The nature and timing of movement along the Purcell Thrust, Rocky Mountain Trench, British Columbia
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AbstractThe Purcell Thrust (PT) is a classic out-of-sequence thrust, a phenomenon that is not well understood. In the study area, the Purcell Thrust lies in and near the Rocky Mountain Trench (RMT). The Purcell Thrustjuxtaposes the eastern margin of the Selkirk Fan Structure (Dogtooth Duplex) of the Omineca Crystlline Belt over the western margin of the Porcupine Creek Anticlinorium in the western margin of the Rocky Mountain Foreland Belt. Throughout most of the current study area, from the Trans-Canada Highway northwest of Golden, BC to the Big Bend region of the Columbia River, the Purcell Thrust is offset by the Trench Normal Faults (TNF) of the RMT. In both the footwall and hanging wall of the Thrust, metamorphic grade increases from sub-garnet grade in the southeast to migmatites in the northwest (towards the PT). Detailed petrographic studies coupled with electron microprobe analyses determined that garnet is the only mineral that is consistently zoned. Generally, Xpe and Xivig values decrease from the rim to core, while Xca and Xmb values increase from the rim into the core. Thermobarometric calculations using THERMOCALC v3.1 combined with the garnet-biotite thermometer and GASP barometer failed to observe a statistically significant barometric break across the Purcell Thrust. It is possible that movement along a Trench Normal Fault may have disguised this barometric break. Calculations of diffusional penetration distances demonstrated that kyanite + staurolite grade garnets smaller than 5 mm in radius probably had their internal chemistry altered by the affects of retrograde cation diffusion. Fault plane reconstructions of movement along the Purcell Thrust and the Trench Normal Faults demonstrated the existence of at least 10 km of dextral movement along the Purcell Thrust. Geochronology demonstrated that peak regional metamorphism was between 159 and 179 Ma, corresponding to the Jurassic collision of the Intermontane Superterrane with the western margin of ancestral North America. Movement along the Purcell Thrust occurred between 107 and < 85 Ma. This is when the Cretaceous collision of the Insular Superterrane collided with the western margin of ancestral North America. Early Eocene extension coupled with movement along the Trench Normal Fault from 61 to 54 Ma.
Bibliography: p. 455-479