The Olympic Mountains

A Wedge of Fun

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Olympic Mountains, Wilapa Hills & Oregon Coast Range

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Geology Map

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Major Rock Units

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1. Peripheral Rocks

2. Core Rocks

Separated by reverse faults

Peripheral Rocks: Mostly the Crescent Fm

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Age:Mostly Eocene (60-50 Myrs)

RocksPillow BasaltsSome Vesicular Basalts15-30 km thick

Limestone(Red)w/deep water forams

Crescent Formation

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Basalt of the Crescent Formation on Klahhane Ridge above Port Angeles (USGS).

Core Rocks

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Age:Eocene-Miocene (40-20 Myrs)Metamorphosed 14-12 Myrs

RocksSandstones

w/graded bedsShale

MetamorphismEast - Low Grade

(11 km burial)West - Not Met.

Youngest Oldest

Core Rocks

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Sandstone beds with thin shale interbeds on Windfall Peak. (USGS)

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Complexly disrupted sandstone and shale beds of Olympic Core on north ridge of Mount Fitzhenry. (USGS)

Olympic Structures

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Peripheral RocksCore Rocks

Note faults place older rock over younger rock

Interpretation: Peripheral Rocks

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Hot Spot Island

Terrane (fault bounded)

Basalt Chemistry = Hot Spot & Divergent Zone

Modern Analogy - Iceland

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Possible Eocene ReconstructionE-1

55 Myrs

35 Myrs

Plate Reconstruction: 55 Myrs

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Supporting Evidence: Rock Age

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Interpretation: Core Rocks

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Trench Sediments

Accretionary Wedge

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What is an Accretionary

Wedge?

Olympic Accretionary Wedge

Olympic History

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Western Core rocks accreted first & were buried deepest

(explains why they are oldest and most metamorphosed)

But Why is the North “Wedge” (Olympics) so much larger than the

South “Wedge” (Wilipa Hills/Oregan Coast Range)?

1. Shoved in the Corner

Insular ST

WashingtonOlympics

CoastRange Oregon

But Why is the North “Wedge” (Olympics) so much larger than the

South “Wedge” (Wilipa Hills/Oregan Coast Range)?

2. There is a bend in the Juan de Fuca Plate under the Olympics

N. America

Cross SectionNorth ------ South

Juan de Fuca

Olympics

Evidence for the Bend1. NW Volcanoes

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100 Depth

Steep Subduction Shallow Subduction

Evidence for the Bend2. NW Earthquakes

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Quakes occur in the subducting plate

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Why is there a Bend at all?Basin & Range Extention to the South

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