COMPARING A PLATE-TECTONIC SCALE MODEL OF SW LAURENTIA DURING PENNSYLVANIAN TO PERMIAN TIME TO INDEPENDENTLY MADE PALEOGEOGRAPHIC MAPS OF LANDSCAPES, BASINS, AND UPLIFTS

Dr. Chuck Kluth was a pioneer in using plate tectonic scale models to understand the late Paleozoic tectonics of SW Laurentia, and the Ancestral Rocky Mountains (ARM) more specifically. The Pennsylvanian to early Permian is one of the least understood tectonic episodes of North American Phanerozoic geology, and yet it is one of the most important periods for energy resources. Decades of research have resulted in a reasonable understanding of the development of individual ARM uplifts and associated basins, and the Pacific and southern margins of Laurentia, but the plate tectonic setting of SW Laurentia and the ARM remain uncertain. We use two independent tools of regional tectonic analysis to explore the development of SW Laurentia during the Pennsylvanian to early Permian, and the ARM embedded within that region: a plate tectonic-scale model and regional paleogeographic maps of the landscapes, basins, and mountains, both covering the same time interval. None of the proposed models fully accounts for the geometry and kinematics of ARM uplifts and the lack of magmatic activity. A presentation at this meeting (Leary and others) proposes a new model for this unusual 3-sided orogeny: the western and SE margins of the Laurentian continent are both the lower plates within convergent boundaries, and the SW margin is proposed to have been dominantly a transpressional plate boundary. The model proposes that ARM deformation was driven by amagmatic sinistral transpression of the Caborca Block along the SW margin from southern California to northern Mexico, and the Ouachita-Marathon belt and western Nevada margins affected only the regions up to 400 – 500 km from the plate boundary and not the ARM. Further, a major plate tectonic change at ~280 Ma (early Permian) resulted in the end of transpression and the beginning of a subduction boundary along the SW and western margins of Laurentia that was then the upper plate. This time also marked the final suturing of the Ouachita/Marathon - South America convergent boundary. We show the compatibility (and lack) of this new tectonic model with an independent set of paleogeographic maps of SW Laurentia by Blakey. We highlight the timing of uplifts and basin formation, the fluctuating shorelines through space and time, and the differences between ARM basins and plate-margin basins through time.