Updated Interpretation of Magnetic Anomalies and
Seafloor Spreading Stages in the South China Sea :
Implications for the Tertiary Tectonics of Southeast Asia
Anne Briais, Philippe Patriat, Paul Tapponnier
To cite this version:
Anne Briais, Philippe Patriat, Paul Tapponnier. Updated Interpretation of Magnetic Anomalies
and Seafloor Spreading Stages in the South China Sea : Implications for the Tertiary Tectonics
of Southeast Asia. Journal of Geophysical Research : Solid Earth, American Geophysical
Union, 1993, pp.VOL. 98, NO. B4, PAGES 6299-6328.
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JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 98, NO. B4, PAGES 6299-6328, APRIL 10, 1993
UpdatedInterpretationof MagneticAnomaliesandSeafloorSpreading
Stagesin the SouthChinaSea' Implicationsfor
the Tertiary Tectonicsof SoutheastAsia
ANNEBRIAIS1, PHILIPPE
PATRIAT,ANDPAULTAPPONNIER
Institutde Physiquedu Globede Paris
We presentthe interpretationof a new set of closelyspacedmarine magneticprofilesthat complements
previousdatain the northeastern
andsouthwestern
partsof the SouthChinaSea(Nan Hai). This interpretation
shows that seafloor spreadingwas asymmetricand confirms that it included at least one ridge jump.
Discontinuitiesin the seafloorfabric, characterizedby large differencesin basementdepth and roughness,
appearto be relatedto variationsin spreadingrate. Betweenanomalies11 and7 (32 to 27 Ma), spreadingat an
intermediate,averagefull rate of •50 mm/yr createdrelatively smoothbasement,now thickly blanketedby
sediments.The ridge thenjumpedto the southandcreatedroughbasement,now muchshallowerandcovered
with thinnersedimentsthanin the north. This episodelastedfrom anomaly6b to anomaly5c (27 to •16 Ma)
andthe averagespreadingratewas slower,•35 mm/yr. After 27 Ma, spreadingappearsto havedevelopedfirst
in the easternpartof the basinandto havepropagated
towardsthe southwestin two majorsteps,at the time of
anomalies6b-7, and at the time of anomaly6. Each stepcorrelateswith a variationof the ridge orientation,
from nearlyE-W to NE-SW, and with a variationin the spreadingrate. Spreadingappearsto have stopped
synchronously
alongthe ridge, at about15.5 Ma. From computedfits of magneticisochrons,we calculate10
polesof finite rotationbetweenthe timesof magneticanomalies11 and5c. The polespermitreconstruction
of
the Oligo-Miocenemovementsof SoutheastAsianblocksnorthand southof the SouthChinaSea. Using such
reconstructions,
we testquantitativelya simplescenariofor the openingof the seain whichseafloor spreading
resultsfrom the extrusionof Indochinarelative to SouthChina, in responseto the penetrationof India into
Asia. This alone yields between500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear
zone,with crustalshorteningin the SanJiangregionandcrustalextensionin Tonkin. The offsetderivedfrom
the fit of magneticisochronson the SouthChinaSeafloor is compatiblewith the offsetof geologicalmarkers
northand southof the Red River Zone. The first phasesof extensionof the continentalmarginsof the basin
are probablyrelatedto motionon the Wang ChaoandThree PagodasFaults,in additionto the Red River Fault.
That Indochinarotatedat least 12ø relative to South China implies that large-scale"domino"modelsare
inadequateto describethe Cenozoictectonicsof SoutheastAsia. The cessationof spreadingafter 16 Ma
appearsto be roughlysynchronous
with the final incrementsof left-lateralshearandnormaluplift in theAilao
Shan(18 Ma), aswell aswith incipientcollisionsbetweenthe Australianandthe Eurasianplates. Henceno
other causesthan the activation of new fault zones within the India-Asia collision zone, north and east of the
Red River Fault, andperhapsincreasedresistanceto extrusionalongthe SE edgeof Sundaland,appearto be
requiredto terminateseafloorspreadingin the largestmarginalbasinof the westernPacificandto changethe
senseof motionon the largeststrike-slipfault of SE Asia.
INTRODUCTION
havebeenbasedona limitednumberof marineprofiles,on structural
evidence
alongthemargins,
andonthedirections
of magnetic
The SouthChinaSea (Nanhai),andbasinscontiguous
to it,
part of the sea. All these
covera surface
of 2.32106km2. Theyaretheresultof a large anomaliesidentifiedin the easternmost
early attemptsconcuron a roughly N-S directionof seafloor
amountof extension,includingthe creationof seafloor,within a
spreading,
andon a fixedpositionof BorneoandIndochinarelative
continental
massthatmayhaveextended
fromtheMalayPeninsula
to
South
China
duringtheopeningof thesea. Combining
predicto the Philippines between South China and the Sunda shelf.
tionsderivedfrom laboratorymodelsof the India-Asiacollision
Extensionstartedin the Paleoceneand stoppedin the middlewith
geological
evidencesuggesting
thattheCenozoictectonics
of
Miocene[TaylorandHayes,1980, 1983;Hinz andSchhiter,1985;
regions
surrounding
the
South
China
Sea
were
dominated
by
leftRu andPigott, 1986]. A significantamountof shortening
followed,mostlyalongtheeasternandsouthern
marginsof thesea, lateralstrike-slipfaulting,Peltzeret al. [1982],Tapponnieret al.
[1988]haveadvocated
a
fromthemiddleMioceneto thepresent[Holloway,1982;Fricaut, [1982,1986]andPeltzerandTapponnier
differentview. Theysuggested
thatthecollisionbetweenIndiaand
1984]. Becausethe SouthChinaSeais largeandsurrounded
on
AsiahadrotatedandpushedIndochinaandBorneotowardstheSE,
threesidesby largecontinental
blocks,a quantitative
understanding
leadingto theopeningof theSouthChinaSeaandrelatedbasinsas
of itsopening
historyisimportant
forunderstanding
thetectonics
of
terminal
pull-apart
basins
attheextremities
of theRedRiver,Wang
Southeast
Asia in Mid-Tertiarytime.
ChaoandThree Pagodasfaults. None of thesestudies,however,
Previousattemptsto describethe formationandgrowthof the
provided
a complete,
quantitative
reconstruction
of theopening
of
SouthChinaSea[TaylorandHayes,1980,1983;Holloway,1982]
theSouthChinaSeausingthepowerfulconstraints
givenby magnetic isochrons and seafloor fabric.
1NowatObservatoire
Midi-Pyrfinfies,
GRGS,Toulouse,
France.
In thispaper,we presentanupdated
interpretation
of themagCopyright
1993by theAmerican
Geophysical
Union.
neticdatain thebasin,madepossible
by thedetailedanalysis
of a
densenewsetof profiles[S.Chen,1987]anddiscuss
theimplicaPapernumber92JB02280.
tionsof thisinterpretation
for the evolutionof the SouthChinaSea
0148-0227/93/92-JB02280505.00
6299
6300
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
spreading
ridge. After recallingpreviousidentifications
of themagnetic anomalies in the basin, we describe our own identification of
magneticlineations.We thenpresentthekinematicparameters
of
spreadingcomputedfrom the fit of the magneticisochrons
andanalyze thecharacteristics
of the spreading
andits evolutionin space
andtime. Finally,we investigate
whetherthecomputed
reconstruction of the openingof thebasinis consistent
with what is knownof
the deformationof the adjacentcontinentalblocks. From thiscom-
parativeanalysis,we derive a sequenceof schematicTertiary
palinspastic
reconstructions
of SoutheastAsia. By includingthe
geologicalevidenceon sedimentarybasinssurroundingthe area
flooredby oceaniccrust,thereconstructions
maybe extrapolated
to
the initial stagesof crustalextension,whichled to the formationof
the pull-apartsand rifts of the Sundashelfand of the SouthChina
andNorthBorneomargins.Our palinspastic
scenariois compared
to scenariosbasedon differentsetsof data,concerningeitherthe
evolutionof the west Pacific [Jolivetet al., 1989], or that of the
India-Asiacollisionzone[Peltzerand Tapponnier,1988].
PREVIOUS
STUDIES
Sincethereexistsno deepseadrilling corein the SouthChina
Basin,the identificationof the magneticanomaliesprovidesthe
mostimportantconstrainton the age of the seafloor.Bowinet al.
[1978]initiallyrecognized
magneticlineations
trendingN70øEona
few profilesnearLuzonIsland. TaylorandHayes[1980]thencorrelatedmagneticprofilesin theeasternpartof thebasinwith a geomagneticreversaltime scale. They identifiedmagneticanomalies
The Sea Beam bathymetricdata collectedduringthe 1985
Charcot
cruises
revealed
predominantly
NE andNW strikingtopographicscarpsin the 200-km-wideaxial regionof theentireSouth
ChinaBasin. This homogeneous
fabricsuggested
thatwhetherin
theeastor in thesouthwest
theaxialseafloorwasgenerated
by a
spreading
axisconsisting
of segments
strikingNE-SW [Pautotet
al., 1986],dissected
in theeastby numerous
right-lateral
transform
faultsthatmaintain
theoverallE-W trendof theaxisthere[Briaiset
al., 1989]. Sinceno detailedinformationon thestructuralfabricof
theseafloor
existsfartheroff-axisto thenorthor south,correlating
themagnetic
anomalies
between
closelyspaced
profilesis theonly
way to constrain
thedirectionof spreading
anditsevolutionin time.
MAGNETIC ISOCHRONSIN THE SOUTH CHINA SEA
New Data Set
Themostvaluable
newsource
of magnetic
datais themapof
closelyspacedprofilescompiledby S. Chen[1987]at the Second
MarineGeologicalInvestigation
Brigade(SMGIB) of the Chinese
Ministryof GeologyandMineralResources
(Figure1). We digitizedthemagnetic
anomalyprofilesfromthemap,to obtaina setof
datathat couldbe easilyprojectedand processed.With a mean
spacingof 10 nauticalmiles(•18 km) betweenprofiles,thisdata
set is the first to provideresolutionsufficientfor a detailedand
quantitative
analysisof the evolutionof the spreading
ridge. The
locations
of theSMGIB, NANHAI andMASIN profilesareshown
in Figure1. In additionto thesenew data,our analysisincludes
the previouslypublishedConradandVernaprofiles[Hayesand
Taylor,1978;TaylorandHayes,1980,1983],whicharenotrepresentedin thefiguresto keepthemreadable.Themagneticprofiles
drawnfromtheSMGIB mapshowgoodconsistency
with previous
profilesguidedby accurate
satellitenavigation.In particular,the
SMGIB magneticdatamatchdatafrom othercruisesat crossing
points,implyingthattheprocessing
of thedataandtheirdraftingon
themaparecorrect.
11 to 5d, thusdatingthe seafloorto be between32 and 17 million
years old. With additionaldata, the sameauthors[Taylor and
Hayes, 1983] revisedtheir distributionof fracturezones,and discardedanomaly5d asreflectingonly the disturbance
by seamounts
of themagnetics
closeto theridgeaxis.
Insufficientevidencein thesouthwestern
partof theSouthChina
Seaat thattimepreventeddatingof thatpartof thebasin. Taylor
and Hayes [1983] nevertheless
inferredthe magneticlineationsto
trendNE-SW, andheatflow measurements
[Watanabeet al., 1977;
TaylorandHayes,1983] suggested
an earlyMioceneage. Since Methods
then, new magneticprofileshave been collectedby the R/V J.
CharcotduringtheFrenchNANHAI andMASIN cruises,andby
Our analysisof the magneticanomaliesin the SouthChina Sea
vesselsfrom Chineseinstitutions
(Figure1). Theseprofilesarean differsfrom previousanalysesin threeways. First, we chosea
importantadditionto the set of availabledata,especiallyin the geomagnetictime scalespecificallyadjustedfor ridgeswith half
northwestern and southwestern subbasins, where the seafloor spreadingratesvarying from less than 10 mm/yr to 30 mm/yr,
fabric and age were not constrained. Nevertheless,while all suchasthe Mid-IndianandSouth-AtlanticRidges. The half rates
previousstudiesconcuruponthe Oligo-Mioceneageof theeastern of 20-30 mm/yrinferredby TaylorandHayes[1980, 1983] in the
basin,with minor differencesconcerningthe orientationof the SouthChinaSeafall withinthisrange. Second,sincethemagnetic
magneticlineationsand the existenceof a jump at the time of profilesarenumerous,butthe magneticanomaliessometimesdiffianomaly7 [Taylor andHayes, 1980, 1983; Watanabeet al., 1977; cultto identifydueto asymmetric
spreading
ratesandridgejumps,
Lu et al., 1987], thereis still no consensus
aboutthe age of the we systematically
testedthemagneticisochrons
by fittingidentified
southwesternbasin. Using data from new R/V R. D. Conrad conjugate
isochrons.The goalwasto obtaina goodsuperposition
cruises RC2612 and RC2614, Hayes et al. [1987] identify of magneticisochrons
fromeithersideof theaxis,andto geta conanomalies
6 to 5d andthusinfera Mioceneagefor thesouthwestern sistentseriesof isochrons,
undertheassumption
thatno majordifpartof the basin. In contrast,Lu et al. [1987], usingthe sameset ferential strain occurred within the oceanic crust since its creation.
of Chinesedata that we use in this study, infer a large age Thiscombination
of identifyingtheanomalies
by comparison
with
discrepancy
betweentheeasternandsouthwestern
subbasins.
They syntheticprofiles,and checkingthe identificationby fitting the
identify anomalies32 to 27 (70-63 Ma), oriented NE-SW, isochrons,
helpeduschoosebetweenalternativesolutions
in certain
southwestof MacclesfieldBank, and anomalies11 to 5D (32- areas.Finally,we complemented
themagneticdatawith thestratig17 Ma) to the east, with orientationsswingingfrom ENE for raphyof sediments
coveringthe oceanicfloor or deposited
on the
anomalies11-8 to E-W for anomalies7-5d. Consequently,
they margins,the depthand structuralfabric of the seafloor,the evoludistinguishthree episodesof spreadingin the evolutionof the tion of the marginsas suggested
by wells and subsidence
studies,
basin,thefirstin Cretaceous-Paleocene
time,thelasttwo in Oligo- the heatflow and the free air gravityanomalies.Suchadditional
Miocenetime. The mostpeculiarfeatureof theiranalysisis the30- dataservedto guide our final identificationsand choosebetween
m.y.-longlapsein seafloorspreading
in theearlyTertiary.
varioussequences
implying different agesfor the oceaniccrust.
