University of Alabama Repository :: Browsing by Author "Stowell, Harold Hilton"

Browsing by Author "Stowell, Harold Hilton"

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Bulk chemical composition and mineral effects on grain conductivity and ice nucleation affinity of volcanic ash
(University of Alabama Libraries, 2017) Cloer, Shelby; Genareau, Kimberly D.; University of Alabama Tuscaloosa
Volcanic lightning is a common phenomenon during explosive eruptions, occurring as vent discharges, near-vent discharges, and plume lightning. Plume lightning is most similar to thunderstorm lightning, where volcanic ash may act as ice nuclei, leading to charging from ice-ice or ice-particle collisions. Volcanic ash samples were used to evaluate the role of ash mineralogy and bulk composition in the intrinsic electrical behavior and ice nucleation efficiency of ash. Samples from 8 volcanoes were used: Augustine, Crater Peak, Katmai, Okmok, Redoubt (Alaska, U.S.A.), Lathrop Well (Nevada, U.S.A.), Taupo (New Zealand), and Valles Caldera (New Mexico, U.S.A.). Five to nine resistance measurements were performed on all ash samples using an Electro-Tech Systems Model 828/863 current amplifier and resistance meter in a controlled environment. Depositional and immersion-mode ice nucleation experiments were performed using a Nicolet Almega XR Dispersive Raman spectrometer, following the methods of Schill et al. (2015). Depositional nucleation experiments were conducted from 225-235 K, and immersion-mode nucleation experiments were conducted from 233-278 K. A JEOL JSM 6010 Plus/LA scanning electron microscope (SEM) and Image-J freeware were used to quantify the number density of mineral phases in backscattered electron images. An x-ray diffractometer (XRD) was used to determine bulk mineralogy and an x-ray fluorescence (XRF) spectrometer was used to determine bulk ash composition. Resistance measurements with SEM analyses reveal that bulk ash composition and mineralogy do not control ash grain electrical conductivity. However, bulk composition and mineralogy do control the frozen fractions generated in immersion-mode ice nucleation experiments, with amounts of MnO, TiO2, and percentage of Fe-oxide phases showing a negative correlation with the frozen fraction. This study adds to our knowledge base on volcanic lightning dynamics and adds new implications for global climate models, which currently only address effects of mineral dust as ice nuclei and overlook the potential role of volcanic ash.
Effects of grain size and shape on volcanic ash electrical conductivity
(University of Alabama Libraries, 2017) Woods, Taylor Watson; Genareau, Kimberly D.; University of Alabama Tuscaloosa
The occurrence of volcanic lightning during explosive eruptions is a common phenomenon, yet the exact mechanisms of electric charge transmission in explosive eruption columns and plumes are poorly understood. Ash is a probable charge carrier and so the physical properties of ash may factor into charge transmission, specifically, the shape and size of ash grains. To examine the relationship between volcanic ash electrical conductivity and particle size and shape, this research compares conductivity measurements to grain size distributions and shape parameters from several volcanic centers. Grain size distributions were measured using a laser diffraction particle size analyzer (LDPSA); grain shapes (e.g., aspect ratios, solidity) were characterized using backscattered electron images. The volume resistivity of minimally compressed samples was measured at controlled temperature (25-30 ℃) and relative humidity (25 %) using a current amplifier and values were converted to conductivity. A general effective media (GEM) equation was then applied to account for variations in grain packing, grain shape, and sample porosity. Although all ash samples are electrically insulating, homogenized samples provided maximum conductivity measurements that ranged over roughly four orders of magnitude (10-9 – 10-13 S/cm); non-homogenized bulk Alaskan sample conductivity values ranged over roughly six orders of magnitude (10-9 – 10-15 S/cm). Results indicate that size has a greater effect on ash conductivity than shape. Results also suggest that particle size distribution controls ash conductivity, as samples with a wide distribution of particle sizes and larger surface areas, due to a combination of fine and coarse grains, will have the highest conductivity values. Therefore, the efficiency of magma fragmentation during explosive eruptions, and the resulting proportions of fine and coarse ash particles generated, controls the bulk electrical conductivity of volcanic ash and may contribute to charge transmission mechanisms in eruptive columns and plumes.
