Quantifying Postrift Lower Crustal Flow in the Northern Margin of the South China Sea

Abstract

Postrift subsidence of sedimentary basins in the northern margin of the South China Sea exceeds more than 2,.000 m, which points toward anomalous postrift crustal deformation. Previous studies have proposed lower crustal flow to explain this observation; however, this hypothesis has never been confirmed quantitatively. Here, we calculate the initial crustal structure and thermal lithospheric thickness of the northern margin of the South China Sea on the basis of recently measured heat flow data, tectonic subsidence curves, and present‐day crustal structure. Crustal thinning processes during rifting and reduced thickness of the lower crust in the postrift are also calculated by the strain rate inversion method and thermal isostasy. Our results show that the initial (Early Cenozoic) crustal thickness of the northern margin of the South China Sea varies between 27.7 and 36.3 km, and the average proportion of the lower crust is 67%. The initial thermal lithospheric thicknesses decrease from ~118 to ~81 km from the shelf to the sea, which indicates that the offshore margin has high temperatures and low strength and is more easily stretched and deformed. The average stretching factor the margin during rifting is 1.24. The lower crust reduces in thickness during the postrift phase by values between 1 and 11 km, which increases gradually from the shelf to deep water. We suggest that the thicker weak crust and hot lithospheric structure therefore make positive contribution to the lower crustal flow, the direction of which is from the oceanic basin to the shelf.

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

“…When assessing the symmetry of the SCS margins, the pre-rifting crustal thickness, crustal rheology and structures should be taken into consideration. End-member modeling indicates that the pre-rifting crustal thickness cannot vary dramatically in the east-west direction (Hayes and Nissen, 2005), which is further confirmed by a recent study (Dong et al, 2020). Subduction of the Izanagi plate under the South China Block induced magma upwelling, which heated the lithosphere and thereby reduced the crustal viscosity (Clift et al, 2002;Brune et al, 2017).…”

“…Subduction of the Izanagi plate under the South China Block induced magma upwelling, which heated the lithosphere and thereby reduced the crustal viscosity (Clift et al, 2002;Brune et al, 2017). The pre-rift lithospheric thermal structure behavior along the east-west direction was similar to that of the pre-rifting crustal thickness (Dong et al, 2020). A series of Mesozoic NE-trending sinistral faults has been described on the South China Block (Gilder et al, 1996).…”

Extension Discrepancy of the Hyper‐Thinned Continental Crust in the Baiyun Rift, Northern Margin of the South China Sea

“…Multiple studies suggested thickness of pre‐rift brittle crust ranges from 9.14 to 12 km (Chen, 2014; Dong et al., 2020; Zhang et al., 2008). A comparison between Z uc and the suggested thickness of 12 km provides further evidence of pre‐rift inheritance of the basement crust.…”

“…The initial crustal thickness of the Cenozoic NSCS had been determined by multiple studies: 30 km based on the crustal thickness of the South China block (Zhou et al., 2012), 30–35 km based on seismic refraction modeling (Hu et al., 2009; Li et al., 2006; Nissen et al., 1995), 33 km in the eastern NSCS while 30.5 km in the western NSCS based on thermal‐subsidence modeling (Dong et al., 2020). The episodic Cenozoic rifting formed an uneven thickness of continental crust wedging oceanwards across the NSCS.…”

“…As for the lower crustal flowing model, even it could produce crustal thickness variation, the ductile materials usually flow along the extension direction (McKenzie & Jackson, 2002). In the NSCS, a few of numerical simulations did propose that the lower crustal flowing occurred across the entire margin of the NSCS along the NW‐SE extension direction (Clift, 2015; Dong et al., 2020). However, this study indicates an inhomogeneous thinning of the crust in an NE‐SW direction, which is perpendicular to the extension direction (Figure 8).…”