The tectonic evolution of the Qiongdongnan Basin in the northern margin of the South China Sea

The tectonic evolution of the Qiongdongnan Basin in the northern margin of the South China Sea

Journal of Asian Earth Sciences 77 (2013) 163–182 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.el...

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Journal of Asian Earth Sciences 77 (2013) 163–182

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

The tectonic evolution of the Qiongdongnan Basin in the northern margin of the South China Sea Bo Hu a, Liangshu Wang a,⇑, Wenbo Yan a, Shaowen Liu b, Dongsheng Cai c, Gongcheng Zhang d, Kai Zhong c, Jianxiang Pei e, Bin Sun a a

State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, No.163 Xianlin Road, Qixia District, Nanjing 210046, Jiangsu, China Key Laboratory of Coast and Island Development, MOE, School of Geographic and Oceanographic Sciences, Nanjing University, No.163 Xianlin Road, Qixia District, Nanjing 210046, Jiangsu, China c China National Offshore Oil Corporation, 25 Chaoyangmenbei Dajie, Dongcheng District, Beijing 100010, China d CNOOC Research Center, 6 Dongzhimenwai Xiaojie, Dongcheng District, Beijing 100027, China e CNOOC Nanhai West Corporation, P.O. Box 11, Potou District, Zhanjiang, Guangdong Province 524057, China b

a r t i c l e

i n f o

Article history: Received 23 January 2013 Received in revised form 9 August 2013 Accepted 15 August 2013 Available online 30 August 2013 Keywords: Qiongdongnan Basin Fault pattern Rift segment Pre-existing structural fabrics South China Sea

a b s t r a c t Qiongdongnan Basin is a Cenozoic rift basin located on the northern passive continental margin of the South China Sea. Due to a lack of geologic observations, its evolution was not clear in the past. However, recently acquired 2-D seismic reflection data provide an opportunity to investigate its tectonic evolution. It shows that the Qiongdongnan Basin comprises a main rift zone which is 50–100 km wide and more than 400 km long. The main rift zone is arcuate in map view and its orientation changes from ENE– WSW in the west to nearly E–W in the east. It can be divided into three major segments. The generally linear fault trace shown by many border faults in map view implies that the eastern and middle segments were controlled by faults reactivated from NE to ENE trending and nearly E–W trending pre-existing fabrics, respectively. The western segment was controlled by a left-lateral strike-slip fault. The fault patterns shown by the central and eastern segments indicate that the extension direction for the opening of the rift basin was dominantly NW–SE. A semi-quantitative analysis of the fault cut-offs identifies three stages of rifting evolution: (1) 40.4–33.9 Ma, sparsely distributed NE-trending faults formed mainly in the western and the central part of the study area; (2) 33.9–28.4 Ma, the main rift zone formed and the area influenced by faulting was extended into the eastern part of the study area and (3) 28.4–20.4 Ma, the subsidence area was further enlarged but mainly extended into the flanking area of the main rift zone. In addition, Estimates of extensional strain along NW–SE-trending seismic profiles, which cross the main rift zone, vary between 15 and 39 km, which are generally comparable to the sinistral displacement on the Red River Fault Zone offshore, implying that this fault zone, in terms of sinistral motion, terminated at a location near the southern end of the Yinggehai Basin. Finally, these observations let us to favour a hybrid model for the opening of the South China Sea and probably the Qiongdongnan Basin. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The knowledge of the opening mechanism of the South China Sea is vital to understanding the tectonic evolution of the Southeast Asia in the Cenozoic. Up to now, several tectonic models have been proposed for it, and these models largely fall into two main types: (1) the pull-apart model (Briais et al., 1993; Gilley et al., 2003; Leloup et al., 1995, 2001; Tapponnier et al., 1982, 1990, 2001) and (2) the slab-pull model (Hall, 2002; Lee and Lawver, 1995; Morley, 2002, 2012; Rangin et al., 1995a; Taylor and Hayes, 1980, 1983). In the pull-apart model, the opening of the South Chi-

⇑ Corresponding authors. Tel.: +86 25 83593561; fax: +86 25 83686016. E-mail addresses: [email protected] (B. Hu), [email protected] (L. Wang). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.08.022

na Sea is kinematically linked with the extrusion of the Indochina Block and associated left-lateral slipping of the Red River Fault Zone during 35–17 Ma, driven by the collision between India and Asia (Gilley et al., 2003; Leloup et al., 1995, 2001; Tapponnier et al., 1982, 1990, 2001). The slab-pull model, however, suggests that it is the subduction of a proto-South China Sea at the north Borneo Trench that results in the extension east of Vietnam and thus the opening of the South China Sea (Hall, 2002; Lee and Lawver, 1995; Morley, 2002, 2012; Rangin et al., 1995a; Taylor and Hayes, 1980, 1983). The northern passive continental margin of the South China Sea, composed of a series of rift basins, is an important region to document the evidences for the opening of the South China Sea. In particular, the well preserved pre-, synand post-spreading structures plus relatively high resolution Cenozoic biostratigraphic time scale and the well constrained spreading

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pattern of the South China Sea make it an ideal place to investigate the Cenozoic tectonic processes (Clift et al., 2001). There are many published studies of crustal structure (Kido et al., 2001; Nissen et al., 1995a, 1995b; Tsai et al., 2004), Cenozoic structures (zones) in basement (Zhou et al., 1995; Zhu et al., 2007) and crustal thinning mechanism (Clift et al., 2001; Davis and Kusznir, 2004; Hayes et al., 1995; Tsai et al., 2004; Yeh and Hsu, 2004). Despite this, more detailed structural studies are still required to define a reasonable mechanism for the opening of the South China Sea. The Qiongdongnan Basin, a Cenozoic rift basin located between the Red River Fault Zone and the northwest spreading centre of the South China Sea, may preserve some structural characteristics that could shed some light on the problem. The Qiongdongnan Basin is one of the Cenozoic sedimentary basins located in the northern margin of the South China Sea (Fig. 1a). Based on over 30 years of hydrocarbon exploration and research, there are already many published works on sequence stratigraphy (Gan et al., 2008; Wang et al., 1998; Wei et al., 2001; Xie and Ge, 1997), sedimentary fill (Lin et al., 2010; Xiao et al., 2003; Zhong et al., 2004a), faults and relay structures (Li and Zhu, 2005; Li et al., 2006; Xie et al., 2007), boundary fault growth (Yin et al., 2010; Yu and Duan, 2008), graben style (Long et al., 2010) and regional tectonics (Li et al., 2006; Long et al., 2010; Xie et al., 2007; Zhong et al., 2004b). In addition, Li et al. (2006), Sun et al. (2005) and Xie et al. (2008) have used scaled analogue models to investigate its structure and kinematic evolution. These previous studies provide an outline of the evolution of the Qiongdongnan Basin both spatially and temporally. However, we know few details about the rifting process of the Qiongdongnan Basin. For example, how did the rift zone in Qiongdongnan Basin initiate and propagate? Which

direction was the main extension direction of the rifting? And which tectonic processes were responsible for driving the rifting? In this paper, with the help of the updated 2-D seismic reflection data gathered mainly within the past 20 years, we try, for the first time, to resolve these questions, based on a detailed geometrical analysis of the fault system in the Qiongdongnan Basin. The results may also benefit hydrocarbon exploration in the Qiongdongnan Basin. 1.1. Geological setting Southeast Asia is a complex assembly of allochthonous continental lithospheric fragments (terranes), including the South China Block (Yangtze and Cathaysia), Indochina Block, Simao Block, Sibumasu Block and West Burma Block (Metcalfe, 2002, 2006). During the Mesozoic, the southeastern part of the Southeast Asia experienced two periods of tectono-magmatism movement: the Indosinian movement (251–205 Ma) and the Yanshanian movement (180–67 Ma) (Zhou et al., 2006). The Yanshanian movement was driven by the subduction of the paleo-Pacific Plate, and structurally dominated mainly by NE–SW-trending normal or strike-slip faults (Faure et al., 1996; Shu et al., 2006; Zhou et al., 2006). The Indosinian movement, however, was dominated by the suture between the South China Block and the Indochina Block (Metcalfe, 2006). The suture zone is composed of two main segments: the Somg Ma Suture Zone (from Late Permian to Early Triassic) and the Dian-Qiong Suture Zone (from Late Permian to Early Triassic) (Cai et al., 2008; Cai and Zhang, 2009). A small part of the DianQiong Suture Zone is suggested to lie in the central Hainan Island with a nearly E–W trend (Cai and Zhang, 2009; Li et al., 2002; Li

