For more than two decades, the paradigm of large-magnitude (~250 km), northwest-directed (~N70°W) Neogene extensional lengthening between the Colorado Plateau and Sierra Nevada at the approximate latitude of Las Vegas has remained largely unchallenged, as has the notion that the strain integrates with coeval strains in adjacent regions and with plate-boundary strain. The paradigm depends on poorly constrained interconnectedness of extreme-case lengthening estimated at scattered localities within the region. Here we evaluate the soundness of the inferred strain interconnectedness over an area reaching 600 km southwest from Beaver, Utah, to Barstow, California, and conclude that lengthening is overestimated in most areas and, even if the estimates are valid, lengthening is not interconnected in a way that allows for published versions of province-wide summations. We summarize Neogene strike slip in 13 areas distributed from central Utah to Lake Mead. In general, left-sense shear and associated structures define a broad zone of translation approximately parallel to the eastern boundary of the Basin and Range against the Colorado Plateau, a zone we refer to as the Hingeline shear zone. Areas of steep-axis rotation (ranging to 2500 km 2 ) record N-S shortening rather than unevenly distributed lengthening. In most cases, the rotational shortening and extension-parallel folds and thrusts are coupled to, or absorb, strike slip, thus providing valuable insight into how the discontinuous strike-slip faults are simply parts of a broad zone of continuous strain. The discontinuous nature of strike slip and the complex mixture of extensional, contractional, and steep-axis rotational structures in the Hingeline shear zone are similar to those in the Walker Lane belt in the west part of the Basin and Range, and, together, the two record southward displacement of the central and northern Basin and Range relative to the adjacent Colorado Plateau. Understanding this province-scale coupling is critical to understanding major NS shortening and westerly tectonic escape in the Lake Mead area. One north-elongate uplift in the Hingeline shear zone is a positive flower structure along a strike-slip fault, and we postulate that most other large uplifts are diapiric, resulting from extension-normal inflow of ductile substrate, rather than second-order isostatic responses to tectonic unloading. We also postulate that large steep-axis rotations, and some small ones as well, result from basal tractions imparted by gradients in southerly directed subjacent ductile flow rather than by shear coupling imparted by laterally variable elongation strains. The shortening strain recorded in the rotations and related structures probably matches or exceeds the magnitude of lengthening, even for the Lake Mead area where we do not question local large (~65 km) west-directed lengthening. We assess the results of extensive recent earth-science research in the Lake Mead area and conclude that previously published models of N-S convergence, westerly tectonic rafting, and N-S occlusion are valid and record unique tectonic escape accommodation for south-directed displacement of the Great Basin sector of the Basin and Range. Genetic ties between the south-directed displacement and plate-interaction forces are elusive, and we suggest the displacement results from body forces inherent in the Basin and Range.

The eastern Lake Mead region, central Basin and Range Province, contains an abrupt boundary between the Colorado Plateau and Basin and Range, west of which decades of tectonic studies have documented extreme (at least 60 km) westerly translation of the Frenchman Mountain structural block away from the boundary, currently at a distance of ~95 km. Detachment-style faulting and large lengthening in the eastern Lake Mead region are generally accepted (eight of ten papers in a 2010 compilation of recent research) as integral to the large province-wide lengthening at this latitude. Presented here is field geologic evidence of the contribution of karsting and tilting on multiple fixed axes to the Miocene strain history. Together with newly recognized strike slip on northerly striking faults, associated steep-axis bending, and evidence against fault listricity, the region probably contributes little to Province-wide lengthening, but estimates of westerly translation of the Frenchman Mountain block remain unchanged.

The Lake Mead region contains major Miocene disruptions of structures formed during Mesozoic tectonic shortening. Erosion by the Colorado River and its tributaries has produced exceptional exposures of diverse structures and basin deposits recording the disruptions. Here we provide an overview of the results of studies of these features that started in earnest in 1934 when Chester Longwell began assessing the geology of the reservoir floor prior to impoundment of Lake Mead. The analysis was reinvigorated in the 1970s and early 1980s with geological mapping and structural and stratigraphic studies by Ernie Anderson and Bob Bohannon, as well as geochemical and volcanological studies by Gene Smith and his students, and has culminated in numerous subsequent studies.

