JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. B4, PAGES 8147-8172, APRIL 10, 2000
Transpression between two warm mafic plates:
The Queen Charlotte Fault revisited
Kristin
M. M. Rohr •
Geological Surveyof Canada, Ottawa
Maren
Scheidhauer 2 and Anne
M. Trehu
College of Oceanography,Oregon State University,Corvallis
Abstract. The Queen CharlotteFault is a transpressive
transformplate boundary
betweenthe Pacificand North AmericanplatesoffshorewesternCanada.Previousmodels
for the accommodation
of transpression
includeinternal deformationof both plates
adjacentto the plate boundaryor obliquesubductionof the oceanicplate; the latter has
been the preferredmodel.Both platesare warm and mafic and have similarmechanical
structures. New multichannel seismic reflection data show a near-vertical Queen Charlotte
Fault downto the first water bottom multiple, significantsubsidence
east of the Queen
CharlotteFault, a large melangewhere the fault is in a compressive
left step,and faulting
which involvesoceanicbasement.Gravity modelingof profilesindicatesthat Moho varies
fairly smoothlyacrossthe plate boundary.Isostaticanomaliesindicatethat the Pacific
plate is flexeddownwardadjacentto the Queen Charlotte Fault. Upward flexure of North
America alongwith crustthickenedrelative to crustin the adjacentbasincreates
topographyknown as the Queen Charlotte Islands.Combinedwith other regionalstudies,
theseobservationssuggestthat the plate boundaryis a vertical strike-slipfault and that
transpression
is taken up within eachplate.
1.
Introduction
Atwater,1989;Furlong,1993].In comparison,the QCF hashad
a relatively simple plate historyfor the last 20 Myr or more
[Rohrand Currie,1997] and providesa goodlocationto study
the poorlyunderstoodgeologiceffectsof transpression
in both
oceanicand continentalplates.
Two mainmodelshavebeenpresentedfor plateinteractions
on the QCF: one in which transformfaulting alternateswith
subductionthrusting [Hyndmanand Ellis, 1981] and one in
Ideason accommodation
of transpression
havebeenheavily
influencedby studiesof oblique subductionzones [e.g., McCaffrey, 1996; Burbidgeand Braun, 1998]. How a transform
plate boundarywith a near-verticaldip changesinto a shallow
dippingsubductionzone is not obvious.At the onsetof and
duringtranspression
the relativemechanicalpropertiesof the
platesand geometryof the plate boundaryare criticalto un- which transform motion is accommodated on the QCF and
derstandingsubsequentdeformation.In the casestudiedhere, compressionby deformation of the oceanic plate and the
the initial conditions are that of a well-established transform
Queen Charlotte Islands[Mackieet al., 1989]. The latter was
fault separatingplateswith similarmechanicalstructures:
thin, consideredan unlikely solution because they believed that
warm, and mafic.
hundredsof kilometersof compressionhad to be accommoThe QueenCharlotteFault (QCF) separatesa youngPacific dated (Figure 1) [Yorathand Hyndman, 1983;Mackie et al.,
plate from an anomalouslymaficand recentlythinnedsection 1989]. Their tectonicreconstructions
of the Queen Charlotte
of the North American plate in westernCanada (Figure 1). region and the Explorer plate placed the commencementof
Reconstructions
of global plate motionsindicatea smallbut transpression
just north of VancouverIsland.The lack of comsignificantcomponentof compression
duringthe Pliocene(5 pressivestructuresin Queen Charlotte Sound and abundant
Ma) [Engebretson,1985; Norton, 1995; Stock and Molnar, compressivestructuresin Hecate Strait [Rohr and Dietrich,
1988], which may have begun at 8 Ma [Atwaterand Stock, 1992] indicate that significantongoingtranspressionbegins
1998].A more southerlysectionof the Pacific-NorthAmerica north of the Tuzo Wilson Seamounts.Since then, Hyndman
plate boundary,the central San Andreas Fault, is thoughtto andHamilton[1993]usedthe polesof StockandMolnar [1988]
absorb 14-72 km of estimated transpressionsolely by in- and Engebretson
[1985]for a simplecalculationthat 80 km of
traplate deformation[Crouchet al., 1984; Wallace,1990]. In compression
had occurredacrossthe QCF, althoughthe tecCalifornia,studyinggeologiceffectsof relativeplate motionsis tonic model of a subductedslab [Hyndmanand Ellis, 1981;
complicatedby migrationof triple junctionswhich produces Hyndman et al., 1982; Yorath and Hyndman, 1983] was not
significant
variationsin mechanicalpropertieswith time [e.g., revised.Primset al. [1997] placedthe relativeplate motions'
vectoron a map to showthat the averagenet predictedoverlap
1Nowat RohrGeophysics,
NorthSaanich,
BritishColumbia,
Canada. of the plates is only tens of kilometers;the value gradually
2Nowat Institutde Geophysique,
Universit6de Lausanne,
Lau- increases from zero at the Tuzo Wilson Seamounts to a maxsanne, Switzerland.
Copyright2000 by the AmericanGeophysicalUnion.
Paper number1999JB900403.
0148-0227/00/1999JB900403 $12.00
imum of 80 km over a lateral distance of 350 km. Large
amountsof compression
mightwellneeda subduction
zoneto be
accommodated;
tensof kilometersmaynot [Crouchet al., 1984].
Other studiesin the last 10 years such as microseismicity,
8147
8148
ROHR
134
ET AL.: TRANSPRESSION
132
130
128
North
Dixon
American
plate
.54
Pacific
plate
52.
52
Tuzo
Wilson
Seamounts
/•/ XExporer
•
-
-
1•4
0
50
ß
132
100
150
130
128
200 km
Figure 1. The Queen Charlotteregion,westernCanada.Stippledarea indicatesthe amountof compression
that wasthoughtto haveoccurredin the last 5 Myr [afterMackieet al., 1989]and darker stipplingshowsthe
amountof overlapif transpression
startsat the Tuzo Wilson Seamountsat 5 Ma. If relativeplate motions
beganto changeat 8 Ma [AtwaterandStock,1998],thenthe easternboundaryof the predictedoverlapsimply
projectslinearlyacrossDixon Entranceand Alaska.Whenevertranspression
began,net predictedtranspres-
sionreaches
a maximum
at GrahamIsland;the instantaneous
angleof transpression
decreases
to the north
asthe QueenCharlotteFault (QCF) trendsmorenortherly.Insetshowslocationof figurein regionalcontext.
Graham and MoresbyIslandsare the principalislandsof the Queen Charlottegroup.Theseislands,a region
of thickercrust,lie eastof the QCF in the regionof predictedplate overlap.Their easternshoreis subparallel
to the azimuthof relativeplatemotionasshownby arrow.The relativeplatemotionvectorwascalculated
from NUVEL-1 [DeMetset al., 1990].Box in northwestcornershowsapproximatearea of Figure2.
refraction, and heat flow data, geologicmapping and paleomagneticstudyof the Queen Charlotte Islands,modelingof
gravityanomaliesand flexure of the Pacificplate, and reconstructionsof sealevel duringthe lastglacialretreat suggestthat
a rethinkingof lithosphericinteractionsalong the QCF is in
order. The regional implicationsof this work are combined
with newmultichannelseismicreflectiondata and gravitymodels
acrossthe northernQCF to infer that compression
is mostlikely
accommodated
by deformationwithin both platesand that the
QCF continuesto accommodatemostof the strike-slipmotion.
2.
Overview of Regional Tectonics
2.1.
Queen Charlotte Fault
Between the Tuzo Wilson Seamounts
and the Alaska border
the QCF is clearly imaged by GLORIA side-scandata as a
distinctline in the seafloor[Brunset al., 1992];the QCF between 54ø and 55øN is shown in Figure 2. In the northern
region,wherethe multichanneldata were collected,two linear
segmentscan be observed;the northern segmentstrikes338ø,
and the southernsegmentstrikes328ø. The differencein angle
ROHR
135•001N
ET AL.' TRANSPRESSION
8149
134030'W
134(•00•
Kilometers
.
