LaMaskin et al 2015 J Geology

36 Pages·2015·3.34 MB·English

Westward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and
Western Idaho, United States
Author(s): Todd A. LaMaskin, Rebecca J. Dorsey, Jeffrey D. Vervoort, Mark D. Schmitz, Kyle
P. Tumpane and Nicholas O. Moore
Source: The Journal of Geology, (-Not available-), p. 000
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/10.1086/681724 .
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Westward Growth of Laurentia by Pre–Late Jurassic Terrane
Accretion, Eastern Oregon and Western Idaho, United States
Todd A. LaMaskin,1,* Rebecca J. Dorsey,2 Jeffrey D. Vervoort,3 Mark D. Schmitz,4
Kyle P. Tumpane,4 and Nicholas O. Moore1
1. DepartmentofGeography and Geology, University of North Carolina Wilmington, 601 South CollegeRoad,
Wilmington, North Carolina 28403-5944, USA; 2. Department of GeologicalSciences, University ofOregon,
1272 University of Oregon, Eugene, Oregon 97403-1272, USA; 3. School of theEnvironment, Washington
State University, Pullman, Washington 99164-2812, USA; 4. DepartmentofGeosciences, Boise
State University, 1910University Drive, Boise, Idaho 83725-1535, USA
ABSTRACT
NewU-PbandSm-NddatafromtheBlueMountainsprovince,easternOregonandwesternIdaho,clarifyterranecor-
relationsandregionalevolutionofthewesternLaurentianplatemarginduringMesozoictime.WereportanEarlyJu-
rassicageforaredtuffunitatPittsburgLanding,Idaho,whichis25m.yr.olderthanpreviousMiddleJurassicestimates.
IntheCoonHollowFormationatPittsburgLandingandatthetypelocationontheSnakeRiver,chemicalabrasion
thermal ionization mass spectrometry U-Pb zircon ages on interbedded tuff and detrital zircon U-Pb maximum de-
positionalagesindicatethatdepositionspannedca.160–150Ma,entirelyduringLateJurassictime.DetritalzirconU-Pb
ages represent local Wallowa arc basement and regional magmatic sources spanning ca. 290–140 Ma. Mudrock Nd
isotopecompositionsoftheCoonHollowFormationrecordanincreaseinjuvenilemagmatismconsistentwithregional
LateJurassictrendsinwesternNorthAmericanmagmaticsystems.ThesedatashowthattheCoonHollowFormation
isnotpartofaMiddleJurassicoverlapassemblage,ashasbeenhistoricallyinterpreted.Instead,weproposethatthe
Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest
Jurassictimeduetoawell-documentedperiodoftrenchretreatinthewesternUnitedStates.Ournewdatarequirethat
theunderlyingWallowaterranewasaccretedtoandreceiveddetritusfromwesternNorthAmericabyca.160Ma(early
Late Jurassic). This minimum estimate forthe age of terraneaccretion in western Idaho and eastern Oregon is sub-
stantially earlier than previous estimates (∼135–118 Ma). In the Blue Mountains region, westward expansion of
LaurentiawasaccomplishedbyaccretionofarcterranestotheNorthAmericancratonpriortoLateJurassictime.
Online enhancement:supplementarydata.
Introduction
Late Paleozoic to early Mesozoic terranes in the aseitherbeforeorafterLateJurassictime(e.g.,Davis
western United States (fig. 1) record westward ex- et al. 1978; Harper and Wright 1984; Wright and
pansionofcontinentalcrustbyaccretionofvolcanic Fahan 1988; Oldow et al. 1989; McLelland et al.
arclithosphere.Whileauthorsagreethataccretion 1992;Hackeretal.1995;Mooresetal.2002;Dorsey
of these late Paleozoic to early Mesozoic terranes and LaMaskin 2007, 2008; Dickinson 2008; Ernst
records Mesozoic westward expansion of the Lau- et al. 2008; Hildebrand 2009, 2013; Schwartz et al.
