JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B8, PAGES 6980-6994, AUGUST 10, 1984 Recent Movements of the Juan de Fuca Plate System ROBIN RIDDIHOUGH! Earth PhysicsBranch, Pacific GeoscienceCentre, Departmentof Energy, Mines and Resources Sidney,British Columbia Analysis of the magnetic anomalies of the Juan de Fuca plate system allows instantaneouspoles of rotation relative to the Pacific plate to be calculatedfrom 7 Ma to the present.By combiningthese with global solutions for Pacific/America and "absolute" (relative to hot spot) motions, a plate motion sequencecan be constructed.This sequenceshows that both absolute motions and motions relative to America are characterizedby slower velocitieswhere younger and more buoyant material enters the convergencezone: "pivoting subduction."The resistanceprovided by the youngestportion of the Juan de Fuca plate apparently resulted in its detachmentat 4 Ma as the independentExplorer plate. In relation to the hot spot framework, this plate almost immediately began to rotate clockwisearound a pole close to itself such that its translational movement into the mantle virtually ceased.After 4 Ma the remainder of the Juan de Fuca plate adjusted its motion in responseto the fact that the youngest material entering the subductionzone was now to the south. Differencesin seismicityand recent uplift betweennorthern and southernVancouver Island may reflect a distinction in tectonicstyle betweenthe "normal" subductionof the Juan de Fuca plate to the south and a complex "underplating"occurring as the Explorer plate is overriddenby the continent.The history of the Explorer plate may exemplifythe conditionsunder which the self-drivingforces of small subductingplates are overcomeby the influenceof larger, adjacent plates. The recent rapid migration of the absolutepole of rotation of the Juan de Fuca plate toward the plate suggeststhat it, too, may be nearingthis condition. INTRODUCTION plexity of movementsnear the two triple junctions at the The Juan de Fuca plate is one of the last continuing rem- north and south endsof the Juan de Fuca plate system(Figure nants of the Farallon plate, which convergedwith the western 1) [e.g.,Davis and Riddihough,1982; Silver, 1971; Carlson and margin of North America over the last 150 million years.The Stoddard,1981] is certainlyindicative that in the extremecase, collisionof the Pacific-Farallonridge with the American plate platemotions respond in a complexinteractive manner. between25 and 30 Ma led to the independenceof the Juan de However, as emphasizedby Hyndrnan[1972] and recently Fuca plate at about 20 Ma. (Ma is usedhere to denote age in discussedin more detail by Dewey [1980], Uyeda and Kana- million years before present.)Since that time its area has pro- mori [1979], and Molnar and Atwater [1978], many aspectsof gressivelydiminished as the bounding,southern triple junction plate motion can only be examinedthrough their motions has migrated northward (for a recent review, see Riddihough relative to the underlying asthenosphere.In a subduction [1982b]). A seriesof detailed magnetic anomaly maps [Raft zone,plates move down into the mantleso that at leastpart of and Mason, 1961; Potter et al., 1974; Curtie et al., 1982] pro- the driving or resistingbuoyancy force is a function of the vide a uniquely comprehensivedata basewith which to study speedof the plate relativeto the asthenosphere.A full exami- the seafloorspreading and plate motions of the Juan de Fuca nation of local plate interactionsmust thereforeinvolve docu- mentation of "absolute" as well as "relative" motions. plate region since 10 Ma. In general,the ridge and spreading changesare characterizedby reduction in spreadingrate and POLES OF RELATIVE MOTION OF clockwiseridge and plate rotation [Riddihough,1977, 1980; THE JUAN DE FUCA SYSTEM Carlson, 1981a]. The principal period of change appears to have occurred between 6 and 3 Ma. Assessmentsof the geometryof presentand recent conver- The pattern of ridge segmentationand rotation observedin gencebetween the Juan de Fuca plate and the Americaplate the magnetic anomalies was first suggestedby Menard and [e.g.,McKenzie and Parker, 1967;Atwater, 1970; Silver,1971; Atwater [1968] as being a responseto interaction betweenthe Riddihough,1977] have usuallybeen basedon completinga oceanic plates and the continental (America) plate. Certainly, planevector triangle between Juan de Fuca/Pacificspreading the proximity of the Juan de Fuca plate convergencezone to assumedas parallel to the Blanco fracture zone and Paci- the ridge (Figure 1) makessome form of interactionextremely fic/Americamotion derivedfrom global solutionsor the San likely. Examinations of the forcesdriving plate motions [e.g., Andreas-Gulf of California system.To the extent that the