Keywords

1 Introduction

The South China Sea (SCS), which spans from Singapore and the Malacca Strait in the south to the Strait of Taiwan in the north, and from Borneo and the Philippines on the east to Vietnam and south-eastern China on the west, is one of the largest marginal seas in the world. It is located at the convergence of three major plates (the Eurasian, Pacific-Philippine and Indo-Australian plates) along two super-convergent zones (the circum-Pacific and Tethyan zones) (Wang et al. 2000; Li et al. 2013; Zhou and Yao 2009). It is surrounded by passive margins (rifting) in the north and south, an active margin (subduction) in the east, and a transform margin in the west. Hence the evolution of SCS can be thought of as an integrated result of a complex tectonic history. Here, the SCS refers a collective feature which includes the deep-sea plain and surrounding slopes and shelves (Fig. 1).

Fig. 1
figure 1

Map depicts major structures in and around the South China Sea Basin (modified after Sun et al. 2009). Sub-basins in the SCS basin are shown (ESB; East sub-basin, NWSB: North-west sub-basin and SWSB: South-west sub-basin). Location of Zhongnan fault is plotted after Ziying et al. 2019 (indicated as No. 1). In the SWSB, ribbon like structures (indicated as No. 2) are highlighted with arrow-head line segments which implies a decollement (Hayes et al. 1995; Ding et al. 2013). The round shapes (indicated as No. 3) in the northern margin are also highlighted with a dotted circle, these structures are compared to the large granitic bodies cropping out onland in China and Vietnam (Savva et al. 2014). Distinct delineation of the basin is marked by large gradient in the bathymetry

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The SCS developed from magma-poor continental margin rifting to subsequent seafloor spreading in the latest Cretaceous to Early Paleocene (Taylor and Hayes 1983; Cullen et al. 2010; Franke 2013) and thus oceanic crust covers the central portion of the basin. Evolution of the SCS is largely believed to be induced by the India-Eurasia collision to the northwest (Morley 2002; Tapponnier et al. 1986), the subduction of the Pacific plate and then the compression from the Philippine sea plate in the east (Li 1994; Zhou et al. 2002), as well as the slab-pull of the proto-South China Sea in the south (Hall 1996; Morley 2002; Taylor and Hayes 1983).

The SCS oceanic basin formed during the early Oligocene to middle Miocene (32–15 Ma) (Li et al. 2015; Taylor and Hayes 1983). Its age is mature enough to experience the entire history of marginal sea development as well as young enough to preserve and exhibit the complete evolutionary records from being distorted (Zhou, and Yao 2009) and moreover it is relatively small so the entire basin is easily accessible. Thus, the basin is considered to be a key site and natural laboratory for studying continental break-up, sedimentary basin formation, mantle and lithosphere evolution, Asian monsoonal climate dynamics and other land-ocean interactions (Li et al. 2012). The lack of salt and Seaward Dipping Reflector (SDRs) and its potential for oil and gas have caught more attention for studying the transition between highly stretched continental and oceanic crust.

Even though studies had been focused primarily on the sea-floor dating and tectonic evolution through modelling (Taylor and Hayes 1980, 1983; Briais et al. 1993; Barckhausen and Roeser 2004), the evolutionary history of the SCS has been in debate for decades (Barckhausen et al. 2014; Briais et al. 1993; Cullen et al. 2010; Franke et al. 2013; Li et al. 2012; Taylor and Hayes 1980, 1983; Zhou et al. 1995). However, in the recent years, with different geophysical data the basin has been well-imaged and scientific drilling expeditions (ODP Leg 184; IODP-349, 367, 368 and 368X) (Wang et al. 2000; Li et al. 2015; Sun et al. 2018 and Childress 2018) have provided crucial clues in constraining the processes of rifting and eventual rupturing of the continental crust during breakup, at this magma-poor rifted margin (drilling locations are shown in Fig. 1). In this paper, we would like to highlight a glimpse of the present scenario of our understanding on the evolution of SCS.

2 Regional Geological Setting

The SCS basin is a ‘V’-shaped basin (characteristic of a propagation opening; Courtillot 1982; Taylor et al. 1999) and shows an irregular triangular shape with the major apex towards south-west which reveals a propagating rift (Huchon et al. 2001). The SCS basin has been subdivided into an eastern sub-basin (ESB), northwest sub-basin (NWSB) and southwest sub-basin (SWSB) (Hutchison 1989; Li et al. 2008) (Fig. 1.). The ESB is separated from the SWSB by Zhongnan Fault Zone (Sun et al. 2009) (Figs. 1 and 2). Briais et al. (1993) proposed that the SWSB and the ESB began to spread in a N-S orientation at 30 Ma after which the spreading ridge jumped towards south where the spreading episode lasted from 25 to ~16 Ma and the spreading appears to propagate from northeast to southwest.

