The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular terrane to the south and the Farewell and other pericratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard pericratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and mid- to Late Cretaceous arc-related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at ca. 81 Ma. They were deformed together ca. 80 Ma by southeast-verging fold-and-thrust–style deformation interpreted to represent final accretion of the Alexander-Wrangellia-Peninsular terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench–Nankai Trough on the east side of central Japan.
The Kahiltna assemblage in southern Alaska (Fig. 1) and associated basins along strike to the east and southeast (Nutzotin, Dezadeash, and Gravina basins) are part of a coeval group of thick, mostly deep-water, deformed mid-Cretaceous (115–80 Ma) basins (some with strata as old as Late Jurassic) in Alaska and northwestern Canada that includes the Kuskokwim basin (Cady et al., 1955; Box and Elder, 1992; Miller and Bundtzen, 1994; Kalbas, 2006), Yukon-Koyukuk basin (Patton and Box, 1989; Nilsen, 1989; O’Brien et al., 2017), Manley basin (Silberling et al., 1992; Weber et al., 1992), Kandik basin (Brabb and Churkin, 1969; Johnsson, 2000), Colville basin (Bird and Molenaar, 1992; Houseknecht et al., 2009), and the Eagle-Mackenzie basin (Miall, 1973; Schröder-Adams, 2014). Following Ingersoll (2012), we use the term “basin” to refer to an accumulation of sedimentary or volcanic rocks, with no inference as to its genesis or origin. These basins occur within and on both flanks of a 6000-km-long belt of mid-Cretaceous subduction-related magmatic rocks (Fig. 1; Rubin et al., 1995; Hart et al., 2004; Pease et al., 2017). Much of the structural evolution of these basins is not clearly understood, including their tectonic settings, particularly those in the continental interior, the timing, rates, and causes of basin subsidence, or the terrane uplifts that provided sediment. Likewise, the broader tectonic setting(s) of the deformational histories of these interior basins (Moore and Box, 2016) is(are) not known. Mid-Cretaceous time represents an important episode in the crustal assembly of Alaska, and the basins hold key information about crustal assembly in space and time.
The Kahiltna assemblage (Ridgway et al., 2002; Kalbas et al., 2007; Hampton et al., 2007, 2010; Hults et al., 2013) consists of a 100+-km-wide northeast-trending belt of deformed clastic strata of Jurassic and Cretaceous age that crop out over 700 km from the eastern Alaska Range in south-central Alaska to the Bristol Bay lowlands in southwestern Alaska (Fig. 1). As the rocks are generally intensely folded and faulted, it has not been possible to break out or name specific formations, and thus the stratified rocks were classified as an “assemblage” by Nokleberg et al. (1994) and Ridgway et al. (2002). Following these authors, we refer to the basinal strata as the Kahiltna assemblage and subdivide it based on age and detrital composition. The assemblage is bounded to the southeast by Triassic and older rocks of the Wrangellia terrane and Triassic and Jurassic rocks of the Peninsular terrane (Jones et al., 1977; Nokleberg et al., 1994; Detterman et al., 1996; Rioux et al., 2007; Greene et al., 2010; Beranek et al., 2014). Exposures on the northwest side of the belt for much of its extent are Paleozoic and Mesozoic carbonate and clastic strata of the Farewell terrane (Dumoulin et al., 2018a, 2018b). At its northeastern end, Kahiltna assemblage occurs on both sides of the Denali fault and is bounded to the north and northwest by poorly understood rocks of the McKinley-Windy terrane. The Hines Creek fault separates the northwesternmost outliers of Kahiltna strata from metamorphic rocks of the Yukon composite terrane farther north.
The Kahiltna assemblage is part of a nearly 2000-km-long belt of similar strata that continues to the east and west (Fig. 1). East of a gap in the central Alaska Range, deformed strata of similar age occur discontinuously farther east in the Nutzotin basin in the eastern Alaska Range near the Canadian border (Manuszak et al., 2007), as the Dezadeash Formation and the adjacent Kluane schist (offset by Cenozoic dextral displacement across the Denali fault: Fig. 1) in Yukon, Canada (Eisbacher, 1976; Lowey, 1998, 2019; Mezger et al., 2001; Israel et al., 2011), and then to the southeast through southeastern Alaska in the Gravina basin (Berg et al., 1972; Brew and Karl, 1988; Rubin and Saleeby, 1991). Southwest of the Alaska Range, the northwestern part of the Kahiltna assemblage strikes southwest into the Upper Cretaceous Kuskokwim Group, whereas the southeastern part of the Kahiltna assemblage trends southwest into Upper Jurassic and Lower Cretaceous strata of the Koksetna River sequence (Wallace et al., 1989; Wilson et al., 2015). The southwestern ends of both the Koksetna River sequence and the Kuskokwim Group are covered by Quaternary sediments of the lowlands at the eastern apex of Bristol Bay.
With the advent of the terrane concept (Jones et al., 1977, 1982; Coney and Jones, 1985), it was recognized that deformed Kahiltna assemblage strata separate distinct crustal domains to the south (Wrangellia and Peninsular terranes, which had earlier amalgamated to form the Alexander-Wrangellia-Peninsular terrane; Beranek et al., 2014) and north (Farewell and Yukon composite terranes) and appear to mark a zone of relative movement between those domains. This broad zone of deformation has been termed a “flysch-filled suture” (Jones et al., 1982) or the Alaska Range suture zone (Ridgway et al., 2002). Deformation of strata in the suture zone is interpreted by many to record the emplacement of the non-Laurentian Alexander-Wrangellia-Peninsular terrane against the Laurentian continent in the late Mesozoic, although interpretations differ as to details of timing and kinematics of motion along structures within the zone (Pavlis, 1982; McClelland et al., 1992; Ridgway et al., 2002; Gehrels et al., 2009; Hults et al., 2013; Yokelson et al., 2015).
In this paper, we report new detrital zircon data from Kahiltna assemblage strata along a transect across the western Alaska Range in south-central Alaska (Fig. 2). Complementary data on sandstone compositions, as well as new ages of interbedded tuffs, have been previously reported (Karl et al., 2016). Structural data acquired along this transect were used to generate a northwest-southeast structural cross section. Detrital zircon geochronological data are reported for a total of 33 new samples, which we compared to previously published data from 10 additional samples in the map area. These data, along with previously reported fossil and interbedded ash age data, were used to constrain depositional age ranges and provenance of six newly defined petrofacies units within the two distinctive successions that make up the Kahiltna assemblage. Detrital zircon data were compared to igneous and detrital zircon age patterns in potential regional source terranes to provide constraints on sediment sources for the six petrofacies units. The ages of the youngest deformed units and of crosscutting plutonic bodies allowed us to tightly constrain the age of deformation of these strata in the western Alaska Range. Using all these data, we present a tectonic model for the depositional and deformational evolution of these sedimentary sequences and suggest an active tectonic analogue.
From the earliest geologic reconnaissance studies along river traverses across the western Alaska Range (Spurr, 1900; Brooks, 1911), clastic rocks of the Kahiltna assemblage were spatially divided into a southeastern group with volcanic and volcaniclastic compositions (“Skwentna Series”; later Skwentna Group of Brooks, 1911; Martin, 1926) and a northwestern group with a more quartzose composition (“Tordrillo Formation”). However, discussion of contrasting compositional units was generally downplayed by later workers (Capps, 1935; Csejtey et al., 1978; Jones et al., 1982; Reed and Nelson, 1980; Nelson et al., 1983), who lumped all the late Mesozoic strata into an undivided Jurassic–Cretaceous stratigraphic map unit.
Working to the southwest of the area of Figure 2, Wallace et al. (1989) referred to the southern, volcaniclastic succession as the Koksetna River sequence, whereas he considered the northwestern, quartzose succession to be part the Kuskokwim Group, which continues well to the north (Fig. 1). Farther northeast in the western Alaska Range, Hults et al. (2013) reported detrital zircon ages from both the southeastern volcaniclastic and northwestern quartzose successions and referred to the northwestern strata as Kahiltna flysch and to the southeastern strata as the flysch of the Tordrillo Mountains. Both Wallace et al. (1989) and Hults et al. (2013) indicated that the contact between these two distinct successions is abrupt; these authors interpreted that contact as a fault of uncertain displacement. Near the northeast end of exposure, Ridgway et al. (2002), Eastham and Ridgway (2002), Trop and Ridgway (2007), and Hampton et al. (2010) also recognized these two compositionally distinct successions as part of the Kahiltna assemblage and interpreted the quartzose strata to be younger than and to occur stratigraphically above the volcaniclastic strata.
The stratigraphic age ranges of the two successions are locally constrained by a small number of known fossil occurrences. The volcaniclastic succession has locally preserved Late Jurassic (Kimmeridgian), Early Cretaceous (Valanginian), and Late Cretaceous (Turonian) fossil occurrences (Wallace et al., 1989; Csejtey et al., 1992; W. Elder, 1989, written commun.), while the quartzose succession has sparse Valanginian, Hauterivian–Barremian, and Cenomanian(?) macrofossil occurrences (Csejtey et al., 1992; Bundtzen et al., 1997; Reed and Nelson, 1980). However, Dumoulin et al. (2018b) suggested that some, and perhaps all, of the Valanginian and Hauterivian–Barremian fossil localities along or near the northwestern margin of the northwestern succession are part of the underlying Mystic subterrane of the Farewell terrane, and not the Kahiltna assemblage.
Detrital zircon geochronological data for sandstone samples from the Kahiltna assemblage have been published from the central Alaska Range (Hampton et al., 2007, 2010) and from the western Alaska Range (Kalbas et al., 2007; Bradley et al., 2009; Hults et al., 2013). Similar analyses have also been published for the Nutzotin basin (Hults et al., 2013), the Dezadeash basin (Lowey, 2019) and the adjacent Kluane schist (Israel et al., 2011), and for Gravina basin strata (Yokelson et al., 2015, and references therein). All these studies identified a distinct contrast in detrital zircon populations between the more volcanogenic succession in the southeastern portion of the Kahiltna assemblage (as well as from the Nutzotin, Dezadeash, and western Gravina basins) and the more quartzose strata in the northwestern part of the Kahiltna assemblage (as well as in the Kluane schist and eastern Gravina basin). These studies interpreted the volcanogenic facies to be derived (primarily) from the Alexander-Wrangellia-Peninsular terrane to the south. The northwestern succession is considered to have been derived primarily from the Yukon composite and Farewell terranes, and possibly others, to the north and east (Hampton et al., 2007, 2010; Kalbas et al., 2007). Hampton et al. (2007, 2010) and Kalbas et al. (2007) also interpreted some input from the Alexander-Wrangellia-Peninsular terrane to the northwest facies and superposition such that the northwestern strata stratigraphically overlie the southeastern succession. However, Hults et al. (2013) interpreted the data to indicate that the northwestern succession lacks any input from the Alexander-Wrangellia-Peninsular terrane and that the two facies are time-correlative, at least in part. Some of the Kahiltna assemblage samples of Hults et al. (2013) and Bradley et al. (2009) have youngest statistical age peaks (maximum depositional age [MDA]) that are Late Cretaceous (as young as ca. 87 Ma and 99 Ma, respectively), whereas those of Hampton et al. (2010) and Kalbas et al. (2007) have youngest MDAs as young as earliest Cenomanian and late Aptian, respectively (ca. 98 and 114 Ma, respectively).
