bluuebunny/crossref_metadata_2025 huggingface dataset:
Velocity bounds from statistical estimates of τ (p) and X (p)
Authors: Stark Philip B. , Parker Robert L.
Published: February 2008

We present a new technique for constructing the narrowest corridor containing all velocity profiles consistent with a finite collection of τ(p) data and their statistical uncertainties. Earlier methods for constructing such bounds treat the confidence interval for each τ datum as a strict interval within which the true value might lie with equal probability, but this interpretation is incompatible with the estimation procedure used on the original travel time observations. The new approach, based upon quadratic programming (QP), shares the advantages of the linear programming (LP) solution: it can invert τ(p) and X(p) data concurrently; it permits the incorporation of constraints on the radial derivative of velocity for spherical earth models; and theoretical results about convergence and optimality can be obtained for the method. We compare P velocity bounds for the core obtained by QP and LP. The models produced by LP predict data values at the ends of the confidence intervals; these values are unlikely according to the proper statistical dstribution of errors. For this reason the LP velocity bounds can be wider than those given by QP, which takes better account of the statistics. Sometimes, however, the LP bounds are more restrictive because LP never permits the predictions of the models to lie outside the confidence intervals even though occasional excursions are expected. The QP bounds grow narrower at lower levels of confidence, but the corridors at 95% and 99.9% are virtually indistinguishable: The data must be improved substantially to make a significant change in the velocity bounds.
Trace element analyses of spheres from the melt zone of the Greenland Ice Cap using synchrotron X ray fluorescence
Authors: Chevallier P. , Jehanno C. , Maurette M. , Sutton S. R. , Wang J.
Published: February 2008

Synchrotron X ray fluorescence spectra of unpolished iron and chondritic spheres extracted from sediments collected on the melt zone of the Greenland ice cap allow the analysis of Ni, Cu, Zn, Ga, Ge, Pb, and Se with minimum detection limits on the order of several parts per million. All detected elements are depleted relative to chondritic abundance with the exception of Pb, which shows enrichments up to a factor of 500. An apparent anticorrelation between the Ni‐content and trace element concentration was observed in both types of spherules. The fractionation patterns of the iron and chondritic spheres are not complementary and consequently the two iron spheres examined in this study are unlikely to result from the ejection of globules of Fe/Ni from parent chondritic micrometeoroids.
Azimuth determination of and from horizontal ocean bottom seismic sensors
Authors: Duennebier F. K. , Anderson P. N. , Fryer G. J.
Published: February 2008

A number of factors must be considered when using particle motion information to determine azimuths to source or azimuths of horizontal geophone axes on or below the ocean floor. These factors include anisotropy, unmatched sensor response, incorrect instrument location, tilted sensors or dipping structure, and poor coupling. Accurate determination of azimuths requires identification and understanding of these factors and their effects on the data.
A note on “Noncohesive critical Coulomb wedges: An exact solution” by F. A. Dahlen
Authors: Barcilon Victor
Published: February 2008

The state of stress and taper in a critical noncohesive Coulomb wedge, which were first proposed by F. A. Dahlen, are systematically derived as solutions of the governing equations.
Velocity of ice streams B and C, Antarctica
Authors: Whillans I. M. , Bolzan J. , Shabtaie S.
Published: February 2008

The essential difference between ice stream and inland ice and the cause of ice streaming are not known. In order to study these problems, velocities have been measured on and near ice streams B and C by repeated tracking of TRANSIT satellites. The results confirm the difference in mode of flow between fast on ice stream B and slow on the interstream ice ridges. Also as expected, ice stream C is nearly stagnant, which with other evidence of formerly fast flow confirms its recent slowing and current thickening. The drainage of ice stream B has a negative balance equivalent to thinning by 0.15 ± 0.05 m yr−1 over the entire catchment. The lower portion of the ice stream is thickening at about 2 m yr−1, but the overall behavior is thinning. This general thinning is probably partly due to progressive extension of the ice stream into inland ice. The transition from inland ice to ice stream seems to occur irregularly both spatially and in time, with rafts of inland ice being carried into the ice stream and slowly incorporated.
Use of a new finite element continuity model to study the transient behavior of ice stream C and causes of its present low velocity
Authors: Fastook J. L.
Published: February 2008

The finite element technique is used to solve the continuity equation for two modeling experiments which investigate the response of Ice Stream C to changes in the boundary conditions. By comparison of the results of these experiments it is possible to delineate the mechanism responsible for the anomalously low observed velocity. These experiments are (1) the sudden capture of a major portion of Ice Stream C's catchment area by neighboring Ice Stream B and (2) the sudden removal of a sliding bed condition along a major portion of Ice Stream C. The results of these experiments favor the second scenario over the first as being responsible for the present state of Ice Stream C and suggest that the sliding condition on Ice Stream C disappeared approximately 2000 years ago.
Analysis of time series of glacier speed: Columbia Glacier, Alaska
Authors: Walters Roy A. , Dunlap William W.
Published: February 2008

