bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic refraction study of upper crustal structure in the Lassen Peak Area, northern California
Authors: Berge Patricia A. , Stauber Doug A.
Published: February 2008

In September 1982, the U.S. Geological Survey recorded a 125‐km‐long refraction profile through the Lassen Peak area of the Cascade Range. This profile extended northwestward to the Klamath Mountains and southeastward to the Sierra Nevada foothills. Interpretation of the refraction data determined the upper‐crustal structure in this volcanic region of northern California. Near‐surface velocities (1.5–5.55 km/s) are consistent with a thickness of 2–4 km for the Cascade volcanic rocks. Between 0.5 km above and 1.8 km below sea level, velocities are about 4.7–5.55 km/s. The highest velocities are modeled below the Devils Kitchen‐Terminal Geyser area, possibly indicating hydrothermal alteration of the volcanic rocks. The absence of low velocities (less than about 3–4.5 km/s) deeper than 1 km above sea level suggests that the Cretaceous sedimentary rocks exposed west of Lassen Peak do not underlie the volcano. Southeast of Lassen Peak, at 1.8 km below sea level, a 0.3‐km‐thick layer with a velocity of 6.1 km/s overlies a lower‐velocity (5.7–5.8 km/s) 2.9‐km‐thick layer. Because these modeled velocities do not change across the boundary between the Sierra Nevada foothills and the Cascade Range, Sierran basement could continue under the Cascade volcanic rocks as far northwest as Lassen Peak. The velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust.
Relation of the spectroscopic reflectance of olivine to mineral chemistry and some remote sensing implications
Authors: King Trude V. V. , Ridley W. Ian
Published: February 2008

Using high‐resolution visible and near‐infrared diffuse spectral reflectance, we have systematically investigated apparent wavelength shifts as a function of mineral chemistry in the Fe/Mg olivine series from Fo11 to Fo91. The study also shows that trace amounts of nickel can be spectrally detected in the olivine structure. We show that significant compositional information can only be extracted at relatively high resolution, because the overall spectral characteristics of the olivines change only subtly as a function of the Fe/Mg ratio. Significant spectral variation as a function of grain size is also demonstrated, adding a further complication to the interpretation of remotely sensed data from olivine‐rich surfaces. This laboratory study is expected to aid in the interpretation of remotely sensed data from both terrestrial and extraterrestrial bodies. Terrestrial applications may include the recognition of ultramafic, ultrabasic, and basaltic terrains which in themselves may have mineral potential. Among extraterrestrial applications, the asteroids are obvious candidates for further examination if instrumentation can provide the necessary wavelength coverage, resolution, and signal‐to‐noise ratio so that spectra can be compared to those laboratory data discussed here. Some permutations of Fe‐Mg‐Ni relations in olivines are discussed as they apply to the interpretation of asteroid surfaces and other extraterrestrial bodies.
Short‐period variations in the length of day: Atmospheric angular momentum and tidal components
Authors: Luo Shifang , Zheng Dawei , Robertson D. S. , Carter W. E.
Published: February 2008

Using very long baseline interferometry (VLBI) observations of extragalactic radio sources, it is now possible to determine the rotational phase angle of the Earth (UT1) with an accuracy of 0.1 ms in an observing period of about 1 hour. We analyzed a 3‐month series of daily length‐of‐day (LOD) values produced from VLBI observations collected under project International Radio Interferometric Surveying (IRIS) during April–July 1984. Comparison of the IRIS LOD values with variations in the atmospheric angular momentum (AAM) during the same time period revealed quasi‐periodic components in both series at about 9, 33, and 58 days, with closely matching phases. We removed the AAM components from the IRIS LOD values, filtered the resulting series to remove variations with periods longer than 35 days, and analyzed the resulting time series to estimate the scaling parameter k/C from the 9.12, 13.66, and 27.56‐day tidal components. The results are consistent with similar results from other methods based on much longer time series.
A review of Australian experience in the evaluation of helium as a pathfinder in exploration
Authors: Butt C. R. M. , Gole M. J. , Hart M. K. W.
Published: February 2008

Orientation surveys were conducted to test the potential of using 4He as a pathfinder in mineral and hydrocarbon exploration. Groundwaters were sampled from drill holes and stock wells. Overburden gas samples were collected via fixed tubes emplaced at 6 m depth in backfilled drill holes. While He concentrations in groundwaters associated with most U mineralization were far in excess of the atmospheric equilibrium value of 0.044 μL/L, equivalent, or higher, concentrations were found in groundwaters from unmineralized areas around the deposits. These latter concentrations appear to be due to accumulation of He in groundwaters with long residence times and by leakage from deep sources, particularly granitoid basement. Only at the Koongarra U deposit, where groundwaters are annually recharged, did anomalous He concentrations appear to indicate the presence of the deposit. No credible He anomaly was detected in overburden gas over any U deposit. However, a marked overburden gas He anomaly was detected over the Gingin gas field, although normalization to constant 20Ne suggested much of this to be a relative enrichment due to near‐surface biological activity related to the leakage of hydrocarbons altering the major gas composition, particularly the CO2/O2 ratio of the soil atmosphere. It is concluded that He, when used in conjunction with other gases, may be useful in near‐surface geochemical exploration for hydrocarbons but that it does not appear to be an effective pathfinder for U deposits.
Frequency dependent polarization analysis of high‐frequency seismograms
Authors: Park Jeffrey , Vernon Frank L. , Lindberg Craig R.
Published: February 2008

