bluuebunny/crossref_metadata_2025 huggingface dataset:
Stratigraphic relations and source areas of ash‐flow sheets of the Black Mountain and Stonewall Mountain Volcanic Centers, Nevada
Authors: Noble Donald C. , Vogel Thomas A. , Weiss Steven I. , Erwin John W. , McKee Edwin H. , Younker Leland W.
Published: February 2008

Recent work has resulted in major changes in the stratigraphic correlations and source‐area assignments of ash‐flow sheets of the late Miocene Black Mountain and Stonewall Mountain volcanic centers, southern Nevada. Ash‐flow tuffs exposed around Stonewall Mountain have different trace element patterns and thermoremanent magnetization directions than do ash‐flow sheetsof the Thirsty Canyon Tuff, erupted from the Black Mountain center, with which they were previously correlated. Hafnium concentrations are particularly useful in distinguishing the megascopically and petrographically similar tuffs ofthe two centers. The ash‐flow sheet overlyingthe Spearhead Member in the Stonewall Mountainarea, herein named the Civet Cat Canyon Member, can be traced northward continuously from outflow facies south of Stonewall Mountain to near‐vent facies on the southern flank of Stonewall Mountain. This, in conjunction with distribution, thickness, and facies relations show that these units are the outflow sheets of the Stonewall Mountain volcanic center; they are herein assigned to a new formation, the Stonewall Flat Tuff of late Miocene age. Tuffs previously mapped near Black Mountain as the Labyrinth Canyon Member (herein abandoned) are correlated on the basis of trace‐element, paleomagnetic, petrographic, and radiometric age data with the type Spearhead Member at Goldfield. Tuffs previously assigned to the Spearhead Member in the Black Mountain frea are reassigned to the Pahute Mesa Member (new) of the Thirsty Canyon Tuff. The Stonewall Mountain volcanic center is thus slightly younger than the Black Mountain center, and the period of time over which the Black Mountain center was active probably was appreciably shorter than was previously inferred. Recognition of the Stone‐wall Flat Tuff as a separate stratigraphic and genetic entity enables us to explain a number of geological inconsistencies, including the marked asymmetric distribution previously attributed to the Thirsty Canyon Tuff. Moreover, the inferred original volume of the Thirsty Canyon Tuff is significantly reduced, increasing the importance of lava relative to tuff in the eruptive history of the Black Mountain center.
Measurement requirements for glacial uplift detection of nonadiabatic density gradients in the mantle
Authors: Fjeldskaar W. , Cathles L. M.
Published: February 2008

Recent isotope studies have rekindled interest in the possibility that the mantle may be chemically stratified and therefore nonadiabatic. Previous calculations of the last ice age are compatible only with a ∼1022P mantle. Calculations of the isostatic response of an adiabatic, partially nonadiabatic, and fully nonadiabatic 1022P mantle to the load cycle of the last ice age show that the uplift responses of an adiabatic and even partially nonadiabatic mantle are very different. However, geoid perturbations mask these differences, and the emergence curves of the two mantle types are nearly identical except in the regions near and under the ice loads at the early stages of isostatic readjustment. The calculations suggest that we must seek shoreline emergence data around glaciated areas more than 6000 years before present if nonadiabatic density gradients in the mantle are to be detected from emergence data.
Introduction to Seamount Special Section
Authors: Watts A. B.
Published: February 2008

This special section is the outcome of a symposium held at the Lamont‐Doherty Geological Observatory November 17–19, 1982, on the origin and evolution of seamounts. The topic for the symposium arose from the realization that although there is now a wealth of new ideas on the geology, geophysics, and geochemistry of the ocean floor, the study of seamounts has been relatively neglected despite their great importance to plate tectonics.

One of the most interesting features of the ocean floor is the large number of small volcanoes or seamounts. Usually, these features are circular in plan view and have a sharp summit. There has been extensive debate in the literature about the significance of the different morphological types of seamounts. A traditional view has been that flattopped seamounts (e.g., guyots) formed as a result of subaerial erosion when the volcanoes were above sea level. Seamounts covered by fringing reefs or sediments (e.g., atolls), on the other hand, are believed to have formed when the original volcanic foundation subsided below sea level.
Intraplate seismicity and stresses in young oceanic lithosphere
Authors: Wiens Douglas A. , Stein Seth
Published: February 2008

We have conducted a systematic study of oceanic intraplate seismicity in the period 1964–1983 occurring in lithosphere younger than 35 million years. Focal mechanisms determined for 26 events using first motions, P wave modeling and surface wave amplitude radiation patterns, along with 13 previously determined mechanisms, indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere. There is no evidence for a worldwide transition from extension to compression at a specific lithospheric age. Tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction. Congressional axes of thrust events show a weak preferred orientation in the spreading direction, presumably due to ridge push stresses. However, stress axes of thrust events in very young lithosphere (age <9 Ma) do not show a correlation with the spreading direction, suggesting that the ridge push force is small near the ridge. These observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis, in agreement with models which assume that the axial region has very little tensile strength. Normal faulting events occur at greater depths and temperatures than thrust events, possibly because rocks under tensional stress have strength maxima at higher temperatures. Alternatively, the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere, if thermoelastic stresses are important in generating this seismicity. Differential thermal contraction along fracture zones does not seem to be a significant source of near‐ridge intraplate seismicity. High seismicity rates in young lithosphere, noted previously on a global scale, seem characteristic of all three major ocean basins. Additionally, most of the total oceanic intraplate moment release takes place in young lithosphere. However, most of the events and more than 96% of the total moment release of the near‐ridge intraplate seismicity are contained in five areas. Two zones in the Indian Ocean, one near Chagos Bank and the other in the Amsterdam‐St. Paul region, indicate tension parallel to the trend of the plate boundary. A region of high seismicity near the East Pacific Rise between 3° and 7°N shows both normal and thrust faulting, while another region, on the Cocos plate west of the Panama Fracture Zone, is characterized by strike‐slip faulting. Thrust, strike‐slip, and normal faulting events are found in a small seismically active zone of the South‐Central Pacific. The extremely uneven distribution of seismicity, if not a result of the relatively short time period studied (20 years), indicates local perturbations strongly affect near‐ridge seismicity.
Early differentiation of the Moon: Evidence from trace elements in plagioclase
Authors: Palme H. , Spettel B. , Wänke H. , Bischoff A. , Stöffler D.
Published: February 2008

Bulk samples and plagioclase mineral grains from cataclastic ferroan anorthosites (mainly from North Ray crater, Apollo 16) were analysed for trace elements by instrumental neutron activation analysis (INAA). Some pure plagioclase crystals have lower concentrations of Cr, Co, Sc, and heavy rare earth elements (REE) than previous analyses indicated. Concentrations of Sr, Eu, Na, Ga, and the REE in plagioclases from different anorthosites vary in a systematic way. Plagioclases with the lowest contents of these elements could have crystallized from a magma with chondritic relative abundances of Ca, Sr, Eu, and the REE and a tenfold enrichment relative to type 1 carbonaceous chondrites. Plagioclases with higher trace element concentrations crystallized from a more evolved magma. The trace element patterns of plagioclases from pristine norites and troctolites are very different from the plagioclase pattern of anorthosites. Besides having higher concentrations of incompatible elements, noritic, and probably also troctolitic plagioclases have much higher contents of Mg, Cr, and Co. This simultaneous enrichment in incompatible elements and Cr and Co can be explained by the mixing of a magma rich in incompatible elements (KREEP‐rich) with a Mg‐, Cr‐, and Co‐rich component supporting previous arguments made on bulk rock chemical grounds. The mafic component could either be late infalling material of bulk moon composition or may be derived from the large mafic layer that must exist in the interior of the moon. Mixing of KREEP and the mafic material probably occurred in the interior of the moon, because norites and troctolites could have formed from a magma of this composition. A similar mixture of KREEP and Mg‐rich material was distributed over the front side of the moon 3.9 to 4 b.y. ago, by large impacts and/or by volcanic eruptions. Anorthosites have Mg/Cr, Fe/Sc, and Co/Fe+Mg ratios different from all other highland rocks, indicating either that average highland rocks are not representative of the bulk moon system or that these ratios change during fractional crystallization of a magma of bulk moon composition.
Lunar luminescence and the filling‐in of Fraunhofer lines in moonlight
Authors: Potter A. E. , Mendell W. , Morgan T.
Published: February 2008

The filling‐in of Fraunhofer lines in moonlight has been attributed to lunar luminescence. In order to test mechanisms proposed for this effect, measurements were made of the filling‐in of the Hα line, the Na D doublet, and a number of lines near the doublet. The degree of filling‐in was not correlated with wavelength in a way expected from a broadband luminescence, such as thermoluminescence. However, it was correlated with equivalent width of the Fraunhofer line, such that it increased as equivalent width decreased. This suggests that the Fraunhofer line filling at the moon's surface is caused by inelastic scattering of sunlight with a small wavelength shift, as for example, photon‐phonon scattering.
Lithospheric stresses due to radiogenic heating of an ice‐silicate planetary body: Implications for Ganymede's tectonic evolution
Authors: Zuber M. T. , Parmentier E. M.
Published: February 2008

Thermal evolution models of differentiated and undifferentiated ice‐silicate bodies containing longlived radiogenic heat sources are examined. Lithospheric stresses arise due to volume change of the interior and temperature change in the lithosphere. For an undifferentiated body, the surface stress peaks early in the evolution, while in the differentiated case, stresses peak later and continue to accumulate for longer periods of time. The variation of near‐surface stress with depth shows that stresses for the undifferentiated body initially penetrate to great depths, but rapidly concentrate within a few kilometers of the surface. For the differentiated body, elastic stresses never accumulate at a depth greater than a few kilometers. These models are applied to consider long‐term radioactive heating as a possible mechanism of tectonic activity and bright terrain formation on Ganymede.
Introduction to Special Section on the Valles Caldera and Jemez Mountains Volcanic Field
Authors: Heiken Grant
Published: February 2008

The Jemez Mountains volcanic field, New Mexico, is one of the best known volcanic centers in the world because of pioneering efforts by C. S. Ross, R. L. Smith, R. A. Bailey, and their coworkers in the U.S. Geological Survey [Ross et al., 1961; Bailey et al., 1969; Smith et al., 1970]. The field contains roughly 2000 km3 of basaltic through rhyolitic rocks, dominated volumetrically by andesites. The most significant activity was formation of the Toledo and Valles calderas during eruption of over 600 km3 of rhyolitic ash flow tuffs, the Bandelier tuff, during two phases 1.5 and 1.1 m.y. ago [Smith and Bailey, 1966; Doell et al., 1968; Izett et al., 1981]. Many concepts of pyroclastic flows and ignimbrites were based upon work in this volcanic field by Ross and Smith [1961].
A possible explanation of gravity anomalies over mid‐ocean ridges
Authors: Hall S. A. , Casey J. F. , Elthon D. L.
Published: February 2008

Gravity fields over the crests of slow spreading mid‐ocean ridges are characterized by small amplitude free air gravity anomaly lows (30–70 mGal) with wavelengths of 40–60 km, flanked by smaller gravity highs. In general, the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until, at fast spreading ridges (> 5 cm/yr), free air gravity highs (10–20 mGal) are observed. Previous explanations of these anomalies, which involve uncompensated ridges, thermally expanded ridges and elastic plate compensation models, are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest. In particular, the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid‐ocean ridge basalts, which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more. In addition, similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers. Geometrical models of mid‐ocean ridges derived from detailed structural and petrologic studies of the Bay of Islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates. Modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly: the ridge topography, the crustal level magma chamber, the configuration of the isotherms near the ridge crest, and a low‐density, partially gabbroic cumulate root extending vertically into the mantle beneath the ridge. Systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers. Major contributions are found to come from the topography and from the lowdensity root, which broadens with slower spreading rates, resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate. At extinct spreading centers, contributions from the magma chamber, isotherms, and topography gradually diminish with time once the ridge ceases to spread, whereas the low‐density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field. Seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the Bay of Islands ophiolite and the gravity analysis presented here.
Impact craters of Venus: Analysis of Venera 15 and 16 data
Authors: Ivanov B. A. , Basilevsky A. T. , Kryuchkov V. P. , Chernaya I. M.
Published: February 2008

