Abstract

The Alaska Range, the topographic signature of the Denali fault, has an unusual physiography, with the Nenana River sourced from the south side of the divide and traversing along the range front some distance before heading north across the mountain range. Previous researchers suggested that a change from south-flowing to north-flowing drainage occurred at ca. 6 Ma, or early-middle Miocene, during initial phases of Alaska Range uplift. We applied 40Ar/39Ar dating of detrital micas from modern river sediment (proxy for basinwide source) and strata of the Neogene Tanana Basin (sink for the paleo–Nenana River), located along the northern front of the Alaska Range, to further investigate this hypothesis. In addition, we acquired 40Ar/39Ar muscovite ages from bedrock for additional source constraints and compared our results to regional geochronology data sets and geological mapping. During the earliest Miocene, the paleo–Nenana River likely flowed south. By the early Miocene, the paleo–Nenana River flowed to the north. During the middle Miocene, drainage reorganization continued, suggesting a variable history of rock uplift in the Alaska Range. By the late Miocene, sediment recycling occurred as the southern extent of the Tanana Basin was uplifted and eroded. The modern Nenana River near Cantwell has a muscovite age signature different than the Tanana Basin strata, implying continued drainage reorganization after the deposition of the Pliocene Nenana Gravel. In summary, the Nenana River drainage changed direction to north-flowing by ca. 18 Ma, driven by tectonism. Drainage reorganization continues today, demonstrating that strike-slip fault transpressive orogens can have complex paleodrainage histories.

INTRODUCTION

There have been numerous detrital geochronology studies examining drainage reorganization due to tectonism, with a focus on drainage changes linked to flat-slab subduction (e.g., Western Interior United States; Sharman et al., 2017) and continent-continent collision (e.g., Tibet; van Hoang et al., 2009). The application of detrital geochronology to investigate drainage reorganization of transpressive orogens along continental strike-slip faults has received less attention. Given that transpressive orogens are associated with complex topographic development histories (e.g., Spotila et al., 2007), and strike-slip fault–related orogenesis can lead to drainage reversals along incised canyons (Brocard et al., 2011), more case studies documenting the drainage evolution and potential river reversals linked to these dynamic mountain ranges are warranted.

The Alaska Range, located along the strike-slip Denali fault, has an unusual physiography, with two north-flowing river systems, the Nenana and Delta Rivers. These anomalous rivers are currently sourced from the south side of the topographic divide, indicating a potentially complex drainage reorganization history along a transpressive orogen (Fig. 1). These rivers traverse along the range front some distance before heading north and cutting through the mountain range (Fig. 2). Drainage reorganization of the Nenana River system, in particular, has long been linked to the timing of the development of the Alaska Range (Moffit, 1915; Wahrhaftig et al., 1969; Ridgway et al., 1999, 2007; Thoms, 2000; Brennan and Ridgway, 2015; Finzel et al., 2015, 2016).

Researchers have argued that there was a change from a generally south-flowing to north-flowing Nenana River at ca. 6 Ma, corresponding with the assumed initial uplift of the Alaska Range based on paleoflow data and clast composition (Wahrhaftig et al., 1969; Ridgway et al., 1999). Based on a more modern detrital zircon U-Pb geochronology study, Brennan and Ridgway (2015) inferred significant drainage reorganization was occurring much earlier, by the early-middle Miocene, but their source-to-sink detrital zircon U-Pb geochronology correlation was potentially nonunique. An early-middle Miocene timing of drainage reorganization is also ∼10 to ∼15 m.y. after the extensively documented initiation of Oligocene to present uplift of the Alaska Range, calling into question their conclusion (Benowitz et al., 2011, 2012a, 2014; Lease et al., 2016).

We built on Brennan and Ridgway’s (2015) approach of using detrital zircon U-Pb geochronology to study the Nenana River drainage history by applying 40Ar/39Ar dating to detrital muscovite from strata of the Neogene Tanana Basin (sink for the paleo-Nenana River), located along the northern front of the Alaska Range (Figs. 1 and 3), and to detrital muscovite from modern river sediment (proxy for basinwide source; Fig. 1). In addition, we applied 40Ar/39Ar dating to muscovite from select bedrock samples for additional source constraints and compared our results to regional geochronology data sets and geological mapping (Figs. 4 and 5). Our results indicate an earlier (ca. 18 Ma) and overall more prolonged history of Nenana River drainage reorganization than previously captured, but these results still support the general conclusions of Brennan and Ridgway (2015). This interpretation aligns well with published evidence of a complex temporal-spatial history of Oligocene to present uplift for the Alaska Range and further demonstrates the utility of combining modern river sediment detrital geochronology with dating of ancient strata and regional bedrock samples.

GEOLOGICAL BACKGROUND

Alaska Range Suture Zone, the Denali Fault, and the Alaska Range

The Mesozoic to Cenozoic Alaska Range suture zone is the tectonostratigraphic boundary between the Precambrian–Paleozoic Yukon-Tanana composite terrane of Laurentian affinity to the north and the accreted, primarily Mesozoic, oceanic rocks of the Wrangellia composite terrane of southern Alaska to the south (Figs. 3 and 6; Trop and Ridgway, 2007). These terranes are intruded and overlain by Devonian to Cenozoic igneous intrusive and volcanic rocks (Fig. 5; e.g., Wilson et al., 2015). The Alaska Range suture zone itself consists of the Kahiltna Basin, which is composed primarily of Late Jurassic to Late Cretaceous marine sedimentary strata (Ridgway et al., 2002), and the Late Cretaceous to Eocene terrestrial Cantwell Basin (Fig. 6; e.g., Tomsich et al., 2014; Salazar-Jaramillo et al., 2016). The Alaska Range suture zone is intruded and overlain by Mesozoic and Cenozoic igneous intrusive and volcanic rocks (Fig. 5) (e.g., Wilson et al., 2015). More recent work has led to the conclusion that these “composite” terranes and Kahiltna nomenclatures are an oversimplification of regions with very complex geologic histories (Hults et al., 2013; Dusel-Bacon et al., 2017; Dumoulin et al., 2018a, 2018b). For the goals of this paper, these vestigial terms are used because they clearly convey and delineate the geologic history of sediment sources north, south, and within the Alaska Range, and they are sufficiently different to reconstruct sediment source histories for the Nenana River system.

