bluuebunny/crossref_metadata_2025 huggingface dataset:
The response of stratospheric ozone to volcanic eruptions: Sensitivity to atmospheric chlorine loading
Authors: Tie XueXi , Brasseur Guy
Published: February 2004

Model calculations suggest that the ozone decrease observed a few years after the eruptions of Mt. Pinatubo and El Chichon may have been unique in the Earth's history, and is directly linked to the emission in the atmosphere of industrially manufactured chlorofluorocarbons. For chlorine loadings typical of the pre‐1980 period, the ozone column abundance should have increased after a large volcanic eruption. After 1980, as a result of growth in chlorine loading, the response of ozone became negative in winter at mid‐ and high latitudes. In the future, the response of ozone is expected to become positive again, if the production of chlorofluorocarbons is sufficiently reduced. The calculations also show that, under low chlorine loadings, the response of ozone is insensitive to the magnitude of the eruption, while, under present conditions (high chlorine loading), the ozone depletion increases with the amount of SO2 injected in the stratosphere by the volcano.
Equatorial oscillations in the middle atmosphere generated by small scale gravity waves
Authors: Mengel J. G. , Mayr H. G. , Chan K. L. , Hines C. O. , Reddy C. A. , Arnold N. F. , Porter H. S.
Published: February 2004

A realistic parameterization scheme for the deposition of gravity‐wave momentum in the middle atmosphere has been incorporated into the 2D version of a global‐scale Numerical Spectral Model of the Earth's middle atmosphere. Here we present early results, obtained with only the simplest assumptions for the incident gravity‐wave spectrum—that it is azimuthally isotropic (i. e., identical flux in the four cardinal directions), globally uniform, and unchanging with season—and with essentially “untuned” values of tunable parameters. This model reproduces reasonably well the observed anomalous latitudinal temperature distribution and the zonal circulation of the upper mesosphere during solstice, just as other models do. It also produces relatively large oscillations in the mean zonal circulation of the middle atmosphere at low latitudes, descending in altitude with time. In the mesosphere and upper stratosphere, the dominant period is semi‐annual and the maximum amplitude is about 20 m/s near a height of 50 km. At lower levels, the dominant period is about 20 months and the maximum amplitude is about 8 m/s near 25 km. These values resemble those associated with the observed Semi‐Annual Oscillation and Quasi‐Biennial Oscillation, respectively, leading us to conclude that small scale gravity waves may contribute significantly to both.
A Late Quaternary magnetic record of Tunisian Loess and its climatic significance
Authors: Dearing John , Livingstone Ian , Zhou L. P.
Published: February 2004

Magnetic measurements for a 10 m loess‐palaeosol sequence from the Matmata Plateau in southern Tunisia show significant down‐profile variations in concentrations of pedogenic ferrimagnetic minerals. The sequence appears to correlate with regional and global records of climate change over the past 70 kyr. There is strong evidence from comparisons of mineral magnetic properties in modern soils and palaeosols that the sequence holds a proxy record of winter precipitation.
Construction of an inter‐tidal digital elevation model by the ‘Water‐Line’ Method
Authors: Mason D. C. , Davenport I. J. , Robinson G. J. , Flather R. A. , McCartney B. S.
Published: February 2004

Progress in the development of a method for constructing a Digital Elevation Model (DEM) of an inter‐tidal zone using remote sensing and hydrodynamic modelling is described. The method allows the construction of an inter‐tidal DEM over large areas relatively quickly as well as frequent subsequent monitoring of the DEM to detect changes, and is particularly suited to SAR satellite data because of the all‐weather day‐night capability of SAR. The resulting DEM may be used for the development of improved hydrodynamic models, and for studying sediment mass transport in the inter‐tidal zone.
Vertical‐axis block rotations in southwestern China since the Cretaceous: New Paleomagnetic results from Hainan Island
Authors: Li Zheng‐Xiang , Metcalfe Ian , Wang Xiaofeng
Published: February 2004

A paleomagnetic study of Early Cretaceous redbeds from Hainan Island, South China, reveals a pre‐folding remanence from six sites, with a mean direction of D=4.3°, I=41.8°, k=51.5, α95=9.4°. The results suggest that Hainan Island may have been rotated ∼10° counterclockwise about a vertical axis since the Early Cretaceous. This rotation is likely to have occurred during the mid‐Tertiary, when large scale left‐lateral motion occurred along the Red River Fault as a result of the indentation of the Indian Plate into Eurasia causing extrusion of the Indochina Block, and opening of the South China Sea. A comparison of Early Cretaceous paleomagnetic results from South China indicates that local rotations are widespread in the western part of the South China Block. Such rotations have to be taken into account when examining pre‐Cenozoic tectonic kinematics of South China, and constructing apparent polar wander path(s) for the region.
On the distribution of mesospheric molecular hydrogen inferred from HALOE measurements of H2O and CH4
Authors: Harries J. E. , Ruth S. , Russell J. M.
Published: February 2004

