Monger, J. and Price, R. (2002). The Canadian Cordillera: Geology ...

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February, 2002 CSEG Recorder 17

INTRODUCTION

The Canadian Cordillera (Figure 1A) is a relatively youthfulmountain belt. However, its origins extend back some 750 millionyears (or 750 Ma1) to the episode of rifting that marked the firststages in the break-up and dispersal of “Rodinia”, aNeoproterozoic supercontinent that had existed since about1,000 Ma. The late Neoproterozoic-earliest Cambrian break-up(between 750-540 Ma) led to the opening of a new ocean basinthat was the distant ancestor of the present Pacific Ocean basinand to the formation of a continent-ocean boundary that ispreserved today within the eastern Canadian Cordillera. Theprotracted evolution of the Canadian Cordillera is dominated byinteractions between the margin of the old, stable NorthAmerican continent and the oceanic lithosphere located outboardof it. The initial intra-plate continent-ocean boundary was analo-gous to that of the present boundary between eastern NorthAmerica and the Atlantic Ocean basin. It persisted until theMiddle Devonian (~390 Ma), when a convergent, inter-plateboundary formed, along which magmatic arcs were generatedwithin the edge of the North American Plate by subduction ofoceanic lithosphere beneath it. Arc magmatism has persisted to agreater or lesser extent until the present but has varied in char-acter. In the late Paleozoic and early Mesozoic (between ~355 and185 Ma), the convergent plate boundary apparently lay welloffshore, and involved chains of island arcs separated from theold continental margin by back arc basins. A modern analoguemay be the western Pacific Ocean basin with its chains of intra-oceanic volcanic islands separated by basins, such as thosecontaining the Japan and Philippine seas, from eastern Asia.Starting in the Jurassic (~185 Ma), the North American continentconverged with the offshore subduction zones. The rocks of theback arc basins and of the offshore arcs were accreted to the orig-inal continental margin, and new continental arcs were built onboth the accreted material and parts of the ancient continentalmargin. By the Late Cretaceous (~90 Ma), a new continentalmargin was located near its present position, about 500 kmoceanwards of the position of the original margin, and boundeda mountain belt that probably resembled the present Andes. Therecord of the 750 million year long evolution is preserved both inrocks of the Cordillera and in those of the Western CanadaSedimentary Basin on its eastern margin and the plains, whichevolved hand-in-hand with the Cordillera (Price, 1994).

Below, we first note the nature and expansion of the earth-science knowledge base for the Canadian Cordillera, then providea simple geological overview, and explore the influence exerted byplate tectonic theory on interpretations of Cordilleran geology.This material provides background for the last half of the article,which is a summary review of the tectonic evolution of theCanadian Cordillera.

DATA ACQUISITION: TWO-, THREE-, AND FOUR-DIMENSIONAL MAPPING

Systematic geological studies in the Canadian Cordillera startedabout 130 years ago, largely stimulated by the search for metallicmineral deposits and coal. It took approximately 100 years togeologically map the entire region mainly on a reconnaissance scaleof ~1:250 000, so that by the 1960s the surface distribution of therocks and many of their ages were reasonably well known (e.g.Gunning, 1966). Since then the amount of information has increasedenormously. Many newer geological mapping projects, generally ata scale of 1:50 000, employ teams of specialists including geophysi-cists, focused on the common goal of unraveling the geology of aregion (e.g. Struik and MacIntyre, 2001). For the third dimension, theresults of surface geological mapping have been extrapolated down-ward with the aid of results from Lithoprobe seismic reflection andrefraction profiles. Lithoprobe profiles in the southern and northernCanadian Cordillera have been interpreted as “vertical geologicalmaps” that show the structure and distribution of rocks in verticalslices through the crust and uppermost mantle (Figure 2; Clowes etal., 1995; Clowes and Hammer, 2000). For the fourth dimension,time, geological mapping today is accompanied by abundant andprecise multi-system isotopic dates and biostratigraphic dates usingmicrofossils such as radiolarians and conodonts, in addition to themacrofossils that have been used for dating for over 130 years.Recent studies refine global correlations between the isotopic andbiostratigraphic time scales by isotopically dating volcanic rocksinterbedded with fossiliferous sedimentary rocks (e.g. Pálfy et al.,2000). On a global scale, the Canadian Cordillera is merely onesegment of the circum-Pacific mountain belts. International cooper-ative research and compilation projects, catalyzed in part by theneed to discover new energy and mineral deposits, attempt to inte-grate the geology, tectonic evolution, energy and mineral resourcesof the Pacific basin margin (Halbouty, 1981; Drummond, 1983;Nokleberg et al., 2000; Scotese et al., 2001).

THE CANADIAN CORDILLERA:Geology and Tectonic EvolutionJim Monger,

Geological Survey of Canada, Vancouver, B.C.and

Department of Earth Sciences, Simon Fraser University, Burnaby, B.C.

Ray Price, Department of Geological Sciences

and Geological Engineering,Queen’s University, Kingston, Ontario

1 Ma (Milliard d’annees) is the conventional “shorthand” symbol for millions of years before the present Continued on 8

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18 CSEG Recorder February, 2002

TABLE 1: MORPHOGEOLOGICALBELTS OF THE CANADIANCORDILLERA (refer to Figure 1A)

Foreland BeltPhysiographic units: include Rocky,

Mackenzie, and Franklin mountainsGeology: mainly sedimentary rock: (1)

Mesoproterozoic (locally Paleoproterozoic)clastics, carbonates and minor magmaticrocks deposited in continental basins; (2)Neoproterozoic-earliest Cambrian shale,sandstone, conglomerate, minor carbonateand local mafic magmatic rock depositedduring the rifting and initial breakup of theRodinian supercontinent that created theancient continental margin of westernNorth America; (3) Cambrian to Jurassic

shelf and slope, carbonate and shaledeposited on and near the ancient conti-nental margin of North America margin;(4) Late Jurassic to early Cenozoic marineto non-marine clastics eroded from theuplifting Omineca and Foreland belts;rocks folded and thrust eastwards over theancient continental margin in Late Jurassic-early Tertiary time.

Omineca BeltPhysiographic units: include Purcell,

Selkirk, Monashee, Cariboo, Omineca,Cassiar and Selwyn mountains

Geology: sedimentary, volcanic andgranitic rock, typically metamorphosed upto high grades: (1) local Paleoproterozoiccontinental crust; (2) Neoproterozoic rift

related clastics and volcanics; (3) Paleozoicpericratonic, off-shelf clastic, and volcanicrocks; (4) local accreted late Paleozoic toEarly Jurassic volcanic and sedimentaryrocks formed in island arcs and marginalbasins; (5) early Cenozoic continentalvolcanic and sedimentary rocks; (6)Paleozoic to early Tertiary granitic rocks;the rocks were complexly deformed mainlyby compression between Middle Jurassicand early Tertiary time, and (in the south)by extension in early Tertiary time.

Intermontane BeltPhysiographic units: include Interior,

Stikine, and Yukon plateaus and SkeenaMountains

Geology: volcanic, sedimentary, andgranitic rocks: (1) Devonian to EarlyJurassic sedimentary and volcanic rocksformed in island arcs and chert-rich accre-tionary complexes; (2a) Middle Jurassic toearly Cenozoic volcanic rocks formedmainly in continental arcs and (2b) marineand non-marine clastics eroded mainlyfrom the uplifting Omineca Belt; (3)granitic rocks ranging in age fromDevonian to Cenozoic; rocks deformedmainly by compression in Mesozoic andextension-transtension in early Cenozoictimes.

Coast BeltPhysiographic units: Coast and Cascade

mountainsGeology: mainly granitic rock: (1)

Jurassic through Cenozoic granitic rock; (2)remainder latest Proterozoic(?), Paleozoicto Holocene volcanic and sedimentaryrocks formed mainly in magmatic arcs butlocally in accretionary complexes, meta-morphosed up to high grades andcomplexly deformed mainly in mid-Cretaceous through early Cenozoic time.

