Marine Geophysical

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Marine Geophysical ResearchAn International Journal for the Study ofthe Earth Beneath the Sea ISSN 0025-3235 Mar Geophys ResDOI 10.1007/s11001-013-9171-y

The sedimentary, magmatic and tectonicevolution of the southwestern South ChinaSea revealed by seismic stratigraphicanalysis

Lu Li, Peter D. Clift & Hung TheNguygen

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ORIGINAL RESEARCH PAPER

The sedimentary, magmatic and tectonic evolutionof the southwestern South China Sea revealed by seismicstratigraphic analysis

Lu Li • Peter D. Clift • Hung The Nguygen

Received: 16 November 2012 / Accepted: 20 February 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The southwestern South China Sea represents an

area of continental crust frozen immediately before the onset

of seafloor spreading. Here we compile a grid of multi-

channel seismic reflection data to characterize the continent-

ocean transition just prior to full break-up. We identify a

major continental block separated from the shelf margin by a

basin of hyperextended crust. Oligocene-Early Miocene

rifting was followed by mild compression and inversion

prior to 16 Ma, probably linked to collision between the

Dangerous Grounds, a continental block to the east of the

study area, and Borneo. The timing of inversion supports

models of seafloor spreading continuing until around 16 Ma,

rather than becoming inactive at 20 Ma. The off-shelf banks

experienced uplift prior to 16 Ma in an area, which had

previously been a depocenter. The off-shelf banks continued

to extend after this time when the rest of the region is in a

phase of thermal subsidence. Post-rift magmatism is seen in

the form of scattered seamounts (*5–10 km across) within

or on the edge of the deeper basins, and are dated as Late

Miocene and Pliocene. They are not clearly linked to any

phase of tectonic activity. Further inversion of the off-shelf

banks occurred in the Pliocene resulting in a major uncon-

formity despite the lack of brittle faulting of that age. We

speculate that this is part of a wider pattern of scattered

magmatism throughout the South China Sea at this time.

Prograding clinoforms are seen to build out from the shelf

edge in the south of the study area during the Pliocene, after

5.3 Ma, and then more towards the north and east during the

Pleistocene. At the same time a trough south of the off-shelf

banks is filled with [1.35 km of mostly Pleistocene sedi-

ment. While we expect the bulk of the sediment to come

from the Mekong River, we also suggest additional sediment

supply from Borneo and the Malay Peninsula via the

Molengraaff River and its predecessors.

Keywords Seismic stratigraphy � Extension � Inversion �Seamounts � Clinoforms

Introduction

The rifting and break-up of continental crust to form oceanic

basins is a fundamental stage in the Wilson cycle of plate

tectonics, which also generates passive continental margins

that are valuable archives of sediment that record the evo-

lution of continental weathering and erosion conditions, as

well as being important locations for hydrocarbon accumu-

lations. While much research has been done on the processes

of rifting, the transition from continental extension into

seafloor spreading remains poorly understood. Although

there is an understanding that some rifted margins are very

magmatic and associated with flood basalt provinces (e.g.,

Northeast Atlantic, South Atlantic, NW Indian Ocean)

(White 1997; Franke et al. 2008), others are believed to be

almost amagmatic (e.g., Iberia-Newfoundland, South

Australia) (Tucholke et al. 2007; Peron-Pinvidic and

Manatschal 2009). While these differences have been linked

to mantle asthenospheric temperatures, finite element mod-

eling of the lithosphere extension now suggests that rates of

L. Li (&)

School of Geosciences, University of Aberdeen,

Aberdeen AB24 3UE, UK

e-mail: [email protected]

P. D. Clift

Department of Geology and Geophysics, Louisiana State

University, Baton Rouge, LA 70803, USA

H. T. Nguygen

Vietnam Petroleum Institute, Hanoi, Vietnam

123

Mar Geophys Res

DOI 10.1007/s11001-013-9171-y

Author's personal copy

extension are also critical (Huismans and Beaumont 2007).

The nature of strain accommodation during break-up is also

controversial and also potentially linked to strain rates

(Huismans et al. 2005). Amagmatic margins are often linked

to development of low-angle detachments prior to final

break-up (Reston et al. 1996), while extension may be more

uniform or even depth-dependent as extension increases

prior to break-up in other settings (Driscoll and Karner 1998;

Davis and Kusznir 2004). The nature of crust in the conti-

nent-ocean transition zone is often obscure and has variously

been described as continental mantle lithosphere, lower

crust, volcanic rocks, or upper continental rocks intruded by

syn-rift magmatism (White et al. 1987; Perez-Gussinye et al.

2001; Whitmarsh et al. 2001). Moving beyond these end-

member margin types, it is increasingly recognized that not

all margins fit neatly into these categories.

The South China Sea (Fig. 1) shows evidence for break-

up that does not neatly fit either magmatic or amagmatic end

members (Clift et al. 2001; Yan et al. 2001). It is, however, an

excellent natural laboratory for looking at the process of

break-up because the conjugate passive margins can be

readily matched and most importantly, the propagating rift is

preserved in the southwestern part of the basin (Huchon et al.

2001). In this area, seafloor spreading is evidenced by

magnetic anomalies, but also passes through a transition

zone between southern Vietnam and Borneo (Fig. 1). This

zone offers the opportunity to examine the transition

between moderately rifted crust under the Sunda Shelf and

the hyper-extended crust ahead of the seafloor spreading

center. While studies have been made of the propagating tip

and the basins of southern Vietnam (Huchon et al. 2001; Lee

et al. 2001; Fyhn et al. 2009), this study is the first that

extends the marine geophysical data coverage to survey the

transition zone between these crustal domains. In addition,

the region provides the opportunity to examine well devel-

oped, but enigmatic volcanism that is apparently not strongly

linked to the basin opening processes (Tu et al. 1991; Li et al.

1994; Shi et al. 2011).

The region is also well placed to examine the history of the

Mekong Delta. The Mekong River is one of the great rivers of

eastern Asia and originates in the Tibetan Plateau, flowing

through Indochina to the modern delta. However, it has been

suggested that the development of rivers in this region has been

influenced by the uplift of the topography since the start of

India-Asia collision (Brookfield 1998; Clark et al. 2004), likely

in the Eocene. In this type of model retilting of eastern Asia

towards the east since the Eocene (Wang 2004) has caused

major drainage reorganization, whose timing remains con-

troversial (Clift et al. 2006; Kong et al. 2012; Yan et al. 2012).

It has been suggested that the upper reaches of the Mekong

used to flow into the headwaters of the Red River and has

subsequently been captured away from that system (Clark et al.

