GSA Bulletin: Structural and kinematic evolution of the ... · 1995). Lithologic divisions (Churkin...

ABSTRACT

The Yukon-Tanana terrane, the largesttectonostratigraphic terrane in the northernNorth American Cordillera, is polygeneticand not a single terrane. Lineated and foliated(L-S) tectonites, which characterize the Yukon-Tanana terrane, record multiple deforma-tions and formed at different times. We docu-ment the polyphase history recorded by L-Stectonites within the Yukon-Tanana upland,east-central Alaska. These upland tectonitescompose a heterogeneous assemblage of de-formed igneous and metamorphic rocks thatform the Alaskan part of what has been calledthe Yukon-Tanana composite terrane. Webuild on previous kinematic data and estab-lish the three-dimensional architecture of theupland tectonites through kinematic andstructural analysis of more than 250 orientedsamples, including quartz c-axis fabric analy-sis of 39 samples.

Through this study we distinguish alloch-thonous tectonites from parautochthonoustectonites within the Yukon-Tanana upland.The upland tectonites define a regionally co-herent stacking order: from bottom to top,they are lower plate North American parau-tochthonous attenuated continental margin;continentally derived marginal-basin strata;and upper plate ocean-basin and island-arcrocks, including some continental basementrocks. We delineate three major deformationevents in time, space, and structural levelacross the upland from the United States–Canada border to Fairbanks, Alaska: (1) pre-Early Jurassic (>212 Ma) northeast-directed,apparent margin-normal contraction that af-fected oceanic rocks; (2) late Early to earlyMiddle Jurassic (>188–185 Ma) northwest-directed, apparent margin-parallel contrac-tion and imbrication that resulted in juxta-

position of the allochthonous tectonites withparautochthonous continental rocks; and (3)Early Cretaceous (135–110 Ma) southeast-directed crustal extension that resulted in expo-sure of the structurally deepest, parautoch-thonous continental rocks. The oldest eventrepresents deformation within a west-dipping(present coordinates) Permian-Triassic sub-duction zone. The second event records Earlyto Middle Jurassic collision of the arc and sub-duction complex with North American crust,and the third event reflects mid-Cretaceoussoutheast-directed crustal extension. Eventsone and two can be recognized and correlatedthrough southern Yukon, even though this re-gion was affected by mid-Cretaceous dextralshear along steep northwest-striking faults.Our data support a model of crustal assemblyoriginally proposed by D. Tempelman-Kluitin which previously deformed allochthonousrocks were thrust over parautochthonousrocks of the attenuated North American mar-gin in Middle Jurassic time. Approximately 50m.y. after tectonic accretion, east-centralAlaska was dissected by crustal extension, ex-posing overthrust parautochthonous strata.

INTRODUCTION

The Yukon-Tanana terrane, the largest tectono-stratigraphic terrane in the northern North Ameri-can Cordillera, was originally defined as an as-semblage of rocks containing a strong L-S(lineated and foliated) tectonite fabric in contrastwith adjacent rocks (e.g., Coney and Jones, 1980).Early studies showed that the Yukon-Tanana ter-rane includes rocks that have distinctly differentgeologic histories (e.g., Churkin et al., 1982; Fos-ter et al., 1985; Aleinikoff et al., 1987), leadingHansen (1990a) to suggest that the terrane be con-sidered as polygenetic rather than as a single co-herent crustal entity. Structural studies of thesetectonites show that they record multiple defor-mations, and their characteristic L-S fabric formed

at different times (e.g., Hansen et al., 1991; Dusel-Bacon et al., 1995). Thus we refer to these rocksas the Yukon-Tanana composite terrane. Wide-spread mid-Paleozoic to Mesozoic magmaticcrystallization and metamorphic cooling ages(Mortensen, 1992) indicate that this was an im-portant period in the assembly of this large tract ofcrust. Despite evidence of a complicated poly-phase history, however, some workers, on the ba-sis of geochemical and isotopic characteristics,treat the Yukon-Tanana composite terrane as auniform assemblage with a simple geologic his-tory (e.g., Gehrels et al., 1990, 1991; Mortensen,1992; Nelson and Mihalynuk, 1993; Mihalynuket al., 1994). Although geochemical or isotopictracers provide useful information about crustalorigin and average mantle-separation ages of theircontinental rocks, these data do not explain howcrust is assembled, or reassembled.

Our goal is to understand the late Paleozoic toMesozoic assembly of east-central Alaska asrecorded by L-S tectonites of the Yukon-Tananaupland, east-central Alaska (referred to hereafteras the upland tectonites, on the basis of their geo-graphic exposure in the Yukon-Tanana upland).The upland tectonites compose a heterogeneousassemblage of deformed igneous and metamor-phic rocks that together form the Alaskan part ofwhat has been called the Yukon-Tanana compos-ite terrane. We document the polyphase historyrecorded within the L-S tectonites and establisha structural framework within which pressure-temperature-time (P-T-t) and geochemical datacan be examined. We build on the kinematic database initiated by Hansen et al. (1991) and estab-lish the three-dimensional architecture of the up-land tectonites through kinematic and structuralanalysis of more than 250 oriented samples, in-cluding quartz c-axis fabric analysis of 39 sam-ples. From these data we define kinematic do-mains and determine temporal relations withinand between domains. This kinematic frame-work, coupled with published thermobarometricand thermochronometric studies, allows us to

211

Structural and kinematic evolution of the Yukon-Tanana upland tectonites,east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly

V. L. Hansen* Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275C. Dusel-Bacon U.S. Geological Survey, M.S. 901, 345 Middlefield Road, Menlo Park, California 94025

GSA Bulletin; February 1998; v. 110; no. 2; p. 211–230; 8 figures.

*e-mail: [email protected]

constrain the geometric and temporal stackingorder of the tectonites, and to identify contrac-tional versus extensional relations within and be-tween the kinematic domains through time.

Although we are ultimately interested in under-standing the spatial and temporal domains withinall of the upland tectonites, we begin our analysisin the eastern Eagle-Tanacross region (Figs. 1 and2). After establishing a structural framework forthat region, we extend our discussion to specificareas to the west. Published thermobarometricand thermochronometric data allow us to refinethe geologic history of the entire upland and cor-relative tectonites in Canada (Fig. 1).

BACKGROUND

The Yukon-Tanana upland consists of moder-ately dissected hills and mountains that escapedregional glaciation. The metamorphic bedrockcomprises a mass of lower greenschist to am-phibolite facies L-S tectonites, intruded by Meso-zoic and Tertiary granitoids and overlain by klip-pen of oceanic rocks (Seventymile terrane) (e.g.,Foster et al., 1994). Our term upland tectonite isgenerally synonymous with Yukon-Tanana ter-rane of older literature, but we introduce this newnomenclature to emphasize the heterogeneouscharacter of this association of crystalline rocks.For clarification we refer to specific terrane des-ignations where appropriate; however, we use theterm assemblage because terrane has implica-tions that may not be appropriate. Early workersrecognized a stacking order among the tectonites,rocks of continental affinity being at the base andoceanic-arc rocks being at high structural levels(e.g., Coney et al., 1980). Initially, these rockswere interpreted as a single structural packagebecause they display pronounced L-S tectonitefabrics, which inhibit identification of primarystructures and lithology. Therefore, early studiesfocused on determining the age of ductile defor-mation, with interpretations ranging from Paleo-zoic (e.g., Dusel-Bacon and Aleinikoff, 1985) toCretaceous (e.g., Nokleberg et al., 1989). Despitetheir apparent structural similarity, kinematic andthermochronometric studies within the uplandtectonites revealed significantly different tectonichistories (e.g., Hansen, 1990a; Hansen et al.,1991; Pavlis et al., 1993; Dusel-Bacon et al.,1995). Lithologic divisions (Churkin et al., 1982)correspond, in general, with metamorphic, kine-matic, and temporal domains, illustrating thatthese rocks are not a single tectonic entity with acommon history.

Upland tectonites are divisible into five distinctpackages (Fig. 2) on the basis of lithology and ge-ologic history (e.g., Churkin et al., 1982; Hansenet al., 1991): (1) oceanic volcanic and sedimen-tary rocks of the Seventymile assemblage; (2) tec-

tonites of oceanic and marginal basin origin in theTaylor Mountain assemblage; (3) tectonites de-rived from siliciclastic marginal-basin strata com-posing the Nisutlin assemblage; (4) paragneissand peraluminous orthogneiss of continentalaffinity (herein referred to as the orthogneiss as-semblage); and (5) tectonized parautochthonouscontinentally derived metasedimentary rocks ofthe Chena River and Fairbanks subterranes.

Seventymile rocks (Fig. 2), correlative withthe Slide Mountain terrane in Canada (Fig. 1)(Harms et al., 1984), include weakly metamor-phosed greenstone, Mississippian to Triassicmetasedimentary rocks, and serpentinized perid-otite (Foster, 1992; Foster et al., 1994). They areinterpreted as disrupted mid-Paleozoic to lowerMesozoic oceanic crust (Harms, 1986; Coney,1989; Roback et al., 1994) and parts of an upperPaleozoic oceanic arc (Nelson et al., 1989).Rocks of the Seventymile assemblage are gener-ally not penetratively deformed.

Tectonites of the Taylor Mountain assemblage,correlative with the Teslin and Anvil assemblagesin Yukon (Hansen, 1990a), consist of high-P am-phibolite facies garnet-amphibolite, biotite ±hornblende ± garnet gneiss and schist, marble,quartzite-metachert, and pelitic schist, and smallbodies of hornblendite, metadiabase, pyroxenite,and serpentinized peridotite (Dusel-Bacon et al.,1995). Slide Mountain and Teslin rocks arebroadly correlative, differing mostly in crustallevel of deformation and grade of metamorphism(Hansen, 1990a; Wheeler and McFeeley, 1991).We regard the Seventymile and Taylor Mountainassemblages as sharing protolith types, but dif-fering in crustal level of deformation. The TaylorMountain assemblage yields latest Triassic toEarly Jurassic metamorphic cooling ages, andthey are intruded by the postkinematic TaylorMountain batholith (titanite U-Pb age of 212 Ma)(Hansen et al., 1991; Dusel-Bacon et al., 1995).

The Nisutlin assemblage (Birch Hill and ButteYukon-Tanana composite subterranes east of Fair-banks; Pavlis et al., 1993), interpreted as tec-tonized marginal-basin strata (Tempelman-Kluit,1979), includes a sequence of greenschist faciessiliciclastic metasedimentary rocks, felsic tomafic metavolcanic rocks, and marble. Limitedbiostratigraphic data indicate that protoliths are inpart Devonian to Mississippian and contain EarlyPennsylvanian to Early Permian and Late Triassicconodonts (Tempelman-Kluit, 1979; Mortensen,1992; Dusel-Bacon et al., 1993; Foster et al.,1994). Metavolcanic rocks are Devonian in age(U-Pb ages, J. N. Aleinikoff and W. J. Nokleberg,1989, unpublished data, cited in Dusel-Bacon etal., 1993; Smith et al., 1994).

The orthogneiss assemblage (also called theLake George subterrane of the Yukon-Tananacomposite terrane, Robinson et al., 1990; Pavlis

et al., 1993; Dusel-Bacon et al., 1995) consists ofpelitic schist and gneiss, quartzite, and minoramphibolite that have been intruded by Devo-nian to Mississippian peraluminous granitoids(now augen orthogneiss). Granitoids show Pre-cambrian U-Pb inheritance and yield εNd valuesconsistent with derivation from cratonal NorthAmerican basement (Aleinikoff et al., 1981,1986; Mortensen, 1992; Helsop et al., 1995;Dusel-Bacon and Aleinikoff, 1996). In general,these tectonites yield Early Cretaceous coolingages (Rb-Sr, 40Ar/39Ar, hornblende, biotite, mus-covite; e.g., Hansen, 1990a; Hansen et al., 1991;Pavlis et al., 1993).

Strata of the Fairbanks subterrane of theYukon-Tanana composite terrane are interpretedas deposited within a continental margin setting,correlative with Selwyn basin strata in Yukon,and hence parautochthonous to North America(Murphy and Abbott, 1995).

Generally, massive Late Triassic to Early Ju-rassic granitoids (e.g., Taylor Mountain batholithand Mount Veta pluton, Fig. 3) only intruderocks of the Seventymile, Taylor Mountain, andNisutlin assemblages over the entire upland(Foster, 1992). These bodies represent arc granit-oids within an oceanic or marginal-basin settingand have been correlated with the Stikinia arc onthe basis of Pb isotopes (Aleinikoff et al., 1987;Dusel-Bacon et al., 1995).

Mid-Cretaceous plutons, generally massive,intermediate in composition, and corundum-normative, are most abundant in the central up-land; they intrude each of the tectonite assem-blages, and display mylonitic textures only alongmajor high-angle faults (Foster et al., 1994). Lat-est Cretaceous to early Tertiary plutons intrudeprimarily the northwestern upland, are generallyfelsic (including some two-mica granites), andform small massive plutons that have well-developed contact aureoles (Dusel-Bacon et al.,1989, 1993). Post-mid-Cretaceous, northeast-striking, high-angle faults disrupt the upland lo-cally; left-lateral offset along most faults is rela-tively minor (Griscom, 1979), but dip-slipmovement may be significant in preservinghigh-level assemblages (Newberry et al., 1995;Solie et al., 1995).

