The Department of Geosciences is a result of the merger of the Geography and Geology & Geophysics Departments, effective July 2014.

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  • Effects of target properties on the formation of lunar impact craters in the simple-to-complex transition

    Chandnani, Mitali; Herrick, Robert; Kramer, Georgiana; Larsen, Jessica; Dehn, Jonathan (2019-12)
    The transition from simple to complex crater morphology in impact craters with increase in crater size has been modelled and observed in planetary bodies across the Solar System. The transition diameter depends upon the strength and gravity of the planetary body. On the Moon, this transition takes place over a diameter range of several kilometers. This range spans a diversity of crater morphologies including simple, transitional and complex craters. The diameter range of 15 20 km falls within the lunar simple-to-complex transition. All other impactor properties held constant, the 15-20 km range corresponds to a factor of three in the magnitude of impact kinetic energy. I conducted detailed geologic investigation of 244 well-preserved craters in this diameter range to elucidate the root causes of morphological variations. I used panchromatic data for observing crater and surface morphology, Digital Elevation Models (DEMs) for evaluating crater morphometry and topographic variation of pre-impact terrain, near-infrared (NIR) bands for determining the composition of crater cavity and surrounding terrain, thermal infrared bands for examining rock abundance, and Synthetic Aperture Radar (SAR) data for detecting impact melt deposits. The results of my investigation indicate that the morphological differences are primarily governed by target properties. Simple craters are confined to the highlands, and the mare are more abundant in complex craters. The mare are composed of solidified basaltic lava flows interlayered with regolith. The layering creates vertical strength heterogeneities that drive the destabilization of the transient cavity and its collapse, causing the transition to complex craters at smaller diameters in the mare. The non-layered highlands are more vertically homogeneous in strength and therefore favor simple crater formation. Eight atypically deep simple craters were identified in the highlands near the mare highlands boundaries, the most porous terrains on the lunar surface. After detailed examination of these craters in comparison to their normal-depth counterparts, I conclude that part of the energy from impact on porous target was spent in target compaction. The higher the porosity of the target, the deeper the crater and greater its volume, due to increased compaction. That only some of the craters in the high porosity terrains are deep suggests that those craters are on locally extreme-high porosity patches. However, an unusual impactor property, such as a high velocity impact, a high density impactor, or a near-vertical impact may also be a contributor. The simple craters in the highlands were observed to be located on flat or gradually sloping surfaces or degraded rims and terraces of pre-existing craters. Most craters with localized slumps superpose sharp topographic breaks such as well-developed rims and terraces of pre-existing craters. However, the topographic settings of 35% of the craters with localized slumps appeared to be similar to that of the simple craters. More detailed topographic study of the pre-impact terrains of these two morphologies revealed that the pre-impact terrains of 35% of the craters with localized slumps are gradually sloping or have subtle topographic breaks. Both sharp and subtle breaks are characterized with similar sloping directions as the adjacent craters' walls, which led to over steepening of the transient cavity walls around this part of the rim and their collapse, thereby causing the accumulation of localized slumped material. Several simple craters were also identified to have formed on pre-impact topographic breaks. However, the simple craters' walls that superpose these breaks were observed to be sloping in directions opposite to that of the breaks. So the ejecta around these walls was deposited along the break slopes, and thus syn-impact mass wasting occurred external (and not internal) to the crater cavity.
  • Constraining the H₂O/CO₂ molar ratio, the volume fraction of exsolved volatiles, and the magma compressibility of the 2006 Augustine eruption, Alaska

    Wasser, Valerie; Lopez, Taryn; Izbekov, Pavel; Larsen, Jessica; Anderson, Kyle; Freymueller, Jeffrey (2019-08)
    Geodetic modeling of volcano deformation can be used to estimate the volume of magma presumed to be mobilized within a volcanic system. These geodetically modeled subsurface reservoir volume changes are commonly much smaller than simultaneous eruptive volumes, where the eruptive volume is estimated based on geological mapping of units, their thicknesses, and their densities. This discrepancy is thought to be at least partially due to magma compressibility, which describes the phenomena where the volume of a given mass of magma changes as pressure increases/decreases primarily due to the presence of highly compressible exsolved volatiles. In this study, I combine deformation, volcanic gas, and petrologic constraints acquired prior to and during the 2006 eruption of Augustine volcano, Alaska, to estimate the amount of exsolved volatiles present in the magma storage region prior to the eruption and calculate the resulting compressibility of the magma. By doing so, I am able to constrain the H₂O/CO₂ molar ratio of the syn-eruptive gas emissions to between 24 and 59, with my best estimate of 28. My results suggest that for the specific parameters of Augustine's magmatic system, including a pressure of 120-170 MPa, a temperature of 880 ± 13 °C, and 40 ± 2% phenocrysts by volume, an exsolved volatile phase of about 8.2 vol% and a magma compressibility of ~7.1 x 10⁻¹⁰ 1/Pa are required to explain the observed eruptive volume to deformation volume ratio equal to three. The exsolved volatile volume and magma compressibility values determined here agree with results of previous studies of volatile-rich volcanic systems. This study reiterates that magma compressibility is an important factor that must be considered when interpreting deformation data within volatile-saturated volcanic systems.
  • Multi-sensor techniques for the measurement of post eruptive volcanic deformation and depositional features

