bluuebunny/crossref_metadata_2025 huggingface dataset:
Deglaciation‐induced vertical motion of the North American continent and transient lower mantle rheology
Authors: Peltier W. R.
Published: February 2008

The last deglaciation event of the current ice age, which began approximately 18,000 years ago and ended about 7000 years ago, continues to exert a profound influence on the vertical motion of the solid earth in and surrounding the regions which were once ice covered. Observations of such vertical motions may be employed to constrain the radial viscoelastic structure of the planet and therefore to contribute in an important way to the understanding of geodynamic processes in general. In this paper both present‐day rates of vertical motion as recorded on tide gauges and long time scale records of vertical motion as recorded in 14C‐controlled relative sea level histories are employed to refine previously constructed models of the radial viscoelastic structure. An interesting sidelight to this analysis is the demonstration that the lower mantle viscosity inferred from the vertical motion data could represent a transient rather than the steady state value, a fact which would help considerably to reconcile the small viscosity stratification required by the glacial rebound data with the large stratification which seems to be required by some recent analyses of the geoid height anomalies associated with internal density heterogeneity of the mantle which has been revealed by application of seismic tomographic techniques. A further product of these new analyses is obtained by the application of the glacial isostatic adjustment model to filter this signal from the secular trends of relative sea level observed on tide gauges from both the east and west coasts of the North American continent. These results may be important in the context of recently proposed scenarios of climatic change.
Mobile Very Long Baseline Interferometry and Global Positioning System measurement of vertical crustal motion
Authors: Kroger Peter M. , Davidson John M. , Gardner Elaine C.
Published: February 2008

Mobile very long baseline interferometry (VLBI) and Global Positioning System (GPS) geodetic measurements have many error sources in common. Calibration of the effects of water vapor on signal transmission through the atmosphere, however, remains the primary limitation to the accuracy of vertical crustal motion measurements made by either technique. We have evaluated the two primary methods of water vapor calibration currently in use for mobile VLBI baseline measurements: radiometric measurements of the sky brightness near the 22–GHz emission line of free water molecules and surface meteorological measurements used as input to an atmospheric model. Based upon a limited set of nine baselines, it is shown that VLBI data calibrated with water vapor radiometer measurements can, in some cases, provide a significantly better fit to the theoretical delay model than the same data calibrated with surface meteorological measurements. A detailed error model for the vertical baseline component indicates that current mobile VLBI technology should allow accuracies of order 5 cm with water vapor radiometer calibration and 10 cm when surface meteorological calibration is used. A statistical analysis of the results of repeated measurements of the 336‐km baseline from Big Pine, California, to Pasadena, California, shows the scatter in the vertical baseline component to be consistent with this model. A similar model for a hypothetical GPS baseline measurement is presented. A covariance analysis based upon this model shows current GPS technology to be capable of accuracies in the vertical baseline component comparable to present mobile VLBI measurements. Expected improvements in system components for both technologies should allow reduction of the uncertainty in the vertical component to less than 3 cm by 1989.
A Global Positioning System baseline determination including bias fixing and water vapor radiometer corrections
Authors: Ware Randolph H. , Rocken Christian , Hurst Kenneth J.
Published: February 2008

We report a Global Positioning System (GPS) baseline determination which includes bias fixing (carrier phase ambiguity solutions) and tropospheric corrections based on simultaneous water vapor radiometer (WVR) observations. Measurements were obtained using single‐wavelength codeless GPS receivers and dual‐wavelength WVRs at each end of a 22‐km baseline near Boulder, Colorado, during three evenings in July 1983. Local thunderstorm activity prevailed during these observations. The WVRs were pointed toward each satellite sequentially at 15‐min intervals during GPS data acquisition. Radio path delay corrections as large as 10 cm were estimated from the WVR observations. Baseline vectors were determined using tropospheric corrections based on WVR and surface meteorological (met) measurements and by resolving phase ambiguities (bias fixing). A repeat measurement of the same baseline was performed without WVRs during clear weather conditions in September 1984. If WVR corrections were not applied, we found that time varying corrections based on surface met measurements improved baseline length repeatability by a factor of 2, compared to corrections based on default surface met values. Applying WVR corrections in bias‐fixed baseline solutions, we found a 4‐cm decrease in relative height and a corresponding factor of 3 improvement in the repeatability (to 0.6 ppm). The WVR corrections also enhanced our ability to resolve phase ambiguities. Bias‐free solutions, with the WVR correction, showed essentially the same 4‐cm decrease in the vertical component (with the repeatability improved to 1 ppm). Baseline solutions using simulated data that include only the WVR correction confirm this decrease in the vertical. These results suggest that during disturbed weather conditions WVR (and surface P and T) measurements may be useful in achieving reliable bias fixing as well as optimum vertical accuracy.
Effects of sample‐electrode interface polarization on the electrical properties of partially molten rock
Authors: Sato Hiroki , Manghnani Murli H. , Lienert Barry R. , Weiner Adam
Published: February 2008

