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Home , 1980 Eureka earthquake, Explorer Plate, Explorer Ridge, Juan de Fuca Plate, Juan de Fuca Ridge

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B8, PAGES 6980-6994, AUGUST 10, 1984

Recent Movements of the 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 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 ."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 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 , which convergedwith the western 1) [e.g.,Davis and Riddihough,1982; Silver, 1971; Carlson and margin of 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 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 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 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, 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

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 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 detail. small samples.With the slopeand spreadingrate at the mean Although transform fracture zones commonly representone point (with confidencelimits) known, the colatitudeand rota- of the most important constraintsin determining plate mo- tion rate of the pole can be uniquelydetermined. From this tions, the Sovanco and Blanco fracture zones (F.Z.) (Figure 1) and from the greatcircle the pole can be calculated. presenta number of problems.The SovancoF.Z. is not clearly Instantaneousepoch poles determined in this manner,rela- definedby bathymetry,magnetic anomalies, or seismicity[e.g., tive to the Pacificplate, are shownin Figure 4. The resulting Milne et al., 1978]. The Blanco F.Z. may have had an ex- "sector-shaped"95% confidencelimits are a consequenceof tremely complex history [Hey and Wilson, 1982] and appar- the independentdetermination of colatitudeand azimuth. ently contains a number of different segmentsof varying orientation including "pull-apart" basins(R. W. Embley, per- Juan de Fuca/PacificPoles sonal communication, 1984). For the purposes of examining Figure 5 comparesthe instantaneouspoles derived in this the recenthistory of the Juan de Fuca system,neither fracture paperwith thoseof Hey and Wilson[1982] calculatedfrom zone providesa clear record of past spreadingdirections. the finite rotationsnecessary to rotate isochronson the Juan For this study, all data were determined from magnetic de Fuca plate onto the correspondingisochrons on the Pacific anomaly orientation and spacing.Anomaly ageswere assigned plate.Considering the uncertaintiesin the techniques,the re- usingthe work by Ness et al. [1980], and individual anomalies sults comparequite closely.One significantdeparture is in or sequencesdisturbed by transforms or known "pseudo- timing. Hey and Wilson [1982] considerthat there was a faults" were discounted.Anomalies were grouped into epochs "drastic shift" in Juan de Fuca/Pacific motion betweenthe of 1 million years duration, and measurementswere made as times of anomalies3a and 3 (around 5 Ma). In fact, detailed in Figure 2 (right). studies of anomaly azimuths [Riddihough,1977, Figure 3; For Juan de Fuca relative to Pacific (JDF/PAC) plate Carlson, 1981a, Figure 5] show that while there may be a spreading,it was assumedthat the great circle containing the slightincrease in the rate of clockwiseanomaly rotation at 5

Spreadingi! Strike rate difference decrease gives to pole angular W angular rotation r rotation z' B

A ,,

I i • t , x+x'>¾+¾'>z+z' to pole topole towardspole ( B relative to A ) Fig. 2. Pole determination parameters from magnetic anomalies used in this study. (Left) Theoretical geometry of spreading.(Right) Applicationto observedmagnetic anomalies. RIDDIHOUGH.' MOVEMENTS OF THE JUAN DE FUCA PLATE 6983

[1982] reconstructionsassume that spreadingremains oblique after a pole change(anomalies are thuscreated parallel to the '--.]. Epoch3.5Ma old ridge)until a propagatorpasses through and reorientsthe ridge system.A difficultywith this is that, as noted earlier, anomaly azimuthsof the same age along the are remarkablyconsistent. Further, the most striking 60- contemporaryridge propagationin the region(the Cobb pro- pagator)separates ridge sections which differ in orientationby no more than 1 ø or 2 ø. Studiesof detailedridge structure[e.g., Macdonald,1982] 5o,1i i i i i indicate that the "crustal " and "neovolcanic" zone COl'ATITU'DE ø wherespreading actually occurs may be lessthan 2 km wide, Fig. 3. Spreadingrates along the Juan de Fuca ridge at 3.5 Ma. althoughoff-axis volcanism does exist and the activefaulting Colatitude is measuredin degrees(111 km) along strike of magnetic zonemay existto +__10 km of the axis.The speedwith whicha anomalieson the Pacific plate; error bars are 95% confidenceinter- ridge can changeorientation without propagationand the vals on rates and the mean (open circle and bar); dashed lines are extent to which preexistingstructure constrainschanges are 95ø/,,confidence limits on the slope. unclear.The fact that propagatorsbreak into old crust in an apparentlycontinuous manner [Hey and Wilson, 1982] sug- Ma, the major change is between 3 and 4 Ma (Figure 6). At geststhat the constraintsmay not be strong.For the present this time, most "fanning"ceases. Nishirnura et al. [1984] con- studies, therefore, it is assumed that anomaly orientations sidered that the major change occurred between 2.4 and 5.4 averagedover 1-million-yeartime periodsare perpendicular Ma with the instantaneouspole remaining in its presentposi- to plate motionsduring the sameperiod and not thoseof a tion since 2.4 Ma. significantlyearlier time. The key to the timing discrepancymay lie in the argument Significantfeatures of the pole positonsin Figure 4 are the that when plate motions change,it may take a finite amount eastwarddrift up to 4.5 Ma and then the rapid increasein of time for the centralrift creatingthe magneticanomalies to distanceaway from the plate. This movementresults in the adjustto the new directionof spreading.Hey and Wilson's

o• / o

25-11.5

ß JDF/AM 0 JDF/HS ß JDF/PAC • Hey & Wilson (1982) ß Nishimura et al. (1981) /x Nishimura et al. (1984)

