Interpretations of over 1,500 km (900 mi) of industry-related reflection data in the Cordillera have revealed the following styles of late Cenozoic deformation: (1) the widespread development of asymmetric eastward-tilted basins that are bounded by low to moderate-angle planar and listric faults, and (2) five en echelon, low-angle reflections interpreted as regional detachments. Some steeply dipping planar and listric normal faults may be partly controlled by the presence of Mesozoic thrust structures, but this hypothesis is not applicable universally. In some cases, ends of normal fault segments are apparently determined by the positions of sidewall ramps and other cross-strike displacement transfer zones of Mesozoic age. Alternatively, several major normal faults, parti ularly those in Tertiary volcano-tectonic complexes, have no obvious relationship to Mesozoic structures. The low-angle reflections interpreted as a set of detachments extend east-west at least 200 km (125 mi) and dip gently westward from 3 km (2 mi) beneath the western Colorado Plateau to over 10 km (6 mi) at the Utah-Nevada border. The structural style of low-angle and listric faults cannot be reconciled easily with classic brittle failure theory, but the interpreted termination of normal faults at or above the frictional/quasiplastic transition may occur as shallow as ^approx 7 km (4 mi). Rheologic models of an extending upper crust suggest a vertically stratified model: brittle from the surface to as shallow as 7 km (4 mi), then variably ductile. The shallow depth of the upper ductil layer has important implication for controlling fault geometry and therefore the locations of fault-related basins.

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