The Denali fault traverses the Alaska Range suture zone and has been an active strike-slip fault since at least ca. 65 Ma (e.g., Benowitz et al., 2014). Ancient and modern horizontal slip rates decrease to the west from ∼12 mm/yr to ∼5 mm/yr (Benowitz et al., 2012b; Haeussler et al., 2017a), with slip taken up on contractional structures on both sides of the Denali fault (Riccio et al., 2014; Bemis et al., 2015; Waldien et al., 2015; Burkett et al., 2016), supporting evidence of a highly transpressive fault system (Fig. 3). Cenozoic magmatism was also associated with the trace of this major structure, with the last major period of plutonism at ca. 40 Ma (e.g., Wilson et al., 2015).

Thermochronology research has constrained the initial timing of uplift of the topographically high modern Alaska Range to ca. 30 to 25 Ma (Benowitz et al., 2014; Riccio et al., 2014; Burkett et al., 2016; Lease et al., 2016). These thermochronology studies and the aforementioned structural studies outline a history of asymmetrical uplift and unroofing of the Alaska Range through time and space along the Denali fault system. The way in which the complex topographic development history of the Alaska Range affected the paleodrainage history of the Nenana River system is the focus of this study.

Tanana Basin

The ancient and still-active Tanana Basin lies north of the Alaska Range, where the southern portion of the basin has been incorporated into the uplifted Alaska Range as the mountain range has encroached northward with time (Ridgway et al., 2007; Bemis and Wallace, 2007; Bemis et al., 2012, 2015). Approximately 2000 m of Neogene strata are exposed at the Suntrana type section located near Healy, Alaska (Fig. 1). These strata were deposited on the muscovite-rich Birch Creek schist basement rock of the Yukon-Tanana terrane (Fig. 6). The Tanana Basin strata are divided into the Miocene Usibelli Group and the Pliocene Nenana Gravel (Fig. 7; Leopold and Liu, 1994). At the type section, the Usibelli Group is ∼800 m thick and is divided into five formations based on facies analysis (Wahrhaftig, 1987; Ridgway et al., 2007). These formations are the Healy Creek (earliest Miocene), Sanctuary (earliest Miocene), Suntrana (early-middle Miocene), Lignite Creek (late Miocene), and the Grubstake (latest Miocene) Formations, after Brennan and Ridgway (2015). The ∼1200 m Pliocene Nenana Gravel unit conformably overlies the Grubstake Formation at the type section location (Wahrhaftig, 1987).

Paleodrainage History of the Nenana River System

Previous researchers have proposed three different interpretations of the paleodrainage history of the Nenana River. (1) Based on clast composition and paleoflow indicators, Wahrhaftig et al. (1969) and Ridgway et al. (1999) inferred that the Nenana River reversed direction from dominantly south-flowing to north-flowing shortly after the deposition of the Grubstake ash at ca. 6 Ma, based on 40Ar/39Ar analysis by Triplehorn et al. (2000). (2) Brennan and Ridgway (2015) conducted detrital zircon U-Pb geochronology analysis of six samples from the Suntrana type section and inferred a change in dominant paleoflow direction from south-directed to north-directed flow by the time of deposition of the middle Miocene (ca. 15 Ma) upper Suntrana Formation. (3) Based primarily on observations of past glacial distributions and topography, Moffit (1915) proposed a rerouting of the Nenana River due to inferred configurations of glaciers during the waning phase of the last glaciation.

PALEODRAINAGE RECONSTRUCTION TECHNIQUES

A diverse set of tools and techniques has been applied to modern rivers and ancient strata to identify tectonically driven drainage reorganization. Sedimentology features can provide information on paleoflow direction (Smith, 1972). Remote sensing has been applied to delineate potential paleodrainage systems (Wiedmer et al., 2010), as has analysis of variations in genetic diversity (Zhang and Sun, 2011). Arguably, the most commonly applied tool to study paleodrainage source-to-sink linkages is high-throughput detrital mineral geochronology (e.g., Cawood et al., 2012; Chirouze et al., 2013; Blum and Pecha, 2014).

Reconstruction of paleodrainage reorganization from detrital mineral age signatures is complicated due to the potential removal of source material through unroofing. Long-distance (>100 km) tectonic translation of basins away from source areas can also complicate interpretation. More foundational, the requirement for nonunique sources is not always met and can lead to equivocal findings. These concerns can be mitigated by: (1) applying provenance source-to-sink geochronology to minerals and radiometric systems that represent different mineral densities, potentially unique lithologies, and crustal depths, such as U-Pb geochronology on zircons and 40Ar/39Ar geochronology on muscovite grains, and (2) complementary dating of source areas through geochronology analysis of bedrock samples and modern river sediment.

Detrital 40Ar/39Ar muscovite geochronology can capture different source lithologies than detrital U-Pb zircon geochronology due to the factors listed below. Fertility or mineral abundance has a strong effect on detrital geochronology source populations. Detrital U-Pb zircon geochronology is well suited for capturing the age signatures of regional plutonism (e.g., Cawood et al., 2012); detrital 40Ar/39Ar muscovite geochronology is well suited for capturing the age signatures of regional metamorphism (e.g., Hodges et al., 2005). The density of minerals also has a strong effect on detrital geochronology source populations. Zircon is a dense mineral (specific gravity: 4.9 g/cc), and so source area populations can be fractionated during transport and deposition as they move primarily as bed load (van Hoang et al., 2010; Lawrence et al., 2011). Muscovite is a lighter mineral (specific gravity: 2.8 g/cc), but muscovite age populations still can be fractionated during transport and deposition (e.g., Garzanti et al., 2008). Muscovite is more likely to be transported as suspended sediment compared to zircon, due to the lower density of muscovite and basal cleavage properties.

By combining detrital 40Ar/39Ar muscovite geochronology (this study) with the interpretations based on detrital U-Pb zircon geochronology techniques (Brennan and Ridgway, 2015), an increased understanding can be obtained of drainage system reorganization in response to the development of a transpressional orogen. In the framework of previous U-Pb zircon work on the paleodrainage history of the Alaska Range (Brennan and Ridgway, 2015), we applied 40Ar/39Ar geochronology to detrital muscovite from Neogene strata of the Tanana Basin at the Suntrana type section, to modern river sediment from rivers south and north of the Suntrana type section, and to select bedrock samples to better evaluate the paleodrainage history of the Nenana River.

