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Stalagmites from Goshute Cave, located in the Great Basin of the western United States, preserve ~20,000 yr of millennial-scale oxygen isotopic variability during marine isotope stages 5c and 5b, similar in timing and structure to Dansgaard-Oeschger (D-O) events 23-21 from the Greenland Ice Sheet Project 2 record. That D-O interstades 23-21 were of longer duration than many of the later D-O events, coupled with the asymmetric shape of the D-O oxygen isotope curve, and the direct U-Th dating of the Goshute Cave stalagmites, allows for an improved understanding of the synchroneity of climatic changes between the western continental United States and the North Atlantic. Eastern Pacific-atmosphere interactions are a likely mechanism for transmission of millennial-scale climate variability into the Great Basin. Keywords: speleothem, Great Basin, last interglacial, oxygen, Dansgaard-Oeschger event.

Abstract: Bearriverops n. gen. is a distinctive clade of small, vaulted trilobites from the Lower Ordovician (Ibexian Series; Stairsian Stage) of Utah and Idaho. The genus includes at least seven [...]

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland--created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada--shed sediments eastward across the Antler foreland, burying the unconformity. Postorogenic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations. Keywords: Paleozoic, tectonics, stratigraphy, Antler, foreland, lithostratigraphy.

The north-northwest--trending White-Inyo Range locally defines the western edge of the Great Basin. The northeast-trending Barcroft structural break lies astride the province boundary. Along this preintrusive, high-angle reverse fault, middle Mesozoic White Mountain Peak alkaline volcanic and intercalated volcaniclastic rocks on the north are separated from uppermost Proterozoic-Lower Cambrian miogeoclinal quartzite and carbonate strata on the south by the 165 [+ or -] 1 Ma (SHRIMP-RG, [sensitive, high-resolution ion microprobereverse geometry], U-Pb zircon) Barcroft pluton. Although locally faulted, the subparallel southeast and northwest borders of the body display intrusive contacts. Finer-grained comagmatic metadiorite occurs as early-stage dikes in the wall rocks. Eastward, the 100 Ma McAfee Creek granite intrudes the Barcroft pluton. Tertiary diabase dikes crosscut the section. The two granitic series, reflecting local evolution of the Mesozoic arc system, are described in this paper: (1) mafic granitoid rocks of the calc-aikaline Barcroft series, including chemically intergradational granodiorite, gabbro/diorite, metadiorite, and rare alaskite-aplite--all rich in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and (2) granite of the felsic McAfee Creek series, which has even greater enrichment of LILEs and greater depletion in HFSEs than the Barcroft pluton. Rocks rich in hornblende clinopyroxene and belonging to the Ca-rich, metaluminous Barcroft series exhibit a broad range of chemical and mineral compositions and represent products of both mixing between high- and low-silica members of the series to generate intermediate compositions and fractional crystallization to generate the most mafic (cumulate) rocks. The younger, more homogeneous, K-rich McAfee Creek-type muscovite granite possesses mildly peraluminons, minimum-melt compositions. Field evidence for magma mixing and isotopic data for the Barcroft series ([[epsilon].sub.Nd(t)] = -1.53 to -5.50, [sup.87]Sr/[sup.86][Sr.sub.(i) = 0.7053 - 0.7063) and the McAfee Creek granite ([[epsilon]Nd(t)] = -5.64 to -9.76, [sup.87]Sr/[sup.86][Sr.sub.(i) = 0.7062 - 0.7116) require open-system processes and involvement of preexisting crustal rocks in their genesis. The Barcroft series is one of several shallow-level magma systems now recognized in the Sierran arc that include significant amounts of reworked, preexisting crustal material. These magma series are distinct from those that represent demonstrably deeper levels of Mesozoic magma systems and lack isotopic evidence for significant involvement of older crust. Despite these differences in magma sources and evolution, both magma series evolved along indistinguishable petrologic and chemical trends. Igneous rocks in the White Mountains record post-Paleozoic growth of this part of the Californian margin: (1) Jurassic and younger subduction, partial fusion of the overlying mantle wedge, and/or deep-seated mafic crust, ascent of mildly alkaline and later calc-aikaline magmas attending Andean-style voicanic-plutonic arc production, with significant involvement of preexisting crustal rocks; (2) Late Cretaceous thickening, heating, and crustal contamination, followed by rise of peraluminous granite-minimum melts; and (3) Neogene Basin and Range lithospheric transtension, tapping of upper-mantle diabasic melt, and dike emplacement. Keywords: Barcroft pluton, caic-alkaline magmas, metaluminous granodiorite, peraluminous granite, Andean arc, Mesozoic margin.

A detailed description, history and analysis are presented on the Basin and Range, or the Great Basin, region between Sierra Nevada and the Colorado Plateau, focusing on plate tectonic movement during the Cenozoic era. Findings support the theory of a large, eastward flow of deep crust from the Sierra to the Plateau during extension

The origin of Carlin-type or sediment-hosted, disseminated gold deposits of the Great Basin, the major source of gold in the United States, is poorly understood. We propose that Eocene magmatism was the heat source that drove the hydrothermal systems that generated these deposits in the Carlin trend and Independence Mountains in northern Nevada. This interpretation is based on a strong spatial and temporal association of Eocene intrusive-volcanic centers with the gold deposits of this region. Our new work and published 40Ar/30Ar dates indicate that magmatism was particularly intense between 39 and 40 Ma throughout northeastern Nevada, especially in and around the area of gold deposits. Carlin-type deposits may have formed preferentially during Eocene magmatism because it was (1) more intense in the area than other magmatic episodes, (2) somehow compositionally distinct, or (3) accompanied by extension that promoted hydrothermal flow. However, large-scale extension does not appear to have been a factor in generating Carlin-type deposits.

Gravitational body forces (i.e., buoyancy forces) have come to be seen as critical to the evolution of orogens. Nevertheless, constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation. By separating the calculation of buoyancy forces from the calculation of the resulting deformation, models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints. Among these are the importance of paleoelevation in driving syn- and postcontractional extension and in localizing contractional strain. Although such phenomena have been considered in more complex models of continental deformation, the simpler analysis presented here clearly establishes first-order limits on lithospheric structures and paleoelevations consistent with buoyancy-driven deformation. In the early Cenozoic Great Basin of the western United States, we show that the low elevations inferred in much of the geologic literature are inconsistent with a body-force origin for observed extensional tectonism. East of the Colorado Plateau, localization of Laramide deformation coincides with pre-Laramide subsidence of the Western Interior seaway. This subsidence prestressed the lithosphere, making the Southern Rocky Mountains the weak link in responding to regional compressional stress.

