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Jgr-Chile-Ridge-Tectonic-Framework.Pdf JOURNALOF GEOPHYSICAL RESEARCH, VOL. 102, NO. B6, PAGES 12,035-12,059, JUNE 10, 1997 The Chile ridge: A tectonic framework $.F. Tebbens,1 S.C. Cande,2 L. Kovacs,3 J. C. Parra,4,5 J.L. LaBrecque,6 and H. Vergara7 Abstract.A newChile ridge tectonic framework isdeveloped based on satellite altimetry data, shipboardgeophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.- Chileanaeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), includingtwo complex systems (the Chile FZ andValdivia FZ systems),have been mapped betweenthe northern end of theAntarctic-Nazca plate boundary (Chile ridge) at 35øSand the Chile margintriple junction at 47øS.Chile ridge spreading rates from 23 Ma to Presenthave been determinedand show slowdowns in spreading rates that correspond totimes of ridge-trench collisions.The ValdiviaFZ system,previously mapped as two FZs withan uncharted seismicallyactive region between them, is now recognized to bea multiple-offsetFZ system composedof six FZs separatedby shortridge segments 22 to 27 km long. At chron5A (-12 Ma),the Chile ridge propagated from the Valdivia FZ systemnorthward into the Nazca plate throughcrust formed 5 Myr earlierat thePacific-Nazca ridge. Evidence for this propagation event includesthe Friday and Crusoe troughs, located at discontinuitiesin the magnetic anomaly sequenceand interpreted as pseudofaults. This propagation event led to theformation of theFriday microplate,which resulted in thetransferal of crustfrom the Nazca plate to theAntarctic plate, and ina 500-kmnorthward stepwise migration of thePacific-Antarctic-Nazca triple junction. Rift propagation,microplate formation, microplate extinction, and stepwise triple junction migration arefound to occurduring large-scale plate motion changes and plate boundary changes in the southeastPacific. Introduction improved due to the acquisition and analysis of additional geophysical traverses of the Chile ridge [Herron, 1972; The Chile ridge is a major branch of the global ridge Klitgord et aI., 1973; Handschumacher, 1976; Herron et al., system,extending from the junction of the Pacific, Antarctic, 1981] and Chile FZ [Anderson-Fontanaet al., 1987]. In the andNazca plates at the JuanFernandez microplate [Larson et immediate vicinity of the Chile margin triple junction, the al., 1992, and referencestherein] to the triple junctionof the chartingof ridge axis segments,FZs, and magneticisochrons Antarctic,Nazca, and SouthAmerican plates near 46øS along was furtherimproved by detailedshipboard surveys [Cande and the southernChile trench [Herron et al., 1981; Cande et aI., Leslie, 1986; Cande et al., 1987a]. 1987b](Figure 1). In January 1990, a team of scientistsfrom the U.S. Naval Theexistence of a mid-ocean'ridge between Rapa Nui (Easter Research Laboratory (NRL), Lamont-Doherty Earth Island)and southern South America was first predicted, on the Observatory(LDEO), the Servicio Nacional de Geologiay basisof seismicevidence, by Ewing and Heezen[1956]. The Mineria,and the Servicio Hidrografico y Oceanografico dola presenceof the Chile ridge was confirmedby bathymetricdata Armada de Chile conductedan aeromagneticsurvey of the collectedon the Scripps Institution of Oceanography Chile ridge. The aeromagneticsurvey and satellitealtimetry Downwindexpedition [Menard et al., 1964]. The spreading data [Sandwell,1993; Smith and Sandwell,1995] form the patternon the Chile ridge was first mappedby Herron and basis for an improvedcharting of the tectonicfeatures of the Hayes[1969] and Herron [1971], using shipboard data, and by Chile ridge and its flanks. Morganet aI. [1969],using aeromagnetic data. Knowledgeof This paper discussesthe tectonicfeatures observed along magneticanomalies and fracture zone (FZ) locationshas the Chile ridge and its flanks. The most significantnew feature is the Friday microplatewhich formedduring a chron 5A (-12 Ma) plate boundaryreorganization. A companion IDepartmentofMarine Science, University ofSouth Florida, St. paper[Tebbens and Cande,this issue]presents reconstructions Petersburg,Florida. 2ScrippsInstitution ofOceanography, LaJolla, California. of the southeastPacific sincethe late Oligocene,showing the 3NavalResearch Laboratory, Washington, D.C. evolution of the Chile ridge within the framework of the 4ServicioNacional deGeologia yMineria, Santiago, Chile. southern Pacific Ocean. 5NowatGEODATOS, Santiago, Chile. %amont-DohertyEarthObservatory ofColumbia University, Palisades,New York. Data 7ServicioHidrografico y Oceanografico dola Armada, Valparaiso, Chile. Tectonicinterpretations are presentedbased on all available topographic,magnetic, and satellite altimetry data in the Copyright1997by the American Geophysical Union. surveyregion, including the 1990 aeromagneticsurvey of the Papernumber 96JB0258 !. Chile ridge. Bathymetryand magneticdata from a 1988 RRS 0148-0227/97/96JB.02581$09.00 Charles Darwin survey are presented,including four ridge- 12,035 12,036 TEBBENSET AL.: CHILE RIDGE TECTONIC FRAMEWORK 120"W 115"W 11OøW 105"W 1O0"W 95øW 90"W 85øW 80"W 75øW 70"W 30øS Nazca PlateFigures 7a & 7b Figures 4a & 4b 35øS Pacific Plate 40øS ures 6a & 6 ,divi&..F.Z...