bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of hematite and its associated remanence in the Copper Harbor Conglomerate (Keweenawan), upper Michigan
Authors: Elmore R. Douglas , Van der Voo Rob
Published: February 2008

To study further the origin remanence in red beds, the volcanogenic Copper Harbor Conglomerate has been investigated using both paleomagnetic and petrographic techniques. Paleomagnetic results suggest the presence of two magnetic components, both possessing a westerly and shallow or normal Keweenawan direction. One component (in the vicinity of declination 294°, inclination −7.6) comprises approximately 30% of the total natural remanent magnetization, is removed by alternating field demagnetization following chemical demagnetization, and shows a consistent inclination error that is controlled in sense and amount by the dip of the cross beds in which the samples were taken. The low coercivities of this component suggest that the magnetization resides in magnetite. This and the inclination error due to the bedding indicate that the component is a detrital remanent magnetization.

The predominant component (declination 286.5°, inclination 0.0°, based on six sites) is removed by chemical demagnetization and shows no consistent relationship between inclination and dip of the cross beds, which suggests that it is a postdepositional magnetization. This component resides in hematite that is removed during the chemical demagnetization procedure. Petrographic studies show that hematite is present authigenically as pigment and as partly to wholly martitized grains. Martitization may have occurred before or after deposition; petrographic results are at present inconclusive. A pole position calculated from this postdepositional component (168.3°E, 11.2°N) is similar to poles from the overlying Nonesuch Shale and Freda Sandstone, which suggests that the remanence was acquired before the end of Freda deposition.
Comparative geochemistry of Apollo 16 surface soils and samples from cores 64002 and 60002 through 60007
Authors: Korotev Randy L.
Published: February 2008

New data are presented for the concentrations of 23 to 27 elements in nine Apollo 16 surface soil samples, two subsplits each of 12 splits of core 60002‐7, and one subsplit each of 10 splits of drive tube 64002. The surface soils are generally similar to other surface soils collected at their respective stations. All ten 64002 samples are similar to each other and to the station 4 surface soils and distinctly different from soils at other stations. High iron concentrations previously reported for these same subsplits of 60002‐7 result primarily from meteoritic metal. One sample from each core is contaminated by stainless steel. The recently proposed ferroan anorthositic norites are required to explain the soil compositions. A yet uncharacterized component with high concentrations of Na, Sr, and Eu is also needed.
Domain wall resonance and stability in magnetite
Authors: Smith Guy M. , Merrill Ronald T.
Published: February 2008

An understanding of the factors governing domain wall stability is crucial to any attempt to model remanence in multidomain materials. This paper introduces a technique to the study of multidomain remanence which has not previously seen application in rock magnetism: domain wall resonance (DWR). We show that the variation of the DWR spectra with different RF power levels gives useful, and hitherto unavailable, information about the nature of the potentials which govern domain wall motion. In particular, we demonstrate that these potentials must rise less rapidly than a harmonic potential, contrary to previous models. The possible mechanisms for domain wall pinning are discussed and it is concluded that the likeliest candidates for pinning sites in our samples, at least, are large nonuniform dislocation arrays. Finally, we suggest a possible method for selectively demagnetizing multidomain grains based on DWR techniques.
Venus gravity anomalies and their correlations with topography
Authors: Sjogren W. L. , Bills B. G. , Birkeland P. W. , Esposito P. B. , Konopliv A. R. , Mottinger N. A. , Ritke S. J. , Phillips R. J.
Published: February 2008

This report provides a summary of the high‐resolution gravity data obtained from the Pioneer Venus Orbiter radio tracking data. Gravity maps, covering a 70° latitude band through 360° of longitude, are displayed as line‐of‐sight and vertical gravity. Topography converted to gravity and Bouguer gravity maps are also shown in both systems. Topography to gravity ratios are made over several regions of the planet. There are markedly different ratios for the Aphrodite area as compared to the Beta and Atla areas.
Statistical averaging of marine magnetic anomalies and the aging of oceanic crust
Authors: Blakely Richard J.
Published: February 2008

Visual comparison of Mesozoic and Cenozoic magnetic anomalies in the North Pacific suggests that older anomalies contain less short‐wavelength information than younger anomalies in this area. To test this observation, magnetic profiles from the North Pacific are examined from crust of three ages: 0–2.1, 29.3–33.1, and 64.9–70.3 m.y, B.P. For each time period, at least nine profiles were analyzed by (1) calculating the power density spectrum of each profile, (2) averaging the spectra together, and (3) computing a ‘recording filter’ for each time period by assuming a hypothetical seafloor model. The model assumes that the top of the source is acoustic basement, the source thickness is 0.5 km, and the time scale of geomagnetic reversals is according to Ness et al. (1980). The calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short‐wavelength information as the crust ages. These results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer, corresponding to pillow basalt of seismic layer 2A, acts as a source of noise to the magnetic anomalies. As the ocean crust ages, this noisy contribution by the pillow basalts becomes less significant to the anomalies. Consequently, magnetic sources below layer 2A must be faithful recorders of geomagnetic reversals.
Changes in compressional and shear wave velocities and dynamic moduli during operation of a hot dry rock geothermal system
Authors: Pearson Christopher F. , Fehler Michael C. , Albright James N.
Published: February 2008