110ø
112ø
114ø
,
116ø
118ø
120øE
N
N
22 ø
22 ø
20 ø
20 ø
+
A
18 ø
\
18 ø
+
16 ø
16 ø
14 ø
14 ø
12 ø
12 ø
ee
ß
ß
ß
ß
eeee
eeeeeeeeeee
eeeee
10 ø
10 ø
N
N
110 ø
112 ø
114 ø
116 ø
118 ø
120øE
Fig1. Location
andnumber
of magnetic
anomaly
profilesusedin thisstudy.SolidlinesareChinese
data(mostlyfromS.Chen
[1987]),dashed
linesFrench
data.Magnetic
anomalies
identified
byTaylorandHayes[1983]areshown
asboldlines.Dottedline
is theapproximate
limit of oceanic
crust.Areas1, 2, 3 areeastern,
northwestern
andsouthwestern
subbasins,
respectively,
as
referred
to in text. Bathymetry
is in meters.Majorseamounts
areshaded.BoxesA toD showlocations
of Figures
3a,3b,6a,and
6b, respectively.
6302
BRIAIS ET AL.' RECONSTRUCTIONSOF THE SOUTH CHINA SEA
Thisapproach
permitsa coherent
interpretation
in smallbasins
such
asmarginal
seas,eventhough
profilesareshortanddifficultto correlatewith a uniquesequence
of magneticreversals,especially
TABLE 1. MagneticReversalTime ScaleUsedin This Study
NormalPolarityIntervals,Ma
whentheaxial ageis unknown.
10.36-
10.42
TaylorandHayes[1980,1983]choseto basetheirinterpretation
on themagnetictimescaleof LaBrecqueet al. [1977]. We have
chosen
to usea slightlymodifiedversionof themagnetic
timescale
described
by Patriat [1987](Table1). In particular,thesequence
of anomalies5 to 13 was adjustedby Patriat [1987] so that
synthetic
magnetic
anomaly
profilesresemble
profilesobserved
not
only on fast and mediumspreadingridges,but also on slow
spreading
ridges. The reversaltime scaleusedhere(Table1,
Figure2) was mostly obtainedby fixing the agesof major
!0.54
14.74-
14.84
reversals
in thesuccession
of reversals
described
by Patriat [1987],
sothatin termsof absolute
agesit is comparable
to thatof Berggren
et al. [1985] (ends of anomaly 5:10.42 Ma, anomaly 6:
20.45 Ma, anomaly 8:27.74 Ma; beginningof anomaly 12:
15.01-
15.15
15.38-
15.42
Anomaly
5
- !0.60
10.74-
10.77
11.04-
11.11
11.63-
11.70
11.94-
12.19
12.55-
12.61
12.88-13.03
13.2713.76-
!4.23
13.49
14.04
- !4.53
15.61-15.64
15.84-
15.88
16.08 - 16.33
5c
32.46Ma). For the sequence
of anomalies5c to 13, both the
16.47-
16.57
numberof reversals
andtheirrelativeagesdifferbetweenthescales
16.73-
16.91
ofLaBrecque
et al. [1977]andPatriat [1987](Figure2, Table1).
BothLaBrecque
et al. [1977]andPatriat [1987]addedshort-period
intervals (< 30,000 years) to change the shape of certain
17.11-
17.14
18.05-
18.14
anomalies,in order to obtain a better fit with observations.Short
18.44-
18.70
normalandreversepolarityintervalshavebeenaddedat thetimes
of anomalies5c and 5e, respectively,in Patriat'stime scale,
resulting
in a change
in therelativepositions
of reversals
relativeto
LaBrecque's
scale(Figure2). Shortreversepolarityintervalshave
alsobeenaddedto thelargenormalintervalof anomaly6, implying
a lowerrelativeamplitudefor anomaly6 andmakinganomalies
5e
and 6a more distinct from anomaly 6 at slow spreadingrates
[Patriat, 1987](Figure2, Table1). Botheffectsareobserved
on
profilesin the Indian Ocean,where the spreadingrate varies
between15 and60 mm/yr,whichsuggests
thatPatriat'stime scale
is morereliablethanpreviousonesfor interpreting
profilesfrom
ridgesspreadingat ratesof lessthan60 mm/yr. In calculating
synthetic
profiles,we alsohavetakenintoaccount
thefactthatthe
changefrom one anomalyto the next alonga magneticprofile
generallyresultsfroma progressive,
ratherthansharp,contrast
of
18.74-19.00
5e
!9.26
- !9.40
6
19.44-
19.54
6
19.58-
19.72
6
19.76-
19.96
6
19.99-
20.19
6
20.22-
20.45
6
20.73-
21.02
6a
21.31 - 21.73
6a
!7.36 - !7.58
17.63 - 17.81
5d
5e
22.00 - 22.18
22.46-
22.56
22.86-
23.03
23. !0 - 23.44
6b
23.77-
23.94
24.07-
24.32
24.59-
24.77
25.74-
25.85
7
25.91 - 26.10
7
magnetization
betweennormalandreverse-polarity
blocks[e.g.,
26.5 ! - 26.69
$chouten, 1971; Blakely and Cox, 1972; Tisseauand Patriat,
1981]. We usedthemethodof artificialratesdeveloped
by Tisseau
andPatriat [1981],in whichanartificialspreading
rateslowerthan
thatcorresponding
to themodelis chosen,andthehorizontalscale
adjustedto restorethepredictedlengthof the modeledmagnetic
profile. A prominenteffectof thisfilteringis to produceloweramplitudeanomalies,which are mostoften observedon slowspreadingridges.
The fit of conjugatemagneticisochrons
is the bestcheckof a
goodidentification
andyieldsthespherical
parameters
thatdescribe
the spreadingquantitatively.We usedtwo methodsto fit the
isochrons
andcalculatethe polesandanglesof rotation. The first
method,introduced
by Patriat [1987]anddiscussed
by $loanand
Patriat [1992], is basedon minimizingthe misfit areaobtained
when matchingthe two linesdefinedby the picksof conjugate
magneticanomalies.Representative
pointsare chosenon each
26.84-
26.95
8
27.02-
27.74
8
28.28-
28.82
9
28.88-
29.32
9
29.75 - 30.05
10
10
30.10-
30.32
31.34-
31.62
11
31.72-
32.09
11
32.46-
32.96
12
35.51 - 35.71
35.77-
36.12
13
Bold:pickedanomalies.
lastselectedleadsto a largermisfit [Patriat, 1987]. The second
methodis an inversemethodfirst described
by Hellinger [1981],
andrefinedby Chang[1987] andRoyerand Chang[1991], who
developed
thesoftwarewe used.Thepointsat whichisochrons
are
to definesegments
of eithermagnetic
lineations
isochron, to avoid disturbed areas such as transform offsets. pickedaregrouped
andtreatedasarcs
Startingwith a "firstguess"pole,we computethe corresponding or transformfaults,pairedbeforecomputation,
misfit, then thoseassociatedwith a seriesof poles at a chosen of greatcircles. The greatcirclesandthe rotationto fit themare
by leastsquares.The misfitbetweenthepointsandthe
distance(here 1ø) from the first one. The pole that yieldsthe determined
thecorresponding
95%confidence
smallestmisfitis retainedandtheoperationrepeated.The searchis greatcirclesis usedto compute
thesetof admissible
polesandanglesof
complete
whenanypoleat a chosen
distance
(here0.1o)aroundthe region,whichrepresents
BRIAISET AL.: RECONSTRUCTIONS
OFTHE SOUTHCHINA SEA
This Study
6303
LaBrecque(1977)
s
N
symmetrical
part
Patfiat
(1987)
15 Ma
Berggrenet al.
(•985)
-
5d
5e
20 Ma
-
6
6a
6b
6c
25 Ma
7
8
9
30 Ma
-
lO
11
10
11
12
35 Ma
-
Fig2. Comparison
of magnetic
reversal
timescales.Normalpolarityblocksaresolid.Symmetric
synthetic
profiles
calculated,
forscales
ofLaBrecque
etal. [1977]andthisstudy,assuming
a 400-m-thick
magnetized
layerundera ridgeoriented
E-W at 15øN,
117øE since its creation.
rotations.The confidenceregionmay be visualizedasan ellipsein Stage poles, describingthe rotation between two consecutive
thelatitude-longitude
plane. It becomesvery largewhenthe angle magneticanomalies,were thencomputedto characterize,stepby
of rotation is small.
We used this method to visualize
the
step,theevolutionof therateanddirectionof spreading.
Structuralobservations
were usedto separatethe eastern,northconfidenceregionsof the poles of rotations. A finite pole of
rotationwas obtainedindependentlyfor eachmagneticanomaly. westernandsouthwestern
oceanicsubbasins
(Figure1), andto re-
6304
BRIAISET AL.: RECONSTRUCTIONS
OFTHE SOUTHCHINA SEA
late the characteristics
of the fabricof the oceaniccrustin eachbasin
Old crustin theeasternbasin.The oldestmagneticlineations
that
to ourmodelof magnetic
isochrons.
The locationof therelict couldbe identifiedareanomaly11 in thenorthand10 in thesouth
32 Ma and 30 Ma, Figures3 and 4). Magnetic
spreading
axiscorresponding
to thelastspreading
episode
wasde- (respectively,
havea loweramplitudethanin thenorthducedfromthebathymetry
andthegeometryof inward-facing
nor- anomaliesin thesoutheast
east,
and
anomalies
8
to
10
are not observednear the southeastern
mal faultsobservedon seismicprofilesandSeaBeamswaths.In
margin(Figures3 and4a). We discuss
possiblecauses
the southwestsubbasin,this relict axis is alsomarkedby a promi- continental
in thesectionconcerning
reconstructions.
nentfreeairgravityanomalylow [TaylorandHayes,1983;Pautot for thisasymmetry
In the easternbasin, there is a greater number of magnetic
et al., 1986,1990;Hayeset al., 1987]. Alongtheaxialpartof the
eastern
basin,theScarborough
seamount
chain(Figure1) probably anomalies north than south of the Scarborough relict axis
lieson or closeto therelictaxis[TaylorandHayes,1983;Briais et (Figure4a). This asymmetricdistributionimpliesthat the ridge
al., 1989].Thefabricof the200-km-wide
axialpartof thatbasinis jumpedto the south,as Taylor and Hayes [1983] first inferred.
characterized
by a blockybasement,
with normalfaultsstriking Placingthisjumpjustafteranomaly7 providesthebestfittingsynENE to NE andfracturezonesstrikingNW, coveredby a sedimen- thetic magneticprofile (Figures3 and 4a). The two conjugate
tarylayerabout0.5-sthick(two-waytraveltime) [Pautotet al., anomalies7 are observedin the north. To the southeast,anomaly7
anomalies6b and8 arejuxta1986, 1990;Briais et al., 1989]. North and southeast
of the in- is missing,andthe large-amplitude
ferredrelictspreading
axis,thesediment
thickness
increases
signifi- posed(Figures3 and 4a). A fanningof the magneticlineations,
cantly,to 1.5-2 s (two-waytraveltime)andthebasement
becomes spanningabout20ø, is observednorthof the ridge,revealinga
center,fromN70øEfor
muchsmoother
[TaylorandHayes,1983]. To thesouthwest,
the changein theoveralltrendof thespreading
fabricis characterized
by normalfault scarpsstrikingmorehomo- anomaly10, to N80øE for anomaly8, and to E-W for the abancenter(Figure3). This reorientation
of theridge
geneously
NE andfracture
zonescarps
striking
NW [Pautotetal., donedspreading
by a reorganization
of theridgeaxis,which
1986;Briais et al., 1989]. The northwestern
subbasin
is a deep, systemis accompanied
thicklysedimented
oceanic
troughbetween
thepassive
continentalwe shall discussin the sectionon the evolutionof the spreading
marginof SouthChinaandthecontinental
blocksof Macclesfieldsystem.
Bank andParacelsIslands. To the east,this troughis continuous
Northwestern basin. Previousdata were too scarceto identify
in thenorthwestern
subbasin,
althoughseismic
withthedeepest
northern
partof theeastern
basinwhereTaylorand magneticanomalies
data
suggested
that
it
is
underlain
by
oceanic
crust.
The new proHayes [1980, 1983] identifiedmagneticanomalies8 to 11
(Figure1). Thelargeandsometimes
sharpchanges
observed
both files confirmthisview andrevealsymmetricallymagnetizedcrust.
in the seafloorfabric and in the sedimentthickness,which suggest Becausethisoceanictroughis particularlynarrow(•150 km), the
of observed
profileswith a magneticmodelis notunique
thatthe characteristics
of the spreading
changedabruptlyin time correlation
duringthe openingof the basin,clearlyrepresentfirst-order (Figure5). Nevertheless,only three sequences,consistingof
features
to be explained
by anykinematicmodelderivedfromthe anomalies10-11, 13-15 or 21-22, providean acceptablematchto
the observedprofiles. Of thesethreepossibilities,
the 10-11 semagneticdata.
quenceis mostconsistent
with structural
andstratigraphic
observations,becauseit allowscontinuous
spreadingoverthewholenorthIdentification
ofMagneticAnomalies
ernbasinin theearlyOligocene(Figures3, 4a and5).
Ridgejump at the timeof anomaly7 and correlativeridgereorEstimationof theageandoverallanomalysequence
in theeastern
basin. The severalattemptswe madeto fit the anomaliesobserved ganization. To the east, the ridge jump after anomaly7 is
observations
of its effecton theridge
on thelongestmagneticprofilesfrom theeasternsubbasin
with se- confirmedby morphological
quencesof the geomagnetic
time scaleconfirmthe Oligocene-early system. Since the conjugateanomalies7, which bound the
ridgeaxis,areseparated
by thesamedistance
all along
Mioceneageof the sequence
of anomalies,with spreadingratesof abandoned
the order of 20-30 mm/yr, as first inferredby Taylor and Hayes that ridge, we infer that the jump was simultaneousfor all
[1980, 1983] (Figures3 and 4a). Theseagespostdatethe rifting segments. The easternspreadingsegments,however,jumped
estimated
to havestartedin thePaleocene-Eocene
alongthenorthern farthersouththanthe westernones(Figures3 and 4a). The age
and southerncontinentalmarginsof the easternSouthChina Sea differenceat the boundarybetweenthe old and new crustis thus
[e.g.,Holloway,1982;Hinz andSchliiter,1985;Fricaut, 1984;Ru greatereastof 117øEthanto the west,as reflectedby the larger
stepobserved
betweenthesecrusts[Rea,1978;Patriat,
andPigott, 1986;Suet al., 1989]. The heatflow measurements
are topographic
alsoconsistent
with an earlyto mid-Tertiaryagefor theoceanfloor 1987]on seismicprofiles(Figure4b). Thejumpwasaccompanied
of theridgeaxis,characterized
by thedemiseof
[Watanabeet al., 1977;Anderson,1980; TaylorandHayes,1980, by a reorganization
1983]. The mosttypicalsequence
to be recognizedin the basinis the previousleft-steppinggeometry,and the formationof more
mostlyright-stepping
axialridgesegments
asobserved
the6b-6 anomalysequence.It is especiallyprominentin thenorth- continuous,
easternpart of the basin(Figures3 and 4). Nowhereelse in the on isochron6b (Figure3).