POTENTIAL MUSCOVITE SOURCES FOR THE NENANA RIVER SYSTEM

To the south of the Alaska Range suture zone, the Jurassic Talkeetna arc plutons (153–201 Ma) are a dominant lithology with abundant muscovite (Fig. 5; Hacker et al., 2011; Terhune et al., 2017). To the north of the Alaska Range suture zone, the basement schist, which has predominantly Cretaceous metamorphic ages, is the identifying lithology of the Yukon-Tanana composite terrane (Fig. 6; Hansen et al., 1991; Dusel-Bacon et al., 1996). The Yukon River drainage encompasses terranes with Jurassic 40Ar/39Ar muscovite age signatures (Gehrels et al., 2009; Staples et al., 2016; Jones et al., 2017), but by the Eocene, it was likely separated from the Tanana River drainage by the high topography of the Yukon-Tanana Highlands (Dusel-Bacon et al., 2016). The Mesozoic Kahiltna and early Cenozoic Cantwell Basin units contain sparse large-grained (>250 µm) primary metamorphic and detrital muscovite, and so they cannot be fully discounted as potential sources for recycled muscovite for the Neogene strata of the Tanana Basin.

Numerous ca. 40 Ma plutons are mapped on the north side of the Denali fault zone in proximity to the Suntrana type section, but they are not present regionally to the north or south (Figs. 5 and 6; Roeske et al., 2012; e.g., Wilson et al., 2015). These plutons provide a potential unique source for both detrital zircon and muscovite grains for the Tanana Basin, but zircons of this age have not been documented in the ancient strata of the Tanana Basin (Brennan and Ridgway, 2015). The lack of zircons from these plutons is likely not related to a mineral fertility concern, because of their felsic lithology. Additional ca. 40 Ma plutons south of the Denali fault, located >100 km east of the Suntrana type section, contain abundant muscovite. However, these plutons were likely translated ∼250 km into place during the Neogene, thus discounting them as a source for muscovite in the Neogene Tanana Basin located north of the Denali fault (Fig. 5; Nokleberg et al., 1985, 1992; Benowitz et al., 2012b).

To the north, strata of the Oligocene to Quaternary Tanana Basin overlie the Yukon-Tanana composite terrane (Ridgway et al., 2007). The well-dated strata of the Tanana Basin generally contain abundant and large-grained (>250 µm) muscovite, providing a potential geochronology fingerprinting tool with which to reconstruct the Oligocene to Quaternary paleodrainage reorganization history of the Nenana River system. The Oligocene to Quaternary strata of the Tanana Basin are also a potential source of progressively recycled muscovite, as the basin has been inverted variably through time and space (Ridgway et al., 2007).

METHODS

Tanana Basin Sampling Strategy

We collected muscovite-rich sandstone, mudstone, and sand samples from the Usibelli Group and Nenana Gravel at the Suntrana type section (Healy Creek site) using the formation descriptions and published photos to select sample locations within the section (Fig. 7; e.g., Ridgway et al., 2007; Brennan and Ridgway, 2015). Given that the change from north-derived to south-derived sediment provenance was documented between the lowermost strata of the Suntrana Formation (ca. 20 Ma) and the uppermost strata of the Suntrana Formation (ca. 15 Ma) by Brennan and Ridgway (2015), we collected three spaced samples from this formation to better delineate when the change in sediment provenance occurred. A total of ∼20 L of generally muscovite-rich material was collected from the lowermost strata of the Suntrana Formation (sandstone collected), middle strata of the Suntrana Formation (mudstone collected), upper strata of the Suntrana Formation (sandstone collected), and the Grubstake Formation (mudstone collected). For the thicker Nenana Gravel unit, we collected a lower sand sample and upper sand sample within the section. The lower Nenana Gravel sample had limited muscovite. All other ancient strata samples had abundant muscovite grains of desired size (∼500 to ∼250 µm).

Modern River Sampling Strategy

We collected ∼20 L of generally muscovite-rich modern river sediment from rivers north and south of the Alaska Range suture zone (Fig. 1). All samples contained substantial quantities of muscovite, except for the two samples from the upper stretches of Nenana River, as noted below. The west-flowing upper (community of Delta site) and lower (community of Fairbanks site) Tanana River samples primarily capture sediment sourced from the Yukon-Tanana Highlands watershed. The north-flowing Delta River sample potentially captures sediment sourced from the Yukon-Tanana composite terrane and the Alaska Range suture zone. The west-flowing Yukon River samples (town site of Circle City and the Dalton Highway Bridge) capture local watershed sediment sources and potentially sediment sourced from Canada. The south-flowing Nenana Glacier outlet sample (headwaters of the Nenana River) captures sediment sourced from the Alaska Range suture zone. This river sample had very limited muscovite. The west-flowing Nenana River sample (Cantwell Bridge) captures sediment primarily sourced from the Alaska Range suture zone in the Nenana Glacier area. This river sample also had very limited muscovite. The north-flowing Nenana River samples at the Tatlanika Bridge site and at the townsite of Nenana potentially capture sediment derived from the Yukon-Tanana composite terrane, the Alaska Range suture zone, and potentially recycled sediment from the Tanana Basin.

Bedrock Sampling Strategy

Bedrock samples (∼4 kg per sample site) were collected to better delineate potential sources for the muscovite of the Tanana Basin and the exhumation history of the Alaska Range. We sampled the muscovite-rich Birch Creek schist basement rock at the Suntrana type section at the base of the Healy Creek Formation to evaluate a local bedrock source for muscovite in the Tanana Basin strata. In total, 16 muscovite-bearing deformed igneous and metasedimentary samples from the Alaska Range suture zone north of the Denali fault were collected. We divided these samples into granitoid or metasediment. Many of these granitoid samples are orthogneiss or mylonites; hence, the muscovite closure temperature for these samples may be below ∼400 °C (e.g., Martin et al., 2015). We complemented our bedrock data with existing bedrock geochronology data sets (Hansen et al., 1991; Nokleberg et al., 1992; Gehrels et al., 2009; Greene et al., 2010; Perry et al., 2010; Hacker et al., 2011; Benowitz et al., 2011, 2014) and geological maps (Dusel-Bacon et al., 1996; Wilson et al., 2015). Additionally, one sample was collected from the ca. 70 Ma strata (Tomsich et al., 2014) of the Cantwell Formation (Fig. 1).

40Ar/39Ar Muscovite Geochronology Methods

We applied 40Ar/39Ar single-grain fusion detrital geochronology (Hodges et al., 2005; Broussard et al., 2018) to the modern river sediment samples and the Tanana Basin strata samples. We applied 40Ar/39Ar muscovite geochronology incremental step-heating analysis (McDougall and Harrison, 1999) to pure muscovite separates (1000–250 µm grain size) from the 16 bedrock samples from the Alaska Range suture zone and the single Birch Creek schist basement sample at the Suntrana type section.