Previous studies demonstrated that the uplandtectonites, and correlative rocks in Yukon, recordtwo distinct phases of ductile deformation: a pre-Early to Early Jurassic and an Early Cretaceousevent. Early deformation is dominantly contrac-tional with evidence of dextral strike-slip shearpreserved locally in Yukon. The Early Cretaceousdeformation records crustal extension in easternAlaska and dextral strike-slip in southern Yukon(Hansen et al., 1989, 1991; Pavlis, 1989; Pavlis etal., 1993). Our data illustrate that the upland tec-tonites also variably record Early to Middle

V. L. HANSEN

212 Geological Society of America Bulletin, February 1998

Geological Society of A

merica Bulletin,February 1998

213

**

* *

*

*

✪

✪

✪

✪

200 km

Yukon B.C.

WhWl

Fb

Alas

kaYu

kon

141°

60°

Cretaceous granites

Triassic to Early Jurassic granitoids

marginal-basin siliciclastic rocks (Dorsey assemblage)

oceanic & arc rocks—upper-plate (Seventymile, Chatanika, & Slide Mountain assemblages)

sheared and metamorphosed siliciclasticmarginal basin sediments & volcanics—could be either upper- or lower-plate, or both (Nisutlin assemblage)

sheared & high P-T metamorphosed oceanic and marginal basin rocks—upper-plate(Teslin-Taylor Mountain assemblage)

parautochthonous North American continental margin, locally hosts Devonian to Mississippian orthogneiss; orthogneiss assemblage-O;Cassiar assemblage-Ca (lower-plate; yields Cretaceous metamorphic cooling ages)

EXPLANATION

volcanic arc (pre-Permian host rocks and Permian to Early Jurassic volcanic rocks; Stikinia)

Quesnel volcanic arc

blueschist/eclogite (Erdmer, 1987, 1992; Erdmer et al., 1996)

Devonian-Mississippian granitoids with pre-mid Jurassic metamorphic cooling ages (Nisling-Tracy Arm)

Devonian-Mississippian granitoids with post-mid Jurassic metamorphic cooling ages (parautochthonous)

Cache Creek accretionary complex & related rocks

continental affinty host rocks with Devonian to Mississippian granitoids, local Permian(?) & Triassic-Early Jurassic arc rocks (upper-plate; yield pre mid-Jurassic metamorphic cooling ages; Nisling-Tracy Arm assemblage)

✇

✇

✇

✇✇ ✇

✇ Tintina fault

Denali fault

MACKENZIE FOLDAND

THRUSTBELT

YUKON

AK

map area B.C.

YK

***up

per-p

late

lowe

r-pla

te

Little Salmon Lake

Laberge-Quiet LakeTeslin

Present study area

CaTSZ

Tectonic assemblage map

O

OO

O

O

O

✪

✪

✪

Figure 1. Simplified tectonic assemblage map (modified from Hansen et al.,1991, and data sources listed therein). Abbreviations: Fb—Fairbanks, Wh—Whitehorse, Wl—Watson Lake, TSZ—Teslin suture zone (see text for explana-

tion). Unlabeled area around Fairbanks is alluvium and unlabeled area northof Denali fault in Alaska comprises various terranes not discussed in this paper.Inset map shows location of detail.

Jurassic deformation that is kinematically distinctfrom the previously recognized deformations.

STRUCTURAL AND KINEMATICRELATIONS

Scope of This Study and Methods of Analysis

We describe the structural relations in fourmain areas; from east to west, these are (1) theeastern Eagle-Tanacross quadrangles; (2) MountWarbelow; (3) Molly Creek; and (4) the southeastBig Delta quadrangle (Figs. 2, 3, and 4). The east-ern Eagle-Tanacross region, the largest and best

exposed, preserves three distinct kinematic do-mains. Data from the other regions allow us to ex-tend the spatial limits of these three domains, andthey provide additional evidence for the geomet-ric, kinematic, and temporal relations between thedomains. The southeast Big Delta area allowsclose spatial correlation with published kinematicand thermochronometric data to the west.

Lithologic contacts generally parallel folia-tion (S), which dips moderately to gently and de-scribes open to gentle upright warps with east-west axial traces (Cushing and Foster, 1984;Foster et al., 1985, 1994). These warps affecteach of the assemblages and therefore they

formed late with respect to final structural juxta-position. Local preservation of the structurallyhighest rocks of the Seventymile assemblage islikely due to their position in broad synformalwarps. Similarly, the deepest structural level thathas orthogneiss assemblage is best exposed inthe core of broad antiformal warps.

The tectonites contain monoclinic or triclinicfabrics composed of foliation (S), elongation lin-eations (Le), textural asymmetries, and locally,mesoscopic folds. Compositional layering andalignment of platy minerals define S; roddedquartz, smeared or crenulated mica, and elongateor aligned feldspar or amphibole define Le. Le (Xaxis) is contained within S (XY plane).

Because the upland tectonites comprise a massof L-S tectonites, in order to understand theirevolution we delineated structural domains as de-fined by Le orientation and associated shear-sense indicators. Because S dips gently to mod-erately throughout the upland, and because Le iswithin S, Le orientation can be represented by aline in map view. We determined Le distributionfrom published maps, unpublished U.S. Geolog-ical Survey field notes, and our field measure-ments. We examined rocks for kinematic indica-tors in the field after carefully determining thattectonite exposures were structurally in situ andnot affected by frost heaving. We collected ori-ented samples for microstructural analysis usingmethods described by Hansen (1990b).

Kinematic Indicators

Mesoscopic, microscopic, and petrofabrickinematic indicators viewed within the motionplane (XZ) of the upland tectonites allow us todefine kinematic domains.

Microstructures. Fabric asymmetry withinthe motion plane records the direction of tec-tonic transport during ductile deformation (e.g.,Eisbacher, 1970; Berthé et al., 1979; Hanmer andPasschier, 1991). No asymmetric fabrics indica-tive of noncoaxial strain were observed in theYZ plane. Mesoscopic and microscopic motion-plane asymmetries used in this study includeasymmetric folds with fold axes perpendicular toLe; shear bands (Platt and Vissers, 1980); rare ro-tated clasts; type I and II S-C fabrics (Lister andSnoke, 1984); mica and amphibole fish; asym-metric pressure shadows; sigma porphyroclasts;obliquity between spaced foliation; and grain-shape–preferred orientation of dynamically re-crystallized quartz.

Quartz c-axes. Asymmetric quartz c-axis fab-rics also record ductile shear sense. We selected36 of the most quartz-rich samples for quartzpetrofabric analysis, which was carried out opti-cally on XZ and XY sections from each sampleusing a universal stage (Turner and Weiss, 1963).

V. L. HANSEN


oceanic, arc, & marginal basin tectonites, high P-T (Taylor Mountain assemblage)

Tok

Delta Junction

Fairbanks

63°

141°142°

Eagle

64°

143°

144°

64°

147°

146°

145°

147°

146°

144°

145°

143°

142°65°

0 30

kilometers

Big Delta

ch

Quaternary surficial deposits

Cretaceous-Tertiary igneous rocks & sedimentsCretaceous granitoids

Late Triassic to Early Jurassic granitoidsoceanic & associated sedimentary strata (Seventymile [S] and Chatanika [ch] assemblages)

marginal-basin strata (Nisutlin assemblage east of ~144°; Birch Hill & Butte "subterranes" west of ~144°)continental affinity paragneiss & peraluminous augen orthogneiss ~340 Ma orthogneiss shown by x-pattern in Figs. 2 & 3 (orthogneiss assemblage)parautochthonous continental strata (Chena River & Fairbanks "subterranes")

stru

ctur

al le

vel

upla

nd te

cton

ites

lowest

highest

ContactFault, barbs on upper-plateDisplacement direction in structurally high rocks

TownsMajor roads

parautochthonous Wickersham assemblage

parautochthonous pre-Tertiary sediments

EXPLANATION

Alaska Yukon British Columbia

Tintina fault

S

TM

N

O

S

SS

S

S

TM

TM

N

N

N

N

N

N

N

N

O

O

O

O

O

O

O

O

Fig. 2Fig. 2

S

O

Figure 2. Simplified tectonic assemblage map of the Yukon-Tanana upland (modified fromFoster, 1992; Pavlis et al., 1993; Dusel-Bacon et al., 1995, and data sources therein) and kine-matic data. Kinematic data west of 145° are from Pavlis et al. (1993); data east of 145° are fromHansen et al. (1991) and this study. Inset shows location in Alaska; irregular box shows area ofFigure 3.

We measured a minimum of 250 c-axes in eachsection, unless noted. Data from both sections areshown with c-axes rotated into a single equal-area, lower-hemisphere composite plot (n > 500)with the XZ plane as the perimeter circle. We at-tempted to achieve coverage both regionally andwith respect to structural domains. The c-axis fab-rics from each sample (Fig. 5) indicate that Le isan elongation lineation (i.e., that it is parallel to X)because the fabric girdle for each sample inter-sects S normal to Le (Behrmann and Platt, 1982).

Quartz c-axis patterns are a function of intra-crystalline slip mechanism, temperature, watercontent, strain rate, deformation path, and totalstrain (Fig. 5kk; Tullis et al., 1973; Nicolas andPoirer, 1976; Lister and Hobbs, 1980; Hobbs,1985; Law, 1990). Asymmetric single-girdle fab-rics can form as the result of noncoaxial compo-nent of strain, the sense of asymmetry recordingthe sense of bulk noncoaxial shear (Fig. 5ll),whereas fabric symmetry indicates bulk pureshear (Fig. 5mm; Schmid and Casey, 1986). Be-

cause c-axis plots record finite strain history, thecomplete fabric diagram could be a result of sev-eral nonunique strain histories. For example, abulk coaxial fabric could record a single coaxialflattening event, or two sequential, coplanar butopposite vergence, noncoaxial ductile shear de-formations. Quartz slip systems also show a rela-tive temperature dependence, that is, low temper-ature basal <a> slip, rhombohedral <a>, prism<a>, and the rarely observed prism <c> slip atprogressively higher temperatures (Fig. 5kk;

YUKON-TANANA UPLAND TECTONITES, ALASKA

Geological Society of America Bulletin, February 1998 215

64°

64°30'

63°30'

141°142°143°

144°63°30'

144°

143°

142°

64°

TanacrossTok

Eagle

145°

ET221ET222

TnMH

EaBD

ET137

ET136Taylor Mtn. batholith

Mt. Warbelow

* Mt. Veta

*

St

ET224

110Ms111Bt

southeastBig Delta

St,Ky

St,Ky

St,Ky

StSt

St

Ky

Ky

St

St,Ky,Si

St

Ky

Eagle-TancrossFig. 4a

Molly CreekFig. 4c

S

S

S

S

TM

TM

TM

TM

N

N

NN

N

N

NN

O

O

O

O

O

OO

O ?

?

Taylo

r Hwy

Figure 3. Simplified tectonic assemblage map of east-central Alaska, including parts of the Big Delta (BD), Eagle (Ea), Mount Hayes (MH), andTanacross (Tn) 1° × 3° sheets (modified from Foster, 1992; Dusel-Bacon et al., 1993, and sources therein) and kinematic data, quartz c-axis local-ities, and metamorphic cooling ages (see Fig. 4 legend). Additional quartz c-axis locations are shown in Figure 4. Pelitic mineral assemblagesshown: Ky—kyanite, Si—sillimanite, and St—staurolite (Dusel-Bacon et al., 1995). Dark gray depicts ultramafic (peridotite or serpentine) bod-ies. Map units and symbols are the same as in Figure 2.


ET234

TH87

64°30'

64°15'

64°

141°141°30'

63°45'

TH7TH60TH106

TH23

TH124

TH80

TH85

ET117TH89

ET102

ET110

ET5

ET25ET20

ET23ET18

ET122

ET100

0 15km

204Hbl187Bt

TH11

EXPLANATIONLe trend

direction of displacement in structurally higher rocks; may be composite from several adjacent field stations

sample location & displacement of strucutrally higher rocks(determined from asymmetric quartz c-axis fabrics)sample location & trend of elongation for coaxial strain (determined from symmetric quartz c-axis fabrics)

TH60

TH85

204Hbl187Bt

location of metamorphic cooling ages; Hbl = hornblende, Ms = white mica, Bt = biotite, (40Ar/39Ar); Ttn = titanite (U/Pb) (sources in Dusel-Bacon et al., 1995)

10.2-12.7 kbar 690-740°C location & values of P-T (Dusel-Bacon et al.,1995)

low-angle fault, teeth on upper plate

within-assemblage low-angle fault, teeth on upper plate; dashed where inferred (from Cushing et al., 1984)

possible low-angle fault, or gradational metamorphic contact; teeth on structurally higher/lower grade rock

low-angle ductile shear zone; exposed overbroad area, therefore approximately located

ET100 locations referred to in text

8.6-9.7 kbar665-730°C

ET50TH51

ET46

10.8-11.7 kbar, 595-635°C6.5-8.8 kbar, 520-560°C

A

A'

4500'

2500'

0 2km 0 2mi

109Ms109Bt ET46ET50

~7.5kbar, 540°C 166Bt

Penetrative L-S fabric domains

NW/SE Le; SE-vergent shear

NE/SW LeNW/SE Le; NW-vergent shear

discrete SE-vergent shear zones (youngest shear)trace of foliation

A(SW) A'(NE)

No vertical exaggeration

orthogneiss paragneiss

~10kbar; 600°C 119HblTH51

6.8-9.0 kbar640-700°C 107Bt, K-Ar

8.9-10.8 kbar630-670°C

64°

143°30'64°15'

ET206

ET204

ET211

106h

ET208

ET158

ET162

ET216

ET157

0 10km

135 Ttn

ET16

ET2ET17

9.0-11.8 kbar 570-640°C

A

10-12 kbar, 645-715°C7.5-10 kbar, 555-660°C

N

NN

N

S

TM

TM

TMTM

TM

S

S

S

OO

O

C

B

Alas

kaYu

kon

?

