JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B5, PAGES 8275-8291, MAY 10, 1993

Deformation of the Between the North American and South American Plates

R. DIETMAR MOLLER

ScrippsInstitution of ,La Jolla, California

WALTER H.F. SMITH 1

Instituteof Geophysicsand PlanetaryPhysics, Scripps Institution of Oceanography,La Jolla, California

Fracturezone trends and •nagneticanomalies in the AtlanticOcean indicate that the NorthAmerican plate must have movedwith respectto the SouthAmerican plate during the openingof the Ariantic. A comparisonof platetectonic flow lineswith fracturezones identified from Geosatand Seasat altimeter data suggests that the NorthAmerican-South American plateboundary nilgrated northward from theGuinea-Demarara shear margin to theVerna Fracture Zone before chron 34 (84 Ma), to north of the DoldrumsFracture Zone before chron22 (51.9 Ma), and to north of the MercuriusFracture Zone betweenchron 32 (72.5 Ma) and chron13 (35.5 Ma). The palcoridgeoffset through time identifiedfrom magnetic anomaliesand the computedcmnulative strike slip motionin theplate boundary area, indicate that the triplejunction may havebeen located between the Mercuriusand the Fifteen-Twentyfracture zones after 67 Ma (chron30). Platereconstructions indicatea DateCretaceous phase of transtension,followed by transpressionin the Tertiary for theTiburon/Barracuda Ridge areasouth of theFifteen-Twenty Fracture Zone. The oceanfloor in thisarea is characterizedby a seriesof ridgesand troughs withlarge Bouguer gravity anomalies (up to ~135 regal). We usesmoothing spline estimation to invertBouguer anomalies for crustallayer structure. Our modelresults suggest that the Mohois uplifted2-4 km overshort wavelengths (-70 ksn)at the Barracudaand Tiburon ridges and imply large anelastic strains. The severelythinned crust at thetwo ridgesimplies that crustalextension must have taken place before they were uplifted. We proposethat the North-SouthAmerican plate boundarymigrated to the latitudeof theTiburon Ridge, bounded by theVerna and Marathon fracture zones, before chron 34 (84 Ma). Post-chron34 crustalthinning during a transtensionaltectonic regi•ne may have been localized at preexisting structuralweaknesses such ,as the Vema,Marathon, Mercurius, and Fifteen-Twenty fracture zone troughs, but reachingthe Fifteen-TwentyFracture Zone and future Barracuda Ridge area only after chron 32 (72.5 Ma). Tlfisinterpretation concurs with our crustalstructural model, wlfich shows stronger crustal thinning underneath the TiburonRidge than at the Barracuda Ridge. Subsequenttranspression may have continued along the existingzones of weaknessin the Tertiary,creating the presentlyobserved crustal deformation and uplift of the Moho,accompanied by anela.stic failure of thecrust. Middle-Eocene- UpperOligocene turbidites on theslope of theTiburon Ridge, now located 800 m abovethe abyssalplain, suggest that most of its uplift occurredat post-Oligocenetimes. The unusuallyshallow Moho underneath the Tiburon and Barracuda ridges representsan unstabledensity distribution, which may indicate that compressivestresses are still presentto maintainthese anomalies,and that the NorthAmerican-South American plate boundary may still be locatedin thisarea.

INTRODUCTION and SouthAmerica southof the Guinea-Demararamargin startedin the Early Cretaceousand the first NOAM-SOAM-African A densepattem of inostlyInedium to largeoffset fracture zones oceanictriple junction is thoughtto have formed at about 110 Ma aswell asa seriesof unusualridges and troughs with largegravity (Late Albian-Early Cenomanian)along the Guinea-Demararashear anomaliesis found in the regionbounded by the Fifteen-Twenty margin[Mascle et al., 1988], as shownon a tentativereconstruction Fracture Zone to the north and the Romanche Fracture Zone to the for 100 Ma (Figure 2). All fracturezones identified froln satellite south(Figure 1), in thispaper referred to as the equatorialAtlantic. altimetry data older than about 100 Ma preservedon the African One reasonfor the tectoniccomplexity of this areamay be that it plate show North American-African spreadingdirections (Figure recordedthe migration of theplate boundary between the Northand 2), confirmingthat the was locatedat the Guinea- SouthAmerican plates. However, the tectonic history of thisarea is Delnararashear Inargin at thatthne. notwell lmowndue to thelack of identifiedmagnetic anomalies. A comparisonof tectonicflow lines computedfroln published It has been suggestedby severalauthors [Roest and Collette, platemodels with the fracturezones in the equatorialNorth Atlantic 1986;Roest, 1987; Candeet al., 1988] thatthis plate boundary was showsthat North American-Africanflow linesclearly mismatch all locatedin theequatorial Atlantic for a significantthne span between fracturezones south of the Fifteen-TwentyFracture Zone younger theLate Cretaceousand the Late Tertiary;this suggestion is based than chron 13 (36 m.y.) (Figure 3a), and that the South Atlantic mainly on evidencefrom fracturezone trends. Rifting between flow lines mismatchthe fracturezone segmentsolder than chron5 (10 Ma) north of the MarathonFracture Zone (Figure 3b). This means that the NOAM-SOAM plate boundarymust have been INowat GeosciencesLaboratory, Ocean and Earth Sciences, National OceanService, NOAA, Rockville, Maryland. locatedin the area boundedby the Marathonand Fifteen-Twenty fracture zones between chrons 13 and 5, and that it migrated

Copyfight 1993 by the AmericanGeophysical Union. northwardfrom its initial positionat the Guinea-Demararashear inarginto this areasolne thne beforechron 13. Paper number92JB02863. South Atlantic flow lines are cotnpatiblewith fracture zones 0148-0227/93/9ZIB-02863 $05.00 southof the Kane and north of the Fifteen-Twentyfracture zone

8275 8276 MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

60øW 40øW 20øW 0 ø 20øN

AFR

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0 o

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18øN ,en-Twen' ,-----. Royal Trough M 16øN __Barracuda Rid: t 5øN Tiburon Ridge 25øW 20øW 15øW 14øN ResearcherRidg• ! Fig. 2. Tentativereconstruction for 100 Ma, when rifting betweenAfrica Toe of Barbados and Souill America south of file Guinea-Demarara margin had started. 12øN Accretionary Complex The first Norill-South American-African oceanic triple junction is thoughtto haveformed at about110 Ma (Late Albian-EarlyCenomanian) alongfile Guinea-Demararashear margin [Mascle et al., 1988]. Evidence 60øW 58øW 56øW 54øW 52øW 50øW 48øW 46øW for the locationof the NOAM-SOAM plate boundaryat that time is Fig. 1. (Top) Tectonicmap of the centralAtlantic Ocean. Heavy lines providedby fracturezones older than about 100 Ma northwestof the indicateidentified plate boundaries(Mid-Atlantic Ridge, and the toe of Guinea Plateau, which show North American-African spreading the Barbadosaccretionary complex). Continentalmargins are dotted. directions. Central North Atlantic and South Atlantic isochrons(shown ,as dfin lines) formedby North American-Africanand SouthAmerican-African plate tnofions. Central North Atlantic isochronsare constructedfor chrons5, 6, gravity data to obtain a more detailedimage of fracturezone trends 13, 21, 25, 30, 32, 33y, 330, 34, M-0, M-4, M-10N, M-16, M-21, M-25; South Atlantic isochronsare drawn for chrons5, 6, 13, 18, 21, 25, 31, 34. in the equatorialAtlantic andcompare these with tectonicflow lines Magnetic anomaliescannot be identified near the magnetic equator. computedfrom publishedfinite rotationpoles, as summarizedby 03ottore) Insetmap silowing anotnalous ridge and trotlghfeatures which Miiller and Roest [ 1992]. We calculateestimates for the cumulative we suggestwere formed by tectonism at the North American-Soulh transpressio•fftranstensionalong the plate boundaryand compare the Americanplate boundary. The Fifteen-TwentyFracture Zone runsalong predicted strike slip componentwith reconstructedpaleoridge theBarracuda Ridge and Royal Trough and meets the MAR at 15ø20'N. offsetsat the Fifteen-Twenty transformto better confine the time intervalwhen the plate boundarymay have beenlocated in this area. Second, we combine ship and satellite gravity data with ship only for the youngeststage (chron 5, 10 Ma, to present). This bathymetrydata to model the crustaldeformation underneath the h•dicatesthat the NOAM-SOAM-African triplejunction may have Barracudaand Tiburon ridges by inversionof smoothedBouguer migratedto northof the Fifteen-TwentyFracture Zone after chron 5. anomaliesfor relief in a multilayermodel of oceancrust. Another bathymetric feature of this region in addition to the SUMMARY OF PREVIOUS INVESTIGATIONS densefracture zone pattern is a seriesof ridgesand troughsin the OF THE NOAM-SOAM PLATE BOUNDARY vicinity of the Fifteen-TwentyFracture Zone: the BarracudaRidge andTrough, the TiburonRidge eastof the LesserAntilles Arc, and The tectoniccomplexity of the equatorialAtlantic, in particular, the ResearcherRidge and Royal Troughwest of the Fifteen-Twenty the history and timing of the NOAM-SOAM plate boundary transform fault (Figure 1). The Barracuda and Tiburon ridges migration, has been the subjectof controversyin the past. Roest exhibitunusually large gravity anomalieswhich may be indicative and Collette [1986] suggestedthat the plate boundarymigrated of plate boundarydeformation processes. The Royal Trough northwardfrotn south of 10ø N to the Fifteen-Twenty Fracture exhibits en 6chelonshaped tectonic fabric and fresh basaltson a Zone at about10 Ma, creatingthe BarracudaRidge andTrough by basementcharacterized by spreadingcenter type faultingidentified means of N-S compressionin the vicinity of the North-South from GLORIA data,and the ResearcherRidge has a largemagnetic American rotation pole. Their model is basedon a survey of the anomaly;these are interpreted as extensional features [Collette et al., Fifteen-Twenty FractureZone between30 ø and 60øW as well as 1984; Roest and Collette, 1986]. mappedfossil spreading directions between the Fifteen-Twentyand In this studywe combinetwo approachesto betterdetermine the the Vema fracturezones. Roest[ 1987] usedSeasat altimetry data in history of the NOAM-SOAM plate boundary. First, we use the equatorialAtlantic for a comparisonof tectonicflow lineswith combinedGeosat ERM (44 stackedcycles) and Seasatalong-track fracture zone gravity anomalies. He concludedthat the North- MOLLERAND SMITH' DEFORMATIONOF THE OCF..ANICCRUST 8277

