Publication Date

Fall 2020

Degree Type


Degree Name

Master of Science (MS)




Robert Miller

Subject Areas



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.