Ancient Northwest

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Ancient Northwest
Logan Pass, Glacier National Park

• The Foundation of a Continent
• and the Belt Supergroup

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Exposed Rocks of the Precambrian
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A-3
Location in Lecture Handouts
Cordillera
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Cordillera
Part of the North American Mobile Belt
Mobile Belt
Compared to Craton
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Anatomy of a Continent
Craton
Continental Shelf
Mobile Belt
Basement Rock
Exposed ancient basement rock
Sedimentary rock covering basement rock
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How do we study basement rock?

• Windows(Basement exposed at the surface)
• Xenoliths/Magma(Carrying pieces of basement to
  the surface)
• Drill Cores Samples(Drilling to the basement)
• Geophysics(Imaging the basement)

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Basement Rock Windows in NA
A-1
Note Not many in the west
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How do we study basement rock?

• Windows(Basement exposed at the surface)
• Xenoliths/Magma(Carrying pieces of basement to
  the surface)
• Drill Cores Samples(Drilling to the basement)
• Geophysics(Imaging the basement)

Mantle Xenolith
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A-4
Old Basement
Young Basement
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How can we use magma chemistry to tell old/young
basement rocks?

• Rubidium-87 (Rb-87) is radioactive and decays to
  form Strontium-87 (Sr-87)
• Rb-87 is common in felsic rocks (continental
  crust)
• Young crust - most Rb had not decayed(not much
  Sr-87)
• Old crust - lots of Rb has decayed into Sr-87
• When crust melts - these chemicals end up in
  magma and magma carries chemical signal to the
  surface
• High Sr-87 old basement belowLow Sr-87 young
  basement below

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The Edge of the Old Basement
A-4
706 Line
gt 0.706 (Old Basement Rock)
lt 0.704 (Young Basement Rock)
Measure Sr-87 relative to Sr-86
Sr87/Sr86 in Plutons
gt 0.706 (high)
lt 0.704 (low)
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How do we study basement rock?

• Windows(Basement exposed at the surface)
• Xenoliths/Magma(Carrying pieces of basement to
  the surface)
• Drill Cores Samples(Drilling to the basement)
• Geophysics(Imaging the basement)

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How do we study basement rock?

• Windows(Basement exposed at the surface)
• Xenoliths/Magma(Carrying pieces of basement to
  the surface)
• Drill Cores Samples(Drilling to the basement)
• Geophysics(Imaging the basement)

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Seismic Profile
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BasementProvinces of North America
A-2
A.K.A. The United Plates of America
GF
Archean Basement (gt2.5 billion years)
Proterozoic Basement (2.5-0.5 billion years)
Mobile Belts
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Northwest Basement
A-2
Wyoming Provenance 2.8-3.5 Gyrs old Includes
rocks found in Beartooth Mtns and Grand Teton
National Park
GF
Hearne Provenance 2.8-3.5 Gyrs old
Collision Zone Great Falls Tectonic Zone
(GF) 2.8 Gyrs ago
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Northwest Basement
A-2
Collision Zone Trans-Hudson Belt 1.9-1.8 Gyrs
ago Wyoming/Hearne collide with Superior
Provenance
GF
By 1.8 Gyrs the basement rock of NA was in place