50øW 40øW 30øW 20øW 10øW

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PaulFZ

o

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etna FZ

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SierraLeone FZ 'orth FZ

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Fig. 3. Fracturezones identified from Geosat and Seasat altimetry data and plate tectonic flow linesfrom (a) NorthAmerican- Africanstage poles and (see Miiller amt Roest [ 1992]for summary)and (b) fromSouth American-African stage poles [Shaw and Cande,1990]. Flowlines in Figure3a areconstructed for chrons5 (10.0 Ma), 6 (20.0 Ma), 13 (35.5 Ma), 21 (49.5 Ma), 25 (59.0 Ma), 30 (67.5Ma), 32 (72.5Ma), 33y(74.3 Ma), 330 (80.2Ma), 34 (84.0Ma), M-O(118.0 Ma), 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 SouthAtlantic flow linesin Figure3b are computedfor 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) and 100Ma (extrapolatedfrom the stage 330-34). Seetext for discussion.The centralNorth Atlanticflow lines(a) mismatchall fracturezones south of the SierraLeone Fracture Zone (SLFZ) for timesyounger than chron 34 (84 Ma) andmismatch all fracturezones south of theFifteen-Twenty Fracture Zone (Fifteen-Twenty Fracture Zone) for times youngerthan chron 13 (36 Ma); the SouthAtlantic flow lines(b) deafly mis•natchfracture zones north of theFifteen-Twenty FractureZone. The NorthAmerican-South American plate boundary must have been located in the areabetween the Fifteen- TwentyFracture Zone and the SLFZ throughmost of thespreading history in thisarea. Hatchuresindicate stretched continental crest. 8278 MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

South Atnerican plate boundary was located south of the Four ComparisonBetween Fracture Zones and SyntheticFlow Lines North FractureZone betweenchrons 34 and 13 and subsequently In order to use the dense fracture zones between the Marathon migratednorthward, passing the Doldrums FractureZone. Roest and Sierra Leone fracture zones for obtaining evidence about the [1987] found that the Verna Fracture Zone shows the southern plate boundarymigration we have plotted both South-American- spreadingdirections since about 20 Ma, whereas the Mercurius, African (Figure 4a) and North American-Africanflow lines (Figure Marathonand Fifteen-Twenty Fracture Zones display the 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 althnetrydata to interpretthe locationof fracturezones between the zonessouth of the Fifteen-Twenty FractureZone for times after Fifteen-Twentyand North Doldrumsfracture zones, which they chron13 (35.5 Ma). As a result,the partsof theseflow lines older comparedwith Southand central North Atlanticflow lines. They found that the South Atlantic flow lines follow the trends of the thanchron 13 are virtuallymeaningless because they havebeen so far offset from the fracture zones that they are meant to model Marathon, Vema, and Doldrums fracture zones as far back as chron (Figure 3a). 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 lines as shownin Figure 4a 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), but may havemigrated north of chron 13. the MarathonFracture Zone as early aschron 21 (50 Ma). The MarathonFracture Zone reflectsSouth Atlantic spreading

FRACTURE ZONES from chron 20 to present day (Figure 4a). The fracture zones betweenthe Mercuriusand the SierraLeone fracture zones are fairly D•a consistentwith the SouthAtlantic spreading directions back to about chron 34 (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-trackhorizontal gravity anomalies)to (72.5 Ma) alsogive a reasonablefit to thesefracture zones (Figure obtainshort-wavelength vertical gravity anomalies along-track by 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 short- There are four densely spaced fracture zones south of the wavelengthfracture zone topography [McKenzie and Bowin, 1976; DoldrumsFracture Zone (Figure3) that are highly segmentedand 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 post-chron22 (51.9 Ma) times the SouthAtlantic flow lines•natch along-trackGeosat and Seasatgravity data by picking the centerof the trends of these fracture zones quite well, while the trend of the gravity troughs correspondingto the deepestportion of the centralNorth Atlantic flow lines clearly mismatchesthese fracture central fracture valleys. We chosethis methodologybecause the zones. This demonstratesthat the triplejunction migrated to north topographyof fracture zones formed in slowly spreadingocean of the Doldrums Fracture Zone some thne before chron 22. crustis associatedwith large-amplitude(2-3 lcm) median valleys SouthAtlantic flow linesfor the CretaceousMagnetic Quiet Zone and a basementtrough at the age offset [Tucholkeand Schouten, (chron M-0 (118.7 Ma) to chron 34 (84 Ma)) do not match the 1988]. Also igneousrocks dredgedfrom similar depthsfrom the observed fracture zones south of the Vema Fracture Zone, but facingwalls of fracturezones in slowly spreadingocean crust in the match the Verna and Marathon fracture zones reasonablywell Indian Ocean differ in type, e.g. gabbros versus , (Figure 4a). This indicatesthat the triple junction was located serpentinitesversus basalts [Engel and Fisher, 1975], indicatinga betweenthe Guinea-Demararashear margin and the Vema Fracture location of the fracture zone at the bottom of the central fracture Zone from its initiation to chron 34. valley. This contrastswith large-offset fracture zones in fast A regionof oceancrust older thanchron M-0 (118.7 Ma) westof spreadingocean crust that exhibit a stepmorphology modulated by the Guinea Plateau is located east of the termination of the South flexural topography from differential subsidenceand thermal Atlanticflow lines (Figure4a), and the few fracturezone segments bendingwith a fracturezone locationat the steepestslope of the interpretedin this area fit central North Atlm•tic flow lines (Figure geoidstep [Wessel and Haxby, 1990]. Miiller et al. [ 1991] showed 4b). Clearly, this portionof oceancrust was createdat the central that the average mismatch between the short-wavelengthgeoid North Atlantic ridge before the onset of South Aanerican-African trough and the basementtrough of 37 profiles crossingthe Kane spreading. FractureZone in the centralNorth Atlanticis about5 kan,providing In summary,the interpretedfracture zone trends in theequatorial "ground truth" for using satellite altimetry data for identifying Atlanticare consistent with a northwardmigration from theGuinea- fracture zones in this area. Demararashear margin to the Verna FractureZone beforechron 34 Large offset fracture zones (Romanche,St. Paul), which are (84Ma), to north of the Doldrums Fracture Zone before chron 22 mainly characterizedby a depth-agestep, were locatedby usingthe (51.9 Ma), and to north of the Mercurius FractureZone between inflection point of the vertical gravity signal, correspondingto the chron32 (72.5 Ma) and chron 13 (35.5 Ma). steepestslope of the depth-agestep. The interpretedfracture zones are shownon Figures3 and4. Flow linesfor centralNorth Atlantic TRANSPRESSION/TRANsTENSION ALONG (Figure 3a, 4a) and South Atlantic (Figure 3b, 4b) seafloor THE NOAM-SOAM PLATE BOUNDARY spreadingwere computedfrom recently publishedfinite rotation poles (seeMiiller and Roest [1992] for a smmnary). All agesof The area between the Fifteen-Twenty and Marathon fracture magneticanomalies referred to in the following are basedon the zones appears to be a likely candidate for plate boundary thne scaleby Kent and Gradstein[ 1986]. deformation,because it is accompaniedby theBarracuda Ridge and MOLLERAND SMITH: DEFORMATIONOF THE OCEANICCRUST 8279

