Download Recent Movements of the Juan de Fuca Plate System

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 anomaliesof the Juan de Fuca plate systemallows instantaneouspoles of
rotation relative to the Pacific plate to be calculatedfrom 7 Ma to the present.By combiningthesewith
global solutions for Pacific/America and "absolute" (relative to hot spot) motions, a plate motion
sequencecan be constructed.This sequenceshowsthat both absolutemotions 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 adjustedits 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"occurringas
the Explorer plate is overriddenby the continent.The history of the Explorer plate may exemplifythe
conditionsunder which the self-drivingforcesof smallsubductingplatesare overcomeby the influenceof
larger, adjacentplates. The recent rapid migration of the absolutepole of rotation of the Juan de Fuca
plate toward the plate suggests
that 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 remnants of the Farallon plate, which convergedwith the western
margin of North America over the last 150 million years.The
collisionof the Pacific-Farallonridge with the American plate
between25 and 30 Ma led to the independenceof the Juan de
Fuca plate at about 20 Ma. (Ma is usedhere to denoteage in
million years beforepresent.)Sincethat time its area has progressivelydiminishedas the bounding,southerntriple junction
has migrated northward (for a recent review, see Riddihough
[1982b]). A seriesof detailed magnetic anomaly maps [Raft
and Mason, 1961; Potter et al., 1974; Curtie et al., 1982] provide a uniquely comprehensivedata basewith which to study
the seafloorspreadingand plate motions of the Juan de Fuca
plate region since 10 Ma. In general,the ridge and spreading
changesare characterizedby reduction in spreadingrate and
clockwiseridge and plate rotation [Riddihough,1977, 1980;
Carlson, 1981a]. The principal period of change appears to
north and south endsof the Juan de Fuca plate system(Figure
1) [e.g.,DavisandRiddihough,
1982;Silver,1971;Carlsonand
Stoddard,1981] is certainlyindicativethat in the extremecase,
platemotionsrespondin a complexinteractivemanner.
However, as emphasizedby Hyndrnan[1972] and recently
discussedin more detail by Dewey [1980], Uyeda and Kanamori [1979], and Molnar and Atwater [1978], many aspectsof
plate motion can only be examinedthrough their motions
relative to the underlying asthenosphere.In a subduction
zone,platesmovedowninto the mantleso that at leastpart of
the driving or resistingbuoyancyforce is a function of the
speedof the plate relativeto the asthenosphere.
A full examination of local plate interactionsmust thereforeinvolvedocumentation of "absolute" as well as "relative" motions.
POLES OF RELATIVE MOTION
OF
THE JUAN DE FUCA SYSTEM
Assessments
of the geometryof presentand recentconvergence
between
the
Juande Fuca plateand the Americaplate
The pattern of ridge segmentationand rotation observedin
the magnetic anomalieswas first suggestedby Menard and [e.g.,McKenzieand Parker, 1967;Atwater,1970;Silver,1971;
Atwater [1968] as beinga responseto interaction betweenthe Riddihough,1977] have usuallybeen basedon completinga
oceanic plates and the continental (America) plate. Certainly, planevectortrianglebetweenJuande Fuca/Pacificspreading
assumedas parallel to the Blanco fracture zone and Pacithe proximity of the Juan de Fuca plate convergencezone to
the ridge (Figure 1) makessomeform of interactionextremely fic/Americamotion derivedfrom global solutionsor the San
have occurred
between 6 and 3 Ma.
likely. Examinations of the forcesdriving plate motions [e.g.,
Forsyth and Uyeda, 1975; Solomonet al., 1975] concludethat
the buoyancyof the subductedslab and the resistanceto subduction at the convergencezone are important componentsof
the forcescontrolling plate movement.The probable effecton
small plates closeto triple junctions was detailed by Menard
[1978] in the idea of "pivoting subduction."The local com-
Andreas-Gulf of California system.To the extent that the NE
Pacificregionis small,this approachgivessolutionswhichare
essentiallycorrect but which mask any geographicalvariations in interaction
rates.
Attemptsto establishpolesof relativerotation betweenthe
Farallon (later Juan de Fuca) and Pacificplatesusingspreading rate variationsand fracturezoneorientationsfor periods
before25 Ma [Franchetauet al., 1970; Handschurnacher,
1976]
• Now at Collegeof Oceanography,
OregonStateUniversity,Corvallis.
Copyright 1984 by the AmericanGeophysicalUnion.
Paper number 4B0686.
0148-0227/84/004B-0686505.00
6980
showedthat suchpoleslay in the BeringSea-CanadaBasin
region.One of the first attemptsto determinea contemporary
poleof motionfor the Juande Fucaplatewasthat of Morgan
[1972], whichwasinterpretedby Carlson[1976] as indicating
a pole of relativeJuan de Fuca/Pacificmotion to the southwest of the plate near 30øN, 140øW. By contrast, Carlson
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
•
Fuca
de
Plate.
-
I
I
I
I
I
I
I
N
I
Gorda
South
Plate
Mendocino
PACIFIC
-
PLATE
San
Francisco
Fig. 1. Locationmap of the Juande Fuca platesystem.
and Blanco fracture zone orientation [Riddihough, 1980] and
numerical inversion via magnetic anomaly superposition[Nishimuraand Hey, 1980; Nishimura et al., 1981] have located
119øW.
the present Juan de Fuca/Pacific pole near 15øS, 150øW, to
Independentdeterminationsusingspreadingrate variations the southwestof the plate. Hey and Wilson [1982] have more
[1976] usedfour earthquakefault plane solutions(two on the
Blanco fracture zone and two on the Medocino fracture zone)
to determine a pole to the northeast of the plate at 57øN,
6982
RIDDIHOUGH' MOVEMENTS OF THE JUAN DE FUCA PLATE
recently calculated a sequenceof pole positions to form the
basisfor their study of propagating rifts (seealso Nishimuraet
al., [1984]). This is further discussedbelow.