During the summers of 1984 and 1985, laser measurements were made of the distance from a reference location to markers on the surface of the lower reach of Columbia Glacier, Alaska. The data have numerous gaps, mostly because of inclement weather. The laser measurements were corrected for variations in atmospheric temperature and pressure and for the trajectory of the marker. The marker speed was calculated by using a combination of cubic splines and digital filters to arrive at a data set with 1‐hour time intervals. The speed varies from 7 to 15 m/d and has three noteworthy components: (1) a low‐frequency perturbation in speed with a time scale of days related to increased precipitation, (2) semidiurnal and diurnal variations related to sea tides, and (3) diurnal variations related to glacier surface melt. The low frequency and tidal period variations are separated by low‐pass filtering and subtracting the filtered data from the original data. The variations from melt runoff and tides have the same frequency range and are examined using harmonic analysis and multiple regression with the various forcing functions. The high‐frequency portion of the ice speed signal is dominated by tidal influences and to a lesser extent by meltwater influences. The low‐frequency portion of the signal is dominated by the effects of precipitation.
Sliding phenomena in a steep section of Balmhorngletscher, Switzerland
Authors: Röthlisberger H.
Published: February 2008

Annual aerial photographs of Balmhorngletscher, available since 1973, have furnished data on large ice avalanches originating in one particular location. The avalanches characteristically occur during an active phase of fast sliding. The analysis of a photogrammetric 1:2000 bedrock map, of longitudinal ice surface profiles from photographs taken at various times, and of velocity measurements by theodolite has shown that the fast sliding and the release of the avalanches in an area of about 250 by 100 m is quite independent of ice thickness. It is suggested that besides the slope of the bed, rock roughness, and topographical features, the particular structure of the rock surface in the fashion of tiles on a roof is a factor which influences the processes of fast sliding and avalanche formation. It is suggested that meltwater gains access to cavities at the rock steps, from where it is dragged along on the ice‐rock interface, getting slowly worked into the basal ice layer. Many weeks after the beginning of the melt season the evolving acceleration becomes apparent, characteristically at the beginning of September. Once the surface meltwater gains direct access to the bed through fresh crevasses, the glacier accelerates progressively as water enters cracks, faults, and microfissures. Water pressure effects may become important as cavities grow. While the gradual development of the active phase permits to forecast the growing avalanche risk, the prediction of the event in time is a long way off, if practical at all.
A new method for the determination of in situ hydraulic properties by pressure pulse tests and application to the Higashi Hachimantai Geothermal Field
Authors: Hayashi K. , Ito T. , Abé H.
Published: February 2008

A new method is proposed to evaluate in situ hydraulic properties of formations using the so‐called pressure pulse test over short test intervals. The analysis utilizes the continuous distribution of instantaneous ring sources of fluid over the well bore surface in order to reduce the problem to a set of singular integral equations for a homogeneous and isotropic porous medium. When the ratio of the test interval length to well bore diameter is small, or the specific storage is small, the flow parallel to the well bore axis becomes significant, and the shape of the type curves obtained from the present analysis is different from that of the conventional type curves. Use of the conventional type curves should be restricted to intervals that are more than 20 times longer than the well bore diameter. Finally, the present method is applied to evaluate the in situ hydraulic properties of the Higashi Hachimantai geothermal model field of Hot Dry Rock, Tohoku University.
Overview of the tectonic setting and recent studies of eruptions of Mount St. Helens, Washington
Authors: Weaver Craig S. , Malone Stephen D.
Published: February 2008

Mount St. Helens lies along the western front of the Cascade Range between Mount Rainier, Washington, and Mount Hood, Oregon. This is an area of transition, both geologically and geophysically. North of Mount Rainier, late Cenozoic volcanism is limited to the major stratovolcanoes, whereas south of Mount Hood the volcanic cover from both stratovolcanoes and monogenetic vents is nearly continuous for 300 km. The dominant stratovolcanoes in the southern Washington Cascade Range are physio‐graphically similar to those in the North Cascades, but the volcanic cover is similar to the Oregon Cascades. A saddle occurs in the Bouguer gravity data over the Columbia River, interrupting a west‐east gravity gradient observed along the rest of the Oregon Cascade Range. Heat flow values are intermediate, being lower than those observed in Oregon but higher than those observed north of Mount Rainier. Seismicity is mostly concentrated along the St. Helens zone (SHZ), and all crustal earthquakes greater than magnitude 5 since 1960 in Washington and northern Oregon have occurred between Mount Rainier and Mount Hood. Little seismicity is known within the Cascade Range south of Mount Hood, and few earthquakes occur within the North Cascades of Washington. Magnetotelluric studies indicate a conductor within the area bounded by Mount Rainier, Mount Adams, and Mount St. Helens, and the conductor is interpreted as an east dipping, compressed marine terrain in the upper plate. Mount St. Helens is located at a dextral offset in the SHZ where the SHZ intersects a set of older, mapped fractures that are preferentially aligned with the contemporary regional principal stress direction. Aeromagnetic and gravity data suggest the presence of an intrusive body beneath the volcano; petrogenesis studies favor multiple intrusions of small batches of magma into the shallow volcanic system. Studies of pyroclastic flows indicate that turbulence within the head of pyroclastic flows may be important for the generation of ash clouds and in forming basal deposits. Observations of the May 18, 1980, eruption and subsequent erosion of pyroclastic flow deposits has allowed a revision of the chronology of the eruption. A numerical study of the slope failure of May 18 suggests that gravity alone was sufficient to trigger the avalanche that initiated the cataclysmic eruption. Hundreds of similar earthquakes recorded during eruptions in 1984 and 1985 suggest that repeated failure or slip within a small volume occurs around the magma supply conduit because of very high strain rates.
Variations of the amplitudes of reflections from old oceanic basement in the North Atlantic
Authors: Rohr K.
Published: February 2008