?

Taylor

Hwy

186Ms191Ms

TH39186Bt188Hbl

185Ms188Bt

194Hbl

185Bt, 187Hbl

201Hbl

213Hbl

166Bt

119Hbl

109Ms109Bt

185Ttn179Hbl

Figure 4. Geologic and kinematic maps of the regions identified in Figure 3. (A) Eastern Eagle and Tanacross map area. (B) Cross section A–A′in southeast corner of A; note that the patterns in B are structural facies (legend shown), not lithologic units, and the scale is not the same as thatin A. (C) Molly Creek region. Units are the same as in Figures 2 and 3. Pressure-temperature and age data from Dusel-Bacon et al. (1995) andsources therein.


ET2 (600) 0.5; 0.0-5.0N22W,18

TTSE

ET5 (500) 0.5; 0.5-5.5N50W,17

TTSE

ET20 (700) 1; 0.5-5.5N45W,15

TTSE

ET17 (550) 0.5; 0.5-3.5N22W,8

COX

ET23 (600) 1; 0.5-7.5N30W,15

TTNW

ET46 (500) 1; 0.5-7.5N10W,0

TTNW

ET102 (550) 0.5; 0.5-4.5N40W,0

TTNW

ET25 (600) 1; 0.5-6.6N30W,20

COX

S47E,3ET-50 (500) 1; 0.5-6.9

ET122 (501) 0.5; 0.5-4N75W,8

TTNW

S80E,20

COX

ET158 (600) 0.5; 0.5-3.4ET136 (551) 0.5; 0.5-4.9N70W,3

COX

N42W,10

TTNW

ET117 (400) 1.5; 0.5-11.4

N52E,12

COX

ET157B (600) 0.5; 0.5-2.5

N25E,20

TTNE

ET216 (600) 0.5; 0.5-4.8S84E,25

TTSE

ET206 (600) 1; 0.5-10.6N40W12

TTSE

ET204 (600) 0.5; 0.5-4.3N0E,11

TTNE

ET221 (513) 0.75; 0.5-5.4

TH60 (500) 0.5; 0.5-3.5S40W,11

TTNE

TH51 (450) 0.5; 0.5-5N30W,0

TTNW

TH39 (500) 0.5; 0.5-4.0S84E,18

COX

TH85 (600) 0.5; 0.5-4.4N56W,11

COX

TH80 (550) 1; 0.5-6.0N67W,0

TTNW

TH89 (700) 0.5; 0.5-4.0N5E,20

TTNE

TH106 (447) 1; 0.5-9.5S35W,4

TTNE

TH7 (450) 0.5; 0.5-4.5N50E,20

TTNE

S9W,28

TTNE

ET222 (600) 0.5; 0.5-3.7

TH23 (500) 0.5; 0.5-3.9S65E,58

TTNWCOX

ET18 (600) 0.5; 0.5-4.0N85W,0

COX

N50W,6

TTNWCOX

TH11 (501) 0.5; 0.5-4.5

ET137 (380) 0.5; 0.5-4.0N50W,32

TTNW

ET110 (500) 0.5; 0.5-5.5S70E,55

TTNW

S70E,21

TTSE

ET162 (600) 0.5; 0.5-5.0

TH124 (550) 0.5; 0.5-4.1S57W,8

TTNECOX

N40W,17

TTSE

ET208 (523) 0.5; 0.5-5.0

N15E,2

TTNE

ET224 (281) 1; 0.5-12.5

TTSETTNW

a b c d e f

kjihg l

r

w

cc

hh ii jjggffee

dd

xs

n

t

y z

u

o p

v

aa bb

qm

basal <a> (low T)

rhomb <a> (medium T)

prism <a> (med-high T)

prism <c>(high T, or2 ⊥ sheardeformations)

foliation

slip planes

Le

kk ll mm

fabric asymmetry fabric symmetry

Figure 5. (a–jj) Equal-area, lower-hemisphere stereonet projections of quartz c-axis petrofabrics viewed within the motion plane (plane nor-mal to S, which contains elongation lineation Le); sample locations in Figures 2 and 3. Foliation shown by horizontal line, Le shown by dot; graymarks pole free (<0.5%) region; arrows show shear sense interpreted from fabric asymmetry (noncoaxial) or fabric symmetry (coaxial shear).Data at top right of each diagram includes: sample number, number of c-axes measured (in parenthesis), contour interval (%), range of contourvalues (%), and trend and plunge of Le. Letters in the lower-left corner give interpreted shear sense in geographic terms: TTNE—top-to-north-east; TTNW—top-to-northwest; TTSE—top-to-southeast; COX—coaxial shear; no shear-sense determined. Plots were contoured usingSTEREOPLOTII (Mancktelow, 1993). (kk) Lower-hemisphere stereoplot showing the location of quartz c-axes with respect to foliation andelongation lineation corresponding to different active slip systems (modified from Schmid and Casey, 1986). (ll) Asymmetric fabric indicative ofsimple shear or high strain; (mm) symmetric fabric indicative of coaxial shear.

Hobbs, 1985). Prism <c> slip occurs at high-temperature subsolidus conditions (Jessel andLister, 1990), although it might also be activatedunder greenschist facies conditions as a result ofpolyphase shear (Oliver, 1996).

Eastern Eagle-Tanacross Region

Our most detailed analysis focuses on the east-ern Eagle-Tanacross region, which is relativelywell exposed, has a low volume of Cretaceousgranitoids, and is accessible along the TaylorHighway (Fig. 4). The Taylor Mountain assem-blage dominates the area; the Seventymile andNisutlin assemblages are exposed in the north,east and west, and the orthogneiss assemblage isexposed in the south.

The Seventymile assemblage is in fault contactwith structurally lower rocks, except east of theTaylor Mountain batholith, where Seventymilegreenstone, metachert, and phyllite may gradeinto higher grade Taylor Mountain rocks (Figs. 2and 3). As discussed by Dusel-Bacon et al.(1995), this contact is shown as a fault in manyrecent papers; however, Foster (1969) noted thatthe contact could be either a gradational meta-morphic contact or a structural contact.

The Nisutlin–Taylor Mountain contact con-sists of a regionally extensive low-angle brittle-ductile thrust fault (Foster et al., 1985). Structuralinterleaving of ultramafic, Taylor Mountain, andNisutlin rocks near the international border at64°N indicates the local complexity of this con-tact (Fig. 4). The contact as exposed along theTaylor Highway is marked by a 1–3-km-wide(map view) moderately south dipping fault zone(Foster et al., 1985). Displacement on the faultzone postdates penetrative ductile shear of theTaylor Mountain and Nisutlin assemblages. A186 ± 2 Ma 40Ar/39Ar muscovite plateau agefrom an undeformed dike that crosscuts tectonitefoliation of the Taylor Mountain assemblage(Cushing et al., 1984; Cushing, 1984) establishesa minimum age for ductile deformation. Evi-dence for the timing of fault juxtaposition of Tay-lor Mountain tectonites above Nisutlin tectonitesis provided by a biotite plateau age of 187 ± 2 Mafrom actinolite-biotite schist formed within ashear zone that is parallel to the south-dippingthrust fault (Cushing et al., 1984).

The nature of the Taylor Mountain–ortho-gneiss assemblage contact is controversial (seediscussion in Dusel-Bacon et al., 1995), in partbecause of poor exposure, yet it is a critical lineof evidence in the tectonic history. If the ortho-gneiss assemblage is structurally above the Tay-lor Mountain assemblage, it provides evidencefor major Early Cretaceous contraction withinthe upland (Nokleberg et al., 1989; Mortensen,1992). However, if Taylor Mountain assemblage

is structurally above the orthogneiss assemblage,then evidence for Early Cretaceous contractionin this part of the upland diminishes (Hansen,1990a; Hansen et al., 1991). If the Taylor Moun-tain assemblage grades stratigraphically down-ward into the orthogneiss assemblage with nosignificant discontinuity (e.g., Mortensen, 1992),then these two tectonic units would appear toshare a common tectonic history. Early MiddleJurassic metamorphic cooling ages from theTaylor Mountain assemblage and mid-Creta-ceous cooling ages from orthogneiss assemblage(e.g., Hansen et al., 1991) are difficult to recon-cile with an upper plate orthogneiss position.Structural and kinematic data presented in thefollowing favor a lower plate position for the or-thogneiss assemblage, and a lack of stratigraphiccontinuity between the orthogneiss and TaylorMountain assemblages.

Two Le populations are found in the easternEagle-Tanacross region (Fig. 4). A domain ofnortheast (to north-northeast) Le is confined tothe Taylor Mountain and Nisutlin assemblages,and is present locally within the Seventymile as-semblage. Northwest Le is present at the deepeststructural level of the Taylor Mountain assem-blage and occurs in the orthogneiss assemblage,locally along the contact between Nisutlin andTaylor Mountain assemblages, and at the base ofsome Seventymile klippen. Notably, intrusivebodies, including Late Triassic plutons, lack tec-tonite fabrics, with the exception of the peralumi-nous orthogneiss units.

Le orientation changes as a function of localand regional structural level. Both northeast andnorthwest Le are present at ET100, an ~10-m-high tectonite exposure that dips gently south(Fig. 4). Here Le trends northeast at the top of theoutcrop but northwest at the base, where asym-metric folds and sheared quartz veins record top-to-the-northwest shear. Regionally, northeast Leis preserved at high structural levels within theTaylor Mountain assemblage (Fig. 4A). North-east Le is also preserved structurally high in theaxis of an open synform near the contact betweenTaylor Mountain and orthogneiss assemblages(Fig. 4B), and northwest Le occurs structurallylow within antiformal warps.

Because the two domains are defined by Le itis difficult to envision how both domains mighthave formed in a single deformation event. Thusthe question becomes how many deformationsare represented, and how are the deformations re-lated in time and space? Determination of shearsense within each domain allows us to addressthese questions.

Northwest-trending slickenside lineations(Lss) are present along brittle faults at the top ofthe Nisutlin assemblage (where it underlies theTaylor Mountain assemblage), and northeast Le

is internally penetrative. Within the same out-crop, penetrative Le formed earlier than spaced,less-penetrative Lss; early formed brittle struc-tures would be erased by younger ductile shear.

Northwest Lss is developed locally alongbrittle fault contacts between the SeventymileNisutlin assemblages, and northwest Le is de-veloped at the deepest exposed structural levelwithin the Taylor Mountain assemblage. Pene-trative northwest Le is developed throughout or-thogneiss assemblage tectonites—neither north-east Le nor northeast Lss have been observed inthese tectonites.

Shear-Sense Indicators. Shear sense for theeastern Eagle-Tanacross region varies spatially(Fig. 4) and can be divided into four regions,from north to south.

1. Along the Taylor Highway north of 64°15′S,the shear sense generally dips gently and definesbroad west-trending warps; the Taylor Mountainand Nisutlin assemblages display a penetrativenortheast Le. Shear-sense indicators are difficultto find in these rocks, probably due to their fine-grained polymineralic nature, but where present,they record top-to-the-northeast shear. Asymmet-ric double- or single-girdle quartz c-axis fabricsalso show top-to-the-northeast shear (TH7,TH60, TH106; Fig. 5, z, ee, and ii). Kinematic in-dicators are rare in the fault zone between theTaylor Mountain and Nisutlin assemblages ex-posed along the Taylor Highway. However,TH124 (Fig. 5jj) displays a slightly asymmetricdouble-girdle quartz petrofabric reflecting a com-ponent of top-to-the-northeast shear, and TH23(Fig. 5bb) shows a slightly asymmetric double-girdle c-axis fabric indicating a component oftop-to-the-northwest shear. These quartz fabricdata and top-to-the-northeast microstructures in-dicate that displacement within the boundaryzone was both top-to-the-northeast and top-to-the-northwest. Because both the Taylor Mountainand Nisutlin assemblages contain northeast Le,we interpret them to have shared top-to-the-northeast deformation. Evidence for top-to-the-northwest shear is localized along the contact, asis northwest Lss, and thus we interpret top-to-the-northwest shear as younger than the penetrativelydeveloped top-to-the-northeast shear.

