Crustal structure across the post-spreading magmatic ridge of the East Sub-basin in the South China Sea: Tectonic significance

Abstract

The 140-km wide last phase of opening of the South China Sea (SCS) corresponds to a N145° direction of spreading with rift features identified on swath bathymetric data trending N055° (Sibuet et al., 2016). These N055° seafloor spreading features of the East Sub-basin are cut across by a post-spreading volcanic ridge oriented approximately E-W in its western part (Zhenbei-Huangyan seamounts chain). The knowledge of the deep crustal structure beneath this volcanic ridge is essential to elucidate not only the formation and tectonic evolution of the SCS, but also the mechanism of emplacement of the postspreading magmatism. We use airgun shots recorded by ocean bottom seismometers to image the deep crustal structure along the N-S oriented G8G0 seismic profile, which is perpendicular to the Zhenbei-Huangyan seamounts chain but located in between the Zhenbei and Huangyan seamounts, where topographic changes are minimum. The velocity structure presents obvious lateral variations. The crust north and south of the Zhenbei-Huangyan seamounts chain is ca. 4-6 km in thickness and velocities are largely comparable with those of normal oceanic crust of Atlantic type. To the south, the Jixiang seamount with a 7.2-km thick crust, seems to be a tiny post-spreading volcanic seamount intruded along the former extinct spreading ridge axis. In the central part, a 1.5-km thick low velocity zone (3.3-3.7 km/s) in the uppermost crust is explained by the presence of extrusive rocks intercalated with thin sedimentary layers as those drilled at IODP Site U1431. Both the Jixiang seamount and the Zhenbei-Huangyan seamounts chain started to form by the intrusion of decompressive melt resulting from the N-S post-spreading phase of extension and intruded through the already formed oceanic crust. The Jixiang seamount probably formed before the emplacement of the E-W post-spreading seamounts chain. Highlights ► The 2D velocity structure along a profile perpendicular to the Zhenbei-Huangyan seamounts chain corresponds to oceanic crust affected by post-spreading magmatism. ► The low velocity body (3.3-3.7 km/s) located in the uppermost crust might correspond to extrusive rocks intercalated with thin sedimentary layers. ► The Jixiang seamount with thicked crust (7.2 km), a tiny post-spreading volcanic seamount, maybe formed by the buoyancy-driven upwelling mechanism.

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

“…The 41 OBS data processing includes instrument relocation and band‐pass filtering (3–12 Hz; Zhang et al, ). As examples of OBS recordings, picking phases, PmP phases, ray tracing, and travel times simulation are published for Profile P4 in He et al (); other examples will not be displayed in this paper. P wave velocity models along these four profiles were first derived by forward modeling using the Rayinvr code of Zelt and Smith ().…”

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

“…The main material composition within a seamount is generally igneous rocks, whose geophysics feature is obviously different from the submarine sediment or mud volcano [ Davies and Stewart , ; Chiu et al ., ; Chen et al ., ; He et al ., ]. Therefore, we identified and selected seamounts in the study area according to the following criteria: (1) rising from the seafloor to more than 0.1 km in height [ Kim and Wessel , ], (2) having high external amplitude seismic reflections, (3) having low amplitude and chaotic internal seismic reflections, (4) having obvious uplift of the adjacent strata at the flanks [ Zhang et al ., ], (5) having higher positive free‐air gravity anomalies [ Tomoda and Fujimoto , ; Wessel et al ., ; Kim and Wessel , ; He et al ., ], and (6) having higher (positive or negative) magnetic anomalies [ Heirtzler Amp and Hadley , ]. Criteria 1–5 were necessary for each seamount, but criterion 6 was not rigorously required for identification due to the geomagnetic polarity reversal in the Earth's history [ Hildebrand and Staudigel , ].…”

The Latest Spreading Periods of the South China Sea: New Constraints From Macrostructure Analysis of IODP Expedition 349 Cores and Geophysical Data

“…Controversy in the relict ridge location (Barckhausen et al, ; Briais et al, ; Li et al, ; Sibuet et al, ; Taylor & Hayes, ; Zhao et al, ) prevents the recognization of spreading scenarios. Recent OBS surveys revealed a low P wave velocity zone in the upper crust across the rift valley of the SWRR (Zhang et al, ) and the seamount chain of the East Subbasin (He et al, ; white circles in Figures , , and ). Previous studies have found that relict ridges typically exhibit low velocities in the upper crust because of the existence of extrusive high‐porosity rocks, volcaniclastic sediments, and shallow faults (Grevemeyer et al, ; Weigel & Grevemeyer, ).…”

“…In this study, core samples, seismic profiles, multibeam bathymetry, free‐air anomalies (FAAs) of gravity (Sandwell et al, ), magnetic anomaly data (Ishihara & Kisimoto, ), P wave velocity (Zhang et al, ; He et al, ), and earthquake focal mechanisms were used.…”