Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

Marine Geology 418 (2019) 106043

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Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

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Lijie Wanga,c, Zhen Suna, , Jinhai Yangb, Zhipeng Sunb, Jitian Zhub, Haiteng Zhuoa, Joann Stockd a

Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China Zhanjiang Branch of China National Offshore Oil Corporation Limited, Zhanjiang 524057, China c University of Chinese Academy of Sciences, Beijing 100049, China d California Institute of Technology, 252-21, Pasadena, CA 91125, USA b

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Editor: Michele Rebesco

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6–10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2–6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults.

Keywords: Post-rift igneous complexes Hydrothermal vents Magma plumbing system Sill complexes South China Sea

1. Introduction Igneous complexes are composed of intrusive sills, dykes, and extrusive lava flows and vent-like volcanos (Lee et al., 2006), which may or may not partly have linkage to hydrothermal vents (Reynolds et al., 2017a). The observation of the widespread magmatic activity starts from the volcanic/magma-rich margins which were proven to play a critical role in the continental rifting and breakup (Hodges et al., 1999;

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Davies et al., 2002; Planke et al., 2005; Cartwright and Møller, 2006). However, magmatism is limited in the magma-poor margins before the breakup (Franke, 2013). Most of the detected igneous bodies on these margins are formed during the post-rift stage (e.g. Yan et al., 2006; Schofield and Totterdell, 2008; Peron-Pinvidic et al., 2010). Study of the morphology, distribution, and characteristics of igneous complexes thus has great significance for the understanding of magma plumbing processes, geodynamics, and tectonic evolution of the continental

Corresponding author. E-mail address: [email protected] (Z. Sun).

https://doi.org/10.1016/j.margeo.2019.106043 Received 28 October 2018; Received in revised form 4 September 2019; Accepted 7 September 2019 Available online 09 September 2019 0025-3227/ © 2019 Elsevier B.V. All rights reserved.

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second rifting stage (Late Oligocene), new faults developed from these faults due to stress field variation, e.g. the F2-1 fault, derived from the F2 fault, controlled the sedimentation of the Lingshui sag (Lei et al., 2011). We call F2 and F2-1 together the F2 fault zone. After these two rifting stages, the central depression had formed a hyperextended continental crust which was thinned to < 3 km, but it failed to develop into a spreading ridge system (Nissen et al., 1995; Qiu et al., 2001; Huang et al., 2011). Following the breakup of the southwest sub-sea basin at 25 or 23 Ma (Briais et al., 1993; Barckhausen and Roeser, 2004; Li et al., 2014), the Qiongdongnan basin ceased rifting at ca. 23 Ma and entered the post-rifting phase (Ru and Pigott, 1986; Briais et al., 1993; Zhou et al., 1995). The major part of the central depression is located in an area with water depth > 300 m and is composed of six sags, including Ledong, Lingshui, Beijiao, Songnan, Baodao, and Changchang (Sun et al., 2015; Fig. 1a). The Ledong, Lingshui, and Beijiao sags are NE-NEE trending, west of the central depression, while the Songnan, Baodao, and Changchang sags trend NEE-EW in the east (Sun et al., 2015; Li et al., 2016; Fig. 1a). In the Lingshui sag, the activity of major faults stopped in the postrifting stage, with only a few faults being still active for a long time. The F2 fault zone strikes NE to E-W as a north boundary of the central depression in map view (Lei et al., 2011), constituting the northern boundary fault of the Lingshui sag (Fig. 2b). F2 is a listric fault and the F2-1 fault is a high angle planar fault (Fig. 2b). The F2 fault was active in the early rift phase, while the F2-1 fault was a continuously active fault from late Oligocene to late Neogene time (Yu et al., 2010; Fig. 2b). Bounded by the breakup unconformity, the strata within the Lingshui sag can be divided into syn-rift and post-rift sequences. In total, nine major horizons, from Tg to T0, have been recognized and dated, subdividing the whole succession into eight major formations from Lingtou to Ledong (Fig. 2a; Xie et al., 2012). Based on different tectonic subsidence and sedimentation rates, four main sequences can be recognized. Syn-rift Sequence I is mainly characterized by lacustrine to transitional facies, which dominated Eocene to lower Oligocene Formations (45–28.1 Ma). Syn-rift Sequence II is neritic to shallow marine clastic strata deposited during late Oligocene time (28.1–23.03 Ma). Post-rift Sequence I, the sediment of the early to middle Miocene (23.03–11.6 Ma), is characterized by shallow marine to bathyal facies in the central sag intercalated with delta facies at the edge of the sag. This sequence is composed of claystone and sandstone in different regions of the sag (Fig. 2a). Post-rift Sequence II, which formed during rapid subsidence from middle Miocene to Quaternary, is mainly deposited in the bathyal to the abyssal environment in the Lingshui sag (Fig. 2a). The Lingshui sag is our main study area, where high-quality 3D seismic and volcaniclastic rock samples have been acquired by CNOOC (Guo et al., 2017).

margin. Seismic-based interpretation has provided excellent insight into the morphologies, emplacement mechanisms, and interconnectivity of different types of igneous complexes in the rifted basins. However, in the magma-rich margins, the igneous bodies cannot be well imaged due to deep burial depths, especially when the post-rift magmatism is still active. Therefore, previous studies of the magmatic complexes have mainly focused on the seismic expressions of intrusions (Davies et al., 2002; Smallwood and Maresh, 2002; Planke et al., 2005), the identification of sill-linked volcanic mounds and hydrothermal vents (Reynolds et al., 2017a), the discrimination of magmatic plumbing systems (Holt et al., 2013; Schofield et al., 2015; Reynolds et al., 2017b), and the roles of igneous bodies on the development of petroleum systems (Delpino and BermúDez, 2009; Monreal et al., 2009; Rateau et al., 2013; Reynolds et al., 2016; Senger et al., 2017). Seismic data also suggested that the lateral magma flow of sill complexes plays a vital role in controlling the magma plumbing systems in the magma-rich margins (Magee et al., 2016). The field outcrops of the volcanic basins and modeling studies also indicate sill-sill abutment or crosscutting formed sill-sill junctions for magma transport between the sill complexes (Galland et al., 2009, 2018; Galerne et al., 2011). However, the situation for the magma-poor rifted margins is different because the magmatic activity mainly occurs in the post-rift stage. The post-rift igneous complexes show a distinct geophysical and morphological property contrast with the host rocks, and thus can be easily discriminated from the sedimentary basins within seismic data. For example, post-rift submarine igneous complexes and plumbing systems were commonly observed in the Newfoundland margin (Peron-Pinvidic et al., 2010) and offshore Australia margins (Schofield and Totterdell, 2008; Jackson, 2012; Jackson et al., 2013; Magee et al., 2013). However, most of the previous studies in the magma-poor margins were restricted to studying the shallow level magma emplacement (< 3 s TWT) due to limited data availability. Even at magma-poor margins, the shielding effect of the igneous bodies directly impedes imaging of the underlying magma plumbing system (e. g. Schofield and Totterdell, 2008; Lester et al., 2014; Gao et al., 2015). Therefore, further studies of the different levels of igneous complexes and magma plumbing systems in the magma-poor margins are still needed. To address these issues, we present the post-rift igneous complexes in the Lingshui sag, which is one of the largest sub-basins in the Qiongdongnan basin, northwestern South China Sea (SCS) margin. The area is characterized by the presence of voluminous igneous segments such as volcano craters, lava flows (Ying et al., 2012), and igneous intrusions (Zhao et al., 2016a, 2016b). Since 2008, CNOOC collected a new suite of 3D seismic data in the study area, with detailed coverage of these igneous components. Complementary well-processed 2D long cable seismic data clearly images the Moho reflections. In addition, well LSB-2W-1, which is located in the 3D seismic survey, happens to penetrate the edge of one cone-shaped structure (Guo et al., 2017). All these data provide an unprecedented opportunity to investigate the structure, magnitude, and extent of the post-rift magmatism in the basin, on a crustal scale.

2.2. Igneous activity history After a long rifting history since the late Cretaceous (ca. 65–33 Ma), the diachronous breakup of the SCS occurred at ca. 33 Ma and ceased at ca. 16 Ma, and the subsequent post-rift subsidence resulted in the largest marginal seas in the west Pacific (Ru and Pigott, 1986; Sun et al., 2011; Li et al., 2014). Different from many other rifted margins, the SCS margin has been defined as a magma-poor margin, given the occurrence of limited magmatism during the syn-rift and early post-rift stage (Li and Liang, 1994; Zou et al., 1995; Zhou et al., 1995; Yan et al., 2001; Hayes and Nissen, 2005; Qiu et al., 2011; Li et al., 2012). However, some recent studies have reported numerous seamounts and large-scale intrusions that are aligned along the extinct spreading ridge of the subsea basin in the post-rift stage (e.g. Yan et al., 2006). The geochemical dating of the dredged samples from seamounts suggested a long period of post-rift basaltic magmatism from ca. 23.8 Ma in the NW sub-sea basin (Li et al., 2015) to 11–3.5 Ma in the central sea basin (Taylor and Hayes, 1983; Tu et al., 1992; Yan et al., 2008; Wang et al., 2009; Yang et al., 2011). The post-rift igneous bodies are also found on the lower

