Midcretaceous Thrusting in the Southern Coast Belt, British

Home , Chilcotin Ranges, Jack Mountain, Mount Tatlow, Oval Peak, Shulaps Range, Stikine River

TECTONICS, VOL. 15, NO. 2, PAGES, 545-565, JUNE 1996

Mid-Cretaceous thrusting in the southern Coast Belt, British Columbia and Washington, after strike-slip fault reconstruction

Paul J. Umhoefer Departmentof Geology,Northern Arizona University, Flagstaff

Robert B. Miller Departmentof Geology, San JoseState University, San Jose,California

Abstract. A major thrust systemof mid-Cretaceousage Introduction is presentalong much of the Coast Belt of northwestern. The Coast Belt in the northwestern Cordillera of North North America. Thrusting was concurrent,and spatially America containsthe roots of the largest Mesozoic mag- coincided,with emplacementof a great volume of arc intrusives and minor local strike-slip faulting. In the maticarc in North America, which is cut by a mid-Creta- southernCoast Belt (52ø to 47øN), thrusting was followed ceous,synmagmatic thrust system over muchof its length by major dextral-slipfaulting, which resultedin significant (Figure 1) [Rubin et al., 1990]. This thrust systemis translationalshuffling of the thrust system. In this paper, especiallywell definedin SE Alaska [Brew et al., 1989; Rubin et al., 1990; Gehrels et al., 1992; Haeussler, 1992; we restorethe displacementson major dextral-slipfaults of the southernCoast Belt and then analyze the mid-Creta- McClelland et al., 1992; Rubin and Saleeby,1992] and the southern Coast Belt of SW British Columbia and NW ceousthrust system. Two reconstructionswere made that usedextral faulting on the Yalakom fault (115 km), Castle Washington(Figure 1)[Crickmay, 1930; Misch, 1966; Davis et al., 1978; Brown, 1987; Rusrnore aad Pass and Ross Lake faults (10 km), and Fraser fault (100 Woodsworth, 199 la, 1994; Miller and Paterson, 1992; km). The reconstructionsdiffer in the amount of dextral offset on the Straight Creek fault (160 and 100 km) and Journeayand Friedman, 1993; Schiarizza et al. , 1990]. how much the NE part of the Cascadescrystalline core ex- The mid-Cretaceousthrust system in the southernCoast panded(30 km and 0 km) during Eocene extension. Belt is the focus of this paper. This deformationepisode ReconstructionA producesthe best match of lithotectonic (~ 100 - 80 Ma) is the major tectonicevent in the belt, during which there was voluminousmagmatism and meta- units and thrust systems. Our synthesisshows that the southernCoast Belt thrust system was >250 - 180 km morphismand basinssubsided and were inverted. Defor- wide after thrusting. The thrust system was mainly mation continuedafter ~80 Ma, but it changed signifi- southwestvergent but had a belt of northeastvergent back cantly to a narrower belt dominatedby strike-slip and thrusts on the northeast side associated with the thrustfaulting and lessvoluminous magmatism. Thus the Tyaughton-Methowbasin, which may indicatelarge-scale southernCoast Belt is a prime example of a complex, tectonic wedging. Thrust faults are commonly low to synmagmaticthrust belt that was overprintedby major moderateangle, but high anglefaults also occur,especially strike-slipfaults. as late stage,out-of-sequence, structures involving plutons. The cause of the mid-Cretaceous contraction in the The amount of thrust displacementacross the system is Coast Belt is controversial.In one hypothesis,the final unknown but must be at least 100 km and may be many accretionof the Insular superterraneinvolved an oceanic hundredsof kilometers. Most thrusting occurredfrom arc-continentalarc collision that causedregional contrac- -100 to -80 Ma and did not migrate systematicallyuntil tion as the outerplate (Insularterrane) continued to subduct after-90 Ma, whenthrusting and magmatism shifted to the and collide with North America [Monger et al., 1982; northeastfor a few million years. Widespreadthrusting Thorkelson and Smith, 1989]. In a second model, the occurredboth near plutons and where there are no (or small) regionalcontraction results from the telescopingof a small plutons,which strongly suggests that thrustfaulting was backarc or transtensionalbasin between the Insular super- causedby regional-toplate-scale forces such as rapidplate termnearc to the westand the westernedge of North Amer- convergenceand/or arc-continent collision. ica [McClelland et aL, 1992; van der Heyden, 1992]. A third model relates thrustingto contractionalstep over zonesin a major dextral strike-slipfault system [Brown, 1987; Maekawa and Brown, 1991]. Another model sug- Copyright1996 by the AmericanGeophysical Union. geststhat the western and easternparts of the southern CoastBelt were part of the sameJurassic arc systemthat Papernumber 95TC03498 wasduplicated by majorEarly Cretaceoussinistral faulting 0278-7407/95TC-03498512.00 [Mongeret al., 1994]. Our studydoes not favor a single

545 546 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

apparentcontradictions within the strike-slipfault system. We also make an explicit effort to tie togetherthrust faults from the southern Coast Mountains of British Columbia to 60øN the North Cascadesof Washington, two areas that were B.C. linked togetherearly [e.g., Crickmay, 1930] but have not always been viewedas the samebelt. We feel this syn- SE Ak. thesis of the reconstructed southern Coast Belt makes a sensible,coherent pattern out of what often seemsto be an inscrutablecollage. We alsobriefly explorethe relation of thrusting and magmatism in the southernCoast Belt. It hasbeen increasingly recognized that plutons in many arcs were emplacedduring regional contraction [e.g., Hutton and Reavy, 1992, Miller and Paterson, 1992; Tobisch et al., 1995], but thrust belts in magmatic arcs are less well studiedin comparisonto foreland fold-and-thrustbelts, where the now classicrelationships of folds to faults and fault geometry were described[e.g., Dahlstrom, 1969; 49øN Boyer and Elliot, 1982; Suppe, 1983]. In addition,none of

200 km the previouslydescribed contractional arcs have such a wide thrust system exposedat differentcrustal levels as in the Cascade thrustsystem reported here in the southernCoast Belt. Plate2 core

Figure 1. Regional tectonicsetting of the Coast Belt of NW Washington, British Columbia, and SE Alaska showingother geophysiographic belts, major terranes, mid- Cretaceous Tectonic Setting of Southern Cretaceousthrust systems outside the southernCoast Belt, Coast Belt and Late Cretaceousto early Tertiary dextral, strike-slip faults. Abbreviations are AX, Alexander terrane; BC, Bella The southernCoast Belt includesthe Wrangelliaterrane along the western edge and terranesof the Intermontane Coola; Cf, Chatam Strait fault; CSZ, Coast Shear Zone; superterranealong the eastern margin and to the east Ef, Entiat fault; Ff, Finlay fault; FRf, Fraserfault; H-Rf, (Figure 1). Making up mostof the southernCoast Belt are Hozameen and Ross Lake faults; M, unassigned numeroussmall terranesthat had a complexlate Paleozoic metamorphicbelt; Pf, Pinchi fault; PR, Prince Rupert; to Mesozoichistory beforeaccreting to North America in RMTf, Rocky MountainTrench fault; SCf, StraightCreek fault; bold SE, southeasternCoast Belt; ST, Stikine Jurassicto Cretaceoustime (Table 1) [e.g., Monger Journeay, 1994]. We include the Coast Mountains of SW terrane; bold SW, southwesternCoast belt; Tf, Teslin fault; TU, Taku terrane;WCf, West Coast fault; and WR, BritishColumbia and the North Cascadesof Washingtonin the Coast Belt. Historically, these two regions were Wrangelliaterrane. Map is compiledafter Wheeleret al. [1991], Rubin et al. [1990], Journeayand Friedman[1993], separated,because the westernparts of these rangeshave substantiallydifferent geology at the surface. We note, Evenchick[1991], and Gabrielse [1985]. however, following others[Crickmay, 1930; Misch, 1966; Monger et al., 1982], that the geologyof the easternparts of both regions has much in common and that one Meso- model,but we concludethat plate-boundary changes and/or zoic terranein the westernpart of the Coast Belt in SW conditionsare importantin explainingthe thrustsystem British Columbia is exposedin a thrust window in the becauseof its greatlength and width. northwesternWashington Cascades (Plate 1). These rela- Theprimary difficulties in analyzingthe mid-Cretaceous tionshipsstrongly suggest that the westernblock in British thrustsystem in the southernCoast Belt arethe lackof a Columbia structurally underlies much of northwestern coherentstratigraphy in muchof the belt andthe presence Washington. of the majordextral, strike-slip faults of latestCretaceous Herein we commonly use three terms for geo- and early Tertiary age that disruptedthe thrustsystem logic/physiographicdivisions of the Coast Belt. We use (Plate1). The wealthof newresearch in thepast decade on southernCoast Belt to refer to that part of the Coast Belt thesestrike-slip faults, mid-Cretaceous thrusts and folds, southof 52øN (Figure 1). This usageis both geologically correlationsof rock units, and crustal structurebased on sensibleand practical because there have been many studies seismicreflection data allows us to better analyze the in the Coast Belt south of Bella Coola but fewer recent thrust system. studiesfrom there to the Prince Rupert region (Figure 1). In this paper,we presenttwo reconstructionsof the Geologically, the region south of 52øN includes the strike-slipfaults (Plate 2 andFigure 2), and usethese to Tyaughton- Methow basinson the northeastand the NW analyzethe style, spatial relations, and timing of thrusting Cascades- San Juan Islands belt on the southwest and in the southernCoast Belt. We favor one (Plate 2) [after numerousterranes, all of which have no counterpartnorth Umhoeferand Schiarizza,1996] that nicely reconciles of 52øN. We further divide the southern Coast Belt into UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING,. SOUTHERN COAST BELT 547

:'•':•'"':':-"."- Granitoid-80-60 plutonMa

121øW Granitoidpluton • •50øN -115-80 Ma

-] Granitoid-185-115 plutonMa

i• Cadwalladerterrane 125øW me.tamorphic • rOCKSCadwallader

i• terraneBridge River BridgeRiye. r

•-• rOCKSme,tamorpmc

-.._-•Niut""Pemberton- terrane • Peliticschist

...::.-. . .. Metshow V .....-"'.. .t.•:.?.'•-_.I I''.'' me.tamorpmc • ...... c;:.'.':.'a.'•i.6•.._ -' .-.*• I .'•.-.ø'.•.!: . -JMt:.. [• rOCKSunassigne.d.