We separated out ∼50–200 grains of 500–250-µm-sized muscovite from each modern river sediment sample and from each sandstone/mudstone sample from the Suntrana type section. In addition to standard mineral separation techniques (floating muscovite in water, paper shaking, Frantz magnetic separation), we occasionally used a stainless-steel rolling pin to “roll” samples in order to break up quartz flakes, which were then removed with repeated sieving. Only two detrital muscovite grains were separated from the Cantwell Basin sample, and so those results are not very robust, but since this data set is likely the first from detrital muscovite from this Alaska Range suture zone Cretaceous basin, we present the data in the Supplemental Material1. The 40Ar/39Ar age determinations were performed by J. Benowitz at the Geochronology Facility at the University of Alaska–Fairbanks. Refer to Supplemental Text for detailed 40Ar/39Ar geochronology methods.

40Ar/39Ar GEOCHRONOLOGY RESULTS

Bedrock 40Ar/39Ar Geochronology

The 40Ar/39Ar analysis of the 16 bedrock samples provided robust results with plateau ages and integrated ages generally overlapping within error. Preferred age determinations and isotopic data are provided in Table 1 and Tables S1 and S2 (see footnote 1). Example age spectra are presented in Figure 8, with all age spectra, Ca/K, and Cl/K data presented in Figure S1. Isochron age determinations were not always possible due to the generally homogeneous nature of the gas release for many samples. When an isochron age determination was possible, the age and isotopic information are presented in Table 1 and Figure S1. The isochron regressions to 40Ar/36Ar indicate no prevailing evidence of excess 40Ar. We plotted integrated ages on Figure 4 because the integrated age of an incremental step-heating analysis is the equivalent of a single fusion age, which is the method we used on the detrital samples from the ancient Suntrana type section strata and modern river sediment muscovite. Details of representative Alaska Range bedrock samples are outlined below.

A muscovite separate from the basement Birch Creek schist was analyzed, and the integrated age (104.3 ± 1.1 Ma) and the plateau age (104.8 ± 1.5 Ma) were within error (Table 1; Figs. 4 and 8; Fig. S1 [see footnote 1]). A muscovite separate from metasediment sample 10CH05B, collected at the headwaters of the Nenana River, was analyzed, and the integrated age (36.7 ± 0.3 Ma) and the plateau age (36.4 ± 0.2 Ma) were within error (Table 1; Figs. 4 and 8; Fig. S2). A muscovite separate from 18BAL, a granitoid sample along the north side of the Denali fault, was analyzed, and the integrated age (23.7 ± 0.3 Ma), plateau age (23.7 ± 0.2 Ma), and isochron age (23.5 ± 0.2 Ma) were all within error (Table 1; Figs. 4 and 8; Fig. S1). A muscovite separate from 45RAP, an orthogneiss sample along the south side of the Denali fault, was analyzed, and the integrated age (31.5 ± 0.1 Ma) and the plateau age (31.6 ± 0.1 Ma) were within error (Table 1; Figs. 4 and 8; Fig. S1).

Overall Detrital Muscovite 40Ar/39Ar Geochronology

Not all of the detrital single-grain fusion technique muscovite grains that were analyzed produced useable results. We applied a 20% error filter to eliminate muscovite grains with (1) low precision, (2) high atmospheric 40Ar content, which is associated with alteration and leads to high % error, and (3) detrital grains that may not have been muscovite grains (>1 Ca/K), which can also lead to high % error. Each sample yielded between two (Cantwell Basin strata) and 92 single-grain muscovite ages. For the middle Suntrana Formation, we ran a second aliquot based on the documented first presence of unique Alaska Range–sourced ca. 37 Ma muscovite grains in the Tanana Basin strata and thus dated 163 middle Suntrana Formation muscovite grains total. River locations are presented in Table S3 (see footnote 1). Modern river sediment and ancient strata muscovite single-grain fusion age determinations, probability density plots, and isotopic data are provided in Tables 2, S4–S6, Figures 9, S2, and S3.

We binned each muscovite grain age into subdivisions of 0–50 Ma (Alaska Range), 50–120 Ma (source area that is nonunique), 120–153 Ma (Yukon-Tanana Highlands), 153–201 Ma (Wrangellia composite terrane age and Stikine terrane potential source), and Triassic period (201–250 Ma) based on regional geochronology data sets and mapping, our new bedrock data set, and the modern river sediment age signatures. Four grains (251, 265, 269, and 550 Ma) from the combined analysis of all the strata samples fell outside of these age bins and were not assigned a source or used in population comparisons because of their limited presence.

Modern River Sediment Detrital Muscovite 40Ar/39Ar Geochronology

We used the modern river sediment detrital muscovite signatures to tune both our age bins and source signatures. Muscovite-dominant unique age populations from the Delta River, upper Tanana River, and lower Tanana River sites all exhibit primarily a Yukon-Tanana Highlands source (120–153 Ma), with limited Wrangellia composite terrane and Stikine terrane (153–201 Ma) input (Table 2; Figs. 1, 9A, 9B, and 9C). The Nenana Glacier outlet and Nenana River at the Cantwell Bridge sites both are dominated by an intra–Alaska Range source (0–50 Ma; Table 2; Figs. 1, 9D, and 9E). We recovered very few muscovite grains from the Nenana Glacier sample (10 successfully dated), reflecting the low fertility of muscovite in this predominantly granodiorite lithology watershed. There were no unique dominant age populations of muscovite from the Nenana River sample at the Tatlanika Bridge nor the sample site from the town of Nenana (Table 2; Figs. 1, 9F, and 9G). The dominant unique age populations of muscovite from the Yukon River samples collected at the Circle City townsite and the Dalton Highway Bridge were derived from either the Wrangellia composite terrane and/or Stikine terrane (153–201 Ma) and Yukon-Tanana Highlands sources (120–153 Ma; Table 2; Figs. 1, 9H, and 9I).

Tanana Basin Strata Detrital Muscovite 40Ar/39Ar Geochronology

The detrital muscovite age populations of the Tanana Basin strata (Table 2; Fig. 9) generally matched all the components present in the modern river sediment samples (Table 2; Fig. 10), but the individual age percentages varied greatly between the individual formations (Figs. S2 and S3 [see footnote 1]). We did not have any detrital muscovite data from rivers draining Wrangellia composite terrane watersheds. During the deposition of the earliest Miocene lower Suntrana Formation, the dominant unique age population of muscovite showed a Yukon-Tanana Highlands source age signature (21%; 120–153 Ma), with limited Wrangellia composite terrane and Stikine terrane–aged muscovite present (3%; 153–201 Ma; Table 2; Figs. 7 and 10A). During the deposition of the early Miocene middle Suntrana Formation, the dominant unique age population of the muscovite showed a Wrangellia composite terrane and Stikine terrane source age signature (31%; 153–201 Ma) and an additional population unique to the Alaska Range (4%; 0–50 Ma) with limited Yukon-Tanana Highlands source–aged muscovite (12%; 120–153 Ma; Table 2; Figs. 7 and 10B).