2. Geological setting 2.1. Tectonic-stratigraphic framework Offshore Hainan Island, the Qiongdongnan basin is a component of the NEE-trending Cenozoic rifted basins in the SCS (Fig. 1a). Three basic tectonic elements of the basin developed from north to south, including the northern depression, the central depression, and the southern uplift (Fig. 1b). The basin started rifting in the middle Eocene and experienced two episodes of rifting (Lei et al., 2011). In the first rifting stage (middle Eocene-early Oligocene), SE-dipping boundary faults, e.g. F5 and F2, controlled the development of the northern depression and the central depression (Lei et al., 2011; Fig. 2b). In the 2

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Fig. 1. (a) Regional structure map of the Qiongdongnan basin. The bathymetric data is from ETOPO1 (Amante and Eakins, 2009). The study area is marked by the red rectangle. The red dots labelled the wells drilled into post-rift igneous related rocks in and around the Qiongdongnan basin. The black dotted rectangle is the 3D seismic area. For sags in the central depression: LD-Ledong, LS-Lingshui, SN-Songnan, BJ-Beijiao, BD-Baodao, CC-Changchang. (b) Map illustrating the geological setting of the South China Sea region (Lü et al., 2013). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

detachment faults (Lei and Ren, 2016; Zhao et al., 2016b). More direct evidence from joint inversion of gravity and magnetic anomaly data revealed high-density igneous magma accreted below the lower crust during the final rifting and early spreading stage, which can serve as a potential source for the post-rift magmatism in the east Qiongdongnan basin (Qiu et al., 2013). Compared with the eastern region, the western part of the basin developed only small-scale post-rift igneous components (Zhao et al., 2016b). Identified extrusive lava flows are mostly present in the middle Miocene sequence in the southwest slope of the basin (Ying et al., 2012; Ying and Liu, 2012). Furthermore, various shaped sills that intruded into the syn-rift sequences have been reported by Zhao et al. (2016b), also in the southwestern part of the basin. However, the distribution, morphology, and relationship of igneous components, the magmatic plumbing system, and emplacement time in this basin are still poorly understood.

slope of the continent-ocean transition zone on both sides of the SCS margin (Clift and Lin, 2001; Franke et al., 2011; Gao et al., 2015; Song et al., 2017). In the Xisha and Nansha area, post-rift igneous bodies appear as isolated seamounts (Kudrass et al., 1986; Zhang et al., 2016; Wang et al., 2018). Constrained by the relationship between seamount and the onlapping stratigraphic units of known age within the seismic data (Zhu et al., 2017), the magma emplacement in the Xisha block was dated to 15.5 Ma and Pliocene (Zhang et al., 2016; Wang et al., 2018). The dredged basalts revealed a Pliocene or younger period of magmatism in the Nansha area (Kudrass et al., 1986; Yan and Liu, 2004). A large number of post-rift magmatic edifices were also identified in the continental marginal basins, especially in the deep-water area, such as the Pearl River Mouth basin (Sun et al., 2014; Zhao et al., 2014; Zhao et al., 2016a), the Zhongjiannan/Phu Khanh basin (Sun et al., 2012; Savva et al., 2013; Vu et al., 2017), and offshore Taiwan (Lester et al., 2014). Located in the northwest margin of the SCS, the Qiongdongnan basin has been proved to host substantial post-rift igneous edifices. The post-rift magmatism has been directly evidenced by the volcanic rocks and mantle sourced CO2 that were sampled in the post-rift sequence in different locations of the basin (Li et al., 1998; He and Liu, 2004; Guo et al., 2017). Interpretation from 2D seismic data (Mao et al., 2015; Zhao et al., 2016b; Guo et al., 2017) and in-situ measured heat flow (Shi et al., 2017) indicate that the distribution of post-rift magmatic bodies is preferentially confined to the eastern area of the Qiongdongnan basin, with very few occurrences in the west. For the origin of the postrift magmatism in the eastern region, some authors suggest that it may be closely related with the Hainan Plume (Zhang et al., 2016) or the

3. Dataset and method 3.1. Dataset 3.1.1. Seismic data The 3D seismic data used in this study covers an area of ca. 3000 km2 around the Lingshui sag and vertically records 8 s in two-way travel time (TWT) (Fig. 1a). Bin spacing of the 3D volumes is 25 m in the cross-line and 12.5 m in the in-line direction. In the sag fringe area where there is no 3D seismic data coverage, the pre-stacked time-migrated 2D seismic profiles were collected in a density of 1 km by 1 km to 3 km by 8 km. The 2D lines were mainly collected with a 7.5 km long 3

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Fig. 2. (a) Comprehensive chronstratigraphic and stratigraphic column (referenced Xie et al., 2012； Du, 2013).The geological time scale is from Ogg et al., 2016. The stratigraphic location of intrusive and extrusive components of the Qiongdongnan basin referenced from Guo et al., 2017; Li et al., 1998. (b) Geo-seismic interpretation from seismic line 11e30795. The interpretation modified from Li et al., 2016; Ren et al., 2014. The geographical location of igneous extrusion referenced Ying et al. (2012). The location of the section is shown in Fig. 1a.

tuffaceous claystones are distinct from those of the host rocks. The host rocks have a density of 2.3–2.47 g/cm3, while the tuffaceous claystones show a density of 2.0–2.15 g/cm3 (Fig. 3a). From the DT curves (sonic differential time curves) in Fig. 3, the calculated P-wave velocities of tuffaceous claystones are 3020–3340 m/s, a bit higher than the overlying layers and ca. 160–480 m/s lower than the underlying claystones. In contrast, wireline logging data shows high gamma-ray values for host rocks of the tuffaceous claystones (Fig. 3a). The tuffaceous claystones are embedded in the Meishan Formation, which is bounded by unconformities T40 (3389 m) and T50 (3888 m) in the well (Figs. 3 and 5; Guo et al., 2017). From the drilled well data and check shots, the measured depth of drilled tuffaceous claystones is about 3483–3536 m (Fig. 3a). The tuffaceous intervals represent the development and emplacement of the cone-shaped structure. Wellpreserved Calcareous nannofossils S. heteromorphus were identified from 3706 m to 3375 m (last natural occurrence in the well; Fig. 3a). The intervals most likely comprise the NN5 zone of the middle Miocene (ca. 15–13.61 Ma) (Xie et al., 2010; Ogg et al., 2016; Fig. 3a). Additionally, the planktonic foraminifera data in Fig. 3a show that the tuffaceous claystones fall in the N10 zone. The labelled species were Globorotalia peripheroronda identified from 3556 m to 3466 m (the last natural occurrence in the well；Fig. 3a).

streamer, a 12.5 m trace interval, a 2 ms sample interval, and their record length is 12 s in TWT. All the seismic data are displayed with zerophase, pre-stack depth-migrated and SEG normal polarity, which is downward increasing in acoustic impedance. The domain frequency of 3D seismic data is 35–45 Hz from 2.5 s to 4.5 s TWT at the middle Miocene sequence levels, where the interval velocity is ca. 2600 m/s according to the logging data of well LSB-2W-1. The domain frequency is attenuated to 18–25 Hz in the deep layer from 5.7 s to 7.0 s TWT, where the interval velocity is about 3100 m/s, according to LS33-1-1 logging data. These data result in a vertical resolution of 15–19 m for the middle Miocene sequence, and 31–43 m for the Eocene to Oligocene sequences. These parameters will be essential to identify and calculate the thickness of the igneous bodies in different stratigraphic levels.

3.1.2. Well data Well LSB-2W-1 was drilled in the northwestern margin of the Lingshui sag, penetrating two layers of tuffaceous claystones. They are 28 m and 20 m thick respectively, intercalated with a thin layer of claystone (Fig. 3a). Below the lower tuffaceous deposit, 411 m of early middle Miocene claystones were drilled. Cuttings sampled within the tuffaceous layers were described from 3483 m to 3511 m in the well LSB-2W-1. The primary mineral in the tuffaceous layers is clay. The secondary minerals are analcime, calcite, potassium feldspar and barite. A few grains of pyrite and glauconite were found in the rocks. Dark green rock debris was identified at a depth of 3500 m. X-ray diffraction analyses on-site showed that the tuffaceous cuttings are composed of 12% quartz, 8% potassium feldspar, 15% calcite, barite 5%, 54% analcime, and 6% clay minerals. Wireline logging suggests that the petrophysical properties of the

3.2. Methods 3.2.1. Seismic interpretation Based on the stratigraphic framework developed by CNOOC (Zhang et al., 2015) and the seismic well-tie to well LSB-2W-1, nine seismic horizons were interpreted regionally (Figs. 2a, 3a, and 5b). The 4

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Fig. 3. (a) Correlation between seismic and borehole data from well LSB-2 W-1. Synthesis of well LSB-2 W-1, key-interpreted units, and lithostratigraphic data were shown in columns of second, third, and seventh respectively. The light blue curve in the second column is synthesis of well LSB-2 W-1. The fourth to sixth columns show borehole logging data of well LSB-2 W-1: sonic differential time curve (DT；μs ft.−1) is shown in the fourth column, which can be calculated to the P-wave velocity. Gamma-Ray (GR) curve is shown in the fifth column, and density curve (RHOB) is shown in the sixth column. The right two columns show the biostratigraphic zones above and below the igneous rocks. (b) A table to explain the calculations about the tuffaceous claystone formed age. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

part (edifice/mound) and the lower part (chimney-like plumbing system) of these structures (Planke et al., 2005). We introduced seismic volcanostratigraphy, igneous seismic facies, and igneous seismic geomorphology (Planke et al., 2017) based methods to describe the geomorphology and internal mound structures. The typical upper part of the mound structures usually appears as crater-, dome-, or eye-shaped (Planke et al., 2005; Hansen and Cartwright, 2006a). Based on this character, we defined the upper part of each structure as a mound-

interpretation focused on the igneous complexes that disturbed sequences from Eocene to upper Miocene and the corresponding seismic interfaces, in particular, the T40 (top Middle Miocene), T50 (top lower Miocene), TM (top mounds horizons), and BM (bottom mound horizon). All these horizons are well defined, with continuous reflections in the study area and are used to guide the interpretation of igneous complexes. Hydrothermal vent and volcanos complexes consist of the upper 5