Fig. 5. Mean instantaneousJuan de Fuca/Pacific(JDF/PAC), Juan de Fuca/America(JDF/AM), and Juan de Fuca/hot spot Fig. 4. Juande Fuca/Pacificpoles determined from this study. (JDF/HS) polesderived in this paper.Numbers are epochdates in Numbersare epochdates in millionyears before present (Ma). The Ma. Polesderived by otherauthors are shownfor reference,together 95% confidencelimits are keyed to each pole positionby symbols. with contemporaryPacific poles from Minster and Jordan[1978]. For derivation, see text. Coastline is as at present. 6984 R•VV•HOUGH:MOVEMENTS OF THEJUAN DE FUCA PLATE

mated that it came into existencearound 7 Ma (seealso Riddi- hough[1977]). However, determination of the pole of rotation .øi sequencepresented here shows that this may not be geo- metrically necessaryand that the 3-4 Ma initiation is to be preferred. 20 ø- A sequenceof mean Explorer/Pacific (EX/PAC) poles from EXPLORER 3.5 Ma to the present is shown in Figure 7 and listed in Table 10 ø- 1. These were calculated using spreadingrates and anomaly azimuths for the [Riddihough,1977] combined ß ' ' ' ' with the average Explorer/Pacific rotation rate determined from the fanning shown in Figure 6. As there is insufficient 20ø'JUAN DE I/•"•o--• ß• Explorer ridge length to observethe decreaseof spreadingrate uc^,. o . toward the pole, the latter providesthe necessarythird param- 10 oo o o•_.o/• eter to determine the pole. This processintroduces a number of errors, not the least 0ø1 ' ' I ' ' I being the assumption of a constant rotation rate. Figure 8 30 ø- shows the EX/PAC pole error space for epoch 0.5 Ma esti- O o o mated using + 5ø great circle error and 95% confidencelimits 20 ø_ ß oo on the rotation rate from the fanningregression.

Gorda/Pacific Poles 10 ø- GORDAR.(N) • dl Riddihough [1980] confirmed that the South beganto spread at a significantlydifferent rate from the North . , , Gorda ridge betweeen2 and 3 Ma. Spreadingon the North lO 5 o MaBP Gorda ridge can be fitted to the same pole of rotation as the ß ANOMALIES ON PACIFIC PLATE remainder of the Juan de Fuca plate up to the present. The o ANOMALIES ON EXPLORER, JUAN DE FUCA South Gorda ridge must thereforehave had a separatepole of AND GORDA PLATES rotation relative to the Pacific plate sinceat least 3 Ma. Riddi- Fig. 6. Magnetic anomaly azimuths plotted against age for three hough [1980] showed that significant anomaly fanning be- sectionsof the Juan de Fuca ridge system.Open circlesare azimuths tween Gorda South and the Pacific plate stopped at about 1 on the plates to the east of the ridge; solid circlesare from the Pacific plate. Ma. Thus the pole must have become much more distant for epoch 0.5 Ma. The question of whether or not the anomaly distortion of the is produced by fanning or someform of inter- nearestemergent pole of the rotation axis being to the south nal deformation is being actively debated [e.g., Silver, 1971; rather than the north of the plate sinceapproximately 2 Ma. Riddihough,1980; Carlson and Stoddard, 1981]. At this stage, Confidencelimits for the epoch-3.5-Ma pole put the closest the poles of Figure 7 and Table 1 (calculated as for the Ex- pole as definitely to the north; limits for the 0.5-Ma pole plorer ridge) are presented as the geometrical consequenceof constrainit to the south of the plate by that time. the fanning interpretation rather than the final answer to this problem. Explorer/Pacific Poles The change in Pacific plate anomaly (ridge) rotation at "Absolute"Poles of Motion around 3 Ma can be seen in Figure 6 to occur for all three Calculation of instantaneousabsolute or "hot spot" poles major sectionsof the ridge system(Gorda, Juan de Fuca, and for the Juan de Fuca systeminvolves reconstructing the posi- Explorer). However, for the Explorer ridge it involves an tion of the instantaneousJDF/PAC pole at the epoch con- almost 10ø jump in azimuth followedby continuingrotation, cerned relative to the hot spot framework and solving the not a cessation.It appearsthat at this time the Explorer plate vector addition with the Pacific/ (PAC/HS) poles. A becameindependent and no longer pivoted around the same numberof PAC/HS poleshave beencalculated using hot spot pole relative to the Pacific plate as the rest of the Juan de traceswithin the Pacific plate [e.g., Minster and Jordan, 1978; Fuca system.This is supportedby the followingfacts: Minster et al., 1974; Chase, 1978; Turner et al., 1980; Duncan, 1. All anomalieson the easternside of the Explorer ridge 1981; McDougall and Duncan,1980]. In comparingthe orien- are "fanned"clockwise in relation to thoseon the west,indica- tation of seamount chains and features within the immediate ting a nearby pole to the southwestwith a rapid angular region (in particular the Cobb, Scott, and Dellwood seamount rotation. systems; Figure 1), model AM1-2 of Minster and Jordan 2. The Explorer ridge spreadingrate for epoch 3.5 Ma is [1978] seemsto producethe closestfit. This is the pole usedin much larger than that of the northernJuan de Fuca ridge. all absolutemotion calculationsin this paper.The resulting This is inconsistentwith a singlepole to the north. mean absolutepoles calculatedin this manner for the Juan de 3. The Pacificplate anomaly azimuth changesof Figure 6 Fuca, Explorer, and Gorda South systemsare listed in Table 1 couldonly be accommodatedwithin a singleplate systemby a and are shownin Figures5 and 7. suddenwestward Explorer ridgejump followedby continuing The main featuresof the Juan de Fuca/hot spot (JDF/HS) westward migration with a rotation pole to the north. This is polesare that they occur closeto the Juan de Fuca plate to inconsistentwith point 2. the northwest until 4.5 Ma, then shift to the southeast, and Hyndrnan et al. [1979] named the transform between the progressivelymigrate northwestwardtoward the plate. The Explorer and Juan de Fuca plates the Nootka Fault and esti- Explorer/hot spot (EX/HS) and Gorda/hot spot (GO/HS) RIDDIHOUGH' MOVEMENTSOF THE JUAN DE FUCA PLATE 6985