' ,-,• ••" {,SCf.-• I' '&":':""'",r_ .•.':. ':'.%• strike-slipfault )t.'-t . lil, S •-_ , ' "•--'"--"-"•:•:' thrust fault • ß . .. ••J•''':•2• '-'S•___' '"I[II .! '":'"!•' '"::;•''"•"'""'i•![". • mid-K TSd --O"'i'•'"":ii:' 8ø':...... ' Q reconstructedpart of thrustbelt ß,-- ' "'C-::5f:.i:'iJ-:-'JJ•..:[Illl Widefaultsymbols NWSanJuan-•11Casca •••x'"/I•-:'"•::•!•ii•ii•: Ef•'•'•:-':-.•....-;•.....-i: ....[I reconstructedindicateboundariesbeltsof

xSCf, / •2! /o,w Stua.....

Figure 2. ReconstructionB' a reconstructionof the southernCoast Belt using the offset and timing on major dextralslip faults in Table 3. See text for discussion.Unit designationsand abbreviationsare as in Plates1 and2. Thin dashedlines are present lines of latitudeand longitude for reference.

two tectonic domains following Monger and Journeay Rock Units [1994], the southwestern and southeastern Coast Belt (Figure 1 and Plate 1). The Northwest Cascadesdo not The older rocks of the southwestern Coast Belt include neatly fit this scheme; we chooseto includethem in the the Wrangellia and Harrison Lake terranes in British southwesternCoast Belt becauseit is geographicallysen- Columbia(Table 1, Plate 1). Voluminousplutonic rocks sible and they containlocal exposuresof rocks similar to intrude theseterranes (Table 1, Plate 1), which were linked the southwesternbelt of British Columbia, but they also by -165 Ma by a suiteof similarplutons [Mahoney, 1994; includerocks potentially correlableto rocksof the south- Friedmanand Armstrong,1995]. The southwesternCoast eastern belt. Belt in Washington (Northwest Cascadesand San Juan 548 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

Table 1. Summaryof Major RockUnits in the SouthernCoast Belt

Rock Unit Age Description Refer-

Methow terrane M-U Jurassic volcanicrocks and overlying sandstone and mudstone 1,2,3 L Jurassic deepmarine sandstone and shale 2,3 Triassic oceanic basalt 4

Methow basin UCretaceous nonmarine red beds and andesites 5 L Cretaceous marine clastic rocks 5,6,7

Cadwallader terrane L-M Jurassic shallowto deepmarine sandstone and shale 8 (CascadeRiver unit) U Triassic volcanic arc and shallow marine and nonmarine 9,10 sandstone and limestone Permian ophiolitecomplex, serpentinite melange 11,12

Tyaughtonbasin UCretaceousL nonmarineconglomerate and andesites 13 Cretaceous marineclastic rocks-complex facies 13 MU Jurassic-L shallowmarine to marginalmarine sandstone, siltstone 14 Cretaceous and conglomerate

BridgeRiver terrane Miss. to oceanicrocks: argillite, chert, basalt, and 15,111 (CogbumGroup, Napeequa unit, U Jurassic minor sandstone,limestone, ultramafite 6,12 Elbow Lake Formation, Deadman Bay unit)

Stikine terrane L Cretaceous conglomerate,sandstone 17,18 M Jurassic volcanicarc, in southonly 19 U Triassic volcanic arc and clastic rocks 20,18

Pelitic schist Triassic ??? Peliticschist, lesser psammitic schist, amphibolite and 21 (Chiwaukumschist, Settler Jurassic ??? ultramafite schist,Chism Schist,Cayoosh L Cretaceous assemblage)

Harrison Lake terrane L Cretaceous sandstone and volcanic rocks 22 M-U Jurassic volcanic arc and sandstoneand shale 22 U Triassic clastic rocks 22

Darringtonphyllite/ U Jurassic phylliteand greenstone and blueschist 23 Shuksan Greenschist L Cret. ?? 24 Helena-Haystackmelange U Jurassic- serpentinitematrix with blocks of ophioliticrocks, 25 (Manastashunit) Cretaceous tonalite,amphibolite, felsic metavolcanic, and sandstone

IngallsComplex U Jurassic ophioliticcomplex 26

Swakaneterrane Proterozoic? biotitegneiss 12,27 Paleozoic? 1, Mahoney,[1994]; 2, Coates,[1974]; 3, O'Brien,[1986]; 4, Ray, [1986];5, McGroder,[1989]; 6, Trexler,[1985]; 7, Barksdale,[1975]; 8, Umhoeferand Tipper,[1996]; 9, Rusmore,[1987]; 10, Umhoefer,[1990]; 11, $chiarizzaet al., [1996]; 12, Taboret al., [1989]; 13, Garver,[1989, [1992]; 14, Umhoefer,[1989]; 15, Potter,[1986]; 16, Cordeyand Schiarizza,[1993]; 17, Umhoeferet al., [1994]; 18, Journeayand Freidman, [1993]; Riddell, [1991]; 19, J.M. Journeay, (pers.comm., 1994); 20, Rusmoreand Woodsworth, [1991b]; 21, Plummer,[1980]; Mahoney and Journeay, [1993]; 22, Arthuret al., [1993];Mahoney et al., [1995];23, Misch, [1966];24, Brown,[1987]; 25, Tabor,[1994]; 26, Miller, [1985]; 27, Rasburyand Walker, [1992]. UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 549

Islands)is a region dominatedby various oceanicand arc (mid-Albian) and lasted until -85-80 Ma [Brandonet al., terranesand lacking plutons (Table 1, Plate 1) [Misch, 1988; McGroder, 1989; Gamer, 1989; Umhoefer, 1989]. 1966; Brown, 1987; Brandon et al., 1988]. An exception to this time of termination is the narrow To the east, the southeasternCoast Belt consists of a regionof major dextral, strike-slipfaults in the southeast- group of terranesof arc and oceanicaffinity distributedin ern domain, where local thrust faulting was commonly an elongatefashion: Cadwallader,Bridge River, Methow associatedwith strike-slip faulting [Journeayet al., 1992; terranes,and a Pelitic belt (Table 1 and Figure 2). Overly- Schiarizzaet al., 1990; Miller and Bowring, 1990], and in ing the Cadwalladerand Methow terranesare late Middle easternparts of the Cascadescore where contractioncon- Jurassicand early Late Cretaceousmarine clastic and vol- tinued until at least 72 Ma [Paterson and Miller, 1995]. canic strata of the Tyaughton - Methow basin (Plate 1) Where relations can be demonstrated,southwest vergent [Jeletzkyand Tipper, 1968; Coates,1970; Barksdale, 1975; thrustfaulting always began before northeastvergent fault- Kleinspehn, 1985; Glover et al., 1988; Gamer, 1992; ing and outlastedit [McGroder, 1989; Schiarizza et al., Mahoney, 1992]; the BridgeRiver terraneis only demon- 1990]. strably overlain by mid-Cretaceousclastic rocks [Carver, The southwestern Coast Belt in British Columbia con- 1992]. Gamer [1992] and McGroder [1989] have argued sistsof widely spaced,northeast dipping, thrust faults cut- that the uppermostLower Cretaceous(mid-Albian, -105- ting abundantplutonic bodiesand low-graderocks of the 100 Ma) stratawere depositedin front of active thrusts Wrangellia and Harrison Lake terranes [Monger, 1990; [Trexler, 1985]. Journeayand Friedman, 1993]. The southwesternbelt in The northeasternpart of the Cascadescore (the area Washingtonis the northwestCascades- San Juan Islands between the White River shear zone on the southwest and thrust belt [Misch, 1966; Brandon et al., 1988; Brown, the RossLake fault to the northeast,Figure 2) is a Creta- 1987], which includesdiverse low-grade thrust sheets, with ceousand Paleogeneplutonic and metamorphicbelt that no synthrustingplutons, that were thrust over the has commonlybeen considered a separateentity than the Wrangellia and Harrison Lake terranes[Brandon et al., CoastBelt in SW British Columbia. In recent years,how- 1988]. The southeasternCoast Belt in British Columbia ever, it has beenproposed that most of the metamorphic containsa narrowzone of medium- to high-graderocks of rocks are equivalentto the lower-gradeCadwallader and the Stikineterrane, Bridge River terrane,and Settler Schist BridgeRiver terranes[Monger, 1986; Tabor et al., 1989; [JourneaylandFriedman, 1993]. Theserocks are cut by a Miller et al., 1993a], and we thus assignthe Cascadecore complex system of ductile thrust faults and intrudedby to the southeasternbelt. One possibleexception to this numeroussynthrusting and postthrusting plutons. Meta- assignmentis the enigmaticSwakane terrane, which is a morphic gradeincreases across strike (structurallyupward) monotonoussequence of largelybiotite gneiss that may be to the east and decreasesalong strike to the northwest a displacedfragment of Paleozoicor Proterozoiccrust [e.g., [Journeayand Friedman,1993]. The southernextension of Haugerudet al., 1994]. thesethrusts in Washingtonin the Cascadescore is an area Much of the southeastern Coast Belt in British of metamorphicrocks that recordmultiple generationsof Columbia has been intrudedby only scatteredplutons, folds,penetrative foliation and mineral lineation, and local which in generalare youngerto the east [Friedmanand ductilethrust shearzones [e.g., Misch, 1966; Brown and Armstrong,1995]. Mid-Cretaceousplutons are widely dis- Talbot, 1989; Miller and Paterson, 1992; R.B. Miller and tributed across the region (Plate 1) [Friedman and S. R. Paterson,unpublished data, 1995]. The bulk of the Armstrong,1995]. The latest Cretaceousto early Tertiary southeasternCoast Belt lies to the east of the high-grade plutonicbelt is solelyin the east. rocks and consistsof low-gradeslices of the Cadwallader, Bridge River, and Methow terranesand strata of the Tyaughton- Methow basin [Schiarizza et al., 1990; Structural Geology Journeayand Friedman, 1993]. These rocks are cut by both northeastand southwestdipping thrusts. Virtually the entire southernCoast Belt is cut by a A major system of dextral, strike-slip faults cut the complex thrust systemwith both northeastand southwest southeasternCoast Belt from -80 to -40 Ma (Plate 1) dippingthrusts; northeast dipping thrusts are the dominant [Monger, 1985; Kleinspehn, 1985; Miller and Bowring, structure(Plate 1) [Misch, 1966; Tabor et al., 1987, 1989; 1990; Umhoeferand Schiarizza, 1996]. These structures Brandon et al., 1988; Brown, 1987; Miller and Paterson, wereused in our reconstructions.This strike-slipfaulting !.992;Journeay and Friedman, 1993]. However,the dip was probablycaused by dextral-obliquesubduction of the and densityof thrustsv •arygreatly throughoutthe belt. Crustal-scaleseismic reflection PrOfiles across the s'outhern Kula plate beneathNorth America [Engebretsonet al., Coast Belt are interpretedto define many kilometer-scale tectonicwedges bounded by both northeastand southwest Was the Southern Coast Belt a Coherent Block dippingthrusts in the southeasternCoast Belt [Varseket in Mid-Cretaceous Time? al., 1993]. U-Pb zircon data from synthrustingand post- thrustingplutons[Journeay and Friedman, 1993; Rusmore Our reconstructions result in the southern Coast Belt and Woo&worth, 1994; Paterson et al., 1994] indicatethat forming a•.coherentblock with many throughgoingbelts thrustfaulting occurred from 97 to -80 Ma. Structuraland (Plate 2). Paleomagneticdata also suggestthe block was stratigraphicrelations in the basinson both sidesof the contiguousduring the mid-Cretaceous,not widely separated Coast Belt suggestthat thrustingbegan -105-100 Ma into smaller blocks. 550 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