epochpole of Juan de Fuca/Pacificplate motion could be
simple manner (with orthogonal and symmetric spreading),
then spreadingrates along a ridge systemwould decreasesystematically toward the pole of rotation, fracture zones would
always occur along small circlescenteredon the pole of rotation, and ridge segmentsand linear magneticanomalieswould
alwaysfollow great circlespassingthrough the pole. Further,
the senseand differencesin azimuth of equivalent anomalies
on either side of a ridge system(fanning) would indicate the
direction and amount of cumulative angular rotation between
the two platesinvolved(Figure 2, left). In practice,divergences
from such ideal behavior are widespread. Nevertheless, the
geometrical concepts have formed the foundation of many
plate tectonic analysesand often offer the only route to the
determination of individual plate motions. Over the Juan de
Fuca system,accuratecontouredmaps of magneticanomalies
with predominantly north-south orientation in a high magnetic latitude provide an opportunity to attempt to use such
conceptsin detail.
Although transformfracture zonescommonly representone
of the most important constraintsin determining plate motions, the Sovancoand Blanco fracture zones(F.Z.) (Figure 1)
presenta number of problems.The SovancoF.Z. is not clearly
definedby bathymetry,magneticanomalies,or seismicity[e.g.,
Milne et al., 1978]. The Blanco F.Z. may have had an extremely complex history [Hey and Wilson, 1982] and apparently contains a number of different segmentsof varying
orientation including "pull-apart" basins(R. W. Embley, personal communication, 1984). For the purposesof examining
the recenthistory of the Juan de Fuca system,neither fracture
zone providesa clear record of past spreadingdirections.
For this study, all data were determined from magnetic
anomaly orientation and spacing.Anomaly ageswere assigned
usingthe work by Ness et al. [1980], and individual anomalies
or sequencesdisturbed by transforms or known "pseudofaults" were discounted.Anomalies were grouped into epochs
of 1 million years duration, and measurementswere made as
in Figure 2 (right).
For Juan de Fuca relative to Pacific (JDF/PAC) plate
spreading,it was assumedthat the great circle containing the
limit on mean azimuth within all epochs except 4.5 Ma
determined from the azimuths of anomalies on the Pacific
plate.(Thoseon the Juande Fuca platewill havebeencumulativelyrotated.)This methodassumes
orthogonalspreading;
Method: Magnetic Anomaly Analysis
however,any randomdeviationsfrom this conditionshould
by sampling.In practice,the 95% confidence
If plate tectonicswere always to occur in a geometrically be compensated
Spreading
i!
rate
decrease
(-t-3.1ø)was -t-2øor less.Nevertheless,
an arbitraryconfidence
limit of _+5ø was assignedto this determination.Clearly,this
methoddoesnot allow for systematicobliquespreadingalong
the wholeridgefor periodsof 1 million yearsor greater.
Spreadingratesweremeasuredperpendicular
to anomalies
as in Figure 2 (right) and projectedonto the Pacific plate
anomaliesrepresenting
the epochgreatcircle(ridge)in swaths
of approximately1ø of colatitude.An exampleof the results
for epoch3.5 Ma is shownin Figure 3. Plate tectonicgeometry requiresthat ideallythesespreadingrateswill conformto
a sine function of colatitude.A simple test showsthat for all
pointson sucha curvethe linearslopeovera distanceof less
than 7ø will differ from the slope of the sine function at the
meancenterpoint by lessthan 0.5%. For the presentanalysis,
therefore,the slopewasdeterminedby linear regression,
95%
confidencelimits beingassignedby the standardmethodsfor
small samples.With the slopeand spreadingrate at the mean
point (with confidence
limits)known,the colatitudeand rotation rate of the pole can be uniquelydetermined.From this
and from the greatcirclethe polecan be calculated.
Instantaneousepochpolesdeterminedin this manner,relative to the Pacificplate,are shownin Figure4. The resulting
"sector-shaped"
95% confidencelimits are a consequence
of
the independentdeterminationof colatitudeand azimuth.
Juan de Fuca/PacificPoles
Figure 5 comparesthe instantaneous
polesderivedin this
paperwith thoseof Hey and Wilson[1982] calculated
from
the finite rotationsnecessary
to rotate isochronson the Juan
de Fuca plateonto the corresponding
isochronson the Pacific
plate.Considering
the uncertainties
in the techniques,
the resultscomparequite closely.One significantdepartureis in
timing. Hey and Wilson [1982] considerthat there was a
"drastic shift" in Juan de Fuca/Pacific motion betweenthe
times of anomalies3a and 3 (around 5 Ma). In fact, detailed
studiesof anomaly azimuths [Riddihough,1977, Figure 3;
Carlson, 1981a, Figure 5] show that while there may be a
slightincreasein the rate of clockwiseanomalyrotationat 5
Strike
difference
gives
to pole
angular
W angular
rotation
r rotation
z'
A
B
,,
i
I
t
to pole
•
,
topole
x+x'>¾+¾'>z+z'
towards
pole
( B relative to A )
Fig. 2. Pole determination parametersfrom magnetic anomalies used in this study. (Left) Theoretical geometry of
spreading.(Right) Applicationto observedmagneticanomalies.
RIDDIHOUGH.' MOVEMENTS OF THE JUAN DE FUCA PLATE
'--.].
6983
[1982] reconstructions
assumethat spreadingremainsoblique
aftera polechange(anomaliesare thuscreatedparallelto the
old ridge)until a propagatorpassesthroughand reorientsthe
ridge system.A difficultywith this is that, as noted earlier,
anomaly azimuthsof the same age along the Juan de Fuca
ridge are remarkablyconsistent.Further, the most striking
contemporaryridgepropagationin the region(the Cobb propagator)separates
ridgesectionswhichdifferin orientationby
Epoch
3.5Ma
60-
no more than 1ø or 2 ø.
