Geological development of the Central and South Vietnamese margin: Implications for the establishment of the South China Sea, Indochinese escape tectonics and Cenozoic volcanism

Tectonophysics 478 (2009) 184–214

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Geological development of the Central and South Vietnamese margin: Implications for the establishment of the South China Sea, Indochinese escape tectonics and Cenozoic volcanism Michael B.W. Fyhn a,⁎, Lars O. Boldreel a, Lars H. Nielsen b a b

Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark Geological Survey of Denmark and Greenland, GEUS, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark

a r t i c l e

i n f o

Article history: Received 5 January 2009 Received in revised form 21 July 2009 Accepted 3 August 2009 Available online 8 August 2009 Keywords: Escape tectonics Seismic analysis Continental margin Volcanism South China Sea Vietnam

a b s t r a c t The Vietnamese margin forms a key region to the understanding of escape tectonics and the development of the South China Sea (SCS). The existing geological reconstructions of the region are restricted to studies of single basins, are based on limited amounts of geophysical data or analysis of onshore geological features. These models are critically assessed on the basis of interpretation of the most comprehensive 2-D digital seismic database published to date within the area combined with a thorough analysis of existing literature, and a new model is presented. The Vietnamese margin is underlain by a series of Paleogene rift basins established through southeastward extrusion of Indochina. The East Vietnam Boundary Fault (EVBF) forms the almost 1000 km long seaward continuation of the left-lateral Ailao Shan-Red River Shear Zone (ASRRSZ). Toward the southern half of the Phu Khanh Basin the EVBF breaks up into discrete segments and splays into the SE-directed Tua Hoa Fault Zone. Paleogene faults splayed from the EVBF and the Mae Ping Shear Zone and accommodated the coeval motion of these two major left-lateral structural lineaments. During the late Oligocene, basin inversions offshore occurred contemporaneously with initial right-lateral inversion along the Mae Ping Shear Zone and the onset of major uplift of the metamorphic core complexes along the ASRRSZ. It is suggested that a dramatic change of the regional stress pattern occurred in response to the northward movement of India and the effective coupling of the West Burma Block and India, the later resulting in broadening of the indenting continental mass. After the mid-Oligocene, left-lateral movements across the offshore EVBF decreased and eventually ceased. Later, onshore sinistral movements were accommodated by internal shortening and local clockwise block rotations within the Shan-Thai Terrain. Renewed rifting offshore south Vietnam resulted from the jump of the SCS spreading axis and subsequent Neogene southwestward propagation of continental break-up. The Neogene opening of the SCS is viewed as a consequence of a slab-pull associated with subduction of the proto-SCS underneath Borneo as offshore Neogene escape tectonism had virtually ceased. During this phase of rifting, right-lateral transtension is inferred along N to NW oriented fault zones in the Phu Khanh and the Nam Con Son basins. Rifting and by inference seafloor spreading continued until the end of middle Miocene time although at significantly reduced rates during the final 5–10 Ma. Termination of seafloor spreading is marked by a distinct latest middle Miocene unconformity in the Nam Con Son and the southern Phu Khanh basins. Neogene volcanism resulted in the build-up of several magmatic structures across the central and South Vietnamese margin and induced reactivation of older faults in the Phu Khanh Basin. The offshore magmatism heralded widespread volcanism onshore southern Indochina associated with a latest Neogene uplift and denudation event. The uplift led to significantly increased siliciclastic accumulation rates offshore, which repressed widespread carbonate deposition. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Present address: Geological Survey of Denmark and Greenland, GEUS, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark. Tel.: +45 3814 2718; fax: +45 3814 2050. E-mail addresses: [email protected] (M.B.W. Fyhn), [email protected] (L.O. Boldreel), [email protected] (L.H. Nielsen). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.08.002

The central and south Vietnamese continental margin forms the transition from the continental Indochina Block to the SCS, which makes the margin a key area for understanding the complex Cenozoic development of Indochina and the SCS (i.e., Tapponnier et al., 1986). The study is therefore of significance to the understanding of the

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geological development of both Indochina and the SCS and thus brings important insight for the understanding of the local petroleum systems and their distribution, although we only briefly deal with the hydrocarbon aspect in this paper. The studied part of the margin is floored by the Phu Khanh Basin in the north and farther south by the Cuu Long and the Nam Con Son basins; all deep Cenozoic rift and sag basins flanking the Con Son Swell (Fig. 1). The establishment of the basins have been linked to continental-scale strikeslip faulting or to an Atlantic-type continental break up by previous studies (Taylor and Hayes, 1980, 1983; Holloway, 1982; Tapponnier et al., 1982, 1986; Khy, 1986; Leloup et al., 1995, 2001a; Tan, 1995; Rangin et al., 1995b; Marquis et al. 1997; Matthews et al., 1997; Roques et al., 1997a,b; Huchon et al., 1998, 2001; Lee & Watkins, 1998; Lee et al., 2001; Pubellier et al., 2005; Clift et al., 2008; Fyhn et al., 2009a). This pronounced divergence in opinion probably reflects 1) the modest data availability and quality; 2) that only a few published integrated basin studies offshore Vietnam exist; and 3) that most previous basin studies fail to fully incorporate the complex geological setting of the region. This article presents an integrated study of the Phu Khanh and the northern parts of

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the Cuu Long and the Nam Con Son basins extending ca. 500 km from north to south, which is incorporated into the complex coeval regional geological development. The basin analysis is based on ca. 20,000 km of high-quality 2-D seismic reflection and well data (Fig. 2) and together with the regional geological analysis aims to illuminate the Cenozoic development of the region viewed in a larger tectonic perspective. As the results of this study markedly differ from those of previous studies a short critical overview of earlier works is presented initially. 2. Geological setting The outline of the Vietnamese continental margin was formed through Cenozoic rifting (Fig. 1). Dating of the rift-onset remains uncertain owing to the scarcity of data from wells penetrating the lower successions of the basins and the predominance of Paleogene terrestrial sedimentary sequences that make biostratigraphic dating imprecise. Rifting in the Song Hong Basin and in the largest basins of the Gulf of Thailand commenced during the Eocene (Fina, 1994; Bustin and Chonchawalit, 1995; Nielsen et al., 1999; Andersen et al., 2005; Clift and Sun, 2006;

Fig. 1. Location of major Cenozoic basins and areas underlain by oceanic crust as well as a simplified structural outline of the region. THFZ = Tua Hoa Fault Zone. The study area outlined in the box is shown in detail in Fig. 2. Basin and fault outline modified after Shaoren et al. (1994); Mat-Zin & Swarbrick (1997); Morley (2002); Huyen et al. (2005); Hall et al. (2008); Morley et al. (submitted for publication). Areas underlain by oceanic crust modified after Briais et al. (1993); Huchon et al. (2001); Raju et al. (2004) and Curray (2005).

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2.1. Paleogene–mid-Oligocene Paleogene rifting along the Vietnamese margin has been interpreted to be associated with both the continental collision of India and Eurasia and a slab-pull from subduction of the proto-SCS underneath Borneo (Rangin et al., 1995a; Hall, 1996; Marquis et al., 1997; Matthews et al., 1997; Roques et al., 1997a,b; Lee and Watkins, 1998; Lee et al., 2001; Pubellier et al., 2005; Clift and Sun, 2006; Clift et al., 2008; Fyhn et al., 2009a). Great controversy exists on this subject. The following sections presents an overview of the two proposed driving mechanisms.

Fig. 2. Location of seismic reflection data along with wells and basins as well as bathymetry derived from seismic interpretation.

Meijun et al., 2008; Morley et al., submitted for publication). Based on the comparable geological setting an Eocene age is also interpreted for the onset of rifting in the study area. This conforms with Eocene sediments drilled in the Cuu Long Basin (Hoa, 1996; Binh et al., 2007) and an 800 m thick Eocene conglomeratic succession that crops out along the shores (Areshev et al., 1992). Regional Late Paleocene–Early Eocene compression and uplift in southern Indochina prior to rifting further narrow the riftonset age to between the middle and late Eocene.

2.1.1. Escape tectonics The Indian and Eurasian collision commenced some time during the Eocene (ca. 55–34 Ma)(e.g., Patriat and Achache, 1984; Aitchison and Davis, 2004; Aitchison et al., 2007). In response to the collision, the Indochina Block and the region south of it were extruded several hundred kilometers to the southeast through a series of major left-lateral fault zones (Tapponnier et al., 1982, 1986, 1990; Lacassin et al., 1993, 1997; Lee and Lawver, 1994, 1995; Leloup et al., 1995; 2001a; Wang and Burchfiel, 1997; Replumaz and Tapponnier, 2003; Wang et al., 2006; Akciz et al., 2008). Southward displacement and rotation of the Indochina Block indicated by paleomagnetic data are viewed as a consequence of the extrusion across these left-lateral fault zones (Huang and Opdyke, 1993; Yang and Besse, 1993; Yang et al., 1995; Chung et al., 1998; Chi and Dorobek, 2004; Takemoto et al., 2005; Charusiri et al., 2006). The rotation of Jurassic, Cretaceous and earliest Paleogene strata within the Shan Thai Terrain seems to have been particularly strong but varies locally, suggesting internal block rotations as well in the area. Similarly, variation in rotation south of the Shan Thai Terrain suggest that Indochina did not behave as a single rigid block in between the major left-lateral fault zones transecting the area, and only southward displacement seems to have taken place in south Vietnam (Chi & Dorobek, 2004). The largest of the left-lateral fault zones extends from the East Himalayan Syntaxis to the eastern margin of Vietnam as the ASRRSZ and its seaward continuation the EVBF (Fig. 1)(Leloup et al., 1995, 2001a; Rangin et al., 1995b, Nielsen et al., 1999; Andersen et al., 2005; Fyhn et al., 2009a). Onshore, left-lateral motion is generally regarded to have initiated during the Eocene/early Oligocene judged by thermochronological evidence and tentatively dated syntectonic sediments along the shear zone (Schärer et al., 1994; Leloup et al., 1995, 2001a,b, 2007; Zhang and Schärer, 1999; Jolivet et al., 2001; Gilley et al., 2003, Shoenbohm et al., 2005; Viola and Anczkiewicz, 2008). Others have inferred a late Oligocene or Miocene onset age of the left-lateral motion (Chung et al., 1997; Wang et al., 1998, 2000; Searle, 2006), although these ages are in clear opposition to the premid-Oligocene onset ages of left-lateral pull-apart rifting in the Song Hong Basin that forms the seaward continuation of the ASRRSZ (Rangin et al., 1995a,b; Nielsen et al., 1999; Sun et al., 2003; 2004; Andersen et al., 2005; Huyen et al., 2005; Clift and Sun, 2006). Left-lateral shearing along the southern part of the ASRRSZ is mainly considered to have taken place in a transtensive regime along the onshore metamorphic core complexes (Figs. 1, 3)(Leloup et al., 1995, 2001a; Jolivet et al., 2001; Anczkiewicz et al., 2007; Viola and Anczkiewicz, 2008). Transtensional faulting has thus been invoked to explain the exhumation of the lower to mid-crustal core complexes along the ASRRSZ (Leloup et al., 2001a; Jolivet et al., 2001; Anczkiewicz et al., 2007). Only scattered Paleogene deposits exist along the trace of the shear zone reflecting either non-deposition throughout the Paleogene or subsequent erosion. The Song Hong Basin forms one of the worlds largest pull-apart basins located on- and offshore Vietnam (Clift and Sun, 2006). The basin lies in direct continuation of the Dai Nui Con Voi metamorphic core complex of the ASRRSZ and is like the core complex to the north

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Fig. 3. Overview of the Indochina block showing the main middle and late Cenozoic structures. The heavily faulted Shan–Thai Terrain makes up the northern part of the Indochina block southeast of the Ailao Shan-Red River shear zone (ASRRSZ) that accommodated most post mid-Oligocene movement across the shear zone. The four metamorphic core complexes cropping out along the ASRRSZ are indicated. AS= Ailao Shan Core Complex, DCS = Dian Chang Shan Core Complex, DNCV= Day Nui Con Voi Core Complex, XLS = Xue Long Shan Core Complex, THFZ= Tuy Hoa Fault Zone, BWB= Beibuwan Basin, CLB = Cuu Long Basin, NCSB = Nam Con Son Basin, PKB = Phu Khanh Basin, QDB = Qiongdongnan Basin.

