W. Scott Baldridge

• Ph. D. California Institute of Technology 1979
• Researcher at Los Alamos National Laboratory

Seismic and gravity data collected by the Summer of Applied Geophysical Experience (SAGE), along with energy-industry seismic data, well data, and geologic maps, go into the construction of geologic cross sections transecting the Rio Grande rift in the area of the Española basin. These cross sections reveal several key structures within the basin i...

Energy and natural resources are crucial to the sustainability of worldwide economies, security, and overall well-being. However, the future workforce in the energy and natural-resources sector is at risk, and meeting the challenges of this dwindling workforce requires well-educated geoscientists in exploration and applied geophysics and related ge...

We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a westdipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo-Santa Clara-Pajarito fault system, a collection of no...

Experts inform that the Summer of Applied Geophysical Experience (SAGE) will complete its 30th field season. SAGE is a unique educational program that combines teaching and research as a partnership between universities, industry; government agencies, and professional societies. It includes a four-week period based in Santa Fe, New Mexico, and one-...

Experts inform that the Summer of Applied Geophysical Experience (SAGE) will complete its 30th field season. SAGE is a unique educational program that combines teaching and research as a partnership between universities, industry; government agencies, and professional societies. It includes a four-week period based in Santa Fe, New Mexico, and one-...

New seismic refraction/wide-angle reflection and CMP seismic reflection profiles were recorded by the SAGE (Summer of Applied Geophysical Experience) program across the eastern boundary of the Santo Domingo basin, Rio Grande rift, New Mexico in 2010 and 2011. In addition, new gravity data were acquired along the seismic lines, in the area of the ea...

SAGE (Summer of Applied Geophysical Experience) is a unique program of education and research in geophysical field methods for undergraduate and graduate students from any university and for professionals. The core program is held for 4 weeks each summer in New Mexico and for an additional week in the following academic year in San Diego for U.S. u...

The La Ristra experiment consisted of a northwest trending linear deployment of 72 broadband seismic stations from West Texas to western Utah with 15 to 20 km spacing. The resulting 1400 km seismic line provides unprecedented images of crust and mantle along a cross section that crosses the Colorado Plateau from near Mount Taylor in the east to the...

Seismic reflection and refraction data acquired in the Rio Grande rift near Santa Fe, New Mexico, in 2009 and 2010 by the SAGE (Summer of Applied Geophysical Experience) program imaged the La Bajada fault (LBF) and strata offset across the associated, perpendicular Budagher fault (BF). The LBF is a major basin-bounding normal fault, offset down to...

After maintaining elevations near sea level for over 500 million years, the Colorado Plateau (CP) has a present average elevation of 2 km. We compute new receiver function images from the first dense seismic transect to cross the plateau that reveal a central CP crustal thickness of 42–50 km thinning to 30–35 km at the CP margins. Isostatic calcula...

The Colorado Plateau in the southwestern U.S. is characterized by a bowl-shaped high elevation, a large gradient in seismic wave velocity across its margins, and relatively low lithospheric seismic wave velocities in comparison to the lithosphere of the Great Plains. Late Neogene-Quaternary magmatism is found on the margins of the Plateau, which ha...

Over the past 4years, controlled field experiments have taken place in Bozeman, MT, USA where pure CO2 has been released at known rates and depths to quantify the detection limits of various monitoring tools and techniques for the use of CO2 seepage detection. As part of this study, new tools engineered at Los Alamos National Laboratory were deploy...

After maintaining elevations near sea level for hundreds of millions years, the Colorado Plateau (CP) of the southwestern United States has a present average elevation of 2 km. However, the sources of buoyant support for this high elevation have long been unclear. We apply receiver function joint velocity analysis and imaging to data from the LA RI...

Latest Cretaceous and Tertiary magmatism in the southwestern U.S. initially recorded subduction of the Farallon plate beneath the North American. With foundering of the Farallon plate and initiation of lithospheric extension in the middle Tertiary, magmatism gave way in the late Tertiary to normal intraplate genesis of mainly basaltic rocks from li...