Insular BeltPhysiographic units: Insular

Mountains, Saint Elias Ranges, coastaldepressions, continental shelf and slope

Geology: volcanic, sedimentary, andgranitic rocks: (1) latest Proterozoic to mid-Cretaceous volcanic and sedimentary rockformed mainly in island arc and lesscommonly oceanic plateau settings; (2)mid-Cretaceous and younger clasticseroded from the Coast Belt; (3) Paleozoic toTertiary granitic intrusions; (4) LateJurassic to Holocene clastic-rich accre-tionary complexes, deformed mainly inLate Cretaceous to Holocene time

Figure 1. The Canadian Cordillera and adjoining parts of southeastern Alaska showing:A Location of the five morphogeological belts (details in Table 1); red lines show approximate positions of thenorthern (N) and southern (S) Canadian Cordilleran Lithoprobe transects, details of which are in Figure 2.B Simplified metamorphic map of the Canadian Cordillera, showing the close correspondence between thedistribution of higher grade metamorphic rock facies and granitic rocks and Omineca and Coast belts. The maplegend below is a pressure-temperature diagram whose colours correspond with those on the map; metamorphicfacies are: Sg subgreenschist; G greenschist; A amphibolite, and B blueschist (blue dots on map); box labelledGr denotes granitic rock.

THE CANADIAN CORDILLERA: Geology and Tectonic EvolutionContinued from Page 17

THE SIMPLE FRAMEWORK: THE FIVEMORPHOGEOLOGICAL BELTS

The Canadian Cordillera is divided into five morphogeologicalbelts, that from east to west are called Foreland, Omineca,Intermontane, Coast, and Insular belts. Each belt is characterizedby a distinctive combination of land forms, rock types, metamor-phic grade and structural style (Gabrielse et al., 1991). The distri-bution of the belts is shown in Figure 1A, and their distinguishingattributes and geographic relationships to major mountain ranges,plateaus, and to the adjoining continental shelf and slope, aresummarized in Table 1. Each belt reflects the sum of all processesthat have shaped the Cordilleran region since the lateNeoproterozoic (750 Ma), but each is dominated by structuralfeatures that formed during Middle Jurassic through early Tertiarymountain building, involving compressional deformation between~185-60 Ma followed by extensional deformation between ~59-40Ma. The match between physiography and bedrock geology isclose, but not perfect, mainly because of late Tertiary andQuaternary (~≤20 Ma) differential vertical movements.

The relationship between the five belts and geology is mostclearly seen by reference to a metamorphic map (Figure 1B). Themap is based on the distribution of minerals that form under

experimentally determined conditions of temperature and pres-sure; these minerals are used to define the metamorphic facies shownon the map. Higher grade metamorphic rocks, of amphibolite andgreenschist facies, and associated granitic rocks, are common in theOmineca and Coast belts, and separate mainly sub-greenschistfacies rocks in the other three belts. An implication of this is thatmany rocks now at the surface in Omineca and Coast belts wereonce buried to depths of 25 km or more. Their presence at thesurface reflects the far greater crustal thickening, uplift, erosion,and tectonic exhumation due to crustal stretching, of the rocks inthose two belts than in the flanking Foreland, Intermontane, andInsular belts, where most exposed rocks were never buried todepths of more than ~10 km.

GEOSYNCLINES, PLATE TECTONICS, ANDACTUALISTIC INTERPRETATIONS

In the mid-19th century, geologists recognized that stratifiedrock sequences in the Appalachians were far thicker thansequences of the same age on the stable continent to the west. Thisled to the concept that mountain belts formed on the sites of enor-mous linear crustal depressions called geosynclines. By the late 19thcentury in western Canada, it was known that there were thickPaleozoic through mid-Mesozoic (~540-160 Ma) sedimentary

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Figure 2. Comparison of interpreted simplified lithospheric structures along the northern (N) and southern (S) Cordilleran Lithoprobe transects, whose locations are shownby the red lines in Figure 1. In both profiles, the heavy green line is the crust-mantle boundary (Moho). AW accreted wedge; AX Alexander terrane; BB Bowser Basin; CACassiar terrane; CC Cache Creek terrane; CD Cadwallader terrane; FS, Fort Simpson (a Precambrian terrane in the craton); KO Kootenay terrane; MS1 undividedPrecambrian (1200-800 Ma) metasedimentary rocks; MS2 undivided Precambrian (1800-1200 Ma) metasedimentary rocks; MT-SH undivided Methow and Shuksanterranes; QN Quesnel terrane; SM Slide Mountain terrane; ST Stikine terrane; WR Wrangellia. Most terrane descriptions are in Table 2. Figure modified by P.T.S. Hammerfrom Clowes and Hammer, 2000.

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sequences in the Foreland Belt. By contrast, the remainder of the Cordillera contained abun-dant marine volcanic and sedimentary rocks of similar ages (and their metamorphic equiva-lents), as well as widespread granitic intrusions. These differences eventually led to thedivision of the Cordillera into a narrow eastern miogeosyncline and a broad western eugeosyn-cline (Kay, 1951), and these terms are sometimes still used as “geological shorthand”.However, one problem, vexing in the light of the old geological adage that “the present is thekey to the past”, was that no present-day geosyncline had been positively identifiedanywhere.

In the late 1960s, with the advent of the plate tectonic theory, it became evident that moun-tain belts form where lithospheric plates converge and collide with one another. It also becameevident that assemblages of different “geosynclinal” rock types could be correlated withspecific plate tectonic settings. When plate tectonic concepts first were applied to Cordillerangeology in about 1970, present-day, actualistic, plate tectonic analogues of components of thegeosyncline were identified. At least two of the tectonic settings recognized in today’s worldwere identified in what formerly had been called the eugeosyncline. Basalt, radiolarian chert,pelite and alpine-type ultramafic rocks were recognized as former ocean floor rocks. Volcanicrocks ranging from basalt to rhyolite that were interbedded with marine sedimentary strataand associated with granitic rocks of the same age, were interpreted as remnants of formerisland arcs. The miogeosynclinal (or miogeoclinal in more current usage), shallow water,carbonate-dominated, sedimentary successions in the Foreland Belt, were identified asancient continental shelf deposits. They are laterally equivalent to the far thinner and strati-graphically incomplete successions deposited to the east on the stable continental basement(or craton) below the plains.

In general, the early interpretations of plate tectonic settings, which were based largely onrock associations with minimal input from geochemistry, have survived later geochemicalscrutiny and have been strengthened further by isotope geochemistry using Sri and εNd values(Armstrong, 1988; Samson and Patchett, 1991; Patchett and Gehrels, 1998). The magmaticrocks in the Omineca Belt and some younger rocks in the Intermontane Belt show evidence ofvarying amounts of contamination by old continental crust (Sri ≥0.705 and –ve εNd). Bycontrast, the older magmatic rocks in the Intermontane Belt and most magmatic rocks inCoast and Insular belts are isotopically juvenile (Sri ≤0.704 and +ve εNd). This means they arederived largely from mantle sources, which supports the earlier interpretations that they areremnants of former oceanic island arcs and ocean floor rocks. Although the westward extentin the subsurface of the old continental basement and of sedimentary rocks eroded from itremains a topic for debate, isotopic data combined with current interpretations of the verticaldistribution of components of the crust (e.g. Figure 2) suggest that much of the crust of thewestern Cordillera is derived from mantle sources. From this, it appears that the CanadianCordillera provides an excellent example of the formation and growth of new continentalcrust.

ADDITIONAL IMPLICATIONS OF PLATE TECTONICS

The above interpretations, which are based on observations of rock associations formed indifferent modern plate tectonic settings, are relatively straightforward. However, inherent inplate tectonic theory are other implications that make interpretations of Cordilleran geologyand its tectonic evolution far less certain.