Fig. 1. Simplified regional tectonic map, based on Briais et al. (1993), Cai and Zhang (2009) and Zhu et al. (2007). RRFZ = Red River Fault Zone; BBGB = Beibu Gulf Basin; YGHB = Yinggehai Basin; QDNB = Qiongdongnan Basin; PRMB = Pearl River Mouth Basin; SCS = South China Sea; BBGRZ = Beibu Gulf Rift Zone; QDNRZ = Qiongdongnan Rift Zone; PRMRZ = Pearl River Mouth Rift Zone.

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et al., 2000a, 2000b) (Fig. 1). However, no data show how it extends into the offshore region to the east of the Hainan Island. The Red River Fault Zone is a first-order tectonic structure in the region around the study area. Published geochronologic studies of the onshore part of this fault zone show that at least two deformation phases occurred since it was formed: (1) a phase of left-lateral slipping during 35–17 Ma (Gilley et al., 2003; Harrison et al., 1992, 1996; Leloup et al., 1995, 2001; Leloup and Kienast, 1993; Scharer et al., 1994; Tapponnier et al., 1990; Wang et al., 2000; Zhang and Scharer, 1999) or after 21 Ma (Searle, 2006) and (2) a phase of right-lateral slipping from 8 to 5 Ma to present (Allen et al., 1984; Leloup and Kienast, 1993; Replumaz et al., 2001; Schoenbohm et al., 2006; To et al., 2000; Zhao, 1995). Studies of the offshore segment of the Red River Fault Zone in the Yinggehai Basin suggest a sinistral motion from at least 30 Ma to 16 Ma (Zhu et al., 2009) or to 5.5 (Rangin et al., 1995b), and a dextral motion (Zhu et al., 2009) or no dextral motion (Rangin et al., 1995b) to present. Compared with the broad agreement from geochronologic studies concerning timing of movements on the onshore and offshore segments of the Red River Fault Zone, estimates of their displacements are still controversial. The estimated sinistral strike-slip displacement of the onshore segment ranges from 100 to 1400 km, while the estimated dextral strike-slip displacement varies between 9 m and 40 km (Allen et al., 1984; Replumaz et al., 2001; Schoenbohm et al., 2006). There is still no evidence to indicate the amount of the strike-slip displacement on the offshore segment of the Red River Fault Zone. The opening of the South China Sea is considered to be the result of interactions of the Eurasian Plate, Pacific Plate and IndoAustralian Plate (Zhu et al., 2009). The southern continental margin of China is a passive continental margin formed during the opening of the South China Sea (Barckhausen and Roeser, 2004; Briais et al., 1993; Taylor and Hayes, 1980, 1983). Based on an interpretation of marine magnetic data, the timing of seafloor spreading in the South China Sea is interpreted as 32–15.5 Ma (Briais et al., 1993) or 31–20.5 Ma (Barckhausen and Roeser, 2004). 1.2. Major rift zones in the northern margin of the South China Sea The extension in the South China Sea is believed to have started in the Late Cretaceous to Early Paleocene (c. 75–60 Ma) and did not evolve into seafloor spreading until Oligo-Miocene (Clift et al., 2001). At the same time, at least four major rift basins were formed in the northern margin of the South China Sea, including the Pearl River Mouth Basin, Beibu Gulf Basin, Qiongdongnan Basin and Yinggehai Basin. Except for the Yinggehai Basin, which is suggested to be a pull-apart basin formed as a result of the sinistral displacement of the offshore part of the Red River Fault Zone (Zhong et al., 2004b; Zhu et al., 2009), the other three basins all are typical rift basins, and at least three typical rift zones, which are termed Pearl River Mouth Rift Zone, Beibu Gulf Rift Zone and Qiongdongnan Rift Zone respectively in this study (Fig. 1), are recognized to underlie the thick sediments in these basins (Zhou et al., 1995; Zhu et al., 2007). These three rift zones are oriented ENE and dominated by ENE-trending normal faults and grabens (Zhu et al., 2007), and are offset and separated from each other by un-faulted or relatively weakly faulted basement rocks (Fig. 1). These four rift basins can be grouped into two generations in terms of their formation times (Fig. 1). The first generation consists of the Pearl River Mouth Basin and Beibu Gulf Basin, in which rifting commenced in the Paleocene (Zhu et al., 2007). The second generation includes the Qiongdongnan Basin and the Yinggehai Basin where rifting initiated around 40 Ma (Li and Lu, 1984; Liu et al., 2009). Although obviously earlier than the onset of the seafloor spreading in the South China Sea, the second generation of rift zone began about 20 my later than the first generation (Table 1).

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1.3. Material and data quality The 2-D seismic reflection profiles and interpretation of fault populations and sedimentary sequences used in this study were provided by the Chinese National Offshore Oil Company (CNOOC) during 1979–2007 (Fig. 2). Except for few wells located in the southern margin of the Qiongdongnan Basin (Rift Zone), all other wells were unevenly distributed in the northern margin, and thus the interpretation of the sedimentary sequences on seismic profiles was mostly based on cuttings rather than cored material. However, 2  2 km to 8  8 km spaced 2-D seismic reflection profiles ensure the credibility of the interpretation of the main structure units and seismic sequences (Fig. 2). The dataset used for the fault population analysis in this paper is picked up from the interpreted seismic profiles which extend to a depth (in second) ranging between 6 s and 12 s two-way travel time (TWT). Except for the oldest sedimentary sequence (Upper Eocene), the age of all other sedimentary sequences is defined by planktonic foraminifera and calcareous nanoplankton data (Li and Lu, 1984; Liu et al., 2009), with an uncertainty of ±0.5 my (Clift et al., 2001). Although the oldest syn-rift section has not yet been sampled, its existence is supported by seismic reflection data and oilsource correlation analysis, and is generally considered to be Upper Eocene (Li et al., 2006; Wei et al., 2001; Zhong et al., 2004a). 1.4. Method The temporal evolution of fault populations can be studied by analyzing syn-sedimentary fault systems where sediment thickness changes across faults record faults displacements through time (Childs et al., 2003; Meyer et al., 2002). Application of this analysis requires particular conditions which most likely occur on small- to moderate-sized faults: (1) sedimentation rates should be equal to or exceed the displacement rates; (2) little or no erosion of the footwalls of faults (Meyer et al., 2002). However, when large-sized normal faults with fault displacement up to hundreds to thousands of metres are involved, the flexural unloading of footwall and the heating of subcrustal lithosphere will lead to significant footwall uplift, which in turn inevitably results in conspicuous erosion of the footwall (Kusznir et al., 1991; van der Beek et al., 1994, 1995). In areas lacking thermochronological data, such as sedimentary basins that are covered by thick sediments, an accurate estimate of the fault throw is impossible because of the difficulty in estimating the amount of the erosion on the footwall. As indicated by Fig. 3a, a normal fault throw consists of two components: the footwall uplift and the hanging wall subsidence. On the one hand, if the accommodation generated by the hanging wall subsidence is fully filled with sediments, then the thickness of the sediment layer can be considered as the amount of the hanging wall subsidence (Fig. 3b). On the other hand, the uplift of the footwall above the sea level will inevitably result in the erosion of the footwall and the formation of an escarpment system (Kusznir et al., 1991; van der Beek et al., 1994, 1995), therefore the amount of the footwall uplift can be calculated by adding the height of the escarpment and the amount of the erosion together (Fig. 3c). Among the three components of a normal fault throw, only the hanging wall subsidence is usually well preserved and can be estimated from the changes in sequence thicknesses across faults, while the height of escarpment and the amount of erosion are more ambiguous and usually impossible to estimate. Given the difficulty in determining the height of escarpment and the amount of footwall erosion and thus the uplift of the footwall, we use increments of hanging wall subsidence instead of increments of fault throw to study the temporal evolution of the

