Transpression between two warm mafic plates: The Queen Charlotte Fault revisited

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B4, PAGES , APRIL 10, 2000 Transpressin between tw warm mafic plates: The Queen Charltte Fault revisited Kristin M. M. Rhr Gelgical Survey f Canada, Ottawa Maren Scheidhauer 2 and Anne M. Trehu Cllege f Oceangraphy, Oregn State University, Crvallis Abstract. The Queen Charltte Fault is a transpressive transfrm plate bundary between the Pacific and Nrth American plates ffshre western Canada. Previus mdels fr the accmmdatin f transpressin include internal defrmatin f bth plates adjacent t the plate bundary r blique subductin f the ceanic plate; the latter has been the preferred mdel. Bth plates are warm and mafic and have similar mechanical structures. New multichannel seismic reflectin data shw a near-vertical Queen Charltte Fault dwn t the first water bttm multiple, significant subsidenceast f the Queen Charltte Fault, a large melange where the fault is in a cmpressive left step, and faulting which invlves ceanic basement. Gravity mdeling f prfiles indicates that Mh varies fairly smthly acrss the plate bundary. Isstatic anmalies indicate that the Pacific plate is flexed dwnward adjacent t the Queen Charltte Fault. Upward flexure f Nrth America alng with crust thickened relative t crust in the adjacent basin creates tpgraphy knwn as the Queen Charltte Islands. Cmbined with ther reginal studies, these bservatinsuggesthat the plate bundary is a vertical strike-slip fault and that transpressin is taken up within each plate. 1. Intrductin Ideas n accmmdatin f transpressin have been heavily influenced by studies f blique subductin znes [e.g., Mc- Caffrey, 1996; Burbidge and Braun, 1998]. Hw a transfrm plate bundary with a near-vertical dip changes int a shallw dipping subductin zne is nt bvius. At the nset f and during transpressin the relative mechanical prperties f the plates and gemetry f the plate bundary are critical t understanding subsequent defrmatin. In the case studied here, the initial cnditins are that f a well-established transfrm Atwater, 1989; Furlng, 1993]. In cmparisn, the QCF has had a relatively simple plate histry fr the last 20 Myr r mre [Rhr and Currie, 1997] and prvides a gd lcatin t study the prly understd gelgic effects f transpressin in bth ceanic and cntinental plates. Tw main mdels have been presented fr plate interactins n the QCF: ne in which transfrm faulting alternates with subductin thrusting [Hyndman and Ellis, 1981] and ne in which transfrm mtin is accmmdated n the QCF and cmpressin by defrmatin f the ceanic plate and the Queen Charltte Islands [Mackie et al., 1989]. The latter was fault separating plates with similar mechanical structures: thin, cnsidered an unlikely slutin because they believed that warm, and mafic. hundreds f kilmeters f cmpressin had t be accmm- The Queen Charltte Fault (QCF) separates a yung Pacific dated (Figure 1) [Yrath and Hyndman, 1983; Mackie et al., plate frm an anmalusly mafic and recently thinned sectin 1989]. Their tectnic recnstructins f the Queen Charltte f the Nrth American plate in western Canada (Figure 1). regin and the Explrer plate placed the cmmencement f Recnstructins f glbal plate mtins indicate a small but transpressin just nrth f Vancuver Island. The lack f cmsignificant cmpnent f cmpressin during the Plicene (5 pressive structures in Queen Charltte Sund and abundant Ma) [Engebretsn, 1985; Nrtn, 1995; Stck and Mlnar, cmpressive structures in Hecate Strait [Rhr and Dietrich, 1988], which may have begun at 8 Ma [Atwater and Stck, 1992] indicate that significant nging transpressin begins 1998]. A mre sutherly sectin f the Pacific-Nrth America nrth f the Tuz Wilsn Seamunts. Since then, Hyndman plate bundary, the central San Andreas Fault, is thught t and Hamiltn [1993] used the ples f Stck and Mlnar [1988] absrb km f estimated transpressin slely by in- and Engebretsn [1985] fr a simple calculatin that 80 km f traplate defrmatin [Cruch et al., 1984; Wallace, 1990]. In cmpressin had ccurred acrss the QCF, althugh the tec- Califrnia, studyin gelgic effects f relative plate mtins is tnic mdel f a subducted slab [Hyndman and Ellis, 1981; cmplicated by migratin f triple junctins which prduces Hyndman et al., 1982; Yrath and Hyndman, 1983] was nt significant variatins in mechanical prperties with time [e.g., revised. Prims et al. [1997] placed the relative plate mtins' vectr n a map t shw that the average net predicted verlap 1Nw at Rhr Gephysics, Nrth Saanich, British Clumbia, Canada. f the plates is nly tens f kilmeters; the value gradually 2Nw at Institut de Gephysique, Universit6 de Lausanne, Lau- increases frm zer at the Tuz Wilsn Seamunts t a maxsanne, Switzerland. imum f 80 km ver a lateral distance f 350 km. Large Cpyright 2000 by the American Gephysical Unin. amunts f cmpressin might well need a subductin zne t be Paper number 1999JB accmmdated; tens f kilmeters may nt [Cruch et al., 1984] /00/1999JB $12.00 Other studies in the last 10 years such as micrseismicity, 8147

2 8148 ROHR ET AL.: TRANSPRESSION Dixn Nrth American plate.54 Pacific plate 52. Wilsn Seamunts Tuz 52 / / XExprer ß km Figure 1. The Queen Charltte regin, western Canada. Stippled area indicates the amunt f cmpressin that was thught t have ccurred in the last 5 Myr [after Mackie et al., 1989] and darker stippling shws the amunt f verlap if transpressin starts at the Tuz Wilsn Seamunts at 5 Ma. If relative plate mtins began t change at 8 Ma [Atwater and Stck, 1998], then the eastern bundary f the predicted verlap simply prjects linearly acrss Dixn Entrance and Alaska. Whenever transpressin began, net predicted transpressin reaches a maximum at Graham Island; the instantaneus angle f transpressin decreases t the nrth as the Queen Charltte Fault (QCF) trends mre nrtherly. Inset shws lcatin f figure in reginal cntext. Graham and Mresby Islands are the principal islands f the Queen Charltte grup. These islands, a regin f thicker crust, lie east f the QCF in the regin f predicted plate verlap. Their eastern shre is subparallel t the azimuth f relative plate mtin as shwn by arrw. The relative plate mtin vectr was calculated frm NUVEL-1 [DeMets et al., 1990]. Bx in nrthwest crner shws apprximate area f Figure 2. refractin, and heat flw data, gelgic mapping and palemagnetic study f the Queen Charltte Islands, mdeling f gravity anmalies and flexure f the Pacific plate, and recnstructins f sea level during the last glacial retreat suggesthat a rethinking f lithspheric interactins alng the QCF is in rder. The reginal implicatins f this wrk are cmbined with new multichannel seismic reflectin data and gravity mdels acrss the nrthern QCF t infer that cmpressin is mst likely accmmdated by defrmatin within bth plates and that the QCF cntinues t accmmdate mst f the strike-slip mtin. 2. Overview f Reginal Tectnics 2.1. Queen Charltte Fault Between the Tuz Wilsn Seamunts and the Alaska brder the QCF is clearly imaged by GLORIA side-scan data as a distinct line in the seaflr [Bruns et al., 1992]; the QCF between 54 ø and 55øN is shwn in Figure 2. In the nrthern regin, where the multichannel data were cllected, tw linear segments can be bserved; the nrthern segment strikes 338 ø, and the suthern segment strikes 328 ø. The difference in angle

3 . ROHR ET AL.' TRANSPRESSION N 'W 134( 00 Kilmeters Figure 2. GLORIA data [Bruns et al., 1992]. The QCF is evident as a linear feature n the seaflr. The nrthern segment trends 337 ø and the suthern 328 ø. The NUVEL-1 vectr trends abut 345 ø here. They meet in a cmpressive left step imaged by line Fine white lines shw lcatin f seismic data. and left step between them creates a cmpresslye bend. Striatins cnsistent with cmpresslye structures frmed by nrthwest directed shear are bserved in the bend. In ther lca- tins, canyns and striatins appear t be ffset right laterally by a few kilmeters Plate Mrphlgy This ceanic-cntinental transfrm fault is characterized by an abrupt transitin between the tw plates (Figure 3). The cntinental shelf under Hecate Strait is fairly flat with tw main incised channels. West f the Queen Charltte Islands the shelf is <10 km wide, and the cntinental slpe cnsists f a terrace, rughly 30 km wide and m deep bund by tw scarps. The uter scarp f the terrace steps dwn t water depths f m, the Queen Charltte Trugh. Alng strike the terrace changes frm a single blck defined by ne main scarp int several blcks defined by scarps which appear t splay ff the QCF [Scheidhauer, 1997]; this change in mrphlgy may be related t decreased bliquity f transpressin t the nrth (A.M. Trehu and M. Scheidhauer, manuscript in preparatin, 2000). The trugh, f prbable flexural rigin [Hyndman et al., 1982; Prims et al., 1997], is several hundred meters deeper than the abyssal plain and begins 90 km nrth f the inceptin f the QCF at the Tuz Wilsn Seamunts. Nrth f the Alaska brder the terrace is less distinct [Bruns and Carlsn, 1987], and there is little bvius cntemprary defrmatin in the Nrth American plate [e.g., Gehrels et al., 1987]. Suth f the Tuz Wilsn Seamunts the ceanic plate is very yung and bradly defrmed [Rhr and Furlng, 1995],

4 8150 ROHR ET AL.: TRANSPRESSION Figure 3. Reginal mrphlgy with lcatin f new multichannel seismic reflectin data: lines 1250, 1262, 1263, and T is the Queen Charltte terrace. Arrw shws directin f relative plate mtin calculated frm NUVEL-1 [DeMets et al., 1990]. Lcatin f QCF was picked frm GLORIA data (Figure 2). Light shaded line is lcatin f crss sectin shwn in Figure 4. Shrt line segmentshw lcatins f multichannel seismic reflectin lines 5, 6, and 7 displayed in Figure 6. and suth f the Queen Charltte Islands the Nrth American plate is simply subsiding in respnse t Micene extensin [Rhr and Dietrich, 1992]. The age f the Pacific plate increases frm 7 Ma at the suthern end f the Queen Charltte Islands t 15 Ma ff nrthern Dixn Entrance. Magnetic anmalies f the Pacific plate strike apprximately nrth-suth and fade ut under the uter scarp f the terrace [Atwater, 1989]. Curtie, 1997] with a cmpnent f transpressin in the last 5 Myr. Prir t 45 Ma the plate bundary was a subductin zne, but its cnfiguratin and histry are prly knwn. A single fault, the Queen Charltte Fault, is thught t take up mst f the past transcurrent mtin; distributed defrmatin may have ccurred during Micene extensin [Rhr and Dietrich, 1992]. Plicene inversin f Micene basins indicates that cmpressin was distributed acrss the cntinental margin 2.3. Plate Histry ver a width f at least 150 km [Rhr and Dietrich, 1992]. The amunt f predicted cmpressin changes alng strike The tectnics f western Canada are dminated by the bundary between the Pacific and Nrth American plates (Figure 1). Relative mtin between the tw plates has been dminantly.strike slip fr at least 20 Myr, if nt 45 Myr [Rhr and because f the angle f relative mtin and changes in strike directin f the QCF. The difference between the strike f the Queen Charltte Fault ff Mresby Island (320 ø, Figure 1) and the directin f relative mtin f plates is ø in the suth

5 ROHR ET AL.: TRANSPRESSION 8151 = 50 t O0... bserved gravity E -50 f ?,,, / 2 20 ½½?*. ':..-:-:...:,..:..? :: ' ;m * ' ' d " / :; "' * m' VE 4:1 [ 1.49; 0,001,[ 2.20; I 2.35; ; Velcity (km/s); Gradient (km/slkm) 3.80; ; ' ß ' ; ' ' Figure 4. (bttm) Refractin results f Dehler and Clwes [1988]; nte thickening f material with velcities greater than 5.3 km/s in terrace. (tp) Gravity interpretatin f this refractin line cmbined with line 6 [after Spence and Lng, 1995]. Lcatin f prfile is shwn in Figure 3. The white layer crrespnds t crust with velcities greater than 5.3 km/s (crystalline basement); light shading crrespnds t water, medium shading crrespnds t velcities km/s (sediments and upper ceanic crust); and darker shading crrespnds t velcities f km/s (mantle). Numbers are density values mdeled (kg/m3). Triangle marks lcatin f OCF. Dtted line is drawn frm the tp f layer 3 in ceani crust acrss the terrace t shw hw much f the terrace wuld be ccupied by a simple undefrmed slab. The vlume abve the dashed line must then be defrmed ceanic crust and sediments. and decreases nrthward t 7 ø ff Dixn Entrance. Average velcities are nrth-nrthwest cm/yr west f Graham Island and Dixn Entrance. The amunt f predicted verlap f the Pacific and Nrth American plates (Figure 1) was calculated assuming n internal defrmatin f either plate and using the NUVEL-1 vectr [DeMets et al., 1990] ver 5 Myr. Predicted verlap acrss the OCF increases frm zer at the Tuz Wilsn Seamunts t a maximum f 80 km at nrthernmst Graham Island and then rapidly decreases again. Atwater and Stck [1998] have recently suggested that the Pacific Nrth America relative plate mtins changed gradually frm trans- current t transpressive beginning at 8 Ma. This wuld increase the amunt f verlap predicted in Dixn Entrance t 100 km but des nt affect the rest f the map shwn Structure A multichannel survey in 1977 imaged flded sediments in the terrace and inverted basins f nrthern Dixn Entrance [Snavely et al., 1981]. The terrace's deeper structure was studied in tw refractin transects at abut 52øN [Hrn et al., 1984] and 53øN [Dehler and Clwes, 1988] (Figure 4, bttm); interpretatins featured crust in the terrace which is lwer in seis-

6 ß 8152 ROHR ET AL.' TRANSPRESSION -5 -t W-E '0 QCF Trugh Terrace Islands _. - ' ß ' ' ' i ' ' "'... "' '... '--. i'l. _ '-.._... Present bathymetry... Te =! 5 km (25 km f underthrusting) Te = 15 km (N underthrusting) - 'Z, Observed tp f plate l 5 Distance (km) Figure 5. Flexural mdeling by Prims et al. [1997] shws that the weight f the terrace alne is sufficient match bserved flexure assuming a density f 2000 kg/m 3. Under the QCF, heavy bar represents tp f putative plate in gravity mdels (see Figure 15). Thrusting f a slab under Nrth America predicts a greater amunt f dwnwarp than is allwed by gravity mdels. Dwnwarp is taken frm a reflectin prfile west f Graham Island [Rhr et al., 1992]. mic velcities than crust n either side. Velcities graded frm 5.3 t >7 km/s abve Mh and crust thickened frm 7 t km tward the cntinent. Mst f the rays which defined Mh under the terrace als traversed the Queen Charltte Islands, where crustal structure was nt cnstrained. Several gravity mdels based n these refractin studies as well as refractin interpretatins in the Queen Charltte Basin interpreted that Mh dips eastward under the terrace and then flattens under the islands (Figure 4, tp) [Hrn et al., 1984; Mackie et al., 1989; Sweeney and Seemann, 1991; Spence and Asudeh, 1993; Spence and Lng, 1995]. High velcities ( km/s) in the lwer cntinental crust [Spence and Asudeh, 1993] indicate that its cmpsitin is dminantly mafic. This is cnsistent with the surface gelgy f the Queen Charltte Islands, which includes 7-10 km f Karmutsen basalt flws and shws that they are part f the Wrangellia terrane [P. D. Lewis, et al., 1991], which was accreted in the Cretaceus [Gabrielse and Yrath, 1991] Flexure and Vertical Mtins Bathymetry and isstasy indicate that the Pacific plate is being flexed [Harris and Chapman, 1994]. In three dimensins the wavelength and amplitude f flexure vary with the strength f the plate as it increases with age. A flexural trugh is nt evident in the suthernmst 90 km f the fault, indicating that until this pint the lad f the terrace r the cmpressive stresses are insufficient flex the plate. Calculatins f flexure (Figure 5) [Prims e! al., 1997] fund that a mdel f underthrusting predicted t much dwnwarp f the Pacific plate. Instead, a clean break at the QCF matched dwnwarp bserved in a reflectin prfile and a depth f 12 km t the tp f a putative undefrmed ceanic plate at the QCF. In the Micene, extensinal faults and subsidence were active in Queen Charltte Sund and Hecate Strait frming the Queen Charltte Basin [Rhr and Dietrich, 1992; Hyndman and Hamiltn, 1993; Lwe and Dehler, 1995; Dehler et al., 1997; Rhr and Currie, 1997]. The basin cnsists f a netwrk f half grabens and grabens acrss a 200-km-wide regin. Plate mtins have been fairly cnstant between 40 and 5-8 Ma; a previusly pstulated time f transtensin is n lnger thught t have ccurred [Nrtn, 1995; Atwater and Stck, 1998]. The mde and cause f extensin remain enigmatic, but cincident acceleratin f uplift f the Cast Muntains led Rhr and Currie [1997] t prpse that extensin was accmmdated n a large lw-angle detachment. Transpressin has inverted subbasins inbard f the Queen Charltte Islands (Figure 6) [Rhr and Dietrich, 1992] but nt in Queen Charltte Sund. Cmpressin defrmed sme upper Micene sediments (e.g., sht pints (SP) , Figure 6a) and was fllwed by brad flding and tilting up t the west which included Plicene pstrift sediments. Strike-slip faults and flwer structures cut the flds. Glacial scuring and fast tidal currents have planed ff the upthrust sediments (Figures 6b and 6c); nly the nrthernmst basins (e.g., Figure 6c) are seismically active [Bird, 1997]. N quantitative wrk has been dne t assess the uplift histry f the Queen Charltte Islands, yet their tpgraphy and gelgy indicate uplift which increases suthward alng strike. Muntains n the western Queen Charltte Islands exceed elevatins f 1000 m. In Mresby Island, upper crustal Palegene bathliths are expsed [Andersn and Reichenbach, 1991], but n Graham Island, Negene rcks frmed at sea level are just abve current sea level. Yrath and Hyndman [1983] suggested that the Queen Charltte Islands were flexed abve sea level in the last 6 Myr by underthrusting; they did nt cnsider alng-strike variatins in flexure. Quaternary histry includes peridic glacial lading ver the last 2 Myr accmpanied by rapid sea level changes caused by eustatic variatins as well as significant reginal flexural effects. A glacier in Dixn Entrance came within 12 km f the QCF nly 11,000 years ag; as it retreated, sea level rse by 100 m in nly 1000 years [Barrie and Cnway, 1996]. This rapid rise has been dcumented as a reginal phenmenn [Barrie et al., 1991; Jsenhans et al., 1995] and suggests a viscelastic time cnstant f a few 100 years r less (T. James, persnal cmmunicatin, 1998), which is significantly smaller than time cnstants used fr mst f Nrth America. The rapid reginal sea level changes may nly be pssible given the warm state f the cntinental lithsphere and if the QCF is a free edge (T. James, persnal cmmunicatin, 1998) Seismicity The Queen Charltte Fault is seismically active (Figure 7); Canada's largest recrded earthquake, a magnitude 8.1, c- curred n it in Fr that event, Bstwick [1984] fund a rupture length f-490 km (300 km nrth and 190 km suth f the epicenter) with an average cseismic displacement f m. The azimuth f the vertical fault plane crrespnds with the strike f the Queen Charltte Fault, implying that n cnvergence was taken up by this event. A shrt cean bttm seismmeter (OBS) survey defined a near-vertical zne f seismicity t depths f 20 km under the QCF cutting dwn t Mh [Hyndman and Ellis, 1981]. A study f reginal seismicity between 1982 and 1996 [Bird, 1997] als fund that events clustered arund the QCF lie in a near-vertical band. This study nly had arrivals recrded frm the eastern side f the QCF fr each event, but events lcated n the QCF alng the middle uter edge f the array did nt significantly change lcatin slutins when different depths were used t lcate a given event. This implies that the earthquakes are distributed vertically. This study culd nt, hwever, determine the maximum depth f faulting.

7 ROHR ET AL.: TRANSPRESSION 8153 c) Figure 6. Inversin f basins frmed in the Micene; see Figure 3 fr lcatin [after Rhr and Dietrich, 1992]. P, Plicene sediments; um, upper Micene; and 1M, lwer Micene sediments as determined frm well ties. B indicates prbable basement rcks. (a) Line 5 shwing basement faults belw flding in sedimentary sectin. (b) Line 6 imaging vertical faults, flding f Micene and Plicene sediments as well as tilting f basement, and Micene/Plicene sectin up t the west. Nte strike-slip fault underneath shtpint lcatin 420. (c) Line 7 shwing flding f sedimentary sectin and strike-slip fault which is active tday (sht pint (SP) 1445). A magnitude 5.3 strike-slip earthquake ccurred km belw this basin in 1990.

8 8154 ROHR ET AL.: TRANSPRESSION 135øW 133øW 131øW 129øW ø z i ø :!!:: -: ''" ': :-:.:.. :. ' ' i ii7..: :......::.,,/.....,. ;..-.. ' '..."4'-. : øW 133w 131w 12Sw Figure 7. Micrseismicity frm Bird [1997] pltted n bathymetry; magnitude 8.1 event is shwn as large pen star. Seismicity ccurs alng the QCF; east f the QCF micrseismicity is cncentrated under nrthern Graham Island and Hecate Strait. White square dentes lcatin fm event in Nine statins used t lcate the events were n the Queen Charltte Islands (QCI); three were acrss Hecate Strait n the mainland. Fcal mechanism slutins f grups f micrearthquakes, 3. New Seismic Reflectin Data and individual events [B rub et al., 1989; Bird, 1997] shw that pure strike-slip as well as blique, thrust, and extensinal 3.1. Acquisitin and Prcessing events ccur near the QCF, beneath the terrace and nrthern Graham Island. The axis f maximum cmpressin strikes nrth t nrth-nrtheast. Events in Figure 7 were lcated assuming a depth f 20 km. Events lcated nrth f the statin and 3) [Scheidhauer array are slightly east f the QCF prbably because the as- sumed depth is t deep. Nevertheless, the spread f activity ver the physigraphic terrace and the variety f mechanisms sugges that the terrace is seismically active. Treated as a grup, 90% f the events under Graham Island ccur at depths <16 km, and 90% f the events under Hecate Strait ccur at depths <20 km. Data cllected in 1994 as part f the Accrete prgram [Andrnics et al., 1999] crss the QCF in tw places; tie lines image mildly defrmed Pacific plate and the terrace (Figures 2 et al., 1999]. The surce, an array f 20 air guns, had a ttal vlume f 138 L (8400 cubic inches). The streamer had 224 channels with a grup spacing f 12.5 m fr a ttal length f 2800 m. The data were recrded with a sampling rate f 4 ms fr 16.5 s. An anti-aliasing high-cut filter f 125 Hz was applied in the field prir t digitizatin. Using Glbal Psitining System (GPS) navigatin, the shtpint data were srted int cmmn midpint (CMP) frmat with a

9 ROHR ET AL.: TRANSPRESSION 8155 bin size f 12.5 m resulting in CMPs with an average fld f 60. Prcessing fllwed the basic steps f editing sht pints, band-pass filtering, analyzing velcities, and stacking [Scheidhauer, 1997]. The data were migrated using a finite difference technique Line 1262 Line 1262 (Figure 8) crsses the QCF at latitude 54.5øN ff Dixn Entrance. The Pacific plate is Myr ld here, and the western edge f the terrace is a ridge which runs nrth-suth, parallel t the magnetic anmalies (Figure 2). Sedimentary unit B thins t the west and nlaps unit A, implying that the ceanic plate was already flexed befre depsitin f unit B. We have n independent estimate f age f sediments, but it seems likely that, similar t the Cascadia basin, the bulk f the sediments are Pleistcene. The uter ridge f the terrace is s defrmed that nly shrt segments f cherent reflectins are visible (CMP ); the standard prcessing used was unable t reslve its internal structure. The terrace is --45 km wide; tw turbidite channels which carry sediments frm Alaska and the Cast Muntains have been fcused here int ne channel by the uter ridges f the terrace. Bradly flded sediments underlie the seaflr; the intensity f flding increases tward the uter ridge and with depth. An incherent unit, F, abuts the QCF; its incherence and assciatin with a majr fault suggesthat it is a melange. Farther t the west, sedimentary layers are flded, and their cntact with the melange is nt defined. Whether these layers have been caught up in the melange r if they nlap an lder melange cannt be distinguished here. The data shw striking differences in structure and stratigraphy acrss the QCF; marked tilting n the Nrth American plate des nt ccur n the Pacific (Figures 8-10). On the Nrth American side the tp 1.0 s f sedimentary layers dip dwn tward the fault and are underlain by a nearly flat event at 4.5 s (Figure 9). West f the fault, reflectins are subparallel t the seaflr and are higher in amplitude than t the east. The cnfiguratin f reflectins at the present seaflr suggests that tilting the Nrth American plate relative t the Pacific creates a depressin which wuld be filled by the next sedimentary depsit. Dips exceed thse typically bserved n cntinental slpes. The dip f the seaflr is 5 ø, and dips f reflectins belw that are as much as 12 ø and 20 ø assuming an interval velcity f 2.0 km/s. In cntrast, sediments immediately seaward f the shelf break n passive margins typically dip 3 ø t 5 ø n tday's Atlantic cntinental slpe and up t 7.5 ø n Micene prgrading clinfrms ff New Jersey [Fulthrpe and Austin, 1998]. shw that subsidence was faster than the rate f depsitin since layers nlap the substrate. Abve U2, layers dwnlap; the sedimentary units are building utward indicating a faster rate f depsitin. Such variatins in sedimentary rate are expected t ccur during glacial cycles and may have ccurred during the last majr glacial cycle (0.125 Ma). This wuld imply that the Pacific plate was <6 km t the suthwest during depsitin f the layers n the Nrth America plate. Plate flexure, sea level, and sedimentatin rate changes are reginal in effect, and ne wuld expect them t be cherent ver distance scales f 5 km. Yet thse effects are nt evident n the Pacific plate Line 1263 Line 1263 (Figure 11) shws mild, apparently inactive defrmatin f the Pacific plate west f the terrace. This line was sht at a lw angle t the strike f the terrace and crsses the uter terrace fault imaged by line 1262 where it has plunged belw the seaflr (CMP ). In this area, which has undergne less defrmatin, the internal structure is mre evident; a small anticline is curved up suth f a fault. Dip mveut prcessing shws that it dips at an angle f at least 52 ø and that it cuts basement. Cmpressin has tilted bth sediments and basement as ppsed t just thickening the sediments, indicating that basement is invlved in the defrmatin. An angular uncnfrmity is assciated with the crest f this anticline. On either side f this structure, reflectrs dip smthly tward the terrace. Steep faults between CMPs 4300 and 4500 are als currently inactive Line 1250 Line 1250 (Figure 12) was cllected acrss the terrace 30 km suth f line The trugh sectin shws layers similar t thse seen in line 1262; althugh the lwer layers are thicker, they still shw unit B nlapping unit A. In the terrace a set f fault-bend flds is cut by nearly vertical faults. The gemetry f these sediments suggests that a cmpressive fault has cut up thrugh the sediments pssibly n a set f ramps and flats [e.g., Shaw and Suppe, 1994], reaching the surface at the westernmst edge f the terrace. Subsequent tilting f the ramps suggests that later faulting was deeper, perhaps because the brittle layer thickened ver time. Much f the recent sedimentatin was depsite during frmatin f the flds since layering nlaps the fld crest (CMPs ). The near-vertical faults culd be the result f later strike-slip defrmatin. On this line the plate bundary is visible, nt as a distinct fault, but as an incherent, apparently uplifted unit (CMPs ). The lack f cherent reflectin energy and weak returns frm this regin suggesthat the sediments have been On the Nrth American plate the sedimentary recrd shws tw different prcesses at wrk: tectnic subsidence and flexdefrmed enugh t destry riginal bedding, i.e., cnstitute a melange. This prfile crssed a restraining bend in the QCF ure frm glaciisstatic frces. The latter entails rapid changes which is ---3 km wide and 30 km lng. Nrthwest trending which are superimpsed n the frmer, a lnger-term, presumably steadier prcess. Dips which exceed even thse bserved during massive sediment buildup suggesthat subsidence is high-amplitude reflectins in the GLORIA data lie between verlapping traces f the fault (Figure 2); they are mst likely uplifted melange. nging. Subsidence culd be caused by lcal thinning f Nrth America r simple flexure. Cunterclckwise rtatin 3.5. Line 1264 between tw active right-lateral strike-slip faults can create extensin in ne sectin and cmpressin in anther. Dixn Entrance is brken by a series f nrthwesterly trending en echeln faults [Rhr and Dietrich, 1992]; shear distributed frm the QCF nt ne f these faults culd easily induce rtatin adjacent t the QCF. The angular relatins abve uncnfrmity U3 (Figure 10) On line 1264 (Figure 13) the melange can be traced frm line 1250 t line 1262, but it is nt as extensive in the shallw sectin f line 1262 as in line The melange culd have frmed lcally n the QCF r at the restraining bend and been dragged nrth 30 km. This wuld imply an age f 0.64 Ma fr the tp f melange in line 1262 using an alng-strike velcity f 47 km/myr. Flding f the sediments at the nrthernmst end

10 8156 ROHR ET AL.' TRANSPRESSION 1 _-IOO (0es) em!l leaml AeM-OM/

11

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13 ROHR ET AL.' TRANSPRESSION JJJ (O S) W!l. I A JI./ em-omi

14 .., 8160 ROHR ET AL.: TRANSPRESSION,, c z (es) ew!t IOAeJ1 em-om/

15 / ROHR ET AL.: TRANSPRESSION 8161 (005) OW!l. IOAeJI. AeM-OM/

16 8162 ROHR ET AL.' TRANSPRESSION õ : -20 -a ' -50 ': -55" -60 '"' -65 ' ': -75 -OO io { 0'0 t '50 millilals Figure 14. Free-air gravity anmaly data. A strng negative-psitive anmaly pair cincides with the QCF; therwise, the anmaly field is relatively smth. In the basin mst anmalies can be explained by knwn sedimenthicknesses [Lwe and Dehler, 1995]. Seismic prfiles mdeled in ther studies and here are marked: line EX1 [Hrn et al., 1984]; lines 5 and 6 (Figure 4) [Spence and Lng, 1995] and line plate bundaries, namely, the nrthern San Andreas Fault [Henstck et al., 1997] and the Alpine Fault [Stern and McBride, 1998]. Kilmeters f relief n the Mh caused by a subducted slab wuld create a significant increase in the gravity field, unless its anmaly is exactly ffset by lateral density changes in the cntinental crust. That kind f cincidence alng 400 km f plate bundary seems implausible. Therefre it seems mst likely that if an ceanic slab exists east f the QCF, it cannt create mre than a few kilmeters f tpgraphy n the Mh Isstasy Immediately adjacent t the QCF, bth plates are ut f lcal isstatic equilibrium. Isstasy was calculated t a depth f 40 km at 20-km intervals alng bth mdels fr lines 1262, EX1 [Hrn et al., 1984], 5, and 6 [Spence and Lng, 1995] (Figure 16). Isstatic anmalies were calculated relative t a reference clumn which was 90 km east f the QCF fr mdels 6, 1262s (the subductin mdel f 1262), and EX1. Calculatins fr mdels 5 and 1262t (the analgus transfrm mdel f line 1262) used the same reference value as mdel 6. The lcatin f the reference clumn is an arbitrary chice based n the length f the mdeled prfiles. Bth plates are prbably flexed t sme small degree ver distances f several hundred kilmeters either side f the plate bundary [e.g., Harris and Chapman, 1994]. A cnsistent pattern between the mdels cmputed by three different studies allws sme generalizatins t be made. The

17 ROHR ET AL.: TRANSPRESSION 8163 Table 1. Densities Used fr Sediments in Gravity Mdels Interval Density, Unit Velcity, m/s kg/m 3 AB CD E F G H III 2700 Frm Scheidhauer [1997] magnitude f the peak t trugh values n these prfiles is similar: 3.4%, 2.2%, 3.6%, 2.9%, and 5.1% frm suth t nrth. The psitive deviatin n Nrth America indicates mre mass, and the negative deviatin ver the Pacific plate indicates less mass than the reference clumn. The isstatic anm- aly ver the Pacific plate becmes wider frm suth t nrth, as ne wuld expect fr cnstant frce alng strike flexing an ceanic plate which is aging and increasing in strength alng strike. Thickness f the brittle regin grws frm - 13 t 20 km (see belw) ver the length f the QCF. EX1 was sht acrss the suthernmst prtin f the terrace and cincides with an area with n bathymetric trugh. Its prfile has <20 km width f negative values, all f which are directly under the terrace. The isstatic anmalies under the Queen Charltte Islands are f similar widths, implying that the Nrth American plate des nt significantly change mechanical prperties alng strike, assuming a cnstant frce alng strike. This rules ut the pssibility that the Queen Charltte Islands are created purely by flexural frces. If they were, they wuld have the same width alng strike instead f widening t the nrth. The isstatic prfile f the transfrm mdel 1262t is interesting in that it seems t imply that mst f Dixn Entrance is ut f isstatic equilibrium. The isstatic prfile f the subductin mdel 1262s, hwever, is similar t that f prfile EX1, which is the basis fr its density values. A. 1262t 5 lo "= 15 C 20' B. 1262s > :'- bserved calculated _... :-:...: ß.- ß ---: _-.. :... _ -._.:.:---- :-r-.. : - -: :.:- =====================.:_: ::- :.:._:_--:.-_-::: ,._..,_. 5 lo m 15 V.E. = 1, Distance (kin) Figure 15. Gravity mdels fr line (a) Transfrm plate bundary using densities similar t thse f Spence and Lng [1995](Figure 4). (b) Subducted slab. Densities in the terrace are based n refractin data and assumed fr the cntinent [Hrn et al., 1984]. Densities fr units A-H are listed in Table 1.

18 8164 ROHR ET AL.: TRANSPRESSION O kilmeters -2% -3% Figure 16. Isstatic anmalies were calculated relative t a clumn 90 km east f the QCF based n gravity mdels f lines EX1 [Hrn et al., 1984], 5, 6 [Spence and Lng, 1995], and Mdel 1262s refers t a subductin gemetry fr the plate bundary and mdel 1262t refers t a transfrm gemetry, as in Figure 15. The negative values west f the QCF indicate a mass deficit in the Pacific plate, and the psitive values east f the QCF indicate a mass excess. 5. Mdel f Accmmdatin f Transpressin A better understanding f reginal defrmatin as well as a number f data sets acquired in the last few years suggesthat the QCF is a vertical thrughging plate bundary which separates tw elastically independent plates (Figure 17). Placing the initiatin f transpressin just nrth f the Tuz Wilsn Seamunts results in <80 km f predicted net plate verlap between 52øN and 54øN in the last 5-8 Myr. Each f the plates absrbs transpressin independently; brittle cmpressive and strike-slip defrmatin are bserved in bth plates. The threedimensinal interactins f the plate bundary and the relative mtin vectr results in Nrth America absrbing mre cmpressin than the Pacific plate. By implicatin the ductile prtins f each plate thicken and/r are deflected dwnward int the asthensphere t accmmdate transpressin Vertical Fault Seismicity recrded by OBSs indicates that the QCF is a vertical fault which cuts dwn t Mh at 21 km [Hyndman and Ellis, 1981; Hrn et al., 1984]. A magnitude 8.1 earthquake n the QCF was a pure strike-slip event; n cmpressin was accmmdated during the event. Studies f micrseismicity als shw high levels f activity n the QCF in a vertical zne [Bird, 1997]. The cmbinatin f the flexural study f the Pacific plate [Prims et al., 1997] and reginal rapid sea level changes during glaciatin [Barrie et al., 1991] further indicate that the tw plates are elastically independent. In ther wrds, the QCF is a clean break between the plates, and there is n underthrusting f the Pacific plate under the Nrth American plate. The QCF has a lng histry f strike-slip mtin; the fact that much f it is still parallel t the directin f relative plate mtin between 40 and 8 Ma (320 ø) ff Mresby Island [Rhr and Curtie, 1997] implies a cntinuus histry f the fault, as well as resistance t changing the plate bundary. When the plate directins began t change t transpressin, the QCF cntinued t accmmdate strike-slip mtin ( km/ Myr). Since the plates n either side f the fault are warm and thin, cmpressin (6-20 km/myr) was mre easily accmmdated within each plate rather than frcing the ceanic plate int the cntinental plate in a subductin zne gemetry Mechanical Structure and Defrmatin Frm the Tuz Wilsn Seamunts t Alaska the tw plates are similar in mechanical prperties, and they are bth defrming internally. Bth plates are mafic s there is n strength discntinuity at the mantle; the depth t the brittle-ductile transitin is a key feature in mafic plates. Micrseismicity [Bird, 1997] shws that the brittle-ductile transitin (the depth abve which 90% f the events ccur [Sibsn, 1982]) begins between 16 and 20 km depth under Graham Island and nrthern Hecate Strait. Fr the rest f the regin we use thermal mdels and heat flw data t assess the depth t the brittleductile transitin. Whether that transitin begins at 650øC r 750øC in mafic plates is nt strictly knwn, but the high heat flw in bth plates means that the vertical separatin between the 650øC and 750øC istherms is nly -3 km. We will arbitrarily use the number 700øC since the relative differences between the tw plates is mre imprtant than pegging the transitin t the nearest kilmeter. Heat flw values average 70 mw/m 2 in the Queen Charltte Islands and nrthern Hecate Strait, indicating a depth f 22 km t the 700øC istherm [T. J. Lewis et al., 1991]. Values f mw/m 2 which have been measured in Hecate Strait [T. J. Lewis et al., 1991] indicate a depth t the 700øC istherm f km again assuming a linear gradient. In Nrth America, depths t the brittle-ductile transitin predicted frm heat flw data are in brad agreement with the km depths inferred frm micrseismicity.

19 :: ROHR ET AL.' TRANSPRESSION ('tu ) qi'd...'e(]

20 8166 ROHR ET AL.: TRANSPRESSION A thermal mdel f yung ceanic plates [Chen and Mrgan, 1990] shws that the depth f the 700øC istherm changes frm 13 t 20 km depth in the Pacific plate as it ages alng the QCF. The yungest ceanic plate is warmer than Nrth America, but simple thermal mdels indicate that the mechanical structure f bth plates is bradly similar. There is n reasn t suppse that ne plate wuld defrm exclusively leaving the ther un- tuched. When transpressin began, the Queen Charltte basin wuld have been warmer than it is nw and the brittle-ductile transitin wuld have been clser t that f the yungest ceanic plate alng the QCF. As defrmatin prceeded, the thermal field wuld be disturbed; bulk strain heating might be imprtant in highly defrmed regins. Mtin n the fault might be an additinal imprtant surce f heat tens f kilmeters frm the fault itself [Chen, 1988]. Nevertheless, the generalizatin that the tw plates have similar mechanical behavir is supprted by current bservatins. Transfrm plate bundaries have been imaged as near vertical znes f lwered velcities in reflectin and refractin experiments as summarized by Stem and McBride [1998] and Detrick et al. [1993]. Fault guge and enhanced pre pressures are thught t ccupy regins 5-10 km wide and up t km deep. Experiments shw that nce frmed such a regin tends t carry mst f the nging strike-slip defrmatin [e.g., Wilcx et al., 1973]. Sme defrmatin can be distributed within the plates especially in relatively weak plates, as here, and/r when the relative plate mtins change. Nevertheless, a magnitude 8.1 strike-slip event is mre likely t ccur n the main fault strand than at sme randm lcatin within either 1984; Dehler and Clwes, 1988; Spence and Lng, 1995] interpreted crustal velcity and density material (>5.3 km/s and 2800 kg/m 3) 3-4 km belw the terrace seaflr fr ttal thicknesses f km. These values are interpreted as arising frm highly defrmed crystalline material. Vlumetrically there is rm fr km f 5.5-km-thick gabbric ceanic crust t be incrprated int the terrace alng line 1262, 20 km n lines 5 and 6, and 9 km n EX1. A simple tw-cmpnent system f 25% sediments (2500 kg/m 3) and 75% ceanicrust (2900 kg/m 3) can prduce the mdeledensity f 2800 kg/m 3. Oceanic crust r mantle altered t serpentinite ( kg/m 3) culd als be a cnstituent f the terrace. If 75% f the vlume f the deepest layer in the terrace is ceanic crust, then 6-20 km f cmpressin culd be accmmdated by duplexing. Because the mechanical prperties f the ceanic plate vary alng strike, the amunt f cmpressin absrbed within the plate may nt be linearly crrelated with the amunt f predicted verlap Brittle defrmatin: Nrth America. The rest f the cmpressin must be accmmdated within the cntinental plate. The amunt varies gradually alng strike frm 0 t 60 km. Early interpretatins cncluded that little defrmatin f Nrth America ccurred in the Plicene [Hrn et al., 1984], but images f flding and uplift f the basins under Hecate Strait give direct evidence f widespread Plicene defrmatin (Figure 6), and micrseismicity prvides ample evidence f recent defrmatin in Graham Island. Evidence discussed belw suggests that the cmpressin is riented nrth-suth and is pervasive within the islands. The islands, then, are the result f crustal thickening and uplift during transpressin. Pssible mechanisms respnsible fr the defrmatin are als discussed. Cmplex and multistage cmpressive defrmatin affects many f the Micene basins inbard f the Queen Charltte Islands but nt Queen Charltte Sund [Rhr and Dietrich, 1992] (Figure 6). In mst cases, the sense f mtin n nrmal faults has been inverted t create uplift. The intensity f defrmatin increases tward the islands. There are several ki- plate Defrmatin adjacent t the QCF. Structures n the QCF vary frm subsidence in Nrth America (line 1262) t flding n the Pacific plate just 10 km t the nrth (line 1264). Rapid subsidenceast f the QCF n line 1262 may indicate that extensin is ccurring. Thinner crust under the subsiding prtin f line 1262 is ne f several mdels which can fit the gravity data. Althugh apparently cntradictry t the reginal stress regime, extensin culd ccur with cunterclckwise rtatin f a segment f Nrth America between tw rightlateral faults. Rtatin simultaneusly creates regins f lcal extensin and cmpressin. The east-west riented flds crgsed by line 1264 are in the right lcatin t be the cmpressinal cunterpart the apparent extensin; flds farther west in the terrace are riented nrthwesterly. Shearing must be distributed int the Nrth American plate and is nt carried slely by the QCF; any ne f the nrthwesterly Micene extensinal faults in Dixn Entrance culd be active, althugh little seismicity has been detected n them in the last 50 years. An alternate explanatin fr the subsidence bserved east f the QCF n line 1262 is that it is caused by flexure and des nt entail plate thinning. The apex f the uplift in Nrth America is 20 km east f the QCF n the cntinental shelf which wuld create a westward dip n the plate under the slpe. Hwever, the magnitude f the dip seems t large fr this prcess and lmeters f vertical relief between basement rcks expsed n Mresby Island and basement rcks imaged under western Hecate Strait, a distance f nly 20 km, indicating that a majr structure is prbably respnsible fr this relief. Tw-dimensinaline length calculatins can estimate the amunt f cmpressin absrbed in the basin but must be used with cautin because the structures vary significantly in all three dimensins, cverage f the regin is sparse, and the lines are nt riented ptimally with respect t each structure. In additin, there were difficulties in crrelating reflectins acrs strike-slip faults and basement highs; strike-slip mtin renders the tw-dimensinal assumptin invalid. Nevertheless, line length cmpressin was cmputed fr the Micene/ Plicene reflectr as a rugh indicatin f the amunt f cmpressin. Tw kilmeters f shrtening were accmmdated alng 63 km f line 5 (3%), 7-9 km f shrtening alng 118 km f line 4 (6-7%), and 9 km f shrtening alng 70 km f line 7 (13%). These estimates d nt include cmpressin, which is such dips have nt been bserved n the available seismic evident immediately prir t depsitin f the reflectr r reflectin prfiles Brittle defrmatin: Pacific plate. The Pacific plate is being defrmed and duplexed in the terrace. Reflectin prfiles acrss the terrace image flded and cmpressed sedcmpressin taken up in basement structures which the reflectr des nt verlie. Line 5 is rughly perpendicular t the QCF, and lines 4 and 7 are subparallel t the QCF. Higher percentages f shrtening are evident in the directin f maximents which are presumably riding a substrate f cmpressed imum cmpressive stress (nrth-suth) predicted in a nrthbasement; line 1263 shws that the uter terrace fault cuts west shear stress regime. upper basement. This study and several thers [Hrn et al., The amunt f recent defrmatin in the islands is hard t

