The Growth of Western North America - Part

IIThe Insular Superterrane, North Cascades & San Juan

Islands

Our Story So Far…

C-3

Arrival of the Insular Superterrane

IntermontaineSuperterrane

InsularSuperterrane

MethowBasin

C-3

C-3

IntermontaineSuperterrane

InsularSuperterrane

MethowBasin

NorthCascades

After Accretion

Subduction zone relocates

North Cascades at 100 Myrs

Insular Superterane: Wrangellia Terrane

Rocks:Basalts

top: vesiclesbottom: pillows

Limestone

Age:Triassic/Jurassic

Source:Fossil clams and forams

western PacificPaleomagnetics

±15° N/S

Type of Terrane:Hot Spot Islands

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Photo: Dr. ML.Bevier

ObliqueCollision Continent

Terrane

ObliqueCollision Continent

ObliqueCollision Continent

ObliqueCollision Continent

Terrane is “smeared out” along coast

Insular Superterra

ne:Wrangellia

C-2

Wrangellia & the North Cascades

C-6

IntermontaineSuperterrane

InsularSuperterrane

MethowBasin

“Microterranes”

Modern Day

Analogy for

North Cascade Rocks

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Cont.

C-6IntermontaineSuperterrane

InsularSuperterrane

North Cascades &San Juan Islands

Time

Many Microterranes of the North Cascades & San Juan Is.

(See list on C-9)

C-4

• Skagit• Chelan Mountains• Nason• Shuksan• Nooksak River• Chilliwack• Haro• Turtleback• Deadman Bay• Garrison• Decatur• Western Melange• Eastern Melange• …and the rest…

Major Terrane Groupings

• Skagit• Chelan Mountains• Nason• Shuksan• Nooksak River• Chilliwack• Haro• Turtleback• Deadman Bay• Garrison• Decatur• Western Melange• Eastern Melange• …and the rest…

Skagit Crystalline Core

NW Cascades andSan Juan Thrust System

Melanges

Major Terrane Groupings

C-4

Skagit Crystalline Core

NW Cascades andSan Juan Thrust System

Melanges

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Skagit Crystalline Core

Skagit Gneiss

Cascade River Schist

HighGrade

NW Cascades ThrustsDarrington Phyllite

Shuksan Blueschist

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Protolith:Shale

Protolith:Basalt

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-- Subducted Ocean Crust --

Low to MediumGrade

Twin Sisters Dunite

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Major Terrane Groupings

Skagit Crystalline Core

NW Cascades andSan Juan Thrust System

Melanges

High Grade Met.

Medium Grade Met.

Low Grade Met.

C-4

Skagit Crystalline Core

NW Cascades & San Juan Islands Thrusts

Separated by thrust faults

C-5 & 6

San Juan Islands

C-7

San Juan Islands

C-7

Decatur TerraneJurassic

ChertBasaltGabbroSerpentinite

Ocean Crust

Similar to Terrane rocks in Northern California

San Juan Islands

C-7

Deadman Bay TerranePermian to Jurassic

ChertLimestoneW/Tethyan ForamsBasalt

Ocean Crust

Similar to Cache Creek Terrane in Intermontaine Superterrane

San Juan

Thrust Sheets

C-7

San Juan Island Thrust Sheets

C-8

NW Cascades/San Juan Islands

C-5

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Nanaimo Formation:Overlap Sequence

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Mt. Stuart - Stitching Pluton

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90 Myrs

Timing of the North CascadesCollision

C-1

Age of theMicroterranes

Age of Accretion115-90 Myrs

C-10

Meanwhile in Oregon…

Terranes of the

Wallowa Mountai

ns

C-11