Gravity, heat flow, and seismic constraints on the processes of crustal extension: Northern margin of the South China Sea

Abstract

Multichannel seismic data and gravity data have been used to construct crustal thickness profiles for three transects (eastern, central, western) across the rifted northern margin of the South China Sea. The present‐day crustal configuration of the margin is then interpreted by modeling the effects of two end‐member classes of extension processes, pure shear and simple shear. The applicability of each of these processes to the extension of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow with observations across the margin. Neither of these end‐member models satisfactorily fits the observed data on the eastern and central transects across the south China margin when typical values for standard input parameters are used; the resulting heat flow is significantly underestimated by both models. In the case of a pure shear model, heat flow observations may be matched either by assuming an uncommonly thin initial steady state lithospheric thickness (∼60 km) or by assuming an unusually large crustal radiogenic heat production within the original, unextended continental crust. A perhaps more reasonable alternative scenario presumes the existence of an initially slightly thinner than “normal” steady state lithosphere (thicknesses of ∼90–100 km) in conjunction with a significant amount of upper crustal radiogenic heat production. Such heat production could be accommodated by the presence of Cretaceous granitic bodies (hypothesized) within the basement beneath the south China margin. In the case of a simple shear model, however, the observed high heat flow on the rifted south China margin may only be matched if the steady state lithospheric thickness is assumed to be uncommonly thin (∼60 km). Because the observed geophysical data characterizing lithospheric extension may be matched using more realistic input parameters in the pure shear case, pure shear extension is preferred over simple shear extension as the dominant mechanism for explaining the large‐scale rifting of the south China margin. For extension within the crust, however, combinations of both processes are not only possible, but probable, given published seismic evidence for through‐going crustal faults on the south China margin.

Rifted margins: Ductile deformation, boudinage, continentward-dipping normal faults and the role of the weak lower crust

“…Here, the ductile behavior is explained by the high heat flow resulting from the rapid inversion of a former magmatic arc (Nissen et al, 1995).…”

“…9). This pattern is generally considered as an extreme, preoceanisation, crustal thinning resulting in a boudinage of the whole continental crust (Nissen et al, 1995;Franke et al, 2014;Savva et al, 2013Savva et al, , 2014.…”

Crustal Thinning of the Northern Continental Margin of the South China Sea

“…Several failed spreading centers have been found (boxes A1, B1, C1 in Figure 8a) (Taylor and Haye, 1983). Faults across the entire crust have been suggested near the failed spreading centers (Nissen et al, 1995b). For comparison, we use the Moho depth defined by Nissen et al (1995a) to calculate the synthetic Bouguer gravity anomaly (the heavy dashed line in Figure 8a).…”

“…Taking into account the results of Nissen et al (1995b) and q m ¼ 3.28 g/cm 3 , respectively. We use the satellite-derived gravity anomalies of Sandwell and Smith (1997) (Figure 9c and 9d).…”

Evolution of deep‐water rifted margins: Testing depth‐dependent extensional models

“…These results, and those in Appendix B, show that for most well-imaged nonvolcanic margins, a simple isostatic model based on a constant crustal density adequately describes the final subsidence of the margin as a function of crustal thinning and is a justification for the modeling we discuss in this paper. Data references are Horsefield et al [1994], Lau et al [2006], Wu et al [2006], Moulin et al [2005], Nissen et al [1995a], and Schnabel et al [2008]. [10] In conjunction with equation (1), this second rule provides an important pin which constrains paleobathymetry at the distal end of a continental margin where estimates are usually poorest.…”