Previous Models for the Tectonic Setting of the Kahiltna Assemblage
In the 1970s, several lines of evidence were discovered to suggest that a large geologic domain from coastal British Columbia to south-central Alaska represented one or more distinct crustal blocks of oceanic plate origin (Wrangellia and Alexander terranes) that were emplaced against continental North America in the late Mesozoic (Jones et al., 1972, 1977; Richter and Jones, 1973; Csejtey, 1976). Highly deformed Jurassic–Cretaceous strata of the Kahiltna assemblage and Gravina-Nutzotin belt were interpreted by these authors to occur along this crustal boundary in southern Alaska. Working in southeastern Alaska, Berg et al. (1972) interpreted the Gravina-Nutzotin basin to have developed along or within a subduction-related volcanic arc overlying the eastern, inboard side of this crustal block (Alexander-Wrangellia-Peninsular terrane), with arc magmatism generated by east-dipping subduction beneath the west side of the block. Although their two-dimensional models suggested that Gravina-Nutzotin strata were deposited in a two-sided basin flanked on the east by North American continental rocks, in the text they suggested that “the initiation, filling and final collapse” (Berg et al., 1972, p. D22) of the basin might somehow have been tied to the displacement of the adjacent oceanic crustal block. Farther north in southern Alaska, the partly correlative Kahiltna assemblage was conceptualized as a “flysch-filled suture” that included rootless, fault-bounded upper-crustal “terranes” of disparate origin, caught up in a Cretaceous zone of convergence between the oceanic Wrangellia terrane and the North American continental Yukon composite terrane to the north (Jones et al., 1982 [p. 3716], 1986). However, the plate-tectonic setting of these deformed sedimentary assemblages and the ways in which their depositional and deformational evolution reflect that setting were not specifically addressed.
McClelland et al. (1992) proposed that the Alexander-Wrangellia-Peninsular terrane migrated northward after the Late Triassic and was emplaced by either oblique subduction or dextral strike-slip faulting against the North American Cordilleran margin in the Middle Jurassic (see also van der Heyden, 1992). Following initial accretion and renewed subduction along the western side of the accreted block, northward displacement of the accreted block continued under dextral transtension, resulting in the initiation of a two-sided, deep-marine transtensional back-arc basin (Gravina-Nutzotin belt and, by inference, the Kahiltna assemblage), with local arc magmatism along the southwestern margin of the basin. Finally, north- and east-directed underthrusting of the Alexander-Wrangellia-Peninsular terrane and the Kahiltna and Gravina-Nutzotin assemblages beneath the Yukon composite terrane resulted in contractional closure of the basin in mid-Cretaceous time.
Ridgway et al. (2002), Kalbas et al. (2007), and Hampton et al. (2007, 2010) refined the tectonic model for the Kahiltna assemblage based on stratigraphic, petrographic, and detrital zircon data. They interpreted the volcaniclastic flank of the basin to represent Late Jurassic–Early Cretaceous (Kimmeridgian–Valanginian) sedimentation into an oceanic basin against the back-arc flank of an intra-oceanic crustal block (Alexander-Wrangellia-Peninsular terrane). Subduction of the northern continuation of that oceanic basin beneath continental Alaska led to diachronous collisional emplacement of the back-arc flank of the Alexander-Wrangellia-Peninsular terrane, beginning in the Early Cretaceous in the eastern Alaska Range and propagating both to the southwest (Kalbas et al., 2007) and to the southeast (Haeussler, 1992). Syncollisional, southwest-flowing quartzose submarine fan deposits of Valanginian–Cenomanian age were interpreted to emanate from a growing collisional welt and to stratigraphically overlie the older volcaniclastic sequence. Kalbas et al. (2007, his figure 13) proposed that sediment was deposited in a scissoring wedge ahead of a propagating zone of convergence and lateral displacement during the final crustal impingement of the Alexander-Wrangellia-Peninsular terrane against North America. Based on the first appearance of major Proterozoic detrital zircons in the Chugach accretionary complex on the south side of the Alexander-Wrangellia-Peninsular, Amato et al. (2013) argued that the Alexander-Wrangellia-Peninsular terrane had accreted to continental North America between ca. 90 and 80 Ma.
Hults et al. (2013) presented detrital zircon data from the Kahiltna assemblage in the western Alaska Range and argued that the two contrasting belts represent partly coeval depositional sequences that have compositions derived entirely from one or the other side of the crustal boundary. They interpreted that the contrasting successions were deposited independently prior to collision and were structurally juxtaposed in the Late Cretaceous along a north-dipping subduction zone by a combination of convergence and strike-slip faulting. Yokelson et al. (2015) presented new detrital zircon age and Hf isotopic data for the Gravina belt in southeastern Alaska that show distinct sources for the volcaniclastic western and quartzose eastern Gravina belts, with no obvious mixing. They (see also Monger et al., 1994; Gehrels et al., 2009) suggested that the two belts were juxtaposed by up to 1000 km of Late Cretaceous sinistral offset.
APPROACH OF THIS STUDY
To better understand the development and deformation of the Kahiltna assemblage, we collected over 200 sandstone samples along a 120-km-long structural transect across the strike of the basinal successions in the western Alaska Range (Fig. 2). We collected and compiled structural data along the transect (Fig. 3) and developed deformational age constraints using dated crosscutting plutons. Sandstone petrographic data were analyzed in detail (Karl et al., 2016), and those data are summarized here. We selected a subset of 33 sandstone samples for detrital zircon analyses, and those data (along with previously published data from 10 other samples in the map area) are presented in Supplemental Items 2–41 and summarized herein. We integrated the structural, stratigraphic, compositional, and detrital zircon data to provide constraints on the provenance, tectonic setting, and stratigraphic and structural evolution of the Kahiltna assemblage. Finally, we developed a tectonic model for the evolution of the Kahiltna assemblage and compared it to a potential modern tectonic analogue in central Japan.
Detrital zircon U-Pb age determinations were done by Apatite to Zircon, Inc. (now GeoSep Services, Inc.), using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. The Data Repository contains a detailed description of analytical methods (Supplemental Item 1), complete data tables including U-Pb concordia diagrams for all samples (Supplemental Item 2), and a summary of calculated age populations for each sample (Supplemental Item 3). Raw, uninterpreted U-Pb data were also published as a U.S. Geological Survey (USGS) Data Release (Jones et al., 2019). We calculated concordia ages (including decay constant uncertainties) for each analysis and used this value as the preferred age of the associated detrital zircon grain, instead of using either the 206Pb/238U or 207Pb/206Pb age (Ludwig, 1998; Nemchin and Cawood, 2005). The concordia age makes optimum use of both decay schemes and obviates the need to choose an arbitrary age threshold for selecting the 206Pb/238U or 207Pb/206Pb age as the “preferred age” for an individual grain (Ludwig, 1998). Additionally, the concordia age calculation gives probability of concordance (POC) for each analysis, which provides a useful means of assessing concordance for all grains regardless of age. After calculating the concordia age and associated statistics for each analysis, we screened the data for uncertainty and POC. Analyses with greater than 10% age uncertainty (at 1σ) were excluded or “filtered” from plots and statistical treatments. Grains with a POC <0.1 were also excluded, unless the grain was older than 1000 Ma and had a calculated concordance (comparison of 206Pb/238U and 207Pb/206Pb ages) between 80% and 105%. Data that were excluded are reported in Supplemental Item 2 but are highlighted to indicate that they were not considered further. Where possible, we also calculated concordia ages for data from samples previously published by Kalbas et al. (2007), Bradley et al. (2009), and Hults et al. (2013) and screened these data using the same criteria described above to ensure consistency throughout the entire sample set considered herein. Age probability diagrams were generated using the kernel density estimation (KDE) method of Vermeesch (2012), and all KDE diagrams were generated using adaptive kernel density estimation, wherein the bandwidth varies depending on data density (Vermeesch, 2012). Histogram bins, where shown, generally represent ∼25 m.y. Age populations were identified using the AgePick macro for Microsoft Excel (Gehrels, 2009), and results for all samples are shown in Supplemental Item 2. The MDA for each sample was determined using the youngest age population made up of three or more grains with ages that overlapped within 1σ uncertainty and defined a distinct population with a mean square of weighted deviates (MSWD) of 2 or less. The MDA as determined from detrital zircon data for each sample is labeled with 2σ uncertainty and the number of grains contributing to the population in Supplemental Item 3. Other notable age populations determined using the AgePick macro are also tabulated in Table 1 and Supplemental Item 3 and discussed in the text below.
After screening all the data, we used the DZstats tool (version 2.2) of Saylor and Sundell (2016) to plot the cumulative distribution function (CDF) for different sample sets and to calculate a variety of comparative metrics, including Kolmogorov-Smirnoff (K-S) and Kuiper tests and cross-correlation, likeness, and similarity coefficients of probability density plots (PDPs) for all samples. The results of the statistical comparisons for all samples are shown in Supplemental Item 4, and selected results are discussed in the text. We also applied multidimensional scaling (MDS) using the R code of Vermeesch et al. (2016) as another means of assessing the similarity between our samples and other published data. MDS is a statistical technique that uses pairwise calculated dissimilarities between samples to produce a map of points on which more similar samples cluster closely together (Vermeesch, 2013). The MDS plots of the U-Pb data sets used the K-S effect size as a dissimilarity measure. The reported stress value is a measure of the “goodness of fit” per Vermeesch (2013). Again, more similar samples cluster together in the plots.
Titanite was analyzed by secondary ion mass spectrometry (SIMS) U-Pb geochronology at the USGS–Stanford University Ion Probe Laboratory. The analyses were made using a sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) instrument. Energy filtering of the secondary beam was used to minimize any possible molecular interferences. Concentration data for titanite were standardized against the BLR reference material relative to intensities for 90Zr216O+; U-Pb ages were calculated by calibration against titanite reference material BLR (1047 ± 0.4 Ma; Aleinikoff et al., 2007), which was analyzed repeatedly throughout the duration of the analytical sessions. Khan (522 ± 2 Ma; Heaman, 2009) titanite grains were analyzed from a separate standard mount as a secondary standard. Data reduction followed the methods described by Williams (1998) and Ireland and Williams (2003) and used the Squid 2 and Isoplot programs of Ken Ludwig (Ludwig, 2001, 2003). The mean age uncertainty is reported at the 2σ level and incorporates the 2σ external spot-to-spot uncertainty of the BLR reference material.
The Kahiltna assemblage strata can be divided into two successions that underlie the northwestern and southeastern parts of the basin, respectively, based on sandstone petrographic analysis (Karl et al., 2016; summarized in Fig. 4 here) and on detrital zircon geochronology (this paper). The northwestern and southeastern successions have distinct compositional and detrital zircon characteristics that are described in detail next. Furthermore, both successions can be further divided into multiple petrofacies units, which, themselves, have distinct lithologic and detrital zircon characteristics. The two successions and their respective petrofacies units are described in the following sections from northwest to southeast. We focused our data collection along a transect across the strike of the Kahiltna assemblage in the western Alaska Range (line A-B in Figs. 2 and 5). We present a northwest-southeast structural cross section in Figure 3. The cross-section line begins at the northwest end along the trace of the western Denali fault, an active right-lateral fault, and ends at the southeast end at the trace of the Castle Mountain fault, another active right-lateral fault (Haeussler et al., 2017).