Compressional and shear wave velocities measured during cross well seismic surveys are found to decrease during operation of a hot dry rock geothermal system. The relative P and S wave velocity decrease is very close to values predicted by O'Connell and Budiansky (1974), who modeled velocities in rocks containing fluid‐saturated and dry cracks. Because our data fit their model quite well, we attribute the velocity decrease to an increased density of fluid‐filled fractures in the reservoir. Fracture density parameters of O'Connell and Budiansky range as high as 0.365 near the main heat transfer areas of the geothermal reservoir. The dynamic shear and Young's modulus are significantly lower near the principal heat transfer surfaces, but the bulk modulus is largely unaffected. These observations suggest that the velocity differences are caused by fully saturated fractures. We demonstrate that the fractures inferred by this survey may have lengths as large as 70 cm. These fractures may have been caused by shear failure, microcracking associated with thermal drawdown, or thermal stress cracking.
A semiempirical method for analysis of the reflectance spectra of binary mineral mixtures
Authors: Johnson Paul E. , Smith Milton O. , Taylor‐George Susan , Adams John B.
Published: February 2008

A simple semiempirical method is presented for determination of the spectral reflectance of a powdered binary mineral mixture. This technique uses a two‐stream radiative transfer model (a modified Kubelka‐Munk model) on a particulate medium of isotropic scatterers. The particles are assumed to be much larger than the wavelength of light under consideration. This same method can be used to determine the relative proportion of components in a mixture for which the spectral reflectance is known. Binary mixtures of olivine, two pyroxenes, and magnetite are used to test this model. The theoretical and empirical results agree approximately within experimental errors.
Shear and compression wave measurements in shocked polycrystalline Al2O3
Authors: Gupta Y. M.
Published: February 2008

Experimental techniques have been developed to study the propagation of large‐amplitude, one‐dimensional shear and compression waves in shocked solids of geophysical interest. Shear wave velocities and amplitudes can be directly measured along with the usual measurements on longitudinal waves. Although shear wave amplitude measurements do not approach the quality of the compression wave data, shear wave velocities can be measured to a precision of better than two percent. Experimental measurements to 90 kbar compressive shock stresses are reported in Al2O3. Within experimental scatter (±2%), our shear wave velocity data are in good agreement with the extrapolation of the ultrasonic measurements. The longitudinal measurements are in good agreement with earlier work. The experimental developments reported here are expected to be important to high pressure geophysics applications because they provide a direct measure of the shear modulus in the shocked state. The shear modulus is not only a more sensitive indicator of the solid state than the longitudinal modulus but in conjunction with the longitudinal wave data can provide the mean stress‐volume relations for comparison with static data.
The accuracy of the low‐degree geopotential: Implications for ocean dynamics
Authors: Wagner C. A.
Published: February 2008

The low‐degree portion of a number of recent global gravitational fields have been tested for accuracy using precise accelerations measured on 24‐hour satellites. These measurements are independent of the data in the field models. The results show that the most accurate of the recent solutions have degree error variances significantly less than 100 mm (geoid height equivalent) for all degrees less than five. The broad and lasting features of ocean circulation should now be visible when comparing the geoid computed from this model with the average sea surface computed from satellite altimetry. However, before accurate mapping is possible, the equivalent accuracy of the mean altimetric surface at long wavelengths needs to be verified.
Seismic imaging of the Elberton Granite, Inner Piedmont, Georgia, using COCORP southern Appalachian data
Authors: Jurdy Donna M. , Phinney Robert A.
Published: February 2008

Seismic imaging of the Elberton granite in the Georgia Inner Piedmont was undertaken to define its shape and to determine its relation to the southern Appalachian thrust. The recent Consortium for Continental Reflection Profiling (COCORP) lines in the southern Appalachians suggest a major thrusting of a crystalline block over sediments, although the extent of this thrust is subject to interpretation. The fine‐grained Elberton, of Late Devonian age, is the only major pluton crossed by the seismic traverse and is not well defined on the unmigrated common depth point (CDP) seismic section. Migration by beam‐forming reveals considerable variation in the direction of returning signal and provides a clearer image of the laminated reflections, ubiquitous in the southern Appalachians, which have been interpreted as the equivalents of the Valley and Ridge sediments beneath an allochthonous sheet. The upper 5 km portions of the migrated and CDP sections lack events, probably because of the necessary stripping (mute) of a large‐amplitude, early arrival from the seismic records before further processing. Filtering, and transformation of the data to the intercept‐slowness domain, to eliminate this interfering arrival are attempted to enhance the upper portion of the seismic sections Although these approaches do not give a consistent picture of the shallow section, a pattern of scatterers located at depths of 2–3.5 km emerges; these may correlate with the base of the intrusive body. This identification of the granite base, along with the presence of the thrust‐related reflectors beneath the pluton, suggests a prethrust or synthrust emplacement of the granite. However, tectonic interpretations of the Inner Piedmont based on the Elberton granite are severely limited by assumptions: that the pluton age corresponds to time of cooling, that the pluton is not fed by a narrow (seismically elusive) feeder, and that the thrust has not been reactivated after emplacement of the intrusive body. If all these assumptions hold, then the Late Devonian age of the granite is the earliest possible time of thrusting.
Microcrack study of granitic cores from Illinois deep borehole UPH 3
Authors: Kowallis Bart J. , Wang Herbert F.
Published: February 2008