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Table 1 Geological time chart of the sedimentary strata of the Qiongdongnan Basin and the Yinggehai Basin, and the timing of rifting in the Pearl River Mouth Basin and Beibu Gulf Basin, sinistral motion on the Red River Fault Zone onshore and offshore and seafloor spreading in the South China Sea. The strata ages are delimited by planktonic foraminifera and calcareous nanoplankton data (Li and Lu, 1984; Liu et al., 2009). The Geologic Time Scale refers to the Gradstein et al. (2004). QDN = Qiongdongnan Basin; YGH = Yinggehai Basin; PRM = Pearl River Mouth Basin; BBG = Beibu Gulf Basin; RRFZ = Red River Fault Zone; SCS = South China Sea; Plank. Fora. = Planktonic foraminifera; Calca. Nano. = Calcareous nanoplankton. References are: 1, Li and Lu (1984); 2, Liu et al. (2009); 3, Zhu et al. (2007); 4, Gilley et al. (2003); 5, Harrison et al. (1992, 1996); 6, Leloup et al. (1995, 2001); 7, Leloup and Kienast (1993); 8, Scharer et al. (1994); 9, Tapponnier et al. (1990); 10, Wang et al. (2000); 11, Zhang and Scharer (1999); 12, Searle (2006); 13, Rangin et al. (1995a, 1995b); 14, Zhu et al. (2009); 15, Briais et al. (1993); 16, Barckhausen and Roeser (2004).

* Upper Eocene has not yet been sampled, but its existence is supported by seismic profile interpretation and oil-source correlation and is generally suggested as Later Eocene by Chinese researchers (Li et al., 2006; Wei et al., 2001; Zhong et al., 2004a).

fault systems in the Qiongdongnan Basin in this paper, although this method obviously degrades the ability of the relevant dataset to provide a detailed quantitative analysis. In spite of this, we believe that a semi-quantitative analysis of the temporal evolution of fault systems is still possible, because it is reasonable to consider that, for faults of same generation in an area, the relatively larger subsidence will indicate, for the first order, a relatively larger amount of normal fault throw, especially when the amount of hanging wall subsidence varies between tens of metres and thousands of metres. Fig. 4 demonstrates ideal geometries of normal faults and related syn-faulting sedimentary layers, and shows how we determine the increment of hanging wall subsidence in this paper. If

sediment compaction is not taken into consideration, the increment of the hanging wall subsidence during the deposition of the layer Li is Osi 1 (the vertical offset of the horizon Hi 1) minus Osi (the vertical offset of the horizon Hi) (Fig. 4a and b). If compaction is considered, a backstripped offset Osi (the vertical offset of horizon Hi) is used to calculate the increment of the hanging wall subsidence (Fig. 4c). The hanging wall subsidence data used for studying the rifting history of the Qiongdongnan Basin in this paper are picked from 273 data-collecting points, which are intersection points of the seismic profiles and the faults (Fig. 5), and consist of four groups of hanging wall subsidence increments (in seconds, two-way travel time, and thus without calculations of compaction) corresponding

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Fig. 2. Interpreted fault system in the study area and relevant seismic profile network. The black thick lines represent the seismic profiles shown in Fig. 6, and thick dashdotted lines represent the seismic profiles used for extension estimate shown in Table 2.

to four successive periods including the Late Eocene, Early Oligocene, Late Oligocene to early Early Miocene and late Early Miocene to present. As the datasets are used to show how the faults grow laterally, they are not converted from time (seconds, two-way travel time) to depth (metres), but this has no influence on the conclusion. 1.5. Terminology Transfer zones are considered to be important structural features in rift zones, through which adjacent fault segments and basin segments link and interact with each other (Morley et al., 1990; Peacock, 2002). In addition, transfer zones are potential hydrocarbon sources, migration channels and traps (Coskun, 1997; Dou and Chang, 2003; Fossen et al., 2010; Morley et al., 1990), thereby attracting considerable attention recently (Fossen et al., 2010; Hus et al., 2006; McClay et al., 2002; Peacock, 2002; Tesfaye et al., 2008). In order to make description and discussion more convenient, we here adopt the classification of transfer zones proposed by Tesfaye et al. (2008), which uses ‘‘accommodation zone’’ to define the region between adjacent overlapping rift segments and uses ‘‘relay zone’’ to define the area between interacting, overlapping fault segments. Moreover, relay zones can be further sub-divided into relay ramp, small narrow horst and small narrow graben (Fossen et al., 2010). 1.6. Outlines of the Qiongdongnan Basin In cross-sections, the basement of the Qiongdongnan Basin is cut mainly by high angle normal faults, and no strike-slip faults are identified, indicating that the Qiongdongnan Basin is a typical rift basin (Fig. 6). Recent studies indicate that the evolution of the Qiongdongnan Basin can be divided into two stages: the rifting stage from 40.4 Ma to 20.4 Ma and the post-rifting stage from 20.4 Ma to the present (Clift and Sun, 2006; Li et al., 2006; Wei et al., 2001; Xie et al., 2007, 2008; Yuan et al., 2008; Zhong et al., 2004a). During the rifting stage, the basin subsided under the control of the active normal faults, while during the post-rifting stage,

the basin further subsided but only relatively very weak faulting activity occurred (Fig. 6). In cross-sections, it can be seen that the wedge-shaped syn-rift strata are overlain by the relatively uniform and flat post-rift strata which gently thin to the basin margin, and the separation surface (S60) between them is usually shown as an angular unconformity at the basin margins and a conformity at the basin centre (Wei et al., 2001) (Fig. 6). Both of the syn- and post-rift sediments with a thickness up to 10,000 m and 8000 m respectively have been sampled by wells, and their ages are delimited by planktonic foraminifera and calcareous nanoplankton data, ranging from Early Oligocene to present (Li and Lu, 1984; Liu et al., 2009). The Qiongdongnan Basin, although filled by more than 10 km thickness of sediment, is now a typical deep-water basin with the water depth greater than 2500 m. Basement rocks have been sampled by many wells in the study area, and include of granite, migmatite, hornfels, tuff, agglomerate, dolomite, andesitic porphyrite, dacite and rhyolite, which are all pre-Cenozoic in age (Wei et al., 2001; Zhong et al., 2004a). The basement is separated from the sedimentary cover by a large unconformity (Wei et al., 2001) (S100 in Fig. 6), which is shown as prominent high amplitude reflections that separate chaotic seismic units below from relatively continuous reflections above in seismic profiles (Fig. 6). The syn-rift strata consist of three sedimentary layers: the ‘‘Upper Eocene’’, Yacheng Formation and Lingshui Formation (Wei et al., 2001; Zhong et al., 2004a) (Table 1). The sedimentary layer below the Yacheng Formation in seismic profiles has not yet been encountered by wells within the study area untill now (Li et al., 2006; Zhong et al., 2004a). As in all wells in study area, including ones which were drilled into the basement rocks, no sediments dated as Paleocene are found, so this layer of sediments is generally viewed as ‘‘Upper Eocene’’. The ‘‘Upper Eocene’’ is supposed to primarily consist of lacustrine deposits (Li et al., 2006; Zhong et al., 2004a), and in seismic profiles, it is mainly shown as wedge shaped layers (Fig. 6). The Yacheng Formation was sampled by many wells and consists of alluvial deposits, coastal plain deposits, shore face deposits and shallow marine deposits (Zhong et al., 2004a). In seismic pro-

