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

Peter D Clift

doi:10.1186/s40562-015-0029-9

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.

Highlights

The process of continental crustal extension and the formation of sedimentary basins have been quantified and described using a number of different approaches that explain how strain is accommodated in the continental lithosphere (McKenzie 1978; Royden and Keen 1980; Tankard and Welsink 1987; Wernicke 1985)
It has long been recognized that uniform extension models do not apply well to passive margin systems and that excess subsidence is often recognized, exceeding that which would be predicted based on brittle upper crustal normal faulting
Efforts to apply simple shear models to passive margins have had limited success as they often predict contrasting subsidence patterns, which are rarely observed in the predicted fashion observe in practice

Summary

Introduction

The process of continental crustal extension and the formation of sedimentary basins have been quantified and described using a number of different approaches that explain how strain is accommodated in the continental lithosphere (McKenzie 1978; Royden and Keen 1980; Tankard and Welsink 1987; Wernicke 1985). *Correspondence: pclift@lsu.edu 1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA Full list of author information is available at the end of the article in a continental plate) to passive margins has resulted in the identification of subsidence anomalies, usually in the form of greater subsidence than would be expected from the degree of extension as measured on normal faults identified within the upper crust in seismic profiles (Driscoll and Karner 1998; Davis and Kusznir 2004; Clift et al 2002; Boillot et al 1988; Lister et al 1991).

Results

Conclusion