rentianmargin,significantcontroversiesremainun- 2010, 2011a, 2011b, 2014; LaMaskin 2012; Sigloch
resolved,includingtheoriginoftheseterranesasei- andMihalynuk2013).
therfar-traveledorfringing-arcsuccessionsandthe Afterdecadesofresearchintheaccretedterranes
timingoftheiraccretiontowesternNorthAmerica of the Blue Mountains province (BMP) of eastern
OregonandwesternIdaho(figs.1,2),twocontrast-
ing end-member tectonic models for the Jurassic
Manuscript received January 9, 2014; accepted March 19,
tectonicsettingandevolutionhaveemerged(fig.3).
2015;electronicallypublishedMay29,2015.
* Authorforcorrespondence;e-mail:[email protected]. Thefirstmodel(fig.3A)proposesEarly–MiddleJu-
[TheJournalofGeology,2015,volume123,p.000–000]q2015byTheUniversityofChicago.
Allrightsreserved.0022-1376/2015/12303-00XX$15.00.DOI:10.1086/681724
000
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000 T. A. LAMASKIN ET AL.
Figure1. Simplifiedpre-TertiarygeologyofthewesternUnitedStates,modifiedfromWyldetal.(2006).BMpBlue
Mountains province; KM p Klamath Mountains; MSNI p Mojave–Snow Lake–Nevada–Idaho fault (afterWyld and
Wright 2005); NWCp northwestern Cascades; SNp SierraNevada.
rassic noncollisional double subduction, followed and accretion in this region with implications for
by Late Jurassic offshore arc-arc collision (Dickin- the dynamic early Mesozoic tectonic setting of
son 1979, 2004; Avé Lallemant 1995; Schwartz western North America. We show that early Me-
et al. 2010, 2011a, 2011b, 2012). We use the term sozoic westward growth of Laurentia occurred by
“collision” specifically for convergent plate inter- tectonic accretion of previously amalgamated arc
actions in which buoyant crust is drawn into a terranes prior to 160 Ma, significantly earlier than
trenchzone,effectivelyjammingsubduction,which proposed in previous models for the Blue Moun-
leads to crustal shortening, thickening, metamor- tains. Our new data also permit direct correlation
phism, and uplift (sensu Cloos 1993; Dorsey and to other terranes and basin-fill successions of sim-
LaMaskin 2007; Dickinson 2008). A second model ilaragealongthewesternUSCordilleraandprovide
(fig.3B)proposesEarly–MiddleJurassicaccretionof new insight into the tectonic evolution of subduc-
thepreviouslycollidedBMPterranestothewestern tionplatemargins.
North American margin by closure of a marginal
basin, followed by postaccretion outboard subduc-
Geologic Setting
tion in the BMP (e.g., Dorsey and LaMaskin 2007,
2008;LaMaskinetal.2008,2011a,2011b;LaMaskin BMPTerranes. TheaccretedterranesoftheBMP
2012). (figs.1,2)includeislandarcsandarc-relatedbasins
Inthisarticle,wepresentnewU-Pbgeochronol- that formed in proximity to the North American
ogy and mudrock Nd isotope compositions from craton (Olds Ferry, Izee, and Baker terranes) and
EarlyandLateJurassicrocksoftheBMP(figs.1,2, offshore from North America (Wallowa terrane).
