bluuebunny/crossref_metadata_2025 huggingface dataset:
Uranium‐lead isotopic ages from the Sierra Nevada Batholith, California
Authors: Chen James H. , Moore James G.
Published: February 2008

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U‐Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc‐alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U‐Pb ages of the Giant Forest‐alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U‐Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U‐Pb, sphene U‐Pb, hornblende K‐Ar, and biotite K‐Ar suggests that zircon U‐Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U‐Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K‐Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.
The distribution of natural fractures and joints at depth in crystalline rock
Authors: Seeburger Donald A. , Zoback Mark D.
Published: February 2008

This paper presents the results of studies of the natural fracture distribution encountered in 10 test wells drilled in three areas of the United States. Seven of the wells were drilled to depths of 200–250 m, while three were drilled to depths of about 1 km. Using an ultrasonic borehole televiewer, fracture depths, strikes, and dips were determined. Steeply dipping fractures were found throughout each of the wells, and in general, few horizontal fractures were observed. Statistically significant fracture pole concentrations were found for each well which were basically invariant with depth, although some variation of fracture orientation with depth was found in two wells. The significant fracture orientations were not found to be the same in wells only several kilometers apart in a given region. In none of the wells did the number of observable fractures decrease markedly with increasing depth. No simple relationship of fracture orientation or fracture density with major structural features such as the San Andreas fault were observed, and no simple relation between the significant fracture orientations and either past or present regional stress fields could be determined.
Mean Earth ellipsoid determined from SEASAT 1 altimetric observations
Authors: West Gladys B.
Published: February 2008

The radius and the origin of a mean earth ellipsoid have been determined using geoid heights derived from SEASAT‐1 radar altimetric observations of the ocean surface and potential coefficients. While assuming that the altimetric geoid is the best available, adjustments are made to the geoid derived from potential coefficients so that the geoid differences are minimized. This adjustment is then applied to the ellipsoid which best fits the geoid derived from potential coefficients. Solutions using two different sets of altimetric geoid heights and geoid heights based on the DOD WGS 72 and GEMl0B potential coefficients gave a mean ellipsoid with radiusa= 6378134.9 m and origin shifts less than l m in thexandycomponents and approximately −2.5 m in thezcomponent. A description of the geoid height data used in the solutions for the ellipsoid parameters, results of the solutions, and a summary are presented.
Theory for thermal microwave emission from a homogeneous layer with rough surfaces containing spherical scatterers
Authors: Shin R. T. , Kong J. A.
Published: February 2008

The radiative transfer theory is used to solve the problem of thermal microwave emission from a homogeneous layer containing spherical scatterers. To model volume scattering effects, we use the Mie phase functions. To model rough top and bottom interfaces, we use the bistatic coefficients for a randomly rough surface obtained using a combination of Kirchhoff theory and geometrical optics approach. Because the bistatic coefficients violate energy conservation, there is ambiguity in the emissivity. However, using two alternate formulations, the upper and lower limits of the emissivity are calculated. The effect of a rough surface is incorporated into the radiative transfer theory by modifying the boundary conditions for the intensities. The radiative transfer equations are then solved numerically by using a Gaussian quadrature method, and the results are illustrated by plotting the brightness temperatures as a function of observation angle for different polarizations. It is shown that the presence of a bottom rough surface increases the brightness temperature except at high angles for vertical polarization. The rough surface at the top boundary makes the angular behavior flatter and displays smaller differences between the horizontal and vertical polarizations.
Lithology and structure of the volcanic sequence in eastern Iceland
Authors: Robinson Paul T. , Mehegan James , Gibson Ian L. , Schmincke Hans‐Ulrich
Published: February 2008

The Reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99.7%. Subaerial lava flows comprise 54% of the core, dikes 41%, and clastic rocks about 5%. In addition, a 1.3‐km‐thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled. The lava flows are chiefly low‐magnesia basalts and ferrobasalts with lesser amounts of icelandite, basaltic andesite, and olivine tholeiite. In the drill core, complete flows range from 1.2 to 19.7 m in vertical thickness and average 7.1 m; in the exposed section, flow thicknesses range from 1 to 30 m and average 5.2 m. Most of the flows are characterized by an upper scoriaceous zone 1–3 meters thick, grading downward through a zone of fractured basalt into a massive flow interior. Some flows also have a thin basal breccia. Olivine tholeiites and some low‐magnesia basalts are commonly mottled, whereas ferrobasalts and icelandites have well developed flow banding. Most of the flows are fine‐ to medium‐grained, aphyric to sparsely phyric, and moderately vesicular. A few are moderately plagioclase phyric; others have scattered phenocrysts of clinopyroxene and olivine. Over 100 intrusive units are recognized in the core, and they span approximately the same compositional range as the flows. They are characterized by having steeply inclined chilled contacts, a uniform massive aspect, and an absence of vesicles or brecciation. Most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures. Many of the dikes are also aphyric, but some contain notable plagioclase or olivine phenocrysts. Groundmass textures range from fine‐grained, intersertal to coarse‐grained, ophitic. Clastic units occur typically as thin, varicolored layers between lava flows, rarely exceeding 0.5 m in thickness. These are chiefly fine‐ to coarse‐grained, poorly‐ to well‐bedded lithic tuffs showing varying degrees of reworking. Most are silicic to intermediate in composition, but basaltic units are also present. Two rhyolitic ash flow tuffs occur in the section, one of which is 30 m thick. We interpret the lavas to be the flanking portions of broad low‐angled central volcanoes centered on silicic complexes. Some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms. Most of the dikes were probably intruded laterally into the lava pile; thus they do not represent direct vertical feeders from deep magma bodies.
Secondary mineralization and hydrothermal alteration in the Reydarfjordur drill core, eastern Iceland
Authors: Mehegan James M. , Robinson Paul T. , Delaney John R.
Published: February 2008

Deep crustal drilling in eastern Iceland has allowed study of a fossil hydrothermal system at a constructive plate margin. The drilled sequence consists of partly to completely altered subaerial lava flows, basaltic dikes, and minor clastic material. Alteration and secondary mineralization are most intense in the flow top breccias where water/rock ratios are presumed to have been the highest. In the upper portion of the cored sequence (to a depth of about 1200 m) alteration is characterized by the deposition of clay minerals ± calcite ± quartz ± laumontite into open spaces such as vugs and vesicles. Low‐temperature zeolites, such as stilbite, epistilbite, mordenite, and heulandite, are also present but are restricted to the upper 500 m of the drill core. Below 1200 m, alteration is characterized by the dissolution and replacement of both primary minerals and earlier authigenic minerals, followed by partial filling of dissolution cavities. Early mineral assemblages consist of epidote ± quartz ± prehnite ± chlorite ± albite, and a later superimposed assemblage consists of calcite + laumontite ± anhydrite. Authigenic sphene, pyrite, chalcopyrite, pumpellyite, actinolite, and wairakite also occur sporadically in the cored sequence. Secondary mineral assemblages and temperature measurements of fluid inclusions suggest a maximum temperature of alteration of about 300°C. Fluid inclusion compositions indicate that the geothermal fluid was meteoric water with very low salinities and high calcium activities. Iron activities and oxygen fugacities were highest in the deeper portions of the systems. The mineral paragenesis suggests that the fluid composition, temperature, and PCO2 varied significantly with time. The thermal energy for the geothermal system was probably derived from a high‐level magma chamber associated with nearby Thingmuli volcano. Local contact metamorphism, indicated by the formation of garnet, occurred during late stage emplacement of dikes into the lava pile. Chlorite ± calcite ± laumontite assemblages were also deposited along subvertical fractures at this time.
Alteration in the IRDP drill hole compared with other drill holes in Iceland
Authors: Kristmannsdóttir Hrefna
Published: February 2008

The overall alteration pattern in the drill hole at Reydarfjördur is very similar to alteration patterns observed in Icelandic geothermal areas and in low‐grade metamorphosed basalts in deep crustal sections elsewhere in Iceland. However more detail is obtained by the study of the IRDP drill core than by study of drill cuttings sampled in previous drill holes in Iceland. A comparatively high fossil thermal gradient is obtained at Reydarfjördur by a combination of mineral stability data and the observed occurence of secondary minerals. This high gradient is consistent with the measured dike dilation at the drill site and the location of the drill site adjacent to a central volcano.
Hydrofracturing stress measurements in the Iceland Research Drilling Project drill hole at Reydarfjordur, Iceland
Authors: Haimson Bezalel C. , Rummel Fritz
Published: February 2008