The best-fittingsyntheticprofile alsosuggests
a changein half
basin,to the southor southwest,is it found with the sametypical
rateat thetimeof thejump,from24 mm/yr(in thewest)
shape(Figures6 and 7). A priority in our interpretationof the spreading
anomaliesin the easternbasinwas to respectthe broad shapeof to 29 mm/yr (in the east)beforethejump, to 19 mm/yr afterthat
of
anomaly6, which is characteristicin mostoceanicbasins[e.g., jump (Figure4a). We infer that the increasein the roughness
Patriat, 1987], aswell asthe largeamplitudeof anomaly6b, which the seafloor observed on the seismic lines is due to the decrease in
rate(Figure4b). A similarincreasein the localroughis prominenton bothsidesof theridge(Figures3, 4a, 6 and7). It spreading
shouldbe notedthat the skewnessis suchthat a giventime in the nessof the oceaniccrustwith decreasingspreadingrate hasbeen
magnetictime scalecorresponds,
within theuncertaintyin picking observedalong the Indian Ocean ridges, which display half
of the anomalies,to a negativeanomalyto the northof the profile, spreadingratesvaryingfrom lessthan10 mm/yr to 20 mm/yrand
CentralandSoutheast
IndianRidges,
andto a positiveanomalyto the southof theprofile(Figures2, 4 30 mm/yrfor the Southwest,
respectively
[Tapscott
et al., 1980;Patriat, 1987].
and7).
BRIAIS ET AL.: RECONSTRUCTIONSOF THE SOUTH CHINA SEA
114 ø
113 ø
115ø
116ø
117ø
6305
118ø
119ø
19 ø
19 o
18 ø
18 ø
17 ø
17 ø
/,
•
-200
120ø
0 +200 nT o
O
Anomaly 11
Anomaly 10
Anomaly 9
Anomaly 8
0
Anomaly 7
ß
Anomaly 6b
ß
Anomaly 6a
ß
Anomaly 6
'•
Anomaly 5e
,I,
Anomaly 5d
ß
k
Anomaly 5c
Locahon of the ridge lump
16 ø
115ø
116ø
116ø
117ø
118ø
119ø
120ø
117ø
118ø
119ø
! 20ø
14ø30'
14ø 30'
14 ø
13 ø
12 ø
11;
16
ø
117
ø
118
ø
119 ø
-2010
0 +2100
nT •,,,Anomaly
Anoma!y
11
10
120
lø
1o
ß Anomaly
6a
,e Anomaly
6
• •3
Anomaly
9
•'Anomaly
5e
0 Anomaly
8
+ Anomaly
5d
O Anomaly7
ß
Anomaly 6b
'•
Anomaly5c
!•i•::•!•::•½•!:.!•
Location of the ridge lump
Fig.3.Magnetic
profiles
plotted
along
shiptracks
in(a)northern
and(b)southern
parts
ofeastern
basin.Frames
A andB in
Figure
1. Barbed
lineistheManila
subduction
zone.
Dotted
lineistheinferred
location
ofcontinent-ocean
boundary.
Symbols
are
thepicks
ofanomalies
identified
inthis
study
(and
picks
ofanomalies
onVerna
profiles
from
Taylor
and
Hayes
[1983]).
Note
the
fanning
ofanomalies
11to7,thesouthwar
dridge
jtimp
and
thereorganization
ofspreading
axis
attime
ofanomaly
7. Gray
dashed
lineistheapproximate
boundary
between
thepieces
ofcrust
created
before
andafter
theridge
jump.
Short segmentsof isochron7 are tentativelyidentifiedto the
Axialeastern
area. In theeastern
area,aftertheridgejump,the
northwestand to the southwest,along the continentalmargins magnetic
isochrons
appear
tobemoredisrupted
whenapproaching
(Figures3 and4). Theridgetherefore
startedto propagate
towards theaxis(Figures
3 and6). TaylorandHayes[1983]inferredthat
the southwest,between Macclesfield Bank and Reed Bank, at that
time.
disruption
to resultfromthe magnetization
of the Scarborough
seamounts,
whichwereemplaced
shortlyafterspreading
stopped
6306
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
19mm/yr
] 29mm/yr• N
Synthetic Prohie
ß-•.--
N
29mmlyr I
19mmlyr
Synthetic
Profile
••
12 11
10 9
7 7 jump 6b 6a 6
I--IC]O OOcDr--1
r---IOO
C3on, onoOOill)OOo]•m•lDI
2oo]
FIT0
NS20
-200
NS!9
NS1;
NS15
NS14
NS13
NS12
NS11
100krr
I
0
I,
50
100km
•
I
I,
I
Synthetic Profile
5d
5e
Ha
6b
7
v
8
9
10
11
I•D•oDODODOOO
• OO• • DO O•
12 11 10 9 8 77jump 5b 6a 6
no jump
C)OO DOC]C][•)OO
[] DO]00000DOOOrr113CDD 19mm/yr
24mm/yr
24mm/yr
19mm/yr
Fig.4.(a)Identification
ofmagnetic
anomalies
in(left)northern
and(right)
southern
parts
oftheeastern
basin,
showing
theridge
jumpjustafteranomaly
7. Anomaly
7 isalsotentatively
identified
onthesouthern
parts
ofprofiles
NS11toNS15.Dataprofiles
areprojected
onN180øE
direction.
Shown
belowthesynthetic
profiles
arecorresponding
normal
blocks
in thetimescale.(b)
Seismic
profiles
[fromTaylor
andHayes,
1983]andlocation
ofridgejumpidentified
frommagnetics,
showing
thecorrelation
of
decreasing
spreading
ratewithincreasing
basement
roughness.
[e.g.,Pautotet al., 1990]. Thishypothesis,
however,
canonlyac- for the northernsideof the ridgeand=12 mm/yr for the southern
countfor an abnormalmagneticsignaturecloseto the seamounts, side. We havea slightpreferencefor the formermodelbecauseit
while the anomalies seem to be distorted farther off-axis as well
accounts
bestfor theobservation
thatanomalies
6 and6a arerepre(Figure6a). The disruptionis thusmorelikely attributedto the sentedby a totalof 3 picksin thesoutheast
andonly2 in thesouthgeometryof thespreading
axisitself,whichincludesnumerous
dis- west,andthatanomaly6 is particularlylargeto thenorthwest
(near
continuities
[Briaiset al., 1989].
N15ø30,Ell6 ø , Figures6 and7). Thischangein thebehaviorof
For the sequenceof anomalies6b to 6 in the easternsubbasin thespreading
centeralongstrikeprobablyexplainswhy the 6b-6
(Figure7a), theeasternandwesternsegments
appearto havedis- sequence
is moredifficultto recognize
southof theridgethannorth
tinctbehaviors.Eastof 117øE,thebest-fitting
synthetic
magnetic of it, asinitiallynotedby TaylorandHayes[1980,1983].
profilesuggests
thatthemagnetic
sequences
arerathersymmetrical, Southwestern
basin. With the integrationof the new profiles,
with a half spreading
rateof about18-20 mm/yr. Westof 117øE, the magneticdata in the southwesternsubbasinhave become
thebestfit is obtained
with a modelinvolvinga smallridgejump especiallydense,whichfrequentlypermitseasycorrelationof the
towards
thesouth
atanomalies
6a-6time,
andahalfspreading
rate anomaliesbetweenthe profiles. That sucha correlationis easier
of 19 mm/yr (Figure 7a). Another possible model involves thanin the centraleasternbasinsuggeststhat the seafloorin the
asymmetric
spreading,
with half spreading
ratesof 18-20 mm/yr southwestern
subbasin
hasbeencreatedalonga spreading
axiswith
BRIAIS ET AL.: RECONSTRUCTIONS
OF THE SOUTHCHINA SEA
6307
AXIS
8
RIDGEJUMP
6b
6a
Spreading
rate I
,• 2.5cm/yr
6
5©
Spreading
rate
-• 2cm/yr
Smooth basement
rough basement
East
Preferred
•
model
12
11
10
I
Alternative
.., ..:.]!::.
....,,
i:
.
.,.
I
.•
.
16
15
13
23
22
2
11
30-32
Ma
12
•
13
34-38
Ma
15
16
models
•
N
I
24
21
47-53
Ma
22
23
24
a
b
!.','
:'"
Fig. 4. (continued)
moresimplegeometrythanthatof theaxisin theeasternbasin.The
prominent
freeairgravityanomalyprovides
thelocationof therelict
spreading
axis[TaylorandHayes,1983;Pautotet al., 1990]. As
in the northwest, the narrowness of the oceanic crust in this
113 ø
a majortranscurrent
zoneexistedneartheboundarybetweenthe
easternandsouthwestern
subbasins.
A modelin whichtheridge
wassuccessively
linkedto eastward-jumping
strike-slip
faultscould
haveaccounted
for sucha patternof spreading
(Figure9 inset).
Theuncontroversial
identification
of magneticanomalies
morerecentthananomaly7 in the easternbasin,andthe reconstruction
of
itscentralpartmaybe usedto yieldanimageof theentirebasinat
the time when spreadingwould haveceasedin the southwest,in
115 ø
116 ø
117 ø
'. '. "./'. '-'- '•"" "'-:A
foø
oø
oø
,ø
oø
•x•
øoø
.ø
•øßoO
oø
oø
oø
oø
oø
oø
oø
oø
,,o::
::oø
::oø
:::
.::
::'
subbasin,
lessthan 150 km from the relict axis to the margin,
makesthe matchof observedanomalieswith a syntheticmodel
nonunique.Four differentsequences
of anomalies,6b-5c, 13-8,
18"
19-13 and 26-21, of the reversaltime scaleprovidesynthetic
profilesthatresemble
theobserved
magnetic
profiles,with similar
spreading rates, from 12.5 to 20mm/yr (FigureS).
Reconstructionof the geometryof the whole basin, however,
permitsusto tentatively
choose
betweenthesesequences.
Inspectionof the shapesof the anomaliesinitially led us to
choose
thesequence
of anomalies
13 to 8, with spreading
ratesof
17 mm/yrin the northandabout11 mm/yrin the south. This interpretation
impliedthatspreading
hadceasedin the southwestern
subbasin10 m.y. beforeit did in the easternone,andthereforethat
114 ø
t'. '! '. '. '-'-'"'-•/'.
18 ø
' "' -" '" -' ti
113 ø
114 ø
115 ø
116 ø
117 ø
Fig. 5. (a) Identificationof magneticanomaliesin the northwestern
riff.
Alternativesynthetic
profileswith anacceptable
matchwith dataareshown.
Data profilesare projectedon a N160øE direction. Gray dashedline is
approximate
locationof the continent-ocean
boundary.(b) Locationof
identifiedmagneticanomalypicks,showingNE-SW orientationof rift.
Bathymetryin meters.
ofridge
segments,
inwhich
thecentral
area
isoccupied
both'by
an
isolated,abandoned
rift to the northandan interveningcontinental
block(MacclesfieldBank)to the south.Clearly,ourdataareinsufthatinterpretation.
Thisimage(Figure9) revealsa complex
system ficientto arguefor suchcomplexity.
117o30
' 118
ø
119
ø
17o30 '
120;7o30
,
17 ø
17 ø
114o30 '
117 ø
116 ø
11,5ø
16o30 '
16 ø
16 ø
15 ø
15 ø
14 ø
14 ø
117 ø
13 ø
118 ø
13ø
Anomaly 11
Anomaly10
n
Anomaly 9
o
O
Anomaly 8
Anomaly 7
ß
Anomaly 6b
&
Anomaly 6a
ß
Anomaly 6
-20[00 +2100
nT
'A' Anomaly 5e
12 ø
12 ø
ß•
•
13030 '
120 ø
119 ø
,I.
Anomaly 5d
ß
A. Anomaly 5c
•
11 ø30'
114ø30 '
111 ø
112 ø
15 ø
Location of the ridge lump
11 ø30'
116030 '
1 16 ø
115 ø
113 ø
114 ø
115 ø
116 ø
15 ø
14 ø
13 ø
13 ø
12 ø
12 ø
•
Anomaly 11
•
Anomaly 10
O
Anomaly 9
o
Anomaly8
11 ø _
ß
Anomaly
6b
Anomaly 6a
Anomaly
6
'•
:"
o.
Anomaly
7
."'
.......................
'"'
-200
11 ø
0 +200 nT
I
i
Anomaly 5e
ß Anomaly 5d
W Anomaly 5c
Locabon of the ridge lump
10o
111 ø
b
J
112 ø
113 ø
1 14 ø
1 15 ø
116 ø
Fig. 6. Magneticprofilesplottedalongshiptracksin axial area,(a) eastand(b) southwest.FramesC andD in Figure1. Dotted
line is theinferredlocationof continent-ocean
boundary.Barbedline is the Manila subduction
zone. Symbolsarethe anomalies
identifiedin thisstudy.Notethestepwisesouthwestward
propagation
of oceanicspreading
afteranomaly7.
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
I
Synthetic
6b
6a
65e
5d
5•
profile
rICO
CI]I
AXIS
200
3
nT0
!
PN59
-200
o
/
I 100km
I
50
i,/•?•
•
PN57-74
NS20
•
NS20
NS19
NS18
NS17
ß
PN65-66
/
NS16
NS15
NS12
6
6b
NS11
PN18
•J•
I
NS10
I
•
NS9
NS8
Synthehc
profile
6b
6a
6
small
jump
0O0O0• O0D•
5e
5d
5c
5c
5d 5e
6 6a
small
6b
Jump
o•-tOlO-tt'•'røø'O
•
• I)OlD
OO
0
Fig.7. Identification
of magnetic
anomalies
in axialareaof basin,(a)east,(b)intermediate,
(c) southwest.
Shown
belowthe
synthetic
profiles
arecorresponding
normal
blocks
intimescale.All models
arecalculated
forhalfspreading
rateof 19mm/yr.
Pooridentification
onprofiles
NS5-8andNWl-13reflects
thecomplexity
of theintermediate
zonebetween
Macclesfield
andReed
Banks.Thesynthetic
profiles
atthebottom
of Figures
7aand7binclude
ridgejumpsatthetimeof anomalies
6a-6and5e-Sd,
respectively,asdiscussed
in the text.
6309
6310
BRIAISETAL.: RECONSTRUCTIONS
OFTHESOUTH
CHINASEA
•
N
AXIS
--E'•'•5'30'
•
NS8
NW1
•.•,•..•'•NW3
ß
6
6a
6b
7
•
••j•,..'"•,
NW
5
.
