Abstract

The Jurassic–Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic–Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ∼380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America.

INTRODUCTION

Sedimentary strata along convergent margins provide a record of subduction-related magmatism, oceanic plateau and arc collisional processes, and regional sediment transport pathways (e.g., Dickinson, 1974; Ingersoll, 1979; Plafker and Berg, 1994; Surpless et al., 2014; Stevens Goddard et al., 2018). Recent advances in techniques such as detrital zircon geochronology and Hf isotope analyses have proved insightful for studying the tectonic and sedimentary configuration of ancient convergent margins (Hampton et al., 2010; Yokelson et al., 2015; Reid et al., 2018; Orme and Surpless, 2019). In this study, we applied these techniques to better evaluate the Mesozoic geologic development of the Northern Cordillera. This convergent margin was the product of multiple protracted collisional events involving allochthonous oceanic plateau and island-arc assemblages, and postcollision strike-slip deformation (Plafker and Berg, 1994; Pavlis and Roeske, 2007; Nelson et al., 2013). In southern Alaska, Mesozoic sedimentary basins are interpreted to be related to the collision of an oceanic plateau, the Wrangellia composite terrane, with the former North American continental margin (Fig. 1; Nokleberg et al., 1985; Plafker and Berg, 1994; Trop and Ridgway, 2007). The Mesozoic tectonic configuration of this convergent margin is debated, with the debate centered on the polarity of subduction (i.e., eastward-dipping vs. westward-dipping subduction) and the tectonic setting of sedimentary basins that formed on the upper plate (e.g., Sigloch and Mihalynuk, 2013, 2017; Monger, 2014; Pavlis et al., 2019).

This study presents new U-Pb detrital zircon geochronology and Hf isotope data from the Jurassic–Cretaceous Nutzotin basin, Wrangell Mountains basin, and Wellesly basin in south-central Alaska. We integrated our data with previously published detrital zircon data from coeval accretionary prism, forearc basin, and retroarc basin assemblages exposed in southern Alaska. Results of our analysis have a bearing on the Mesozoic timing of deposition and deformation in the upper plate, the Mesozoic spatial configuration of tectonic elements of the upper plate, and the regional provenance of sediment. Our study also presents one geologic test for important ongoing discussions regarding the Mesozoic configuration of the Cordilleran margin, which center on the timing of terrane collision, the tectonic setting of sedimentary basins, the polarity of subduction along the margin, and the amount of postcollisional displacement on regional strike-slip faults—all of which are important for understanding the tectonic growth of western North America.

GEOLOGIC AND TECTONIC FRAMEWORK

Wrangellia Composite Terrane

The Wrangellia terrane extends discontinuously from south-central Alaska to Vancouver Island in southwestern British Columbia (Fig. 1; Jones et al., 1977; Nokleberg et al., 1994; Plafker and Berg, 1994). The terrane consists of late Paleozoic island arc–type rocks and marine strata, and early Mesozoic oceanic plateau basaltic flows and carbonate rocks (Plafker and Berg, 1994; Greene et al., 2009; Beranek et al., 2014). Wrangellia, along with the Peninsular terrane of southwestern and south-central Alaska, and the Alexander terrane of southeastern Alaska and British Columbia, make up the Wrangellia composite terrane (Insular terrane of Monger et al., 1982; Plafker and Berg, 1994). Intruding the Wrangellia composite terrane in south-central Alaska, there are series of plutonic rocks that represent the remnants of magmatic arcs that ranged in age from Pennsylvanian to the latest Cretaceous (Fig. 2; Monger et al., 1982; Plafker et al., 1989; Plafker and Berg, 1994). The age ranges and tectonic setting of these plutonic rocks are discussed in more detail later in the “Potential Sources of Sediment” section. The specific timing of collision of the Wrangellia composite terrane with the Mesozoic continental margin remains controversial, with studies suggesting that collision occurred sometime between the Middle Jurassic to Late Cretaceous and may have been diachronous along strike (McClelland et al., 1992; Ridgway et al., 2002; Trop et al., 2002, 2005; Hampton et al., 2010; Beranek et al., 2017; Stevens Goddard et al., 2018).

Wrangell Mountains Basin

Middle Jurassic–Upper Cretaceous strata of the Wrangell Mountains basin are exposed in the southern Wrangell Mountains (WMB in Fig. 2). The Wrangell Mountains basin is composed of an ∼7-km-thick succession of marine siliciclastic strata that rest disconformably upon Triassic–Early Jurassic carbonate strata of Wrangellia (MacKevett, 1969; MacKevett et al., 1978; Trop et al., 2002). The stratigraphy and geologic configuration of the Wrangell Mountains basin are shown in Figures 3 and 4, and the reader is referred to MacKevett (1969, 1978) and Trop et al. (2002) for detailed lithologic descriptions and biostratigraphic age determinations. Marine Middle to Upper Jurassic strata in the basin, which include the Nizina Mountain and Root Glacier Formations and the Kotsina Conglomerate, were deformed by Late Jurassic–Early Cretaceous shortening (Trop et al., 2002). An angular unconformity separates the Jurassic strata from the relatively undeformed Lower Cretaceous shallow-marine strata of the Berg Creek and Kuskalana Pass Formations (Figs. 3 and 4B; MacKevett et al., 1978). Another unconformity separates Lower Cretaceous strata from Lower to Upper Cretaceous marine strata in the basin, which include the Kennicott, Moonshine Creek, Shultze, Chititu, and MacColl Ridge Formations (Fig. 3; Jones and MacKevett, 1969; MacKevett, 1978; Trop et al., 1999). Age constraints for the majority of the Cretaceous strata are based primarily on biostratigraphic ranges of marine megafauna, whereas the age of the MacColl Ridge Formation, the stratigraphically youngest Mesozoic unit in the basin, is based on 40Ar/39Ar dating of volcanic tuffs (Jones and MacKevett, 1969; Trop et al., 1999). Jurassic strata of the Wrangell Mountains basin have been previously interpreted as having formed in a marine retroarc basin, based on their position inboard (north) of Early Jurassic–Early Cretaceous plutons (Fig. 4A). Cretaceous strata of the Wrangell Mountains basin have been previously interpreted as having formed in a forearc basin, based on their position outboard (south) of late Early to Late Cretaceous plutons and volcanic rocks (Fig. 4A; Trop et al., 2002).

Nutzotin Basin

The Nutzotin basin consists of an ∼6-km-thick succession of Upper Jurassic–Lower Cretaceous sedimentary and volcanic strata exposed north of the Wrangell Mountains (Figs. 2 and 5A; Richter, 1976; Manuszak et al., 2007). As shown in Figure 3, these strata include the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence, the Lower Cretaceous Chisana Formation, and the Lower Cretaceous Beaver Lake formation (Fig. 3). The Nutzotin Mountains sequence consists of ∼3 km of marine mudstone, sandstone, conglomerate, and volcaniclastic strata that were deposited in mainly submarine fan systems (Manuszak et al., 2007). An Oxfordian–Valanginian age for the Nutzotin Mountains sequence was established based on the presence of sparse marine fossil assemblages (Richter and Jones, 1973). Locally, the Nutzotin Mountains sequence is in disconformable contact with underlying Triassic volcanic and carbonate rocks of Wrangellia (Figs. 5B and 6). Jurassic–Cretaceous strata of the Nutzotin basin are interpreted to have been formed in a marine retroarc basin along the inboard margin of the Wrangellia composite terrane, based on their position inboard (north) of Early Jurassic–Late Cretaceous plutons (Trop et al., 2002; Manuszak et al., 2007). The Nutzotin Mountains sequence shares a gradational upper contact with the ∼2-km-thick Chisana Formation (Fig. 3; Richter, 1976). The Chisana Formation consists of intermediate-mafic lavas and volcaniclastic deposits that formed in subaqueous to subaerial environments (Fig. 5A; Manselle et al., 2020). Above the Chisana Formation, there is the ∼400-m-thick Beaver Lake formation (Fig. 3; Trop et al., 2020). The Beaver Lake formation consists of fluvial strata with well-preserved plant macrofossils, in situ fossilized trees, and dinosaur trackways, indicative of forested terrestrial depositional environments (Fiorillo et al., 2012; Trop et al., 2020).

Wellesly Basin

The Jurassic–Cretaceous Wellesly basin is exposed north of the Denali fault in east-central Alaska (Figs. 1, 2, and 5A). Strata of the Wellesly basin consist of ∼1.5 km of poorly sorted, matrix-supported, pebble-boulder conglomerate interbedded with coarse-grained sandstone deposited in a nonmarine(?) setting (Richter, 1976). These poorly exposed strata locally overlie metamorphic rocks of the Yukon composite terrane (Richter, 1976). Prior to our study, strata of the Wellesly basin had only been studied as part of a regional geologic mapping project that tentatively interpreted them as the possible shoreline equivalent to deeper-marine strata of the Nutzotin basin (Richter, 1976).

Yukon Composite Terrane

The Yukon composite terrane is an amalgamated assemblage of Neoproterozoic(?)–late Paleozoic arc-type and metasedimentary, metaplutonic, and metamorphic rocks that formed along the pericratonic Laurentian margin (Mortensen, 1992; Dusel-Bacon et al., 2006; Nelson et al., 2013). The Yukon composite terrane (Intermontane terrane of Monger et al., 1982; Colpron et al., 2007) is composed of the Cache Creek, Quesnellia, Stikine, and Slide Mountain terranes located in Yukon and British Columbia, and the Yukon-Tanana terrane located in Alaska (Fig. 1). A series of plutonic rocks that range in age from Devonian to Cretaceous intrude the Yukon composite terrane in Alaska (Dusel-Bacon et al., 2006, 2015; Dusel-Bacon and Williams, 2009). The specific age ranges of these plutonic rocks are discussed in more detail later herein.

