bluuebunny/crossref_metadata_2025 huggingface dataset:
Ice and debris in the Fretted Terrain, Mars
Authors: Lucchitta Baerbel K.
Published: February 2008

Viking moderate‐ and high‐resolution images along the northern highland margin were studied monoscopically and stereoscopically to contribute to an understanding of the development of fretted terrain. Results suport the hypothesis that the fretting process involved flow facilitated by interstitial ice. The process apparently continued for a long period of time, and debris‐apron formation shaped the fretted terrain in the past as well as the present. Interstitial ice in debris aprons is most likely derived from ground ice obtained by sapping or scarp collapse. Debris aprons could have been removed by sublimation if they consisted mostly of ice, or by deflation if they consisted mostly of debris. To remove the debris, wind erosion was either very intense early in martian history, or was intermittent, perhaps owing to climatic cycles. The location of debris aprons in two midlatitudinal belts can be attributed to atmospheric conditions that permit ice to remain in the ground at moderate depth in those latitudes, and climatic factors that permit temperatures to be warm enough for ice to flow. Channels have minimum gradients of 5–8 m/km, which permits flowing ice to deform by dislocation creep at exceedingly low strain rates; alternately, ice‐charged debris may have been buoyed by brines. By analogy with the fretted channels, ice could have flowed through the outflow channels if the atmospheric pressure was somewhat higher in the past and permitted ice to exist in the equatorial area.
Origin of spinel‐rich chondrules and inclusions in carbonaceous and ordinary chondrites
Authors: Kornacki Alan S. , Fegley Bruce
Published: February 2008

Our evaluation of three models of the origin of spinel‐rich chondrules and inclusions includes new calculations of the major element refractory mineral condensation sequence from a gas of solar composition over a wide pressure interval. Our condensation calculations show that spinel‐rich chondrules did not crystallize from metastable liquid condensates, and that spinel‐rich inclusions are not aggregates of refractory nebular condensates. We propose that spinel‐rich objects are fractionated distillation residues of small aggregates of primitive dust that lost Ca, Si‐rich partial melts by evaporation, ablation, or splashing during collisions. This model also explains why spinel‐rich chondrules and inclusions: (1) are usually smaller than melilite‐rich chondrules and inclusions; (2) often have highly fractionated trace‐element compositions; and (3) usually do not contain Pt‐metal nuggets even when they are more enriched in the Pt‐group metals than nugget‐bearing melilite rich objects.
John C. Jamieson January 5, 1924, to June 26, 1983
Authors: Schock R. N.
Published: February 2008

John Calhoun Jamieson (Figure 1) was a rare individual who combined unusual scientific talents with a profound respect for his fellow scientists. This special section pays respect to his memory in a way that he truly would have enjoyed—a collection of papers on some of his favorite subjects.

John devoted a major part of his life to high‐pressure physics and geophysics. His achievements in this regard are extraordinary and form the basis for much of what we now expend a great deal of effort trying to better understand. To gain insight into the intimate connection between John and high‐pressure science, we must go back to his early career. He was born and raised in St. Joseph, Missouri, and entered the University of Chicago during the early days of World War II. Like many of the students of that day, he subsequently enlisted in the military service. He served as an army weather observer in the Far East. After the war, he returned to Chicago and took his first degree in 1947. He received his Master's degree in 1951 and received one of the first National Science Foundation awards for graduate work in 1952.
Comments on “Deformation of clinopyroxenite: Evidence for a transition in flow mechanisms and semibrittle behavior” By S.H. Kirby and A.K. Kronenberg
Authors: Boland J. N.
Published: February 2008

Attempts to improve the accuracy of experimentation in the Griggs‐type piston‐cylinder deformation apparatus are most welcome and the recent study by Kirby and Kronenberg [1984] on the deformation of clinopyroxenite has the potential for useful quantitative manipulation of rheological data. However, there are many apparent inconsistencies and data‐handling inaccuracies in this work that require clarification before further use can be made of the data. The following comments relate specifically to (1) the presentation and/or selection of data, (2) rheological relationships, (3) semibrittle behavior, reproducibility and instability, and (4) single versus multiple mechanisms of deformation.
A magnetotelluric survey over the East Bull Lake gabbro‐anorthosite complex
Authors: Kurtz R. D. , Ostrowski J. A. , Niblett E. R.
Published: February 2008

