Dissertations / Theses on the topic 'Apatite (U-Th)/He'

Une première partie de cette thèse est méthodologique: elle vise à améliorer la thermochronométrie (U-Th-Sm)/He sur apatite et les modèles de diffusion actuels. Nous avons étudié le cas du Massif Armoricain (France), et celui des Alpes Suisses. Nos travaux montrent que la rétentivité en hélium des apatites est plus élevée que prévu par les modèles traditionnellement utilisés. Une approche statistique comprenant des algorithmes d'apprentissage montre que la composition chimique des grains ne semble pas jouer sur la rétentivité des apatites en hélium. Le principal paramètre contrôlant la dispersion des âges hélium est l'état d'endommagement des cristaux. Nous proposons une approche de physique/minéralogie expérimentale pour le caractériser à l'échelle nanométrique. Dans la dernière partie de la thèse, nous avons appliqué la méthode (U-Th-Sm)/He sur apatite au cas de la marge algérienne. Nous mettons en évidence une phase majeure de dénudation dans la région des Petites Kabylies au cours du Tortonien. Cette phase marque probablement le début de l'inversion de la marge, bien plus précocement que suggéré jusqu'alors
The first part of the thesis aims at improving the methodology and the models involved in apatite (U-Th-Sm)/He thermochronology. For this purpose, we studied two geological cases: the Armorican Massif (France) and the Swiss Alps. Our work suggests that apatite helium retentivity is higher than predicted in traditional models. A statistical approach using machine learning algorithms evidences that the apatite chemical composition of grains does not influence helium retentivity. The key parameter is the parent radionuclide and the crystal damage contents. We suggest an experimental procedure in order to characterize damage in apatite at the sub-micrometer scale. In the last part of the thesis, we applied the (U-Th-Sm)/He method to the Algerian Margin. We evidenced a major denudation phase in Petite Kabylie ("Lesser Kabylia") during the Tortonian times. This phase likely corresponds to the initiation of the margin inversion, earlier than previously suggested

The uniquely low temperature sensitivity of the apatite (U-Th)/He system makes it an invaluable tool for studying shallow crustal processes which are not accessible through other techniques. Major advancements in both the theoretical and practical aspects of the technique have taken place over the past decade or so, however the routine application of the process is often held back by the perceived problem of single grain age ‘over dispersion’, particularly when applied to old, slowly cooled geological settings. There persists a misconception that age dispersion is indicative of a problem with the apatite (U-Th)/He system. A significant component of single grain age dispersion is inherent to the natural system, and therefore beneficial to reconstructing robust thermal histories. Variations in crystal grain size, accumulated amounts of radiation damage and changes to the helium concentration gradient within a grain due to fragmentation all contribute positively to age dispersion. Other, imposed factors such as crystal zoning and 4He implantation (which are undesirable) can also contribute to dispersion, however in the vast majority of cases their effects are negligible and only contribute noise to the inherent natural dispersion signal. The Ballachulish Igneous complex (BIC) in western Scotland has been used as a case study to demonstrate the range of age dispersion which should be expected when analysing large numbers of single grain aliquots per sample. Where 20+ grains are analysed, total dispersion will often be well in excess of 100% for old, slowly cooled samples, indeed dispersion in excess of 200% is possible. Such dispersion will often be as a consequence of outlying or apparently anomalous ages, however such ages should not be discounted unless there is sound analytical justification to do so. Apparent anomalous ages will often be ‘swallowed up’ by the data if more, or even different sized/shaped grains are analysed. Due to the competing effects of the three main causes of inherent natural dispersion, it should not be expected that large, well dispersed data sets will show any significant correlation between single grain age and either grain size or eU concentration. However a lack of correlation does not indicate poor quality data. Brown, Beucher and co-workers (Brown et al., 2013; Beucher et al., 2013) proposed a new modelling approach to account for the common occurrence of broken crystals in apatite separates, demonstrating that the additional inherent natural age dispersion arising from analysing fragments can be exploited when reconstructing thermal histories. A new inversion technique – HelFRAG was developed, based on a finite length cylinder diffusion model. The model is computationally demanding, therefore sampling based inversion methods requiring many forward model simulations become less practical. Consequently, an approximation of the finite cylinder diffusion model has been incorporated into the modelling software QTQt (Gallagher, 2012). Here, the approximation – QFrag has been demonstrated capable of returning comparable results to the full HelFRAG inversion technique when given the same synthetic data set, enabling the more routine application of the fragment model. Both QFrag and HelFRAG modelling techniques have been used to model the new BIC AHe dataset. The purpose is twofold: to demonstrate the importance of the fragment model with a real dataset, and to provide a new thermochronological interpretation for the BIC. When using this dataset, modelling samples individually shows only subtle differences (if any) between modelling broken grains correctly as fragments, verses modelling them incorrectly as whole grains. A far greater difference in the model output is seen when only modelling 3-6 grains compared to 20+, irrespective of whether fragments are treated correctly or not. When multiple samples are modelled together in a vertical profile, the fragment effect becomes much more important. A very different thermal history interpretation arises when any broken grains are modelled incorrectly as whole grains compared to when modelled as fragments. The new thermal history interpretation for the BIC involves a four stage cooling history from the time of intrusion (c. 424Ma). Very rapid cooling and uplift occurred immediately after intrusion over the first c. 20Myrs of the history (Phase 1). This brought the complex from c. 10km depths to within 2-3km of the surface. There followed much slower continued uplift between c. 404Ma and c. 300Ma, resulting in up to 1km of denudation (Phase 2). Over the next c. 150Myrs only a small volume of uplift occurred, however the geothermal gradient increased towards the end of this time period, suggesting crustal thinning (Phase 3). A final, rapid period of cooling and uplift occurred at c. 140Ma, bringing the top of the profile very near to the surface (Phase 4). No significant denudation has occurred since the end of this rapid uplift phase (10’s to 100’s of meters at most). The first two phases of cooling are interpreted as the final stages of the Caledonian orogeny, with erosion driven isostatic uplift causing continued denudation after the cessation of collisional tectonics. The end of phase three and the subsequent rapid uplift (Phase 4) are interpreted as the beginnings of crustal thinning and continental rifting which ultimately led to the opening of the North Atlantic Ocean.