Microcracks were examined in five granite samples ranging in depth of origin from 696 to 1572 m by optical and scanning electron microscope observations and by elastic property measurements. The samples were obtained from the continuous crystalline core from Illinois deep borehole UPH 3 located in Stephenson County, Illinois. The core is a fine‐grained granite at the top and it becomes progressively coarser with depth. Cracks and fractures can be seen with the unaided eye in most of the core. Many of the larger fractures are coated or filled with secondary minerals. Long, narrow, tapered cracks which are open have not been observed in other midcontinent granites. These narrow cracks have sharp, matching edges and appear to be fresh. They are attributed to stress relief due to removal of the core by drilling. This interpretation is supported by measurements of velocity and strain as a function of confining pressure. The velocity and static bulk modulus at 1‐bar pressure were smaller for samples originating from greater depths, indicating progressively greater stress relief cracking for deeper samples. The crack spectrum derived from the strain measurements showed that the majority of the crack porosity was for cracks with closure pressures between 150 and 225 bars. The conclusion that stress relief microcracks occur with greater concentrations in cores from greater depths implies that cautious interpretation must be made of laboratory measured physical properties which are sensitive to open microcracks.
The 1982 Control Network of Mars
Authors: Davies Merton E. , Katayama Frank Y.
Published: February 2008

Summary and entire article are available on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B93‐005; $2.50. Payment must accompany order.

A new planet‐wide control network of Mars was computed in September 1982 using a large single‐block analytical triangulation with 47,524 measurements of 6853 control points on 1054 Mariner 9 and 757 Viking pictures. A total of 19,139 normal equations were solved, and the resulting standard error of measurement was 18.06 μm. Appendix A lists those control points that are identified by name and letter designation; Appendix B gives the areographic coordinates of the control points. Both appendices are available on microfiche. The coordinates of the Viking 1 lander site are latitude 22.480°, longitude 47.962°, and radius 3389.32 km.
The Olympus Mons Aureole: Formation by gravitational spreading
Authors: Francis P. W. , Wadge G.
Published: February 2008

New observations of the aureole deposits of Olympus Mons are in agreement with previous suggestions that the aureole lobes could have been formed by movement under gravity away from the observed scarps. The characteristic corrugated morphology of the aureole is probably secondary, formed by erosion along structural inhomogeneities. Geometries of the corrugations and translational faults in the aureole deposits favor a mechanism of emplacement that involves material with substantial shear strength rather than viscous flow. Estimates of the aspect ratios of terrestrial landslides, nappes, and ash flows suggest that the aureole deposits are more similar in overall shape to slide‐emplaced bodies than to pyroclastic flows. Although Martian physical conditions permit the formation of ash flows, there is no positive evidence for their existence. A gravitational spreading mechanism is proposed that involves imbricate thrusting in distal regions, analogous with terrestrial thrust sheets. Corrugations in the aureole developed by erosion along imbricate or listric faults. Basal shear stresses on the decoupling surface may have been of the order of 106 Pa (10 bars). The nature of the decoupling surfaces in the proximal regions is uncertain but is likely to have been the topographic surface in distal parts. The aureole deposits were probably emplaced as a series of sheets by spreading under gravity from an ancestral Olympus Mons, which extended farther to the northwest than does the present volcano, as a result of the accumulation of lavas under the then prevailing stress conditions. Recent lava eruptions from the volcano have been concentrated in northeast and southwest sectors, indicating a changed stress regime since aureole emplacement.
A microcrack model for the deformation and failure of brittle rock
Authors: Costin L. S.
Published: February 2008