z 8280 MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

Troughand the TiburonRidge as well as the ResearcherRidge and TABLE la. North-South American Finite Reconstruction Poles Royal Troughwest of the present-dayFifteen-Twenty Transform Anomaly Age,Ma Latitude•øN Longitude,øE Angle,deg. 5 10.4 17.2 -53.5 2.00 Fault. All these bathymetric features show elements of plate 6 20.5 15.1 -54.1 3.91 boundarydeformation, either compressionalor extensional[Roest 13 35.5 16.7 -53.7 5.72 and Collette,1986]. Followingour fracturezone analysis,we start 21 50.3 15.5 -53.8 7.12 with the working hypothesisthat this area may have been the 25 58.6 13.8 -52.0 8.31 location of the plate boundaryafter chron 13 (35.5 Ma), and 31 68.5 12.2 -53.4 9.50 34 84.0 6.4 -58.0 9.31 possiblybefore that time. North-South American finite reconstructionpoles and stage poles. We can test the hypothesisthat the plate boundarymay in fact Calculatedfrom the rotationpoles summarizedby Miiller and Roest have been located at the Fifteen-Twenty Fracture Zone by [1992]. A positiverotation angle indicates counterclockwise rotation. calculatingthe strike slip motion along the NOAM-SOAM plate boundaryat this fracturezone through thne andcomparing it with the reconstructedoffsets of the Fifteen-TwentyFracture Zone. If TABLElb. North-SouthAmerican Stage Poles for NorthAmerica this hypothesisis correct, the present-dayoffsets of magnetic Age1, Ma Age2, Ma Latitude:øN Longitude,,øE Angle:deg. 0.0 10.4 17.2 -53.5 -2.00 anomalies north and south of the fracture zone would reflect the 10.4 20.5 12.9 -54.6 -1.91 cumulativedifferential strike slip alongvarious portions of the plate 20.5 35.5 20.2 -53.0 -1.82 boundary. 35.5 50.3 10.6 -53.9 -1.41 Only a few magneticanomalies in theequatorial Ariantic south of 50.3 58.6 4.4 -41.2 -1.23 58.6 68.5 0.5 -61.7 -1.23 the Fifteen-TwentyFracture Zone have beenidentified because of 68.5 84.0 -57.1 -147.8 -1.22 the locationof thisroughly east-west spreading area at the magnetic See Table la footnotes. equator,where inclination approaches zero. The magneticanomaly picksidentified in this areaare from Klitgord and Schouten[ 1986]. boundarywas not located at the Fifteen-Twenty FractureZone If we reconstructthe relative positionof the magneticpicks north during the entire time interval but migratedto this locationsome andsouth of the Fifteen-TwentyFracture Zone by usingthe North thne after chron 32. American-African and South American-African rotation Betweenchron 32 and 30 (67.5 Ma) the fracturezone offset is parameters,respectively, we obtain the paleo-offsetof the Mid- reducedfrom 145 to 105 km, althoughthe plate model predicts Atlantic Ridge at the triple junction throughtime. We test our continuedleft-lateral strike slip of 15 km. This observationmay working hypothesisby plotting the paleo-ridgeoffset againstthe reflecta ridgejmnp in responseto the continuedlateral separation of cumulativestrike slip motion throughtime (Figure 5), calculated the two ridge segments bounding the fracture zone. In the from the finite rotationpoles in Tables la and lb. following time (chron 30 to chron 5 (10 Ma)) the fracture zone The reconstructedoffset of the Fifteen-TwentyFracture Zone offset increasesfrom 105 to 215 kan until chron 5, while 90 kan of increasesfrom 95 to 145 kan from chron 34 (84 Ma) to chron 32 cmnulativestrike slip are predicted. Hence for this periodthere is (72.5 Ma). The predictedstrike slip for the sametime is 115 km, an excellent agreement between the predicted strike slip and overtwice asmuch as the observedoffset of magneticlineations at observedincrease in offset, yielding evidence that the plate the fracture zone that varies between 25 and 50 kin. This boundarymay have been located at the Fifteen-Twenty Fracture discrepancyis mostreadily explained by the inferencethat the plate Zone from chron 30 to chron 5. However, we point out that a directcorrelation between observed changes in offsetand predicted 300 strike slip cannot be expected, because transform offsets also changein responseto changesin directionof platemotion. To furthertest whether the plate boundarymay havebeen located ZOO in the Fifteen-TwentyFracture Zone area, we now computethe cumulativecompression/extension along this fracture zone by using thecentral North Atlantic and the South Ariantic rotation parameters. Two phasesof differing tectonicregimes can be distinguishedfor 100 e Present day offset (km) the differential motion history along the plate boundary. From o---- Reconstructedoffset (km) chron 33o to 30 the entire plate boundaryfrom crustas old as 84 --' Cumulativestrike-slip (km) Ma to the mid-oceanridge was subjectedto left-lateraltrm•stension, resulting in up to ~50 kan extensionin the area of the western 0 10 20 30 40 50 60 70 80 90 BarracudaRidge. After chron30 the tectonicregime changed to Age (•a) transpressionalong the entireplate boundary.At chron21 (49 Ma), transtensionreplaced transpression along the easternportion of the Fig. 5. The paleoridgeoffset versus the cumulativestrike-slip motion at plate boundary. Since we rely on publishedreconstructions for the Fifteen-TwentyFracture Zone yields strongevidence that the plate computingthe differential motion along the plate boundary,we boundarywas locatedhere from chron30 (67.5 Ma) to chron5 (10 Ma). For ti•nesafter chron33o (80.2 Ma) thereis a goodagreement between 45 restrict ourselvesto computing the cumulative compressionand km of totalincrease in fracturezone offset and 65 k•n of predictedstrike- extensionfor post-chron30 thnes. The calculatedtotal deformation slip from chron33o to chron 32 (72.5 Ma). Betweenchron 32 and 30 from chron30 to chron5 alongthe plate boundaryamounts to 70 (67.5 Ma) the fracture zone offset is reduced from 145 to 105 km, km of compressionin the westernBarracuda Ridge area, 30 km of althoughthe plate model predictscontinued left-lateral strike-slip of 15 compressionin the easternBarracuda Ridge area,and about30 km k•n, probablyreflecting a ridgejump. In the followingtime (chron30 to chron5 (10 Ma)) the fracture zone offset increasesfrom 105 to 215 k•n of extensionin the Researchel;Ridge-Royal Trough area, if the until chron 5, in good agreementwith 90 km of predictedcmnulative plate boundarywas in fact locatedat the Fifteen-TwentyFracture strikeslip. Zone duringthis time (Figure6). MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST 8281

30 13 18N 25 21 Royal 34 16N 30 25 21 13 i•es• erRidge 14N

62W 60W 58W 56W 54W 52W 50W 48W 46W

Fig. 6. Magneticanomaly picks from Norill and (open squares), and anomaly picks rotated from Africa(crosses) in a reconstructionfor chron5 (10 Ma). Fracturezone (FZ) picksfrom NOAM andSOAM aresolid squares; FZ picksrotated fromAfrica are solid triangles. Light gray area near Barracuda Ridge indicates cumulative transpression from chron 30 (67.5Ma) to chron5. Dark gray areanear Royal Troughindicates cumulative transtension from chron21 (49 Ma) to chron5. The calculatedtotal deformation from chron 30 to chron5 alongthe plate boundary amounts to 70 km of compressionin thewestern BR area,30 lanof compressionin the eastern BR area,and 30 km of extensionin theRT, if theplate boundary was in factlocated in this area.

Roestand Collette [1986] speculatedthat both the topography typesare griddedat 2.5 arc rain of latitude and longitudeusing and the gravity anomaly of the Barracuda Ridge indicate continuouscurvature splines in tension[Smith and Wessel,1990]. compressionat a time whenthe lithospherehad gainedconsiderable Becauseship gravity data have DC offsets [Wessel and Watts, thickness. However, they could not clearly distinguishbetween 1988], we adjustedthe DC level of eachship to matchthe satellite compressionaland extensionaldeformation from seismicprofiles gravity field preparedfrom combinedSeasat, Geosat, and ERS-1 acrossthe BarracudaTrough. Seisnficdata froth the sedimentsin altimeterdata [Sandwelland Smith,1992]. For eachship survey, the trough south of the Barracuda Ridge and from the Tiburon we form thedifference between the shipand satellite data and adjust Ridge(Peter and Westbrook,1976; Mascle and Moore, 1990) show the ship data so that the mean difference becomeszero. The widespreadnormal faults but few reversefaults. Therefore these adjusteddifferences are thengridded and addedto the satellitedata seismicreflection data do not seemto provideunequivocal evidence to form the gravity field shownin Figure 8. The accuracyof the for the styleof tectonicdeformation in thisarea. shipbathymetry data was assessed by Smith[1993]. The computedtranstension east of anomaly21 coincideswith the The topographyand gravity data in Figure 8 are strongly occurrenceof the ResearcherRidge andRoyal Trough,which Roest anticorrelatedin the southwestcorner of the maps,near Barbados. and Collette[1986] interpretedas a constructionalvolcanic ridge The topographyrises to shallowlevels on theBarbados accretionary andextensional basin, respectively. The easterntermination of the complex;the gravityshows large negative values, presumably ResearcherRidge is located west of anomaly 6. Transtension becausethe depth to the Moho is increasingon the subductingplate. younger than chron 6 apparentlydid not cause constructional We find that this anticorrelationproduced spurious trends in our volcanism,but resultedin formationof the Royal Trough. The gravity modeling, and therefore we detrend the gravity and absenceof any volcanicridge east of the ResearcherRidge and the topographydata. Trendsare computedby griddingthe data in a presenceof the Royal Troughto the northmay indicatethat some larger area than shown in Figure 8, and using a 400-1an median timebetween chrons 6 and5 theplate boundary junaped to northof filter overthe observeddata grids to definethe trend(Figure 8c and theFifteen-Twenty Fracture Zone. 8d). The trendsare essentially"flat" over muchof the map area outsidethe accretionarywedge, and so the detrendingmakes little TOPOGRAPHY AND GRAVITY ANOMALIES change (other than mean level) in the data in this area. The detrendeddata are shownas Figure8e and 8f.