Table 2. Fault Scenario Used for Reconstruction A

Stage of Faulting Total Offset Offset on Individual Faults

Mid-late Eocene 110 km 110 km, StraightCreek fault (South) -44- >34 Ma 100 km, FraserRiver fault (North) 10 km, Marshall Creek/NW Yalakom fault (North) Middle Eocene 50 km 50 km, StraightCreek fault -47 - 44 Ma 30 km, N-S closingof NE coreof North Cascades 20 km, Marshall Creek/NW Yalakom faults Early Eocene 85 km 85 km, Yalakom/Hozameen/FoggyDew faults -58 - -47 Ma Latest Cretaceous - 10 km 10 km, Castle Pass/Downton Creek/Ross Lake faults Paleocene -80- 60 Ma (255 km total)

Paleomagneticdata from the southernCoast Belt sug- and Schiarizza [1993, 1996] (Table 2, Plate 2). Recon- gest that it may have moved northward2500 - 3000 km structionA is original in that it (1) accountsfor a new, after-90 Ma. If partsof the block movednorthward >600 refined estimate of timing and displacementon the km more than otherparts (the strikelength of the southern Yalakomand related faults, (2) incorporatesthe conclusions Coast Belt), then the block would have been in separate of Coleman and Parrish [1991] on the timing and piecesin mid-Cretaceoustime. However, resultsof pale- kinematicsof the Yalakom andMission Ridge faults and omagneticstudies from four widely spacedlocations within their relation to other faults, (3) includesrecent conclusions the southernCoast Belt (Plate 1) containing 90-Z_10Ma on the Ross Lake fault [Miller, 1994], (4) accountsfor rock units all have similar northward translation within penetrativeextension in partof the Cascadecore [Haugerud error, which suggeststhat the southern Coast Belt was et al., 1991], and (5) resolvesan outstandingconflict in the contiguous:Mt. Stuart batholith, 3100+600 km [Beck et timing and offset of the Fraserand Straight Creek faults, al., 1981]; Spuzzumpluton, 2300-Z-_700km [Irving et al., segmentsof the samefault zone. 1985]; Porteau pluton, 3200-Z-_500km or 1600-Z-_400km ReconstructionA has the following history [from [Irving et al., 1995]; SilverquickFormation/Powell Creek Umhoeferand Schiarizza,1996] (Table 2, Figure3). From volcanics,3000-Z-_500 km [Wynne et al., 1995] (Plate 1). -85 to-58 Ma, the Ross Lake- Downton Creek- Castle The Silverquick/PowellCreek studyis the only one done Pass faults were one zone that had 10 km of dextral offset in layeredrocks with unambiguouspaleohorizontal. These (Figure 3b). Dextral, strike-slip, and thrust faulting (the resultssuggest that the block was coherentand that if a latter not accountedfor in the reconstruction)were also large-offsetfault zone is present,it is east of the southern occurringat the sametime in this zone and northeastof the Coast Belt. Entiat fault [Hurlow, 1993; Paterson and Miller, 1995]. This transpressivestage of deformationended about 60-57 Ma, near the end of the Paleocene, when a dextral + exten- Strike-Slip Fault Reconstructions for 80 Ma sionalstage began accompanied by widespreaddevelopment of basinswithin the Coast Belt, which are conspicuously In order to analyze the thrust system, we must first absentin the latest Cretaceousto Paleocenestage. This reconstructthe major dextral,strike-slip faults, which was secondstage from-58 to 47 Ma saw -80 km of dextral done in two ways (Tables 2 and 3, Plate 2, Figure 2). slip on the Yalakom fault and subsidiaryfaults and related ReconstructionA is basedon a recent analysisof strike- extensionon the Tatla Lake metamorphiccore complex slip faulting in the southeasternCoast Belt by Umhoefer [Friedmanand Armstrong, 1988] (Figure 3c). The Hoza-

Table 3. Fault Scenario Used for Reconstruction B

Stageof Faulting Total Offset Offset on Individual Faults

Mid-late Eocene 100 km 100 km, StraightCreek fault (South) -44- >34 Ma 90 km, Fraser River fault (North) 10 km, Marshall Creek/NW Yalakom fault (North) Middle Eocene 20 km 20 km, Marshall Creek/NW Yalakom faults -47 - 44 Ma Early Eocene 85 km 85 km, Yalakom/Hozameen/FoggyDew faults -58 - -47 Ma Latest Cretaceous - 10 km 10 km, Castle Pass/Downton Creek/Ross Lake faults Paleocene -80- 60 Ma (215 km total) 52 ø E. Waddington tbxust belt

Taseko-BridgeR. 'thrust belt MT:• Castle Pass fault

;Rmb

125ø ":*'*"...:i :.•, :::..-.-.-.:-:...... ß",'o,C.X>•X• .... •...... ::.:'"* .::.::•:-•'••• . "...., ?:::'. Pem•on '•..- -::% .....•:::::.....:> -' ... '"'..?'"?•-- :•:'..;:.•• - •:-.4 •::.')':.;:...:::•'"'"*:':?' •;'" w,k :.'-,....{}:{?c,{,• 'q , •'{ '*:::"':::::.:]::"Z'•:*::.:" Kt'•s t :-' '(,:..;>...... 7;-*% •.:.::7•,..•:: ...... :.::_:•. •' , fault '•:,.'"'::::..':•?:. • ".-.':..::::.?,i'-•:•"..:.::.'-' •?:':•:'i.""•'::•':"'?-':':xXX '.,.N W.':% •.. ""%.•,, . " 5½• v '../:::b:.-.:•:. [q'..-:::..•:':'.•;.:.•, ½' ' ';.::•,, ' Z'::::....;....• • 121ø

-80-60Ma X * • . P' ":'"""""":'"v-..'4;,c.:•.•_•.',,, Methow Granitoidpluton•;:•, ••'?UU:U•U•j•_spt.•' te,ane&asin(SE subbelt)

-115-80M • H•sonthrust beltL•e •(f•]•:•Ho Granitoidpluton Vancouver /

Cadwallader • fault terraBe-185-115 Ma 49123• • • / Pasayten Cadwallader loan - metamorphicrocks .axaCascades BridgeRiver HHm '-.':%::/::,meta- terrane Paleomagnetism '-' Methow :• studysite " •oq4 '•": Bridge River .... metamorphicrocks • Majorstrike-slipdextral fault Stikineterranek mid-Kthrust fault 48ø122q-ø Peliticschist Anticline 173 Syncline • .,•_ Harrison Lake • '•'. .... terrane 50 km DS"MR'"':"::i..:74:..::.. "::..'"'':": unassigned w' . metamorphic MtStuart Cas Core rocks batholith(SE subbelt)

Plate 1. Geologicmap of the southernCoast Belt (52ø to 47øN) showingmajor lithotectonic belts, mid-Cretaceous thrust belts, andmajor early Tertiary dextral,strike-slip faults. The Pasaytenfault is shownas a thrustfault followingthe interpretationof Varseket al. [1993]. Abbreviationsare CCBd, centralCoast Belt detachment;Ct, Chuiwauntenthrust fault; Dp, DicksonPeak pluton;DS, Darrington- Shuksanunits; Etb, Eldoradothrust belt; HHm, Helena-Haystackmelange; JMt, JackMountain thrust; MR, ManastashRidge; MT, Mount Tatlow; NC-FDf, North Creek - Foggy Dew fault; nRLf, northRoss Lake fault; Pp, Porteau pluton;SPb, Spuzzum batholith; RLsz, Rock Lake shearzone: SRmb, Shulaps Range metamorphic belt; Tp, Tenpeakpluton; TSd, Twin Sistersdunite; WPt, Windy Passthrust. Map is compiledfrom Tabor et al. [1987, 1993, 1994], McGroder [1989], Rusmoreand Woodsworth[1994], Schiarizzaet al. [1993], Journeayet al. [1992], andJourneay and Friedman[ 1993]. Yf t 121øW "'• 50øN --] Granitoid-80-60 plutonMa

Granitoidpluton - 1 ! 5-80 Ma

-• Granitoid-185-115 plutonMa Pf

49øN i• terraneCadwallader Cadwallader metamorphic rocks

Bridge River terrane BridgeRi.ver met.amorpnic rOCKS

Stikine terrane

Pelitic schist

.•..'C.•terraneHarrison Lake

metamorphic unassignedrocks

strike-slipMajordextralfault •-• Ef

thrust fault MR ! ++++ mid-K

121/øwMt.Stuart partreconstructed of thrustbelt HHmI batholith San Juan - Wide fault symbols NW Cascades indicate boundaries of reconstructed belts

50 km

SCf

Plate 2. ReconstructionA: our preferredreconstruction of the southernCoast Belt usingthe offset and timing on major dextral slip faults in Table 2 after Umhoeferand Schiarirza [ 1996]. See text for discussion.Unit designationsand abbreviationsare as in Plate 1; additional abbreviations are Btb, Bralome thrust belt; CPf, Castle Pass fault; CPt, Castle Pass thrust, EWtb, eastern Waddingtonthrust belt; Ef, Entiat fault: Ff, Fraserfault; HLtb, HarrisonLake thrustbelt; HZf, Hozameenfault; Ktb, Kwoiek thrustbelt; MCf, MarshallCreek fault; NCFDf, North Creek-FoggyDew fault; Pf, Pasaytenfault; RLf, RossLake fault; RLsz, Rock Lake shearzone: SCf, StraightCreek fault;TBtb, Taseko-BridgeRiver thrustbelt; WRsz, White River shearzone; Yf, Yalakom fault. Thin dashedlines are presentlines of latitudeand longitudefor reference. UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 553

A. 80 Ma B. 80-~58Ma

X te•ane•asin

• fid•••• Ca••••e•asin Va••'.X• 100 km te•ane••3 fault--'•.••• • • Hgz•eenfault

oatholi•puzzl:z• • •'! Mr.Stu• •au•t • • batholith

C. 58-47 Ma D. 47-44 Ma E. 44->34 Ma Tatla Lake metamorphic complex N•q4oTy basin • Creek - ShulapsRange •$,x• •0 • Mmshall/ metamorphicbelt • Mission• fault

te•ane•asin

Cascades core NEp 50 •p• of '••••••3• Cascadescore •W•. • • o •.

Figure 3. A five-step sequentialmap-view depictionof reconstructionA (after Umhoefer and Schiarizza, 1996). A few of the major geologicfeatures of the southernCoast Belt are shown for referencewith patternslike Plate 2, exceptfor the verticalchevron pattern, which indicatesregions wheremiddle crustalrocks were deformedand later uplifted within the Eocene. Thin long dashedlines are faultsthat move after the particularstage of the reconstruction;wide solidlines are faults that were active during that stage;thin solid lines are faults that were inactive by that stage. Large numbers next to active faults are the amount of dextraloffset in kilometers restoredduring that stage; large underlinednumbers are the radiometricage of the adjacentpluton that gives constraintson age of faulting. The large screwin Figure3d is to help visualizethat if that block is relativelystable, then the amountsof offset in the Skagit part of the Cascadescore, Straight Creek fault, and Yalakom fault are compatible(that is, StraightCreek offset= Yalakom + Skagitoffset).

meen fault was probablythe major southernextension of for-20 km of dextral offset. Adjacentto, and below, this the Yalakom, althoughthe RossLake fault may have been major upper crustal strike-slip fault zone, the Shulaps active as well with lesser offset [Miller, 1994]. At 47 Ma, Range and northeasternpart of the Cascadescore were the system changedas movementon most of the Ross evolving from broad mid and lower crustal (respectively) Lake and the Hozameenfaults died, the StraightCreek fault dextral shear zones to unroofing metamorphiccomplexes beganactivity in the south and the Marshall Creek fault boundedby obliqueand normalslip brittle faults [Coleman became a link between the Straight Creek and NW and Parrish, 1991; Haugerudet al., 1991]. The transten- Yalakom fault in the north (Figure 3d). From -47 to 44 sion and unroofingof the Cascadescore are interpretedto Ma, this new Straight Creek- Marshall Creek- Yalakom involve-30 km of N-S extension in this reconstruction; fault systemwas the major dextralfault zone and accounted this accountsfor another30 km of offset on the Straight 554 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

Creek fault that did not occur on the northern faults. The rocks in the southwest [Rusmore and Woodsworth, 1994]. final stageof strike-slipactivity from 44 to 34 Ma began A well-developedschistosity and downdip elongation with a "Y" configurationas the Fraserfault beganactivity lineation formed during the main phase of deformation. as the major northernextension of the Straight Creek fault Northeastvergent, late-stagemap-scale folds, small-scale and offset of 10 km continued on the Marshall Creek - structures,and map relations all indicate top-to-the- Yalakom fault system (Figure 3e). Deformation soon northeastthrusting. Shorteningacross the thrustbelt was shiftedsolely to the Straight Creek- Fraserfault system, at least 50% (-40 km) but may have been significantly whichaccumulated 110 and 100 km of offset,respectively, more as this estimateis only from the frontal part of the duringthis stage. belt [Rusmoreand Woodsworth, 1994]. The contrast between reconstructions A and B is in the Taseko Lake- Bridge River and Bralorne- interpretationsof the Straight Creek fault. Reconstruction Kwoiek Creek thrust belt. The Taseko Lakes - B usesthe alternateinterpretation of 90- 100 km of dextral BridgeRiver and Bralorne- Kwoiek thrust belts (Plate 1) slip on that fault (Table 3, Figure 3). The Straight Creek are contiguous and show similar structural histories fault does not have an early history independentof the [Schiarizzaet al., 1990; Journeayet al., 1992]. The region Fraser fault in this model, and therefore the two faults have also experiencedperiodic and spatiallydiscontinuous mag- 100 and 90 km of offset,respectively (10 km is still trans- matism in latest Cretaceousand early Tertiary time, but fered from the StraightCreek to Marshall Creek fault). In thereare no plutonsof mid-Cretaceousage, the ageof most addition, in this reconstructionno significant extension of the thrusting. The complex faulting can be dividedinto occursin the Skagitpart of the Cascadescore. three stages,only the first of which is relevant to our Our two reconstructionsdo not incorporatetwo different analysishere: southwestand northeastvergent thrusting interpretationsof the Fraserfault. Friedmanand van der and associatedfolding from 96 Ma (or as early as -105 Ma) Heyden [1992] demonstratedthat two plutonsnow sepa- to -86 Ma; oblique dextral-thmstand dextral faulting rated - 140-160 km across the Fraser fault have a similar between86 and68 Ma; anddextral and extensionalfaulting Permianage. We note that the rock units surroundingthe in Paleocene(?) and Eocene time [Schiarizzaet al., 1990; two plutons are very differentand theseplutons and the Journeayet al., 1992]. The pre-86 Ma thrustsform two rockssurrounding them have experienced multiple deforma- main systems. The Bralorne system consistsof anas- tions. Thus the validity of a simple match acrossthe tomosing, moderatelyto gently northeastdipping faults Fraserfault is questionable.Monger and Journeay[1994] that show sou•.hwest directed motion based on small-scale use 125 km of offset on the Fraser fault, basedon match- structures.This thrust systemcuts rocksas youngas the ing the Yalakom and Ross Lake faults, belts of Eocene Albian Taylor Creek Groupand is locally overlainby -90- extensionand plutonism [Coleman and Parrish, 1991], and 80 Ma volcanicrocks and truncatedby the 92 Ma Dickson latestCretaceous plutons. We note,however, that the belt Peak pluton (Plate 1) [Journeayet al., 1992]. The Eldo- of Eoceneextension and plutonism is wide enoughthat it rado system of thrusts is moderatelysouthwest dipping, is still a coherent,though more elongate,belt using our shows widespreadevidence for northeastdirected motion, 100 km offset on the Fraser fault. In summary,we favor and cuts faults of the Bralornethrust system. Theselate- the offset on the Fraserfault that makesthe belts of geol- stagethrusts are interpretedto havebeen active duringgold ogy in the Cretaceous,before the strike-slip faulting, the mineralization in the Bralorne district, where veins are 91 most coherent, and that is -90-100 km of offset on the to 86 Ma [Leitch et al., 1991]. In the Bralorne to Kwoiek Fraserfault [Monger, 1985]. area, this thrust system separatesrocks with polyphase deformation in the western footwall from rocks with one Local Thrust Fault Relations episodeof deformationin the hanging wall, and therefore Eastern Waddington thrust belt. Thrust faults in these thrusts are out-of-sequencerelative to the Harrison this belt cut Triassic volcanic and sedimentary rocks Lake belt discussedbelow [Journeayet al., 1992]. assignedto the Stikine terraneand a volcanicunit and over- Harrison Lake thrust belt. The westernpart this lying clastic unit of Early Cretaceousage [Rusmoreand belt is cut by widely spacednortheast dipping thrust faults Woodsworth,1994; Umhoefer et al., 1994]. The north- of the foreland that have brittle-ductile character and easternfront of the thrust belt also cuts the Albian Taylor downdiplineations (Plate 2 and Figure 4, B-B') [Journeay Creek Group and Upper CretaceousPowell Creek vol- and Friedman, 1993]. Thrustsof an imbricatezone (on the canics. Ages from synthrustingand postkinematicplutons boundaryof the southwestern- southeasternCoast Belt) are and metamorphicpatterns indicate that thrustingbegan by low angle with mylonites and downdip stretchinglin- 87-84 Ma and, endedsoon after and the areacooled steadily eations, and they expose high-grademetamorphic rocks into the early Tertiary [Rusmoreand Woodsworth,1994]. [Journeayand Friedman, 1993]. Kinematic indicatorsin The presenceof the Taylor Creek Groupin the forelandof the westconsistently indicate top-to-the-southwest motion. the belt suggeststhrusting began in Albian time, about This thrustingis the early stageof a two-stagehistory and 110-100 Ma, becausethat unit is synorogenicin its type was active after eruptionof 102 Ma volcanicrocks, during area-100-150 km to the southeast[Garver, 1992]. intrusionof a 97 Ma pluton, and beforeintrusion of a 96 The easternWaddington thrust belt (Figure 4, A'A') is a +6/-5 Ma pluton [Journeayand Friedman, 1993]. The coherentgroup of northweststriking and southwest dipping steeply northeast dipping, ductile central Coast Belt thruststhat displacelow-grade to unmeta-morphosedrocks detachmentis a major late-stagereverse fault that separates in the northeastand mid amphibolite faciesmetamorphic the imbricatezone from the easternpart of the thrustbelt UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 555