2.5 ...... 5

EX/HS

3.5

2.5

1.5 EX/AM

2.5

EX/PAC

-4oo-•_ l•e3'5 J--

GS/ 2.5

• b

Fig. 7. Mean instantaneouspoles for the Explorerand Gorda Southplates, PAC, Pacific;HS, hot spot framework; AM, Americaplate framework;EX, Explorer;GS, Gorda South' JF, Juan de Fuca. Numbersare epochdates in Ma; directionsof plate rotation are shownby arrows.Coastline is as at present.

polesmay occurextremely close to or within the Explorer and (JDF/AM) pole at 29.11øN, 112.72øWat 1.05ø/Ma; this com- Gorda platesand consistentlyinvolve clockwiserotation. paresclosely with the 0.5-Ma pole shownin Table 1.)

Poles of Motion Relative to the America Plate ConfidenceLimits for Pole Determinations Calculation of mean poles relative to the American plate and Relative Motions can be carried out through reconstructionand vector addition The asymmetric,"sector-shaped" confidence limits for in- usinga Pacific/Americapole. A numberof authorshave calcu- stantaneousJDF/PAC and EX/PAC polesshown in Figures4 lated such a pole for the present, but there remains some and 8 result from the calculation methods. To estimate confi- uncertaintyas to its applicabilityfor periodsolder than 5 Ma dence limits for JDF/AM, JDF/HS, Explorer/America [e.g., Molnar and Atwater, 1978; En•tebretson,1982]. This un- (EX/AM), and EX/HS poles, the bounding and mean pole certainty is of undoubted importance in assessingmotions of positions(and rotation rates) were convolvedwith the quoted the Juan de Fuca plate relative to the America plate. However, mean poles and 2a limits on latitude, longitude,and rotation for consistencythe poles listed in Table 1 (shown in Figures 5 rate for RM1 and AM1-2. Ninety-five percent confidence and 7) have been calculatedassuming model RM2 of Minster limits on the resultingmatrices of polesare shownin Figures and Jordan [1978] back to 6.5 Ma. (Using RM2, Nishirnuraet 8 and 9 for epoch0.5 Ma. (It should be noted from Figure 4 al. [1984] recentlycalculated a presentJuan de Fuca/America that these0.5-Ma confidencelimits are probably the smallest 6986 RIDDIHOUGH' MovsMs•rrs OF THE JUAN DE FUCA PLATE

TABLE 1. Mean InstantaneousRotation Polesof the Juan de Fuca Plate System

Relative to Relative to Relative to Pacific Plate* Hot Spot Framework• America• Epoch, Ma øN øW ø/m.y. øN øW ø/m.y. øN øW ø/m.y.

Juan de Fuca 6.5 73.5 121.8 + 1.42 57.3 195.0 +0.59 57.0 200.3 +0.83 5.5 69.5 124.0 + 1.54 56.1 176.4 +0.69 56.9 185.9 +0.93 4.5 65.7 118.3 + 1.87 58.8 150.2 +0.98 60.6 162.5 + 1.19 3.5 74.9 80.0 +0.96 - 19.8 82.6 -0.22 42.1 244.9 +0.43 2.5 73.6 12.3 +0.71 21.0 1Q6.1 -0.47 -4.7 92.9 -0.50 1.5 - 11.7 145.3 -0.61 38.5 117.7 - 1.16 28.4 110.2 - 1.07 0.5 -9.9 146.1 -0.62 39.2 119.4 - 1.18 29.4 111.7 - 1.09