Structural connections (accommodation zones) between basins in continental rifts control distribution and facies of syntectonic sediments and magmatic rocks, and therefore resources such as groundwater, hydrocarbons, and minerals. These zones are commonly buried beneath rift-filling sediments, thus geophysical methods are of paramount importance in...

The increasing world demand and record-high costs for energy and mineral resources, along with the attendant environmental and climate concerns, have escalated the need for trained geophysicists to unprecedented levels. This is not only a national need; it's a critical global need. As Earth scientists and educators we must seriously ask if our geop...

Shear-wave splitting measurements from SKS and SKKS phases show fast polarization azimuths that are subparallel to North American absolute plate motion within the central Rio Grande Rift (RGR) and Colorado Plateau (CP) through to the western rim of the CP, with anisotropy beneath the CP and central RGR showing a remarkably consistent pattern with a...

Teleseismic traveltime data are inverted for mantle Vp and Vs variations beneath a 1400 km long line of broadband seismometers extending from eastern New Mexico to western Utah. The model spans 600 km beneath the moho with resolution of ~50 km. Inversions show a sharp, large-magnitude velocity contrast across the Colorado Plateau-Great Basin transi...

The Table Mountain Quadrangle lies on the southeast margin of the Colorado Plateau, about 30 km northwest of Magdalena, New Mexico. Rocks exposed on the quadrangle range in age from Late Triassic to Pliocene, and unconsolidated deposits of Quaternary age are widespread. The quadrangle is transected by the Rio Salado, a major east-flowing tributary...

Shear-wave splitting measurements are determined using data collected from LA RISTRA 1 and 1.5 (Colorado pLAteau RIo Grande Rift/Great Plains Seismic TRAnsect) to study the origin of seismic anisotropy in the mantle beneath the Colorado Plateau and the Colorado Plateau-Great Basin transition. Results show that, on the average, the fast polarization...

The Volcanology Summer Field Course, taught jointly by volcanologists from the University of New Mexico and Los Alamos National Laboratory, has instructed over 140 undergraduate and graduate students from 15 countries since 1992. The course consists of nine graded field exercises conducted in diverse volcanic rocks of the Miocene to Quaternary-age...

Finite volume calculations of magma flow in dikes are presented that tie observational markers available during effusive eruptions to the stability of the event. We use a new model for heat and mass transfer of a melt with a temperature dependent viscosity and the potential to undergo phase change. Magma driving pressures are coupled with dike geom...

We analyse active-experiment seismic data obtained by the 1993 Jemez Tomography Experiment (JTEX) programme to elucidate the heterogeneous structure of the Jemez volcanic field, which is located at the boundary between the Colorado Plateau and the Rio Grande Rift. Using a single isotropic scattering assumption, we first calculate the envelope Green...

We conducted a time-domain airborne electromagnetic (AEM) survey of part of the semiarid Pajarito Plateau of northern New Mexico to determine depths and lateral extent of perched aquifers in the vadose zone and depths and pathways of infiltration to the regional aquifer. The electrical resistivity of the plateau ranged over three orders of magnitud...

Investigation and characterization of groundwater resources beneath the Pajarito Plateau of northern New Mexico is important because of contamination resulting from historical operations of the Los Alamos National Laboratory and recognition that the Plateau is an area of recharge to the regional groundwater reservoir. Multidisciplinary efforts incl...

The transport of basaltic magma through brittle and elastic crust occurs primarily via planar dikes. Host rock composition and temperature, dike width, advective vigor, and magma supply rate all affect dike evolution. Previous models addressed these effects, typically for uniform host rock compositions appropriate to specific case studies. We expan...

SAGE, a field-based educational program in applied geophysical methods has been an REU site for 16 years and completed its 23rd year of operation in July 2005. SAGE teaches the major geophysical exploration methods (including seismics, gravity, magnetics, and electromagnetics) and applies them to the solution of specific local and regional geologic...

