bluuebunny/crossref_metadata_2025 huggingface dataset:
Inference of the angular velocity of plasma in the Jovian magnetosphere from the sweepback of magnetic field
Authors: Khurana Krishan K. , Kivelson Margaret G.
Published: February 2008

Particle observations from the dayside magnetosphere of Jupiter have shown that the plasma subcorotales beyond a radial distance of ≈20 RJ. However, no information is available on the azimuthal velocity of plasma on the nightside of Jupiter owing to the unfavorable viewing geometries of the Voyager spacecraft. In this paper, we follow the approach of Vasyliunas (1983) to calculate the torque applied by the magnetic field on the plasma and calculate the resulting plasma angular velocity for various assumed outflow rates. Thus for a given value of the outflow rate we can calculate the sector‐averaged angular velocity of plasma from the observed values of the azimuthal component of the magnetic field (Bϕ) in the lobes and the normal component (Bz) in the plasma sheet. We develop new techniques to determine the normal and azimuthal components of the magnetic field in a plasma sheet coordinate system. From the Voyager 2 data, it appears that the corotation in the postmidnight quadrant of the magnetosphere can be maintained up to a radial distance of ≈50 RJ if the outflow rate in that quadrant does not exceed 2.5×1029 amu/s. For outflow rates exceeding 2.5×1029 amu/s in that quadrant partial corotation would result. Our conclusions differ from those of Vasyliunas because we have reanalyzed the Voyager data and have improved the estimates of Bϕ and Bz. A comparison of results from Voyager 1 and 2 is presented.
The ponderomotive force of standing Alfvén waves in a dipolar magnetosphere
Authors: Allan W.
Published: February 2008

Nonlinear effects associated with standing Alfvén waves may be significant in planetary magnetospheres. The field‐aligned second‐order force and acceleration generated by standing Alfvén waves in a dipolar magnetosphere are derived using previously published perturbation series for toroidal mode eigenfunctions and eigenvalues. The field‐aligned ponderomotive force and ponderomotive acceleration (PMA) are then derived by time integration. The second‐order acceleration and the PMA are independent of the plasma mass density distribution in the equatorial plane, and are only weakly dependent on the density structure along magnetic field lines. The second‐order acceleration has an oscillation at twice the standing wave frequency superimposed on the PMA at a given position. The amplitude and phase of this oscillation are dependent on ionospheric conductance and field‐aligned density structure. The PMA has an approximate L5 dependence, and can have relatively large values in the outer magnetosphere. For example a fundamental standing Alfvén wave with equatorial electric field amplitude 5 mV m−1 gives a maximum PMA of 12 g toward the equatorial plane at L = 9. An approximate upper limit on the energy of particles accelerated by this mechanism appears to be of the order of 1 keV, attained by O+ ions accelerated from the ionosphere to the equatorial plane at high L values.
Mechanistic investigations of shuttle glow
Authors: Caledonia G. E. , Holtzclaw K. W. , Krech R. H. , Sonnenfroh D. M. , Leone A. , Blumberg W. A. M.
Published: February 2008

A series of laboratory measurements have been performed in order to provide a mechanistic interpretation for the visible shuttle glow. These studies involved interactions of an 8 km/s oxygen atom beam with both contaminant dosed surfaces and gaseous targets. We conclude that visible shuttle glow arises from surface mediated O + NO recombination via a Langmuir‐Hinshelwood mechanism and that the gas‐phase exchange reaction O + N2 → NO + N provides a viable source of precursor NO above surfaces oriented in the ram direction.
Comparisons of spectral thermospheric general circulation model simulations and E and F region chemical release wind observations
Authors: Mikkelsen I. S. , Larsen M. F.
Published: February 2008

High‐latitude chemical release wind measurements were carried out in February and March 1978, in March 1985, and in March 1987. In each of the experiments, wind profiles were obtained covering heights in both the E and the F region. Three of the release experiments were carried out on the evening side of the auroral oval and one on the morning side. Two sets of measurements were carried out in disturbed conditions at solar maximum, while the other two were carried out during quiet periods at solar minimum. The spectral thermospheric general circulation model that has been developed at the Danish Meteorological Institute is used to simulate the conditions appropriate to each of the four experiments and detailed comparisons between the model predictions and the measurements are presented. Considering the uncertainties in the various external sources of forcing, such as the plasma convection patterns, the model adequately reproduces the major features of all the wind profiles. However in the E region the relative wind maxima from the model are, in general, above the heights of the observed wind maxima, possibly due to the oversimplified auroral precipitation used in the model, with the electrons being represented by single Maxwellian energy spectra only. The uncoupled neutral and ionized atmospheric compositions used in the model may also explain part of the unrealistic simulated winds. The upward propagating tides are found to modify the E region winds significantly, even under disturbed conditions when the plasma forcing might be expected to dominate the dynamics. In our results the latter is shown by the sensitivity of the simulated flows to the lower boundary condition which is the imposed tidal oscillation structure.
Evidence for ion jets in the high‐speed solar wind
Authors: Feldman W. C. , Gosling J. T. , McComas D. J. , Phillips J. L.
Published: February 2008