Electrical impedance in a partially molten gabbro was measured with the two‐electrode method in the frequency range of 0.032 Hz to 10 kHz. Samples of different thicknesses were used to separate the bulk polarization effect from the sample‐electrode interface polarization effect. The measurements show that both real and imaginary impedances increase linearly with increasing sample thickness. When the impedance data are extrapolated to zero thickness, the real impedance is as small as the experimental error, whereas the imaginary impedance has a finite value. This result indicates that the real impedance is mainly controlled by the electrical properties of the partially molten gabbro itself and that only small amounts of electrode interface polarization contribute to the imaginary impedance. After correction for the interface polarization effect, the present data were analyzed by the theoretical expression that describes the electrical impedance of partially molten rocks. The results show that frequency dispersion of electrical impedance in partially molten gabbro is associated with the polarization effect of ions in the partial melt.
A model of propagating brittle failure in heterogeneous media
Authors: Katz J. I.
Published: February 2008

I consider a nonelastodynamic model for the propagation of brittle failure in a heterogeneously stressed medium. Numerical simulation shows a power law distribution of failure sizes, with a slope not far different from the size distribution of seismic events. The time history of calculated failure activity is not consistent with observed seismic behavior, and some explanations and implications are discussed.
North American Jurassic Apparent Polar Wander: Implications for plate motion, paleogeography and Cordilleran tectonics
Authors: May Steven R. , Butler Robert F.
Published: February 2008

Eight paleomagnetic poles are considered to be reliable Jurassic reference poles for cratonic North America. These poles form a consistent chronological progression defining two arcuate tracks of apparent polar wander (APW) from Sinemurian through Tithonian time (203–145 Ma). Combined with reliable Triassic and Cretaceous reference poles, the resulting path is well modeled by paleomagnetic Euler pole (PEP) analysis and is significantly different from previous APW compilations. These differences reflect differences in original data sets, modes of analysis, and geologic time scales and translate into substantial and important differences in paleolatitude estimates for cratonic North America. PEP analysis reveals two cusps, or changes in the direction of APW: one in the Late Triassic to Early Jurassic (Jl) and one in the Late Jurassic (J2). The Jl cusp represents the change in North American absolute plate motion associated with rifting of the central Atlantic and Gulf of Mexico, while the J2 cusp correlates temporally with the marine magnetic anomaly M21 plate reorganization and to various North American intraplate tectonomagmatic events (e.g., Nevadan Orogeny). Analysis of pole progression along the Jl to J2 and J2 to Cretaceous APW tracks indicates constant angular plate velocity of 0.6°–0.7°/m.y. from 203 to 150 Ma followed by significantly higher velocity from 150 to 130? Ma. Late Triassic‐Jurassic reference poles indicate more southerly paleolatitudes for cratonic North America than have previous compilations requiring modification of displacement scenarios for suspect terranes along the western Cordillera.
Geochemistry of sulfur in Mount Etna plume
Authors: Martin D. , Ardouin B. , Bergametti G. , Carbonnelle J. , Faivre‐Pierret R. , Lambert G. , Le Cloarec M. F. , Sennequier G.
Published: February 2008

Aircraft measurements of sulfur compounds and trace elements including Po210 were carried out in the plume of the Mount Etna volcano, at distances 10 to 260 km from the crater. The experiment was performed in September 1983 following the large lava emission of March–August 1983. Trace elements, particularly Po210 and Si, were measured with the aim of calculating their atmospheric dilution and evaluating their outputs. This study enabled us to determine the SO2 to SO4 conversion rates to be between 2.2 × 10−6 and 4.0 × 10−5 s−1 in a cloud free anticyclonic situation. The main process of SO2 removal in a volcanic plume should be a heterogeneous process of adsorption of this gas onto large particles. Emission fluxes of SO2 were found to vary between 27 and 38 kg s−1, leading to a mean Po210 output of 0.4 Ci/d.
Using volatile constituents of soils and soil gases to determine the presence of copper‐zinc ore bodies at Johnson Camp, Arizona
Authors: Hinkle Margaret E.
Published: February 2008

Soils and soil gas samples were collected along three traverses across the copper‐zinc ore bodies at Johnson Camp, in southeastern Arizona. The copper‐zinc ore occurs as isolated sulfide and oxidized sulfide replacement bodies in limestone host rocks covered by 10–225 m of alluvium. Samples were analyzed for helium using mass spectrometry and for carbon dioxide and sulfur compounds by gas chromatography. Above average concentrations of carbon dioxide and helium in soil gases, as well as above average concentrations of carbon dioxide degassed from soils, occurred over and adjacent to all three ore bodies. Carbonyl sulfide and carbon disulfide degassed from soils occurred in the vicinity of ore bodies as deep as 150 m, but not to 225 m depth. Volatile constituents in soil gas samples collected by hollow probe defined the ore bodies better than volatile constituents degassed from surficial soil samples.
Sedimentary deposits in the Northern Lowland Plains, Mars
Authors: Lucchitta B. K. , Ferguson H. M. , Summers C.
Published: February 2008