We present a multitaper algorithm to estimate the polarization of particle motion as a function of frequency from three‐component seismic data. This algorithm is based on a singular value decomposition of a matrix of eigenspectra at a given frequency. The right complex eigenvector
corresonding to the largest singular value of the matrix has the same direction as the dominant polarization of seismic motion at that frequency. The elements of the polarization vector
specify the relative amplitudes and phases of motion measured along the recorded components within a chosen frequency band. The width of this frequency band is determined by the time‐bandwidth product of the prolate spheroidal tapers used in the analysis. We manipulate the components of
to determine the apparent azimuth and angle of incidence of seismic motion as a function of frequency. The orthogonality of the eigentapers allows one to calculate easily uncertainties in the estimated azimuth and angle of incidence. We apply this algorithm to data from the Anza Seismic Telemetered Array in the frequency band 0 ≤ ƒ ≤ 30 Hz. The polarization is not always a smooth function of frequency and can exhibit sharp jumps, suggesting the existence of scattered modes within the crustal waveguide and/or receiver site resonances.
Strategies for high‐precision Global Positioning System orbit determination
Authors: Lichten Stephen M. , Border James S.
Published: February 2008

High‐precision orbit determination of Global Positioning System (GPS) satellites is a key requirement for GPS‐based precise geodetic measurements and precise low‐Earth orbiter tracking. We explore different strategies for orbit determination with data from 1985 GPS field experiments. The most successful strategy uses multiday arcs for orbit determination and incorporates fine tuning of spacecraft solar pressure coefficients and stochastic station zenith tropospheric delays using the GPS data. Average rms orbit repeatabilities for five of the GPS satellites are 1.0, 1.2, and 1.7 m in altitude, cross‐track, and downtrack components, when two independent 5‐day fits are compared. Orbits predicted up to 24 hours outside a 7‐day arc show average rms component differences of 1.5–2.5 m when compared to independent solutions obtained with a separate, nonoverlapping 5‐day arc. For a 246‐km baseline, with 6‐day arc carrier phase solutions for GPS orbits, baseline repeatability is 2 parts in 108(0.4–0.6 cm) for east, north, and length components and 8 parts in 108for the vertical component. For a 1314‐km baseline with the same orbits, baseline repeatability is about 2 parts in 108for the north component (2.5 cm) and 4 parts in 108or better for east, length, and vertical components. When GPS carrier phase is combined with pseudorange, the 1314‐km baseline repeatability improves further to 5 parts in 109for the north (0.6 cm) and 2 parts in 108for the other components (2–3 cm).
Crystallization and vesiculation of the 1984 eruption of Mauna Loa
Authors: Russell James K.
Published: February 2008

Calculations based on existing thermodynamic models for silicate melts are used to model the physical and chemical evolution of the 1984 Mauna Loa magma. Specifically, the calculations simulate the vesiculation‐induced crystallization of the ascending magma. The calculations provide a physiochemical model to explain the increase in crystallization under isothermal conditions. The evolution of the Mauna Loa magma is modeled in three separate stages corresponding to (1) the subsurface crystallization of the phenocryst assemblage, (2) the crystallization and vesiculation of the magma during its ascent from depth, and (3) the crystallization and vesiculation of the lavas at the surface. The preeruptive crystallization of olivine and orthopyroxene phenocrysts began at ≃ 1155°C and 0.2 GPa. Olivine and plagioclase alone crystallized during the ascent of the magma. The bulk of the crystallization and vesiculation of the Mauna Loa magma occurred at pressures less than 20 MPa. Under lithostatic pressures this corresponds to depths as shallow as 600–700 m. Ancillary calculations have established the net heat effect of the prescribed ascent path and demonstrate that the heat associated with vesiculation easily compensates for the heats of crystallization. The calculations suggest that the near‐surface processes can be quite endothermic depending on the initial H2O content of the magma. For a magma with 1 wt % dissolved H2O the heat balance demands approximately 5–10°C cooling. Further calculations predict the liquid line of descent for this ascent path and document the corresponding variations in the physical properties of the melt phase.
Magmatic crystal stratigraphy and constraints on magma chamber dynamics: Laser interference results on individual phenocrysts
Authors: Pearce T. H. , Griffin M. P. , Kolisnik A. M.
Published: February 2008

