NORTH-SOUTH DIFFERENCES IN TIMING AND STYLE OF ANCESTRAL ROCKY MOUNTAIN DEFORMATION, SOUTHWESTERN NORTH AMERICA

The Ancestral Rocky Mountains (ARM) can be broadly divided into two geographic domains that differ in structural style and timing of peak deformation. The northern domain comprises basins and uplifts of the Wichita-Uncompahgre trend. Sedimentation within basins of this domain records a prominent pulse of subsidence during early to middle Pennsylvanian, particularly in Desmoinesian time. In the Paradox basin and central Colorado trough, Desmoinesian strata are volumetrically dominant (Mallory, 1972). Structural style along the Wichita-Uncompahgre trend is characterized by NW- or WNW-striking, moderate to steeply dipping faults with mostly reverse separations. Previous workers have suggested that left-lateral components of slip on these faults may have been important, especially in the Wichita uplift area.

In contrast, basins in the southern ARM domain of central and southern New Mexico and adjacent areas of Texas and Chihuahua experienced peak subsidence rates in Virgilian-early Wolfcampian time. Deformation was mostly accommodated by slip along high-angle, N-striking faults. Most faults display normal separation; examples of both left and right lateral faults are present. Because major deformation in the Wichita-Uncompahgre trend predated peak deformation in central and southern New Mexico, regional balancing considerations suggest that lateral components of slip on faults in the southern domain were not large.