The lowland plains on Mars have surfaces marked by large polygonal fracture patterns. It was recently proposed that the fracture patterns were developed on sedimentary deposits from outflow channels. We support this hypothesis because of the following observations. (1) Polygonal fracture patterns tend to occur in low areas on Mars that apparently received influx of sediments; the area of northern plains, in which the fractures occur, also forms a regional low. (2) The fracture patterns are concentrated near reentrants of the plains projecting into the southern highlands. Not only are these reentrants associated with outflow channels, but channels and polygonally fractured deposits have similar ages. (3) Sinuous ridges at the mouths of Martian outflow channels resemble ridges in Antarctic ice streams and ice shelves; the similarity suggests that the Martian material was transported from the southern highlands toward the northern plains, as the Antarctic material is transported from the continent toward the adjacent ocean. Overall, it appears that a massive transfer of material took place midway in Martian history, either through the outflow channels or elsewhere along the northern highland scarp, and that this material probably formed the deposits now characterized by polygonal fracture patterns.
Venus rock composition at the Vega 2 Landing Site
Authors: Surkov Yu. A. , Moskalyova L. P. , Kharyukova V. P. , Dudin A. D. , Smirnov G. G. , Zaitseva S. Ye.
Published: February 2008

The Vega 2 lander determined the composition of rock in the area of the northern slope of the high‐mountain massif Aphrodite Terra, which had never previously been analyzed. The rock was analyzed by X ray fluorescence analysis employing instrumentation similar to that on Venera 13 and 14. However, our experience with those probes made it possible to introduce some changes in the design of the device and in the program of its operation; as a result, the reliability of the device was improved. At the same time, the analytical characteristics of the instrument were improved: the range of the elements determined was expanded and the sensitivity was increased. The X ray fluorescence spectrometer operated on the Venusian surface in a preprogrammed mode. The abundances of the major rock‐forming elements from Mg to Fe, inclusively, and also upper limits for some heavier rare elements, were determined. Comparison of these data with analyses of terrestrial rocks enables us, with certain reservations, to suggest that the analyzed Venusian sample is olivine gabbro‐norite.
Transverse stress: Green's functions
Authors: Merriam J. B.
Published: February 2008

The theory required to handle the deformations of the Earth by surface tractions is developed. Green's functions for displacement, strain, tilt, gravity, and potential are produced. Preliminary results show that transverse stresses caused by ocean tide loading of seafloor relief, principally, the continental slope, can generate continental strain tide perturbations of a few parts in × 1010. This is a few percent of the total tidal strain and is comparable in magnitude to perturbations from local topography, cavity, and structural effects.
East Olkaria Geothermal Field, Kenya: 2. Predictions of well performance and reservoir depletion
Authors: Bodvarsson Gudmundur S. , Pruess Karsten , Stefansson Valgardur , Bjornsson Sveinbjorn , Ojiambo Sebastian B.
Published: February 2008

Performance predictions are presented for the East Olkaria geothermal field, using a three‐dimensional well‐by‐well model calibrated against 6.5 years of production history. Various reservoir development schemes are investigated to study the effects of different well spacing on well deliverabilities, power production of 45 and 105 MWe (megawatts electric), and the effects of injection on well performance and reservoir depletion. It is shown that the present well density at Olkaria (20 wells/km2) is too high; recommended well density for future wells is 11 wells/km2 (300‐m well spacing). The present production area at East Olkaria (2 km2) is capable of 45 MWe power production for a 30‐year period, but 105 MWe power production requires a well field area of about 9.5 km2, which may not exist. Injection can help sustain steam flow rates from wells, thus reducing the need for new development wells.
A fractal interpretation of topography and geoid spectra on the Earth, Moon, Venus, and Mars
Authors: Turcotte Donald L.
Published: February 2008

Global spectra are available for topography and geoid on the earth, Venus, Mars, and the moon. If the spectral energy density has a power law dependence on wave number a fractal is defined. The topography spectrum for the earth is a well‐defined fractal with D = 1.5; this corresponds to Brown noise with the amplitude proportional to the wave length. Although there is more scatter for the other planetary bodies, the data for Mars and the moon correlate well with the data for the earth. Venus topography also exhibits a Brown noise behavior but with a smaller amplitude. The power law dependence of the earth's geoid is known as Kaula's law. We show that uncompensated Brown topography gives a geoid with a power law dependence that is in quite good agreement with Kaula's law. However, the required amplitude is only 8% of the observed topography. A similar result is found for the other bodies with the ratio of the amplitude of topography required to explain the geoid to the observed topography increasing to 72% for the moon.
Neutron‐induced gamma ray spectroscopy: Simulations for chemical mapping of planetary surfaces
Authors: Brückner J. , Wänke H. , Reedy R. C.
Published: February 2008

