Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40Ar/39Ar...

For permission to copy, contact [email protected] 2003 Geological Society of America200

GSA Bulletin; February 2003; v. 115; no. 2; p. 200–216; 7 figures; 1 table; Data Repository item 2003031.

Cooling and exhumation of the Shuswap Metamorphic Core Complexconstrained by 40Ar/39Ar thermochronology

Olivier Vanderhaeghe†

Universite Henri Poincare Nancy, UMR G2R, BP 239, 54506 Vandoeuvre, France, Department of Geology and Geophysics,University of Minnesota, Pillsbury Hall, Minneapolis, Minnesota 55455, USA and Research School of Earth Sciences,Australian National University, Canberra, ACT 0200, Australia

Christian Teyssier‡

Department of Geology and Geophysics, University of Minnesota, Pillsbury Hall, Minneapolis, Minnesota 55455, USA

Ian McDougall§

W. James Dunlap#

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

ABSTRACT

The exhumation history of the ShuswapMetamorphic Core Complex (ShuswapMCC) is deciphered from structural anal-ysis and 40Ar/39Ar thermochronology. Withthe exception of a few samples contaminat-ed by excess argon, the analytical resultsindicate a consistent range of early Tertiary40Ar/39Ar ages throughout the area. Horn-blende yields ages ranging from ;59–54Ma; muscovite and biotite ages are clusteredbetween 49.5 and 48 Ma, and are consistentwith the timing of closure of K-feldspars,typically ranging from 50 to 43 Ma, exceptfor samples in the immediate footwall of theColumbia River fault that yield ages asyoung as 26 Ma. These 40Ar/39Ar ages, com-bined with existing U/Pb ages on zirconsand monazites, constrain the cooling histo-ry at several localities in the Shuswap MCCbetween Trans Canada Highway 1 and theThor-Odin dome. The migmatitic core ofthe complex is affected by a rapid coolingevent (;700 8C to ;300 8C), from crystal-lization of zircons through closure of argondiffusion in hornblende and micas, between;56 and ;48 Ma. Based on our previousstructural analysis, we propose that thisfirst period of rapid cooling follows an ex-humation event associated with the forma-tion of the Shuswap MCC by activation of

†E-mail: [email protected].‡E-mail: [email protected].§E-mail: [email protected].#E-mail: [email protected].

a low-angle detachment and ductile thin-ning of a previously thickened and partiallymolten crust. Multidiffusion-domain ther-mal modeling of K-feldspar constrains thelower temperature history from ;350 8C to;150 8C and indicates a period of thermalstability from ;48 to ;45 Ma, and a coolingevent at ;45 Ma, except for the K-feldsparssampled in the immediate footwall of theColumbia River fault, which indicate rapidcooling at ;33 Ma. These cooling eventsare attributed to exhumation accommodat-ed by local activation of high-angle normalfaults.

Therefore, we distinguish two periods ofexhumation in the Canadian Cordillera: (1)Exhumation in Paleocene time related tocrustal thinning and tectonic denudation byactivation of detachment zones at ;60–50Ma, which corresponds to gravitational col-lapse of the thickened and partially moltenCordilleran crust; and (2) Exhumation re-lated to Eocene regional extension, Basin-and-Range style, at ;45 Ma, with reacti-vation of the eastern boundary fault of thecore complex, the Columbia River fault, inOligocene time at ;33 Ma.

Keywords: Argon thermochronology, Ca-nadian Cordillera, exhumation, metamor-phic core complex, Shuswap, tectonics.

INTRODUCTION

Metamorphic Core Complexes (MCC) arecharacterized by the exhumation of metamor-

phic rocks from beneath upper crustal unitsalong low-angle detachment zones (Arm-strong, 1982; Coney, 1980). The metamorphiccore complex style of crustal extension re-flects the presence of a relatively thin, brittleupper crust overlying a thick ductile crust andcontrasts with ‘‘wide rift’’ or ‘‘narrow rift’’styles of extension as defined by Buck (1991).Assessing the thermal-mechanical evolutionof the crust during extension requires a mul-tidisciplinary approach combining structuralgeology to reconstruct the geometry and es-timate the rheological behavior, petrology andthermobarometry to determine the pressure-temperature conditions, and geochronologyand thermochronology to constrain the timingof events.

The tectonic history of the North AmericanCordillera is marked by crustal extension in-terpreted as the result of gravitational collapseof a previously thickened crust (Coney andHarms, 1984). Crustal extension is accom-modated by the formation of a relatively nar-row belt of metamorphic core complexes fromAlaska to Mexico (Armstrong, 1982; Critten-den et al., 1980), followed by widespreadcrustal extension in the Basin and RangeProvince of the western United States (Liu,2001). These two stages can be compared tothe metamorphic core complex and wide riftstyles of extension (Buck, 1991). The Shus-wap MCC is the largest of the North Ameri-can MCCs, and being to the north of the Basinand Range Province, it is relatively little af-fected by widespread extension (Liu, 2001).Thus, it is a good target to evaluate the impact

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COOLING AND EXHUMATION OF THE SHUSWAP METAMORPHIC CORE COMPLEX

of the various styles of extension affecting thecrust associated with exhumation of metamor-phic rocks.

The aim of this paper is to present the re-sults of an 40Ar/39Ar and K/Ar thermochron-ologic study that constrains the cooling his-tory of the Shuswap MCC. The comparisonof these data with previously published geo-chronologic data and structural analysis al-lows a reconstruction of the thermal and me-chanical evolution of the Cordilleran crustduring the formation of the Shuswap MCC asmetamorphic rocks are transferred through thebrittle-ductile transition. The significance ofthese results is discussed in the framework ofthe regional geodynamic evolution.

GEOLOGIC SETTING OF THESHUSWAP MCC

The Shuswap MCC in the CanadianCordillera

The formation of the Canadian Cordillera isthe result of Mesozoic accretion of magmaticarcs to the western edge of the North Ameri-can craton (Monger et al., 1982; Wheeler andMcFeely, 1991). Eastward thrusting of alloch-thonous terrains over the sedimentary se-quences accumulated on the North Americanpaleomargin (Brown et al., 1986; Price, 1986)resulted in the formation of a 50–60-km-thickcrustal welt (Coney and Harms, 1984). Crustalthickening and burial of the sediments werefollowed by thermal relaxation associatedwith widespread high-temperature metamor-phism and crustal anatexis (Carr, 1992; Ny-man et al., 1995; Sevigny et al., 1989; Van-derhaeghe and Teyssier, 1997). The southernCanadian Cordillera (Fig. 1) is affected by anarray of brittle normal faults linked by largestrike-slip faults (Ewing, 1981; Struik, 1993).The Shuswap MCC is cross-cut on its north-ern tip by the dextral strike-slip fault thatmarks the Rocky Mountain Trench and con-nects with the Wolverine MCC a few hundredkilometers to the north (Struik, 1993). Previ-ous geochronologic studies based on the U/Pb,Rb/Sr, and K/Ar methods provide a generalframework for the timing of events in theShuswap MCC (Carr, 1991; Crowley, 1997a,1997b; Johnston, 1998; Parkinson, 1991,1992; Parrish, 1995; Parrish and Armstrong,1987; Parrish and Wheeler, 1983). These stud-ies have shown that the high-grade metamor-phic rocks forming the crustal root of the oro-genic belt were exhumed during Paleogenetime, with the development of the ShuswapMCC in the hinterland of the Canadian Cor-dillera (Fig. 1). During this period, major

changes in relative plate motion velocity andazimuth occurred between the North Ameri-can plate and the subducting Kula plate, prob-ably as a result of global-scale plate reorga-nization (Engebretson et al., 1985; Stock andMolnar, 1988).