Explorer 3.5 40.2 134.9 - 3.1 45.7 125.4 - 3.9 43.0 123.4 - 3.8 2.5 42.1 137.1 - 3.1 47.6 127.6 - 3.9 45.2 125.3 - 3.8 1.5 43.5 136.3 - 3.1 49.0 127.3 - 3.9 46.7 124.7 - 3.8 0.5 44.2 135.7 - 3.1 50.0 127.4 - 3.9 47.8 124.5 - 3.8

Gorda Southe: 2.5 38.1 127.8 - 13.5 40.0 126.1 - 14.5 39.3 125.4 + 14.2 1.5 39.0 128.0 - 13.2 40.9 126.2 - 14.0 40.2 125.5 - 14.0 0.5 30.0 129.2 - 1.1 46.9 114.5 - 1.9 41.2 108.0 - 1.7

Poles are given in relation to Pacific, hot spot framework,and America consideredfixed in their presentpositions. Pole namedis the emergentpole closestto the Juan de Fuca plate. Rotationsare -, clockwise,and +, anticlockwiseviewed from the pole. *For confidencelimits, seeFigure 4. •'Calculatedusing Pacific plate motion as determinedby RM1 and AM1-2 of Minster and Jordan [1978]. :[Polesassuming no internaldeformation of Gorda Southplate (see text).

of the set being considered.)These simplifiedconfidence limits ment with the conceptof Menard [1978] that the youngest tend to be slightly smaller than the conventional elliptical material(where the ridge is closestto the subductionzone) limits [e.g., Minster and Jordan, 1978] and are also non- providesthe greatestresistance to subduction.In effect,the symmetricabout the third pole determinedby the vectoraddi- platetends to pivot about sucha point. tion of the two mean poles (e.g., JDF/PAC and Paci- Between 4 and 3 Ma there is a dramatic change. The fic/America (PAC/AM)). In terms of the significanceto be youngestnorthern portion of the platethat wasproviding the givento the resultingmotions of the Juande Fucaplate maximum resistancebreaks off and beginsto move indepen- system,they neverthelessprovide at least a minimum assess- dentlyas the Explorerplate. (The approachof the easternend ment of the probable errors. For Juan de Fuca plate motions of the SovancoF.Z. to the margin may have contributedto at 0.5 Ma, they suggesterrors of _+7 mm a-• and __+7ø rela- this.)It immediatelybegins to moveabout a verylocal pole so tive to America and + 10 mm a-• and + 20 ø relative to the that its motion into the convergencezone is severelyreduced. hot spot framework for a point in the center of the plate. Even with the probableerror limits of pole positions,by 2.5 Explorer plate motions are lesscertain, having probable errors Ma it may be rotating about a point within itselfsuch that in for 0.5 Ma of __+15 mm a-• and _+12 ø relative to America and a translational senseit is no longer moving systematically _+15 mm a-• and +__100 ø (for a point near BrooksPeninsula toward the former subductionzone. This type of "spinning" that is within the confidencelimits of the absolutepole posi- motion continuesto 0.5 Ma. The pole uncertaintylimits in- tion (Figures1 and 8)). By comparison,similar calculations for cludethe possibilitythat in the hot spotframework it couldbe the errorsof JDF/AM motionat 3.5 Ma give _+13 mm a-• moving away from its subductionzone with the America and _+17 ø. plate. At the time that the Explorer plate becomesindependent, ABSOLUTE MOTIONS OF TI-• JUAN DE FUCA SYSTEM the Juan de Fuca plate becomesnotably smaller, and the Figure 10 presentsa sequentialpicture of the absolute(rela- point of maximumresistance to movementprovided by the tive to hot spots)plate motions of the Juan de Fuca plate youngestmaterial entering the subductionzone shifts to some- systemsince 7 Ma. The motionsare shownon a geometryof wherein the Cape Blanco area. By epoch2.5 Ma, the mean the plate systemrelative to the presentcoastline and use the absolutevelocity of the plate has droppedto 40% of its pre- mean calculatedpoles of Table 1 subjectto the uncertainties vious value, and the slowestmotion has shiftedto the south- discussed above. This reconstruction does not include the ac- eastin conformitywith Menard's[1978] concept.The velocity cretinghistory of the Blanco F.Z. by accumulatingpropagat- of the plate continuesto reduceuntil epoch0.5 Ma. The abso- ing offsetsas proposedby Hey and Wilson [1982]. For the lute velocitydetermined from the meanpole now variesfrom understandingof the motions of the rigid plate systemover 20 mm a-• near VancouverIsland to 10 mm a-• near Cape the mantle and relativeto the margin,this may not be critical. Blanco,the pole of absoluterotation beingin northernCali- America absoluteplate motion is as givenby AM1-2. fornia. However, considering the uncertaintiesdiscussed For the period from 7 to 4 Ma, motion of the plate is to the above,it is now possiblethat the velocityof the southernpart northeast with the slowest motion off . The of the plate relativeto the mantleis closeto zero. occurrenceof the slowestrate at the northern end is in agree- A separatepole of motion for the southernGorda plate is RIDDIHOUGH: MOVœM•NXS OF • JUAN DE FUCA PLATE 6987

50ø ,EX/HS.•::..:'••ii/i:•11:•!iii•i•i•::