Tensor and scalar magnetotelluric and audiomagnetotelluric (MT/AMT) data have been acquired on a gabbro‐anorthosite pluton in northern Ontario, about 100 km west of Sudbury. The scalar data from 102 sites were used to map resistivity contrasts associated with near‐surface geology and overburden. Data from stations located away from possible geological inhomogeneities provided a structural interpretation which was subsequently confirmed by tensor MT soundings and by drilling. The scalar data also provided a means of interpreting the bedrock structure associated with a major fault that bisects the pluton. The apparent resistivity curves from the eight tensor stations ranged from nearly isotropic to strongly anisotropic with large shifts in apparent resistivity between the different locations. However, the measured impedances, both modulus and phase, strongly reflected the predominant northwest‐southeast trend of faulting and diabase dykes in the region. Modelling indicates the presence of an anisotropic, subhorizontal conducting layer beginning at a depth of about 800 m. The direction of fluid transport within this layer is expected to be parallel to the structural trends and to the E polarization axis of the impedance tensor. The presence of this conductor has been confirmed by borehole resistivity measurements. A conductive zone in the lower crust has also been detected, commencing at a depth of about 18 km. Finally, the tensor measurements at the longest periods are consistent with an increase in conductivity in the upper mantle suggesting the existence of an electrically conducting asthenosphere in this part of the Canadian Shield.
Strain accumulation in the Shumagin Seismic Gap, Alaska
Authors: Savage J. C. , Lisowski M.
Published: February 2008

Strain accumulation in the Shumagin seismic gap was estimated from repeated trilateration surveys made between 1980 and 1985. The measured rate of shear‐strain accumulation (0.01 ± 0.03 μrad/a) was an order of magnitude less than the rate (0.18 μrad/a) predicted for seismic subduction. The absence of measurable shear‐strain accumulation in the Shumagin Islands suggests that the main thrust zone there may not be locked but rather may slip stably. If this is indeed the case, the probability of a great, shallow thrust earthquake in the Shumagin gap is greatly diminished. A moderate earthquake (mb = 6.0) occurred at the seaward end of the trilateration network 4 months after the most recent survey. However, no significant anomalies that could be interpreted as precursors were detected in that survey.
Geodesy by radio interferometry: Evidence for contemporary plate motion
Authors: Herring T. A. , Shapiro I. I. , Clark T. A. , Ma C. , Ryan J. W. , Schupler B. R. , Knight C. A. , Lundqvist G. , Shaffer D. B. , Vandenberg N. R. , Corey B. E. , Hinteregger H. F. , Rogers A. E. E. , Webber J. C. , Whitney A. R. , Elgered G. , Ronnang B. O. , Davis J. L.
Published: February 2008

Analysis of 211 very long baseline interferometry observing sessions carried out between November 1979 and August 1984 has yielded estimates of the distances between various radio telescopes located in North America and Europe. The average rate of change of the distances between four radio telescopes in North America (Haystack Observatory, Massachusetts; Westford Radio Telescope, Massachusetts; George R. Agassiz Station, Texas; and Owens Valley Radio Observatory, California) and one in Europe (Onsala Space Observatory, Sweden) obtained from the analysis of these data is 19 ± 10 mm/yr, where the (68% confidence interval) standard deviation is for the estimate of the rate of change of the Haystack‐Onsala baseline length, the one determined most accurately from these data. This estimate of the standard deviation is dominated by the effects of correlated systematic errors due mostly to errors in the model used for the atmospheric delay which we infer introduces errors in each baseline length estimate of 40 mm standard deviation and 60 days correlation time. (By contrast the statistical standard deviation is only 2 mm/yr.) The estimated geologic rates of change of these baseline lengths, averaged over ∼ 106 years, are 15 to 17 ± 3 mm/yr for the various North American sites to Onsala. Improvements in our model of the atmosphere, and continued monitoring of the distances between North American and European telescopes, will allow the uncertainty of the rate estimates to be reduced over the next few years to a value small compared to our estimated rate of change of these baseline lengths. The use of multiple radio telescopes in Europe will allow us also to separate possible local site motions from plate motions.
Global nondynamic orbit improvement for altimetric satellites
Authors: Sandwell David T. , Milbert Dennis G. , Douglas Bruce C.
Published: February 2008

The largest source of error in satellite altimetry is in the radial position of the satellite. Radial orbit errors of more than a few decimeters prohibit basin‐scale studies of sea surface height variability. We explore nondynamic techniques for reducing this error. Sea surface height differences at intersections of satellite altimeter profiles (crossover data) provide a strong constraint on radial orbit error but do not uniquely define it. The portion of orbit error that is a function of latitude and longitude only produces no crossover differences and therefore cannot be recovered with crossover data. Using mathematics (inclination functions) originally developed for satellite dynamics, we determine the entire class of orbit error functions not recoverable with crossover data. These functions are mappings of surface spherical harmonics into the orbit plane. For example, the l = 1, m = 0 surface harmonic maps into sinusoidal orbit error with a frequency of once per orbit. Nonzonal harmonics map into linear combinations of three or more frequencies that are linked by the inclination functions. Between frequencies of 0 and 2.2 cycles per orbit there are nine orbit error components that cannot be recovered using crossover data. These components are uniquely defined, however, by nine globally distributed radial tracking points. Fewer tracking points are sufficient if a smoothness criteria is applied to the orbit correction curve. Our findings suggest that radial orbit error can be significantly reduced by including a few globally distributed radar reflectors (or transponders) in the tracking network.
Helium soil‐gas survey of the aurora uranium deposit, McDermitt Caldera Complex, Oregon
Authors: Reimer G. Michael
Published: February 2008

