bluuebunny/crossref_metadata_2025 huggingface dataset:
Orogeny and large‐scale strike‐slip faulting on Venus: Tectonic evolution of Maxwell Montes
Authors: Bruegge R. W. Vorder , Head J. W. , Campbell D. B.
Published: February 2008

The Ishtar Terra highland region of Venus contains mountain belts morphologically similar to terrestrial orogenic belts. Akna Montes and Freyja Montes are long, linear mountain belts with about 4 km of relief above the 3‐ to 4‐km‐high Lakshmi Planum. The Maxwell Montes mountain belt, however, is more rectangular in plan view and has relief of approximately 8 km. We have investigated the nature of this latter range using complementary radar images obtained by the Arecibo Observatory and the Venera 15/16 spacecraft, as well as allimetry data obtained by the Venera 15/16 and Pioneer Venus spacecraft. Geological and structural maps have been compiled on the basis of these data, the individual features and their stratigraphic relationships interpreted, and a model for the origin and evolution of Maxwell Montes is proposed. The dominant features of this mountain range are long, parallel ridges and valleys that are intepreted to be anticlines and synclines and to have resulted from ENE‐WSW oriented compression. Two major shear zones bound Maxwell Montes on the north and south, converging toward the west. Nine narrow linear parallel features several hundreds of kilometers long (identified as cross‐strike discontinuities, CSDs) cut obliquely across the strike of the ridges and valleys in a NW‐SE direction and disrupt the structural fabric of the mountain range, dividing Maxwell Montes into 10 crustal domains. Among several possible origins, we find that the data are most consistent with their interpretation as right‐lateral strike‐slip faults with offsets of up to 125 km. Retrodeformation and reconstruction of Maxwell Montes using the offsets determined for individual domains produces a long linear mountain range similar to Akna Montes. Using geologic unit maps and topographic maps of the present configuration of Maxwell, similar reconstructions were made; these reconstructions restored several linear tectonic elements, topographic trends, and sinuous unit boundaries to more contiguous positions. On the basis of these data and observations we suggest that Akna Montes may represent the initial form of compressional orogenic belts on Venus, while Maxwell Montes attained its present morphology through a second stage of deformation involving large‐scale strike‐slip faulting. We examine several models for this second stage and favor the interpretation that it occurred as the Akna Montes‐like proto‐Maxwell Montes was transported to the west between two converging shear zones, accompanied by rotation of the entire mountain range. Thirty percent shortening of the mountain range is implied by the geometries of the present and reconstructed Maxwell Montes. On the basis of the deforamation in Maxwell Montes and the evidence for large‐scale transport we conclude that large‐scale horizontal a movement of crustal materials is a significant aspect of the tectonics of Venus.
Structure of the Nankai Trough Accretionary Zone from multichannel seismic reflection data
Authors: Moore G. F. , Shipley T. H. , Stoffa P. L. , Karig D. E. , Taira A. , Kuramoto S. , Tokuyama H. , Suyehiro K.
Published: February 2008

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12–16 km wide and 350–750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by “kink” folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthrusting sediments form fault‐bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5–2.5 km. The second thrust cuts up from an inferred décollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothrust zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal décollement. The underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the décollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thrusts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the western area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured décollement in the eastern area.
Contrasting geothermal regimes of the Barbados Ridge Accretionary Complex
Authors: Langseth Marcus G. , Westbrook Graham K. , Hobart Michael
Published: February 2008

Three E‐W transects of closely spaced heat flow measurements across the Barbados Accretionary Complex are reported. One transect, about 100 km in length at 14°20′N and a shorter 15‐km line at 14°35′N show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone. This decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge. Except for anomalously high heat flow over a mud volcano on the eastern end of the 100‐km transect, fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region. An increase of heat flow is observed at the western end of the 100‐km transect where high‐angle arcward vergent thrusting is occurring. We attribute this increase primarily to a decrease in the arcward thickening of wedge material, and heat generated by friction on the décollement. In strong contrast a short line of stations at 15°30′N (the Deep Sea Drilling Project/Ocean Drilling Project (DSDP)/(ODP) drilling transect) found anomalously high heat flow (a 40% increase over the heat flux from the lithosphere) with large variability over the most recently accreted material and also in the adjacent basin. The thermal regime in the vicinity of the DSDP/ODP transect is dominated by a strong flow of pore fluids along the décollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments. Heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13°50′N. The thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one‐dimensional model, which showed that the resulting effects are too small to produce detectable heat flow anomalies. Consequently, the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow.
Thrust‐related mudstone fabrics from the Barbados Forearc: A backscattered scanning electron microscope study
Authors: Prior David J. , Behrmann Jan H.
Published: February 2008

