A detrital record of Mesozoic island arc accretion and ...record of exhumation associated with...

A detrital record of Mesozoic island arc accretion and exhumationin the North American Cordillera: U‐Pb geochronologyof the Kahiltna basin, southern Alaska

Brian A. Hampton,1 Kenneth D. Ridgway,2 and George E. Gehrels3

Received 1 June 2009; accepted 19 February 2010; published 3 August 2010.

[1] The stratigraphic record of Mesozoic arc accretionin the North American Cordillera is preserved in adiscontinuous belt of clastic strata that are exposedinboard (cratonward) of the allochthonous Wrangelliacomposite terrane in southern Alaska, western Canada,and Washington State. LA‐ICPMS analyses of eightsamples (n = 714 detrital zircon grains) collected atdifferent stratigraphic intervals from the Jurassic‐Cretaceous Kahiltna assemblage in southern Alaskareveals a bulk U‐Pb age distribution of Precambrian‐Mesozoic age grains (Mz 74%, Pz 11%, Pc 15%). Acomparison of U‐Pb ages from older to younger strati-graphic intervals within the Kahiltna assemblagereveals three stages of exhumation and basin develop-ment during arc accretion. Stages include (1) an initialLate Jurassic–Early Cretaceous pre/early collisionalphase during which detritus was derived almost solelyfrom Middle–Late Jurassic and Early Cretaceous mag-matic sources of the outboard Wrangellia compositeterrane (Mz 100%, Pz 0%, Pc, 0%), (2) a second EarlyCretaceous syncollisional phase that reflects the intro-duction of Paleozoic and Precambrian detritus from theinboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%)and an upsection increase in older detrital zircongrains compared to Mesozoic age grains (Mz 65%,Pz 11%, Pc 24%), and (3) a final Early‐Late Creta-ceous late/postcollisional phase that represents contin-ued detrital contributions from Precambrian–Mesozoicsource areas (Mz 19%, Pz 22%, Pc 59%) locatedinboard and outboard of the Kahiltna basin. Similarbulk trends in detrital zircon age populations havebeen reported from along‐strike, age‐equivalent strataof the Gravina belt (Mz 74%, Pz 20%, Pc 6%) insoutheastern Alaska suggesting that similar provenancetrends may exist in basins along this >2000 km ‐longcollisional zone. Citation: Hampton, B. A., K. D. Ridgway,and G. E. Gehrels (2010), A detrital record of Mesozoic island

arc accretion and exhumation in the North American Cordillera:U‐Pb geochronology of the Kahiltna basin, southern Alaska,Tectonics, 29, TC4015, doi:10.1029/2009TC002544.

1. Introduction[2] The addition of allochthonous material to a continental

margin represents a punctuated tectonic event in the historyof an orogenic belt that is often best preserved in thestratigraphic record of a sedimentary basin. In collisionalplate margin settings defined by continent‐scale suturezones and strike‐slip faults, both the obliquity of conver-gence during subduction and the overall protracted nature ofaccretion can result in a progressive pattern of ocean basinclosing and subsequent exhumation of a suture zone over ascale of millions of years [e.g., Graham et al., 1975;Ingersoll et al., 1995, 2003]. Exhumation associated withaccretionary events may be long‐lived (>30 Myr) and resultin thick successions of synorogenic clastic strata (>5 km)that record the provenance and depositional history ofaccretion through time. The application of U‐Pb dating ofdetrital zircons from sedimentary basins in suture zonesettings provides a sensitive tool for understanding long‐term interactions between accreting allochthonous materialalong the outboard margin of a basin and sources areasalong the inboard (cratonward) margin of a basin [e.g.,Gehrels et al., 1995; DeGraaff‐Surpless et al., 2003;Weislogel et al., 2006; Leier et al., 2007]. Comparison ofrelative contributions from exhuming source areas adjacentto long‐lived, regional‐scale, sedimentary basins provides avaluable approach to constrain spatial and temporal detritaltrends throughout a basin and progressive exhumationhistory along continental margins.[3] An unresolved problem in the evolution of the North

American Cordillera is the accretionary history along thenorthern Cordillera continental margin since the end of theTriassic. Mesozoic clastic marine strata are exposedthroughout western Washington, southwestern BritishColumbia and Yukon Territory, and southern Alaska(Figure 1) and occur along the suture between metamorphicand igneous units of the Intermontane belt and allochtho-nous igneous island arc material of the Wrangellia com-posite terrane [e.g., Berg et al., 1972; Brandon et al., 1988;Rubin and Saleeby, 1991; McClelland et al., 1992; Cohen etal., 1995; Kapp and Gehrels, 1998; Kalbas et al., 2007].The accretion of the Wrangellia composite terrane to theNorth American Cordillera represents the largest addition ofjuvenile crust to the western margin of North American inthe past 200 Myr [Coney et al., 1980; Plafker and Berg,

1Department of Geological Sciences, Michigan State University, EastLansing, Michigan, USA.

2Department of Earth and Atmospheric Sciences, Purdue University,West Lafayette, Indiana, USA.

3Department of Geosciences, University of Arizona, Tucson, Arizona,USA.

Copyright 2010 by the American Geophysical Union.0278‐7407/10/2009TC002544

TECTONICS, VOL. 29, TC4015, doi:10.1029/2009TC002544, 2010

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http://dx.doi.org/10.1029/2009TC002544

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HAMPTON ET AL.: A DETRITAL RECORD OF ARC ACCRETION TC4015TC4015

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1994]. The detrital record of this accretionary event ispotentially preserved in a >2000 km long discontinuous beltof Jurassic–Cretaceous strata that are located along theinboard (cratonward) side of the Wrangellia composite ter-rane (Figure 1).[4] In southern Alaska, exhumation associated with

accretion of the Wrangellia composite terrane is recorded inUpper Jurassic–Cretaceous strata of the Kahiltna assemblage[Ridgway et al., 2002; Hampton et al., 2007], the NutzotinMountains sequence [Manuszak et al., 2007], and theGravina belt [Kapp and Gehrels, 1998; Gehrels, 2001].Upper Jurassic–Cretaceous sedimentary basins in southernAlaska and farther south along the margin are closely linkedboth stratigraphically and compositionally to the Wrangelliacomposite terrane, and potentially contain the stratigraphicrecord of exhumation associated with island arc accretion tothe inboard Intermontane belt.[5] A key debate surrounding the Mesozoic exhumation

history of the northern Cordilleran is the classification ofUpper Jurassic–Cretaceous strata as related to either pre-collisional, syncollisional, or postcollisional processes. Arange of different models have been proposed for thetectonic setting of these sedimentary basins that include(1) preaccretionary basins that are related to the oceanicWrangellia composite terrane and formed and evolved faroutboard of the continental margin [Jones et al., 1977, 1982;Coney et al., 1980], (2) syncollisional basins that are relatedto collisional processes between the Wrangellia compositeterrane and the Mesozoic continental margin [Pavlis, 1982;McClelland et al., 1992; Ridgway et al., 2002; Trop andRidgway, 2007], and (3) syncollisional to postcollisionalsedimentary basins associated with strike‐slip faulting in thesuture zone and translation between the arc and margin[Wallace et al., 1989]. This study summarizes the spatialand temporal distribution of U‐Pb detrital zircons ages in theUpper Jurassic–Cretaceous Kahiltna assemblage in south-western and south central Alaska. We present an overviewhere on how detrital zircon provenance comparisonsbetween different stratigraphic intervals in synorogenicstrata can be used as powerful tool for constraining the

timing and nature of exhumation and basin fill during pro-tracted accretionary events along collisional convergentmargins.

