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This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of these data were used by Gupta (1973b, c) as the basis for his claim that large premonitory changes in the extent of stress-induced S-wave splitting are observed for moderate-sized earthquakes in Nevada. Analysis of particle motion for an earthquake sequence near Slate Mountain indicates that changes in S-wave splitting did not occur during that sequence. For the Mina area, comparison of S-wave signatures for 158 events occurring over a 3-year period resulted in the identification of numerous events that would be considered anomalous by Gupta's criteria, but these were not followed by larger earthquakes. The present state of knowledge on crustal structure and seismic source parameters in the western Basin and Range province is not sufficient to discriminate between stress-induced velocity anisotropy and the many other factors that contribute to the complexity of S-wave signatures.

Nine heat flow values from Baja California and Sonora represent the first determinations of heat flux from Mexico and, in general, justify southerly extensions of the Basin and Range and Southern California thermal provinces of the southwestern United States. Heat flow in northern Baja California is low (0.84 hfu), consistent with known values from the Sierra Nevada Batholith, but a value of 1.23 hfu was recorded in the central area of the peninsula. Seven measurements from Sonora, averaging 2.5 hfu, are similar to Basin and Range values. Reduced heat flow values ( q* ) have been calculated for Sonora and are interpreted as indicative of a separate Basin and Range thermal subprovince in Mexico. A tectonic model developed for northwestern Mexico attributes the high heat flow of the Basin and Range to behind arc extension associated with the termination of Cenozoic subduction. Opening of the Gulf of California and sea-floor spreading therein has provided a vent for the release of heat accumulated under northwestern Mexico. Thus, the southern portion of the Basin and Range province (in Mexico) appears to be cooling and narrowing and a subprovince of high reduced heat flow is defined east and perhaps southwest of the Gulf of California.

Western North America between the coastal ranges and the Great Plains, and from the Mexican-United States border to the Trans-Canada Highway has been studied by means of three large two-dimensional arrays of recording magnetometers. The paper discusses the conductive structures found by these arrays in the upper mantle of the region. Highly conductive mantle material at least 100km thick for conductivity 0.2S/m underlies the Basin and Range Province and ridges of still higher conductivity (or greater thickness for a given conductivity) underlie the Wasatch Fault Belt and Southern Rockies. Under the Great Plains the upper mantle is more resistive and is of sub-shield type. North of the Basin and Range the Cordillera have a thin layer (10-20km at 0.2S/m) of conductive material in the upper mantle and the conductivity indicates a lower level of tectonic activity than in the midlatitudes of the U. S. A. Under the Colorado Plateau the conductivity is of Great Plains type or intermediate between this and that beneath the Basin and Range. Alternative conductive models satisfying variation-field anomalies are discussed with special reference to the poor depth and excellent lateral resolution of magnetometer arrays. The distribution of heat-flow is compared with that of electrical conductivity and it is shown that there is generally close correspondence and full support for association of high conductivity with high temperature. Seismological velocity-depth profiles, station terms in P and S teleseismic times and other parameters are considered and are again consistent with the conductivity and geothermal information. The case for partial melting is considered. It is strongly supported by seismological parameters in the low-velocity layer and gives the most plausible hypothesis for the contrasts of order 100 in conductivity required by the magnetometer array results. Three tentative suggestions are made concerning tectonics of the region.

Nearly one hundred years ago, Gilbert (23, 24) and other geologic pioneers introduced the idea that much of the seelIling jumble of mountains and valleys in western North America was the result of far different processes than fold mountain systems such as the Appalachians or Alps. After a century of geologic and geophysical investigations in the region, it is now generally accepted that the physiography of the Basin and Range province (Figure 1) is one of sculptured and partially buried fault-bounded blocks that have been produced by the extension of the region during late Cenozoic time. Crustal blocks composed of complexly deformed, diverse pre-Cenozoic rocks and relatively undeformed, predominantly nonmarine volcanic rocks of early and middle Cenozoic age have been variously uplifted, tilted, and dropped along numerous normal faults throughout a broad region from Mexico to Canada-from as far west as California and Oregon to as far east as western Texas (e.g. Cook, 13; Gilluly, 27 ; Thompson, 76). The distribution of late Cenozoic normal faults in the western United States is shown on Figure 2 (note that the regional extent of faulting is somewhat larger than the Basin and Range physiographic province of Figure 1). The recent seismicity (Figure 3) shows that small earthquakes are widespread in what Atwater (5) called a wide soft zone accommodating oblique divergence between the Pacific and North American plates. The net effect of fault movements within this region is a crustal extension oriented roughly WNW-ESE. The actual motion on individual faults is quite variable, however, and appears to be controlled by the orientation of faults with respect to this principal extension (Thompson & Burke, 78). In the northern portion of the region -across Nevada and western

Several hundred xenoliths of Cr-diopside-spinel peridotite from basanitic lavas in the western Grand Canyon along the eastern margin of the Basin and Range Province can be grouped into essentially three distinct textural categories with correlative contrasts in mineral composition. At least two of these textures seem unique to this locality. Xenoliths display varying degrees of depletion in basaltic constituents and last equilibrated between 900 and 1000°C. A few contain coarse exsolution intergrowths of diopside and enstatite whose reconstructed composition has about 15 wt.% CaO corresponding to an original temperature of crystallization of approximately 1400°C. The genetic relation between the three types — if such exists — is uncertain, but overall they indicate a vertically inhomogeneous upper mantle. The variety of textures found worldwide in mantle-derived Cr-spinel peridotite xenoliths of more or less equivalent composition suggests varied and probably complexly superposed histories.