Backscattered electron imaging on the scanning electron microscope has been used to examine the nature of microfabrics in mudstones from the Barbados forearc. Samples from two important tectonic junctions in hole 671B of Ocean Drilling Program leg 110 have been examined in detail. At thrust A (128 m depth) clays and clasts are pervasively aligned at about 50° to the thrust surface, and a macroscopic scaly fabric is defined by anastamosing fractures. In the décollement zone (500–550 m depth) there is no preferred alignment of clays or clasts even though an irregular, macroscopic scaly fabric is present. Scaly fabric in hole 671B is in part an artifact of specimen desiccation but may also reflect the release of anisotropic, elastic strain relating to in situ stress conditions. The disclosure that scaly fabric does not relate to grain alignment has profound implications for the nature of the Barbados décollement seismic reflector. The reflector does not correspond to a fabric anisotropy and is most likely to relate to the documented fluid flow anomaly in the décollement. In contrast to scaly fabrics, the microfabrics are the product of sediment deformation history. The differences between the microfabrics at thrust A and those in the décollement probably relate to contrasts in deformation conditions. In the décollement, deformation probably occurred by paniculate flow under high pore fluid pressures, where the individual panicles were aggregates of phyllosilicates. At thrust A, high pore fluid pressures were not maintained, due to surface proximity, enabling preconsolidation stresses to be exceeded. As a result, a pervasive phyllosilicate alignment has developed.
Doppler radar imaging of spherical planetary surfaces
Authors: Hudson R. Scott , Ostro Steven J.
Published: February 2008

A new approach to making radar reflectivity images of spherical planetary objects uses echo spectra acquired as a function of rotational phase and at an arbitrary number of subradar latitudes. If only equatorial views are used then the image will have a north‐south ambiguity. If non‐equatorial views are used then unambiguous images are possible. The technique is tailored for depolarized or diffuse (nonspecular) polarized backscatter and works best when the limb darkening is minimal. In developing the Doppler‐radar imaging system, the target's reflectivity distribution is expanded in a truncated spherical harmonic series and the distribution of echo power in rotational phase and Doppler frequency is obtained as a linear, analytic function of the series coefficients. To estimate the coefficients from an observed phase‐Doppler distribution, the inversion is cast as a least‐squares problem and solved using singular value decomposition. The result is a linear imaging system whose capabilities and sensitivity to such factors as subradar latitude coverage and signal‐to‐noise ratio are easily explored with simulations. Doppler‐radar imaging can be used with existing radar telescopes to map the diffuse component of echoes from the inner planets and to make north‐south ambiguous reflectivity maps of the icy Galilean satellites. SNRs needed for Doppler‐radar imaging of the largest asteroids, Io, and Titan would be accessible upon implementation of upgrades proposed for the Arecibo telescope.
Seismic fracture anisotropy in the Earth's crust: An overview
Authors: Leary Peter C. , Crampin Stuart , McEvilly Thomas V.
Published: February 2008

Fluid‐filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement. If we are to comprehend the role of fractures and fluids in tectonic processes, to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure, or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways, we must understand how seismic waves interact with the fracture, crack, and microcrack structures within the rock mass. A salient feature of fracture‐related rock structure is the effective seismic anisotropy of aligned fractures and microcracks. With the advent of large‐volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust. Crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10–100 km scale of crustal stress orientation and preseismic stress buildup, through the 100–1000 m scale of fluid reservoir structure and characterization, and the 1–10 scale of mining and geoengineering rock strain monitoring, to millimeter and micrometer or possibly smaller fluid‐filled inclusions determining the elastic response of the intact rock mass. To focus developments in seismic anisotropic studies of crustal fracture phenomena, the American Geophysical Union and the Society of Exploration Geophysicists sponsored a Chapman Conference on Seismic Anisotropy in the Earth's Crust, held May 31 to June 4, 1988, at the Lawrence‐Berkeley Laboratory of the University of California. This was the Third International Workshop on Seismic Anisotropy (3IWSA). Nineteen conference papers are presented in this special section of the Journal of Geophysical Research.
Normalized remanence in sediments of the western equatorial Pacific: Relative paleointensity of the geomagnetic field?
Authors: Tauxe Lisa , Wu Guoping
Published: February 2008

Piston cores from the Ontong‐Java Plateau collected during the Eurydice expedition have been used to estimate regional geomagnetic paleointensity. Rock magnetic measurements of the sediments show that (1) the remanence is carried by stable pseudo‐single domain magnetite, (2) the grain sizes and concentrations are relatively uniform with respect to the normalization technique and (3) susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field. We therefore have measured the rémanent intensity (both natural and demagnetized to 15 mT) and susceptibility of archive quarters of three cores at centimeter intervals using an automated through‐bore magnetometer. A further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores. One of our cores displays high coherence over a broad range of frequencies, while for two others, coherence is insignificantly different from zero at the 95% level of confidence. Oxygen isotopic records, paleontological datum levels and the Brunhes‐Matuyama boundary provide an independent temporal framework for inter‐core comparison. When the two cores which pass our acceptance criteria are placed on a common time scale, the coherence between the normalized intensity records is significantly different from zero at well above the 95% level of confidence, suggesting some common input signal over the same band of frequencies. Furthermore, there is a great deal of similarity between the records from the Ontong‐Java Plateau and a record from core RC10‐167 taken some 3000 km to the north. The distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows (0–5 Ma), with a slightly smaller standard deviation. The inter‐core agreement within a single region, the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field. Spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years, over the period 0–350 ka; this feature is however lacking for the period 350–700 ka.
Combined inversion for the three‐dimensional Q structure and source parameters using microearthquake spectra
Authors: Scherbaum Frank
Published: February 2008

The estimation of Q values and/or source corner frequencies fc from single‐station narrow‐band recordings of microearthquake spectra is a strongly nonunique problem. This is due to the fact that the spectra can be equally well fitted with low‐Q/high‐fc or a high‐Q/low‐fc spectral models. Here, a method is proposed to constrain this ambiguity by inverting a set of microearthquake spectra for a three‐dimensional Q model structure and model source parameters seismic moment (Mo ) and corner frequency (fc ) simultaneously. The inversion of whole path Q can be stated as a linear problem in the attenuation operator t* and solved using a tomographic reconstruction of the three‐dimensional Q structure. This Q structure is then used as a “geometrical constraint” for a nonlinear Marquardt‐Levenberg inversion of Mo and fc and a new Q value. The first step of the method consists of interactively fitting the observed microearthquake spectra by spectral models consisting of a source spectrum with an assumed high‐frequency decay, a single‐layer resonance filter to account for local site effects, and additional “whole path attenuation” along the ray path. From the obtained Q values, a three‐dimensional Q model is calculated using a tomographic reconstruction technique (SIRT). The individual Q values along each ray path are then used as Q starting values for a nonlinear iterative Marquardt‐Levenberg inversion of Mo and fc and a “new” Q value. Subsequently, the “new” Q values are used to reconstruct the next Q model which again provides starting values for the “next” nonlinear inversion of Mo, fc, and Q. This process is repeated until the “goodness of fit measure” indicates no further improvement of the results. The method has been tested on a set of approximately 2800 P wave spectra (0.9 < M < 2.0) from the recordings of 635 microearth‐quakes from the Kaoiki seismic zone in Hawaii (Big Island) which were recorded at up to six stations. The hypocenters are distributed within a volume of approximately 18×l8×l5km (depth). The Q model uncertainties have been estimated on the basis of several different tests: Self‐consistency, constraining the comer frequencies, and additionally splitting the data set. The standard deviation of the final Q model which used a grid size of 1.5×1.5×2.0 km (depth) was less than 3% for the depth range 0–5 km, less than 5% between 5 and 7 km, and 7% between 7 and 9 km. The simulation of strong attenuation effects close to the surface shows that site effects may cause a corruption of the resulting Q model at shallow depths. For the given data set and depths below 3–5 km, the method is believed to be able to resolve the model dependent attenuation structure on a scale down to 1–2 km with a resolution of a few percent.
Seismotectonics of thin‐ and thick‐skinned deformation in the Andean Foreland from local network data: Evidence for a seismogenic lower crust
Authors: Smalley Robert , Isacks Bryan L.
Published: February 2008