5o,1
i
Studiesof detailedridge structure[e.g., Macdonald,1982]
i
i
COl'
ATITU'DE
ø
i
i
Fig. 3. Spreadingratesalong the Juan de Fuca ridgeat 3.5 Ma.
Colatitudeis measuredin degrees(111 km) along strike of magnetic
anomalieson the Pacific plate; error bars are 95% confidenceintervals on rates and the mean (open circle and bar); dashedlines are
95ø/,,confidencelimits on the slope.
Ma, the major change is between 3 and 4 Ma (Figure 6). At
this time, most "fanning"ceases.Nishirnuraet al. [1984] considered that the major change occurred between 2.4 and 5.4
Ma with the instantaneouspole remaining in its presentposition since 2.4 Ma.
The key to the timing discrepancymay lie in the argument
that when plate motions change,it may take a finite amount
of time for the centralrift creatingthe magneticanomaliesto
adjustto the new directionof spreading.Hey and Wilson's
o•
/
indicate that the "crustal accretion" and "neovolcanic" zone
wherespreadingactuallyoccursmay be lessthan 2 km wide,
althoughoff-axisvolcanismdoesexistand the activefaulting
zonemay existto +__
10 km of the axis.The speedwith whicha
ridge can changeorientation without propagationand the
extent to which preexistingstructure constrainschangesare
unclear.The fact that propagatorsbreak into old crust in an
apparentlycontinuousmanner [Hey and Wilson,1982] suggeststhat the constraintsmay not be strong.For the present
studies,therefore, it is assumedthat anomaly orientations
averagedover 1-million-yeartime periodsare perpendicular
to plate motionsduringthe sameperiodand not thoseof a
significantlyearlier time.
Significantfeaturesof the pole positonsin Figure 4 are the
eastwarddrift up to 4.5 Ma and then the rapid increasein
distanceaway from the plate. This movementresultsin the
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 instantaneous
Juan de Fuca/Pacific(JDF/PAC),
Juan de Fuca/America(JDF/AM), and Juan de Fuca/hot spot
Fig. 4. Juande Fuca/Pacific
polesdetermined
from this study. (JDF/HS) polesderivedin this paper.Numbersare epochdatesin
together
Numbersare epochdatesin millionyearsbeforepresent(Ma). The Ma. Polesderivedby otherauthorsare shownfor reference,
Pacificpolesfrom Minster and Jordan[1978].
95% confidence
limits are keyedto eachpole positionby symbols. with contemporary
For derivation, see text.
Coastline is as at present.
6984
R•VV•HOUGH:MOVEMENTS
OF THEJUANDE FUCA PLATE
.øi
EXPLORER
20 ø
-
10 ø
-
ß '
' ' '
20ø'JUAN
DE
10
I/•"•o--• ß•
uc^,.
ooo o•_.o/•
0ø1 '
'
o.
I
'
'
I
30 øO
o
20 ø
_
10 ø-
ß
o
oo
GORDA
R.
(N)•dl
mated that it came into existencearound 7 Ma (seealso Riddihough[1977]). However, determination of the pole of rotation
sequencepresented here shows that this may not be geometrically necessaryand that the 3-4 Ma initiation is to be
preferred.
A sequenceof mean Explorer/Pacific (EX/PAC) poles from
3.5 Ma to the present is shown in Figure 7 and listed in Table
1. These were calculated using spreadingrates and anomaly
azimuths for the Explorer ridge [Riddihough,1977] combined
with the average Explorer/Pacific rotation rate determined
from the fanning shown in Figure 6. As there is insufficient
Explorer ridge length to observethe decreaseof spreadingrate
toward the pole, the latter providesthe necessarythird parameter to determine the pole.
This processintroduces a number of errors, not the least
being the assumption of a constant rotation rate. Figure 8
shows the EX/PAC pole error space for epoch 0.5 Ma estimated using + 5ø great circle error and 95% confidencelimits
on the rotation rate from the fanningregression.
Gorda/Pacific Poles
Riddihough [1980] confirmed that the South Gorda ridge
beganto spread at a significantlydifferent rate from the North
Gorda ridge betweeen2 and 3 Ma. Spreadingon the North
lO
5
o
Gorda ridge can be fitted to the same pole of rotation as the
MaBP
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. RiddiFig. 6. Magnetic anomaly azimuthsplotted againstage for three hough [1980] showed that significant anomaly fanning besectionsof the Juan de Fuca ridge system.Open circlesare azimuths tween Gorda South and the Pacific plate stoppedat about 1
on the platesto the east of the ridge; solid circlesare from the Pacific
Ma. Thus the pole must have becomemuch more distant for
plate.
epoch0.5 Ma.
The question of whether or not the anomaly distortion of
the Gorda plate is produced by fanning or someform of internearestemergentpole 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 sinceapproximately2 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 (calculatedas for the Expole as definitely to the north; limits for the 0.5-Ma pole plorer ridge) are presentedas the geometricalconsequenceof
constrainit to the southof 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 seenin Figure 6 to occur for all three
major sectionsof the ridge system(Gorda, Juan de Fuca, and
Explorer). However, for the Explorer ridge it involves an
almost 10øjump in azimuth followedby continuingrotation,
not a cessation.It appearsthat at this time the Explorer plate
becameindependentand no longer pivoted around the same
pole relative to the Pacific plate as the rest of the Juan de
Fuca system.This is supportedby the followingfacts:
1. All anomalieson the easternsideof the Explorer ridge
are "fanned"clockwisein relation to thoseon the west,indicating a nearby pole to the southwestwith a rapid angular
Calculation of instantaneousabsolute or "hot spot" poles
for the Juan de Fuca systeminvolvesreconstructingthe posi-
tion of the instantaneousJDF/PAC pole at the epoch concerned relative to the hot spot framework and solving the
vectoraddition with the Pacific/hotspot (PAC/HS) poles.A
numberof PAC/HS poleshave beencalculatedusinghot spot
traceswithin the Pacific plate [e.g., Minster and Jordan, 1978;
Minster et al., 1974; Chase, 1978; Turner et al., 1980; Duncan,
1981; McDougall and Duncan,1980]. In comparingthe orientation of seamount chains and features within the immediate
rotation.
region (in particular the Cobb, Scott, and Dellwood seamount
systems; Figure 1), model AM1-2 of Minster and Jordan
2. The Explorer ridge spreadingrate for epoch 3.5 Ma is
much larger than that of the northernJuan de Fuca ridge.
