Yukon-Tanana Upland Research Articles

Indicator mineral analyses of stream-sediment samples using automated mineralogy and mineral chemistry: Applicability to exploration in covered terranes in eastern Alaska, USA

In the past two decades, significant research efforts have been devoted to porphyry copper indicator mineral (PCIM) identification and mineral chemistry to assist exploration. Such studies are important in the Yukon-Tanana upland region of eastern Alaska because well-established geochemical exploration techniques, such as the geochemistry of stream-sediment samples, are less effective in the search for mineral resources owing to cover materials that subdue geochemical signatures. The unglaciated Yukon-Tanana upland region is well-endowed with mineral resources, including porphyry Cu-Mo-Au deposits at Taurus, Bluff, Dennison, and Oreo. Using automated scanning electron microscope (SEM) techniques, numerous PCIMs have been identified in rocks and stream-sediment samples that would likely not have been observed using visual binocular microscope methods. Sulfide minerals such as chalcopyrite, chalcocite, covellite, bornite, molybdenite, and pyrrhotite were identified in rock samples, and more than half of the stream-sediment samples from drainages containing known porphyry mineral occurrences contain chalcopyrite with or without bornite. Many drainages without known occurrences also contain these minerals, which suggests potential for additional occurrences. Svanbergite, an aluminum phosphate sulfate (APS) mineral [SrAl3(PO4)(SO4)(OH)6], was identified in some of the altered or mineralized rock samples from Taurus, and in stream drainages containing porphyry occurrences and may represent the best indicator mineral for Taurus-like porphyry deposits. Apatite chemistry has great potential as a tool for assessing the presence of porphyry mineralization. Grains from rock and stream-sediment samples that contain high Cl (>∼0.4 wt%), Fe (>0.4 wt%), and Mn (>2000 ppm) clearly distinguish apatite from other sources (metamorphic, unmineralized igneous rocks). This study is one of the first to document new streamlined indicator mineral sample collection and processing methods and to combine automated SEM techniques and chemistry of minerals derived from both bedrock and surficial (stream) sediments as they apply to porphyry exploration.

Yedoma Cryostratigraphy of Recently Excavated Sections of the CRREL Permafrost Tunnel Near Fairbanks, Alaska

Recent excavation in the new CRREL Permafrost Tunnel in Fox, Alaska provides a unique opportunity to study properties of Yedoma — late Pleistocene ice- and organic-rich syngenetic permafrost. Yedoma has been described at numerous sites across Interior Alaska, mainly within the Yukon-Tanana upland. The most comprehensive data on the structure and properties of Yedoma in this area have been obtained in the CRREL Permafrost Tunnel near Fairbanks — one of the most accessible large-scale exposures of Yedoma permafrost on Earth, which became available to researchers in the mid-1960s. Expansion of the new ∼4-m-high and ∼4-m-wide linear excavations, started in 2011 and ongoing, exposes an additional 300 m of well-preserved Yedoma and provides access to sediments deposited over the past 40,000 years, which will allow us to quantify rates and patterns of formation of syngenetic permafrost, depositional history and biogeochemical characteristics of Yedoma, and its response to a warmer climate. In this paper, we present results of detailed cryostratigraphic studies in the Tunnel and adjacent area. Data from our study include ground-ice content, the stable water isotope composition of the variety of ground-ice bodies, and radiocarbon age dates. Based on cryostratigraphic mapping of the Tunnel and results of drilling above and inside the Tunnel, six main cryostratigraphic units have been distinguished: 1) active layer; 2) modern intermediate layer (ice-rich silt); 3) relatively ice-poor Yedoma silt reworked by thermal erosion and thermokarst during the Holocene; 4) ice-rich late Pleistocene Yedoma silt with large ice wedges; 5) relatively ice-poor fluvial gravel; and 6) ice-poor bedrock. Our studies reveal significant differences in cryostratigraphy of the new and old CRREL Permafrost Tunnel facilities. Original syngenetic permafrost in the new Tunnel has been better preserved and less affected by erosional events during the period of Yedoma formation, although numerous features (e.g., bodies of thermokarst-cave ice, thaw unconformities, buried gullies) indicate the original Yedoma silt in the recently excavated sections was also reworked to some extent by thermokarst and thermal erosion during the late Pleistocene and Holocene.

