bluuebunny/crossref_metadata_2025 huggingface dataset:
An interpretation of the alteration assemblages at Sulphur Springs, Valles Caldera, New Mexico
Authors: Charles R. W. , Buden R. J. Vidale , Goff Fraser
Published: February 2008

Sulphur Springs, located near the western edge of the Valles caldera, north central New Mexico, is the only vapor‐dominated surface manifestation of any significance found in the Valles. The percolating acid sulfate waters and fumarolic gases have leached and oxidized the country rock near the hot springs to a bleached wasteland. The near‐surface mineral assemblages at 90°C are restricted to one and two phases occurring in contact with a variety of solutions and gases. The conditions of local equilibrium at 90°G are approximately halotrichite + alunogen, log fO2 −45, log fS2−12, pH 0.3; szomolnokite + pyrite, log fO2 −46, log fS2−16, pH 0.8; alunite, log fO2−50, log fS2−10, pH 1–2; kaolinite + sulphur, log fO2 −53, log fS2 −10, pH 1–2, log aK+ = 0.0012 m, log aSiO2 = 0.0033 m; and pyrite, log fO2 −60, log fS2 −19, pH 4.3. This study has demonstrated that local equilibrium is closely approached even under the low‐temperature and ‐pressure conditions found near the surface.
Constraints on the age of heating at the Fenton Hill Site, Valles Caldera, New Mexico
Authors: Harrison T. Mark , Morgan Paul , Blackwell David D.
Published: February 2008

Subsurface samples and temperature data from the Fenton Hill drilling site, adjacent to the Valles caldera, New Mexico, allow an investigation of the thermal history of this site through analyses of the behavior of isotopic dating systems in changing thermal regimes and through thermal modeling. Estimates of the age of the heating event at Fenton Hill responsible for the current elevated temperatures vary from about 1 to 4 Ma. However, new 40Ar/39Ar analyses and thermal modeling indicate that the age of heating may be much younger than previously believed. The 40Ar/39Ar analyses of microcline separates from five samples, ranging in drill hole depth from 1.13 to 4.56 km and in situ temperatures from 110°C to 313°C, indicate thermal events at the site at around 1030 and 870 Ma and a very recent thermal event related to the thermal development of the caldera. The maximum estimates of peak heating duration during this recent thermal event are between 3 and 60 ka. Two distinct thermal perturbations are recognized in the subsurface temperature data, a shallow disturbance, probably related to heating from a lateral flow of hot water just above the Precambrian surface at a depth of about 730 m and a deep thermal disturbance, the source of which is unknown. Model studies indicate the age of the shallow thermal disturbance to be about 10 ka. With reasonable constraints on the source of the deep thermal disturbance, it must lie within a few kilometers of the site, and its maximum age is estimated to be less than about 40 ka. Thus heating at the Fenton Hill site appears to be much younger than the main caldera event at about 1 Ma and is probably related to a magmatic and/or hydrothermal event very close to Fenton Hill during the last few tens of thousands of years.
Tectonics and evolution of the Juan Fernandez Microplate at the Pacific‐Nazca‐Antarctic Triple Junction
Authors: Anderson‐Fontana Sandra , Engeln Joseph F. , Lundgren Paul , Larson Roger L. , Stein Seth
Published: February 2008

In this study, magnetic and bathymetric data from an R/V Endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the Juan Fernandez microplate at the Pacific‐Nazca‐Antarctic triple junction. Magnetic lineations along the East Ridge of the microplate fan to the south, indicating that the Nazca‐Juan Fernandez pole is near the northern end of this feature. Relative motions of the Juan Fernandez‐Pacific‐Nazca‐Antarctic four‐plate system are determined using relative plate motion inversions. The resulting tectonic model predicts that the northern boundary of the Juan Fernandez microplate is a zone of compression and that the West Ridge and southwestern boundary are spreading obliquely. The southeastern boundary is predicted to be obliquely divergent, but present data fail to constrain its geometry. We also present a schematic evolutionary model which relates the formation of the Juan Fernandez microplate to differential spreading rates at the triple junction, and discuss related complexities. The future evolution of the fourplate system depends on whether one or both ridges remain active.
Small‐angle scattering studies of the pore spaces of shaly rocks
Authors: Hall Peter L. , Mildner David F. R. , Borst Roger L.
Published: February 2008

