bluuebunny/crossref_metadata_2025 huggingface dataset:
Venus: The nature of the surface from Venera panoramas
Authors: Garvin James B. , Head James W. , Zuber Maria T. , Helfenstein Paul
Published: February 2008

Images of the surface of Venus obtained by the Soviet Venera 9, 10, 13, and 14 landers have been analyzed to provide a basis for understanding the nature of geologic processes operating there. The four spacecraft landed in the Beta‐Phoebe region at median elevations in the upland rolling plains province. The landing points are each separated by distances of more than a thousand kilometers. The Venera panoramas were digitized and transformed into various perspectives in order to facilitate analysis and comparison with other planetary surfaces. Bedrock is exposed at the Venera 10, 13, and 14 sites and is characterized by semicontinuous, flat polygonal to subrounded patches up to several meters in width. The bedrock surface is often dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters and abundant linear and polygonal vertical fractures. Angular to subangular layered to platy blocks in the 5‐ to 70‐cm range dominate the Venera 9 site and occur much less frequently at the other sites. Blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it. Soils (particles <1 cm) are abundant at the Venera 9, 10, and 13 sites but are uncommon at Venera 14. Features indicative of a strong eolian influence (moats, dunes, wind tails) are not observed. A striking aspect of the Venera landing sites is their extreme similarity despite separation distances of thousands of kilometers. Several hypotheses are considered for the origin of the bedrock surfaces, and we investigate in detail the hypothesis that bedrock originated from surface lava flows. In this interpretation, the broadly platy nature of the surface is analogous to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust. The layering is interpreted to be formed by a combination of upper thermal boundary layer formation and horizontal sheets formed by cooling and shearing during flow emplacement. Vertical fractures are attributed largely to joint patterns formed during cooling. This interpretation made on the basis of surface morphology is consistent with Venera 13 and 14 geochemical results which reported high potassium basalt and tholeiitic basalt compositions, respectively. If this interpretation is correct, large regions of the Beta‐Phoebe area are likely to be characterized by lava flows. The relative freshness of features observed by Venera 14 suggests that some bedrock surfaces are geologically young or that erosion rates are low.
Point defect chemistry of minerals under a hydrothermal environment
Authors: Hobbs B. E.
Published: February 2008

The kinetics of rock/water interactions are sufficiently rapid that most hydrothermal systems in nature will be in equilibrium with the adjacent rock mass. The bulk rock chemistry buffers the fugacity of oxygen, which in turn fixes the fugacities of water and of hydrogen for a given pressure and temperature. Systems in which only water, oxygen, and hydrogen are present as fluid phases are considered here. Variations in the fugacity of oxygen by several orders of magnitude are possible locally, controlled by variations in local rock chemistry; these lead to relative small variations in the fugacity of water. Incorporation of a hydrogen defect that is capable of acting as an acceptor into silicates leads to a strong dependence of point defect chemistry upon the fugacities of both water and oxygen. The strong dependence on the fugacity of water is capable of explaining the hydrolytic weakening effect, but in view of the strong dependence on oxygen fugacity, the question should also be raised whether it is an oxygen effect that is observed in the classical hydrolytic weakening process or solely a dependence on changes in the fugacity of water. Examples are given for impure natural quartz, olivine, and albite with trace amounts of calcium.
Crustal studies, velocity inversions, and fault tectonics: Results from a microearthquake survey in the New Madrid Seismic Zone
Authors: Nicholson C. , Simpson D. W. , Singh S. , Zollweg J. E.
Published: February 2008

A dense network of portable seismographs was operated in the Ridgely area of western Tennessee in May of 1978. During 32 days of operation, 122 earthquakes were detected, of which 90 were located. The high quality P and S arrivals recorded exhibit travel time ratios (ts/tp) that decrease significantly with depth. This decrease is most rapid in the upper few kilometers and necessarily reflects similar changes in the velocity ratio (Vp/Vs). Based on laboratory and theoretical studies, such observations are consistent with a decrease in crack density with increasing confining pressure. A special study of Vp, Vs, and Vp/Vs revealed strong variations with depth that also correlate with rock type. Inversion of the data for both P and S velocity structures confirms the presence of a low‐velocity zone in the depth range 2 to 5 km; a high‐velocity lid, with high Vp/Vs and thus, high crack density; and a boundary at the near surface for S to P conversion, associated with the unconsolidated saturated sediments of the Mississippi embayment. Relocated hypocenters using independent velocity structures for P and S are systematically deeper and define a seismogenic layer between 4 and 13 km depth. The earthquakes near Ridgely show that most of the activity is occurring on a northwest striking reverse fault that dips to the southwest. Minor off‐fault activity within the hanging wall was also observed representing motion on a series of small normal faults dipping to the northeast. Other composite focal mechanisms were determined for nearby regions. These solutions are all in general agreement with a horizontal compressive stress field striking northeast to east, favoring right‐lateral strike‐slip motion on left‐stepping northeast trending faults, or reverse motion on faults trending northwest and north. These results are consistent with the stress field found in a large area interior to the United States, and imply a rotation of the regional stress field since the late Cretaceous when extensional faulting occurred in the area.
Recent movements of the Juan de Fuca Plate System
Authors: Riddihough Robin
Published: February 2008

