ABSTRACT Determining the origin and evolution of basin-and-range geomorphology and structure in the western United States is a fundamental problem with global implications for continental tectonics. Has the extensional tectonic development of the Great Basin been dominated by steeply dipping (horst and graben) faulting or detachment faulting? The purpose of this paper is to provide evidence that attenuation due to multiple coalescing detachment faults has been a significant or dominant upper-crustal process in at least some areas of the Great Basin. We present mapping at a scale of 1:3000 and seismic refraction profiling of an area at the discontinuity between the White Pine and Horse Ranges, east-central Nevada, USA, which indicate the existence of a detachment rooted in an argillaceous ductile unit. This fault, which we call the Currant Gap detachment, coalesces with the previously mapped regional White Pine detachment. Our data suggest that the Currant Summit strike-slip fault at the surface, previously proposed to explain a nearly 2500 m east-west surface offset between the two ranges, likely does not exist. If a discontinuity exists at depth, it could be manifested at the surface by the undulating topography of the two coalescing detachments. On the other hand, offset domal uplifts in the two ranges would obviate the need for any lateral discontinuity at depth to explain the observed surface features. Our previous mapping of the White Pine detachment showed that it extends over the White Pine, Horse, and Grant Ranges and into Railroad Valley (total of 3000 km 2). Accordingly, we propose a model of stacked, coalescing detachments above the metamorphic infrastructure; these detachments are regional and thus account for most of the basin-range relief and upper-crust extension in this area. An essential feature of our model is that these detachments are rooted in ductile units. Detachments that have been observed in brittle units could have initiated at a time when elevated temperatures or fluid flow enhanced the ductility of the rocks. The Currant Gap and White Pine detachments exhibit distinctive types of fluid-genetic silicified rocks. Study of such rocks in fault contacts could provide insights into the initiation and early history of detachment faulting as well as the migration of fluids, including petroleum.

Abstract Cleavage refraction angles are used to estimate effective viscosity contrasts between interlayered quartzites and phyllites within the Baraboo Syncline, Wisconsin, USA. Both types of layers contain two major phases, quartz and pyrophyllite, with minor amounts of hematite. Quartz (with minor hematite) behaves as the strong phase and pyrophyllite acts as the weak phase. Cleavage refraction directly relates to mineralogy with a linear relationship between bedding/cleavage angle and strong-phase concentration. Mineralogy exerts first-order control over effective viscosity contrasts, which are generally small, in most cases <10. Effective viscosity contrasts are consistent across the fold, so are likely not to be highly strain dependent and indicate approximate linear viscous rheology. Microstructures suggest deformation was dominated by dislocation creep in layers with high quartz concentrations and diffusive mass transfer in layers with lower quartz concentrations, and that the transition of the deformation mechanism is gradual. Thus, the rheological flow laws at the small scale may not reflect the bulk flow law at the large scale over the span of the deformation. Effective viscosity contrasts allow an evaluation of samples compared to theoretical two-phase mixtures. The analysed samples most closely resemble the Reuss bound of two-phase mixtures, regardless of the mineralogy.