bounded by the left-lateral Song Hong Fault to the west and the Song Lo fault to the east (Figs. 1, 3). Major Paleogene–mid-Oligocene leftlateral strike-slip motion across the northern EVBF associated with movements across the ASRRSZ has been documented by Rangin et al. (1995a), Nielsen et al. (1999); Sun et al. (2003, 2004); Andersen et al. (2005); Huyen et al. (2005) and Clift & Sun (2006). Paleogene leftlateral movements and pull-apart rifting have been interpreted across the EVBF in the Song Hong Basin based on: 1) the presence of pervasive WNW to NW trending faults that outline extensional fractures along the NNW to NW trending outline of the EVBF in the basin, 2) the dominant right-stepping fault-splay geometry towards the Beibuwan and the Qiongdongnan basins, 3) the results of analogue modeling, and 3) the connection to the coeval left-lateral ASRRSZ (Figs. 1 and 3). Mapped eastward fault splays from the EVBF towards the Beibuwan and Qiongdognan basins were suggested to have taken up a considerable amount of the left-lateral motion (Rangin et al., 1995a; Nielsen et al., 1999; Sun et al., 2003, 2004; Andersen et al., 2005; Clift and Sun, 2006). According to Roques et al. (1997a) the left-lateral EVBF continued as far south as to the Phu Khanh Basin where it merged with a rightlateral fault zone. A right-lateral offset has been invoked across the EVBF offshore central and south Vietnam south of the fusion point (Marquis et al., 1997; Roques et al., 1997a; Clift et al., 2008). However, the fault merging remains undocumented in the Phu Khanh Basin and Fyhn et al. (2009a) interpreted a significant Paleogene left-lateral offset across the EVBF in the northern Phu Khanh Basin. The continuation of the EVBF south of central Vietnam has for a long time remained speculative. The fault was suggested to splay

seaward and thus to have controlled continental rifting and SCS seafloor-spreading by pull-apart (Tapponnier et al., 1982, 1986; Briais et al., 1993; Leloup et al., 1995, 2001a; Replumaz and Tapponnier, 2003). In accordance with this model, Matthews et al. (1997) suggested that the approximately E–W trending Paleogene rift system in the Nam Con Son Basin could be associated with undocumented NW–SE trending sinistral shear zones located nearby. In the southern part of the Indochina Block, escape tectonics resulted in Eocene–early Oligocene left-lateral motion along the NW– SE-trending Three Pagodas and the Mae Ping (Wang Chao) shear zones (Lacassin et al., 1993, 1997; Morley et al., 2007; Smith et al., 2007). Based on left-laterally offset geological markers, Tapponnier et al. (1986) and Peltzer & Tapponnier (1988) inferred a total left-lateral offset of ca. 300 km across the two shear zones. Left-lateral motion and pull-apart along the Three Pagodas Shear Zone was suggested to have opened the Pattani, Khmer, Malay and the West Natuna basins south of Indochina although this remains to be documented (Tapponnier et al., 1982, 1986; Leloup et al., 1995, 2001a; Lacassin et al., 1993, 1997; Replumaz and Tapponnier, 2003). Farther east the Cuu Long and the Sarawak basins were similarly interpreted as Paleogene pull-apart basins linked with the Mae Ping Shear Zone. However, a major NW-trending left-lateral fault zone transecting the southeast Vietnamese shelf has not been documented and data from the Sarawak Basin does not support this model as Eocene and younger rifting seems associated with NW–SE-oriented right-lateral shear zones instead of left-lateral (Mat-Zin and Swarbrick, 1997; Morley, 2002). In the Cuu Long Basin, Paleogene rifting occurred across NE– SW- and E–W-trending normal faults that splay toward the W and the

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WNW below the modern Mekong Delta, which could reflect the culmination of the Mae Ping Shear Zone or one of its fault strands into the basin (Lee et al., 2001; Hung et al., 2003; Thang, 2005). Regional paleogeographic reconstructions incorporate ca. 150 km of left-lateral offset across the southeasternmost part of the Mae Ping Shear Zone between late Eocene and mid-Oligocene time (Hall, 2002). It is difficult to reconcile such large offset given the apparent continuity of the Late Jurassic–earliest Paleogene magmatic belt straddling Vietnam's southeast coast where it should be transected by the suggested trace of the Mae Ping Shear Zone (Fig. 1). Gravimetric studies indicate that the Cuu Long Basin could not have taken up more than ca. 30 km left-lateral displacement (Huchon et al., 1998), and boudinage restoration may indicate as little as few tens of kilometers of leftlateral motion along the Mae Ping Shear Zone although a larger offset was suggested by Lacassin et al. (1997). The formation of the Cuu Long Basin is ascribed to a pull-apart mechanism (Tapponnier et al., 1982; Leloup et al., 1995, 2001a,b; Huchon et al., 1998; Morley, 2002; Hung et al., 2003), although, Lee et al. (2001) suggested a simple rift history of the basin independent from escape tectonism related to the initial break-up of the SCS. Many tectonic models suggest the Three Pagodas and the Mae Ping shear zones as continuous fault lineaments stretching more than 1000 km (Tapponnier et al., 1982, Briais et al., 1993; Leloup et al., 1995, 2001a; Lacassin et al., 1997; Replumaz and Tapponnier, 2003). However, better defined parts of the shear zones are composed of branching fault networks up to 250 km wide with important N–S and NW–SE trends forming large-scale, nested strike-slip-duplex structures (Morley, 2002, 2004; Morley et al., 2007, submitted for publication; Smith et al., 2007), and most of the fault zones are poorly defined leaving the interpretation of parts of the faults speculative such as that of the Mae Ping Shear Zone southeast of Thailand (Fig. 1).

2.1.2. Slab-pull tectonics Subduction of a proto-SCS and subsequent continental collision along northern Borneo is indicated by the presence of widespread accretionary complexes, ophiolite emplacement, volcanic activity, extensive compressional deformation, uplift and crustal thickening in northern Borneo and subducted proto-SCS slab imaged through seismic tomography underlying northwestern Borneo and Palawan. (Hamilton, 1979; Taylor and Hayes, 1980, 1983; Holloway, 1982; James, 1984; Williams et al., 1988; Hutchison, 1989, 1996, 2004, 2005; Hutchison et al., 2000; Bénard et al., 1990; Rangin et al., 1990; Hall, 1996, 2002; Hall et al., 2008; Curtis et al., 1998; Honza et al., 2000; Clift et al., 2008). During the latest Cretaceous and the Paleogene the proto-SCS and the continental Dangerous Ground moved toward a subduction zone located along the coast of Borneo (Fig. 1)(Hutchison, 1996, 2004, 2005; Hutchison et al., 2000). During Eocene time, after collision of the Luconian microcontinent and Borneo, subduction continued farther east and the associated southward drift of the proto-SCS and the Dangerous Grounds occurred along a several hundred kilometer wide zone transected by a series of right-lateral transforms in and east of Luconia (Fig. 1) (Hazebroek and Tan, 1993; Mat-Zin & Swarbrick, 1997; Morley, 2002; Hutchison, 2004; Liu et al., 2004; Clift et al., 2008; Hall et al., 2008). The slab-pull combined with fault splays from the transform controlled rifting prior to the break-up of the SCS as well as the subsequent seafloor spreading. A right-lateral transform fault was speculated to continue northward along the central Vietnamese coast, but remains undocumented (Taylor and Hayes, 1980, 1983; Holloway, 1982; Marquis et al., 1997; Roques et al., 1997a,b; Morley, 2002; Clift et al., 2008; Hall et al., 2008), whereas, a seismic study of the central Vietnamese margin demonstrates the continuation of the left-lateral ASRRSZ as far south as the northern Phu Khanh Basin (Fyhn et al., 2009a).

Latest Cretaceous and Paleocene rifting farther east along the south China margin preceded the collision age of India and Eurasia and has been regarded as either back arc extension or a slab-pull effect associated with the southward subduction of the proto-SCS (Holloway, 1982; Ru and Pigott, 1986; Wang and Sun, 1993; Pigott and Ru, 1994; Pinglu and Rao, 1994; Yu, 1994; Zhou et al., 1995; Morley, 2002; Clift et al., 2008). 2.2. Late Oligocene–earliest Miocene 2.2.1. Ailao Shan-Red River shear zone and the East Vietnam boundary fault Rapid mid-Oligocene to earliest Miocene uplift and exhumation are documented by thermobarometric investigations and radiometric dating of the metamorphic core complexes along the ASRRSZ (Schärer et al., 1990; 1994; Tapponnier et al., 1990; Leloup et al., 1995, 2001a,b; Nam et al., 1998; Wang et al., 1998, 2000; Zhang and Schärer, 1999; Gilley et al., 2003; Viola and Anczkiewicz, 2008). The exhumation of the northwestern-most metamorphic core complexes was interpreted as a consequence of up to hundreds of kilometers late Oligocene to early Miocene sinistral transpression, whereas the Ailao Shan and the Dai Nui Con Voi core complexes was suggested to have been exhumed through transtension (Fig. 3)(Leloup et al., 2001a,b). Transpressional uplift as far south as the Ailao Shan Core Complex was documented by Shoenbohm et al. (2005), and a similar uplift mechanism for the Dai Nui Con Voi Core Complex in Vietnam was suggested by Searle (2006). Large scale transpression is in accordance with the numerous brittle transpresional and thrust faults within the Ailao Shan Core Complex and in the neighboring Shan-Thai Terrain (Fig. 3)(Tapponnier et al., 1990; Leloup et al., 1995; Wang and Burchfiel, 1997; Wang et al., 2006; Akciz et al., 2008), and is also indicated in the adjacent shear zone farther southeast along the Dai Nui Con Voi Core Complex with the presence of several thrust faults subparallel to the shear zone (Fig. 3)(Tien, 1991; Lacassin et al., 1998; Leloup et al., 2001a). A large post mid-Oligocene left-lateral offset is contradicted by seismic studies of the Song Hong Basin that document an early phase of major strike-slip faulting followed by late Oligocene–early Miocene gentle rifting to post-rift sagging (Rangin et al., 1995a; Nielsen et al., 1999; Sun et al., 2003; Andersen et al., 2005; Huyen et al., 2005). During the mid-Oligocene a pronounced basin inversion marked by a distinct angular unconformity occurred in the Song Hong Basin, which appears to have been most severe in the northwestern part of the basin near the onshore metamorphic core complexes. Restricted leftlateral faulting continued in a narrow belt between the Song Lo Fault and the EVBF after mid-Oligocene time; although, the total sinistral offset hardly exceeded a few tens of kilometers (Rangin et al., 1995a; Nielsen et al., 1999; Andersen et al., 2005). This suggests either, that the general perception of major left-lateral movements along the onshore ASRRSZ postdating the mid-Oligocene is wrong or that leftlateral motion was taken up by onshore deformation north of the Song Hong Basin. There are no indications of hundreds of kilometers of leftlateral motion accommodated between the Vietnamese Dai Nui Con Voi metamorphic core complex and the modern shoreline ca. 150 km farther to the southeast. This strongly suggests that post midOligocene left-lateral offsets across the southern part of the ASRRSZ was modest, which contradicts the regional reconstructions proposed by e.g., Lee & Lawver (1994, 1995) Leloup et al. (1995, 2001a,b), Hall (1996, 1997, 2002), Wang et al. (1998, 2000), Replumaz & Tapponnier (2003), Hall & Morley (2004), and Searle (2006). Farther to the NW, a considerable part of the Indochinese extrusion was accommodated by shortening, local block rotation and transpresion within the Shan– Thai Terrain (Fig. 3)(Wang and Burchfiel, 1997; Huang and Opdyke, 1993; Haihong et al., 1995; Sato et al., 1999, 2001, 2007; Hall, 2002; Akciz et al., 2008). Most left-lateral shearing along the ASRRSZ following the mid-Oligocene may therefore have been taken up within the Shan–Thai Terrain.