Major Strike-Slip Faults

Postcollisional strike-slip faults led to reorganization of the southern Alaska convergent margin since the Cretaceous and need to be considered in studies of this convergent margin. The southernmost major fault system is the Border Ranges fault, which juxtaposes the Chugach accretionary prism against the Wrangellia composite terrane (Fig. 2). The Border Ranges fault is a reactivated Mesozoic subduction zone thrust that accommodated at least ∼130 km of dextral strike-slip motion between the Cretaceous to Paleogene (Pavlis, 1982; Roeske et al., 2003; Pavlis and Roeske, 2007). Bounding the Wrangellia composite terrane to the north, there is the Denali fault, which juxtaposes the Wrangellia composite terrane against the Yukon composite terrane. The Denali fault is a dextral strike-slip fault that spans ∼2000 km across the Northern Cordillera. Estimates of Late Cretaceous–Cenozoic displacement for the Denali fault are ∼370–450 km, which are based in large part on the interpreted correlation between the Nutzotin basin and the Dezadeash basin (Fig. 1). These estimates assume that two distinct outcrop belts represent a single sedimentary basin that was subsequently offset by the Denali fault during Late Cretaceous–Cenozoic time (Eisbacher, 1976; Nokleberg et al., 1985; Lowey, 1998). Recent studies also indicate that more than ∼150 km of this displacement may have occurred since the Oligocene (Trop et al., 2019). The Totschunda fault merges with the Denali fault in eastern Alaska (Fig. 2). The Totschunda fault is also a dextral strike-slip fault that extends for at least 200 km in a southeastward direction from the Denali fault (Fig. 5A; Richter and Matson, 1971). The total displacement history along the Totschunda fault is unclear; however, the fault is interpreted to have been active since ca. 25 Ma, coinciding with the collision of the Yakutat microplate with the southern Alaska convergent margin (Milde et al., 2013). Mid-Cretaceous dikes injected into the fault zone record an earlier phase of deformation interpreted as coinciding with accretion of Wrangellia (Trop et al., 2020). Finally, the structural feature separating the Yukon composite terrane from the parautochthonous rocks of the ancestral continental margin is the Tintina fault (Fig. 1). The Tintina fault records dextral strike-slip motion, with Late Cretaceous to Paleogene displacement estimates of ∼400–430 km (Gabrielse et al., 2006).

METHODS

Sample Collection

In total, 14 samples were collected for detrital zircon analysis. These included: eight samples from the Nutzotin Mountains sequence in the Nutzotin basin, five samples from strata in the Wrangell Mountains basin (Root Glacier Formation, Kotsina Conglomerate, Kennicott Formation, Moonshine Creek Formation, and MacColl Ridge Formation), and one sample from the Wellesly basin. Locations of collected samples are denoted in Figures 3, 4A, and 5A. A summary of analyzed samples and corresponding geochronology data are listed in Table 1. With the exception of two samples (062718CF-02 and 071718KR-01), the samples collected for detrital zircon analysis are “composite” samples, meaning that they represent multiple fist-sized sandstone samples collected from one measured section, rather than a single sandstone bed. These “composite” samples were originally collected for sandstone petrography studies (Manuszak, 2000; Trop et al., 2002) prior to the advent and application of laser-ablation–inductively coupled plasma–mass spectrometry to U-Pb geochronologic studies of detrital zircons. It is also important to note that sample CG-DZ-1/2/3 is a composite of three individual samples collected from the Moonshine Creek Formation in the Wrangell Mountains basin.

U-Pb Detrital Zircon Analysis

U-Pb detrital zircon analysis of sample CG-DZ-1/2/3 was conducted using laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia using a Resonetics RESOlution M-50-LR attached to an Agilent 7700x quadrupole ICP-MS. Ablation of individual zircon grains resulted in a 34 µm spot diameter and a 15 µm pit depth in each grain. U-Pb detrital zircon analysis of the rest of the samples was conducted at the University of Arizona LaserChron Center. U-Pb detrital zircon ages were obtained using LA-ICP-MS with a Thermo Element2 single-collector ICP-MS following the methods outlined by Gehrels et al. (2008). A 20 µm spot diameter resulted from laser-ablation analysis, leaving a 15 µm pit depth in each grain. The number of analyses varied between 79 and 315 per sample, depending on the number of zircon grains in each sample. For zircon grains with ages younger than 1.0 Ga, the 206Pb/238U ages are reported. Conversely, the 206Pb/207Pb ages are reported for zircon grains older than 1.0 Ga, due to the 206Pb/207Pb ages being less sensitive to Pb loss, which is more common in older systems (Gehrels et al., 2008; Gehrels, 2012). Results from the analysis are plotted on normalized age distribution plots (relative age probability plots; Figs. 7A–7C). Peaks shown on the age distribution plots were determined using the program Age-Pick from the program Isoplot designed by Ludwig (2003). Peak ages were calculated using clusters of three or more grains of overlapping age within 2σ error, with the youngest peak age representing the maximum depositional age (MDA) for the sample. In this approach, we used the youngest graphical detrital zircon age peak on the age probability plot, as controlled by multiple U-Pb grain ages (single-grain-age peaks were ignored; e.g., YPP (youngest graphical peak) method of Dickinson and Gehrels, 2009). We decided on this approach because it best agrees with the existing biostratigraphic control for the basins we studied. In a later section, we discuss the general agreement between the calculated MDAs and the previously determined biostratigraphy. We also prefer this approach for MDA calculation because some of the more recent detrital zircon studies from the basins/accretionary prism with which we compared our data used a similar approach. These studies were: Reid et al. (2018) for the Matanuska forearc basin strata, and Amato et al. (2013) for the Chugach accretionary prism. We note that the true depositional age of our sample may be younger than or equal to the detrital zircon MDA (e.g., Dickinson and Gehrels, 2009; Coutts et al., 2019). In total, 2777 new U-Pb detrital zircon ages are reported for strata of the Wrangell Mountains, Nutzotin, and Wellesly basins (see Supplemental Data1).

Hf Detrital Zircon Isotope Geochemistry

Hf isotope geochemical analysis of detrital zircons was conducted using a Nu multicollector ICP-MS following the methods presented in Gehrels and Pecha (2014). Single zircon grains were laser ablated with a 40 µm spot diameter over the preexisting pit used for U-Pb analysis, so that the Hf compositions correspond to a U-Pb crystallization age. Hf analyses were conducted for zircons from each of the main age groups in selected samples. For our analysis, positive εHf(t) values are considered “juvenile” (derived from magmas associated with mantle components), and negative εHf(t) values are considered “evolved” (derived from magmas associated with older continental components; Bahlburg et al., 2011). In total, 124 new Hf isotope compositions for the Nutzotin and Wellesly basins are reported in this study. Hf isotope compositions were not analyzed for strata of the Wrangell Mountains basin. Results from the analyses were plotted on an Hf evolution diagram (Fig. 8). The Hf evolution diagram shows the εHf(t) values for all analyzed samples, which represent the 176Hf/177Hf ratio at the time of crystallization relative to the chondritic uniform reservoir (CHUR; Bouvier et al., 2008).

RESULTS

Nutzotin Basin

Samples LC1-COMP and 062718CF-02 were collected from thin sandstone beds within black shale that characterize the stratigraphically lowest part of the Nutzotin Mountains sequence, which is dominated by black shale (FA2; Fig. 6). LC1-COMP (n = 312) contained predominantly Mesozoic detrital zircon age signatures with subordinate Paleozoic ages (Precambrian [pC] = 0%, Paleozoic [Pz] = 2%, Mesozoic [Mz] = 98%). The sample yielded an MDA of 155 Ma (Fig. 7A). Hf isotope compositions were not analyzed for sample LC1-COMP. Sample 062718CF-02 (n = 315) yielded detrital zircons with predominantly Mesozoic ages and sparse Paleozoic and Precambrian ages (pC = <1%, Pz = 2%, Mz = 98%). The MDA of this sample is 154 Ma (Fig. 7A). The εHf(t) analysis of sample 062718CF-02 (n = 19) yielded juvenile compositions (+14.9 to +3.0) for detrital zircons between 169 and 149 Ma (Fig. 8).

Sample BR1-COMP (n = 119) was collected from normally graded sandstone beds that represent FA3 of the Nutzotin Mountains sequence (Fig. 6). The sample contained detrital zircon ages that were primarily Mesozoic in age, with subordinate Paleozoic and Precambrian ages (pC = <1%, Pz = 1%, Mz = 99%). BR1-COMP exhibited predominantly Jurassic detrital zircon ages, with an MDA of 154 Ma (Fig. 7A). The results of the εHf(t) analysis of sample BR1-COMP (n = 18) are shown for zircons with ages from 171 to 148 Ma in Figure 8 and range from juvenile to slightly evolved (+11 to −0.5). Sample 071718KR-01 (n = 315) was collected from an exposure of the Nutzotin Mountains sequence in the Slate Creek region of the eastern Alaska Range (Fig. 5A). Prior to our study, this exposure had not been assigned a facies association based on the stratigraphic framework of Manuszak et al. (2007). Based on the lithology of the exposure, which consists predominantly of shale with minor sandstone, we assigned this sample to facies association 2 (FA2) of the Manuszak et al. (2007) classification. The sample contained a detrital zircon age distribution that was entirely Mesozoic (pC = 0%, Pz = 0%, Mz = 100%). All detrital zircon ages in the sample ranged from 164 to 143 Ma. The MDA of sample 071718KR-01 is 151 Ma (Fig. 7A). The εHf(t) analysis of detrital zircon ages between 160 and 145 Ma in sample 071718KR-01 (n = 16) yielded juvenile compositions (+11.9 to +4.0; Fig. 8).

Samples MC1-COMP and MM1-COMP were collected from two sections of predominantly conglomerate and sandstone in the Nutzotin Mountains sequence (FA1; Fig. 6). Sample MC1-COMP (n = 312) yielded detrital zircon ages that were predominantly Mesozoic, with a few Paleozoic and Precambrian ages (pC = <1%, Pz = 1%, Mz = 99%). The MDA of sample MC1-COMP is 145 Ma (Fig. 7A). Hf compositions were not analyzed for this sample. Sample MM1-COMP (n = 230) contained detrital zircon with ages that were predominantly Mesozoic and Paleozoic (pC = 0%, Pz = 24%, Mz = 76%). MM1-COMP yielded an MDA of 144 Ma. This sample also exhibited a larger population of Paleozoic ages than the other samples, with a Pennsylvanian peak age of 301 Ma (Fig. 7A). The results for the εHf(t) analysis of sample MM1-COMP (n = 23) are reported for both Mesozoic and Paleozoic zircon populations. Mesozoic zircons between 154 and 138 Ma yielded ϖ values that were entirely juvenile (+14.3 to +5.7), whereas Paleozoic zircons between 306 and 297 Ma yielded εHf(t) values that were juvenile to slightly evolved (+6.2 to −0.6; Fig. 8).