2. Geologic Background[6] The Kahiltna assemblage crops out in a >400 km long

belt throughout south central and southwestern Alaska(Figures 1 and 2) and has been mainly described fromprevious studies in the context of regional geologic mappingprojects [e.g., Csejtey et al., 1978, 1992; Reed and Nelson,1980; Jones et al., 1982; Smith et al., 1988; Bundtzen et al.,1997]. The unit consists of a 3–5 km thick succession ofmainly submarine fan strata [e.g., Wallace et al., 1989;Eastham, 2002; Hampton et al., 2007; Kalbas et al., 2007]that represent >45 Myr of continuous clastic sedimentation.Thick successions of synorogenic strata together withoccurrences of Late Jurassic‐Cretaceous (Kimmeridgian–Cenomanian) marine macrofossils from the Kahiltna as-semblage [Eakins et al., 1978; Reed and Nelson, 1980;Jones et al., 1980, 1983; Wallace et al., 1989; Bundtzen etal., 1997] are cited as evidence that sedimentation per-sisted in the Kahiltna basin from Late Jurassic to at least thebeginning of Late Cretaceous time.[7] The Wrangellia composite terrane consists primarily

of volcanic, sedimentary, and metasedimentary strata of thePeninsular terrane (southwestern and south central Alaska),theWrangellia terrane (south central and southeastern Alaskaand southwestern British Columbia), and the Alexanderterrane (southeastern Alaska and northwestern BritishColumbia) (Figure 1) [Jones et al., 1977; Gehrels andSaleeby, 1987; Plafker and Berg, 1994]. Magmatic eventsassociated with the Wrangellia composite terrane arerepresented by a number of arc‐related rocks that are out-lined in Figure 1c. In southern Alaska, the Kahiltnaassemblage crops out in the suture zone between oceanic totransitional crust of the Wrangellia composite terrane andpredominately pericratonic crust of the Yukon‐Tananacomposite terrane (Figure 2).[8] The Yukon‐Tanana composite terrane in east central

and southeastern Alaska consists primarily of Paleozoic and

Figure 1. Location map, generalized geologic map, and summary of significant and long‐lived magmatic source areas innorthern parts of the North American Cordillera. (a) Location map showing the occurrence Upper Jurassic–Cretaceous strataexposed along the inboard (cratonward) margin of the Wrangellia composite terrane in southern Alaska and westernCanada. Note that the Intermontane belt is confined to regions inboard of the Wrangellia composite terrane and Denali faultand south and west of the Tintina fault. (b) Generalized geologic map specific to significant magmatic source areas andtectonostratigraphic terranes of the North American Cordillera. (c) Summary of significant magmatic source areas fromthe Wrangellia composite terrane and Intermontane belt in southern Alaska and western Canada. Key references for mag-matic source areas associated the Wrangellia composite terrane include Muller [1980], Aleinikoff et al. [1988], Brandon etal. [1986], Nokleberg et al. [1986, 1994], Gehrels and Saleeby [1987], Gardner et al. [1988], Dodds and Campbell [1988],Beard and Barker [1989], Onstott et al. [1989], Plafker et al. [1989], Gehrels [1990], Friedman et al. [1990], Monger andMcNicoll [1993], Pálfy et al. [1999, 2000], DeBari et al. [1999], Rioux et al. [2007], Roeske et al. [2003], and Snyder andHart [2007]. Key references for magmatic source areas associated with the Intermontane belt include Aleinikoff et al.[1981, 1986], Okulitch [1985], Dusel‐Bacon and Aleinikoff [1985], Wilson et al. [1985], Gehrels [2001], Mortensen[1990, 1992], Mortimer et al. [1990], Greig et al. [1992], Mihalynuk et al. [1992], Sevigny and Parrish [1993], Fosteret al. [1994], Friedman and Armstrong [1995], Ghosh [1995], Greig and Gehrels [1995], Childe [1996], Johnston et al.[1996], Childe and Thompson [1997], Currie and Parrish [1997], Pálfy et al. [1997, 2000], Klepeis et al. [1998],Deyell et al. [2000], Gehrels [2001], MacIntyre et al. [2001], Villeneuve et al. [2001], Butler et al. [2002], Dusel‐Bacon etal. [2002], Erdmer et al. [2002], Day et al. [2003], Gehrels et al. [2009], and Mahoney et al. [2009].


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Mesozoic metasedimentary and igneous rocks that representpericratonic crustal material that have been inboard (cra-tonward) of the Wrangellia composite terrane since middleMesozoic time. The Yukon‐Tanana composite terrane,together with the Stikinia, Quesnellia‐Slide Mountain, andCache Creek terranes of southwestern Yukon Territory andwestern British Columbia, have been referred to collectivelyas the Intermontane belt or “superterrane” in previousstudies [Monger et al., 1982; Saleeby, 1983; Wheeler andMcFeely, 1991]. The inboard, northern and eastern mar-gins of the Intermontane belt are bounded by the Tintinafault while the outboard, southern and western margins arebordered by the Wrangellia composite terrane (Figure 1).Amalgamation of terranes that make up the Intermontanebelt and subsequent accretion of these terranes to the NorthAmerican margin may have begun as early as Permian timewith the final stages of accretion by Middle Jurassic time[Monger and Berg, 1987; Mihalynuk et al., 1992; Plafkerand Berg, 1994; Gehrels, 2001]. Thus, the southern andwestern outboard margin of the Intermontane belt in Alaskaand western Canada likely represented the Cordilleracontinental margin by Middle Jurassic time. Precambrian‐Cretaceous magmatic source areas of the Intermontane beltare outlined in Figure 1c and provide a general summary ofpotential sources for detrital zircons that are present in theKahiltna assemblage.

3. Methodology[9] This study presents U‐Pb ages from detrital zircon

grains with magmatic origin (inferred here by U/Th values<5) that were collected within the context of eight mea-sured stratigraphic sections from throughout the UpperJurassic‐Cretaceous Kahiltna assemblage of southern Alaska(Figures 2 and 3). Thick stratigraphic successions (3–5 kmthick) and regional lateral extent (>400 km), together withthe long‐lived depositional history of the Kahiltna assem-blage (>45 Myr) make it one of the more ideal basins in theNorth American Cordillera for examining the stages ofexhumation and deposition associated with Mesozoic islandarc accretion. A comparison of detrital trends from theKahiltna assemblage with known magmatic sources in theNorth American Cordillera allows for an initial test of exist-ing precollisional, syncollisional, and postcollisional modelsfor basin evolution and ultimately provides a first‐orderconstraint on the timing and extent of exhumation associatedwith accretion of the Wrangellia composite terrane.[10] Detrital zircons were separated from eight sandstone

samples of the Kahiltna assemblage in the Alaska Rangesuture zone (Figures 2 and 3). Samples were collected atdifferent stratigraphic intervals in the Kahiltna assemblagealong a >250 km long transect across the northern TalkeetnaMountains, the Clearwater Mountains, and the southern and

western Alaska Range. Refer to Figure 2 for a geographiclocation of these samples in the Alaska Range suture zoneand Figure 3 for the stratigraphic location of samples in thecontext of measured sections.[11] A total of 714 detrital zircons were analyzed using

laser ablation inductively coupled plasma mass spectrom-etry (LA‐ICPMS) at the Arizona LaserChron Center (http://sites.google.com/a/laserchron.org/laserchron/home) utilizingmethods described by Gehrels et al. [2006, 2008]. Indi-vidual grains were analyzed with a Micromass Isoprobemulticollector ICPMS equipped with nine faraday collec-tors, an axial Daly detector, and four ion‐counting channels.The Isoprobe is equipped with a New Wave DUV 193 laserablation system with an emission wavelength of 193 nm.The analyses were conducted on 35–50 micron spots withan output energy of ∼40 mJ and a repetition rate of 8 Hz.Each analysis consisted of one 20 s integration on thebackgrounds (on peaks with no laser firing) and twenty 1 sintegrations on peaks with the laser firing. The depth of eachablation pit is ∼20 mm. The collector configuration allowssimultaneous measurement of 204Pb in a secondary electronmultiplier while 206Pb, 207Pb, 208Pb, 232Th, and 238U aremeasured with Faraday detectors. All analyses were con-ducted in static mode.[12] Correction for common Pb was done by measuring

