JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B9, PAGES 10,421-10,437, SEPTEMBER 10, 1988
TectonicEvolution of theExplorer-Northern Juan de Fuca Region From 8 Ma to the Present
MONABOTROSl AND H. PAULJOHNSON
Schoolof Oceanography,University of Washington,Seattle
New magneticanomaly data, in conjunctionwith SeaBeam and SeaMARC H a_,_ts,has been used to schematicallymodel the tectonic development of the Explorer-Northern Juan de FucaRegion with a seriesof fiveridge-offset propagators during the last 8 m.y. TheSovanco Fracture Zone played a majorrole in the evolutionof thisarea since its originas a spreadingcenter offset between the Explorer and Juan de Fuca Ridgesapproximately 7.4 Ma. Between• time and2.5 Ma, the transformmigrated southwards due to southwardpropagation of Explorer Ridge, and lengthened bythe addition of threepropagating ridge offsets as wellas by asymmetricspreading tothe east on Explorer Ridge. Since the independence of Explorer Plate at 4 Ma, therehas been clockwise rotation of theExplorer Spreading Center relative to the Juan de Fuca Ridge. In responseto forcesacting at the convergentmargin, the SovancoFracture Zoue also underwent clockwise reorientation,and in theprocess developed into a broadzone of shear.The tectonic deformation associated withthis rotation distorted the previously existing N-S magneticanomaly pattern. Explorer Seamount, a majorgeological feature in theregion, is modeledas a presentlyextinct southern spreading segment of the ExplorerRidge system which was isolated by the northwardmigration of the westend of the Sovanco FractureZone. The recent( INTRODUC'rlON propagatingrift theory[Shih and Molnar, 1975; Hey, 1977],Hey and Wilson[1982] showedthat the obliquemagnetic anomaly In comparisonto the extensivelystudied Juan de FucaRidge, offsetswhich had been recognizedin the northeastPacific the ExplorerSpreading Center, which lies to the north of the anomalypattern, and which appearedto be a contradictionto the SovancoFracture Zone (Figure1), hasreceived relatively little theoryof rigid plate tectonics[Raft and Mason, 1961; V/he, attention. Elucidationof the role of rift propagationin the 1968],could result from the propagation of oneridge segment at evolutionof ExplorerRidge has been hindered in thepast by the the expenseof another. Wilson et al. [1984] modeled the lackof detailedmagnetic and bathymetric data. The objectiveof evolutionof theJuan de Fuca Ridge during the past 17 m.y. with this studyis to integratea new magneticanomaly compilation a seriesof 7 propagatingrifts. A detailedstudy of the recent with recentlyacquired Sea Beam bathymetry and SeaMARCII tectonicevolution of theyoungest propagator on the Juan de Fuca sidescan sonar data [Davis eta/., 1984a;Sawyer eta/., 1984],in Ridgewas presented by Johnsonet al. [1983]. a schematicmodel for the tectonicevolution of the Explorer- Integrationof magneticanomaly data with other geophysical northernJuan de Fucaarea from 8 Ma to thepresent (Ma hereis andgeological studies [e.g., $rivastava eta/., 1971;Barr, 1972; usedto denoteage in million yearsbefore present). Further, the McManuset al., 1972;Barr andChase, 1974; Hyndman et al., implicationsof this model for regionalplate interactionsare 1978;Hyndman et al., 1979;Riddihough et al., 1980;Davis and discussed. Riddihough,1982] has produced a general picture of theregional The studyof marinemagnetic anomalies has proven to be one platetectonic geometry which is summarizedin Figure1. The of the most useful techniquesfor decipheringthe tectonic spreadingsegments of ExplorerRidge, together with the Sovanco evolutionof oceanicplat•.s. In thenortheast Pacific, the history FractureZone, comprise the boundary between the Pacific Plate of theJuan de Fucaplate system has been developed by a number andthe much smaller Explorer Plate. To theeast, the Explorer of authorsincluding Wilson [1965], Vine and Wilson [1965], Plateconverges with the NorthAmerican Hate. The northeast- Pavoni[1966], Vine [1968],Atwater [1970], Riddihough [1977], southwesttrending Nootka Fault Zone is a transformboundary Menard [1978], and Carlson [1981]. Recentcomprehensive betweenthe Explorerand Juan de Fucaplates [Hyndman et al., analysesof both relative and absoluteplate motionsin the 1979]. The northernmostsegments of ExplorerRidge terminate northeastPacific were presentedby Riddihough[1984] and againstthe Revere-DellwoodFracture Zone, a transformfault Nishimuraet al. [1984]. Following the developmentof the that extends northwestward to the Dellwood and J. Tuzo Wilson spreadingcenters [Riddihough et al., 1980;Currie et al., 1984]. 1Nøwatthe Graduate School ofOceanography, University ofRhode An earlystudy of thenorthern Juan de FucaRidge by Barr Island,Narraganseu, Rhode Island. [1972], suggestedthat the SovancoFracture Zone initiated between8 and9 Ma, andthat the Explorer Plate has been moving Copyright1988 by the American Geophysical Union. largelyindependently since that time. In his analysisof the Papernumber 7B6015. marinemagnetic anomaly pattern, Riddihough [1977] argued that 0148-0227/88/007B-6015505.00 relativemotion between the Juan de Fucaand Explorer plates is 10,421 10,422 BoT•os ANDJOHNSON: TBCr0NIC EVOLUTION OF JUAN DE FUCA I•ION Thereforethe boundariesof the magneticanomalies can be TUZOKNOLLSW.SO.• •, REVERE- '•.•"'"'•.'-':' NORTH AMERICA \.DELLWOODEZ. "4•. P L AT E reasonablydated at the age of the correspondingreversal DELLWOOD•/'\ .,• •'•:.j..':...... :-•.•: boundary. KNOLLS ß I • ('•:•'"-•••'•::•'" .. :...-::•.•-:'.:-'.':- - •x ::•-" Havingidentified the isochrons,the spreadingrates and 50* ER•• ;'?"•'::Z:':':::.:..'..:.. anomaly azimuths were then measuredfrom the magnetic anomalymap using the method described by Riddihough [1977; 1984]. Spreadingrates were measured perpendicular tothe strike % PLATE-X• '"-•i:.':."'•4;'".•.•''•...'-". •=:• "•'-'.:..'.. of the anomalies, and between reversal boundariesthat representedan intervalof approximately1 m.y. Thespreading 48 ø ratewas considered to be representativeof themedian time of the RiD(•E I_ DE FUCA x • I• PLATE \ interval across which the measurementwas made. No ,JUANDEFUCATJUAN X,'• i•:,':././.:::..__0.....• measurements were made across fracture zones or linear I$O ø 126 o discontinuitiesin the magnetic anomaly pattern. An assumption Fig.1. Locationdiagram showing the spreading centers in theNE Pacific inherentin thistechnique, and one thatis criticalin thisarea of near Vancouver Island. The wide lines define the axis of current or mc•mtspreading on the Northam Juan de Fuca and Explorer Ridges, and frequentspreading readjustments, is that of orthogonalspreading. the muchsmaller Dellwood and Tuzo Wilsonspreading segments. Although random deviationsfrom this conditionshould be ExplorerRidge is boundedto the northand southby the Revere- compensatedby the closely spaced measurements along axis, the Dellwoodand Sovanco Fracture Zones, reSl•Ctively. Explorer Plate is separatedfrom the Juande FucaPlate to the southeastby the Nootka techniquewould not identify continuous oblique spreading along Fault. thewhole ridge for periodsof onemillion years or more.The trendsof individualmagnetic anomalies were determined by drawinga straightline throughthe peaks located at thecenters of requiredfor at leastthe last 7 m.y. However,as a resultof more theanomalies. Azimuths were measured in degreeseast of north recentanalyses of the polesof rotation,Riddihough [1984] and,using the orthogonal spreading assumption, a spreading concludedthat transformmotion between these two plateswas directionwas applied to themedian age of thecorresponding notinitiated until 3 to4 Ma. Aroundthis time, the Explorer area anomaly, wasthe youngestportion of the Juande FucaPlate entering the Thehigh-resolution bathymetric Sea Beam data [Earth Physics subductionzone. In his 1984model, Riddihough proposed that Branch,Energy, Mines and Resources,1984a, b, 1985a,b; the resistanceto subductionprovided by the buoyancyof this GeologicalSurvey of Canada,1984a, b], in conjunctionwith the materialresulted in its detachmentat 4 Ms as theindependent side-scanSeaMARC II data[Dav/s et al., 1984b;Geological ExplorerPlate. Sincethen, the plate has been rotating at a slower Surveyof CanadaMaps 12 and 14, 1987],allowed an accurate rateabout a