bluuebunny/crossref_metadata_2025 huggingface dataset:
Three dimensional modelling of chlorine activation in the Arctic stratosphere
Authors: Kettleborough J. A. , Carver G. D. , Lary D. J. , Pyle J. A. , Scott P. A.
Published: February 2004

The UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Model has been used to study the chemical evolution of the northern hemisphere polar vortex. A run that includes a parametrisation of the heterogeneous reactions occurring on polar stratospheric clouds (PSCs) has been initialised with the ECMWF analyses of the 5th of January 1992. This run produces high values of active chlorine within the polar vortex, for example mixing ratios of ClOx (=Cl+ClO+2×Cl2O2) of 1.8ppbv are calculated after six days of integration, with correspondingly low values of HCl and ClONO2. During warm periods of this model run, when the PSC reactions are not active, the ClOx is converted slowly back into ClONO2. Chemical loss of ozone in January in the model is small.
Continuous high velocity aseismic zone beneath the Izu‐Bonin Arc
Authors: Ohtaki Toshiki , Kaneshima Satoshi
Published: February 2004

Since 1904, eleven unusually deep earthquakes have been reported near the southern end of the Izu‐Bonin arc (about 24°N, 142°E). These isolated events are separated from ordinary deep earthquakes in the Wadati‐Benioff (W‐B) zone of the Izu‐Bonin (I‐B) arc by a distance of about 300 km parallel to the trench axis. They also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them. Using 3‐D ray tracing, a detailed analysis is performed of the difference in travel time residuals between the anomalous events and the deep events within the W‐B zone to the north. This analysis yields higher reliability and resolution than can be obtained from ordinary tomographic imaging. We find that these unusual events occurred in a high velocity zone which is connected to the northern W‐B zone of the I‐B arc. The aseismic high velocity zone has a P‐velocity at least 3% faster than the surrounding mantle. Both our result and previous evidence for a horizontally lying slab west of the W‐B zone at 27°N (e.g., Okino et al., 1989) are consistent with the existence of a prominent contortion of the subducted slab in the I‐B region.
Volcanic aerosol and polar stratospheric clouds in the winter 1992/93 North Polar vortex
Authors: Rosen James M. , Kjome N. T. , Fast H. , Larsen N.
Published: February 2004

Backscatter profiles were measured at 940 and 490 nm in the winter north polar vortex using balloon borne backscattersondes. There was no mid‐season breakdown of the vortex, and temperatures were sufficiently cold to expect the formation of polar stratospheric clouds (PSCs). The mass loading from the Mount Pinatubo aerosols inside the vortex was observed to be larger than that of the previous year and equivalent to about 20 megatonnes if spread uniformly over the earth. The positive identification of typical PSCs was made more difficult by the presence of considerable volcanic aerosol background. An unusual scattering layer that can probably be identified as some type of PSC was not associated with as much backscatter as observed in previous non‐volcanic years with similar stratospheric temperatures. The layer apparently was composed of comparatively large particles with a relatively low, highly variable number concentration and was easily identified by the enhancement in fine structure of the scattering properties rather than an outstanding increase in average scattering values. The arctic volcanic aerosol layer was found to produce a small but significant depolarized backscattered signal. This preliminary result could be interpreted as indicating the presence of some non‐spherical particles (perhaps due to freezing effects) and/or spherical particles containing inclusions of irregular shapes with differing refractive index.
In situ measurements of Pinatubo aerosol over Kiruna on four days between 18 January and 13 February 1992
Authors: Deshler Terry
Published: February 2004

Vertical profiles of aerosol were measured on 18 and 27 January and 4 and 13 February, 1992, using balloonborne particle counters released from Esrange (68°N, 21°E) near Kiruna, Sweden. Condensation nuclei (CN) and aerosol with radii ≥ 0.15 µm, in 8 size classes, were measured. In the lower stratosphere concentrations of the larger particles were greater, by factors of 10 to 100, than those measured on flights from Esrange in January and February 1991. CN concentrations in the two years showed no pronounced differences. The bulk of the particles were volcanic aerosol from the eruption of Mt. Pinatubo.

On the first three flights, in air polewards of the stratospheric jet stream, there was little volcanic aerosol above 20 km, although a highly structured polar stratospheric cloud (PSC) was detected above the volcanic layer on 27 January. In the main volcanic layer, below about 16 km, the aerosol constituted 10 to 60 ppbm sulfuric acid, with a surface area density of 5 to 10 µm² cm−3, based on lognormal size distributions fit to the concentration measurements. These are respectively 50 and 10 times larger than the mass and surface area densities found in 1991. On the last flight in air equatorward of the jet stream, volcanic aerosol was detected up to 27 km, with peak values, near 20 km, of 100 ppbm sulfuric acid and 20 µm² cm−3 surface area density. These are similar to observations in the mid‐latitudes at that time.
Pressure‐temperature range of reactions between liquid iron in the outer core and mantle silicates
Authors: Song Xi , Ahrens Thomas J.
Published: February 2004