Two soil gas helium surveys were carried out in a section of the McDermitt caldera complex of mineralized volcanic rocks in Oregon. A regional helium anomaly was found and is thought to be associated with uranium‐rich tuffaceous fill of the caldera and the Aurora uranium deposit, which occurs near the northeastern rim of the caldera. Local hydrology may have an effect on the displacement of the helium anomaly from the uranium deposit and be a carrier of helium from sources at depth. This study suggests that helium surveys may be useful in a volcanic environment by helping to select areas for exploratory drilling for uranium deposits.
Asymptotic normal modes of a laterally heterogeneous Earth: 2. Further results
Authors: Henson I. H. , Dahlen F. A.
Published: February 2008

In the geometrical optics approximation the split eigenfrequencies δω of an isolated multiplet on a laterally heterogeneous earth are certain eigencontours of the great circular average
of the local eigenfrequency perturbation δωlocal. The normal
to a surface wave orbital plane, averaged over an orbit, precesses around the contours of
, and the quantization condition that selects the eigencontours is
where
is the orbital angular momentum and
is averaged over a precession cycle. All but one of the split eigenfrequencies in this approximation are degenerate doublets because of the even parity of
, and the spacing between adjacent doublets along a given branch of
versus
is Δωadjacent ≈ 2π/Tprecess, where Tprecess is the precession period. There is a separate branch of the action
versus
associated with every local minimum or maximum of
on the surface of the earth, and the resulting interleaving of doublets can lead to very irregular line spacing. The doublets are split even on a nonrotating earth by tunneling between surface waves propagating in opposite directions about otherwise identical ray paths. Ordinarily, the tunneling time Ttunnel is much longer than Tprecess so the spacing Δωtunnel = 2π/Ttunnel between the singlets in a quasi‐degenerate doublet is extremely small. In general, the line spacing in a split multiplet is most complicated near the separatrix or slow‐fast transition, since it is there that the precession period is the greatest and the tunneling time is the least.
Imaging radar polarimetry from wave synthesis
Authors: Zebker Howard A. , van Zyl Jakob J. , Held Daniel N.
Published: February 2008

An imaging radar polarimeter measures the dependence of radar backscatter intensity and relative phase as a function of both the transmitted and received wave polarization states. We have built such an instrument by adding a dual‐polarized antenna and a four‐channel data system to a conventional airborne synthetic aperture radar system. We measure directly the amplitude and phase of all elements of the scattering matrix corresponding to each individual pixel in a radar image. Subsequent data processing combines these elements to synthesize any desired combination of transmit and receive antenna polarizations, and thus we can measure the variation of intensity or phase with polarization. Different scattering models predict different functional dependences of intensity on polarization: observation of this dependence for actual targets permits identification of the dominant scattering mechanisms contributing to the measured backscatter. For example, we find that Bragg scattering closely approximates both the observed scattering from the ocean and, surprisingly, from lava fields in Idaho. Urban areas can be modeled as two‐bounce dielectric corner reflectors. Scatter from forests appears to have two main components: one is a two‐bounce corner reflector term and the other is a diffuse component that we attribute to multiple scatter. Knowledge of the polarization signature of an object permits the selection of optimum radar polarizations to enhance certain classes of terrain. For example, we can choose a set of polarization states to optimize contrast in an image. We have applied this technique to mapping and differentiation of lava flows and to differentiation of forested and clear‐cut areas.
Tektite‐like bodies at Lonar Crater, India: Implications for the origin of tektites
Authors: Murali A. V. , Zolensky M. E. , Blanchard D. P.
Published: February 2008

Homogeneous, dense glass bodies (both irregular and splash form) with high silica contents (∼67% SiO2) occur in the vicinity of Lonar Crater, India. Their lack of microlites and mineral remnants and their uniform chemical composition virtually preclude a volcanic origin. They are similar to tektites reported in the literature. While such a close association of tektite‐like bodies with impact craters is already known (Aouelloul Crater, Mauritania; Zhamanshin Crater, U.S.S.R.), the tektite‐like bodies at Lonar Crater are unique in that they occur in an essentially basaltic terrain. Our geochemical data are consistent with these high silica glass bodies being impact melt products of two‐thirds basalt and one‐third local intertrappean sediment (chert). The tektite‐like bodies of the impact craters Lonar, Zhamanshin, and Aouelloul are generally similar. Strong terrestrial geochemical signatures reflect the target rock REE patterns and abundance ratios and demonstrate their terrestrial origin resulting from meteorite impact, as has been suggested by earlier workers.
Carbon in olivine: Results from nuclear reaction analysis
Authors: Mathez E. A. , Blacic J. D. , Beery J. , Hollander M. , Maggiore C.
Published: February 2008