Craton cores far from plate boundaries have traditionally been viewed as stable features that experience minimal vertical motion over 100-1000Ma time scales. Here we show that the Fennoscandian Shield in southeastern Sweden experienced several episodes of burial and exhumation from similar to 1800Ma to the present. Apatite, titanite, and zircon (U-Th)/He ages from surface samples and drill cores constrain the long-term, low-temperature history of the Laxemar region. Single grain titanite and zircon (U-Th)/He ages are negatively correlated (104-838Ma for zircon and 160-945Ma for titanite) with effective uranium (eU=U+0.235xTh), a measurement proportional to radiation damage. Apatite ages are 102-258Ma and are positively correlated with eU. These correlations are interpreted with damage-diffusivity models, and the modeled zircon He age-eU correlations constrain multiple episodes of heating and cooling from 1800Ma to the present, which we interpret in the context of foreland basin systems related to the Neoproterozoic Sveconorwegian and Paleozoic Caledonian orogens. Inverse time-temperature models constrain an average burial temperature of similar to 217 degrees C during the Sveconorwegian, achieved between 944Ma and 851Ma, and similar to 154 degrees C during the Caledonian, achieved between 366Ma and 224Ma. Subsequent cooling to near-surface temperatures in both cases could be related to long-term exhumation caused by either postorogenic collapse or mantle dynamics related to the final assembly of Rodinia and Pangaea. Our titanite He age-eU correlations cannot currently be interpreted in the same fashion; however, this study represents one of the first examples of a damage-diffusivity relationship in this system, which deserves further research attention.

The integration of low-temperature thermochronometers, including apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission-track (AFT) methods, allows for a quantification of the thermal history experienced by rocks as they heat and cool through upper crustal temperature regimes (<200°C). Whereas these methods are practical in geologic terranes that have undergone rapid cooling, application to strata with protracted cooling histories is complicated by the enhanced role of grain-specific parameters (volume, chemistry, radiation damage) on the kinetics of helium diffusion and fission track annealing. The effects of these variables are most prevalent in sedimentary samples, where natural variance in detrital accessory mineral populations results in a broad range of diffusion kinetics and great dispersion in corresponding cooling dates. This thesis integrates contemporary thermochronometer diffusion and annealing kinetics to investigate the burial and exhumation history of two natural laboratories. In the Mackenzie Mountains and Plain of the Northwest Territories, long-term radiation damage accumulation in zircon from Neoproterozoic siliciclastic units produces ZHe dates that track Albian to Paleocene burial and exhumation in front of the foreland-propagating fold-thrust belt. For the Phanerozoic stratigraphic section, AFT annealing kinetics are calculated from Devonian and Cretaceous samples, and are incorporated into multi-kinetic AFT modeling. These kinetics also constrain AHe date-radiation damage trends, and when combined allow for an estimation on the magnitude of eroded sediment across regional pre-Albian and post-Paleocene unconformities. Finally, conodont (U-Th)/He data from Anticosti Island, Québec in the Gulf of the St. Lawrence are compared with ZHe, AHe and AFT data to test their utility as a thermochronometer for carbonate basin analysis. These data evince a Mesozoic thermal history previously unattributed to the region. Ultimately, this thesis provides a novel assessment on the ways in which thermochronometer date dispersion can be quantified to assess the thermal evolution of sedimentary basins from burial through to inversion.

Uplift of the Teton Range is primarily controlled by displacement across the range-front Teton normal fault. The Tetons comprise the footwall block while the hanging wall encompasses Jackson Hole valley and a portion of the Snake River. Relative to the rest of the Rocky Mountains, the Tetons experienced the majority of uplift very recently, substantiating the need for a detailed investigation integrating structural analysis and bedrock thermochronometry. New low-temperature cooling ages are documented in three vertical transects across the Teton Range and at low elevations parallel to the Teton fault. Samples adjacent to the Teton fault are consistently young (~9 Ma) and represent a minimum estimate for the onset of Teton fault-related uplift. Modeling of time-temperature histories supports a ~9-11 Ma onset of rapid uplift, indicating that the Teton fault likely originated as a Basin and Range-type structure. A maximum throw of ~8 km occurs proximal to the Grand Teton, while the average throw for the entire ~100 km along-strike fault length is ~3.3 km. Thus, the geometry of the Teton fault is comparable to traditional scaling relationships dictating a correlation between fault length and displacement. Inversion of the typical (U-Th)/He (AHe) and fission track (AFT) relationship in a few of the Teton Range samples is a result of intense zoning, primarily in apatite from Precambrian layered gneisses. Nonetheless, both the AHe and AFT ages consistently indicate slight differential uplift of the Tetons between the Late Oligocene and Middle Miocene. HeFTy models indicate that doming of the Precambrian-Paleozoic unconformity occurred prior to ~50 Ma. However, by ~15 Ma, rapid cooling of the Mount Moran section essentially â flattenedâ the unconformity. Thus, the modern domed shape is a result of displacement across the Teton fault, allowing the unconformity to be used as a proxy for fault deformation. Moreover, reconstruction of the unconformity and volume calculations produced an average depth to incision of ~0.3 km and a long-term erosion rate of 0.18 mm/yr. Compared to the long-term uplift rate of 0.22 mm/yr, this provides a quantitative explanation for the modern Teton topography.
Master of Science

The Pacific-North American plate boundary is typified by transpression and convergence, yet the relationship between interplate deformation and long-term crustal shortening is not fully understood. The continuous belt of rugged topography that extends along the entire plate boundary is generally associated with oblique tectonic plate motion, strong interplate coupling, and terrane accretion, but relating plate boundary orogenesis to variations in plate geometry and behavior requires detailed case studies. The northern San Gabriel Mountains along the San Andreas fault and the Chugach-Kenai Mountains above the Aleutian subduction zone are located along highly tectonically active sections of the Pacific-North American plate boundary and have not been studied from the context of long-term landscape development. To determine whether mountain building along these sections of the plate boundary reflects recent, rapid exhumation as observed in bordering mountain belts, low-temperature thermochronometry and topographic analyses were applied to each area. In the northern San Gabriel Mountains, apatite (U-Th)/He ages are >10 Ma along narrow crystalline ridges topped by low-slope erosional surfaces located within ~5 km of the San Andreas fault zone. In the Chugach-Kenai Mountains, the youngest apatite (U-Th)/He ages (~5 Ma) are an order of magnitude older than those from the Yakutat collision zone to the east, despite the presence of a continuous swath of glaciated, rugged topography between the two areas. Exhumation rates inferred from these ages are <1 mm/yr, suggesting that there has been minimal recent denudation in the northern San Gabriel and Chugach-Kenai Mountains. The lack of evidence for recent mountain building in both of these case studies implies that interplate deformation is heterogeneous and that other factors (secondary structures, climate) besides plate kinematics and topographic character must be considered for understanding landscape development.
Ph. D.

A evolução da margem continental do sudeste do Brasil tem sido discutida por diversos autores desde meados do século passado até os dias atuais, especialmente no contexto da origem e evolução dos escarpamentos e das bacias tafrogênicas. Buscou-se contribuir com novos dados sobre a evolução da área a partir da aplicação da termocronologia de baixa temperatura (U-Th)/He em apatita, que oferece uma sensibilidade significativa para registrar movimentações tectônicas na crosta superior. Foi possível obter idades em 107 cristais de apatita de 18 amostras do embasamento coletadas no perfil com orientação NW-SE, numa seção entre a Serra da Mantiqueira e o Gráben da Guanabara. As idades corrigidas variam entre 250,1 8,7 Ma e 43,5 1,9 Ma (2 σ) e as não corrigidas entre 174,13 3,03 Ma e 27,07 0,60 Ma (1 σ). O Neocretáceo, o Eocretáceo e o Paleoceno são os principais registros no conjunto de dados, em ordem de importância. No Neocretáceo, o intervalo entre 83,6 e 72,1 Ma (Campaniano) representa o maior destaque nos registros termocronológicos, embora os outros registros (Maastrichtiano e Santoniano) também estejam presentes e sejam importantes. As idades do Neocretáceo destacam a importância dos eventos tectonomagmáticos e soerguimento regional na história térmica dessa área, inclusive com idades (~86 Ma) atribuídas ao contexto de soerguimento da Serra do Mar. As idades do Eocretáceo indicam o registro de eventos térmicos mais antigos, vinculados à evolução pré-rifte. Já os dados do Paleoceno estariam associados ao evento de reativação responsável pela implantação do sistema de riftes continentais (~65 Ma) e as idades do Eoceno, restritas à borda de falha da bacia de Resende (49,7 Ma e 43,5 Ma), à reativação do sistema de riftes nessa área. A dispersão de idades foi interpretada como efeito dos danos de radiação já que muitos grãos apresentam correlação entre idade e concentração de urânio (eU). Os padrões de tempo-temperatura (t-T), definidos a partir dos modelos HeFTy calibrados para o modelo de difusão que considera os efeitos de danos de radiação nos cristais, registraram eventos de resfriamento rápido, os quais mostram correlação direta com episódios de reativação e soerguimento na margem continental e com registros nas bacias continentais e marginais. O padrão de aumento das idades com a elevação, assim como da costa em direção ao interior é observado, mas mostra-se alterado pela ocorrência de idades mais jovens associadas à complexa evolução dessa margem continental com desnivelamentos de blocos vinculados à tectônica pós-rift, numa situação que ressalta a influência dos episódios de reativação. As estimativas de denudação total variam entre 1,2 e 2,8 km. As taxas de erosão variam entre 15,2 e 35,3 m/Ma. A evolução da área indica não apenas a influência de um evento específico mas, possivelmente, uma combinação de episódios que se alternaram e/ou atuaram em conjunto em determinados períodos. Os eventos de reativação mais antigos, combinados com os mais recentes, exibem os seus remanescentes na paisagem (serras da Mantiqueira e do Mar e os grábens e bacias sedimentares) e assumem papel fundamental na evolução da área. Os registros de tais episódios podem ser observados nas histórias térmicas das rochas e nos depósitos correlativos nas bacias sedimentares marginais e intracontinentais.
This doctoral dissertation involves low-temperature thermochronologic investigations to constrain the Mesozoic and Cenozoic tectonic reactivation of the continental margin of southeast Brazil. The study area is located in a segment of the Neoproterozoic Ribeira belt in southeastern Brazil, which occupied a central position in Western Gondwana. Lately, during the Mesozoic and Cenozoic, fault zones related to the development of the continental rifts in southeast Brazil reactivated. Using apatite (U-Th)/He thermochronology (AHe), we have presented the results of analysis on 107 apatite crystals of basement samples collected from a NWSE transect in the Mantiqueira Mountains to the Guanabara Graben, as well as from the NESE transverse faults. The data range from 27.07 0.60 Ma to 174.13 3.03 Ma (1 σ) for uncorrected ages, and from 43.5 1.9 Ma to 250.1 8.7 Ma (2 σ) for corrected ages. The Neo-Cretaceous, Eo-Cretaceous, and Paleocene are the main recorded AHe ages, in order of importance. The Eo-Cretaceous ages indicate the occurrence of older thermal events related to a pre-rifting phase (~121 Ma). The Neo-Cretaceous ages signify the importance of tectonic and magmatic events, and regional uplifting for the thermal history of the study area, including ages related to the Serra do Mar Mountains uplift (~86 Ma). Paleocene ages seem to be related to the reactivation (~65 Ma), which was responsible for the continental rifts in the southeastern Brazil. Finally, the Eocene ages (49.7 Ma and 43.5 Ma), which are from samples restricted to the Resende Basin border faults, indicate a continental rift reactivation. We investigated the age dispersion data, which were interpreted as an effect of radiation damage. Several samples show correlations between apatite (U-Th)/He age and effective U concentration (eU). We have applied HeFTy thermal modeling to obtain timetemperature (t T) paths using a radiation damage diffusion model. Inverse modeling of (U-Th)/He age data suggests rapid cooling episodes for all samples. The main thermal events recorded by the HeFTy models show a direct correlation with the timing of regional tectonic events: reactivation phases, continental margin uplift, and the sedimentary record. Apatite (U-Th)/He ages increase with distance from the coast and with elevation. However, these patterns are discontinued by samples of younger ages as a result of the reactivation process of pre-existing structures in the Brazilian continental margin. The total estimated denudation range from 1.2 to 2.8 km. The erosion rates range from 15.2 to 35.3 m/My. Thus, the multi-episodic thermal events, which led to the formation of important regional tectonic and geomorphological features (Mantiqueira Mountains, Serra do Mar Mountains, grabens, and sedimentary basins), seem to play a fundamental role in the evolution of the Brazilian continental margin. The records can be found in the thermal history of rocks and its correlated deposits in the continental margin.

The St. Lawrence Platform (SLP) and Humber Zone (HZ) of the southern Quebec Appalachians has historically been explored as a potential hydrocarbon reservoir. Extensive vitrinite reflectance studies on the basin resolved the degree of thermal maturation yet the timing of the thermal maximum is not well undertood. Determining the timing of such low temperature events can allow for a better understanding of the shallow crustal processes that may have allowed for the generation and entrapment of oil and gas. We have employed apatite (AHe) and zircon (ZHe) (U-Th)/He thermochronmetry across a network of late Cambrian to late Ordovician siliciclastic and Grenvillian basement samples in order to resolve the history within the ~210-35°C window. Single crustal dates from individual samples show age dispersion by as much as 300 m.y. with a strong positive to negative correlation with increasing eU concentration. A similar positive correlation can be observed when significant intra-sample grain size variation is present. AHe and ZHe data in the southwestern portion of the basin, near Montreal, allow for thermal maxima of up to 200°C to occur either during the late Ordovician, as a result of the Taconic orogeny, or from the continued sedimentation into the Devonian as a result of the Acadian orogeny. Regional burial trends deduced from these thermal maxima along with local paleo-geothermal gradients indicate that if sedimentation continued after the late Ordovician there was no significant increase in burial in southwestern portion of the SLP as previously suggested. Maximum heating is followed by a protracted cooling through the ZHe partial retention zone (PRZ) into the late Jurassic and early Cretaceous where the cooling rate increases by an order of magnitude through the AHe PRZ until ca. 100 Ma. The timing of this accelerated cooling is coeval with the passage of the Great Meteor Hot Spot across the area; the cooling may be a result of increased erosion from thermal uplift. Within the HZ, both the external and internal sections experienced rapid cooling through the Silurian after the Taconic thermal maximum. The timing of relatively rapid cooling coincides with documented normal faulting and back-thrusting in the orogen, which is the likely cause of exhumation. The HZ witnessed protracted cooling through the late Jurassic, when there is a one order of magnitude increase in cooling rate until surface conditions are attained. Increased recognition of these low temperature events has augmented our understanding of the evolution of accretionary orogens and consequently reduces the risks associated with oil and gas exploration.

Three studies on Cordilleran foreland basin deposits in the western U.S.A. constitute this dissertation. These studies differ in scale, time and discipline. The first two studies include basin analysis, flexural modeling and detailed stratigraphic analysis of Upper Cretaceous depocenters and strata in the western U.S.A. The third study consists of detrital zircon U-Pb analysis (DZ U-Pb) and thermochronology, both zircon (U-Th)/He and apatite fission track (AFT), of Upper Jurassic to Upper Cretaceous foreland-basin conglomerates and sandstones. Five electronic supplementary files are a part of this dissertation and are available online; these include 3 raw data files (Appendix_A_raw_isopach_data.txt, Appendix_C_DZ_Data.xls, Appendix_C_UPb_apatite.xls), 1 oversized stratigraphic cross section (Appendix_B_figure_5.pdf), and 1 figure containing apatite U-Pb concordia plots (Appendix_C_Concordia.pdf). Appendix A. Subsidence in the retroarc foreland of the North American Cordillera in the western U.S.A. has been the focus of a great deal of research, and its transition from a flexural foreland basin, during the Late Jurassic and Early Cretaceous, to a dynamically subsided basin during the Late Cretaceous has been well documented. However, the exact timing of the flexural to dynamic transition is not well constrained, and the mechanism has been consistently debated. In order to address the timing, I produced new isopach maps from ~130 well log data points that cover much of Utah, Colorado, Wyoming and northern New Mexico, producing in the process, the most detailed isopach maps of the area. These isopach maps span the Turonian to mid-Campanian during the Late Cretaceous (~93–76 Ma). In conjunction with the isopach maps I flexurally modeled the Cordilleran foreland basin to identify when flexure can no longer account for the basin geometry and identified the flexural to dynamic transition to have occurred at 81 Ma. In addition, the dynamic subsidence at 81 Ma is compared to the position of the hypothesized Shatsky Oceanic Plateau and other proposed drivers of dynamic subsidence. I concluded that dynamic subsidence is likely caused by convection over the plunging nose of the Shatsky Oceanic Plateau. Appendix B. The second study is a detailed stratigraphic study of the Upper Cretaceous, (Campanian, ~76 Ma) Sego Sandstone Member of the Mesaverde Group in northwestern Colorado, an area where little research has been done on this formation. Its equivalent in the Book Cliffs area in eastern Utah has been rigorously documented and its distal progradation has been contrastingly interpreted as a result of active tectonism and shortening in the Cordilleran orogenic belt ~250 km to the west and to tectonic quiescence, flexural rebound in the thrust belt and reworking of proximal coarse grained deposits. I documented ~17 km of along depositional dip outcrops of the Sego Sandstone Member north of Rangely, Colorado. This documentation includes measured sections, paleocurrent analysis, a stratigraphic cross section, block diagrams outlining the evolution of environments of deposition through time, and paleogeographic maps correlating northwest Colorado with the Book Cliffs, Utah. The sequence stratigraphy of the Sego Sandstone Member in northwest Colorado is similar to that documented in the Book Cliffs area to the south-southwest, sharing three sequence boundaries. However, flood-tidal delta assemblages between fluvio-deltaic deposits that are present north of Rangely, Colorado are absent from the Book Cliffs area. These flood-tidal-delta assemblages are likely caused by a large scale avulsion event in the Rangely area that did not occur or was not preserved in the Book Cliffs area. In regards to tectonic models that explain distal progradation of the 76 Ma Sego Sandstone Member to be caused by tectonic quiescence and flexural rebound in the thrust belt, the first study shows that at 76 Ma, flexural processes were no longer dominant in the Cordilleran foreland, so it is inappropriate to apply models driven by flexure to the Sego Sandstone Member. Dynamic processes dominated the western U.S.A. during the Campanian, and flexural processes were subordinate. Appendix C. In order to test the tectonic vs. anti-tectonic basin-filling models for distal coarse foreland deposits mentioned above, the third study involves estimating lag times of Upper Jurassic to Upper Cretaceous conglomerates and sandstones in the Cordilleran foreland basin. Measuring lag time requires a good understanding of both the stratigraphic age of a deposit and the thermal history of sedimentary basin. To further constrain depositional age, I present twenty-two new detrital zircon U-Pb (DZ U-Pb) sample analyses, spanning Upper Jurassic to Upper Cretaceous stratigraphy in Utah, Colorado, Wyoming and South Dakota. Source exhumation ages can be measured using thermochronology. To identify a thermochronometer that measures source exhumation in the North America Cordillera, both zircon (U-Th)/He, on eleven samples, and apatite fission track (AFT) thermochronology, on eleven samples was performed. Typically, the youngest cooling age population in detrital thermochronologic analyses is considered to be a source exhumation signal; however, whether or not these apatites are exhumed apatites or derived from young magmatic and volcanic sources has been debated. To test this, I double dated the detrital AFT samples, targeting apatites with a young cooling age, using U-Pb thermochronology. Key findings are that the maximum depositional ages using DZ U-Pb match existing biostratigraphic and geochronologic age controls on basin stratigraphy. AFT is an effective thermochronometer for Lower to Upper Cretaceous foreland stratigraphy and indicates that source material was exhumed from >4–5 km depth in the Cordilleran orogenic belt between 118 and 66 Ma, and zircon (U-Th)/He suggests that it was exhumed from <8–9 km depth. Double dating apatites (with AFT and U-Pb) indicate that volcanic contamination is a significant issue; without having UPb dating of the same apatite grains, one cannot exclude the possibility that the youngest detrital AFT population is contaminated with significant amounts of volcanogenic apatite and does not represent source exhumation. AFT lag-times are 0 to 5 Myr with relatively steady-state to slightly increasing exhumation rates. We compare our data to orogenic wedge dynamics and subsidence histories; all data shows active shortening and rapid exhumation throughout the Cretaceous. Our lag-time measurements indicate exhumation rates of ~.9–>>1 km/Myr.

