Thrust faulting in the Laramie Mountains, Wyoming, from reanalysis of COCORP data

Detailed analysis of COCORP seismic reflection data from the Laramie Mountains, Wyoming, shows that processing artifacts produce confusing patterns in a profile that crosses the mountain front. Elimination of artifacts in reprocessing allows for interpretations of residual reflections that are consistent with mapped geology. Reprocessing and model analysis image a near-surface fault-zone reflection and constrain its apparent west dip to be 30°-35°W. Bands of east- and west-dipping seismic events from basement rocks do not appear to offset the basement surface and therefore probably are not from faults directly related to uplift of the range. Multiple Laramide structures east of the exposed fault contact between Precambrian and Paleozoic rocks contribute to uplift of the range, and an exposed basement arch, which extends in the subsurface below Line 3, provides evidence for macroscopic folding of crystalline basement rocks. Lack of continuous fault-zone reflections extendable to depth on the seismic data suggests the absence of major mylonite zones analogous to those associated with the Wind River thrust. Such mylonites are probably better reflectors than brittle deformation zones associated with faults beneath the Laramie Mountains. Although fault-zone reflectivity depends on the physical properties of individual fault zones, the Wind River thrust and Laramie Mountains frontal fault zone may approach end members of fault-zone reflectivity.