Tungsten skarn potential of the Yukon-Tanana Upland, eastern Alaska, USA—A mineral resource assessment

Tungsten (W) is used in a variety of industrial and technological applications and has been identified as a critical mineral for the United States, India, the European Union, and other countries. These countries rely on W imports mostly from China, which leaves them vulnerable to supply disruption. Consequently, the U.S. government has a current initiative to understand domestic resource potential. The eastern Alaska portion of the Yukon-Tanana Upland (YTU), is prospective for W skarn deposits, the major source of global W supply. The regional geology consists of juxtaposed Paleozoic lithotectonic packages that were reaccreted to North America in the Mesozoic. Multiple subsequent episodes of arc-related magmatism intruded the lithotectonic packages, accompanied by W skarn formation mostly associated with 100–90 Ma intrusions; major W skarn deposits in Canada are part of the same metallogenic event (e.g., Mactung, Cantung). In this paper, we present an assessment for undiscovered W skarn resources for parts of the lesser-explored western (Alaskan) portion of the YTU.We used GIS proximity analysis to map the intersection of pluton and carbonate-bearing rocks to define three permissive tracts for W skarn deposits. The permissive tracts were qualitatively assessed by mineral potential mapping using region-wide sediment geochemistry and mineral concentrate datasets. This analysis showed that much of the western YTU has high potential for undiscovered W skarn deposits, whereas the eastern and southern YTU had only isolated areas of medium to high potential. Historical production and the quality of the geochemistry data of the western YTU tract (ca. 9200 km2) permitted a quantitative assessment of undiscovered W resources. Probabilistic estimates by a panel of 20 experts predicted a 70% chance of one to three undiscovered W skarn deposits in the western YTU tract. The rationale for favorability employed by the expert panel included favorable lithology, previous production, clustering of previously mined deposits, W placers in the area, lack of recent exploration, pan concentrates containing W minerals, and W geochemical anomalies. Estimates were combined with a global grade and tonnage model for W skarns in a Monte Carlo simulation and provided a median estimate of undiscovered resources of 94 kt WO3. If the undiscovered W skarn deposits are located close to infrastructure (e.g., near Fairbanks, or close to roads and/or power grid), application of an economic filter indicates that the median total economically recoverable WO3 is 63 kt with a net present value (NPV) of $330 million USD (2008 dollars). Whereas if deposits are far from infrastructure, median recoverable WO3 is only 30 kt and the NPV is $44 million.Our models for contained WO3 resources and NPV estimates for the western YTU tract are considerably lower than the known resources in skarns in adjacent areas in Canada. Estimates for the western YTU are also lower than preliminary estimates for undiscovered W skarn deposits in areas of the western conterminous United States. We speculate that lower permeability and continuity of favorable carbonate rock horizons in the relatively higher-grade metamorphic country rocks in the Alaska portion of the YTU may explain some of the differences in prospectivity. More detailed geologic mapping, modern geochemistry, and geophysical surveys are needed to refine the resource potential of the whole YTU. Regardless, quantitative mineral resource assessment provides a useful tool for making first-order regional estimates of undiscovered resources, identifying target areas for new data acquisition, and guiding research on the fundamental controls of district-scale metallogenic endowments.

Hydrogeochemistry in the Yukon-Tanana upland region of east-central Alaska: Possible exploration tool for porphyry-style deposits

