Tectono-stratigraphic terranes and the metamorphic history of the northeastern part of the crystalline core of the north Cascades: evidence from the Twisp Valley Schist

The Twisp Valley Schist (TVS) is a chaotically mixed unit of mainly siliceous schist with significant amounts of metabasite, calc-silicate rock, and marble, and minor ultramafite and metapsammite. The TVS contains both ocean-island basalts and mid-ocean-ridge basalts. The TVS is lithologically similar to the Napeequa unit of the Chelan Mountains terrane in the Cascades core and is broadly correlative with units outside of the core. The TVS experienced polyphase deformation and greenschist to middle amphibolite facies metamorphism during the interval from 90 Ma to ca. 50 Ma. Paleocene dynamothermal metamorphism is the best documented event. Major crustal loading of the TVS is inferred from the replacement of andalusite by kyanite, the presence of garnets and the deeper emplacement levels of younger plutons. Loading may record thrusting in the northeastern core that is bracketed between 88 and 65 Ma and is younger than previously recognized, major contractional deformation in the North Cascades. -from Authors