Movement History of the Pasayten Fault Zone: Insights into Large-Scale Transport Across the North American Continental Margin
Master of Science (MS)
Controversial paleomagnetic data implies that at 90 Ma much of the Coast Mountains and Insular superterrane were 1200-3000 km to the south. The eastern boundary of these allochthonous rocks is near the Pasayten fault in southern British Columbia. This study covered an ~40 km segment of the fault, and the Eagle Plutonic Complex and Eagle shear zone to the east. In the 157-123 Ma Eagle tonalite, NE-vergent reverse shear zones involve rafts of the host Nicola Group; overall, the Eagle shear zone has flattening fabrics. Plutonic screens in the tonalite yielded a U-Pb zircon age of 207.9±2.1 Ma, implying rocks related to the Triassic Mount Lytton Complex extend ~100 km SE into the tonalite. Greenschist-grade mylonites of the ~110 Ma Fallslake Plutonic Suite of the Eagle Plutonic Complex formed in the Pasayten fault zone. Kinematic indicators, including quartz fabrics from Electron Backscatter Diffraction, record sinistral slip with a west-side-down normal component. Cooling ages indicate motion at ~110-104 Ma, which is some of the youngest documented sinistral shear in the northern Cordillera. West of the Pasayten fault, detrital zircon ages from Eocene clastic rocks have a major peak at 93 Ma, which does not match Eocene basins east of the fault or adjacent Cretaceous strata of the Methow basin. Brittle slip with a presumed normal component occurred after the sinistral shear, but no dextral slip is recognized, casting serious doubt that the Pasayten fault is a major structure in the Baja BC hypothesis.
Lee, John Daniel, "Movement History of the Pasayten Fault Zone: Insights into Large-Scale Transport Across the North American Continental Margin" (2020). Master's Theses. 5152.