Explaining discontinuous garnet zoning using reaction history p-t models: an example from the Salmon River suture zone, west-central Idaho
(University of Alabama Libraries, 2015) Bollen, Elizabeth Marie; Stowell, Harold Hilton; University of Alabama Tuscaloosa
Discontinuously zoned or two-stage garnet has been observed in numerous locations and geologic settings worldwide. These garnets are characterized by sharp breaks in inclusion density and compositional zoning, and often, these sharp breaks are interpreted as a hiatus in growth, change in growth rate, change in bulk rock composition, chemical diffusion, or absorption and new growth of garnet. During accretion of terranes and microplates, thermal pulses and thrust fault movements occur, which drive metamorphism and therefore the growth of garnet. Multiple garnet growth events could produce a discontinuously zoned garnet and each growth stage could be interpreted to represent a separate metamorphic event. Two-stage garnet is common in the Salmon River suture zone (SRSZ) and multiple tectonic models have been proposed based on the two-stage garnet. Getty et al. (1993) and Selverstone et al. (1992) proposed multiple accretion and metamorphic events based on the estimates for pressure, temperature, and age of these garnets. Recently, McKay (2011) proposed that heating after several major fault displacements caused the growth of two-stage garnet. This study uses compositions of garnet cores and rims on isochemical phase diagrams to construct new garnet growth P-T paths. Core and rim P-T estimates combined with observed mineral assemblages indicate an initial garnet growth reaction, followed by a reaction consuming and then growing garnet, e.g., chlorite + garnet = amphibole + H2O and amphibole = garnet + Al2SiO5 (kyanite) + H2O. Isochemical P-T modeling of garnet modal percentages, mineral compositions, and petrologic observations supports the occurrence of these reactions in the SRSZ garnet. The proposed reaction history would produce two-stage garnet along a single prograde path, which does not require multiple thermal and tectonic events. This interpretation supports the single terrane accretion hypothesis proposed by McKay (2011).
Geochronology and pressure-temperature conditions of mid-to-lower crustal processes in a Cretaceous magmatic arc, Fiordland, New Zealand
(University of Alabama Libraries, 2010) Parker, Karen Ann; Stowell, Harold Hilton; University of Alabama Tuscaloosa
Exposures of mid-to lower-crust in Fiordland contain evidence of high temperature and pressure processes occurring beneath magmatic arcs. This thesis presents new U-Pb zircon ages that constrain emplacement of the Malaspina Pluton, Western Fiordland Orthogneiss (WFO) to 116-114 Ma and new Sm-Nd garnet ages of 115-110 Ma for partial melting of the Malaspina near the Doubtful Sound Shear Zone (DSSZ), central Fiordland. U-Pb igneous zircon ages, from west to east across Doubtful Sound (DS), are 114.2 ± 2.2, 115.4 ± 2.2, and 116.1 ± 2.4 Ma. This compares to a contact metamorphic zircon rim age of 114.8 ± 2.4 Ma for the 119.8 ± 2.9 Ma Misty Pluton. From west to east across DS, Sm-Nd garnet ages are 113.6 ± 2.5, 113.2 ± 2.8, 115.6 ± 2.6, 111.6 ± 3.1, 110.6 ± 1.9, 113.3 ± 2.6, and 111.9 ± 3.0 Ma. Hence partial melting and garnet granulite-facies metamorphism lasted ≥ 5 m.y. in DS and occurred within 5.5 m.y. of pluton emplacement. These ages compare to Sm-Nd garnet ages for garnet granulite of 126-109 Ma in northern Fiordland (Pembroke) and ca. 110 Ma in southern Fiordland (Resolution Island). Mineral assemblages in DS migmatite include Amp + Cpx + Grt + Qtz + Pl ± Bt + Rt + Ilm ± relict OPx ± relict Czo. Euhedral, peritectic garnet occurring in leucocratic veins and reaction zones, has little zoning in composition and ranges from Alm_53 Prp_27 Sps_3 Grs_17 in the west to Alm_44 Prp_36 Sps_1.5 Grs_18.5 in the east. Ilmenite with exsolved rutile and oriented exsolution needles of rutile in garnet indicate re-equilibration after high T and P. Phase diagram sections for melted WFO and thermobarometry using garnet, clinopyroxene, and plagioclase compositions indicate 700-840°C and 10.6-14.2 kbar for metamorphism on DS. Similar methods for Pembroke Granulite of >750°C at 12-16 kbar indicate diachronous widespread high P metamorphism and partial melting for this ca. 4,375 km^2 section of mid-to lower-crust with these processes occurring early in the north and later in the south (126-105) Ma.
Investigating ultra-low velocity zones at the core-mantle boundary beneath the Southern Hemisphere using an Antarctic dataset
(University of Alabama Libraries, 2018) Carson, Sarah; Hansen, Samantha E.; University of Alabama Tuscaloosa
The core-mantle boundary (CMB) represents the largest absolute density contrast on our planet, and it is associated with significant heterogeneities. The CMB structure focused on in this study are ultra-low velocity zones (ULVZs), laterally-varying, 5-50 km thick isolated patches seen in some locations just above the CMB that are associated with increased density and reduced seismic wave velocities. The variable characteristics associated with ULVZs have led to many questions regarding their origins, but only about 17% of the CMB has been surveyed for the presence of ULVZs given limited seismic coverage of the lowermost mantle. Therefore, investigations that sample the CMB with new geometries are critical to further our understanding of ULVZs and their potential connection to other deep Earth processes. The Transantarctic Mountains Northern Network (TAMNNET), a 15-station seismic array that was deployed in Antarctica from 2012-2015, provides a unique dataset to further study ULVZ structure with new and unique path geometry. Core-reflected ScP phases recorded by TAMNNET well sample the CMB in the vicinity of New Zealand in the southwestern Pacific, providing coverage between an area to the northeast where ULVZ structure has been previously identified and another region to the south, where ULVZ evidence is inconclusive. This area is of particular interest because the data sample across the boundary of the Pacific Large Low Shear Velocity Province (LLSVP). The Weddell Sea region near Antarctica is also well sampled in this study, providing new information on this area that has not been previously studied. By identifying and modeling energy associated with the ScP waveform, new portions of the CMB have been explored and evidence for ULVZs in both regions has been found. A correlative scheme between 1-D synthetic seismograms and observed data demonstrate that ULVZs are required in the study regions, but modeling uncertainties limit the ability to definitively define ULVZ characteristics. Given that ULVZs are detected within, along the edge of, and far from the Pacific LLSVP, the results support the hypothesis that ULVZs are compositionally distinct from the surrounding mantle, and thus may be ubiquitous along the CMB; however, they may be thinner than can be resolved by seismic detection in some locations. Mantle convection currents may sweep the ULVZs into thicker piles in some areas and may push these anomalies toward the boundaries of LLSVPs.