Small‐angle neutron and X ray scattering (SANS and SAXS) measurements have been performed on shaly rocks from a variety of oil field locations. Thin core sections cut parallel to their bedding planes give circularly symmetric scattering patterns. On the basis of the model of Debye et al., the data indicate characteristic pore dimensions of 9–15 nm. Sections cut normal to the bedding planes exhibit elliptically symmetric scattering indicative of pores flattened in the direction of sedimentary compaction. Fourier inversion of the symmetric SANS data in all cases yields a broad distribution of pore diameters having peaks typically in the range 4–6 nm. These distributions are in reasonable agreement with distributions derived from nitrogen adsorption isotherms, although often in disagreement with nitrogen desorption or mercury porosimetry data. At higher scattering vectors, in the Porod region, differences between the SAXS and SANS data are observed which are explained in terms of the rather different sensitivities of the two techniques to empty and filled pores. The data indicate that a significant component of the total porosity is due to pores of less than 2 nm in diameter, in which water is retained under ambient conditions. In several cases, pore dimensions measured by SANS increase after extraction of bitumens by methylene chloride solution, suggesting an intimate microscopic association of hydrocarbon and fine mineral grains.
Heat flow observations on the Bermuda Rise and thermal models of midplate swells
Authors: Detrick R. S. , Von Herzen R. P. , Parsons B. , Sandwell D. , Dougherty M.
Published: February 2008

The Bermuda Rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western North Atlantic. The magnitudes of the depth and geoid anomalies associated with the Bermuda Rise are similar to the anomalies associated with other swells surrounding recent volcanic islands (e.g., Hawaii), suggesting that despite the lack of recent volcanism on Bermuda, the rise has a similar origin to other midplate swells. Results are reported from 171 new heat flow measurements at seven carefully selected sites on the Bermuda Rise and the surrounding seafloor. Off the Bermuda Rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict, with the measured heat flow apparently reaching a uniform value of about 50 mW m−2 on 120 m.y. old crust. On the Bermuda Rise the heat flow is significantly higher (57.4±2.6 mW m−2) than off the swell (49.5±1.7 mW m−2). The magnitude of the anomalous heat flux (8–10 mW m−2) is comparable to that previously found along the older portion of the Hawaiian Swell near Midway. The existence of higher heat flow on both the Hawaiian Swell and Bermuda Rise indicates that these features fundamentally have a thermal origin. The differences in the shape, uplift, and subsidence histories of the Hawaiian Swell and Bermuda Rise can be quantitatively explained by the different absolute velocities of the Pacific and North American plates moving across a distributed heat source in the underlying mantle. Two‐dimensional numerical convection models indicate that the observed depth, geoid, and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection.
High‐pressure, high‐temperature geophysics: A memorial to John C. Jamieson
Authors: Halleck P. M. , Graham E. K.
Published: February 2008

John Jamieson's career in geophysics spanned more than 30 years and encompassed an impressive range of subjects. Although best known for his developments in the field of high‐pressure X ray crystallography, he was also active in high‐pressure ultrasonics and in shock wave technology. He determined phase relationships, equations of state, and high‐pressure structures of numerous minerals and inorganic analogs but took most delight in applying these data to the physics of planetary interiors. His students and colleagues remember him best for the contagious enthusiasm with which he approached geophysics, as well as an incurable penchant for puns.
Elasticity of muscovite and its relationship to crystal structure
Authors: Vaughan Michael T. , Guggenheim Stephen
Published: February 2008

The 13 adiabatic elastic stiffness moduli of natural muscovite have been measured using Brillouin scattering. Following the standard Institute of Radio Engineers convention, these constants are (in gigapascals) C11 = 181.0 ± 1.2, C22 = 178.4 ± 1.3, C33 = 58.6 ± 0.6, C44 = 16.5 ± 0.6, C55 = 19.5 ± 0.5, C66 = 72.0 ± 0.7, C23 = 21.2 ± 1.8, C13 = 25.6 ± 1.5, C12 ± 48.8 ± 2.5, C15 = −14.2 ± 0.8, C25 = 1.1 ± 3.7, C35 = 1.0 ± 0.6, and C46 = −5.2 ± 0.9. The main observed features are the very great acoustic anisotropy due to the weak interlayer bonding and the small, but not insignificant, deviation from hexagonal symmetry (monoclinicity) in the velocity patterns.
closure of rock joints
Authors: Brown Stephen R. , Scholz Christopher H.
Published: February 2008

