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[1] Simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western North America. Using ArcGIS, we retrodeformed the NAVDat (North American Volcanic Database, navdat.geongrid.org) using the western North America reconstruction of McQuarrie and Wernicke (2005). From these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age, composition, and migration of Cenozoic volcanism from 36 Ma to present. These relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time. Western North America is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism. A first-order observation from this study is that magmatism throughout the Basin and Range appears to be primarily driven by plate boundary effects, notably subducting and foundering slabs as well as slab windows. Exceptions include the Yellowstone hotspot system along the northern border of our study area and late-stage (

[1] The 86-km-long Surprise Valley normal fault forms part of the active northwestern margin of the Basin and Range province in northeastern California. We use trench mapping and radiocarbon, luminescence, and tephra dating to estimate displacements and timing of the past five surface-rupturing earthquakes on the central part of the fault near Cedarville. A Bayesian OxCal analysis of timing constraints indicates earthquake times of 18.2 ± 2.6, 10.9 ± 3.2, 8.5 ± 0.5, 5.8 ± 1.5, and 1.2 ± 0.1 ka. These data yield recurrence intervals of 7.3 ± 4.1, 2.5 ± 3.2, 2.7 ± 1.6, and 4.5 ± 1.5 ka and an elapsed time of 1.2 ± 0.1 ka since the latest surface-rupturing earthquake. Our best estimate of latest Quaternary vertical slip rate is 0.6 ± 0.1 mm/a. This late Quaternary rate is remarkably similar to long-term (8–14 Ma) minimum vertical slip rates (>0.4–0.5 ± 0.3 mm/a) calculated from recently acquired seismic reflection and chronologic and structural data in Surprise Valley and the adjacent Warner Mountains. However, our slip rate yields estimates of extension that are lower than recent campaign GPS determinations by factors of 1.5–4 unless the fault has an unusually shallow (30°–35°) dip as suggested by recently acquired seismic reflection data. Coseismic displacements of 2–4.5 ± 1 m documented in the trench and probable rupture lengths of 53–65 km indicate a history of latest Quaternary earthquakes of M 6.8–7.3 on the central part of the Surprise Valley fault.

[1] Changes in the volumes of ice caps considerably alter the stress state of the lithosphere by generating a transient signal that is added to the tectonic background stress field. These stress field changes, in turn, affect crustal deformation and in particular the slip behavior of existing faults. Here we use three-dimensional finite element models to investigate how arrays of normal and thrust faults near a growing and subsequently melting ice cap are influenced in their slip evolution. The results show that regardless of fault dip, both types of faults experience a decrease in their slip rate during ice cap advance and an increase in their slip rate during ice cap retreat if they are located beneath the ice cap. In contrast, faults outside the ice cap that are loaded on their footwall or hanging wall only show the opposite pattern: their slip rate increases during glacial loading and decreases during subsequent unloading. If the load is located along strike of the fault; that is, at one of its tips, the slip behavior of normal and thrust faults is different: The normal fault shows a slip rate increase during unloading, the thrust fault during loading. Our results explain the location and timing of deglaciation-induced paleoearthquakes in Scandinavia and the contrasting slip histories reported from normal faults in the Basin and Range Province, which are located at different positions relative to the former Yellowstone ice cap. More generally, our findings imply that a uniform slip behavior of faults in formerly glaciated regions should not be expected.

Received 30 July 2007; revised 12 April 2008; accepted 2 July 2008; published 14 October 2008. [1] We utilized teleseismic receiver function techniques complemented by active source refraction seismology to study the crustal structure and continental rift processes responsible for the development of the northwest corner of the Basin and Range province in northwest Nevada. Our passive seismic array of 28 short-period stations, spanning 70 km west to east, and 5 broadband USArray transportable array stations that extended our aperture to 230 km provided data on crustal properties independent of the results of our active source refraction experiment. Combining data from the two experiments provides better constraints on Moho depth, Vp/Vs ratio, and the structure and composition of the crust beneath the northwestern Basin and Range province than possible from either experiment alone. Our new data indicate Moho depths that vary from 29.5 on the east to 36.5 km in the west along the passive source array and Vp/Vs ratios that systematically span a wide range from 1.68 to 1.83. Velocity modeling of data collected from the 300 km, wide-angle refraction/reflection survey provides comparable Moho depths of 32–37 km under the same region. Decreasing Vp/Vs is correlated with decreasing crustal thickness, with Vp/Vs ratio > 1.80 in northeast California and Vp/Vs

[1] Extension across the southern Great Rift of the Eastern Snake River Plain (ESRP), Idaho, was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes. Cumulative rift-perpendicular extension ranges from 0.64 to 4.50 m along the 14 km long Kings Bowl segment, from 1.33 to 4.41 m along the 14 km long New Butte segment, and from 0.74 to 1.57 m along the 4 km long Minidoka segment. Along strike of each segment, extension increases toward coeval vents. Each rift segment is interpreted to be underlain by a subsurface dike, whose dimensions are calculated using buoyancy equilibrium and boundary element models. Dikes are calculated to have tops that are 950–530 m deep, bottoms that are 23–31 km deep, and widths that taper to zero from a maximum of 2–21 m. Modeling suggests that the Kings Bowl dike has a maximum probable width of ∼8 m and a volume of ∼2 km3, about 400 times the volume of its coeval lava flow. Dike widths and ages at the southern Great Rift provide evidence for a Holocene ESRP strain rate of about 1 to 3 × 10−16 s−1, which is as much as an order of magnitude slower than strain rates in the adjacent, seismically active Basin and Range province. Eruptive fissures are present where rift width is

Received 13 July 2005; revised 26 October 2006; accepted 31 January 2007; published 8 June 2007. [1] Large lakes can both produce and record significant crustal deformation. We present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake Lahontan, in western Nevada, using a layered Maxwell viscoelastic model. The inferred viscosity structure depends on loading history. We use three variants of a well-documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure. A simple two-layer model, with an elastic plate over an inviscid half-space, fits the observed elevation pattern quite well, with a residual variance of 32% of the data variance. Using multilayered, finite viscosity models, the residual variance is reduced to 20% of the data variance, which is very near to the noise level. In the higher-resolution models, the viscosity is below 10 18 Pa s over the depth range from 80 to 160 km. The minimum viscosity is very similar to the value that has been seen in the eastern Great Basin, from similar analyses of Lake Bonneville shorelines, but the lowviscosity zone is thinner beneath Bonneville. Making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations. Additionally, we find that small variations in proposed loading models can result in presumably spurious density inversions, and suggest that this modeling approach provides a test for loading histories.

[1] Global Positioning System (GPS) data collected in campaigns in 2000 and 2004 were processed and interpreted with other GPS data in the western Basin and Range province to provide new constraints on the rate, style, and pattern of deformation of the central and northern Walker Lane (WL), which lies near the western boundary of the Basin and Range. Across the central WL, near 38N latitude, the velocities with respect to North America increase westward by � 10 mm/yr inducing dextral shear. Farther north between 40 and 41N latitude, a western zone of � 7 mm/yr relative motion undergoes dextral shear, and an eastern zone of � 3 mm/yr relative motion undergoes extension and shear. These data show that the northern WL is essentially a dextral shear zone experiencing minor net dilatation (eD = 2.6 ± 0.8 nstrain/yr). Near most Holocene normal faults, dilatation inferred from the velocity field is not greater than the uncertainties. However, near the central Nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts (eD = 23.0 ± 3.9 nstrain/yr), some of which is attributable to transient postseismic deformation following large historic earthquakes. A block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the Honey Lake, Warm Springs, east Pyramid fault system, and Mohawk Valley faults is � 7 mm/yr. The WL is a zone whose width and dilatation rate increase northwestward, consistent with counterclockwise rotation of the Sierra Nevada microplate and transfer of deformation into the Pacific Northwest.

[1] We installed 5 broadband and 45 short-period temporary seismic stations, distributed partly as a dense, 100-km-long, N-S linear array and partly as a regional network, throughout the Menderes Massif of western Turkey in order to study crust-upper mantle structure and seismicity. In this study, we have combined teleseismic waveform data from these stations with data from several permanent seismic stations to determine crustal thickness variations in the Aegean region. Receiver function studies at seven broadband stations, using the H-k stacking method, have yielded crustal thicknesses and V-p/V-s ratios over a broad region of the Aegean. A more detailed crustal image was obtained in the central Menderes Massif, where we applied common conversion point stacking to receiver functions obtained from the N-S linear array. The results show a general trend of westward crustal thinning from 36 km in central Anatolia to 28 - 30 km in the central Menderes Massif to 25 km beneath the Aegean Sea. The results also indicate that crustal thinning in the Aegean is not uniform in the N-S extensional direction. The crust is thinner in the central Menderes Massif ( 28 - 30 km of crustal thicknesses) and the Cycladic Massif ( 25 26 km) than in surrounding regions where crustal thicknesses are 32 - 34 km. The long-lived elevated Moho under the metamorphic core complexes suggests that the lower crust in the Aegean region is at least 3 times more viscous than that in the Basin and Range Province, where the Moho is much flatter.

