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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.