EVALUATING THE RELATIVE IMPORTANCE OF BOUNDARY DISPLACEMENTS AND CONTINENT SHAPE IN GOVERNING INTERIOR STRESS AND STRAIN DURING THE ANCESTRAL ROCKY MOUNTAIN OROGENY WITH A 3-DIMENSIONAL FINITE ELEMENT METHOD

Hypotheses for the generation of the Ancestral Rocky Mountain Orogeny attribute its origin to the arrangement of tectonic boundary forces (styles of tectonism) upon Laurentia. Different sequences, orientations, and magnitudes of stress on the Laurentian perimeter during assembly of Pangaea have been assigned as the cause of the uplifts and sedimentary basins comprising evidence for this late Paleozoic orogeny. In addition to the different theories of tectonism, multiple workers give credence to preexisting weaknesses generated by late Proterozoic-early Cambrian rifting as controls upon the local positions of Ancestral Rocky Mountain uplifts.

The issue addressed in this study is how confidently the style of tectonism along the continental domain can be tied to a particular distribution of interior stress and strain given the uncertainty in initial shape of that domain. For example, there exist multiple geometric restorations for the shape of Laurentia prior to the Ancestral Rocky Mountains, but it is unclear how different possibilities for the pre-Ancestral Rocky Mountain shape of the continent could mitigate the abilities of distinct styles of tectonism to generate unique patterns of interior stress and strain.

Here, by utilizing a 3-dimentional geomechanical finite element method, the degree to which the shape of Laurentia could influence the distribution of interior stress and strain is determined relative to changes in the governing boundary tectonism. Preliminary results show that changes in the shape of the boundary can influence the velocity, stress, and strain in the domain over twice as much as changing the node resolution of the model. However, velocity, stress, and strain between distinct initial domain shapes for the same styles of tectonism are more like each other than those for varying styles of tectonism upon one constant boundary shape. So, in the pre-Ancestral Rocky Mountain case, the pattern of stress and strain depends most strongly upon the style of tectonism, but domain shape has a significant influence, particularly upon depth (“z”) directed strain. This work suggests determining the shape of the pre-Ancestral Rocky Mountain continent is important, but within the present geological uncertainty, it is a subordinate forcing compared to competing theories for boundary tectonism.