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contour lines become sparse and show the shape of the lateral lobes (Fig. 4). All the mounds are capped by the upper Meishan and/or lower Huangliu Formation (Fig. 5). The mounds and the onlapping layers are separated by the Top Mound Horizons. The Bottom Horizon of the mounds lies within the middle Miocene interval. Results of paleontological fossils in the well LSB-2 W-1 in the Lingshui sag show that the sediments around the mounds were deposited in a shallow marine environment (Fig. 3a), which suggests that these mound structures formed in a submarine environment. Twenty-one of these mounds are isolated, but five others have apparently linked lobes and form compound mounds (Figs. 5a, 6, and 8a). Most of the mounds are distributed in the northern slope of the Lingshui sag, nine of which are around the axis of the F2 and F2–1 faults. Ten other mounds are located in the central Lingshui sag (Fig. 4). The number and size of mounds decrease toward the center of the Lingshui sag. No mound was distinguished in the southeast part of the 3D seismic data (Fig. 4). Mounds on the northern slope, like M1 and M26, show NW-SE trending long axes, parallel to the dip direction of the slope. Near the faults, the mounds trend mainly NE, the same as the fault orientation (Fig. 4). In the central Lingshui sag, mounds display no orientation preference. According to the Top Mound reflections, three crater-shaped, six dome-shaped, and twelve eye-shaped isolated mounds were recognized. The compound mounds (M25) are composed of two or more separate concentric mound structures, which predominantly display as dome shape (Fig. 8a, b). The crater-shaped mounds in the Lingshui sag show little erosion toward the bottom sequence and appear in angular unconformity with the overlying strata (Figs. 5a and 6b). All of these mounds are unconformably downlapped by the overlying strata. The top reflections of the mounds exhibit moderate to high positive continuous amplitude, e.g. M17 (Fig. 6b). Three crater-shaped mounds have a relatively wider base (Figs. 5a,7b, and d), with basal diameters of 4.08–6.18 km, central summit heights of 0.27–0.63 km, and average flank dips of ≤13.3° (Fig. 7a). The summit height shows a moderate, negative correlation with basal diameter and volume (Fig. 7d). As shown in Fig. 6a and c, the M14 and M24 mounds are domeshaped. They display a flat-lying and concordant relationship with the underlying layers. The basal reflections are coincident with moderate to high positive amplitude. The upper boundaries of all the dome-shaped mounds are convex (Fig. 6a, c). However, the reflections of the overlying strata display three different kinds of features: 1) the overlying reflections are characterized by a subtle divergence, like M25 in Fig. 8a; 2) the overlying sequences gradually prograde toward the mound, such as M14 and M26 (Figs. 6a and 8a); 3) the capped reflections exhibit concordant deformation with the mound, e.g. M24 (Fig. 6c). In addition, as the acoustic impedance is changing at different locations between the mounds and the overlying strata, the top reflections of the dome-shaped mounds exhibit distinctively variable amplitude from low to high, from a trough to a peak event, from smooth to rough, and from continuous to semi-continuous for different mounds (Figs. 5a and 6). These varied reflections indicate that the composition is changing at the different parts of the mound. Moreover, the dome-shaped mounds have relatively smaller volume, ranging from 0.13 to 0.98 km3. The volume increases with the basal diameter and summit height (Fig. 7e, f). In contrast, the average flank dip shows a weak negative correlation with the basal diameter and volume (Fig. 7b, c). The eye-shaped mound is more common than the other two types of mounds in the Lingshui sag. Mound M19, shown in Fig. 6d, is characterized by eye-shaped reflections and semi-continuous to chaotic internal reflections. The top reflections appear moderate amplitude and semi-continuous (Fig. 6d). The mound was subtly truncated by the submarine channels on the top (Fig. 6d). The base of this mound is concave-upward. It shows slight truncation toward the underlying layers. The base horizons of eye-shaped mounds are characterized by high and positive amplitude. The volumes of the eye-shaped mounds

shaped region that was covered by the TM horizon and exhibited downlapping internal reflections on the BM horizon (Fig. 5). The internal architecture of the upper part of the mounds was clearly visible in the seismic reflections of interval tuffaceous claystones on the well-tie to LSB-2W-1. The volcanostratigraphy and hydrothermal vent seismic facies reviewed by Planke et al. (2000, 2015), Magee et al. (2013), and Reynolds et al. (2017a) were also used for guiding the interpretation. Following methods we used, a coherence map along 20 ms above the Bottom Mounds Horizon was mapped to image the internal mounds and surrounding sediments. Root Mean Square (RMS) amplitude extraction was used to highlight the different amplitude values between mounds and the surrounding strata. We interpreted sills according to their distinct reflection characteristics, with laterally limited high amplitude reflections, terminating abruptly, cutting across or/and influencing the surrounding layers on the seismic profile (Brown, 2004; Planke et al., 2005; Thomson and Schofield, 2008; Jackson et al., 2013). We defined several Top Intrusion Horizons that are the uppermost peak events of similar reflection properties. Because the sills could be composed of several individual or merged units, we separate out these reflections if they do not overlap in depth and are not laterally continuous. Together, we interpreted the Top Sill Horizons using guided 2D auto tracking, with ten-line interval inline/crossline spacing. The subsequent deeper trough reflection was picked (ten-line interval inline/crossline) as an estimate for the base of the sill. Considering the prominent response of sills on 3D seismic data, automatic tracking, Root Mean Square (RMS) amplitude highlighting, and 3D visualization were used to depict and map the morphology and structure of the intrusive sills in this study. 3.2.2. Calculating geometrical parameter of igneous bodies We quantified the geometries of the igneous complexes, including the basal diameter, summit height, average flank dip, and volume of mounds and the emplaced depth, volume, and thickness of intruded sills. All of these geometries were calculated in the depth domain. Since different seismic facies represent the discrepancy of composition of intra-mounds (Magee et al., 2013), the intra-mound velocity is also changing. The final well calibrated 3D pre-stack domain migration (PSDM) internal velocity was used for time-to-depth conversion by Liu et al. (2014). The emplacement depth of the sills was calculated from the distance from depth-converted grids of the Top Intrusion Horizons minus the paleo-seabed horizons when the sills intruded. The volume of the sill is calculated by time thickness of the isochron multiplied by the internal velocity of the sill. The isochron of time thickness is calculated from time grids of the Top Intrusion and Bottom Intrusion Horizons. A commonly accepted internal velocity of 5500 m/s was used to calculate the sills' volume (Berndt et al., 2000; Bartetzko et al., 2005; Magee et al., 2015). The average thickness of each sill was calculated by the volume and area. 4. Results Based on the abovementioned seismic related methods, as well as the seismic well-tie to LSB-2W-1, significant horizons were picked in detail to constrain the stratigraphic framework and including T40 (top middle Miocene), T50 (top lower Miocene), TM (top mounds horizons), and BM (Bottom mound horizon), Top Sill Horizons, Bottom Sill Horizons, and the Eocene-Oligocene boundary. A range of igneous complexes were recognized and mapped in the Lingshui sag in detail (Figs. 4–11). 4.1. Mound characteristics 4.1.1. Morphology and geometry of mounds Twenty-six mound structures were identified on seismic profiles in the middle Miocene sequence, most of which show concentric structures on isopach maps (Fig. 4). Toward the fringe of the mounds, the 6

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Fig. 4. Distribution of mounds, F2 fault zone, overlain on the Bottom Mound Horizon's depth-structure map. The thickness map of Bottom Mound Horizon and Top Mound Horizons (grey to white scale) shows the distribution of the mounds in the Lingshui sag. The mounds with red label numbers are distributed in the central section of the sag, while the mounds with numbers are distributed along the F2 fault zone. In the map view, the F2 fault zone is NEE trending and includes: (1) Preexisting faults (black dashed line) underlying mounds; (2) Black line are long period active faults that continued to be active after the mounds formed. Colors indicate the depth grid map of the Bottom Mounds Horizon (see Fig. 1a for the area). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

characterized by a flat base and concordant reflections with the underlying layers. Like the isolated dome-shaped mounds (such as M14), the compound mounds are directly capped by the overlying sediments (Fig. 8a). The top reflections of the compound mounds also show different amplitudes due to differences in acoustic impedance.

are similar to those of the dome-shaped mounds, ranging from 0.06 to 1.54 km3. Nearly two-thirds of the eye-shaped mounds are smaller than 0.3 km3 (Fig. 7c). Similar to the dome-shaped mounds, although independent from the variation of summit height (Fig. 7a), the average flank dip of the eye-shaped mounds is 8.4°–22.69°, which shows a weak negative correlation with basal diameter and volume (Fig. 7b, c). Moreover, the summit height and the basal diameter of the eye-shaped mounds range from 0.13 to 0.51 km and from 0.94 to 4.56 km respectively. Only this type of mounds exhibits a positive correlation between the summit height and basal diameter (Fig. 7d). The compound mounds, like M25 shown in Fig. 8a, are