The evolution of mountain landscapes is controlled by complex interactions between large-scale tectonic, surficial and climate conditions. Dominant processes are attributed to creating characteristic features of the landscape, but topographic features are the cumulative result of coupled surficial processes, each locally effective in a different climate or elevation regime. The focus of erosion by glacial, fluvial, or mass wasting processes is highly sensitive to small changes in boundary conditions, therefore spatial and temporal variability can be high when observed over short time scales. This work evaluated methods for dissecting the history of complex alpine landscapes to understand the role of individual processes influenced by changing climate and underlying bedrock. It also investigated how individual and combined mechanisms of surficial processes influenced the evolution of topography in the Teton Range in Wyoming. Detrital apatite (U-Th)/He thermochronology and cosmogenic radionuclide erosion rates were applied to determine spatial and temporal variability of erosion in the central catchments of the range. Spatial variability existed between the glacial and fluvial systems, indicating that sediment erosion and deposition by these processes was controlled by short-term variability in climate conditions. Effective glacial incision also controlled other processes, specifically enhancing rock fall activity and inhibiting fluvial incision. Short-term erosion rates were highly variable and were controlled by stochastic processes, particularly hillslope failures in response to slope oversteepening due to glacial incision and orientation and spacing of bedrock fractures. Erosion rates averaged over 10 ky time scales were comparable to long-term exhumation rates measured in the Teton Range. The similarity of spatial erosion patterns to predicted uniform erosion and the balance between intermediate and long-term erosion rates suggests the landscape of the Teton Range is approaching steady-state, but frequent stochastic processes, short-term erosional variability and coupled processes maintain rugged topographic relief.
Ph. D.

La question scientifique principale de ce travail de thèse est la compréhension des moteurs et des taux d’érosion, à l’échelle de plusieurs dizaines de millions d’années, d’un archipel océanique volcanique. Une approche thermochronologique basse et moyenne température sur des roches intrusives est utilisée, afin de contraindre les trajets d’exhumation de roches plutoniques dans la croûte océanique de l’archipel de Kerguelen (sud de l’Océan Indien). Cet archipel, dont la formation a débuté il y a 30 Ma, est la partie émergée du plateau océanique du même nom, formé suite à l’activité du point chaud de Kerguelen, qui commence il y a 120 Ma. La particularité de l’archipel de Kerguelen, en comparaison aux îles océaniques classiques, est la présence de roches plutoniques différenciées, cibles de choix pour l’utilisation de la thermochronologie basse et moyenne température. Ces méthodes permettent de reconstituer les trajets thermiques de refroidissement, et donc d’exhumation des plutons, depuis leur formation en profondeur jusqu’à leur exhumation à la surface. Dans ce travail, trois thermochronomètres ont été utilisés : la méthode ⁴⁰Ar/³⁹Ar sur biotite ainsi que les méthodes (U-Th)/He sur apatite et zircons. Les données obtenues à Kerguelen ont permis de mettre en évidence un modèle d’érosion différentielle sur l’archipel, avec pulses d’érosion locaux à l’échelle d’un complexe plutonique, qui contrastent avec les taux régionaux faibles depuis 25 Ma. Cette observation met en évidence le rôle prédominant de l’activité magmatique, et la tectonique locale associée, sur l’exhumation des plutons et la dénudation en surface
The main scientific aim of this work is the understanding of driving forces and erosion rates, at the scale of several dozens of millions of years, of an oceanic volcanic island. A low and mid temperature thermochronological approach on intrusive rocks is used, in order to reconstruct exhumation paths in the oceanic crust of the Kerguelen archipelago (southern Indian Ocean). This archipelago was created 30 Ma years ago and is the emerged part of the eponymous oceanic plateau, formed following the activity of the Kerguelen hot spot, which began 120 Ma ago. The singularity of the Kerguelen archipelago, compared to other oceanic islands, is the occurrence of acid plutonic rocks, on which we can perform thermochronology on. Such methods allow reconstructing the thermal cooling paths, and thus, exhumation paths of the plutons since their deep formation to their surface exhumation. In this work, three thermochronometers were used: the ⁴⁰Ar/³⁹Ar on biotite method, and the (U-Th)/He on apatite and zircon methods. The data obtained on Kerguelen Islands indicate a differential erosion over the archipelago, with local erosion pulses within a plutonic complex, contrasting with regional low erosion rates since 25 Ma. This observation points to the predominant role of magmatic activity, and the associated local tectonic activity, on the exhumation of plutonic bodies and surface denudation

