Continents are made and deformed by plate motion
Continents are older than ocean rocks
Lithosphere floats on a viscous layer below the continents
Shields
central, older portions of continents
low elevation and relatively flat
basement complex of metamorphic and igneous rocks
Stable Platforms
shields covered with a series of horizontal sedimentary rocks
many transgressions and regressions
sedimentary rocks are now preserved in large basins
Mountain belts
relatively narrow zones of folded, compressed rocks with associated magmatism
formed at convergent plate boundaries
two major active belts: Cordilleran (Rockies-Andies) and Alps-Himalaya
older examples include Appalachians and the Urals
Appalachian Mountains - eroded because they formed during assembly of Pangaea more than 270 m.y. ago. Consists of
Plateau - undeformed but uplifted sedimentary rocks
Valley and Ridge - folded and faulted sedimentary rocks
Blue Ridge - intensely deformed metamorphic complex
Piedmont and Coastal Plain - intensely deformed metamorphic complex with igneous intrusions
Multiple zones because Applachians formed from multiple collisions.
North American Cordillera - Complex geologic history from multiple episodes of deformation and magmatism over the past 500 million years. Includes Rocky Mountains, Basin and Range, and Sierra Nevada mountains. Western margin of U.S. was a subduction zone which was converted to a transform fault starting about 30 Ma as the Farallon plate completely subducted and a middle ocean ridge and a subduction zone met.
Magmatic differentiation: magma transferred to continents at subduction zones
Continental accretion: buoyant fragments of continents attached to continents as the result of plate motions
Accretion of Fragments
Accretion of Island Arcs
Accretion Along Transform Fault
Accretion by Continental Collision/Rifting
Suspect Terranes of Western North America - Multiple accretions of older island arcs, oceanic plateaus, oceanic crust, and marine sedimentary rocks during the subduction and eventual disapperance of Farallon plate
How Continents are Modified
The Himalayan Orogeny
60-40 Ma - The Indian Plate subducts under the Eurasian Plate. Passive margin on Northern India and active margin on Southern Asia
40-20 Ma - India subcontinent collides with Tibet and breaks along the Main Central Thrust fault. Passive and active margin sediments are folded, uplifted and partially eroded. Plastic thickening of southern edge of Asian continent
20-10 Ma - A second thrust fault forms, lifting the first fault. Himalayas mountains from from folded and faulted sediments and crust. Ganges plain subsides and fills with sediments from erosion of Himalayas. Tibetan Plateau forms from plastic thickening of southern edge of Asian continent.
Formation of Appalachian and Urals Mountains during assembly of Pangea
510 Ma - after breakup of Rodina supercontinent, U.S is part of Laurentia continent
450 Ma (Taconic orogeny) - Laurentia collides with island arcs
400 Ma ( Acadian and Caledonian orogenies) - Laurentia collides with Baltica and forms contintent of Laurussia.
340 Ma (Variscan orogeny) - Collision of Laurussia and Gonawana starts in what is now Europe
300 Ma (Appalachian, Ural and Hercynian orogenies) - collision of Laurussia and Gonawana continues between what is now Europe and Africa and extends into what is now North America, Siberia collides with Laurussia forming the Ural mountains.
270 Ma - Assemby of Pangaea is complete.
Wilson Cycle - Edges of many cratons have experienced multiple episodes of a plate tectonic cycle
Rifting during breakup of a supercontinent
Passive margin cooling and sediment accumulation as ocean basin opens
Active margin volcanism and terrane accretion during subduction and ocean closure
Orogeny during continent-continent collision that forms next supercontinent