4.1.2. Intra-mound Two different intra-mound architectures were revealed by seismic facies between the concentric structure and lobes of the mounds. The concentric structure of the mound exhibits chaotic internal reflections on the seismic profile (such as M14 in Fig. 6a), which is characterized 7

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Fig. 5. Seismic profile (a) and sketch interpretation (b) illustrating the spatial and stratigraphic relationships between sills, associated feeders, and extrusive mounds of the middle Miocene igneous complexes in the basin. TM, the Top Mound Horizons. BM, the Bottom Mounds Horizon. Detailed descriptions of the components of the igneous complexes are shown in Figs. 8a, and 9b, d. Location of the profile is shown in Fig. 1a.

coherence map along 20 ms above the Bottom Mounds Horizon (Fig. 8c) shows that the concentric structure of the mounds displays high variance (low coherence), which is markedly different from the surrounding lobate reflections and linear faults (Fig. 8c). On the RMS amplitude extraction map, the lobes appear high amplitude and can be easily distinguished from the mound and the surrounding sediments (Fig. 8d). However, most of the concentric structures are smaller than the actual size of mounds both on the top mound time map and the seismic coherence map (Fig. 8b, c). The RMS amplitude extraction between Top and Bottom Mounds Horizon shows that the intra-mounds

by a package of various amplitude, middle-high frequency, and discontinuous reflections. However, the lobes of the mound usually show concordant reflections. The concordant reflections are distributed around the concentric structure and constitute the lobes, which typically have one or several parallel high-amplitude and continuous reflections (such as M14 in Fig. 6a). According to the drilling results in well LSB-2W-1, the low impedance, high negative amplitude, and concordant reflections of M1 are composed of tuffaceous claystones and a thin layer of claystone (Figs. 3 and 8a). In contrast, the concentric structure of M1 shows chaotic reflections (Fig. 8a). The seismic

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Fig. 6. Seismic profiles and interpretive sketches of the dome-shaped (a, c), crater-shaped (b), and eye-shaped (d) mounds and the upper part of the plumbing system. The locations of the sections are indicated in Figs. 1a, 4, and 5a.

mounds (Bottom Mounds Horizon) as the emplacement depth at the time of intrusion (Fig. 9a; Table 1). No high amplitude reflections of intruded sills can be identified in the sediment layers deeper than 7000 ms (TWT) from either 3D data or 2D long cable profiles of the Lingshui sag (Figs. 2b, 5a, and 9a). The intrusive sills are mainly concentrated around the central part of the Lingshui sag (Fig. 10a), where the crust is hyperextended (Fig. 10b; Qiu et al., 2013). Seismic data revealed these sills in the detected resolution. When the sills are thick enough (over 86 m), they exhibit a series of highamplitude lower frequency seismic reflections (Fig. 5a). The top of the intrusions is represented by the high positive amplitude, while the bottom of the sills is a negative phase with moderate-high amplitude (Fig. 5a). When the intrusive sill is too thin to be discriminated, the sill may show a single peak-trough doublet (Fig. 9b, d, and e). However, limited by seismic resolution (see Section 3.1.1), the sill will not be detected if the thickness is < 8–10 m thick in shallow layers or 16–22 m thick in deeper layers. The reflection on the edge and bottom of the sill usually shows lower amplitude than the central part (Fig. 9b), which

correspond to low amplitude anomalies and the lobes display high amplitude anomalies (Fig. 8d). The profile and plan view maps suggest that the lobate reflections were distributed around the concentric structure of the mounds or flowed into the low-lying space, which can be exemplified by M1 (Figs. 6a and 8a, c, d). Both chaotic and concordant reflections existed commonly in the dome-shaped mounds (Figs. 6a and 8a), while the chaotic reflections predominated in the eyeand crater-shaped mounds (Fig. 6b, d). 4.2. Intrusive sills Compared with juxtaposed deposits, abnormally high-amplitude reflections in the deep sequences were interpreted as intrusive sills. In total, ninety-eight sills were identified from lower Oligocene to lower Miocene sequences in the Lingshui sag (Fig. 5). Surrounding the intruded sills, no obvious syn-emplacement ‘forced folding’ was observed (Fig. 5). The sills are identified between 4000 and 7000 ms, which is approximately equal to 1.2 to 6.3 km beneath the shallow extrusive 9

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Fig. 7. Graphs highlighting the statistic relationships between the different morphometric parameters measured for the 26 observed mounds.

sills comprise 17.3% of all the mapped intrusions (Fig. 9a; Table 1). They show slightly flat inner saucer and gently inclined rims from crosssection and 3D perspectives (Fig. 9b, c). These sills vary from 0.45 to 9.6 km in width, and 1.1–15.7 km in length (Table 1). The volume of

may suggest interbedded thin layered clay and dolerite (e.g. Planke et al., 2005). Three geometric types of sills were observed in the study area, including saucer-shaped, inclined, and compound sills. Saucer-shaped 10

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Fig. 8. (a) Enlarged map of the black square in Fig. 5a to display the morphology and architecture of the mound structures. (b) Time map of the Top Mound surface in part of the 3D survey area illustrating the distribution of mounds (black line with arrow). (c) Seismic coherence map along 20 ms above the Bottom Mounds Horizon highlighting discontinuities within the concentric structure of mounds (the location was shown in white line with arrow) and faults (the location was shown in light green line with arrow). The outlines of overlying mounds were shown in white dashed lines). The concentric of mounds, faults, and numerous fractures show low coherence and are highlighted on the map. The lobes distributing between the outlines and the concentric structure of the mounds are characterized by high coherence, which exhibit the same feature as the surrounding sediment layer. (d) RMS map was extracted from the Top and Bottom Mounds Horizon illustrating the reflection magnitude of the strata and mounds. The lava flows of mounds show high amplitude anomalies. The concentric structures of mound (the same location of the low coherence region in the mound as shown in the Fig. 8c) appear low amplitude reflections. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the saucer-shaped sills varies from 0.06 to 10 km3, and the thickness of the sill decreases slightly from the inner saucer to the inclined rims (Fig. 9b). The estimated thickness of saucer-shaped sills varies from 39 to 175 m. The RMS map shows that the amplitude of sills is high inside the saucer and moderate to low at the inclined rims (Fig. 9c). Inclined sheets constitute 61.2% of all the intruded sills (Fig. 9a; Table 1). Most of the inclined sheets are elongate and have concave-up shapes. The margins of sills have a relatively irregular shape from a 3D perspective view (Fig. 9a). The sills are 0.06–10 km3 in volume, while the thickness ranges from 56 to 377 m. In the shallow intervals, sheetshaped sills are parallel and weakly discordant with the surrounding layers (Fig. 9d). In the deep layers, the inclined sheets are steeper. In profile and spatial view, the other 21.5% of sills consist of multiple inclined sheets or/and saucer-shaped lobes (Fig. 9a; Table 1). These types of sills are called compound sills (Hansen and Cartwright, 2006b; Reynolds et al., 2017c). They connect to each other by step faults (Fig. 9b and d), with ‘T’ or/and ‘F’ shaped junctions, like the intruded sills documented in the volcanic margins (e.g. Thomson and Hutton, 2004). Fig. 9d–f illustrate the discrepant shape of compound sills in different directions. In the P-Q section (Fig. 9d), the sill shows several different dips where inclined sheet lobes are connected by a junction. In contrast, the S-T section (Fig. 9e) displays an elongate

discordant sheet shape. The amplitude varies at different parts of sills in the RMS map, indicating that the compound sills are composed of multiple fingers. The connected junction usually displays a fault that cut off the sill (Fig. 9f). Though the compound sills are laterally interconnected by junctions and formed a larger intruded area, other sills are isolated, especially in the margin of the sag (Fig. 9a). The seismic profile, 3D view map, and plan view illustrate the distribution and connection of the sills in the Lingshui sag (Figs. 5, 9a, 10, and 11). Our results indicate that 17% of the sills are not vertically or laterally connected to other sills, and occur as isolated features. These sills mainly intruded in the edge of the sag, for example, the sills in the west region (Figs. 5 and 9a). The remaining 83% of the sills display a complex nature, vertically stacked or linked by vertical dykes or by a laterally extensive series of interconnected inclined sheets, saucer-shaped sills, or/and compound sills. These form several groups of interlinked sill complexes mainly in the central and eastern part of the Lingshui sag at a large scale (Figs. 9a and 10). The seismic profile crossing the biggest sill complex in the central part of the Lingshui sag shows that most of the sills are connected to others by vertical dykes and lateral junctions, and a few of them are partially or entirely overlapped (Fig. 11). Almost every sill is linked to one or more vertical dykes or faults at the edge of the sill, forming the sill complexes (Fig. 11b and c).

