bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of magnetic orientation in driver gas associated −Bz events
Authors: Hoeksema J. Todd , Zhao Xuepu
Published: February 2008

Can the magnetic orientation of driver gas in the interplanetary medium observed at Earth be predicted when its solar source is identified? Based on the exploratory assumption that the free magnetic energy present when a coronal mass ejection (CME) is initiated is transferred almost totally to other forms of energy by the time the CME stops acceleration, the potential field model can appropriately be used to calculate the magnetic orientation in the ejected plasma. As a first test of this hypothesis we have investigated the source regions of five strong −Bz events detected at 1 AU for which solar sources have been identified (Tang et al., 1989; Tsurutani et al., 1992). Three were traced to flare associated CMEs, one to an eruptive prominence associated CME, and one to three possible solar sources. The computed magnetic orientations at the candidate “release height” (the height where the front of a CME ceases to accelerate) above the flare sites associated with CMEs show the existence of the expected southward field component. On the other hand, the predicted field orientation at the candidate release height above the erupting prominence associated with a CME does not have a dominant southward field component. The result shows that the exploratory assumption may be proper only for the case of flare CMEs. With such a severely limited number of cases the results are only suggestive, but the magnetic orientation in flare associated CME generated driver gas may be predictable.
A semikinetic model for early stage plasmasphere refilling: 2, Effects of wave‐particle interactions
Authors: Lin J. , Horwitz J. L. , Wilson G. R. , Ho C. W. , Brown D. G.
Published: February 2008

We treat the early stages of plasmasphere refilling along an initially depleted L=4 magnetic flux tube through a semikinetic model. The companion paper by Wilson et al. (this issue) describes the plasma evolution for a “baseline” refilling situation in which Coulomb collisions play a central role. Here we focus on the effects of wave‐particle interactions in which stochastic diffusion of ions in perpendicular velocity due to equatorially concentrated electromagnetic ion cyclotron waves plays a central role. We examine characteristic individual ion trajectories, as well as the evolution of bulk parameters and ion distribution functions when equal “polar wind” streams are injected at the northern and southern ionospheres. In the ion trajectories, it is found that relatively modest and realistic perpendicular electric field power levels lead to decreased mirror latitudes, substantial acceleration, and equatorial entrainment of these ions. After about 8 hours, significant general accumulation of plasma occurs all along the flux tube, with equatorial densities attaining levels of 15 ions/cm³, and equatorial parallel and perpendicular temperatures of around 2.4 and 60 eV, respectively. Incorporating an electron temperature distribution which is calculated as a fraction of the local ion temperature, and therefore also peaks at the magnetic equator, leads to an ambipolar electric field which acts to reflect incoming ions back toward the ionospheres. In this case, we find results strikingly similar to those of Olsen et al. (1987), wherein trapped 50 eV ion distributions are seen at the magnetic equator, while incoming and reflected, cool, 1–2 eV, field‐aligned streams are seen on both sides of the equator. A substantial equatorial density depletion is also found, in good accord with the results of Olsen (this issue).
Small‐scale electrodynamics of the cusp with northward interplanetary magnetic field
Authors: Basinska Ewa M. , Burke William J. , Maynard Nelson C. , Hughes W. J. , Winningham J. D. , Hanson W. B.
Published: February 2008

We report on possible, low‐altitude field signatures of merging occurring at high‐latitudes during a period of strong, northward directed interplanetary magnetic field. Our study of the small‐scale electrodynamics in a vicinity of the poleward cusp boundary is based on multisensor measurements from a single DE 2 satellite pass over northern hemisphere cusp/cleft regions. Observed sunward convection over the polar cap and presence of the reverse ion dispersion feature are in agreement with expectations for merging occurring at the magnetopause, on the field lines mapping to the poleward boundary of the cusp. Large electric and magnetic field spikes detected at the poleward edge of the magnetosheathlike particle precipitation are interpreted as field signatures of the low‐altitude footprint of such merging line locations. A train of phase‐shifted, almost linearly polarized electric and magnetic field fluctuations was detected just equatorward of the large electromagnetic spike. These may be due to either ion cyclotron waves excited by penetrating magnetosheath ions or transient oscillations in the frame of convecting plasma, brought about by the sudden change in the flow at the magnetospheric end of the field line.
On the thermodynamics of diamagnetic plasma expansions
Authors: Gisler Galen R. , Onsager T. G.
Published: February 2008

Particle heating in a diamagnetic plasma expansion is studied by means of well‐diagnosed simulation with an axisymmetric particle‐in‐cell code. Moments of the particle distribution function are obtained for spatially distinct subsets of the particles to examine temperature and density histories for different regions of the expanding plasma. The simulation is followed through one expansion‐contraction cycle. While adiabatic behavior is observed during much of the cycle, significant deviations from the adiabatic result in strong particle heating. Anomalous ion heating occurs throughout the plasma during the expansion phase. This is manifested earliest in the ion parallel temperature, which increases first in the expanding plasma's outer reaches and last in the center. This heating originates from the entropy that is generated at the barrel ends of the plasma as the initial expansion along the ambient field is essentially free. Later, after a diamagnetic cavity is formed, ions within the cavity are reflected by the magnetic mirrors at the necks and transport some of the generated entropy back into the center of the cavity. The ion heating that occurs can easily raise the bulk plasma temperature by an order of magnitude over the initial adiabat.
Detection of artificially created negative ion clouds with incoherent scatter radar
Authors: Sultan Peter J. , Mendillo Michael , Oliver William L. , Holt John M.
Published: February 2008