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in some case, it is difficult to be sure if there is any hard linkage between neighbouring faults or fault segments. Such cases are considered to no hard-linkage between faults in map view. 2.1. Major faults Several major faults or fault zones dominated the evolution of the Qiongdongnan Basin during the rifting stage. In the following descriptions, these major faults are divided into three groups, as discussed below.

Fig. 3. Schematic illustrations show the components of a normal fault throw. (a) The throw of a normal fault can be considered as the sum of the footwall uplift and the hanging-wall subsidence. (b) A normal fault may experience several phases of faulting, 3 times, for example. When the footwall erosion is not taken into consideration, the total throw is the sum of the three phases of throw increment, each of which is composed of an uplift increment and a subsidence increment. (c) The footwall uplift is the sum of the footwall erosion and the height of the escarpment on the foot wall.

files, it is mainly wedge shaped and bounded by normal faults at its thickest end, and its top surface (S70) is usually shown as a truncation surface at structural highs and a conformity at structural lows (Fig. 6). The Lingshui Formation is also found in many wells and is mainly composed of delta deposits, shore face deposits and shallow marine deposits (Zhong et al., 2004a). Its top surface (S60) is interpreted as one of the large unconformities in the basin (Wei et al., 2001), and indicates the cessation of the rifting in the Qiongdongnan Basin (Fig. 6). The stratigraphic units formed during the post-rifting stage are outside the scope of this work and do not affect the conclusions. 2. Structures in the main rift zone A main rift zone is found at the centre of the Qiongdongnan Basin (light yellow area in Figs. 7 and 8). Fig. 7 maps the fault system as well as the relief of the basement (S100), and shows the spatial relationship between them. Fig. 8 is a 3-D version of Fig. 7. However, owing to the large spacing between the 2-D seismic profiles,

2.1.1. Fault Group A This group of faults is located in the westernmost part of the Qiongdongnan Basin, and includes the No. 1 Fault, No. 3 Fault, No. 20 Fault, Faults F, Fault G and Fault H (Figs. 7 and 8). The No. 1 Fault is a boundary fault that separates the Yinggehai Basin to the southwest from the Qiongdongnan Basin to the northeast (Figs. 7 and 8). In cross-sections, it is a southwest-dipping, high angle fault with some normal displacement (Clift and Sun, 2006; Zhu et al., 2009). However, studies from Clift and Sun (2006), Zhong et al. (2004b) and Zhu et al. (2009) show that the No. 1 Fault is a strike-slip fault which has experienced several phases of deformation in the Cenozoic, including both sinistral and dextral movements. The slip sense history of the No. 1 Fault and other parallel or sub-parallel strike-slip faults found in the Yinggehai Basin indicates that they might be major offshore extensions of the Red River Fault Zone (Clift and Sun, 2006; Zhong et al., 2004b; Zhu et al., 2009). Because it is deeply buried by sediments and badly imaged by seismic profiles in the Qiongdongnan Basin, the No. 1 Fault is traceable on seismic profiles but shows few structural details of the strike-slip movement (Fig. 6). The Fault F, No. 3 Fault, Fault G, No. 20 Fault and Fault H are E– W- or nearly E–W-trending faults located to the northeast of the No. 1 Fault (Figs. 7 and 8). They terminate to the west against or nearly against the No. 1 Fault (Figs. 7 and 8). The first four faults dip to south while the last one dips to north, and thus a full graben formed between the last two faults (Fig. 8). The No. 3 Fault, Fault G, No. 20 Fault and Fault H have a fault length ranging between 30 and 60 km, and have a maximum normal displacement of 4800, 3700, 5400, 2700 m respectively. 2.1.2. Fault Group B This group of faults is situated in the middle part of the Qiongdongnan Basin, and includes the No. 2 Fault, No. 5 Fault, No. 6 Fault, No. 11 Fault, No. 14 Fault, Fault C, Fault D and Fault E (Figs. 7 and 8). In cross-sections, each of these faults has a high angle fault plane with normal displacement and shows no indication of strikeslip movement. In map view, they are NE or nearly NE trending, except the No. 14 Fault which is NW striking. The Fault C, Fault D, Fault E and No. 14 Fault have lengths around 40 km and a maximum normal displacement of 1800, 3700, 4900 and 4200 m respectively. The No. 2 Fault, No. 5 Fault, No. 6 Fault and No. 11 Fault have lengths ranging between 70 and 140 km and a maximum normal displacement of 9700, 5600, 4200 and 8000 m respectively. Both in map view and cross-sections, some of these major faults have multiple fault branches (Figs. 6 and 7). The No. 2 Fault has a branch that extends off the main fault at a point near its centre, which divides into even smaller branches (Fig. 7). The No. 5 Fault has many small branches along the main fault (Fig. 7). The No. 11 Fault shows bifurcated fault tips on both ends (Fig. 7). Another feature of these faults is that, in map view, almost all of their tips are curved, in most cases, towards the dip direction of the main faults (Fig. 7). However, except for small bends along these major faults, their fault traces in map view are quite straight (Fig. 9).

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Fig. 4. Schematic illustrations show geometries of ideal normal faults and relevant syn-faulting sedimentary layers in cross-sections. (a) The footwall as well as the hanging wall had successively received three phases of syn-faulting sediment before the deposition of the post-faulting sedimentary layer L4. (b) Nondeposition or erosion occurred to the footwall during the deposition of L1. (c) Two strata columns C1 and C2, according to the sedimentary strata on the footwall and the hanging wall respectively, are used for backstripping the hanging-wall subsidence. Keep the basements of the footwall and the hanging wall fixed, strip off the sedimentary layers above the target layer, and then decompact the left layers.

