Orogen‐parallel extension in the North Cascades Crystalline Core, Washington

Metamorphic stretching lineations throughout the Crystalline Core of the North Cascades, Washington, show a strong preferred orientation in a horizontal NW-SE direction. Foliations strike NW and dip steeply. Preliminary observations of shear sense features indicate noncoaxial right-lateral motion. We interpret these observations to indicate that the Crystalline Core was a broad, right-lateral shear zone. The ages of the fabrics, as recorded by metamorphic minerals and syntectonic plutons, range from pre 85-Ma in the south to post 45-Ma in the north; thus the shear regime was active during an extended interval of metamorphism and plutonism in the orogen. A similar kinematic pattern is suspected for at least part of the Coast Plutonic Complex, British Columbia, which lies on strike with the Crystalline Core to the northwest.