The Halogen Occultation Experiment (HALOE) is in orbit on NASA's Upper Atmosphere Research Satellite (UARS), and has been used to make measurements of a number of stratospheric and mesospheric constituents since October 1991. These include, amongst others, water vapour, H2O, and methane, CH4, two principal components of the total hydrogen budget of the middle atmosphere. The third main component is molecular hydrogen, H2, which is not measurable by HALOE or any other UARS sensor. By making the assumption that the total hydrogen content of the middle atmosphere is a conserved quantity, and that these three constituents dominate the budget, it is possible to infer the H2 fields in the mesosphere from the HALOE H2O and CH4 measurements.
Non‐diagonal ion pressure in nearly‐perpendicular collisionless shocks
Authors: Gedalin M. , Zilbersher D.
Published: February 2004

We study the influence of the gyrophase‐bunched ions on the tensorial structure of the ion pressure in the front of a nearly‐perpendicular supercritical shock. Presence of reflected ions results in the appearance of significant off‐diagonal elements of the pressure tensor. Behavior of the pressure tensor in the foot is analyzed within the model of the specular reflection of ions. The off‐diagonal elements of the pressure tensor are directly related to the potential electric field in the shock front and can substantially contribute to the cross‐shock potential.
Dual seismogenic behavior: The 1985 Central Chile Earthquake
Authors: Barrientos Sergio E.
Published: February 2004

Repeated leveling lines evidenced almost half a meter of uplift at coastal localities nearby the epicentral region of the March 3, 1985, Ms=7.8, Central Chile earthquake. A tide gauge at Valparaiso and two limnigraphs, 27 km apart, situated at the extremes of Rapel Lake, to the south of the leveling line, have been recording continuously sea level and the equipotential lake level for more than 10 years, providing a permanent very‐long base tiltmeter. Water level difference at the two limnigraphs as a function of time resembles a ramp function, beginning approximately at the time of the earthquake occurrence; it has a rise time of approximately 10 months with a maximum amplitude of 120 mm or 4.4 µradians in tilt. The overall shape of the time‐dependent tilt is mimicked by the sea level signal recorded at Valparaíso, about 100 km away from Rapel Lake, showing a maximum coastal subsidence of 0.6 m. Gravity surveys carried out in three different pre and postseismic epochs, along the segment of the leveling line which shows major coseismic uplift, indicate that the whole region has subsided postseismically, 10 cm in five years. Simple two‐dimensional models for both co and postseismic stages indicate that at least 1.2 m of average fault displacement in ten months of postseismic movement along the contact between Nazca and South America plates is responsible of the time dependent elevation changes, 25% less of what was deduced with the same method for the coseismic stage.
Infrared background of spacecraft in low‐Earth orbit attributable to ion chemistry
Authors: Dressler Rainer A. , Murad Edmond
Published: February 2004

A diurnal variation in the infrared background radiation observed on Spacelab 2 suggests that ion chemistry plays a part in the generation of those infrared emissions. Because water is the major contaminant in the environment of spacecraft in low earth orbit, it is thought to be the source of vibrationally excited and infrared active H2O+ formed in O+(4S) + H2O charge‐transfer reactions. H2O+ infrared luminescence spectra (0.01 µm FWHM) are calculated for the predicted nascent vibrational distribution as well as for steady‐state conditions assuming a charge‐transfer relative velocity of 7.8 km s−1, the low‐Earth orbital velocity. The calculated luminescence spectra exhibit an intense band between 3 and 3.5 µm, attributable to symmetric stretch deexcitation, and a minor band between 6 and 8.5 µm associated with bending vibrational transitions. The effect of charge‐transfer product rotational temperature, H2O partial pressure, and motional electric field on the infrared background spectra are investigated.
Interpretation and consequences of large‐scale magnetic variances observed at high heliographic latitude
Authors: Jokipii J. R. , Kóta J. , Giacalone J. , Horbury T. S. , Smith E. J.
Published: February 2004

We consider recent Ulysses observations of the large‐scale variances in the transverse components of the interplanetary magnetic field. A previously‐suggested theory is shown to provide a good fit to the observed spatial variation and level of the fluctuations. The transport of cosmic rays in the heliosphere will be significantly affected by these fluctuations. In addition to impeding the inward, radial diffusive and drift access of cosmic rays over the poles, the magnetic fluctuations imply a large latitudinal diffusion, caused primarily by the field‐line mixing, or random walk.
Reflection of shear Alfvén waves at the ionosphere and the divergent Hall current
Authors: Yoshikawa A. , Itonaga M.
Published: February 2004