Shapes of proton and alpha particle velocity distribution functions in the high‐speed solar wind are examined for the time period from 1973 to 1987. Whereas enhanced high‐energy extensions are a near universal feature of the proton distributions, no such enhancements are observed in alpha particle distributions. Such distributions are examined and documented in detail. An interpretation of this observation is presented in terms of a two‐component model of coronal heating and acceleration. We suggest that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. This model predicts that the ambient flow of plasma from coronal holes is characterized by a low‐speed and a low‐mass flux. In particular, the proton flux is insufficient to drag alpha particles out of the solar gravitational potential well. This should lead to a considerable enhancement of the helium abundance close to the Sun. Plasma jets rich in helium and accelerated by reconnection close to the Sun buffet the ambient plasma from below. The slow‐moving ambient plasma is coupled to the faster‐moving jets by Coulomb friction close to the Sun, and then through microinstabilities far from the Sun. The net effect of this interaction is a high‐speed, double‐streaming proton population and a single component, but faster alpha particle flow at large heliocentric distances.
Precipitation of solar wind like ions in the polar cap during northward interplanetary magnetic field
Authors: Nishida A. , Mukai T. , Hayakawa H. , Kaya N. , Fujimoto M.
Published: February 2008

At altitudes of 8000 to 10,000 km EXOS D (Akebono) has observed several cases of an almost monoenergetic band of ions that extends over spatial ranges of several degrees or more poleward of the cusp. The energy spectra of these ions vary little over large distances in each case, and are very similar to the high‐energy component of the ion spectrum in the cusp. The ion spectra are compatible with the field‐aligned beam where the energy of the bulk motion (0.7 ‐ 2 keV) is about 10 times the thermal energy (0.04 ‐ 0.2 keV). Number fluxes are of the order of 106 (cm² s)−1, and densities are of the order of 10−2 cm−3. These ion bands are observed during intervals of northward IMF, and are sometimes accompanied by electron precipitation of the polar rain type. These ions are likely to be the energetic component of the solar wind ions which are introduced into the magnetosphere together with MHD waves that are transmitted from the magnetosheath.
Long‐term variations of interplanetary magnetic field spectra with implications for cosmic ray modulation
Authors: Bieber John W. , Chen Jiasheng , Matthaeus William H. , Smith Charles W. , Pomerantz Martin A.
Published: February 2008

Yearly averaged power spectra of interplanetary magnetic field turbulence at 1 AU are calculated over the period 1965‐1988 for fluctuations in the frequency range 5.8 × 10−6 to 4.6 × 10−5 Hz, corresponding to periods of 6‐48 hours. The spectra are well described by power laws with a typical spectral index of −1.2. Averaged over 24 years, the amplitudes of the component spectra are in the proportion 1.8∶1.5∶1 for magnetic field components, respectively, in the north‐south direction, perpendicular to the Parker spiral in the ecliptic plane, and parallel to the Parker spiral. The amplitudes of the spectra vary with the sunspot cycle and are inversely correlated with the intensity of ∼10‐GeV cosmic rays. Using the observed spectra, we calculate a lower limit to the cosmic ray scattering mean free path using resonant magnetostatic quasi‐linear theory for both “slab” and isotropic geometries of the turbulence. The mean free paths thus obtained are typically ∼0.1 AU in the slab model and ∼0.3 AU in the isotropic model, but they are not significantly correlated with the modulated galactic cosmic ray intensity recorded by neutron monitors. This suggests that the scattering processes described by resonant magnetostatic theory play, at best, a very minor role in the solar modulation of ∼10‐GeV cosmic rays.
Testing the isotropic boundary algorithm method to evaluate the magnetic field configuration in the tail
Authors: Sergeev V. A. , Malkov M. , Mursula K.
Published: February 2008