A continuum model derived from the mechanics of tensile microcracks is presented which describes the deformation of brittle rock. The model employs the assumption that stress and time‐dependent microcrack growth is responsible for the inelastic deformation. Microcrack growth is assumed to occur by two mechanisms: stress‐induced crack growth (time independent) and stress corrosion (stress and time dependent). From the analysis of individual cracks a criterion for the initiation of damage (crack growth) is derived. This results in the specification of initial and subsequent damage surfaces in stress space which are similar to yield surfaces in the theory of plasticity. When the stress state is below the damage surface, no stress‐induced crack growth can take place. For stress states on the damage surface, crack growth accompanies any increase in loading, thus expanding the damage surface. By generalizing the results obtained from the analysis of single cracks, a continuum description of the behavior of an ensemble of cracks in an otherwise elastic body is derived. The resulting constitutive equation is essentially elastic but accounts for material behavior due to microcrack growth through the inclusion of an internal state variable which is a measure of the crack state. The form of the evolutionary equation for the crack state parameter is determined from the fracture mechanics analysis of single cracks and experimental results on time‐dependent crack growth in rock. Model simulations of quasi‐static uniaxial and triaxial compression tests are presented, and the results are compared to the results of a similar laboratory test on Westerly granite.
Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift, Inca Transform, and Ecuador Rift: 3. Trace element abundances and petrogenesis
Authors: Perfit Michael R. , Fornari Daniel J. , Malahoff Alexander , Embley Robert W.
Published: February 2008

Glassy to sparsely phyric submarine lavas were recovered from nine Alvin dive sites located along the eastern Galapagos rift and at the intersection with the Inca transform. Samples include quartz‐normative tholeiitic basalts (MORB), numerous Fe‐ and Ti‐enriched basalts (FeTi basalts) and a smaller number of oceanic andesites (55.9–64.3 wt % SiO2). MORB have light rare earth element (REE) and large ion lithophile (LIL) element depletions but exhibit slight REE fractionation (CeN/YbN 0.6–1.0) and increasing negative Europium anomalies (0.96–0.66) with progressive differentiation. Andesites have sixfold to tenfold enrichments of incompatible elements and volatiles compared to the least fractionated (Mg number < 60) basalt recovered. REE and LIL enrichments in the FeTi basalts and andesites are up to 70% greater than those predicted from closed‐system fractional crystallization models computed using major and trace element data. Trace element data indicate that extreme fractional crystallization of MORB liquids (40–65%) has occurred in order for the most evolved FeTi basalt to be generated and further crystallization (40–50%) of FeTi basalt residual liquid is required to produce andesites. Magma mixing has occurred on a small scale in the evolved liquids and can partially explain the chemical characteristics of basaltic andesites. All data point to extensive amounts of fractional crystallization during the evolution of these lavas; however, mantle heterogeneity and other processes such as partial melting, open‐system fractional crystallization, and convection‐driven thermogravitational diffusion may also play minor roles in influencing magmatic evolution. The principal tectonic controls which influence magmatic evolution along this accretionary boundary are (1) the presence of the cold, thick lithospheric edge of Nazca plate exposed at the intersection with the Inca transform, and (2) attempted rift propagation across the Inca transform intersection, which has resulted in excess magmatism and the eruption of only FeTi basalts. Three subrift, accretionary magmatic domains result from the interplay of transform effects and attempted propagation. The chemistry of lavas is a direct consequence of the magmatic domain that prevails along an accretionary boundary.
Inversion of batch melting equations and the trace element pattern of the mantle
Authors: Albarède F.
Published: February 2008

The inversion of trace element data for batch melting models consists of using concentrations measured in a series of cogenetic lavas to derive the trace element pattern of their source. The necessary minimum assumptions include the absence of late crystallization/contamination processes (primary liquids), the identification of the possible residual phases, and a homogeneous equilibrium melting situation for which partition coefficients can be estimated. The problem is shown to be, in general, over‐determined provided the number of samples exceeds the number of the elements for which source concentration is desired. There are several important restrictions: (1) transition elements are of little value because they are sensitive to subtle crystallization effects, (2) olivine and orthopyroxene reject almost perfectly all the remaining trace elements and their abundance must be constrained independently from petrological information. After the reliability of the theory was checked using synthetic examples, three case studies were carried out on Grenada alkali basalts and Kohala and Honolulu basaltic series (Hawaii). Rare earth elements, Th, and Ba data were inverted and suggest that except for Honolulu, the mantle source is LREE depleted in accord with Nd isotope data. Garnet behaves generally as a perfectly residual phase. Some tentative mineralogical compositions of the mantle, from which the different series are derived, are proposed. There is no indication that the present theory could not be used for major elements as well.
Paleomagnetism of two Late Pleistocene lake basins in Colorado: An evaluation of detrital remanent magnetization as a recorder of the geomagnetic field
Authors: Rosenbaum J. G. , Larson E. E.
Published: February 2008

