Symmetry of the South China Sea conjugate margins in a rifting, drifting and collision context

Abstract

The symmetry of conjugate rifted margins is a first-order observable feature reflecting the geodynamic processes acting during and after continental rifting. The symmetry of the South China Sea (SCS) rifted margins can be deduced from comprehensive geophysical datasets on a continental-margin scale. Here, we combine three key approaches: (1) lateral fault distributions are delineated according to free-air gravity anomalies, (2) crustal stretching styles are mapped based on gravity inversion constrained by deep seismic profiles, and (3) total extensions of conjugate margins are estimated according to margin restorations and tectonic settings. We infer a predominantly symmetric rifting style caused by pure shear extension, with only narrow domains of asymmetric deformation in continent-ocean transition (COT) regions that have undergone simple shear and where the lower crust of the highly thinned distal margin is embrittled before continental breakup. However, our analysis also suggests that this symmetry has been modified by post-rift geodynamic processes. Southward lower crustal flow, which occurred only on the southern margin due to the low viscosity of the lower crust and the slab pull induced by the subducting Proto-SCS plate during seafloor spreading, shifted the crustal stretching styles from symmetric to asymmetric. The collision between the southern margin and Borneo thickened the lower crust more than the upper crust at the southern end of the southern margin and shortened the southern margin. This event had a large impact on the SE margin but a small impact on the SW margin. We conclude that (1) for the SE and NE margins, the crustal stretching styles shifted from asymmetric to approximately symmetric, and the total extensions shifted from symmetric to asymmetric; (2) for the SW and NW margins, the crustal stretching and total extensions remained asymmetric and symmetric, respectively.

New zircon radiometric U/Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, east Malaysia): Time to reconsider the geological evolution of Southeast Asia? — REPLY

“…We suggested extension of the spreading axis only few kilometers onshore into Sabah consistent with data from Pilia et al (2021). ( 4) The free-air gravity anomaly map of Bai et al (2020) shows a negative anomaly at the extension of the Sulu trench in Sabah. This could be an indication of significant crustal thinning and asthenosphere upwelling similar to the Sulu and South China Sea, in accordance with our interpretation.…”

Crustal architecture and evolution of the southwestern South China Sea: Implications to continental breakup

“…A ductile layer is probably required underneath the detachment fault to accommodate the subsidence at the depocenter (Fig. 13b) as interpreted previously in adjacent Qiongdongnan Basins and Xisha Trough (Clift et al, 2002;Davis and Kusznir, 2004;Pichot et al 2014, Lei andRen, 2016;Xie et al, 2019;Lei et al, 2019;Bai et al, 2020). This could favor the depth-dependent decoupling as suggested in several simulations in the case of wide rifted margins (Huismans and Beaumont, 2011;Lin, 2001, Brune et al, 2017;Ros et al, 2017).…”

“…In the north, Xisha Islands margin based on correlation with seismic profiles tied with wells in adjacent Phu Khanh and Qiongdongnan basins (Vu et al, 2017;Savva et al, 2013;Lei and Ren, 2016), which is identified an unconformity around 32 Ma (T70) and the BU at 23 Ma (T60). In the south, it shares a similar rifting history with the northern margin during the Paleogene (Holloway, 1982;Liang et al, 2019;Bai et al, 2020;Zhang et al, 2020b). The age constraint is established on the base of the well Sampaguita-1 continuously documenting Cenozoic strata and encountering the Mesozoic strata (location S-1 on Figs.…”

“…The NW Borneo margin rift basins associated with rift basins extends over 600 kilometers southward to Sabah as seen on the seismic and gravity data (Cullen et al, 2010;Bai et al, 2020).…”

New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern Malaysia): Time to reconsider the geological evolution of Southeast Asia? — COMMENT

“…(4) Regional gravity data (Fig. 1) do not support extending the Sulu Sea southwest rift under Sabah (Cullen et al, 2012;Bai et al, 2020). ( 5 Ultramafic rocks are normally devoid of primary zircons, but zircon can crystallize in ophiolitic rocks during hydrothermal alteration (Dubińska et al, 2004;Schaltegger, 2007).…”