206Pb/204Pb, with the composition of common Pb fromStacey and Kramers [1975] and uncertainties of 1.0 for206Pb/204Pb and 0.3 for 207Pb/204Pb. Fractionation of206Pb/238U and 206Pb/207Pb during ablation was monitoredby analyzing fragments of a large concordant zircon crystalthat has a known (ID‐TIMS) age of 563.5 ± 3.2 Ma [Gehrelset al., 2008]. Typically this reference zircon was analyzedonce for every four unknowns. The uncertainty arising fromthis calibration correction, combined with the uncertaintyfrom decay constants and common Pb composition, contributes∼1% systematic error to the 206Pb/238U and 206Pb/207Pb ages(2‐sigma level). The preferred ages are based on 206Pb/238Uratios for <1.0 Ga grains and on 206Pb/207Pb for >1.0 Gagrains. A summary of measured isotopic ratios and ages arereported in Data Set S1 in the auxiliary material.1 Analysesare shown on an Pb/U Concordia diagram (Figure 4) and onan relative probability plots (Figure 4, 5, and 6) (see Ludwig[2008] for use of plotting program). The latter is generatedby summing the individual probability distributions forall grains in a sample. U and Th concentrations are cali-brated by analysis of NBS SRM 610 trace element glass.

3.1. U‐Pb Detrital Zircon Age Distribution

[13] Detrital zircon age data from the Kahiltna assemblagereveal numerous occurrences of concordant to slightly dis-

1Auxiliary materials are available in the HTML. doi:10.1029/2009TC002544.

Figure 2. Generalized geologic map of the Alaska Range suture zone in south central and southwestern Alaska (modifiedfrom Wilson et al. [1998]). The suture zone is defined as the broad region (>100 km in places) that occurs between theWrangellia composite terrane (north of the Talkeetna fault) and the inboard (cratonward) Intermontane belt (south of theDenali and Hines Creek faults). Numbered field sites depict locations of measured stratigraphic sections and U‐Pb detritalzircon samples collected from the Upper Jurassic–Cretaceous Kahiltna assemblage in the southern Alaska Range, northernTalkeetna Mountains, and Clearwater Mountains.


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cordant Mesozoic, Paleozoic, and Precambrian age zircons(Figure 4a). Mesozoic (Mz) age grains are the most abun-dant with less abundant occurrences of Paleozoic (Pz) andPrecambrian (Pc) age grains (Mz 74%, Pz 11%, Pc 15%;Figure 4b). The majority of Mesozoic grains have ages thatfall between 250 and 100 Ma with primary peak occurrencesat 201, 156, and 123 Ma (Figures 4b and 5). The majority ofPaleozoic age grains are between 450 and 300 Ma with aprimary peak occurrence at 364 Ma (Figures 4b and 5). Themajority of Precambrian age grains fall between 2.1 and

1.7 Ga and 2.8–2.3 Ga with isolated peak occurrences at1793, 1929, and 2647 Ma. The remainder of Precambriangrains are scattered between 0.7 and 0.6, 1.7–1.1, and 2.8–3.1 Ga (Figures 4b and 5). All age comparison is based onthe geologic time scale of Gradstein et al. [2004].

3.2. U‐Pb Maximum Depositional Ages inStratigraphic Context

[14] The thickness (3–5 km thick), regional extent (>400 km),and long‐lived depositional history (>45 Myr) of the

Figure 4. Summary of bulk detrital zircon data from eight samples (n = 714) collected from the UpperJurassic–Cretaceous Kahiltna assemblage in southern Alaska. (a) Pb/U concordia diagram for detritalzircon grains from the Kahiltna assemblage. Cutaway portion of concordia diagram depicts concordancefor Phanerozoic age zircon grains. (b) Relative age probability diagram for concordant detrital zirconsfrom the Kahiltna assemblage reflecting distributions of Precambrian (15%), Paleozoic (11%), andMesozoic (74%) age zircon grains. Phanerozoic ages distributions are represented in the cutaway ageprobability plot. All age comparison is based on the geologic time scale of Gradstein et al. [2004].


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Figure 5


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Kahiltna assemblage allow for detrital zircon age popula-tions to be evaluated within a basin‐scale context. By usingexisting age constraint from the Kahiltna assemblagetogether with maximum depositional ages determined fromthe youngest cluster of zircon grains, samples can beassigned to a given stratigraphic interval in the Kahiltnabasin (Figures 3 and 6). The rationale behind this approachis that, in addition to the total bulk distribution of detritalzircon ages from the Kahiltna assemblage (Figure 4), itallows for a spatial and temporal comparison of detritalzircon provenance throughout the history of the basin bycomparing age populations between individual samples atdifferent stratigraphic intervals. Ultimately, a comparison ofupsection changes in detrital zircon contributions offersinsight into regions that were being exhumed throughoutvarious stages Mesozoic arc accretion.[15] In order to promote reproducibility and determine the

most reliable maximum depositional age (MDA) for eachsample at a given stratigraphic interval, we report threealternate measures of maximum depositional age as outlinedby Dickinson and Gehrels [2009], that include the youngestsingle grain age (YSG), the youngest graphical peak age(YPK), and calculated weighted mean age (WMA) of theyoungest cluster of three or more grains (Figures 3 and 6).The youngest graphical age peak represents a maximum inage probability that comprises contributions (at 2‐sigma)from three or more grain analyses. The weighted mean ageincorporates both analytical and systematic error and isreported at 2‐sigma uncertainty using the youngest cluster ofthree or more grain ages. Although it has been demonstratedthat the youngest detrital age in a sample can be a statisti-cally robust and valid procedure to constrain maximumdepositional age [Dickinson and Gehrels, 2009], we rely onthe youngest graphical age peak and weighted mean age as aconservative, first‐order constraint on the depositional ageof each sample. It is worth noting that for the majority ofsamples from the Kahiltna assemblage, the youngest grain,peak age, and weighted mean age fell within a 10 Myr agerange (Figures 3 and 6). Maximum depositional ages foreach sample are presented in context with previouslyreported, biostratigraphic ages determined from marinemacrofossil occurrences from throughout the Kahiltnaassemblage (Figures 3, 5, and 6).[16] The oldest sample in this study (BC‐16) was

collected from the easternmost exposures of the Kahiltnaassemblage in the northeastern Talkeetna Mountains‐Clearwater Mountains region (Figures 2 and 3). LateJurassic‐Early Cretaceous (Kimmeridgian‐Valanginian;155.7–136.4 Ma) marine fossils have been used to constrainthe age of strata in this region [Jones et al., 1980; Silberling

et al., 1981a, 1981b; Smith et al., 1988; Csejtey et al., 1992](Figure 3). We report a maximum depositional age for thissample as follows: for sample BC‐16, youngest single grainage is 139 ± 2 Ma, youngest peak age is 141 Ma, andweighted mean age is 140 ± 2 Ma (Figure 6). The maximumdepositional age from the youngest peak and weighted meanage is earliest Early Cretaceous (Berriasian–Valanginian;145.5–136.4 Ma). This maximum depositional age is theoldest that we report from the Kahiltna assemblage andcorresponds with the upper age range from Late Jurassic–Early Cretaceous macrofossils reported from this region.This sample was likely collected from the basal‐most stra-tigraphy in the Kahiltna basin and effectively representssome of the oldest known strata of the Kahiltna assemblagein southern Alaska.[17] West of the oldest sample from the northeastern