Paleomagnetic studies of magnetite‐bearing lacustrine sediments from two late Pleistocene sections (age range about 12,000–23,000 14C years), one at Devlins Park in the Front Range, the other at Triangle Park in the northern end of the Sawatch Range, Colorado, indicate that detrital remanent magnetization of fine‐grained (silty clay) sediments at both locations was capable of recording changes in the magnetic field over periods as short as 100–200 years, whereas, coarser grained silts in the Triangle Park section seem to be less efficient recorders of the geomagnetic field. The remanence of the fine‐grained sediments contains an apparent inclination error of about 9° at both localities. In addition, locally operative depositional and/or postdepositional processes have produced both intrabasinal and interbasinal systematic differences in the remanence records of up to about 30° in declination and 10° in inclination. Comparison of the directional data from Devlins Park with those from four other broadly contemporaneous North American lake basins reveals that although there are abundant minor and several major discrepancies, there are general similarities in the longer wavelength features, particularly of declination. The differences in the records are attributable to depositional and postdepositional factors, variable and secondary overprinting, local field differences, sampling techniques, and incorrect stratigraphic correlations. The overall similarity in the records probably reflects the influence of the geomagnetic field. The greatest difficulty in determining the significance of the similarities and differences between the records from the various lakes is the present lack of precise age and stratigraphic control.
Cosmogenic neon from procompaction irradiation OF Kapoeta and Murchison
Authors: Caffee M. W. , Goswami J. N. , Hohenberg C. M. , Swindle T. D.
Published: February 2008

Neon from hand‐picked Murchison and Kapoeta grains, selected on the basis of the presence or absence of solar flare particle tracks, was analyzed in order to delineate the precompaction history of this material. The irradiated grains showed large enrichments of cosmogenic neon relative to the unirradiated grains. Galactic cosmic ray (GCR) exposure ages for the unirradiated grains yield the nominal values reported for the recent exposure history of these meteorites. Apparent minimum precompaction galactic exposure ages of 28 m.y. and 56 m.y. would have been obtained for Murchison and Kapoeta, respectively, if the cosmogenic effects in the irradiated grains were due to GCR irradiation. Since this seems unreasonably long, the cosmogenic neon in the irradiated grains may be due to spallation by solar cosmic rays. This, however, would require a more active early sun. The isotopic composition of the cosmogenic neon in these grains suggests a harder energy spectrum than is characteristic of present solar flares. Lack of apparent solar wind effects may require some kind of shielding, such as nebular gas.
Chemical and physical studies of type 3 chondrites: 2. Thermoluminescence of sixteen type 3 ordinary chondrites and relationships with oxygen isotopes
Authors: Sears Derek W. G. , Weeks Karen S.
Published: February 2008

Thermoluminescence (TL) sensitivity values for sixteen type 3 ordinary chondrites, fourteen of them from Antarctica, have been measured. The values obtained (normalized to the TL sensitivity of the Dhajala meteorite) range from 1.6 (Allan Hills A77216) to 0.010 (Allan Hills A77176), and include two (Reckling Peak A80207 and Allan Hills A77176) that are particularly low. They fill a hiatus in the TL distribution that previously existed between St. Mary's County and Bishunpur, the latter being a meteorite with one of the lowest TL sensitivities known. The histogram of TL sensitivity values now shows a single distribution with higher values preferred; it resembles the histogram for L chondrites occupying the petrologic types 3, 4, 5, and 6. There is a tendency for the TL sensitivity of meteorites to decrease as δ18O increases; equilibrated meteorites have TL sensitivities of 5‐50 and δ18O = 4.5 to 5.0%, while the most primitive meteorites with TL <0.01 have δ18O = 5.7 to 6.2‰. Meteorites with intermediate TL have intermediate δ18O. Theoretically it is possible that the range of δ18O values observed may reflect progressive loss of O in the form of CO at very low temperatures, but very restrictive physical conditions and a complex history seem to be required.
Experimental study of segregation in plane front solidification and its relevance to iron meteorite solidification
Authors: Sellamuthu R. , Goldstein J. I.
Published: February 2008

A directional solidification technique was developed and applied to the problem of fractional crystallization of an iron meteorite parent body. Samples of Fe‐Ni alloys close to meteorite compositions and containing S, P, and C were made. The solidified structures contain secondary phases such as sulphides within the proeutectic single crystal austenite (taenite). As a result of these experiments, we propose that the secondary phases observed in iron meteorites were formed during primary solidification of austenite (taenite). The measured composition profiles of Ni, P and C in the alloys were used to explain the elemental distribution within a chemical group of iron meteorites. An analytical procedure was applied to determine the equilibrium distribution coefficients as a function of fraction solidified for Ni and P from the composition profiles. The distribution coefficients of Ni and P agree with previous values. These distribution coefficients are of particular interest in the determination of the elemental distributions in iron meteorites.
Siderophile trace elements in the Earth's oceanic crust and upper mantle
Authors: Chou Chen‐Lin , Shaw D. M. , Crocket J. H.
Published: February 2008

