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 madethat 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, dur- ing 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