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Fig. 9. (a) Time structure map of Top Intrusion Horizons (sill) in the 3D space to show the sill morphologies in the Lingshui sag, where saucer-shaped, inclined sheets, compound sill, and sill complexes are displayed in different levels with different dip angles. The sills are more isolated in the edge of the Lingshui sag, while the overlapped vertically and interconnected sills formed sills complex network occur in the central of the Lingshui sag. The region of the map see 3D seismic area in Fig. 1a. (b), (d), and (e) Seismic cross sections to show the shape of the sills. The saucer-shaped sills and inclined sheets formed as lobes of the compound sills, which are interconnected laterally by step junctions. The path of the seismic cross-sections is indicated in Fig. 9 c and f. (c) and (f) artificially illuminated 3D views of compound sills produced by mapping the conventionally picked top sill reflections overlying the RMS map of sill reflections. Vertical axis is TWT downward and counters line represent deeper parts of the sills. Red colors represent high amplitude of the sills whilst purple colors are low amplitude. The region of the maps showed in Fig. 9a with different directions. (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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Fig. 10. (a) Map illustrating the distribution of post-rift igneous complexes in the Lingshui sag. The same sequence volcanic craters and related lava flows in the south slope of the Lingshui sag referenced from Ying et al. (2012). (b) Isochron map of the continental crust in the Lingshui sag. The isochron map was calculated by the one-way travel time of reflections of the continental crust (Time grid of Moho (interpreted from 2D long-cable seismic profiles) minus Time grid of Tg (the Top Basement Horizons)) plus the average crustal velocity of 6.2 km/s according to Huang et al. (2011).

basement). Part of these dykes penetrated to the sill and pinch out at the lower part of mound (Fig. 5a), while the others pinch out near the bottom of the sills (e.g. Fig. 9d and e). In addition to the F2 fault zone, numerous fault-like steps were observed in the central Lingshui sag (Fig. 11). These faults were found on the edge or crest of the sill complexes and extended to the deep layer beneath the sills (Fig. 11a and b). The faults are narrow and sub-vertical in section view (Fig. 11a, b). Most of the faults contain several branches when they extend from deep to shallow layers. Some of the faults obviously cut the sills, while others just slightly affect the imaging of the sills in the deep layer. Some faults can also connect sills in different levels and make them overlap vertically, while some of them ascended to the shallow layer and disappeared in the fracture sequence without any shallow sills (Fig. 11a).

4.3. Feeders and faults Field-, modeling-, and seismic reflection-based studies indicate that feeders provide a pathway for magma or fluids originating from magma intrusions to transport magma vertically to different levels (e.g. Jamtveit et al., 2004; Kavanagh et al., 2006; Galerne et al., 2011; Jerram and Bryan, 2015; Eide et al., 2016). In the shallow layer, the feeders may show a few different features on the seismic profile, e.g. funnel-shaped conduit, “wash-out” shaped conduit, and “pull-up” shaped pipes are observed under the mounds (Figs. 6 and 8a). Funnelshaped conduits partly disturbed the surrounding strata. Inside the conduits, the reflections are mostly chaotic and exhibit various dip directions (Fig. 6a and c). The diameter of the conduit increases gradually from the lower part to the top part (Fig. 6a, c, and d). “Wash-out” shaped conduits are often detected beneath the crater-shaped mounds (Fig. 6b), which exhibit low amplitude, semi-continuous, and blurred reflections (Fig. 9b). The feeder under M14 in Fig. 6a is characterized by “pull-ups” which distorted the amplitude and reflections. In the concentric center of the pull-up, the reflections are more chaotic (Fig. 5a). On the seismic profile, most of the feeders beneath the mounds extended to the deep layer (Fig. 5a). Some of them are narrower in the lower part and seem to originate from the crest of sills (Figs. 5a and 9), while the others are almost connected to fault planes at depth (e.g. feeder beneath M14 in Fig. 5a). Not all the feeder zones display a traceable path on the seismic profile from the mound down to their origin, especially when the deeper parts of the conduits are cut off by subsequent faults or intruded sills (e.g. feeders beneath M25 in Fig. 5). The uncertain factors of feeders and linking to the sills in the deep layer also is related to the limitations of seismic detection (see Section 5.3 for more details). At the northwestern edge of the study area, several possible dykes can be identified as sub-vertical discontinuities that can be traced below or between the horizontal sills (Fig. 5). Internal dykes exhibit disrupted, altered, or brecciated zones on the seismic profile, which is similar to the feeders beneath the mounds (Fig. 5a). The dykes seem to vertically originate from the crust or fault plane below 6–7 s (near the sediment

5. Discussion The igneous intrusions and related vent structures provide evidence for studying the post-rift fluid migration and magma plumbing system in the Lingshui sag. A few questions arise from our mapping, including aspects of the detailed origin and formation of seismically-interpreted mounds, magma emplacement age, and the igneous plumbing system in the Lingshui sag. The key to these questions can be constrained by examination of the seismic and well data, and the comparison of field observations, laboratory and numerical modeling in other basins. The same layered volcanic system found in the southern Lingshui sag was also formed in a similar middle Miocene igneous complex as one complete system (Fig. 10; Ying et al., 2012). The discussion below includes mound origins, age correlations, uncertain interpretation of igneous plumbing systems, and the evolution model. 5.1. Origin of mounds By comparing the seismic characters of the mounds in this study with reported volcanic mounds and hydrothermal vents elsewhere, such as in the northern Atlantic margins, offshore Australia, and the northern SCS margin (Jamtveit et al., 2004; Svensen et al., 2004; Planke 13

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Fig. 11. Diagram illustrating key features of intrusive sill complex distribution and connectivity. (a) Seismic cross section of sill complexes in the Lingshui sag. (b) RMS map of the same seismic cross section in Fig. 11a. (c) Interpretational sketch of sill complexes and dykes. See Fig. 1 for the line location.

documented in the Bass basin (Magee et al., 2013). No volcanic lava flows were found connecting the sills in this study. Only two of the isolated mounds are not circular in map view: M1 and M26 (Fig. 4). They seem to have a large area of lobes that could be lava flows. However, the lobes show high negative amplitudes (indicating low impedance), unlike the high positive amplitude (high impedance) of lava flows found on the southern slope of the sag (see locations in SE corner of Fig. 10a; Ying et al., 2012). This means the lava flow-like morphology may correspond to hydrothermal venting fluid flows with lower density and velocity. In contrast, all the mounds in the northwest of Lingshui sag are commonly found at a consistent stratigraphic level (Bottom Mounds Horizon) like the hydrothermal vents along the northeast Atlantic margin (Svensen et al., 2004), whereas the volcanism usually occurs in multiple episodes on different pre-eruptive surfaces. Contextual arguments also favor the interpretation of these mounded structures as being of fluidization origin. The small-scale intrusions and large emplacement depth of sills indicate that few extrusive volcanoes should be formed. Most of discovered sill-fed volcanoes are emplacement at a depth of < 1.2–1.5 km (Bell and Butcher, 2002), especially in the magma-poor margins (Magee et al., 2013; Zhao et al., 2014, 2016a; Reynolds et al., 2017b). When the magma is enough to produce a large scale of sill complex, sill-fed volcanoes may be formed (Planke et al., 2005). However, hydrothermal venting can occur when the sill complexes are emplaced at depths from 1 to 9 km (Planke et al., 2005; Schofield et al., 2015; Reynolds et al., 2017b). The intrusive emplacement depth is 1.2–6.3 km in this study area, and the limited intrusive volume suggests to us that the extrusive mounds are hydrothermal vents. This wider evidence also supports a hydrothermal vent origin for the cone-shaped structures.

et al., 2005; Hansen, 2006; Svensen et al., 2006; Grove, 2013; Magee et al., 2013; Zhao et al., 2016a; Reynolds et al., 2017a), we suggest that there are two possible interpretations for the origins of the mound structures, both of which are linked to underlying sill intrusions. We argue that the cone-shaped structures in this study could be interpreted either as volcanic mounds or hydrothermal vents. Compared with these vent features in the other margins, the cone-shaped mounds found in the Lingshui sag have similar morphology, volumes, and dimensions to the volcanic mounds and hydrothermal vents (Fig. 7; Zhao et al., 2014; Reynolds et al., 2017a). However, the direct well calibration, petrophysics of the mound samples, and seismic expressions of mounds indicate the origins are more consistent with hydrothermal vents than volcanic mounds. The composition of the mounds described here are predominantly sedimentary rocks from the well LSB-2W-1 with minor components of pyroclastic rocks (see Section 3.1.2). In contrast, the volcanic mounds usually have a high content of volcanic materials, such as hyaloclastite, pillow lavas, or basalts (Bell and Butcher, 2002; Magee et al., 2013; Reynolds et al., 2017a). The composition of mounds in this study resemble those causing venting and hydrothermal degassing that can involve significant disruption of the overlying rocks, but not always specifically involving the eruption of volcanic material, similar to those demonstrated onshore (e.g. Svensen et al., 2006; Angkasa et al., 2017). In particular, the velocities of tuffaceous intervals in this study (3020–3340 m/s) are smaller than those documented from mafic shield volcanoes (e.g. 3300–5500 m/s; Calvès et al., 2011; 3300–4600 m/s; Zhao et al., 2016a; 2090–4025 m/s; Reynolds et al., 2017a). The chaotic internal mounds (e.g. M1, M25, and M26 in Fig. 8a) probably composed by mixture of the remobilized sediment, fluids, or breccias (e.g. Jamtveit et al., 2004; Planke et al., 2005; Grove, 2013) that usually shows lower velocity than pure mudstone and volcanic rocks. The lower densities (2.0–2.15 g/cm3) of tuffaceous intervals also indicated the mounds contain limited volcanic materials but they have been sources of fluid generations. Furthermore, well-tie and seismic facies also suggest the mounds are hydrothermal vents rather than volcanoes. The well-tie of LSB-2W-1 shows the lobate of mounds are low impedance, with negative amplitude, which is different from the moderate-high positive amplitude on normal upper volcanic mound reflections (Bell and Butcher, 2002; Magee et al., 2013; Zhao et al., 2016a; Reynolds et al., 2017a). Internally, most of mounds herein have parallel negative moderate-high amplitude reflections in their flanks (e.g. M14 in Fig. 6a) instead of positive moderate-high amplitude hummocky reflections that are more typical of volcanoes (Reynolds et al., 2017a). The dome-shaped mounds also have the layer-parallel, low-moderate amplitude reflections that are similar to the flank reflections of hydrothermal vents in other volcanic basins (e.g. Kjoberg et al., 2017; Reynolds et al., 2017c). Additionally, the eye-shaped hydrothermal vents described by Planke et al. (2005) have inwardly dipping reflections in their lower parts, like the funnel-shaped conduits beneath M24 (Fig. 6c). These reflection features are unlike the ‘pull-up’ or blurred reflections beneath the cone-shaped volcanoes (Reynolds et al., 2017a). Even through ‘pullup’ reflections were found at the lower part of some of the dome-shaped mounds (e.g. M14 and M25 in Fig. 5a), we consider these reflections to be artifacts, due to the lower wave impedance of central hydrothermal vents than host rocks, and the disturbed reflections exhibit a higher negative amplitude than volcanoes with a high wave impedance (Jackson, 2012). Similarly, the morphology and volume of mounds also indicated the mounds are more likely to be interpreted as hydrothermal vents. Most of the mounds have steeper flanks (7–23°), smaller summit heights (< 0.19 km), and lower volumes (< 3 km3) than the shield volcanoes