A general expression for the reflection and mode conversion of MHD waves at the ionosphere is derived. On the basis of the expression, the effect of ionospheric divergent Hall current (Hall part of the induction current associated with the inductive electric field of fast magnetosonic wave) on localized toroidal oscillation is examined. When the horizontal scale of localized oscillation is of the order of several times of the height of ionosphere, the inductive electric field can play a significant role in the reflection of shear Alfvén waves with longer periods (e.g., ∼100 s) in the high‐latitude region, especially, in the auroral zone. Then, the contribution of the divergent Hall current to the field‐aligned one can be no longer neglected and so the eigenfrequency of localized toroidal oscillation is effectively controlled by the height‐integrated Hall conductivity in the ionosphere.
The effect of volcanic aerosols on ultraviolet radiation in Antarctica
Authors: Tsitas Steven R. , Yung Yuk L.
Published: February 2004

Volcanic eruptions can inject large amounts of aerosol into the atmosphere, and, at large solar zenith angles, scattering by these aerosols can actually increase the flux of UV‐B (290–320 nm) radiation reaching the surface. This is surprising since aerosols increase the reflection of sunlight to space. As previous explanations of this phenomenon are heuristic and incomplete, we first provide a rigorous and complete explanation of how this surprising effect occurs. This phenomenon makes Antarctica during spring the most susceptible place on Earth to the scattering effect of volcanic aerosols, due to the combined effect of the spring ozone hole and the large solar zenith angles characteristic of this time of year. We show that an aerosol layer lying above Antarctica during spring will decrease the integrated daily dose of biologically weighted irradiance, weighted by the erythema action spectrum, by only up to 5%. Hence the effects of any significant destruction of ozone induced by volcanic aerosols will not be offset by aerosol scattering. Thus after a volcanic eruption, life in Antarctica during spring will suffer the combined effects of the spring ozone hole and ozone destruction induced by volcanic aerosols, with the latter effect only slightly offset by aerosol scattering.
Deep electrical conductivity structures of the Appalachian Orogen in the southeastern U.S.
Authors: Ogawa Yasuo , Jones Alan G. , Unsworth Martyn J. , Booker John R. , Lu Xinyou , Craven Jim , Roberts Brian , Parmelee Jennifer , Farquharson Colin
Published: February 2004

Long period magnetotelluric data across the southeastern Appalachians image deep crustal and upper mantle relics of ancient orogenic events. Inversions of the responses show: (1) Beneath the Appalachian mountains there is a sub‐horizontal conductor at 15–20 km depth which dips to the southeast at the surface trace of the Brevard fault. (2) At the location of the Central Piedmont suture, there is a crustal conductor which dips towards the southeast, interpreted as a structure related to the Acadian suture. (3) Upper mantle conductors were found at 80 km depth northwest of the Blue Ridge and at 140 km depth southeast of the Eastern Piedmont. Between these, there is a northwest‐dipping resistive gap, possibly representing the remnant structure of the Alleghanian collision.
Mesopause temperature profiling by potassium lidar
Authors: von Zahn Ulf , Höffner Josef
Published: February 2004

We report on initial measurements of potassium density and temperature profiles of the mesopause region by a containerized and hence transportable lidar instrument. The centerpiece of this lidar is a new, all‐solid‐state laser system. We employed a cw seeded and pulsed alexandrite ring laser working at the 770 nm wavelength of the K(D1) finestructure line. We show that, different from the Na case, in K soundings the effects of a minor isotope have to be taken into account in the data analysis. We point out that we observe on average considerably smaller K column densities than those published in the late 1970s by a French group. We study quantitatively the influence of the Hanle and potential saturation effects on the derived temperatures. We give an example of a mesopause temperature profile taken with our transportable K lidar.
Velocity structure of the Andes of central Peru from locally recorded earthquakes
Authors: Dorbath Catherine
Published: February 2004

Arrival‐times of local events recorded during two short term seismic fied investigations in central Peru are used to determine the P and S velocity structures above the slab, in a segment of the Andes where the subduction is horizontal. The velocities in the upper‐crust can be related to the sedimentary cover. The Moho depth is estimated to be about 50 km below the western part of the sub‐Andean zone, and deepens gently to the west, reaching 60 km under the Western Cordillera. An uplift of high velocity material is observed under the coastal zone, associated with a cluster of earthquakes in and above the slab. The asymmetrical structure of the crust below the Eastern Cordillera reflects the convergence process in this part of the Andes: the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera, the compression then decreases toward the east.
A scenario for Jovian S‐bursts
Authors: Zarka Philippe , Farges Thomas , Ryabov Boris P. , Abada‐Simon Meil , Denis Laurent
Published: February 2004

