Tectonophysics 476 (2009) 397–417

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Tectonophysics

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The Red River Fault zone in the Yinggehai Basin, South China Sea

Mangzheng Zhu a,⁎, Stephan Graham b,1, Tim McHargue c,2

a 6001 Bollinger Canyon Road, D 1236, San Ramon, California, 94583, USAb Braun Hall GES Building 320, Room 118, Department of Geological and Environmental Sciences, Stanford, California, 94305, USAc 6001 Bollinger Canyon Road, D 1174, San Ramon, California, 94583, USA

⁎ Corresponding author. Tel.: +1 925 842 3710; fax: +E-mail addresses: mzhu@chevron.com (M. Zhu), sag

(S. Graham), timmchargue@gmail.com (T. McHargue).1 Tel.: +1 650 723 0507; fax: +1 650 725 2099.2 Tel.: +1 925 842 6255; fax: +1 925 842 2076.

0040-1951/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tecto.2009.06.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 December 2007Received in revised form 13 May 2009Accepted 11 June 2009Available online 21 June 2009

Keywords:Red River FaultYinggehai BasinSouth China SeaSlip reversalIndia–Asia collision

The Red River Fault, a major strike-slip fault of onshore Southeast Asia, is suggested to be closely related tothe formation of the South China Sea and uplift of the Tibetan plateau, yet published documentation of theoffshore segment of the Red River Fault is sparse. High-resolution seismic reflection and borehole dataobtained by the petroleum industry along the northwestern margin of the South China Sea permit mappingof the offshore segment of the Red River Fault in the Yinggehai Basin, as well as demonstration of the timingof its shear movement and slip reversal. Two boundary faults and two basin-center faults are mapped. Threesuccessive deformation phases for the offshore Red River Fault zone include sinistral movement from ~30to16Ma, slip reversal between 16 and 5.5 Ma, and dextral movement with low rates after 5.5 Ma. Althoughno robust piecing points were observed for the sinistral movement period, the horizontal displacement ofdextral movement is estimated to be tens of kilometers. This study supports a two-stage model for theevolution of the India–Asia collisional orogen. Before the middle Miocene, sinistral movement of the RedRiver Fault likely was linked to spreading of the South China Sea, consistent with the concept of continentalextrusion. After the middle Miocene, distributed shortening better explains a low rate of dextralaccommodation of the Red River Fault in response to the continuing India–Asia collision.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The Tibetan plateau is the highest and largest plateau on Earth, andit is an ideal place to study continent–continent collision dynamics.Understanding the evolution of the Tibetan plateau and its relation-ship to regional patterns of structure are of great importance. Twomajor hypotheses about the mechanisms for uplift of the Tibetanplateau are the continental extrusion model (Tapponnier et al., 1982,2001) (Fig. 1) and the distributed shortening model (England andHouseman, 1986; Molnar et al., 1993). A central prediction of thecontinental extrusion model is that sinistral movement of the RedRiver Fault caused spreading of the South China Sea at its termination(Tapponnier et al., 1990). Alternatively, the Red River Fault also playsan important role in the distributed shortening model, with Tibetrotating around the eastern Himalayan syntaxis by introduction of aweak crustal layer through lower crustal flow southeastward (Roydenet al., 1997; Wang et al., 1998a; Clark et al., 2005). Hence, study of the

1 925 842 2076.raham@stanford.edu

l rights reserved.

Red River Fault is critical for evaluating these two hypotheses of thetectonic evolution of Southeast Asia and for postulating mechanismsof the India–Asia collision.

Compared with the sizable amount of published work about theonshore segment of the Red River Fault (e.g., Allen et al., 1984;Tapponnier et al., 1990; Scharer et al., 1994; Leloup et al., 1995;Harrison et al., 1996; Chung et al., 1997; Wang et al., 1998a; Searle,2006), relatively few subsurface images and supporting data havebeen published for assessing the character of the offshore segment ofthe Red River Fault (Rangin et al., 1995; Gong et al., 1997; Clift and Sun,2006; Huyen et al., 2007).

High-resolution reflection seismic and borehole data collected bythe petroleum industry along the northern margin of the South ChinaSea provide an excellent opportunity to study the offshore segmentof the Red River fault and its implications for the India-Asia collision.Our specific motivations in studying the offshore segment of the RedRiver Fault are fourfold. Our first goal is to investigate the strike-slipdistribution and structure of the Red River Fault in the YinggehaiBasin, providing a more complete view of the fault by combiningboth onshore and offshore data. Secondly, we document the timingand strike-slip movement of the offshore Red River Fault, in order toassess its slip reversal history during the middle and late Miocene.The third objective is to investigate the relationship between thefault and seafloor spreading of the South China Sea, as a test ofplausibility of the continental extrusion model. The final goal is to

Fig. 1. Tectonic and geologic map of Southeast Asia. (A) Modified after Tapponnier et al. (1990). It shows the continental extrusion model proposed by Tapponnier et al. (1990). 1: TheAltyn Tagh Fault. 2: The Kunlun Fault. 3: The Karakoram Fault. (B) Modified after Gilley et al. (2003) and Leloup et al. (1995). The onshore segment of the Red River Fault is exposed infour ranges about 10–20 km wide, including Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS), and the Day Nui Con Voi (DNCV).

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assess possible driving mechanisms for strike-slip movement of theRed River Fault.

2. Background

2.1. The Tibetan plateau

The Tibetan plateau has elevation in excess of 5000 m and greatcrustal thickness. The timing of its uplift is still in debate (Molnar andEngland,1990;Harrison et al.,1995; ColemanandHodges,1995; Blisniuket al., 2001;Kirbyet al., 2002; Spicer et al., 2003; Clark et al., 2004; Currieet al., 2005; Clark et al., 2005; Clift et al., 2006; Rowley and Currie, 2006;Ritts et al., 2008), as shown in Fig. 2. Besides timing, mechanisms ofplateau uplift and the amount of shortening in the plateau are alsocontroversial issues about the Tibetan plateau. To address these issues,twomajor contrastinghypotheseshavebeendeveloped. The continentalextrusion model (Tapponnier et al., 1982, 2001) emphasizes that upliftof the Tibetanplateauwas caused by thickening of the Asian crust aboveIndia's obliquely subducting mantle. Convergence was principallyabsorbed by crustal thickening and extrusion of the left lateral strike-slip faults between coherent lithospheric blocks (Tapponnier et al.,1982,2001). On the other hand, the distributed shortening hypothesissuggests that the lithosphere continuously thickened as a widespreadviscous sheet, and convective removal of the lower lithosphere of theTibetanplateaubuoyantly bounded its crust and triggeredextensionandvolcanism (England and Houseman, 1986). This model-based hypoth-esis suggests that broadly distributed shortening of both crust andmantle accommodated plate convergence (Molnar et al., 1993).

2.2. South China Sea

The South China Sea developed through the interaction of theEurasian, Pacific, and Indo-Australian plates. As the South China Seaopened, several Cenozoic basins formed along its northern margin,including the Yinggehai Basin, Qiongdongnan Basin, Beibu Gulf Basin,and Pearl River Mouth Basin (e.g., Holloway, 1982; Zhou,1997; Lin andZhang, 1997).

