Coupled onshore erosion and offshore sediment loading as causes of lower crust flow on the margins of South China Sea

Abstract

Hot, thick continental crust is susceptible to ductile flow within the middle and lower crust where quartz controls mechanical behavior. Reconstruction of subsidence in several sedimentary basins around the South China Sea, most notably the Baiyun Sag, suggests that accelerated phases of basement subsidence are associated with phases of fast erosion onshore and deposition of thick sediments offshore. Working together these two processes induce pressure gradients that drive flow of the ductile crust from offshore towards the continental interior after the end of active extension, partly reversing the flow that occurs during continental breakup. This has the effect of thinning the continental crust under super-deep basins along these continental margins after active extension has finished. This is a newly recognized form of climate-tectonic coupling, similar to that recognized in orogenic belts, especially the Himalaya. Climatically modulated surface processes, especially involving the monsoon in Southeast Asia, affects the crustal structure offshore passive margins, resulting in these "load-flow basins". This further suggests that reorganization of continental drainage systems may also have a role in governing margin structure. If some crustal thinning occurs after the end of active extension this has implications for the thermal history of hydrocarbon-bearing basins throughout the area where application of classical models results in over predictions of heatflow based on observed accommodation space.

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

“…However, magmatic crustal thickening results in uplift rather than subsidence. In any case, there is no clear evidence of underlying magma intrusion(s) in some areas with rapid postrift subsidence, such as Baiyun Sag (Clift, ).…”

“…On the one hand, rapid accumulation promotes lower crustal flow in the northern margin of the South China Sea. On the other hand, erosion on the shore also creates a regional uplift that provides a path for the lower crust to flow to the continent (Clift, ), causing the lower crust to flow from the sea to the land (Figure b), which provides the dynamic conditions for the lower crustal flow.…”

“…In recent years, the implementation of a number of scientific projects, including the Integrated Ocean Drilling Program and South China Sea Deep Plan, has provided a range of useful data for studying the evolution of this rifted margin (Larsen et al, ; Li et al, ). Anomalous postrift subsidence is also observed in basins such as the Yinggehai, Qiongdongnan, and Pearl River Mouth basins along the northern margin of the South China Sea (Clift & Sun, ; Clift et al, ; Clift, ; Shi, Jiang, et al, ; Xie et al, ). After the Late Miocene, some parts of this area (e.g., Xisha) experienced large‐scale inundation of carbonate platforms (Wu et al, ).…”

Crustal structure and rifting of the northern South China Sea margin: Evidence from shoreline‐crossing seismic investigations

“…The notion that magmatic underplating interacted with lower crustal flow during the rifting process is still vague and requires verification by a thermal evolution model of the rifted margin of the SCS, which would indicate a thermal anomaly on the continental shelf. If the lower crust is mafic, then the crustal flow may be more likely to be ductile mid crust (Clift, ). In general, thinned continental crust of the Hainan Rise and the rift and faulted depression zones is interpreted as the rift belt similar to the “Hinge zone” at other rifted margins (Tréhu et al, ; Watts, Rodger, Peirce, Greenroyd, & Hobbs, ) as described below.…”

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

“…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).…”