localpole such that its motioninto the convergence identificationof the current plate boundaries in the region and zonehas been sharply reduced. alsoprovided details of the structure of seafloor as old as 2.5 m.y. Previousplate reconstructionsin the Explorer area Thisinformation was integrated with the magnetic anomaly data [Riddihough,1977; Hyndman eta/., 1979;Riddihough et al., in developinga tectonic history for the Explorer-NorthernJuan 1980;Riddihough, 1984] have relied, for themost part, on the de Fucaregion. magneticssurvey of Raft and Mason [1961]. New and more detailedsea surfacemagnetics data have becomeavailable over DATA thelast few yearsas a resultof a seriesof multiparametermarine Themagnetics data compiled in thisstudy incorporated the geophysicalsurveys off Canada'swest coast conducted by the resultsfrom six differentsurveys whose tracklines, overlain on PacificGeoscience Center in conjunctionwith other Canadian themagnetic anomaly map, are shown in Figure2. Thedata in governmentagencies [Curtie et al., 1983]. In additionto this thenortheast region was integratedfrom a mappreviously largedata set, detailed magnetics information was obtained in the publishedby the GeologicalSurvey of Canada[1979]. The vicinityof the currentaxis of spreadingon a numberof recent majorityof thecoverage isrepresented bythe ENE trending lines cruisesto ExplorerRidge. The newly compiledmagnetic whichwere surveyed by CSSParizeau in 1983 and 1985. The anomalymap for the ExplorerPlate area constitutes the primary magneticsdata along the NNE-oriented tracklinesin the database for thisstudy. northwestarea of the map was collectedon CSS Parizeauin 1979. Thespacing of these1979, 1983, and 1985 lines is 10 km 'I••QUE [Currieet al., 1983]. Theaverage water depth in theregion is The platetectonic reconstruction of theExplorer-Northern Juan about3 km. Moredetailed coverage over Explorer Ridge was de Fucaregion was developed by correlatingthe linear magnetic obtainedon surveysconducted aboard CFV Endearour(1979 polarityintervals. The anomalieswere examinedboth in contour and1985) and M.V. PandoraII (1984). A Barringermodel OM form and profile and, following two-dimensionalinversion 104proton precession magnetometer was used to collectthe data. modeling,identified using the time scaleof Nesset al. [1980]. The magneticanomalies were computed using the spherical As is the casein thisstudy area, for magneticanomalies formed harmoniccoefficients of theIGRF for 1980. The valuesalong by the magnetizationof N-S trendingbodies in a directionclose tracklineswere computerplotted on a transverseMercator to the presentEarth's field at high latitudes,the geographical projectionat a scaleof 1:125,000,and then contoured by hand. displacementof the anomaliesfrom theirsource is insignificant. Crossovererrors for the 1983and 1985 CSS Parizeau surveys BOTROSAND JOHNSON: TECTONIC EVOLUTION OFJUAN DB FucA RsO•ON 10,423 50ø0 ' 49o0 ' 4800 ' •52 øO' •5•ø O' ••5OO O' •2900 ' Fig. 2. Shiptracklines superimposed on the newly contouredmagnetic anomaly data. Anomaliesin the regionbounded by latitudes49oN and 50oN, andeast of longitude130ow, wereincorporated from a previouslycompiled data set [Geological Surveyof Canada,1979]. Magneticcontour interval is 200 nT. Dashedtracklines are partialprofiles that weresubjected to inversion.The resultsof thismodeling are shownin Figures5a and5b. Dottedtracklines la and2a aresegments over Explorer Seamountwhich were modeled at a largerscale. were approximately•0 nT, and occasionallyup to +60 nT. The IGRF doesnot appearto accuratelyrepresent the average However,high crossover values (about +70 nT andrarely as high field overthis region because the net anomalybaseline in thearea as+150 nT) existin the northwestarea of the map betweenthe remainslargely positive rather than zero. The +100 nT contour 1979and 1985 data sets. Sincethe 1985coverage in thisregion waschosen as the baselinefor themap since the background zero is more extensive,the valuesof thissurvey were used wherever level varied from about +30 nT to +150 nT. Similar results have possiblefor internalconsistency, and the 1979 data was used beenreported in othermagnetic field studiesin thisarea [Haines only in the northwestcorner of the map where no other et al., 1971; Srivastavaet al., 1971; Ricldihoughet al., 1980; informationwas available. The tectonichistory proposed in this Johnsonet al., 1983], whichmay indicatethat an entireregion in paperdoes not hingeon the anomaliesin thisNW areaof the the northeastPacific is underlainwith anomalouslymagnetic map and thereforeinaccuracies resulting from the disagreement lower crustor uppermantle. of these two data sets would not alter the interpretation The SeaBeam echo-soundingsystem aboard the NOAA ship significantly. Surveyorwas used to mapthe seafloorin the areaof thenorthern 10,424 BOT•OSAND JOHNSON: TECTONIC EVOLUTION OFJUAN DE FUCA RBOION Juande Fucaand Explorer Ridges in 1983 and 1984. SeaBeam centerconfiguration [Macdonald and Fox, 1983;Lonsdale, 1983; is a multinarrowbeam, bathymetricsurvey systemwhich Johnsoneta/., 1983;Macdonald et al., 1984]. providesa three-dimensionalview of the bottom [Renard and ExplorerRift is situatedto the northwestof SouthernExplorer Allenou, 1979]. The high-resolutionbathymetric charts Ridge and consistsof two en echelonsegments which, like interpretedin thisstudy were produced at a 10-mcontour interval ExplorerDeep, are bathymetric lows (Figure 3a). The spreading by Energy, Mines and Resources,Canada, and the National centersare about21 km in lengthand are orientedat N42oE Oceanicand AtmosphericAdministration, United States[Earth (northernExplorer Rift) andN37oE (southem Explorer Rift). An Physics Branch, Energy, Mines and Resources,1984a,b, unnamedlinear pull-apartbasin with approximatedimensions of 1985a,b;Geological Survey of Canada,1984a, b]. 18 km by 5 km trendsN159oE, and appears to connectExplorer Informationregarding the sedimentaryversus volcanic nature Rift to SouthernExplorer Ridge. of the seafloor,which is lacking in Sea Beam data, can be The fourspreading segments of ExplorerRidge, as defined by obtainedusing side-scan sonar instruments such as SeaMARC II. Sea Beam, can also be identified in the SeaMARC II data. Thisis a long-rangeside-scan imaging and bathymetric mapping Strongreflections produced by the scarpsbounding the axial too1that producesgeometrically correct images of the seafloor valley of SouthernExplorer Ridge can be resolvedin the side- [Blackintonet al., 1982]. The SeaMARC II data was collected scan sonar images,which show the entire ridge segmentto by the Pacific GeoscienceCenter in cooperationwith the consist of hard acoustic reflectors indicative of unsedimented Universityof Hawaiiusing the R/V KanaKeold. The sidescan seafloorand young volcanics. Explorer Deep, on theother hand, sonarimages discussed in this study were published by Daviset has substantialsediment cover suggestinga lack of very recent al. [1984b],and by theGeological Survey of Canada[1987]. volcanic activity [Geological Survey of Canada, 1987]. Continuous seismic reflection profiles revealed sediment RBGIONALGEOMORPHOLOGY thicknessesof 100 to 200 m in ExplorerDeep [Grill et al., 1981]. The SeaBeam acoustic imaging of theNorthem Juan de Fuca In contrastto ExplorerDeep, the two rift segmentscomprising and Explorer Ridges reveals a complexplate boundary ExplorerRift, aswell asthe pull-apart basin, are flooredby hard configurationin thisarea (Figures 3a,3b). The ExplorerRidge acousticreflectors that presumablyrepresent recently erupted systemappears to consistof fourseparate segments (Figure 3a). basalts. This would support the previous hypothesisthat The locusof spreadingin thesouth is definedby a linear,6$-krn- ExplorerRift is very young,and that spreading has jumped from longbathymetric high named Southern Explorer Ridge. Between ExplorerDeep to ExplorerRift within the last million years 49o32'N and 49o46'N, a well-definedaxial valley trending [Hindmanet al., 1978;Davis and Riddihough, 1982]. Basedon N28oE is locatedat the crestof SouthernExplorer Ridge. therecovery of freshbasalts from both of the northernspreading Beyondthe northern and southernf terminations of thevalley, the centers,Cousens et al. [1984] estimatethat the ridge jump