Source and sink characteristics of the continental slope-parallel Central Canyon in the Qiongdongnan Basin on the northern margin of the South China Sea

Source and sink characteristics of the continental slope-parallel Central Canyon in the Qiongdongnan Basin on the northern margin of the South China Sea

Journal of Asian Earth Sciences 134 (2017) 1–12 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.else...

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Journal of Asian Earth Sciences 134 (2017) 1–12

Contents lists available at ScienceDirect

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

Full length article

Source and sink characteristics of the continental slope-parallel Central Canyon in the Qiongdongnan Basin on the northern margin of the South China Sea Chao Li a, Chengfu Lv a,⇑, Guojun Chen a, Gongcheng Zhang b, Ming Ma a,c, Huailei Shen b, Zhao Zhao b, Shuai Guo b a

Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China Research Institute of China National Offshore Oil Corporation, Beijing 100028, China c University of Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e

i n f o

Article history: Received 7 January 2016 Received in revised form 24 October 2016 Accepted 25 October 2016 Available online 26 October 2016 Keywords: Central Canyon Ledong submarine fan Shuangfeng submarine fan Source-conduit-sink system Qiongdongnan Basin

a b s t r a c t The ‘‘source-conduit-sink” model is crucial for studying deep-water sedimentary systems along a continental margin. Using seismic data, bulk rare earth element compositions of sediments and zircon U-Pb age data, we examined the supply and deposition (i.e., the source and sink) of the sediments in the Central Canyon of the South China Sea. Five phases of secondary canyon fill are present in the Central Canyon. The natural levees developed at the head of phase 1 of the secondary canyon deposits indicate that the Central Canyon initially developed at 10.5 Ma. The sediments in the Central Canyon were supplied by the Ledong submarine fan, and the provenance of the material in the Ledong submarine fan and Central Canyon was eastern Vietnam. Large amounts of sediments were transported through the Central Canyon to the Shuangfeng Basin and deposited during four phases of submarine fan development. Phases 1–3 of the Shuangfeng submarine fans are composed of deep-water branching channel and interchannel sediments. Phase 4 of the Shuangfeng submarine fan consists of deep-water channel and lobe sediments. Tectonic events, including the broad uplift of the Tibetan Plateau and central-southern Vietnam during the late Miocene, reversal of the strike-slip Red River Fault, and rapid subsidence in the Qiongdongnan Basin at approximately 5.5 Ma, provided favourable conditions for the growth of the Ledong submarine fan, Central Canyon and Shuangfeng submarine fan system. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Deep-water canyons are conduits through which gravity flows can transport shelf sediments to deep-sea plains (Babonneau et al., 2013; Gales et al., 2014; Martín-Merino et al., 2014; Pickering et al., 2015). Consequently, they represent intermediate links in the ‘‘source-to-sink” system along continental margins (Parra et al., 2012; Matenco and Andriessen, 2013; Prizomwala et al., 2014; Bentley et al., 2015). Gravity flow deposits in deepwater canyons are usually composed of coarse-grained sediments, including gravelly sandstones and massive/graded sandstones, and relatively fine-grained sediments, including siltstones, silty mudstones and mudstones (Beaubouef, 2004; Mayall et al., 2006; Kane et al., 2009; Jobe et al., 2010). These sediments can not only

⇑ Corresponding author. E-mail address: [email protected] (C. Lv). http://dx.doi.org/10.1016/j.jseaes.2016.10.014 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved.

accumulate large amounts of oil and gas resources, but can also archive enormous amounts of climatic and regional structural information (Bourget et al., 2010; Talling, 2014; Gales et al., 2014). The Central Canyon in the Qiongdongnan Basin has a length, width and depth of 520 km, 2–15 km and 270–800 m, respectively. The sediments in the Central Canyon contain hundreds of billions of cubic metres of natural gas (Huang et al., 2016), record the tectonic evolution of the Tibetan Plateau and the Red River Fault, and archive information on regional environmental and climate changes. Previous studies have documented the Central Canyon in terms of its morphology (Li et al., 2013; Su et al., 2014), sequence stratigraphic framework (Gong et al., 2011), filling sediments (Gong et al., 2011; Su et al., 2014, 2015), genetic mechanism (Yuan et al., 2009; Gong et al., 2011), influencing factors (Gong et al., 2011; Su et al., 2014, 2015) and conditions of natural gas accumulation (Huang et al., 2016). The formation of the Central Canyon appears to be closely related to the rapid uplift of the Tibetan Plateau and the activity of the Red River Fault zone (Yuan et al.,

