Décollements, Detachments, and Rafts in the Extended Crust of Dangerous Ground, South China Sea: The Role of Inherited Contacts

Abstract

We investigate the crustal structure of the Dangerous Ground (South China Sea) through processing and interpretation of coincident wide‐angle reflection and refraction seismic data. Continental crust of Dangerous Ground has been moderately thinned, down to 15 km, so that most of the structures accompanying the early opening of the South China Sea from Cretaceous to Miocene have been preserved. Subbasement reflectors as well as refraction velocities image an interpreted dismantled Mesozoic metamorphic unit in the southernmost section of our study area. A rollover structure indicates that the reflective base of the unit was used as a décollement where low‐angle normal faults root and blocks rafted. The metamorphic unit is discontinued in a nearby basin located immediately to the north, where the refraction velocity model shows thinning of the crust from 20 to 15 km, with the presence of a 5‐km‐high mantle dome. In this deeper basin, mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an ~15 km offset crustal detachment surface that we link down to the mantle dome. We infer that the detachment reactivated an inherited low‐angle contact most probably related to the Yanshanian belt. In map view, the reactivated structure forms a half‐graben basin oriented NNE‐SSW oblique to the generally accepted direction of extension. This orientation follows the general trend of a granitic belt that spanned the South China margin prior to extension, related to the subduction of the Paleo‐Pacific.

The Succession of Upper Eocene- Upper Miocene Limestone Growth and Corresponding Tectonic Events in Luconia Shelf, Sarawak, Malaysia

“…The faults rooted deep into the boundary of upper and lower crust, originated from the earlier formed faults during Upper Cretaceous-Eocene when rifting was active in Palawan and surrounding. The deep seated listric faults seen in the northern part of Central Luconia and North Luconia ( Figure 12B (2)) are similar to observation made by Pichot et al, (2014), Franke et al (2014) and Liang et al, (2019) in the other SW sub-basins of the SCS, particularly in the Dangerous Grounds and Nam Con Son Basin. The listric faults exhibit common patterns in the whole SW sub-basins of the SCS, indicating a consistent degree of deformation that affected the whole southern South China Sea.…”

Seismic Imaging of an Intracrustal Deformation in the Northwestern Margin of the South China Sea: The Role of a Ductile Layer in the Crust

“…Strikingly, the crust with velocities lower than 6.0 km/s is dismantled along a small window about 20 km wide in the Zhongsha Trough (near OBS23), with rotated basement on both flanks that might also represent rollover structures. Such crustal structures show strong similarities on several wide‐angle seismic profiles (Figure 7), including OBS2011‐1 in the southwest extension of the Zhongsha Trough (H. B. Huang et al., 2019) and OBS2014s in the Liyuexi Trough (Basin C in Liang et al., 2019). In all profiles, the seaward dipping detachment faults are recognized along the basement highs, beneath which the crust is stretched to be ∼10 km and the crust above 6.0 km/s is highly stretched or dismantled.…”

“…Such crustal structures show strong similarities on several wide-angle seismic profiles (Figure 7), including OBS2011-1 in the southwest extension of the Zhongsha Trough (H. B. Huang et al, 2019) and OBS2014s in the Liyuexi Trough (Basin C in Liang et al, 2019). In all profiles, the seaward dipping detachment faults are recognized along the basement highs, beneath which the crust is stretched to be ∼10 km and the crust above 6.0 km/s is highly stretched or dismantled.…”

“…From our model, the highly reflective top of the lower crust indicates a prominent velocity/density contrast, which could have been enhanced by a front of intruded sills blocked at the ductile boundary (Figures 5d, and 10c, and 10d). Previous studies suggest that a weak middle crustal layer is important for compensating the upper crustal extension, while (Liang et al, 2019). (c) The model of OBS2011-1 (H. B.…”

“…Other profiles are labeled and shown as black lines, OBS2011-1 (H. B. Huang et al, 2019); OBS-2014 (Liang et al, 2019;Pichot et al, 2014); OBS1996-4 (X. L. Qiu et al, 2001); OBS2006-1 (Wu et al, 2011); OBS2006-2 (Ao et al, 2012;Wang et al, 2020); OBS2013-3 (Guo et al, 2016); and OBS-Zhang (J. Zhang et al, 2016a).…”