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2.2.2. Mae Ping shear Zone In south Indochina left-lateral motion within the Mae Ping Shear Zone ceased around mid-Oligocene time (Lacassin et al., 1997). Moderate latest Oligocene to early Miocene right-lateral inversion is indicated by local pull-apart basins formed at right-lateral releasing bends along the Mae Ping and parallel shear zone together with rapid along-strike uplift (Lacassin et al., 1997; Morley, 2002; Morley et al., 2007, submitted for publication; Smith et al., 2007). Morley et al. (submitted for publication) and Searle et al. (submitted for publication) link the inversion and the related rifting in Thailand to midOligocene coupling of the West Burma Block and India and the following substantial northward drift of west Burma along the Sagaing strike-slip fault, as indicated by transtensional rifting of the Mergui Basin and subsequent seafloor spreading in the Andaman Sea (Polachan and Racey, 1994; Bertrand and Rangin, 2003; Raju et al., 2004; Curray, 2005). The block coupling and the activation of the Sagaing Fault resulted in dextral wrenching along N–S and NW–SE striking fault networks immediately east of it (Morley et al., submitted for publication). Huchon et al. (1994) suggested the inversion of the Mae Ping Shear Zone as the result of the northward indentation of India, which according to their model resulted in a series of time transgressive inversion events that commenced in the southern part of the region. 2.2.3. South China Sea seafloor spreading Identification of the oldest magnetic anomaly 12 or 11 shows that the initial break-up of the SCS occurred around ca. 29–31 Ma following the revised time scale of Cande and Kent (1995) (Taylor and Hayes, 1980, 1983; Briais et al., 1993; Barckhausen and Roeser, 2004). The initial seafloor spreading occurred across an approximately E–W trending spreading axis located in the northern half of the SCS. At the time of anomaly 10 (ca. 28–29 Ma) spreading ceased in the westernmost part of the basin but continued farther east (Briais et al., 1993). Following anomaly 7 (ca. 24 Ma) at the Paleogene–Neogene boundary, spreading jumped south and started to propagate toward the southwest. The reorientation was accompanied by a shift in spreading direction from ca. N–S to more NW–SE directed in the southwestern SCS. 2.2.4. The Cuu Long, Nam Con Son and Phu Khanh Basins In the Cuu Long Basin sporadic late Oligocene compression resulted in basement uplift and inversion of Paleogene normal faults (Hung et al., 2003; Cuong et al., 2005; Thang, 2005). The probable link between the Mae Ping Shear Zone and the Cuu Long Basin leads us to speculate that the basin inversion was a consequence of the contemporary inversion of the shear zone. Rifting in the Cuu Long Basin continued until the end Oligocene time where a distinct unconformity marks the Oligocene/Miocene boundary and the onset of post-rift sagging, interpreted as a break-up unconformity related to the southward jump and reorientation of the SCS-spreading axis (Khy, 1986; Tan, 1995; Lee et al., 2001). The unconformity was traced seaward into the Nam Con Son Basin and here indicates the onset of a second rift phase in the basin (Matthews et al., 1997; Lee et al., 2001). In the northern Phu Khanh Basin, Fyhn et al. (2009a) reported a distinct mid-Oligocene unconformity caused by uplift along the EVBF and suggested a link between the formation of this unconformity and the coeval inversion in the Song Hong Basin. Rifting and left-lateral transtension along the EVBF continued during the late Oligocene in the Phu Khanh Basin, and a distinct unconformity marks the Paleogene–Neogene boundary across which rifting decreased and changed significantly. This was linked with the termination of leftlateral movements in the EVBF and a moderate right-lateral inversion (Fyhn et al., 2009a). Lee and Watkins (1998) noted the same unconformity but assigned it a mid-Oligocene age and interpreted it

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as a break-up unconformity caused by the initial onset of seafloor spreading farther northeast. 2.3. Early Neogene 2.3.1. Ailao Shan-Red River Shear Zone and the East Vietnam Boundary Fault Along the Vietnamese margin, major tectonic and depositional changes occurred from about the Paleogene–Neogene boundary. Onshore, rapid exhumation of the ASRR-metamorphic core complexes continued during the early Miocene, which was interpreted as the result of major left-lateral faulting along the shear zone (Leloup et al., 1995, 2001a; Wang et al., 1998, 2000; Nam et al., 1998; Searle, 2006). Left-lateral shearing along the ASRRSZ is interpreted to have stopped around the early–middle Miocene boundary based on the temporary slow-down of uplift of the metamorphic core complexes and the completion of seafloor spreading suggested to have been governed by the kinematics of the ASRRSZ (Leloup et al., 1995, 2001a). In the Song Hong Basin, a change from modest transtension to transpression occurred around the early/middle Miocene boundary (Rangin et al., 1995b; Nielsen et al., 1999; Andersen et al., 2005; Clift and Sun, 2006), attributed to the onset of right-lateral shearing (Lee and Lawver, 1994, 1995; Nielsen et al., 1999; Andersen et al., 2005). However, Rangin et al. (1995a,b) inferred a later onset of right-lateral inversion around the end of the Miocene. 2.3.2. Final subduction of the proto-South China Sea Subduction along the northwest coast of Borneo and Palawan gradually closed the remaining part of the proto-SCS during the early Miocene (James, 1984; Levell, 1987; Hall, 1996, 2002). During the early Miocene, the Dangerous Ground micro plate collided with northwestern Borneo and Palawan in response to the closure of the proto-SCS. The collision resulted in regional uplift and is marked by intra-plate shortening. The elimination of the proto-SCS has been associated with the formation of the Top Crocker Unconformity of e. early Miocene age (Hall et al., 2008) or the younger Deep Middle Miocene Unconformity that mark a significant latest early–earliest middle Miocene shift in depositional style in and offshore northern Borneo (Levell, 1987; Bérnard et al., 1990; Rangin et al., 1990; Balaguru and Nichols, 2004; Hutchison, 2004, 2005). Crustal shortening continued in places long into the late Neogene, which has been attributed both to the plate collision and to gravity driven toe-of-slope compression (Hinz et al., 1989; Bérnard et al., 1990; Rangin et al., 1990; Morley et al., 2003; Hutchison, 2004; Franke et al., 2008). 2.3.3. South China Sea seafloor spreading During the early Neogene, the continental break-up in the southern part of the South China Sea area propagated southwestward and thus triggered seafloor spreading in the area between present day Vietnam and Borneo (Pautot et al., 1990; Briais et al., 1993; Barckhausen and Roeser, 2004). Briais et al. (1993) interpreted the southwestward propagation to have ceased around anomaly 6 corresponding to ca. 19–20 Ma, a few million years before the final halt of seafloor spreading. Later, Huchon et al. (2001) demonstrated the southwestern spreading arm as a propagating V-shaped zipperlike oceanic rift indicating that the propagation continued to the end of seafloor spreading. The magnetic anomaly pattern of the youngest ocean floor is poorly defined (Taylor and Hayes, 1983; Briais et al., 1993). Based on an unchanged spreading rate after the formation of the youngest identifiable anomaly (5D) Briais et al. (1993) assumed that seafloor spreading terminated around anomaly 5C corresponding to ca. 16– 17 Ma (Cande and Kent, 1995), whereas, a revised study of the magnetic anomaly-pattern suggested that seafloor spreading terminated as early as ca. 20.5 Ma during anomaly 6A1 (Barckhausen and Roeser, 2004).

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Fig. 4. Well tie located north of the Phu Khanh Basin. The well is located immediately seaward of the southernmost part of the Song Hong Basin (also known as the Quang Ngai Graben or the Qui Nhon Basin), and drilled a Neogene section flooring in a middle to lower Miocene basaltic volcano dated through apatite fission track analysis. A = Recent– Pliocene, B = upper Miocene, C = middle–lower Miocene. The Triton Carbonate Platform imaged seaward of the well was intersected in other wells.

2.3.4. The Cuu Long, Nam Con Son and Phu Khanh Basins An important early Miocene structural change, accompanied by a distinct transgression occurred in the basins offshore south and central Vietnam (Khy, 1986; Tan, 1995; Matthews et al., 1997; Roques et al., 1997a; Lee et al., 2001; Fyhn et al., 2009a). In the Cuu Long Basin, Neogene post-rift sagging lead to a basin-wide transgression interpreted by the upward change from continental to marine dominated siliciclastic deposits during early Miocene time (Khy, 1986; Tan, 1995; Lee et al., 2001). In the Nam Con Son Basin, strong rifting revitalized along NW–SEto N–S-trending faults after the Paleogene in response to the southwestward propagation of continental break-up toward the basin (Matthews et al., 1997; Huchon et al., 1998, 2001; Lee et al., 2001). In response to the approaching southwestern SCS spreadingarm, a gradual transgression occurred in the basin, which resulted in the dominance of lower Miocene marine siliciclastic deposits with structurally controlled facies belts (Matthews et al., 1997; Lee et al., 2001). As transgression continued, a gradual deepening took place across the basin depocenters and widespread carbonate deposition commenced (Matthews et al., 1997; Lee et al., 2001). During the late middle Miocene, a relative sea-level lowstand temporarily interrupted carbonate sedimentation. The associated unconformity pronouncedly truncates the underlying formations and marks a distinct decrease in rifting (Matthews et al., 1997). In the northern Phu Khanh Basin, N–S- to NW–SE-trending rifts influenced subsidence along the basin margin (Lee and Watkins, 1998; Clift et al., 2008; Fyhn et al., 2009a). Huchon et al. (1994) and Rangin et al. (1995a) presented evidence for similarly striking dextral faults immediately onshore and suggested the onset of right-lateral movement during the early Neogene. In the northern Phu Khanh Basin, widespread carbonate growth commenced as subsidence regained after the halt of left-lateral motion in the EVBF indicating that the early Miocene transgression affected this part of the region too (Lee and Watkins, 1998; Fyhn et al., 2009a). Part of

the carbonates connects to the Triton Carbonate Platform (Da Nang– Paracel Carbonate Platform), making up a major carbonate platform that continues north of the basin (Holland et al., 1992; Roques et al., 1997a,b; Fyhn et al., 2009a). Early Neogene volcanism in the Phu Khanh Basin was reported by Fyhn et al. (2009a), which is supported by an early to middle Miocene alkaline basaltic volcano drilled immediately north of the basin (Fig. 4) (BHP, 1993). Widespread volcanism onshore south and central Indochina postdate the early Neogene, although, a small number of basalts from central Vietnam yield middle and early Miocene ages suggesting an earlier inception of volcanism (Barr and Macdonald, 1981; Rangin et al., 1995a; Hoang et al., 1996; Chung et al., 1998; Hoang and Flower, 1998; Lee et al., 1998; Wang et al., 2001). 2.4. Late Neogene 2.4.1. Volcanism, uplift and denudation of central and south Vietnam Extensive areas of southeastern Indochina are covered by large basalt plateaus of mainly late Neogene age, which has been ascribed to the emplacement of a diffuse mantle plume, crustal thinning or the regionally hot upper mantle underneath this part of Indochina (Barr and Macdonald, 1981; Rangin et al., 1995a; Hoang et al., 1996; Chung et al., 1998; Hoang and Flower, 1998; Lee et al., 1998; Wang et al., 2001; Lebedev and Nolet, 2003; Hall and Morley, 2004). In places, extensional faulting accompanied the volcanism as suggested by the injection of basalts along normal-fault planes and by faults offsetting late Miocene to sub-Recent basalts (Rangin et al., 1995a). The faults show N–S and NE–SW orientations, most likely following inherited structural fabric. The volcanism led to extensive uplift and accelerated denudation of the southeastern part of Indochina (Carter et al., 2000). Although the onset of magmatism occurred during the early Neogene, the associated rapid uplift only commenced around middle late Miocene time as onshore volcanism intensified. The uplift has since affected the local climate and oceanography, which together with the related increase in offshore sediment supply influenced the basin development in and

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Fig. 5. Well tie located in the northern Cuu Long Basin. A = Recent–Pliocene, B = upper Miocene, C = middle Miocene, D = lower Miocene, E = Oligocene, F = Mesozoic granitic basement. Shallow marine — delta plain deposits dominate the Neogene succession with the greatest abundance of brackish to non-marine deposits in the lowest part. The upper Oligocene synrift sequence is drilled and consists dominantly of non-marine to brackish accumulations (DEMINEX, 1979). The Paleogene–Neogene unconformity marks the termination of rifting and seal off most faults.