Sample KNC1-COMP was collected from tabular sandstone beds in the middle part of the Nutzotin Mountains sequence (FA4; Fig. 6). KNC1-COMP (n = 87) gave detrital zircon ages that were primarily Mesozoic, with subordinate Paleozoic ages (pC = 0%, Pz = 2%, Mz = 98%). The age distribution of the sample yielded an MDA of 144 Ma (Fig. 7A). Hf compositions were not measured for this sample. Sample BON-COMP was collected from the stratigraphically highest known section of the Nutzotin Mountains sequence (FA5; Fig. 6). BON-COMP (n = 314) yielded detrital zircon ages that were predominantly Mesozoic, with sparse Paleozoic and Precambrian ages (pC = <1%, Pz = 2%, Mz = 98%). The sample exhibited two Cretaceous peak ages at 140 Ma and 133 Ma, the younger of which is interpreted as the MDA of the sample (Fig. 7A). The εHf(t) values for detrital zircons between 152 and 127 Ma in BON-COMP (n = 24) yielded entirely juvenile compositions (+15.1 to +6.8; Fig. 8).

Prior to our study, the depositional age of the Nutzotin Mountains sequence was based primarily on biostratigraphic ages of limited marine megafauna (Richter, 1971, 1976; Richter and Jones, 1973; Richter and Schmoll, 1973) and a limited amount (n = 10) of detrital U-Pb zircon ages (Manuszak et al., 2007). The stratigraphic configuration illustrated in Figure 6 was constructed based on the geologic mapping of Manuszak et al. (2007). Our new detrital zircon ages from the Nutzotin Mountains sequence provide an opportunity to evaluate and strengthen the chronostratigraphic framework for this 3-km-thick stratigraphic section. Two samples (LC1-COMP and 062718CF-02) were collected from the lower part of the Nutzotin Mountains sequence in the hanging wall of the regional thrust belt (FA2), whereas two other samples (MC1-COMP and MM1-COMP) were collected from the lower part of the Nutzotin Mountains sequence that is in depositional contact with the Wrangellia composite terrane (FA1; Fig. 6). Biostratigraphic ages reported for FA1 and FA2 of the Nutzotin Mountains sequence range between Oxfordian and Tithonian (164–145 Ma; Richter, 1971; Richter and Schmoll, 1973). MDAs from FA2 and FA1 range between 155–154 Ma and 145–144 Ma, respectively, and lie within the established biostratigraphic age range. Two samples were collected from the middle part of the Nutzotin Mountains sequence (BR1-COMP from FA3 and KNC1-COMP from FA4; Fig. 6). Biostratigraphic ages from this part of the sequence are between Tithonian and Valanginian (152–133 Ma; Richter, 1976). The MDA for FA3 is 154 Ma, which is slightly older than the established biostratigraphic age, whereas the MDA for FA4 (144 Ma) falls within the biostratigraphic age range. Finally, one sample (BON-COMP) was collected from the stratigraphically youngest part of the Nutzotin Mountains sequence (FA5), which has a biostratigraphic age between Tithonian and Valanginian (152–133 Ma; Fig. 6; Richter, 1971; Richter and Jones, 1973). The MDA for FA5 is 133 Ma, which agrees with the biostratigraphic age range. The detrital zircon MDAs reported here for the Nutzotin Mountains sequence are generally consistent with the previously reported biostratigraphic ages and stratigraphic interpretation of Manuszak et al. (2007). This indicates that our approach using detrital zircon ages to approximate depositional ages is reasonable for the Nutzotin Mountains sequence.

Wrangell Mountains Basin

ROOT1-COMP was collected from the Upper Jurassic Root Glacier Formation in the Wrangell Mountains basin (Fig. 3). Sample ROOT1-COMP (n = 97) yielded a predominantly Mesozoic detrital zircon age distribution, with minor Paleozoic and Precambrian grains (pC = 1%, Pz = 1%, Mz = 98%). A peak age of 152 Ma is interpreted as the MDA of ROOT1-COMP (Fig. 7B). Sample KOT-COMP came from the Upper Jurassic Kotsina Conglomerate (Fig. 3), which is age-equivalent to the Root Glacier Formation (Trop et al., 2002). KOT-COMP (n = 79) contained detrital zircon ages that were almost entirely Mesozoic with subordinate Paleozoic ages (pC = 0%, Pz = 1%, Mz = 99%). The age distribution yielded an MDA of 159 Ma (Fig. 7B).

Sample KUS1-COMP was collected from the Lower Cretaceous Kennicott Formation (Fig. 3). KUS1-COMP (n = 107) yielded detrital zircon that had primarily Mesozoic and Paleozoic ages with subordinate Precambrian ages (pC = 4%, Pz = 30%, Mz = 66%). The age distribution in the sample yielded three major peaks at 307, 300, and 153 Ma (Fig. 7B). The MDA for this sample is interpreted as 153 Ma.

Sample CG-DZ-1/2/3 is a composite of three individual samples collected from the Upper Cretaceous Moonshine Creek Formation (Fig. 3). CG-DZ-1/2/3 (n = 239) displayed primarily Mesozoic and Paleozoic detrital zircon ages with few Precambrian grains (pC = <1%, Pz = 35%, Mz = 64%). The sample yielded four major peak ages at 299, 150, 116, and 104 Ma, with subordinate peaks at 463, 439, and 224 Ma (Fig. 7B). The MDA for sample CG-DZ-1/2/3 is 104 Ma. Finally, sample MAC-COMP was collected from the Upper Cretaceous MacColl Ridge Formation (Fig. 3). Detrital zircon ages from sample MAC-COMP (n = 100) yielded primarily Mesozoic ages with subordinate Paleozoic and Precambrian ages (pC = 3%, Pz = 5%, Mz = 92%). MAC-COMP yielded a total of nine peak ages, with the youngest peak age at 76 Ma, and older subordinate peak ages of 196, 157, 153, 146, 142, 119, 104, and 85 Ma (Fig. 7B). The peak age of 76 Ma is interpreted to be the MDA for MAC1-COMP.

Published age constraints for strata of the Wrangell Mountains basin are based on biostratigraphic ages of abundant marine megafauna as well as radiometric age dating of plutonic clasts and volcanic tuff beds (MacKevett, 1978; Trop et al., 1999, 2002). Biostratigraphic ages for the Root Glacier Formation indicate an Oxfordian–Tithonian depositional age (164–145 Ma; MacKevett, 1969). The MDA for the unit based on detrital zircons is 152 Ma, which lies in the biostratigraphic age range for the formation. Age constraints for the Kotsina Conglomerate are based on: (1) radiometric age dating of hornblende in a granitic clast in the unit, which yielded a 40Ar/39Ar age of 152.8 ± 1.1 Ma, and (2) stratigraphic relationships that indicate the Kotsina Conglomerate is age-equivalent to the better-dated and more fossil-rich Root Glacier Formation (Oxfordian–Tithonian; Trop et al., 2002). The detrital zircon MDA for the Kotsina Conglomerate is 159 Ma, which lies within the established stratigraphic age constraints. Biostratigraphic ages for the Lower Cretaceous Kennicott Formation indicate an Albian (113–100.5 Ma) age for the formation. The MDA based on detrital zircons for the Kennicott Formation is 153 Ma, which is much older than the biostratigraphic age range. Given the abundance of marine megafauna in the Kennicott Formation in the sampled region (Grantz et al., 1966; Winkler et al., 1981), an Albian depositional age seems likely. Biostratigraphic ages for the Upper Cretaceous Moonshine Creek Formation indicate a Cenomanian (ca. 100–94 Ma) depositional age. The detrital zircon MDA for the Moonshine Creek Formation is 104 Ma, which is older than the biostratigraphic age range, but it is still generally consistent with prior age constraints. Finally, age constraints from the Upper Cretaceous MacColl Ridge Formation are based primarily on: (1) palynology, which yielded a Campanian (84–72 Ma) biostratigraphic age, and (2) radiometric age dating of volcanic tuff beds near the top of the formation, which yielded 40Ar/39Ar ages of 79.4 ± 0.7 and 77.9 ± 2.1 Ma (Trop et al., 1999). The detrital zircon MDA for the MacColl Ridge Formation is 76 Ma, which is consistent with palynologic and radiometric ages for the unit. Overall, our detrital zircon MDAs for the Wrangell Mountains basin are broadly consistent with published biostratigraphic ages. However, our MDAs for the basinal strata may be older than the true depositional ages, especially in the case for the Kennicott Formation, which will be discussed in greater detail in a later section.

Wellesly Basin

Sample WM1-COMP (n = 150) was collected from sandstone interbedded with conglomerate in the Wellesly basin (Fig. 3). Due to the limited exposures of the Wellesly strata, the stratigraphic position of the sample is uncertain. WM1-COMP showed a detrital zircon age spectrum from Precambrian to Mesozoic (pC = 13%, Pz = 31%, Mz = 56%). In total, 19 peak ages were present in the sample: two Cretaceous peak ages at 133 and 126 Ma; four Jurassic peak ages at 200, 187, 172, and 147 Ma; one Triassic peak age at 218 Ma; six Paleozoic peak ages at 413, 382, 368, 328, and 265 Ma; and six Precambrian peak ages at 1849, 1649, 1482, 1223, 1135, and 1037 Ma (Fig. 7C). The MDA of sample WM1-COMP is interpreted as 126 Ma. There had been no previous age data reported from the Wellesly basin. Richter (1976) had tentatively assigned it a Jurassic–Cretaceous age due to its geographic proximity to the better-dated Nutzotin Mountains sequence (Fig. 2). The εHf(t) values for Mesozoic detrital zircon grains with ages from 197 to 158 Ma yielded an array of juvenile to evolved signatures (Fig. 8; +9.6 to −14.4). The εHf(t) values for Paleozoic zircons with ages of 390, 355, and 326 Ma yielded slightly juvenile to evolved signatures (+1.5 to −6.3). Finally, the εHf(t) values for two Paleoproterozoic grains with ages of 1853 and 1820 Ma yielded slightly evolved to slightly juvenile signatures (−2.0 and +2.4, respectively; Fig. 8).