In this project apatite fission track and (U-T)/He thermochronometers are used to determine the cooling history of rocks from the coastal (south eastern New South Wales) and the interior (Bathurst area) regions of the eastern Australia high elevation passive margin. Two traverses across the coastal lowlands, escarpment and plateau top are used to determine the tempo and styles of response of the landscape to the continental breakup and sea-floor spreading of the Tasman Sea (85 Ma). The three prevailing models of escarpment evolution namely retreat into a downwarped rift shoulder, escarpment retreat and excavation in place on a high elevation rift shoulder with flexural rebound are described and tested using a previously untested combination of apatite fission track and (U/Th)/He data. The thermochronological data indicate that the coast was affected by a denudational pulse that peaked around 120-100 Ma and that was extinguished by the time of sea-floor spreading. The rapid denudational event caused the removal of 3-4.5 km (depending on the geothermal gradient) of crust at the coast and of approximately 2 km at the present base of the escarpment. The thermochronological data are inconsistent with the downwarped rift shoulder model and the apatite (U-Th)/He data indicate that, while the coast was denuded very rapidly, the coastal lowlands were excavated in place at a much lower pace, and the escarpment reached its present position no later than 60 Ma. This suggests that during continental extension and breakup, rates of denudation at the coast were approximately 80-30m/Myr (depending on the geothermal gradient), whereas at the base of the present escarpment they were about 10-5 m/Myr. The period after sea-floor spreading was characterised by stability and low rates of erosion. The pre-breakup topography, reconstructed using the backstacking technique, is characterised by a considerable relief in the area of the present escarpment. This result confirms the hypothesis that the escarpment evolved pinned to its present position.

A new apatite (U-Th)/He (AHe) dataset from subvertical transects collected in the Teton and Gallatin Ranges in the Teton-Yellowstone region provides insight for the slip history and length of the Teton fault. Along the northernmost segment of the Teton fault, inverse thermal history modeling of AHe data from Eagles Rest Peak yield a ~9 Ma age for onset of fault slip. This age supports previous interpretations that Mount Moran may be the true center of the Teton fault. This refined interpretation coupled with lengthdisplacement fault scaling analysis and previous estimates of total fault displacement (~6 km) indicates that the Teton fault may extend 50-90 km north of Mount Moran. However, this new data precludes the possibility that the Teton and East Gallatin faults represent the same structure. Yet, because these systems share a similar structure trend and initial slip ages (13 Ma and 16 Ma, respectively), they may still be related at a larger scale. To the south, the Teewinot transect yields the oldest onset age of ~32 Ma, however a >500 m vertical data gap in this transect leads us to cautiously interpret the results of this model, particularly as this age conflicts with four other transects along-strike.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Neste trabalho são apresentados os resultados obtidos por meio de análises geomorfológicas (Mapa de Níveis de Paleosuperfícies) e termocronológicos (traços de fissão em apatitas e U-Th/He em apatitas) ao longo da Serra do Mar nos setores (1), (2), (3) e (4). As correlações entre as analises geomorfológicas e termocronológicas evidenciaram uma geológica e geomorfológica compreendida entre o Cretáceo Superior e Paleoceno, demonstrando que a evolução das morfologias que compõem a área de estudo estiveram associadas a eventos tectônicos e sucedidos por uma intensa atividade erosiva. De acordo com as datações realizadas utilizando termocronômetros com temperatura de fechamento distintas, estes indicaram que as configurações dos relevos que compõem a Serra do Mar não podem ser associadas apenas aos efeitos das atividades erosivas (recuo de escarpa) e isostáticas, para poderem explicar a homogeneidade entre as idades de traços de fissão e (U-Th)/He em apatitas, sem a presença da atuação tectônica soerguendo e desnivelando parte destes relevos.
In this paper the results obtained geomorphological analysis (Map of levels of Palaeosurfaces) and thermochronogical analysis (apatite fission-track and UTh/ He). The correlation between the geomorphologica and the thermochronological analysis evidenced a geological and geomorphological evolution from the Upper Cretaceous to the Palaeocene, showing that the evolution of the morpholoies composing the study area were associated to tectonics events and preceded by intense erosive activity. According to the datings done using thermocronometers with distinct closing temperatures the configuration of the relieves that compose the Serra do Mar can not be associated only to the effects of the erosive (escarpment retreate) and isostatic activities but also to the tectonic motion uplifting and unlevelling part such morphologies, in order to explain the homogeneity between the ages of the fission-track and U-Th/He of apatites.