Simultaneous measurements of the low‐altitude energetic particle flux by NOAA spacecraft and the geostationary magnetic field by GOES 2 spacecraft are used to test the recently proposed isotropic boundary algorithm (IBA) method to evaluate the instantaneous magnetospheric configuration. According to the IBA method, the equatorward boundary of the isotropic proton precipitation, in brief the isotropic boundary (IB), corresponds to the boundary separating adiabatic and chaotic regimes of particle motion in the tail current sheet and is controlled by the properties of the equatorial magnetic field. In this study we confirm some of the fundamental features of the IBA method. First, we show that the low‐altitude IB position of 30‐ to 300‐keV protons is strongly controlled by the equatorial magnetic field in the tail. (The corresponding correlation coefficient exceeds 0.9.) Second, the MLT dependence of the nightside IB latitude is in good agreement with that computed using magnetospheric models. Third, the observed magnetic field and the field predicted by the IBA method using the measured IB position have similar values and are well correlated with a correlation coefficient of at least 0.84 for the main components and a standard deviation of only about 10% of the dynamic range of these components. This shows that the threshold condition separating the two particle motion regimes is fulfilled in the proximity of the IB field line. We argue that the remaining inconsistencies between the calculated and observed magnetic fields are mainly due to the fact that the available magnetospheric models seem to underestimate the amount of tailward stretching of both the tail field lines during active conditions as well as field lines starting from the dayside. In view of its good capabilities to remotely determine the instantaneous magnetic field, we expect that the IBA method will find wide applications in the mapping of magnetic field lines and in testing of existing and new magnetospheric models.
Ion transport and loss in the Earth's quiet ring current: 1. Data and standard model
Authors: Sheldon R. B. , Hamilton D. C.
Published: February 2008

We have studied the transport and loss of ions in the Earth's quiet time ring current, comparing the standard radial diffusion model developed for the higher‐energy radiation belt particles with measurements of the lower‐energy ring current ions. We compiled a data set with full local time coverage from the quietest days seen by the AMPTE/CCE/CHEM instrument in near‐equatorial orbit at L=2‐9 RE. This data set provides, for the first time, ionic composition information in an energy range that includes the bulk of the ring current energy density, 1‐300 keV/e. Protons were found to dominate the quiet time energy density at all altitudes, peaking near L∼4 at 60 keV cm−3, with much smaller contributions from O+ (1‐10%), He+ (1‐5%), and He++ (<1%). The proton densities were azimuthally symmetric excepting a small dawn‐dusk distortion caused by the cross‐tail electric field, and a plasma sheet contribution for L>6 near midnight. Thus the standard radial diffusion model, which incorporates an outer source boundary at 7.5 RE from the Earth, and diffuses ions earthward while undergoing charge exchange and Coulomb energy loss, should fit the data. We improved on previously used model loss processes by incorporating the latest atomic physics cross sections from the literature, updating the last survey done 15 years ago. We also included the effects of finite electron temperature on Coulomb drag. A χ² minimization procedure was used to fit the amplitudes of the standard electric radial diffusion coefficient, giving DLLE = 5.8 × 10−11 RE²/s. Yet the model was unable to fit the data (to within a factor of 10) over 50% of the energy and radial ranges of the data set, particularly at L<4 or E<30 keV. Assuming that the loss terms in the model are correct, the data can be inverted to extract a radial diffusion coefficient that had nearly constant amplitude from 2‐7 RE. This suggests that another transport mechanism is operating in the ring current region, which is strongest at smaller radial distances. We speculate that fluctuating ionospheric electric fields may be the source of this additional diffusion.
A numerical ionosphere‐magnetosphere coupling model with variable conductivities
Authors: Janhunen P. , Huuskonen A.
Published: February 2008

We describe a numerical ionosphere‐magnetosphere coupling model with a relationship between the field‐aligned potential drop and the ionospheric conductivities. A two‐dimensional incompressible magnetohydrodynamic (MHD) model for the magnetosphere, field lines with finite resistivity, a “2 1/2” dimensional ionosphere, as well as a phenomenological modeling of the ionization process are integrated in a time‐dependent fluid simulation code. We discuss the algorithm and the validity of the results and give an example run with possible application to eastward drifting Ω bands of the morning sector. Our model is valid for time scales longer than the Alfvén bounce time and for spatial scales roughly between 10 and 1000 km. It is found that the inhomogeneous ionospheric conductivities affect the system dynamics substantially. This feature is essential when the model is used to study the development of auroral forms.
Present‐day stress orientations adjacent to active strike‐slip faults: California and Sumatra
Authors: Mount Van S. , Suppe John
Published: February 2008

