Paleozoic tectonic evolution of the rifted margins of Laurentia

ABSTRACT The Neoproterozoic to Cambrian rifting history of Laurentia resulted in hyperextension along large segments of its Paleozoic margins, which created a complex paleogeography that included isolated continental fragments and exhumed continental lithospheric mantle. This peri-Laurentian paleogeography had a profound effect on the duration and nature of the Paleozoic collisional history and associated magmatism of Laurentia. During the initial collisions, peri-Laurentia was situated in a lower-plate setting, and there was commonly a significant time lag between the entrance of the leading edge of peri-Laurentia crust in the trench and the arrival of the trailing, coherent Laurentian landmass. The final Cambrian assembly of Gondwana was followed by a global plate reorganization that resulted in Cambrian (515–505 Ma) subduction initiation outboard of Laurentia, West Gondwana, and Baltica. Accretion of infant and mature intra-oceanic arc terranes along the Appalachian-Caledonian margin of the Iapetus Ocean started at the end of the Cambrian during the Taconic-Grampian orogenic cycle and continued until the ca. 430–426 Ma onset of the Scandian-Salinic collision between Laurentia and Baltica, Ganderia, and East Avalonia, which created the Laurussian continent and closed nearly all vestiges of the Iapetus Ocean. Closure of the Iapetus Ocean in the Appalachians was followed by the Devonian Acadian and Neoacadian orogenic cycles, which were due to dextral oblique accretion of West Avalonia, Meguma, and the Suwannee terranes following the Pridolian to Lochkovian closure of the Acadian seaway and subsequent outboard subduction of the Rheic Ocean beneath Laurentia. Continued underthrusting of Baltica and Avalonia beneath Laurentia during the Devonian indicates that convergence continued between Laurentia and Baltica and Avalonia, which, at least in part, may have been related to the motions of Laurentia relative to its converging elements. Cambrian to Ordovician subduction zones formed earlier in the oceanic realm between Laurentia and Baltica and started to enter the Arctic realm of Laurentia by the Late Ordovician, which resulted in sinistral oblique interaction of the Franklinian margin with encroaching terranes of peri-Laurentian, intra-oceanic, and Baltican provenance. Any intervening seaways were closed during the Middle to Late Devonian Ellesmerian orogeny. Exotic terranes such as Pearya and Arctic Alaska became stranded in the Arctic realm of Laurentia, while other terranes such as Alexander and Eastern Klamath were translated further into the Panthalassa Ocean. The Middle/Late Devonian to Mississippian Antler orogeny along the Cordilleran margin of Laurentia records the first interaction with an outboard arc terrane built upon a composite block preserved in the Northern Sierra and Eastern Klamath terranes. The Carboniferous–Permian Alleghanian-Ouachita orogenic cycle was due to closure of the vestiges of the Rheic Ocean and assembly of Pangea. The narrow, continental transform margin of the Ouachita embayment of southern Laurentia had escaped accretion by outboard terranes until the Mississippian, when it collided with an outboard arc terrane.

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