ANALOGUE OF THE U.S. CORDILLERA ON THE NORTHERN MARGIN OF NEOPROTEROZOIC GONDWANA

The evolution of the U.S. Cordillera from terrane accretion to magmatic arc development, ridge-trench collision, transform margin formation and, finally, continental rifting, is geodynamically linked to the amalgamation and breakup of Pangea. Broad similarities between this history and that of the northern Gondwanan margin, as reflected in the late Mesoproterozoic-Early Paleozoic evolution of Avalonia and related peri-Gondwanan terranes, suggest a similar linkage to the assembly and dispersal of Rodinia. (1) Just as the evolution of primitive Cordilleran arcs in Panthalassa coincided with the Late Paleozoic assembly of Pangea, so the evolution of primitive Avalonian arcs (proto-Avalonia) at 1.2-1.0 Ga coincides with the assembly of Rodinia. (2) In the same way that accretion of mature oceanic arcs to western North America and the development of the main phase of Cordilleran arc magmatism followed the Early Mesozoic breakup of Pangea, so the development of mature oceanic arcs at 750-650 Ma (early Avalonian magmatism), their accretion to Gondwana at ca. 650 Ma, and the formation of a continental margin arc at 635-570 Ma (main Avalonian magmatism), followed the breakup of Rodinia at ca. 755 Ma. (3) In the same way that North America's collision with the East Pacific Rise in the Oligocene led to the diachronous initiation of a transform margin, so the diachronous termination of subduction in the absence of continental collision and its transition to an intracontinental transform regime at ca. 590-540 Ma is attributed to ridge-trench collision. (4) Just as the interaction of the East Pacific Rise with the North American continental margin in the Pliocene led to the separation of Baja California, so Avalonia separated from Gondwana in the Early Ordovician. In a reciprocal view, the Late Orovician-Early Silurian sinistral accretion of Avalonia to eastern Laurentia emulates the Cenozoic dispersal of Cordilleran terranes and may mimic the paths of future terranes transferred to the Pacific plate.