CRUSTAL FLOW AND A COMPARISON OF THE EOCENE EVOLUTION OF THE NORTH CASCADES CRYSTALLINE CORE AND EOCENE METAMORPHIC CORE COMPLEXES TO THE EAST

Eocene metamorphism, ductile deformation, and magmatism in the crystalline core of the North Cascades broadly coincide with events in metamorphic core complexes in the Omineca belt and eastward into Idaho. The magmatic arc represented by the Cascades core probably reached a crustal thickness of ≥55 km in the Cretaceous and may represent the western margin of a broad Late Cretaceous to Eocene orogenic plateau that included the Omineca rocks. Eocene collapse of the arc was marked by exhumation with nearly isothermal decompression of deep-crustal rocks in two domains coeval with rapid subsidence and deposition in nearby transtensional non-marine basins. These domains are the orthogneiss-dominated, partially migmatitic, 8–10 kb Skagit Gneiss Complex, and the metaclastic, 9–12 kb Swakane Gneiss. In the Skagit rocks, gently to moderately dipping foliation, subhorizontal NNW-SSE to NW-SE lineation, and constrictional domains are compatible with ductile transtension linked to dextral-normal displacement on the Eocene Ross Lake fault system, the northeastern boundary of the Cascades core. Swakane gneisses record Eocene, top-to-N to -NNW, subhorizontal non-coaxial shear. Flow in both domains is oblique to subparallel to the orogen, and contrasts with the largely E-W extensional flow in the Omineca core complexes to the east. Similarly, Eocene extensional basins to the east contrast with the transtensional basins of the Cascades. These differences in part reflect the position of the thick crust relative to the plate margin. Extension was presumably driven by orogenic collapse in a largely extensional regime in the Omineca complexes. In contrast, dextral shear associated with the oblique plate boundary, and along-strike gradients in crustal thickness and thermally controlled rheology, modified the strain field and resulted in orogen-parallel to subparallel, transtensional flow in the North Cascades. Any linked deep-crustal Eocene deformation must thus be marked by a major swing in the direction of ductile flow.