Present‐day stress directions interpreted from well bore breakouts adjacent to two crastal‐scale, active, strike‐slip faults (the San Andreas fault in California and the Great Sumatran fault in Sumatra) indicate that the maximum horizontal stress direction (SH) is oriented at a high angle (70°–90°) to both faults. The regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to SH and are therefore in the thrust‐fault orientation. These observations indicate a decoupling of the strike‐slip and compressional components of the deformation within these broadly transpressive zones. Borehole breakouts in 118 wells in western California indicate a regionally consistent stress pattern with SH generally oriented NE‐SW, nearly perpendicular (80°–90°) to the strike of the San Andreas fault. The orientation of SH nearly perpendicular to the San Andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the Coast and Transverse Ranges indicating active compressional deformation, fault plane solutions for recent dip‐slip‐style earthquakes, principal stress directions determined from inversion of earthquake focal mechanisms, and induced hydraulic fracture orientations. A stress trajectory map for western California is computed using an iterative statistical algorithm in which observed directional data, such as breakout directions, are used to obtain a model regional stress field. Analysis of well bore breakouts in 25 wells within the central and southern oil districts of Sumatra indicates that the regionally defined SH adjacent to the active Great Sumatran strike‐slip fault is oriented at a high angle (70°–80°) to the fault This orientation of SH is consistent regionally with geologic stress indicators and focal mechanisms of dip‐slip earthquakes. Preliminary analysis of the stress field in the vicinity of the Philippine and Alpine faults suggests SH is also oriented at a high angle to these active strike‐slip faults. Similarly, the minimum horizontal stress Sh is oriented at a high angle to the the Kane and Dead Sea transforms. The observation of SH and Sh in the vicinity of active strike‐slip faults being oriented nearly perpendicular and parallel to the faults suggests that large, crustal‐scale strike‐slip faults may, in general, be inherently weak surfaces.
Petrology and magma chamber processes at the East Pacific Rise ∼ 9°30′N
Authors: Batiza Rodey , Niu Yaoling
Published: February 2008

We present new major and trace element data for a set of closely spaced (< 1.8 km) dredges along a well‐studied portion of the East Pacific Rise (EPR) axis near 90°30′N (9°17′N to 90°50′N). With the exception of enriched mid‐ocean ridge basalt (E‐MORB) at 9°35′N, the lavas are all normal mid‐ocean ridge basalt (N‐MORB) with a limited range of MgO (8.40–6.22 wt %). Major element and trace element data favor derivation of the melts from a single parental composition by low pressure crystallization of olivine, plagioclase and clinopyroxene in the ratio of 16:62:22. This model is consistent with liquid line of descent models but inconsistent with petrographic observations in that most of the N‐MORB lavas have only plagioclase phenocrysts. We ascribe this to gravitational crystal settling of mafic phases and flotation of plagioclase, supported by crystal size distribution data and density relations. Most likely this occurred in the axial magma chamber (AMC) that underlies the EPR in this area. The chemistry of axial lavas varies along axis and correlates roughly with elevation of the axis and depth to the AMC. We interpret these correlations as favoring an AMC that is chemically zoned along‐axis, with Fe‐rich melts at its distal ends. This favors a central injection of MgO‐rich melt with lateral along‐axis shallow transport. The height of eruptions along axis is apparently controlled by magma density such that least dense MgO‐rich melts build local volcanic constructs of the highest elevation. The E‐MORB lavas are older than the N‐MORB and probably erupted at a time when the AMC was absent or was much smaller in size than presently. E‐MORB could have originated by deep melting processes or very shallow contamination of N‐MORB. Its presence in the 9°30′N area supports the notion that magma chambers are not truly steady state. Instead they probably come and go on a time scale of 3000–6000 years. A single parental magma seems to supply melts to the AMC along the entire 60 km segment of the EPR, suggesting that central supply injection sites are widely spaced. Based on our data, there is no evidence for petrologic segmentation corresponding to 4th‐order tectonic segmentation in this area; however, it may be present below the resolution of our sampling.
Postextensional mantle healing and episodic extension in the Canning Basin
Authors: Braun Jean
Published: February 2008

Regions of the lithosphere that undergo finite crustal extension but are subsequently allowed to thermally relax during a period of reduced tectonic activity may become regions of relative strength and therefore resist any further extension. The strengthening is a consequence of the temperature dependence of rock rheology and the local thinning of the crust bringing strong upper mantle olivine‐rich rocks closer to the surface where they replace weaker lower crustal plagioclase‐feldspar‐amphibole bearing rocks. This concept, which I call postextensional mantle healing, is illustrated by the predictions of a numerical model. Although the model is based on the results of rock testing in the laboratory, the results do not depend on a particular choice of creep parameters for the lower crust. The Palaeozoic tectonic history of the Canning Basin (northwestern Australia) is characterized by a series of extensional episodes separated by periods of relative quiescence. In this paper, I suggest that postextensional mantle healing is responsible for the continuous shifting through time of tectonic activity between the several major depocenters. The tectonic development of the East African rift system follows a scenario similar to that of the Canning Basin: extensional structures inherited from two bursts of extension in the Early Mesozoic and Cretaceous have not been reactivated during the Tertiary rifting which is responsible for the present‐day rift morphology.
The contact between opposing fault surfaces at Dixie Valley, Nevada, and implications for fault mechanics
Authors: Power William L. , Tullis Terry E.
Published: February 2008