The latest results concerning the depth variation of Q, P- and S-wave velocities obtained by various seismological methods are reviewed with special emphasis on the regional variation. The body-wave travel time and the slope, dT/dΔ, have been inverted to obtain P- and S-wave velocity distributions within the mantle. The P velocity is now believed to be known with an accuracy of ±1% in the mantle below 1000km. In the upper mantle, a considerable difference has been found among different regions. The long-period surface waves have been used to reveal the regionality of the upper mantle. Based upon the surface wave and body-wave results, it is now agreed that the mantle beneath shields has much higher seismic velocities than those beneath ocean and tectonically active regions. At such tectonically active regions as Japan, basin and range province of the western U. S., and the Pacific ocean side of South America, the upper mantle velocities have been found to be significantly lower than the average. Teleseismic P waves are delayed by 1.0 to 1.5 sec at tectonically active regions as compared with shields. The existence of the low-velocity zone for S waves in the sub-oceanic mantle is almost certain. The shape of the low-velocity zone varies markedly beneath continents. The distribution of Q with depth is not known in detail. However, it is almost certain that the value of Q is higher by more than one order of magnitude in the mantle below 1000km than in the mantle above. The average values of Qα (Q for P-waves) and Qβ (Q for S-waves) for the upper mantle range from 100 to 300. Horizontal heterogeneities as large as a few per cent in velocity and one order of magnitude in Q have been found beneath mid-oceanic ridges and island arcs. However, the uniqueness of the structure derived is still questionable.

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.

Microseismicity, composite focal-mechanism solutions, and previously-published focal parameter data are used to determine the current tectonic activity of the prominent zone of seismicity in western Nevada and eastern California, termed the Nevada Seismic Zone. The microseismicity substantially agrees with the historic seismicity and delineates a narrow, major zone of activity that extends from Owens Valley, California, north past Dixie Valley, Nevada. Focal parameters indicate that a regional pattern of NW-SE tension exists for the western Basin and Range and is now producing crustal extension within the Nevada Seismic Zone. An eastward shift of the seismic zone along the Excelsior Mountains and left-lateral strike-slip faulting determined from a composite focal mechanism indicate transform-type faulting between Mono Lake and Pilot Mountain. Based on these results and other data, it is suggested that the Nevada Seismic Zone is caused by the interaction of a westward flow of mantle material beneath the Basin and Range Province with the boundary of the Sierra Nevada batholith.

Data obtained during recent detailed mapping of intrusive areas in the eastern Great Basin combined with an examination and review of 107 mining districts in the Basin and Range Province of the United States containing hydrothermal deposits of Au, Ag, Pb, Zn, Cu, Mo, W, Kg, and U has led to the recognition of significant differences between "productive" intrusives, (those associated with hydrothermal mineralization) and "barren" intrusives, (without significant mineralization). For 86 percent of the deposits of Cretaceous and younger age there is a significant amount of associated intrusive porphyry or aphanitic rock present as large dikes, sills, and breccias with aphanitic matrix or more commonly as stocks or pipes occurring with or without granitoid textured rock. These rock types with few exceptions, are exposed within 2 miles of the ore deposit. The great majority of barren intrusives are of granitoid rocks only, except for the presence of smaller porphyry dikes and sills. Deposits of tungsten show an association preference with granitoid intrusives. These statistics indicate that in finding or developing a new ore deposit of the above metals other than tungsten, it is necessary to have a large intrusive aphanite porphyry existing within 2 miles of the exploration area in order reasonably to expect (for 86% of the cases) the existence of a significant ore body. Because barren intrusive porphyries do exist in the Basin and Range Province, further investigation of the differences and similarities between productive and barren porphyry plutons is contemplated.

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 geographic and hypsometric relations of Rush Valley and Tooele Valley, Utah, are described. The forms of the valley floor and peculiar features of Rush Lake and of the alluvial fans of Rush Valley are interpreted as evidences of the probable integration of the drainage of this valley with that of Tooele Valley, the first stage leading toward the coalescence of the basins. Two examples of capture, in the restricted sense, in other parts of the Basin and Range province are cited. The conclusion seems justified that this stage of the arid cycle can now be established inductively and need rest no longer on a purely deductive basis.

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.

Biotites from several Basin and Range monzonitic stocks have been analyzed for Cu, Pb and Zn. These data have been compiled for biotites from the Tintic, Bingham, Park City-Little Cottonwood, Iron Springs, San Francisco and related districts, the Gold Hill mining district, also the Mineral Range stock, and the Ibapah stock, all in Utah. Similar data have been compiled for the Ely district and the Whitehorse Pass stock in Nevada. The biotites were analyzed spectrochemically on an Applied Research Laboratories, 1.5 m, 24,000 line per in. grating spectrograph. In was used as an internal standard. The F test was used to establish the equivalence the variances of base metal concentrations in biotite from groups of individual stocks or districts. A graphical comparison of geometric means and standard deviations of the base metal content of biotites from the individual stocks and districts was examined and visual estimates of differences and similarities were noted. The comparisons show the following correlations: 1) Each stock or district belongs to its individual trace base-metal concentration population. 2) Contrasting correlations exist between base-metal concentration of biotite and base-metal production in the sampled areas. High Cu content is associated with Cu production, whereas low Pb and Zn content is associated with Pb and Zn production. 3) Biotites from hydrothermally altered stocks contain more Cu and less Pb and Zn than unaltered stocks. 4) Trace base metal content of biotite is an indicator of base metal mining districts in the Basin and Range province.