Structural and Metamorphic DataBetween Trans Canada Highway 1 andthe Thor-Odin Dome

The Shuswap Metamorphic Core Complexis characterized by a low-angle detachmentzone that separates remnants of a dismem-bered upper unit from the exhumed metamor-phic core (Brown and Journeay, 1987; Carr,1992; Johnson and Brown, 1996; Johnston etal., 2000; Parrish et al., 1988; Vanderhaegheet al., 1999b).

Below the detachment zone, a middle andlower tectonic unit can be distinguished (Van-derhaeghe et al., 1999b). The middle unit con-sists of amphibolite-facies gneisses and abun-dant leucogranite laccoliths. The lower unit,composed of diatexite-migmatites and gran-ites, appears in the core of structural domes,such as the Thor-Odin dome (Fig. 2). Thesehigh-grade metamorphic rocks are affected bya ductile fabric characterized by a shallowly-dipping composite foliation (Brown and Jour-neay, 1987; Johnson and Brown, 1996; John-ston et al., 2000; Vanderhaeghe et al., 1999b)and a dominantly east-west trending mineraland stretching lineation (Fig. 2). Boudinageand fracturing provided pathways for melt mi-gration, forming a network of leucograniticdikes and sills that permeates the middle unitsand is structurally connected from the mig-matites of the lower unit with leucogranite lac-coliths emplaced within the detachment zone(Vanderhaeghe, 1999). Qualitatively, the re-sultant finite strain ellipsoid is indicative offlattening over most of the MCC, with a ver-tically oriented direction of shortening and anE-W dominant direction of extension. Ther-mobarometry indicates that conditions of par-tial melting, with temperatures of 600–700 8Cat 4–6 kbar, were reached in the core of thedome (Duncan, 1984). Thermobarometric dataobtained in the middle unit along Trans Ca-nadian Highway 1 indicate that partial meltingprevailed coeval with the development of theductile fabric with a pressure ranging from 6to 9 kbar and temperature estimates between620 and 820 8C (Ghent et al., 1977; Lane etal., 1989; Norlander et al., 2002; Nyman etal., 1995). The significance of the ductile fab-ric and whether it is mostly related to crustalthickening or thinning is still debated (Brownet al., 1992; Johnson and Brown, 1996; John-

ston et al., 2000; Mattauer et al., 1983; Parrishet al., 1988; Vanderhaeghe et al., 1999b). Inparticular, the Monashee Decollement, an am-phibolite-facies top-to-the-east shear zone, hasbeen interpreted as a crustal-scale thrust(Brown et al., 1992; Brown et al., 1986; John-ston et al., 2000; Mattauer et al., 1983) or asthe root zone of the low-angle detachments(Vanderhaeghe et al., 1999b).

The detachment zone bordering the Shus-wap MCC (Figs. 1 and 2) is a composite struc-ture with the Okanagan-Eagle River detach-ment to the west (Johnson and Brown, 1996;Tempelman-Kluit and Parkinson, 1986) andthe Columbia River detachment to the east(Read and Brown, 1981; Vanderhaeghe et al.,1999b). The detachment zones are character-ized by progressive overprinting of the high-grade fabric of the middle unit by a retrogradegreenschist facies mylonitic fabric, as well aspseudotachylite and cataclasite zones, indicat-ing decreasing pressure (P) and temperature(T) during deformation. The leucogranite lac-coliths are typically homogeneously deformedover several hundred meters, and they are spa-tially associated with the detachments, sug-gesting that they intruded synkinematically(Gapais, 1989). At Trinity Hills (Fig. 2), theC/S mylonitic fabric (Berthe et al., 1979) inthe leucogranite is defined by quartz ribbonsand boudinaged muscovite porphyroclasts en-veloping fractured porphyroclasts of K-feldspars.

Above the detachment, tilting of blocks isassociated with the formation of pull-apart andextensional basins filled by Eocene sedimen-tary sequences and volcanic rocks. A secondset of high-angle faults affecting all unitstrends dominantly north-south and is associ-ated with dip-slip or slightly oblique-slip nor-mal movement (Fig. 2). These faults are con-nected by east-west–trending steep strike-sliptransfer faults (Johnson and Brown, 1996).The highest peaks of this region are borderedto the east by the east-dipping Columbia Rivernormal fault (Lane, 1984) and to the west bythe west-dipping Victor Lake normal fault.The Columbia River normal fault, whichcross-cuts the Columbia River detachment,causes tilting of the hanging wall and delimitsa small graben basin of unknown age to theeast of Mount Hall (Vanderhaeghe et al.,1999b). In the southern continuation of theColumbia River fault, the Slocan Lake fault(Fig. 1) is identified on the Lithoprobe seismicprofile, where it is inferred to affect at leastthe upper half of the crust (Cook et al., 1992).

Previous Geochronology

Ages obtained on plutonic suites in thehanging wall of the detachments have been

202 Geological Society of America Bulletin, February 2003

VANDERHAEGHE et al.



Figure 1. (A) Geologic map of the southern Cordillera modified after Wheeler and McFeely (1991). The numbers refer to the Lithoprobeseismic lines. (B) Crustal scale cross section based on geologic information and on our interpretation of the Lithoprobe profile.N

summarized in Parrish (1995). The youngestare U/Pb ages of zircons and monazites fromcalc-alkaline batholiths that range from 174 to161 Ma and are interpreted as the Mid-Jurassicemplacement of these plutons in an island arcsetting (Carr, 1991; Parrish and Armstrong,1987; Parrish and Wheeler, 1983). Mesozoiccooling from ;500 8C–;300 8C is recordedby K/Ar and 40Ar/39Ar data for hornblende andbiotite (Archibald et al., 1983; Colpron et al.,1996; Mathews, 1981). Thermal modeling ofK-feldspars from the Selkirk allochthon in thehanging wall of the Columbia River fault in-dicates that Mesozoic cooling was followedby cooling at a rate of 1–3 8C/Ma until Eocenetime (Colpron et al., 1996). Sedimentary se-quences, deposited in extensional basins in thehanging wall of the western Okanagan-EagleRiver detachment, are intruded and capped byrhyolitic to basaltic volcanic rocks dated at49–47 Ma by the K/Ar method (Mathews,1981). The sediments contain pebbles fromthe underlying calc-alkaline batholith andgreenschists, and also from the leucogranitesand gneisses exhumed in the footwall of thedetachments, indicating that rocks of the MCCwere exposed to erosion before 49 Ma.