Dc•a The largestgravity anomalies in the westernequatorial Atlantic are associatedwith the Barracudaand the Tiburonridges (Figures If the NOAM-SOAM plate boundaryhad beenlocated in the 8a and 8e). The Barracuda Ridge has a clear topographic Fifteen-TwentyFracture Zone areaand the oceanfloor in that area expression(Figure 8b and 8f); the Tiburon Ridge is not easily were subjectto initial transtensionand subsequenttranspression, definedtopographically because of its proximityto the Barbados we might expectto find evidencefor crustaldeformation. Here we accretionarycomplex but appearsas a distinct structuralhigh in examine bathymetryand gravity anotnaliesfor clues to crustal seistnicreflection profiles [Mauffret et al., 1984]. deformation.The westernequatorial Atlantic is relativelydensely BouguerGravity Anomalies coveredby ship tracks(Figure 7). We combineship and satellite free-airgravity data with shipbathymetry data to constructgridded We obtainthe Bouguer gravity anomaly grid shown in Figure9 topographyand gravity data sets(Figures 8a and 8b). Both data byremoving the gravity effect of thedetrended topography (Figure 8282 MOLLERAND SMITH: DEFORMATIONOF THE OCEANIC CRUST

20øN 20øN

19øN 19øN

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16øN 16øN

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12øN 12øN 60øW 59øW 58øW 57øW 56øW 55øW 54øW 60øW 59øW 58øW 57øW 56øW 55øW 54øW

Fig. 7. (a) Ship bathymetrytrac 'ks and (b) gravitytracks from shipand satellitealfimetry data in the westernequatorial Atlantic. The Barracudaand Tiburon ridges (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 varyingelevation h(x), x is a 2-vectorindicating (normally) horizontallylayered crust (e.g., layer 2, layer 3, Moho). a positionin the plane: Most modelsof marine gravity anomaliesare constructedusing a layered oceaniccrust [cf. Watts, 1978]. Gravity interpretationis •'[g(x)] =21t'G Apext•-IM Z) In' n!1 •' [{h(x))n]. (1) nonunique,and any numberof structurescan be devisedwhich will fit the observedgravity anomaly. If we assumethat the Bouguer Here•' [] indicatesa Fourier transform, g(x) is thepredicted anomalyshown in 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 depth to the densitycontrast (5200 meandepth to 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 anomalies are 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 separatethese two difficultiesby definingm, the Bougueranomaly the Bouguergravity reaches+ 100 mGal. hmnediatelyto the north at the meandepth of the Moho: in the Barracuda Trough, the Bouguer gravity is negative (-60 ooNn. l mGal) while the bathymetryis flat. The largestpositive Bouguer anomaly (135 mGal) is over the Tiburon Ridge. This feature is at the limit of the area affectedby detrending. Southof the Tiburon Equation(2) is a nonlinearproblem for h, given m, which can be Ridge there is another negative Bouguer anomaly (-80 mGal), solvedby iterativeapproximation [Oldenburg, 1974]. Substitution indicatinganother trough. of (2) into (1) gives

Fig. 8. We combinedship and satellitefree-air gravity data with shipbathymetry data t.o construct gridded topography and gravitydata sets (Figures 8a and 8b). The topographyand gravity data are strongly anticon'elated in the southwestcorner of the maps,near Barbados. We foundthat this anticorrelation produced spurious trends in our gravitymodeling, and so we detrended thegravity and topography data. Trendswere computed by griddingfl•e datain a largerarea than shown and using a 400-kin medianfilter overfl•e observed data grids to definethe trend(Figures 8c and8d). The trendsare essentially "flat" over much of fl•e maparea outside the accretionarywedge, and so fl•e &trendingmakes little change(other titan mean level) in the datain this area. The derrendeddata are shown as Figures8e and8f.. MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST 8283

z z z Z Z Z Z Z Z Z

i i i i i i 8284 MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

g(k) = exp[ - Iklz] re{k) (3) wherev is a parameterthat adjusts the relative weight of E andL. Asv --->0, ffi will fit g exactly,while as v --->o% t'• --->0. Some which is the unstableinverse problem of downwardcontinuation of intermediatechoice of v producesa smooth ffi with a reasonablefit g to obtain m. tothe data. The estimate ffi whichminhnizes the sum (6), given v, Stable estimatesof m may be obtainedfrom (3) by low-pass is filtering of g beforedownward continuation. We use a filte,' which minimizes the curvature of m, because the curvature of h is (7) indicative of deformation in the crust. Estimation of a minimum- •(k)=g{k} [.xp [ -exp[-[Iqz] 2 z] + v ]. curvature.. m from noisyg is shnilarto theproblem of deconvolution by smoothingspline estimation [Constable and Parker, 1991]. Equation(7) is a filterthat is applied to g toobtain t•i. Becausev Morgan and Blackman [Inversion for crustal structure from has units of k -4, it is convenient to define a characteristic combinedgravity and bathymetry data: a prescriptionfor downward wavelength3, Ofthe filter (7) as continuation,subre. to J. of Geophys.Res.] suggesteda similar approachfor modeling the Moho at mid-oceanridges. Let the • = 2/rv1/4. (8) es[hnatedBouguer gravity anomaly at theMoho level be t•i. Then In Figure10a, we showthe root-mean-square (RMS) misfit(in theintegrated squared curvature of t•i. is milliGals),as a functionof 3,(in kilometers).The RMS is foundby dividingE It2 by thenumber of gridelements. L = JJ Ikl4[•(k)] 2 dk (4) Weobtained ffi for thevalues of 3, shownin Figure10a and andthe integrated squared misfit of ffi. to the data g is inverted (2) for the Moho elevation h in each case. We then computedthe absolutevalue of the curvatureof the Moho elevation, E = Jl[g{k)- exp[-lkl z]ffi(k}] 2 dk (5) Iv2 hi, to assessthe extent of crustaldeformation hnplied by •. Curvaturesup to approximately10 -6 m-1 can be sustainedby Nowwe may choose t•. soas to minimize the sum elasticflexure of theocean lithosphere, while curvatures exceeding this value are expectedto result in failure [McNutt, 1984]. At E + vL (6) crustallevels, this failure is expectedto takeplace by brittlefaulting. Figure10b shows the percentof the map areaof Figure9 over which the Moho curvatures exceed the elastic lhnit, for various valuesof theparameter 3,. Figure 10 is an illustration of the classictrade-off between -245 -210 -175 -140 -105 -70 -35 0 35 70 105 resolutionand variance [Backus and Gilbert, 1970;Menke, 1989]. 60øW 59øW 58øW 57øW 56øW 55øW 54øW If we choosea smallvalue for 3,,we fit thedata very well, but the 20øN model implies a Moho which is faulted nearly everywhere;

Filter wavelength, (km) 19øN 100 200 300 3O ...... i ...... I i i i i i i i i i I 18øN I E 20 I 17øN I • 1o I

16øN

0

...... i I ...... i i i [ i i i i i i i i i 15øN 8O - I . I 14øN ß I •o 40

13øN c• 20

12øN I 60øW 59øW 58øW 57øW 56øW 55øW 54øW ß Fig. 10. Root meansquare misfit and percentof faultedcrustal area Fig. 9. Bouguergravity anomalies in the westernequatorial Atlantic. versuswavelength of the smoothingspline filter, showingthe classic Notethe positive Bouguer anomalies at tl•eBarracuda Ridge (up to 100 trade-off between resolution and variance. If we choosea small value for regal),and at theTiburon Ridge (up to 135mGal). NegativeBouguer 1,we fit thedata very well, butthe modelimplies a mohowhich is faulted anomaliesare found in the BarracudaTrough (-60 regal) andtl•e Somhof nearlyeverywhere; conversely, a largevalue of 1 yieldsa modelwith a theTiburon Ridge (-80 mGal),indicating another trough. mostlyunbroken Moho but a poorfit to the data. MOLLERAND SMITH: DEFORMATION OFTHE OCEANIC CRUST 8285