A study was conducted on deformational features around the strike-slip fault systems in the Basin and Range Province, southeast Nevada. Results showed that the basin north of Las Vegas, NV, its intense contractional deformation and the southward shift of deposition during inversion may have been produced by the Las Vegas Valley shear zone and the Bitter Spring Valley fault. These results suggest that basin formation and localized vertical tectonics can arise from strike-slip fault systems.

Recent geophysical studies in the southern Sierra Nevada found no significant Airy-type crustal root; the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere. We suggest that the late Cenozoic uplift of the Sierra Nevada may have resulted from mantle upwelling under the Basin and Range province, which tends to push ductile material within the surrounding lithosphere, causing it to flow away and downward. Numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the High Sierra and lithospheric thickening under the western Sierra Nevada, comparable to the observed structure.

Two key questions are raised by the presence in the Basin and Range Province of subhorizontal normal faults, which were active in and around the Miocene. First, how did they develop, when their existence seems to be precluded by elementary rock-mechanics arguments? Second, if they did slip in the recent geological past, why do normal faults now active in the brittle upper crust have steeper dips, both in the western USA and elsewhere? One may now answer both questions, by considering the stress field in regions where the isostatic response during extension involves horizontal flow in the plastic lower crust. This stress tensor will have inclined principal axes, making it possible for normal faults to form with either very steep or subhorizontal dips depending on the sense of flow. This can be controlled by environmental conditions, which influence rates of hanging-wall sedimentation and footwall erosion and thus determine lateral variations in lithostatic pressure in the lower crust. Regional-scale lateral variations in the geothermal gradient caused by subduction can also influence lower-crustal flow regimes in adjacent zones of continental extension, enabling extension to occur on low-angle normal faults dipping away from the active continental margin. Keywords: Basin and Range Province, Aegean Sea, normal faults, lower crust, isostasy.

The latest Cenozoic (

A large, global positive excursion in [Delta]13C (from -0.5 to 4.5[per thousand]) during the Late Cambrian Pterocephaliid biomere/Steptoean Stage (Aphelaspis-Elvinia zones) is documented at high stratigraphic resolution in three sections in the eastern Great Basin. The excursion, which we refer to as the Steptoean Positive Carbon Isotope Excursion, or SPICE excursion, began coincident with a world-wide extinction event. The [Delta]13C data from the Great Basin reflect global seawater values in a wide range of lithologies, including oolitic grainstones, wackestones, thrombolitic boundstones, and flat-pebble conglomerates. We use a section at Shingle Pass in the southern Egan Range to divide the Pterocephaliid biomere into eight isotope steps that represent equal divisions of the [Delta]13C rise and fall ([+ or -]4[per thousand]). This provides a basis for recognition of a revised chronostratigraphic framework for the Pterocephaliid biomere/Steptoean Stage. Strata deposited during the beginning of the SPICE excursion record a major change in the pattern of sedimentation in the eastern Great Basin. This is reflected in a siliciclastic-carbonate transition at Shingle Pass, Nevada, and a carbonate-siliciclastic transition at the House Range and Lawson Cove sections in Utah. A regional siliciclastic influx recognized throughout the Great Basin occurs near the peak of the SPICE excursion. Carbon isotope analyses from cratonal sections in Wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the SPICE excursion. The correlated changes in [Delta]13C, relative sea level, and the marine biota during the SPICE excursion provide remarkably detailed records of a major paleoceanographic event. We speculate that changes in sea level, climate, or tectonics may have triggered the SPICE excursion and coeval extinction event. Subsequent burial of organic carbon caused the increase in [Delta]13C and may have led to an interval of global cooling. The results of this study lend confidence to carbon-isotopic studies of pre-Mesozoic rocks.

Streams erode new basins in extensional mountain ranges where tectonic asymmetry facilitates divide migration. Ranges in the northern Basin and Range of the United States form a space-for-time series that illustrates stream incision and divide migration following footwall uplift, accelerating basin growth, and changes in the amount and direction of volume loss. During basin expansion, stream erosion exceeds hillslope retreat, which elevates summit plateaus.

Dense aggregations of Oryzopsis hymenoides (Indian ricegrass) and Bromus tectorum (cheatgrass) seeds occur in rodent caches in the Great Basin. Rodent caching behavior may influence establishment and persistence of these two desert grasses. Seed caches of rodents clearly introduce exaggerated seedling competition in these species. Greenhouse experiments were used to determine if establishment and persistence of these species were affected differently by densities of seeds in caches. Germination and establishment of Indian ricegrass, a native perennial grass, were less affected by high seed densities than germination and establishment of cheatgrass, an introduced annual weed. The different natural histories of these species, as well as data presented here, suggest that the high seed densities introduced by caching behavior of desert rodents may be beneficial to Indian ricegrass and harmful to cheatgrass., INTRODUCTION Birds, harvester ants, and rodents (primarily of the family Heteromyidae) are major granivores of North American deserts. Like some other rodents, species of the family Heteromyidae cache seeds in [...]

The Nd isotopic compositions of Middle Proterozoic to Early Cambrian terrigenous sedimentary rocks in the Great Basin, United States, were determined to identify the source areas of the siliciclastic detritus and to constrain models for the early tectonic evolution of the western margin of the continental United States. In the southwestern Great Basin and vicinity, Nd isotope data from the intracratonic, Middle to Late Proterozoic Pahrump Group reveal that the coarsegrained detritus from the Crystal Springs and the younger Kingston Peak Formations, including apparent glacial dropstones, was derived from the local Proterozoic basement (province 1, as defined by Nd isotope data). Fine-grained detritus derived from other Proterozoic crustal terranes (provinces 2 or 3) was identified in the southernmost exposed portions of the Pahrump Group, and corresponds to more distally derived material originally eroded from Proterozoic basement farther to the south and east. Coarse- and fine-grained sedimentary material composing the overlying Early Cambrian miogeoclinal strata were also derived primarily from local Province 1 basement rocks, but the occurrence of detritus derived from Province 2 or 3 crust is widespread geographically. The deposition of sediment derived from province 2 or 3 across the developing continental shelf presumably reflects thermal subsidence and progressive marine encroachment of the ancient continental margin. Within the Middle Proterozoic to Early Cambrian section, only the arkosic middle member of the Wood Canyon Formation and the correlative deeper marine Andrews Member of the Campito Formation have measured [[Epsilon].sub.Nd] values greater than-15 (-6 to -4). These units were apparently derived from a local, [approximately]1 b.y. old, granitic source exposed during the Early Cambrian Period to the northeast of present-day southern Nevada. The tectonic settings of the formation and subsequent uplift of this granitic terrane are unknown, but erosion of the granite generated excellent Nd isotopic marker beds that should prove useful in correlating Early Cambrian miogeoclinal sedimentary rocks throughout southwestern North America. In the north-central Great Basin, Late Proterozoic miogeoclinal sedimentary rocks of the Trout Creek and McCoy Creek groups show a marked shift from province 2 or 3 to province 1 sources through time. This shift is consistent with a change in topography related to the emergence of the Tooele-Uinta arch as an area of positive relief. The Nd data provide the first evidence that this structure was present in this region as early as latest Proterozoic time. As in the southern Great Basin, however, the isotope data indicate that sediments deposited in the Late Proterozoic Era in the northern regions were dominantly derived from sources in the local Precambrian basement.

A rough crust-mantle boundary can produce amplitude versus offset characteristics similar to those of a second-order discontinuity (i.e., a gradient), namely a reduction in precritical PmP. Basin and Range reflection and refraction data can be simulated as well with rough Moho models as with gradient models. A rough Moho presents a different view of the interaction between the mantle and crust, where tectonic and magmatic structures are preserved and the crust and mantle are distinct. The rough Moho supports a crustal evolution model in which the crust is injected by mantle-derived mafic dikes and sills, but not to the extent that the lower crust is a continuous sequence of mafic/ultramafic intrusions, as commonly inferred from gradient models. As such, in the Basin and Range the Moho represents a well-defined chemical boundary.

Basin and Range extension, which began in the Tertiary and continues today, is well documented in Mexico north of the Trans-Mexican volcanic belt. In contrast, evidence for Basin and Range extension in southern Mexico is largely limited to the Oaxaca basin, a north-northwest-trending Miocene graben. We discovered another north-northwest-trending Tertiary basin, the Arcelia graben, approximately 200 km west of the Oaxaca basin and 50 km south of the Trans-Mexican volcanic belt. Arcelia graben subsidence began in the early Tertiary and mostly ended prior to accumulation of upper Oligocene volcanic rocks, indicating Basin and Range extension in this area was limited to a

Folds are able to form in highly extended areas where stratified cover rocks respond to basement fault offsets. The response of cover rocks to basement faulting can be studied especially well in plunging structures that expose large structural relief. The southern Basin and Range province contains plunging folds kilometres in amplitude at the corners of domino-like tilt blocks of basement rocks, where initially steep transverse and normal faults propagated upward toward the layered cover rocks. Exposed tilted cross sections, as much as 8 km thick, display transitions from faulted basement to folded cover that validate laboratory models of forced folds. The folded cover masks a deeper extensional style of brittle segmentation and uniform steep tilting.

Surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff. Although many geologists have briefly alluded to 'broken' phenocrysts, none have addressed their origin in any detail. Petrographic study of 117 cooling units in the middle Tertiary ash-flow province of the Great Basin, United States, provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash-flow tuffs. Although not proven to be wholly ineffective, breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons. First, the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes (e.g., only 1% probability for 10% phenocrysts); collision is less likely if pyroclasts move by laminar rather than turbulent flow. Second, the coating of glass and/or melt on the phenocrysts provides a cushion that absorbs the impact force. Third, plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in Great Basin tuffs. Fourth, euhedral phenocrysts of feldspar are commonplace in many Great Basin tuffs, and in some they constitute a significant proportion of the phenocrysts, indicating that mutual impact does not modify all intratelluric crystals during explosive eruption. The two most populated categories of phenocryst shape in Great Basin tuffs probably correspond to what has been previously called 'broken' phenocrysts. Somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral. These are similar in shape, size, and composition to grains in polycrystalline aggregates within the same thin section. Kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption. More than half of the plagioclase and all of the quartz phenocrysts in Great Basin tuffs consist of irregularly shaped fragments with cuspate, embayed outlines, resembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclusions of glass are common and are especially evident in larger, more or less whole crystals. Textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma. As the erupting magma decompressed, vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart, forming the phenoclasts. Shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement. Euhedral phenocrysts are common in ash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnostic of ash-flow tuffs, because they do not occur in Plinian ash-fall deposits and are rare in lava flows. These textural contrasts are useful for interpretation of generally older, but in any case altered and recrystallized, volcanic rocks. In such rocks, critical groundmass features and field relations that could provide clues to their origin have been obscured, but the shapes of relict phenocrysts are commonly well preserved.

Seismic reflection profiles, drill-hole data, and geologic maps delimit the form of normal faults and Tertiary sedimentary rocks in the southern half of the Cache Valley basin in northern Utah. Dips of faults and sedimentary rocks were estimated from time-migrated reflection profiles by using the stacking velocities for the data. At the southern end of the basin, the East Cache fault zone is listric; it shows 50 [degrees] W dips near the surface and [approximately equal to]20 [degrees] W dips at depth. The fault zone has been the site of [approximately equal to]5.6 km of net dip slip. Tertiary rocks dip 18 [degrees] E-25 [degrees] E and exhibit a rollover geometry above the fault zone. No faults are interpreted on the western side of the basin. In the central part of the basin in Utah, the East Cache fault is a single fault that dips at least 45 [degrees] W near the surface and is the site of 4.5-6.4 km of net dip slip. Here, the basin is bounded to the west by the West Cache fault, which is the site of at least 1 km of net slip that increases northward to 2 km of net slip. Slip on the East Cache fault resulted in planar, east-dipping, older Tertiary rocks near the bottom of the basin. Younger Tertiary strata, with southwest, west, and northwest dips, reflect complex tilting due to slip on the West and East Cache faults. Anticlines in the Tertiary basin-fill deposits are present in the central part of the basin and may reflect changes in normal-fault geometry at depth. Northward, dip slip on the East Cache fault zone decreases to 2.5 km. The basin is broad, shallow, and filled with nearly flat lying Tertiary rocks. This area, near the north-south midpoint of the basin, is bounded by the West and East Cache faults, but slip on the West Cache fault appears to diminish northward. A north-trending reflection profile tied to both drill-hole data and the east-trending seismic profiles indicates that the basin is deeper in the southern end. The along-strike changes in fault geometry, the amount of fault-slip, the subsurface form of the basin-filling sedimentary rocks, and the form of the basin indicate a complex history of faulting and deposition during its formation. This study and other recent ones from the Basin and Range province indicate that such complexities may be typical of many Tertiary basins in the region.