Migmatitic gneisses exposed in the Mon-ashee complex comprise paragneisses yieldingRb/Sr and U/Pb zircon ages of ;2.2 Ga in-truded by orthogneisses yielding ages from1.9 Ga to 1.8 Ga (Armstrong et al., 1991;Crowley, 1997a, 1997b; Parkinson, 1991;Wanless and Reesor, 1975). Precambrian U/Pbages on zircons are slightly discordant andsuggest an affinity with the North Americanbasement (Armstrong et al., 1991; Crowley,1997a, 1997b; Parkinson, 1991). Leucogran-ites of the Ladybird suite (Fig. 2), intruded atmidcrustal levels, exhibit U/Pb monazite andzircon ages ranging from 60 to 55 Ma (Carr,1991; Carr, 1992; Johnston et al., 2000; Par-kinson, 1992). Parkinson (1992) reported agesas young as 50 Ma on zircons from pegmatitesintruded along the detachment zone. Vander-haeghe et al. (1999b) conducted a U/PbSHRIMP study of zircon populations of twogranitic veins of the migmatitic core of theThor-Odin dome and one sample of the leu-cogranite laccolith along the western detach-ment at Trinity Hills (sample 97046 of thisstudy). Cathodoluminescent imaging of zircongrains from the migmatite samples displaysinherited cores, with Paleoproterozoic ages,and U/Pb-rich rims that yield ages of 56.4 6

1.4 Ma and 55.9 6 3.1 Ma. Zircons of theleucogranite with magmatic oscillatory zoningyield an age of 59.8 6 1.0 Ma. Some zirconsof the leucogranite contain inherited cores ofPaleoproterozoic age. These results indicatethat early Tertiary melting was prevalent inthe Thor-Odin dome and are consistent withthe idea that the underlying migmatite may bethe main source of leucogranites (Vander-haeghe et al., 1999b).

Cooling ages of metamorphic rocks of theShuswap MCC obtained from Rb/Sr K-feldspar/muscovite mineral isochrons and K/Ar datingof hornblende, white mica, and biotite rangefrom 64 to 45 Ma (Mathews, 1981; Parrish etal., 1988). The spread of K/Ar ages and theirmutual inconsistency in a number of casessuggest that contamination by excess argon ispresent at some localities (Mathews, 1981;Parrish et al., 1988).

40Ar/39Ar THERMOCHRONOLOGY

The goal of the present study is to constrainthe cooling and exhumation history of thehigh-grade metamorphic rocks of the middle andlower units exposed in the footwall of the de-tachments that bound the Shuswap MCC. Usingdifferent K-bearing mineral phases such as horn-blende, muscovite, biotite, and K-feldspar, withdifferent argon closure temperatures, the 40Ar/39Ar thermochronologic method allows the re-construction of temperature-time curves,which are an important guide in decipheringthe exhumation history. At each locality, sam-ples belonging to the same tectonic unit at theoutcrop scale were collected from various li-thologies in order to obtain amphibole, mica,and K-feldspar. Hornblende was found in am-phibolite boudins likely derived from maficintrusives transposed and disrupted in the fo-liation. Biotite flakes delineating the fabric ofthe rock were separated from paragneisses. Inthe vicinity of the detachment zone, muscoviteand K-feldspar were found in separable quan-tities in mylonitic leucogranites. At deeperstructural levels, K-feldspar was separatedfrom leucosomes contained in the foliation.

Sample Descriptions

The migmatitic lower unit is represented bysamples collected to the south of Mount Odinand to the east of Mount Thor. Samples south-east of Mount Odin comprise a hornblende-

biotite-plagioclase 6 quartz and calcite gneiss(97005) and a garnet leucogranitic vein en-closed in the synmigmatitic foliation of themetatexites (97013). Samples northeast ofMount Thor comprise a massive amphiboliteboudin (97019) and a tonalitic garnet leuco-some (97020). In the thin section, the two am-phibolites exhibit lepidoblastic textures withalternating layers of hornblende and biotite. Inamphibolite 97005, the hornblende containssmall plagioclase inclusions. The graniticsamples show an equigranular coarse texturewith some myrmekitic intergrowths. The fo-liation is delineated by biotite. Granite 97013contains primary muscovite, garnet, fibrolite,and kyanite included in K-felspar, suggestinga peraluminous composition. In contrast, sam-ple 97020 does not contain any K-feldspar andis more tonalitic in composition.

The middle unit is represented by samplescollected on Mount Symonds and along TransCanada Highway 1, to the east of Victor Lake(Fig. 2). Samples from Mount Symonds com-prise a massive amphibolite boudin (97029),a metapelitic paragneiss folded with a garnetleucogranite (97023, 97025). The amphibolite97029 has a grano-lepidoblastic texture withlayers of hornblende and biotite, alternatingwith layers of quartz and plagioclase. The leu-cogranites (97023, 97025) contain muscovite,biotite, and garnet arranged in an equigranularcoarse texture. Quartz shows undulatory ex-tinction and is elongated in a direction con-sistent with the axial plane of a minor foldaffecting the rock at the scale of the thin sec-tion. The metapelitic layer, which is surround-ing the leucogranite, exhibits a first fabricwarped around the granite and delineated bybent biotites. A second foliation, parallel tothe one defined by the quartz in the leuco-granite, is delineated by smaller biotite grains.The K-feldspar in the leucogranite is alteredand contains micron-scale white micas. Sam-ples from Victor Lake comprise a massive am-phibolite composed essentially of hornblende(97090) and a mylonitic metapelitic schist(97091). The mylonitic fabric of the metape-lite is delineated by the alignment of biotiteand sillimanite. Asymmetric tails around gar-net porphyroclasts and antithetic shear bandsindicate a top-to-the-east sense of shear.

The Columbia River detachment zone wassampled along the western shore of the UpperArrow Lake, at the latitude of the dam, northof Revelstoke, and also on Highway 23, east


VANDERHAEGHE et al.

Figure 2. (A) Geologic map of the Shuswap metamorphic core complex in the region of the Thor-Odin dome. (B) Cross sections ABCand DE, located in Figure 2A, show the approximate projection of samples used in the 40Ar/39Ar study. Sample 97106 is located betweenthe two cross sections. Samples 97029, 97023, and 97025, from Mount Symonds are located south of Mount Thor on the DE crosssection.



Figure 2. (Continued.)

of Mount Hall. The samples from Revelstokecomprise a massive amphibolite boudin(97071), a cataclasitic schist (97070), and amylonitic quartzo-feldspathic schist (97073).The cataclasitic schist 97070 consists of close-ly spaced cataclasite zones composed of a ma-trix of crushed quartz, feldspar, and whitemica in which clasts of ;0.1 mm are en-closed. These zones separate rock fragmentsthat can be several centimeters in diameter. Inthese clasts, K-feldspar is altered and containssmall white mica flakes no longer than a fewmicrons. The quartzo-feldspathic schist exhib-its a C/S fabric with S planes defined by freshbiotite. The sample from Mount Hall is a my-lonitic leucogranite (97106). In the thin sec-tion, the mylonitic fabric is delineated byquartz ribbons and mica fishes surroundingfractured K-feldspar porphyroclasts. Asym-metric tails around clasts indicate a top-to-the-east sense of shear.

