Browsing by Subject "Deformation"

Now showing 1 - 6 of 6
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    The Development Of Olivine Textures In Complex Deformation Geometries
    (2023) Wagner, Nicole
    The analysis of crystallographic preferred orientations (CPOs) in olivine is a crucial tool in our understanding of the Earth’s upper mantle. The development of CPOs is controlled by the activation of slip systems, which are sensitive to various thermochemomechanical conditions such as stress, temperature, and water content. Laboratory experiments have offered insight on the conditions in which different olivine CPOs develop. However, the relationship between these thermochemomechanical conditions and CPOs is complicated. Recent studies have challenged some of our current understanding on olivine CPO development and have brought attention to the importance of kinematics. Here, we aim to experimentally investigate the role of kinematics in the development of olivine CPOs, particularly pertaining to complex deformation geometries involving simultaneous pure and simple shear. To explore this topic, cylindrical samples of dry, polycrystalline San Carlos olivine were deformed in either simultaneous extension and torsion or simultaneous shortening and torsion at a temperature of 1523 K and confining pressure of 300 MPa in a Paterson gas-medium apparatus at the University of Minnesota. Following deformation, the CPO was measured at different sections along the sample radius using electron backscatter diffraction analysis. For each section, the kinematic vorticity number, the equivalent strain, the J-index, and the M-index were determined. The fabric index angle was also calculated to classify the resulting CPO for each section. In total, two samples were deformed in extension and torsion and three samples in shortening and torsion. Of the two extension and torsion experiments, one sample localized thus allowing for multiple sections along the length to be analyzed. Moreover, the samples subjected to extension and torsion went out to axial strains of 0.15, 0.26, and 0.36 and to shear strains between 1.5 and 2.2. These samples generally produced stronger textures and displayed an evolution from C-type to E-type to D-type CPO with increasing kinematic vorticity. The presence of an E-type CPO aligns with numerical simulations of CPO development in simultaneous extension and simple shear geometries and suggests that olivine does not necessarily develop as a function of water content. Conversely, the samples deformed in shortening and torsion went to axial strains of -0.12, -0.28, and -0.35 and shear strains between 1.0 and 1.7. These samples had weaker textures and typically evolved from an AG-type to an A-type CPO with increasing kinematic vorticity, which also reasonably aligns with numerical simulations of this given geometry. Although different textural evolutions were observed between these two deformation geometries, the fabric index angles began to converge as kinematic vorticity increased for both sets of experiments, ultimately resulting in textures expected in simple shear. The geometries achieved in this study were then simulated using a modified director textural model. The simulations produced from this model reasonably reproduced the CPO characteristics observed in the experimental samples, but the CPO classification, as determined by the fabric index angle, did not align as well. All in all, the findings presented in this study highlight the role of kinematics in CPO development, emphasize the caution needed when interpreting the thermochemomechanical conditions of the mantle from seismic anisotropy and exhumed peridotites, and contribute valuable information on how we can use olivine CPOs as a field tool.
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    Isotopy of nodal symplectic spheres in rational manifolds
    (2013-11) Chang, Ching-Hao
    In 1985, M. Gromov proved that any symplectic sphere of degree 1 in CP2 is isotopic to an algebraic line. J. Barraud extended Gromov's work to show that any symplectic sphere of degree d in CP2 with only positive ordinary double point singularities is symplectically isotopic to an algebraic curve. In this paper, We imitate Barraud's approach and further extend the result to the nodal symplectic spheres in rational manifolds. We prove that if (M, w) is a rational symplectic 4-manifold, and A a homology class in H2(M, Z) with Kw(A) < 0, then the space of nodal symplectic spheres in the homology class A has only finitely many isotopy classes.
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    Magnetic and physical characteristics of magnetite associated with deformation and exsolution.
    (2011-10) Till, Jessica Lynn
    This thesis contains a collection of laboratory-based studies designed to characterize the magnetic properties and physical aspects of magnetite that result from deformation or high temperature growth. In Chapter 2, a detailed rock magnetic characterization of rocks containing nanoscale magnetite exsolved from volcanic glass identifies the location of domain-state thresholds through distinct transitions in remanence and susceptibility properties. This unique material is an excellent candidate for standard material to be used in studies of magnetite granulometry. In Chapter 3, theoretical timescales for the growth of sub-microscopic magnetite needles during exsolution from plagioclase are calculated using results of diffusion experiments. Measured diffusivities are modeled to calculate the amount of diffusion-limited growth possible under different conditions of nucleation temperature and cooling rate. In Chapters 4 and 5, the development and evolution of magnetic fabrics are investigated through deformation experiments on synthetic rock-analogues at high temperatures and ductile conditions. Stress-induced changes in rock magnetic properties after deformation are significant. Examination of deformation-induced remagnetization demonstrates that a primary remanence can survive conditions equivalent to moderate metamorphism in certain cases and that petrofabric can play an important role in determining the remanence stability. High-temperature deformation experiments result in a pattern of anisotropy development that indicates plastic deformation of magnetic grains, which is distinct from anisotropy development resulting from different magnetite strain responses. Experimental data are combined with theoretical magnetic anisotropy models and used to estimate effective magnetite strains and strain partitioning from magnetic fabric data in deformed samples. Finally, observations of strong shape-preferred orientation and deformation-induced microstructures in magnetite grains from high-temperature shear experiments indicate plastic deformation of magnetite. Microstructural observations place constraints on the rheological behavior of magnetite and the conditions in which dislocation creep is dominant. These observations prompt a re-examination of the previously established magnetite flow laws which are modified and used to construct new deformation mechanism maps.
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    The role and fate of fluid inclusions in natural and experimental deformation
    (2013-10) Carter, Matthew James
    Fluid circulation is essential for several geological processes. In the upper crust, brittle fractures and faults can provide pathways for surface fluids to reach ductilely deforming regions in the mid to lower crust where fluids can substantially weaken the rocks. Although this phenomenon is well established, the mechanism(s) that transport and redistribute fluids in the ductilely deforming regime of the lithosphere, and the resulting rheological consequences are incompletely known. This dissertation combines field observations and experiments to reach a better understanding of how fluids may penetrate into and be redistributed in low permeability, ductilely deforming rocks. The field-based part of this project (chapter 2) focuses on observations from the northern Snake Range metamorphic core complex where meteoric fluids are known to have interacted with ductilely deformed footwall rocks below the detachment fault. Samples collected from a well-exposed, 150 m thick section of footwall quartzite mylonite contain abundant fluid inclusions (FIs) related to micro- and outcrop-scale structures. Higher salinity, miscible CO2 and H2O fluids were trapped along healed fractures at conditions between 270 and 345 °C and 1 kbar confining pressure, and lower salinity CO2 and H2O FIs formed later along healed fractures. Results suggest that meteoric fluids infiltrated this detachment system at pressure and temperature conditions above CO2-H2O miscibility during the latest stages of ductile deformation, but the precise pathway of infiltration, through brittle fracture or along grain boundaries, is unclear. Principal exhumation of the detachment system occurred along a high geothermal gradient (~ 70 °C/km), consistent with a previous estimations, and with fluid circulation driven by rapid exhumation of hot, mid to lower crustal rocks. This field-based study is complemented by an experimental approach to the problem of hydration of ductilely deforming rocks. This approach necessitates the use of mineral aggregate that can deform to high finite strain at high temperature under accelerated laboratory conditions. Torsion experiments of fine-grained olivine aggregates at 1200 °C fulfill these conditions: olivine deforms in the dislocation creep regime, and the assemblage olivine + water is stable at that temperature. Therefore, olivine is used in the experimental part of this project; results obtained from olivine experiments can help understand the physical processes that operate in other mineral species, such as quartz, during plastic deformation in the presence of excess water. A suite of anneal (static condition) and torsion experiments on olivine were designed to test various hypotheses for sample hydration and to determine the fate of FIs during high temperature deformation (i.e. dislocation creep). Talc was used in experiments to add water to the sample (via dehydration at elevated temperature), and was fitted as a sleeve around the outer cylinder of samples or inserted as a cylinder into cored samples. Results from anneal experiments (chapter 3) indicate fluids infiltrated the sample along grain boundaries, and in some cases along fluid-filled fractures. Samples annealed with talc contain FI-depleted areas representing olivine grains that recrystallized before becoming water saturated, whereas FI-rich parts of the sample were over-saturated with water during recrystallization. FIs restrict grain growth by Zener pinning, and are more abundant in samples with higher water content, particularly along grain boundaries. High temperature torsion experiments (chapter 4) were performed on wet and dry olivine aggregates with and without talc. Samples deformed with talc (whether initially dry or wet) are substantially weaker owing to the presence of water. All samples display reduced grain size, shape preferred orientation of olivine grains, and a pervasive C'-fabric defined by the alignment of FIs. Like in the anneal experiments, samples that were hydrated by talc contain FI-rich and FI-depleted domains, but in this case domain boundaries have been distorted by deformation. Olivine pole figures obtained from EBSD-based crystallographic fabric analysis confirm deformation of olivine was accommodated by dislocation creep by activation of the (010)[100] slip system in the recrystallization regime of grain boundary migration. Results indicate that shearing accommodated by dislocation creep rearranges and aligns FIs into bands at a low angle to the shear plane. The arrangement of FIs into low-angle bands is interpreted to have established under a pressure gradient that focuses fluids in low-pressure bands, locally enhancing permeability, and creating high diffusivity pathways in the sample. Results of this work demonstrate that microstructures may be highly affected where fluids are introduced in excess into ductilely deforming rocks. Abundant FIs may prevent grain growth, which in turn may affect several grain size sensitive processes, and FIs may also be reorganized by deformation accommodated by dislocation creep (i.e. grain boundary migration recrystallization). These micro-scale results have important rheological implications for a broad range of tectonic settings where water may be available in excess in the Earth's lithosphere.
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    Stress-driven melt redistribution in partially molten rocks deformed in torsion: from pressure shadows to base-state segregation
    (2014-10) Qi, Chao
    The redistribution of melt in partially molten rocks during deformation plays an important role in the evolution and dynamics of Earth's mantle. Previous studies discovered different scales of melt redistribution: melt alignment and melt segregation to form melt-enriched bands , both of which have demonstrated their importance to the deformation of the mantle. In this dissertation, two new forms of stress-driven melt redistribution in deformed partially molten rocks are produced: a formation of pressure shadows around rigid particles and a large-scale, base-state melt segregation. For pressure shadows, observations on the microstructure around the rigid particles revealed the melt distribution and solid flow field, which will provide a constraint on the bulk viscosity of the partially molten rock, if associated with theoretical studies. The presence of base-state melt segregation validated a hypothesis of viscous anisotropy, which provides explanations for melt segregation processes and will cause a significant impact to the dynamic of the mantle. Therefore, the studies of stress-driven melt redistribution in this dissertation are of great significance that will influence the future studies of Earth's mantle.
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    Timescales of migmatization, metamorphism, and deformation in a collapsed Orogenic Plateau.
    (2009-01) Gordon, Stacia Michelle
    Migmatites play an important role in the evolution of mountain systems by inducing rheological contrasts and focusing strain. In modern orogenic plateaux, investigations have suggested that a layer of partially molten crust is located in the mid- to lower crust. To understand the role of partially molten crust in orogenic systems, it is important to determine how much of the crust was partially molten for how long, and to link the conditions, timing, and consequences of partial melting to tectonic processes at different crustal levels during construction and collapse of orogens. The Skagit Gneiss, in the highgrade core of the North Cascade continental magmatic arc of Washington, USA and BC, and the Valhalla complex, in the Shuswap metamorphic core complex of southeastern British Columbia, Canada, both contain abundant migmatites and represent the western and eastern margins, respectively, of a proposed orogenic plateau that was once present in western North America during the Late Mesozoic-Early Cenozoic. In the Valhalla complex, samples of migmatite were collected from the dome core to the bounding detachment fault for conventional, in situ, and depth profiling ion microprobe analyses. The conventional and in situ analyses of stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U-Pb zircon ages that cluster near 60 Ma, interpreted as the timing of melt crystallization. Monazite Th-Pb ages range from 57-49 Ma. Patchy zoning and the range of dates suggest that the monazite was recrystallized under fluid-mediated conditions. To better understand the late history recorded in the monazite, depth profiling U-Pb ages were obtained from the outermost rims of zircons and yield a consistent age of 51 Ma. Oxygen isotopic measurements of the unpolished crystal faces systemically yield heavier δ18O (up to 9.0 ‰) relative to interior compositions (down to 5.5 ‰). Furthermore, Ti concentrations of unpolished crystal faces and grain interiors yield temperatures of ~650 ºC. The depth profiling zircon results and the conventional Th-Pb monazite results indicate that deformation- and fluidmediated recrystallization of zircon and monazite occurred at high-T conditions as late as 51 Ma. The Ar cooling ages overlap with the youngest monazite and zircon results and cluster from 51 to 49 Ma. The geochronometric, geochemical and trace element results from the Valhalla complex, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, show that a large region of orogenic crust in this part of the Cordillera was partially molten in the early Tertiary. Rapid cooling is associated with extension and exhumation of migmatites in the domes. In the Skagit Gneiss, monazite and zircon were dated using the U-Pb TIMS method from migmatites in 3 localities. Zircons from the mesosome of the westernmost locality commonly yield Cretaceous dates, with younger dates clustering at 61 Ma. Leucosomes yield zircon with concordant dates that range from 68 to 47 Ma, interpreted as representing the timing of melt crystallization. In comparison, monazite reveal bimodal results, with one group clustering near 48 Ma and a second set of older dates from 69 to 65 Ma. The latter monazite dates are consistently older than the zircons from the same leucosome, consistent with the possibility that the older monazites record the timing of prograde to possibly peak metamorphism. The Eocene zircon and monazite dates are at the young end of the age spectrum for the North Cascades arc system and overlap with the timing of transtensional basin formation, suggesting that partial melting was an active process during at least the initial stages of extension and exhumation of the high-grade rocks. In addition, in the Skagit Gneiss, a detailed study of part of the eastern bounding strike-slip fault zone suggests that a dynamic system was present between the high-grade Skagit rocks, the fault, and the adjacent basin. A step-over zone in the strikeslip fault may have developed during transpression and allowed part of the basin to be incorporated into the high-grade core and undergo metamorphism and deformation with the Skagit Gneiss. Although ~ 300 km separate the North Cascades from the Shuswap metamorphic core complex today, the two regions share many similarities: 1) both areas expose deformed high-grade gneiss that underwent isothermal decompression; 2) both areas contain abundant deformed migmatites that crystallized at similar times; and 3) Ar cooling ages from the two regions are similar. In both the Skagit Gneiss and the Valhalla complex, the partially molten crust played a significant role in the decompression and exhumation of the terranes. The similarities in P-T-t-d between the two regions strongly indicate that the North Cascades and the Omineca belt were dynamically linked and that the two areas represent the collapsed margins of an orogenic plateau. The migmatites in both areas are evidence of the layer of partially molten crust that once flowed beneath the proposed plateau. The data from the two areas suggest that partial melting must play a major role in the tectonic evolution of orogenic systems that contain abundant melt (e.g., Himalaya-Tibet; Andes-Altiplano-Puna; Cordillera).