Thesis (Ph. D.)--Oregon State University, 2006. / Printout. Includes maps in pocket. Includes bibliographical references (leaves 389-409). Also available on the World Wide Web

Thesis (M.S.)--University of Delaware, 2008. / Principal faculty advisor: George W. Luther, III., College of Marine & Earth Studies. Includes bibliographical references.

Large Igneous Provinces (LIPs) represent geologically rapid emplacements of vast quantities of igneous material into/onto Earth's crust. There is a marked correlation in the known ages of LIPs and Mesozoic extinctions and other environmental perturbations, suggesting a possible causal link between these events. However, uncertainties in matching LIP basalt ages to the stratigraphic record of Mesozoic events mean that a sedimentary tracer of volcanism would better indicate a precise coincidence between the two phenomena. Mercury (Hg) has shown potential as such a proxy. Volcanism is a major source of Hg to the natural environment, and its relatively long atmospheric residence time (0.5–2 years) allows global distribution of the element before it is deposited to sediments. However, questions remain about how the manner of LIP emplacement might influence its impact on the Hg cycle, as well as how sedimentary processes may locally overprint any global signal. Here, the Hg records of three Mesozoic events are investigated: the end-Triassic extinction (TJ: ~201.5 Ma), Cretaceous Oceanic Anoxic Event 2 (OAE 2: ~94 Ma), and the latest Cretaceous (K–Pg: ~67–66 Ma). These events coincided with markedly different LIPs: OAE 2 with multiple submarine LIPs; the K–Pg and TJ with subaerial LIPs; with the TJ also featuring release of additional thermogenic volatiles from intrusion of organic-rich lithologies by LIP sills. Additionally, mercury is used with osmium and carbon isotopes to study the temporal relationships between volcanism, weathering, and the carbon cycle during the Toarcian Oceanic Anoxic Event (~183 Ma). This work highlights the links between LIP volcanism and other surface processes, and shows that subaerial LIPs featuring thermogenic emissions are most likely to perturb the global Hg cycle, with the record of such perturbations dependent on the nature of the sedimentary archive.

The last several years have seen renewed interest in the origin of silicic magmas thanks to the developments of new microanalytical techniques allowing the measurement of the isotopic and trace element compositions of erupted magmas on sub-crystal length scales. Concurrently, there has been rapid improvement in the sophistication of computer modeling of igneous systems. This dissertation is an interdisciplinary study of the rhyolites of the Yellowstone hotspot track using both techniques. Chapters II-IV, which have all been published in existing journals, are a detailed study of the O and Hf isotopic compositions of zircon phenocrysts from large rhyolitic eruptions in the central Snake River Plain, and from rhyolites which erupted in Oregon, Idaho, and Nevada coeval with the Columbia River flood basalts. They show that rhyolites are derived from combinations of fractionates of mantle-derived basalts and of different crustal end-members which are identifiable by their distinct isotopic end-member compositions. In the Snake River Plain and Yellowstone, they recognize a common trend where early erupted rhyolites have a strong signature of melting of ancient Precambrian crust, whereas later erupted rhyolites more closely resemble the mantle in their radiogenic isotopes and are more likely to be depleted in oxygen isotopes. Diversity in zircon grain compositions also documents a batch mixing process in which multiple compositionally distinct magma bodies are assembled into a larger common magma body prior to eruption. In Chapters V and VI, the former of which has been published with the latter in preparation, a new series of magmatic-thermomechanical models is presented which assume that melts rising through the crust are arrested by strong rheological contrasts. The strongest such contrast occurs at the brittle-ductile transition at 5-10 km depth, leading to the formation of a 10-15 km thick mafic mid-crustal sill, which separates upper and lower-crustal zones of partial melt, corroborating previous geophysical imaging studies. In Chapter VI, the above isotopic trends are replicated in the modeling scheme, which shows that the source depth of crustal melts tends to shallow with time through a combination of crustal heating and repeated caldera collapses. This dissertation includes both previously published co-authored material.

One of the defining characteristics of hotspot volcanism is the presence of a long-lived, linear chain of age-progressive volcanoes created by the movement of the lithosphere over a stationary melting anomaly. However, the spatial distribution of volcanism at hotspots is often complex and highly variable suggesting that the relationship between magma generation and magma transport at hotspots is poorly understood. Here, I present the results of the first systematic quantitative characterization of the spatial distribution of volcanism at oceanic hotspots. In the first study I develop a novel methodology to characterize the across-strike distribution of volcanism at hotspots and apply it to a catalog of 40 oceanic hotpots. I find that only 25% (10/40) of hotspots exhibit the simple single-peak profile predicted by geodynamic models of melt generation in mantle plumes. The remaining 75% (30/40) of hotspots exhibit a dual- or multi-peak pattern. In the second study, I focus on the across-strike distribution of volcanism at the oceanic hotspots that are sourced by a deep-rooted mantle plume. 14 out of the 15 consensus plume-fed hotspots exhibit a dual-peaked across-strike profile. The spacing between these peaks display a strong negative correlation with lithospheric age, in direct contrast to models of inter-volcanic spacing controlled by elastic plate thickness. This relation suggests a different mechanism controls volcanic spacing at plume-fed hotspots. In the third chapter, I investigate variations in the average topographic profiles over time along the two longest and best-constrained oceanic hotspot tracks: Hawaii and Louisville. I find that the dual-peak across-strike profile of volcanism is a persistent feature at the Louisville hotspot over the entire length of the track examined (spanning a period of more than 65 Myr). In contrast, the dual-peak profile of volcanism at Hawaii is only evident along the most recent portion of the track (i.e., over the last 5 Myr). In total, this thesis represents a significant step foreword in the collective understanding of hotspot volcanism, and introduces a new diagnostic tool for analysis of hotspot influenced seafloor topography.