Talkeetna Mountains‐Clearwater Mountains region, foursamples (EFC‐071102‐04, HPC‐072802‐02, TCB‐205,AC‐072002‐01) were collected from the northwesternTalkeetna Mountains (Figures 2 and 3). Late Jurassic–EarlyCretaceous (Kimmeridgian–Valanginian; 155.7–136.4 Ma)marine fossils have been used to constrain the age of theKahiltna assemblage in the northwestern Talkeetna Moun-tains [Jones et al., 1980; Silberling et al., 1981a, 1981b;Smith et al., 1988; Csejtey et al., 1992] (Figure 3). For eachof these samples, maximum depositional ages are asfollows: for sample EFC‐071102‐04, youngest single grainage is 106 ± 7 Ma, youngest peak age is 115 Ma, andweighted mean age is 116 ± 1 Ma; for sample HPC‐072802‐02, youngest single grain age is 117 ± 6 Ma, youngest peakage is 121 Ma, and weighted mean age is 123 ± 2 Ma; forsample TCB‐205, youngest single grain age is 119 ± 2 Ma,youngest peak age is 124 Ma, and weighted mean age is125 ± 2 Ma; for sample AC‐072002‐01, youngest singlegrain age is 119 ± 2 Ma, youngest peak age is 119 Ma, andweighted mean age is 120 ± 3 Ma (Figure 6). The range ofmaximum depositional ages from these samples spanspart of the Early Cretaceous (Barremian–Aptian; 130.0–112.0 Ma) and is younger than the previously reportedbiostratigraphic age range of Late Jurassic–Early Cretaceous(Kimmeridgian–Valanginian; 155.7–136.4 Ma). These newage constraints suggest that the upper age for the Kahiltnaassemblage in the northwestern Talkeetna Mountains maybe as young as latest Early Cretaceous (Aptian–Albian;125.0–99.6 Ma) [Hampton et al., 2007].[18] One sample (BC3‐792) was collected from the

western end of the Kahiltna assemblage in the southwesternAlaska Range. Early Cretaceous (Valanginian–Barremian;140.2–125.0 Ma) marine fossil occurrences have been usedto determine the age of the Kahiltna assemblage in the

Figure 5. Relative age probability diagrams for each individual sample from the Kahiltna assemblage. Plots are shown bytheir spatial distribution from west to east throughout the Alaska Range suture zone. Percentage distribution of zircon ageshas been determined after grouping samples by their geographic location (e.g., Alaska Range southwest, Alaska Range cen-tral, northwestern Talkeetna Mountains, and northwestern Talkeetna Mountains‐Clearwater Mountains). Preexisting bio-stratigraphic age ranges are included for each geographic location. All age comparison is based on the geologic timescale of Gradstein et al. [2004]. Refer to Figures 2 and 3 for samples locations in the southern Alaska Range, northernTalkeetna Mountains, and Clearwater Mountains.


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southwestern Alaska Range [Reed and Nelson, 1980;Bundtzen et al., 1997]. The maximum depositional age forthis sample is as follows: for sample BC3‐792, youngestsingle grain age is 106 ± 4 Ma, youngest peak age is199 Ma, and weighted mean age is 117 ± 5 Ma (Figure 6).Due to the relatively low number of Early Cretaceous agegrains (n = 4) in this sample there is a discrepancy betweenthe Early Jurassic peak age as compared to the Early Cre-taceous youngest single grain and weighted mean ages. Inthis instance, given that this sample was collected within thecontext of a measured stratigraphic section that contains thebiostratigraphic occurrences of Early Cretaceous macro-fossils, we have adopted the Early Cretaceous weightedmean age to reflect the maximum depositional age for thissample. Thus these strata are taken to be Early Cretaceous(Aptian; 125.0–112.0 Ma) and are inferred to representthe upper age equivalent to Early Cretaceous strata ofthe Kahiltna assemblage in the northwestern TalkeetnaMountains.[19] Two samples (OC1‐630, RG‐146) were collected from

Kahiltna basin strata exposed in the central Alaska Range(Figures 2 and 3). Late Jurassic–Cretaceous (Kimmeridgian–Cenomanian; 155.7–93.5 Ma) marine fossils that occur inthis part of the Kahiltna assemblage have been used toconstrain the age of the Kahiltna assemblage in this region[Jones et al., 1980, 1983]. The maximum depositional agesfor this part of the Kahiltna assemblage are as follows: forsample OC1‐630, youngest single grain age is 112 ± 1 Ma,youngest peak age is 112 Ma, and weighted mean age is113 ± 2 Ma; for sample RG‐146, youngest single grain ageis 85 ± 2 Ma, youngest peak age is 98 Ma, and weightedmean age is 102 ± 2 Ma (Figure 6). Based on the occurrenceof peak ages of Early–Late Cretaceous and the occurrence ofmacrofossils as young as earliest Late Cretaceous, we reporta maximum depositional age of Early Cretaceous (Aptian;125.0–112.0 Ma) for sample OC1‐630 and Early–LateCretaceous (Albian–Cenomanian; 112.0–93.5 Ma) for sam-ple RG‐146. Sample OC1‐630 corresponds with the EarlyCretaceous age sample from the southwest Alaska Range(BC3‐792) and the four coeval samples from the northwest-ern Talkeetna Mountains (EFC‐071102‐04, HPC‐072802‐02, TCB‐205, AC‐072002‐01). The youngest sample fromthe study (RG‐146) is from the uppermost stratigraphicintervals of the Kahiltna basin and is taken to represent the

Figure 6. Relative age probability plots showing Phanero-zoic zircon ages and a summary of maximum depositionalages (MDA) for each individual sample from the Kahiltnaassemblage. Plots are shown by their spatial distributionfrom west to east throughout the Alaska Range suture zone.Preexisting biostratigraphic age ranges are included for eachgeographic location and are presented with maximum depo-sitional ages that include the youngest single grain (YSG),the youngest graphical peak age (YPK), and weighted meanage (WMA) from the youngest cluster of three or moregrains. All age comparison is based on the geologic timescale of Gradstein et al. [2004]. Refer to Figures 2 and 3for sample locations in the southern Alaska Range, north-ern Talkeetna Mountains, and Clearwater Mountains.


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youngest known part of the Kahiltna assemblage in southcentral Alaska.

4. Provenance[20] Previous studies have provided a wealth of isotopic

age constraint for many of the regional and long‐lived mag-matic events in the North American Cordillera (Figure 1c)making it one of the better constrained orogenic belts in termsof Phanerozoic magmatic history. Low ratios of uranium‐thorium (<5) recorded from detrital zircon grains within theKahiltna assemblage suggest that ages reflect timing ofmagmatism and initial crystallization rather than thermalresetting from regional metamorphism. By correlating U‐Pbdetrital zircon ages from the Kahiltna assemblage withpreviously reported ages from magmatic provinces through-out the Cordillera we are able to examine the relative con-tributions from sources located inboard and outboard of theKahiltna basin during accretion of the Wrangellia compositeterrane. Detrital contributions of recycled zircon grains fromPrecambrian‐Mesozoic sedimentary and metasedimentarysources from throughout the Cordillera likely make up asignificant percentage of grains from the Kahiltna assem-blage and will be addressed later in this text.[21] It is important to note that the Kahiltna basin occurs

in a suture zone setting in southern Alaska and the northernmargin of the basin is bound by the ∼1200 km long Denalistrike‐slip fault. Up to 400 km of dextral displacement hasbeen proposed for the Denali fault since Late Cretaceoustime based on geologic studies [Eisbacher, 1976; Nokleberget al., 1985; Plafker and Berg, 1994;Wyld et al., 2006; Tropand Ridgway, 2007] and conservative estimates frompaleomagnetic data sets indicate 1650 ± 890 km of dis-placement relative to stable North America between 80 to55 Ma [Stamatakos et al., 2001]. Given this wide range ofproposed displacement for regions outboard of the Tintinaand Denali fault systems (Figure 1a), the following textattempts to account for all known significant, long‐lived,pre‐Cenozoic magmatic sources in northwestern BritishColumbia, southwestern Yukon Territory, and southernAlaska.

