The Cretaceous–Paleogene Ross Lake fault system: Relationships between magmatism and faulting during large-scale translation in the northern North American Cordillera of Washington and British Columbia

ABSTRACT The ≥10-km-wide, long-lived Ross Lake fault system is part of a ≥500-km-long zone of Paleogene, NW-striking, high-angle faults in the northern North American Cordillera. This fault system forms the eastern boundary of the exhumed Late Cretaceous–Eocene North Cascades magmatic arc and is marked by metamorphic discontinuities and focused magmatism. The Ross Lake system consists of two major sets of faults. The eastern set of the Hozameen–North Creek fault forms the western boundary of the Jurassic–Cretaceous Methow sequence and in part separates the sequence from its likely metamorphic equivalents. Dextral slip along this segment of the fault system is only constrained to between ca. 87 Ma and 48 Ma. The northernmost strand of the western fault set, the Ross Lake fault (sensu stricto), is a vertical zone of horizontally lineated mylonite that separates upper-amphibolite-facies rocks of the Skagit Gneiss Complex from lower-grade metamorphic rocks to the east. Traced to the SE, the Ross Lake fault is interpreted to step westward ~5 km across a gently to moderately dipping zone containing rocks buried to >30 km depth. The fault continues along strike for ~80 km as the Gabriel Peak tectonic belt, which is a moderately to steeply NE-dipping mylonite zone that records dextral shear in the north and reverse shear farther south. Movement in this transpressional zone occurred from potentially as early as ca. 82 Ma to ca. 58–55 Ma. The tectonic belt is truncated in the south by the Foggy Dew fault zone, which contains higher-grade fault rocks than the on-strike Hozameen–North Creek fault. The Foggy Dew fault zone was active from at least 63–55 Ma and ended by 49 Ma. It records oblique dextral-normal slip (down-to-the-E) marking the regional transition in northern Washington and southern British Columbia from transpression to transtension, and extension farther to the east. The Foggy Dew fault zone is truncated to the SE by the ca. 49–48 Ma Cooper Mountain batholith. South of this intrusion, slip is potentially transferred across strike westward to the Eocene dextral Entiat fault within the North Cascades arc. Field relations and geochronologic data are interpreted to indicate that during the active duration of the fault system, numerous magmatic intrusions led to complex rheological transitions that influenced the location of shear and helped the Ross Lake system respond to changing tectonic regimes. Melt probably initially weakened the shear zones, and strain was subsequently focused along rheologically strong pluton margins after pluton crystallization. Offsets of contacts in the northern extent of the Ross Lake fault system suggest that cumulative dextral slip on the fault system was likely ≥125 km but <400 km. This dextral slip occurred at a time when paleomagnetic data indicate large northward translation of the Insular superterrane. The fault system disrupted and shuffled rocks within the eastern part of paleomagnetically defined Baja–British Columbia and is important for paleogeographic reconstructions of the North American Cordillera.

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