New thermobarometric and thermochronometric data from the Illecillewaet synclinorium, a broad regional southwest-verging structure in the western flank of the Seikirk fan structure, indicate that accretion of the Intermontane superterrane resulted in deep burial (20-25 km) of the outer margin of Ancestral North America and that subsequent southwest-verging folding and thrusting were accompanied by rapid exhumation. Southwest-verging folding of lower Paleozoic rocks in the Iliecillewaet synclinorium was initiated during regional peak-metamorphic conditions at 6-7 kbar. Crystallization of late-synkinematic granitoid plutons that intrude the Illecillewaet synclinorium also occurred at pressures of 6-7 kbar; however, equilibration of the contact aureoles of the plutons occurred at 3-3.5 kbar, during the latter stages of the southwest-verging deformation. Evidently, a decompression on the order of 3 kbar occurred concurrently with southwest-verging folding and thrusting in the Illecillewaet synclinorium. Thermochronometric analyses further indicate that plutonic rocks from the Illevillewaet synclinorium cooled rapidly between times of crystallization and times of Ar closure in biotite. Regional relationships indicate that the accretion of the Intermontane superterrane to the edge of the North American margin in southeastern British Columbia occurred between 187 and 173 Ma (Toarcian-Bajocian). Subsequent southwest-verging deformation occurred approximately between 173 and 168 Ma (Bajocian-Bathonian). Thus, the removal of at least 10 km of rocks from above the Illecillewaet synclinorium, inferred from thermobarometric data, occurred approximately between 173 and 168 Ma. Rapid denudation of the Selkirk fan structure was probably the result of synorogenic extension (or extensional collapse) contemporaneous with the development of southwest-verging structures at deeper levels of the orogen. The data presented here indicate that all significant deformation and most of the exhumation along the western flank of the Selkirk fan structure were complete by late Middle Jurassic time. After the Middle Jurassic, the Selkirk fan structure must have been transported passively northeastward along the basal decollement of the Cordilleran foreland thrust-and-fold belt, together with the Intermontane superterrane, as the fold-and-thrust belt tectonically overrode the margin of the North American craton.

Lower-Middle Devonian carbonates (270-400 m thick) of the eastern Great Basin were deposited along a low-energy, west-ward-thickening carbonate platform. Six regional facies representing peritidal, shallow subtidal, stromatoporoid biostrome, deep subtidal, slope, and basin environments are recognized. Four third-order ([approximately equal to]1.5-2.5 m.y. durations), transgressive-regressive sequences are identified across the platform-to-basin transition based on deepening and shallowing patterns in regional facies, intensity and stratigraphic distribution of subaerial exposure features, and stacking patterns of fourth- to fifth-order, upward-shallowing peritidal and subtidal cycles. Transgressive systems tracts along the basin/slope are characterized by upward-deepening successions of proximal through distal turbidites overlain by fine-grained, hemipelagic deposits. Shallow-platform transgressive systems tracts are composed of stacks of thicker-than-average peritidal cycles overlain by subtidal cycles or noncyclic deep subtidal facies. Maximum flooding zones along the shallow platform are composed of stacked peritidal cycles dominated by subtidal facies, noncyclic deep subtidal facies, or distinct deeper subtidal units within successions of restricted shallow subtidal or peritidal facies. Highstand systems tracts along the basin/slope are composed of hemipelagic deposits overlain by distal through proximal turbidites. High-stand systems tracts along the shallow platform are characterized by upward-shallowing succession of cyclic peritidal through shallow subtidal facies. Sequence boundary zones (2-16 m thick) along the shallow platform are composed of exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and increases in the intensity of cycle-capping subaerial exposure features. Sequence boundary zones along the basin/slope (6-20 m thick) are composed of upward-shallowing successions of proximal turbidites or by platform-margin peloid shoal deposits; the absence of exposure features and meter-scale cycles within basin/slope sequence boundary zones indicates that the combined rates of third- through fifth-order sea-level fall rates were less than tectonic subsidence rates. Sequence stratigraphic correlations between contrasting facies belts of the basin/slope (section NA) and the edge of the shallow platform (section TM) were independently verified with high-resolution conodont and brachiopod biostratigraphy. Correlation of sequences 1-4 with transgressive-regressive sequences of similar age in the western, midwestern, and eastern United States, western Canada, and Europe indicates they are eustatic in origin. Systems-tract scale correlations across the study area indicate that the platform evolved from a homoclinal ramp to a distally steepened ramp, then into a flat-topped platform (sequences 1-2). An incipiently drowned, intraplatform basin developed during sequence 3 as the result of third-order sea-level rise and differential sediment accumulation rates between the platform margin and intraplatform basin. During deposition of highstand systems tract 3, progradation infilled the intraplat-form basin, resulting in a flat-topped platform. A distally steepened ramp developed during transgressive systems tract/maximum flooding zone 4 and evolved into a fiat-topped platform during highstand systems tract 4 deposition. The four sequences stack in an aggradational to slightly progradational pattern ('keep-up' style sedimentation) and are bound by sequence boundary zones rather than unconformities, suggesting that greenhouse climate modes and second-order accommodation gains related to the lower portion of the second-order Kaskaskia sequence controlled sequence-scale stacking patterns.

A mantle plume may be responsibile for the 800 m topographic step between Tonopah and Las Vegas. Various measuures point to a combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source. The isostatic balance for the difference in elevation between the high northern and the low southern Basin and Range is not found within the crust. The mantle plume explanation is also consistent with isotopic, gravity, seismic, and heat flow observations.

High-precision 40Ar/39Ar isotopic ages obtained from Cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike-slip faulting in the western Basin and Range Province. In the northern Wassuk Range, faulting began between ca. 26 and 24.7 Ma; both normal and strike-slip faults are bracketed between 23.1 and 22.2 Ma, and between 15 and 14 Ma. These ages document inception of the Ancestral Walker Lane, a northwest-trending zone of right-transtensional faulting in western Nevada that separated extending crust on the east from the unextended Sierra Nevada block on the west at lat 39 [degrees] N. We speculate that the southwesterly migrating, episodic Oligocene - early Miocene, east-west extensional faulting in the Basin and Range thinned and weakened the crust, allowing right-slip faults to develop in the Walker Lane in response to San Andreas right-shear in California. Southwest of the Walker Lane there was no faulting prior to 15 Ma. Here, in the Yerington district, andesitic magmatism began at ca. 15 Ma and was followed by >150% east-west extension along closely spaced (1-2 km) normal faults with up to 4 km of offset each (Proffett, 1977). These faults tilted older Cenozoic rocks 35 degrees]-40 [degrees] W. Our new 40Ar/39Ar ages substantially revise earlier K-Ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13.8-15 Ma, and that these faults are intruded by 12.6-13.0 Ma dacites. Rapid extension is thus bracketed to a 0.7-1.7 m.y. interval at 95% confidence, indicating local, east-west strain rates of 2-4 X [10.sup.-14]/s (5-10 mm/yr). Following this period, lower rates of extension prevailed near Yerington along more widely spaced normal oblique-slip faults that localized clastic sedimentation of the Wassuk Group between 11 and 8 Ma. These faults and sedimentary rocks are more abundant southwest of Yerington in a belt parallel to the Walker Lane in previously little-extended crust. From 7 Ma to present, normal right-oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near Yerington and extend 100 km southwest of the Walker Lane, which continues to be the locus of strike-slip faulting. Thus, since 15 Ma the margin of the Basin and Range has moved progressively 100 km west creating the broad Walker Lane belt and lower strain rates near Yerington.

The flexural subsidence history recorded in Cenomanian to early Campanian (97 to 80 Ma) strata in the Cretaceous U.S. Western Interior basin was studied with two-dimensional flexural backstripping techniques. Results indicate that the flexural subsidence resulting from thrust loading was superimposed on epeirogenic subsidence in the foreland basin. The flexural component exhibits significant spatial and temporal variations along both the strike and dip relative to the Sevier thrust belt. The greatest cumulative subsidence occurred in southwestern Wyoming and northern Utah. Concurrent subsidence in northwestern Montana and southern Utah was insignificant. Temporal trends in subsidence also show a distinct regional pattern. From the Cenomanian to late Turonian (97 to 90 Ma), subsidence rates were high in Utah and much lower in Wyoming and Montana. In contrast, during the Coniacian and Santonian (90 to 85 Ma) subsidence accelerated rapidly in Wyoming, increased slightly in Montana, and decreased in Utah. We suggest that these spatially and temporally varying subsidence patterns reflect the interplay of several geo-dynamic factors, including: (1) temporal and spatial variation in emplacement of the thrust loads, (2) segmentation of the basement into adjacent blocks with different rheological properties, (3) reactivation of basement fault trends, and (4) regional dynamic topographic effects.

To reconcile differences between the assessments of crustal composition in the southern Basin and Range province on the basis of seismic refraction and reflection data and lower-crustal xenoliths, we measured velocities of xenoliths from the Cima volcanic field in southern California. Lower-crustal samples studied included gabbro, microgabbro, and pyroxenite. We find that the mafic xenolith velocities are compatible with regional in situ measurements from seismic refraction studies, provided that a mixture of gabbro and pyroxenite is present in the lower crust. Supporting this model are observations that many of the lower-crustal xenoliths from the Cima volcanic field are composites of these rock types, with igneous contacts. Vertical incidence synthetic seismograms show that a gabbroic lower crust with occasional pyroxenite layering can produce a reflective lower crust that is similar in texture to that shown by seismic reflection data recorded nearby.

Fossil spring deposits are common in the southern Great Basin, and their distribution provides important constraints on the hydrologic response of the regional water table to climate change. This information is crucial, because the proposed high-level nuclear waste repository at Yucca Mountain will be located [approximately]200-400 m above the modern water table. Water tables will rise in response to a future return to glacial climates, but the magnitude of the change - and the consequences for radionuclide travel times and overall repository integrity - are key uncertainties. Increased recharge during past pluvial periods in the Spring Mountains and Sheep Range caused water tables to rise and ground water to discharge over broad expanses of the Las Vegas Valley system, and in nearby Pahrump, Sandy, and Coyote Springs Valleys. In contrast, other valleys in the region contain only small areas of Pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium. In these instances, which include the Valley Wells area, and Piute and northern Coyote Springs Valleys, smaller ranges such as the Clark and New York Mountains supplied the moisture. The change in water-table levels since the last full glacial period varies between and within valleys, from as little as 10 m in several areas to 95 m in the Coyote Springs Valley. At Yucca Mountain, the water table has probably changed by [less than or equal to]115 m in response to climate change. The spring deposits and the mollusk faunas found with them, often misinterpreted as lacustrine in origin, share many essential features with active spring systems in northeast Nevada. Deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high. Pale green mud, containing a mix of aquatic, semiaquatic, and moist terrestrial mollusks, accumulates in wet meadows and marshes associated directly with spring discharge. The record in subbasins of the Las Vegas Valley system is dominated by late Wisconsin-age sediments, although pond sediments and alluvium belonging to at least one older (pre-Wisconsin?) pluvial period are also locally exposed. Deposits from two even earlier episodes of spring discharge, both of which also occurred during Rancholabrean time (10 to

When viewed with stress transformation laws and an idealized physical model, observations of oblique slip and slip partitioning in the Basin and Range (western United States) are interpreted to show that (1) separate regions with the same net extension direction are not necessarily characterized by the same regional stress field, (2) fault systems exhibiting partitioning where one of the faults is near vertical generally do not require temporal changes in the stress field to explain the disparate slip vectors on the adjacent faults, and (3) the relative strengths of active fault zones may vary by more than an order of magnitude.

Estimates of surface extension in the southern Basin and Range province and transition into the Colorado Plateau range from a few percent to several hundred percent locally, yet the crustal thickness varies perhaps only 10-15 km across these provinces. Within the southern Basin and Range and the metamorphic core complex belt, extremely extended crust is directly juxtaposed against equally thick (or thinner) crust that underwent far milder extension. Unless preextension crustal thickness varied dramatically over a short distance, the crust must have maintained its thickness during extension, through mechanisms that involve crustal flow and magmatism. We employ a 300-km-long profile of seismic refraction and coincident vertical-incidence reflection data to investigate the geophysical signature of these processes from the extended southern Basin and Range province to the unextended Colorado Plateau. By integrating the seismic velocity with the pattern of reflectivity along the profile, we estimate the amounts of Tertiary magmatism and flow that have occurred. We estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the Transition Zone and southern Basin and Range province, respectively. We emphasize that this 8-km estimate is strictly an upper bound, and that the actual amount of magmatism was probably less (3 to 4 km). We further speculate that several kilometers of silicic rock was added to the metamorphic core complex belt via ductile flow. As suggested by numerous numerical models of crustal extension, we conclude that a mobile, felsic midcrustal layer accommodated most of this crustal flow. This ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended Transition Zone and Colorado Plateau. In contrast, the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension.