Theoretical incoherent scatter (IS) spectra from collisionless F region plasma are calculated taking into account the effect of negatively charged ions. Such ions can significantly change the composition of the local ionosphere when they are created artificially during active chemical release experiments. If IS radar observations are conducted upon such a modified plasma, the interpretation of the scattered radar signal will be complicated by the negative ions, which are not found in the natural (e.g., O+/e−) ionosphere. Specifically, the presence of heavy negative ions will broaden the IS spectrum, making the shoulder peaks sharper, and creating an additional small central hump. A radar spectral analysis program that assumes only the naturally occurring plasma constituents will erroneously attribute such spectral changes to changes in other plasma parameters, such as the temperature ratio Te/Ti. In order to detect heavy negative ions we fix the temperature structure of the ionosphere to a preevent average measurement and interpret any changes in spectral shape during the experiment as being caused by changes in composition, and not by changes in Te/Ti. The Millstone Hill 440‐MHz IS radar was used to observe the spatial and temporal development of heavy negative ion plasma clouds created during four active chemical release experiments: the Ionospheric Modification Study (IMS) in 1983, the Space‐Plasma Negative Ion Experiments (SPINEX 1 and 2) in 1984 and 1986 (all SF6 releases), and NICARE 1 in 1989 (CF3Br release). Each experiment led to the creation of a region of heavy, negatively charged ions. Concentrations of 10–40% SF6− (146 amu) are reliably detected in the SPINEX 1, SPINEX 2 and IMS data sets. An average uncertainty of ±10% SF6− is present in all three experiments. Concentrations of 30% Br− (80 amu) are further detected in the NICARE 1 release, with uncertainties of ±4%.
Birkeland currents in an anisotropic, magnetostatic plasma
Authors: Birmingham Thomas J.
Published: February 2008

An expression for the parallel current density is derived for a plasma characterized by negligible bulk flow (magnetostatic) velocity and a two‐component (anisotropic) pressure tensor by expanding the equilibrium Vlasov equation for each species in the adiabatic parameter until such point as a nonvanishing moment j∥ = ∫ d³ υυ∥ ƒ is identified. The result is a nonlocal one: it relates j∥ at one point s along a field line to j∥ at another (reference) point s0 plus an integral function of the pressure and magnetic field between them. It is a generalization and elaboration of results obtained by Bostrom (1975), Heinemann (1990), and Heinemann and Pontius (1991). The expression could have been obtained by integrating the current continuity equation with −▽ · j⊥ as a source term and j⊥ given by perpendicular momentum balance. We explicitly show the equivalency. The widely used Vasyliunas (1970) equation follows when P⊥ is set equal to P∥ and s and s0 are taken to be at the ionosphere and the equator. An extended discussion of the relationship of results derived here to others in the literature is carried out in an effort to bring unity and perspective to this problem area.
Langmuir turbulence and ionospheric modification
Authors: Stubbe P. , Kohl H. , Rietveld M. T.
Published: February 2008

Experimental results, obtained with the Heating facility and the EISCAT UHF and VHF radars at Tromsø, Norway, have been used to test existing Langmuir turbulence theories and to provide further observational facts to be taken into account in a new theoretical formulation of Langmuir turbulence. It is found that none of the available theories is able to describe the diversity of observational facts. The dominance of propagating Langmuir and ion acoustic waves in the observed spectra, and the fact that these waves closely follow their linear dispersion relations, lend support to a weak turbulence description. On the other hand, the observation that the number of cascade steps is limited to two, independent of input power, strongly contradicts such an approach. Unmistakable indications for the occurrence of caviton collapse are not seen in the experimental results at Tromsø. The low‐frequency turbulence is found to consist of subsonic and sonic contributions, showing widely independent properties. The sonic turbulence is seen to evolve from the natural, thermally excited, ion acoustic background.
Precipitation of ions induced by magnetotail collapse
Authors: Delcourt D. C. , Moore T. E.
Published: February 2008

The precipitation of ions triggered by dipolarization of magnetospheric field lines during substorm expansion phase is examined by means of single‐particle codes. As large but short‐lived electric fields develop and particles experience transient nonadiabatic motions, it is demonstrated that “de‐trapping” of inner plasma sheet populations can be achieved either via damping of magnetic moment or impulsive accelerations in the parallel direction. It is shown that the former precipitation mechanism results from phasing between gyromotion and the surging electric field. It preferentially affects heavy and low‐charge state ions which can display large de‐energization rates within a cyclotron period. The latter mechanism rather favors the loss of lighter and/or higher‐charge state ions which are less sensitive to temporal nonadiabaticity. In this case, injection into the loss cone may result from enhanced curvature related acceleration. As this latter acceleration occurs on the dipolarization time scale, it affects a limited amount of plasma sheet populations, namely those intercepting the region of maximum curvature at the time of peak induced electric field.
An estimate of the heat flow on the meteor rise, subantarctic South Atlantic
Authors: Nobes David C. , Mienert Jürgen , Mwenifumbo C. Jonathon
Published: February 2008

