Cenozoic tectonic subsidence in the Qiongdongnan Basin, northern South China Sea

Abstract

A number of major controversies exist in the South China Sea, including the timing and pattern of seafloor spreading, the anomalous alternating strike-slip movement on the Red River Fault, the existence of anomalous post-rift subsidence and how major submarine canyons have developed. The Qiongdongnan Basin is located in the intersection of the northern South China Sea margin and the strike-slip Red River fault zone. Analysing the subsidence of the Qiongdongnan Basin is critical in understanding these controversies. The basin-wide unloaded tectonic subsidence is computed through 1D backstripping constrained by the reconstruction of palaeo-water depths and the interpretation of dense seismic profiles and wells. Results show that discrete subsidence sags began to form in the central depression during the middle and late Eocene (45-31.5 Ma). Subsequently in the Oligocene (31.5-23 Ma), more faults with intense activity formed, leading to rapid extension with high subsidence (40-90 m Myr À1 ). This extension is also inferred to be affected by the sinistral movement of the offshore Red River Fault as new subsidence sags progressively formed adjacent to this structure. Evidence from faults, subsidence, magmatic intrusions and strata erosion suggests that the breakup unconformity formed at ca. 23 Ma, coeval with the initial seafloor spreading in the southwestern subbasin of the South China Sea, demonstrating that the breakup unconformity in the Qiongdongnan Basin is younger than that observed in the Pearl River Mouth Basin (ca. 32-28 Ma) and Taiwan region (ca. 39-33 Ma), which implies that the seafloor spreading in the South China Sea began diachronously from east to west. The post-rift subsidence was extremely slow during the early and middle Miocene (16 m Myr À1 , 23-11.6 Ma), probably caused by the transient dynamic support induced by mantle convection during seafloor spreading. Subsequently, rapid post-rift subsidence occurred during the late Miocene (144 m Myr À1 , 11.6-5.5 Ma) possibly as the dynamic support disappeared. The post-rift subsidence slowed again from the Pliocene to the Quaternary (24 m Myr À1 , 5.5-0 Ma), but a subsidence centre formed in the west with the maximum subsidence of ca. 450 m, which coincided with a basin with the sediment thickness exceeding 5500 m and is inferred to be caused by sediment-induced ductile crust flow. Anomalous post-rift subsidence in the Qiongdongnan Basin increased from ca. 300 m in the northwest to ca. 1200 m in the southeast, and the post-rift vertical movement of the basement was probably the most important factor to facilitate the development of the central submarine canyon.

Clinoform growth and sediment flux into late Cenozoic Qiongdongnan shelf margin, South China Sea

“…The SCS region is composed by the Yinggehai Basin (YGHB) to the west of Hainan Island, Qiongdongnan Basin (QDNB) to the south of Hainan Island, and Pearl River Mouth Basin in the east (Figure a,b). The general evolution of the entire region includes a post‐rift thermal subsiding stage (Early‐Middle Miocene) and then an accelerated subsidence stage to the present (Wang et al, ; Zhao, Sun, Sun, Wang, & Sun, ). In the Pliocene, the YGHB and the QDNB merged and synchronously experienced relatively rapid subsidence (Figure a,b; Cao, Jiang, Wang, Zhang, & Sun, ; Clift & Sun, ).…”

“…(a) Modern topographic continental shelf margin of QDNB, SCS with the shelf edge location of T40, T30, T27, T20 and T0 (Gong et al, ; Ren et al, ; Shi et al, ; Sun et al, ; Zhao et al, , ). AA′, BB′ and CC′ mark the 3 illustrated seismic profiles out of a group of 28 parallel seismic lines available to this study.…”

“…(a) Northwest‐southeast orientated shelf slope seismic profile of QDNB shelf margin, SCS (see also Gong et al, ; Su et al, ; Sun et al, ; Zhang et al, ; Zhao et al, ; Zhao et al, ) (see the location AA′ in the base map Figure a). Y ‐axis is the TWTT (two‐way travel time in milliseconds).…”

“…The growth trajectory made by the shelf break during the entire growth period of the margin is somewhat irregular, and shows several flat (progradation) and rising (aggradation) segments. Within the three chosen seismic inlines (Figure , Figures A and B in Supplementary material), the well‐known T40 surface marks the beginning of an accelerated subsidence stage, separating the overlying passive continental shelf margin from underlying post‐rift interval of thermal subsidence (Wang et al, ; Zhao et al, ).…”

Along‐strike variability in shelf‐margin morphology and accretion pattern: An example from the northern margin of the South China Sea

“…For the YGHB and QDNB, the BU is ca. 23 Ma (Zhao et al, ). Other than the BU, the timing of the initial formation of the shelf–slope physiography was also distinct in different parts along the northern SCS margin.…”