Abundances of Re, Ir, and Au have been determined in a suite of oceanic basalts using radiochemical neutron activation analysis. Mean abundances of highly siderophile elements in the oceanic crust have been estimated. Siderophile elements are fractionated during basalt genesis. Iridium, Os, Pd, Pt, Ru, and Rh are depleted in basalts relative to peridotites. Gold is only slightly enriched in basalts. Rhenium is favorably partitioned into the liquid during partial melting of peridotites. Concentrations of siderophile elements in the earth's upper mantle are higher than that expected from core‐mantle equilibrium. Six platinum‐group elements, Re, and Au have chondritic ratios in the upper mantle, indicating the presence of a meteoritic component. The abundances of eight highly siderophile elements can be accounted for by 0.74% of CI‐chondritelike material in the upper mantle. The low‐Ti mare basalts have lower Re contents and lower Re/Ir ratios than oceanic basalts, which may be interpreted by a lower partition ratio of Re under extremely dry and reducing conditions in the lunar interior.
Thermal stresses in planetary elastic lithospheres
Authors: Turcotte Donald L.
Published: February 2008

The role of thermal stresses in the tectonics of the moon is reexamine in this paper. A model is used that considers a spherical elastic shell overlying a fluid core. It is shown that the thermal stresses generated by temperature changes within the shell usually dominate over the thermal contraction or expansion of the core. During the entire evolution of the moon the cooling of the lithosphere is likely to have dominated the cooling of the interior and the result would be tensional lithospheric thermal stresses. However, during the recent evolution of the moon the change in the near surface thermal stresses is compressional. It is argued that the surface compressional features on Mercury are not due to the thermal contraction of the interior.
Magnetic paleointensity determination on lunar sample 62235
Authors: Sugiura Naoji , Strangway David W.
Published: February 2008

There have been very few determinations of the ancient lunar magnetic field using the full Thellier technique. Samples studied range in age from 4.0 b.y. to very young. Four samples show values less than 0.1 oersteds. The fifth sample reported on earlier by Collinson et al. [1973] has a reported value of 1.3 oe. In this paper, we have repeated this determination and found essentially the same very high value. It remains to be assessed by further sample studies whether this value is anomalous and somehow associated with impact or whether this reflects a high intensity field in early lunar history.
High magnetic coercivity of meteorites containing the ordered FeNi (Tetrataenite) as the major ferromagnetic constituent
Authors: Nagata T.
Published: February 2008

Three chondrites, Yamato‐74160 (LL6), ALH‐77260 (L3), and St. Séverin (LL6), contain a considerable amount of the ordered FeNi, tetrataenite, in their metallic components. Natural remanent magnetization (NRM) of the three chondrites contains a highly stable component which can hardly be demagnetized by alternating magnetic fields up to 1800 Oe peak. The highly stable NRM component is identified to the NRM of the tetrataenite phase which has a large magnetic and optical anisotropy. By heating up to about 800°C, however, the tetrataenite phase is broken down to the ordinary disordered taenite which has a much weaker magnetic coercivity.
Impact of an asteroid or comet in the ocean and extinction of terrestrial life
Authors: Ahrens Thomas J. , O'Keefe John D.
Published: February 2008

Finite difference calculations describing the impact mechanics associated with a 10 to 30 km diameter silicate or water object impacting a 5 km deep ocean overlying a silicate solid planet at 30 km/sec demonstrate that from 12 to 15% of the bolide energy resides in the water. In the gravity field of the earth some 10 to 30 times the impactor mass of water is launched on trajectories which would take it to altitudes of 10 km or higher. This ejecta launched on trajectories which can achieve stratospheric heights is 101 to 102 projectile masses, similar to that resulting from impact of objects on an ocean‐free silicate half‐space (continent). As in the case of impact directly onto a continent, only the ejecta composed of impactor material, launched on trajectories which would carry it to stratospheric heights, matches the fraction (10−2 to 10−1) of bolide (extraterrestrial) material found in the platinum‐metal‐rich Cretaceous‐Tertiary and Eocene‐Oligocene boundary layers. Oceanic impact results in impulsivelike giant tsunamis initially having amplitudes of ∼4 km, representing the solitary waterwave stability limit in the deep ocean, and containing 10−2 to 10−1 of the energy of the impact. Using the constraint of no observed turbidites in marine sediments in the Cretaceous‐Tertiary and Eocene‐Oligocene boundary materials (calculated maximum water‐sediment interface particle velocity ∼10° m/sec) implies a maximum impactor energy of ∼1028 to ∼1029 erg corresponding to a maximum diameter for a silicate impactor of ∼2 km (at 11 km/sec). Minimal global tsunami run‐up heights on the continents corresponding to impacts of this energy are 300–400 m. We speculate that such waves would inundate all low altitude continental areas and strip and silt over virtually all vegetation. As a result, the terrestrial animal food chain would be seriously perturbed. This, in turn, could have caused extinction of large terrestrial animals including the archosaurs.
Cratering experiments in sands and a trial for general scaling law
Authors: Mizutani Hitoshi , Kawakami Shin‐ichi , Takagi Yasuhiko , Kato Manabu , Kumazawa Mineo
Published: February 2008