Well‐exposed normal fault surfaces from Dixie Valley, Nevada, provide a unique opportunity to study the contact properties of fault surfaces, because it is possible to examine both the hanging wall and footwall surfaces of a fault zone which was once seismically active. The topography of the individual surfaces, as well as the aperture, or gap between the two surfaces, was measured using surface profiling instruments and a resin replication technique. These measurements enable frictional parameters of the surfaces, most notably the characteristic decay distance, Dc, to be calculated using contact theory. Although the individual surfaces are approximately self‐similar and fractal, the aperture between the opposing surfaces is a stationary quantity because the surfaces are well correlated at dimensions larger than approximately 2 mm. Consequently contact theory can be used to calculate an average size for contact spots between the surfaces, and by inference, a characteristic decay distance, Dc, for frictional slip between the surfaces. Direct inspection of resin replicas of the aperture allows an independent estimate of average contact spot size. Both approaches give contact spot diameters of ≈0.16 mm, and Dc of 0.01–0.1 mm. Because the individual surfaces of the fault are fractal and approximately self‐similar, correlation between the surfaces should decrease significantly during slip events, particularly if slip rates are high (seismic events). Decreased correlation between the surfaces should lead to larger contact spot sizes and hence larger Dc. Thus critical slip distances for natural faults should evolve concurrently with seismic fault slip.
Paleostress adjacent to the Alpine Fault of New Zealand: Fault, vein, and styolite data from the Doctors Dome Area
Authors: Nicol Andrew , Wise Donald U.
Published: February 2008

Doctors Dome, 75 km north of Christchurch, New Zealand, is an early Pleistocene to Recent structure being deformed along the southeast edge of the Pacific‐Australian plate boundary. Paleostress in the area has been determined in basement rocks of the Mesozoic meta‐graywacke Torlesse Supergroup which lies unconformably beneath Cretaceous and younger cover rocks. Pre‐Late Cretaceous multiple folding and bedding plane faulting produced a complex, low grade, metasedimentary basement with predominantly steeply plunging fold hinges and largely isotropic overall behavior with respect to later deformation. Probable Late Cretaceous WNW‐ESE shortening and NNE‐WSW extension of the basement produced widespread laumontite, calcite, and silica veins. Widespread but sparse slickensided minor fault surfaces are probably related to the same stress system. During the late Cenozoic dome stage, basement was uplifted, rotated and severely thrust along some of the range fronts to produce local high concentrations of small slickensided surfaces in the frontal areas. Inversion of basement fault data for the area indicates a general northwest compression with two peaks, one WNW‐ESE parallel to the shortening suggested by the older vein system and the other parallel to southeast‐northwest stylolite columns in the cover rocks. This direction is approximately parallel to regional indicators of contemporary deformation in and adjacent to the Alpine Fault Zone and suggests that the stress field affecting these rocks has not changed significantly since the late Pliocene‐early Pleistocene. Like the San Andreas system, this compression is at a high angle to the strike of the zone as a whole, but is compatible with the direction of plate convergence and motion of the major faults. Between the overlapping ends of the Alpine Fault and the Hikurangi Subduction Zone the Alpine Fault may become subhorizontal at middle‐lower crustal levels, partially decoupling the crust from underlying structures, and thus allowing oblique motion to be transferred directly onto the fault from the subduction complex, while aiding the change from subduction to continental collision.
Multibeam bathymetric, gravity and magnetic studies over 79°E fracture zone, Central Indian Basin
Authors: Raju K. A. Kamesh , Ramprasad T. , Kodagali V. N. , Nair R. R.
Published: February 2008

A regional scale bathymetric map has been constructed for the 79°E fracture zone (FZ) in the Central Indian Basin between 10°15′S and 14°45′S latitudes and 78°55′E and 79°20′E longitudes using the high‐resolution capabilities of the Hydrosweep multibeam swath bathymetric survey system. The bathymetric map clearly reveals the 79°E FZ, depicting a ridge and trough topography with an elevation difference of over 300 m. Prominent E‐W bathymetric lineations and several new features such as seamounts are conspicuous on the map. The E‐W bathymetric lineations bend at the fracture zone toward south resulting in oblique extensional features. These features are more prominent in the middle and northern portion of the study area and are less significant in the southern portion. The prominence of these features in the north and their subtle presence in the south are probably related to the strong and weak shear coupling at the RTI, respectively, during the various stages of FZ evolution. Gravity and magnetic data were analyzed together with the bathymetric data. The observed free‐air gravity across the 79°E FZ is modeled in terms of the lithospheric thickness, taking into consideration the thermal evolution of the oceanic lithosphere. A lithosphere thickness of about 100 km is inferred from these studies.
Gravity map of Brazil: 2. Regional and residual isostatic anomalies and their correlation with major tectonic provinces
Authors: Ussami Naomi , de Sá Nelsi Côgo , Molina Eder Cassola
Published: February 2008

