Twenty reversed, two‐ship expanding spread profiles (ESPs) with maximum source‐receiver offsets of ∼100 km were collected in three transects across the rifted northern margin of the South China Sea. Source‐receiver offset versus two‐way travel time (X‐T) data were mapped into the intercept time versus ray parameter (τ‐p) domain, and velocity‐depth solutions were obtained by a combination of τ‐sum inversion in the τ‐p domain and ray tracing in both the τ‐p and X‐T domains. Arrivals from the Moho were detected on 17 of the ESPs. The depths to Moho determined for individual ESP interpretations have reproducibilities of ±0.1 km to ±3 km; in most cases the Moho depth has been determined to within ±1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data. Variations in present‐day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin. The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent‐ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (VP< 6.4 km/s) and lower crust (VP> 6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures. Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure. Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.

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.

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A study of the rifled northern margin of the South China Sea, based on new single‐ship and two‐ship multichannel seismic data, has revealed the presence of highly reflective, steeply dipping normal faults which cut through the entire crust. These faults, with dips of ∼25°–40°, are associated with localized crustal thinning of ∼5–15 km and represent horizontal extension of the order of tens of kilometers. A range of ∼5–20 km of horizontal extension has been measured along individual faults. Although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin (only about 15% or less), the amount of localized crustal thinning which takes place along these fault surfaces is quite significant. At one fault location, the crustal thickness is reduced by ∼40%. Although the faults described have been recognized at three separate locations along the margin, their trends appear to differ at each. In two examples, the faults are oblique to the inferred, general north‐south extension direction of the margin. This characteristic, taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust, suggests that at least some of these faults were present prior to rifting. Such faults are postulated to have been former thrust faults, perhaps associated with a Triassic suture zone and/or Cretaceous subduction margin, which have been reactivated as normal faults during subsequent rifting. Alternatively, some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension.