Browsing by Subject "Gneiss dome"

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    Late-stage exhumation of metamorphic core complexes and landscape development during orogenic collapse of the North American cordillera
    (2014-02) Toraman, Erkan
    Exogenic and endogenic processes control the exhumation of partially molten crust in extending orogens. Their relative contribution to total denudation is critical to evaluate different tectonic models. Therefore, assessing the timing, rates, style, and conditions of events from melt crystallization to cooling at near-surface temperatures is significantly important for understanding the thermo-mechanical evolution of orogenic crust and linking deep-to-shallow processes. Metamorphic core complexes (MCC) and gneiss domes located within the hinterland of orogenic belts expose a significant quantity of former partially-molten mid-to-lower crust in the form of migmatites. The Thor-Odin, Frenchman's Cap, and Okanogan domes are exposed in the biggest Cordilleran-style metamorphic core complex, the Shuswap MCC, where a series of migmatitic gneiss domes formed during collapse of the thickened crust in the Cenozoic. The Thor-Odin and Frenchman's Cap domes form the Shuswap MCC's narrow northern end where the present day topographic relief reaches up to 2.5 km with deeply incised valleys and ubiquitous glacial features. The Okanogan dome, on the other hand, represents the wider, lower-relief (≤ 1km) southern termination. In the Thor-Odin and Frenchman's Cap domes, zircon U-Th/He ages range from 45 to 37 Ma. Apatite fission track ages range between 48 to 14 Ma and increase with increasing sample elevation. Thermal modeling of samples from higher altitudes (~2100 to 1800 m) verify only rapid Eocene cooling, whereas the lower-elevation samples (~1800 to 500 m) reveal an additional Plio-Quaternary cooling event. The presence of the top of a fossil Eocene partial annealing zone at ~1800 m indicates that the migmatite dome reached near-surface depths (1-2 km) during its initial exhumation mainly by detachment tectonics. Apatite U-Th/He chronometry of these samples yields Miocene (26-5 Ma) ages. A number of low elevation (~500 m) samples collected from valleys reveal intra-sample single grain U-Th/He ages. Combined with the results of thermal modeling, these age variations indicate a rapid exhumation pulse at ca. 3 Ma, possibly related to continental glaciation. In the Okanogan dome, zircon U-Th/He ages range from 51 to 41 Ma and decrease towards the detachment fault zone, emphasizing up to 3.7 km/myr slip rate on the detachment zone. Apatite fission track and U-Th/He ages vary from 51 to 23 Ma, recording a very slow phase of erosional exhumation of the dome that removed ~ 2 km of rocks subsequently after its initial rapid ascent facilitated by detachment tectonics in the Eocene. Low-temperature data also document different cooling paths of rocks depending on their structural level; rocks closer to the detachment zone display rapid cooling rates (≥ 100 °C/myr), whereas deeper structural levels cool slowly (10-30 °C/myr). As in the North American Cordilleran hinterland, a series of migmatite-cored metamorphic core complexes is exposed in the Hellenides, where the geodynamic context of migmatitic dome formation is well known from previous research. Multiple low-temperature thermochronologic techniques combined with existing structural, geo- and thermochronologic, and petrologic data from Cordilleran and central Aegean migmatitic gneiss domes document rapid ascent of partially molten mid-to-lower crustal rocks, facilitating significant mass and heat advection to the shallow crust. Heat advection results in a high-geothermal gradient in shallow crust and shifts the brittle-ductile transition zone close to the surface. Percolation of surface-derived fluids through fault and fracture zones enables rapid cooling of rocks and enhances the brittle rheology.
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    Mechanical analysis of continental rifting and orogenic collapse
    (2019-06) Korchinski , Megan Steele
    Extension of the continental lithosphere occurs during orogenic collapse, rifting of continents, and development of back arcs, and is therefore a critical process during the evolution of tectonic plates. The accommodation of extension is a function of mechanical coupling within the continental lithosphere, and research presented here employs 2D numerical modeling to explore extension-driven dynamic interactions that occur between layers of differing rheological properties. Extension rate exerts a strong control on mechanical coupling between brittle faulting and flow within the deep crust, and therefore on the localisation of deformation during extension. This thesis presents 2D numerical experiments with a 100 km thick lithosphere (normal thickness continental crust, 40 km) in which the extension rate (1 to 2 cm/yr), viscosity of the deep crust, and density of the rift basin fill are systematically varied. Experiments under slow extension (1 cm/yr) exhibit homogeneous strain across the lithosphere for 20-25 My followed by rapid localization of deformation into a rifting instability, whereas experiments under fast extension (2 cm yr-1) exhibit development of a rifting instability within 2 My. These results capture a critical transition from instantaneous rifting to prolonged thinning under lithospheric extension. Experimental results illustrate that the viscosity of the deep crust will dramatically impact mechanical coupling within lithosphere, thereby influencing formation of passive margins and exhumation of near-Moho material within gneiss domes. Numerical experiments address the role of the deep crust by systematically varying the viscosity and density of the deep crust in continental lithosphere under extension (40 km and 60 km crust). The low viscosity deep crust end member produces significant exhumation by flow into a metamorphic core complex, while the high viscosity deep crust end member exhibits formation of lithospheric-scale shear zones and minimal deep crustal flow. In order to further explore these results, pressure-temperature-time-deformation paths along cross sections in a subset of experiments (60 km crust), which illustrate that convergent flow within channels at near-Moho depths is not restricted to systems with low viscosity deep crust. Results indicated that the viscosity of the deep crust is a primary control on the accomodation of extension, and highlight a dynamic feedback between deep, ductile deformation and shallow, brittle deformation during core complex development. Finally, I address the competition between vertical mass transport as a result of (1) deposition of sediments and (2) exhumation of the deep crust. Numerical experiments in which deep crust viscosity and basin infill density are systematically varied illustrate that this competition can strongly impact the tempo of deformation localisation in extended continental lithosphere. Experiments exhibit approximately similar timing and extent of deformation localization between sediment-present and sediment-absent experiments. For a basin infill that is lighter then the deep crust, experiments with a strong deep crust exhibit distributed extension in the shallow crust and burial of the exhumed deep crust under basins, and experiments with a weak deep crust exhibit localized deformation and efficient exhumation of the deep crust to the near-surface with limited basin deposition. However, experiments with a density inversion exhibit a gravitational instability with rapid sinking of the basin fill; this instability is accompanied by exhumation of deep crust on the sides of the basin. If the viscosity of the deep crust is sufficiently low, a drip of cool, dense basin infill ponds at the Moho, dramatically impacting the geotherm at the center of the experiment.