Two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the Iceland Research Drilling Project deep hole at Reydarfjordur, east Iceland. As indicated by the continuously extracted drill core, the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes. The density of the basalt was used to estimate the vertical stress σV (in megapascals) at depth D (in meters): σV = 0.027D. In the 33‐ to 270‐m depth range the horizontal principal stresses (σHmax and σHmin) as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress: σHmin = 2.1 + 0.016D; σHmax = 3.3 + 0.016D. In the 300‐ to 550‐m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m. Conditions favoring normal faulting (σV σHmax σHmin) were established in the 540‐ to 580‐m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of σHmax and σHmin This would be in accord with the predominant fault type in east Iceland. The relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near‐surface high compressive stresses previously determined by Hast (1973) in east Iceland. The large increase in horizontal stress in the range of 300‐ to 500‐m depth is not understood. A correlation appears to exist between the zone of almost unintruded thick lava flows (200–500 m) and that of high stresses. An intuitive model suggesting ‘locked in’ stresses in this zone is proposed. The hydrofracturing‐determined overall mean direction of σHmax is N50°E, while a more selective sampling of results yields N40°E. The measured stress directions appear to be reasonably compatible with the local structure such as dike direction (NNE) and lava dip (WSW). Significantly, σHmax direction is subparallel to the N20°E trend of the nearest fissure swarms in the axial rift zone. This almost coincidence of directions, like the measured stress magnitudes, is also in agreement with the model of spreading lithospheric plates being pulled apart.
Viscoelastic‐gravitational deformation by a rectangular thrust fault in a layered Earth
Authors: Rundle John B.
Published: February 2008

Previous papers in this series have been concerned with developing the numerical techniques required for the evaluation of vertical displacements which are the result of thrust faulting in a layered, elastic‐gravitational earth model. This paper extends these methods to the calculation of fully time‐dependent vertical surface deformation from a rectangular, dipping thrust fault in an elastic‐gravitational layer over a viscoelastic‐gravitational half space. The elastic‐gravitational solutions are used together with the correspondence principle of linear viscoelasticity to give the solution in the Laplace transform domain. The technique used here to invert the displacements into the time domain is the Prony series technique, wherein the transformed solution is fit to the transformed representation of a truncated series of decaying exponentials. Purely viscoelastic results obtained are checked against results found previously using a different inverse transform method, and agreement is excellent. The major advantage in using the Prony series technique is that deformations can be computed for arbitrary time intervals. A series of results are obtained for a rectangular, 30° dipping thrust fault in an elastic‐gravitational layer over viscoelastic‐gravitational half space. Time‐dependent displacements are calculated out to 50 half space relaxation times τa, or 100 Maxwell times 2τm = τa. Significant effects due to gravity are shown to exist in the solutions as early as several τa. The difference between the purely viscoelastic solution and the viscoelastic‐gravitational solutions grows as time progresses. Typically, the solutions with gravity reach an equilibrium value after 10–20 relaxation times, when the purely viscoelastic solutions are still changing significantly. Additionally, the length scaling which was apparent in the purely viscoelastic problem breaks down in the viscoelastic‐gravitational problem. Two independent length scales, one of which changes with time, are now seen to characterize the problem.
Single‐crystal X ray study of γ Mg2SiO4
Authors: Sasaki Satoshi , Prewitt Charles T. , Sato Yosiko , Ito Eiji
Published: February 2008

The crystal structure of the spinel polymorph of Mg2SiO4 has been refined using a single crystal synthesized at 220 kbar and 1000°C. The u parameter of the structure with a = 8.0649(1) Å is 0.3685(6). Although the refinement indicates some cation disorder in Mg2SiO4 spinel, the results are not definitive because of the similarity in the scattering factors of Mg and Si. The average Si‐O and Mg‐O distances are 1.655(4) and 2.070(4) Å, respectively. The final R and Rw values are 0.048 and 0.040 for 41 reflections up to sin θ/λ = 1.0.
Identification of a Precambrian Rift through Missouri by digital image processing of geophysical and geological data
Authors: Guinness E. A. , Arvidson R. E. , Strebeck J. W. , Schulz K. J. , Davies G. F. , Leff C. E.
Published: February 2008

