We report structural, paleomagnetic, and magnetic fabric data for mid-Cretaceous plutons of the Peninsular Ranges batholith along a transect at ~30°N latitude. Four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite. In this sector, El Milagro, Aguaje del Burro, La Zarza, and San Telmo plutons yield a combined paleopole at 82.1°N, 169.7°E (K = 137.6, A 95 = 7.9°; n = 4–38 sites), which, rotated for closure of the Gulf of California, falls at 79.3°N, 179.5°E, and it is concordant with the North America reference pole. Plutons in the transition zone, between the eastern and western sectors of the Peninsular Ranges, have magnetizations residing in hematite. El Potrero and San José plutons yield highly discordant paleopoles, indicating apparent clockwise rotation (R) and flattening (F) of 33.0° ± 5.1° and −27.6° ± 6.1°, respectively (San José), and 46.1° ± 5.9° and −31.0° ± 7.0° (El Potrero). The discordance is best explained by west-down tilt of the crustal block between the Main Mártir thrust and the Rosarito fault, which are major compressional structures parallel to the trend of the Peninsular Ranges. The San Pedro Mártir pluton, a large La Posta–type pluton on the eastern sector of the transect, has magnetizations that reside primarily in hematite. The mean paleomagnetic pole (71.3°N, 335.5°E; K = 40.7 and A 95 = 7.2°) is slightly discordant, indicating westward tilt of ~15°. The different paleopoles obtained for individual plutons convincingly show that the Peninsular Ranges batholith has suffered internal deformation, which is more intense along the transition zone. The magnetic fabric for plutons representative of the western, eastern, and transitional sectors of the range show marked contrasts in the deformation recorded by anisotropy of magnetic susceptibility (AMS). Anisotropy is weakly developed in the western sector (El Milagro), very strongly developed in the transition zone (San José), and moderately developed in the eastern sector (Sierra San Pedro Mártir). Within the plutons, El Milagro fabrics record emplacement-related stress. In contrast, San José and San Pedro Mártir appear to record regional stress linked to evolution of the Main Mártir thrust. Overall, our data are consistent with rotation of the crustal block where Potrero and San José plutons are located; rotation was accommodated by major crustal faults in a compressional stress field, as the crustal block moved to occupy the space abandoned by the ascending (and westward expanding) San Pedro Mártir diapir batholith. The rotation could be related to interaction between the large Sierra San Pedro Mártir pluton and the Main Mártir thrust, or to mechanical controls such as wedging against a rigid salient.

Abstract The Cañones fault zone in north-central New Mexico is a boundary between the Colorado Plateau to the west and the Rio Grande rift to the east. It consists of a major fault, the Cañones fault, and a series of synthetic and antithetic normal faults within the Abiquiu embayment in the northwestern Española basin. The Cañones fault is a southeast-dipping high-angle normal fault, striking ~N20°E in the south, N40°E in the middle, and east-west at its northern end. The synthetic and antithetic faults are sub-parallel to the major fault. Detailed fault kinematic studies from the master fault reveal that the trends of slickenlines range S85°E - S70°E, and average approximately S76°E. Slickenlines on antithetic faults trend S20°W – N30°W, clustering at ~ N70°W. The attitude of fault surfaces and slickenlines indicate east-southeast/west-northwest extension within the Cañones Fault Zone. The sense of motion on the major fault is normal dominantly with left-slip. Fault throw is at least 225 m, based on Mesozoic units as hanging wall and footwall cutoffs. Thus, the heave is as ~143 m and the left-lateral displacement is ~60 m, given the averaged fault attitudes. In contrast to sub-horizontal Permian-Triassic units in its footwall, hanging wall strata of the Cañones fault zone dips in two directions: west-dipping Jurassic Entrada, Todilto, and Morrison formations; and south-east-dipping Eocene El Rito, Oligocene Ritito, and Oligocene-Miocene Abiquiu formations. Tilted Jurassic strata suggest that the overall structure is monoclinal, probably resulting from Laramide orogeny shortening. The Eocene-Miocene basin fill sediments, surprisingly, dip 10° – 30° away from the Cañones Fault, instead of dipping northwest towards the fault. This phenomenon, in contrast to the prediction of the rollover structure, suggests a different mechanism on this fault zone. Field observation provides direct evidence that basinward tilting is accommodated by multiple antithetic normal faults that cut through Permian to Miocene units. We propose that extensional fault-propagation folding model is a possible mechanism to result in the regional tilting of the basin fill. During upward propagation of the fault tip, horizontal-axis rotation and antithetic and synthetic faulting occur within a triangle zone above the fault tip. Alternatively, a buried large-scale low-angle normal fault can also generate such basinward tilting. In this scenario, the Cañones fault and other southeast-dipping normal faults are antithetic faults that grow on the detachment. These hypothetical mechanisms take into account the antithetic faulting within a rift-bounding fault zone and can be indicative of the evolution of other rift basins in which basin fills dip to the axis, such as the eastern Española basin and San Luis basin in northern New Mexico and southern Colorado.