A study of the amplitudes of wide‐angle reflections from the sediment‐basement interface of old oceanic crust shows that the uppermost portion of layer 2 is homogeneous on the 0.15–1.00 km scale but not on the 3.0–8.0 km scale. The amplitude versus range patterns of the basement reflections recorded on 30 seismic lines were examined; most patterns are affected by the rough topography of the sediment‐basement interface. Three lines have smooth travel time curves and appear to be unaffected by topography. The amplitude patterns of these lines are different from each other; they can be modeled by synthetic seismograms computed from velocity structures which have different thicknesses of a transition zone between the limestone sediments and the basaltic crust.
Paleomagnetic evidence of reversals resulting from helicity fluctuations in a turbulent core
Authors: Clement Bradford M.
Published: February 2008

Detailed transition records of the upper Olduvai (N‐R) and lower Jaramillo (R‐N) reversals from deep‐sea sediment cores in both the northern and southern hemispheres constrain the presence of large scale symmetries in these transitional fields. These records, obtained from core RC14‐14 (35.91°S, 59.97°E) and Deep Sea Drilling Project hole 609B (49.86°N, 335.77°E), may be modeled by specifying the geometry of the nondipole field while allowing the axial dipole to vary according to Olson's (1983) dipole time evolution equations. The resulting synthetic records of inclination, declination, and intensity account for the major features of the observed records. Good fits are obtained for all four transition records by partitioning the energy lost by the dipole field, with 40% going to −G°2 and 55% going to +G°4, and maintaining an equatorial dipole at 5% of the dipole strength. Both reversals are modeled as full (two component) reversals; the only difference between the two models is the duration of the helicity fluctuation. Thus both of these transitional fields were dominated by low, even degree terms, suggesting similarities in these N‐R and R‐N reversal processes which result in symmetries about the equator. The successful modeling of these records demonstrates that these data are consistent with Olson's hypothesis that helicity fluctuations can result in reversals of an axial dipole field generated by a homogenous, α2 dynamo.
The demagnetizing factors in multidomain grains
Authors: Xu Song , Merrill Ronald T.
Published: February 2008

Experiments and theory are combined in an attempt to determine how well a demagnetizing factor, N, characterizes the internal magnetic field of a multidomain (MD) grain. It is shown that all previous experimental estimates of N are suspect and that only under unusual circumstances can one have much confidence in determining a value for N that characterizes the internal magnetic field. One of those circumstances is when the sample is nearly stoichiometric, as is the case of a large synthetic magnetite crystal found here to have an MD value of N essentially identical to its single‐domain value. However, calculations show that the value for N in small MD grains depends on the domain and magnetization state of the sample. It is shown that the demagnetizing factor cannot be uniquely determined for small MD grains, but it can often be well approximated by the single‐domain demagnetizing factor for large MD grains. A value of 10 μm is shown to be roughly the linear dimension that separates “large” from “small” grains in the case of equidimensional magnetite.
Static compression of MnO manganosite to 60 GPa
Authors: Jeanloz Raymond , Rudy Abbey
Published: February 2008

Static compression measurements to 60 GPa yield a room temperature isotherm for MnO that is described by a bulk modulus and pressure derivative K0 = 162 (± 17) GPa, K0′ = 4.8(± 1.1). These values are compatible with ultrasonically determined moduli, and our data show that an Eulerian rather than a Lagrangian finite‐strain equation of state is preferable. We find no indication of a structural transition to 60 GPa. Hence we interpret the variation of c44 with pressure that has been observed ultrasonically as reflecting magnetic interactions between Mn ions. A simple model of these interactions yields pressure derivatives c44′ ∼ 0.1 and c44″ ⪝−0.03 GPa−1, which are close to the ultrasonic values.
Terrestrial helium investigations in earth science
Authors: Reimer G. Michael , Kurz Mark D.
Published: February 2008

Technological advancements provide the opportunity to explore the earth in greater detail than ever before. Yesterday's theories presently are being tested and certainly will be modified tomorrow. In the last half century, we have witnessed such a development arriving at today's understanding of plate tectonics. We believe that such is the case with the investigations of helium and helium isotopes; they will have a major role in the understanding of the dynamics of the earth.
Helium distribution pattern of measured and corrected data around the “Eldingen” Oil Field, NW Germany
Authors: van den Boom G. P.
Published: February 2008