Timing for the spreading of the South China Sea has been inter-preted to have occurred during 32–15.5 Ma based on magneticanomalies (Taylor and Hayes, 1980, 1983; Briais et al., 1993).Subsequently, Barckhausen and Roeser (2004) interpreted thatseafloor spreading of 5.6 cm/year full rate began at ~31Ma in thecentral part of the South China Sea. At 25 Ma spreading accelerated to7.3 cm/year full rate, contemporaneously with activation of a secondspreading center in the southwestern part of the South China Sea.Formation of oceanic crust ended at 20.5 Ma simultaneously at bothspreading centers (Fig. 2).

2.3. Red River Fault

The Red River Fault is a first-order tectonic structure that maybe followed from the east margin of the Tibetan Plateau to theSouth China Sea, separating the South China Block to the north and theIndochina block to the south, as shown in Fig. 1. The onshore segmentof the Red River Fault is exposed in four ranges about 10–20 km wideunderlain by mylonitic gneiss, metamorphosed to amphibolite grade(Leloup et al., 1995). These high-grade metamorphic massifs are the

Fig. 2. Stratigraphic column of the Yinggehai Basin and major tectonic events affecting the Tibetan plateau and South China Sea. Stratigraphic column of the Yinggehai Basin is fromHuang et al. (2004); Tectonic subsidence and sedimentation rates are from Profile 914540 by Lin and Zhang (1997); Wang (1997); Barckhausen and Roeser (2004).

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Xuelong Shan in China, Diancang Shan in China, Ailao Shan in China,and the Day Nui Con Voi in Vietnam.

The onshore segment of the Red River Fault reflects at least twosuccessive deformation phases illustrated in Fig. 3 and Table 1, butreversal of slip on the Red River Fault from 17 to 5 Ma is not welldocumented. Pliocene–Quaternary right-lateral movement of the RedRiver Fault is documented onshore by sharp geomorphic fault traces,large river offsets, and well-preserved cumulative scarps. Dextralmotion on the Red River Fault likely started around 8–5 Ma, and thetotal slip has been estimated to be 9 m–54 km (Allen et al.,1984;Wanget al., 1998a; Replumaz et al., 2001; Schoenbohm et al., 2006). Inaddition, the Red River Fault was characterized by left-lateral move-ment about 34–17 Ma, with widely varying estimation of total slip of100–1400 km(Tapponnier et al.,1990;Harrison et al.,1992; Leloup andKienast, 1993; Scharer et al., 1994; Leloup et al., 1995; Harrison et al.,1996; Chung et al., 1997; Wang et al., 1998b; Zhang and Scharer, 1999;Wang et al., 2000; Leloup et al., 2001; Gilley et al., 2003; Searle, 2006).

Following the pioneering work of Tapponnier et al. (1990), manygeologists have posited that the ductile, left-lateral deformation of theRed River Fault absorbed hundreds of kilometers of slip and asubstantial part of Asian shortening during the Cenozoic India–Asiacollision, and lead to the opening of the South China Sea (e.g., Schareret al., 1994; An et al., 2001; Leloup et al., 2001). However, someevidence is incompatible with the continental extrusion model: 1)Lack of recent significant shortening within sediments adjacent to theRed River Fault (Schoenbohm et al., 2006); 2) The Red River Faultmay extend across the Yinggehai Basin along the east Vietnammargin,but dextral motion of the east Vietnam margin is synchronous withspreading of the South China Sea (Roques et al., 1997b). Huyen (oralcommunication, 2007) also argued that sinistral movement may have

occurred during 32–17 Ma using unpublished seismic data offshore ofeastern Vietnam, which is synchronous with sea floor spreading of theSouth China Sea.

3. Dataset and age control

The dataset used in this study consists of 2-D and 3-D seismicreflection data, and 26-boreholes with wireline-logs provided byChevron Corporation, as well as published 2-D seismic lines in theYinggehai–Qiongdongnan Basin (Rangin et al., 1995; Gong et al., 1997;Roques et al., 1997a, b; Ying, 1998; Clift and Sun, 2006), as depicted inFig. 4. Thewest and central Yinggehai Basin are coveredwith a regionalgrid of 2-D seismic reflection profiles with 20–80 km spacing, whereasthe eastern parts of the Yinggehai Basin and Qiongdongnan Basin arecovered by 2×2 km–4×4 km spaced 2-D seismic reflection profiles.The 3-D seismic reflection dataset covering 1200 km2 is located at thejunction of the Yinggehai and Qiongdongnan basins. The 3-D seismicdata were acquired with spacing of 25 m in both inline and crosslinedirections. Published seismic lines are located in the Tonkin Gulf(Rangin et al., 1995), near the Zhongjian Ridge (Marquis et al., 1997;Roques et al., 1997a,b), and the Beibu Gulf basin (Ying, 1998). We mapstructures with greatest confidence in areas covered by the dense 2-Dseismic lines with spacing from 2×2 km to 4×4 km and the 3-Dseismic survey in the Qiongdongnan and southeastern Yinggehaibasins. In contrast, correlation of faults and folds between profiles isreasonable but less certain in the central andwestern Yinggehai Basin,where our 2-D seismic lines and published seismic lines are sparse.

Age controls on seismically mapped horizons of 21, 16, 13.8, 11, 5.5,3.6, 2.6, and 2 Ma aremainly supported by nannofossil biostratigraphyprovided by Chevron Corporation from 19 drilling sites and by ages

Fig. 3. Geochronologicaldata for theRedRiver Fault andspreadingof the SouthChinaSea. Theonshore segmentof theRedRiver Fault is exposed in four ranges about10–20 kmwideunderlainbymylonitic gneiss,metamorphosed to amphibolite grade (Leloup et al.,1995). These high-grademetamorphicmassifs are the Xuelong Shan in China, Diancang Shan in China, Ailao Shan in China,and theDayNuiConVoi inVietnam(Fig.1). YGH:YinggehaiBasin.Referencesare1, ZhangandScharer (1999);2,Gilleyet al. (2003);3, Leloupetal. (2001);4, LeloupandKienast (1993);5, Leloupet al. (1995);6, Schareret al. (1994); 7, Tapponnieret al. (1990), 8,Harrisonet al. (1992,1996);9, Searle (2006); 10,Allenet al. (1984); 11, Replumazet al. (2001);12, Schoenbohmet al. (2006);13,Zhao (1995); 14, To et al. (2000); 15, Wang et al. (1998a); 16, Wang et al. (2000); 17, Rangin et al. (1995); 18, Briais et al. (1993); 19, Barckhausen and Roeser (2004); 20, Lu (1997b).