Jovian S‐bursts are intense impulsive decameter radio spikes drifting in frequency in tens of milliseconds over several hundreds of kHz up to a few MHz. Their generation scenario has been much debated for 30 years. The automated analysis of an extensive set of digital radio observations at very high temporal and spectral resolutions is presented here. It strongly suggests that S‐bursts are the cyclotron‐maser emission of electron populations with ∼5 keV energy, accelerated near Io and then in quasi‐adiabatic motion along magnetic field lines connecting the Io torus to Jupiter's auroral regions. This scenario is consistent with Voyager observations of Alfvèn waves near Io's wake and with recent infrared observations of the Io flux tube footprint. The total energy, velocity and pitch angle of the radiating electrons, and the extent of the bursts radiosources are deduced from the radio measurements, which appear as a promising remote sensing tool for the Jovian magnetosphere, and possibly that of other “radio” planets.
Biological micro‐particles in rain water
Authors: Casareto Beatriz E. , Suzuki Yoshimi , Okada Kikuo , Morita Masataka
Published: February 2004

This work constitutes the first report on the morphology and chemical identification of biological micro‐particles in rain water at two sites (Shizuoka and Tsukuba) from Japan. Micro‐particles in rain water were deposited directly onto electron microscopic grids using ultracentrifugation. Particles with diameters more than 0.2 µm were examined using an electron microscope equipped with an energy‐dispersive X‐ray analyzer. Biological particles in rain water were found to be bacteria and leaf debris, and most of those particles were associated with minerals. Peaks of phosphorus and potasium in the X‐ray spectra, which are characterized as chemical components of bacteria or leaves, were observed in all the biological particles. These particles in rain water were present in number percentages of 23 to 34% of the total insoluble particles, with concentrations of 1.6×104 to 2.4×104 cm−3 of rain water. This finding suggests that these biological particles are captured effectively in cloud water and/or precipitation, and that they may play a role in cloud condensation and/or ice nucleation.
Dissolution behavior and surface tension effects of organic compounds in nucleating cloud droplets
Authors: Shulman Michelle L. , Jacobson Michael C. , Carlson Robert J. , Synovec Robert E. , Young Toby E.
Published: February 2004

Solubilities and surface tensions were measured for difunctional organic acids in various concentrations of (NH4)2SO4 and NH4HSO4 aqueous solutions. Model results using these data indicate that the organic compounds affect cloud droplet growth by two mechanisms: by gradual dissolution in the growing droplet which affects the shape of the Köhler growth curve, and by lowering of surface tension which decreases the critical supersaturation.
Space‐time structure of optical flashes and ionization changes produced by lighting‐EMP
Authors: Inan Umran S. , Sampson Wesley A. , Taranenko Yuri N.
Published: February 2004

Intense electromagnetic pulses (EMPs) released by lightning discharges produce bright optical emissions at 80–95 km altitudes emitted in a thin (∼30 km) cylindrical shell expanding to radial distances of up to >150 km, lasting for ∼400 µs, and appearing in limb‐view as a thin layer with ∼400 km lateral extent.
Low‐Ca garnet harzburgites from Kimberley, South Africa: Abundance and bearing on the structure and evolution of the lithosphere
Authors: Schulze Daniel J.
Published: February 2004

Based on composition and abundance of garnets from heavy mineral concentrates and from ultramafic nodules, it is estimated that low‐calcium garnet harzburgites, thought to be the host rocks of most diamonds in the upper mantle, form 2% of the mantle (5% of the garnet‐bearing portion) sampled by Group I kimberlites at Kimberley, South Africa. As both sampling approaches (xenocryst abundance and xenolith abundance) yield virtually identical estimates, there is no need to invoke the presence of magnesite or interstitial liquid to cause disaggregation of low‐Ca garnet harzburgites during eruption. Six of the eleven low‐Ca garnet harzburgite xenoliths identified contain zoned garnets and although there is considerable variation between samples, typically Ca increases and Cr decreases from core to rim. All other minerals are homogeneous (except in one rock) and magnesian (e.g., Mg/(Mg + Fe) in olivine = 0.929–0.949). Four of the eleven samples contain magnesio‐chromite (59.7–62.6 wt % Cr2O3), and two have primary phlogopite. These rocks have apparently equilibrated at pressure and temperature conditions equivalent to those on a conductive subcontinental geothermal gradient, mostly in the diamond stability field, and within the range of equilibration of the majority of garnet Iherzolites from Kimberley. Zoning is interpreted as due to re‐equilibration of the Ca‐poor domains toward the local bulk composition of the diopside‐saturated lherzolite upper mantle. Introduction of Ca may have been fluxed by the presence of a fluid (also introducing phlogopite), but uniformly low TiO2 values argue against interaction with a silicate melt. Such Ca metasomatism, if post diamond formation, could ultimately result in transformation of low‐Ca garnet harzburgites into garnet Iherzolites that could contain diamonds with low‐Ca garnet harzburgite paragenesis inclusions.
Seismic anisotropy of shales
Authors: Johnston Joel E. , Christensen Nikolas I.
Published: February 2004

The seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure, X ray diffraction techniques, and electron microprobe backscatter (BSE) imaging. The velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding. Anisotropy, at elevated pressures caused mainly by preferred orientation of clays (illite) parallel to bedding, ranges from 20% (Vp) and 19% (Vs) for a sample of New Albany Shale to 30% (Vp) and 35% (Vs) for a sample of Chattanooga Shale. The degree of clay mineral alignment in the shales is constrained by “orientation indices” produced using simple X ray diffraction techniques. A strong positive correlation is found between the degree of preferred orientation, as expressed in the orientation indices, and seismic anisotropy. BSE images of the shale fabrics confirm in a qualitative manner the results of the X ray study. To investigate wave propagation in the shales, elastic constants of each sample are calculated and used to produce phase and group (wave) velocity surfaces, which describe variation in velocity as a function of angle to the bedding normal. The calculated velocity surfaces, constrained by independent velocity measurements, display a lack of shear wave splitting at “near‐normal incidence” in even the most anisotropic shales. For the highly anisotropic Chattanooga shales, group velocity surfaces differ significantly from corresponding phase velocity surfaces.
Mantle plume interaction with an endothermic phase change
Authors: Schubert Gerald , Anderson Charles , Goldman Peggy
Published: February 2004

High spatial resolution numerical simulations of mantle plumes impinging from below on the endothermic phase change at 660‐km depth are used to investigate the effects of latent heat release on the plume‐phase change interaction. Both axisymmetric and planar upflows are considered, and the strong temperature dependence of mantle viscosity is taken into account. For plume strengths considered, a Clapeyron slope of −4 MPa K−1 prevents plume penetration of the phase change. Plumes readily penetrate the phase change for a Clapeyron slope of −2 MPa K−1 and arrive in the upper mantle considerably hotter than if they had not traversed the phase change. For the same amount of thermal drive, i.e., the same excess basal temperature, axisymmetric plumes are hotter upon reaching the upper mantle than are planar upwellings. Heating of plumes by their passage through the spinel‐perovskite endothermic phase change can have important consequences for the ability of the plume to thermally thin the lithosphere and cause melting and volcanism.
Southernmost South America‐Antarctic Peninsula relative plate motions since 84 Ma: Implications for the tectonic evolution of the Scotia Arc region
Authors: Cunningham W. Dickson , Dalziel Ian W. D. , Lee Tung‐Yi , Lawver Lawrence A.
Published: February 2004

We have attempted to quantify the relative motion history between southernmost South America (SSA) and the Antarctic Peninsula (AP) by calculating and comparing SSA‐Africa, AP‐Africa and SSA‐AP synthetic flow lines for 84–0 Ma. The flow lines were created using published poles of rotation and plate reconstruction software. The results indicate that since 84 Ma, SSA and AP have moved approximately westward relative to a fixed Africa; however, SSA's rate of westerly motion in that reference frame has been significantly more rapid than AP's rate. Approximately 1320 km of east‐west, left‐lateral strike‐slip displacement and 490 km of north‐south, divergent displacement have occurred between the southern tip of SSA and the northern tip of AP since 84 Ma. Increased rates of SSA‐AP interplate separation and a change in the angle of plate divergence at approximately 55–40 Ma marked the onset of accelerated continental separation that eventually led to seafloor spreading in the western Scotia Sea at 30 Ma and the development of the Scotia Arc. Increased separation rates between SSA and AP at 55–40 Ma may be related to a global Eocene plate reorganization event. The northeast‐southwest oriented western Scotia Sea spreading centers appear to have accommodated all of the SSA‐AP interplate motion between 30 and 9 Ma. We suggest that prior to 30 Ma and the opening of Drake Passage, components of interplate strike‐slip and divergent motion were accommodated by intracontinental deformation that included strike‐slip faulting, counterclockwise tectonic rotation, and continental extension in the southernmost Andes. The results indicate that the opening of the Scotia Sea was caused by plate‐scale motions as SSA and AP drifted away from Africa at different velocities along different, nonparallel trajectories. Subduction retreat along the South Scotia Ridge and South Sandwich arc and back arc spreading in the Scotia Sea contributed to the width of separation between SSA and AP across Drake Passage. The results place limits on how SSA‐AP relative motion has been temporally and spatially partitioned in the Scotia Arc region.
Two‐phase opening model for the Okinawa Trough inferred from paleomagnetic study of the Ryukyu arc
Authors: Miki Masako
Published: February 2004

Tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central Ryukyu arc and the southern Ryukyu arc for a paleomagnetic and geochronological study, in an attempt to understand the opening of the Okinawa Trough back arc basin. Stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central Ryukyu arc and from 12 sites of two geological units of the southern Ryukyu arc. The paleomagnetic results were compared with the Tertiary apparent pole wander path of Eurasia in order to investigate the tectonic movement of the Ryukyu arc with respect to the Asian continent. The comparison and the radiometric ages indicate that the southern Ryukyu arc rotated clockwise through 25° between 10 Ma and 6 Ma. In contrast, the central Ryukyu arc experienced little rotational motion since 17 Ma. We propose the following two‐phase opening model for the Okinawa Trough opening. The first stage took place between 10 Ma and 6 Ma. The southern part of the trough opened by means of a “wedge” mode to cause the clockwise rotation of the southern Ryukyu arc. In the central part the “parallel” opening occurred, and the Ryukyu arc drifted without any rotation. The second phase started recently in the whole Okinawa Trough. It is observed as the present activities in the trough.
Geochemical and isotopic disequilibrium in crustal melting: An insight from the anatectic granitoids from Toledo, Spain
Authors: Barbero L. , Villaseca C. , Rogers G. , Brown P. E.
Published: February 2004

In the Hercynian Anatectic Complex of Toledo (ACT) the anatectic granitoids include leucogranitic leucosomes, leucogranitic massifs, and restite‐rich granites. They show a broad range of initial Sr and Nd isotope ratios from 0.711 to 0.720 and 0.51164 to 0.51203, respectively, which clearly indicate the absence of isotopic homogenization in the melts. Broadly, the ranges reflect the isotopic variation of the metapelitic protoliths. If crustal melting occurs under water‐undersaturated conditions, as is the case of the ACT, the generated melts do not isotopically and chemically equilibrate with the granulitic residuum. The preservation of heterogeneities could arise through a number of processes. (1) duration of the process:in which the presence of melts with disequilibrium features, and the high solid content of several of the granites in the ACT point to a very short‐lived magmatic system, (2) limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization, and (3) magma dynamics in which the low melting rates, the high restite and low water contents, and the silica‐rich composition of these granites, together with the small volume and high viscosity values of the melts, seriously restrict the physical processes leading to homogenization.
San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry
Authors: Luhr James F. , Aranda‐Gómez Jose J. , Housh Todd B.
Published: February 2004

The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate‐type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, εNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation‐elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate‐type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.
A comparison of Holocene paleomagnetic secular variation records from North America
Authors: Lund Steve P.
Published: February 2004

The comparison of nine Holocene (0–12,000 years B.P.) records of paleomagnetic field secular variation (PSV) obtained from across North America indicates that distinctive field features in inclination and declination often can be traced for more than 4000 km without significant change in pattern. Independent age determinations corroborate these correlations while also suggesting that westward (or eastward) drift is not significant. On the other hand, northward drift may be a significant process at least between 1500 and 5500 years B.P. Several methods of analyzing the overall PSV character point to circularity (vector looping) as a preferred means of defining coherent space‐time vector variations which are termed “waveforms”. A few distinctive waveforms, each discernible for about 1400 years, dominate PSV across North America during the Holocene; 4.5 loops and 2 loops being associated with clockwise and counterclockwise circularity, respectively. The clockwise loops appear to be periodic, occurring every 2400 years, and most likely result from the recurrence of a distinctive regional dynamo source in the outer core. The pattern of circularity noted in North America during the last 3000 years correlates in time with similar circularity changes seen globally. This may indicate that there is some degree of coupling between different regional dynamo sources within the outer core. The fact that some North American waveforms are almost identical to waveforms observed elsewhere around the world at different times may indicate that similar distinctive regional dynamo sources recur randomly throughout the outer core.
Arrhenius behavior of aftershock sequences
Authors: Marcellini Alberto
Published: February 2004