The solubility behavior of C in olivine has been investigated using the 12C(d, p)13C nuclear reaction analysis technique. The technique provides an in situ microanalysis of concentration as a function of depth below the surface. In the case of olivine the depth profile extends to approximately 2.6 μn. Microbeam capability is achieved by focusing the deuteron beam with a superconducting solenoid. However, thermal degradation of the sample imposes a practical limit on the lateral spatial resolution of approximately 150 μm. The detection limit for C in forsteritic olivine is 65 ppm (atomic) and limited mainly by important background contributions from both Mg and Si. Olivines of similar compositions (Fo90–92) but from diverse geologic environments were studied, including (l) olivine crystallized in the presence of saline hydrothermal fluid in the shallow crust, (2) upper mantle megacrysts and crystals separated from spinel Iherzolite xenoliths from alkali basalts, (3) natural crystals equilibrated with graphite and magnesite at 30 kbar and 1400°C, and (4) olivine from a diamond‐bearing garnet Iherzolite xenolith from a kimberlite. Contrary to expectation based on previous experiments, none of the samples were found to contain C in concentrations greater than the detection limit. Concentration profiles generated by the use of combined sputtering and Auger techniques demonstrate that the adsorbed C‐rich layer on polished surfaces is typically restricted to the upper 100 Å of the surface.
Multifrequency and multipolarization radar scatterometry of sand dunes and comparison with spaceborne and airborne radar images
Authors: Blom Ronald , Elachi Charles
Published: February 2008

Understanding the unusual radar scattering characteristics of sand dunes is necessary in the analysis of radar images of aeolian landscapes of the earth and of other planets. In this paper we report on airborne radar scatterometer data of sand dunes, acquired at multiple frequencies and polarizations. Radar backscatter from sand dunes is very sensitive to the imaging geometry. At small incidence angles the radar return is mainly due to quasi‐specular reflection from dune slopes favorably oriented toward the radar. A peak return usually occurs at the incidence angle equal to the angle of respose for the dunes. The peak angle is the same at all frequencies as computed from specular reflection theory. At larger angles the return is significantly weaker. The scatterometer measurements verified observations made with airborne and spaceborne radar images acquired over a number of dune fields in the United States, central Africa, and the Arabian peninsula. The imaging geometry constraints indicate that possible dunes on other planets, such as Venus, will probably not be detected in radar images unless the incidence angle is less than the angle of repose of such dunes and the radar look direction is approximately orthogonal to the dune trends.
Thermohydrological conditions and silica redistribution near high‐level nuclear wastes emplaced in saturated geological formations
Authors: Verma A. , Pruess K.
Published: February 2008

Evaluation of the thermohydrological conditions near high‐level nuclear waste packages is needed for the design of the waste canister and for overall repository design and performance assessment. Most available studies in this area have assumed that the hydrologic properties of the host rock are not changed in response to the thermal, mechanical, or chemical effects caused by waste emplacement. However, the ramifications of this simplifying assumption have not been substantiated. We have studied dissolution and precipitation of silica in liquid‐saturated hydrothermal flow systems, including changes in formation porosity and permeability. Using numerical simulation, we compare predictions of thermohydrological conditions with and without inclusion of silica redistribution effects. Two cases were studied, namely, a canister‐scale problem, and a repository‐wide thermal convection problem and different pore models were employed for the permeable medium (fractures with uniform or nonuniform cross sections). We find that silica redistribution in water‐saturated conditions does not have a sizeable effect on host rock and canister temperatures, pore pressures, or flow velocities.
New laboratory method for analyzing deformation and displacement in creeping fluid: Examples from Stokes Flow and a thermal plume
Authors: Weijermars Ruud
Published: February 2008

Until now, it has been impossible to visualize and quantify the finite strains and rotation components of deformation in flowing fluids by other than numerical methods. A new practical method is presented for studying both the displacement and deformation of fluid volumes in low inertia flows from physical laboratory experiments. The philosophy behind the method is outlined, and its potential is concisely illustrated with some preliminary results. These consider the finite deformation of fluid volumes creeping past a falling cylinder (Re = 3 × 10−9) and through a thermal convection cell with axial symmetry (Ra = 104, Pr = 108). Their respective deformation fields have been summarized in contour maps of the finite strain intensity, finite strain ellipsoid shapes, and their finite rotation angles. Laboratory studies of progressive finite deformation of fluid volumes in low inertia flows are of particular interest to geophysics and structural geology, but might also open new avenues for other disciplines.
Microplate and shear zone models for oceanic spreading center reorganizations
Authors: Engeln Joseph F. , Stein Seth , Werner John , Gordon Richard G.
Published: February 2008