A hydrogeochemical study using high resolution ICP-MS was undertaken at the Taurus and other porphyry Cu–Mo(-Au) occurrences and Ag–Au–Cu (+/− Pb, Zn) occurrences with epithermal-style characteristics in the Yukon-Tanana upland region of eastern Alaska. Surface water samples were collected from 30 sites on creeks that drain known deposits and occurrences and surrounding presumably unmineralized areas. Water samples for the entire ~9 km length of McCord Creek, which drains the Taurus deposit, and those from streams draining the areas at and near the Bluff and Dennison porphyry occurrences have high conductivity values (492–1250 μS/cm) and consistently high concentrations of B (3–250 μg/L), Co (2.3–42 μg/L), Mn (339–4750 μg/L), Re (0.012–0.1 μg/L), and SO42− (>200 mg/L), all of which are well above the median value for this data set and significantly greater than concentrations in water samples from the unmineralized areas. These are the best pathfinder elements specifically for porphyry style deposits because most of them are not anomalous in waters near epithermal occurrences. Copper concentrations are high (up to 115 μg/L) in some low-pH water samples from McCord Creek and drainages around Bluff, and a few near neutral pH waters have high molybdenum (>1 μg/L), but neither element is consistently anomalous in close vicinity to the porphyry occurrences, possibly due to a metal-poor, sulfide-poor leached cap (average of ~50 m) that overlies supergene and hypogene mineralized zones and is the dominant rock at surface. High concentrations of Bi and/or As occur in many waters associated with mineralized areas, particularly the Bluff and Dennison occurrences. In general, the element associations related to porphyry deposits reflect the deposit mineralogy, as well as size of the footprint related to alteration and mineralization.

Can Siberian alder N-fixation offset N-loss after severe fire? Quantifying post-fire Siberian alder distribution, growth, and N-fixation in boreal Alaska.

Fire severity affects both ecosystem N-loss and post-fire N-balance. Climate change is altering the fire regime of interior Alaska, although the effects on Siberian alder (Alnus viridis ssp. fruticosa) annual N-fixation input (kg N ha-1 yr-1) and ecosystem N-balance are largely unknown. We established 263 study plots across two burn scars within the Yukon-Tanana Uplands ecoregion of interior Alaska. Siberian alder N-input was quantified by post-fire age, fire severity, and stand type. We modeled the components of Siberian alder N-input using environmental variables and fire severity within and across burn scars and estimated post-fire N-balance using N-loss (volatilized N) and N-gain [biological N-fixation and atmospheric deposition]. Mean nodule-level N-fixation rate was 70% higher 11-years post-fire (12.88 ± 1.18 μmol N g-1 hr-1) than 40-years post-fire (7.58 ± 0.59 μmol N g-1 hr-1). Structural equation modeling indicated that fire severity had a negative effect on Siberian alder density, but a positive effect on live nodule biomass (g nodule m-2 plant-1). Post-fire Siberian alder N-input was highest in 11-year old moderately burned deciduous stands (11.53 ± 0.22 kg N ha-1 yr-1), and lowest in 11-year old stands that converted from black spruce to deciduous dominance after severe fire (0.06 ± 0.003 kg N ha-1 yr-1). Over a 138-year fire return interval, N-gains in converted black spruce stands are estimated to offset 15% of volatilized N, whereas N-gains in burned deciduous stands likely exceed volatilized N by an order of magnitude. High Siberian alder density and nodule biomass drives N-input in burned deciduous stands, while low N-fixer density (including Siberian alder) limits N-input in high severity black spruce stands not underlain by permafrost. A severe fire regime that converts black spruce stands to deciduous dominance without alder recruitment may induce progressive N-losses which alter boreal forest ecosystem patterns and processes.

Cosmogenic10Be and36Cl geochronology of cryoplanation terraces in the Alaskan Yukon-Tanana Upland

ABSTRACTCryoplanation terraces are prominent but enigmatic landforms found in present and past periglacial environments. Geomorphologists have debated for more than a century over processes involved in the formation of these elevated, step-like, bedrock features. Presented here are the first numerical surface exposure ages and scarp retreat rates from cryoplanation terraces in the Yukon-Tanana Upland (YTU) in Alaska, part of unglaciated eastern Beringia, obtained from terrestrial cosmogenic nuclides (TCN) in surface boulders. Ages comprise six10Be TCN ages from two terrace treads near Eagle Summit and six36Cl ages from two treads on Mt. Fairplay. Based on these exposure ages, scarps at both locations were last actively eroding from 49 to 22.4 ka. Both locations exhibit time-transgressive development, particularly near scarp-tread junctions. Boulder exposure ages and distances between sampled boulder locations were used to estimate scarp retreat rates of 0.11 to 0.56 cm/yr. These numerical exposure ages presented here demonstrate that the cryoplanation terraces in the YTU are diachronous surfaces actively eroding during multiple cold intervals. With these results, hypotheses for cryoplanation terrace formation are discussed and evaluated for the YTU, including those based on geologic structure, nivation, and the influence of permafrost.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Origin of last-glacial loess in the western Yukon-Tanana Upland, central Alaska, USA

