Geological Sciences Theses and Dissertations
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Item Open Access Characterizing Landslide Movement at the Boulder Creek Earthflow, Northern California, Using L-band InSAR(University of Oregon, 2009-09) Stimely, Laura Lyn, 1982-Spatial and temporal patterns of movement of the Boulder Creek earthflow were investigated using 26 interferograms derived from ALOS satellite radar images acquired between February 2007 and February 2008. Persistently unstable hillslopes in Northern California are ideally suited to the study of the dynamics and morphological signature of earthflows, as the deeply sheared melange lithology, high seasonal rainfall, and fast uplift rates promote widespread deep-seated landsliding. In addition to identifying multiple active landslides in the region, L-band InSAR reveals varying deformation rates in the accumulation, transport, and toe regions of the Boulder Creek earthflow. Downslope displacement rates up to 1.8 m/yr are observed on the earthflow over a I-year period. The pattern of deformation is similar to that observed from 1944-2006 inferred from aerial photography. Interferograms highlight spatially variable rates controlled by lithology and gullies, and movement correlates with seasonal rainfall with a phase lag of ~2 months.Item Open Access Climate and Ecological Change in Oligo-Miocene Mammals(University of Oregon, 2011-12) Orcutt, John D.Whether or not a causal relationship exists between climate and mammal body size is one of the longest-standing and most intractable questions in ecology. The classic model of body size evolution (Bergmann's Rule) holds that body size is driven by temperature, but more recent hypotheses have suggested that other climatic variables or biotic interactions may play a more important role. The use of paleoecological data to address this question allows variables that are tightly correlated in modern ecosystems to be teased apart and allows body size patterns to be observed through time, adding an extra dimension to analyses. This dissertation details the findings of two paleoecological tests of Bergmann's Rule in the Oligo-Miocene (30-5 Ma), one tracking body size and climate through time in the northwestern United States and another tracking geographic body size trends through time along the west coast of North America. In both cases, body size was analyzed in three representative families of mammals: equids, canids, and sciurids. Such large-scale analyses are dependent on fossils that can be placed in a reliable taxonomic, geologic, and temporal context, and this dissertation also focuses on a reevaluation of the canid fauna of Oregon's Juntura Formation that places a critically important Late Miocene carnivore fauna in just such a context. Two genera of canids - Epicyon and Carpocyon - are described from the fauna for the first time, with important implications for regional biostratigraphy. The body size analyses show no consistent relationship between body size and any climatic variable. Further, body size patterns vary widely between taxa at several levels, suggesting that one universal driver of body size evolution does not exist. Not only is there no evidence for Bergmann's Rule in Oligo-Miocene mammals, but comparative analyses of geographic body size patterns in the modern genera Odocoileus, Canis, and Spermophilus fail to show the latitudinal gradients upon which Bergmann's Rule is predicated. The apparent existence of such trends in some taxa may be the result of anthropogenic extirpation at low latitudes, further underscoring the importance of including paleontological data when formulating models predicting the response of biotic variables to environmental change.Item Open Access Constraints on Eruption Dynamics, Mount St. Helens, WA, 2004-2008(University of Oregon, 2009-09) Schneider, Andrew Daniel, 1982-Different models have been proposed for the "drumbeat" earthquakes that accompanied recent eruptive behavior at Mount St. Helens. Debate continues as to whether seismicity is related to brittle failure during the extrusion of solid dacite spines or is the result of hydrothermal fluids interacting with a crack buried in the volcanic edifice. My model predictions of steady-state conduit flow confirm the strong control that degassing exerts on eruptive behavior. I discuss the necessary role of degassing for extruded material to attain the high density (low vesicularity) of the observed spine material and discuss the implications for generating seismicity. A brittle-failure source of seismicity