Magmatism in the South China Basin

P wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East subbasin of the South China Sea (SCS). Four types of crust have been defined: (a) thin oceanic crust (<5 km), (b) typical oceanic crust (5–6 km), (c) thick oceanic crust hosting postspreading volcanoes (>6 km) with significant intrusive roots, and (d) thick oceanic crust with enhanced spreading features (>6 km) but without significant roots. Within the central East subbasin, the thin oceanic crust, only identified inside an 80 km wide zone, is located within an overall 150 km wide domain characterized by N055° seafloor spreading trends. The postspreading volcanoes were formed during a N‐S tensional episode around 6–10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and postspreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e., at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the postspreading volcanism might have been influenced by the Hainan plume activity through a buoyancy‐driven partial melting mechanism.

Section: Geological Setting Of the Postspreading Volcanism In The Cenmentioning

confidence: 99%

Section: Accuracy and Resolution Testsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Postseafloor Spreading Volcanism in the Central East South China Sea and Its Formation Through an Extremely Thin Oceanic Crust

Zhao

Sibuet

et al. 2018

“…However, the crust and the Cenozoic sediments in the Northwest Sub-basin were strongly affected by buried magmatic seamounts (Figures 4); therefore, the density of the buried magmatic seamounts should be assigned to a separate value distinct from those of the crust and the sediments. According to geochemical data for major elements and some trace elements of the basalts sampled at the seamounts and the islands of previous studies [Tu et al, 1992;Yan et al, 2008;Li et al, 1991], this study calculated the density of the basalts sampled at adjacent Daimao, Xianbei, Zhangzhong and Zhenbei seamounts using the (Figure 10). Generally, we can see these density values largely distributed on both sides of 2.65 g/cm 3 , which we assign as the buried magmatic seamounts in the gravity model.…”

Section: Gravity Modelingmentioning

confidence: 99%

“…But to the east, the crustal thickness along profile ESP-W, which crossed the coast line, the PRMB (including the structural units of NFT, Zhu III D, SHASR, Zhu II D and SR) and the QDNB (including CCS), did not decrease monotonically from the continent to the oceanic basin. Rather, the crustal thickness changes from 28-30 km under the coast line, to 15 km below the Zhu III D, to 21-24 km Densities are derived from the geochemical data for major elements and some trace elements of the basalts sampled at the seamounts (such as Daimao, Xianbei, Zhangzhong, and Zhenbei) of previous studies [Tu et al, 1992;Yan et al, 2008;Li et al, 1991] using the calculated method proposed by Zhang et al [2010]. Generally, these density values evenly distribute on both sides of 2.65 g/cm 3 .…”

Section: Crustal Thinningmentioning

confidence: 99%

Crustal structure and extension mode in the northwestern margin of the South China Sea

Gao

McIntosh

et al. 2016

Combining multi‐channel seismic reflection and gravity modeling, this study has investigated the crustal structure of the northwestern South China Sea margin. These data constrain a hyper‐extended crustal area bounded by basin‐bounding faults corresponding to an aborted rift below the Xisha Trough with a subparallel fossil ridge in the adjacent Northwest Sub‐basin. The thinnest crust is located in the Xisha Trough, where it is remnant lower crust with a thickness of less than 3 km. Gravity modeling also revealed a hyper‐extended crust across the Xisha Trough. The postrift magmatism is well developed and more active in the Xisha Trough and farther southeast than on the northwestern continental margin of the South China Sea; and the magmatic intrusion/extrusion was relatively active during the rifting of Xisha Trough and the Northwest Sub‐basin. A narrow continent‐ocean transition zone with a width of ∼65 km bounded seaward by a volcanic buried seamount is characterized by crustal thinning, rift depression, low gravity anomaly and the termination of the break‐up unconformity seismic reflection. The aborted rift near the continental margin means that there may be no obvious detachment fault like that in the Iberia‐Newfoundland type margin. The symmetric rift, extreme hyper‐extended continental crust and hotter mantle materials indicate that continental crust underwent stretching phase (pure‐shear deformation), thinning phase and breakup followed by onset of seafloor spreading and the mantle‐lithosphere may break up before crustal‐necking in the northwestern South China Sea margin.

New insights into the magmatism in the northern margin of the South China Sea: Spatial features and volume of intraplate seamounts

Fan

Xia

Zhao

et al. 2017

The extensive intraplate seamounts are obvious features in the northern South China Sea (SCS). However, the distribution, volume, and origin of these seamounts are not well understood, which greatly hinders our understanding of magmatism in the SCS. Based on high‐resolution bathymetric data and 147 seismic profiles, and combining gravity and magnetic data, we first identify 45 seamounts in the northern margin of the SCS and simulate their shape with elliptical cones. Results show that the total volume of these 45 seamounts above seafloor is estimated at about 1885–3078 km3 and the total volume of intrusive magma above Moho is about 0.15 Mkm3, which is close to the estimates for classic large igneous provinces across the world. These seamounts are mostly located on the continental slope with thin crust (approximately 12–18 km), which reduces the overlying pressure and shortens the magmatic conduits. The dominant azimuth of elliptical major axis in seamounts is consistent with the synrift and synspreading fault strikes (NE‐NEE), indicating that these preexisting faults provide magmatic conduits for the subsequent postrift intraplate seamounts. Based on three existing clues, i.e., (1) the intraplate seamounts, high velocity layer and Hainan mantle plume are contiguous in 3‐D space, (2) the high‐velocity layer is thicker beneath the continental shelf but thinner beneath the slope, and (3) the basalts dredged from certain seamounts show OIB‐type geochemical features, we propose a magmatic upwelling pattern which contains Hainan mantle plume to explain the spatial and morphological characteristics of these intraplate seamounts.