JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B5, PAGES 8275-8291, MAY 10, 1993
Deformation
of the Oceanic Crust Between the North American
and South American Plates
R. DIETMAR
MOLLER
ScrippsInstitutionof Oceanography,La Jolla, California
WALTER H.F.
SMITH 1
Instituteof Geophysics
and PlanetaryPhysics,ScrippsInstitutionof Oceanography,La Jolla, California
Fracturezonetrendsand•nagnetic
anomalies
in the AtlanticOceanindicatethatthe NorthAmericanplatemusthave
movedwith respectto the SouthAmericanplateduringthe openingof the Ariantic. A comparison
of platetectonicflow
lineswith fracturezonesidentifiedfrom GeosatandSeasataltimeterdatasuggests
thatthe NorthAmerican-South
American
plateboundarynilgratednorthward
fromtheGuinea-Demarara
shearmarginto theVernaFractureZonebeforechron34 (84
Ma), to northof the DoldrumsFractureZone beforechron22 (51.9 Ma), and to northof the MercuriusFractureZone
betweenchron32 (72.5 Ma) and chron13 (35.5 Ma). The palcoridgeoffsetthroughtime identifiedfrom magnetic
anomalies
andthe computed
cmnulative
strikeslipmotionin theplateboundaryarea,indicatethatthe triplejunctionmay
havebeenlocatedbetweenthe Mercuriusandthe Fifteen-Twentyfracturezonesafter67 Ma (chron30). Platereconstructions
indicatea DateCretaceous
phaseof transtension,
followedby transpression
in theTertiaryfor theTiburon/Barracuda
Ridge
areasouthof theFifteen-Twenty
Fracture
Zone. Theoceanfloorin thisareais characterized
by a seriesof ridgesandtroughs
withlargeBouguergravityanomalies
(upto ~135regal). We usesmoothing
splineestimation
to invertBougueranomalies
for crustallayerstructure.Our modelresultssuggest
thattheMohois uplifted2-4 km overshortwavelengths
(-70 ksn)at
theBarracuda
andTiburonridgesandimplylargeanelastic
strains.Theseverely
thinnedcrustat thetworidgesimpliesthat
crustalextensionmusthavetakenplacebeforethey were uplifted. We proposethat the North-SouthAmericanplate
boundarymigratedto the latitudeof theTiburonRidge,boundedby theVernaandMarathonfracturezones,beforechron34
(84 Ma). Post-chron
34 crustalthinningduringa transtensional
tectonicregi•nemayhavebeenlocalizedat preexisting
structural
weaknesses
such,astheVema,Marathon,Mercurius,andFifteen-Twenty
fracturezonetroughs,butreachingthe
Fifteen-Twenty
FractureZoneandfutureBarracuda
Ridgeareaonlyafterchron32 (72.5 Ma). Tlfisinterpretation
concurs
with our crustalstructuralmodel,wlfichshowsstrongercrustalthinningunderneath
the TiburonRidgethanat the Barracuda
Ridge. Subsequent
transpression
mayhavecontinued
alongthe existingzonesof weakness
in the Tertiary,creatingthe
presentlyobserved
crustaldeformation
andupliftof the Moho,accompanied
by anela.
sticfailureof thecrust.Middle-EoceneUpperOligoceneturbiditeson theslopeof theTiburonRidge,nowlocated800 m abovetheabyssalplain,suggest
thatmost
of its upliftoccurred
at post-Oligocene
times. The unusually
shallowMohounderneath
theTiburonandBarracuda
ridges
represents
an unstabledensitydistribution,whichmayindicatethat compressive
stresses
are still presentto maintainthese
anomalies,
andthatthe NorthAmerican-South
Americanplateboundarymaystillbelocatedin thisarea.
INTRODUCTION
A densepattemof inostlyInediumto largeoffsetfracturezones
aswell asa seriesof unusualridgesandtroughswith largegravity
anomaliesis foundin the regionboundedby the Fifteen-Twenty
Fracture Zone to the north and the Romanche Fracture Zone to the
south(Figure1), in thispaperreferredto astheequatorialAtlantic.
One reasonfor the tectoniccomplexityof this areamay be that it
recordedthemigrationof theplateboundarybetweentheNorthand
SouthAmericanplates.However,thetectonichistoryof thisareais
notwell lmowndueto thelackof identifiedmagneticanomalies.
It has been suggestedby severalauthors[Roestand Collette,
1986;Roest,1987; Candeet al., 1988]thatthisplateboundarywas
locatedin theequatorialAtlanticfor a significantthnespanbetween
theLateCretaceous
andtheLateTertiary;thissuggestion
is based
mainly on evidencefrom fracturezone trends. Rifting between
INowat Geosciences
Laboratory,
OceanandEarthSciences,
National
OceanService, NOAA, Rockville, Maryland.
Copyfight 1993 by the AmericanGeophysicalUnion.
Paper number92JB02863.
0148-0227/93/9ZIB-02863
$05.00
8275
Africa and SouthAmerica southof the Guinea-Demararamargin
startedin theEarly Cretaceousandthe first NOAM-SOAM-African
oceanictriple junction is thoughtto have formed at about 110 Ma
(Late Albian-Early Cenomanian)alongthe Guinea-Demararashear
margin[Mascleet al., 1988], asshownon a tentativereconstruction
for 100 Ma (Figure 2). All fracturezonesidentifiedfroln satellite
altimetry data older than about 100 Ma preservedon the African
plate showNorth American-African spreadingdirections(Figure
2), confirmingthat the triplejunctionwas locatedat the GuineaDelnararashearInarginat thatthne.
A comparisonof tectonicflow lines computedfroln published
platemodelswith the fracturezonesin the equatorialNorth Atlantic
showsthatNorth American-Africanflow linesclearlymismatchall
fracturezonessouthof the Fifteen-TwentyFractureZone younger
than chron 13 (36 m.y.) (Figure 3a), and that the SouthAtlantic
flow linesmismatchthe fracturezonesegmentsolderthanchron5
(10 Ma) north of the MarathonFractureZone (Figure 3b). This
meansthat the NOAM-SOAM plate boundarymust have been
locatedin the areaboundedby the Marathonand Fifteen-Twenty
fracture zones between chrons 13 and 5, and that it migrated
northwardfrom its initial positionat the Guinea-Demararashear
inarginto thisareasolnethne beforechron13.
South Atlantic flow lines are cotnpatiblewith fracture zones
southof the Kane and north of the Fifteen-Twentyfracturezone
8276
MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST
60øW
40øW
20øW
0ø
20øN
AFR
15øN
0o
10øN
NAM
18øN
,en-Twen'
,-----.
__Barracuda
16øN
Rid:
M
Royal Trough
5øN
t
Tiburon Ridge
14øN
25øW
Researcher
Ridg•
20øW
15øW
!
Toe of Barbados
12øN
Accretionary Complex
60øW
58øW
56øW
54øW
52øW
50øW
48øW
46øW
Fig. 1. (Top) Tectonicmap of the centralAtlantic Ocean. Heavy lines
indicateidentifiedplate boundaries(Mid-Atlantic Ridge, and the toe of
the Barbadosaccretionarycomplex). Continentalmarginsare dotted.
Central North Atlantic and South Atlantic isochrons(shown,asdfin lines)
formedby North American-Africanand SouthAmerican-Africanplate
Fig. 2. Tentativereconstruction
for 100 Ma, whenrifting betweenAfrica
and Souill America south of file Guinea-Demarara margin had started.
The first Norill-South American-African oceanic triple junction is
thoughtto haveformedat about110 Ma (Late Albian-EarlyCenomanian)
alongfile Guinea-Demarara
shearmargin[Mascleet al., 1988]. Evidence
for the locationof the NOAM-SOAM plate boundaryat that time is
providedby fracturezonesolderthan about 100 Ma northwestof the
Guinea Plateau, which show North American-African spreading
directions.
tnofions. Central North Atlantic isochronsare constructedfor chrons5, 6,
gravitydata to obtain a more detailedimage of fracturezonetrends
in
the equatorialAtlanticandcomparethesewith tectonicflow lines
South Atlantic isochronsare drawn for chrons5, 6, 13, 18, 21, 25, 31, 34.
Magnetic anomaliescannot be identified near the magneticequator. computedfrom publishedfinite rotationpoles,as summarizedby
03ottore) Insetmapsilowinganotnalous
ridgeandtrotlghfeatureswhich Miiller and Roest [ 1992]. We calculateestimatesfor the cumulative
we suggestwere formed by tectonism at the North American-Soulh transpressio•fftranstension
alongtheplateboundaryandcomparethe
Americanplateboundary.The Fifteen-TwentyFractureZone runsalong predicted strike slip componentwith reconstructedpaleoridge
theBarracudaRidgeandRoyalTroughandmeetstheMAR at 15ø20'N.
offsetsat the Fifteen-Twenty transformto better confinethe time
intervalwhentheplate boundarymay havebeenlocatedin thisarea.
Second, we combine ship and satellite gravity data with ship
only for the youngeststage(chron5, 10 Ma, to present). This
bathymetrydata to model the crustaldeformationunderneaththe
h•dicatesthatthe NOAM-SOAM-African triplejunctionmay have
Barracudaand Tiburon ridgesby inversionof smoothedBouguer
migratedto northof the Fifteen-TwentyFractureZone after chron
anomaliesfor relief in a multilayermodelof oceancrust.