ridgetopography curves to the east. The axial valley shoals occurred about 0.2 Ma. northwardsto a broadplateau which rises to a depthof 1800m at The SeaMARCII dataindicates that Explorer Rift andExplorer 49047'N. This latitudecoincides with the southernextent of Deep are oriented approximatelyorthogonal to the Revere- ExplorerDeep, the segment to thenortheast of SouthernExplorer DellwoodFracture Zone, which forms the northernboundary of Ridge.The shallowest, northern portion of theaxial valley lies at the Explorerspreading center system. At the intersectionsof theprojected intersection of a seamountchain on the PacificPlate thesetwo segmentswith thefracture zone, features characteristic with SouthernExplorer Ridge, and is a knownsite of extensive of other ridge-transformintersections [Fox and Gallo, 1984], hydrothermalactivity [Tunnicliffe et al., 1986]. The occurrence suchas the curvingof rift-parallelfaults into the transform,are of hydrothermalventing at the shoalestportion of a ridge observed[Geological Survey of Canada,1987]. In contrast,the segmentin coincidencewith the projectedintersection of a southernintersection of SouthernExplorer Ridge with the seamount chain has also been observed elsewhere on the Juan de SovancoFracture Zone is muchmore ambiguous. The difficulty Fuca Ridge [Chaseet al., 1985; Tivey and Delaney,1985; in identifying the exact location of the ridge-transform Karsten et al., 1986]. To the north of the summitof Southern intersectionlies primarilyin the poor def'mitionof the Sovanco ExplorerRidge, the orientationchanges to a trend of about FractureZone. Neitherbathymetry, seismicity, or magneticsdata N40oE, and the ridge appearsto curve eastwardstowards reveal a narrow, linear principal displacementzone for the ExplorerDeep. At its southernend, the ridge topography transformmotion [e.g., Milne et al., 1978; Cowanet al., 1986]. graduallydiminishes to a broadlow within which lies a line, The limitedSea Beam coverage in the Sovancoarea displays a trendingapproximately N15oE, of smallvolcanic cones about 1 number of left-laterally offset, elevated blocks which are km in diameter. boundedby northwestand northeast trending linearions (Figures In contrastto the elevatedridge topographyof Southern 3a,3b). A seriesof NW-SE orientedfaults are locatedto the ExplorerRidge, the remainingthree spreading segments of the southof theblocks in theeastern part of theSovanco (Figure 3b). Explorersystem appear to berifted valleys (Figure 3a). Explorer Basedmainly on this data,Cowan et al. [198(5]interpret the Deepconsists of a 53-kin-longrift whichtrends N36OE and has a transformto consistof a broad (12-18 km wide) zone of maximumdepth of 3300m. The valleyshoals and narrows to the deformation within which crustal blocks have rotated clockwise south, and at the latitude of the northern termination of Southern in responseto right-lateraltranscurrent motion. ExplorerRidge, Explorer Deep beginsto curvesouthwestward. Spreadingat the northernend of the Juande FucaRidge is Thiscurvature of ExplorerDeep and Southern Explorer Ridge concentratedin a N20OEtrending graben named West Valley towardone another is characteristicof an overlapping spreading (Figure3b). As is thecase for NorthernExplorer, spreading has BOTROSAND JOHNSON; TBCTONIC EVOLUTION OFJUAN DB FUCA RBOION 10,425 13•ø00 ' 130ø30 ' •30ø00 ' 129030 ' I I ,,• I ½z00f %oo zOO0 ER ,vpøø 50o00 ' - 50o00 ' ED •50( SER 49ø30 ' -- 49o30 ' • 0C •600' ,8% 49ø00 ' - 49o00 ' / ESMT I I ! 131ø00' 130 ø30' 130ø00 ' 129 ø30' Fig. 3a. Sea Beambathymetry over ExplorerRidge and the westemportion of the SovancoFracture Zone adaptedfrcm GeologicalSurvey of Canada[1987] map 4. Contourintezwal is 100m, exceptover Explorer Seamount where the Jmezwal J•200 m. Terminologyused: ER, ExplorerRift; ED, ExplorerD,'..ep; SER, SouthernExplorer Ridge; ESMT, ExplorerSeamount; WSFZ, WestemSovanco Fracture Zone. recentlyjumped westward fxom Middle Valley to West Valley SovancoFracture Zone and the Nootka fault [Hyndmanet al., [Davisand Lister, 1977], andKarsten eta/. [1986] constrainthe 1979]; the exactposition of this triplejunction, however, is not timingof thisjump to lessthan 0.2 Ma. At its northernend, West clear. Valley terminatesat a triple junction intersectionwith the In additionto allowinga more accurateidentification of the 10,426 BoT•os AND•OHNSON: TECI'ONIC EVOLUTION OF •UAN Dlt FucA REGION 129ø30 ' 129ø00 ' 128ø:30 ' I I I 49ø00 ' 49ø00 ' •½00•00 2500-• 2700 48050 ' 48ø$0 ' wv 48ø00 ' 48ø00 ' I 129ø30 ' 129ø00 ' 128 ø 30' Fig.3b. SeaBeam bathymetry over the eastern portion of the Sovanco Fracture Zone and Northern Juan de Fuca Ridge adapted fromGeological Survey of Canada[1987]. Contourinterval is 100m. Terminologyused: ESFZ,Eastern Soyanco Fracture Zone;WV, WestValley. currentplate boundaries, the SeaBeam and SeaMARC II data trendinglinealion, and to the west by a constructionalflank provideinformation on off-axis geomorphology. In the areaof similarto thatof SouthernExplorer Ridge, is similarto the"ridge SouthernExplorer Ridge, the bathymetryis extremely bows"described by Kappeland Ryan [ 1986]. Two moredistinct asymmetricabout the ridge axis. Immediatelyto the westof NE-SWtrending lineations are visible in theSea Beam coverage SouthernExplorer Ridge is a broadvalley within which lies a to the west. The orientationof the easternone is N3 loE, while line of smallvolcanic cones trending approximately N24oE thelineament furthest to the weststrikes N43oE and comprises (Figure3a). The valleyfloor is whiteto light greyin the theeastern boundary of anotherbroad topographic high. If these SeaMARCII images,suggesting that it is sedimented.To the lineafionsrepresent previous trends of SouthernExplorer Ridge, westof the valleyis a broadtopographic high that is almost thenit appearsthat the axis of spreadinghas shifted progressively identicalto thewestern half of SouthernExplorer Ridge (Figure counterclockwisein the recent past (<1 Ma). 3a). This'tmlf-ridge," which is boundedto theeast by a N28oE Thereare no equivalentNE-SW trending ridges to theeast of BOTROSAND JOHNSON: T!•CTONIC EVOLUITON OFJUAN DS FUCA RBOION 10,427 SouthernExplorer Ridge and instead, a broadplateau which rises inherentin this techniqueare as follows: there is no vertical to a depthof 1800 m is situatedeast of the southernend of variationin magnetization,the topographyis two-dimensional, SouthernExplorer Ridge (Figure 3a). Thisfeature is boundedto the magnetizationis parallelto a geocentricaxial dipolefield the northand westby steeptectonic scarps which meet at the direction,and the upperboundary of a constantthickness source northwestcomer in an orthogonalconfiguration. The eastern layeris the observedbathymetry. The high- andlow-pass filter boundaryof theplateau is located beyond the Sea Beam coverage usedin themodeling of eachprofile was the approximateprofile but availabledata suggests that it is constructional. lengthand the averagewater depth,respectively. The crustal ExplorerSeamount, situated to the southwestof Southern magnetizationderived from the Fourierinversion is nonunique. ExplorerRidge and with approximatehorizontal dimensions of Parkerand Huestis [1974] showed that for anygiven topography 80 km by 40 kin, is a majorgeological feature of the region thereexists a magnetizationdistribution known as the annihilator (Figure3a). Althoughthe general trend of theseamount is NW- that will produceno externalmagnetic field on the level of the $E, the high-resolutionSea Beam data revealsExplorer observations.Therefore any multipleof it may be addedto the Seamountto consistof a seriesof narrowcurvilinear tidies magnetizationsolution and the sumwill alsobe a solutionwhich orientedatbetween N150E and N17OE, which are superimposed reproduces the observed field. on thebroader elevated topography. The steepeastern edge of The resultsof the inversionmodeling, together with the ExplorerSeamount is alsothe westernboundary of a NW-SE anomalyidentifications, are shownin Figures5a and 5b. For trendingrifted valley. The valley has a sedimentedappearance in eachprofile, the upper solid curve is theobserved magnetic field. the SeaMARCH images[Geological Survey of Canada,1987], The dotted line, being the field producedby the forward similarto thatof ExplorerDeep. The