    McAlpin, David B.; Meyer, Franz J.; Begét, James; Webley, Peter W.; Dehn, Jonathan (2019-08)
    Remote sensing of volcanic activity is an increasingly important tool for scientific investigation, hazard mitigation, and geophysical analysis. These studies were conducted to determine how combining remote sensing data in a multi-sensor analysis can improve our understanding of volcanic activity, depositional behavior, and the evolutionary history of past eruptive episodes. In a series of three studies, (1) optical photogrammetry and synthetic aperture radar are combined to determine volumes of lahars and lava dome growth at Redoubt Volcano, Alaska; (2) applied data from multiple synthetic aperture radar platforms are combined to model long-term deposition of pyroclastic flow deposits, including past deposits underlying current, observable pyroclastic flow deposits at Augustine Volcano, Alaska; and finally (3) combined, low-spatial-resolution thermal data from Advanced Very High Resolution Radiometer sensors are combined with high resolution digital elevation models derived from the microwave TanDEM-X mission, to increase the accuracy of eruption profiles and effusion rates at Tolbachik Volcano on the Kamchatka Peninsula, Russian Far East. As a result of this study, the very diverse capabilities of multiple remote sensing instruments were combined to improve the understanding of volcanic processes at three separate locations with recent eruptive activity, and to develop new methods of measurement and estimation by merging the capabilities of optical, thermal, and microwave observations. With the multi-sensor frameworks developed in this study now in place, future efforts should focus on increasing the diversity of sensor types in joint analyses, with the objective of obtaining better solutions to geophysical questions.
  • Paleobiology of ichthyosaurs: using osteohistology to test hypotheses of growth rates and metabolism in a clade of secondarily aquatic marine tetrapods

    Anderson, Katherine L.; Druckenmiller, Patrick; Erickson, Gregory; Horstmann, Lara; Fowell, Sarah (2019-08)
    Ichthyosaurians (Ichthyosauria) are one of the most prominent groups of secondarily aquatic Mesozoic marine reptiles. Over their 160 million years of evolution, the clade evolved a streamlined body plan with paddle-like limbs, convergent with modern cetaceans. Despite the fact that ichthyosaurians have been studied by paleontologists for over a century, very little is known about aspects of their biology, including quantification of their age structure and growth rates. Multiple lines of evidence, including oxygen isotope, swimming modality, and body shape analyses suggest that ichthyosaurians experienced elevated growth rates and likely maintained an elevated body temperature relative to ambient sea water. In this dissertation, I test these hypotheses using osteohistological methods. In the first manuscript, we describe new material of the small-bodied Upper Triassic ichthyosaurian Toretocnemus from the Nehenta Formation and the Hound Island Volcanics (both Norian, Upper Triassic) of Southeast Alaska. During the Upper Triassic, ichthyosaurians experienced their greatest size disparity, with large-bodied species rivaling the size of modern blue whales (Balaenoptera musculus; 20+ m body length) living alongside small-bodied species (1 m body length) like Toretocnemus. Prior to this study, Toretocnemus was known from Carnian deposits of California and possibly Sonora, Mexico. The referred material described here expands its geographic and temporal range. There are very few known ichthyosaurians from the Norian; thus, this material sheds light on the clade's diversity before the end Triassic extinction event. In the second and third manuscripts, we use osteohistological methods to describe the microstructure of various skeletal elements of two species of Stenopterygius from the Posidonia Shale (Lower Jurassic) of Germany. The Posidonia Shale is a Konservat-Lagerstätten that preserves over 3000 ichthyosaurian specimens, approximately 80 percent of which are referable to Stenopterygius. First, we sampled over 40 skeletal elements from one individual specimen referred to Stenopterygius quadriscissus to 1) describe the mineralized tissues across the skeleton, 2) infer relative growth rate, and 3) identify elements with growth marks. Almost all elements described demonstrate fibrolamellar primary bone, indicative of a rapid growth rate. We also identify growth marks in several elements, including the dentary and premaxilla, that will be used in future growth studies. In the third manuscript, we sample a scleral ossicle from Stenopterygius triscissus to describe its microstructure and investigate the use of ossicles for skeletochronology. The use of scleral ossicles for determining age structure has been documented in extant sea turtles as well as dinosaurs. We sectioned one ossicle in three planes and document conspicuous growth banding in the short axis section. Although this method requires further testing, we tentatively determine a minimum age of 7 years at the time of death for this individual. This dissertation lays critical groundwork for future studies of the paleobiology of ichthyosaurians. We are already in the preliminary stages of using these results to 1) quantify age structure and growth rates of an ichthyosaurian (Stenopterygius quadriscissus) for the first time, and 2) test the use of scleral ossicles for skeletochronology of ichthyosaurians. Through addressing these basic aspects of ichthyosaurian biology, we can begin to investigate how ichthyosaurian development and physiology changed over time and space and develop a greater understanding of this clade's 160 million years of evolution.
  • Description, phylogenetic analysis and taphonomy of a new Thalattosaur from the Brisbois member of the Vester Formation (Carnian/Norian) of central Oregon

    Metz, Eric T.; Druckenmiller, Patrick; Fowell, Sarah; Whalen, Michael (2019-05)
    Thalattosaurs are a poorly understood group of exclusively Triassic, secondarily aquatic tetrapods. Despite being first described over a century ago, their morphology and evolutionary history has been clouded by a spotty fossil record and poor specimen preservation. Here, a new thalattosaur genus is established on the basis of newly-discovered three dimensional cranial and postcranial elements from multiple individuals of different ontogenetic stages. The specimens were found within a single calcareous conglomerate nodule from the Brisbois Member of the Vester Formation in central Oregon, USA. The Brisbois Member thalattosaur is a relatively large taxon with an estimated total length of 4 - 5 m. Numerous cranial autapomorphies help diagnose the new taxon, including a ventrally deflected rostrum bearing a rugose ornamentation. The first three dimensionally preserved thalattosaur braincase anatomy is also described along with elements from nearly every region of the postcranium. Using high resolution surface laser scans, the first three-dimensional digital reconstruction of any thalattosaur skull is presented. Phylogenetic analysis indicates the Brisbois Member thalattosaur is a basal member of Thalattosauroidea, a clade that includes other taxa with highly modified rostra from Europe and China.
  • Metal and mineral zoning and ore paragenesis at the Kensington Au-Te deposit, SE Alaska