_
NS6
66a6b
NW
7
200•
nT
oI
NS5
- 200 •
•
NW9
•
NW11
0 50 100km
J•
I
•
•_•
•
NW13
v
••'•
•'ltl
N•15
a
/ /
Synthetic
profile sI5 •c
mp
AXIS
Fig. 7.
(continued)
6a
BRIAIS ET AL.' RECONSTRUCTIONS OF THE SOUTH CHINA SEA
6311
AXIS
5c
5d
5e
6
200-1
nTO-200'-'
o
I,
5O
100km
I
/
I !
NW25
NW26
NW28
PN8
//
NW 30
NW 32
PN9
I
i
/
NW
33
/
NW 35
•
NW
38
Syntheticprofile
6b
6a
6
5e 5d
5c
5c
5d 5e
DC]D
ridI 00 rl•DTITDOTD•••••
6
6a
6b
0 01i• ODD
AXIS
Fig. 7.
(continued)
This led usto abandonour initial matchandto opt for a match
corresponding
to the sequenceof anomalies6b to 5c (Figures6
and7). Themainadvantage
of thismatchis thatit yieldsa simple,
continuousgeometry of magnetic isochronsacrossthe whole
central part of the basin. Moreover, fits of anomalies in the
southwestern
part, usingpoles of rotationcomputedonly from
anomaliesidentifiedin theeasternpart,providea positivetestfor
the choiceof the 6b-5c sequenceof reversalsin the southwest.
That we couldobtaina consistentset of Euler polesof rotation,
with reasonable
confidenceellipses,for theseanomaliesis alsoan
argumentin favorof thischoice.A problemthatremainswith this
alternativematch,however,is that the amplitudeand shapeof
anomalies 6 to 6b in the southwestare different from those of the
samesequence
in the east,whereanomaly6b is so easily
recognized
(Figures
6 and7). In fact,thisnewinterpretation
of the
southwestsubbasinnow hinges on the series of anomalies
observed
onthenorthern
sideofprofiles
NW20-23,fromanomaly
5c, closeto the axis,to anomaly6 or 6b, closeto the margin
(Figures6, 7b andc and8).
Thefirstidentifiable
anomalies
alongthemargin
areanomaly
6 in
thesouthwest
(westof 115øE)andanomaly6b in theintermediate
zonebetweenthedisrupted
easternandthesouthwesternmost
axes
(from115øEto 116øE)(Figures
6 and7). Thisarrangement
of the
initial anomalies
impliesthatthe ridgepropagated
towardsthe
6312
BRIAIS ET AL.: RECONSTRUCTIONSOF THE SOUTH CHINA SEA
AXIS
NW20
NW21
NW22
NW23
AXIS
N •
HSR
6c-5b
25-15.5my
1.75cm/yr
13•.8
1.5cm/yr
38-27.õmy
2.0cm/yr
18-13
45.5-'36my
26-20
1.25cm/yr
6o'-46my
Fig. 8. Magneticdataprofilesfrom(top)SW subbasin
compared
with(bottom)synthetic
magnetic
profilescorresponding
to four
differentsequences
of magneticreversaltimescale,showingthattheinterpretation
is nonunique.
southwestin two majorsteps,first at the time of anomalies6b-7,
creatinga newspreading
axisin the intermediate
zone,thenat the
time of anomaly6, whentheridgereachedits longestextent.The
poorlydeveloped
shapeof anomaly6b in thesouthwest
maybe due
to the factthatit represents
the first anomalyalongthe margin,as
doesanomaly11 to the northeast.
The bestfit with a syntheticmagneticprofileis obtainedwith a
spreadingrate of about 18 mm/yr on either sidesof the axis.
Severalprofiles(NW20 to NW25, Figures7b and 7c), however,
displayprominentasymmetry.Syntheticprofilesinvolvingasymmetryin thespreading
ratesdo notmatchtheobservations
aswell
as modelsinvokinga ridgejump. Our preferredmodelthusincludesone moresmallridgejump alongthe segmentdefinedby
theseprofilesat thetimeof anomaly5d. As all otherjumpsinferred
frommagnetic
anomalies
in theSouthChinaSea,thatjumpwasdirected toward the south.
Cessationof seafloorspreading.Becausethe axialpartof the
eastern
basindoesnotdisplayanyidentifiablemagneticanomalies,
the lastepisodeof spreading
theremay only be deducedfrom the
modelfor anomalies
6-5d, assuming
a constant
spreading
rateafter
anomaly5d. Thissuggests
a cessation
of spreading
nearanomaly
5c (Figure7a). The anomaliesobservedin the axial zone show
prominent
variations
of wavelength
alongstrike(Figures4 and7),
which may be interpretedin termsof differentialasymmetric
spreadingrelated to the reorientationof spreadingsegments
[MenardandAtwater,1968;Hey et al., 1988]. In thesouthwestern
part, the end of the spreadingis observedjust after anomaly5c
(e.g.,onprofilesNW19 to NW25,Figures6 and7). Thesimplest
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
112 ø
18 ø
:.
114 ø
116 ø
:.y \::::: :)-/
:(
6313
118 ø
_--
\'::.://--'
_
16 ø
....::::::::::: :-:
.:.:.:.:.:.:.:.:.:.:.:.: .:.......:
14 ø
Fig.9. Computed
reconstruction
of thecentraleastern
basinat timeof theridgejump(-An7) showing
complexities
impliedif
sequence
of anomalies
8-13is chosen.Inset:Sketches
depicting
modelof evolution
of spreading
in SouthChinaSeaassuming
spreading,
andassociated
strike-slip
faulting,ceased
earlierin thesouthwest
(1) thanin theeast,dueto theactivation
of a new
strike-slip
faultto theeast(2).
conclusion
is thusthatthecessation
of spreadingin the SouthChina
Sea was synchronous
all alongthe ridge,just after anomaly5c, at
•15.5 Ma, althoughslight diachronismbetweenthe east and the
floor and obducted onto Mindoro
southwest cannot be excluded.
Finite andStagePolesofRotation
Note that this conclusion is a direct
consequence
of our preferredanomalysequence
match.
RECONSTRUCTION OF SEAFLOOR SPREADING
IN THE SOL/TH CHINA SEA
Island in middle Miocene time
[Ranginet al., 1985].
UsingbothPatriat's[1987] andHellinger's[1981] methods,we
adjusted
theparameters
of rotationto obtaina consistent
setof stage
poles,with no abruptchangein spreadingdirectionandrate. The
isochrons in a small ocean basin like the South China Sea are rela-
tively shortandclosetogether,sothata simplebestfit may leadto
Thesetof all themagnetic
lineations
deduced
fromtheanalysis
described
aboveis presented
in Figure11, alongwith all thetectonicfeatures
observed
onSeaBeamswaths.Thesegmentation
of
theisochrons
derivesfromthefitsof magnetic
anomalies
identified
on the profiles,resultingin the sameshapeof the isochronson
eithersideof theridge.By fittingthesemagnetic
isochrons
we calculatedtheEulerianpolesof rotation.We presentheretheresults
of thefitsandthereconstructions
of theoceanic
partof thebasin,
inconsistencies.
To constrain
the fit of anomalies
older than
anomaly7 in the easternbasin,we thususedthe additionalobservation that the anomaliesabut the continentalmargin, west of
Macclesfield
Bank to the north and of Reed Bank to the south
(Figure11). The fit of magneticanomaliesyoungerthananomaly
7 is constrained
by the elbowshapeof the axis. The greatestuncertaintyis in the centralaxial zoneof thebasin,dueto the poormagneticdataandlargenumberof seamounts.Henceas an additional
withoutconsidering
thesurrounding
continental
areas.We discuss constraintto fit anomalies5e to 5c in this zone,we tried to respect
theparameters
of rotationcalculated
fromthesefits (Tables2-4) theorientationof thefault scarpsobservedon SeaBeambathymetand the major stepsin the evolutionof the spreadingridge ric swaths(Figure11). The only magneticisochrons
thatare sig(Figures10-12). To assess
thelimit of theoceaniccrust,we took nificantly offset by fracturezonesare anomalies10, 6b and 5d
intoaccount
thegravityanomalies
defined
in thegravityprofilesof (Figures3, 6 and11). The factthatwe couldclearlyidentifymost
TaylorandHayes[1983]andin thefreeairgravityanomaly
map of the segmentsof theseanomalieson eithersideof the ridgepro-
published
by B. Chenet al. [1987]. We alsotookintoaccountthe vided a valuable constraint for the fit. Such a constraint could not
magneticprofiles,especiallyalongthe northernandsouthwestern be usedfor otherisochrons,however,becausethey were eithernot
limitsof thebasin.Theoldest,southeasternmost
partof theoceanic disruptedby fracturezones,as in the caseof anomalies9 or 8, or
crustin thebasinappears
tobepartlyconcealed
undera recentcom- too disruptedby short-offsetdiscontinuities,
as for anomalies6 to
pressional
thrustwedgeandthethicksedimentation
derivedfrom it. 5d in the east.
Thatinterpretation
is supported
by seismicevidence
[Taylorand
Becausesmallridgejumpsoccurredat thetimesof anomalies6
Hayes, 1983],andby the existence
of a sliceof Mid-Oligocene and5d (Figure7), the corresponding
reconstructions
are lessceroceaniccrustinferredto havebeenpart of the SouthChinaSea tain than for other anomalies. Since the ridge jumps are small,
6314
BRIAIS ET AL.' RECONSTRUCTIONSOF THE SOUTH CHINA SEA
TABLE
Anomaly
2. Finite Rotation Parameters
Age,Ma
Latitude,
deg
Longitude,
deg
Angle,deg
Patriat's[1987] Method
5c
5d
5e
6
6a
6b
16.56
17.81
19.00
20.45
21.73
23.44
-3.0
5.0
-1.4
0.1
0.1
-1.1
93.6
105.5
88.7
83.3
81.3
75.9
0.7
3.7
2.4
2.8
3.5
3.9
jump
25.91
7.0
87.8
8
9
10
27.74
29.32
30.32
9.3
8.2
7.9
91.2
87.4
85.7
10.3
10.3
10.8
7.5
5d
5e
6
6a
6b
8
9
10
17.81
19.00
20.45
21.73
23.44
27.74
29.32
30.32
105.01
92.26
74.37
78.60
84.61
92.95
94.02
86.79
3.79
2.72
2.32
3.26
4.78
10.98
13.29
11.20
Hellinger's[1981] Method
5.70
0.09
-4.52
-0.50
0.02
9.73
11.12
8.27
Positivelatitudes
andlongitudes
arenorthern
andeastern
hemisphere,
respectively.
TABLE 3. StagePolesandAnglesof Rotation
Anomalies
Age End, Ma
Time Span,m.y.
Latitude,deg
Longitude,deg
Angle, deg
Patriat's [1987] Method
End-
15.64
0.69
16.33
1.48
5d-5e
17.81
1.19
5e - 6
19.00
1.45
5c-
5c
5d
6-6a
6a-
6b
6b - jump
jump - 8
-3.0
6.7
-12.9
7.3
20.45
1.28
21.73
1.71
-7.2
0.4
23.44
25.91
2.47
1.83
14.2
14.3
0.7
3.0
-47.8
58.0
1.6
0.5
72.9
0.7
39.1
0.6
101.0
101.0
3.4
3.3
8-
9
27.74
1.58
2.7
0.7
9-
10
29.32
1.00
4.4
55.8
0.6
30.32
1.77
10.9
84.9
1.4
End - 5d
15.64
2.17
5d-
17.81
1.19
-15.4
-46.6
1.33
5e - 6
19.00
1.45
-11.0
-33.1
0.90
6 - 6a
20.45
1.28
8.8
89.0
6a-6b
21.73
1.71
0.6
97.3
1.57
6b - 8
23.44
4.30
16.2
100.1
6.41
8-
9
27.74
1.58
100.0
2.33
9-
10
29.32
1.00
-51.2
2.65
30.32
1.77
10-
11
-10.7
93.6
108.2
Hellinger's[1981] Method
10-
5e
11
5.7
17.0
-17.3
10.1
105.0
84.1
3.79
0.98
1.36
Polesandanglescomputed
for southern
flankof theridgefromfiniterotationsin Table2. Positivelatitudesandlongitudes
arenorthern
andeasternhemisphere,
respectively.
however,they may be consideredto reflect a mere asymmetryin
spreading.We thereforeincludethe resultsof the fits of anomalies
5d and 6, notingthat the uncertaintyfor themis greater. The parametersof rotationat thetimeof thelargejump afteranomaly7 are
interpolated
fromthosecorresponding
to anomalies8 and6b.
Sinceanomaly11 is only tentativelyidentifiedon bothsidesof
the narrowrift in the northwest,we calculatedthe pole of rotation
corresponding
to the stagebetweenanomaly11 and anomaly10,
which forms the axis of that rift.
Uncertaintieson the rotationparameterswere computedusing
Hellinger's[1981] method. The resultsof the computations
were
unstableandvariedasa functionof thedatapointsconsidered.The
95% confidenceellipsesare shownin Figure12a. For anomaly
5c, no acceptable
confidenceellipsewasobtained,althougha good
fit wasreachedusingPatriat's [1987] method.The confidenceel-
lipsesfor anomalies11 and9 areverylarge. For anomalies
11 and
5c, theconjugate
picksareverycloseto oneanother,implyinga
verysmallrotationangleandthereforea greateruncertainty.For
anomaly9, thegreatuncertainty
is probablyrelatedto theshapeof
theisochron,
whichconsists
of shortsegments
withoutsignificant
offset. For otheranomalies,the confidenceellipsesare smaller
(Figure12a)butarealwayselongated
in a directionparallelto the
isochrons,which reflectsthe absenceof large-offsettransform
faults,andtherelativelyshortlengthof all theisochrons.
The finitepolesof rotationareall locatedto theSW of thebasin,
thepolescorresponding
to anomalies10 to 8 beingcloserto the
basinthan the polescorresponding
to the youngerphasesof
seafloorspreading(Figure12a). The stagepolesof rotationare
muchmorescattered,
butroughlyalignedona greatcircle.Using
the main bathymetricfeaturesin the SouthChina Basin and the
BRIAIS ET AL.: RECONSTRUCTIONS
OF THE SOUTHCHINA SEA
6315
TABLE
4. Directions
(D)andFullRates
ofOpening
forPoints
inSouthwestern
(A)andEastern
(B)Parts
oftheBasin
PointA
Anomalies
Age end,Ma
A,deg
D, deg
15.64
16.56
17.81
19.00
20.45
21.73
23.44
25.91
27.74
29.32
30.32
26
9
163
56
43
78
14
-36
-44
-2
0
-13
-10
11
15.64
17.81
19.00
20.45
21.73
23.44
27.74
29.32
30.32
27.74
12
162
149
26
21
-37
-10
-1
-6
-32
PointB
Fullrate,mm/yr
A,deg
D, deg
Full rate,mm/yr
31
13
167
60
47
82
17
17
118
63
33
-35
-39
7
0
-13
-10
3
4
5
-2
-4
56
52
34
36
44
36
45
60
44
62
93(?)