The middle Miocene upper Suntrana Formation has a large (compared to all the other strata samples) contribution of Triassic-aged muscovite grains (14%; 201–250 Ma) and a single grain (1%) from an age population unique to the Alaska Range (0–50 Ma; Table 2; Figs. 1 and 10C). The Yukon-Tanana Highlands source age signature is still limited (13%; 120–153 Ma). The presence of Wrangellia composite terrane and Stikine terrane–aged muscovite is still higher (22%; 153–201 Ma) compared to the older lower Suntrana Formation (3%; 153–201 Ma). During the deposition of the late Miocene Lignite Creek Formation, the dominant unique age population of muscovite has a Wrangellia composite terrane and Stikine terrane source age signature (31%; 153–201 Ma) and an additional population unique to the Alaska Range (4%; 0–50 Ma), with limited Yukon-Tanana Highlands source–aged muscovite (12%; 120–153 Ma; Table 2; Figs. 1 and 10D).

During the deposition of the latest Miocene Grubstake Formation, there are unique age populations of muscovite with Wrangellia composite terrane and Stikine terrane source ages (26%; 153–201 Ma) and Yukon-Tanana Highlands source–aged muscovite (27%; 120–153 Ma), with an additional population unique to the Alaska Range (2%; 0–50 Ma; Table 2; Figs. 1 and 10E). We did not date >50 muscovite grains for the Nenana Gravel samples, and so our age populations may not be fully reflective of the age fractions present in these strata (Dodson et al., 1988; Vermeesch, 2004). However, the number of grains needed per sample is dependent on the source and sink detrital geochronology complexity (Vermeesch, 2004). Given both the limited number of muscovite age populations present in the modern river sediment and the ancient strata as a whole, dating 36 grains (lower Nenana Gravel sample) ensures a 95% certainty that no fraction greater than 14% was uncaptured from the muscovite detrital population present (Vermeesch, 2004). Dating 46 grains (upper Nenana Gravel sample) ensures a 95% certainty that no fraction greater than 11% was uncaptured from the muscovite detrital population present. During deposition of the Pliocene lower Nenana Gravel unit, the dominant unique age populations of the muscovite are Yukon-Tanana Highlands source–aged muscovite (28%; 120–153 Ma) and Wrangellia composite terrane and Stikine terrane source ages (16%; 153–201 Ma; Table 2; Figs. 1 and 10F). During the deposition of the Pliocene to Quaternary upper Nenana Gravel unit, the dominant unique age populations of muscovite are Yukon-Tanana Highlands source–aged muscovite (17%; 120–153 Ma) and Wrangellia composite terrane and Stikine terrane source age signatures (11%; 153–201 Ma; Table 2; Figs. 1 and 10G).

Cantwell Basin Strata

The single sample from the ca. 70 Ma Cantwell Basin strata yielded two grains with ages of ca. 138 and ca. 135 Ma (Table S6 [see footnote 1]), similar to grain ages from the Yukon-Tanana Highlands. Given the size (<∼250 µm) and limited presence of muscovite in the Cantwell Basin strata observed, it is hard to interpret this limited data set. We present it as possible supporting evidence that flow direction was from the north during the Late Cretaceous (Ridgway et al., 1997) and suggest that further detrital muscovite work on the Mesozoic basins of the Alaska Range suture zone may provide fruitful results.

DISCUSSION

Alaska Range Bedrock Muscovite 40Ar/39Ar Geochronology

The 40Ar/39Ar muscovite age for the mica-rich Birch Creek schist underlying the Suntrana type section is ca. 105 Ma (Figs. 4 and 8). All the examined strata of the Tanana Basin have muscovite age populations overlapping this age. The age of the Birch Creek schist falls into the nonunique age bin of 50–120 Ma, which is present in all our modern river sediments, except the Nenana Glacier outlet sample. These results make it difficult to discern if local schist units outcropping as either topographic highs or the walls of river canyons were a contributing sediment source to the ancient Tanana Basin strata.

As previous researchers have noted, there is a distinct difference in eastern Alaska Range muscovite 40Ar/39Ar cooling ages across the Denali fault (Benowitz et al., 2011). Muscovite 40Ar/39Ar cooling ages to the south of the Denali fault are ca. 30 Ma to ca. 50 Ma. Muscovite 40Ar/39Ar cooling ages to the north of the Denali fault are ca. 15 Ma to ca. 30 Ma, with one metasedimentary bedrock sample north of the Denali fault having an age of ca. 37 Ma (Table 1; Figs. 4 and 11). We interpret this muscovite age to reflect formation, resetting, or recrystallization of mica during the emplacement of the ca. 40 Ma suite of plutons along the Nenana Glacier. Metamorphic and thermal aureoles of large plutons can be over ∼2 km wide (Annen, 2017). The presence of these uniquely sourced muscovites in the Tanana Basin strata, where corresponding ca. 40 Ma zircons are not recorded (Brennan and Ridgway, 2015), can be explained by unroofing of the shallower metamorphic aureole of these plutons before the plutons themselves.

The break in muscovite 40Ar/39Ar cooling ages across the Denali fault reflects the documented history of deeper Neogene exhumation along the north side of the Denali fault in the Mount Hayes segment of the Alaska Range compared to the south side of the fault (Figs. 4 and 12; Benowitz et al., 2011, 2014). The population of muscovite cooling ages at ca. 25 Ma is in line with the initiation of flat-slab subduction of the Yakutat microplate at ca. 30 Ma (Brueseke et al., 2019), northwest convergence of southern Alaska across the Denali fault, and increased slip rates along the Denali fault since this time (Figs. 4 and 12; Benowitz et al., 2012b; Lease et al., 2016; Davis et al., 2017). An increase in Pacific plate–Alaska obliquity and convergence rate at ca. 25 Ma (Jicha et al., 2018) was also likely a factor driving increased rates of deformation along the Denali fault system. The east-to-west variation in 40Ar/39Ar muscovite cooling ages north of the Denali fault supports evidence of variable exhumation through time and space for the high peak region of the eastern Alaska Range (Benowitz et al., 2011, 2014). The muscovite 40Ar/39Ar cooling ages south of the Denali fault likely reflect the timing of regional pluton emplacement, metamorphism, and uplift (Fig. 4; e.g., Riccio et al., 2014).