Older Strata Northwest of the Kahiltna Assemblage
The northwestern area of the cross section (Figs. 2 and 3) exposes deformed strata of the Dillinger and Mystic subterranes of the Farewell terrane (Dumoulin et al., 2018a, 2018b). These thin-bedded carbonates and siliciclastic turbidite strata are cut by top-to-the-northwest thrust faults and have been deformed into northwest-vergent overturned folds (Bundtzen et al., 1997; see subsequent section on “Deformation of the Kahiltna assemblage”). Exposed Farewell strata range from Cambrian–Ordovician to Lower Jurassic, with scattered occurrences of Lower Cretaceous (Valanginian) strata (Reed and Nelson, 1980; Dumoulin et al., 2018a, 2018b). Along the southeastern flank of the exposed Farewell terrane, Lower Jurassic quartz-rich, deep-water clastic strata of the Mystic subterrane with diabasic intrusions (Dumoulin et al., 2018b) are overlain by slaty Kahiltna assemblage shale with rare graded pea-gravel conglomeratic turbidite beds. The nature of this contact is disputed. Bundtzen et al. (1997) interpreted the Kahiltna assemblage to sit on a northwest-vergent thrust fault over Jurassic rocks of the Mystic subterrane. Kalbas et al. (2007) interpreted the contact as a disconformity beneath Kahiltna strata. Our observations were ambiguous, although in one place (sample D13 in Fig. 2), Kahiltna strata within 10 m of the contact dip southeast away from the contact but are upside-down (based on graded bedding), suggesting the contact is at least locally structural.
Northwestern Succession of the Kahiltna Assemblage
The northwestern succession of the Kahiltna assemblage (Fig. 2) consists of interbedded turbiditic sandstones and shales. Sandstones are relatively quartz-rich rocks (>30% quartz), and fine flakes of detrital muscovite typically decorate parting surfaces in shales and slates. Three petrofacies units (Dalzell, Whistler, and Hayes) are distinguished based on the statistical age of the youngest detrital zircon age populations, as well as on small differences in the relative proportions of quartzose grains and lithic fragments (Fig. 4; Karl et al., 2016). The deformational geometry of the older, northwestern petrofacies unit (Dalzell) of the northwestern succession is distinct from that shared by all the other petrofacies units, which will be discussed in a subsequent section (Deformation of the Kahiltna Assemblage).
Dalzell Petrofacies Unit
The Dalzell petrofacies unit is exposed in a northeast-trending swath, ∼20 km wide, immediately south of the Farewell terrane (Fig. 5). A small area of Dalzell unit occurs well within the Farewell terrane exposure area (sample D14 on Fig. 2) in apparent depositional contact. The unit is named for exposures in Dalzell Gorge, an infamously difficult stretch of the crossing of the Alaska Range in the annual Iditarod dog race. Compositionally, the Dalzell petrofacies unit (Fig. 4) is characterized by abundant lithic fragments (45%), which are mostly sedimentary (including low-grade metasedimentary), and an average of 35% quartz grains, with a small but persistent component of white mica (Karl et al., 2016). The clastic composition indicates derivation from sources that include low-grade metasedimentary rocks, felsic plutonic rocks, and higher-grade micaceous schists.
The Dalzell unit was the subject of a detailed stratigraphic study in the area of the transect by Kalbas et al. (2007). They described a composite section over 5500 m thick composed predominantly of deep-water turbidite deposits, with lesser interbedded debris-flow pebbly conglomerates (Fig. 6A). Sparse bidirectional paleocurrent indicators suggest either northeast or southwest paleoflow. Lithofacies reconstruction of the depositional environment was interpreted as a base-of-slope prograding submarine fan system. The presence of a large slump block of Farewell stratigraphy interbedded in the lower, argillaceous part of the section suggests proximity to a steep slope, presumably near the presently exposed Farewell terrane immediately to the northwest.
Fourteen Dalzell unit samples were analyzed for detrital zircon geochronology (samples D1–D14; Fig. 2), including two (D11, D1) previously published (Kalbas et al., 2007; Hults et al., 2013). The detrital zircon age populations in the Dalzell samples (Figs. 7 and 8; Table 1) consist of more than 70% Phanerozoic grains with three age populations: a strong Early Jurassic age probability peak between 200 and 180 Ma (median value of 27% of grain population) and smaller peaks at ca. 350 Ma and 105 Ma. Precambrian grain ages range continuously from 540 to 2700 Ma, with small (3%–5%) populations with median values of ca. 600, 1040, and 1750 Ma, and a smaller late Archean age population ca. 2570 Ma. All MDAs for Dalzell unit samples are older than ca. 100 Ma (Table 1), with MDA values mostly ca. 107–103 Ma (late Early Cretaceous: Albian). A few samples lack the Albian age population but have older Cretaceous MDAs that are either Aptian (117–114 Ma: samples D11, D12) or Barremian (ca. 131 Ma: sample D5) age. Three samples (D9, D10, and D13) lack an Early Cretaceous age population (but do have a few Early Cretaceous grains) and have Early Jurassic MDAs (ca. 194–179 Ma). No geographic pattern to the MDA distribution within the Dalzell unit is evident (Fig. 5). The presence of fossiliferous carbonate beds of Valanginian age in the underlying Farewell terrane (Dumoulin et al., 2018b) suggests that the Dalzell unit is entirely younger than Valanginian (134–139 Ma). Based on the MDAs, we interpret the Dalzell unit to be Lower Cretaceous, and mostly upper Lower Cretaceous (Albian).
The observed spreads of individual sample KDE curves on the CDF (Fig. 8B) and sample locations in the MDS (Fig. 8C) illustrate the variations in age populations and distributions among Dalzell petrofacies unit samples. Five samples (D5, D6, D7, D8, D12) have small to moderate Late Jurassic age probability peaks between ca. 170 and 145 Ma, and small Early Cretaceous peaks between 135 and 111 Ma. These samples have the widest spread of locations in MDS space (Fig. 8C). Several samples (D2, D3, D6, D9, D10, D13) have Devonian age populations (ca. 396–361 Ma), and a few samples (D1, D3, D13) have small Silurian peaks (ca. 456–418 Ma). Four samples (D5–D8) statistically differ from all other Dalzell samples (Fig. 8C), but all but D5 have some affinity to each other and to some Whistler samples (Fig. 9C; Supplemental Item 4). The KDE plots of these four samples (Figs. 7 and 8) show that they share small populations of Precambrian grains (∼10% vs. the mean of 35% for the other Dalzell samples) and significant Late Jurassic and early Early Cretaceous age populations (170–120 Ma) not found in other Dalzell samples.
Whistler Petrofacies Unit
The Whistler petrofacies unit of the northwestern succession, named for a well-known porphyry Cu-Au prospect and prominent air strip at the junction of Happy and Skwentna River valleys (Hames, 2014), occurs in a 30-km-wide, northeast-trending belt to the southeast of the Dalzell petrofacies unit (Fig. 5). The unit consists entirely of medium- to fine-grained quartzose turbiditic sandstone-shale sequences lacking in coarser detritus. The Whistler petrofacies unit is superficially similar to the Dalzell unit in composition (Fig. 4) in its conspicuous quartz and mica detritus but is characterized by lower content of lithic fragments (average-35%) and higher quartz content (average 40%; Karl et al., 2016).
Eleven sandstone samples of the Whistler petrofacies unit were analyzed for detrital zircon geochronology (samples W1–W11, Fig. 2), including one (W7) previously published by Hults et al. (2013). Summaries of the detrital zircon results are shown in Figures 7 and 9 and in Table 1. Detrital zircon populations in these samples consist of more than 85% Phanerozoic grains with three prominent age populations: a strong Early Jurassic age probability peak between 200 and 180 Ma (median value of 35% of grain population), an intermediate Late Cretaceous peak ranging from ca. 99 to 84 Ma (equivalent to MDA), and a smaller peak around ca. 350 Ma. The Precambrian grain population is generally similar to but smaller than that of the Dalzell unit. We interpret the Late Cretaceous MDAs of the Whistler unit samples (Table 1) to be roughly equivalent to their true depositional ages, from Cenomanian to Santonian, reflecting active magmatism in the source region.
The MDS diagram in Figure 9C shows considerable spatial overlap between most Whistler and Dalzell samples, suggesting broadly similar provenance. There are some variations between individual Whistler samples (Figs. 9A and 9B) that have consistent geographical trends. The three samples with the oldest MDAs (ca. 99–95 Ma; Fig. 5) are the most northwesterly Whistler samples in the transect (W9–W11), and they are also the only Whistler samples with small Devonian and/or Silurian peaks. These three samples plot across the MDS diagram together with the entire spread of Dalzell samples, consistent with their location closest to the inferred top of the Dalzell succession. Small Late Jurassic (ca. 168–147 Ma) age populations are present in half the Whistler samples (Table 1), and those samples generally also have late Early Cretaceous (ca. 121–110 Ma) age populations. Sample W11 shows poor correlation with other Whistler samples (Supplemental Item 4) and plots closest to Dalzell samples in Figure 9C. Sample W6 plots closer to the single Hayes unit sample than to other Whistler samples.
Hayes Petrofacies Unit
The Hayes petrofacies unit of the northwestern succession occurs locally in small areas south of the Whistler petrofacies unit, surrounded by and overlying strata of the Old Man petrofacies unit of the southeastern succession, typically in cores of structural synclines (Figs. 3 and 5). Two of these small occurrences are found just northwest of the Hayes River, from which the unit took its name; the third location is along the northwestern boundary of the Old Man unit near its contact with the Whistler unit. In each location, the Hayes petrofacies strata appear to conformably overlie the Old Man or Spring petrofacies strata and to be similarly deformed, although the contact relationships were not observed in detail. At the single detrital zircon sample site (sample H, Fig. 2), 10-m-thick amalgamated turbiditic sandstone beds have abrupt bases on underlying shales with mud rip-up clasts near the base and gradational tops with the overlying shales. Compositionally, the Hayes petrofacies unit is similar to the Whistler petrofacies unit (Fig. 4) but even more quartz-rich (average 50%) and has a lower average abundance of lithic fragments (25%), with a small but persistent component of white mica (Karl et al., 2016).
A single sample of the Hayes petrofacies unit was analyzed for detrital zircon geochronology (sample H, Fig. 2). Although it is very similar to samples of the Whistler petrofacies unit (Figs. 7 and 9) in its Late Cretaceous (35%), Late Triassic–Early Jurassic (35%), and Carboniferous (8%) age populations (Table 1), its youngest age peak (MDA) is distinctly younger than those of the Whistler unit at ca. 81 Ma (early Campanian). It lacks age populations between 180 and 109 Ma and has only a single age population older than Carboniferous: Middle Paleoproterozoic at 1933 Ma.
Southeastern Succession of the Kahiltna Assemblage
The southeastern succession of the Kahiltna assemblage also consists predominantly of interbedded turbiditic sandstone and shale, although it is locally shallow marine in character. However, in contrast to the northwestern succession, sandstones of the southeastern succession are low in quartz, lack muscovite, and are characterized by abundant volcanic lithic fragments and feldspar grains (Fig. 4). Three petrofacies units (Old Man, Spring, and Trimble) are distinguished based on the statistical ages of the youngest detrital zircon peaks, as well as on small compositional differences. The deformational geometry of the southeastern succession will be discussed in a subsequent section (Deformation of the Kahiltna Assemblage).
Old Man Petrofacies Unit
Southeast of and in abrupt contact with the Whistler unit, the Old Man petrofacies unit of the southeastern succession is named for exposures on ridges flanking Old Man Creek, a southern tributary drainage to the lower Skwentna River. It forms an outcrop belt ∼25 km wide near the structural transect, apparently narrowing to less than 10 km at the southwestern edge of the map area (Fig. 5). Sandstone compositions (Fig. 4) are characterized by volcanic detritus, with an average of 50% volcanic lithic clasts, nearly 50% plagioclase, and generally less than 5% monocrystalline quartz and a small but persistent component of clinopyroxene crystals (Karl et al., 2016). Detrital muscovite, ubiquitous in sandstones and slates for the northwestern succession, is entirely lacking in sandstones and in interleaved shales of the southeastern succession. Based on our observations, the Old man petrofacies includes both shallow- and deep-marine deposits.