16
166
152
30
25
18
18
169
34
109
-35
-3
5
-8
-31
9
14
-26
-5
-2
55
30
32
42
44
52
52
55
92(?)
50
Patriat's[1987] Method
End-
5c
5c-5d
5d-
5e
5e-6
6 - 6a
6a - 6b
6b - jump
jump- 8
8-9
9-10
10-
11
49
35
43
34
41
36
36
Hellingerg[1981]Me•od
End-5d
5d- 5e
5e- 6
6 - 6a
6a- 6b
6b- 8
8 -9
9-10
10-11
8- 10
40
39
36
37
36
Directions
and
rates
computed
from
stage
poles
obtained
with
Patriat's
[1987]
method
and
Hellinger's
[1981]
method.
Points
A and
Bshown
in
Figure
11.Dtaken
positive
eastward.
SymbolA,
distance
between
stage
pole
ofrotation
and
point
ofmeasurement.
Data
forHellinger's
[1981]
method,PointB, is usedfor Figure12b.
geometry
oftheRedRiver
Fault
asfirst-order
constraints,
Peltzer uncertaintyon the spreadingdirection,about20ø, leadingto an
uncertaintyof about5 mm/yr in the spreadingratebecauseof the
inducedobliquity.
Seafloorspreadingbeganin the SouthChinaSeaat anomaly11
7.5øS,96øE. This pole is locatedfartherfrom the basinthanthe
poleswe calculated,anda few degreessouthof them(Figure12a). (32 Ma). Anomaly 11 is not uniformlyobservedto the north,and
In theiranalysisof thepaleogeographic
evolutionof the Southwest not clearlyobservedin the south(Figures3 and 10a), which sugPacific,Jolivet et al. [1989] estimateda pole of rotation(N39ø, geststhat the incipientridge probablyconsistedof discontinuous
centers.A correlativeexplanation
maybe thattheridge
E176.25ø) for the stageof spreading
afteranomaly6 (20 Ma), the spreading
present-day
traceof theRed River Faultbeingtakenasa smallcir- jumpedto the southbetweenthe times of anomalies11 and 10,
cle. This polelies northandeastof thebasin,very far from all the leavingbothsidesof theincipientridgeto thenorth. Sucha pattern
polescalculated
here. The pole(13.54øS,2.63øE)theyassumed
for of incipientoceanicspreading,sourceof atypicalmagneticsignathe first stageof spreading(32-20 Ma), on the other hand, is tures, is observed in the northern Red Sea, where discontinuous
within a confidenceellipseof ourresults(Figure12a). Le Pichon volcaniccentersdisposeden •chelonare creatingthe first basalt
[1988] relatedall Cenozoicstrike-slipmovementwithin theAsian flowsof the thinningcontinentalmargin[Cochran,1983;Dixon et
of the ridgesegments
in the South
continentto rotationabouta singlepole, situatedat 11øN, 95øE, al., 1987]. The orientation
China Sea, as drawnfrom the isochrons,varied from NE-SW to
closeto ourresults(Figure12a).
WNW-ESE. The directionof spreading
wasslightlywestof north
Major Stagesof SeafloorSpreading
(Table4, Figure12b).
and Tapponnier [1988] inferred the pole of finite rotation of
Indochina
relative to South China between 45 and 15 Ma to lie at
Theparameters
of rotation
arecomputed
foreachstage
between The spreading
ceasedin the northwestern
subbasin
just after
successive
anomalies
fromthesequence
of finitepoles.Therates anomaly10 (30 Ma). It continuedfartherto the east,however.
andorientation
of spreading
for two points(A andB, Table4, The isochrondrawnfor anomaly10 suggests
thattheridgewas
Figure12b)nowbelonging,
respectively,
tothesouthwestern
and segmented
andthatoverall,thesegments
werearranged
en6chelon,
eastern
relictspreading
axesarethencomputed
fromthesestage into a right-steppingarray (Figure10b). The directionof
poles.Thedirections
of spreading
shownin Table4 arethose spreading
wascloseto N-S (Table4, Figure12b).
computed
fromtherotation
parameters,
andareconsistent
withthe
Betweenanomalies10 and 7 (30-26 Ma), fanningof the
isochrons
10to 8 (Figures
3 and10c)impliesa reorientation
of the
lies6 to 5c, followingourchoiceto usethisorientation
asa con- spreading
system
fromanaverage
ENE-WSWtoanE-Wdirection,
straint.Fromthebeginning
of theoceanic
spreading
(32 Ma) to alsosuggested
bythecomputed
directions
of spreading
(Table4,
anomaly
5d(17.8Ma), thefull spreading
ratedecreases
frommore Figure12b). Anomaly8 appears
tohavebeenthemostcontinuous
than60 mm/yrto about35 mm/yr(Figure12b,Table4). It in- isochronbecausethe small fracturezonesor discontinuities
obcreases
backto about55 mm/yrbetweenanomaly5d andtheces- served
atanomalies
9 and10nolonger
disrupt
it.
sationof the spreading(15.5Ma). The differencesin the
Between
anomalies
7 and6b(26-24Ma),theridgejumped
to
directions
of spreading
estimated
fromthedifferentmethods
of thesouth,
andthenrapidly
propagated
tothesouthwest,
resulting
in
calculation
of thepolesof rotation(Table4) giveanideaof the a drastic
change
in theaxisconfiguration.
Thebeginning
of the
orientationof the fabricmeasuredon the structuralmapsfor anoma-
6316
BRIAIS
ETAL.:RECONSTRUCTIONS
OFTHE
SOUTH
CHINA
SEA
114 ø
20,1
115 ø
'
............
116 ø
117 ø
118 ø
119 ø
[.....
•..... • ..... •.....
ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß. ß
112ø
113ø
I
I
190
114ø
20 Ma - Anomaly
6
...........
115ø
116ø
117ø
118ø
119ø
J.......... I..... F..... I.'.'.'.'.',
'. '' '' '' '' '' '' '' ß" ......
"""
'
19 ø
18 ø
ß .................................................... .........................-.............................-..
ß. '. '. '. [. [. [. [. [. [. •.
......
.
17 ø
.......
[. [. •a[nk•
ß' •
....................
........
.................
........
20113
ø
114
ø
115
ø
116
ø
117
ø
118
ø
ß.............'.'.'.'.'.'.'.'.'
-/
119
a
' . ß . ß . ß . ß . ' . ß . ß . ' ßß ß .....
6b
...........
.
.
..................
..........
..........
ß .................
19 ø
.........
........
.........
12 ø
113ø
ß
114ø
115ø
•
116ø
I .....
26 Ma - Anomaly
7
117ø
•.....
• .....
118ø
' ....
119ø
•
' i'. i"'.. .............
;' •' i' i' ;' i' ;' i' i' i' ;' i' i' ''" ""
ß
19 ø
18 ø
112ø
113ø
114ø
115ø
116ø
117ø
119 ø
118ø
.
17ø ':':';':';'.
...........
•4
-
18Ma- Anomaly
5d
...........
ß'-'.'.'.'-'""""""""
ii I ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ....
--'":-:':':':':':':':':"
16"
15"
ßß .-.
I 3ø
114ø
i
115ø
116ø
117ø
118ø
ß. ß. ß. ß. ß. ß. ß.
ß ' ß' ß 'B&nk.
'
119ø
... ß.ß.ß..i.ß.ß.ß.ß.ß.I.. ß.ß.ß.ß.1.ß.ß.ß."'1
ß:.:.:.:.:.:"':"
' '•:.
:.:.:.:.:.:.(•
_
::
•
••--•o
6b• 6a
ß
17 ø
.]]::]: :.x.._•.......,,
.' '..B•n.
N..:. •
16ø
•_ •
8
Fig.10.(a)-(f)
Fitsofmagnetic
isochrons
andapproximate
location
ofthecontinent-ocean
boundary
attime
ofAnll,An10,
ridge
jump
(-An7),
An6b,
An6and
An5d,
respectively.
Circles
and
triangles
aretheanomalies
identified
onthenorthern
andsouthern
sides,
respectively.
Dotted
area
iscontinental
orstretched
continental
crust.
Northern
continental
margin
isfixed.
.
BRIAIS ET AL.: RECONSTRUCTIONS
OFTHE SOUTHCHINA SEA
112 ø
113 ø
20"
114 ø
\
115 ø
116 ø
117 ø
6317
118 ø
'.'.'.'.'
''" "•'•
•2_•TM
,•
•::.....?•.:•••,.,••
•v .....................
'C•:::::::;:•v•l?•-•'
'"•
'•-,,
- ' '• !
!'.'.'.'.'.'.'.'.'.'.'.'./._......•-••
.' .' .' .' .' .' .' .' .'/•
•
• •.,,••,,
-----'-•'-•'-
...........
P
g
!'.' .'.' .'. •"
•ø •..
ß:.:.:.'.'.'.'.'.'.'.''.'.'.'.'i/•
ß'. '. '. '. '. '. '.2.'. '. '. '. '.•
•o_'.'.'.'.'.'.'.':'.'.'.'.j/•
.......
' ....
........
•
......
." ß
ß '
• ci
,...1•ø•
• ß. .
....
•
ß. .•'•.... •
•
•
•
•
•..•
6e
.- •
- • t'l)••
•^
•
"
ß
..........
.......
120 ø
ß......I..
ß.........!.........
,i'..I..............•.....-.......
•.............
,.........-..
t
........
119 ø
::....:•
f::'"'•.',':,,,:z•]
ov•
•
•........... .:•:.•......--".'::•
•.i:....•
•.
'•/'
?;,
• • s •'•"..'•.
......
:..11"'•W'
-' •
•
-•-- •
-•
•
•
!• 5..•'•_•z
6a/
• :..-.':..••5•11•ii'.'".'"'..'•:..:'•:..•:...............:...•,,,
-- ••--•_
--\
'". ......... i::i::i::i::!::i::ii?.':•
........"
•-
..
•,.v
•
5d
:.:."
....:.::
.........
60
'::'•
."• :'::';•i•
........
3
--•
-
•
•/"•..."•
.-
............................................:.:...........:.7.•
5e x.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'
.........
' ....
_
y:.:.:.:.:.:
11 ø
Fig.11.Magnetic
lineations
in SouthChinaSeafromthisstudyandfaultscarps
in axialareaobserved
onSeaBeamswaths
[from
Briaiset al., 1989]. Inset:Sketchrepresenting
theevolution
of ridgegeometry
in theeastern
partof SouthChinaSea,showing
thatsegmentation
accompanies
changes
in thedirection
of spreading
whiletheoverallridgetrendremains
E-W.
propagationat the time of anomaly7 is suggested
by the identification of a smallridgesegmentbetweenMacclesfieldBank andReed
Bank (Figures3 and lec). The 6b isochronis well-definedin the
east,with a configurationinheritedfrom the ridgejump. To the
southwest,
it formsonly shortsegments
betweenMacclesfieldand
Reed Banks(Figure led). We cannotreconstructthe spreading
geometryin the extremelydisruptedzonearoundEll5 ø. Sincea
large offset is observedbetween anomaliesin the east and in the
southwest,
this'zonewasprobablycharacterized
eitherby transform
faultslinkedby shortridgesegments,
or by obliquesegments
with
right-lateral
normalslip. A high-resolution
studyof thebathymetry
of this areawould be necessary
to choosebetweenthesegeometries.
Anomaly6 (20.5 Ma) corresponds
to a secondstepof rapid
propagation
to thesouthwest,
in whichtheridgereaches
itsmaximum extension.Given the availabledata,this corresponds
to the
maximumextension
of thespreading
system.The computeddirectionof spreading
aftertheridgejump(anomalies
6b-6a) is NW-SE,
becomescloserto N-S aroundanomaly6, then changesback to
NW-SE after anomaly5e (Table4, Figureslee, lef and 12b).
6318
BRIAIS
ETAL.:RECONSTRUCTIONS
OFTHESOUTH
CHINA
SEA
Z
L•
o
4-
Z
BRIAIS ET AL.' RECONSTRUCTIONS
OF THE SOUTH CHINA SEA
6319
ridgein the area,a fact thathasstrongimplications
for the
reconstruction scenario.
-8O
6O
E
4O
'-' 40•
_
- 2O
20-
I i
I
I
I
I
I
I
Ages (Ma)
o
2ow
•
•-•5e-5d
_s__.
40•,/
10-9
6b-6a,,
•
20E
'10-8"- -.•
j!9-8
,•,OE
,
Vo.
/•d-.end
•
/
•
/
/
\
/
bI
•
Fig. 12. (continued)
The time of anomaly6 is alsocorrelatedwith a reorientationof the
spreading
segments
in the east,fromN80øEto N60øEon average
[Pautotet al., 1986]. At thatstage,theridgeappearsto havebeen
segmented,probablymore to the eastthan to the southwest. In
both areas, however, the Sea Beam bathymetryreveals clear
NE-SW normalfault scarps,but it doesnot showasmanyfracture
zones as the offset of the magnetic isochronsmight suggest
(Figure11). This impliesthat the spreadingaxismay havebeen
offsetalongzonesof ratherdiffusedeformation,which do not have
a distinctbathymetric
expression
[Pautotet al., 1990]. At thetime
of anomaly5d (18 Ma) the orientationof the spreadingsegments
was still changing,from N70øE to N50-40øE, which lead to disruptedanomaliesin the east(Figures10f and 11; seeBriais et al.
[1989]). The directionof spreading
wascloseto NW-SE (Table4,
Figure 12b).
Seafloor spreading probably stopped after anomaly 5c
(15.5 Ma) in the entirebasin. In the southwest,
that age is well
constrained
by thebest-fitting
synthetic
magneticprofiles,giventhe
anomalysequence
we chose.In theeast,theaxialareais occupied
by the Scarborough
seamounts
chain. The emplacementof the
seamounts
is datedas 10-15 Ma from dredgedvolcanics[Hekinian
et al., 1989],andmayhaveoccurredalongtherelictspreading
axis
becauseit represented
a weakzonein the oceaniccrust[Briaiset
al., 1989]. The cessationof spreadingseemsto have occurred
roughlyat the sametime throughoutthe basin. The extension
stoppedcompletelyand was not takenup by anotherspreading
Characteristics
of theEvolutionof theSpreading
System,a Discussion
Direction of spreadingand segmentation
of the ridge. As mentionedearlier,anomalies
10 to 7 displaya 20øfanningthatreflectsa
progressivechangein the directionof the spreadingaxis. Also,
while at the time of anomaly10 the ridgeappearsto be segmented
by severalfracturezones,mostof thesezonesdisappearduringthe
reorientationof the ridge, since anomaly8 is more continuous.