This work reports progress in current studies of zoning of magmatic crystals from basalts and andesites. A simple law of normal growth is tested using zoning profiles of olivine and plagioclase crystals and is shown to be compatible with the zoning profiles of some olivines and parts of complexly zoned plagioclases. Using a convenient linearizing technique (in which the ratio of compositional end‐members is plotted against volume), we can identify those parts of the zoning profile which fit a simple normal growth law. This technique permits easy identification of those parts of a complexly zoned crystal which require a more complex law of growth. Calculations used in this modeling are similar to those previously used for olivine (Pearce, 1984c, 1987) and are now extended to plagioclase. It is concluded that it is both theoretically possible and practically feasible to make inferences about the liquid in the local system from which a crystal grew by examining the detailed zoning pattern of the crystal.
Depth distribution of local stress inhomogeneities in the Vrancea Region, Romania
Authors: Trifu Cezar‐Ioan
Published: February 2008

Corrected displacement amplitude spectra of weak and moderate (ML = 2.5–5.0) Vrancea earthquakes (h = 0–201 km) exhibit an obvious shape with two comer frequencies and two slopes. This behavior is related to local inhomogeneities within the seismic zone, and a heterogeneity parameter, ε is defined as the fraction of the complete stress drop released during earthquake (barrier model) or the ratio of the asperity to source radii (asperity model). The parameter ε is evaluated as the ratio of the first corner frequency versus the second one and it ranges from 0.50 to 0.15. The heterogeneity appears to increase with increasing magnitude (ε decreases), suggesting that the earthquakes represent the failure of asperities. This behavior is not evident for the events in the 30–50 km depth interval. A mostly crack origin of earthquakes in this zone is suggested. This region is found to be more homogeneous and able to decouple the seismic activities in the crust and sinking slab. The depth distribution of local inhomogeneities within the region is heterogeneous. The crust and two intermediate depth intervals, 90–110 km and 120–160 km, are the most heterogeneous and have occurrence potential for moderate earthquakes. The two intermediate zones show the highest activities, which are slightly decoupled due to a more homogeneous layer (110–120 km).
Bilateral topographic symmetry patterns across Aphrodite Terra, Venus
Authors: Crumpler L. S. , Head James W.
Published: February 2008

Western Aphrodite Terra, Venus, is characterized by a series of parallel linear structural discontinuities 2000–4000 km in length and 100–200 km wide, which strike at high angles to the general topographic trend of the Aphrodite Terra highlands. The broad characteristics of the cross‐strike discontinuities (CSDs) are similar to both strike‐slip fault zones and terrestrial oceanic fracture zones. In an effort to distinguish between these two hypotheses, topographic profiles were taken across Aphrodite Terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on Earth. In addition to a broad bilateral symmetry at a range of angles across Aphrodite Terra, detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities. In addition, within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding CSDs and terminates against them. Furthermore, linear rise crests are offset in a right‐lateral sense along the CSDs between domains by 200–1200 km. If the discontinuities represent strike‐slip movement, then retrodeformation of the axes of symmetry along the CSDs to form a straight line should form a broadly symmetrical topographic high. Instead, when the topography is retrodeformed, the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the CSDs. The nature and configuration of the CSDs, the characteristics of the topography and topographic patterns, including bilateral topographic symmetry generally parallel to CSDs within domains, the existence of linear rise crests defined by the axes of bilateral symmetry within domains, and the offsets of these linear rise crests between domains, all suggest that western Aphrodite Terra on Venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross‐strike discontinuities are analogous to oceanic fracture zones rather than strike‐slip faults.
Helium 4 and radon 222 concentrations in groundwater and soil gas as indicators of zones of fracture concentration in unexposed rock
Authors: Banwell G. M. , Parizek R. R.
Published: February 2008

The presence of zones of fracture concentration in unexposed rock may be partially assessed by measuring the concentrations of 4He and 222Rn in groundwater and soil gas. Helium 4 in groundwater and soil gas and 222Rn in groundwater were measured in the vicinity of two intersecting lineaments in Lehigh County, Pennsylvania. The lineaments are thought to mark deep fracture systems which provide conduits for the migration of 4He and 222Rn. High groundwater 4He concentrations revealed the presence of the two lineaments. Low groundwater 222Rn activities are found near the lineaments and qualitatively support a numerical model which relates low 222Rn activity to high fracture transmissivity. Radon 222 activity correlates most strongly with the presence of H2S in groundwater, implying that geochemical controls or groundwater circulation patterns strongly influence radon concentration. Soil gas concentrations of 4He barely exceeded background levels and did not reliably indicate the lineament zones. The data show that groundwater 4He concentrations may be used to identify fracture zones in unexposed bedrock.
Shock compression of molten silicate: Results for a model basaltic composition
Authors: Rigden S. M. , Ahrens Thomas J. , Stolper E. M.
Published: February 2008

A technique has been developed for measurement of the shock wave, pressure‐density equation of state of molten silicates initially at temperatures of up to ∼2000 K. A 40‐mm propellant gun apparatus accelerates metal flyer plates to speeds of up to 2.5 km s−1; these flyer plates are capable of driving shock waves with amplitudes of 35–40 GPa (350–400 kbar) into molten silicate samples. Modifications to the standard equation of state experiments that are described here include design of a molybdenum sample container for the molten silicate; use of a 10‐kW radio frequency induction heater to melt the sample prior to impact; implementation of shuttering systems to protect the optical system and prevent preexposure of the film in the rotating‐mirror, continuously writing, streak camera; and reduction of Hugoniot data taking into account the effect of the sample capsule. Data for a model basaltic liquid (36 mol % anorthite, 64 mol % diopside) at an initial temperature of 1673 K and initial density of 2.61 Mg m−3, yield a shock velocity‐particle velocity (US‐UP) relation given by US = 3.06 + 1.36 UP km s−1 up to values of UP = 1.7 km s−1. The zero‐pressure, bulk sound speed is in good agreement with ultrasonic measurements. The best fit Birch‐Murnaghan equation of state for this model basaltic liquid is K0S = 24.2 GPa and K′ = 4.85 based on Hugoniot points at low pressures (<25 GPa). Within the resolution of our data set, density increases smoothly with pressure over the 0–25 GPa pressure range, suggesting that structural rearrangements take place gradually in response to pressure in this pressure interval. At high pressures (≳ 25 GPa) the Hugoniot data suggest that the liquid stiffens considerably. This may indicate that the gradual structural changes characteristic of the lower‐pressure regime, such as changes of Al3+ and Si4+ coordination by oxygen from fourfold to sixfold, are essentially complete by ∼25 GPa. These high‐pressure Hugoniot data are fit by US = 0.85 + 2.63 Up km s−1. The high‐pressure regime is similar to that obtained in initially solid silicates upon shock compression. Shock temperature calculations yield values of 2400–2600 K at 25 GPa, and the states achieved are believed to lie metastably in the liquid field.
Elasticity of enstatite and its relationship to crystal structure
Authors: Duffy Thomas S. , Vaughan Michael T.
Published: February 2008

The nine adiabatic single‐crystal elastic moduli of a natural orthopyroxene of enstatite composition (Mg0.94Fe0.06SiO3) have been measured at ambient conditions by Brillouin spectroscopy. The single‐crystal elastic stiffnesses in gigapascals (GPa) are C11 = 229.3 ± 1.1; C22 = 167.0 ± 1.1; C33 = 193.9 ± 1.5; C44 = 79.7 ± 0.7; C55 = 76.1 ± 0.6; C66 = 77.1 ± 0.9; C12 = 73.6 ± 1.7; C13 = 49.8 ± 1.6; and C23 = 46.6 ± 1.6. The average Hashin‐Shtrikman bulk and shear moduli are 102.2 and 73.9 GPa, respectively. The results of this study are generally consistent with those obtained for other Mg‐rich pyroxenes. Some of the individual moduli along the enstatite‐ferrosilite join can be related to specific chemical and structural features. The most variable modulus is C33, and it appears to be a complicated function of both chemistry and structure. Its magnitude relative to C11 is most easily explained by the presence or absence of more than one tetrahedral chain. For orthopyroxenes of intermediate composition C11 is higher than expected, probably due to a combination of the effects of site ordering and impurities in the Ml site. The third longitudinal modulus, C22, is observed to increase linearly as a function of enstatite composition from En0 to En100. An alternative interpretation for C11 and C22 is that between En100 and En50, C11 is constant and C22 decreases linearly with increasing iron content, while between En50 and En0, C11 decreases linearly and C22 is constant. The shear and off‐diagonal moduli vary nonsystematically but tend to vary in similar ways. With the exception of C12, the averaged moduli also show nonsystematic variations. Cation site ordering appears to strongly affect C44. The modulus C11 is also nonsystematic but for reasons which are not obvious. Our results are consistent with the interpretation that the bulk modulus does not vary with composition whereas the shear modulus does so in a linear fashion.
The Sou Hills: A barrier to faulting in the central Nevada Seismic Belt
Authors: Fonseca Julia
Published: February 2008

Bedrock zones transverse to regional structures may indicate barriers at depth that inhibit the propagation of fault ruptures in the Basin and Range Province. The Tertiary bedrock of the Sou Hills separates Dixie and Pleasant valleys and is transverse to the trends of physiography and historic surface faulting in central Nevada. Four lines of evidence indicate that the Sou Hills are a barrier to faulting in the seismic belt. First, total late Cenozoic vertical displacement on range‐bounding faults decreases toward the Sou Hills. Second, analyses of landforms that reflect rates of relative uplift show that Quaternary uplift decreases where range‐bounding faults meet the Sou Hills. Third, the most recent prehistoric faulting south of the transverse zone is several thousand years younger than faulting to the north. Fourth, patterns of late Quaternary fault scarps in the Sou Hills are similar to rupture patterns observed at the termination of faults elsewhere.
Sequential geomagnetic reversals recorded in Upper Tortonian marine clays in western Crete (Greece)
Authors: Valet Jean‐Pierre , Laj Carlo , Langereis Cornelis G.
Published: February 2008

We report a very detailed study of four different geomagnetic reversals of upper Tortonian age, recorded in three marine sections in western Crete. Multiple records have been obtained for two of the reversals, and both the directional and the relative field intensity behavior have been studied. The intensity decreases to about 15% of its pretransitional value in all cases. For the three oldest reversals the sequences of changing field directions of reversals of opposite sense are almost antipodal, suggesting that the process of reversal from either starting polarity stays unchanged for about 1.3 m.y. A change then occurs for the fourth reversal. The occurence of perturbations in the sequence of changing field directions in two records either at the onset or at the end of the reversal are interpreted in terms of a recently suggested mechanism for triggering reversals.
Wetting of enstatite by basaltic melt at 1350°C and 1.0‐ to 2.5‐Gpa Pressure
Authors: von Bargen Nikolaus , Waff Harve S.
Published: February 2008

The results of partial melting experiments of olivine, enstatite, and basalt PG‐16A at 1350°C and pressures from 1.0 to 2.5 GPa indicate that the melt wets enstatite with a dihedral angle of 20°–40° in the presence of small amounts of H2O. This angle is significantly less than the critical dihedral angle of 60°, and thus the melt can distribute itself in an interconnected network of prismatic channels along the grain edges, where enstatite is the dominant solid phase. The dihedral angles from olivine‐enstatite‐melt triple junctions are such that an interconnected network of melt channels is also possible where enstatite and olivine occur in similar proportions. Together with the results of previous partial melting experiments in the olivine‐basalt systems, the results of this study suggest that in the Earth's upper mantle the melt will occur largely in networks of interconnected channels along the intergranular edges, where morphologies are controlled by thermodynamic equilibrium. We conclude that the presence of enstatite will have no significant effect on the permeability of partially molten zones in the mantle, if small amounts of H2O are present.
Frequency distribution and quantification of deep earthquakes
Authors: Giardini Domenico
Published: February 2008

The frequency distribution of deep earthquakes (d > 350 km) is analyzed on a regional scale, using magnitude data from ∼4660 events reported in the International Seismological Center Bulletin (1964–1982) and 391 centroid moment tensor solutions (1977–1987). We use the moment magnitude Mw to define b values comparable with studies of shallow seismicity; scaling laws relating Mw, mb, and Mo are derived (Mw = 1.6mb − 3.2; mb = [log Mo − 4.3]/2.4) which compare very favorably with published laws. Uncertainties and biases of the mb and Mo data sets are considered. The slope of the frequency distribution is evaluated over a restricted magnitude range; the results are very robust and independent from the magnitude range used. From the mb magnitude data we obtain regional b values in the range 0.4 < b < 1.22, with a worldwide average value b = 0.87; these results are confirmed by the analysis of the seismic moment data. The steepest frequency distribution is observed in the Tonga Islands region (b = 1.22); although ∼60% of the worldwide deep seismicity occurs here, only few events since 1977 had seismic moment exceeding 1019 N m. The high fractal dimension (D = 2.4) implies a volume partially filled with fault planes, in agreement with the observations on the seismic morphology of the region. At the other extreme. South America is characterized by the largest recorded deep events and an almost complete absence of small ones (b = 0.4).
Aperture correlation of a fractal fracture
Authors: Wang J. S. Y. , Narasimhan T. N. , Scholz C. H.
Published: February 2008

A rough‐walled fracture is modeled by fractal geometry. In the fractal fracture model, the rock surfaces are characterized by a fractal dimension D between 2 and 3, with lower D for smoother surfaces and higher D for rougher surfaces. The mismatch due to shear displacement between two mirror‐image fractal surfaces determines the fracture aperture distribution. An analytic equation is derived for the variogram γƒ(r) describing the spatial correlation of the aperture of a fractal fracture. The aperture of a smooth fracture with low D is highly correlated over distances much larger than the shear displacement. The aperture of a rough fracture with high D becomes uncorrelated within a range shorter than the shear displacement. Near the origin, r = 0, of the variogram, the variogram is proportional to r6–2D. For the special case of Brownian fractal with D = 2.5, the variogram is proportional to r, which is the same r dependence exhibited by the spherical model widely used in geostatistical analyses.
The Mesozoic tectonic history of the Magellan Microplate in the western central Pacific
Authors: Tamaki Kensaku , Larson Roger L.
Published: February 2008

Magnetic lineation mapping in the western central Pacific has revealed a pair of opposite‐sensed, fanned lineation patterns that define the accretionary boundaries of the fossil Magellan microplate. This tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations. The entire evolutionary history of the Magellan microplate is well constrained to a 9‐m.y. period in the Early Cretaceous by synchronous spreading patterns and associated geologic data. During this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km×600 km with spreading centers on two opposing sides and transform faults on the other two sides. The lifetime and size of the Magellan microplate are somewhat longer and larger, respectively, than presently active microplates on the East Pacific Rise. However, these modern structures are still evolving and growing, and the tectonic behavior of the modern and Cretaceous systems appears to be similar. Study of both active and fossilized microplates should provide additional insights on their common tectonic histories. In particular, we show that the Magellan Trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history. The Magellan Trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the Pacific, Farallon, and Phoenix plates at Ml0N time. Microplate activity ceased when the microplate became welded to the Pacific plate at M9 time.
The mathematician paleoplate
Authors: Mammerickx J. , Naar D. F. , Tyce R. L.
Published: February 2008

The Pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations. One of the clearest examples of such a reorganization is documented in the Mathematician Seamounts region, where a distinctive geomorphology and well‐developed magnetic anomalies are present. This reorganization involved a short‐lived microplate between the failing Mathematician Ridge and a new propagating spreading center: the East Pacific Rise. It produced a transfer of a fragment of lithosphere from the Farallon to the Pacific plate, and also created a number of landforms and magnetic patterns, within and on the margins of the captured fragment; these make up the Mathematician paleoplate. In many cases, two sides of a microplate are active spreading ridges. A microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge. We define a paleoplate as the area of the seafloor, from the axis of a failed rift to the boundary of resumed, full spreading. It includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading, prior to complete abandonment of the failed rift. The Mathematician paleoplate has the following boundaries and components from west to east: the axis of the Mathematician failed rift, the fragment of captured Farallon plate, a complex rift initiation site at the Moctezuma Trough, a zone of slow spreading, and an as yet ill‐defined eastern boundary where dual spreading stopped and full spreading resumed. The northern boundary of the paleoplate is the Rivera fracture zone; its southeastern boundary a now‐inactive transform fault, the West O'Gorman fracture zone. In this case, as well as in other more poorly documented ones, relict landforms and magnetic patterns are carried on the aging lithosphere, away from the spreading ridge, recording a former geometry.
Anomalous shear wave attenuation in the shallow crust beneath the Coso Volcanic Region, California
Authors: Sanders Chris , Ho‐Liu Phyllis , Rinn David , Kanamori Hiroo
Published: February 2008

We use seismograms of local earthquakes to image relative shear wave attenuation structure in the shallow crust beneath the region containing the Coso volcanic‐geothermal area of eastern California. SV and P wave amplitudes were measured from vertical component seismograms of earthquakes that occurred in the Cososouthem Sierra Nevada region from July 1983 to August 1985. Seismograms of 16 small earthquakes show SV amplitudes which are greatly diminished at some azimuths and takeoff angles, indicating strong lateral variations in S wave attenuation in the area. Three‐dimensional images of the relative S wave attenuation structure are obtained from forward modeling and a back projection inversion of the amplitude data. The results indicate regions within a 20 by 30 by 10 km volume of the shallow crust (one shallower than 5 km) that severely attenuate SV waves passing through them. These anomalies lie beneath the Indian Wells Valley, 30 km south of the Coso volcanic field, and are coincident with the epicentral locations of recent earthquake swarms. No anomalous attenuation is seen beneath the Coso volcanic field above about 5 km depth. Geologic relations and the coincidence of anomalously slow P wave velocities suggest that the attenuation anomalies may be related to magmatism along the eastern Sierra front.
Remanence rotation due to rock strain during folding and the stepwise application of the fold test
Authors: Kodama Kenneth P.
Published: February 2008

Numerical modeling was conducted to determine the effects of simple models of folding strain on prefolding Fisherian distributions of magnetization. I found that the constant volume, simple shear associated with flexural slip/flexural flow folding can rotate a prefolding magnetization to have a maximum precision parameter K value at partial unfolding and thus appear to be synfolding in age. If the magnetization is treated as a passive line marker, this behavior only occurs for steep initial magnetizations. The initial homogeneity of the Fisherian precision parameters from each limb is degraded by this type of deformation and may be used to detect a strain modified remanence. If the magnetization is carried by rigid, mechanically active ellipsoidal or spherical particles, a maximum K value at partial unfolding will occur for any initial inclination, with the special case that rigid spheres will be rotated to cause a maximum K value at 50% unfolding for any initial inclination. Homogeneity of limb precisions is not affected in this case. A discontinuous model of strain during folding, in which pressure solution removes material at an axial‐planar, spaced cleavage and causes limb dips steeper than bedding dips, could also produce a maximum K value at partial unfolding. Strain and remanence studies of flexural slip folding in the Allentown Formation dolomite and the Bloomsburg Formation red sandstones and mudstones suggest that (1) the rigid grain model may be a more realistic model of magnetic particle behavior during shear strain and (2) about one quarter to one third of the theoretically predicted, simple shear strain occurs in these rocks. This amount of strain cannot explain the synfolding behavior observed in the Allentown Formation but could explain the maximum K values at 80%–90% unfolding seen in the Bloomsburg Formation and, perhaps, other Appalachian clastic rocks.
Magnetic domains on unpolished and polished surfaces of titanium‐rich titanomagnetite
Authors: Moskowitz Bruce M. , Halgedahl Susan L. , Lawson Charles A.
Published: February 2008