5.
13, 21, 25, 30, 32, 33y, 330, 34, M-0, M-4, M-10N, M-16, M-21, M-25;
Another bathymetricfeature of this region in addition to the
densefracturezonepattern is a seriesof ridgesandtroughsin the
vicinity of the Fifteen-TwentyFractureZone: theBarracudaRidge
andTrough,the TiburonRidgeeastof the LesserAntillesArc, and
theResearcherRidgeandRoyal Troughwestof the Fifteen-Twenty
transform fault (Figure 1). The Barracuda and Tiburon ridges
exhibitunusuallylargegravity anomalieswhichmay be indicative
of plate boundarydeformationprocesses. The Royal Trough
exhibitsen 6chelonshapedtectonicfabric and fresh basaltson a
basementcharacterized
by spreadingcentertypefaultingidentified
from GLORIA data,andtheResearcherRidgehasa largemagnetic
anomaly;theseareinterpreted
asextensional
features[Colletteet al.,
1984; Roest and Collette, 1986].
In this studywe combinetwo approaches
to betterdeterminethe
history of the NOAM-SOAM plate boundary. First, we use
combinedGeosatERM (44 stackedcycles)and Seasatalong-track
SUMMARY
OF PREVIOUS INVESTIGATIONS
OF THE NOAM-SOAM
PLATE BOUNDARY
The tectoniccomplexityof the equatorialAtlantic,in particular,
the history and timing of the NOAM-SOAM plate boundary
migration,has been the subjectof controversyin the past. Roest
and Collette [1986] suggestedthat the plate boundarymigrated
northwardfrotn south of 10ø N to the Fifteen-Twenty Fracture
Zone at about10 Ma, creatingtheBarracudaRidgeandTroughby
means of N-S compressionin the vicinity of the North-South
Americanrotationpole. Their model is basedon a surveyof the
Fifteen-TwentyFractureZone between30ø and 60øW as well as
mappedfossilspreadingdirectionsbetweentheFifteen-Twentyand
theVema fracturezones. Roest[ 1987] usedSeasataltimetrydatain
the equatorialAtlantic for a comparisonof tectonicflow lineswith
fracture zone gravity anomalies. He concludedthat the North-
MOLLERAND SMITH' DEFORMATION
OF THE OCF..ANIC
CRUST
50øW
40øW
30øW
8277
20øW
10øW
20øW
10øW
20øN
10øN
Doldrums FZ
SierraLeoneFZ •
Four-NorthFZ
PaulFZ
o
50øW
40øW
30øW
20øN
FZ
etna FZ
10øN
Doldrums FZ
SierraLeoneFZ
'orth FZ
St.F
FZ
o
Fig. 3. FracturezonesidentifiedfromGeosatandSeasataltimetrydataandplatetectonic
flow linesfrom(a) NorthAmericanAfricanstagepolesand(seeMiilleramtRoest[1992]for summary)
and(b)fromSouthAmerican-African
stagepoles[Shawand
Cande,1990]. Flowlinesin Figure3a areconstructed
for chrons5 (10.0Ma), 6 (20.0Ma), 13 (35.5 Ma), 21 (49.5 Ma), 25 (59.0
Ma),30 (67.5Ma),32 (72.5Ma), 33y(74.3Ma),330(80.2Ma),34 (84.0Ma), M-O(118.0Ma), M-4 (126.0Ma),M-10N (131.5
Ma), M-16 (141.5 Ma), M-21 (149.5 Ma), andM-25 (156.5 Ma); The SouthAtlanticflow linesin Figure3b are computed
for
chrons5 (8.9 Ma), 6 (19.4 Ma), 8 (26.9 Ma), 13 (35.3 Ma), 20 (44.7 Ma), 22 (51.9 Ma), 25 (58.6 Ma), 30 (66.7 Ma), 33y (74.3
Ma), 330 (80.2 Ma), 34 (84.oMa) and100Ma (extrapolated
fromthestage330-34). Seetextfor discussion.
ThecentralNorth
Atlanticflow lines(a) mismatchall fracturezonessouthof the SierraLeoneFractureZone(SLFZ) for timesyoungerthanchron
34 (84 Ma) andmismatch
all fracturezonessouthof theFifteen-Twenty
FractureZone(Fifteen-Twenty
FractureZone)for times
younger
thanchron13 (36 Ma); theSouthAtlanticflow lines(b) deafly mis•natch
fracturezonesnorthof theFifteen-Twenty
FractureZone. The NorthAmerican-South
Americanplateboundarymusthavebeenlocatedin the areabetweenthe FifteenTwentyFractureZoneandtheSLFZ throughmostof thespreading
historyin thisarea. Hatchures
indicatestretched
continental
crest.
8278
MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST
BetweenFractureZonesandSynthetic
Flow Lines
South Atnerican plate boundary was located south of the Four Comparison
North FractureZone betweenchrons34 and 13 and subsequently
In order to use the dense fracture zones between the Marathon
migratednorthward,passingthe DoldrumsFractureZone. Roest
and Sierra Leone fracture zones for obtaining evidence about the
[1987] found that the Verna Fracture Zone shows the southern
plate boundarymigrationwe have plottedboth South-Americanspreadingdirections since about 20 Ma, whereas the Mercurius,
African (Figure 4a) and North American-Africanflow lines (Figure
MarathonandFifteen-TwentyFractureZonesdisplaythe southern
4b) for comparisonwith fracturezone trends. Figure 3 illustrates
pattern since 10 Ma. Cande et al. [1988] also utilized Seasat
that North American-Africanflow lines clearly mismatchfracture
althnetrydatato interpretthe locationof fracturezonesbetweenthe
zonessouthof the Fifteen-Twenty FractureZone for times after
Fifteen-Twentyand North Doldrumsfracturezones,which they
chron13 (35.5 Ma). As a result,the partsof theseflow linesolder
comparedwith SouthandcentralNorth Atlanticflow lines. They
than
chron13 are virtuallymeaninglessbecausethey havebeenso
found that the South Atlantic flow lines follow the trends of the
far offset from the fracture zones that they are meant to model
Marathon, Vema, and Doldrums fracture zones as far back as chron
(Figure3a). In order to utilize the older North American-African
21 (50 Ma), althoughthe North and South American flow line
flow line information, we have used South American-African flow
trendsfor times prior to chron 13 are relatively similar. Hence
linesasshownin Figure4a to obtainthe correctflow line positions
Cande et al. [1988] concludedthat the NOAM-SOAM plate
at chron 13. We used these positions as new "seedpoints" to
boundaryhad migratednorthward(to the Fifteen-TwentyFracture
computeNorth American-African flow lines for times older than
Zone) at leastat chron13 (35 Ma), butmayhavemigratednorthof chron 13.
theMarathonFractureZone asearlyaschron21 (50 Ma).
The MarathonFractureZone reflectsSouthAtlantic spreading
from chron 20 to present day (Figure 4a). The fracture zones
FRACTURE ZONES
betweentheMercuriusandthe SierraLeonefracturezonesarefairly
D•a
consistent
with the SouthAtlanticspreading
directionsbackto about
chron34 (Figure 4a). However, many North American-African
We utilized combined Geosat and Seasat deflection of the vertical
flow lines for times between chron 13 (35.5 Ma) and chron 32
data (equivalentto along-trackhorizontalgravity anomalies)to (72.5 Ma) alsogive a reasonablefit to thesefracturezones(Figure
obtainshort-wavelength
verticalgravityanomaliesalong-trackby 4b). Therefore it appearsthat the northwardmigration of the
Hilbert transformation.We high-passfiltered the verticalgravity NOAM-SOAM plate boundaryto northof the MercuriusFracture
along-trackwith a Gaussianfilter (o= 11.2 s, Z=400 lcm)to isolate Zone must have occurred after chron 32 and before chron 13.
wavelengths related to isostatically uncompensated shortThere are four densely spaced fracture zones south of the
wavelengthfracturezonetopography[McKenzieand Bowin, 1976; DoldrumsFractureZone (Figure3) that are highly segmented
and
Mulder and Collette, 1984].
cannot be traced as far back as the fracture zones to the north. For
We identified
small- and medium-offset
fracture zones from the
along-trackGeosatand Seasatgravity data by picking the centerof
the gravity troughscorrespondingto the deepestportion of the
central fracture valleys. We chosethis methodologybecausethe
topographyof fracture zones formed in slowly spreadingocean
crustis associatedwith large-amplitude(2-3 lcm)medianvalleys
and a basementtroughat the age offset [Tucholkeand Schouten,
1988]. Also igneousrocks dredgedfrom similar depthsfrom the
facingwalls of fracturezonesin slowly spreadingoceancrustin the
Indian Ocean differ in type, e.g. gabbros versus basalts,
serpentinites
versusbasalts[Engeland Fisher, 1975], indicatinga
location
of the fracture
zone at the bottom
of the central
fracture
valley. This contrastswith large-offset fracture zones in fast
spreadingoceancrustthatexhibit a stepmorphologymodulatedby
flexural topography from differential subsidenceand thermal
bendingwith a fracturezone locationat the steepestslopeof the
geoidstep[Wesseland Haxby, 1990]. Miiller et al. [ 1991]showed
that the average mismatch between the short-wavelengthgeoid
troughand the basementtrough of 37 profiles crossingthe Kane
FractureZone in the centralNorth Atlanticis about5 kan,providing
"groundtruth" for using satellite altimetry data for identifying
fracture zones in this area.
