Print this pageBasin-Range Extensional Architecture: Mormon Mountains Region, Nevada
CARPENTER, JAMES A. and DANIEL CARPENTER
A Shallow Dip (5 to 25 degree) normal fault model was applied to the area based on a geologic map covering part of the Mormon Mountains. Proponents maintain that the range is an incipient metamorphic core complex. Klippen of Paleozoic carbonate rock are interpreted as hanging wall remnants of a rooted Mormon Peak low-angle normal fault ("detachment"). The Shallow Dip model was applied elsewhere in the region such that the continental crust is purported to be comprised of an imbricate stack of slab-like extensional allochthons separated by 5 to 25 degree west dipping faults having dip separations exceeding 20 kilometers, and with regional extension greater than 100%. Serious problems have been identified with this model.
New geophysically-based structural analyses suggest the crustal architecture is dominated by high-angle normal faults dipping 60 plus or minus 10 degrees in the upper 5-10 km. Regional extension is approx. 20%. The steep fault geometry was determined using reflection seismic (utilizing diffractions) and borehole data. Bouguer gravity gradients require modeling steep fault dip. Fault dips range from 55 to 68 degrees in piedmont exposures. Structural analyses initiating the use of seismic, gravity, and subsurface well data in the region led us to a High-angle (60 plus or minus 10 degrees) normal fault model and an enigma: Evidence was not found to support the existence of the purportedly rooted low-angle normal faults. They are not imaged on the seismic data (e.g., where they would be required to cut reflectors) and not penetrated, as the Shallow Dip model predicts, where deep subsurface well control exists. Subsequent research by the U.S.G.S. also integrated geophysical and field data. The High-angle Fault model was corroborated, and serious concerns were echoed about the Shallow Dip model.
The disparate High-angle and Shallow-dip normal fault models cannot coexist in 3-D space. The Shallow-dip model is based solely on interpreting klippen in the Mormon Mountains, Tule Springs Hills, and Beaver Dam Mountains as remnants of extensional allochthons. Down-slope vergence and subcrop field relations of the klippen exhibit evidence of rootless gravity sliding. Many klippe lie upon Quaternary clastics. Gravity slide detachment systems form as rootless surficial features. This explains why they do not continue in the subsurface - thus resolving the enigma of why they are not seismically imaged or drilled. Thus, the High-angle Fault model is integral to the first-order extensional architecture of the crust in the region, and the Shallow Dip model is obviated because it is based on the misinterpretation of rootless gravity slide systems.