Cosmic rays interact with the surface of a planetary body and produce a cascade of secondary particles, such as neutrons. Neutron‐induced scattering and capture reactions play an important role in the production of discrete gamma ray lines that can be measured by a gamma ray spectrometer on board an orbiting spacecraft. These data can be used to determine the concentration of many elements in the surface of a planetary body, which enables us to recognize individual geological units and provides clues to the bulk composition and in turn the origin and evolution of the body. To investigate the gamma ray fluxes induced by accelerator neutrons, experiments were carried out by irradiating thin targets with neutrons of energies from 14 MeV to 0.025 eV. The neutron fluxes at target position were measured by foil activation techniques. The ratio of the epithermal to thermal neutron flux was determined to be 2.0, a value that is similar to that in the moon. Gamma rays in the energy range of 0.1 to 8 MeV emitted by the targets and the surrounding material were measured by a high‐resolution germanium detector. Most of the gamma ray lines that are expected to be used for planetary gamma ray Spectroscopy were found in the recorded spectra. These spectra were unfolded, background was subtracted, and gamma ray attenuation corrections were made to obtain the corresponding gamma ray fluxes from the targets. The majority of gamma ray lines were narrow without noticeable Doppler broadening except for the very broad 4.4‐MeV line of carbon and five asymmetric germanium lines produced by the detector itself. The agreement of measured gamma ray flux ratios with calculated flux ratios for neutron‐capture reactions showed that thermal neutron data can be used for theoretical calculations of low‐energy neutron‐induced gamma ray fluxes. This study was a first step toward a more realistic simulation of cosmic‐ray‐induced gamma‐ray production and it indicates the importance of accelerator irradiation experiments to future planetary missions.
Melt segregation and flow in mantle diapirs below spreading centers: Evidence from the Oman Ophiolite
Authors: Rabinowicz M. , Ceuleneer G. , Nicolas A.
Published: February 2008

A mantle diapir of 8‐km radius has been recognized by systematic structural mapping of the Oman ophiolite in the Maqsad district. This diapir chilled while still active under the ridge crest. Streamlines rotated in the diverging part of the diapir a few hundred meters under the Moho. This implies a decrease by several orders of magnitude in the effective viscosity of the peridotites to a depth of 1 km beneath the Moho. We present dynamic models for diapiric flow across such an interface. A step of 0.5 MPa in dynamical pressure is found along the high/low viscosity interface; the highest pressure occurs within the uppermost low‐viscosity layer. This implies that magma percolating through the porous peridotite network is retained below this interface. This promotes a rapid increase of the melt/rock ratio, hence also a dramatic decrease in the effective viscosity of the peridotites. The feedback between magma percolation and decreasing viscosity produces the steep rotation of the mantle flow under the Moho.
Synthetic Pn and Sn phases and the frequency dependence of Q of oceanic lithosphere
Authors: Sereno Thomas J. , Orcutt John A.
Published: February 2008

The oceanic lithosphere is an extremely efficient waveguide for high‐frequency seismic energy. In particular, the propagation of the regional to teleseismic oceanic Pn and Sn phases is largely controlled by properties of the oceanic plates. The shallow velocity gradient in the sub‐Moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle. The confinement of Pn/Sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed. The rapid disappearance of Sn and the discontinuous drop in Pn/Sn group velocity beyond a critical distance, dependent upon the local thickness of the lithosphere, are interpreted as a shadowing effect of the low Q asthenosphere. Wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere. The results were compared to data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific. The Pn/Sn coda is successfully modeled as a sum of leaky organ‐pipe modes in the sediment layer and oceanic water column. While scattering is present to some degree, it is not required to explain the long duration and complicated nature of the Pn/Sn wave trains. The presence of extremely high frequencies in Pn/Sn phases and the greater efficiency of Sn than Pn propagation are interpreted in terms of an absorption band rheology. A shorter high‐frequency relaxation time for P waves than for S waves results in a rheology with the property that Qα > Qβ at low frequency while Qβ > Qα at high frequency, consistent with the teleseismic Pn/Sn observations. The absorption band model is to viewed as only an approximation to the true frequency dependence of Q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed.
Ocean borehole horizontal seismic sensor orientation determined from explosive charges
Authors: Anderson P. N. , Duennebier F. K. , Cessaro R. K.
Published: February 2008

High‐quality seismic data recorded by a three‐component, ocean borehole seismometer are used to determine the horizontal geophone azimuths to within ±1.5°. Analysis of water waves and first arrivals from 22 explosive charges fired to the downhole instrument show the effects of tilted sensors, shot mislocation, and dip of sediment‐basement interface. Analysis also shows that the ocean subbottom seismometer (OSS IV) borehole package is not positioned below the sediment‐basalt interface as previously believed but is actually locked in the sediments at some distance above the basement. The results of this study indicate that carefully located shots can be used to obtain geophone sensor azimuths to within ±1.0°, a precision adequate for most seismic and acoustic purposes.
Seasat Altimetry and the South Atlantic Geoid: 1. Spectral analysis
Authors: Gibert Dominique , Courtillot Vincent
Published: February 2008

