Reinterpretation of Mormon Peak Detachment in Mormon Mountains, Southern Nevada

James A. Carpenter

Brian Wernicke proposed that the Meadow Valley Mountains are part of a west-directed Mormon Peak extensional allochthon and that low-angle normal faults control basin-and-range structure. He suggested the Meadow Valley Mountains detached off the Mormon Mountains and moved 20 km during the middle to late Miocene, along a low-angle normal fault--the Mormon Peak detachment--dipping 20°-25° west. In this interpretation, the updip source area for the Paleozoic klippen of the Mormon Peak extensional allochthon now exposed in the Mormon Mountains requires an origin from heights greater than 9 km (30,000 ft) above the present Virgin Valley basin.

K-Ar and seismic reflection data document syntectonic sedimentary in-filling of the Virgin Valley basin beginning during the early Oligocene in response to the high-angle (60°) Virgin-Beaver Dam Mountains normal fault. Therefore, a source area (9 km high) for klippen of the Mormon Peak extensional allochthon did not exist. The Virgin Valley basin had already accumulated Tertiary basin-fill sediments for about 20 m.y. prior to the proposed time of initiation of the Mormon Peak detachment.

If the Meadow Valley Mountains had detached off the Mormon Mountains, the stratigraphy of the ranges should be similar. The stratigraphic thickness from the Ordovician Pogonip Group to the Pennsylvanian-Permian Bird Spring Formation in the Mormon Mountains is only 1,409 m, whereas in the Meadow Valley Mountains the equivalent stratigraphic interval is 3,421 m, 243% thicker. Also, several key formations that exist in the Meadow Valley Mountains die out into unconformities to the east and do not exist in the Mormon Mountains. The striking contrast in stratigraphy demonstrates that the Mormon Mountains were not a source area for the Meadow Valley Mountains.

Klippen of the purported Mormon Peak extensional allochthon that veneer the Mormon Mountains represent gravity-slide blocks that have source areas within the Mormon Mountains and require only 0.5-4 km displacements. These gravity-slide blocks moved in various downslope directions on rootless denudational faults resulting from the loss of lateral support because of high-angle basin-range faulting and associated erosion. Detailed mapping of Wernicke's easternmost klippe of the Mormon Peak extensional allochthon reveals east-vergent structural features rather than west-vergent, as necessary in his uniform sense-core complex model.

Geologic and geophysical data show that high-angle normal faults exert primary control over basin-and-range structure, whereas gravitational sliding is a secondary surficial (rootless) feature of minor significance.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.