Analysis of the magnetic anomalies of the Juan de Fuca plate system allows instantaneous poles of rotation relative to the Pacific plate to be calculated from 7 Ma to the present. By combining these with global solutions for Pacific/America and “absolute” (relative to hot spot) motions, a plate motion sequence can be constructed. This sequence shows that both absolute motions and motions relative to America are characterized by slower velocities where younger and more buoyant material enters the convergence zone: “pivoting subduction.” The resistance provided by the youngest portion of the Juan de Fuca plate apparently resulted in its detachment at 4 Ma as the independent Explorer plate. In relation to the hot spot framework, this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased. After 4 Ma the remainder of the Juan de Fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south. Differences in seismicity and recent uplift between northern and southern Vancouver Island may reflect a distinction in tectonic style between the “normal” subduction of the Juan de Fuca plate to the south and a complex “underplating” occurring as the Explorer plate is overridden by the continent. The history of the Explorer plate may exemplify the conditions under which the self‐driving forces of small subducting plates are overcome by the influence of larger, adjacent plates. The recent rapid migration of the absolute pole of rotation of the Juan de Fuca plate toward the plate suggests that it, too, may be nearing this condition.
Lindemann Melting Law: Anharmonic correction and test of its validity for minerals
Authors: Wolf George H. , Jeanloz Raymond
Published: February 2008

High‐pressure experimental data are used to evaluate the validity of the Lindemann melting criterion for minerals. Because anharmonicity can be significant at the high temperatures associated with melting, leading order anharmonic contributions to the mean‐square atomic vibrations have been included in the present lattice dynamical formulation of the Lindemann criterion. The parameters required in this Lindemann expression can be determined from an inversion of thermodynamic data so that a direct comparison with experimental melting data can be made without resort to model potentials. In general, we find that although atomic vibrations can be significantly anharmonic near the melting point, the shape of the melting curve predicted from the Lindemann criterion is not much influenced by anharmonicity. We demonstrate that the Lindemann expression is inconsistent with experimental melting data for a variety of substances, particularly at low pressures. In some cases (argon, sodium chloride and potassium chloride), the melting slopes observed at high pressures are consistent with a Lindemann expression. In general, we find that a Lindemann criterion is not an adequate description of the melting behavior of minerals. Therefore, an application of the Lindemann law to obtain the melting temperatures of minerals at high pressures, and to determine the fusion curve of the earth's deep interior is, at present, speculative.
Sr isotopic variations along the Juan de Fuca Ridge
Authors: Eaby Jacqueline , Clague David A. , Delaney John R.
Published: February 2008

Sr isotopic ratios of 39 glass and microcrystalline basalt samples along the Juan de Fuca Ridge and 1 glass sample from Brown Bear Seamount are at the lower end of the range for normal mid‐oceanic ridge basalt (MORB); the average 87Sr/86Sr ratio is 0.70249±0.00014 (2‐σ). Although subtle variations exist along strike of the ridge, the Sr isotope data do not show systematic variation relative to the proposed Cobb Hotspot. The isotopic data are inconsistent with an enriched mantle‐plume origin for the Cobb‐Eickelberg Seamount chain, as has been proposed for Iceland, the Azores, and the Galapagos spreading center. Sr isotopic ratios of samples collected north and south of the Cobb offset are identical, although minor element ratios indicate that these regions have chemically distinct mantle sources. These distinct mantle sources may not have been separated long enough to develop isotopic differences.
Implementation of electrostatic feedback with a LaCoste‐Romberg Model G Gravity Meter
Authors: Harrison J. C. , Sato Tadahiro
Published: February 2008

We have devised a simple method for implementing electrostatic feedback with a LaCoste‐Romberg model G gravity meter. Earth tide observations made with this instrument in the feedback mode should be superior to those currently being made without feedback, because of the increased stability of the scale factor, improved calibration, and the elimination of instrumental lags due to spring hysteresis. Other possible applications of our technique include the determination of the short‐range periodic errors in the measuring screw‐lever system and improved accuracy in measuring small gravity differences.
Amatitlan, An actively resurging cauldron 10 km south of Guatemala City
Authors: Wunderman Richard L. , Rose William I.
Published: February 2008