Investigation of natural weathering processes and artificial treatment techniques in the attenuation of toxic metals from coal fly ash
(University of Alabama Libraries, 2010) Bhattacharyya, Sidhartha; Donahoe, Rona Jean; University of Alabama Tuscaloosa
Coal fly ash contains high levels of hazardous trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V, which may have a negative impact on the environment due to potential leaching by acid rain and groundwater. This study seeks to develop new fly ash management techniques by determining the effects of natural weathering on trace element mobility in fly ash and by evaluating the potential use of surfactant-modified zeolite and ferrous sulfate treatment for attenuating the mobility of trace elements associated with fly ash. The effects of weathering on trace element mobility in fly ash were studied using batch competitive adsorption experiments. Fresh fly ash shows high adsorption capacity for As, V and Mo, while weathered fly ash shows high affinity for Ni, Sr, As and V. Both fresh and weathered fly ash show low adsorption capacity for Se and B. Weathering reduced the adsorption capacities of fresh fly ash for As, B, Cr, Mo, Se and V, indicating increased mobility in ash disposal environments. The effectiveness of surfactant-modified zeolite (SMZ) as a PRB material was studied using batch experiments under competitive adsorption conditions. The results showed that SMZ preferably adsorbed V, Mo and Cr over As and Se. Unmodified zeolite (UMZ) showed high adsorption capacities for Ni and Sr. Both SMZ and UMZ failed to remove B from solution. The use of SMZ as a PRB material in coal fly ash management will be limited by its low affinity for B as well as its relatively low affinity for As, Se and cations. Ferrous sulfate treatment of coal fly ash successfully reduced the mobility of oxyanionic trace elements. The unbuffered 1:30 FS treatment was highly successful; oxyanion mobility reductions were: As (24-91%), Cr (82-97%), Mo (79-100%), Se (41-87%) and V (55-100%). On the other hand, the 1:30 FS fly ash treatment failed to reduce the mobility of B, Ni and Sr. Ferrous sulfate treatment is cost effective and can be applied directly to fresh fly ash produced in electric power plants, as well as to the fly ash already placed in the ash disposal facilities.
Magmatism, metamorphism, and deformation in the Mountain Home Metamorphic Complex, Blue Mountains Province, Oregon, and its role in late Jurassic deformation in the western North American Cordillera
(University of Alabama Libraries, 2013) Anderson, Bryan Scott; Schwartz, Joshua; University of Alabama Tuscaloosa
Orogenesis in the Blue Mountains Province (BMP) of NE Oregon involved the accretion of various tectonostratigraphic terranes in concert with syn-tectonic magmatism. Timescales of deformation, metamorphism and magmatism in this region of the Cordillera are poorly constrained, hindering detailed understanding of the timing of tectonic events and the magmatic and metamorphic processes accompanying them. The Mountain Home Metamorphic Complex (MHMC) is a sequence of medium-grade metavolcanic and metasedimentary rocks that borders the Wallowa and Baker terranes of the BMP. The MHMC is intruded by two groups of plutons distinguished based on their respective presence and absence of crystal-plastic deformational textures. This study utilizes U-Pb zircon dating on both crystal-plastically deformed and undeformed pluton suites to bracket the timing of metamorphism and deformation in the MHMC. The whole rock geochemistry of the MHMC was compared to the adjacent Baker and Wallowa terranes to assists in determining the relationship between these terranes and the MHMC. In addition, the petrogenesis of plutonic and metamorphic rocks in the MHMC is determined by Lu-Hf isotopic ratios of zircons. Two distinct plutonic suites are present in the MHMC: 1) deformed hornblende (±clinopyroxene) gabbro, hornblende tonalite and trondhjemite (Group 1), and 2) undeformed biotite-hornblende tonalite, biotite-hornblende quartz diorite, and hornblende gabbronorite (Group 2). 206Pb/238U zircon ages for Group 1 plutons range from 159.46 ± 0.67 to 149.5 ± 1.6 Ma; whereas Group 2 plutons yield ages ranging from 149.4 ± 2.1 Ma to 145.66 ± 0.50 Ma. These ages bracket deformation to between 149.5 to 149.4 Ma, which contrasts with the timing of previously documented tectonic events in the BMP. Epsilon Hf values in the MHMC range from +16.3 to +10.2 in Group 1 plutons and +13.8 to +4.2 in Group 2 plutons. These values exclude correlation between the MHMC and the Baker terrane, but not the Wallowa terrane. On a large scale, the MHMC is metamorphosed to amphibolite facies, a significantly higher grade than any other regional scale metamorphism in the BMP. The timing of deformation constrained in this study does not correspond with any of the previously proposed deformational models for the BMP. The geographically closest known deformational event that corresponds with the timing of deformation in the MHMC is the 150 ±1 Ma Nevadan Orogeny of the Klamath Mountains. The unique timescale of deformation and higher grade metamorphism imply that the MHMC does not belong to the Baker or Wallowa terranes, but may be a new terrane in the BMP.
