Carpenter, James A.1
(1) Crain Energy, Ltd. / R. Lacy, Inc, Longview, TX
ABSTRACT: The Cutting Edge of Debate in Extensional Tectonics
G.K. Gilbert's (1875) revolutionary Block-Fault model for the Basin-Range was
supplanted by Wernicke (1981-Present) with a Shallow-Dip Detachment model, based on mapped
klippen composed of Paleozoic carbonates in the Mormon and Beaver Dam mountains
(NV-UT-AZ). Wernicke views the klippen as upper plate remnants of rooted low-angle normal
faults ("detachments"), projecting basal fault surfaces to depth. In his model
the crust is comprised of thin slabs (~5 km thick; a.k.a. "extensional
allochthons") separated by shallow dipping (3-20 degrees west) detachments that
experienced tens of kilometers of slip; accommodating >115% regional extension. Is this
klippen-based metamorphic core complex-derived model valid?
The author (and D.G. Carpenter) has analyzed reflection seismic (>260 km), subsurface wells and Bouguer gravity data (in concert with new geologic mapping), and arrived at an alternative model: a Steep-Dip Block-Fault model; supporting Gilbert's 1875 model of horst-and-graben structure. In this alternative model Basin-Range faults dip 60 +/-10 degrees. The magnitude of regional extension is 15-25%. Wernicke's three purportedly rooted shallow-dipping detachments are not observed by the author (1988-Present) or the USGS (1993-Present) on reflection seismic data, nor are they encountered where there is important subsurface well control.
So, what's the skinny on those klippen? Nine klippen display folding indicative of downslope vergence at toe areas; four exhibit overturning; one-dozen Beaver Dam klippen lie upon Cenozoic basin-fill sediments over a N-S expanse >4km and E-W expanse >4km. Carpenter and Carpenter (1988), Anders et al. (1998) and Hintze (2001) interpret the klippen as remnants of rootless gravity sliding.
Wernicke's Shallow-Dip Detachment model is defeated. However, diverse and inclusive written and verbal public debates with gentle hearts and clear heads are encouraged.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.