POTENTIAL SOURCES OF SEDIMENT

Detrital zircon ages from the Nutzotin basin, Wrangell Mountains basin, and Wellesly basin overlap in age with magmatic sources from both the accreted oceanic rocks of the Wrangellia composite terrane and the continental margin rocks of the Yukon composite terrane. This section summarizes the main magmatic sources from both composite terranes that may have contributed sediment to these basins. The Wrangellia composite terrane, and subsequently the overlying sedimentary basins, has undergone significant dextral displacement since the Late Cretaceous. Paleomagnetic studies of the MacColl Ridge Formation in the Wrangell Mountains basin (for stratigraphic location, see Fig. 3) indicate that the unit formed at a more southerly latitude, around southern British Columbia (Stamatakos et al., 2001). Conversely, other studies have postulated that the Wrangellia composite terrane was positioned ∼3000 km to the south around present-day Baja California during Late Cretaceous time; this interpretation is known as the Baja–British Columbia hypothesis (Wynne et al., 1995; Irving et al., 1996; Cowan et al., 1997; Hollister and Andronicos, 1997; Garver and Davidson, 2015). The majority of the detrital zircon ages in our data set can be accounted for by magmatic sources in the Northern Cordillera, so we focus on sediment sources in British Columbia, Yukon, and Alaska for the provenance of detrital zircons in the studied units.

Potential Magmatic Sources of Sediment on the Wrangellia Composite Terrane

The majority of magmatic sources associated with the Wrangellia composite terrane range between late Paleozoic to late Mesozoic in age. Late Paleozoic magmatic sources in south-central Alaska include Pennsylvanian–Permian plutons of the Skolai arc (320–290 Ma), which are exposed in the Wrangell Mountains, as well as parts of the eastern Alaska Range (SKA in Fig. 2). U-Pb zircon analyses of the Skolai arc plutons yielded ages between 310 and 284 Ma (Gardner et al., 1988; Beard and Barker, 1989; Beranek et al., 2014). Nd isotopic compositions from Pennsylvanian plutons associated with the arc, exposed in the St. Elias Mountains, yielded signatures indicative of mixing of mantle and crustal material (Beranek et al., 2014).

Jurassic–Early Cretaceous magmatic sources of the Wrangellia composite terrane in south-central Alaska include volcanic-plutonic rocks associated with the Talkeetna (201–153 Ma), Chitina (175–135 Ma), and Chisana (130–115 Ma) arcs. Talkeetna arc plutons are exposed in the southern Talkeetna Mountains (TKA in Fig. 2) and yielded U-Pb zircon ages of 201–181 Ma and 177–153 Ma (Rioux et al., 2007, 2010). Nd isotopic compositions of Talkeetna arc plutons indicate a juvenile source for the magmas, which is consistent with an oceanic arc interpretation (Rioux et al., 2010). Plutons of the Chitina arc are primarily exposed in the Chitina Valley south of the Wrangell Mountains (CTA in Fig. 2). U-Pb zircon analyses from the Chitina arc yield ages of 153–150 Ma, while detrital zircon grains from modern river sands draining from Chitina arc plutons yield ages between 156 and 130 Ma (Plafker et al., 1989; Roeske et al., 2003; Day et al., 2016; Trop et al., 2016). The Chitina arc is interpreted as an oceanic arc that is the southeastern continuation of the Talkeetna arc to the northwest (Plafker et al., 1989; Trop et al., 2005). Nd isotopic compositions of sedimentary and volcaniclastic strata in the Dezadeash Formation in southwestern Yukon, which are interpreted to have been chiefly sourced from the Chitina arc, indicate that Chitina arc magmas were derived primarily from mantle material with mixing of older crustal material (Lowey, 2011, 2018). The Chisana arc is exposed north of the Wrangell Mountains (CSA in Fig. 2). U-Pb zircon ages of Chisana arc plutons range between 126 and 113 Ma, and isotopic compositions indicate that magma was derived from primitive melts in a subduction-related arc setting (Snyder and Hart, 2007; Graham et al., 2016; Manselle, 2019; Manselle et al., 2020). Late Cretaceous magmatic sources in the Talkeetna Mountains have U-Pb zircon ages of 79–67 Ma (mlK in Fig. 2; Bleick et al., 2012). The latter suite is part of an extensive Late Cretaceous arc system (Kluane arc of Plafker and Berg, 1994) that may be the northern continuation of the Coast Mountains batholith exposed in southeastern Alaska and British Columbia (Fig. 1).

Precambrian magmatic sources are lacking on the Wrangellia terrane, but Neoproterozoic–Cambrian plutons are exposed on the Alexander terrane, which is located southeast of our study area (Fig. 1; Gehrels and Saleeby, 1987). Sedimentary strata related to the Alexander terrane also contain Precambrian zircons (White et al., 2016).

Potential Magmatic Sources of Sediment on the Yukon Composite Terrane

Magmatic sources that intrude the Yukon composite terrane in Alaska, Yukon, and British Columbia range in age from Paleozoic to Mesozoic. Paleozoic magmatic sources include Devonian–Mississippian (376–320 Ma) plutons that intrude the Yukon-Tanana, Quesnellia–Slide Mountain, and Stikinia terranes (Mortensen, 1990; Dusel-Bacon et al., 2004, 2006). Nd isotopic compositions from these plutons indicate evolved signatures representing melting of older crustal material (Mortensen, 1992). Permian plutonic sources include 275–255 Ma plutons that intrude the Quesnellia–Slide Mountain terrane in Yukon and British Columbia (Mortensen, 1990).

Mesozoic magmatic sources include Late Triassic–Early Jurassic (215–175 Ma) plutonic rocks that intrude the Yukon-Tanana terrane (Fig. 2; Aleinikoff et al., 1981). U-Pb zircon analyses from the Taylor Mountain batholith exposed in eastern Alaska yielded ages between 216 and 181 Ma (Dusel-Bacon and Williams, 2009; Day et al., 2014; Dusel-Bacon et al., 2015). Pb isotopic compositions of these plutons indicate a mixture of mantle and crustal components during melting (Dusel-Bacon et al., 2015). Cretaceous plutons also intrude the Yukon-Tanana terrane in eastern Alaska and yielded ages between 112 and 66 Ma (Richter, 1976; Foster et al., 1994; Dusel-Bacon et al., 2015).

A potential more distal source, based on the current position of the study area, also includes the Coast Mountains batholith of southeastern Alaska and British Columbia (Fig. 1). Plutons of the Coast Mountains batholith intrude both the Wrangellia and Yukon composite terranes; plutons intruding the Wrangellia composite terrane have yielded U-Pb zircon ages of ca. 160–140 Ma, ca. 120–100 Ma, and ca. 100–80 Ma, whereas plutons intruding the Yukon composite terrane have yielded U-Pb ages of 190–110 Ma and 100–50 Ma (Gehrels et al., 2009). The εHf(t) values of plutons of the Coast Mountains batholith range between +9 and +2 for plutons intruding the Wrangellia composite terrane and between +13 and +1 for plutons intruding the Yukon composite terrane; these values indicate derivation from primarily juvenile melts with minor crustal input (Cecil et al., 2011). The Coast Mountains batholith was positioned closer to the basins in this study, due to several hundred kilometers of post–Late Cretaceous dextral displacement along the Denali fault system (Nokleberg et al., 1985). These plutons also document a prominent magmatic lull between 140 and 120 Ma that is also present throughout the Northern Cordillera (Armstrong, 1988; Gehrels et al., 2009).

Secondary sources for Precambrian detrital zircons may include reworked detrital zircons from the Yukon-Tanana terrane in southeastern Alaska (Pecha et al., 2016), as well as from primary magmatic sources in western Canada and sources on the Canadian Shield (Gehrels et al., 1995; Linde et al., 2017). Recycled zircons from the Grenville orogen may also have served as a secondary source, as previous studies have noted Grenville-aged (1350–950 Ma) detritus in the Northern Cordillera (Rainbird et al., 1997). Grenville-aged detritus is interpreted to have been reworked from continental-scale clastic wedges that accumulated across western Laurentia during the Neoproterozoic (Rainbird et al., 2012). There are other potential sources of the same age range, such as the 1000–950 Ma orthogneiss units and their sedimentary derivatives in the Canadian Arctic (Estrada et al., 2018).

DISCUSSION

New Insights on Basin Development and Provenance of the Nutzotin Basin

Our new detrital zircon data from the Nutzotin Mountains sequence offer new insight into the timing of Mesozoic basin development, and the depositional and deformational history of the Nutzotin basin. The data also provide a test for the original stratigraphic model for the basin proposed by Manuszak et al. (2007), which was primarily based on geologic mapping, stratigraphic correlation, and limited biostratigraphic data. The oldest part of the stratigraphy, based on detrital zircon MDAs, is represented by the distal submarine fan strata of facies association 2 (FA2) from the northwestern part of the basin, which have MDAs of 155–154 Ma (062718CF-02 and LC1-COMP in Fig. 6). These strata contain abundant detrital zircon ages between 169 and 145 Ma (95% of total detrital zircons ages of samples from FA2; Fig. 7A) that overlap in age with plutonic rocks of the Chitina arc (175–135 Ma) exposed to the south of the basin (CTA in Fig. 2). The εHf(t) values from detrital zircons from these strata are juvenile to intermediate (+14.9 to +3.5; 062718CF-02 in Fig. 8), consistent with a magmatic source for plutons of the Chitina arc that was derived from both mantle and crustal sources (Lowey, 2011). The overlap in detrital zircon ages and isotopic compositions, and the regional proximity of the arc to the Nutzotin Mountains sequence make the Chitina arc plutons the likely predominant source for detrital zircons in the basin. Other potential sources of sediment include the Jurassic Talkeetna arc (201–153 Ma) exposed to the southwest of the basin in the Talkeetna Mountains (TKA in Fig. 2). Ages from the Chitina arc (175–135 Ma) overlap with reported ages from the Talkeetna arc (201–153 Ma). In terms of detrital zircons that could strictly be sourced only from the Talkeetna arc (201–175 Ma), less than 1% of all grains overlap with this age range. Thus, we do not consider the Talkeetna arc as a major contributor of sediment to the Nutzotin basin compared to the Chitina arc.