Similarly, the prominentdisruptionof the spreadingaxis starting
after anomaly6a, with no clearly identifiableisochronsegment
youngerthananomaly6, seemsto be relatedto changesin the directionof spreading.The segments
thatformthesuccessive
ridges
appearto becomeshorterastheirorientation
changes
fromN80øEat
thetimeof anomaly6a,to N50øEat thetimeof anomaly5d. Sucha
segmentation
of the axis is consistentwith the observationof truncatedridgesontheSeaBeambathymetryin theeasternaxialregion,
as well as with the observationthat seamountsin this region are
elongated
in theN50øEandN140øEdirections[Briaiset al., 1989;
Pautot et al., 1990]. Hencein the easternbasin,segmentation
appearsto accompanychangesin the spreadingdirection,while the
overallridgeorientationremainsE-W (Figure11, inset). This is
probablybecausethe overalldirectionof the ridgeis inheritedand
controlledby thegeometryof themargin,whichis closeto E-W.
Differentmodelshavebeenproposedfor small-scalereorientation of spreadingcenters.Onemodel[MenardandAtwater,1968]
invokes progressiverotation of the ridge, with shorteningor
lengtheningof the segmentsaccordingto the geometrybeforeand
after reorientation.Anothermodelsuggeststhat reorientationoccursasa new spreadingdirectionforcesspreadingcentersto propagate along a new trend while those correspondingto the old
spreadingdirectiongraduallydie out [Hey, 1977;Hey et al., 1988].
The fact that the ridgejumpedto the southat the time of anomaly
6a-6 in the easternSouthChinaSea(Figure7a) may imply thatthe
reorientation
of theridgepreferentiallyinvolvedsmall-scale
propagationof spreadingcenterstowardsthe southwest,therebytransferringpartof theoldercrustto thenorthernplate.
The evolutionof the spreading
ridgederivedfrom the magnetic
anomalies confirms changes in the direction of spreading
(Figure 12b), that had been suspectedbut not demonstrated
by
earlierstudies[Pautotet al., 1986, 1990;Hayeset al., 1987]. The
computationof the parametersof spreadingat two pointsof the
spreadingsystem(Table4), however,suggests
that,in general,the
ridgesegments
wereprobablynotperpendicular
to thespreading
direction,evenin the laststagesof thespreading.A detailedstudyof
the topography
wouldbe necessary
to modelfurtherthe evolution
of the ridge,especiallyin the areayoungerthananomaly6b, where
a progressive
reorientation
of the spreadingsystemis stronglysuspected.
Asymmetricspreading,ridgejumpsand ridgepropagation. In
additionto themajorridgejumpjust afteranomaly7, theevolution
of the SouthChinaspreadingcenterseemsto havebeencharacterizedby smallridgejumps,alwaysto thesouth,andby a systematic
asymmetryof spreading,ratesto the northgenerallybeinggreater
thanto the south. Relativeto SouthChina,this asymmetryreflects
a southwardmotion of the ridge faster than that due only to
spreading. Stein et al. [1977] have shownthat migration of a
spreading
ridgewith respectto thedeepmantlemightinduceasymmetry,with fasterspreadingon the trailingside. In all modelsof
6320
BRIAIS ET AL.: RECONSTRUCTIONS
OF THE SOUTH CHINA SEA
openingof the SouthChina Sea, the smallersouthernplatesare
implicitlyassumed
to havea greaterabsolutemotion(relativeto the
deep mantle) than the larger northernplate (Eurasia). In other
words,as the SouthChina Sea spreadingcenterwas movingsouth
relativeto SouthChina(Eurasia),it alsomovedsouthrelativeto the
deepmantle. The asymmetryobservedin thebasinis thusthatexpectedfrom thequalitativemodelof Steinet al. [1977].
The ridge jump observedat the time of anomaly7 seemstoo
largeto be linkedwith the asymmetryof the extensionin thebasin.
Rather,it probablyreflectsa changein the geodynamicboundary
conditionsprior to the propagationof the ridgetowardsthe southwest,that may havetriggeredboth thejump andthe propagation.
An attemptto reconstruct
the ridge at the time of thejump reveals
that the old and new crustsoverlap,implying that the old and the
new ridgeswere simultaneously
spreadingfor sometimebeforethe
old onediedout. That ridgejumpsareprogressive
andnot instantaneousis widely documented
in otheroceanicbasins.
Oneconsequence
of thepropagation
of a rift is thewedgeshape
of the continent-ocean
boundarydue to the progressivetearingof
the continent[e.g., Courtillot, 1982, Courtillotet al., 1984]. The
irregular,stepwisedecreaseof the areaflooredby oceaniccrusttoward the SW suggests
thatridgepropagationoccurredin a discon-
CHINA
•
/•,
EARLY
MIOCENE
:HINA
/TAIWAN
• [•
,:
•
ß
'
•'.
f
/
{•
0
BASIN
tinuous fashion. It was fast between anomalies 7 and 6b, rather
slowbetweenanomalies6b and 6a, andthenacceleratedagainbefore anomaly6 (Figures10 and 11).
COMPATIBILITY
WITH SURROUNDING
DISCUSSION
AND IMPLICATIONS
EVOLUTION
CONTINENTAL
TECTONICS:
FOR THE TERTIARY
108'
110'
I 150
120
ø
OF SOUTHEAST ASIA
In this section we try to relate quantitatively the unusual
evolution of the South China Sea spreadingsystem with the
Tertiarytectonicevolutionof the continentalareassurrounding
that
sea. In regionssuchasthe westernPacific,it hasbecomeclearthat
theopeningof marginalbasinsis a three-dimensional
problem,and
that the local relationshipbetweenback arc extensionand nearby
subductionhasto be integratedinto a larger-scaleplate tectonic
framework [e.g., Tapponnier et al., 1982, 1986; Peltzer and
Tapponnier,1988;Jolivet et al., 1989]. An importantproblemin
reconstructing
seafloorspreadingin the SouthChinaSeais thatthe
spreadingcentersthat led to its opening formed, evolved for
•15-20 m.y. and "peacefully"died out while being surroundedto
the north, west and southby large continentalblocks. Clearly, a
prerequisite
to constrainthetectonichistoryof the SouthChinaSea
is a three-dimensionalunderstandingof the large-scaleTertiary
tectonicsof its continentalsurroundings.
Openingof theSouthChina Sea WithintheFramework
of Asian Tectonics
Given that the present-dayorientationof extensionaltroughs
northandsouthof the basin,andof the relict spreadingaxis,is almostperpendicular
to the Vietnamcoast,all modelsof openingof
the SouthChinaSeapostulatethe existenceof a majortranscurrent
systemwestof thebasin(Figure13). Sinceseveraldeepsedimentarytroughsparallelthe Chinesemargin,initial riftingis thoughtto
haveinvolvedmostlynormalfaultsalongthismargin,hencemostly
strike-slipfaultsalongtheVietnammargin. From a kinematicpoint
of view, it is possibleeitherthattheriftsterminatedagainsta rightlateral fault following that margin (Figure 13a), with only small
fragmentsdetachedfrom the Chinesecontinentslidingsouthalong
it [Holloway, 1982; Taylor and Hayes, 1983;Lu et al., 1987], or
that left-lateralstrike-slipfaultsfrom within the continentpropa-
Fig. 13.Fundamental
difference
between
previous
modelsfor theopening
of the SouthChinaSea.(a) Taylor and Hayes [1983] (Early-Miocene
stage):Right-lateralfaultingalongthe Vietnamtransform,with Indochina
andBorneofixedrelativeto SouthChina,(b) Tapponnier
etal. [1986]:leftlateralfaultingbetweenSouthChinaandIndochina,
with Borneomoving
with Indochina. BB: BeibuBasin,DG: DangerousGrounds(Spratly
Islands),MB: MacclesfieldBank, NBB: North BorneoBasin, NP: North
PalawanBlock,PA: Paracels(Xisha)Islands,RB: ReedBank,SA: Sabah,
SCAR: ScarboroughSeamountchain, SP: South Palawan Block, XT:
XishaTrough,YB: YinggehaiBasin.
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
gated along that margin into the rift system (Figure 13b)
[Tapponnieret al., 1982;Peltzerand Tapponnier,1988]. The secondprocessis now observedat the appropriatescalebut at a more
incipientstagein North China (Figure15) [e.g., Tapponnierand
Molnar, 1977]. Thesetwo hypotheses
drasticallyopposethe two
typesof kinematicmodelsof openingthat havebeenput forward
andrepresenttwo mechanisms
thatinvolvedistinctdrivingforces.
In thefirsttypeof model,theopeningof theSouthChinaBasinis
only drivenby the inferredsubductionof older oceaniccrustinto
the North Borneotroughto the south. In the secondtypeof model,
thecontinental
riffs first developat thetip of propagating
left-lateral
strike-slipfaults,andtheextension
is drivenby therelativemotion
of Indochina,or of slicesof it, that are pushedtowardsthe SE or
SSE relativeto SouthChinaby the penetrationof India into Asia.
Subductionzones around the southeasternedge of Indochina
mostlycontributeto makethatmotionpossible.
To our knowledge,work in regionssouthof the South China
Seahasnotyieldedcompellingevidencefor hundreds
of kilometers
of shortening,consistentwith subduction,followed by collision,
duringthe period between30 and 15 Ma. On seismicreflection
profiles,the faultedOligocene-EarlyMiocenecarbonateplatform
that includesDangerousGrounds,Reed Bank and underliesthe
North Palawantrough,apparentlyextendsunderSouthPalawanto
the NW Sulu basin with a southwarddip of only a few degrees
[Hinz et al., 1985;Rangin and Silver, 1991]. This impliesthatthe
North Palawantroughmerelyresultsfrom elasticdownwarpingin
responseto loading by a thrust wedge along the NW edge of
Palawan[Hinz and Schl•iter,1985;Hinz et al., 1985]. That thrust
wedge may extendeastwardsto Mindoro, where Mid-Oligocene
oceaniccrust,perhapsformely part of the South China seafloor,
was obductedaround15 Ma [Ranginet al., 1985]. Thuswhile up
to a few tensof kilometersof shorteningalong the northwestern
edgesof Palawan[Ranginand Silver, 1991] andNE Borneo,particularlyafter the Mid-Miocene,havebeendocumented,
the existence of a subductionzone along the NW edge of Borneo and
Palawan [Hamilton, 1979; Taylor and Hayes, 1980, 1983] is in
doubt. The existenceof manyhundredsof kilometersof left-lateral
movementalongtheRedRiver Faultin the Oligo-Miocene,on the
otherhand,is now corroborated
by structuraland paleomagnetic
evidence,andby radiometricdating. For thisreason,we shallfocuson testingthe secondtypeof model,usingthe resultsof the reconstruction
presented
earlierin thepaper.
The similaritybetweenthe large-scalepatternof fault zonesin
southeastern
Asia andin indentationexperiments
suggests
thatthe
localizationof strainalonga smallnumberof longandnarro•
strike-slipshearzonesthatpropagate
rapidlywithin thecontinental
lithosphereis an inescapable
elementof continentaltectonics[e.g.,
Tapponnieret al., 1986;Peltzer and Tapponnier,1988;Davy and
Cobbold,1988;Peltzer, 1988]. To a first approximation,
thevarious blocksboundedby thesezones(Malaysia, Indochina,South
ChinaandNorth China)may thusbe seento behaveasrigid rotating blocks,themotionof whichmaybe described
by platetectonic
parameters.We examinethe mechanics
of openingof the South
ChinaSeain thislight.
6321
ductileshearalongthe 1000-km-longRed River Faultzone. That
shearhasformeda 10-to-20-km-wide
beltof high-grade
gneisses,
withnearverticalfoliationandnearhorizontal
lineation
[Tapponnier
et al., 1990a;Leloup,1991]. Theshearzoneincludes
leucogranites
derivedfrom late tectonic,anatecticmeltingwithin the gneisses,
whichprobably
implyshearing
of thelithospheric
mantleaswellas
thecrust[FleitoutandFroidevaux,1980]. Radiometric
datingof
leucogranites
in twolocations
alongthebelt(AilaoShan)yieldsan
ageof 23.0-,-0.2 Ma for the late ductiledeformationalongthe
shearzone[Schiireret al., 1990]. Majorupliftof thecentralAilao
Shangneisses
hadoccurred
by 18 Ma [Harrisonet al., 1992]. The
present
offsetbetween
theUttaradittandtheJinshasutures
suggests
thattheamount
of premiddleMiocenedisplacement
alongthefault
hasbeenat least500-700 km [Tapponnieret al., 1986, 1990a;
PeltzerandTapponnier,
1988;Leloup,1991]. Fromcomparisons
of the paleolatitudes
of coevalred bedsin Thailandand China,Z.
YangandJ. Besse(Paleomagnetic
studyof PermianandMesozoic
sediments
fromNorthernThailandsupports
theextrusion
modelfor
Indochina,submitted
toEarth andPlanetaryScience
Letters,1992,
hereinafter
referredto asYangandBesse(submitted
paper))suggestas much as 1500 *_.800 km of post middle Cretaceousdisplacementbetweenthe Indochinablock andthe SouthChinablock.
The WangChaoandThreePagodas
Faults,thatslicethrough
Burma,Thailandand Cambodia,left-laterallyoffsetthe Early
Mesozoicgranitebatholithof centralThailandby about300 km
[Tapponnier
et al., 1986]. Thesefaultsalsodisplaceanddeform
Jurassic
andCretaceous
rocks[Departmentof Mineral Resources,
1982], an observation
which is compatiblewith Cenozoicmovement on them. In the field, they showleft-lateralshearin lowgrade mylonitesand ultramylonites[Lacassinet al., 1992].
Paleomagnetic
evidencesuggests
a 10-25øclockwiserotationof the
Khorat plateau,the stablecore of Indochina,relative to Eurasia
[Achache
et al., 1983;ChenandCourtillot,1989;YangandBesse,
submitted
paper].Finally,boththeRedRiverandtheWangChao
faultshavechanged
sense
fromleft-lateral
in Oligo-Miocene
timeto
right-lateral
in theQuaternary
[Allenet al., 1984;Tapponnier
et al.,
1986],whichhasprobablyreducedthepresent-day
left-lateraloffsetsof geologicalmarkersacrossthem.