Metamorphism of the Wenatchee Ridge orthogneiss: a combined application of geochronology and phase equilibrium modeling
(University of Alabama Libraries, 2013) Holler, Robert; Stowell, Harold Hilton; University of Alabama Tuscaloosa
Pressure and temperature estimates from phase diagram models indicate that the Wenatchee Ridge orthogneiss is the deepest exposed segment of the crust in the Nason terrane of the Cascades crystalline core, WA. Peak metamorphic P-T conditions of 8-11.5 kbar and ~600-700 ºC exceed predictions for the Chiwaukum Schist and Mt. Stuart batholith to the west-southwest and are compatible with a gradient of increasing P and T from the southernmost Nason terrane to the NRMG and WRO. Wenatchee Ridge orthogneiss zircon U-Pb ages (88.9-84.8 Ma) are identical to other zircon ages, interpreted as ages of intrusion, from the Pear Lake orthogneiss. Additional similarities in external grain morphology and internal zoning suggest an igneous origin to the Wenatchee Ridge orthogneiss. The P-T results and ages support the interpretation of the Wenatchee Ridge orthogneiss origin as one of many intrusive bodies within the Nason Ridge Migmatitic Gneiss, all of which intruded the Chiwaukum Schist. In addition, the results are compatible with a proposed southwest oriented thrust loading resulting in 1) older metamorphic ages and 2) elevated pressure and temperature conditions in the northeastern Nason terrane.
Pressure-temperature-time paths, prograde garnet growth, and protolith of tectonites from a polydeformational, polymetamorphic terrane: Salmon River Suture Zone, West-Central Idaho
(University of Alabama Libraries, 2011) McKay, Matthew P.; Stowell, Harold Hilton; University of Alabama Tuscaloosa
The metamorphic rocks of the Salmon River suture zone (SRSZ) in west-central Idaho provide a unique glimpse into mid-lower crustal processes during continental growth by island arc accretion. The SRSZ, which separates island arc terranes of the Blue Mountains Province (BMP) from the Mesozoic margin of North America, contains medium to high grade tectonites that record multiple metamorphic and deformation events. The SRSZ is divided by the Pollock Mountain thrust fault (PMtf) into two structural blocks: the higher-grade Pollock Mountain plate (PMp), and the lower-grade, underlying Rapid River plate (RRp). Previous studies interpreted pre-144 Ma metamorphism within the SRSZ related to assembly of the BMP. Counter-clockwise P-T paths for metamorphism within the RRp [peak=8-9 kbar ~600°C, retrograde=5-7 kbar, 450- 525°C] were inferred to include prograde garnet growth during pre-144 Ma loading followed by garnet growth during rapid cooling due to lithospheric delamination. The PMp was interpreted to have subsequently been buried to increasing depth and metamorphosed again at 128 Ma as a result of the BMP docking with North America. New P-T-t paths for the RRp and PMp constructed from geochronology, geothermobarometry, pseudosections, and petrography suggest that after loading, slow cooling rates caused diffusion in garnet rims, which produced counter-clockwise P-T paths. Garnet Sm-Nd ages of 112.5±1.5 Ma from the RRp, and 141-124 Ma from the PMp suggest that metamorphism within the SRSZ is diachronous and that crustal thickening was protracted occurring between 141-112 Ma. P-T-t paths between both plates indicate that the PMp reached peak metamorphism prior to peak metamorphism of the RRp. This suggests that the PMp was buried prior to the development of the PMtf. The RRp was subsequently buried along the PMtf, which was followed by development of the Rapid River thrust fault, which juxtaposed RRp schists onto the Wallowa terrane of the BMP. This model suggests that metamorphism in the SRSZ was controlled by individual thrust faults instead of recording collisions between terranes and is consistent with a prolonged burial of rocks in the SRSZ followed by slow cooling that does not require lithospheric delamination to account for retrograde P-T estimates.