A 14×16 km diameter collapse caldera has been recognized 10 km south of Guatemala City, Guatemala. The caldera is north of the presently active volcano Pacaya and west of Agua, a large stratovolcano. The caldera was not previously recognized because its eastern and western margins coincide with faults that outline the Guatemala City graben and because the northern margin of the caldera is buried by pyroclastic rocks. The existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults, hot springs, well‐log data, and lithological changes in sedimentary rocks. A sequence of nine silicic pyroclastic deposits, totaling a volume of more than 70 km 3 dense rock were erupted from the caldera. The ages of these eruptions are mainly between about 300,000 years B.P. to less than 23,000 years B.P. The rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow, surge, and fall deposits, mainly of rhyolitic to dacitic composition. Successive pyroclastic eruptions have generally become smaller in volume and more silicic with time. Major and minor element chemistry distinguish Amatitlan pyroclastics from those of other nearby calderas. The caldera lies at the intersection of an offset of the volcanic chain (the Palin Shear) and the faults along the volcanic front (Jalpatagua fault zone). The caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben. The graben's alignment with the Jalpatagua fault zone suggests a genetic relationship. The longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents. Available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome, near a young silicic intrusion. The caldera is active and will probably erupt again. Over 1 million people live within 20 km and would be threatened in the event of a moderate eruption. Suggestions for future research focus on hazard assessment and forecasting
Spall observations and mechanisms in alluvium
Authors: Stump Brian W. , Reinke Robert E.
Published: February 2008

Spall, as identified by minus 1 g dwells on vertical accelerometers followed by impulsive rejoin signals, has been observed in ground motion data recorded in alluvium. These data resulted from explosions ranging from fully contained to surface burst configurations. The variability of the loading environments and the observational data support the conclusion that the phenomena result from multiple mechanisms. The data show that relatively low accelerations (<20 g) accompany the process, but owing to the long dwell times involved, resulting displacements may be large. Five spall models are discussed and shown to operate in some aspect of the data base. These mechanisms include (1) simple tensile reflection from the free surface, (2) Rayleigh/shear waves from surface sources, (3) spherical and cylindrical wave field divergence, (4) positive and negative phase airblast effects on porous materials, and (5) material properties of porous particulate matter.
A geometric model for excavation and modification at terrestrial simple impact craters
Authors: Grieve R. A. F. , Garvin J. B.
Published: February 2008

A geometric model for the excavation and modification of simple crater development is presented. Modification is modelled by considering the relative dimensions and geometries of the final crater and the transient cavity formed by excavation and displacement. The input parameters required are the rim crest diameter of the final crater, the depth to autochthonous basement, the rim crest height and the outer rim slope. The model calculates the diameter of the final crater at the original ground plane, the dimensions of the transient cavity and the rim crest volume difference between the final and transient cavities. This volume is considered to be the material which slumped off the unstable, overheightened transient cavity wall during modification and corresponds to the observed volume of the breccia lens within the final crater. Excavation is approximated by a steady state Z model, with the diameter of excavation equivalent to the previously modelled diameter of the transient cavity at the original ground plane, the center of flow taken as 1 projectile diameter, and Z = 2.5–3.0. The Z model predicts such parameters as the depth and volume of excavation, which can be compared with observational data. The applicability of the model has been tested with data from Meteor and Brent craters. In the case of Meteor, the modelled final crater diameter at the original ground plane is within 26 m of the observed value and the modelled breccia lens and the rim crest volumes of the final true crater are within 9.5% and 2.5%, respectively, of observed values. The results of the Z model for Meteor crater, with Z = 2.8 ± 0.1, give results consistent with observational data on the depth and volume of excavation. The correspondence for the more degraded Brent crater is less precise, as there is greater uncertainty in the input parameters. It is possible, however, to match the model parameters with the breccia lens volume and published estimates of original dimensions. The modelled excavated volumes for Meteor and Brent are 58% and 55%, respectively, of the transient cavity volume below the original ground plane. In the absence of additional observational data for further testing, it is concluded that the model may provide first‐order spatial information on the formation of simple craters.
Composition and origin of enstatite in E chondrites
Authors: McKinley Susan G. , Scott Edward R. D. , Keil Klaus
Published: February 2008