A helium survey in the area of the oil‐bearing structure of Eldingen (NW Germany) was carried out in 1984 and 1985. In the survey area of approximately 70 km2, 196 overburden samples were collected from 15 to 25 m depth. The measured helium concentrations were corrected by using 15 additional field and laboratory parameters. A comparison of the regional distribution patterns of measured and corrected He values are shown, and the influence of structural pattern and groundwater movements in the area is discussed. The application of the helium method in the investigation of the Eldingen area leads to positive results. The helium distribution pattern is recognizable at the surface around the hydrocarbon deposit. Information on the structural situation and hydrogeological conditions has to be taken into account for interpretation.
Tectonic motion in the western United States inferred from Very Long Baseline Interferometry measurements, 1980–1986
Authors: Kroger Peter M. , Lyzenga Gregory A. , Wallace Karen S. , Davidson John M.
Published: February 2008

We use over 6 years of mobile very long baseline interferometry (VLBI) baseline measurements between 12 sites in the western United States to infer their velocities relative to the North American plate. We find these velocities to be generally consistent with those determined from geologic data and contemporaneous satellite laser ranging measurements in the same region. The discrepancy between the largest velocities determined from the VLBI measurements of 40–48 mm yr−1 and the relative plate velocity of 50–56 mm yr−1 predicted from plate motion models is found to be consistent with a broadened distribution of interseismic strain from cyclic activity on the San Andreas and subsidiary faults. The VLBI data are best explained by a cumulative rate of strike‐slip motion near the plate boundary of approximately 48 mm yr−1, although exclusion of competing values of 56 and 41 mm yr−1 is based upon very few data. The rates of offshore fault slip inferred from this study range from ∼15 mm yr−1 in central California to negligible amounts in the San Francisco region. Finite element calculations of multiple fault strain distributions show good agreement with systematic variations in the distribution of shear strain along the San Andreas system, as revealed by previous geodetic measurements.
On the dynamics of mid‐ocean ridges
Authors: Ribe Neil M.
Published: February 2008

Most extant flow models for ocean ridges suffer either from unrealistic geometry (vertical conduit models) or a stress singularity at the ridge axis (corner flow models). I present a model which avoids these difficulties by incorporating a weak “plate boundary zone” (PBZ) between diverging surface plates. An analytical solution for the flow of a constant‐viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates, and used to predict the dynamically supported topography (axial valley) on the PBZ, the force required to pull the plates apart, and the rate of melt extraction at the ridge. Stress in the asthenosphere increases linearly with spreading rate, but decreases as the plates become thinner, and the dynamics of ridge crests is controlled by the balance between these opposing effects. The depth of the axial valley is controlled by the plate thinning effect, and is therefore a decreasing function of spreading rate. The depth of the axial valley on the mid‐Atlantic ridge can be explained if the viscosity of the asthenosphere is 6×1019–1021 Pa s and the lower surface of the plate is the 600°C isotherm. The plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate. About five times as much force is required to maintain a fast spreading ridge (9 cm yr−1 half rate) as a slow spreading ridge (1 cm yr−1). Melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere (≪ total degree of melting) ranges from about 2.5–4.0 % for a slow spreading ridge to 4.0–6.5 % for a fast spreading ridge. The dynamics of melt extraction does not depend significantly on the finite width of the PBZ.
Paleomagnetism of Proterozoic and Cambrian Strata, Adrar de Mauritanie, CRatonic West Africa
Authors: Perrin M. , Elston D. P. , Moussine‐Pouchkine A.
Published: February 2008

Four paleomagnetic poles are reported from red beds of the Adrar de Mauritanie, cratonic West Africa. Thirteen localities were occupied, and 224 samples were collected in seven units from a middle Proterozoic to early Paleozoic sedimentary section. Two poles are reported from middle and late Proterozoic strata; a third pole is reported from Cambrian strata. The magnetizations for these reside in stable high‐temperature components, revealed by progressive thermal demagnetization analysis to 675°C. The fourth pole is interpreted to reflect a Permian overprinting in the Cambrian strata. Two Cambrian‐Ordovician units sampled contained directions too scattered to allow the calculation of any pole. The pole from middle Proterozoic strata (998±32 Ma; Rb‐Sr date on argillite), derived from four localities (52 samples, single polarity), plots at 48.7°N, 206.9°E (dp = 1.1°, dm = 2.9°). The pole from late Proterozoic strata (775 ± 52 Ma; Rb‐Sr date), derived from two localities (15 samples, mixed polarity), plots at 34.0°N, 246.9°E (dp = 4.6°, dm = 8.6°). Neither has correlatives in age in the paleomagnetic data set for cratonic West Africa, nor are there obvious correlatives with the paleomagnetic data set from the remainder of Africa. The pole that we consider to reflect a Cambrian direction comes from three localities (23 samples, mixed polarity), plotting at 0.3°S, 249.3°E (dp = 5.7°, dm = 11.5°). Its antiparallel pole corresponds with one group of Cambrian poles reported from Africa but differs markedly from another group of Cambrian poles. The late Paleozoic overprint direction obtained from three units (50 samples from four localities, single polarity) provides a (reversed polarity) pole that plots at 47.2°S, 42.6°E (dp = 1.5°, dm = 3.0°), which is in good agreement with Permian poles reported from elsewhere in Africa. In spite of the large number of poles already reported from Africa, additional data are needed before a reliable apparent polar wandering path can be constructed for the Paleozoic and Proterozoic of Africa.
Regional magnetic overprinting of Witwatersrand Supergroup Sediments, South Africa
Authors: Layer Paul W. , Kröner Alfred , McWilliams Michael , Clauer Norbert
Published: February 2008

