EASTERN PACIFIC ANALOGUE FOR AVALONIA AND CORRELATIVE PERI-GONDWANAN TERRANES

The northeastern Pacific margin is used as an actualistic plate tectonic model for the evolution of Avalonia and correlative peri-Gondwanan terranes. This evolution includes: (1) development of primitive oceanic arcs in Panthalassa and circum-Rodinian oceans immediately following the amalgamation of Pangea and Rodinia, respectively; (2) accretion of these arcs to the continental margins and development of continental margin arcs following the breakup of the supercontinents: Early Mesozoic off western Laurentia, and c.755 Ma off Rodinia; (3) diachronous termination of subduction as a consequence of ridge-trench collision (Kula-Farallon ridge and East Pacific Rise in the eastern Pacific: Merlin-Morgana and Morgana-Mordred ridges in the ocean bordering northern Gondwana), and transition to an intracontinental wrench system in the Late Mesozoic-Tertiary in the eastern Pacific region, and Neoproterozoic off Gondwana, (4) separation of borderland terranes: Late Mesozoic in Baja BC and Oligocene in Baja California in wesern Laurentia, and Cambro-Ordovician in peri-Gondwanan terranes in the Appalachian-Caledonian orogen; (5) dispersal of these exotic terranes by transporting them on oceanic plates: during the Late Mesozoic and Tertiary on the Kula and Pacific plates in the eastern Pacific, and during the Ordovician on the Mordred and Morgana plates in the Iapetus Ocean; (6) accretion of these exotic terranes to the Laurentian continental margin: Baja BC in the Early Tertiary (Baja California will accrete at some time in the future), and peri-Gondwanan terranes in the Late Ordovician-Early Silurian. Western Avalonia appears to have been rotated through c.90° between separation from Gondwana and its accretion to eastern Laurentia. This may have occurred either during separation by analogy with the Transverse Ranges in California; during oceanic transport; or during sinistral accretion to eastern Laurentia.