The Atascosa Lookout lava flow is a mid-Tertiary trachyandesite flow that caps the rhyolitic to dacitic volcanic sequence exposed in the Atascosa Mountains of southern Arizona. The flow erupted near the beginning of extension in the southern Basin and Range following the floundering of the Farallon plate and during the development of the San Andreas fault. The flow hosts a variety of disequilibrium crystals and textures including resorbed and overgrown feldspar phenocrysts with inclusion-rich zones, quartz-bearing enclaves, and clusters of plagioclase +/- chromium diopside, magnesian augite, quartz, hornblende, and orthopyroxene crystals and envelopes of contrasting composition with both the groundmass and the enclaves. Current evidence suggests that magma generation and differentiation commonly take place mainly in the lower crust and batches of magma are emplaced and equilibrate across a range of crustal levels. Crystallization depths and temperatures of various phases in the flow were obtained with new and revised geothermometers and geobarometers to examine the petrogenesis of the lava flow. Major elements of parental melts for most of the mineral phases were estimated using thermobarometry equilibrium tests and rare earth and trace element concentrations of parental melts of hornblendes and clinopyroxenes were calculated using known partition coefficients elements. Thermobarometry shows distinct ranges of temperatures and pressures for each component of the flow and calculated parental melts of various phases are distinct from one another. Orthopyroxenes crystallized at depths greater than 25 km, at the highest temperatures from the most mafic parent, estimated to be a picro-basalt. Clinopyroxenes crystallized at 11.5 – 30 km, lower temperatures and a more evolved parent of basalt or trachybasalt composition. Plagioclase crystallized throughout the crust from a range of intermediate melts and hornblendes crystallized 12 – 13 km from a parental melt similar in composition to the groundmass. This study demonstrates the lava flow hosts minerals that crystallized from different parent melts at various crustal levels. Extension and previous magmatism provided a rapid path for magma to ascend, subduing crustal assimilation and enhancing the probability of a diverse crystal cargo that retains the record of the plumbing system beneath a volcanic complex.

The deformation mechanisms responsible for the extension and rifting in Basin and Range extension over the past ~36 Ma, and their relative importance remain debated. Slab rollback, lithospheric body forces, and relative plate motions have all been shown to contribute, but the relative importance of each mechanism is not fully understood. Here, we build three-dimensional (3D) steady state geodynamic models to simulate the full tectonic reconstruction of Basin and Range extension and compare these results with known geologic field observations and other detailed reconstructions of surface deformation. Our modeling approximates lithospheric deformation through Stokes flow in a spherical cap of variable viscosities. By applying reconstructed boundary conditions, crustal thickness, and surface elevation at 17 Ma, and varying lithospheric viscosity we map out the predicted response of the surface motions and lower crustal flow for different assumed lithospheric viscosity contrasts and investigate the origin of core complex formation. Comparisons between predicted model deformation and geologic field observations from metamorphic core complexes and exposed fluorite deposits indicate: (1) The primary driving force of the formation of geologic features in the western US is regional gravitational collapse focused in the lower crust. Plate motions are second order by comparison at this time period and act to rotate velocities near the plate boundary. (2) A weak lower crust facilitates metamorphic core complex formation and extension in the Nevadaplano. Lateral extrusion of the lower crust serves as a mechanism for both core complex formation and the flattening of the Moho that is observed at present day. (3) Lower crustal flow is a contributes to the rotation and tilt of the Colorado Plateau and formation of the Rio Grande Rift.

This dissertation investigates the crustal structure of the eastern Basin and Range Province in the western United States and its relationship with the present-day extensional regime governing this region. The use of combined results from different geophysical methods provide a better understanding of the subsurface crustal structure and the processes involved in this extensional deformation. Teleseismic receiver functions were used to create a uniformly sampled map of the crustal thickness variations and stacked images of the crust beneath the majority of the state of Utah, which provide additional constraints on the seismic characteristics of the crust and upper mantle. These results reveal crustal variations characterized by a distinct change in crustal thickness that closely follows the surface trace of the Wasatch fault, with differences in depth of up to 10 km across a distance of less than 55 km. Analysis of seismic reflection profiles, horizontal and vertical crustal velocities from continuous GPS, and surface geology provide new constraints on the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults. Seismic reflection data show recent activity along high-angle normal faults that become listric with depth, sole into preexisting décollements, reactivating them, and appear to be connected at depth with a regionally extensive detachment horizon. GPS data reveal present-day crustal extension of ~3 mm/yr and no net vertical motion between the Colorado Plateau and eastern Basin and Range. Inverse modeling results of the crustal deformation data include a low-angle dislocation (~8-20°) at a locking depth of ~7-10 km, consistent with the interpreted seismic data, and slipping at 3.2±0.2 mm/yr, suggesting an active regionally extensive sub-horizontal surface beneath the eastern Basin and Range. A test of this hypothesis using seismic data interpretation as the basis for a forward strain accumulation model shows that displacement across a deep low-angle detachment imaged seismically is also consistent with geodetic velocities. Seismic and geodetic data support a model for eastern Basin and Range mechanics wherein diffuse permanent strain of the upper crust by multiple discrete faults is facilitated by displacement along a single low-angle detachment at midcrustal depth.