Arrhenius expression of static fatigue is used to model aftershock sequences. Assuming that the initial stress condition σ0 at the mainshock origin time t0 is the superposition of the stress before the mainshock and of a stress step σod produced by the dynamic rupture of the mainshock, we may express a general Arrhenius aftershock model as Δ∑(τi) = σod + (RT/γ)Δti where Δ∑(τi) is the cumulative stress drop of mainshock and aftershocks at time τi = ti ‐ t0 ti is the origin time of ith aftershock and Δti = ln ti ‐ ln t0. The fit of the model to the aftershock sequences of May 6, 1976, ML = 6.3 Friuli earthquake (northeastern Italy) and of November 23, 1980, ML = 6.6 Campania‐Basilicata earthquake (southern Italy) is rather good: the coefficient of variation obtained by regression analysis is around 3% and indicates that, at least for the time window length considered (around 780 hours since the mainshock), the cumulative stress drop is entirely coseismic. The present aftershock model is derived from the empirical model t = s exp ((U ‐ γσ)/RT), considering the aftershock origin time to be equivalent to time to fracture t. Results show the validity of this approach. As an example, the derivatives of stress with respect to time to fracture obtained by the analysis of the two sequences are in good agreement with those obtained on laboratory samples; in particular, when the laboratory conditions (confining pressure, characteristics of the specimen) are similar to field conditions, the scatter is between 5 and 50%. The model appears also to be consistent with experimental evidences of direct correlation of p (exponent of Omori law) on surface heat flow.
Seamount volcanism at the Reykjanes Ridge: Relationship to the Iceland hot spot
Authors: Magde Laura S. , Smith Deborah K.
Published: February 2004

The axial zone of the Reykjanes Ridge is covered with small (0.5–3 km in diameter) volcanoes that pile together to form larger axial volcanic ridges. This style of volcanism is similar to that at the Mid‐Atlantic Ridge (MAR) and may be common to slow spreading ridges despite proximity of the Reykjanes Ridge to the Iceland hot spot. In this study we quantitatively investigate the population of seamounts in three study areas at the Reykjanes Ridge. Areas A and B are centered at 62°N and 60°N, respectively. Area C is centered at 58°N and is located south of the transition in ridge morphology from an axial high to an axial graben. Using multibeam bathymetry data, 541 seamounts (summit height H > 50 m) were identified in the three areas, and their size and shape statistics were compiled. Additionally, 105 seamounts in areas B and C were recognized in deep‐towed side scan images, and their surface morphologies (hummocky or smooth) were recorded. On the basis of estimated population parameters, we find that seamounts at the Reykjanes Ridge are more abundant (310±20 per 103 km2), on average, than at the MAR between 24° and 30°N (200±10 per 103 km2). Significant along‐axis variations exist at the Reykjanes Ridge, however, which are not simply related to distance from the hot spot: area B has nearly twice the abundance of seamounts as either area A or area C. Variation in the characteristic height of the seamount population is also observed between the Reykjanes Ridge (68±2 m) and the MAR (58±2 m), but no significant variation is found between our three study areas. A dramatic change in seamount surface morphology occurs between areas B and C (there are no side scan data from area A). Area C has 78% hummocky seamounts (similar to the proportion observed at the MAR), while area B has 83% smooth seamounts. On the basis of these results, we present a conceptual model for building the shallow crust at the slow spreading Reykjanes Ridge that takes into account the possible influence of the Iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances, sizes, shapes, and surface morphologies. In this model we suggest that the increased seamount production and proliferation of smooth seamounts in area B may be associated with a pulse of hot spot material, in the form of asthenosphere of higher temperature, that has recently affected area B.
Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — SdS travel time residuals
Authors: Vasco D. W. , Johnson Lane R. , Pulliam Jay
Published: February 2004

On the basis of P, PP, S, SS arrival times and SS ‐ S410S, SS ‐ S660S differential times, we construct models of mantle P and S velocity structure and boundary topography of the 410‐km and 660‐km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topography at the bounce point. The cap‐averaged PP ‐ P and SS ‐ S differential time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well with PP ‐ P and SS‐S differential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robust lp inversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled by SS ‐ S410S and SS ‐ S660S differential times, the velocity estimates are not seriously contaminated by the topography of the 410‐ and 660‐km discontinuities. However, estimates of boundary deflections may be influenced by extensive P and S velocity variations of 3% or greater. We find the 410‐km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660‐km discontinuity is quite distinct from that of the 410‐km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660‐km discontinuity with known subduction zones. The elevated topography in the 410‐km discontinuity beneath Europe is underlain by a trough in the 660‐km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. Negative P and S velocity anomalies, underlying back‐arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion of PP and SS travel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35–660 km beneath the Northwest Pacific we observe high P velocity. Where they are resolved, mid‐ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid‐Atlantic Ridge is underlain by negative S velocity anomalies. In the Pacific, the East Pacific Rise is an extensive low S velocity anomaly.
Seismological evidence for magmatic and hydrothermal structure in Long Valley Caldera from local earthquake attenuation and velocity tomography
Authors: Sanders C. O. , Ponko S. C. , Nixon L. D. , Schwartz E. A.
Published: February 2004