[1] The relationship of strain accumulation to strain release over different timescales provides insight to the dynamics, structural development, and spatial and temporal pattern of earthquake recurrence in regions of active tectonics. The Great Basin physiographic province of the western United States is one of the Earth’s broadest regions of ongoing continental extension, encompassing an area reaching 800 km in width between the Sierra Nevada to the west and Wasatch mountains to the east. We present observations arising from excavations, scarp profiling, optically stimulated luminescence, and radiocarbon dating to place limits on the late Pleistocene paleoseismic history of faults bounding eight ranges across the interior of the northern Great Basin, specifically, the Shawave, Hot Springs, Humboldt, Sonoma, Shoshone, Tuscarora, Dry Hills, and Pequop ranges. Combining the observations with similar previously published studies within and at the margins of the Great Basin yields a transect that extends eastward across the basin between the 40th and 41st parallels. The sum of observations provides a picture of the patterns and rates of earthquake recurrence over the region during the last 20–45 kyr that may be compared to patterns of contemporary seismicity and recently reported measures of strain accumulation across the area using GPS. The recurrence rate of large surface rupture paleoearthquakes along ranges at the margins of the Great Basin is systematically greater than observed along ranges in the interior. The pattern is similar to seismological and geodetic measurements that show levels of background seismicity and strain accumulation are also concentrated along the margins of the Great Basin. An east-west extension rate across the interior of the Great Basin on the order of 1/2 mm yr 1 (strain rate of 1 nstrain yr 1 ) over the last 20–45 kyr is estimated by summing the record of paleoseismic displacements across the 400 km breadth of the transect, as compared to 2m m yr 1 of strain accumulation indicated by a recently reported analysis of a collinear GPS survey. The comparison is hindered by significant uncertainties coupled to the geologic rate estimate. The transect also crosses the northern limit of the central Nevada seismic belt. The central Nevada seismic belt is defined by a north-northeast trending alignment of historical surface rupture earthquakes, increased levels of background seismicity, and strain accumulation rates greater than observed elsewhere in the interior of the Great Basin. The reported recurrence rate of late Pleistocene surface rupture earthquakes within the central Nevada seismic belt is also generally greater than observed along our transect. The observations when taken together suggest that the characteristics of strain release observed historically within the central Nevada seismic belt have been operative over the latest Pleistocene and that the apparently greater rates of strain accumulation and release in the central Nevada seismic belt are diminished or less localized in regions to the north and east. Thus, while the historical alignment of surface ruptures that defines the central Nevada seismic belt remains a unique clustering of earthquakes in time and space, the likelihood of the cluster at its observed location appears greater than would be expected to the north or eastward in the interior of the Great Basin.

[1] We use geodetic velocities obtained with the Global Positioning System (GPS) to quantify tectonic deformation of the northwest Basin and Range province of the western United States. The results are based on GPS data collected in 1999 and 2003 across five new quasi-linear networks in northern Nevada, northeast California, and southeast Oregon. The velocities show ∼3 mm/yr westward movement of northern Nevada with respect to stable North America. West of longitude 119°W the velocities increase and turn northwest, parallel to Sierra Nevada/Great Valley microplate motion, and similar to velocities previously obtained to the south. The observations are explained by a kinematic model with three domains that rotate around Euler poles in eastern Oregon and western Idaho. Northeast California experiences internal dextral shear deformation (11.2 ± 3.6 nstrain/yr) subparallel to Pacific/North America motion. Relative motions of the domains imply 2–5 mm/yr approximately east-west extension in northwest Nevada and 1–4 mm/yr approximately north-south contraction near the California/Oregon border. The northward decreasing approximately east-west extension in northwest Nevada is consistent with the northern termination of Basin and Range deformation, faulting and characteristic topography. No significant extension is detected in the Oregon Basin and Range. The Oregon Cascade arc moves north at ∼3.5 mm/yr and is possibly influenced by the approximately eastward motion of the Juan de Fuca plate. These results disagree with secular northwest trenchward motion of the Oregon forearc inferred from paleomagnetic rotations. South of latitude 43°, however, trenchward motion exists and is consistent with block rotations, approximately east-west Basin and Range extension, and northwest Sierra Nevada translation.

[1] We develop a map of crustal thickness variations across the Great Basin, Colorado Plateau, Rocky Mountain, and Great Plains Provinces of the western United States using common conversion point stacking of teleseismic receiver functions. Below the Rocky Mountains and High Plains in Colorado we find the thickest crust in the region at 45–50 km thick. Beneath the Basin and Range, thinner, between 30 and 40 km, crust is found. Thin, 30 km thick, crust is present in the northern portion of Nevada and Utah despite elevations similar to those farther south. Crustal thickness across the Colorado Plateau can be characterized as a broad transitional region between the thin crust of Basin and Range to the thicker crust of the Rocky Mountains. The impedance contrast across the Mohorovicic discontinuity decreases below the Colorado Plateau, as converted arrivals recorded in this region appear weak compared to surrounding areas. Variations in VP/VS across the region indicate higher values along the western boundary of the Basin and Range, in the Rocky Mountains, and in the western Great Plains. We are not able to characterize VP/VS in the Colorado Plateau. We find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole. Differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western United States.

[1] A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide the most reliable measurements possible of geodetic strain patterns across the nation’s only proposed permanent repository for high-level radioactive waste. The network lies astride a boundary between the geodetically stable central Great Basin and the active western Great Basin, which at the latitude of Yucca Mountain is undergoing distributed right-lateral shear at a rate of � 60 nstrain/yr. Monitoring from 1999 to 2003 (3.75 years) yields a velocity field characterized by nearly homogenous N20� W rightlateral shear of 20 ± 2 nstrain/yr (net velocity contrast of � 1.2 mm/yr across a 60 km aperture) in the vicinity of the proposed repository site. Comparison of time series of continuous results with earlier campaign surveys indicating � 50 nstrain/yr of westnorthwest extension from 1991 to 1997 suggests that the more rapid rates were in part transient motions associated with the 1992 Ms 5.4 Little Skull Mountain earthquake. Postseismic motions do not appear to affect the 1999–2003 velocity field in either campaign or continuous data. The magnitude of the velocity contrast across the area, the overall linearity of the gradient, and the large area of undeforming crust to the east of Yucca Mountain are difficult to explain by elastic bending of the crust associated with the Death Valley fault zone, a major right-lateral strike-slip fault about 50 km west of the repository site. These observations, along with apparent local variations in the velocity gradient, suggest that significant right-lateral strain accumulation, with displacement rate in the 1 mm/yr range, may be associated with structures in the Yucca Mountain area. The absence of structures in the area with equivalent late Quaternary displacement rates underscores the problem of reconciling discrepancies between geologic and geodetic estimates of deformation rates. INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements— intraplate (8110); 1243 Geodesy and Gravity: Space geodetic surveys; 8107 Tectonophysics: Continental neotectonics; 8109 Tectonophysics: Continental tectonics—extensional (0905); KEYWORDS: geodesy, tectonics, Yucca Mountain

[1] We have modeled the broad postseismic uplift measured by geodetic leveling in the epicentral area of the 1959 Mw = 7.3 Hebgen Lake, Montana earthquake, a normal faulting event in the northern Basin and Range province. To fit the observed uplift we calculate synthetic postseismic deformation using the relaxation response of a gravitational viscoelastic Earth to the earthquake. For a model with an elastic plate overlying a viscoelastic half-space, we find that the elastic thickness is 38 ± 8 km, which is close to the local crustal thickness. The half-space viscosity is estimated at 4 × 1018±0.5 Pa s. The leveling data do not require a viscous lower crust but permit a lower bound viscosity of 1020 Pa s. The observed broad uplift cannot be explained by physically plausible afterslip on and below the coseismic fault. However, local deformation across the coseismic surface rupture requires shallow afterslip reaching the surface. The postseismic deformation induced by the estimated viscoelastic structure decays exponentially with a time constant of ∼15 years. Because of coupling between the elastic layer and the viscoelastic substrate, this relaxation time is significantly longer than the 2 year Maxwell relaxation time of the viscous half-space itself. Our result suggests the importance of postseismic relaxation in interpreting high-precision global positioning system velocities. For example, our model results suggest that postseismic transient velocities from both the 1959 Hebgen Lake and the 1983 Mw = 6.9 Borah Peak earthquakes are currently as large as 1–2 mm/yr.