Digital free air and Bouguer gravity anomaly images have been constructed from approximately 287,000 station readings for the region bordered by 25°–49°N latitude and 80°–110°W longitude. The technique used to interpolate between station locations was based on a two‐dimensional spatial filter, where the average of the anomaly values located within the filter area was computed. The images contain as many as 256 contours (values in byte variable), so that subtle anomaly patterns can be identified and traced with much greater certainty than on most contour maps. A newly discovered feature in the midcontinent is a gravity low that begins at a break in the midcontinent gravity high in SE Nebraska, extends across Missouri in a NW‐SE direction, and intersects the Mississippi Valley graben to form the Pascola arch. The anomaly varies from 120 to 160 km in width, extends about 700 km, and is best expressed in southern Missouri, where it has a Bouguer amplitude of approximately −34 mGal. The magnitude of the anomaly cannot be explained on the basis of a thickened section of Paleozoic sedimentary rock. The gravity data and the sparse seismic refraction data for the region are consistent with an increased crustal thickness beneath the gravity low. Some of the discrete positive magnetic anomalies in Missouri are located along the borders of the gravity low. Digitally enhanced thermal infrared images from the Heat Capacity Mapping Mission show a distinct alignment of linear structures with the gravity feature. The linear features in some cases correspond to mapped high‐angle normal faults, to drape folds over relief within the Precambrian basement, and in some cases to extensions of mapped structures. The gravity anomaly also cuts across the major Precambrian boundary in SE Missouri, marking the change from older, sheared granites and metasedimentary rocks to younger granites and rhyolites. Given the cumulative evidence, the gravity anomaly is probably the present expression of a failed arm of a rifting event, perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern Missouri.
Source mechanism of volcanic tremor
Authors: Ferrick M. G. , Qamar A. , St. Lawrence W. F.
Published: February 2008

Low‐frequency (<10 Hz) volcanic earthquakes originate at a wide range of depths and occur before, during, and after magmatic eruptions. The characteristics of these earthquakes suggest that they are not typical tectonic events. Physically analogous processes occur in hydraulic fracturing of rock formations, low‐frequency icequakes in temperate glaciers, and autoresonance in hydroelectric power stations. We propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low‐frequency volcanic earthquakes (tremor). The fluid dynamic source mechanism explains low‐frequency earthquakes of arbitrary duration, magnitude, and depth of origin, as unsteady flow is independent of physical properties of the fluid and conduit. Fluid transients occur in both low‐viscosity gases and high‐viscosity liquids. A fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid, material properties, geometry and roughness of the conduit, and boundary conditions. To demonstrate the analytical potential of the fluid dynamic theory, we consider a single‐phase fluid, a melt of Mount Hood andesite at 1250°C, in which significant pressure and velocity variations occur only in the longitudinal direction. Further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations. For a simple, constant pressure reservoir‐conduit orifice fluid system, a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour. Fluid kinematic viscosity (0.412 m2/s) is both included and neglected in the analysis to illustrate its effect upon system damping. Tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures. The entire volcanic fluid system does not generally experience a transient flow condition at a given time. Closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system. Total fluid movement may not be accurately monitored by tremor data alone, since steady state and slowly changing flows are aseismic.
Gravity field model of mars in spherical harmonics up to degree and order eighteen
Authors: Balmino G. , Moynot B. , Valès N.
Published: February 2008

A new model of Mars gravity field in spherical harmonics up to eighteenth degree and order has been computed from all available Doppler tracking data of Mariner 9 and Viking 1 and 2 orbiters. It represents an achievement in the knowledge of Mars global gravitational potential, since no more data will be available for such a derivation before the next mission to the planet. The model has been extensively tested from the point of view of orbit representation over periods of time ranging from a few hours to 12 days, and it is currently used for geophysical studies of the Martian lithosphere.
Explosive volcanism on Hecates Tholus, Mars: Investigation of eruption conditions
Authors: Mouginis‐Mark Peter J. , Wilson Lionel , Head James W.
Published: February 2008

Evidence is presented supporting the identification of a discrete, well‐preserved air fall deposit generated by explosive volcanism on Mars. The deposit is located immediately to the west of the summit caldera of Hecates Tholus (32°N, 209°W), the northernmost of the three volcanic constructs in the Elysium Planitia region. The absence of superposed impact craters larger than the resolution limit of the Viking images (40 m per pixel) indicates a very young age for this eruption, which evidently postdates even the most recent collapse episode of the Olympus Mons caldera (∼300 m.y. B.P.; Neukum and Hiller (1981)). From the distribution of craters smaller than 2 km in diameter, it appears that the air fall material mantles an area about 50×75 km in extent to an estimated thickness of about 100 m. By adapting numerical models for terrestrial volcanic eruptions to the Martian environment, these dimensions imply an eruption cloud height of about 70 km. In order to attain such an altitude, the mass eruption rate must have been ∼107 kg/s and the volatile content either ∼1 wt % for H2O or ∼2 wt % for CO2. Although it is not possible to distinguish between silicic and basaltic volcanism on Hecates, were the volatile species CO2, the depth of most recent storage prior to eruption of the magma would have to be 50–100 km (due to the solubility of CO2 in the melt). If H2O were the driving volatile, a minimum depth of storage would be 0.2–4 km, thereby permitting the possibility of ground water assimilation by the magma.
Rate of wind abrasion on Mars
Authors: Greeley Ronald , Leach Rodman N. , Williams Steven H. , White Bruce R. , Pollack James B. , Krinsley David H. , Marshall John R.
Published: February 2008

