APPALACHIAN, GONDWANAN, CORDILLERAN INTERACTIONS: A NEW GEODYNAMIC MODEL FOR THE PALEOZOIC TECTONIC EVOLUTION OF THE NORTH AMERICAN CORDILLERA

A number of Cordilleran terranes have detrital zircon age populations which suggest to us a Gondwanan or peri-Gondwanan source rather than a Laurentian source. In addition, we suggest that the peculiar detrital zircon ages of the Ordovician Cordilleran miogeocline were for the most part not derived from the Peace River Arch as proposed by Gehrels and others (1995) but instead reflect the Taconic Orogeny in the Appalachian Orogen and were in large part derived from a Gondwanan source. As pointed out by Gehrels and others (1995) the Ordovician of the Cordilleran miogeocline is distinct from other time periods. In particular, the local Precambrian basement cannot have supplied the detrital zircons now found within the Sonora and Nevada segments of the miogeocline. Dominant age populations are in the 1.8-2.1 Ga range whereas local Precambrian crystalline basement is <1.8 Ga. In addition to this age population, the southern British Columbia miogeocline contains a significant component of Grenville age zircon. We propose that the Taconic Orogeny initiated dispersal of eastern Laurentian (Grenville) age detritus into the Cordilleran miogeocline and that Gondwana (South America) sourced the Sonora and Nevada detrital zircon populations. Following Dalla Salda and others (1992) we suggest that Gondwana and Laurentia were close enough in the Ordovician for dispersal of Amazonian craton detrital zircon onto the Laurentian margin. Unusual Neoproterozoic detrital zircon age populations are present in the Roberts Mountain Allochthon. Elements of The Shoo Fly Complex of the northern Sierra terrane also have Neoproterozoic detrital zircon populations. There is no recognized source for significant amounts of Neoproterozoic zircon along the Cordilleran margin. The Antelope Mountain quartzite of the Klamath Mountains may also have a non Laurentian sediment source. We suggest that much of the detritus in these terranes was derived from either Gondwana or from peri-Gondwanan terranes such as Avalon and the Carolina terrane of the Appalachian system. Following and during accretionary events in the Silurian-Devonian, terranes now found in the Cordillera migrated along the eastern Laurentian margin until being carried up the western Laurentian margin from south to north in a Caribbean/Scotia type arc setting.