    Heinchon, Sarah H.; Newberry, Rainer; Severin, Kenneth; Keskinen, Mary (2019-05)
    The Kensington gold mine is a structurally controlled mesothermal gold deposit at the northern end of the Juneau Gold Belt. The Kensington is the only known gold deposit where over 90% of the gold occurs as calaverite (AuTe₂) rather than native gold (Au, Ag). Calaverite and native gold occur as microscopic inclusions in pyrite. Very little geochemical work had been performed on the ore at Kensington. This project generated a large geochemical data set of metals in sulfide concentrations of the ore. To better understand this unique gold deposit I collected and analyzed 214 sulfide concentrates by X-ray fluorescence. The concentrates were collected from various locations, vein types, and sulfide textures to examine possible correlations between the different sample types and between trace metals associated with sulfides and tellurides. Kensington sulfide concentrates have a consistent and strong correlation between Au and Te that indicates 90% of the gold occurs as calaverite. I surveyed 26 polished sulfide grain mounts. There are varying amounts of native gold and calaverite, depending on the presence of non-Au telluride minerals (petzite, hessite, coloradoite, altaite, tellurobismuthite, and melonite) and Cu sulfide minerals (chalcopyrite, tetrahedrite, and bornite). The corresponding sulfide concentrates still had a consistent Au to Te ratio despite the increased observed ratio of native gold to calaverite independent of Cu or non-Au telluride associated elements. The consistent Au to Te ratio indicates a single Au-Te event, however the variable native gold to calaverite ratios and variable inclusion textures in pyrite and Cu-sulfides suggest two Au-Te events: a pre/syn-pyrite and a separate post-pyrite Au-Te event. I used an electron microprobe to generate Co maps of pyrite with varying gold and/or calaverite inclusion textures. There are multiple pyrite growth phases with two distinct pyrites: Co-rich pyrite generations (> 2 weight % Co) and Co-poor pyrite generations (< 10 ppm). Co maps distinctly show multiple generations of native gold and calaverite inclusions relative to pyrite growth. The majority of non-Au tellurides and Cu-sulfide are post-pyrite. There are three main stages of metal mineralization in the Kensington ore: Early barren pyrite; Calaverite, minor native gold, and more pyrite; Cu-sulfides and non-Au tellurides.
  • Rock and age relationships within the Talkeetna forearc subduction complex in the Nelchina area, Southern Alaska

    Barefoot, John D.; Nadin, Elisabeth; Newberry, Rainer; Keskinen, Mary; McCarthy, Paul (2018-12)
    Subduction-zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. In south-central Alaska, a remarkably well-preserved exposure of subduction-related outcrops is located at the foot of Nelchina Glacier. Here, the crystalline basement of the Talkeetna volcanic arc is in contact with the mélange of its associated accretionary complex along the Border Ranges fault. A new zircon U-Pb age of an amphibolite from the Talkeetna arc mid-crustal basement just north of the fault is 188.9 ± 2.2 Ma, coincident with previously published dates from the mafic section of the arc. A new amphibole ⁴⁰Ar/³⁹Ar age from the same outcrop yields a plateau age of 182.6 ± 1.3 Ma, reflecting cooling/exhumation of this part of the arc. The mélange south of the arc and the Border Ranges fault, known as the McHugh Complex, comprises sheared metasedimentary rocks, metavolcanic rocks, and chert, and in the Nelchina area it includes a roughly 100-m-diameter block of pillow lavas that are undeformed but altered. Detailed compositional data show that the pillow lava block formed in an intraplate setting. New whole-rock ⁴⁰Ar/³⁹Ar analyses of two pillow-lava samples yielded irregular plateaus with an approximate age of 60 Ma, which we interpret to be largely reset due to reheating. Hypabyssal dikes crosscut the mélange, as well as younger accretionary prism deposits in the area, and provide a new zircon U-Pb age of 53.0 ± 0.9 Ma, which coincides with ages of near-trench plutonism across southern Alaska. This plutonism has been ascribed to subduction of a spreading ridge that migrated eastward along the southern Alaska margin. These new ages constrain the McHugh Complex formation and subsequent hydrothermal alteration to pre-55 Ma. We suggest that the pillow lava was originally part of a Triassic (or earlier) seamount that was decapitated and incorporated into the mélange as the oceanic plate entered the subduction zone. The pillow lava subsequently underwent extensive hydrothermal alteration that almost completely reset its age during the ridge subduction event. We further posit that the Talkeetna volcanic arc and its associated accretionary prism sediments were in their current configuration during the ca. 55 Ma plutonism that was common throughout southern Alaska.
  • Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: multiple topographic development drivers through time

    Terhune, Patrick J.; Benowitz, Jeffrey; Freymueller, Jeffrey; Gillis, Robert (2018-08)
    Intraplate mountain ranges can have polyphase topographic development histories reflecting diverse plate boundary conditions. We apply ⁴⁰Ar/³⁹Ar, apatite fission track (AFT) and apatite (U-Th)/He (AHe) geochronology-thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling and inferred exhumation patterns as proxies for the deformation history of this long-lived intraplate mountain range. High-temperature ⁴⁰Ar/³⁹Ar geochronology on muscovite, biotite and K-feldspar from Jurassic granitoids indicates post-emplacement (~158-125 Ma) cooling and Paleocene (~61 Ma) thermal resetting. ⁴⁰Ar/³⁹Ar whole rock volcanic ages and AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene-Eocene, suggesting that the Range is partially paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain Fault suggest ~2-3 km of vertical displacement that also contributed to mountain building, likely in response to the inboard progression of the subducted Yakutat microplate. Paleocene-Eocene volcanic and exhumation ages across interior southern Alaska north of the Border Ranges Fault System are similar and show no N-S or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that slab breakoff sub-parallel to the trench and subsequent mantle upwelling drove magmatism, exhumation and rock cooling synchronously across south-central Alaska and played a primary role in the development of the southern Talkeetna Mountains.
  • Gold and base metal mineralization of the Dolphin intrusion-related gold deposit, Fairbanks Mining District, Alaska