The reorganization of oceanic spreading centers is often accomplished through the process of rift propagation. We first examine general rift propagation geometries for the simple two‐plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously, such that the propagating and dying rifts never overlap. A velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements, and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics. We then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation, such that a technically independent overlap region, like the Easter or Juan Fernandez plates along the East Pacific Rise, is produced. We examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers. The tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear. Rigid plate models are suggested by the reasonable success of Euler vectors derived from inversions of plate motion data (spreading rates, transform azimuths, and slip vectors) both in describing available data and in predicting subsequently acquired data. On the other hand, recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing, rather than rigid behavior, in the interior of the Easter microplate. We use schematic evolutionary simulations for an Easter‐style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models. In a microplate model, rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region, while structures within the overlap zone rotate rigidly. In contrast, in a shear model, relative motions at the boundaries differ from those predicted by rigid plate tectonics, and the angular relationships of structures within the overlap region are altered by internal deformation. Thus inversion of relative motion data yields different results for rigid and shear behavior. Similarly, under favorable circumstances, structural trends could be diagnostic of the different cases. We find that much of the Easter structural data suggestive of shearing are also consistent with rigid plate tectonics. We conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics. Furthermore, the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations.
Triple junction reorganization
Authors: Kleinrock Martin C. , Morgan Jason Phipps
Published: February 2008

For at least the past 20 m.y. the Bouvet triple junction appears to have existed predominantly in a ridge‐fault‐fault rather than a ridge‐ridge‐ridge configuration when it has an isosceles velocity vector triangle that allows either configuration to be stable. In this paper two conceptual models originally developed to explain the general orthogonality of ridges and transform faults in spreading center systems are examined to see if they can explain the observed predominance of the ridge‐fault‐fault geometry for this triple junction. The first hypothesis is that a ridge axis will form and evolve to remain oriented perpendicular to the direction of the maximum local tensile stress at the ridge axis; at a triple junction this can be strongly modified by the local plate boundary geometry. The second hypothesis is that plate boundary geometries adjust to minimize the power dissipation associated with plate boundary motions. Both hypotheses successfully predict the observed predominance of the ridge‐fault‐fault geometry. We suspect that the minimum dissipation hypothesis is successful for spreading center problems, despite its questionable theoretical underpinning, because for these problems it is equivalent to plate geometries assuming a minimum resistive force configuration. Since a minimum resistive force configuration is a partial corollary of a local stress‐controlled ridge axis geometry, the resulting minimum dissipation hypothesis plate geometry predictions will be similar to stress‐controlled plate boundary geometries. Because of its simplicity, the minimum dissipation hypothesis may remain a useful tool for predicting plate boundary configurations provided it is realized that this is more an approximate rule of thumb than a hard physical theory. Observations of the occurrence and orientation of episodic overshooting of ridge segments past the stable triple junction location should provide a better test that both fine‐ and large‐scale triple junction tectonics are stress controlled.
Correcting gravity measurements for the effects of local air pressure
Authors: Niebauer T. M.
Published: February 2008

A simple exponential atmosphere is used to derive the gravitational changes caused by the direct attraction of the local air pressure. This approach yields a Green function which can be used with surface air pressure readings near and including the observation point to correct gravity measurements for the gravitational attraction of the atmosphere. This model predicts a lower local air pressure admittance than the standard value (−0.43 μGal/mbar) obtained assuming a plane parallel atmosphere. The curvature of the Earth and the scale height of the atmosphere reduce this admittance by about 7–8%.
Wrinkle ridge assemblages on the terrestrial planets
Authors: Watters Thomas R.
Published: February 2008

The wrinkle ridge assemblage is a group of associated features that have been observed on the Moon, Mars, and Mercury. Members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first‐, second‐, or third‐order ridges on the basis of dimensions. The ridge assemblages described in this study are known to occur in mare basalts on the Moon and smooth plains units that may be volcanic in origin on Mars and Mercury. The structures that occur in the continental flood basalts of the Columbia Plateau in the northwestern United States are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets. The anticlinal ridges of the Columbia Plateau and many first‐order ridges on the Moon, Mars, and Mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling (flexure‐fracture) or buckling as a consequence of reverse or thrust faulting (fracture‐flexure). Estimates of the unit shortening across first‐order ridges, using simple geometric forms to approximate the ridge morphology, are lower than those determined for the anticlines of the Columbia Plateau, where displacements are the results of folding and reverse to thrust faulting. Evidence of layer‐parallel extension that often accompanies folds developed by fracture‐flexure mechanisms (i.e., drape or fault‐bend folds) is not found in the anticlinal ridges of the Columbia Plateau and has only been observed in one ridge on the Moon. A kinematic model involving buckling followed by reverse to thrust faulting (flexure‐fracture) is favored for the origin of the anticlinal ridges of the Columbia Plateau and many of the first‐order ridges on the Moon, Mars, and Mercury.
Geophysical considerations in the fifth force controversy
Authors: Stacey Frank D. , Tuck Gary J. , Moore G. Ian
Published: February 2008

