New Insights Into the Rift‐To‐Drift Process of the Northern South China Sea Margin Constrained by a Three‐Dimensional Wide‐Angle Seismic Velocity Model

Abstract

A three‐dimensional (3D) P‐wave seismic velocity (Vp) model of the crust in the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first‐arrival travel‐time tomography using wide‐angle seismic data from a network of 49 OBSs and 11 air‐gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ∼8 to 20 km, a ∼20 km‐width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ∼4.5 to 9 km. A high‐velocity (7.0–7.5 km/s) lower crust (HVLC) ranges in thickness from ∼1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn‐rift and syn‐breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) < ∼3, whereas gentler thinning associates to thinner HVLC at β > ∼4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.

3D Crustal Architecture and Along‐Strike Variation in the Mid‐Northern South China Sea Rifted Margin

“…A crustal velocity profile revealed by OBS data (Figure 3a) published by Zhang, Zhao, Ding, et al. (2023), of which the location coincides with the MCS profile in Figure 3b is cited here to compare the Top HVLC interpreted from MCS data and the Top HVLC interpreted from OBS data. The Top HVLC and Moho were identified by Zhang, Zhao, Ding, et al.…”

“… (a) The OBS profile H2 (extracted from OBS‐3d) crossing from U1499 to U1502 (modified from Zhang, Zhao, Ding, et al., 2023) and imaging the whole crustal structure. The 7.5 km/s and 7.0 km/s velocity contours have been taken as the Moho and top HVLC, respectively.…”

“…The Top HVLC and Moho were identified by Zhang, Zhao, Ding, et al. (2023) as the 7.0 km/s and 7.5 km/s velocity contours, respectively. Along this seismic line, we randomly select 10 locations at equal intervals (∼48 km) to measure the Top HVLC height above the Moho on the OBS velocity profile and the time difference between the interpreted Top HVLC and the Moho at the corresponding locations on the MCS profile (Figures 3a and 3b).…”

“…Seismic data support the presence of a continuous strike‐slip (transfer) zone in the mid‐northern SCS that may have been active during continental rifting (Zhang, et al., 2021; Zhao et al., 2020). Furthermore, wide‐angle seismic profiles constrain changes in the seismic structure of the crust along the margin and show an uneven distribution of high‐velocity lower crust (HVLC) (Figure 1a) (Ao, et al., 2012; Ding et al., 2012; Eakin et al., 2014; Huang et al., 2021; Liu et al., 2018; Liu et al., 2021; Nissen et al., 1995; Qiu et al., 2001; Wan et al., 2017; Wan et al., 2019; Wang et al., 2006; Wang, Zhao, Zhang, et al., 2022; Wei et al., 2011; Wu et al., 2012; Yan et al., 2001; Zhang, Zhao, Sun, et al., 2023; Zhang, Zhao, Ding, et al., 2023).…”

“…Our study area has been marked by a yellow dashed box. The tracks of the main published OBS/OBH seismic lines are also marked: ESP‐E (Nissen, et al., 1995), OBS1993 (Yan, et al., 2001), OBH1996 (Qiu, et al., 2001), OBS2001 (Wang, et al., 2006), OBS2006‐1 (Ding, et al., 2012; Wu et al., 2012), OBS2006‐2 (Ao, et al., 2012), OBS2006‐3 (Wei, et al., 2011), OBS2012 (Wan, et al., 2017), OBS2013‐1 (Huang et al., 2021), OBS2015‐2 (Liu, et al., 2018), OBS2016‐2 (Wan, et al., 2019), OBS2017‐BY (Zhang, Zhao, Sun, et al., 2023), OBS2019‐1 (Liu, et al., 2021), OBS‐3d (Zhang, Zhao, Ding, et al., 2023), OBS‐T1/T2 (Eakin, et al., 2014). The orange OBS lines reveal the high‐velocity lower crust (HVLC), and the black OBS lines reveal the absence of HVLC.…”