The possibility of reaction between the liquid iron of the Earth's outer core and crystalline silicates ((Mg, Fe)SiO3) of the mantle, Mg0.9, Fe0.1SiO3(pv) + 0.15 Fe (ϵ) = 0.9MgSiO3 (pv) + 0.2FeO (hpp) + 0.05 FeSi (ϵ) + 0.05 SiO2 (st), was proposed by Knittle and Jeanloz (1989,1991) on the basis of exploratory experiments. We calculate Gibbs free‐energies for the reactants and the products of the above reaction in order to constrain the pressure‐temperature range. Upon application of thermal expansion, equation of state and heat capacity data, we demonstrate that this reaction can occur at pressures as low as 30 GPa at 3500 K, and at temperatures as low as 900 K at the 130 GPa pressure of the present core‐mantle boundary. Furthermore, we propose a modified equation of reaction at the core‐mantle boundary: Mg0.9Fe0.1SiO3 (pv) + 0.3Fe (ϵ) = 0.9MgSiO3 (pv) + 0.3FeO(hpp) + 0.1FeSi. Our results imply that similar reactions may occur during the early accretion history of the Earth.
EUVE photometric observations of the Moon
Authors: Gladstone G. Randall , McDonald John S. , Boyd William T. , Bowyer Stuart
Published: February 2004

During its all‐sky survey, the Extreme Ultiaviolet Explorer (EUVE) satellite observed the Moon several times at first and last quarters, and once immediately following the Dec. 10, 1992 lunar eclipse. We present here a portion of this data, in the form of extreme‐ultraviolet (EUV) images of the Moon and derived geometric albedos. Extreme ultraviolet observations of the Moon are of considerable interest, since it has been speculated that lunar EUV emissions may be dominated by L‐ and M‐shell X‐ray fluorescence and may thus provide a useful diagnostic of surface elemental abundances. From the EUVE photometer data we obtain average geometric albedos of 0.15% (±0.03%), 3.1% (±0.3%), and 3.5% (±0.3%), over wavelength intervals of 150–240 Å, 400–580 Å, and 550–650 Å, respectively. The standard deviations listed result from photon counting statistics only—uncertainties arising from the assumed solar EUV flux and the correction to zero phase angle are not included. An upper limit geometric albedo of 0.13% is obtained for the wave‐length interval 75–180 Å. Also, using previously published ROSAT data, we estimate a lunar geometric albedo of 0.014% (±0.002%) over the wavelength interval 50–80 Å. These EUV albedos (and previously published far‐ultraviolet albedos) are well fit by the scaled reflectivities of SiO2 and Al2O3. Over the wavelength ranges of the EUVE photometers, the observed brightness of the Moon seems to be largely consistent with reflected sunlight rather than X‐ray fluorescence. Since the L‐ and M‐shell fluorescence signal is expected to be carried by only small number of emission lines, however, it will require EUV observations of higher spectral resolution (≲5 Å) to determine their exact contribution, if any, to the lunar EUV spectrum.
Estimating thermal forcings of greenhouse gases from ancient climates: The problem of statistical confounding
Authors: Loehle Craig
Published: February 2004

Data from ice cores show that CO2 and air temperature are highly correlated over the last 157,000 yr. Although this correlation can be taken as evidence that CO2 amplifies orbital forcing of temperature and is thus a strong greenhouse gas, this paper argues that estimating the strength of the CO2 warming effect from statistical evaluations of past climates based on CO2 and orbital forcing is hampered by strong multiple correlations between CH4, CO2, ocean currents, ice volume (and therefore albedo), dust, and nonseasalt sulfate. To estimate the strength of the greenhouse warming effect of CO2 from historical data, these correlations and multiple forcings should be taken into account.
Mechanism of low‐frequency radio emissions in the heliosphere
Authors: Macek Wieslaw M.
Published: February 2004

We present a model for the generation of Langmuir waves in the outer heliosphere. We further invoke the nonlinear interaction between the Langmuir waves resulting in electromagnetic radiation, at the second harmonic of the plasma frequency, which is able to leave the source region. This model can provide a quantitative explanation of the remote observation of radio emissions at 2–3 kHz by the Voyager plasma wave instrument in the outer heliosphere. If these radio emissions are generated by Langmuir waves, the Langmuir wave electric fields at the remote source are estimated to be 100–200 μV m−1. Location of the source of the emissions at the meridian passing through the apex is consistent with the recent direction finding on Voyager 1.
Variability of dayside electron temperature at Venus
Authors: Mahajan K. K. , Ghosh S. , Paul R. , Hoegy W. R.
Published: February 2004