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2009; Gong et al., 2011). Turbidity currents, debris flows, slumps and hemi-pelagic deposition occurred in the Central Canyon, and its filling patterns have been previously summarized (Yuan et al., 2009; Gong et al., 2011; Su et al., 2014, 2015). However, with regard to the genetic mechanism of the Central Canyon, two key questions remain unresolved. (1) The starting position of the Central Canyon is located at the juncture between the Yinggehai Basin and the Qiongdongnan Basin (Fig. 1). How did the sediments from the uplifting zone in the western and northwestern parts of the Yinggehai Basin traverse the basin to the Central Canyon? (2) Because the Central Canyon was a large-scale conduit for sediment transport, where and in what form were the large amounts of sediment that were transported through the canyon deposited? This paper identified the developmental phases of the Central Canyon and constrained the ages of each phase of secondary canyon development. Using seismic data, bulk rare earth element (REE) compositions of sediments and zircon U-Pb age data, we examined the characteristics of sediments deposited at the head and terminus of the Central Canyon and their provenance. Our results indicate that the sediments originated from eastern Vietnam and were first deposited in the Ledong submarine fan. Large amounts of sediment were then remobilized to and through the Central Canyon and were deposited in the Shuangfeng Basin, forming a submarine fan over the course of four phases.

2. Geologic background 2.1. The Yinggehai Basin The Yinggehai Basin, located between Hainan Island (China) and Vietnam, is a Cenozoic high-temperature and high-pressure

gas-bearing basin (Lei et al., 2011, 2015; Cao et al., 2015). The basin is connected to the Beibuwan Basin via the Red River Fault to the northeast and to the Qiongdongnan Basin via the Red River Fault to the southeast (Fig. 1). The basin consists of the Central Depression, in which numerous diapirs have developed (Fig. 3A) (Lei et al., 2011, 2015). Between the late Cretaceous and Palaeocene, the basin began to rift, and the sedimentary facies included fluvial deposits of conglomerate and sandstone (Sun et al., 2014). Between the Eocene and early Oligocene, the basin was characterized by extensive fault subsidence, and the sedimentary facies included alluvial fans and fluvial and lacustrine deposits (Sun et al., 2014). During the middle to late Oligocene, the rifting decreased, and littoral neritic facies units were deposited (Lei et al., 2011; Sun et al., 2014). In the Miocene, the basin developed a post-rift subsidence phase, and littoral or deltaic facies to neritic facies were deposited. The deposition in the central and southern parts of the basin transitioned to marine facies (Lei et al., 2011; Sun et al., 2014). After the Pliocene, the southern part of the basin experienced a period of rapid subsidence, and neritic-bathyal facies were deposited (Lei et al., 2011, 2015; Sun et al., 2014).

2.2. The Qiongdongnan Basin The Qiongdongnan Basin, located between Hainan Island and the Xisha Islands, has an area of 82,900 km2 (Hu et al., 2013). The basin is bounded by the Hainan Uplift via faults to the north, adjoins the Yinggehai Basin by the Red River Fault to the west, and is connected to the Pearl River Mouth Basin by faults to the northeast (Fig. 1). The structural framework of the basin features a pattern of ‘‘two uplifts and three depressions” and has characteristics of northern faulting and southern overlap (M. Zhao et al.,

