The southeastern Asia was principally under control from two of the most critical geodynamic domains: the Paleo-Pacific subduction zone to the east and the Tethys subduction zone to the west (Carter et al., 2001; Li and Powell, 2001; Hall, 2002, 2012; Lepvrier et al., 2004; Zhou et al., 2008; Wallace et al., 2009; Li et al., 2011). A general consensus is reached that a well-developed Andean-type of volcanic arc existed along the southeastern Asian margin (e.g. South China southeastern margin) lasting from Triassic-Jurassic to Cretaceous (Maruyama, 1997; Zhou and Li, 2000; Li and Li, 2007; Li et al., 2012). The Paleo-Pacific subduction beneath the eastern Asian margin led to extremely drastic and complex tectonic activities across the South China Block. Multi-stage structural deformation, magmatism as well as local stress field changes took place in this region (Jahn et al., 1990; Zhou et al., 2006; Li et al., 2014). Further west, the Indochina Block collided obliquely with the South China along the Song-Ma Suture and Red River Fault Zone (Leloup et al., 2001; Lepvrier et al., 2004; Metcalfe, 2011, 2013). Therein, narrowing and closure of the Meso-Tethys was likely responsible for the late Triassic regional folding, uplifting and orogeny over the Indochina Continent (Metcalfe, 1996; Roger et al., 2000; Lepvrier et al., 2008; Sone and Metcalfe, 2008). Subsequently, the convergent Andean-type continental margin was transferred into the divergent western Pacific margin prior to the opening of the SCS (Zhou et al., 2006; Schellart et al., 2006; Li et al., 2012, 2014; Shi and Li, 2012). This sudden “transpression-transtension” switching process might be largely triggered by oceanward retreat of the Paleo-Pacific subduction zone in the late Mesozoic (Jahn et al., 1990; Lapierre et al., 1997; Li et al., 2014). The plate driving forces experienced fundamental changes during this time. Large-scale rifting started around the coastal South China, which resulted in the thinning of continental margin (Lei et al., 2019a). During the Eocene–early Miocene, southward slab pull of the oceanic crust led to elimination of the Proto-SCS (Hall, 2002, 2012; Morley, 2002; Hayes and Nissen, 2005). The present SCS was eventually generated, displaying a sequential pattern of continental margin–transitional crust–oceanic crust. During the long-term extensional and thermal-subsidence stages, thick sedimentary strata were formed over the SCS Basin basement during Cenozoic, and were influenced from different source-to-sink systems (Shao et al., 2015, 2017, 2019; Cao et al., 2018; Lei et al., 2018, 2019b). These sedimentary basins mainly include the Beibu Gulf, Yinggehai, Qiongdongnan and the Zhujiang River Mouth Basin (Briais et al., 1993; Rangin et al., 1995; Yao, 1996; Morley, 2002, 2016) (Fig. 1a).
Previous studies show that a number of East Asian and Southeast Asian continental terranes, e.g. the North China Block and the South China Block, the Indochina Block, have the origin of the northern Gondwana supercontinental margin (Veevers, 2004; Metcalfe, 2011; Cocks and Torsvik, 2013; Metcalfe, 2013). Separation, drifting and amalgamation of the Gondwana-related continental blocks as well as the accompanying oceans might have largely shaped the southeastern Eurasian margin before Cenozoic (Metcalfe, 1996). As a key region of the southeast Asia, the SCS Basin basement has been the focus of numerous plate geodynamic reconstructions (Zhou et al., 2008; Metcalfe, 2011; Morley, 2012; Zhu et al., 2017). However, there are still hot debates on its tectonic affiliation. Some researchers propose that the pre-Cenozoic lithospheric layer was partly derived from the South China terrain, i.e., discrete micro-fragments of the Beibu Gulf Basin together with the Hainan Island. These micro-terranes were inferred to commonly develop on a Proterozoic folded continental massif (Liu et al., 2006, 2011; Li et al., 2008c; Sun et al., 2014). Other scholars suggest that the SCS Basin basement is likely to be a missing linkage (or an individual tectonic domain) between the Cathaysia Block and Indochina Block, which was dated by scattered Precambrian ages (Lan et al., 2003; Wan et al., 2006; Lu et al., 2011; Yu et al., 2012). To be noted, the early Paleozoic metamorphism and magmatism have been confirmed within the Cathaysia Block and Indochina Block by massive geochronological and thermochronological studies (Lepvrier et al., 2008; Charvet et al., 2010; Wang et al., 2011; Vượng et al., 2013; Faure et al., 2014). Therefore, it also suggested that the SCS Basin basement dominated by Paleozoic strata, i.e., Yinggehai Basin, the east of Hainan Island, etc. was mainly sourced from the Paleozoic outer extension of the Cathaysia Block or Indochina Block. Previous studies mainly concentrated on the Zhujiang River Mouth Basin of the northeastern SCS were also reanalyzed (Qiu et al., 1996; Li et al., 1999; Xu et al., 2017). As complementary information, the data synthesis will largely enable us to obtain a more comprehensive interpretation across the whole basement (refer to Fig. 1 and Table 1 for geographical locations and more details of the published data).