Estimation of the rate of aeolian abrasion of rocks on Mars requires knowledge of (1) particle flux, (2) susceptibilities to abrasion of various rocks, and (3) wind frequencies on Mars. Fluxes and susceptibilities for a wide range of conditions were obtained in the laboratory and combined with wind data from the Viking meteorology experiment. Assuming an abundant supply of sand‐sized particles, estimated rates range up to 2.1×10−2 cm of abrasion per year in the vicinity of Viking Lander 1. This rate is orders of magnitude too great to be in agreement with the inferred age of the surface based on models of impact crater flux. The discrepancy in the estimated rate of abrasion and the presumed old age of the surface cannot be explained easily by changes in climate or exhumation of ancient surfaces.We consider the primary reason to be related to the agents of abrasion. Either windblown grains are in very short supply, or the grains are ineffective as agents of abrasion. At least some sand‐sized (∼100 μm) grains appear to be present, as inferred from both lander and orbiter observations. High rates of abrasion occur for all experimental cases involving sands of quartz, basalt, or ash. However, previous studies have shown that sand is quickly comminuted to silt‐ and clay‐sized grains in the martian aeolian regime. Experiments also show that these fine grains are electrostatically charged and bond together as sand‐sized aggregates. Laboratory simulations of wind abrasion involving aggregates show that at impact velocities capable of destroying sand, aggregates form a protective veneer on the target surface and can give rise to extremely low abrasion rates.
Spectral evidence for the mineralogy of high‐albedo soils and dust on Mars
Authors: Singer Robert B.
Published: February 2008

Because of the lack of direct mineralogic data available for Mars, spectral remote sensing techniques and, in particular, earth‐based reflectance spectroscopy remain the primary source of this information. Presented here are laboratory observations which further constrain the mineralogy and origin of the high‐albedo soils and dust. Earth‐based observations show the reflectance spectra of classical bright regions to be fairly uniform, with a strong Fe3+ → O2− charge transfer absorption edge extending from the near UV through the visible. This absorption is relatively smooth, unlike those observed for crystalline ferric oxides which have superimposed Fe3+ crystal field bands. Dilution of ferric oxide with a spectrally more neutral medium (montmorillonite) weakens the charge transfer and crystal field absorptions together and does not serve to make the ferric oxide spectrum more Mars‐like (smoother). Nontronite (ferric‐iron‐bearing smectite clay) was also investigated for spectral agreement with telescopic observations. Pure nontronite has Fe3+ absorptions analogous to but different from ferric oxides. As in the previous case, the absorptions are very distinct and inappropriate for Mars. Admixture of neither montmorillonite nor ferric oxide serves to improve the nontronite spectrum. The conclusion is that nontronite is not a major component of martian soils, although the presence of other iron‐poor clays cannot be totally excluded based on currently available observational data. The best spectral analogs known for high‐albedo soil and dust are a specific type of palagonite from Hawaii: X‐ray amorphous weathering products of mafic volcanic glass. The indication is therefore that ferric iron on Mars is likely to occur in poorly defined crystallo‐graphic sites such as found in these amorphous materials. These materials form slowly under semiarid conditions at ambient temperature. Even low‐temperature hydrothermal alteration of glass might provide enough ion mobility to favor some formation of crystalline clays over amorphous gels and may therefore not be the primary mechanism responsible for soil formation on Mars. The amorphous Hawaiian soils exist metastably for thousands of years on Earth. Similar materials on Mars would be expected to survive considerably longer under the present cold and dry climatic conditions.
Remote sensing evidence for regolith water vapor sources on Mars
Authors: Huguenin Robert L. , Clifford Stephen M.
Published: February 2008