5.2. Age for post-rift magma emplacement Although we lack isotopic dates from tuffaceous claystones or other igneous samples, a rough age constraint can be achieved using seismicbased techniques. Previous studies suggest that there are two seismicbased techniques to elucidate the magma emplacement time: 1) the relative age between post-intrusion onlapping strata and the deformational structures at sill-sediment contacts, i.e., their stratigraphic relationships (Davies et al., 2002; Trude et al., 2003; Hansen and Cartwright, 2006a); and 2) dating the base of the sill-fed mound structures (Davies et al., 2002; Planke et al., 2005). The timing of magma emplacement can be further constrained when biostratigraphic information from drilled wells can be acquired within the surrounding successions, which is the case in this study. Although syn-emplacement deformations are not obvious in the study area, biostratigraphic data from clastic rocks above and/or below the tuffaceous claystones in well LSB-2W-1 can provide excellent conditions to use the second method to constrain the relative igneous age by dating the basal boundary of the hydrothermal vents (Fig. 3a). On the seismic profile crossing the well LSB-2 W-1, the bottom boundary of the tuffaceous claystones displays high-amplitude and positive seismic reflections (Fig. 3a), which corresponds to the Bottom Mounds Horizon. In addition, all mounds appear to have their base sitting on this horizon (Fig. 5). These stratigraphic relationships indicate that fluid release probably occurred after deposition of the strata of the Bottom Mounds Horizon, and the Bottom Mounds Horizon should represent the contemporaneous paleo-seabed (c.f. Schofield and Totterdell, 2008; Jackson, 2012; Zhao et al., 2014). In this study, the age of mound development was calculated by using the biostratigraphic data focusing on the middle Miocene sediments below the tuffaceous claystones, sedimentation rates, and third-order sequence boundaries 15

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Table 1 Measurements of intrusion dimensions. Intrusion number

Intrusive type

Part of sill complex

Length (km)

Width (km)

Emplacement depth (m)a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

Compound Saucer Inclined sheets Inclined sheets Saucer Inclined sheets Compound Inclined sheets Saucer Saucer Saucer Inclined sheets Saucer Inclined sheets Saucer Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Compound Inclined sheets Inclined sheets Inclined sheets Saucer Saucer Inclined sheets Inclined sheets Compound Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Saucer Compound Inclined sheets Inclined sheets Inclined sheets Inclined sheets Saucer Compound Saucer Inclined sheets Inclined sheets Inclined sheets Inclined sheets Compound Inclined sheets Inclined sheets Compound Inclined sheets Compound Saucer Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Compound Saucer Compound Compound Inclined sheets Compound Inclined sheets Saucer Compound Inclined sheets Inclined sheets

No Yes Yes No Yes Yes No No No Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes yes Yes yes Yes Yes Yes No Yes Yes No No yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No Yes No No Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes No No No No No Yes Yes

9.07 13.2 1.45 2.9 4.67 2.16 4.03 2.23 2.76 4.08 1.75 2.12 8.45 4.84 2.32 3.31 1.1 2.7 1.73 1.72 2.08 2.21 1.78 3.48 4.22 6.34 4.87 4.99 4.11 1.86 2.26 2.9 3.52 2.62 2.53 2.72 3 7.27 2.29 1.45 2.33 2.29 5.59 3.7 3.03 6.08 8.92 5.4 4.57 3.52 1.71 1.66 10.64 3.45 4.48 4.33 1.6 2.83 3 3.4 3.09 2.76 3.6 3.16 3.8 5.24 8.9 4.48 3.38 8.57 3.89 4.77 6.48

5.2 9.6 0.93 1.6 2.02 1.2 2.38 1.54 2.01 2.25 1.38 1.21 4.75 1.58 1.74 2.55 0.45 0.89 1.45 1.2 1.7 2.1 1.23 1.36 3.23 3.48 2.56 2.88 2.8 0.58 1.49 0.83 1.59 2.44 1.52 1.25 1.61 3.06 1.22 0.89 1.57 1.63 4.91 2.81 2.84 4.35 6.33 1.54 3.21 3 1.62 1.44 5.24 2.35 2.86 3.39 1.44 2.66 1.63 2.8 2.65 2.36 2.1 2.14 2.28 2.83 4.98 2.97 2.51 6.21 2.79 3.76 2.79

1235 1837 1963 2134 2218 2348 2383 2483 2585 2767 2900 3047 3097 3100 3137 3147 3175 3353 3409 3454 3497 3586 3590 3625 3650 3662 3663 3668 3686 3725 3728 3851 3860 3983 4096 4115 4130 4131 4163 4207 4324 4336 4387 4451 4451 4543 4568 4692 4724 4724 4725 4743 4793 4807 4852 4873 4960 5006 5017 5068 5089 5116 5136 5155 5193 5211 5229 5417 5456 5459 5561 5675 5707

Volume (km3)

4.55 6.82 0.10 0.32 0.36 0.27 0.60 0.24 0.18 0.62 0.16 0.21 4.58 0.51 0.25 0.51 0.41 0.15 0.19 0.15 0.17 0.36 0.18 0.36 0.53 3.69 0.98 0.87 0.87 0.13 0.49 0.19 3.39 1.13 0.41 0.34 0.35 2.85 0.23 0.10 0.34 0.24 2.03 0.81 0.63 1.05 5.01 0.56 2.63 1.14 0.33 0.16 5.36 0.06 2.29 0.78 0.18 0.73 0.45 0.81 0.77 0.39 1.11 0.70 1.26 1.28 4.04 0.58 0.77 3.48 0.94 1.76 1.65

Average thickness (m)a

Fed mounds

119 134 105 116 125 105 113 114 39 117 98 113 141 98 87 118 137 129 107 103 91 102 103 131 50 301 109 105 89 103 377 109 100 295 139 115 160 145 97 103 113 102 100 131 119 56 145 96 153 154 137 103 125 90 199 175 113 125 130 136 123 120 213 144 175 121 160 61 163 103 148 149 126

No Yes No No No No No No No No No No Yes Yes No No No No Yes No No Yes Yes No No No No Yes No Yes No No Yes No No Yes No Yes No Yes Yes No No No Yes No No Yes Yes Yes Yes No No No No No No Yes Yes No Yes Yes No Yes No Yes Yes No Yes Yes Yes No Yes

(continued on next page) 16

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Table 1 (continued) Intrusion number

Intrusive type

Part of sill complex

Length (km)

Width (km)

Emplacement depth (m)a

Volume (km3)

Average thickness (m)a

Fed mounds

74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Compound Inclined sheets Inclined sheets Inclined sheets Saucer Compound Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Saucer Inclined sheets Inclined sheets Inclined sheets Inclined sheets Inclined sheets Compound Inclined sheets Compound Compound Inclined sheets Compound Inclined sheets

Yes No Yes No No No No No Yes No Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes

5.02 11.26 2.9 2.97 3.14 5.32 15.17 2.14 4.57 9.51 2.41 3.35 4 4.12 5.04 5.13 3.99 4.14 5.44 2.74 8.28 6.57 4.23 7.87 2.41

2.48 5.29 1.82 1.43 1.9 2.94 8.61 1.65 3.78 6.56 1.55 1.67 3.7 2.15 2.37 1.57 2.38 3.33 2.39 1.24 4.43 2.66 2.28 4.75 1.55

5751 5844 5847 5862 5961 6032 6131 6301 6142 5919 4015 6075 4139 3775 5211 4385 3843 4146 5528 4272 4388 4223 4238 5413 4015

1.16 0.37 0.58 0.31 0.78 1.50 10.00 0.41 2.93 5.43 0.39 0.58 1.24 0.64 1.16 0.64 0.88 0.97 1.51 0.45 5.25 2.04 1.45 3.57 0.39

112 126 149 134 175 113 135 173 253 148 123 145 132 104 119 87 116 137 106 118 154 126 174 135 128

Yes Yes No Yes Yes Yes Yes Yes No No No No No No No No No No No No No No No No No

a

Well calibrated 3D Pre-stack domain migration (PSDM) internal velocity was used for depth conversion. The emplacement depth is not decompacted.

