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Tectonic Evolution of the Explorerт•'Northern Juan De Fuca Region 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(<l Ma) tectonicevolution of ExplorerRidge involved both northward and southwardpropagation of Southern Explorer Ridge, as well as an eastward ridge jump during the Bmnhes. In thenorthern portion of thestudy area, the axis of spreadingjumped 40 kmto the NW around0.3 Ma, fromthe easternExplorer Deep to thewestern Explorer Rift. 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 Juande 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
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