limitedside-scan data over modelingof the observedbathymetry and the calculatedcrustal the easternpart of Explorer Seamount,on the other hand, magnetization,represents a control on the validityof the crustal suggeststhat it consistsof roughacoustic reflectors with local magnetizationgenerated by the inversiontechnique. The middle patchesof smoothreflectors. Explorer Seamount, which rises to profiledisplays the calculatedmagnetization as a solidline, the a depthof 900 m, is theshallowest feature in thestudy area. annihilatoras a dottedline, and the anomaly identifications as the Thereis no bathymetricedifice equivalent in scaleto Explorer numbersassociated with the peaks. The lower curve is the Seamounton theeastern side of therifted valley, although the bathymetricprofile, and the assumedthickness of the source broadplateau situated to theeast of SouthernExplorer Ridge may layerin kilometersis indicatedby theletter t at thetop. representan easternportion of ExplorerSeamount that was Figure 5a showsfive inversionprofiles over Explorer movednorthwards during past eventsinvolving major plate Seamountwith lines 1A and2A representingexpanded portions boundaryreadjustments. The similarityof theeastern scarp of of lines1 and2, respectively.The resultsindicate that a coherent ExplorerSeamount and the steepwestern boundary of the reversalsequence overlies Explorer Seamount independent of the plateau,as well astheir equivalence in topographicscale, would bathymetry.Comparison of thissequence with thegeomagnetic supportthis idea. timescale suggests that the largest positive peak, also interpreted Fromthe Figure 4 overlayof thestructural features interpreted asthe youngest part of the seamount,is besttentatively identified fromthe Sea Beam and SeaMARC H dataon the new magnetic as anomaly2.1'. If this interpretationis correct,Explorer anomalypattern, it is apparentthat all four currentand recent Seamountis not youngerthan 2.5 m.y. The fact thatanomaly Explorerspreading segments lie withina lobster-shapedBrunhes reversals can be seen to continue within the seamount to the east boundary.This magnetic boundary is stronglyasymmetric about of the 2.1' anomalypeak may imply that the youngestmaterial theaxis of currentspreading of SouthernExplorer Ridge, being lies within the seamountand not at the easternedge. NRM approximatelythree times wider to the west than to th,?east. measurementsof five basaltsamples from ExplorerSeamount Asynunetryof the Bmnhes is alsoobserved about Explorer Deep, averaged3.8 A/m, consistentwith an olderage for the seamount butin thiscase the axis of recentspreading is situatednear the sinceNRM valuesof basaltsfrom the current Explorer spreading westernedge of the anomaly. The northernand southern axisrange from 30 A/m to 50 A/m. A singleCurie temperature topographicboundaries of Explorer Seamountappear to from one of the seamountbasalt sampleswas 425oC, and correspondto NW-SEtrending discontinuities in the magnetic exhibitedthe irreversibleheating and cooling curves associated anomalypattern. The left-laterallyoffset and elevated blocks of with the unmixingof oxidizedtitanomaghemite, properties that theSovanco Fracture Zone, with the exception of thevery eastern are alsoconsistent with the olderage for the seamount. andwestern blocks, are overlain by a broad,positive, generally Lines4 through8, shownin Figure5b, are profilesacross E-W trendingmagnetic anomaly. SouthernExplorer Ridge and ExplorerRift. In line 4, which overliesthe southernend of SouthernExplorer Ridge, the MASTIC ANOMALYINVF•_SIONS asymmetryof the Bnmhesboundary about the ridgeis evident, AN• ANo•v InmrrmcAuoN with SouthernExplorer Ridge situated towards the eastern edge. The tectonicscarp bounding the high plateauto the east of Eightcoincident magnetic field and bathymetryprofiles, SouthernExplorer Ridge appearsto be a magneticreversal representedby thedashed lines in Figure2, werechosen for two- boundaryas well as a tectonicboundary, indicating that this dimensionalinversion modeling. Using the observedfield and pronouncedstructural feature is at least0.? m.y. The olderage topography,a Fourier method for calculatingpotential anomalies wouldbe consistentwith thehypothesis that the plateau may be [Parker,1973] was appliediteratively to solvefor the source an easternportion of ExplorerSeamount which was displaced magnetization[Parker and Huestis,1974]. The assumptions fromthe area now occupied by thevalley to theeast of Explorer 10,428 BomosAND JOHNSONI TBCI'ONIC EVOLUTION OFJu.• Dl•Fucn REGION 50ø( 49o0 ' 48o0 ' 132ø0' 131o0' 130ø0' 129ø0' Fig. 4. Linedrawing interpretation of themajor tectonic features in the areasuperimposed on the schematicmagnetic anomaly compilation.Stippled areas are positively magnetized, while white areas are negatively magnetized. Dashed lines represent the approximateboundaries of topographic highs. The left-laterally offset and rotated blocks of theSovaneo Fracture Zone are shown at approximately49oN. Terminologyused: RDFZ, Revere-DellwoodFracture Zone; Ell, ExplorerRdt; ED, ExplorerDeep; SEll, ScuthemExplorer Ridge; ESMT, ExplorerSeamcunt; SFZ, Sovanco Fracture Zone; WV, WestValley. Seamcunt(Figure 5a, line 2). Line 5 in Figure 5b overliesa presenceof a thinveneer (<250 m) of sedimentsand anomalously positive magnetic anomaly on the Explorer Plate which is thick crust over a considerable area which extends 60 km east of continuousfor approximately50 km in an E-W direction(Figure Explorer Ridge. The anomalousthickness of the crest was 2). The bathymetricprof'fie for line 5 revealsno positive attributedto crustalcompression within ExplorerPlate as had topographicfeature associated with this anomalyalthough the been predictedby the tectonicmodel of Riddihough[1977]. depthis 500 m to 700 m shallowerthan depthsat equivalent Inversionof themagnetic field andbathymetry data in thisprofile distancesfrom theridge on the PacificPlate (see lines 6 through resultedin a positivemagnetization for thecrest. 8, Figure5b). It is unlikelythat the increasedsediment influx Line 6 crossesthe peakof SouthernExplorer Ridge as well as fromthe adjacentcontinental margin can entirely account for this the two "half ridges" lying to the west (see Figure 3a). depthdifference between the Pacificand ExplorerPlates. A Anomalies1', 2', 3, and3' canbe identifiedon the Pacific Plate; seismicstudy by Malacek and Clowes [1978] indicatedthe however,anomaly 2 is missingwest of SouthernExplorer Ridge. BOT•OS• JOHNSON:TsCrON•C EVOLUTION OFJUAN DB FUC• ]•d•OION l 0,429 LINE IA; EXPLORER SEAMOUNT LINE I-' EXPLORER SEAMOUNT t =1.0 KM t=I.5KM 'øøør- -I / /t MAGNETICFIELD/ ••o0 MAGNETICß FIELD . -•ooo ,o.o• +3 •' •3 •' z •' -IO0O / ' ' •' MAGNETIZATION/ IO'OF MAGNETIZATION1 2.2 '"'""' '""...... " ...... -I0.0 10.01.5 ::5.0 I I I I I I I I 0 IS 30 45 60 75 90 I I I I 48.88 e N DI STANCE ( K M ) 49.06*N 0 15' ::50 131.68*W 13030*W DISTANCE ( KM ) LINE 2: EXPLORER SEAMOUNT LINE 2A EXPLORER SEAMOUNT t=I.OKM t=I.SKM 1øøøF 1000F MAGNETIC FIELD 1 o• / A MAGNETICFIELD/ -I000 L_ _j -IOO0 •0.o 2.1' I0O F , ,•"• MAGNETIZATION1 ß ION P , / • ,..,,,...... ,,...... ,,...... -IO.Ot- • J -IO.O t.5 METR ,"sVV•sFBA•Yrt METR¾] 3.0-- • _/// _.1 o 15 30 45 60 75 o IS 30 49.16ON OISTANCE(KM) 49. 28ON DISTANCE KM I$1.18øW 130.08oW LINE 3: EXPLORER SEAMOUNT t =1.5 KM Iø00r- I . MAGNETICFIELD AGE(MY) ANOMALY NO. ;• ..* '--; ß 2 •... • % ...... O: 1 _1 • -•00"" "'"'? ' '"'" '-"- 2 2 'ø-øB MAGNETIZATION • ...... -...... O.o,. o...... 12' 4 -IQ B TRY 6 x SEA 4 • I I I I I I 0 15 $0 45 60 75 I00 115 49.17ON DISTANCE( KM ) 49.