4). These data place important new constraints on The Olds Ferry and Izee terranes are the most in-
thetimingandstyleofMesozoicterraneevolution board (farthest east in restored coordinates) of the
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nnyrp
datapresentediplutonsareshowterraneboundarunitsoftheBakeCreekfault;WB
includeAgesofsand/orne-levelPoison
to3).onrap
ed01atiterCF
fibal.(2011)andmodi2012),andAnderson(2fiuncertainterraneaflirehereconsideredsubandLaMaskin2007);P
askinetketal.(ndicateschistaDorsey
ceslistedinLaMŽrzetal.(2011),áQuestionmarksidtheBurntRivergasequence(after
ourKue).anme
msb),agne,p
edfroa,2011zirconeterrast;MS
fi 1bnhi
ce,modital.(201pzU-PGrindstoRiversc
provinwartzenage;s,theBurnt
ntains1),Schm-argoterranepBRS
BlueMouetal.(201ppotassiunhornsubbatholith;
GeologicmapoftheFigure2.Garwoodetal.(2008),Northrupwheredataareavailable(k-arlocations.TheBourneandGreepterrane.BMBBaldMountainWallowabatholith.
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Figure3. ModelsforJurassictectonicsettingandevolutionoftheBlueMountainsprovince(BMP).A,Early–Middle
Jurassicnoncollisionaldoublesubduction,followedbyLateJurassicoffshorearc-arccollision(Dickinson1979,2004;
AvéLallemant1995;Schwartzetal.2010,2011a,2011b,2012).B,Early–MiddleJurassicaccretionofthepreviously
collidedBMPterranestothewesternNorthAmericanmarginbyclosureofamarginalbasin,followedbypostaccretion
outboard subduction in the BMP (e.g., Dorsey and LaMaskin 2007, 2008; LaMaskin et al. 2008a, 2011a, 2011b;
LaMaskin2012).BpBakerterrane;FTBpfold-thrustbelt;N.A.pNorthAmerica;OFpOldsFerryterrane;S.L.psea
level;TBpthrustbelt;WapWallowaterrane.
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Journal of Geology WESTWARD GROWTH OF LAURENTIA BY ACCRETION 000
Figure4. Samplelocationsplottedonachronostratigraphiccorrelationchartforsedimentaryandvolcanicrocksin
theBlueMountains.ModifiedfromDorseyandLaMaskin(2007).SeeDorseyandLaMaskin(2007)foradditionaldata
sources.Notethatformationtimerangesareplottedaccordingtonewdatapresentedinthisarticle.CA-ID-TIMSp
chemical abrasion isotope dilution thermal ionization mass spectrometry; CH p Coon Hollow Formation; DC p
Doyle Creek Formation; HW p Hurwal Formation; LA-ICP-MS p laser ablation inductively coupled plasma mass
spectrometry; MBLS p Martin Bridge limestone.
BMP terranes and contain Permian?–Jurassic plu- continent-fringing volcanic arc system (LaMaskin
tonic,volcanic,andvolcaniclasticsedimentaryrocks et al. 2008a, 2011b; Tumpane 2010; Kurz 2010;
(figs.2,4).IntheOldsFerry–Izeesuccession,LateTri- Northrupetal.2011).
assicsedimentaryrockscontainabundantArchean– TheBakerterranesubduction-accretionarycom-
Paleozoicdetritalzircongrainsderivedfromrecycled plex separates the Wallowa and Olds Ferry–Izee
late Paleozoic arcs, whereas Jurassic deposits con- terranes and represents deformed Olds Ferry–Izee
tainabundantProterozoic–Mesozoicgrainssourced forearc crust and the associated accretionary prism
from continental rocks to the east (LaMaskin et al. (fig. 2; Brooks 1979; Dickinson 1979; Ferns and
2011b). Enriched sedimentary trace element com- Brooks 1995; Schwartz et al. 2010). Rocks of the
positionsandpre-MesozoicdetritalzirconU-Pbages Baker terrane display both juvenile and evolved iso-
indicatethatOldsFerry–IzeeJurassic-agearc-related topic compositions (Schwartz et al. 2010) and con-
basinsreceivedcontinentallysourcedsediment,sup- tain Archean and Proterozoic detrital zircon grains
porting the interpretation that they represent a (AlexanderandSchwartz2009).TheBakerterraneis
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000 T. A. LAMASKIN ET AL.