SW A I I A' NE km -, --- _•__-- •"• _•'"•• fault Level '• '• '• '• '• ...... •...... EWaddington2'.'-' :'•''•--' thrustbelt_ • Ty.•h 1ton basin Metbow \basin •TvPasayten • • % 9 1.0 2p Sea-- • w T• No Vertical Exaggeration 87 faultzone r terrane???ff}i•) 20 < RW • Yalakømn' ' ! 20 BendorBreak insection ::• fault Frase•River fault SWCoast Belt iI SECoast Belt

. LillooetLake - H•'rison L e EC,B Kwoi•V E Cascadesfold- Pasayten Serfm 91 thrustbelt 94 ,'•84 _thrus•69k 50 thrustbelt fault km Level ..... *':•..... ' • l[•'"""'"':'"'•::•''•:?.•'• basin •J x• 1

Wrangelliaterrane??? HarrisonLake '• %1,... I • ! • I???I • •/r•.I•E'• •i 1999' i I i I i 20 terrane???'• - Ii_•l• !??',,',, ::: ...,..,•i•1• ,•i ', •?.:i ' i Ti I I I I I I I • • Fraser •ver fault iHoZ•n •i•• faul ' ]2o v SW Coast Belt SE Coast Belt

C NWCascades thrustsystem WindyI Pass CascadesCore C• km • "- •" '-' • • • th st 93•93. RLsz WRsz92 72 91 km Sea ß .e.__ 0

10 - - 10 enozoic Level20 •C'- oceanicterranes Chiwau 20 StraightCreek fault Entiatfault Foggy Dew • CB • <• T • fault

Figure 4. Cross sectionsfrom the northern(A-A'), central(B-B'), and southern(C-C') portions of reconstructionA (Plate 2). Part of sectionA-A' is from Rusmoreand Woodsworth[1994], (RW), part of sectionB-B' is from Journeayand Friedman[1993], (JF), McGroder[ 1989], (M), and Varsek et al. [1993], (V), andpart of sectionC-C' is from Cowanand Bruhn [1992], (CB), and Tabor et al. [1987], (T). Short dashedlines are taken directly or conceptuallyfrom the interpretationsfrom seismic reflectiondata [Varseket aL, 1993]. Patternsare sameas Plates2, 3, and Figure 2. Thin lines with no patternsare beddingin A-A' andfoliation in C-C'. Numbersabove dots in sectionsare agesof the displayedplutons: underlined numbers are postthrusting plutons; others are synthrustingplutons. See Plate 2 for location of the sections.

[Joumeayand Friedman, 1993]. Fabrics in mylonites The folds are open to generally tight and range from along the detachmentindicate east-side-updisplacement. upright to overturned. Thrusts are east vergent in the Structuresin the high-gradehanging wall of the detachment northernpart of the Methow basin, whereaswest vergent include early tight folds and late folds overturnedto the thrustsmay dominatein the southernpart (easternpart of southwest.Late-stage structures are synkinematicwith the B-B' in Figure 4) [McGroder,1989] (althoughHaugerud et detachment and demonstrate that the southeastern belt al. [ 1994] statethat evidencefor the latter thrustsis equiv- includeda series of low-angle thrusts that were cut by ocal). The largest structuresare east vergent, the most higher angle faults. In the west, late and postkinematic important of which is the Chuwantenthrust, which puts plutons of the late stage are 94 and 91 Ma, and in the Lower to Middle JurassicLadner Group on the Upper Cre- imbricate zone a postkinematicpluton is 94 +6/-5 Ma taceousPasayten Group [McGroder, 1989; Monger a•l [Journeayand Friedman, 1993]. Thusthrusting in the Har- McMillan, 1989]. McGroder[1989] utilized the regional rison Lake belt occurredin two stagesfrom -100 to -90 southeastplunge of the Methow basin to constructbal- Ma, similar to the southwest vergent thrusting of the ancedcross sections from which he interpretsthe overall Bralornesystem. structureof the basin as an east vergent tectonic wedge. Eastern Cascades Fold belt. This belt deforms McGroderhas estimatedthat wedgingresulted in - 50 km the predominantlysedimentary rocks of the Methow basin of ENE-WSW shortening near the international border, into north to northwesttrending, moderately to steeply with shorteningdecreasing southward in the fold belt. dippingthrusts and igently plunging map-scale folds, which Contractional structures in the southwestern were active between - 100 and 88 Ma [McGroder, 1989]. part of the Cascades core. The thrust belt in the 556 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT southwesternpart of the Cascadescore (that part southwest possiblylarge plutons[Brown and Walker, 1993], over the of the Entiat fault, Plate 1) involves progressivelydeeper Chiwaukum Schist. Thermobarometric data indicate that a structurallevels from SW to NE. The southernmargin of major pressuregradient occurs in the southernpart of the the coreis markedby the Windy Pass thrust, which carries schistand presumablyreflects either the thrust front or the ophiolitic rocks of the Middle to Late JurassicIngalls boundaryof the overlyingpluton(s). The deepeststructural Complex of the NorthwestCascades System in its hanging levels (P - 9 kbar) in the northeasternpart of the wall [Miller, 1985]. Metamorphic grade in the Ingalls Chiwaukum Schist lie in the footwall to the White River Complex increasesfrom subgreenschistto amphibolite shear zone (Plate 1) [Magloughlin, 1993; Brown and faciesas this fault is approached.Ductile defom•ationin- Walker, 1993]. This shearzone dips moderatelyto steeply tensifies near imbricate thrusts [Miller, 1985, 1988], and north to NE, has a downdip lineation and reverse-slip the Windy Pass thrust has beenfolded on the map scale. kinematicindicators, and carriesin its hangingwall the 91- The vergenceof the thrust is unknown,but regional rela- 92 Ma Tenpeak pluton and high pressureamphibolite- tions are most compatiblewith the Ingalls Complex hav- faciesrocks of the Napeequaunit [Van Diver, 1967; Tabor ing originatedto the south [Miller et al., 1993b], and the et al., 1987; Magloughlin, 1993]. The Napeequais proba- fault possibly representsa northeastdirected back thrust. bly the high-gradecorrelative of theBridge River terraneof Regardlessof the vergence,the overlapof the Ingalls and the southeasternCoast belt [Tabor et al., 1989; Miller et the Chiwaukum Schist indicatesa minimum displacement of 14 km [Paterson et al., 1994]. The thrust was active al., 1993c, 1994]. The Tenpeakand severalother mid-Cre- between96 and 93 Ma, but the age of initiation is uncer- taceousplutons in the hanging wall crystallizedat high- tain [Miller, 1985; Paterson et al., 1994]. pressures(7-9.5 kbar) [Dawes, 1993] in contrastto plutons The Chiwaukum Schist is the unit in the Cascades core of similar age elsewherethroughout the Cascadescore, that bestpreserves the mid-Cretaceousstructures. It records includingthe Mount Stuartbatholith and otherplutons that at least five transpositioncycles [Patersonet al., 1994], intrude the Chiwaukum Schist [Miller et al., 1993a]. The most of which reflect regional progressivedeformation involvement of the Tenpeak pluton and the retrograde marked by NE-SW contractionand synchronousNW-SE metamorphismof the Chiwaukum Schist in the White extension. Contractionis manifestedby SW vergentfolds River shearzone indicatethat it postdatesmost of the con- that have gentle NW-SE trendingfold axesand moderately traction in the Chiwaukum Schist and movement on the NE dipping axial-planarcleavages and are associatedwith Windy Pass thrust. Thus the shearzone is probably a weak NW-SE trending stretchinglineations [Miller and majorout-of-sequence structure. We also note that specu- Paterson, 1992; Paterson et al., 1994]. Most of this latively the shearzone flattensupward and was responsible deformationoccurred before and during emplacementof the for the loading of the Chiwaukum Schistto the southwest. 93 Ma Mount Stuart batholith. Latest recognizedcontrac- In summary,contraction in the southernpart of the Cas- tional structuresin the southernmostpart of the core are cadescore is markedby repeatedfolding in the Chiwaukum steep southwest-directedductile shear zones that are Schist, movement on late reverse-slipshear zones, and synemplacementto postemplacementof the Mount Stuart possibly a major back thrust. Significant contraction [Miller and Paterson, 1992; Magloughlin, 1994]. These beganby 96 Ma, was active at 93 Ma and after 91 Ma, and shearzones may mark significant breaksin metamorphic ductiledeformation ended by 80 Ma, basedon the youngest pressure within the Chiwaukum Schist [Magloughlin, K-Ar biotite ages [Engels et al., 1976] in the schist near 1994]. the White River shear zone [Miller and Paterson, 1992; The Chiwaukum Schist records NE-SW shortening Paterson et al., 1994]. normal to the subhorizontalstretching lineation. Brown Northeast Part of Cascades Core. The most dif- and Talbot [1989] interpretedthe lineations in the Chi- ficult part of the North Cascadesin which to evaluatemid- waukum and elsewherein the Cascadescore to recordper- Cretaceous contraction is in the Cascades core northeast of vasive orogen-paralleldextral shear. In contrast, others the Entiat fault (Plate 1). Much of the evidencethere for [Miller and Paterson, 1992; Paterson et al., 1994; Paterson mid-Cretaceoustectonics has been obscuredby 75-45 Ma and Miller, 1995] have emphasizedthe lack of evidencefor plutonism, metamorphism, and penetrative deformation displacementparallel to this lineation. Brandonet al. [e.g., Miller et al., 1989; Haugerudet al., 1991]. The [1994] have madea similar argumentfor lineationsin the main manifestations of Cretaceous contraction are at least Northwest Cascadessystem, and we infer that the well- three cyclesof approximatelycoaxial, generallySW ver- documentedNE-SW contractionalong the length of the gent folds of gently to moderatelydipping foliations Coast Belt [Rubin et al., 1990] indicatesthat dextral shear [Patersonand Miller, 1995]. This folding in part occurred was at most a minor compontentof the mid-Cretaceous between 74 and 60 Ma [Paterson and Miller, 1995], but deformation. some of the earliest folds may recordthe mid-Cretaceous The Chiwaukum Schist experiencedearly low P/T event. Other possiblemajor mid-Cretaceouscontractional metamorphism in the aureole of the Mount Stuart structuresinclude the pre-84 Ma folded,but generallygen- batholith and was subsequentlyloaded by up to 4 kbar tly dippingtectonic contactbetween the Swakaneterrane [Evans and Berti, 1986; Brown and Walker, 1993; Paterson (Table 1) and the overlying Napeequaunit [Tabor et al., et al., 1994]. This loadingprobably records emplacement 1987; Hurlow, 1992] and Late Cretaceousor older inferred of higherlevel mid-Cretaceous(-93-85 Ma) thrusts[Evans thrust boundariesbetween componentsof the Napeequa and Berti, 1986; McGroder, 1991; Paterson et al., 1994], or unit (probablemetamorphosed Bridge River terrrane)and UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 557