Fig. 1. Topographic map showing the locations of the main basins and the distribution of the Central Canyon (outlined by the yellow line), the regional drainage systems in the continental margin of the northwestern South China Sea (1-Red River, 2-Ma Song River, 3-Blue River, 4-Well DF1-x, 5-Lishui River, 6-Qiubin River, 7-Zhubi River, 8Changhua River, 9-Lihe River, 10-Ganen River, 11-Wanglou River, 12-Ningyuan River), and the locations of the utilized boreholes and seismic profiles. (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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2015; Z.X. Zhao et al., 2015; Hu et al., 2013). The tectonic evolution of the basin involved rifting and post-rifting phases (Fig. 3B). Multiple episodes of rifting occurred during the early stages. The first episode of rifting occurred during the late Cretaceous to early Eocene. The second episode of rifting occurred during the Eocene to early Oligocene. This phase can be divided into two stages: rapid subsidence occurred during the middle to late Eocene and stable subsidence occurred during the late Eocene to the early Oligocene. The third episode of rifting occurred in the late Oligocene (M. Zhao et al., 2015; Z.X. Zhao et al., 2015; Morley, 2016). In the Neogene, the basin developed a thermo-subsidence stage. During the stage of rifting subsidence, the marine-continental transitional facies of the Yacheng Formation and the marine littoral-neritic facies of the Lingshui Formation were deposited in the basin. During the post-rift stage, marine littoral-neritic and bathyal facies, including the strata of the Sanya Formation, Meishan Formation, Huangliu Formation and Yinggehai Formation, were deposited in the basin (Fig. 2) (Gong et al., 2014; Wang et al., 2015; M. Zhao et al., 2015; Z.X. Zhao et al., 2015). The Central Canyon developed during the deposition of the Huangliu Formation through to the deposition of the second member of the Yinggehai Formation.

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2.3. The Shuangfeng Basin The Shuangfeng Basin is located in the transitional area between the north continental slope and the Central Sea Basin of the South China Sea. It is adjacent to the Pearl River Mouth Basin to the north and adjoins the Zhongsha Uplift to the south and the Xisha Trough to the west (Fig. 1). The Shuangfeng Basin is a filled oceanic basin, and its flat surface dips to the east (Fig. 3C) with a gradient of 0.3–0.4  10 3 (Ding et al., 2011; Chen et al., 2015). From 32 to 30 Ma, with the expansion of the South China Sea, the Shuangfeng Basin experienced ‘‘scissor-type” extension from east to west. Followed the marine transgression, the sedimentary environment in the basin was transitioned from terrestrial to transition facies. At approximately 28 Ma, the extension of the Shuangfeng Basin decreased, and the sedimentary environment transformed into deep-sea conditions. From 25 to 23 Ma, the spreading axis of the South China Sea transitioned to the south, and the Shuangfeng Basin ceased to expand and began to experience the initial stages of thermal subsidence. After 10.5 Ma, the strata in the Shuangfeng Basin reached a thickness of 1000 m (Ding et al., 2011).

Fig. 2. Sketch showing the sequence classification, lithologic characteristics, biological fossils, depositional environment, relative change in sea level and tectonic stage of the Qiongdongnan Basin. The ages of the sequence boundaries, formations and relative changes in sea level in the Qiongdongnan Basin were provided by the Research Institute of China National Offshore Oil Corporation; the biological data were adopted from Gong and Li (1997); and the global eustatic curve was taken from Haq et al. (1987).

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Fig. 3. (A) A seismic profile across the Central Depression, showing the sequence framework of the Yinggehai Basin. (B) A seismic profile across the Songdong Sag and Lingshui Sag, showing the sequence stratigraphic framework of the Qiongdongnan Basin. (C) A seismic profile oriented NW-SE, showing the sequence stratigraphic framework of the Shuangfeng Basin. Note the thick strata deposited after 10.5 Ma (corresponding to the sequence surface of T40).