Langmuir probe measurements on Pioneer Venus Orbiter show that electron temperature (Te) profiles exhibit two distinct regions. The lower, but more extended region is in the main ionosphere where Te increases slowly with altitude. The other, less extended region is in the ionopause, where Te rises sharply with altitude. If horizontal magnetic fields and flux ropes in the ionosphere inhibit vertical thermal conductivity sufficiently, then the observed Te profile could be explained with EUV as the major heat source [Cravens et al., 1980]. The rise in Te in the ionopause region has generally been attributed to solar wind heating [Brace and Kliore, 1991]. We suggest that this sharp rise in Te is due primarily to the steep fall in electron density, Ne. If the heating rate is essentially unchanged and heat conduction is not of primary importance, then a steep rise in Te will maintain a constant electron cooling rate for a steeply falling Ne. We have observed large orbit to orbit variations in Te in the ionopause region which are found to be inversely related to changes in Ne. Variations in solar wind dynamic pressure do not seem to have a direct effect on Te, rather the effect is indirect coming through the sharp decrease in Ne.
The effect of subionospheric propagation on whistlers as deduced from direction finding measurements
Authors: Ohta Kenji , Shimizu Akio , Hayakawa Masashi
Published: February 2004

The characteristics of subionospheric propagation of magnetospheric whistlers have been investigated by means of both digital spectral analysis with high resolution and our field‐analysis direction finding measurement for whistlers observed in South China. It is found that very low latitude whistlers, propagated in the Earth‐ionosphere waveguide over a distance of the order of 1000km, exhibit very clear additional dispersion effects near the cutoff frequencies of the subionospheric 1st‐ and higher‐order modes. The additional traces are also found to be left‐handed polarized, and when the frequency approaches the cutoff frequencies, the wave polarization becomes very close to exactly left‐handed circular and the incident angle becomes nearly vertical. While, the whistler components at frequencies away from the cutoff frequencies are linearly polarized. Finally, we emphasize the important use of the subionospheric propagation characteristics in the study of the whistler duct structure, ionospheric transmission of a downgoing whistler and its excitation of subionospheric modes.
Pondermotive versus mirror force in creation of the filamentary cavities in auroral plasma
Authors: Singh Nagendra
Published: February 2004

Recently rocket observations on spikelets of lower‐hybrid waves along with strong density cavities and transversely heated ions were reported. The observed thin filamentary cavities oriented along the magnetic field in the auroral plasma have density depletions up to several tens of percent. These observations have been interpreted in terms of a theory for lower‐hybrid wave condensation and collapse. The modulational instability leading to the wave condensation of the lower‐hybrid waves yields only weak density perturbations, which cannot explain the above strong density depletions. The wave collapse theory is based on the nonlinear pondermotive force in a homogeneous ambient plasma and the density depletion is determined by the balance between the wave pressure (pondermotive force) and the plasma pressure. In the auroral plasma, the balance is achieved in a time τwc ≤1 ms. It is shown here that the mirror force, acting on the transversely heated ions at a relatively long time scale, is an effective mechanism for creating the strong plasma cavities. We suggest that the process of wave condensation, through the pondermotive force causing generation of short wavelength waves from relatively long wavelength waves, is a dominant process until the former waves evolve and become effective in the transverse heating of ions. As soon as this happens, mirror force on ions becomes an important factor in the creation of the density cavities, which may further trap and enhance the waves. Results from a model of cavity formation by transverse ion heating show that the observed depletions in the density cavities can be produced by the heating rates determined by the observed wave amplitudes near the lower‐hybrid frequency. It is found that the creation of a strong density cavity takes a few minutes.
Variations in layer 2A thickness and the origin of the central anomaly magnetic high
Authors: Gee Jeff , Kent Dennis V.
Published: February 2004

The seismically determined off‐axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high typically associated with the ridge crest. The positive magnetization contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales (2–3 km) comparable to that of the proposed Layer 2A thickening. We present magnetic remanence data from basalts dredged on and near the East Pacific Rise axis at 12°N which are compatible with a rapid magnetization reduction (∼20 k.y. to decay to 1/e). Together with near bottom magnetic profiles from the ultra‐fast‐spreading East Pacific Rise at 19.5°S, these data suggest that previous estimates of the time constant of alteration inferred from slow‐spreading ridges (0.5 m.y.) may be more than an order of magnitude too high.
Imaging of subsurface contaminations using dipole EM systems
Authors: Sharma S. P. , Verma S. K.
Published: February 2004

Major contemporary applications of the dipole EM methods deal with the problems of imaging subsurface contaminated or polluted zones, mapping of saline‐water incursion zones in coastal areas, foundation engineering and waste disposal etc. Because such problems are mostly encountered in populated and/or industrialized areas, inductive electromagnetic (EM) methods employing small dipolar transmit‐receive (T‐R) systems provide efficient tools to tackle them. A number of T‐R orientations, measuring different primary and resultant fields, are currently employed in EM exploration.

