J. M.G. Glen

• PhD
• United States Geological Survey

The West Flank FORGE (Frontier Observatory for Research in Geothermal Energy) site is located immediately west and outside of the Coso geothermal field, eastern California. Coso is a fluid-dominated, high temperature geothermal system that has been producing power continuously since 1987. The reservoir is composed of highly faulted, fractured and h...

The relative contributions of tectonic and magmatic processes to continental rifting are highly variable. Magnetic, gravity, and seismic reflection data from Surprise Valley, California, in the northwest Basin and Range, reveal an intrabasin, fault-controlled, ~10-m-thick dike at a depth of ~150 m, providing an excellent example of the interplay be...

A remarkable characteristic of earthquakes is their clustering in time and space, displaying their self-similarity. It remains to be tested if natural and induced earthquakes share the same behavior. We study natural and induced earthquakes comparatively in the same tectonic setting at the Coso Geothermal Field. Covering the pre-and co-production p...

Increased levels of seismicity coinciding with injection of reservoir fluids have prompted interest in methods to distinguish induced from natural seismicity. Discrimination between induced and natural seismicity is especially difficult in areas that have high levels of natural seismicity, such as the geothermal fields at the Salton Sea and Coso, b...

We use seismic, tilt, Lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geyser's eruption cycle. Previously we described measurements and analyses associated with the geyser's erupting jet dynamics [Karlstrom et al., 2013]. Here w...

Yellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high-resolution airborne and ground magnetic surveys and measurements of remanent and...

Faults and fractures play an important role in the circulation of geothermal fluids in the crust, and the nature of that role varies according to structural setting and state of stress. As a result, detailed geologic and geophysical mapping that relates thermal springs to known structural features is essential to modeling geothermal systems. Publis...

Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geosphere www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or...

HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic histo...

HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic histo...

The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America. The primary goal of this project is to evaluate geothermal potential in three distinct settings: (1) Kimama site: inferred high sub-aquifer geothermal gradient associated with t...

We performed continuous acoustic measurements during four days at Lone Star Geyser, Yellowstone National Park, USA. The microphone was located at 10 meters from the geyser's cone, and the acoustic signal was sampled at 1000 Hz. The 3-hour-long eruptive cycle at Lone Star Geyser contains several water fountaining episodes followed by the main erupti...

Geysers are intermittently discharging hot springs that are driven by steam and non-condensable gas. They provide unique opportunities to study multiphase eruption processes and the geophysical signals they induce. In September 2010 we carried out a four-day experiment at Lone Star Geyser in Yellowstone National Park. The geyser is located about 5...

Yellowstone National Park (YNP) hosts a very large hydrothermal system with over 10,000 thermal features. Although hydrothermal alteration in YNP has been extensively studied with field observations, remote-sensing imagery, and core drilling, the volume and geometry of hydrothermal systems at depth remain poorly constrained. Magnetic surveys can he...

New field, laboratory, and modeling results from the Sheep Creek transition zone suggest that a directional change with an average rate of at least 0.1 °/day occurred during a 15 month long interval toward the end of this 15.2 Ma reverse-to-normal geomagnetic polarity switch. The evidence for this brief episode of rapid transitional field change co...

Geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava-flow records. By splicing together paleomagnetic results from earlier studies at Steens Mountain with those from three newly studied sections of Oregon Plateau flood basalts at Catlow Peak and Poker Jim Ridge 70–90 km to the s...

Ground magnetic data contain information, not pre-sent in aeromagnetic data, which may be useful for precisely mapping near-surface faults and contacts, as well as constraining or aiding interpretation of other geophysical methods. However, collecting ground magnetic data on foot is labor-intensive and is therefore limited to small surveys. In this...

The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America, and an area with the highest calculated geothermal gradients. This makes the SRP one of the potentially highest producing geothermal districts in the United States. Elevated hea...

Yellowstone National Park (YNP) hosts a very large hydrothermal system with over 10,000 thermal features. Hydrothermal alteration in YNP has been mapped with field observations and remote-sensing imagery, but these methods can only detect alteration at the ground surface. Magnetic surveys are useful for detecting buried hydrothermal alteration as d...