widelyconsideredtobecorrelativetosimilarrocks placestheBMPattheapproximatelatitudeofsouth-
oftheCacheCreekterraneinBritishColumbia(e.g., western Arizona and northern Mexico adjacent to
Kaysetal.2006;Wyldetal.2006). the Jurassic continental arc (Saleeby and Busby-
TheWallowaterraneincludesPermian–EarlyJu- Spera1992;DickinsonandLawton 2001).
rassic arc-related rocks and occupies an outboard Salmon River Fold-Thrust Belt and Western Idaho
(oceanward) position relative to other terranes in ShearZone. InwesternIdaho,theWallowaterrane
the BMP (fig. 2).Permian–Triassicrocks of the Wal- isstructurallyoverriddenbytheSalmonRiverfold-
lowa terrane show relativelyflat rareearth element thrust belt, and the inboard Baker, Izee, and Olds
patternsandnegligibleEuanomalies,suggestingthat Ferry terranes of the BMP appear to be absent be-
duringmostoftheTriassictimetheWallowaterrane cause of regional tectonic truncation (fig. 2). The
did not receive sediment from continental sources Salmon River beltis azoneof strong crustal short-
andlikelyrepresentsajuvenile,intraoceanicvolcanic ening and thickening that consists of greenschist
arc (LaMaskinet al. 2008a, 2008b; Kurz2010).Wal- toamphibolitefaciesrocksexposedinabeltoftop-
lowaarcbasementincludesPermian–LowerTriassic to-the-west thrust nappes where the metamorphic
plutonic rocks (ca. 265–249 Ma) and Late Triassic grade increases eastward to higher structural lev-
rocks(ca.229and215Ma;Walker1995;Kurzetal. els(fig.2;e.g.,LundandSnee1988;Selverstoneetal.
2011).VolcaniccoverrocksoftheWildSheepCreek 1992;Gettyetal.1993;GrayandOldow2005;Blake
and Doyle Creek Formations are Late Triassic in et al. 2009; Gray et al. 2012). The western struc-
age(ca.230Ma;Vallier1995;LaMaskinetal.2008b; turalboundaryoftheSalmonRiverbeltistheHeav-
Kurz et al. 2011). At isolated localities along the ensGatefault,aneast-dippingthrustfaultthatwas
Oregon-IdahoborderandinfarwesternIdaho,Wal- reactivated as a down-to-the-east normal fault dur-
lowaterranerocksaredepositionallyoverlainalong ingTertiaryextension(GrayandOldow2005;Blake
an angular unconformity by fluvial to deep-marine etal.2009;Grayetal.2012).TheHeavensGatefault
sedimentary rocks of the Coon Hollow Formation directly juxtaposes hanging-wall greenschist-grade
(CHF;figs.2,4,5A,6;Morrison1963;Vallier1977, tectonitesoftheSalmonRiverfold-thrustbeltagainst
1995;Goldstrand1994;LaMaskinetal.2008a;Kauff- unmetamorphosed to low-grade footwall rocks of
man et al.2009; Schmidt etal. 2009).The CHF has the Wallowa terrane, including the CHF. Rocks in
traditionallybeenconsideredtheuppermostunitof theSalmonRiverfold-thrustbeltappeartobemeta-
theWallowaterraneandthereforeiscommonlybe- morphosed equivalents of BMP supracrustal rocks
lieved to be allochthonous relative to North Amer- (Baker,Izee,andOldsFerryterranes),althoughpro-
ica(Vallier1977,1995). tolith ages and terrane affinities remain poorly un-
BMPterranesmayhavebeenaffectedbyunknown derstood(e.g.,LundandSnee1988;Blake1991;Sel-
amounts of dextral-transpressivedisplacementsdur- verstoneetal.1992;Gettyetal.1993;GrayandOldow
ing Mesozoic time, as inferred from regional paleo- 2005;Lundetal.2007;Blakeetal.2009;Schmidtetal.