-6 -5 -4 -3 -2 -1 0 I 2 3 4 5 6 < 10 -6 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 from the ! = 70km model with a densitycontrast of (pro=3300kgm -3, @c=2800 kgm-3). The Moho is elevated underneath theridges and deflected downward under the adjacent troughs,resulting insevere crustal flfinning and flfickening reflecting thelarge positive and negative Bouguer gravity anomalies overthe ridges and troughs (Figure 1la). Themodeled Moho shows large curvatures inthe area of the Tiburon and Barracuda ridgesand troughs ,aswell ,as in file Barbados accretionary prism, while curvatures larger than 10 '6 are less common outside of fl•eseareas (Figure 11 b). conversely,a large valueof X, yieldsa modelwith a mostly anddeflected downward under the adjacenttroughs, resulting in unbrokenMoho but a poorfit to the data. Thereis no obvious severecrustal thinning and thickening, reflecting the large positive choiceof X,from Figure 10. We choseX, based on two criteria: and negativeBouguer gravity anomalies over the ridgesand Firstit is our expectationthat large Moho curvatures should be troughs. generallyconfined to the-area of theTiburon and Barracuda ridges The smoothing-splinemodel results in a fairly goodfit of the andtroughs and the subductionzone complex, where the large model-gravityanomalies tothe observed gravity anomalies (Figure Bouguergravity anomalies are observed. Second is ourdesh'e that 12) with an RMS misfit of 7.4 regal. Althoughthe one-layer themodel should fit thegravity anomaly data as well aspossible at crustalmodel satisfies the observed gravity field well, it is doesnot theTiburon and Barracuda ridges and troughs. Using .these criteria, provideus with a realisticmodel for the crustal structure, because it we chose X, = 70 km. doesnot consider the large variations in sedimentthickness in this Figure 11a showsthe Moho elevationand Figure lib the area. Publishedseismic profiles [Peter and Westbrook,1976; distribution of elastic failure from the X, = 70 kan model with a Mauffretet al., 1984]show deep sediment-filled troughs bordering densitycontrast of Pro-Pc(Pro =3300 kg m-3, pc=2800kg m-3). theBarracuda and Tiburon ridges. The density we usefor inverting The modeledMoho showslarge curvaturesin the area of the theBouguer anomalies is toolarge in ourl-layer model is toolarge, Tiburon and Barracudaridges and troughsas well as in the becausesediments are less dense than upper crustal rocks. Barbadosaccretionary prism, while curvatures larger than 10 -6 are Consequently,we overestimatethe amplitude and curvature of the less commonoutside of theseareas (Figure I lb). The modeled modeledMoho underneath these troughs by usingcrustal densities. Mohowe•t of thetoe of theaccretionary complex, shown as a In order to construct a more realistic model, we designed a dashedline in Figures 11a and llb, is not relevant for our nonlinear inversion method that considerssediments, as well as an objectivesand cannot be interpreted, because we detrend the data in uppercrustal and a lower crustal layer. In thismodel we iteratively thisarea. Cross-sectional profiles through the model at theTiburon perturbinitially flat-lying crustal layers by using smoothed residual Ridge(A-A') andthe BarracudaRidge and Trough (B-B') are gravityanomalies. These are smoothedusing the splinefilter shownin Figure12. The Mohois elevatedunderneath the ridges (equation7) withz = 5200m to downwardcontinue the residual 8286 MOLLER AND SMITH: DEFORMATIONOF THE OCEANIC CRUST gravity to the mean seafloordepth. We use the densitiesand layer Latitude (deg N) thicknessesgiven in Table 2 for the layered model, which are 12 13 14 15 16 17 18 19 20 constrainedby regionalseismic data. Sincewe do not haveenough • I [ I [ I • I • I [ I • I • seismicdata to constructa completesedhnent thickness map for our 150 model area, we start our model with a sedimentlayer of 500 m thickness. This is a reasonablevalue for the average sediment 100 thicknessin this area, which varies between 100 and about 1000 m, 50 exceptfor evenlarger sediment thiclmesses in theBarracuda Trough 0 andthe troughsouth of the TiburonRidge [Tucholke et al., 1982]. -50 In the multilayer model, we computea residualanomaly grid ' I ' I ' I ' I ' I ' I ' I ' from theBouguer gravity anomalies by settingall gravity anomaly values smaller than 7.5 mGal to zero. We chose this value, because gravitycross-over errors show uncertainties of this order[Wessel and Watts,1988]. We usethe residualgravity anomaliesto pel•urb all crustal layers in concert. By downward continuing to the sediment-waterinterface only, we perturbthe crustallayers too little at each model step, but create a stable, converging inversion process.We createa layer perturbationp(x) from the smoothed residualgravity s(x) givenby

] I , I , I [ I [ I [ I [ I ] p{x}= S{X} / 2/tG{pm- ps} (9) 150 ß wherewe usethe entire density contrast in thelayered model (Pm 1O0 -Ps), becausethe perturbation will be addedto all layers.Hence 50 thelayers maintain their thickness, except that we haveto satisfythe constraintthat no layer may rise above the seafloor. While a o negativesmoothed residual s(x) will driveall subseafloorhorizons -50 down,with a sedimentlayer growing thicker as seismic layer two is ß I ' I ' I ' I ' I ' I ' I ' depressed,a positives(x) will drive contactsup until they are truncatedagainst the seafloor,which may expose deeper levels of [ I [ I ] I [ I • I • I • I the section.

Using the new perturbedcrustal layer structure,we computea •' XX XXX Xx new gravitymodel from (1) by summingthe contributionsfrom *-, -10 each densitycontrast in the layer structure. We computethe l•• ,N / • ======.•: ',:•:•:/ N/ ,•N/ X / N / maximumabsolute gravity residual as the observedminus the new model. Now we againcompute a residualanomaly grid by setting ' I ' I ' I ' ! ' I ' I ' I all valuessmaller than 7.5 regal to zero and smoothit by usingthe 12 13 14 15 16 17 18 19 20 smoothingspline filter (7). We stopthe modeliterations, if the maximumabsolute value of the smoothedresidual gravity anomalies Fig. 12. Cross-section•profiles through the •tal single-layermodel at the TiburonRidge (A-A' on Figure 11) and the BarracudaRidge and is smallerthan 5 mGal. This is equivalentto usingthe Chebychev Trough•-B' on Figure11). •e Mohois elevatedunderneath fi•e ridges norm as convergencecriterion. and deflecteddownw•d underthe adjacent•oughs, resultingin severe The differencebetween the linear model usingonly one crustal crustal•inning and thickeningreflecting •e large positiveand negative layerand the multilayer model to invertresidual gravity anomalies Bouguergravity anomaliesover fi•e ridgesand •oughs. Althoughthe for Moho topographyis illustratedin Figure 13. We designeda one-layer•stal modelsatisfies •e observedgravity field well, it is does syntheticgravity anomalywith a positiveand a negativepeak not provideus with a realisticmodel for fi•e crustalstructure, because it do• notco•ider •e l•ge vadafionsin s•iment •ickn•s in tldsarea. (Figure13) to showthe effectof inversionfor layer topography usingthe single-layer (Figure 13a) and the multilayer model (Figure 13b). The single-layermodel results in a symmetricridge/trough 3 are ntissing,and a smaller-amplitudetrough, resultingfrom Moho topography(Figure 13a), whereas the inversion for tnodelingthe sediment fill in the trough. multilayertopography results in an asymmetriclayer structure with The Moho elevation(Figure 14a) andthe distributionof elastic a large-amplituderidge, underneath which layer2 andparts of layer failure(Figure 14b) from the multilayermodel is not drastically differentfrom the single-layer model results (Figure 11). However, TABLE2. Thicknessesand Densities of LayersUsed in Model althoughwe have usedthe samewavelength (70 km) for the Layer T!dckness•in Density,kg/m 3 smoothingfilter in bothmodels, we havecreated smoother Moho Water colmnn 5200 1030 Sediments 500 a 2300 topographyunderneath the sediment-filled troughs in themultilayer Layer2 2300b 2800c model. This is reflected in the total area of elastic Moho failure, Layer3 4700b 2950c whichis 30% in the multilayermodel, as opposedto 38% in the Mantle 3400d single-layermodel. Figure 15 showstwo modelcross sections a Mean undisturbedsediment thickness around the BarracudaRidge from over the Barracudaand Tiburon ridgesand troughs. Comparison Tucholkeet al. [1982]. with Figure 12 reveals that the elevated Moho topography bPurdy [1983]. c Carlson and Herrick [1990]. underneaththe Barracuda and Tiburon ridges, resultingfrom d Watts[1978]. positiveresidual gravity anomalies, is quitesi•nilar in bothmodels. MOLLERAND SMITH: DEFORMATIONOF THE OCEANIC CRUST 8287

a Distance (km) b Distance(km) 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 • I i I • I • I i I • I • I • I • -5 rho = 1025 = ,?•r•rho = 2300 ...... ,...... ,...... ,...... •...... ,...... ,...... h = 500-•... -5 ,•, ' x rho= 2800• ß %%%'.-..,!l ;, ;, ;, ;, ;, ;, %;; __•-10 'x xßx/x x';rho=2950x'.xß ßßßßßßßßß ßß ß'h=4700.x -10 ..•x/•/ß-ß-ß-ß-ß• ..... x ß x / ß x x ß / x x ß ß, , , , ½

•2 15 -15 ...... "":'"' ...... i...... ;...... i...... ;...... i ...... ;...... :...... i...... ":'"'% ...... :...... i...... :......

