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.

Over recent decades, numerous studies have highlighted the importance of opal, chalcedony and quartz varieties, chiefly in volcanic, but also in metamorphic and sedimentary environments. The focus is to define accurately their structures, composition and properties, as well as to identify the factors controlling the formation and the ageing of different forms of silica. In the field of archaeological sciences efficient discriminants are the bases from which the origin and provenance of materials may be traced. Substantial efforts were made in the attempt to combine geochemical, mineralogical, petrographic and geological features with archaeological and archaeometric information. However the results show that data integration is complicated, and several unanswered questions remain. On the one hand, archaeological research has focused on technological and ethnographic aspects, mainly concerning use-wear and heat-treatment studies. Mineralogical characterization has often been limited to the identification of the material, frequently by Raman microspectroscopy alone. On the other hand, the Earth sciences have provided basic mineralogical, crystal-chemical and geological knowledge, but failed to provide a systematic data collection of sources and their geochemistry. As a consequence, large gaps persist in the identification of archaeological opals, chalcedonies and quartz varieties, and in the geographic mapping of possible sources. In this context, the present review aims to summarize the current academic debate on such issues, possibly to encourage further work in the field. After a brief introduction to terminology, the structure of opals, their colours and properties are discussed, followed by an introduction to silica dissolution/precipitation and opal-formation processes. The next section reviews the information available on use of opals and provenance from historical sources, mainly Pliny the Elder, followed by a short list of ancient and modern opal supply areas, together with a (necessarily incomplete) summary of the geological and geochemical information. The discussion then encompasses chalcedony, agate and chalcedony varieties (carnelian, sard, onyx, sardonyx, chrysoprase, Cr-chalcedony, ‘gem silica’ or ‘chrysocolla chalcedony’ and heliotrope), following the same scheme as was adopted for opals. Terminology, distinguishing features, formation conditions, information derived from Pliny’s books, past and current supply areas and, finally, archaeometric provenance issues are addressed for each type of material. As for chalcedony, a comprehensive note on moganite has been included. The next section focuses on chert, flint and jasper. Given the large amount of materials available on this topic, the present review must necessarily be considered introductory and partial. The discussion aims to provide useful indications on how to distinguish chert from flint and chert from jasper; secondly, the information provided by Pliny and the archaeometric state of the art on these materials is reviewed. The last section examines quartz varieties: hyaline quartz (rock crystal), milky quartz, smoky quartz, rose and pink quartz, amethyst, citrine, prasiolite and blue quartz. An exhaustive mineralogical discussion on quartz is beyond the scope of this review; conversely a review of the historical information is provided, together with a brief list of major supply areas, a summary of the archaeometric studies performed on these materials, as well as an indication of the geological literature which can be used proficiently for provenance studies.

The 45 km 2 map area is situated at the south end of the Hot Creek Range in central Nevada, ~16 km east of the buried leading edge of the Mississippian Roberts Mountains thrust. Three eastward-trending left-slip faults divide the area into four structural blocks. The southernmost block is occupied solely by upper Oligocene volcanic rocks. The narrow northernmost block, now occupied surficially by valley fill and volcanic rocks, represents the south end of the main part of the Hot Creek Range, from which the study area is offset. The middle two blocks display different aspects of the eastward-traveled outer crater rim created by the ca. 382 Ma (early Late Devonian, middle Frasnian) Alamo impact. The Alamo impact was produced by a 5-km-diameter bolide, most likely a comet, which excavated a transient submarine crater 44–65 km in diameter. Comparison of thin (8–12 m) Alamo Breccia deposits in the northern of the middle two blocks with a more easterly, thick (35–42 m) Alamo deposit in the main Hot Creek Range, 4 km north of the map area, suggests that these blocks traveled many kilometers eastward. The northern of the middle two blocks contains a large olistolith capped by the thin breccia, whereas the southern block contains a larger olistolith lacking an Alamo Breccia cap. Three Devonian pulses of the Antler orogeny are better documented in the chapter on the Bisoni-McKay area. Here, the first Antler pulse in latest Middle Devonian time is obscured within an ~9 m.y. hiatus enlarged by excavation of the Alamo impact crater. The second Antler pulse is recorded by the ~4 m.y. hiatus produced by the regional unconformity between the lower and upper members of the Woodruff Formation. The third Antler pulse is documented by an ~8 m.y. regional hiatus between the Mississippian Webb Formation and Upper Devonian Woodruff Formation. In previous papers, we had interpreted this pulse to initiate the Antler orogeny.