Throughgoing crustal faults along the northern margin of the South China Sea and their role in crustal extension

Abstract

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.

Late Mesozoic compressional folds in Dongsha Waters, the northern margin of the South China Sea

“…In Taylor and Hayes' (1983) reconstruction model, the proto-Pacific oceanic crust subducted northwestward along the south side of North Palawan and Liyue Bank with the southwest continuation unspecified. Hayes et al (1995) declared recognition of some crustal through-going faults over a vast region beyond Dongsha Waters through western Pearl River Mouth Basin to Xisha Trough, and postulated them as former thrust faults associated with a Triassic suture and/or Cretaceous margin which have been activated as normal fault during subsequent rifting. Other workers, e.g., Li et al(2008), Liu et al (1992), Yao et al(2011), Zhao et al (2012 and Zhou et al (2008), put the Late Mesozoic subduction trench or collisional suture within the northern margin of the South China Sea with more or less variations of the specific position, which implied that the South China Sea basin developed along the previous subduction trench and/or suture (mechanically weak) zone.…”

Crust and upper mantle structure and its tectonic implications in the South China Sea and adjacent regions

“…14b) perfectly matches with the tectonic weak zone as expected. It has been assumed that once the lithospheric extension (e.g., Hayes et al, 1995;Rangin et al, 1995;Zhou et al, 1995) could not sustain the block displacement, a breakup along the weak zone would be developed. Thus, the South China margin has become a stress-free boundary.…”

South China Sea Tectonics

“…This mapping activity has already started producing 2-D seismic grids around our drill sites. Other MCS and magnetic data were collected near the drill sites by the R/Vs Vema, Conrad, and Haiyang IV Hayes, 1980, 1983;Yao et al, 1994;Hayes et al, 1995) (Figure F2). Two stages of Sino-US cooperation in the early 1980s added more dense geophysical data coverage, which includes sonobuoy measurements, two-ship expanding spread profiles, and piston cores (Taylor and Hayes, 1983;Yao et al, 1994;Hayes et al, 1995).…”

“…Other MCS and magnetic data were collected near the drill sites by the R/Vs Vema, Conrad, and Haiyang IV Hayes, 1980, 1983;Yao et al, 1994;Hayes et al, 1995) (Figure F2). Two stages of Sino-US cooperation in the early 1980s added more dense geophysical data coverage, which includes sonobuoy measurements, two-ship expanding spread profiles, and piston cores (Taylor and Hayes, 1983;Yao et al, 1994;Hayes et al, 1995). The German R/V Sonne carried out five cruises in 1987 (SO-49 and SO-50B), 1990(SO-72A), 1994(SO-95), and 2008 (Franke et al, 2011) and collected >10,000 km of MCS data and high-resolution echograms (Lüdmann and Wong, 1999;Lüdmann et al, 2001).…”

“…Mesozoic subduction of the paleo-Pacific plate under the Eurasian plate partially occurred along the presentday northeastern SCS continental margin (Jahn et al, 1976;Hilde et al, 1977;Hamilton, 1979;Holloway, 1982;Taylor and Hayes, 1983;Hayes et al, 1995;Zhou and Li, 2000;Yang and Feng, 2003;Xiao and Zheng, 2004;Zhou et al, 2008;Li et al, 2008a). This subduction resulted in the emplacement of igneous rocks and the formation of a wide orogenic belt in Southeast Asia (Zhou and Li, 2000;Shi and Li, 2012).…”