Number in Fig. 1a Sample No. Depth/m Isotopic ages/Ma Analytical method Lithology References 1 HF28-2-1 3942.0–3943.6 109.25±2.40 K-Ar cataclastic granodiorite Li et al. (1999) 2 LF2-1A 2480.0–2483.5 100.38±1.46 K-Ar cataclastic dimicaceous granite Li et al. (1999) 2 LF2-1A 2480.0–2483.5 94.83±1.89 Rb-Sr cataclastic dimicaceous granite Li et al. (1999) 3 XJ24-3-1AX 4318–4319 98 K-Ar cataclastic granite Li et al. (1999) 4 HZ25-2-1X 3196.4 99.80±1.53 K-Ar cataclastic granite Qiu et al. (1996) 5 XJ24-1-1X 3851.0–3851.5 84 K-Ar granite Li et al. (1999) 6 HZ35-1-1 2218.9 105 K-Ar cataclastic quartz diorite Li et al. (1999) 7 HZ32-1-1 2791 88.5±3.6 K-Ar cataclasitc granite Li et al. (1999) 8 LH1-1-1A 1836.5 90.62±1.49 K-Ar cataclastic granodiorite Li et al. (1999) 8 LH1-1-1A 1 822.0–1837.5 72.78±1.37 Rb-Sr cataclastic granodiorite Li et al. (1999) 9 ZHU1 1 846–1 847 73–76 K-Ar granite Li et al. (1999) 10 ZHU2 2379–2380 70.5 K-Ar biotite granite Li et al. (1999) 11 ZHU5 3261.8–3262.3 75 K-Ar granite porphyry Li et al. (1999) 12 PY3-1-1 3192 90.7±3.3 K-Ar granite Li et al. (1999) 13 PY4-1-1 3160 130±5 K-Ar granite Qiu et al. (1996) 14 PY21-3-1 4068.0–4019.5 89.83±1.32 K-Ar cataclastic biotite granite Li et al. (1999) 15 EP18-1-1A 3448.25 100.5±1.7 K-Ar granite Qiu et al. (1996) 16 PY27-1-1 3607–3609 118.9±2.1 K-Ar quartz monzonite Li et al. (1999) 17 XJ17-3-1 2122–2124 79.2±2.8 K-Ar cataclastic granite Li et al. (1999) 18 YJ26-1-1 1700–1702 89.20±1.58 K-Ar rhylote porphyry Li et al. (1999) 19 SH2-1-1 3641.2 118 K-Ar biotite hornblende diorite Li et al. (1999) 20 KP9-1-1 1662–1774 153±6 K-Ar cataclastic granite Li et al. (1999) 21 LF354 2472.3 196.4±1.4 U-Pb granite Xu et al. (2017) 22 LF3521 2443.5 195.0±2.2 U-Pb diorite Xu et al. (2017) 23 S08-18-2 2700 159.1±1.6 U-Pb tonalite Yan et al. (2010) 24 S08-18-4 2700 157.8±1.0 U-Pb tonalite Yan et al. (2010) 25 S08-32-1 3100 153.6±0.3 U-Pb monzogranite Yan et al. (2010) 26 S08-32-3 3100 127.2±0.2 U-Pb monzogranite Yan et al. (2010)
Table 1. Summary of published data of SCS pre-Cenozoic basement (refer to Fig. 1 for detailed sample locations)
In order to verify the complicated nature of the basement structure and its regional tectonic significance, this study collected one biotite granite and eight metamorphic clastic rock samples for geochemistry and geochronology analysis together with petrography identification (refer to Table 2 for detailed sample information on detrital zircon U-Pb geochronology study). These samples were straight drilled through the pre-Cenozoic basement by the China National Offshore Oil Corporation.
Borehole No. Geographical location Sampling depth/m Depth of basement/m Previous geological
Lithology Y1 Yinggehai Basin 3021–3023 3021 ? 1264±83 Precambrian metamorphic biotite granite WC1711 Qiongdongnan Basin 2216 2216 Paleozoic 114±4 early Cretaceous meta-tuff CC211 Qiongdongnan Basin 1116 1076 Paleozoic 129±7 early Cretaceous meta-tuff KP111 Zhujiang River Mouth Basin 1893–1897 1884 Paleozoic 124±3 early Cretaceous volcaniclastic rock BD2311 Qiongdongnan Basin 2166 2166 Paleozoic 85±3 late Cretaceous metamorphic quartzose sandstone CC111 Qiongdongnan Basin 1172 1117 Paleozoic 125±4 late Cretaceous metamorphic sandstone YJ3511 Zhujiang River Mouth Basin 4321–4339 4321 Paleozoic 88±2 late Cretaceous metamorphic siltstone YJ3611 Zhujiang River Mouth Basin 3565–3580 3484 Paleozoic 88±2 late Cretaceous metamorphic mudstone and siltstone Note: ? represents not have been dated before.