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from recent publications (Rangin et al., 1995; Lu, 1997b; Thompsonand Abbott, 2003; Clift and Sun, 2006). There are no deep boreholes toconfirm the ages of sedimentary rocks older than 30 Ma in the central

Table 1Previous studies about the horizontal displacement and slip duration of the Red River Fault

Movement Duration Displacement Displacement rate Evidences

Sinistral Tertiary 700(+/−200) km ~3.3 cm/yr (35–17 Ma) Large-scallater Paleo

Sinistral 36–17 Ma 1400(+/−400)km

4–5 cm/yr (27–17 Ma) Zipper tec

Sinistral ~23 N500km N/A Estimated

Sinistral N/A ~600km N/A Correlatiohigh-pota

Sinistral N/A ~600km N/A CorrelatioCavinh Co

Sinistral 34–17 Ma 600 km 6 cm/yr (27–22 Ma) 40Ar/39ArVietnam

Sinistral 30~5.5 Ma Ten of kilometers N/A SubsurfacSinistral 36~5 Ma 200 km N/A Axial depoSinistral After 21 Ma Unknown N/A No robustDextral Holocene 9 m–6 km 2–5 mm/yr GeomorphDextral Plio-Quaternary 25 (+/− 0.5) km ~5mm/yr Large riveDextral Late Miocene to

present~40 km N5 mm/yr Field map

Dextral Present 1.7(+/−0.2) m GPS obserDextral Present 1–5 mm/yr GPS obser

Yinggehai Basin. Ages of horizons 30 and 26 Ma are extrapolated frombiostratigraphic data from the deep wells in Tonkin Gulf (Rangin et al.,1995; Thi and Quan, 1997).

.

Authors

e geological markers: late Mesozoic red-bed basins;zoic–Mesozoic sutures, intrusive rocks, and volcanic belts

Leloup et al. (1995)

tonics model supported by Paleomagnetism Leloup et al. (2001)

by rotation of the Indochina Block Tapponnier et al.(1990)

n of Permian–Triassic basalt successions and late Paleogenessic magmas in northwest Yunnan and northwestern Vietnam

Chung et al. (1997)

n with the late Archean Kangding Complex (China) andmplex (Vietnam)

Lan et al. (2001)

thermochronological study of the metamorphic massif at Wang et al. (1998a,b,2000)

e seismic data Rangin et al. (1995)center migration in the Yinggehai basin Sun et al. (2003)markers used to prove 500–1000 km offsets Searle (2006)ic fault trace, large river offsets Allen et al. (1984)r offset in Yunnan Replumaz et al. (2001)ping Schoenbohm et al.

(2006)vation, Yunnan, China Zhao (1995)vation in Vietnam To et al. (2000)

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4. Framework of the Yinggehai Basin

The NW-oriented Yinggehai Basin is located on the continentalshelf at water depths of 50–200 m in the northwestern South ChinaSea. The elongate Yinggehai Basin is more than 500 km long and150 km wide (Fig. 5). The Yinggehai Basin perhaps developed at areleasing bend of the Red River Fault (Gong et al., 1997), and it displaysseveral features typical of strike-slip basins, as described below.

TheYinggehai Basin is boundedbyhigh-angle faultswithnormal andreverse separation, consistent with strike-slip systems (Christie-Blickand Biddle, 1985; Harding, 1990), and geological mismatches of seismicfacies occur across the major faults, especially basin boundary faults inthe study area. Furthermore, the basin fill reflects cycles of subsidenceand uplift, contributing to its longitudinal and lateral basin asymmetry,aswell asmigration of depocenterswith time (Christie-Blick and Biddle,1985; Harding, 1990). Depocenter migration and the thickness of basin

Fig. 4. Location of subsurface data, Yinggehai–Qiongdongnan Basins, South China Sea. Locat80×20 km in the Qiongdongnan and Yinggehai basins. Bin spacing of the 3D seismic survey isincluding Tonkin Gulf (Rangin et al., 1995), Beibu Gulf (Ying, 1998), and Offshore Da Nang a

filling are shown in Figs. 5–8. During 30–21 Ma, NE–SWoriented depo-centers were localized in the northwestern basin shown in Figs. 5 and 6.Fig. 5 shows that depocenters migrated to the southeastern basinbetween 21 and 16 Ma. Between 16 and 5.5 Ma, the depocentersgradually migrated ~200km northwestward, and paused in the north-western basin. After 5.5 Ma, the depocenters rapidly migrated into theQiongdongnan Basin to the east. Depocenter migration was alsopreviously recognized by Gong et al. (1997) and Clift and Sun (2006).

The Yinggehai Basin is also characterized by rapid basin filling (Linand Zhang, 1997; Huang et al., 2004), episodic rapid basin subsidence(about 50–270 m/m.y.; Lin and Zhang, 1997), overpressure, and hightemperature (Chen et al., 1998; Hao et al., 1998; Hao et al., 2000; Dongand Huang, 1999; Dong and Huang, 2000; Zhang and Huang, 1991;Fig. 2). In addition, diapiric structures occur in the middle of the basin(Gong et al., 1997), associated with faulting and natural gas. Naturalgaseswith high contents of CO2 ranging between 15% to85% occur near

ion of this map is in Fig. 1. Spacing of the 2D seismic data ranges from 2×2–4×4 km to25×25 m in both inline and crossline directions. 2-D seismic lines cited from literature,rea (Roques et al., 1997a,b).

Fig. 5. Depocenter migration after 30 Ma in the Yinggehai–Qiongdongnan Basins, northwestern South China Sea. Numbers of the depocenters are compacted thickness (m).

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the diapirs, andmore than 10% of CO2 samples are of inferred inorganiccrustal origin with heavy carbon isotope values from −0.56 to −8.16and lower 3He/4He ratio (Huang et al., 2004).

5. Stratigraphy

Although the Yinggehai Basin is associated with strike-slip tecton-ism, it also resembles a typical rift basin during its early history, asimaged by 2-D seismic-reflection profiles shown in Fig. 9. The faultedsyn-rift sequences are overlain by relatively uniform and flat post-riftstrata that gently thin to the basinmargin. The Eocene–Oligocene synriftand Neogene–Quaternary post-rift megasequences are separated by anangular unconformity at the basin margins and a traceable high-amplitude reflection in the basin. The synrift strata rest on Paleozoic andMesozoic metamorphic basement rocks (Lu, 1997a). Deeply buriedsynrift strata are not imaged on seismic sections in the center of thebasin, where their thickness is estimated to be about 5000–7000 m

(Huang et al., 2004). The post-rift megasequence is about 9000 m thick(Huang et al., 2004). The Cenozoic stratigraphic succession is about17 km thick in total in the Yinggehai Basin (Gong et al., 1997).

Following the nomenclature of Prosser (1993) for rifting phases,the widely distributed Paleogene synrift megasequence consists of riftinitiation and rift climax phases, characterized by abrupt facieschanges. Eocene strata were deposited in topographic lows duringthe rapid rift initiation phase. Eocene strata are characterized bysubparallel and chaotic reflections with low continuity, interpreted aslacustrine, fluvial, alluvial, and marine environments (Lu, 1997a).During the subsequent rift climax phase in the Oligocene, accom-modation rapidly increased and sedimentation was outpaced bysubsidence. The Oligocene synrift sequence is dominated by diver-gent, subparallel/continuous seismic reflections, consistent withneritic/bathyal environments (Lu, 1997a).