    Raymond, Luke M.; Newberry, Rainer; Larsen, Jessica; Keskinen, Mary (2018-08)
    The Dolphin deposit is an intrusion-related gold deposit (IRGD) located approximately 30 km north of Fairbanks, Alaska. The deposit is in--and adjacent to--a composite mid-Cretaceous pluton intruding amphibolite facies metamorphic rocks. An NI43-101 compliant gold resource estimation for the deposit (utilizing a 0.3 g/t cut-off grade) is 61.5 Million tonnes (Mt) at 0.69 g/t indicated (1.36 million oz = Moz) and 71.5 Mt at 0.69 g/t inferred (1.58 Moz). Due to extensive hydrothermal alteration of the intrusion, identifying rock types in hand sample and thin section, as well as by standard compositional techniques (e.g., SiO₂ vs. Na₂O + K₂O), has proven problematic. By plotting wt % TiO₂ vs. P₂O5 obtained from XRF analyses and four-acid digest ICP-MS data, two distinct population clusters appear. By comparison with least-altered intrusive rock analyses from the Fairbanks district, the igneous units were originally granite and tonalite. Because there is no gradational transition through an intermediate granodiorite unit, they were most likely derived from two separate magmatic bodies rather than in-situ fractionation from a single parent. Tonalite is concentrated along the northern and eastern margins of the stock with granite composing the rest of the body. Tonalite xenoliths in granite and granite dikes intruding tonalite prove that tonalite is the older unit. Investigations of hydrothermal alteration (based on chemical analyses, X-ray diffraction, and thin section examination) show albitic and advanced argillic (kaolinite-quartz) alteration are the dominant styles with sericite common throughout. Advanced argillic is a low temperature (<300°C) low pH alteration style that has not been previously identified in intrusion related gold deposits (IRGDs) in interior Alaska. Albitic alteration probably resulted from higher temperature, more neutral pH fluids. Gold investigations show that gold occurs as coarse-grained Au°, aurostibite, and maldonite in quartz + sulfide veins; fine-grained Au° in the oxide zone; and in many forms in disseminated sulfide. These forms include Au° inclusions in pyrite and arsenopyrite; solid-solution Au within compositionally zoned arsenopyrite; and as Au° nanoparticles in pyrite and probably arsenopyrite. Using UAF's JEOL JXA-8530F microprobe, I found that solid-solution gold occurs only in arsenopyrite with strong compositional zoning. Such grains are always small (< 0.2 mm) and commonly have low As cores; gold- bearing mantles with moderate % As; and higher As rims. In contrast, compositionally homogenous arsenopyrite does not contain detectable solid-solution gold. Pyrite is commonly arsenian and carries dissolved gold (if any) near detection limits. Gold mineralization has not been tied to any one lithology or alteration style; however, gold does seem to correlate with abrupt changes in alteration, especially between sericite + albite and kaolinite + sericite alteration. Gold-bearing, zoned arsenopyrite is predominantly associated with advanced argillic alteration and apparently represents a rapid growth, disequilibrium phenomenon.
  • Spatial and temporal variations in slip behavior beneath Alaska-Aleutian subduction zone

    Li, Shanshan; Freymueller, Jeffrey T.; Christensen, Douglas; Tape, Carl; West, Michael (2018-08)
    Characterizing spatial and temporal variations of slip behavior observed along subduction faults is of great significance for understanding the dynamics of subduction zones, features of great subduction zone earthquakes and deformation patterns across the subduction plate boundary through the seismic cycle. The Alaska-Aleutian subduction zone is one of the most tectonically active margins in the world. Great earthquakes and slow slip events recorded in this area are closely related in space. An increasingly dense array of Global Positioning System (GPS) receivers measures surface deformation at sites with high accuracy and provides a perfect tool for estimating the slip distribution on the plate boundary. GPS observations show that the motion of the Earth is not entirely linear: the long-term steady motion is interrupted by events like earthquakes, slow slip events (SSEs) and deformation of volcanoes, etc. Two long-term SSEs were detected in Lower Cook Inlet, Alaska (1992.0-2004.8 and 2009.85-2011.81) by inverting the slip distributions from GPS site velocities. The occurrence of SSEs based on the estimated slip distribution patterns provides strong evidence for the transition from stick-slip behavior to episodes and continuous aseismic creep on the subduction plate interface. Coulomb stressing rate changes (CSRC) due to the two detected long-term SSEs indicate that regions in the shallow slab (30-60 km) that experience significant increase in CSRC show an increase in seismicity rate during SSE periods. The modified quantitative rate/state stress transfer model suggests that the SSEs increase stress on surrounding faults, thereby increasing the seismicity rate even though the ratio of the SSE induced stressing rate to the background stressing rate is small. The SSEs were shown to cause significant stress changes in the seismogenic zone. This highlights the importance of exploring the relationship between SSEs and earthquakes, as well as how this relationship impacts the strain accumulation in the subduction zone. A repeat survey of the existing campaign GPS sites combined with continuous GPS sites provided a > 20 year time span for estimating the interseismic velocities of the Alaska Peninsula. From this I inferred a more precise model for the location and spatial extent of the change from locked to creeping behavior across the Alaska Peninsula. Given this more detailed distribution of the slip behavior, the results suggest that slip behavior correlates with the pre-existing plate fabric on the downgoing plate, seismic behavior, the reflection character of the slab interface itself and the rupture history of past great earthquakes.
  • History of the Chukchi borderland and the Amerasia basin, Arctic Ocean