requires that the gouge elastic properties accommodate some strain, since the magma compressibility in the upper conduit is too low to do so on its own. I also report on a novel method for generating high-resolution digital elevation models of fault surface textures.Item Open Access The contribution of large, slow-moving landslides to landscape evolution(University of Oregon, 2009-12) Mackey, Benjamin HunterThis dissertation discusses the contribution of deep-seated landslides and earthflows to the morphology, erosion, and evolution of mountainous landscapes, focusing on the northern California Coast Ranges. In active landscapes, channel incision is necessary to create relief but also increases stresses in adjacent hillslopes, ultimately leading to slope failure. While conceptually simple, the spatial relationships between channel incision and landsliding have not been well quantified. Along the South Fork Eel River, I mapped the distribution of deep-seated landslides using light detection and ranging (LiDAR) derived maps. Landslide density increases in regions subject to late Pleistocene-Holocene channel incision and particularly in response to lateral incision at the apex of meander bends. Wavelet analysis of channel sinuosity reveals hillslopes are most sensitive to meander wavelengths of 1.5 km. Argillaceous lithology generates abundant earthflow activity along the main stem Eel River, yet spatial and temporal patterns of earthflow movement are poorly understood. I undertook a detailed study of the Kekawaka Earthflow using LiDAR, meteoric 10 Be in soil, orthorectified historical aerial photographs, and field surveys. Inventories of 10 Be in soil pits increase systematically downslope, indicate an average movement rate of 2.1 ± 1.3 m/a over the past 150 years, and establish a minimum earthflow age of 1700 years. The Kekawaka earthflow has a systematic history of movement, both spatially, with greatest movement in the narrow transport zone, and temporally, as velocities peaked in the 1960's and have slowed since 1981. I used LiDAR and aerial photographs to map earthflow movement and calculate sediment flux across 226 km 2 of the main stem Eel River. From 1944-2006, 7.3% of the study area was active, and earthflows account for an erosion rate of 0.53 ± 0.04 mm/a, over half the regional average sediment yield. Velocity time series on 17 earthflows suggest temporal earthflow behavior is influenced by decadal-scale changes in precipitation, temperature, and river discharge, although local topographic factors can overwhelm this climatic signal. When active, earthflows erode an order of magnitude faster than surrounding terrain; however, source supply limitations appear to govern long- term earthflow evolution. This dissertation includes previously published coauthored material.Item Open Access Crustal and upper mantle structure beneath the Galapagos arechipelago from seismic tomography(University of Oregon, 2010-12) Villagomez Diaz, Darwin R., 1973-To explain the origin of several distinct aspects of the Galápagos volcanic hotspot, such as the broad geographical extent of recent volcanism and the unusual pattern of geochemical anomalies, we conducted seismic tomography studies of the upper mantle and crust beneath the Galápagos Archipelago. The studies combine measurements of group and phase velocities of surface waves and delay times of body waves. We find that upper mantle seismic velocities are lower than those beneath other regions of comparable age in the Pacific and consistent with an excess temperature of 30 to 150°C and ∼0.5% melt. We attribute the excess temperature and presence of melt to an upwelling thermal mantle plume. Crustal seismic velocity is up to 25% lower than that of very young crust at the East Pacific Rise (EPR) and is comparable to that of Hawaii, which we attribute to heating by increased intrusive activity above the Galápagos plume and the construction of a highly porous volcanic platform. In addition, we find that the Galápagos hotspot is underlain by a high-velocity region whose thickness varies from 40 to 100 km. The tomographic images reveal that the upwelling mantle plume tilts northward (towards the nearby Galápagos Spreading Center) as it rises and then spreads laterally when it reaches the bottom the lid. The lid, which we attribute to residuum from melting, is thickest where it is farthest from the spreading center, suggesting that ridge processes may affect the generation and amount of thinning of the residuum layer. In addition, the thickness of the lid correlates well with the geographical