around the Cuu Long, the Nam Con Son and Phu Khanh basins (Carter et al., 2000, Murray and Dorobek, 2004; Clift et al., 2008; Fyhn et al., 2009b). 2.4.2. The Cuu Long, Nam Con Son and Phu Khanh Basins Rifting declined after the late middle Miocene uplift and thermal sagging came to dominate in the offshore areas, although late Neogene faulting are documented (Fyhn et al., 2009a,b). During the earliest late Neogene, carbonate deposition in the Nam Con Son Basin area mainly took place on a number of structural highs (Matthews et al., 1997; Fyhn et al., 2009b). Following a middle late Miocene relative sea-level fall linked with the onset of onshore uplift, carbonate platforms along the margin further retreated and eventually perished (Matthews et al., 1997; Fyhn et al., 2009b). Consequently, siliciclastic deposition came to dominate the latest Neogene, and a substantial broadening of the central and south Vietnamese shelf occurred in response to increased terrigenous sediment supplies (Matthews et al., 1997; Lee and Watkins, 1998; Lee et al., 2001; Clift et al., 2008; Fyhn et al., 2009a,b).

profiles in the northern Cuu Long and Nam Con Son basins were available to this study. Similarly, three wells immediately north of the Phu Khanh Basin were used to establish a stratigraphic age control in the study area (Figs. 2, 4, 5). The chronostratigraphic framework was refined by correlating to the stratigraphy of the Nam Con Son Basin proposed by Matthews et al. (1997) and by correlating a dated onshore flood basalt province with its offshore continuation in the Phu Khanh Basin (Rangin et al., 1995b; Chung et al., 1998; Lee et al., 1998). The depositional strata was subdivided into seismic sequences that reflect the structural and stratigraphic development of the area. Based on the seismic interpretation, 4 structural maps and five facies maps were produced. The facies are based on analysis and mapping of seismic facies such as reflection amplitudes, continuities, and reflection shapes, frequencies, reflector-termination patterns, geometry of the units as well as the mutual relationships between the individual seismic facies. All “depths” and “thicknesses” in the following sections are given in seismic two-way-travel times (TWT) unless otherwise stated. 4. Structural and stratigraphic interpretation

3. Data and methods 4.1. Pre-Cenozoic basement and the Paleogene development The present study is based on ca. 20,000 km of commercial 2-D digital reflection seismic data (Fig. 2) that were acquired between 1974 and 2003, with the majority collected in 1993 and 2003. Chronostratigraphic interpretations of four wells, three of which are tied to seismic

4.1.1. Pre-Cenozoic basement The top of the acoustic basement is a strong positive reflector and the deepest regionally traceable seismic horizon offshore south

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Fig. 7. Seismic transect across the East Vietnam Boundary Fault (EVBF) in the northern Phu Khanh Basin. West of the fault zone, the pre-Tertiary is composed of crystalline basement whereas a sedimentary succession floors the Cenozoic east of the fault zone. A tectonically disturbed Paleogene syn-rift succession fills the structural low along the EVBF. Lower Miocene carbonate platforms caps the igneous basement and the Paleogene rift sequence in areas. The carbonates are buried beneath a prograding late Neogene siliciclastic dominated shelf and shelf slope succession.

and central Vietnam, the depth of which outlines the major basins and highs in the region. The horizon that corresponds to the top of the pre-Tertiary is the rift-onset unconformity and is offset by numerous extensional faults. The faults outline three rift basins: the Phu Khanh Basin, the Nam Con Son Basin, and the Cuu Long Basin (Fig. 6a), and the Con Son Swell, a structural high that separates the Cuu Long and the Nam Con Son basins and the Phu Khanh and the Cuu Long basins. The Nam Con Son and the Phu Khanh basins connect to the east. In the Phu Khanh Basin, the faults mainly strike N–S to NW–SE. The fault strike changes to a more E–W-trending direction in the eastern part of the basin, but it was generally difficult to map these faults in detail, as their strike is parallel to most of the seismic lines. In addition these faults are found at a considerable depth and in places below a thick volcanic cover complicating the recognition of faults (Fig. 6a). The southernmost part of the Song Hong rift continues into the Phu Khanh Basin and is outlined by a narrow graben in the northern part of the basin (Fig. 7). The narrow graben is underlain by only 2–5 km thick crust and outline a moho uplift immediately below the depression as

indicated by gravimetric modeling (Fyhn et al., 2009a). This indicates that the graben-confining faults cut the entire crust in a steep path along the depression. The eastern fault of the narrow graben continues to the south and forms the main structure in the Phu Khanh Basin making up the mapped continuation of the coast parallel EVBF south of the Song Hong Basin. The strait EVBF lineament continues more than 150 km to the south and breaks into discrete extensional fault segments that splays into the NW–SE-trending Tua Hoa Fault Zone, which confines the Phu Khanh Basin to the southwest (Fig. 6a). Landward of the EVBF and the Tua Hoa Fault Zone, the acoustic basement is characterized by a chaotic reflection pattern, in places dominated by multiples (Fig. 7). This part of the acoustic basement likely corresponds to the Triassic to earliest Paleocene crystalline basement similar to that cropping out onshore central and south Vietnam few km landward of the seismic sections (Tien, 1991; Tinh, 1998). In contrast, the top of the acoustic basement east of the EVBF is characterized by a strong angular unconformity that caps a thick prerift sedimentary succession (Figs. 7, 8). The pre-rift sedimentary succession is traceable in a ca. 100 km long zone along the EVBF, but

Fig. 6. a) Time-depth structure map to the pre-Tertiary acoustic basement. Dotted areas represent areas with basement concealed beneath thick volcanic successions or situated below seismic penetration (7–8 s TWT). EVBF = East Vietnam Boundary Fault, THFZ = Tuy Hoa Fault Zone. b) Time-depth structure map to near the top of the Paleogene. The EVBF does not offset the mapped surface, whereas numerous faults located in the shoreward part of the Phu Khanh Basin and across the Con Son Swell and in the Nam Con Son Basin intersect the surface. In contrast, the Paleogene–Neogene boundary is only mildly faulted in the Cuu Long Basin. c) Time-depth structure map of the late middle Miocene unconformity. Faulting dropped markedly after the middle Miocene. d) Time-depth structure map of an intra late Miocene surface.

M.B.W. Fyhn et al. / Tectonophysics 478 (2009) 184–214 Fig. 8. Seismic section located east of the East Vietnam Boundary Fault in the Phu Khanh Basin showing a distinct sedimentary pre-rift succession (the acoustic basement). The pre-Tertiary unit is down faulted beneath seismic maximum depth toward the center of the basin and buried underneath a thick Paleogene succession. The syn-rift unit is covered by marine Neogene deposits dominated by late Neogene deep marine sediments. A prominent Neogene volcano is transected farthest south.

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continues north of the study area and is down faulted beneath seismic maximum depth to the south. The changing nature of the pre-Tertiary acoustic basement across the EVBF could well be a result of lateral juxtaposition of different pre-Tertiary units. Together with the unbroken length and the straightness of the EVBF, the intensity of syn-rift deformation along the fault and the northward connection to the strike-slip fault system in the Song Hong Basin and the ASRRSZ this is interpreted to suggest a significant lateral offset across the EVBF in the Phu Khanh Basin. A left-lateral sense of motion is suggested by the linkage with the left-lateral Song Hong Basin fault system and the ASRRSZ further north. Northwest trending extensional fault splays in the southern part of the Phu Khanh Basin and in the Tua Hoa Fault Zone suggest a Paleogene axis of lengthening compatible with left-lateral strike-slip across the EVBF. Left-lateral strike-slip across the EVBF is also suggested by the northwestward horsetail fault splays from the EVBF in the northeastern part of the basin and is supported by right-stepping, extensional faults interpreted as synthetic fractures corresponding to ridel shears that form the offshore equivalent of the pervasive sinistral fault system onshore described by Rangin et al. (1995a). The pre-rift juxtaposition has been mapped in the northern ca. 100 km of the area, but seems to continue north of the data-covered study area, and farther south is down faulted and concealed by the great water depth and Cenozoic sedimentary column below conventional seismic recording depth (mostly 7–8 s TWT)(Fig. 8). Assuming that this juxtaposition owes to the left-lateral strike-slip offset along the EVBF a minimum of 100 km of left-lateral offset is inferred across the EVBF in the Phu Khanh area. The lateral motion was most likely constrained to the Eocene and Oligocene as indicated by the age of the synrift succession along the EVBF, which is

supported by the basement age west of the EVBF indicating a postCretaceous juxtaposition. In various places in the Cuu Long Basin, the top of the acoustic basement overlies a chaotically reflected interval, whereas the interval is partially transparent in other places. The extensively drilled acoustic basement is composed of Late Mesozoic granitic intrusives, related effusives as well as low-grade meta-sediments (Fig. 5)(Areshev et al., 1992; Lee et al., 2001; Hung et al., 2003; Cuong et al., 2005). Extensional NE–SW to N–S-verging faults crosscut the basin basement and outline the up to ca. 4 s TWT deep depocenter located in the southwestern-most part of the study area (Fig. 9). The basin shallows landward, toward the Tua Hoa Fault Zone, and seaward along the Con Son Swell. The studied northern part of the Nam Con Son Basin forms a distinct down to ca. 6 s TWT deep NE–SW-trending depression in the acoustic basement (Fig. 6a). The depression is defined by NE–SW to N–S trending normal faults, many of which are listric with heaves up to a few tens of kilometers (Figs. 6a, 10). The top of the acoustic basement is characterized by a strong reflection that caps an interval of chaotic and/or sub-transparent reflections as in the Cuu Long and the western part of the Phu Khanh basins. This pattern most likely corresponds to a crystalline basement that may include low-grade meta-sediments as tested farther southwest in the basin (Matthews et al., 1997). The basin axis trends toward the northeast and forms the southwestern prolongation of the southern spreading center of the SCS. Landward, the basin is bordered by the Con Son Swell. Across the Con Son Swell, a strong reflection marks the top of the almost reflection free acoustic basement that corresponds to the late Mesozoic granitic basement encountered in wells. To the south, the Con Son Swell forms a shallow buried slightly rifted basement high, whereas farther north it projects into the SCS. Here it is transected by extensional

Fig. 9. Seismic transect of the deeper part of the Cuu Long Basin located in the southwestern-most part of the study area. Two distinct major synrift sequences make up the Paleogene separated by a distinct inversion unconformity. The oldest Eocene to early Oligocene sediments appears distinctly tectonized compared with the late Oligocene sequence. The changing upper Oligocene depocenters as well as distinct internal onlap and truncational surfaces indicate a disturbed late Oligocene tectonic environment in the region with frequent structural shifts. The Oligocene is capped by a later inversion unconformity that merges with the mid-Oligocene unconformity toward the basin flanks.

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Fig. 10. Seismic transect across the Nam Con Son Basin. Major listric faults displace the acoustic basement and the Cenozoic successions. Miocene platform carbonates upward grades into deep marine siliciclastics filling in the basin relief.

faults mostly trending N–S to NE–SW that in places downfault the basement below 3 s TWT (Fig. 6a). 4.1.2. Paleogene rifting (Eocene–mid-Oligocene) Cenozoic deposits unconformably overly the acoustic basement in the area, and Lower Oligocene deposits are proven in wells outside depocenters. Eocene deposits exist in the Cuu Long and adjacent rift basins (Areshev et al., 1992; Fina, 1994; Bustin and Chonchawalit, 1995; Nielsen et al., 1999; Binh et al., 2007, Meijun et al., 2008; Morley et al., submitted for publication), and most likely also in the Phu Khanh and the Nam Con Son basins (Matthews et al., 1997; Lee et al., 2001; Clift et al., 2008; Fyhn et al., 2009a). In the Cuu Long Basin the Eocene to mid-Oligocene package fills NE–SW trending grabens and half grabens (Figs. 5, 9, 11a). The oldest Cenozoic deposits in the Phu Khanh Basin are syn-rift sediments mainly located seaward of the EVBF and in the fault bounded depression that connects the Phu Khanh Basin with the Song Hong basins to the north (Figs. 6a, 7, 8). The substantial offsets across the main faults displace the base of the Paleogene synrift deposits below the recording depth of the seismic data in part of the basin (Figs. 6a, 8). Along the EVBF, the Paleogene succession is deformed and broken relative to coeval syn-rift strata farther from the EVBF, which is interpreted as deformations caused by the considerable lateral movement along the EVBF (Fig. 7). To the south, the Tua Hoa Fault Zone borders the Paleogene deposits of the Phu Khanh Basin (Fig. 6a). Syn-rift deposition in grabens and half grabens indicate Paleogene activity along the fault zone. The distribution of Paleogene sediments in a narrow belt between the Cuu Long and the Phu Khanh basins suggests a Paleogene linkage between the basins (Fig. 11a). The connecting corridor may have been broader than suggested by the preserved Paleogene deposits as subsequent erosion has narrowed the connecting sedimentary prism (Fig. 12).