$$ ø N 131-70øW 130 15øW 10,430 BOTROSAND JOHNSON: TECTONIC EVOLUTION OFJUAN D]t FucA REGION The relativemagnetic low immediatelyto the westof Southern asynunetricspreading, with fasterspreading to the eastthan to ExplorerRidge overlies the sedimentedvalley discussedin the thewest, on Northern Explorer [Riddihough, 1977; Riddihough et previoussection and couldrepresent hydrothermally leached a/., 1980; Davis and Riddihough,1982]. It is apparentfrom Brunhes-agecrust [Levi and Riddihough,1986], ratherthan a Figure6 that the SoyancoFracture Zone has lengthenedwith magneticreversal boundary. The valley overlainby a positive time,and currenfiy offsets the northern Juan de FucaRidge from magneticanomaly to the eastof SouthernExplorer Ridge in this SouthernExplorer Ridge by approximately120 kin. profileis the southernend of ExplorerDeep. Line 7 overlies Spreadingrates and anomalyazimuths measured from the SouthernExplorer Rift anda seamountchain lying to thewest on magneticanomaly compilation are shown in Figure7. The map thePacific Plate. Thenarrow peak associated with ExplorerRift wasdivided into five separateregions for thispurpose: anomalies is identifiedas the Brunhes. The bathymetriceffects of the eastand westof NorthernExplorer, anomalies east and westof seamountshave been removed in the magnetizationprofile, SouthernExplorer, and the anomalieswest of the Juande Fuca allowinga moreconfident identification of theanomaly sequence Ridge. Dataolder than 6.5 m.y.were obtained from the Raft and althoughthe generalshape of the magnetizationprofile indicates Mason[1961] compilation.Spreading has been asymmetric in inadequatelow-frequency filtering. Line 8 crossesNorthern NorthernExplorer, about 1 cm/yrfaster to the eastthan to the ExplorerRift andshows the valleyto be overlainby a positive west,since at least 3 Marestilting inthe northwestward m•,grafion anomalypeak which was identified as the Brunhes. of ExplorerRidge. On the otherhand, spreading since the Havingused the crustalmagnetizations calculated by the Jaramilloappears to havebeen 3 timesfaster to the westthan to inversiontechnique to constrainthe isochronidentifications in theeast on SouthernExplorer Ridge. Spreadingrates measured profile,this information cotfid then be transferredto themagnetic west of Explorer Ridge are consistenfiyslower than those anomalymap (Figure6). The mostcomplete reversal sequence measuredwest of the Juande FucaRidge for the last 7.5 m.y., north of the Sovanco Fracture Zone is found to the east and west and the rateshave generallydeclined for both ridge systems of NorthernExplorer, where the anomalyidentifications shown duringthis time interval. in Figure 6 are similarto thoseof Riddihoughet al. [1980]. The graphson theright sideof Figure7 showthat clockwise Figure6 showsa distortedanomaly pattern in a large, wedge- reorientationof the anomaliesfrom a N-S trendbegan about 8 shapedregion which extendsfrom southand eastof Explorer Ma. Rotationcontinued at approximatelythe same rate on both Seamount,northward to the area east of SouthernExplorer the Juande Fuca andExplorer Ridges until 4 Ma. Sincethis Ridge. A coherentreversal sequence lies to thewest of thiszone time,the Juande FucaRidge has remained at an orientationof and appearsto originatewithin ExplorerSeamount. The new aboutN20oE, whileExplorer Ridge has continuedto rotateat a dataover ExplorerSeamount is the mostsignificant difference ratesignificantly faster than that prior to 4 Ma. For example,a from the older Raft and Mason [1961] map, in which the changein azimuthof about12o is observedin the Explorer ExplorerSeamount area was overlain by a broad,poorly deftned anomaliesbetween the start and end of anomaly2', whichis negativeanomaly. The new compilationreveals a numberof equivalentto a rotationrate of 13O/m.y.during the time interval linear discontinuitiesin the magnetic anomalieswhich run of 3.4 Ma to 2.5 Ma. Thisclockwise motion allowed the ridge to obliqueto the generalN-S fiend. Theseare interpretedas the be orientedmore perpendicularto the convergencezone (see tracesof five propagatingoffsets with A, B, C, andD beingright- Figure1). The currentspreading segments of NorthernExplorer lateral,and E theonly left-lateral offset (Figure 6). PropagatorC are oriented between N36oE and N42oE, while Southern has been previously identified as a magnetic anomaly ExplorerRidge has an orientationof N28oE. Total spreading discontinuityby Riddihoughet al. [1980,Figure 9]. rateson ExplorerRidge are plotted at thebottom of Figure7 and As has beenpreviously observed [Riddihough, 1977, 1984; showconsiderably faster spreading in thenorth than in thesouth: Carlson, 1981], clockwiserotation of the anomalieswith time 4.4 cm/yrand 3.0 cm/yr,respectively, during the Brunhes. This occurredon boththe Explorerand Juande Fucasegments. In differenceis consistentwith a currentpole of rotationfor addition, the new data conrums earlier observations of Pacific/Explorermotion calculatedby Riddihough[1984] as lying to the southwest.Similar restfits for spreadingrate and anomalyazimuths were obtained by Riddihough [1977, Figure 3; Fig.5a. (opposite)Results of inversionof themagnetic data from lines shownin Figure2. Theupper solid line is theactual magnetic field data. 1984,Figure 6] for theExplorer and Juan de FucaRidges, and a while the upperdotted line is the magneticfield generatedby the similarrotation history was detem•ed•by Carlson [1981, Figure calculatedmagnetization and bathymetryand used as a consistency 5] for theJuan de FucaRidge. check. The middlegraph shows the invertedcrustal magnetization in unitsof ampsper meter as a solidline and the annihilator as a dottedline. The numbersassociated with the magnetizationpeaks are anomaly E¾OI.,UTIONOF THE EXPLO•-NOR• identifications.An asteriskin front of an anomalynumber indicates crust Ju•.• D]•FUCA REOION formedat the Juande FucaRidge. The bottomcurve is thebathymetric profilewith a depthscale in kilometers.The assumedthickness of the A plate tectonichistory for this areamust accountfor the magneticlayer in kilometersis indicatedby the lettert at thetop. SER definesthe location of thecurrent axis of spreadingon Southern Explorer following features: (1) the observedlinear offsets in the Ridge. Filtersused in theinversion program were average water depths magneticanomaly pattern; (2) clockwiseanomaly rotation; (3) for short wavelengthsand approximatetrackline lengths for long asymmetricspreading with faster spreadingto the east on wavelengths. The time scale used in the magneticanomaly identifications,based on Ness et al. [1980],is shownin the lower right ExplorerRidge; (4) a magneticreversal sequence originating corner. within Explorer Seamount;(5) lengtheningof the Sovanco BoT•osAND JOHNSON: TECTONIC EVOLUTION OFJUAN DE Fuc• ]•EGION 10,431 LINE 4; SOUTHERN EXPLORER RIDGE f = 0.7 K M • . -I000 30.Or- • • --• -30.0 1.8 2.8 I I I I i I 0 15 30 45 60 •5 49 44•N DISTANCE ( KM ) 49. 54•N 131.O•W 129.98 • W LINE ,5: SOUTHERN EXPLORER RIDGE t =I.OKM LINE 6-' SOUTHERN EXPLORER RIDGE t=0.7 KM 'øøør' Iø00r MAGNETICFIELD / .... MA6NETIC FIELD (/) :-' ...... ½g ß ...... ,.• .. • '.,...... ; ,-; ...•._ • '•....:.': ...... ß..._:.•.-: • -- -Iooo DO 20. Or- • N -20.0 25 -0•" SER i I I I 0 35 70 105 140 :3.2- 'J• Y ,.9fI •RI I I I I I 49.56'N DISTANCE ( KM ) a,9.29'N i;50 lSeW 128.71eW 0 17 :34 51 68 85 lOP. 119 49. 58•N DISTANCE(KM) 49.70• N 1:32.00'W 130.0õ LINE 7= SOUTHEXPLORER RIFT t= 0.7 KM LINE 8:NORTH EXPLORER RIFT t--O.7KM MAGNETIC FIELD MAGNETIC FIELD ß{ . • ;.. -..- •..... • .. '...-'-' -IOOO 40'01-' -'l -I000 ! MAGNETIZATION! 15.0r- _ . A MA•N•T,ZAT,ON / • ...... -4o.o .150 •- .-, 2.1r- - -,•TR , , , :30(- I I I I I 0 18 :36 54 72 90 108 126 '0 18 :36 54 72 90 49.98eN D,STANCE( KM ) 50.04eN 50. 