Fig. 2
figure 2

It shows the free-air gravity anomaly variation in the South China Sea Basin in which major features are highlighted. Zhongnan fault (indicated as No. 1) is distinctly visible. The map reflects high gravity anomalies in the oceanic basin and lows in the surrounding islands (after Yu et al. 2017). The region of ribbon like structures is indicated as No. 2

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The conjugate margins of the SCS exhibit various morphological features (Yu et al. 2017) which are shown in Fig. 1. The bottom of the basin is fairly flat with seamounts, sea knolls and depressions of varying sizes. The elongation direction of the seamounts is mainly in E-W (Yu et al. 2017). In the deep area of the SCS, there are more than 50 seamounts that are over 500 m above the seafloor. Compared with those in the central basin, the seamounts in the southwest sub-basin are much smaller and their sails are relatively small.

The south-western part of the SCS shows ribbon like structures (Hayes et al. 1995; Ding et al. 2013) distributed over a wide “Basins and Range-like” province which implies a decollement level in the crust (Gilder et al. 1991; Franke 2013; Pichot et al. 2014) (Fig. 2). The central parts of both the northern and southern margins display a different morphology underlined by large round shapes of around 50–100 km in size. The round shapes in the northern margin may be compared to the large granitic bodies cropping out onland in China and Vietnam (Savva et al. 2014) (Fig. 1). Zhongnan Fault Zone is a striking feature in the middle part of the SCS basin.

Across the South China Sea, the crustal thickness varies significantly (Nissen et al. 1995; Qui et al. 2011; Pichot et al. 2014). The northern margin has an average of 30 km (Nissen et al. 1995) which generally thins towards the South with 10–15 km thick (Qui et al. 2011; Pichot et al. 2014). The coastal zone of the northern margin is marked by a much rougher morphology with elongated flakes composed of granite and Palaeozoic sediments (Shen 2008; Pichot et al. 2014). Basin portions closer to COT boundaries are composed of a thinner crust (between 3 and 5 km on the northern margin and 3 km on the southern side) (Savva et al. 2014).

Satellite gravity images clearly reflect most of the primary tectonic features and many other secondary structural details in the basin (Fig. 2). The gravity map depicts high gravity anomalies in the oceanic basin and lows in the surrounding islands, indicating that the crust of the basin is very thin and characteristic of typical oceanic crust. NE-trending low-intensity anomaly stripes are observed in the high-gravity background of the southwest sub-basin (Yu et al. 2017). The region of dangerous grounds is marked by elongated gravity high and low anomalies.

3 Evolution History

The south-west tapered South China Sea is a backarc basin rifted off the South China Block (Sun 2016) which resulted from the late Cretaceous to Paleogene rifting, late Eocene to middle Miocene seafloor spreading, and post-spreading subduction and closing since the late Miocene. The complex geological evolution of the SCS resulted in a variety of structures across the basin, including the abundant extensional structures and limited magmatic activity that developed in the northern margin of the SCS during and after the rifting and seafloor spreading (Franke et al. 2013). The opening of the SCS was mainly induced by the extrusion of Indochina subcontinent due to Tibet uplift and slab pull due to subduction of the Asian continental crust into the Philippine Sea Plate. Sun et al. (2009) viewed that a Proto-SCS existed before rifting and spreading occurred mainly along the present south-eastern margin although its size and connection with other paleo-oceans are not clear. Based on sea floor spreading magnetic anomaly data from the northeast, the SCS first opened at ~37 Ma, in the late Eocene. IODP Expedition 349 core (Li et al. 2014) shows that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1–2 Ma along the northern continent-ocean boundary (COB), and the terminal age of seafloor spreading is ~15 Ma in the East sub-basin and ~16 Ma in the Southwest sub-basin. Larsen et al. (2018) also proposed that narrow and fast rift-to-drift transitions along the northern SCS has resulted due to major rifting events within thin lithosphere permitting mantle upwelling, yielding abundant MORB-type melts during final breakup and early seafloor spreading.

The rift stages (in which the basin rifted from the South China Block) that resulted in the formation of the SCS started with an initial uplift of the rift shoulders accompanied by widespread erosion and peneplanation in the Late Cretaceous to Early Paleocene (Taylor and Hayes 1980, 1983; Ru and Pigott 1986; Schlüter et al. 1996; Pubellier et al. 2003). According to Sun et al. (2009), the proto-margins of the SCS experienced three main stages of deformation as the rifting propagated from North to South and then from East to West. By the end of rifting of the NE part of the SCS, spreading began and propagated gradually toward the SW. In the north eastern portion of the SCS, Hsu et al. (2004) and Yeh et al. (2010) interpreted Late Eocene/Early Oligocene oceanic crust and seafloor spreading (37.8–30.1 Ma). However, the nature of the crust there is ambiguous as McIntosh et al. (2013) proposed a hyper-extended crust with a possible mantle exhumation. The timing of seafloor spreading in the central South China Sea has been revised to 32–20.5 Ma (Barckhausen and Roeser 2004) from 32 to 15.5 Ma (Taylor and Hayes 1980; Briais et al. 1993). The post Middle Miocene evolution is considered to be devoid of important tectonic activity except for the Northwest Borneo wedge vertical motions (Sapin et al. 2011).