The Con Son Swell seems generally to have been subjected to uplift and erosion during the Paleogene and deposits of that age has only been firmly identified in a small half-graben next to the Cuu Long Basin (Fig. 11a). A restricted, relatively thin seismic unit in the northern part of the swell may tentatively be ascribed a Paleogene age as the package is characterized by strong, continuous reflections and is capped by an erosional unconformity similar to the Paleogene deposits in the basin areas. However, it is not possible to correlate the unit with the adjacent Paleogene strata in the basins with certainty as they do not connect and as the deeper seismic signal deteriorates below a younger karst surface in the area (Fyhn et al., 2009b). Likewise in the Nam Con Son Basin, it was not possible to correlate with adjacent Paleogene strata with certainty, and the interpreted distribution of the Paleogene deposits are tentative (Fig. 11a). Like further southwest in the Nam Con Son Basin (Matthews et al., 1997), the Paleogene synrift succession seems to be patchily distributed only. The oldest succession overlying the acoustic basement was deposited in restricted NE to NNE trending grabens. The graben fill generally has relative strong, continuous reflections and is capped by a characteristic truncational unconformity (Fig. 13). These successions therefore resemble Paleogene counterparts in the Cuu Long, the Phu Khanh and the Nam Con Son basins. Based on the patchy distribution of Paleogene deposits, widespread areas across the studied part of the Nam Con Son Basin may have been subjected to uplift throughout the Paleogene (Fig. 11a). 4.1.3. Basin inversions (late Oligocene) A mid-Oligocene unconformity generally separates deformed Eocene–lower Oligocene synrift intervals from less deformed deposits in the Cuu Long Basin (Figs. 5, 9), and marks a depositional widening from sedimentation in discrete grabens to deposition in more continuous sag depressions. Extensional rifting continued throughout the period, but

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Fig. 11. Facies maps of the study area with main structures marked. a) Paleogene facies map illustrating the dominance of non-marine to restricted marine deposits. b) Early Miocene facies map mirroring the early Neogene transgression resulting from the opening of the SCS. This resulted in widespread carbonate accumulations in the north and alluvial to shallow marine siliciclastics farther south. c) Late early Neogene facies map. During this period more open marine conditions caused by the propagating continental breakup in the southeastern part of the study area promoted carbonate growth in the area. Shallow marine siliciclastic sedimentation prevailed farther landward and to the north where magmatism caused local uplift and retreat of carbonate deposition. d) Early late Neogene facies map. A transgression followed in the wake of a late middle Miocene uplift in the southeastern part of the study area. This resulted in widespread carbonate deposition across the northern Con Son Swell and deeper marine deposition in the eastern part of the study area. Volcanism in the Phu Khanh Basin and alluvial to shallow marine deposition in the Cuu Long Basin continued during the period. e) Latest Miocene–Recent facies map. Siliciclastic supplies increased during the most recent part of the basin evolution due to onshore uplift. This promoted the buildup of a prominent shelf slope and inhibited carbonate production in the region. Magmatism in the Phu Khanh Basin dropped during the period whereas volcanism initiated in the southernmost part of the study area.

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Fig. 12. Transect of the narrow belt that attached the Phu Khanh Basin to the Cuu Long Basin during Eocene–Oligocene times. Truncation at the top of the Paleogene suggests a once broader connection.

compressional deformation inverted older structures in places, and structural highs with fractured basement developed. Some of these have later been charged with petroleum sourced from the Paleogene

succession (Hung et al., 2003; Cuong et al., 2005). Late Oligocene tectonism also caused frequent depocenter shifts and resulted in shifting internal onlap and truncational unconformities (Fig. 9).

Fig. 13. Seismic transect of the Nam Con Son Basin illustrating the distinct truncation at the upper–lower Neogene transition as well as Paleogene(?) deposits flooring the basin.

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The upper Oligocene is capped by a distinct truncational unconformity that separates Paleogene from Neogene deposits and seals off most faults. The unconformity marks the termination of rifting in the Cuu Long Basin, and merge with the mid-Oligocene unconformity toward the Cuu Long Basin margin as the Paleogene synrift sequences wedge out (Fig. 9). Along the Tua Hoa Fault Zone, the merged unconformity is well developed and marked by a strong reflection suggesting a protracted uplift comparable to that in the Cuu Long Basin farther south. The unconformity seals a minor part of the faults along the fault zone; but most faults resumed rifting during the early Neogene. In the Nam Con Son Basin, a distinct unconformity caps the oldest Cenozoic deposits and is tentatively interpreted as the Oligocene/Miocene unconformity observed in the Cuu Long and the Nam Con Son basins by Matthews et al. (1997) and Lee et al. (2001). North of the Tua Hoa Fault Zone the seismic unconformity fades indicating the presence of upper Oligocene deposits in the Phu Khanh Basin. A tectonic shift seems to have taken place during the late Oligocene in the Phu Khanh Basin as a distinct unconformity separates the lower from the upper Paleogene synrift succession in the central part of the basin (Fig. 14). The unconformity probably corresponds to the mid-Oligocene unconformities in the Cuu Long Basin and in the Song Hong Basin (Fyhn et al., 2009a). Truncation increases toward the EVBF indicating a mid-Oligocene change in tectonic style along the fault. Renewed syn-rift subsidence is registered by the presence of a thick upper Oligocene syn-rift prism along the fault zone. The subsidence pattern changed after the mid-Oligocene uplift. Extensional subsidence occurred regionally east of the EVBF before midOligocene time, whereas late Oligocene subsidence was concentrated in a narrow zone along the EVBF due to decreasing fault activity in the area (Figs 8, 14). 4.1.4. Paleogene depositional facies Along the central and south Vietnamese margin, the Paleogene is predominantly composed of alluvial/fluvial and lacustrine sediments

with occasional restricted marine intercalations as indicated by well data and seismic interpretations (Matthews et al., 1997; Todd et al., 1997; Lee et al., 2001; Hung et al., 2003; Thang, 2005; Fyhn et al., 2009a). The Paleogene synrifts contain prolific carbonaceous lacustrine mudrocks and coal intervals (Bojesen-Koefoed et al., 2005; Matthews et al., 1997; Lee et al., 2001; Hung et al., 2003; Thang, 2005; Fyhn et al., 2009a). The synrift succession includes intervals of continuous strong reflections most common in depocenters and onlapping toward the basin margins (Fig. 15). These intervals probably represent lacustrine mostly mudprone intervals. The lacustrine facies intervene with successions of weaker reflections having poorer continuities interpreted as more alluvial-dominated deposits. Fan-shaped bodies interpreted as alluvial fans and fan deltas occasionally flank faults and inclined surfaces (Fig. 15). In the lower part of the synrift section, intervals of disrupted strong reflections are widely distributed and represent tectonised continental deposits. 4.2. The early Neogene development 4.2.1. Early Neogene rifting (earliest–middle Miocene) Resting on the Paleogene–Neogene unconformity, the lower Neogene package characterizes a general widening of the depositional area that resulted in the burial of most of the Con Son Swell and the Nam Con Son Basin (Fig. 11b). In addition, deposition expanded landward to include a larger region along the present Vietnamese coast that experienced uplift and basement exposure during Paleogene time. In the Nam Con Son Basin, extensive faulting renewed during early Neogene time. Northeast–southwest- to N–S-trending faulting during the period resulted in the formation of the conspicuous deep southwest-trending basement depression that forms the continuation of the SCS-spreading axis (Figs. 6b, 10, 13). Lower and middle Miocene onlaps toward the Con Son Swell indicate that the basement-ridge geometry amplified during the early Neogene as rifting caused the Nam Con Son Basin to subside. Early

Fig. 14. Seismic transect across the deeper part of the Phu Khanh Basin showing the pronounced mid-Oligocene unconfomity. Truncation along the unconformity increases toward the East Vietnam Boundary Fault (EVBF) and indicate up to 1–1.5 s TWT basin fill removed during the uplift corresponding to ca. 1.5–3 km of denudation during the uplift phase. The rift induced subsidence pattern changes after the uplift from being characterized by broad subsidence east of the East Vietnam Boundary Fault before mid-Oligocene time to narrow subsidence along the trail of the Boundary Fault during the late Oligocene.

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Fig. 15. Paleogene deposition took place in rift controlled depocenters. Continues relatively strong reflectors often fill in depocenters typical of lacustrine dominated sequences. These in turn may be flanked by wedges composed of more discontinuous reflectors interpreted as alluvial fans and fan deltas that fringe inclined rift margins. Less strongly reflected and more discontinuous reflector packages also occur in the graben center and generally become more frequent toward the base of the grabens. These probably also represent successions with a high fluvial content.

Neogene faulting across the northern Con Son Swell occurred along more NNE–SSW-trending faults, as compared to the Nam Con Son Basin. A few E–W- to ENE–WSW-trending faults outline some of the main structural depressions across the Con Son Swell differing from the general fault trend. Further north, older structures were reactivated and major NWtrending faults formed along the Tua Hoa Fault Zone during the earliest Neogene (Fig. 6b). In the Phu Khanh Basin, faulting was largely restricted to the western basin margin where NW–SE- to N–S-trending rifts formed a structural belt that connects to the Tua Hoa Fault Zone (Fig. 6b). In the Paleogene rift center across the seaward part of the basin, only minor faulting took place including modest compression in a few steep faults active during the earliest Miocene. The EVBF was virtually sealed by the lower Miocene succession due to the termination or near cessation of lateral movements along the fault. However, fast postrift subsidence dominated across the EVBF influencing the local depositional pattern. In the Cuu Long Basin only a few minor NE–SW-trending faults transect into the lower Neogene succession (Figs. 5, 6c, 9). 4.2.2. Early Neogene depositional facies In the Cuu Long Basin, a reflection package dominated by relatively continuous, internal parallel reflections with intermediate amplitudes characterizes the lower Miocene section, although subtle channel features exist. The seismic facies is interpreted as dominantly lacustrine to shallow marine deposits with alluvial elements, in accord with well data (Figs. 5 and 11b). The package thins shoreward and toward the Con Son Swell, and intervals of less continuous, weaker reflections become more dominant. This most likely corresponds to an increase in alluvial deposits in high-lying settings close to the provenance areas, suggesting a decreased marine and/or lacustrine affinity toward the Con Son Swell and the Vietnamese mainland. Strongly reflected mounds a few kilometers across and few hundred ms high interpreted as volcanoes overly the base of the Miocene (Fig. 16). This accords with lowermost Miocene basaltic successions encountered e.g., in the Ruby well (Hamid, 1994).

In the Nam Con Son Basin and across the Con Son Swell, a comparable lower Miocene seismic facies exists suggesting that alluvial to shallow marine deposition prevailed in this area as well, with an increasingly marine dominance in the rapidly subsiding basin (Figs. 11b, 13, 17). In most of the Nam Con Son Basin and the seaward part of the Con Son Swell, the lower Neogene strata grade into a lower– middle Miocene succession of stronger and relatively continuous reflectors (Figs. 11c, 13, 17). This lower–middle Miocene succession is generally well-developed on structural highs and near escarpments and is interpreted as platform carbonates with platform margins located along major fault escarpments. A similar lower Neogene facies association exists in the neighboring part of the Nam Con Son Basin, where lowest Miocene alluvial to shallow marine deposits grade upward into lower–middle Miocene carbonates and marine siliciclastics (Matthews et al., 1997).

Fig. 16. Seismic transect across the Cuu Long Basin showing high amplitude volcanic structures at the Paleogene–Neogene boundary. The presence of stratigraphically equivalent basalt and related intrusions in nearby wells substantiate the volcanic nature of these build-ups.