The seismic structure of the crust and upper mantle of the southwestern United States is examined using receiver functions calculated from teleseismic arrivals recorded in the Colorado Plateau–Rio Grande Rift–Great Plains Seismic Transect (LA RISTRA) experiment. We apply receiver function estimation and filtering methods developed by Wilson and Ast...

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere...

Magmatic compositions in large, long-lived volcanic fields typically evolve through a variety of petrogenetic processes. Determining their relative roles may be difficult because (1) multiple processes may act simultaneously at a single volcano, (2) different processes may produce similar compositional results, and (3) individual processes may diff...

Volcanism of Pliocene to Quaternary age in New Mexico is similar in volume, eruptive style, and compositional range to that of much of the southwestern U. S. during the same period of time. Typically, volcanism comprises isolated small volcanic fields consisting of monogenetic cinder cones, domes, and small shield cones, together with associated la...

Cambridge Core - Sedimentology and Stratigraphy - Geology of the American Southwest - by W. Scott Baldridge

Surface wave phase velocities from 29 earthquakes are used to map the shear velocity structure to ∼350 km depth across the 950-km-long Rio Grande Rift Seismic Transect Experiment (LA RISTRA) seismic array in the southwest United States. Events from a range of back azimuths minimize the effects of multipathing. The resulting velocity model reveals a...

We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deploye...

High mountains and extensive middle to late Cenozoic magmatism rim the margins of the Colorado Plateau (CP) resulting in a bowl-shaped morphology, with the interior of the CP being relatively unaffected by Laramide compression and Cenozoic extension and magmatism. We present new passive seismic imaging and modeling results from the RISTRA 1.0-1.5 t...

Results from the Colorado Plateau-Rio Grande Rift-Great Plains seismic transect (LA RISTRA) experiment are consistent with a pure shear extension mechanism for the Rio Grande rift (RGR). LA RISTRA was a 950 km-long PASSCAL broadband seismic line with approximately 18 km station spacing deployed during 1999-2001 along a great circle from Lake Powell...

This paper reviews the significant advances made in research into the Jemez volcanic field since 1980. The fields of study include: tectonic history; stratigraphy; volcanic petrology; volcanic activity; subsurface structure; caldera resurgence; geophysical evidence for a magma body; and hydrothermal activity. The geochronology of volcanic, hydrothe...

El Porticito is the approximately 70-m high eroded remnant of a 7.08 ± 0.25 Ma volcanic vent located in the transition zone between the Colorado Plateau and the Basin and Range province in west-central New Mexico. It is distinguished by the strikingly white, vertical and horizontal, centimeter- to meter-scale, magmatic veins that cut across the bla...

Shear-wave splitting parameters (fast polarization direction and delay time) are determined using data from LA RISTRA (Colorado pLAteau RIo Grande Rift/Great Plains Seismic TRAnsect), a deployment of broadband seismometers extending from the Great Plains, across the Rio Grande Rift and the Jemez Lineament, to the Colorado Plateau. Results show that...

We present new images of the P and S wave seismic structure of the upper mantle beneath a transect extending from the Colorado Plateau across the Rio Grande Rift to the Great Plains of western Texas. The upper mantle structure was determined using travel times from the LA RISTRA passive seismic array deployed from July 1999 to May 2001. Large seism...

The RISTRA array extends from the Colorado Plateau across the Rio Grande Rift into the Great Plains. We explore the state of the uppermost mantle beneath these three distinct geologic provinces to understand how the lithosphere and asthenosphere are related to the tectonic evolution of these regions. 57 three-component broadband seismometers of the...

Background noise power spectral density (PSD) estimates for 54 PASS- CAL Colorado Plateau/Rio Grande Rift/Great Plains Seismic Transect (LA RISTRA) stations were computed using data from 1999 to 2000. At long periods (0.01-0.1 Hz), typical vertical noise levels are approximately 12 dB higher than the nearby Global Seismic Network (GSN) borehole sta...