40 ø

30.... •i•!ii::::•:• o0• b b

Fig. 8. The 95% latitude and longitudeconfidence limits for epoch-0.5-Mapole determinationsfor the Explorer plate. Circlesare pole positionsderived from vector addition of mean EX/PAC and PAC/AM or PAC/HS poles,not the mean of the covariance of confidence limits. probably significantat least from 3 Ma. As with the Explorer ing with the America plate. Over the succeeding2 million plate, the resultantmotion is clockwiserotation around a pole years,motions remain largely the same. closeto or within the plate. A similar situation develops,that Even taking into account the large uncertainties,the inde- of little or no motion toward the Gorda/America convergence pendenceof the Explorer plate by 3.5 Ma probably resultsin a zone, perhaps even motion away from the convergencezone. sharp reduction in convergencerates along Vancouver Island The changein the Gorda South pole for epoch 0.5 Ma retains (Figure 12). In relation to the America plate, the Explorer the clockwise motion but could result in movement that is plate again rotatesclockwise around a local pole probably to significantly away from the subduction zone and the Men- the southeast(see Figure 7), which appears to be migrating docino escarpment. northward and becomingcloser to the Explorer plate itself. After the breakaway of the Explorer plate at 3.5 Ma, the remainder of the Juan de Fuca plate behavesin relation to the America plate much as it did when consideringabsolute mo- MOTIONS OF THE JUAN DE FUCA SYSTEM tions. The convergencerate probably declinesfrom its 4.5-Ma RELATIVE TO AMERICA value at all points along the convergencezone (Figure 12). By epoch 2.5 Ma, the JDF/AM pole has shifted to the southeast Figure 11 presentsa sequentialpicture of the motions of the of the system,so that the slowestconvergence is in the Cape Juan de Fuca plate systemrelative to the America plate on the Blanco area where the youngestmaterial is entering the sub- same plate layout as Figure 10. Again, these are the motions duction zone. Convergenceprobably continues to decline to given by the mean poles of Table 1 and subject to consider- the present,and the pole moves toward the plate. able uncertainties. At 6.5 Ma, relative motion is to the north- If, as before,the southernpart of the Gorda plate is treated eastat meanspeeds varying from 60 mm a-• alongthe Van- as rigid, this plate apparently rotates clockwisearound a local couverIsland margin to 70 mm a-• south of Cape Men- pole close to or within the plate through epochs2.5 and 1.5 docino. As with the absolute motions, the slowest movement is Ma. For epoch 0.5 Ma, convergencemay cease,and the plate off Vancouver Island where the youngestmaterial is converg- may moveto the northwestat approximately40 mm a- •. 6988 RIDDIHOUGH:MOVEMENTS OF THE JUAN DE FUCA PLATE

Fig. 9. The 95% latitudeand longitude confidence limits for epoch-0.5-Mapole determinations for the Juande Fuca plate.Dashed oval is 95% confidence interval for JDF/PAC pole from Nishimura et al. [ 1984].

DISCUSSIONOF PLATE MOTIONS that therewas a strikingcorrelation between velocity and the In summary,the plate motions of theJuan de Fucasystem length of trench with subductedslab. Chappieand Tullis as shownby the mean rotation polesboth in an absolute [1977] alsoconcluded that themajor driving forces are associ- (relativeto hot spots)and in a relative(to America)sense atedwith subduction, a pull due to thedowngoing slab acting show similar characteristics: on both upper and lower plates,and a resistanceto conver- 1. Therate of motiondeclines beginning at 4 Ma. gence.Recently, Carlson [1981b] and Carlsonet al. [1983] 2. Themotion pattern is suchthat the youngest converg- assessedthe balanceof forcesusing multiple linear regression ing materialmoves slowest; this patternis reestablishedfor and concludedagain that is the dominantforce in theJuan de Fucaplate after 4 Ma whenthe Explorerplate determiningplate velocities. becomesindependent. The data assembledin Figure 10 can be usedto calculate 3. After 4 Ma, all closestrotation poles lie to the south themean integrated velocities of the Juande Fucaplate rela- andprogressively move toward the plates concerned. tive to the hot spot framework since 7 Ma. These velocities Theerrors of polepositions and rotation rates are probably decreasefrom around 45 mm a-• at 6.5 Ma to 17 mm a-• at suchas to reducethe significance of characteristic1 above, but 0.5 Ma. Allowing for uncertaintiesof q-20 mm a- • for the both characteristics2 and 3 remain true at the 95% confi- olderepochs and q-10 mm a-x for thepresent, this suggests dence limit. that the velocityhas either remained similar or decreasedby up to 50% duringthis period. Using the geometry of theplate systemshown in Figures10 and 11,the total areaof theplate Driving Forces hasdecreased by approximately50% duringthe same period. The enigmaof platedriving forces has been examined, both The effectiveridge length (normalized by totalperimeter) has theoreticallyand by the empiricalapproach of comparing decreasedby approximately25%, and the effectivetrench platespeeds with parameters such as area, ridge length, trench lengthhas remainedalmost identical. In qualitativeterms, length,etc. Forsythand Uyeda [1975] concludedthat there therefore,the resultsmay be compatiblewith otherglobal wasno obviousrelation between plate velocityand area but studies. , RIDDIHOUGH: MOVEMENTS OF THE JUAN DE FUCA PLATE 6989 6990 RIDDIHOUGH' MOVEMENTSOF Wl-IEJUAN DE FUCA PLATE RIDDIHOUGH'MOVEMENTS OF THE JUAN DE FUCA PLATE 6991

at 50øN 128øW declinedto near zero suggeststhat it is its motion into the mantle which provided the predominantresistance. The absenceof any fault plane solutions showing under-