Modern River Sediment Detrital Muscovite 40Ar/39Ar Geochronology Source Classification

The Nenana River at its headwaters (Nenana Glacier outlet sample) and at Cantwell (Table 2; Figs. 1 and 9) has clear intra–Alaska Range suture zone signatures. Downstream, this signal is drowned out by primary schist sources and possibly recycled Tanana Basin strata (Table 2; Figs. 1 and 9). The Delta River, upper Tanana River, and lower Tanana River samples have primarily Yukon-Tanana Highlands 40Ar/39Ar muscovite age signatures, which we refer to as north-sourced in the following discussion (Table 2; Figs. 1 and 9).

The Yukon River samples have dominantly Wrangellia and Stikine terrane 40Ar/39Ar muscovite age signatures (Table 2; Figs. 1 and 9). Logically, these 153–201 Ma–sourced muscovites did not come from south of or within the Alaska Range suture zone, but are more likely sourced from the Stikine terrane in Canada or local sources in the Yukon-Tanana composite terrane (Jones et al., 2017). Hence, 153–201 Ma 40Ar/39Ar muscovite ages are not in themselves unique to south or north of the Alaska Range suture zone, nor are the U-Pb zircon ages (Brennan and Ridgway, 2015). Changes in the proportion of Yukon-Tanana Highlands 40Ar/39Ar muscovite ages and the presence of 40Ar/39Ar muscovite ages unique to the Alaska Range suture zone are needed to discern if the 153–201 Ma 40Ar/39Ar muscovite ages are likely south and intra–Alaska Range suture zone–sourced or north-sourced.

Nenana River Drainage Reconstruction

We did not sample the earliest Miocene Healy Creek Formation. Based on detrital U-Pb zircon geochronology and clast composition, Brennan and Ridgway (2015) concluded that the main source for sediment in the Tanana Basin during this time period was from the Yukon-Tanana composite terrane located to the north. This interpretation adds support to the Yukon-Tanana Highlands being a topographic high by this time period, if not earlier (Eocene; Dusel-Bacon et al., 2016).

Neither Brennan and Ridgway (2015) nor this study sampled the earliest Miocene Sanctuary Formation. Ridgway et al. (2007) interpreted the Sanctuary Formation as reflecting a regional early Miocene rapid subsidence event. Rapid subsidence is often driven by tectonic changes (Beaumont, 1981), and so the paleogeographic interpretation of Ridgway et al. (2007) is consistent with findings by other researchers (e.g., Lease et al., 2016) indicating that the Alaska Range was developing by early Miocene time.

During the deposition of the early Miocene lower Suntrana Formation, the paleo–Nenana River system likely derived sediment from north of the Alaska Range suture zone, based on the dominance of muscovite from the Yukon-Tanana Highlands to the north (21%; 120–153 Ma) and limited Wrangellia terrane–aged muscovite (3%; 153–201 Ma; Fig. 10A). This interpretation aligns well with the detrital U-Pb zircon geochronology and sedimentological conclusions of Brennan and Ridgway (2015), who stated that the main source for sediment for the Tanana Basin during this time period was from the north. The lack of potentially Stikine-aged muscovite (3%; 153–201 Ma) also adds support to the Yukon-Tanana Highlands being a topographic high by this time period, if not earlier (Eocene; Dusel-Bacon et al., 2016).

By the time of deposition of the early Miocene middle Suntrana Formation, sediment sources included significant mica from south of the Alaska Range suture zone and within the Alaska Range, based on: (1) the observed large increase in 153–201 Ma 40Ar/39Ar muscovite grains (3% to 31%) compared to the underlying lower Suntrana Formation, (2) a corresponding decrease in north-sourced Yukon-Tanana Highlands–affinity 40Ar/39Ar muscovite grains (21% to 12%) compared to the underlying lower Suntrana Formation, and (3) an additional population of ages unique to the Alaska Range suture zone (4%; 0–50 Ma; Table 2; Fig. 10B). Alternatively, the large increase in 153–201 Ma 40Ar/39Ar muscovite grains in the middle Suntrana Formation may have been derived from the Yukon River watershed to the north. However, this is unlikely due to (1) the documented high topography between the Yukon River and the Tanana River system (Dusel-Bacon et al., 2016), (2) the limited number of grains (<1%) in this age range present in the modern Tanana River samples (Table 2), (3) the decrease in Yukon-Tanana Highlands–affinity 40Ar/39Ar muscovite grains compared to the lower Suntrana Formation, and (4) the presence of grains that only have a known source from within the Alaska Range.

One factor of note, Brennan and Ridgway (2015) did not sample the middle Suntrana Formation, limiting comparisons between the detrital geochronology methods applied here. Ridgway et al. (2007) interpreted an increase in volcanic and plutonic/greenstone clasts in the Suntrana Formation compared to the Healy Creek Formation as reflecting a contribution of sediment from the south. Brennan and Ridgway (2015) documented a change in provenance between the lowermost Suntrana Formation and the upper Suntrana Formation, which supports our interpretation that by the time of middle Suntrana Formation deposition, there was a change in sediment source from north-dominated to south and intra–Alaska Range sourced.

The middle Miocene upper Suntrana Formation shows a continued dominant presence of 153–201 Ma 40Ar/39Ar muscovite grains compared to north-sourced 40Ar/39Ar muscovite grains (22% to 13%), with an additional limited population unique to the Alaska Range (1%; 0–50 Ma; Table 2; Fig. 10C). The middle Miocene upper Suntrana Formation also has a large contribution of Triassic 40Ar/39Ar muscovite grains (14%; 201–250 Ma) compared to the other strata samples analyzed. This indicates a likely variable history of rock uplift in the Alaska Range, which has also been documented through bedrock thermochronology studies (e.g., Benowitz et al., 2014; Lease et al., 2016; this study). This is also the stratum and time period in which Brennan and Ridgway (2017) observed a change in sediment provenance, supporting additional drainage reorganization at this time.

The late Miocene Lignite Formation has a continued dominant presence of 153–201 Ma 40Ar/39Ar muscovite grains compared to north-sourced 40Ar/39Ar muscovite grains (29% to 14%), with an additional population unique to the Alaska Range (4%; 0–50 Ma; Table 2; Fig. 10D). During this time period, Brennan and Ridgway (2015) inferred that the focus of exhumation in the Alaska Range varied and included sediment sourced from the exhuming Cantwell Basin (Figs. 5 and 6).

