Finite extension across the South Vietnam basins from 2D

Marine and Petroleum Geology 04 "0887# 508Ð523

S9153Ð7061:87:,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reservedPII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 2 0 Ð 1

Finite extension across the South Vietnam basins from 2Dgravimetric modelling] relation to South China Sea kinematics

P[ Huchon�\ T[ N[ H[ Nguyen0\ N[ Chamot!RookeLaboratoire de Ge�ologie\ Ecole Normale Supe�rieure "CNRS URA 0205#\ 13 rue Lhomond 64120 Paris Cedex 94\ France

Received 16 February 0887^ revised 05 May 0887^ accepted 17 May 0887

Abstract

We derive a crustal thickness map over the south Vietnam basins from a 2D inversion of gravity data that takes into account thesedimentary in_ll and its density variation with depth[ This map is used to estimate the total amount of stretching across thesebasins\ that varies from 089 km to the east "at 000>E# to 29 km to the west "at 096>E#[ Comparison with South China Sea kinematics"amount and timing of oceanic opening# implies large decoupling with respect to the Indochina block and supports the hypothesisof formation of the South China Sea in upper Oligocene to lower Miocene by southward subduction of the proto!South China Sea[Þ 0887 Elsevier Science Ltd[ All rights reserved[

Keywords] Extension^ Vietnam^ Gravimetric modelling

0[ Introduction

The South China Sea is one of the largest marginalbasins in the western Paci_c "Fig[ 0#[ According to theanalysis of magnetic anomalies\ it opened from 21 to 04Ma "Briais et al[\ 0882^ Taylor + Hayes\ 0879\ 0872#[While its eastern margin has disappeared by subductionbeneath the Philippines islands\ its northern and southernmargins are typical passive margins which have recordedthe rifting history of the basin[ Since the oceanic basinhas propagated toward the south!west\ the timing forstretching of the continental crust to the west must bediachronous[ In turn\ this diachronicity is a key for under!standing the mechanism of opening of the oceanic basin[

Several opening models have been proposed "Jolivet etal[\ 0878^ Lee + Lawver\ 0883^ Rangin\ 0885^ Rangin etal[\ 0889^ Tapponnier et al[\ 0871\ 0875^ Taylor + Hayes\0879\ 0872#[ They all correspond to two end!membermodels\ implying either sinistral or dextral motion alongthe western margin\ o}shore central Vietnam[ Taylor andHayes "0879# _rst identi_ed the magnetic anomalies inthe South China Sea and observed their relative sym!metry and their EÐW orientation in the eastern part ofthe basin[ Huchon et al[ "0883# and Hall "0885# argued

� Corresponding author[ Tel[] ¦22 0 33 21 11 43^ Fax] ¦22 0 33 2119 99^ E!mail] huchonÝgeodyn[ens[fr

0 Now at Department of Geology\ Vietnam National University inHanoi\ 89 Nguyen Trai\ Dong Ha\ Hanoi\ Vietnam

that extension was caused by slab!pull of a subductingproto!South China Sea located further south[ Based onthe analysis of the continental tectonics and on small!scale experiments\ Peltzer "0872#\ Peltzer and Tapponnier"0877# and Tapponnier et al[ "0871\ 0875# suggestedinstead that the Indochina block was extruded south!eastwards along the left!lateral Red River Fault duringthe India!Eurasia collision[ According to radiometricdata "Leloup et al[\ 0884^ Sha�rer et al[\ 0889#\ the left!lateral motion was active up to early Miocene time andwas coeval with the formation of the South China Seawhich is then interpreted as a pull!apart basin related tothe motion along the Red River Fault[

The northern margin of the South China Sea is rela!tively well known[ The Beibu basin "Tonkin gulf# riftedmainly in the Oligocene as the termination of the RedRiver Fault\ but its western part\ close to the Red RiverFault remained active until 4[4 Ma ago "Rangin et al[\0884b#[ South of the Beibu basin\ the structure and ageof the Song Hong "Yingghehai# basin is still debated\leading to divergent interpretations as a left!lateral "LePichon et al[\ 0884# or right!lateral "Harder et al[\ 0881#pull!apart basin during the Oligocene to lower Miocene[Further east\ the Pearl River mouth basin also riftedduring the Oligocene "Li Pinglu + Rao\ 0883^ Pigott +Ru\ 0883^ Ru + Pigott\ 0875^ Wang + Sun\ 0883^ Yu\0883#[ Deep seismic soundings support the interpretationof the basin as located above a northward dipping crustalscale detachment "Hayes et al[\ 0884^ Nissen et al[\ 0884a\

P[ Huchon et al[:Marine and Petroleum Geolo`y 04 "0887# 508Ð523519

Fig[ 0[ Location map of the studied area[ Rectangle] area of Figs 1\ 2 and 3[

0884b#[ The geometry and timing of extension in thesebasins thus apparently supports Tapponnier et al[|shypothesis[ However\ it has been shown that the oceanicspreading kinematics drastically changed around ano!maly 6 "15 Ma# "Briais et al[\ 0878^ Pautot et al[\ 0875#\when the ridge reoriented from NÐS to NWÐSE spreadingand started to propagate toward the south!west "Briaiset al[\ 0882#\ until it reached the present!day location ofthe oceanic tip\ near 8>N of latitude and 009>E of longi!tude "Coulon et al[\ 0884#[ The timing and kinematics ofextension in the western margin of the South China Seaare thus crucial elements in determining the relationshipof oceanic spreading to continental tectonics "Hayes\0874#[

Recent onshore and o}shore data acquired in the QuiNhon and Nha Trang basins "Marquis et al[\ 0886^Roques et al[\ 0886a\ 0886b# support the Hayes| hypoth!esis of a dextral NÐS margin o}shore central Vietnam[On the basis of these data\ Rangin et al[ "0884a# haveproposed that sinistral motion along the Red River fault

system may be restricted to the area north of Danang\while partly coeval with dextral motion along the Viet!nam margin south of Danang[