2004). Knowing the timing of growth of the Mekong Delta

would be important for testing models for drainage evolution

and understanding how it might be linked to tectonic processes

in SE Asia. Because the submarine delta of the Mekong lies on

the outer Sunda Shelf, we also have the opportunity to provide

a minimum age for the timing of delta initiation and thus

potential headwater capture, which is presently unclear (Clift

et al. 2004; Murray and Dorobek 2004).

Geologic setting

The cause of opening of the South China Sea has long been

debated with the arguments falling into two categories. Some

workers propose that the South China Sea formed as the result

of the extrusion and rotation of Indochina relative to southern

China in response to the collision of India and Eurasia (Tap-

ponnier et al. 1986; Briais et al. 1993; Replumaz et al. 2001;

Replumaz and Tapponnier 2003; Leloup et al. 2007; Fyhn

et al. 2009). In contrast, others have argued that the South

China Sea is a marginal basin formed by subduction-related

stresses, similar to the Sea of Japan (Holloway 1982; Taylor

and Hayes 1983; Clift et al. 2008). In this case the extension

that formed the modern South China Sea was caused as the

southern margin was pulled south, away from southern China

by subduction of a paleo-South China Sea under northern

Borneo (Ru and Pigott 1986; Morley 2002). Most recently

Cullen et al. (2010) argued in favor of an origin incorporating

aspects of both extrusion and subduction related processes.

The rifting of the South China Sea is dated as being Eocene to

Early Oligocene (Ru and Pigott 1986) or maybe earlier in the

Cretaceous-Eocene (Su et al. 1989; Rangin and Silver 1991;

Schluter et al. 1996; Clift and Lin 2001). The extrusion model

is consistent with the left-lateral strike-slip motion along the

Red River Fault (Tapponnier et al. 1986; Briais et al. 1993;

Replumaz et al. 2001; Replumaz and Tapponnier 2003),

whereas the subduction model requires right-lateral motion

(Holloway 1982; Taylor and Hayes 1983).

In the Middle Oligocene (*30 Ma) rifting was followed

by seafloor spreading along the northern margin, although in

the Late Oligocene the ridge jumped to the south, out of the

Xisha Trough and seafloor spreading then continued, prop-

agating towards the southwest until the Middle Miocene,

although there is some dispute about whether spreading

continued until only 20 Ma (Barckhausen and Roeser 2004)

or whether it lasted until 16 Ma (Briais et al. 1993). Recently

published magnetic data suggest that the start of seafloor

spreading dates from *37 Ma in the most northerly parts of

the basin, immediately SW of Taiwan (Hsu and Sibuet 2004).

Our study area (Fig. 1) lies in the southwestern South

China Sea, straddling the edge of the continental margin

and extending into deep water. To the northwest the area is

bounded by the southern Vietnamese margin, and to the

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northeast there is the continent-ocean transition zone linked

to the now inactive seafloor spreading center. To the south

lies central Borneo and to southeast is Sunda Shelf char-

acterized by two major NW–SE trending structures named

the West Baram Line and Lupar Line (Hutchison 2004),

Our study area mainly overlaps two tectonic provinces,

the Dangerous Grounds and the eastern shelf edge of the

Nam Con Son margin and Sunda Shelf (Fig. 1). The

Dangerous Grounds lie north of Borneo and is separated

from the island by the North Borneo Trough (Hutchison

2010). The main part of the Dangerous Grounds is com-

posed of continental crust and was extended during the

Eocene–Oligocene (Taylor and Hayes 1983; Schluter et al.

1996). The sediment deposited on the Dangerous Grounds

is supplied both from Borneo and from the Mekong River,

but is quite thin so that the tectonic structures are still well

exposed in the bathymetry (Hutchison and Vijayan 2010).

The Nam Con Son basin, together with the smaller Cuu

Long Basin to the north represent rift structures formed as a

result of the extension prior to seafloor spreading in the

southwestern South China Sea. These are mostly filled by

sediment close to sealevel and are presently supplied with

sediment by the Mekong River. The data for our study

cover the junction of the East Vietnam Transform margin

with the rifted Dangerous Grounds and the Sunda Shelf

(Figs. 1, 2). As a result the basin’s evolution has been

controlled by a number of processes, most notably strike-

slip forces driven by the East Vietnam Transform Zone,

and extensional stresses linked to the propagation of sea-

floor spreading during the Early to Middle Miocene

(Matthews et al. 1997; Huchon et al. 2001; Lee et al. 2001).

The early rifting stage of the Nam Con Son Basin starts in

the Eocene-Early Oligocene. Rifting accelerated during the

Miocene (Matthews et al. 1997; Lee et al. 2001), but in the

Fig. 1 Topographic and shaded bathymetric map of the South China

Sea showing the study area and its relationship with the Red River

faults Zone, Central Highlands of Vietnam, Nam Con Son basin and

with the Sunda Shelf. Solid white line indicates approximate location

of the continent-ocean transition (COT) based on seafloor spreading

anomalies of Briais et al. (1993). The numbers represent the seafloor

spreading anomalies. Blue lines on Sunda Shelf show route of

Molengraaff River system. Map generated by GeoMapApp

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Middle Miocene extension and subsidence were interrupted

by inversion that also affected the West Natuna Basin and

Indonesian East Java Sea Basin (Matthews et al. 1997;

Hutchison 2004; Clift et al. 2008).

Although active extension has effectively ceased since

16 Ma, the South China Sea has been affected by moderate

degrees of magmatism of uncertain origin. Thick basalts were

emplaced in the Central Highlands of Vietnam around 8 Ma

(Carter et al. 2000) and Plio-Pleistocene volcanism has been

documented from Hainan Island and the adjacent Leizhou

Peninsula (Tu et al. 1991; Shi et al. 2011). Furthermore, young

volcanic seamounts have been observed on the northern margin

of the basin in seismic profiles, especially towards the northeast

edge of the basin (Yeh et al. 2012). While some workers have

suggested that this young, post-rift magmatism is the product of

a South China Sea plume, this has been hard to demonstrate

because the magmatism is much more widespread and without

the typical age progression seen in classic plume tracks such as

Hawaii. Moreover, analysis of dynamic topography in this area

indicates that the crust is depressed over a region of colder than

normal upper mantle, inconsistent with the presence of a typical

plume (Lithgow-Bertelloni and Gurnis 1997).