Heat flow determinations require more than one reliable temperature measurement to obtain an estimate of the temperature gradient, and subsequently the heat flow. Two temperature readings were taken on leg 114 of the Ocean Drilling Program, both in hole 704B on the Meteor Rise in the subantarctic South Atlantic. One of these readings appears to be reliable, but the other appears to be invalid. Because any temperature gradient and estimate of heat flow derived using these two readings will be questionable, we suggest a different approach to determine heat flow. We use the one reliable temperature measurement to calibrate the temperature derived from the induction resistivity log and the laboratory porosity measurements. The temperature and heat flow depend on the shape factor used in the modified Archie's law. The temperature gradient empirically obtained from the resistivity and porosity is 38±4 mK/m and the average heat flow is 64±8 mW/m2, which is consistent with the age of the Meteor Rise (approximately 60 to 65 Ma). These values are in agreement with the computed heat flow when we assume a simple linear temperature gradient through the sediment section.
Temporal variation of the Earth's low‐degree zonal gravitational field caused by atmospheric mass redistribution: 1980–1988
Authors: Chao B. Fong , Au Andrew Y.
Published: February 2008

Temporal variations in the low‐degree zonal harmonics of the Earth's gravitational field have recently been observed by satellite laser ranging. A host of geophysical processes contribute to these variations. The present paper studies quantitatively a prime contributor, atmospheric mass redistribution, using global surface pressure data from the European Centre for Medium Range Weather Forecasts for the period of 1980–1988. Seasonal variations are focused upon: We compute the annual and semiannual amplitudes and phases of the zonal Jl coefficient with degree l = 2–6 with and without the oceanic inverted‐barometer (IB) effect and obtain the predicted effects on the orbit nodal residuals δΩ of Lageos and Starlette. These predicted δΩ are then compared with the observed δΩ. It is found that the atmospheric influence, combined with the hydrological influence (from Chao and O'Connor, 1988), agree well with the Lageos observation for the annual term. The corresponding match appears poorer for Starlette. The comparisons for the semiannual term, however, show little agreement. The discrepancies can be attributed primarily to the uncertainty arising from the IB assumption, the systematic errors in the hydrological estimates and in the observations, as well as other geophysical processes that influence global mass distribution, particularly those in the oceans. Interannual and decadal variations, with their impact on the observations of “secular” changes are also discussed.
Waveform inversion for three‐dimensional density and S wave structure
Authors: Tanimoto Toshiro
Published: February 2008

Long‐period seismic data (longer than about 100 s) are sensitive to density as well as P wave and S wave structure in the Earth. Seismograms containing overtone and fundamental mode signals are inverted directly for density and S wave velocity structure. A simple relation between P wave and S wave velocity perturbations is assumed in order to reduce the number of unknowns. A model is parameterized by spherical harmonics up to degree and order 8 and by five knots in depth (at 30, 220, 450, 670 and 882 km) with linear interpolation between adjacent knots. Density resolution is good only at shallow depths; in fact, only the average in the upper 200 km is marginally resolved. S wave resolution is generally good throughout the depth range. S wave results show good correlation to surface geological features and are similar to previous studies. Density results show quite different patterns from S wave maps; high‐density regions appear along the subduction zones surrounding the Pacific Ocean and low‐density regions appear in most major continents. Most continents have low density and high S wave velocity, which suggests that continental mantle consists of compositionally different materials since thermal effects can never produce such perturbations. High S wave velocity also suggests a relatively cold geotherm for continents, and thus continents should consist of significantly low‐density material such as depleted mantle rocks, as proposed by many researchers before. The density map also correlates well with a geoid map for spherical harmonic components ℓ=4–6 but not for ℓ=2,3. It suggests that sources for these geoid undulations ℓ=4–6 are in the upper mantle, probably in the upper 300–400 km, but those for ℓ=2,3 are in deeper regions.
Pore pressures and permeabilities measured in marine sediments with a tethered probe
Authors: Davis E. E. , Horel G. C. , MacDonald R. D. , Villinger H. , Bennett R. H. , Li H.
Published: February 2008