If there are non‐Newtonian components of gravity, now popularly dubbed the “fifth force,” as several observations indicate, then the favored representation is in terms of Yukawa potentials with ranges that make them accessible to geophysical observation. We must now consider at least two Yukawa terms of opposite signs, so that the observed effects may be subtle. Measurements in different crustal structures (continental mines and boreholes, oceans and ice sheets) could help to resolve the details, but it is possible for fortuitous cancellations to invite misleading conclusions where measurements are made in a layer that is underlain by a much denser layer. However, with currently favored parameters of a pair of Yukawa terms, both ice and ocean measurements should give effects of the sign expected from mine measurements, but with amplitudes reduced by partial cancellation due to the layered structures. We also reexamine conventional interpretations of the mine gravity anomalies and reassert that uncertainties in density estimates must be discounted. A new inversion of the broad scale gravity anomalies in the area of the north Queensland mines that we have used confirms the earlier conclusion that the mine gradient anomaly is not a consequence of a regional free‐air gradient anomaly, although this conclusion is not as secure as the dismissal of density error.
Thermal demagnetization of partial thermoremanent magnetization
Authors: Worm H.‐U. , Jackson M. , Kelso P. , Banerjee S. K.
Published: February 2008

Three well‐characterized glass‐ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations (pTRMs) in the temperature interval 400°–350°C for varying lengths of time (10 min to 5 days) and in magnetic fields from 0.5 to 5 Oe (0.05–0.5 mT). The thermal demagnetization behavior of the pTRMs changes with domain state in the sense that with decreasing coercivity (increasing grain size), demagnetization curves become less steep and may not completely demagnetize until heated to the Curie temperature. Numerical calculations based on Néel's single domain (SD) theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures. We observe that experimental results do not match any of the theoretical predictions. There is no discernible field dependence of demagnetization curves for the SD and pseudo‐single domain samples and even for the “soft” multidomain (MD) sample in the field range investigated. An important consequence for thermally derived paleointensity data is that they may not be entirely reliable, especially if the sample remanence is due to MD grains.
Geology and petrology of the Woods Mountains Volcanic Center, southeastern California: Implications for the genesis of peralkaline rhyolite ash flow tuffs
Authors: McCurry Michael
Published: February 2008

The Woods Mountains Volcanic Center is a middle Miocene silicic caldera complex located at the transition from the northern to the southern Basin and Range provinces of the western United States. It consists of a trachyte‐trachydacite‐rhyolite‐peralkaline rhyolite association of lava flows, domes, plugs, pyroclastic rocks, and epiclastic breccia. Volcanism began at about 16.4 Ma, near the end of a local resurgence of felsic to intermediate magmatism and associated crustal extension. Numerous metaluminous high‐K trachyte, trachydacite, and rhyolite lava flows, domes, and pyroclastic deposits accumulated from vents scattered over an area of 200 km2 forming a broad volcanic field with an initial volume of about 10 km3. At 15.8 Ma, about 80 km3 of metaluminous to mildly peralkaline high‐K rhyolite ash flows were erupted from vents in the western part of fhe field in three closely spaced pulses, resulting in the formation of a trap door caldera 10 km in diameter. The ash flows formed the Wild Horse Mesa Tuff, a compositionally zoned ash flow sheet that originally covered an area of about 600 km2 to a maximum thickness of at least 320 m. High‐K trachyte pumice lapilli, some of which are intimately banded with rhyolite, were produced late in the two later eruptions, Intracaldera volcanism from widely distributed vents rapidly filled the caldera with about 10 km3 of high‐K, mildly peralkaline, high‐silica rhyolite lava flows and pyroclastic deposits. These are interlayered with breccia derived from the caldera scarp. They are intruded by numerous compositionally similar plugs, some of which structurally uplifted and fractured the center of the caldera. The center evolved above a high‐K trachyte magma chamber about 10 km in diameter that had developed and differentiated within the upper crust at about 15.8 Ma. Petrological, geochemical, and geophysical data are consistent with the idea that a cap of peralkaline rhyolite magma formed within the trachyte chamber as a result of fractional crystallization within chemical boundary layers.
Present‐day motion along the Owen Fracture Zone and Dalrymple Trough in the Arabian Sea
Authors: Gordon Richard G. , DeMets Charles
Published: February 2008