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

More than 100 new K-Ar dates for volcanic and intrusive rock specimens from the eastern Basin and Bange Province provide numerous tests of the dating technique based on comparisons between samples from the same and sequentially related units and between mineral separates from the same or related samples. Using single samples, Tertiary volcanic units may be dated with a precision (σ) of 4 per cent. Between-sample variance is larger than analytical error. With multiple samples the precision of age determination may reach 1–2 per cent. In order of decreasing suitability (increasing deviation from average dates for multiply dated units) for dating volcanics are: biotite, sanidine, hornblende, plagioclase, pyroxene and whole rock samples, but none of these materials shows serious Ar deficiencies or excesses. Unweathered but hydrated glass gives dates consistently about 10 per cent low. Biotite and Muscovite are suitable for dating plutons; K-feldspar shows small Ar deficiencies hornblende appears to be acceptable, but pyroxene separates may contain extraneous Ar, and are thus considered unreliable. All available dates show a consistent regional pattern. Scattered igneous centers existed in late Eocene-early Oligocene time in the Wasatch region of Utah, in northern Nevada and probably at Gold Hill, Utah. Intense igneous activity occurred in and near east-central Nevada during the Oligocene; activity shifted towards the eastern and southern margins of the Great Basin during Miocene and Pliocene time and gradually decreased in intensity so that at present only sporadic eruption of mafic lavas occurs. In individual areas, dates on intrusive and extrusive igneous rocks are in close agreement and activity is notably episodic (within a period of about 5 million years). Eruption of silicic volcanics is quickly followed by mafic volcanism that marks the end of one of the short episodes. Regionally the short episodes are smeared out into a broad pulse of Tertiary igneous activity between 40 and 5 million years ago.

It has been statistically shown that a great majority of the larger Cretaceous and younger hydrothermal ore deposits of Au, Ag, Cu, Pb, Zn, Mo, Hg, and U in the Basin and Range province are associated with intermediate to acidic intrusive aphanitic or porphyritic rocks of near similar age. Recent work indicates that, except for the Pacific Coast Ranges and the Mother Lode area, similar ore-porphyry relationships apply to deposits over the entire western United States, however, many intrusive porphyries exist that have little or no associated hydrothermal deposits. Some intrusive porphyries, therefore, seem to be genetically related to ore deposits while others are not. These then may conveniently be classified into 2 categories as barren and productive. To aid further the ore finder, an attempt is made here to offer criteria that will serve to distinguish barren from productive porphyries. Among the many suggestive distinctions the following seem to be of most importance. Productive acid porphyry bodies generally are of considerable size and seem to cross cut enclosing rock structures with very little or no disturbance due to intrusive action. On the other hand barren porphyries may be of various sizes but on the whole have a tendency to either be concordant with wall rock structures or, where cross cutting relations exist, show obvious intrusive structural effects such as brecciation, folding, or up arching. Other criteria are tabulated in order of importance.

The Great Basin section of the Basin and Range Physiographic Province of Nevada and adjoining states is a semiarid to arid region that is completely cut off from the sea. Valleys of the Great Basin are partly or completely surrounded by mountains in contrast to the Basin and Range Province of southern Arizona where isolated mountains are completely surrounded by valleys. Valleys completely surrounded by mountains are described as topographically closed whereas valleys that have surface connections with nearby valleys are considered to be topographically open. However, the topographic characteristics of individual valleys are not necessarily indicators of the hydrologic characteristics. The valley fill may be saturated with water to the ground surface or it may be entirely drained, depending upon the position of the controlling outlet. The playa on the valley floor may be dry or wet depending upon the depth to water in the valley. A classification of valleys of the Great Basin is presented, based ...

A program has been written for the University of Nevada's IBM 1620 computer which locates epicenters of seismic events that occur within the Nevada region. The program utilizes readings of P and S waves at up to eight stations in the Basin and Range province, rejects readings for events which are outside the region of interest or events recorded at less than three Nevada stations, establishes trial epicenters for locatable events by means of a table-lookup routine which utilized P-time differences between pairs of stations, and adjusts the trial epicenter by a least-squares procedure based on a sloping-layer crustal model derived from recent U. S. Geological Survey seismic refraction profiles. Epicenters calculated by the program are found to be affected by near-surface structural irregularities in the vicinity of receiving stations, but are not affected either by near-surface features in the source region or direction of the propagation paths with respect to regional structural trends. Traveltime delays calculated for Nevada seismographic stations appear to correlate with Bouguer gravity anomalies at the station sites.

Summary Geomagnetic substorm fields were simultaneously recorded during the summer of 1968 at stations of a two-dimensional array of variometers covering western Texas, New Mexico, Arizona and eastern California. This work formed part of a continuing study of upper mantle conductive and thermal structure under the western United States. Magnetograms and maps of Fourier spectral components of two substorms are used to describe conductive structures in the upper mantle and crust. Conductive structures under the Southern Rockies and Wasatch Fault Belt, discovered in a similar array study in 1967, are shown to continue southward. The Basin and Range Province as a whole overlies a highly conducting (and, by inference, hot) region in the mantle. Conditions are discussed which must be satisfied by any assumed regional field used to isolate and normalize the anomalous fields. Anomalous fields satisfying these conditions are derived from the 1967 array data supported by the 1968 array. Numerical calculations are used to show that the anomalies cannot be associated with conductors of limited thickness within the crust, unless these have improbably high conductivity. Two-dimensional ridges on the surface of a semi-infinite conductor, on the other hand, satisfactorily model the observed fields. The conductive structures can therefore be assigned with some confidence to the upper mantle. The anomalies are well represented by induction in a half-space of conductivity 0.2 (ohm m)−1 with a ridge under the Southern Rockies and a step with superposed ridge under the Wasatch Fault Belt. The depths to the surface of the conductor in our model are 190 km under the Basin and Range Province, 120 km under the Wasatch Fault Belt, 150 km under the Southern Rockies and 350 km under the Colorado Plateau and the Great Plains. Such models are naturally not unique.

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.