AbstractLoess is widespread over Alaska, and its accumulation has traditionally been associated with glacial periods. Surprisingly, loess deposits securely dated to the last glacial period are rare in Alaska, and paleowind reconstructions for this time period are limited to inferences from dune orientations. We report a rare occurrence of loess deposits dating to the last glacial period, ~19 ka to ~12 ka, in the Yukon-Tanana Upland. Loess in this area is very coarse grained (abundant coarse silt), with decreases in particle size moving south of the Yukon River, implying that the drainage basin of this river was the main source. Geochemical data show, however, that the Tanana River valley to the south is also a likely distal source. The occurrence of last-glacial loess with sources to both the south and north is explained by both regional, synoptic-scale winds from the northeast and opposing katabatic winds that could have developed from expanded glaciers in both the Brooks Range to the north and the Alaska Range to the south. Based on a comparison with recent climate modeling for the last glacial period, seasonality of dust transport may also have played a role in bringing about contributions from both northern and southern sources.

Apatite fission-track evidence for regional exhumation in the subtropical Eocene, block faulting, and localized fluid flow in east-central Alaska

The origin and antiquity of the subdued topography of the Yukon–Tanana Upland (YTU), the physiographic province between the Denali and Tintina faults, are unresolved questions in the geologic history of interior Alaska and adjacent Yukon. We present apatite fission-track (AFT) results for 33 samples from the 2300 km2 western Fortymile district in the YTU in Alaska and propose an exhumation model that is consistent with preservation of volcanic rocks in valleys that requires base level stability of several drainages since latest Cretaceous–Paleocene time. AFT thermochronology indicates widespread cooling below ∼110 °C at ∼56–47 Ma (early Eocene) and ∼44–36 Ma (middle Eocene). Samples with ∼33–27, ∼19, and ∼10 Ma AFT ages, obtained near a major northeast-trending fault zone, apparently reflect hydrothermal fluid flow. Uplift and erosion following ∼107 Ma magmatism exposed plutonic rocks to different extents in various crustal blocks by latest Cretaceous time. We interpret the Eocene AFT ages to suggest that higher elevations were eroded during the Paleogene subtropical climate of the subarctic, while base level remained essentially stable. Tertiary basins outboard of the YTU contain sediment that may account for the required >2 km of removed overburden that was not carried to the sea by the ancestral Yukon River system. We consider a climate driven explanation for the Eocene AFT ages to be most consistent with geologic constraints in concert with block faulting related to translation on the Denali and Tintina faults resulting from oblique subduction along the southern margin of Alaska.

Edaphic and microclimatic controls over permafrost response to fire in interior Alaska

Discontinuous permafrost in the North American boreal forest is strongly influenced by the effects of ecological succession on the accumulation of surface organic matter, making permafrost vulnerable to degradation resulting from fire disturbance. To assess factors affecting permafrost degradation after wildfire, we compared vegetation composition and soil properties between recently burned and unburned sites across three soil landscapes (rocky uplands, silty uplands, and sandy lowlands) situated within the Yukon Flats and Yukon-Tanana Uplands in interior Alaska. Mean annual air temperatures at our study sites from 2011 to 2012 were relatively cold (−5.5 ° C) and favorable to permafrost formation. Burning of mature evergreen forests with thick moss covers caused replacement by colonizing species in severely burned areas and recovery of pre-fire understory vegetation in moderately burned areas. Surface organic layer thickness strongly affected thermal regimes and thaw depths. On average, fire caused a five-fold decrease in mean surface organic layer thickness, a doubling of water storage in the active layer, a doubling of thaw depth, an increase in soil temperature at the surface (−0.6 to +2.1 ° C) and at 1 m depth (−1.7 to +0.4 ° C), and a two-fold increase in net soil heat input. Degradation of the upper permafrost occurred at all burned sites, but differences in soil texture and moisture among soil landscapes allowed permafrost to persist beneath the active layer in the silty uplands, whereas a talik of unknown depth developed in the rocky uplands and a thin talik developed in the sandy lowlands. A changing climate and fire regime would undoubtedly influence permafrost in the boreal forest, but the patterns of degradation or stabilization would vary considerably across the discontinuous permafrost zone due to differences in microclimate, successional patterns, and soil characteristics.