The Neogene–Quaternary post-rift megasequence consists of twoelements. During the Miocene early post-rift stage, local differential

Fig. 6. Isochron map between 30 and 21 Ma, Yinggehai–Qiongdongnan Basins, northwestern South China Sea. Isochron contours are two way traveltime (TWT) in seconds.

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subsidence across faults was greatly decreased, and regional sub-sidence was mainly due to lithospheric thermal cooling effects (Linand Zhang, 1997; Lu, 1997a). This element of the basin fill,characterized by continuous parallel seismic reflections and localprograding clinoforms, suggesting siliciclastic sedimentation in neriticmarine environments along with basin-margin fan-delta develop-ment (Gong et al., 1997). After 10.5 Ma, the Yinggehai and Qiong-dongnan basins experienced coeval subsidence, and strata of the twobasins can be correlated. During the Pliocene–Quaternary late post-rift stage, faulting reactivated but rapid sedimentation rates exceededsubsidence rates. Pliocene–Holocene strata are relatively undeformedin the Yinggehai Basin. Parallel seismic reflections and regionalprograding clinoforms with high continuity reflect a neritic marineenvironment and deltaic progradation (e.g., Zhu et al., in press).

6. Structures of the offshore Red River Fault system

Two boundary faults and two basin-center faults in the YinggehaiBasin are likely the major offshore extensions of the Red River Fault

zone (Fig. 10). Two boundary faults trend along the northeast (fault A)and southwest (fault D) margins of the Yinggehai Basin are shown asbasin boundaries in Chinese literature. Fault A is namedNo.1 Fault, andpart of Fault D is called Yingxi Fault in Chinese literature. The other twobranches, C andD, are likely the twoprincipal faults of the basin center.Faults C and D are newly documented in this study. We alphabeticallyname of themajor faults in this study to avoid confusionwith the faultnomenclature in Chinese and Vietnamese publications.

The four faults are slightly curvilinear inmap view in the YinggehaiBasin (Fig. 10). In the northern basin, the Red River Fault system isnarrow and consists of four main strands that diverge southward outof the Tonkin Gulf. Near the south end of the Yinggehai Basin, thesefaults converge again and possibly connect with the east Vietnammargin faults to the south. In this paper, we examine in detail thesefour major faults and associated deformation during three phases ofstrike-slip movement (~30–16 Ma, 16–5.5 Ma, and b5.5 Ma). Due tolimited 2-D seismic survey coverage in the middle of the basin,correlating and connecting northern segments of Faults B, C, and Dwith their southern segments is tentative.

Fig. 7. Isochron map between 21 and 16 Ma, Yinggehai–Qiongdongnan Basins, northwestern South China Sea. Isochron contours are two way traveltime (TWT) in seconds.

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6.1. Oligocene–early Miocene structure of the offshore Red River Fault(~30–16 Ma)

The late Paleogene through early Neogene was a period of majorstrike-slip and associated deformation on faults of the Red RiverSystem in the Yinggehai Basin, based on pronounced thickness con-trasts across the faults.

6.1.1. Fault AIn map view (Figs. 10 and 11), Fault A is a boundary fault,

separating the Yinggehai Basin from the Beibu Gulf and Qiongdong-nan Basins. The linear trace of Fault A is mapped for about 420 kmfrom Tonkin Gulf to the Zhongjian ridge (Fig. 11). The strike of Fault Achanges from NE–SW in the north to N–S in the south.

In cross-sections (Fig. 12), southwestward-dipping Fault A is a highangle normal fault with displacement decreasing with time. It becameactive during 36–30 Ma (Lu, 1997a), peaked around 30–26 Ma, andthen faded around 21–16 Ma, based on stratal growth patterns. Inaddition, Fault A created great accommodation space during the syn-rift stage of sedimentation, and greatly deformed the rift and part ofthe post-rift megasequences.

6.1.2. Fault BIn plan view (Figs. 10 and 11), the curvilinear trace of Fault B has

been mapped for about 450 km from the Tonkin Gulf, throughout thecenter of the Yinggehai Basin. It possibly extends to near theZhongjian Ridge. The northern segment of Fault B strikes N–S, but itbends to NW–SE in the southern basin.

In cross-sections (Fig. 13), the northern segment of Fault B is asouthwestward-dipping, high angle fault with reverse slip oftenassociated with an anticline (L9 and L11 in Fig. 13). The wavelengthof the fold adjacent to the fault decreases from 19 km to 11 kmsoutheastward. Displacement across Fault B diminished upward untilaround 26 Ma in the northern basin, thereby defining the upper agelimit of its activity.

The southern segment of Fault B is localized along the axis of thebasin (L19 in Fig. 13), accompanied by diapiric structures. The chaoticseismic facies that characterize diapir-like structures apparently areinfluenced by gas effects. High content of crustal gas (Huang et al.,2004) found in these diapir-like structures suggests that Fault B is adeep-rooted fault. Initial movement of these diapiric structuresoccurred in the Oligocene (He, 1990). Thermogenic gas migrationalong Fault B occurred after 21–10 Ma, as suggested by isotope data

Fig. 8. Isochron map between 16 and 5.5 Ma, Yinggehai–Qiongdongnan Basins, northwestern South China Sea. Isochron contours are two way traveltime (TWT) in seconds.

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(Huang et al., 2004). In addition, the diapirs formed earlier in themiddle of the basin and later in the south basin (Lu, 1997a). Thisdiachronous development of diapir structures possibly suggests thatmovement of Fault B propagated from north to south.

6.1.3. Fault CFault C extends about 480 km from the Tonkin Gulf, cuts across the

center of the Yinggehai Basin, and approaches the Zhongjian Ridge(Figs. 10 and 11). In general, the trend of Fault C is NW–SE. Theseparation sense of Fault C changes significantly from reverse sepa-ration in the northern basin to normal separation in the southernbasin (Fig. 14).

In the northern basin, the magnitude of reverse faulting decreasessouthward along Fault C, as does related stratal folding. In Tonkin Gulf,the steeply southwestward dipping Fault C is characterized by positiveflower structures (L1 and L5 in Fig.14)with intense reverse faulting andoblique N–S en echelon folds (Rangin et al., 1995). Deformation wasmost intense during the Oligocene and then stopped around 16 Ma.After the deformedOligocene–Miocene stratawere exhumed and partlyeroded until 5.5 Ma, the fault was unconformably overlapped by flat-lying Pliocene–Holocene deposits (L1 in Fig. 14). Oligocene strata

adjacent to Fault C are separated bymore than1500 mof structural reliefin Tonkin Gulf. Further southward, Fault C is associated with a largeanticline (L5, L9, and L13 in Fig. 14). The wavelength of the anticlinedecreases southward from ~30km to ~10km.

6.1.4. Fault DFault D extends ~500km along the west margin of the Yinggehai

Basin (Figs. 10 and 11), where its trend varies from N–S in its northernsegment to NW–SE in its southern segment. The dip direction andseparation sense apparently alternate along the strike of this faultfrom a northeastward dipping fault segment with reverse separationin the northern basin, to a northeastward dip with normal separationin the south basin (Fig. 15).

