Enkelmann, EvaDamant, Kade A.2023-04-042022-07-29Damant, K. A. (2022). Prolonged Post-Orogenic Extension in the Southeastern Canadian Cordillera: Miocene Reactivation of the Columbia River Fault (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.https://prism.ucalgary.ca/handle/1880/116033https://dx.doi.org/10.11575/PRISM/dspace/40879Post-orogenic extension in the southeastern Canadian Cordillera manifests in the form of normal faults and metamorphic core complexes exhumed during the Paleocene–Eocene, but new thermochronology data suggest this interval may be longer. The Columbia River fault is an east-dipping brittle-ductile normal fault bounding the eastern margin of the Monashee Complex. Apatite fission track and (U-Th)/He data were collected from granitic and metamorphic bedrock along two elevation profiles across the Columbia River fault north of Revelstoke, BC. Ten apatite fission track ages range from 29.3–17.7 Ma and eleven apatite (U-Th)/He ages range from 26.6–7.0 Ma. Inverse thermal history modelling suggest the footwall underwent rapid cooling between 18–16 Ma. Whereas, the hanging wall experienced more protracted cooling from 40 Ma to present with two small cooling pulses at 20 and 10 Ma. These new data suggest the Columbia River fault was reactivated in the early Miocene and exhumed the Monashee Complex an additional ~2 km. Combining all thermochronology data from the region suggests three phases of accelerated cooling in the Monashee Complex: 1) Eocene (56–33 Ma), 2) Miocene (20–17 Ma), and 3) Pliocene to recent (2.8–0 Ma).Inverse thermal history modelling of apatite and zircon (U-Th)/He and apatite fission track data elsewhere in the southern Canadian Cordillera also exhibit an early Miocene cooling pulse. Widespread exhumation at this time coincides with an early Miocene plate tectonic reorganization of the Pacific-North American margin which resulted in an increase in transtension in North America. The Columbia River fault is proposed to be reactivated as a splay fault associated with dextral strike slip faulting along the Rocky Mountain Trench in the early Miocene, driven by a change in plate boundary forces. Building on previous models, pre-existing gravitational instabilities in the southeastern Canadian Cordillera aided in lateral extension of the upper crust along unstable inherited structures throughout the Shuswap domain.EnglishThermochronologyapatite (U-Th)/Heapatite fission tracktectonicsextensionCanadian CordilleraGeologymaster thesis