An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.
Assessed data on heat capacity, thermal expansion, and compressibility for some oxides and silicates
Authors: Saxena Surendra K. , Shen Guoyin
Published: February 2008

Thermochemical and thermophysical data for several oxides and silicates are assessed and refined from the available data on Cp (heat capacity at constant pressure), thermal expansion (α) and compressibility (β) from the relation: Cp = Cν + α2VTT/βT, where Cν (heat capacity at constant volume) at low‐temperatures is modelled from Kieffer's (1979a, b, c) model. For five minerals, periclase, forsterite, pyrope, lime, and corundum, high temperature data are available and a comparison between the available and assessed data shows a good match. The available data on thermal expansion and bulk modulus, the calculated and assessed Cν data on 44 oxides and silicates are made internally consistent satisfying the above relationship between the heat capacity at constant pressure and at constant volume, thermal expansion and bulk modulus. These data are useful in phase equilibrium computations.
Electrical conductivity of San Carlos Olivine along [100] under oxygen‐ and pyroxene‐buffered conditions and implications for defect equilibria
Authors: Wanamaker B. J. , Duba A. G.
Published: February 2008

The electrical conductivity along [100] of single crystal San Carlos olivine was measured as a function of temperature between 1100° and 1200°C and oxygen fugacity between 10−6 and 10+0.5 Pa (at 1200°C), and either with (“pyroxene‐buffered”) or without (“self‐buffered”) an added natural pyroxene buffer from a San Carlos lherzolite. Under these temperature and ƒO2 conditions, electrical conduction in the self‐buffered sample is attributed to polarons (Fe*) and electrons (e′) and in the pyroxene‐buffered sample is attributed to polarons(Fe*) and magnesium vacancies(V″Mg). Over the range of temperature and ƒO2 investigated, the electrical conductivity of the self‐buffered sample is given by σsb[100] = 2.27(S/m)e−0.55(eV)/kT ƒO20.18+306.3(S/m)e−2.25(eV)/kT ƒO2−0.18 and for the pyroxene‐buffered sample by σpb[100] = 0.18(S/m)e−0.34(eV)/kT ƒO20.17+15.2(S/m)e−1.3(eV)/kT where k is Boltzmann's constant, T is in Kelvin, and ƒO2 is in atmospheres. The conductivity of the pyroxene‐buffered sample is lower than that of the self‐buffered sample, primarily as a result of a decrease in the polaron concentration. the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wüstite‐magnetite buffer. Electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes. A series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to <0.1 μm. Mantle temperature profiles based on laboratory measurements of self‐buffered samples predict temperatures of the order of between 25° and 150°C colder, depending on the ambient oxygen fugacity, than those based on measurements of pyroxene‐buffered samples.
Solidification and morphology of submarine lavas: A dependence on extrusion rate
Authors: Griffiths Ross W. , Fink Jonathan H.
Published: February 2008

The results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity. The experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter, the ratio of the time taken for the surface to solidify, and a time scale for lateral flow. For submarine basalts a solution of the cooling problem (which is dominated by conduction in the lava but convective heat transfer in the water) and estimates of lava viscosities place this parameter within the empirically determined “pillowing” regime over a wide range of extrusion rates. This result is consistent with the observation that pillow basalts are the most common products of submarine eruptions. Smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma. Still higher eruption rates in regions of low topographic relief may produce submarine lava lakes. Minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type.
Effects of the melting regime on the composition of the oceanic crust
Authors: Plank Terry , Langmuir Charles H.
Published: February 2008

The physical form of the melting regime and the mechanisms of melt extraction influence the composition of magmas erupted at ocean ridges. We investigate aspects of this relationship, beginning with the assumption that melts can be extracted from the melting regime without significant reequilibration during their passage to the surface. The ocean crust thus represents a mixture of the individual melts. Many melting regimes lead to the same “residual mantle column” (RMC), defined as a vertical section through the mantle external to the melting regime. The RMC is the integrated result of melt extraction and is useful in evaluating the geochemical effects of many different types of melting regimes. Consideration of the RMC shows that the “shape” of the melting regime is not necessarily an important parameter in affecting the composition of the ocean crust. The important parameters are the way mantle flows through the melting regime and the relationship between melt fraction and pressure during adiabatic melting. Calculating the volume and composition of the ocean crust can be reduced to a simple mixing problem. Virtually all ridge models predict continuous mixing of melts from the solidus to the maximum extent of melting. Given these boundary conditions, even complex melting regimes lead to geochemical results that are similar to those produced by batch melting. Thus batch melting may approximate the net effects of the melting process remarkably well. An important exception to these generalizations is binary mixing between melts of very different composition. This is possible beneath ocean ridges if very low degree melts, formed at the volatile‐present solidus, mix with higher‐degree melts formed directly beneath the ridge. There are limitations to the effectiveness of such a mixing process because the source volume for the low‐degree melts is constrained by the finite pressure interval between the dry and volatile‐present solidi of the mantle. These constraints place an upper limit of a factor of 5 on the incompatible element enrichment that can be explained by such mixing. This is a small factor relative to the global variability of mid‐ocean ridge basalts. A few local regions, however, show major and trace element covariations that may be consistent with this type of mixing. Adequate data sets to fully test the possibility are lacking. If such mixing occurs, there must be a physical mechanism to focus the lowest degree melts from the furthest reaches of the melting regime into the mantle directly beneath the ridge axis. The physical difficulties associated with horizontal transport of low‐degree melts over tens to hundreds of kilometers are imposing and suggest that alternative models should be seriously considered.
40Ar/39Ar Geochronology of Post‐Valles Caldera Rhyolites, Jemez Volcanic Field, New Mexico
Authors: Spell Terry L. , Harrison T. Mark
Published: February 2008

