Re-examination of Field
Evidence for Extreme Crustal Extension in the Mormon Mountains, Southeastern
Nevada.
Christopher D. Walker, Mark H. Anders and Nicholas
Christie-Blick, Lamont Doherty Earth Observatory, Columbia University,
Palisades, NY10964, chrisw@ldeo.columbia.edu
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 pseudo-horizontal to overturned, and downslope vergence is
also seen. Many areas are highly faulted in a chaotic fashion, with high-angle
normal and reverse faults.
Kinematic indicators on the MPD itself show that movement occurred in
the same plane as dip, in a range of directions from 000¡ to 300¡, at a high
angle 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 which 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.