We examine the seismic structure of the lithosphere in the southwestern United States using receiver functions calculated from teleseismic arrivals recorded in the LA RISTRA (Colorado PLAteau, Rio Grande RIft, Great Plains Seismic TRAnsect) experiment. LA RISTRA was a 950 km-long PASSCAL broadband seismic line with approximately 18 km station spaci...

We have inverted P and S wave teleseismic delay times recorded by the LA RISTRA (Colorado PLateau, Rio Grande Rift, Great Plains Seismic TRansect) array. The RISTRA array consisted of 57 broad band seismic stations most of which were deployed in a line extending 950 km from Lake Powell, Utah to Pecos, Texas. P, PKP, S, and SKS travel time delays we...

We computed a reflectivity image for the Earth beneath the Jemez volcanic field, New Mexico, using a novel adaptation of petroleum exploration seismic imaging. This image was obtained by applying the Kirchhoff wave field imaging method to digitally recorded teleseismic data. The volume imaged has a lateral extent of 30 km, extends to 45 km depth, a...

The UNM-LANL Program in Volcanology was a vision of Wolf Elston in the late 1980s. Finally established in mid-1992, the program takes advantage of the extensive volcanic record preserved in northern New Mexico, and of the unique expertise and exceptional research facilities existing at the two institutions. Courses are directed toward upper divisio...

We have inverted P and S wave teleseismic delay times recorded by the LA RISTRA (Colorado PLateau, Rio Grande RIft, Great Plaines Seismic TRAnsect) array. The RISTRA array consisted of 57 broadband seismic stations most of which were deployed in a line extending 950 km from Lake Powell Utah to Pecos Texas. P, PKP, S, and SKS travel time delays were...

Project LA RISTRA (Colorado Plateau Rio Grande Rift Great Plains Seismic Transect) has completed one year of deployment of a broadband seismic array across the central Rio Grande Rift. The NW-SE trending, 950 km long linear array extends approximately from Lake Powell, Utah to Pecos, Texas. We have calculated receiver functions from teleseismic arr...

Summer of Applied Geophysical Experience (SAGE) is a unique educational program in geophysics where close interaction among students, teachers and industry visitors take place. SAGE enables undergraduates and graduate students to share the excitement of modern field geophysical research and learning. They collect data with modern equipment and proc...

Measurements of {sup 238}U-{sup 230}Th-{sup 226}Ra and {sup 235}U-{sup 231}Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that ({sup 230}Th/{sup 238}U) {gt} 1 indicates that garnet is required as a residual phase in the...

``The second AGU Excellence in Geophysical Education Award was presented to the faculty of the Summer of Applied Geophysical Experience (SAGE): Scott Baldridge, Shawn Biehler, Larry Braile, John Ferguson, Bernard Gilpin, and George Jiracek. The persistence and commitment of this group has provided the geophysical community with a superb educational...

Measurements of ²³⁸U-²³°Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (²³°Th/²³⁸U) < 1 indicates that garnet is required as a residual phase in the magma sources for al...

New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative P wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km. The three principle features of our model for Valles caldera are: (1) ne...

Investigations conducted during the 1997 Summer of Applied Geophysical Experience (SAGE) field course at one site at Los Alamos National Laboratory (LANL) successfully delineated a waste disposal trench dug in the 1940s. The survey, which was popular with the students, provided them with important experience in “real world” geophysical problems and...

After a lengthy battle against opposition by specific interest groups, researchers with the Jemez Tomography Experiment (JTEX) investigated the crustal structure beneath a large, continental silicic magmatic system in the Jemez Mountains of New Mexico (Figure 1) in 1995. The seismic component involved both active and passive sources as well as a mo...

Six different techniques were used to delineate a 40 year old trench boundary at Los Alamos National Laboratory. Data from historical aerial photographs, a magnetic gradient survey, airborne multispectral and thermal infra-red imagery, seismic refraction, DC resistivity, and total field magnetometry were utilized in this process. Each data set indi...