0
10
20
30
40
50
60
Figure 2. GLORIA data [Brunset al., 1992].The QCF is evidentas a linear feature on the seafloor.The
northernsegment
trends337øandthesouthern
328ø.The NUVEL-1 vectortrendsabout345øhere.Theymeet
in a compressive
left stepimagedby line 1250.Fine white lines showlocationof seismicdata.
and left stepbetweenthem createsa compresslye
bend. Striationsconsistent
with compresslye
structures
formedby north-
strikethe terracechangesfrom a singleblockdefinedby one
main scarpinto severalblocksdefinedby scarpswhichappear
west directed shear are observed in the bend. In other locato splayoff the QCF [Scheidhauer,
1997];this changein mortions,canyonsand striationsappearto be offsetrightlaterally phologymaybe relatedto decreasedobliquityof transpression
by a few kilometers.
to the north (A.M. Trehu and M. Scheidhauer,
manuscriptin
preparation,2000). The trough, of probableflexural origin
2.2. Plate Morphology
This oceanic-continental
transformfault is characterized
by [Hyndmanet al., 1982;Primset al., 1997],is severalhundred
plainandbegins90 km northof
an abrupt transitionbetweenthe two plates(Figure 3). The metersdeeperthanthe abyssal
the
inception
of
the
QCF
at
the Tuzo Wilson Seamounts.
continentalshelf under Hecate Strait is fairly flat with two
main incised channels. West of the Queen Charlotte Islands North of the Alaskaborder the terraceis lessdistinct[Bruns
the shelfis <10 km wide,and the continentalslopeconsists
of and Carlson,1987], and there is little obviouscontemporary
a terrace,roughly30 km wideand 1500-2000m deepboundby deformationin the North Americanplate [e.g.,Gehrelset al.,
two scarps.The outerscarpof the terracestepsdownto water 1987].Southof the Tuzo WilsonSeamounts
the oceanicplate
depthsof 2500-3000 m, the Queen CharlotteTrough.Along isveryyoungandbroadlydeformed[RohrandFurlong,1995],
8150
ROHR
ET AL.: TRANSPRESSION
Figure 3. Regionalmorphologywith locationof new multichannelseismicreflectiondata:lines 1250, 1262,
1263,and 1264.T is the QueenCharlotteterrace.Arrow showsdirectionof relativeplate motioncalculated
from NUVEL-1 [DeMetset al., 1990].Locationof QCF waspickedfrom GLORIA data (Figure2). Light
shadedline is locationof crosssectionshownin Figure4. Shortline segments
showlocationsof multichannel
seismicreflectionlines5, 6, and 7 displayedin Figure 6.
and south of the Queen Charlotte Islands the North American
Curtie, 1997]with a componentof transpression
in the last 5
plate is simply subsidingin responseto Miocene extension Myr. Prior to 45 Ma the plate boundarywasa subductionzone,
[Rohrand Dietrich, 1992].
but its configurationand historyare poorly known.A single
The age of the Pacificplate increasesfrom 7 Ma at the fault, the QueenCharlotteFault, is thoughtto takeup mostof
southern end of the Queen Charlotte Islands to 15 Ma off
the past transcurrentmotion; distributeddeformationmay
northernDixon Entrance.Magnetic anomaliesof the Pacific have occurredduring Miocene extension[Rohrand Dietrich,
plate strikeapproximatelynorth-southand fade out under the 1992]. Pliocene inversion of Miocene basins indicates that
outer scarpof the terrace[Atwater,1989].
compressionwas distributed acrossthe continentalmargin
over a width of at least 150 km [Rohrand Dietrich,1992].
2.3. Plate History
The amountof predictedcompression
changesalongstrike
The tectonicsof western Canada are dominated by the becauseof the angleof relative motion and changesin strike
boundarybetweenthe PacificandNorth Americanplates(Fig- direction of the QCF. The difference between the strike of the
ure 1). Relativemotionbetweenthe two plateshasbeendom- QueenCharlotteFault off MoresbyIsland(320ø,Figure1) and
inantly.strikeslip for at least20 Myr, if not 45 Myr [Rohrand the directionof relativemotionof platesis ---26ø in the south
ROHR
ET AL.: TRANSPRESSION
t O0
8151
......
=
50
E
-50
f• -100
...................................
o
?, •,,
observed
gravity
/
2•20
•
½½?*.
':..-:-:.•..:,..:..?•::•
'.•....
.. . ..... ........
.........
50 ..............
•..................................................
;m
•*• • '•' •d•"•/•:;
"'•*•........
m'•• •
0
50
100
40
150
200
80
120
•
•1
300
160
2÷0
350
200
VE 4:1
Velocity (km/s); Gradient (km/slkm)
[•
,[•
I•
•
1.49;0,001
2.20;0.530
2.35;0.700
3.80;0.350
3.80;0.350
3.40;0.500
4.10' 0.400
5.00ß0.180
•
6.70'0.012
5.30;0.200
6.50' 0.030
6.10-0.095
7.90' 0.005
Figure 4. (bottom)Refractionresultsof Dehlerand Clowes[1988];notethickeningof materialwith velocitiesgreaterthan5.3 km/sin terrace.(top) Gravityinterpretationof thisrefractionline combinedwith line 6
[afterSpence
andLong,1995].Locationof profileis shownin Figure3. The whitelayercorresponds
to crust
with velocitiesgreaterthan 5.3 km/s (crystallinebasement);light shadingcorresponds
to water, medium
shadingcorresponds
to velocities2.2-4.7 km/s (sedimentsand upper oceaniccrust);and darker shading
corresponds
to velocities
of 7.9-8.1km/s(mantle).Numbers
aredensity
values
modeled
(kg/m3).
Triangle
markslocationof OCF. Dottedline is drawnfromthe top of layer3 in oceaniccrustacrossthe terraceto show
howmuchof the terracewouldbe occupiedby a simpleundeformedslab.The volumeabovethe dashedline
must then be deformed
oceanic crust and sediments.
and decreasesnorthward to 7ø off Dixon Entrance. Average
velocitiesare north-northwest4.7-4.8 cm/yrwest of Graham
Island and Dixon Entrance.The amountof predictedoverlap
of the Pacificand North Americanplates(Figure 1) wascalculatedassumingno internal deformationof either plate and
usingthe NUVEL-1 vector [DeMetset al., 1990] over 5 Myr.
Predictedoverlapacrossthe OCF increasesfrom zero at the
Tuzo Wilson Seamounts to a maximum
of 80 km at northern-
currentto transpressive
beginningat 8 Ma. Thiswouldincrease
the amountof overlappredictedin Dixon Entranceto 100 km
but doesnot affect the rest of the map shown.
2.4.
Structure
A multichannelsurveyin 1977 imagedfolded sedimentsin
the terrace and inverted basins of northern
Dixon
Entrance
[Snavely
et al., 1981].The terrace'sdeeperstructurewasstudmostGraham Islandand then rapidlydecreasesagain.Atwater ied in two refractiontransectsat about52øN[Hornet al., 1984]
andStock[1998]haverecentlysuggested
that the PacificNorth and53øN[Dehlerand Clowes,1988](Figure4, bottom);interAmericarelativeplate motionschangedgraduallyfrom trans- pretationsfeaturedcrustin the terracewhichis lowerin seis-
8152
ROHR
W-E
'
TRANSPRESSION
QCF
ß
' '
'
i
''
"'
.....•"'
' ..........
•'--.•
•i'L
.
.•
_•'-.._•
ß
-5
-to
-15 ........
Present bathymetry
.....
-20 -25
Te= ! 5 km(25kmofunderthrusting)
-- --. - Te = 15 km (No underthrusting)
15'0
'Z
previouslypostulatedtime of transtension
is no longerthought
to haveoccurred[Norton,1995;Atwaterand Stock,1998].The
mode and causeof extensionremain enigmatic,but coincident
accelerationof uplift of the Coast Mountains led Rohr and
Currie[1997]to proposethat extensionwasaccommodated
on
a large low-angledetachment.
Transpressionhas invertedsubbasins
inboardof the Queen
CharlotteIslands(Figure 6) [Rohrand Dietrich,1992]but not
in Queen Charlotte Sound.Compressiondeformedsomeupper Miocenesediments(e.g.,shotpoints(SP) 600-800, Figure
6a) andwasfollowedbybroadfoldingandtiltingup to thewest
which included Pliocenepostrift sediments.Strike-slipfaults
and flower structurescut the folds. Glacial scouringand fast
tidal currentshaveplanedoff the upthrustsediments(Figures
6b and 6c); onlythe northernmost
basins(e.g.,Figure6c) are
seismicallyactive [Bird, 1997].
No quantitativework has been done to assessthe uplift
historyof the Queen Charlotte Islands,yet their topography
and geologyindicate uplift which increasessouthwardalong
Trough
Terrace
• Islands
_
-
ET AL.'
, Observed top of plate
loo
5o
o
Distance (km)
Figure 5. Flexuralmodelingby Primset al. [1997]showsthat
the weight of the terrace alone is sufficientto match observed
flexureassuming
a densityof 2000kg/m3. Under the QCF,
heavybar representstop of putative plate in gravitymodels
(see Figure 15). Thrustingof a slab under North America
predictsa greater amount of downwarpthan is allowed by strike. Mountains on the western Queen Charlotte Islands
gravitymodels.Downwarpis taken from a reflectionprofile exceedelevationsof 1000m. In MoresbyIsland,upper crustal
west of Graham Island [Rohret al., 1992].
Paleogenebatholithsare exposed[Andersonand Reichenbach,
1991], but on Graham Island, Neogene rocksformed at sea
level are just above current sea level. Yorath and Hyndman
mic velocitiesthan cruston either side.Velocitiesgradedfrom [1983]suggested
that the Queen CharlotteIslandswere flexed
5.3 to >7 km/s above Moho and crust thickened from 7 to
abovesealevel in the last 6 Myr by underthrusting;they did not
18-20 km towardthe continent.Most of the rayswhichdefined consideralong-strikevariationsin flexure.
