Publication Date

Fall 2022

Degree Type


Degree Name

Master of Science (MS)




Robert Miller

Subject Areas



The Pasayten fault extends ~250 km from Washington to British Columbia (BC), roughly following the controversial, paleomagnetically defined “Baja BC” boundary. This study investigated the deformation history of the Pasayten fault zone through mapping contacts, measuring orientations of foliations, lineations, and folds, and examining kinematic indicators in the northern segment in BC and in the southern projection of the fault in Washington. In the northern area, the ~110-105 Ma Pasayten fault separates the Jurassic-Cretaceous Methow basin from the Middle Jurassic to mid-Cretaceous Eagle Plutonic Complex and Permian Eastgate-Whipsaw metamorphic belt to the northeast. Structural orientations and kinematic indicators in outcrops and thin sections demonstrate ductile sinistral movement of the fault with a small component of reverse or normal slip. Any dextral slip was brittle. The southern area is on strike with the projection of the fault where the Cretaceous Okanogan Range batholith intrudes undated Leecher Metamorphics. Based upon the lack of offset contacts and foliations which strike discordant to the projection of the fault, the Pasayten fault does not continue south of its previously mapped termination; its slip was likely transferred to the Cretaceous Red Shirt and Methow River thrusts. Therefore, the Pasayten fault is not a major structure responsible for postulated dextral transport of the Baja BC hypothesis.