The continent‐ocean transition on the northwestern South China Sea

Abstract

Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent-ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental-crust domain is characterized by tilted fault blocks generally overlain by thick syn-rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8-10 s twtt. The thickness of the continental crust changes greatly across the basin, from 20 to 25 km under the shelf and uppermost slope, to~9-6 km under the lower slope. The oceaniccrust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a~5-10 km wide zone where oceanic-type features directly abut or lap on continental-type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along-strike variation in the tectonic structure of the continental margin. The NE-most lines display~20-40 km wide segments of intense faulting under the slope and associated continental-crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across ã 100-110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.

Seismic Evidence for Tectonically Dominated Seafloor Spreading in the Southwest Sub‐basin of the South China Sea

“…In our MCS profile NH973-1, the M-reflectors mainly follow the 6.8-7.2 km/s velocity contours (Figure 11). In addition, the layer 2/3 boundary has also been barely seen from the MCS profiles in oceanic basins, such as the west Pacific Ocean (Tsuji et al, 2007) and the ESB and NWSB of the SCS (Cameselle et al, 2017;McIntosh et al, 2014;Sun et al, 2016). The velocities below these M-reflectors increase from 6.8-7.2 km/s to 7.5 km/s with a velocity gradient of 1.0/s on average (Figure 9), which is much higher than the velocity gradient in normal oceanic layer 3.…”

“…It is commonly accepted that the seafloor spreading occurred first in the ESB and then propagated southwestward to the SWSB with a southward ridge jump and a ridge reorientation from E-W to NE-SW (Briais et al, 1993;Franke et al, 2014;Li et al, 2014;Taylor & Hayes, 1983). The exceptions are observed in few localities where postspreading seamounts stand, and the crust is relatively uniform with an average thickness of 6.0 km defined by clear Moho reflectors on multichannel seismic (MCS) profiles (Cameselle et al, 2017;Ding et al, 2018;McIntosh et al, 2014;Sun et al, 2016). The exceptions are observed in few localities where postspreading seamounts stand, and the crust is relatively uniform with an average thickness of 6.0 km defined by clear Moho reflectors on multichannel seismic (MCS) profiles (Cameselle et al, 2017;Ding et al, 2018;McIntosh et al, 2014;Sun et al, 2016).…”

The Moho Depth of the South China Sea Basin From Three‐dimensional Gravity Inversion With Constraint Points and Its Characteristics

“…10 and 11 show that, unlike the East Basin and Southwest Basin, the Northwest Basin dose not exhibit thickened spreading center. Using the re-processed multichannel seismic profile and referring to the gravity gradient data, Cameselle et al (2015) suggested that the Northwest Basin is not the result of thinning due to crustal rifting and is formed because of the movement of the spreading center from northeast to southwest to cause the instantaneous crustal rifting. The thinned crust is bounded by the Shuangfeng Seamount.…”

“…Ding et al (2009) also suggested that the scale of spreading of the Northwest Basin is stronger in the east than that in the west where the continental rift basin seems to dominate in this region. Additionally, the studies of OBS reflect the small scale and short-term of spreading of the Northwest Basin and that layer 2 experienced asymmetric overflow of basal magma that blurred the magnetic anomaly belts (Wu et al, 2011), which proves that Cameselle et al (2015)'s conclusion is possible, but the double spreading centers are hard to explain. The Northwest Basin had not experienced the complete spreading process and moved to the east gradually.…”

“…In the Northwest Basin, the OBS2006-1 profile perpendicular to the spreading center and the OBS 2006-2 along the spreading center are explained by Wu et al (2011) and Ao et al (2012) and they described the COB. In addition, Cameselle et al (2015) identified the COB in the Northwest Basin, based on the reprocessed multi-channel seismic profiles and gravity gradient data. The number of seismic profiles in the Southwest Basin is limited, but the COB is identified using the OBS profiles Ruan et al, 2011;Pichot et al, 2013).…”