The active rift system along the postulated landward extension of the East Pacific Rise into western North America (Heezen, 1960; Menard, 1960; Cook, 1961) consists of several active rift zones which follow belts of seismicity and probable high heat flow and include: (a)the belt of grabens along the eastern margin of the Sierra Nevada, the Walker Lane, and the Churchill Arc; (b)the system of great trenches extending through western Arizona, central Utah, southeastern Idaho, western Wyoming, western Montana, and British Columbia; and (c)the Rio Grande rift belt. The present paper reviews the following additional recent geophysical evidence that defines in more detail the active rift system in the eastern part of the Basin and Range province: 1. (1) In north central Utah, gravity surveys, together with refraction and/ or reflection seismic data along some profiles, indicate that the Wasatch Trench is a northward-trending belt of grabens about 160 miles long and up to 13 miles wide. 2. (2) In south central and southwestern Utah, gravity surveys indicate grabens adjacent to the northward-trending Sevier and Hurricane fault zones, which lie along the rift belt. 3. (3) In New Mexico, several gravity profiles across the Rio Grande rift belt indicate the width and depth of the grabens. 4. (4) Sparse data tentatively indicate high heat flow in the rift belts. The possible mechanism of the Late Cenozoic vulcanism and diastrophism in the Basin and Range province is explained by a postulated mantle convection current (associated with the formation of the Rise) that rose beneath the eastern part of the Basin and Range province (or perhaps the Colorado Plateau) and flowed westward (Cook, 1961). The active rift system is in the region of tension over the upwelling convection currents. To the west, in the region of translation and compression, it is postulated that the westward-moving mantle convection current dragged along the continental crust in the California region. The crustal drag caused a large continental-type block to move westward into the Pacific Ocean about 50–100 km in the California region between the landward projections of the Mendocino and Murray fracture zones, which were probably major lines of weakness existing before the formation of the East Pacific Rise. On the northern and southern margins of the block, respectively, the movement occurred along transcurrent and/or transform faults in: 1. (1) the Cape Mendocino area (Gorda escarpment). 2. (2) the Garlock fault, Big Pine fault, Channel Islands. Transverse Ranges, and Pinto Mountain fault areas. The westward movement of the block, which occurred along transcurrent and/or transform faults associated with these two fracture zones, probably caused: (a) the folding of the Coast Ranges in California along the frontal (i.e., west) segment of the block and (b) the bending of the San Andreas fault zone in two areas along the southern margin of the block. The right-lateral movement of the San Andreas fault zone is explained by a postulated second, northwestward-trending mantle convection current with a velocity gradient increasing southwest; this movement has apparently moved into the western Basin and Range province recently to cause transcurrent faulting there (Billings, 1960), and a change in the direction of the mantle convection currents there is postulated. The concept that a westward-moving continental-type crustal block has overridden the oceanic crust in the Pacific Ocean and that the driving force probably originated in the upper-mantle spreading beneath the Basin and Range province (or perhaps the Colorado Plateau) is fundamental to the interpretation of the Late Cenozoic tectonics in the southwestern United States. If the United States, along with the North American continent, has drifted westward as a consequence of the sea-floor spreading along the Mid-Atlantic Ridge and overriden the East Pacific Rise, as postulated by Vine (1966), the westward movement of the above-mentioned crustal block would constitute about 50–100 km of horizontal displacement in addition to the “normal” westward drift of the North American continent as a whole.

of the Rio Grande where, according to the earliest Spanish accounts (Winship 1896), there were over forty villages in seven districts: Tiguex, with twelve pueblos located in the vicinity of modern Albuquerque and probably represented by the surviving Tiwa towns of Isleta and Sandia; seven pueblos in the Galisteo and along the eastern foothills of the Sandia Mountains; the Keres district with seven pueblos; Jemez, also with seven pueblos; three villages at Aguas Calientes, exact location unknown; Yunque-yunque, probably corresponding to San Juan, one of the present Tewa villages; Picuris; and Taos. Closely related to these groups, but situated in the adjacent Pecos drainage, was the pueblo of Cicuye, or Pecos, the easternmost occupied pueblo (Figure 1). As an archeological region, the northern Rio Grande may be defined as a roughly rectangular area extending from a line through Isleta in the south, and the Colorado line on the north; and from the Canadian River on the east, to the Rio Puerco and Rio Chama on the west (Figure 1). Physiographically this area is complex, containing features of the Basin and Range, Southern Rocky Mountain, and Colorado Plateau provinces. The aboriginal occupation was confined largely to the broad alluvial valley found within the Rio Grande Depression, a tectonic unit belonging to the Basin and Range province, and the adjacent tributary streams. The Rio Grande Depression extends northward well up into southern Colorado, separating the southern Rockies into two prongs: an eastern prong-the Sangre de Cristo Range, a steep anticlinal uplift, and a western prong-the Nacimiento-Brazos-Conejos-San Juan system. In part, the structure of the western prong is obscured by extensive volcanics (Northrop in Colbert and Northrop 1950: 41). The higher elevations (above 7,500 feet) were not permanently occupied, so far as is presently known, but were utilized for hunting. The Upper Sonoran life zone embraces almost all of the occupied area. It is characterized by extensive open country of grama grass and sagebrush, and forested areas of pinyon and juniper. Precipitation is meager-averaging less than twenty inches in much of the area-and specialized farming techniques, either floodwater farming or irrigation, are required for the successful practice of agriculture.

The amount of trace Pb in potash feldspars can be correlated with the Pb ore deposits associated with igneous rocks. The Pb content of K feldspars separated from several quartz monzonite intrusives of the Basin and Range province has been determined spectrochemically. Samples were collected from the Tintic, West Mountain (Bingham), Park City-Little Cottonwood, and Iron Springs mining districts in Utah, and from the Robinson mining district and the Whitehorse Pass area in Nevada.

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.

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2000 stations in the western United States as a part of the Vela Uniform program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The velocity of P waves in the upper mantle ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province, and, in general, the velocity tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The thickness of the crust tends to be large or small as the P wave velocity (and presumably the density) in the upper mantle is large or small. Within the Basin and Range province, the thickness of the crust seems to vary directly with regional altitude above sea level. Evidence that rocks of intermediate P wave velocity exist in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as from secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper-mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.