Local network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca plate. We find two areas of foreland seismicity, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold‐thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. In both areas, events are concentrated between 15 and 35km depth and have volumetric, rather than planar, fault‐like distributions. The depth distribution is unusually deep for intraplate earthquakes and suggests a brittle‐ductile transition near 30–35 km. It also suggests the upper plate may have a geotherm similar to stable cratonic regions, rather than a steeper one that may be expected due to a lithospheric thinning implied by the location of the subducted plate. This can be understood as a transient effect of a Neogene flattening of the dip of the subducted plate, where a rapid thinning of the upper plate was followed with underplating by the cool subducted plate, leaving the upper plate geotherm unchanged while producing a thinned, mechanically weaker lithosphere. Finally, a spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31°S may be related to subduction of the Juan Fernandez ridge.
Crustal thickness and upper mantle velocities in the Tibetan Plateau Region from the inversion of regional Pnl waveforms: Evidence for a thick upper mantle lid beneath southern Tibet
Authors: Holt William E. , Wallace Terry C.
Published: February 2008

The crustal thickness and upper mantle velocity variation in Tibet and surrounding regions was mapped through the inversion of 130 individual long‐period Pnl waveforms. The 130 paths crisscross the Tibetan Plateau, Hindu Kush, Karakoram, southwest China, Burma, and northern India regions. We regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values. Crustal thicknesses in the Tibetan plateau, Karakoram, and Hindu Kush regions range from 63 to 72 km and Pn velocities are 0.10–0.20 km/s faster beneath these regions than those beneath northern India. We observed impulsive first arrivals (evidence for positive upper mantle gradients) on seismograms recorded at distances between 1000 and 1400 km. The paths of these waves crossed portions of Tibet, the Hindu Kush, and east India regions. Simple modeling of the long‐period Pn wave‐form and amplitude requires fairly high upper mantle gradients of 0.18–0.25 km/s per 100 km beneath the Pamir‐Hindu Kush region and requires an upper mantle lid in excess of 100 km thickness. The southern half of Tibet also requires a lid of about 100 km thickness. A thinner upper mantle lid (50 km) provided a poor match to the Tibetan observations, even with a very high gradient of 0.2 km/s per 100 km. The upper mantle structure of east India appears to be similar to the southern half of Tibet (100‐km‐thick positive gradient zone). Both the increase in Pn velocity from India into Tibet and the evidence for a thick upper mantle lid beneath Tibet are consistent with the model in which Indian lithosphere has underthrust southern Tibet. Due to poor path coverage, the structure beneath northern Tibet cannot be resolved, and thus underthrusting beyond central Tibet cannot be constrained. The rapid increase in upper mantle velocity from northern India into Tibet can be explained by the pressure increase alone, produced by the double‐thick crust, and indicates that the Indian lithosphere, which underlies Tibet, is out of thermal equilibrium.
Circulation near Axial Seamount
Authors: Cannon G. A. , Pashinski D. J.
Published: February 2008

Year‐long moored current measurements from 1987 to 1988 on the saddle (1860 m) between Axial Volcano and Brown Bear Seamount show an oscillating flow with a broadband frequency centered at about 4 days and a long‐term average flow of about 4 cm/s southward. The measurements at sill depth also suggest a possible alternating exchange of water north and south of the Cobb‐Eickelberg seamount chain with warmer water (0.05°C) coming from the north. However, measurements near the top of the volcano at about 1400 m show a weaker, but northward, mean flow, and the 4‐day oscillation barely exists. Conductivity, temperature, and depth (CTD) observations on the east side of the volcano infer possible intensification of southward flow adjacent to the Juan De Fuca Ridge, similar to that found with combined CTD and current meter observations about 100 km to the south. Southward flow is hypothesized along the entire east side of the ridge. On the west side the Axial‐Brown Bear saddle may be near a convergence zone between southward and northward currents which then flow westward. The highly energetic environment on the saddle suggests other gaps in the ridge, and seamount chains may be important in determining the fate of the hydrothermal effluents. The general oceanic flow at the top of the caldera, however, is more tenuous, and the depth of change between these two regimes cannot be determined.
Observations on the effects of sampling on hydrothermal vent habitat and fauna of Axial Seamount, Juan de Fuca Ridge
Authors: Tunnicliffe Verena
Published: February 2008