Our petrologic and electron‐probe study of enstatite in six EH3, 4, and 5 chondrites, including the previously undescribed Allan Hills A77156, provides no support for Leitch and Smith's [1982] model in which crystals and liquids from two chemically distinct, partially molten planetesimals are mixed. We find that Cr and Mn concentrations in enstatite grains in matrix and chondrules are not uniquely defined by their cathodoluminescence color. Wide variations in enstatite compositions among chondrules may partly reflect heterogeneities in the precursor material. However, correlated variations of Cr, Mn, and Fe concentrations in enstatite are probably largely due to fractionation during chondrule crystallization. The inverse correlation of petrologic type with minor element concentrations in enstatite appears to be a metamorphic effect.
Isotope geochemistry of thermal and nonthermal waters in the Valles Caldera, Jemez Mountains, Northern New Mexico
Authors: Vuataz François D. , Goff Fraser
Published: February 2008

Over 100 stable isotope and 45 tritium analyses from thermal and nonthermal waters of the Jemez Mountains region, New Mexico, have been used to define the hydrodynamics of the Valles caldera (Baca) geothermal system and related geothermal fluids of the region. Evaluation of 36 cold meteoric waters yields an equation for the Jemez Mountains meteoric water line of δD = 8δ18O + 12, while further evaluation of nine cold meteoric waters yields an equation relating recharge elevation to deuterium content of E(meters) = −44.9 (δD) ‐ 1154. Based on the deuterium content of five Baca well waters (223°–294°C), the average recharge elevation of the Valles geothermal system ranges from 2530 to 2890 m. This range of elevations falls between the elevations of the lowest point of the caldera floor (2400 m) and the summit of the resurgent dome inside the caldera (3430 m). Thus stable isotopes indicate that the caldera depression probably serves as a recharge basin for the deep geothermal system. Although cold spring waters of the Jemez Mountains region consist of meteoric water, tritium analyses show that most of them contain water between 20 and 75 years old. The two major streams draining the Valles moat zone, San Antonio Creek and East Fork Jemez River, may contain more than 50% of this relatively old groundwater depending on the season. In contrast, streams draining the central resurgent dome of the caldera contain present‐day meteoric water. Using piston flow and homogeneously mixed reservoir models as end‐member cases, the tritium contents of the Baca fluids (0.18–1.10 tritium units (TU)) indicate that the mean residence time of water in the reservoir is between 60 and 10,000 years old. Deep geothermal fluids display a positive oxygen 18 shift of not less than 2‰ because of rock‐water isotopic exchange at 220°–300°C. The Valles geothermal system is capped by a vapor zone that is roughly 600 m thick and best developed at Sulphur Springs. Fumarolic steam at Sulphur Springs is unusually depleted in oxygen 18, suggesting that it is probably derived by boiling of near‐surface groundwater at 200°C. Surface acid‐sulfate waters are mixtures of condensed steam and surficial groundwaters that display the isotopic processes of evaporation and exchange between H2O and CO2. A lateral outflow plume discharges from the Valles geothermal system down the Jemez fault zone and feeds two sets of thermal springs in San Diego Canyon. Isotopic evidence shows that these springs consist of three components: (1) deep geothermal fluid, (2) surficial and/or near‐surface groundwater, and (3) relatively old, but cold, mineralized water. This latter component presumably circulates in Paleozoic strata underlying the western caldera flank. Hot, Precambrian, pore fluid brine occurs beneath the main Valles caldera hydrothermal system and may be generated by metamorphic processes in the relatively impermeable conductive regime above the magmatic heat source of the caldera.
Fast rift propagation along the East Pacific Rise near Easter Island
Authors: Naar David F. , Hey R. N.
Published: February 2008

During a 1983 Seabeam cruise, we surveyed the Easter‐Nazca spreading center which forms the eastern boundary of the Easter microplate. We have analyzed all available bathymetry and magnetic anomaly data to determine the present fine‐scale tectonic configuration and recent tectonic evolution of this spreading center. The narrowing of the axial (Brunhes) anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm/yr assuming an average spreading rate of 120 mm/yr. There are indications of a short episode of faster propagation (350 mm/yr) prior to 0.4 Ma. These are the fastest rates of rift propagation yet documented. The tip of the propagating rift is at about 25°S, 112°25′W, ∼180 km south of the previously hypothesized position. The recent spreading direction is N85°W. A large well‐defined “sheared zone” to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the Easter‐Pacific spreading center (360 km to the west) is being replaced by the fast propagator.
Multibeam study of the Flores Backarc Thrust Belt, Indonesia
Authors: Silver Eli A. , Breen Nancy A. , Prasetyo Hardi , Hussong Donald M.
Published: February 2008