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.

This paper reevaluates the geometry and processes of extension in the boundary zone between the western Colorado Plateau and the Basin and Range Province. Based on new mapping of extensional detachment faults, restored cross sections, and 40 Ar/ 39 Ar K-feldspar thermochronology, we present an alternative to the previously published model that the Gold Butte block is a tilted 15–18-km-thick intact basement crustal section. Mapping of windows of crystalline basement at 1:12,000 scale delineates a bedding-parallel detachment fault system that parallels the Great Unconformity in the Tramp Ridge block, just north of the Gold Butte block. Above this detachment fault, extensional allochthons containing Upper Paleozoic through Tertiary (>18 Ma) rocks exhibit tilting due to westward translation and tilting. We project this geometry above the Gold Butte block itself based on restoration of slip across the Gold Butte fault. This reconstruction suggests that the detachment system extended over lateral distances of >1000 km 2 , helping define a region of relatively modest extension (~25% for cover; 10% for basement) within the Nevada transition zone between the Colorado Plateau and Basin and Range. In agreement with previously published mapping and structural cross sections, our restored cross sections suggest that extensional deformation initiated with formation of hanging-wall anticlines above a listric Grand Wash fault system and evolved via a combination of both listric faulting and domino-block translation and tilting. New data presented in this paper document that extension was also facilitated by slip on bedding-subparallel detachment zones in the Bright Angel Shale, along the basement unconformity, and along other zones of weakness, such that the extended Paleozoic cover was partly decoupled from less-extended basement. This detachment system ramps down into basement to merge with the South Virgin–White Hills detachment at the west end of Gold Butte, the principal extensional detachment of the region. Our mapping and structural model suggest that movement on these detachment faults initiated at low angle. Further, using the geometry from restored cross sections, we infer that the deepest rocks now exposed in the western Gold Butte block resided at depths of ~4 km below the Great Unconformity (~8 km below the surface) rather than the previously published 15 km below the unconformity (~19 km below the surface). New 40 Ar/ 39 Ar K-feldspar thermochronology from the Gold Butte block, added to a compilation of published thermochronologic data, is used to help evaluate alternative models. K-feldspar multiple diffusion domain (MDD) modeling suggests that rocks throughout all but the westernmost part the block had cooled through 150–200 °C before the Phanerozoic and resided at temperatures <200 °C prior to onset of rapid Miocene extension at 17 Ma. Pre-extensional (pre–17 Ma) 100 °C and 200 °C isotherms were located near the east and west ends of the basement block, respectively. Muscovite, biotite, and K-feldspar from a 70 Ma Laramide pluton deep in the block give 40 Ar/ 39 Ar ages of 70, 50, and 30 Ma, respectively. MDD modeling of K-feldspar from this sample is compatible with cooling the westernmost part of the block from 225 °C to 150 °C between 17 and 10 Ma. Available thermochronology can be explained by either structural model: our model requires pre-extensional geothermal gradients of ~25 °C/km, rather than 15–20 °C/km as previously published.

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.