magnetic inclinations, geologic correlations, and ki- 2013).
nematicfabricspreservedinregionalplutonicrocks, The Salmon River fold-thrust belt is bounded
callingintoquestiontheiroriginalpaleolatitude(e.g., ontheeastbythewesternIdahoshearzone(fig.2),
Wyld and Wright 2001; Housen and Dorsey 2005; a narrow, subvertical zone of tectonized Late
Wyldetal.2006).Differenttectonicreconstructions Cretaceous–Early Tertiary plutons coincident with
have relied on different amounts of displacement: asharpSr-isotopicboundarythatdefinestheedgeof
i
(1) !100 km of displacement (e.g., Dickinson 2004, the preaccretion North American continental mar-
2006;GrayandOldow2005)impliesthatduringpre- gin (Armstrong et al. 1977; Giorgis et al. 2005). Late
CretaceoustimetheBMPwasoutboardofan enig- Cretaceous deformation in the western Idaho shear
matic Pacific Northwest continental margin and zone resulted in sharp structural and isotopic trun-
that the history of this margin has been subse- cationoftheNorthAmericanmarginandcompletely
quently obscuredbyplutonemplacementandCre- obscures original contact relationships at the con-
taceous shortening; (2) ∼400 km of displacement vergent margin between accreted terranes of the
juxtaposes the Triassic–Jurassic western US fring- BMPandtheNorthAmericancraton(LundandSnee
ingarccomplexoftheBMP(OldsFerryterrane)with 1988;McClellandetal.2000;Giorgisetal.2005,2008;
island arc and back-arc basin rocks of northern Ne- Blakeetal.2009).
vada(BlackRockterraneandAuldLangSyneGroup Jurassic Tectonic Setting. The traditional model
in western Nevada; e.g., Oldow 1984; Wyld and (fig. 3A) for the Middle–Late Jurassic tectonic set-
Wright2001;Wyld2002;Wyld et al. 2003, 2006; La- tingoftheBMPconsistsof(1)Early–MiddleJurassic
Maskin et al. 2011a); and(3)11000kmofdisplace- noncollisionaloffshoresubductionbeneaththefac-
ment (e.g., Housen and Dorsey 2005, after 94 Ma) ingWallowaandOldsFerryarcterranesand(2)Late
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Journal of Geology WESTWARD GROWTH OF LAURENTIA BY ACCRETION 000
Figure5. FieldphotographsoftheCoonHollowFormationandtheredtuffunit.A,Angularunconformitybetween
theSevenDevilsGroupandoverlyingCoonHollowFormation,IdahosideofSnakeRiversouthofBigCougarCreek
(rivermile179.5).B,Thin-beddedtolaminatedammonite-bearingshalewithinterbeddedfinesandstone(arrows)in
the Coon Hollow Formation lower flysch unit, type locality. C, Chert-clast conglomerate in the Coon Hollow For-
mation upper flysch unit, type locality. D, Medium-bedded, normally graded beds of tuffaceous volcanic-clast con-
glomerateandvolcaniclasticsandstone,redtuffunit,PittsburgLanding,Idaho.E,MassiveconglomerateoftheCoon
HollowFormationfluvialunit,PittsburgLanding,Idaho.F,DetritalclastofcoralintheCoonHollowFormationfluvial
unit,PittsburgLanding,Idaho.Acolorversionofthisfigureisavailableonline.
JurassicoffshorecollisionoftheWallowaterranewith western North American craton ca. 128Ma during
the Olds Ferry, Izee, and Baker terranes (Dickinson strong crustal shortening and peak metamorphism
1979,2004;AvéLallemant1995;Schwartzetal.2010, intheSalmonRiverbelt(Gettyetal.1993;Schwartz
2011a, 2011b, 2014), followed by (3) later accretion et al. 2011a). Age constraints for Late Jurassic de-
of the amalgamated Blue Mountain terranes to the formation come from the youngest detrital zircons
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000 T. A. LAMASKIN ET AL.