[1] Two GPS geodetic surveys across the Basin and Range (BR) province of western North America have detected an anomalous compression east of the Central Nevada Seismic Belt (CNSB). The first network, installed by the U.S. Geological Survey along an east-west trending transect across the BR at approximately 40°N, consisted of 63 monuments observed campaign style in 1992, 1996, and 1998. The second network, installed by the California Institute of Technology, consisted of 50 continuously operating GPS receivers, 18 of which were roughly along the same transect as the campaign survey. We have used a viscoelastic postseismic relaxation model of the four largest earthquakes in the CNSB (on the Pleasant Valley fault in 1915, the Cedar Mountain fault in 1932, and on the Fairview Peak and Dixie Valley faults in 1954) to interpret the geodetic velocities observed across the CNSB. A model of postseismic relaxation with a lower crustal Maxwell viscosity of 5–50 × 1018 Pa s can explain the apparent compression regardless of the assumed mantle viscosity. The geodetic velocities that we determined after removing the postseismic contribution indicate that within current GPS detection limits the BR is stable east of the CNSB and is undergoing rapid right-lateral shear west of the CNSB.

Postextensional uplift and tilting of Quaternary lake sediments in the Halloran Hills, eastern Mojave desert, California, suggest that regional-scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late Miocene time. The concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional-scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting. An analytic solution, developed in this paper, combines viscous flow in the lower crust, flexural bending within the midcrust, and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension. Assuming that there was no topographic relief across the Halloran Hills region prior to extension, inversion of modern topographic data indicates that crustal thinning above the Halloran Hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of Clark Mountain, in good agreement with the known geometry of the fault and the location of its breakaway zone. Incorporation of Quaternary tilting data indicates that the modern viscosity beneath the Halloran Hills (assuming a 10-km lower crustal channel) is a maximum of 1019 Pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 Ma. This corresponds to a temperature increase of at least 75° to 100°C at the Moho. In our opinion, the most likely source of this temperature increase is a regional-scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust. If correct, these results have important implications for the way in which crustal extension is linked to mantle heating within the Basin and Range Province.

Geologic mapping and structural analysis of the Lost River and Lemhi normal faults in east central Idaho suggest that many rupture segment boundaries, previously inferred from geomorphic data coincide with two classes of structural complexities: (1) zones of distributed faults and steps or jogs in the range front faults, and (2) intersections with preexisting Eocene to Oligocene(?) cross faults at 10–15 km depth. Segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5–6 km wide (in plan view). In contrast, simple fault traces characterize the range front faults in the interiors of segments. Thus segment boundaries of normal faults are broad zones of intersecting faults, not discrete points. The structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults. Intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the Lost River and Lemhi faults. Downdip projections of preexisting Eocene to Oligocene(?) normal faults in the footwall of the Lost River and Lemhi faults may coincide with four or five segment boundary zones at about 15 km depth, the depth at which large earthquakes nucleate in the Basin and Range province. Structural analyses also show that some cross faults, which coincide with segment boundary zones at the surface, are distant from these zones at depth. Conversely, some potentially significant structures present at depth are far removed from the segment boundary zone at the surface. Thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near-surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones. Where cross faults define segment boundaries, the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults.

Resistivity structure to about 25 km depth is defined from two-dimensional modeling of 29 magnetotelluric (MT) soundings (0.002–5 Hz) that traverse 280 km of the southwestern Colorado Plateau, transition zone, and Basin and Range provinces in Arizona. From the surface to 5 km depth, the MT model suggests structural relationships between low-resistivity sedimentary and volcanic rocks (50–300 ohm m) and high-resistivity granitic and gneissic basement (500–9000 ohm m). In the Basin and Range province, the MT model and a seismic reflection section show a generally consistent distribution of supracrustal rocks that have relatively low to moderate resistivity (MT) and relatively strong, locally coherent reflectivity. The supracrustal zone defined by these physical properties is inferred to be composed of upper plate rocks above a middle Tertiary detachment fault system which has been mapped in surrounding ranges. Some low-angle fault zones inferred from seismic reflections to extend into high resistivity basement below the supracrustal rocks are not resolved by the MT model. A low-resistivity zone with a conductance of 500 S or more is modeled in the crust at an average depth of about 15 km in the Basin and Range province and transition zone and may deepen below the southwestern part of the Colorado Plateau. In the Basin and Range province, the top of the low resistivity may correspond to a reflective layer with a 6-s two-way-travel time. This deep low-resistivity zone might be caused by a small fraction of connected hydrous solutions or silicic melts.

Where there is isostasy, the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load, whether compensation is in crustal roots or in the mantle. The result is a Poiseuille flow (planar channel flow) in the weak lower crust, which removes crust from under mountains and smooths and levels the topography. Assuming cubic power law creep, the flux of crust is proportional to the third power of the topographic gradient, to the usual Arrhenius term, to the tenth power of the absolute temperature of the lower crust, and to the negative fifth power of the geothermal gradient. The result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped, with a flat central plateau and steeper flanks spreading outward. When adjacent “pancakes” merge, most of the relief is eliminated. Flow may either roughen or smooth the Moho, depending on whether there are lateral density contrasts in the mantle lithosphere or not. Although it is difficult to find analytic solutions for this evolution, it is easily simulated numerically. Using various published flow laws for plausible lower crustal rocks, lateral extrusion is shown to be insignificant under the oceans, marginally significant under shields and platforms, important under elevated plains, and dominant beneath high plateaus and/or hot, delaminated regions. In particular, the Basin and Range province of the western United States cannot maintain short-wavelength (100 km) Moho relief of more than 1 km for times greater than 10–20 m.y. at most. If the Tibetan Plateau of China has been delaminated, as recently suggested, then it may flatten even faster, reducing 100-km-wavelength topographic features to 200 m relief in no more than 0.03–0.13 m.y., and leaving only features with wavelengths over 400 km at the present. Therefore the lower crust of Tibet may resemble a hydraulic reservoir, as previously suggested. Because the flatness of the Moho is self-maintained in these regions, information on past tectonics is continually lost, and the present Moho shape cannot be used to balance cross sections for times in the past. The flatness of the Moho today is not a constraint on the mechanisms of extension or shortening in these regions, nor is it evidence for underplating by intrusions. It also follows that very intense and inhomogeneous dilational strains may have occurred locally, as in the metamorphic core complexes of the Basin and Range province.

Rayleigh wave phase and group velocities and attenuation coefficients in the period range 18 to 120 s have been determined for three regions along nine paths across the western United States using a two-station technique. The attenuation coefficients were found to increase from east to west between the Rocky Mountains and the Pacific coast. Rayleigh wave group and phase velocities were inverted, using a differential procedure, to obtain shear wave velocity models for three regions. These velocity models were then used in an inversion process where Qβ−1 as a function of depth was obtained from observations of Rayleigh wave attenuation. The inversion results show that Qβ values in the upper mantle of the western United States are lowest for the coastal regions and westernmost Basin and Range and highest for the Rocky Mountains and western Great Plains. Intermediate Qβ values appear to occur in the upper mantle beneath the Basin and Range province and the Columbia and Colorado plateaus, although uncertainties in the data prevent a clear separation between that region and coastal regions. Low values of Qβ occur in the upper crust, higher values in the lower crust, and highest values in the uppermost 20 km of the mantle of all these regions. These overlie an upper mantle low-Q zone in which Qβ values decrease from east to west, lowest values (15–20) being similar to those observed above descending slabs in the western Pacific. The regional pattern of upper mantle shear wave Q values is similar to that observed for upper crustal shear wave Q values inferred from surface wave studies and for Lg coda Q values. The patterns of upper crustal and upper mantle Q variations correlate with the temporal sequence of tectonic and magmatic activity in the western United States, lowest Q values in both depth ranges occurring where tectonic activity has been most recent. Partial melting and/or enhanced dislocation motion in the mantle and variations in the volume of interstitial hydrothermal fluids in cracks in the upper crust may both be ultimately due to processes which occurred during plate consumption and interaction near the western margin of the United States.

The extensional wedges that overlie low-angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold-and-thrust belt, but with an opposite sense of shear on the basal fault. Such wedges are common in zones of crustal extension, such as the Basin and Range province of the North American Cordillera, the North Sea, and the U.S. Gulf Coast. This paper examines the mechanics of these upper crustal extensional wedges, using a modification of the compressional Coulomb critical taper model developed for fold-and-thrust belts and accretionary wedges. If a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer, then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline. A critically tapered extensional wedge is one that is on the verge of Coulomb failure everywhere. A wedge whose taper is narrower may be slid stably down the same incline without internal deformation; a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical. After testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges, we apply the theory to a pair of active extensional wedges in the Brazos area, offshore Texas. Both wedges are underlain by well-imaged basal detachment faults dipping at 17 o down to depths of 10 km; one wedge exhibits relatively little internal deformation, whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults. Direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical. Regional values for the coefficients of basal and internal friction in excess of 0.3 together with vertical cohesion gradients in excess of 1 kPa/m are consistent with the observed pore pressures and the geometry of the wedges, as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults. Such values are comparable to values inferred from mechanical and thermal models of compressional wedges, and they are consistent with laboratory rock mechanics data.

