift basins that formed during initial Eocene(?)-Oligocene rifting. Following cessation of rifting at the end of Oligocene time, these basins were subjected to spatially and temporally variable, complex inversion events during Miocene time. Fault orientations on inversion structures in the West Natuna Basin and the Western NCSB closely parallel the western side of the Natuna Arch, which may have served as a regional "buttress" where stress was concentrated and strain was deflected from Early to Late Miocene time. Early to Middle Miocene basin inversion across the Western NCSB was coincident with the most intense phase of basin inversion in the West Natuna and Malay basins. Contraction in the Western NCS, West Natuna, and Malay basins was accommodated through reactivation of major basin-bounding fault systems that resulted in asymmetric fault-bend folding of syn- and early post-rift strata. Inversion of western Sunda Shelf basins progressed from the West Natuna and Western Nam Con Son basins into the southern Malay Basin from Early to Middle Miocene time. The most intense inversion was recorded in the West Natuna Basin during Early Miocene time with regional uplift of the southern Malay and West Natuna basins during Middle Miocene time. Whereas both the Eastern and Western NCS sub-basins experienced fault reactivation during Miocene time, the timing and styles of inversion are different. Unlike the Western NCSB, the Eastern NCSB experienced only mild positive reactivation of pre-existing synthetic and antithetic hanging-wall faults, causing simple amplification of pre-existing rollover in the hanging-wall fill during Middle Miocene time. Basin inversion of the West Natuna, Western Nam Con Son, and Malay basins is attributed to collision-induced clockwise rotation of Borneo and the attached, rigid Natuna Arch and Natuna Basement Ridge, beginning during Early Miocene time. This accounts for: 1) the south to north progression of inversion from Early to Late Miocene time, 2) magnitudes of inversion documented within each basin, 3) the suggested NW-SE orientation of []₁,4) the approximately N-S azimuth of compression that caused observed styles of inversion to form.