Previously we verified experimentally that a generalized form of the Greenwood‐Williamson theory describing the elastic contact of two random surfaces adequately predicts the closure of joints in fused silica glass samples. Here we present a similar analysis for experiments on rock. We have tested the theory quantitatively by comparing joint closure from experiment with joint closure predicted by the theory. These comparisons were made using topography data from the surfaces of the joints used in the closure experiments. Three types of tests were done: (1) Granite and quartzite samples were studied where the joints consisted of surfaces ground with #80 and #60 grits. Here the surfaces were random and uncorrelated with each other. These experiments permitted a direct test of the theory. As with the experiments on glass, the theory agrees with experiment. However, uncertainty in the values of the elastic constants for rock result in larger possible errors in predicting joint closure. (2) Tension fractures in granite and quartzite samples were studied. Here the surfaces were highly correlated with each other. The amount of closure expected for correlated surfaces depends on the degree of correlation, since this determines the effective roughness of the joint. Here the surface topography was repeatedly high‐pass filtered and the theory compared with experiment at each stage to determine the effective scale over which the surfaces were mismatched. (3) Ground surfaces of marble were subjected to the same loading conditions. Here the elastic assumption was no longer appropriate. These experiments are an example that the limit of elastic deformation is reached for all materials for surface asperities below a given dimension. This dimension is related to the indentation hardness of the material, which is lower for calcite than for silicate minerals.
Equation of state and thermodynamic parameters of NaCl to 300 kbar in the high‐temperature domain
Authors: Birch Francis
Published: February 2008

Data of several kinds (ultrasonic, volumetric, shock compression) are combined to produce a set of isotherms for NaCl in the temperature‐pressure region between 25° and 500°C and 0–300 kbar. A preliminary examination of the Boehler‐Kennedy measurements of linear compression to 30 kbar and 500°C supports the provisional assumption that (∂P/∂T)v is independent of P and possibly also of T in the “high‐temperature” domain. The equation of state may consequently take the form P(V, t) = P25(V) + (t ‐ 25)(∂P/∂T)v, where t is Celsius temperature. P25(V) is the 25°C isotherm of an earlier paper, with small adjustments; this equation was controlled at 300 kbar by the shock measurements and at zero pressure by ultrasonic measurements. Isotherms of the BE2 form are constructed for the five additional temperatures of the Boehler‐Kennedy data with (∂P/∂T)v = 0.0286 kbar/deg. These are in nearly as good agreement with the volumetric data as were the curves fitted to these measurements alone. At the same time, the parameters of these isotherms show a regular dependence upon temperature. Various thermo dynamic parameters are tabulated, with the aid of handbook values of CP and zero‐pressure thermal expansion coefficients. In particular, Grüneisen's ratio γ is found to be nearly independent of temperature, while γ/V is nearly independent of pressure.
Global radar units on Venus derived from statistical analysis of Pioneer Venus Orbiter radar data
Authors: Davis Philip A. , Kozak Richard C. , Schaber Gerald G.
Published: February 2008

Unsupervised, linear‐discriminant cluster analysis of Pioneer Venus reflectivity and rms‐slope data has produced a classification of 15 radar units. Examination of four hypsometric provinces previously established for Venus has been used to show that, although the highland (6053 to 6055.5 km) and mountainous (greater than 6055.5 km) provinces are distinctly different in their radar unit compositions from each other and from the lowlands (less than 6051 km) and rolling plains (6051 to 6053 km) provinces, the lowlands and rolling plains provinces are very nearly the same in their radar properties. This observation indicates that hypsometry alone is not a reliable criterion for quantitative examination of the surface geology on Venus. A classification unit encompassment scheme has been developed that shows the presence of three distinct trends: two toward elevations higher than the planetary mean elevation, and the other toward elevations lower than the mean. Venera lander data and earth‐based radar images have been used to infer that seven units within the two higher‐elevation unit encompassment trends represent materials of mostly basaltic composition that differ only in their surface morphologies. The physical characteristics of these materials indicate that they form terrains ranging from banded through rift valley to relatively flat and that the materials are composed of bedrock and “soil” in variable ratios. Three other radar classification units appear to be morphologically transitional forms between units of these two higher‐elevation unit encompassment trends. The five remaining units are associated with the lower‐elevation unit encompassment trend; their geologic significance cannot be determined with any certainty with the existing Pioneer Venus radar data. The radar units that correspond to those of an existing radar unit map, produced by supervised analysis, display similar areal distributions on Venus. However, our radar unit map shows more detail and is relatively unbiased in terms of unit definition. This global radar classification image of the Venusian surface provides a working data base from which more exacting geologic interpretations can be made when higherresolution imagery becomes available.
Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy: 3. Inversion
Authors: Nataf Henri‐Claude , Nakanishi Ichiro , Anderson Don L.
Published: February 2008

Lateral heterogeneity in the earth's upper mantle is investigated by inverting dispersion curves of long‐period surface waves (100–330 s). Models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies. We also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities. The data are from the observation of about 200 paths for Love waves and 250 paths for Rayleigh waves. For both the regionalized and the spherical harmonic inversions, corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters. These corrections are found to be important, especially at low spherical harmonic order the “trench region” and fast velocities down to 250 km under shields. Below 200 km under the oceans, both S velocity and S anisotropy support a model of small‐scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough. The spherical harmonic models clearly demonstrate (a posteriori) the relation between surface tectonics and S velocity heterogeneities in the first 250 km: all shields are fast; most ridges are slow; below 300 km, a belt of fast mantle follows the Pacific subduction zones. However, at greater depths, large‐scale heterogeneities that seem to bear no relationship to surface tectonics are observed. The most prominent feature at 450 km is a fast‐velocity region under the South Atlantic Ocean. Smaller‐scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths: an anomalously slow region centered in the south central Pacific is possibly linked to intense hot spot activity; a very fast region southeast of South America may be related to subduction of old Pacific plate. Between 200 and 400 km, a belt of SV>SH anisotropy follows part of the ridge and subduction systems, indicating vertical mantle flow in these regions. The spherical harmonic results open new horizons for the understanding of convection in the mantle. Perspectives for the improvement of the models presented are discussed.
Absolute gravity measurements in California
Authors: Zumberge M. A. , Sasagawa G. , Kappus M.
Published: February 2008

We have constructed an absolute gravity meter that determines the local gravitational acceleration by timing a freely falling mass with a laser interferometer. The instrument has made measurements at 11 sites in California, four in Nevada, and one in France. The uncertainty in the results is typically 10μGal (1 Gal ≡ 1 cm/s2; 1 μGal = 10−6 Gal = 10−8 m/s2). Repeated measurements have been made at several of the sites; only one shows a substantial change in gravity.
Time‐dependent profiles of lava flows
Authors: Baloga Stephen , Pieri David
Published: February 2008

This work investigates the combined effects of a time‐dependent effusion rate and a spatially varying viscosity on the thickness profile of a flowing lava. We formulate relatively simple governing equations as a primitive dynamical model for these two influences on lava flow morphology. Free boundary solutions for the profile of the flow and its extent are presented and analyzed for different boundary conditions at the source of the flow and several models for the spatial dependence of the viscosity. The results presented here suggest that time dependence in the flow depth at the source and the form of the viscosity variation are significant influences on the morphology and dimensions of lava flows. Moreover, this analysis implies that time‐dependent source conditions may contribute to the scatter in plots of eruption rate versus flow length and disparities between field and laboratory estimates of lava viscosity.
Origin of convex accretionary wedges: Evidence from Barbados
Authors: Zhao W.‐L. , Davis D. M. , Dahlen F. A. , Suppe J.
Published: February 2008

The surface slopes of submarine accretionary wedges generally decrease away from the toe or deformation front toward the arc. This paper presents evidence that this characteristic convex cross‐sectional shape is a direct mechanical response to the lithification of sediments during accretion. The recently accreted sediments near the toe, which are typically unconsolidated and therefore very weak, must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression. This critical wedge taper, and thus the bathymetric slope of the accretionary prism, is relatively high for the porous, weak sediments near the toe. In contrast, the compacted sediments farther back in the wedge are stronger, so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression. The result is a convex cross‐sectional wedge shape. We investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity. A straightforward generalization of the exact critical taper model is combined with empirical cohesion‐porosity‐velocity relations to infer the distribution of porosity and P wave velocity within the Barbados accretionary wedge. We estimate that the porosity decreases from its near‐toe value of 70% to approximately 30% at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge.
A kinematic model for the Gorda Deformation Zone as a diffuse southern boundary of the Juan de Fuca Plate
Authors: Wilson Douglas S.
Published: February 2008

Several lines of evidence indicate that the space problem caused by southeastward motion of the Juan de Fuca plate toward the east‐west Mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform. A kinematic model for this deformation based on the assumption of nondivergent flow (conservation of area) within the area known as the Gorda plate, and on the assumption of no subduction along the Mendocino transform, successfully matches the isochron pattern inferred from magnetic anomalies. The model assumes strain rate decreasing to zero approaching the rigid Juan de Fuca plate, with no sharp boundary or separate rigid area that would imply a discrete Gorda plate. This interpretation allows magnetic anomalies from south of the Blanco Fracture Zone to be used to constrain the kinematics of the Juan de Fuca plate. These anomalies imply a substantial change in the distance to the rotation pole at roughly 3 Ma, in addition to the clockwise shift in motion direction at 5 Ma.
Gaussian analysis of temperature effects on the reflectance spectra of mafic minerals in the 1‐μm region
Authors: Roush Ted L. , Singer Robert B.
Published: February 2008