The southern margin of the South China Sea has amore complex history\ probably because it recorded notonly the Oligo!Miocene rifting\ but also the Paleoceneevents "Hinz + Schlu�ter\ 0874^ Shaoren et al[\ 0883#[Hamilton "0868# _rst suggested that the opening of theSouth China Sea caused the separation of the DangerousGrounds\ Reed Bank and Palawan blocks from mainlandChina[ From a comparison between the northern andsouthern margins of the South China Sea\ Kanyuan etal[ "0883# concluded with striking di}erences\ due\however\ to the overprint of compressional events to thesouth[ Although of the scope of this paper\ we may noticethat most authors now recognize the former existenceof a proto!South China Sea which has been subductedbeneath northern Borneo "e[g[\ Holloway\ 0871#[ InSabah\ the main compressional event was initiated inMiddle Miocene "Rangin et al[\ 0889# and corresponds

P[ Huchon et al[:Marine and Petroleum Geolo`y 04 "0887# 508Ð523 510

to the collision that followed the complete close of theproto!South China Sea "Tongkul\ 0883#[

Oil exploration has extensively covered the southernVietnam basins since the early 69s and their general struc!ture is well known\ except for the deepest parts whichhave never been reached by exploration wells due to thevery thick sedimentary in_ll "more than 09 km#[ Thethickness and nature of the sediments\ including theirdensities\ are thus reasonably well known\ although notyet published[ In addition\ satellite altimetry now pro!vides an homogeneous data set of the gravity _eld[ Itsresolution "a few km# is enough for regional studies andcan therefore be used to estimate the thickness of thecrust underlying the basins\ and therefore provides newconstraints on their stretching mechanism[

1[ Structure and history of the southern Vietnam basins

The southern Vietnam o}shore is characterised by ashallow platform\ less than 199 m of water depth\ formedby the prograding Plio!Quaternary sediments of theMekong delta "Figs 1 and 2#[ Beneath this recent sedi!mentary cover\ oil industry exploration has revealed thepresence of two main basins\ the northern Cuu Long

Fig[ 1[ Bathymetric map of the southern Vietnam shelf and adjacent oceanic basin "location] see inset in Fig[ 0#[

basin and the southern Nam Con Son basin\ separatedby a basement high named Con Son ridge[ Further southare the Natuna basins\ which are located west and southof the southern Vietnam shelf[

1[0[ The Cuu Long basin

The Cuu Long basin is the continuation of the onshoreMekong basin and is about 399 km long and 099 kmwide\ with a NEÐSW trend "Fig[ 2# "Le\ 0875#[ It mainlyshows NEÐSW normal faults "Mai\ 0884# active until theend of Oligocene "Truong et al[\ 0880#\ and EÐW trendingfaults in the central part of the basin[ The syn!riftsequence is divided into the Ca Coi formation of Eoceneage and the Oligocene Tra Tan formation "Do et al[\0882#[ It comprises ~uviatile to deltaic sediments] sand!stones\ shales and silts "CCOP\ 0880#[ The post!riftsequence is divided in the Bac Ho "Lower Miocene#\ ConSon "Middle Miocene#\ Dong Nai "Upper Miocene# andBien Dong "Plio!quaternary# formations\ all marine butwith a slight uplift in Middle Miocene\ due to depo!sitional rates exceeding the thermal subsidence in thebasin[ The total sediment thickness reaches 8 km in thecentre of the basin "Fig[ 2#[ The general structure of the


)

)CMYK Page 511

Fig[ 2[ Basement depth map "compiled from various industrial sources and from the PONAGA|82 cruise#[

basin is attributed to the termination of the left!lateralWang Chao fault "Tapponnier et al[\ 0875#[

1[1[ The Nam Con Son basin

The Nam Con Son basin is deeper and more complexthan the Cuu Long basin "Fig[ 2#[ It shows EÐW\ NEÐSW\ NNEÐSSW as well as NÐS normal faults[ The oldestsediments are attributed to Eocene and Early Oligocene\but only the Late Oligocene "Cau Formation or com!posite sequence T19 of Matthews et al[\ 0886# has beenreached by exploration wells on structural highs[ As inthe Cuu Long basin\ the Oligocene sediments which haveaccumulated in EÐW trending half!graben are ~uviatileto deltaic\ but here the Oligo!Miocene unconformity isnot well marked "Matthews + Todd\ 0882#[ The syn!riftformations are Early to Middle Miocene\ thus youngerthan in the Cuu Long basin[ The Dua formation "EarlyMiocene\ composite sequence T29# is marine "sub!lit!toral# and constituted by black shales intercalated withsandstones[ It was deposited in NEÐSW to NÐS graben"Matthews et al[\ 0886# as were the Thong and Mang Cauformation "Middle Miocene\ composite sequences T39and T59#\ which contain mainly sandstones associated toshallow water limestones "reefs#[ The boundary between

the Dua and Thong formations is an unconformity whichseals most of the NEÐSW normal faults[ However\ someof the faults continue into the Middle Miocene up toanother unconformity dated at about 09 Ma[ The LateMiocene "Nam Con Son formation\ composite sequenceT74# is represented by deeper marine sediments which\with the prograding Plio!quaternary Bien Dong forma!tion\ form the post!rift sequence with average subsidencerates of 49 m Ma−0 typical of thermal subsidence\ con!trasting with the faster subsidence rates "³699 m Ma−0#recorded from 08[4Ð05 Ma[

A major di}erence in age therefore exists between theCuu Long "Oligocene# and the Nam Con Son "Lowerto Middle Miocene# basins[ Their respective structuralsettings are also di}erent] while the Cuu Long basin seemsto be associated with NWÐSE trending left!lateral faults"with more or less NNWÐSSE extension#\ the structureof the Nam Con Son basin\ although also characterisedby NEÐSW normal faulting\ is not[ Rather\ several NEÐSW to NNEÐSSW normal faults appear to branch in ahorse!tail fashion onto NÐS faults[ This structuralrelationship suggest that the NÐS faults are right!lateralstrike!slip faults[ This tectonic pattern is also compatiblewith NNWÐSSE extension\ but no obvious structural linkexists between the two basins[ Finally\ it is worthwhile


noting that the structure of the Nam Con Son basinappears to be in the continuation of the deeper oceanicbasin located further east "Fig[ 1#[ This basin\ up to 3999m deep "water depth# corresponds to the tip of the south!westward propagating South China Sea\ which reacheslongitude 009>E about 04[4 Ma ago\ according to theinterpretation of the magnetic anomalies[