CascadesRiver unit (probablemetamorphosed Cadwallader favorthe model that, overall,thrusting was directedto the terrane) [e.g., Brown et al., 1994]. SW. Northwest Cascades - San Juan Islands thrust The magnitudeof thrusting is uncertainbut must be system. This system, which was active between- 100 large. Severaltens of kilometersare required by the struc- and 84 Ma [Brandon et al., 1988], is a thick stack of turaloverlap, regardless of thetransport direction [Haugerud regional-scalelow-angle nappesof mostly low-grade,late et al., 1994]. McGroder [1991] states that structural Paleozoicand Mesozoic arc-type and oceanicstrata [Misch, overlaprequires a minimum of 47 km and 25 km on two 1966; Monger, 1966; Brown, 1987; Brandonet al., 1988], thrustsof the belt. The presenceof a large number of which are thrust over Middle Jurassic to Lower Cretaceous exotic slices in the thrust system [e.g., Brandonet al., stratathat are probablythe extensionof the HarrisonLake 1988] alsoimplies large displacement. terraneof the southwesternCoast belt (westernpart of C-C' in Figure 4) [Misch, 1966; Monger and Journeay, 1994]. Extendingthe thrustsheets west to the San Juan Islandsis Regional Synthesis more difficult, although close similarities between San Reconstruction A. Reconstruction A results in six Juan units and rocks in the NW Cascades have been noted coherent lithotectonic belts with distinct thrust fault zones [e.g., Brown and Vance, 1987; Brandonet al., 1988]. formingmost of their boundaries(numbered on Plate 2 and The thrustscommonly have shallow dips, but there are Figure 2). The coherenceof the belts and similarity of many exceptions. The overall map pattern suggestsNW large faults bounding the belts acrossreconstruction A trending folds above ramps in a southwestor northeast (Plate 2) are what gives us confidencethat it is viable. vergent thrust system, but this pattern in part reflects Many of these contactsare also terraneboundaries, which Eoceneand younger(?)folds [e.g., Brandonet al., 1988; the mid-Cretaceousthrusting and strike-slip deformation Haugerudet al., 1994]. The thrustsformed at lower tem- have largely obscured. We feel that if a coherentrecon- peraturesthan those to the east in the Cascadescore, as struction of the thrust belt can be obtained(Plate 2), this fault breccias,scaly fault-zone fabrics, and Riedel-type will greatly aid the analysisof theseterrane boundaries. shearpatterns are characteristic [Cowan and Brandon,1994; On the northeast, belt 1 is the Methow terrane and the Haugerudet al., 1994]. Low-T (< 200øC), high-P meta- overlying Methow basin. To the southwest,belt 2 con- morphismaccompanied to slightly postdatedthrusting and sistsof Cadu/alladerterrane and overlying Tyaughtonbasin waspresumably related to rapidstructural burial and subse- in the north and Bridge River terrane, its metamorphic quentuplift within an advancingthrust wedge [Brandon et equivalent(Napeequa unit of Cascadescore), and the proba- al., 1988]. ble metamorphosedCadwallader terrane (Cascade River unit The kinematicsof the thrust system are controversial. of Cascadescore) in the south. The two easternbelts are Southwest vergence is supportedby the (1) regional not separatedby major thrusts,but by the youngerdextral, northweststrike of the system, includingthe inferredfore- strike-slip Yalakom, Fraser, and Hozameen-NorthCreek- land basin [e.g., Brandon et al., 1988]; (2) brittle fault Foggy Dew faults. In fact, the presenceof identicalrocks geometries[Cowan and Brandon,1994]; (3) orientationof and thrustsin the northernpart of the two belts is the pri- asymmetric folds [Misch, 1966; Cowan and Brandon, mary evidencefor the 115 km of dextral offset on the 1994]; and (4)evidence for southwestdirected thrusting Yalakom fault [Riddell et al., 1993]. Gamer [1989, 1992] along strike to the northwestin the Coast Belt for - 1200 has demonstratedbased on detailedprovenance and strati- km [Rubin et al., 1990]. Misch [1966] proposeda slightly graphicevidence that the mid-Cretaceous sedimentary rocks different and more complex kinematic model with of the Methow and Cadwalladerbelts were coniguousat overturned folds and associated faults that record SE ver- that time. Both belts have early southwestvetgent thrusts genceon the south,western vergence near the International that formed from --100 to 90 Ma. Northeast vetgent boundary,and NW vergenceon the north [cf. Crickmay, thrusts cut these structures and were active from -95/90 to 1930; Monger, 1966]. These NW vergentstructures may 85 Ma. McGroder [1989] suggestedthat in belt 1, both providethe connectionbetween the thrusts in the North- groupsof faults were activesimultaneously and formeda west Cascades and the Coast Mountains of British tectonic wedge (B-B' in Figure 4) and northeastvetgent Columbia. The bend in the thrust system may reflect the faults outlasted the southwest vergent ones. Seismic presenceof the rigid massof Wrangelliaand the pre-mid- reflectiondata further suggesttectonic wedging [Varsek et Cretaceousplutons of the southwesternCoast Belt in al., 1993]. In belt 2, at presentexposure levels the north- British Columbia [Crickmay, 1930; Monger and Journeay, eastvetgent thrusts consistently cut the southwestvetgent 1994]. thrusts. Belt 2 contains the belt of Eocene deformation A markedlycontrasting model is that the systemis a correlatedacross the Fraser- StraightCreek fault [Coleman broad, northwest striking dextral transpressivezone in and Parrish, 1991]. which thrustsformed in contractionalstep over zones. The Belt 3 is the pelitic Chism - Settler - Chiwaukum evidencefor this interpretationis largely the regional Schists(Plate 2). The CayooshAssemblage of the Bridge developmentof northwest trending lineations [Brown, River terrane[Mahoney and Journeay,1993] may also be 1987;Maekawa and Brown, 1991; Haugerudet al., 1994], partof the peliticschist belt [Mongerand Journeay,1994]. althoughthe kinematicsare commonlyinconclusive. We These pelitic schistshave been correlatedby previous areimpressed by the continuityof the NorthwestCascades workersbased on similarities in lithology, geochemistry, systemwith structureselsewhere in the CoastBelt andthus and metamorphichistories [e.g., Misch, 1977: Mon•,r 558 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