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5 degrees N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the southward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

As part of the development effort of a regional climate model (RCM) for the southern Great Basin, this paper presents a validation analysis of the climatology generated by a high-resolution RCM driven by observations. The RCM is a version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, version 4 (MM4), modified for application to regional climate simulation. Two multiyear simulations, for the periods 1 January 1982 to 31 December 1983 and 1 January 1988 to 25 April 1989, were performed over the western United States with the RCM driven by European Centre for Medium-range Weather Forecasts analyses of observations. The model resolution is 60 km. This validation analysis is the first phase of a project to produce simulations of future climate scenarios over a region surrounding Yucca Mountain, Nevada, the only location currently being considered as a potential high-level nuclear-waste repository site. Model-produced surface air temperatures and precipitation were compared with observations from five southern Nevada stations located in the vicinity of Yucca Mountain. The seasonal cycles of temperature and precipitation were simulated well. Monthly and seasonal temperature biases were generally negative and largely explained by differences in elevation between the observing stations and the model topography. The model-simulated precipitation captured the extreme dryness of the Great Basin. Average yearly precipitation was generally within 30% of observed and the range of monthly precipitation amounts was the same as in the observations. Precipitation biases were mostly negative in the summer and positive in the winter. The number of simulated daily precipitation events for various precipitation intervals was within factors of 1.5- 3.5 of observed. Overall, the model tended to overestimate the number of light precipitation events and underestimate the number of heavy precipitation events. At Yucca Mountain, simulated precipitation, soil moisture content, and water infiltration below the root zone (top 1 m) were maximized in the winter. Evaporation peaked in the spring after temperatures began to increase. The conclusion drawn from this validation analysis is that this high-resolution RCM simulates the regional surface climatology of the southern Great Basin reasonably well when driven by meteorological fields derived from observations.

In the northwestern to central Nevada Basin and Range, there are correlations between velocity and specific geologic structures of the crust. Mapped range-bounding faults at the surface can be traced to appreciable (10 km) depths based on velocity variations and are consistent with subsurface projections of the faults based on seismic reflection images. The limiting depth of the faults, as indicated by the velocity variations, corresponds to the maximum depth of earthquakes along the seismic profile. Correlations between velocity and the surface geology show that in the upper crust the pre-Cenozoic rocks are underlain by high-velocity (6.0 km/s) rocks, whereas the Tertiary ranges are underlain by lower-velocity (4.0-5.7 km/s) rocks to depths as great as 10 km. Although the Tertiary rocks differ in composition from the Mesozoic rocks, the lower-velocity Tertiary rocks may also be attributed to rock masses which are broken (4.0-5.7 km/s), and the higher-velocity Mesozoic rocks (6.0 km/s) may be attributed to largely unbroken rock masses. The regional seismicity pattern is consistent with this interpretation, as earthquakes are largely confined within or near the base of the low-velocity rocks. These low-velocity, highly fractured rocks are laterally distributed in discrete zones, suggesting that extension is not uniformly distributed but occurs in discrete, highly extended zones. Beneath these highly extended zones, the lower-crustal layers show structural evidence of extension, and velocity measurements suggest that the lowermost crust has been magmatically underplated. The superposition of Tertiary volcanic rocks, highly fractured upper crust, and lower-crustal magmatic underplating suggests that the Tertiary volcanic rocks originated from lower-crustal magmas that migrated to the surface via the highly extended zones. The velocity structure of one of the highly extended zones and the Lahonton Basin resembles that of many continental rifts. The velocity structure beneath central Nevada, however, is much more like normal continental crust. On the basis of isotopic studies, it is concluded that the transition between highly extended crust and more normal crust occurs in the area inferred to be the edge of the North American craton.

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low Si[O.sub.2] and high abundances of incompatible elements (Ti[O.sub.2], Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high Si[O.sub.2] and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristic of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystalization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Much of northern and central Mexico underwent east-northeast extension in the mid- to late Cenozoic; this area constitutes a major but little-recognized part of the Basin and Range province. The extended region is bounded on the east by the Laramide thrust front of the Sierra Madre Oriental. On the west, the relatively unfaulted Sierra Madre Occidental separates the extended area in central Mexico from that around the Gulf of California. Extension occurred as far south as what is now the Trans-Mexican volcanic belt, and in Oaxaca, south of the belt. The Basin and Range province in Mexico constitutes approximately half of the 19 x [10.sup.5] [km.sup.2] of Western North America that underwent mid- to late Cenozoic extension. North-northwest orientations of numerous epithermal vein systems indicate that east-northeast extension began as early as 30 Ma in areas north of the volcanic belt. Major episodes of faulting began at about 23 to 24 Ma and 12 to 13 Ma, both in Mexico and in the southwestern United States. Faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting. Widespread Quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the Trans-Mexican volcanic belt. In contrast, the tectonics in and south of the belt are now probably related to subduction of the Rivera plate. Contemporaneity of the 12 Ma episode with early extension around the Gulf of California attributed to Pacific-north American plate boundary reorganization suggests that extension is related predominantly to plate boundary effects. The beginning of extension at 30 Ma may be related to initial encroachment of the East Pacific Rise upon the trench that lay off western North America.

Conditions responsible for vegetative differences between the Great Basin and Great Plains are poorly understood. Two hypotheses for the differences are (1) the geophysical barrier of the Rocky Mountains and a slow rate of dispersal of taxa restrict interchange of taxa between the Great Basin and Great Plains, even where gaps in the mountains occur; and (2) differences in climate between the two regions limit floristic intermixing. We examined relationships for both hypotheses using vegetative data from 14 study sites in Idaho, Colorado, Wyoming and Montana. Canopy cover of graminoids, forbs and shrubs was regressed against selected climatic variables and distance of each study site from the Rocky Mountains. Shrub cover diminished, graminoid cover increased and forb cover remained roughly constant on a west-to-east gradient. Graminoid cover increased and forb cover remained roughly constant on a west-to-cast gradient. Graminoid cover was positively correlated with summer precipitation, and shrub cover was positively correlated with winter precipitation and negatively correlated with summer precipitation. These vegetative variables were more highly correlated with climatic data than with distance from a broad gap in the Rocky Mountain chain. Current distributions of Great Plains and Great Basin taxa suggest that the Rocky Mountains have not been an effective barrier to plant dispersal in Wyoming. Our data suggest that seasonality of precipitation restricts dispersal of taxa between the Great Basin and Great Plains.