Using the SeaMARC II seafloor mapping tool in conjunction with closely spaced seismic reflection profiles, we have mapped a segment of the Flores back arc thrust zone. Structural irregularities along the deformation front of the thrust zone result from changing stratigraphy and basement structure of the lower plate. NE trending faults cutting the outer slope of the Flores basin are easily mapped because they truncate dense drainage patterns obliquely. Mud diapirs, probably indicating elevated fluid pressures, have formed throughout the accretionary wedge but appear to be concentrated (as do back arc thrust earthquakes) at the ends of thrust faults. The overall orientation of the deformation front of the accretionary wedge is 100°, suggesting a NNE sense of thrust motion and supporting an origin of the thrust zone by collision of the arc with Australia rather than by magmatic forcing or gravitational sliding or spreading. Orientations of faults and folds close to the arc are not consistent with the trends of the frontal thrusts, however, and the difference may be due to either different initial orientations or to later rotations of structural features as the accretionary wedge grew.
Regional dust deposits on Mars: Physical properties, age, and history
Authors: Christensen Philip R.
Published: February 2008

Major dust storms on Mars play an important role in the deposition and removal of fine dust material. Thermal, radar, and visual remote sensing observations provide important constraints on the Martian regolith which have been used to determine the location and physical properties of regional dust deposits. These deposits are located in three northern equatorial regions, Tharsis (−20°S to 50°N, 60° to 190°W), Arabia (−5°S to 30°N, 300° to 360°W), and Elysium (10° to 30°N, 210° to 225°W). They are covered by fine (∼2–40 μm), bright (albedo > 0.27) particles, with fewer exposed rocks and coarse deposits than found elsewhere. Dust is currently deposited uniformly throughout the equatorial region at a rate of ∼40 μm/global storm. Over geologic time the rate of accumulation may vary from 0 to 250 μm/yr due to changes in atmospheric conditions produced by orbital variations. Dust deposited during global storms is subsequently removed only from dark regions, resulting in a net accumulation in the low‐inertia, bright regions. The thickness of these current dust deposits is 0.1–2 m. The thermal inertia places a lower limit of ∼0.1 m on the thickness of these deposits, while the sparse but ubiquitous presence of exposed rocks and the degree of visible mantling indicate that the thickness is less than 5 m. Dual‐polarization radar observations of a very rough texture in Tharsis are consistent with this model, with a ∼2‐m‐thick dust layer burying most of the surface rocks but permitting radar sampling of the rough subsurface. Based on their thickness and rate of accumulation, the age of these deposits is 105–106 years, suggesting a cyclic process of deposition and removal. One possible cause may be cyclic variations in the magnitude and location of maximum wind velocities related to variations in Mars' orbit. At present, perihelion and maximum wind velocities occur in the south, whereas regional dust deposits occur in the north, suggesting net transport from south to north. Orbital parameters oscillate with periods ranging from 5×104 to 106 years. The agreement between these periods and the dust deposit age suggests a possible link. At different stages in orbit evolution, maximum wind velocities will occur in the north, with subsequent erosion and redistribution of the accumulated fines. Based on this model, much of the uppermost Martian surface is very young and is being continually reworked.
Nonuniform magnetization of seamounts: A least squares approach
Authors: McNutt Marcia
Published: February 2008

Residual total magnetic field anomalies, relative to the best fitting uniform magnetization models, are large and spatially coherent over three small Tertiary seamounts on the Cocos plate. Furthermore, the paleomagnetic parameters determined from the uniform magnetization models in some cases disagree significantly with those predicted from models of absolute motion for the Cocos plate. Inversion of the sea surface magnetic anomaly using a least squares procedure which allows magnetization to vary among different sections of each seamount shows clear polarity reversals in two of the seamounts studied. The reversed zones constitute about 10% of the seamount volume. For the third seamount the nonuniform magnetization solution more correctly determines the paleomagnetic inclination and declination for the only section of the dipolar anomaly not obscured by a magnetic lineation traversing the area. The polarities indicated in the least squares solutions were confirmed by magnetic gradiometer measurements from the submersible Alvin. For all three seamounts the paleomagnetic latitudes and/or rotation angles in the nonuniform magnetization solutions changed appreciably (by as much as 20°) to agree better with plate motion models. The nonuniform magnetization models reveal that growth rates for such volcanoes approach 1000 km3/Ma, with the final phase of volcanism consisting of summit, as opposed to flank, eruptions. The largest of the three volcanoes formed off‐ridge on lithosphere at least 0.5 Ma old and may have remained active for 1 Ma. Hydrothermal activity apparently persists at least 0.7 Ma after the last major phase of volcanism.
Possible Mars brines: Equilibrium and kinetic considerations
Authors: Zent Aaron P. , Fanale Fraser P.
Published: February 2008

