Abstract

The Twisp Valley Schist (TVS) is important for both the correlation of terranes in the Cascades crystalline core and for the determination of the metamorphic pressure–temperature–time history of the northeastern part of the core. The TVS is a chaotically mixed unit of mainly siliceous schist (metachert) with significant amounts of metabasite, calc-silicate rock, and marble, and minor ultramafite and metapsammite. Geochemical analyses of metabasites indicate that the TVS contains both ocean-island basalts (OIB) and mid-ocean-ridge basalts (MORB). 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, including the Mississippian–Jurassic Bridge River – Hozameen terrane of the eastern Coast belt, and coeval OIB-bearing terranes in the Northwest Cascades thrust system. Much of the northeastern Cascades core thus consists of oceanic rocks that probably originally lay between the Insular and the Intermontane superterranes, or were part of the latter superterrane.The TVS experienced polyphase deformation and greenschist to middle amphibolite facies metamorphism during the interval from 90 Ma (and possibly earlier) to ca. 50 Ma. Paleocene (ca. 65–58 Ma) dynamothermal metamorphism is the best documented event and in part resulted from forcible emplacement of plutons and from broadly distributed deformation in the Ross Lake fault zone. Major crustal loading of the TVS is inferred from the replacement of andalusite by kyanite, the presence of garnets that record an increase in pressure of 1.5–3.0 kbar (1 kbar = 100 MPa) from cores to rims, 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.

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