The provenance and stratigraphic architecture of basin-filling Miocene sediments around the Gold Butte area, southern Nevada, and adjacent highlands record the erosion of fault blocks that progressively tilted during extension. This study focuses especially on upper Miocene correlatives of the red sandstone unit and the Muddy Creek Formation that were deposited during waning stages of extension. Upper parts of the underlying middle Miocene Horse Spring Formation are also addressed. The large east-tilted South Virgin–White Hills block, including the Gold Butte block, was the primary source of coarse detritus into the adjacent half-graben basins on both sides. Voluminous, very coarse-grained sediments were shed eastward down the back slope of this tilt block into the Grand Wash Trough. This suggests that there were large middle and late Miocene catchments on that side of the block, possibly inherited from a gentler dip slope early in the tilting history. The block uplifted and tilted during slip on the west-dipping South Virgin–White Hills normal fault that bounds the west side of the block. Its exposed footwall shed coarse-grained debris to the west. While the fault was active, this debris included rock-avalanche megabreccias. Longitudinal transport of coarse-grained sediment also occurred along the axes of basins on both sides of the block. In the late Miocene, fault death at ca. 10 Ma followed rotation of the South Virgin–White Hills fault, and the along-strike Quail Spring fault, from initial dips >55° to dips <30°. This cessation of faulting coincided with and likely caused an eastward shift in locus of faulting to the steeper Wheeler fault system. Coarse sediment shed from the South Virgin–White Hills tilt block gradually declined as deformation waned and limestone-rich sedimentation expanded onto the basin margins against the block. Where the rising sedimentary fills eventually bridged across the block and connected basins on either side, these bridge sites served to focus later integrated regional drainage—the Pliocene Colorado River. Progressive Miocene tilting of the highland block would have broadened its structural footwall on the west and narrowed its east-dipping back slope. Migration of the drainage divide by erosion and piracy, influenced by changing tilt slopes, can explain the modern position of the divide in the Gold Butte block as one that separates drainage roughly equally down the two sides.

The Lost Basin Range in the eastern Lake Mead domain consists of Proterozoic rocks that bound the west side of the Grand Wash Trough. Exhumation of the Proterozoic rocks of the Lost Basin Range occurred from ca. 18 to 15 Ma based on seven apatite fission-track ages that range from 20 to 15 Ma. The Lost Basin Range fault lies along the west side of the Lost Basin Range and steps to the east to the southern end of the Wheeler fault, which then runs north for 60 km, where it joins the Grand Wash fault. The geometry of the southern Wheeler–Lost Basin Range fault system is that of a relay ramp between two, west-dipping, high-angle normal faults. The intervening area of the fault step over, Gregg Basin, is interpreted as a relay ramp basin. New interpreted ages from stratigraphic units on the north and east sides of the Lost Basin Range integrated with existing structural data from the eastern Lake Mead domain reveal that faulting, sedimentation, and tilting of hanging-wall and footwall blocks along the southern Wheeler–Lost Basin Range fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the southeastern Lost Basin Range, while the age of continuing sedimentation in Gregg Basin is poorly constrained. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and minor breccia between ca. 15.3 and ca. 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic strata of the south Wheeler Ridge area tilted east 20°–30° more than the Miocene strata that overlie them, and therefore this tilting occurred before ca. 14 Ma. Upward-decreasing (fanning) bedding attitudes in the overlapping Miocene conglomerate indicate that Paleozoic strata were being tilted along with the Miocene strata by ca. 14 Ma. Gentle (5° and less) east dips in the lower beds of the Hualapai Limestone above and east of the paleocanyon suggest that most tilting in the western Grand Wash Trough ceased by ca. 11 Ma. The lower conglomerate of Gregg Basin lies below, and interfingers with, the limestone of Gregg Basin, which is undated but correlates with the 11–7 Ma Hualapai Limestone in the adjacent Grand Wash Trough. The syncline in upper Gregg Basin strata is linked spatially to the Wheeler and Lost Basin Range faults and indicates that these faults were likely active at 11–7 Ma. The two faults appear to cut the Gregg Basin limestone, and therefore post–7 Ma fault activity at lower rates is likely.