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3. Data and methodology The data used in this study include seismic profiles, bulk REE compositions of sediments and zircon U-Pb ages. Seismic data from the Yinggehai Basin, Qiongdongnan Basin and Shuangfeng Basin were collected by the China National Offshore Oil Corporation. The density of the 2-D seismic lines in the Yinggehai Basin and Qiongdongnan Basin is 4  4 km. The 2-D seismic data from the Shuangfeng Basin were collected recently, with a traverse density of 12  12 km. The interpretation of the 2-D seismic profiles was conducted in the IESX module of the Geoframe workstation. The bulk REE composition data and zircon U-Pb dating data from sediments for provenance analysis were cited from previous studies for different regions. Specifically, the REE compositions of sediments from the Red River were cited from Y.F. Wang et al. (2011), Y.M. Wang et al. (2011), M. Zhao et al. (2015) and Z.X. Zhao et al. (2015). The REE compositions of sediments from Hainan Island were cited from Shao et al. (2010). The REE compositions of sediments from eastern Vietnam were cited from M. Zhao et al. (2015) and Z.X. Zhao et al. (2015). The REE compositions of sediments from three wells in the Central Canyon were cited from Li et al. (2015). The bulk REE compositions adopted in this paper were the mean results of the above regions. Zircon U-Pb dating data were cited from Zuo et al. (2015), and the samples used for dating include drill cores from the Central Canyon and eastern Vietnam, samples from four modern rivers in eastern Vietnam, and samples from six modern rivers on Hainan Island.

4. Results and discussion 4.1. Age of the Central Canyon development The age of initial deposition in the Central Canyon has been suggested to be 5.5 Ma, 10.5 Ma and 15.5 Ma (Yuan et al., 2009; Gong et al., 2011; Su et al., 2014). These age estimates differ because the erosional depth of the Central Canyon increases gradually from west to east, and its bottom cuts through the sequence boundaries T30 (5.5 Ma) (Fig. 4A), T40 (10.5 Ma) (Fig. 4B) and T50 (15.5 Ma) (Fig. 4C). The Central Canyon is commonly considered to have developed at 5.5 Ma. There are two lines of evidence for this age: (1) At 5.5 Ma, the Tibet Plateau was uplifted, and large amounts of sediments were supplied to the Yinggehai and Qiongdongnan basins. (2) At 5.5 Ma, the strike-slip movement of the Red River Fault transformed from left-lateral to right-lateral, inducing gravity flows (Yuan et al., 2009; Gong et al., 2011). To constrain the developmental age of the Central Canyon, we identified phases of canyon filling based on the following criteria. (1) If a later secondary canyon eroded the fill of an earlier canyon, a contact relationship involving truncation, erosion surface, and onlapping fill would form with the earlier canyon (Fig. 4D). (2) If the later secondary canyon was highly erosive, it would cause a deep incision in the earlier canyon. The amplitude of the seismic reflector on the walls of the later secondary canyon would therefore be higher and regular in shape (Fig. 4E). (3) If the later secondary canyon was weakly erosive, it would cause little or no incision. This appearance was commonly found in the later filling stage of the Central Canyon. (4) Different dip angles exist in different secondary canyons (Fig. 4F). Firstly, each secondary canyon phase was identified based on high-resolution 3-D seismic profiles, and the identifying marks and models between the secondary canyon phases were established. Secondly, each secondary canyon phase was identified on the 2-D seismic profile with the use of the previously constructed model. Finally, five phases of secondary canyon formation were identified between the sequence surfaces of 10.5 Ma and 4.2 Ma in the Central Canyon (Fig. 4G).