In most of the above problems the target zone can be simulated by a thin conductive or resistive layer. The present work quantifies the resolving capabilities of various dipole EM systems for such a target layer. The target layer is considered embedded in different types of commonly occuring geoelectrical situations. Comparative study of the resolutions obtained for the target layer by different dipole systems reveals that the perpendicular loop system provides the best resolution due to its unique advantage of being free from primary field interference.
True polar wander affects the Earth dynamic topography and favours a highly viscous lower mantle
Authors: Spada Giorgio , Sabadini Roberto , Boschi Enzo
Published: February 2004

True Polar Wander (TPW), the global motion of the Earth's mantle relative to the axis of rotation, is considered for its impact on the degree 2 surface dynamic topography induced by mass anomalies in a viscously stratified mantle. These sources, responsible for a geoid and topographic signal at the Earth's surface, mimic the effects of subductions and other thermal instabilities on long‐term Earth's rotation. A rotating, viscous planet has the ability to displace the axis of rotation with respect to internal mass anomalies in order to maintain, on the average, the excess of non‐hydrostatic geoid along the equator, affecting at the same time the degree 2 pattern of the dynamic topography which supports the geoid. By means of a time series of synthetic mass anomalies, randomly distributed in order to avoid any bias due to the clustering of the sources on time scales of 107 yr, we show that TPW introduces a selection rule which favours in the dynamic topography the order m=0 for an isoviscous mantle and the order m=2 for lower mantle viscosities at least a factor 30 higher than in the upper mantle. The dominance of the order m=2 in the dynamic topography of the real Earth, induced by the mass anomalies inferred from seismic tomography, agrees with the requirements of a rotating planet deformed by internal sources, characterized by a substantial viscosity increase in the lower mantle. This agreement seems to indicate that the intimate physical reason of the high order 2 content in the topography stands on the interplay between the dynamic response of the mantle to internal sources and Earth rotation.
BrO as a sink for dimethylsulphide in the marine atmosphere
Authors: Toumi Ralf
Published: February 2004

Dimethylsulphide (DMS) is the major biogenic sulphur source gas. It is known to be oxidised by OH and NO3 to ultimately yield sulphate particles which are important in acid deposition and are also involved in the atmospheric radiative balance. In the laboratory BrO has also been shown to oxidise DMS. Here I present atmospheric model calculations based on inorganic Bromine measurements and find that oxidation by BrO is a sink for DMS.
The impact of freezing of sulfate aerosols on the formation of polar startospheric clouds
Authors: Larsen Niels
Published: February 2004

In simulation models, used to describe the formation of Polar Stratospheric Clouds (PSC), it is usually assumed that the background population of sulfate aerosols acts as nucleation centers for the PSCs. At normal stratospheric temperatures the aerosols are in the liquid phase, not suitable for PSC nucleation, but still it has been assumed that the particles will freeze above the nitric acid trihydrate (NAT) condensation temperature. Homogeneous freezing of aerosol particles into sulfuric acid tetrahydrate and ice has been incorporated in a detailed microphysical PSC‐model to investigate the impact of freezing among the sulfate aerosols on the PSC size distributions. It is demonstrated here that freezing of the largest particles might explain the observed high HNO3/NAT saturation ratios and denitrification by particle sedimentation in the Arctic stratosphere. Comparisons have also been made to the PSC formation under Antarctic temperature conditions.
Characteristic aerosol optical depths during the Harmattan Season on sub‐Sahara Africa
Authors: Pinker Rachel T. , Idemudia G. , Aro T. O.
Published: February 2004

Aerosol optical depths were derived from sun photometer measurements in three narrow bands centered at 500, 675 and 875 nm, taken at Ilorin, Nigeria, located in sub‐Saharan Africa. Selected results will be presented on the diurnal, seasonal, and interannual variability of spectral aerosol optical depth for two Harmattan seasons, during 1987–1989. At 500 nm values between 0.9–1.6 were quite common while the averages were between 1–1.2.
Yield strength at high pressure and temperature
Authors: Weidner Donald J. , Wang Yanbin , Vaughan Michael T.
Published: February 2004

Yield strength is measured at high pressure and temperature using a large volume, high pressure apparatus (SAM85) with synchrotron radiation. A macroscopic deviatoric stress is manifest as a uniform deviatoric strain that is oriented by the geometry of the pressurizing medium. Microscopic deviatoric stress is identified as the elastic broadening of diffraction lines. The deviatoric stress reaches the yield point as evidenced by the uniformity, the saturation, and the temperature dependence of the deviatoric stress. Yield strengths, which correspond to the stress saturation level at a few per cent strain, are determined for NaCl and MgO up to 8 GPa and 1200°C. The results are consistent at room temperature with previous diamond anvil studies and demonstrate the effect of pressure on yield strength. These data demonstrate the feasibility of determining high pressure, high temperature yield strengths for mantle phases.
Deep structure of the U.S. Atlantic continental margin, offshore South Carolina, from coincident ocean bottom and multichannel seismic data
Authors: Holbrook W. Steven , Reiter E. C. , Purdy G. M. , Sawyer D. , Stoffa P. L. , Austin J. A. , Oh J. , Makris J.
Published: February 2004