2. The Taylor Mountain assemblage between64°15′ and 63°55′N displays top-to-the-north-west shear at more than 25 stations. Quartz c-axis stereo plots of ET110, ET117, ET122, andTH80 show fabric asymmetry indicating top-to-the-northwest shear consistent with microstruc-tural kinematic indicators (Figs. 3a and 4, k, l,m, and ff). Quartz c-axis fabrics TH11, TH39,and TH85 show relatively symmetric fabricswith girdles perpendicular to Le (Fig. 5, aa, cc,and gg), confirming that the northwest Le is anelongation lineation.

V. L. HANSEN


3. Microstructures and quartz c-axis fabricsfrom the Nisutlin assemblage near the interna-tional border at ~64°N record top-to-the-north-east and top-to-the-northwest shear, dependingon structural level. TH89 displays a quartz fabricasymmetry indicating top-to-the-northeast shear(Fig. 5hh) at structurally high levels, whereasstructurally low ET102 yields an asymmetric fab-ric recording top-to-the-northwest shear, con-firming field kinematic interpretations (Fig. 5j).

4. Kinematic indicators are best developed orpreserved south of 63°55′ (Fig. 4). This region,which includes the contact between the tectonitesof the Taylor Mountain and orthogneiss assem-blages, records four different shear directions:top-to-the-northeast and rare top-to-the-south-west, top-to-the-northwest, and top-to-the-south-east shear. We sampled across the Taylor Moun-tain–orthogneiss assemblage contact in fourswaths; in each area, northeast Le is preserved athigher structural levels than northwest Le.

The westernmost traverse through the contactzone is the most detailed; northeast Le-bearinggarnet-biotite gneiss and mica quartz schist arepreserved in the axis of a gentle, upright, north-west-trending synform with northwest Le-bear-ing pelitic rocks on either limb (Fig. 4B). S-Cfabrics and mica fish show top-to-the-southwestshear parallel to northeast Le. On the limbs of thesynform, S-C fabrics, microstructures, and quartzc-axis fabrics variably record top-to-the-north-west and top-to-the-southeast shear. Top-to-the-northwest shear is recorded on the northern limband at structurally high levels on the southernlimb. Top-to-the-southeast S-C fabrics are pre-served at the structurally deepest exposed levelsof the southern limb (Fig. 4b). Quartz c-axis fab-rics (ET46, ET50, and TH51) show top-to-the-northwest shear (Fig. 5, h, i, and dd). TH51 has adouble girdle with faint asymmetry; the asymme-try could reflect a small component of noncoax-ial top-to-the-northwest shear having a largecomponent of coaxial shear, or a combination oftop-to-the-northwest and top-to-the-southeastshear. Both explanations are reasonable and pos-sible given that strain can be partitioned into lo-cally dominant coaxial and noncoaxial shear, andconsidering that the region underwent both top-to-the-northwest and top-to-the-southeast shear.The region north of, and including, TH51 recordsdominantly top-to-the-northwest shear, althoughdiscrete top-to-the-southeast shear zones cut pen-etratively developed top-to-the-northwest fabrics,indicating that top-to-the-southeast shear isyounger than top-to-the-northwest shear (Fig.4B). South of TH51 top-to-the-southeast sheardominates. Shear sense is not directly correlativewith lithology; top-to-the-northwest shear is pre-served in the metasedimentary and metavolcanicrocks as well as the orthogneiss; both top-to-the-

northwest and top-to-the-southeast shear arerecorded in the orthogneiss (Fig. 4B). On the ba-sis of lithology, we interpret the northeast Le-bearing garnet-mica gneiss and schist in the syn-form axis as the Taylor Mountain assemblage,and the northwest Le-bearing pelitic schist as theorthogneiss assemblage. This interpretation isconsistent with lithologic mapping of Foster(1970, 1976). Therefore, as elsewhere, the TaylorMountain assemblage contains both northeastand northwest Le, and the orthogneiss assem-blage records both top-to-the-northwest and top-to-the-southeast shear. Because the tectonites inboth the Taylor Mountain and orthogneiss as-semblages contain northwest Le, but only theTaylor Mountain assemblage contains a north-east Le, we propose that the top-to-the-north-west shear fabric represents a tectonic contactbetween the two assemblages, and that top-to-the-northwest deformation was responsible fortheir juxtaposition. It is also possible that the twoassemblages could have been juxtaposed duringtop- to-the-northeast shear and then locally over-printed by top-to-the-northwest shear such thattop-to-the-northeast shear within the orthogneissassemblage was completely overprinted by top-to-the-northwest shear. However, the require-ment that top-to-the-northeast shear be every-where overprinted only within the orthogneissassemblage seems coincidental, and thus we fa-vor the former interpretation.

Temporal relations between the top-to-the-northeast and top-to-the-northwest shear eventsare best recorded ~5–10 km to the east of sectionA–A′ (Fig. 4A). In this area, a tectonite lozengeof clinozoisite-chlorite gneiss preserves top-to-the-northeast S-C fabric. The lozenge is en-closed in northwest Le-bearing pelitic schist(ET20). The northeast Le gneiss lozenge hosts aquartz vein (ET23) that itself bears northwest Leand yields an asymmetric top-to-the-northwestquartz c-axis girdle (Fig. 5f). The enclosingnorthwest Le tectonites also record top-to-the-northwest shear, as shown by microstructuresand by quartz petrofabric in ET20 (Fig. 5e).Therefore, top-to-the-northeast shear affectedthe lozenge, which became incorporated into atop-to-the-northwest shear zone, at which time aquartz vein in the lozenge deformed ductilely,reflecting top-to-the-northwest shear. Adjacenttectonites also contain northwest Le but recordsymmetric quartz petrofabrics (ET17, ET18, andET25) indicative of coaxial finite strain (Fig. 5,c, d, and g). ET25 shows a slight fabric asymme-try reflecting top-to-the-northwest shear (Fig.5g). The history gained from quartz petrofabricsis consistent with independent kinematic dataacquired at ET2 and ET16.

Temporal relations between top-to-the-north-west and top-to-the-southeast shear can be dis-

cerned at ET2. Here, peraluminous augen or-thogneiss forms a sill-like sheet in gently north-west-dipping metasedimentary host rocks; wecollected ~30 samples at 10 stations along a de-tailed transect (~250 m) normal to structuralthickness. Although Le trends consistently north-west, both top-to-the-northwest and top-to-the-southeast shear fabrics are recorded in outcropand in thin section. As in the western section, top-to-the-northwest shear dominates to the north, inthe structurally high part of the section, and top-to-the-southeast shear dominates to the south,structurally lower in the section. A quartzitepetrofabric records top-to-the-southeast shear(ET2, Fig. 5a). Although no sharp division is ob-served between domains of opposite shear, less-penetratively developed top-to-the-southeastshear bands cut, and are therefore younger than,more penetratively developed top-to-the-north-west fabrics in a schistose tectonite.

At ET16, about 10 km west of line A–A′ (Fig.4A), well-developed S-C fabrics in peralumi-nous augen orthogneiss consistently indicatetop-to-the-northwest shear. However, the maintectonite fabric is folded by tight, northeast-trending folds that are asymmetric to the south-east, indicating top-to-the-southeast shear. Simi-lar temporal relations are documented along theinternational boundary near ET5 (Fig. 4). Heremetasedimentary and metavolcanic tectonitescould belong to the Taylor Mountain assemblageand/or the orthogneiss assemblage. The foliationdips gently northeast and Le trends northwest.Field and microstructures record top-to-the-southeast shear. The quartz c-axis diagram forET5 shows top-to-the-southeast shear (Fig. 5b).To the north, structurally higher, two additionalstations record top-to-the-southeast shear. Yetfarther north, we observed top-to-the-northwestshear. Temporal relations between top-to-the-northwest and top- to-the-southeast shear at thislocation are unconstrained.

In summary, the distribution of kinematic do-mains with respect to tectonic packages andstructural level provides clues about how eachdomain formed (Fig. 6). In the eastern Eagle-Tanacross region, three main deformation fab-rics are recognized. (1) Top-to-the-northeastfabrics (and rare top-to-the-southwest fabrics)occur within the structurally high oceanic andsiliciclastic marginal-basin strata (Seventymile,Taylor Mountain, and Nisutlin assemblages);they are least developed in low-grade rocks(Seventymile) and best developed in high-graderocks (Taylor Mountain). (2) Top-to-the-north-west fabrics occur along brittle (Lss) and ductile(Le) fault zones between lithotectonic packages.(3) Top-to-the-southeast fabrics dominantly cutthe structurally low orthogneiss assemblagerocks. Mid-Cretaceous granites crosscut all of



the upland tectonites. Top-to-the-northeastshear is the oldest of the three shear deforma-tions, and top-to-the-southeast shear is theyoungest. Top-to-the-northeast fabrics havenowhere been observed to affect the orthogneissassemblage.

Mount Warbelow

Mica schist, marble, quartzite, and horn-blende-biotite schist of the Taylor Mountain as-semblage are exposed in the Mount Warbelowarea. The foliation dips gently, and Le trendsnorth-northeast and northwest (Fig. 3). We spotsampled eight locations across ~200 km2.Field, microstructural, and petrofabric data all

indicate top-to-the-northwest and top-to-the-north-northeast shear (Figs. 2 and 5, w, x, andy). We interpret this region as one of spatialoverlap of top-to-the-northeast and top-to-the-northwest ductile shear; we have no age rela-tions from this area.

Molly Creek

The Molly Creek area, just west of Mount Veta(Figs. 3 and 4C), preserves evidence for similargeometric and temporal relations among thethree domains documented in the eastern Eagle-Tanacross region, and it affords another view ofthe top-to-the-southeast shear. The structurallyhighest tectonites contain northeast Le, and a

quartz c-axis fabric records top-to-the-northeast(ET216, Fig. 5v). Structurally lower tectonitesshow northwest Le and top-to-the-northwest.Quartz c-axes from ET157, a (chlorite)-garnet-feldspar quartzite, and ET158, a (chlorite)-amphibole-garnet-white mica quartzite, showsymmetric girdles reflecting coaxial bulk strain(Fig. 5, p and q). At ET157, different parallel fo-liations preserve either northeast or northwest Le.The ET157 c-axis diagram (northwest motionplane) shows possible prism <c> slip (Fig. 5p; ro-tation of the data into the northeast motion planealso shows possible prism <c> slip). Althoughprism <c> slip is interpreted to occur at high tem-peratures (i.e., granulite facies, Jessel and Lister,1990), it can occur at greenschist facies condi-

V. L. HANSEN


NENW

??

?191 Ms

186±2 Ms187±2 Bt

204±4 Hbl

194±2 Hbl

201±5 Hbl213±2 Hbl

177 Ms

110 Ms, 111 Bt

166±1 Bt

119 Hbl, 109 Ms, 109 Bt

135±1 Ttn

212 Ttn209±3 Hbl204±9 Bt

188 Bt, 185 Ms187 Hbl, 185 Bt188 Hbl, 186 Bt

10-12 kbar, 645-715°C7.5-10 kbar, 555-660°C

8.6-9.7 kbar, 665-730°C

8.9-10.8 kbar, 630-670°C

9-11.8 kbar, 570-640°C6.8-9 kbar, 640-700°C

6.5-8.8 kbar, 520-560°C10.8-11.7 kbar, 595-635°C

thermochronologic datathermobarometric data

Taylor Mtn.Batholith

Figure 6. Schematic block diagram of the upland tectonites looking north; cut away northeast and northwest faces illustrate the distributionof kinematic domains with respect to lithologic packages. Map patterns are the same as in Figures 2–4. Heavy black lines mark faults; teeth indi-cate brittle fault, wavy line indicates ductile shear. Stick arrows represent brittle deformation (Lss); arrow shows upper plate displacement. Largearrows and circles represent penetrative ductile shear and displacement direction of structurally high rocks; white—top-to-the-northeast (andlocal top-to-the-southwest) shear; black—top-to-the-northwest shear; gray—top-to-the-southeast shear. The effects of late east-trending uprightopen folds are removed. Thermochronologic and thermobarometric data are from sources given in Dusel-Bacon et al. (1995).

tions as a result of consecutive and coplanar, butperpendicular, shear events (Oliver, 1996).ET157 shows no evidence of high-temperatureshear, but northeast Le and northwest Le pre-served in different foliations probably recordconsecutive, coplanar, perpendicular shearevents. Thus we favor this explanation for theprism <c> fabrics. Top-to-the-northeast shearprobably predates top-to-the-northwest shear onthe basis of relations described from the easternEagle-Tanacross area.