In the south, rifting delayed until the Eocene-Miocene and often a breakup unconformity of 3–5 Ma is observed underlying strata of 16 Ma and older ages. Since the middle Miocene, the eastern margin has been subducting, subsequently causing the closure of the sea basin and rise of Taiwan in the last 5 Ma (Li et al. 2013).

4 Dynamics of the South China Sea

Researchers have given considerable attention to understand the dynamics and related mechanisms for the evolution of the SCS. Different models have been proposed (Sun 2016) for the evolution of SCS. Of these the models that are widely accepted include (a) Tectonic extrusion model (Tapponnier et al. 1990; Briais et al. 1993); (b) Backarc extension model (Hilde et al. 1977); (c) Two-stage rifting model (Yao 1999); (d) Proto-SCS dragging model (Holloway 1982; Taylor and Hayes 1983; Hall 1996), (e) Models that involve extension induced by mantle plume (Tamaki and Hoang 1998); and (f) Combinations of proto-SCS pull and extrusion of the Indochina Peninsula, and/or mantle flows (Tamaki 1995; Morley 2002; Zhou et al. 2002; Sun et al. 2006), etc.

4.1 The Tectonic Extrusion Model

This model is based on physical modelling experiments (Tapponnier et al. 1990; Briais et al. 1993) and considered to be the most famous model. The model depicts the major deformation in the Eurasian crust due to the collision between the Indian and Eurasian continents, leading to southward extrusion of the Indo-China Peninsula along the Ailaoshan-Red River sinistral fault. The model was supported by the synchronicity between the strike-slip movement of the fault (Tapponnier et al. 1990) and the spreading of the SCS (Taylor and Hayes 1980, 1983). However, problems emerged with the extrusion model in terms of age discrepancy, compression and extension of the SCS, failing to reflect the real geodynamic conditions and failing to explain the wider spreading in the east of SCS than in the west.

4.2 Backarc Extension Model

This model (Hilde et al. 1977) proposes that the SCS was a back-arc basin rifted from the passive margin of the Eurasian continent. In the model, the subduction direction of the Philippine Sea and the Pacific Plate is not compatible with the extension direction of the SCS even though the subduction of the Philippine Sea Plate was taken as the primary driving force (Karig 1971; Ben-Avraham and Uyeda 1973; Guo et al. 1983). Also, the SCS was assumed as a backarc basin related to subduction of the Neotethys Plate between the Australian and Eurasian continents (Hilde et al. 1977). The model shows that the extension of the SCS started at 100 Ma (Hilde et al. 1977; Stern and Bloomer 1992) without any evidence.

4.3 Two-Stage Rifting Model

Considering geomorphological/geophysical features like water depths, extensional basins surrounding the SCS, and magnetic anomalies in the SWSB of the SCS, this model proposes two extension events in the SCS-the first extension, which occurred in the Late Eocene to Early Oligocene (42–35 Ma), formed the southwest-northeast trending SWSB and the second extension occurred in the Late Oligocene to Early Miocene, resulting in the formation of east–west trending ESB (Yao 1999). Despite the two-stage opening of the SCS predicted by this model is confirmed by recent dating results, the timing is not consistent (Li et al. 2014).

4.4 Proto-SCS Dragging Model

According to this model, a proto-SCS existed south of the current SCS and disappeared through south-eastward subduction beneath the Luzon and Sulu islands in the Late Cretaceous to Paleocene (Holloway 1982; Taylor and Hayes 1983; Lee and Lawver 1994, 1995) which resulted in extension and rifting along the southeast margin of the South China Block and the formation of the SCS (Holloway 1982; Taylor and Hayes 1983; Hall 1996). But, paleomagnetic results suggest that the proto-SCS was very small (Lee and Lawver 1994) to have pulled apart the thick continental lithosphere of South China.

4.5 Models that Involve Extension Induced by Mantle Plume

In this model, a mantle plume is considered as the driving force for the evolution of the SCS (Tamaki and Hoang 1998) which is apparently supported by seismic tomography showing high temperature anomalies beneath the SCS (Huang and Zhao 2006; Zhao 2007). However, absence of large scale magmatism and other geochemical data on some basaltic rocks (Yan et al. 2008; Zou and Fan 2010; Xu et al. 2012; Huang et al. 2013) are against this plume induced model.

4.6 Combined Model

In this model, several events/features like proto-SCS subduction/pull plus mantle flow induced by collision between the Indian and Eurasian continents (Sun et al. 2006); proto-SCS subduction/pull plus southward extrusion of the Indochina Peninsula (Morley 2002); or multiple plate subduction, shearing, and collisions (Tamaki 1995; Zhou et al. 2002) are put together to explain the opening of the SCS. However, neither extrusion nor proto-SCS subduction/pull can explain the formation of the SCS nor can it explain the two-stage extension. As of now, this model is more widely used than the other models.

Remarks

From various perspectives the SCS has been an interesting region for geoscience researchers. Existence of different types of geological features which preserves the complete evolutionary records of the SCS makes it a fascinating area for understanding the geological journey that was taken by a rifted marginal sea which has been influenced by various plate tectonic processes. Studies with the help of integrated geophysical and geological data would be able to find a conclusive mechanism for the initiation, evolution and formation of the South China Sea.