1[2[ The West Natuna basin

The West Natuna basin is located to the west of theNatuna arch and is the south!eastward extension of theMalay basin[ It is controlled by two main fault directions]NEÐSW to ENEÐWSW normal faults and WNWÐESEfaults considered as left!lateral strike!slip faults "Letou!zey et al[\ 0877^ Mingard + Willett\ 0882^ Nga et al[\0885#[ Extension on NEÐSW normal faults started duringthe Oligocene "near the top of the Benua Lama for!mation# and continued during the Early Miocene[ Nearthe top of Barat formation "about 10 Ma ago#\ normalfaults show numerous inversions sealed by the 09 Maunconformity "Ginger et al[\ 0882#[

1[3[ The East Natuna basin

The East Natuna basin is located to the south of theNam Con Son basin and is separated from the WestNatuna basin by the NÐS trending Natuna arch "Fig[ 0#[It has a similar history to the West Natuna basin duringthe Oligocene and Early Miocene\ but no inversion isrecorded in this basin[ Rather\ the NEÐSW normal faultsare systematically associated with NÐS faults on whichthey branch in a horse!tail fashion\ suggesting a dextralmotion on these NÐS faults[ The most developed NÐSdextral graben is the Komodo graben[

Three di}erent types of basins thus characterise thecontinental margin west of the tip of the oceanic crust ofthe South China Sea] "0# the Cuu Long basin\ Oligocenein age^ "1# the West Natuna basin\ Oligocene to EarlyMiocene and inverted in Middle Miocene^ "2# the NamCon Son and East Natuna basins\ mainly of EarlyMiocene age and without Middle Miocene inversion\excepted a few localised gentle inversions in its south!eastern part[ Their period of tectonic subsidence and theirstructural patterns form the basis for the interpretation ofthe crustal structure that we shall derive from gravimetricmodelling in the next section[

2[ Gravimetric modelling

2[0[ 2D gravity inversion for Moho depth

The inversion procedure is a two!step processing ofthe free!air gravity anomaly[ The _rst step consists ofcalculating a Bouguer anomaly reduced to the mantle by

evaluating the contribution of the various bodies fromthe observation plane "sea surface# to the top of thecrystalline basement "water and sediment contributions#[In a second step\ the obtained Bouguer anomaly isinverted to recover undulations of the Moho disconti!nuity[ Details of the procedure can be found in Chamot!Rooke et al[ "0886#[ The mantle Bouguer is calculatedusing a 2D analytic solution for thin sheets contribution"Talwani + Ewing\ 0859#[ The inversion is then per!formed using Fourier domain gravity expansion "Parker\0861#[

In practice\ the mantle Bouguer calculation requiresknowledge of the geometry and density contrast "withrespect to crystalline basement# of the various surfacebodies[ Since density increases with depth\ it is necessaryto consider several sedimentary layers[ At this stage\ andprovided the geometries and densities of sedimentary lay!ers are correctly estimated\ the only unknown parameteris the density of the basement[ We used a constant densityof 1[64 g cm−2 for the basement density throughout thecalculations\ which is a typical mean value for continentalcrust[

The second step consists in determining the geometryof the Moho from the mantle Bouguer[ The main assump!tion is that the residual Bouguer anomaly "free!air cor!rected for surface bodies# can be totally attributed tovariations of the Moho depth[ The inversion is based onFourier transform methods[ The gravimetry e}ect `"x\ y#produced by the periodic undulation h"x\ y# of an inter!face separating two layers with a density contrast Dr atdepth dŁ h"x\ y# is]

`"x\ y#� 1pGDrh"x\ y#e−kd "0#

where G is the gravitational constant and k the wavenumber\ and high order terms in the gravity expansionhave been ignored[ In the Fourier domain\ this equationbecomes]

G"kx\ ky#� 1pGDrH"kx\ ky#e−kd "1#

where H"kx\ ky# is the Fourier transform of h"x\ y#[ Onecan then obtain the geometry h"x\ y# of the Moho by]

h"x\ y#�TF−06TFð`"x\ y#Ł

1pGDre−kd7 "2#

where TF and TF−0 denotes the Fourier and inverseFourier transforms\ respectively\ and where Dr is thedensity contrast between mantle and basement[ Noticethat the inversion procedure requires a reference level dfor Moho undulations[ This level can be determined byadditional constraints at some speci_c location "such asMoho depth obtained from seismic refraction#[ However\mathematically\ the Fourier expansion requires d to beas close as possible to the mean depth of the undulations[A full discussion of this problem is found in Chamot!Rooke et al[ "0886#[ A safe procedure is to recalculate a


)

)CMYK Page 513

posteriori the Moho contribution using the above men!tioned 2D analytical solution[ The residual "observedminus calculated gravity# can in turn be resolved throughsome minor adjustments of the Moho depth around theinverted solution[

2[1[ Data set used

Data compilation has been done from latitude 5[34>Nto 01>N and from longitude 095[34>E to 000>E[ Thebathymetric grid "Fig[ 1# is a combination of the ETOPO4database complemented with high accuracy multibeamdata obtained over the propagating tip of the SouthChina Sea during the PONAGA|82 cruise of R:V L|At!alante "Coulon et al[\ 0884#[ Over the southern Vietnamshelf\ the water depth is generally less than 199 m[ Theshelf edge is marked by a 0999 m scarp\ with a generalNÐS trend[ East of it\ the water depth increases towardthe oceanic basin\ reaching nearly 3999 m[

Sediment isopachs were obtained from various indus!trial sources and merged after checking regional mul!tichannel seismic lines[ The two!way travel times T "in s#obtained from seismic data were converted to depth Z"in m# using the following relationship\ based on thecompilation of data from about 29 wells]

Fig[ 3[ Free air gravity anomaly map\ derived from altimetry "Sandwell\ version 6[1#[

Z� 05[7¦648[4T¦136T1 "3#

The obtained basement depth map is shown in Fig[ 2[The basement depth varies from 1Ð7 km in the Cuu Longbasin and from 2Ð02 km in the Nam Con Son basin[ It isless than 0 km on the Con son ridge[

The most recent version "6[1# of the free air anomalymap derived from satellite altimetry "Sandwell + Smith\0881# was used "Fig[ 3#[ Although less precise at shortwavelength than shipboard measurements\ the satellitederived map is su.cient for our purpose[ Comparisonwith ship data on small areas shows a fair agreement atwavelengths greater than 09 km[

Finally\ sediment densities were obtained from a com!pilation of several industrial wells\ from which we con!structed a density versus sediment thickness chart "Fig[4#[ The density contrast between the basement and theoverlying sediments becomes negligible below 4Ð5 kmthickness[