(1) Enormous lateral displacements concealed in the western Cordillera?

Very large horizontal displacements of pieces of lithosphere are possible in the worldgoverned by plate tectonics. An excellent demonstration of this is provided by the enormousamount of oceanic lithosphere that during the last ~150 million years has disappeared into themantle in subduction zones located along the western margin of the North American Plate.

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TABLE 2: SUMMARYDESCRIPTIONS OF CANADIANCORDILLERAN TERRANESAbbreviations are those on Figure 3; accreted terranesdenoted by (A), pericratonic terranes by (P)

AX = ALEXANDER TERRANE (A): LatestProterozoic to Triassic mafic to felsic volcanic rocks,terrigenous clastic and carbonate rocks, and earlyPaleozoic granitic rock; pre-Devonian rocks arc-related; Paleozoic paleomagnetic data discordantwith those of North America.

BR = BRIDGE RIVER TERRANE (A): Disrupted,variably metamorphosed, Missippian to LateJurassic chert, argillite, sandstone, basalt, ultra-mafics, and minor carbonates; contains Triassicblueschist; early to late Meszoic accretionarycomplex

CA = CASSIAR TERRANE (P): Neoproterozoic toDevonian platformal carbonate rocks, sandstone,and graptolitic shale; a sliced-off fragment of theancient continental margin deposits transportednorthwards ≥500 km on Tintina, Northern RockyMountain Trench strike-slip faults.

CC = CACHE CREEK TERRANE (A): DisruptedMississppian to Early Jurassic chert, argillite, basalt,and carbonate and ultramafic rocks; local Triassicblueschist. Late Permian shallow water faunasresemble those of western Panthalassa; earlyMesozoic accretionary complex accompanyingQuesnellian and Stikinian arcs.

CD = CADWALLADER TERRANE (A): Permianbasalt, gabbro and ultramafic rock, overlain byTriassic arc-related basalt, carbonate and clastics,overlain by Upper Triassic to mid-Cretaceous clasticrocks. The overlying Late Cretaceous arc volcanicscarry paleomagnetic signatures indicating ~35°northward displacement w.r. to continental interior.

CG = CHUGACH TERRANE (A): inboard:metachert, basalt and local carbonate with EarlyJurassic blueschist; outboard: Cretaceous greywackeand argillite; melange with a matrix of UpperJurassic to Lower Cretaceous cherty argillite andblocks of mafic volcanics, chert, limestone, andultramafics; a long-lived Mesozoic accretionarycomplex.

CK = CHILLIWACK TERRANE (A): Devonian toPermian clastics, carbonate, and arc-relatedvolcanics overlain by early Mesozoic volcanogenic clastics, Jurassic arc-related arc-related volcanics and Jurassic to mid-Cretaceous clastics; Permian faunas resemblethose of Quesnellia and Stikinia and in southwestern U.S.

KO = KOOTENAY TERRANE (P): Variably metamorphosed, Neoproterozoic andPaleozoic strata comprising continent-derived clastic rocks, rift-related volcanicrocks, and Devonian arc-related volcanic and granitic rocks; these rocks probablywere deformed prior to deposition of late Paleozoic pelite, conglomerate, sand-stone, limestone, and basic volcanic rocks; overlapped and/or overthrust by SlideMountain and Quesnellian strata.

MT = METHOW TERRANE (A): Permian basalt, gabbro and ultramafics repre-senting oceanic lithosphere; Lower Jurassic to Cretaceous marine clastic sedimen-tary strata containing local Middle Jurassic arc volcanic rocks; may = Cadwalladerterrane, but lacks the Triassic arc rocks.

PR = PACIFIC RIM TERRANE (A): Disrupted, mainly Upper Jurassic and LowerCretaceous greywacke, argillite, chert, and basalt.

OL = OLYMPIC TERRANE (A): early Tertiary basalt, Tertiary greywacke, shale,melange and broken formation. QN = QUESNEL TERRANE (or QUESNELLIA)(A): Upper Devonian to Permian clastics, arc related volcanics and carbonate;Ordovician(?) to Permian ultramafics, basalt, chert, pelite and minor carbonateocean floor rocks (=?Slide Mountain terrane); overlapping early Mesozoic arc-related volcanic and intrusive rocks, argillite, sandstone, and local carbonate;Permian, Triassic and Early Jurassic faunas differ from coeval faunas at same lati-tude on craton; comparable to those in Stikine and Chilliwack terranes and W USAand NW Mexico; overlying mid-Cretaceous continental arc volcanics on south-

western edge displaced ~10°? northward with respect to craton

SM = SLIDE MOUNTAIN TERRANE (A): Upper Paleozoic mafic volcanics,ultramafic and local mafic intrusions, chert, clastics and local carbonate;Pennsylvanian and Permian paleomagetic record suggests northward displace-ments of 20° w.r.to craton; overlapped by Upper Triassic fine clastics.

ST = STIKINIA TERRANE (or STIKINIA) (A): Devonian to Jurassic mafic tofelsic, arc-related volcanic rocks and clastic, and upper Paleozoic carbonate;Permian, Triassic and Jurassic faunas differ from those at same present latitude oncraton, but are similar to those in Chilliwack and Quesnel terranes and in thewestern United States; however paleomagnetic studies on Permian, Late Triassicand Early Jurassic rocks suggest little displacement w.r. to craton.

WR = WRANGEL TERRANE (or WRANGELLIA) (A): Middle Paleozoic toJurassic mafic to felsic volcanic rocks and comagmatic intrusions, limestone, pelite,with conspicuous Middle(?) and Upper Triassic plume related(?) tholeiitic basaltoverlain by carbonate. Paleomagnetic data on Triassic strata on Vancouver Islandindicate little or no displacment, whereas those from southern Alaska about20°northward displacement; this is interpreted as late separation betweennorthern and southern regions.

YA = YAKUTAT TERRANE (A and P): Upper Mesozoic pelite, greywacke, andmelange, and Cenozoic marine and continental clastic rocks.

YT = YUKON-TANANA TERRANE (A and P): Heterogenous metamorphicterrane comprising sedimentary and magmatic rocks of Late Proterozoic,Paleozoic, and Mesozoic protolith ages. Similar to Kootenay Terrane in manyaspects but may include rocks related to Slide Mountain Terrane.


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Figure 3. Terrane map of the Canadian Cordillera and adjacent parts of Alaska; most rocks shown are of Paleozoic andearly Mesozoic ages. Map shows locations of: (1) rocks (NAM) that were deposited on the ancient continental margin;Mo is part of the craton exposed in a structural window; (2) proximal (CA) and distal (KO, YT) pericratonic terranesthat formed along the margin but in uncertain paleogeographic relationship to it; (3) accreted terranes of (3a) (mainly)island arc affinity; “inner terranes" (QN, ST) accreted in the Jurassic; "outer terranes" (AX, WR) accreted in theCretaceous; (3b) accretionary complexes; "chert-rich" (BR (part), CC, SM) are pre-Middle Jurassic; "clastic-rich" (BR(part), CG, PR) include Late Jurassic to Recent rocks. The terranes are named and their nature summarized in Table2. White areas, mainly in the Coast Belt, feature voluminous Middle Jurassic and younger granitic rocks; JFR is theJuan de Fuca Ridge (modified from Monger and Nokleberg, 1996 and Nokleberg et al., 2000).

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A basic tenet of plate tectonics is that ocean-floor spreading is symmetrical about the mid-oceanic ridges. However, the present distribution of the ages of the Pacific Ocean floor ismarkedly asymmetric. The main spreading centre (East Pacific Rise) is in the southeasternPacific Ocean, and its northern extensions (Juan de Fuca and Gorda ridges) either lie close tothe North American continent or else have been replaced by transform plate boundaries(Queen Charlotte and San Andreas faults). At latitude 40°N, the Pacific Ocean is about 7000km wide and floored entirely by the enormous Pacific Plate, which increases in age west-ward from Oligocene (~25 Ma) near northern California to Jurassic (~180 Ma) southeast ofJapan. In the northeastern segment of the Pacific Ocean margin, almost all of the “mirrorimage” of the vast Pacific Plate has disappeared by subduction beneath western NorthAmerica.

