Elsevier

Marine Geology

Volume 436, June 2021, 106475
Marine Geology

Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea

https://doi.org/10.1016/j.margeo.2021.106475Get rights and content

Highlights

  • Oceanic crust is imaged from the edge to the extinct ridge of the East Sub-basin.

  • The crust becomes thick from 3.9 km at the edge to 8.4 km at the abandoned ridge, but thin to 3.7 km at the extinct ridge.

  • Tectonic extension continued from the magma-poor rifting to initial spreading.

  • Magmatic budget culminated around the intervening ridge jump.

  • Tectonism dominated the last stage of seafloor spreading in the South China Sea.

Abstract

Following magma-poor continental rifting, the South China Sea (SCS) experienced seafloor spreading from ~33 Ma to ~15 Ma with an intervening southward ridge jump at ~25 Ma, generating the oceanic East, Northwest and Southwest Sub-basins. However, the magmatic processes during spreading remain poorly known owing to the lack of oceanic crustal images. Here we present a composite profile comprising three seismic lines that run from the northern continent-ocean transition (COT) to the extinct spreading center of the East Sub-basin to reveal the crustal structures. In the northernmost oceanic portion, the first oceanic crust featuring rough basement with numerous faults, and diffuse and weak Moho reflector, is only 3.9–4.5 km thick, implying relatively low magmatic budget and protracted tectonic extension continued from the magma-poor continental rifting to initial spreading. From the northern COT ocean-ward to the abandoned ridge where the jump event occurred, the basement becomes smooth with subdued faults. The Moho reflector tends to be continuous and strong, and the igneous crust gradually becomes thick (locally up to 8.4 km), suggesting considerably increasing magmatic budget during the early spreading stage. However, in the central portion spanning from the abandoned ridge to the extinct spreading axis, the basement becomes rough with crustal faults again. And the crust gradually becomes thin, to only 3.7 km at the extinct spreading center where a deep median valley is still visible, though severely altered by post-spreading volcanism, implying decreasing magmatic budget and increasing tectonic extension during the late spreading stage. Therefore, the crustal structure variations in the East Sub-basin reflects a prominent increasing to decreasing change of magmatic budget during spreading with the peak around the intervening ridge jump. Thin, tectonized crust and deep median valley around the extinct spreading center further support slow seafloor spreading during the last spreading stage of the SCS which ended at ~15–16 Ma, and also suggest that magma contribution from the Hainan mantle plume might be very limited if any.

Introduction

The South China Sea (SCS) has experienced high to hyper continental extension, seafloor spreading, and post-spreading magmatism since the Cenozoic, generating the wide continental margins and the SCS oceanic basin which includes the major East Sub-basin (ESB), and the pronged Northwest Sub-basin (NWSB) and Southwest Sub-basin (SWSB) (Fig. 1) (Taylor and Hayes, 1983; Briais et al., 1993; Barckhausen and Roeser, 2004; Barckhausen et al., 2014; Expedition 349 Scientists, 2014; Li et al., 2014; Qiu et al., 2016; Ding et al., 2018; Yu et al., 2018a). The continental margins of the SCS feature numerous crustal faults, highly extended blocks, and weak syn-rift magmatism and volcanism, and the rifting process there has been identified as magma-poor or non-volcanic (e.g., Yan et al., 2001; Yan et al., 2006; McIntosh et al., 2013; Franke et al., 2014). Recently, Larsen et al. (2018) and Ding et al. (2020) speculated that magma supply became abundant during the final continental breakup and oceanic spreading onset in the northern SCS. This view was based on the discovery of narrow continent-ocean transition (COT) revealed by the drilling of International Ocean Discovery Program (IODP) Expeditions 367–368. Such a rapid transition from magma-poor continental rifting to magma-rich seafloor spreading is rare if any.

