Abstract

The South Virgin Mountains and Grand Wash trough comprise a mid-Miocene normal fault system that defines the boundary between the unextended Colorado Plateau to the east and highly extended crust of the central Basin and Range province to the west. In the upper 3 km of the crust, the system developed in subhorizontal cratonic strata in the foreland of the Cordilleran fold and thrust system. The rugged topography and lack of vegetation of the area afford exceptional three-dimensional exposures. Compact stratigraphy and well-defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system. Reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1:12 000 shows that the fault system accommodated more than 15 km of roughly east-west–directed Miocene extension. Extension was initially accommodated on moderately to steeply dipping listric normal faults. As the early faults and fault blocks tilted, steeply to moderately dipping faults initiated within the fault blocks, soling into the early faults. Some of the early faults were active at dips of <20°. Isostatically driven tilting is superimposed on tilting due to active slip and domino-style rotation of the fault blocks. Collectively these processes rotated originally steeply dipping faults to horizontal orientations. The kinematics are inconsistent with the widely accepted view that many near-horizontal normal faults were rotated to their present orientations by later, crosscutting normal faults. However, reexamination of other areas suggests that the evolutionary sequence seen in the South Virgin Mountains may, in fact, be widely applicable.

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