The western detachment was sampled westof Mabel Lake, both to the east and to the northof Trinity Hills, and also to the south of Sica-mous on Highway 97A. Sampled amphibolitelayers (97041, 97050, 97068) are massive andmostly composed of hornblende. Leucogranitesamples (97046, 97049) exhibit a recrystallizedmicrofabric delineated by elongated quartz rib-bons and biotite. Asymmetric tails of fine-grained quartz-feldspar layers around K-feldsparporphyroclasts and muscovite aligned in the Cplanes indicate a top-to-the-west sense of shear.South of Sicamous, a mylonitic migmatitic par-agneiss (97067) was sampled, having a grano-lepidoblastic texture marked by the alternationof biotite layers and quartz layers containingsome garnet, K-feldspar, and plagioclase.

Analytical Procedures

Mineral separates (.99% pure) were pre-pared using conventional heavy liquid and mag-

netic separation techniques. Potassium-argonages were determined by analytical techniquesoutlined in McDougall and Schmincke (1977),with potassium measured by flame photome-try and argon measured by isotope dilutionfollowing extraction of the argon by fusion ofan aliquot of the sample in vacuo.

Samples analyzed by the 40Ar/39Ar methodwere irradiated for 140 or 196 hours in posi-tion X33 or X34 of the HIFAR nuclear reactorat Lucas Heights, New South Wales. Cadmi-um shielding was used to reduce the thermalneutron dose received by the samples. Correc-tion factors utilized were those determined byTetley et al. (1980). Biotite standard GA1550of K/Ar age 97.9 Ma (McDougall and Harri-son, 1999; McDougall and Roksandic, 1974)was employed as the fluence monitor. Step-heating experiments were undertaken in adouble-vacuum, resistively heated furnace.The heating schedule utilized depended upon


VANDERHAEGHE et al.

TABLE 1. K/AR AGES OF SAMPLES FROM THE SHUSWAP METAMORPHIC CORE COMPLEX

Sample location Rock type ANU# Latitude Longitude Mineral Grain size(micron)

K(%)

40Ar*(10210 mol/g)

40Ar*(of total 40Ar)

Age(Ma, 1s error)

S. Odin, lower unit Amphibolite boudin 97–5 50832.359 118809.259 hornblende 124/251 0.8279 0.8174 1.023 86.3% 70.3 6 1.1E. Thor, lower unit Amphibolite boudin 97–19 508389 118802.29 hornblende 353/500 0.5015 0.5001 1.064 86.3% 118.5 6 1.3Symonds, middle unit Amphibolite boudin 97–29 50827.89 118804.19 hornblende 124/178 0.3824 0.3889 0.439 72.3% 64.5 6 0.9Mabel lk, W. detachment Amphibolite boudin 97–41 50828.259 118843.19 hornblende 124/251 0.8447 0.8508 0.908 57.8% 60.7 6 0.7Trinity hills, W. detachment Amphibolite boudin 97–50 50833.29 118852.29 hornblende 124/251 0.9398 0.9375 0.928 87.6% 54.0 6 0.6Sicamous, W. detachment Amphibolite boudin 97–68 50847.859 118858.49 hornblende 124/251 1.1439 1.1187 1.096 70.3% 55.0 6 1.0W. Revelstoke, E. detachment Amphibolite boudin 97–71 51803.559 118812.159 hornblende 124/251 0.8643 0.8615 0.974 84.6% 59.6 6 0.6Victor lk, MD, middle unit Amphibolite boudin 97–90 50857.19 118824.259 hornblende 251/422 0.4544 0.4524 2.411 94.3% 283.1 6 3.0S. Odin, lower unit Migmatitic leucosome 97–13 50832.49 118808.89 K-feldspar 124/251 11.32 11.24 13.30 88.1% 66.7 6 0.7Symonds, middle unit Leucogranite 97–25 508289 118804.99 K-feldspar 124/251 9.590 9.703 8.696 54.8% 51.2 6 0.6Mabel lk, W. detachment Leucogranite 97–46 50826.99 118846.39 K-feldspar 124/251 9.077 9.141 7.778 91.3% 48.6 6 0.5Trinity hills, W. detachment Leucogranite 97–49 50833.259 1188529 K-feldspar 251/500 10.93 11.07 9.810 78.9% 50.7 6 0.6Victor lk, MD, middle unit Leucogranite 97–91 50857.19 118824.259 K-feldspar 124/251 12.03 12.01 22.24 81.7% 103.7 6 1.1Mt Hall, E. detachment Leucogranite 97–106 50843.259 118800.659 K-feldspar 178/251 11.03 11.02 12.55 91.8% 64.5 6 0.7S. Odin, lower unit Amphibolite boudin 97–5 50832.359 118809.259 biotite 178/500 7.140 7.099 6.903 88.1% 55.1 6 0.6E. Thor, lower unit Metapelitic gneiss 97–20 508389 118802.29 biotite 124/251 7.450 7.536 7.355 94.5% 55.7 6 0.6Symonds, middle unit Metapelitic gneiss 97–23 50827.89 1188069 biotite 124/251 7.678 7.654 7.141 62.6% 52.9 6 0.6Mabel lk, W. detachment Leucogranite 97–46 50826.99 118846.39 muscovite 152/251 8.444 8.436 7.348 93.6% 49.5 6 0.6Mabel lk, W. detachment Amphibolite boudin 97–41 50828.259 118843.19 biotite 124/251 6.754 6.776 5.871 73.3% 49.4 6 0.4Sicamous, W. detachment Metapelitic gneiss 97–67 50847.859 118858.49 biotite 124/251 7.591 7.590 6.519 65.9% 48.9 6 0.5W. Revelstoke, E. detachment Leucogranite 97–70 51803.559 118812.159 muscovite 251/500 8.345 8.169 6.898 94.1% 47.5 6 0.8W. Revelstoke, E. detachment Metapelitic gneiss 97–73 51803.559 118812.159 biotite 178/500 6.327 6.238 5.334 93.6% 48.3 6 0.6Victor lk, MD, middle unit Metapelitic gneiss 97–91 50857.19 118824.259 biotite 178/353 7.636 7.826 6.877 84.2% 50.6 6 1.0

the sample being analyzed and is shown withthe results presented in the supplementarydata tables. We note here, however, that forthe K-feldspars, the heating schedule com-prised 43 steps at 50 8C intervals, starting at450 8C and ending at 1450 8C, generally withat least two extractions at each temperature.Following purification of the argon, isotopicanalyses were made using a VG1200 or aVG3600 mass spectrometer. All ages are cal-culated with the decay constants recommend-ed by Steiger and Jager (1977).

Results

Results of the K/Ar dating are given in Ta-ble 1, and the thermochronology data are sum-marized in Figures 3, 4, and 5.1

HornblendeThe K/Ar dating of hornblende yielded ages

ranging from 283 to 54 Ma, but five of thesamples have calculated ages between 65 and54 Ma. The step-heating experiments on horn-blende yielded rather discordant and irregularage spectra (Fig. 3), which will be discussedbelow. As expected, the 40Ar/39Ar total fusionages are similar to the K/Ar ages. The K/Caratios found for each hornblende are fairlyconstant and lower than 0.5, suggesting thatthe hornblendes are not contaminated signifi-cantly by other mineral phases.