The pattern of historical ground deformation, seismicity, and crustal structure near Medicine Lake volcano illustrates a close relation between magmatism and tectonism near the margin of the Cascade volcanic chain and the Basin and Range tectonic province. Between leveling surveys in 1954 and 1989 the summit of Medicine Lake volcano subsided 389±43 mm with respect to a reference bench mark 40 km to the southwest (average rate = 11.1±1.2 mm/yr). A smaller survey across the summit caldera in 1988 suggests that the subsidence rate was 15–28 mm/yr during 1988–1989. Swarms of shallow earthquakes (M ≤ 4.6) occurred in the region during August 1978, January–February 1981, and September 1988. Except for the 1988 swarm, which occurred beneath Medicine Lake caldera, most historical earthquakes were located at least 25 km from the summit. The spatial relation between subsidence and seismicity indicates (1) radially symmetric downwarping of the volcano's summit and flanks centered near the caldera and (2) downfaulting of the entire edifice along regional faults located 25–30 km from the summit. We propose that contemporary subsidence, seismicity, and faulting are caused by (1) loading of the crust by more than 600 km3 of erupted products plus a large volume of mafic intrusives; (2) east-west extension in the western Basin and Range province; and, to a lesser extent, (3) crystallization or withdrawal of magma beneath the volcano. Thermal weakening of the subvolcanic crust by mafic intrusions facilitates subsidence and influences the distribution of earthquakes. Subsidence occurs mainly by aseismic creep within 25 km of the summit, where the crust has been heated and weakened by intrusions, and by normal faulting during episodic earthquake swarms in surrounding, cooler terrain.

Very rapid subduction of the Farallon plate under southwestern North America between 60 and 40 Ma was accompanied by a relatively low volume of magmatism throughout the southwestem United States and north- em Mexico. Between 40 and 20 Ma, when subduction slowed significantly and in one area may have even stopped, magmatism became widespread and voluminous from Nevada and Utah to central Mexico. This correlation of rapid subduction with a relatively low volume of magmatism can be explained by the observa- tion that subduction-related andesitic arc volcanism, often formed in a Laramide-style compressional regime, is relatively low volume compared to continental volcanism. The shallow roots of arc volcanic systems are clearly exposed in the porphyry copper deposits found in currently active arcs and common throughout southwestern North America between 60 and 50 Ma. By 43 Ma, worldwide plate motions changed, the Pacific plate began moving away from North America, and subduction of the Farallon plate slowed. *By around 36 Ma, the easternmost part of the East Pacific Rise, which was located between the Pioneer and Murray fracture zones, approached the trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone. Uplift on land became widespread. Voluminous continental magmatism formed the Sierra Madre Occidental (SMO) of Mexico, one of the largest batholiths in the world, as well as volcanic centers now exposed in the San Juan Mountains of Colorado and the Rio Grande Rift of New Mex- ico. Vectors of motion of the Pacific plate relative to the North American plate determined by Stock and Molnar (1988) are consistent with formation of a transtensional environment along the plate boundary sufficient to create a 100- to 200-km-wide void just landward of the old voIcanic arc. While the SMO batholith was forming within this void, the Monterey and Arguello microplates just offshore to the west were broken off from the Farallon plate and rotated so that the East Pacific Rise in this immediate area became nearly perpendicular to the trench and perpendicular to the vector of motion of the Pacific plate relative to North America. Formation of the SMO batholith was followed between 24 and 20 Ma by a major increase in the rate of subduction of the Guadalupe plate, a fragment of the former Farallon plate, and by increasing mylonitization, extension, and uplift in the metamorphic core complexes that extend northwestward through southern Arizona from the northem end of the SMO batholith. The plate margin underwent another major change between 12.5 and 10 Ma when subduction again stopped, strike slip faulting became dominant along the coast, the Basin and Range Province opcned, and numerous tectonostratigraphic terranes in southern Cali- fornia underwent large rotations. By 3 Ma a large, new terrane had been severed from North America immediately west of the SMO batholith as the Gulf of California opened. These observations can be explained by a model for the weakening and ultimate falling apart of the uppermost part of the subducted oceanic plate in the 20-30 m.y. after the end of rapid subduction. As the plate falls apart, not only is compressional stress relieved, but significant backslip along the old subduction zone is also possible, perhaps bringing blueschists rapidly upward from 20- to 30-km depths.

Fission track ages of apatite and zircon from metamorphic, plutonic, and sedimentary rocks along a 80-km transect across the Colorado Plateau-Basin and Range boundary in west central Arizona show differences in the low-temperature cooling histories between the provinces. The transect extends from Cypress Mountain in the Colorado Plateau transition zone to the eastern Buckskin Mountains in the Basin and Range. Along the northeast margin of the Basin and Range province, metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16–10 Ma. These ages are similar to K-Ar fusion ages of biotite and age minima of K-feldspar on 40Ar/39Ar age spectra and collectively indicate rapid cooling. One K-feldspar age spectrum has an age maximum of about 22.8 Ma, an age minimum of 11.8 Ma, and a spectrum whose shape is suggestive of reheating, possibly in middle Miocene time. The heating event was probably related to hydrothermal activity during emplacement of Cu and Mn deposits in and above the detachment fault zone. Effects of this heating are only locally detected in rocks above the detachment fault. In the Poachie Range fission track ages of apatite and zircon are 60–50 and 80–70 Ma, respectively. The disparity between the apatite and zircon ages indicates that the rocks cooled slowly in Late Cretaceous and early Tertiary time, probably due to gradual uplift and erosion. Total uplift and denudation in the area of the Poachie Range since Cretaceous time is 6 km or more. To the northeast of the range, fission track ages of apatite and zircon increase and diverge, indicating that apparent uplift decreased in that direction. The apatite ages from the Poachie Range are concordant with early Tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect. The ages represent cooling of crystalline rocks after Cretaceous regional metamorphism and magmatism. Near Bagdad, 20 km northeast of the Poachie Range, 2 km or less of erosion has occurred since intrusion of high-level plutons and dikes and caldera formation in Late Cretaceous time. Remnants of an erosion surface that developed in middle Tertiary time are preserved in the transition zone. Volcanic and sedimentary rocks at least as old as early Miocene were deposited on the erosion surface and filled valleys cut into it. Dissection of these deposits began about 8 Ma. We interpret these data combined with those from other studies to indicate that in Cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust. The Cretaceous igneous rocks at Bagdad are high-level manifestations of this magmatism. Uplift and slow cooling occurred in Late Cretaceous and early Tertiary time. In late Oligocene and early Miocene time during northeast-southwest extension, middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone. Tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern Buckskin and Harcuvar mountains. Middle Miocene reheating occurred locally in the lower plate, along the detachment, and in nearby parts of the upper plate.

Magnetic properties of mid-Tertiary volcanic rocks west of Jiggs in northeast Nevada were investigated for the purposes of interpreting igneous, structural, and tectonic processes in this part of the Basin and Range province. Anisotropy of magnetic susceptibility (AMS) patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs. Paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north. New stratigraphic, chemical and K-Ar isotopic data support these interpretations. Dacite to rhyolite ash flow tuffs of the Indian Well Formation were subdivided into two main units: the lower and predominant tuff of Jiggs (35.8–37.0 Ma) and the unconformably overlying but areally restricted tuff of Hackwood (30.8 Ma). The Jiggs unit has two polarities whereas the Hackwood has only a reversed polarity remanence. Together these units with tilt correction define a pole (92°E, 87°N, dp = 6°, dm = 8°) for approximately 30–37 Ma. This pole is concordant with coeval North American reference poles, indicating that this zone of approximately 30 km NS extent has not undergone significant vertical axis rotation relative to the North American reference. Andesite lavas of normal magnetic polarity and of 32.5-Ma age characterize the Diamond Hills immediately to the south. We interpret this region, from both structural evidence and discordant paleomagnetic direction, to have rotated approximately 25° counterclockwise relative to the Indian Well volcanic units to the north. The apparent rotation of the Diamond Hills is possibly the result of drag on the left-lateral Garcia fault which limits the Diamond Hills on the southwest. Analysis of AMS data suggests, by patterns of the K1 axes, two distinct sources for the Jiggs unit: a northern buried source and a central partially buried source. Lithologic evidence consistent with proximal vent facies is found near the latter source. An unusual pattern in the AMS K3 axes, suggested as resulting from the tilt correction applied to initial dip, is consistent with the interpreted flow directions. Tight age constraints can be placed on the deformation of the main ash flows of the Jiggs unit, in the 35.8- to 30.8-Ma interval. Because the Diamond Hills unit is slightly younger and appears to have been affected by the same deformative episode as the Jiggs, the regional deformation affecting both units can be more narrowly confined to the 32.5–30.8 Ma interval. This deformation is inferred to coincide with the major early Tertiary episode of crustal extension in this region of the Basin and Range.