In cross-sections (Fig. 15), the northern segment of Fault D is a highangle thrust fault associatedwithpositiveflower structures (L3 andL7 inFig. 15), with deformational magnitude decreasing southward. Theunbackstripped fold's amplitude is about 300–700 m in the Tonkin Gulf,and it decreases to ~200 m southward. Fault Dmotion climaxed around26Ma; thereafter, its deformationdecreasedand stopped around21 Ma.

The southern segment of Fault D is a high angle normal fault(L19 in Fig. 15), separating the Yinggehai Basin and Hue basin.

Fig. 9. Transverse 2-D seismic sections L1, L7, L19 and L25, Yinggehai Basin, South China Sea. Panel a is an uninterpreted transverse 2-D seismic sections. Panel b is an interpretedtransverse 2-D seismic sections. A, B, C, and D are fault names. L1, L7, L19 and L25 are numbers of 2-D seismic reflection profiles. Location of the 2-D seismic lines is in the inset Fig. 2.0–30: Ages of horizons calibrated by borehole information (Ma).

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Fault distribution is highly complex here, likely owing to the mergingof the Red River Fault and offshore Song Ma fault in the Hue basin(Fig. 11).

6.2. Middle–late Miocene structural inversion of the offshore Red RiverFault (16–5.5 Ma)

Most of the basement-involved faults were quiescent during thepost-rift phase between 16 and 5.5 Ma (Figs. 16, and 17). During thistime frame, Fault B was inactive and structural inversion prevailed inthe Yinggehai Basin, as described below.

6.2.1. Fault AA large positive structural inversion is demonstrated across a

30 kmwide zone between points X and Y (L3 in Fig. 12). This inversionstructure was associated with syn-tectonic sedimentation, enhancedby uplift of the previous sag. There are at least three periods ofdeformation associated with this inversion structure. Thick Oligoceneto lower Miocene sediments were deposited in the previous sag,thinning along the sag margins on the adjacent fault's hanging wall.Later, the Oligocene–lower Miocene deposits in the previous sag weredeformed and folded by thrust faulting. It is followed by uplift anderosion to form an unconformity dated at 16 Ma. This change is apositive inversion structure (cf. Williams et al., 1989), indicating the

polarity of structural relief from a previous low to high. Positivestructural inversion is consistent with a shift from a transtensionalregime to a transpressional regime.

To the south, Fault A is associated with slightly positive inversionstructure with reverse reactivation of normal faults (L5 in Fig. 12).Reverse-slip separation and folding of sedimentary reflections occurslocally. Reactivation of this fault occurred between 16–13.8 Ma andstopped around 3.6 Ma.

6.2.2. Fault CFault C displays intense structural inversion in Tonkin Gulf (L1 in

Fig. 14), as recognized by Rangin et al. (1995). Oligocene–Miocenestrata were uplifted by structural inversion between 16 and 5.5 Ma,and were eroded and overlain by Pliocene–Holocene deposits.

6.2.3. Fault DStrata that accumulated after 26 Ma are thin in the vicinity of Fault

D (L1 in Fig. 14). Undeformed strata at 5.5 Ma indicate thatdeformation ended by that time, as also recognized by Rangin et al.(1995). In addition, the hanging wall of Fault D was slightly upthrownon line L15 (Fig. 15), indicating the development of a positiveinversion after 16 Ma. Sediments deposited in the hanging wall arethinner than those at the footwall during 5.5–3.6 Ma, which suggeststhat the reverse rejuvenation of this fault ended prior to 3.6 Ma.

Fig. 10. Timemap of 26 Ma structure, Yinggehai–Qiongdongnan basins, northwestern South China Sea. Dash lines are faults modified from published literature, including Tonkin Gulf(Rangin et al., 1995), Beibu Gulf (Gong et al., 1997; Ying, 1998), Offshore Da Nang area (Marquis et al., 1997; Roques et al., 1997a,b), and Onshore Vietnam (Clift and Sun, 2006).

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6.3. Pliocene–Holocene structure of the offshore Red River Fault(after 5.5 Ma)

6.3.1. Fault AFault Awas reactivated near Hainan Island after 5.5 Ma (Fig. 17). Its

reactivation is indirectly supported by present fluid expulsionobserved near Fault A. More than 100 modern gas seepages occuradjacent to Fault A, as detected by sidescan sonar (Xie, 2003).Reactivation of Fault A is also accompanied by reactivation ofbasement faulting and submarine channel incision (Figs. 17 and 18).

Slope parallel submarine channel A formed at the basin floor, andfilled with more than 350 m of thick massive turbidite sandstones(Gong et al., 1997), as shown by high amplitudes in the 3-D seismicamplitude extraction map (Fig. 18).

6.3.2. Fault BFault B, with subvertical reverse slip, diverges and extends upward

into Pliocene and Holocene strata (L18 and 19, Fig. 13), and intersects

the sea floor in some locations. Fault B accompanies en echelon faultsstriking N–S, which are related to diapiric structures.

6.3.3. Faults C and DFaults C and D were locally reactivated after 5.5 Ma with little

vertical slip. Associated Pliocene–Holocene strata are relativelyundeformed.

7. Strike-skip movement of the offshore Red River Fault zone

The Red River Fault system in the Yinggehai Basin displays featurescharacteristic of strike-slip fault systems as defined by Christie-Blickand Biddle (1985), Harding (1990), and Sylvester (1988), including1) four long and likely through-going deformation zones shown inFig. 10; 2) subvertical to moderate dip of these four master faults;3) abrupt changes in seismic stratigraphic facies across the fault sys-tem; 4) coexistence and simultaneous development of both transpres-sional and transtensional structures in individual transverse profiles

Fig. 11. Timemap of 21 Ma structure, Yinggehai–Qiongdongnan basins, northwestern South China Sea. Dash lines are faults modified from published literature, including Tonkin Gulf(Rangin et al., 1995), Beibu Gulf (Gong et al., 1997; Ying, 1998), Offshore Da Nang area (Marquis et al., 1997; Roques et al., 1997a,b), and Onshore Vietnam (Clift and Sun, 2006).

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(e.g., Fault B with reverse slip adjacent to Fault A with normal sepa-ration; L1 in Fig. 9); 5) flower structures in transverse profile (FaultsC and D; L1 in Fig. 9), 6) coeval en echelon flanking structures;7) structural inversion along master faults (Faults A, C, and D),8) basement involvement, and 9) possibly inconsistent dip directionand separation sense along a single fault (Faults C and D). In addition,thesemajor faults project intomappedhorsetail splays of the RedRiverFault onshore and in Tonkin Gulf (Rangin et al., 1995). Therefore, weconclude that these four faults are the major extension of the offshoreRed River Fault zone. Slip motion and magnitude of the offshore RedRiver Fault zone are examined as follows.