A new probe for measuring pore fluid pressures in marine sediments has been constructed and tested in two hydrologic environments on the Juan de Fuca Ridge. The instrument utilizes a highly incompliant high‐sensitivity differential pressure transducer, and a small (4mm) diameter sensing probe that telescopes within a larger diameter 1.5‐m‐long (currently 2.5 m) strength member. The small probe diameter and the low compliancy serve to minimize the time required for pressures to approach equilibrium after penetration. Observed decay times are short enough to permit the instrument to be used in a tethered mode. The mechanical configuration enables the small diameter probe to penetrate the sediments without buckling and to be pulled out without bending. This allows multiple penetrations to be completed during a single instrument lowering. Ten measurements were made during the development and testing of the instrument in fine‐grained turbidite sediments on the Juan de Fuca Ridge. Four penetrations were uninterrupted by mechanical disturbances or instrumentational problems, and although equilibrium conditions were approached during only one penetration (2.7 hours maximum undisturbed decay period), all provided useful constraints on equilibrium pore pressures. Values at two sites where sediment cover is thick and heat flow relatively low were unresolvably different from hydrostatic. Values estimated from two penetrations located within a few hundred meters of a hydrothermal vent field within the sedimented axial valley of the northern Juan de Fuca Ridge, Middle Valley, exceeded hydrostatic and yielded pressure gradients of roughly 0.5 kPa m−1. Permeabilities were estimated from the rates of decay of the penetration transients. Values ranged from 5×10−16 to 1×10−14 m2 and agreed well with values measured on core samples collected near the probe penetrations which ranged from 8×10−16 to 1×10−14 m2. Using the values determined from the probe measurements from Middle Valley, the implied rate of fluid flow upward through the sediments in the vicinity of the vent field is roughly 5×10−10 m s−1, or 15 mm yr−1. With the current practical limit of resolution of roughly 10−10 m s−1 (3 mm yr−1) this direct measurement technique provides a means of determining pore fluid flow rates that is roughly 1 order of magnitude more sensitive than the method of estimating the rate of flow from the perturbation of the conductive thermal regime measured by typical marine heat flow instruments.
Precaldera lavas of the southeast San Juan Volcanic Field: Parent magmas and crustal interactions
Authors: Colucci M. T. , Dungan M. A. , Ferguson K. M. , Lipman P. W. , Moorbath S.
Published: February 2008

Early intermediate composition volcanic rocks of the Oligocene (circa 34–29 Ma) southeast San Juan volcanic field, southern Colorado, comprise the Conejos Formation. Conejos lavas include both high‐K calc‐alkaline and alkaline magma series (54–69% SiO2) ranging in composition from basaltic andesite (basaltic trachyandesite) to dacite (trachydacite). The subsequent Platoro caldera complex (29–27 Ma) was superimposed on a cluster of broadly precursory Conejos stratocones. Precaldera volcanism occurred in three pulses corresponding to three time‐stratigraphic members: (1) the Horseshoe Mountain member, (2) the Rock Creek member, and (3) the Willow Mountain member. Each member exhibits distinctive phenocryst modes and incompatible trace element contents. Horseshoe Mountain lavas (hornblende‐phyric) have relatively low alkali and incompatible element abundances, Rock Creek lavas (anhydrous phenocrysts) and ash‐flow tuffs have the highest abundances, and Willow Mountain lavas (diverse mineralogy) are intermediate. All Conejos lavas exhibit low ratios of lead (206Pb/204Pb = 17.5 to 18.2) and neodymium (εNd = −8 to −4) isotopes and high 87Sr/86Sr (0.7045 to 0.7056) compared to depleted asthenospheric mantle. These values lie between those of likely mantle compositions and the isotopic composition of Proterozoic crust of the southern Rocky Mountains. Mafic lavas of the Horseshoe Mountain member have the lowest Pb and Nd isotope ratios among Conejos members but trend toward higher isotopic values with increasing degrees of differentiation. Compositions within the Rock Creek series trend toward higher Pb and lower Nd isotope ratios with increasing SiO2. Willow mountain volcanic sequences define diverse chemical‐isotopic correlations. We interpret the chemical and isotopic differences observed between mafic lavas of each member to reflect derivation from compositionally distinct mantle derived parent magmas that have experienced extensive deep level crustal contamination. Chemical and isotope correlations within each member indicate Conejos magmas differentiated via staged polybaric, multiprocess evolutionary paths. Stage I occurred near the base of the crust as mantle‐derived basalt evolved to basaltic andesite by the MASH mechanism. Stage II followed ascent to shallower crustal levels where basaltic andesite differentiated to more evolved compositions by combined fractionation and assimilation of heterogeneous crust.
Mechanics of emplacement and tectonic implications of the Ramon Dike Systems, Israel
Authors: Baer Gidon , Reches Ze'ev
Published: February 2008