pattern of geochemical anomalies of erupted lavas, suggesting that the lid may control the final depth of decompression melting. We conclude that many of the distinct characteristics of the Galápagos can be attributed to the interaction of the upwelling plume with the lid and the nearby ridge. We further suggest that the ridge affects the geometry of plume upwelling in the upper mantle and also the pattern of lateral spreading of the plume due to its effect on the thickness of the residuum layer. This dissertation includes previously published co-authored material.Item Open Access Diverse Oxygen Isotope Values and High Magmatic Water Contents within the Volcanic Record of Klyuchevskoy Volcano, Kamchatka, Russia(University of Oregon, 2007-12) Auer, Sara Lynn, 1983-Klyuchevskoy volcano, located in Kamchatka's subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest 8180 values for olivines and basalts. I present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of 14C and tephrochronologically-dated tephra layers and lavas spanning the eruptive history and the AI-Mg compositional range of the basalt to basaltic andesites found at Klyuchevskoy. A hybrid model of moderately-high 8180 silicate melt from the subducting slab, coupled with high 8180 fluid is provided to best explain Klyuchevskoy's 1) voluminous, high-rate volcanism, 2) unusual high-8180 signature, 3) two endmember basaltic magma, and, 4) hydrous high-AI, high-8180 component.Item Open Access The early Miocene Cape Blanco flora of coastal Oregon(University of Oregon, 2009-09) Emerson, Lisa Francis, 1979-This dissertation establishes the age, depositional environment, composition, and climatic conditions for the Cape Blanco flora. The paleotemperature estimated by the Cape Blanco flora, the Temblor flora of California, and the Seldovia flora of Alaska are then compared with sea surface temperatures estimated from oxygen isotope analysis of benthic foraminifera. The unconformity-bound shallow marine sandstone of Floras Lake includes a redeposited tuff bed which contains fossil leaves at Cape Blanco. An 40 Ar/ 39 Ar age of 18.26 ± 0.86 Ma is presented for the tuff as well as a paleomagnetic stratigraphy of the sandstone. Sedimentary structures of the tuff bed are evidence that the tuff was deposited at or just above the strand line. The depth of tuff deposition was shallower than the adjacent marine sands, and this short-lived shoaling may have been a result of increased sediment supply. The fossil flora was an oak forest with numerous species of Fagaceae. Additional components include lanceolate Salicaceae leaves, entire margined Lauraceae, fragmentary Betulaceae, and lobed Platanaceae. Coniferous debris, charcoal, Equisetales, and Typhaceae forms are also figured. Ten leaf forms could not be confidently assigned to established names but are described, figured, and called angiosperm forms 1-10. In total 44 unique forms are identified. The size and margin type of the dicot specimens are quantified, and by comparison with known modern floras, a former mean annual precipitation of 201 (+86, -61) cm and a former mean annual temperature of 18.26 ± 2.6°C are estimated. The paleotemperature of the ∼17.5 Ma Seldovia Flora and the ∼17.5 Ma Temblor Flora are estimated using the same method, establishing a ∼0.7°C per degree of latitude temperature gradient for the northern hemisphere temperate zone. The leaf based gradient is steeper than the sea surface temperature gradient, of ∼0.26°C per degree of latitude as estimated from oxygen isotopic composition of foraminifera collected from ocean sediment cores. Both fossil leaf and isotope methods suggest that the early Miocene was ∼5°C warmer than today. This thesis includes unpublished co-authored material.Item Open Access Effects of noise on teleseismic T* estimation and attenuation tomography of the Yellowstone region(University of Oregon, 2009-06) Adams, David C., 1952-Studies on seismic attenuation are an important complement to those on seismic velocity, especially when interpreting results in terms of temperature. But estimation of attenuation (t*) is more computationally involved and prone to contamination by noise, especially signal-generated noise. We have examined the effects of various forms of synthetic noise on t* estimation using time and frequency domain methods with varying window lengths and data frames of reference. We find that for S-waves, error due to noise can be reduced by rotating the data into the estimated polarization direction of the signal, but unless the exact nature of the noise is