Gaussian band‐fitting analysis has been applied to the reflectance spectra of mafic silicates, which are of geologic importance throughout much of the solar system. Reflectance spectra obtained over a sample temperature range of ≈80 to 448 K of olivine, clinopyroxene, and orthopyroxene were used in order to characterize the spectral changes in the 1‐μm wavelength region of these minerals as a function of temperature. Four Gaussians are required to characterize the olivine, while two Gaussians are necessary to characterize both the orthopyroxene and clinopyroxene. The trends of the individual Gaussian parameters (integrated area, width, and center position) are presented as a function of temperature for each mineral. The characterization by Gaussian analysis of the 1‐μm absorption feature of all minerals yields consistent trends in center position, integrated area, and width that may prove useful in quantitative determination of these minerals in mineral mixtures. The Gaussian analysis of olivine, for the three primary band components, is qualitatively consistent with previous crystal field transition assignments of Fe2+ in the M(1) and M(2) crystallographic sites. The trends of the determined olivine Gaussian center positions imply that site symmetry is more important in determining transition energies than metal‐oxygen distances. The result of the Gaussian analysis for orthopyroxene is consistent with the need for two Gaussians to describe an asymmetric band. The Gaussian analysis of the clinopyroxene is consistent with previous assignments of crystal field transitions due to Fe2+ in the M(1) and M(2) crystallographic sites.
Magnetic structure of the upper kilometer of the marine crust at Deep Sea Drilling Project Hole 504B, eastern Pacific Ocean
Authors: Smith Guy M. , Banerjee Subir K.
Published: February 2008

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low‐temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200‐m transition zone, consisting of a mixture of extrusives and dikes. High‐temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation‐exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe‐rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation‐exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magnetic properties.
The effects of chemical density differences on convective mixing in the Earth's mantle
Authors: Gurnis Michael
Published: February 2008

Subducted lithosphere and crust are chemically distinct from ambient mantle, and some have hypothesized that the resulting anomalous buoyancy may affect mixing and, in particular, may cause chemical heterogeneity to persist. To explore this effect, two‐dimensional convective mixing calculations have been carried out. Boundary conditions mimicking features of plate kinematics have been imposed and negatively buoyant tracers placed beneath converging plate margins to simulate subduction. The principal parameter of interest is the density anomaly ratio: Δρ/(ρo α ΔT), the ratio of the chemical to the thermal buoyancy. For a density anomaly ratio appropriate to complete separation of the oceanic crust from the rest of the lithosphere, it is marginally possible for the tracers to segregate into the warm region near the bottom and neutralize the thermal buoyancy there, for internally heated flows, and to segregate to the base of the uprising limbs of convection, for bottom heated flows. However, for the more likely scenario of oceanic crust and lithosphere maintaining their layering after subduction, there is neither segregation nor neutralization of thermal buoyancy. The average residence time is the average time tracers remain in the box from subduction to sampling beneath ridges and could be increased by 4% from the passive case at the very most if the crust and lithosphere remain layered. Even in the more extreme case of separation of crust from lithosphere, the residence times are only increased by 15% from the passive case.
Paleomagnetism of Jurassic volcanic rocks in the Patagonia Mountains, southeastern Arizona: Implications for the North American 170 Ma reference pole
Authors: May Steven R. , Butler Robert F. , Shafiqullah Muhammad , Damon Paul E.
Published: February 2008

Paleomagnetic study of Middle Jurassic volcanic and volcaniclastic rocks in southeastern Arizona yields a paleomagnetic pole located at 61.8°N, 116.0°E (k = 49.6, α95 = 6.2°). These rocks from Corral Canyon represent an autochthonous segment of the Jurassic Cordilleran magmatic arc and were deposited directly on Upper Paleozoic carbonates of the cratonic miogeocline. Rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism. Rb/Sr isotope geochemistry on whole‐rock samples from eight welded ash flow tuffs yields an isochron age of 172.2 ± 5.8(2σ) Ma. The Corral Canyon pole falls between poles from Newark Trend Group II (179 ± 3 Ma) and Glance Conglomerate (151 ± 2 Ma).
The role of channels in the transport of radon from the soil
Authors: Schery S. D. , Siegel D.
Published: February 2008