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A high-amplitude seismic reflection associated with gullwingshaped progradation is present at the top of phase 1 of the secondary canyon fill (Fig. 4H). This reflection represents deposition of the natural levees that formed when turbidity currents overflowed the channel. On the seismic profile, the seismic reflector of the levee dips away from the canyon at a high angle, and the dip angle decreases gradually toward the floodplain and downlaps on the T40 sequence boundary. The growth of the levees indicates that the development age of phase 1 of the secondary canyons is consistent with the T40 sequence boundary, whose geologic age is 10.5 Ma (Fig. 4I). The parallel sheet-shaped reflector with a weak amplitude developed on the levee represents the deposition of hemi-pelagic mudstone. This thick bed of mudstone experienced erosion during phases 2–5 of the secondary canyons and forms a unifiedboundary that is consistent with the T30 sequence boundary (5.5 Ma) (Fig. 4H). These results indicate that phases 2–5 of the secondary canyons occurred at and/or after 5.5 Ma. In addition, the relative sea level of the Qiongdongnan Basin was at its lowest level at 10.5 Ma (Fig. 1). This condition was especially favourable to the development of the Central Canyon. In contrast, at 5.5 Ma, the relative sea level in the Qiongdongnan Basin was at its highest level (Fig. 1), which was unfavourable to the formation of the Central Canyon. As a result, the initial development age of the Central Canyon was 10.5 Ma. 4.2. Deposition of the sediments transported through the Central Canyon The Central Canyon crosses the Xisha Trough and enters the Shuangfeng Basin. Using the latest seismic data collected from the Shuangfeng Basin, we recognized a large number of deepwater channels and lobes on the T40 sequence boundary. These deposits constitute a submarine fan with deep-water branching channels with inter-channel areas and deep-water channels with lobes (Fig. 5). Four phases of submarine fan deposition were identified in the vertical section. In this paper, this fan is termed the ‘‘Shuangfeng submarine fan” due to its development position (Fig. 6). Phases 1–3 of the Shuangfeng submarine fan consist of deepwater branching channel and inter-channel sediments (Fig. 5A). As the branching channels advanced to the Central Sea Basin in the South China Sea, the sediment transport distances and the area of the fan increased. Phase 1 of the Shuangfeng submarine fan was deposited in the section between the Xisha Trough and the Shuangfeng Basin and is the smallest unit, with an area of approximately 3500 km2 (Fig. 6A). Phase 2 of the Shuangfeng submarine fan was deposited in the southwestern part of the basin and has an area of 5600 km2 (Fig. 6B). Phase 3 of the Shuangfeng submarine fan was deposited in the middle and southern parts of the basin and is the largest in scale, with the longest sediment transport distances and an area of approximately 8600 km2 (Fig. 6C). The heads of phases 1–3 of the Shuangfeng submarine fan are linked to the Central Canyon in map view. Based on the seismic profiles, the development of a synchronous deep-water channel is not observed in the northern and southwestern portions of phases 1–3 of the Shuangfeng submarine fan (Fig. 5B). Therefore, the existence of a source to the north or southwest can be excluded. Meanwhile, the southeastern part of the Shuangfeng Basin is connected to the Central Sea Basin and is not supplied with sediment from the southeast. Hence, the sediments in phases 1–3 of the Shuangfeng submarine fan were transported by the Central Canyon and are the result of the final deposition of Central Canyon sediments. Phase 4 of the Shuangfeng submarine fan consists of a deepwater channel and lobes (Fig. 5C–F). The channel developed to the west of the lobes (Fig. 5D) and is connected to the terminus

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Fig. 4. Seismic profiles showing that the increasing depth of erosion has cut through the sequence surfaces of T30, T40 and T50 successively from the head to the middle reaches of the Central Canyon (A–C). The division of phases of secondary canyon formation in the Central Canyon (D–G). (D) Contact relationships of truncation, erosional boundary and onlapping deposits between secondary channels. (E) The wall of a new channel is evidenced by the high-amplitude seismic reflection. (F) The dip angles of formations differ among the secondary channel phases. (G) Five phases of secondary channels within the Central Canyon were identified. (H) Seismic profile showing the high-amplitude seismic facies with a gullwing-shaped configuration. This facies is considered to be levee deposit and can be observed along both sides of the phase 1 secondary canyon. (I) Schematic diagram showing that the bottom of the levee deposits corresponds to the seismic reflector T40 (10.5 Ma), which indicates that the initial development of the Central Canyon likely occurred during the same period (10.5 Ma). The fill of the secondary canyons associated with phases 2–5 only occurs within a single complete canyon boundary, which coincides with seismic reflector T30 (5.5 Ma). Therefore, the development of phases 2–5 of the secondary canyons occurred at 5.5 Ma or later.

of the Central Canyon (Fig. 5E). Phase 4 of the Shuangfeng submarine fan was separated from phases 1–3 by the Central Seamount (Fig. 6D). The seismic profile shows that the thicknesses of the two phases of lobes gradually decrease eastward (Fig. 5D) and that their widths decrease to the north until the lobes disappear (Fig. 5F). These observations indicate that the phase 4 sediments in the Shuangfeng submarine fan are not from the north. The deep-water channel that lies west of the lobes is connected to the Central Canyon, thereby demonstrating that phase 4 of the Shuangfeng submarine fan is also the result of the final deposition of Central Canyon sediments. 4.3. Provenance of the sediments in the Central Canyon As the REE compositions of sediments are highly stable and can retain information on the provenance of the sediments, REE analyses can be used to trace the evolution of the sediment source. Shao et al. (2010) analysed the REE compositions of sediments from Hainan Island, and Y.F. Wang et al. (2011), Y.M. Wang et al. (2011) and Li et al. (2015) analysed the REE compositions of sediments within the Central Canyon. M. Zhao et al. (2015) and Z.X. Zhao et al. (2015)

