Crustal Structure across the Northwestern Margin of South China Sea: Evidence for Magma‐poor Rifting from a Wide‐angle Seismic Profile

Abstract

We present results from a 484 km wide‐angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi‐channel seismic line (SO49–18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean‐bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust‐mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high‐velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma‐poor rifting. Both, the NWSB and the Southwest Sub‐basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi‐channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub‐basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east‐west thicken trend of the crust below the continent slope. This might be contributed by the east‐west sea‐floor spreading along the continental margin.

New Insights Into the Rift‐To‐Drift Process of the Northern South China Sea Margin Constrained by a Three‐Dimensional Wide‐Angle Seismic Velocity Model

“…The location of the Mesozoic volcanic arc and the Mesozoic forearc basin identified in previous work (Li et al, 2018;Zhao et al, 2019) are shown in gray and yellow dotted line polygons, respectively. Seismic lines of OBS1993 (Yan et al, 2001), OBS2006-1 (Ding et al, 2012), OBS2006-3 (Wei et al, 2011), and OBS2010-1 (Cao et al, 2014) are shown in gray. White lines correspond to the magnetic lineations identified by Briais et al (1993).…”

Along‐Margin Variations in Breakup Volcanism at the Eastern North American Margin

“…Magma‐poor margins include the conjugate Newfoundland, Canada, and Iberia Peninsula margins (e.g., Boillot et al, 1980; Shillington et al, 2006; Whitmarsh et al, 2001) and the Gulf of Aden (Autin et al, 2010; d'Acremont et al, 2005). Intermediate examples, with elements of both end‐members, include the South China Sea (e.g., Franke, 2013; Weiwei et al, 2012) and the Gulf of California (Lizarralde et al, 2007).…”

Seismic structure of the northwestern margin of the South China Sea: implication for asymmetric continental extension

“…However, existence of the HVL is still uncertain due to inadequate constraints from the refracted arrivals in the lower crust and lack of the reflections from top of the HVL. By far, studies on the crustal structures of the NW margin suggest an less prominent or absent HVL (Qiu et al 2001;Wu et al 2011;Ding et al 2012;Pichot et al 2014;Yu et al 2017;this work), as compared with the NE margin where a HVL with velocity of 7.3-7.5 km s -1 and thickness of several kilometres was detected along the OCT (Yan et al 2001;Wang et al 2006;Wei et al 2011). Distribution of the HVL may imply that the northern margin experienced different magmatic history laterally (Pichot et al 2014).…”

“…A distinct high velocity layer (HVL) has been revealed at the bottom crust by refraction seismic surveys (Yan et al 2001;Wei et al 2011), restricted to slope areas in the NE margin. In comparison, existence of the HVL in the NW margin has long been controversial since wide-angle seismic modelling reported no such finding (Qiu et al 2001;Ding et al 2012;Wu et al 2011), while a few seismic surveys and gravity modelling revealed scattered thin HVL (Qiu et al 2013;Pichot et al 2014). The transverse discrepant crustal structures along the northern margin were suggested to be associated with the progressive continental rifting and opening of the SCS (Zhou et al 1995).…”