We report results of vertical impacts of aluminum projectiles into quartz sand. The impacts were performed at velocities of 35 to 830 m/sec with a single stage powder gun. Morphology of craters formed in loose sands is generally conical, whereas morphology of craters formed in self‐compacted sands varies from a flat‐floor type to a double ring type, with increasing impact velocity. The present data together with previous experimental data and Dienes and Walsh's ‘late‐stage equivalence’ indicate that crater diameter is expressed by a function of the ‘late‐stage effective energy’ but not of the impact kinetic energy. The diameter of a crater formed in a noncohesive sand was found to be proportional to one‐fourth the power of the late‐stage effective energy. For a general impact cratering in a target with a finite strength, the diameter, D, versus impact velocity, v, relation is written as follows:

urn:x-wiley:01480227:media:jgrb4659:jgrb4659-math-0001

where v* and D* are normalizing values of the impact velocity and diameter, and K is a constant related to target and projectile properties. The relation suggests that there are four regimes in crater diameter versus the kinetic energy relation.
Homogeneity of lava flows: Chemical data for historic Mauna Loan eruptions
Authors: Rhodes J. M.
Published: February 2008

Chemical analyses of basalts collected from the major historic eruptions of Mauna Loa volcano show that many of the flow fields are remarkably homogeneous in composition. Despite their large size(lengths 9–85 km), large areal extents (13–114 km2 ), and various durations of eruption (1–450 days), many of the flow fields have compositional variability that is within, or close to, the analytical error for most elements. The flow fields that are not homogeneous vary mainly in olivine content in an otherwise homogeneous melt. Some are composite flow fields made up of several, apparently homogeneous subunits erupted at different elevations along the active volcanic rifts. Not all volcanoes produce lavas that are homogeneous like those of Mauna Loa. If we are to use studies such as this to evaluate compositional diversity in lavas where there is a lack of sampling control, such as on other planets, it is necessary to understand why some flow units and flow fields are compositionally homogeneous and others are not, and to develop criteria for distinguishing between them.
Viscous flow behavior of four iron‐containing silicates with alumina, effects of composition and oxidation condition
Authors: Klein Lisa C. , Fasano Benjamin V. , Wu Jenn Ming
Published: February 2008

The viscosity of four iron‐containing silicates with alumina has been measured under oxidizing and reducing conditions over the temperature range 450° to 700°C. Initially, the compositions contain approximately 2, 5, 10, and 20 mol.% of trivalent metal oxide. The viscosities of compositions with approximately equimolar Fe3+ and Al3+ are compared to those determined for compositions with trivalent cation all Fe or all Al. Then the batch composition calculated to replace one half of the Fe3+ by Al3+ under oxidizing conditions is corrected for the divalent iron present under mildly reducing conditions in forming gas (95% N2‐5% H2) and strongly reducing conditions with carbon. In general, the viscosity measured in air was higher than the viscosity measured in forming gas with or without carbon for a given temperature and composition. The glasses prepared under oxidizing conditions had Fe2+/Fe total ratios less than 0.25 and the glasses prepared under reducing conditions had Fe2+/Fe total ratios between 0.27 and 0.91. The viscosities for these glasses can be represented by a Vogel‐Fulcher relation for each of the three oxidation conditions.
Recent changes in the dynamic condition of the Ross Ice Shelf, Antarctica
Authors: Jezek Kenneth C.
Published: February 2008

Variations in the amplitude of radar echoes from the bottom of the grid western half of the Ross Ice Shelf have been analyzed. Contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf, bands of low signal strength downstream from both Crary Ice Rise and the Siple Coast do not correlate with modern flow lines. The difference in direction between the radar bands downstream of Crary Ice Rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around Crary Ice Rise retreated within the last 1000 years. This hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of Crary Ice Rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank. We interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise. Similar but less detailed evidence found downstream of the Siple Coast suggests that there was a regional retreat of the West Antarctic grounding line.
Calcite fabric and acoustic anisotropy in deep‐sea carbonates
Authors: Schaftenaar C. H. , Carlson R. L.
Published: February 2008

Several investigators have proposed that compressional wave anisotropy in indurated, deep‐sea carbonates is caused by a preferred orientation of calcite c axes normal to bedding. To test this hypothesis, we have used X ray pole figure goniometry to measure calcite fabrics in three anisotropic, calcareous sediment samples from Deep‐Sea Drilling Project hole 516F, on the Rio Grande Rise. Anisotropy in these samples ranges from 3.4 to 12.0%. The suggested concentration of c axes is present but very weak; concentrations of c axes normal to bedding are in the range 1.1–1.3% per 1% area. We used the Voigt‐Ruess‐Hill averaging scheme to calculate velocities from the X ray fabric data and found that even in nonporous, pure carbonate rocks, having the observed concentrations of c axes, the acoustic anisotropy would be less than 1%. We conclude that preferred orientation of calcite is not a significant contributor to compressional wave anisotropy in deep‐sea sediments.
A detailed view of the South Pacific geoid from satellite altimetry
Authors: Sandwell David T.
Published: February 2008

