Alaska Department of Natural Resources

New geochemical, U-Pb geochronology, and Sr-Nd-Hf isotope data provide evidence for the tectonic evolution of the Seventymile terrane in interior Alaska, USA. Ultramafic and mafic rocks of the Seventymile terrane are thought to represent components of a dismembered ophiolite and provide unique constraints on regional terrane evolution and accretion. The Seventymile ophiolite represents fragments of the Devonian to Permian Slide Mountain Ocean (SMO) that separated allochthonous and parautochthonous continental fragments of western North America. It now occurs as multiple thrust sheets containing Permian mafic and ultramafic rocks overlying and/or possibly imbricated with amphibolite-facies supracrustal rocks of allochthonous Yukon-Tanana terrane and parautochthonous North America. Seventymile klippen contain variably serpentinized peridotite, primarily harzburgite, low-grade meta-mafic rocks, and minor oceanic sedimentary rocks (argillite, chert, limestone, and metasandstone). Mafic rocks include gabbro to diabase, typically as dikes, veinlets, or rare massive stocks intruding peridotite. Mafic rocks also include greenstones of the Seventymile assemblage in klippen structurally underlying, and in shear zone contact with, ultramafic klippen. New trace element and radiogenic isotope data from mafic magmatic rocks associated with the Seventymile ultramafic bodies show evidence for a weakly subduction-modified mantle source, like the mantle source of normal mid-ocean-ridge basalt (N-MORB) or back-arc basin basalt (BABB). Seventymile assemblage greenstones are more heterogeneous. They range from N-MORB to enriched mid-ocean-ridge basalt (E-MORB) and ocean-island basalt (OIB), with a subset of samples indicative of continental arc affinity. These geochemistry results indicate that distinct tectonic environments are represented by at least two, and possibly three, lithological and structural units comprising the Seventymile terrane. Hf-Nd isotope systematics are consistent with a depleted MORB mantle (DMM)−like component that overlaps with Pacific MORB. Primary zircon is rare, but new in situ U-Pb data for gabbro and greenstone indicate ca. 274−272 Ma peak zircon and titanite crystallization. Scattered younger zircons define a ca. 255 Ma zircon peak and correspond to secondary crystallization associated with baddeleyite reaction of high-Si fluids during low-grade metamorphism. If Seventymile suites are contemporaneous, obduction associated with the closure of the SMO resulted in the stacking of ophiolitic packages representing distinct tectonomagmatic settings across the transition from pericontinental, to epicontinental, to distal ocean back-arc. Intrusions hosted in klippe of ultramafic rocks, plus the least subduction-modified greenstones underlying them, geologically and compositionally resemble Slide Mountain rocks of the Campbell Range formation in eastern Yukon and may provide a new piercing point across the Tintina fault.