analysed the REE compositions of sediments from the Red River and the eastern part of Vietnam. We compiled and compared the mean values of the REE compositions of sediments from different regions in the literature to determine the provenance of the sediments in the Central Canyon. When normalized to the Post-Archean Average Shale (PAAS) REE patterns (Fig. 7), the sediments from the Hainan Island display negative Eu anomalies, whereas those from the Red River and eastern Vietnam display positive Eu anomalies. However, the sediments from eastern Vietnam have much higher Eu concentrations than those from the Red River (Fig. 7A). The REE patterns of core samples from wells in the headstream, upstream and midstream sections of the Central Canyon have positive Eu anomalies (Fig. 7B). These REE patterns differ from those of the Hainan Island sediments but are similar to those of the Red River and eastern Vietnam sediments. Furthermore, the Eu concentrations in the sediments from the Central Canyon are similar to those from eastern Vietnam. The zircon age spectrum for the Hainan Island provenance has two peaks at 100 Ma and 300 Ma (Fig. 8). The zircon age spectrum for the Red River provenance has three zircon age peaks, 35 Ma,

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Fig. 5. (A) A seismic profile perpendicular to depositional strike, showing typical deep-water branching channels in the Shuangfeng Basin. Synchronous deep-water channels were not found in the seismic profile from the southwest region of the Shuangfeng Basin (B). Therefore, a sediment source to the southwest can be excluded. Seismic profiles that are perpendicular to the depositional strike (C and E) and parallel to (D and F) the depositional strike showing typical deep-water channel and lobe deposits. The channel lies to the west of the lobes, which were deposited during two phases. The thicknesses of the two lobe phases decrease gradually eastward, and the widths gradually decrease northward. See Fig. 1 for profile locations.

255 Ma and 730 Ma. The zircon age spectrum for the Ma Song River and Blue River provenance in the northern part of Vietnam has

three zircon age peaks at 35 Ma, 255 Ma and 435 Ma. Zircon ages from the eastern part of Vietnam range from 110 to 2800 Ma,

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Fig. 6. Sketch showing the distribution of the Shuangfeng submarine fan. Phase 1 (A) is distributed in the section between the Xisha Trough and Shuangfeng Basin and is the smallest in terms of area. Phase 2 (B) is distributed in the southwestern basin and is larger than phase 1, with an area of 5600 km2. Phase 3 (C) is the largest, with the longest sediment transport distance and an area of 8600 km2. Phase 4 (D) is separated from phases 1–3 by the Central Seamount and consists of two phases of lobes.

including four peak groups: 110–170 Ma, 200–350 Ma, 420– 500 Ma, and 800–1000 Ma. The zircon age spectrum for the Lishui River and Qiubin River provenance in the southern part of Vietnam has two zircon age peaks at 255 Ma and 435 Ma. The age spectrograms of the two wells in the Central Canyon are similar and range from 20 to 2800 Ma, including five peak groups at 25–60 Ma, 70– 170 Ma, 220–270 Ma, 420–460 Ma and 800–850 Ma. These values are similar to the age range of zircons from eastern Vietnam. Therefore, we conclude that the provenance of the Central Canyon sediments was eastern Vietnam. 4.4. The source-to-sink process in the Central Canyon The initial development of the Central Canyon occurred at 10.5 Ma, and the length of the initial Central Canyon was only 400 km, with its head located within the Ledong-Lingshui depression transition zone. By analysing the sedimentary characteristics of the headstream (Ledong Sag) of the initial Central Canyon, this paper sought to ascertain how the sediments from eastern Vietnam entered the Central Canyon. The results indicate that, at 10.5 Ma, the Ledong Sag developed a large-scale submarine fan (termed the Ledong submarine fan). The seismic reflection of this submarine fan reveals a wedge-shaped progradational structure with

