North Cascades Western Domain Metamorphic Core Methow Domain

Western Domain Nooksack terrane- volcaniclastic sediments deposited in submarine fans (Jurassic to Cretaceous age) Chilliwack River terrane- Volcanic and sedimentary rocks from a volcanic arc- slightly metamorphosed- some greenstone pillow basalts, phyllites, marbles– strongly folded and faulted- Devonian to Triassic

Western Domain (cont.) Bell Pass Melange-(French word for mix)- associations of very unusual rocks in the same unit (Precambrian to Triassic) –Yellow Aster Complex-granulite facies metamorphic rocks –Twin Sisters Dunite- dunite is a rock composed almost entirely of olivine. Interpretation is that this is upper mantle rock

Easton Terrane (formerly called Shuksan Greenschist) Darrington phyllite- many of the phyllites in our collection are from the Darrington phyllite. Locally graphitic. Originally deep ocean sediments Shuksan greenschist- originally oceanic basalts. Metamorphosed into greenschists and blueschists. Jurassic age

Metamorphic Core Domain Chelan Mountains Terrane- contains many different units, but a major unit is the Cascade River schist- many of our biotite, garnet schists are Cascade River schist. Originally this terrane was both oceanic and arc environments. There are metamorphosed plutonic roots to the volcanic arc. – Triassic in age

Metamorphic Core Domain (cont.) Nason terrane- Metmorphosed submarine arc sediments. Mostly schists and gneisses derived from the schists. Original sediments deposited in Triassic. Swakane terrane-If you drive north of Wenatchee along the Col. River, this terrane is exposed on west side of river. Primarily gneisses derived from volcanic or sedimentary arc rocks. Probably Precambrian in age.

Metamorphic Core Domain (cont.) Little Jack terrane- small terrane north and east of Ross Lake. Metamorphosed sediments from a submarine fan at the base of a volcanic arc. Also contains blocks of metamorphosed ultramafics.

Younger gneisses associated with the Metamorphic Core Domain Skagit Gneiss Complex- gneisses and migmatites derived from plutons intruded into the assembled terranes of the Meta. Core. Domain. Probably not intruded in the present location. Othrogneiss- gneiss derived from a granitic intrusive rock Paragneiss- gneiss derived from sedimentary rocks

Methow Domain Methow terrane- thick sequence of marine sediments as old as Jurassic, sediments deposited on land (rivers, lakes, etc), and some volcanics and associated plutonic rocks (Jurassic to Cretaceous)

Methow Domain (cont.) Hozomeen terrane- Crater and Jack Mountains on the east side of Ross Lake. Composed of deep marine basalts, cherts, and associated sediments. Mississippian to Triassic in age.

Metamorphism, Faulting and Intrusion The terranes of the North Cascades were accreted about 90 million years ago (Cretaceous). Many of the terranes show metamorphism associated with this accretion. The thrust faults in the Western Domain formed at this time. Strike-slip faulting occurred after the thrust faulting, some faults may have been Paleocene, others were active through Eocene. Intrusion of granitic batholiths throughout the Cascades