Large offset fracture zones (Romanche,St. Paul), which are
mainly characterizedby a depth-agestep,were locatedby usingthe
inflectionpoint of the vertical gravity signal, correspondingto the
steepestslopeof the depth-agestep.The interpretedfracturezones
areshownon Figures3 and4. Flow linesfor centralNorth Atlantic
(Figure 3a, 4a) and South Atlantic (Figure 3b, 4b) seafloor
spreadingwere computedfrom recently publishedfinite rotation
poles(seeMiiller and Roest [1992] for a smmnary). All agesof
magneticanomaliesreferred to in the following are basedon the
thnescaleby Kent and Gradstein[ 1986].
post-chron22 (51.9 Ma) timesthe SouthAtlantic flow lines•natch
the trends of these fracture zones quite well, while the trend of
centralNorth Atlantic flow lines clearly mismatchesthesefracture
zones. This demonstrates
that the triplejunctionmigratedto north
of the Doldrums
Fracture Zone some thne before chron 22.
SouthAtlanticflow linesfor the Cretaceous
MagneticQuietZone
(chron M-0 (118.7 Ma) to chron 34 (84 Ma)) do not match the
observed fracture zones south of the Vema Fracture Zone, but
match the Verna and Marathon fracture zones reasonablywell
(Figure 4a). This indicatesthat the triple junction was located
betweenthe Guinea-Demararashearmarginandthe Vema Fracture
Zone from its initiation
to chron 34.
A regionof oceancrustolderthanchronM-0 (118.7 Ma) westof
the Guinea Plateau is located east of the termination
of the South
Atlanticflow lines(Figure4a), andthe few fracturezonesegments
interpretedin this area fit central North Atlm•tic flow lines (Figure
4b). Clearly, this portionof oceancrustwas createdat the central
North Atlantic ridge before the onsetof South Aanerican-African
spreading.
In summary,theinterpretedfracturezonetrendsin theequatorial
Atlanticareconsistent
with a northwardmigrationfrom theGuineaDemararashearmarginto the Verna FractureZone beforechron34
(84Ma), to north of the Doldrums Fracture Zone before chron 22
(51.9 Ma), and to north of the Mercurius FractureZone between
chron32 (72.5 Ma) and chron 13 (35.5 Ma).
TRANSPRESSION/TRANsTENSION
THE NOAM-SOAM
ALONG
PLATE BOUNDARY
The area between the Fifteen-Twenty and Marathon fracture
zones appears to be a likely candidate for plate boundary
deformation,
becauseit is accompanied
by theBarracudaRidgeand
MOLLERAND SMITH: DEFORMATION
OFTHE OCEANICCRUST
z
8279
8280
MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST
Troughandthe TiburonRidge aswell asthe Researcher
Ridge and
Royal Troughwestof the present-dayFifteen-TwentyTransform
Fault. All these bathymetric features show elements of plate
boundarydeformation,either compressional
or extensional[Roest
and Collette,1986]. Followingour fracturezoneanalysis,we start
with the working hypothesisthat this area may have been the
location of the plate boundaryafter chron 13 (35.5 Ma), and
possiblybeforethattime.
We can test the hypothesisthat the plate boundarymay in fact
have been located at the Fifteen-Twenty Fracture Zone by
calculatingthe strike slip motion along the NOAM-SOAM plate
boundaryat thisfracturezonethroughthne andcomparingit with
the reconstructed
offsetsof the Fifteen-TwentyFractureZone. If
this hypothesisis correct, the present-dayoffsets of magnetic
anomalies
north and south of the fracture
zone would
reflect
the
cumulativedifferentialstrikeslip alongvariousportionsof theplate
boundary.
Only a few magneticanomaliesin theequatorialArianticsouthof
the Fifteen-TwentyFractureZone have beenidentifiedbecauseof
thelocationof thisroughlyeast-westspreadingareaat themagnetic
equator,whereinclinationapproaches
zero. The magneticanomaly
picksidentifiedin thisareaare from Klitgordand Schouten[ 1986].
If we reconstructthe relative positionof the magneticpicks north
andsouthof the Fifteen-TwentyFractureZone by usingthe North
American-African
and
South
American-African
rotation
parameters,respectively,we obtain the paleo-offsetof the MidAtlantic Ridge at the triple junction throughtime. We test our
workinghypothesisby plottingthe paleo-ridgeoffset againstthe
cumulativestrike slip motion throughtime (Figure 5), calculated
from thefinite rotationpolesin Tablesla and lb.
The reconstructedoffset of the Fifteen-TwentyFractureZone
increasesfrom 95 to 145 kanfrom chron 34 (84 Ma) to chron32
(72.5 Ma). The predictedstrikeslip for the sametime is 115 km,
overtwiceasmuchastheobservedoffsetof magneticlineationsat
the fracture
zone
that varies
between
25 and 50 kin.
This
discrepancy
is mostreadilyexplainedby the inferencethattheplate
300
ZOO
100
0
e
Present day offset (km)
o----
Reconstructedoffset (km)
--'
Cumulativestrike-slip(km)
10
20
30
40
50
60
70
80
90
Age
(•a)
Fig. 5. The paleoridgeoffsetversusthe cumulativestrike-slipmotionat
the Fifteen-TwentyFractureZone yields strongevidencethat the plate
boundarywas locatedherefrom chron30 (67.5 Ma) to chron5 (10 Ma).
For ti•nesafterchron33o (80.2 Ma) thereis a goodagreementbetween45
km of totalincreasein fracturezoneoffsetand65 k•nof predictedstrikeslip from chron33o to chron32 (72.5 Ma). Betweenchron32 and 30
(67.5 Ma) the fracture zone offset is reduced from 145 to 105 km,
althoughthe plate model predictscontinuedleft-lateralstrike-slipof 15
k•n, probablyreflectinga ridgejump. In the followingtime (chron30 to
chron5 (10 Ma)) the fracturezone offset increasesfrom 105 to 215 k•n
until chron 5, in good agreementwith 90 km of predictedcmnulative
strikeslip.
TABLE la. North-South
Anomaly
Age,Ma
American Finite Reconstruction Poles
Latitude•øN Longitude,øE Angle,deg.
5
10.4
17.2
-53.5
2.00
6
20.5
35.5
50.3
58.6
68.5
84.0
15.1
16.7
15.5
13.8
12.2
6.4
-54.1
-53.7
-53.8
-52.0
-53.4
-58.0
3.91
5.72
7.12
8.31
9.50
9.31
13
21
25
31
34
North-South American finite reconstructionpoles and stage poles.
Calculatedfrom the rotationpolessummarizedby Miiller and Roest
[1992]. A positiverotationangleindicatescounterclockwise
rotation.
TABLElb. North-South
AmericanStagePolesfor NorthAmerica
Age1, Ma
Age2, Ma
Latitude:
øN Longitude,,
øE Angle:deg.
0.0
10.4
20.5
35.5
50.3
58.6
10.4
20.5
35.5
50.3
58.6
68.5
68.5
84.0
17.2
12.9
20.2
10.6
4.4
0.5
-57.1
-53.5
-54.6
-53.0
-53.9
-41.2
-61.7
-2.00
-1.91
-1.82
-1.41
-1.23
-1.23
-147.8
-1.22
See Table la footnotes.
boundarywas not located at the Fifteen-Twenty FractureZone
duringthe entire time interval but migratedto this locationsome
thne after chron 32.
Betweenchron32 and 30 (67.5 Ma) the fracturezone offset is
reducedfrom 145 to 105 km, althoughthe plate model predicts
continuedleft-lateralstrikeslip of 15 km. This observation
may
reflecta ridgejmnp in responseto thecontinuedlateralseparation
of
the two ridge segments bounding the fracture zone. In the
following time (chron 30 to chron 5 (10 Ma)) the fracturezone
offset increasesfrom 105 to 215 kan until chron 5, while 90 kan of
cmnulativestrikeslip are predicted. Hence for thisperiodthereis
an excellent agreement between the predicted strike slip and
observedincreasein offset, yielding evidence that the plate
boundarymay have been located at the Fifteen-Twenty Fracture
Zone from chron 30 to chron 5. However, we point out that a
directcorrelationbetweenobservedchangesin offsetandpredicted
strike slip cannot be expected, because transform offsets also
changein responseto changesin directionof platemotion.
To furthertestwhetherthe plate boundarymay havebeenlocated
in the Fifteen-TwentyFractureZone area, we now computethe
cumulativecompression/extension
alongthisfracturezoneby using
thecentralNorthAtlanticandtheSouthArianticrotationparameters.