[1978] seemsto producethe closestfit. This is the pole usedin
all absolutemotion calculationsin this paper.The resulting
This is inconsistentwith a singlepole to the north.
3. The Pacificplate anomalyazimuth changesof Figure 6
mean absolutepolescalculatedin this manner for the Juan de
Fuca, Explorer, and Gorda South systemsare listedin Table 1
and are shownin Figures5 and 7.
couldonly be accommodated
within a singleplatesystemby a
suddenwestwardExplorer ridgejump followedby continuing
westward migration with a rotation pole to the north. This is
inconsistentwith point 2.
Hyndrnan et al. [1979] named the transform between the
Explorer and Juan de Fuca platesthe Nootka Fault and esti-
The main featuresof the Juan de Fuca/hot spot (JDF/HS)
polesare that they occurcloseto the Juan de Fuca plate to
the northwest until 4.5 Ma, then shift to the southeast,and
progressivelymigrate northwestwardtoward the plate. The
Explorer/hot spot (EX/HS) and Gorda/hot spot (GO/HS)
RIDDIHOUGH' MOVEMENTSOF THE JUAN DE FUCA PLATE
2.5
...........
6985
5
EX/HS
3.5
2.5
EX/AM
1.5
2.5
EX/PAC
-4oo-•_ l•e3'5
J--
GS/
2.5
•
b
Fig. 7. Mean instantaneous
polesfor the Explorerand Gorda Southplates,PAC, Pacific;HS, hot spotframework;
AM, Americaplate framework;EX, Explorer;GS, Gorda South' JF, Juan de Fuca. Numbersare epochdatesin Ma;
directionsof plate rotation are shownby arrows.Coastlineis as at present.
polesmay occurextremelycloseto or within the Explorerand
Gorda platesand consistentlyinvolveclockwiserotation.
(JDF/AM) pole at 29.11øN, 112.72øWat 1.05ø/Ma; this comparescloselywith the 0.5-Ma pole shownin Table 1.)
Poles of Motion Relative to the America Plate
ConfidenceLimitsfor Pole Determinations
Calculation of mean poles relative to the American plate
can be carriedout through reconstructionand vector addition
usinga Pacific/Americapole. A numberof authorshave calculated such a pole for the present, but there remains some
uncertaintyas to its applicabilityfor periodsolder than 5 Ma
[e.g., Molnar and Atwater, 1978; En•tebretson,
1982]. This uncertainty is of undoubted importancein assessing
motions of
the Juan de Fuca plate relative to the America plate. However,
for consistencythe poles listed in Table 1 (shownin Figures 5
and 7) have been calculatedassumingmodel RM2 of Minster
and Jordan [1978] back to 6.5 Ma. (Using RM2, Nishirnuraet
al. [1984] recentlycalculateda presentJuan de Fuca/America
and Relative
Motions
The asymmetric,"sector-shaped"confidencelimits for instantaneousJDF/PAC and EX/PAC polesshownin Figures4
and 8 result from the calculation
methods. To estimate confi-
dence limits for JDF/AM, JDF/HS, Explorer/America
(EX/AM), and EX/HS poles, the bounding and mean pole
positions(and rotation rates)were convolvedwith the quoted
mean polesand 2a limits on latitude, longitude,and rotation
rate for RM1 and AM1-2. Ninety-five percent confidence
limits on the resultingmatricesof polesare shownin Figures
8 and 9 for epoch0.5 Ma. (It shouldbe noted from Figure 4
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
Epoch,
Ma
Relative to
Relative to
Relative to
Pacific Plate*
Hot Spot Framework•
America•
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
3.5
65.7
74.9
118.3
80.0
+ 1.87
+0.96
58.8
- 19.8
150.2
82.6
+0.98
-0.22
60.6
42.1
162.5
244.9
+ 1.19
+0.43
2.5
73.6
+0.71
21.0
1Q6.1
-0.47
1.5
0.5
- 11.7
-9.9
145.3
146.1
12.3
-0.61
-0.62
38.5
39.2
117.7
119.4
- 1.16
- 1.18
-4.7
28.4
29.4
110.2
111.7
92.9
-0.50
- 1.07
- 1.09
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
0.5
43.5
44.2
136.3
135.7
- 3.1
- 3.1
49.0
50.0
127.3
127.4
- 3.9
- 3.9
46.7
47.8
124.7
124.5
- 3.8
- 3.8
2.5
1.5
0.5
38.1
39.0
30.0
127.8
128.0
129.2
- 13.5
- 13.2
- 1.1
- 14.5
- 14.0
- 1.9
39.3
40.2
41.2
125.4
125.5
108.0
+ 14.2
- 14.0
- 1.7
Explorer
Gorda Southe:
40.0
40.9
46.9
126.1
126.2
114.5
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.
•'CalculatedusingPacificplate motion as determinedby RM1 and AM1-2 of Minster and Jordan
[1978].