Shales from the late Archean Witwatersrand Supergroup of South Africa yield stable, wellgrouped remanent magnetizations after thermal and alternating field demagnetization. Virtual geomagnetic poles from 17 sites located throughout the central and western parts of the basin indicate that rocks of the basin were remagnetized after folding. A paleomagnetic fold test shows that the Fisherian precision parameter, K, decreases from 22 to 3 after restoring the beds to paleohorizontal. Illite crystallinity indices show that the remagnetized samples were subjected to at least middle greenschist facies temperatures, whereas magnetic blocking temperature estimates indicate upper greenschist facies temperatures. A K‐Ar age obtained from clays of 1945±40 Ma (2σ) is assigned to this metamorphic event. The new paleomagnetic pole obtained from the Witwatersrand Basin sites (19°N, 46°E, K = 22, A95 = 8°) is consistent with other 1950–2050 Ma poles from Southern Africa, confirming independently the K‐Ar age. The distinctive overprint direction seen in shale specimens may be useful as a directional marker for orienting deep bore cores.
Sulfur isotope study of chimney minerals and vent fluids from 21°N, East Pacific Rise: Hydrothermal sulfur sources and disequilibrium sulfate reduction
Authors: Woodruff Laurel G. , Shanks Wayne C.
Published: February 2008

Sulfur isotopic analyses of hydrothermal chimneys and coexisting fluids collected from hot vent areas at 21°N, East Pacific Rise, indicate δ34S values for sulfide minerals of 0.9–6.2‰ and for aqueous H2S of 1.3–5.5‰. The chemical processes resulting in mineral zoning of chimney structures are a controlling influence on 34S variation within individual chimneys. In general, coarsely crystalline chalcopyrite, which lines chimney interiors, is depleted in δ34S relative to exterior ZnS (either sphalerite or wurtzite). Chalcopyrite which formed by replacement of other sulfides has higher sulfur isotope values than chalcopyrite which precipitated directly from hydrothermal fluid. Bornite and other Cu‐rich sulfides, which occur as thin layers replacing the outer margins of massive chalcopyrite zones, have the highest δ34S values of any 21°N samples, ranging from 5.2‰ to 6.2‰. The significant 34S enrichment in bornite requires reduction of isotopically heavy seawater‐derived sulfate in the chimney walls. Vent fluid δ34SH2S values are similar to δ34S values of inner wall chimney sulfides for seven of 11 samples. In other samples, aqueous H2S is enriched isotopically in δ34S compared with coexisting sulfides, suggesting admixture of H2S derived from reduced seawater sulfate. The δ34S value of end‐member hydrothermal fluid at 21°N is about 1.0–1.5‰, which is somewhat higher than basaltic sulfide, the main source of sulfur in the hydrothermal system. This 34S enrichment apparently is due to sulfate reduction in the deep hydrothermal system during conversion of basaltic pyrrhotite to pyrite. The South West (SW) vent field has significantly higher average δ34S values (3.1‰) compared with the other 21°N vent fields. The SW field is situated on the edge of a lava collapse structure, promoting development of an extensive feeder zone which is the site of near‐surface sulfate reduction during redissolution of previously deposited anhydrite. The δ34S values for chimney sulfides and vent fluid H2S which are higher than about 3‰ for SW samples and higher than about 2‰ for the other vents reflect incorporation of additional reduced seawater sulfate in the chimney environment.
Experimental “sytectonic” dehydration of serpentinite under conditions of controlled pore water pressure
Authors: Rutter E. H. , Brodie K. H.
Published: February 2008

We have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine + talc + water, at temperatures ranging between 300° and 600° and at total pressures from 100 to 300 MPa. Constant stress, stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed. Effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system. Stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction. At 500° and 600°C, however, the strength was linearly dependent on the inverse of the deformation rate, and independent of effective confining pressure over the range investigated. Microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine‐grained (0.1 µm olivine produced in the breakdown reaction. The enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion‐accommodated grain boundary sliding. These experimental results suggest that metamorphic transformations affect the deformability of rocks, and demonstrate enhancement of deformability through the transient existence of fine‐grained reaction products. The results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain‐size reduction by dynamic recrystallization in plastically deforming rocks.
A strontium isotope study of the VC‐1 core hole and associated hydrothermal fluids and rocks from Valles Caldera, Jemez Mountains, New Mexico
Authors: Vuataz François‐D. , Goff Fraser , Fouillac Christian , Calvez Jean‐Yves
Published: February 2008