2.1.3. Fault Group C This group of faults, located in the eastern part of the Qiongdongnan Basin, include En echelon Fault Zone A and En echelon Fault Zone B (Figs. 7 and 8). Both of the fault zones are composed of several E–W-trending or nearly E–W-trending, isolated fault segments, which, in map view, have a largely en echelon pattern within each fault zone (Figs. 7 and 8). Owing to the resolution of our data, it seems that the neighbouring fault segments in these fault zones are soft-linked, and no obvious breaching fault is found in the relay zones between them (Fig. 6). In cross-sections, all the fault segments are typical normal faults. Most of the fault segments in En echelon Fault Zone A dip to south while most of the fault segments in En echelon Fault Zone B dip to north, thus forming a complex full graben (Changchang Sub-basin) between the two fault zones (Fig. 6). Figs. 1 and 7 show that this group of faults is located in an area just between the Fault Group B and the northwest spreading centre of the South China Sea. The En echelon Fault Zones A and B are connected to the Fault group B by a soft linkage (Fig. 7). However, because the seismic profiling surveys only cover the western part of the Fault Group C, the structural features of its eastern part and the

way they are connected to the northwest spreading centre of the South China Sea remain uncertain. The portions of the En echelon Fault Zone A and B imaged by seismic data are nearly 150 and 200 km long respectively (Fig. 7). The maximum normal displacement is 3200 m for the En echelon Fault Zone A and 5100 m for the En echelon Fault Zone B. In map view, most of the fault segments demonstrate local bends along strike and curved fault tips which, in most cases, bend to varying degrees to the centre of the Changchang Sub-basin (Fig. 7). On the other hand, however, if all the curved tips and local bends along fault plane are excluded, it can be seen that some of the fault segments show a linear or slightly curved geometry in map view (Fig. 9). However, there are also some fault segments that are curved. 2.2. Major sub-basins in the main rift zone Five major subsidence centres are recognized in the main rift zone, and they are termed the Ledong Sub-basin, Lingshui Sub-basin, Beijiao Sub-basin, Baodao Sub-basin and Changchang Sub-basin in unpublished Chinese reports (Figs. 7 and 8). They all

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Fig. 5. The distribution of the data-collecting points and relating faults and seismic profiles.

formed during the rifting stage, bounded and controlled by the major faults described above, and separated from each other by accommodation zones (Figs. 7 and 8). In the areas flanking the main rift zone, numerous smaller half-grabens and full-grabens developed coevally with the five major sub-basins, but they are usually separated from each other as well as the main rift zone by surrounding structural highs with little or no deformations (Fig. 6). The Ledong Sub-basin is an E–W-oriented full graben which is controlled mainly by two normal faults (the No. 20 Fault and Fault H) to the north and south respectively, and by a strike-slip fault (the No. 1 Fault) to the southwest (Figs. 7 and 8). The Lingshui Sub-basin is a complicated half-graben which is mainly controlled by the Fault D and No. 14 Fault on the northwest and northeast respectively. The Beijiao Sub-basin is a half-graben which is controlled by the No. 11 Fault on the southeast (Figs. 7 and 8). The Baodao Sub-basin is a complicated NE–SW-extending half-graben which is controlled by the No. 2 Fault on the northwest (Figs. 7 and 8). The Changchang Sub-basin is a complex E–W-oriented full-grabenw that is controlled by two en echelon fault zones (the En echelon Fault Zone A and B) on the north and the south respectively, and its eastern part is beyond the coverage of our dataset (Figs. 7 and 8). 2.3. Accommodation zones in the main rift zone According to the definitions in Section 1.5, there are several major accommodation zones that are recognized in the main rift zone, including AZ 1, AZ 2, AZ 3, AC 1 and AC 2 (Figs. 7 and 8). AZ 1 is an accommodation zone that is quite similar to a kind of low-strain accommodation zone suggested by McClay et al. (2002), which is formed by interlocking arrays of oppositely dipping faults. As shown in Fig. 8, AZ 1 is a NW–SE-trending structural zone into which many faults extend from surrounding areas and terminate. For example, Fault C, Fault D, Fault G and No. 3 Fault all have one tip terminated in AZ 1 (Figs. 7 and 8). In addition, within the AZ 1, the northwest-dipping Fault C and south-dipping Fault G

nearly encounter each other, and the same happens again between the southeast-dipping Fault D and north-dipping F1 (a small fault in the footwall of the No. 20 Fault) (Fig. 7). Given the resolution of our data, it seems that no hard linkage takes place on the two pairs of approaching faults. AZ 2 is an anticlinal-like high relief zone separating the Lingshui Sub-basin from the Baodao Sub-basin (Figs. 7 and 8). It starts from an area between the Fault D and No. 2 Fault or an area between the Fault C and No. 6 Fault, and terminates at an area near the No. 11 Fault. The NW–SE-striking AZ 2 is normal to or at a high angle to the local trend of the main rift zone (Figs. 7 and 8). Structurally similar to AZ 1, a series of faults extend into and terminate in the AZ 2, such as the Fault C, Fault D, No. 2 Fault and No. 6 Fault (Figs. 7 and 8). However, different from AZ 1, AZ 2 is partially breached by the NW–SE-trending No. 14 Fault which is largely parallel to it. Given the resolution of our data, it appears that hard linkage between neighbouring faults rarely takes place. AZ 3 is a transfer zone of different preferred fault trends, across which the dominating trends of faults change from NE–SW in the Baodao Sub-basin to nearly E–W in the Changchang Sub-basin (Figs. 7 and 8). Unlike AZ 1 and AZ 2 which are fault-complicated structural-high zones, AZ 3 is shown to be a typical graben both in map view and in cross sections. AC 1 and AC 2 are a kind of low-relief accommodation zone suggested by Rosendahl (1987), which resulted from the subsidence of a pair of competing face-to-face half-grabens and is shown to be an anticlinal-like structure. The major difference between the origins of AC 1 and AC 2 is the extent to which the relative face-to-face half-grabens overlap with each other. In the case of AC 1, the extent of the overlapping between the Beijiao Sub-basin and the Lingshui Sub-basin is less than 20%, when taking into consideration the relative geometrical relationship between the graben-controlling faults of the two sub-basins, i.e. between the No. 11 Fault and No. 14 Fault or Fault D, and the trend of AC 1 is at a large angle to the trend of these graben-controlling faults (Fig. 8). In the case of AC 2, the extent of the overlapping between the Beijiao Sub-basin and the Baodao Sub-basin is more than 50%, i.e. the overlapping

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Fig. 6. Interpreted seismic profiles show the major faults and relating syn-faulting sedimentary sequences. See Fig. 2b for location.

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Fig. 7. Schematic map shows the main rift zone, major border faults, major sub-basins and main accommodation zones. The base map is a shaded relief map of the basement surface. The green line represents the No. 1 Fault which is the only one strike-slip fault in the study area. The black lines represent the normal faults dipping to northwest, north or northeast. The grey lines represent the faults located in the shoulders of the main rift zone. The red lines represent the normal faults dipping to southwest, south or south east. The yellow area represents the main rift zone and its borderline is the 7000 m contour (depth below sea-level) of the basement surface. The purple area represents the shoulders of the main rift zone. The blue rectangular areas are major accommodation zones in the main rift zone. LDSB = Ledong Sub-basin; LSSB = Lingshui Sub-basin; BJSB = Beijiao Sub-basin; BDSB = Baodao Sub-basin; CCSB = Changchang sub-basin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. A 3D version of Fig. 7 shows the main structural features of the main rift zone. The spatial relationships among the subsidence area, major border faults and major accommodation zones are shown. The green line represents the No. 1 Fault which is the only one strike-slip fault in the study area. The black lines represent the normal faults dipping to northwest, north or northeast. The red lines represent the normal faults dipping to southwest, south or south east. The green rectangles and the triple-junction-like polygon represent major accommodation zones. LDSB = Ledong Sub-basin; LSSB = Lingshui Sub-basin; BJSB = Beijiao Sub-basin; BDSB = Baodao Sub-basin; CCSB = Changchang sub-basin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.1. Faulting during Late Eocene Fig. 10a shows the distribution of the hanging wall subsidence during the beginning (40.4–33.9 Ma) of rifting in the Qiongdongnan Basin. At many data-collecting points, the value of the hanging wall subsidence is zero or nearly zero, suggesting that many faults or fault segments did not form or were beginning to form at the end of this period, while the value at the other data-collecting points ranges from small to moderate (100–1240 s). In this stage, active faults were primarily located in the western and the central parts of the study area, and the dominant trend was NE to NEE. The faults were sparsely scattered and uniformly distributed, and both the number and the length of the active faults were relatively small. It can be seen that most border faults of the main rift zone initiated during this period.