Paleomagnetic analysis of a northern Colorado River extensional corridor sheds light on the structure of a Miocene accommodation zone. Little vertical axis rotation is evident in the zone indicating that neither distributed strike-slip displacement nor major movement between opposing tilt blocks occured. Lower plate rock and mylonite distribution indicates shoaling of structural levels toward the accommodation zone. Upper crustal extension also declines in the same direction. The results suggest that the zone is a consistent barrier between conjugate Miocene normal faults.

Seismic reflection records of the 1986 Nevada PASSCAL seismic experiment in the roughly east-west direction (dip line) show a highly reflective crust with a broad (diffuse) Moho which is characterized at normal incidence by strong, subparallel, laterally discontinuous, multicyclic events lacking observable diffractions. Recordings in the roughly north-south direction (strike line) show a transparent crust with short scattered events and a strong continuous Moho. Elastic finite-difference modeling indicates that (1) effects due to the topography of the reflecting interfaces and/or the existence of heterogeneities increase the traveltimes, thus lowering the velocity estimates, and (2) the seismic response of a layered structure can be disrupted from a strong, laterally continuous event to a series of short multicyclic events by increasing the dimensions of the overlying heterogeneities. Finite-difference modeling demonstrates that large-scale anisotropy, achieved by roughly east-west alignment of strongly elongated lenses (boundings) located at mid- and lower-crustal depths, can explain the observed differences in the seismic response along the perpendicular lines.

The influence of tectonic setting and age on the variation of isotopic signatures of granitic plutons in the northern Great Basin has, in general, not been apparent from previous investigations. Although Elison et al. pointed out isotopic differences between Jurassic and younger plutons near the 0.706 Sr isopleth, and Farmer and DePaolo noted that there might be a difference between Cenozoic vs. Mesozoic plutonic rocks in the eastern part of the northern Great Basin, neither of these studies revealed the remarkable correlation between isotopic signature, age, and tectonic setting shown by our expanded Sr, Nd, and Pb isotopic data ase. Jurassic-Early Cretaceous plutons in the northern Great Basin have a limited range of Sr and Nd isotopic values that cluster near bulk earth. All but one of these plutons have [epsilon.sub.Nd] values less negative than -7 despite their location both to the west and east of the [epsilon.sub.Nd]=-7 line. Construction of Sr 0.706 and [epsilon.sub.Nd]=-7 isotopic boundaries is virtually impossible for plutons of this age range. In contrast, Upper Cretaceous peraluminous granites east of the [epsilon.sub.Nd]=-7 line have very negative [epsilon.sub.Nd] values and high initial Sr ratios, and they appear to represent essentially pure crustal melts. The data favor a model that equates generation of these plutons via crystal thickening associated with the Sevier thrsut belt. Cenozoic plutons appear to be mixtures of mantle and crustal reservoirs, and their isotopic systematics, along with those of the Late Cretaceous age plutonic suite, define a previously unrecognized, approximately east-west-trending crustal boundary between predominantly Archean crust to the north and predominantly Proterozoic crust to the south. The isotopic data from the Jurassic-Early Cretaceous plutonic suite do not reflect the presence of this boundary, suggesting that the isotopic systematics of this plutonic suite may not have been controlled by the same variations in crustal and/or mantle lithospheric structure at depth.

Uranium concentrations of 42 to 169 ppm in fibrous calcite from spring-deposited tufa in the middle Miocene Barstow Formation, California, are among the highest reported for calcite. Fission-track maps of multiple bands of uniformly dull-luminescent fibrous calcite show that the concentration of U increases in the outward growth direction of the calcite of each individual band. Homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated. It is proposed that the cyclic pattern of increase in U concentration reflects a cyclical change in the U/Ca ratio in the fluid. Episodic mixing between Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkaline lake water with a high U/Ca ratio could produce the episodes of formation of fibrous calcite with increasing U concentrations. The spring water supplies the Ca for calcite precipitation, and the U concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water. These cycles reflect a variable recharge of groundwater into the lake by springs. The large variation in U concentrations suggests mixing of 0% to 45% spring water with the lake, while the narrow range in [Delta][sup.18]O of -6.37 [salinity] to -6.87 [salinity] limits the variation of the proportions of spring and lake water to 5%. Keywords: carbonates, Great Basin, U, O-18, paleohydrology.

Significant effort has been expended in an attempt to date hydrothermal activity that generated Carlin-type gold deposits (CTDs) in the Great Basin of Nevada. Thus far, these efforts have been only partially successful, because the relationship(s) between the dated mineral and hydrothermal activity are equivocal in many cases. Galkhaite, a trace component of at least four CTDs in Nevada, contains significant amounts of Rb and virtually no Sr, making it an ideal candidate for radiometric dating. At the Getchell deposit, galkhaite is paragenetically late, but clearly associated with gold mineralization. Our data place gold mineralization at Getchell at 39.0 [+ or -] 2.1 Ma. This is the first unequivocally gold-related date produced for any of the Carlin-type systems. Galkhaite also has been reported at the Carlin, Rodeo, and Betze deposits and is likely present in other CTDs in Nevada. This mineral may provide a solution to the conundrum of dating of CTDs. Keywords: galkhaite, Getchell mine, Carlin-type deposits, Rb-Sr dating, gold.