2[3[ Results

The Bouguer anomaly map reduced to mantle is shownin Fig[ 5[ A _rst approximation of the Moho depth wasobtained through the inversion procedure described


Fig[ 4[ Density vs depth determined from about 29 wells in the South Vietnam basins[ The bold line is the mean curve that we used for the gravimetriccomputations[

above[ The re!calculated signal "forward modelling basedon 2D exact solution rather than Fourier approximation#showed di}erences locally reaching 29 mgal[ Theseresiduals "di}erences between observed and recomputedgravity signals# were in turn used to adjust the _rst ordersolution by minor depth corrections of the Moho[ The_nal Moho depth map "Fig[ 7# leads to residuals less than3 mgal "rms#\ with maximum not exceeding 01 mgal "Fig[6#[ The crustal thickness map "Fig[ 8# shows that theminimum thickness corresponds to the oceanic pro!pagating tip of the South China Sea "less than 7 km#\while continental crust is thinnest "09Ð01 km# beneaththe Nam Con Son basin[ By contrast\ the Cuu Long basinis much less thinned "19 km#[

3[ Kinematics of continental stretching

3[0[ Amount of stretching across the Nam Con Son basin

The crustal thickness over the South Vietnam basins"Fig[ 8# obviously shows a close relationship to the tipof the South China Sea oceanic crust[ In spite of thecomplexity introduced by the presence of two sub!basinsin the Nam Con Son basin\ the deepest part of thesebasins clearly lies at the south!western continuation ofthe fossil oceanic propagating tip "Coulon et al[\ 0884#[Subsidence analysis\ both in the Nam Con son basin andin the Zengmu basin\ shows that the tectonic subsidencephase started 10Ð11 Ma ago "Academia Sinica\ 0878#\


Fig[ 5[ Computed mantel Bouguer anomaly "explanation] see text#[

and ended shortly after spreading ceased in the SouthChina Sea\ 04Ð05 Ma ago[

To further compare the synchronous amounts of oce!anic opening in the South China Sea and of continentalextension over the South Vietnam basins\ we sys!tematically computed from the crustal thickness map thetotal amount of stretching along small circles about Briaiset al[|s "0882# South China Sea pole of opening for thisperiod "10Ð04 Ma# "Fig[ 09#[ This was done by assumingan initial crustal thickness of 29 km before stretching\inferred from the observed crustal thickness along thecoast of Vietnam "Bui\ 0882#\ and assuming further con!servation of crustal volume during stretching[ Ru andPigott "0875# emphasized that the South China Sea regionrecords several episodes of stretching\ so that the assump!tion of a 29 km constant crustal thickness\ prior to theMiocene stretching\ may not be valid[ However\ we showin a further section that the stretching across the CuuLong basin\ which records the Oligocene stretching\ ismuch less than in the Con Son basin[ Note that we didnot include the Cuu Long basin in the calculations sincethis basin is older than the period into consideration "10Ð04 Ma#[

The poles of rotation of Briais et al[ "0882# for thecorresponding period of time*anomalies 5 "19[4 Ma1#\5a "10[6 Ma# and 5b "12[3 Ma#*are rather close to eachother\ and far from the South Vietnam basins\ and theyresult in nearly the same small circles[ Figure 00 showsthe total amount of stretching computed from the crustalthickness map along these small circles\ as a function oflongitude[ It ranges from 29 km to the south!west to 089km to the north!east[ This south!westward decrease isfairly linear and would correspond to a pole of rotationlocated very close to the south!west "at about 5>29?N\095>29?E#\ assuming that stretching accommodates arigid rotation of Vietnam with respect to the southernmargin of the South China Sea[ The formation of thebasins consequently does not obey the same kinematics asthe oceanic opening of the South China Sea\ the westwarddecrease in continental stretching being faster than thatpredicted by the oceanic kinematics[

This rapid decrease of stretching toward the south!west is also in good agreement with the basin inversion

1 Using Harland et al[|s "0889# timescale[


Fig[ 6[ Residual free air gravity anomaly "di}erence between the free air anomaly computed from the model and the observed anomaly#[

which occurred 10 Ma in the West Natuna basin "Nga etal[\ 0885^ Ginger et al[\ 0882#[ Extension to the north!eastof Natuna arch and slight compression to the south!westbeing kinematically compatible\ it suggests a commoncause\ which\ concerning the inversion of the WestNatuna basin\ can be considered as the propagation ofcompression in the Rajang!Crocker belt of north Borneo"Be�nard et al[\ 0889^ Prouteau et al[\ 0885#[ We shall seein the last section that this explanation is consistent withthe hypothesis of Taylor and Hayes "0879# for the for!mation of the South China Sea[

3[1[ Comparison with the kinematics of the South ChinaSea spreading ridge

We now compare our estimate of continental stret!ching to the predicted amount of oceanic opening duringthe same period of time "Fig[ 01#[ The oceanic motionsince anomalies 5\ 5a and 5b was obtained using thecorresponding poles of rotation of Briais et al[ "0882#[ Tothe north!east\ close to the oceanic tip\ the predictedoceanic motion since anomaly 5 "19[4 Ma# is smaller bymore than 39 km than our continental stretching esti!

mate[ On the other hand\ the predicted motion sinceanomaly 5b "12[3 Ma# is larger by about 29 km than ourestimate[ A good _t is obtained for anomaly 5a "10[6Ma#\ suggesting that continental stretching is kine!matically compatible with oceanic spreading if extensionstarted 10[6 Ma ago[ This falls into the range of ages forthe onset of tectonic subsidence in the South Vietnambasins[ In other words\ the present day crustal thicknessestimated in the north!eastern region\ close to the oceanictip\ is fully explained by the kinematics describing theoceanic spreading in the South China Sea since 10[6 Ma[

However\ we showed that further west continentalstretching decreases more rapidly and does not conformwith the oceanic kinematics[ This observation impliesstrong decoupling between the South China Sea and thecontinental block west of Natuna arch[ The estimatedamount of o}set "about 059 km# is simply the di}erencebetween the continental stretching to the south!west "notmore than 29 km# and the relative motion predicted bykinematics "089 km#[ This decoupling may either cor!respond to a northward motion of Vietnam with respectto the southern margin of the South China Sea\ if thelatter is attached rigidly to the continental block west of


)

)CMYK Page 517

Fig[ 7[ Computed Moho depth[

Natuna arch\ or to a southward motion of the southernmargin of the South China Sea with respect to Vietnamif the latter is attached rigidly to the continental blockwest of Natuna arch[