known, no method or window size is clearly preferable. We recommend the use of multiple estimation methods including a careful assessment of the uncertainty associated with each estimate, which is used as a weight during inversion for 1/Q. Our synthetic tests demonstrate that the misfit between actual and estimate-predicted traces or spectra correlates with t* error, and a similar relationship is suggested for real data. Applying this approach to data from the Yellowstone Intermountain Seismic Array, we employ two important constraints during inversion. First, we scale the misfit values so that the resulting weights are comparable in magnitude to the squares of the eventual data residuals. Second, we smooth the model so that the maximum attenuation (1/Q) does not exceed a value which would totally explain the observed velocity anomaly. The tomographic models from all the estimation methods are similar, but in the vicinity of the Yellowstone mantle plume, S-wave models show greater attenuation than do P-wave models. We attribute this difference to greater focusing by the plume of S-waves. All models show relatively high attenuation for the plume at depth, but above 250 km attenuation in the plume drops rapidly to values less than those of the surrounding mantle. We attribute this drop to the onset of partial melting, which dehydrates the olivine crystals, suppressing dislocation mobility and thereby attenuation. These attenuation models suggest excess plume temperatures at depth which are too low to support a plume origin in the lower mantle. This dissertation includes unpublished co-authored material.Item Open Access Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise(University of Oregon, 2011-06) Durant, Douglas Troy, 1965-Results from a recent mid-ocean ridge tomography study along the fast-spreading, northern East Pacific Rise (EPR) reveal that the axis of mantle upwelling beneath the ridge is skewed with respect to the spreading axis, giving rise to regions of both rise-centered and off-axis mantle melt accumulation. Here, we investigate the effects of off-axis melt accumulation on the architecture of overlying crust as well as off-axis melt delivery on crustal construction along the ridge axis. We first present evidence for off-axis magmatism 20 km from the spreading center in 300-ka-old crust overlying a region of off-axis melt supply. Seismic data reveal an intrusive complex ∼2 km beneath the seafloor that is limited in lateral extent (<5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust, which provides the necessary conditions to drive off-axis volcanic and hydrothermal activity. We next present results from thermodynamic modeling that show systematic, along-axis variations in the depth of crystallization and degree of differentiation of magma produce crustal density variations of ∼0.1 g/cm 3 . These density anomalies are on the order inferred from a recent study that shows increasing axial depth along the northern EPR correlates with an increase in crustal density and offset of mantle upwelling with respect to the ridge axis. Our results, along with geophysical and geochemical data from the 9°-10°N region of the EPR, suggest that along-axis deeps correspond with magmatic systems that have significant near-Moho (i.e., crust-mantle transition) crystallization, which we attribute to off-axis delivery of mantle melt. As this investigation is motivated by the EPR tomography results, we conclude with a numerical study that examines the travel time sensitivity of Pn , a sub-crustal head wave commonly used in local travel time tomography, to crustal and mantle heterogeneity. Our results indicate that Pn travel times and Fresnel zones are insensitive to normal sub-axial crustal thickness anomalies, mantle velocity gradients and crust-mantle velocity contrast variations and that mantle low-velocity zones must be at least 3 km thick to produce significant, near-constant Pn delay times. Our data support the validity and interpretation of the EPR tomography results. This dissertation includes both previously published and unpublished co-authored material.Item Open Access Eruptive Processes of Mafic Arc Volcanoes – Subaerial and Submarine Perspectives(University of Oregon, 2011-09) Deardorff, Nicholas D., 1980-Mafic arc volcanoes have eruption styles that range from explosive to effusive. In a broad sense, eruption style is controlled by the rate of magma supply to the vent. In this dissertation I examine relationships between eruption rate and style in two separate studies: (1) an investigation of ongoing