Datasets and methods

This study is based on a number of reflection seismic

profiles, totally *4,800 km collected from the southwest-

ern South China Sea. On the southern Vietnamese margin,

data (red solid lines) were released from Halliburton

Geophysical Services, Inc. The survey lines (black solid

lines) across the shelf edge of the Nam Con Son margin

and the adjacent deep water and slope area were provided

by PetroVietnam (Fig. 2). Age control is provided to the

seismic stratigraphy through biostratigraphy from drill sites

located on the shelf, as shown in Fig. 2 and provided by

Total. These ages were transferred to the seismic profiles

after converting drilling depth to two-way travel time

Fig. 2 Bathymetric map of the

southwestern South China Sea

showing the total seismic

reflection data coverage

analyzed in this study. Whitebold lines show those linespresented in this paper. Image is

from GeoMapApp. The greendots show the well locations

used to provide biostratigraphic

age control. Contours are in

1,000 m intervals. VB Vanguard

Bank, GB Grainger Bank, ABAlexandra Bank, PCB Prince

Consort Bank

Mar Geophys Res

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(TWT) using the stacking velocities derived from pro-

cessing of the MCS data. These dated horizons were then

correlated across the entire study area, with well-based age

picks being projected on to the nearest seismic profile from

where they were correlated over wide areas (Fig. 3). Ages

were converted from the original biostratigraphic zones

provided in the drilling reports into numerical ages using

the timescale of Gradstein et al. (2004). The oldest age pick

from the drilling was the base of the Miocene in well

11-1-CH-1X and 11-CH-CPD. Below that level we have no

age control but follow the study of Matthews et al. (1997)

in assigning the syn-rift strata largely to the Oligocene.

Matthews et al. (1997) had the additional benefit of more

drilling data that penetrated deeper in the section compared

to this study. Readers are referred to this earlier study

concerning detailed discussion of age uncertainties.

The base of Oligocene strata is inferred by the basement

reflector, which can be followed over long distances across

the study area. Age resolution is best for Miocene strata

because a number of wells have penetrated these forma-

tions. In contrast, the Plio-Pleistocene was not cored by

industrial wells so that we do not have a detailed dated sub-

division of those units. It is impossible to estimate the ages

within the uncored Plio-Pleistocene because earlier studies

of this time interval in South and SE Asia (Clift 2006)

indicate significant changes in mass accumulation rates

during a period of rapidly changing monsoon strength, so

that it is not reasonable to extrapolate recent accumulation

rates over the past 5 Ma.

We construct a sediment budget for our study area using

the interpreted seismic reflection profiles. These were first

converted to eroded rock volumes by converting the sec-

tions from a vertical time axis into depth. Of all stages in

the sediment budget estimation this process is the most

prone to error and results in uncertainties of up to 20 % due

to variations in the velocity-depth conversion within indi-

vidual units (Clift 2006). Decompaction methods (Sclater

and Christie 1980; Kusznir et al. 1995) were then applied

to the sections in order to restore each dated sediment body

to its original thickness prior to burial. Knowledge of the

sediment type is important to this calculation because

shales experience much greater loss of porosity during

burial than do sandstones (Sclater and Christie 1980) and in

this case we used lithological data from the drilling wells or

assumed an average silty composition where data were

missing. The decompaction process involves accounting

for the loss of porosity of the sediment during burial, which

would otherwise result in an underestimation of deposited

volumes for the older, deeper buried sediment packages.

After the original, uncompacted volume of sediment in

each dated interval has been determined, the mass of rock

delivered during that time period can be calculated. Errors

Fig. 3 Composite seismic profile made from Lines CN-27 and S9-D, respectively covering the shelf and slope regions, showing the location of

the wells that provide age control to our seismic stratigraphy. See Fig. 2 for location. TWT two-way travel time

Mar Geophys Res

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in estimating sediment age, lithology and compaction his-

tory are much smaller than the time-depth conversion and

rarely exceed 5 %.

The process of decompaction was described in detail by

Clift et al. (2004), and followed routine basin analysis

methods, whose results are generally considered robust at the

first order level. In this study two-dimensional decompaction

was calculated using the program Flex-DecompTM (Kusznir

et al. 1995). A basin wide budget was then calculated by

adding together the five sections from across basin and

normalizing the rates for each dated interval in order to

match the calculated total volume of the basin. It must be

assumed that the analyzed profiles were representative of the

total mass flux on to the margin since the Oligocene. This is a

reasonable assumption because our lines cut across all the

major depocenters of the basin and we believe that our choice

of lines does yield a representative image of the total basin

volume. In the absence of 3-D seismic data any volume

calculations would be based on these long lines and no sig-

nificant error was introduced by taking this two dimensional

compiling approach, which follows the methodology laid out

in the regional mass calculation study of Clift (2006).

Results: seismic stratigraphic analysis

The sections were interpreted using the classic seismic

stratigraphic principles of Vail et al. (1977), with the major

depositional packages being defined by erosional sequence

boundaries, typically formed by sealevel fall, although in

this case also by erosion caused by tectonically-driven

uplift, most notably the Mid Miocene Deep Regional

Unconformity (DRU, Reflector B400) (Hazebroek and Tan

1993; Hutchison 1996). We define the following units.

Oligocene (*34–24.5 Ma; maximum

thickness *4.4 km)

This unit is defined as being between our seismic basement

(B600) and a poorly defined reflector within the tilted pre-

rift sequence (B500). The upper surface was picked on the

basis of a strong reflector observed on Profile S9D (Fig. 4)

and followed as well as possible across the survey region.

The unit is often poorly imaged but where possible it is

seen to be well stratified and apparently pre-dating the

faulting that cuts it.

Lower Miocene (24.5–16.4 Ma; maximum

thickness *4.0 km)

This unit lies above reflector B500 and under the most

prominent reflector (B400) seen across the area, which

shows clear evidence for tectonic inversion, often related to

the truncation of faults against the top of the B400 surface.

Like the underlying Oligocene this unit is characterized by

well-stratified reflector that do not clearly thickened

towards faults and are therefore considered to be primarily

pre-rift. Imaging of the unit is not very clear and in general

little can be said about seismic facies.

Middle Miocene (16.4–8.6 Ma; maximum

thickness 1.7 km)

This unit lies unconformably on top of reflector B400 and is

generally a rather thin unit that infills the topography over-

lying the deep regional unconformity. It was picked on the

basis of a rather strong reflector that forms the top of the unit

B300. Strong reflectors within this unit suggest that large sand

bodies may form an important part of the stratigraphy. The

unit appears to be gently deformed across the offshelf banks.