E thrusting[Milne et al., 1978; Rogers,1979] in the convergence zone of the Juan de Fuca plate system,the low-energyrelease of Juan de Fuca/America convergence[Hyndman and Wei- i ! I ' I ' I I ' I ' I ' I chert, 1983], and the steady "back tilting" of coastal areas at 46' N 125'W [Ando and Balazs, 1979; Riddihough, 1982a; Reilinger and Adams,1982] have all been suggestedas indicating that Juan de Fuca/America interaction in this region is continuously aseismic.Although these observationscannot necessarilybe extendedto long-term processes,they do indicate that at least + on a short time scale, conditions of low resistanceto Juan de Ma Ma ß total Fuca/America convergencecan exist. o orthogonal The fact that once the Explorer plate is detached,the re- Fig. 12. Interaction rates (in millimeters per year) at two points mainder of the Juan de Fuca plate adjustsits motion so that near the continentalmargin versustime. Solid circles,movement rela- its southern (youngest)part moves slowestis an apparently tive to America; open circles, convergenceperpendicular to the strongconfirmation of the pivoting subductionconcept. How- margin. Error bars are estimated and may be minimum for older ever,a major dilemmais posedby the data of Figure 10: If the epochs;see text for explanation. Explorer plate was providing the maximum resistanceto Juan de Fuca motion, why doesthe mean velocity of the remainder Nevertheless,it is clear that many of the quantitative rela- of the Juan de Fuca plate decreaseand not increasewhen the tions determined from global studiesdo not fit the Juan de Explorer plate becomesdetached? One solution to this prob- Fuca plate. In particular, the model IV regressionof Carlson lem may be that the resulting decreasein plate size is the [1981b] predictsthat the mean velocity of the Juan de Fuca dominant factor in determiningplate speed.Another solution platewould be 98 mm a-• at 6.5 Ma and 94 mm a-• at 0.5 may involve other componentsof plate motion. Ma. Figure 8 of Forsyth and Uyeda [1975] also suggeststhat to fit in with the global plate observations,the Juan de Fuca Subduction, Overriding,and "Rollback" Velocities plate, with a present effectivetrench length of 40%, should Dewey [1980] and Uyeda and Kanamori [1979] rationalized havea velocityof 100mm a- • or greater.Using a plateage at the movementsof plates in a convergencezone into three the margin of 10 Ma, the relation derived by Carlson et al. components.Following Dewey's notation, these components [1983] predicts an absolute velocity for the Juan de Fuca are plateof 25-35 mm a-•. This is closeto the absolutevelocities calculatedfrom Figure 10. However, plate reconstructionsas Vo absolutevelocity of the overridingplate; in Figure 10 (see also Engebretson,[1982]) suggestthat the V, absolutevelocity of the underlyingplate (its rate of inser- ridge has stayed approximately the same distance offshore tion into the asthenosphere); since10 Ma. The ageof theplate entering the trench has thus V, rate of rollback at which a trench movesoceanward due remainedsimilar. If the mean speedhas decreasedmarkedly in to the rate of "fall" of the subductingplate, V0, into the thisperiod, age seems unlikely to be a significantfactor. asthenosphere.(This is distinct from what might be Quantitatively, it would seem that the balance of forces termed the "forced" rollback due to the advance of an controllingJuan de Fuca plate motions may not be the same overlyingplate.) balanceas for other, larger plates.As plates becomeas small as the Juan de Fuca, the simplelinear relationsbetween forces Both Dewey [1980] and Uyeda and Kanamori [1979] argue such as those proposedby Carlson et al. [1983] may change that the tectonicsof a convergencezone are highly dependent or ceaseto apply. upon the relativevalues of V, and Voand are much lessdepen- dent upon V,. Pivoting Subduction Explorerplate. The rapid declineof Explorer plate veloci- The processsummarized in Figures 10 and 11 seemsto ty and the movement of its pole of rotation has produced a supportMenard's [1978] conceptof pivoting subducti0nand situation where its present rate of insertion along its own providesa strikingexample of the rate and styleof responseof length into the mantle, V•, may be near zero. Although there a small plate system.Some form of resistanceat the conver- is, presumably,a sectionof subductedExplorer plate beneath genceor subductionzone is apparentlyan importantfactor in Vancouver Island, it is not coherentlymoving down into the determiningplate motion in this region. Ironically, because mantle. It seemsreasonable to argue that, in fact, the only both absolute and relative to America motions show the same reason that this portion of the plate was previously moving pattern, it is not immediately obvious whether the predomi- down into the asthenospheremay have been becauseit was nant factor is the motion of the Juan de Fuca plate into the attachedto the older and larger Juan de Fuca plate. Once it mantle or