The Old Man unit was the subject of a 1:63,360 mapping study by Solie et al. (1993) and an associated 40Ar/39Ar dating study (Layer and Solie, 2008). Strata of the Old Man unit consist predominantly of interbedded dark-gray shale, siltstone, and dark-green feldspatholithic sandstone. Solie et al. (1993) identified an upper unit of sandstone and granule to boulder conglomerate underlain and overlain by predominantly shale units. Conglomerates are clast-supported features (Fig. 6C); clasts are rounded and include mafic volcanic and intermediate to felsic intrusive rocks, as well as limestone, chert, quartzite, and shale. Sandstones interbedded with the conglomerate are generally graded but locally have meter-scale cross-bedding. Repeated 5–15-m-thick thinning- and fining-upward cycles in channelized bases and apparent westward fining are tentatively interpreted to record westward-prograding, shallow-marine channels. Three porphyritic andesite clasts in the conglomerate yielded 40Ar/39Ar plateau or weighted mean hornblende ages from 94.8 ± 1.4 Ma to 101.0 ± 2.0 Ma, giving a maximum depositional age for the unit (Layer and Solie, 2008). A single Turonian fossil locality was reported on the Skwentna River west of sample S1 on Figure 2 (W. Elder, 1989, written commun.).
Six samples of the Old Man petrofacies unit were analyzed for detrital zircon geochronology (OM1–OM6; Figs. 2, 7, and 10), including one previously published (OM3; Bradley et al., 2009). OM5 is from a sandstone interbedded with the conglomerate described by Solie et al. (1993). The samples are characterized by a prominent Late Cretaceous age probability peak that is either the only age peak or is accompanied by a broad ca. 160 Ma age probability peak (Fig. 10A; Table 1). All MDAs are Late Cretaceous (ca. 99–89 Ma; Cenomanian–Turonian). Based on the presence of interbedded air-fall tuffs in the Old Man section (dated at ca. 97 and 93 Ma; Karl et al., 2016), we interpret the similar MDAs to reflect a Cenomanian–Turonian depositional age, because of contemporaneous volcanism in the source area. The Old Man samples generally cluster in the CDF and MDS diagrams (Figs. 10B and 10C), and the spread is attributed to the presence or absence and relative abundance of the Jurassic age population.
Spring Petrofacies Unit
The Spring petrofacies unit, named for occurrences near Spring Creek, a tributary to the Hayes River, occurs sporadically within the Old Man unit outcrop area (Fig. 5) but has not been identified within the area of the Trimble petrofacies unit (Karl et al., 2016). The unit was not originally recognized in the field but was distinguished later by its sandstone petrography and its detrital zircon signature. No fossils have been identified from the unit. The Spring unit is characterized by volcaniclastic sandstones and slates (Fig. 4), some with turbidite features, but others with 30-cm-scale cross-beds. Sandstone contains up to 20% monocrystalline quartz and 40% plagioclase and is distinguished from other petrofacies of the southeastern succession by prominent (up to 11%) detrital potassium feldspar.
Three samples of the Spring petrofacies unit were analyzed for detrital zircon geochronology (S1–S3; Figs. 2, 7, and 10; Table 1), including one previously published sample (S2; Bradley et al., 2009). The Spring petrofacies samples are characterized by broad Late Jurassic–Early Cretaceous detrital zircon age population peaks, ranging from ca. 171 to 142 Ma (Table 1). The MDAs range from ca. 146 to 103 Ma and are intermediate between Old Man and Trimble MDAs. Two samples (S1, S3) have Early Jurassic age populations (ca. 197–180 Ma). Two samples (S2, S3) have up to 20% Precambrian grains with small but statistically significant age populations at ca. 940, 1390, 1610, 1692, and 2721 Ma (Table 1; Supplemental Item 3). Sample S3 contains more than 5% Paleozoic grains that define a distinct Silurian age population. The other Spring sample (S1) has a late Early Cretaceous MDA (late Albian: ca. 103 Ma) that is only slightly older than the oldest Old Man MDA (ca. 99 Ma). On the MDS diagram in Figure 10C, the Spring samples show spread among themselves and are distinctly separated from all but one of the Old Man samples. The Spring samples plot much closer to Trimble samples in Figure 11C. We interpret the spread to suggest that the Spring unit is transitional between the Old Man and Trimble petrofacies units.
Trimble Petrofacies Unit
The Trimble petrofacies unit of the southeastern succession is the southeasternmost unit of the Kahiltna assemblage (Figs. 3 and 5), forming an outcrop belt up to 75 km wide. The unit is named for exposures flanking the broad lower valley of the Trimble River. The Trimble unit sandstones are composed primarily of volcanic detritus, with ∼10% monocrystalline quartz, 50% feldspar, and 40% volcanic and sedimentary lithic fragments (Fig. 4; Karl et al., 2016). Only turbidite- and debris-flow facies sedimentary structures have been observed, including channelized sandstones (Fig. 6D).
Eight samples of the Trimble petrofacies unit were analyzed for detrital zircon geochronology (T1–T8; Figs. 2, 7, and 11; Table 1), including four previously published samples (T2, T3, T5 from Bradley et al., 2009; T8 from Hults et al., 2013). The Trimble petrofacies unit is characterized by a single broad Late Jurassic age probability peak at ca. 160 Ma, but individual MDAs range from ca. 168 to 147 Ma (Tithonian–Oxfordian; Table 1). Seven of the 10 Trimble samples also have Early Jurassic (ca. 196–171 Ma) age populations, and two have a small Late Triassic (ca. 215–211 Ma) age population. Trimble samples generally have fewer than 1% Precambrian grains (median value), although two samples have 4% Precambrian grains. The Trimble unit samples are distinct from all but the Spring unit samples in the MDS diagram, and most Trimble samples cluster together (Fig. 11C). One sample (T6) plots separately from the others because of its strongly unimodal character and distinct MDA relative to other unimodal samples, but it does have strong similarity with one of the Spring unit samples (S1; Fig. 11C). However, the sample also had the lowest detrital zircon yield of all samples in the study, which could have influenced the overall statistical comparison.
Deformation of the Kahiltna Assemblage
Kahiltna strata are tightly folded (Figs. 12B–12D), typically with axial planar slaty cleavage. Most outcrops show abundant faults with typically uncertain offset, and a shear fabric was locally observed (Fig. 12E). A structural cross section across the strike of the Kahiltna assemblage in the western Alaska Range (line A–B in Figs. 2 and 5) is illustrated in Figure 3. Map locations of the photos of deformation shown in Figure 12 are given in Figure 5.
Deformed strata of the Dillinger and Mystic subterranes of the Farewell terrane (Dumoulin et al., 2018a, 2018b) underlie the northwestern area of the cross section (Figs. 2 and 3). These thin-bedded carbonates and siliciclastic turbidite strata are cut by top-to-the-northwest thrust faults and have been deformed into overturned folds verging to the northwest (Bundtzen et al., 1997; see also Figs. 12A and 13B herein) with an associated axial planar slaty cleavage (Fig. 13C). The disputed nature of the contact of the Dalzell petrofacies unit over the Farewell terrane was discussed above. However, the Dalzell petrofacies unit is deformed similarly to the adjacent Farewell terrane, by tight, northwest-verging, overturned folds (Fig. 13A) with gently southeast-dipping axial planar slaty cleavage (Fig. 13C). Although locally the Farewell terrane preserves evidence of an earlier deformation phase (e.g., Bundtzen et al., 1997), the Farewell terrane and the Dalzell unit share a deformation fabric for ∼20 km on either side of their contact. This deformation phase postdated the youngest MDA of the Dalzell unit (103 Ma) and predated the oldest crosscutting pluton (65.1 ± 0.4 Ma Hartman pluton; Gamble et al., 2013).
We recovered metamorphic titanite from a mafic dike in the Farewell terrane just west of and beneath the Dalzell petrofacies unit (Fig. 5). The titanite is associated with calcite bands in banded calcite-chlorite veins, some with an internal shear fabric (photomicrograph in Supplemental File S5). The veins cut the unfoliated plagioclase-pyroxene microgabbro dike with pseudomorphic pyroxene replaced by fine calcite-epidote-chlorite-quartz aggregates. Based on the chemistry of the dike (reported in Dumoulin et al., 2018b), it appears to be a feeder to Late Triassic–Early Jurassic lavas with rift-type chemistry, locally exposed within the Farewell terrane. Titanite from this sample yielded a SHRIMP U-Pb age of 96.8 ± 2.3 Ma (Supplemental File S5). However, the titanite age is younger than the youngest MDA of the depositionally overlying Dalzell unit of the Kahiltna assemblage. We interpret the titanite to have grown during repeated veining associated with northwest-vergent thrusting and folding that affected the Dalzell unit and the underlying Farewell terrane on the northwest flank of the Kahiltna assemblage.
In contrast to the Dalzell petrofacies unit, the Whistler petrofacies unit is characterized by tight, south-southeast–vergent overturned folds (Figs. 12B–12D and 13D) with moderately north-northwest–dipping, axial planar slaty cleavage (Fig. 13E). Bedding-parallel shearing is apparent within some folds (Figs. 12B and 12C). As the compositional distinction between the Dalzell and Whistler petrofacies units is not sharp, and the nature of the change from northwest- to southeast-vergent folding is poorly understood and imprecisely located, the nature and character of the boundary between the two petrofacies units are poorly understood. Deformation of the Whistler petrofacies unit predates the oldest crosscutting plutonic body (Whistler pluton: 76.1 ± 0.1 Ma; Hames, 2014) and postdates the youngest Whistler MDA (ca. 87 Ma near the pluton, ca. 84 Ma elsewhere). This deformation is constrained to the interval 87–76 Ma, i.e., distinctly younger than that affecting the Dalzell petrofacies unit (97 ± 2 Ma), as interpreted from its underlying metamorphic titanite age.
The Old Man and Spring petrofacies units, like the Whistler unit, are also deformed into south-southeast–vergent overturned folds (Fig. 13F) with moderately northwest-dipping axial planar cleavage (Fig. 13G). The similarity of deformation of the strata of the Old Man/Spring petrofacies units to that of the Whistler strata (Figs. 13D–13G) suggests that the three units were deformed together or during the same progressive deformational event. Stratal dismemberment by a scaly anastomosing fabric (Fig. 12E) is locally observed. Deformation must have postdated deposition of the Old Man unit, which has Turonian marine fossils (ca. 93.9–89.8 Ma; Walker et al., 2018) and MDAs from 99 to 89 Ma (Table 1). Felsic tuff layers locally interlayered with Old Man petrofacies strata yielded U-Pb zircon ages of ca. 97 Ma and 93 Ma, or Cenomanian to Turonian (Karl et al., 2016). The unit is intruded by a syn- to postdeformational granodioritic pluton that yielded a zircon U-Pb age of ca. 80 Ma (Fig. 5; Jones et al., 2013, 2014; Wilson et al., 2015). Thus, deformation is constrained to the interval ca. 89–80 Ma.
