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 transpressivetransform plate boundary betweenthe Pacificand North American platesoffshore western Canada. Previous models for the accommodationof transpressioninclude internal deformationof both plates adjacentto the plate boundaryor obliquesubduction of 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 down to the first water bottom multiple, significantsubsidence east of the Queen Charlotte Fault, a large melangewhere the fault is in a compressiveleft step, and faulting which involvesoceanic basement. Gravity modelingof profilesindicates that Moho varies fairly smoothlyacross the plate boundary.Isostatic anomalies indicate that the Pacific plate is flexed downwardadjacent to the Queen Charlotte Fault. Upward flexure of North America alongwith crustthickened relative to crust in the adjacentbasin creates topographyknown as the Queen Charlotte Islands.Combined with other regionalstudies, these observationssuggest that the plate boundaryis a vertical strike-slipfault and that transpressionis taken up within each plate. 1. Introduction Atwater,1989; Furlong, 1993]. In comparison,the QCF hashad a relatively simple plate history for the last 20 Myr or more Ideas on accommodationof transpressionhave been heavily [Rohrand Currie,1997] and providesa goodlocation to study influencedby studiesof oblique subductionzones [e.g., Mc- the poorlyunderstood geologic effects of transpressionin both Caffrey, 1996; Burbidgeand Braun, 1998]. How a transform oceanicand continentalplates. plate boundarywith a near-verticaldip changesinto a shallow Two main modelshave been presented for plate interactions dipping subductionzone is not obvious.At the onset of and on the QCF: one in which transformfaulting alternateswith duringtranspression the relativemechanical properties of the subductionthrusting [Hyndman and Ellis, 1981] and one in platesand geometryof the plate boundaryare critical to un- which transform motion is accommodated on the QCF and derstandingsubsequent deformation. In the casestudied here, compressionby deformation of the oceanic plate and the the initial conditions are that of a well-established transform Queen Charlotte Islands[Mackie et al., 1989]. The latter was fault separatingplates with similarmechanical structures: thin, considered an unlikely solution because they believed that warm, and mafic. hundredsof kilometersof compressionhad to be accommo- The Queen CharlotteFault (QCF) separatesa youngPacific dated (Figure 1) [Yorathand Hyndman, 1983;Mackie et al., plate from an anomalouslymafic and recentlythinned section 1989]. Their tectonicreconstructions of the Queen Charlotte of the North American plate in western Canada (Figure 1). region and the Explorer plate placed the commencementof Reconstructionsof global plate motionsindicate a small but transpressionjust north of VancouverIsland. The lack of com- significantcomponent of compressionduring the Pliocene(5 pressivestructures in Queen Charlotte Sound and abundant Ma) [Engebretson,1985; Norton, 1995; Stock and Molnar, compressivestructures in Hecate Strait [Rohr and Dietrich, 1988], which may have begun at 8 Ma [Atwaterand Stock, 1992] indicate that significantongoing transpression begins 1998].A more southerlysection of the Pacific-NorthAmerica north of the Tuzo Wilson Seamounts. Since then, Hyndman plate boundary,the central San Andreas Fault, is thought to andHamilton [1993]used the polesof Stockand Molnar [1988] absorb 14-72 km of estimated transpressionsolely by in- and Engebretson[1985] for a simplecalculation that 80 km of traplate deformation[Crouch et al., 1984; Wallace,1990]. In compressionhad occurredacross the QCF, althoughthe tec- California,studying geologic effects of relativeplate motionsis tonic model of a subductedslab [Hyndmanand Ellis, 1981; complicatedby migration of triple junctionswhich produces Hyndman et al., 1982; Yorath and Hyndman, 1983] was not significantvariations in mechanicalproperties with time [e.g., revised.Prims et al. [1997] placedthe relativeplate motions' vectoron a map to showthat the averagenet predictedoverlap 1Nowat RohrGeophysics, North Saanich, British Columbia, Canada. of the plates is only tens of kilometers;the value gradually 2Nowat Institutde Geophysique,Universit6 de Lausanne,Lau- increases from zero at the Tuzo Wilson Seamounts to a max- sanne, Switzerland. imum of 80 km over a lateral distance of 350 km. Large Copyright2000 by the American GeophysicalUnion. amountsof compressionmight well needa subductionzone to be Paper number 1999JB900403. accommodated;tens of kilometersmay not [Crouchet al., 1984]. 