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.

Introduction The Basin-and-Range physiographic province, as outlined by a committee of the Association of American Geographers and the Physiographic Committee of the U. S. Geological Survey,[2][1] extends in the United States from southern Oregon to trans-Pecos Texas, and contains not less than 125 closed drainage basins—that is, basins which have no surface outlets for the water that falls upon them. The mountainous rims of these basins, with their great relief and remarkable exposures of geologic formations and structures, have, from the beginning of explorations in the region, compelled the attention of geologists. In contrast, the interior depressions of the basins, mantled by recent sediments and with surface features that are hardly discernible except by an experienced observer, have received relatively little attention and are commonly regarded as lacking in geologic interest. However, the serious work that has been done in these interior depressions, or basin valleys, has shown that they . . . [1]: #fn-2

Combined radioactivity and heat flow measurements in pluionie rocks at 38 localities in the United States define three heat flow provinces; the eastern United States, the Sierra Nevada, and a zone of high heat flow in the western United States which includes the Basin and Range province. In each of these provinces heat flow ( Q ) and heat production ( A ) are related by an equation of the form Q = a + bA . The simplest interpretation of this linear relation is that the radioactivity measured at the surface is constant from the surface to depth b but varies from place to place. Thus the fraction of heat flow from the lower crust and upper mantle, a , remains constant within a heat flow province while the variable upper crustal radioactivity generates the variable heat flow observed at the surface. In the eastern United States b = 7.5 km and a = 0.79 cal/cm 2 sec, in the Sierra Nevada b = 10.1 km and a = 0.40 cal/cm 2 sec, and in the Basin and Range province b = 9.4 km and a = 1.4 cal/cm 2 sec. The line characteristic of the eastern United States may have broad applicability to stable portions of continents and thus be considered the reference curve for normal continental heat flow. The similarity of all the slopes indicates that most local variability of heat flow is due to sources in the uppermost 7–11 km of the earth's crust, and that the contribution from the lower crust and upper mantle is quite uniform over large regions. The intercept values can be used to infer the proportion of heat flow from the mantle and to map provinces with different mantle heat flow. These heat flow provinces correlate closely with surface geological provinces.

Spring Creek Cave, a single-component, habitation site in the Big Horn Basin near Ten Sleep, Wyoming, yielded an assemblage of stone artifacts diagnostic of the Late Middle Prehistoric period in context with a considerable variety of hitherto undescribed perishable items. Charcoal produced a radiocarbon date of A.D. 225 ? 200. The Spring Creek material considerably amplifies present knowledge of Late Middle period technology and economy in the Northwestern Plains and provides opportunities for comparison of Late Middle period perishable items with similar items in Basketmaker assemblages of the Southwest and in other assemblages from variou.s parts of the Basin-Plateau region. S PRING CREEK CAVE (48 WA 1) is a dry cave inside the bend of the Big Horn Mountains in northern Wyoming. It has produced a single level of occupation with material diagnostic of the Late Middle Prehistoric period. The Big Horn Basin is a syncline about 50 mi. wide and 90 mi. long that connects with the Northwestern Plains through a narrow gap to the north in Montana. Except for this gap, the basin is completely surrounded by mountains. To the east are the Pryor Mountains and the Big Horn Mountains which connect with the Owl Creek Mountains to the south. The Carter and Absaroka mountains form the western border. The entire Big Horn Basin is drained by the Big Horn River which, before entering the basin, is known as the Wind River. This river flows northward to the Yellowstone River in Montana. Although the Big Horn Basin is considered an enclave of the Northwestern Plains, conditions are reminiscent of the Basin and Range Province further to the southwest. Rainfall is usually less than 7 in. annually at lower elevations. Great stretches of alkaline soil support mainly greasewood, shadscales, saltbushes, rab, bit brush, and small sagebrush. In areas of better soil there is a sparse grass cover with some juniper and yucca. It is a typical badland topography relieved by many fertile river valleys that lie along live streams marked by stands of cottonwoods and willows. Except for the narrow gap to the north, one approaches mountains in every direction. Rainfall increases rapidly with elevation and results in related changes in floira. The mountain front is dissected by deep, narrow canyons, many of which contain live streams and provide an environment that contrasts sharply with the nearby badlands.

INTRODUCTION Ajo lies in the Papago country of southwestern Arizona, in the Sonoran desert section1 of the Basin and Range province (Fig. 1). The region, which displays a wide variety of topographic forms, offers unusual opportunity for study to those interested in the physiography of arid regions because the United States Geological Survey has completed topographic maps, one of the Ajo quadrangle on the scale of 1: 48,000, and one of a few square miles in the mining area near Ajo on the scale of 1: 12,000. The Ajo quadrangle includes the small mountain masses of the Little Ajo Mountains, Childs Mountain, and part of the Batamote Mountains. Much of the area is occupied by desert plains, of which the largest is the Valley of the Ajo. Altitudes range from 1,150 to 3,200 feet. Local relief is moderate and only exceptionally reaches 1,000 feet in a square mile. The climate . . .

The extant reptilian fauna of the Columbian Plateau and the northern half of the Basin and Range Province lacks turtles. Their absence appears to be of relatively recent occurrence, for turtles persist in the surrounding areas and nave been found in Plio-Pleistocene deposits, Glenn Ferry Formation, of this region. The latest faunal list for the Glenn Ferry Formation (Taylor, 1966, p. 75) includes Kinosternon sp. and Pseudemy idahoensis . Through the effort of Dr. C. W. Hibbard and his field parties, a large collection of chelonian shell fragments has been amassed from the vicinity of Hagerman, Twin Falls County, Idaho. Unless indicated otherwise, the specimens reported herein are from the Hagerman area.