26øN 151.17eW50'25•N D,STANCE (KM)150.19•W Fig.Sb. Inversion ofadditional profiles from the trackUnes ofFigure 2. Thesame format as Figure Sa is used. 10,432 BtymosAND JOHNSON: TBCTONIC EVOLUTION OVJUA• DS FUC• RBGION 50o0 ' 49o( 48o0' 1•2o0 ' 131ø O' 130ø0' 129ø0' Fig. 6. Contourmap of magneticanomalies for the new datacompilation overlain with anomalyidentifications. Solid lines representproposed propagator wakes and dashedlines representhypothetical propagator wakes which are neededin a comprehensivemodel but are not clearly supported by thedata. As in Figure4, greystippled areas are regions of positively magnetizedcrest and white areas are negatively magnetized. Labels A, B, C, D, andE areused to identifypropagators discussed in the text. Transform with time; (6) a predominanceof E-W trending B, propagatedin the samedirection as A at a slowerrate of about anomaliesin theSovanco Transform region. I cm/yr. The originof the Sovancoby a southwardpropagating The time of the earliestdiscontinuous N-S anomalyon the offsethas beenpreviously proposed by Wilsonet al. [1984], PacificPlate west of theJuan de FucaPlate is 7.4 Ma (Figure8); althoughthe timeand place of initiation,as well as therate of until the endof anomaly4' (7.8 Ma), the Explorerand Juan de propagation,differs from this interpretation of the combined Raft FucaRidges were essentiallyone continuousspreading center. andMason [1961] and the new magnetics data. Propagator5 of We suggestthat two offsets(A and B in Figure6) developed Wilsonet al. [1984] first appearedat 8.56 Ma at a latitudeof aroundthe beginningof anomaly4 time (7.4 Ma), as canbe seen about51oN and propagatedsouthward at approximately3.9 in theoriginalRaff and Mason (1961) data. OffsetA represents cm/yr,whereas offset A originatedat 7.4 Ma at a latitudeof the first appearanceof the Soyanco Fracturezone, which then 49.6oN andpropagated at a rateof 1.9 cm/yr. beganto propagatesouthward at 1.9 cm/yr. The northernoffset, Half-spreadingrates measured from PacificPlate anomalies BOT•OSA• JOI-•SON:T•cro•c EVOLUTIONOFJUAN D• FUCAl•oiON 10,4•• NORTHERN EXPLORER 2 time,or 4.8 Ma. The oneto thenorth on ExplorerRidge, offset C in Figure6, beganto propagateto thesouth at a rateof 2.9 cm/yr. w 40ø Eø• •oo w_•. Offset D developedon the Juande Fuca Ridge and beganto I --eEXPLORERDEEP 2 :N EXPLORER RIFT propagatenorthward at aboutI cm/yr. The traceor "pseudofault" fir' I I I ( I I I I ß'• 6 4 2 0 0o I • I3 = 4.? EXPLO•RER 2 RIFT•0 of propagatorD in themagnetic anomaly pattern lies to thenorth o SOUTHERN EXPLORER of thatof propagator7 in Wilsonet al. [ 1984]. ,,i 4 By 3.4 Ma or the beginningof anomaly2' (Figure8), Explorer Platehas clearlyseparated from the Juande FucaPlate as a result of its resistanceto subductionat the convergentmargin r• i i I co 6 4 2 0 [œiddihough,1984]. Continuedclockwise rotation, with a faster •' JUANDE FUCA total spreadingrate in the north thanin the south,indicates that 40 ß <5 Explorer/Pacificmotion was centeredon a nearby pole of rotation to the southwest. Relative motion between the Juan de 20ø _:_- . --• W . Fuca and Explorerplates is taken up by the Nootka fault i I I I 08 6 4 2 0 08o 6 4 , :• I •) [ttyndman et al., 1979]. At this time, the three northern MILLION YEARS BP MILLION YEARS bP propagatorscontinued to movesouth while offsetD propagated •: EXPLORERRIDGE to the north. There is evidence for the eastern branch of the • 6 N northwardpropagator on the Juan de Fuca in the magnetic anomalycompilation of Connardet al. [1984]. As is thecase on the Pacific Plate, the discontinuityin the Juan de Fuca Plate i magneticanomaly pattern first appears at theend of anomaly3', <{ I I I I I I I I o 6 4. 2 0 or 4.8 Ma. MILLION YEARS BP By 2.5 Ma (Figure8), propagatorD will have intersectedthe Fig. 7. Spreadingrates and azimuthsof the axis of spreadingfor the Sovanco Transform Fault, thereby increasingits length. Explorerand Northern Juan de Fuca Ridges shown as a functionof time. Spreadingrates are shownon theleft sideof thefigure and azimuths on Assumingthat spreadingwas roughly perpendicularto the theright. Spreadingrates have generally decreased on all segmentsover N17oE trendingridges of ExplorerSeamount, then the direction the last 6 m.y. whilethe axesof spreadinghave rotatedin a dockwise of spreading on the Explorer Seamount Segment was direction. The latter movementallowed the ridge to becomemore orthoganalto the zoneof activesubduction (see Figure 1). in the lower approximatelyparallel to thaton the Juande FucaRidge at 2.5 left curve,N, NorthernExplorer and S, SouthernExplorer. Error bars on Ma. NorthernExplorer Ridge was by this time, however, spreadingrates are standarddeviations obtained from at least four oriented12o moreclockwise than the Juande FucaRidge. A separatemeasurements, Azimuths have an average error of +2ø . major plate boundaryreorganization, in which the Sovanco FractureZone plays a significantrole, began to takeplace at this were slightlyslower on the ExplorerRidge than on the Juande time. We proposethat the transform,in responseto forcesacting FucaRidge at the beginningof anomaly4 time. The portionof at the convergencezone, began to reorient in a clockwise ridge that would eventuallyproduce Explorer Seamount can be direction.This led to a situationof greatertectonic equilibrium modeledas thesouthernmost spreading segment of the Explorer [Menard and Atwater, 1968], since the clockwise movement Ridgesystem (ESS or ExplorerSeamount Segment in Figure8), allowedthe SovancoFracture Zone to approachan orthogonal boundedby offsetA to thesouth and offset B to thenorth. If one configurationwith the more clockwise strike of Northern extrapolatesthrough time and assumes that spreading on Explorer ExplorerRidge. The northwardmovement of the transform hasbeen asymmetricwith fasterspreading to the eastsince the boundarywould then have isolatedthe Explorer Seamount time of the initiation of the Sovanco Fracture Zone, then the segmentfrom theremainder of ExplorerRidge. It is possiblethat independenceof the ExplorerPlate is not requireduntil 4 Ma as duringthis periodof plateboundary readjustment, the plateau proposedby Riddihough[1984]. At this time the anomaly now situatedto the eastof SouthernExplorer Ridge may have azimuthson theExplorer and Juan de Fucaridges begin to differ beenmoved northward from its position as the eastern portion of significantly,and correspondingplate motioncan no longerbe ExplorerSeamount. However,an exact mechanismfor sucha describedby the samepole of rotationrelative to the Pacific translocation,such as a major, throughgoing,NE-SW-oriented Plate. strike-slipfault, is not evidentin thedata. Having madethe assumptionof asymmetricspreading to the Duringthe migration of theSovanco Fracture Zone through the eastsince 7.4 Ma, total spreadingrates on ExplorerRidge are crust,the transformwill have produceda broad shearzone. only slightlyslower than those on the Juande FucaRidge at 4.8 Tectonicdeformation within this shifting zone will havedistorted Ma, consistentwith a previouslyidentified pole of rotationfor the previouslyexisting N-S magneticanomaly pattern, possibly Pacific/Juande Fuca motion to the north [Riddihough,1984]. replacingit with anE-W magneticfabric. In thisregard, the most However,asymmetric spreading with approximately30% faster likely processto have producedthe presentE-W magnetic accretionto the east thanto the westmay have resultedin the anomaly trend involves coherentalteration associated with northwestwardmigration of ExplorerRidge, and lengtheningof serpenfinizationof ultramaficsalong E-W orientedfractures the SovancoTransform Fault. We suggestthat two new withinthe shearzone. The secondarymagnetite formed during magneticanomaly offsets developed at the end of anomaly3' this alterationprocess would have been magnetizedin the 10,434 Bomos • JOHnSOn:TacroNlC EvournON o• JUAND- FUCAR•o•o• 0 •, •:• 4.8Mo PA • ESS PA ½,• JDF c •:•3.4Mo ESS c• ,•By• Ex ß • ESS•-'•--- A sFz o/T__ :/•,D JDF i 2.5Me • ///?PRESENT •, •--_ EX ESS •'•JDF PAJ•;• JDF Fig.8. A conceptfor the tectonic evolution ofthe Explorer and Northern $uan de Fuca Ridges during the :last 8 m.y. Size of the arrowsis roughlyproportional to the half-rate of spreading.Thin dashed lines are propagator wakes, the broad dashed line representstheNootka Fault. Terminology used: PA, Pacific Plate; •DF, $uan de Fuca Plate; EX, Explorer Plate; ESS, Explorer SeamountSegment; EES, Extinct Explorer Seamount;SFZ, Soyanco Fracture Zone; N-F, Nootka fault. Letters A, B, C, D, andB inbold identify propagators interpreted from equivalently labelled linear discontinuities inthe magnetic anomaly pattern shown in Figure6. Dashedarrow at 2.5 Ma indicates direction ofSFZ re. orientation between 2.5 Ma and 1 Ma. Similarly,the dashed arrow at1 Ma shows counterclockwise direction ofmotion for the SFZ between 1Ma and the present. directionof thecontemporaneous field. Fox andOpdyke [1973] The magneticanomaly "pseudofaults" at the northernend of showedthat serpentinizedintrusive rocks can be strongly SouthernExplorer Ridge (Figure 6) do notshow the classic form magneticand are commonly associated with, or areproducts of, of a V pointingtowards the propagator tip asdescribed by Hey the tectonicsof fracturezones. The presenceof a roughlyE-W andWilson [1982]. Assuming the half-ridges lying to the west of orientedgravity low in thisarea [Earth Physics Branch, Energy, SouthernExplorer Ridge (Figure 3a)were producedby this Mines,and Resources, Canada, 1985a] supports this