Interpretations of past global plate motions are based on magnetic anomaly patterns inoceanic lithosphere and on hot spot tracks (which are linear belts of intra-plate volcanics thatrecord the passage of a plate over a zone of partial melting within the deeper mantle).Reconstruction of the movements of the North Pacific Ocean floor suggests that during thelast 150 million years a vast area of ocean floor ~13 000 km wide – a distance equal to one-third Earth’s circumference – has disappeared beneath the western margin of the NorthAmerican plate (Engebretson et al., 1992). Seismic tomography indicates that the subductedoceanic lithosphere descends through the entire mantle to accumulate near the core-mantleboundary at depths of ~2900 km below the western North Atlantic (Grand et al., 1997).

In western Canada and adjoining parts of Alaska, two kinds of geology record this enor-mous amount of plate convergence. One is the relatively minor amount of ocean floor mate-rial that has been scraped off the various subducting oceanic plates in the last 150 millionyears and transferred to the overriding North American plate. It forms accretionary (or subduc-tion) complexes, which mainly underlie the continental shelf and slope southwest ofVancouver Island and off southern Alaska, and are exposed on-land near the coast in thoseareas (Figure 2; CG, OL, PR, YA of Figures 3, 4; Table 2). The other record is the accompa-nying arc magmatism that has penetrated the overriding plate. The arc is represented by theabundant late Mesozoic and younger granitic and volcanic rocks in and to the east of theCoast Belt (Figure 4).

Reconstruction of plate kinematics using ocean floor magnetic anomalies and hot spottracks can directly establish amounts of plate convergence at best only as far back as Jurassictime, because no ocean floor older than ~180 million years is known to exist. Older recordsof plate convergence are preserved only by the ancient accretionary complexes embeddedwithin mountain belts and by arc magmatic rocks. In the Canadian Cordillera, the olderaccretionary complexes (BR, CC and? SM of Figure 3; Table 2) and the accompanying earlyMesozoic (~230-185 Ma) arc rocks (in QN, ST of Figure 3; Table 2) occur mainly in theIntermontane Belt. Late Paleozoic (~350-250 Ma) arc rocks are widespread in Intermontaneand Coast belts (in QN, ST of Figure 3; Table 2), and are as old as Middle Devonian (~390Ma) in the Omineca Belt (in KO, YT of Figure 3; Table 2). Still older arc rocks, of earlyPaleozoic and locally latest Proterozoic age (~600-400 Ma), are found in the Insular Belt (inAX of Figure 3; Table 2) although the paleogeographic relationship of the latter rocks to theNorth American continental and/or plate margin is unknown. Thus, a cryptic record ofpotentially enormous amounts of plate convergence appears to be present within theCordillera. Accordingly, the reconstruction of successive relative paleogeographic positionsof components of the western Cordillera presents major challenges; it cannot be assumed,but must be proven, that two dissimilar but contiguous rock units of the same age alwayswere side by side.

Amounts of lateral displacement have been determined traditionally by identifying offsetcounterparts of linear structures or distinctive rock units across thrust faults in thrust and

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fold belts such as the Foreland Belt, or across strike-slip faults suchas the Tintina and Denali faults in Yukon and Alaska (DE; TI ofFigure 4). Total displacements on such structures are a fewhundreds of kilometres at most. Plate-scale lateral displacementsmay amount to many thousands of kilometres, and in the on-landgeological record these must be inferred from paleomagnetic andpaleobiogeographic studies, which unfortunately provide contra-dictory stories in some cases.

Paleolatitudes may be determined by measuring the anglebetween the magnetic inclination that was frozen into, for example,a volcanic rock as it cooled, and an assumed paleohorizontal surface(e.g. bedding, flow layering). At low latitudes the angle is small, andat high latitudes the angle is large, thus giving a quantitativemeasure of the latitude at which the rock cooled. Paleolatitudes ofrocks in the western Cordillera are compared with those of rocks of

the same age on the old North American continent in order to estab-lish the relative amounts of offset between the two (e.g. Irving andWynne, 1991; Harris et al., 2000). Paleomagnetic studies suggest thatthere was mostly south-to-north displacement, in amounts rangingup to about 4000 kilometres, of some rocks in the western Cordillerarelative to those in the interior of the continent. Paleomagneticstudies are likely to give little or no indication of the amounts ofdisplacement across lines of longitude. This presents a particularproblem in the Cordillera, as most global reconstructions of the posi-tions of continents (e.g. Scotese, 2001) suggest that the westernmargin of North America has remained orientated approximatelynorth-south over the last 300 million years.

Paleobiogeographic studies use the inferred time-space distri-bution of ancient faunas and floras which, by analogy with themodern world, were controlled by variations in paleoclimate and


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Figure 4. Locations of featuresof the Canadian Cordillera andadjacent parts of Alaska thatformed during the period ofterrane accretion and moun-tain building, mainly fromMiddle Jurassic through earlyTertiary time (~180-40 Ma).(1) Middle Jurassic to EarlyCretaceous continental arcsthat are emplaced across CC,QN, KO; (2) Middle Jurassicthrough Early CretaceousGravina-Gambier (Gg) islandarc, emplaced across WR, AX;(3) Mid-Cretaceous throughearly Tertiary continental arcsemplaced across all terranes,with exception of the accompa-nying accretionary complexes;filled circles denote plutons toosmall to show on the map; (4)clastic sedimentary basins (BoBowser Basin; Fb ForelandBasin, Gb Georgia Basin; notshown are basins on the conti-nental shelf filled with mate-rial eroded from adjoining,uplifted fold and thrust belts.(5) Major faults include (5a)active subduction zones (ALAleutian; CS Cascade); (5b)active transform fault (QSQueen Charlotte); (5c) majorthrust fault systems of (a)Jurassic age (KS King Salmon;WA Waneta) and (b) mid-Cretaceous and early Tertiaryages (FO Foreland; PAPasayten); (5d) major strike-slip faults of mainly LateCretaceous and Tertiary ages(DE Denali; NR NorthernRocky Mountain Trench; TETeslin; TI Tintina); (5e) earlyTertiary normal faults.


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also the distribution of continents. Paleobiogeographic studiespotentially can address questions of amounts of translation acrosslines of longitude as well as latitude. About 70 years ago, the distri-bution of certain terrestrial dinosaurs found in South America andAfrica was cited as evidence supporting the then controversial, butnow accepted, idea that those continents were together in theMesozoic. One problem with paleobiogeographic studies is thatthey are difficult to quantify, although attempts to do this havebeen made (e.g. Smith and Tipper, 1986; Belasky and Runnegar,1994). In the Intermontane, Coast and Insular belts of theCanadian Cordillera, marine fossils of Permian, Triassic and EarlyJurassic ages (~300-185 Ma) mostly appear to be displaced north-wards relative to those that lived on the ancient continental margin(respectively, Monger and Ross, 1971; Tozer 1982; Smith andTipper, 1986). In addition, some Permian to Middle Triassic (~280-230 Ma) fossils in the Cache Creek accretionary complex (CC ofFigure 3) are similar to fossils found today in Japan, China, south-eastern and central Asia, and the Mediterranean region. They aredifferent from comparable fossils of the same age range found else-where in North America. Thus, they appear to be truly exotic,which can be explained if they were rafted into the NorthAmerican plate margin on a subducting oceanic plate, the scraped-off remnants of which form the Cache Creek accretionary complex.