We present the results of a combined multichannel seismic reflection (MCS) and wide‐angle, ocean bottom seismic profile collected in 1988 across the Carolina Trough on the U.S. Atlantic continental margin. Inversion of vertical‐incidence and wide‐angle travel time data has produced a velocity model of the entire crust across the continent‐ocean transition. The margin consists of three structural elements: (1) rifted continental crust, comprising 1–4 km of post‐rift sedimentary rocks overlying a 30–34 km thick subsedimentary crust, (2) transitional crust, a 70‐ to 80‐km‐wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10‐ to 24‐km‐thick subsedimentary crust, and (3) oceanic crust, comprising 8 km of sedimentary rocks overlying an 8‐km‐thick crystalline crust. The boundary between rifted continental and transitional crust, marked by the Brunswick magnetic anomaly, represents an abrupt change in physical properties, with strong lateral increases in seismic velocity, density, and magnetic susceptibility. The transitional crust contains mid‐crustal seaward‐dipping reflections observed on the MCS section and has seismic velocities of 6.5–6.9 km/s in the midcrust and 7.2–7.5 km/s in the lower crust. Modeling of potential field data shows that transitional crust also produces the prominent, margin‐parallel gravity anomaly and the Brunswick and East Coast magnetic anomalies. These observations support the interpretation that the transitional crust was formed by magmatism during continental breakup. The prodigious thickness (up to 24 km) of igneous material rivals that interpreted on continental margins of the North Atlantic (e.g., Hatton Bank and Vøring Plateau), which formed in the vicinity of the Iceland hotspot. These observations, when combined with other transects across the margin, confirm previous suggestions that the U.S. Atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long‐lived mantle plume.
Volcanic and hydrothermal processes associated with a recent phase of seafloor spreading at the northern Cleft segment: Juan de Fuca Ridge
Authors: Embley R. W. , Chadwick W. W.
Published: February 2004

The northern portion of the Cleft segment, which is the southernmost segment of the Juan de Fuca Ridge, is the site of a seafloor spreading episode during the mid‐1980s that was originally discovered by the occurrence of anomalous hydrothermal bursts (megaplumes) and later documented by seafloor mapping of new pillow mounds (NPM) that were erupted. Several field seasons of investigations using sidescan sonar, a deep‐tow camera system, and the submersibleAlvinreveal that about 30 km of the ridge crest is hydrothermally active and/or has experienced recent volcanic and tectonic activity associated with this episode. The most intense hydrothermal activity within this area and all the known high‐temperature vents lie along a fissure from which a young sheet flow (YSF) erupted. Extinct chimneys located within 100–200 m on either side of the fissure system represent an older (>100 years) and probably less intense, hydrothermal regime. The bathymetry and the morphology of the YSF suggest that this eruption occurred over a 1–2 km section of the fissure system that forms its eastern boundary and that it flowed to the south. Fields of lava pillars concentrated at the margins of the YSF where lava probably formed when the lava stagnated near the edges of the flow. A comparison of sidescan data sets collected in 1982 and 1987 implies that the YSF was erupted at least 7 months prior to the NPM, consistent with analysis of bottom photographs that suggests that the eruptions of the YSF and NPM were only separated by a few years. The low hydrothermal flux over the NPM relative to the YSF suggests a rapidly cooled underlying heat source beneath the former. We propose that the NPM were erupted from a dike or dikes injected laterally to the north from a magma body lying beneath the YSF. Recent evidence of a decrease in the intensity of the overlying hydrothermal plumes suggests that the system is continuing to cool down.
Age variations of oceanic crust Poisson's ratio: Inversion and a porosity evolution model
Authors: Shaw Peter R.
Published: February 2004

Porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities. Porosity is particularly important in the uppermost young crust, where rapid variations in velocities with depth and crustal age are observed. Knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of Poisson's ratio are available from observations of compressional and shear seismic velocities υp and υs. In this paper I present a joint seismic waveform inversion for υp and υs; velocities are found while maximizing or minimizing Poisson's ratio using a hypothesis‐testing mechanism. I apply this method to ocean bottom hydrophone data in 140 Ma Atlantic crust; the resulting solution corridor agrees with laboratory measurements without the low Poisson's ratio anomalies at depths of 0.8–1.5 km found by Spudich and Orcutt (1980) and Au and Clowes (1984) on younger (<15 Ma) Pacific crust. Compiling other published υp and υs solutions, an age‐dependent pattern emerges: none of the solutions for crust older than 60 Ma display the Poisson's ratio anomaly. I propose a simple crustal evolution model, using thin and thick cracks, to explain these observations: thin cracks preferentially close at shallow depths in the crust, producing the localized Poisson's ratio anomaly. Sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements. This model is consistent with similar models of crack populations and their evolution from shallow measurements.
Fine‐scale magnetic anomaly field over the southern Juan de Fuca Ridge: Axial magnetization low and implications for crustal structure
Authors: Tivey Maurice A.
Published: February 2004