The Hayes petrofacies unit, with petrographic and detrital zircon character of the northwestern succession, appears to depositionally overlie Old Man strata of the southeastern succession in three areas (Figs. 3 and 5). Hayes strata are also deformed with a slaty cleavage, and bedding attitudes cluster with those of the underlying Old Man strata (Fig. 13F). Like the underlying Old Man unit, we interpret the deformation of the Hayes unit to have occurred prior to intrusion by the crosscutting pluton with a U-Pb zircon age of ca. 80 Ma (Fig. 4; Jones et al., 2013, 2014; Wilson et al., 2015). Our sample of the Hayes unit (25 km away from this pluton) has an MDA of ca. 81 Ma (Table 1), so significant deformation must have occurred locally between ca. 81 and 80 Ma (Fig. 5). This very tight age constraint on overlap of northwestern petrofacies strata over strata of the southeastern succession, followed quickly by deformation, is an important constraint on the evolution of the Kahiltna assemblage.
The farthest southeast Kahiltna unit (Trimble petrofacies unit) is strongly deformed by tight west-northwest–trending folds with no clear vergence (Fig. 13H), and with steep west-northwest-striking axial planar slaty cleavage (Fig. 13I). The Trimble unit is separated from the Old Man petrofacies unit to the northwest by the linear Hayes River valley, and the contact between the units there has been interpreted to be a normal fault with northwest-side-down displacement (Solie and Layer, 1993). Deformation of the unit must have postdated the age of the strata, the youngest MDA of which is ca. 147 Ma (Table 1). The unit is crosscut by a postdeformational pluton that has yielded a zircon U-Pb age of 79.5 ± 1.1 Ma (Fig. 5; Jones et al., 2013, 2014). Thus, deformation of the Trimble unit is constrained to the interval 147–80 Ma. The broad timing constraints allow the possibility that it shares the deformation of the Whistler and Old Man units, but the evidence is less compelling.
Regional Correlation of Kahiltna Assemblage Successions
Based on sandstone grain petrography and on detrital zircon age populations, the Kahiltna assemblage strata can be clearly separated into two populations: a northwestern succession with quartzose compositions and southeastern succession with volcaniclastic compositions (Figs. 2 and 4). Based on the youngest age populations, those two successions can be further divided (Fig. 5) into three petrofacies units each (northwestern: Dalzell, Whistler, and Hayes; southeastern: Trimble, Spring, and Old Man). Based on the progression in the MDAs within each of the compositional groups (Table 1), we suggest that the units have stratigraphic significance within each succession.
The northwestern succession has been mapped as Kuskokwim Group at the southwestern edge of Figure 2 (Wallace et al., 1989; Wilson et al., 2015), but quartzose strata farther northeast were mapped differently as part of the Kahiltna assemblage (Ridgway et al., 2002; Hampton et al., 2007, 2010; Kalbas et al., 2007; Hults et al., 2013). However, samples from the mapped Kuskokwim Group (samples D1, D3, and D4 in Figs. 2 and 10) are indistinguishable in petrography (Karl et al., 2016) and detrital zircon age populations from those of the northwestern succession farther northeast (sample D2 and D5–D14). We propose that all strata with similar composition (characterized by their monocrystalline quartz and white mica contents; Karl et al., 2016) and detrital zircon age populations farther northeast also be considered part of the Kuskokwim Group. The Kuskokwim Group thus encompasses samples H, W1–W11, and D1–D14 in Figures 2 and 7, and in Table 1. These samples show a strong detrital zircon age probability peak at ca. 200 Ma, a small but persistent peak at ca. 350 Ma, and more variable Cretaceous peaks ranging from ca. 130 to 80 Ma. Precambrian grains are an important component to all the samples, ranging from just over 50% down to ∼8%.
The three petrofacies units of the Kuskokwim Group (Figs. 14A and 14B) differ primarily in the ages of their youngest detrital zircon age population peaks or MDAs (Fig. 5). The Dalzell petrofacies unit in the north has MDAs ranging from 194 to 103 Ma, with no discernible geographic trend. All but three of the samples have Early Cretaceous MDAs, and we infer that most or all of the unit is Lower Cretaceous (primarily Albian). The Whistler petrofacies unit records MDAs from 99 to 84 Ma (early Late Cretaceous: Cenomanian to Santonian), and samples are generally arrayed with the oldest MDAs to the northwest and the youngest MDAs close to the southeastern edge of the unit. The Hayes petrofacies unit, which depositionally overlies the Old Man unit with MDAs as young as ca. 89 Ma, yielded a Campanian MDA of ca. 81 Ma, the youngest MDA yet found in the Kahiltna assemblage. We infer that the MDAs in the Whistler and Hayes petrofacies approximate their depositional ages, resulting from active volcanism in their source regions. Depositional ages in the Kuskokwim Group strata within the Kahiltna assemblage broadly show a trend that gets younger from northwest to southeast.
The strata of the southeastern succession, with their volcaniclastic compositions (Trimble, Spring, and Old Man petrofacies units), occur mostly in the Skwentna River drainage, and we informally refer to them as the Skwentna River sequence (Figs. 14A and 14B). These strata occur in the same general area as rocks first referred to as the Skwentna Series (Spurr, 1900) and later the Skwentna Group (Brooks, 1911; Martin, 1926). However, these authors incorrectly identified the lithologies as volcanic rocks (rather than clastic rocks composed of volcanic detritus) and incorrectly correlated them with the volcanic Lower Jurassic Talkeetna Formation to the south and east, so we do not continue the usage of “Series” or “Group” but retain the geographic name. These strata trend southwest along strike into the Koksetna River sequence (Wallace et al., 1989; Gamble et al., 2013; Hults et al., 2013), and to the northeast toward occurrences of similar strata in the northern Talkeetna (Eastham and Ridgway, 2002) and Clearwater Mountains (Smith, 1981; Hults et al., 2013). These are shown on the Alaska State geologic map as Upper Jurassic and Lower Cretaceous Gravina-Nutzotin unit (Wilson et al., 2015), yet the long distance to those basins (Fig. 1) makes us hesitant to use that terminology here. Because of the prior usage by Wallace et al. (1989) and Smith (1981), we refer to this belt of volcaniclastic strata in the Kahiltna assemblage as the Koksetna-Clearwater belt. We propose that all strata with similar composition (characterized by low quartz content and abundant plagioclase and volcanic rock fragments) and similar detrital zircon age populations to the southwest and northeast of the Skwentna River sequence (“southern flysch belt” of Hults et al., 2013) be considered part of the Koksetna-Clearwater belt of the Kahiltna assemblage.
The Koksetna-Clearwater belt is represented by samples T1–T8, S1–S3, and OM1–OM6 in Figures 2 and 7. Samples are composed predominantly of Mesozoic grains, with a prominent ca. 165–140 Ma age probability peak, and only a few samples have more than a trace of Precambrian-age grains (Table 1). In the western Alaska Range, there are three petrofacies units of the Koksetna-Clearwater belt, which differ primarily in the age of the youngest detrital zircon peak or MDA (Fig. 5). The Trimble unit has Middle Jurassic (Callovian) to Early Cretaceous (Berriasian) MDAs ranging from ca. 168 to 147 Ma. Kimmeridgian (middle Late Jurassic) and Valanginian (early Early Cretaceous) fossils are documented in the Koksetna River sequence to the southwest (Wallace et al., 1989). The Spring unit samples have MDAs ranging from ca. 146 to 103 Ma, so it must at least locally be as young as late Albian in age. The Old Man unit has a single Turonian fossil, MDAs ranging from ca. 99 to 89 Ma, and tuffs dated at ca. 93 and 97 Ma (Karl et al., 2016), indicating deposition during the Cenomanian and Turonian Stages of the Late Cretaceous. Based on MDAs, significant age overlap occurs between the adjacent but compositionally contrasting Old Man and Whistler petrofacies units.
Detrital Sources of the Petrofacies Units of the Northwestern Succession (Kuskokwim Group, Kahiltna Assemblage)
The abundance of sedimentary lithic fragments (including slate), monocrystalline quartz, and muscovite grains in the Kuskokwim Group samples indicates derivation from clastic sources that included low-grade metasedimentary rocks, felsic plutonic rocks, and higher-grade micaceous schists. Since the Dalzell unit, at least locally, sits depositionally on the low-grade metasedimentary strata of the Farewell terrane (e.g., sample D14; Fig. 2), we first looked to the Farewell terrane as a possible sediment source. Figure 14A shows composite detrital zircon probability plots for the Kahiltna assemblage petrofacies units in comparison to probability plots for sandstones of the Farewell terrane (divided into pre– and post–middle Silurian strata), as well as several other northwest North American sources. The Farewell terrane is distinguished from other northwest North American sources by its prominent 700–600 Ma detrital zircon population peak (Dumoulin et al., 2018a, 2018b), particularly in pre–middle Silurian strata. This potentially is the source of the 700–600 Ma age peak in Dalzell sandstones, and its lesser prominence in Whistler sandstones. Middle Silurian and younger strata of the Farewell terrane have smaller Precambrian detrital zircon populations with broad continuous distributions from 2000 to 600 Ma, but also a prominent Silurian (ca. 430 Ma) age population (typically 15%–30%). However, a minor (3%) Silurian population is present in only five of the 26 Dalzell-Whistler-Hayes sandstones (Fig. 7). The infrequency of a Silurian peak in the Cretaceous Dalzell-Whistler-Hayes sandstones indicates that middle Silurian and younger strata of the Farewell terrane had limited contribution to the zircon populations of the Cretaceous sandstones. The subtle change in Paleozoic and older detrital zircon populations between Dalzell and Whistler units (e.g., loss of a minor Silurian peak, general reduction in Precambrian grains with only the 1900–1700 Ma peak remaining) could reflect decreasing erosional contributions from Farewell strata with time. The other 2750–1000 Ma grain populations have many possible sources in northwestern North America (Gehrels and Pecha, 2014; Lane and Gehrels, 2014; Dusel-Bacon et al., 2017).
The three prominent Phanerozoic detrital zircon age components in the Kuskokwim Group samples match well with igneous age ranges from granitoid rocks in the Yukon composite terrane of east-central Alaska and the adjacent Yukon province (Interior AK+YK in Fig. 14A) and farther south in southeastern Alaska and the Canadian province of British Columbia (BC+SE AK in Fig. 14A), although the relative peak abundances differ between these sources and the Kuskokwim samples. The prominent Early Jurassic peaks (ca. 199–177 Ma) in Dalzell-Whistler-Hayes sandstones correlate temporally with the age of widespread granodioritic plutonic rocks cutting the Fortymile River assemblage, the amphibolite-facies, upper structural plate of the Yukon composite terrane (Dusel-Bacon et al., 2015). The Carboniferous–Late Devonian peak (ca. 376–316 Ma) in the Kuskokwim Group samples correlates approximately with the igneous age range from augen gneisses in the Yukon composite terrane (Dusel-Bacon and Williams, 2009). Cretaceous peaks in the Kuskokwim Group samples (ca. 121–84 Ma) also match with the approximate age range of widespread Mesozoic granites in the Yukon composite terrane (Aleinikoff et al., 2000; Dusel-Bacon et al., 2015). In the Talkeetna Mountains to the northeast, Hampton et al. (2017) reported negative epsilon Hf isotopic values from late Early Cretaceous detrital zircons in quartzose Upper Cretaceous nonmarine strata overlying deep-marine Kahiltna-Kuskokwim strata, indicative of a component of Proterozoic or older continental crust in the sources of these Cretaceous magmatic sources. The Cretaceous granitoids in the Yukon composite terrane (Aleinikoff et al., 2000) have negative epsilon Nd isotopic values that are expected to correlate with negative epsilon Hf isotopic values (Vervoort et al., 1999). Abundant coarse micaceous schists in the Yukon composite terrane are a suitable source of the ubiquitous muscovite flakes in Dalzell-Whistler-Hayes sandstones.