The ages and distribution of porphyry copper deposits of the southern Basin and Range Province suggest an origin influenced by a mantle hot spot as this part of the North American plate moved N35–40°W at 3.57 ± 0.65 cm/yr during the interval of the Laramide Orogeny 72 to 52 my ago. This motion coupled with that of the hot spot interpretation of the Corner Seamounts region of the North Atlantic during the same time interval suggests that the North American plate rotated clockwise at an average rate of 0.35 degrees per million years. The apparent pole of rotation is located near 50°N latitude and 64°W longitude in the region of Anticosti Island in the Gulf of St. Lawrence. The apparent diameter of the hot spot is 450 km. Northwesterly motion and clockwise rotation of the North American plate are compatible with existing concepts of plate tectonics, paleomagnetism, and paleoclimatology. The development of early Tertiary blankets of supergene enrichment and their partial destruction during the late Tertiary can be explained by a change from a warm and wet climate during the early Tertiary to a cooler and drier climate during the late Tertiary as this part of the North American plate moved northwesterly from a more tropical climate into a more temperate and arid zone. Orographic rain shadows produced by middle and late Tertiary mountain building in southern California were undoubtedly influencial in enhancing late Tertiary aridity.

For many years the terms orogeny and mountain building have been used so loosely that they no longer convey definite meanings. Because of this lack of precision, hypotheses of diastrophism are often based on false premises. Folding and thrusting, often referred to as tectogenesis or orogeny, is best displayed by stratified rocks. Some geologists believe that folds and thrusts are due to vertical movements accompanied by lateral spreading, that the strata are stretched an amount commensurate with the intensity of the folding, and that the opposite sides of the sedimentary packet do not move toward each other. Most geologists, however, agree that during folding and thrusting the strata are shortened and that the two opposite sides of the sedimentary packet move toward each other by an amount commensurate with the intensity of the folding. But it is not clear to what extent the entire crust in the deformed belt is shortened. In one extreme view, the two opposite ends of the deformed belt do not move toward each other, and the folds and thrusts result from gravity sliding. At the other extreme, the entire crust is believed to be shortened to the same extent as the sedimentary strata. An excellent example of broad vertical movements (epeirogenesis) that has never been sufficiently emphasized is in eastern North America, involving the Appalachians, Coastal Plain, and Continental Shelf. During the Mesozoic and Cenozoic the Fall-Line surface was depressed as much as 20,000 feet beneath the Continental Shelf, whereas it was uplifted at least 8000 feet over the Appalachians. The northeastern two-thirds of the Appalachian-Ouachita belt has been going up since the Jurassic, whereas the southwestern third has been going down. Thus the present Appalachians are unrelated to the late Paleozoic folding. Certain metamorphic minerals—such as kyanite—or certain metamorphic assemblages—such as jadeite and quartz—form at confining pressures found only at depths of tens of miles, implying tremendous erosion wherever such minerals or assemblages are exposed. Broad vertical movements, accompanied by high-angle faulting, are illustrated by the fault-block mountains of the Basin and Range Province and by such features as the Rhine graben and the Rift Valleys of Africa. Although the nature of the displacement along the San Andreas fault has been known for more than 50 years, only in recent decades have other large strike-slip faults been recognized. But it is now the fad to assign all kinds of faults and even nonexistent faults to the strike-slip category. In recent years seismologists have emphasized the importance of strike-slip faults. The author suggests that in the Fairview Peak-Dixie Valley, Nevada, earthquakes there is evidence that strike-slip movements are invading an area previously characterized by Basin-Range structure. The geologic record indicates that large strike-slip faults have been distinctly subordinate to folding, broad vertical movements, and broad vertical movements accompanied by high-angle faulting. Possible displacement of the crust or the entire earth relative to the axis of rotation has been emphasized in recent years. Continental drift, if it occurs, is one such type of movement. Slipping of the entire crust over the mantle is another. Much research has been accomplished in paleomagnetism in recent years, but the data are too scanty and conflicting to permit any definite conclusions. Among the possible causes of diastrophism are (1) contraction of the earth, (2) convection currents, (3) formation of large pockets of magma, (4) sialic material leaking out of the mantle, (5) conversion of sial to mantle by change of low-pressure minerals to high-pressure minerals, or the reverse, and (6) serpentinization or deserpentinization of the upper part of the mantle. Mountain building is merely one manifestation of vertical movements of the crust. In the past mountain building has been erroneously considered by many to be primarily the result of folding and thrusting. Certainly many modern ranges are the result of vertical uplift unrelated to folding. Many such uplifts are accompanied by high-angle faulting to produce fault-block mountains. Blocks caught between strike-slip faults may be squeezed upward if the blocks move toward each other. Mountainous uplifts have also resulted from folding and thrusting. Conversely, in some folded belts erosion appears to have kept pace with the rise of the folds so that no mountains developed.

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.

The variation of shear velocity with depth in the upper mantle for the Basin and Range province of western North America has been studied with direct measurements of dT/dΔ for S waves in the distance range 14° < Δ < 40°. Three orthogonal components of digital data were used and onset times were determined using the product of the horizontal radial and vertical components of motion and particle motion diagrams. A linear LRSM array in Arizona was used for the measurement of dT/dΔ. An S-wave velocity distribution is derived, compatible with P-wave velocity models for the same region. The derived model consists of a thin lid zone of shear velocity 4.5 km/sec overlying a low-velocity zone and a change in velocity gradient at a depth of 160 km. Two regions of high-velocity gradient are located at depths beginning at 360 km and 620 km.