Figure6. ImagesoftypeandPittsburgLandinglocalitiesshowingstratigraphicandstructuralsetting.A,CoonHollow
typelocation(GoogleImageq2011DigitalGlobe).ViewlookingnortheastacrosstheSnakeRiverintoIdaho.Adapted
fromMorrison(1963)andunpublishedmapping.B,PittsburgLanding.ViewlookingsoutheastalongWestCreek,Idaho,
acrosstheSnakeRiverintoOregon.AdaptedfromVallier(1998),Schmidtetal.(2009),andunpublishedmapping.C,View
of Early Jurassic red tuff unit overlain by the Late Jurassic Coon Hollow Formation fluvial. View looking north. Loca-
tionandagesofchemicalabrasionisotopedilutionthermalionizationmassspectrometryU-Pbzircongeochronology
samplesareshown.Acolorversionofthisfigureisavailableonline.
in deformed sedimentary rocks of the Izee terrane cordatransitionfromlowSr/Ymagmatismca.162–
(ca. 159 5 2 Ma) and crystallization ages of post- 157 Ma to high Sr/Y magmatism ca. 148–145 Ma,
kinematic plutons (ca. 154 5 1 Ma) in the Baker whichhasbeeninterpretedtorepresentcrustalthick-
terrane (Schwartz et al. 2011a). Plutonic rocks in eningduetoarc-arccollision(Schwartzetal.2011a,
the Baker terrane bracket this time frame and re- 2014).
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Journal of Geology WESTWARD GROWTH OF LAURENTIA BY ACCRETION 000
A different tectonic model (fig. 3B) involves two distinct metamorphic stages are recorded in
(1) Late Triassic–Early Jurassic offshore collision SalmonRiverrocks:(1)theBlueMountainterranes
(amalgamation) of the Wallowa terrane with the were amalgamated offshore prior to ca. 144 Ma,
Olds Ferry, Izee, and Baker terranes and (2) Middle and(2)regionalpeakmetamorphismintheSalmon
Jurassic accretion and collision of the previously Riverfold-thrustbeltwasreachedca.128Ma.Sub-
amalgamated Wallowa–Baker–Olds Ferry–Izee ter- sequent 40Ar/39Ar studies in Idaho have been inter-
ranes to the western US plate margin, followed by pretedtosuggestthattheoldestdeformationinthe
(3) Late Jurassic postaccretion outboard subduction regionoccurredca.130Ma(Sneeetal.1995,2007).
(e.g., Dorsey and LaMaskin 2007, 2008; LaMaskin Finally, a U-Pb age of 136 5 1 Ma on a deformed
et al. 2008a, 2011a, 2011b; LaMaskin 2012). This pluton in the hanging wall of the Heavens Gate
second model proposes that collisional tectonics fault has been interpreted to reflect post–136 Ma
played a significant role in the development of the accretion(Grayetal.2012;Gray2013).
BMPbutthatcollisionwascompletebyca.160Ma Field-based stratigraphic and structural analysis
(earliestLateJurassic). hasbeenusedtoarguethatpre-Cretaceousdocking
A long-standing point of agreement in the geol- of the Wallowa terrane is recorded in widespread
ogy of the region has been correlation of the CHF LateJurassiccontractionalstructuresbracketedbe-
inHellsCanyon(Wallowaterrane)withMiddleJu- tween 157 and 145 Ma (Gray and Oldow 2005). In
rassicIzeeterranerocksoftheJohnDayOregonre- this view, Late Jurassic accretionary deformation
gion(e.g.,PessagnoandBlome1986;Whiteetal.1992; is superposed on an earlier deformation history in
Dickinson2004,2008;DorseyandLaMaskin2007, the Salmon River fold-thrust belt that is related to
2008;LaMaskinetal.2008a).Thiscorrelationhasled structuralclosureofafringingarc(Izee–OldsFerry–
to the interpretation of a regional Middle Jurassic Bakerterranes)andanow-consumedback-arcbasin
sedimentary overlap assemblage (Blue Mountain in Oregon and Idaho (Gray and Oldow 2005).