We compare the results of six different tomographic inversions for the three‐dimensional seismic attenuation and velocity structure in Long Valley caldera, California, in order to constrain interpretations on the location and character of magmatic and geothermal structure in the caldera crust.PandSwave amplitudes and arrival times obtained in previous studies from seismograms of local earthquakes recorded on vertical‐ and three‐component seismographs were inverted forQSV−1,QSH−1, ΔQS−1, ΔQP−1, and ΔVP/VSin the upper 9 km of the caldera crust. Detailed comparisons of the results of the inversions suggest significant locations of high temperatures and perhaps magma and hydrothermal zones in the caldera. A region of lowQswith locally variableQPandVP/VSis imaged in the 7–9 km depth range beneath the resurgent dome. Overlying and adjacent to this anomaly are significant low‐QPand low‐VP/VSanomalies as well as smaller zones of lowQSand highVP/VS. Together, these suggest zones of high temperature/magma below and adjacent to zones of supercritical hydrothermal fluids. At 4–5 km depth along the southern rim of the caldera just east of Mammoth Mountain a significant anomaly with relatively low‐QPand relatively low‐VP/VSis imaged and is interpreted as a supercritical hydrothermal zone. Poorly resolved relatively lowQPand relatively highVP/VSare seen at 4–5 km depth beneath the Inyo craters, suggesting possible high temperatures and perhaps magma. Because of lack of data in the individual inversions the resultant images have spatially variable error and resolution. However, by comparing multiple images of different seismic parameters we have improved our ability to determine the significance of and constrain interpretation of seismic structure anomalies.
Gold‐quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting
Authors: Robert François , Boullier Anne‐Marie , Firdaous Karima
Published: February 2004

Gold‐quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime. In the Val d'Or district of the Abitibi greenstone belt, Canada, quartz‐tourmaline‐carbonate veins form a vein field (30 × 15 km) in the hanging wall of a crustal‐scale fault zone, which was the main channelway for upward migration of the deeply generated fluids. The veins occur in small high‐angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks. They have formed incrementally during active reverse faulting in response to crustal shortening, at depths corresponding to those at the base of the seismogenic zone in actively deforming crust. Detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures (ΔPƒ of the order of 200 MPa) and for cyclic stress reversals during vein formation and provide good support for the fault valve model. A comparison of vein characteristics with “standard” earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake (4 > M > 3 or less). The large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal‐scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system. It is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes (M > 6) nucleating near the base of the seismogenic regime along the nearby crustal‐scale fault, either as aftershocks or as a precursory smarm.
Origin of compositional heterogeneities in tuffs of the Timber Mountain Group: The relationship between magma batches and magma transfer and emplacement in an extensional environment
Authors: Cambray F. William , Vogel Thomas A. , Mills James G.
Published: February 2004

Compositionally zoned ash flow sheets provide convincing evidence for chemically zoned magma bodies. Most workers have assumed that the high‐silica portions of these magma bodies evolved largely by differentiation processes that occurred within the magma chamber. However, chemical heterogeneities within some ash flow sheets are not consistent with these differentiation processes. The chemical variation of pumice fragments in the large volume (>1200 km3), Rainier Mesa ash flow sheet ranges from 55 to 76.3% silica. These pumice fragments occur in three distinct chemical groups. A low‐ and high‐silica group is separated by a compositional gap at about 72% silica, and within the high‐silica group there are two distinct populations based on trace element variations. There is little overlap between populations. These three magma types have been resident in same magma chamber at the same time and cannot be produced by any differentiation process of a single magma body. They must reflect discrete magma batches generated in the source area. Furthermore, the lower silica portion (<72% SiO2) of the Rainier Mesa ash flow sheet is chemically distinct from the lower silica portion of the overlying Ammonia Tanks ash flow sheet, even though they erupted within 200,000 years of each other. These ash flow sheets from the SW Nevada volcanic field are associated in time and place with Basin and Range extension, and all models for extension involve detachment surfaces that extend to great depth. A model for the relationship of these compositional heterogeneities and the regional extension involves (1) the generation of magma batches by either continuous melting of the source at different temperatures, or by melting of different sources, (2) the use of faults (shears) as conduits for transport of magma, and (3) the use of a dilatant releasing step on a detachment as storage chamber for the magma. We first address the evidence for separately generated magma batches and then propose a model for their coalescence in a single chamber prior to eruption.