Isostasy, flexure and dynamic processes all influence the shape of the Earth’s surface. While the first two processes are well understood, dynamic topography remains controversial. On the continents, dynamic uplift is often expressed by positive long-wavelength gravity anomalies, radial drainage patterns, and slow seismic velocity anomalies within the upper mantle. Volcanic activity and elevated heat flow are also often observed. The aim of this study is to investigate the link between geochemical compositions of intracontinental magmatism and geophysical, geomorphological and geodetic observations of dynamic uplift. Three volcanic regions are considered in detail: western North America, northeast Brazil and Madagascar. The combined database includes 348 new whole-rock geochemical analyses. Rare earth element concentrations of mafic, asthenospheric-derived volcanic samples are exploited to calculate the depth and temperature of melt generation by inverse modelling. A sensitivity test of this modelling scheme is carried out. Lithospheric thickness and mantle temperature are independently determined from shear wave velocity models. Beneath western North America, a negative correlation between shear wave velocities at depths of 70–150 km and degree of melting is observed. Temperatures obtained from igneous compositions and from shear wave velocity profiles beneath volcanic fields closely agree. Melts are produced within, or close to, the spinel-garnet transition zone at depths shallower than $\sim$70 km, yielding mantle potential temperatures of up to 1380$^{\circ}$C. Calculated uplift and heat flow based upon these results match observed surface elevation and heat flow measurements. In northeast Brazil, Jurassic, Cretaceous and Cenozoic phases of mafic igneous activity are recognised. Jurassic magmatic activity probably resulted from spinel-field melting at potential temperatures of $\sim$1380$^{\circ}$C. This episode is associated with regional magmatism during break-up of the Central Atlantic Ocean. Cretaceous compositions record melting at potential temperatures of 1330–70$^{\circ}$C at similar depths. This activity is linked to extension at the time of break-up of the equatorial and South Atlantic Ocean. Cenozoic volcanism comprises low-degree melts within the spinel-garnet transition zone at ambient potential temperature. Shear wave velocity models support these results. Cenozoic volcanism in Madagascar is predominantly alkaline and records small-degree melting with minor temperature anomalies within the spinel-garnet transition zone. Rare tholeiitic basalts record temperatures up to 1360$^{\circ}$C. Analysis of global and regional shear wave velocity models closely matches these results. The principal control on continental magmatism appears to be temperature anomalies within the upper mantle beneath thin lithosphere. Highest mantle potential temperatures correlate with largest dynamic uplift. Mantle potential temperatures $ < $1350$^{\circ}$C are matched with minimal or negative dynamic topography.