activity at NW Rota-1, a submarine volcano in the Mariana arc, and (2) a morphologic study of the Collier Cone lava flow field in the Central Oregon Cascades. The eruptions of NW Rota-1 range from effusive to moderately explosive; eruptions are effusive when mass eruption rate (MER) is low and explosive when MER increases. The explosivity of submarine eruptions is suppressed by seawater because of increased hydrostatic pressure, rapid cooling, and the high viscosity of water relative to air (which limits expansion). The combination of seawater and relatively low MERs limit pyroclast deposition to within meters to tens of meters of the vent. In fact, many pyroclasts fall back into the vent and are recycled. Evidence for recycling includes microcrystalline inclusions within erupted pyroclasts and elevated Cl and Na concentrations in matrix glass. Enrichment of Cl and Na suggests that seawater assimilation provides a geochemical signature of recycling. Recycling is limited to low MER explosive eruptions and is not observed in either effusive lava or deposits from high MER explosions. Direct observations of eruptions allow measurements of eruption rate. However, it is more challenging to estimate MERs of eruptions that were not observed. To address this problem, I develop and test methods of constraining the eruption rate (and duration) of the c. 1600 year old Collier Cone lava flow using the flow morphology. To quantify flow morphology I combine field observations with GIS analysis of Lidar-derived digital topography. Channel dimensions constrain emplacement rates; dominant wavelengths and amplitudes of surface folds constrain spatial and temporal changes in flow rheology. Three videos of eruption activity accompany this dissertation as supplemental files. This dissertation includes both previously published and unpublished co-authored material.Item Open Access ETS in Tidal Records(University of Oregon, 2011-12) Alba, Sequoia Kia MarieUplift rates associated with 12 episodic tremor and slip events on the Cascadia Subduction Zone occurring between 1997 and 2010 have been determined from hourly water level records from 4 NOAA tide gauges (Neah Bay, Port Angeles, Port Townsend, and Seattle). Displacements inferred from water levels generally agree with displacements inferred from modeling GPS data. Examination of uplift between events shows an inter-event deformation rate approximately equal in magnitude, with ETS events, on average, releasing strain accumulated between events, suggesting that ETS is consistent with the elastic rebound theory. Additionally, while the GPS record only extends to the late 1990s and the tremor record includes only recent decades for Cascadia, tidal records in the Pacific Northwest and around the world span many decades. Thus, by showing that ETS can be resolved in tidal records we open up the possibility that tidal records could be used to study ETS where other tools are not available. This thesis contains unpublished coauthored material.Item Open Access An Examination of Surface Displacement at the Portuguese Bend Landslide, Southern California, Using Radar Interferometry(University of Oregon, 2008-09) Calabro, Max David, 1984-SAR data constrain deformation of the Portuguese Bend landslide in space and time, and its movement is found to correlate with seasonal rainfall. Traditional InSAR methods estimate an average downslope displacement rate of 1.01 à ± 0.44 m/yr during the summer months. Interferograms show a consistent loss in phase coherence over the landslide for epochs greater than six months, especially in the winter. Interferograms become incoherent as a result of large displacement, and incoherence is used to map the spatial extent of the slide through time. The displacement rate increases several weeks after the beginning of the rainy season as rainwater percolating into the slide elevates pore pressure. SAR data processed using the permanent scatterer technique are partially successful on the rapidly moving landslide. Scatterers are identified on the landslide, but rates are underpredicted due to unwrapping uncertainty. A model based on rainfall-initiated landslides constrains diffusivity using InSAR observations.Item Open Access Extension between Major Faults, Central Oregon Basin and Range(University of Oregon, 2009-09) Treerotchananon, Anuwat, 1979-I present an alternative approach to determine the magnitude and direction of extension in the Basin and Range Province at the north end of Summer Lake basin using GIS techniques. Offset across 161 faults and tilting of 56 fault blocks