:[Polesassumingno internaldeformationof Gorda Southplate(seetext).
of the set being considered.)Thesesimplifiedconfidencelimits
tend to be slightly smaller than the conventional elliptical
limits [e.g., Minster and Jordan, 1978] and are also nonsymmetricabout the third pole determinedby the vectoraddition of the two mean poles (e.g., JDF/PAC and Pacific/America (PAC/AM)). In terms of the significanceto be
givento the resulting
motionsof the Juande Fucaplate
system,they neverthelessprovide at least a minimum assessment of the probable errors. For Juan de Fuca plate motions
at 0.5 Ma, they suggest
errorsof _+7 mm a-• and __+
7ø relative to America and + 10 mm a-•
and + 20 ø relative to the
hot spot framework for a point in the center of the plate.
Explorer plate motionsare lesscertain,having probableerrors
for 0.5 Ma of __+
15 mm a-• and _+12ørelativeto Americaand
_+15 mm a-• and +__
100ø (for a point near BrooksPeninsula
that is within the confidencelimits of the absolutepole position (Figures1 and 8)). By comparison,similarcalculationsfor
the errorsof JDF/AM motionat 3.5 Ma give _+13 mm a-•
and _+17ø.
ABSOLUTE MOTIONS OF TI-• JUAN DE FUCA SYSTEM
Figure 10 presentsa sequentialpictureof the absolute(relative to hot spots)plate motions of the Juan de Fuca plate
systemsince7 Ma. The motionsare shownon a geometryof
the plate systemrelativeto the presentcoastlineand usethe
mean calculatedpolesof Table 1 subjectto the uncertainties
discussed above. This reconstruction
does not include the ac-
cretinghistory of the Blanco F.Z. by accumulatingpropagating offsetsas proposedby Hey and Wilson [1982]. For the
understandingof the motions of the rigid plate systemover
the mantle and relativeto the margin,this may not be critical.
Americaabsoluteplate motion is as givenby AM1-2.
For the periodfrom 7 to 4 Ma, motion of the plate is to the
northeast with the slowest motion off Vancouver
Island. The
occurrenceof the slowestrate at the northern end is in agree-
ment with the conceptof Menard [1978] that the youngest
material(wherethe ridgeis closestto the subduction
zone)
providesthe greatestresistance
to subduction.
In effect,the
platetendsto pivot about sucha point.
Between 4 and 3 Ma there is a dramatic change. The
youngestnorthernportionof the platethat wasprovidingthe
maximum resistancebreaks off and beginsto move independentlyas the Explorerplate.(The approachof the easternend
of the SovancoF.Z. to the margin may have contributedto
this.)It immediatelybeginsto moveabouta verylocalpoleso
that its motion into the convergence
zone is severelyreduced.
Even with the probableerror limits of pole positions,by 2.5
Ma it may be rotating about a point within itselfsuchthat in
a translational senseit is no longer moving systematically
toward the former subductionzone. This type of "spinning"
motion continuesto 0.5 Ma. The pole uncertaintylimits includethe possibilitythat in the hot spotframeworkit couldbe
moving away from its subductionzone with the America
plate.
At the time that the Explorer plate becomesindependent,
the Juan de Fuca plate becomesnotably smaller, and the
point of maximumresistance
to movementprovidedby the
youngest
materialenteringthe subduction
zoneshiftsto somewherein the Cape Blanco area. By epoch2.5 Ma, the mean
absolutevelocityof the plate has droppedto 40% of its previous value, and the slowestmotion has shiftedto the southeastin conformitywith Menard's[1978] concept.The velocity
of the platecontinuesto reduceuntil epoch0.5 Ma. The absolute velocitydeterminedfrom the meanpole now variesfrom
20 mm a-• near VancouverIslandto 10 mm a-• near Cape
Blanco,the pole of absoluterotation beingin northernCalifornia. However, considering the uncertaintiesdiscussed
above,it is now possiblethat the velocityof the southernpart
of the platerelativeto the mantleis closeto zero.
A separatepole of motion for the southernGorda plate is
RIDDIHOUGH:
MOVœM•NXS OF •
JUAN DE FUCA PLATE
6987
,EX/HS
50ø
.•::..
:'••ii/i:•11:•!iii•i•i•::
40 ø
30....
•i•!ii::::•
:• o0•
b
b
Fig. 8. The 95% latitude and longitudeconfidencelimits for epoch-0.5-Mapole determinationsfor the Explorer plate.
Circlesare pole positionsderivedfrom vectoraddition of mean EX/PAC and PAC/AM or PAC/HS poles,not the mean
of the covariance of confidence limits.
probably significantat leastfrom 3 Ma. As with the Explorer
plate, the resultantmotion is clockwiserotation arounda pole
closeto or within the plate. A similar situation develops,that
of little or no motion toward the Gorda/America convergence
zone, perhapseven motion away from the convergencezone.
The changein the Gorda South pole for epoch0.5 Ma retains
docino. As with the absolute motions, the slowest movement is
ing with the America plate. Over the succeeding2 million
years,motions remain largely the same.
Even taking into account the large uncertainties,the independenceof the Explorer plate by 3.5 Ma probably resultsin a
sharp reduction in convergencerates along Vancouver Island
(Figure 12). In relation to the America plate, the Explorer
plate again rotatesclockwisearound a local pole probably to
the southeast(see Figure 7), which appears to be migrating
northward and becomingcloserto 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 motions. The convergencerate probably declinesfrom its 4.5-Ma
value at all points along the convergencezone (Figure 12). By
epoch 2.5 Ma, the JDF/AM pole has shifted to the southeast
of the system,so that the slowestconvergenceis in the Cape
Blanco area where the youngestmaterial is entering the subduction zone. Convergenceprobably continues to decline to
the present,and the pole movestoward the plate.
If, as before,the southernpart of the Gorda plate is treated
as rigid, this plate apparently rotates clockwisearound a local
pole close to or within the plate through epochs2.5 and 1.5
Ma. For epoch 0.5 Ma, convergencemay cease,and the plate
off Vancouver Island where the youngestmaterial is converg-
maymoveto the northwestat approximately
40 mm a- •.
the clockwise
motion
but could result in movement
that is
significantlyaway from the subductionzone and the Mendocino escarpment.