Direct comparison of low-fold, high-energy explosive and high-fold, lower-energy Vibroseis methods for acquiring deep crustal seismic reflection data in the Basin and Range Province suggests that the high-fold common midpoint (CMP) method there does not provide the best possible image of lower crustal structure. During the recent acquisition of a Vibroseis profile in the Basin and Range Province we fired single deep shot holes to obtain a coincident single-fold explosive section. Within the upper crust (upper 3 s) the explosive source and Vibroseis records are nearly equivalent. For record times below 3 s, however, comparison of the explosive source gathers and the coincident final 60-fold Vibroseis section demonstrates that low-fold explosive profiling provides a higher-quality image of the midcrust to lower crust (3–10 s). The higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives, making them less sensitive to common noise sources. Whereas deeper than 4–5 s the Vibroseis energy levels on individual source efforts fall to that of ambient noise levels, the explosions provide signal-generated energy exceeding ambient noise levels down to 18–19 s. Although individual reflections can be correlated on explosion and Vibroseis shot gathers, reflection events on the 60-fold Vibroseis stack do not correlate to those on the single-fold explosion profile, suggesting that the high-fold CMP method in our study did not maintain the integrity of the weak lower crustal reflected arrivals. Reasons why the high-fold CMP method apparently failed include complex, even time-varying, statics, nonhyperbolic moveout at long offsets, and the difficulty in resolving stacking velocities with data having low signal-to-noise ratios. Reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower-energy Vibroseis section.

The character of deep reflections recorded in wide-angle seismic experiments often suggests fine-scale layering in the structure of the reflecting target. The lateral continuity of wide-angle reflections is enhanced, however, because energy arriving at a long-off set receiver is confined to a narrow range of apparent slowness. The distribution of energy with slowness was studied by Levander and Gibson (this issue), who show that the restricted range amounts to a dip filter. Their energy-slowness distribution is used here to relate lateral correlation in a reflected wave field to the correlation properties of a randomly heterogeneous target. Tests with finite difference synthetic data from Levander and Gibson confirm a simple convolutional relation between the statistics of the wave field and those of a target with small-magnitude velocity variations. Field data recorded in the Basin and Range Province, Nevada, show a progressive increase in reflection continuity with increasing source-receiver offset, as expected. Interpretation of reflector heterogeneity for this data set, however, is complicated by noise contamination and scattering above the target.

I present a model of lithospheric P-wave velocity structure in the northwestern Basin and Range province, Nevada, based on wide-angle seismic data collected in 1986 under the auspices of the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL). The lithosphere in the Basin and Range consists of (1) 2- to 3-km-thick low-velocity (2.5–4.0 km/s) sedimentary basins and high-velocity (>4.5 km/s) ranges overlying a basement of velocity 5.5–6.1 km/s, (2) a mid-crust (6.1–6.2 km/s) which consists of rocks of probable granitic to granodioritic composition in greenschist to amphibolite facies and which includes two local low-velocity zones (5.5–5.7 km/s), (3) a laterally varying lower crust which consists of rocks of probable gabbroic composition in upper amphibolite to granulite fades and which comprises layers of 6.6 and 7.0–7.2 km/s average velocity, (4) a complex crust-mantle transition best modeled as a velocity gradient zone 1–2 km thick, and (5) an upper mantle with velocities of 7.7–7.9 km/s which includes a local low-velocity zone (7.5 km/s). Crustal thickness in the region varies laterally from 28 to 37 km, significantly thicker than several previous estimates. The upper mantle velocities and velocity gradients differ slightly on the two perpendicular profiles, suggesting possible azimuthal anisotropy. A wedge-shaped, eastward-thickening lower crust of high seismic velocity (7.0–7.2 km/s) on the eastern pan of the survey may represent the westward edge of “rift-stage” Precambrien continental crust or, alternatively, mafic material underplated and intruded into the crust during Cenozoic extension. Comparison of the velocity model with coincident seismic reflection data shows that the reflection and refraction Mohos are coincident across the entire profile, thus indicating that deep reflections (4–10 sec two-way travel time) seen on COCORP and PASSCAL-piggyback reflection profiles originate entirely within the crust. The top of the reflective zone, however, lies consistently above the mid-crustal increase in average velocity, indicating that the reflectivity is not restricted to the mafic part of the crust; on PASSCAL piggyback profiles, in fact, the middle (silicic) crust appears more reflective than the lower (mafic) crust This can be explained as the result of either mafic, sill-like intrusions or ductile strain fabrics. Both of these processes are likely to have accompanied Cenozoic extension in the region.

Heat flow data for the state of Washington are presented and discussed. The heat flow in the Okanogan Highland averages 75 mW m−2, and the gradient averages 25°C km−1. The heat flow in the Columbia Basin averages 62 mW m−2, and the mean gradient is 41°C km−1. Both of these provinces are interpreted to have a mantle heat flow of about 55–60 mW m−2, the same value as in the Basin and Range province to the south and the intermountain region of Canada to the north. These areas comprise the high heat flow, back arc region of the Cordillera. In the coastal provinces and the western part of the southern Washington Cascade Range the heat flow is below normal and averages 40 mW m−2 with an average gradient of 26°C km−1. This low heat flow is related to the absorption of heat by the subducting slab, part of the Juan de Fuca plate, that is beneath the Pacific Northwest. Thus the low heat flow area represents the outer arc part of the subduction zone. Within the volcanic arc, the Cascade Range, the heat flow pattern is complicated. The heat flow is best characterized in the southern Washington Cascade Range. The heat flow there averages 75 mW m−2 and the gradient averages 45°C km−1. The heat flow peaks at over 80 mW m−2 along the axial region that coincides with the Indian Heaven, Mount Adams, and Goat Rocks centers of Quaternary volcanism. As is the case in northern Oregon and southern British Columbia, the western edge of the region of high heat flow has a half width of 10 km, implying a heat source no deeper than about 10 km. In the northern Washington Cascade Range the data are too sparse to determine the average heat flow. There are two saddles in the heat flow pattern in Washington, along the Columbia River and in central Washington. The origin for the contrasting heat flow may be segmentation of the heat source or some more local effect. The heat flow of the Cascade Range is well characterized in several locations, and the pattern is similar at all localities. The most striking feature is the 10 km half width of the western side of the high heat flow region where it abuts the low heat flow outer arc region. The axial heat flow ranges from about 80 to greater than 100 mW m−2. The midcrustal temperatures are interpreted to range from about 400°C to 800°C. The source of these high temperatures is interpreted to be a long-lived midcrustal zone of magma residence that is characteristic of the Cascade Range regardless of erustal type, rate of volcanism, or composition of volcanism. For example, in southern Washington the region of high heat flow spans the width of the Quaternary zone of volcanism with Mount St. Helens and Mount Adams at the west and east edges, respectively. On the other hand, most of the Oregon stratovolcanoes are near the center of anomalous region.

Two widespread lower Miocene rhyolite ash flow tuffs in the Kofa and Castle Dome mountains of southwestern Arizona are products of caldera-forming eruptions. These closely erupted tuffs, the tuff of Yaqui Tanks and the tuff of Ten Ewe Mountain, are approximately 22 Ma in age and their eruptions culminate a 1- to 2-m.y.-long burst of calc-alkaline volcanic activity centered on the northern Castle Dome Mountains. Exotic blocks of Proterozoic and Mesozoic crystalline rocks up to 20 m across are present in exposures of the tuff of Yaqui Tanks exposed in the central Castle Dome Mountains and the southern Kofa Mountains. A single, thick cooling unit of the tuff of Ten Ewe Mountain that includes thick lenses of mesobreccia marks the location of the younger caldera that extends from Palm Canyon in the western Kofa Mountains eastward more than 7 km along strike to the central part of the range. The tuffs show rapid thinning away from their inferred sources. They were probably associated with high-volume (100 kms) eruptions. Large residual Bouguer gravity anomalies, one beneath each inferred caldera, are interpreted as batholithic rocks or low-density caldera fill. Caldera-related volcanism in the Kofa region occurred during a transition in extensional tectonic regimes: from a regime of east–west trending uplifts and basins to a regime manifest primarily by northwest striking normal faults. A narrow corridor of folding and strike-slip faulting formed during volcanism in the southern Kofa Mountains. Upper Oligocene or lower Miocene coarse sedimentary rocks along the southern flank of the Chocolate Mountains anticlinorium in the southern Castie Dome Mountains mark the periphery of a basin similar to other early and middle Tertiary basins exposed in southern California. The volcanic section of the Kofa region was dissected by high-angle normal faults related to northeast-southwest oriented crustal extension typical of the southern Basin and Range province.