From the shape of the age spectra and from

1GSA Data Repository item 2003031, 40Ar/39Ardata tables, is available on the Web at http://www.geosociety.org/pubs/ft2003.htm. Requestsmay also be sent to [email protected].

the apparent ages, it would appear that severalof the age spectra are affected by excess ar-gon. This is particularly apparent for samples97005 and 97019 from the Thor-Odin dome(Fig. 3), as they exhibit saddle shaped agespectra. The age spectrum of hornblende97090 from Victor Lake is very irregular andyields a total fusion age of 305 Ma, which isdifficult to interpret geologically. The remain-ing five samples, which show less irregularage spectra and even flat sections (Fig. 3),yield 40Ar/39Ar ages more consistent with geo-logical and other geochronological data. How-ever, even in these samples, the determinationof precise ages is difficult and somewhat prob-lematic and arbitrary.

To illustrate these problems of interpreta-tion, consider the step-heating experiment onhornblende 97071 from Revelstoke in thefootwall of the Columbia River fault. Thisyields an irregular age spectrum (Fig. 3) thatcan be interpreted as indicating closure to ar-gon diffusion at a maximum age of ;59.0 Ma,which is a weighted mean age for all stepsexcept for those showing much higher ages atthe beginning and end of the experiment. Al-ternatively, it could be argued that the mini-mum in the spectrum, 56.4 Ma, might also beregarded as a maximum age for closure to ar-gon diffusion in the hornblende. Similarly,hornblende 97068, from the vicinity of thewestern detachment near Sicamous, gives aweighted mean age of 53.7 Ma from 97% ofthe gas released, with the lowest age recordedin the spectrum, 50.8 Ma. Samples from Trin-ity Hills and Mount Symonds yield similarages, with a total fusion age of 54.7 6 0.5 Ma

for hornblende 97050 and a weighted meanage of 56.0 6 0.2 Ma for 82% of the gasreleased from hornblende 97029. Hornblende97041, from the eastern shore of Mabel Lake,shows a saddle shape, but younger concordantages are found for three steps in the centralpart of the age spectrum, comprising 11% ofthe gas released, giving a weighted mean ageof 54.9 6 0.2 Ma. This might reasonably beinterpreted as a maximum age estimate forclosure of the hornblende to argon. Overall,for these five hornblendes, the age spectra canbe interpreted as indicating cooling throughtheir respective closure temperatures some-where in the approximate time interval of 59–51 Ma, with best estimates of ;56–54 Ma forfour of the samples.

MuscoviteMuscovite separates from three different lo-

calities in the Shuswap MCC are characterizedby flat age spectra for most of the gas release;mean ages of plateau or flat sections are notvery different from total fusion or K/Ar ages(Fig. 3, Table 1). In addition, 40Ar/39Ar agesare consistent throughout the core complexand lie in a very narrow range from 49.5–48.2Ma. In the vicinity of the western detachmentzone, muscovite separate 97046 yielded a totalfusion age of 49.5 6 0.1 Ma. On the otherside of the Shuswap MCC, in the footwall ofthe Columbia River fault, muscovite separates97106 and 97070 yield mean ages of 49.0 60.1 for 87% of the gas released and 48.2 60.1 Ma for 92% of the gas released, respec-tively, from step-heating experiments. Al-though the errors of each step do not strictly



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VANDERHAEGHE et al.

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Figure 5. Thermal modeling of K-feldspar 97025 treated as an example. See discussion in text. (A) Measured age spectra compared withfour modeled age spectra, in dotted lines. (B) Determination of the activation energy using a least-squares fit of the low-temperaturepoints on an Arrhenius plot. (C) Estimation of the domain size distribution using the computer program of Lovera (1992) in which thevolume fraction and the size of each domain are modified to achieve the best fit of the experimental data on the Arrhenius diagram.(D) Cooling histories that result in the four synthetic age spectra shown in Figure 5A. Only cooling history 2 produces a modeled agespectrum that fits the measured one reasonably well. The shaded area shows the range of cooling histories that produce similar goodfits to the data.

overlap, the small error in the age indicatedby the weighted mean reflects the regularityof the age spectra.

BiotiteBiotite separates also are characterized by

flat age spectra for most of the gas release,although they tend to show more scatter thanmuscovite at the beginning and toward the endof the step-heating experiments (Fig. 3). Theyounger ages at the beginning of the experi-ments probably reflect minor argon loss. Meanages obtained from the flat sections of the agespectra cluster at ca. 48 Ma and, in most cases,are similar to the K/Ar ages. Biotite 97041,from the eastern shore of Mabel Lake, was notdated by 40Ar/39Ar but yields a K/Ar age of49.1 6 0.4 Ma, consistent with the other agesobtained in the area. The plateau or plateau-like 40Ar/39Ar ages range from a mean age of

47.7 6 0.1 Ma for 86% of the gas releasedfor biotite 97067, sampled south of Sicamous,to a mean age of 49.4 6 0.1 Ma for 88% ofthe gas released for biotite 97023, sampled atMount Symonds, and a mean age of 49.5 60.1 Ma for 89% of the gas released for biotite97091, sampled at Victor Lake. For the onelocality (Revelstoke) where both muscoviteand biotite were analyzed, the biotite (97073)yields a mean age of 48.0 6 0.1 Ma for 82%of the gas released, virtually indistinguishablefrom the muscovite age. Biotite 97020 fromthe Thor-Odin dome yields an older mean ageof 55.4 6 0.2 Ma for 88% of the gas released.Interpretation of the 40Ar/39Ar apparent age ofthis sample is somewhat problematic becausethe age spectrum does not show any obvioussigns of excess argon. Nevertheless, the 40Ar/39Ar age of 55 Ma seems too old when com-pared with the younger U/Pb ages on zircon

and monazite from leucosomes sampled in thearea (Carr, 1991a; Carr, 1992; Parkinson,1992; Vanderhaeghe et al., 1999b).

K-feldsparThe age spectra obtained on K-feldspar sep-

arates generally are characterized by a gradualincrease in 40Ar/39Ar age with increasing tem-perature of release, typically covering an agerange of several million years (Fig. 4). Incon-gruent melting of the sample in the vacuumsystem occurs at ;1150 8C and is marked bya drastic decrease in the effective diffusionlength scale, as illustrated in the log(r/r0) plot.At this stage, from 80 to 90% of the gas hasbeen released. The first of a series of isother-mal steps tends to yield an apparent 40Ar/39Arage significantly older than the others. Thisfeature is especially apparent for the first10%–20% of gas release and is generally at-


VANDERHAEGHE et al.

tributed to excess argon sited in fluid inclu-sions (Harrison et al., 1993). There is a ten-dency for a sample showing high ages duringthe first 10%–20% of degassing also to exhibitevidence of excess argon in the latter stagesof gas release. Such a correlation is particu-larly apparent for sample 97013 and, to a less-er extent, for sample 97091. For sample97013, excess argon is suggested by steps be-tween 40 and 80% of gas release yielding ap-parent 40Ar/39Ar ages that are older than agesfound for steps performed at higher tempera-tures. Excess argon is the most likely causefor the elevated 40Ar/39Ar apparent ages de-rived from those parts of the age spectra ofK-feldspars 97013, 97091, and 97106 that aremuch older than the ages obtained from horn-blende and micas from the same localities orolder than published U/Pb ages on zircons andmonazites from the same area (Carr, 1991;Carr, 1992; Parkinson, 1992; Vanderhaeghe etal., 1999b). Foster et al. (1990) have reportedon similar K-feldspar age spectra, and theyhave shown that the excess argon released inthe higher temperature steps is likely locatedwithin large diffusion domains.