Pyroclastic evidence of syn-eruptive degassing at the explosive/effusive transition
(University of Alabama Libraries, 2016) Matthews, Emily Suzanne; Genareau, Kimberly D.; University of Alabama Tuscaloosa
The 2010 eruption of Merapi (Java, Indonesia) initiated with an uncharacteristic explosion, followed by rapid lava dome growth and collapse, all of which generated deadly pyroclastic density currents (PDCs). PDC samples from the initial explosion on October 26th were collected from several locations surrounding the edifice. Plagioclase phenocrysts represent the primary component of the dominant ash mode due to the elutriation of the finer ash fraction during PDC transport. Secondary electron images of 45 phenocrysts were taken using the scanning electron microscope (SEM) to examine preserved glass coatings on phenocrysts, which represent the interstitial melt within the magma at the point of fragmentation. Using these images, the bubble number densities (BNDs) were determined, and the decompression rate meter of Toramaru (2006) was used to calculate the decompression rate during the initial explosion of the 2010 Merapi eruption. Calculated decompression rates range from 6.08x10^7 Pa/s to 1.4x10^8 Pa/s. Decompression rates have shown to correlate with eruption column height; therefore Merapi’s rates should be similar to those of other Vulcanian explosions, because the eruption column was 8-9 km in height. Sakurajima volcano (Japan) experienced decompression rates from 7.0 × 10^3 to 7.8 × 10^4 Pa/s during the later phase of the fall 2011 Vulcanian explosions. Plinian explosions, such as at the 1991 eruption of Mt. Pinatubo and the 1980 eruption of St. Helens had much higher column heights compared to the initial 2010 Merapi explosion; 35 km, 19 km, and 8-9 km, respectively, but decompression rates in a comparative range (10^8 Pa/s). Higher decompression rates during the 2010 initial explosion at Merapi likely resulted from increased overpressure in the shallow conduit, as revealed through previous geochemical analyses of the erupted crystals Results indicate that decompression rates may be underestimated for Vulcanian explosions.
Rifting and subduction in the Papuan peninsula. Papua New Guinea: the significance of the Trobriand tough, the Nubara strike-slip fault, and the Woodlark Rift to the present configuration of Papua New Guinea
(University of Alabama Libraries, 2014) Cameron, Milo Louis; Goodliffe, Andrew M.; University of Alabama Tuscaloosa
The calculated extension (~111 km) across the Woodlark rift is incompatible with the > 130 km needed to exhume the Metamorphic Core Complexes on shallow angle faults (< 30°) using N-S extension in the Woodlark Basin. High resolution bathymetry, seismicity, and seismic reflection data indicate that the Nubara Fault continues west of the Trobriand Trough, intersects the Woodlark spreading center, and forms the northern boundary of the Woodlark plate and the southern boundary of the Trobriand plate. The newly defined Trobriand plate, to the north of this boundary, has moved SW-NE along the right lateral Nubara Fault, creating SW-NE extension in the region bounded by the MCC's of the D'Entrecasteaux Islands and Moresby Seamount. Gravity and bathymetry data extracted along four transect lines were used to model the gravity and flexure across the Nubara Fault boundary. Differences exist in the elastic thickness between the northern and southern parts of the lines at the Metamorphic Core Complexes of Goodenough Island (Te_south = 5.7 x 103 m; Te_north = 6.1 x 103 m) and Fergusson Island (Te_south = 1.2 x 103 m; Te_north = 5.5 x 103 m). Differences in the elastic strength of the lithosphere also exist at Moresby Seamount (Te_south = 4.2 x 103 m; Te_north = 4.7 x 103 m) and Egum Atoll (Te_south =7.5 x 103 m; Te_north = 1.3 x 104 m). The differences between the northern and southern parts of each transect line imply an east-west boundary that is interpreted to be the Nubara Fault. The opening of the Woodlark Basin resulted in the rotation of the Papuan Peninsula and the Woodlark Rise, strike slip motion between the Solomon Sea and the Woodlark Basin at the Nubara Fault, and the formation of the PAC-SOL-WLK; SOL-WLK-TRB triple junctions. The intersection of the Woodlark Spreading Center with the Nubara Fault added the AUS-WLK-TRB triple junction and established the Nubara Fault as the northern boundary of the Woodlark plate
Role of changing effective bulk composition in the formation of grossular zoning during garnet granulite metamorphism in the Western Fiordland Orthogneiss, NZ
(University of Alabama Libraries, 2015) Norton, Rebecca Ann; Stowell, Harold Hilton; University of Alabama Tuscaloosa
Garnet compositions and zoning, which are important records of crustal processes, are often used to constrain P-T-t paths for deep crustal rocks now exposed at the surface. These P-T-t paths require knowledge of the stable minerals and effective bulk composition during each increment of garnet growth, because effective bulk composition may change due to compositional fractionation by mineral growth and/or mass transfer. The importance of changing effective bulk composition as the result of partial melting is shown for two samples from the Western Fiordland Orthogneiss, New Zealand. Both samples experienced partial melting during garnet granulite metamorphism. High grossular garnet rims in granulite from Fiordland and elsewhere have been interpreted to result from pressure increases during garnet growth. Pseudosection models presented here indicate that changes in effective bulk composition from host, to garnet reaction zone, and then to leucosome, could produce peritectic garnet with high-grossular rims which grew along an isobaric path during melting and/or melt injection.