a moderate amplitude. The maximum thickness of the upper fan is approximately 2000 m, and the horizontal extent of the lower fan is 100 km. At the toe of the lower fan, the head of the initial Central Canyon appears, suggesting that the Ledong submarine fan was the source of material for the Central Canyon. By analysing the spatio-temporal evolution of the submarine fan, Y.F. Wang et al. (2011) and Y.M. Wang et al. (2011) found that the Ledong submarine fan gradually retreated to the Yinggehai Basin (Fig. 9A). This paper analysed the evolution of phases 1–5 of the secondary canyons and found that these canyons experienced the same retreat process as the Ledong submarine fan (Fig. 9B). Therefore, the Central Canyon and the Ledong submarine fan developed synchronously, and the Ledong submarine fan, located at the head of the Central Canyon, was the direct sediment source for the Central Canyon (Fig. 10). Based on these observations, the Ledong submarine fan, Central Canyon and Shuangfeng submarine fan developed synchronously. The sediments derived from eastern Vietnam were first deposited in the Ledong submarine fan, which gradually expanded to a length of more than one hundred kilometres between the Yinggehai Basin and Qiongdongnan Basin. Then, the sediments in the Ledong submarine fan were transported through the Central Canyon and deposited in the Shuangfeng Basin as a submarine fan (Fig. 10).

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(10.5 Ma) and present (i.e., from the T40 sequence surface to the seabed, Fig. 3b) is much larger than that deposited during the preceding period (i.e., between the T100 and T40 sequence surfaces, Fig. 3b). The sedimentation rate for the Huangliu Formation (10.5–5.5 Ma) is considerably higher than the rates for the Meishan Formation (15.5–10.5 Ma) and the Yinggehai Formation (5.5– 1.9 Ma) (Wang et al., 2013). These findings are consistent with the timing of the development of the Ledong submarine fan and initial Central Canyon at 10.5 Ma. The rapid offshore sediment accumulation in the Huangliu Formation is directly related to the increase in erosion rates caused by the uplift of southern and central Vietnam from 10.5 to 5.5 Ma. The rapid increase in the sedimentation rate during this period led to the regional development of steep slopes, high pore pressures and weak cementation within the sediments (Wang et al., 2013), thereby providing the basic conditions for the formation of the Ledong submarine fan and the Central Canyon.

Fig. 7. The REE distribution patterns of the Hainan Uplift, Red River, eastern Vietnam and Central Canyon sediments. The sediments from Hainan Island exhibit negative Eu anomalies, whereas those from the Red River and eastern Vietnam exhibit positive Eu anomalies (A). The sediments from the Central Canyon exhibit positive Eu anomalies and are similar in terms of REE composition to those from eastern Vietnam (B).

4.5. The tectonic mechanism of the source-to-sink system in the Central Canyon 4.5.1. The rapid uplift of the Tibetan Plateau during the late Miocene The Tibetan Plateau is generally acknowledged to have experienced multiple phases of uplift, and some studies have shown that extremely rapid uplift of the Tibetan Plateau initiated during the late Miocene (Amano and Taira, 1992; Zhong and Ding, 1996; Wang and Ding, 1998). Li (1995) proposed that the uplift of the Tibetan Plateau was divided into three stages based on palaeomagnetic data, sedimentary data and the architecture of the surrounding basins. A final period of rapid uplift of the Tibetan Plateau occurred from the latest Miocene to Quaternary. Cui et al. (1996) argued that the Tibetan Plateau underwent three periods of uplift. According to a karst denudation plane, the present altitude of the Tibetan Plateau has mainly resulted from rapid uplift since the latest Miocene (10.5 Ma). The rapid uplift of the Tibetan Plateau during the late Miocene may have caused high rates of erosion and increased the rate of sediment supply to the Yinggehai Basin. The development age (10.5 Ma) of the Ledong submarine fan and initial Central Canyon corresponds to the period of rapid Tibetan Plateau uplift. The material eroded from the plateau was sufficient for the growth of the system containing the Ledong submarine fan, Central Canyon, and Shuangfeng submarine fan.