Given the relative abundance of Phanerozoic grains in the northwestern Kahiltna assemblage strata (Figs. 8B and 9B), we estimate that igneous rocks of the Yukon composite terrane contributed more than 50% of the zircon populations to the Dalzell-Whistler sandstones. As mentioned above, the contribution from the Farewell terrane decreased through time between ca. 105 and 85 Ma during the progression from Dalzell to Whistler deposition, while the contribution from the Yukon composite terrane detritus increased.
Detrital Sources of the Petrofacies Units of the Southeastern Succession (Skwentna River Sequence of the Koksetna-Clearwater Belt, Kahiltna Assemblage)
The Trimble unit is characterized by a single broad Late Jurassic (Tithonian–Oxfordian) detrital zircon age probability peak ranging from ca. 168 to 147 Ma, with a lesser Early Jurassic peak between ca. 196 and 171 Ma and an even rarer Late Triassic peak at ca. 215 Ma (Fig. 7; Table 1). Trimble samples generally have 0%–5% Precambrian grains, and no samples have a statistically significant Precambrian age population. The Late Jurassic age populations are correlative with volcanic and plutonic rock ages within the “Chitina arc” sequence of the Alexander-Wrangellia-Peninsular terrane from the Wrangell Mountains down into southeastern Alaska (Plafker et al., 1989; Trop and Ridgway, 2007; Beranek et al., 2017; Lowey, 2019). The Early Jurassic ages are well represented in both intermediate volcanic and felsic plutonic rocks within the Alexander-Wrangellia-Peninsular terrane (“Alexander-Wrangellia-Peninsular igneous” in Fig. 14B; Reed and Lanphere, 1973; Rioux et al., 2007; Wilson et al., 2015). Late Jurassic uplift of the Peninsular terrane is recorded along the southeast-vergent Bruin Bay fault by syntectonic sedimentary rocks in the associated Upper Jurassic Naknek Formation (Trop et al., 2005), as well as by Late Jurassic 40Ar/39Ar cooling ages for hornblende in Early Jurassic plutonic rocks (Hacker et al., 2011). Late Jurassic tectonism is interpreted by some as recording a collisional event between the combined Alexander-Wrangellia terranes and the Peninsular terrane (Clift et al., 2005; Rioux et al., 2007; Amato et al., 2013; Moore and Box, 2016) to form the combined Alexander-Wrangellia-Peninsular terrane. However, Beranek et al. (2014) argued that the Alexander-Wrangellia-Peninsular terrane was amalgamated earlier in the Permian.
Spring unit samples are similar to Trimble samples, but each of the three samples has something that sets them apart from the Trimble samples. They each have a prominent Late Jurassic age probability peak and a subordinate Early Jurassic peak (although the age populations in sample S2 are slightly younger), and one has a Late Triassic peak as well. S1 is distinguished from Trimble samples by its prominent late Early Cretaceous age populations (ca. 109 and 103 Ma). Also, two samples (S2, S3) have a significant (20%) component of Precambrian grains (with small but distinct age populations at 940, 1390, 1610, 1692, and 2721 Ma; Table 1), and one of those has a peak on the Silurian-Devonian boundary. These age populations are similar to those contained in clastic rocks within the Tlikakila complex, a pre-Jurassic basement complex within the Alexander-Wrangellia-Peninsular terrane (Fig. 14B; Amato et al., 2007a). The lack of a significant early Proterozoic (e.g., 2000–1700 Ma) detrital zircon population in the Spring samples (Fig. 14B; Table 1), a feature shared with potential Alexander-Wrangellia-Peninsular terrane sources, contrasts with its prominence in both North American and Farewell terrane sources (Fig. 14A) and supports the interpretation of an Alexander-Wrangellia-Peninsular terrane source.
The Old Man unit is characterized by two detrital zircon age probability peaks: ca. 99–89 Ma (Late Cretaceous: Cenomanian–Turonian) and ca. 172–139 Ma (Middle Jurassic–Early Cretaceous: Aalenian–Berriasian stages). Plutonic rocks in both age ranges are documented in the adjacent Alexander-Wrangellia-Peninsular terrane (Plafker et al., 1989; Hart et al., 2004, 2010; Olson et al., 2017). Volcanic rocks within the older age range are known in the Alexander-Wrangellia-Peninsular terrane (Lowey, 2007), but none has been identified yet in the younger age range except for recently identified ca. 93 Ma air-fall tuffs within the Old Man unit (Karl et al., 2016). We infer that Late Cretaceous detrital zircons from the Old Man units in the study area were supplied to the deep-sea sedimentary unit by fluvial dispersion of similar tuffs that must have blanketed the subaerial Alexander-Wrangellia-Peninsular terrane. One of the Old Man samples (OM3) has more than 10% early Paleozoic and Proterozoic detrital zircons with a ca. 1694 Ma age population. This sample lacks a 2000–1800 Ma population peak characteristic of North American sources (Fig. 14A); however, the predominance of middle Proterozoic over early Proterozoic population peaks is characteristic of the Tlikakila basement complex within the Alexander-Wrangellia-Peninsular terrane (Fig. 14B; Amato et al., 2007a). We suggest that Tlikakila basement may have been the source of Precambrian detrital zircons in the southeastern succession of the Kahiltna assemblage.
Evidence for Discrete Sources of the Two Successions of the Kahiltna Assemblage
An important question is whether any detritus from the Alexander-Wrangellia-Peninsular terrane can be detected in the northwestern succession of the Kahiltna assemblage, or if detritus from the continental flank can be detected in the southeastern succession. Given the prominent Late Jurassic (ca. 168–142 Ma) age populations in the Trimble and Old Man units, significant input from the Alexander-Wrangellia-Peninsular terrane into the northwestern succession should be recognizable by the presence of peaks in that range. Moderate (6%–12%) Late Jurassic peaks occur in 4 of the 14 Dalzell samples, and a small peak (3%) occurs in one other, and small (3%–5%) Late Jurassic peaks occur in half of the Whistler sandstones, and these potentially could be derived from the Alexander-Wrangellia-Peninsular terrane. Igneous rocks of this age are also found on the continental (north and eastward) side of the Kahiltna-Gravina basinal assemblage, although their distribution appears to be limited to an area in the Coast Range near latitude 52°N in western British Columbia (Gehrels et al., 2009). There does not seem to be any spatial or temporal pattern to the occurrence of those detrital zircon age peaks in the northwestern succession that might suggest increased input of Late Jurassic detrital zircon populations closer to the Alexander-Wrangellia-Peninsular terrane or an increasing input with decreasing depositional age. The Hayes unit, depositionally overlying the Old Man unit, lacks a ca. 168–142 Ma detrital zircon age population, suggesting little or no input from the southeastern petrofacies source.
In the northern Talkeetna Range and eastern Alaska Range, Huff et al. (2011) and Hampton et al. (2017) interpreted mixed positive and negative epsilon Hf values from Early Jurassic and late Paleozoic detrital zircons from the northwestern succession of the Kahiltna assemblage to indicate provenance from both the oceanic Alexander-Wrangellia-Peninsular terrane to the south and older continental terranes to the north. However, Early Jurassic igneous rocks with positive epsilon Nd isotopic values are known to occur on both flanks of the Kahiltna assemblage (north Farewell terrane—Dumoulin et al., 2018b; south Alexander-Wrangellia-Peninsular terrane—Rioux et al., 2007). Late Paleozoic continental (Piercey et al., 2006) as well as oceanic (Box et al., 2015) sources are also known from the north side of the Kahiltna assemblage. These apparently mixed continental and oceanic source terranes could either have been derived solely from the north side of the Kahiltna assemblage or mixed from both northern and southern sources.
Several lines of evidence suggest that Cretaceous zircons from the Whistler and Old Man units were derived from different, contrasting sources. Late Cretaceous zircons of the Whistler and Old Man units in this study have distinct geochemical compositions (e.g., median U and Th contents of Whistler zircons are more than twice those of Old Man zircons; Supplemental Item 2), suggesting derivation from distinctly different sources. In the northern Talkeetna Range, late Early Cretaceous detrital zircons in northwestern succession strata in the Kahiltna assemblage yield only highly negative Hf isotopic data (Hampton et al., 2017), which indicate derivation of these grains solely from continental sources to the north. Late Cretaceous tuffs are interbedded in the Old Man unit (Karl et al., 2016), possibly derived from contemporaneous surface eruptions during magmatism of that age known sporadically along the Alexander-Wrangellia-Peninsular terrane (Anderson et al., 2013), and, at the Pebble Cu-Au deposit (Fig. 1), they have Nd isotopic values that indicate a much less evolved, “oceanic” source (Ayuso et al., 2013). Clarification of this important issue of the mixed provenance of Kahiltna assemblage sandstones from both flanks of the basin and their spatial and temporal evolution must await further isotopic study of both Kahiltna assemblage components and presumed source components.
Timing of Deformation of the Kahiltna Assemblage
Deformation across the Kahiltna assemblage in the western Alaska Range can be grouped into distinct structural domains. The northern domain, which includes the Dalzell unit and the underlying Farewell terrane, is characterized by southeast-dipping thrust faults and tight, overturned folds verging to the northwest with moderately southeast-dipping axial planar cleavage (Figs. 13A–13C). Deformation occurred after the youngest involved strata (D2; MDA = ca. 103 Ma) and prior to intrusion of the oldest crosscutting plutons at ca. 67 Ma (Jones et al., 2013, 2014; Wilson et al., 2015). We interpret a ca. 97 Ma metamorphic titanite (Fig. 5; Supplemental File S5) from a deformed mafic dike in the underlying Farewell terrane adjacent to its contact with the Kahiltna assemblage to record the age of this north-verging fold-and-thrust episode.
The central structural domain, which includes deformation of the Whistler, Old Man, Spring, and Hayes petrofacies units, consists of south-southeast–vergent folds (and presumed thrusts) with moderately north-northwest–dipping axial planar cleavage (Figs. 13D–13G). Deformation of the Old Man, Spring, and Hayes units occurred after deposition of the youngest involved strata (Hayes petrofacies unit: MDA = ca. 81 Ma) and prior to intrusion of a crosscutting 80.3 ± 0.1 Ma pluton (Fig. 5; Wilson et al., 2015). Timing of the deformation of the Whistler unit is a little more loosely constrained, with the youngest involved strata (sample MDAs) progressing southward from ca. 99 to 87 Ma, with the youngest strata crosscut by the 76.7 ± 0.2 Ma Whistler pluton (Hames, 2014). Regionally, such deformation timing is supported by ca. 85–75 Ma detrital zircon fission-track age populations from zircons in stream sediments eroded from Mesozoic and Paleozoic rocks, which indicate Late Cretaceous exhumation from 5 to 12 km depths (Lease et al., 2016).
A southern structural domain, which deforms the Trimble petrofacies unit, consists of south-southwest–vergent folds with steep west-northwest–striking axial planar cleavage (Figs. 13H and 13I). Deformation occurred after deposition of the youngest involved strata (MDA = ca. 147 Ma) and prior to intrusion of crosscutting Late Cretaceous plutons as old as ca. 80 Ma (Fig. 5; Jones et al., 2013, 2014).