The latest Miocene Grubstake Formation has the reappearance of a high percentage of 40Ar/39Ar muscovite grains sourced from north of the Alaska Range (26%), with the addition of a contribution of grains sourced from south of the Alaska Range (27%) and a population unique to the Alaska Range (2%; 0–50 Ma; Table 2; Fig. 10E). We interpret this to indicate that by this time, the Healy Creek Formation strata located to the south and rich in clasts from Yukon-Tanana composite terrane–derived schist (Ridgway et al., 2007) were being exhumed and recycled, based on the increase of Yukon-Tanana Highlands 40Ar/39Ar muscovite grains in the Grubstake Formation strata. Lower Suntrana Formation strata to the south were also potentially being recycled by this time, based on our 40Ar/39Ar muscovite results, demonstrating that this stratum had a primarily north-derived muscovite age signature.

Brennan and Ridgway (2015) did not sample the Grubstake Formation, limiting comparisons between methods applied, but our interpretation of a latest Miocene inversion of the Tanana Basin to the south providing recycled sediment is consistent with theirs for the occurrence of recycling during deposition of the Pliocene Nenana Gravel unit. A clast petrology and provenance study of the Usibelli Group and Nenana Gravel also supports recycling of sedimentary basins to the south of this depositional zone during the latest Miocene to Pliocene, based on a dramatic increase in the presence of sandstone clasts in the Grubstake Formation compared to lower down in the section (Ridgway et al., 1999).

During the deposition of the Pliocene lower Nenana Gravel unit, the dominant age population is north-sourced compared to 153–201 Ma 40Ar/39Ar muscovite grains (28% to 16%). During the deposition of the Pliocene upper Nenana Gravel unit, the dominant age population is north-sourced compared to 153–201 Ma 40Ar/39Ar muscovite grains (17% to 11%). Both these Nenana Gravel samples produced limited 40Ar/39Ar muscovite ages, but the detrital U-Pb zircon study of Brennan and Ridgway (2015) provided addition support for our interpretation. Brennan and Ridgway (2015), as we do, inferred that this was a time of continued inversion and recycling of the Healy Creek and lower Suntrana Formation strata to the south.

The modern Nenana River near Cantwell has an 87% contribution of muscovite grains from the Alaska Range suture zone and no 153–201 Ma 40Ar/39Ar muscovite grains. The lack of Wrangell composite terrane–aged muscovite grains in the modern Nenana River implies that bedrock south of the Alaska Range is no longer a sediment source for the Tanana Basin. Based on this observation, we conclude that there was continued drainage reorganization after the deposition of the Nenana Gravel unit, likely driven by slip on the Broad Pass fault or other possible inferred thrusts similar to the Susitna thrust, which were active during the Quaternary (Fig. 1; Riccio et al., 2014; Haeussler et al., 2017b).

A compilation of apatite fission-track data from 17 igneous clasts collected from the Suntrana type section (Nenana Gravel unit) provides additional insight into the regional history of Alaska Range unroofing (Perry, 2014). Apatite fission-track data broadly represent the time a rock sample cooled through an ∼100 °C isotherm, which is inferred to represent a depth of ∼3 to ∼5 km (e.g., Beamud et al., 2011). Nine of the 17 measured apatite fission-track Nenana Gravel clast ages are ca. 20 Ma, implying this was a period of regional unroofing upstream in the Alaska Range. These thermochronology timing constraints fit well with our interpretation that the upper Nenana River reversed direction by ca. 18 Ma due to tectonic activity in the Alaska Range. The overall apatite fission-track clast data set of Perry (2014) also indicates a complex regional unroofing history that aligns well with our interpretation, i.e., that the Alaska Range has experienced a prolonged and complex topographic development history.

An examination of the sediment provenance of Cenozoic strata from Cook Inlet (Fig. 13) provides an independent test of the interpretation that there was an Oligocene river system draining into Cook Inlet that originated north of the Alaska Range. Finzel et al. (2015) concluded, based on detrital U-Pb zircon provenance analysis, that there was an Oligocene regional drainage system with headwaters north of the Alaska Range. They inferred that sediment derived from the Yukon-Tanana Highlands was transported across the Alaska Range and deposited in Cook Inlet until the Miocene. Combined zircon U-Pb geochronology and fission-track double-dating of Cenozoic strata in Cook Inlet (Fig. 13) also demonstrated that rivers likely crossed the Alaska Range and transported sediment derived from the Yukon-Tanana Highlands into Cook Inlet during pre-Miocene Cenozoic times (Finzel et al., 2016). The same studies make the case that by Miocene time, the Alaska Range was a topographic high affecting sediment routing. These results and interpretations (Finzel et al., 2015, 2016) are in general agreement with the Nenana River ancestral drainage reorganization findings of our study and that of Brennan and Ridgway (2015).

Given the complex nature of strike-slip fault sediment routing systems, where rivers can be sequentially lengthened and captured as water gaps are translated in and out of a drainage’s reference frame (Duvall and Tucker, 2015), and given that dominant sediment sources can vary with time as different regions of a transpressive mountain belt can experience focused exhumation with time (Dorsey and Roering, 2006; this study), it is difficult to infer a transpressive river’s drainage history from one outcrop location. Putting the documentation of the Tanana Basin change in sediment source—from north of the Alaska Range to south and within the Alaska Range during early Miocene times (Brennan and Ridgway, 2015; this study)—into a broader view of regional Neogene paleodrainage studies of southern Alaska can provide additional insight. Finzel’s et al. (2015, 2016) regional findings concluded that sediment in the pre-Miocene Cook Inlet Basin was derived from north of the Alaska Range, and during Miocene times, sediment derived from north of the range ceased to be a source. Additionally, given that the location of the headwaters of the Nenana River system has been generally pinned since ca. 18 Ma (this study), it is reasonable to infer that at least a segment of the upper ancestral Nenana River system experienced a drainage reversal in early Miocene times. Further work is needed to determine the full paleodrainage history of the Nenana River system.

The lag between the initial development of the Alaska Range (ca. 30 Ma to ca. 25 Ma; Benowitz et al., 2014; Lease et al., 2016) and the paleodrainage reversal of the Nenana River system (ca. 18 Ma) deserves further discussion. Both Benowitz et al. (2011, 2014) and Lease et al. (2016) documented potential asymmetrical topographic development in the Alaska Range. The drainage reorganizational response of transpressive mountain ranges along strike-slip fault systems likely is delayed as topography dynamically varies as crustal blocks are advected through geometric complexities along the strike-slip fault system (e.g., Burkett et al., 2016) and as the overall extent of the range expands (Lease et al., 2016). The process of rivers along strike-slip faults being sequentially lengthened and captured as water gaps are translated into and out of a drainage’s reference frame likely also plays a role (Duvall and Tucker, 2015) in the response time between initial increase in surface uplift rates and drainage reversals along transpressive mountain belts.