7.1. Slip sense history of the Red River Fault zone

7.1.1. Oligocene–early Miocene sinistral slip (30–16 Ma)Based on our work and published data (Rangin et al.,1995; Gong et

al.,1997; Clift and Sun, 2006; Huyen et al., 2007), regional structuralpatterns and strain analysis are consistent with left-lateral slip for theRed River Fault zone in the Yinggehai Basin before 16 Ma: 1) The

Yinggehai Basin is a strike-slip basin, formed by N–S stretching and anE–W transtensional stress field; 2) In plan view, N–S trending enechelon folds (Rangin et al., 1995) are oblique to the NW–SE sheardirection of the traces of the Red River Fault zone in the Tonkin Gulf.They represent the shortening component of the strain locally; 3) NE–SW trending extensional faults are present at the junction of theYinggehai and Qiongdongnan Basins, and they are linked to strike-slipmovement of the faults in the Yinggehai Basin (Fig. 11; Zhong et al.,2004); 4) Depocenters migrated southeastward from 30 to 16 Maalong the central master faults in the Yinggehai Basin. In summary,structural and stratigraphic features are consistent with a sinistralstrain ellipse with NW principal displacement (Wilcox et al., 1973).

Our interpretation of sinistral slip in the Yinggehai Basin before16 Ma is consistent with other evidence. Results from sandboxanaloguemodeling of the Yinggehai Basin are consistent with sinistralextrusion of the Indochina block (Sun et al., 2003). Many onshoregeological markers in China and the Indochina block along the RedRiver Fault imply left-lateral slip sense, as summarized in Table 1.Moreover, left-lateral kinematic indicators are ubiquitous within the

Fig 12. Transverse 2-D seismic sections of Fault A, Yinggehai Basin, South China Sea. Panel a is Panel b is an interpreted transverse 2-D seismic sections of Fault A. A, B, C, and D are faultnames. L3–L25 are numbers of 2-D seismic reflection profiles. Location of these 2-D seismic lines is in Fig. 6. 2.0–30: Ages of horizons calibrated by borehole information (Ma).

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mylonites in the onshore segment of the Red River Fault (Leloup et al.,1995).

Relationships between the orientation of the Red River Fault andslip direction determines whether releasing bends or restrainingbends developed along local fault segments (cf. Crowell, 1974). Thenorthern segments of Faults C and D define restraining bends (L1 inFig. 14), and are dominated by positive flower structures with theirassociated thrust faults and en echelon folds. The southern segment ofFault A likely also defines a restraining bend. In contrast, releasingbends dominate the northern segment of Fault A (L3 and L5 in Fig. 12)and southern segments of Faults C and D (L21 in Fig. 14; L19 in Fig. 15),which are characterized by transtensional faulting.

7.1.2. Middle–late Miocene structure inversion (16–5.5 Ma)Strike-slip is often associatedwith inversion tectonics, and location

of inversion often suggests the location of a master fault (Harding,1990). It is difficult to reactivate steeply dipping normal faults byinversion, but it is possible to reactivate steep faults by oblique orstrike-slip movements, especially where the minimum compressivestress is horizontal (Coward, 1994).

Likemany other strike-slip inversions in theworld (e.g., Hsiao et al.2004), slip sense reversed from left lateral to right lateral along theRed River Fault zone in the Yinggehai Basin during 16–5.5 Ma(although reversion was also interpreted during 21–15 Ma by Cliftand Sun, 2006). This view is supported by structural and stratigraphic

evidence. For example, divergent bends were reactivated by con-vergent structures in the study area. For instance, in the northeastreleasing bend of Fault A, regional subsidence ceased with develop-ment of a large-scale structural inversion around 16 Ma near TonkinGulf (L3 in Fig. 12), and this event likely marks the onset time of slipreversal. To the south, normal faulting ceased and thrust faultingdeveloped along Fault A (L5 in Fig. 12). The onset of slip reversalappears to have occurred 16–13.8 Ma. At the southwest releasing bendof Fault D, slip reversal initiated after 16 Ma, climaxed around 5.5–3.6 Ma, and persists until the present in themiddle of the basin (L15 inFig. 15). Moreover, inversion is supported by stratigraphic reorganiza-tion in the Yinggehai Basin. Depocenters shifted northwestward by~200km from 16 to 5.5 Ma, shown in Fig. 5. In general, inversion waslikely caused by dextral movement of the Red River Fault zone, whichdeveloped earlier in the northern basin and later in the southernbasin.

7.1.3. Pliocene–Holocene dextral slip (after 5.5 Ma)The Red River Fault reactivated in a right lateral sense along the

present-day trend of the Red River Fault zone in the Yinggehai Basin.Our results show this movement likely climaxed around 5.5–3.6 Maand has persisted until the present time.

Pliocene–Holocene structures are consistent with dextral strain(Wilcox et al., 1973) based on the following evidence. 1) Positivestructural inversion along Faults A (e.g., L5 in Fig.12) and D (e.g., L15 in

Fig. 13. Transverse 2-D seismic sections of Fault B, Yinggehai Basin, South China Sea. Panel a is an uninterpreted transverse 2-D seismic sections of Fault B. Panel b is an interpretedtransverse 2-D seismic sections of Fault B. A, B, C, and D are fault names. L5–L19 are numbers of 2-D seismic reflection profiles. Location of these 2-D seismic lines is in Fig. 10. 2.0–30:Ages of horizons calibrated by borehole information (Ma).

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Fig. 15) suggest strain shifting from transtension to transpressionlocally, and changing the slip sense of faulting in the study area. 2) E–W stretching and a N–S transtensional stress field occurs in theYinggehai Basin (Luo et al., 2003). 3) NNW–SSE trending en echelonfaults with diapir-like structures occur along Fault B in the center ofthe basin (Li and Zhang, 1997), although an alternative interpretationof these en echelon faults is overpressure and diapirism. Becauseangular deviation and shear direction of en echelon structurescommonly ranges from 10° to 35° (Harding and Lowell, 1979),regional slip lies along a NW–SE trend in the Yinggehai Basin. 4)Pliocene structures are consistent with right-lateral slip sense in thenorthwestern Yinggehai Basin (Rangin et al., 1995; Gong et al., 1997;Huyen et al., 2007).

Our interpretation is also supported by other evidence for dextralslip from the onshore Red River Fault zone. They include: earthquakefocal mechanisms from onshore segments of the Red River Fault(Allen et al., 1984), GPS measurement along the onshore Red RiverFault in China (Zhao, 1995) and Vietnam (To et al., 2000), and dextraloffsets estimated by large river offset and fault scarps along the RedRiver (Replumaz et al., 2001).

7.2. Magnitude of strike slip

We have not recognized robust piercing points as defined byCrowell (1974) in our dataset and cannot constrain rates of lateral slipon any of these faults. However, the magnitude of strike slip on the

Red River Fault system can be inferred from the reasoning used toexplain the geological mismatches along major faults.