were estimated to calculate extension as a function of azimuth in this area. The orientation of a representative set of slickenlines was collected in the field to assign average values for the GIS analysis. Azimuthal variation of extension is consistent with a strain ellipse indicating plane strain with extension of 1.5 to 5.5 percent along the maximum extension direction of N75E and no extension along the minimum N15W axis. Blocks tilt on average 60° from the maximum extension direction, suggesting the underlying detachment dips -N15E. This technique allows strain associated with the numerous small faults to be added to the sparse large faults for a complete regional analysis.Item Open Access Gas Migration Through Crystal-Rich Mafic Volcanic Systems and Application to Stromboli Volcano, Aeolian Islands, Italy(University of Oregon, 2011-09) Belien, Isolde L.M.B. (Leo Maria Beatrijs), 1985-Crystals influence the migration of gas through magma. At low concentrations, they increase the bulk fluid properties, especially viscosity. At concentrations close to maximum packing, crystals form a rigid framework and magma cannot erupt. However, erupted pyroclasts with crystal contents close to the packing concentration are common at mafic volcanoes that exhibit Strombolian behavior. In this dissertation, I study the influence of solid particles on gas migration. I apply my results to Stromboli volcano, Italy, type locality of the normal Strombolian eruptive style, where gas moves through an essentially stagnant magma with crystallinity ∼50%. Specifically, I investigate the effect of crystals on flow regime, gas content (Chapter II), bubble concentration (number densities), bubble shapes, bubble sizes (Chapter III), and bubble rise velocities (gas flux) (Chapter IV). I find that gas-liquid flow regimes are not applicable at high particle concentrations and should be replaced by new, three-phase (gas-liquid-solid) regimes and that degassing efficiency increases with particle concentration (Chapter II). In Chapter III, I show that crystals modify bubble populations by trapping small bubbles and causing large bubbles to split into smaller ones and by modifying bubble shapes. In Chapter IV, I model Stromboli's crystal-rich magma as a network of capillary tubes and show that bubble rise velocities are significantly slower than free rise velocities in the absence of particles. In each chapter, I use analogue experiments to study the effect of different liquid and solid properties on gas migration in viscous liquids. I then apply my analogue results to magmatic conditions using simple parameterizations and/or numerical modeling or by comparing the results directly to observations made on crystal-rich volcanic rocks. Chapter V proposes a mechanism for Strombolian eruptions and gas migration through the crystalrich magma in which the effect of crystals is included. This model replaces the current twophase "slug" model, which cannot account for the high crystallinity observed at Stromboli. There are three appendices in this dissertation: a preliminary study of the influence of particles on gas expansion, image analysis methods, and the numerical code developed in Chapter IV. This dissertation includes previously published and unpublished co-authored material.Item Open Access Investigating Groundwater Arsenic Contamination using Aquifer Push-Pull Tests(University of Oregon, 2010-06) Daigle, Ashley R., 1986-The bedrock aquifer of the Southern Willamerte Basin, Oregon, USA, is contaminated with arsenic at concentrations as high as several ppm. Single-well push-pull tests were conducted to investigate how microbial metabolisms control arsenic occurrence and levels in the aquifer. Test solutions containing ethanol were injected into the aquifer; dissolved gases, groundwater, and sediments were then sampled to monitor the speciation of carbon, iron, sulfur, and arsenic. Ethanol amendment stimulated a series of microbial metabolisms, including arsenate reduction, iron reduction, and sulfate reduction. Arsenate reduction converts arsenate to arsenite; iron reduction produces ferrous iron; sulfate reduction releases sulfide. Arsenite and ferrous iron then combine with sulfide and form arsenic sulfide and iron sulfide minerals. Results of the experiments demonstrate that the interactions among microbial metabolisms and mineral precipitation influenced arsenic contamination in the aquifer. These results shed new light on potential bioremediation strategies in the area.Item Open Access Isotopic and Petrologic Investigation and Model of Genesis of