Magnetic time‐variations between Tucson, Arizona and Sweetwater, Texas indicate that a zone of high electrical conductivity underlies the southwestern United States. The interpretation of this zone by Schmucker as a rise of the isotherms in the upper mantle is supported by six heat flow observations along the line of the geomagnetic profile. These and other observations indicate a high but variable heat flow in the Basin and Range Province which contrasts strongly with the uniform values of [Formula: see text] reported for the Texas Foreland. The width of this high heat flow anomaly, which may extend across the entire Basin and Range Province, suggests anomalously high temperatures in the upper mantle. This interpretation is further supported by magnetotelluric data between Phoenix, Arizona and Roswell, New Mexico and by the low seismic [Formula: see text] velocity and negative gravity anomaly. It is suggested that the “anomalous mantle” may be related to the tectonic evolution of the western United States and the late Cenozoic fault system.

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.

The triangular highland known as the Llano de Albuquerque lies in Bernalillo and Valencia counties, New Mexico, between the Rio Grande and the Rio Puerco. Westward it breaks off in a low escarpment called the Ceja del Rio Puerco, which in part forms the western boundary of the Basin and Range Province. The northern 30 miles of the Ceja is a fault-line scarp which marks a series of nearly north-south en echelon faults between the late Tertiary Santa Fe formation and the older rocks of the Plateau Province. Part I.-The Santa Fe consists of fine-grained, alluvial fan deposits. Part of the material was derived from areas of volcanic rocks, but the major portion of it came from the sedimentary rocks of the Plateau Province to the northwest. The boundary of the basin of deposition lay some 10 miles or more northwestward. Ten miles east of this area the Santa Fe consists of river gravel, indicating that the fan deposits were laid down in the western portion of a basin containing a through-flowing river comparabl...

Summary Magnetic field time variations were observed in September 1967, with a two-dimensional array of 42 three-component variometers between latitudes 36" and 43" N and longitudes 101" W and 116" W. Fourier analysis of a polar substorm and of a storm shows that the former has a smooth spectrum and the latter a complex spectrum with many maxima. Upper mantle conductivity structure can be seen qualitatively in the original variograms, but is far more sharply defined in maps of Fourier spectral component amplitudes and phases. A ridge of high conductivity runs at a depth no greater than 200 km under the Southern Rocky Mountains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. A strong conductivity anomaly runs north-south along the Wasatch Front through central Utah, and indicates the presence of an upwelling of highly conductive material at depth no greater than 120 km along the edge of a step structure which brings the conductive mantle to shallower depth under the Basin and Range Province than under the Colorado Plateau. Long-period maps from the storm suggest a rise in the conductive mantle between the northsouth structures, from the Colorado Plateau southward to the Basin and Range. The daily variation shows the conductivity structures and indicates their great extent in depth. The geomagnetic deep sounding anomalies are found to be in excellent agreement with existing heat flow data, and this supports correlation of electrical conductivity with temperature. There is also good correlation with the available seismic velocity information for the upper mantle. 1. Introdaction Geophysical observations of several kinds indicate that the upper mantle of the Earth under North America is laterally inhomogeneous. Upper mantle seismic velocities of 8.0 km s-' or larger are characteristic of the eastern United States and the Great Plains Province, while velocities decrease to values of 7.9 km s-' or lower west of the Rocky Mountains (Herrin & Taggart 1962). A similar pattern is shown by travel-time anomalies of seismic waves at vertical incidence. P and S waves arrive early at stations in the eastern United States; late arrivals are predominant in the western United States (Cleary & Hales 1966; Doyle & Hales 1967; Hemn & Taggart 1968). As the differences between the P travel-time residuals and the gravity anomalies in the central and western U.S. cannot be explained by the Birch (1961) relation between velocity and density, Hales & Doyle (1967) suggested that tempera

Eight deep resistivity probes have been made along a traverse extending from southwest Colorado to southern California to determine the electrical properties of the deep crust. The dipole method for measuring resistivity was used, and dipole spacings as great as 50 to 100 km were obtained at each survey location. In the Colorado Plateaus province, the electrical basement usually does not coincide with the top of the Precambrian crystalline basement, but rather, corresponds with the top of the carbonate rocks of Paleozoic age. Because of the large electrical conductance in the sedimentary column, resistant basement rock was detected at only three of the five sounding locations in the Colorado Plateaus province. At these three locations, the minimum possible basement resistivity was 2000 to 6000 ohm‐m. Electrical basement was detected at the survey locations in the Basin and Range province. The minimum possible basement resistivity was 600 to 1000 ohm‐m. A survey near Barstow, California, indicates that the resistant portion of the crust may be 9 to 10 km thick and may have a resistivity of 600 to 700 ohm‐m. A conductive zone underlies the resistant portion of the crust.

A gravity survey covering 250 square miles and including the Walnut Gulch watershed at Tombstone, Arizona has been conducted. The watershed is one of the experimental areas of the Southwest Watershed Research Center, Agricultural Research Service in Tucson, Arizona. The gravity survey method was selected as the geophysical method that would give a regional picture of the subsurface geology before making extensive surveys by the seismic. refraction method. Basically, the gravity survey method detects and measures variations in the earth's gravitational force. These variations are associated with changes in rock and alluvium density near the surface. Many geologic structures of interest in watershed ground-water hydrology cause disturbances in the normal density distribution which give rise to anomalies. The geology of the watershed is typical of Basin and Range province and contains deep basin fill deposits surrounded on the east, south, and southwest by igneous intrusives, volcanics, and sedimentary rocks. Concealed border faults may have a decided effect on the hydrology of the watershed. Four base stations were established and 360 gravity stations occupied by a Worden Educator* gravimeter loaned from the Geophysics Laboratory of the University of Arizona. Bench marks and large scale topographic maps served as control. Raw field data were reduced to the simple Bouguer anomaly values through the use of a computer program. The gravity survey reveals a 16-mgal. gravity low over the east-central portion of the watershed. This low is interpreted as indicating that about 3200 feet of low density alluvium underlies the area at this point. Gravity highs are associated with mountain ranges and igneous plugs where the more dense rocks occur.

