abstract: A mineral’s helium content reflects a balance between two competing processes: accumulation by radioactive decay and temperature-dependent diffusive loss. (U-Th)/He dating of zircon and other uranium and thorium-bearing minerals provides insight into the temperature histories of rocks at or near Earth’s surface that informs geoscientists’ understanding of tectonic and climate-driven exhumation, magmatic activity, and other thermal events. The crystal structure and chemistry of minerals affect helium diffusion kinetics, recorded closure temperatures, and interpretations of (U-Th)/He datasets. I used empirical and experimental methods to investigate helium systematics in two minerals chronometers: zircon and xenotime. The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM. Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage. Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2019

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The Nepalese Himalaya, one of the most active regions within the Himalayan Mountain belt, is characterized by a thick succession of Miocene age Siwalik sedimentary rocks deposited at its foreland basin. To date, much of its tectonic evolution, including exhumation in the Nepalese Siwalik, is poorly understood. This study of a quantitative analysis of the bedrock river parameters should provide crucial information regarding tectonic activities in the area. The study investigated geomorphic parameters of river longitudinal profiles from 54 watersheds within the Siwalik section of the Nepalese Himalaya, for the first time. A total of 140 bedrock rivers from these watersheds were selected using stream power-law function and 30-meter resolution ASTER DEM. The quantitative data from the river longitudinal profiles were integrated with published exhumation ages. Results of this study show, first, a presence of major and minor knickpoints, with a total of 305 knickpoints identified, of which 180 were major knickpoints and the rest were minor knickpoints. Further classifications of knickpoints were based on structures (lineaments extracted from SRTM DEM), lithology, and possible uplift. Second, the Normalized Steepness index (ksn) values exhibited a range from 5.3 to 140.6. Third, the concavity index of streams in the study area ranged from as low as -12.1 to as high as 31.1 and the values were consistently higher upstream of the knickpoints. Finally, integration of the river profile data with the published exhumation ages show that the regions with a high ksn value correspond to the regions with higher incision and, therefore, are likely to have high uplift. The presence of a break in ksn in the eastern section of the study area suggests that the incision is likely accelerated by Main Frontal Thrust (MFT) movements. Erosion of the thrust sheet could have influenced the rapid uplift of the Siwalik due to isostatic processes. Thus, the timing of the source-region exhumation and its rate suggests that MFT-related tectonics, and/or climate processes, likely influenced the landscape evolution of the study area. The results of this study should help in comprehending the neo-tectonic deformation of the Nepalese Himalaya.

River incision into a widespread, upland low-relief landscape, and related patterns of exhumation recorded by low-temperature thermochronology, together underpin geodynamic interpretations for crustal thickening and uplift of the eastern Tibetan Plateau. We report results from a suite of 11 (U-Th-Sm)/He cooling-age samples. Eight samples comprise a 1.2 km relief section collected from elevations up to 4800 m in the Jiulong Shan, an elevated, rugged region located in the hinterland of the Yalong-Longmen Shan Thrust Belt, and surrounded on three sides by upland low-relief landscape surfaces. Zircon and apatite cooling ages record two episodes of rapid exhumation in the early Oligocene and late Miocene, that were separated by a period of stability from similar to 30 to 15 Ma. The first episode is consistent with a similar pulse evident from the Longmen Shan. The second episode is ongoing, and when integrated with adjacent cooling-age data sets, shows that doming of the Jiulong Shan has resulted in 2 to 4 km of differential exhumation of the plateau interior. We show from a compilation of glacial landform-mapping that the elevation of the plateau surface closely tracks global last glacial maximum equilibrium line altitude. We hypothesize that smoothing of highlands by efficient glacial and periglacial erosion, coupled with potential river captures and conveyance of sediments via external drainage, can yield an apparently continuous low-relief plateau landscape formed diachronously at high elevation. (C) 2016 Elsevier B.V. All rights reserved.