Seismic investigations of the northern Transantarctic Mountains
(University of Alabama Libraries, 2017) Graw, Jordan Hunter; Hansen, Samantha E.; University of Alabama Tuscaloosa
Stretching ~3500 km across Antarctica and reaching elevations of ~4500 m, the Transantarctic Mountains (TAMs) are the largest non-compressional mountain chain on Earth. The TAMs show no evidence of folding or reverse faulting as is typically seen in contractional mountain building, calling the origin of the mountain range into question. Using data from the recent Transantarctic Mountains Northern Network seismic deployment, this dissertation integrates Rayleigh wave surface wave tomography, downward continuation and wavefield decomposition, and seismic anisotropy studies to better characterize the structure beneath the northern TAMs and to assess uplift. Surface wave tomographic images indicate a previously unidentified low shear wave velocity anomaly beneath the northern TAMs, with faster seismic velocities behind the TAMs front. The low shear wave velocity anomaly is interpreted as reflect rift-related decompression melting associated with Cenozoic extension. Uplift for the TAMs is attributed to a thermal buoyancy force associated with this anomaly. When trying to assess crustal structure, ice coverage is typically troublesome as reverberations in the ice layer can complicate the P-wave response. Downward continuation and wavefield decomposition removes the effect of ice layers on the P-wave response, resulting in signal that can be directly modeled for Earth structure. Inversion solution models agree well with results from previous studies based on S-wave receiver functions and tomography, confirming relatively thin crust beneath the northern TAMs. Upper mantle structure can also be assessed with seismic anisotropy. I performed shear wave splitting analyses on PKS, SKS, and SKKS phases to obtain the splitting parameters (fast axis directions φ and delay times δt). Behind the TAMs front, the anisotropic signature is interpreted as relict fabric “frozen” into the lithosphere from tectonic processes in the geologic past. Near the Ross Sea coastline, the signature is interpreted as a result from rift-related decompression melting, creating active upper mantle flow. Results highlight heterogeneity in the uplift along the TAMs front. The degree of uplift in the northern TAMs is similar to that in the central TAMs; however, the northern TAMs appear to have a stronger thermal buoyancy component, creating more pronounced topography.
Structural and kinematic evolution of the Himalayan thrust belt, central Nepal
(University of Alabama Libraries, 2014) Khanal, Subodha; Robinson, D. M.; University of Alabama Tuscaloosa
Over the last two decades, several competing dynamic models have been proposed to explain the kinematics of the Himalayan thrust belt. The accuracy of dynamic and kinematic models is limited by poorly documented geologic structures. With increased accessibility of the thrust belt and advances in analytical techniques, several new data sets greatly improve our understanding and provide a background to reevaluate the kinematics of the Himalayan thrust belt. In this dissertation, I integrate structural mapping, microstructural analysis, detrital and igneous zircon geochronology, low-temperature thermochronology, Nd isotopic analysis, and structural reconstructions in central Nepal to determine the evolution of the Himalayan thrust belt. Because the role and evolution of the Main Central thrust, the Ramgarh-Munsiari thrust, and the Lesser Himalayan duplex are highly debated, I emphasize these systems to provide a comprehensive structural evolution of the Himalayan thrust belt. U-Pb dating of metamorphic rims of igneous zircons and crystallization ages of cross-cutting pegmatite veins suggest that deformation on the Himalayan thrust belt started with slip on an intra-Greater Himalayan thrust active at ~20-29 Ma that emplaced the now erosionally isolated Kathmandu klippe. These ages predate the slip on the Main Central thrust. Absence of a fault contact between the Greater Himalaya and Tethyan Himalaya in the klippe suggests the South Tibetan Detachment system may have activated after the slip started on the intra-Greater Himalayan thrust. Ductile motion on the South Tibetan Detachment system may have ended prior to the activation of the Main Central thrust. This result and observations contradict the extrusion model that advocates contemporaneous activity with thrust sense shear on the Main Central thrust and normal sense shear on the South Tibetan Detachment system. In addition, there is another orogenic scale thrust, subparallel to the Main Central thrust, the Ramgarh-Munsiari thrust, that only carries lower Lesser Himalayan Paleoproterozoic rock over other Lesser Himalayan rock and accommodates a magnitude of shortening similar to the 100's km of slip on the Main Central thrust. I construct an orogenic scale balanced cross-section along the Marsyangdi River where the entire Lesser Himalayan duplex is exposed, particularly focusing on the architecture of the duplex to determine whether the duplex is forward dipping or hinterland dipping and the presence/absence of an orogenic scale, out-of-sequence thrust. I integrate quartz-feldspar deformation temperatures and zircon (U-Th)/He thermochronology and present a kinematic model that provides the structural context for geophysical, petrological, and geochronological studies in central Nepal. Collectively, this study helps to determine partitioning of strain among the various thrust sheets that account for over 2000 km of shortening in a compressional continental tectonic setting. The results suggest that deformation in the Himalaya began with the activation of an intra-Greater Himalayan thrust and successively moved south with the activation of Main Central thrust, Ramgarh-Munsairi thrust, Lesser Himalayan duplex, and finally the Subhimalayan thrust system. Although there was minor out-of-sequence thrusting in the hinterland, the bulk of the Himalaya evolved in-sequence thrusting from north to south.