collisional basin, Izee basin) as evidence for pre– StratigraphyoftheCHF. Atitstypelocality(figs.2,
MiddleJurassicamalgamationoftheBlueMountain 4, 6A), the CHF is divided into three conformable
terranes. map units: (1) basal volcaniclastic sandstone and
Incontrast,theageofaccretionoftheBlueMoun- limestone-clast conglomerate (∼20–60 m thick),
tain amalgamated arc terranes to western North (2) volcaniclastic lower flysch unit (∼80 m thick;
America remains a long-standing point of conten- figs.5B,7A–7C),and(3)chert-richupperflyschunit
tion, with interpretations ranging from ca. 165 to (∼300 m thick; figs. 5C, 7D; Morrison 1963; Gold-
118 Ma, an uncertainty of 45 m.yr. (e.g., Lund and strand 1994). The lower flysch unit at the type lo-
Snee1988;Selverstoneetal.1992;Gettyetal.1993; cality has been assigned an early Late Jurassic, Ox-
Snee et al. 1995, 2007; Gray and Oldow 2005; Dor- fordianage(ca.163.551.1to157.351.0Ma)onthe
seyandLaMaskin2007,2008;Schwartzetal.2010, basis of the ammonite Cardioceras (Scarburgiceras)
2011a,2011b,2014).Earlyworkinterpretedca.118 martinicollected∼125mabovethecontactwiththe
Ma ages on deformed hornblende from the Salmon underlyingSevenDevilsGroup(table1).
River fold-thrust belt as metamorphic crystalliza- At the Pittsburg Landing locality (figs. 2, 4, 6B),
tionages,suggestingthatsuturing(collision)ofBMP deposits traditionally mapped as the CHF are di-
terranes and North American continental rocks videdintothreeunits:(1)redtuffunit(∼120mthick;
occurred between ∼118 and 95 Ma during regional figs.5D,6C),(2)fluvialunit(∼420mthick;figs.5D–
metamorphism,crustalshortening,thickening,and 5F,6C,7E),and(3)marineunit(∼100mthick;fig.6C;
mountain building (Lund and Snee 1988). In con- White 1994; White and Vallier 1994). The red tuff
trast,Selverstoneetal.(1992)arguedthattheca.118 unitislithologicallydominatedbypinkvolcaniclas-
Maageisacoolingage.Theyconcludedonthebasis tictoepiclastic,monomictsandstoneandconglom-
of geothermometry and geobarometry that (1) deep erate;includesweldedtuffinitsupperparts(White
tectonicburialofBlueMountainterranesviaunder- 1986);andisoverlainbypolymictbrownsandstone
thrusting beneath the North American margin oc- andconglomerateandyellow-weatheringmudstone
curredduringEarlyCretaceoustimeca.13055Ma of the fluvial unit along a moderate angular un-
and (2) rapid exhumation and cooling due to de- conformity(figs.5E,6B,6C,7F).Thefluvialunitis
tachment and sinking of mantle lithosphere be- gradationallyoverlainbyblackshaleandmudstone
gan ca. 120 5 5 Ma. Selverstone et al. (1992) also of the marine unit, which in turn grades upsec-
suggested that collision-related thrusting in the tionintochert-richvolcaniclasticsandstone(fig.6B,
SalmonRiverfold-thrustbeltmayhavebeenunder 6C). The marine unit contains a fauna including
way as early as 160 Ma. On the basis of Sm-Nd coraltaxaidentifiedbyStanleyandBeauvais(1990)
dating of garnet, Getty et al. (1993) proposed that as Middle Jurassic in age (Bajocian, 170.3 5 1.4 to
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