All plate motion data available for describing the present‐day motion between the Arabian and Indian plates are analyzed to understand the present‐day tectonic role of the presumed plate boundary: the Owen fracture zone and Dalrymple trough in the northern Arabian Sea. All prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary. In particular, published global plate motion models predict some convergence across the Owen fracture zone and Dalrymple trough. Convergence along the Owen fracture zone is contradicted by earthquake focal mechanisms, which show right‐lateral strike slip. Convergence along the Dalrymple trough is contradicted by observed normal faulting in the trough, and by normal and strike‐slip focal mechanisms for earthquakes occurring along or near the trough. The sense and rate of motion is further constrained by spreading rates along the Sheba Ridge (northwest of the Owen fracture zone) and along the Carlsberg Ridge (southeast of the Owen fracture zone). To estimate the present‐day motion, we determine 43 3‐m.y.‐average spreading rates from Sheba Ridge, Carlsberg Ridge, and Central Indian Ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the Arabia‐India boundary. We determine an Arabia‐India Euler vector that predicts 2 mm/yr of right‐lateral strike‐slip motion along the Owen fracture zone, with a 95% confidence interval of 0–7 mm/yr, which excludes the faster rates of slips predicted by several prior models. The direction of motion is 35°–50° counterclockwise from that predicted by published global plate motion models. If we omit the data along the Arabia‐India boundary, the motion differs insignificantly from zero, suggesting that magnetic profiles from the Carlsberg and Sheba ridges give spreading rates too imprecise to detect the slow Arabia‐India motion. If the Owen fracture zone and Dalrymple trough are regarded as a plate boundary, it is the slowest slipping plate boundary known on Earth, with motion about 100 times slower than the fastest slipping faults along the East Pacific Rise.
Evolution of a Chemically Zoned Magma Body: Black Mountain Volcanic Center, southwestern Nevada
Authors: Vogel Thomas A. , Noble Donald C. , Younker Leland W.
Published: February 2008

Rocks of the Black Mountain volcanic center consist of four ash flow sheets and units of lava that underlie, interfinger with, and overlie the sheets. Rocks from the center represent three magma types. Magma type c was present through the history of the center, whereas types a and b were available after the eruption of the Rocket Wash Member, during the eruptions of the Pahute Mesa and Trail Ridge members. The magma types are defined by trace element ratios; for example, magma types a, b, and c have La/Th values of 1.0–3.5, >7.5, and 3.5–7.5. Silica contents in the magma types a, b, and c range from 71.5 to 74.1, from 65.8 to 69.2, and from 55.6 to 73.8 wt %, respectively. The stratigraphic distribution of chemically distinct pumice fragments within the ash flow sheets is used to show that magma type a was located in the uppermost part of the chamber and was underlain successively by magma types b and c. Because pumice fragments that belong to all three magma types occur in individual cooling units, a zoned magma body must have existed during this period. Magma mixing is indicated by the disequilibrium phenocrysts which are common in pumice fragments from all magma types; however, this mixing did not destroy the original zoning of the upper part of the magma body. Most of the chemical variation of magma type c is consistent with fractionation of feldspar, olivine, and pyroxene, but abundant disequilibrium, mafic phenocrysts indicate that magma replenishment and mixing were common. Magma type b had much higher La/Th and light rare earth element (LREE)/heavy rare earth element values and must have originated independently from magma type c. Most likely the two types were derived from different source material. The low La/Th values of magma type a can be explained by separation of a phenocryst assemblage containing both a LREE‐bearing phase and zircon from either magma types b or c, or possibly by the partial melting of source material containing these phases.
Paleomagnetic and cooling constraints on the duration of the Pahute Mesa‐Trail Ridge Eruptive event and associated magmatic evolution, Black Mountain Volcanic Center, southwestern Nevada
Authors: Weiss Steven I. , Noble Donald C. , McKee Edwin H.
Published: February 2008

Two successive ash‐flow cooling units of the Black Mountain volcanic center, the Pahute Mesa and Trail Ridge members of the late Miocene Thirsty Canyon Tuff, possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178° declination and −10° inclination. These are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition, or possibly from a field oriented near the limit of secular variation. Regardless of the type of geomagnetic field behavior, the Pahute Mesa Member was apparently overlain by the Trail Ridge Member before appreciable shift of the anomalous geomagnetic field. This strongly suggests that the time between eruption of the two ash‐flow sheets was very short, perhaps less than 100 years, a conclusion that is independently supported by the local absence of a complete cooling break between the two units. The interval between eruption of the Pahute Mesa and Trail Ridge members was too brief for reestablishment of chemical stratification in the Black Mountain magma system. Nevertheless, a small volume of highly evolved comendite magma developed and was erupted as lava within the Black Mountain caldera and as a component of the Trail Ridge Member. The short repose interval and rapid development of the comendite, together with evidence for mixing of magmas of silicic to near‐mafic composition (Vogel et al., this issue), and the presence of air‐fall tuff at the base of the Trail Ridge, lead us to conclude that eruption of the Trail Ridge Member was probably triggered by injection of hot, relatively unevolved magma into the Black Mountain magma body.
P wave attenuation of the Yellowstone Caldera from three‐dimensional inversion of spectral decay using explosion source seismic data
Authors: Clawson Steven R. , Smith Robert B. , Benz Harley M.
Published: February 2008