0148-0227/00/1999JB900403 $12.00 Other studiesin the last 10 years such as microseismicity, 8147 8148 ROHR ET AL.: TRANSPRESSION 134 132 130 128 Dixon North American plate .54 Pacific plate 52. 52 Wilson SeamountsTuzo /•/ XExporer• - ß - 1•4 132 130 128 0 50 100 150 200 km Figure 1. The Queen Charlotteregion, western Canada. Stippled area indicatesthe amountof compression that was thoughtto haveoccurred in the last 5 Myr [after Mackie et al., 1989] and darker stipplingshows the amountof overlapif transpressionstarts at the Tuzo Wilson Seamountsat 5 Ma. If relativeplate motions beganto changeat 8 Ma [Atwaterand Stock,1998], then the easternboundary of the predictedoverlap simply projectslinearly across Dixon Entranceand Alaska.Whenever transpression began, net predictedtranspres- sionreaches a maximumat GrahamIsland; the instantaneousangle of transpressiondecreases to the north asthe QueenCharlotte Fault (QCF) trendsmore northerly.Inset showslocation of figurein regionalcontext. Graham and MoresbyIslands are the principalislands of the Queen Charlottegroup. These islands, a region of thickercrust, lie eastof the QCF in the regionof predictedplate overlap.Their easternshore is subparallel to the azimuthof relativeplate motion as shownby arrow.The relativeplate motion vector was calculated from NUVEL-1 [DeMetset al., 1990].Box in northwestcorner shows approximate area of Figure2. refraction, and heat flow data, geologicmapping and paleo- 2. Overview of Regional Tectonics magneticstudy of the Queen Charlotte Islands,modeling of gravity anomaliesand flexure of the Pacificplate, and recon- 2.1. Queen Charlotte Fault structionsof sealevel duringthe last glacialretreat suggestthat Between the Tuzo Wilson Seamounts and the Alaska border a rethinkingof lithosphericinteractions along the QCF is in the QCF is clearly imaged by GLORIA side-scandata as a order. The regional implicationsof this work are combined distinctline in the seafloor[Bruns et al., 1992]; the QCF be- with new multichannelseismic reflection data and gravitymodels tween 54ø and 55øN is shown in Figure 2. In the northern acrossthe northernQCF to infer that compressionis mostlikely region,where the multichanneldata were collected,two linear accommodatedby deformationwithin both platesand that the segmentscan be observed;the northern segmentstrikes 338 ø, QCF continuesto accommodatemost of the strike-slipmotion. and the southernsegment strikes 328 ø . The differencein angle ROHR ET AL.' TRANSPRESSION 8149 135•001N 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 trends 337 ø and the southern328 ø. The NUVEL-1 vectortrends about 345 ø here. They meet in a compressiveleft step imagedby line 1250.Fine white lines showlocation of seismicdata. and left stepbetween them createsa compresslyebend. Stri- strike the terracechanges from a singleblock defined by one ationsconsistent with compresslyestructures formed by north- main scarpinto severalblocks defined by scarpswhich appear west directed shear are observed in the bend. In other loca- to splayoff the QCF [Scheidhauer,1997]; this changein mor- tions,canyons and striationsappear to be offsetright laterally phologymay be relatedto decreasedobliquity of transpression by a few kilometers. to the north (A.M. Trehu and M. Scheidhauer,manuscript in 2.2. Plate Morphology preparation,2000). The trough, of probableflexural origin This oceanic-continentaltransform fault is characterizedby [Hyndmanet al., 1982;Prims et al., 1997], is severalhundred an abrupt transitionbetween the two plates(Figure 3). The metersdeeper than the abyssalplain andbegins 90 km northof continentalshelf under Hecate Strait is fairly flat with two the inception of the QCF at the Tuzo Wilson Seamounts. main incised channels. West of the Queen Charlotte Islands North of the Alaska border the terraceis lessdistinct [Bruns the shelfis <10 km wide,and the continentalslope consists of and Carlson,1987], and there is little obviouscontemporary a terrace,roughly 30 km wideand 1500-2000m deepbound by deformationin the North Americanplate [e.g.,Gehrels et al., two scarps.The outer scarpof the terracesteps down to water 1987].South of the Tuzo WilsonSeamounts the oceanicplate depthsof 2500-3000 m, the Queen CharlotteTrough. Along is veryyoung and broadly deformed [Rohr and Furlong, 1995], 8150 ROHR ET AL.: TRANSPRESSION Figure 3. Regionalmorphology