Abstract. Sound geologic reasoning underpins detrital zircon (DZ) maximum depositional ages (MDAs) via the principle of inclusions, although interpreting in situ U–Pb date distributions requires many geologically, analytically, and statistically driven decisions. Existing research highlights strengths and challenges of various algorithm approaches to deriving MDAs from DZ dates, yet community consensus on best practices remains elusive. Here, we first address new laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology for five DZ samples from a ∼1 km thick section of mid-Cretaceous strata in Alaska's Colville foreland basin. Youthful DZ yields are extremely sparse, and the MDAs are n=1. LA-ICPMS and CA-ID-TIMS dates from the same grains (i.e., tandem dating) adhere to a uniform pattern: laser ablation dates are younger than paired isotope dilution dates, with in situ offsets ranging from −0.3 % to −6.4 %. Existing biostratigraphic constraints suggest a ∼110–94 Ma sedimentation window for the sampled section, but the CA-ID-TIMS MDAs reduce by ∼8.5 Myr the maximum geologic time recorded by the stratigraphy. A simple age–depth analysis incorporating the CA-ID-TIMS MDAs and correlation of a new CA-ID-TIMS tephra zircon age yields geologically reasonable minimum stratigraphic accumulation rates, but an LA-ICPMS-based interpretation would render an improbable and inaccurate chronostratigraphy. We then explore the new tandem data and two previously published Mesozoic tandem DZ datasets for their broader MDA research implications, focusing on tandem-date pair relations and youthful-population sampling densities rather than conducting the typical MDA algorithm output assessment. Percent-offset plots document impactful (∼2 %–3 % on average) and pervasive (∼87 %–100 % of pairs per study) young bias for the laser ablation dates, likely reflecting a complex combination of analytical dispersion, low-temperature Pb loss, and matrix effects, which are topics we review in detail. Deconvolving offset sources without elaborate geochronologic experiments is difficult, but our tandem-date analysis provides critical context, and follow-up CA-ID-TIMS can diminish or eliminate analytical, systematic, and geologic offset sources. We also (1) redefine the reference value for MDA accuracy as the crystallization age of the youngest analyzed DZ population in a sample and (2) reframe LA-ICPMS-based DZ MDA algorithm evaluations around validity – how capable are the metrics of accurately measuring what they are intended to measure? – rather than MDA benchmarking by existing age constraints. These new perspectives follow straightforward geochronologic and stratigraphic principles, and our synthesis intends to identify and clarify opportunities to further refine DZ MDA research.

Global climate change creates geologic hazard cascades as the cryosphere experiences warming. The rapid retreat of Barry Glacier, a tidewater glacier in Prince William Sound, Alaska, has destabilized the cliff walls adjacent to the fjord, including a large landslide, approximately 2-km-wide, 1-km-tall, and ∼500 Mm3 in volume. The Barry Arm landslide was first identified in 2019 but has since been noted in photographs dating back to the 1930s. Catastrophic failure of the landslide has the potential to generate a tsunami with life-threatening waves in nearby fjords, including the port town of Whittier, 60 km from the landslide. Since monitoring began in 2021, slow downslope movement with short periods of acceleration has been observed. In this study, we refine the observations of landslide acceleration and correlate these periods with meteorological observations to assess the potential for further acceleration and catastrophic failure. We use ground-based synthetic aperture radar data (GBInSAR) collected sub-hourly from a location across the Barry Arm fjord since May 2022 with a line of sight that captures ~90% of the downslope landslide vector movement to generate time series of the landslide’s three main kinematic elements (distinct regions of deformation). This time series shows landslide-wide motion from late August to early November 2022 (2 months) at rates of 20-80 mm/day, then again from late September to mid-October 2023 (1.5 months) at 10-20 mm/day. No landslide-wide motion was detected in 2024. The Cascade Glacier sits stratigraphically above and to the northwest of the landslide and has been identified as a potential source of water for the landslide system. Ice-penetrating radar data collected in 2024 show an over-deepened section of Cascade Glacier adjacent to the most active kinematic element of the landslide, the Kite, suggesting melt water might pool and subsequently seep into the Kite kinematic element. Two full meteorological stations, each with additional node stations, monitor weather near the landslide and provide 15-minute precipitation and temperature data. We combine a simple positive degree-day factor melt model with precipitation analysis to show that the timing of movement of the Kite is correlated with the effects of seepage into the landslide subsurface, which are primarily driven by snow and ice melt. Understanding links between landslide displacement and melting of snow and ice could potentially lead to the use of meteorological conditions or forecasts as an additional risk assessment tool for identifying when the hazard of failure could be most severe. Our study accompanies others’ analyses of the Barry Arm Landslide using lidar, satellite InSAR, seismic, and infrasound data and contributes to our limited but critical understanding of landslide hazards in Alaska.