4.5.2. The broad uplift of Vietnam during the late Miocene The karstification of the Phan Rang carbonate platform documents broad uplift of southern and central Vietnam during the late Miocene (10.5 Ma) (Fyhn et al., 2009). This uplift and the development of a river system in the newly uplifted mountains may have resulted in a voluminous supply of sediments to the nearby study area. The thickness of material deposited between the late Miocene

4.5.3. The strike-slip movement of the Red River Fault The Red River Fault, which developed along the western boundary of the Qiongdongnan Basin, accommodated large amounts of sinistral strike-slip deformation (over 700 km) between 30 and 16 Ma. The direction of slip on the fault reversed between 16 and 5.5 Ma, and dextral faulting has occurred since 5.5 Ma (Zhu et al., 2009). The development of phases 2–5 of the secondary canyons at 5.5 Ma is consistent with the timing of the Red River Fault reversal. In association with the reversal from sinistral to dextral motion along the Red River Shear Zone, high-frequency seismicity occurred in the Qiongdongnan Basin and Yinggehai Basin (Zhu et al., 2009, 2011), and a rapid subsidence event occurred in the Qiongdongnan Basin at approximately 5.5 Ma (Yuan et al., 2008; Song et al., 2011). This seismicity may have triggered large-scale gravity currents, and the rapid subsidence resulted in conditions that were conducive to gravity currents entering the Qiongdongnan Basin and forming the phases 2–5 of the secondary canyons and the Shuangfeng submarine fan.

5. Conclusions (1) The sediments within the Central Canyon can be subdivided into five phases of secondary canyon filling. The natural levees in the headstream of phase 1 of the secondary canyon deposits and the relative change in the sea level of the Qiongdongnan Basin indicate that the initial development age of the Central Canyon was 10.5 Ma instead of 5.5 Ma. (2) The Shuangfeng submarine fan was deposited over the course of four phases. Phases 1–3 are composed of deepwater branching channel and inter-channel sediments. During these phases, the sediment transport distances increased gradually, and the area of the fan progressively expanded from the Xisha Trough to the Central Sea Basin. Phase 4 of the Shuangfeng submarine fan consists of deep-water channels and lobes. The sediments in the Shuangfeng submarine fan were transported through the Central Canyon. (3) The provenance of the sediments in the Ledong submarine fan and Central Canyon was eastern Vietnam. The Ledong submarine fan directly supplied sediments to the Central Canyon. (4) Tectonic events, including the rapid and broad uplift of the Tibetan Plateau and central-southern Vietnam during the late Miocene, provided sufficient sediments for the growth of the Ledong submarine fan and the initial Central Canyon at 10.5 Ma. The reversal in fault motion along the strikeslip Red River Fault and a rapid subsidence event occurred in the Qiongdongnan Basin at approximately 5.5 Ma and cre-

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Fig. 8. The zircon age spectrograms of material from surrounding rivers. The results indicate that the provenance of the Central Canyon sediments was eastern Vietnam. See Fig. 1 for the location of samples.

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Fig. 9. The spatial evolution of the Ledong submarine fan (A) (Y.M. Wang et al., 2011) and Central Canyon (B). The Ledong submarine fan gradually retreated to the Yinggehai Basin, and the Central Canyon exhibits similar characteristics.

Fig. 10. Schematic diagram illustrating the source and sink characteristics of the Central Canyon. The sediments derived from eastern Vietnam first formed the Ledong submarine fan between the Yinggehai Basin and the Qiongdongnan Basin. Then, the sediments of the Ledong submarine fan were transported through the Central Canyon and were deposited in the Shuangfeng Basin, forming the Shuangfeng submarine fan. In addition, the sediments supplied by the Hainan uplift (Gong et al., 2014; Li et al., 2015) and northern continental slope (Su et al., 2014, 2015) are also cited in this model diagram.

ated favourable conditions for gravity currents, which formed phases 2–5 of the secondary canyon fill and the Shuangfeng submarine fan. The development of the Central Canyon System has important implications for the tectonic activity of the Tibetan Plateau, Vietnam and Red River Fault.

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