21 ROHR ET AL.: TRANSPRESSION 8167 estimate because few Negene rcks are expsed n Mresby Island and expsure f rcks n Graham Island is nly 3% [Hicksn, 1991]. Steep Tertiary blck faults are cmmn, but sense and age f mtin n the faults are difficult t quantify [e.g., Thmpsn et al., 1991; P. D. Lewis et al., 1991]. Vigrus micrearthquake activity in nrthern Graham Island indicates that present-day defrmatin is ccurring. The shape f the islands suggests that they were frmed by the nrthward push f transpressin and cnsequent brittle and/r ductile thickening f the crust ver the last 5-8 Myr. Their western edge is determined by the OCF, and the eastern edge is subparallel t the current relative plate mtin vectr. Their nrthern edge runs east-west which is the predicted strike directin f cmpressive structures in a nrthwestern directed shear stress regime. The shape f the islands is als cincident with the net predicted verlap. If simple underthrusting was ccurring, the regin f uplift shuld cntinue nrth int Dixn Entrance instead f terminating in an eastwest edge which is seismically active. Shallw sediments just nrth f Graham Island are nt cmpressively defrmed [Rhr and Dietrich, 1992]. The cincidence f seismic activity and the castline indicates that the defrmatin creating the seismic activity has been active fr sme time and has uplifted the Micene basalts and sediments which cmprise Graham Island. If the Queen Charltte Islands were an intact fault-bund blck [Rhr and Dietrich, 1992] which has been squeezed upward and nrthward between the QCF and a fault which splays ff the QCF, then pervasive earthquake activity under the islands' eastern and nrthern shres shuld be ccurring. The splay culd be blind but apprach the surface near the eastern shre f the islands creating the structural relief between the islands and the basin. The Luscune Inlet and Sandspit Faults are subparallel t the eastern shre f the islands, but mapping has nt identified evidence fr recent mtin n these faults Charltte Sund t 7 km ver a distance f 75 km [Spence and Lng, 1995]. If such crust were present alng the QCF at the nset f transpressin, it wuld easily defrm and culd have absrbed significant cmpressin t reach the islands' current thickness f 27 km. Cmpressinal defrmatin active ver perids f time has a tendency t expand laterally [e.g., Masek and Duncan, 1998]; new faults and flds carry defrmatin ut frm the regin f maximum uplift. The apparent greater uplift f the suthern islands suggests that defrmatin f Nrth America began in Mresby Island and prgressed nrthward creating Graham Island. This mechanism explains vigrus seismic activity in nrthern Graham Island and Hecate Strait and relative quiescence elsewhere. Palemagnetic data indicate that mid-tertiary dikes in the Queen Charltte Islands have been tilted dwn t the nrth 9ø-16 ø (and allw tens f kilmeters r even 100 km f nrthward mtin f the islands since the dikes frmed (13-54 Ma). As nted by Prims et al. [1997], the directin f tilt cincides with the current plates' relative mtin. Mre recent wrk (E. Irving, persnal cmmunicatin, 1999) reveals that lcatins just nrth f the seismgenic zne in Graham Island are nt tilted. This substantiates ur idea that the seismgenic zne is a defrmatin frnt advancing nrthward. Irving et al. [1992] interpreted the tilting t ccur n pervasiv east-west Micene extensinal faults; in ne lcatin, such faults have been mapped but nt dated [Lewis, 1991]. Nr has the tilting been independently dated. The amunt f tilting is sufficiently unifrm that it must ccur n numerus faults; therwise, very deep rcks wuld be expsed alng strike. This unifrmity is at dds with the variety f fcal mechanisms and fault planes interpreted by Bird [1997] but culd be explained if, ver the lng term, reverse r ne-sided flwer structures dminated defrmatin. [P. D. Lewis et al., 1991]. If 50 years f earthquake recrding is Cmpressin may als be accmmdated within the Cast representative f lng-term defrmatin, then the idea f an Muntains which brder the basin n the east. Inbard f intact blck is nt viable because little micrearthquake activity ccurs n the east cast f the islands [Bentbe et al., 1989; Bird, 1997]. Nrth American crust adjacent t the QCF may have been quite thin and been pervasively and prgressively thickened during the Plicene n cmpressive r flwer structures which are blind. Cmpressinal flwer structures can cnsist f many Queen Charltte Sund and nrthern Vancuver Island, single-crystal fissin track data detected an acceleratin f uplift rates in the Cast Muntains after 5 Ma [O'Sullivan and Parrish, 1995]. N cmpressive defrmatin is bserved in Queen Charltte Sund, but GPS data indicate that nrthern Vancuver Island is mving at a rate f abut 3 mm/yr t the nrthwest [Drageft and Hyndman, 1995; Hentn et al., 1998], faults which flatten tward the surface but d nt necessarily implying that a small amunt f Pacific-Nrth American relabreak the surface. Such structures have the ptential t create reginal tilting and crustal thickening with few visible surface tive plate mtin is distributed acrss the margin here. Uplift f the muntains in respnse t cmpressin is easily underfaults. Micrseismicity suggests that current defrmatin c- std if the muntains are a weak and warm zne as discussed curs n pervasive faults frm 3 t 20 km depth; mechanisms by Rhr and Curtie [1997]. This style f defrmatin might be can be blique, extensinal, thrust, r strike-slip, but pressure axes are generally cnsistent with the nrth-nrtheast cmpressin directin 'which ccurs in a nrth-nrthwest shear [Bird, 1997]. N surface faults n Graham Island which might be caused by micrearthquakes have been reprted [Hicksn, 1991], but the expsure f the surficial gelgy abve the seismgenic regin is nly 3%. In Hecate Strait the largest strike-slip event (M = 5.3) is assciated with a strike-slip fault which cuts flded Micene sediments (Figure 6c). Evidence that Nrth American crust adjacent t the QCF may have been quite thin can be fund in a reflectin/ refractin prfile that runs just suth f Mresby Island and the Tuz Wilsn Seamunts, line 3. The sedimentary basins ccurring inbard f Hecate Strait; single-crystal fissin track measurements culd easily test this idea Ductile defrmatin. Intraplate cmpressin is prbably accmmdated by flw and thickening within the ductile prtins f the plates. The biggest difference between the tw plates mechanically is that the ductile regin cnsists purely f mantle material in the Pacific plate but includes sme crust in Nrth America. Thickening f the crustal ductile prtin under the islands culd result in sme uplift, whereas thickening f the denser mantle ductile prtin will tend t cause subsidence. In Figure 17 we have schematically drawn the defrmatin f the ductile prtins f the plates as ccurring mstly in shw n cmpressive defrmatin [Rhr and Dietrich, 1992], Nrth America based n the inference that mst f the brittle and crystalline crust thins seaward frm 23 km under Queen defrmatin ccurs within Nrth America. We have n direct