The Lake Mead fault system is a northeast-striking, 130-km-long zone of left-slip in the southeast Great Basin, active from before 16 Ma to Quaternary time. The northeast end of the Lake Mead fault system in the Virgin Mountains of southeast Nevada and northwest Arizona forms a partitioned strain field comprising kinematically linked northeast-striking left-lateral faults, north-striking normal faults, and northwest-striking right-lateral faults. Major faults bound large structural blocks whose internal strain reflects their position within a left step-over of the left-lateral faults. Two north-striking large-displacement normal faults, the Lakeside Mine segment of the South Virgin–White Hills detachment fault and the Piedmont fault, intersect the left step-over from the southwest and northeast, respectively. The left step-over in the Lake Mead fault system therefore corresponds to a right-step in the regional normal fault system. Within the left step-over, displacement transfer between the left-lateral faults and linked normal faults occurs near their junctions, where the left-lateral faults become oblique and normal fault displacement decreases away from the junction. Southward from the center of the step-over in the Virgin Mountains, down-to-the-west normal faults splay northward from left-lateral faults, whereas north and east of the center, down-to-the-east normal faults splay southward from left-lateral faults. Minimum slip is thus in the central part of the left step-over, between east-directed slip to the north and west-directed slip to the south. Attenuation faults parallel or subparallel to bedding cut Lower Paleozoic rocks and are inferred to be early structures that accommodated footwall uplift during the initial stages of extension. Fault-slip data indicate oblique extensional strain within the left step-over in the South Virgin Mountains, manifested as east-west extension; shortening is partitioned between vertical for extension-dominated structural blocks and south-directed for strike-slip faults. Strike-slip faults are oblique to the extension direction due to structural inheritance from NE-striking fabrics in Proterozoic crystalline basement rocks. We hypothesize that (1) during early phases of deformation oblique extension was partitioned to form east-west–extended domains bounded by left-lateral faults of the Lake Mead fault system, from ca. 16 to 14 Ma. (2) Beginning ca. 13 Ma, increased south-directed shortening impinged on the Virgin Mountains and forced uplift, faulting, and overturning along the north and west side of the Virgin Mountains. (3) By ca. 10 Ma, initiation of the younger Hen Spring to Hamblin Bay fault segment of the Lake Mead fault system accommodated westward tectonic escape, and the focus of south-directed shortening transferred to the western Lake Mead region. The shift from early partitioned oblique extension to south-directed shortening may have resulted from initiation of right-lateral shear of the eastern Walker Lane to the west coupled with left-lateral shear along the eastern margin of the Great Basin.

Scattered remnants of highly diverse stratigraphic sections of Tertiary lacustrine limestone, andesite flows, and 23.8–18.2 Ma regional ash-flow tuffs on the north flank of the Mormon Mountains record previously unrecognized deformation, which we interpret as pre–17 Ma uplift and possibly weak extension on the north flank of a growing dome. Directly to the north of the Mormon dome, 17–14 Ma ash-flow tuffs and rhyolite are interstratified with landslides, debris avalanches, debris flows, and alluvial-fan deposits that accumulated to a thickness of more than 2 km in an extension-parallel basin. The source for the landslides and debris avalanche deposits is unknown, but it was probably an adjacent scarp along a transverse fault bounding an early part of the Mormon dome. An average 45° of easterly tilt of the entire Tertiary basin-fill succession represents the major post–14 Ma deformation event in the region. We question the basis for the published estimate of 22 km of westerly displacement on the Mormon Peak detachment fault and, on the basis of landslides in the upper plate having a probable source in the adjacent Mormon dome, constrain the heave to ~4 km. We interpret the dome and basin as coupled strains similar to others in the region and suggest that these strains reflect a waveform pattern of extension-normal lateral midcrustal ductile flow. Previously, doming was interpreted as an isostatic response to tectonic unloading by large-displacement detachment faults or as pseudo-structural highs stranded by removal of middle crust from adjacent areas. Moreover, we argue that the strong thinning of upper-plate rock successions throughout the Mormon Mountains and Tule Springs Hills resulted from a loss of rock volume by protracted fluid flow, dissolution, and collapse, seriously limiting the usefulness of upper-plate strain in evaluating extension magnitude. We present a geohydrologic model that couples uplift driven by ductile inflow with dissolution driven by fluid infiltration, possibly augmented by mantle-derived CO 2 -rich fluids. Karsting in the uplands led to carbonate sedimentation in adjacent lowlands. Whether or not our downward revision of extension in the Mormon Mountains is valid, extension at that latitude is isolated from extension in the Lake Mead area by a low-strain corridor between the two areas. Recognition of the isolated and potentially diminished strain impacts estimates of maximum finite elongation of the Basin and Range Province because one of three vector paths used in those estimates passes through the Mormon Mountains.