Continental mafic volcanic rocks commonly contain partially resorbed grains of quartz surrounded by reaction rims of granular pyroxene. δ180 values for single grains and for grain-composites of quartz from a lherzolite-bearing flow from Mount Taylor, NM, are 8.18–10.54‰ and 9.77–10.68‰, respectively, indicating that the quartz is crustally derived....

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic...

The thermal history of the lower crust and upper mantle of the Colorado Plateau region is reconstructed on the basis of Nd and Sr isotopes in minerals and whole rock xenoliths hosted by Tertiary minette and kimberlite. The mineral data (garnet and clinopyroxene) indicate that lower crustal granulite and amphibolite (equilibration depth ≈25 km; equi...

This chapter describes the formation of extensional stresses in the Earth's lithosphere that is a global tectonic phenomenon, occurring in all plate settings. When initiated in continental lithosphere, extension give rise to a range of tectonic features called “continental rifts.” Continental rifts preserve the most complete, and possibly the only,...

This chapter focuses on the Oslo rift formed in response to a combination of regional stretching caused by dextral strike-slip movements along the Sorgenfrei-Tornquist Zone and a positive temperature anomaly in the asthenosphere. The earliest manifestation of the Oslo rifting event was the formation of a shallow depression in late Carboniferous tim...

Seismic and gravity data, acquired by the SAGE program over the past twelve years, are used to define the geometry of the Espanola basin and the extent of pre-Tertiary sedimentary rocks. The Paleozoic and Mesozoic units have been thinned and removed during Laramide uplift in an area now obscured by the younger rift basin. The Espanola basin is gene...

This chapter discusses the Rio Grande rift, which is a part of a broad region of the western United States, including the Basin and Range province, that has undergone lithospheric thinning and crustal extension during the middle to late Cenozoic. The present extensional setting of the rift is generally related to plate boundary forces acting along...

This chapter deals with the Southern Oklahoma aulacogen, which is a major structural element of interior North America. As a rift, this feature involved extensive volcanism and modification of the crust. When reactivated by later events at the continental margin, vertical displacements alone are approximately 15 km. This aulacogen provides the stru...

Proton-induced X-ray emission (PIXE) microanalysis is shown to be capable of semiquantitative to quantitative standardless determination of trace element abundances in coal macerals and sulfides using an internal normalization protocol. PIXE spectra are quantified using a least-squares data reduction method by normalization to S determined by elect...

Teleseismic P-wave relative arrival-time data, collected from a temporary array during the 1993 Jemez Tomography Experiment (JTEX), have been inverted to image velocity anomalies beneath the Valles caldera in northern New Mexico. Instruments were deployed in two 30-km-long profiles, one of 8 and one of 9 stations. These profiles crossed the caldera...

Measurements of uranium/thorium and samarium/neodymium isotopes and concentrations in a suite of Hawaiian basalts show that uranium/thorium fractionation varies systematically with samarium/neodymium fractionation and major-element composition; these correlations can be understood in terms of simple batch melting models with a garnet-bearing perido...

This chapter discusses the principal petrologic techniques that have been applied to igneous rocks in rifts. The objective is to provide the necessary background for the comparative analysis of rifts. Petrological and geochemical studies of rift-associated igneous rocks can provide information on pressure, temperature, compositions of crust and man...

Characterization of a well-exposed boundary of the Rio Grande rift was undertaken in order to document structures associated with the initial stages of crustal extension. Combined interpretations of seismic reflection, seismic refraction, gravity, and geologic data acquired along a profile perpendicular to this boundary define the geometry of fault...

The geometry of major basin-bounding faults in the central Rio Grande rift indicates that extension is characterized by overlapping and nested half grabens linked together by accommodation zones and bounded by tilted block uplifts. The Albuquerque-Belen Basin is composed of two separate, opposite-facing, structurally complex basins linked by an acc...

The crustal contributions to 12 early Oligocene to Pleistocene rhyolite systems located throughout the Cordillera was estimated. Crustal contributions to large-volume rhyolite systems decrease from the Oligocene to the Miocene. Rhyolite systems younger than 20 Ma are dominated by mantle components. Regional cooling of the lower crust, which progres...