Sea surface, deep tow, and submersible magnetic surveys show that the spreading axis of the Cleft segment of the southern Juan de Fuca Ridge (SJDF) is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent Vance segment and most mid‐ocean ridge spreading centers. Forward iterative thickness modeling, based on the near‐bottom magnetic data, shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the Cleft axial valley. Thick crust is required to produce the magnetic anomaly high over the Vance axial valley. This magnetic modeling demonstrates that variations in the base of the magnetic source layer, which is assumed to be equivalent to the base of the extrusive lavas, can produce a significant proportion of the observed short‐wavelength magnetic anomaly signal. These magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2A thins beneath the axial valley of the Cleft and is thicker beneath the Vance axial valley. Recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading, which could produce significant magnetic contrasts. The results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration, first‐order change in upper crustal thickness is one of the most important variables to consider in the production of fine‐scale magnetic anomalies in young ocean crust.
Jasper Seamount structure: Seafloor seismic refraction tomography
Authors: Hammer Philip T. C. , Dorman LeRoy M. , Hildebrand John A. , Cornuelle Bruce D.
Published: February 2004

The velocity structure of Jasper Seamount was modeled using one‐ and three‐dimensional inversions of P wave travel times. The results represent the first detailed seismic images of a submerged, intraplate volcano. Two seismic refraction experiments were completed on Jasper Seamount, incorporating ocean bottom seismometers and navigated seafloor shots. The P wave travel times were first used to compute a one‐dimensional velocity profile which served as a starting model for a three‐dimensional tomographic inversion. The seamount P velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths. This suggests that Jasper Seamount is constructed predominantly of extrusive lavas with high average porosity. The velocity models confirm morphological predictions: Jasper Seamount is a shield volcano with rift zone development. High seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks. Comparisons between P velocity models of Jasper Seamount and the island of Hawaii reveal that these two shield volcanoes are not structurally proportional. Jasper Seamount is far smaller than Hawaii, yet both volcanoes exhibit an outer extrusive layer of similar thickness. This suggests that seamount size influences the intrusive/extrusive proportions; density equilibrium between melt and country rock may explain this behavior.
Gravity changes and deformation due to a magmatic intrusion in a two‐layered crustal model
Authors: Fernández José , Rundle John B.
Published: February 2004

We develop and extend theoretical and computational methods for the calculation of the deformation, gravity and potential change due to a point source of magma injection into a multilayered, elastic‐gravitational earth model. In our calculations, which are based upon the method outlined by Rundle, two distinct layers overlying a half‐space may be incorporated. The source can be located in either of the layers or the half‐space. The method is quite general, and can be readily adapted to calculations in which stresses in either the layers or the half‐space relax by viscoelastic flow. The results obtained indicate that the use of homogeneous half‐space to represent the Earth may in some cases be too simple a model and that variations in elastic moduli have a more significant effect than variation in reference density on both the surface displacements and gravity changes. As an example, we calculate the displacement and gravity changes due to a subsurface mass injection in a crust‐mantle model appropriate to the volcanic island of Lanzarote, which is presently the subject of numerous geophysical experiments. Both historical and recent data indicate that Lanzarote may be subject to some risk of volcanic eruption in the future, thus our calculations may be useful in interpreting observations of preemption phenomena. The results are discussed in terms of prediction versus measurement capabilities.
Theory of partial thermoremanent magnetization in multidomain grains: 1. Repeated identical barriers to wall motion (single microcoercivity)
Authors: Dunlop David J. , Xu Song
Published: February 2004

We extend the Néel (1955) theory of thermoremanent magnetization (TRM) in multidomain grains to include (1) the acquisition of partial TRM produced by cooling from T2 to T1 in an applied field H0 and (2) the thermal demagnetization of partial and total TRM. If T2 is ≥1°C below the Curie temperature Tc, the internal demagnetizing field −NMs (Ms is saturation magnetization and N is demagnetizing factor) is larger than geophysically reasonable values of H0. The initial state for partial TRM blocking is then close to a demagnetized state and is quite different from the near‐saturation initial state modeled by Néel for TRM blocking. As a result, a partial TRM Mptr(T2, T1,H0) usually has a considerably lower intensity than total TRM Mtr(Tc,T0,H0) or partial TRM Mptr(Tc,T1,H0). However, during thermal demagnetization, Mtr or Mptr(Tc,T1,H0) will begin to disappear at lower temperatures than Mptr(T2,T1,H0) and exactly the same stable remanence will be isolated at high temperatures in all three cases. Blocking and unblocking do not occur at the same temperature as in the case of single‐domain grains: TRM or partial TRM blocking is a sharp process (apart from possible reequilibration of walls when H0 → 0), but thermal unblocking proceeds gradually toward a demagnetized state by continuous wall reequilibration and is complete only close to Tc. Above a threshold temperature Tcrit which is dependent on the type of partial TRM, the remanence intensity decreases in proportion to Hc (T) for continuous thermal demagnetization or ∝ Hc(T)IMs(T) for step wise thermal demagnetization, where Hc is average microcoercivity. Because a grain with a single microcoercivity can acquire partial TRMs in many different blocking temperature ranges, each with a different set of wall displacements, Thellier's Law of Additivity of partial TRMs is only very approximately obeyed and the various partial TRMs are not independent.
The elastic anisotrophy of shales
Authors: Sayers C. M.
Published: February 2004