The distribution of intrusive rocks throughout the Great Basin is important because many plutons are associated with base and precious metal mineral deposits, and may provide insights on regional magmatism and tectonism. Combined information on their physical properties and geophysical signatures will allow improvements on their inferred horizontal...

The Late-Miocene Stanislaus Group of lavas and ignimbrites were deposited across a region of diverse tectonic rates and style. Distributed east of Sonora Pass across the Walker Lane Belt into Nevada, and extending as far west as Knights Ferry along the Stanislaus River in the central Sierran foothills, the Stanislaus Group's distinctive lithologic...

Since 2006 a collaborative effort between Stanford University, the USGS, and UC Berkeley has maintained five ultra-low frequency electromagnetic (UFLEM) stations along the San Andreas Fault system. The standard site is equipped with three orthogonal coil magnetometers and two sets of orthogonal 100m electrode pairs, provides data in the 0.01- 20 Hz...

Surprise Valley, located in the northeast corner of California, constitutes the westernmost basin of the Basin and Range Province (BRP) and acts as a transition zone between the unextended Cenozoic volcanic rocks of the Modoc Plateau to the west and the extended BRP to the east. Previous seismic experiments undertaken in Surprise Valley to examine...

A new paleomagnetic result from a lava flow with a distinctive, two-part remanence reinforces the controversial hypothesis that geomagnetic change during a polarity reversal can be much faster than normal. The 3.9-m-thick lava (``Flow 20'') is exposed in the Sheep Creek Range (north central Nevada) and was erupted during a reverse-to-normal (R-N) g...

The Columbia River Basalt Group (CRBG) eruptions have a well-defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale. 40Ar/39Ar ages are presented from lavas erupted in the R0 through N1 magnetozones of the CRBG and in the transition between R0 and N0. Four ages from transitionally magne...

The Amphitheater Mountains synform on the southern flank of the Alaska Range in south-central Alaska provides one of the most complete sections of a Large Igneous Province worldwide, revealing, in near continuous section: basal mafic and ultramafic sill complexes, associated mafic and ultramafic rocks, lower submarine lavas, and overlying subaerial...

Surprise Valley in northeastern California offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late Miocene to recent activity in isolation from other major normal fault-bound basins. Seismic velocity and potential field modeling help determine the nature of basin fill and identify intra-basin...

The Columbia River Basalt Group (CRBG) eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale. Fifteen 40Ar/39Ar ages from lavas erupted in the R0 through R1 magnetozones of the CRBG, in conjunction with the geomagnetic polarity time scales (GPTS) of Lourens et al....

The southern flank of the Alaska Range in south central Alaska exposes a thick, fresh sequence of Middle to Late Triassic Nikolai Greenstone. At least 5 ultramafic intrusive complexes and numerous gabbroic sills of similar age intrude Pennsylvanian to Triassic volcanogenic and sedimentary rocks below the Nikolai. The 2 largest and best exposed ultr...

New audiomagnetotelluric (AMT), gravity, and magnetic data were collected in Surprise Valley, northwestern Basin and Range, in order to investigate the role that the Lake City Fault Zone (LCFZ) may play in controlling geothermal circulation in the area. Surprise Valley hosts an extensional geothermal system currently undergoing exploration for deve...

Stanford-USGS and UC Berkeley have been maintaining five ultra-low frequency electromagnetic (ULFEM) stations along the San Andreas Fault system over a multi-year period. The standard site is equipped with two sets of orthogonal 100-meter electrodes, as well as a set of 3 orthogonal magnetometers. These sites are intended to monitor and record ULFE...

The Blue Mountain geothermal field, located about 35 km northwest of Winnemucca, Nevada, is situated along a prominent crustal-scale fracture interpreted from total-intensity aeromagnetic and gravity data. Aeromagnetic data indicate that this feature is related to the intrusion of mafic dikes, similar to the Northern Nevada Rift (Zoback et al., 199...

We have revisited the problem of mapping depth to the Curie temperature isotherm from magnetic anomalies in an attempt to provide a measure of crustal temperatures in the western United States. Such methods are based on the estimation of the depth to the bottom of magnetic sources, which is assumed to correspond to the temperature at which rocks lo...