Large-Volume High-Silica Rhyolites in Arc Environments: Karymshina Caldera, Kamchatka, Russia(University of Oregon, 2011-12) Shipley, Niccole KiyomiLarge-volume calderas are responsible for producing large deposits of rhyolite and high-silica rhyolite, but the mechanisms by which these deposits are produced are still poorly understood. The Kamchatka Peninsula of Russia contains several large calderas and is one of the most volcanically active areas on Earth. Karymshina Caldera, the largest (25 km x 15 km) caldera in Kamchatka, produced an estimated 800 km 3 of high-silica rhyolitic ignimbrites and post-caldera extrusions, which erupted 1.78 and 0.5-0.8 Ma, respectively. SiO2 content ranges from 66.27-71.89 wt% in the ignimbrites and 70.16-77.70 wt% in the post-caldera extrusions studied. Crystal content is primarily quartz and plagioclase, 0.5-2 mm in size, with other minerals. Values of δ18 O, δD, 87 Sr/86 Sr, and 144 Nd/143 Nd indicate little assimilation of crustal material, in contrast to modeling results. XRF analysis indicates a homogeneous magma. The rhyolite-MELTS program was used to model crystallization of a basaltic source with addition of amphibolite partial melt and hydrothermally-altered silicic rock to reproduce the observed compositions. This thesis contains both previously published and co-authored material.Item Open Access Laboratory Experiments in Cold Temperature Rock Deformation(University of Oregon, 2011-12) Van Alst, Laura JaneThe physical weathering of rock in cryogenic regions through a process called ice segregation is important for understanding subglacial processes, landscape evolution and cold region engineering. Ice segregation was examined by freezing water-saturated cores of Eugene Formation sandstone at temperatures between -15° and -2°C. Cores between -8° and -5°C took 30-45 minutes to crack, while cores at warmer or cooler temperatures took either more than 90 minutes or did not crack at all. Numerical modeling shows that cores break under isothermal conditions. The results of this study suggest that previous models in which temperature gradients are held responsible for driving flow towards growing cracks are incomplete. I introduce a new model of ice segregation to explain how premelted liquids from smaller pores can migrate and contribute to the growth of large cracks. This dissertation includes unpublished material.Item Open Access Large-Volume Rhyolite Genesis in Caldera Complexes of the Snake River Plain(University of Oregon, 2011-06) Watts, Kathryn Erin, 1983-Caldera-forming eruptions are dramatic and destructive natural phenomena, causing severe and sustained consequences to society. This dissertation presents new geochemical and geochronologic data for caldera-forming tuffs and pre- and post-caldera rhyolites of the two youngest caldera complexes in the Snake River Plain (SRP) in the western USA: Heise (6.6-4.5 Ma) and Yellowstone (2.1-0.6 Ma). Caldera complex evolution at Heise and Yellowstone can be described by formation of 3-4 spatially overlapping "nested" calderas, successive collapse of intracaldera fill, and development of a large hydrothermal system. Comparison between Heise and Yellowstone reveals that late-stage rhyolite eruptions have drastic depletions in 18 O that require remelting of large volumes (1,000's of km 3 ) of hydrothermally altered rock. Archean xenoliths and Phanerozoic rocks of the crustal basement beneath the SRP province are not depleted in 18 O and therefore cannot be a source of these rhyolites. Isotopic mixing models indicate that early large-volume rhyolites are produced by melting and hybridization of the crust by mantle-derived basalt, and late-stage rhyolites tap hydrothermally altered portions of intracaldera rocks from previous eruptions. Caldera-forming eruptions at Heise culminated 4.45 Ma with eruption of the 1,800 km 3 Kilgore Tuff, the most voluminous 18 O-depleted rhyolite in the SRP and worldwide. O, Sr, and Nd isotope geochemistry, zircon ages, mineral and whole-rock geochemistry, and liquidus temperatures for Kilgore Tuff samples erupted >100 km apart are similar and/or overlapping within error, indicating derivation from a remarkably homogeneous low-δ 18 O magma reservoir (δ 18 O=3.4[per thousand]). Caldera-wide batch assembly and homogenization of variably 18 O-depleted melt pockets with diverse zircon populations can explain the Kilgore Tuff's genesis. Central Plateau Member (CPM) rhyolites at Yellowstone have the same timing (∼2 million years after the initiation of volcanism), magnitude of δ 18 O depletion (∼3[per