Exhumational history of the central Rocky Mountain Trench using low-temperature thermochronology

Date
2021-01-18
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Abstract
The Rocky Mountain Trench (RMT) is a 1600 km long valley extending from Montana northwest into northern British Columbia. The RMT’s impressive length and continuous nature have been interpreted to be the result of faulting, and localized erosion, along a continent scale crustal weakness located beneath the RMT, possibly the ancient continental margin. Despite the continuous nature of the RMT, the faults within the RMT vary along strike from dextral strike-slip faults in the north to normal faults in the south. The central RMT, located near Valemount, BC, is thought to be the transition zone between dextral strike-slip faulting in the north and normal faulting in the south. I use low-temperature thermochronology on 32 bedrock samples collected on both sides of the central RMT and from two elevation profiles along a ~300 km long section of the central RMT, from McBride southeast to Donald, to compare the cooling histories across the central RMT. I use apatite fission track (AFT), which records cooling through the apatite partial annealing zone (PAZ)(~60–120 °C), and apatite (U-Th)/He (AHe), which records cooling through the apatite partial retention zone (PRZ) (~40–80 °C). I report AFT ages from 25 samples and AHe ages from 28 samples along with thermal-history models. In the north, near McBride, data along the two elevation profiles and from thermal-history models suggest ~2 km of west-side down normal faulting along the RMT in this area since Miocene time (<20 Ma). Further south thermal-history models reveal rapid exhumation of the Malton Gneiss Complex from at least ~4.8 km depth since Miocene time (<20 Ma). Slower cooling outside of the Malton Gneiss Complex, west of the North Thompson Albreda Fault (NTAF) and east of the RMT suggests west-side down and east-side down normal faulting, respectively. Later, since 10 Ma a combination of ~1.6 km of west-side down displacement across the RMT near Valemount, increased erosion due to glacial carving, and isostatic rebound ue to lithospheric mantle delamination drives additional exhumation of the Malton Gneiss Complex and footwall of the RMT to surface from ~1.2 km and ~2.8 km depths, respectively.
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Citation
Fraser, K. (2020). Exhumational history of the central Rocky Mountain Trench using low-temperature thermochronology (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.