1 oo 100

• 50 5o

,• o 0 < -50 < -50 • -100 -100

• I t I • I t I t I t I • I -5 'ß:/ß/X'/ß//////////////////////// ßß•ßß•ßß ßß••ß ßßßßß ßß• • x/X/ß/ßß)X ß ß ß ß F'"x:'"?'"'"•k ß x/xx X/x/xx/x/x/x/x/• . • •ß•ß' ß' ß' • x•'•ßV.".":.--."....'•••:•,•l•_ß'_ß' ß'ß' ß' ß' ß' ß' ß' ß' x -10 ß / ßßßßßßßßßX/'>"/ / xf-•'•:::•!•'.::A'•/ß/•ß ß/ •/ß/• ßß ßßßß/• ß ß ß

-15 -15

' I ' I ' I ' I ' I ' I ' I ' 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800

Fig. 13. Illustrationof the perturbationof initiallyflat-lying layers by residualgravity anomalies using (a) fi•esingle-layer and (b) the multilayermodel. The single-layermodel restilts in a symmetricridge/trough Moho topography(Figtire 13a), whereasthe inversionfor multilayertopography results in an asymmetriclayer structure with a largeamplitude ridge, underneath which layer 2 andparts of layer3 aremissing, and a smaller-amplitudetrough, restilting from modelingthe sediment fill in the trough.

However,the layer structuremodeled frotn large negative residual 15. Layeredmodels are convenient to calculate,but they may not gravity anomalies,as over sediment-filledtroughs south of the always be appropriate. For example,thinning of layer three is Tiburon Ridge and on both sidesof the BarracudaRidge, is observedat fracturezones [Detrick et al., 1982; Sinha and Louden, smootherthan in the single-layermodel, resulting in a morerealistic 1983] and has beeninvoked to explainthe Blake Spur Fracture Moho curvature. Zonegravity anomaly [Morris et. a/., 1991],although this anomaly In orderto checkour model against seismic data from our study is an order of magnitude smaller than the anomaly over the area, we show a crosssection interpreted from a single-channel BarracudaRidge. Sincethe formation of theBarracuda Ridge and seismicprofile by Mauffret et al. [1984] that crossesthe Tiburon Troughseems related to theFifteen-Twenty Fracture Zone, it might andwestern Barracuda ridges (Figure 16a), and compare it with our be appropriateto draw a modelwith a thinnedlayer 3 alongthe modeledlayer structure sampled along this ship-track (Figure 16b). Barracuda Ridge/Trough. However, because this area has Note that the seismicsection has a verticalaxis in two-way travel undergonean episodeof extension,followed by compression,we thne,while themodel section is in kilometers.Both profiles show a really have no idea what sort of crustalsection is reasonable.The deep sedimentfilled trough north of the BarracudaRidge and displacementof subseafloorlayers shownin Figures 12 and 15 outcroppingbasement at the Tiburon Ridge. The top of the neednot be thoughtof asrepresenting the actualconfiguration of Barracuda Ridge in our model section appears to consist of thecrust; they may be interpretedas merely a proxyfor changesin sediment(Figure 16b). This is a result of invertingsmoothed densityor thicknessas well aslayer displacement. For example, residualgravity anomalies.We startour modelwith a 500m thick upliftof the Moho may be seenas indicativeof increaseddensity; sedimentlayer, and use smoothed residual gravity anomalies ()• > thisincrease need not actuallyoccur by Moho uplift. Nevertheless, 70kin)to perturbthe layer structure. Topographic ridges of widths thereis independentevidence for uplift of materialfrom deep smallerthan 35 km may remain coveredwith sediment,because structurallevels at the BarracudaRidge: mantleperidotites have short-wavelengthgravity an9maliesthat may accompanythese beendredged from the Fifteen-Twenty Fracture Zone during cruises featuresare not invertedfor layerstructure. of the Soviet researchvessel Boris Petrov in 1989 and 1990 [J. Casey,pers. communication, 1991].

DISCUSSION Compressionaldeformation by elasticbuckling has been invoked to explain gravity anomalies in the Indian Ocean [Karner and It is important to bear in mind the nonuniquenessof gravity Weissel,1990]. This is not appropriatefor the gravityanomalies interpretationwhen examining the caa•stal sections in Figures12 and studiedhere. A topographiccompensation mechanism for the 8288 M(JLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 < 10 -6 10 -6 > 10 -6

60øW 59øW 58øW 57øW 56øW 55ow 54ow 60ow 59øW 58øW 57øW 56øW 55øW 54øW

ß 20øN ß ...... 20øN

19øN 19øN

18øN 18øN

17øN 17øN

:::::::::::::::::::::::.:...... 16øN

''::?:i! ....,, ':'•i;'•':'"•"- ..... 15øN ...i.i.i.!:i•.. 15øN

14øN ili!"'"""?,...... i'"i...... }::i:' "!:"...... 13ON i:,,:••' 13øN

12øN ß ß 12øN 60ow 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 smoothingspline filter asin the single-layermodel but havecreated smoother Moho topography underneaththe sediment filled troughsin themultilayer model. This is reflectedin thetotal area of elasticMoho failure, which is 30% in the multilayermodel, compared with 38% in thesingle-layer model.

Barracuda and Tiburon ridges can be excluded, for they are 6). It is unlikely that the shorteningpredicted frownthe plate uncompensated.We have alsoshown that elasticbuckling cannot reconstructionsis grosslyin error. The central North Atlantic and ,. accountfor the crustalstructure of the ridgesand troughs,because the SouthAtlantic are amongthe oceanicregions best covered by the large, short-wavelengthBouguer gravity anomaliesrequire magneticanomaly and fracturezone data. Even if the kinematic curvaturesexceed the elasticlitnit of about 10-6 m-1 [McNutt, shorteningwere more than50% less,of the orderof 10-20 km, we 1984]. Our inversion procedure yields smoothly undulating would still be left with an order of tnagnitudemore than that contactswhich resemblegeological folds. Although we cannot resolvedby gravityinversion. Hence the inversionsperformed here directlydescribe faults in our models,the largecurvatures of these are not a good method for estimatingstrain. Becausethe plate contactssuggest that the crustunderneath the anomalousridges and kinematic analysis suggests large shortening and the Moho troughsin this areais brokenand faulted. Deep seisnficreflection curvaturesunder the ridgesexceed the elasticlitnit, we believethat data from this area are not available to us to test this inference. large strainsare localized in theseareas. Zuber and Parmentier Shallowreflection profiles show a prevalenceof normalfaults in [1991] have made a careful analysis of the compressional sedhnentssouth of the BarracudaRidge and on the Tiburon Ridge deformationof the ocean lithosphere.They fh•d that once strain [Peter and Westbrook, 1976; Mcz•cle and Moore, 1990]. In the exceedsthe linear elasticlimit, a runawayinstability occurs which BarracudaTrough seismicdata show disturbedsediments but do rapidlylocalizes large strains. Sucha processmay describewhat not allow a distinctionbetween normal and reversefaults [Roestand we seeat theseridges. Collette, 1986]. However, the question remains how the cumulative North We calculate the north-south crustal shortening along our Panerican-SouthAmerican transtension and transpressioncalculated multilayermodel profiles A-A' andB-B' overthe 900-kmlength of from plate kinematic modelsmay be reconciledwith the crustal theprofiles. The shorteningwe obtainis of theorder of 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 BarracudaRidge is calculatedfrom plate kinematic models (Figure followingmodel resultsand observations: MOLLER AND SMITH: DEFORMATIONOF THE OCEANIC CRUST 8289

Latitude(deg N.) a 12 13 14 15 16 17 18 19 20 • I • I [ I • I • I • I [ I

150 a

1 O0

50 9 0

-50 ' I ' i ' i ' i ' i ß I ' i ß b Distance •om I I I I ' I .... I .... I .... i

t...... •' . -'•. .'••.-.'•:-••:._., ':':i._.':_.5.2:.• .,.•---•-c' ß . , ß , ...... x,.x 15.0 15.5 16.0 16.5 17.0

, I i I • I , I • I [ I [ I t 150 _ Latitude(deg N.) b 100 Fig. 16. (a) Comparisonof a seisnficprofile from Mat•?et et al. [1984] thatcrosses the Tiburonand western Barracuda ridges (C-C' in Figtire 14) 5O (b) with our modeledlayer structure sampled along the sametrack. Note 0 that the seismicsection has a verticalaxis in two-way travel fiine, while the modelsection is in kilometers.Both profiles show a deepsediment- . -50 filled troughnorth of the BarracudaRidge and outcroppingbase•nent at the TiburonRidge. The top of the BarracudaRidge in our modelsection appearsto consistof sediment. This is a result of inverting smoothed , I , I ] I , I , I , I , I , residual gravity anomalies. We start our model with a 500 m-thick seditnentlayer and usesmoothed residual gravity anotnalies (1 > 70 kin) :.:...... ':.'.'. ß to perturbthe layer structure.Topographic ridges of widthssmaller than 35 km may remaincovered with sedi•nent,because short-wavelength -]0 gravityanomalies that may accompany these features are not inverted for layer structure.