Moho under the terrace also traversed the Queen Charlotte
Quaternaryhistoryincludesperiodicglacialloadingoverthe
Islands, where crustal structure was not constrained. Several last 2 Myr accompaniedby rapid sea level changescausedby
gravity modelsbased on these refraction studiesas well as eustaticvariations as well as significantregional flexural efrefractioninterpretationsin the Queen Charlotte Basininter- fects.A glacier in Dixon Entrance came within 12 km of the
preted that Moho dips eastwardunder the terrace and then QCF only 11,000 years ago; as it retreated, sea level rose by
flattensunder the islands(Figure 4, top) [Horn et al., 1984; 100 m in only 1000years[Barrieand Conway,1996].This rapid
Mackie et al., 1989; Sweeneyand Seemann,1991; Spenceand risehasbeendocumentedasa regionalphenomenon[Barrieet
Asudeh,1993;Spenceand Long, 1995].
al., 1991;Josenhans
et al., 1995]and suggests
a viscoelastic
time
High velocities(6.5-7.3 km/s) in the lower continentalcrust constantof a few 100 yearsor less(T. James,personalcom[Spenceand Asudeh, 1993] indicate that its compositionis munication,1998),whichis significantly
smallerthantime condominantlymafic.This is consistent
with the surfacegeologyof stantsusedfor mostof North America.The rapid regionalsea
the Queen Charlotte Islands, which includes 7-10 km of Karlevel changesmay onlybe possiblegiventhe warm stateof the
mutsen basalt flows and shows that they are part of the continentallithosphereand if the QCF is a free edge (T.
Wrangellia terrane [P. D. Lewis, et al., 1991], which was ac- James,personalcommunication,1998).
creted in the Cretaceous[Gabrielseand Yorath, 1991].
2.6.
2.5.
Flexure
and Vertical
Motions
Bathymetry and isostasyindicate that the Pacific plate is
beingflexed [Harrisand Chapman,1994].In three dimensions
the wavelengthand amplitudeof flexurevarywith the strength
of the plate as it increaseswith age. A flexural trough is not
evidentin the southernmost90 km of the fault, indicatingthat
until this point the load of the terrace or the compressive
stresses
are insufficientto flex the plate. Calculationsof flexure
(Figure 5) [Primse! al., 1997] found that a model of underthrustingpredictedtoo much downwarpof the Pacificplate.
Instead, a clean break at the QCF matched downwarpobservedin a reflectionprofile and a depth of 12 km to the top
of a putativeundeformedoceanicplate at the QCF.
In the Miocene, extensional faults and subsidencewere ac-
tive in Queen Charlotte Soundand Hecate Strait formingthe
Queen CharlotteBasin[RohrandDietrich,1992;Hyndmanand
Hamilton, 1993; Lowe and Dehler, 1995; Dehler et al., 1997;
Rohr and Currie,1997].The basinconsists
of a networkof half
grabensand grabensacrossa 200-km-wideregion. Plate motions have been fairly constantbetween 40 and 5-8 Ma; a
Seismicity
The QueenCharlotteFault is seismically
active(Figure7);
Canada'slargest recordedearthquake,a magnitude8.1, occurred on it in 1949. For that event,Bostwick[1984] found a
rupturelengthof-490 km (300 km northand 190km southof
the epicenter)with an averagecoseismic
displacement
of 4-7.5
m. The azimuthof the verticalfault planecorresponds
with the
strike of the Queen Charlotte Fault, implyingthat no convergencewas taken up by this event.A short oceanbottom seismometer(OBS) surveydefineda near-verticalzoneof seismicity to depthsof 20 km under the QCF cuttingdown to Moho
[Hyndmanand Ellis, 1981].A studyof regionalseismicitybetween 1982 and 1996 [Bird, 1997] alsofound that eventsclustered around the QCF lie in a near-verticalband. This study
onlyhad arrivalsrecordedfrom the easternsideof the QCF for
each event,but eventslocatedon the QCF along the middle
outer edge of the array did not significantlychangelocation
solutionswhen different depthswere used to locate a given
event.This impliesthat the earthquakesare distributedvertically.This studycouldnot, however,determinethe maximum
depth of faulting.
ROHR
ET AL.:
TRANSPRESSION
c)
Figure 6. Inversion of basinsformed in the Miocene; see Figure 3 for location [after Rohr and Dietrich,
1992].P, Pliocenesediments;
uM, upperMiocene;and1M, lowerMiocenesedimentsasdeterminedfromwell
ties.B indicatesprobablebasementrocks.(a) Line 5 showingbasementfaultsbelowfoldingin sedimentary
section.(b) Line 6 imagingverticalfaults,foldingof Mioceneand Pliocenesedimentsas well as tilting of
basement,and Miocene/Pliocenesectionup to the west.Note strike-slipfault underneathshotpointlocation
420. (c) Line 7 showingfoldingof sedimentary
sectionand strike-slipfault whichis activetoday(shotpoint
(SP) 1445).A magnitude5.3 strike-slipearthquakeoccurred--•20km belowthisbasinin 1990.
8153
8154
ROHR
135øW
133øW
ET AL.: TRANSPRESSION
129øW
131øW
z............................
o
..............
ø :-:.:o..
:. '
•ø ii-• ...... - .......
ii7..:•
......
•...........
....:...•.•..::•.,,/•.•.......
__•
...•
.. .•,•.•. •: •!!::•-:
''"•':
'......
..•'•'.
135øW
133ow
. ."4'- •
131ow
;..-•
.__________________•:.•...
12Sow
Figure 7. Microseismicityfrom Bird [1997] plotted on bathymetry;magnitude8.1 event is shownas large
openstar.Seismicityoccursalongthe QCF; eastof the QCF microseismicity
is concentratedundernorthern
GrahamIslandand Hecate Strait.White squaredenoteslocationofM - 5.3 eventin 1990.Nine stationsused
to locate the eventswere on the Queen Charlotte Islands(QCI); three were acrossHecate Strait on the
mainland.
Focal mechanismsolutionsof groupsof microearthquakes, 3. New Seismic Reflection Data
and individualevents[B•rub•et al., 1989;Bird, 1997]showthat
pure strike-slip as well as oblique, thrust, and extensional 3.1. Acquisition and Processing
events occur near the QCF, beneath the terrace and northern
Data collectedin 1994 as part of the Accreteprogram[AnGraham Island. The axis of maximum compressionstrikes dronicoset al., 1999] crossthe QCF in two places;tie lines
north to north-northeast.Events in Figure 7 were locatedas- imagemildlydeformedPacificplate and the terrace(Figures2
et al., 1999].The source,an arrayof 20 air
suminga depth of 20 km. Eventslocatednorth of the station and 3) [Scheidhauer
array are slightlyeast of the QCF probablybecausethe as- guns,had a total volume of 138 L (8400 cubicinches).The
sumeddepth is too deep. Nevertheless,the spreadof activity streamerhad 224 channelswith a groupspacingof 12.5 m for
over the physiographic
terraceand the varietyof mechanisms a total length of 2800 m. The data were recorded with a
suggestthat the terrace is seismicallyactive. Treated as a samplingrate of 4 ms for 16.5s.An anti-aliasinghigh-cutfilter
group,90% of the eventsunderGrahamIslandoccurat depths of 125 Hz was appliedin the field prior to digitization.Using
<16 km, and 90% of the events under Hecate Strait occur at Global PositioningSystem(GPS) navigation,the shotpoint
dataweresortedinto commonmidpoint(CMP) formatwith a
depths <20 km.
ROHR
ET AL.:
TRANSPRESSION
8155
show that subsidencewas faster than the rate of deposition
sincelayersonlap the substrate.Above U2, layersdownlap;the
sedimentaryunits are buildingoutwardindicatinga faster rate
of deposition.Suchvariationsin sedimentaryrate are expected
to occur during glacial cyclesand may have occurredduring
the last major glacialcycle(0.125 Ma). This would imply that
3.2.
Line 1262
the Pacificplate was<6 km to the southwestduringdeposition
Line 1262(Figure 8) crosses
the QCF at latitude54.5øNoff of the layers on the North America plate. Plate flexure, sea
Dixon Entrance.The Pacificplate is --•15 Myr old here, and the level, and sedimentationrate changesare regional in effect,
westernedge of the terrace is a ridge which runs north-south, and onewould expectthem to be coherentover distancescales
parallel to the magneticanomalies(Figure 2). Sedimentary of 5 km. Yet thoseeffectsare not evidenton the Pacificplate.
unit B thins to the west and onlapsunit A, implyingthat the
3.3.
Line 1263
oceanicplate was alreadyflexed before depositionof unit B.
We have no independentestimateof age of sediments,but it
Line 1263 (Figure 11) showsmild, apparentlyinactivedeseemslikely that, similarto the Cascadiabasin,the bulk of the formationof the Pacificplate westof the terrace.This line was
sediments are Pleistocene.
shot at a low angle to the strike of the terrace and crossesthe
The outer ridge of the terraceis so deformedthat only short outer terrace fault imaged by line 1262 where it has plunged
segmentsof coherent reflectionsare visible (CMP 10200- belowthe seafloor(CMP 2300-2500). In this area,whichhas
10700);the standardprocessing
usedwasunableto resolveits undergone less deformation, the internal structure is more
internal structure. The terrace is •--45 km wide; two turbidite
evident; a small anticline is curved up south of a fault. Dip
channelswhich carry sedimentsfrom Alaska and the Coast moveoutprocessing
showsthat it dipsat an angleof at least52ø
Mountains have been focusedhere into one channel by the and that it cuts basement.Compressionhas tilted both sediouter ridgesof the terrace.Broadlyfolded sedimentsunderlie ments and basementas opposedto just thickeningthe sedithe seafloor;the intensityof folding increasestoward the outer ments,indicatingthat basementis involvedin the deformation.
ridgeandwith depth.An incoherentunit, F, abutsthe QCF; its An angularunconformityis associatedwith the crest of this
incoherenceand associationwith a major fault suggestthat it anticline. On either side of this structure, reflectors dip
is a melange. Farther to the west, sedimentarylayers are smoothlytoward the terrace. SteepfaultsbetweenCMPs 4300
folded, and their contact with the melange is not defined. and 4500 are also currentlyinactive.
