Walker, Christopher D.¹, Mark H. Anders¹, Nicholas Christie-Blick¹ 
(1) Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY

ABSTRACT: Re-examination of Field Evidence for Extreme Crustal Extension in the Mormon Mountains, Southeastern Nevada

Extreme amounts of crustal extension have been attributed to the Nevada, Arizona, Utah tristate area, with 200-300% extension accommodated by low-angle normal faulting. We have re-examined the field evidence in the Mormon Mountains of southeastern Nevada on which this model was based, and have found that an alternative model better explains the features seen. 
The models hinge on differing interpretations of the most prominent feature in the mountain range - a 10° dipping surface that generally places younger rocks over older rocks. This surface is called the "Mormon Peak Detachment" (MPD) in the literature. Where we examined the MPD in the NW Mormons it dips to the NW. Beneath the MPD, the beds are relatively flat-lying, disturbed by Cretaceous thrusting and later pressure-solution. Above the MPD, the rocks vary from near-horizontal to overturned. In many areas, the rocks are highly faulted in a chaotic fashion, with both normal and reverse high-angle faults. Kinematic indicators on the MPD itself show that movement occurred in the local dip direction, between 300° and 000°, and transverse to the southwesterly (225°) direction predicted by the extreme extension model. 
These lines of evidence lead to the conclusion that the MPD is not a continuous, seismically cycled fault, but rather represents the basal surface to a series of coherent blocks that slid radially away from a central dome. The surface therefore accommodates no crustal extension, and estimates of crustal extension over the region must be re-evaluated.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.