A radial system comprising more than 200 basaltic and trachytic dikes and two minor systems of parallel dikes intruded the Ramon area, southern Israel, during the Early Cretaceous. Field relations between dikes and fractures in the radial system indicate that the dikes intruded self‐generated fractures, and thus indicate the directions of the tectonic stresses. Other field observations indicate that the dikes propagated in subhorizontal directions up to distances of 15 km from their source. Our analysis of the emplacement mechanics of these dikes shows that the horizontal propagation is best explained by the density differences between the intruding magma and the host rocks. The measured mean density for basement rocks at depths greater than 2.4 km is 2.55±0.07 g/cm3, and it is 2.36±0.21 g/cm3 for the sedimentary cover above. For the magma to be propagating horizontally at its neutral buoyancy level requires a mean magma density of about 2.5 g/cm3. The large distance of horizontal propagation requires a low viscous pressure drop, below 0.1 MPa/km within the dike, and an overpressure of about 1 MPa in the magma chamber. We computed stress trajectories using a two‐dimensional elastic model for a pressurized hole in a regional stress field and compared it to the Ramon radial system. The model reveals that the dikes originated at a central intrusion of about 3 km diameter and intruded under a predominantly radial state of stress with negligible regional stress field. This period of weak tectonic stresses and intensive magmatism falls between the early Mesozoic extensional regime and the late Mesozoic‐Cenozoic compressional regime in Israel. The calculated center of the radial system is offset from a large magnetic anomaly south of the Ramon area, suggesting a 3 km right‐lateral displacement along the Ramon fault after the intrusion of the radial system.
The shock wave equation of state of brucite Mg(OH)2
Authors: Duffy Thomas S. , Ahrens Thomas J. , Lange Manfred A.
Published: February 2008

New equation of state (EOS) data for brucite Mg(OH)2shocked between 12 and 60 GPa are reported. When combined with earlier data of Simakov et al. (1974), it is found that brucite EOS data between 12 and 97 GPa can be fit with a single linearUs‐uprelationship:Us= 4.76(0.11) + 1.35(0.05)up. The third order Birch‐Murnaghan equation parameters are:Kos= 51 ± 4 GPa andK′os= 5.0 ± 0.4. The lack of aUs‐updiscontinuity indicates that no phase transformation with a significant volume change occurs to at least 97 GPa. However, thermodynamic and theoretical Hugoniot calculations suggest brucite may dehydrate with only a small volume change. A lower bound for this dehydration pressure under shock conditions is inferred to be 26 GPa. We report the first partial release states measured for this material. The data are in quantitative agreement with earlier shock recovery experiments (Lange and Ahrens, 1984). Volatilization upon release begins at pressures as low as 12 GPa, much less than predicted by the shock entropy method. Calculated phase boundaries using the present EOS data are consistent with experimental data and indicate that brucite is unlikely to be stable under lower mantle conditions. However, brucite data, in conjunction with data for silicates and oxides, can be used to infer the effect of H2O on lower mantle properties. At high pressure, bulk sound velocities calculated for MgO and Mg(OH)2are very similar, indicating that the presence of hydrous assemblages in the lower mantle may not produce anomalous bulk seismic velocities. Comparison of densities in brucite and other high‐pressure phases under mantle conditions indicates that the water content of the lower mantle is between 0 and 3 wt %.
An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California
Authors: Wilson Jeffery M. , McCarthy Jill , Johnson Roy A. , Howard Keith A.
Published: February 2008

A 135‐km‐long, NW‐SE trending, seismic refraction/wide‐angle reflection profile provides a unique along‐strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two‐dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (< 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s−1) overlying a fairly homogeneous basement with velocities of 6.0 km s−1; (2) a localized, high‐velocity (6.4 km s−1) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s−1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s−1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a Pn velocity of 8.0±0.10 km s−1. The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s−1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle‐derived magmatism and lateral ductile inflow in the crust are proposed.
Introduction to Middle Tertiary Cordilleran Volcanism: Magma Sources and Relations to Regional Tectonics
Authors: Lipman Peter W. , Glazner Allen F.
Published: February 2008

Cenozoic magmatism in the North American Cordillera has attracted the attention of volcanologists, structural geologists, and petrologists for more than a century. From horseback reconnaissance in the 1800s, when a hand lens was the analytical instrument of choice [e.g., von Richthofen, 1868; King, 1878] to today's studies, where mass spectrometers, electron microprobes, and computers are essential tools, attention has focused on the broad regional extent, large volumes, and complexly shifting space‐time‐composition patterns of Tertiary magmatism in the Cordillera.

Cordilleran igneous rocks provide key information on regional geologic history, primary geologic constraints on many types of ore deposits, and the basis for development of broad concepts applicable to global magmatism and plate tectonics. Studies of Cordilleran igneous rocks have been especially intense in the past few decades, fueled by improved analytical techniques and by societal concerns such as the search for geothermal and mineral resources and evaluation of volcanic hazards.
Petrophysical analysis of the Cajon Pass Scientific Well: Implications for fluid flow and seismic studies in the continental crust
Authors: Vernik Lev , Nur Amos
Published: February 2008