The long-term (>1 Ga) thermal histories of cratons are enigmatic, with geologic data providing only limited snapshots of their evolution. We use zircon (U-Th)/He (zircon He) thermochronology and age composition correlations to understand the Proterozoic-Phanerozoic thermal history of Archean Wyoming province rocks exposed in the northern Laramide ranges of western North America. Zircon He ages from the Wind River Range (54 dates) and Bighorn Mountains (32 dates) show negative correlations with effective uranium (eU), a proxy for radiation damage. Zircon dates from the Bighorns are between 960 Ma (low-eU) and 20 Ma (high-eU) whereas samples from the Wind Rivers are between 582 Ma (low-eU) and 33 Ma (high-eU). We applied forward modeling using the zircon radiation damage and annealing model ZrDAAM to understand this highly variable dataset. A long-term t-T path that is consistent with the available geologic constraints successfully reproduced age-eU correlations. The best fit to the Wind Rivers data involves two phases of rapid cooling at 1800-1600 Ma and 900-700 Ma followed by slower cooling until 525 Ma. During the Phanerozoic, these samples were heated to maximum temperatures between 160 and 125 degrees C prior to Laramide cooling to 50 degrees C between 60 and 40 Ma. Data from the Bighorn Mountains were successfully reproduced with a similar thermal history involving cooler Phanerozoic temperatures of similar to 115 degrees C and earlier Laramide cooling between 85 and 60 Ma. Our results indicate that age-eU correlations in zircon He datasets can be applied to extract long-term thermal histories that extend beyond the most recent cooling event. In addition, our results constrain the timing, magnitude and rates of cooling experienced by Archean Wyoming Province rocks between recognized deformation events, including the >1 Ga period represented by the regionally-extensive Great Unconformity.

A região de estudo localiza-se na porção da Bacia do Camaquã, sendo classicamente conhecida com “Janela Bom Jardim”. Esta área já foi intensamente estudada, principalmente em decorrência dos depósitos minerais amplamente distribuídos. Apesar disso ainda não existe um consenso com relação à evolução tectônica da região, desde processos formadores da bacia até eventos pósdeposicionais, como também com relação ao seu posicionamento estratigráfico dentro da sequência deposicional da Bacia do Camaquã. Dessa forma, este trabalho vem contribuir e acrescentar aos modelos anteriores com dados geocronológicos e termocronológicos, aplicados diretamente às rochas sedimentares da região, bem como, novos dados de estrutural. Para tanto, submetemos seis amostras de rochas sedimentares da região de Minas do Camaquã à análise pelo método U-Pb com LA-ICP-MS e SHRIMP, aplicado a zircões detríticos, no intuito de mapear possíveis áreas fonte dos sedimentos como também, propor uma idade máxima de deposição para a sequência. Estas mesmas amostras foram utilizadas para separação de apatitas para análise termocronológicas (traços de fissão em apatitas), com o objetivo de caracterizar eventos de soerguimento e denudação relacionados à tectônica que afetou a área. O estudo de proveniência possibilitou identificar uma grande contribuição de zircões de idade Brasiliana, principalmente do final do Neoproterozóico e secundariamente populações de zircões de idade Paleoproterozóica, principalmente relacionadas ao Ciclo Transamazônico. A população principal foi correlacionada às intrusões graníticas do Terreno Taquarembó, e possivelmente contribuições do Batólito Pelotas, como também, das sequências vulcano-sedimentares mais antigas da própria Bacia do Camaquã. As idades Paleoproterozóicas foram atribuídas á rochas do Complexo Granulítico Santa Maria Chico e Complexo Encantadas, como também, zircões retrabalhados das rochas metassedimentares do Complexo Metamórfico Porongos. Ainda foi possível estimarmos a idade máxima de deposição da sequência em 554 Ma a partir do grão de zircão mais jovem datado posicionando estas rochas dentro do intervalo deposicional do Grupo Santa Bárbara. A análise por traços de fissão em apatitas permitiu identificar quatro populações de idades principais. Estas populações foram atribuídas à influência de dois eventos orogênicos na margem sul – ocidental do Gondwana: Famatiniana e Gondwanides. O primeiro evento é bem documentado pelas populações de 407-362 Ma e o segundo pelas populações de 302 Ma e 242-211 Ma. A quarta população (133 Ma) foi registrada em uma única amostra indicando um evento térmico ocasionado por intrusões vulcânicas associadas ao vulcanismo Paraná-Etendeka (Cretáceo inferior). Por fim, dados estruturais permitiram identificar uma evolução tectônica complexa, marcada por processos relacionados formação da bacia até reativações pósdeposicionais. Os eventos D1 e D2 foram associados a esforços gerados durante os estágios finais do Ciclo Brasiliano, com campos de tensão S10E-N10W e SE-NW, respectivamente. Um evento D3 com campo de tensão W-L registra reativações causadas pela Orogenia Famatiniana. O evento D4 marcado por uma compressão NNE-SSW e uma extensão NE-SW são relacionados aos processos tectônicos Triássicos (Orogenia Gondwanides). Um último evento D5 extensional (L-W) foi correlacionado aos processos de separação do Gondwana. / The study area is located in the southern portion of the Camaquã Basin, being classically known as "Bom Jardim Window". This area has been intensely studied, mainly due to the ore deposits widely distributed. Yet there is still no consensus regarding the tectonic evolution of the region, since basin inception processes until postdepositional events, but also regarding its stratigraphic position within the depositional sequence Basin Camaquã. Thus, this work contributes to previous models with geochronological and thermochronological data, applied directly to the sedimentary rocks of the region, as well as new structural data. Therefore, we submitted six samples of sedimentary rocks from the Camaquã Mines region to U-Pb method with LA-ICP-MS e SHRIMP applied to detrital zircons in order to map possible source areas of the sediments, as well as, proposing a maximum depositional age for the sequence. These same samples were used for apatite separation for thermochronological analysis (apatite fission tracks), with the aim of characterizing uplift and denudation events related to tectonics process that affected the area. Provenance study enabled us to identify a large contribution of Brasiliano zircon ages, especially from Late-Neoproterozoic and secondarily, zircons populations of Paleoproterozoic age, mainly related to Trans-Amazonian Cycle. The main population was correlated with granitic intrusions from Taquarembó Terrene and possibly contributions from Pelotas Batholith, as well as, from older volcano-sedimentary sequences of the Camaquã Basin. Paleoproterozoic ages were assigned to rocks of Santa Maria Chico Granulitic Complex and Encantadas Complex, as well as, reworked zircons from metasedimentary rocks of Porongos Metamorphic Complex. Still it was possible to estimate the maximum deposition age of the sequence on 554 Ma from the youngest zircon grain dated. Previous isotopic data allow positioning these rocks within the depositional range of Santa Bárbara Group. Apatite fission track analysis identified four main ages populations. These populations were attributed to the influence of two orogenic events on the Southwestern margin of Gondwana: Famatinian and Gondwanides. The first event is well documented by the 407-362 Ma age populations and the second by the 302 Ma and 242-211 Ma. The fourth population (133 Ma) was recorded in only one sample indicating a thermal event caused by volcanic intrusions associated with volcanism Paraná-Etendeka (Lower Cretaceous). Finally, structural data allowed identifying a complex tectonic evolution, characterized by basin inception processes until post-depositional reactivations. The events D1 and D2 were associated with stresses generated during the final stages of the Brazilian Cycle with stress fields of S10E-SE-NW and SE-NW, respectively. A third event (D3) with W-L stress field records the reactivation Famatinian Orogeny. The D4 event marked by a NNE-SSW compression and a NE-SW extension related to Triassic tectonic processes (Gondwanides Orogeny). One last extensional (W-L) event (D5) was correlated to the processes of separation of Gondwana.