The40Ar/39Ar dating of postcollapse rhyolites in the Valles Caldera (1140 kyr) defines five intervals during which rhyolite domes, flows, and pyroclastic rocks were emplaced from ring fracture vents. Previously, ages were based on limited K‐Ar dating and stratigraphic relations. The 40Ar/39Ar dates are concordant with the previous dates for about half of the samples; others have ages significantly older (7–25%) despite evidence for small amounts of xenocrystic material. Eruptive activity occurred during relatively brief periods at 1133, 973‐915, 800‐787, 557‐521, and ∼300‐170 kyr separated by quiescent intervals of ∼115–300 kyr. Isochrons defined by single‐crystal sanidine fusions indicate trapped 40Ar/39Ar ranging from ∼282 to 326. Sanidine analyses yield reliable and consistent results, whereas biotite and glass data may be ambiguous. Biotite may have low radiogenic yields, contain variable amounts of extraneous argon, or define apparent isochrons discordant with cogenetic sanidine. Unaltered, unhydrated glass in one case yields an isochron concordant with coexisting sanidine, whereas in another is undateable due to large amounts of non‐radiogenic argon. The revised chronology has implications for the evolution of the Jemez magmatic system as well as for the geomagnetic polarity timescale. The discrete periods of volcanism, relations among rhyolites erupted within these intervals, and the duration of the intervening quiescent intervals are consistent with the generation, evolution, and eruption of several distinct magmas following caldera collapse. In addition, the 40Ar/39Ar data suggest revision of the Brunhes ‐ Matuyama polarity reversal boundary from the currently accepted 730 kyr to 780 kyr.
The Pictou Red Beds' Pennsylvanian Pole: Could Phanerozoic rocks in the interior United States be remagnetized?
Authors: Pan H. , Symons D. T. A.
Published: February 2008

The Pennsylvanian red beds of the Pictou Group on Prince Edward Island, Canada, retain a stable remanence in hematite that gives a paleomagnetic pole position at 42°N, 131°E (dp=2°, dm=4°). Positive conglomerate and contact tests along with dual polarities show that the remanence is primary and of Stephanian age (286–306 Ma). For elongated pole distributions through time, Bingham statistics is more suitable than Fisher statistics. Bingham statistics shows that there is a consistent discordance in the Pennsylvanian to Late Triassic poles from the Appalachian orogenic belt and interior craton of North America. This discordance is most readily explained by younger partial remagnetization, or possibly complete remagnetization, in many rock units, especially red beds of the interior craton. Bingham analysis also shows that there is a significant difference between the Colorado Plateau and interior craton that is most readily explained by a Laramide 5.5° ± 2.9° clockwise rotation of the plateau relative to the craton.
Crack models for a transversely isotropic medium
Authors: Cheng C. H.
Published: February 2008

A commonly used model for a transversely isotropic cracked rock is that by Hudson (1980, 1981). This model is based on a simplified analysis of a thin circular crack, with displacement and stress conditions specified on the boundary. These papers have a second‐order correction in addition to the first‐order term in porosity/crack density. In this paper we compare the results of Hudson with those of Anderson et al. (1974) and Cheng (1978) using the long‐wavelength static approximation and the ellipsoidal crack model first proposed by Eshelby (1957). We show that the Hudson model and those based on the complete Eshelby theory agree for small‐aspect‐ratio cracks and small crack densities, as expected, provided the “weak material” version of Hudson's (1981) model is used. For larger crack densities but small aspect ratios, Hudson's first‐order term agrees with the Eshelby solution. The expansion in the second‐order term in crack density is an asymptotic series and not a uniformly converging series. Thus there is no general statement one can make about the accuracy of the second‐order expansion that is valid for a variety of situations. A new expansion based on the Padé approximation is proposed which is identical to Hudson's expansion up to second‐order in density. This expansion avoids some of the problems associated with Hudson's second‐order expansion such as increasing moduli with crack density at relatively small crack densities.
Geothermal heat flux from hydrothermal plumes on the Juan de Fuca Ridge
Authors: Bemis Karen G. , Von Herzen Richard P. , Mottl Michael J.
Published: February 2008