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Fig. 17. Seismic transect of the Nam Con Son Basin illustrating the lower Miocene siliciclastic succession overlain by lower–middle Miocene carbonates. Note also the listric nature of the confining faults.

Fig. 18. Seismic transect across the southern Phu Khanh Basin. The lower Neogene succession is comprised by siliciclastic dominated deposits and volcanics filling in rift topography. Volcanism continued into the late Neogene contemporary to the build up of the Phan Rang Carbonate Platform. The carbonate platform is buried underneath a pronounced shelf– shelf–slope and basin-floor complex.

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2009a). In between and adjacent to the two carbonate platforms a strongly reflected seismic package is located that most likely corresponds to off-platform marls, part of which may posses source rock quality as seep oils derived from Tertiary marly source rocks is found along the Vietnamese coast immediately up-dip of this facies (Traynor and Sladen, 1997; Bojesen-Koefoed et al., 2005; Fyhn et al., 2009a). A decrease in reflection amplitudes of the lower Miocene carbonate succession in the Phu Khanh Basin indicates an upward increase in siliciclastic content. The northeastern carbonate platform persisted during the middle Miocene while the shoreward platform succumbed due to subaerial exposure as suggested by its truncated upper surface and the widespread hiatus marking the platform top. The uplift and contemporaneous rifting was most likely governed by local volcanism as implied by mounded and diffuse high-amplitude intervals across the basin area and basalts encountered in the 121CM-1X well (Figs. 4, 8, 11c, 18) (BHP, 1993). Fig. 19. Seismic section from the Phu Khanh Basin illustrating a distinctly truncated earliest Miocene carbonate sequence. The truncation most likely resulted from a magmatic induced uplift related to early to middle Miocene volcanism nearby. The unconformity is buried underneath upper Neogene shelf and shelf slope deposits.

4.3. The late Neogene development

In the southern part of the Phu Khanh Basin, relatively continuous reflections with intermediate amplitudes characterize the lower Miocene section and are interpreted to reflect mainly alluvial to shallow marine siliciclastic deposits (Fig. 18). Along the western margin of the basin and farthest to the north, strong lower Miocene reflectors outline carbonate platforms overlying the acoustic basement and Paleogene deposits across structurally high-lying areas (Fig. 7). In the southern part of the area, the platform outline has been strongly modified by deep erosion as suggested by reflector truncation, which has obscured escarpment-like platform margins observed farther north (Fig. 19). The platform succession to the northeast forms the continuation of the oldest part of the Triton Carbonate Platform drilled north of the Phu Khanh Basin (Figs. 4 and 11b)(BP, 1990; Holland et al., 1992; BHP, 1993; Roques et al., 1997a,b; Fyhn et al.,

4.3.1. Late Neogene rifting and sagging (middle Miocene–Recent) The upper Neogene rests on an unconformity and its correlative conformity dated as late middle Miocene by Matthews et al. (1997) (Figs. 13 and 18). In great parts of the Neogene rift center in the Nam Con Son Basin and in the southern Phu Khanh Basin, the surface forms a pronounced unconformity with deep truncation of both rift shoulders and hanging-wall blocks (Figs. 13 and 18). The unconformity weakens shoreward and northward and is represented by the correlative conformable section across the central parts of the Cuu Long and the northern Phu Khanh basins. The considerable depth to the base of the upper Neogene suggests a distinct late Neogene increase of the regional subsidence rates in the Phu Khanh and the Nam Con Son basins in comparison to the thickness of the lower Neogene succession deposited in a relatively shallow environment. The great thickness of the upper Neogene

Fig. 20. Seismic transect across the latest middle Miocene to late Miocene Phan Rang Carbonate Platform. Platform growth resumed on the northern Con Son Swell after the latest middle Miocene uplift and in places buried early to middle Miocene platform carbonates. Platform growth declined after a second uplift phase during the late Miocene, which eventually led to the drowning of the carbonate platform. Turbidite and current related channel- and canyon-like features incise the siliciclastic dominated interval that overlies the carbonate platform.

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Fig. 21. Seismic transect across the northern Con Son Swell. Latest Neogene hummocky and shingled clinoforms reflecting delta to pro-delta deposits dominate shoreward and seaward grades into a ramp-like succession of draping hemi-pelagic sediments that buries the Phan Rang Carbonate Platform.

succession that regionally exceeds 2 s TWT implies significantly increased sedimentation rates in the Phu Khanh Basin throughout the late Neogene (Figs. 7, 8). The upper middle Miocene unconformity represents a marked decrease in rift activity across the Con Son Swell and in the Nam Con Son and the southern Phu Khanh basins (Fig. 6c). Although rifting in this area decreased markedly, moderate extension continued throughout late Miocene time (Fig. 6c, d). In the northwestern Phu Khanh Basin, rifting continued until the middle late Miocene when volcanism in the area declined suggesting a causal link (Fig. 11 d, e). The faulting produced and reactivated a series of half grabens and resulted in landward widening of the depositional area (Fig. 11d). In the Cuu Long Basin post-rift sagging continued throughout the late Neogene with only few minor active faults in the area (Fig. 6c, d). 4.3.2. Late Neogene depositional facies Distinct lateral variations of the upper Neogene seismic facies occur across the study area. In the Cuu Long Basin and across the southwestern Con Son Swell, the upper Neogene succession is characterized by relatively continuous, intermediate-amplitude reflections interrupted by patchy channel features suggesting contin-

ued alluvial to shallow marine deposition in the area in accordance with well data (Figs. 5 and 11d, e). Across the southwestern Con Son Swell, strongly reflective Pliocene–Recent mounds exist and in places pierce the modern seafloor (Fig. 5). The Phy Quy Island (Iles des Cendres) located in this area represents the subarially exposed Pleistocene to Holocene volcanic part of such mound indicating the post-Miocene volcanic origin of these mounds. Magmatic activity on the island continues in modern time as demonstrated by the 1923eruption (Hoang Nguyen et al., 1996; Hoang and Flower, 1998; Lee et al., 1998). Across the northern part of the Con Son Swell, a platform unit with a distinct steeply inclined seaward margin make up the upper Miocene succession in the area constituting the Phan Rang Carbonate Platform (Figs. 11d and 20)(Fyhn et al., 2009b). After a middle late Miocene uplift that resulted in widespread and strong karstification, the platform split up and drowned during the subsequent latest Miocene and Pliocene reinundation of the area. The overlying succession is dominated on the shoreward side by hummocky and shingled clinoforms of weak to intermediate-amplitude reflections, indicative of delta to pro-delta deposits (Fig. 21). The pro-delta facies grades seaward into a ramp-like succession overlying the carbonate

Fig. 22. Seismic transect across the shoreward part of the Phu Khanh Basin. The strongly reflected succession represents the continuation of onshore dated late Miocene flood basalts. Subtle clinoforms visible in the seaward part of the basalt succession reflect basalt deltas.

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platform. The facies is composed of draping reflections including lowamplitude intervals interpreted as hemipelagic drapes deposited at a few hundred meters to ca. 1 km depth. Numerous channel features incise the drapes and are interpreted as submarine channel-incisions. Seaward, the ramp deposits grade into steeper slope deposits with numerous up to 5 km wide and several 100 ms deep turbidite feedercanyons cut perpendicular into the slope (Fig. 20). North of the Con Son Swell and farthest to the south, the ramp and slope grade into high and steep shelf slopes (Figs. 11e and 18). In the Nam Con Son and the southern Phu Khanh basins, the seismic facies basinward of the ramp and shelf slope is characterized by continuous, weak and intermediate reflections that fill-in the structural lows and onlap the basin margins and structural highs, typical of mixed deep-marine turbidites and hemipelagic deposits (Figs. 10 and 13). On these intra-basinal highs, strongly reflective upper Neogene platform units grade upward into weaker reflective upper Miocene to Pliocene draping successions interpreted as drowned carbonate platforms overlain by hemipelagic sediments. Farther north, in the shoreward part of the Phu Khanh Basin, an upward broadening shelf developed during late Neogene time comprises deltaic to shallow marine deposits (Fig. 7). A diffuse highamplitude interval with a strongly reflected, rough upper surface and numerous c. 100–300 ms high mounds compose part of the shoreward succession (Figs. 11d and 22). The interval is interpreted as the seaward continuation of local onshore basalts with ages ranging in between 7.5 and 10.5 Ma (Chung et al., 1998; Lee et al., 1998; Rangin et al., 1995b). Subtle clinoforms with steeply dipping foresets in the seaward part of the basalt interval most likely corresponds to volcanic deltas along the paleo-shore line known from e.g., the North Atlantic (Fig. 22)(Boldreel & Andersen, 1994). Farther seaward, a number of distinct mounds up to several hundred ms high most likely reflect volcanoes, in places fringed by more horizontal volcanic flows and related volcanoclastic deposits (Fig. 11d). High-amplitude, saucer-shaped structures within the upper Neogene form the base of gentle domes (Fig. 23). The structures are interpreted as igneous sills analogue to intrusive complexes described along the Atlantic margin (e.g., Hansen and Cartwright, 2006; Rocchi et al., 2007; Polteau et al., 2008). The overlying folds indicate the relative age and depth of in placement. A coast-parallel shelf slope established in the Phu Khanh Basin during the late Neogene and build up as the shelf prograded across the deeper part of the basin (Figs. 7, 11e, 18). Farthest north, the shelf-slope facies grades into a ramp and slope facies comparable to that observed above the seaward part of the Con Son Swell. In the northern and central Phu Khanh Basin, the basinward upper Neogene facies onlaps older relief of the area and grades shoreward into slope deposits composed of low-relief lenses of slope-front fill with thin interjacent drapes (Fig. 18). The facies is generally characterized by weak to intermediate reflections of variable continuity interrupted by numerous channel features on the edge of seismic resolution and is interpreted as a mixed succession of turbidites and hemipelagic sediments. 5. Discussion 5.1. Escape tectonics Evidence for large-scale left-lateral strike-slip along the EVBF before mid-Oligocene time is presented here and in previous studies of the Song Hong and the Phu Khanh basins (Rangin et al., 1995b; Nielsen et al., 1999; Andersen et al., 2005; Sun et al., 2003, 2004; Clift and Sun, 2006; Fyhn et al., 2009a). Arguments against left-lateral motion across the northern EVBF have focused on the ca. NNW–SSE- to N–S bend of the EVBF zone along the Song Hong Basin even though a more linear through-going cross-basin fault may well lie concealed underneath the extremely thick post-rift succession in the central part of the basin. However, more convincing evidence for left-lateral motions come from the presence of 1) pervasive WNW- to NW trending extension

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Fig. 23. Seismic section from the Phu Khanh Basin intersecting saucer shaped intrusion. The emplacement of magma caused a doming of the above-lying deposits, which are onlapped by latest Neogene deep marine sediments indicating the relative age and depth of emplacement.

faults along the basin, 2) the dominating horsetail fault-splay geometry towards the Beibuwan and the Qiongdongnan basins, 3) the internal WNW- to NW trending, right-stepping extensional fault style in the basin, and 4) the unbroken and immediate connection with the ASRRSZ — one of Earths most prominent left-lateral shear zones. In addition, analogue modeling of left-lateral pull-apart rifting in general (Smit et al., 2008) and of the Song Hong Basin in specific allowing for clockwise rotation of the extruded Indochina Block (Sun et al., 2003, 2004) have produced fault geometries comparable to those observed in and around the basin. The bending geometry of the Song Hong Basin does therefore not conflict with left-lateral motion across the EVBF, rather it reflects clockwise rotation of the extruding block combined with left-lateral shearing across less prominent NWtrending faults immediately southwest of the ASSRRSZ and the northern EVBF (Figs. 1, 3, 24a). Left-lateral shearing across such faults may help explain why southeastern Indochina did not behave as a single rigid block, thus allowing for southward extrusion of southeast Indochina parallel with the adjacent EVBF in the southern Song Hong Basin and the Phu Khanh Basin. Based on gravimetric modeling, the southward continuation of the EVBF in the Phu Khanh Basin has been argued to merge with a prominent right-lateral fault zone in the northern part of the basin (Roques et al., 1997a). This right-lateral fault zone has not been identified in this or other seismic studies of the Phu Khanh Basin and

