Cascadia Subduction Zone (“Cascadia Megathrust”) 1

"The earth, instead of appearing as an inert statue, is a living, mobile thing.”, J. Tuzo Wilson, 1968

Tectonics 102 (fall 2017 supplement to Geology of NPs): Part 1, More on Tectonics

Gary Oberts, Nov. 1, 2017

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Mid-Atlantic Ridge

East Pacific RiseMarianna and Aleutian Trenches

Hawaiian Islands Archipelago

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Plate boundaries and movement speed

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Range of movement: 1.3 – 18.3 cm / Yr = 0.5 – 7.2 in. / Yr

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Structure and Processes of the Tectonic Earth

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Precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches

and submarine mountain ranges

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1) Divergent boundaries (spreading centers, ridges, rises)

In ocean along spreading ridges

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On land

Iceland

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Afar Region, Horn of Africa

(East Africa Rift Valley)

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Ocean Depth

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Isotopic age-dating

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Repeated magnetic reversals of the Earth magnetic field in the geologic past

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2) Convergent boundaries (subduction zones)

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a) Oceanic-Continental – most major subduction zones like Juan de Fuca (below), Alaska, Chile, Indonesia

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Alaska

Cascadia Subd. Zone

Aleautian Chain Subd. Zone

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Nazca Plate subducting under So. American Plate (~8 cm or 3” per year)

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Farallon Plate (and Juan de Fuca

remnant)

< Likely the current situation

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Farallon subduction.mpg

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b) Oceanic-Oceanic – ocean-

located subduction zones like the Mariana

Trench (~11,000 m deep), usually with volcanic

island arc behind subduction zone

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“Accretion” (add-on, append, paste in variable shapes)

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Formation of an Accretionary Wedge

Graphics from Nat. Geog. DVD #734

c) Continental-Continental –Major mountain building

often results, like Himalayans/Hindu Kush

Ranges and Tibetan Plateau

Moving at a rate of about

*29.5’/100 yrs.😱😱😱

* 61 mm/yr 35

3) Transform boundaries (lateral, slip)

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~3”/Yr

~1”/Yr

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“Triple Junctions” (3 types)

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Spreading

Colliding

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Gorda Plate >

Complex (spreading and colliding)

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Mantle “Hot Spots”

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Continental Environments

Super-volcano

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Oceanic Environments

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New seamount (submerged island) of Lo’ihi• 400,000 years old

• 3,000’ below sea level• 10,000’ above sea floor

• Will emerge from sea in 10,000 – 100,000 years

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