Orientador: Peter Christian Hackspacher
Coorientadora: Marli Carina Siqueira Ribeiro
Banca: Ana Olivia Barufi Franco-Magalhães
Banca: Daniel Françoso de Godoy
Resumo: A termocronologia de baixa temperatura tem sido uma ferramenta importante para a quantificação de processos geológicos em margens continentais passivas. Neste contexto, a margem angolana mostra evidências de uma evolução pós-rifte policíclica marcada por eventos de soerguimento, inversão de bacias e mudanças nas taxas de sedimentação para as bacias marginais, os quais têm controlado a tectônica salina e a localização dos depósitos de hidrocarbonetos. Para entender a evolução pré, sin e pós-rifte da margem sudoeste angolana, amostras de afloramentos para análises por traço de fissão e (U-Th)/He em apatitas foram coletadas em um perfil entre as cidades de Namibe e Lubango variando de 79 m à 1675 m de altitude. A área está situada no limite entre os segmentos Central e Austral do Atlântico Sul a poucos kilômetros à norte da cordilheira Walvis e compreende as rochas arqueanas à paleoproterozóicas do cráton do Congo. As idades TFA variam de 120.6 ± 8.9 Ma à 328.8 ± 28.5 Ma e apresentam uma tendência de envelhecimento com o aumento da altitude. O comprimento médio dos traços (MTLs) varia entre 11.77 ± 1.82 µm e 12.34 ± 1.13 µm com uma distribuição unimodal (TLDs). As idades HeA variam entre 128.8 ± 3.9 Ma e 196.6 ± 5.9 Ma e apresentam um padrão de envelhecimento em direção ao Grande Escarpamento, com idades mais antigas do que as respectivas idades TFA, interpretadas como o resultado do acúmulo de danos radioativos nas apatitas. As modelagens das histórias térmicas foram construídas com os parâmetros cinéticos Dpar e com o ângulo em relação ao eixo-c por meio do software Hefty. Ambas as modelagens TFA e HeA indicam três principais eventos de resfriamentos associados à soerguimento/denudação: do Jurássico ao Eocretáceo, no Neocretáceo e do Eocêno - Oligocêno ... (Resumo completo, clicar acesso eletrônico)
Abstract: The low-temperature thermochronology has been an important tool to quantify geological processes in passive continental margins. In this context, the Angolan margin shows evidence of a polycyclic post-rift evolution marked by different events of uplift, basin inversion and changes in sedimentation rates to the marginal basins, which have controlled the salt tectonics and the hydrocarbon deposits. To understand the pre, sin and post-rift evolution of the southwestern Angolan margin, outcrop samples were collected for apatite fission track and apatite (U-Th)/He analysis ranging in elevation from 79 m to 1675 m from the coast toward the interior plateau in a profile between Namibe and Lubango cities. The area lies on the edge of Central and Southern Atlantic segments, a few kilometers northward the Walvis ridge, and encompasses the Archean and Proterozoic basement rocks of the Congo craton. The AFT ages range from 120.6 ± 8.9 Ma to 328.8 ± 28.5 Ma and they show a trend of increasing age with increasing elevation. The partial main track lengths (MTLs) vary between 11.77 ± 1.82 µm to 12.34 ± 1.13 µm with unimodal track length distributions (TLDs). The AHe ages range from 128.8 ± 3.9 Ma to 196.6 ± 5.9 Ma and they show a trend of increasing ages toward the Great Escarpment, older than the respective AFT ages, which could be interpreted as a result of the radiation damage accumulation in the apatites. The thermal histories modelling have been constrained with the kinetic parameters Dpar and c-axis angle by the Hefty software. Both AFT and AHe thermal histories modelling indicate three main episodes of denudation/uplift driven cooling: from Jurassic to Early Cretaceous, in the Late Cretaceous and from Oligocene-Miocene to recent times, which are compatible with the sedimentary fill interpreted from the offshore Namibe basin geophysical data ... (Complete abstract click electronic access below)
Mestre

Orientador: Peter Christian Hackspacher
Banca: Norberto Morales
Banca: Sandro Guedes
Banca: Cláudio Riccomini
Banca: Pedro José Nunes
Resumo: Neste trabalho são apresentados os resultados obtidos por meio de análises geomorfológicas (Mapa de Níveis de Paleosuperfícies) e termocronológicos (traços de fissão em apatitas e U-Th/He em apatitas) ao longo da Serra do Mar nos setores (1), (2), (3) e (4). As correlações entre as analises geomorfológicas e termocronológicas evidenciaram uma geológica e geomorfológica compreendida entre o Cretáceo Superior e Paleoceno, demonstrando que a evolução das morfologias que compõem a área de estudo estiveram associadas a eventos tectônicos e sucedidos por uma intensa atividade erosiva. De acordo com as datações realizadas utilizando termocronômetros com temperatura de fechamento distintas, estes indicaram que as configurações dos relevos que compõem a Serra do Mar não podem ser associadas apenas aos efeitos das atividades erosivas (recuo de escarpa) e isostáticas, para poderem explicar a homogeneidade entre as idades de traços de fissão e (U-Th)/He em apatitas, sem a presença da atuação tectônica soerguendo e desnivelando parte destes relevos.
Abstract: In this paper the results obtained geomorphological analysis (Map of levels of Palaeosurfaces) and thermochronogical analysis (apatite fission-track and UTh/ He). The correlation between the geomorphologica and the thermochronological analysis evidenced a geological and geomorphological evolution from the Upper Cretaceous to the Palaeocene, showing that the evolution of the morpholoies composing the study area were associated to tectonics events and preceded by intense erosive activity. According to the datings done using thermocronometers with distinct closing temperatures the configuration of the relieves that compose the Serra do Mar can not be associated only to the effects of the erosive (escarpment retreate) and isostatic activities but also to the tectonic motion uplifting and unlevelling part such morphologies, in order to explain the homogeneity between the ages of the fission-track and U-Th/He of apatites.
Doutor