Chemical and strontium isotopic analyses have been performed on 10 geothermal waters from the Valles caldera/Jemez Mountain region, as well as 10 rock samples collected in the VC‐1 scientific core hole. The 87Sr/86Sr values of other rocks sampled in the Jemez Mountains have also been used in this study to understand relations between the fluids and rocks of the volcanically driven geothermal system and the lateral outflow plume, discharging down the Jemez fault zone. The strontium content of groundwater is linked to the host rock composition, the aquifer temperature, and therefore the solubility of Sr‐containing minerals. Low Sr concentrations in the fluid are found with a high temperature and/or volcanic host rocks, whereas high Sr concentrations are characteristic of a low temperature and/or a Paleozoic sedimentary aquifer. Important variations of the 87Sr/86Sr value in hydrothermal waters (0.70842–0.72193) are related to chemical composition, Sr content, and host rocks. For VC‐1 core samples, the Sr isotopic ratios are also markedly variable (0.70459–0.75686), low for the Quaternary volcanics and high for the Paleozoic sedimentary rocks. The same observation is made for other Jemez Mountains rocks (0.70465–0.8163) with, in addition, high 87Sr/86Sr values for Precambrian crystalline rocks. A good correspondence is generally found between the waters and the reservoir rocks studied here, which indicates that isotopic equilibrium is rapidly attained. The following suite of hydraulically related fluids displays a progressive enrichment in radiogenic strontium: Baca wells (87Sr/86Sr = 0.7084–0.7094), VC‐1 core hole at 483 m (0.7152–0.7154), Main Jemez Spring (0.7217), and Soda Dam (0.7219). This trend is associated with flow through Paleozoic rocks and with the length of the flow path from the center of Valles caldera (primary geothermal fluids) toward San Diego Canyon, along the Jemez fault zone (derivative waters).
Uranium‐series age determination of calcite veins, VC‐1 drill core, Valles Caldera, New Mexico
Authors: Sturchio Neil C. , Binz Carl M.
Published: February 2008

Uranium‐series analysis (238U‐234U‐230Th) of 13 calcite veins from the hydrothermally altered Madera Limestone in the VC‐1 drill core was performed to determine the ages of the veins and their relation to the Valles hydrothermal system. Thermal water from VC‐1 and two hot springs in San Diego Canyon was analyzed for U and (234U/238U) to help evaluate the constancy of initial (234U/238U). The (230Th/234U) age of one of the veins is ∼95 kyr, and those of two other veins are ∼230 and ∼250 kyr. Five of the veins have near equilibrium (230Th/234U) and are probably older than ∼0.3 m.y. Uranium concentrations in the remaining veins are too low for analysis by the α‐spectrometry techniques employed in this study. Of the five veins near (230Th/234U) equilibrium, four are also near (234U/238U) equilibrium, suggesting ages greater than ∼1.0 m.y., but one has (234U/238U) = 1.15, suggesting an age between ∼0.3 and ∼1.0 m.y. Calculated initial (234U/238U) of the veins yielding relatively young ages are neither equal to each other nor to (234U/238U) in thermal water from VC‐1, indicating inconstancy of initial (234U/238U) that may be related to variations in groundwater mixing proportions. Three of the four veins that yield relatively young ages consist of coarse, sparry, vuggy calcite, suggesting that this may be the type of calcite vein which forms under conditions resembling those encountered presently in VC‐1. The analytical data are consistent with closed‐system behavior of U and Th in the VC‐1 calcite veins.
Constraining aspherical structure with low‐degree interaction coefficients: Application to uncoupled multiplets
Authors: Ritzwoller Michael , Masters Guy , Gilbert Freeman
Published: February 2008

