COMPARING MODELS OF THE ACCRETIONARY HISTORY OF WESTERN NORTH AMERICAN TERRANES -- IDEAS FOR ADVANCING UNDERSTANDING WITH PALEOGEOGRAPHIC MAPS

No aspect of paleogeographic reconstruction is more critical than the assessment of the origin and past configuration of terranes and geologic elements that comprise a given region. The Paleozoic and Mesozoic geologic history of the Cordilleran region of western North America has been punctuated by a series of orogenic events, each of which resulted in regional deformation and reconfiguration of the western margin of the continent. For each of these events, geologic, geophysical, and paleontologic data have resulted in equivocal and contradictory conclusions regarding origin of terranes, which in turn has generated contrasting models and differing paleogeography. Because maps force a holistic approach to showing various data sets that are linked but geographically separate, individual aspects of competing scenarios can be readily compared and contrasted. For example, thermochronology data from uplifted areas, paleocanyon locations, basin dispersal patterns, and provenance data for the same time slices could be superimposed on contrasting maps; the combination of these datasets on a potential paleogeographic map would force future mapmakers to choose between or modify competing models regarding the location of key geologic/geomorphic elements such as: major rivers that access interior parts of the Cordillera, major deltas, major drainage divides, uplifted mountain ranges versus low ranges, and size and shape of large marine embayments.

We use several hypothetical examples of maps for the same time interval (160-145 Ma Nevadan orogeny and 90-85 Ma late Sevier orogeny) that can form discrete tests of competing tectonic scenarios. For example, the accretion of Stikinia has been modeled as both an exotic, far-traveled terrane and as a fragment of North America rifted from the continent and reattached during the Nevadan orogeny. Wrangellia, a consensus exotic terrane, has been modeled as an indenter during both orogenies and as not involved with either. Baja BC has recently been shown west of southern California at 90 Ma or adjacent to the Klamaths. Current paleogeographic reconstructions of the contrasting hypotheses dramatically illustrate these competing models; however, not until more data are gatherered and analyzed, can additional constraints be placed on regional paleogeography.