We calculate the fate of postulated near surface brines on Mars. No stable brine system can exist on Mars today. The integrated H2O flux from a subsurface H2O reservoir is greater than zero at all latitudes where the highest recurring temperature exceeds the eutectic of chemically reasonable brines. Nonequilibrium brine systems or brine systems in dynamic equilibrium are possible. We calculate the rate of H2O mass loss from subsurface brines as a function of latitude, depth, regolith porosity, eutectic temperature, and pore size. Some brines may exist in the near‐equatorial subsurface of Mars for periods on the order of 107 years. Seasonally variable radar reflectivity of the Martian surface has been interpreted as indicative of melting of subsurface brines (Zisk and Mouginis‐Mark, 1980), We present a model for a chemically reasonable brine that could reproduce radar results and estimate the escape rate of H2O molecules from such a brine. The presence of a low‐permeability duricrust may be required to preserve such a brine for reasonable periods and to prevent detection of an areally extensive subsurface system by the Viking MAWD instrument. A porosity no lower than 20–30% should suffice to reduce H2O escape fluxes to the required rates.
Hydrous alteration of amorphous silicate smokes: First results
Authors: Nuth Joseph A. , Donn Bertram , Deseife Rudy , Donn Allen , Nelson Robert
Published: February 2008

Results of the initial studies of the hydrous alteration of amorphous Mg‐SiO smokes indicate that although these materials readily adsorb water, the silicate structure is much more stable at 360°K than expected. We have observed drastic changes in the relative absorption strengths of the 10‐ and 20‐micron “silicate” features that appear quite rapidly at 750°K; these observations might have important implications for the interpretation of cometary dust spectra. Observations of the development of 3.4–3.5 micron features possibly due to hydrocarbons in the spectra of processed Mg‐SiO smokes have raised the exciting possibility that these amorphous condensates could act as Fischer‐f ropsch type catalysts to produce hydrocarbons in the primitive solar nebula.
Acoustic and static compression experiments on the elastic behavior of hematite
Authors: Liebermann Robert C. , Maasch Kirk A.
Published: February 2008

Ultrasonic data for a high‐purity single‐crystal of hematite (α‐Fe2O3) as a function of decreasing temperature at P = 1 bar indicate a 10% decrease in the bulk modulus K across the second‐order Morin magnetic transition at Tm = 261 K. The Morin temperature increases with pressure and occurs at 298 K near 30 kbar. Thus a theoretical pressure‐volume trajectory for the isothermal compression of hematite at 298 K has a change in slope at P = 30 kbar, reflecting the discontinuous decrease in K across the Morin transition. Static compression data for a single‐crystal specimen and for a coarse‐grained powder do indicate such a discontinuity in slope and agree very well with the theoretical P‐V trajectory. In contrast, data for fine‐grained powder or polycrystalline specimens do not reflect such magnetoelastic behavior, since they are single‐domained and their elastic properties are not influenced by domain wall‐stress interactions. Such discontinuities in the bulk modulus are difficult to observe in static compression experiments but are commonly associated with second‐order phase transitions and may help to explain anomalous compression data for other magnetic oxides.
Long‐term variations of the external and internal components of the Earth's magnetic field
Authors: Gavoret J. , Gibert D. , Menvielle M. , Le Mouël J. L.
Published: February 2008

The present study applies classical techniques of time series analysis to the separation of the external and internal signals from the geomagnetic field. We first differentiate the data with respect to time in order to get rid of the large secular variation over the long time span considered (up to 100 years). Next we filter the resulting data to eliminate short‐period components of the magnetic signals. We then proceed to identify the external signal by comparing the long‐term variations of the processed components of the geomagnetic field, as measured at different European observatories, with the long‐term variations of geomagnetic indices devised to characterize the fields from external sources. Finally, we remove the identified external signal from the geomagnetic field series and get a satisfactory estimate of the internal signal. This procedure is facilitated by the different behaviors of the two signals over the considered time span. The external signal is inherently of recurrent nature linked to solar‐terrestrial interaction; it oscillates around zero with a maximum amplitude of about 5 nT/yr. The internal signal, on the other hand, displays the characteristics of a secular trend, combining sustained monotonous behavior over periods of several decades with sudden slope variations and reversals; the total range of this internal secular signal is of the order of 50 nT/yr, far larger than any external contribution. Using the 1883–1983 magnetic series at Chambon‐la‐Forêt, the 1890–1983 series at Niemegk, and shorter series at the United Kingdom observatories of Eskdalemuir and Hartland, we have been able to get a coherent overall picture of the secular variation as measured in Europe. For instance, the first‐order time derivative of the Y (east) component essentially displays an increase in two steps from 1900 to 1925, a monotonous decrease from 1925 to 1969 with a regular steplike substructure, and a rapid increase since 1969, followed by a marked reversal of slope in 1979. These results emphasize the internal origin of the 1969 jerk and single out, in Europe, a 1979 event of opposite sign, these two features being quite reminiscent of the behavior of the secular variation during the first quarter of the century.
Extension of continental lithosphere: A model for two scales of basin and range deformation
Authors: Zuber M. T. , Parmentier E. M. , Fletcher R. C.
Published: February 2008