MOTIONS OF THE JUAN DE FUCA SYSTEM
RELATIVE TO AMERICA
Figure 11 presentsa sequentialpicture of the motions of the
Juan de Fuca plate systemrelativeto the America plate on the
same plate layout as Figure 10. Again, these are the motions
given by the mean poles of Table 1 and subjectto considerable uncertainties. At 6.5 Ma, relative motion is to the north-
eastat meanspeedsvaryingfrom 60 mm a-• alongthe VancouverIsland margin to 70 mm a-• south of Cape Men-
6988
RIDDIHOUGH:
MOVEMENTS
OFTHEJUANDEFUCAPLATE
Fig. 9. The95% latitudeandlongitude
confidence
limitsfor epoch-0.5-Ma
poledeterminations
for theJuande Fuca
plate.Dashed
ovalis95%confidence
interval
forJDF/PACpolefromNishimura
etal.[ 1984].
DISCUSSIONOF PLATE MOTIONS
that therewasa strikingcorrelation
betweenvelocityandthe
In summary,
theplatemotionsof theJuande Fucasystem length of trenchwith subductedslab. Chappieand Tullis
thatthemajordrivingforcesareassocias shownby the mean rotationpolesboth in an absolute [1977] alsoconcluded
a pulldueto thedowngoing
slabacting
(relativeto hot spots)and in a relative(to America)sense atedwithsubduction,
show similar characteristics:
on both upper and lower plates,and a resistanceto conver-
gence.Recently,Carlson[1981b] and Carlsonet al. [1983]
the balanceof forcesusingmultiplelinearregression
2. Themotionpatternis suchthattheyoungest
converg- assessed
ing materialmovesslowest;this patternis reestablished
for and concludedagain that slab pull is the dominantforcein
1. Therateof motiondeclines
beginning
at 4 Ma.
theJuande Fucaplateafter4 Ma whentheExplorer
plate determiningplate velocities.
The data assembled
in Figure 10 can be usedto calculate
velocities
of theJuande Fucaplaterela3. After 4 Ma, all closestrotationpoleslie to the south themeanintegrated
tive to the hot spot framework since7 Ma. Thesevelocities
andprogressively
movetowardtheplatesconcerned.
becomesindependent.
Theerrorsof polepositions
androtationratesareprobably decreasefrom around 45 mm a-• at 6.5 Ma to 17 mm a-• at
suchasto reducethesignificance
of characteristic
1 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-10mma-x for thepresent,
thissuggests
that the velocityhaseitherremained
similaror decreased
by
up to 50% duringthisperiod.Usingthegeometry
of theplate
dence limit.
Driving Forces
The enigmaof platedrivingforceshasbeenexamined,both
systemshownin Figures10and 11,the total areaof theplate
hasdecreased
by approximately
50%duringthesameperiod.
Theeffective
ridgelength(normalized
by totalperimeter)
has
theoretically
and by the empiricalapproachof comparing decreasedby approximately25%, and the effectivetrench
platespeeds
withparameters
suchasarea,ridgelength,trench lengthhas remainedalmostidentical.In qualitativeterms,
length,etc. Forsythand Uyeda [1975] concludedthat there therefore,
the resultsmay be compatible
with otherglobal
wasno obviousrelationbetweenplatevelocityand area but
studies.
,
RIDDIHOUGH: MOVEMENTS OF THE JUAN DE FUCA PLATE
6989
6990
RIDDIHOUGH' MOVEMENTSOF Wl-IEJUAN DE FUCA PLATE
RIDDIHOUGH'MOVEMENTS
OF THE JUANDE FUCA PLATE
at 50øN 128øW
E
i
!
I
'
I
'
I
I
'
I
'
I
'
I
at 46' N 125'W
Ma
+
Ma
ß total
o orthogonal
Fig. 12. Interaction rates (in millimeters per year) at two points
near the continentalmargin versustime. Solid circles,movementrelative to America; open circles, convergenceperpendicular to the
margin. Error bars are estimated and may be minimum for older
epochs;seetext for explanation.
Nevertheless,it is clear that many of the quantitative relations determined from global studiesdo not fit the Juan de
Fuca plate. In particular, the model IV regressionof Carlson
[1981b] predictsthat the mean velocity of the Juan de Fuca
platewouldbe 98 mm a-• at 6.5 Ma and 94 mm a-• at 0.5
Ma. Figure 8 of Forsyth and Uyeda [1975] also suggeststhat
to fit in with the global plate observations,the Juan de Fuca
plate, with a presenteffectivetrench length of 40%, should
havea velocityof 100mm a- • or greater.Usinga plateageat
the margin of 10 Ma, the relation derived by Carlson et al.
[1983] predicts an absolute velocity for the Juan de Fuca
6991
declinedto near zero suggeststhat it is its motion into the
mantle which providedthe predominantresistance.
The absenceof any fault plane solutionsshowing underthrusting[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 Weichert, 1983], and the steady "back tilting" of coastal areas
[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-termprocesses,
they do indicatethat at least
on a short time scale, conditions of low resistanceto Juan de
Fuca/America convergencecan exist.
The fact that once the Explorer plate is detached,the remainder of the Juan de Fuca plate adjustsits motion so that
its southern (youngest)part moves slowestis an apparently
strongconfirmationof the pivotingsubductionconcept.However,a major dilemmais posedby the data of Figure 10: If the
Explorer plate was providingthe maximum resistanceto Juan
de Fuca motion, why doesthe mean velocityof the remainder
of the Juan de Fuca plate decreaseand not increasewhen the
Explorer plate becomesdetached?One solutionto this problem may be that the resulting decreasein plate size is the
dominant factor in determiningplate speed.Another solution
may involve other componentsof plate motion.