4.1. Primary Magmatic Source Areas of the NorthernCordillera

[22] Pre‐Cenozoic magmatic source areas in the westernparts of the North American Cordillera range from latestProterozoic to Late Cretaceous in age (Figure 1). However,with the exception of latest Neoproterozoic‐Cambrian agezircons (555–520 Ma) from the Alexander terrane insoutheastern Alaska [Gehrels and Saleeby, 1987; Gehrels,1990], as of yet no magmatic source areas have beenreported from the Wrangellia composite terrane that areolder than Paleozoic. Some of the oldest Paleozoic andMesozoic sources consist of Ordovician‐Early Devonian(480–410 Ma) and Permian‐Triassic plutonic suites (∼280–220 Ma) from the Alexander terrane in southeastern Alaska[Gehrels and Saleeby, 1987; Gehrels, 1990]. AdditionalLate Paleozoic source areas associated with the Wrangelliacomposite terrane include isolated Devonian plutonic rocks

[Muller, 1980; Brandon et al., 1986; Dodds and Campbell,1988] as well as Pennsylvanian–Permian plutonic suites(320–285 Ma) associated with the Skolai arc and Strelnaassemblage [Nokleberg et al., 1986; Aleinikoff et al., 1988;Dodds and Campbell, 1988; Gardner et al., 1988; Beardand Barker, 1989; Plafker et al., 1989].[23] The oldest known magmatic source areas associated

with the Intermontane belt include an isolated suite ofCambrian–Ordovician rocks in western British Columbia[Okulitch, 1985]. Some of the most widespread Paleozoicmagmatic sources areas in the Intermontane belt consist ofLate Devonian–Mississippian rocks (365–320 Ma) associ-ated with the Quesnellia‐Slide Mountain terrane in south-west Yukon and western British Columbia and Stikiniaterrane of southwest British Columbia [Mortensen, 1990;Greig and Gehrels, 1995; Johnston et al., 1996]. AdditionalPaleozoic sources include Late Devonian–Mississippianaugen gneiss and orthogneiss (380–330 Ma) of the Yukon‐Tanana composite terrane in east central Alaska and westernYukon [Dusel‐Bacon and Aleinikoff, 1985; Aleinikoff et al.,1986; Mortensen, 1990; Johnston et al., 1996; Day et al.,2003]. Permian age igneous rocks (275–255Ma) are exposedin the Quesnellia‐Slide Mountain terrane of southwestYukon and western British Columbia [Mortensen, 1990,1992].[24] Documented occurrences of Mesozoic magmatic

source areas in the Intermontane belt include Middle Triassic–Late Jurassic igneous rocks (245–145 Ma) of the Stikine,Cache Creek, and Quesnellia‐Slide Mountain terranes insouthwestern Yukon and western British Columbia, and theStikinia terrane in western British Columbia [Mortimer et al.,1990;Greig et al., 1992;Mihalynuk et al., 1992; Sevigny andParrish, 1993; Ghosh, 1995; Greig and Gehrels, 1995;Johnston et al., 1996; Currie and Parrish, 1997; Childe,1996; Childe and Thompson, 1997; Deyell et al., 2000;Pálfy et al., 1997, 2000; MacIntyre et al., 2001; Villeneuveet al., 2001; Erdmer et al., 2002]. Coeval Mesozoic pluton-ism is recorded in Late Triassic–Early Jurassic age igneousrocks (215–175 Ma) of the Yukon‐Tanana compositeterrane of east central Alaska [e.g., Aleinikoff et al., 1981;Wilson et al., 1985; Foster et al., 1994; Dusel‐Bacon et al.,2002]. Early–mid Mesozoic magmatic source areas associ-ated with the Wrangellia composite terrane consist primarilyof the Late Triassic–Jurassic Talkeetna arc (201–153 Ma) ofthe Peninsular terrane [Onstott et al., 1989; Pálfy et al.,1999; Rioux et al., 2007] in south central and southwest-ern Alaska and equivalent rocks of the Bonanza arc (202–160 Ma) of the Wrangellia terrane in southwestern BritishColumbia [Friedman et al., 1990; Monger and McNicoll,1993; DeBari et al., 1999; Pálfy et al., 2000]. While theWrangellia composite terrane does contains some Permian‐Triassic magmatic sources areas, there are currently noreported magmatic events associated with Wrangellia thatcan account for detrital zircon ages in the Kahiltna assem-blage that occur between 220 and 205 Ma.[25] The remainder of Mesozoic magmatic sources areas

in the northern Cordillera consist primarily of the MiddleJurassic–Early Cretaceous Chitina arc (175–135 Ma)[Plafker et al., 1989; Nokleberg et al., 1994; Roeske et al.,


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2003] and Early Cretaceous Chisana arc (140–115 Ma)[Snyder and Hart, 2007] associated with the Wrangelliaterrane in south central Alaska and a suite of Cretaceousplutons (110–85 Ma) of the Yukon‐Tanana composite ter-rane in east central Alaska and southwestern Yukon [Fosteret al., 1994]. Jurassic–Paleogene age‐equivalent igneousrocks of the Coast Mountains Batholith (also referred to asCoast Plutonic Complex) (180–50 Ma) occur along theinboard margin of the Wrangellia composite terrane andoutboard margin of the Intermontane belt in western BritishColumbia, southwestern Yukon, and southeastern and eastcentral Alaska [Friedman and Armstrong, 1995; Mongerand McNicoll, 1993; Klepeis et al., 1998; Gehrels, 2001;Butler et al., 2002; Gehrels et al., 2009; Mahoney et al.,2009].

4.2. Primary and Recycled Source Areas for theKahiltna Basin

[26] Approximately ∼12% of the Mesozoic populationfrom the Kahiltna assemblage contains detrital zircon grainsthat have ages younger than 135 Ma, and of these, 8.5% ofgrains fall between 135 and 115 Ma. The best match to apotential source area for these grains is likely some com-bination of igneous rocks of the Chisana arc (140–115 Ma;Figure 1) associated with Wrangellia composite terrane insouthern Alaska and Jurassic–Cretaceous plutons of theCoast Mountains Batholith in southwest Yukon, BritishColumbia, southeast Alaska, and east central Alaska (Figure 1).The remaining ∼3.5% of Mesozoic grains fall between 110and 85 Ma and were most likely derived from Cretaceoussources in the Coast Mountains Batholith and equivalentLate Cretaceous plutons (110–85 Ma; Figure 1) exposed inthe Yukon‐Tanana composite terrane of east central Alaska.[27] The majority of Mesozoic detrital zircons from the

Kahiltna assemblage have age ranges between 215 and135 Ma (Figure 4). These grains were likely derived in partfrom the Chitina arc (175–135 Ma), the Talkeetna arc (201–153 Ma), and the oldest rocks of the Chisana arc (140–115 Ma) all of which are considered part of the Wrangelliacomposite terrane in southern Alaska (Figure 1). Age‐equivalent rocks of the Bonanza arc (202–160) associatedwith Wrangellia terrane in southwest British Columbia arealso a potential source for latest Triassic–Jurassic age grains.Triassic–Jurassic plutonic rocks (245–145 Ma) also occurthroughout the Intermontane belt in association with theQuesnellia–Slide Mountain terrane, Cache Creek terrane,and Stikinia terrane in southwestern Yukon, and westernBritish Columbia (Figure 1) and could be possible sourceareas for the Kahiltna assemblage. Mesozoic detrital zircongrains of the Kahiltna assemblage that are older than 205 Maare best matched as having been derived from Triassicplutonic rocks of the Intermontane belt. An additional Tri-assic source may have been the younger parts of Permian‐Triassic plutons (280–220 Ma) of the Alexander terrane insoutheast Alaska and southwest Yukon (Figure 1).[28] Paleozoic age detrital zircons make up ∼11% of the

total grains from the Kahiltna assemblage (Figure 4) andwere most likely derived from source areas associated withthe Intermontane belt and Wrangellia composite terrane.