Shales constitute about 75% of the clastic fill of sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure. The elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients Wlmn in an expansion of the clay‐particle orientation distribution function in generalized Legendre functions. Application is made to the determination of the anellipticity of shales. For transverse isotropy the anellipticity quantifies the deviation of the P wave slowness curve from an ellipse and is shown to depend on a single coefficient W400 in the expansion of the clay‐particle orientation distribution function. If W400 is small, the anellipticity may be neglected, as is apparently the case for a near‐surface late Tertiary shale studied by Winterstein and Paulson. Strongly aligned clay particles result in a positive value of W400 and a positive anellipticity, in agreement with the majority of field measurements. However, less well ordered shales could have a significantly positive second moment W200 but only a small positive or even negative value of W400 For such shales the anellipticity would be small or negative despite a preferred alignment of clay particles in the bedding plane. Numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given.
An introduction to the Special Section on Oceanic Crustal Evolution
Authors: Johnson H. Paul , Becker Keir
Published: February 2004

The concept that the rock formations which make up the ocean floor can change with time is relatively recent and is an idea that has itself undergone considerable evolution. Prior to the hypothesis of plate tectonics, the seafloor was thought of as unchanging and essentially ageless. The identification of a specific “age” for a particular portion of oceanic basement developed naturally from the concept of seafloor spreading early in the 1960s. Field studies during this period began to show that in fact, the upper oceanic crust was far from uniform and that some of this variability could be associated with systematic changes in crustal age.
Extent of partial melting beneath the Cascade Range, Oregon: Constraints from gravity anomalies and ideal‐body theory
Authors: Blakely Richard J.
Published: February 2004

The spatial correlation between a horizontal gradient in heat flow and a horizontal gradient in residual gravity in the Western Cascades of central Oregon has been interpreted by others as evidence of the western edge of a pervasive zone of high temperatures and partial melting at midcrustal depths (5–15 km). Both gradients are steep and relatively linear over north‐south distances in excess of 150 km. The Western Cascades gravity gradient is the western margin of a broad gravity depression over most of the Oregon Cascade Range, implying that the midcrustal zone of anomalous temperatures lies throughout this region. Ideal‐body theory applied to the gravity gradient, however, shows that the source of the Western Cascades gravity gradient cannot be deeper than about 2.5 km and is considerably shallower in some locations. These calculations are unique determinations, assuming that density contrasts associated with partial melting and elevated temperatures in the crust do not exceed 500 kg m−3. Consequently, the gravity gradient and the heat flow gradient in the Western Cascades cannot be caused directly by the same source if the heat flow gradient originates at midcrustal depths. This conclusion in itself does not disprove the existence of a widespread midcrustal zone of anomalously high temperatures and partial melting in this area, but it does eliminate a major argument in support of its existence. The gravity gradient is most likely caused by lithologic variations in the shallow crust, perhaps reflecting a relict boundary between the Cascade extensional trough to the west and Tertiary oceanic crust to the west. The boundary must have formed prior to Oligocene time, the age of the oldest rocks that now conceal it.
Rheological evolution of the ocean crust: A microstructural view
Authors: Agar Susan M.
Published: February 2004

Rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal, mechanical, and experimental models. Modeling studies help to constrain an average crust and mantle rheology and the deep‐seated processes that control regional‐scale strength and stress field variations. The smoothing of fine‐scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust. Examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling. Examples of structures and deformation histories in diabases from Deep Sea Drilling Project/Ocean Drilling Program site 504B, the Hayes and Atlantis fracture zones, (Mid‐Atlantic Ridge), and the Troodos ophiolite, (Cyprus), are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years. A survey of brittle, quasi‐plastic, and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust. Geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures. Melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies. A schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates. Although microstructural studies of ocean crust are still in their infancy, they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures.
Ocean crust formation processes at very slow spreading centers: A model for the Mohns Ridge, near 72°N, based on magnetic, gravity, and seismic data
Authors: Géli Louis , Renard Vincent , Rommevaux Céline
Published: February 2004