We have studied three Permian–Triassic (PT) localities from China as part of a combined magnetostratigraphic, 40Ar/39Ar and U–Pb radioisotopic, and biostratigraphic study aimed at resolving the temporal relations between terrestrial and marine records across the Permo-Triassic boundary, as well as the rate of the biotic recovery in the Early Triass...

New magnetic and seismic surveys investigate a subsurface structure in eastern Surprise Valley, NE California, that may have significance to both the geothermal system and the mode of accommodation of extension across the valley. Surprise Valley is located in the transition zone between unextended Cenozoic volcanic rocks of the Modoc Plateau to the...

We revisit the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm in the Great Basin, assuming that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose their spontaneous magnetization (e.g., 580°C for magnetite). The Cenozoic evolution of the Great Basin is a complex product...

Detailed thermal demagnetization of samples from three vertical profiles through a 3.9 m thick lava flow provide evidence of very rapid field change during a geomagnetic polarity transition at 15.6 Ma. The lava flow, referred to here as "Flow 20", occurs near the top of a stack of ~50 lavas in the Sheep Creek Range (north central Nevada) that recor...

As part of the PBO (Plate Boundary Observatory) mission we have installed three ultra-low frequency electromagnetic (ULF-EM) recording sites in northern California. We hope to use this data to better understand physical processes associated with earthquakes and to find evidence for the generation of subsurface electromagnetic fields, including the...

The Steens Basalt, now considered part of the Columbia River Basalt Group (CRBG), contains the earliest eruptions of this magmatic episode. Lava flows of the Steens Basalt cover about 50,000 km2 of the Oregon Plateau in sections up to 1000 m thick. The large number of continuously exposed, quickly erupted lava flows (some sections contain over 200...

We consider the origin and character of a prominent large-scale geophysical feature in north-central Nevada that is coincident with the western margin of the northern Nevada rift—a mid-Miocene rift that includes mafi c dike swarms and associated volcanic rocks expressed by a NNW-striking magnetic anomaly. The geophysical feature also cor- relates w...

The central section of the Surprise Valley, NE California, in the westernmost section of the Basin and Range province contains a geothermal system related to extensional tectonics. Most geophysical research related to this geothermal system has focused primarily on the Lake City hot springs. Our work includes several other hot springs within the va...

Combined geological and geophysical investigations are used to characterize intra-basin and basin-bounding faults, constrain basin geometry, study fault interactions, and ultimately to identify areas favorable to hydrothermal flow in the geothermal system in Surprise Valley, California. We utilize high-resolution gravity and ground-magnetometer dat...

The backarc region of the Cascadia forearc in Oregon and northern California displays a northeast-trending crustal fabric evident in regional gravity data, seismicity, fault distribution, Cretaceous plutonism, Mesozoic accreted terranes, and Quaternary volcanism. These associations involve rocks of diverse ages and origins and indicate that long-li...

We have revisited the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm and tested our new methodology in an attempt to provide an independent estimate of heat flow in the Great Basin. Such methods typically assume that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose th...

One of the best records of magnetic field behavior during a geomagnetic polarity reversal is found in the Steens basalts of the Oregon Plateau. Lavas preserving transitional field directions were first discovered near the top of a 1000 m flow-on-flow section at Steens Mountain, Oregon. Other Steens lavas of equivalent age (16.5 Ma) recording transi...

Electromagnetic (EM) signals are one of the most commonly claimed precursors to earthquakes. The most cited of these proposed EM events is the report of ultra-low frequency (ULF) EM anomalies prior to the 10/17/1989 magnitude 7.1 Loma Prieta earthquake (Fraser-Smith et al., 1990, GRL 17, 1465) recorded at Corralitos (7 km from the epicenter). Altho...

We have discovered new evidence relating to the hypothesis of very rapid jumps in direction during the 16.6 Ma reversal recorded by Steens Basalt flows. The benchmark record of the polarity transition is punctuated by three large directional gaps (Mankinen et al., 1985; Prevot et al., 1985). An important question is whether any of the gaps mark imp...

The best known record of the earth's magnetic field behavior during a geomagnetic polarity reversal preserved in volcanic rock is the reverse to normal (R-N) polarity reversal found in the Steens Basalts of SE Oregon. At three locations where reverse to normal sections are found (Steens Mountain, Catlow Peak, and Poker Jim Ridge), four high precisi...