Abstract Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea where they form long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean (see Fig. 1 ). The concept of plate tectonics explains the locations of volcanoes and their relationship to other large-scale geologic features. The Earth's surface is made up of a patchwork of about a dozen large plates and a number of smaller ones that move relative to one another at <1 cm to ~10 cm/yr (about the speed at which fingernails grow). These rigid plates, with average thickness of ~80 km, are separating, sliding past each other, or colliding on top of the Earth's hot, viscous interior. Volcanoes tend to form where plates collide or spread apart ( Fig. 2 ) but can also grow in the middle of a plate, like the Hawaiian volcanoes ( Fig. 3 ). Of the more than 1,500 volcanoes worldwide believed to have been active in the past 10,000 years, 169 are in the United States and its territories ( Ewert et al., 2005 ) (see Fig. 4 ). As of spring 2007, two of these volcanoes, Kilauea and Mount St. Helens, are erupting, while several others, including Mauna Loa, Fourpeaked, Korovin, Veniaminof, and Anatahan, exhibit one or more signs of restlessness, such as anomalous earthquakes, deformation of the volcano's surface, or changes in volume and composition

The eastern Transverse Ranges provide essentially continuous exposure for >100 km across the strike of the Mesozoic Cordilleran orogen. Thermobarometric calculations based on hornblende and plagioclase compositions in Mesozoic plutonic rocks show that the first-order distribution of rock units resulted from differential Laramide exhumation. Mesozoic supracrustal rocks are preserved in the relatively little exhumed eastern part of the eastern Transverse Ranges and south-central Mojave Desert, and progressively greater rock uplift and exhumation toward the west exposed rocks originating at mid-crustal depths. The eastern Transverse Ranges thus constitute a tilted, nearly continuously exposed crustal section of the Mesozoic magmatic arc and framework rocks from subvolcanic levels to paleodepths as great as ~22 km. The base of this tilted arc section is a moderately east-dipping sheeted magmatic complex >10 km in width by 70 km in length, constructed structurally beneath, yet synchronous with Late Jurassic and Cretaceous upper-crustal plutons. Geochronology and regional structural relations thus suggest that arc magmas generated in the lower crust of this continental arc interacted in a complex mid-crustal zone of crystallization and mixing; products of this zone were parental magmas that formed relatively homogeneous upper crustal felsic plutons and fed lavas and voluminous ignimbrites.

The upper Miocene to lower Pliocene Bouse Formation in the lower Colorado River trough of the American Southwest was deposited in three basins—from north to south, the Mohave, Havasu, and Blythe Basins—that were formed by extensional faulting in the early to middle Miocene. Fossils of marine, brackish, and freshwater organisms in the Bouse Formation have been interpreted to indicate an estuarine environment associated with early opening of the nearby Gulf of California. Regional uplift since 5 Ma is required to position the estuarine Bouse Formation at present elevations as high as 555 m, where greater uplift is required in the north. We present a compilation of Bouse Formation elevations that is consistent with Bouse deposition in lakes, with an abrupt 225 m northward increase in maximum Bouse elevations at Topock gorge north of Lake Havasu. Within Blythe and Havasu Basins, maximum Bouse elevations are 330 m above sea level in three widely spaced areas and reveal no evidence of regional tilting. To the north in Mohave Basin, numerous Bouse outcrops above 480 m elevation include three widely spaced sites where the Bouse Formation is exposed at 536–555 m. Numerical simulations of initial Colorado River inflow to a sequence of closed basins along the lower Colorado River corridor model a history of lake filling, spilling, evaporation and salt concentration, and outflow-channel incision. The simulations support the plausibility of evaporative concentration of Colorado River water to seawater-level salinities in Blythe Basin and indicate that such salinities could have remained stable for as long as 20–30 k.y. We infer that fossil marine organisms in the Bouse Formation, restricted to the southern (Blythe) basin, reflect colonization of a salty lake by a small number of species that were transported by birds.