New P and S wave seismic data from the Basin and Range province - Colorado Plateau transition zone allow a better determination of the composition of the lower crust than would be possible from P wave data alone. The data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the US Geological Survey's Pacific to Arizona Crustal Experiment. Significant seismic observations about the lower crust include (1) strong P wave reflections at its top, (2) moderate-strength S wave reflections from the entire lower crust, (3) an average P wave velocity of 6.6 {plus minus}0.15 km s{sup {minus}1}, and (4) an average Poisson's ratio of {sigma} = 0.27 {plus minus} 0.02. Using these constraints and physical property measurements from laboratory rock samples, the authors conclude that the average composition of the lower crust in this region is intermediate-to-mafic and that mafic underplating has been minor. Consequently, the Cenozoic-age mafic and ultramafic xenoliths that are found in the Transition Zone must have resided in the upper mantle or in a thin transition zone at the base of the crust since these rock have higher P wave velocities and Poisson's ratio than observed seismically.

Hundreds of short-lived, small- to moderate-volume, mostly mafic volcanoes occur throughout the Lassen region of NE California and surround five longer-lived, large-volume, intermediate to silicic volcanic centers younger than 3 Ma. Volcanic rocks older than 7 Ma are scarce in the Lassen region. We identify 537 volcanic vents younger than 7 Ma, and we classify these into five age intervals and five compositional categories based on SiO2 content. Maps of vents by age and composition illustrate regionally representative volcanic trends. By 2 Ma, the eastern limit of voicanism had contracted westward toward the late Quaternary arc. Late Quaternary volcanism is concentrated around and north of the silicic Lassen volcanic center. The belt of most recent volcanism (25–0 ka) has been active since at least 2 Ma. Most mafic volcanism is cakalkaline basalt and basaltic andesite. However, lesser volume of low-potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Thus models of the mantle source and tectonic control of LKOT magmatism should be applicable both within and behind the subduction-related arc. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. The large, long-lived volcanic centers developed just west of a zone of closely spaced NNW trending normal faults. Within that zone of faulting, pervasive ENE extension has precluded growth of large, long-lived crustal magma systems. We interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. In our view, the Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. An episode of ENE extension that began in the late Miocene in the northwestern Basin and Range province appears to have triggered widespread late Miocene to Quaternary mafic volcanism in the Lassen region. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.

The Basin and Range province of the western United States is characterized by active east-west extension at strain rates of-- 1 cm yr -1 , high regional heat flow of--90 mW m -2, and widespread late Cenozoic bimodal rhyolitic-basaltic magmatism. The 1986 Program for Array Seismic Studies of the Continental Lithosphere Basin and Range seismic experiment imaged a portion of northwestern Nevada to determine the crustal structure and to assess reported differences between refraction versus reflection determinations of Moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms. Our interpretation of the refraction/wide-angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east-west profile and 3 km over its 220 km north-south length. We characterize the velocity structure by five layers: (1) an upper most crust, composed of sedimentary rocks and basement that has an average velocity of 5.7 km s -i , (2) a middle crust that extends to a depth of 18-22 with an average velocity of 6.1 km s -1 , (3) a 10-12 km thick lower crust with an average velocity of 6.6 km s -1 , (4) a 2-5 km thick transitional crust-mantle boundary defined by a 7.6 km s -l velocity, and (5) an upper mantle with an average Pn velocity of 7.9-8.0 km s -1 . These data show a uniformily thick crust of 28 km +5 km beneath northwestern Nevada and a lack of evidence for significant low-velocity crustal layers. In general, the entire crustal column exhibits uniform positive velocity gradients of 0.02-0.04 s- . The midcrustal discontinuity is interpreted as a small velocity increase of 0.2 km s- l at -- 18 km depth. The absence of a significant midcrustal reflection and the presence of large velocity gradients throughout the crust arises either from (1) a systematic increase in mafic composition with depth, or (2) possibly high lower crustal pore pressure that acts to decrease the velocity contrast across the midcrustal boundary. Our interpreted seismic velocity structure is consistent with a broad range of bulk compositions for the crust. Upper crustal velocities are consistent with bulk compositions that range from metasediments and granites at shallow depth to diorites at midcrustal levels, while lower crustal velocities suggest bulk compositions that range from diorites and amphibolities, at midcrustal depths, to mafic granulites and gabbros at the base of the crust. The uniform crustal thickness of 30-34 km in northwestern Nevada is close to the crustal average for the Basin and Range province but is thin when compared to surrounding regions, such as the Sierra Nevada and Colorado Plateau where crustal thickness is greater than 40 kin. Similarily, northwestern Nevada Pn velocities of 7.9-8.0 km s -1 are near the province-wide average of 7.9 km s -l . Temperature-corrected Pn velocities, from a 90 mW m -2 geotherm for the Basin and Range to 60 mW m -2 for the stable interior of the United States, are within one standard deviation of the continental average of 8.05 km s-i and suggest a uniform upper mantle composition across the Basin and Range. In addition to these observations, the homogeneity of the velocity structure beneath the western Basin and Range argues for a youthful Moho and crust that has been reworked by province-wide late Cenozoic extension, episodic magmatism, and underplating.

Data on satellite laser ranging (SLR) to Lageos aquired during the period 1978-1988 are analyzed on the basis of the precise modeling of the orbit dynamics of Lageos, producing estimates of tectonic motion for 22 sites located on seven major plates. It was estimated that intraplate motion within northern Europe is below the 2 mm/yr level in absolute rate, in agreement with conclusions of Zoback et al. (1989) regarding the stress across the region. A comparison of SLR geodesic rates with those from NUVEL-1 and AMO-2 models showed high correlations between tracking sites that are well within plate interiors, but displayed small but significant departures from unity in slope which are attributed to the possibility of recent changes in relative velocities or geologic time scale uncertainties. For lines crossing the Nnorth Atlantic, the San Andreas fault, and within the Basin and Range province, the geodesic rates determined by SLR are in good agreement with those determined by VLBI.

Recent heat flow measurements at 10 separate localities in southern New Mexico and West Texas range from 0.9 to 3.6 HFU (1 HFU = 1 × 10−6 cal/cm2 s). Radiogenic heat production determinations at eight of these localities range from 2.2 to 4.9 HGU (1 HGU = 1 × 10−13 cal/cm3 s). When the heat flow measurements are reduced by the heat from bedrock radioactivity, the Rio Grande rift and immediately adjacent areas are characterized by very high reduced flux (≥1.8 HFU), which contrasts strongly with 1.2 to 1.5-HFU values in bordering portions of the Basin and Range province and a 0.8-HFU value assumed for the southern Great Plains. Analyses of 2000 gravity observations in the Basin and Range province and the Great Plains reveal that a positive gravity ridge of about +30 mGal occurs along the Rio Grande rift in the Franklin and San Andres mountains and the Tularosa Valley. This relative gravity high can be caused by crustal thinning and/or shallow sills and dikes of mafic rocks in the crust under the rift. Gravity anomalies and a thin crust are compatible with a low-density upper mantle beneath most of the Basin and Range province. The new heat flow and radioactivity data indicate that the Basin and Range-Great Plains heat flow transition is marked by unusually high observed and reduced flux in the neighborhood of the Rio Grande rift. The combined thermal and gravity observations may be explained by shallow crustal intrusions, a hot abnormal upper mantle, and relative crustal thinning beneath the rift.

Heat flow and heat production measurements have been made in the vicinity of Long Valley from 0–30 km from the rim of the caldera and up to 30 km on either side of the boundary of the Basin and Range province at the eastern scarp of the Sierra Nevada. The search for a thermal anomaly associated with magma is complicated by the location of the caldera at the boundary between these two provinces with strongly contrasting regional heat flows and by unknown effects of hydrothermal circulation. The data show no conspicuous effect of the province transition, possibly a small local heat flow anomaly near the east rim of the caldera, and a very substantial anomaly near the west rim. Simple heat conduction models suggest that Long Valley caldera is the surface expression of a deep magmatic system; an upper crustal magma chamber could not have sustained molten material throughout the 2-m.y. eruptive history unless it were resupplied with heat from deep crustal or subcrustal magmatic sources. If the heat were supplied by crustal intrusion of mantle basalt, the crust would thicken rapidly unless magmatic activity were accompanied by accelerated local crustal spreading. To generate a viable silicic magma chamber by sill injection in the upper 5–8 km of crust, minimum intrusion rates of the order of l m per century are probably required. Thermal models for the near-normal heat flow at the east rim suggest that magma beneath the eastern part of Long Valley caldera might have been exhausted during eruption of the Bishop Tuff 0.7 m.y. ago and that the resurgent dome, which subsequently formed in the west central caldera, marks the location of a residual chamber more circular in plan. High heat flow indicated by the single measurement hear the west rim can be attributed to a simple shallow magma chamber beneath the western caldera or to recent local magmatism along the Sierra frontal fault system. Additional heat flow and hydrologic measurements are necessary for a confident interpretation of the thermal history and the present state of the caldera region.