Upper Miocene (8.6–5.3 Ma; maximum

thickness 1.8 km)

This unit is defined as lying between Reflector B300 below

and the prominent Reflector B200, which is widely rec-

ognized across the entire area on the basis of its charac-

teristic strong reflectance. In the offshelf banks the unit is

characterized by a typical lighter colored reflectance,

almost transparent in nature. The B200 reflector appears to

be very slightly inverted under the Vietnamese continental

margin usually above regions where earlier stronger

inversion had occurred prior to Reflector B400. In the

offshelf banks B200 forms the base of younger basins and

is clearly uplifted across the top of the offshelf banks. The

seismic facies appear to be quite distinctive in being poorly

reflective and parallel laminated in the eastern part of the

area (Fig. 5), although in the absence of drilling data we

are unable to determine the nature of the sedimentary rocks

composing this unit.

Lower Pliocene (5.3–3.6 Ma: maximum

thickness 1 km)

This unit is best developed on the western side of the study

area and progrades across the reflector B200 which appears

to act as an unconformity. The unit is effectively the basal

deltaic strata of the Mekong Delta. The reflectors within

the unit show a gradual slope down towards the east and

are generally quite continuous and well-defined. They are

interpreted as being consistent with large clastic sedimen-

tary wedges, at least partly comprised of sandstones. The

top Reflector B180 is a strong reflector on the western side

of the study area that pinches out against Reflector B200

under the continental slope.

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Middle Pliocene (unknown age: maximum

thickness 0.7 km)

This unit is defined on the basis of two strong reflectors

(B160 and B180). This deposit is again largely concen-

trated under the continental slope of Vietnam and is also

present but thinnly developed in the basins north and south

of the offshelf banks. On the western side of the study area

the unit shows well-developed steeply dipping clinoforms

prograding towards the east. It has downlapping relation-

ship along its bottom surface suggestive of an unconfor-

mity or erosional discordance.

Upper Pliocene ([1.8 Ma: maximum thickness 0.8 km)

Defined between the strong reflectors B150 and B160 this

unit forms one of the older parts of the submarine Mekong

Delta and is effectively not present under the offshore tanks

and associated basins. It’s upper reflector dips steeply

towards the east where it also downlaps on to reflector

B180. Internally the unit shows well-developed, steepdip-

ping internal reflectors consistent with a clastic sedimen-

tary origin at the delta front.

Pleistocene (\1.8 Ma: maximum thickness 2 km)

The youngest units in the seismic stratigraphy we have

generated form a eastward prograding set of clinoforms,

which are not controlled by any age picks from the wells but

are instead defined only through the recognition of strong

reflectors within the shelf and are interpreted to represent a

series of erosional unconformities separating the different

lobes of the prograding delta. Each of these units steps fur-

ther and further to the east representing the gradual building

of the delta front through time. The youngest parts of the

Pleistocene are also found in the deep basins especially to the

south of the offshelf banks. Each of the units picked here was

chosen on the basis of a particularly prominent and usually

an unconformable reflector found especially in the top sets of

the delta. As well as building out towards the east we note

that there is significant aggradation of the units on top of one

another infilling the accommodation space that existed on

the edge of the continental margin.

The stratigraphy described above divides readily into two

sections, a lower syn-rift and early post-rift sequence that is

marked by faulting and inversion structures and a younger

sequence that features growth of clinoforms and the east-

ward migration of the shelf edge. In profile S9-D the

Fig. 4 Seismic profile S9-D

across the shelf edge of the Nam

Con Son margin with

interpretation shown in the

lower panel. See Fig. 2 for

location. Note the clear

inversion structure at *65 km

shown in the uplift of the green,

Middle Miocene reflector

(B400)

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123


prograding clinoforms are especially well imaged and show

both aggradation and progradation to the east (Figs. 3, 4).

The clinoform sequences appear to be based on the reflector

B200, dated at 5.3 Ma. This seems to represent a relatively

uniform surface over which the clinoforms were built.

The results of our sediment budget are shown in Fig. 6,

with the separate budgets for the shelf edge and the off-

shelf banks shown separately in Fig. 6A, B, respectively.

Our analysis indicates high values of sediment flux in the

syn-rift period of the Oligocene, increasing further into the

Early Miocene, followed by a sharp decrease in the Middle

Miocene but rebounding to a peak after 8 Ma. Pliocene

rates are very low at the shelf edge but less so on the off-

shelf banks. Pleistocene rates then increase along the shelf

edge to slightly exceed the high values seen in the Middle

Miocene. We further use our budget to modify that of Clift

(2006) in order to revise the regional mass flux to the basin

(Fig. 6D). The basin-wide budget does not decrease in the

Middle Miocene as the study area does. Instead the most

obvious features of the entire offshore are increased sedi-

mentation rates after 24 Ma and moderate increases after 8

and 1.8 Ma. We attribute differences in sediment budget

across the region to the tendency of the Mekong to fill

basin closest to the coast first before overspilling into the

study area along the continental margin.

Results: regional structure

The regional structure across the continental margin is best

demonstrated by Line S9D (Fig. 4) which crosses the edge

of the Sunda Shelf and runs in deep water, approximately

following the rift axis towards the tip of the proposed

propagating rift system. This means that this line does not

Fig. 5 Seismic profile S74-A-1

showing raw data and

interpretation in the lowerpanel. Note prominent inversion

at *45 km mark. See Fig. 2 for

location. TWT two-way travel

time

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cross many of the extensional faults that are believed to

accommodate much of the extension in the area (Huchon

et al. 2001). The oldest units of Early Miocene-Oligocene

age that underlie Reflector B400 are quite thick and show

on moderate thinning towards the NE. They are cut by

high-angle normal faults (typically *80–90�), as well as

by a small diaper (at *90 km). The older units thin

towards a pronounced seamount volcano at *120 km and

are capped by a sub-horizontal, undulating erosive

unconformity. The seamount itself deforms Relector B400

and older units, with younger ones onlapping the upturned

strata around the seamount. The strata of the Lower

Miocene-Oligocene are locally steeply dipping, especially

around 75 km. The B400 DRU surface is onlapped by a

thin Upper Miocene sequence. The youngest part of the

section is dominated by a large set of eastward migrating

clinoforms identified as the submarine Mekong Delta and

that build out into the basin across Reflector B200, dated as

base of the Pliocene. A number of sub-units are identified

within the Plio-Pleistocene, representing different stages of

delta growth. The toe of the delta foresets is now close to

the volcanic seamount edifice.

The margin structure to the north of the surveyed region is

shown by Line S74-A-1 (Fig. 5), which runs N–S sub-parallel

to the shelf break. The same units can be traced from the south

into this line. As before the Lower Miocene-Oligocene is cut

by high angle faulting and is relatively thick, although slightly

thinner than observed on Line S9-D (Fig. 4). This unit thins

sharply to the north onto a structural high. The DRU (B400) is

again well developed and clear with strong reflectors seen

under the surface compared to lighter reflectance in the

younger part of the section. The strata under B400 are also

clearly seen to be folded on a large scale, wavelength of

*10 km. This folding does not affect the strata lying above

Reflector B400. The Late Miocene infills the topography

above B400 and is itself deformed in an event post-dating

B200, base Pliocene. The youngest reflectors, B100 and B70

can be seen to onlap onto B200 suggesting that this has been

deformed and gently folded after the end of Late Miocene

sedimentation. The Pleistocene sediments are much reduced

in this section compared to that seen in Section S9-D.