From a regional mapping perspective (Richter, 1976; Manuszak et al., 2007), strata of FA2 are located in the hanging wall of a thrust fault system that juxtaposes these strata with proximal submarine fan strata of facies association 1 (FA1) that have MDAs between 145 and 144 Ma (MM1-COMP and MC1-COMP in Fig. 6). Strata of FA1 exhibit a similar Late Jurassic to Early Cretaceous detrital zircon age population of 159–137 Ma (86% of total detrital zircon ages for samples from FA1 strata) that is of similar age to plutons of the Chitina arc. Detrital zircons from these strata also exhibit similar juvenile to intermediate εHf(t) values (+14.6 to +5.7; MM1-COMP in Fig. 8) that are consistent with mantle- and crustal-derived magma sources for plutons of the Chitina arc. One sample from FA1 (MM1-COMP) documented a significant population of Paleozoic detrital zircons (315–295 Ma) that overlaps in age with plutons of the Pennsylvanian–Permian Skolai arc (320–290 Ma) located south of the basin (SKA in Fig. 2). The εHf(t) values for these Paleozoic detrital zircons yielded predominantly intermediate compositions (+6.2 to −0.6; Fig. 8), which overlap with Nd isotopic compositions of Skolai arc–equivalent plutons in the St. Elias Mountains, which in turn indicate a mixture of mantle and crustal melts (e.g., Beranek et al., 2014). Northward-directed paleoflow measurements for strata of FA1 also support the interpretation of the Chitina and Skolai arcs as the major sources of sediment during this time interval (Kozinski, 1985; Manuszak et al., 2007). The proximal submarine fan strata of FA1 consist of thick-bedded, cobble conglomerate with outsized, meter-scale limestone clasts (Manuszak et al., 2007). The clast composition of conglomerate beds, in conjunction with our new detrital zircon data, indicate that by ca. 145 Ma, exhumation and erosion of the Wrangellia terrane and associated magmatic arcs, along with subsidence of the Nutzotin basin, had commenced.

The middle part of the Nutzotin Mountains sequence consists of over 1000 m of tabular sandstone and subordinate clast-supported, pebble conglomerate of facies associations 3 and 4 (FA3 and FA4; BR1-COMP and KNC1-COMP in Fig. 6). Clast compositions in conglomerate in these facies associations record a relative up-section increase in metabasalt relative to limestone clasts, reflecting exhumation and erosion of deeper levels of the Wrangellia terrane (Manuszak et al., 2007). Our detrital zircon data from FA3 and FA4 exhibit dominant Late Jurassic to Early Cretaceous detrital zircon ages (169–135 Ma; Fig. 7A) and juvenile to slightly evolved εHf(t) values (+11.0 to −0.5; BR1-COMP in Fig. 8), indicating that the sediment contribution from igneous rocks of the Chitina arc, built along the southern margin of the Wrangellia terrane, was still an important source of sediment to the Nutzotin basin. The youngest MDA that we documented from FA3 and FA4 is 144 Ma (Fig. 7A). When combined with the oldest MDA of 155 Ma from the Nutzotin Mountains sequence, this basin contains a record of a major influx of sand- and gravel-size detritus from 155 to 144 Ma; most of this sediment was derived from Wrangellia and related plutons of the Chitina arc. The final stage of basin development recorded by the Nutzotin Mountains sequence is represented by fossiliferous shale with minor, thin interbeds of sandstone of facies association 5 (FA5; BON-COMP in Fig. 6). This facies association is characterized by abundant nontransported, open-marine bivalve macrofauna, and it has been interpreted as being deposited in shallow-marine environments above storm wave base (Manuszak et al., 2007). The MDA of this upper part of the stratigraphy is 133 Ma (Fig. 6). Detrital zircon ages between 152 and 135 Ma from one sample from FA5 indicate that the Chitina arc was still a dominant source of sediment for the basin (Fig. 7A). Our data from FA5 also document a small population of 130–127 Ma detrital zircons that overlap with the Chisana arc (130–115 Ma); volcanic and plutonic rocks of this age are best exposed along the southern and eastern margins of the Nutzotin basin (CSA in Fig. 2; Fig. 5A). The εHf(t) values for detrital zircons overlapping with the Chisana arc (135–127 Ma) yield predominantly juvenile signatures (+15.1 to +6.8; Fig. 8), which are consistent with previous interpretations that the Chisana arc developed as an oceanic arc built upon overthickened crust of Wrangellia (e.g., Snyder and Hart, 2007; Manselle, 2019). Based on these ages and compositions, we interpret this to indicate that during Cretaceous deposition in the Nutzotin basin, the Chitina arc was still a major source of detrital zircons, while the Chisana arc initiated as a minor contributor of detrital zircons during the latest stages of deposition of the Nutzotin Mountains sequence.

The upper part of the Nutzotin Mountains sequence stratigraphy, FA5, is interbedded with thin basaltic to andesitic lava flows that increase up section toward the gradational contact with the Chisana Formation (Fig. 3; Manuszak et al., 2007). The Chisana Formation consists of 3 km of Lower Cretaceous (Hauterivian–Aptian) lava flows, tuff, mudstone, and volcaniclastic breccia (Richter, 1976; Sandy and Blodgett, 1996) with depositional ages between 121 and 117 Ma (Manselle et al., 2020; Trop et al., 2020). The Nutzotin Mountains sequence and the Chisana Formation are deformed by thrust faults (Fig. 5B) and are intruded by undeformed plutons with U-Pb ages between 126 and 113 Ma (Richter, 1976; Manuszak et al., 2007; Graham et al., 2016). The youngest strata in the Nutzotin basin are the fluvial strata of the Beaver Lake formation, which are deformed into broad open folds. These strata consist chiefly of conglomerate with minor sandstone and mudstone (Trop et al., 2020). Biostratigraphic age determinations and U-Pb ages of detrital zircons indicate that these strata were deposited between ca. 117 and 98 Ma and were derived chiefly from Chisana arc sources (Trop et al., 2020). These ages, in conjunction with our detrital zircon data set from the Nutzotin Mountains sequence, indicate that deposition and volcanism in the Nutzotin basin occurred between 155 and 98 Ma.

New Insights on Basin Development and Provenance of the Wrangell Mountains Basin

Our detrital zircon data, combined with previously reported sedimentological and structural data (Trop et al., 2002), provide additional insights into the depositional and tectonic history of the Wrangell Mountains basin. The Upper Jurassic Root Glacier Formation and the Kotsina Conglomerate have MDAs based on detrital zircons of 152 and 159 Ma, respectively (Fig. 7B). Previous biostratigraphic data indicate that these strata are Oxfordian–Tithonian in age (MacKevett, 1969), suggesting that the MDAs are a close representation of the depositional ages. The Root Glacier Formation consists of mudstone, sandstone, and conglomerate representing submarine fan and slope environments, whereas the Kotsina Conglomerate consists of pebble-boulder conglomerate representing fan-delta environments (Trop et al., 2002). Clast compositions of conglomerate in both units consist primarily of metabasalt and limestone derived from volcanic and carbonate strata of the Wrangellia composite terrane (Trop et al., 2002). Our detrital zircon ages from both units yielded a combined major age population between 166 and 144 Ma (∼90% of total detrital zircon ages of both samples), which matches closely with ages of plutonic rocks of the Chitina arc (175–135 Ma) exposed directly to the southeast of the basin (CTA in Fig. 2). This interpretation is consistent with northward-directed paleoflow indicators in the Upper Jurassic strata (Trop et al., 2002). Volcanic flows and tuffs in the Root Glacier Formation as well as Jurassic granitic clasts in the Kotsina Conglomerate are also indicative of a proximal arc source (Trop et al., 2002). We interpret the Chitina arc as being the predominant source of detrital zircons for the Wrangell Mountains basin during deposition of the Root Glacier Formation and Kotsina Conglomerate. Other potential sources are plutons of the Talkeetna arc (201–153 Ma) in the Talkeetna Mountains exposed along-strike to the west and northwest of the basin (TKA in Fig. 2). In terms of detrital zircon grains that could only be sourced from the Talkeetna arc, only 2% of the grains fall within this range (grains between 201 and 175 Ma); this suggests to us that the arc was not a major source of sediment to the basin. The Root Glacier Formation and Kotsina Conglomerate are interpreted to have formed coeval with displacement on the Chitina thrust belt (CTB in Fig. 2). This northwest-trending thrust belt juxtaposes Wrangellia terrane rocks against Jurassic strata of the Wrangell Mountains basin (Gardner et al., 1986; Trop et al., 2002). During Late Jurassic shortening, the Chitina thrust belt exhumed rocks of the Chitina arc and Wrangellia terrane that were then eroded and deposited in the retro-foreland setting of the Wrangell Mountains basin (Trop et al., 2002).

Lower Cretaceous strata of the Kennicott and Moonshine Creek Formations overlie deformed Jurassic strata along an angular unconformity (Fig. 4B) and consist of conglomerate, sandstone, and mudstone that were deposited in nearshore to offshore marine environments (Trop et al., 2002). Clast compositions of conglomerate from both formations indicate derivation from the Wrangellia composite terrane, and paleoflow measurements indicate southward to southeastward sediment dispersal (Trop et al., 2002). Detrital zircon ages for the Lower Cretaceous Kennicott Formation yielded two major age populations: 310–289 Ma and 160–142 Ma (Fig. 7B), which overlap in age with plutons of the Skolai (320–290 Ma) and Chitina (175–135 Ma) arcs, currently exposed southeast of the basin (SKA and CTA in Fig. 2). Another potential source for the Jurassic detrital zircons in the Cretaceous strata are underlying Jurassic intrabasinal strata that were deformed and eroded prior to deposition of the Cretaceous strata across the angular unconformity (Fig. 4B). Note that detrital zircon ages from the Root Glacier Formation and Kotsina Conglomerate are similar to ages for the Kennicott Formation (Fig. 7B). Jurassic strata are currently exposed north of the Kennicott Formation (Fig. 4A), which, combined with southward paleoflow measurements (Trop et al., 2002), suggests that erosion and reworking of Jurassic strata in the basin during the Cretaceous very likely occurred. Detrital zircon ages from the Moonshine Creek Formation exhibit similar Pennsylvanian–Permian and Late Jurassic–Early Cretaceous populations between 317 and 284 Ma and between 169 and 136 Ma, which overlap in age with plutons of the Skolai and Chitina arcs, as well as detrital zircon ages in the older intrabasinal strata. Major Cretaceous populations range between 125 and 113 Ma and between 111 and 96 Ma, which overlap with plutons of the Chisana arc (CSA in Fig. 2) as well as mid- to Late Cretaceous plutons that intrude the Yukon composite terrane in south-central Alaska north of the basin (mlK in Fig. 2). These populations, combined with southeastward paleoflow indicators, document the introduction of Cretaceous arc detritus from both inboard and outboard terranes into the basin, while remnant Paleozoic and Jurassic plutons, as well as older intrabasinal strata, were being eroded.