As revealed by the 2-D seismic profiles in Fig. 9, the magnitude ofvertical displacement and stratal wedging associated with the fourmajor faults indicates that these faults experienced rapid sinistralmovement before 21 Ma. In contrast, the major faults show much lessleft lateral motion during 21–16 Ma, with only amoderate thickness ofunfaulted strata. After 16 Ma, the major faults were relativelyquiescent, as suggested by faulted strata with minor vertical dis-placement. These observations support the interpretation that thedextral slip episode accumulated much less slip than the sinistralepisode in the Yinggehai Basin.

At least 20 km of sinistral slip occurred along the offshore segmentof the Red River Fault in the Yinggehai Basin. No convincing piercingpoints are observed in our dataset to constrain the upper limit ofhorizontal displacement, which likely is much greater than 20 km,based on onshore evidence summarized in Table 1. However, thelower limit of the sinistral slip can be estimated from structuralrelationships along Fault D in the Hue Subbasin. Seismic line L19(Figs. 9 and 15) shows that strata with southwest and northeast dipwere separated before 21 Ma, and now they are juxtaposed againsteach other. The distance between this section and the nearest sectionwith reasonably well documented faults (L21 in Fig. 14) is ~20km,which is a minimum estimated distance of sinistral slip. Subsequently,the hanging wall of Fault D was translated at least 20 km from thecurrent location, in order to avoid the problem of oppositely dipping

Fig. 14. Transverse 2-D seismic sections of Fault C, Yinggehai Basin, South China Sea. Panel a is an uninterpreted transverse 2-D seismic sections of Fault C. Panel b is an interpretedtransverse 2-D seismic sections of Fault C. A, B, C, and D are fault names. L1–L21 are numbers of 2-D seismic reflection profiles. Location of these 2-D seismic lines is in Fig. 6. 2.0–30:Ages of horizons calibrated by borehole information (Ma).

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strata. However, the complexity of the fault system makes ourestimate uncertain, because structures in the Hue basin are alsopossibly influenced by the Song Ma fault, as shown in Fig. 11.

We estimate that there have been tens of kilometers of dextraldisplacement along the major faults in the Yinggehai Basin. Ourreasoning is based on the juxtaposition of similar sedimentarypackages withminor vertical slip alongmajor faults during the dextralepisode after 16 Ma.

8. Discussions

8.1. Distribution and structure of the offshore Red River Fault zone

Through this study, two boundary faults (Faults A and D) and twobasin-center faults (Faults B and C) have been documented using agrid of seismic reflection data in the Yinggehai Basin. These four faultsare likely the offshore branching extension of the Red River Faultbecause: 1) they project toward the Red River Fault zone onshore andin the Tonkin Gulf, and 2) they show typical features of strike-slipfaults. The structural features of the offshore Red River Fault zone aregeneralized as follows: 1) These four faults are long, steep, likelythoroughgoing faults with normal or reverse slip; 2) reverse faultingoccurs at the restraining bends with positive flower structures, enechelon folds, and positive structural inversion; 3) transtensionalstructures are developed at the releasing bends; and 4) dip directionand separation sense apparently alternate along a single fault.

Different from previous structural mapping (Gong et al., 1997), thisstudy suggests that two major fault branches (Faults C and D; Figs. 11,13, and 14) axially cut the center of the Yinggehai Basin. These twofaults likelywere not recognized in previous research because they areoften deeply buried, especially in the basin center. Besides directlydocumenting faults with 2-D seismic data, our interpretation of faultlocation is supported by the presence of medium to large anticlinesand seeps of thermogenic gas along these central faults, and as well asby migration of depocenters along these central faults.

8.2. Tectonics of the offshore Red River Fault zone

From our studies we infer three successive deformation phases forthe offshore Red River Fault zone. From ~30 to 16 Ma, the Red RiverFault was characterized by sinistral movement in the Yinggehai Basin.Rapid sinistral movement of the Red River Fault was coeval with syn-rift deposition before 21 Ma. Slow sinistral movement of a few faultscharacterized the period of 21–16 Ma in the Yinggehai Basin. Duringthe second stage between 16 and 5.5 Ma, the Red River Faultexperienced slip reversal in the Yinggehai Basin, and slip sensechanged from sinistral to dextral. Slip reversal likely happened earlierin the north basin than in the south basin, which was previously notedin Clift and Sun (2006). Diachroneity of slip reversal is evidenced bythe timing of structural inversion and diapir development, as well asnorthwestward migration of the depocenters in the Yinggehai Basin.Dextral movement of the Red River Fault appears to have climaxed

Fig. 15. Transverse 2-D seismic sections of Fault D, Yinggehai Basin, South China Sea. Panel a is an uninterpreted transverse 2-D seismic sections of Fault D. Panel b is an interpretedtransverse 2-D seismic sections of Fault D. A, B, C, and D are fault names. L3–L19 are numbers of 2-D seismic reflection profiles. Location of these 2-D seismic lines is in Fig. 10. 2.0–30:Ages of horizons calibrated by borehole information (Ma).

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around 5.5–3.6 Ma and diminished to the present. A large submarinechannel developed at about 3.6 Ma along the basin axis, parallel to theRed River Fault strands (Figs. 17 and 18).

Our timing results are consistent with most thermochronologicalstudies of metamorphic rocks along the onshore Red River Fault(Fig. 3), although there is a lack of age controls older than 30 Ma inthis study. Geochronological information regarding the onshore RedRiver Fault zone comes from about 40 U/Pb ages constraining high-temperature changes, as well as more than 100 40Ar/30Ar agesconstraining its cooling history (Tapponnier et al., 1990; Harrisonet al., 1992; Leloup and Kienast,1993; Scharer et al., 1994; Leloup et al.,1995; Harrison et al., 1996; Chung et al., 1997; Wang et al., 1998b;Zhang and Scharer, 1999; Wang et al., 2000; Leloup et al., 2001; Gilleyet al., 2003; Searle, 2006). A few samples collected along the onshoreRed River fault in the Ailao Shan are dated between 17 and 20 Ma, butstill other samples gathered in the Ailao Shan and Day Nui Con Voimassif and most samples in Xuelong Shan and DianCang Shan showsinistral movement before 20 Ma.

8.3. Role of the Red River Fault in the formation of the South China Sea

The role of the Red River Fault in the opening of the South ChinaSea is a key prediction of the continental extrusionmodel (Tapponnieret al., 1982, 2001). Our interpretation of structures, shear timing andkinematics is consistent with a linkage between South China Sea

spreading and sinistral shear of the Red River Fault. We infer that theoffshore Red River Fault zone experienced fast sinistral movementfrom ~30 to 21 Ma, with cessation of most basement-involved faultslip around 21 Ma. Our inferred timing for sinistral movement of theoffshore Red River Fault is consistent with the majority of geochro-nological ages onshore, especially those that display rapid cooling inthe Day Nui Con Voi massif in Vietnam and the southern segment ofthe Ailao Shan. In the South China Sea, timing of seafloor spreadinghas been revised to 31–20.5 Ma (Barckhausen and Roeser, 2004), from32 to 15.5 Ma (Briais et al., 1993) by incorporating new magneticanomaly data. Thus, opening of the South China Sea is coeval withrapid sinistral movement of the major faults in the Yinggehai Basinand most geochronological results onshore.