We now consider the structure across the major bathy-

metric high that projects in a SW-NE direction from the edge

of the Sunda Shelf at around 8�N. Line TC93010 (Fig. 7)

crosses the ridge over the Vanguard Bank (Fig. 2), extending

into deep-water on either side. As before the Lower

Miocene-Oligocene is cut by high-angle (70–80�) faulting

and although it is thick under the central platform area these

units are seen to thin towards the south. Strong multiples

make it hard to clearly identify the basement under the

shallowest parts of the Vanguard Bank. The DRU at

Reflector B400 is identified and marks the end of strong

upper crustal faulting. The Late Miocene-Recent sequence is

relatively reduced in this area, especially over the crest of the

Vanguard Bank, and around a volcanic seamount observed at

*145 km, which is seen to have deformed Reflector B200

and all overlying sediments. The Pleistocene B70 and B100

reflectors lie undeformed with the sediments of these units

infilling the topography on either side of the bank by simply

ponding in the lowest parts of the basin.

30 km east of Line TC93010, Line TC93014-2 crosses

both the eastern end of Vanguard Bank and the larger

Prince Consort Bank to the north (Fig. 8). The overall

structure is one of horsts and grabens separated by high

angle faults. While these mostly affect the Lower Miocene-

Oligocene this section is the first seen in which the fault

show major offset across Reflector B400, DRU. Again the

Oligocene is relatively thick (*1.5 s TWT) under modern

bathymetric highs and thins to the south. There is very little

Lower Miocene on Prince Consort Bank (*0.2 s TWT),

with the thickest sections seen north of the bank. Although

Fig. 6 Sedimentary budgets for A the area of the Sunda Shelf margin

considered in this study, B the region of the continental shelf edge

offshore the Nam Con Son basin, C the region of the off-shelf banks

and D the revised sediment budget for the entire Mekong Delta

offshore region, modified from Clift (2006) and adjusted with the new

volumes surveyed in this study

Mar Geophys Res

123


Reflector B400 is faulted and folded, we note that Reflector

B200 is also deformed and uplifted, especially on the

southern edge of the bank (Figs. 8, 9, 10). The basin,

south of the Vanguard Bank contains thicker sediment

(*2.5 s TWT) than observed in Line TC93010. The sea-

floor is quite flat and unformed in that region compared to

the rougher topography north of Prince Consort Bank

where minimal Pleistocene sediment covers Reflector

B200.

Another 62 km towards the northeast Line TC93022

crosses Grainger and Alexandra Banks (Fig. 9). The structure

is again dominated by high-angle normal faulting that cuts the

Lower Miocene-Oligocene and in places also shows major

offset of Reflector B400 (DRU), most notably at around

165 km where a large tilted normal fault block is seen to define

the southern edge of the shallow bathymetry. The central part

of Grainger Bank shows the Pliocene B200 unconformity

truncating the B400 Mid Miocene Unconformity. Only the

Pleistocene that has accumulated south of the bank is unde-

formed and no Plio-Pleistocene is found on top of the banks or

in the basin north of the Grainger Bank. The section is again

marked by the presence of a volcanic seamount at 120 km that

clearly deforms reflector B400. Reflector B300 is also tilted on

the south side of the volcano but it is unclear whether that is a

depositional dip or has been deformed during intrusion.

Line TC93026 is the most northeasterly of the profiles

we highlight here and it is located at the tip of Alexandra

Bank (Fig. 10). This section shares many of the same

features seen in the other profiles cutting the continental

promontory but is especially good at showing the

differences between the north and the south sides of the

bank. The basin to the south of Alexandra Bank is a deep

basin (*1.5 s TWT) composed entirely of Pliocene and

younger material that onlaps the deformed B200 surface,

which is strongly uplifted across the bank. The older B400

DRU surface generally postdates faulting, although some

major structures are again seen to cut this reflector. The

Upper Miocene has a distinctive, light reflective pattern

compared to the dark, stronger reflectors in the underlying

Lower Miocene-Oligocene at least where the quality of the

seismic reflection data is sufficiently good to allow this

difference to be observed. Without local drilling data it is

unclear to us what the significance of this reflective dif-

ference is. The seafloor north of the bank is marked

effectively by exposure and erosion of the Upper Miocene,

with incision of small channels into the section that shows

a gradual slope spanning at least 70 km from the crest of

the bank and suggesting large scale tilting after the Late

Miocene (Figs. 9, 10).

Having defined the major reflectors and mapped them

over the entire area we are able to generate both maps of

depth to any given horizon and isopachs showing unit

thicknesses. Figure 11A–C show the depths to the basement,

top of the Oligocene (B500) and Mid Miocene (B400) across

the study area. These maps are shown in depths measured in

kilometers and are depth converted based on stacking

velocities. All three depth maps show a similar pattern, with

the shallowest depths in the northwest and across the various

banks that extend from the shelf at 8�N. It is noteworthy that

the banks represent an essentially coherent, upstanding

Fig. 7 Seismic profile

TC93010 showing raw data and

interpretation in the lowerpanel. Note the tectonic

inversion structure at *35 km

and the volcanic edifice at

150 km. See Fig. 2 for location.

TWT two-way travel time

Mar Geophys Res

123


continental fragment that is separated from the main edge of

the Sunda Shelf by deep sedimentary basins. This is not

apparent in the bathymetry, where the Vanguard Bank is

essentially in contact with the shelf regions of the Sunda

Shelf immediately to the west. Our mapping reveals that the

banks are a separate entity, removed from the shelf edge.

Figure 11D shows the isopach map for the complete

Oligocene-Mid Miocene rift sequence. Not surprisingly the

thickest rift sediments lie in the deep basin between the shelf

edge and the offshore banks, *109–110�E. The thick of the

syn-rift is not strongly affected by the topography of the

modern banks. There is also a prominent syn-rift depocenter

at 10–11�N, north of the banks. As noted in the seismic

profiles, the syn-rift becomes quite thin towards the south-

east. Figure 11E, F show how the thicknesses within the syn-

rift compare between the Oligocene and later Early Miocene

Fig. 8 Seismic profile showing

raw data and interpretation in

the lower panel. The basement

is affected by high-angle

(80�–90�) extensional faults and

the basin is relatively sediment

starved over the top of the

structure, which must postdate

the Oligocene-Lower Miocene.