The study of hydrothermal vents is conducted mostly through samples collected by submersible. Two vent fields on Axial Seamount, Juan de Fuca Ridge, have been visited at least three times each between 1983 and 1988 by Canadian, American, and Soviet workers. This study offers some comparative observations, but controlled experimentation was not conducted. After 5 years, unsampled vents appeared unchanged in gross aspects such as animal abundance, but those that had been sampled were depleted or gone. Changes at a sulphide deposit inhabited by tube worms were documented by a time‐lapse camera over two years during which time both sulphide area and tube worm coverage decreased most dramatically immediately after sampling with submersibles. Vestimentiferans displaced from fluid flows were able to survive several weeks but were rarely seen emergent. Surfaces denuded of tube worms were repopulated by polychaete species previously known at this vent. Recruitment by tube worms to either the denuded mound or to settling plates was negligible. Some vent animals may be highly sensitive to disruption despite the naturally unstable nature of their habitat. Effects of sampling on long‐term studies of vent animals, sulphide deposits, and vent effluents need serious consideration.
Infrared radiative transfer in the dust‐free Martian atmosphere
Authors: Crisp David
Published: February 2008

Gases in the Martian atmosphere, including CO2, H2O, CO, and O3 combine to produce some absorption at most infrared wavelengths. Line‐by‐line and quasi‐random models are used to derive synthetic spectra of dust‐free Martian atmospheres. These spectra show where gases absorb most strongly and provide a baseline for comparison with the results from more complete models that include the effects of dust. Gas absorption and emission features at many infrared wavelengths provide a source of contamination that must be removed from remote sensing observations of the Martian surface. For example, the weak reflectance minimum observed at wavelengths near 2.35 μm, which has been interpreted as evidence for a variety of surface materials, is produced almost entirely by atmospheric CO and CO2 absorption. Isotopic CO2 bands near 7 and 8 μm and near‐infrared water vapor absorption bands partially overlap strong carbonate and hydrate features and frustrate systematic spectroscopic searches for these important candidate surface materials on Mars. In other spectral regions, gas absorption bands provide opportunities to study the structure and composition of the Martian atmosphere. Computed radiances within the strong CO2 15‐μm band are incorporated into an atmospheric retrieval algorithm to derive the atmospheric temperature structure from Mariner 9 IRIS observations. Absorption and emission by gases also contributes to the energetics of the Martian atmosphere. Near‐infrared CO2 bands absorb enough sunlight to produce globally‐averaged solar heating rates that vary from 1 K/Earth day at the surface, to 10 K/Earth day at pressures near 0.01 mbar. Other gases contribute 1–5% of the heating at some levels.
Martian south polar cap 1988
Authors: Iwasaki Kyosuke , Saito Yoshikazu , Nakai Yoshihiro , Akabane Tokuhide , Panjaitan Edward , Radiman Iratius , Wiramihardja Suhardja D.
Published: February 2008

The regression of the Martian south polar cap during the 1988 apparition is examined with the measurements of the photographs of Mars obtained at the Bosscha Observatory, at the Hida Observatory, and at the Kwasan Observatory. Around LS = 230°, the regression of the south polar cap in 1988 appears similar to those in other years. After LS = 240° the regression of the south polar cap in 1988 seems slower than those in 1956, in 1971, in 1973, or in 1977. However, in the period LS = 240°–250°, it seems faster than that in 1939 which is the slowest reported or than that in 1986 which is also very slow. The 1988 regression seems to be one of the slowest in the period LS = 240°–290°. Like the case of the south polar cap in 1986, the regression of the south polar cap in 1988 supports interannual difference of the regression of the south polar cap. The data from 1988 also support the proposition that the size of the south polar cap around LS = 250° becomes large in the year when global dust storms do not occur.
An outgassing release factor for nonradiogenic volatiles on Mars
Authors: Scambos Ted A. , Jakosky Bruce M.
Published: February 2008