Far-Field Deformation Effects in Interior Alaska
Late Cretaceous deformation was widespread across western Alaska northwest from the Alaska Range to the southern flank of the Brooks Range, constrained to roughly the same time interval as deformation of the Kahiltna assemblage (Moore and Box, 2016). Deformation is best recorded in mid- to Upper Cretaceous clastic sedimentary strata of the Kuskokwim and Yukon-Koyukuk basins to the northwest of the Kahiltna assemblage (Fig. 15). In the Kuskokwim basin, the timing of deformation is constrained by the youngest marine fossils of the deformed strata (Turonian: 93.9–89.8 Ma; Elder and Box, 1992; Walker et al., 2018) and the oldest ages of unconformably overlying volcanic rocks (74.3 ± 2.2 Ma by 40Ar/39Ar dating; Box et al., 1993) or crosscutting dikes (U-Pb zircon SHRIMP-RG age of 80.2 ±1.3 Ma; Bradley et al., 2017). In the Yukon-Koyukuk basin, deformed Turonian strata are overlain by lavas and crosscut by hypabyssal intrusive rocks, the oldest K-Ar age of which is 68.8 ± 3 Ma (Patton and Moll-Stalcup, 1996). The shortening directions represented by folds and thrusts are dominantly northwest-southeast, perpendicular to the trend of the Alexander-Wrangellia-Peninsular terrane and the Kahiltna assemblage in southern Alaska (Fig. 15). The temporal and spatial coincidence of this widespread deformation far into the continental interior (even with reversal of Cenozoic dextral faulting on the Denali, Tintina, and ancillary faults) with the arrival and collision of the Alexander-Wrangellia-Peninsular terrane against a subduction zone dipping under continental Alaska, is strongly suggestive of a causal relationship. The progressive Late Cretaceous northward sweep of subduction-related magmatic rocks across western Alaska (Moll-Stalcup, 1994) represents the onset of underthrusting of the Pacific plate beneath consolidated Alaska after collisional emplacement of the Alexander-Wrangellia-Peninsular terrane.
Tectonic Model for Deposition and Deformation of Kahiltna Assemblage
The Kahiltna assemblage has been interpreted to overlap the mid-Cretaceous tectonic boundary between continental Alaska and the oceanic Alexander-Wrangellia-Peninsular terrane. The nature of the mid-Cretaceous boundary has been interpreted as (1) a subducting Pacific basin oceanic plate dipping north and east (Ridgway et al., 2002; Kalbas et al., 2007; Hampton et al., 2007, 2010), (2) a subduction zone dipping south and west (Reed et al., 1983), (3) a major dextral strike-slip fault (Hults et al., 2013), or (4) a major sinistral strike-slip fault (Monger et al., 1994; Gehrels et al., 2009; Yokelson et al., 2015). Three lines of evidence favor the first interpretation (although a strike-slip component of motion is not excluded). The presence and continuity of a 6000-km-long mid-Cretaceous magmatic belt with subduction-related geochemical character, a few hundred kilometers inland from and parallel to the mid-Cretaceous continental margin from southwestern Canada through central Alaska (Fig. 1) to the Okhotsk-Chukotsk volcanic belt in eastern Siberia (Rubin et al., 1995; Hart et al., 2004; Pease et al., 2017), are taken as strong evidence for an oceanic plate subduction zone dipping north and east beneath continental Alaska prior to emplacement of the Alexander-Wrangellia-Peninsular terrane. Northerly underthrusting of Kahiltna assemblage strata beneath the Yukon composite terrane to the north is also indicated from magnetotelluric data (Stanley et al., 1990). Within the Kahiltna assemblage outcrop area, a belt of serpentinite bodies (Jones et al., 1980; Roeske et al., 2005) and chert and argillite mélange with Cenomanian fossils (Bier, 2010) have been interpreted as reflecting emplacement in an accretionary wedge above a north-dipping subduction zone within the Kahiltna assemblage.
Three structural and stratigraphic features of the Kahiltna assemblage that we have documented here can be readily interpreted to support a model of a south-facing subduction zone beneath continental Alaska into which was carried a south-facing juvenile island arc. First, the Kahiltna assemblage records southward vergence and fold overturning that involved both the strata flanking and derived from the continental margin (except along its northwestern margin, i.e., Dalzell petrofacies unit; see below) and the strata of the juvenile island arc. Second, the continent-flanking northwestern succession records younger depositional ages toward the south (from Dalzell to Whistler to Hayes petrofacies units) in the direction of structural vergence. Third, the youngest petrofacies unit (Hayes) of the northwestern succession depositionally overlies the youngest petrofacies unit (Old Man) of the southeastern (volcaniclastic) succession, and it was deformed together with the underlying volcaniclastic succession, within approximately 1 m.y. of deposition. These features are discussed below in terms of their relevance to the model of a collision between the back-arc margin of a juvenile arc and a north-dipping subduction zone beneath continental Alaska.
Structural vergence of the upper plate of a subduction zone toward the incoming, subducting oceanic plate is typical of subduction zones around the world, as originally shown by study of ancient uplifted accretionary wedges (e.g., southern Alaska—Moore, 1973; Plafker et al., 1994; Amato et al., 2013; western California—Ernst, 1975; Blake et al., 1988; Wakabayashi, 1992), as well as by geophysical and drilling transects across active subduction zones (Moore et al., 1988; Westbrook et al., 1988; Yamada et al., 2011). Both ancient and modern examples of subduction zone accretionary wedges also show younger depositional ages in the direction of structural vergence, generally reflecting the progressive deposition and accretion of trench sediments to the toe of the accretionary wedge.
A model of trench sedimentation for the northwestern succession is supported by the progressive younger MDAs southward across the Whistler petrofacies unit (Fig. 5), as well as the broader trend of younger ages southward from Dalzell to Whistler to Hayes petrofacies units. Although our data also indicate a relative northwest to southeast decrease in the age of deformation between the Dalzell and the Hayes petrofacies units, our constraints on the age of deformation within the Whistler petrofacies unit are insufficient to test whether its record of younger depositional ages southward is matched by progressively younger deformational ages southward. The capping of the southeastern succession by the northerly derived Hayes petrofacies unit at or shortly after ca. 81 Ma and the short time (constrained to about 1 m.y.) between its deposition and subsequent deformation prior to the 80 Ma postkinematic emplacement of a crosscutting pluton (Fig. 5) are most easily explained as sedimentation of continental-sourced debris in a trench atop the subsided back-arc flank of the Alexander-Wrangellia-Peninsular oceanic arc as it was subducted beneath the forearc of the north-dipping subduction zone.
The opposite structural vergence (northwestward) of the older unit of the northwestern succession (Dalzell unit) is interpreted to have predated the southeast structural vergence recorded by the rest of the Kahiltna assemblage, based on the ca. 97 Ma metamorphic titanite (Fig. 5; Supplemental File S5) within the deeper part of the northwest-verging structural pile. Such landward-vergent fold/thrust deformation is uncommon in sedimented forearcs, but it has long been noted in the offshore Washington sector of the Cascadia subduction zone (MacKay et al., 1992) and is known from several other subduction zones (Byrne and Hibbard, 1987). We infer that this episode of continent-ward vergence predated the subsequent seaward vergence recorded by the adjacent, younger Whistler petrofacies unit.
Our model for the stratigraphic and structural evolution of Kahiltna assemblage is presented in three stages in Figure 16. The model envisions the Kahiltna assemblage to represent the structural juxtaposition of contrasting sedimentary sequences on two flanks of a subducting oceanic plate (Fig. 16A). Sedimentary strata on the eastern flank of the closing ocean basin (early Dalzell petrofacies in this paper and eastern Gravina belt; Kapp and Gehrels, 1998; Gehrels, 2001; Yokelson et al., 2015) were derived from the continental interior (Yukon composite and Farewell terranes) and were initially deposited on either older basement of the upper plate of the north-dipping subduction zone or in an adjacent trench on the oceanic plate before being structurally accreted to the upper plate. Sedimentary strata on the southern flank of the closing ocean basin (Trimble petrofacies in this paper, and Nutzotin Mountains sequence, Dezadeash Formation, and western Gravina belt farther east) were deposited on the back-arc flank of an active volcanic arc (Alexander-Wrangellia-Peninsular terrane) generated by subduction of oceanic crust of the Pacific basin beneath its southern flank. Initial underthrusting of the back-arc flank of the Alexander-Wrangellia-Peninsular terrane beneath the north-dipping continental subduction zone deformed the Nutzotin Mountains sequence with south-verging folds and thrust faults prior to crosscutting plutons dated by K-Ar method between 117 and 105 Ma (Manuszak et al., 2007). Subaerial exposure of the sediment surface by 117 Ma (Koepp et al., 2017) may indicate initial uplift related to collisional tectonics, or, alternatively, aggradation of the underlying volcanic pile. Given the caveat of the uncertainty of the accuracy of K-Ar ages, we used the oldest crosscutting age to infer that collisional deformation had begun in the Nutzotin Mountains area by 117 Ma. Younger deformation of mid-Cretaceous strata to the west (Hampton et al., 2010; this paper) and to the southeast (McClelland et al., 1992; Yokelson et al., 2015) of the Nutzotin Mountains suggests the colliding arc was not parallel to the continental subduction zone and that the collision propagated both to the west (Ridgway et al., 2002) and to the east (Haeussler, 1992) from the initial point of collision in or near the present Nutzotin Mountains.
By ca. 97 Ma, the collision had propagated to the west into the area of the northwestern Talkeetna and Clearwater Mountains (Fig. 16B). Geophysical data (Glen et al., 2007) suggest that strata like the Nutzotin Mountain sequence (Hults et al., 2013) were thrust southward over older rocks in the Alexander-Wrangellia-Peninsular terrane. Farther north, intensely metamorphosed Dalzell petrofacies–equivalent strata (Maclaren metamorphic belt) were thrust southward over Trimble petrofacies–equivalent strata (Smith, 1981; Davidson et al., 1992) along the collisional suture until at least 75 Ma (Ridgway et al., 2002). The structural vergence is reversed farther to the north, with continentally derived Kahiltna strata and their Farewell terrane basement involved in north-verging folds and thrusts (Hampton et al., 2007). This deformation preceded uplift and unconformable deposition of late Albian(?)–Cenomanian nonmarine strata (Caribou Pass Formation [CPF]; Fig. 16C). Similar deformation continued to the northwest, where the Dalzell petrofacies and the adjacent Farewell terrane were deformed by north-vergent deformation at ca. 97 Ma.
After ca. 97 Ma, normal oceanic plate subduction continued to the west of the propagating collision, accreting continentally derived trench sediments (Whistler petrofacies) with consistent southward structural vergence (Fig. 16C). Sedimentation on the back-arc flank of the approaching Alexander-Wrangellia-Peninsular terrane records active pyroclastic volcanism and erosion of distinctive intermediate lavas (Old Man petrofacies). By ca. 81 Ma, the back-arc flank of the Alexander-Wrangellia-Peninsular terrane slid under the trench and was overlapped by continentally derived trench sediments from the north. This sequence was deformed and intruded by ca. 80 Ma arc-related magmas as Alexander-Wrangellia-Peninsular magmatism migrated northward across the suture and began stitching the accreted terranes together.