Whether theselayershavebeen caughtup in the melangeor if
3.4.
Line 1250
they onlap an older melangecannotbe distinguishedhere.
The data show strikingdifferencesin structureand stratigLine 1250(Figure 12) wascollectedacrossthe terrace30 km
raphyacrossthe QCF; markedtilting on the North American southof line 1262.The trough sectionshowslayerssimilar to
plate does not occur on the Pacific (Figures 8-10). On the thoseseenin line 1262; althoughthe lower layersare thicker,
North American side the top 1.0 s of sedimentarylayersdip they still showunit B onlappingunit A. In the terracea set of
down toward the fault and are underlainby a nearly flat event fault-bendfoldsis cut by nearlyverticalfaults.The geometryof
that a compressive
fault has cut up
at 4.5 s (Figure9). West of the fault, reflectionsare subparallel thesesedimentssuggests
possiblyon a setof rampsand flats[e.g.,
to the seafloorand are higher in amplitudethan to the east. throughthe sediments
The configurationof reflectionsat the present seafloorsug- Shaw and Suppe,1994], reachingthe surfaceat the westerngeststhat tilting the North American plate relative to the most edge of the terrace. Subsequenttilting of the ramps
Pacificcreatesa depressionwhichwould be filled by the next suggeststhat later faulting was deeper, perhapsbecausethe
sedimentarydeposit.Dips exceedthosetypicallyobservedon brittle layer thickenedover time. Much of the recentsedimencontinentalslopes.The dip of the seaflooris 5ø, and dips of tationwasdepositedduringformationof the foldssincelayering
reflectionsbelow that are as much as 12ø and 20ø assumingan onlaps the fold crest (CMPs 3500-3650). The near-vertical
interval velocity of 2.0 km/s. In contrast,sedimentsimmedi- faults could be the result of later strike-slipdeformation.
On this line the plate boundaryis visible,not as a distinct
ately seawardof the shelf break on passivemarginstypically
dip 3øto 5øon today'sAtlantic continentalslopeand up to 7.5ø fault, but as an incoherent,apparentlyuplifted unit (CMPs
on Miocene progradingclinoformsoff New Jersey[Fulthorpe 4900-5100). The lack of coherentreflectionenergyand weak
returnsfrom this regionsuggestthat the sedimentshave been
andAustin, 1998].
On the North American plate the sedimentaryrecord shows deformedenoughto destroyoriginalbedding,i.e., constitutea
two different processesat work: tectonicsubsidenceand flex- melange.This profile crosseda restrainingbend in the QCF
ure from glacioisostatic
forces.The latter entailsrapid changes which is ---3 km wide and 30 km long. Northwest trending
whichare superimposedon the former, a longer-term,presum- high-amplitudereflectionsin the GLORIA data lie between
ably steadierprocess.Dips which exceedeven thoseobserved overlappingtracesof the fault (Figure 2); they are mostlikely
during massivesedimentbuildup suggestthat subsidenceis uplifted melange.
ongoing.Subsidencecould be causedby local thinning of
3.5.
Line 1264
North America or simple flexure. Counterclockwiserotation
between two active right-lateral strike-slipfaults can create
On line 1264 (Figure 13) the melangecan be traced from
extensionin one sectionand compressionin another. Dixon line 1250 to line 1262, but it is not as extensive in the shallow
Entrance is broken by a seriesof northwesterlytrending en sectionof line 1262 as in line 1250. The melange could have
echelonfaults[RohrandDietrich,1992];sheardistributedfrom formed locallyon the QCF or at the restrainingbend and been
the QCF onto one of thesefaults could easilyinduce rotation draggednorth 30 km. This would imply an age of 0.64 Ma for
the top of melangein line 1262usingan along-strikevelocityof
adjacentto the QCF.
The angularrelationsaboveunconformityU3 (Figure 10) 47 km/Myr. Foldingof the sedimentsat the northernmostend
bin sizeof 12.5m resultingin CMPs with an averagefold of 60.
Processingfollowed the basic steps of editing shot points,
band-passfiltering, analyzingvelocities,and stacking[Scheidhauer,1997].The datawere migratedusinga finite difference
technique.
8156
ROHR
ET AL.'
TRANSPRESSION
1
_-IOO
o
o
(0es) em!l leAmlAeM-OM/
ROHR
o
ET AL.'
TRANSPRESSION
8159
•
o
o
o
o
o
o
o
00
o
o
JJJ o
o
•
(O•S)•W!l.I•A•JI./•eM-OMI
8160
ROHR
ET AL.: TRANSPRESSION
,,
..,
c•
z
(oes) ew!t IOAeJ1
•eM-OM/
ROHR ET AL.: TRANSPRESSION
/
(005) OW!l.IOAeJI.
AeM-OM/
8161
8162
ROHR
ET AL.'
TRANSPRESSION
80
•
75
70
65
60
õ0
415
40
35
30
115
-15
.-•:•
-20
-ao
-40
-45
'
-50
':
-55"
-60
•'"'
-65
'
-70
':
..
-75
-OO
-90
129
..
-95
-iO0
0
50
{ 0'0
t 50
200
2'50
millilals
Figure 14. Free-air gravityanomalydata. A strongnegative-positiveanomalypair coincideswith the QCF;
otherwise,the anomalyfield is relativelysmooth.In the basinmost anomaliescan be explainedby known
sedimentthicknesses
[Loweand Dehler,1995].Seismicprofilesmodeledin other studiesand here are marked:
line EX1 [Horn et al., 1984];lines5 and 6 (Figure 4) [Spenceand Long, 1995] and line 1262.
plate boundaries,namely, the northern San Andreas Fault
[Henstocket al., 1997]and the Alpine Fault [SternandMcBride,
1998].Kilometersof relief on the Moho causedby a subducted
slab would create a significantincreasein the gravity field,
unlessits anomalyis exactlyoffsetby lateral densitychangesin
the continentalcrust.That kind of coincidencealong400 km of
plate boundary seemsimplausible.Therefore it seemsmost
likely that if an oceanicslab existseast of the QCF, it cannot
createmore than a few kilometersof topographyon the Moho.
4.3. Isostasy
Immediately adjacent to the QCF, both plates are out of
localisostaticequilibrium.Isostasywascalculatedto a depthof
40 km at 20-km intervalsalongboth modelsfor lines 1262,EX1
[Horn et al., 1984],5, and 6 [Spenceand Long, 1995] (Figure
16). Isostaticanomalieswere calculatedrelativeto a reference
column which was 90 km east of the QCF for models 6, 1262s
(the subductionmodel of 1262), and EX1. Calculationsfor
models5 and 1262t (the analogoustransformmodel of line
1262) usedthe samereferencevalue as model 6. The location
of the reference column is an arbitrary choice based on the
lengthof the modeledprofiles.Both platesare probablyflexed
to some small degree over distancesof severalhundred kilometerseithersideof the plateboundary[e.g.,Harrisand Chapman, 1994].
A consistentpattern betweenthe modelscomputedby three
different studiesallowssomegeneralizationsto be made. The
ROHR
ET AL.:
TRANSPRESSION
Table 1. DensitiesUsed for Sedimentsin Gravity Models
Unit
Interval
Density,
Velocity,m/s
kg/m3
AB
CD
E
F
4700
3300
2100
4880
2500
2300
2100
2550
G
H
III
3960
2300
2450
2150
2700
as one would expectfor constantforce along strike flexingan
oceanicplate which is aging and increasingin strengthalong
strike.Thicknessof the brittle regiongrowsfrom - 13 to 20 km
(seebelow) over the lengthof the QCF. EX1 wasshotacross
the southernmostportion of the terraceand coincideswith an
area with no bathymetrictrough. Its profile has <20 km width
of negativevalues,all of which are directlyunder the terrace.
The isostatic anomalies under the Queen Charlotte Islands are
From Scheidhauer[1997]
magnitudeof the peak to trough valueson these profiles is
similar: 3.4%, 2.2%, 3.6%, 2.9%, and 5.1% from south to
north. The positivedeviationon North America indicatesmore
mass,and the negative deviation over the Pacific plate indicates less mass than the reference
8163
column. The isostatic anom-
aly over the Pacificplate becomeswider from southto north,
of similarwidths,implyingthat the North Americanplate does
not significantlychange mechanicalproperties along strike,
assuminga constant force along strike. This rules out the
possibilitythat the Queen Charlotte Islandsare createdpurely
by flexuralforces.If theywere, theywould havethe samewidth
along strike instead of widening to the north. The isostatic
profile of the transform model 1262t is interestingin that it
seemsto imply that most of Dixon Entrance is out of isostatic
equilibrium. The isostatic profile of the subductionmodel
1262s,however,is similar to that of profile EX1, which is the
basisfor its densityvalues.
A. 1262t
o
5
•
lO
"=
15
C• 20'
25
0
20
40
60
80
100
120
140
B. 1262s
>
o -100--...- ..
:'- observed
.............
_......................... :-:
..........:.......
calculated
....__
ß .-ß ---:..... . -•.-._-..•: .....
_•-._.:.:----•:-r.•.•: •- -:__:.:-=====================
.:_:
::- :.:._:_--:
.-_-:::..-...,._..,_.
o
5
•
m
lO
15
0
V.E.= 1,-•
20
•
60
80
100
120
140
160
Distance
(kin)
Figure 15. Gravity modelsfor line 1262. (a) Transformplate boundaryusingdensitiessimilarto thoseof
Spenceand Long [1995] (Figure 4). (b) Subductedslab.Densitiesin the terraceare basedon refractiondata
and assumedfor the continent[Horn et al., 1984]. Densitiesfor unitsA-H are listedin Table 1.