    Ilhan, Ibrahim; Coakley, Bernard J.; Johnson, Christopher A.; Houseknecht, David W.; Whalen, Michael T. (2018-08)
    Structural and stratigraphic interpretation of 2D multi-channel seismic (MCS) reflection profiles through recognition of the sub-surface reflection patterns and integration of the seismic interpretation with the other geophysical and geological data reveal the history of the Chukchi Borderland. This investigation provides new constraints for the tectonic development of the Amerasia Basin. North-striking normal faults of the Chukchi Borderland dissect the continental basement into the Chukchi Plateau, Northwind Basin and Northwind Ridge from west to east. A well-developed angular unconformity (Au) separates the stratigraphic section into sub and super-Au seismic units. Sub-Au units include: (1) seaward dipping reflections (SDRs) observed in the juncture between the North Chukchi-Toll Basins and Chukchi Plateau; (2) growth and folded strata in the Northwind Basin; (3) thrust faults in the Northwind Basin and over the Northwind Ridge; and (4) a clinoform sequence that downlaps onto the extended continental crust of the Canada Basin, supported by presence of SDRs and diapiric reflections within the crust. Au is inferred to correlate to the Hauterivian (LCu) and the Middle Jurassic (Ju) unconformities of the Alaska North Slope. The SDRs indicate that the southwestern margin of the Chukchi Borderland may be a rifted continental margin. Loosely constrained age control of a super-Au unit (inferred condensed section, perhaps correlative to Hauterivian pebble shale or the Jurassic upper Kingak shale units of Alaska North Slope) implies that the rifted margin subsided no later than the earliest Cretaceous, providing a plausible time constraint for Middle Jurassic-earliest Cretaceous rifting in the North Chukchi Basin. The growth strata and north-striking normal faults of the Northwind Basin are continuous with the extensional structures of the Mississippian Hanna Trough, providing a geologic linkage between the two. The folding and thrust faults reveal a phase of contraction confined to sub-Au units of the south and eastern Northwind Basin and Northwind Ridge. The clinoform sequence of the Northwind Ridge-Canada Basin is inferred to correlate with the Upper Jurassic-Lower Cretaceous Kingak shale unit of Alaska North Slope, implying that the extension of the crust beneath the western Canada Basin occurred no later than the Middle Jurassic. Super-Au strata (~16 km) onlap the condensed section, SDRs, growth and passive margin strata from west to east, tapering down to a few kilometers north and eastward across the seismic grid. These are part of the Aptian through Cenozoic Brookian megasequence, a series of clinothems, deposited across the foreland of the Chukotka and Brooks Range orogens. These strata were deposited by northward-migrating depositional systems that progressively filled the North Chukchi Basin and buried the southern flank of the Chukchi Borderland, and deposited along the Northwind margin of the Canada Basin. Another unit of growth strata is observed in the Northwind Basin, indicating another phase of extension of the Boderland. The Upper Cretaceous section of the Brookian megasequence is displaced by normal faults over the Chukchi Plateau and inferred age-equivalent strata over the Northwind Ridge. These constrain the second phase of extension of the interior Borderland to the Late Cretaceous to Paleocene. The recognition of the sub-Au units and continuity of the super-Au units across the area, north-striking normal faults, and the absence of east-directed thrust faults between the Northwind Ridge and Canada Basin invalidate one model proposed for tectonic development of the Amerasia Basin. Models that require significant relative motion between the Chukchi Shelf and Borderland since the Middle Jurassic are precluded by these observations.
  • Earthquake source mechanisms and three-dimensional wavefield simulations in Alaska

    Silwal, Vipul; Tape, Carl; Christensen, Douglas; West, Michael; Ruppert, Natalia; Freymueller, Jeffrey (2018-08)
    This thesis presents: (1) a set of earthquake source mechanism catalogs for Alaska and (2) a threedimensional seismic velocity model of Alaska. The improved earthquake sources are used within the velocity model for generating synthetic seismograms, which are then compared with recorded seismograms to assess the quality of the velocity model. An earthquake source mechanism can be modeled as a moment tensor, which is a 3 × 3 symmetric matrix. We estimate the moment tensor for earthquakes by comparing observed waveforms (body waves and surface waves) with synthetic waveforms computed in a layered model. The improved moment tensor solutions are obtained by utilizing both the body waves and surface waves at as many broadband stations as possible. Further improvement in the inversion technique is obtained by (1) implementation of L1 norm in waveform misfit function and (2) inclusion of first-motion polarity misfit in the misfit function. We also demonstrate a probabilistic approach for quantifying the uncertainty in a moment tensor solution. Moment tensors can be used for understanding the tectonics of a region. In the Cook Inlet and Susitna region, west of Anchorage, we determined moment tensor solutions for small-tointermediate magnitude (M ≥ 2.5) crustal earthquakes. Analyzing these small earthquakes required us to modify the misfit function to include first-motion polarity measurements, in addition to waveform differences. The study was complemented with the probabilistic hypocenter estimation of large historical earthquakes (Mw ≥ 5.8) to assess their likelihood of origin as crustal, intraslab, or subduction interface. The predominance of thrust faulting mechanisms for crustal earthquakes indicate a compressive regime within the crust of south-central Alaska. Wavefield simulations are performed in three regions of Alaska: the southern Alaska region of subduction, the eastern Alaska region with the accreting Yakutat microplate, and the interior Alaska region containing predominantly strike-slip faulting, including the Minto Flats fault zone. Our three-dimensional seismic velocity model of Alaska is an interpolated body-wave arrival time model from a previous study, embedded with major sedimentary basins (Cook Inlet, Susitna, Nenana), and with a minimum shear wave velocity threshold of 1000 m/s. Our comparisons between data and synthetics quantify the misfit that arises from different parts of each model. Furtherwork is needed to comprehensively document the regions within each model that give rise to the observed misfit. This would be a step toward performing an iterative adjoint tomographic inversion in Alaska.
  • Mechanisms and implications of changes in the timing of ocean freeze-up

