The tectono-stratigraphic and magmatic evolution of conjugate rifted margins: Insights from the NW South China Sea

Highlights

• The tectono-stratigraphic architecture of a section across the NW South China Sea rift propagator is described and restored.

• The sedimentary and magmatic evolution of crustal thinning is documented in a Wheeler Diagram.

• Distinction between stratigraphic and tectonic top basement has fundamental consequences for kinematic restorations.

Abstract

This study is based on a careful analysis of high-quality reflection seismic sections located at the tip of the NW South China Sea V-shaped rift basin. Using the CGN-1 section, a seismic line imaging the complete sedimentary and magmatic architecture of conjugate rifted margins, we: (1) provide a detailed description of the crustal architecture; (2) define extensional domains, which we relate to specific deformation phases; and (3) determine the tectono-stratigraphic evolution linked to rifting. Based on these, we propose a kinematic restoration and quantify the amounts of extension and associated strain rates. We discuss the link between the kinematic evolution and the sedimentary and magmatic record and illustrate it in a Wheeler Diagram. Relying on the identification and characterization of distinct stratal patterns and crustal architectures, we propose qualitative and quantitative criteria to interpret two critical rift events that are necking and hyperextension. These two events are linked to the individualization and subsequent dismembering of a so-called keystone, here referred to as H-block. It is the first time such an approach is used to decipher the tectono-stratigraphic evolution of a complete syn-rift mega-sequence across present-day conjugate rifted margins. This study differs from previous interpretations of correlative surfaces in the distinction between: (1) different types of top basement; and (2) syn- and post-tectonic packages within the syn-rift record. It leads to new interpretations of the tectono-stratigraphic evolution of the NW South China Sea and has the potential to be used as a new approach to analyze, quantify and correlate events recorded in seismic sections across rifted margins.

Introduction

In the last two decades, the understanding of rifted margins, in particular of their most distal parts has significantly evolved (Abdelmalak et al., 2015, Brune et al., 2014, Manatschal et al., 2015; Péron-Pinvidic et al., 2019; Péron-Pinvidic and Osmundsen, 2018; Sapin et al., 2021). The access to high-resolution, long-offset seismic and drill hole data enabled the development of new concepts to explain crustal and lithospheric thinning, breakup and onset of seafloor spreading (Deng et al., 2020, Larsen et al., 2018, Nirrengarten et al., 2020, Osmundsen and Ebbing, 2008; Péron-Pinvidic et al., 2019; Reston, 2007; Sun et al., 2018; Sutra and Manatschal, 2012; Wang et al., 2018; Wang, 2019; Quirk et al., 2014). Recently, detailed studies that evaluated the spatial and temporal evolution of rifting at conjugate margins and documented it in a Wheeler Diagram have been proposed for fossil margins (Masini et al., 2013, Ribes et al., 2019). However, such an approach has not been applied to seismically imaged present-day rifted margins yet. The documentation of the tectono-sedimentary and magmatic evolution requires a proper kinematic restoration of a complete transect across conjugate margins, and a first-order quantification of extension rates and accommodation creation during the necking and hyperextension stages.

At present, there are still many open questions related to when, how, and under what conditions major crustal thinning (necking and hyperextension) and crustal separation occur, and how these processes are recorded in the stratigraphy. To answer to these questions, understanding the stratigraphic record is fundamental. However, it is at present both poorly understood and barely documented, in particular in the most distal parts of rifted margins. The efficient record of syn-rift deformation in seismic data requires high sedimentation rates, which are, however, rare at distal margins (e.g., Epin et al., 2021; Sapin et al., 2021). Moreover, most geophysical acquisitions and geological studies document only one of the two conjugate margins, which hampers unraveling the evolution of entire rift systems. High-quality seismic images of well-preserved pairs of conjugate margins displaying high sedimentation rates during rifting and breakup are rare and exist only in a handful of places such as the Red Sea (Ligi et al., 2018), the Gulf of Aden (Leroy et al., 2012), the Woodlark Basin (Taylor et al., 1999), the Gulf of California (Lizarralde et al., 2007) and the South China Sea (SCS).

In this study, we focus on the tip of the NW-SCS, where seafloor spreading propagated towards the Xisha Trough before failing (Fig. 1). There, numerous, high-quality, long offset seismic reflection surveys provide a good imaging of the sediment-rich and magma-poor conjugate margins, offering an excellent and rare opportunity to explore and describe the complete rift evolution. Besides, the International Ocean Discovery Program (IODP) Expeditions 367/368/368X drilled several sites at the Ocean–Continent Transition (OCT) approximately 150–200 km east of our study area (Fig. 1) (Larsen et al., 2018).

The most recent studies showed that rifting and breakup along the SCS are, in many regards, different from classical Atlantic-type magma-poor or magma-rich margins (Sun et al., 2018; Ding et al., 2020; Luo et al., 2021; Nirrengarten et al., 2020). Indeed, neither evidence for exhumed mantle, nor Seaward Dipping Reflections (SDRs) have been found in the SCS. The occurrence of long-offset extensional detachment faults (Savva et al., 2013; Clerc et al., 2017; Yang et al., 2018; Zhao et al., 2018; Deng et al., 2020; Zhang et al., 2020) and syn-rift magmatic systems (Ding et al., 2020; Zhang et al., 2021) show complex interactions between extensional and magmatic systems during rifting and breakup. These results highlight the need for a more detailed analysis of the spatial and temporal tectono-magmatic and sedimentary evolution, which is the aim of this study.

The present study is based on the careful analysis of a dense, high-quality seismic reflection dataset that images the conjugate margins at the tip of the NW-SCS V-shaped rift basin. The overall margin architecture is best imaged in the CGN-1 reflection seismic section provided by CNOOC (Fig. 2). This study aims to: (1) provide a detailed description of the crustal architecture, define extensional domains to characterize the margin architecture, and determine the tectono-sedimentary and magmatic evolution of this rift system; (2) understand the interplay between deformation, sedimentation and magmatic processes during rifting and breakup in 2D; (3) analyze the evolution of deformation through time and space by linking the tectono-stratigraphic evolution with crustal thinning and synthetize it in a Wheeler diagram, and (4) quantify the amount of strain and the strain-rate accommodated along these conjugate margins. For the first time, we propose a Wheeler Diagram that describes and quantifies the temporal and spatial tectono-stratigraphic evolution at a pair of conjugate rifted margins during a complete syn-rift mega-sequence, from necking, to hyperextension, to crustal separation and proto-oceanic seafloor spreading. Moreover, based on the identification and characterization of seismic sequences, distinctive stratal packages and specific crustal architectures, we propose qualitative and quantitative criteria that allowed us to interpret the processes linked to two critical rift phases, which are the necking and hyperextension phases.