Nine new values of heat flow in Colorado and New Mexico range from 1.22 μcal/cm2 sec to 2.7 μcal/cm2 sec. These data indicate that high heat flow is characteristic of all the Southern Rocky Mountains and the Basin and Range province in southwestern New Mexico. In the Southern Rocky Mountains, high and variable heat flow may be due to radioactivity in the upper part of a thick crust. In the Basin and Range province, differences in crustal structure suggest that high flux may be related to additional factors besides crustal radioactivity.

Eleven new heat-flow determinations in the northwestern United States, based on data from twenty-one drill holes and two mine shafts, together with previously available data, suggest a heat-flow pattern similar to that observed in the southwestern United States. In particular the high heat flow found in the Basin and Range province, about 2.0 μcal/cm2 sec, is characteristic of the Northern Rocky Mountains province and possibly the Columbia Plateaus as well. This region of high heat flow is referred to in this paper as the Cordilleran thermal anomaly zone. Most of central Wyoming, central Montana, and western Washington may have normal heat flow. A heat-flow determination in the Black Hills is high. The Northern Rocky Mountains and Basin and Range provinces have similar regional heat flow, crustal structure, upper mantle Pn velocity, and Cenozoic geologic history. Thus, tectonic schemes that emphasize the Basin and Range province as a unique area on the North American continent must be reevaluated.

The amplitude spectra of critically refracted waves are sensitive to small velocity gradients in the refracting horizon. Negative gradients and anelasticity (Q−1) result in similar amplitude decays with distance for narrow bandwidth data; positive gradients result in a net amplitude gain with distance with respect to the zero gradient, infinite Q case. Application of these results to published Pg and P* amplitude data indicates that negative velocity gradients of the order of 10−3 km sec−1 km−1 may be present in the crystalline crust and intermediate layer in the high heat flow Basin and Range province, and that positive velocity gradients are present in corresponding horizons in the provinces having normal heat flow in the eastern United States and west coast. Velocity gradients inferred from laboratory measurements of velocity variations in granites and basic igneous rocks with temperature and pressure, together with published geothermal gradients for the Basin and Range province and the eastern United States, are consistent with this result.

The results of a study of seismic waves generated by eight nuclear explosions and recorded at 31 locations between the Nevada test site (NTS) and Ordway, Colorado, are discussed. The line of recording stations crosses the eastern part of the Basin and Range province, the Colorado plateaus, and the southern Rocky Mountains, and it extends into the Great Plains. In the eastern Basin and Range province and the western part of the Colorado plateaus (0≤Δ≤385 km), the time-distance curves for Pg and Pn can be expressed, respectively, as T1 = 0.8 + Δ/6.0 and Ts = 5.8 + Δ/7.6. A third phase, tentatively identified as P*, is represented by the equation T2 = 3.8 + Δ/6.5. Using the crustal structure and Pn velocity (7.9 km/sec) found for the NTS region by other authors, we find that the above relations indicate that the thickness of the crust increases from about 25 km at NTS to about 42 km in the western part of the Colorado plateaus. East of this boundaiy the velocity of P in the upper mantle increases to 8.0 km/sec; depth to the M discontinuity in the Colorado plateaus is approximately constant over the range 435

Seismic-refraction measurements from chemical explosions near San Francisco, California, and Fallon and Eureka, Nevada, were made along a line extending nearly 700 km inland from San Francisco across the Coast Ranges, Great Valley, Sierra Nevada, and Basin and Range province. The velocity of Pg in the Basin and Range province was found to be 6.0 km/sec. Between Fallon and Eureka the velocity of Pn is 7.8 km/sec, and just east of the Sierra Nevada it is about 7.9 km/sec. Two prominent phases closely following the first arrival between 50 and 250 km from the source in the Basin and Range province were interpreted as reflections from an intermediate layer and from the M discontinuity. The velocity of P in the possible intermediate layer, deduced from the reflected phases because the refracted wave expected from this layer is nowhere a first arrival, seems to be 6.6 km/sec at the top of the layer and probably increases with depth. In the Coast Range northeast of San Francisco, the velocity of P in the crust, 5.4 to 5.6 km/sec, is substantially smaller than in the Basin and Range province. When corrected for abnormal delays in the sediments, first arrivals from San Francisco across the Great Valley suggest a Pn velocity of 7.9 km/sec. The low apparent velocity of first arrivals on this profile beyond the valley indicates that the M discontinuity dips downward beneath the Sierra Nevada. From a depth of about 20 km beneath the Coast Ranges and Great Valley, the M discontinuity descends to at least 40 km beneath the high Sierra Nevada. East of the Sierra Nevada it rises rapidly to about 22 km beneath Carson Sink and then dips downward toward Eureka, where it reaches a depth of 32 km. If the intermediate layer is real, the foregoing depths are increased by 2 to 5 km.

From the pendulum base stations at Mystic and Truckee, California, a line of gravimeter stations, with side loops, was extended eastward through Hazen, Fallon, Eastgate, and Austin, Nevada, crossing the area displaced by faults in 1954. Regionally, the Bouguer anomaly is about −160 mgal from Hazen eastward through Fallon to the Stillwater Range. Farther east the values decrease to −215 mgal near Austin, a change that can be accounted for by isostatic compensation for the increase in average elevation of about 1700 ft. Low-density sedimentary and volcanic debris is abundant enough in the intermontane basins and in parts of the mountains to make the average density of the whole elevated land mass abnormally low with respect to the conventional value, 2.7 g/cc. Consequently, 10 to 15 pct of the compensation for this high region is in the superficial materials themselves. The Basin Ranges show no evidence of being individually compensated. Locally, each of the basins has a negative Bouguer anomaly relative to the adjacent ranges, reflecting thick sedimentary and volcanic fill. Dixie and Fairview Valleys, in the area displaced by faults in 1954, are characterized by local negative anomalies of about 30 mgal, indicating that these valleys contain several thousand feet of lightweight Cenozic sediments and that their bedrock floors lie below sea level. The 1954 faulting is thus only the latest of many displacements that produced not only the visible topographic relief of 5000 ft but also a buried relief of comparable magnitude. Romney's [1957] seismic studies of the 1954 faulting and Whitten's [1957] geodetic measurements agree with direct observations of fault surfaces to indicate a horizontal extension of about 5 ft normal to the trace of the fault. Independent of a strike-slip component of faulting, the region is expanding in area. If the total structural relief was produced by displacements comparable to that of 1954, the extension across Dixie and Fairview Valleys amounts to about 1½ mi in roughly 15 million years, and if this is taken as a fair sample of the Basin and Range Province, the rate of distension in the Province is about 1 ft/century, a rate well within that of historical fault displacements. At the time of the 1954 faulting, the Bouguer gravity anomaly either did not change or decreased algebraically by an amount no greater than 1.0 mgal.

The earth's lithosphere and asthenosphere are modeled as a thin elastic sheet and a fluid substratum, respectively; the physical principles involved are briefly described. The flexural rigidity of the lithosphere is deduced from observations of the wavelength and amplitude of bending in the vicinity of supercrustal loads. Data from Lake Bonneville given by M. D. Crittenden, Jr., are reinterpreted to give a value for the flexural rigidity of the lithosphere in the Basin and Range province of the western United States of 5×1022 Newton meters. Observations of loading in Canada give values for the flexural rigidity of greater than 3×1020N m for the Caribou Mountains in Northern Alberta; about 4×1023 N m for the topography over the Interior Plains; about 1023 N m for the Boothia uplift in arctic Canada; and about 1025 N m for the bending of the beaches of Pleistocene Lakes Agassiz and Algonquin. The flexure of the lithosphere at Hawaii and the bending of the oceanic lithosphere near island arcs give values of about 2×1023 N m. For short-term loads (103–104 years) the flexural rigidity of the continental lithosphere is almost two orders of magnitude larger than for long-term loads, indicating nonelastic behavior of the lithosphere with a viscous (about 1023 N sec m−2) as well as an elastic response to stress. From the values of the flexural rigidity, the thickness of the continental lithosphere is inferred to be about 110 km and that of the oceanic lithosphere about 75 km or more. The anomalously low flexural rigidity of the lithosphere of the Basin and Range province may be due to a very thin lithosphere, only about 20 km thick, with hot, lower crustal material acting as an asthenosphere.