This paper presents a discussion of the spectral characteristics of geoid profiles derived from Seasat alitmetry in the South Atlantic. Individual profile spectra are computed and averaged in seven separate geodynamic provinces (Mid‐Atlantic Ridge, flanks, Walvis and Rio Grande aseismic ridges, and Argentine and Cape basins). Three distinct wavebands can be identified in most spectra. For wavelengths shorter than 35 km the spectra are typical of white noise with a 7‐cm rms amplitude typical of Seasat altimeter measurement noise. For wavelengths between 300 and 800 km (our uppermost limit, since original profiles are 1700 km long) the spectra are in reasonable agreement with Kaula's empirical rule and are compatible with a Pratt (thermal boundary layer) compensation mechanism, with an ill‐defined compensation depth of 30–60 km. The paper centers on the description of the 35–300 km part of the spectra. Computation of an impedance for each province requires good knowledge of its bathymetry, which is far from satisfactory as yet in the South Atlantic. We show that for wavelengths longer than 80 km the squared amplitude spectrum of bathymetry varies as the inverse of wave number. We propose that this relationship might also hold at shorter wavelengths, although it is not observed due to presently insufficient data. Using these spectral estimates, and correcting for the mean depth of the topography, we derive impedance (actually response function) estimates separately for each province. The data are well accounted for by a regional compensation scheme based on elastic flexure models. Elastic thickness, the best resolved parameter, ranges from 2 to 5 km for the ridge and flanks, and the crustal thickness from 4 to 13 km. Crustal thickness appears to be larger on the east flank, demonstrating a general lack of symmetry between east and west in the South Atlantic, which appears to hold at all wavelengths and is also expressed in the bathymetry. The Rio Grande rise involves low (but nonzero) elastic thickness and large crustal thickness (25–35 km) in agreement with previously proposed models of local (Airy) compensation. The Walvis ridge appears to imply a larger elastic thickness, in excess of 8 km if the crustal thickness is less than 20 km, which may be due to emplacement on older lithosphere. Therefore, despite a lack of sufficient bathymetric control, geoid data can be used at the scale of the entire South Atlantic to provide reasonable information on thermomechanical compensation in the various provinces. The spectral structure emphasizes the 35–300 km spectral window and should be used in constructing band‐pass filters that can separately enhance features related to plate kinematics or sublithospheric convection.
The ammonia‐water phase diagram and its implications for icy satellites
Authors: Johnson Mary L. , Nicol Malcolm
Published: February 2008

We have used a Holzafpel‐type diamond anvil cell to determine the NH3‐H2O phase diagram in the region from 0 to 33 mol % NH3, 240 to 370 K, and 0 to 5 GPa. The following phases were identified: liquid; water ices Ih, III, V, VI, VII, and VIII; ammonia monohydrate, NH3·H2O; and ammonia dihydrate, NH3·2H2O. Ammonia dihydrate becomes prominent at moderate pressures (less than 1 GPa), with planetologically significant implications, including the possibility of layering in Titan's magma ocean.
Hydrological properties of Topopah Spring Tuff: Laboratory measurements
Authors: Daily William , Lin Wunan , Buscheck Thomas
Published: February 2008

The purpose of this work is to study the transport of water (both the liquid and vapor phases) in tuff from the Topopah Spring welded unit under conditions expected in the near‐field environment of a high‐level nuclear waste container. A naturally fractured sample of Topopah Spring tuff (approximately 8 cm in diameter and 10 cm long) from Yucca Mountain at the Nevada Test Site, Nevada, was studied using, as a pore fluid, natural groundwater recovered from a well in which the principal producing horizon is the Topopah Spring Member. Confining pressure, sample temperature, and pore pressure were held at values that simulate expected in situ near‐field conditions shortly after emplacement of the container. Results of this work are comparable with results from previous similar experiments on 2.54‐cm‐diameter samples of Topopah Spring tuff. During the more than 6‐month experiment duration, water permeability decreased about 3 orders of magnitude. The most rapid measured permeability change (between about 600 and 100 μdarcy) occurred when the sample temperature was increased from room temperature to 89°C. Subsequently, water permeability decreased in a fairly monotonie manner to a value of about 1 μdarcy. This behavior probably resulted from a decrease in the aperture of the natural fracture, possibly reflecting transport and redeposition of silica (which was already present in the fracture). This process also resulted in a weak bonding of the sample halves. Previous experiments on two smaller naturally fractured Topopah tuff samples resulted in fracture healing presumably by the same mechanism. The degree of healing, indicated by bonded tensile strength and change in fluid permeability, seems to be sample dependent. The distribution of electrical resistivity indicates that the sample dehydrates and rehydrates nonuniformly. This is consistent with our findings on smaller samples. Computed impedance tomographs indicate that during each of the five subsequent dehydrations of the sample, water first left the matrix adjacent to the fracture surface, then escaped the sample through the fracture aperture. Drying then progressed into the matrix around the fracture. Rehydration of the dry sample was not quite the reverse of the dehydration process. Water initially entered the fracture along the fracture edge, and the matrix was then wetted progressively from these points. However, the matrix was also wetted along paths apparently unrelated to fracture flow.
Explicit time‐dependent solutions and numerical evaluations for penny‐shaped hydraulic fracture models
Authors: Advani S. H. , Torok J. S. , Lee J. K. , Choudhry S.
Published: February 2008