The stratigraphically youngest Mesozoic unit in the basin, the MacColl Ridge Formation, is exposed in the southeastern part of the Wrangell Mountains basin (Figs. 3 and 4A). The MacColl Ridge Formation consists of conglomerate, sandstone, mudstone, and tuff deposited in submarine fan environments (Trop et al., 1999). Paleoflow measurements indicate primarily northward sediment dispersal, and clast compositions of conglomerate beds consist of metabasalt, granite, and limestone that are indicative of derivation from plutonic, volcanic, and carbonate rocks of the Wrangellia composite terrane (Trop et al., 1999, 2002). Volcanic tuff beds in the MacColl Ridge Formation are interpreted to represent derivation from a newly established Cretaceous arc source north of the basin (Kluane arc of Plafker and Berg, 1994; Trop et al., 1999, 2002). The MDA based on detrital zircons for the MacColl Ridge Formation is 76 Ma, which is consistent with 40Ar/39Ar radiometric ages of 79.4 ± 0.7 Ma and 77.9 ± 2.1 Ma for volcanic tuffs in the unit (Trop et al., 1999). The large Cretaceous detrital zircon age population (94–75 Ma; Fig. 7B) is similar to large populations of Upper Cretaceous detrital zircons that have been documented in coeval Upper Cretaceous strata of the Matanuska basin and Chugach accretionary prism (see locations in Fig. 2). These large populations are interpreted to reflect a regional introduction of sediment from a Late Cretaceous arc system (Kluane arc) built along the southern Alaska convergent margin (e.g., Amato et al., 2013; Stevens Goddard et al., 2018).

New Insights on Provenance of the Wellesly Basin and Implications for Strike-Slip Displacement on the Denali Fault

Wellesly basin strata are exposed directly east of the Denali fault in the study area, and they stratigraphically overlie Paleozoic metasedimentary strata of the Yukon composite terrane (Figs. 1 and 2). This basin contains major detrital zircon peak ages between 218 and 187 Ma that overlap with Late Triassic–Early Jurassic plutons of the Taylor Mountain batholith (216–181 Ma). These plutons are exposed north of the Wellesly basin in the Yukon composite terrane (Fig. 2). The εHf(t) values of detrital zircons with ages between 197 and 182 Ma yielded juvenile to evolved compositions (+9.6 to −14.4; Fig. 8), which are consistent with mantle- and crustal-derived magmas of the Taylor Mountain batholith (e.g., Dusel-Bacon et al., 2015). More distal potential sources include volcanic rocks and plutons associated with the Talkeetna arc south of the Denali fault (Fig. 2). Nd isotopic compositions from the Talkeetna arc plutons, however, are primarily juvenile (Rioux et al., 2007), whereas the εHf(t) values from the Wellesly basin display a wide range of isotopic compositions, making the Talkeetna arc an unlikely source of sediment for the basin. For this reason, we consider the Taylor Mountain batholith as a dominant source of sediment for the Wellesly basin.

Late Jurassic–Early Cretaceous (167–126 Ma) detrital zircon ages from the Wellesly basin account for 13% of the total age distribution (Fig. 7C). Plutons of this age are rare in the Yukon composite terrane of south-central Alaska, but a pluton of this age (ca. 163–161 Ma) has been recently documented in along-strike rocks in the Yukon (M. Colpron, 2019, personal commun.). Another potential distal source may be plutons of the eastern portion of the Coast Mountains batholith. On a regional scale, pluton ages of 140–120 Ma are rare in the Coast Mountains batholith (Armstrong, 1988; Gehrels et al., 2009), but locally there are plutons of this age in the eastern Coast Mountains batholith (Mahoney et al., 2009). Conversely, documented magmatic sources of this age are located south of the Denali fault and are associated with the Wrangellia composite terrane, including both the Chitina (175–135 Ma) and Chisana arcs (130–115 Ma; Fig. 2). The εHf(t) values of detrital zircons between 167 and 158 Ma from the Wellesly basin, however, have juvenile to evolved signatures (+6.7 to −5.5; Fig. 8), which are inconsistent with predominantly juvenile isotopic compositions from plutons intruding the Wrangellia composite terrane (e.g., Snyder and Hart, 2007). We interpret the Late Jurassic–Early Cretaceous detrital zircons to have been derived from plutons associated with the Yukon composite terrane.

Consistent with sediment sources north of the Denali fault, we interpret Paleozoic detrital zircons (498–254 Ma) from the Wellesly basin to have been likely sourced from Devonian–Mississippian (376–320 Ma) and Permian (275–255 Ma) plutons that intrude the Yukon composite terrane; the εHf(t) values (0 to −6) from these detrital zircons are consistent with more evolved Paleozoic sources (e.g., Mortensen, 1992). Precambrian zircons account for 13% of the total age distribution in the Wellesly basin, with sparse ages ranging between 1.8 and 0.5 Ga (Fig. 7C). These zircons were likely recycled from older sedimentary strata in the Northern Cordillera, such as Paleozoic–Triassic passive-margin strata, where similar-aged Precambrian detrital zircon populations have been recognized (Fig. 1; Gehrels et al., 1995; Gehrels and Pecha, 2014; Romero et al., 2020).

The Wellesly basin had been interpreted to be a possible proximal equivalent to the more distal Nutzotin basin west of the Denali fault (Fig. 2; Richter, 1976). Our comparison of the detrital zircon ages and Hf compositions from the two basins demonstrates that the basins have a very different provenance. Detrital zircon ages from the Nutzotin basin are dominated by unimodal peak ages between 155 and 133 Ma (Fig. 7A) and Hf compositions that are predominantly juvenile to slightly evolved (+15.1 to −0.6; Fig. 8). Conversely, detrital zircon ages from the Wellesly basin are dominated by multiple peak ages that are clustered between 218 and 172 Ma (Fig. 7C), display a higher concentration of Precambrian detrital zircons, and have Hf compositions that range from juvenile to highly evolved (+9.6 to −14.4; Fig. 8). Based on this comparison, we propose that the Wellesly and Nutzotin basins are unrelated. Instead, the Wellesly basin appears to be more genetically linked to the Jurassic–Cretaceous Kahiltna basin, currently exposed in the Talkeetna Mountains and Alaska Range in south-central Alaska (Figs. 2 and 9). Detrital zircon ages from the Wellesly basin and Kahiltna assemblage in the Talkeetna Mountains and Alaska Range display dominant age populations between 215 and 170 Ma and similar subordinate Paleozoic ages (Supplemental Fig. S12). The Kahiltna assemblage in the Alaska Range, however, displays significantly more Precambrian ages (38%; see Hampton et al., 2010; see also Supplemental Fig. S1) than the Wellesly basin (13%). We suggest that the Wellesly basin is more similar to the Kahiltna assemblage in the Talkeetna Mountains, which contains 5% Precambrian ages, as opposed to the Kahiltna assemblage in the Alaska Range. Maximum depositional ages for the Kahiltna strata in the Talkeetna Mountains range between 124 and 115 Ma (Supplemental Fig. S1; Hampton et al., 2010), which are slightly younger than the MDA of 126 Ma for the Wellesly basin (Fig. 9). This difference may suggest that the Wellesly strata may represent the older part of the interpreted Kahiltna-Wellesly basin system. If so, this might also account for the more prominent, younger peak between 120 and 100 Ma in the Kahiltna basin of the Talkeetna Mountains compared to the Wellesly basin (Fig. 9).

Both the Wellesly and Kahiltna basins are currently situated adjacent to the Denali fault, which is interpreted to have undergone ∼370 km of Late Cretaceous–Cenozoic offset based on the correlation of the Nutzotin and Dezadeash basins to the south (Fig. 9; Eisbacher, 1976; Nokleberg et al., 1985; Lowey, 1998), with ∼150 km of displacement possibly occurring since Oligocene time (Trop et al., 2019). Our new data also allow for a comparison of detrital zircon ages from the Nutzotin and Dezadeash basins as an independent test of previous correlations interpreting that these two now-offset basins formed as a single basin. These basins have been previously interpreted as proximal-distal equivalents within the same sedimentary system (Eisbacher, 1976; Nokleberg et al., 1985; Lowey, 1998, 2018). Detrital zircons ages from both basins exhibit unimodal age distributions with major Late Jurassic populations between 165 and 145 Ma (Fig. 9; Lowey, 2018). This is consistent with both basins sharing a similar source of sediment, and thus it corroborates previous interpretations suggesting that these basins were offset along the Denali fault. Restoration of the Wellesly and Kahiltna basins along the Denali fault results in a minimum offset estimate of ∼330–390 km. Given the wide spatial extent of the Kahiltna assemblage in the Talkeetna Mountains, we include a range of possible offset amounts in Figure 9. Future work needs to be undertaken to better confirm and tighten the correlation between the Wellesly and Kahiltna basins; however, our initial comparisons are consistent with the previously postulated ∼370 km of Late Cretaceous–Cenozoic offset along the Denali fault east of our study area.

Our data also have implications regarding displacement along the Totschunda fault system. Isolated exposures of the Nutzotin basin occur along the west side of the Denali-Totschunda fault system in the eastern Alaska Range (e.g., Slate Creek area in Fig. 5A). We speculate that these isolated exposures of the Nutzotin Mountain sequence formed as part of the main basin, currently located to the southeast between the Totschunda and Denali faults (Fig. 5A), and were subsequently offset and transported northwestward along the Denali-Totschunda fault system. Detrital zircon ages from one of these exposures at the Slate Creek locality (sample 071718KR-01 in Fig. 7A; see also Fig. 5A) compare favorably to detrital zircon ages from the Nutzotin basin proper (Fig. 7A). Restoring the Slate Creek location southeastward to the junction of the Totschunda and Denali faults (Fig. 5A) provides a minimum offset estimate of ∼75 km. This estimate is generally consistent with recent studies suggesting ∼85 km of offset since ca. 18 Ma (Berkelhammer et al., 2019), although some studies postulate that much of this offset could have occurred since ca. 6 Ma (Allen et al., 2018).