We develop a model for deformation in an extending continental lithosphere that is stratified in density and strength, assuming a rheology consistent with seismic focal depths and experimental flow laws. The model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust. The magnitudes of the instabilities are directly related to strength contrasts within the lithosphere, while the dominant wavelengths of necking are controlled mainly by the thicknesses of the strong layers. The results are applied to the Basin and Range Province of the western United States where two scales of deformation can be recognized, one corresponding to the spacing of ranges and the other to the width of tilt domains. A Bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized. For plausible density and strength stratifications, our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges, while that of the longer wavelength necking is consistent with the width of tilt domains. We thus suggest that Basin and Range deformation may be controlled by two scales of extensional instability. Extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension. The resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low‐angle extensional detachments.
Role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges, California: Evidence for intracontinental subduction
Authors: Sheffels Barbara , McNutt Marcia
Published: February 2008

Bouguer gravity anomalies in the Transverse Ranges of southern California show a pronounced asymmetry in north‐south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike. While individual gravity/topography profile pairs could be consistent with a number of isostatic compensation mechanisms, the unusual relationship between gravity and topography in the San Bernardino Mountains severely limits the class of acceptable models if the Bouguer gravity field is caused by flexure of the crust/mantle boundary. The simplest solution consistent with the data in the Western Transverse Ranges, the San Gabriel Mountains, and the San Bernardino Mountains models the Great Valley and Mojave provinces to the north as a 50‐km‐thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south. The best fit to the gravity data in the San Bernardino Mountains requires a terminal bending moment on the southern plate beneath the mountains. The magnitude of the terminal bending moment decreases to the west in the San Gabriel and Western Transverse Ranges. We equate the load responsible for this terminal moment with the high‐velocity, slablike feature mapped in the upper mantle beneath the Transverse Ranges using seismic tomography. The coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small‐scale convective feature.
Thermal stresses in the oceanic lithosphere: Evidence from geoid anomalies at fracture zones
Authors: Parmentier E. M. , Haxby W. F.
Published: February 2008

Models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate. However, there has so far been little direct evidence for the magnitude and distribution of thermal stresses. We present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones. Specifically, our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones. Including the effect of varying thermal bending moments, thin plate theory predicts vertical, nonisostatic displacements of the lithosphere by plate flexure. The predicted amplitude of the resulting geoid anomaly is large enough to be observed in Seasat altimeter profiles. Furthermore, the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable. The anomaly due to thermal stresses has been clearly identified in geoid profiles across the Clarion and the Udintsev fracture zones. The amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700°C, consistent with the maximum depth of seismicity in the oceanic lithosphere. The distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes.
Surface deformation due to inflation of an arbitrarily oriented triaxial ellipsoidal cavity in an elastic half‐space, with reference to Kilauea volcano, Hawaii
Authors: Davis Paul M.
Published: February 2008

Approximate expressions for surface displacements due to fluid inflation of an arbitrarily oriented ellipsoidal cavity in an elastic half‐space are found using Eshelby's elastic inclusion theory and Mindlin's half‐space point force solution. The cavity is replaced by material having the same properties as the surrounding medium. The point force distribution on its surface is determined which exerts a uniform pressure on the medium immediately outside its boundary. Then the ellipsoid and its forces may be removed and replaced by material strained to generate the same pressure without affecting the solution in the remainder of the medium. The half‐space point force solutions were found which satisfy boundary conditions exactly on the free surface but approximately on the ellipsoid. The approximation becomes exact for a deeply buried ellipsoid and is reasonably accurate if the depth to its center is greater than twice its dimension. The far‐field solution is a weighted combination of displacements from nine double forces located at the ellipsoid center. The model was applied to measured displacement on Kilauea volcano taken over a period of intrusion and eruption, with the result that a vertically elongated ellipsoidal pressure center fits the data significantly better than a spherical one. However, earthquake swarm activity spread over the zone during the inflation episode, which demonstrates that the medium does not respond elastically. The volume of the slip found by summing seismic moments is of the same order as the surface uplift, suggesting that anelastic accommodation of the intruded material by the surrounding medium is significant. The pressure center is therefore a combination of magma pressure and failed matrix. Its vertical elongation confirms the contention that vertical elongation is expected when the volcano undergoes extreme stressing beyond the elastic limit.
Orientation, relative age, and extent of the Tharsis Plateau Ridge System
Authors: Watters Thomas R. , Maxwell Ted A.
Published: February 2008