Seismogenically active faults (those that produce earthquakes) are very complex systems that constantly change through time. When an earthquake occurs, the rocks surrounding a fault (the “fault rocks”) become altered or damaged. Studying these fault rocks directly can inform what processes operated in the fault and how the fault evolved in space and time. Examining these key aspects of faults helps us understand the earthquake hazards of active fault systems. The Mecca Hills, southern California, consist of a set of hills adjacent to the southernmost San Andreas Fault. The topography is related to motion on the San Andreas fault, which poses the largest seismic hazard in the lower forty-eight United States. The southernmost San Andreas fault, and the Mecca Hills study location may be reaching the end of its earthquake cycle and is due for a major, potentially catastrophic earthquake. The seismic hazards of the region, coupled with its proximity to major populated areas (Coachella Valley, Los Angeles Basin) make it a critical research area to understand fault zone evolution and the protracted history of fault development. The goal of this thesis was to directly examine the fault rocks in the Mecca Hills to understand how San Andreas-related faults in this area have evolved and behaved through time. This study integrates a variety of field and laboratory techniques to characterize the structural, geochemical, and thermal properties of the Mecca Hills fault rocks. The results herein document two distinct phases of deformation in the rocks exposed in the Mecca Hills, one around 24 million years ago and the other in the last one million years. This more recent phase of deformation is characterized by fault block exhumation and fluid flow in the fault zones, likely related to changing dynamics of the southernmost San Andreas Fault system. The older event informs how and when these rocks came close to Earth’s surface before the San Andreas Fault initiated.