INTEGRATING DIVERSE CONSTRAINTS TO RESOLVE COMPLEX ROCKY MOUNTAIN TECTONIC PROBLEMS

Rick Groshong has shown the power of open-minded integration of 3D geometries and strains with the conditions and mechanisms of deformation. This can be a difficult approach to emulate - areas of current structural and tectonic debate often involve a bewildering array of multi-disciplinary observations. To help provide a timely resolution of scientific controversies, we have developed a computer spreadsheet that (1) organizes observations relating to a scientific problem, (2) helps evaluate their consistency with each hypothesis, and (3) tabulates any special conditions needed for consistency with each hypothesis so that both conflicting conditions and critical unknowns are identified.

This spreadsheet was used to organize and evaluate observations related to the age of dextral strike-slip faulting in the Rocky Mountains. In northern New Mexico, truncated Precambrian lithologic belts and their aeromagnetic signatures indicate 80 to 170 km of dextral strike slip along N-S fault zones. Recent publications have attributed these dextral displacements to Laramide, Ancestral Rocky Mountain and Precambrian orogenic events. Each observation of local and regional structural geometry, stratigraphy, kinematic indicator, temperature indicator, paleomagnetic pole, and isotopic equilibration can be discounted on some grounds. When viewed in aggregate, however, Precambrian dextral slip provides an explanation with far fewer special conditions than Laramide or Ancestral Rocky Mountain dextral slip. In addition, deep-seated Precambrian brittle faulting does not require contradictory special conditions of (1) higher temperatures indicated by the lack of clay alteration in the indurated microcline-qtz breccias and (2) lower Phanerozoic temperatures indicated by conodonts and Precambrian Ar ages from highly-strained microcline.

In contrast, application of the spreadsheet to the generation of the Pennsylvanian Ancestral Rocky Mountains (ARM) suggests that neither of the leading hypotheses is correct. Strike-slip deformation during continental collision to the SE does not explain the kinematics of ARM faults and basement-involved deformation during low-angle subduction to the SW does not explain the lack of ARM igneous rocks.