Crustal structure and rifting of the northern South China Sea margin: Evidence from shoreline‐crossing seismic investigations

Abstract

Seismic reflection and refraction data acquired from both onshore and offshore in the South China area and on the continental shelf of the northern South China Sea passive margin provide new constraints on the P‐wave velocity structure beneath the continental shelf and contribute to constructing a continental rift evolution model. Based on the crustal thicknesses and P‐wave velocities derived from the velocity structure of the seismic profile OBS2010‐2 across the continental shelf, two distinct domains are identified: (a) the thinned/extended continental crust and (b) the transitional crust. The thinned continental crust has a thickness of 25–27 km across the Hainan Rise, decreasing to 21–23 km further south in the rift and faulted depression zone. The transitional crust is characterized by a thickness of 20–21 km. High‐velocity values (>7.0 km/s) in the lower crust of the transitional domain may indicate possible magmatic underplating during rifting. Our results show that the Cathaysia Block extends from onshore South China to offshore into the South China Sea and is bounded by the transitional crust of the continental shelf. The initial rifting of the continental margin occurred in the rift and faulted depression zone and might be explained by the upwelling of the lithosphere based on a depth‐dependent extension mechanism during the late Cretaceous or Paleocene rifting.

Crustal thinning and extension in the northwestern continental margin of the South China Sea

“…The average P-wave velocity of the upper crust varies from 5.4-6.3 km/s, and the lower crust is constant at 6.8 km/s (Nissen et al, 1995a, b). The newly acquired wide-angle OBS velocity model shows the entire velocity structure from the South China coast area to the continental shelf part (Zhu et al, 2015, Fig. 10a).…”

“…3). The interval of geophones is 25 m, and the shot interval is 20 s at 5 knots average ship velocity, resulting in the shot interval of 50 m. The detailed P-wave velocity structure across the continental shelf coincident with the seismic reflection Line 1 is indicated in the profile OBS2010-2 (Zhu et al, 2015). The recorded time is 10 s, recording length and the interval of geophones is 25 m and the shot interval is 120 s, with 5 knots average ship's speed, resulting in shot intervals of 300 m. The seismic Line 2 is a 24-channel reflection profile recorded with 10 s record length.…”

“…The classical rifted margin is composed of thinned continental crust, transitional crust (continent-ocean transition zone) and normal oceanic crust, such as the rifted margin in the Gulf of California (Lizarralde et al, 2007), the southeast Greenland margin (Korenaga et al, 2000) and the Grand Banks-Newfoundland Basin rifted margin (Reid, 1994). The upwelling of the lithosphere with a depth-dependent extension mechanism potentially explains the initial rifting during the late Cretaceous or Paleocene (Zhu et al, 2015). The upwelling of the lithosphere with a depth-dependent extension mechanism potentially explains the initial rifting during the late Cretaceous or Paleocene (Zhu et al, 2015).…”

“…3). The seismic Line 1 is a four-channel reflection seismic profile, coincident with the offshore part of a wide-angle refraction profile OBS2010-2 (Zhu et al, 2015) and was shot using Bolt guns towed at 10 m depths. The recorded time is 10 s, recording length and the interval of geophones is 25 m and the shot interval is 120 s, with 5 knots average ship's speed, resulting in shot intervals of 300 m. The seismic Line 2 is a 24-channel reflection profile recorded with 10 s record length.…”

“…The recorded time is 10 s, recording length and the interval of geophones is 25 m and the shot interval is 120 s, with 5 knots average ship's speed, resulting in shot intervals of 300 m. The seismic Line 2 is a 24-channel reflection profile recorded with 10 s record length. The interval of geophones is 25 m, and the shot interval is 20 s at 5 knots average ship velocity, resulting in the shot interval of 50 m. The detailed P-wave velocity structure across the continental shelf coincident with the seismic reflection Line 1 is indicated in the profile OBS2010-2 (Zhu et al, 2015).…”