The governing nonlinear equations for the hydraulically induced propagation of a circular crack in a poroelastic medium are derived. A Lagrangian formulation is utilized with the crack maximum width and radius selected as generalized coordinates. Explicit time‐dependent solutions for the crack geometry and fluid pressure associated with Newtonian and non‐Newtonian fracturing fluids are derived. The classical expression relating fracture width in terms of fracture radius is deduced as a special case. In addition, parameter sensitivity responses and comparison of results are presented.
Vibrational spectrum of MgSiO3 perovskite: Zero‐pressure Raman and mid‐infrared spectra to 27 GPa
Authors: Williams Quentin , Jeanloz Raymond , McMillan Paul
Published: February 2008

MgSiO3 in the perovskite structure, thought to be the dominant mineral phase of the earth's mantle, has been studied by both zero‐pressure Raman spectroscopy and by infrared spectroscopy to pressures of 27 GPa. A total of eight bands are observed: four in the Raman spectrum between 250 and 500 cm−1, and four in the mid‐infrared between 500 and 800 cm−1. These bands are divisible into three separate groupings: high frequency (∼800 cm−1), caused by asymmetric silicon‐oxygen stretching motions; intermediate frequency (370–690 cm−1), modes predominantly due to octahedral stretching and bending vibrations; and low frequency (200–300 cm−1), involving Mg2+ motion. The pressure shift of the four mid‐infrared modes yields an average mode Grüneisen parameter of 1.36 ± 0.15. Systematic trends observed between mode Grüneisen parameters and mode frequency are used to derive an estimate of the thermodynamic Grüneisen parameter from the spectroscopic data. In this way, we derive an estimated value of ∼1.9, in good agreement with the thermodynamic value of 1.77 measured at zero pressure.
Geochemistry, mineralogy, and petrogenesis of basalt from the Gorda Ridge
Authors: Davis Alicé S. , Clague David A.
Published: February 2008

Basalt pillow lava with glassy rims was dredged from 17 sites along the Gorda Ridge between latitudes 43°N and 40.8°N. All samples are low‐K2O mid‐ocean ridge tholeiitic basalt having a narrow compositional range with Mg numbers clustered around 60–62; more primitive and evolved compositions are present but rare. None of the more fractionated samples is as evolved as ferrobasalt from the Juan de Fuca Ridge. Incompatible trace element ratios are typical of normal mid‐ocean ridge basalt but show considerable scatter. The 87Sr/86Sr ratios, ranging from 0.70233 to 0.70267, are near the low end of the range for normal mid‐ocean ridge basalt. The phenocrysts present are abundant plagioclase, less abundant olivine ± spinel, and traces of sulfide. Clinopyroxene is absent in the glasses. The mineral chemistry is typical of ocean floor basalt. However, unusually anorthitic plagioclase in some highly plagioclasephyric samples, complex zoning in plagioclase and olivine, and other disequilibrium features indicate complex petrogenetic processes involving magma mixing. Glass inclusions in olivine and plagioclase indicate primitive trapped melts. The chemical diversity of Gorda Ridge lava appears to have been primarily caused by variable degrees of partial melting, but crystal fractionation and magma mixing were also important processes. Compositional variations along the strike of the ridge indicate that lava from the segment south of the offset at latitude 41.6°N was generated by the smallest percentage of partial melting of a source less depleted than that of the two northern segments. Lava from the central and northern segments was generated by variable percentages of partial melting of a uniformly depleted source. Lava from the northern segment shows the greatest diversity with a large range in percentage of melting and both relatively primitive and evolved compositions in spatially and temporally closely associated lava. The diversity of lava from the northern segment may result from thermal perturbations caused by proximity to the Blanco Fracture Zone. Gorda Ridge lava is more similar to that from the Mid‐Atlantic Ridge at 22°–25°N than to that from other spreading centers in the Pacific Ocean. Magma chambers under the Gorda Ridge appear to be small, discontinuous in time and space and possibly at greater depth than those beneath other Pacific spreading centers.
Metals and isotopes in Juan de Fuca Ridge hydrothermal fluids and their associated solid materials
Authors: Hinkley Todd K. , Tatsumoto Mitsunobu
Published: February 2008