(2) Orogenic collages: their analysis and synthesis – tectonicassemblages and terranes

In 1974, Helwig proposed the term orogenic collage for thegeological complexity that is the aftermath of superimposedsystems of subduction, sea-floor spreading, and transformfaulting. An outstanding example of an orogenic collage that isforming today is the seemingly chaotic geology of Indonesia whereseveral oceanic arc systems are being swept together in thecomplex zone of convergence between Australia, Asia and thePacific basin (Hamilton, 1979). The Canadian Cordillera is a goodexample of an older orogenic collage. Paleozoic and earlyMesozoic arc and accretionary complex systems are overprinted byyounger arcs that are, in turn, further disrupted by large, lateMesozoic-Tertiary strike-slip faults and related crustal extension,so that older geological relationships have generally been obscuredor destroyed by younger events (c.f. Figures 3, 4).

Two methods have evolved to aid analysis of the CanadianCordilleran collage. The first, noted earlier, involves groupingdifferent rock types associated in space-time as actualisticanalogues of the probable tectonic settings in which they formed,such as island and continental arcs, accretionary complexes, andcontinental shelves and slopes. These groupings, called tectonicassemblages, are major units on newer geological maps of the entireCanadian Cordillera (Wheeler and McFeely, 1991). The secondmethod involves identification of regions within the Cordillerancollage, called terranes, that are distinguished by having geologicalrecords distinct from those of other parts of the collage, and alsofrom strata deposited on and near the ancient continental margin(Figure 3; Table 2). Where not obscured by younger intrusions or

cover, terrane boundaries appear to be faults. In addition, someterranes contain paleomagnetic and/or paleontological recordsthat are different from those of other terranes and/or from rocks ofsimilar age that now are at the same latitude on the ancient conti-nental margin. From this it is inferred that adjacent terranes shouldbe interpreted not as merely stratigraphic facies of one another, butinstead they should be suspected of having been widely separatedduring their formation and far removed from their present posi-tions along the continental margin. For this reason, they werecalled suspect terranes by Coney et al. (1980). Other qualifiers thatmerge the two methods include accreted for terranes composedmainly of isotopically juvenile material and added to the old conti-nental margin, and pericratonic for terranes that formed around themargin of an old stable continent but in uncertain paleogeographicrelationship to it. Qualifiers that emphasize the dominant tectonicsetting in which the rocks of a terrane formed, such as arc or accre-tionary complex terrane, are also used.

Terranes are the “building blocks” of the Cordilleran orogeniccollage and as such are major tectonic entities shown on interpreta-tions of the vertical distribution of rock types in the crust of thewestern Cordillera (Figure 2). The timing of the assembly of terranesto form the present crust may be deciphered by the following simplegeological relationships: (1) stratigraphic units that overlap two ormore terranes; (2) magmatic belts common to two or more terranes;(3) dated bounding faults which juxtapose one terrane with another;and (4) clastic rocks identified as having been eroded from oneterrane and deposited upon another (Figure 4).

(3) Seeing the big picture: far distant causes of Cordillerantectonic changes?

In a world whose outer solid layer consists of lithosphericplates, changes of plate interactions or kinematics in one place onthe surface may have far reaching effects. For example, theHimalayas are located on the site of a convergent plate boundarythat coincides with the northern boundary of the Indian sub-conti-nent. The oceanic lithosphere that formerly separated India fromcentral Asia disappeared into the mantle over 50 million years ago.The subsequent prolonged collision of the Indian continent withcentral Asia causes earthquakes, folding and faulting over a vastregion that extends northwards for about 3000 km through centralAsia as far as southern Siberia, and east-west for over 4000 km,from China through the Middle East.

Closer to home, it has long been suspected that Cordilleranmountain building is related to the opening of the North AtlanticOcean basin following the break-up of the late Paleozoic-earlyMesozoic (~300-200 Ma) supercontinent called Pangea (Daly, 1926;Coney, 1972). The opening of the North Atlantic Ocean basin beganwith a period of rapid sea-floor spreading between northwestAfrica and eastern North America about 170 million years ago,which followed several tens of millions of years of rifting (but notdrifting). On the western side of North America, the time of initia-tion of this spreading event coincided with the initiation of the


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transition from a convergent plate margin featuring island arcs andback arc basins, to one featuring continental arcs. The search fordistant causes of change in Cordilleran tectonic activity may befraught with uncertainty, but cannot be excluded in a worldgoverned by plate tectonics.

THE TECTONIC EVOLUTION OF THE CANADIANCORDILLERA

In the following section, we expand upon the summary of thetectonic evolution given in the Introduction and attempt to explainsome of the events that have taken place in the region over the last~750 million years that resulted in the mountains of westernCanada.

(1) The global paleogeographic background

The oldest known ocean floors are about 180 million years old,whereas the age of the Earth is about 4,500 million years, so that lessthan four percent of the geological record of Earth’s tectonic evolu-tion is preserved within the ocean basins. Global paleogeographicreconstructions for periods older than 180 million years are basedentirely on evidence from the continents. Moreover, these recon-structions must attempt to match rock units and the remains offormer mountain belts that now are fragmented and dispersedamong different continents, and to identify rock associations that arethe vestiges of former ocean basins now incorporated within conti-nents. The paleolatitudes of continents are derived from paleomag-netic studies and from paleoclimatic indicators such as ancientglacial, desert, or coal deposits and the distribution of different

types of fossils. There is general agreement on the relative positionsof continents back into late Paleozoic time (~300 Ma), although thereare some minor controversies. Reconstructions of the locations of thecontinents for the early Paleozoic are more controversial, and thosefor much of the Precambrian (≥545 Ma) are mainly speculative.

About 1,000 Ma, all of the continents evidently amalgamatedinto the supercontinent called Rodinia (Figure 5A; see Scotese, 2001for global paleogeographies). Rifting, fragmentation and dispersalof components of Rodinia were underway by about 750 Ma. As thecontinental fragments began to disperse, an ocean basin developedon the margin of the large fragment called Laurentia, whichincluded present North America, Greenland and(?) the north-eastern-most part of Russia. This Laurentian margin eventuallybecame the site of the present Cordillera. The adjacent ocean basinwas the distant ancestor of the present Pacific Ocean basin and hasbeen called Panthalassa. By about 300 Ma, Panthalassa hadexpanded to occupy more than a hemisphere, and all of theformerly dispersed continental fragments had re-amalgamated ina new configuration in the other hemisphere to form superconti-nent called Pangea. About 200 Ma, Pangea started to break-up andto separate into the present continents. Eventually, as the newcontinents dispersed, Panthalassa contracted in size and becamethe Pacific Ocean basin, which was bounded on its northeast side bythe North American continent.

(2) Rift and drift: formation of the ancient continental marginof western North America

Figure 5: Evolution of the CanadianCordillera cartooned on space-timediagrams. Horizontal coordinate:west to east (in present geographiccoordinates); geographic position(after Early Devonian time) is fixedrelative to the trench; vertical coor-dinate: numbers show age inhundreds of millions of years beforepresent. A gives names of conti-nents and ocean at different timesand the names applied to each atthose times. B shows features relatedto the plate tectonic activity atdifferent times.




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The rifting within Rodinia that preceded sea-floor spreading,continental drift, and the creation of the ancient western margin ofthe North American continent is recorded by a distinctive suite oflate Neoproterozoic (≤750 Ma) immature coarse clastic and glacio-genic sedimentary rocks and associated mafic volcanic rocks. Itextends along the Cordillera from northern Alaska to southernCalifornia (Stewart, 1972) and in the southern Canadian Cordillerait comprises the lower part of the Windermere Supergroup. Theeastern limit of the late Neoproterozoic rifting is the main controlon the configuration of the eastern margin of the CanadianCordillera, with its series of broad eastward-convex arcs in thesouthern Canadian Rockies, the northern Canadian Rockies innortheastern British Columbia, and the Mackenzie Mountains. Inaddition, the eastern limit of rifting coincided with the easternedge of the lower Paleozoic continental shelf (or miogeocline),which now is located within the Foreland Belt.