Nevertheless, spatial relationships between the Red River Fault andthe South China Sea cannot be conclusively demonstrated by ourstudies. Our data donot reveal (but do not exclude) convincing evidencefor horizontal sinistral offset of hundreds of kilometers in the YinggehaiBasin, as suggested byother researchers summarized inTable 1. Onshorestudies that have shown a positive correlation between the Red RiverFault and spreading of the South China Sea, include: 1) more than 14°clockwise rotation of themid-Cretaceous Indochina block relative to theSouth China Block along the Red River Fault zone by paleomagneticstudies (Yang and Besse, 1993; Yang et al., 2001); 2) high-grademylonitic gneisses of the Red River Fault zone indicating sinistral slip(Leloup et al., 1995, 2001; Gilley et al., 2003); and 3) concordant

Fig. 16. Time map of 16 Ma structure, Yinggehai–Qiongdongnan basins, northwestern South China Sea. Isochron contours are two way traveltime (TWT) in seconds.

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displacementof severalMesozoic geologicalmarkers by ~700km,whichis about the size of the South China Sea in its direction of extension(Tapponnier et al., 1990; Leloup et al., 1995, 2001).

8.4. Implications for regional tectonics

Our offshore studies are consistent with a linkage in timing andkinematics between Red River Fault sinistral motion and opening of theSouth China Sea, thereby permissively supporting the continentalextrusion model before the middle Miocene. The continental extrusionmodel (Tapponnier et al., 1982, 1990) holds that the left-lateralmovementof theRedRiver Fault, the largest strike-slip fault in SoutheastAsia, was triggered by the shear stress component from obliquesubduction of the Indian plate relative to the Eurasian plate and rotationof the Indochina block. Parts of the Tibetan plateau achieved currentelevationbyat least themiddleMiocene(e.g., Blisniuk et al., 2001; Spiceret al., 2003), if not considerably earlier (e.g., Rowley and Currie, 2006;Wang et al., 2008). Thus, eastward extrusion and uplift of the Tibetanplateau were probably most important before the middle Miocene.

However, certain observed features along the Red River Fault anddeformation of the eastern Tibetan plateau after the middle Miocene

appear to be inconsistent with the continental extrusion model in atleast threeways: 1) The horizontal displacement rate ismuch less thanthe minimum requirements for the continental extrusion model alongthe Red River Fault after the middle Miocene (Allen et al., 1984; Zhao,1995; Wang et al., 1998a; To et al., 2000; Replumaz et al., 2001;Schoenbohm et al., 2006). 2) The eastern margin lacks large lateCenozoic shortening structures commonly associated with crustalthickening (e.g., Royden et al., 1997). 3) The extrusion model does notindicate clockwise rotation of crustal material around the easternHimalayan syntaxis (Roydenet al.,1997). This suggests that continentalextrusion might be only a relatively minor tectonic factor after 16 Maduring the later stages of the India–Asia collision, and of considerablyless importance after the middle Miocene than before it (cf. Yue andLiou, 1999; Yue et al., 2004; Darby et al., 2005; Ritts et al., 2004, 2008).

Due to apparent deficiency of the continent extrusion model after16 Ma, a different model is needed. Increasing evidence suggests thatdistributed shortening played a more important role in the later stageof India–Asia collision. The distributed shortening model holds thatthe partially molten middle crust underlying extruded southeastwardas Poiseuille-type channel flow (Royden et al., 1997; Beaumont et al.,2004; Klemperer, 2006). Southeastern Tibet has rotated clockwise

Fig. 17. Time map of 5.5 Ma structure, Yinggehai–Qiongdongnan basins, northwestern South China Sea. The location of the onshore Red River Fault is from published literature (Cliftand Sun, 2006), and distribution of the turbidite channel is from Gong and Xie (1997). Isochron contours are two way traveltime (TWT) in seconds.

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without major crustal shortening around the eastern Himalayansyntaxes, with initiation and prolonged history of eastward extrusionof crustal material from the plateau around the strong crust of theSichuan basin (Royden et al., 1997; Clark et al., 2005). This modelbetter explains the deformation record in response to the continuingIndia–Asia collision, specifically the regional low rate of dextralaccommodation of the Red River Fault zone. In addition, this modelmay better explain slip reversal of the Red River Fault, as well asinitiation of right-lateral displacement along the Gaoligong fault (Hsu,2000), Sagaing fault (Bertrand et al., 1999; Vigny et al., 2003; Bertrandand Rangin, 2003; Fig. 1) and Jiali fault (Armijo et al., 1989; Lee et al.,2003) after the middle Miocene.

In general, we favor a hybrid model for the evolution of the RedRiver Fault zone. It appears that continental extrusion was dominantin India–Asia collision before themiddleMiocene, and coincident withgrowth of an orogen whose height may have been comparable to themodern plateau. During the second stage after the middle Miocene,distributed shortening became more important than continental

extrusion along the Red River Fault, while crustal materials of theTibetan plateau grew eastward and dextrally rotated around the eastHimalayan syntaxis.

9. Conclusions

This study describes the structure of the offshore Red River Fault inthe Yinggehai Basin, documents timing of its strike-slip movementand slip reversal, and addresses mechanisms of the India–Asiacollision.

Two boundary faults and two previously undocumented, basin-center faults have been described. These four faults display typicalfeatures of strike-slip faults, and they are likely the offshoreextensions of the Red River Fault. Three deformation phases for theoffshore Red River Fault zone include sinistral movement from ~30 to16 Ma, slip reversal between 16 and 5.5 Ma, and dextral movementafter 5.5 Ma. Slip reversal also is indicated by diachronous basininversion from north to south, thereby partially filling an important

Fig. 18. Amplitude extraction map along the 3.6 Ma horizon, Yinggehai–Qiongdongnan basins. Location of the 3-D seismic survey is shown in Figs. 4 and 17.

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gap in the history of the Red River Fault during the middle to lateMiocene. Although no robust piercing points are observed during thesinistral movement period, the horizontal displacement of dextralmovement is estimated to be tens of kilometers.

Our analysis suggests timing and kinematic linkages betweensinistral movement of the offshore Red River Fault and opening of theSouth China Sea before the middle Miocene, consistent with thenotion that extrusionwas dominant along the southeastern margin ofthe Tibetan plateau before the middle Miocene. After the middleMiocene, distributed shortening better explains the deformationrecord in response to the continuing India–Asia collision, especiallythe regional low rate of dextral deformation of the Red River Faultzone.

Acknowledgements

We gratefully acknowledge Chevron Corporation for providing thesubsurface dataset, and we thank Chevron and China NationalOffshore Oil Company for permission to publish this work. We arealso grateful for financial support for this paper from the industrialaffiliate companies of the Stanford Project On Deepwater DepositionalSystems (SPODDS) and a software grant to Stanford University from

Landmark. In addition, we benefited from Simon Klemperer, DjordjeGrujic, Bui Huyen Bradley Ritts, and Zhen Sun for insightfulsuggestions for this paper.

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