3.2. Faulting during Early Oligocene Fig. 10b maps the increment of the hanging wall subsidence from 33.9 to 28.4 Ma. It shows that the number, density, length, fault throw and distribution area of the active faults are obviously larger than shown by Fig. 10a. The fault traces were extended by both the gradual propagation of fault tips and the linkage of neighbouring fault segments (Figs. 10b and 11). The geometry of all the major faults or fault zones was established during this period, and thus the main rift zone was fully formed (Fig. 10b). The preferred orientations of the active faults show differences between different regions, i.e., for the faults in the region to the west of the 111°E, the preferred trend is NE to NEE, while it becomes nearly E–W for that to the east of the 111°E (Fig. 10b). The increments of the hanging wall subsidence range from small to large (100–2873 s).

3.3. Faulting during Late Oligocene to early Early Miocene Fig. 10c maps the hanging wall subsidence increment between 28.4 and 20.4 Ma. It shows that, at some data-collecting points, the value of the hanging wall subsidence decreased to zero or nearly zero, accordingly, the fault trace of a few faults retreated or became resegmented, and several small faults even stopped faulting during this period (Fig. 10c). The hanging wall subsidence ranges from small to large (100–3160 s).

3.4. Faulting during late Early Miocene to the present Fig. 9. The border faults or fault zones that show generally linear shape in map view.

between the No. 11 Fault and No. 2 Fault, and the trend of AC 2 is at a small angle to the trend of the relevant graben-controlling faults (Figs. 7 and 8).

During this period (from 20.4 to 0 Ma), faulting activity was much weaker than before (Fig. 10d). Fault tip retreat and fault extinction are more obvious than the former period (Fig. 10d), and the hanging wall subsidence ranges from small to moderate (100–1272 s).

4. Discussion 4.1. Tectonic evolution during rifting stage 3. Evolution of the fault system Based on the dataset recording the increments of hanging wall subsidence during the different phases of the rifting, the evolution history of the fault system in the Qiongdongnan Basin is described in this section. The increments of hanging wall subsidence are mapped in Fig. 10. Fig. 10a–d shows the hanging wall subsidence and the suite of figures shows the whole evolutionary history of fault growth and strain distribution throughout the rifting stage.

As shown in Section 1.6, the basement of the Qiongdongnan Basin is composed of pre-Cenozoic rocks, and the oldest sedimentary layer sampled by wells in the study area is of Early Oligocene age (Wei et al., 2001; Zhong et al., 2004a), so there is a time gap of at least 30 my between the two strata. On the other hand, seismic profile interpretation indicates that there is a layer of sediments below the Lower Oligocene and above the basement in the basin centre (Fig. 6). Because of its wedge shapes shown in cross-sections and an indication from oil-source correlation study, it is generally

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Fig. 10. An evolutionary suit of fault maps show the evolution of the fault system in the Qiongdongnan Basin. The black polygons present the faults or fault segments with the width being scaled to the estimated hanging-wall subsidence increment (in second) at the data-collecting points. DCP = data-collecting points. (a) The Late Eocene fault map. (b) Early Oligocene fault map. (c) Late Oligocene to early Early Miocene fault map. (d) Late Early Miocene to present fault map.

considered to be the oldest syn-rifting sedimentary layer of Late Eocene age (Wei et al., 2001; Zhong et al., 2004a). These arguments imply that the basement of the Qiongdongnan Basin had been standing above sea level and suffering denudation before the onset of the rifting in the Late Eocene, and this situation possibly can be traced back to the Paleocene. The onset of the rifting was around 40 Ma (Table 1). From around 40.4 to 33.9 Ma, rifting activity and associated subsidence took place within a limited region mainly located in the western and central part of the study area, as shown by the distribution of the active faults at that time (Fig. 10a). Sparsely distributed normal faults and half grabens are mainly NE–ENE trending (Fig. 10a).

The syn-rift deposits in these half grabens are dominated mainly by lacustrine facies (Li et al., 2006; Zhong et al., 2004a). However, the outline of the main rift zone was not clear at the time, and there were large areas of structural highs standing above sea level within or around the rift zone. From 33.9 to 28.4 Ma, the active rift zone and the subsided area were enlarged. The area prominently influenced by active faults at the time extended from the western and central part into the eastern part of the study area (Fig. 10b). Accordingly, fault patterns changed greatly. Except for the central part of the study area where the dominant trend of faults remained NE to ENE, both the western and the eastern parts were dominated by nearly E–W-trending

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Fig. 10 (continued)

faults (Fig. 10b). At the end of this period, the border fault system of the main rift zone was already well developed, and nearly all faults in the study area reached their greatest length (Fig. 10b). Fluvial, open shoreline and shallow marine environments are recorded in the basin during this period (Zhong et al., 2004a). A few structural highs, such as the accommodation zone AZ 2 and some parts of the shoulders of the main rift zone, remained above or near sea level (Fig. 6), indicating that fault-controlled subsidence was still the most important mechanism for the crust thinning at the time. In particular, at around 30 Ma the Qiongdongnan Basin started to be bounded by a few NW-trending, left-lateral strike-slip faults on the west till 16 Ma, such as the No. 1 Fault (Clift and Sun, 2006; Zhu et al., 2009).

From 28.4 to 20.4 Ma, subsidence took place in a further enlarged area, except for few structural highs on the shoulders of the main rift zone, as shown by the distribution of the Lingshui formation (Fig. 6). Accordingly, the fault system changed its style again, i.e. fault extinction and retreating of fault tips happened on some faults while the rest of the faults maintained their trace length (Fig. 10c). River mouth environments and shallow marine environments are recorded (Zhong et al., 2004a). The deposition of the Lingshui formation on some active horsts implies either thermal contraction-derived subsidence started to play an important role in the subsiding or crustal thinning by some mechanisms other than normal faulting took place and overcame the uplift by the faulting (Fig. 6).

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Fig. 11. The throw profiles of several major faults or fault segments. In these profiles, the hanging-wall subsidence is served as a proxy for the fault throw to plot the chart. The locations of these normal faults are shown in Fig. 5.

From 20.4 Ma to the present, the Qiongdongnan Basin was in a phase of thermal subsidence with only minor normal faulting (Fig. 10d). 4.2. Pre-rift structures in the basement Before further discussion of the initiation of the Qiongdongnan Basin, a knowledge of pre-existing structures in the basement is needed. However, due to the poor quality of the seismic data below horizon S100, interpretation of pre-existing structures in the basement of the Qiongdongnan Basin is not possible, so some other indirect methods of analyzing the nature of pre-existing structures are needed. We here suggest that the trace of structural features in map view would shed some lights on the problem. Rosendahl (1987) suggests that graben-bounding faults usually tend to be arcuate in plan view, and provides three possible explanations for this. Firstly, it is a consequence of the combination of unequal development of four ideal fault sets and trackline aliasing of an original orthorhombic fracture pattern. Secondly, it is formed by trackline aliasing of composite pull-apart fracture geometries. The last explanation is simply that the arcuate forms are real. Martel (1999) proposes that mechanical controls, such as variation in stress drop or in fault strength, heterogeneity in host-rock stiffness and interaction with other faults, may cause non-uniform rotation along a fault, thus curving the faults. In addition, when the slip of a fault is in the order of metres and the length of it is hundreds of metres or larger, the mechanisms of the fault growth will be dominated by linkage of neighbouring faults (Crider and Peacock, 2004). A fault growing in this way usually shows along-strike bends or even a corrugated fault trace in map view (Ferrill et al., 1999; McClay et al., 2002; Peacock, 2002). In a conclusion, faults naturally have a strong trend to grow into corrugated or arcuate geometries. However, not all faults are arcuate or corrugate, that is to say, there must be some mechanisms that may prevent faults and relat-