Experimental measurements of permeability as a function of scale as well as direct detection of flow indicate macroscopic channels are present at a soil site. A model for heterogeneous media is developed that provides a dimensionless number S, an estimate of the ratio of the exhalation from the channels to the exhalation from the soil matrix, to indicate whether or not channels are important for transport of radon to the atmosphere. The model considers the effects of flow and transverse diffusion in the channels, flow and diffusion in the soil matrix, and coupling between the channels and matrix due to diffusive exchange. Specific predictions depend on soil properties, the geometry of the channels, and the pressure gradient. The model indicates that channels are important for the present soil at times of higher pressure gradients such as can occur with fronts, wind, and atmospheric turbulence. The radon concentration exhausted for steady state upward flow is greater than that exhausted for transient upward flow. On a time‐averaged basis, the contribution of channels to overall exhalation of radon appears moderate. The modeling suggests channeling in soils could be a factor enhancing exhalation of radon to the atmosphere at other locations. The distance of subsurface radon transport is increased by significant channeling, but transverse diffusion into the surrounding matrix is a strong mitigating influence.
Dislocation model for aseismic crustal deformation at Hollister, California
Authors: Matsu'ura Mitsuhiro , Jackson David D. , Cheng Abe
Published: February 2008

Geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes. We model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust. Frictional drag is represented by uniform dislocation on a fault surface. We assume that friction occurs only on the upper “brittle zone” of each block boundary and that frictionless sliding occurs below this zone. Using a Bayesian inversion procedure, we invert geodetic data to determine the parameters of the block and dislocation model. Parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate, dip angle, slip angle, and width of each rectangular fault patch. For Hollister we assume five blocks and nine rectangular fault patches. Rates of length change on 92 lines observed by trilateration make up the data. Triangulation, leveling, very long baseline interferometry, satellite ranging, and global positioning system data can also be easily included in our model. Our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data. The block parameters are estimated from geological observations of fault displacement rates, while the widths of the fault patches are estimated from earthquake hypocentral depths. We choose prior estimates of the dislocation rate to match observed creep rates. The estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates. This suggests that the block motion is steady on time scales from 10 years to 1 m.y. The net motion across the San Andreas‐Calaveras fault system is 38 ± 3 mm/yr oriented S38°E. Thus some additional displacement, possibly offshore, is needed to match the rate of 56 mm/yr predicted by plate tectonic models. The depth of the transition from frictional to free sliding varies considerably, from 0.4 km on the southern Calaveras to 11 km on the central San Andreas fault. Two segments of the San Andreas, north of 36°50′ and south of 36°40′, have the greatest potential for moderate to large earthquakes.
Compressibility of porous rocks
Authors: Zimmerman Robert W. , Somerton Wilbur H. , King Michael S.
Published: February 2008

Four compressibilities are defined for porous rock, relating changes in the bulk and pore volumes to changes in the pore and confining pressures. Using a micromechanical theory based on classical linear elasticity, three relations are found between these compressibilities. Two of these relations are verified experimentally for Berea and Bandera sandstone. Bounds are derived for these compressibilities, involving only the porosity and the elastic moduli of the rock‐forming minerals. For the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures. This dependence is verified for Berea, Bandera, and Boise sandstone. While the strains cannot be expressed as functions of an “effective stress,” regardless of how it is defined, the (elastic) volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure.
Anomalous magnetic properties in rocks containing the mineral siderite: Paleomagnetic implications
Authors: Ellwood Brooks B. , Balsam William , Burkart Burke , Long Gary J. , Buhl Margaret L.
Published: February 2008