22 8168 ROHR ET AL.: TRANSPRESSION measurements t indicate that such thickening is ccurring r 6. Discussin hw that defrmatin is arranged at depth; seismic measure- The hypthesis favred in this paper is that the QCF is a ments f plate thickness culd test the idea. If an asymmetry vertical plate bundary and that transpressin has been accmarises such that ne plate is deeper than anther, it might mdated by significant defrmatin within bth the ceanic initiate a subductin-type gemetry. and the cntinental plates. In this mdel the Queen Charltte 5.3. Plate Flexure Islands are the direct result f pervasive transpressive defrmatin. Previus mdels have favred a gemetry in which the Pacific plate. Flexure f the Pacific plate des nt ceanic plate is being subducted, the plate bundary switches begin until 90 km nrth f the inceptin f the QCF. Mdeling between thrust and strike-slip gemetries [Hyndman and Ellis, by Prims et al. [1997] shwed that the lad f the terrace is 1981; Yrath and Hyndman, 1983], and the islands are a cmre than sufficient t cause the degree f flexure bserved herent blck. A mdel in which bth plates were defrmed has further nrth in the Pacific plate (Figure 5). Densities used t been presented by Hrn et al. [1984] but was dismissed because lad the terrace abve assumed basement were lwer than evidence f defrmatin within Nrth America was lacking at thse fund in gravity mdels; increasing them abve 2000 the time. kg/m 3 increased the predictedwnward flexure t unrealistic values. Using a density f 2800 kg/m 3 in the terrace t create a 6.1. Cmparisn t Previus Mdels lad f similar mass indicates that a smaller vlume f the terrace (---70%) is lading the plate. This in turn implies that the plate is brken west f the QCF, prbably by the fault system which defines the uter edge f the terrace Nrth American plate. The relatively cnsistent pattern f isstatic anmalies assciated with the Queen Charltte Islands n the Nrth American plate shws that the plate is flexed upward alng the QCF and, assuming that the frces alng the fault are relatively unifrm, that the plate has relatively cnstant mechanical prperties alng the QCF. The prfiles crssed the fault where predicted amunts f cnvergence vary frm t 50 km and where basement is abve and belw sea level. Sme shrt-wavelength variatins exist and may be related t the shrt-wavelengthetergeneities in plate thickness predicted by Dehler et al. [1997] and/r variatins in within the OBS array and are prbably reliable; a velcity mdel similar t that f a cincident refractin survey [Hrn et al., 1984] was used. N event mechanisms culd be cmputed, but the verwhelming prbability is that earthquakes under the QCF are the result f strike-slip mtin n the QCF. If this is accepted as the mst likely explanatin, then a Sumatra-type the thermal field. mdel is nt pssible here. Flexure mdels and refractin data The lack f any uncnfrmity in the upper sediments west f the QCF n line 1262 (Figure 10) argues that the Pacific plate further argue against the existence f an undefrmed subducting slab. did nt experience the same flexural respnse t glacieustatic Previus mdels tended t assume that all defrmatin clading that Nrth America did. If the tw sedimentary sectins verly the same subducting slab which intrudes Nrth America, then glacial lading f Nrth America affects the slab dip and cnsequently the dip f all sediments depsited n it. This is readily seen east f the QCF. If there is n slab and Nrth America is cmpletely decupled frm Pacific by a vertical fault, then the Pacific plate wuld experience n flexure during glacial cycles. Glacial tilting in cntinental crust where the depth t the brittle-ductile transitin is 30 km deep is typically 1 m/km [Thrsn, 1989]. In thinner plates and if the QCF is free edge, then the amunt f flexure may be significantly greater than this value. Lading f this margin by a curred clse t the QCF. Mackie et al. [1989] cnsidered the pssibilities f distributed defrmatin in the islands but rejected this prcess because the crust is thinner than the glbal average. We prefer t take the perspective that crust under the islands is thicker than adjacent crust in the mildly cmpressed cntinental shelf. On the basis f reginal structural trends, Rhr and Dietrich [1992] first prpsed that distributed defrmatin frm the plate bundary was an imprtant prcess in Queen Charltte Basin and that increased cmpressive defrmatin tward the nrthern end f the basin argued fr a three-dimensinal distributin f stress, nt a simple twdimensinal rthgnal partitining f cmpressin. Cm- glacier affrds an interesting test f the mechanical nature f a transfrm plate bundary. The pssible psitive flexure in Dixn Entrance indicated by the isstatic anmalies f prfile 1262t is reinfrced by the bservatin that crust f similar thickness in Hecate Strait has A plate gemetry similar t that fund in znes f blique subductin is at first glance a pssible slutin t the gemetry f blique transcurrent mtin. In Sumatra, strike-slip mtin is accmmdated by a strike-slip fault in the upper plate 300 km east f the trench [Fitch, 1972]. The OBS survey f Hyndman and Ellis [1981], althugh shrt, did lcate several earthquakes underneath the surface expressin f the QCF and near Mh at depths f km. Mst event lcatins were pressive defrmatin f Nrth America in the Plicene was recgnized by Hyndman and Hamiltn [1993], but they cnsidered the amunt f cmpressin absrbed by Nrth America t be negligible. The mdel, which prpses a subducting slab, includes a plate bundary which changes dip by 60 ø r mre at sme unspecified time interval [Hyndman et al., 1982; Yrath and Hyndman, 1983]. Such changes wuld create large changes in the flexural respnse f the plates [Prims et al., 1997] if they ccurred n timescales lnger than the viscelastic time cnstant, a few hundred years. If the ceanic plate is nly 10 km under Nrth America, its surface under the terrace wuld be depressed by 6 km; unlading it by rupture alng a strike-slip subsided and carries up t 1 km f pstrift sediments. In Dixn Entrance, n such subsidence r pstrift sequence is visible [Rhr and Dietrich, 1992]. Lateral heat lss during stretching may have cled it enugh that there was little pstrift subsidence, r it culd be actively held up by the cmpressive frces develped by the nrthern push f the Queen Charltte Islands int the mre cmpetent Alexander terrane f the Alaska panhandle. Flding in the nrthern ends f the subbasins f Dixn Entrance crrbrate the ntin f significant fault wuld release this depressin. A prcess which invlves nrth-suth directed cmpressin within Nrth America [Rhr cyclic lading and unlading shuld create a recgnizable cyand Dietrich, 1992]. clic pattern f angular uncnfrmities n the ceanic plate.

23 ROHR ET AL.: TRANSPRESSION 8169 Tw angular uncnfrmities are recgnizable in the seismic vide n independent infrmatin n whether r nt a subducdata; ne appears t be assciated with nset f flexural bend- tin slab exists. Such a gemetry is permissible if the slab ing r heavy sedimentatin beginning in the Pleistcene and merges smthly with a flat cntinental Mh alng all 400 km anther is assciated with defrmatin n a fault defining the f the plate bundary; a vertical fault separating the tw plates uter edge f the terrace. A cyclic pattern is nt currently is equally permissible. recgnized. If the majr fault plane switched dip at rates f less A plate bundary mdel in which a subductin slab takes up than a few hundred years, we wuld nt necessarily expect t mst f the cmpressin predicts that the western side f the see a flexural respnse in either plate. Queen Charltte Islands is substantially uplifted and erded Creating a new transfrm bundary every s ften thrugh [Yrath and Hyndman, 1983], deepening the level f expsure the entire Pacific plate seems mechanically inefficient and imf crustal rcks t the west. This is nt bserved [Thmpsn et plausible. Labratry experiments n rupturing new strike-slip al., 1991; P. D. Lewis et al., 1991]; instead, the expsure deepfaults in transpressive stress regimes [e.g., Wilcx et al., 1973] ens t the suth. Cupled with the up-t-the-suth palemagshw that the new faults frm in small irregular t en echeln netic tilts and fcal mechanismshwing nrth t nrtheasterly segments. The current surface expressin f the QCF as seen cmpressin, the case fr histrical and present-day nrtherly in GLORIA data is f segments nearly 100 km lng; the suthern segments are parallel t the directin f the pre-plicene cmpressive defrmatin f the islands is clear. plates' relative mtin [Hyndman and Hamiltn, 1993; Rhr and Currie, 1997]. Thus the large-scale mrphlgy f the QCF 6.2. Strength f Plate Bundary and Seismicity argues against this majr fault being recreated dwn t 20 km On many transfrm plate bundaries, defrmatin appears every nw and then. t be partitined between pure strike-slip and pure cmpres- The three-dimensinal cnsequences f flipping plate sive mtin, indicating that the main fault itself is weak relative bundaries by 60 ø have nt been explred. The 1949 magni- t the plate [Munt and Suppe, 1987]. In the Queen Charltte tude 8.1 event ruptured rughly 500 km f the Fairweather/ regin, fault mechanisms frm the Pacific and Nrth American QCF, but nt the entire QCF, implying that segments f the plates have blique slip as well as pure thrust, extensin, and fault wuld be ut f synch with regard t transcurrent r strike-slip mtin. The evidence given abve that the Queen subductin faulting. Gelgic cnsequences f this phenme- Charltte Islands are being pushed nrth int Dixn Entrance nn shuld be bservable. which is being pushed int the Alaska panhandle suggests that The structural and mechanical cnditins f the plates exa simple tw-dimensinal partitining f mtin is nt ccuristing at cmmencement f transpressin are extremely imprring in this prtin f Nrth America. tant. A subductin zne with a well-develped slab which un- Seismicity can als indicate the relative strength f a fault. derges a change in relative mtin t blique underthrusting will prbably retain its subductin gemetry. Transcurrent Bird [1997] has calculated that between 1951 and 1981 the faulting in the upper plate is a cmmnly bserved mechanical mment release rate was abut 4 x 1028 N m/yr. If seismicity slutin t this stress regime. A transcurrent plate bundary, takes up all f the relative plate mtin, then the QCF frm hwever, will nt instantaneusly transmgrify int a subduc- the Tuz Wilsn Seamunts t the Alaska brder (400 km) has tin zne because the relative mtin vectr changes gradually a shear mdulus f 10 GPa fr a 20-km-deep seismgenic by ø. Initiatin f subductin is nt a trivial matter espe- zne, which is quite weak. If the shear mdulus has values f cially in yung buyant ceanic plates. The age f the Pacific GPa, a value cnsidered typical fr ht mafic plates plate adjacent t the Queen Charltte Basin at 5 Ma was nt [Kreemer et al., 1998], then the amunt f strain accmmdated significantly different than it is nw. In the case f the QCF in by earthquakes is 30-50%. It may be that the difference bewhich bth plates adjacent t the fault are warm and weak, it tween the strength f Nrth America and the main fault zne seems lgical that defrmatin within bth f these plates will is nt great enugh t prduce strain partitining. take up the cmpressin, as is bserved. Large amunts f net The cncept that strain partitining (r lack theref) is a cmpressin may well initiate subductin, but tens f kilme- unique indicatr f fault strength may be versimplified. Preters d nt seem t be large enugh. If the brittle-ductile existing gemetry f the plate bundary and hetergeneities transitin in Nrth America was significantly shallwer than it within the plates may verride such effects during defrmatin. is nw when transpressin began, then it is cnceivable that The fact that cmpressin within Hecate Strait ccurs mst brittle Pacific plate may have intruded the Nrth American visibly by inversin f Micene extensinal faults indicates that ductile zne. Once started, that prcess wuld prbably cnpreexisting structures have strngly influenced reginal defrtinue. Hwever, current seismicity and flexural studies argue matin. against there nw being a subducted slab. The mdel prpsed here argues that a "megathrust" event Refractin studies f the Queen Charltte Basin mapped between underthrust ceanic and verriding cntinental plates thinned cntinental crust and relatively flat Mh. Basement under the eastern prtins f the islands is -4 km thicker than as predicted by Yrath and Hyndman [1983] and mdeled by under the basin; there was n direct evidence fr a subducted Smith et al. [1998] is highly unlikely. The mdel prpsed here slab f Pacific crust in the wrk f Spence and Asudeh [1993] can explain current seismicity in Graham Island and adjacent and Hle et al. [1993]. In the rughly 30 km between the QCF Hecate Strait as defrmatin which is prpagating nrthward and the westernmst limit f these studies the authrs pstu- frm the hinterland, Graham Island. T date, intraplate earthlated that a subducted slab might exist, but it wuld have t dip quakes are typically smaller than magnitude 5. It shuld be at angles >20ø-26 ø. nted, hwever, that blind thrusts in the "big bend" f the San Gravity mdels acrss the QCF cncur with refractin data Andreas Fault system have prduced earthquakes f magnithat Mh is nearly flat under the islands. Hwever, similar tude 6.7 [Suthern Califrnia Earthquake Cmmittee and U.S. crustal densities in bth plates means that gravity mdels pr- Gelgical Survey Staff, 1994].