The release factor for outgassing of nonradiogenic volatiles (e.g., water) on Mars since the end of accretion and heavy bombardment is estimated to be 0.017–0.112. The estimate is based on the 40Ar release factor (which is 0.012–0.040) and a correction coefficient derived from the history of volcanism on Mars. The 40Ar release factor is calculated from the bulk K content of the planet and its current atmospheric inventory of 40Ar. The range of Martian bulk K contents considered is 150–500 ppm, based on the geochemistry of the shergottites, nakhlites, and Chassigny (SNC) meteorites and the thermal history of the planet. The correction coefficient (termed the nonradiogenic release coefficient) compensates for the variable abundance of 40Ar in the Martian mantle through time versus the relatively constant nonradiogenic volatile abundance and for the variable rate at which outgassing has occurred through time on Mars. Volcanism is used as a measure of outgassing activity, and studies of Martian volcanic activity through time by Greeley (1987) and Tanaka et al. (1988) are used to calculate the correction coefficient. The calculated nonradiogenic release coefficients are 1.4–2.8; i.e., the release of nonradiogenic volatiles is 1.4–2.8 times the 40Ar release factors. The calculation assumes that Mars has not lost 40Ar since accretion ended and that the nonradiogenic volatiles are released with equal efficiency and by the same events as 40Ar. Interpretation of the result is highly dependent on the fate of early accreted surficial volatiles. If the present atmosphere and surficial volatile inventory is entirely secondary, i.e., released by outgassing after accretion, the release factors calculated may be used as an approximate ratio of present surficial inventory to total volatile content. If Mars retained some primary surficial inventory in addition to secondary volatiles, then the release factors pertain to the secondary fraction only, and estimates of the contribution of volatiles released by volcanism since accretion may be used to constrain the volatile content of the postaccretionary Martian mantle.
Seismic source theory in stratified anisotropic media
Authors: Ben‐Menahem Ari , Sena Arcangelo G.
Published: February 2008

We extend seismic source theory to general anisotropic media for the numerical evaluation of spectral amplitudes of point sources in multilayered anisotropic crustal models. First, we obtain an explicit representation of the spectral elastodynamic Green's tensor in general homogeneous anisotropic media as a sum of three integrals over the corresponding three slowness surfaces. The multidimensional stationary phase principle is then applied to derive an asymptotic approximation at the far field. In the case of azimuthally isotropic media, we offer an alternative representation of the Green's tensor and its ensuing displacements fields in the form of an exact Hankel transform over the horizontal wave number variable. The total field is specified here in terms of two potentials: an SH potential and a mixed quasi‐transverse/quasi‐longitudinal potential, both of which assume the role of two scalar Green's functions. The availability of the Green's tensor in analytical form enables one to obtain readily numerical solutions for a wide selection of media and sources. It is shown that the radiation field of an explosion has the following new features: (1) Quasi‐transverse waves are created with four and eight lobe patterns; (2) quasi‐longitudinal waves are generated for the collatitudinal displacement with four lobe patterns; (3) the energy ratio SV/P may reach the value of 20 for more than 50% of the azimuths in crustal structures such as tuff and shales; and (4) radiation patterns for vertical shear waves are created which are indistinguishable from corresponding waves produced by earthquake faults. Our formalism allows us to establish a Haskell‐type matrix algorithm for a multilayered azimuthally isotropic half‐space, which enables us to calculate body waves and surface waves in real‐earth crustal models.
Field‐impressed anisotropies of magnetic susceptibility and remanence in minerals
Authors: Potter David K. , Stephenson Alan
Published: February 2008

The application of alternating (AF) or direct (DF) magnetic fields to samples containing dispersed particles of magnetite, titanomagnetite, or maghemite has been found to alter significantly the measured low‐field susceptibility anisotropy even though the particle orientation is unchanged. The acquisition of isothermal remanent magnetization (IRM) is also strongly dependent on the previous AF treatment given to the samples. A method of quantifying the changes brought about by application of the fields is described. The samples studied were intrinsically isotropic or only weakly anisotropic, and the computed impressed susceptibility or IRM ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis. The size and shape of the field‐impressed ellipsoids were strongly dependent upon particle size. These effects might thus form the basis of a rapid, nondestructive means of estimating particle size in rocks. The results also have important implications for magnetic fabric analysis and the static AF demagnetization of rocks. In particular, to ensure that the intrinsic magnetic fabric is measured without any field‐impressed component of anisotropy, samples should not be subjected to static AF demagnetization prior to the determination of either low field susceptibility anisotropy or IRM anisotropy. Samples should also not carry a significant remanence (such as an IRM acquired in a field greater than about 5 mT) prior to low field susceptibility anisotropy analysis.
Permeability estimation from velocity anisotropy in fractured rock
Authors: Gibson Richard L. , Toksöz M. Nafi
Published: February 2008