ing structures from evolving into arcuate forms, and pre-existing structural fabrics, which had formed in the basement before rifting, might be the commonest. Taking Devils Lane Graben as an example, its graben-bounding faults are controlled by a regional set of pre-existing, N–S-trending joints, and thus each fault shows a linear geometry in map view (Fossen et al., 2010). The linear grabenbounding faults are also shown by an analogue modelling from McClay et al. (2002). The modelling shows that a significant portion of the graben-bounding faults are formed by the linkage of precursory coplanar fault segments which are parallel to the basement structural fabrics, and show a broadly linear geometry after linkage (McClay et al., 2002). In addition, a straight narrow horst is formed at the stepover of two oppositely dipping graben-bounding faults in the Devils Lane Graben under the control of the preexisting joints (Fossen et al., 2010). We suggest that pre-existing structural fabrics had also played a significant role in the evolution of the Qiongdongnan Basin, because not all the major graben-bounding faults are as exactly as predicted by the models of Rosendahl (1987) and Martel (1999), instead, many of them consist of linear or very gently curved segments in map view when local curvatures along strike are overlooked, typically the No. 2 Fault, the No. 5 Fault, the No. 6 Fault and the No. 11 Fault (Fig. 9). In addition, there are some other structures that also show a linear geometry, such as the Horst 1 in Fig. 9. Horst 1 is a narrow zone of structural high located between the No. 2 Fault and the No. 6 Fault, with a maximum length up to 100 km and a width ranging between 1 and 20 km (Fig. 12). The western part of the Horst 1 is a typical narrow horst which shows a quite linear form on the whole if local curvatures along strike are disregarded (Figs. 9 and 12). These examples imply that a set of NE–SW- to ENE–WSW-trending, pre-existing structural fabrics existed in the basement before the rifting and controlled the formation of at least part of NE–SW- to ENE–WSW-trending faults in the Qiongdongnan Basin. Similarly, a set of nearly E–Wtrending pre-existing structural fabrics was speculated to control

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the formation of the En echelon Fault Zone A and En echelon Fault Zone B (Fig. 9). Because NE–SW-trending normal faults and wrench faults are typical structural features of the Jurassic–Cretaceous Yanshanian event (Faure et al., 1996, 2009; Ren et al., 2002; Yin, 2010; Zhou and Li, 2000), we tentatively suggest that the NE–SW- to ENE– WSW-trending, pre-existing structural fabrics are the relics of this event. The nearly E–W-trending, pre-existing structural fabrics, on the other hand, are tentatively suggested to be the relics of the Indosinian event, as implied by the nearly E–W-trending DianQiong Suture Zone exposed on the central Hainan Island (Cai and Zhang, 2009; Li et al., 2002; Li et al., 2000a, 2000b). 4.3. Segmentation and formation of the main rift zone The main rift zone can be sub-divided into three segments, as shown by the structural patterns (Figs. 7 and 8). The western segment is dominated mainly by nearly E–W-trending structures, such as the Fault F, Fault G, Fault H, No. 3 Fault, No. 20 Fault and

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Ledong Sub-graben (Figs. 7 and 8). The middle segment is mainly composed of NE–NEE-trending structures, including the Fault C, Fault D, Fault E, No. 5 Fault, No. 6 Fault, No. 2 Fault, No. 11 Fault, Lingshui Sub-graben, Beijiao Sub-graben and Baodao Sub-graben (Figs. 7 and 8). The eastern segment is again dominated mainly by nearly E–W-trending structures, such as the En echelon Fault Zone A, En echelon Fault Zone B and Changchang Sub-graben (Fig. 7a and b). In addition, these three rift segments are separated from each other by two accommodation zones, AC 1 and AC 3 (Figs. 7 and 8). Taking into consideration the arguments discussed in previous section, it can be suggested that the fault formation of the middle segment of the main rift zone as well as the flanking areas have been significantly influenced by NE–SW- to ENE–WSW-trending, pre-existing structural fabrics, and the fault formation of the eastern segment have been mainly controlled by the nearly E–Wtrending, pre-existing structural fabrics. These faults were either reactivated pre-existing fabrics or newly formed faults but influenced by it. In fact, NE–SW- to ENE–WSW-trending normal faults

Fig. 12. The No. 2 Fault, No. 6 Fault and Host 1 in map view (top) with their interpretations on seismic profiles (bottom).

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Fig. 13. A schematic tectonic model shows the evolution of the main rift zone. (a) Before the rifting taking place, at least two sets of structural fabrics had already existed in the basement of the Qiongdongnan Basin; (b) at around 40 Ma, the Xisha Block started to move toward southeast plus a slight anti-clockwise rotation. From 40 to 33.9 Ma, the rifting process concentrated mainly in the western and central parts of the study area, and normal faults were mainly parallel to the NE- to ENE-trending, pre-existing structural fabrics; (c) during 33.9–28.4 Ma, the rifting extended from the western and central parts to the eastern part of the study area, in which the rifting was influenced by the nearly E–W-trending, pre-existing structural fabrics and two en echelon fault zones was formed. The sinistral No. 1 Fault was also formed during this period, and constructed a local extensional stress field with the maximum principle tensile stress in nearly north–south direction in the western part of the study area where a group of nearly E–W-trending faults was formed; (b-i) and (c-i) show a fault pattern developed by analogue model in orthogonal extension (McClay et al., 2002); (c-ii) show a fault pattern developed by analogue model in 60°oblique extension (McClay et al., 2002); (c-iii) The plan view arrangement of en echelon normal faults associated with an idealized left lateral strike-slip fault (modified from Allen and Allen (2005)).

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Table 2 Estimates of extension in the Qiongdongnan Basin for different rifting stages. SD = Stretched distance, back-stripped sum of the heaves of the interpreted faults on each seismic profile. SF = Average stretch factor of the whole profile. See the location of the profiles in Fig. 2b. Profiles

Profile length (m) 40.4–33.9 Ma 33.9–28.4 Ma 28.4–20.4 Ma

Time

SD (m) SF SD (m) SF SD (m) SF

SP02

SP04

SP08

SP09

SP011

SP12

SP14

154,651 7468 1.06 19,961 1.16 27,470 1.22

194,028 4995 1.03 14,251 1.08 23,099 1.14

220,106 7822 1.06 23,016 1.17 38,898 1.29

243,943 3083 1.01 16,880 1.08 29,901 1.14

192,461 685 1.00 8517 1.06 15,129 1.10

257,850 3513 1.02 9261 1.04 16,926 1.07

273,825 10,507 1.05 24,466 1.12 35,632 1.18

and nearly E–W-trending normal faults also dominate the Pearl River Mouth Basin and Beibu Gulf Basin (Lister et al., 1986; Sun et al., 2009, 2010; Zhou et al., 1995; Zhu and Jiang, 1998). The fact that there are similar preferred trends for normal faults in different generations of the rift basins is evidence that pre-existing structures did play important roles during their formation processes. Except for being possibly influenced by nearly E–W-trending, pre-existing structural fabrics, as indicated by the linear-shaped No. 3 Fault (Fig. 9), the formation of the western segment of the main rift zone, however, might also have been affected by the No. 1 Fault which is a left lateral strike-slip fault and bounds the basin on the southwest. The reasons include: (1) the major faults in this segment are nearly E–W-striking normal faults, which is largely compatible with the sinistral motion of the NW-trending No. 1 Fault both chronologically and spatially (Figs. 10 and 13); (2) although its length (along strike) is much smaller than that of the other two rift segments, its width (vertical to strike) is obviously larger than them (Figs. 7 and 8); (3) unlike the other two rift segments where extensional strain was obviously concentrated on the single boundary fault (or fault zones) on each side, extensional strain in the western segment was more averagely distributed onto several parallel or sub-parallel major faults, as indicated by the maximum normal displacements described in Section 2.1 and the dataset of increments of hanging wall subsidence mapped in Fig. 10.