Based on the analysis of magnetic lineations, it is commonly agreed that seafloor spreading began firstly from the ESB at about 33 Ma, then jumped southward at about 25 Ma and propagated to the SWSB (e.g., Taylor and Hayes, 1983; Briais et al., 1993; Barckhausen and Roeser, 2004; Barckhausen et al., 2014; Li et al., 2014). However, controversy remains on the cessation age of seafloor spreading in the SCS. Most researches (e.g., Taylor and Hayes, 1983; Briais et al., 1993; Li et al., 2014; Qiu et al., 2016) proposed that seafloor spreading ended at about 15–16 Ma, but Barckhausen and Roeser (2004) and Barckhausen et al. (2014) suggested an earlier cessation age of 20.5 Ma which calls for faster spreading rate and higher magmatic budget during the last spreading stage. In addition, Yu et al. (2018b), Zhang et al., 2018a, Zhang et al., 2018b and Yang et al. (2019) speculated that the Hainan mantle plume activity has influenced magmatism of the last spreading to post-spreading stages in the central ESB. This inference was based on the geochemical analysis of the basaltic rocks drilled by IODP Expedition 349 and dredged from post-spreading seamount. But the cessation of seafloor spreading under context of a growing mantle plume is puzzling. In fact, these latest interpretations about magmatism over the oceanic regime were mainly based on the sparse drilling and dredging, when magmatic budget was poorly known. In recent years, a few of deep seismic surveys were conducted in the SCS oceanic basin, providing us with great possibility to establish a continuous record of magmatic budget from the COT to the extinct spreading ridge.

In this study, we present and analyze a composite profile comprising three multichannel seismic (MCS) lines that run from the northern COT to the extinct spreading center of the ESB (Fig. 1a). Our objective is to reveal the oceanic crustal structures, expound the temporal variations of magmatic budget, and discuss the tectono-magmatic interaction during seafloor spreading.

Section snippets

Geological setting

The SCS oceanic basin was generated in between the conjugated margins (Fig. 1) by approximately N-S to NW-SE oriented seafloor spreading driven by the southward subduction of the proto-SCS oceanic crust under the Borneo (Taylor and Hayes, 1983; Hall, 1996, Hall, 2002). The continental margins are wide with a number of scattered Cenozoic rifted basins, such as Pearl River Mouth Basin in the northern margin, Zhongjiannan Basin in the northwestern margin and Zhenghe Basin in the southern margin (

MCS data acquisition and processing

Three MCS lines (1555, East-1, and 97301) in the northern ESB are analyzed in this study (Fig. 2, Fig. 3, Fig. 4). Lines 1555 and East-1 crossing the northern part of the ESB in NW-SE direction (Fig. 1a) were collected by China National Offshore Oil Corporation (CNOOC) and Guangzhou Marine Geological Survey (GMGS), respectively. Line 97301 crossing the central part of the ESB in approximatively N-S direction (Fig. 1a) was acquired by South China Sea Institute of Oceanology, Chinese Academy of

Results

In Fig. 2, Fig. 3, Fig. 4 we present MCS lines 1555, East-1 and 97301 to show the reflection seismic images of the ESB from its northern COT to extinct spreading center (Fig. 1a). From these seismic images, it can be shown that the oceanic crustal structures of the ESB vary considerably along the spreading direction.

In the northernmost oceanic portion of the ESB (~65–110 km distance along line 1555 and ~0–50 km distance along line East-1; Figs. 2a–b and 3a–b), the oceanic basement is rough as

Time-varying magmatic budget during spreading in the ESB

Magmatic budget during spreading correlates well with the seismically determined thickness of igneous crust (White et al., 1992; Yu et al., 2018a). Generally, the crust in magma-rich spreading domain is thick (>5.5 km) (Fig. 6), e.g., ~6 km in the Pacific Ocean (Aghaei et al., 2014). On the contrary, it is thin (<5 km) and more or less mixed with serpentinized mantle rocks in magma-poor spreading domain, e.g., only 1.3–4 km off the New found land and Iberia margins (Whitmarsh et al., 1996;

Conclusions

Deep seismic profiles crossing the northern half of the ESB of the SCS document complex oceanic crustal structure variations, reflecting prominently varying magmatic budgets during seafloor spreading. The primary findings and conclusions are as follows:

  • (1)

    In the northernmost oceanic portion that formed firstly after ~33 Ma, the basement is rough and offset by several faults, the Moho reflector is diffuse and weak, and the crust is only 3.9–4.5 km thick, thinner than normal oceanic crust,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We acknowledge insightful discussions with Prof. Xiaolong Huang, Prof. Guoliang Zhang, and Dr. Fan Yang. We also thank the editor-in-chief, Prof. Shu Gao, for his editorial handling. Prof. Joe Cartwright and one anonymous reviewer are acknowledged for their crucial and constructive comments which helped to improve the paper. This work is financially supported by CAS Special Research Assistant Funding Program to Junhui Yu, Open Fund of Key Laboratory of Marine Geology and Environment, CAS (Grant

References (69)

  • M.C. Schmidt-Aursch et al.