Cracks in a rock mass subjected to a uniaxial stress will be preferentially closed depending on the angle between the fracture normal vectors and the direction of the applied stress. If the prestress fracture orientation distribution is isotropic, the effective elastic properties of such a material after application of the stress are then transversely isotropic due to the overall alignment of the cracks still open. Velocity measurements in multiple directions are used to invert for the probability density function describing orientations of crack normals in such a rock. This is accomplished by expanding the crack orientation distribution function into generalized spherical harmonics. The coefficients in this expansion are functions of the crack density and the crack aspect ratio distribution. The information on fracture distribution obtained from the velocity inversion allows an estimation of the anisotropic permeability of the fractured rock system. Permeability estimates are based on the number of cracks open of each aspect ratio, and the contribution of a given crack is weighted by the cosine of the angle between the crack and the direction of the applied pressure gradient. This approach yields a prediction of permeability as a function of the angle from the uniaxial stress axis. The inversion for crack orientation is applied to ultrasonic velocity measurements on Barre granite, and permeability predictions for this sample are presented. The inversion results are good and reproduce velocity measurements well, and the permeability predictions show some of the expected trends. Initial comparisons of the predictions with available permeability data, however, show deviations, suggesting that further information on partial crack closure and connectivity of cracks should be included into the permeability model.
The effect of H2O and CO2 on the viscosity of sanidine liquid at high pressures
Authors: White Bradford S. , Montana Art
Published: February 2008

As part of our continuing research on the nature of silicate liquids at high pressures and to provide insight into the flow properties of granitic magmas at depth, we determined the viscosity (η) of liquid KAlSi3O8, and the viscosities of liquids in the systems KAlSi3O8‐CO2 and KAlSi3O8‐H2O, as a function of pressure and temperature using the falling‐sphere method. At 1500°C, log η of KAlSi3O8 decreases from 5.0 at 1 bar to 4.36 and 3.96 at 15 and 20 kbar, respectively, consistent with the negative (∂η/∂P)T of other highly polymerized silicate liquids. However, log η isothermally increases to 4.23 at 25 kbar. Adding 0.50 wt % CO2 to the liquid produces an isothermal decrease in log η to 4.06, 3.79, and 3.41 at 15, 20, and 25 kbar and 1500°C. An equimolar amount of H2O (0.21 wt %) reduces log η to 3.45, 3.40, and 3.34 at the above conditions. At 25 kbar, 0.21 wt % H2O and 0.50 wt % CO2 produce nearly equal reductions in the viscosity of KAlSi3O8 at 1500° and 1600°C. These data indicate that H2O and CO2 disrupt the flow mechanism of KAlSi3O8 by breaking bridging‐oxygen bonds, either by reaction of the molecular volatile species with the tetrahedral network to form OH− and CO32−, or by CO2 reacting with a non‐bridging oxygen to form CO32−, which disrupts the network of the liquid by rearrangement of the local balance in charges. This is in accord with previous studies of the viscosities of H2O‐ and CO2‐bearing NaAlSi3O8 liquids. CO2 is more effective in reducing the viscosity of KAlSi3O8 relative to NaAlSi3O8 at 25 kbar, indicating that the former liquid may dissolve a larger percentage of total carbon as CO32−. This conclusion is consistent with our published feldspar‐CO2 phase equilibria. At 20 and 25 kbar and 1500°C, log η decreases to 2.30 as H2O increases to 2.00 wt %, with a highly positive (∂2(log η)/∂χ2)T,P. In contrast to KAlSi3O8‐H2O, KAlSi3O8‐CO2 exhibits a viscosity minimum between 0.50 and 1.00 wt % at 20 kbar and 1.00 and 1.50 wt % at 25 kbar, perhaps reflecting a concentration‐dependent speciation of the CO2 molecule similar to that for H2O, or a change toward non‐Newtonian, pseudoplastic behavior as the liquid becomes saturated with carbon.
Geochemistry and petrology of spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico: Partial melting and mantle metasomatism
Authors: Liang Yan , Elthon Don
Published: February 2008