    Rolph, Rebecca J.; Mahoney, Andrew; Walsh, John; Eicken, Hajo; Winsor, Peter; Loring, Philip (2018-08)
    The shift to an Arctic seasonal sea ice cover in recent years motivates a deeper understanding of freeze-up processes and implications of a lengthened open water season. As the sea ice boundary between the Arctic ocean and atmosphere covers a smaller area, the effects of enhanced wind mixing become more pronounced. Winds are important for ocean circulation and heat exchange. Ultimately, they can influence when freeze-up can occur, or can break up new ice as it forms. The chapters of this thesis are motivated by the substantial social and geophysical consequences of a lengthening open water season and linked through discussion of what controls freeze-up timing. Implications of a declining sea ice cover as it pertains to the three Arctic Alaska coastal communities of Kotzebue, Shishmaref, and Utqiaġvik are explored in depth. Indices of locally-relevant metrics are developed by using physical climate-related thresholds found by other studies to impact Alaska communities and coastal erosion rates. This allows for a large-scale climate dataset to be used to define a timeseries of these indices for each community. We found a marked increase in the number of false freeze-ups and break-ups, the number of days too windy to hunt via subsistence boat, and in Utqiaġvik, an approximate tripling of erosion-capable wind events from 1979-2014. The WRF-downscaled ERA-Interim dataset (ERA-Interim for sea ice) was also used in the analysis of all chapters. The cumulative wind energy input into the upper ocean was calculated for the Chukchi, southern Beaufort, and northeast Bering Seas at time periods up to three months prior to freeze-up, and then correlated with the timing of freeze-up. We have found that increased wind energy input into the upper ocean 2-3 months prior to freeze-up is positively and most strongly correlated with the date of freeze-up in the Chukchi Sea. Analysis of wind climatology shows winds are increasing in the period prior to freeze-up as a delayed freeze-up moves into the fall storm season. A negative correlation is found in the Bering Sea over shorter timescales, suggesting that storms promote the arrival of sea ice there. Case studies are evaluated for the Chukchi Sea and Bering Sea, to illustrate mechanisms at play that cause the positive and negative correlations in these seas, respectively. Ice advection and high winds from northerly directions are shown to hasten the timing of freeze-up in the Bering Sea. In the Chukchi Sea, higher winds from the dominant northeasterly direction promote upwelling of warm and salty water up onto the shelf, which suggests a mechanism for why high winds are associated with a delayed freeze-up there. We next examine the effect of winds on freeze-up timing by using a 1-D vertical column model of the mixed layer. The model is initialized using temperature and salinity profiles obtained from a freeze-up buoy deployed in 2015 in the north-east part of the Chukchi Sea. The meteorological forcing used to drive the model experiments comes from a WRF-downscaled ERA-Interim Reanalysis dataset. Our results show that vertical wind-driven mixing leads to enhanced heat loss. In light of the previously found positive correlation between wind energy input and freeze-up timing, the mixing model results suggest horizontal advection not captured by the 1-D column model can dominate wind-driven vertical mixing to promote freeze-up.
  • Remote sensing of erosion and shallow water bathymetry to aid river navigation on the Colville River, Nuiqsut AK

    Payne, Cole S.; Panda, Santosh; Prakash, Anupma; Brinkman, Todd (2018-08)
    The Colville is the longest river (~600 km) in Arctic Alaska. Nuiqsut is an established Alaska Native community of ~400 people on the Colville River. Its residents rely heavily on the Colville for subsistence needs, however, changing river dynamics caused by accelerated bank erosion, river siltation, low water, and shifting and drying channels are causing concern and making boat travel increasingly difficult and dangerous. Recently, local residents have reported increased erosion at bluff sites along the Colville, which threatens existing infrastructure. Also reported are unexpected shallow water sections along the main channel of the Colville, limiting their access to subsistence food sources. Residents have expressed a need for monitoring erosional rates on the Colville as well as a map product that could aid in river navigation. These concerns shaped the main goals of this Thesis: 1) To use remote sensing techniques to map and quantify erosion rates and the volume of land loss at selected bluff sites along the main channel of the Colville, and to assess the suitability of automated methods of regional erosion monitoring. 2) To use optical satellite images for mapping river bathymetry and generate GIS map products that show potential shallow water sections (<2m) and poor channel connections, and to assess the feasibility of future monitoring based off our methods that rely on extracting relative water depth values from publicly available optical remote sensing images. For our erosional study we used orthomosaics from high resolution aerial photos acquired in 1955 and 1979/1982, as well as high resolution WorldView-2 images from 2015 to quantify long-term erosion rates and the cubic volume of erosion. We found that, at the selected sites, erosion rates averaged 1 to 3.5 m per year. The erosion rate remained the same at one site and increased from 1955 to 2015 at two of the four sites. We estimated the volume of land loss to be in the magnitude of 166,000 m³ to 2.5 million m³ at our largest site. We also found that estimates of erosion were comparable for manual hand-digitized and automated methods, suggesting our automated method was effective and can be extended to monitor erosion at other sites along river systems that are bordered by bluffs. For our bathymetry study we used summer 2017 scenes from three optical sensors (PlanetScope 3m, Sentinel 2 10m, and Landsat 30m) along with field measurements on the river to map shallow water bathymetry along a 45 km stretch of the Colville. We found a strong correlation (R²=0.89) between field-measured water depths and image-derived reflectance quantity (natural log ratio of green over red bands). We analyzed the two essential criteria for suitable bathymetry mapping from optical images: clear weather and clear water conditions. We expect several days (≈16) of suitable conditions during the ice-free season to facilitate reliable bathymetry mapping and remote monitoring of shallow water sites. We also discuss a relative depth mapping technique which is useful for boat navigation in the absence of ground truth measurements. We deliberately employed simple and robust empirical techniques that could serve as a basis for a fully developed river monitoring project in the near future led by local community residents. An implementation of our methods by the community, in order to develop a river depth monitoring program, would be an important step forward for the advancement of community-based science and the co-production of knowledge. Our technique may help address emerging environmental and societal issues in other regions where sufficient river navigation fosters local livelihoods.
  • Evidence for the presence of mesic herbaceous shrub tundra with isolated stands of trees in south central Beringia during glacial stages 2 - 16

    Morris, Emily P.; Fowell, Sarah; Bigelow, Nancy; Wooller, Matthew (2018-08)
    Palynological analysis of assemblages from the Integrated Ocean Drilling Program (IODP) Expedition 323, Bering Sea Expedition, site U1343, located in deep water adjacent to the Bering Sea shelf edge, permit reconstruction of the terrestrial vegetation of the southern margin of central Beringia. Previous research done by Rachel Westbrook on marine isotope stages (MIS) 1-6 indicates that the southern coast of central Beringia was a glacial refugium for boreal forest vegetation. This study extends and augments Westbrook's research by analyzing additional samples from IODP site U1343 spanning the last 258 - 615.3 kya, during glacial stages 8, 10, 12, 14, and 16. Grass (Poaceae) and sedge (Cyperaceae) pollen dominate the assemblages with small but persistent amounts of boreal forest taxa, such as alder (Alnus), birch and dwarf birch (Betula and Betula nana), and spruce (Picea). A set of modern surface samples forming a transect from the southwestern margin of the Bering Sea shelf to Nome, AK were obtained and analyzed as potential modern analogs for the palynofloral assemblages from site U1343. Low percentages of boreal forest taxa in nearshore samples from this Bering Sea shelf transect reflect the vegetation of the coastal Seward Peninsula and the Yukon/Kuskoskim delta, regions dominated by herbaceous tundra with isolated stands of trees. Comparison of modern and fossil assemblages via canonical community ordination indicates that the IODP samples from site U1343 and modern samples from the Bering Sea shelf are most similar to surface samples from lakes and bogs surrounded by moist herbaceous/shrub tundra. These data suggest that boreal forest taxa persisted throughout MIS 8, 10, 12, 14, and 16 on the southcentral Beringian coast, where the vegetation was primarily moist herbaceous/shrub tundra with intermittent stands of trees.
  • Shear wave splitting and mantle flow in Alaska