Two phasesof differing tectonicregimescan be distinguished
for
the differential motion history along the plate boundary. From
chron33o to 30 the entire plate boundaryfrom crustas old as 84
Ma to the mid-oceanridge wassubjectedto left-lateraltrm•stension,
resulting in up to ~50 kan extensionin the area of the western
BarracudaRidge. After chron30 the tectonicregimechangedto
transpression
alongthe entireplateboundary.At chron21 (49 Ma),
transtension
replacedtranspression
alongthe easternportionof the
plate boundary. Since we rely on publishedreconstructions
for
computingthe differential motion along the plate boundary,we
restrict ourselvesto computingthe cumulative compressionand
extensionfor post-chron30 thnes. The calculatedtotaldeformation
from chron30 to chron5 alongtheplate boundaryamountsto 70
km of compression
in thewesternBarracuda
Ridgearea,30 km of
compression
in the easternBarracudaRidge area,and about30 km
of extensionin the Researchel;
Ridge-RoyalTrougharea,if the
plate boundarywas in fact locatedat the Fifteen-TwentyFracture
Zone duringthistime (Figure6).
MOLLER AND SMITH:
DEFORMATION OF THE OCEANIC CRUST
30
8281
13
25
18N
21
Royal
34
30
16N
25
21
13
i•es•
erRidge
14N
62W
60W
58W
56W
54W
52W
50W
48W
46W
Fig. 6. MagneticanomalypicksfromNorillandSouthAmerica(opensquares),
andanomalypicksrotatedfromAfrica(crosses)
in a reconstruction
for chron5 (10 Ma). Fracture
zone(FZ) picksfromNOAM andSOAMaresolidsquares;
FZ picksrotated
fromAfricaaresolidtriangles.LightgrayareanearBarracuda
Ridgeindicates
cumulative
transpression
fromchron30 (67.5Ma)
to chron5. Dark grayareanearRoyalTroughindicatescumulativetranstension
from chron21 (49 Ma) to chron5. The
calculated
totaldeformation
fromchron30 to chron5 alongtheplateboundary
amounts
to 70 km of compression
in thewestern
BR area,30 lanof compression
in theeastern
BR area,and30 kmof extension
in theRT, if theplateboundary
wasin factlocated
in this area.
Roestand Collette [1986] speculatedthat both the topography
and the gravity anomaly of the Barracuda Ridge indicate
compression
at a time whenthe lithospherehad gainedconsiderable
thickness. However, they could not clearly distinguishbetween
compressionaland extensionaldeformationfrom seismicprofiles
acrossthe BarracudaTrough. Seisnficdata froth the sedimentsin
the trough south of the BarracudaRidge and from the Tiburon
Ridge(Peterand Westbrook,1976;Mascleand Moore, 1990) show
widespreadnormal faults but few reversefaults. Thereforethese
seismicreflectiondatado notseemto provideunequivocal
evidence
for the styleof tectonicdeformationin thisarea.
The computedtranstension
eastof anomaly21 coincides
with the
occurrence
of theResearcher
RidgeandRoyalTrough,whichRoest
typesare griddedat 2.5 arc rain of latitude and longitudeusing
continuouscurvaturesplinesin tension[Smithand Wessel,1990].
Becauseship gravity data have DC offsets [Wessel and Watts,
1988], we adjustedthe DC level of eachshipto matchthe satellite
gravity field preparedfrom combinedSeasat,Geosat,and ERS-1
altimeterdata[Sandwelland Smith,1992]. For eachshipsurvey,
we formthedifferencebetweentheshipandsatellitedataandadjust
the ship data so that the mean difference becomeszero. The
adjusteddifferencesare thengriddedand addedto the satellitedata
to form the gravityfield shownin Figure8. The accuracyof the
shipbathymetry
datawasassessed
by Smith[1993].
The topographyand gravity data in Figure 8 are strongly
anticorrelatedin the southwestcornerof the maps,nearBarbados.
and Collette[1986] interpreted
as a constructional
volcanicridge The topography
risesto shallowlevelson theBarbadosaccretionary
andextensional
basin,respectively.The easternterminationof the complex;
the gravityshowslargenegativevalues,presumably
thedepthto the Mohois increasing
on thesubducting
plate.
ResearcherRidge is locatedwest of anomaly6. Transtension because
youngerthan chron 6 apparentlydid not cause constructional We find that this anticorrelationproducedspurioustrendsin our
volcanism,but resultedin formationof the Royal Trough. The gravity modeling, and therefore we detrend the gravity and
absence
of any volcanicridgeeastof the Researcher
Ridgeandthe topographydata. Trendsare computedby griddingthe data in a
presenceof the Royal Troughto the northmay indicatethat some larger area than shownin Figure 8, and using a 400-1anmedian
timebetweenchrons6 and5 theplateboundary
junapedto northof filter overtheobserveddatagridsto definethe trend(Figure8c and
theFifteen-TwentyFractureZone.
8d). The trendsare essentially"flat" over muchof the map area
outsidethe accretionary
wedge,andso the detrendingmakeslittle
TOPOGRAPHY AND GRAVITY ANOMALIES
change (other than mean level) in the data in this area. The
detrendeddataare shownasFigure8e and8f.
The largestgravityanomaliesin the westernequatorialAtlantic
are associated
with the Barracudaandthe Tiburonridges(Figures
If the NOAM-SOAM plate boundaryhad beenlocatedin the 8a and 8e). The Barracuda Ridge has a clear topographic
Fifteen-TwentyFractureZone areaandthe oceanfloor in that area expression(Figure 8b and 8f); the Tiburon Ridge is not easily
were subjectto initial transtension
and subsequent
transpression, definedtopographically
becauseof its proximityto the Barbados
we mightexpectto find evidencefor crustaldeformation.Here we accretionarycomplexbut appearsas a distinct structuralhigh in
examinebathymetryand gravity anotnaliesfor clues to crustal seistnicreflectionprofiles[Mauffretet al., 1984].
Dc•a
deformation.The westernequatorial
Atlanticis relativelydensely
BouguerGravityAnomalies
coveredby shiptracks(Figure7). We combineshipand satellite
free-airgravitydatawith shipbathymetry
datato construct
gridded
We obtaintheBouguer
gravityanomaly
gridshownin Figure9
topographyand gravity data sets(Figures 8a and 8b). Both data byremoving
thegravityeffectof thedetrended
topography
(Figure
8282
MOLLERANDSMITH: DEFORMATION
OFTHEOCEANIC
CRUST
20øN
20øN
19øN
19øN
18øN
18øN
17øN
17øN
16øN
16øN
15øN
15øN
14øN
14øN
13øN
13øN
12øN
60øW
12øN
59øW
58øW
57øW
56øW
55øW
54øW
60øW
59øW
58øW
57øW
56øW
55øW
54øW
Fig. 7. (a) Ship bathymetrytrac'ksand (b) gravitytracksfrom shipandsatellitealfimetrydatain the westernequatorialAtlantic.
The BarracudaandTiburonridges(Figure 1) are indicatedby the gray areas.
8f) from the detrendedgravity (Figure 8e), assuminga densityof
Inversionof BouguerAnotncdies
for LayeredStructure
2800 kg m-3 for the topography. In this calculation,we use
Bouguer anomalies indicate changes in crustal thickness or
Parker's [ 1972] expressionfor the gravity effect of a density
density, or vertical displacement of structural contacts in a
contrastAp with a varyingelevationh(x), x is a 2-vectorindicating
(normally) horizontallylayeredcrust(e.g., layer 2, layer 3, Moho).
a positionin theplane:
Most modelsof marine gravity anomaliesare constructedusinga
layered oceaniccrust [cf. Watts, 1978]. Gravity interpretationis
nonunique,and any numberof structurescan be devisedwhich will
fit the observedgravity anomaly. If we assumethat the Bouguer
Here•' [] indicates
a Fouriertransform,
g(x)is thepredicted anomalyshownin Figure 9 is causedby undulationsof a density
gravity anomaly, G is Newton's gravitational constant, k is a contrastat the Moho, then equation(1) can be usedto calculatethe
Fourierwave vector(kx =2•r/)•x,ky =2•r/)•y,where)• is a Bouguergravity anomaly,g, given the Moho elevation,h, and the
wavelength),and z is the mean depthto the densitycontrast(5200 meandepthto the Moho, z.
m for the seafloortopography). We took four terms in the series
If we attempt to invert the Bouguer gravity anomaly for the
(1).
topography of the Moho surface, we have two difficulties:
Bouguer gravity anomaliesare usually negative over positive Equation (1) is nonlinear in h, and its inverse also involves
elevations of the seafloor, because these are isostatically downward continuation, which is unstable. It is convenient to
compensated.The oppositeis true at the BarracudaRidge, where separatethesetwo difficultiesby definingm, the Bougueranomaly
theBouguergravity reaches+ 100 mGal. hmnediatelyto the north at themeandepthof theMoho:
in the Barracuda Trough, the Bouguer gravity is negative (-60
ooNn.l
mGal) while the bathymetryis flat. The largestpositiveBouguer
anomaly(135 mGal) is over the Tiburon Ridge. This featureis at
the limit of the area affectedby detrending. Southof the Tiburon Equation(2) is a nonlinearproblemfor h, given m, which can be
Ridge there is another negative Bouguer anomaly (-80 mGal), solvedby iterativeapproximation
[Oldenburg,1974]. Substitution
indicatinganothertrough.
of (2) into (1) gives
•'[g(x)]
=21t'G
Ap
ext•-IM
Z) In'
n!1•' [{h(x
))n].(1)
Fig. 8. We combinedshipandsatellitefree-airgravitydatawith shipbathymetry
datat.oconstruct
griddedtopography
and
gravitydatasets(Figures8a and8b). The topography
andgravitydataarestronglyanticon'elated
in thesouthwest
cornerof the
maps,nearBarbados.We foundthatthisanticorrelation
produced
spurious
trendsin ourgravitymodeling,andso we detrended
thegravityandtopography
data. Trendswerecomputed
by griddingfl•edatain a largerareathanshownandusinga 400-kin
medianfilter overfl•eobserved
datagridsto definethetrend(Figures8c and8d). Thetrendsareessentially
"flat"overmuchof
fl•emapareaoutsidethe accretionary
wedge,andsofl•e&trendingmakeslittle change(othertitanmeanlevel) in the datain this
area. The derrended
dataareshownas Figures8e and8f..
MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST
z
z
z
Z
Z
Z
i
Z
i
8283
Z
i
Z
i
Z
i
i
8284
MOLLER AND SMITH:
g(k) = exp[ - Iklz] re{k)
DEFORMATION OF THE OCEANIC CRUST
(3)
wherev is a parameterthatadjuststherelativeweightof E andL.
Asv --->0, ffiwill fit g exactly,
whileasv --->o%t'• --->0. Some
whichis the unstableinverseproblemof downwardcontinuationof
intermediate
choice
of
v
produces
a smooth
ffiwitha reasonable
fit
g to obtainm.
to
the
data.
The
estimate
ffi
which
minhnizes
the
sum
(6),
given
v,
Stableestimatesof m may be obtainedfrom (3) by low-pass is
filtering of g beforedownwardcontinuation.We use a filte,' which
minimizes the curvature of m, because the curvature of h is
•(k)
=g{k}
[.xp[ -exp[-[Iqz]
2 z] + v ].
indicative of deformation in the crust. Estimation of a minimum-
(7)
curvaturem from noisyg is shnilarto theproblemof deconvolution
Equation
(7)is afilterthatisapplied
tog toobtain
t•i. Because
v
by smoothingspline estimation [Constable and Parker, 1991].
Morgan and Blackman [Inversion for crustal structure from has units of k-4, it is convenient to define a characteristic
3,Ofthefilter(7) as
combinedgravityandbathymetrydata:a prescription
for downward wavelength
continuation,subre.to J. of Geophys.Res.] suggesteda similar
• = 2/rv1/4.
(8)
approachfor modeling the Moho at mid-oceanridges. Let the
..
es[hnated
Bouguer
gravity
anomaly
attheMoholevelbet•i. Then
theintegrated
squared
curvature
oft•i.is
L = JJ Ikl
4[•(k)]
2 dk
(4)
Weobtained
ffifor thevalues
of 3, shown
in Figure10aand
inverted (2) for the Moho elevation h in each case. We then
computedthe absolutevalue of the curvatureof the Moho elevation,
andtheintegrated
squared
misfitofffi.tothedatag is
E = Jl[g{k)-exp[-lkl
z]ffi(k}]
2 dk
In Figure10a,we showtheroot-mean-square
(RMS) misfit(in
milliGals),
asa function
of 3,(inkilometers).
TheRMSisfoundby
dividing
EIt2bythenumber
of gridelements.
(5)
Iv2hi,to assess
theextentof crustal
deformation
hnplied
by•.
(6)
crustal
levels,thisfailureis expected
to takeplacebybrittlefaulting.
Figure10bshowsthe percentof themapareaof Figure9 over
Nowwemaychoose
t•. soastominimize
thesum
Curvatures
up to approximately
10-6 m-1 canbe sustained
by
elasticflexureof theoceanlithosphere,
whilecurvatures
exceeding
this value are expectedto result in failure [McNutt, 1984]. At
E + vL
which the Moho curvatures exceed the elastic lhnit, for various
-245 -210 -175 -140 -105
60øW
59øW
58øW
-70
57øW
-35
0
56øW
35
55øW
70
105
54øW
20øN
valuesof theparameter3,.
Figure 10 is an illustrationof the classictrade-off between
resolution
andvariance[Backusand Gilbert,1970;Menke,1989].
If we choose
a smallvaluefor 3,,we fit thedataverywell,butthe
model implies a Moho which is faulted nearly everywhere;
Filterwavelength, (km)
19øN
100
3O
.........
200
i
.........
I
......
300
I
i
i
i
i
i
i
i
i
i
[
i
i
i
i
i
i
i
i
i
I
18øN
I
E 20
I
17øN
I
•
I
1o
16øN
0
15øN
8O
........
i
-
i
i
i
I
.
14øN
I
ß
•o
40
c•
20
I
13øN
12øN
I
60øW
59øW
58øW
57øW
56øW
55øW
54øW
Fig. 10. Root meansquaremisfit and percentof faultedcrustalarea
Fig. 9. Bouguergravityanomalies
in the westernequatorialAtlantic. versuswavelengthof the smoothingsplinefilter, showingthe classic
NotethepositiveBouguer
anomalies
at tl•eBarracuda
Ridge(upto 100 trade-off between resolution and variance. If we choosea small value for
regal),andat theTiburonRidge(upto 135mGal). NegativeBouguer 1,we fit thedataverywell, butthemodelimpliesa mohowhichis faulted
anomalies
arefoundin the Barracuda
Trough(-60 regal) andtl•eSomhof
nearlyeverywhere;
conversely,
a largevalueof 1 yieldsa modelwith a
theTiburonRidge(-80 mGal),indicating
anothertrough.
mostlyunbrokenMohobuta poorfit to the data.
ß
MOLLER
ANDSMITH:DEFORMATION
OFTHEOCEANIC
CRUST
-6
-5
-4
-3
-2
-1
0
I
2
3
4
5
6
< 10-6
8285
10-6
> 10-6
55øW
54øW
20øN
19øN
18øN
17øN
i16ON
15øN
'l
................................
::•"
....
14 øN
13øN
60øW 59øW 58øW 57øW 56øW 55øW 54øW60øW59øW 58øW 57øW 56øW 55øW 54øW
Fig.11. (a)Moho
elevation
and(b)distribution
ofelastic
failure
fromthe! = 70kmmodel
witha density
contrast
of
(pro=3300
kgm-3,@c=2800
kgm-3).TheMoho
iselevated
underneath
theridges
anddeflected
downward
under
theadjacent
troughs,
resulting
insevere
crustal
flfinning
andflfickening
reflecting
thelarge
positive
andnegative
Bouguer
gravity
anomalies
over
theridges
andtroughs
(Figure
1la). Themodeled
Moho
shows
large
curvatures
inthearea
oftheTiburon
andBarracuda
ridges
andtroughs
,as
well,as
infileBarbados
accretionary
prism,
while
curvatures
larger
than10'6areless
common
outside
of
fl•eseareas(Figure11b).
conversely,
a largevalueof X, yieldsa modelwith a mostly anddeflecteddownwardunderthe adjacenttroughs,resultingin
reflecting
thelargepositive
unbrokenMoho but a poorfit to the data. Thereis no obvious severecrustalthinningandthickening,
over the ridgesand
choiceof X,fromFigure10. We choseX,basedon twocriteria: andnegativeBouguergravityanomalies
Firstit is ourexpectation
thatlargeMohocurvatures
shouldbe troughs.
modelresultsin a fairly goodfit of the
generally
confined
tothe-area
of theTiburon
andBarracuda
ridges The smoothing-spline
anomalies
totheobserved
gravity
anomalies
(Figure
andtroughs
andthe subduction
zonecomplex,wherethelarge model-gravity
Bouguergravityanomalies
areobserved.Second
is ourdesh'e
that 12) with an RMS misfitof 7.4 regal. Althoughthe one-layer
themodelshouldfit thegravityanomalydataaswell aspossible
at crustalmodelsatisfiestheobservedgravityfield well, it is doesnot
uswitha realistic
modelforthecrustal
structure,
because
it
theTiburonandBarracuda
ridgesandtroughs.Using.these
criteria, provide
doesnotconsider
thelargevariations
in sediment
thickness
in this
we chose X, = 70 km.