Apart from these problems, the age spectraof the various K-feldspar separates show com-parable trends. The youngest 40Ar/39Ar appar-ent ages are obtained from K-feldspar sepa-rates 97070 and 97106 sampled in thefootwall of the Columbia River fault, with firststeps at ca. 25 Ma and a flat section at 32–33Ma. The remainder of the age spectrum of K-feldspar 97070 exhibits a gradual increase ofage with temperature, up to ca. 40 Ma. In con-trast, the age spectrum of K-feldspar 97106displays a wider range in ages, and the lastmelting step yields ca. 75 Ma. These old agesare not consistent with the muscovite age of49 Ma and probably reflect excess argon, al-though it is not possible to unequivocally de-tect it from the shape of the age spectrum.Except for slight differences, on the order of1–2 Ma, in the 40Ar/39Ar apparent ages record-ed, the age spectra obtained for K-feldspars97046, 97049, 97067, and 97025 exhibit re-markably similar shapes and range of ages.With increasing temperatures, the 40Ar/39Arapparent ages of these samples increase grad-ually but the total range of age is limited toonly a few million years from the beginningof the experiment to melting of the K-feldspargrains. The ages for the low-temperature steps(up to ;800 8C) range from 43 to 47 Ma(without taking into account the age spikes).The age spectra are then marked by an in-crease in the slope and an increase in 40Ar/39Arapparent ages to 52 Ma before melting of thesample. For these samples, the ages obtained

from the last steps before melting are gener-ally younger than or similar to the 40Ar/39Arages derived from micas from the samelocalities.

Multidiffusion Domain Thermal Modeling:Example of K-feldspar 97025

In order to derive cooling histories from theK-feldspar 40Ar/39Ar data, we followed themultidiffusion domain thermal modelingmethod described in detail elsewhere (Lovera,1992; Lovera et al., 1989; Richter et al.,1991). In this section, we outline the thermalmodeling procedure using K-feldspar 97025as an example (Fig. 4).

The first step of the thermal modeling is todetermine the activation energy by using aleast-squares fit to the first low-temperaturepoints on the Arrhenius plot (Fig. 5). With in-creasing temperature, departure from a straightline is interpreted to correspond to exhaustionof the smallest domains. We obtain, in thisexample, an activation energy of 48.1 kcal/mol for points 2–8 (Fig. 5B) with a precisionon the order of 6 2 kcal/mol, which translatesinto about 6 20 8C in the models below. It isassumed that this value is constant for all thedomains.

The domain-size distribution was estimatedby a trial-and-error calculation, using the com-puter program of Lovera (1992), in which thevolume fraction and the size of each domainare modified in order to achieve the best fit ofthe experimental data on the Arrhenius dia-gram. For K-feldspar 97025, the Arrheniusdata are best fitted with six diffusion domains,although it is possible to get a reasonablematch of the actual and modeled age spectrawith only three domains (Fig. 5). On the log(r/r0) plot, near-horizontal segments correspondto sections where degassing is dominated byone diffusion domain. However, because thecooling rate is variable, the shape of the agespectrum is controlled by the combined effectsof the diffusion domain-size distribution andthe cooling history. Consequently, a flat seg-ment of the age spectrum could correspondeither to a period of rapid cooling or to closureof a given diffusion domain, or both.

Once a domain size distribution has beenchosen, it is possible to produce synthetic agespectra by applying different cooling historiesto the multidiffusion domain model (Fig. 5D).The range of possible cooling histories thataffected the sample corresponds to the onesthat yield synthetic age spectra that reproducethe experimental age spectra. The domain sizedistribution is modeled for the total amount ofgas released. The drastic decrease in the log(r/r0) plot at ;90% gas release (Fig. 5C) reflects

melting of the sample (.1150 8C). This de-crease is expected because the diffusivity ofargon is much higher in melts. Therefore, thesynthetic age spectra do not utilize data fromthe melting steps.

In the case of K-feldspar 97025, the agesincrease regularly during the course of the ex-periment from 43 Ma to 54 Ma, except for afew steps at ;60% of gas released and for thefirst of the isothermal steps. These featuressuggest contamination of the sample by someexcess argon. Therefore, we choose to pro-duce a synthetic age spectrum that fits onlythe minimum values in the measured agespectrum. A range of cooling histories thatproduce a synthetic age spectrum that matchesthe measured age spectrum is indicated bydashed lines on Figure 5D. Modifications tothe cooling history of ;30 8C or 1 Ma yieldsynthetic age spectra that depart significantlyfrom the experimental data, and the same fourexamples are given in Figure 5A.

We followed the same procedure for theother samples. For those affected by excessargon, we tried first to reproduce the entire agespectrum for each, and then we applied a dif-ferent set of cooling histories aimed at mod-eling only the sections of the age spectrum,which are consistent with other geologicalconstraints. The results of thermal modelingof all the K-feldspars are indicated on Figure6, where the polygon delimits the range ofpossible cooling histories that provide a goodfit to experimental data. Although the detailsof the cooling history vary from one K-feldsparto another, they share some general features.Most K-feldspars close with respect to argonin a closure temperature window ranging from350–150 8C with the exception of K-feldspar97067, which exhibits an unusually high clo-sure temperature for its larger domains at;400 8C. The cooling history derived fromthis K-feldspar is slightly different from thatof neighboring localities (97046 and 97049).In addition, the cooling history derived fromthe larger domains of this K-feldspar is notconsistent with the 40Ar/39Ar apparent ages ofa biotite from the same sample, so that wesuspect that the activation energy used, 51.3kcal/mol, may be too high and uncharacteris-tic of the sample (Lovera et al., 1997).

Notice that we choose to apply thermal his-tories dominated by progressive cooling.However, it is conceivable that distribution ofargon in the K-feldspar could be significantlymodified by a thermal spike, and it must berealized that equally good fits of the age spec-tra could be obtained for such cases.



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VANDERHAEGHE et al.

DISCUSSION AND CONCLUSIONS

Cooling and Exhumation During theFormation of the Shuswap MCC

The cooling and exhumation histories of theShuswap MCC (Fig. 7) are reconstructed onthe basis of existing U/Pb, 40Ar/39Ar, and fis-sion track ages combined with thermobaro-metric and structural data. The shape of thecooling curve is marked by rapid coolingevents separated by periods of relative thermalstability.