Potential source areas associated with the Wrangellia com-posite terrane include the Skolai arc and Strelna assemblage(320–285 Ma) in south central Alaska as well as latestNeoproterozoic–Cambrian (555–520 Ma), Ordovician–earliest Devonian (480–410 Ma), and Permian–Triassicplutonic rocks (280–220 Ma) associated with the Alexanderterrane in southeast Alaska and southwest parts of theYukon Territory (Figure 1). The Devonian age Steele Creekand Mount Constantine gabbro and Saltspring plutons arealso possible sources associated with Wrangellia and theAlexander terrane [Muller, 1980; Brandon et al., 1986;Dodds and Campbell, 1988]. Paleozoic source areas asso-ciated with the Intermontane belt include Permian ageplutonic rocks (275–255 Ma) of the Quesnellia‐SlideMountain terrane in southwest Yukon and British Columbia,Devonian–Mississippian plutonic suites (380–330 Ma) ofthe Quesnellia‐Slide Mountain terrane and Yukon‐Tananacomposite terrane in southwest Yukon and east centralAlaska (Figure 1).[29] Precambrian grains make up ∼15% of the total

detrital zircon population from the Kahiltna assemblage(Figure 4). Up to 7% of total grains fall between an agerange of 2.1–1.7 Ga and 4% fall between 3.1 and 2.5 Ga.Ages >1.6 Ga were likely originally derived from northernlocations within the North American Cordillera (north oflatitude 40°N) that included the Canadian Shield, theWindermere Supergroup, and the Belt‐Purcell Group [e.g.,Gehrels et al., 1995; Mahoney et al., 1999]. Isolatedoccurrences of younger Precambrian detrital zircons (agesyounger than 1.6 Ga) likely originated south of latitude 40°Nand may have come to their present location by multiplerecycling events throughout the Phanerozoic tectonicdevelopment of the Cordillera [e.g., Ross et al., 1992;Gehrels et al., 1995]. A majority of Precambrian andPaleozoic age detrital zircons in the Kahiltna assemblagelikely have undergone multiple cycles of exhumation andsedimentation. In addition to Precambrian and Paleozoicmagmatic source areas throughout the North AmericanCordillera, Paleozoic metasedimentary strata that areexposed throughout the Intermontane belt are likely sourcesfor recycled zircon grains. Recycled detrital zircon grainsranging in age from 3.0 to 1.0 Ga have also been reportedfrom the Alexander terrane [Gehrels et al., 1996].

5. Discussion[30] The bulk occurrence of Precambrian–Mesozoic age

zircons from the Kahiltna assemblage (Figure 4) infers thatexhumation along the northern Cordillera throughout theJurassic‐Cretaceous sedimentary cycle of the Kahiltna basininvolved detrital contributions from the Intermontane beltand the Wrangellia composite terrane. Moreover, a com-parison of detrital zircon age populations from individualsamples at specific stratigraphic intervals throughout theKahiltna assemblage reveals an upsection trend in prove-nance suggestive of differential contributions from outboardand inboard source areas from the early to late stage of basindevelopment.


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5.1. Upsection Trends in Provenance as a Proxy forMesozoic Exhumation

[31] Existing biostratigraphic age constraint and newmaximum depositional ages from the Kahiltna assemblageallow us to examine changes in provenance during indi-vidual stages of sedimentation and evolution of the Kahiltnabasin (Figure 7). The oldest stratigraphic interval in theKahiltna assemblage that was sampled for this study islocated in the easternmost part of the Alaska Range suturezone (Figures 2 and 3) and is interpreted here to representthe lowermost, basal stratigraphy in the Kahiltna basin(Figure 7). Detrital zircon age populations from this strati-graphic interval reveal that 100% of zircon grains areMesozoic in age and fall between an age range of 140–180 Ma (Figures 5, 6, and 7). Possible sources for thisdetritus include the Talkeetna, Chitina, Chisana arcs of theWrangellia composite terrane in southern Alaska as well as

age‐equivalent magmatic source areas of the Yukon‐Tanana,Quesnellia‐Slide Mountain, Cache Creek, and Stikinia ter-ranes of the Intermontane belt (Figure 1).[32] It could be argued that the occurrence of 100%

Mesozoic age zircons at this stratigraphic interval representsthe initial exhumation of Mesozoic source areas bothinboard and outboard of the Kahiltna basin. However, giventhe multiphase history of exhumation in the North AmericanCordillera throughout the Phanerozoic it seems unlikely thatregions inboard of the Kahiltna basin have not undergoneextensive phases of exhumation and hence, would havebeen exhumed to much deeper levels to expose Paleozoicand Precambrian age sources by Late Jurassic–Early Creta-ceous time. This argument together with the lack of Paleo-zoic and Precambrian age detrital zircons in the basal partsof the Kahiltna assemblage as compared to younger strati-graphic intervals leads us to infer that detritus at this

Figure 7. Summary of relative age probability plots and age percent distribution showing the temporal,upsection trends in detrital zircon ages from base to top of the Upper Jurassic–Cretaceous Kahiltna assem-blage. Samples have been grouped according to their maximum depositional age. Trends in detrital zirconages from base to top of the Kahiltna basin show a relative decrease upsection in Mesozoic age grainswith a relative increase in Paleozoic and Precambrian age grains. Geologic time scale based on that ofGradstein et al. [2004].


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stratigraphic interval was derived almost solely from theWrangellia composite terrane.[33] The four samples collected from the northwestern

Talkeetna Mountains are inferred to be stratigraphicallyyounger than the Late Jurassic–Early Cretaceous stratadescribed above based on Early Cretaceous (Barremian–Aptian; 130.0–112.0 Ma) maximum depositional ages(Figures 3, 6, and 7). Detrital zircon age populations fromthis stratigraphic interval of the Kahiltna assemblage reveala range of Mesozoic, Paleozoic, and Precambrian age zircongrains (Mz 84%, Pz 11%, Pc 5%) (Figure 7). Similarly, twosamples from the Alaska Range share roughly coeval EarlyCretaceous (Aptian 125.0–112.0 Ma) maximum deposi-tional ages with these samples and reflect a similar upsectiontrend in varying percentages of Mesozoic, Paleozoic, andPrecambrian age zircons (Mz 56%, Pz 11%, Pc 24%)(Figure 7). Collectively, these six samples are interpreted torepresent a younger stratigraphic interval of the Kahiltnaassemblage where detritus was being derived from a muchmore extensive drainage network and diverse set of sourceregions that include both the outboard Wrangellia compositeterrane and inboard Intermontane belt.[34] The youngest stratigraphic interval sampled from

the Kahiltna assemblage is from the central Alaska Range(Figure 3) and is interpreted to represent the uppermoststratigraphy of the Kahiltna basin exposed in the AlaskaRange suture zone based on a Early–Late Cretaceous (Albian–Cenomanian; 112.0–93.5 Ma) maximum depositional age(Figures 3, 6, and 7). Detrital zircon age populations fromthe uppermost stratigraphic interval in the Kahiltna basinshow a relative decrease in Mesozoic age zircons andincrease in the percentage of Paleozoic and Precambrian agegrains (Mz 19%, Pz 22%, Pc 59%) compared to olderstratigraphic intervals (Figure 7). Provenance trends fromthis uppermost stratigraphic interval of the Kahiltna assem-blage are interpreted to reflect significant detrital contribu-tions from extensive source network throughout the inboardIntermontane belt and relative decreased contributions fromthe outboard Wrangellia composite terrane.