The 87Sr/86Sr ratio of the hydrothermal solution (HTS) (0.7034) is larger than that of basalt (0.7025) at the southern vent field of the Juan de Fuca Ridge (SJFR). Both the Sr isotopic ratio for HTS and the water/rock interaction ratio lie between those at two sites farther south on the East Pacific Rise, 13°N and 21°N. These parameters may be closely related to subsurface temperatures and rates of magma ascent and to extent of faulting and surface areas of the frameworks of the hydrothermal systems. For these three Pacific Ocean sites there is no steady geographical progression of these measured parameters, nor of reported spreading rate, with increasing latitude northward. Pb and Nd isotopic measurements are uniform for all samples from the SJFR, ranging only from 18.43 to 18.58 for 206Pb/204Pb (fluids and associated solids) and centering near 0.5131 for 143Nd/144Nd (only fluids measured). Values for basalts and sulfides from the site have similar values. Relatively high 206Pb/204Pb values at the SJFR suggest the potential for the existence of an anomalous radiogenic heat source in the underlying mantle material. A Nd/Sm mass ratio of about 5 suggests that the system of the SJFR, like those at 13°N and 21°N, may be light rare earth element enriched. The availability of three types of solid material associated with each fluid sample has provided an opportunity to study chemical distribution between fluids and solids. Sr isotopic compositions provide strong evidence that at least two of the types of solids are precipitates from their individually associated fluids. For most metals measured, the masses in the fluids were greater than the masses in solid materials. However, some metals or the solids that contain them may not have been conserved, fractions possibly being lost during sampling or earlier. A Sr isotopic method of determining extent of seawater dilution of the fluid samples yields different results from the traditionally used method based on Mg concentrations, perhaps due to differing degrees of precipitation of the two metals.
Eruptions of Pavlof Volcano and their possible modulation by ocean load and tectonic stresses
Authors: McNutt S. R. , Beavan R. J.
Published: February 2008

All nine magmatic eruptions at Pavlof Volcano in six of the years from 1973 to 1984 have occurred between September 9 and November 20. Volumes of erupted material range from 0.1 to 15.9 × 106 m3 (dense rock equivalent) at an average rate of ∼3 × 106 m3 yr−1. The volumes are estimated from eyewitness reports for two eruptions; the others are estimated from a relationship that we derive between eruption volume and harmonic tremor duration and amplitude. The volume of lava erupted is approximately time predictable. A significant correlation exists between the eruptions and yearly nontidal variations in sea level and may result from ocean loading. Calculated volume changes beneath the volcano due to ocean loading are from 0.02 to 2 times eruption volumes, and we postulate that the volcano acts as a long‐period (several month) volume strain meter, with lava being preferentially erupted when strain beneath the volcano is compressive. The volcano did not erupt during the period 1978–1980, when tilt, seismic data, and sea level data indicate that deep aseismic slip may have occurred. Models of this event predict a volume increase beneath the volcano that might have compensated strain from magma injection. These observations indicate that Pavlof Volcano may be responsive to small, slow changes in ambient stresses or strains and that these changes may modify or trigger eruptions.
Impact craters of Venus: A continuation of the analysis of data from the Venera 15 and 16 spacecraft
Authors: Basilevsky A. T. , Ivanov B. A. , Burba G. A. , Chernaya I. M. , Kryuchkov V. P. , Nikolaeva O. V. , Campbell D. B. , Ronca L. B.
Published: February 2008

Additional studies of Venusian impact craters have been made based on an analysis of Venera 15/16 radar imagery and altimetry. The crater population on Venus has been subdivided into groups representing different morphological classes and types. The craters display the size‐dependent variations in morphology which are well known from other planets. Assuming a crater production rate based on estimates by Hartmann et al. (1981), their areal density indicates an age for the total population of approximately 1 b.y. Alternative estimates based on the assumption that the recent cratering rates on the earth and Venus were similar would give a significantly lower age. The areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed, and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b.y. in age. A suggested model of the effect of the atmosphere on the crater diameter‐frequency curve indicates that cometary impacts played only a minor role in forming the contemporary Venus environment. A comparative study of craters in the area where the Arecibo earth based radar and Venera 15/16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle Arecibo images correspond to the flat bottoms of the craters seen in the Venera images.
Introduction to Special Section on How Volcanoes Work: Part 1
Authors: Tilling Robert I.
Published: February 2008

More than 80% of the Earth's surface is of volcanic origin, testifying that volcanism was a dominant process in the creation and evolution of our planet. From a human standpoint, volcanic eruptions have engendered fear, superstition, fascination, and scientific curiosity. Since the emergence of volcanology as a science in the nineteenth century, the question of “How Volcanoes Work” has been considered by many minds, particularly in the wake of the well‐publicized and documented catastrophic 1883 eruption of Krakatau (Indonesia) recently reviewed by Simkin and Fiske [1983]. However, many more decades were to pass before scientists could begin to address seriously this fundamental question: by undertaking the needed systematic, long‐term studies of volcanoes, in addition to short‐term expeditions in response to specific volcanic events.
An experimental study of subduction and slab migration
Authors: Kincaid Chris , Olson Peter
Published: February 2008