Figure11a showsthe Moho elevationand Figure lib the area. Publishedseismicprofiles[Peter and Westbrook,1976;
distribution of elastic failure from the X, = 70 kan model with a
Mauffretetal., 1984]showdeepsediment-filled
troughs
bordering
andTiburonridges.Thedensitywe usefor inverting
density
contrast
of Pro-Pc
(Pro
=3300kg m-3,pc=2800
kg m-3). theBarracuda
The modeledMoho showslarge curvaturesin the area of the
Tiburon and Barracudaridges and troughsas well as in the
theBouguer
anomalies
istoolargein ourl-layermodelistoolarge,
becausesedimentsare less dense than upper crustal rocks.
weoverestimate
theamplitude
andcurvature
of the
Barbados
accretionary
prism,whilecurvatures
largerthan10-6are Consequently,
thesetroughsby usingcrustaldensities.
less commonoutsideof theseareas(Figure I lb). The modeled modeledMohounderneath
In order to construct a more realistic model, we designed a
Mohowe•tof thetoeof theaccretionary
complex,
shown
asa
dashedline in Figures 11a and llb, is not relevant for our
nonlinearinversion method that considerssediments,as well as an
objectives
andcannot
beinterpreted,
because
wedetrend
thedatain
thisarea.Cross-sectional
profilesthrough
themodelat theTiburon
Ridge(A-A') andthe Barracuda
RidgeandTrough(B-B') are
shownin Figure12. The Mohois elevatedunderneath
theridges
upper
crustal
andalowercrustal
layer.In thismodel
weiteratively
perturb
initiallyflat-lying
crustal
layers
byusing
smoothed
residual
gravityanomalies.Theseare smoothed
usingthe splinefilter
(equation
7) withz = 5200m to downward
continue
theresidual
8286
MOLLER AND SMITH: DEFORMATIONOF THE OCEANIC CRUST
Latitude (deg N)
gravityto the mean seafloordepth. We usethe densitiesandlayer
thicknessesgiven in Table 2 for the layered model, which are
constrainedby regionalseismicdata. Sincewe do not haveenough
seismicdatato constructa completesedhnentthicknessmapfor our
model area, we start our model with a sedimentlayer of 500 m
thickness. This is a reasonablevalue for the average sediment
12
13
14
15
16
17
18
19
20
•
I
[
I
[
I
•
I
•
I
[
I
•
I
•
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
]
I
,
I
,
I
[
I
[
I
[
I
[
I
]
'
150
100
50
thicknessin this area, which varies between 100 and about 1000 m,
exceptfor evenlargersediment
thiclmesses
in theBarracuda
Trough
andthe troughsouthof theTiburonRidge[Tucholkeet al., 1982].
In the multilayermodel, we computea residualanomalygrid
from theBouguergravityanomaliesby settingall gravity anomaly
0
-50
values smaller than 7.5 mGal to zero. We chosethis value, because
gravitycross-over
errorsshowuncertainties
of this order[Wessel
and Watts,1988]. We usethe residualgravityanomaliesto pel•urb
all crustal layers in concert. By downward continuingto the
sediment-waterinterfaceonly, we perturbthe crustallayerstoo little
at each model step, but create a stable, converging inversion
process.We createa layer perturbationp(x) from the smoothed
residualgravitys(x) givenby
p{x}= S{X
}/ 2/tG{p
m- ps}
(9)
150
wherewe usetheentiredensitycontrastin thelayeredmodel(Pm
ß
1O0
-Ps), because
theperturbation
will be addedto all layers.Hence
thelayersmaintaintheirthickness,
exceptthatwehavetosatisfythe
50
o
constraintthat no layer may rise above the seafloor. While a
negativesmoothed
residuals(x) will driveall subseafloor
horizons
down,with a sedimentlayergrowingthickerasseismiclayertwo is
depressed,
a positives(x) will drive contactsup until they are
truncated
againsttheseafloor,whichmayexposedeeperlevelsof
-50
ß
I
'
I
'
I
'
I
'
I
'
I
'
I
[
I
[
I
]
I
[
I
•
I
•
I
•
I
the section.
Usingthe new perturbedcrustallayer structure,we computea
new gravitymodelfrom (1) by summingthe contributions
from
eachdensitycontrastin the layer structure. We computethe
maximumabsolutegravityresidualastheobservedminusthe new
model. Now we againcomputea residualanomalygridby setting
all valuessmallerthan7.5 regal to zero and smoothit by usingthe
•'
*-,
l•
•
XX
XXX
Xx
-10
,N/ •
'
12
=========================
/ N/
,•N
/ X / N/
.•: ',:•:•:
I
13
'
I
14
'
I
15
'
!
16
'
I
17
'
I
18
'
I
19
20
smoothingsplinefilter (7). We stopthe modeliterations,if the
Fig. 12. Cross-section•profilesthroughthe•tal single-layermodelat
the TiburonRidge (A-A' on Figure 11) and the BarracudaRidge and
Trough•-B' on Figure11). •e Mohois elevatedunderneath
fi•eridges
and deflecteddownw•d underthe adjacent•oughs, resultingin severe
crustal•inning and thickeningreflecting•e large positiveand negative
layerandthemultilayermodelto invertresidualgravityanomalies Bouguergravity anomaliesover fi•e ridgesand •oughs. Althoughthe
gravityfield well,it is does
for Moho topography
is illustratedin Figure 13. We designeda one-layer•stal modelsatisfies•e observed
notprovideus with a realisticmodelfor fi•e crustalstructure,
because
it
syntheticgravity anomalywith a positiveand a negativepeak
do• notco•ider •e l•ge vadafionsin s•iment •ickn•s in tldsarea.
maximumabsolutevalueof thesmoothed
residualgravityanomalies
is smallerthan5 mGal. This is equivalentto usingthe Chebychev
normasconvergence
criterion.
The differencebetweenthe linear model usingonly one crustal
(Figure13) to showthe effectof inversionfor layertopography
usingthesingle-layer
(Figure13a)andthemultilayermodel(Figure
13b). The single-layermodelresultsin a symmetricridge/trough 3 are ntissing,and a smaller-amplitudetrough,resultingfrom
Moho topography(Figure 13a), whereas the inversion for tnodelingthesedimentfill in thetrough.
multilayertopography
resultsin anasymmetric
layerstructure
with
The Mohoelevation(Figure14a)andthedistribution
of elastic
a large-amplitude
ridge,underneath
whichlayer2 andpartsof layer failure(Figure 14b) from the multilayermodelis not drastically
differentfromthesingle-layer
modelresults(Figure11). However,
TABLE2. Thicknesses
andDensitiesof LayersUsedin Model
althoughwe have usedthe samewavelength(70 km) for the
Layer
T!dckness•
in
Density,
kg/m
3
smoothing
filter in bothmodels,we havecreatedsmoother
Moho
Water colmnn
5200
1030
topography
underneath
the
sediment-filled
troughs
in
the
multilayer
Sediments
500 a
2300
Layer
2
Layer
3
Mantle
2300b
4700b
2800c
2950c
3400d
a Mean undisturbedsedimentthicknessaroundthe BarracudaRidgefrom
Tucholkeet al. [1982].
bPurdy
[1983].
c Carlson and Herrick [1990].
d Watts
[1978].
model. This is reflected in the total area of elastic Moho failure,
whichis 30% in the multilayermodel,as opposedto 38% in the
single-layermodel. Figure 15 showstwo modelcrosssections
over the BarracudaandTiburonridgesand troughs. Comparison
with Figure 12 reveals that the elevated Moho topography
underneaththe Barracudaand Tiburon ridges, resultingfrom
positiveresidualgravityanomalies,
is quitesi•nilarin bothmodels.
MOLLERANDSMITH: DEFORMATION
OFTHEOCEANIC
CRUST
a
100
•
•
200
I
i
300
I
•
400
I
•
500
I
i
600
I
•
700
I
•
800
I
•
,•,
Distance
(km)
0
100
200
300
400
500
600
700
800
I
rho = 1025
-5
=
,?•r•rho = 2300 ....... ,....... ,....... ,....... •....... ,....... ,....... h = 500-•...
!l
' x rho= 2800•
__•-10 'x
•2
b
Distance (km)
0
8287
ß
%%%'.-.., ;,
x';rho=2950x'.xß
xßx/x
;, ;, ;, ;,
;, %;;
ßßßßßßßßß
ßß ß'h=4700.x
•x/•/ß-ß-ß-ß-ß•
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400
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I
500
'
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600
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I
700
'
800
0
100
200
300
400
500
600
700
800
Fig. 13. Illustrationof the perturbation
of initiallyflat-lyinglayersby residualgravityanomaliesusing(a) fi•esingle-layerand(b)
the multilayermodel. The single-layermodelrestiltsin a symmetricridge/troughMoho topography(Figtire 13a), whereasthe
inversionfor multilayertopography
resultsin an asymmetric
layerstructure
with a largeamplituderidge,underneath
whichlayer
2 andpartsof layer3 aremissing,anda smaller-amplitude
trough,restiltingfrom modelingthesedimentfill in the trough.
However,thelayer structuremodeledfrotnlargenegativeresidual
gravityanomalies,as over sediment-filledtroughssouthof the
Tiburon Ridge and on both sidesof the BarracudaRidge, is
smoother
thanin thesingle-layer
model,resultingin a morerealistic
15. Layeredmodelsareconvenient
to calculate,
buttheymaynot
alwaysbe appropriate. For example,thinningof layer three is
observedat fracturezones[Detrick et al., 1982; Sinha and Louden,
1983] and has beeninvokedto explainthe Blake SpurFracture
Zonegravityanomaly[Morriset. a/., 1991],although
thisanomaly
In orderto checkourmodelagainstseismicdatafromourstudy is an order of magnitude smaller than the anomaly over the
Ridge. Sincetheformationof theBarracuda
Ridgeand
area,we show a crosssectioninterpretedfrom a single-channel Barracuda
seismicprofile by Mauffret et al. [1984] that crossesthe Tiburon Troughseems
relatedto theFifteen-Twenty
FractureZone,it might
andwesternBarracudaridges(Figure16a),andcompareit with our be appropriate
to drawa modelwith a thinnedlayer 3 alongthe
modeledlayerstructuresampledalongthisship-track(Figure16b). Barracuda Ridge/Trough. However, because this area has
an episodeof extension,followedby compression,
we
Note that the seismicsectionhasa verticalaxisin two-waytravel undergone
thne,whilethemodelsectionis in kilometers.Bothprofilesshowa really have no idea what sortof crustalsectionis reasonable.The
deep sedimentfilled troughnorth of the BarracudaRidge and displacementof subseafloorlayers shownin Figures 12 and 15
the actualconfiguration
of
outcroppingbasementat the Tiburon Ridge. The top of the neednot be thoughtof asrepresenting
asmerelya proxyfor changes
in
Barracuda Ridge in our model section appearsto consist of thecrust;theymaybeinterpreted
as well aslayerdisplacement.For example,
sediment(Figure 16b). This is a resultof invertingsmoothed densityor thickness
density;
residualgravityanomalies.We startour modelwith a 500m thick upliftof theMohomay be seenasindicativeof increased
this
increase
need
not
actually
occur
by
Moho
uplift.