A survey of ground‐based and spacecraft observations at visible wavelengths of condensate activity in the Martian atmosphere is presented. The observations reveal that Solis Lacus has been a center of condensate activity throughout the observational record, along with sites in Noachis‐Hellespontus and in border regions of Syrtis Major. The condensate activity did not appear to be controlled by the distribution of aerosol dust, surface albedo, thermal inertia, or topography. Rather, the patterns of cloud and fog occurrence, together with observed and inferred surface winds, point to these sites as primary H2O vapor source regions for condensate development. Specifically, it is proposed (1) that the unusual distribution of condensates around Solis Lacus at the onset of the 1973 dust storm may have resulted from the transport of moisture from the central plume site in northern Solis Lacus (near Sinai Planum) by prevailing daytime winds and nighttime drainage winds, (2) that the recurrent winter Tharsis and Hellas condensate features may have developed from moisture derived primarily from the upwind Solis Lacus and Noachis‐Hellespontus source regions, respectively, and (3) that the recurrent summer Elysium clouds may have developed as a result of moisture from upwind Syrtis Major border sites. The estimated outgassing rate from Solis Lacus during 1973 was ∼10−1 km3 yr−1, and a similar rate is suggested for the Noachis‐Hellespontus site. These rates would probably not have been high enough to be detected by the Viking MAWD experiment, but they extrapolate to the possible loss of a meter of ice from the sites in only 103 years. It is proposed that the ice is present in the surface layer and that it could undergo melting during a portion of the day over an appreciable fraction of the year. Radar, thermal, and water vapor data would be consistent with the possible occurrence of soil moisture at these sites.
Microearthquakes in the Black Smoker Hydrothermal Field, East Pacific Rise at 21°N
Authors: Riedesel Mark , Orcutt John A. , MacDonald Kenneth C. , McClain James S.
Published: February 2008

In July and August 1980, an array of five ocean bottom seismographs was deployed within 3 km of the 350°C hydrothermal vents at the Rivera submersible experiment (RISE) site at 21°N, on the East Pacific Rise. Two of these instruments were placed within 600 m of the vents, using a transponder navigation network. The array detected four basic types of events. The first type consisted of local, very small microearthquakes. Locations obtained for 11 of these events place three within 1 km of the vents, with the others elsewhere along the rise crest. They appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area. A study of the S‐P times of this type indicates a maximum hypocentral depth of 2–3 km, implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents. The second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor. The frequency of these events falls in the 1–5 Hz range and are similar in appearance to those seen at Mount St. Helens prior to and during its May 1980 eruption. They may be either hydrothermal or volcanic in origin. The third type of event produced a very monochromatic, high‐frequency seismogram, with the energy concentrated at 20 Hz. These events also appear to have a local origin. The particle motion of these earthquakes shows almost pure Stonely wave propagation, and they appear to originate from a regularly oscillating dilatational source. The nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood. The fourth type of earthquake recordings were regional events, apparently from the nearby Rivera and Tamayo fracture zones The low‐frequency acoustic noise in the water was 16–64 times higher within 300 m of the vents than it was 2 km away. This implies that the black smoker vents may be generating a large acoustical disturbance as the 350°C water pours into the ocean. The observations are among the first of seismicity on moderate to fast spreading ridges, and many of the events appear to be related to the hydrothermal activity found there.
Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study
Authors: Reasenberg P. , Ellsworth W. L.
Published: February 2008

Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14‐km portion of the northeastern sheet between 4‐ and 10‐km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3‐ and 7‐km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements.

Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐006; $1.00. Payment must accompany order.
Viscosity of the lower mantle as inferred from rotational data
Authors: Yuen David A. , Sabadini Roberto , Boschi Enzo V.
Published: February 2008

Until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts, and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters, such as those associated with the surface loading. In this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth's rotation axis with theoretical results from a layered viscoelastic, rotating earth, which has been subjected to glacial forcing. Our model, consisting of an elastic lithosphere, a two‐layer, adiabatically stratified viscoelastic mantle, and an inviscid core, is essentially analytical, and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon. Both sets of rotational data, polar wander and nontidal deceleration of the length of the day, have been employed to constrain the viscosity structure. On the basis of this type of investigation, the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4×1022 P. Our results, which just rely on the second degree harmonic of the strain field, also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere. This value, ranging between 130 and 190 km, suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle.
Contents of natural radioactive nuclides in soil in relation to their surface area
Authors: Megumi Kazuko , Oka Takashi , Yaskawa Katsumi , Sakanoue Masanobu
Published: February 2008