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Fig. 24. Conceptual reconstruction of the middle to late Cenozoic Indochinese development showing the approximate position of the modern coastline as reference. See Figs. 1 and 3 for geographic nomenclature. a) Larger parts of Southeast Asia were extruded to the southeast due to the Indian Eurasian collision during Eocene to early Oligocene time. This led to major sinistral strike-slip in the ASRRSZ and in the EVBF as well as in the Mae Ping and the Three Pagodas shear zones, which splayed into the extensional fault systems that form the backbone of the rift basins surrounding Indochina. The proto-SCS was subducted underneath Borneo and Palawan, the slab-pull from which probably triggered rifting in areas farther east of the Indochinese margin. b) As the northward indentation of India went on and the West Borneo block coupled with India during late Oligocene through earliest Miocene times, left lateral faulting across the Mae Ping and the Three Pagodas shear zone terminated and was mildly inverted. The Indochinese extrusion farther north was taken up within the Shan–Thai Terrain and the EVBF only experienced moderate sinistral offset. The shear sense of the southernmost part of the EVBF inverted around the beginning of the Miocene related to collision tectonics and/or the subducting proto-SCS. Continental break up and seafloor spreading was forced by the slab-pull from the subducting proto-SCS. c) Extrusion along the ASRRSZ continued to be taken up within the Shan–Thai Terrain during the early and middle Miocene but gradually decreased and eventually ceased. Rifting and seafloor spreading offshore south Indochina resulted from the southward drift of the Dangerous Grounds associated with the subducting proto-SCS. Consequently, the N–S- to NW–SEtrending faults along southeastern Indochina may have had a pronounced right-lateral component like faults offshore Borneo. Volcanism along the southeastern Indochina margin marks the inception of magmatism in the region. d) Southeastward extrusion of the South China Block eventually exceeded the Indochinese by late Neogene time; causing the sense of strike-slip across the ASRRSZ and the northwestern-most part of the EVBF to become dextral. Farther southward, widespread magmatism took place across the southern Indochina, which led to onshore uplift and increased offshore sedimentation. The proto-SCS had closed and the subsequent continental shortening along Borneo had peaked before late Neogene time. Consequently, seafloor spreading in the SCS and the propagation of continental breakup ceased around the end of the early Neogene. e) Modern outline of the region.

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does probably not exist. The EVBF in the Phu Khanh Basin thus only connects with the major left-lateral fault zone from the Song Hong Basin, and the northwest trending extensional faults associated with the southward break up of the EVBF together with the right-stepping horsetail fault splays from the EVBF in the northeastern part of the basin and the synthetic extensional fault pattern analogue to onshore left-lateral faults suggest left-lateral motion across the southward continuation of the EVBF in the Phu Khanh Basin as well. Left-lateral shearing along the EVBF controlled Paleogene rifting along the western basin margin in the Phu Khanh Basin, and significantly influenced rifting farther east (Fig. 25). A pronounced lateral offset along the EVBF is suggested both by the superimposition of different pre-Tertiary units across the fault and by the remarkable continuity and linearity of the structure. The segmentation and the southeastward splaying of the EVBF indicate a reduced amount of left-lateral displacement toward the Tuy Hoa Fault Zone, which attest to the accommodation of major left-lateral motion in the coeval NW– SE- to E–W-trending rift system eastward of the transform. A comparable structural configuration exists farther north between the EVBF and the western Beibuwan and Quingdongnan basins suggesting take-up of left-lateral motion up to hundreds of km offshore (Fig. 3)(Rangin et al., 1995a,b; Roques et al., 1997a; Nielsen et al., 1999; Sun et al., 2004; Andersen et al., 2005; Huyen et al., 2005; Clift and Sun, 2006). We suggest that most of the early Oligocene and

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older strike-slip motion across the ASRRSZ and the EVBF was taken up by rifting in the offshore basins through extensional fault splays into the Beibuwan, the Qiongdongnan and the Phu Khanh basins (Fig. 24b), which accounts for hundreds of kilometers of Paleogene left-lateral extrusion. A continuation of the EVBF south of the Phu Khanh Basin toward Borneo seems unlikely given the structural segmentation of the fault in the Phu Khanh Basin. This demonstrates that the theory of an unbroken transform structure from NW Borneo along the shores of Vietnam to the East Himalayan Syntaxis is incorrect (Fig. 24a), as otherwise frequently invoked (e.g., Holloway, 1982; Briais et al., 1993; Leloup et al., 1995, 2001a,b; Replumaz and Tapponnier, 2003; Clift et al., 2008). Slab-pull forces from the subducting proto-SCS most likely contributed to the tensile stress seaward of the EVBF and resulted in rifting from the Chinese margin in the north to offshore Borneo in the south (Fig. 24a). South of the Phu Khanh Basin, the only patchily distributed Paleogene strata indicate that the Con Son Swell and part of the Nam Con Son Basin formed areas with high-lying basement (Fig. 25). Farther south in the Nam Con Son Basin approximately E–W trending Paleogene grabens underlie the Neogene sediment cover suggesting a Paleogene rift-mechanism distinct from the Neogene propagation of the SCS spreading axis (Matthews et al., 1997). Although a splay from the Mae Ping Shear Zone underneath the Mekong Delta remains to be documented, the W to WNW-ward

Fig. 25. Simplified stratigraphic columns for the basins along the Vietnamese margin with main regional tectonic events indicated. N. CSS = North Con Son Swell, NE. CLB = Northeast Cuu Long Basin, NE. MB = Northeast Malay Basin (the Vietnamese part of the basin), N. NCSB = North Nam Con Son Basin, PKB = Phu Khanh Basin, SHB = Song Hong Basin. Stratigraphy of the Song Hong Basin is inspired after Huyen et al. (2005) combined with unpublished in-house data. The stratigraphy of the Vietnamese part of the Malay Basin is based on unpublished in-house data.

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fault bends in the eastern Cuu Long Basin south of the study area is consistent with rifting in response to escape tectonics (Fig. 24a). Huchon et al. (1998) estimated approximately 30 km of stretching perpendicular to the Cuu Long Basin axis. Such an offset along the Mae Ping Shear Zone is reconcilable with the relative unbroken Late Jurassic–earliest Paleogene granite belt along the coast of SE Vietnam — a larger offset is not (Fig. 1). The Paleogene rifting along the Vietnamese margin (Fig. 25) is thus in accordance with the original model of Tapponnier et al. (1986), which suggested that the rift basins along the margin were formed as splays from the left-lateral EVBF and the Mae Ping Shear Zone. Coeval rifting farther east was more likely governed by slab-pull forces related to subduction below Borneo as suggested by Hall (1996) (Fig. 24a). A distinct tectonic shift signified by basin inversions occurred along the entire Vietnamese margin after early Oligocene time contemporary to the onset of exhumation of the core complexes along the ASRRSZ (Fig. 25). Huchon et al. (1994) ascribed the latest Oligocene/ earliest Neogene tectonic reorganization to changing stress patterns related to the northward indentation of India. In addition, the reorganization occurred simultaneous to the tectonic shift in the adjacent Thailand and Myanmar (Burma) area, associated with the effective coupling of the West Burma Block and India, which amongst others resulted in strike-slip inversion of the Mae Ping Shear Zone (Lacassin et al., 1997; Morley et al., 2007; Smith et al., 2007; Morley et al., submitted for publiction). Coupling of the West Burma Block and India broadened the continental indenter toward the east, which obviously had a large impact on escape tectonism in neighboring Indochina. The effect are manifested in the severe crustal shortening and uplift of the Shan–Thai Terrain that constitutes the northern part of the Indochina Block that border the less deformed West Burma Block (Fig. 3)(Tien, 1991; Leloup et al., 1995; Wang and Burchfiel, 1997; Upton, 1999; Wang et al., 2006; Akciz et al., 2008). This clearly indicates that continental coupling and northward movement of India led to prominent compression in the northern part of the Indochina Block as well as transpression along the ASRRSZ (Fig. 24b). Left-lateral shearing offshore Indochina evidently declined after mid-Oligocene time and in the Phu Khanh Basin completely stopped following the Paleogene (Fig. 25). Nevertheless, most onshore thermochronologic studies are interpreted to indicate major Neogene left-lateral motions in the onshore part of the shear zone, and more limited pre mid-Oligocene left-lateral deformation (e.g., Leloup et al., 1995, 2001a; Nam et al., 1998; Wang et al., 1998, 2000; Gilley et al., 2003; Searle, 2006). As a consequence of this, most regional paleoreconstructions incorporate the main left-lateral shear across the East Vietnam Margin or part of this after the mid-Oligocene (e.g., Hall, 1996, 2002; Morley, 2002; Replumaz and Tapponnier, 2003). Following the mid-Oligocene onshore crustal shortening of northern Indochina accommodated virtually the entire left-lateral offset along the ASRRSZ (Fig. 24b, c). This explains why major leftlateral displacement is reported to have continued onshore while significantly decreased in the offshore part of the fault zone. A recent study by Viola and Anczkiewicz (2008) conclude that little left-lateral faulting in the Day Nui Con Voi metamorphic core complex occurred after the Oligocene. This fits with the coeval modest left-lateral motion in the offshore continuation of the shear zone nearby. Hence, structural shortening within the Shan–Thai Terrain must have accommodated most post mid-Oligocene left-lateral motion northwest of the Song Hong Basin. 5.2. Formation of the SCS Left-lateral extrusion tectonics along the Vietnamese margin virtually ceased around the late Oligocene to the earliest Miocene. Consequently, Miocene spreading in the SCS was not associated with escape tectonics as otherwise widely argued (Fig. 24c)(Tapponnier

et al., 1982, 1986; Peltzer and Tapponnier, 1988; Briais et al., 1993; Leloup et al., 1995, 2001a; Replumaz and Tapponnier, 2003). Throughout the earliest spreading phase, N–S tension related to the EVBF-extensional splays in and south of the Qiongdongnan Basin may have contributed to the Oligocene seafloor spreading farther east. However, the earliest seafloor-spreading phase was like the Neogene probably governed by the slab-pull resulting from subduction beneath Borneo as maximum extension (evidenced by the formation of oceanic crust) occurred several hundred kilometers east of the EVBF (Taylor and Hayes, 1980, 1983; Briais et al., 1993). The eastward retreat of active seafloor spreading after early Oligocene time (Briais et al., 1993) and the coeval decrease in left-lateral faulting across the EVBF precludes a pull-apart mechanism for the late Oligocene seafloor spreading as this would have required several hundred kilometers of left-lateral strike-slip across the EVBF (Fig. 25) (Tapponnier et al., 1986; Peltzer and Tapponnier, 1988; Briais et al., 1993) and thus lends support to the slap-pull model. The Neogene onset and renewal of rifting along the south and central Vietnamese margin reflect the jump of seafloor spreading and the subsequent propagation of continental break-up around the Paleogene–Neogene boundary (Fig. 25). The most intense rifting occurred in the Nam Con Son Basin and in the southernmost part of the Phu Khanh Basin located at the tip of the propagating spreading axis. The direction of seafloor spreading and the NE–SW trending basin axis in the Nam Con Son Basin suggest NW–SE directed extension compatible with the slab-pull from subduction underneath northwest Borneo. Regional northwest–southeast extension could indicate a right-lateral component across the N–S to NNW–SSE verging faults in the study area similar to south of the spreading zone and in the southern Nam Con Son Basin (Figs. 1 and 24c)(Shaoren et al., 1994; Schlüter et al., 1996; Mat-Zin & Swarbrick, 1997). This is in accordance with onshore observations suggesting right-lateral faulting and reactivation across N–S- to NNW–SSE- trending faults. However, the right-lateral component does not seem to have exceeded a few kilometers as indicated by the generally moderate fault length and the moderate offset of geological markers on- and offshore (Tien, 1991; Tinh, 1998). The lower Neogene facies succession in the region indicates a gradual transgression that may be viewed in the light of the Miocene opening of the SCS. As the spreading axis jumped and subsequently propagated southwestward during early Miocene time, the study area came in close contact with the oceanic part of the SCS. As a result, marine conditions gradually came to dominate, which in the Phu Khanh Basin resulted in development of carbonate platforms, whereas siliciclastic deposition, possibly promoted by a high terrigenous input, prevailed farther south. During the later part of early Neogene time, open-marine conditions established in the northern part of the Nam Con Son Basin and the adjacent northeastern Con Son Swell, which together with a favorable climate and structural setting promoted carbonate production. Northwest–southeast directed dilatation and associated rifting continued unabated until near the end of middle Miocene time in the Nam Con Son and the southern Phu Khanh basins and across the northern Con Son Swell (Fig. 25). Assuming a zipper-like southwestward propagation of the spreading axis toward the Nam Con Basin as demonstrated by Huchon et al. (2001), the unabated rifting until the end of middle Miocene time in the Nam Con Son Basin suggests continued propagation of continental break-up and seafloor spreading throughout most of the middle Miocene. This indicates a significant prolongation of the seafloor-spreading phase in contrast to previous estimates (Taylor and Hayes, 1980, 1983; Briais et al., 1993; Barckhausen and Roeser, 2004) and imply slowed spreading rates after anomaly 5D time (18 Ma (Cande and Kent, 1995)) following the anomaly pattern proposed by Briais et al. (1993). This divergence in interpretation likely stems from the poor age resolution of the youngest oceanic crust that lacks a clear magnetic anomaly pattern