Note the flat-topped basement

blocks within the banks that

may indicate subaerial erosion

and exposure. See Fig. 2 for

location. TWT two-way travel

time

Fig. 9 Seismic profile showing

raw data and interpretation in

the lower panel. A volcanic

edifice is seen at 115 km. Note

the erosive channels and the

ponding of Pleistocene sediment

against the tilted fault blocks on

the southern side of the banks.

Fault motion deforms sediment

up to Lower Pliocene age. The

erosive channels are believed to

represent scour by bottomcurrents See Fig. 2 for location.

TWT two-way travel time

Mar Geophys Res

123


phases. These phases have several features in common but

also some important differences. The early syn-rift is par-

ticularly thick along the edge of the modern shelf, while the

later rift sequence shows sedimentation had moved slightly

westward by that time (*109�E) and that a basin 2.5 km

deep had developed in the northwestern corner of the survey

area.

Discussion

Timing of rifting

The analysis presented above shows that much of the

extension, brittle faulting predates the B400 DRU. We

thus consider the units under this reflector to represent a

syn-rift package, albeit one disturbed by later faulting and

compression. In this region almost all the faults continue

up to the base of the Middle Miocene (*16 Ma) and are

then truncated by an unconformity. In some of the profiles

across the offshelf banks we noted that extensional

faulting has continued after the DRU, which is cut and

tilted after that time. The timing of extension correlates

reasonablely well with the proposed 16 Ma end of sea-

floor spreading, as dated by Briais et al. (1993). In this

scenario extension ahead of the propagating seafloor

spreading center affected the continental crust and both

oceanic and continental crusts stopped major extension

after 16 Ma as the regional stresses changed, probably as

the Dangerous Grounds block came into collision with the

North Borneo Trough (Hutchison 2004; Clift et al. 2008).

In contrast, our extension age is harder to explain if

seafloor spreading terminated at 20 Ma (Barckhausen and

Roeser 2004). In this case extension of the continental

margin would have continued after the end of seafloor

spreading. Although we consider this unlikely, it is pos-

sible that the mechanically weak lithosphere of the con-

tinental margins was susceptible to renewed extensional

deformation compared to the mechanically stronger oce-

anic lithosphere. The weakest part of the plate would be

at the ridge crest but if this were allowed to become

inactive and cool then it is possible that new extensional

stresses would tend to affect the continental margins

rather than the rigid oceanic lithosphere. In general we do

not favor this option because of the rather short duration

between the proposed end of the seafloor spreading and

the DRU, which would not allow much thermal maturity

to develop. Furthermore, we have seen no evidence to

suggest rifting prior to 16 Ma occurred in two distinct

phases. It is hard to understand why extension would only

affect the continental margins and not the oceanic litho-

sphere too.

Our analysis revealed evidence of limited Mid and even

Late Miocene extension. The off-shelf banks in particular

show evidence of strong faulting after 16 Ma. Line

TC93022 (Fig. 9) shows an impressive large, tilted block

on its southern side in which the DRU is faulted and tilted

and lesser faulting affects much of the crust in the banks

area. Uplift of the banks relative to the surrounding basins

appears to have continued for some time after the DRU.

Fig. 10 Seismic profile

showing raw data and

interpretation in the lowerpanel. Pleistocene sediment is

ponded on the south but not

north side of this section. These

deposits are believed to be distal

turbidites based on strong

reflectors but lack of channel-

levee complexes. They are

presumed to be largely sourced

from the west, i.e., from the

Mekong and Molengraaff

Rivers. The section provides

strong evidence for inversion

dating after the Early Pliocene.

See Fig. 2 for location. TWTtwo-way travel time

Mar Geophys Res

123


Fig. 11 A Depth to basement (B600), uncorrected for sediment

loading, note the great depth between the offshelf continental

fragment and the edge of the Sunda Shelf located in the NW of the

study area, B Depth to the base of the Early Miocene (B500,

*24 Ma), C Depth to the base of the Middle Miocene (B400,

*16 Ma), note how the shelf edge appears to be controlled by major

normal faults, D Isopach of the sediment thickness deposited between

the basement and B400 representing the syn-rift sequence, E Isopach

of the sediment thickness deposited between the basement and B500,

representing the thickness of the early syn-rift phase, and F Isopach of

the sediment thickness deposited between the B500 and B400

reflectors, representing the thickness of the late syn-rift phase. Major

faults are shown as red lines with tick marks indicating the

downthrown side. Note the overall SW–NE structural trend of the

rift. The thickness of the rift sequences is only roughly governed by

the location of the major faults

Mar Geophys Res

123


We note that the B200, Lower Pliocene reflector is

deformed and uplifted over the banks, but also floors the

basin to the south, suggesting that much of the topography,

at least on the southern edge of the banks has been gen-

erated during the Pliocene, although there is not much

evidence for large scale faulting of that age, or younger. It

is possible that much of the faulting could be at a sub-

seismic scale. Walsh et al. (1991) estimated that around

40 % of the total extension in faults is too small to be

imaged, but in this case we do not see any significant major

faults of Pliocene and younger ages, yet the onlapping

relationships between the basin fill and Reflector B200

require deformation in the Early Pliocene. Quite what is

driving that deformation is unknown but could be linked to

the young magmatism, discussed below.

There is clear evidence for compression and basin

inversion in many of the sections. The presence of thick

strata of Oligocene and Lower Miocene age in high fault

blocks in the center of the banks argues strongly that major

inversion has happened. The faults themselves indicate that

much of this inversion occurred just prior to the Reflector

B400, the DRU. Figure 5 shows a fold structure generated

within the Oligocene and Lower Miocene in the north of

the study area. Folding affects the two syn-rift sequences

equally indicating that the timing of folding must be at the

end of the active extensional period, *16 Ma. This

inversion event is by far the strongest seen in our survey

and correlates with other Mid Miocene inversion-related

unconformities throughout the southern South China Sea

(Matthews et al. 1997; Hutchison 2004, 2005). Later

inversion is less obvious but we do see evidence for uplift

across the Prince Consort Bank dating from the Pliocene

(Fig. 9). The erosional truncation of the B400/DRU surface

by Reflector B200 suggests that this bank was uplifted

above sealevel at that time, despite this being a time of

post-rift thermal subsidence. We note that our Pliocene

inversion correlates in time with that seen in the northern

margin of the South China Sea, where strike-slip faulting

has been invoked to generate the structural high of the

Dongsha Rise (Ludmann and Wong 1999) (Fig. 1). Quite

what is triggering this motion is unclear but was linked to

magmatism by Ludmann and Wong (1999). Hainan Island

is another dramatic example of linked magmatism and

uplift seen in the northern parts of the sea (Shi et al. 2011)

that result in major sediment flux to the ocean in the form

of large prograding foresets (Clift and Sun 2006). While

that mechanism may be appropriate in this area we can at

least discount the impact of mountain building in Taiwan

as a trigger, as has been suggested on the northern margin.