Sampling and magnetic measurement of a 1‐m bed in a new road cut of the Upper Cretaceous Austin Chalk (northeastern Texas) has yielded anomalous magnetic results. Initial measurement of the anisotropy of magnetic susceptibility (AMS) indicated unusually high anisotropies and low bulk susceptibilities characteristic of a magnetocrystalline anisotropy which might be expected for siderite (FeCO3). Natural remanent moments (NRM) for these samples were low (<1 × 10−4 A m2), and directions were typical for samples which had acquired a normal geomagnetic field overprint at the site. Periodic remeasurement of the NRM yielded an increase in moment for some samples and periodic reversals of RM in a direction parallel or antiparallel to the ambient field in the laboratory. Isothermal remanent moments of these samples saturates at low induction values (∼200 mT). AMS remeasurement over a period of weeks to months revealed a general decrease in the anisotropy magnitudes, an increase in susceptibility, and a change in principal axis orientations. These data were compared with heating (oxidation) experiments on Austin Chalk, siderite ore, and clastic sediments with siderite cements. All of these results are consistent with the X‐ray diffraction and Mössbauer data, which indicate the presence of siderite in the Austin Chalk samples. The Mössbauer spectra of the samples obtained at room temperature and 78°K indicate the presence of approximately 80% pyrite, 10% siderite, and 10% of a clay component (possibly chlorite) when only the iron‐bearing mineral components are considered. After sampling, exposure to the air, and subsequent oxidation in the laboratory, the siderite in Austin Chalk samples appears to have altered to γFe2O3 (maghemite) or Fe3O4 (magnetite), increasing the magnetic moment and changing the NRM and AMS directions in the samples. The continuing changes appear to reflect a conversion from the less stable γFe2O3 to αFe2O3 (hematite) or oxidation of Fe3O4 to αFe2O3. We infer from our study that oxidation of siderite‐bearing limestone (e.g., during the formation of unconformities or due to low‐magnitude tectonic events) may produce a very stable secondary magnetic moment in the rock. Such changes may be very important when interpreting paleomagnetic data from limestones.
Synfolding and prefolding magnetizations in the Upper Devonian Catskill Formation of eastern Pennsylvania: Implications for the tectonic history of Acadia
Authors: Miller John D. , Kent Dennis V.
Published: February 2008

The Upper Devonian Catskill Formation was sampled for paleomagnetic study in east‐central Pennsylvania (41°N, 76°W). In one area the dominant component of magnetization (SE) is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660°C. A conventional fold test is positive at the 99% confidence level. However, statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3/4 unfolding. The magnetization is therefore secondary, with a mean direction of D = 166.6°, I = −1.8°. The corresponding pole position (48.1°N, 124.1°E, a95 = 4.0°) is indistinguishable from paleopoles from earlier studies of the Catskill, which therefore can also be regarded as representing Permo‐Carboniferous remagnetizations. A second component of magnetization (SW, pole position 32.8°N, 90.0°E, a95 = 7.2°) with discrete unblocking temperature spectra and southwesterly declination was isolated in a few samples from the first area and in most samples from a second area. A prefolding origin of this magnetization is supported by a positive fold test on five samples from the first area. If this magnetization does represent a Devonian magnetization then the true paleolatitude for east‐central Pennsylvania is 16° +/− 7.2°S, which is consistent with the paleolatitude observed in many of the Upper Devonian rock units in the Acadia region, although inconsistent with others. The question of the position of Acadia relative to North America in the Upper Devonian is therefore still open.
Shock‐implanted noble gases: An experimental study with implications for the origin of Martian gases in shergottite meteorites
Authors: Bogard Donald D. , Hörz Friedrich , Johnson Pratt H.
Published: February 2008

The existence of trapped gases in the EETA 79001 shergottite meteorite that were apparently shock‐implanted from the Martian atmosphere raises important questions as to the mechanism of gas implantation and whether the implanted gas has been mass fractionated. To study the phenomenon of shock‐implantation of gases, we artificially shocked whole‐rock and powder samples of a terrestrial basalt to pressures of 2–40 GPa in the presence of controlled gas mixtures ranging from 10−4 to 3 atmospheres. Argon, Kr, and Xe, and to a lesser extent Ne, were readily implanted into the silicate under a wide range of experimental conditions. As exemplified by Ar, the amount of implanted gas is linearly proportional to its partial pressure over a partial pressure range of 0.0001 to 0.1 atmosphere, the former value being similar to the partial pressure of Ar on Mars. The implanted gas showed no evidence of isotopic fractionation, and for Kr and Xe fractionation limits of 0.1% per mass unit could be set. The elemental composition of shock‐implanted gas also closely resembles the ambient gas phase, except that Ne is only lightly retained and readily leaks from the samples after the shock. Stepwise temperature releases of gas implanted at 2, 5, 20, and 35 GPa indicate two or more lattice sites for the implanted gas that are possibly related to microcracks and other lattice defects. Activation energies, Q, for diffusion of shock‐implanted gas from the samples increase dramatically with increasing shock; values of Q for Ar diffusion for the four shock levels 2, 5, 20, and 35 GPa are 7, 9, 14, and ∼25 kcal/mol, compared to 30–45 kcal/mol for typical radiogenic Ar. Diffusion of Kr and Xe is considerably slower than Ar. The amounts of gas that would have been implanted with 100% efficiency were calculated from the measured porosities of the powder samples and were compared to observed abundances. The implantation efficiencies were approximately 0.5% at 2 GPa, 7% at 5 GPa, and greater than 50% at both 20 and 35 GPa. These high gas implantation efficiencies occur for shock pressures where modest amounts of sample melting at grain boundaries begins, and they suggest that higher shock pressures generating substantial melting could not achieve appreciably greater efficiencies. These experimental data are consistent with shock‐implantation of Martian gases without mass fractionation into shock‐melted phases of EETA 79001; the progenitor of these melts may have been porous, fragmental material.
The water vapor content profile in the Venusian atmosphere according to the results of experiments from Vega 1 and 2
Authors: Surkov Yu. A. , Shcheglov O. P. , Ryvkin M. L. , Sheynin D. M. , Davydov N. A.
Published: February 2008