U-Pb zircon and monazite geochronology and hafnium isotopic geochemistry of neoacadian and early alleghanian plutonic rocks in the Alabama Eastern Blue Ridge, Southern Appalachian Mountains
(University of Alabama Libraries, 2012) Ingram, Stanton B.; Schwartz, Joshua; University of Alabama Tuscaloosa
The Alabama eastern Blue Ridge (EBR) of the Southern Appalachian Mountains hosts a variety of felsic plutonic rocks, which intrude multiply deformed Neoproterozoic to Ordovician metasedimentary rocks. Plutons consist of two distinct suites based on geochemical composition and degree of deformation: pre- to syn-kinematic Neoacadian, low Sr/Y plutons (ca. 380-360 Ma) and late- to post-kinematic, Early Alleghanian high Sr/Y plutons (ca. 350-330 Ma). Here, I report new whole rock geochemistry, U-Pb zircon SHRIMP-RG (Sensitive High Resolution Ion Micro Probe-Reverse Geometry) ages, and Hf isotope data for 6 plutons in the Alabama EBR. Low Sr/Y plutons are predominantly biotite-muscovite granites and granodiorites and include the Rockford Granite (376.6 ± 1.5 Ma) and the Bluff Springs Granite (363.8 ± 2.9 Ma). The Enitachopco trondhjemite dike also displays a Neoacadian age of 366.5 ± 3.5 Ma. Zircon Hf isotope data from the low Sr/Y suite range from -11.2 to +2.0. These plutons are in general strongly deformed, and display geochemical characteristics consistent with mid crustal (<35 km) partial melting of pre-existing continental crust. By contrast, high Sr/Y plutons are deformed to undeformed, and consist of low-K tonalites and trondhjemites (e.g., Almond trondhjemites and Blakes Ferry pluton) with geochemical characteristics suggestive of deep-crustal partial melting of a garnet amphibole-bearing source. Two samples of the Almond trondhjemite (Wedowee pluton and Almond pluton) yielded ages of 334.6 ± 3.2 Ma and 343.4 ± 3.4 Ma, respectively. An additional peak at 324.4 ± 3.3 may represent a Pb-loss event. Another sample of Almond trondhjemite yielded complex ages with a peak at 349.1 ± 1.8 Ma The undeformed Blakes Ferry pluton also yielded complex results with Grenville-age cores (ca. 1000-1080 Ma), and rim ages ranging from ca. 350 to 330 Ma with peaks at 343.1 ± 3.3 Ma and 331.1 ± 3.8. Igneous monazite yielded an age of 345.9 ± 3.1 Ma supporting a ca. 345 Ma crystallization age. Hf isotope data from the high Sr/Y suite range from -14.6 to +5.6. We propose that the transition from Neoacadian, low Sr/Y, mid-crustal partial melting to Early Alleghanian high Sr/Y deep crustal partial melting reflects thickening of the EBR during Neoacadian deformation. Hf isotope values also transition from crustal values (-εHf) to a mixed signature (+εHf and -εHf), reflecting both mantle and lower crustal melting. This transition may be related to slab break off following Neoacadian deformation.
U-Pb zircon geochronology, Hf isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)
(University of Alabama Libraries, 2012) Jeffcoat, Charles Ryan; Schwartz, Joshua; University of Alabama Tuscaloosa
Macquarie Island, located in the Southern Ocean, is a section of subaerially exposed oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. Macquarie Island is unique among oceanic ophiolite sequences in that it is still located in the basin in which it formed. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite, and therefore provides a unique window into lower oceanic crust. Here, we report integrated Pb/U zircon ages and Lu-Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three mantle-hosted gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. ^206 Pb/^238 U SHRIMP-RG zircon error weighted average ages for the lower crustal gabbros are 8.7 ± 0.3 Ma (N = 9) and 9.0 ± 0.2 Ma (N = 13), whereas the mantle-hosted gabbro dikes/dikelettes yield overlapping error-weighted average ages of 8.7 ± 0.2 Ma (N = 9), 8.7 ± 0.3 Ma (N = 8), and 8.9 ± 0.2 Ma (N = 11) (all errors 2σ). The phlogopite vein yielded a slightly younger error weighted average age of 8.5 ± 0.1 Ma (N = 11). Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N = 28), +7.0 to +16.4 for the gabbro dikes/dikelettes (N = 24), and +8.4 to +12.2 for the phlogopite vein (N = 12). The wide range in values (particularly from the gabbro dikes/dikelettes) is consistent with a heterogeneous source region composed of depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. No pattern of enrichment with time is observed within the resolution of the U-Pb zircon dates. We interpret these results to indicate that magmatic construction of Macquarie Island occurred between 8.7 and 9.0 Ma and involved sampling of at least two distinct mantle sources.