8164
ROHR
ET AL.:
TRANSPRESSION
-2%
200
150
1 O0
50
0
-3%
kilometers
Figure 16. Isostaticanomalieswere calculatedrelative to a column90 km eastof the QCF basedon gravity
modelsof lines EX1 [Horn et al., 1984], 5, 6 [Spenceand Long, 1995], and 1262. Model 1262srefers to a
subductiongeometryfor the plate boundaryand model 1262t refersto a transformgeometry,as in Figure 15.
The negativevalueswest of the QCF indicatea massdeficitin the Pacificplate, and the positivevalueseast
of the QCF indicate a mass excess.
well as resistanceto changingthe plate boundary.When the
plate directionsbegan to changeto transpression,the QCF
continuedto accommodatestrike-slipmotion (43-46.6 km/
A better understandingof regionaldeformationaswell as a
Myr). Sincethe plateson either sideof the fault are warm and
numberof data setsacquiredin the lastfew yearssuggestthat
thin, compression
(6-20 km/Myr) was more easilyaccommothe QCF is a vertical throughgoingplate boundarywhich sepdated within each plate rather than forcingthe oceanicplate
aratestwo elasticallyindependentplates(Figure 17). Placing
into the continentalplate in a subductionzone geometry.
the initiation of transpression
just north of the Tuzo Wilson
Seamountsresultsin <80 km of predictednet plate overlap
5.2.
Mechanical
Structure and Deformation
between52øNand 54øNin the last5-8 Myr. Each of the plates
From the Tuzo Wilson Seamountsto Alaska the two plates
absorbstranspressionindependently;brittle compressiveand
strike-slipdeformationare observedin both plates.The three- are similar in mechanicalproperties,and they are both dedimensionalinteractionsof the plate boundaryand the relative forminginternally.Both platesare maficsothere is no strength
motion vector resultsin North America absorbingmore com- discontinuityat the mantle; the depth to the brittle-ductile
pressionthan the Pacificplate. By implicationthe ductilepor- transition is a key feature in mafic plates. Microseismicity
tionsof eachplate thickenand/orare deflecteddownwardinto [Bird,1997]showsthat the brittle-ductiletransition(the depth
abovewhich 90% of the eventsoccur [Sibson,1982]) begins
the asthenosphereto accommodatetranspression.
between16 and 20 km depth under Graham Island and north5.1.
Vertical
Fault
ern Hecate Strait. For the rest of the region we use thermal
Seismicityrecorded by OBSs indicatesthat the QCF is a modelsand heat flow data to assessthe depth to the brittleverticalfault whichcutsdownto Moho at 21 km [Hyndmanand ductile transition.Whether that transitionbeginsat 650øCor
Ellis, 1981;Horn et al., 1984].A magnitude8.1 earthquakeon 750øCin mafic plates is not strictlyknown, but the high heat
the QCF was a pure strike-slip event; no compressionwas flow in both platesmeansthat the verticalseparationbetween
accommodatedduring the event. Studies of microseismicity the 650øCand 750øCisothermsis only -3 km. We will arbialso showhigh levelsof activityon the QCF in a vertical zone trarily use the number 700øC since the relative differences
[Bird, 1997]. The combinationof the flexural study of the between the two plates is more important than peggingthe
Pacificplate [Primset al., 1997] and regional rapid sea level transition to the nearest kilometer.
Heatflowvaluesaverage
70mW/m2in theQueenCharlotte
changesduring glaciation[Barrieet al., 1991] further indicate
that the two platesare elasticallyindependent.In otherwords, Islandsand northernHecate Strait,indicatinga depthof 22 km
the QCF is a clean break between the plates, and there is no to the 700øCisotherm [T. J. Lewis et al., 1991]. Values of
underthrustingof the Pacificplate under the North American 87-112 mW/m2 which have been measured in Hecate Strait
plate.
[T. J. Lewiset al., 1991]indicatea depthto the 700øCisotherm
The QCF has a long historyof strike-slipmotion; the fact of 17-21 km againassuminga linear gradient.In North Amerthat much of it is still parallel to the directionof relative plate ica, depthsto the brittle-ductiletransitionpredictedfrom heat
motionbetween40 and 8 Ma (320ø) off MoresbyIsland[Rohr flow data are in broad agreementwith the 16-20 km depths
and Curtie, 1997] impliesa continuoushistoryof the fault, as inferred from microseismicity.
5.
Model
of Accommodation
of Transpression
ROHR ET AL.' TRANSPRESSION
o
::
...
..
('tu•)qi'd...'e(]
8165
8166
ROHR
ET AL.:
TRANSPRESSION
A thermalmodelof youngoceanicplates[ChenandMorgan,
1990]showsthat the depthof the 700øCisothermchangesfrom
13 to 20 km depthin the Pacificplate asit agesalongthe QCF.
The youngestoceanicplate iswarmer than North America,but
simplethermal modelsindicatethat the mechanicalstructure
of both platesis broadlysimilar.There is no reasonto suppose
that one plate would deform exclusively
leavingthe other untouched.
When transpressionbegan, the Queen Charlotte basin
would have been warmer
than it is now and the brittle-ductile
transitionwould have been closerto that of the youngestoceanic plate along the QCF. As deformation proceeded,the
thermal field would be disturbed;bulk strainheatingmight be
important in highly deformed regions.Motion on the fault
might be an additionalimportantsourceof heat tens of kilometers from the fault itself [Chen, 1988]. Nevertheless,the
generalizationthat the two plateshavesimilarmechanicalbehavior is supportedby current observations.
Transformplate boundarieshave been imagedas near vertical zones of lowered
velocities
in reflection
and refraction
1984;Dehler and Clowes,1988; Spenceand Long, 1995] interpreted crustalvelocityand densitymaterial (>5.3 km/s and
2800kg/m3) 3-4 kmbelowtheterraceseafloor
fortotalthicknessesof 5.5-15 km. These valuesare interpretedas arising
from highly deformed crystalline material. Volumetrically
there is room for ---30 km of 5.5-km-thickgabbroicoceanic
crustto be incorporatedinto the terracealongline 1262,20 km
on lines5 and 6, and 9 km on EX1. A simpletwo-component
system
of 25%sediments
(2500kg/m3) and75%oceanic
crust
(2900kg/m3) canproduce
themodeled
density
of 2800kg/m3.
Oceaniccrust or mantle altered to serpentinite(2550-2600
kg/m3)couldalsobea constituent
of theterrace.
If 75%of the
volumeof the deepestlayerin the terraceis oceaniccrust,then
6-20 km of compression
could be accommodated
by duplexing. Becausethe mechanicalpropertiesof the oceanicplate
vary along strike, the amount of compressionabsorbedwithin
the plate may not be linearly correlatedwith the amount of
predictedoverlap.
5.2.3.
Brittle
deformation:
North
America.
The
rest of
the compression
mustbe accommodated
within the continenexperimentsas summarizedby Stem and McBride [1998] and tal plate. The amountvariesgraduallyalongstrikefrom 0 to 60
Detricket al. [1993].Fault gougeand enhancedpore pressures km. Early interpretationsconcludedthat little deformationof
are thoughtto occupyregions5-10 km wide and up to 10-20 North Americaoccurredin the Pliocene[Hornet al., 1984],but
km deep. Experimentsshowthat once formed sucha region imagesof folding and uplift of the basinsunder Hecate Strait
tendsto carrymostof the ongoingstrike-slipdeformation[e.g., givedirectevidenceof widespreadPliocenedeformation(FigWilcox et al., 1973]. Some deformation can be distributed ure 6), and microseismicity
providesampleevidenceof recent
below sugwithin the platesespeciallyin relativelyweak plates,as here, deformationin Graham Island. Evidencediscussed
and/orwhenthe relativeplate motionschange.Nevertheless,a geststhat the compression
is orientednorth-southand is permagnitude8.1 strike-slipevent is more likely to occuron the vasive within the islands. The islands, then, are the result of
main fault strand than at some random location within either
crustalthickeninganduplift duringtranspression.
Possible
mechanismsresponsiblefor the deformationare also discussed.
plate.
5.2.1. Deformation adjacent to the QCF. Structures on
Complex and multistagecompressivedeformation affects
the QCF varyfrom subsidence
in North America(line 1262)to manyof the Miocene basinsinboardof the Queen Charlotte
foldingon the Pacificplatejust 10 km to the north (line 1264). Islandsbut not Queen Charlotte Sound [Rohr and Dietrich,
Rapid subsidence
eastof the QCF on line 1262 may indicate 1992](Figure6). In mostcases,the senseof motionon normal
that extensionis occurring.Thinner crustunder the subsiding faultshas been invertedto create uplift. The intensityof deportion of line 1262 is one of severalmodelswhich can fit the formation increases toward the islands. There are several kigravitydata.Althoughapparentlycontradictoryto the regional lometersof verticalrelief betweenbasementrocksexposedon
stressregime,extensioncouldoccurwith counterclockwise
ro- Moresby Island and basementrocks imaged under western
tation of a segmentof North America between two right- Hecate Strait, a distanceof only 20 km, indicatingthat a major
lateral faults.Rotation simultaneously
createsregionsof local structureis probablyresponsiblefor this relief.
