REVISION OF THE PALEOGEOGRAPHY OF THE ANCESTRAL ROCKY MOUNTAINS

The Ancestral Rocky Mountains (ARM) were enigmatic uplifts and basins in western Pangaea, centered in Colorado. Review of new and existing data indicate that the models of the paleogeography of the ARM is in need of revision. Most of the structures had a NW-SE orientation, and the uplifts were tilted fault blocks. The largest structures are concentrated on a zone that trends from the Wichita Mountains in Oklahoma through the ARM and the Oquirrh Basin to the Cordilleran margin. This zone may have been partly inherited. N-S depictions on earlier maps is probably a product of bias imparted by later structures. The mountain fronts were shortened but the region also has evidence of extension. Left slip has been suggested along NW and E-W zones. The development of ARM is interpreted to have been caused by the collision of a irregular southern margin of Laurentia with Gondwana to form Pangaea. A southwestward peninsula of Laurentia was pushed northward and rotated slightly clockwise in two phases by the collision resulting in shortening but also strike-slip and extension. The clockwise rotation resulted in the left slip motion on several faults. The earliest phase of large scale deformation was in mid-Pennsylvanian ( Atokan-Desmoinesian) and caused the Ancestral Front Range and San Luis Uplift and associated basins. The second phase was early Permian (Wolfcampian) around a different pole of rotation and resulted in the Uncompaghre and Central Basin Platform and smaller structures.