The rifting that marked the onset of Windermere depositionbegan about 750 Ma (Figure 5B). This is about 200 million yearsbefore the onset of deposition of the thick succession of latestNeoproterozoic-earliest Cambrian mature quartz sandstones andoverlying shallow-water carbonates with interbedded shales thataccumulated along the ancient continental shelf in a miogeoclinalsetting. Bond and Kominz (1984) analysed variations in the rates ofsediment accumulation with geologic age in the miogeocline. Usingan algorithm that compensates for the effects of sedimentcompaction and isostatic subsidence due to sediment loads, to esti-mate the rates of the underlying tectonic subsidence beneath themiogeocline. They discovered a distinctive pattern of exponentialdecrease in the rate of “tectonic” subsidence of the Cordillleranmiogeocline during Cambrian to mid-Ordovician time that is indica-tive of thermal contraction due to cooling of underlying hot mantlerocks. By extrapolating the exponential rate of subsidence back intime they showed that the onset of thermally driven subsidence, andtherefore the transition from rifting to seafloor-spreading and conti-nental drift along this margin of Laurentia, must have occurred inearliest Cambrian or latest Neoproterozoic time (~545 Ma).

The rift-drift transition evidently occurred at different timesalong different parts of the margin, as it did along the westernmargin of the North Atlantic Ocean basin between the easternUnited States and Greenland (Sears and Price, 2000). A lateNeoproterozoic carbonate platform and associated slope depositswhich occur in the Mackenzie Mountains of the northernCanadian Cordillera indicate that the continental margin is some-what older there (Dalrymple and Narbonne, 1996) than in thesouth. However, by the earliest Cambrian (~540 Ma) a persistentcontinental shelf-slope boundary was established along the site ofwhat eventually became the eastern Canadian Cordillera (Figure5B). Today, the stratigraphic record of that ancient boundary, andof the lateral changes of rock units across it, is preserved in theForeland and Omineca belts of the Canadian Cordillera. Thepassive margin deposits, dominated by thick deposits ofcarbonate, can be traced eastwards into the far thinner and incom-plete successions that overlie the stable continental basement of the

Western Canada Sedimentary Basin in the western Great Plains.Westwards, the carbonate passes laterally into calcareous shale (theabrupt transition can be observed above Highway 1, near Field,British Columbia). West of the town of Golden, which is located inthe southern Rocky Mountain Trench, age-equivalent rocks aredark shale, minor carbonate, and local associated mafic volcanicrocks, all of which presumably were deposited in deep water onthe ancient continental slope. The east-to-west facies changes canbe correlated with changes of thickness of the underlying conti-nental basement that presumably resulted from the normalfaulting, horizontal extension, and crustal thinning that occurredduring rifting and break-up of the Rodinian supercontinent.

(3) Plate convergence: subduction, arc magmatism, andeventually mountain building

The passive, intra-plate continental margin persisted for at least150 million years, until Middle Devonian time (~390 Ma) when itwas succeeded by an inter-plate, mainly convergent plate marginthat apparently has persisted more-or-less continuously until thepresent (Figure 5B). The inter-plate margin involved a succession ofmagmatic arcs, represented by volcanic rocks and accompanyingplutons, and also by accretionary complexes associated with the arcs(Figures 3, 4; Armstrong, 1988; Monger and Nokleberg, 1996).

Today, the western margin of the North American Plate featuresconvergent and transform plate boundaries. Between latitudes~25°N and 40°N, the boundary is dominated by transform faults(e.g. the San Andreas Fault). Between latitudes ~40° and 50°N, theboundary is convergent (the Cascadia subduction zone), betweenlatitudes ~50°- 60°N it is a transform margin (the Queen CharlotteFault), and north of latitude 60°N it is convergent (the Aleutiansubduction zone; respectively CS, QC, AL of Figure 4). It is difficultto know how often convergent-transform boundaries of this typeexisted in the past. Ancient transform faults may leave little tracein the rock record; their presence may be indicated by the absenceof contemporary arc magmatism, but this also could be caused bythe temporary cessation of subduction.

3.1 Initial arc magmatism on the North American platemargin: In the Middle and Late Devonian (~390-360 Ma), graniticand volcanic arc-related rocks were emplaced in and on older rift-related and continental margin deposits and the pericratonicterranes. The arc rocks occur in a belt that extends discontinuouslyfrom central California to Arctic Alaska, and in the CanadianCordillera they are located mainly in the Omineca Belt (in KO, YTof Figure 3). Because the Devonian arc rocks occur in and on rocksthat formed on or near the ancient continental margin, and becauseno contemporary subduction complex is found continentward ofthem, they evidently reflect subduction of oceanic lithospherebeneath the edge of the North American plate.

Today, nowhere on Earth is there firm evidence of subductionbeing initiated, and the cause of the change from a passive, intra-plate margin to a convergent, inter-plate, margin is uncertain.


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Following continental rifting and drift, the newly formed oceaniclithosphere cools, densifies and loses buoyancy. Modelling (Faccennaet al. 1999) suggests that long-term, local and far-field compressionalstresses become focused on the boundary, already weakened by rift-related faults, between buoyant, high-standing continental litho-sphere and denser, low-standing, oceanic lithosphere. The stressesmay be enhanced by erosion of material from the high-standingcontinent and its deposition on the adjoining ocean floor. Eventually,a trench becomes nucleated at the continent-ocean boundary, and theoldest, densest oceanic lithosphere is subducted.

In the Cordilleran region, the oldest oceanic lithosphere wasthat which formed immediately after latest Neoproterozoic-earliestCambrian break-up and dispersal of Rodinia, and which layclosest to the continental margin. It is tempting to link the time ofchange from an intra-plate to an inter-plate boundary to the majorsub-Middle Devonian unconformity which extends from theForeland Belt through the plains region, and to evidence for intra-Middle Devonian folding (Root, 2001). In addition, the time ofchange corresponds, perhaps not fortuitously, with the end of theAcadian Orogeny in the Appalachian Mountains on the other sideof the continent.

3.2 Arc-facing directions: The question of whether oceanic lith-osphere was subducted beneath the North American plate, oraway from it, can be resolved by considering the facing directionsof magmatic arcs relative to the North American continentalmargin. Arc-facing directions may be determined from the lateralvariations in rock type and geochemistry of the magmatic rockswithin the arc, and also from the external lateral relationships ofthe arc to coeval accretionary complexes on one side and back arcbasins and/or the continental margin on the other. Mid-Mesozoicand younger arcs evidently faced away from the North Americancontinent because accretionary complexes of the same ages lieoceanward of the arcs. The facing directions of Triassic and Jurassicarc rocks of Quesnel terrane (Figure 3; Mortimer, 1987) and theMiddle Jurassic arc of Stikinia (Anderson, 1993) were away fromthe continent, but the facing directions of early Mesozoic and olderarcs of Stikine, Wrangellia and Alexander terranes are unknown.The facing direction of the late Paleozoic arc of Quesnel terrane insouthern British Columbia apparently was away from the craton(Roback et al., 1994). Contrarily, that of a Permian arc in Yukon-Tanana terrane, north of latitude 60°N, where there are hints of latePaleozoic subduction involving Slide Mountain terrane (SM of

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Figure 3), is thought to be towards the craton (Mortensen, 1992).The occurrence of Middle and Late Devonian arc rocks within peri-cratonic and craton margin deposits with no contemporary accre-tion complex to the east suggests that at that time the dip of thesubduction zone was beneath rather than away from the continent.