24 8170 ROHR ET AL.: TRANSPRESSION 6.3. Cmparisn t Other Regins changes in relative plate mtins [Pckalny et al., 1997]; the The Pacific-Nrth America plate bundary t the suth f adjacent very yung plate will defrm easily, and the ldest the regin that we cnsidered lies entirely within yunger, plate alng the transfrm will be mre resistant defrmatin weaker ceanic material. Adjustment t the change in relative during transpressin. The OCF strikes at a lw angle t the mtins frm 8 t 5 Ma has ccurred by ridge migratin, trend f magnetic anmalies resulting a slw change in plate jumps, and, currently, defrmatin in a wide shear zne [Rhr age alng strike, and the flexural anmalies f Nrth America and Furlng, 1995; Kreemer et al., 1998]. T the nrth the suggest that little change in mechanical behavir ccurs alng strike. amunt f transpressin is smaller; the terrace is less distinct [Bruns and Carlsn, 1987], and there is little bvius defrmatin in Nrth America [Gehrels et al., 1987]. The Pacific plate 7. Cnclusins is lder and strnger, and in Nrth America the crust changes frm the Wrangell t the Alexander terrane, which did nt Transpressin acrss the Pacific-Nrth America plate experience the same degree f warming and extensin in the bundary f western Canada is mst likely accmmdated by Micene [Gabrielse and Yrath, 1991]. strike-slip mtin alng the OCF and bth strike-slip and cm- Regins f transpressin which are currently being studied pressive defrmatin within each plate. The QCF is a lngwith mdern seismic techniques are New Zealand and Califr- lived transfrm plate bundary separating tw warm mafic nia. Net cmpressin alng the central San Andreas Fault has plates. The ceanic plate is 7-15 Myr ld and Nrth America been estimated t be km, and all f it is thught t be cnsists f the mafic Wrangell terrane that was heated by accmmdated by intraplate defrmatin n bth sides f the Micenextensin. The brittle-ductile transitin in bth plates San Andreas Fault [Cruch et al., 1984] as well as delaminatin is currently between 13 and 21 km depth. The three- [Wallace, 1990], althugh the exact structures are nt well dimensinal interactin f the plate bundary and the relative knwn. Inversin structures similar t thse in Hecate Strait mtin vectr puts mst f the defrmatin in Nrth America. are bserved east f the San Andreas Fault in the Kettleman A small amunt f transpressin in the ceanic plate is accm- Hills and Calinga Anticline [Namsn and Davis, 1988]. In the mdated within the terrace, an km-wide regin f du- Transverse Ranges a large high-velcity anmaly in the mantle plexed ceanic plate and flded sediments; lwered crustal velcities attest t the defrmatin f basement material. In [Humphreys et al., 1984; Khler, 1999] has been interpreted t be ductile cntinental plate bwed int the mantle by Nrth America, uplift and cmpressin f Micene extentranspressin. In New Zealand, interpretatins have included sinal basins attest distributed defrmatin. The prtin f the idea that a plate can be delaminated [Walctt, 1998]; its the plate lying in the regin f predicted verlap east f the upper brittle part is peeled ff and frced upward, while the OCF is under nrtherly directed cmpressin and its crust is being thickened t frm the Queen Charltte Islands. lwer sectin sinks under the adjacent plate. Mre recently, extensive seismic measurements shw bth extensive anistrpy [Klsk et al., 1999] and a high-speed velcity anmaly in Acknwledgments. We wuld like t thank the tireless effrts f the mantle beneath New Zealand [Mlnar et al., 1999]. These Master Ian Yung and the crew f the R/l/Maurice Ewing fr the data results have been interpreted t indicate that the mantle is acquisitin and Jhn Diebld and Linc Hllister fr their genersity in defrming a wide shear zne and nt in a subductin gesharing the data. Supprt fr this wrk has been prvided by NSF, EAR EAR and EAR Interpretatins metry. It shuld be nted that bth regins are dminantly we much t discussins with Kevin Furlng and Tm James. Helpful silicic in the crustal dmain and the plate may have a layer f reviews f the manuscript have been prvided by Rn Clwes, Ralph very weak crustal material abve strnger mantle. This me- Currie, Snya Dehler, Kevin Furlng, Rb Gvers, Tm James, and an chanical prfile, as cntrasted with the simpler prfile f mafic Assciate Editr. This is Gelgical Survey f Canada paper plates wuld affect the plates' respnse t transpressive stresses. References Oceanic transfrms are the mst abundant examples f Andersn, R. G., and I. Reichenbach, U-Pb and K-Ar framewrk fr strike-slip interactin between mafic plates, but little is knwn Middle t Late Jurassic (172-> 158 Ma) and Tertiary (46-27) plutns abut pssible crustal thickening r uplift caused by transpres- in the Queen Charltte Islands, in British Clumbia, in Evlutin sinal frces. Transfrm ridges in lng ffset fracture znes and Hydrcarbn Ptential f Queen Charltte Basin, British Clumcan be km wide, but uplift may als be caused by bia, edited by G. J. Wdswrth, Pap. Gel. Surv. Can., 90-10, 59-88, extensinal frces [Pckalny et al., 1996]. Lcal thickening un- Andrnics, C. L., L. S. Hllister, C. Davidsn, and D. Chardn, der a transfrm ridge at the Clippertn Fracture Zne has Kinematics and tectnic significance f transpressive structures been attributed t transpressinal frces [I/an Avendnk et al., within the Cast Plutnic Cmplex, British Clumbia, J. Struct. 1998; Pckalny et al., 1997]. Krauset al. [1998] suggest that the Gel., 21, , Atwater, T., Plate tectnic histry f the nrtheast Pacific and western 400-km-lng Mendcin ridge frmed by bductin during Nrth America, in The Gelgy f Nrth America, vl. N, The Eastern transpressin in the last 19 Myr. Anmalusly aged basalts Pacific Ocean and Hawaii, edited by E. L. Winterer, D. M. Hussng, testify t transfer f crust frm ne plate t anther, althugh and R. W. Decker, pp , Gel. Sc. f Am., Bulder, Cl., the exact nature f the structures which accmmdate the Atwater T., and J. M. Stck, Implicatins f recent refinements transfer is unknwn. Previus uplift f the ridge is shwn by Pacific-Nrth America plate recnstructins, Int. Gel. Rev., 40, the presence f wave-wrn cbbles n the crest f the ridge , A significant difference between the Queen Charltte regin Barrie, J. V., and K. W. Cnway, Sedimentary prcesses and surficial and an ceanic transfrm is that between spreading centers the gelgy f the Pacific margin f the Queen Charltte Islands, British plates age and change mechanical behavir alng strike mre Clumbia, Gel. Surv. Can., Current Res., 1996-E, 1-6, Barrie, J. V., B. Brnhld, K. W. Cnway, and J. L. Luternauer, quickly than in the example studied here. Plate being created Surficial gelgy f the nrthwestern Canadian cntinental shelf, at the ridge itself is expected t adjust instantaneusly t Cnt. Shelf Res., 11, , 1991.

25 ROHR ET AL.: TRANSPRESSION 8171 Bdrubd, J., G. C. Rgers, R. M. Ellis, and E. O. Hasselgren, A micrseismicity study f the Queen Charltte Island regin, Can. J. Earth Sci., 26, , Bird, A., Earthquakes in the Queen Charltte regin: , M.Sc. thesis, Univ. f Victria, Victria, B.C., Canada, Bstwick, T. K., A re-examinatin f the 1949 Queen Charltte earthquake, M.Sc. thesis, Univ. f B.C., Vancuver, Canada, Bruns, T. R., and P. R. Carlsn, Gelgy and petrleum ptential f the sutheast Alaska cntinental margin, in Gelgy and Resurce Ptential f the Cntinental Margin f Western Nrth America and Adjacent Ocean Basins: Beaufrt Sea t Baja Califrnia, Earth Sci. Set., vl. 6, edited by D. Schll, A. Grantz, and J. G. Vedder, pp , Circum-Pac. Cunc. f Energy and Miner. Resur., Tulsa, Okla., British Clumbia: Structure f the crust, Can. J. Earth Sci., 30, , Hrn, J. R., R. M. Clwes, R. M. Ellis, and D. N. Bird, The seismic structure acrss an active ceanic/cntinental transfrm fault zne, J. Gephys. Res., 89, , Humphreys, E., R. W. Claytn, and B. H. Hager, A tmgraphic image f mantle structure beneath suthern Califrnia, Gephys. Res. Lett., 11, , Hyndman, R. D., and R. M. Ellis, Queen Charltte fault zne: micrearthquakes frm a temprary array f land statins and cean bttm seismgraphs, Can. J. Earth Sci., 18, , Hyndman, R. D., and T. S. Hamiltn, Queen Charltte area Cenzic tectnics and vlcanism and their assciatin with relative plate mtins alng the Nrtheastern Pacific margin, J. Gephys. Res., 98, Bruns, T. R., A. J. Stevensn, and M. R. Dbsn, GLORIA investi- 14,257-14,277, gatin f the Exclusive Ecnmic Zne in the Gulf f Alaska and ff Hyndman, R. D., T. J. Lewis, J. A. Wright, M. Burgess, D. S. Chapman, sutheast Alaska: M/V Farnella Cruise F7-89-GA, June 14-July 13, and M. Yaman, Queen Charltte fault zne: Heat flw measure- 1989, U.S. Gel. Surv. Open File Rep., , ments, Can. J. Earth Sci., 19, , Burbidge, D. R., and J. Braun, Analgue mdels f bliquely cnver- Irving, E., J. G. Suther, and J. Baker, Tertiary extensin and tilting in gent cntinental plate bundaries, J. Gephys. Res., 103, 15,221- the Queen Charltte Islands: Evidence frm dyke swarms and their 15,237, palemagnetism, Can. J. Earth Sci., 29, , Chen, Y., Thermal mdel f ceanic transfrm faults, J. Gephys. Res., Jsenhans, H. W., D. W. Fedje, K. W. Cnway,, and J. V. Barrie, Pst 93, , glacial sea-levels n the western Canadian cntinental shelf: Evi- Chen, Y., and J. Mrgan, A nn-linear rhelgy mdel fr mid-cean dence fr rapid change, extensive subaerial expsure, and early ridge axis tpgraphy, J. Gephys. Res., 95, 17,583-17,604, human habitatin, Mar. Gel., 125, 73-94, Cruch, J. K., S. B. Bachman, and J. T. Shay, Pst-Micene cmpres- Klsk, E. R., F. T. Wu, H. J. Andersn, D. Eberhart-Phillips, T. V. sinal tectnics alng the central Califrnia margin, in Tectnics and McEvilly, E. Audine, M. K. Savage, and K. R. Gledhill, Upper Sedimentatin Alng the Central Califrnia Margin, Annu. Meet. Pap., mantle anistrpy in the New Zealand regin, Gephys. Res. Lett., 26, , vl. 38, edited by J. K. Cruch and S. B. Bachman, pp , Pac. Sect., Sc. f Ecn. Palentl. and Mineral., Bakersfield, Calif., Khler, M., Lithspheric defrmatin beneath the San Gabriel Mun tains in the Suthern Califrnia Transverse Ranges, J. Gephys. Res., Dehler, S. A., and R. M. Clwes, The Queen Charltte Islands refrac- 104, 15,025-15,042, Krause, D.C., R. A. Duncan, and M. R. Fisk, Origin f Mendcin tin prject, part I, The Queen Charltte Fault Zne, Can. J. Earth Sci., 25, , Ridge, NE Pacific, thrugh bductin f the Juan de Fuca plate, Es Dehler, S. A., C. E. Keen, and K. M. M. Rhr, Tectnic and thermal Trans. AGU, 79(45), Fall Meet. Suppl., F859, Kreemer, C., R. Gvers, K. P. Furlng, and W. E. Hlt, Plate bundary evlutin f Queen Charltte Basin: Lithspheric defrmatin and defrmatin between the Pacific and Nrth America plates in the subsidence mdels, Basin Res., 9, , Explrer regin, Tectnphysics, 293, , DeMets, C., R. G. Grdn, D. F. Argus, and S. Stein, Current plate Lewis, P. D., Dextral strike-slip faulting and assciated extensin alng mtins, Gephys. J. Int., 101, , the suthern part f the Luscne Inlet fault system, suthern Detrick, R. S., R. S. White, and G. M. Purdy, Crustal structure f Queen Charltte Islands, British Clumbia, Pap. Gel. Surv. Can., Nrth Atlantic fracture znes, Rev. Gephys., 31, , A, , Dragert, H., and R. D. Hyndman, Cntinuus GPS mnitring f Lewis, P. D., J. W. Haggatt, R. G. Andersn, C. J. Hicksn, R. I. elastic strain in the nrthern Cascadia subductin zne, Gephys. Thmpsn, J. R. Dietrich, and K. M. M. Rhr, Triassic t Negene Res. Lett., 22, , evlutin f the Queen Charltte regin, Can. J. Earth Sci., 28, Engebretsn, D., Relative mtins between ceanic and cntinental , plates in the Pacific Basin, Spec. Pap. Gel. Sc. Am., 205, Lewis, T. J., W. H. Bentkwski, and J. A. Wright, Thermal state f the Fitch, T. J., Plate cnvergence, transcurrent faults and internal defr- Queen Charltte Basin, British Clumbia: warm, in Evlutin and matin adjacent t sutheastern Asia and the western Pacific, J. Hydrcarbn Ptential f Queen Charltte Basin, British Clumbia, Gephys. Res., 77, , edited by G. J. Wdswrth, Pap. Gel. Surv. Can., 90-10, , Fulthrpe, C. S., and J. A. Austin Jr., Anatmy f rapid margin pr gradatin: Three-dimensinal gemetries f Micene clinfrms, Lwe, C., and S. A. Dehler, Crustal thickness beneath the Queen New Jersey margin, AAPG Bull., 82, , Charltte Basin, Canada: Results f a seismically cnstrained gravity Furlng, K. P., Thermal-rhelgic evlutin f the upper mantle and inversin, J. Gephys. Res., 100, 24,331-24,345, the develpment f the San Andreas fault system, Tectnphysics, 223, , Gabrielse, H., and C. J. Yrath (Ed.), The Gelgy f Nrth America, vl. G-2, Gelgy f the Crdilleran Orgen in Canada, Gel. Sc. f Am., Bulder, Cl., Gehrels, G. E., J. Saleeby, and H. C. Berg, Gelgy f Annette, Gravina and Duke Islands, Can. J. Earth Sci., 24, , Mackie, D. J., R. M. Clwes, S. A. Dehler, R. M. Ellis, and P. Mrela-l'Huissier, The Queen Charltte Islands refractin prject, part II, Structural mdel fr transitin frm Pacific plate t Nrth American plate, Can. J. Earth Sci., 26, , Masek, J. G., and C. C. Duncan, Minimum-wrk muntain building, J. Gephys. Res., 103, , McCaffrey, R., Estimates f arc-parallel strain rates in fre-arcs, Gelgy, 24, 27-30, Mlnar, P., H. J. Andersn, E. Audine, D. Eberhart-Phillips, K. R. Gledhill, E. R. Klsk, T. V. McEvilly, D. Okaya, M. K. Savage, T. Stern, and F. T. Wu, Cntinuus defrmatin versus faulting Harris, R. N. and D. S. Chapman, A cmparisn f mechanical thickness estimates frm trugh and seamunt lading in the sutheastern Gulf f Alaska, J. Gephys. Res., 99, , Henstck, T., A. Levander, and J. A. Hle, Defrmatin in the lwer crust f the San Andreas Fault system, nrthern Califrnia, Science, thrugh cntinentalithsphere f New Zealand, Science, 286, , , , Hentn, J. A., H. Dragert, R. D. Hyndman, and K. Wang, Gedetic Munt, V. S., and J. Suppe, State f stress near the San Andreas fault: mnitring f crustal defrmatin n Vancuver Island, Es Trans. Implicatins fr wrench tectnics, Gelgy, 15, , AGU, 79(45), Fall Meet. Suppl., F192, Namsn, J., and T. L. Davis, Seismically active fld and thrust belt in Hicksn, C. J., The Masset Frmatin n Graham Island, Queen the San Jaquin Valley, central Califrnia, Gel. Sc. Am. Bull., 100, Charltte Islands, British Clumbia, in Evlutin and Hydrcarbn , Ptential f Queen Charltte Basin, British Clumbia, edited by G. J. Nrtn, I., Plate mtins in the Nrth Pacific: The 43 Ma nnevent, Wdswrth, Pap. Gel. Surv. Can., 90-10, , Tectnics, 14, , Hle, J. A., R. M. Clwes, and R. M. Ellis, Interpretatin f three- O'Sullivan, P. B., and R. Parrish, The imprtance f apatite cmpsidimensinal seismic refractin data frm western Hecate Strait, tin and single-grain ages when interpreting fissin track data frm