Images of sea surface undulations in the South Pacific have been constructed from GEOS 3 and SEASAT altimeter data. Height discrepancies at crossover points, associated with long‐wavelength radial orbit error, were suppressed by taking along‐track derivatives of the ascending and descending profiles. These geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical. Finally, the results are displayed by using the hill shading technique, where gray‐tone images represent the innner product of the deflection vector with an assigned sun vector. Less apparent sea surface undulations can be enhanced by varying the sun's zenith and azimuth. Shorterwavelength sea surface undulations reflect seafloor topography. For instance, fracture zones (FZ's) appear as elongated sharp steps in the sea surface, while seamounts produce circular bumps. Since large areas of the South Pacific are unsurveyed, many previously undetected features appear on the images. Comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to Easter Island's expression. The dominant features in the images, however, are the large age‐offset FZ's such as the Eltanin and Udintsev FZ's. The images reveal that the Eltanin FZ is connected to the Louisville Ridge; combined they produce a continuous geoid signature across most of the South Pacific. This supports the hypothesis of Hayes and Ewing (1968) that the Louisville Ridge is the northwest extension of the Eltanin FZ.
Horizontal strain across the Wasatch Front near Salt Lake City, Utah
Authors: Snay Richard A. , Smith Robert B. , Soler Tomas
Published: February 2008

Two adjacent geodetic networks span a 105‐km extent of the Wasatch fault zone in north‐central Utah. One of the networks, located in the vicinity of Salt Lake City, was observed by the National Ocean Service/National Geodetic Survey (NOS/NGS) in 1962–1963 (triangulation mode) and again in 1973–1974 (combined triangulation‐trilateration mode). To the north, in the vicinity of Ogden, the U.S. Geological Survey (USGS) performed five separate trilateration surveys of the second network during the 1972–1981 interval. When fit to a model assuming both temporal and spatial homogeneity for the strain rate, the two independent data sets yield mutually consistent strain rate tensors. Subsets of the NOS/NGS data, however, suggest spatial variation in the strain field, and a subset of the USGS data indicates a possible temporal variation in the strain rate. The quality of the data is such that we are uncertain whether or not these apparent variations represent actual tectonic signals. The strain rate tensors for the two complete data sets suggest an underlying deformational pattern characterized by east‐west (E‐W) maximum principal strain (extension positive). This pattern may also be inferred from regional seismicity and Quaternary geology. The suspected variations would imply that the area to the north of Salt Lake City experienced significant E‐W compression preceding 1978, in contradiction to the underlying strain pattern. This compression may be caused by the mountainous block, eastward adjacent to the fault zone, displacing westward along a shallow‐angle, west dipping detachment surface. We speculate on the existence of an E‐W trending zone of weakness, for example, secondary faults, located north of Salt Lake City to explain how the stress field to the north may be decoupled from that to the south where E‐W compression has not been detected. While seismic and geologic information contribute some plausibility to our speculations, the evidence for the existence of the suspected variations is inconclusive.
Constraints on plate motions in southern Pakistan and the northern Arabian Sea from the focal mechanisms of small earthquakes
Authors: Quittmeyer Richard C. , Kafka Alan L.
Published: February 2008

The focal mechanism and depth were determined for nine small earthquakes (M0<1025 dyn cm, M<5.5) that occurred in southern Pakistan and the northern Arabian Sea from an analysis of the vertical component of Rayleigh waves in combination with limited first‐motion data. Focal parameters were determined from the Rayleigh waves by using an event‐pair method of analysis. For earthquakes that are located very close to each other (<≈ 50 km), the event‐pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest (20–50 s). For events separated by more than ≈ 100 km the propagation effects are reduced for only the longer periods (≈ 40–50 s). The earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern Pakistan triple junction. The Owen fracture zone is a transform fault that accommodates right‐lateral motion between the Indian and Arabian plates. The plate boundary in the vicinity of the Murray ridge is also partially made up of transform segments that strike subparallel to the Owen fracture zone. Spreading centers may also exist in the vicinity of the Murray ridge but were not documented by seismic or other evidence. The slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion. The Arabian plate is being subducted beneath the Eurasian plate along the southern coast of Pakistan. Slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions. Left‐lateral motion is documented along the boundary between the Indian and Eurasian plates in southern Pakistan. The predicted direction of relative motion between these plates is not significantly different from that observed. Two of the earthquakes studied appear to be intraplate in nature. The depth and focal mechanism of one intraplate event, which may have occurred within the accretionary prism along the Makran coast, however, are based on limited data. The other intraplate earthquake occurred within the sedimentary deposits along the western edge of the Indian subcontinent near the Indian‐Eurasian plate boundary.

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