Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 11
Presentation Time: 4:50 PM


WHITMEYER, Steve, Dept. of Geology & Environmental Science, James Madison University, Harrisonburg, VA 22807,

The “classic” orogenic cycle of 1: rift-drift, 2: subduction initiation/arc formation, 3: microcontinent accretion, and 4: ocean closure/continental collision has been applied with some success to the eastern margin of Laurentia for the period between the breakup of Rodinia and the assembly of Pangaea. A similar cycle is recognized for the western margin of Gondwana, where major tectonic events are broadly coeval with the more familiar eastern Laurentia sequence. East-directed subduction (present-day coordinates) initiated along the western margin of Gondwana in the late Proterozoic to early Cambrian. This was followed by late Ordovician emplacement of arc-related tonalites and granites, and Devonian accretion of tectonothermal terranes, including the Laurentia-derived Precordillera terrane.

Evidence for Paleozoic orogenic events along the western margin of Gondwana is well documented in the Eastern Sierras Pampeanas of central Argentina. Extensive NNE-trending ductile shear zones, some of which are interpreted as major terrane sutures, reach up to several kilometers in thickness. Detailed analyses of metamorphic and deformation fabrics indicate predominantly dextral oblique transpression with a strong east over west component. Fabrics preserve evidence for the Famatinian orogeny (late Ordovician to early Devonian), collision of the Precordillera terrane (middle Devonian), and possibly the later emplacement of Chilenia. Similarly-timed tectonic events associated with the development of eastern Laurentia include Taconic (Ordovician) arc accretion and Acadian (Devonian) accretion of Gondwana-derived microcontinents, such as West Avalon and Carolina.

The occurrence of broadly coeval cycles of accretion on both sides of the Iapetus during the Paleozoic suggests that a complete depiction of an orogenic cycle includes continental rifting, oceanic spreading, subduction on both margins of the oceanic basin, and final closure of the ocean with continental collision. Local complexities such as accretion of arc systems and/or microcontinents can occur simultaneously on opposing margins of a closing ocean basin. The Rodinia to Pangaea sequence suggests that a complete orogenic cycle can be accomplished in ~500 m.y., with the present-day Atlantic in the drift phase and the Pacific in the subduction phase.