its interaction with the overlying continental plate. becamedetached, the sum of forcesdriving the Explorer plate The latter may provide frictional resistanceto convergence; changedradically, and it could no longer continue any down- theformer may provide buoyancy resistance as the material of ward movement.Dewey [1980] stressesthat rollback velocity the oceanicplate moves downward. However, if the detach- V, is independentof V•. However,if one of the dominantforces ment of the Explorer plate reflecteda balance point between drivingplate motion is the negative buoyancyof subducted its resistanceto motion and the strengthof its attachmentto slabs,then it seemslikely that, although independent,both V, the remainder of the Juan de Fuca plate, then the fact that its and V• will approachzero if this buoyancyceases to be nega- absoluterather than relative translational movement rapidly tive. In the case of the Explorer plate, V• is near zero, and it 6992 RIl•l•II-IOUC;I-I: MOVEMENTS OF THE JUAN DE FUCA PLATE would seemprobable that V. the "free" rollback velocity, may A seriesof cross sectionsderived from seismicity,seismic thus also be near zero. refraction,and gravity modeling were constructedby Riddi- In tectonic and structural terms, the result of this situation hough [1979] (see also Ellis et al. [1983] and Michaelson could be as in Figure 13. The Explorer plate, having ceasedto [1983]). All showed an increase in dip or "bend" in the descendinto the mantle under its own body forces, is still downgoing slab beneath the Georgia Strait and the Puget convergingwith the America plate and has been overridden Sound. One of these is shown in Figure 14 (top). Figure 14 by up to 70 km in a southwesterlydirection. It may thus be (bottom three panels)suggests how sucha bend could be pro- effectively underplating the northern part of Vancouver ducedby an increasein V•and V• relativeto Vo.Previous to 4 Island. Such a process would, presumably, involve an in- Ma, the relation between Vo(America absolutevelocity, ~ 23 creasedstress coupling with the overlyingAmerica plate and a mm a -•) and V• may have beensuch that the marginwas contrastin recent vertical movementhistory. These conditions "compressional"[Dewey, 1980]. At 4 Ma an increasein V•and plus the complexityof the interactionwith the America plate V• produced by the detachmentof the Explorer plate could producedby the local pole of relative movementwould seem cause the bend in the plate at the trench to begin to "fall." to ensurethat there should be a strong contrast betweenthe This increasein V• might change the characterizationof the tectonic processesaffecting northern Vancouver Island and margin at this time to something closer to that defined by those affecting southern Vancouver Island and the Wash- Dewey [1980] as "neutral." ington and Oregon margins.This seemsto be true of present The aseismicnature of the Juan de Fuca plate convergence seismicity[Milne et al., 1978; Rogers,1979], in particular in zone and the accretionarynature of the Washington-Oregon the zone where the subductedNootka Fault [Hyndman et al., margin [e.g., Silver, 1972] seemto be in accordwith a neutral 1979] marks the boundary between the two regimes.Rogers concept [Dewey, 1980]. The initiation of high Cascadevol- [1983] has also pointed out that the vertical movement pat- canism [e.g., McBirney, 1978] in the late Pliocenecould also tern [Riddihough,1982a] and topographyof the northern part be a responseto a changein asthenosphericflow betweenthe of Vancouver Island are distinct from the areas to the south. downgoingand overlyingslabs in a similar way to that sug- Juan de Fuca plate. The dilemma of the apparently con- gestedby Barazangi and lsacks [1976] for the . tinuing reduction in motion of the Juan de Fuca plate after Certainly, the existenceof the bend in the downgoingslab in the detachmentof the resistiveExplorer plate may be resolv- the appropriatelateral position(100-120 km from the margin) able if the independenceof V• and V• suggestedby Dewey to have been produced near 4 Ma seemsto be an indication [1980] does hold true for the nonzero condition. As shown that a change in rollback velocity at about that time is a (Figure 10), V• for the Juan de Fuca plate apparentlycontinues possibleoccurrence. to decline after 4 Ma. However, if the Explorer plate was However,this explanationdoes not accountfor the fact that providingbuoyancy resistance to descentinto the mantle, then globally,most subductingslabs have a "knee bend" at a depth its removalmay have resulted in an increasein V•and thus V•, of between30 and 50 km [Pennington,1983]. The association the trench rollback velocity.Such an increasewould not show of the bend in the Juan de Fuca plate with seismicitydue to in Figure 10 but might be detectablein a verticalcross section phasechanges [Rogers, 1983] seemsto supportthe conceptof throughthe Juan de Fuca convergencezone. Pennington[1983] that the bend is a responseto the increase