4.4. The extension direction and tectonic model In addition to pre-existing structural fabrics, the extension direction with respect to it is an important factor that can influence the formation of a rift. Based on an analysis of the fault patterns in the main rift zone, a rough estimate of extension direction for the rifting is made in below. As described in previous sections, the middle segment is characterized by long, linear, NE-ENE-trending, extensional boundary faults which was formed by along-strike linkage of coplanar or nearly coplanar fault segments, as can be inferred from the local bends along strike, bifurcated fault tips and the throw profiles for some major faults (Figs. 2, 11a and f). In addition, some internal faults also trend ENE–WSW (Figs. 2 and 7). It can be seen that this fault pattern shows many similarities to the scaled analogue model in orthogonal extension studied by McClay et al. (2002) (Fig. 13b-i and c-i), and thus implies that the extension direction of the middle segment was largely orthogonal to the NE- to ENE-trending, pre-existing structural fabrics and thus dominantly SE to SSE. On the other hand, the eastern segment of the main rift zone is characterized mainly by two boundary fault zones, the En echelon Fault Zone A and B, both of which consist of largely left-stepping, E–W-trending or nearly E–W-trending fault segments, as well as by E–W-trending or NE-trending internal faults (Figs. 2 and 7). This fault pattern shows strong resemblances to that developed in the

60° oblique rift model in McClay et al. (2002), which is characterized by en echelon rift marginal-fault systems oblique to the extension direction (Fig. 13c-ii), and is similar to an extent to the Tertiary rift basins of Thailand where the en echelon fault patterns arranged along pre-existing structural fabrics are found to be an important structural feature of oblique extension regimes (Morley et al., 2004). This circumstance implies that the extension direction for the eastern segment was oblique and at a high angle to the nearly E–W-trending, pre-existing structural fabrics and thus dominantly SSE, which is largely as same as that predicted from the middle segment. Based on the analysis above, the opening of the Qiongdongnan Basin is suggested to be driven by a largely SE-oriented extension, implying that a SE-directed displacement plus perhaps a slight anti-clockwise rotation were happened on the Xisha Block. Consequently, a structural model can be constructed for the Qiongdongnan Basin, as shown in Fig. 13.

4.5. Implications for regional tectonics The issue about the opening mechanism of the South China Sea has been in dispute for a long time, and the models largely fall into two main types: (1) the pull-apart model (Briais et al., 1993; Gilley et al., 2003; Leloup et al., 1995, 2001; Tapponnier et al., 1982, 1990, 2001) and (2) the slab-pull model (Hall, 2002; Lee and Lawver, 1995; Morley, 2002, 2012; Rangin et al., 1995a; Taylor and Hayes, 1980, 1983). These models emphasize the dominant role of extrusion of the Indochina Block and sinistral motion on the Red River Fault Zone (Briais et al., 1993; Leloup et al., 2001; Tapponnier et al., 1982), or the subduction of a proto-South China Sea at the North Borneo Trench (Hall, 2002; Taylor and Hayes, 1980, 1983), in the opening of the South China Sea. Morley (2002), on the other hand, uses a modified model to try to make a kinematic linkage between the South China Sea spreading centre and the Red River Fault Zone. Royden et al. (2008) suggests that large-scale extrusion of lithospheric fragments beyond the borders of the Tibetan plateau needs not only the continental collision and crust thickening in the plateau on one side of the extruding lithospheric fragments but also a trench rollback on the other side. To decide which of these models work for the South China Sea, there are several key questions need to answer, and two of them are: (1) how far does Red River Fault Zone extend offshore in terms of sinistral motion, and (2) how much left-lateral strike-slip displacement occurred on this portion of the fault zone. As far as this study is concerned, it is difficult to determine from the structure patterns in the basin whether the opening of the Qiongdongnan Basin was driven by the extrusion of the Indochina Block and sinistral motion on the Red River Fault Zone or by slab pull from the North Borneo Trench, even though the western segment of the main rift zone shows many structural features indicative of the influence of the left-lateral strike-slip No. 1 Fault, as

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discussed in Section 4.3. However, the estimate of the amount of rifting in the Qiongdongnan Basin may suggest the termination of the Red River Fault Zone in terms of sinistral motion at the southern end of the Yinggehai Basin. The estimate is made from several NW–SE-trending seismic profiles which cross the main rift zone (Fig. 2 and Table 2). It is simply a sum of the heaves of the interpreted faults on each seismic profile, and thus is a lower limit of the total extension for each seismic profile as well as a lower limit of the whole extension of the rift zone. The estimate ranges between 15 and 39 km (Table 2), which largely matches an estimate of tens of kilometres of sinistral motion along the strike-slip faults in the Yinggehai Basin (Rangin et al., 1995b; Zhu et al., 2009). If the tens of kilometres of sinistral motion is close to the fact, it may imply that nearly all the left lateral displacement of the Red River Fault Zone in the Yinggehai Basin before the middle Miocene can be totally absorbed by the extension in the Qiongdongnan Basin, and thus implies that the Red River Fault Zone may terminate here or a nearby area, which may further imply that a mechanism other than the extrusion of the Indochina Block is needed to explain the opening of the South China Sea during the Oligocene and Early Miocene. In general, we favour the modified model suggested by Morley (2002) for the opening of the South China Sea and the Qiongdongnan Basin.

5. Conclusions Updated seismic reflection data allows a detailed structural analysis of the Qiongdongnan Basin. The Qiongdongnan Basin is suggested to be a rift basin formed in the Cenozoic and a main rift zone is found in the central part of the basin and underlying thick post-rift sediments. The main rift zone is 50–100 km wide and over 400 km long. It is composed of five sub-basins or three major segments, which are separated from each other by transfer zones. It has an arcuate shape, and its trend turns from NE in the western part to nearly E–W in the eastern part. The border fault system of the main rift zone is highly segmented by different kinds of relay structures. All the sub-basins and rift segments were controlled mainly by NE-trending and nearly E–W-trending normal faults. Geometrical analysis of the border faults of the main rift zone suggests that the NE-trending border faults of the middle segment and the nearly E–W-trending border faults of the eastern segment are controlled by a set of NE-striking, pre-existing structures and a set of E–W-trending, pre-existing structures respectively, while the border faults of the western segment are suggested to be controlled by the sinistral strike-slip movement on the No. 1 Fault that is a branch of the Red River Fault Zone as suggested by some previous studies. The evolution history of the fault system in the Qiongdongnan Basin is reconstructed as follows: During the Late Eocene, sparsely distributed NE-trending faults and half grabens formed mainly in the western and the central part of the basin. Up to the end of the Early Oligocene, accompanied by an increasing in the number and the length of the normal faults, the border fault system of the main rift zone formed and the area influenced by faulting extended into the eastern part of the study area. During the Late Oligocene to early Early Miocene, many small faults ceased to be active and thus the number of active faults decreased, while the subsidence area was further enlarged, at this time, mainly into the flanking area of the main rift zone. In addition, estimates of extensional strain along NW–SE-trending seismic profiles, which cross the main rift zone, vary between 15 and 39 km, and are generally comparable to the sinistral displacement on the Red River Fault Zone offshore, implying that this

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