Table 2. List of samples analyzed for Laser-ICP-MS detrital zircon U-Pb dating (refer to Fig. 1a for detailed sample locations)
Geochemical analyses for the SCS Basin basement metamorphic clastic rock samples were carried out at the State Key Laboratory of Marine Geology of Tongji University, China. After being washed by deionized distilled water, bulk sediments were then dried at 50°C for 48 h to prevent external contamination. Meta-sedimentary rocks were heated to 600°C for 2 h to remove the organic matter and interlayer water. Dissolved in a 1:1 HF-HNO3 mixture, samples were finally measured for their major and trace elemental concentration. The instrumentation was constituted by an inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher Scientific, USA) and Thermo Elemental X-Series inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific, USA), respectively. Three certified materials (GSR-5, GSR-6, and GSD-9 from the Institute of Geophysical and Geochemical Exploration, China) were repeatedly used as unknown samples for the precision and accuracy assessment. The external precision (1 σ) was generally better than 5% for trace elements (Fig. 2). And results were usually in agreement with the reference materials (modified from Li et al. (2003) and Wei et al. (2006)).
Figure 2. Primitive mantle-normalized trace element spider diagrams (a) and chondrite-normalized rare earth elements (REE) distribution patterns (b) for the SCS Basin basement samples. Trace elements of the primitive mantle and chondrite REE are from Sun and McDonough (1989) and Taylor and McLennan (1985), respectively. Average continental crust (ACC) are from Rudnick and Gao (2003).
In this study, zircon grains were separated from meta-sedimentary rocks using standard density and magnetic separation techniques at the laboratory of the Institute of Regional Geology and Mineral Resources, Hebei, China. Over 200 chosen zircons were randomly pasted on adhesive tape and then cast in an epoxy mount prior to the polishing process. Zircons were examined with cathodoluminescence images (CL) to determine their internal structures and microzonation positions (20–30 µm) (refer to Fig. 3 for CL images of representative samples). Measurements of U, Th and Pb were later conducted at the State Key Laboratory of Marine Geology of Tongji University, China. The instrumentation is composed of a Thermo Elemental X-Series ICP-MS coupled to a New Wave 213 nm laser ablation system (Shao et al., 2016). Typically, 5-scan cycles were conducted for each analysis with a spot diameter of 30 μm. The 91500 external standards ((1065.4
$ \pm $0.3) Ma) were interspersed with tested samples to calculate isotopic ratios by ICPMSDataCal (Wiedenbeck et al., 1995; Liu et al., 2010). The glass NIST 610 was used to optimize the machine as an internal standard. Accuracy of our analytical results was validated by reference material Plešovice with an age of (337.1±0.4) Ma (Sláma et al., 2008). Measured compositions have been corrected for common Pb using the method of Andersen (2002). The 206Pb/238U and 207Pb/206Pb ages were finally adopted for younger or older zircons of 1 000 Ma, respectively. Wherein, uncertainties on individual U-Pb analyses are quoted at the 1 σ level. Age distributions in this study and other published documents are visualized as histograms and kernel density estimation plots (Vermeesch, 2012).
Both gravimetric and magnetic measurements have been conducted in the study area. Free-air gravity anomalies were originally extracted from satellite-derived data of 2-min grid (Sandwell and Smith, 1997). Since there is an obvious density contrast around the Cenozoic-Mesozoic regional unconformity, free-air gravity anomalies are sensitive to undulations caused by rifting and faulting structures in the pre-Cenozoic basement. The total field magnetic anomalies also contain rich geological implications. However, they could possibly be biased at middle-to-low latitudes because of the oblique inclinations of induced magnetizations. To facilitate the precision of our data interpretation, this study processed the magnetic anomalies calculation (source from compilation by Geological Survey of Japan and Coordinating Committee for Coastal and Offshore Geoscience Programmes in East and Southeast Asia (CCOP), 1996) by reducing to the pole (RTP magnetic anomalies), which are sharper to the existence of magmatic bodies. Here this study has also compiled seismic profiles and reconstructed the major stratigraphic framework to generalize a set of models between travel time and depth. In addition, this study obtained the relationship between travel time and rock density by conducting an integrated investigation on different types of rocks over the northern SCS Basin basement. This gravity-magnetic-seismic joint inversion shows less uncertainties and artificialities, which is largely discriminative from both traditional stripping method and interactive interpretation model. By recognizing the existence of geological anomaly bodies, this study is able to better describe the pre-Cenozoic SCS Basin basement.