Since this decoupling implies left!lateral or right!lateralstrike!slip motions\ respectively\ it can be compared toknown tectonic features which may accommodate sucha motion[ While the Cuu Long basin might be interpretedas a horse!tail termination of the Wang Chao fault\ andthe Malay basin as connected to the Three Pagodas faultsystem "Tapponnier et al[\ 0875#\ no NWÐSE trendingfaults appear to connect with the Nam Con Son basin[On the other hand\ NÐS trending faults along the Natunaarch are good candidates for accommodating dextralmotion\ as well as NWÐSE trending strike!slip faultsrecognised o} Sarawak "Mat!zin + Swarbrick\ 0886#"Fig[ 02#[ It suggests that\ at least from 11Ð04 Ma\ southVietnam was attached to the continental block west ofNatuna arch[ Consequently\ our analysis suggests that atotal of 059 km of dextral NÐS motion may have occurred"from 11Ð04 Ma# in a distributed way to the south!westof the Nam Con Son basin[

Furthermore\ rifting of the south Vietnam basins iscoeval with the southward subduction of the proto!SouthChina Sea beneath Borneo as inferred from the 07Ð08

Ma Sintang adakites "northwest Borneo# "Prouteau etal[\ 0885# that mark the early stage of subduction\ fol!lowed by calc!alkaline magmatism and collision in Mid!dle Miocene "Rangin et al[\ 0889#[ This time relationshipthus favors the model in which the slab pull is the drivingforce for the opening of the South China Sea "Hall\0885#[

Independent estimates based on gravity modelling inthe Nha Trang basin "Marquis et al[\ 0886# suggest that089 km of dextral motion occurred along the NÐS Viet!nam scarp from 15Ð18 Ma to 19[4 Ma[ In total the sou!thern margin of the South China Sea seems to have beendisplaced with respect to Vietnam by about 249 km from15Ð18 Ma up to the end of spreading\ a value close to thetotal amount of opening of the South China Sea since15Ð18 Ma "214Ð339 km measured along the margin\according to Briais et al[|s "0882# kinematics#[

3[2[ Amount of stretching across the Cuu Long basin

The same analysis of stretching has been done for theCuu Long basin[ However\ South China Sea kinematicsbefore 29 Ma would predict NEÐSW trending extension\incompatible with the overall structure of the basin "Fig[2# and its link with the NWÐSE trending left!lateral Wang


)

)CMYK Page 518

Fig[ 8[ Computed crustal thickness[

Chao fault[ Crustal sections perpendicular to the basinaxis indicate a maximum amount of stretching of 29 km"Fig[ 00#\ in good agreement with the 19Ð16 km computedby "Mauri\ 0882#[ As can also be seen from the crustalthickness map\ the stretching across the Cuu Long basinis by far less than across the Nam Con Son basin[ More!over\ it bears no obvious link with the early phase ofopening of the South China Sea\ both in terms of direc!tion and amount of extension[ Recent estimates "Hayeset al[\ 0884^ Nissen et al[\ 0884a# of the extension acrossthe Pearl River Mouth basin\ along the northern marginof the South China Sea\ show large values\ up to 299 km\consistent with possible hundreds of kilometres ofleft!lateral displacement along the Red River Fault"Tapponnier et al[\ 0871\ 0889#[ By contrast\ the limitedamount of extension across the Cuu Long basin does notfavour the 039 km of displacement along the Wang Chaofault postulated by Tapponnier et al[ "0875#[

4[ Conclusion

Our analysis suggests that a clear link exists betweenthe formation of the Nam Con Son basin and the oceanicopening of the South China Sea between 10 and 04 Ma\

the period during which the spreading centre rapidlypropagated toward the south!west[ However\ the morerapid decrease in stretching toward the south!west\ com!pared to the prediction of oceanic kinematics\ has twoimportant consequences] _rst\ it explains the inversion ofnormal faults in the West Natuna basin\ which occurred10 Ma ago[ Second\ it leads us to conclude that thesouthern margin of the South China Sea moved by some059 km with respect to the Indochina block\ in the waysuggested by Taylor and Hayes "0879# in their dextralVietnam margin model\ as opposed to the left!lateralmodel of Tapponnier et al[ "0871#[

Acknowledgements

This work was initiated in the framework of a col!laborative research project between Ecole NormaleSupe�rieure\ University Paris 5 and several Vietnameseinstitutions\ including the National University of Viet!nam in Hanoi[ We warmly thank Prof[ Tong Dzuy Thanhand Xavier Le Pichon for leading the project[ TOTALOil + Gas provided us with invaluable help\ includinglogistic support and access to data[ This work bene_tedfrom discussions with P[ Ge�rard and B[ Simon "TOTAL#\


Fig[ 09[ Location of {sections| "small circles about Briais et al[|s "0882# rotation pole for anomaly 5a# used for the computation of the amount ofstretching in Fig[ 00[ The area used for the computation is shaded[ Bold line] x!axis of Fig[ 00[


Fig[ 00[ Amount of stretching across the Nam Con Son basin\ computed from Fig[ 8 along small circles about the rotation poles of "Briais et al[\0882# corresponding to magnetic anomalies 4d\ 5\ 5a and 5b "see Fig[ 09#[ The amount of stretching across the Cuu Long basin is also shown[

Fig[ 01[ Average amount of stretching across the Nam Con Son basin compared to the motion predicted by the oceanic kinematics for anomalies 4d\5\ 5a and 5b[


Fig[ 02[ Structural sketch showing the relationship between the south Vietnam basins and the South China Sea[ Black arrows] Oligocene to EarlyMiocene strike slip motions[ Open arrows] Early to Middle Miocene strike slip motions[ Star] pole of opening of the South Vietnam basin[

and C[ Rangin[ The assistance of PetroVietnam in com!piling the database is greatly acknowledged[ Commentsby R[ Hall and an anonymous referee helped to improvethe manuscript[

References

Academia Sinica "0878#[ Report on Comprehensive Investigation andresearch of Nansha Islands and Adjacent Maritime Regions[ Beij!ing] Academia Sinica Publishing House[

Be�nard\ F[\ Muller\ C[\ Letouzey\ J[\ Rangin\ C[\ + Tahir\ S[ "0889#[Evidence of multiphase deformation in the Rajang!Crocker Range"northern Borneo# from Landsat imagery interpretation] geo!dynamic implications[ Tectonophysics\ 072\ 210Ð228[