1] The northwestern margin of the Basin and Range Province is characterized by a transition from low-magnitude ($20%) extension in northwestern Nevada to relatively unextended volcanic plateaus in northeastern California. Seismic-velocity and potential-field modeling provides new control on the Mesozoic-to-present tectonic evolution of this poorly...

A three-dimensional (3D) geologic model of part of the northern Nevada rift encompassing the Beowawe geothermal system was developed from a series of two-dimensional (2D) geologic and geophysical models. The 3D model was constrained by local geophysical, geologic, and drill-hole information and integrates geologic and tectonic interpretations for t...

Recent gravity and aeromagnetic investigations of the Talkeetna Mountains of south-central Alaska (61.5-63.75°N, 145 151°W) were undertaken to study the region's framework geophysics and to reinterpret crustal structures and composition. Aeromagnetic data for this study were compiled from 13 available regional- and localscale surveys. Over 400 new...

Recent investigations of the Talkeetna Mountains in south-central Alaska were undertaken to study the region's framework geophysics and to reinterpret structures and crustal composition. Potential field (gravity and magnetic) and magnetotelluric (MT) data were collected along northwest-trending profiles as part of the U.S. Geological Survey's Talke...

A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25° north and extend downward to depths between 20 and 25 km. These reflections reveal what...

Three sections of Steens type basaltic lavas were erupted on the Oregon Plateau during a reverse to normal (R-N) switch of the earth's magnetic field. These sections are almost certain to have been erupted during the same reversal, considering the similarity of field behavior, stratigraphic relationships, and rock types. We obtained four 40Ar/ 39Ar...

A new three-dimensional (3D) geologic model of part of north-central Nevada, including parts of Battle Mountain, Shoshone Mountains, and the Sheep Creek Range, was developed from a series of two-dimensional (2D) geophysical models. The D model was constrained by geologic and drill-hole information, and calculated depth to pre-Cenozoic basement. The...

New results demonstrate strong lateral inhomogeneity in the paleomagnetic and rock magnetic properties of a 15.6 my old lava flow from the Sheep Creek Range (north central Nevada) that was erupted during a geomagnetic polarity transition. The 4 to 5 m thick flow lies stratigraphically between two flows with remanence directions (D=349, I=-48 and D=...

Pliocene lavas and sediments of Wild Horse Mesa in the Coso Range, CA exhibit clockwise vertical-axis rotation of fault-bounded blocks. This indicates localization of one strand of the Eastern California shear zone/Walker Lane Belt within a large-scale, transtensional, dextral, releasing stepover. We measured rotations paleomagnetically relative to...

The fluviolacustrine sedimentary sequence of the Chemeron Formation exposed in the Barsemoi River drainage, Tugen Hills, Kenya, contains a package of five successive diatomite/fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the Central Kenya Rift. The overwhelming abundance in the diatomite of pla...

We present an overview of the regional geophysical setting of the Yellowstone Hotspot track along the Snake River Plain (SRP) and surrounding regions in support of a proposed scientific drilling program for one of the world's youngest, best-preserved intra-continental hotspots. The preliminary scientific drilling plan is to core a series of interme...

A simple relation is generally presumed to exist between an igneous rock's anisotropy of magnetic susceptibility (AMS) fabric and magma flow. In such cases (termed " normal "), the magnetic fabric displays a magnetic foliation pole, given by the minimum AMS axis, that lies perpendicular to the flow plane (i.e., dike walls, or lava flow top or botto...

The Steens Basalts of the Columbia River Basalt Group erupted about 16-17 Ma covering southeastern Oregon, northern Nevada, and southwest Idaho with up to 600m of basaltic lavas. Later Basin and Range faulting created exposures containing up to 200 lava flows. At Steens Mountain these lavas were discovered to have recorded the most detailed volcani...

We demonstrate an efficient method of rigorously separating difficult-to-distinguish lavas into eruptive units based on paleomagnetic remanence direction and rapid X-ray fluorescence spectroscopy (XRF) for Rb, Sr, Y, Zr, and Nb of polished paleomagnetic core samples (called PC XRF). Combined use of paleomagnetic remanence and PC XRF for lava finger...