5.2. Mesozoic Record of Exhumation and Basin FillDuring Arc Accretion

[35] The Jurassic–Cretaceous tectonic history of arc ac-cretion along the western margin of the North AmericanCordillera has received a considerable amount of study overthe past several decades and remains a topic of ongoingdebate [e.g., Berg et al., 1972; Pavlis, 1982; Gehrels andSaleeby, 1987; Brandon et al., 1988; Rusmore et al.,1988; Wallace et al., 1989; Rubin et al., 1990; McClellandet al., 1992; Monger et al., 1994; Plafker and Berg, 1994;Umhoefer et al., 2002; Cohen et al., 1995 Kapp and Gehrels,1998; Mahoney et al., 1999; Gehrels, 2001; Ridgwayet al., 2002; Trop et al., 2002, 2005; Dickinson, 2004; Cliftet al., 2005; Hampton et al., 2007; Kalbas et al., 2007;Manuszak et al., 2007; Trop and Ridgway, 2007]. A range ofend‐member models have been proposed to explain thetectonic significance of Upper Jurassic–Cretaceous silici-clastic strata that occur inboard of the Wrangellia compositeterrane and outboard of the Intermontane belt. Proposed

depositional models for the Kahiltna basin and age equiv-alent strata in parts of southern Alaska include sedimen-tation occurring prior to and unrelated to arc accretion,sedimentation occurring coeval and as a direct result ofsubduction and arc accretion, and sedimentation occurringafter arc accretion as a result of strike‐slip basin develop-ment in a suture zone setting [e.g.,Wallace et al., 1989; Kappand Gehrels, 1998; Gehrels, 2001; Ridgway et al., 2002;Hampton et al., 2007; Kalbas et al., 2007; Manuszak et al.,2007; Trop and Ridgway, 2007].[36] Collectively, data from individual stratigraphic

intervals in the Kahiltna assemblage show distinct temporaltrends in provenance. One of the more noticeable of thesetrends is the relative decrease from base to top of theKahiltna assemblage in the occurrence of Mesozoic agedetrital zircons as compared to Paleozoic and Precambrianage zircons (Figure 7). Preaccretionary models for basindevelopment would have the Kahiltna basin closely asso-ciated with the Wrangellia composite terrane and far out-board of the Intermontane belt and North AmericanCordillera [Jones et al., 1977, 1982; Coney et al., 1980].Detrital trends from the Kahiltna could support this model inthe earliest stages of basin development, however theoccurrence of Paleozoic and Precambrian detritus associatedwith the Intermontane belt and North American miogeoclinein the younger parts of the Kahiltna basin require somecontributions from an inboard source by at least the middlestages of the Early Cretaceous (Hauterivian‐Barremian;136.4–125.0 Ma).[37] Postaccretionary models for basin development

would have Upper Jurassic–Cretaceous strata developing ina series of strike‐slip basins in the suture zone between theWrangellia composite terrane and Intermontane belt. In thismodel, sedimentation would record postaccretionary exhu-mation of the Wrangellia composite terrane and Intermon-tane belt within narrow, strike‐slip related basins. Theoccurrence of Mesozoic, Paleozoic, and Precambrian detritalzircons in the younger stratigraphic intervals from theKahiltna assemblage would support the unroofing patternexpected from a postaccretionary model. However, whilethe occurrence of 100% Mesozoic age detrital zircon grainsin the basal stratigraphic intervals of the Kahiltna assem-blage does not discount this type of model, it does requiresome explanation as to why the Wrangellia composite ter-rane was the sole source for detritus during the earlieststages of basin development if the terrane had already col-lided with the continental margin.[38] Based on the upsection trends in detrital zircon age

distribution from the Upper Jurassic–Cretaceous strata of theKahiltna assemblage we suggest a three‐stage model forbasin development, exhumation, and sediment dispersalduring the Late Jurassic–Cretaceous accretion of the Wran-gellia composite terrane to the Intermontane belt and NorthAmerican Cordillera (Figures 7 and 8).5.2.1. Stage 1: Late Jurassic–Early Cretaceous[39] Detrital zircon ages from the basal stratigraphic

intervals of the Kahiltna assemblage reveal a narrow rangeof possible source regions that appear to be limited toMiddle Jurassic–Early Cretaceous magmatic sources in the


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Figure 8. A three‐stage conceptual model for sedimentary basin evolution associated with the accretionof the Wrangellia composite terrane to the Intermontane belt and North American Cordillera duringJurassic–Cretaceous time. The tectonic configurations of two parallel, north dipping subduction zones arebased on reconstructions of Trop and Ridgway [2007].


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Wrangellia composite terrane (Figures 1, 7, and 8a). Duringthe initial Late Jurassic–Early Cretaceous phase of basindevelopment the proximity of the Wrangellia compositeterrane to source areas of the Intermontane belt is poorlyconstrained. Provenance trends suggest that during LateJurassic–Early Cretaceous time the Kahiltna basin was inclose proximity to, and receiving detritus solely fromMesozoic sources associated with the Wrangellia compositeterrane (Figures 7 and 8). The Late Jurassic–Early Cretaceousstage of deposition is interpreted to be part of a retroarcforeland basin that formed along the inboard margin of theWrangellia composite terrane just prior to and/or during theinitial stages of exhumation associated with arc collision andsuturing to the Intermontane belt [Ridgway et al., 2002;Trop and Ridgway, 2007] (Figures 8a and 8b).5.2.2. Stage 2: Early Cretaceous[40] Detrital zircon ages from Lower Cretaceous strata of

the Kahiltna assemblage reflect a wide range of possiblesource areas during arc accretion that include Precambrian,Paleozoic, and Mesozoic magmatic source areas from theinboard Intermontane belt and outboard Wrangellia com-posite terrane (Figures 1, 7, and 8b). Occurrences ofPaleozoic and Precambrian grains in younger stratigraphicintervals of the Kahiltna assemblage provide a strongargument to link the Kahiltna basin to inboard source areasin the Intermontane belt by at least the middle stages of theEarly Cretaceous (Hauterivian–Barremian; 136.4–125.0Ma).The upsection increase in Paleozoic and Precambriandetritus recorded at this stratigraphic interval is interpreted torepresent the initial detrital contributions from the inboardIntermontane belt to the Kahiltna basin during the EarlyCretaceous phase of exhumation and basin development.Provenance trends from Lower Cretaceous strata of theKahiltna assemblage are interpreted to reflect a collisionalstage in the development of the Kahiltna basin in whichsource regions in the Wrangellia composite terrane andIntermontane belt were being exhumed, eroded, and subse-quently deposited in the Kahiltna basin (Figure 8b). In thisinterpretation, the Kahiltna basin would have transitionedfrom a retroarc foreland basin to a remnant ocean basinwhere it was receiving detritus from an along‐strike suturebetween the Wrangellia composite terrane and Intermontanebelt (Figure 8b).5.2.3. Stage 3: Early Cretaceous–Late Cretaceous[41] Detrital zircon ages from the youngest stratigraphic

intervals of the Kahiltna basin are reflective of source areasthat include both the inboard Intermontane belt and outboardWrangellia composite terrane (Figures 1, 7, and 8c). Prov-enance trends from Upper Cretaceous strata of the Kahiltnaassemblage are interpreted to reflect exhumation during the

final stages of accretion of the Wrangellia composite terraneto the Intermontane belt (Figure 8c). By the beginning ofLate Cretaceous time, the Kahiltna basin was likely in themature stages of remnant ocean basin development in whichthe basin was receiving detritus from exhumed regions ofthe Intermontane belt with relative decreased contributionsfrom the Wrangellia composite terrane. If the Kahiltnaassemblage represents sedimentation in a closing oceanbasin between the Wrangellia composite terrane and Inter-montane belt, then Late Cretaceous basin development waslikely characterized by the transition from the final stages ofa closing remnant ocean basin to a flexure‐induced colli-sional foreland basin and/or strike‐slip basin (Figure 8c). Inthis scenario, Upper Cretaceous nonmarine strata that havebeen reported at the top of the Kahiltna assemblage[Hampton et al., 2007] may represent the initial stages ofcrustal shortening, peripheral foreland basin development,and subsequent strike‐slip faulting in the suture between theWrangellia composite terrane and Intermontane belt.

