THE CHALLENGE: RECONSTRUCTING CORDILLERAN PALEOGEOGRAPHIES

Advent of plate tectonic theory in the 1960s had an immediate impact on our understanding of Cordilleran evolution. Deterministic geosynclinal settings were discarded for "actualistic" ones, with the corollary that the western Cordillera possibly recorded enormous, but cryptic, tectonic displacements across then newly-recognized former plate boundaries and within weak arc lithosphere sandwiched between strong oceanic and continental lithospheres moving on different trajectories. Paleobiogeographic and paleomagnetic data were needed to explore the amounts of such potentially large displacements; structural evidence alone was insufficient. Such data, together with provenance studies using dated detrital zircons, when "tagged" to disparate parts of the Cordillera called terranes, potentially can tell us where a terrane orginated, and how it migrated and accreted to the pre-Cordilleran continental margin to build the Cordilleran orogenic collage.

Where are we today? The abundant record of arc magmatism and its spatial relationship to coeval accretionary complexes indicate that for much of the past 390 million years vast amounts of oceanic lithosphere were subducted beneath what is now western North America. However, with exception of material in accretionary complexes, there is little evidence for latitudinal transport of Paleozoic arcs for great distances across the ancestral Pacific Ocean or Panthalassa. Instead, arc terranes in the interior of the northern Cordillera were founded in part on material detached from the pre-Cordilleran margin, and evolved in eastern Panthalassa but at latitudes south of where they are today along that margin. Modern analogues may be Japan and "Zealandia" in the western Pacific, separated from nearby continents by back-arc basins. Conversely, other northern Cordilleran arc terranes, today flanking the Pacific, evidently migrated southward from regions near the present Arctic Ocean and probably did not enter Panthalassa until Permo-Triassic time. By the Middle Jurassic, most terranes were accreted - somewhere - to the western North American Plate margin. Subsequently, they were shuffled, first southward and then, during and following latest Early Cretaceous time, northward within that margin.