abstract: The Byrd Glacier region of Antarctica is important for understanding the tectonic development and landscape evolution of the Transantarctic Mountains (TAM). This outlet glacier crossing the TAM marks a major discontinuity in the Neoproterozoic-early Paleozoic Ross orogen. The region has not been geologically mapped in detail, but previous studies have inferred a fault to exist beneath and parallel to the direction of flow of Byrd Glacier. Thermochronologic analysis has never been undertaken across Byrd Glacier, and little is known of the exhumation history of the region. The objectives of this study are to assess possible differential movement across the inferred Byrd Glacier fault, to measure the timing of exhumation, and to gain a better overall understanding of the structural architecture of the TAM. Apatites and zircons separated from rock samples collected from various locations north and south of Byrd Glacier were dated using single-crystal (U- Th)/He analysis. Similar cooling histories were revealed with comparable exhumation rates of 0.03 ± 0.003 and 0.04 ± 0.03 mm/yr north and south of Byrd Glacier from apatite data and somewhat similar rates of 0.06 ± 0.008 and 0.04 ± 0.01 mm/yr north and south of Byrd Glacier from zircon data. Age vs. elevation regressions indicate a vertical offset of 1379 ± 159 m and 4000 ± 3466 m from apatite and zircon data. To assess differential movement, the Kukri Peneplain (a regional unconformity) was utilized as a datum. On-site photographs, Landsat imagery, and Aster Global DEM data were combined to map Kukri Peneplain elevation points north and south of Byrd Glacier. The difference in elevation of the peneplain as projected across Byrd Glacier shows an offset of 1122 ± 4.7 m. This study suggests a model of relatively uniform exhumation followed by fault displacement that uplifted the south side of Byrd Glacier relative to the north side. Combining apatite and zircon (U-Th)/He analysis along with remote geomorphologic analysis has provided an understanding of the differential movement and exhumation history of crustal blocks in the Byrd Glacier region. The results complement thermochronologic and geomorphologic studies elsewhere within the TAM providing more information and a new approach.
Dissertation/Thesis
M.S. Geological Sciences 2011

The formation and evolution of metamorphic core complexes has been widely studied using low temperature thermochronometry methods. Interpretation of these data has historically occurred through the lens of the traditional slip rate method which provides a singular rate that unroofing occurs at temporally as well as spatially, and assumes unroofing is dominated by motion on a single master detachment fault. Recently, several new studies have utilized (U-Th)/He ages with a higher spatial density and greater nominal precision to suggest a late-stage rapid increase in the rate of unroofing. This analysis is based on the traditional slip rate method interpretation of broad regions of core complexes that display little to no change in age along the slip direction. An alternative interpretation is presented that instead of a change in slip rate, there may have been a change in the style of unroofing, specifically caused by the transfer of displacement from low-angle detachment faulting to high-angle normal faults. Apatite fission-track (AFT), and apatite and zircon (U-Th)/He (AHe and ZHe) analyses were applied to samples from the Santa Catalina-Rincon (n=8 AHe, and n=9 ZHe) and Harcuvar (n=12 AFT, n=16 AHe, and n=17 ZHe) metamorphic core complexes in an attempt to resolve the possible thermal effects of high-angle normal faulting on core complex formation. Samples from the Harcuvars were taken along a transect parallel to slip direction with some samples specifically targeting high-angle normal fault locations. The AFT data collected here has the advantage of improved analysis and modeling techniques. Also, more than an order of magnitude more data were collected and analyzed than any previous studies within the Harcuvars. The AFT ages include a trend from ~22 Ma in the southwest to ~14 Ma in the northeast and provide a traditional slip rate of 7.1 mm/yr, similar to previous work. However, two major high-angle, detachment-parallel normal faults were identified, and hanging-wall samples are ~3 m.y. older than the footwalls, indicating high-angle normal faults rearranged the surface expression of the distribution of thermochronometer ages to some extent. AHe ages range from 8.1 Ma to 18.4 Ma but in general decrease with increasing distance in the slip direction. ZHe ages generally range between 13.6 Ma and 17.4 Ma. A series of unexpectedly young AFT ages (10-11 Ma), given by three complete samples and distinct population modes in others, suggest that some parts of the range underwent a later-stage unroofing event possibly caused by high-angle faulting. Confined fission-track length distributions were measured for Harcuvar samples and modeled using the modeling software HeFTy to infer thermal histories and calculate local cooling rates. These imply a component of steady cooling in some parts of the range, evidence of a different departure from a single-detachment dominated model.
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The Aegean is a classic setting for studying exhumation of high-pressure (HP) metamorphic rocks. Two end-member models are proposed to explain the uplift of these rocks: core-complex style extension along low-angle normal faults and extrusion-wedge uplift. Extrusion-wedge underplating is the mechanism that exhumed HP rocks on Evia whereas Tinos hosts several detachments varying in age from 30-9 Ma. Andros, situated between them, may be the geological manifestation of the interplay of these processes and provides an opportunity to test these models. Detachments on NW Tinos and on Andros and the enigmatic low-angle Makrotantalon Unit contact on Andros were insufficiently dated prior to this study. Geo- and thermochronometrycombined with structural observations from sampling transects in the transport direction from (1) lower plate Cycladic Blueschist Unit on Andros and Tinos, (2) middle plate Makrotantalon Unit on Andros, and (3) hanging wall Upper Unit address these issues. Maximum depositional ages from detrital zircon U-Pb geochronometry and structures reveal Paleocene-Eocene syn-HP metamorphism thrusting resulted in an inversed-age relationship between the Permian Makrotantalon Unit and the underlying Triassic-Eocene Cycladic Blueschist Unit on Andros. The Makrotantalon Unit has an internal inversed stratigraphy whereas the Cycladic Blueschist Unit on Andros and Tinos appear stratigraphically intact. Structures and zircon and apatite (U-Th)/He ages in transects from NW Tinos (~12-8 Ma) and central Andros Cycladic Blueschist Unit (~13-7 Ma) indicate rapid cooling due to exhumation associated with the Livada Detachment. Older cooling ages (~16-10 Ma) and structures in the Makrotantalon Unit indicate later brittle strain localization on the Makrotantalon Thrust contact is accommodated by rheologically weaker serpentinites and calc-schists, resulting in slivering of the footwall under the Livada Detachment on Andros. Estimated mean cooling slip rates of the Livada Detachment on Andros of ~3.8 (+1.2/-1.3) km/Myr and 2.1 (+0.2/-0.2) km/Myr on NW Tinos resulted in minimum vertical exhumations of 15 km and 4 km, respectively. The NCDS here accommodated ~12-25% of 60 km of HP-rock exhumation from ~30-7 Ma. We present a tectonic model to elucidate the evolution of the Makrotantalon Unit and the magnitude, temporal, and spatial variability of exhumation via detachments on these islands.