Results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction: slab penetration through the transition zone and horizontal slab migration. Cold, negatively buoyant molded slabs of concentrated sucrose solution, with viscosities of 3–5 × 106 P were introduced into a more dilute, two‐layered sucrose solution representing the upper and lower mantle. The transition zone was modeled by a step increase in both density and viscosity. Ratios of slab thickness to upper layer depth, the relative viscosities in each layer, and the effective Rayleigh number were close to mantle values. The initial configuration consisted of two plates separated by a trench gap, with one plate attached to a dipping slab, simulating a developing subduction zone with an overriding plate. Two different boundary conditions were used at the trailing (ridge) end of the subducting plate: (1) fixed end, corresponding to zero ridge motion, and (2) free end, corresponding to zero ridge resistance. Based on results from 15 experiments with various combinations of upper layer, lower layer, and slab densities (ρU, ρL, ρS) and viscosities, we find that slab penetration depth is sensitive to both the normalized slab density anomaly R = (ρS ‐ ρL)/(ρS ‐ ρU) and the dip angle. Three penetration regimes are recognized: R ≳ 0.5, in which the slab sinks into the lower layer without distortion; −0.2 ≲ R ≲ 0.5, which permits limited penetration, the amount dependent on dip angle (in this regime the slab develops a thick root just beneath the transition zone); and R < −0.2, in which the slab is deflected by the transition, and little or no penetration occurs. Retrograde slab motion and trench migration occurred in nearly every case, with the overriding plate moving parallel to but faster than the trench, causing the gap between subducting and overriding plates to close with time. Prograde trench motion was never observed, and the trench remained stationary only in the extreme case (free ridge condition with equal upper and lower layer densities). These results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone.
How Thick Are the Continents?
Authors: Lerner‐Lam Arthur L. , Jordan Thomas H.
Published: February 2008

Fundamental and higher‐mode Rayleigh waves propagating across northern Eurasia and the western Pacific Ocean have been used to construct a lower bound on zmax, the maximum depth extent of seismic velocity differences between old continents and old oceans. The waveform inversion procedure of Lemer‐Lam and Jordan has been modified to search for optimum path‐averaged elastic models whose significant structural differences are “squeezed” above some chosen depth z*. An inversion with z* unconstrained yielded two reference models, EU0 for northern Eurasia and PA0 for the western Pacific. The reference models provide good fits to the vertical component waveform data out to the fourth overtone, but they are characterized by substantial velocity differences extending into the lower mantle. Resolving‐power calculations suggest that such deep variations are not required by the data. Using EU0 and PA0 as starting models for additional inversions and their fit to the data to define noise levels, we constructed a pair of acceptable models, EU2 and PA2, whose shear velocity differences are squeezed above z* = 400 km and compressional velocity differences squeezed above z* = 220 km. However, we were unable to obtain a successful fit to the data with z* = 220 km for shear velocity and thus reject the hypothesis Zmax ≤ 220 km in favor of the alternative zmax > 220 km. The EU2‐PA2 differences in shear velocities are generally consistent with other recent seismological models of continent‐ocean heterogeneity, including the regional models SNA and ATL of Grand and Helmberger and the global model M84C of Woodhouse and Dziewonski. The average upper mantle shear velocities in our P‐SV models are substantially lower than Grand and Helmberger's SH models, however, this discrepancy cannot be explained by simple path differences and points to the importance of polarization anisotropy. The high‐velocity zones underlying the ancient cratonic nuclei are inferred to be part of the tectosphere, translating coherently with the continents during plate motions. This and other studies of upper mantle structure are more consistent with a thick plate, compositionally stratified model of the cratons than with the conventional thin plate models of continental thermal structure.
Thermal and mechanical properties of the Cape Verde Rise
Authors: McNutt Marcia
Published: February 2008

The Cape Verde Rise represents an extreme case of the process of oceanic midplate volcanism. The regional depth and geoid anomalies relative to the plate model are the largest observed, peaking at 2.4 km and 12m, respectively. The late Tertiary volcanoes rest on Mesozoic‐aged seafloor, indicating that the lithosphere was about 130 m.y. old when loaded. Depth and geoid anomalies in the Cape Verde region are examined using both forward filtering and admittance techniques to determine the thermomechanical properties of the swell. The effective elastic thickness for the plate supporting the volcanoes is 28±4 km and the compensation depth for the broad swell is 69±10 km. These two results, combined with heat flow observations from the rise, are consistent with a model for reheating of the lithosphere beneath Cape Verde such that the average thermal gradient is increased by ∼35%. While there is no doubt that the Cape Verde lithosphere has been reheated, it is unlikely that the entire depth anomaly is explained by thermal anomalies confined to the lithosphere. A linear programming inversion of the geophysical data places an upper bound of 1.7 km on the portion of the depth anomaly due to lithospheric reheating. There are no permissible thermal models fitting all of the geophysical data that do not require either dynamic support from or elevated temperature within the asthenosphere.
Changes in direction of seafloor spreading revisited
Authors: Hey R. N. , Menard H. W. , Atwater T. M. , Caress D. W.
Published: February 2008

Rotation and propagation have both been proposed as mechanisms by which spreading centers can reorient following changes in direction of seafloor spreading. The rotation hypothesis predicts a smooth asymmetric fanning of magnetic anomaly and abyssal hill orientations, whereas the propagation hypothesis predicts that the orientation changes will occur at abrupt discontinuities which are either failed rifts or pseudofaults. To test between these hypotheses, we conducted a detailed Sea Beam/magnetics/seismic reflection survey of the area where the direction of Pacific‐Farallon spreading changed between anomalies 24 and 22, just north of the major bend in the Surveyor fracture zone. The change in trend of seafloor fabric is abrupt, supporting the rift propagation hypothesis for spreading center reorientation.