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The global distribution, setting, and dynamic implications of Ordovician orogenesis are reviewed. Evidence for true Ordovician continent-continent collision is absent. Orogenesis is principally due to accretion of arc terranes and/or ribbon microcontinents. Most arc terranes are ensialic and separated from the adjacent continents by backarc or marginal basins, the episodic closure of which commonly was responsible for orogenesis. Little evidence is preserved for true intra-oceanic juvenile arcs during the Early to Middle Ordovician. Instead, subduction appears to have been localized near the margins of Laurentia, Gondwana, Baltica, and Siberia, forming extensive linear orogenic belts during relatively short periods when the upper plate switched from extension to compression. Such tectonic switching appears to have taken place along the entire length of the Pacific and Iapetan margins of Gondwana (>10,000 km) from Middle–Late Cambrian to Early Ordovician time. The onset of orogenesis along Gondwana's Pacific margin during the end of the Early Cambrian (ca. 513 Ma) coincided with subduction initiation along both margins of the Iapetus Ocean. Orogenesis and subduction initiation are causally related to a global-scale plate reorganization, probably induced by terminal amalgamation of Gondwana. During the Paleozoic, Laurentia's Iapetan margin steadily grew in size and expanded southward owing to continuous accretion of suprasubduction zone oceanic crust, peri-Gondwanan arc terranes, and ribbon microcontinents. In contrast, the Pacific, Iapetan, and Rheic margins of Gondwana saw little addition of new, allochthonous crust. Accretion mainly involves reattachment of previously rifted-off arc terranes and small slivers of the intervening marginal basin crust.

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