The effect of even‐order aspherical structure on displacement for each isolated multiplet in the Earth's seismic free oscillation spectrum can be represented by a discrete set of coefficients which characterize the interaction between singlets. Each interaction coefficient is linearly related to aspherical structure of a given harmonic degree and azimuthal order. Although interaction coefficients are nonlinearly related to displacement, they can be estimated iteratively by Newton's method, a technique that we call spectral fitting. We have applied spectral fitting to approximately 350 recordings taken from 18 large or deep events and have estimated the degrees 2 and 4 interaction coefficients for 38 low harmonic degree (≤9) multiplets. An error analysis, based on misfit, is also presented. These coefficients and errors provide linear constraints on aspherical structure and are tabulated for use in future inversions. The estimated coefficients partition naturally into two subsets: those for the 10 anomalous multiplets and those for the 28 multiplets dominantly sensitive to mantle heterogeneity. On average, the degree 4 coefficients lie below the error estimates, and we interpret only the degree 2 coefficients. The degree 2 coefficients for the mantle sensitive multiplets behave smoothly, within observational error, along low radial order dispersion branches (0S, 1S, 2S, 5S), indicating that a simple mantle model exists that will fit them. Comparison between the estimated interaction coefficients and those predicted by models expressed as perturbations to νs or νp is complicated by the necessity of assuming empirical scaling relationships between νs, νp, and ρ. The numerical values in these relationships are currently subject to debate, but in any reasonable case the degree 2 coefficients predicted from the models M84A and L02.56 of Dziewonski and Woodhouse agree qualitatively with the estimated coefficients. A wide range of mantle models will quantitatively fit the mantle sensitive coefficients, the fundamental mode coefficients computed from multiplet center frequency observations (0S20‐0S40), and the geoid simultaneously. The characteristics of these models are strongly dependent on allowed model characteristics with trade‐offs between volumetric and boundary perturbations being particularly important. In agreement with the results described in an earler paper using a different technique, the anomalous multiplets are dominated by degree 2, axisymmetric structure somewhere in the core. Inferences concerning the nature and location of this heterogeneity are also dependent upon the scaling relationships and constraints placed on mantle structure. Not surprisingly, the resolution of the cause of anomalous splitting awaits confidence in long‐wavelength mantle models and more and different kinds of data.
Glacial isostatic adjustment and mantle viscosity
Authors: Officer Charles B. , Newman Walter S. , Sullivan John M. , Lynch Daniel R.
Published: February 2008

The determination of mantle viscosity conditions from observations of relative sea level changes associated with glacial unloading of the Laurentide and Fennoscandia ice sheets has been reexamined. The conclusion is reached that the adjustment has been an upper mantle process associated with the low‐seismic‐velocity, low‐viscosity asthenospheric layer rather than a whole mantle process of essentially uniform mantle viscosity. The critical data for resolving this dichotomy are those from the zone of peripheral subsidence adjacent to the region of central uplift of the Laurentide and Fennoscandia ice sheets.
Crustal structure in the ROSE area of the East Pacific Rise: One‐dimensional travel time inversion of sonobuoys and expanded spread profiles
Authors: Vera Emilio E. , Mutter John C.
Published: February 2008

We study the variability of the young oceanic crust as a function of age by analyzing 11 digitally recorded sonobuoys and three expanded spread profiles over 0.5‐, 2.5‐, and 4.5‐m.y.‐old oceanic crust. The data were collected during the Rivera Ocean Seismic Experiment at the East Pacific Rise between 11° and 13°N. We analyze the data within a one‐dimensional travel time scheme using the tau‐p method and travel time forward modeling. We perform the tau‐p mapping by using a polynomial least squares technique that is fast and can be applied even to poor quality data as in this case. We do not see large changes in the velocity structure between 0.5‐and 4.5‐m.y.‐old oceanic crust. The overall structure consists of a rapid increase in velocity from about 2.5–6 km/s within the uppermost 1 km of crust (layer 2A), a zone ≤1 km in thickness with little or no velocity gradient (layer 2B), a thin transition layer with a relatively rapid increase in velocity from about 6.2 to 7 km/s (layer 2C), and a thick nearly homogeneous basal layer with velocities around 7.2 km/s (layer 3). We find that the low velocities at the seafloor and large velocity gradients within the shallow crust can be explained by the hypothesis of large‐scale cracks and their behavior under confining pressure. For the variation of velocity V versus depth below seafloor z, this hypothesis yields V = (V02 + Bz)1/2, a relationship that fits our solutions.
Asymptotic approximations for normal modes and surface waves in the vicinity of the antipode: Constraints on global earth models
Authors: Romanowicz Barbara , Roult Geneviève
Published: February 2008

An asymptotic formalism developed earlier for the case of observations away from the epicenter and its anitpode is here extended to near‐source and near‐antipodal observations. This formalism is valid in the limit of large angular orders and for smooth models of the Earth. We show that for a given dispersion branch the variation of the eigenfrequency shift as a function of angular order l is smooth to order 1/l, contrary to the case of nonpolar observations. If the source is isotropic, this frequency shift depends on the even poloidal part of lateral heterogeneity, as seen with respect to the source‐receiver axis. For a more general moment tensor source, terms s even, m = ± 2 are also present, weighted by the radiation pattern of the source. In the case of propagating wave trains we show that the phase shift at the antipode is π/2 for individual wave packets (R1R2, R3R4, · · ·) and that the remaining phase term can be converted into a phase velocity related to the global Earth structure. The measured phase velocities should be equal from one wave packet to the next and are directly convertible into the corresponding normal mode frequency shifts. We illustrate this using observations from the GEOSCOPE network. We thus show that antipodal observations of free oscillations eigenfrequencies or alternatively of individual wave trains can be used to put additional constraints on the even‐order large‐scale lateral heterogeneity in the Earth's mantle.
Single‐domain theory of remanent magnetization
Authors: Moon Thomas , Merrill Ronald T.
Published: February 2008