Briais\ A[\ Patriat\ P[\ + Tapponnier\ P[ "0882#[ Updated interpretationof magnetic anomalies and sea~oor spreading stages in the South

China Sea] implications for the Tertiary tectonics of Southeast Asia[J[ Geophys[ Res[\ 87"B3#\ 5188Ð5217[

Briais\ A[\ Tapponnier\ P[\ + Pautot\ G[ "0878#[ Constraints of Seabeamdata on crustal fabrics and sea~oor spreading in the South ChinaSea[ Earth Planet[ Sci[ Lett[\ 84\ 296Ð219[

Bui\ C[ Q[ "0882#[ Some characteristics of the deep crustal structure andthe geodynamics in the territory of Viet Nam and neighbouring seaarea[ Journ[ Geol[ "GS Vietnam#\ 0Ð1\ 28Ð49[

CCOP[ "0880#[ Total sedimentary isopach maps o}shore southeastAsia[ CCOP Tech[ Bull[\ 12\ 44Ð89[

Chamot!Rooke\ N[\ Gaulier\ J[ M[\ + Jestin\ F[ "0886#[ Constraints onMoho depth and crustal thickness in the Liguro!Provencžal basinfrom a 2D gravity inversion] geodynamic implications[ Rev[ Inst[ fr[Pe�tr[\ 45"5#\ 446Ð472[

Coulon\ O[\ Huchon\ P[\ Hai\ N[ T[ N[\ Marquis\ G[\ Chamot!Rooke\N[\ Le Pichon\ X[\ Thi\ P[ T[\ Rangin\ C[\ + Team\ T[ P[ "0884#[Tectonic structure and age of the South China Sea fossil propagatingridge[ In Cenozoic evolution of the Indochina peninsula\ Vietnam]Hanoi + Do Son[ pp[ 40Ð40[


Do\ B[\ Phan\ H[ Q[\ Pham\ H[ Q[\ + Tran\ L[ D[ "0882#[ Tertiarystratigraphy of continental shelf of Vietnam[ In 0st Intern[ seminaron the stratigraphy of the southern shelf of Vietnam[ Ho Chi MinhCity\ pp[ 0Ð03[

Ginger\ D[ C[\ Ardjakusumah\ W[ O[\ Hedley\ R[ J[\ + Pothecary\ J["0882#[ Inversion history of the West Natuna basin] example fromthe Cumi!Cumi PSC[ In Proceedin`s of the 11nd annual convention\Vol[ 0\ "pp[ 524Ð547#[ Jakarta] Indonesian Petroleum Association[

Hamilton\ W[ "0868#[ Tectonics of the Indonesian Region[ U[S[ Govern!ment Printing O.ce\ U[S[ Government Printing O.ce\ 234[

Hall\ R[ "0885#[ Reconstructing Cenozoic SE Asia[ In R[ Hall + D[Blundell\ "Eds#\ Tectonic evolution of southeast Asia\ Vol[ 095\ "pp[042Ð073#[ London] Geol[ Soc[ Spec[ Publ[

Harder\ S[ H[\ Mauri\ S[ J[\ + McCabe\ R[ "0881#[ Miocene right lateralMotion on the Red river Fault and in the Song Hong!YinggehaiBasin\ Viet Nam[ Trans[ Amer[ Geophys[ Union[ EOS\ 62"32#\ 423Ð424[

Harland\ W[ B[\ Amstrong\ R[ L[\ Cox\ A[ V[\ Craig\ L[ E[\ + Smith\A[ G[ "0889#[ A geologic time scale 0878[ Cambridge UniversityPress[

Hayes\ D[ E[ "0874#[ Margins of the southwest sub!basin of the SouthChina Sea*A frontier exploration target< Ener`y\ 09\ 262Ð271[

Hayes\ D[ E[\ Nissen\ S[ S[\ Buhl\ P[\ + Diebold\ J[ "0884#[ Through!going crustal faults along the northern margin of the South ChinaSea and their role in crustal extension[ J[ Geophys[ Res[\ 099"B00#\11\ 324Ð11\ 335[

Hinz\ K[\ + Schlu�ter\ H[ U[ "0874#[ Geology of the Dangerous Grounds\South China Sea\ and the continental margin o} Southwest Pal!awan] Results of Sonne cruises SO!12 and SO!16[ Ener`y\ 09\ 171Ð177[

Holloway\ N[ H[ "0871#[ North Palawan Block\ Philippines\ Its relationto Asian mainland and role in evolution of the South China Sea[Am[ Assoc[ Petr[ Geol[ Bull[\ 55\ 0244Ð0272[

Huchon\ P[\ Le Pichon\ X[\ + Rangin\ C[ "0883#[ Indochina peninsulaand the collision between India and Eurasia[ Geolo`y\ 11\ 16Ð29[

Jolivet\ L[\ Huchon\ P[\ + Rangin\ C[ "0878#[ Tectonic setting of westernPaci_c marginal basins[ Tectonophysics\ 059\ 12Ð36[

Kanyuan\ X[\ Ciliu\ H[\ Shaoren\ J[\ Yixiang\ Z[\ Daquan\ S[\ Sigao\X[\ + Zhongrong\ C[ "0883#[ Comparison of the tectonics and geo!physics of the major structural belts between the northern andsouthern continental margins of the South China Sea[ Tecton!ophysics\ 124\ 88Ð005[

Le Pichon\ X[\ Huchon\ P[\ Rangin\ C[\ + Coulon\ O[ "0884#[ Formationof Indochinese continental margin and of the South China Sea] factsand questions[ In Cenozoic evolution of the Indochina peninsula\ "p[099#[ Vietnam] Hanoi + Do Son[

Le\ V[ K[ "0875#[ The structure of the Mekong trough[ InternationalGeoloìcal Review\ 17"0#\ 76Ð84[

Lee\ T[!Y[\ + Lawver\ L[ A[ "0883#[ Cenozoic plate reconstruction ofthe South China Sea region[ Tectonophysics\ 124\ 038Ð079[

Leloup\ P[ H[\ Lacassin\ R[\ Tapponnier\ P[\ Scha�rer\ U[\ Dalai\ Z[\Xiaohan\ L[\ Liangshang\ Z[\ Shaocheng\ J[\ + Trinh\ P[T[\ "0884#[The Ailao Shan!Red River shear zone "Yunnan\ China#\ Tertiarytransform boundary of Indochina[ Tectonophysics\ 140\ 2Ð73[