the time, such as errors in the sedimentation rate. The calculated age of 13.2 Ma clearly conflicts with the top of the NN5 zone, which is 13.6 Ma. However, the tuff interval is just 32.6 m above the bottom of the N10 zone. That means the other uncertain factors have relatively little effect on the calculated results. Therefore, the clear occurrences of Globorotalia peripheroronda (N10 zone marker species) are used to support an early middle Miocene (ca. 14.6 Ma) age for the hydrothermal vent development in the study area. The occurrences of Globorotalia peripheroronda also have mixed age uncertainties because the fluidization effects may carry some older sediments to the paleo-surface along with their biomarkers. Despite the different intrusion depths, the different intrusion thickness implying different cooling times, the sparse points of biostratigraphic data, and the errors of sedimentation rate, all of which cause uncertainties in constraining the magma emplacement time, we suggest that these sills probably were emplaced at ca. 14.6 Ma. The magma emplacement during the middle Miocene in the Lingshui sag can be further understood when placed into the tectonic framework of the entire Qiongdongnan basin. Based on the chronology outlined by CNOOC (Xie et al., 2012) and the stratigraphic contact between igneous bodies and surrounding strata, it is found that massive magmatism began to occur since the end of early Miocene in the Qiongdongnan basin (Zhang et al., 2016; Zhao et al., 2016b), which also corresponds to the cessation of seafloor spreading of the SCS (Taylor and Hayes, 1983; Li et al., 2014). In addition, the middle Miocene magmatism in the Lingshui sag also coincides with Neogene long-term deep thermal upwelling (Shi et al., 2017), continuous fault activity (Li et al., 2016), and the stage of relatively slow post-rift subsidence of the northwestern margin of the SCS, especially east of the central depression of the Qiongdongnan basin (Li et al., 1998; Xie et al., 2006; Zhao et al., 2013). We conjecture that the magmatism began to be active in a broader zone after the seafloor spreading ceased in the SCS (ca. 16 Ma), when post-spreading magmatism was seen both in the sub-sea basin and the continental margins.

established by the CNOOC (Xie et al., 2010; Liu et al., 2009). The tuffaceous intervals represent the development and emplacement of vents. From the drilled well data and check shots, the measured depth of drilled tuffaceous claystones is about 3536–3483 m (Fig. 3). Well preserved Calcareous nannofossils of Sphenolithus heteromorphus were identified from 3706 m to 3375 m (last natural occurrence in the well), which suggests that the tuffaceous interval most likely lies within the NN5 zone of the middle Miocene (ca. 15–13.61 Ma) (Xie et al., 2010; Ogg et al., 2016; Fig. 3a). The last occurrence of Sphenolithus heteromorphus was common in the drilled wells and is widely used as a marker species for the middle Miocene in the Qiongdongnan basin (Xie et al., 2010). In addition, the planktonic foraminifera data in Fig. 3 show that the volcaniclastic rocks fall in the N10 zone (14.78–14.16 Ma). The labelled species were Globorotalia peripheroronda identified from 3556 m to 3466 m (the last natural occurrence in the well). The bottom depths of the NN5 and N10 zones are 3706 m and 3556 m respectively. That means the sediment thickness (ignoring compaction) from the bottom of tuffaceous claystones to the biostratigraphic markers of known age is 170 m and 20 m respectively. Taking into account the compaction, the initial sediment thickness is calculated to be ca. 276.2 m and ca. 32.6 m by using the back-stripping method (Watts and Ryan, 1976; Steckler and Watts, 1978; Sclater and Christie, 1980; Zhao et al., 2013; Fig. 3b). Assuming the average sedimentation rate in the sequence of the Meishan Formation is ca. 150 m/ m.y. (Zhao et al., 2015), the sediment period from the biostratigraphic markers to the bottom of tuffaceous claystones can be calculated and the results are 1.8 m.y. and 0.22 m.y. (Fig. 3b). Therefore, the age of mound eruption is probably 13.2 Ma or 14.6 Ma from the Calcareous nannofossils and planktonic foraminifera in well LSB-2W-1, respectively (Fig. 3b). Below the NN5 and N10 zones in the well LSB-2W-1, no younger species were found (Fig. 3a). We can suggest that the tuffaceous intervals may therefore represent the initial appearance of species from zone N10. Although the marker species of NN5 is common and widespread in the study area, the zone represents a relatively long geologic interval from 15 to 13.6 Ma and a thicker sediment layer (3706–3375 m, not accounting for compaction) in the well location. It will introduce errors if there are some uncertain factors for constraining 17

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from lower Miocene to the deep layer (Fig. 11a). These steep reflections seem to cut off the sills and formed step junctions, which is analogous to geologic observations at the sand intrusions that were arrested by the polygonal faults in the Faroe-Shetland basin (Bureau et al., 2013). On the right hand side of Fig. 11a where identified sills are limited or none, however, these steps are still numerous. Several seismic variance time slices from 7000 to 4500 ms reveal that these steps are unoriented polygonal fractures which are more chaotic and unoriented in the shallow layer (Song, 2012). Additionally, these steps are different from the dykes known from outcrops worldwide. The dyke swarms usually have an orientation or radial pattern with igneous underplating that is indicated by seismic data and outcrops (Wall et al., 2010; Minakov et al., 2017). Seismic modeling reveals that presence of multiple, stacked, and interconnected sills may also generate complex small-scale faults recognized by their interference patterns (Rabbel et al., 2018). These interference patterns are mostly distributed surrounding the sills, tens to two hundred meters away (Eide et al., 2016). This phenomenon is not like these steps that can be traced vertically over a long time scale (ca. 0.5–2 s, TWT) and have a clear fault plane reflection in this study. The connections visible as steps on the seismic data between sills are probably polygonal fractures rather than the actual junctions, especially when the frequency is lower in the deep layer (e.g. Fig. 11). Even though the vertical magma transport is not clearly revealed through the seismic data, these numerous possible vertically extensive small faults, including these coexisting with wave interference reflections in the shallow layer (Fig. 11), and the tectonic faults, such as the F2 fault zone, probably provide convenient pathways for magma transport from depth and interconnection of sills.

5.3. Uncertain interpretation of the magma transport system from the seismic imaging The volume, distribution, and connection of sill complexes are indicators of magma transport in the sedimentary basins that possibly influence the scale, extrusive volcanoes, and fluid flows. Seismic modeling, constrained by well data and outcrops, suggest that the seismic frequency content and impedance contrast can influence the igneous seismic imaging (Magee et al., 2015; Eide et al., 2017). The models revealed that numerous thin sills have little to no obvious amplitude anomaly on the seismic profile (Rabbel et al., 2018), although they could generate wave interference similar to small faults on the seismic profile. The modeling studies also indicate that significant amounts of intruded sills, locally up to 88%, could be missing when interpreting seismic data in volcanic basins (Magee et al., 2015; Schofield et al., 2015). Well-logs of LS33-1-1 (see location in SE area of Fig. 10a) revealed that host rocks in the deep layer have an increasing impedance with depth (Du, 2013). However, these host rocks still showed a lower P-wave and lower density than the doleritic sills (Skogly, 1998). This indicates that frequency content is a critical factor to affect detection of sills. Like the sill complexes in the magma-rich basins (Smallwood and Maresh, 2002; Thomson and Hutton, 2004), we can traditionally map parts of sills with a large thickness (> 20 m) on the seismic data by picking the high-amplitude reflections (Figs. 9 and 11). Limited by seismic resolution (see Section 3.1.1), the sills will not be detected if the thickness is < 8–10 m at the shallow depths or 16–22 m at greater depths. Additionally, in any of our 2D or 3D data (i.e. Figs. 2a and 11a) no sills are observed at depths > 7000 ms TWT in this study. We conclude that either the sills are too thin to image at these depths, or else that magma transport at depth was via dykes that are too steep to be imaged at such depths. On the seismic profiles, there are several sills that are interconnected through junctions. These junctions are similar to the broken bridges that were shown cross-connecting sills in outcrops in Jameson Land, East Greenland (Eide et al., 2016). Sill-sill junctions for magma transport between the sill complexes were confirmed by outcrops and analogue modeling (Galland et al., 2009; Galerne et al., 2011). However, the component of the junctions is fragments between host rocks that are artifacts of magma-filled fractures (Rabbel et al., 2018). Seismic modeling also showed these junctions may be seismic interference between two isolated sills (Magee et al., 2015). Additionally, geochemical data between two connected sills showed that even when sills were in contact, it does not imply that they were feeding each other (Galerne et al., 2011). Internal disturbed feeders are imaged beneath or/and surrounding the sills complexes in this study (Figs. 9 and 11). We compare seismic expressions of faults and dykes observed in other locations to discuss their presence and uncertainties in the interpretation of these steps in this study. Sub-vertical dykes have rarely been interpreted using seismic data, due to the fact that the method was designed for imaging the sub-horizontal structures. Wall et al. (2010) used 3D seismic data and magnetic data to interpret the 0.5–2 km wide vertical disturbance of seismic reflections as dykes with negative magnetic anomalies. Most of the dykes were directly linked to collapse craters in the shallow layer. The downward thinning internal ‘pull-up’ reflections between sill tips and hydrothermal pipes are probably related to fluidization of the rock due to pressure build-up in the metamorphic aureoles of underlying sills (e.g. Kjoberg et al., 2017; Reynolds et al., 2017b). Similarly, in our data, reflections between sills and underlying sills with a range of dip angle probably can be interpreted as dykes (Fig. 9d and e). These vertical and sub-vertical disturbed reflections are like the field observations of dykes (Eide et al., 2016) and the sill interconnections in the Barents Sea (Minakov et al., 2017). The dykes mainly are distributed surrounding the isolated sills at the edge of the sag (Fig. 9). However, some of the sub-vertical and vertical reflections have branches that formed surrounding the identified sills that are extending