Using thermochronology to test the validity of two balanced cross sections, Karnali River, far-western Nepal
(University of Alabama Libraries, 2019) Battistella, Claire; Robinson, D. M.; University of Alabama Tuscaloosa
A balanced cross section provides a viable subsurface geometry; however, multiple subsurface cross section geometries may be possible to match the observed surface geology. Low-temperature thermochronologic data and thermokinematic models can be used to test these multiple subsurface geometries. In this study, I use muscovite 40Ar/39Ar, apatite and zircon fission track, and zircon (U-Th)/He ages in conjunction with two balanced cross sections along the Karnali River in far western Nepal to determine if the cross sections are viable. The balanced cross sections are reconstructed to determine the kinematic sequence and then sequentially deformed in ~10 km increments with flexural loading and erosional isostatic unloading applied to each step. Ages are assigned to each 10 km increment of displacement and then used to create a 2-D thermokinematic model, which predicts the cooling ages along the section. By varying the location of the décollement ramp, shortening rates, and radiogenic heat production, the model is modified to better match the existing observed age data. Comparison of the modeled cooling ages to the thermochronologic data indicates that although the cross sections are valid and balanced, the subsurface fault geometry does not reproduce the existing observed cooling ages over the young uplift imparted by the active ramp in the décollement below the Lesser Himalayan duplex. Thus, a new balanced cross section is needed with a different location and geometry of the ramp to produce the existing cooling ages.
Utilizing high-precision geochronology to evaluate the rates of geologic processes
(University of Alabama Libraries, 2013) Gatewood, Matthew Patrick; Stowell, Harold Hilton; University of Alabama Tuscaloosa
High-precision U-Pb zircon and Sm-Nd garnet isotopic ages are used to investigate the rates of geologic processes over timespans of millions of years and spatial scales ranging from mountain belts (km) to thin-sections (microns). New techniques are presented to improve the precision and spatial resolution of the isotopic data used for calculating garnet Sm-Nd ages. Established analytical methods are used to determine U-Pb ages for complex zircon. These techniques are used to address: (1) a tectonic problem in the North Cascades crystalline core; and (2) to evaluate the fundamental process of chemical equilibrium in a garnet-bearing schist from the central Appalachians. Previously-published whole-grain detrital zircon ages for the Swakane Gneiss have been interpreted to reflect post ca. 73 Ma deposition of the protolith sediments and used to propose rapid loading of the Swakane protolith to ca. 35 km in < 5 m.yr. However, Swakane Gneiss zircons have polyphase growth zoning and preserve a complex depositional and metamorphic history. The rapid loading model is tested by correlating new garnet Sm-Nd ages with spot ages of metamorphic zircon overgrowths to establish the timing of metamorphism. I also construct P-T-t paths for the depositional and loading history of the Swakane Gneiss. Garnet-rock Sm-Nd isochrons of 73.5±1.2, 71.3±2.8, and 65.8±0.7 Ma, document the timing of garnet growth. Homogeneous metamorphic zircon rims have high U/Th (> 5) and define an array of concordant U-Pb ages from 75 to 63 Ma. P-T-t paths for six Swakane Gneiss samples compliment these new ages by documenting a metamorphic history involving: (1) loading of the protolith sediments to 24-33 km, likely via overthrusting of arc rocks; (2) heating to peak temperatures of 650-710°C; and (3) decompression and cooling during exhumation. The new P-T-t dataset documents that regional metamorphism of the Swakane Gneiss protolith occurred during thrust loading and subsequent heating between 75 and 63 Ma. A new model for pre-75 Ma deposition and 75-63 Ma metamorphism of the Swakane Gneiss is proposed that does not require rapid loading rates. To evaluate the process of chemical equilibrium in rocks, I test the idea that Mn contents and high-precision Sm-Nd ages of compositionally-equivalent garnet crystal segments should correlate from garnet to garnet throughout an equilibrium volume of metamorphic rock from Townshend Dam, VT. Major element zoning in garnet is concentric, with Mn-rich cores and Mn-poor rims. Garnets are HREE-enriched overall and show little-to-no zoning except for increased MREE and HREE near the rims. Similar garnet compositions and zoning profiles throughout the sample suggest that garnet growth occurred at the same P-T-X-M conditions over the sample volume. Garnet-rock isochrons calculated for 35 compositionally-specific garnet crystal segments range from 383.3±7.4 Ma to 374.9±1.8 Ma. Sampled Mn contents and Sm-Nd age uncertainties make it difficult to evaluate the correlation of Mn and age for each compositionally-equivalent crystal segments throughout the sample volume. However, 7 garnet cores, 14 mantle zones, and 8 rims, define three distinct multi-point isochrons of 380.5 ± 2.0 Ma, 377.4 ± 1.2 Ma, and 376.6±0.9 Ma, respectively, yielding a garnet growth duration of 3.9±2.2 m.yr. These three composite Mn-age zones make up a Mn vs. age curve that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Grouped isotopic ages and Mn contents suggest that major element and isotopic equilibrium were generally maintained throughout the short duration of garnet growth in the sample. However, a detailed correlation of Mn and garnet age is precluded by the short duration of garnet growth in this sample.

Browsing by Author "Stowell, Harold Hilton"

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