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 . Accessed: 01/06/2015 17:42 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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. This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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. This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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). This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions 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 This content downloaded from 128.135.181.171 on Mon, 1 Jun 2015 17:42:41 PM All use subject to JSTOR Terms and Conditions