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Alluvial groundwater systems are an important source of water for communities and biodiverse riparian corridors throughout the arid and semi-arid Basin and Range Geological Province of western North America. These aquifers and their attendant desert streams have been depleted to support a growing population, while projected climate change could lead to more extreme episodes of drought and precipitation in the future. The only source of replenishment to these aquifers is recharge. This dissertation builds upon previous work to characterize and quantify recharge in arid and semi-arid basins by characterizing the intra-annual seasonality of recharge across the Basin and Range Province, and considering how climate change might impact recharge seasonality and volume, as well as fragile riparian corridors that depend on these hydrologic processes. First, the seasonality of recharge in a basin in the sparsely-studied southern extent of the Basin and Range Province is determined using stable water isotopes of seasonal precipitation and groundwater, and geochemical signatures of groundwater and surface water. In northwestern Mexico in the southern reaches of the Basin and Range, recharge is dominated by winter precipitation (69% ± 42%) and occurs primarily in the uplands. Second, isotopically-based estimates of seasonal recharge fractions in basins across the region are compared to identify patterns in recharge seasonality, and used to evaluate a simple water budget-based model for estimating recharge seasonality, the normalized seasonal wetness index (NSWI). Winter precipitation makes up the majority of annual recharge throughout the region, and North American Monsoon (NAM) precipitation has a disproportionately weak impact on recharge. The NSWI does well in estimating recharge seasonality for basins in the northern Basin and Range, but less so in basins that experience NAM precipitation. Third, the seasonal variation in riparian and non-riparian vegetation greenness, represented by the normalized difference vegetation index (NDVI), is characterized in several of the study basins and climatic and hydrologic controls are identified. Temperature was the most significant driver of vegetation greenness, but precipitation and recharge seasonality played a significant role in some basins at some elevations. Major contributions of this work include a better understanding of recharge in a monsoon-dominated basin, the characterization of recharge seasonality at a regional scale, evaluation of an estimation method for recharge seasonality, and an interpretation of the interaction of seasonal hydrologic processes, vegetation dynamics, and climate change.

abstract: A fundamental gap in geomorphic scholarship regards fluvial terraces in small desert drainages and those terraces associated with integrating drainages. This dissertation analyzes four field-based case studies within the Sonoran Desert, south-central Arizona, with the overriding purpose of developing a theory to explain the formative processes and spatial distribution of fluvial terraces in the region. Strath terraces are a common form (Chapters 2, 3, 4) and are created at the expense of bounding pediments that occur on the margins of constraining mountainous drainage boundaries (Chapters 1, 2, 3). Base-level fluctuations of the major drainages cause the formation of new straths at lower elevations. Dramatic pediment adjustment and subsequent regrading follows (Chapter 3), where pediments regrade to strath floodplains. This linkage between pediments and their distal straths is termed the pediment-strath relationship. Stability of the base level of the major drainage leads to lateral migration and straths are carved at the expense of bounding pediments through an erosional asymmetry facilitated by differential rock decay between the channel bank and bed. Fill terraces occur within the Salt River drainage basin as a result of the integration processes that connect formerly endorheic basins (Chapter 4). The topographic, spatial, and sedimentologic relationship of the Stewart Mountain terrace (Chapter 4) points to a different genetic origin than the lower terraces in this basin. The high Stewart Mountain fill terrace records the initial integration of this river. The strath terraces inset below the Stewart Mountain terrace are a result of the pediment-strath relationship. These case studies also reveal that the under-addressed drainage processes of piracy and overflow have significant impacts in the evolution of drainages the lead to both strath and fill terrace formation in this region. / Dissertation/Thesis / Ph.D. Geography 2013

La mise en place de laves et d'ignimbrites rhyolitiques de nature hyperalcaline marque le début de la rupture du bloc continental qui a précédé l'ouverture du Golfe de Californie. Ces roches affleurent à l'est du grand volcan bouclier quaternaire du Pinacate ainsi que dans la région d'Hermosillo. Les données de terrain, couplées à des déterminations géochronologiques 40Ar/39Ar, à une étude minéralogique fine, aux données chimiques et isotopiques (Sr, Nd et Pb) ont permis d'établir l'âge Miocène moyen du volcanisme hyperalcalin et de préciser les conditions de genèse et d'évolution de ce magmatisme. Les données isotopiques marquées par une grande variabilité des rapports en strontium, soulignent une contamination par la croûte précambrienne. Les modélisations effectuées sont en accord avec une origine des liquides acides hyperalcalins par cristallisation fractionnée à partir de basaltes transitionnels, dans des réservoirs superficiels, avec une contamination de l'ordre de 10 %. Des évidences de mélanges magmatiques montrent que ces liquides n'ont pas évolué en système fermé. Les affleurements de la région du Pinacate constituent le prolongement méridional des manifestations miocènes de même nature situées sur la bordure du craton nord-américain, depuis le Nevada jusqu'à la Californie. Le volcanisme du Sonora central est l'équivalent des formations qui affleurent dans la région de Puertecitos, en Basse Californie. Ces deux ensembles sont situés là aussi sur une limite lithosphérique majeure entre le craton nord-américain et les terrains accrétés des Guerrero terranes. Ce volcanisme est lié à des remontées de matériel asthénosphérique (slab window ?) qui ont favorisé les phénomènes d'amincissement lithosphérique et de production de magmas basaltiques de type transitionnel.

Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. / Graduation date: 2012 / In order for the .age files to run, the add-in called ArArCalc for Excel (version 200 or 2003) must be installed. ArArCalc is available from the website Earthref.org

The Hurricane Fault is a 250-km long, west dipping, Basin and Range-bounding normal fault in SW Utah and NW Arizona that initiated in the mid-Miocene to Pliocene. It has been primarily active in the Quaternary, with slip rates of 0.2 – 0.6 mm/yr. There are multiple hot springs along its 250-km length and multiple late Tertiary-Quaternary basaltic centers broadly parallel the fault. Possible sources of hot spring fluids include deeply-circulated meteoric water that experienced water-rock exchange at high temperatures (>100 °C) and deep-seated crustal fluids. Aside from the source of modern hot spring fluids and heat, questions about the spatio-temporal history of fluid flow along the Hurricane Fault remain unaddressed. Abundant damage zone veins, cements, and host rock alteration are present, indicative of past fluid flow. Carbonate veining and cementation is a key feature of the Hurricane Fault zone, and is the primary feature exploited to characterize the thermochemical history of fault-related paleofluids. A combination of macroscopic and microscopic carbonate observations, chemical composition, and precipitation temperature of calcite veins was used to determine past water-rock diagenetic interaction and vein evolution in the Hurricane Fault zone. Calcite iv in concretions and veins from the damage zone of the fault shows a wide range of carbon and oxygen stable isotope ratios, with δ13CPDB from -4.5 to 3.8 ‰ and δ18OPDB from -17.7 to -1.1‰. Fluid inclusion microthermometry homogenization temperatures range from 45 to 160 °C, with fluid salinities of 0 to 15 wt% NaCl calculated from melting temperatures. Combining the two datasets, two main fluids that interacted with the fault zone are inferred: (1) basin brines with a δ 18OSMOW of 9.2 ‰ and (2) altered meteoric fluids with a δ 18OSMOW of -11.9 to -8.3 ‰. Calculated dissolved CO2 δ 13CPDB (-8.5 to -1.3 ‰) indicates mixed marine carbonate and organic or magmatic sources. Fault zone diagenesis was caused by meteoric water infiltration and interaction with carbonate-rich rocks, mixed with upwelling basin brines. Fluid-rock interaction is concentrated in the damage zone, where fracture-related permeability was utilized for fluid flow. A distinct mineralization event punctuated this history, associated with basin brines that were chemically influenced by nearby basaltic magmatism. This implies a hydrologic connection between the fault and regional magmatism.

A geochemical study of major springs and wells in Spring Valley, Nevada, and Snake Valley, Utah-Nevada was initiated in response to the Clark, Lincoln and White Pine Counties Groundwater Development Project proposed by the South Nevada Water Authority (SNWA). Water budget estimates suggest that interbasin flow accounts for a significant portion (~25%) of the water budgets in Spring and Snake Valleys. Although interbasin flow is possible in some areas, alternative plausible explanations place significant uncertainty on water budget allocations. To examine the plausibility of local and interbasin flow paths the groundwater flow in Spring and Snake Valleys was evaluated using solute and isotopic data. Evidence for local flow paths includes: 1) stable isotope values in local areas which are similar to isotope values in adjacent recharge zones; 2) measurable 3H content and 14C activities ≥ 50 pmc in most samples which suggests short residence times; and 3) plausible geochemical models of local flow paths. Previously defined interbasin flow paths in southern Spring Valley are marked by samples that have low 14C activities (mean = 20.14 pmc), which are consistent with long residence times and can be explained by either interbasin flow from adjacent basins or deep circulation in the basin-fill sediments of Spring Valley. Interbasin flow from southern Spring Valley to southern Snake Valley cannot be confirmed or rejected based on the current data and modeling constraints, which result in plausible models involving both local flow paths and interbasin flow paths. Interbasin flow from northern Spring Valley to northern Snake Valley is unlikely and can be explained by the deep circulation of groundwater that is mixed with modern recharge. The plausibility of alternative explanations to describe previously defined interbasin flow paths suggests that water budget allocations in Spring and Snake Valleys should be redistributed or reevaluated. The use of existing water budgets that allocate large components of water to interbasin flow to determine the distribution of water resources may result in incorrect estimations of available resources.