5.4. Middle Miocene igneous complexes and evolution model After comparison of all of the distribution of igneous bodies, tectonic stress, and pre-existing tectonic structures in the Lingshui sag, it is clear that the magma plumbing system here requires different formative models from the magma-rich basins and active margins. Similar to the magma-rich rifted basins (e.g. Cartwright and Hansen, 2006; Schofield et al., 2015), the distribution of the identified Middle Miocene igneous complexes in the Lingshui sag is related to the thinning of continental crust and pre-existing faults. Fig. 10 shows that most of the sills are distributed in the central part of the Lingshui sag, where the continental crustal thickness is 6–10 km. From the center to the margin of the Lingshui sag, the number of intrusive sills decreases, and the sills become more isolated, where the continental crust thickens to 10–12 km. Note that no sills can be identified at the edge of the Lingshui sag from either 2D or 3D seismic data, where the crustal thickness is over 12 km. This means that the intruded sills are mainly concentrated in the center of the hyperextended continental crust. In contrast to the sills, the hydrothermal vents and volcanic bodies are distributed mainly along the pre-existing fault zones on the rift shoulders of the Lingshui sag (Fig. 10). Both the number and the size of the volcanic extrusions and vents decrease toward the central part of the Lingshui sag. Most of the hydrothermal vents were detected on the tips or crests of intruded sills along the F2 fault zone, the northern rift shoulder of the sag (Fig. 10). These hydrothermal vents were on-lapped or draped by the upper Miocene or/and Pliocene bathyal mudstone in parallel blank reflections (Figs. 3 and 5). In contrast, volcanic complexes are preferentially present in the southern slope of the Lingshui sag, where the volcanic craters are distributed along the pre-existing faults in an NEE orientation (Ying et al., 2012). The crustal thickness of the southern slope is 14–16 km. Note that no sills were found under the volcanoes in the southern slope of the sag (Fig. 10). In the magma-rich rifted margins, lateral magma flows are primarily transported within the widespread sill complexes influenced by the host-rock lithology and the state of stress, instead of vertical dykes (Magee et al., 2016). Sills in the magma-rich margins often form interconnected complexes and tend to extend from the basement to the 18

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entire basin fill (Cartwright and Møller, 2006). Sill complexes are capable of transporting magma to tens of kilometres vertically and hundreds of kilometres laterally (e. g. Cartwright and Møller, 2006; Leat, 2008; Schofield et al., 2015). In our study, most sills are of small volume (< 10 km3), and were emplaced at a large depths of 1.2–6.3 km with a total coverage area of ca. 885 km2 (Table 1). The total volume of identified sills is ca. 127.8 km3 with the above feeding vent deposits of ca. 26.8 km3 (data from Table 1 and Fig. 7). Most of the sill complexes are distributed in the central part of Lingshui sag (Fig. 10a). However, the seismic artifacts between sills and isolated distribution of sills indicate that the intrusions are distributed individually. Most of them are not laterally and vertically interconnected. Some sub-vertical dykes beneath the isolated sills and numerous uncertain lateral connections indicate that vertical dykes possibly played a role in transporting magma from depth. Furthermore, the smaller diameter of sills in this study than those documented at the volcanic rifted margins also indicates that lateral magma transport was confined in the Lingshui sag. Different genetic models of magmatism have been proposed to control the post-rift magma activities in the northwest margin of the SCS, especially in the central Qiongdongnan basin where the continental crust was hyperextended. These models include the warmer asthenosphere (Shi et al., 2017), lower crustal/mantle flow caused by depth-dependent extension (Zhao et al., 2016b; Lei and Ren, 2016), and the Hainan mantle plume (Mao et al., 2015; Zhang et al., 2016). Due to the absence of related geophysical and geochemical information, the size, location, and depth of melt source are still not clear in the Lingshui sag. The 2D long-cable seismic profiles are very clear, showing good continental crustal structure in the Lingshui sag, not only showing the clear Moho (Ren et al., 2014), but also revealing the magma interconnection pathways and magma plumbing systems. Compared to the magma-rich margins, intruded sills in the Lingshui sag are much smaller, more isolated, and distributed in a more dispersed fashion (Fig. 11). This suggests the crucial role of the existent vertical dykes. In the central sag, abundant vertical polygonal faults beneath the sill intrusions seem to extend downward to the deep layer below the thinned crust. These faults possibly provide good conduits for magma transport, sourced from the melt at the crust-mantle boundary. Additionally, the faults along both shoulders of the sag may also form another pathway for magma ascending from depth. Outcrops in the Neuquen basin, Argentina show that the orientations of dyke swarms may follow pre-existing faults, in which case they may not systematically relate to the principal tectonic stress directions (Spacapan et al., 2017). Similarly, all the analysis above suggests that the pre-existing tectonic faults have exerted an important control on the magma emplacement and distribution in the shallow layer (see Fig. 12), which provide a preferential, high-permeability pathway for magma flowing and erupting onto the paleo-seafloor (Valentine and Krogh, 2006; Thomson, 2007; Valentine and Gregg, 2008). Indeed, sills emplaced in rifted basins show clear evidence that normal faults affect the morphology of sills, as concordant sill segments are connected by intrusive segments emplaced along faults (Magee et al., 2013). In foreland basin settings, sill emplacement may also be controlled by thrust faults and ramps (Galland et al., 2007; Ferré et al., 2012). In sum, faults probably provide vertically weak zones in the deep layer and help magma ascend through the thinning continental crust and inject into the rift basin for magma transport and storage. Taking into account all the aspects together, we propose a 3D model that illustrates the mechanism and evolution of post-rift magma emplacement in the magma-poor rifted margins (Fig. 12). In the deep layer, the magma plumbing system may be dominated by vertical dykes, which can transport magma and control the sill distribution and connection patterns in the basin. In the shallow layer, thick sedimentary sequences (related to the emplacement depth), and the polygonal faults in the central depression confine the sills' distribution, with limited hydrothermal vents developed above them. Fault systems can also serve as a good pathway for magma to ascend at high speed from melt source to the paleo-seabed and form cone-shaped

Fig. 12. The 3D conceptual cartoon model showing the distribution, magma plumbing system, and a possible mechanism of the post-rift igneous complex in the Lingshui sag, one of the classical magma-poor rifted basins.

volcanoes. This is especially the case in the southern slope (Fig. 10) and Fig. 12. Though characterized by a similar morphology and component of the igneous complex, our work shows a more diversified organization of magma plumbing systems similar to those known from magma-rich margins. This model also highlights the importance of the underlying continental crust in influencing the sites and development of igneous complexes, and the distribution of magma intrusion and eruption within extensional rift systems. 6. Conclusions A series of sill complexes and hydrothermal vents are identified and studied in detail in the Lingshui sag, central western area of the Qiongdongnan basin for the first time, using well samples and 2D/3D seismic data. After discriminating all the identifiable post-rift igneous complexes, we carried out a spatial relationship comparison and drew the following main conclusions: (1) Twenty-six mounds have been mapped individually or in compound bodies that have crater-, dome-, and eye-shaped structures in the study area. Most of the mounds occurred above the crests of intruded sills, which are distributed preferentially along the F2 fault zone in a NEE orientation. Diversely shaped feeders connect the mounds to deeply buried sills, which may act as the feeders of the mounds and suggest that the mounds have a genetic linkage with sills in the deep layer through the feeding system. (2) The mounds found in the Lingshui sag show similar geometries and dimensions to those found in the magma-rich margins. However, the direct well calibration, petrophysics of the mound samples, and seismic expressions indicate that the cone-shaped mounds are hydrothermal vents. (3) Intruded sills have also been recognized as sheet, saucer or/and compound shapes in the seismic dataset. In the central sag, most sills tend to connect with each other by junctions to form sill complexes, while the sills are much isolated at the edge of the sag. The sills are distributed mainly in the Lingshui sag where the continental crust is 6–10 km thick, suggesting that post-rift magmatism is related to thinning continental crust. (4) Biostratigraphic data and the calculated sedimentation rate from 19

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the well LSB-2 W-1 are used to constrain the age of igneous activity to ca. 14.6 Ma. This age correlates with the intense magmatism in the east part of the Qiongdongnan basin since the end of the early Miocene (ca. 16 Ma), and might be affected by the cessation of SCS spreading and the continuous fault activities. (5) Numerous thin sill intrusions and stepped dykes probably cannot be seen by seismic imaging in contrast to the outcrops and seismic modeling. Compared to the magma-rich rifted margins, igneous complexes in the Lingshui sag are of small scale and are more sparsely distributed. Their isolated distribution indicates that scattered vertical feeders may play an important role in delivering magma from depth. (6) Sill-linked hydrothermal vents tend to be present along the F2 fault zone, while volcanoes linked with no sills tend to occur on the southern slope of the sag along pre-existing faults. The intruded sills are mainly confined within the hyperextended crustal center, where few overlying venting structures were discriminated. The distribution of magmatic bodies and hydrothermal vents indicates sediment thickness, and pre-existing faults may influence the middle Miocene small-scale magma transport, storage, and venting in the Lingshui sag.

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