The basic principles underlying the single‐domain theory developed by Néel are examined using a modem statistical mechanics approach. It is shown that the quasi‐equilibrium theory of Néel is in good shape but there are problems with the dynamic part of the theory. The physical assumptions made with regard to “thermal fluctuations” appear oversimplified, and Néel's theory, and a similar theory by Brown, violate the detailed balance theorem of statistical mechanics. These problems deal only with the attempt frequency factor used in estimations of the relaxation time and not with the important exponential factor, which appears to have been correctly determined by Néel. In the absence of a convincing theory for the attempt frequency factor, it is recommended that experimental values be used (107 to 108 s−1 for magnetite). Although Néel's theory does appear to be correct in identifying the critical exponent dependence of the relaxation time as a function of activation energy and temperature, the activation energy values used by paleomagnetists in applying Néel's theory are often incorrect, as has been previously pointed out by Smith. Consistent with Smith's suggestion, lower activation energies are obtained by modeling noncoherent reversal modes with the passage of a domain wall. This leads to a significant lowering of the activation energy in most cases. However, the resulting activation energy still appears to be somewhat too high based on experimental evidence, indicating that either defects play an important role and/or other modes of noncoherent rotation occur (including other possible domain wall structures) in practice.
San Jacinto Intrusive Complex: 2. Geochemistry
Authors: Hill R. I. , Chappell B. W. , Silver L. T.
Published: February 2008

Rocks from three large (>1002 km) tonalitic intrusions exposed in the San Jacinto Mountains of southern California show a restricted compositional range of between 63 and 68 wt % SiO2 for all but volumetrically minor felsic differentiates (with Si02≈70 wt %). All rocks with less than 65.5 wt % SiO2 show linear element‐element covariation. Felsic differentiates have characteristics (higher SiO2, K2O, Rb, Ba, U; higher and variable rare earth elements) consistent with derivation by in situ fractionation; rocks with between 65.5 and 70 wt % SiO2 have intermediate characteristics and are interpreted as derived from liquids formed by mixing “primitive” liquids with fractionated liquids within an intermittently recharged, continuously solidifying magma chamber. Mafic inclusions extend the compositional trends of the mafic tonalites to 55 wt % SiO2. The chemical variations of both inclusions and more mafic tonalites are interpreted as resulting from processes acting before injection of their parental liquids into the observed crustal magma chambers. Effects of chamber processes are minor for all but the most felsic rocks. The major effect of recharge is to buffer the thermal and chemical properties of liquids within the magma chambers, yielding large volumes of relatively homogeneous tonalite. For those elements where the bulk distribution coefficient is between about 0.5 and 2, concurrent recharge and solidification produces rocks that closely approximate the composition of the added liquids. Estimated Rayleigh numbers for these liquids are high (>1010), implying convection throughout much of the solidification history of each chamber. Existence of trace element variations within analyzed rocks imply that convection was not totally efficient at homogenizing the various batches of liquid added to each chamber.
San Jacinto Intrusive Complex: 3. Constraints on crustal magma chamber processes from strontium isotope heterogeneity
Authors: Hill R. I. , Silver L. T.
Published: February 2008

Strontium isotope data from three large plutons interpreted as derived from dynamic magma chambers (Hill, this issue; Hill et al., this issue) show the existence of pronounced isotopic heterogeneity within each unit. Ranges of calculated initial 87Sr/86Sr are unit I, 0.7060–0.7076; unit II, 0.7060–0.7074; unit III, 0.7058–0.7073. A limited sample of mafic inclusions and dyke rocks, interpreted as samples of liquid added to the various chambers during inflation, have initial 87Sr/86Sr of 0.7068–0.7084. These variations are regular at the kilometer scale within each pluton and show no identified correlation with any other observed geocheinical or petrological parameter. Slightly older small intrusions that span the compositional range olivine gabbro to granite show a systematic increase in initial 87Sr/86Sr from 0.7057 in the southwest to 0.7077 in the northeast, expressing the pattern previously reported for the northern 600 km of the Peninsular Ranges batholith. The Sr isotope data indicate that melt production, aggregation, transport, and crystallization processes were far from capable of completely homogenizing initial variations in initial 87Sr/86Sr within the liquids from which these rocks crystallized. Whole chamber convection is apparently ruled out by these data; more complex patterns, including double‐diffusively induced horizontal stratification, are permitted.
The scientific work (1957–1987) of Allan Cox
Authors: Banerjee Subir K.
Published: February 2008

During the summer of 1951 Allan Cox, then a 25‐year‐old chemistry major at the University of California, Berkeley, became curious about geology and worked as a geology field assistant to Clyde Wahrhaftig. Thus did Allan Cox begin a momentous career in geomagnetism, paleomagnetism, and plate tectonics which left its indelible mark in the sciences themselves, in the workings of earth sciences through his leadership roles in the American Geophysical Union, the Geological Society of America and the National Academy of Sciences, and most importantly, in the lives and works of many junior scientists in the United States and throughout the world. Twenty‐three years ago I was one of those junior scientists whose life was touched and transformed by this unique individual over the course of ensuing years. It is quite possible that but for a chance meeting with Allan at the 1965 IAGA Assembly in Pittsburgh, I would have not seen for a long time the important ways by which basic rock magnetism can and does affect progress in geomagnetism and paleomagnetism.