Estimates of the heat output of hydrothermal vents, identified along the Endeavor and Southern segments of the Juan de Fuca Ridge, are used to evaluate the total heat flux associated with hydrothermal circulation for the ridge segment. A 50‐m array carried by DSV Alvin sampled the temperature and vertical velocity structure of hydrothermal plumes from individual vents. These measurements are used to estimate the thermal flux associated with such plumes. The maximum heat flux calculated for a single vent is 50 MW (1 MW = 1×106 W). The median heat flux per vent is 9 MW and 3 MW, respectively, for the Endeavour Segment (18 vents) and Southern Segment (18 vents). The estimates for any given vent may vary over an order of magnitude. This uncertainty is due mainly to the difficulty of locating the centerline of the plume relative to the point of measurement, although the uncertainties in the constants for the appropriate equations based on laboratory experiments also contribute significantly to the net error. For the Endeavor Segment, the minimum total geothermal heat flux due to hydrothermal circulation exceeds 70 MW. The minimum estimate for the Southern Segment is 16 MW. The maximum estimate is probably closer to the total heat flux from high‐temperature venting (239 MW and 66 MW respectively). High‐temperature hydrothermal venting accounts for only a small fraction of the heat available according to steady state predictions of conductive heat flux; other hydrothermal phenomena (e.g., diffuse flow) probably account for a greater proportion of the total hydrothermal heat flux.
Lithospheric structure of Tibet and western North America: Mechanisms of uplift and a comparative study
Authors: Beghoul Noureddine , Barazangi Muawia , Isacks Bryan L.
Published: February 2008

We seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the Cenozoic uplift of the Tibetan plateau located behind the Himalaya collision zone and the western United States located behind the San Andreas transform/convergent plate boundary. Selected first P arrivals of International Seismological Centre (ISC) data at distances less than 22° are analyzed by the two‐station method to constrain the upper mantle structure of the Basin and Range (BR), Colorado Plateau (CP), Great Plains (GP) and Tibetan Plateau (TP). The data are presented as plots of apparent velocity versus distance to the second station. We show that these observations of apparent velocities versus distance as well as those that concern the attenuation of Sn provide estimates of the mantle lid thicknesses. We obtain the following mantle lid thicknesses: BR 20–40 km; CP 35–50 km; GP 150–195 km; and TP 135–180 km. Sn is not observed for paths beneath the BR and CP, while Sn efficiently propagates beneath the GP; Sn is also observed beneath most of Tibet except in the north central part of the TP. A possible interpretation of our observations is that Indian continental lithosphere underthrusts only about the southern 2/3 of Tibet.
A teleseismic study of the Altiplano and the eastern Cordillera in northern Bolivia: New constraints on a lithospheric model
Authors: Dorbath Catherine , Granet Michel , Poupinet Georges , Martinez Claude
Published: February 2008

The Altiplano, one of the world's great plateaus, is the most significant geomorphological unit of the Central Andes. Knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region. In order to study this, we performed a teleseismic field experiment in northern Bolivia. During a 4‐month period, 34 short‐period seismic stations were installed along a 320‐km‐long profile, crossing the Altiplano and the Eastern Cordillera, from the volcanic arc zone to the sub‐Andean zone, perpendicular to the main structural direction. A careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns; the maximum amplitude of these residuals along the profile reaches 3 s for P phases and have a strong azimuthal dependence. The residuals show a noticeable sudden increase, clearly associated with the fault system bordering the Eastern Cordillera to the west, the Cordillera Real fault system. The high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur. The origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill, with the maximum thickness found under the Altiplano basin. The velocity perturbations in the lower crust are interpreted as variations of the Moho depth which decreases from about 60 km below the Altiplano to 50 km below the Eastern Cordillera. In the upper mantle, a high‐velocity zone is observed below the Eastern Cordillera. The major feature of this model is a subvertical boundary, dipping slightly to the southwest, which separates two strongly contrasting velocity units throughout the model, from the surface down to 140 km. This boundary coincides with the Cordillera Real fault system and is interpreted as an old suture. As the Altiplano is characterized by low velocities in the crust, the high velocities beneath the Eastern Cordillera down to 120 km may correspond to the Brazilian craton; in the region studied, the western limit of the underthrusting of the craton corresponds to the western border of the Eastern Cordillera.