Large hydrothermal deposits of halloysite clay occur in the Terraced Hills, Washoe County, Nevada and similar bodies probably are present elsewhere in the Basin and Range province. The host rock, an andesitic tuff, is underlain and overlain by volcanic flows; all these rocks are late Miocene to Pliocene in age. The clay bodies are composed mostly of halloysite with some iron oxides, variable amounts of feldspar and quartz, and locally some montmorillonite. Commonly all the pyroclastic unit is altered to halloysite material. In one locality, however, the halloysite body is restricted to the upper part and it is in sharp contact with underlying, partly montmorillonitized tuff. The contact of a clay body with the overlying basalt is distinct. Generally some halloysite is present in the lower part of the basalt and montmorillonite occurs in both materials near their contact. The solutions that altered the tuff were generated during volcanism, rose along high-angle faults, and were restricted to the permeable and otherwise favorable vitric tuff by the capping of relatively impermeable basalt.

The largest rift zone of Europe and Asia is located in the region of Lake Baikal. In 1968–1970 deep seismic measurements were carried out along a number of profiles with a total length of about 2000 km within the rift zone and in the adjacent parts of the Siberian platform and the region of the Baikal Mountains. These investigations were of a reconnaissance nature, and therefore the point sounding method was used. A low-velocity region for compressional waves (7.6–7.8 km/sec) has been found and could be traced over a large area in the upper parts of the mantle. The width of this anomalous zone is 200–400 km. The Baikal rift lies in its northwestern part. Within the studied part of the Siberian platform the thickness of the earth's crust is 37–39 km, while in the rift zone it is 36 km, and further to the southeast the crust-mantle boundary lies at a depth of 45–46 km. The Baikal rift proper is bounded in the northwest by a deep fracture zone and does not seem to be associated with any significant “root” or “antiroot” in the relief of the Mohorovicic discontinuity. The reduced compressional velocity in the upper parts of the mantle beneath the Baikal zone is considered to correspond to the same phenomena found under the mid-oceanic ridges and the extended rift system in the Basin and Range province of North America. The Baikal rift in the narrow sense of the word lies over the northwestern edge of the anomalous mantle region. This asymmetric position seems to be its main peculiarity.

Several noteworthy examples of magnetic evidence of strike‐slip faults have been published in recent years, including examples in the Pacific Ocean (Vacquier et al. 1961) and the western Atlantic Ocean (Drake and Woodward, 1963). Aeromagnetic data indicate vertical and horizontal movement along the southern edge of the Hueco Mountains, El Paso and Hudspeth Counties, Texas. The amount of slip is small compared to the published examples, but displacement is of interest because it occurs at the junction of two large, vastly different geologic provinces, the Basin and Range province to the north and the thrust belt of northern Mexico to the south.

Travel times of P waves from 25 earthquakes to stations in North America, in the distance range 32° to 100°, have been analyzed by a least squares technique similar to the “time term” method of refraction seismology. In this way, deviations from the Jeffreys-Bullen tables have been separated from the effects of station and source. Differences from J-B times were calculated at 2° intervals, and a smooth curve was fitted to the results with an average scatter of 0.07 sec. The station residuals show a marked regional trend, with arrivals up to a second early in the central United States, and up to a second late in the Basin and Range Province. On the assumption that the derived travel time curve is universal in character, station residuals were calculated for stations not in the United States. Although this part of the analysis is less exact, consistent regional trends are apparent. P arrivals are early in shield areas, late in areas of recent tectonic uplift, and early along parts of the Pacific margin where deep-focus earthquakes are common. Station residuals for stations in southern California agree well with those obtained in a study by Press and Biehler. Carder9s data from nuclear explosions in the Pacific show broad agreement with the present findings, but discrepancies exist which require further investigation.

The deformed beds of the Llano de Albuquerque are beveled and overlain by a cover of gravel, caliche, soil, and wind-blown sand 5-70 feet thick. Topographically the Llano de Albuquerque is a plain which slopes gently eastward toward the Rio Grande and is surrounded by escarpments and erosional slopes; it is a remnant of a once widespread erosion surface made up of coalescing pediments graded to the Rio Grande. At the time of formation of this surface, here named Ortiz, the Rio Grande had a grade about 500 feet above the present. On it were poured the great basaltic flows of the Mount Taylor region. The Ortiz surface has been dissected and largely destroyed by several successive lowerings of the grade of the Rio Grande. In the first period of dissection following Ortiz time the Rio Puerco gained its north-south course by piracy of certain western tributaries of the Rio Grande flowing in a southeasterly direction out of the Plateau Province. The vulcanism and erosion of later cycles are briefly summarized.

Summary The compressional velocity distribution of the upper mantle beneath the Basin and Range province is interpreted from a large group of shortperiod, long-range recordings of nuclear explosions at the Nevada Test Site. The interpretation includes a low-velocity channel characterized by a top at a depth of 50 ± 10 km and a velocity increase in the depth range from 120 to 155 ± 10 km, with a discontinuity in velocity near the bottom of this depth range. The compressional velocity beneath the Basin and Range province apparently increases very slowly at depths between about 170 and 290 km, as inferred from weak arrivals along a 8.6±0.1 km s−1 refraction branch for distances of about 1520 km and greater. Sharp increases of velocity were found for depths of 315 ± 15 km and 422± 15 km; the velocity contrast at the deeper discontinuity is about twice as large as that near 315 km. Compressional velocities beneath the central United States are inferred to be similar to those beneath the Basin and Range province at depths greater than 170 km although small velocity differences may possibly persist to depths of the order of 400 km.

Joint sets, veins, dikes and faults, comparison of structural trends in productive and nonproductive stocks, Basin and Range Province