-15

12 13 14 15 16 17 18 19 20 Between chron 30 (67 Ma) to chron 5 (10 Ma) right-lateral transpressionis predictedfrom plate kinematicsfor the Barracuda Fig. 15. Cross-sectionalprofiles from the multilayer model at the same Ridgearea, with 90 km of cumulativestrike slip, and 30-70 kanof locationsas shownin Figure 12. Comparisonwith Figtire 12 revealsthat the elevated Moho topographyunderneath the Barracudaand Tiburon compression. ridges,resulting from positiveresidual gravity anomalies, is quitesinfilar 3. The BarracudaTrough is coincidentwith the western in bofl•models. However, the layer structuremodeled from largenegative extensionof the Fifteen-TwentyFracture Zone [Roestand Collette, residualgravity anomalies,as over sediment-filledtroughs south of the 1986]. The deep trough southof the Tiburon Ridge may be TiburonRidge and on bothsides of the BarracudaRidge, is smootherthan bounded by the western extensionsof the Mercurius and Verna in the single-layermodel, resulting in a morerealistic Moho curvature. FractureZones (Figure 4). 4. The largegravity anomaly over the Tiburon Ridge requh'es a 1. The North American-SouthAmerican plate boundarywas crustalthickness of only 1.5-2kan, indicating a very shallowMoho initiated at the Guinea-Demarara shear margin in the Late (Figure15a). The Mohodepth underneath the Barracuda Ridge is Cretaceous.A comparisonof fracturezones with syntheticplate modeledto be 4 kan below seafloor(Figure 15b). In both cases flow lines showsthat it migratedto the area betweenthe Marathon layer2 is infen'edto beabsent; underneath the Tiburon Ridge, even and Fifteen-Twenty fracture zones, where the Barracuda and the thicknessof layer 3 is reducedconsiderably. This indicates Tiburonridges are located,some time betweenchron 32 (72.5 Ma) severecrustal thinning along these uncompensated ridges and active andchron 13 (35.5 Ma). Moho uplift. Sedimentfilled troughsseveral kilometers deep are 2. Differential motion between and South foundadjacent to theridges America calculated for the Barracuda Ridge area from plate 5. Moho curvaturesunderneath the ridges and troughsin the kinematicdata suggests a phaseof left-lateraltranstension between studyarea exceed the elastic lhnit 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 100 kanof strikeslip and-50 lan extension.However, a large 6. Normal faultsare cormnonin sedhnentson the TiburonRidge discrepancybetween the changein offset of the Fifteen-Twenty and south of the Barracuda Ridge [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 locatedsouth of Troughcould be due to eitherextension or compression[Roest and the Fifteen-TwentyFracture Zone/Barracuda Ridge during this thne. Collette, 1986]. 8290 MOLLER AND SMITH: DEFORMATION OF THE OCEANIC CRUST

7. Focal mechanismsof 2 large intraplateearthquakes north of CONCLUSIONS the Barracuda Ridge have composite thrust and strike-slip A comparisonof fracture zone trendsand syntheticflow lines mechanisms[Bergman, 1986] andindicate N-S compression. revealsthat the North American-SouthAmerican-African triple 8. Heat flow on the TiburonRidge from 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 the Vema FractureZone beforechron 34 (84Ma), to timesas high as expected on normalocean crust of thisage (about north of the Doldrums Fracture Zone before chron 22 (51.9 Ma), 80 Ma). and to north of the Mercurius Fracture Zone between chron 32 9. A 220m thick middle Eocene-upperOligocene sequence of (72.5 Ma) and chron13 (35.5 Ma). Two anomalousridges on old turbiditeswas found on the slopeof the Tiburon Ridge at Ocean oceancrust (-84-70 Ma) betweenthe Mercuriusand Fifteen-Twenty Drilling Programsite 672A [Mascle and Moore, 1990]. The fracture zones, the Barracudaand Tiburon ridges, exhibit large, turbiditesare foundabout 800 tn abovethe adjacentabyssal plain. short-wavelengthBouguer gravity anomalies(up to 135 regal), which require mantle densitiesat shallow levels under the ridges Theseobservations and modelingresults lead to the following (Moho uplift of up to 4 lcm). Inversion of Bouguer gravity geologicalinterpretation: when transtension started between North anomaliesfor smoothcrustal structure yields layers with curvatures America and South America in the Late Cretaceous(we can only exceeding10-6 m-1 underneaththeridges and adjacent troughs. calculatethis for the time after chron 34 (84 Ma) due to the absence This exceedsvalues permissible under elastic buckling and suggests of magneticanomalies during the CretaceousMagnetic Quiet that this area has been subjectto severedeformation as a result of Period), the Fifteen-Twenty, Marathon, Mercurius, and Verna North American-SouthAmerican plate motions. Plate kinematic fracturezones may have representedstructural weaknesses in the data suggesta Late Cretaceousphase of transtension,followed by oceaniccrust. Along theirfracture troughs the crustmay havebeen transpressionin the Tertiary for the Tiburon/BarracudaRidge area. severelythinned already, as has been observedat other fracture We proposethat crustalthinning during the transtensionalperiod zonesin the centralNorth and equatorialAtlantic [Detrick et al., was localized at preexisting structural weaknessessuch as the 1982, Sinha and Louden, 1983, Morris et. al., 1991]. Verna, Marathon, and Mercurius fracture zone troughs. A Most likely the transtensionlocalized strain along these structural comparisonof fracturezones with syntheticflow linessuggests that weaknessesso that the crustwas thinnedeven more. Our analysis duringmost of this period the plate boundarywas locatedbetween of the changein offsetthrough thne of the Fifteen-TwentyFracture the Verna and the Marathon fracturezones, but may have included Zone versuscomputed strike slip for the thneof transtension(84-67 the Fifteen-Twenty FractureZone after chron 32 (72.5 Ma). This Ma) suggeststhat the locationof the plate boundarywas southof interpretationconcurs with our crustal structural model, which the Fifteen-TwentyFracture Zone. Hencewe wouldexpect to find showsmore severecrustal thinning underneath the TiburonRidge more evidencefor extensionin the Tiburon Ridge area than in the than at the BarracudaRidge. After chron30 (67 Ma) transtension BmTacudaRidge/Trough area. This line of reasoningis supported wasreplaced by transpressionin this area,localizing the resulting by our crustalmodel sections from inversionof gravityanomalies, strain along the already thinned and weakened crustal sections, which show more severe thinning under the Tiburon Rise than creating the ridge/trough topography observed today in the underneaththe BarracudaRidge. Tiburon/BarracudaRidge area. Middle Eocene-upperOligocene When the tectonicreghne changed to transpressionin the Early turbidiresdrilled on the flank of the Tiburon Ridge far abovethe Tertiary, the deformationcaused by compressionalstresses may adjacentabyssal plain suggestthat muchof its uplift hasoccmxed haveremained localized at the alreadythinned crustal areas. This is after the Oligocene. High heat flow at the Tiburon Ridge and the also likely becauseof the additionalstrike slip componentof the unstable density distribution underneath the Tiburon and the differentialplate motion that may havecontributed to lateralmotion Barracuda ridges indicate that uplift due to compressional andfaulting of the crustalong the fracture zones. Oncethis process deformationin this area may still be ongoingtoday, and that the had started a combination of compressional and strike slip North American-SouthAmerican plate boundary may still be located componentschanging throughout the Tertiary may have caused here. initial crustalbuckling that resultedin elasticfailure after strain Given this scenario,we would expectto find frequentreverse exceededthe linearelastic limit, resultingin runawaydeformation as faults in the plate deformation area. From published seismic found by Zuber and Parmentier [1991]. That strain must have sectionsit appearsthat noHnalfaults in sedhnentsare moreconunon exceededthe elastic lhnit in the large parts of the area from the than reversefaults, but Bull and Scrutton(1992) have interpreted BarracudaTrough to southof the Tiburon Ridge is suggestedby sedimentonlap patternsin the BarracudaRidge area as indicators ourmap of Mohocurvatures exceeding 10-6m -1 (Figure 14). for rotation and reverse faulting. A combination of seismic Publishedplate kinematic data cannot resolve the thne within the reflectionand refractiondata will be necessaryto investigatethe Tertiary when the Barracudaand Tiburon Ridges were uplifted. styleand amount of deformationin thisarea and compare these data However,the Eocene-upperOligocene sequence of turbiditesfound with the North American-South American differential motions and on the slopeof the Tiburon Ridge 800 m abovethe abyssalplain theiruncertainties predicted from platereconstructions. documentsthat this ridge must have risen significantlyafter the Oligocene. The high heat flow on the TiburonRidge alsofits the Acknowledgements. The gridding, contouring, grid sampling, scenariowell, andmay be accountedfor by the very shallowMoho detrending,and graphic88oftware u•ed in thi• paper ½ome•from the (1.5-2 kan)underneath the ridge and not by fluid dischargefrom the GMT •y•tom (Wesseland Smith, 1991). JasonPhipp• Morgan •ugge•tod the •moothingspline inversion approach. The authorswould like to thank nearbyaccretionary wedge, as suggestedby Fisher and Hounslow Andrew D. Walker, Kevin M. Brown, Waltor R. Roe•t, David T. [1990]. The shallow Moho underneaththe Tiburon and Ban'acuda Sandwell,and ChristopherSmall for manyhelpful di•cusdon•. We thank Ridgesreflects an unstabledensity distribution and indicatesthat Han• Schoutenand an anonymou•reviewer for critical review• of the compressivestresses may be requiredat the presenttime to maintain manuscript.R.D.M. wa• •upportedby a graduateresearch a•si•tant•hip these anomalies. and W.H.F.S. by a postdoctoral•cholarship from the C.H. and I.M. MOLLERAND SMITH: DEFORMATIONOF THE OCEANIC CRUST 8291