3.3 Different types of arcs: Late Paleozoic through Triassic arcmagmatism (~350-180 Ma) clearly differed from the preceding andsucceeding arc magmatism in its spatial relationship to the ancientcontinental margin (Figure 5B). The differences between theancient arc types in the Cordillera may be explained by observa-tions on modern arcs (Hyndman, 1972; Uyeda and Kanamori,1979; Jarrard, 1986). Most island arcs tend to be in a state of exten-sion and the accompanying subduction zone dips steeply into themantle. Conversely, most continental arcs tend to be undercompression and the accompanying subduction zone has a shal-lower dip. Factors evidently contributing to the different arc typesinclude: (1) variable rates of plate convergence; (2) the age (andthus density) of the subducting oceanic plate; and (3) the relativedirections of motion of the upper plate either toward or away fromthe oceanic trenches, which are the surface traces of subductionzones that extend into the mantle. When the rate of advance of theupper plate relative to the trench is less than the rate of retreat ofthe lower plate due to trench roll back as the oceanic plate sinksinto the mantle, island arcs and back arc basins form, like those inthe western Pacific Ocean. When the rate of advance of the upperplate is greater than the rate of retreat of the lower plate due totrench roll back, the upper plate collides with the oceanic litho-sphere (Russo and Silver, 1996). In this case, compression in the hotand weak arc lithosphere causes thickening of the crust by folding,thrusting and flow, and eventually mountainous regions cappedby continental arcs, such as the Andes, are formed.

Based on the relationship of the arcs (or plate margin) to theancient continental margin, it is possible to divide arc evolution inthe Canadian Cordillera into three stages (Figure 5B).

First, as noted above, Middle and Late Devonian (390-355 Ma) arcrocks were emplaced in and on continental margin deposits andpericratonic terranes from California to Arctic Alaska.

Second, the Devonian arc evolved into a late Paleozoic-earlyMesozoic (~355-185 Ma) island arc and back-arc system. The arcs(represented by rocks in QN and probably ST of Figure 3) werelocated on the margin of the North American plate, outboard of abasin of unknown width largely floored in the late Paleozoic byoceanic rocks (SM of Figure 3) and in the Triassic by fine-grainedclastic sedimentary rocks. The basin probably formed by back-arcrifting and spreading during subduction and arc extension,although there is evidence for compressional deformation (andsubduction?) in the basins in Devono-Mississippian (~355 Ma) andPermo-Triassic (~250 Ma) times. The setting of the late Paleozoic-early Mesozoic arcs probably was similar to that of the present-dayJapanese and Philippine islands. Viewed in a global context, the

change from the setting of the Devonian continental margin arc tothe late Paleozoic offshore arc occurred during amalgamation ofthe Pangean supercontinent, possibly because the continental partof the North American plate retreated from the trench on itswestern boundary.

Third, the Middle Jurassic and Early Cretaceous (~170-0 Ma) arcsin the Intermontane and Omineca belts were continental whereasthose in the northern Insular Belt and southwestern Coast Beltbelong to the Gravina-Gambier island arc (Gg of Figure 4). LateCretaceous and younger arc rocks are entirely continental. Thecontinental arc rocks mostly were emplaced across older island arcrocks previously accreted to the ancient continental margin and thepericratonic terranes. Locally they were deposited on the conti-nental margin; tuff as old as Early Jurassic (~185 Ma) isinterbedded with sedimentary strata in the Foreland Belt (Hall etal., 1998). The time of transition from island arc magmatism tocontinental arc magmatism apparently coincides with the break-up of Pangea and advance of the North American Plate towardsthe trench on its western boundary (Figure 5B).

3.4 Collage complexities caused by oblique subduction? Thedirection of subduction probably was mostly oblique, rather thanorthogonal, to the orientation of the long-lived convergent plateboundary. Oblique subduction results in the formation of arc-parallelstrike-slip faults in hot, weak arc lithosphere (Fitch, 1972; Jarrard,1986; Oldow et al., 1990). Such faults greatly increase thecomplexity of the Cordilleran collage.

Several major Late Cretaceous and Tertiary right-hand strike-slip faults lie well within the Canadian Cordillera and adjacentparts of Alaska (e.g. DE, NR, TI, FS on Figure 4). The sense ofmovement on these faults is consistent with the oblique right-handconvergence of offshore oceanic plates relative to the margin of theNorth American plate, as reconstructed by Engebretson et al.(1985). Furthermore, that part of the Cordillera underlain by theaccreted terranes contains Permian through Early Jurassic faunasand paleomagnetic inclinations in rocks ranging in age fromPaleozoic to Late Cretaceous that indicate that many rocks in thewestern Cordillera were displaced latitudinally northward relativeto the North American continent. Local structural evidence for left-hand and right-hand faults that range in age from Devonian(?) toEarly Cretaceous provides hints of earlier orogen-parallel move-ments.

With the possible exceptions of pre-Jurassic arc rocks ofAlexander terrane and Wrangellia, all of the arc terranes in theCanadian Cordillera arguably are pieces of arcs that formed some-where along the North American plate margin (Figure 3; Samsonand Patchett, 1991; Monger and Nokleberg, 1996). After their forma-tion, the arcs may have been cut by intra-arc strike-slip faults and thepieces dispersed or doubled-up along the margin. The MiddleJurassic-Early Cretaceous (~170-100 Ma) Gravina-Gambier islandarc (Gg, which overlaps AX and WR; c.f. Figures 3, 4) once may have


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been the offshore continuation of theJurassic-Cretaceous continental arcpreserved in the southern interior ofBritish Columbia and in the conterminouswestern United States. The arc may havebeen cut acutely by left-hand strike-slipfaults, for which there is field evidence,and the northern part displaced south-wards by ≥800 km in Early Cretaceoustime to juxtapose it with the continental arcin the southern half of British Columbia(Monger et al., 1994). Stratigraphic andfaunal similarities suggest that Stikineterrane (ST of Figure 3) is a fragment of thesame late Paleozoic-early Mesozoic islandarc as Quesnel terrane (QN), with theCache Creek terrane (CC) as the accre-tionary complex accompanying both arcterranes. Stikine terrane underthrustCache Creek terrane in the Middle Jurassic(~170 Ma) on the King Salmon Fault (KS ofFigure 4). How did Stikine terrane come tolie outboard of the Cache Creek Complex?One suggestion is that the arc was foldedoroclinally about a vertical axis to enclosethe Cache Creek accretionary complexbetween Quesnel and Stikine terranes(Mihalynuk et al., 1994). An alternativesuggestion, supported by local fieldevidence for latest Triassic-Early Jurassicleft-hand strike-slip faulting (C.J.R. Hart,pers. comm.,1999) is that Stikine terranewas emplaced by strike-slip faultingoutboard of the Cache Creek Complex(Scotese et al., 2001). It seems that intra-plate arc-parallel (and orogen-parallel)displacements thus may be responsible forpositioning large offshore arc terranes (ST;and AX and WR overlapped by Gg;Figures 3, 4) prior to their final accretionsto the ancient continental margin.

(4) Cordilleran mountain-building

Middle Jurassic to Recent (~170-0 Ma)clastic deposits preserved in basins within,and external to, the Cordillera provide anunequivocal stratigraphic record ofregional uplift above sea-level and accom-panying erosion on the site of the formercontinental margin and its fringingaccreted terranes. That sedimentation inthe basins was related to compression isshown by nearby and contemporaneous

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thrust and fold belts (Figure 4). Initially, Middle Jurassic (~175 Ma)clastic strata of Bowser Basin in northern British Columbia (Bo ofFigure 4) were eroded from Cache Creek strata that had been thrustsouthwestwards (on KS of Figure 4) on to Stikine terrane. By thelatest Jurassic (~150 Ma) detritus eroded from the site of theOmineca Belt was being deposited to the east in the latest Jurassic-Early Cretaceous foreland basin which overlies the ancient conti-nental shelf strata in the Foreland Belt. By the Late Cretaceous (≤80Ma), clastic deposits eroded from the mountains were deposited forhundreds of kilometres to the east on the stable continental platformto form the youngest part of the Western Canada Sedimentary Basin(Fb of Figure 4; Price, 1994). In the western Cordillera, EarlyCretaceous and older marine deposits within the Coast Belt werebeing uplifted and eroded by early Late Cretaceous time (~90 Ma).The products of erosion were deposited mainly to the west in theInsular Belt (e.g. Georgia Basin, Gb of Figure 4). They also spilled outon to the Pacific Ocean floor, from whence they eventually werecarried northward on the oceanic plate to be accreted as majorcomponents of Chugach and younger accretionary terranes insouthern Alaska (CG of Figure 4).