Concentrations of uranium series nuclides (238U, 226Ra, 210Pb), thorium series nuclides (228Ra, 212Bi), and potassium in soil of a granitic belt were examined in relation to their surface area of soil, clay mineral composition, and the SiO2 and Fe contents. In soil which is less weathered and has a surface area of around 10 m2/g, the 238U, 226Ra, 210Pb, 228Ra, and 40K contents increase with an increase the SiO2 content related to the elemental compositions of their parents rocks. In homologous soils resulting from progressive weathering, the 238U, 226Ra, 210Pb, and 228Ra contents are found to increase with increasing surface area. For an homologous soil which is free from the clay consisting of three‐layer units, the 40K content decrease with increasing surface area. However, in an homologous soil which contains a clay consisting of three‐layer units, the 40K content is not surface area dependent.
The Apollo 14 Regolith: Chemistiy of cores 14210/14211 and 14220 and soils 14141, 14148, and 14149
Authors: Laul J. C. , Papike James J. , Simon Steven B.
Published: February 2008

Chemical data for 31 major, minor, and trace elements were obtained by instrumental neutron activation analysis (INAA) for bulk and size fractions of drive tubes 14210/11 (39 cm) and 14220 (16.5 cm) and soils 14141, 14148, and 14149. Both cores are KREEPy and homogeneous in chemical composition. The meteoritic component varies from 3 to 7% in both cores. The heterogeneous Ni profiles suggest little in situ mixing of the cores since their emplacements. The bulk soils on the average contain 20% low‐K‐Fra Mauro basalt (LKFM) (62295), 10 to 15% mare basalt ( 14072), 6%anorthosite‐norite‐troctolite (ANT) and 59% high‐K KREEP. There are no significant intrasite chemical variations at the Apollo 14 site. Among the grain sizes, the coarse 1000–90, 90–20, and 20–10 μm fractions are similar in chemistry, but quite different from the < 10 μm fine fractions. The < 10 μm fine fractions comprise 5 to 15%of the bulk soils and are consistently more feldspathic and enriched in incompatible (LIL) elements relative to the coarse fractions. In the mature soils, agglutinates dominate the chemistry of the coarser fractions, and the similarity of LIL contents between the 1000–90 μm fractions and the < 10 μm fine fractions strongly suggests that agglutinates are derived from the fine fraction, which supports the F3 model of Papike et al [1981] Relative to the bulk soil, the < 10 μm fine fraction is enriched in Al2O3, CaO, Na2O, K2O, Ba, Sr, La(REE), Eu, and Th and depleted in MgO, FeO, Mn, and Sc. The observed varying degrees of enrichments and depletions can largely be explained by simple comminution of inesostasis and feldspar and their preferential incorporation into the finest fraction. Lateral mixing over short distances, simple comminution, and vertical mixing of the regolith are the dominant soil‐forming processes.
I‐XE ages of individual Bjurbole chondrules
Authors: Caffee M. W. , Hohenberg C. M. , Swindle T. D. , Hudson B.
Published: February 2008

We have obtained initial iodine isotopic compositions for 10 individual chondrules from the meteorite Bjurbole, with a total range in well‐determined values of 6.5%, which would correspond to an age difference of 1.6 m.y. These results can be interpreted as distinct formation ages, metamorphic ages or isotopic inhomogeneities in the early solar system, and can serve to constrain models for chondrule formation. Two of the chondrules show evidence of either isotopic inhomogeneties or disturbances of their I‐Xe systems.
Strain dependent attenuation: Observations and a proposed mechanism
Authors: Stewart Robert R. , Toksoz M. Nafi , Timur A.
Published: February 2008