Thermochronology of the Yukon-Tanana Terrane, West-Central Yukon: Evidence for Jurassic Extension and Exhumation in the Northern Canadian Cordillera

Within the McQuesten area of the central Yukon region of the Canadian Cordillera, the >100-km-long Willow Lake fault juxtaposes two crustal domains: (i) on the northeast, the Reid Lakes complex, an unmetamorphosed and predominantly undeformed Devono-Mississippian assemblage of plutonic and volcanic rocks; and (ii) on the southwest, typical Yukon-Tanana terrane (YTT) with deformed greenschist to amphibolite facies metamorphosed pre–late Devonian to late Permian rocks. In this study we investigate the tectonic history of these two crustal domains and present sensitive high-resolution ion microprobe U-Pb ages that delineate middle to late Paleozoic arc magmatism and fit key units into the YTT’s regional lithotectonic framework. Notably, magmatic zircons possess ca. 193 Ma overgrowths, and patches of recrystallized zircon with low Th/U ratios occur in samples proximal to the fault. Mississippian (ca. 349−325 Ma) biotite and hornblende 40Ar/39Ar cooling ages of the Reid Lakes complex suggest cooling immediately following volcanism and batholith emplacement. Plutonic samples near the fault trace, exhibiting weak to high strain, yield younger, ca. 193−191 Ma biotite 40Ar/39Ar ages. Zircon (U-Th)/He ages from the magmatic complex reflect ca. 216−214 Ma cooling through the uppermost crust of structurally higher volcanogenic rocks, followed by ca. 159–130 Ma cooling of the deeper-seated plutonic phase. Across the Willow Lake fault, YTT metamorphic rocks yielded ca. 197−165 Ma biotite and hornblende 40Ar/39Ar ages and ca. 113−90 Ma zircon (U-Th)/He ages. The marked step in cooling ages across the fault and the juxtaposition of the deformed rocks against undeformed rocks strongly indicates the Willow Lake fault is an extensional structure with top-down-to-the-northeast displacement, which exhumed the middle crust commencing in the early Jurassic. Aishihik suite magmatic rocks of the Stikinia/Quesnellia arc in the McQuesten area yielded a zircon age of ca. 205 Ma and 205−196 Ma40Ar/39Ar cooling ages. These ages and the presence of magmatic epidote suggest that the Aishihik suite cooled rapidly and was exhumed from midcrustal levels, further illustrating the significance of the Jurassic event that exhumed the central belt of YTT rocks. A similar tectonothermal evolution in the nearby Stewart River area, along strike to the northwest from McQuesten, was attributed to transtensional exhumation and cooling during slab rollback, leading to unroofing of the outboard regions of the terrane. We suggest that early Jurassic extensional exhumation of the YTT domain was mechanically linked to the Teslin fault system and more widespread that previously considered, likely accommodating crustal thinning from the Yukon-Tanana upland of eastern Alaska to the Glenlyon region of central Yukon. Parts of this system may have been episodically reactivated and overprinted, perhaps even as part of the Eocene Tintina fault, masking the early history.

Recovery and Trends of Peregrine Falcons Breeding in the Yukon-Tanana Uplands, East-Central Alaska, 1995–2003

Abstract We conducted intensive helicopter-supported surveys to assess the distribution, occupancy, and reproductive rate of Peregrine Falcons (Falco peregrinus anatum) in the Yukon-Tanana uplands of interior Alaska 1995–2003. We surveyed specific reaches of Birch Creek and the Goodpaster and Salcha rivers in this remote area to monitor this nesting species. Each year, we visited potential cliff-nesting areas and known nesting territories during early incubation (late May) to assess occupancy and nesting activities, and during the late-nestling period (mid-July) to count young, document nesting success, and measure reproductive rate. We identified 55 different nest cliffs occupied by Peregrine Falcons during at least one year of our survey. The number of occupied nesting territories detected each year increased from a low of 12 in 1995 to a high of 38 in 2002. Reproductive rate averaged 1.6 young/occupied territory and 2.4 young/successful territory for the nine-year period. The percentage of successful t...