Seismic refraction, gravity, and phase velocity data are generally consistent with an interpretation of crust and upper mantle structure from the Pacific basin across the edge of the continent, the Coast Ranges, the Great Valley of California, the Sierra Nevada, and part of the Basin and Range province. The derived structure consists of a thin oceanic crust beneath the Pacific basin, thickening at the continental slope to a little more than 20 km under the Coast Ranges and Great Valley, and thickening further under the Sierra Nevada and Basin and Range province. The ocean basin is underlain by a normal high-velocity mantle, but the entire continental area is underlain by an anomalous upper mantle whose velocity and density are about 3% less than normal. The anomalous mantle extends to a depth of about 50 km and its lower boundary may be gradational. The crust under the Sierran highland region (Sierra Nevada and western Basin Ranges) is thicker than that in the area to the east; this ‘root’ of the highland is not limited to the Sierra Nevada proper. The root and the voluminous plutonic rocks constitute the core of the Cordilleran eugeosyncline. The root derived from a simple gravity model is about 32 km deep and, from seismic refraction data, more than 40 km deep. To reconcile the two a denser root is required, and it is suggested that this represents a residue of partial melting to form the plutonic rocks. The anomalous upper mantle, which is probably composed of plagioclase peridotite derived by a phase change from garnet peridotite or pyroxene peridotite, can explain no more than about 1 km of the Cenozoic uplift in the region. To explain greater uplift by this means requires renewal of the anomalous mantle, possibly by differentiation into basalt and peridotite followed by convective overturn to bring fresh garnet peridotite to the upper mantle. An anomalous upper mantle characterizes many regions of recent tectonic activity. Superimposed upon the regional gravity anomalies are anomalies caused by large anomalous masses in the upper part of the crust: (1) a negative anomaly of about 50 mgal caused by sedimentary rocks of the Great Valley, (2) positive anomalies associated with the ‘greenstone belt’ beneath the Great Valley and the western Sierra Nevada, (3) negative anomalies due to granitic batholiths, and (4) a negative anomaly suggesting thick sedimentary rocks on the continental slope. The positive gravity and magnetic anomaly of the greenstone belt is comparable with anomalies marking the mafic belts of other geosynclines.

Likely source material for the Sierra Nevada batholith is represented by exposures of Paleozoic rocks on the western flanks of the Sierra Nevada, Paleozoic sedimentary rocks in roof pendants east of the Sierran crest, and both Precambrian and Paleozoic rocks in the western portion of the Basin and Range province. The radiogenic heat production of these rocks has been calculated from γ-radiometric determinations of their U, Th, and K contents. The abundances of these radioisotopes in carbonate rocks are roughly an order of magnitude lower than those in adjacent siliceous sedimentary rocks; therefore, the radiogenic heat production in the Paleozoics depends mainly on the relative amounts of carbonate and siliceous material. Heat production varies from east to west, with weighted values (expressed in units of μcal/g yr) of 3.7 in Precambrian rocks of the Death Valley area, 3.5 in Paleozoics of the Inyo and White mountains, 3.8 in Paleozoic pendants, and 2.1 in rocks of the Calaveras formation on the western slope. A difference, similar to that in the Paleozoics, is observed between Mesozoic rocks exposed near the crest (4.4) and the Mesozoics of the western foothills (2.4). The geographic distribution of heat production matches (though not proportionally) that in the batholithic rocks. This suggests that there was a regional distribution of radioactivity in likely source material for the granitic rocks that now, strongly accentuated by the effects of erosion, is reflected in the batholith.

Between 1962 and late 1970, subsurface temperature measurements were attempted at more than a thousand drilling sites in the western United States. Temperatures from over 150 boreholes at about 100 distinct sites were suitable for estimates of the vertical geothermal flux. These results more than double the data from the western United States and confirm that heat flow is variable but generally high in this region. Within the over-all pattern of high heat flow, there are several distinct geographical regions, each occupying several hundred square kilometers, characterized by low-to-normal heat flow. Normal values were measured in the Pacific northwestern coastal region and the northwestern Columbia Plateaus. Additional results confirm the previously reported trend of very low heat flow in the western Sierra Nevada, increasing to normal near the crest of the range. The present work also confirms that heat flow is high in the northern and southern Rocky Mountains and somewhat lower in the central Rockies. The north-central part of the Colorado Plateau is a region of normal heat flow with higher values near its eastern border with the southern Rockies. The Basin and Range province as a whole is characterized by high heat flow that extends to within 10 to 20 km of the eastern scarp of the Sierra Nevada. The abrupt thermal transition between the Sierra Nevada and the Basin and Range province may occur partly in the Sierra Nevada physiographic province. Between Las Vegas and Eureka, Nevada, there is a large previously undetected zone of low-to-normal heat flow that is most probably the result of a systematic, regional water circulation to depths of a few kilometers. North of this zone, there is an area of several hundred square kilometers characterized by heat flows of 2.5 HFU (μcal/cm2 sec) or greater. In central California and adjoining western Nevada, a preliminary contour map suggests a heat-flow pattern with alignment parallel to the strike of the major geologic structures.

Indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western North America. The San Andreas, East-Central California (ECC), Mendocino Triple Junction (MTJ), and Pacific Northwest (PNW) domains are found to have similar stress fields, although the style and rate of deformation in each is different. We propose that all of these domains are generated by shear between the Pacific and North American plates. In the MTJ and PNW domains this shear may be evidence for the initiation of a transform fault connection between the San Andreas and the Queen Charlotte Island fault systems. The ECC, a region including much of the Sierra Nevada, appears to result from the relatively stiff nature of the San Andreas fault system. The Big Bend and region of relatively new faulting in northern California act to make the San Andreas fault system appear stiff on a regional scale. Two other domains, the Northern Basin and Range (NBR) and the Intermountain Seismic Belt (ISB), show evidence of being controlled by more local forces. The NBR has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the Juan de Fuca-North America plate boundary. The ISB can be explained by readjustments between the Colorado Plateau and the Basin and Range Province, which may be caused by the lowering of the Basin and Range due to extension.

During the Cenozoic Era, volcanism in the Basin and Range Province of the western United States changed from predominantly intermediate (andesitic and dacitic) to predominantly basaltic. Accompanying this shift was a general decrease in the amount of contamination suffered by basalts en route to the surface. Existing models relate these progressive changes to plate tectonic events at the continental margin of western North America, and assume that extensional tectonism favors eruption of basalt. However, in many areas, synextensional rocks are predominantly intermediate or silicic in composition. Here we propose that the nature of volcanism in an area undergoing extension is a strong function of the density contrast between a magma and the overlying crust. Subtle variations in this density difference will determine the level of neutral buoyancy for a magma, and the height to which a column of magma may rise by hydrostatic pressure. Increases in crustal density favor eruption of dense magmas. Contamination of stagnated basalts by crustal material is more efficient than olivine fractionation in reducing the density of residual liquids, and a stagnated magma may ascend buoyantly after relatively small amounts of contamination. A progressive increase in mean crustal density, caused by crustal extension and consequent thinning of the low-density upper crust coupled with emplacement of mafic plutons into the crust, may account for the observations that (1) the proportion of basalt erupted in the Basin and Range in the Cenozoic has increased with time, (2) younger basalts are generally less contaminated than older basalts, (3) major young silicic centers are apparently restricted to areas of low-density crust, and (4) midcrustal magma bodies are generally located at prominent P wave velocity discontinuities.

Constraints on the current stress regime of the actively extending northern Basin and Range province are provided by deformation data (focal mechanisms and fault slip studies), hydraulic fracturing in situ stress measurements, borehole elongation (“breakouts”) analyses, and alignment of young volcanic vents. The integrated data indicate significant variations both in principal stress orientations and magnitudes. An approximately E-W least principal stress direction appears to characterize both the eastern and western margins of the Basin and Range province, whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented NW to N60°W. The contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins. Active deformation along the southeastern and western province margins is characterized by a combination of strike-slip and normal faulting. Focal mechanisms along northeastern province margin (Wasatch front) and in central Nevada indicate a combination of normal and oblique-normal faulting. Temporal, regional, and depth-dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles. However, some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated. Evidence for significant temporal variation (or multiple cycles of variation) in relative stress magnitude comes from the Sierran front-Basin and Range boundary region where recent earthquakes are predominantly strike slip, whereas the profound relative vertical relief across the Sierra frontal fault zone in the last 9–10 m.y. implies a normal faulting stress regime. Using the best data on stress orientation, relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements. Analysis of well-constrained slip vectors in the Owens Valley, California, area indicate that large temporal variations in the magnitude of the approximately N-S oriented maximum horizontal stress are required to explain dominantly dip-slip and strike-slip offsets on subparallel faults. Similar faulting relations are observed throughout much of the boundary zone between the Basin and Range-Sierra Nevada (including the Walker Lane belt). Along the eastern province margin in the Wasatch front area in Utah, available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present-day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so-called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin-and-range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (

As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area, and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho (32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province, and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.