In summary, during earliest Miocene times, the ancestral Nenana River system flowed to the south into Cook Inlet as the Alaska Range was rising (Fig. 13A), but the Alaska Range was less extensive than today (Bill et al., 2018; Waldien et al., 2018). By the early Miocene (ca. 18 Ma), the upper Nenana River drainage system changed direction from south-directed to north-directed (Fig. 13B). This drainage change was likely due to regional topographic growth exceeding subsidence and/or incision rates in combination with drainage reorganization related to strike-slip translation of water gaps and topographic barriers. By the middle Miocene, the Nenana River drainage system continued to be reorganized as the Alaska Range rose asymmetrically along and across the Denali fault system (Benowitz et al., 2014; Lease et al., 2016). During the latest Miocene and Pliocene, the Healy Creek Formation and lower Suntrana Formation strata to the south of the Suntrana type section locale were likely uplifted, eroded, and recycled into the Tanana Basin. Continued drainage reorganization likely occurred after the inversion of the Tanana Basin at the Suntrana type section location.

Our conclusions do not support a Nenana River reversal at ca. 6 Ma (Wahrhaftig et al., 1969; Ridgway et al., 1999) nor the late Cenozoic timing of an ice stream–driven Nenana River drainage reversal (Moffit, 1915). Our results refine the interpretations of Brennan and Ridgway (2015) in which they inferred there was a change in dominant paleoflow direction from south-directed to north-directed by the time of deposition of the middle Miocene upper Suntrana Formation (ca. 15 Ma; Fig. 13).

The lack of ca. 40 Ma detrital U-Pb zircons in the Usibelli Group and Nenana Gravels (Brennan and Ridgway, 2015) and the presence of ca. 37 Ma 40Ar/39Ar detrital muscovite in many of these strata (this study) can be potentially explained by the regional pluton emplacement history during the Eocene in the Alaska Range. The ca. 37 Ma Nenana pluton metamorphic aureole (e.g., muscovite source) was likely unroofed before the deeper but related granitic pluton (e.g., possible zircon source). Hence, 40Ar/39Ar analyses of detrital muscovite, a mineral representing potentially shallower crustal levels, provide evidence of a change in unroofing patterns before detrital U-Pb zircon analyses in this case.

In this study, we demonstrated the detrital 40Ar/39Ar muscovite geochronology record of paleodrainage reorganization for a long-lived strike-slip fault in a transpressive orogen, reflecting the complex temporal-spatial history of topographic development that is characteristic of these types of mountain ranges. The detrital geochronology signature of paleodrainage reorganization during the development of a continental strike-slip fault transpressive orogen can be expected to vary throughout the life span of the mountain range due in part to (1) the aforementioned complex temporal-spatial topographic history of these mountain ranges and (2) the juxtaposition of diverse source terranes along and across a strike-slip fault–related mountain range.

CONCLUSION

The 17 new bedrock 40Ar/39Ar muscovite ages compiled here with 11 published 40Ar/39Ar muscovite ages support previous interpretations that the modern period of rapid Alaska Range uplift began by ca. 25 Ma (Benowitz et al., 2014; Lease et al., 2016). This new data set also supports these researchers’ interpretations that the focused region of Oligocene to present topographic development in the Alaska Range has varied through time and space due to its nature as a transpressive orogen.

The detrital 40Ar/39Ar muscovite signatures of seven Tanana Basin strata (607 single-grain fusion ages) and modern river sediment samples (575 single-grain fusion ages) track the Miocene to present history of paleodrainage reorganization of the Nenana River system. During the deposition of the earliest Miocene (ca. 20 Ma) lower Suntrana Formations, the paleo–Nenana River likely still flowed southward into Cook Inlet, with the Yukon-Tanana Highlands being a dominant source for sediment (Fig. 13A). During the deposition of the early Miocene (ca. 18 Ma) middle Suntrana Formation, sediment sources included significant mica from south of the Alaska Range, possibly from the Wrangellia composite terrane, along with a contribution of sediment unique to the Alaska Range suture zone (Fig. 13B). The middle Miocene upper Suntrana Formation had a large contribution of Triassic-aged muscovite grains, indicating continued drainage reorganization and a likely variable history of rock uplift in the Alaska Range, as also documented through bedrock thermochronology studies (e.g., Benowitz et al., 2014; Lease et al., 2016; this study). During latest Miocene and through Pliocene time, there is evidence of a large population of muscovite grains sourced from the Yukon-Tanana Highlands, implying sediment recycling as the southern extents of the Healy Creek and lower Suntrana Formations were uplifted and eroded. Drainage reorganization likely continued after the deposition of the Pliocene Nenana Gravel unit due in part to slip on the Broad Pass fault or other regional structures.

In conclusion, the Alaska Range was being unroofed by the early Miocene, the Nenana River drainage changed direction by this time period to northward-flowing, and drainage reorganization continued to near modern times driven by tectonic processes. The application of detrital 40Ar/39Ar muscovite geochronology to Alaska is a nascent domain but may add further constraints for paleodrainage reconstructions when combined with the more popular detrital U-Pb zircon geochronology approach. We recommend applying both detrital 40Ar/39Ar muscovite and U-Pb zircon geochronology to ancient strata and modern river sediment when attempting to reconstruct regional paleodrainage histories, with the addition of select bedrock dating when regional constraints are lacking. Our overall detrital muscovite work on modern river sediment and ancient strata of the Alaska Range demonstrates that strike-slip fault transpressive orogens can have complex paleodrainage reorganization histories.

ACKNOWLEDGMENTS

This work was partially funded by National Science Foundation (NSF) grant EAR-1249885 to Benowitz and NSF grant EAR-0952834 to Roeske and a University of Alaska Fairbanks Vice Chancellor of Research Office publication grant, and an Undergraduate Research & Scholarly Activity grant to Kailyn Davis. We thank Willis Hames, an anonymous reviewer, and Guest Associate Editor Jamie Jones for constructive and helpful reviews and suggestions that improved the manuscript.

1Supplemental Material. Isotopic data tables and figures and sample locations. Please visit https://doi.org/10.1130/GES01673.S1 or access the full-text article on www.gsapubs.org to view the Supplemental Material.
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