Deep seismic structure of the northeastern South China Sea: Origin of a high‐velocity layer in the lower crust

Abstract

We present a 2‐D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent‐ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies.

Possible Spatial Distribution of the Mesozoic Volcanic Arc in the Present‐Day South China Sea Continental Margin and Its Tectonic Implications

“…The average source depths for the magmatic arc are well consistent with the results imaged by deep seismic profiles (Wan et al, 2017). Previous studies have developed general models for the vertical integral of magnetization along the crustal section by assigning constant magnetization values to the magnetic crust (Mayhew et al, 1991).…”

“…Volcanic arc magmas originate from partial melting of the overriding mantle in the subduction zone and accumulate at the continental crust interior, forming large-scale magmatic arc roots (Currie et al, 2015). For example, a thick high-velocity layer is imaged in the lower crust in the northern SCS continental shelf and viewed as a remnant arc (Wan et al, 2017). We note that previous studies also viewed the high-velocity layer as underplating during continental breakup (e.g., Kido et al, 2001;Nissen et al, 1995).…”

Crustal structure of the eastern Piedmont and Atlantic coastal plain in North Carolina and Virginia, eastern North American margin

“…We also compare two examples of rifted crustal structure with our study to better understand the nature of ENAM margin. One is the Kenya Rift of the East African Rift System (EARS), which has a narrow rift mode (Buck 1991); the other is an example of wide rift mode, the South China Sea (SCS) rift (Wan et al 2017). And the SCS rift and the ENAM rift are fully developed rifting margin (Buck 1991;Brune et al 2016;Wan et al 2017), while the Kenya Rift is a youthful narrow rift and appears to be an example of early-stage continental rifting (Buck 1991;Ebinger 1989).…”

Crustal S-wave velocity structure across the northeastern South China Sea continental margin: implications for lithology and mantle exhumation

“…The north margin has experienced multiple episodes of rifting (Taylor and Hayes, ). There are also well‐preserved Mesozoic sequences that can provide rich information on the pre‐rifting geological process of the SCS (Li C‐F et al, ; Shi HS and Li C‐F, ; Fan CY et al, ; Wan KY et al, ).…”

“…Based on numerical modeling, Song TR () suggested that the high‐velocity layer near the continent‐ocean boundary (COB) could represent the exhumation of serpentinized upper mantle. Wan KY et al () suggested that the high‐velocity layer underlying the transitional crust is related to Cenozoic decompression melting, whereas that beneath the Dongsha Rise further north is associated with the Mesozoic subduction of the Paleo‐Pacific plate.…”