Subductio
n, Overriding,and "Rollback" Velocities
Dewey [1980] and Uyeda and Kanamori [1979] rationalized
the movementsof plates in a convergencezone into three
components.Following Dewey's notation, these components
are
plateof 25-35 mm a-•. This is closeto the absolutevelocities
calculatedfrom Figure 10. However, plate reconstructionsas
in Figure 10 (see also Engebretson,[1982]) suggestthat the
ridge has stayed approximately the same distance offshore
since10 Ma. The ageof theplateenteringthetrenchhasthus
remainedsimilar.If the mean speedhasdecreasedmarkedly in
thisperiod,ageseemsunlikelyto be a significantfactor.
Quantitatively, it would seem that the balance of forces
controllingJuan de Fuca plate motions may not be the same
balanceas for other, larger plates.As plates becomeas small
as the Juan de Fuca, the simplelinear relationsbetweenforces
suchas those proposedby Carlsonet al. [1983] may change
or ceaseto apply.
Vo absolutevelocityof the overridingplate;
V, absolutevelocityof the underlyingplate (its rate of insertion into the asthenosphere);
V, rate of rollback at which a trench movesoceanwarddue
to the rate of "fall"of the subducting
plate,V0, into the
asthenosphere.(This is distinct from what might be
termed
the "forced"
rollback
due to the advance
of an
overlyingplate.)
Both Dewey [1980] and Uyeda and Kanamori [1979] argue
that the tectonicsof a convergencezone are highly dependent
upon the relativevaluesof V, and Voand are much lessdependent upon V,.
PivotingSubduction
Explorerplate. The rapid declineof Explorer plate velociThe processsummarizedin 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 strikingexampleof 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 subductionzoneis apparentlyan importantfactorin Vancouver Island, it is not coherentlymoving down into the
determiningplate motion in this region. Ironically, because mantle. It seemsreasonableto 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 immediatelyobviouswhether 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 overlyingcontinental plate. becamedetached,the sum of forcesdriving the Explorer plate
The latter may provide frictional resistanceto convergence; changedradically, and it could no longer continue any downtheformermayprovidebuoyancy
resistance
asthematerialof 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
e buoyancyof subducted
ment of the Explorer plate reflecteda balance point between drivingplate motion is the negativ
its resistanceto motion and the strengthof its attachmentto slabs,then it seemslikely that, although independent,both V,
the remainderof the Juan de Fuca plate, then the fact that its and V• will approachzero if this buoyancyceasesto be negaabsoluterather than relative translational movement rapidly tive. In the caseof 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
thus also be near zero.
In tectonic and structural terms, the result of this situation
could be as in Figure 13. The Explorer plate, havingceasedto
descendinto the mantle under its own body forces, is still
convergingwith the America plate and has been overridden
by up to 70 km in a southwesterlydirection.It may thus be
effectively underplating the northern part of Vancouver
Island. Such a process would, presumably, involve an increasedstresscouplingwith the overlyingAmericaplate and a
contrastin recentvertical movementhistory.Theseconditions
plus the complexityof the interactionwith the America plate
producedby the local pole of relative movementwould seem
to ensurethat there should be a strong contrastbetweenthe
tectonic processesaffecting northern Vancouver Island and
those affecting southern Vancouver Island and the Washington and Oregon margins.This seemsto be true of present
seismicity[Milne et al., 1978; Rogers,1979], in particular in
the zone where the subductedNootka Fault [Hyndman et al.,
1979] marks the boundary betweenthe two regimes.Rogers
[1983] has also pointed out that the vertical movement pattern [Riddihough,1982a] and topographyof the northern part
A seriesof cross sectionsderived from seismicity,seismic
refraction,and gravity modeling were constructedby Riddihough [1979] (see also Ellis et al. [1983] and Michaelson
[1983]). All showed an increase in dip or "bend" in the
downgoingslab beneath the Georgia Strait and the Puget
Sound.One of theseis shown in Figure 14 (top). Figure 14
(bottom three panels)suggestshow sucha bend could be pro-
ducedby an increase
in V•and V•relativeto Vo.Previous
to 4
Ma, the relation between Vo(America absolutevelocity, ~ 23
mm a-•) and V• may have beensuchthat the marginwas
"compressional"
[Dewey,1980].At 4 Ma an increase
in V•and
V• producedby the detachmentof the Explorer plate could
causethe bend in the plate at the trench to begin to "fall."
This increasein V• might changethe characterizationof the
margin at this time to somethingcloser to that defined by
Dewey [1980] as "neutral."
The aseismicnature of the Juan de Fuca plate convergence
zone and the accretionarynature of the Washington-Oregon
margin [e.g., Silver, 1972] seemto be in accordwith a neutral
concept [Dewey, 1980]. The initiation of high Cascadevolcanism [e.g., McBirney, 1978] in the late Pliocenecould also
be a responseto a changein asthenospheric
flow betweenthe
of Vancouver Island are distinct from the areas to the south.
downgoingand overlyingslabsin a similar way to that sugJuan de Fuca plate. The dilemma of the apparently con- gestedby Barazangiand lsacks [1976] for the Nazca plate.
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 producednear 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.
However,this explanationdoesnot accountfor the fact that
to decline after 4 Ma. However, if the Explorer plate was
providingbuoyancyresistanceto descentinto the mantle,then globally,most subductingslabshave a "knee bend" at a depth
its removalmayhaveresulted
in an increase
in V•andthusV•, of between30 and 50 km [Pennington,1983]. The association
the trench rollback velocity.Suchan increasewould not show of the bend in the Juan de Fuca plate with seismicitydue to
in Figure 10 but might be detectablein a verticalcrosssection phasechanges[Rogers,1983] seemsto supportthe conceptof
Pennington[1983] that the bend is a responseto the increase
throughthe Juan de Fuca convergence
zone.
