Understanding the 3D Formation of a Wide Rift: The Central South China Sea Rift System

Abstract

Rifted margins result from continental lithosphere extension, breakup, and transition to seafloor spreading by modes typically described by end-member conceptual models. However, current geophysical data challenge these archetypes showing a more complex spectrum of rifting styles. Here, we integrate geophysical and geological observations to constrain the time and space evolution of the central South China Sea (SCS) rift system. We provide new insights into the continental extension mechanisms and continent-ocean transition (COT) formation from a seismic transect parallel to the extensional direction of the central SCS rift system. We present 850-km-wide region of continental crust where distributed extensional deformation formed a pattern of lithospheric boudinage defined by seven 100-to 200-km-long structural segments. Each segment is formed by an 18-to 24-km-thick crust sector laterally thinning into an interboudin neck with ≤10-km ultrathin crust (ß factors~3.3-6.5). Drill and stratigraphic information support that extension continued at all six interboudin necks in the central SCS until~23 Ma, when breakup by seafloor spreading propagation reached one of them. We propose that the seven segments evolved as discrete subsystems from early rifting to breakup. Structurally equivalent ultrathin crust grabens occur NE and SW of both SCS conjugate margins, supporting a 3D wide rift mode of deformation across the SCS rift system. The six ultrathin crust contemporaneous necks and the abrupt nature and location of the COT support that the breakup was not controlled by further continental lithospheric thinning but rather was determined by the seafloor spreading propagation toward the SW.

A 3D Snapshot of Crustal Breakup Deduced From Seismic Analysis of the Tip of the NW South China Sea

“…In this study, we focus on the termination of the NW V-shaped basin, which separates the Western Pearl River Mouth Basin and China's continental margin in the north from the Macclesfield-Paracel Block (Zhongsha-Xisha) in the south (Figure 1). Cameselle et al (2017Cameselle et al ( , 2020 suggested that the spreading center propagated westwards and died shortly before reaching the Xisha Trough in the late Early Oligocene, an interpretation that is not compatible with the observations reported in the present study. Ding et al (2012); Wu et al (2012); Franke et al (2014); Gao et al (2016); Cameselle et al (2017Cameselle et al ( , 2020; Wang et al (2020); and Chao et al (2021) used different approaches to interpret the NW-SCS.…”

“…Cameselle et al (2017Cameselle et al ( , 2020 suggested that the spreading center propagated westwards and died shortly before reaching the Xisha Trough in the late Early Oligocene, an interpretation that is not compatible with the observations reported in the present study. Ding et al (2012); Wu et al (2012); Franke et al (2014); Gao et al (2016); Cameselle et al (2017Cameselle et al ( , 2020; Wang et al (2020); and Chao et al (2021) used different approaches to interpret the NW-SCS. As a result, they interpreted differently (a) magmatic budgets at breakup time, (b) distributions of the Continent Ocean Boundary (COB) or limits of the Ocean Continent Transition (OCT), and (c) details on how the breakup process was recorded in the study area.…”

“…Cameselle et al. (2017, 2020) suggested that the spreading center propagated westwards and died shortly before reaching the Xisha Trough in the late Early Oligocene, an interpretation that is not compatible with the observations reported in the present study. Ding et al.…”

“…Cameselle et al. (2017, 2020) and Franke et al. (2014) proposed seismic interpretations (for locations of their lines see Figure 1) that partially conflict with the interpretations by Ding et al.…”

“…Based on seismic refraction studies, these authors proposed a crustal model for the NW-SCS that forms the foundation of the present study. Cameselle et al (2017Cameselle et al ( , 2020 and Franke et al (2014) proposed seismic interpretations (for locations of their lines see Figure 1) that partially conflict with the interpretations by Ding et al (2020), in particular about the structure of the ocean-continent transition (OCT). A detailed description of the OCT from the central N-SCS was proposed by Zhang, Sun, Manatschal, Pang, Li, et al (2021) based on IODP drill hole data and seismic reflection lines located more than 250 km east of the area investigated in the present study.…”