Peraluminous orthogneiss records both top-to-the-northwest and top-to-the-southeast ductileshear as defined by S-C fabrics. Top-to-the-southeast shear occurred, at Molly Creek, along amore localized, discrete shear zone than the ear-lier, more penetratively developed top-to-the-northwest fabrics, indicating that top-to-the-southeast shear postdated top-to-the-northwestshear. Here top-to-the-southeast shear, comparedwith top-to-the-northwest shear, probably repre-sents higher localized strain, and/or it occurred ata shallower crustal level. Top-to-the-southeastshear is documented at ET206 in the field (S-Cfabrics), in thin section, and through c-axis fab-rics from a deformed quartz vein (Fig. 5t). The c-axis fabric shows a strong single girdle markedmostly by basal <a> slip, indicative of low-temperature shear (i.e., greenschist facies). AtET208, extremely well-developed S-C fabricsand small (l < 1–3 cm), Le-normal asymmetricfolds that deform the S-C fabrics indicate thattop-to-the-southeast displacement here waslikely accompanied by local high strain. A quartzpetrofabric diagram from this location displays asingle asymmetric girdle (Fig. 5u). To the southat ET211, top-to-the-northwest S-C fabrics occurin peraluminous orthogneiss within both upperand lower plate positions with respect to a gentlynortheast-dipping top-to-the-southeast shearzone. These top-to-the-southeast tectonites typi-cally display transitional ductile-brittle character.The quartz petrofabric of ET204 records top-to-the-southeast shear (Fig. 5s). Orthogneiss withinthe upper level of the top-to-the-southeast shearzone is locally chloritized. Graphitic quartziteET162 records low-T (basal <a>), top-to-the-southeast shear (Fig. 5r).

In summary, the Molly Creek area hosts eachof the three kinematic domains recognized to theeast. Here top-to-the-northeast shear occupies thehighest structural level; top-to-the-northwestshear fabrics overlap spatially with top-to-the-northeast shear fabrics locally; and top-to-the-southeast shear fabrics overprint top-to-the-northwest fabrics and occur in a zone within thetop-to-the-northwest domain. As in the easternEagle-Tanacross region, the orthogneiss assem-blage records only top-to-the-northwest and top-to-the-southeast shear.

Southeast Big Delta

Our work in the southeast Big Delta area fo-cused on three locations (Fig. 3). At the southern-most locality, field and microstructural data fromgently dipping schist and orthogneiss consis-tently record top-to-the-west-northwest shear.S-C orthogneiss fabrics at ET136 record top-to-the-northwest shear. Quartz petrofabrics fromET136 and ET137 show symmetric and top-to-the-west-northwest asymmetric girdles, respec-tively (Fig. 5, n and o). In both cases, prism,rhombohedral, and basal <a> slip are repre-sented, indicating that top-to-the-northwest shearoccurred at moderately high temperature. ET136records bulk coaxial strain (perhaps with a slighttop-to-the-northwest noncoaxial component) thatcould result from coaxial deformation, or fromsequential colinear top-to-the-northwest and top-to-the-southeast shear.

To the northwest, peraluminous orthogneissand quartz-mica schist and gneiss dip gently inmany directions. Ultramafic to mafic rocks, lo-cally foliated and marked by a variably devel-oped Le, are structurally above the orthogneissand host rocks; north-northeast to northeast Le,marked by amphibole, occurs structurally highwithin the ultramafic rocks (not shown in Fig. 3).Structurally deeper, toward the contact with theorthogneiss, Le trends northwest to west, andtop-to-the-northwest shear is recorded by fieldand microstructures. Farther to the northwest,peraluminous orthogneiss displays top-to-the-northwest and top-to-the-west S-C fabrics. Evi-dence for local top-to-the-east-southeast shear isalso preserved. On the basis of lithologic andkinematic similarity with relations documentedto the east, we interpret the ultramafic rocks asklippen correlative with the Taylor Mountain orSeventymile assemblages. Thus, the structuraland kinematic evolution determined for theMount Warbelow, Molly Creek, and southeastBig Delta regions appear to be consistent withthat of the eastern Eagle-Tanacross region.

P-T and Cooling Histories

Published thermobarometric and thermo-chronometric data (U-Pb titanite, 40Ar/39Ar horn-blende, white mica, and biotite; see Dusel-Baconet al., 1995, for references) constrain the physicalconditions and the time of deformation withineach domain (Figs. 4 and 6). Although P-T andthermochronometric data were collected withinthe structural-kinematic framework describedherein, interpretation of the data must be under-taken with caution because kinematics, P-T, andcooling age data cannot be unequivocally corre-lated to a unique time window. Thermometricdata record peak temperature, barometric data

generally record pressure that accompanied peaktemperature, and thermochronometric data recordwhen a specific mineral lattice closed for a par-ticular isotopic system, a process largely depen-dent on temperature. Shear zones anastomoseand can vary in thickness and strain intensity as afunction of total strain, lithology, preexisting fab-rics, crustal level, and other variables. As a result,polydeformed tectonites may preserve variableage relations dependent on the dynamothermalconditions (e.g., peak temperature, length of timeat peak temperature, kinetics of reactions, strainhistory) that affected a specific tectonite sample.With these cautions in mind, thermobarometricand thermochronometric data can place impor-tant constraints on the tectonic evolution of theupland tectonites.

Top-to-the-northeast shear occurred underhigh P-T conditions prior to or during the LateTriassic to Early Jurassic (Fig. 6). Evidence forthis conclusion comes from TH7 and TH60,which contain top-to-the-northeast fabrics andyielded P-T conditions of 10–12 kilobars,645–715 °C, and 7.5–10 kilobars, 555–660 °C,respectively (Dusel-Bacon et al., 1995). Becausethese samples only record top-to-the-northeastshear—the oldest deformation in the area—theP-T conditions recorded probably accompaniedtop-to-the-northeast shear. The top-to-the-north-east tectonites record cooling through ~500 °C at204 Ma (hornblende) and through ~350 °C at 191Ma (white mica). Moreover, top-to-the-northeastshear predated intrusion and cooling of the mas-sive Taylor Mountain batholith, which cooledthrough ~600 °C at 212 Ma and ~500 °C at ca.209 Ma (titanite and hornblende, respectively).Thus, this deformation occurred, at least in part,prior to 209 Ma, and likely prior to 212 Ma.Therefore, these cooling ages do not necessarilydate the age of deformation, but rather reflectpostkinematic cooling.

Top-to-the-northwest tectonites also recordhigh P-T conditions, and they cooled throughabout 300 °C by Middle Jurassic time (Figs. 4aand 6; Dusel-Bacon et al., 1995). P-T data fromfour samples from within the top-to-the-north-west shear domain yield high P-T conditionsranging from 520 to 730 °C and 6.5 to 11.7 kilo-bars. Each of these samples are from the TaylorMountain–orthogneiss assemblage contact zone.P-T constraints from two structurally deep ortho-gneisses yield 570–700 °C and 6.8–11.8 kilobars(Figs. 3A and 5). Top-to-the-northwest tectonitesyield cooling ages from hornblende- biotite pairsof 188 and 186 Ma at high structural levels, and177 Ma (white mica) and 166 Ma (biotite) atdeeper structural levels. High-level cooling agesprobably more closely bracket cessation of de-formation, whereas deep-level cooling ages re-flect posttectonic crustal cooling. This minimum



age constraint on ductile deformation is similar tothe 187 Ma 40Ar/39Ar biotite age from the south-dipping brittle-ductile shear zone that places theTaylor Mountain assemblage above the Nisutlinassemblage (Cushing, 1984; Cushing et al.,1984). These workers interpreted biotite to haveformed during fault-zone formation and thereforeto date imbrication along this zone. Lss and kine-matic data from the fault zone indicate top-to-the-northwest thrusting, consistent with shearsense preserved in the L-S tectonites. Therefore,tectonites affected by top-to-the-northwest shearcooled more quickly at high structural levels rel-ative to deeper structural levels, and the upperstructural levels cooled through their blockingtemperatures at about the same time that dis-placement along intra-assemblage and inter-assemblage fault zones waned. The Taylor Moun-tain batholith records cooling though ~600 °C at212 Ma and through ~300 °C at 204 ± 9 Ma(Dusel-Bacon et al. 1995). Presumably, the baseof the Taylor Mountain batholith records north-west shear, although the region is not exposed.

The timing of top-to-the-southeast shear isconstrained by titanite, hornblende, white mica,and biotite cooling ages of 135, 119, 109, and109 Ma, respectively (Figs. 4A and 6). Top-to-the-southeast tectonites cooled from ~500 °C to300 °C in 10 m.y. Quartz in many top-to-the-southeast tectonites shows microscopic texturesindicative of low-temperature plasticity, includ-ing strong undulatory extinction, ragged grainboundaries, and subgrain growth. The lack of an-nealed textures, together with relatively fast cool-ing rates, indicates that ductile shear accompa-nied, rather than predated, cooling (Hansen andOliver, 1994). Thus, the metamorphic coolingages closely approximate the time of top-to-the-southeast ductile shear.

A peraluminous orthogneiss with the oldertop-to-the-northwest fabric that was sampled in alower plate position relative to a top-to-the-south-east shear domain yielded white mica and biotitecooling ages of 110 and 111 Ma, respectively(Fig. 6). Although it contains earlier-formed top-to-the-northwest shear fabrics, the rock probablycooled through the white mica and biotite block-ing temperatures during tectonic denudation re-lated to top-to-the-southeast shear. Thus, the pre-served kinematic fabrics and cooling ages reflectdifferent events because top-to-the-northwestshear fabrics formed at temperatures above theAr/Ar blocking temperatures for muscovite andbiotite, and the rocks remained buried until thetop-to-the-southeast event.

P-T conditions of top-to-the-southeast shearare difficult to ascertain because top-to-the-southeast shear postdates early high-temperatureevents, making it difficult or impossible to be cer-tain that the P-T determinations from top-to-the-

southeast tectonites record physical conditionsthat accompanied top-to-the-southeast shear andnot the conditions during earlier deformation.P-T determinations from adjacent tectonitesshow high pressure and temperature (top-to-the-southeast fabric: 9–11.8 kilobars, 570–640 °C;northwest Le, no shear sense determined: 6.8–9kilobars, 640–700 °C) (Figs. 4A and 6). Temper-ature probably reached ≥600 °C locally duringtop-to-the-southeast shear because titanite (U-Pbclosure T of 600 °C; Heaman and Parrish, 1991)records cooling at 135 Ma, unless shearing andductile deformation substantially lowers the U-Pbblocking temperature of titanite. Lower tempera-ture conditions (<300–350 °C) during top-to-the-southeast shear are reflected locally in the MollyCreek area, ~20 km to the northwest, where min-erals record transitional brittle behavior. Thus,top-to-the-southeast shear probably occurredover a range of P-T conditions.

Integration of kinematic, P-T, and cooling dataresults in the following conclusions (Fig. 7).(1) Top-to-the-northeast shear, at high P-T condi-tions, occurred prior to 191 Ma and probably priorto 212 Ma, but is now preserved at high structurallevels (Seventymile, Taylor Mountain, and Nisut-lin assemblages). (2) Top-to-the-northwest shearpostdated top-to-the-northeast shear—likely be-ginning before 188 Ma and waning by ca. 185Ma. Top-to-the-northwest shear occurred locallyat high P-T conditions, and placed Seventymile,Taylor Mountain, and Nisutlin assemblages struc-turally above rocks of continental affinity. (3)Top-to-the-southeast shear that occurred from ca.135 to 109 Ma resulted in exposure of the struc-turally deepest parautochthonous continentalrocks. Oceanic and marginal-basin rocks are in-truded by Late Triassic to Early Jurassic arc plu-tons, and all assemblages are intruded by mid-Cretaceous plutons.

REGIONAL CORRELATION

Compilation of our data, together with pub-lished studies, reveals that the general structural,kinematic, P-T, and temporal constraints derivedfrom upland tectonites allow correlation with up-land tectonites to the west and with tectonites inwestern and central Yukon.

Yukon-Tanana Upland

In the western Yukon-Tanana upland north ofFairbanks, Chatanika eclogites are structurallyabove parautochthonous continental strata correl-ative with autochthonous North American rocksof the western Selwyn basin, Yukon (Fig. 1)(Murphy and Abbott, 1995). The eclogites recordhigh P-T metamorphic conditions (Laird et al.,1984; Brown and Forbes, 1986) and have been

correlated with eclogites enclosed in oceanic andmarginal-basin tectonites in Yukon (Erdmer,1987, 1992; Hansen, 1992a, 1992b). Northeast ofBig Delta, oceanic rocks are structurally abovemarginal-basin tectonites. Although few kine-matic indicators have been documented, top-to-the-northeast shear structures are reportedfrom the eclogitic and oceanic rocks, and top-to-the-northwest structures are present locallywithin marginal-basin tectonites, and charac-terize the parautochthonous Wickersham assem-blage (Pavlis et al., 1993) (Fig. 2).

Top-to-the-southeast shear characterizes thecontact between the structurally low orthogneissassemblage and structurally high marginal-basintectonites west of ~145° (Fig. 2). The contact rep-resents a mid-Cretaceous extensional shear zoneon the basis of consistent top-to-the-southeastkinematic data, a sharp transition in metamorphicgrade across the zone, and thermochronometricdata recording fast mid-Cretaceous cooling offootwall rocks (Pavlis, 1989; Pavlis et al., 1993).