Evidence for prolonged mid-Paleozoic plutonism and ages of crustal sources in east-central Alaska from SHRIMP U–Pb dating of syn-magmatic, inherited, and detrital zircon

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of igneous zircons from the Lake George assemblage in the eastern Yukon–Tanana Upland (Tanacross quadrangle) indicate both Late Devonian (∼370 Ma) and Early Mississippian (∼350 Ma) magmatic pulses. The zircons occur in four textural variants of granitic orthogneiss from a large area of muscovite–biotite augen gneiss. Granitic orthogneiss from the nearby Fiftymile batholith, which straddles the Alaska–Yukon border, yielded a similar range in zircon U–Pb ages, suggesting that both the Fiftymile batholith and the Tanacross orthogneiss body consist of multiple intrusions. We interpret the overall tectonic setting for the Late Devonian and Early Mississippian magmatism as an extending continental margin (broad back-arc region) inboard of a northeast-dipping (present coordinates) subduction zone. New SHRIMP U–Pb ages of inherited zircon cores in the Tanacross orthogneisses and of detrital zircons from quartzite from the Jarvis belt in the Alaska Range (Mount Hayes quadrangle) include major 2.0–1.7 Ga clusters and lesser 2.7–2.3 Ga clusters, with subordinate 3.2, 1.4, and 1.1 Ga clusters in some orthogneiss samples. For the most part, these inherited and core U–Pb ages match those of basement provinces of the western Canadian Shield and indicate widespread potential sources within western Laurentia for most grain populations; these ages also match the detrital zircon reference for the northern North American miogeocline and support a correlation between the two areas.

Cosmogenic exposure dating of late Pleistocene moraine stabilization in Alaska

Seventy-three new 1 0 Be/ 2 6 Al ages from 57 moraine boulders and 2 tors, together with 43 previously published cosmogenic exposure ages from 41 moraine boulders, allow us to critique the use of cosmogenic exposure (CE) dating of moraine boulders in Alaska. Boulder exhumation during moraine degradation likely gives rise to the largest uncertainty in constraining the timing of initial moraine stabilization following ice retreat. Isotopic inheritance appears to be most important for moraines deposited close to their cirque headwalls. Boulder-surface (bedrock) erosion rate can be roughly constrained and leads to a range in moraine stabilization ages. Snow-cover history is difficult to constrain, but its effect is thought to be minor for the tall boulders sampled. Despite these complications, the CE ages provide important new information regarding the timing of the last and penultimate glaciations in Alaska. Three penultimate moraines yielded CE ages that overlap with marine isotope stage (MIS) 4/early MIS 3 (45-65 ka) rather than MIS 6 (ca. 140 ka). Based on a combination of our new CE chronologies and existing 1 4 C ages from six study areas, glaciers retreated from their local late Wisconsin maxima: ca. 24-27 ka, Kokrines Hills (west-interior Alaska); ca. 24-26 ka, northeastern Brooks Range (NE Alaska); ca. 21-23 ka, Yukon Tanana Upland (east-interior Alaska); ca. 22 ka, Ahklun Mountains (SW Alaska); ca. 20 ka, western Alaska Range (central Alaska); ca. 16-18 ka, Chuilnuk Mountains (SW Alaska). Overall, glacier retreat was concurrent with the peak of the last global glacial maximum, probably in response to limited moisture availability.

U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleo-zoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376–353 Ma, 378–346 Ma, and 374–358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ± 3 and 362 ± 4 Ma) and associated orthoamphibolite (369 ± 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust and continental-margin arc rocks generated in thick continental crust. Given the absence in our study areas of intermediate-composition magmatic products generally found in most arcs, and the presence of bimodal magmatism, the alkalic within-plate chemistry of the mafic rocks and some of the felsic rocks, and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin, we consider it most likely that this prolonged Late Devonian to Early Mississippian magmatic episode resulted from attenuation of the ancient continental margin of western North America, rather than development of an arc, as proposed by many others.