The measured attenuation (Q−1) of a rock is a function of a number of parameters, one of those being the applied strain amplitude. It is important to understand the effect that strain amplitude has on Q−1 for several reasons: different measurement techniques use differing strain amplitudes and may measure a dissimilar Q−1, near source (large strain) wave propagation may behave highly non‐linearly, and the strain amplitude dependence can provide insight into the attenuation mechanism. A physical model based on the contact friction between crack surfaces in the rock has been developed to describe rock deformation and dissipation under large applied strain. The three‐dimensional crack surfaces are characterized by a statistical distribution of asperity heights. The sliding contact of these spherically‐tipped asperities dissipates frictional energy. Hertzian theory is applied to the average asperity contact and predicts that the large strain attenuation is given by Q−1 = kζε/P4/3, where k is a constant consisting of the matrix elastic parameters, ζ is the crack density, ε is the strain amplitude, and P is the confining pressure. The total attenuation measured appears to be the sum of this strain dependent term and a strain independent term. The results of ultrasonic pulse transmission experiments are compared with the model's prediction. Both P and S waves with strain amplitudes from 10−8 to 10−5 were employed. Frequencies from 0.4 to 1.5 MHz were used in conjunction with rock confining pressures of 2 to 580 bars on dry Berea sandstone and lucite samples. The spectral ratio method and rise time technique were applied to deduce the Q−1 values. The observed data and other observations from the literature compare well with the model's prediction for the dependence of Q−1 on large strain amplitude, crack density, and pressure.
Late Precambrian Keweenawan Asymmetric Polarities as analyzed by axial offset dipole geomagnetic models
Authors: Nevanlinna H. , Pesonen L. J.
Published: February 2008

The Great Logan Paleomagnetic Loop has been interpreted to be a paleomagnetic signature of the North American plate motion during the Keweenawan igneous activity (1200–1000 Ma). However, the presence of three successive asymmetric polarities, along with new geochemical data from the Mamainse Point area, makes this interpretation untenable. A new interpretation is presented according to which the asymmetric polarities represent long‐term zonal nondipole geomagnetic field components. Wilson's single‐dipole offset model can in general explain the inclinational asymmetries, but it yields intensity ratios (reversed/normal) significantly lower than those observed. A coaxial two‐dipole model is proposed that better takes into account the observed intensity data. This model consists of a geocentric dipole and an offset dipole located at the core‐mantle boundary. The offset dipole simulates the long‐term zonal average of the spherical harmonic nondipole field. If the geocentric dipole reverses while the offset dipole retains its constant polarity, nonantiparallel reversal results. A method is presented by which the depth of the offset dipole and the ratio of the dipole strengths can be determined using only the observed inclinations. The two‐dipole model suggests that inclinational asymmetries are due to a different nondipole/dipole ratio during the normal and the reversed polarity. The most probable variant of the two‐dipole models predicts that this ratio is 35% and 49% for the normal and the reversed polarity, respectively. A new interpretation for the evolution of the Logan Loop is described according to which at least 50% of the hairpin‐shaped loop is produced by oscillation of the dipole strength ratio. Apparent polar wander (APW) is still evident, but the shape of the loop and the APW speed along the loop is reduced. The two‐dipole model is tested by worldwide paleomagnetic data of Keweenawan age.
A technique for reducing low‐frequency, time‐dependent errors present in network‐type surveys
Authors: Cloutier James R.
Published: February 2008

At each node of a network‐type survey, two surveyed values are obtained. In the absence of errors, these two values should be equal, providing that the quantity being surveyed is, for all practical purposes, time independent. When a low‐frequency, time‐dependent error enters the system, it can be indirectly observed in the form of surveyed‐value differences occurring at the network nodes. This paper describes a technique for reducing such an error. The method consists of using a conjugate gradient projection algorithm to find the unbiased, discrete function of minimum weighted variation which produces the observed nodal differences. After inspecting the discrete solution, a suitably chosen continuous function can be fitted to the discrete data and used to reduce the time‐dependent error. This technique has been successfully used to reduce the radial ephemeris error present in SEASAT geoid height data (Cloutier, 1981). Future applications include the removal of the diurnal variation present in magnetic surveys and the removal of the nonlinear gravimeter drift present in gravity surveys.
Heat transfer and intraplate deformation in the central Indian Ocean
Authors: Geller Carol A. , Weissel Jeffrey K. , Anderson Roger N.
Published: February 2008

Nineteen new heat flow measurements made across deformed oceanic lithosphere in the Central Indian Ocean, and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region. Many of the temperature‐depth profiles are nonlinear. Upward convection of water is the most likely explanation for the curvature of the temperature profiles, since other possible causes, including variations in bottom water temperatures, conductivity changes with depth in the sediments, and experimental error, can be eliminated. This interpretation reguires water velocities of the order of 7×10−8 m/s, which is unusual because the lithosphere is relatively old (72–82 m.y.) and a thick sedimentary cover (1−2.5 km) is present. These observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment‐water interface. We infer that extra heat is being generated at shallow depths (perhaps less than 35 km) in the plate, although the specific mechanism by which deformational energy is converted into heat is difficult to determine.