    3D gravity modelling reveals off-axis crustal thickness variations along the western Gakkel Ridge (Arctic Ocean)

    Tectonophysics

    (2016)
  • M. Xiao et al.

    Evidence of Early Cretaceous lower arc crust delamination and its role in the opening of the South China Sea

    Gondwana Res.

    (2019)
  • P. Yan et al.

    The temporal and spatial distribution of volcanism in the South China Sea region

    J. Asian Earth Sci.

    (2006)
  • Q.S. Yan et al.

    Petrology and geochemistry of Mesozoic granitic rocks from the Nansha micro-block, the South China Sea: constraints on the basement nature

    J. Asian Earth Sci.

    (2010)
  • Q.S. Yan et al.

    The late Mesozoic–Cenozoic tectonic evolution of the South China Sea: a petrologic perspective

    J. Asian Earth Sci.

    (2014)
  • F. Yang et al.

    Plume-ridge interaction in the South China Sea: thermometric evidence from Hole U1431E of IODP Expedition 349

    Lithos

    (2019)
  • L.L. Yang et al.

    The structure and evolution of Deepwater basins in the distal margin of the northern South China Sea and their implications for the formation of the continental margin

    Mar. Pet. Geol.

    (2018)
  • G.L. Zhang et al.

    Geochemical nature of sub-ridge mantle and opening dynamics of the South China Sea

    Earth Planet. Sci. Lett.

    (2018)
  • O. Aghaei et al.

    Crustal thickness and Moho character of the fast-spreading East Pacific Rise from 9°42′N to 9°57′N from poststack-migrated 3-D MCS data

    Geochem. Geophys. Geosyst.

    (2014)
  • U. Barckhausen et al.

    Seafloor spreading anomalies in the South China Sea revisited

  • A. Briais et al.

    Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: implications for the Tertiary tectonics of Southeast Asia

    J. Geophys. Res.

    (1993)
  • G.Q. Cai et al.

    Mesozoic Northward Subduction along the SE Asian Continental margin Inferred from Magmatic records in the South China Sea

    Minerals

    (2019)
  • A.L. Cameselle et al.

    The continent-ocean transition on the northwestern South China Sea

    Basin Res.

    (2017)
  • S.M. Carbotte et al.

    Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

    Geol. Soc. Lond. Spec. Publ.

    (2015)
  • G.L. Christeson et al.

    Synthesis of Oceanic Crustal Structure from Two-Dimensional Seismic Profiles

    Rev. Geophys.

    (2019)
  • M. Delescluse et al.

    The oceanic crustal structure at the extinct, slow to ultraslow Labrador Sea spreading center

    J. Geophys. Res. Solid Earth

    (2015)
  • H.J.B. Dick et al.

    An ultraslow-spreading class of ocean ridge

    Nature

    (2003)
  • B.-M. Ehlers et al.

    Subsidence and crustal roughness of ultra-slow spreading ridges in the northern North Atlantic and the Arctic Ocean

    Geophys. J. Int.

    (2009)
  • Expedition 349 Scientists

    South China Sea tectonics: opening of the South China Sea and its implications for southeast Asian tectonics, climates, and deep mantle processes since the late Mesozoic

  • T. Funck et al.

    Crustal structure of the ocean-continent transition at Flemish Cap: seismic refraction results

    J. Geophys. Res.

    (2003)
  • S. Gozzard et al.

    South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion

    Petroleum Geosci.

    (2019)
  • I. Grevemeyer et al.

    Structure of oceanic crust and serpentinization at subduction trenches

    Geosphere

    (2018)
  • R. Hall

    Reconstructing Cenozoic SE Asia

  • E.E.E. Hooft et al.

    Asymmetric plume-ridge interaction around Iceland: the Kolbeinsey Ridge Iceland Seismic Experiment

    Geochem. Geophys. Geosyst.

    (2006)
  • Cited by (9)

    View all citing articles on Scopus
    View full text