Deformation of the Oceanic Crust Between the North American and South American Plates

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 ScrippsInstitution of Oceanography,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 the TiburonRidge, 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 the crust.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 Africa 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 triple junction was locatedat the Guinea- SouthAmerican plates. However, the tectonichistory of thisarea is Delnararashear Inargin at thatthne. notwell lmowndue to the lackof 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 15øN 0 o 10øN NAM 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

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