Letouzey\ J[\ Sage\ L[\ + Mu�ller\ C[ "0877#[ Geological and structuralmap of eastern Asia\ Introductory notes[ Amer[ Ass[ Petrol[ Geol[Bull[\ 41[

Li\ Pinglu\ + Rao\ C[ "0883#[ Tectonic characteristics and evolutionhistory of the Pearl River Mouth basin[ Tectonophysics\ 124\ 02Ð14[

Mai\ T[ T[ "0884#[ Seismic!stratigraphic studies of the continental shelfof southern Vietnam[ Journ[ Petrol[ Geol[\ 07"2#\ 234Ð243[

Marquis\ G[\ Roques\ D[\ Huchon\ P[\ Coulon\ O[\ Chamot!Rooke\N[\ Rangin\ C[\ + Pichon\ X[ L[ "0886#[ Amount and timing ofextension along the continental margin o} central Vietnam[ Bull[Soc[ Ge�ol[ France\ 057"5#\ 04Ð13[

Matthews\ S[ J[\ Fraser\ A[ J[\ Lowe\ S[\ Todd\ S[ P[\ + Peel\ F[ J["0886#[ Structure\ stratigraphy and petroleum geology of the south

east Nam Con Son basin\ o}shore Vietnam[ In A[ J[ Fraser\ S[ J[Matthews\ + R[ W[ Murphy\ "Eds#\ Petroleum Geolo`y of SouthEast Asia\ "pp[ 78Ð095#[ Geol[ Soc[ Spec[ Publ[ No[ 015\ London]Burlington House[

Matthews\ S[ J[\ + Todd\ S[ P[ "0882#[ A tectonostratigraphic modelfor the southern Nam Con Son basin\ o}shore Vietnam[ In FirstInt[ Seminar on the Strati`raphy of the souther Vietnam shelf[ HoChi Minh City\ Vietnam[

Mat!Zin\ I[ C[\ + Swarbrick\ L[ E[ "0886#[ The tectonic evolutionand associated sedimentation history of Sarawak Basin\ easternMalaysia] a guide for future hydrocarbon exploration[ In A[ J[Fraser\ S[ J[ Matthews\ + R[ W[ Murphy\ "Eds#\ Petroleum Geolo`yof South East Asia\ "pp[ 126Ð134#[ Geol[ Soc[ Spec[ Publ[ No[ 015\London] Burlington House[

Mauri\ S[ J[ "0882#[ Gravity modeling of cenozoic extensional basins\o}shore Vietnam[ Texas A+M] MS thesis[

Mingard\ H[ F[\ + Willett\ E[ A[ F[ "0882#[ Pre!Miocene structural andstratigraphic development at the junction of the south Con Son andwest Natuna basins*Implications for hydrocarbon prospectivityof continental shelf of Vietnam[ In 0st International seminar on thestratigraphy of the southern shelf of Vietnam[ Ho Chi Minh City\Vietnam[

Nga\ K[\ Madon\ M[\ + Tjia\ H[ D[ "0885#[ Role of pre!Tertiary frac!tures in formation and development of the Malay and Penyu basins[In R[ Hall\ + D[ Blundell\ "Eds#\ Tectonic evolution of southeastAsia\ Vol[ 095\ "pp[ 170Ð178#[ London] Geol[ Soc[ Spec[ Publ[

Nissen\ S[ S[\ Hayes\ D[ E[\ Bochu\ Y[\ Weijun\ Z[\ Yongqin\ C[\ +Xiaupin\ N[ "0884a#[ Gravity\ heat ~ow\ and seismic constraints onthe process of crustal extension] northern margin of the South ChinaSea[ J[ Geophys[ Res[\ 099"B00#\ 11\ 336!11\ 372[

Nissen\ S[ S[\ Hayes\ D[ E[\ Buhl\ P[\ Diebold\ J[\ Bochu\ Y[\ Weijun\Z[\ + Yongqin\ C[ "0884b#[ Deep penetration seismic soundingsacross the northern margin of the South China Sea[ J[ Geophys[Res[\ 099"B00#\ 11\ 396!11\ 322[

Parker\ R[ L[ "0861#[ The rapid computation of potential anomalies[Geophys[ Journ[ Royal astron[ Soc[\ 20\ 336Ð344[

Pautot\ G[\ Rangin\ C[\ Briais\ A[\ Tapponnier\ P[\ Beuzart\ P[\ Leric!olais\ G[\ Mathieu\ X[\ Wu\ J[\ Han\ S[\ Li\ H[\ Lu\ Y[\ + Zhao\ J["0875#[ Spreading direction in the Central South China Sea[ Nature\210\ 049Ð043[

Peltzer\ G[ "0872#[ Naissance et e�volution des de�crochements lors d|unecollision continentale\ approche expe�rimentale\ application a� la tec!tonique de l|est de l|Asie[ Univ[ Paris 6] PhD thesis[

Peltzer\ G[\ + Tapponnier\ P[ "0877#[ Formation an devolution ofstrike!slip faults\ rifts and basins during the India!Asia collision] anexperimental approach[ J[ Geophys[ Res[\ 82"B01#\ 04974Ð04006[

Pigott\ J[ D[\ + Ru\ K[ "0883#[ Basin superposition on the northernmargin of the South China Sea[ Tectonophysics\ 124"0Ð1#\ 16Ð49[

Prouteau\ G[\ Maury\ R[ C[\ Rangin\ C[\ Suparka\ E[\ Bellon\ H[\Pubellier\ M[\ + Cotten\ J[ "0885#[ Les adakites mioce�nes du nord!ouest de Borne�o\ te�moins de la fermeture de la proto!Mer de Chine[Comptes rendus Acad[ Sci[\ 212\ 814Ð821[

Rangin\ C[ "0885#[ Tectonics of cenozoic sedimentary basins in SE Asia[In F[ Roure\ N[ Ellouz\ V[ S[ Shein\ + I[ I[ Skvortsov\ "Eds#\Geodynamic evolution of sedimentary basins\ "pp[ 240Ð256#[ Paris]Technip[