Determinations of the water vapor concentration in Venus's atmosphere from Vega 1 and 2 were carried out by means of the moisture analyzer VM‐4. The analyzer includes two sensitive elements: heating water‐absorbing sensor based on LiCl for measuring the temperature of the condensation point (−30° to +30°), and a coulombmetric sensor for measuring volume water vapor concentration (0.01–0.2%). The ranges of water concentration measurements by both sensors complement each other and partially overlap. The instruments were placed on the landing devices of the spacecraft. The measurements were carried out at altitudes of 62–25 km above the surface of the planet. Two vertical water vapor concentration profiles were measured in Venus's night atmosphere.
Evidence for a complex archean deformational history; Southwestern Michipicoten Greenstone Belt, Ontario
Authors: McGill George E. , Shrady Catherine H.
Published: February 2008

The Michipicoten Greenstone Belt extends for about 150 km ENE from the northeastern angle of Lake Superior. In common with many other Archean greenstone belts, it is characterized by generally steep bedding dips and a distribution of major lithologic types suggesting a crudely synclinal structure for the belt as a whole. Detailed mapping and determination of structural sequence demonstrate that the structure is much more complex. The Archean history of the belt includes formation of at least three regionally significant cleavages, kilometer‐scale overturning, extensive shearing, and diabase intrusion. Most well defined, mappable “packages” of sedimentary rocks appear to be bounded by faults. These faults were active relatively early in the structural history of the belt, when extensive overturning also occurred. Steepening of dips, NW‐SE shortening, development of steep NE cleavage, and pervasive shearing all postdate the early faulting and the regional overturning, obscuring much of the detail needed to define the geometry of the earlier structures. The results obtained so far suggest, however, that the Michipicoten Greenstone Belt underwent an early stage of thrusting and associated isoclinal folding, probably in a convergent tectonic environment.
Estimating the Compensation Depth of the Hawaiian Swell With Linear Filters
Authors: McNutt Marcia , Shure Loren
Published: February 2008

Broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism. The cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere. According to the former explanation, we would expect the low‐density material compensating the swell to lie beneath the conducting lid, whereas the latter model allows compensation at shallower depths. We present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies. Our method does not require an a priori estimate of swell height or width, nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes, which are compensated by flexure of the oceanic Moho. Rather, we exploit the difference in the predicted sign and amplitude of the Moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects. An application of these linear filters to residual topography and geoid data from the region surrounding the Hawaiian hot spot yields a compensation depth of 70 ± 10 km, within the lower lithosphere.
Fractal analysis applied to characteristic segments of the San Andreas Fault
Authors: Aviles C. A. , Scholz C. H. , Boatwright John
Published: February 2008

Fractal theory is applied in a quantitative analysis of the San Andreas fault (SAF) geometry. The method, which directly measures the increase in total fault length with a decrease in ruler size, gives the fractal dimension D and scaling properties for the chosen length band 0.5–1000 km. A physical interpretation of D is that it measures the irregularity of the fault trace in the selected band. The fault is subdivided into six segments of distinctive seismic behavior. A “main” fault trace which shows either maximum coseismic slip or creeping was selected for analysis, with three alternative branches examined for the SAF system south of San Bernardino. Branches of the fault trace are not considered in this work. Fractal dimensions calculated for the different segments range from 1.0008 to 1.0191, and are different from 1.0 to the 95% confidence interval. These small changes reflect the overall smoothness of the main fault trace. Significant variations in D among segments indicate heterogeneities in the fault smoothness along strike. D also changes significantly from the short‐length band to the long‐length band where the demarcation point ranges from 1 to 2 km. The short‐length band has larger D values. A slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting. This indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view.