A study of the rifted northern margin of the South China Sea, based on new single-ship 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 similar to 25 degrees-40 degrees, are associated with localized crustal thinning of similar to 5-15 lan and represent horizontal extension of the order of tens of kilometers. A range of similar to 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 similar to 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.
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