Magnetic domains on unpolished and polished surfaces of synthetic aluminium‐magnesium titanium‐rich titanomagnetite (AMTM60) have been investigated with the dried Bitter method and scanning electron microscopy (SEM). The high magnification and superior resolution of the SEM afforded easy measurements of characteristic domain features, including domain wall widths. The large depth of field of the SEM also allowed domain patterns on unpolished sample surfaces to be studied. It was found that analogous domain structures were observed on unpolished, unpolished but acid‐etched, and ionically polished surfaces. However, even on unpolished surfaces, the domain style in AMTM60 was different from that expected for this material based on its cubic magnetocrystalline anisotropy. Features commonly observed on unpolished surfaces included (1) closely spaced stripe domains, (2) sinusoidally wavy walls, (3) rows of circular spike domains, and (4) complicated wavy wall patterns consisting of star like features enclosed by undulating and branching walls. All these features corresponded to domain structures found in uniaxial materials whose anisotropy Ku and saturation magnetization Ms satisfy the condition 2πMs2/Ku<1. The source of the uniaxial anisotropy in AMTM60 was most likely due to residual stress. Quantitative estimates of the exchange and anisotropy constants in AMTM60 were obtained from analysis of domain patterns using a theoretical model for sinusiodally wavy walls. Furthermore, as predicted by theory, wall widths increased and anisotropy decreased as the geometric complexity of the domain pattern increased from simple stripe domains to complicated wavy wall domains in response to a decrease in local residual stresses. Finally, it was observed that there was an important difference between domain style on polished and unpolished surfaces. On ionically polished surfaces only, many regions exhibited a “patchy” appearance, in which the domain style changed abruptly or in which domain walls disappeared completely at the edge of the patch and suggested that stresses produced by mechanical polishing may not have been removed completely or uniformly by ionic polishing. Consequently, it is uncertain how much of the intrinsic domain structure in titanium‐rich titanomagnetites survives mechanical polishing.
Rheology of subliquidus magmas: 1. Picritic compositions
Authors: Ryerson F. J. , Weed H. C. , Piwinskii A. J.
Published: February 2008

The relationship between the rheology and phase equilibria of a picritic basalt from Kilauea Iki has been investigated at 1 atm along the QFM buffer. Between 1270° and 1180°C olivine and minor spinel are the only liquidus phases, and the melt volume decreases from 85 to 74 vol %. At the ol‐sp‐pc‐cpx cotectic, melt is consumed more rapidly dropping to 47 vol % at 1139°C. The rheology of the magma is non‐Newtonian, is characterized by time‐dependent, pseudoplastic behavior, and is consistent with power law flow or Bingham pseudoplastic behavior. Non‐Newtonian effects are most apparent when the crystal volume is greater than 25% and the shear rate is increasing. Once stirred, the magma approaches Newtonian rheology with decreasing shear rate. At 1170°C (approximately 25% suspended crystals) the apparent activation energy (calculated at unit shear rate) increases from 123 ± 10 kcal mol−1 to 452 ± 21 kcal mol−1. When the change in liquid composition along the liquidus is included, the observed viscosities are in good agreement with those calculated from the Einstein‐Roscoe equation for a serial size distribution of crystals. A quadratic fit to the shear stress‐strain rate data yields a nonzero intercept indicative of a finite yield strength. The apparent yield strength increases with decreasing temperature reaching ≈800 Pa at 1149°C. These extrapolated (and model dependent) values are in good agreement with those determined by other methods (Shaw et al., 1968; McBirney and Murase, 1984). The yield strengths can be fit by a power law expression, σ0 = 6500Φ2.85, where σ0 is the yield strength and Φ the crystal volume fraction. This expression recovers the yield strength estimates for other basalts, suggesting that it may be generally applicable to magmas in which the melt is of broadly basaltic composition. The magmas exhibit heating due to viscous dissipation during shearing. However, this effect is not large enough to produce the observed pseudoplasticity. We conclude that the non‐Newtonian behavior in these magmas is due to reorientation of solids in the flow field and solid‐solid interactions. We develop a model in which the time‐dependent pseudoplasticity and yield strengths are produced by the relationship between solid phase contiguity (the fraction of solid internal surface area snared with other solids) and grain dispersive pressure due to shearing. Increased contiguity is related to increased apparent viscosities and yield strengths, while increased grain dispersive pressure due to shearing acts to diminish contiguity.
Fracture fillings and intrusive pyroclasts, Inyo Domes, California
Authors: Heiken Grant , Wohletz Kenneth , Eichelberger John
Published: February 2008