Compatibility
ofSeafloorSpreading
withLeftLateral
MotionAlong theRed River Zone
Assumingthattheregionsnorthor southof theSouthChinaSea
ridgedid not deformmuchbetween32 and 16 Ma, andthatblocks
on the southernside of the SouthChina Sea spreadingridge
(Dangerous
Grounds,
ReedBankandmuchof Borneo)remained
attached
to Indochina
duringtheopeningof thesea,thedisplacement on the Red River Fault estimated from reconstruction of the
magnetic
isochrons
in theSouthChinaSeais compatible
with the
sinistralshearrecentlydocumented
in Yunnan(Figure14). This
positive
testof kinematic
compatibility
isillustrated
bymovingback
thenortheastern
boundary
of Indochina,
takento lie alongthepresenttraceof theRedRiverFault,relativeto SouthChina,usingthe
rotation
parameters
derivedfromthestudyof theoceanic
partof the
basin(Tables2, 3, Figure12). Overall, the successive
positions
of the tracesuggestthatmuchof the spreadingof the SouthChina
TimingandAmountofMovementonLeft-Lateral
Sea may be related to strike-slip movement on that fault
Southeast Asian Fault Zones
(Figure 14). Table 5 showsthe direction,rate and amountof
The kinematicsandagesof the deformationobservedalongthe finite motionof two points(C, northof the Uttaradittsuture;D,
Red River Faultzoneconfirmthatthiszonehasbeena majormid- near the Gulf of Tonkin) which now belong to the fault trace,
Tertiary left-lateralboundarybetweenSouthChina and Indochina. computed from the stage poles of rotation derived from the
Recentfield work in Yunnanhasrevealedubiquitousleft-lateral magneticisochrons(Table 3). The computeddirectionsand the
6322
BRIAIS ET AL.' RECONSTRUCTIONS OF THE SOUTH CHINA SEA
reconstruction
of Figure 14 showthat to first orderthe Red River
Faultbehavedasa transcurrent
plateboundary,allowingIndochina
and the blocksattachedto it (Borneo,DangerousGrounds,Reed
Bank)to movesoutheast
relativeto SouthChina. After 30 Ma, the
estimated
left-lateralsliprateontheRedRiverFaultzonewasof the
order of 35 mm/yr, a value typical of great strike-slipfaults at
present.Thetotalamountof left-lateralmotionlinkedwith seafloor
spreading(=560 km) (Figure14, Table5) is compatiblewith that
estimated
fromtheoffsetof theJinsha-Uttaraditt
suture[Peitzerand
I
I
•
[
Tapponnier, 1988], which is seen to be nearly restored in
Figure 14.
I
SOUTH
[•
30N
STRIKE PERPENDICULAR
COMPRESSION
CHINA
BLOCK
-
SHORTENING
ZONE
At a moredetailed
level,thereconstruction
predicts
thatstrike:
perpendicular
shortening
(=160km)should
havecharacterlzed
thd
tectonics
ofregions
along
thenorthwestern
segment
of thefaultin
the San Jiang region, while strike-perpendicularextension
(=130 km) shouldhaveoccurredalongthesoutheastern
segmentof
the fault (Figure14, Table5). Thesepredictionsare compatible
with the observedshortening,
by NNW-SSE trendingfolds,of the
Mesozoicred bedsin northwesternYunnan [Tapponnieret al.,
1990a],andwith theapparent
scissor
openingof thewedge-shaped
SAN
JIANG
k• TOTAL=542km
•,.•• --....
25N
STRIKE
PERPENDICULAR-
....•:•..'
'••'• lkmEXTENSION
o lOO
I
:: ß C••
70km
i! ." ..
.•' • 54km
500km
I
:. ß .-
::::
20N
32Ma
......
30Ma
......
.,&-
••
,'•
•••
':: ß •.•'
(
D•:O/
•
•
26Ma oøøø
'
20Ma ***
Present ß ß
,NDOCHNA
R';•
I
15N
85E
95E
90E
100E
105E
110E
STRIKE
NW-SEHanoiandYinggehai
basins.
Suchinferences,
however,
canonlybetakenasqualitative,
sinceit is likelythatlateMioceneto
presentstrainin Yunnanprovince,suchasleft-lateralmotionalong
theN-S trendingXiaojangFaultandcontinuingdextralshearin the
China-Burmaborderregion,hasbent the traceof the Red River
Fault, whose western segmentis now concaveto the north.
Nevertheless,
the strike-perpendicular
compression
andextension
duringTertiaryleft-lateralshearprobablyreflectthe fact thatthe
fault was not a small circle about the Indochina/South China
PERPENDICULAR
COMPRESSION
PURE
STRIKE-SLIP
TRACE
Pole
of
averagepole of rotation(5.3øN, 66.3øE,Figure12a), this pole
beingcloserto the traceof thefault thanif thishadbeenthe case
(Figure14b). Other large active strike-slipfaults of Asia (in
particularthe Altyn Tagh-Haiyuanfault system)alsosliceacross
regionsof shortening
in thewest(Tibet)andregionsof extension
in
the east (Shansi and North China plains) (Figure 15) [e.g.,
Tapponnier
et al., 1986;PeltzerandTapponnier,
1988].
Fig.14.(a)Successive
positions
(relative
to South
China)
of northern
boundary
ofIndochina
(taken
toliealong
thepresent-day
trace
oftheRed
River
Fault
), moved
back
attimes
ofanomalies
6,7 (ridge
jump),
10and
OF
)•, THE
FAULT
-3- rotation
/ 4...• "'• STRIKE
/
•. PERPENDICULAR
11, usingthepolesof rotationlistedin Table1. Blockssouthof the
spreading
ridgeareassumed
tohavebeenpartof theSunda
shelfsince
32
Ma.(b)Strike-perpendicular
compression
andextension
resulting
froma
ß EXTENSIONsmaller
distance
between
thepoleof rotation
andthefaultthanif thefault
were a small circle.
SMALL
CIRCLE
TABLE 5. Directions
(D), RatesandAmountsof Displacement
onTwo Pointsof RedRiverFault
Point C
Point D
(NearNorthernEnd
of UttaradittSuture)
Ageend,
Anomalies Ma
A,
deg
D,
deg
Rate,
(NearImmersion
in Gulf of Tonkin)
Finite
mm/yr Displ.,
km
A,
deg
D,
deg
Rate,
Finite
mm/yr Displ.,
km
End- 5e
5e - 6
15.64
19.00
28
46
-59
-12
37
30
124
43
28
49
-49
-8
37
31
124
45
6 - jump
jump- 8
20.45
25.91
18
9
-40
-69
27
29
147
57
19
8
-26
-38
28
27
154
51
8- 10
10- 11
Total
End - 6
27.74
30.32
78
21
-9
-32
50
62
82
23
-7
-21
50
66
15.64
30
-50
33
128
56
555
157
30
-39
33
130
61
565
157
jump- 10
25.91
22
-27
37
157
24
-17
39
170
Directions
andamounts
computed
fromstage
poles
obtained
withPatriat's
[1987]method.
Points
C andD shown
inFigure
14. D taken
positive
eastward.
Symbol/5,
distance
between
stage
poleof rotation
andpointof measurement.
BRIAISET AL.: RECONSTRUCTIONS
OFTHE SOUTHCHINA SEA
6323
N/
GOBI
DESERT
•
1_2o
ORDOS
PLATEAU
,; ;;•'
_
.:::::::::'
CHNAO ....
:::?
PLAINS
••••••••••••
EAST
ß"
::::•
CHINA
SEA
Fig.15.Present-day
tectonics
ofAltynTagh-Haiyuan-Qinling
left-lateral
faultsystem
withassociated
zonesof compression,
and
en•chelonriftsandright-lateral
faultsin NorthChina.Adapted
fromTapponnier
andMolnar[1977].Faultplanesolutions
of the
1966 Xing Tai, 1969 Bohai,and1976ChanTangearthquakes
areshown. Inferredcounterclockwise
rotationof blocksis after
PeltzerandTapponnier
[1988].Thismaybeviewedastheincipient
analog
oftheRedRiverFaultsystem
during
theOligocene.
RelationshipBetweentheOpeningof theSouthChinaBasin
and SurroundingContinentalStrike-SlipTectonics
First stagesof rifling - Eoceneto Mid-Oligocene(40 to 30 Ma).
Thebeginning
of theriftingis datedasEoceneto earlyOligocene
The followingpalinspastic
scenariorestsuponthecompatibility, by subsidence
studiesalongthe SouthChinamargin[Holloway,
outlinedabove,betweenthe parameters
of spreadingin the South 1982; Ru and Pigott, 1986; Suet al., 1989] and as latest
ChinaSeaandthe kinematicsof faultingin Southeast
Asia, which
suggests
thatthe assumptions
madeareplausible.It alsotakesinto
accountother geologicalconstraintssummarizedby Hamilton
[1979],Holloway[1982],TaylorandHayes[1983],Tapponnieret
al. [1986] andRanginand Silver [1991]. We beginwith a hypotheticalreconstruction
of Southeast
Asia shortlyafterthe onsetof
riftingin theSouthChinaSeaandon theSundashelf(Figure16a).
We thenpresenta seriesof palinspastic
maps(Figures16b-16e),
in which the successiverotationsof SoutheastAsian blocks,and of
observedstructuralmarkersfixedto them,arecomputedonlyfrom
thefit of magneticisochrons
identifiedin the SouthChinaSea. The
final map (Figure16f) showsthe tectonicchangesthat have occurrednearthe southeastern
marginof thebasinbetweenthe endof
spreading
andthe presenttime.
Cretaceous-Eocene
by seismicand well studieson the southern
Sundashelf[Hinz andSchlfiter,1985;Hinz et al., 1985;Rangin
and Silver; 1991]. Becausethe Thai, Malay and northBorneo
(Zengmu)basins,
withup to 10 km of sediments
[Anderson
et al.,
1978]lie alongtheextension
of theThreePagodas
andassociated
strike-slipfaults,we takethe view thatrifting within the Sunda
shelfwasrelatedto left-lateral
motionalongthisfaultsystem
[e.g.
ZhangWenYou,1983;Tapponnier
et al., 1986](Figure16a). As
a resultof suchleft-lateralmotion,it is possiblethatoceaniccrust
formedin thelateEocenein theeasternmost
partof theextensional
system(Figure16), yieldinga sourcefor the obducted
ophiolites
of CentralPalawan(H. Kreuser,unpublished
report,1983,ascited
in the workby Hinz et al. [1985]), and the Cagayanvolcanics
[RanginandSilver,1991].
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
6324
E90
N3•80
El00
E110
E120
E130
El00
E9•0
EURASIA
a' ' ' •&^s,^'
' ' '••
N30
d
\
N25
ß'
N20
_
N15
INDIA
N10
N5
•
0
INDO
INDOAUSTRALIAN
PLATE!
AUSTRALIAN
PLATE
Late Eocene
S15
I
I
N3•80
Anomaly 6-20 Ma
I
E90
I
I
E100
b
I
I
I
E110
I
E120
E130
E90
E130
El00
e
EURASIA
SOUTH
CHINA
EURASIA
P
Philippine
•
Sea
-,,,,••NDOCi4NA
' •••Z.•'•
'"'"'•
/:'t hill
pin
N5
0
S5
INDO AUSTRALIAN
• PLATE
s10
INDO
End of spreading
15.5
Anomaly 10-30 Ma
PLATE
AUSTRALIAN
Ma
I
s15
.............................
N3•80
C
E90
E100
E110
•u.^s,^
E120
E130
•
•^c,•,c
N20
'••••
N25
E90
-•'
E100
E110
"•
i'
o
s5
AUSTRALIAN
•'Z '"•__
s•o
INDO
AUSTRALIAN
PLATE
Anomaly
7-26
Ma
S15
I
I
I
I
I
I
I
I
I
Present
........
PLATE
E120
I
BRIAISET AL.: RECONSTRUCTIONS
OFTHE SOUTHCHINA SEA
6325
Justafter anomaly7 the spreadingsystemjumped to the south,
and the ridge began to propagate towards the southwest
(Figures10 and 16d). We infer that a preexistingcrustalfabricor
the rheologyof the lithospherepreventedthe ridge from jumping
and propagatingin a singlestep,at the time of anomaly10, and
Beibu(Gulf of Tonkin)andYinggehai(westof HainanIsland) maintainedMacclesfieldandReedBankstogetherbetweenanomatroughsformedpull-apartbasinsat the southerntip of the Red lies 10 and 6b (Figures16b, 16c and 16d). With -500 km of
River Fault system[Zhang Wen You, 1983; Tapponnieret al., propagationbetween anomaly 7 (26 Ma) and the cessationof
propagationof the
1986]. Expanding
spreadprofilesperformed
on the SouthChina spreading(16 Ma), the rate of southwestward
marginrevealthatthecrustbeneath
theXishatrough(Figure13) is rift was 50 mm/yr on average. It might have been as fast as
anomalously
thin, suggesting
thatthistroughis alsoa failedrift 83 mm/yr if most of the propagationoccurredbefore anomaly6
(20 Ma). Relativelyrapidpropagation
(30 mm/yr)andlocalization
[HayesandSpangler,1988].
Jumpsandsoutheastward
propagation
of spreading:A resultof of a spreadingcenterinto diffuselyrifted andstretchedcontinental
southwardpropagationof the Red River Fault. At the time of crust is observedat presentin southernAfar along the Gulf of
anomaly10 (30 Ma) spreading
stopped
in thenorthwestern
rift, but Tadjourah[Courtillot,1982;Courtillotet al., 1984]. The Afar area
continuedeastof MacclesfieldBank(Figure16b). Freeair gravity is characterizedby rotationsof blocks of various scales [e.g.,
anomalies [B. Chen, 1987] suggestthat the westernpart of Tapponnieret al., 1990b;Actonet al., 1991] at relativelyfastrates
MacclesfieldBank was affectedby rifting, while the easternhalf (lø/105years),in a regionwhere"buffer"zonesof diffuseextenbehavedasa singleblock. Thatextension
in theSouthChinaBasin sioncoexistwith morelocalized,overlappingvolcanicrifts. In the
was more diffusein the west than in the easthaslong beenrecog- South China Sea, the zone west of and between Macclesfield and
nized[TaylorandHayes,1980,1983;Hayes,1985]. We interpret Reed Banksmay have representedsucha transitionregioninto
thisasa resultof thepropagation
of theRedRiverFaulttip to the which the spreadingcenteronly localizedand propagatedfrom
First stageof spreadingin the basin: Anomaly11 (32 Ma).