Using cluster analysis, a total of 605 vents in San Francisco Volcanic Field are studied over an area of approximately 5000km2. Application of alignment analysis techniques, including the two-point azimuth analysis and Hough transform analysis, demonstrates that cinder cones are aligned along common orientations within larger clusters. These alignments consist of 9-10 cinder cones, are 20-38 km long, and are regional features. The vent alignments indicate the presence of geological features along which magma ascended more readily than elsewhere. The NE-trending Mesa Butte and Oak Creek Canyon-Doney fault systems seem to control the intermediate to silicic centers which are on the intersection of these fault systems with Cataract Creek fault system and affect the development of cinder cone alignments. Geological maps and geophysical surveys indicate that most vent alignments are parallel or subparallel to these large scale fault systems. This suggests that vent alignment patterns are controlled by regional structures.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Coquihalla Volcanic Complex consists of calc-alkaline acid to intermediate extrusive and intrusive rocks which have an areal extent of roughly 30 km2, near Hope, British Columbia. The oldest and most voluminous members of the complex are rhyclitic pyroclastic rocks (variably welded lithic-crystal lapilli tuff, vitric tuff, and crystal-lithic lapilli tuff) , that have an overall thickness of approximately 1600 m. Later igneous activity produced numerous andesite to dacite domes, dykes, and sills- A late stage diorite to quartz-diorite stock forms the core of Coquihalla Mountain. Most pyroclastic rocks rest unconformably en the Jurassic to Cretaceous Eagle pluton. Monolithologic avalanche breccias formed in the southern portion of the map area, where pyroclastic recks were deposited against a fault scarp with uplifted Lower Cretaceous Pasayten Group rocks. In the southeastern part of the area, monolithologic avalanche breccias formed in response to tilting and uplift of the underlying Eagle pluton as the basin subsided. All tuffaceous rocks are characterized by vitroclastic textures, and contain phenocrysts of plagicclase (An 40-20), biotite, quartz, and minor potassium feldspar and titanomagnetite. Andesites are porphyritic with phenocrysts of plagioclase (An 76-30), calcic augite, magnesic- to tschermakitic hornblende, and titanomagnetite. Glomeroporphyritic clots consist of plagioclase, aluminous augite, and titancmagnetite. Porphyritic dacites contain phenocrysts of plagioclase(An 60-35), hornblende, titanomagnetite, and minor apatite. The diorite stock consists of orthopyroxene^ clinopyroxene, plagioclase, titanomagnetite, and ilmenite, with interstitial quartz and potassium feldspar. Three K-Ar dates average 21.4±0.7 Ma, and are concordant with a Eb-Sr isochron (22.3±4 Ma with initial 87Sr/86Sr = 0.70370±0.00008) based on seven whole rock sauries which span the entire compositional range of the suite. These results indicate that the Coquihalla Volcanic Complex is coeval with calc-alkaline centres in the Pemberton Volcanic Belt. The whole rock compositions of members of the Coquihalla Volcanic Complex show a range in silica contents from 54 to 76 weight per cent (volatile-free). In relation to increasing silica content, chemical variations within the suite are characterized by enrichment of K₂0, Na₂0, Rb, and Nb, and depletion in Al₂0₃, Ti0₂/ MgO, MnO, CaO, P₂O₅, Cr, Ni, V, and Sr. The elements Ba, Ce, Nd, and Zr show enrichment throughout most of the suite, but depletion in the most felsic members. Interpretation of chemical variations of whole rocks and constituent phenocrysts suggests that the chemical diversity of the suite is governed by fractional crystallization. The results of quantitative major and trace element modelling indicate that 1) hornblende dacites can be derived from basaltic-andesites by 50% crystallization of a mixture of plagioclase, hornblende, clinopyroxene, titanomagnetite, and apatite, and 2) rhyolites can be derived from dacites by roughly 45% crystallization of a mixture of plagioclase, hornblende, biotite, titanomagnetite, and apatite. Basaltic andesite compositions are consistent with derivation from basaltic liquids (modified by olivine fractionation) that are produced by partial melting of hydrous mantle peridotite above the subducted Juan de Fuca plate. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Unknown

Io, the most volcanically active body in the solar system, also has some of the tallest and steepest mountains. The mountains seem to be tectonic in origin, yet the methods of their formation have not been decisively constrained and their associations with volcanic paterae are yet unclear. We have compiled global spatial statistics on mountain dimensions and orientations, lineations attributed to structures, straight patera margins, and patera dimensions in order to better define their genetic relationships and the mechanisms forming each type of feature. Additionally, we have produced 4 regional structural maps of mountain complexes and have proposed tectonic histories. Global statistics show that paterae and mountains and their associated lineations are more common at low latitudes and that lineations attributed to tectonics have preferred azimuths of 45° and 135°, whereas straight patera margins and azimuths appear more random. Additionally, tectonic lineations tend to cluster to those of similar types and are smaller when closer together. Mountains in general on Io are isolated, varied in size and shape, and have no significant geographic patterns in those variations. These results may indicate that global-scale processes are involved in forming Io's tectonic structures, but that the diversity of mountain characteristics and the collapse of paterae adjacent to mountain complexes may be more regionally controlled. Mapping of the Hi'iaka, Shamshu, Tohil, and Zal regions has shown that Io's mountains reside in large, faulted-bounded crustal blocks, which have undergone modification through local responses of subsurface structures. Strike-slip motion along reactivated faults has led to the formation of both transpressional and transtensional features, creating tall peaks and low basins, some of which are now occupied by paterae. Subsurface structures play a large role in Io's mountain diversity. Based on interpretation of statistical results and on our localized mapping, we propose that Io's mountains result from a combination of crustal stresses involving both global and local-scale processes. Multiple faults and fractures in a variety of orientations formed in Io's lithosphere, created over billions of years by stresses imposed by volcanic loading and tidal flexing. These faults have been progressively buried over time under multiple layers of volcanic material. Stresses continuing from loading and tidal massaging sometimes occur at oblique angles to pre-existing faults, reactivating them as reverse, normal, or strike-slip faults. Because of this, large, cohesive fault-bounded blocks have undergone both transpressional and transtensional modification. Further degradation of mountains has also occurred from extensive mass wasting, gravitational collapse, and erosion by sublimation and sapping of sulfur-rich layers within the crust. This model of fault-bounded blocks being modified by continual stresses and local structural response accounts for the variation and patterns of mountain sizes, shapes, and orientations, along with their isolation and interactions with other features. It presents an explanation for the influence of global and regional tectonics and a more detailed account of the formation of some of Io's remarkable mountains.