Rangin\ C[\ Bellon\ H[\ Benard\ L[\ Letouzey\ J[\ Mu�ller\ C[\ + Sanudin\T[ "0889#[ Neogene arc!continent collision in Sabah\ Northern Bor!neo "Malaysia#[ Tectonophysics\ 072\ 294Ð208[

Rangin\ C[\ Huchon\ P[\ Bellon\ H[\ Nguyen Dinh\ H[\ Le Pichon\ X[\Phan Van\ Q[\ + Roques\ D[ "0884a#[ Cenozoic tectonics of centraland south Vietnam] evidences for superposed tectonic regimes[ Tec!tonophysics\ 140\ 068Ð085[

Rangin\ C[\ Jolivet\ L[\ Pubellier\ M[\ + Tethys Paci_c working group["0889#[ A simple model for the tectonic evolution of Southeast Asiaand Indonesia region for the past 32 m[y[ Bull[ Soc[ è�ol[ France\7"5#\ 778Ð894[


Rangin\ C[\ Klein\ M[\ Roques\ D[\ Le Pichon\ X[\ + Le Van Trong["0884b#[ The Red River Fault system in the Tonkin gulf\ Viet Nam[Tectonophysics\ 132\ 198Ð111[

Roques\ D[\ Matthews\ S[ J[\ + Rangin\ C[ "0886a#[ Constraints onstrike!slip motion from seismic and gravity data along the Vietnammargin o}shore Danang] implications for hydrocarbon pros!pectivity and opening of the East Vietnam Sea[ In A[ J[ Fraser\ S[J[ Matthews\ + R[ W[ Murphy\ "Eds#\ Petroleum Geolo`y of SouthEast Asia\ "pp[ 130Ð242#[ Geol[ Soc[ Spec[ Publ[ No[ 015\ London]Burlington House[

Roques\ D[\ Rangin\ C[\ + Huchon\ P[ "0886b#[ Geometry and senseof motion along the Vietnam continental margin] onshore:o}shoreDa Nang area[ Bull[ Soc[ è�ol[ France\ 057"3#\ 302Ð311[

Ru\ K[\ + Pigott\ J[ D[ "0875#[ Episodic rifting and subsidence in theSouth China Sea[ Am[ Ass[ Petrol[ Geol[ Bull[\ 69\ 0025Ð0044[

Sandwell\ D[ T[\ + Smith\ W[ H[ F[ "0881#[ Global marine gravity fromERS!0\ Geosat and Seasat reveals new tectonic fabric[ EOS\ Trans[Amer[ Geophys[ Union\ 62"32#\ 022[

Shaoren\ J[\ Xiaozhong\ Z[\ Ciliu\ H[\ + Kanyuan\ X[ "0883#[ Thestratigraphy\ structure and evolution of the Zengmu basin[ Tectono!physics\ 124"0Ð1#\ 40Ð51[

Sha�rer\ U[\ Tapponnier\ P[\ Lacassin\ R[\ Leloup\ P[ H[\ Dalai\ Z[\ +Shaocheng\ J[ "0889#[ Intraplate Tectonics in Asia] A precise Agefor Large Scale Miocene movements along the Ailao Shan RiverShear Zone\ China[ Earth Planet[ Sc[ Lett[\ 86\ 54Ð66[

Talwani\ M[\ + Ewing\ M[ "0859#[ Rapid computation of gravitationalattraction of three!dimensional bodies of arbitrary shape[ Geophys!ics\ 14"0#\ 129Ð114[

Tapponnier\ P[\ Lacassin\ R[\ Leloup\ P[ H[\ Scha�rer\ U[\ Dalai\ Z[\Haiwei\ W[\ Xiaohan\ L[\ Shaocheng\ J[\ Liangshen\ Z[\ + Jiayou\Z[ "0889#[ The Ailao ShanÐRed River metamorphic belt] tertiary

left!lateral shear between Sundaland and South China[ Nature\ 232\320Ð326[

Tapponnier\ P[\ Peltzer\ G[\ + Armijo\ R[ "0875#[ On the mechanics ofthe collision between India and Asia[ In M[ P[ Coward\ + A[ C[Ries\ "Eds#\ Collision tectonics\ Vol[ 08\ "pp[ 004Ð046#[ London]Geol[ Soc[ Spec[ Publ[

Tapponnier[ P[\ Peltzer\ G[\ Dain\ A[ Y[ L[\ Armijo\ R[\ + Cobbold\ P["0871#[ Propagating extrusion tectonics in Asia] new insights fromsimple experiments with plasticine[ Geolo`y\ 09\ 500Ð505[

Taylor\ B[\ + Hayes\ D[ E[ "0879#[ The tectonic evolution of the SouthChina Sea basin[ In D[ E[ Hayes\ "Ed[#\ The tectonic and èoloìcevolution of southern asian seas and islands*Part 0\ Vol[ 12\ "pp[78Ð093#\ Washington] Amer[ Geophys[ Union[ Geophys[ Mono`r[

Taylor\ B[\ + Hayes\ D[ E[ "0872#[ Origin and history of the SouthChina Sea basin[ In D[ E[ Hayes "Ed[#\ The tectonic and èoloìcevolution of Southeast Asian Seas and Islands\ Part 1\ Vol[ 16\ "pp[12Ð45#[ A[ G[ U[\ Washington\ D[C[] Geophys[ Mono`r[ Ser[

Tongkul\ F[ "0883#[ The geology of northern Sabah\ Malaysia] itsrelationship to the opening of the South China Sea basin[ Tec!tonophysics\ 124"0Ð1#\ 020Ð036[

Truong\ M[\ Le\ V[ D[\ + Nguyen\ M[ H[ "0880#[ Application of seismicmethod for delineation and prognosis of stratigraphic traps ofhydrocarbons in Cenozoic basins on the continental shelf of Viet!nam[ In 1nd conference on the Geology of Indochina\ vol[ 1\ "pp[ 266Ð278#[ Hanoi] Geological Survey of Vietnam[

Wang\ C[\ + Sun\ Y[ "0883#[ Development of Paleogene depressionsand deposition of lacustrine source rocks in the Pearl River MouthBasin\ northern margin of the South China Sea[ Amer[ Ass[ Petrol[Geol[ Bull[\ 67"00#\ 0600Ð0617[

Yu\ H[!S[ "0883#[ Structure\ stratigraphy and basin subsidence of Ter!tiary basins along the Chinese southeastern continental margin[Tectonophysics\ 124\ 52Ð65[

Documents

Finite extension across the South Vietnam basins from 2D