Spinel Iherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N = 0.10–0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80–4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910°–1070°C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4–8.7% clinopyroxene, 43.8–44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes. The ∑FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2O enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution of group Ib peridotites. Clinopyroxenes in the group Ib xenoliths are reversely zoned with higher Mg # (100×Mg/(Mg+∑Fe)), CaO, and TiO2 and with lower Na2O and Al2O3 in their rims than in their cores. Bulk rock Al2O3 and SiO2 contents of the group Ib xenoliths tend to be lower than those of group Ia at the same MgO contents. The Al2O3 and SiO4 depletion in bulk rocks and Al2O3 and Na2O depletion in the rims of the group Ib minerals result from reaction with an alkaline‐element‐enriched, H2O dominated fluid. The history of group Ib xenoliths appear to be dominated by an early phase of melt extraction followed by interaction with a basanitic melt to produce the FeO, Na2O, and LREE enrichment in clinopyroxenes. These peridotites then appear to have been affected by a H2O‐rich fluid that depleted Al2O3 and Na2O from the rims of clinopyroxenes and produced incipient melt veins in the xenoliths.
High‐latitude paleomagnetic poles from Middle Jurassic Plutons and moat volcanics in New England and the controversy regarding Jurassic Apparent Polar Wander for North America
Authors: Van Fossen Mickey C. , Kent Dennis V.
Published: February 2008

A paleomagnetic study of Middle Jurassic plutonic and volcanic rocks in New England (White Mountains Magma Series) yields high‐latitude pole positions for North America. High unblocking temperature, moderate to high coercivity magnetizations of normal polarity have been isolated in three plutons (White Mountains batholith, Mount Monadnock, and the Belknap Mountains; mean age ∼169 Ma), but the mean pole (88.4°N, 82.1°E, A95 = 6.1°) is not distinguishable from the geographic axis and therefore the hypothesis that the plutons have been contaminated by recent field overprints can not be rejected. However, a dual polarity, high unblocking temperature, and high coercivity magnetization isolated from the Moat volcanics (169 Ma, Rb‐Sr age) was apparently acquired soon after caldera collapse and tilting, at about the time of intrusion and cooling of the Conway granite (reported ages K‐Ar biotite, 168 Ma; zircon fission track, 163 Ma). The Moat volcanics pole position (78.7°N, 90.3°E, dp = 7.1°, dm = 10.2°) calculated using the mean magnetization direction of reversed polarity (the Cr component) falls at high latitude but is distinguishable from the spin axis. Moreover, published Middle Jurassic paleomagnetic poles from Gondwana (Africa, Australia, and East Antarctica) transferred to the North American reference frame also suggest a high‐latitude Middle Jurassic pole position for North America, in agreement with the Moat volcanics pole. The new evidence for a Middle Jurassic loop to high latitudes in the North American apparent polar wander path conflicts by 15°–20° with some key published Jurassic reference poles (e.g., the Newark Trend N2 and the Corral Canyon poles) used to constrain current paleomagnetic Euler pole (PEP) apparent polar wander paths for the Jurassic. We suggest that a plausible explanation for the discrepancy is that the N2 and Corral Canyon magnetizations are in fact secondary and were acquired after tilting. The hypothesis that the North American apparent polar wander path ventured to high latitude in the Middle Jurassic requires further testing, however the results of this study already suggest that the path may be more complicated than that proposed by recently published PEP studies.
Small‐scale disequilibrium in a magmatic inclusion and its more silicic host
Authors: Davidson Jon P. , de Silva Shanaka L. , Holden Peter , Halliday Alex N.
Published: February 2008

Basaltic andesite inclusions and their host dacite from the Purico‐Chascon complex in northern Chile are isotopically distinct. Textural characteristics of the inclusions are typical of those resulting from magma mingling. Serial sectioning across the interface of an inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled us to examine the scale of mixing and chemical (isotopic and trace element) disequilibrium. The results of this work show that (1) the composition of the inclusion is relatively homogeneous; (2) the dacite host is generally higher in 87Sr/86Sr and lower in 143Nd/144Nd than the enclave, but it is heterogeneous on a small scale, and probably a “hybrid”; (3) the isotopic composition in the marginal zone, apparently on both host and inclusion sides of the weakly chilled interface, actually shows the highest 87Sr/86Sr and lowest 143Nd/144Nd; (4) large plagioclase crystals in the inclusions and host are xenocrystic, with higher 87Sr/86Sr than any of the other samples. These observations are reconciled with a model of magma evolution in a crustal magma chamber. In such a scenario the mafic magma is overlain by a cap of rhyolite—a partial melt of the dacitic ignimbrites which now underlie the Purico‐Chascon complex. Overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions.
Plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the Sierra Nevada Batholith, California
Authors: Barbarin Bernard
Published: February 2008

Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.