Metamorphic and structural evidence for significant vertical displacement along the Ross Lake fault zone, a major orogen‐parallel shear zone in the Cordillera of western North America

Results of an investigation of the petrology and structure of the Skymo complex and adjacent terranes constrain the amount, timing, and sense of motion on a segment of the > 600‐km‐long Late Cretaceous ‐ early Tertiary Ross Lake fault zone (RLFZ), a major orogen‐parallel shear zone in the Cordillera of western North America. In the study area in the North Cascades, Washington state, the RLFZ accommodated significant pre‐middle Eocene vertical displacement, and it juxtaposes the Skymo complex with upper amphibolite facies (650°–690°C and 6–7 kbar) Skagit Gneiss of the North Cascades crystalline core to the SW and andalusite‐bearing phyllite of the Little Jack terrane (Intermontane superterrane) to the NE. The two main lithologic units of the Skymo complex, a primitive mafic intrusion and a fault‐bounded block of granulite facies metasedimentary rocks, are unique in the North Cascades. Granulite facies conditions were attained during high‐temperature (> 800°C), low pressure (≤ 4 kbar) contact metamorphism associated with intrusion of the mafic magma. P‐T estimates and reaction textures in garnet‐orthopyroxene gneiss suggest that contact metamorphism followed earlier, higher pressure regional metamorphism. There is no evidence that the Skagit Gneiss experienced high‐T ‐ low‐P contact metamorphism. In the Little Jack terrane, however, texturally late cordierite ± spinel and partial replacement of andalusite by sillimanite near the terrane's fault contact with Skymo gabbro suggest that the Little Jack terrane experienced high‐T (∼ 600°C) ‐ low‐P (≤ 4 kbar) contact metamorphism following earlier low‐grade regional metamorphism. Similarities in the protoliths of metasedimentary rocks in the Skymo and Little Jack indicate that they may be part of the same terrane. Differences in pressure estimates for the Little Jack versus Skymo for regional metamorphism that preceded contact metamorphism indicate vertical displacement of ∼ 10 km (west side up) on the strand of the RLFZ that now separates the two structural blocks. High‐angle faults in the study area are dextral‐reverse mylonitic shear zones that experienced later brittle normal slip. Vertical motion on these shear zones before intrusion of Skymo gabbro can account for metamorphic discontinuities indicated by P‐T results. The terranes have also been internally deformed by nonintersecting but coeval dextral and sinistral shear zones that formed after the terranes were brought together in the RLFZ and intruded by Eocene dikes, These results show that the RLFZ has accommodated significant vertical displacement but perhaps no more than tens of kilometers of early Tertiary lateral movement. Structural evidence for earlier, large‐magnitude strike‐slip displacement is not preserved.