Global Tectonic Cycles

(Supercontinent Cycle)

Working backward from positions and shapes of today's continents, modern tectonicists -- beginning in the early 1900's with Alfred Wegener -- propose that all major continental masses assembled into a supercontinent (Pangea) about 300 million years ago. About 100 million years later, Pangea broke apart, sending its fragments on a 200-million-year journey toward today's configuration.

Evidence suggests that Earth's continents have joined and split not once but many times. Some tectonicists now believe that multiple "Atlantic" oceans have closed and opened while the Pacific Ocean has widened and narrowed in complement. Cycle duration (period) seems to lie in the range of 400 to 500 million years. Boundaries of traditional divisions of geologic time may reflect transitions between phases of this tectonic cycle. Changes in global environment due to this tectonic cycle (e.g. sea level, average ocean depth, ocean circulation patterns) may have caused some major extinctions observed in the geologic record. Assembly of past supercontinents presumably formed metamorphic rock and mountain belts now well within cratonic core regions.

In this scenario:

• Most Georgia rocks north of the Fall Line -- and rocks beneath Coastal Plain sediments -- formed during opening and closure of the Iapetus Ocean, ancestor of today's Atlantic Ocean.
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• The Corbin meta-granite, among Georgia's oldest rocks, may have formed during closure of an "Atlantic" ancestor ocean more than a billion years ago, like Pennsylvanian granites formed during closure of Iapetus.
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• Ocoee meta-sediments (in the Cohutta mountains) probably were deposited in rift basins during breakup of Pangea's ancestor supercontinent.
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• Ridge and Valley sedimentary rocks were deposited on proto-North America's continental shelf -- isolated from metamorphic processes in the Iapetus basin -- from Cambrian through Pennsylvanian time.
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• Triassic/Jurassic diabase dikes intruded into northwest-trending cracks in continental crust during Pangea's breakup.
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• Coastal Plain sediments were deposited on a foundation of metamorphic rock -- apparently a fragment of proto-Africa -- during higher sea levels of late Cretaceous, Tertiary, and Quaternary time.

This table illustrates how Georgia rocks may fit into global tectonic cycles; please regard it as speculative and fuzzy approximation. For simplified presentation, this table shows 450-million-year tectonic cycles, with a rather coarse resolution of 25 million years per line. Careful observers may notice imperfect fit. Cycle period may be other than 450 million years, or perhaps not all cycles have the same duration.

Events of this size do not happen quickly. Presumably, Pangea's assembly took place over many millions of years, probably in stages. Likewise, Pangea evidently rifted in stages to form the Atlantic, Caribbean, and Mediterranean basins. Time boundaries therefore are approximate.

Global Tectonic Cycles
and Georgia Rocks

For further information on tectonic cycles and continental assembly, consult:

• "The Supercontinent Cycle", Scientific American, July, 1988;
  also compiled in Shaping the Earth; Tectonics of Continents and Oceans
• "The Southern Appalachians and the Growth of Continents",
  Scientific American, October, 1980;
  also compiled in Shaping the Earth; Tectonics of Continents and Oceans
• U.S. Geological Survey Professional Paper 1475
• Assembling California, John McPhee, 1993

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