Fractures containing juvenile magmatic pyroclasts were encountered during drilling into a 600‐year‐old feeder dike beneath the Inyo Domes chain, California. The Inyo Domes consist of a north‐south trending, 10‐km‐long chain of domes, rhyolitic tuff rings, and phreatic craters. Boreholes were cored through the 51‐m‐diameter conduit of Obsidian Dome, the largest of the Inyo Domes, and through an unvented portion of the intrusion (dike) 1 km to the south. Pyroclast‐bearing fractures were intersected in both holes: (1) 7‐ to 40‐cm‐thick fractures in welded basaltic scoria and quartz monzonite country rock are adjacent to the conduit at depths of 400–411 m and 492–533 m; they contain gray, clastic deposits, which show truncated cross bedding and convolute bedding; (2) adjacent to the dike, massive fracture fillings occur at depths of 289–302 m (129 m east of the dike) and 366–384 m (95–87 m east of the dike). The fracture fillings consist of mineral clasts derived from the quartz monzonite, quartz monozonitic and basaltic lithic clasts, and juvenile glass pyroclasts. Angular mineral components are present in the same ratio as in the surrounding quartz monzonite country rock. Juvenile glassy and hyalocrystalline pyroclasts make up from less than 1% up to 22% of the deposits. They consist of blocky obsidian clasts, equant, blocky glass pyroclasts with vesicularities of 0–30%, and small pumices with vesicularities of 30–40%. Intrusive pyroclasts differ from erupted pyroclasts in their generally lower vesicularity, higher crystallinity, and the presence of solution pits and clay coatings indicative of prolonged contact with water. The presence, orientation, and texture of fracture fillings strongly resemble those of propped, man‐made hydrofractures. We interpret these fractures as naturally occurring hydrofractures. The apparently horizontal fracture orientations may have been controlled by perturbations of maximum principal stress by the dikes or by preexisting sheet fractures in the quartz monzonite country rock. Assumption of elastic moduli and fracturing properties for the Sierran basement rock allows calculation of fluid overpressures 5 to 9 MPa in excess of overburden stress. These overpressures are consistent with either vapor exsolution from decompressed magma or rapid heating of groundwater. However, the textural and chemical similarity of the pyroclasts to phreatomagmatic tephra that appears late in the explosive eruption sequence suggests that heating of groundwater by the dike/conduit caused the fracturing. Such fracturing around volcanic conduits may play an important role in the development of hydrothermal circulation.
A magmatic model of Medicine Lake Volcano, California
Authors: Donnelly‐Nolan Julie M.
Published: February 2008

Medicine Lake volcano is a Pleistocene and Holocene shield volcano of the southern Cascade Range. It is located behind the main Cascade arc in an extensional tectonic setting where high‐alumina basalt is the most commonly erupted lava. This basalt is parental to the higher‐silica calc‐alkaline and tholeiitic lavas that make up the bulk of the shield. The presence of late Holocene, chemically identical rhyolites on opposite sides of the volcano led to hypotheses of a large shallow silicic magma chamber and of a small, deep chamber that fed rhyolites to the surface via cone sheets. Subsequent geophysical work has been unable to identify a large silicic magma body, and instead a small one has apparently been recognized. Some geologic data support the geophysical results. Tectonic control of vent alignments and the dominance of mafic eruptions both in number of events and volume throughout the history of the volcano indicate that no large silicic magma reservoir exists. Instead, a model is proposed that includes numerous dikes, sills, and small magma bodies, most of which are too small to be recognized by present geophysical methods.
Common characteristics of paired volcanoes in northern Central America
Authors: Halsor Sid P. , Rose William I.
Published: February 2008

Four pairs of active volcanoes along the northern Central American volcanic front have erupted basalt‐andesite magmas that show consistent intrapair behavioral and compositional differences. These differences are found in records of volcanic activity and complete major and minor element data on over 200 samples. From northwest to southeast along the volcanic front the four volcano pairs are Cerro Quemado‐Santa María, Tolimán‐Atitlán, Acatenango‐Fuego, and Santa Ana‐Izalco. The volcano pair relations help explain compositional differences, apart from those reflecting variation in crustal thickness of about 15 km along the volcanic front, providing insight into across‐arc variations and closely spaced subvolcanic plumbing systems. Intravolcano pair spacing is less than 5 km compared with an average intervolcano spacing of 25 km along the entire volcanic front. Within each volcano pair, the seaward volcano has had more frequent historic activity, erupting magmas that are generally more mafic, lower in large ion lithophile elements and higher in Na2O/K2O than magmas erupted from its landward counterpart. Each paired volcano site lies in close proximity to a rhyolitic caldera, situated north or northeast of the volcano pair. However, rare earth element data at the Tolimán‐Atitlán volcano pair imply that mixing between caldera rhyolite and the mafic magma of the paired volcanoes does not occur. Petrographie, isotopic, and other geochemical data from the Tolimán‐Atitlán volcano pair suggest that separate but contemporaneous magma bodies beneath each volcano evolve and pass through the crust at different rates. Atitlán magmas are processed through the crust more efficiently and with greater frequency than Tollman magmas, which undergo longer periods of stagnation interrupted by mafic injection and rapid eruption. This relation appears to hold at the other paired volcano sites and is further evidence that closely spaced volcanoes, with similar subcrustal magma sources, evolve over separate magmatic plumbing systems that traverse the crust. The pairing pattern probably reflects the regional southward migration of the volcanic front.