We further note that the uplift and exhumation of the

Vietnamese Central Highlands, which were dated to start at

*8 Ma by Carter et al. (2000) does not seem to have an

offshore equivalent, as there is little evidence of active

tectonism between Reflectors B200 and B300 in our

surveys.

Growth of the Mekong Delta

Proximity of the shelf to the mouth of the Mekong Delta

suggests that this is a primary source of sediment to the

continental margin and that the progradation represents the

gradual filling of accommodation space across the Sunda

Shelf, as first the Cuu Long and then the Nam Con Son

basins are filled and overspill (Lee et al. 2001).

This observation can be interpreted to indicate the start of

Mekong Delta sedimentation after 5.3 Ma. Conceivably

this could indicate a birth of the Mekong River in the

present location at that time following drainage capture in

Eastern Tibet/Yunnan. However, there may be other rea-

sons for the development of impressive clinoforms in this

place after 5.3 Ma. Deltas tend to prograde with time and it

is possible that the delta was in existence before 5.3 Ma but

located further north. Murray and Dorobek (2004) identi-

fied prograding foresets of presumed Mekong affinity

starting the Late Miocene of the Nam Con Son basin. After

the Cuu Long and Nam Con Son basins were filled, the

submarine clinoforms that represent the delta on the shelf

and which lie seaward of the river mouth would then have

prograded further to the SE into the location of our survey.

It is perhaps interesting to note that sedimentation rates

for the greater Mekong Delta region do not show a rapid

increase at the time at which we identify the start of

clinoform progradation. Instead sediment flux increase

after 24 Ma and again after 8 Ma, around the time that the

earliest clinoforms begin to deposit (Murray and Dorobek

2004). An alternative sediment budget published by

Metivier et al. (1999) favored a more progressive increase

in sediment delivery after 30 Ma with a peak in the Late

Miocene and slight decrease during the Plio-Pleistocene

which is not so dissimilar to our basin-wide budget

(Fig. 6D), given the modest age control available.

Our seismic data allow us to reconstruct how the delta

front of the Mekong has been built since 5.3 Ma. Figure 12

shows a series of isopach maps starting with the oldest

defined unit overlying the B200 Reflector and showing how

the depocenter of the delta front has migrated since 5.3 Ma.

The dates on the units are not well defined but provide a

rough image of where sediment has been preserved at the

shelf edge. Since the shelf edge is the place with the

greatest accommodation space, located close to the river

mouth most of the sediment is stored in that area. Our maps

show that the greatest sediment thicknesses were being

preserved in the southern part of the shelf edge after

5.3 Ma and that the sedimentation had become especially

focused there and away from the north shelf edge by the

Late Pliocene (Fig. 12C). The Pleistocene show a

Mar Geophys Res

123


significant change, with the main depocenter migrating

towards the east to a significant degree (Fig. 12D), as well

as supplying sediment more generally to the deeper water

regions and especially to the basins on the south side of the

Vanguard and Grainger Banks. The SE basin seems to be

particularly important depocenter during the Early

Fig. 12 Isopach maps showing the Pliocene to Recent sediment

thicknesses, largely reflecting building of the Mekong submarine delta

to the east from the shelf edge. The different packages mapped are not

all well dated but show a progressive eastern shift in the depocenter

from the oldest delta package (A) to the youngest mapped unit under

the seafloor (F). The reflectors are shown on Fig. 4. A B180-B200

Lower Pliocene, B B160-B180 Middle Pliocene, C B150-B160 Upper

Pliocene, D B100-B150 Lower Pleistocene, E B070-B100 Middle

Pleistocene, and F Seafloor-B070 Upper Pleistocene

Mar Geophys Res

123


Pleistocene, receiving [13.5 km in that period alone, after

receiving effectively no material during the Pliocene.

Figure 13 shows a second set of isopach maps that

display the progressive construction of the post-5.3 Ma

Mekong Delta. In this case the maps are also based on the

B200 Reflector but instead of showing each depositional

package they show how the total mass has been constructed

progressively. These maps show that the delta started to

Fig. 13 Isopach maps total sediment thickness through time about

the Lower Pliocene reflector at the base of the Mekong Delta

sequence. Each map showing the total thickness of the delta wedge as

it grows progressively through time with A being the first package

deposited and F the present thickness of the Mekong Delta wedge

Mar Geophys Res

123


form at the shelf edge in the southern part of the survey

region and then built towards first the north and then after

the start of the Pleistocene we observe faster growth to the

east and especially filling of the trough to the south of the

Vanguard and Grainger Banks. It is noteworthy that very

little sediment is supplied to the deep water north of the

Prince Consort and Alexandra Banks and that indeed these

is very little supply to the northern shelf edge. This may

reflect the patterns of sediment transport. Indeed, modern

sediment dispersal from the Mekong River Mouth is mostly

towards the SW, where the largest Holocene depocenter is

located (Xue et al. 2010). It is possible that the reversing

monsoon-driven currents on the Sunda Shelf may be

responsible for remobilizing sediment to the shelf edge

after that initial transport. However, it should be considered

that the source of sediment is not the Mekong alone for at

least part of the sealevel cycle because during lowstands

the entire Sunda Shelf was exposed and the margin was

receiving sediment from a SW to NE flowing river, known

the Molengraaff River that brought material close to the

southern edge of the survey region from source areas in

SW Borneo and the Malay Peninsula (Molengraaff and

Weber 1921; Hanebuth et al. 2002). If this system was

important, then this may explain why the basins south of

the off shelf banks are filled with sediment but not those

north of the banks.

Magmatism

Our seismic profiles have revealed the presence of occa-

sional volcanic seamounts found mostly in the basin areas

offshore the shelf break (Figs. 3, 4, 7, 9). The volcanic

Fig. 14 Close-up images of

volcanic seamounts in south,

central part A and central

western parts of the basin

C. Parts B and D respectively

show the interpretation of the

lines and the deformation of the

pre-emplacement reflectors

around the intrusions followed

by passive onlapping of younger

sequences

Mar Geophys Res

123


edifices are apparently of quite limited extent, being

restricted to the seamounts which measured *5–10 km

wide but do not extend over a wider area. This shows that

the degree of melting is quite modest. Seamounts are only

found in the basin areas or on the sides of the off-shelf

banks, not on the bank crests or on the Sunda Shelf, sug-

gesting a link between thinning lithosphere and melting.