Timing of top-to-the-northeast and top-to-the-northwest shear in the western upland is not wellconstrained. Top-to-the-northwest shear is young-er than top-to-the-northeast shear and older thantop-to-the-southeast shear (Pavlis et al., 1993).Muscovite from Chatanika tectonites yielded K-Ar cooling ages of 178 and 166 Ma (Wilson andShew, 1981). Although excess Ar or Ar loss maybe a problem, on the basis of regional upland re-lations we interpret these muscovite K-Ar ages toreflect cooling following top-to-the-northwestshear, which placed Chatanika eclogite aboveparautochthonous continental strata. We interpretthe top-to-the-northeast shear to predate juxtapo-sition of eclogite and parautochthonous conti-nental strata.

Thus, the geologic relations illustrated in Fig-ure 7, correlative across the upland, describe asimple stacking order from bottom to top: NorthAmerican parautochthonous continental margin,to continental marginal-basin strata, to ocean-basin and island-arc rocks. In many regions,oceanic and arc rocks are directly above par-autochthonous continental rocks. The regionallyconsistent stacking order, and the integrated kine-matic, P-T, and cooling age data provide con-straints that must be addressed by any tectonicmodel.

A model in which the orthogneiss assemblageis thrust over the Taylor Mountain assemblage inCretaceous time (Foster et al., 1994) is not sup-ported by the data presented here. First, contrac-tion and juxtaposition of the orthogneiss and Tay-lor Mountain assemblages occurred in Jurassic,not mid-Cretaceous, time. Second, emplacementof the orthogneiss assemblage over the TaylorMountain assemblage is inconsistent with meta-morphic cooling ages. In the case of polyphase

V. L. HANSEN


deformation and polyphase cooling, tectonitesthat yield relatively older cooling ages must bestructurally high (hanging-wall position), andthose that record relatively younger cooling mustoccupy a low structural level (footwall position)regionally, unless imbrication completely post-dates cooling. Third, mid-Cretaceous shearing istop-to-the-southeast, the orthogneiss assemblagebeing in a lower plate position in the Eagle-Tanacross region and in the western upland.

The model of Mortensen (1992) calls on strati-graphic continuity between the orthogneiss andTaylor Mountain assemblages. This model is in-consistent with our data. Top-to-the-northeast de-formation is nowhere documented in the struc-turally deep (stratigraphically in Mortensen’smodel) orthogneiss assemblage, yet evidence forthis older penetrative shear event is documentedin structurally high rocks (stratigraphically highin Mortensen’s model); this relationship is diffi-cult to justify within a stratigraphic continuumhypothesis. In addition, Late Triassic–EarlyJurassic arc granitoids intrude only oceanic and

marginal-basin tectonites (Seventymile, TaylorMountain, and Nisutlin assemblages), and havenowhere been documented to intrude the ortho-gneiss assemblage. If all these assemblages rep-resent a continuous stratigraphic section, it is dif-ficult to justify how intrusive rocks wouldselectively affect younger (and thus higher) andnot older rocks. Furthermore, the top-to-the-northwest deformation, shared by the TaylorMountain and orthogneiss assemblages, con-strains the mode of juxtaposition, and metamor-phic cooling ages constrain the time of juxtaposi-tion (i.e., late Early Jurassic).

Our data also argue against the Cache Creekenclosure model (Nelson and Mihalynuk, 1993;Mihalynuk et al., 1994), as do broad geologic re-lations. The Cache Creek enclosure model wasdeveloped to address apparent “enclosure” of al-lochthonous rocks (Stikinia arc, Quesnelia arc,and Cache Creek oceanic strata) by rocks of con-tinental affinity—a “paradox” disregarded inother models (Mihalynuk et al., 1994). Unfortu-nately, these authors set up a paradox that does

not exist; they assumed that continental margincrystalline rocks wrap around the northern end ofStikinia and Quesnelia. This assumption dissi-pates when one examines geologic and structuralhistories. There are several problems inherent inthis analysis. (1) The authors treat the Yukon-Tanana composite terrane as a single tectonic en-tity with a singular geologic history, thus disre-garding the stacking order of the lithologic unitsand documented differences in the deformationof the assemblages. (2) Continental margin rocksof the orthogneiss assemblage are parautochtho-nous to North America, and the Chena River andFairbanks subterranes are correlative with au-tochthonous Selwyn basin strata, western Yukon.Therefore neither of these packages of rockswere separated from western North America byan oceanic or back-arc basin in Permian to Mid-dle Jurassic time, as required by the enclosuremodel. (3) The Nisutlin assemblage, which iscomposed of continental margin rocks that definethe western limb of the proposed enclosure oro-cline, are separate and distinct from the parau-tochthonous rocks (e.g., Currie, 1995). Tempelman-Kluit (1979) originally proposed that Nislingrocks may be a rifted fragment of North America,and we know of no data that would argue againstthis hypothesis, though this is not required by anymodels. (4) Upland tectonites represent the hingeof the proposed orocline of crystalline rocks inthe Cache Creek model; however, the deforma-tion, timing, and crustal conditions documentedherein, and correlative across the upland andcentral Yukon, are not addressed in the contextof the model. For example, contrary to state-ments by Mihalynuk et al. (1994), the effects ofthe earliest, top-to-the-northeast, shear defor-mation can be spatially and temporally differen-tiated from subsequent deformation related toimbrication in late Early Jurassic time (e.g.,Hansen, 1992b; data presented herein). The en-closure model also requires top-to-the-northeastand top-to-the-southwest contraction of uplandtectonites and correlative rocks in Yukon at ca.185 Ma (Mihalynuk et al., 1994, Fig. 4C), con-trary to our data, which record early top-to-the-northeast shear (not addressed in the enclosuremodel), followed by locally spatially distinct top-to-the-northwest shear, which waned ca. 185 Ma.Thus the enclosure model addresses a paradoxthat does not exist, and the P-T-t displacementhistory recorded in the upland tectonites, locatedwithin the proposed oroclinal hinge, is inconsis-tent with the enclosure model.

Correlation with Yukon Tectonites

Hansen et al. (1991) outlined broad similari-ties in kinematics and cooling ages between theeastern upland tectonites and L-S tectonites



oceanic & arc strata (Seventymile) (Taylor Mountain)marginal-basin strata (Nisutlin)continental rocks (orthogneiss assemblage)

oceanic strata (Seventymile) (Taylor Mountain)possibly marginal-basin strata (Nisutlin)

212 Ma > D2 ≥188-185 Mahigh P-T(collision)

oceanic & arc strata (Seventymile) (Taylor Mountain)marginal-basin strata (Nisutlin)continental rocks (orthogneiss assemblage)

135 Ma > D3 > 109 Ma(crustal extension)

arc magmatismLate Triassic to Early Jurassic

granitic plutonism mid Cretaceous

affected assemblages & stacking order

timing and P-T conditions(tectonic interpretation)kinematics

D1 > 212 Mahigh P-T;local blueschist & eclogite(subduction zone— downflow & backflow)

oceanic & arc strata (Seventymile) (Taylor Mountain)marginal-basin strata (Nisutlin)

(local)

Geologic history of upland tectonites

all assemblages

D3

D2

D1

Figure 7. Geologic history of upland tectonites.

within and east of the Teslin suture zone, Yukon,supporting previous correlation of these units(Tempelman-Kluit, 1979). The northwest-strik-ing Teslin suture zone separates parautochtho-nous ancestral North America from allochtho-nous terranes (Tempelman-Kluit, 1979). Thesuture zone is marked by a 10–15-km-wide zoneof L-S tectonites that have oceanic and marginal-basin lithologies. Metamorphic grade rangesfrom mid-greenschist to albite-epidote amphibo-lite facies and rare eclogite (Erdmer, 1987;Hansen, 1992b). Tectonites are locally intrudedby Late Triassic to Early Jurassic plutons. Suture-zone tectonites have been examined within astructural-kinematic and thermochronometricframework in the Laberge–Quiet Lake and LittleSalmon Lake areas (Fig. 1).

In the Laberge–Quiet Lake area, tectonitesrecord high P-T conditions that reflect subduc-tion-zone metamorphism (Hansen, 1992b), inagreement with P-T conditions recorded by theenclosed Permian Last Peak eclogite (Erdmer,1987; Erdmer and Armstrong, 1989; Erdmer etal., 1996). Tectonites record apparent normal, re-verse shear, and strike-slip shear, which are in-terpreted to record backflow, downflow, andmargin-parallel shear, respectively, within awest- dipping subduction zone (Hansen, 1989,1992a). Following subduction-zone deformation,these rocks, which cooled quickly at ca. 195 Ma,were thrust eastward over parautochthonous con-tinental strata in Early Jurassic time (Hansen etal., 1991; Hansen, 1992b). ParautochthonousNorth American strata of Cambrian(?) andyounger age that host Mississippian peraluminousorthogneisses (Hansen et al., 1989) were ductilelydeformed as a result of eastward overthrusting.These rocks cooled slowly through ~500 to 300°C from ca. 150 Ma to 120 Ma. In mid-Creta-ceous time (ca. 110 to 90 Ma), the area was cutby dextral strike-slip faults broadly parallel tothe Tintina fault, and intruded by granites. Theregional stacking order preserved in tectonites inthis region is the same as that documented forthe Yukon-Tanana upland-parautochthonouscontinental strata; Mississippian orthogneiss arestructurally deepest (although here they are eastof allochthonous rocks in map view), and mar-ginal-basin and oceanic tectonites intruded byarc rocks are at the highest structural level.

At Little Salmon Lake (Oliver, 1996) (Fig. 1),upper plate tectonites record top-to-the-south-west (apparent normal) and top-to-the-northeast(apparent reverse) directed dip-slip shear, inter-preted as having formed within a subductionzone. Dip-slip tectonites are partially overprintedby top-to-the-northwest fabrics. 40Ar/39Ar ther-mochronology and calcite e-twin analysis con-strains each of these ductile shear events to pre-190 Ma. Tectonites cooled slowly following

northeast-directed folding. The cooling-deforma-tion history recorded within the Little SalmonLake tectonites is consistent with tectonite for-mation within a west-dipping subduction com-plex (Oliver et al., 1995).

Deformation in both these regions is broadlycorrelative with deformation documented in theupland tectonites, although Early to early MiddleJurassic imbrication of allochthonous rocks wasnortheast directed in the Laberge–Quiet Lake areaand northwest directed at Little Salmon Lake andin the Yukon-Tanana upland of Alaska. These dif-ferences may be related to paleomargin orienta-tion. Both regions in Yukon lack evidence of mid-Cretaceous extension, although mid-CretaceousRb-Sr and K-Ar cooling ages from Paleozoicorthogneiss in western Yukon could reflect cool-ing related to extension (Hansen, 1990a). Duringmid-Cretaceous time, central Yukon was dis-sected along dextral strike-slip faults and intrudedby granitic plutons (Tempelman-Kluit, 1976;Gabrielse and Dobbs, 1982).

Tectonic Synthesis

Although Cordilleran tectonic analysis has fo-cused for many years on terrane identificationand distinctions between terranes, identifyinglinks between and among terranes is the next stepin understanding crustal assembly processes. Theassemblages with which we are concerned, in theinterior part of the northern North AmericanCordillera north of 57°N, are, from east to west(Fig. 1) (1) ancestral North American strata, (2)parautochthonous North American strata—stratadisplaced by strike-slip faults (orthogneiss as-semblage as defined in this paper, and Cassiarand Kootenay terranes of Wheeler et al., 1991),(3) marginal-basin strata (Nisutlin assemblage[within which we include the Nasina assemblageof Wheeler and McFeely, 1991]), (4) oceanicrocks (Slide Mountain, Seventymile, Teslin–Taylor Mountain, and Chatanika assemblages),(5) pre-Permian host rocks, and Permian to LateTriassic and Early Jurassic volcanic-arc rocks(Stikinia), and (6) rocks of continental affinity(Tracy Arm–Nisling assemblage). Each of theseassemblages forms a band from ~57°N to centralAlaska (Currie, 1995), and each has geologic tiesto adjacent assemblages, except the continentalassemblages. Parautochthonous North Americanstrata, and Nisling–Tracy Arm assemblages in-clude Devonian-Mississippian plutons (nowdominantly orthogneiss) that yield εNd values in-dicative of continental partial melting (e.g.,Aleinikoff et al., 1981, 1986; Bennett andHansen, 1988; Gehrels et al., 1990; McClellandet al., 1991; Jackson et al., 1991; Samson et al.,1991; Mortensen, 1992; Grant et al., 1996). TheCache Creek and Quesnelia terranes are not dis-

cussed in this paper except to outline how ourdata relate to previous interpretations of CacheCreek–Yukon–Tanana composite terrane interac-tions. The Cache Creek and Quesnelia terranesare exposed dominantly south of the assemblageswe are concerned with and a discussion of theseunits is outside the limits of the present work.