OVERRIDING BUT NO 'SUBDUCTION'

OVERRIDING PLUS 'SUBDUCTION'

EXPLORER PLATE AMERICA PLATE

JUAN DE FUCA PLATE

Fig. 13. Block diagram showingproposed plate configuration and contrast beneath Vancouver Island. "Subduction"is usedhere as movementof the underlyingplate downwardinto the asthenosphere. RIDDIHOUGH'MOW•mNTS OF Tim JUANDE FUCA PLATE 6993

Vancouver I. The reasonfor the timingof theindependence of thispart of /'•, Georgia the Gorda plate at 3 Ma is not clear.It was not the youngest 2.3 TøfinøBasin/ •S/• Coast. ßMtns. portionof the subductingJuan de Fuca plate at that time, althoughthe recentdetachment of the Explorerplate may --•__••__ 2.92 •0km have precipitateda redistributionof stresssuch that this cornerof the systembetween the marginand the Mendocino F.Z. becameparticularly vulnerable. Being small, once it had 3.295.• t100 broken away, it would presumablyhave little or no self- , • , • , ;200 drivingforce. It couldbe arguedthat the clockwisemotion from 3 to 1 Ma was determinedeither by northwesterlyPaci- 6 Ma compressional fic absolutemotion or by the northwesterlyright-lateral shear -•-•Vr lessthan ; •Vo appliedby Pacific/Americarelative motion. The increasein JF absolutemotion by 0.5 Ma and the rapid decreasein spread- ing on the southernGorda ridgesuggest that by this time at least,its motion was controlledby the northwesterlymoving Pacificplate. Given the length of the MendocinoF.Z., this seemsreasonable and may be part of the processof northward I I ' I ' migrationof the Mendocinotriple junction as suggestedby Riddihough[ 1980]. 4 Ma change ,Vr increases • ;Vo SUMMARY COMMENTS

. JF Althoughthere are undeniableuncertainties in the poles and rates of rotation determined for the Juan de Fuca plate systemby the methodsused here, the resultscompare well with otherapproaches to the problemand producea coherent patternof platemovements. This patternis valuablein that it

' I ' I ' I may show in detail how the movementsof a small plate systemare affectedby a numberof factors,notably by resist- 0 Ma neutral anceat the convergencezone. It affirmsthat at leastfor small plates,Menard's !-1978] concept of pivoting•subductionis •-•Vr equals• iVø valid. The behavior of the most resistive,Explorer plate por- tion of the systemin eventuallybecoming detached and ceas- bend ingto movedown into the mantle is physically re'asonable and suggeststhat thedominant resistance to motion is in thiscase providedby theasthenosphere. The behaviorof theremainder . ,atVg . , of the Juande Fuca plateafter the Explorerplate detachment 0 km 200 400 600 suggeststhat verticaland horizontal plate movements relative to the mantle could be independent. Fig. 14. (Top panel)Cross section across southern Vancouver Recentwork on the Winona Basin [Davis and Riddihough, Island[from Riddihough, 1979]. (Bottom three panels) Sequential mechanismfor the possibleproduction of an observedbend in the 1982] and continuingdebate on the southernpart of the downgoingslab. Sections are fixed in relationto themantle' America Gorda platetend to confirmthat closeto the triplejunctions, (AM) moveswestward at absolutevelocity V o. At 6 Ma the trench small fragmentsof platesbecome dominated by the move- rollbackvelocity V• is lessthan Vo, and compressionalconvergence mentsof the larger,bounding plates.' In the Juande Fuca occurs.At 4 Ma, V•increases. This results in anincrease in V•such systemit seemsthat an almostcontinuous succession from thatit equalsVo and a steepeningof the downgoing slab. The tectonic styleis nowneutral [Dewey, 1980], and a bendis produced. larger,self-driven plates to small,interplate fragments can be seen.The fact that the absolutepole of motion for the Juande Fuca plate itselfmay now be almostwithin the 'subducted in negativebuoyancy produced by the phasechanges. Its lo- portionof thisplate (Figure 5) suggeststhat it, too,is rapidly cation in the Juan de Fuca plate at a point which could infer slowingand will soonbe controlledby its largerneighbors. changesin convergentstyle at around 4 Ma may thus be a While it is a very smallpart of the globalplate system, it may coincidence. thusprovide a graphicexample of the end points•of the plate Gorda South tectonicprocess. As stressedat the outset,motions for the southernpart of Acknowledgments.The work presentedin this paper benefited the Gorda plate whichapparently moved independently from considerablyfrom discussionswith C. Nishimuraand D. S. Wilson, 3 Ma [Riddihough,1980] havebeen calculated assuming rigid who kindly providedpreprints of their work on the Juan de Fuca plate.I am particularlygrateful for a comprehensivereview from T. platetectonics. The possibilityof nonrigid,or at leastimbri- Atwater, which resultedin a much more realisticappraisal of the cate, distortion within this southernplate sectionwas suggest- errors and uncertaintiesinvolved in reconstructingthe plate motions. ed by Silver[1971] and has beenmore recentlyexamined by Earth PhysicsBranch contribution 1125, Carlson[1976] and Carlsonand Stoddard[1981]. Analysisof the 1980 Eureka [Smith et al., 1981] showsthat REFERENCES northeasterlyoriented left-lateral shear does occur and Ando,M., and E. I. Balazs,Geodetic evidence for aseismicsubduction stronglysuggests that imbricatedeformation is a probable of the Juan de Fuca Plate, d. Geophys.Res., 84, 3023-3028, 1979. explanation. Atwater,T., Implicationsof platetectonics for the Cenozoicevolution 6994 RIDDIHOUGH: MOVEMENTS OF THE JUAN DE FUCA PLATE

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