Author: Thiessen, Eric
One of the largest but poorly understood tectonic boundaries in North America is the Snowbird Tectonic Zone (STZ), which separates the Rae and Hearne cratons. Significant tectonothermal reworking of Neoarchean protoliths in the STZ occurred at ca. 2.55 Ga and 1.9 Ga, however, substantial uncertainty persists with regard to the nature and extent of that metamorphism. This thesis focuses on a poorly studied portion of the STZ and adjacent crustal domains within Northwest Territories in order to better understand the nature of 1.9 Ga tectonometamorphism. New mapping revealed a 300 km long and up to 20 km wide crustal-scale shear zone, the Wholdaia Lake shear zone (WLsz). Analysis of hanging wall metasedimentary units returned a new depositional age of 1.98-1.94 Ga. These metasedimentary units, and much of the STZ in the south Rae, were buried and partially melted by ca. 1.92 Ga and began to exhume 7-10 kbar, granulite-facies rocks between 1.92 and 1.90 Ga. Continued amphibolite-facies exhumation of crustal domains was accommodated by the WLsz and other localized extensional shear zones between 1.90 and 1.86 Ga. Much of this high-grade margin was at the surface by 1.83 Ga when new volcanic supracrustal material was deposited. Petrological investigation and Lu-Hf garnet dating of multiple metamorphic assemblages in a mafic granulite from the WLsz has revealed high-grade age components at 2.11 and 1.87 Ga, whereas zircon U-Pb age components are 2.6 and 1.89 Ga. These results highlight a high-grade event at 2.11 Ga not recorded by U-Pb zircon analysis during a poorly understood time of possible extension in the Rae craton. Mapping, U-Pb zircon and titanite geochronology along the STZ at Kasba Lake demonstrates continuity of rock types, isotopic ages and overall shear zone character for >200 km of strike length. Pervasive high-strain occurred between 1.9 and 1.84 Ga within a panel between the Rae and Hearne cratons without obvious coeval metamorphism; however, Archean metamorphism and deformation are also preserved in this region. This panel may be a sliver of Rae crust that was highly strained but never reached high-grade conditions at 1.9-1.8 Ga but likely contributed to exhumation of the STZ region.
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Thesis advisor: Gibson, H. Daniel
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