Sulfide minerals in Mesozoic replacement, skarn, porphyry, and vein deposits in lower Paleozoic rocks in central and eastern Nevada have sulfur isotope compositions (10% [is less than or equal to] [[Delta].sup.34]S [is less than] 20% and radiogenic lead ratios ([sup.206]Pb/[sup.204]Pb [is greater than] 19) that are elevated relative to the range of S and Pb isotope compositions in eastern Great Basin metal deposits. The S and Pb isotope compositions of central and eastern Nevada Mesozoic metal deposits (e.g., Eureka) are similar to the S and Pb isotope compositions of pyrite disseminated in the thick ([is less than or equal to] 8 km) terrigenous detrital succession (TDS) of siliciclastic rocks of Late Proterozoic-Early Cambrian age subjacent to the deposits. TDS rocks are, therefore, a possible source for most if not all S and Pb in these deposits. To the south and east in southern Nevada, southeastern California, and western Utah, progressively thinner TDS rocks correlate with lower [[Delta].sup.34]S values ([is less than] 10% and lower [sup.206]Pb/[sup.204]Pb ratios ([is less than] 18.6) in overlying Mesozoic metal deposits. These relationships suggest that TDS rocks supplied S and Pb to overlying deposits in amounts proportional to TDS thickness and that some S and Pb in the southern and eastern Great Basin deposits in lower Paleozoic rocks came from more isotopically homogeneous and presumably deeper sources, most likely Early and Middle Proterozoic crystalline rocks. Possible S and Pb sources for eastern Great Basin metal deposits in middle and upper Paleozoic rocks include, in addition to TDS pyrite and Early and Middle Proterozoic crystalline rocks, Paleozoic sedimentary pyrite that has S and Pb isotope compositional ranges similar to, as well as lower than, TDS pyrite isotope ranges. S and Pb isotope compositions of sulfide minerals in metal deposits that are temporally related to middle Tertiary granitic intrusions also vary geographically and are generally lower than isotope compositions of Mesozoic metal deposits, regardless of Paleozoic hostrock age. Compared to the Mesozoic deposits, middle Tertiary deposits in central and eastern Nevada apparently derived significant, but mostly smaller, amounts of S and Pb from TDS rocks and/or Paleozoic rocks. Tertiary metal deposits in western Utah may have obtained nearly all their S and Pb from older Precambrian crystalline rocks or from magmas and virtually none from TDS and Paleozoic rocks. Semiquantification of source-rock contributions of S and Pb to metal deposits is based on average S and Pb isotope compositions of possible source rocks and simple mixing calculations. Possible source rocks are somewhat isotopically inhomogeneous, but their S and Pb isotope compositional ranges largely bracket the S and Pb isotope compositions of metal deposits in the eastern Great Basin, thus facilitating determination of end-member contributions. Geologic factors that cause isotope inhomogeneity in both source rocks and metal deposits include different source-rock provenances, particularly for Pb isotopes, isotope mixing and fractionation by unrecognized hydrothermal processes, metamorphism, and tectonism that has juxtaposed potential source rocks of differing ages and isotope compositions. TDS pyrite formed from processes that produced S with high [[Delta].sup.34]S values--including diagenesis involving seawater sulfate and, at higher temperatures and greater depths, thermochemical sulfate reduction. Radiogenic Pb in TDS pyrite was derived from leaching of quart-zofeldspathic sedimentary rocks. Granitic melts acquired S and Pb, and possibly other ore-forming components, by bulk assimilation of TDS and/or Paleozoic sedimentary rocks, Proterozoic crystalline rocks, and possibly older Precambrian rocks; by volatilization of disseminated pyrite in source rocks during ascent; and by hydrothermal circulation near the sites of ore deposition. The high density of eastern Great Basin metal deposits and the sources of S and Pb for these deposits appear to be a function not only of the large number of granitic intrusions, but also of intrusion age and the thickness and type of Precambrian crust. S and Pb isotope compositions in eastern Great Basin metal deposits support a proposed origin for Jurassic, Cretaceous, and Tertiary intrusions that involves generation of magmas at different crustal levels and variable amounts of magmatic contamination by Precambrian rocks. Keywords: Great Basin, isotopes, lead, mineral deposits, source rocks, sulfur.

Slip rates and slip vector azimuths on major active oblique normal faults are used to invstigate whether circulation associated with the Yellowstone upwelling plume is driving tectonic deformation in the northeast Basin and Range province. Observed deformation is consistent with this suggestion; the plume is sheared to the southwest by motion of the North American plate. Testable predictions are made for structure and evolution of the region.

[sup.18.O]/[sup.16.O] ratios of granitic rocks in western North America reflect source-rock lithologies, providing insight into the geology of the deep continental crust. In a 400-km-wide, 800-km-long, east-west transect through the northern Great Basin, the Mesozoic and Cenozoic plutons define three zones: western law-[sup.18.O] zone (WZ), [[delta]sup.18.O] = +6 to +8.5; central high-[sup.18.O] zone (CZ), [[delta]sup.18.O] = +9 to +13; eastern zone (EZ) (Utah), [[delta]sup.18.O] = +7 to +9. The WZ-CZ boundary is analogous to the [[delta]sup.18.O] 'step' in the Peninsular Ranges batholith (PRB). A similarly abrupt ([sup.87.Sr]/[sup.86.Sr])[sub.i] 'step' (0.710 to the east) defined by Farmer and DePaolo (1983) lies 150-200 km east of the WZ-CZ [sup.18.O]/[sup.16.O] step, dividing the CZ into two subzones: V-type in the west, corresponding to the eastern half of the PRB (but much wider due to regional Cenozoic extension); and S-type to the east. A plausible source for the high-[sup.87.Sr], low-[[epsilon.sub.Nd]], S-type subzone is a late Precambrian (miogeoclinal) metasedimentary section, whereas the V-type source appears to be a Phanerozoic volcanogenic (eugeoclinal) accreted terrane. Both source regions have very high [[delta]sup.18.O] (+9 to +16) but radically different [sup.87.Sr]/[sup.86.Sr] (and [[epsilon.sub.Nd]]); thus, we would not place the buried edge of the >1.5 Ga craton beneath the [sup.87.Sr]/[sup.86.Sr] step, but farther east near the EZ boundary. We suggest instead that this step is a sharp lithologic and 'age' boundary (a suture-zone?) within the giant prism of sediments and volcanics in Nevada.

The Darwin Plateau, in the southwestern Great Basin of California, is underlain by upper Cenozoic basalt flows, pyroclastic rocks, and alluvial deposits. Many of the volcanic units are laterally extensive and can be correlated across the study area, using hand-sample characteristics and magneto-stratigraphy. The age of the section, from potassium-argon dating and magnetic polarities, is largely 5.3 to 5.7 m.y., although the oldest basalt flow is 7-8 m.y. old. Structural, radiometric, and sedimentologic data all suggest that basin-range crustal extension was underway locally by 7-8 Ma, and the earliest high-angle normal faulting predates 5.8 Ma. This chronology is consistent with westward migration of tectonism in the southwestern Great Basin. Additionally, structural data suggest two extension directions, the current west-northwest-east-southeast direction and an earlier west-southwest-east-northeast one. The change in extension direction occurred after 5.7 Ma. Local and regional data also indicate that the maximum compressive and intermediate stresses in the latest stress regime have been approximately equal in magnitude. Paleomagnetic data do not indicate significant rotation of the Darwin Plateau between the many large strike-slip faults in the area.

Craig H. Jones et al. used a simplified analysis of the function of gravitational body forces to evaluate the origin of initial extensions in the Great and Range. They assumed a constant average density of the crust during deformation which may not truly represent reality. Considering the range of crustal thickness suggested for the Great Basin before the Tertiary extension, it is most likely that the lower crust was at least partially eclogitized, and therefore has a higher density.