Two-dimensionalline length calculationscan estimatethe
extension and compression.The east-west oriented folds
crogsedby line 1264 are in the right location to be the com- amountof compression
absorbedin the basinbut mustbe used
pressionalcounterpartto the apparentextension;foldsfarther with caution becausethe structuresvary significantlyin all
west in the terrace are oriented northwesterly.Shearingmust three dimensions,coverageof the region is sparse,and the
be distributedinto the North Americanplate and is not carried linesare not orientedoptimallywith respectto eachstructure.
solely by the QCF; any one of the northwesterlyMiocene In addition, there were difficultiesin correlating reflections
extensionalfaults in Dixon Entrance couldbe active,although acrossstrike-slipfaultsand basementhighs;strike-slipmotion
little seismicityhasbeen detectedon them in the last50 years. rendersthe two-dimensionalassumptioninvalid.Nevertheless,
An alternateexplanationfor the subsidence
observedeastof line length compressionwas computed for the Miocene/
the QCF on line 1262is that it is causedby flexureanddoesnot Pliocenereflectoras a roughindicationof the amountof comentail plate thinning.The apexof the uplift in North America pression.Two kilometersof shorteningwere accommodated
is 20 km east of the QCF on the continental shelf which would
along63 km of line 5 (3%), 7-9 km of shortening
along118km
create a westwarddip on the plate under the slope.However, of line 4 (6-7%), and 9 km of shorteningalong70 km of line
whichis
the magnitudeof the dip seemstoo large for this processand 7 (13%). Theseestimatesdo not includecompression,
such dips have not been observedon the availableseismic evident immediatelyprior to depositionof the reflector or
reflectionprofiles.
compression
taken up in basementstructures
whichthe reflec5.2.2. Brittle deformation:Pacificplate. The Pacificplate tor does not overlie. Line 5 is roughlyperpendicularto the
is being deformed and duplexedin the terrace. Reflection QCF, and lines 4 and 7 are subparallelto the QCF. Higher
profilesacrossthe terrace image folded and compressedsed- percentagesof shorteningare evidentin the directionof maximentswhich are presumablyriding a substrateof compressed imum compressive
stress(north-south)predictedin a northbasement; line 1263 shows that the outer terrace fault cuts west shear stressregime.
The amount of recent deformation
in the islands is hard to
upper basement.This studyand severalothers [Horn et al.,
ROHR
ET AL.:
TRANSPRESSION
estimatebecausefew Neogenerocksare exposedon Moresby
Island and exposureof rocks on Graham Island is only 3%
[Hickson,1991].SteepTertiary blockfaultsare common,but
senseand age of motion on the faults are difficult to quantify
[e.g.,Thompsonet al., 1991;P. D. Lewiset al., 1991].Vigorous
microearthquakeactivityin northernGraham Island indicates
that present-daydeformationis occurring.
The shapeof the islandssuggests
that they were formed by
the northward push of transpressionand consequentbrittle
and/or ductile thickeningof the crust over the last 5-8 Myr.
Their westernedgeis determinedby the OCF, and the eastern
edgeis subparallelto the currentrelativeplate motion vector.
Their northern edge runs east-westwhich is the predicted
strike direction of compressivestructuresin a northwestern
directedshear stressregime.The shapeof the islandsis also
coincidentwith the net predicted overlap. If simple underthrustingwas occurring,the region of uplift shouldcontinue
north into Dixon Entrance insteadof terminating in an eastwest edge which is seismicallyactive. Shallow sedimentsjust
north of Graham Islandare not compressively
deformed[Rohr
and Dietrich,1992].The coincidenceof seismicactivityand the
coastlineindicatesthat the deformation creating the seismic
activity has been active for some time and has uplifted the
Miocenebasaltsand sediments
whichcompriseGrahamIsland.
8167
Charlotte Soundto 7 km over a distanceof 75 km [Spenceand
Long, 1995].If suchcrustwere presentalongthe QCF at the
onset of transpression,it would easilydeform and could have
absorbedsignificantcompressionto reach the islands'current
thickness
of 27 km.
Compressionaldeformationactiveover periodsof time has
a tendencyto expandlaterally[e.g.,Masekand Duncan, 1998];
new faults and folds carry deformationout from the region of
maximumuplift. The apparentgreater uplift of the southern
islandssuggests
that deformationof North America began in
Moresby Island and progressednorthward creating Graham
Island. This mechanismexplainsvigorousseismicactivity in
northernGraham Island and Hecate Strait and relativequiescence elsewhere.
Paleomagneticdata indicate that mid-Tertiary dikes in the
Queen Charlotte Islands have been tilted down to the north
If the Queen Charlotte Islands were an intact fault-bound
9ø-16ø (and allow tensof kilometersor even 100 km of northward motionof the islandssincethe dikesformed(13-54 Ma).
As noted by Primset al. [1997], the directionof tilt coincides
with the currentplates'relativemotion.More recentwork (E.
Irving, personalcommunication,1999) revealsthat locations
just north of the seismogeniczone in Graham Island are not
tilted. This substantiatesour idea that the seismogenic
zone is
a deformationfront advancingnorthward.Irving et al. [1992]
interpretedthe tilting to occuron pervasiveeast-westMiocene
block [Rohrand Dietrich, 1992]which has been squeezedupward and northwardbetweenthe QCF and a fault whichsplays
off the QCF, then pervasiveearthquake activity under the
islands'easternand northernshoresshouldbe occurring.The
splaycouldbe blind but approachthe surfacenear the eastern
shoreof the islandscreatingthe structuralrelief betweenthe
islandsand the basin.The LouscouneInlet and SandspitFaults
are subparallelto the easternshoreof the islands,but mapping
mappedbut not dated [Lewis,1991]. Nor has the tilting been
independentlydated. The amount of tilting is sufficientlyuniform that it must occur on numerousfaults; otherwise,very
deeprockswouldbe exposedalongstrike.This uniformityis at
oddswith the variety of focal mechanismsand fault planes
interpretedby Bird [1997] but could be explainedif, over the
long term, reverse or one-sidedflower structuresdominated
has not identified
deformation.
evidence for recent motion
on these faults
[P. D. Lewiset al., 1991].If 50 yearsof earthquakerecordingis
representativeof long-term deformation,then the idea of an
intactblockis not viablebecauselittle microearthquakeactivity occurson the eastcoastof the islands[Bentbeet al., 1989;
Bird, 1997].
North American crustadjacentto the QCF may have been
quite thin and been pervasivelyand progressivelythickened
duringthe Plioceneon compressive
or flower structureswhich
are blind. Compressionalflower structurescan consistof many
faultswhich flatten toward the surfacebut do not necessarily
break the surface.Suchstructureshavethe potentialto create
regionaltilting and crustalthickeningwith few visiblesurface
faults. Microseismicitysuggeststhat current deformationoccurs on pervasivefaults from 3 to 20 km depth; mechanisms
can be oblique,extensional,thrust,or strike-slip,but pressure
axes are generally consistentwith the north-northeastcompressiondirection 'which occurs in a north-northwestshear
[Bird, 1997]. No surfacefaults on Graham Islandwhich might
be causedby microearthquakes
have been reported [Hickson,
1991], but the exposureof the surficial geology above the
seismogenicregion is only 3%. In Hecate Strait the largest
strike-slipevent(M = 5.3) is associated
with a strike-slipfault
whichcutsfolded Miocene sediments(Figure 6c).
Evidencethat North American crust adjacentto the QCF
may have been quite thin can be found in a reflection/
refractionprofile that runs just southof Moresby Island and
the Tuzo Wilson Seamounts,line 3. The sedimentarybasins
showno compressivedeformation [Rohr and Dietrich, 1992],
and crystallinecrust thins seawardfrom 23 km under Queen
extensional faults; in one location, such faults have been
Compressionmay alsobe accommodated
within the Coast
Mountains
which
border
the basin on the east. Inboard
of
Queen Charlotte Sound and northern Vancouver Island, sin-
gle-crystalfissiontrack data detectedan accelerationof uplift
rates in the Coast Mountainsafter 5 Ma [O'Sullivanand Parrish, 1995].No compressive
deformationis observedin Queen
Charlotte Sound, but GPS data indicate that northern Van-
couver Island is moving at a rate of about 3 mm/yr to the
northwest[Drageftand Hyndman,1995;Hentonet al., 1998],
implyingthat a smallamountof Pacific-NorthAmericanrelative plate motion is distributedacrossthe margin here. Uplift
of the mountainsin responseto compression
is easilyunderstood if the mountains
are a weak and warm zone as discussed
by Rohr and Curtie [1997].This styleof deformationmight be
occurringinboard of Hecate Strait; single-crystal
fissiontrack
measurementscould easilytest this idea.
5.2.4. Ductile deformation. Intraplate compressionis
probably accommodatedby flow and thickeningwithin the
ductileportionsof the plates.The biggestdifferencebetween
the two platesmechanicallyis that the ductile region consists
purelyof mantlematerialin the Pacificplate but includessome
crustin North America. Thickeningof the crustalductileportion under the islands could result in some uplift, whereas
thickeningof the densermantle ductile portion will tend to
cause subsidence.
In Figure 17 we have schematicallydrawn the deformation
of the ductile portions of the plates as occurringmostly in
North
America
deformation
based on the inference
occurs within
North
that most of the brittle
America.