5.3. Spatial Trends in Provenance From MesozoicBasins of Southern Alaska

[42] The Gravina belt of southeastern Alaska is exposedalong strike of the Kahiltna assemblage and is also locatedin the suture between the outboard Wrangellia compositeterrane and inboard Intermontane belt (Figures 1 and 9).Collectively, the Gravina belt consists of Upper Jurassic‐Cretaceous marine siliciclastic and volcaniclastic strata thatare age‐equivalent with the Kahiltna assemblage of south-western and south central Alaska. We present a summary ofpreviously reported U‐Pb detrital zircon data from theGravina belt [Kapp and Gehrels, 1998; Gehrels, 2001] as areference for comparing with provenance trends from theKahiltna assemblage.[43] Detrital zircon data from the Gravina belt include

total of 332 grains and reveal a bulk distribution of Meso-zoic (74%), Paleozoic (20%), and Precambrian (6%) agegrains. The majority of Mesozoic grains have ages between180 and 120 Ma with a primary peak occurrence at 154 Maand a smaller isolated peak at 203 Ma. The youngest grainfrom this sample set is 110 ± 2 Ma and the youngestgraphical peak is at 111 Ma (Figure 9). Paleozoic grainoccurrences are primarily scattered between 350 and 250 Mawith a primary peak at 340 Ma, and a range of 450–400 Mawith a primary peak at 417 Ma. Precambrian grains in theGravina belt are scattered between 2600 and 550 Ma withno elevated occurrences or primary peaks at any particularage range. Note that overall the detrital zircon populationsbetween the Kahiltna assemblage and the Gravina belt are

Figure 9. Comparison of detrital zircon age occurrences from Upper Jurassic–Cretaceous strata of the Kahiltna assem-blage in southwestern and south central Alaska with the Gravina belt in southeastern Alaska. (a) Generalized geologicmap showing the distribution of Upper Jurassic–Cretaceous age strata located inboard (cratonward) of the Wrangellia com-posite terrane. (b) Relative age probability plots and percent occurrence of detrital zircon ages from the Kahiltna assemblage(n = 714) presented in this study. (c) Relative age probability plots and percent occurrence of detrital zircon ages from theGravina belt (n = 332) in southeastern Alaska. All U‐Pb detrital zircon data presented from the Gravina belt have beensummarized from previous work by Kapp and Gehrels [1998] and Gehrels [2001]. All age comparison is based on thegeologic time scale of Gradstein et al. [2004].


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Figure 9


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somewhat comparable with both units having similarpercentages of Mesozoic, Paleozoic, and Precambrian agedetrital zircon grains (Figure 9).[44] The distribution of detrital zircon ages in both the

Gravina belt and Kahiltna assemblage suggest similarprovenance trends that may reflect a shared history ofMesozoic exhumation and basin development. While wepresent one interpretation for stages of exhumation andbasin development based on provenance trends from theKahiltna assemblage, a comparative study of basin prove-nance from Jurassic‐Cretaceous strata that are exposedalong the >2000 km long collisional zone is needed to betterunderstand the timing and mechanisms associated withaccretion of the Wrangellia composite terrane to the NorthAmerican Cordillera.

6. Conclusions[45] U‐Pb LA‐ICPMS analyses of detrital zircons from

synorogenic strata provide a sensitive provenance tool forunderstanding long‐term trends in exhumation and sedimentdispersal along collisional plate boundaries. In this study thetechnique has been applied to address a long‐standingdebate in the timing, trends in exhumation, and basindevelopment associated with Mesozoic arc accretion alongthe western margin of the North American Cordillera. Thisstudy summarizes U‐Pb detrital zircon data from base to topof the Kahiltna assemblage which makes up the northern-most occurrence of a >2000 km long discontinuous belt ofUpper Jurassic–Cretaceous strata that are exposed inboard(cratonward) of the Wrangellia composite terrane fromsouthern Alaska to southwestern British Columbia andnorthwestern Washington.[46] Upper Jurassic–Cretaceous strata of the Kahiltna

assemblage contain Mesozoic, Paleozoic, and Precambrianage detrital zircons (Mz 74%, Pz 11%, Pc 15%) that werederived from source areas that included the outboardWrangellia composite terrane and inboard Intermontanebelt. The majority of grain occurrences are between 250 and100, 450–300, 2100–1700, 2800–2300 Ma with primaryPhanerozoic peak occurrences at 123, 156, 201, and 364 Maand isolated Precambrian peak occurrences at 1793, 1929,and 2647 Ma. Age population trends from the Kahiltnaassemblage are similar in overall percentage distributionwith detrital zircon ages from Upper Jurassic–Cretaceousstrata of the Gravina belt (Mz 74%, Pz 20%, Pc 6%) that areexposed along strike in southeast Alaska.[47] By comparing upsection trends in detrital zircon age

population at different stratigraphic intervals throughout theKahiltna assemblage we are able to examine the range ofpossible regions inboard and outboard of the basin that werebeing exhumed and contributing detritus to the Kahiltna

basin at a given point in time during the accretion of theWrangellia composite terrane. This approach provides avaluable test that can be applied to any sedimentary basinwith the adequate age constrain and is especially useful intesting existing tectonic models for the accretionary historyof the North American Cordillera. Upsection trends indetrital zircon ages from the Kahiltna assemblage are inter-preted here to represent three stages of exhumation and basindevelopment during the Late Jurassic–Cretaceous accretionof the Wrangellia composite terrane to the Intermontanebelt.[48] Stages include an initial Late Jurassic‐Early Creta-

ceous phase of exhumation and basin development duringwhich arc accretion was characterized by retroarc forelandbasin development along the inboard margin of the Wran-gellia composite terrane. During this stage, the Kahiltnabasin was receiving a majority of detritus from Mesozoicmagmatic sources of the Wrangellia composite terrane(Mz 100%, Pz 0%, Pc 0%). This initial stage was followedby a second Early Cretaceous stage of basin developmentthat was characterized by continued exhumation of theoutboard Wrangellia composite terrane and initial detritalcontributions from Precambrian–Mesozoic sources areas ofthe inboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%;Mz 65%, Pz 11%, Pc 24%). The tectonostratigraphic con-text for the Early Cretaceous development of the Kahiltnabasin would represent a remnant ocean basin setting wheredetritus was being transported from the along‐strike suturebetween the Wrangellia composite terrane and Intermontanebelt. The final stage of basin development was characterizedby continued Early to Late Cretaceous exhumation in theIntermontane belt with relative decreased in Mesozoicdetrital contributions from the Wrangellia composite terrane(Mz 19%, Pz 22%, Pc 59%). In the final stage of this col-lisional model, the Kahiltna assemblage was likely transi-tioning from a remnant ocean basin setting to the initialstages of a Late Cretaceous peripheral foreland basin and/orstrike‐slip basin.

[49] Acknowledgments. U‐Pb analyses were conducted at theArizona LaserChron Center with support from the National Science Founda-tion (awards EAR‐0443387 and EAR‐0732436). This project was supportedin part by the U.S. Geological Survey Project “Talkeetna MountainsTransect: Tectonics and Metallogenesis of South‐Central Alaska,” the U.S.Geological Survey EDMAP Program, as well as research funds awardedto Hampton from Michigan State University. This manuscript benefitedfrom discussions with Dwight Bradley, Kevin Eastham, James Kalbas, PaulLandis, Philip Mooney, Sarah Roeske, and Jeff Trop. We thank MicheleGutenkunst and James Kalbas for field assistance as well as the U.S. Geo-logical Survey in Anchorage, Alaska, for aiding with field logistics. We aregrateful to T.L. Pavlis and anonymous reviewers for comments thatimproved the original version of this manuscript.

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G. E. Gehrels, Department of Geosciences,University of Arizona, Tucson, AZ 85721, USA.

B. A. Hampton, Department of GeologicalSciences, Michigan State University, 206 NaturalScience Bldg., East Lansing, MI 48824‐1115, USA.([email protected])

K. D. Ridgway, Depar tment of Ear th andAtmospheric Sciences, Purdue University, WestLafayette, IN 47907‐2051, USA.


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