hypothesis, segment,northward propagation of SouthernExplorer Ridge sinceit indicatesthe presence of lowerdensity material. musthave occurred very rapidlybetween the Jaramilloand the By 1 Ma (Figure8), the SovancoFracture Zone will have beginningof theBrunhes (offset E in Figure6). Sincethe trace reachedits northernmostposition as indicatedby the southern of thepseudofault does not intersect the current northern tip of limit of the Jaramilloto the west of SouthernExplorer Ridge SouthernExplorer Ridge which lies further to the east, it appears (Figure6). Also by this time, offsetsB and C have been thatthe locusof spreadingon SouthernExplorer Ridge has absorbedby the transformfault, therebyincreasing its length. jumpedto theeast during the Brunhes.This mechanismwould The Explorer Seamountsegment now exists as an abandoned accountfor the extremeasymmetry of the Brunhesboundary spreadingcenter on the PacificPlate to thesouthwest of Explorer aboutSouthern Explorer Ridge (Figure 4). Constraintsplaced by Ridge. At 1 Ma, a new left-lateral offset (E in Figure 6) the bathymetryand the magneticanomaly pattern, however, developedin NorthernExplorer. The segmentto thenorth of this suggestthat southward propagation of Southern Explorer Ridge offsetwould be the proto-ExplorerDeep which,on the basisof probablyoccurred after the eastward ridge jump. theJaramillo anomaly on theExplorer Plate (Figure 6), extended Supportfor continuednorthward propagation on Southern about22 km southwardof its presentlimit [Botroset al., 1986]. ExplorerRidge comes from geochemical analyses by Michaelet The segmentsouth of offsetE is the proto-SouthernExplorer al. [1986], who reportedthe occurrenceof iron-enrichedFe-Ti Ridge. Between1 Ma and the present,this southernsegment basaltsat the highestportion of the ridge. This latitude appearsto havepropagated to thenorth as spreading declined in correspondsto a distanceof 18 km, or about600,000 years, ExplorerDeep. Duringthe same time interval, the southern tip of behindthe northernpropagator tip and is consistentwith the SouthernExplorer Ridge may havepropagated to the southas modelof Christieand Sinton [1981] for northwardpropagation indicatedby thepresent southern extent of theBrunhes anomaly of this ridgesegment. In additionto the FeTi enrichment,the overthis segment. The southward propagation of ExplorerRidge axialrocks of ExplorerRidge are substantially more magnetic would have caused the western end of the Soyanco Fracture Zone (30-50A/m) thanthose found on a normalMORB spreading to reverseits northwardmotion throughthe crust. The more center[Johnson and Atwater, 1977]. Coupledwith the high northwardextent of the Jaramilloon the Juan de Fuca Plate, in amplitudemagnetic anomalies shown in Figure6, thismedium comparisonto the presentnorthern limit of West Valley spreadingrate ridge fulfills all of the therequirements for the [Connard eta/., 1984; Karsten eta/., 1986], supportsa magnetotelechemistryhypothesis, i.e., FeTi basalts,highly progressivelymore southwardposition for the transformfauk magneticextrusives, and high amplitudemagnetic anomalies between1 Ma andthe present. [Vogtand Johnson, 1973]. In additionto the rock data in support BOTROSAND IOHNSON: 'I•cromc EVOLUTIONOF•UAN DB FUCA RBOION l 0,43• of recentrift propagation,submersible observations of rifting in of NorthernExplorer Ridge in thesame direction during anomaly theabsence of recentvolcanic activity, together with a northward 2'. This reorientation, like the earlier clockwise rotation of ageprogression of hydrothermal deposits from oldest to youngest ExplorerPlate and Northern Explorer Ridge, is a directresponse at theshoalest portion of theridge, led Tunnicliffe eta/. [1986]to to forcesacting at the convergentmargin. The readjustmentof proposethat the centralaxial rift of SouthernExplorer Ridge is the SovancoFracture Zone is probablydelayed with respectto propagatingnorthward into previously unbroken crust. that of Explorer Ridge since the resistanceto motion on With the timing constrainedby the width of the Bmnhes transforms,which must break throughold crust in order to anomalyover Explorer Rift, spreadingmay have jumped reorientthemselves, is muchgreater than the resistance to motion westwardfrom ExplorerDeep to the two en echelonsegments at spreadingcenters. In our hypothesis,large-scale deformation comprisingExplorer Rift around0.3 Ma, therebyproducing the associatedwith themovement of thisshear zone through the crust presentExplorer spreading center configuration (Figure 8). The is used to account for the lack of coherent, N-S oriented now extinctExplorer Seamount segment is locatedon thePacific anomaliesin thelarge, wedge-shaped region which extends from Plateto thesouthwest of SouthernExplorer Ridge. east of Explorer Seamount,northwards to the area east of SouthernExplorer Ridge (Figure 6). In invokingserpentinization IMI•CATIONS I•ORlhtOIONAL of ultramaficsalong E-W trendingfractures as a mechanismfor PL• •CrlO•qS producingthe observed E-W magneticfabric, this model predicts theoccurrence of highmagnetic intensity and high susceptibility Oneof themajor drawbacks inherent in anyattempt to decipher rocksin theregion of theSovanco Fracture Zone. the tectonicevolution of thiscomplex region is the fact thathalf Anomalydiswrtion within a migratingSovanco Fracture Zone of the storyis missing.In thecase of the ExplorerPlate, no cannotexplain all of theE-W orientedanomalies, however, since marinemagnetic anomalies older than about 3.4 Ma remain. some are found to the north of the northernmost extent of the Thereforean interpretationof the plate tectonicsof the region transform(for example,line 5, Figure 5b). The ambiguous priorto thistime hinges on thePacific Plate anomaly data. In an anomaly pattern in this area may be the result of crustal areawhere rift propagation,ridge rotation, asymmetric spreading, deformationwithin Explorer Plate as suggested by theoccurrence and ridge jumping are not uncommon,the usual simplifying of several intraplateearthquakes (G. C. Rogers, personal assumptionsof symmetricand perpendicularspreading become communications,1986). The observedrecent counterclockwise questionable,and the solutions nonunique. wtationof SouthernExplorer Ridge, which may representan The timingof theindependence of the Explorer Plate from the attempt by this ridge segmentto achieve an orthogonal Juande FucaPlate is not clearand can only be constrainedto one configurationwith the SovancoFracture Zone, hasresulted in a of two epochs;if one assumesthat spreadingwas symmetric considerable difference between its orientation and that of the aboutExplorer Ridge between 8 Ma and 3 Ma, thenthe total northernExplorer spreading segments comprising Explorer Rift. spreadingrate on Explorer Ridge was slower by approximately2 If spreadingis occurringperpendicular to the currenttrends of cn• thanthe contemporaneousspreading rate on the Juande eachof theserift segments,then the approximately12o more Fuca. In this case,Pacific/Explorer motion and Pacific/Juande clockwiseorientation of ExplorerRift comparedwith Southern Fuca motion would have been centeredon differentpoles of ExplorerRidge impliesthat somedegree of compression,and rotationsince approximately 7.5 Ma. On the otherhand, if one henceintraplate deformation, must be takingplace to theeast. assumesthat spreadinghas been asymmetricto the east on ExplorerRidge since 8 Ma, suchthat the total spreading rate on CONCLUSIONS Exploreris not significantlyslower than the total spreading rate on theJuan de Fuca,then the independence of the ExplorerPlate 1. Over the last 8 Ma, the Explorer-NorthernJuan de Fuca is notrequired until the azimuthsof the anomaliesformed at the regionhas had a complextectonic hiswry of northwardand Explorerand Juan de Fuca Ridges, and the assumed directions of southwardpropagating rift offsets,and this history is further spreading,begin to differ significantlyat 4 Ma. The question complicatedby bothasymmetric spreading and rapid spreading ariseswhether it is valid to assumethat spreadingcan be centerjumps. asymmetricfor a period of 7 m.y.? In other words, can 2. The SovancoFracture Zone originatedas an offsetbetween asynunetricspreading occur on a steady-statebasis? Suchan ExplorerRidge and the Juande FucaRidge approximately 7.4 