Cretaceous to early Tertiary shortening of the foreland region occurred along listric thrust faults bounded by conjugate wrench faults that cut through and ductilely deformed the underlying Precambrian basement. Oblique convergence on east-dipping B-type subduction along the western edge of the North American plate coupled with west-dipping A-type subduction along a zone east of the plate margin and marked by muscovite-bearing two-mica granites produced conjugate WNW-trending left-slip and NE-trending right-slip faults and the foreshortening evinced by the decollement faults of the Western Overthrust. Many left-slip faults split into right-turning splays that join other left-lateral faults, creating thrust systems with a regional Z-shaped bend (e.g., thrusts of the Sawtoo h Range, Montana). Conversely, right-slip faults split into left-turning splays that join other right-slip faults creating S-shaped thrust systems. Conjugate pairs of strike-slip faults bound concave-westward zones of decollement thrust faults (e.g., thrusts of the Belt graben, Montana). Complex flower structures mark the loci of many wrench faults (e.g., Arizona and southwestern New Mexico). Late Tertiary to Holocene extensional deformation of western North America produced the north-trending listric normal faults of the Great Basin and reversed the sense of movement on many preexisting strike-slip and thrust faults. These regional relationships are clearly visible on digitally processed Landsat imagery.

This model also accounts for the synchronous intrusion of both two-mica (muscovite) and one-mica (copper porphyry type) granitic rocks in the region. This geologic paradigm will assist the exploration for hydrocarbons and minerals.

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