The geologic, structural, petrologic, and geo-chronometric data outlined here correlate sys-tematically across the upland and into Yukon.They are consistent with the tectonic model ofterrane accretion proposed by Tempelman-Kluit(1979) on the basis of the distribution of litho-tectonic assemblages and sequence of structuralstacking in Yukon. His model has been modifiedby local detailed studies, but the original tectonicframework accommodates, and even predicts, theresults of more than 15 years of research by anumber of workers employing diverse geologictools. In the model, continental affinity Nisling–Tracy Arm rocks rifted away from the westernmargin of North America in Devonian-Mississip-pian time, or shortly thereafter. Prior to, or duringrifting, Devonian-Mississippian plutons intrudedNorth American crust, including rocks that weresoon rifted from North America. The incipient in-tervening Anvil ocean separated the Nisling–Tracy Arm assemblage from their parent NorthAmerica. Slide Mountain–Seventymile assem-blages represent Anvil oceanic strata (Harms,1986; Nelson, 1993), although they have someisotopic similarities to North American marginalbasin strata and North America (Nelson et al.,1989; Smith and Lambert, 1995; Roback et al.,1994). The width and shape of the Anvil oceanare unknown. During Early Permian time, theAnvil ocean began to close by west-dipping,right-oblique subduction (present-day coordi-nates; Hansen, 1989, 1992b) (Fig. 8). The arc,Stikinia, was built in part on oceanic crust (Currieand Parrish, 1993) and in part on crust of conti-nental affinity (Nisling–Tracy Arm; e.g., Rubinand Saleeby, 1991; Jackson et al., 1991). Thus,along-strike variations in marginal basin detritus,isotopic signatures, and magma compositionswould be expected. Slide Mountain–Seventymilestrata were deformed during west-dipping sub-duction that closed the Anvil ocean. Low-gradeSlide Mountain–Seventymile rocks, imbricatedalong brittle faults (e.g., Harms, 1986), and Tes-lin–Taylor Mountain tectonites, metamorphosedand ductilely deformed under high P-T, representAnvil ocean strata that were tectonically trans-ferred from the footwall plate to the hanging-wallplate, at shallow and deep levels, respectively,across the west-dipping convergent margin.Eclogite and high-P-T tectonites of the Teslin–Taylor Mountain assemblage are consistent withsubduction geotherms and deformation. Nisutlinmarginal-basin strata could have been deposited

V. L. HANSEN


adjacent to western North America, or on the op-posite side of the Anvil ocean adjacent to therifted fragment of North America. Either positionfits available data. In east-central Alaska near theYukon border, marginal-basin tectonites recordtop-to-the-northeast deformation, and in easternYukon, Nisutlin tectonites record Early Permianblueschist facies metamorphism (e.g., Erdmerand Helmstaedt, 1983; Erdmer, 1987, 1992).These characteristics suggest an upper plate (out-board of the Anvil ocean) position for theseNisutlin rocks. Farther to the west, however,graphitic schist included in the Nisutlin terrane isthought to grade depositionally and metamorphi-cally into the Chena River sequence (Smith et al.,1994), a lower plate assemblage.

Until recently, there were few direct age con-straints on the onset of subduction-zone defor-mation and metamorphism, but the Last Peakeclogite yields a concordant U-Pb metamorphiczircon date of 268.8 ± 1.5 Ma (Creaser et al.,1996), in good agreement with blueschist coolingages (256 ± 8 Ma, 267 ± 8 Ma, Rb-Sr, whitemica, Erdmer and Armstrong, 1989; ages rangingfrom 260.0 ± 2.6 to 272.7 ± 2.6 Ma, 40Ar/39Ar,white mica, Erdmer et al., 1996), interpreted toreflect initial mid Permian subduction of theAnvil ocean (Erdmer and Armstrong, 1989;Hansen, 1992b). The oldest eclogite and blueschistages constrain the start of subduction becausethey reflect the early high-P-T thermal regimeduring subduction-zone initiation (e.g., Cloos,1986; Cloos and Shreve, 1988). Marginal-basin

strata that originated on the western side of theAnvil ocean (upper plate convergent margin)might have undergone blueschist metamorphismearly in Anvil ocean closure, essentially markingsubduction initiation, whereas marginal-basinstrata adjacent to North America would likely de-form late during Anvil closure.

Permian magmatic-arc activity is documentedin rocks (Mortensen, 1992) that we interpret asallochthonous, upper plate, and correlative withthe Stikinia arc. An Early Permian sill (276 Ma,U-Pb zircon, titanite) also intrudes a part of theimbricated Slide Mountain sequence, indicatingthat some imbrication occurred prior to sill em-placement (Harms, 1986). Harms (1986) notedthat the nappes and sills were folded alongwith Late Permian strata. Therefore, contrac-tional deformation began by at least 276 Ma,and continued beyond Permian time, yet pre-dated thrusting of the rocks over parautochtho-nous Cassiar strata in Jurassic time. Thus, sub-duction-zone metamorphism, arc magmatism,and contraction of oceanic sediments indicatethat during Permian time, the Anvil ocean wasin the process of closing.

As the accretionary complex within the upperplate migrated eastward with time, Stikinia arcgranitoids locally intruded the subduction com-plex as well as Nisling–Tracy Arm crust of theupper plate (e.g., Taylor Mountain batholithand Mount Veta pluton [Aleinikoff et al., 1986;Dusel-Bacon et al., 1995] in Alaska, and theKlotassin and Aishihik calc-alkaline suites in

Yukon [e.g., Mortensen, 1992; Johnston andErdmer, 1995]). With continued subduction, theupper plate would have accreted marginal-basinstrata from a successively more eastern position,and ultimately overrode western North America.As lower plate marginal-basin strata and NorthAmerican rocks were pulled into the subduction-collision zone, these rocks were ductilely de-formed at depth with top-to-the-east-northeastshear in southern Yukon, and top-to-the-north-west shear in central Yukon and east-centralAlaska. The change in displacement from east-northeast–directed margin-normal contraction,to northwest-directed contraction at a low angleto the margin, could reflect a change related tocollision, a change in plate motion, or a bend inthe western margin of North America. North-west-directed deformation further imbricatedupper plate rocks (Stikinia, Slide Mountain–Seventymile, and Teslin–Taylor Mountain assem-blages) and placed them above parautochthonouslower plate rocks (orthogneiss assemblage andSelwyn basin-correlative strata). Ultimately, theentire upper plate was tectonically accreted to thelower plate (North America), which grew laterallyand thickened.

The timing of collision is constrained in threeregions. In the Laberge–Quiet Lake area, fastcooling of upper plate tectonites at ca. 195 Mapostdates high P-T subduction-zone deformationand is interpreted to mark the uplift and coolingof upper plate rocks due to collision; lower platetectonites, which show top-to-the-east-northeastshear presumably related to collision, were tec-tonically buried at ca. 195 Ma and cooled slowlythrough 500 to 300 °C from ca. 145 to 119 Ma(Hansen et al., 1991). Little Salmon Lake upperplate tectonites cooled slowly from ca. 191 to160 Ma; cooling is interpreted to have resultedfrom erosional exhumation following collision(Oliver et al., 1995; Oliver, 1996). In east-centralAlaska, upper plate tectonites cooling through500 to 300 °C from 188 to 185 Ma postdatednorthwest-directed ductile shear, which we in-terpret as forming during collision.

More than 50 m.y. after northwest-directedcollision, upland tectonites in Alaska, and likelyin westernmost Yukon, underwent southeast-directed crustal extension resulting in tectonicdenudation of parautochthonous strata (Hansenet al., 1991; Pavlis, 1989; Pavlis et al., 1993).Synchronous with this extension, the westernNorth American margin in central and southernYukon was cut by steep, northwest-striking dex-tral faults, including the Tintina fault. Dextral dis-placement and northwestward translation ofNorth American margin rocks and imbricatedallochthonous rocks, resulted in dismembermentof the pre-mid-Cretaceous western margin ofNorth America.



Stikinia volcanic arc

Anvil ocean/oceanic crustparautochthonous North American continental margin

marginal-basin strata (lower-plate Nisutlin strata, to be tectonitized upon collision)

high-P subduction zone complex(Teslin-Taylor Mountain tectonitesand upper-plate Nisutlin tectonites— early formed blueschist and eclogite)

low-P accretionary complex(Slide Mountain-Seventymile)

Nisling-Tracy Armcontinental rocksrifted from North Americain the Paleozoic

Paleozoic plutons, to be tectonizedupon collision — will yield post-midJurassic cooling

Paleozoic plutons— will yield pre-mid Jurassic cooling

hanging-wall plate footwall plate

✪

✇

✇

✪

Figure 8. Tectonic model (modified from Hansen et al., 1991); units and symbols as in Figure 1.

Stikinia was also translated to the northwestduring mid-Cretaceous time, although paleomag-netic, paleontologic, and geologic evidence indi-cate that paleolatitudes for Stikinia rocks wereconcordant with those of ancestral North Amer-ica during Triassic and Early Jurassic (and LatePermian?) time (e.g., Irving and Monger, 1987;Irving and Yole, 1987; Irving and Wynne, 1990,1991a, 1991b; Oldow et al., 1989). Northwarddisplacement of Stikinia relative to North Amer-ica in Cretaceous time following southwardJurassic–Cretaceous movement is consistent withplate-tectonic reconstructions (Engebretson et al.,1985), Middle Jurassic sinistral shear along theTally Ho shear zone within the Stikinia rocks ofsouthern Yukon (Hart and Radloff, 1990), andother early Mesozoic shear zones of the southernCordillera (see Oldow et al., 1989, for discus-sion). The importance of this “yo-yo tectonics”with respect to this study is that in Late Permianthrough Early Jurassic time, the Stikinia arc andassociated upper plate rocks were located atabout the same latitude that they are today—forming a band from Fairbanks to ~57°N. Thus,Stikinia apparently collided with North Americaat about the same location that it is today, and af-ter collision Stikinia was translated to the southduring Jurassic–Cretaceous time, and translatednorth again in Cretaceous time.

IMPLICATIONS

The proposed model incorporates along-strikechanges in paleogeography, including the widthof the Anvil ocean, the character of the Stikiniaarc basement, the shape of the western continen-tal margin of North America (poorly constrained),and local deposition of marginal-basin strata.Therefore, the model predicts a multitude ofalong-strike (and across-strike) changes in sedi-mentary facies, intrusive composition and inheri-tance, and preserved structural level. Despite al-lowable along-strike differences, there are a fewtests of the basic model. Two important tests con-cern the structural-kinematic, metamorphic, andthermal history of Devonian–Mississippian plu-tons, and the distribution of Late Triassic–EarlyJurassic plutons.

Paleozoic Orthogneiss

Devonian–Mississippian plutons are includedwithin both the lower plate orthogneiss and theupper plate Nisling–Tracy Arm assemblages,and all of the plutons yield εNd values indicativeof continental crust. The difference between thetwo plutonic assemblages is their postcrystal-lization histories. If the Nisling–Tracy Armrocks are rifted remnants of the western marginof North American (Tempelman-Kluit, 1979),

then Nisling–Tracy Arm and North Americanstrata should have a shared prerift history, butdifferent postrift histories. In Mississippian time,Nisling–Tracy Arm rocks were rifted from NorthAmerica and stranded on a different plate, shed-ding detritus into the enlarging basin to the east,which separated them from western NorthAmerica. In Early Permian time, Nisling–TracyArm rocks locally formed the basement for theStikinia arc, which formed during west-dippingsubduction of the Anvil ocean. The Nisling–Tracy Arm assemblage thus resided in an upperplate position relative to the orthogneiss assem-blage, which remained part of North America.With continued basin closure, and finally colli-sion in Early Jurassic time, the Nisling–TracyArm assemblage, together with the arc and accu-mulated subduction complex, were thrust overtheir pre-Pennsylvanian North American coun-terparts. The Nisling–Tracy Arm assemblageclaimed a structurally high position relative toNorth American strata, which were tectonicallyburied. Mid-Cretaceous crustal extension re-sulted in local exposure of North Americanstrata, which yield postaccretion cooling ages. Inthis scenario, the isotopic signature inheritedfrom plutonic protoliths within these two pack-ages of rocks would be expected to be similar,even though the subsequent structural, metamor-phic, and thermal histories of the rocks are dif-ferent. Without the benefit of integrated kine-matic and thermochronologic data, these twoassemblages appear to be correlative. In our in-terpretation, North American rocks of theattenuated margin yield post-Middle Jurassic(mostly mid-Cretaceous) cooling ages and arenow at deep structural levels as a result of beingoverthrust in Early Jurassic time; whereas theNisling–Tracy Arm assemblage, which has sim-ilar pre-Pennsylvanian characteristics, resides athigh structural levels and yields pre-MiddleJurassic cooling ages.