The Tharsis ridge system is roughly circumferential to the regional topographic high of northern Syria Planum and the major Tharsis volcanoes. However, many of the ridges have orientations that deviate from the regional trends. Normals to vector means of ridge orientations, calculated in 10° boxes of latitude and longitude, show that the Tharsis ridges are not purely circumferential. Intersections of vector mean orientations plotted as great circles on a stereographic net show that the ridges are not concentric to a single point but to three broad zones. These data indicate either that the Tharsis ridge system did not form in response to a single regional compressional event with a single stress center or that the majority of the ridge system did form in a single event but with locally controlled deflections in the generally radially oriented stress field. Superposition relationships and relative ages suggest that the compressional stresses that produced the ridges occurred after the emplacement of the ridged plains volcanic units (early in the Hesperian period) but did not extend beyond the time of emplacement of the Syria Planum Formation units or the basal units of the Tharsis Montes Formation (during the latter half of the Hesperian period). Comparison of topographic data with the locations of ridges demonstrates a good correlation between the topographic edge of the Tharsis rise and outer extent of ridge occurrence. Compressional deformation related to Tharsis is present out to about 5100 km from the regional topographic center located near northern Syria Planum. The innermost extent cannot be determined because the ridged plains units have been buried by more recent volcanic units, although ridge formation appears to have extended farther inward than is presently observed. Models involving a combination of isostatic stresses and stresses resulting from flexural loading appear to explain best the observed tensional features, topography, gravity, and, to a first approximation, the locations and orientations of the compressional ridges.
Limitations to the application of electrostatic feedback in gravity meters
Authors: Hugill A. L. , Valliant H. D.
Published: February 2008

Application of an electrostatic restoring force to the mass of a gravity meter is equivalent to introducing a springlike force working in opposition to the mainspring. This destabilizing effect increases with feedback force and for astatic instruments becomes an important factor in design for operating ranges as small as 5 mGal. With short‐period instruments the operating range can be extended by 2 or 3 orders of magnitude before the destabilizing effect becomes important. Theoretical results are verified by experiments conducted on a LaCoste and Romberg model G gravity meter.
Recent vertical crustal movements near Charleston, South Carolina
Authors: Poley Catherine M. , Talwani Pradeep
Published: February 2008

Reevaluation of first‐order leveling data for the South Carolina Coastal Plain shows evidence of recent vertical crustal movements. These movements do not appear to be due to internal systematic errors in the data but rather to external processes such as near‐surface water withdrawal or neotectonic movements. Dislocation modeling of the observed movements near Charleston provided a nonunique solution; thus the geodetic data do not provide a constraint on the tectonic model for the Charleston area. The movements observed near Charleston are aseismic, and any tectonic movements are masked by the effects of water withdrawal or are buried within the noise level.
A densely spaced array of sea level monitors for the detection of vertical crustal deformation in the Shumagin Seismic Gap, Alaska
Authors: Beavan John , Hurst Ken , Bilham Roger , Shengold Laurence
Published: February 2008

We have installed a network of sea level gauges with ∼40 km spacing in the Shumagin Islands in order to detect relative vertical motion, in particular, possible crustal motion precursory to an expected major earthquake. This required the development and deployment of a pressure sensor sea level gauge suitable for installation on remote and harsh coastlines. Data are collected in near‐real‐time via satellite, both in order to exploit fully any precursors that may be observed and to provide continuous information on the status of the instruments. Using Wiener filtering techniques, we have determined conservatively that no relative vertical crustal motion >0.1 m between stations has occurred during 1981–1985. This is consistent with independent geodetic leveling and trilateration data, though evidence exists for more rapid deformation during 1978–1980. The sea level data collected so far have an rms noise level of ∼0.04 m after processing; this is limited in part by the pressure sensor and associated electronics. Improved pressure gauges with better long‐term stability have recently been installed. A short‐baseline tiltmeter operating in a tunnel has demonstrated that suitably designed and located land‐based tiltmeters have a lower noise level, and hence better precursor detection characteristics than the sea level gauges, at monthly and shorter periods.