Nevertheless,
sedimentlayer,andusesmoothed
residualgravityanomalies
()• >
evidencefor uplift of materialfrom deep
70kin)to perturbthelayerstructure.Topographic
ridgesof widths thereis independent
smallerthan35 km may remain coveredwith sediment,because structurallevelsat the BarracudaRidge: mantleperidotiteshave
FractureZoneduringcruises
short-wavelengthgravity an9maliesthat may accompanythese beendredgedfromtheFifteen-Twenty
Moho curvature.
featuresarenotinvertedfor layerstructure.
DISCUSSION
It is importantto bear in mind the nonuniqueness
of gravity
interpretation
whenexaminingthecaa•stal
sections
in Figures12 and
of the Soviet researchvesselBoris Petrov in 1989 and 1990 [J.
Casey,pers.communication,1991].
Compressional
deformation
by elasticbucklinghasbeeninvoked
to explain gravity anomalies in the Indian Ocean [Karner and
Weissel,1990]. This is not appropriate
for the gravityanomalies
studiedhere. A topographiccompensationmechanismfor the
8288
M(JLLER AND SMITH: DEFORMATIONOF THE OCEANIC CRUST
-6
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-5
-4
-3
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-2
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20øN
1
2
56øW
3
4
55ow
5
6
< 10 -6
54ow 60ow
10 -6
59øW
58øW
57øW
> 10 -6
56øW
55øW
ß
...............
54øW
20øN
19øN
19øN
18øN
18øN
17øN
17øN
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.:.....
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15øN
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•'
12øN
13øN
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60ow
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ß
59øW
58øW
57øW
56øW
55øW
54øW60øW
59øW
58øW
57øW
56øW
55øW
54øW
Fig. 14. (a) Moho elevationand (b) the distributionof elasticfailure from the multilayermodel. We have usedthe same
wavelength(70 km) for the smoothing
splinefilter asin the single-layermodelbut havecreatedsmootherMohotopography
underneath
thesediment
filledtroughsin themultilayermodel.Thisis reflectedin thetotalareaof elasticMohofailure,whichis
30% in the multilayermodel,compared
with 38% in thesingle-layermodel.
Barracuda and Tiburon ridges can be excluded, for they are
uncompensated.
We havealsoshownthatelasticbucklingcannot
accountfor the crustalstructureof the ridgesand troughs,because
the large, short-wavelengthBouguer gravity anomaliesrequire
curvaturesexceedthe elasticlitnit of about 10-6 m-1 [McNutt,
1984]. Our inversion procedure yields smoothly undulating
contactswhich resemblegeologicalfolds. Although we cannot
directlydescribefaultsin our models,the largecurvaturesof these
contactssuggestthatthecrustunderneath
the anomalous
ridgesand
troughsin this areais brokenandfaulted. Deep seisnficreflection
6). It is unlikely that the shorteningpredicted frownthe plate
reconstructions
is grosslyin error. The centralNorth Atlantic and
the SouthAtlantic are amongthe oceanicregionsbestcoveredby
magneticanomalyand fracturezone data. Even if the kinematic
shorteningwere more than50% less,of the orderof 10-20 km, we
would still be left with an order of tnagnitudemore than that
resolvedby gravityinversion.Hencethe inversionsperformedhere
are not a good method for estimatingstrain. Becausethe plate
kinematic analysis suggests large shortening and the Moho
curvaturesunderthe ridgesexceedthe elasticlitnit, we believethat
data from this area are not available to us to test this inference.
large strainsare localized in theseareas. Zuber and Parmentier
Shallowreflectionprofilesshowa prevalenceof normalfaultsin [1991] have made a careful analysis of the compressional
sedhnentssouthof the BarracudaRidge andon the Tiburon Ridge deformationof the oceanlithosphere.They fh•d that once strain
[Peter and Westbrook,1976; Mcz•cleand Moore, 1990]. In the exceedsthe linear elasticlimit, a runawayinstabilityoccurswhich
BarracudaTrough seismicdata show disturbedsedimentsbut do rapidlylocalizeslarge strains. Sucha processmay describewhat
not allow a distinctionbetweennormal andreversefaults[Roestand we seeat theseridges.
However, the question remains how the cumulative North
Collette, 1986].
Americantranstension
andtranspression
calculated
We calculate the north-south crustal shortening along our Panerican-South
multilayermodelprofilesA-A' andB-B' overthe900-kmlengthof from plate kinematic modelsmay be reconciledwith the crustal
theprofiles.The shortening
we obtainis of theorderof 500 m, that structuremodeled from gravity inversion. In summary,plate
is negligible. This is a result of the smoothingin our inversion kinematic modeling, inversion of gravity anomalies, and
method. In contrast,a maximum shorteningof 30-70 kan at the geological/geophysicaldata from the study area include the
Barracuda
Ridgeis calculated
fromplatekinematicmodels(Figure followingmodelresultsand observations:
,.
MOLLER AND SMITH: DEFORMATIONOF THE OCEANIC CRUST
a
Latitude(deg N.)
12
13
•
150
I
14
•
I
15
[
I
16
•
I
17
•
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•
8289
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50
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-50
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Distance
I
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....
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, ':':i._.':_.5.2:.
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15.0
,
150
I
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,
I
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-50
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,
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]
I
,
I
,
I
,
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,
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,
. ..':.'.'.
ß
16.0
16.5
. .x,.x
17.0
Latitude(degN.)
_
100
15.5
i
Fig. 16. (a) Comparisonof a seisnficprofile from Mat•?et et al. [1984]
thatcrosses
the TiburonandwesternBarracudaridges(C-C' in Figtire14)
(b) with our modeledlayerstructuresampledalongthesametrack. Note
that the seismicsectionhas a verticalaxis in two-waytravel fiine, while
the modelsectionis in kilometers.Bothprofilesshowa deepsedimentfilled troughnorthof the BarracudaRidgeandoutcroppingbase•nentat
the TiburonRidge. The top of the BarracudaRidgein our modelsection
appearsto consistof sediment. This is a result of invertingsmoothed
residualgravity anomalies. We start our model with a 500 m-thick
seditnentlayer andusesmoothedresidualgravityanotnalies(1> 70 kin)
to perturbthe layerstructure.Topographicridgesof widthssmallerthan
35 km may remaincoveredwith sedi•nent,becauseshort-wavelength
gravityanomalies
thatmayaccompany
thesefeatures
arenotinvertedfor
-]0
layer structure.
-15
12
13
14
15
16
17
18
19
20
Between chron 30 (67 Ma) to chron 5 (10 Ma) right-lateral
transpression
is predictedfrom platekinematicsfor the Barracuda
Fig. 15. Cross-sectional
profilesfrom the multilayermodel at the same
Ridgearea,with 90 km of cumulativestrikeslip,and30-70 kanof
locationsas shownin Figure 12. Comparisonwith Figtire 12 revealsthat
the elevatedMoho topographyunderneaththe Barracudaand Tiburon compression.
3. The BarracudaTrough is coincidentwith the western
ridges,resultingfrom positiveresidualgravityanomalies,is quitesinfilar
in bofl•models.However,thelayerstructuremodeledfrom largenegative extensionof the Fifteen-TwentyFractureZone [Roestand Collette,
residualgravity anomalies,as over sediment-filledtroughssouthof the 1986]. The deep trough southof the Tiburon Ridge may be
TiburonRidgeandon bothsidesof the BarracudaRidge,is smootherthan
boundedby the western extensionsof the Mercurius and Verna
in the single-layermodel,resultingin a morerealisticMoho curvature.
1. The North American-SouthAmericanplate boundarywas
initiated at the Guinea-Demarara shear margin in the Late
Cretaceous.A comparisonof fracturezoneswith syntheticplate
flow linesshowsthat it migratedto the areabetweenthe Marathon
and Fifteen-Twenty fracture zones, where the Barracuda and
Tiburonridgesare located,sometime betweenchron32 (72.5 Ma)
andchron13 (35.5 Ma).
2.
Differential
motion
between
North
America
and South
FractureZones(Figure4).