Dissolution has removed large volumes of rock at low-angle normal faults, i.e., detachment faults, in the Mormon Mountains and the Tule Springs Hills in the eastern Basin and Range Province, southeastern Nevada. Evidence for major dissolution includes widespread solution-collapse breccias, meter-scale stylolite structures, and high-angle accommodation faults that terminate at or merge with dissolution seams. Chemically reactive fluids moving along the fault zones led to a strong depletion of 18 O in the detachment fault breccias (e.g., a δ 18 O decrease of 8‰ relative to the unaltered rocks). These strong chemical shifts, demonstrated by (1) negative oxygen isotope values and (2) steep compositional gradients marked by metal enrichment in elements such as Au, Ag, Ti, Pb, Zn, and Cu, are generally restricted to the narrow (<1 m to 8 m) microbreccia zones. Extensional faulting and fracturing, accompanying regional uplift, opened conduits for the influx of meteoric waters from above and hydrothermal fluids from below. As the largest, most permeable structures that formed during uplift, detachment faults focused the fluid flow. In this deformation and hydrogeologic model, dissolution-caused stratal thinning is a major complement to detachment faulting and is an important process that resolves void space issues in the reconstruction of cross sections.

Study of more than 1,000 strike-slip and dip-slip faults in Miocene rocks along a 10-km segment of Rainbow Canyon reveals a well-constrained paleoextension direction of 235°, representing a main stage of syndepositional extensional deformation. This direction is consistent with northeast-southwest extension computed or estimated over a large part of the Basin and Range for main-phase extension. It can be computed from separated or combined subsets of strike-slip and dip-slip faults and is independent of fault size or geographic subarea. The deformation is characterized by synfaulting deposition of volcanic and sedimentary sequences that thicken toward predominantly northwest-striking block-boundary growth faults resulting in fan-shaped patterns in northeast-southwest cross section. Concave-upward faults are common, and fault-to-bedding angles in vertical sections containing the extension direction average about 90°. The coefficient of extension at stratigraphically median levels is 1.9 (90 percent), 10 to 20 percent of which is associated with block-interior displacements on sub-map-scale faults. Though the deformation is primarily synvolcanic, it is interpreted to be more closely associated with regional extension and low-angle normal faulting than with volcano-tectonic processes. The study provides evidence for a young stage of west-northwest extensional deformation that primarily utilized existing faults in combined strike-slip, oblique-slip, and dip-slip modes and accounted for less than 5 percent of the total observed deformation. Extension directions are less well constrained than for the early deformation owing to smaller sample sizes. As with the early deformation, those computed from subsets of strike-slip and dip-slip faults are similar to one another (average 290°). The young deformation occurred following a 55° clockwise rotation of σ 3 sometime in the last 10 m.y. The mixture of dip-slip and strike-slip faulting during each of the deformations is interpreted as resulting from vertical and horizontal constriction normal to the extension direction, rather than from alternations of paleostress conditions. An estimated 10 to 30 percent of the total brittle strain is associated with the strike-slip faulting. This study provides strong verification that our understanding of deformation intensity is highly dependent on the scale of investigation, depth of exposure, and knowledge of slip-sense characteristics.

Strong contrasts in the tectonic evolution of the three principal parts of the Intermontane System (Basin and Range, Colorado Plateaus, and High Lava Plains) are responsible for strong contrasts in crustal structure outlined elsewhere in this volume. Extremes in Cordilleran history are represented by the interior of the Colorado Plateaus and the Basin and Range province. The plateaus have enigmatically escaped strong deformation and magmatism since Precambrian time, whereas most parts of the Basin and Range have experienced repeated orogenesis, the youngest of which is continental magmatism and extensional deformation with dimensions and magnitudes that may not be exceeded anywhere in the world. Between these extremes lie the High Lava Plains, parts of which appear to be dominated by the passage of a single major pulse of magmatism and rifting that produced crust no older than late Cenozoic age. Numerous crustal-scale tectonic and magmatic events beginning in Archean time and extending into Cenozoic time are chronicled as events having a potential for molding crustal structure. They include protracted Archean and early Proterozoic south- or southwest-directed continental growth or cratonization by sparsely recorded processes of deformation, sedimentation, magmatism, and metamorphism, followed in middle Proterozoic time by epicratonal basin sedimentation and in the late Proterozoic by reshaping of the western continental margin by passive-margin rifting and foundering. Late Paleozoic and early Mesozoic continent-margin collisional events led to the obduction of the Roberts Mountains and Golconda allochthons and accretion of related terranes. A second event of continent-margin truncation and reshaping occurred in early Mesozoic time. It was followed during Mesozoic time by major east-directed subduction of oceanic lithosphere and associated accretions of volcanotectonic terranes to the west margin of the continent. Genetically related magmatism, metamorphism, and compressional tectonism reached far inboard of the continental margin; these were guided to some extent by preexisting tectonic features. During Cenozoic time, igneous activity of colossal magnitude swept through the Cordillera along regular paths without apparent regard for preexisting tectonic features. Coeval and subsequent extensional tectonism teamed with the magmatism to reshape the geophysical framework of much of the Basin and Range and High Lava Plains. Accordingly, it is a major challenge to recognize the fingerprint of ancient major crustal-scale events within the current geophysical framework.