FINDING A BOUNDARY TECTONISM MODEL GENERATING A CONTINENTAL STRESS FIELD COMPATIBLE WITH ANCESTRAL ROCKY MOUNTAIN UPLIFTS

Tectonic models for explaining the locations and orientations of intraplate Ancestral Rocky Mountain (ARM) uplifts have focused upon the southwestern United States. However, far-field forces may also have been important for driving the orientations of the ARM structures. We used 3D finite element geomechanical modeling to evaluate stress orientation implications from prior and current tectonic hypotheses for the ARM intracontinental orogeny. In general, thrust faults parallel to ARM uplifts (striking NW-SE) require stress to be relieved across the continent in a roughly east-west direction. Otherwise, in prior and current models of tectonism thrust faults should preferentially strike NE-SW. What we have found in the geomechanical models is that it is possible to relieve E-W stress with an extensional Laurentia-Panthalassa (western) boundary, but because of its spatial proximity to the ARM area, the risk is that the region containing ARM uplifts drops out of the thrust faulting regime and into a strike-slip dominated regime. However, we note that rifting is well documented to have been occurring on the east Greenland coast during the Middle Pennsylvanian and this provides another means of relieving east-west pressure in the continent in a position along its perimeter distant from the ARM region. When extension along the east Greenland coast is included in the modeling it permits a NW-SE striking thrust fault-prone area spatially overlapping ARM uplifts. These results suggest that the tectonic explanation for the ARM structures should consider not just the proximal compressional forces generated by tectonism in southern, southwestern, and western North America, but also the coeval strike-slip and extensional forces along the eastern and northeastern continental boundaries.