Regional Sediment Transport, Basin Development, and Tectonics along the Mesozoic Southern Alaska Convergent Margin

The Jurassic–Cretaceous Nutzotin and Wrangell Mountains basins are located along the inboard (continentward) and outboard (oceanward) margins of the Wrangellia composite terrane, respectively (Fig. 2). Correlations between the two basins have been previously made on the basis of (1) the proximity of the two basins, (2) the positions of both basins depositionally overlying the Wrangellia terrane (Fig. 1), (3) the similarity of Upper Jurassic lithologies indicating submarine fan depositional environments, and (4) the detrital compositions, indicating that both basins were being sourced with sediment derived from strata and plutons associated with Wrangellia (Trop et al., 2002; Manuszak et al., 2007; Trop and Ridgway, 2007). Our new detrital zircon geochronologic data set allows us to further explore and evaluate the provenance and temporal links between the two basins.

Our results are consistent with Upper Jurassic strata in the Wrangell Mountains basin (the Root Glacier Formation and Kotsina Conglomerate), which represent more proximal submarine fan facies, being coeval with the Jurassic strata in the Nutzotin basin, which represent more medial to distal submarine fan facies. Detrital zircon ages from Upper Jurassic–Lower Cretaceous strata in both basins are shown in Figure 10A. Note that the age distributions are nearly identical, with major Jurassic peak ages between 160 and 145 Ma in both basins. The detrital zircon populations indicate that the Chitina arc was the dominant source of sediment for both of these basins. Plutons of the Chitina arc are currently exposed south of the Wrangell Mountains and Nutzotin basins (CTA in Fig. 2), suggesting that both basins formed in a retroarc position (Fig. 11A). As shown in Figure 3, Late Jurassic–Early Cretaceous deposition in the Wrangell Mountains basin was marked by the development of several unconformities, whereas deposition in the Nutzotin basin was apparently more continuous. These unconformities have been interpreted to be the product of shortening along the Chitina thrust belt (CTB in Fig. 2; Fig. 4). This thrust-related shortening is interpreted to have produced accommodation space in the Nutzotin basin related to flexural subsidence as well as erosion and sediment transport from the more proximal Wrangell Mountains basin to the more distal Nutzotin basin (Trop et al., 2002).

From a more regional perspective, strata of the Nutzotin and Wrangell Mountains basins are generally equivalent in age to Jurassic–Lower Cretaceous sedimentary strata of the Matanuska and Kahiltna (Clearwater Mountains) basins, as well as metasedimentary and metavolcanic rocks of the Chugach accretionary prism (Potter Creek assemblage; Fig. 3). The Matanuska and Kahiltna basins are exposed to the south and north of the Talkeetna Mountains in south-central Alaska, respectively, and represent forearc/intra-arc and retroarc strata relative to the adjacent Talkeetna magmatic arc (Fig. 2; Trop and Ridgway, 2007). The Chugach accretionary prism is exposed outboard of these strata along the southern Alaska margin, and it represents part of a paleo–subduction complex associated with eastward-dipping (northward in present-day coordinates) subduction underneath the Wrangellia composite terrane (Pavlis, 1982; Plafker et al., 1994). To better document the Mesozoic tectonic configuration and spatial distribution of sedimentary basins along the southern Alaska convergent margin, we integrated our comparison of detrital zircon ages of the Nutzotin and Wrangell Mountains basins with previously reported sedimentological and detrital zircon data from the Jurassic–Lower Cretaceous strata of the Kahiltna and Matanuska basins, and the Chugach accretionary prism, as shown in Figures 10A and 11A.

During Late Jurassic time, the outboard margin of the Wrangellia composite terrane was characterized by marine deposition in the Matanuska and Wrangell Mountains basins (Fig. 11A; Trop et al., 2002, 2005; Trop and Ridgway, 2007). In the Matanuska basin, predominantly coarse-grained fan-delta and shelfal deposits of the Naknek Formation (Fig. 3) are interpreted to have been deposited in a forearc basin setting, due to their position outboard of plutonic rocks of the Talkeetna arc and inboard of the Chugach accretionary prism (Fig. 11A; Trop et al., 2005). Coeval forearc basin strata located along strike in the southern Wrangell Mountains have not been recognized; in this region, plutonic rocks of the Chitina arc are juxtaposed directly against the accretionary prism (Fig. 2). The “missing” forearc basin in this region has been interpreted to have been tectonically removed by a period of subduction erosion that occurred during the Late Jurassic–Early Cretaceous (Trop et al., 2002, 2005; Clift et al., 2005). In contrast, Upper Jurassic strata of the Wrangell Mountains basin (Root Glacier Formation and Kotsina Conglomerate) are located in a retroarc basin position relative to the coeval Chitina plutons (Figs. 2 and 3). These strata also record marine deposition in submarine fan and fan-delta environments (Trop et al., 2002). Detrital zircon ages from Upper Jurassic strata of the Matanuska and Wrangell Mountains basins are shown in Figure 10A. Both basins display unimodal age distributions, with predominantly Late Jurassic–Early Cretaceous zircon ages ranging between 181–136 Ma (Matanuska basin) and 166–144 Ma (Wrangell Mountains basin; Fig. 10A). Conglomerate beds from both basins yield granitic clasts with Late Jurassic isotope ages (Trop et al., 2002, 2005). A similar Late Jurassic detrital zircon age signature occurs in the Middle to Late Jurassic mélange (Potter Creek Assemblage) of the Chugach accretionary prism, with the dominant age population ranging between 203 and 148 Ma (Fig. 10A; Amato et al., 2013). Detrital zircon provenance analyses of the Matanuska basin (Trop et al., 2005; Trop, 2008; Reid et al., 2018; Stevens Goddard et al., 2018), the Wrangell Mountains basin (this study), and the Chugach accretionary prism (Amato et al., 2013) indicate that the primary magmatic sources of sediment for all of these basins during the Jurassic were the Talkeetna and Chitina arcs (Fig. 10A). The Talkeetna and Chitina arcs are interpreted to represent a single magmatic arc system built upon the Wrangellia composite terrane above an eastward-dipping (northward in present-day coordinates) subduction zone (Plafker et al., 1989, 1994; Rioux et al., 2007).

Marine deposition along the outboard margin of the Wrangellia composite terrane during the Late Jurassic was coeval with the beginning stages of marine deposition along the inboard margin, represented by submarine fan strata in both the Kahiltna (Clearwater Mountains) and Nutzotin retroarc basins (Fig. 11A; Trop and Ridgway, 2007). Detrital zircon ages of Upper Jurassic strata in both basins yield primarily Late Jurassic detrital zircon ages that match best with the ages of the Talkeetna-Chitina arc (Fig. 10A). Hf isotope compositions reported in this study from the Nutzotin basin (Fig. 8) also compare favorably to reported compositions from the Matanuska basin, which are consistent with sediment derivation from the Talkeetna-Chitina arc (see figure 4 in Reid et al., 2018). Collectively, the regional detrital zircon database indicates that during the Late Jurassic, strata on both the inboard and outboard margins of the Wrangellia composite terrane (i.e., from the retroarc basins to the accretionary prism) were being supplied from a single arc system (Fig. 10A).

Beginning in the Early Cretaceous, the locus of magmatism migrated inboard, with the establishment of the Chisana arc (Fig. 11B). Detrital zircon ages in the Beaver Lake formation of the Nutzotin basin and Kahiltna assemblage (Alaska Range and Talkeetna Mountains) of the Kahiltna basin overlap with the Chisana arc, indicating that the arc was a primary source of sediment for these inboard basins (Fig. 10B). Conversely, the outboard basins were characterized by a depositional hiatus, with Jurassic strata in both the Matanuska and Wrangell Mountains basins being exhumed due to shortening (Fig. 3; Trop et al., 2002, 2005). This depositional hiatus is marked by prominent Late Jurassic–Early Cretaceous unconformities present in the outboard basins as well as in the Chugach accretionary prism, which represent potentially ∼40–30 m.y. of nondeposition and erosion in the outboard basins and accretionary prism (Fig. 3). The unconformities in the outboard basins have been previously attributed to a period of subduction erosion possibly caused by subduction of an oceanic spreading ridge during the Early Cretaceous (Pavlis et al., 1988; Clift et al., 2005; Trop and Ridgway, 2007; Amato et al., 2013; Stevens Goddard et al., 2018; Mahar et al., 2019).

Deposition in the outboard basins resumed in the Early Cretaceous (early Albian), with marine deposition in the Matanuska basin (lower Matanuska Formation) and the Wrangell Mountains basin (Kennicott Formation), as well as renewed deposition in the Chugach accretionary prism (McHugh Creek Assemblage; Figs. 3 and 11C; Trop and Ridgway, 2007). Detrital zircon ages from the lower Matanuska Formation in the Matanuska basin and from the McHugh Creek Assemblage in the Chugach accretionary prism show more multimodal age distributions than those documented in the Jurassic strata, with a dominant mid-Cretaceous detrital zircon age signature between 110 and 90 Ma as well as minor Jurassic ages (Fig. 10B; Amato et al., 2013; Reid et al., 2018; Stevens Goddard et al., 2018). The multimodal age distributions exhibited by Lower Cretaceous strata in both the Matanuska basin and Chugach accretionary prism are interpreted to reflect (1) continued exhumation and erosion of the remnant Talkeetna-Chitina arc and reworking of related Jurassic sedimentary strata in the forearc basin, and (2) the establishment of a new mid-Cretaceous magmatic arc (middle Cretaceous igneous belt of Plafker and Berg, 1994) that intruded both the inboard and outboard terranes (Fig. 11C; Amato et al., 2013; Stevens Goddard et al., 2018). In contrast to the Matanuska basin and Chugach accretionary prism, Lower Cretaceous strata in the Wrangell Mountains basin (Kennicott Formation) exhibit a detrital zircon age spectrum much like the Upper Jurassic strata in the basin (Fig. 11B). Mid-Cretaceous strata (Moonshine Creek Formation) stratigraphically above the Kennicott Formation document the introduction of sediment from an Early Cretaceous arc (Chisana arc) as well as a mid-Cretaceous arc system (Fig. 10B). We interpret this to suggest that the Wrangell Mountains basin was initially depositionally isolated from the new sources of sediment in the Albian and then ultimately linked with inboard sediment sources provided by the Chisana arc in the Cenomanian.