    McPherson, Amanda M.; Christensen, Douglas; Tape, Carl; Freymueller, Jeffrey (2018-08)
    We explore the nature of mantle anisotropy and flow under Alaska by presenting 2389 SKS shear wave splitting observations from 547 earthquakes recorded at 384 broadband stations deployed in Alaska since 2010. We expand upon the results of Seismic Anisotropy under central Alaska from SKS splitting observations by Christensen and Abers (2010) and Insights into mantle structure and flow beneath Alaska based on a decade of observations of shear wave splitting by Perttu, Christensen, Abers, and Song (2014) to better understand the effect of flat slab subduction on mantle anisotropy and flow. Shear wave splitting is a common tool to investigate anisotropy in the upper mantle, which is often assumed to be caused by mantle flow or preexisting fabrics in the lithosphere. In Alaska, splitting appears to be controlled by the absolute plate motion of the North American and Pacific plates, and the interaction between the two plates. In particular, the subducting Pacific plate acts as a barrier to flow. Directly north of the slab, fast directions are oriented along the strike of the slab with large δts, and are caused by along strike flow in the mantle wedge. Stations further to the north, outside of the influence of the mantle wedge, gradually see fast directions parallel to the absolute plate motion direction of the North American plate. South of the slab, fast directions depend on the geometry of the subducting plate. South of the Alaska Peninsula, the fast directions are parallel to the trench, which represent along strike flow under the Pacific plate. To the east, however, flat slab subduction dominates. Here the fast directions are perpendicular to the trench (parallel to the absolute motion of the Pacific plate) and are indicative of entrained flow from the motion of the Pacific plate. Fast directions near the Fairweather- Queen Charlotte transform system are parallel to the faults, and are likely caused by the deformation associated with large lithospheric blocks moving past each other. The region between the inferred east end of the Pacific plate and the transform boundary is dominated by the collision and accretion of the Yakutat terrane. The tectonics of this region are still in debate and the fast directions are difficult to interpret.
  • Ionospheric correction of interferometric SAR data with application to the cryospheric sciences

    Liao, Heming; Meyer, Franz J.; Freymueller, Jeffrey T.; Tape, Carl; Watkins, Brenton (2018-08)
    The ionosphere has been identified as an important error source for spaceborne Synthetic Aperture Radar (SAR) data and SAR Interferometry (InSAR), especially for low frequency SAR missions, operating, e.g., at L-band or P-band. Developing effective algorithms for the correction of ionospheric effects is still a developing and active topic of remote sensing research. The focus of this thesis is to develop robust and accurate techniques for ionospheric correction of SAR and InSAR data and evaluate the benefit of these techniques for cryospheric research fields such as glacier ice velocity tracking and permafrost deformation monitoring. As both topics are mostly concerned with high latitude areas where the ionosphere is often active and characterized by turbulence, ionospheric correction is particularly relevant for these applications. After an introduction to the research topic in Chapter 1, Chapter 2 will discuss open issues in ionospheric correction including processing issues related to baseline-induced spectrum shifts. The effect of large baseline on split spectrum InSAR technique has been thoroughly evaluated and effective solutions for compensating this effect are proposed. In addition, a multiple sub-band approach is proposed for increasing the algorithm robustness and accuracy. Selected case studies are shown with the purpose of demonstrating the performance of the developed algorithm. In Chapter 3, the developed ionospheric correction technology is applied to optimize InSAR-based ice velocity measurements over the big ice sheets in Greenland and the Antarctic. Selected case studies are presented to demonstrate and validate the effectiveness of the proposed correction algorithms for ice velocity applications. It is shown that the ionosphere signal can be larger than the actual glacier motion signal in the interior of Greenland and Antarctic, emphasizing the necessity for operational ionospheric correction. The case studies also show that the accuracy of ice velocity estimates was significantly improved once the developed ionospheric correction techniques were integrated into the data processing flow. We demonstrate that the proposed ionosphere correction outperforms the traditionally-used approaches such as the averaging of multi-temporal data and the removal of obviously affected data sets. For instance, it is shown that about one hundred multi-temporal ice velocity estimates would need to be averaged to achieve the estimation accuracy of a single ionosphere-corrected measurement. In Chapter 4, we evaluate the necessity and benefit of ionospheric-correction for L-band InSAR-based permafrost research. In permafrost zones, InSAR-based surface deformation measurements are used together with geophysical models to estimate permafrost parameters such as active layer thickness, soil ice content, and permafrost degradation. Accurate error correction is needed to avoid biases in the estimated parameters and their co-variance properties. Through statistical analyses of a large number of L-band InSAR data sets over Alaska, we show that ionospheric signal distortions, at different levels of magnitude, are present in almost every InSAR dataset acquired in permafrost-affected regions. We analyze the ionospheric correction performance that can be achieved in permafrost zones by statistically analyzing correction results for large number of InSAR data. We also investigate the impact of ionospheric correction on the performance of the two main InSAR approaches that are used in permafrost zones: (1) we show the importance of ionospheric correction for permafrost deformation estimation from discrete InSAR observations; (2) we demonstrate that ionospheric correction leads to significant improvements in the accuracy of time-series InSAR-based permafrost products. Chapter 5 summarizes the work conducted in this dissertation and proposes next steps in this field of research.
  • Petrographic and microfacies analysis of the Shublik formation, northern Alaska: implications for an unconventional resource system