In the framework of the Cajon Pass scientific drilling project in the vicinity of the San Andreas fault system, petrophysical analysis of the retrieved core was focused on elastic properties, density, porosity, and permeability. Simultaneous study of rock mineral composition, texture, degree of metamorphism, and fracture filling yielded a petrophysical classification of the studied rock that is convenient for the interpretation of the downhole logging data. Given the small percentage of the hole drilled with coring, the geophysical data were used to construct a lithologic‐petrophysical column of the well. Most of the petrophysical variability seen in this column is the result of laumontite alteration of granitoids during retrograde metamorphism in the zeolite facies. The intensity peaks of the alteration process are spatially confined to the oblique reverse faults (25°–40° dip angles as observed in the core and borehole televiewer images) accompanied by brittle deformation of the adjacent granitoids. The marked drop in density, velocity, and elastic moduli and the noticeable increase in porosity and permeability of these altered fault zone rocks are enhanced owing to in situ stress shielding. The low velocity and density and higher permeability associated with these faults makes them detectable seismic reflectors and hydrologic conduits in the upper crust. By contrast, based on our data analysis, the matrix of the intact crystalline rock in situ is inferred to be essentially free of microcracks and interconnected channels with free water. A detailed analysis of the various factors affecting the microcrack introduction into the cores retrieved from deep and superdeep holes is presented to show that a unique interpretation of in situ stresses from analysis of microcracks in cores is not possible without a better understanding of the mechanisms of crack formation.
Determination of geoid height at Breid Bay, east Antarctica
Authors: Shibuya K. , Fukuda Y. , Michida Y.
Published: February 2008

By the combined observation of satellite Doppler positioning, Global Positioning System (GPS) relative carrier phase measurement, and ocean tide observation, we obtained the geoid height at Breid Bay (70°12′S, 23°47′E) as 16.8 m above the World Geodetic System 1984 (WGS84) Earth ellipsoid. The broadcast ephemeris satellite Doppler positioning was made, and the ellipsoidal height at L0 point on the ice sheet of Breid Bay is estimated to an accuracy of ±4 m from the accepted 95 Navy Navigation Satellite System (NNSS) satellite passes. GPS relative carrier phase measurement was made between L0 point and the deck of the icebreaker Shirase (S point). The height difference between L0 point and S point is determined to an accuracy of ±0.3 m from the analysis of 15‐min carrier phase data from four satellites by the doubly differenced phase method. The recording of sea level variation was made using the sea bottom pressure‐transducer water level recorder for 4 days, together with monitoring of ship's attitude and meteorological data. The separation of S point from the local mean sea level is determined to an accuracy of ±0.3 m. The standard deviation of the obtained geoid height can be considered to be (42 + 0.32 + 0.32)½ ∼ 4 m. Thus obtained geoid height at S point is 3.4 m smaller than the WGS84 model geoid (18 order truncation) and 8 m smaller than the OSU‐86D geoid of 250 order harmonics. Though the discrepancy of a few meters may be explained by local geoid highs in Breid Bay, 3–5 m systematic depression of geoidal contours may be required in the region concerned (20°E to 40°E) in order to explain geoid heights at both S point and Syowa Station. The method we applied here is operationally simple and appropriate for dense installation of geoid height control stations along circum‐Antarctic coastal zone.
Imaging the Juan de Fuca Plate beneath southern Oregon using teleseismic P wave residuals
Authors: Harris Ruth A. , Iyer H. M. , Dawson Phillip B.
Published: February 2008

The Juan de Fuca plate subducts under the western margin of the North American plate in southern British Columbia, Washington, Oregon, and northern California. Benioff zone seismicity delineates shallow parts of the plate in Washington and northern California, whereas in Oregon there is a lack of seismicity. This study images the Juan de Fuca plate in southern Oregon using seismic tomography. We inverted P wave travel time residuals from a 366‐km‐long seismic array operated in southern Oregon in 1982. The southeast striking array extended from the Coast ranges to the Modoc Plateau and crossed the High Cascades at Crater Lake, Oregon. We imaged three features under the array: one high‐velocity zone and two low‐velocity zones. The high‐velocity zone is 3–4% faster than the surrounding upper mantle. It dips steeply at 65° to the east beneath the Cascade Range and extends down to at least 200 km. We propose that this high‐velocity feature is subducted Juan de Fuca plate. We also imaged two low‐velocity zones, both of which are 3–4% slower than the surrounding earth structure. The southeastern low‐velocity zone may be caused by partially molten crust underlying the Crater Lake volcano region.
Three‐dimensional seismic imaging of the Costa Rica Accretionary Prism: Field program and migration examples
Authors: Stoffa Paul L. , Shipley Thomas H. , Kessinger Walter , Dean Donald F. , Elde Rigmor , Silver Eli , Reed Don , Aguilar Alvaro
Published: February 2008

Seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins. On convergent plate margins, widely separated seismic reflection profiles often do not image complex structural features because out‐of‐plane reflections and diffractions obscure the seismic images. Yet the structural features are often critical to understanding the tectonic processes. Thus, the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure. To image the complex geologic structures of an active continental margin, a three‐dimensional (3‐D) seismic survey was conducted off of the Nicoya Peninsula of Costa Rica in April 1987. Over 60,000 shot points of 96‐trace multichannel data were collected in a 9×22 km grid. This detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin. In this paper we outline the data acquisition program, including the navigation processing critical to successful imaging, and the seismic processing methodology and then show several examples of images improved by 3‐D surveying. The resulting 3‐D seismic data were migrated both as independent lines and as a 3‐D volume. Seismic images were produced as conventional depth and time sections and as cross‐sectional slices in depth and time. We present comparisons of the migration results that show significant improvements of the subsurface image using 3‐D techniques. To illustrate these improvements, we present images of two complex geologic areas: a mud volcano and reflections near the top of the accretionary prism, and the top of the subducting oceanic basement beneath the prism. Comparisons between the original stacked data, the data of the two‐dimensional migration, and the results of the 3‐D migration illustrate the value of 3‐D techniques in studies of active margins. In addition to the advantages of the 3‐D imaging which include better structural delineation and improved signal‐to‐noise ratio in the final image, we show that the ability to view the 3‐D data volume in both section and plan view offers significant interpretational advantages.
Partitioning of platinum group elements in the Fe‐O‐S System to 11 GPa and their fractionation in the mantle and meteorites
Authors: Fleet Michael E. , Tronnes Reidar G. , Stone William E.
Published: February 2008