• 45øS -fa'•tao Figures 5a & 5b '?. 5O S 50øS Antarctic to . 120øW 115"W 110øW 105"W 100'W 95øW 90øW 85•W 80øW 75øW 70øW Figure 1. Major tectonic featuresof the SouthPacific includingridge axes (heavy solid lines), FZs (solid and dashed lines are known and inferred locations, respectively), nontransformoffset traces (dash-dotted lines), fossil ridge axes (railroad track pattern), transferredlithosphere (striped areas),PAC-ANT-FAR triple junction trace (TJT) (dotted lines with heavy dots), and other tectonicdiscontinuities (heavy lines with dots). The 1990 aeromagnetic survey lines are shown (light dotted lines). Diagonal shading denotes transferred lithosphere. Location maps for Figures 4, 5, 6, and 7 are shown. perpendicularprofiles south of the Guafo FZ. Fourpreviously The altitude at which aeromagneticdata are collectedaffects unpublishedaeromagnetic lines are included which were the resolutionof the recorded signal. When weather permitted, navigated with a combination of LORAN and inertial lines were flown at approximately 500 m elevation, whichis navigation: two lines cross the Chile ridge axis, one near less than 4.0 km over the seafloor at the ridge axis. At this 38.2øS and the other near 39.5øS; and two are east-west lines low altitude, the magneticsignal is comparableto a shipboard located between 35øS and 36øS, to the north of the Chile ridge survey. A few sectionswere flown at altitudesas high as3 krn, (D. W. Handschumacher,unpublished data, 1980). due to poor weatherand equipmentproblems, resulting in the In January1990, a P-3 Orion aircraftequipped with a proton loss of the short-wavelength information in these track precessionmagnetometer collected a total of 37,500 km of segments. aeromagneticdata (Figures 1 and 2). The survey lines were Magnetic anomalieswere identified by comparingthem to a oriented along seafloorspreading flow lines determinedprior model basedon the known sequenceof geomagneticpolarity to the survey by examining altimetry data for FZ orientations. reversals [Cande and Kent, 1992, 1995]. Representative Total intensity magnetic measurements were reduced to magneticprofiles are easily correlatableand identifiablewhen anomaly form by subtracting the 1985 International comparedto a modelprofile (e.g., Figure 3). We includedor, Geomagnetic Reference Field [International Association of when necessary, modified the magnetic anomaly Geomagnetismand Aeronomy (IAGA), 1987] correctedfor 5 identificationsand/or FZ locationsof Morgan et al. [1969], years of secular variation. The flights were almost entirely Herron and Hayes [1969], Herron [1971], Klitgordet al. navigatedwith Global PositioningSystem (GPS). During two [1973], Handschumacher [1976], Herron et al. [1981], short flight segmentswhen GPS was not available,OMEGA LaBrecque[ 1986], Candeand Leslie [ 1986], Anderson-Fontana navigationwas used. The OMEGA-navigatedsegments are the et al. [ 1986], Molnar et al. [ 1975], and Lonsdale [1994]. sections of the first and fourth aeromagnetic tracks north of Satelliteradar altimetry data were combined with shipboard the Guafo FZ and west of the ridge axis (181 and 355 km long, andaeromagnetic data to mapthe locationof ridgeaxes and respectively)(Figure 2). Small but significantdeviations (of FZs. Color imageswere createdusing the gravityanomaly the order of 4 kin) in the linearity of the magnetic isochrons gridof Smithand Sandwell[1995] whichincorporated satellite acrossthese track segmentsare believed to be artifactsdue to altimetryprofiles from ERS 1, Geosat,and Seasat. The grid inaccuracies in the OMEGA navigation relative to the GPS spacingis 1/20th of a degree in longitude. In this paper, navigation. In other regions with only GPS navigation, "gravityfield" refersto the Smithand Sandwell[1995] gravity offsetsof the order of 4 km are interpretedto be real. grid. TEBBENSET AL.: CHILERIDGE TECTON1C FRAMEWORK 12,037 100øW 95øW 90"W 85"W 80"W 75 øW _ ;-_-___ ß -........ ..--- ................................... ................................ • ............:................... •.•/(.n_,_e_.• ...... =•%:.................... 57"ti,•e 5 .->-<:::'".:'.' .... / ....... d,! .._ ................... i -• -. --••;::.:_v. •: :'............ '.......................... I ...... '............ ' ........... ' .............""""' :'•"1"'"•"• ......... 40 S I'. .......................................................,-....................
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