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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 arcslightly 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