Seafloorspreading
startedto the northin a basinparallelto the
SouthChinamargin(includingthenorthwestern
rift). Thatbasin
was probablypart of a systemof basinsarrangeden •chelon,
branchingat the tip of the Red River Fault (Figure16b). The
south,leadingto transient,diffusestretchingandrifting,which
neverevolveintostableseafloorspreading
because
of theirproximity to thefaulttip. Thisprocess
mayhavebeendominant
in thearea
westof MacclesfieldBank(Figures16c, 16d and17).
anomaly7 onwards. At present,there is no informationon
possible tectonic rotations at the scale of blocks such as
MacclesfieldBank. Our interpretationpredictscounterclockwise
rotations(Figure 17), comparableto those now inferred in
Polea
of rotation not
too far from South
China Sea
directed
•
South
"""•*""••?rd
[
jumps
Strike-slip fault
propagation rate
migration
faster than slip rate
and corresponding
spreading rate
ni wi1
,,,/
,,/
Less extension in
westem basins
Zoneof greatmstab•#ly.
Asymmetric
spreading
More extension in
eastem basins
Ead•erandeaser
d•ffuseextensionalstra,n Stlort
localizationof sprea•ng
livedntisn•gratesouth
centersocearaccrust
Southwardpropagation
of the faulttendsto dragwestemrift
segments
andprovtdes
dnvtngromeforant•clockmse
rotations,
asymmetnc
spreading
andsouthward
ndgejumps,
ruth a slowerrate southof the ridge
Fig.17.(a) Sketchshowing
howstrike-slip
faultpropagation
mayleadto transient
extension
nearthefaultwhilemoresteady
spreading
occursfartheraway.(b) Sketchshowing
howleft-lateralstrike-slip
faultpropagation
mightaccount
for complexities
in
theopeningof the SouthChinaSea.
,1
,
Fig. 16. (a) Hypotheticalreconstruction
of the first stageof extensionof the Sundashelf.Formationof northBorneoextensional
trough.(b) to (e) Reconstruction
of SouthChinaSeaandSoutheast
Asia at timesof anomalies10 (30 Ma), 7 (26 Ma), 6 (20 Ma),
andwhenspreading
stopped(15.5 Ma). (f) Present-day
geometryof SouthChinaSeaandsurrounding
regionsof Southeast
Asia.
Notetherotationof Borneobetween16 Ma andpresent.Speckledareasarestretched
continental
crust;darkareas,oceaniccrust.
6326
BRIAIS ET AL.: RECONSTRUCTIONS OF THE SOUTH CHINA SEA
NortheastChina(Figure15) [Peltzerand Tapponnier,1988],while
interpretations involving right-lateral shear along Vietnam
[Holloway, 1982; Taylor and Hayes, 1983] predict clockwise
rotations. Hence paleomagneticstudiesof orientedcoresmight
allow to testfor suchrotationsand decidewhich interpretationis
morelikely. We notethatTertiarycounterclockwise
rotationshave
been documentedalong the fragmented northwesternedge of
Borneo[Schrnidtke
et al., 1990], eastof the extremityof the leftlateralWang Chao-ThreePagodasfault system,a regionprobably
analogousto thosediscussed
above.
The directionsof spreadingcomputedfrom the finite polesof
rotation are closer to NNW-SSE after the ridge jump (26 Ma).
This leadsto thereorientation
of the spreading
segments
from E-W
to N70øEobserved
in theeast(Figure11), with anomalies
morerecentthananomaly6b all displayinga very disrupted
pattern,while
spreadingsegmentsin the southwest,which are relatively long,
probablyformedroughlyperpendicular
to thespreading
directionas
the ridgesystempropagatedto the southwest.Within a large-scale
tectonic framework dominated by left-lateral motion between
Indochina and South China, the jumps, the readjustmentsin
spreadingdirection,andthe irregularsouthwestward
propagation
of
the spreading
centermaybe viewedasa consequence
of the southsoutheastward
propagation
of the Red River Fault. From a position
southwest
of Hainan(17øN in a referenceframefixed to presentday SouthChina) around30 Ma, the Red River Fault tip might
havepropagated
to a positionsoutheast
of Vietnam(11øN in that
samereferenceframe) around20 Ma, or about600 km in 10 Ma,
at an averagerate of 60 mm/yr. That boththe averagerate of slip
on the RedRiver Faultandof openingin the SouthChinaSea(3550 mm/yr) arelessthanthe averagerateof propagation
of thetip of
that fault is to be expected. If only elasticstressesgovernedthe
propagationof large-scaletensile or sheardiscontinuitiesin the
lithosphere,
a muchfasterrateof propagation
thanof slipor opening, in proportionto the aspectratioof suchdiscontinuities,
would
ensue. Even with viscousstrainin the more ductilelayersof the
lithospheredampingpropagation,it seemslikely that the rate of
propagation
wouldremaingreaterthattherateof slipon thefault.
The propagationof the spreadingsystemstopsat anomaly 6
(20 Ma) (Figures11, 16d, and 16e). It is possiblethatat thistime
a steadystateconnectionwasfinally reachedbetweenthe Red River
strike-slipfault andthe SouthChinaSeaspreadingcenter,although
[Ranginand Silver, 1991]. This deformationprobablyatteststo
more efficient couplingbetweenthe Philippineand the Eurasian
Plates. Similarly, more active terraneaccretion,leadingto the
present-daycollision with Australia, begins along the Eastern
Sundasubductionzones. Both factors may have contributedto
stopthe southeastward
extrusionof Indochina(Figure16f).
CONCLUSION
Clearly, the reconstructions
presentedhere, the choiceof hypotheses
fromwhichtheyderiveandthepalinspastic
scenarios
with
whichtheyarecompatiblearenotunique. Oneparticularlyimportanthypothesis
is the choiceof the magneticanomalysequence
in
the southwestern basin. Nevertheless, because the Red River-
Ailaoshanfaultzoneis a largeOligo-Miocenediscontinuity
thatextendsinto the SouthChina Sea, with apparentlyat least500 to
600 km of left-lateraldisplacement,
andbecausedisplacement
on
thisfault is compatiblewith seafloorspreadingin thatsea,we believethatit hasplayeda leadingrole in the formationandevolution
of the sea. The reconstructions
of Figure16 are the simplestthat
accountfor the existingbody of observations
bothwithin the basin
andin continental
regionssurrounding
it to thenorthandwest.
At a more detailedlevel, the inferencethat the openingof the
SouthChinaSeabasinsis linkedwith the propagation
of a large
left-lateralstrike-slipfault(theRedRiverFault)alongtheirwestern
margin, and the averageposition, relative to this fault, of the
Indochina/South
China rotationpoles,providekinematicallyand
mechanically
acceptable
waysto accountfor thecomplexshapeof
thebasins,thetransitionbetweenseafloorspreading
in theeastand
diffuseextensionin the west,possiblecounterclockwise
rotations
in zonesof diffuse extension,as well as generallyasymmetric
spreadingand systematically
south-directed
ridgejumps. This is
schematically
summarized
in Figures17a and 17b. Thatthe stage
polesof rotationare roughlyalignedon a greatcircle that crosses
the SouthChinaSeasuggests
that the mechanismof openingdid
notchangedrasticallyduringtheopening(Figure12).
At the coreof the extrusionscenarios
proposed
by Peltzeret al.
[1982]andrapponnieret al. [1982,1986]is thehypothesis
thatthe
collisionbetweenIndia andAsia providesan essentialpartof the
forcesdrivingseafloorspreadingin the SouthChina Sea,andthat
subductionalone,southand eastof that sea,althoughrequiredto
that connection is difficult to trace with extant data. The estimated
allow extrusion,would not sufficeto do the job. The work premotion on the fault is then almostpurely strike slip, as seenin sentedhere supportsthis inference. CollisionhasprobablyconFigure14. The strike-slipfault is closestto a transformfault, its tributedto guidethekinematics
of seafloorspreading
in theSouth
horizontalmotionbeingtransmitted
almostentirelyto the spreading ChinaSeain two ways. First,it hasactivatedlarge,concaveto the
system. This scenariomight explainthe increasein the spreading south,left-lateralstrike-slipfaults cuttingeasternAsia andterminorthintoAsia,
rate after anomaly5d. The homogeneous
NE-SW orientationof natingintopull-aparts.Second,asIndiapenetrated
scarps,visible on the Sea Beambathymetry[Pautotet al., 1986, thus moving past Indochina, it probably induced successive,
1990; Briais et al., 1989], suggeststhat this situationand the clockwiserotationsof the large blocks extrudedbetweenthese
N140 ñ 10øEspreadingdirectionremainedsteadyuntil the cessa- faults. The faults are thus less curved than the small circles about
the Euler poles deducedfrom seafloorspreadingreconstruction
tion of the spreading.
Cessationof spreading. After 16 Ma, spreadingapparently (Figure14). Our resultssuggestthat the geometry,kinematics,
fieldsandscaledependence
of thefaultingpatternand
stoppedall alongthe ridgeaxis(Figures11 and 16e). In the inter- displacement
arebestillustrated
by thequantitative
descrippretationdiscussed
here,the end of spreadingis seenas a conse- of theblockrotations
thatwerefirstusedto modelexquenceof processesoccurringwell beyondthe marginsof the tionsof theplasticineexperiments
South China Sea. We suggestit is related to the cessationof trusiontectonicsin Asia [Peltzer, 1983;Peltzer and Tapponnier,
motion on the Red River Fault, itself inferred to result from the
1988],eventhoughsuchexperiments
areon a plane,andnot,ason
deformationand clockwise rotation of the fault system, and to Earth,a sphericalsurface.
Althoughpaleomagnetic
studieshaverevealedcounterclockwise
changesin the stressfield as the Indian indenterpassesMalaysia
and Indochina [e.g., Tapponnier et al., 1986; Peltzer and rotationsof Mesozoicand Cenozoicrocks along the deformed
Tapponnier,1988]. The middle Miocene is alsoa time of crustal southernmarginof the SouthChina Sea [Schmidtkeet al., 1990],
shorteningbetweenPalawanand the CagayanRidgeto the south the clockwiserotationof the Khoratredbeds,whichrestuponthe
BRIAISETAL.: RECONSTRUCTIONS
OFTHESOUTH
CHINASEA
6327
Yashui, Heat flow, thermal conductivity, thermal gradient, in
relativelyundeformedcore of Indochina,atteststo a 10ø to 25ø
Geophysical
Atlas of East andSoutheast
AsianSeas,Map and Charts
clockwisebulk rotationof Indochinarelativeto Asia [Achacheet
Ser., vol. MC 25, edited by D. E. Hayes, Geological Society of
al., 1983; Chen and Courtillot, 1989; Yang and Besse,submitted
America, Boulder, Colo., 1978.
paper]. This rotation is in keeping with the reconstruction Avouac,J.-P.,P. Tapponnier,andG. Peltzer,Activethrustingandfolding
presented
here,whereseafloorspreading
aloneinducesabout12øof
alongthe northeastern
Tien Shan,and Late Cenozoicrotationof the
clockwise
rotation
of Indochina
relative
to South China.
The
existenceof a relativerotationbetweenthesetwo largecontinental
blocksdrawsattentionto a radicalshortcoming
of the plane-strain,
large-scale"domino"modelsthat are now in fashionto describe
strike-slipdeformationin collisionzones[e.g.,Deweyet al., 1989;
EnglandandMolnar, 1990]. Althoughtheparallelstrike-slipfaults
that separatethe adjacent,"domino"-shaped
blocksin suchmodels
leadto fast,coevalrotationof theblocks,theypermitno rotationof
one block relative to the next. Both the analysisof magnetic
isochrons in the South China Sea, and the paleomagnetic
declinations
observed in the late Mesozoic
beds of the Khorat thus
Tarim relativeto DzungariaandKazakhstan,
J. Geophys.
Res.,in press,
1992.
Berggren,W. A., D. V. Kent, J. J. Flynn, and J. A. Van Couvering,
Cenozoicgeochronology,
Geol.Soc.Am. Bull., 96, 1407-1418,1985.
Blakely, R., and A. Cox, Identification of short polarity events by
transformingmarinemagneticprofilesto the pole,J. Geophys.Res.,
77, 4339-4349, 1972.
Bowin, C., R. S. Lu, C. Lee, and H. Schouten,Plate convergenceand
accretionin theTaiwan-Luzonregion,Am.Assoc.Pet. Geol.Bull., 62,
1645-1672, 1978.
Briais,A., P. Tapponnier,
andG. Pautot,Constraints
of Seabeamdataon
crustalfabricsand seafloorspreadingin the SouthChina Sea,Earth
Planet. Sci. Lett., 95, 307-320, 1989.
properties
of estimated
rotations,
J. Geophys.
imply that plane strain, "domino" models provide incorrect Chang,T., On thestatistical
Res., 92, 46319-46329, 1987.
descriptionsof the Cenozoictectonicsof SoutheastAsia. As
Chen,B., Freeair gravityanomalies,
in Atlas of Geologyand Geophysics
convergencebetweenIndia and Asia was progressivelyabsorbed
of theSouthChina Sea, scale1:2,000,000,SecondMar. Geol. Invest.
north of what is now the Gangesplain, it is unlikely that blocks
Brigade of the Minist. of Geol. and Miner. Resour.,Guangdong
located far north of India rotated as fast as those located closer to it.
It is also unlikely that they startedto rotateat the sametime and
rotatedthesameamount.Recentstudies[Chenet al., 1991;Meyer,
1991;Avouacet al., 1992] suggestin fact that the finite Cenozoic
clockwiserotationsof the Tarim andof the Qaidamare only of the
orderof 8øand6ø,respectively,
while partsof westernSichuanand
Yunnan may have rotatedby 40ø or more [Otofuji et al., 1990;
Huang et al., 1992]. It is no surprisethat in the 50-m.y.-long
collisionhistory,IndochinawasthefirstlargeAsianblockto rotate
clockwiseandthat it rotatedrelativeto SouthChinawhile moving
hundredsof kilometersto the SE-SSE [Peltzer and Tapponnier,
1988]. A time-dependent,northwarddecreaseof the clockwise
rotationof blocksseparated
by dominantlystrike-slipfaultsmakes
extrusionan inescapable
component
of the large-scaledeformation
of thezonelying eastof India throughout
thecollision,therateand
amountof rotationof eachblock merelyaffectingthe directionin
whichit escapes.
Acknowledgments.
Particularthanksaredueto DennisHayesfor inviting
A.B. to participateto ConradcruisesRC2613 andRC2614 andto spend4
monthsat the Lamont-DohertyGeologicalObservatory,and for allowing
herto useunpublished
datafrom thesecruisesin herPh.D. thesis.These
datahelpedusinterprettheChinesedatapresented
here. Jean-YvesRoyer
kindlyprovideduswith hisprogramsto fit magneticisochrons.We have
greatlybenefittedfrom discussions
with RolandoArmijo, JeanBesse,
RobinLacassin,Herv• Leloup,BertrandMeyer, and Gilles Peltzer. We
alsoappreciated
Kurt Feigl'scriticalreadingandGuy Aveline'stechnical
support.Remarksfrom BrianTaylor andan anonymous
reviewerhelped
improvethefinalversionof thepaper.Thisis IPGP contribution
1240.
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