Thermobarometric and structural data de-scribed in the geological setting indicate thatmetamorphic rocks of the lower and middleunits exhumed in the Shuswap MCC were af-fected by partial melting under a high geo-thermal gradient at the onset of cooling (Dun-can, 1984; Nyman et al., 1995). Thecrystallization age of these melts, and thuscooling below ;700 8C (Fig. 7) is constrainedby the ca. 56 Ma U/Pb ages obtained bySHRIMP analysis on zircon rims from leu-cosomes sampled in the Thor-Odin dome ofthe lower unit (Vanderhaeghe et al., 1999b)and the youngest U/Pb ages of ca. 55 Ma ob-tained on syntectonic granites emplaced inamphibolite facies rocks of the middle unit(Carr, 1992; Johnston, 1998; Parkinson, 1992;Vanderhaeghe et al., 1999b).

The variability of hornblende 40Ar/39Ar ap-parent ages obtained from samples of the mid-dle and lower units of the Shuswap MCC sug-gests that they are affected by excess argon,and that resetting of these minerals with re-gard to argon before final cooling is only par-tial. As previously discussed, the flatter por-tions of hornblende age spectra yield 40Ar/39Arapparent ages ranging from 59 to 54 Ma, withminimum ages as low as 51 Ma. These horn-blende ages suggest that the temperature wasabove ;500 8C (Harrison, 1981) before ca. 59Ma and probably before ca. 54 Ma, but therather irregular age spectra and the evidencefor excess argon preclude precise determina-tion of the time of closure of hornblende toargon diffusion. Muscovite and biotite showalmost identical 40Ar/39Ar apparent ages (Fig.3). The mean age of muscovite and biotiteover the Shuswap MCC, excluding one incon-sistently old analysis (97020), are 48.9 Maand 48.6 Ma, respectively. If we assume a dif-ference in the closure temperature of 50–1008C between biotite and muscovite (Dunlap,2000; Hames and Bowring, 1994), the smalldifference in the 40Ar/39Ar apparent ages im-plies rapid cooling.

Further support for this rapid cooling eventcomes from K-feldspar models for 97025,

97049, and 97046, which record rapid coolingfor their larger domains at ca. 50 Ma (Figs. 4and 6). Following this rapid cooling event, aperiod of thermal stability is recorded by K-feldspar model cooling histories, indicatingthat the rocks of the Shuswap MCC, at thepresent exposure level in the footwall of thedetachments, were at an almost constant tem-perature of ;300 8C from 48 to 45 Ma (Fig.4). Another rapid cooling event followed thisperiod of thermal stability, as recorded by thelow temperature domains of K-feldspar, whichclosed from ;300–150 8C at ;45–43 Ma atmost localities, and at 33 Ma in two samples(97106, 97070) from the immediate footwallof the Columbia River fault (Figs. 2, 4, and6). Thermal modeling of these latter two sam-ples indicates that they remained at a temper-ature of ;300–350 8C until ;40 Ma. This lastperiod of rapid cooling is followed by anotherperiod of thermal stability with a slow de-crease in temperature from 200 to 150 8C,when closure of K-feldspar with respect to ar-gon diffusion occurred.

The low-temperature cooling history of theShuswap MCC is discussed in a recent study(Lorencak et al., 2001) on the basis of fission-track analysis on zircons and apatite with clo-sure temperatures of 220 8C 6 50 8C and 1008C 6 10 8C, respectively (Corrigan, 1993;Green et al., 1989; Yamada et al., 1995). Asample from the hanging wall of the Okanagan-Eagle River detachment near Salmon Armyields an apatite fission-track age of 66.7 69.6 Ma, which confirms that the Upper Unitcooled before the formation of the ShuswapMCC. In samples from the Middle and LowerUnits, zircon fission-track ages range from 54to 38 Ma, and apatite fission-track ages rangefrom 49 to 28 Ma. Some of these fission-trackages have been obtained on samples that werealso dated using 40Ar/39Ar on K-feldspar. Inthese cases, the fission-track ages confirm andcomplement the cooling curves modeled usingK-feldspar 40Ar/39Ar spectra that indicate arapid cooling event that started at ;45–43 Maat most localities, and after 33 Ma in the im-mediate footwall of the Columbia River andVictor Lake faults. Clasts of Ladybird leuco-granite sampled in the Enderby and Trinity HillsEocene basins yield apparent apatite fission-track ages of 44.5 6 6.4 Ma and 51.1 6 19.6Ma. The modeled thermal history of the En-derby clast based on the fission-track data in-dicates that it was affected by rapid coolingbelow 120 8C at ;50 Ma. The hanging wallof the Columbia River fault, the Clachnacu-dainn complex, yields apatite ages of 39.6 66.2 Ma and 34.4 6 9.6 Ma. Vitrinite reflec-tance obtained on coal-bearing sediments from

these early Tertiary basins indicates that theywere exposed to temperatures ranging from175 to 200 8C (Mathews, 1981). This impliesthat the near-surface geothermal gradient washigh (.50 8C/km) in this shallow basin duringor slightly after its formation. The preserva-tion of relatively old (49 Ma) apatite fission-track ages indicates that such a high geother-mal gradient probably did not last for morethan a few million years after the depositionof the early Tertiary sediments. These featuressuggest that heat was advected toward the sur-face to cause the high near-surface geothermalgradient and that subsequent thermal relaxa-tion was efficient enough to restore the geo-thermal gradient within a few million years.

The two-stage cooling history recorded bythe metamorphic rocks of the Shuswap MCCis interpreted as reflecting two distinct stagesof exhumation. Although erosion was activein the Canadian Cordillera throughout earlyTertiary time, as evidenced by accumulationof sediments in the Alberta foreland basin(Price and Mountjoy, 1970; Yorath, 1991) andin extensional basins formed on top or on thesides of the Shuswap MCC (Church, 1981;Mathews, 1981), the amount of sediment de-posited in these basins cannot account for theremoval of ;15 km of crustal section requiredto exhume the high-grade rocks of the Shus-wap MCC from ;56 Ma to ;45 Ma (Van-derhaeghe et al., 1999a). Therefore, in the ab-sence of a known major basin of this age tothe west of the system, it is likely that exhu-mation was accommodated by tectonicdenudation.

The first cooling event from ;700 8C to;300 8C, recorded in the middle and lowerunits of the Shuswap MCC, occurred after thecrystallization of melt. Structural analysis ofthe lower and middle units shows that the de-velopment of the ductile fabric recording ver-tical thinning occurred in the presence of melt(Vanderhaeghe, 1999; Vanderhaeghe et al.,1999b). The 60–55 Ma syntectonic leucogran-ites emplaced along the detachments constrainthe timing of the onset of extension. The pos-itive dP/dT Clapeyron slope of dehydrationmelting reactions implies that melting shouldbe favored during decompression. In turn,crystallization of the migmatites is likely tooccur after decompression, so that the ;56Ma crystallization age of the leucosome frac-tion in the migmatitic core marks the end ofthe period during which the crust was partiallymolten. The crystallization of the lower unitmigmatites at ;56 Ma is temporally relatedto the first period of rapid cooling constrainedby the hornblende and mica 40Ar/39Ar ages.The thermal stability following the period of



Figure 7. Interpretative cross sections of progressive exhumation of the Shuswap metamorphic core complex, based on cooling historyshown in T-t graphs; each section is a snapshot of the crust at a given time. Two exhumation events are distinguished: One correspondsto orogenic gravitational collapse accommodated by activation of low-angle detachments and ductile thinning of the partially moltencrust between 56 and 48 Ma; the second exhumation event follows a period of thermal stability and is related to regional extensionaccommodated by the activation of high-angle normal faults. Note the possible reactivation of the Columbia River fault, producinglocalized cooling at ca. 33 Ma. MD stands for Monashee Decollement.