thousand] depleted relative to normal rhyolites), and cumulative eruptive volume (∼4,000-4,500 km 3 ) as the Kilgore Tuff of the Heise volcanic field. Isotopic, age, and geochemical data for CPM rhyolites show that they become progressively more homogeneous and evolved from 260 ka to 75 ka. Whereas the Kilgore Tuff erupted climactically as an explosive caldera-forming tuff, CPM rhyolite eruptions record sequential, predominantly effusive, "snapshots" of magma assembly, homogenization, and differentiation. This dissertation includes co-authored materials both previously published and submitted for publication.Item Open Access Late Miocene Extensional Deformation in the Sierra Bacha, Coastal Sonora, Mexico: Implications for the Kinematic Evolution of the Proto-Gulf of California(University of Oregon, 2011-12) Darin, Michael HarrisonThe Gulf of California is an active rift basin formed by late Cenozoic dextral-oblique extension along the Pacific-North America plate boundary. Well exposed volcanic and sedimentary rocks in the Sierra Bacha, coastal Sonora, Mexico, preserve a history of proto-Gulf (late Miocene) deformation and offer insight into the structures and kinematics responsible for localization of the plate boundary and inception of the Gulf at about 6 Ma. Geologic mapping, fault kinematic analysis, and paleomagnetic data suggest that proto-Gulf deformation in the Sierra Bacha occurred primarily by ENE-WSW extension and that vertical-axis rotation related to dextral strain was minor. Lack of significant dextral shear supports an emerging model for proto-Gulf deformation in which dextral strain was not ubiquitous across Sonora but instead became localized during latest Miocene time in a narrow coastal shear zone that mechanically weakened the lithosphere and helped facilitate continental rupture. This thesis includes the "Geologic Map of the Sierra Bacha, Coastal Sonora, Mexico" as supplemental material.Item Open Access Magmatic volatile contents and explosive cinder cone eruptions in the High Cascades: Recent volcanism in Central Oregon and Northern California(University of Oregon, 2011-03) Ruscitto, Daniel M., 1981-Volatile components (H 2 O, CO 2 , S, Cl) dissolved in magmas influence all aspects of volcanic activity from magma formation to eruption explosivity. Understanding the behavior of volatiles is critical for both mitigating volcanic hazards and attaining a deeper understanding of large-scale geodynamic processes. This work relates the dissolved volatile contents in olivine-hosted melt inclusions from young volcanics in the Central Oregon and Northern California Cascades to inferred magmatic processes at depth and subsequent eruptive activity at the surface. Cinder cone eruptions are the dominant form of Holocene volcanism in the Central Oregon segment of the High Cascades. Detailed field study of deposits from three cinder cones in Central Oregon reveals physical and compositional similarities to explosive historic eruptions characterized as violent strombolian. This work has important implications for future hazard assessments in the region. Based on melt inclusion data, pre-eruptive volatile contents for seven calc-alkaline cinder cones vary from 1.7-3.6 wt.% H 2 O, 1200-2100 ppm S, and 500-1200 ppm Cl. Subarc mantle temperatures inferred from H 2 O and trace elements are similar to or slightly warmer than temperatures in other arcs, consistent with a young and hot incoming plate. High-magnesium andesites (HMA) are relatively rare but potentially important in the formation of continental crust. Melt inclusions from a well-studied example of HMA from near Mt. Shasta, CA were examined because petrographic evidence for magma mixing has stimulated a recent debate over the origin of HMA magmas. High volatile contents (3.5-5.6 wt.% H 2 O, 830-2900 ppm S, 1590-2580 ppm Cl), primitive host crystals, and compositional similarities with experiments suggest that these inclusions represent mantle-derived magmas. The Cascades arc is the global end member, warm-slab subduction zone. Primitive magma compositions from the Cascades are compared to data for arcs spanning the global range in slab thermal state to examine systematic differences in slab-derived components added to the mantle wedge. H 2 O/Ce, Cl/Nb, and Ba/La ratios negatively correlate with inferred slab surface temperatures predicted by geodynamic models. Slab components become increasingly solute-rich as slab surface temperatures increase from ∼550 to 950°C at 120 km depth. This dissertation includes previously published and unpublished co-authored material.