Basin-and-Range Border Fault Geometry in NV-UT-AZ Region: Steep-Dip Normal Fault Model Versus Shallow-Dip Detachment Model

A long-standing debate in extensional tectonics centers on Basin-and-Range border faults: The normal faults dip 60° +/- 10° in the NV-UT-AZ region according to a Steep-Dip model proposed by Carpenter and Carpenter (1988, 1989; 1993, 1997, 2004); the faults dip 11°-20° according to a Detachment model proposed by Wernicke (1981, 1982, 2007; Wernicke et al., 1984, 1985; Wernicke and Axen, 1988; Axen, 2004).

The NV-UT-AZ region, encompassing the Mormon and Beaver Dam mountains, is important to the Detachment model: Basal faults of klippen in the mountains are projected down-dip into the subsurface. Wernicke's Detachment model received corroboration in 1987 from Anderson and Bohannon (U.S.G.S.). Supporting evidence for the Steep-Dip model includes observations of range-front faults (dipping 55°-68°), steep gravity gradients at range fronts, and constraints from reflection seismic and subsurface well data. The structural and geophysical research suggests Wernicke's three purported detachments are nonexistent in the subsurface. This finding in concert with discovering variously oriented down-slope verging toe structures in klippen and young basin-fill sediment beneath klippen led Carpenter and Carpenter to conclude the detachments are surfaces of rootless gravity sliding. Macroscopic retrodeformation of klippen proposed by Wernicke is inconsistent with highly variable subcrop relations; whereas such variability is expected for gravity-slide detachment systems.

Carpenter and Carpenter (1988, 1989) stood alone for five years on their side of the debate when in 1993 Bohannon and Anderson changed their views from 1987. They no longer agree with the Detachment model, and instead view border faults as dipping 55°-65°, with initiation dips up to 74°. Additional third-party support for the Steep-Dip model has been forthcoming from Anders et al. (1998, 2004, 2006), Hintze (2001), Langenheim et al. (2000, 2001), and Walker et al. (2004, 2007). Anders' and Walker's studies of klippen support the finding that they are products of rootless gravity sliding.