VANDERHAEGHE et al.

rapid cooling suggests that thermal reequili-bration was achieved by ;48 Ma. Based onthese data, we suggest that the first period ofrapid cooling from 700–300 8C at an averagerate of ;65 8C/Ma corresponds to thermal re-laxation closely following rapid exhumationof the migmatite core accommodated by duc-tile thinning and activation of low-angle de-tachments (Fig. 7).

The homogeneity of the 40Ar/39Ar ages ob-tained throughout the studied area from micas,and to a lesser extent, from hornblende, indi-cates that the footwall of the detachments be-haved, essentially, as a single unit during thiscooling event. Previous K/Ar Mesozoic agesobtained on rocks and minerals from the hang-ing wall of the detachments suggest that thedetachment separates units that experiencedvery different thermal histories. The detach-ment contains fluid inclusions that have oxy-gen isotopic signatures suggesting a dominantinput of meteoric water (Holk and Taylor,1997). Accordingly, the detachments appear tobe an important thermal boundary within thecrust above which heat was efficiently trans-ferred by fluids circulating in the fractured up-per unit. Notice that although basaltic flowsand dikes are reported in the area at similarages of ;50 Ma (Mathews, 1981), their vol-ume does not seem significant enough tocause a regional thermal anomaly, and we donot consider them in our reconstruction of thethermal evolution of the Shuswap MCC.

The second cooling event, from ;300 8Cto ;100 8C, is recorded by small diffusion do-mains of K-feldspar, zircon, and apatite fission-track data. We correlate this younger eventwith local block tilting and related exhumationin the footwall of high-angle normal faultsthat affected all structural units of the Shus-wap MCC from ;45 to ;33 Ma (Fig. 7). TheVictor Lake fault and the Columbia Riverfault are the youngest of these high-angle nor-mal faults and were active sometime after 45Ma. They cross-cut all lithological units of theShuswap MCC and are probably responsiblefor the rugged relief in their footwall. Exhu-mation in the footwall of high-angle normalfaults is not synchronous at the scale of theShuswap MCC, and a west-to-east migrationof the activation of normal faults is indicatedby K-feldspar 40Ar/39Ar ages and fission-trackdata.

Formation of the Shuswap MCC in itsGeodynamic Context

Three possible mechanisms have been in-voked to explain the transition from crustalthickening to crustal extension in the Cana-

dian Cordillera; namely, (1) the increase in thepotential energy by uplift of the crust relatedto asthenospheric upwelling associated witheither delamination of the thickened litho-spheric root (Bardoux and Mareschal, 1994;Liu, 1996; Ranalli et al., 1989), or to the riseof a mantle plume (Gough, 1986); (2) de-crease in the boundary tectonic force causedby plate motion reorganization (Bardoux andMareschal, 1994; Engebretson et al., 1985;Liu, 1996; Parrish et al., 1988); and (3) thedecrease in the strength of the crust due tothermal weakening and to partial melting(Bardoux and Mareschal, 1994; Liu, 1996;Vanderhaeghe and Teyssier, 1997). Thesemechanisms are not mutually exclusive, butthe timing of cooling and exhumation record-ed in the Shuswap MCC compared to the tim-ing of these mechanisms provides a test oftheir relative influence.

Convergence between the Kula-Farallonoceanic plates and the North American con-tinental plate resulted in the accretion of al-lochthonous terranes to the edge of the NorthAmerican continent until at least ;56 Ma(Engebretson et al., 1985). Plate motion mod-els using Euler poles, assuming a fixed hot-spot reference frame (Engebretson et al.,1985), or using the global plate circuit (Stockand Molnar, 1988) indicate that major platereorganization, characterized by a significantdecrease in the convergence rate between theKula-Farallon plates and North America, oc-curs in the Pacific Basin between 56 and 50Ma, and at 45 Ma in the southwestern Pacific.At the latitude of the southern Canadian Cor-dillera, a drastic change in the force balanceis also inferred at the time of the intersectionof the Kula-Farallon ridge with the edge of thecontinent (Engebretson et al., 1985). Later on,after 37 Ma, the total subduction of the Kulaplate is also likely to have modified the forcebalance at the edge of the Canadian Cordillera(Engebretson et al., 1985). Mantle delamina-tion is inferred to occur at ;50 Ma in orderto explain the current high heat flow anomalyabove the southern Omineca belt, but its tim-ing is poorly constrained (Bardoux and Ma-reschal, 1994).

On the other hand, the coincidence in timeand space between development of the ductilefabric of the high-grade rocks exhumed in theShuswap MCC, the presence of melt, crystal-lization of zircons, and rapid cooling throughthe 500 8C and 300 8C isotherms all suggestthat there is a genetic link between the initi-ation of orogenic collapse and mechanicalweakening of the crust due to partial melting.A similar tectonic evolution is reported southof the Shuswap MCC, for the Bitteroot com-

plex in Idaho (Foster and Fanning, 1997), andnorth of the Shuswap MCC, for the Wolverine-Horseranch Range complex (Plint et al., 1992;Struik, 1993). The three MCCs are structurallyconnected by dextral strike-slip faults. Theserelationships indicate that crustal extensionand formation of MCCs occurred at the scaleof the Canadian Cordillera, and that extensionwas accommodated laterally by transfer strike-slip zones. These features could be explainedeither by (1) extension in a context of obliqueconvergence with dextral strike-slip motionbeing associated with opening of large exten-sional corridors, or alternatively by (2) oro-genic collapse of the thickened hinterland ofthe Canadian Cordillera independently of theplate convergence vector. Given the uncertain-ty in the timing of all these events, it is cur-rently difficult to propose a definite model onthe genetic relationships between thinning ofthe Cordilleran orogenic crust and global scaleplate motion or asthenospheric mantleupwelling.

ACKNOWLEDGMENTS

This work was supported by NSF-EAR 9526915.Neutron irradiations were facilitated by the Austra-lian Institute of Nuclear Science and Engineeringand the Australian Nuclear Science and TechnologyOrganization. Reviews from Peter Doughty, DavidFoster, and Randy Parrish helped the clarity and thelength to improve the manuscript. This study ben-efited from the input of many people. Matevz Lor-encak shared a few adventures in British Columbiaand, more importantly, the rock load (threatenedwith a Swiss army knife). O.V is grateful to themembers of the Geochronology and Isotope Geo-chemistry Group at RSES, Shane Paxton and JohnMya for sharing all the tricks of mineral separationthat were worth a few cakes at tea time, and toRobyn Maier for her patience and guiding rolethrough the argon labyrinth. Sue Keay in particular,but also another bunch of Aussie mates, blokes, andchicks are thanked for their warm hospitality andfresh beers.

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Armstrong, R.L., 1982, Cordilleran metamorphic core com-plexes—From Arizona to southern Canada: AnnualReview of Earth and Planetary Science, v. 10,p. 129–154.

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