4. The largegravityanomalyovertheTiburonRidgerequh'es
a
crustalthickness
of only 1.5-2kan,indicatinga very shallowMoho
(Figure15a). The Mohodepthunderneath
theBarracuda
Ridgeis
modeledto be 4 kan below seafloor(Figure 15b). In both cases
layer2 isinfen'ed
tobeabsent;
underneath
theTiburonRidge,even
the thicknessof layer 3 is reducedconsiderably.This indicates
severecrustalthinningalongtheseuncompensated
ridgesandactive
Mohouplift. Sedimentfilled troughsseveralkilometersdeepare
foundadjacentto theridges
America calculated for the Barracuda Ridge area from plate
5. Moho curvaturesunderneaththe ridges and troughsin the
kinematicdatasuggests
a phaseof left-lateraltranstension
between studyareaexceedtheelasticlhnit of about10-6 m-1 andrequire
chron 34 (84 Ma) and 30 (67.0 Ma) that would have resultedin artelastic failure of the crust.
over 100kanof strikeslip and-50 lan extension.However,a large
6. Normalfaultsarecormnonin sedhnents
on the TiburonRidge
discrepancybetweenthe changein offset of the Fifteen-Twenty and south of the BarracudaRidge [Peter and Westbrook,1976;
Fracture Zone with the predicted strike slip during that time Mascle and Moore, 1990]. Deformed sedimentsin the Barracuda
suggeststhat the plate boundarywas more likely locatedsouthof Troughcouldbe dueto eitherextensionor compression
[Roestand
theFifteen-TwentyFractureZone/Barracuda
Ridgeduringthisthne. Collette, 1986].
8290
MOLLER AND SMITH:
DEFORMATION OF THE OCEANIC CRUST
7. Focal mechanismsof 2 large intraplateearthquakesnorthof
the Barracuda Ridge have composite thrust and strike-slip
mechanisms
[Bergman,1986] andindicateN-S compression.
CONCLUSIONS
A comparisonof fracture zone trendsand syntheticflow lines
revealsthat the North American-SouthAmerican-African triple
8. Heat flow on theTiburonRidgefrom OceanDrilling Program junction must have migrated from the Guinea-Demararashear
site 672A is 79øC/kin [Fisher and Hounslow, 1990], about 2.5
marginto the to theVema FractureZone beforechron34 (84Ma), to
timesashighasexpectedon normaloceancrustof thisage(about north of the Doldrums Fracture Zone before chron 22 (51.9 Ma),
80 Ma).
and to north of the Mercurius
9. A 220m thick middle Eocene-upperOligocenesequenceof
turbiditeswas found on the slopeof the Tiburon Ridge at Ocean
Drilling Programsite 672A [Mascle and Moore, 1990]. The
turbiditesarefoundabout800 tn abovethe adjacentabyssalplain.
(72.5 Ma) andchron13 (35.5 Ma). Two anomalousridgeson old
oceancrust(-84-70 Ma) betweenthe MercuriusandFifteen-Twenty
fracturezones,the Barracudaand Tiburon ridges, exhibit large,
short-wavelengthBouguer gravity anomalies(up to 135 regal),
which require mantle densitiesat shallow levels under the ridges
(Moho uplift of up to 4 lcm). Inversion of Bouguer gravity
anomaliesfor smoothcrustalstructureyieldslayerswith curvatures
Theseobservationsand modelingresultslead to the following
geologicalinterpretation:whentranstension
startedbetweenNorth
America and South America in the Late Cretaceous(we can only
calculatethis for the time after chron 34 (84 Ma) due to the absence
of magneticanomaliesduring the CretaceousMagnetic Quiet
Period), the Fifteen-Twenty, Marathon, Mercurius, and Verna
fracturezonesmay have representedstructuralweaknessesin the
oceaniccrust. Along theirfracturetroughsthe crustmay havebeen
severelythinned already, as has been observedat other fracture
zonesin the centralNorth andequatorialAtlantic [Detrick et al.,
1982, Sinha and Louden, 1983, Morris et. al., 1991].
Mostlikely the transtension
localizedstrainalongthesestructural
weaknessesso that the crustwas thinnedeven more. Our analysis
Fracture
Zone
between
chron 32
exceeding
10-6 m-1 underneath
theridges
andadjacent
troughs.
Thisexceedsvaluespermissibleunderelasticbucklingandsuggests
that this area has been subjectto severedeformationas a result of
North American-SouthAmerican plate motions. Plate kinematic
datasuggesta Late Cretaceousphaseof transtension,followed by
transpression
in the Tertiary for the Tiburon/Barracuda
Ridge area.
We proposethat crustalthinningduringthe transtensional
period
was localized at preexisting structural weaknessessuch as the
Verna, Marathon, and Mercurius fracture zone troughs. A
comparison
of fracturezoneswith syntheticflow linessuggests
that
duringmost of this period the plate boundarywas locatedbetween
the Verna and the Marathon fracturezones,but may have included
the Fifteen-Twenty FractureZone after chron 32 (72.5 Ma). This
interpretationconcurswith our crustal structural model, which
showsmore severecrustalthinningunderneaththe TiburonRidge
than at the BarracudaRidge. After chron30 (67 Ma) transtension
wasreplacedby transpression
in this area,localizingtheresulting
strain along the already thinned and weakenedcrustal sections,
creating the ridge/trough topography observed today in the
Tiburon/Barracuda
Ridge area. Middle Eocene-upperOligocene
turbidiresdrilled on the flank of the Tiburon Ridge far abovethe
adjacentabyssalplain suggestthat muchof its uplift hasoccmxed
after the Oligocene. High heat flow at the Tiburon Ridge and the
unstable density distribution underneath the Tiburon and the
Barracuda ridges indicate that uplift due to compressional
deformationin this area may still be ongoingtoday, and that the
NorthAmerican-South
Americanplateboundarymay stillbe located
of thechangein offsetthroughthneof theFifteen-TwentyFracture
Zoneversuscomputedstrikeslipfor thethneof transtension
(84-67
Ma) suggests
that the locationof the plate boundarywas southof
the Fifteen-TwentyFractureZone. Hencewe wouldexpectto find
more evidencefor extensionin the TiburonRidge areathanin the
BmTacuda
Ridge/Trougharea. This line of reasoningis supported
by our crustalmodelsectionsfrom inversionof gravityanomalies,
which show more severe thinning under the Tiburon Rise than
underneaththeBarracudaRidge.
When the tectonicreghnechangedto transpression
in the Early
Tertiary, the deformationcausedby compressionalstressesmay
haveremainedlocalizedat the alreadythinnedcrustalareas. This is
also likely becauseof the additionalstrike slip componentof the
differentialplatemotionthatmay havecontributedto lateralmotion
andfaultingof the crustalongthefracturezones. Oncethisprocess
had started a combination of compressional and strike slip
componentschangingthroughoutthe Tertiary may have caused here.
initial crustalbucklingthat resultedin elasticfailure after strain
Given this scenario,we would expectto find frequentreverse
exceededthelinearelasticlimit, resultingin runawaydeformationas
faults in the plate deformation area. From publishedseismic
found by Zuber and Parmentier [1991]. That strainmust have
sectionsit appearsthatnoHnalfaultsin sedhnentsare moreconunon
exceededthe elastic lhnit in the large parts of the area from the
thanreversefaults,but Bull and Scrutton(1992) haveinterpreted
BarracudaTrough to southof the Tiburon Ridge is suggestedby
sedimentonlap patternsin the BarracudaRidge area as indicators
ourmapof Mohocurvatures
exceeding
10-6m
-1(Figure14).
for rotation and reverse faulting. A combination of seismic
Publishedplatekinematicdatacannotresolvethethnewithin the
reflectionand refractiondata will be necessaryto investigatethe
Tertiary when the Barracudaand Tiburon Ridgeswere uplifted.
styleandamountof deformationin thisareaandcomparethesedata
However,the Eocene-upperOligocenesequenceof turbiditesfound with the North American-South
American differential motions and
on the slopeof the Tiburon Ridge 800 m abovethe abyssalplain
theiruncertainties
predictedfrom platereconstructions.
documentsthat this ridge must have risen significantlyafter the
Acknowledgements. The gridding, contouring,grid sampling,
Oligocene. The high heatflow on the TiburonRidge alsofits the
scenariowell, andmay be accountedfor by the very shallowMoho detrending,and graphic88oftwareu•ed in thi• paper ½ome•from the
(1.5-2 kan)underneath
theridgeandnot by fluid discharge
from the GMT •y•tom (Wesseland Smith, 1991). JasonPhipp•Morgan •ugge•tod
the •moothingsplineinversionapproach.
The authorswouldlike to thank
nearbyaccretionarywedge, as suggestedby Fisher and Hounslow
[1990]. The shallow Moho underneaththe Tiburon and Ban'acuda
Ridgesreflects an unstabledensitydistributionand indicatesthat
compressive
stresses
may be requiredat thepresenttime to maintain
these anomalies.
Andrew D. Walker, Kevin M. Brown, Waltor R. Roe•t, David T.
Sandwell,andChristopher
Smallfor manyhelpfuldi•cusdon•.We thank
Han• Schoutenand an anonymou•reviewerfor critical review• of the
manuscript.R.D.M. wa• •upportedby a graduateresearcha•si•tant•hip
and W.H.F.S. by a postdoctoral•cholarshipfrom the C.H. and I.M.
MOLLERANDSMITH: DEFORMATION
OFTHEOCEANICCRUST
8291
Green Foundation for Earth Sciencesat the Scripps Institution of
Oceanography.
M011er,R. D. and W. R. Roest, Fracture zones in tile North Atlantic from
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(ReceivedJanuary10, 1992;
revisedSeptember21, 1992;
acceptedDecember4, 1992.)