Recurring middle Pleistocene outburst floods in east-central Alaska

Recurring glacial outburst floods from the Yukon-Tanana Upland are inferred from sediments exposed along the Yukon River near the mouth of Charley River in east-central Alaska. Deposits range from imbricate gravel and granules indicating flow locally extending up the Yukon valley, to more distal sediments consisting of at least 10 couplets of planar sands, granules, and climbing ripples with up-valley paleocurrent indicators overlain by massive silt. An interglacial organic silt, occurring within the sequence, indicates at least two flood events are associated with an earlier glaciation, and at least three flood events are associated with a later glaciation which postdates the organic silt. A minimum age for the floods is provided by a glass fission track age of 560,000 ± 80,000 yr on the GI tephra, which occurs 8 m above the flood beds. A maximum age of 780,000 yr for the floods is based on normal magnetic polarity of the sediments. These age constraints allow us to correlate the flood events to the early-middle Pleistocene. And further, the outburst floods indicate extensive glaciation of the Yukon-Tanana Upland during the early-middle Pleistocene, likely representing the most extensive Pleistocene glaciation of the area.

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon–Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215–212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188–186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ~135–115 Ma, and muscovite and biotite ~110–108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Apatite fission-track evidence of widespread Eocene heating and exhumation in the Yukon-Tanana Upland, interior Alaska

We present an apatite fission-track (AFT) study of five plutonic rocks and seven metamorphic rocks across 310 km of the Yukon–Tanana Upland in east-central Alaska. Samples yielding ~40 Ma AFT ages and mean confined track lengths > 14 µm with low standard deviations cooled rapidly from >120°C to <50°C during a 3–5 Ma period, beginning at about 40 Ma. Data from samples yielding AFT ages >40 Ma suggest partial annealing and, therefore, lower maximum temperatures (~90–105°C). A few samples with single-grain ages of ~20 Ma apparently remained above ~50°C after initial cooling. Although the present geothermal gradient in the western Yukon–Tanana Upland is ~32°C/km, it could have been as high as 45°C/km during a widespread Eocene intraplate magmatic episode. Prior to rapid exhumation, samples with ~40 Ma AFT ages were >3.8–2.7 km deep and samples with >50 Ma AFT ages were >3.3–2.0 km deep. We calculate a 440–320 m/Ma minimum rate for exhumation of all samples during rapid cooling. Our AFT data, and data from rocks north of Fairbanks and from the Eielson deep test hole, indicate up to 3 km of post-40 Ma vertical displacement along known and inferred northeast-trending high-angle faults. The predominance of 40–50 Ma AFT ages throughout the Yukon–Tanana Upland indicates that, prior to the post-40 Ma relative uplift along some northeast-trending faults, rapid regional cooling and exhumation closely followed the Eocene extensional magmatism. We propose that Eocene magmatism and exhumation were somehow related to plate movements that produced regional-scale oroclinal rotation, northward translation of outboard terranes, major dextral strike-slip faulting, and subduction of an oceanic spreading ridge along the southern margin of Alaska.

Apatite fission-track evidence of widespread Eocene heating and exhumation in the Yukon-Tanana Upland, interior Alaska

Apatite fission-track evidence of widespread Eocene heating and exhumation in the Yukon-Tanana Upland, interior Alaska

Last Interglacial age of the Eva Forest Bed, Central Alaska, from thermoluminescence dating of bracketing loess

The Eva Forest Bed in the Yukon-Tanana Upland of east-central Alaska represents a frozen, buried, boreal forest. This bed consists of well-preserved peat lenses, sticks, roots, logs, as well as rooted and unrooted stumps of trees. Where preserved, the Eva Forest Bed is overlain by the Goldstream Formation and underlain by Gold Hill Loess. Almost all exposures have a prominent unconformity at the top of the Gold Hill Loess, even where the forest bed is not preserved. Attempts have been made over the last 50 yr to determine the age of this bed numerically. We report here details of thermoluminescence (TL) dating of 14 samples of loess from above and below this unconformity in the Fairbanks area. Together with knowledge about the climatic indicators from the Eva Forest Bed, these TL age estimates: (1) indicate that the most probable age of the Eva Forest Bed is ∼125 ka, the time of the warmest part of oxygen isotope stage 5; and (2) confirm stratigraphic inferences from tephra beds in the upper Gold Hill Loess that erosion has removed variable thicknesses of the original loess complex “just prior” to the development of the Eva Forest Bed.