Snow avalanches are a significant concern across much of Alaska, posing risks to public safety, transportation, and accessibility. Despite their profound effect on society and the natural environment, avalanches remain poorly understood in many parts of Alaska. Public awareness and availability of information to support hazard adaptation, mitigation, and preparedness efforts are limited, even in the state’s most densely populated areas. The development of comprehensive avalanche hazard information is constrained by sparse observations of weather and snowpack conditions, and limited data on avalanche frequency. As climate warming progresses, shifts in temperature and precipitation are expected to modify snowpack conditions and influence the frequency and magnitude of avalanches, potentially changing Alaska’s vulnerability to these hazards. Understanding current avalanche hazards is critical for anticipating future climate-driven avalanche activity and assessing its potential effects on society and landscape dynamics. This project uses advanced avalanche models, originally developed and calibrated in Switzerland, to identify potential release areas and simulate millions of avalanche scenarios using downscaled climate reanalysis data and projections. The result is large-scale avalanche hazard indication maps for southeastern Alaska, both for present and future climate.

Abstract Frozen debris lobes (FDLs) are slow-moving landslides in permafrost. FDL-A, the largest monitored FDL in the Brooks Range of Alaska, has steadily progressed downslope toward the Dalton Highway, which is the only road to the oil and gas fields of the North Slope. To avoid this encroaching landslide, the Dalton Highway was realigned farther downslope in 2018. The abandoned portion of the highway was left in place, providing a unique opportunity for a full-scale field experiment to monitor the impact of a landslide on an engineered structure. In 2020, we conducted a subsurface investigation, drilling and sampling the subsurface soils and installing geotechnical instrumentation within the abandoned highway embankment. Here, we present the integration of multiple datasets to provide a detailed description of the landslide–embankment collision. FDL-A is shearing within weathered bedrock ∼7.6 m below the embankment surface. It horizontally displaced a portion of the embankment ∼0.3 m as of November 1, 2023. Using the infinite slope approach and strength properties determined from laboratory testing, we estimate that FDL-A is impacting the highway embankment and underlying soils with at least 77.8 kN/m width shear force. As this force occurs approximately 4 m below the bottom of the embankment, we postulate that the presence of the abandoned Dalton Highway embankment does little to stop FDL-A’s downslope progression.

Spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), is the most destructive pest of mature spruce (Picea) in western North America. Recent outbreaks in Alaska and other western US states highlight the need for tools to protect Picea from D. rufipennis. The primary antiaggregation pheromone of D. rufipennis (3-methylcyclohex-2-en-1-one, MCH) and various combinations of potential repellents (1-octen-3-ol, exo-brevicomin, endo-brevicomin, ipsdienol, ipsenol, limonene, and verbenone) were tested for their ability to disrupt the response of D. rufipennis to attractant-baited multiple-funnel traps. Two assays were conducted on the Kenai Peninsula, Alaska, in June and July 2021. All treatments significantly reduced the mean number of D. rufipennis caught compared to the baited control. No other significant differences were observed among treatments. Informed by these and other data, tree protection studies were established in Lutz spruce, Picea × lutzii, on the Kenai Peninsula in 2022 and in Engelmann spruce, Pi. engelmannii, in the Uinta Mountains, Utah, in 2021. All experimental trees were baited with frontalin. Repellent treatments included MCH (SPLAT MCH, ISCA Inc., Riverside, CA, USA) and at least 1 additional repellent combination. In Alaska, all treatments significantly reduced colonization (strip attacks + mass attacks) and mortality of individually treated Pi. × lutzii and all Picea within 11.3-m radius of each treated Pi. × lutzii compared to the control. In Utah, all treatments except for SPLAT MCH + octenol significantly reduced colonization compared to the control. Only SPLAT MCH + Acer kairomone blend (AKB) and SPLAT MCH + octenol reduced Pi. engelmannii mortality compared to the control. SPLAT MCH + AKB and SPLAT MCH + acetophenone and green leaf volatiles (PLUS) were the most effective across both studies. The implications of these and other results to the development of an effective semiochemical repellent for D. rufipennis are discussed.