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M w 7.9 Denali fault earthquake. These data and aftershock seis-micity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska...

As a necessary part of the PBO (Plate Boundary Observatory) mission we are developing a network of three ultra-low frequency electromagnetic (ULF-EM) recording sites in northern California to understand earthquake physics and to detect pre-seismic transients if such exist. Each site will have three orthogonal magnetometers and duplicate sets of ort...

Seismic-velocity and potential-field modeling along an E-W profile provides new constraints on the crustal structure of the northwestern Basin and Range transition zone. Our data span the change in tectonic setting from low-magnitude (

Topography and geophysical data suggest that the path of Yellowstone hotspot (YSHS) volcanism was controlled by pre-existing crustal structures associated with the Snake River Plain (SRP), and that Great Basin (GB) extension is intimately tied to hotspot melting. From its point of inception (Glen and Ponce, 2002), the YSHS migrated south to the sou...

Since the end of the 1980's, studies on the anisotropy of magnetic susceptibility (AMS) of dikes and lavas have largely assumed a simple relation between the principal AMS directions and magma flow. In the so-called "normal" magnetic fabric case, the magnetic foliation mimics the flow plane. The AMS minimum -- Kmin, which is normal to the foliation...

The lower Steens Basalt flows of the Oregon Plateau appears to have been erupted earlier than the oldest Columbia River Basalt flows (Hooper et al., 2002), although some contest this conclusion (Baksi, 2004). The extent and timing of the Steens Basalt eruptions are not well understood and knowledge gained in this area will enhance understanding of...

A global Magnetic Petrology Database (MPDB) was developed and continues to be updated at NASA/Goddard Space Flight Center. The purpose of this database is to provide the geomagnetic community with a comprehensive and user-friendly method of accessing magnetic petrology data via the Internet for a more realistic interpretation of satellite (as well...

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska...

Geomagnetic reversals could be far more complex than even the best, most detailed paleomagnetic record in hand. Sequences of lava flows can give accurate spot readings of the field but yield records that are necessarily shot full of holes, whereas sedimentary recordings smooth temporal variations of the field and may also be subject to hiatuses and...

Among the most prominent physiographic features of southern Alaska are a series of nested arcuate lineations, including the Denali fault, that parallel the concave-southward southern coastline of the state. These features are generally interpreted as major dextral shear zones that formed in the Late Cretaceous to early Tertiary in response to stres...

It has been suggested that SE Oregon, NW Nevada and SW Idaho were quickly covered by Steens-type basalts at the beginning of Yellowstone hotspot volcanism. Only Steens Mountain has been studied in detail, but exploratory paleomagnetic sampling at six other sites suggests that the R-N polarity reversal recorded at Steens is common throughout the are...

Mankinen and Gromme (1982) hypothesized that two flows from the region were erupted during the Cobb Mtn event since they yielded dates of 1.07+/-0.14 Ma and 1.07+/-0.12 Ma (Duffield et al., 1980). Given advances in the magnetic polarity timescale, it is now unclear whether to assign these flows to the Jaramillo subchron (0.99-1.07 Ma), the Punaruu...

We present results from a study of a partial magnetic polarity transition record from a 9.7 Ma 40Ar/39Ar dated volcanic section on the island of Gomera (Canary Islands, Spain). The record provides results that bear on the debate over whether long-term persistent features of field behaviour exist during transitions. The sampled section, containing m...

Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single-domain magnetite, consistent wit...

The younger of two closely spaced palaeomagnetic excursions at Pringle Falls, Oregon, is recorded in lacustrine silts that crop out in Long Valley, California. Assigned an age of about 220 000 years, the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 °S latitude east of South America before returning to the...

During middle Miocene time, western North America was subject to flood-basalt volcanism, dike-swarm injection, and broad-scale fracturing and folding of the crust. We propose a simple model to account for these events and for a regional pattern of geologic and geophysical features. Aeromagnetic maps reveal some of the most important elements of thi...

Oriented magnetite inclusions with two suborthogonal elongation directions in the (010) plane of clinopyroxene (CPX) are the dominant source of highly stable remanent magnetization in gabbros of the Messum Complex, Namibia. The inclusions are subparallel to the X ("X" inclusions) and Z ("Z" inclusions) crystallographic axes in the host CPX crystals...