Using explosion source, seismic refraction data, recorded in the 1978 and 1980 Yellowstone‐Snake River Plain seismic experiments, a three‐dimensional inversion of differential P wave attenuation was used to assess the relative variations in Q−1 in and around the volcanically active, 45 km by 70 km, Yellowstone caldera, northwestern Wyoming. Differential attenuation was derived from spectral decay of upper crustal Pg phases, observed from six explosions and recorded at 90 temporary stations. Because of the relatively short time windows used to determine the spectral content, a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance. Differential P wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects. The observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized, 70×105 km, three‐dimensional surface and upper crustal Q−1 model of the Yellowstone caldera and surrounding region. Results showed that the surface layer, to depths of 2 km within the Yellowstone caldera, is characterized by relatively high attenuation with low Q values less than 30, compared to values of 38 to 50 outside the caldera. The higher attenuation in the caldera's surface layer is thought to be associated with Quaternary lake sediments, highly altered rhyolites, and the possible influence of steam in areas of hydrothermal activity. In the crystalline upper crust, at depths of 2 km to 12 km, Q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity. Within the caldera, upper crustal attenuation generally corresponded to Q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material. In comparison, relatively high attenuation, Q = 40, was observed in the upper crust of the northeastern Yellowstone caldera in an area that also corresponds to a 20% reduction in Pg velocity, a prominent negative Bouguer gravity low of −20 mGal, near Yellowstone's largest area of hydrothermal activity and near a Quaternary, volcanic resurgent dome. On the basis of constitutive models of velocity and density, this zone of high attenuation is likely a product of unusual hydrothermal activity, highly altered upper‐crustal rocks, a shallow magmatic source or a highly porous, steam‐saturated body. Outside the caldera, the upper crust is relatively unaffected by high temperatures and thermal alteration, and the corresponding attenuation is low with Q values greater than 200.
Solitary waves in mantle plumes
Authors: Schubert G. , Olson P. , Anderson C. , Goldman P.
Published: February 2008

Numerical calculations in two dimensions show that solitary wave disturbances can propagate along thermal plumes in a homogeneous, viscous fluid with a thermally activated rheology similar to the mantle. Comparison of the numerical results with analytic results from an idealized two‐fluid model indicates that the two‐fluid model can be applied to solitary waves in the thermally activated plume. A two‐fluid model of solitary waves on cylindrical conduits is accordingly applied to mantle plumes to estimate the propagation speeds, time durations, and pulse lengths of solitary waves in mantle plumes as a function of background mantle viscosity, plume flux, plume density deficit, plume viscosity, and the volume of material transported by the solitary wave. It is inferred that mantle plumes may be episodic via solitary waves and that these disturbances might account for observed 0(10 m.y.) variations in the durations and spacings of episodes of enhanced hotspot volcanic activity. Solitary waves in mantle plumes could be generated by interactions among plumes and between plumes and the large‐scale, time‐dependent mantle circulation.
Microseismicity in the middle Okinawa trough
Authors: Ouchi Toru , Kawakami Hisatosi , Nagumo Shozaburo , Kasahara Junzo , Koresawa Sadayuki
Published: February 2008

We conducted a microearthquake survey with ocean bottom seismometers (OBS) in the middle Okinawa trough in 1984. During about one month of operation, very active microseismicity was observed. The microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of OBS stations. This activity was recorded by almost every OBS station but not by land‐based instruments. Swarm activities also occurred at many places. Average activity levels of earthquakes whose durations are longer than 30 s, recorded at individual OBSs, are from 4 to 20 events per day, which seem higher than those of ordinary mid‐oceanic ridge‐fracture zone systems and other back arc basins. Apparently, the number of smaller size earthquakes is fewer than expected. This might indicate the rather high attenuation of seismic waves or low‐Q of the crust around OBS stations, which makes it difficult to detect small size events at a distant range. Attenuation of seismic waves was large beyond the epicentral distance of about 120 km. This seems to indicate low‐Q in the lower crust. Also, a particularly low‐Q area was found at the northern most part of the research area where high heat flow measurements were obtained. This suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies. At least two types of swarm activities are indicated for this area. Some have no dominant events, show high “b” value properties, and seem to be volcanic. Others have dominant events and appear to be tectonic in nature. Among the swarm activity, two kinds of signals closely resembling volcanic tremors were observed. From these features, we conclude that the crust of this area is highly fractured and subject to intense tectonic activities, as well as some kind of igneous processes.