The partitioning of minor amounts of platinum group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) in the Fe‐O‐S system has been investigated to 11 GPa. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid, reported earlier for the Fe‐Ni‐S system at low pressure, is extended to high pressure. The experiments were carried out at 1000°–1375°C and 4.5–11 GPa with graphite capsules using 18‐mm octahedral pressure cells in a uniaxial split‐sphere multianvil apparatus. Oxygen fugacities corresponded to wustite stability. Phases present included sulfide liquid (close to M3S2 stoichiometry), FeS (troilite, pyrrhotite, HPP), alloys (PtFe, PtFeIr, OsIr, FePtIrOs, OsFeIr), and wustite. Partition coefficients for OsIr alloy/Fe1−x S at 1200°C, 11 GPa are Os (1400), Ir (160), Pt (32), Ru (0.6), Rh (0.2), and Pd (0.04) and for FePtOs alloy/liquid at 1100°C, 8.5 GPa are Os (120), Ir (82), Pt (20), Ru (5.0), Rh (1.6), and Pd (0.1). Geochemical processes which involve separation of a metal‐rich phase from sulfide therefore would result in fractionation of PGE, with Os, Ir, and Pt concentrated in the former phase and Pd in the latter. These results are further support for the presence of a discrete metal‐sulfide liquid during the primary differentiation of planetary material and preclude an explanation for the fractionation of PGE in chromitites and mafic crustal rocks based on equilibrium partitioning between refractory alloys in residual assemblages and immiscible sulfide liquid in silicate melts.
Submillimeter horizontal position determination using very long baseline interferometry
Authors: Herring T. A.
Published: February 2008

An analysis of interferometric phase delays from 15 years of Mark I and Mark III very long baseline interferometry (VLBI) experiments carried out with two radio telescopes in Westford, Massachusetts, ∼1.24 km apart, yields weighted root‐mean‐square (WRMS) scatters about the mean locally horizontal coordinates of 1.0 and 2.0 mm in the north and east directions, respectively. The vertical coordinate scatter over this same duration is 3.2 mm. The measurements made during the last decade using the more accurate Mark III VLBI system yield WRMS scatters of 0.7, 0.8, and 2.3 mm, for the north, east, and vertical coordinates, respectively. Analysis of this latter data set using the less accurate, but more commonly used, group delay measurements yields WRMS scatters of 1.8, 2.5, and 2.8 mm for the north, east, and height components, respectively. The estimated rates of change of the coordinates of the baselines from the phase delay solution are −0.12 ± 0.04, −0.09 ± 0.07, and −0.26 ± 0.11 mm/yr for the 15‐year analysis, and −0.11 ± 0.05, 0.08 ± 0.06, and −0.15 ± 0.15 mm/yr for the last decade. From these results, the maximum admissible rate of change of this baseline, either from geophysical causes or from telescope deformation, can be bounded to be less than 0.5 mm/yr (95% confidence interval) in all coordinate directions. We conclude from these studies that VLBI antennas of at least of the structural quality of the pair in Westford satisfy a necessary but not sufficient condition for being able to maintain a global reference system with submillimeter per year accuracy for intervals in excess of a decade. These data are also used to determine an error model for the VLBI group delay measurements, and, for this particular pair of telescopes, they indicate that the WRMS difference between group and phase delays is composed of a constant part (5.4 mm, for the most recent data) and a signal‐to‐noise ratio (SNR) term which is ∼10% larger than that computed theoretically.
Paleomagnetic and rock magnetic study of combustion metamorphic rocks in Israel
Authors: Ron Hagai , Kolodny Yehoshua
Published: February 2008