Green Foundation for Earth Sciencesat the Scripps Institution of M011er,R. D. and W. R. Roest, Fracture zones in tile North Atlantic from Oceanography. combinedGeosat and Seasatdata, J. Geophys.Res., 97, 3337-3350 1992. REFERENCES Mfiller, R.D., D.T. Sandwell, B.E. Tucholke, J.G. Sclater,and P.R. Shaw, Depthto basementand geoid expression of the KaneFracture Zone: A Backus, G. E., and J. F. Gilbert, Uniquenessin the inversion of gross comparison,Mar. Geophys.Res., 13, 105-129,1991. Earth data, Ptu'l.os Trans. R. Soc. London, Ser. A, 266, 123-192, 1970. Oldenburg,D.W., The inversionand interpretationof gravity anomalies, Bergman,E.A., lntraplateearthquakes and file stateof stressin oceanic Geophysics,39, 526-536, 1974. lithosphere,Tectonophysics, 132, 1-35, 1986. Parker,R. L., The rapid calculationof potentialanomalies, Geophys. J. R. Bull, J.M., and R.A. Scrutton, Diffuse Plate boundariesin the Indian and Astron. Soc., 31,447-455, 1972. AtlanticOceans,EOS Trans Amer GeophysUnion, 73, 508-509, 1992. Peter, G., and G.K. Westbrook,Tectonics of southwesternNorth Atlantic Cande,S.C., J. L. LaBrecque,and W. F. Haxby, Plate kinematicsof the and BarbadosRidge complex,Am. Assoc.Pet. Geol. Bull., 60, 1078- SouthAtlantic: Chron 34 to present,J. Geophys.Res., 93, 13,479- 1106, 1976. 13,492, 1988. Purdy,G. M., The seisnficstructure of 140 my old crustin the western Carlson,R. L., and C. N. Herrick, Densitiesand porositiesin the oceanic central , Geophys. J. R. Astron. Soc., 72, 115-137, crustand their variationswith depthand age, J. Geophys.Res., 95, 1983. 9153-9170, 1990. Roest,W. R., Seafloorspreading pattern of the North Atlantic between Collette, B. J., A. P. Slootweg,J. Verhoef, and W. R. Roest,Geophysical 10ø and 40øN, Geol. Ultraiectina,48, 121 pp., 1987. investigationsof file floor of file AtlanticOcean between 10 ø and 38ø Roest,W. R., and B. J. Collette, The Fifteen Twenty FractureZone and N (Kroonvlag-project),Proc. K. Ned. Alcad.Wet., Ser. B, 87, 1-76, the North American-SouthAmerican plate boundary,J. Geol. Soc. 1984. London, 143, 833-843, 1986. Constable,C., and R. Parker, Deconvolutionof long-corepalcomagnetic Sandwell,D.T. and W.H.F. Smith, Global marine gravity from ERS-1, measurements-Splinetherapy for file linear problem,Geot•hys. J. Int., Geosat and Seasat reveals new tectonic fabric, EoS Trans. A GU, 73, 104, 453-468, 1991. 133, 1992. Detrick, R. S., M. H. Corntier, R. A. Prince, and D. W. Forsyth,Seisntic Shaw,P. R., andS.C. Cande,High-resolution inversion for SouthAtlantic constraintsof the crustal structurewithin the Verna Fracture Zone, J. plate kinematicsusing joint altimeterand magneticanomaly data, J. Geophys.Res., 87, 10,599-10,612, 1982. Geophys.Res., 95, 2625-2644, 1990. Engel,C.G., andR.L. Fisher,Granific to ultramaficrock complexes of the Sinha, M. C., and K. E. Louden, The OceanographerFracture Zone - I. Indian Oceanridge system,western Indian Ocean, Geol. Soc.Am. Crustal structurefrom seismic refraction studies, Geophys.J. R. Bull., 86, 1553-1578, 1975. Astron. Soc., 75, 713-736, 1983. Fisher, A.T., and M.W. Hounslow, Heat flow through the toe of the Smith, W. H. F., On the accuracy of digital bathymetric data, Jour. Barbados accretionary complex, ODP leg 110: Tectonic and Geophys.Res., in press,1993. hydrologicsynflmsis, Proc. OceanDrill. Program, Sci. Results,I10, Smith, W. H. F., and P. Wessel, Gridding with continuouscurvature 345-364, 1990. splinesin tension,Geophysics, 55, 293-305, 1990. Karner, G. D., and J. K. Weissel, Factors controlling the location of Tucholke,B. E., R. E. Houtz, and W. J. Ludwig, Sedimentthickness and compressionaldeformation of oceaniclithosphere in the centralIndian depthto basementin westernNorth Atlantic Ocean basin, AAPG Bull, Ocean,J. Geophys.Res., 95, 19,795-19,810,1990. 66, 1384-1395, 1982. Kent, D. V., and F. M. Gradstein,A Jurassicto Recentchronology, in Tl•e Tucholke, B.E. and H. Schouten,Kane Fracture Zone, Mar. Geophys. Geologyof North America, vol. M, The WesternNorth Atlantic Res., I0, 1-39, 1988. Region,edited by P. R. Vogt and B. E. Tucholke, pp. 45-.50, Watts, A. B., An analysisof isostasyin the world'soceans, 1, Haw,'fiian- GeologicalSociety of America,Boulder, Colo., 1986. Emperorseamount chain, J. Geophys.Res., 83, 5989-6004, 1978. Klitgord,K. D., and H. Schouten,Plate kinematicsof the centralNorth Wessel,P. andW.F. Haxby, Thermalstresses, differential subsidence, and Atlantic, in The Geology of North A,nerica, vol. M, The Western flexureat oceanicfracture zones, J. Geophys.Res., 95, 375-391, 1990. North Atlantic Region,edited by P. R. Vogt and B. E. Tucholke,pp. Wessel,P. andW. H. F. Smith,Free software helps map and display data, 351-404, GeologicalSociety of America,Boulder, Colo., 1986. EoS Trans. AGU, 72, 445, 1991. Mascle, A., and J.C. Moore, ODP leg 110: Tectonic and hydrologic Wessel, P., and A. B. Watts, On the accuracy of marine gravity synthesis,Proc. Ocean Drill. Program.,Sci. Results,II0, 409-424, measurements,J. Geophys.Res., 93, 393-4 13, 1988. 1990. Westbrook,G. K., A. Mascle, and B. Biju-Duval, Geophysicsand the Mascle, J., E. Blarez, and M. Marinho, The shallow structures of the structure of the Lesser Antilles Arc, Initial Rep. DSDP, 78A, Guineaand Ivory Coast-Ghanatransform margins: Their bearingon Washington,23-38, 1984. the equatorialAtlantic Mesozoic evolution, Tectonot•hysics, 1.55, 193- Zuber, M. T., and E. M. Parmenfier,Fold growth in a non-Newtonian 210, 1988. viscouslithosphere: Implications for horizontalshortening associated Mauffret, A., G. K. Westbrook,M. Truchan, and J. Ladd, The relief of the with intraplatefolds in Indian Ocean seafloor,EoS Trans. Amer. oceanicbasement and the structureof the front of the accrefionary Geophys.Union, 72, 472, 1991. complexin the regionof sites541,542 and 543, Initial Rep. DSDP, 78A, 49-56, 1984. McKenzie, D., and C. Bowin, The relationshipbetween bafllymetry and gravityin the AtlanticOcean, J. Geophys.Res., 81, 1903-1915, 1976. R.D. Miiller, SIO, UCSD, 9500 Gilman Dr., La Jolla, CA 92093- McNutt, M. K., Lithosphericflexure and thermalanomalies, J. Geophys. 0208. Res., 89, 11,180-11,194, 1984. W.H.F. Smith, GeosciencesLaboratory, Ocean and Earth Sciences, Menke, W., GeophysicalData Atutlysis: DiscreteInverse Theory, 2nd NOS, NOAA, 11400Rockville Pike, Rockville,MD 20852. rev. ed., 285 pp., Acadentic,San Diego, Calif., 1989. Morris, E., R. S. Detrick, T. A. Minschull, R. S. White, P. Bulfi, and J. C. Mutter, Variations in crustal structure: Normal vs. fracture zone crust, EoS Trans. A GU, 72, 431,1991. Mulder, T. F. A., and B. J. Collette, Gravity anomalies over inactive (ReceivedJanuary 10, 1992; fracture zones in the central Norill Atlantic, Mar. Geophys.Res., 6, revisedSeptember 21, 1992; 383-394, 1984. acceptedDecember 4, 1992.)