assumptionmay notbe unreasonablein thiscomplex region, and Ma. Sincethe timeof its formationto the endof anomaly2', the wastherefore invoked in thetectonic history suggested in Figure transformhas migrated southward due to southwardpropagation 8. of ExplorerRidge, and has beenlengthened by asymmetric The proposedhistory for theSovanco Fracture Zone is oneof spreadingto the easton ExplorerRidge, as well as by the anoffset that lengthens and propagates southward from the time additionof threepropagating offsets. Two of theoffsets were of its initiationat around7.5 Ma to approximately2.5 Ma. The southwardmoving propagators from Explorer Ridge and one was idea that the Sovanco offset has moved southward rather than a northwardmoving propagator from the Northern Juan de Fuca remainedstationary is supportedby the presenceof a NW-SE Ridge. trending,as opposedto an E-W trending,linear discontinuityin 3. At the endof anomaly2', theSovanco Transform developed thePacific Plate magnetic anomalies. into a broad zone of shear that underwent first clockwise, and Clockwiserotation of the SovancoFracture Zone began at the then counterclockwiserotation. The currentexpression of the endof anomaly2' time(2.5 Ma), followingconsiderable rotation SovancoFracture Zone, as well as the areato the north and south, 10,43• Boaos AsuJonson: T•cro•c Evoixmo•Ol• JuAs D,• FUCA Rao•o• is overlainby an E-W magneticfabric which may havebeen with Dick Chase,Peter Michael,and Greg Shea concerningthe producedby remagnetizationof ultramafics during shearing and geochemistryof basalts from Explorer Ridge. The inversionprogram usedto model the magneticswas obtainedfrom Ken Macdonaldand serpen•zation. SteveMiller. This paperhas benefittedfrom discussionswith Jill 4. Southwestof ExplorerRidge, ExplorerSeamount is Karsten,John Delaney, Bob Karlin,Maurice Tivey, andDarrel Cowan. composedof a seriesof curvilinearridges that are overlainwith Themanuscript was gready improved by thereviews of RickCarlson and DougWilson. This work was supl0opedby NSF grantOCE 83-09812 lineatedmagnetic anomalies. These anomalies are interpreted as andONR grantN-(X)014-84-01 l l. M. Betresgratefully acknowledges 2 having formed at an abandonedspreading center segment yearsof supportfrom a NaturalSciences and EngineeringResearch originallya part of the ExplorerRidge system. The Explorer Councilof Canadapostgraduate scholarship. University of Washington Schoolof Oceanographycontribution 1755. Seamountsegment may have been isolated from the remainder of Explorer Ridge by the northwardmovement of the Sovanco FractureZone between 2.5 Ma and 1 Ma. Althoughthere is no bathymetricfeature equivalent in scaleto ExplorerSeamount Atwater,T., Implicationsof platetectonics for theCenozoic evolution of immediatelyto the east,the broadplateau situated east of the westernNorth America, Geol. $oc. Am. Bull., 81, 3513-3536, 1970. Barr,S. M., Geologyof thenorthern end of theJuan de Fuca Ridge and southernend of SouthernExplorer Ridge may representan adjacentcon•ental slope, Ph.D. thesis. Univ. of B.C., Vancouver, easternportion of ExplorerSeamount that was moved northwards B.C., 1972. duringthis period of majorplate boundary reorganization. The Barr,S. M., andR. L. Chase,Geology of the northernend of Juande FucaRidge and sea-floor spreading, Can. J. Earth$ci., 1!, 1384- reasonfor themass excess that forms the generalelevation of the 1406, 1974. seamount is not clear. Blackinton,J. G., D. M. Hussong,G. J. Fryer,D. M. Hills,and D. A. Cuddy,SeaMARC II mappingtechniques (abstract), Eos Trans. AGU, 5. Analysisof theanomaly patterns argues for a separationof 65,984, 1982. theExplorer Plate from the Juan de Fuca Plate, but the timing of Boux)s,M., H. P. Johnson,and R. Riddihough,Explorer Spreading this separationis not uniquelydetermined due to a lack of Center:The recent tectonic evolution of a smalloceanic plate magneticanomaly data older than 3.4 Ma onExplorer Plate. The boundary(abstract), Eos Trans. AGU, 67, 362, 1986. Carlson,R. L., LateCenozoic rotations of theJuan de Fuca ridge and the PacificPlate magnetic anomaly pattern will allowthe formation Gordarise: A casestudy, Tectonophysics, 77, 171-188, 1981. of a separateExplorer Plate at theNootka Fault either at 4 Ma or Chase,R. L.,J. R. Delaney,$.L. Karsten,H. P.Johnson, S.K. Juniper, J. ? Ma. The 4 Ma dateis preferredon the basisof the available E. Lupron,S. D. Scott,V. Tunnicliffe,S. R. Hammond,and R. E. McDuff, Hydrothermalvents on an axis seamountof theJuan de Fuca data,but this interpretation requires an assumption of asynunetric Ridge,Nature, $15,212-214, 1985. spreadingto theeast on Explorer Ridge between 4 Ma and? Ma. Christie,D. M., and$. M. Sinton,Evolution of abyssallavas along 6. The recent(<1 Ma) tectonicevolution of the Explorer propagatingsegments of the GalapagesSpreading Center, Earth Planet.$ci. Left., 56, 321-335,1981. SpreadingCenter system involves rapid northward propagation Connard,G., R. Couch,G. Ness,and S. Troseth,Magnetic anomaly of SouthernExplorer Ridge between0.9 Ma and 0.7 Ma as identifications,in Western North American continental margin and ExplorerDeep "retreated"northwards. During the Brunhes,an adjacentocean floor off Oregonand Washington, Atlas 1, Ocean MarginDrilling Program, Regional Atlas Series: MarineScience eastwardjump of SouthernExplorer Ridge took place followed International,sheet 5, editedby L. D. Kulm et al., JOIDES, by southwardand continuednorthward propagation. In the Washington,D.C., 1984. northernExplorer region, Sea Beam and SeaMARC II data Cousens,B. L., R. L Chase,and J. G. Schilling,Basalt geochemistry of theExplorer Ridge area, northeast Pacific Ocean, Can. J. Earth$ci., suggestthat thelocus of spreadinghas recently shifted about 40 21, 157-170,1984. km to the northwest,from the easternExplorer Deep to the Cowan,D. S., M. Botros,and H. P. Johnson,Bookshelf tectonics: westernExplorer Rift. Magneticanomaly data over Northern Rotatedcrustal blocks within the Sovanco Fracture Zone, Geophys. Res.Lett., 13, 995-998, 1986. Explorerindicates that the ridgejump occurredapproximately Curfie,R. G., R. V. Cooper,R. P. Riddihough,and D. A. Seemann, 0.3 Ma. Multiparametergeophysical surveys off the westcoast of Canada: 7. Analysesof rocksamples from Explorer Ridge reveal that, 1973-1982,Current Research, Part A, Geol.$urv. Can. Pap. 85-1A, 207-212, 1983. like spreadingcenters with similar tectonicenvironments where Curfie,R. G., E. E. Davis,R. P. Riddihough,and D. M. Hussong, multipleevents of rift offsetpropagation have takenplace Acousticimagery of the Pacific-America-Explorertriplejunction (Galapagesand Juan de FucaRidges), Explorer extrusive rocks (abstract),Eos Trans. AGUe 65, 1110,1984. Davis,E. E., andC.R.B. Lister,Tectonic structures on theJuan de Fuca areboth FeTi enriched and very strongly magnetized. The along- Ridge,Geol. 5oc. Am. Bull., 88, 346-363,1977. strike pattern of FeTi enrichment shows the maximum Davis,E. E., andR. P. Riddihough,The WinonaBasin: Structureand enrichmentto be 18km (0.6m.y.) behind the most recent passage tectonics,Can. J. Earth$ci., 19, 767-788,1982. Davis,E. E.,R. G.Curfie, R. P. Riddihough,W. B. F. Ryan,K. Kastens, of a propagatingoffset. This further example adds to thegrowing D. M. Hussong,S. R. Hammond,and A. Malahoff,Geological bodyof evidencethat the tectonichistory of a spreadingcenter mappingof thenorthern Juan de Fucaand Explorer ridges using can stronglyinfluence both the geochemicaland geophysical SeaMARCI, SeaMARC II, andSea Beam acoustic imaging (abstracO, EosTrans. AGU, 65, 1110,1984a. propertiesof theupper extrusive crust. Davis,E. E., R. G. Curfie,B. S. Sawyer,and D. M. Hussong,Juan de FucaRidge Atlasof SeaMARCH AcousticImagery, Earth Phys. BranchOpen File Rep. 84-17, Energy,Mines, and Resources, Ottawa, Acinowledgmen•s.M. Betreswould like to thankRobin Riddihough Canada,1984b. for an earlydiscussion which generated the idea for thisstudy, as well as EarthPhysics Branch, Energy, Mines and Resources, Canada, Open File numerousdiscussions afterward. The authors greatly appreciate the help 84-6, 1984a. of RalphCurtie and Earl Davis in providingmagnetics, Sea Beam, and EarthPhysics Branch, Energy, Mines and Resources, Canada, Open File SeaMARCH data. 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