26 8172 ROHR ET AL.: TRANSPRESSION plutnic rcks: A case study frm the Cast Ranges, British Clumbia, Earth Planet. Sci. Lett., 132, , Pckalny, R. A., P. Gente, and W. R. Buck, Oceanic transverse ridges: A flexural respnse t fracture-zne-nrmal extensin, Gelgy, 24, 71-74, Pckalny, R. A., P. J. Fx, D. J. Frnari, K. C. MacDnald, and M. R. Perfit, Tectnic recnstructin f the Clippertn and Siqueirs Fracture Znes: Evidence and cnsequences f plate mtin change fr the last 3 Myr, J. Gephys. Res., 102, , Prims, J., K. P. Furlng, K. M. M. Rhr, and R. Gvers, Lithspheric structure alng the Queen Charltte margin in western Canada: cnstraints frm flexural mdeling, Ge Mar. Lett., 17, 94-99, Rhr, K. M. M., and L. Currie, Queen Charltte Basin and Cast Muntains: Paired belts f subsidence and uplift caused by a lwangle nrmal fault, Gelgy, 25, , Rhr, K. M. M., and J. R. Dietrich, Strike-slip tectnics and develpment f the Tertiary Queen Charltte basin, ffshre western Canada: Evidence frm seismic reflectin data, Basin Res., 4, 1-19, Rhr, K. M. M., and K. P. Furlng, Ephemeral plate tectnics at the Queen Charltte triple junctin, Gelgy, 23, , Rhr, K. M. M., C. Lwe, and P. D. Snavely, Jr., Seismic reflectin, magnetics and gravity data acrss the Queen Charltte fault and Dixn Entrance, Gel. Surv. Can. Open File Rep., , Scheidhauer, M., Crustal structure f the Queen Charltte transfrm fault zne frm multi-channel seismic reflectin and gravity data, M. A. thesis, Oreg. State Univ., Crvallis, Scheidhauer, M., A.M. Trehu, and K. M. M. Rhr, Multi-channel seismic reflectin survey ver the nrthern Queen Charltte fault, Gel. Surv. Can. Open File Rep., , Shaw, J. H., and J. Suppe, Active faulting and grwth flding in the eastern Santa Barbara Channel, Califrnia, Gel. Sc. Am. Bull., 106, , Sibsn, R. H., Fault zne mdels, heat flw, and the depth distributin f earthquakes in the cntinental crust f the United States, Bull. Seisml. Sc. Am., 72, , Smith, A. J., R. D. Hyndman, K. Wang, M. Schmidt, and H. Dragert, Thermal and dislcatin mdeling and great earthquakes f the Queen Charltte Transfrm Fault, Es Trans. AGU, 79(45), Fall Meet. Suppl., F192, Snavely, P. D., Jr., H. C. Wagner, D. H. Tmpkins, and D. L. Tiffin, Preliminary gelgic interpretatin f a seismic reflectin prfile acrss the Queen Charltte Island fault system ff Dixn Entrance, Canada-United States, U.S. Gel. Surv. Open File Rep., , Suthern Califrnia Earthquake Cmmittee and U.S. Gelgical Survey Staff, The magnitude 6.7 Nrthridge, Califrnia, earthquake f January 17, 1994, Science, 266, , Spence, G., and I. Asudeh, Seismic velcity structure f the Queen Charltte Basin beneath Hecate Strait, Can. J. Earth Sci., 30, , Spence, G. and D. I. Lng, Transitin frm ceanic t cntinental crustal structure: Seismic and gravity mdels at the Queen Charltte transfrm margin, Can. J. Earth Sci., 32, , Stern, T. A., and J. H. McBride, Seismic explratin f cntinental strike-slip znes, Tectnphysics, 286, 63-78, Stck, J. M., and P. Mlnar, Uncertainties and implicatins f the Late Cretaceus and Tertiary psitin f Nrth America relative t the Faralln, Kula, and Pacific plates, Tectnics, 7, , Sweeney, J. F., and D. A. Seemann, Crustal density structure f Queen Charltte Islands and Hecate Strait, British Clumbia, in Evlutin and Hydrcarbn Ptential f Queen Charltte Basin, British Clumbia, edited by G. J. Wdswrth, Pap. Gel. Surv. Can., 90-10, 89-96, Telfrd, W. M., L. P. Geldart, and R. E. Sheriff, Applied Gephysics, Cambridge Univ. Press, New Yrk, Thmpsn, R. I., J. W. Haggart, and P. D. Lewis, Late Triassic thrugh early Tertiary evlutin f the Queen Charltte Basin, British Clumbia, in Evlutin and Hydrcarbn Ptential f Queen Charltte Basin, British Clumbia, edited by G. J. Wdswrth, Pap. Gel. Surv. Can., 90-10, 3-30, Thrsn, R. M., Glaci-isstatic respnse f the Puget Sund area, Washingtn, Gel. Sc. Am. Bull., 101, , Van Avendnk, H. J. A., A. J. Harding, J. A. Orcutt, and J. S. McClain, A tw-dimensinal tmgraphic study f the Clippertn transfrm fault, J. Gephys. Res., 103, 17,885-17,899, Walctt, R. I., Mdes f blique cmpressin: Late Cenzic tectnics f the Suth Island f New Zealand, Rev. Gephys., 36, 1-26, Wallace, R. E. (Ed.), The San Andreas Fault system, Califrnia, U.S. Gel. Surv. Prf. Pap., 1515, Wilcx, R. E., T. P. Harding, and D. R. Seely, Basic wrench tectnics, Am. Assc. Pet. Gel. Bull., 57, 74-96, Yrath, C. J., and R. D. Hyndman, Subsidence and thermal histry f Queen Charltte Basin, Can. J. Earth Sci., 20, , K. M. M. Rhr, Rhr Gephysics, 719 Birch Rad, Nrth Saanich, British Clumbia, Canada V8L 5S1. (krhr@hme.cm) M. Scheidauer, Institut de Gephysique, Universitd de Lausanne, Bat. Cllege prp edeutique, CH-1015 Lausanne, Switzerland. (Maren. Scheidhauer@ig.unil.ch) A.M. Trehu, Cllege f Oceangraphy, Oregn State University, Crvallis, OR (trehu@ce.rst.edu) (Received May 10, 1999; revised Octber 28, 1999; accepted Nvember 5, 1999.)