1986; Evansand Berti, 1986]. The Settler Schist possibly Belt 6 is the Harrison Lake and Wrangellia terranes, correlatesto the Darrington - Shuksanunits of the NW which by middle Jurassictime probably were the same Cascadesbased on similar protolith composition,distinc- entify [Monger and Journeay, 1994; Mahoney, 1994]. tive fabric in eachunit, and the samestructural position in Belts 3/4 (in the north) and belt 5 (in the south) are thrust the thrust stack [Monger, 1991]. This interpretationsig- over belt 6 along the (from north to south) centralCoast nificantly enlargesthe pelitic schistbelt (Plate 2). Other Belt detachment, the Church Mountain thrust fault, and the workers, however, argue strongly against this correlation frontal Haro thrust fault of the San Juan thrust belt, which becauseof differencesin Rb-Sr dataand details of lithology may have beena linked setof faults duringmid-Cretaceous between the two units [Duggan and Brown, 1994; thrusting[Crickmay, 1930; Misch, 1966], and mark terrane Haugerud et al., 1994]. boundaries. Belt 4 is lithologically distinctfrom belts 2 and 3 and The position of magmatic belts also supportsrecon- includesthe Stikine terraneof the easternWaddington area structionA. The synthrusting(-100- 80 Ma) magmatic and generallysimilar rocksnear Pemberton. Details of the belt is found in all reconstructed belts but belt 5 and is thus stratigraphyof the two areasdiffer, but they both contain not an adequatetest of the validity of the reconstruction. similar Triassic arc rocks and lack Jurassicsedimentary The postthrust system, -80 - 60 Ma, magmatic belt, strata [Riddell, 1991; Rusmore and Woodsworth, 1991b], however,does have an impressivealignment in reconstruc- in contrastto the type Cadwalladerterrane, which has a tion A. Virtually all 80 - 60 Ma plutons lie in a narrow nearlycomplete Jurassic sedimentary section. belt within reconstructedbelts 2 and4. In additon,many Belt 2 is juxtaposedagainst belts 3 and 4 acrossthe of these plutons are spatially and temporally associated (from NW to SE) easternWaddington thrust belt, Bralome with the early stageof dextral,strike-slip faulting on the - Kwoiek thrust belt, and White River shear zone. This Castle Pass, Downton Creek, and Ross Lake faults. boundarywas probably a prethrustsystem (pre-100 Ma) Reconstruction B. The differences between the two terraneboundary as it separatesvery distinct rock units; reconstructionsare along the Fraser and Straight Creek alternatively,it may be a mid-Cretaceousterrane boundary faults, mainly in the southernhalf of the region. Recon- [cf. Tabor et al., 1987; Umhoeferet al., 1994]. The east- structionB doesnot align lithotectonicand structuralbelts ern Waddingtonthrust belt is NE vergent [Rusmoreand as successfullyas reconstructionA (comparePlate 2 and Woodsworth, 1994], was active from at or before 87 Ma to Figure2), but it doessatisfy some potential problems with about 80 Ma, and shares much in common with the Castle A in the south. The BridgeRiver terranerocks and meta- Pass thrust belt in belt 2. The Bralorne- Kwoiek thrust morphic rocks in the Cascadescore that have been corre- belt and White River shear zones are continuous in the latedto themare not alignedin reconstructionB (Figure 2). reconstructionand are similar in many respects. Both Likewise,the Chism andSettler schists do not align.with zonesare NE dipping and place BridgeRiver terranerocks the similar Chiwaukum Schist in B. The Chiwaukum over pelitic schistsof belt 3. Timing on the White River Schist is juxtaposedagainst blueschistand greenschist shear zone is not well constrained, but latest movement is faciesmeta-basalts of the ShuksanSuite in B (Figure 2). bracketedbetween 91 and 80 Ma (see above). The Bralome Two interpretationsmay obviatethese objections. First, - Kwoiek thrust belt was active from about 100 to 86 Ma, there is some lithologic similarity betweenthe Settler so the availableage datapermit the interpretationthat the Schist and Bridge River rocks, although the Settler to two thrustzones were active simultaneously.The belt 2-4 Chiwaukum and Bridge River to Napeequacorrelations and belt 3-4 boundariesdisplay relatively small, local slices seem much better by lithology [Misch, 1977; Monger, of rockscorrelated to theBridge River terrane. Theseoccur 1986; Tabor et al., 1989; Miller et al., 1993c]. Second, near the central Coast Belt detachment(CCBd) (Plate 2) of McGroder [1991] has advancedan argument that the Journeayand Friedman [1993], a large out-of-sequence Chiwaukum and Settler may be on oppositesides of a reversefault. Theserelations suggest that the BridgeRiver regionalanticlinorium and therefore a specificspatial jux- terranemay underliestructurally parts of belts 2 and 3 tapositonafter restorationof the StraightCreek fault is not (Figure 4b). necessary. Belt 5, in the southwest,is separatedfrom belt 3 by the The latter interpretationsare weaker when the major centralCoast Belt detachmentof Journeayand Friedman structures in reconstruction B are also considered. The belt [1993] and either its hypotheticalextension near the Mt. of steep thrusts near the Mt. Stuart batholith and the Stuart batholithor the Windy Pass thrust fault, which accompanyingpressure gradient do not match the gently placesthe ophiolitic Ingalls Complex over the Chiwaukum inclinedfaults in the NorthwestCascades thrust system to Schist [Miller, 1985]. The centralCoast Belt detachment which it is juxtaposed(Figure 2). Similarily, the major and Rock Lake shear zone within belt 3 near the Mt. Stuart Bralorne- Kwoiek thrustbelt hasno apparentmatch to the batholithare northeastdipping thrust zones across which SE in reconstruction B. The White River shear zone, areabrupt pressure gradients with higher-pressurerocks on however,does approximately line up with the centralCoast the northeast.The Windy Passthrust, however, is mostly Belt detachment,and thesestructures may be similarin size southdipping and may be a major back thrust. This and style,although the White River shearzone doesappear structuralboundary separates very distinct rock units, and it to have been active later. probablyis a terranesuture zone. Belt 5 includesoceanic The weaknesses of reconstruction A relative to B relate unitsthat may correlateto the BridgeRiver terraneto the primarily to restorationsacross the Straight Creek fault in east [Brown, 1987]. the southernpart of the region. The most problematic UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 559 relationshipis the match in reconstructionA betweenser- McGroder's [1991] structural analysis. The most pentinitemelange of ManastashRidge on the eastside of comprehensivekinematic analysisof mid-Cretaceouscon- the Straight Creek fault (MR on Plates 1 and 2) and the traction in the southern Coast Belt is that of McGroder Helena-Haystackmelange on the west side,both of which [1991], who estimated a minimum of 400 km of E-W containsimilar distinctiveblock types. [Miller and Vance, shorteningacross the North Cascadesfrom the Northwest 1981; Miller et al., 1993b; Tabor et al., 1994]. Recon- Cascades system to the eastern Cascades fold belt. structionA also does not accountas well for possible Although acknowledgingthe problems presentedby oro- westwardcontinuations of the ophiolitic Ingalls Complex gen-parallelstrike-slip faults, McGroder [1991, p. 202] in the Northwest Cascades. In reconstruction A, the ignoredthem (exceptfor verticaldisplacement) arguing that Ingalls Complex is on strike with the very different they did not preventa "conceptuallymeaningful geometric southwesternCoast Belt of British Columbia, whereas in B construction." the ophiolite is nearly on strike with the Twin Sisters Our preferredstrike-slip reconstruction indicates signifi- dunite,which is probablythe root of a dismemberedophio- cant specific problems with McGroder's[1991] model, lite of the NorthwestCascades [Vance et al., 1980]. althoughit providesgeneral support for large shortening Along-strike variations in the southern Coast and the style of faulting he portrayed. The major problem Belt. In our favored reconstructionA (Plate 2), the with the McGroder model is that thrusts in the eastern southernCoast Belt thrust systemis 250 km wide in the Cascades foldbelt are far removed from the inferred correla- southand -200 km wide farthernorth after thrusting(Plate tive structures in the Cascades core and Northwest Cascades 2). It would be 300 km wide in the southif we extrapo- system. For example,the Chuwantenthrust appears to die latedthe Methow basin areaalong strike to the southeast. out •- 100 km NW of its presumedcorrelative, the Jack The wider systemin the southis becauseof the greatflap Mountain thrust(Plate 2), and thus it is unlikely that these of low-anglethrusts of belt 5, which appearto have moved thrusts are correlative. Rather, the Chuwanten probably farther southwestover the Wrangellia - Harrison Lake cuts down beneaththe Castle Pass thrust systemin the footwall than the areasto the north (Figure 4). The style BridgeRiver- Bralornearea, and the projectionof the Jack of thrustingis generallysimilar in both the southeastand Mountain thrust is probablybuffed beneath the Columbia northeast.Thrusting is spatiallyassociated with mid-Cret- Plateau. Our reconstructioncauses less severeproblems aceousmagmatism that was active during some part of the for McGroder's [1991] continuation of structuresbetween thrustingin all areasexcept the belt 5. This aspectof belt the Cascadescore and the NorthwestCascades system, 5 may indicatethat a major transferzone is requiredalong althoughthrusts in this systemare •- 75 km south of their its northernmargin. position in McGroder'scross section. We also note that The thrustsystem has deeper crustal levels exposedin his correlationof the Skagit Gneissof the northeastCas- the southand shallower levels in the north,but this may cades core with the southwestern Coast Belt and the inter- be partly an artifact of the areasthat have been studied. pretationthat this terraneunderlies the Cascadescore are High-graderocks are exposed east of HarrisonLake [Brown contraryto our and othersassignment of theserocks to the and Burmester,1991; Journeayand Friedman, 1993] and high-gradeBridge River and Cadwalladerterranes of belt along the westernedge of the easternWaddington thrust two [Tabor et al., 1989; Miller et al., 1993c, 1994; belt in the north [Rusmoreand Woodsworth,1994]. The Haugerud et al., 1994]. lack of studynorthwest of HarrisonLake makesit possible Despite thesecriticisms, the greatwidth (up to 300 km) that high-graderocks exist along the centralpart of the of the thrust system from the NorthwestCascades system whole southern Coast Belt. to the projectionof the easternCascades fold belt supports The areasof mixed northeastand southwestdipping McGroder's[ 1991] interpretationof very large shortening. thrust faults are primarily in the northeast, near the Thus his generalconclusions are probablycorrect, although Tyaughton- Methow basin of belt 1 in our reconstruction manyof the specificcorrelations are not valid. (Plate 2), whereas the central and southern areas of the Shortening across region. Total shorteningacross southernCoast Belt are dominatedby northeastdipping the southern Coast Belt due to mid-Cretaceous thrust fault- thrusts. Analysisof seismicreflection profiles also shows ing is difficult to determine. One attemptto balancea mixed dips of faulting mainly in the southeasternCoast cross section acrossthe belt suggests400 - 500 km of Belt [Varseket al., 1993]. However, the wholly southwest shortening [McGroder, 1991], but as discussedabove, dippingeastern Waddington thrust belt in the northernend McGroder's model did not account for much of the strike- of our study areais in the southeasternCoast Belt; thus slip faulting, especially within the southeasternbelt. there may be a changeto the north to dominantly south- Despite this problem, much of McGroder'sanalysis west dipping thrusts. This interpretationis currentlydiffi- remains valid, the _>50 km of shorteningwithin the cult to judge becauseof the lack of researchsouthwest of Methow basinand the _>75km shorteningin the NW Cas- the easternWaddington thrust belt. Becausemost of the cades. This total of _>125km doesnot accountfor any southwestdipping thrustsare spatially associatedwith the shorteningin reconstructedbelts 2 and 3 of our model Tyaughton- Methow basin, perhapsthe contrastbetween (Plate 2), wheremainly metamorphicrocks make estimates the relatively weak clastic section in the basin and the of shorteningdifficult. We concludethat much more than region to