Studies from three areas in central Yukon pro-vide an opportunity to examine the Mississippianplutons and their postemplacement kinematicand cooling histories. In the Laberge–Quiet Lakearea (Fig. 1), two ca. 355 Ma plutons (now ortho-gneiss) intrude Cassiar strata (Hansen et al.,1989). Klippen of Teslin–Taylor Mountain andNisutlin tectonites are structurally above the or-thogneiss and their host pelitic metasedimentarytectonites. The orthogneiss and host rocks recordtop-to-the-east-northeast ductile shear and slowcooling from Late Jurassic to mid-Cretaceoustime (Hansen et al., 1991; Hansen 1992b). Theserocks are therefore lower plate, parautochthonousNorth American strata. In the Little Salmon Lakearea, an orthogneiss at the highest exposed struc-tural level yields a U-Pb zircon age of 353 Ma,and a biotite 40Ar/39Ar cooling age of 215 Ma

(Oliver, 1996). The Little Salmon Lake ortho-gneiss, like the Laberge-Quiet Lake orthogneiss,yields εNd values consistent with North Ameri-can affinity (Bennett and Hansen, 1988). The Lit-tle Salmon Lake orthogneiss displays apparentnormal dip-slip shear fabrics (interpreted as sub-duction backflow; Oliver, 1996), consistent withupper plate tectonites in the Laberge–Quiet Lakearea to the south. The orthogneiss and its hostrocks are interpreted as upper plate Nisling–Tracy Arm assemblage, consistent with their highstructural position and the pre-Middle Jurassiccooling ages.

In the Teslin map area (Fig. 1), 340–350 Maplutons, locally deformed to orthogneiss, resideat the highest exposed structural level, and yieldεNd ratios consistent with North American con-tinental affinity (Stevens et al., 1996; Stevens andErdmer, 1996; Creaser et al., 1997). Althoughmetamorphic cooling ages are not available inthis area, host tectonites are intruded by a mas-sive hornblende-bearing tonalite to quartz dioritepluton with a concordant 207Pb/206Pb age of 215Ma, and a 40Ar/39Ar hornblende age of 188 Ma(Stevens et al., 1996). The temporal relations be-tween tectonism and Early Jurassic plutonism aresimilar to those of the Taylor Mountain tectonitesand the Taylor Mountain batholith in east-centralAlaska. It is most plausible that the orthogneisscooled at about the same time as the dioritepluton—in Early to Middle Jurassic time. We in-terpret these rocks as upper plate Nisling–TracyArm, similar to the upper plate interpretation putforward by Stevens et al. (1996).

Parautochthoneity of the inboard orthogneissassemblage, which is correlated with Kootenayrocks of southern British Columbia (Wheeler andMcFeely, 1991), is consistent with the assertionthat lower Paleozoic strata of the Kootenay ter-rane, southern British Columbia, are deposition-ally linked to and conformable with rocks of theNorth American miogeocline (Colpron and Price,1995). The interpretation of parautochthoneity isalso strengthened by the recognition of Mississip-pian magmatism in autochthonous miogeoclinalCassiar strata east of the Tintina fault. Petrologi-cal and geochemical data indicate probable for-mation within an extensional setting (Mortensen,1982), consistent with stratigraphic and structuralevidence for mid-Paleozoic extension of the Cas-siar assemblage (Gordey et al., 1987). Ortho-gneiss assemblage metabasalt trace element ratiosshow mid-oceanic-ridge basalt and within-plategeochemical affinities supporting correlation withthe pericratonic Kootenay terranes, and suggest-ing an origin within an attenuated continentalmargin (Dusel-Bacon and Cooper, 1995). Thus, itis possible that Devonian and Mississippian gran-itoids that intruded North American marginal-basin strata represent an intrusive component of

V. L. HANSEN


intermittent early Paleozoic extension (Rubin etal., 1990; Colpron and Price, 1995).

Late Triassic–Early Jurassic Plutons

The Late Triassic–Early Jurassic calc-alkalineplutons, heralded as probable Stikinia (e.g.,Aleinikoff et al., 1986), also provide a criticalpreaccretion link between upper plate assem-blages and a distinction from lower plate assem-blages. Although the Nisling–Tracy Arm assem-blage is geochemically similar to NorthAmerican strata, it has a different post-Mississip-pian geologic history than its pre-PennsylvanianNorth American counterparts. In addition to thedifference in late Paleozoic–early Mesozoic his-tories outlined here, the Nisling–Tracy Arm as-semblage is intruded by Late Triassic–EarlyJurassic plutons (e.g., Johnston and Erdmer,1995), as are the Teslin–Taylor Mountain andSlide Mountain–Seventymile assemblages. Incontrast, to date there are no documented exam-ples of Late Triassic to Early Jurassic calc-alkaline plutons intruding the orthogneiss,Kootenay, or Cassiar assemblages in Yukon orAlaska (see Dusel-Bacon et al., 1995). Thusupper plate rocks (Nisling–Tracy Arm, Stikinia,Teslin–Taylor Mountain, Slide Mountain–Seven-tymile, and locally Nisutlin assemblages) werelikely separate from lower plate parautochtho-nous and autochthonous North American stratain Late Triassic–Early Jurassic time.

Crustal Assembly

The data favor a model in which upper plateassemblages (Nisling–Tracy Arm, Stikinia, SlideMountain–Seventymile, Teslin–Taylor Moun-tain, and Nisutlin rocks) are thrust over lowerplate rocks of the attenuated North Americanmargin (orthogneiss, Kootenay, and Cassiar as-semblages) more than 50 m.y. before crustal ex-tension. Although the crust was extended duringtop-to-the-southeast shear in the Alaskan upland,central and southern Yukon was dissected bynorthwest-striking dextral strike-slip faults. The50 m.y. time gap between crustal thickening andextension, and the synchroneity of extension anddextral translation in Alaska and Yukon, respec-tively, may place constraints on the mechanismof extension.

Pavlis et al. (1993) proposed three end-mem-ber models for mid-Cretaceous upland extension,which are dependent, in large part, on regionaltiming relations and kinematics: (1) a two-phasemodel involving Jurassic collision and Creta-ceous extension, using the Neogene Hellenic arcas an analog; (2) a syncollisional model, employ-ing the Carpathian Mountains as an analog; and(3) a syncollisional plateau uplift model, exten-

sion being driven by gravity spreading, a Hima-layan analog. The Carpathian and Himalayananalogs require Early to mid-Cretaceous colli-sion of allochthonous rocks with the NorthAmerican margin, and near temporal equivalenceof contraction and extension. Our data are con-sistent only with the Hellenic arc analog, al-though it could also fit a model in which exten-sional collapse significantly postdates collisionalthickening (e.g., Rubin et al., 1995).

The Hellenic arc analog for mid-Cretaceousextension in the upland (Pavlis et al., 1993),based on the Neogene collision and related back-arc extension of the eastern Mediterranean, sim-ilar to that proposed by Pavlis (1989), assumeslate Paleozoic to Triassic west-dipping subduc-tion followed by Middle Jurassic collision ofStikinia with North America. Following colli-sion, a subduction zone dipping in the oppositedirection, beneath North America, formed alongthe western margin of North America and re-mained active until the collision of the outboardPeninsular-Alexander-Wrangellia superterranein mid-Cretaceous time. Initial collision of thesuperterrane in Early to mid-Cretaceous time re-sulted in formation of a new subduction zonealong the trailing edge of the superterrane. As aresult of collision, the Andean margin of NorthAmerica was isolated from plate convergence,and convergence across the older, inboard sub-duction zone became slower than the rate of con-vergence between North America and the out-board oceanic plates, resulting in back-arcspreading due to slab rollback. Subduction roll-back, causes in turn, southeast-directed exten-sion in the upland. The biggest problem with theHellenic arc analog, as pointed out by Pavlis etal. (1993), is the lack of clear evidence for amagmatic arc on North America from latestJurassic to mid-Early Cretaceous time (e.g.,Armstrong, 1988).

Rubin et al. (1995) proposed subduction roll-back as a cause for mid-Cretaceous crustal exten-sion within the context of the entire northerncircum-Pacific of western Canada, southeast andnorthern Alaska, and the Chukotka Peninsula,Russia. These workers related rollback to anabrupt change in motion of the subducting Faral-lon plate at ca. 120 Ma, and coincidental openingof the Canada basin by rifting or by strike-slipfaulting. Retreat of the paleo-Pacific margin ac-companied opening of the Canada basin, result-ing in rollback subduction and subsequent south-directed extension across much of interiorAlaska. Either of these models for rollback sub-duction and subsequent south-directed (south-east-directed) crustal extension are consistentwith our data from the upland tectonites; how-ever, synchronous dextral translation along dis-tributed steep northwest-striking faults in central

Yukon are predicted by the large-scale plate in-teractions proposed by Rubin et al. (1995), andmight not be expected as a result of superterraneaccretion proposed by Pavlis et al. (1993).

Our study of the upland tectonite sheds lighton Mesozoic crustal assembly processes in thenorthern Cordillera. With the recognition of ter-ranes, Coney (1981) proposed that huge volumesof new crust were added to western North Amer-ica in a very short time (7.23 × 106 km2 during165 m.y.). Our data indicate that this volume ofnew crust might be greatly overestimated. Al-though a map view of Yukon and Alaska revealsmuch apparent new crust, much of the new crustforms large thin klippen, and thus represent verylittle volume. In addition, large tracts of one ofthe largest previously proposed “accreted ter-ranes”—the Yukon-Tanana composite terrane—is dominantly composed, volumetrically, of thestructurally lowest unit composed of NorthAmerican parautochthonous strata. These rocksare not new to North America; they have simplybeen “repainted” by allochthonous fragments inEarly to Middle Jurassic time, and locally trans-lated along their parent margin. Mid-Cretaceousextension revealed the true nature of the base ofthe crust as allochthonous “paint” was tectoni-cally denuded and eroded. The chipped alloch-thonous “paint” exposes the crustal foundation ofNorth American strata. Furthermore, part of thetectonically accreted crust, the Nisling–TracyArm assemblage, is also not new to the NorthAmerican continent, but is simply rifted frag-ments, now returned, perhaps in a different loca-tion, within the parent continent. Thus the vol-ume of continental crust, which is actually new tothe North American continent within the northCordillera, might include only terrain west of theNisling–Tracy Arm assemblage.

CONCLUSIONS

Integrated kinematic, P-T, and thermochrono-metric data allow us to distinguish allochthonoustectonites from parautochthonous tectoniteswithin the Yukon-Tanana upland. Upland tec-tonites define a regionally coherent stacking or-der; from bottom to top, these are the NorthAmerican parautochthonous continental margin,continental marginal-basin strata, and ocean-basin tectonites locally intruded by arc plutons.We delineate three major deformation events intime, space, and structural level across the Yukon-Tanana upland: (1) pre-Early Jurassic (> 212Ma), northeast-directed, apparent margin-normalcontraction that affected allochthonous oceanic,arc, and marginal-basin rocks; (2) late Early toearly Middle Jurassic (>188–185 Ma), north-west-directed, apparent margin-parallel contrac-tion and imbrication that resulted in juxtaposition



of the allochthonous tectonites with parau-tochthonous continental rocks; and (3) Early Cre-taceous (135–110 Ma), southeast-directed crustalextension that resulted in exposure of the struc-turally deepest, parautochthonous continentalrocks. These events can be correlated across theupland and through south-central Yukon, al-though mid-Cretaceous deformation in the Yukonwas dominated by dextral translation along steepnorthwest-striking faults, in keeping with the re-gional plate kinematics predicted by plate mar-gins and relative plate motions.

The oldest recognized deformation took placewithin a west-dipping middle Permian–Late Tri-assic subduction zone, above the Stikinia arc. De-formation variably affected upper plate rocks, in-cluding local continental basement to the arc, andoceanic and marginal-basin strata. These upperplate tectonites were locally intruded by Late Tri-assic to Early Jurassic calc-alkaline arc plutons.They were then thrust over parautochthonousNorth American strata (lower plate rocks) in Earlyto Middle Jurassic time. Mid-Cretaceous rollbacksubduction may have caused southeast-directedcrustal extension of the upland region, resulting intectonic denudation and exposure of the previ-ously overthrust parautochthonous strata.

Prior to Permian subduction, the Anvil oceanformed due to rifting of the western margin ofnorthern North America. The rifted continentalfragments have North American geochemicaland isotopic fingerprints, although their Pennsyl-vanian to Late Jurassic histories are very differentfrom those of their North American counterparts.Both packages were intruded by Devonian-Mississippian plutons, but the rifted Nisling–Tracy Arm assemblage came to reside in the up-per plate of the convergent margin that broughtthese assemblages back together, whereas theNorth American strata reside in the lower plate.

ACKNOWLEDGMENTS

Hansen’s work was supported by National Sci-ence Foundation grant EAR-9103531. We greatlyappreciate discussions with H. Foster, J. Goodge,R. Law, K. Meisling, J. Mortensen, D. Oliver,T. Pavlis, and F. Weber. We thank N. Mancktelowfor early use of his stereo program; J. Goodge,D. Oliver, and J. Willis for comments on earlyversions of the manuscript; and D. Bradley,R. Miller, T. Pavlis, and S. Roeske for careful re-views. C. Bacon and S. Douglass provided expertfield assistance over several summers. We thankRon Warbelow for expert helicopter piloting, andVicki, June, and Larry Taylor for their wonderfulhospitality. Above all, we thank Helen Foster forher years of objective mapping in east-centralAlaska, her devotion to recording accurate datathat we are only beginning to understand, and her

willingness to share her observations and inter-pretations gained from covering endless miles ofthe upland on foot.

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