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.
University of Minnesotaq Ph.D. dissertation. February 2014. Major: Geology. Advisor: Christian Teyssier and Donna L. Whitney. 1 computer file (PDF); x, 263 pages, appendix p. 204-263.
Late-stage exhumation of metamorphic core complexes and landscape development during orogenic collapse of the North American cordillera.
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