OVERRIDING
BUT
NO 'SUBDUCTION'
OVERRIDING
PLUS
'SUBDUCTION'
EXPLORER
PLATE
AMERICA
PLATE
JUAN DE
FUCA PLATE
Fig. 13. Block diagram showingproposedplate configurationand contrastbeneathVancouver Island. "Subduction"is
usedhereasmovementof the underlyingplate downwardinto the asthenosphere.
RIDDIHOUGH'MOW•mNTSOFTim JUANDE FUCAPLATE
Vancouver
The reasonfor thetimingof theindependence
of thispart of
the Gorda plate at 3 Ma is not clear.It wasnot the youngest
portionof the subducting
Juande Fuca plate at that time,
althoughthe recentdetachmentof the Explorerplate may
have precipitateda redistributionof stresssuch that this
cornerof the systembetweenthe marginand the Mendocino
F.Z. becameparticularlyvulnerable.Beingsmall,onceit had
broken away, it would presumablyhave little or no self-
I.
/'•, Georgia
/ •S/•
2.3 Tøfinø
Basin
6993
Coast
. ßMtns.
--•__••_
_ 2.92 •0km
3.295.•
,
6 Ma
•
,
compressional
•
t100
,
;200
-•-•Vr lessthan ;
drivingforce.It couldbe arguedthat the clockwise
motion
•Vo
JF
from 3 to 1 Ma was determinedeither by northwesterlyPacific absolutemotion or by the northwesterlyright-lateralshear
appliedby Pacific/America
relativemotion.The increasein
absolutemotionby 0.5 Ma and the rapiddecrease
in spreading on the southernGorda ridgesuggest
that by this time at
least,its motion was controlledby the northwesterlymoving
Pacificplate. Given the lengthof the MendocinoF.Z., this
seemsreasonable
and may be part of the processof northward
I
I
4 Ma
'
I
'
Riddihough[ 1980].
change
,Vr increases •
.
migrationof the Mendocinotriplejunctionas suggested
by
;Vo
SUMMARY COMMENTS
Althoughthere are undeniableuncertainties
in the poles
JF
and rates of rotation determined for the Juan de Fuca plate
'
0 Ma
'
I
I
'
I
neutral
•-•Vr equals•
systemby the methodsusedhere,the resultscomparewell
with otherapproaches
to the problemand producea coherent
patternof platemovements.
This patternis valuablein that it
may show in detail how the movementsof a small plate
systemare affectedby a numberof factors,notablyby resistanceat the convergence
zone.It affirmsthat at leastfor small
plates,
Menard's
!-1978]
concept
of pivoting•subduction
is
iVø
valid. The behavior of the most resistive,Explorer plate por-
tion of the systemin eventuallybecomingdetachedand ceas-
ingto movedownintothemantle
isphysically
re'asonable
and
bend
0 km
.
,atVg
. ,
200
400
suggests
that thedominantresistance
to motion is in thiscase
providedby theasthenosphere.
Thebehaviorof theremainder
of the Juande Fuca plateafterthe Explorerplatedetachment
600
Fig. 14. (Top panel)CrosssectionacrosssouthernVancouver
suggests
that verticalandhorizontalplatemovements
relative
to the mantle could be independent.
Recentwork on the Winona Basin [Davis and Riddihough,
Island[fromRiddihough,
1979].(Bottomthreepanels)
Sequential
mechanism
for the possible
production
of an observed
bendin the 1982] and continuingdebateon the southernpart of the
downgoing
slab.Sections
arefixedin relationto themantle'America Gorda platetendto confirmthat closeto the triplejunctions,
(AM) moveswestward
at absolute
velocityVo.At 6 Ma the trench small fragmentsof platesbecomedominatedby the moverollbackvelocityV• is lessthan Vo, and compressional
convergence mentsof the larger,boundingplates.'
In the Juande Fuca
occurs.
At 4 Ma, V•increases.
Thisresults
in anincrease
in V•such systemit seemsthat an almostcontinuoussuccession
from
thatit equalsVoanda steepening
of thedowngoing
slab.Thetectonic
larger,
self-driven
plates
to
small,
interplate
fragments
can
be
styleisnowneutral[Dewey,1980],anda bendisproduced.
seen.The fact that the absolutepoleof motionfor the Juande
in negativebuoyancyproducedby the phasechanges.
Its locationin the Juan de Fuca plate at a point which could infer
changesin convergentstyleat around4 Ma may thus be a
coincidence.
Gorda South
As stressedat the outset,motions for the southernpart of
the Gorda platewhichapparentlymovedindependently
from
3 Ma [Riddihough,
1980]havebeencalculatedassumingrigid
platetectonics.
The possibility
of nonrigid,or at leastimbri-
Fuca plate itselfmay now be almostwithin the'subducted
portionof thisplate(Figure5) suggests
that it, too,is rapidly
slowingand will soonbe controlledby its largerneighbors.
While it is a verysmallpart of the globalplatesystem,it may
thusprovidea graphicexampleof the end points•ofthe plate
tectonicprocess.
Acknowledgments.
The work presentedin this paper benefited
considerably
from discussions
with C. Nishimuraand D. S. Wilson,
who kindly providedpreprintsof their work on the Juan de Fuca
plate.I am particularlygratefulfor a comprehensive
reviewfrom T.
Atwater,which resultedin a much more realisticappraisalof the
cate, distortion within this southernplate sectionwas suggest- errorsand uncertaintiesinvolvedin reconstructingthe plate motions.
ed by Silver[1971] and hasbeenmorerecentlyexaminedby Earth PhysicsBranchcontribution1125,
Carlson[1976] and Carlsonand Stoddard[1981]. Analysisof
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RIDDIHOUGH: MOVEMENTS OF THE JUAN DE FUCA PLATE
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(ReceivedFebruary 14, 1983;
revisedApril 30, 1984;
acceptedMay 1, 1984.)