Among the most prominent physiographic features of southern Alaska are a series of nested arcuate lineations, including the Denali fault, that parallel the convex-southward coastline of the state. These features are generally interpreted as major dextral shear zones that respond to stresses imposed on the western edge of North America by transcurre...

Aeromagnetic and filtered magnetic maps of the western U.S. reveal at least 3 and up to 6 narrow, arcuate anomalies that form a radial pattern, the geometry of which suggests a unifying process. The most notable of these anomalies is the northern Nevada rift (NNRe), first identified as a high-amplitude, linear aeromagnetic anomaly extending from th...

The first appearance of Hindeodus parvus (Kozur & Pjatakova) at the Permian-Triassic (P-T) GSSP level (base of Bed 27c) at Meishan is here confirmed. Hin-deodus changxingensis Wang occurs from Beds 26 to 29 at Meishan and appears to be restricted to the narrow boundary interval immediately above the main mass extinction level in Bed 25. It is sugge...

This new study of the Gauss/Matuyama transition from Searles Lake, California conjoined with other records from the western United States, provides interesting insights into the structure of the reversing magnetic field. The present study employs improved measurement and data reduction techniques, multiple parallel strings of samples, and a finer s...

More than 33 m of 2.5 Ma sediment from Searles Lake, California was studied in order to construct a record of secular variation (SV) across the Gauss/Matuyama (G/M) normal-to-reverse polarity transition. The behavior of the field preceding and following the reversal is considered here, while in a companion paper [Glen et al., this issue] the detail...

Paleointensity experiments were carried out with the Coe and Thellier methods on Early Cretaceous basalt (133 My old) from the Paraná basin in southern Brazil. Paleointensity estimates could be obtained from only six lava flows: three of normal and three of reversed polarity, of the 71 sampled. Moreover, the quality of the determinations is fairly...

The attraction of hexagonal closed packed (hcp) iron to a magnet at 16.9 gigapascals and 261 degrees centigrade suggests that hcp iron is either paramagnetic or ferromagnetic with susceptibilities from 0.15 to 0.001 and magnetizations from 1800 to 15 amperes per meter. If dominant in Earth's inner core, paramagnetic hcp iron could stabilize the geo...

The Paraná-Etendeka igneous province is one of the largest flood volcanic provinces in the world; peak magmatic activity at 132 Ma is believed to have occurred about 5 m.y. before the birth of south Atlantic sea floor and development of rift basins along the Brazilian coastal margin. Anisotropy of magnetic susceptibility (AMS) measurements on 283 s...

Paleomagnetic measurements performed on samples from Owens Lake cores OL-92-1, 2, and 3 reveal that the composite core (OL-92), extending to a depth of 323 m, spans nearly the past 800 k.y. The Matuyama/Brunhes transition is identified near the base of the composite section yielding an average sedimentation rate of ∼40 cm/k.y. The record displays s...

Detailed 40Ar/39Ar laser step-heating analyses of mineral separates from five volcanic units in Namibia and Angola and five intrusions in Namibia yield important geochronological data for the Etendeka igneous province. Ten plateau dates on plagioclase, hornblende, and biotite between 131.7 ± 0.7 and 132.3 ± 0.7 Ma were obtained, and a late syenite...

We report on a high resolution transition record from Searles Valley, California that appears not to be hampered by problems of smoothing or recording breakdown, and, unlike many other sedimentary records, defines a path unconfined in longitude. A prominent feature of the record is the steady oscillation of the pole along a swath stretching from we...

The Paraná-Etendeka flood volcanic event produced ∼1.5 x 106 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40Ar/39Ar data combined with earlier paleomagnetic results indicate that Paraná flood volcanism in southern Brazil began at 133 ± 1 mi...

The birth of the Yellowstone hotspot in middle Miocene time was marked by extensive flood basalt volcanism. Prominent aeromagnetic anomalies (referred to collectively as the Northern Nevada rifts), extending hundreds of kilometers across Nevada, are thought to represent dike swarms injected at the time of flood volcanism. Until now, however, dikes...