Nd-Sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in Tertiary minettes and kimberlitic diatremes of the Colorado Plateau. The objective was to establish the vertical distribution of mantle extraction ages for an undisturbed Precambrian crustal province characterized by normal thickness...

The tectonomagmatic history of the Oslo Rift may be subdivided into 5 main periods. Stage 1 (> 300 Ma): development of a shallow depression. Stage 2 (300-295 Ma): roughly simultaneous onset of widespread basalt volcanism (B1) and vertical movement along NNW-SSE- to N-S-trending faults. Stage 3 (295-275 Ma): main rifting period, accompanied by volca...

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal tem...

Last June and July, for the ninth consecutive summer, geophysics students from universities across the United States and around the world converged on Santa Fe, N.Mex., to participate in a unique geophysical field program (Figure 1). Among a variety of activities, students surveyed, laid cables, buried electrodes, and gathered and interpreted a wea...

The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and eru...

Extension associated with the initial (early Miocene) stages of opening of the Red Sea resulted in intrusion of a widespread system of dikes and smaller bodies, primarily along the northeastern margin of the Red Sea/Gulf of Suez axis. Dikes, some up to more than 800 km in length, were emplaced dominantly parallel to the Red Sea/Gulf of Suez from th...

We document a systematic inverse relationship between the volume of erupted volcanic rocks associated with continental rifts and the seismic compressional wave velocity contrast across the Moho. This relationship is interpreted as evidence that magmatism significantly modifies the composition(s) of the lower crust (and upper mantle) during rifting...

Chinese translation of K. H. Olsen et al. (1987) originally published in Tectonophysics, v. 143, p. 119-139, 1987. The Rio Grande rift of the southwestern United States is one of the world's principal continental rift systems. It extends as a series of asymmetrical grabens from central Colorado, through New Mexico, to Presidio, Texas, and Chihuah...

Since they are two of the prominent continental rifts which are active today, the Kenya and Rio Grande rifts have been the subject of many recent studies. There are many gaps in our knowledge, but the data available make a comparative analysis worthwhile. Although they are part of much larger extensional regimes, these rifts are of similar dimensio...

A model of the subsurface structure of the eastern part of the Espanola Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refract...

Long before the thunderheads towered high above the nearby mountain peaks, the crisp, clear morning air in northern New Mexico reverberated with the sounds of a unique geophysical program. Voice transmitted by radio signaled vibroseis operators to move to the next flag, while freshly acquired seismic records were greeted with excited debate. It was...

In this paper the authors present geologic mapping, K-Ar chronology, major and trace element data, mineral chemistry, and Nd, Sr, and O isotopic data for volcanic rocks of the Mount Taylor volcanic field (MTVF). The MTVF lies on the tectonic boundary between the Basin and Range province and the southeastern Colorado Plateau and is dominated by Moun...

The Rio Grande rift is one of the major late Cenozoic continental rifts of the world, sharing most geophysical, geochemical, and geological characteristics with other rifts. Cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western North American margin: Paleocene-Eocene: Laramide primarily amag...

Presents in several sections the broad plate-tectonic setting of volcanoes and volcanic fields of the western U. S.; and locations and brief descriptions of the Neogene Taos, Ocate, and Lucero volcanic fields of New Mexico. Many others are represented.

Valles caldera has become world famous as an example of a resurgent caldera and as the location of a high-temperature geothermal system of volcanic origin (Smith and Bailey, 1968; Dondanville, 1978). Although the caldera and its modern hydrothermal system result from events that have happened during the last 1.12 Ma, the Jemez volcanic field has ex...

This feature of the SW USA is a region where the lithosphere is being pulled apart. Due to changes in the nature of the plate boundary, the Rio Grande rift area entered a phase of extension, known as the "basin and range event', about 10 million years ago. The anatomy of the rift, and the structure and extension of the lithosphere are described wit...