The age of deformation of the Gravina belt in southeastern Alaska is constrained between ca. 100 and 90 Ma (Rubin and Saleeby, 1992), which is also younger than deformation documented in the Nutzotin Mountains (Haeussler, 1992; Manuszak et al., 2007; Koepp et al., 2017). Deformation of the Gravina belt also consists of oceanward-vergent folds and thrusts (Haeussler, 1992). The western and eastern Gravina belt sequences were derived from the Alexander-Wrangellia-Peninsular terrane and the Yukon composite terrane, respectively (Yokelson et al., 2015). The contrasting provenance of the western and eastern sequences is analogous to the southeastern and northwestern sequences of the Kahiltna assemblage, respectively. With the present constraints from ages of plutons that crosscut the associated deformation, the emplacement of the Alexander-Wrangellia-Peninsular terrane oceanic arc began to the north of the Nutzotin basin at ca. 117 Ma and then progressed both to the west into the Kahiltna assemblage (deforming the inferred trench sediment of the Hayes petrofacies in the western Alaska Range ∼37 m.y. later) and to the southeast into the Gravina basin (where undeformed plutons intruded the collision deformational fabric ∼17–27 m.y. later).
Modern Analogue: Collision of Izu-Bonin-Marianas Arc with Honshu Arc of Central Japan
A possible modern analogue for the collision of the Alexander-Wrangellia-Peninsular terrane with North America is the ongoing collision of the Izu-Bonin-Marianas arc with the Honshu arc of central Japan (Fig. 17; Draut and Clift, 2013). Unlike the longer (hundreds of millions of years) precollisional history of the Alexander-Wrangellia-Peninsular terrane, the Izu-Bonin-Marianas arc developed since 50 Ma along the eastern edge of the Philippine Sea plate above the westward subducting Pacific plate (Stern and Bloomer, 1992). The northwest end of the Izu-Bonin-Marianas arc is being obliquely subducted at the junction of the Japan Trench and the Nankai Trough; collision of the arc with the trench began ca. 15 Ma (Kimura et al., 2008). Northwestward subduction of back-arc crust of the Philippine Sea plate continues along the Nankai Trough to the south of the collision at ∼4–6 cm/yr (Kimura et al., 2008), so the collision should be propagating southward along the Nankai Trough.
In Figure 17, we compare the geometry and scale of the accreted Alexander-Wrangellia-Peninsular terrane (Fig. 17A) of the northwestern North American Cordillera with the Izu-Bonin-Marianas arc in the western Pacific basin (Fig. 17B) at the same scale to show the remarkable geometric similarity of the ongoing collision in Japan with the Late Cretaceous collision of the Alexander-Wrangellia-Peninsular terrane with North America (although mirror images in terms of subduction zone polarity). It is noted that subduction beneath the eastern flank of the Izu-Bonin-Marianas arc continues following the initiation of the collision with the Japan Trench–Nankai Trough subduction zone. On the Izu Peninsula of Japan, trenchward-vergent structures uplift and deform strata of the Izu-Bonin-Marianas arc (Fig. 17C), and Mount Fuji, a subduction-related volcano, is built over the deformed strata (Soh et al., 1998).
Okuzawa and Hisada (2008) discussed the compositional evolution of sedimentary strata atop the uplifted strata of the colliding Izu arc west of Mount Fuji. These authors interpreted a sedimentary compositional transition to date the initial impingement of the Izu-Bonin-Marianas arc against the Japanese forearc. Lower Miocene clinopyroxene-rich volcaniclastic sediments of the Izu-Bonin-Marianas arc are overlain abruptly by quartz-rich Lower Middle Miocene strata with detrital garnets characteristic of source rocks in the Japan forearc. Okuzawa and Hisada (2008) also interpreted the Japan-sourced strata above the transition as trench sedimentation atop the subducting back-arc flank of the Izu-Bonin-Marianas arc. This transition is analogous to the upward stratigraphic transition in the western Alaska Range from clinopyroxene-rich Old Man strata to quartzose Hayes strata.
The question of mixed provenance trench sediments, composed of both continental and juvenile arc components, ahead of the propagating Izu-Bonin-Marianas collision was addressed by Clift et al. (2013). Detrital zircon populations from modern trench sediment in the Nankai Trough (trench) 150 km west of the collision (sample site located in Fig. 17C) lack any Izu-Bonin-Marianas arc contribution, as seen in streams draining uplifted Izu-Bonin-Marianas arc rocks from the ongoing collision at the head of the Nankai Trough. The trench sediment signature instead reflects more orthogonal input from nearby forearc drainages, so input of colliding arc detritus to trench sediment is limited beyond the immediate vicinity of the ongoing collision (Clift et al., 2013). This differs somewhat from the propagating collisional models for the Kahiltna assemblage of Ridgway et al. (2002), Kalbas et al. (2007), and Hampton et al. (2007, 2010). These models (see figure 13 of Kalbas et al., 2007) envision the northwestern succession of the Kahiltna assemblage to represent a sedimentary wedge, derived from and prograding ahead of the collisional complex, which is subsequently deformed in the propagating collisional suture. Instead, the Japan analogue shows minor sediment routing to the trench ahead of the propagating collision. Clarification of when and where mixed provenance sediments are incorporated into the Kahiltna assemblage (Huff et al., 2011; Hampton et al., 2017) will be important to refining the current tectonic model.
Our tectonic model for the Kahiltna assemblage (Fig. 16) closely aligns with our current understanding of the modern, propagating collision of the back-arc flank of the oceanic Izu-Bonin-Marianas arc with the Japan Trench–Nankai Trough in central Japan. In Figure 17D, features of the Cretaceous collision of the Alexander-Wrangellia-Peninsular terrane with North America that are analogous to the collision of the Izu-Bonin-Marianas arc with central Japan are labeled on the same base map from Figure 17C. The back-arc flank of the colliding arc, underthrusting the continentward-dipping subduction zone at a high angle, is progressively accreted to the continental accretionary wedge as the collision propagates laterally. At the point of active underthrusting, the youngest back-arc strata (Old Man petrofacies) are overlapped by continentally derived trench sediments (Hayes petrofacies), followed quickly by deformation within the subduction zone. As the collision progresses, the collisional complex is uplifted, eroded, and capped by nonmarine sediments (analogous to the Caribou Pass Formation). The collision results in juxtaposition of the precollisional back-arc sedimentary wedge against the precollisional continental accretionary wedge, giving the appearance of a single two-sided (albeit deformed) basin (analogous to the Kahiltna assemblage).
Strata of the Kahiltna assemblage in the western Alaska Range are predominantly sub-wave-base turbidites and debris-flow deposits, but they also locally include shallow-marine deposits. They are spatially separated into contrasting northwest and southeast compositional belts or successions, separated by a sharp boundary. We equate the northwestern succession with the Kuskokwim Group along strike to the southwest, with which it shares similar detrital composition and zircon populations. The southeastern succession is continuous to the southwest and northeast with the Koksetna-Clearwater belt, with which it shares similar detrital composition and zircon populations. The name “Kahiltna assemblage” refers to the structural assemblage of Kuskokwim Group and Koksetna-Clearwater belt from the central Alaska Range to the Bristol Bay lowlands.
Each succession is spatially divided into three age-petrofacies units, with the Upper Cretaceous units of each succession in contact in the central part of the basin. The Kuskokwim Group in the Kahiltna assemblage is divided into a northwestern part (Dalzell petrofacies unit) with detrital zircon MDAs between ca. 194 and 103 Ma (late Early Cretaceous) and a southeastern part (Whistler petrofacies unit) with detrital zircon MDAs between ca. 99 and 84 Ma. The local Skwentna River sequence, part of the Koksetna-Clearwater belt of the Kahiltna assemblage, is divided into a southeastern part (Trimble petrofacies unit) with detrital zircon MDAs between ca. 168 and 147 Ma (Late Jurassic) and a northern part characterized by an older unit (Spring petrofacies unit) with detrital zircon MDAs between ca. 146 and 103 Ma that is locally exposed beneath a more widely exposed, younger unit (Old Man petrofacies unit) with detrital zircon MDAs between ca. 99 and 89 Ma. In a few places, strata of northwestern provenance (Hayes petrofacies unit) depositionally overlie the Old Man petrofacies unit. A single sample from the Hayes unit yielded a detrital zircon MDA of 81 Ma, the youngest known MDA in the Kahiltna assemblage.
A cross section across the Kahiltna assemblage in the western Alaska Range shows distinct structural domains. The northwestern domain, which includes the Dalzell unit and the underlying Farewell terrane, is characterized by tight, overturned folds verging to the northwest with moderately southeast-dipping axial planar cleavage. Deformation occurred after the youngest involved strata (103 Ma) and prior to intrusion of crosscutting Late Cretaceous (67 Ma) plutons. This north-verging fold-and-thrust episode is interpreted to have occurred around 97 Ma, based on a SHRIMP U-Pb age of metamorphic titanite in a mafic rock within the underlying Farewell terrane. A central domain, which overprints the Whistler, Old Man, Spring, and Hayes petrofacies units, consists of south-southeast–vergent folds with moderately north-northwest–dipping axial planar cleavage. This deformation is interpreted to have progressed from northwest to southeast across the Whistler unit, following closely after depositional ages of the southeastward-younging unit. The end of southeast-verging deformation occurred after the youngest involved strata (ca. 81 Ma) and prior to intrusion of crosscutting Late Cretaceous (ca. 80–76 Ma) plutons. A southern deformational domain, which overprints the Trimble petrofacies unit, consists of folds without clear vergence and with steep west-northwest–striking axial planar cleavage. Deformation occurred after the youngest involved strata (ca. 147 Ma) and prior to intrusion of crosscutting Late Cretaceous (as old as 80 Ma) plutons. The age and kinematics of Kahiltna assemblage deformation coincided with regional deformation of central and western Alaska as far north as the southern flank of the Brooks Range.
The detrital zircon age populations of the Kuskokwim Group in the Kahiltna assemblage were largely derived from the Phanerozoic igneous rocks of the Yukon composite terrane, with lesser input (mostly recognized by its Precambrian signature) from the Farewell terrane. The detrital zircon age populations of the Skwentna River sequence of the Koksetna-Clearwater belt were initially derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and then show a marked increase in the early Late Cretaceous input from synchronous volcanism in the Alexander-Wrangellia-Peninsular terrane. Although the detrital zircon and compositional contrasts are sharp between the geographically separated successions, small inputs of the southeastern facies into the northwestern facies cannot be definitively ruled out with available data; however, we note that there are no spatial trends that suggest such input, nor are there any temporal trends to suggest increasing input through time. The southeastern succession was depositionally overlain by the northwestern petrofacies unit (Hayes unit) at or following ca. 81 Ma, and both were deformed together by ca. 80 Ma.
We present a tectonic model for the structural geometry of the Kahiltna assemblage as a progression from sedimentary accretion in the forearc of a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane), which drove collisional deformation across western Alaska. The nature or magnitude of any strike-slip component during or after collision is uncertain.
We suggest that an active tectonic analogue for the Late Cretaceous collision of the Alexander-Wrangellia-Peninsular island arc with continental North America is the ongoing collision of the Izu-Bonin-Marianas island arc with the Japanese continental arc at the Japan Trench–Nankai Trough on the east side of Honshu Island in central Japan.
We appreciate the financial support of the Mineral Resources Program (U.S. Geological Survey) for this study. We thank helicopter pilots Cameron Walker, Grayson Morrow, and Patrick Zimmerman of Prism Helicopters for their careful, competent transport through difficult conditions in the western Alaska Range. We are thankful to Kiska Minerals for use of their camp at Whistler, Steve Perrins for lodging at Rainy Pass Lodge, and Rob Jones for lodging at Big River Lodge. Thanks also go to Richard Lease, Julie Dumoulin, Chad Hults, and Brian Hampton for helpful discussions, and we appreciate helpful reviews by Richard Lease, Ken Ridgway, Trevor Waldien, and Grant Lowey.