Finally, during the Late Cretaceous, deposition in the Nutzotin and Kahiltna basins along the inboard margin ceased, and these basins were deformed and exhumed (Fig. 11D; Manuszak et al., 2007; Trop and Ridgway, 2007). Exhumation of the inboard basins was coeval with continued marine deposition in the outboard basins (Upper Matanuska Formation in the Matanuska basin and MacColl Ridge Formation in the Wrangell Mountains basin), as well as rapid and extensive growth of the Chugach accretionary prism (Valdez Group; Fig. 11D; Plafker et al., 1994; Trop, 2008; Amato et al., 2013). Detrital zircon ages from outboard Upper Cretaceous strata display dominant Late Cretaceous age populations as well as minor Early Jurassic–Early Cretaceous ages (Fig. 10C; Stevens Goddard et al., 2018). Granitic clasts in the Matanuska Formation yield Late Jurassic and Late Cretaceous zircon ages (Trop, 2008). These detrital zircon trends suggest that across the outboard margin, erosion of remnant Jurassic–Cretaceous arc sources and older basin strata continued to supply minor sediment to the basins, whereas the Late Cretaceous arc system (Kluane arc of Plafker and Berg, 1994) became a dominant source of sediment along the southern Alaska convergent margin.

Implications for Subduction Polarity along the Mesozoic Convergent Margin

The results of our study have implications for important ongoing discussions concerning the Mesozoic tectonic configuration of the Northern Cordilleran margin. Much of this debate centers on the polarity of subduction along the Late Jurassic–Early Cretaceous margin of western North America. The most accepted interpretation for the last several decades is that this convergent margin was the product of eastward-dipping subduction (northward-dipping from the perspective of Alaska) outboard of the Wrangellia composite terrane (McClelland et al., 1992; Plafker and Berg, 1994; Plafker et al., 1994; Kalbas et al., 2007; Gehrels et al., 2009; Hampton et al., 2010; Pavlis et al., 2019). A new intriguing model based on seismic anomalies in the deep mantle has postulated that the margin formed due to westward-dipping subduction along the inboard margin of the Wrangellia composite terrane (Sigloch and Mihalynuk, 2013, 2017; Spencer et al., 2019). Our new detrital zircon data sets from the Nutzotin and Wrangell Mountains basins, in conjunction with the regional framework from previously published detrital zircon data sets, indicate a strong provenance and temporal link between all major Mesozoic tectonic elements of the southern Alaska convergent margin. This regional provenance link is important because it ties the south-verging accretionary prism (Chugach terrane) to the forearc basin (Matanuska basin), to the Chitina and Chisana arcs, and to the retroarc basins (Wrangell Mountains and Nutzotin basins; Fig. 2). This type of spatial configuration from outboard accretionary prism to retroarc basins is the hallmark for recognizing subduction polarity in ancient convergent margins (e.g., Burchfiel and Davis, 1972; Dickinson, 1974; Plafker and Berg, 1994; Pavlis et al., 2019) and, in the case of Alaska, is most consistent with Late Jurassic–Early Cretaceous northward subduction.

In the west-dipping subduction interpretation, in contrast, one interpretation for the Chugach terrane would be that it represents a retroarc basin related to the Late Jurassic–Early Cretaceous Talkeetna, Chitina, and Chisana arc systems. Decades of geologic studies, however, document that the Chugach terrane is defined by south-verging (present coordinates) thrust faults (Plafker et al., 1994, and references therein) with blueschist-facies and fault-bounded slices of oceanic material with crystalline ages of 204–185 Ma (Sisson and Onstott, 1986; Roeske et al., 1989) and younger blueschist constrained by detrital zircon MDAs ca. 135–100 Ma (Day et al., 2016; Pavlis et al., 2019). The southward structural vergence direction and the presence of blueschist are defining structural and petrologic features of an accretionary prism that formed above a north-dipping (present coordinates for Alaska) subduction zone and are not geologic features characteristic of retroarc basins. Another possibility in the west-dipping subduction interpretation is that the Chugach terrane is unrelated to Late Jurassic–Early Cretaceous magmatic arcs and sedimentary basins of the Wrangellia composite terrane (Sigloch and Mihalynuk, 2017). In this interpretation, the Chugach terrane would be required to have been transported by post–Late Jurassic–Early Cretaceous large-magnitude, strike-slip displacement to its current position. Results from our analysis do not preclude hundreds of kilometers of displacement along major strike-slip faults in our study area (e.g., Border Ranges fault, Denali fault, etc.), but they do require that all the basins and the Chugach terrane were proximal enough to share regional major sources of sediment and to have stratigraphic ties. More research needs to be completed on this critical question, but collectively our findings are most consistent with a tectonic configuration in which the accretionary prism, magmatic arcs, and sedimentary basins formed over a Late Jurassic–Early Cretaceous eastward-dipping subduction zone along the outboard margin of the Wrangellia composite terrane, similar to the modern configuration of the convergent margin.

CONCLUSIONS

  • (1) New U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence yielded a predominant unimodal age distribution with major peak ages between 155 and 133 Ma. These data provide a more rigorous chronostratigraphic framework for this little studied ∼3-km-thick unit. Detrital zircon Hf results displayed primarily juvenile to slightly evolved compositions (+15.1 to −0.6). Our results are consistent with detrital zircons being derived mainly from Late Jurassic–Early Cretaceous arc sources (Chitina and Chisana arcs) that intrude/overlie the Wrangellia composite terrane, and little to no sediment contribution from continental sources.

  • (2) New U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Root Glacier Formation, Kotsina Conglomerate, and Kennicott Formation of the Wrangell Mountains basin yielded unimodal age spectra with major peak ages between 159 and 152 Ma, consistent with derivation from plutons associated with the Wrangellia composite terrane (Chitina arc). Detrital zircon ages from the Lower to Upper Cretaceous Moonshine Creek Formation had major peak ages between 150 and 104 Ma, consistent with derivation from the Chitina arc as well as the Chisana arc. Detrital zircon ages from the Upper Cretaceous MacColl Ridge Formation displayed a multimodal age distribution with a major Late Cretaceous peak age at 76 Ma, and with subordinate peak ages between 196 and 104 Ma. These ages are consistent with detrital zircon derivation from a newly developed Late Cretaceous arc system that intruded both the Wrangellia and Yukon composite terranes as well as sediment derivation from remnant Jurassic–Early Cretaceous arc rocks and reworking of older basinal strata.

  • (3) The similarity in detrital zircon age distributions in Mesozoic strata in both the Nutzotin and Wrangell Mountains basins suggests that the two basins were depositionally connected and were subsequently separated by Mesozoic regional shortening and Cenozoic volcanism.

  • (4) The comparison between our new detrital zircon data and previously published data from strata of Mesozoic forearc basins, retroarc basins, and the accretionary prism exposed along the southern Alaska convergent margin indicates that all these elements shared a strong provenance and temporal link during the Late Jurassic–Late Cretaceous. It also documents a progressive shift in regional sediment sources, from a Late Jurassic–Early Cretaceous provenance characterized by a single arc system (Jurassic–Early Cretaceous Talkeetna-Chitina arc) to a provenance from two distinct Cretaceous arc systems (Chisana and Kluane arcs) and a remnant Jurassic arc system.

  • (5) The new U-Pb detrital zircon ages from the Wellesly basin document a multimodal age distribution with dominant peak ages between 200 and 172 Ma and subordinate peak ages between 147 and 126 Ma. The detrital zircon Hf results displayed a wide range of juvenile to highly evolved compositions (+9.6 to −14.4). These results are consistent with sediment sources from the Mesozoic continental margin represented by the Yukon composite terrane.

  • (6) Detrital zircon data from the Wellesly and Nutzotin basins are inconsistent with a previous tentative interpretation wherein the two basins were considered to be proximal-to-distal equivalents. Instead, the provenance of the Wellesly basin appears to be similar to the Kahiltna basin, indicating that the two basins may have been offset ∼330 km or more along the Denali fault.

  • (7) The strong provenance, temporal, and spatial links between the outboard and inboard basins along the southern Alaska convergent margin are inconsistent with models that invoke Late Jurassic–Early Cretaceous westward-dipping subduction, and they are instead more consistent with an eastward-dipping subduction zone.

ACKNOWLEDGMENTS

This research was funded by a grant from the National Science Foundation (EAR-1550034) to Ridgway, and by the Alaska Geological Society to Fasulo. We benefited from discussions with Chris Andronicos, Jeff Benowitz, Maurice Colpron, Nat Lifton, and Mariah Romero. We thank Wai Allen and Brandon Keough for assistance in the field as well as with sample analysis; the staff scientists at the University of Arizona LaserChron Center for their assistance with geochronological analyses; Jeff Benowitz and Anna Liljedahl for lodging, meals, and other logistical support during field work; the staff at the Wrangell–St. Elias National Park and Preserve for logistical assistance; and Andrew Caruthers, Ben Gill, Martyn Golding, Selva Marroquin, Jeremy Owens, Teddy Them, João Trabucho, and Yorick Veenma for generously providing samples from the Moonshine Creek Formation. We appreciate constructive feedback from Chris Andronicos on an earlier version of the manuscript. Finally, we thank Science Editor Andrea Hampel for handling this manuscript, and Associate Editor Alexander Rohrmann, Devon Orme, and Robinson Cecil for constructive reviews that improved the clarity of the manuscript.

1Supplemental Data. (A) U-Pb analytical results from detrital zircons from the Nutzotin, Wrangell Mountains, and Wellesly basins. (B) Lu-Hf analytical results from detrital zircons from the Nutzotin and Wellesly basins. Please visit https://doi.org/10.1130/GEOS.S.12465842 to access the supplemental material, and contact editing@geosociety.org with any questions.
2Supplemental Figure S1. Normalized distribution plot of detrital zircon ages from the Kahiltna assemblage of the central Alaska Range (Hampton et al., 2010), the Wellesly basin (this study), and the Kahiltna assemblage of the northwestern Talkeetna Mountains (Hampton et al., 2010). Note that the detrital zircon age distribution of ages older than 500 Ma has 10× vertical exaggeration. Please visit https://doi.org/10.1130/GEOS.S.12465950 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: Andrea Hampel
Guest Associate Editor: Alexander Rohrmann
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.