The structural record shows that between the Early Jurassic andlate Paleocene (~185-58 Ma) the arc region was mainly undercompression, accompanied at different times by left-hand andright-hand transpression. This was succeeded by right-handtranstension and extension during the Late Paleocene and Eocene(59 - ?40 Ma). As noted earlier, as the North American plate riftedand separated from Africa and Eurasia during the break up ofPangea, it seemingly advanced towards and overrode the trenchalong its western margin and so collided with the oceanic plate.This process provides a simple explanation for the accretion of thegeochemically juvenile island arc terranes to the ancient westernmargin of the North American continent. The accreted terranesappear to be tectonic flakes, each of which preserves an uppercrustal section ≤10 km thick that apparently was delaminated fromits underlying lower crustal and upper mantle lithosphere byinsertion of the relatively strong wedge of old, cold NorthAmerican continental lithosphere. During the advance of thewedge, the delaminated lower crustal and mantle lithosphere rootsevidently became entrained in the underlying subduction zone(Figure 6). Eventually the westward-tapering wedge of NorthAmerican continental lithosphere overrode the subduction zone.

Both the foreland fold and thrust belt of the Rocky Mountainsand the foreland basin began to form as the orogenic collage to thewest and the deposits of the old continental shelf and slope beganto collide. However, the main event in the formation of the fore-land thrust and fold belt, and in the subsidence of the forelandbasin, occurred during a Late Cretaceous-Paleocene interval ofright-hand transpression. Thick accumulations of Paleozoic andProterozoic supracrustal rocks were scraped off the under-ridingNorth American crystalline basement by the over-riding orogeniccollage of tectonic flakes. They moved up the ramp, which wasformed by the ancient rifted continental margin of North America,on to the flat surface of the old North American continent. The

latter subsided under their weight, trapping detritus eroded fromthe uplifting region to the west in the evolving foreland basin to theeast. Eventually, as the foreland basin fill and the underlying plat-formal cover of the North American craton became incorporatedinto the evolving thrust and fold belt, the Front Ranges andFoothills of the Rocky Mountains were formed.

The ensuing Late Paleocene and Eocene right-hand transten-sion profoundly modifed the Omineca and Intermontane belts.Locally, extension in the Omineca Belt has exposed (or tectonicallyexhumed) metamorphic rocks from depths of more than 25 km, asin the Monashee Complex, west of Revelstoke in southeasternBritish Columbia (Mo of Figure 3), where Paleoproterozoic (~2,000Ma) basement rocks are exposed. These basement rocks are likethose under western Alberta, but they are here exposed in atectonic window through the basal thrust fault zone that separatesthe foreland thrust and fold belt from the underlying rocks.

(5) The Cordillera today

The Cordillera today occupies the leading edge of the NorthAmerican plate. It is bounded southwest of Vancouver Island bythe Cascadia subduction zone (or Cascadia megathrust; CS of Figure4), west of which is the oceanic lithosphere of the small, youngJuan de Fuca Plate. North of this, the plate boundary is the QueenCharlotte transform fault (QC of Figure 4) west of which the enor-mous Pacific Plate is moving northward to descend below Alaskaalong the Aleutian subduction zone (or Aleutian megathrust; AL ofFigure 4). Although the contemporary plate boundary is relativelysharp, on-going tectonic activity manifested by earthquakes,volcanism, and deformation extends into the converging plates,and the limits of the plate margin are diffuse. Earthquake epicen-tres are concentrated along and near the plate boundary, but earth-quakes occur sporadically across the entire Cordillera. Recentlyactive volcanoes such as Mount Garibaldi in southwestern BritishColumbia are at the north end of the Cascade magmatic arc, whichextends southwards into northernmost California, parallel withthe subducting oceanic Juan de Fuca and Gorda oceanic platesoffshore. Other volcanic activity, such as that forming MountEdziza in northwestern British Columbia, may reflect riftingrelated to the offshore Queen Charlotte transform. Geologicalevidence for active deformation is clearly expressed adjacent to theplate boundary where sediments are being scraped off thesubducting Juan de Fuca plate to form a thrust faulted and foldedaccretionary wedge. However, geological evidence of Recentdeformation on land (Figure 6) is more elusive. This could be duein part to the continental glaciation that prior to 12,000 years beforethe present extended just south of latitude 49°N and may haveeroded or obscured the geomorphic expression of small-scaletectonic features. In addition, glacial erosion has created over-steepened topography in some mountainous areas which, unsup-ported after ice retreat, causes gravitational collapse; related faultsmay be difficult to distinguish from faults caused by tectonicactivity. Major questions concern drainage reversals of large riverssuch as the Yukon and Fraser rivers (e.g. Tempelman-Kluit, 1980).


Were these reversals due to blocking by ice in the mountains nearthe coast or to late tectonic uplift of those regions or to both?Regional uplift of up to about 4 km in the last 10 Ma in the southernCoast Belt has been documented from cooling histories derivedfrom fission track studies in the mountains, and from the lateMiocene change from wet to dry floras in the Chilcotin region,central British Columbia, which is in the rain shadow of the CoastMountains (Parrish, 1983).

CONCLUSIONS

Plate tectonic concepts have produced dramatic advances inour understanding of the tectonic evolution of the CanadianCordillera, but they have also unveiled daunting new research

challenges. The large-scale horizontal displacements of lithos-pheric plates that are driven by geologically rapid recycling ofoceanic lithosphere through the Earth’s mantle provide an elegantframework for interpreting the kinematics, dynamics and historyof Cordilleran tectonic evolution. The recognition that certaindistinctive assemblages of rocks and geological structures withinthe Canadian Cordillera can be identified with specific present-dayplate tectonic settings has provided actualistic analogues for platetectonic interpretations of various suites of rocks. This has led totectonic assemblage maps, recognition of terranes, recognition andpartial paleogeographic reconstruction of the Cordilleran orogeniccollage, and new insights on mountain-building and on the gener-ation of new continental crust. It has also unveiled the daunting

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Figure 6: Tectonic wedging and crustal thickening in southeastern British Columbia.A: Early Jurassic (~185 Ma) island arc (of QN, Figure 3) and its early Mesozoic back-arc basin, (mainly) on top of Slide Mountain terrane; onset of convergence of NorthAmerica with trench to west and collapse of back-arc basin. B: Late Early Jurassic (~180 Ma) collapsed basin thrust over old continental margin; flattening of subductionzone and initiation of continental margin arc. C: Early Middle Jurassic (~170 Ma) southwest verging deformation occurred as Kootenay terrane was detached from NorthAmerica and wedged under the old continental margin deposits; North American lithosphere wedged under Quesnel terrane; entrained and consumed in the subduction zone;subduction zone flattened and magmatic arc migrated eastward into the zone of southwest verging deformation.


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challenge of understanding the underlyingprocesses and of resolving the large uncertain-ties about relative displacements of someterranes during the Mesozoic and of allterranes prior to the Mesozoic. Much has beenlearned, but much remains to be learned,particularly about the processes within theEarth that drive the creation, displacementand destruction of the lithospheric plates.

ACKNOWLEDGEMENTS

Steve Gordey of the Geological Survey ofCanada (Vancouver office) and Mike Gray ofRubicon Minerals made constructive reviewsof the article. In addition, we wish to thankPhil Hammer of the Department of Earth andOcean Sciences at the University of BritishColumbia, who modified Figure 2 for thisarticle, and also several members of theGeological Survey of Canada (Vancouveroffice) who assisted the first author with theremaining figures.

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