The 10 kilometer wide Ross Lake fault system (RLFS) is part of a 500 kilometer long zone of high-angle faults in the North American Cordillera. The RLFS consists of two major sets of faults. The eastern set of the Hozameen and Slate Creek faults and more southerly North Creek fault form the western boundary of the Jurassic-Cretaceous Methow River basin and in part separate it from metamorphic equivalents of Methow strata. Minor structures along the North Creek fault record dextral strike-slip events that occurred between approximately 88 and 50 Ma. The same formations lie on both sides of the faults, implying modest slip (10s of km?). The northernmost strand of the western fault set, the Ross Lake fault itself, is a vertical zone of horizontally-lineated mylonite that separates upper-amphi