We have no direct
8168
ROHR
ET AL.:
TRANSPRESSION
measurementsto indicatethat suchthickeningis occurringor 6. Discussion
how that deformationis arrangedat depth; seismicmeasureThe hypothesisfavored in this paper is that the QCF is a
mentsof plate thicknesscould test the idea. If an asymmetry
verticalplate boundaryand that transpression
hasbeenaccomarisessuch that one plate is deeper than another, it might
modated by significantdeformationwithin both the oceanic
initiate a subduction-type
geometry.
and the continentalplates.In this model the Queen Charlotte
Islandsare the direct result of pervasivetranspressive
defor5.3.
Plate Flexure
mation.Previousmodelshavefavoreda geometryin whichthe
5.3.1. Pacific plate. Flexure of the Pacificplate doesnot
oceanicplate is being subducted,the plate boundaryswitches
beginuntil 90 km north of the inceptionof the QCF. Modeling
betweenthrustand strike-slipgeometries[Hyndmanand Ellis,
by Prims et al. [1997] showedthat the load of the terrace is
1981;Yorathand Hyndman,1983], and the islandsare a comore than sufficientto causethe degreeof flexure observed
herentblock.A modelin whichboth plateswere deformedhas
further north in the Pacificplate (Figure5). Densitiesusedto
beenpresentedbyHorn et al. [1984]but wasdismissed
because
load the terrace above assumed basement were lower than
evidenceof deformationwithin North America waslackingat
those found in gravity models;increasingthem above 2000 the time.
kg/m3 increased
thepredicted
downward
flexureto unrealistic
values.
Usinga density
of 2800kg/m3 in theterraceto createa
load of similar
mass indicates
that a smaller
volume
of the
terrace(---70%) is loadingthe plate. This in turn impliesthat
the plate is broken west of the QCF, probablyby the fault
systemwhich definesthe outer edge of the terrace.
5.3.2. North American plate. The relatively consistent
patternof isostaticanomaliesassociated
with the Queen Charlotte Islandson the North Americanplate showsthat the plate
is flexedupwardalongthe QCF and, assumingthat the forces
along the fault are relativelyuniform, that the plate has relativelyconstantmechanicalpropertiesalongthe QCF. The profilescrossedthe fault wherepredictedamountsof convergence
vary from ---20 to 50 km and where basementis above and
below sea level. Some short-wavelength
variationsexist and
maybe relatedto the short-wavelength
heterogeneities
in plate
thicknesspredictedby Dehleret al. [1997] and/orvariationsin
the thermal
field.
The lack of anyunconformityin the uppersedimentswestof
the QCF on line 1262(Figure 10) arguesthat the Pacificplate
did not experiencethe sameflexuralresponseto glacioeustatic
loading that North America did. If the two sedimentarysections overly the same subductingslab which intrudes North
America,then glacialloadingof North Americaaffectsthe slab
dip and consequently
the dip of all sedimentsdepositedon it.
This is readily seen east of the QCF. If there is no slab and
North America is completelydecoupledfrom Pacificby a vertical fault, then the Pacificplate would experienceno flexure
duringglacialcycles.Glacial tilting in continentalcrustwhere
the depth to the brittle-ductiletransitionis 30 km deep is
typically1 m/km [Thorson,1989]. In thinner platesand if the
QCF is free edge,then the amountof flexuremay be significantly greater than this value. Loading of this margin by a
glacieraffordsan interestingtest of the mechanicalnature of
a transformplate boundary.
The possiblepositiveflexurein Dixon Entranceindicatedby
the isostaticanomaliesof profile 1262t is reinforcedby the
observation
that crust of similar thickness in Hecate
Strait has
subsidedand carriesup to 1 km of postriftsediments.In Dixon
Entrance, no such subsidenceor postrift sequenceis visible
[Rohrand Dietrich, 1992]. Lateral heat lossduring stretching
may havecooledit enoughthat there was little postriftsubsidence,or it couldbe activelyheld up by the compressive
forces
developedby the northern push of the Queen Charlotte Islands into the more competent Alexander terrane of the
Alaskapanhandle.Foldingin the northernendsof the subbasinsof Dixon Entrance corroboratethe notion of significant
north-southdirectedcompression
within North America [Rohr
and Dietrich,1992].
6.1.
Comparison to Previous Models
A plate geometrysimilar to that found in zonesof oblique
subductionis at first glancea possiblesolutionto the geometry
of obliquetranscurrentmotion.In Sumatra,strike-slipmotion
is accommodatedby a strike-slipfault in the upper plate 300
km eastof the trench[Fitch,1972].The OBS surveyof Hyndman and Ellis [1981],althoughshort,did locateseveralearthquakes underneath the surface expressionof the QCF and
near Moho at depthsof 18-21 km. Most eventlocationswere
within the OBS array and are probably reliable; a velocity
modelsimilarto that of a coincidentrefractionsurvey[Hornet
al., 1984]wasused.No eventmechanisms
couldbe computed,
but the overwhelming
probabilityis that earthquakes
underthe
QCF are the resultof strike-slipmotion on the QCF. If this is
acceptedas the most likely explanation,then a Sumatra-type
model is not possiblehere. Flexuremodelsand refractiondata
further argueagainstthe existenceof an undeformedsubducting slab.
Previous models tended to assume that all deformation
oc-
curredcloseto the QCF. Mackie et al. [1989]consideredthe
possibilitiesof distributeddeformationin the islandsbut rejected thisprocessbecausethe crustis thinnerthan the global
average.We prefer to take the perspectivethat crustunderthe
islandsis thickerthan adjacentcrustin the mildlycompressed
continental shelf. On the basisof regional structuraltrends,
Rohr and Dietrich[1992]first proposedthat distributeddeformation from the plate boundarywas an importantprocessin
Queen Charlotte Basinand that increasedcompressive
deformation toward the northern end of the basin argued for a
three-dimensionaldistribution of stress,not a simple twodimensionalorthogonal partitioning of compression.Compressivedeformationof North America in the Pliocenewas
recognizedbyHyndmanandHamilton[1993],but theyconsidered the amount of compressionabsorbedby North America
to be negligible.
The model, which proposesa subductingslab, includesa
plate boundarywhich changesdip by 60ø or more at some
unspecifiedtime interval [Hyndmanet al., 1982; Yorathand
Hyndman,1983].Suchchangeswould createlargechangesin
the flexuralresponseof the plates [Primset al., 1997] if they
occurredon timescaleslonger than the viscoelastictime constant, a few hundredyears.If the oceanicplate is only 10 km
under North America, its surface under the terrace would be
depressedby 6 km; unloadingit by rupture alonga strike-slip
fault would releasethis depression.A processwhichinvolves
cyclicloadingand unloadingshouldcreate a recognizablecyclic pattern of angularunconformitieson the oceanicplate.
ROHR
ET AL.:
TRANSPRESSION
Two angular unconformitiesare recognizablein the seismic
data;one appearsto be associated
with onsetof flexuralbending or heavysedimentationbeginningin the Pleistoceneand
anotheris associatedwith deformationon a fault definingthe
outer edge of the terrace. A cyclicpattern is not currently
recognized.If the major fault plane switcheddip at ratesof less
than a few hundredyears,we would not necessarilyexpectto
see a flexuralresponsein either plate.
Creating a new transformboundaryeveryso often through
the entire Pacificplate seemsmechanicallyinefficientand implausible.Laboratoryexperimentson rupturingnew strike-slip
faults in transpressive
stressregimes[e.g., Wilcoxet al., 1973]
showthat the new faultsform in smallirregularto en echelon
segments.The currentsurfaceexpressionof the QCF as seen
in GLORIA data is of segmentsnearly100km long;the southern segmentsare parallel to the directionof the pre-Pliocene
plates' relative motion [Hyndmanand Hamilton, 1993;Rohr
and Currie,1997].Thusthe large-scalemorphologyof the QCF
arguesagainstthis major fault beingrecreateddownto 20 km
every now and then.
The three-dimensional consequencesof flipping plate
boundariesby 60ø have not been explored.The 1949 magnitude 8.1 event ruptured roughly500 km of the Fairweather/
QCF, but not the entire QCF, implyingthat segmentsof the
fault would be out of synchwith regard to transcurrentor
subductionfaulting. Geologicconsequences
of this phenome-
8169
vide no independentinformationon whether or not a subduction slab exists.Such a geometryis permissibleif the slab
mergessmoothlywith a flat continentalMoho alongall 400 km
of the plate boundary;a verticalfault separatingthe two plates
is equallypermissible.
A plate boundarymodelin whicha subductionslabtakesup
most of the compressionpredictsthat the westernside of the
Queen Charlotte Islandsis substantiallyuplifted and eroded
[Yorathand Hyndman,1983],deepeningthe level of exposure
of crustalrocksto the west.This is not observed[Thompsonet
al., 1991;P. D. Lewiset al., 1991];instead,the exposuredeepensto the south.Coupledwith the up-to-the-southpaleomagnetictilts and focalmechanisms
showingnorth to northeasterly
compression,
the casefor historicaland present-daynortherly
compressivedeformationof the islandsis clear.