NE Forsyth and Uyeda, 1975; Solomonet al., 1975] concludethat Pacificregion is small,this approachgives solutions which are the buoyancyof the subductedslab and the resistanceto sub- essentiallycorrect but which mask any geographicalvari- duction at the convergencezone are important componentsof ations in interaction rates. the forcescontrolling plate movement.The probable effect on Attemptsto establishpoles of relativerotation betweenthe small plates closeto triple junctions was detailed by Menard Farallon (later Juan de Fuca) and Pacificplates using spread- [1978] in the idea of "pivoting subduction."The local com- ing rate variationsand fracturezone orientations for periods before25 Ma [Franchetauet al., 1970; Handschurnacher,1976] showedthat suchpoles lay in the Bering Sea-CanadaBasin • Now at Collegeof Oceanography,Oregon State University, Cor- vallis. region.One of the first attemptsto determinea contemporary poleof motionfor the Juande Fucaplate wasthat of Morgan Copyright 1984 by the AmericanGeophysical Union. [1972], whichwas interpreted by Carlson[1976] as indicating Paper number 4B0686. a pole of relativeJuan de Fuca/Pacificmotion to the south- 0148-0227/84/004B-0686505.00 west of the plate near 30øN, 140øW. By contrast, Carlson 6980 RIDDIHOUGH' MOVEMENTS OF THE JUAN DE FUCA PLATE 6981 QUEEN ½ Dellwood Smt, / Scott Smts Explorer Explorer Plate Sovanco F.Z. tka Fault Cobb• \ Sea-• mounts % Juan de I • Fuca Plate. - I I I I I I I N Gorda South Plate Mendocino PACIFIC - PLATE San Francisco Fig. 1. Locationmap of the Juande Fuca platesystem. [1976] usedfour earthquakefault plane solutions(two on the and Blanco fracture zone orientation [Riddihough, 1980] and Blanco fracture zone and two on the Medocino fracture zone) numerical inversion via magnetic anomaly superposition[Ni- to determine a pole to the northeast of the plate at 57øN, shimuraand Hey, 1980; Nishimura et al., 1981] have located 119øW. the present Juan de Fuca/Pacific pole near 15øS, 150øW, to Independentdeterminations using spreading rate variations the southwestof the plate. Hey and Wilson [1982] have more 6982 RIDDIHOUGH' MOVEMENTS OF THE JUAN DE FUCA PLATE recently calculated a sequenceof pole positions to form the epochpole of Juan de Fuca/Pacificplate motion could be basisfor their study of propagating rifts (seealso Nishimura et determined from the azimuths of anomalies on the Pacific al., [1984]). This is further discussedbelow. plate.(Those on the Juande Fuca platewill havebeen cumu- lativelyrotated.) This methodassumes orthogonal spreading; Method: Magnetic Anomaly Analysis however,any randomdeviations from this conditionshould If plate tectonicswere always to occur in a geometrically be compensatedby sampling.In practice,the 95% confidence simple manner (with orthogonal and symmetric spreading), limit on mean azimuth within all epochs except 4.5 Ma then spreadingrates along a ridge systemwould decreasesys- (-t-3.1ø) was -t-2ø or less.Nevertheless, an arbitraryconfidence tematically toward the pole of rotation, fracture zones would limit of _+5 ø was assignedto this determination.Clearly, this always occur along small circlescentered on the pole of rota- methoddoes not allow for systematicoblique spreading along tion, and ridge segmentsand linear magneticanomalies would the whole ridgefor periodsof 1 million yearsor greater. always follow great circlespassing through the pole. Further, Spreadingrates were measured perpendicular to anomalies the senseand differencesin azimuth of equivalent anomalies as in Figure 2 (right) and projectedonto the Pacific plate on either side of a ridge system(fanning) would indicate the anomaliesrepresenting the epochgreat circle(ridge) in swaths direction and amount of cumulative angular rotation between of approximately1 ø of colatitude.An exampleof the results the two platesinvolved (Figure 2, left). In practice,divergences for epoch3.5 Ma is shownin Figure 3. Plate tectonicgeome- from such ideal behavior are widespread. Nevertheless, the try requiresthat ideallythese spreading rates will conformto geometrical concepts have formed the foundation of many a sine function of colatitude.A simple test showsthat for all plate tectonic analysesand often offer the only route to the pointson sucha curvethe linearslope over a distanceof less determination of individual plate motions. Over the Juan de than 7ø will differ from the slope of the sine function at the Fuca system,accurate contoured maps of magneticanomalies meancenter point by lessthan 0.5%. For the presentanalysis, with predominantly north-south orientation in a high mag- therefore,the slopewas determinedby linear regression,95% netic latitude provide an opportunity to attempt to use such confidencelimits beingassigned by the standardmethods for conceptsin