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Chapter 20 Evolution of the Continental Crust

Anatomy of a continent:

Old (billions of years old), deformed craton at center, eroded flat. Craton consists of:

• shield (exposed metamorphic and igneouos rocks) plus
• interior platform (shield rocks covered by a few km of sedimentary rock), with a few deeper sedimentary basins

 

Margins of continents, either:

• Young orogenic zones - areas of young mountain building. Mountain building generally involves plate boundaries and collisions (Active Margins)
• Passive continental margins, that is, a continental boundary that is NOT currently tectonically active (not currently a plate boundary), for example the East Coast of North America

 

Structure and growth of North American continent

• Precambrian (Canadian) Shield- 2 -2.5 billion years old, deformed and eroded igneous and metamorphic rocks exposed at surface
• Interior Platform, Precambrian metamorphic and igneous rocks covered by a few kilometers of sediment and soil
• Appalachian Mountains - East coast, show evidence of several plate tectonic cycles (opening and closing of Atlantic Ocean with associated subduction zones), 300 - 800 million years old
• North American Cordillera - Mountainous western portion of continent, mostly less than 250 million years old, many mountain ranges associated with Cenozoic subduction zone along western North American coast. Also contains accreted microplate terranes
• Coastal plain (Atlantic coast and Gulf of Mexico) - passive margin, thick acculumulation of sediments

Details of Western North America - North American Cordillera (see figure 20.5)

• Coast Ranges - Sediments and low T, high P metamorphic rocks (subduction melange) - evidence of subduction of the Pacific Plate
• Sierra Nevada/Cascade Mountains - Sierras are a giant granitic batholith, Cascades are currently active volcanic arc - evidence of a subduction zone younger than the Rocky Mountains
• Basin and Range - Region of extensional tectonics, normal faults, and fault-block mountains separated by sediment filled basins
• Rocky Mountains - Granitic plutons, evidence of older (100-150 my) subduction zone, now being uplifted for the second time (rejuvenated)
• Colorado Plateau - Piece of interior platform - thick flat-lying sediments (Paleozoic, Mesozoic) overlying Precambrian crystalline basement (exposed at the very bottom of the Grand Canyon)
• Columbia River Plateau - flood basalts, 5 - 15 million years old, hotspot??
• Snake River Plain - Yellowstone Hotspot - intraplate volcano, currently active hotspot

Growth of continents - appears to have occurred throughout geologic time. Oldest continental rocks about 4 billion years old. Oldest cratons - 3 - 3 1/2 billion years old

Regional Vertical Movements of Continents - Ups and Downs

• Ice Loading and Unloading
• Cooling and heating of the lithosphere - for example at mid-ocean ridges
• Heating and Extension - Basin and Range
• Mantle Plumes - uplift
• Sediment deposition, creation of sedimentary basins - subsidence

Arizona Geologic Provinces

• Colorado Plateau - high in elevation with flat lying sedimentary rocks, Mesozoic & Paleozoic in age. Deep down (a few kilometers) are Precambrian basement rocks (for example at the bottom of the Grand Canyon).
• Basin and Range - Generally north trending fault-block (normal fault, tensional forces) mountain ranges separated by deep basins filled with alluvium. Complicated geologic histories. Low elevation.
• Transition zone - Area between the other two geologic provinces. Incised river valleys. Precambrian rocks exposed in places.