6.2. Strength of Plate Boundary and Seismicity
On manytransformplate boundaries,deformationappears
to be partitionedbetweenpure strike-slipand pure compressivemotion, indicatingthat the main fault itself isweak relative
to the plate [Mountand Suppe,1987].In the Queen Charlotte
region,fault mechanismsfrom the Pacificand North American
plates have oblique slip as well as pure thrust, extension,and
strike-slipmotion. The evidencegiven abovethat the Queen
CharlotteIslandsare beingpushednorth into Dixon Entrance
non should be observable.
whichis beingpushedinto the Alaskapanhandlesuggests
that
The structuraland mechanicalconditionsof the plates exa simple two-dimensionalpartitioningof motion is not occuristingat commencementof transpression
are extremelyimporring in this portion of North America.
tant. A subductionzone with a well-developedslabwhich unSeismicitycan also indicate the relative strengthof a fault.
dergoesa changein relativemotionto obliqueunderthrusting
Bird
[1997] has calculatedthat between 1951 and 1981 the
will probably retain its subductiongeometry. Transcurrent
ratewasabout4 x 1028N m/yr.If seismicity
faultingin the upperplate is a commonlyobservedmechanical momentrelease
solutionto this stressregime.A transcurrentplate boundary, takesup all of the relativeplate motion, then the QCF from
to the Alaskaborder(400km) has
however,will not instantaneously
transmogrifyinto a subduc- the Tuzo WilsonSeamounts
a
shear
modulus
of
10
GPa
for a 20-km-deep seismogenic
tion zonebecausethe relativemotionvectorchangesgradually
zone,
which
is
quite
weak.
If
the
shearmodulushasvaluesof
by 70-25ø.Initiation of subductionis not a trivial matter especially in youngbuoyantoceanicplates.The age of the Pacific 20-30 GPa, a value consideredtypical for hot mafic plates
et al., 1998],thenthe amountof strainaccommodated
plate adjacentto the Queen Charlotte Basin at 5 Ma was not [Kreemer
significantlydifferent than it is now. In the caseof the QCF in by earthquakesis 30-50%. It may be that the differencebewhich both plates adjacentto the fault are warm and weak, it tween the strengthof North America and the main fault zone
seemslogicalthat deformationwithin both of theseplateswill is not great enoughto producestrainpartitioning.
take up the compression,
as is observed.Large amountsof net
The conceptthat strain partitioning(or lack thereof) is a
compressionmay well initiate subduction,but tens of kilome- unique indicatorof fault strengthmay be oversimplified.Preters do not seem to be large enough. If the brittle-ductile existinggeometryof the plate boundaryand heterogeneities
transitionin North Americawassignificantly
shallowerthan it within the platesmayoverridesucheffectsduringdeformation.
is now when transpressionbegan, then it is conceivablethat The fact that compressionwithin Hecate Strait occursmost
brittle Pacific plate may have intruded the North American visiblyby inversionof Mioceneextensionalfaultsindicatesthat
ductile zone. Once started,that processwould probablyconpreexistingstructureshave stronglyinfluencedregionaldefortinue. However, current seismicityand flexural studiesargue mation.
againstthere now being a subductedslab.
The model proposedhere arguesthat a "megathrust"event
Refraction studiesof the Queen Charlotte Basin mapped
betweenunderthrustoceanicand overridingcontinentalplates
thinned continentalcrustand relativelyflat Moho. Basement
under the easternportionsof the islandsis -4 km thickerthan as predictedby Yorathand Hyndman [1983] and modeledby
under the basin; there was no direct evidence for a subducted Smithet al. [1998]is highlyunlikely.The modelproposedhere
slabof Pacificcrustin the work of SpenceandAsudeh[1993] can explaincurrent seismicityin Graham Island and adjacent
and Hole et al. [1993].In the roughly30 km betweenthe QCF Hecate Strait as deformationwhich is propagatingnorthward
and the westernmostlimit of thesestudiesthe authorspostu- from the hinterland,GrahamIsland.To date,intraplateearthlated that a subductedslabmightexist,but it wouldhaveto dip quakesare typicallysmaller than magnitude5. It shouldbe
noted,however,that blind thrustsin the "bigbend" of the San
at angles>20ø-26ø.
Gravity modelsacrossthe QCF concurwith refractiondata Andreas Fault systemhave producedearthquakesof magnithat Moho is nearly flat under the islands.However, similar tude 6.7 [SouthernCaliforniaEarthquakeCommitteeand U.S.
crustaldensitiesin both platesmeansthat gravitymodelspro- GeologicalSurveyStaff, 1994].
8170
ROHR ET AL.: TRANSPRESSION
6.3. Comparisonto Other Regions
changes
in relativeplatemotions
[Pockalny
et al., 1997];the
adjacent
veryyoungplatewill deformeasily,andthe oldest
The Pacific-North
Americaplateboundary
to the southof
the regionthatwe considered
liesentirelywithinyounger, platealongthe transformwill be moreresistantto deformation
The OCF strikesat a lowangleto the
weakeroceanic
material.
Adjustment
to thechange
in relative duringtranspression.
anomalies
resulting
in a slowchange
inplate
motionsfrom 8 to 5 Ma hasoccurred
by ridgemigration, trendofmagnetic
jumps,and,currently,
deformation
in a wideshearzone[Rohr age alongstrike,and the flexuralanomaliesof North America
thatlittlechange
in mechanical
behavior
occurs
along
and Furlong,1995;Kreemeret al., 1998].To the norththe suggest
strike.
amountof transpression
is smaller;the terraceis lessdistinct
[Brunsand Carlson,1987],andthereis little obviousdeforma-
tionin NorthAmerica[Gehrels
etal., 1987].ThePacific
plate 7.
isolderandstronger,
andin NorthAmerica
thecrustchanges
Conclusions
Transpression
acrossthe Pacific-NorthAmericaplate
boundary
ofwestern
Canada
ismostlikelyaccommodated
by
strike-slip
motionalongtheOCFandbothstrike-slip
andcomdeformation
withineachplate.The QCFis a longRegions
of transpression
whicharecurrently
beingstudied pressive
from the Wrangellto the Alexanderterrane,whichdid not
experience
the samedegreeof warmingandextension
in the
Miocene[Gabrielse
andYorath,1991].
plateboundaryseparating
two warmmafic
withmodernseismic
techniques
areNewZealandandCalifor- livedtransform
nia.Net compression
alongthe centralSanAndreasFaulthas plates.The oceanicplateis 7-15 Myr old andNorthAmerica
of the maficWrangellterranethat washeatedby
beenestimated
to be 28-72km,andall of it isthought
to be consists
extension.
Thebrittle-ductile
transition
inbothplates
accommodated
by intraplatedeformationon both sidesof the Miocene
SanAndreasFault[Crouch
etal., 1984]aswellasdelamination is currentlybetween13 and 21 km depth. The threeinteraction
of theplateboundary
andtherelative
[Wallace,1990], althoughthe exact structuresare not well dimensional
known. Inversion structuressimilar to those in Hecate Strait
are observed east of the San Andreas Fault in the Kettleman
motionvectorputsmostof the deformationin North America.
A smallamountof transpression
in theoceanic
plateisaccomregionof duHillsandCoalinga
Anticline[Namson
andDavis,1988].In the modatedwithinthe terrace,an ---30-km-wide
Transverse
Ranges
a largehigh-velocity
anomaly
in themantle plexedoceanicplate and folded sediments;loweredcrustal
[Humphreys
etal., 1984;Kohler,1999]hasbeeninterpreted
to velocities attest to the deformation of basement material. In
of Mioceneextenbe ductilecontinentalplate bowedinto the mantleby North America,uplift and compression
basins
attestto distributed
deformation.
Theportionof
transpression.
In New Zealand,interpretations
haveincluded sional
overlapeastof the
theideathata platecanbe delaminated
[Walcott,
1998];its theplatelyingin theregionof predicted
andits crustis
upperbrittlepart is peeledoff andforcedupward,whilethe OCF is undernortherlydirectedcompression
being thickenedto form the Queen Charlotte Islands.
lowersection
sinksunderthe adjacent
plate.Morerecently,
extensive seismic measurements show both extensive anisot-
ropy[Klosko
etal., 1999]anda high-speed
velocity
anomaly
in
Acknowledgments. We would like to thank the tirelessefforts of
themantlebeneath
NewZealand[Molnar
etal., 1999].These MasterIan Youngandthecrewof theR/l/MauriceEwingforthedata
acquisition
andJohnDiebold
andLincHollister
fortheirgenerosity
in
sharing
the
data.
Support
for
this
work
has
been
provided
by
NSF,
deforming
in a wideshearzoneandnotin a subduction
ge- EAR-95-27001
EAR-92-19002
and EAR-92-19870.
Interpretations
ometry.It shouldbe notedthatbothregions
are dominantly owemuchtodiscussions
withKevinFurlong
andTomJames.
Helpful
silicicin thecrustal
domainandtheplatemayhavea layerof reviews
of themanuscript
havebeenprovided
byRonClowes,
Ralph
TomJames,
andan
veryweak crustalmaterialabovestrongermantle.This me- Currie,SonyaDehler,KevinFurlong,RobGovers,
Editor.ThisisGeological
Survey
of Canada
paper1999079.
chanical
profile,ascontrasted
withthesimpler
profileof mafic Associate
resultshave been interpretedto indicatethat the mantle is
plateswouldaffect the plates'responseto transpressive
stresses.
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M. Scheidauer,Institut de Geophysique,Universitdde Lausanne,
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(Maren.Scheidhauer@ig.unil.ch)
A.M. Trehu, Collegeof Oceanography,Oregon State University,
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(ReceivedMay 10, 1999;revisedOctober28, 1999;
acceptedNovember5, 1999.)