The Mohns Ridge, in the Norwegian Greenland Sea, is one of the slowest spreading centers of the mid‐ocean ridge system (8 mm/yr half rate). Sea Beam data acquired with R/V Jean Charcot near 72°N show that its rift valley floor is characterized by en échelon volcanic ridges, oriented obliquely relatively to the average strike of the ridge axis. These ridges are regularly spaced along the axis, about every 40 km, and are separated by nontransform discontinuities. Sharp positive magnetic anomalies, centered over the topographic highs, suggest that they are eruptive centers, considered as the surficial expression of active spreading cells. Over the rift valley, Bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free‐air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface. This body, if it exists, probably corresponds to the zone of low viscosity that can be inferred from the model of Chen and Morgan (1990b), which predicts the existence of a decoupling region, between the upper crust and the asthenophere below. Its width varies rapidly along‐strike, from less than about 5 km to more than 15 km. In plan view, it has a pinch and swell form, which defines a series of spreading cells, the center of one cell being where the Bouguer anomaly is widest. Short wavelength (less than 10 to 20 km) along‐strike variations, such as Bouguer anomaly lows centered on the topographic highs, reflect local effects associated with the presence of the eruptive centers. Seismic tomography data from a 20×10 km active oblique volcanic ridge near 72°22′N tend to indicate that the links between the main, low‐velocity body at depth, and the magma injections centers which lie within the rift valley inner floor are probably complex.
Rapid rupture and complex faulting of the May 12, 1990, Sakhalin deep earthquake: Analysis of regional and teleseismic broadband data
Authors: Kuge Keiko
Published: February 2004

A large (Mw=7.1) deep earthquake (606 km) occurred beneath Sakhalin Island, about 200 km away from the nearest well‐defined Wadati‐Benioff zone in southwestern Kurile. Moment tensors obtained in various frequency bands using the teleseismic data show that the Sakhalin event produced double‐couple radiation over a very broadband frequency range, and the P waveforms appear simple at most teleseismic stations (Δ=30–90°). On the other hand, many broadband seismometers successfully recorded the seismic waves at regional distances (Δ=3–25°) in Japan, showing two significant arrivals in the P wave seismograms whose time interval is about 3 s. The two P wave arrivals have the same polarity, the same apparent velocity, and similar amplitudes, which suggests that the two arrivals are the result of two sub‐events in the source process of the Sakhalin event. Analysis of the regional and teleseismic waveform data suggests an anomalously rapid slip propagation in the source process of the Sakhalin deep event. The difference in P waveform between the regional and teleseismic stations can be explained by having a long distance (>14–15 km) and short time interval (<1.5–2 s) between the two subevents, which implies that the apparent rupture velocity is larger than the S wave velocity unless the two subevents occurred simultaneously. The observation that the source duration of the Sakhalin event is shorter than those of other deep events of the same size supports the notion of rapid slip‐propagation for the Sakhalin event.
Garnet‐pyroxene‐amphibole xenoliths from Chino Valley, Arizona, and implications for continental lithosphere below the Moho
Authors: Smith Douglas , Arculus Richard J. , Manchester Janet E. , Tyner G. Nell
Published: February 2004

Garnet‐pyroxene‐amphibole xenoliths illustrate how P and T histories can be recorded in rocks from the crust‐mantle transition and document the diversity of continental lithosphere below the Mohorovicic discontinuity. The xenoliths are from the Sullivan Buttes Latite in Chino Valley, Arizona, in the Transition Zone of the Colorado Plateau. The most definitive depth assignments depend upon garnet‐pyroxene thermobarometry coupled with analysis of Ca and Al gradients in orthopyroxene. Websterites that record temperatures of 600 to 700°C contain orthopyroxene zoned in Al but not Ca, and these rocks were carried up from depths of at least 43 km. Websterites that record temperatures of 800–900°C contain more homogeneous orthopyroxene, and they were erupted from 70 to 80 km. Most eclogite and amphibole‐rich xenoliths record temperatures in the range bracketed by websterites and so were probably erupted from similar depths. Element abundances and Sr, Nd, and Pb isotope ratios establish that protoliths of most xenoliths formed by crystal‐melt fractionation from basaltic magmas. Diverse Sr and Nd isotopic compositions range from εNd ≈ +8 and 87Sr/86Sr ≈0.7045 for two Websterites to εNd ≈ −9 and 87Sr/86Sr ≈ 0.7064 for both parts of a composite eclogite. Most xenoliths probably have Proterozoic protoliths, although many record more recent thermal and metasomatic events, and a few probably formed from Cenozoic magmas. Observations are consistent with a reconstruction of the lithosphere in which eclogite and amphibole‐rich rock were volumetrically important to depths of at least 70–80 km at 25 Ma. Anhydrous peridotite may not dominate just below the Mohorovicic discontinuity beneath Chino Valley or beneath some other localities on the Colorado Plateau and elsewhere. No evidence was observed in the Chino Valley suite for replacement of continental lithosphere during Phanerozic tectonism or for significant underplating in Cenozoic time.