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within a ca. 50 km wide belt that straddle the extinct spreading ridge (Taylor and Hayes, 1983; Briais et al., 1993). The extensive thickness of this youngest ca. 50 km oceanic crust to the southwest compared to the thinner older crust farther from the spreading center (Trung et al., 2004) could be a result of protracted magmatic activity. In addition, this may explain the presence of the Scarborugh Seamounts aligned along the extinct eastern part of the spreading ridge that expose latest middle to late Miocene basalts (Fig. 26)(Briais et al., 1993; Trung et al., 2004; Braitenberg et al., 2006; Yan et al., 2006). The distinct drop in rifting in the Nam Con Son Basin across the late middle Miocene unconformity is suggested to have resulted from the termination of effective seafloor spreading. As the proto-SCS closed during the early Miocene time (e.g., Hall, 2002), seafloor spreading rates dropped. However, continental shortening along the Borneo margin led to the continuation of slow seafloor spreading and rifting along the South Vietnamese margin (Fig. 26). During the middle Miocene, continental collision culminated and the rate of crustal shortening was reduced, which impeded oceanic spreading in the SCS, and inhibited further propagation of continental break-up. Mantle and melt upwelling associated with the blocking of seafloor spreading induced a regional thermal uplift that affected areas adjacent to the extinct spreading axis including the Nam Con Son Basin and the southern part of the Phu Khanh Basin (Fig. 26). Hence, the latest middle Miocene unconformity marks the break down of seafloor spreading and continental break-up in keeping with distinctly reduced rifting following the uplift.

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(Fyhn et al., 2009b). During the same period onshore studies reveal the onset of major uplift and denudation evidenced offshore by the considerable thickness of the siliciclastic dominated late Neogene succession (Carter et al., 2000). Furthermore, the decreased carbonate deposition after middle Neogene time likely reflect the increased supplies of inorganic nutrients and siliciclastic sediments as well as climatic and oceanographic shifts following in the wake of the onshore uplift (Fyhn et al., 2009b).

5.3. Neogene volcanism, uplift and increased sedimentation rates Neogene volcanism along the central and South Vietnamese margin appears to have taken place during two phases. The earliest phase of latest Oligocene–earliest Miocene age is recognized only in the Cuu Long Basin. Although seismic data suggest only a moderate volume of eruptites from this phase, the numerous volcanic mounds scattered across the basin within a short stratigraphic interval suggest that the latest Oligocene/earliest Miocene was a period of intense magmatism in the Cuu Long Basin. Magmatism probably relate to one of two tectonic events: 1) the late Oligocene inversion or 2) the onset of southwestward propagation of the SCS spreading axis. As the Cuu Long Basin was virtually unaffected by early Miocene rifting and as coeval volcanism did not affect the Nam Con Son Basin we prefer the former explanation. The drilled late Oligocene volcanics formed prior to the SCS-reorganization and therefore lend support to a link to the coeval inversion event (Areshev et al., 1992; Hamid, 1994). The second more widespread and voluminous volcanic phase is linked with the late Neogene onshore volcanism based on their coincident timing and on seismic data that document the offshore continuation of late Neogene volcanic provinces exposed onshore. This documents that the onshore volcanic region centered in southern Indochina continues offshore and that offshore volcanism was and still is of comparable intensity to the onshore. Although early–late Miocene volcanism offshore is firmly established by seismic stratigraphy and drilling (Fig. 4)(BHP, 1993), the precise onset of volcanism is poorly constrained due to the deterioration of the seismic signal within and below the volcanic successions and due to the limited well control. Hence, the intense modern volcanic activity of southeast Indochina commenced offshore during early Neogene time and subsequently broadened and intensified onshore during the late Neogene (Fig. 25) (Hoang and Flower, 1998; Lee et al., 1998). Rangin et al. (1995a) suggested that late Neogene rifting onshore was controlled by simultaneous magmatism. A similar pattern is suggested offshore by rifts localized along coeval volcanic centers in the Phu Khanh Basin (Fig. 11c–e). As onshore, magmatism offshore revitalized older fault systems related to escape tectonism and the subsequent propagation of the southwestern SCS spreading arm. The karstification of the Phan Rang Carbonate Platform documents a broad uplift of the northern Con Son Swell during the late Miocene

Fig. 26. Conceptual reconstruction of the southwesternmost late-stage opening of the SCS. a) (ca. 22 Ma) Subduction of the proto-SCS resulted in zipper-like opening of the SCS. Crustal extension took place ahead of the tip of continental breakup, which caused Neogene rifting in the Nam Con Son and the southern Phu Khanh basins. b) (ca. 17 Ma) As the proto-SCS closed during the late-middle Miocene seafloor spreading and the propagation of continental breakup slowed considerably. Subsequent collisional shortening along Borneo led to ca. 50 km of spreading at reduced pace, which resulted in distinctly thicker middle Miocene oceanic crust without well developed magnetic anomalies as compared to the older crust farther away from the spreading center. c) (ca. 12 Ma) As collisional shortening dropped, effective seafloor spreading and rifting ahead of the continental breakup was blocked. The final rising melt and hot asthenosphere resulted in a pronounced uplift evidenced by the late middle Miocene unconformity in the area surrounding the spreading axis as further southward movement of the Dangerous Grounds and seafloor spreading was blocked. This also accounts for the Scarborough magmatism along the extinct spreading ridge postdating seafloor spreading.

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Fig. 26. (continued).

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Noticeably, magmatism commenced several million years before the middle late Miocene inception of onshore uplift (Carter et al., 2000; Wan et al., 2006). This most likely reflects that the majority of the onshore basalts erupted during the latest Neogene only, following a magmatic intensification onshore (Barr and Macdonald, 1981; Rangin et al., 1995a; Hoang and Flower, 1998). Contrary, most offshore volcanism took place during early to middle late Miocene time. In the shoreward part of the Phu Khanh Basin, a distinct unconformity separates lowest Miocene carbonates from late Miocene clastics reflecting a local uplift induced by nearby magmatism (Figs. 11b–e and 19). Volcanism may have struck through earlier in the offshore areas due to the severe Paleogene attenuated lithosphere. For the same reason a lesser uplift took place offshore as the continental margin already underwent post-rift sagging and compactional subsidence. 6. Conclusion Based on a comprehensive analysis of both new seismic data and existing literature four main conclusions can be drawn concerning: 1) Paleogene rifting along the Vietnamese continental margin, 2) the implications of mid–late Oligocene basin inversions, 3) the Neogene opening of the South China Sea, and 4) volcanism along the margin. Paleogene rifting was related mainly to escape tectonism that exerted a major control on the establishment of the margin, which is in contrast to the prevailing theories concerning the evolution of the Vietnamese South China Sea-margin. The study further shows that the left-lateral East Vietnam Boundary Fault makes up the almost 1000 km long seaward continuation of the Ailao Shan–Red River Shear Zone that straddles the north and central Vietnamese shelf and terminates in the southern Phu Khanh Basin. Major eastward extensional fault splays accommodated the left-lateral movement across the EVBF that amounted to hundreds of km judged by the size and the intensity of crustal stretching in the basins associated with these fault splays. Offset pre-rift lithologies interpreted across the southern part of the EVBF supports a lateral offset of such magnitude. Paleogene rift basins underlying the north and central Vietnamese continental margin formed in response to hundreds of km of left-lateral transtension across the Boundary Fault accommodated within major eastward extensional fault splays. Similarly, the Cuu Long Basin most likely formed through fault splays from a strand of the left-lateral Mae Ping Shear Zone but only accounts for a lateral offset in the order of a few tens of kilometers. The Nam Con Son Basin formed during two periods of rifting, the oldest of which took place during Paleogene time. Segments of the northeastern basin together with the Con Son Swell seem to have constituted part of a larger exposed basement high throughout the Paleogene. The proportions of a regional mid-Oligocene tectonic shift have been overlooked until now. We suggest that this change took place in response to the northward indentation of the Indian continent and to the West Burma Block coupling with India, which resulted in a significantly broadening of the colliding continental mass. As a consequence, the Mae Ping Shear Zone and the Cuu Long Basin inverted during late Oligocene time. Farther north, left-lateral displacement decreased across the East Vietnam Boundary Fault and had virtually ceased around the end of the Oligocene. In the Song Hong and the Phu Khanh basins structural inversion resulted from this tectonic change. Subsequent left-lateral displacement across the Ailao Shan-Red River Shear Zone was most likely taken up onshore by crustal shortening and block rotations within the Shan–Thai Terrain located in between the West Burma Block and the shear zone. The understanding of the development of the South China Sea mainly comes from the studies of magnetic anomalies of the oceanic crust, but this approach has not resolved the age of the final spreading phase unambiguously nor has it provided any conclusive evidence concerning the fundamental driving mechanism of the oceanic

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spreading. We document intense Neogene rifting in the Nam Con Son and the southern Phu Khanh basins, which occurred in response to southwestward propagation of continental break-up and seafloor spreading. Neogene rifting and opening of the South China Sea commenced when the offshore effects of escape tectonics had virtually stopped and thus most likely happened in response to the slab-pull from the subduction of the proto-South China Sea beneath Borneo and Palawan. The continuation of middle Miocene seafloor spreading is suggested by coeval rifting in the Nam Con Son Basin at the propagating tip of the oceanic basin. Seafloor spreading terminated around the middle–late Miocene transition as the protoSouth China Sea closed and crustal shortening related to the closure declined as indicated by the distinct slow-down and eventual halt of rifting at the propagating tip of oceanic spreading. A pronounced latest middle Miocene unconformity formed in the Nam Con Son and the southern Phu Khanh basins due to the halt of seafloor spreading. Previous research has focused on onshore late Neogene Volcanism in southern Indochina. We show that volcanism took place during two phases along the western South China Sea-margin. The first phase took place during the late Oligocene to earliest Miocene in the Cuu Long Basin and was associated with basin inversion. The second volcanic phase occurred more regionally and commenced offshore during early Miocene time. Magmatic activity later widened and caused basaltic volcanism onshore. In parts of the Phu Khanh Basin particularly intense magmatism reactivated older faults and caused the build up of major volcanoes that now underlie the Vietnamese margin. A subsequent late Neogene intensification of volcanism onshore lead to regional uplift and denudation of southeastern Indochina. This in turn enhanced the offshore siliciclastic accumulation rates, and thereby repressed offshore carbonate deposition. Acknowledgement This study was funded by the University of Copenhagen. Additional funding was obtained through the Danida-sponsored ENRECA project “Integrated analysis and modeling of geological basins in Vietnam and assessment of their hydrocarbon potential, phase II”. We thank PetroVietnam and VPI (Vietnam Petroleum Institute) for providing seismic reflection and well data and for permission to publish this paper, as well as VPI and the Geological Survey of Denmark and Greenland, (GEUS) for providing facilities for data interpretation and analysis. Jette Halskov, Eva Melskens and Stefan Sølberg drafted the figures. Thanks are due to VPI for inviting the first author as a guest researcher at VPI in Hanoi, Vietnam. The reviews of R. Hall and C. Morley led to improvements of the manuscript and helped strengthen the argumentation.

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