Paleomagnetic and rock magnetic studies were combined with the mineralogy and metamorphic petrology of a sequence of combustion metamorphic (CM) rocks in Israel. Samples from 12 sites from the three major outcrops of the Hatrurim Formation (“Mottled Zone”) were analyzed. These are originally Cretaceous bituminous chalks and marls which have undergone spontaneous combustion during the Late Neogene. The analyses included natural remanent magnetization (NRM) intensity, alternating field (AF) and thermal demagnetization, and isothermal remanent magnetization (IRM) measurements. Mineralogical phases were identified by X‐ray diffraction and electron microprobe. All metamorphosed rocks have a NRM intensity far above normal sedimentary rocks. This increase in intensity is observed even in rocks where no mineralogical indications of metamorphism are observed. The major contributors to the rock magnetism in these rocks are maghemite, hematite and rare magnesioferrite. The magnetic properties were acquired during the burning process as a consequence of oxidation of Fe‐sulfides. The in situ direction of the mean magnetic vector is Dec = 8.2°, Inc = 49.2°, K = 40 and α95 = 5.2 whereas the stratigraphically corrected mean is Dec = 25.1°, Inc = 46.9°, K = 26 and α95 = 6.5. The stratigraphic mean direction is in accordance with the Pliocene expected field direction for this area as derived from stable Africa poles. Therefore combustion must postdate tectonic tilt or vertical axis rotation associated with Late Cretaceous major folding and faulting and rifting associated with the Dead Sea Transform. Normal and reverse polarity and paleosecular variation indicate that combustion at the different sites was not simultaneous.
Simulations of magma withdrawal from compositionally zoned bodies
Authors: Trial Alain F. , Spera Frank J. , Greer John , Yuen David A.
Published: February 2008

Heterogeneities in the form of continuous (less common) or discontinuous (more common) mineralogical, geochemical and thermal gradients within volcanic deposits are ubiquitous in the geologic record. Examples cover a wide spectrum of eruptive volumes (10−2 km3 to > 103 km3), compositions (felsic to mafic), mass discharges (103 to 109 kg/s) and eruptive styles (Plinian air falls, pyroclastic flows, lava flows). An understanding of the dynamics of the magma withdrawal process is essential for accurate reconstruction of pre‐emptive chemical and thermal fields within crustal magma bodies. A model based on numerical solution of the governing conservation equations has been devised in order to investigate the dynamics and systematics of central vent and ring fracture eruptions from flat‐roofed and sloped‐roof magma bodies strongly zoned in density and viscosity. Scale analysis of the conservation equations together with about 150 numerical experiments enables development of quantitative correlations for the mass discharge,
, and the duration of mixed magma eruption, Δt, with the effective driving pressure, σ. Natural eruptions span the entire range of dynamic regimes from viscous through inertial. For central vent eruptions in the viscous regime,
is proportional to σ/(dη) where d is the conduit length and η is the viscosity. In the transitional and inertial regimes, the intensity varies as (σ/ρ)3/5 and (σ/ρ)½, respectively, where ρ is the density. Similarly, Δt depends on (σ/d)−5, σ−3/5 and σ−½ in the viscous, transitional, and inertial regimes, respectively. For systems with layered viscosity structure, Δt is nearly independent of the viscosity ratio. Pressures (above hydrostatic∥ needed to drive eruptions are proportional to
,
5/3, and
2 in the three dynamic regimes. Typical eruptions in the viscous and transitional regimes require pressure anomalies of order 102)103 Pa whereas high intensity (inertial) eruptions demand anomalies of order 106 Pa. Because the lower values are less than or equal to maximum tidally induced stresses, Earth tides appear capable of exciting eruptions. Several scenarios for producing variable discharge during a single eruptive phase were also studied. These include arbitrary changes in the basal pressure and eruptions controlled by density and viscosity gradients. It is demonstrated quantitatively that compositional gaps and episodes of mixed‐magma eruption can originate due to the withdrawal process itself and are not necessarily related to pre‐eruptive gradients within the magma body.
Optical variability and pigment biomass in the Sargasso Sea as determined using deep‐sea optical mooring data
Authors: Smith R. C. , Waters K. J. , Baker K. S.
Published: February 2008

Optical measurements from an untended mooring provide high‐frequency observations of in‐water optical properties and permit the continuous estimation of important biological parameters. A 9‐month time series of optical data, composed of three separate deployments, from the Biowatt 1987 deep‐sea mooring located in the oligotrophic waters of the Sargasso Sea at 32°N, 70°W, is presented. These data have been analyzed using several bio‐optical models for the purpose of providing a continuous estimate of the spectral diffuse attenuation coefficient and pigment biomass. Results from the first 70‐day deployment are emphasized to demonstrate the utility of optical observations as proxy measures of biological parameters, to present preliminary analysis of the temporal variations, and to intercompare our bio‐optical observations with concurrent physical observations. Bio‐optical properties exhibit variation in response to physical forcings, such as diel variations of incident solar irradiance, episodic wind forcing, advective mesoscale eddy events, seasonal variability corresponding to changes in solar radiation, the shoaling of the mixed layer depth, and succession of phytoplankton populations. The mooring data are discussed in the context of contemporaneous shipboard observations as well as in terms of future ocean color satellite observations.
Refraction of sound waves at polar latitude
Authors: Munk Walter
Published: February 2008

Horizontal refraction in the ocean sound channel is a function of the acoustic mode number and frequency (chromatic aberration), and may lead to wide separations of long‐range transmission paths. We consider the 1960 antipodal transmission from Perth, Australia, to Bermuda. The path has a southernmost point in the Indian Ocean, which depends sensitively on horizontal refraction associated with the north‐to‐south shoaling of the sound axis across the Antarctic Circumpolar Current. (Refraction by the current velocity is relatively small.) This southernmost point is at about 40°S for low modes of relatively high frequency, and at about 50°S for high modes of low frequency.