Figure 14 shows close-up seismic images of two volcanic

features in order to show their structure in detail. The

seismic does not provide much detail concerning the

internal structure but aprons of sediment around the vol-

canoes are visible. What is more obvious is the deforma-

tion of the surrounding strata, with sediment deposited

prior to intrusion bowed upwards against the side of the

seamount, while younger sediments lie passively against

the structure, allowing the timing of emplacement to be

determined (Figs. 4, 7, 9). The relationship between the

seamount volcanic rocks and the normal continental base-

ment is unclear. In Fig. 14A all reflectors from basement

(B600) to Lower Pliocene (B200) have been tilted against

the sides of the seamount, which suggests that the

seamount was generated after the Early Pliocene. In con-

trast, Fig. 14C shows that Reflector B300 (Late Miocene)

is gently ponded upon Reflector B400 (Middle Miocene).

In that case Reflector B300 is almost flat, which suggests

this volcano is relatively old and generated between the

Middle Miocene and Late Miocene. Upturned strata around

the seamounts may suggest syn-depostional growth of the

volcano, whereas nearly flat strata and may suggest real-

tively quick growth/formation of the seamount.

In all cases the volcanism is constrained as post-dating

the end of active seafloor spreading in the adjacent oceanic

lithosphere (Briais et al. 1993; Barckhausen and Roeser

2004) and we see that it also postdates the active extension

we have documented in the study area, with the exception

of some extensional faulting, mostly limited to the off-shelf

banks. We speculate that some of the later uplift of the

banks could linked to the magmatism. How any magma is

generated in such a setting is unclear because under normal

mantle conditions (i.e., asthenosphere *1,300 �C) signif-

icant extension is required to drive adiabatic upwelling,

with melting occurring when extension exceeds *200 %

(b = 3) (McKenzie and Bickle 1988). Our sections show

negligible extension at the time of emplacement after the

Middle Miocene. It is noteworthy that even when extension

was significant prior to 16 Ma there was no seamount

volcanism, while a brief inspection of the seismic profiles

provided in this study shows that even in the offshelf banks

deformation mostly involved folding and effectively zero

extension. In this respect the volcanic rocks in this region

are similar to those found in the regions between the

Dongsha Rise and Taiwan, as well as on Hainan Island and

the neighboring Leizhou Peninsula (Tu et al. 1991;

Shi et al. 2011). Our observations require either (1) hotter

than normal asthenospheric conditions, (2) thinning of the

mantle lithosphere that is not related to upper crustal

extension, (3) the presence of asthenospheric sources that

are more fertile than normal under the basins or (4) the

presence of aqueous fluids in the mantle that depress the

solidus in this region. With our present data set it is not

possible to distinguish between these alternative models,

only to note that the enigmatic South China Sea neo-

volcanic province extends as far south as the deep-water

Nam Con Son margin.

Conclusions

In this study we have made a structural and stratigraphic

synthesis of the southwestern parts of the South China Sea

(Fig. 15), immediately to the east of the Nam Con Son basin,

based on a grid of industry seismic reflection data tied to

boreholes near the shelf edge. The area is located immedi-

ately in front of a propagating oceanic spreading center and

allows us to examine the deformation related to both the

advancing extension and to the strike slip motion along the

East Vietnam margin. Our analysis shows that the sedi-

mentary section comprises a thick sequence of Oligocene-

Lower Miocene strata, which are affected mostly by rather

high-angle normal faults. Most of these faults terminate at an

unconformity dated as Middle Miocene (*16 Ma). Map-

ping of the depth to basement reveals the presence of a sig-

nificant continental block that is separated from the thicker

crust under the Sunda Shelf by a tract of hyperextended crust

that is mostly indistinguishable from the crust near the

propagating rift tip, at least in terms of its depth. This off-

shore block now takes the form of a series of shallow-water

banks that connect with the edge of the Sunda Shelf.

Extension in these off-shelf banks continued after the ces-

sation of brittle faulting within the main shelf, lasting well

into the Late Miocene. The off-shelf banks appear to have

been a depocenter during the Oligocene-Early Miocene but

were affected by an uplift event prior to the Middle Miocene

DRU, which we interpret to be a response to the collision

between the Dangerous Grounds and Borneo. There is also

evidence for late-stage uplift during the Pliocene when a

major unconformity is observed on the Prince Consort Bank.

This uplift does not affect the main Sunda Shelf. This late

stage uplift may be linked to continued magmatism in the

form of scattered seamounts, which postdate the period of

brittle upper crustal faulting and which are not clearly linked

to any regional extensional event. The trigger for this mag-

matism is presently unclear but this appears to be part of a

wider diffuse magmatic province throughout the South

China Sea.

Mar Geophys Res

123


Our survey also shows the eastward progradation of the

Mekong Delta, which we constrained to date from at least

5.3 Ma, slightly younger than the age of onset noted within

the more proximal Nam Con Son and Cuu Long Basins

(Murray and Dorobek 2004). This age is somewhat

younger than the proposed ages of major drainage capture

in southeastern Tibet in the Early Miocene (Clift et al.

2006; Hoang et al. 2009) and suggests that the present

location of the Mekong Delta is not principally governed

by drainage capture driven by continental tectonics,

although we cannot discount entirely the influence of uplift

during the Late Miocene of the Central Highlands of

Vietnam (Carter et al. 2000). Pliocene and younger sedi-

mentation is preferentially focused towards the southern

end of our study area, migrating slightly to the north but

mostly towards the east since 5.3 Ma. The most dramatic

progradation occurs during the Pleistocene when a deep

trough to the south of the off-shelf banks is filled with

sediment [1.35 km thick. The delta comprises a series of

clinoforms lobes whose primary source is presumed to be

the Mekong River although the filling of the deep-water

trough south of the off-shelf banks is suggestive of sig-

nificant sediment flux also from further east and south,

potentially via the Molengraaff River that dominates the

Sunda Shelf during sea level lowstand periods.

Acknowledgments We thank PetroVietnam and Total Exploration

and Production for providing data used in this study. The study

benefited from comments by Jean-Luc Auxietre and Gwang Hoon

Lee.

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Fig. 15 Diagram showing the

tectonic and sedimentary events

affecting the study area, based

on the results of this study. Start

of motion on Red River Fault is

from Leloup et al. (2001), the

start of seafloor spreading is

from Briais et al. (1993), the end

of extension in Pearl River

Mouth Basin is from Clift and

Lin (2001)

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