the west of voluminousplutons promoted thrust- 125 km of shorteningtook place across the southernCoast ing toward and within the basin. Belt duringthe mid-Cretaceousthrust faulting. We note Implications of reconstructions for that the two areaswhere shortening estimates are calculated 560 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT constituteonly -50% of the width of the belt andtherefore action betweenplutonism and contraction[e.g., Hollister we might infer that >250 km of shorteningacross the belt and Crawford, 1986; Miller and Paterson, 1992; Rusmore is a reasonableminimum estimateif shorteningwas fairly and Woodsworth, 1994; Ingram and Hutton, 1994; evenly distributedand that the 400 - 500 km shortening Paterson et al., 1994]. estimatedby McGroder [ 1991] is possible. A strikingfeature of the southernCoast Belt is that mid- Cretaceous thrusts in the flanks of the belt, such as the Discussion Northwest Cascades- San Juan Islands system, are relatively gently dippingbut steepensignificantly within the Why Rotation in the Reconstructions? zoneof abundantplutons (Figure 4). This doesnot repre- The Coast Belt is divided across a line from 49øN on the sent a simple foreland-hinterlandtransition, as most con- west to 50 ø 30' N on the east; north of that line the belt tractional faults dip steeply NE in the forelandto the trends about due northwest, and south of that line it trends Harrison Lake thrust belt, which is composedlargely of more northerlyor north-northwest(Plate 1). Becauseof plutons. In additionto their steepness,some of the major this changein strike,restoring blocks along dextral,strike- structures in the southeastern Coast Belt domain are late- slip faults createslarge gaps, which we minimizedby let- stage,out-of-sequence thrusts that carry synthrustingplu- ting the blocksrotate as we reconstructthem. But what is tonsin their hangingwall [Journeayand Freidman,1993]. the geologicexplanation for this changein orientationof This suggeststhat the steepnessof the thrusts may be the southern Coast Belt? relatedto the intrusion processor to the presenceof these We proposethat two tectonic mechanismsat different relatively rigid bodies. timesin the Cenozoiccan explain the relativerotation and The effect of magmatismon the mechanicsand location argue that becauseof this rotation the belt was not bent of thrusts in the Coast Belt is still poorly understood. duringmid-Cretaceous thrusting (Plate 2). Hollister and Crawford[1986] emphasizedthat melt genera- 1. The most compellinganswer may be that the large tion resultedin major tectonicsurges in the northernCoast bend occurredduring regional Eocene extensionin the Belt and that magmatismled to thermalweakening of the southeastern Coast Belt and the southern Intermontane and belt. We have alsonoted above that the sparse,high-angle Omineca Belts to the east. In the Eocene,-100 km of thrustsactive after 90 Ma were localized along pluton mar- WNW-ESE extension occurred in the southern Omineca gins. Many synthrustingplutons, however, apparently Belt [Tempelman-Kluitand Parkinson, 1986; Parrish et al., experiencedonly minor subsolidusdeformation [e.g., 1988] directly eastof the Methow basin. At the sametime Miller and Paterson, 1994; Paterson et al., 1994]. On a in the Eocene, the southwestern Coast Belt was largerscale, thin-skinned faulting ended in the southwestern experiencingnorthward translation along dextral, strike-slip domainduring widespread magmatism but continuedin the faults and local extension. The net effect of these two pluton-poorsoutheastern domain. regionalstrain patterns may have beento rotatethe south- An ongoing controversyis the relationshipof pluton ern andeastern parts of reconstructionA clockwiserelative emplacementand major crustal loading within the Cascades to the northernpart, becausethe northernand westernareas core. Several workershave documentednearly isothermal were moving northwestalong dextral-slipfaults while the pressureincreases of 2 to 6 kbar during Cretaceousmeta- southernand eastern areas were within the zone of major morphism[Evans and Berti, 1986; Brown and Burmester, WNW-ESE extension. 1991; Brown and Walker, 1993; Miller et al., 1993a; 2. The bendis close to alignment with the bendin the Sawyko, 1994] and attributedthem to either loading by present outer margin of the Pacific Northwest at the thrustsof the NorthwestCascades system [e.g., McGroder, Olympic Mountains. Brandon and Calderwood[1990] 1991] or to loadingby now erodedplutons [Brown and suggestedthat crustalstructural patterns in the Olympic Walker, 1993]. The considerablewidth (> 250 km) of the Mountains and the bend in the margin may best be mid-Cretaceousthrust system, as indicatedby our recon- explainedby clockwise rotation of the Pacific Northwest structions,supports the interpretationthat thrust loading south of the Olympics due to Miocene to Recent Basin- was the more importantmechanism. Crustal thickening and-Rangeextension that is presenteast of the Oregon by thrustingoccurred in areaswith no or few Cretaceous coast and dies out to the north. This mechanism is the plutons,such as in the NorthwestCascades system, where same as what we envision for the Eocene, but in a more structuralburial is manifestedby low-temperature/high- southernposition. pressuremetamorphism that accompaniedthrusting or was slightlypostdated thrusting [Brandon et al., 1988]. Never- Relationships Between Plutonism and Thrust- theless,the relativelycircular isobaric surfaces constructed ing in the Southern Coast Belt by Brownand Walker[ 1993]for partsof the Cascadescore It has been increasinglyrecognized that plutons in the aredifficult to explainby thrustloading and more work is roots of some arcswere emplacedduring regional contrac- neededto evaluatethe magmaloading hypothesis. tion [e.g., Tobischet al., 1995; Hutton, 1992]. The Coast Southern Coast Belt Thrust System and Plate Belt representsperhaps the largestsuch recognized arc, as Tectonics mid-Cretaceousplutonism and thrustingoccurred for a dis- tanceof over 1200 km along strike. Most studiesof this Our studyand a review of the southernCoast Belt geol- arc have focusedon tectonostratigraphicrelationships, but ogy strongly reinforces previous interpretations[e.g., severalrelatively recentpapers have documentedthe inter- Engebretsonet al., 1985; Umhoefer, 1987] that a major UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT 561 changein tectonicstyle at -85-80 Ma was causedby a 5. The suggestionfor the earliest thrusting in mid- changefrom the Farallon to Kula plate along the southern Albian time (- 105-100 Ma) is from stratigraphicobserva- Coast Belt or a major change within the Kula plate tionsof local unconformitiesand locally sourcedconglom- [Engebretsonet al., 1995]. Widespread- 110-80 Ma mag- erate clasts in the Tyaughton - Methow basin [Garver, matism and thrust faulting was followed from 80 to -60 1989; McGroder, 1989], but elsewhere the initiation of Ma by a narrowzone of magmatismand dextral-slip+thrust thrustingis uncertain. Structuresin synkinematicplutons faulting on the east side of the southernCoast Belt. In all indicatethat thrusting was widespreadby 96 Ma. Major areasof the southernCoast Belt, thereis a similarityin the southwestvergent faulting ceasedat 92-91 Ma in many timing of the cessationof major, widespreadthrusting at areas;relations interpreted on seismicreflection lines sug- 90 to 80 Ma. Coeval (110 - 80 Ma) arc magmatism is gest that northeastvergent faulting accompaniedthis and widespreadand voluminousin the region, andsedimentary formedlarge-scale tectonic wedges. basins lay on the northeastside of the belt. From 80 to 6. After-90 Ma, the former areasof southwestvergent -60 Ma, thereare no basinalrocks preserved in the south- faulting had either a few major, high-angle,out-of-sequence ern Coast Belt and magmatismis foundonly in a narrow thrusts that were still southwestvergent or had a domi- belt in the southeastern Coast Belt. nanceof northeastvergent thrusts that cut the olderfaults. The prof6undchange in the southernCoast Belt at 85-80 This secondstage lasted until 85 - 80 Ma. Ma was from contractional deformation oriented normal to 7. Magmatism was widespreadin the southwestern the orogen(WSW-ENE) to dextral-transpressionaldeforma- CoastBelt from -96 to -80 Ma, or throughoutmost of the tion with dominantlyNW to north strikingfaults. This is time of thrusting. Magmatism in the southeasternbelt exactly what is expectedif the Farallon plate subducted wassporadic and scattered before 90 Ma, and then from 90 normal to the southernCoast Belt (North America)during to 80 Ma a majorvolcanic sequence was erupted over much the-110 to 85 Ma interval and the Kula plate subducted of the southeasterndomain (Plate 1). This magmaticpat- obliquely northwardafter-80 Ma as predictedin plate tern and the timing of thrusting suggestthat low-angle, motion models [Engebretsonet al., 1985; Engebretson,et thin-skinnedfaulting died in the southwesternand western al., '1995]. The continuity of the Coast Belt from Wash- part of the southeasternbelts at ~90 Ma becauseof the ingtonto SE Alaska [Rubinet al., 1990] suggeststhat the large volumeof magma that intrudedthe middleand upper entireCoast Belt lay along the Kula plate after80 Ma and crust. Note that the few major, high-anglethrusts that the Kula-Farallon-NorthAmerica triple junction lay at the becameactive after 90 Ma are locatedalong the marginsof southernend of the Coast Belt. The location along western largeplutons, which truncateearlier structures. The post- North America of the major plate boundarychange at -80 90 Ma northeastvergent thrusting was accompaniedby Ma summarized here from evidence in the southern Coast eruption of thick andesitesin the Tyaughton - Methow Belt is dependenton resolution of the Baja British basin. Columbiacontroversy [e.g., Cowan, 1994]. 8. Estimatesof shorteningare >125 km from two seg- ments in the southwesternand northeasternparts of the belt, while estimatesof shorteningfor the central part of the belt are not yet possible. Theselocal estimatessuggest Conclusions that approximately>250 km of shorteningoccurred across the entire southern Coast Belt and that the 400 - 500 km 1. The southernCoast Belt thrust system consistspri- marily of northeastdipping thrust faults with southwest suggestedby McGroder[1991] is possible. vergence. Nearly all faults of the southeasternand south- 9. We infer from a comparisonto plate motion studies westerndomains and early stagefaults of the southeastern [e.g., Engebretson,et al., 1995] that the Farallonplate was domainare SW vergent. beingsubducted under the southernCoast Belt duringmid- 2. An important belt of southwestdipping thrust faults Cretaceousthrusting and magmatism. At -85-80 Ma, the with northeastvergence is in the northeastadjacent to, and Kula platestarted to convergewith very oblique,northward within, the Tyaughton- Methow basin. Where both motion along the southernCoast Belt as indicatedby the northeastarid southwest dipping faults are documented, the dramaticdecrease of magmatismand switch to more local- northeastdipping faults are older. ized dextral+thrustfaulting. 3. Much of the thrust system has thin-skinnedfaulting and folding, especially at the eastern and southwestern margins. However, thick-skinned,moderate to high-angle thrustfaults are common,particularly as late-stage,out-of- Acknowledgments. This research was supported by sequencefaults in the centralpart of the belt. Thesediffer- NSF grantsEAR-8904383 and EAR-9304426 to Umhoefer and encesare probably relatedto synthrustingmagmatism in EAR-8917343 and EAR-9219536 to Miller. We thank N. the interior of the belt and not in the margins. Brown, D. Cowan, R. Freidman, J. Garver, K. Glover, R. 4. Tectonicwedging at the tens of kilometer to kilome- Haugerud, S. Johnson, M. Journeay, K. Kleinspehn, M. McGroder, J. Monger, S. Paterson, C. Potter, M. Rusmore, P. ter scaleis stronglysuggested in seismicreflection sections Schiarizza R. Tabor and G. Woodsworth for constructive dis- [Varseket al., 1993] and from local geometricrelations in cussions over the years and J. Monger, D. Cowan, and H. the Methow basin [McGroder, 1989]. Tectonic wedgingis Hurlow for very helpful reviews. Colored copies of Plates 1 difficult to prove in most places becausethe northeast and 2 at the same size as herein can be ordered from the first vergentthrusts cut the southwestvergent thrusts. author (Umhoefer) for $4.00 each. 562 UMHOEFER AND MILLER: MID-CRETACEOUS THRUSTING, SOUTHERN COAST BELT

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