    Knox, April Rae; Whalen, Michael; Hanks, Catherine; Wartes, Marwan (2018-08)
    The North Slope of Alaska includes a world-class conventional petroleum system that has been producing for approximately 40 years. While the regional stratigraphy includes multiple proven source and reservoir rocks, the Middle to Upper Triassic Shublik Formation (Fm.) is the most prolific. The decline in conventional hydrocarbon production on the North Slope and the presence of high quality source rocks inspire the evaluation of the Shublik Fm. as an unconventional petroleum system where oil or gas are produced directly from source rocks. The Shublik Fm. is a heterogeneous calcareous shale and limestone interval that has been interpreted to indicate deposition influenced by marine upwelling. Lithofacies observed in outcrop consist of intervals of non-resistant organic-rich packages that are interbedded with resistant coarsening upward rhythmic depositional successions of phosphatic and carbonate parasequences. The heterogenous characteristics of the microfacies is one of the challenges in the unconventional exploration of the Shublik Fm. Detailed microfacies descriptions, as part of this study, document the complex lithologies and identify patterns in the occurrence of the microfacies. Microfacies descriptions are the building blocks to identify stacking patterns that define the parasequences. The parasequences observed in core, outcrop, and petrographic analysis are calibrated to well logs to map relevant stratigraphic intervals on a regional scale. A targeted interval for the unconventional exploration of the Shublik Fm. is identified using a relative brittleness index. Flat clam and phosphatic parasequences are the primary packages present within the targeted interval. The parasequences are comprised of small-scale brittle and ductile couplets. High resolution pore imaging and mechanical stratigraphy characteristics are documented within the interval. Pore types and networks provide potential storage and migration pathways within unconventional resource systems. Pore types were imaged at a high resolution using scanning electron microscopy. Pore types that are present within the Shublik Fm. include interparticle, intraparticle, porous floccules, moldic, microfracture, and microchannel. The phosphatic limestone facies contains the greatest amount of porosity including interparticle, intraparticle, and moldic within the phosphate nodules and matrix and fractures surrounding phosphate nodules. Microfracture, microchannel, and intraparticle porosity are the primary pore types within the flat clam facies. Mechanical stratigraphy has a direct impact on the success of hydraulic fracturing. The microfacies descriptions identify laminae scale mechanical stratigraphy characteristics which include erosional surfaces, laminations, graded bedding, mineralogical variation, and textural anisotropy and isotropy. A regional correlation of the targeted interval was developed building upon previous work and utilizing sequence stratigraphic models. Significant parasequence surfaces are readily identified in the well log signatures. The challenge of the heterogeneity of the Shublik Fm. is simplified first by identifying a targeted interval consisting of brittle and ductile packages enriched in TOC. Then recognizing the parasequences contained as part of the interval which include significant characteristics pertaining to pore networks and mechanical stratigraphy. Combining methods and results of this study provides an important component to evaluating the Shublik Fm. as a potential unconventional resource system.
  • Decadal scale vegetation maps for the boreal forest surrounding Fairbanks, Alaska

    Huhman, Hannah E.; Prakash, Anupma; Rosselló, Jordi Cristóbal; Dewitz, Jon (2018-08)
    Vegetation maps of a selected area within the boreal forest surrounding Fairbanks, Alaska, have been generated for the nominal years of 1985, 1995, 2005, and 2015 using Landsat 4 and 5 Thematic Mapper and Landsat 8 Operational Land Imager surface reflectance products at 30 meter spatial resolution using a decision tree classification. The maps include 9 U.S. Geological Survey (USGS) vegetation classes, as well as barren land, open water, and ice/snow classes that are consistent with the classes identified in the 2001 National Land Cover Database (NLCD) map of Alaska generated by the USGS. Classification steps are based on USGS methodology, with refinements for the boreal forest, to ensure further comparison to the 2001 USGS NLCD map for Alaska. The overall weighted accuracies of first order estimates of data quality using cross validation are 93.2%, 88.4%, 93.3%, and 86.9% for the nominal years of 1985, 1995, 2005, and 2015 maps, respectively, compared to 81.8% accuracy for the USGS NLCD 2001 product. This study demonstrates that the spatial and spectral resolution of Landsat data is the best available for mapping the vegetation of Alaska's boreal forest at 1:50,000 scale. It also shows that the boreal forests surrounding Fairbanks, Alaska have witnessed a decrease in the growth of evergreen forests, an expansion of shrub and an increase in wetland distribution, all of which have been reported as impacts of a warming climate in the Arctic and Sub-arctic.
  • Crustal and upper mantle velocity structure in Alaska

    Searcy, Cheryl Kaye; Christensen, Douglas; Layer, Paul; Stringer, William; Kawasaki, Koji; Stone, David (1996)
    The crustal and upper-mantle velocity structure of Alaska testifies to a complex tectonic framework. Much of the structure and history of this framework remains to be conclusively determined. This thesis presents the results of three independent investigations of velocity structure in Alaska in an attempt to provide some insight into its tectonic development. The first study involved the analysis of receiver functions to determine velocity structure beneath College Station (COL), located in Fairbanks, Alaska. Receiver functions from several back azimuths facilitate a fairly detailed analysis of deep crustal velocity structure beneath COL, including an indication that Moho dips to the northeast. The second study also employed receiver function methods to investigate velocity structure for four temporary three-component seismic stations placed in the Brooks Range. Due to the short deployment of these stations in the Brooks Range only a rough estimate of crustal velocities were obtained. Nevertheless, crustal thickening beneath the Brooks range is clearly indicated by an increase in the depth to Moho. The final study undertaken was a three-dimensional tomographic P-wave velocity inversion for the subduction zone region of south central Alaska. Data for the tomographic inversion consisted of local and teleseismic ray paths. The resulting velocity perturbations indicate a positive velocity anomaly associated with the subducting Pacific plate. Furthermore, the tomographic images clarify physical characteristics of the subducting plate such as structure, thickness, and depth of penetration into the mantle.

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