From description of impact craters on Venus and crater size‐and‐depth distributions, models are developed that analyze the size‐distribution curve and radar‐bright haloes around fresh craters. The dense Venusian atmosphere has a strong influence on cratering. The estimated age of the surface of the area under study is from 0.5 to 1.0×109 years. The estimated rate of resurfacing during this time interval is about 0.04 to 0.8 cm/106 years. The lithosphere of Venus seems to be sufficiently rigid to sustain craters of 100–140 km diameter and 0.8–2.5 km depth for about 108 years.
Crater relaxation as a probe of Europa's interior
Authors: Thomas Paul J. , Schubert Gerald
Published: February 2008

Viscous relaxation rates of craters are examined to discriminate among models of Europa's interior. It is shown that the presence of an insulating surface regolith is required for ice lithospheres that do not contain a liquid H2O layer. Without the presence of such an insulating regolith, relaxation times for craters ≈100 km in diameter are excessively long (>1 b.y.) and thus inconsistent with the absence of such features in Voyager images. Finite element calculations indicate that a thick insulating surface frost layer (raising surface temperatures ≳ 40 K above the solar radiation ambient temperature of 92 K) allows a 25‐km‐thick surface viscous layer to relax ≈ 100‐km‐diameter craters on the short timescales (≃100 m.y.) required. If the icy lithosphere is 100 km thick (and without a liquid H2O layer, but containing a subsolidus convecting region), surface temperatures need to be raised 20 K by insulation to allow relaxation of craters in this size range to occur within the required time.
Discovery of solar wind neon in the Allende meteorite
Authors: Heymann D. , Palma R. L.
Published: February 2008

Inert gases have been measured in seven sieve fractions of a disaggregated sample of the Allende meteorite. The disaggregation was done by ultrasonic treatment in water and by freeze‐thawing. This sample constitutes the first gas‐rich portion know to occur in the Allende meteorite. The composition of the trapped neon is solar, i.e., Neon‐B, and the gas‐rich samples contain more trapped 20Ne than 36Ar. The set of sieve fractions show an anticorrelation of 20Ne content and grain size. Gas‐richness seems to be quite common among the CV3 meteorites with Allende added to the earlier known cases of Mokoia, Vigarano, and Efremovka.
Volatile solubilities in magmas: Transport of volatiles from mantles to planet surfaces
Authors: Holloway John R. , Jakobsson Sigurdur
Published: February 2008

Experimental measurements of the solubility of CO2, CO, CH4, H2, and H2O in silicate melt were made at pressures from 10 to 20 Kb and temperatures of 1200°C. The C‐O‐H fluid phase was buffered by graphite, iron, and wüstite. Detailed measurements were made on melts of NaAlSi3O8 composition and the results confirmed for alkalic and tholeiitic basalt liquids. The melt volatiles have dramatically lower H2/H2O and higher CO/CO2 ratios than the fluid. The result is large differences between the C‐O‐H compositions of the melt and the fluid (fluids about 16 atm % C, 6% O, and 78% H; melts about 4% C, 32% O, and 64% H). The measured solubilities allow calculation of activity coefficients for volatile species in the melt. They are, roughly, CO = 0.1, H2O = 0.6, CO2 = 1, H2 = 3, and CH4 = 60. This two‐order‐of‐magnitude variation in activity coefficients causes the significant volatile fractionation observed between fluid and melt. We argue that the same fractionation will occur in the more usual case of fluid‐absent partial melting of planetary mantles. The fractionation is such that reducing mantles will form magmas that yield volcanic gases about 1.5 orders‐of‐magnitude more oxidizing than the mantle source, while oxidizing mantles will form magmas with more reduced volcanic gases. We conclude that a volcanic gas composition cannot be directly used to estimate either the oxygen fugacity or volatile composition of its magma source region. The results suggest that volcanic gases will normally lie in the “neutral” range of oxygen fugacity, near that of the quartz‐fayalite‐magnetite buffer. These gases are predominately H2O with minor CO2, CO, H2, and CH4.
John C. Jamieson, Pioneer in experimental mineralogy
Authors: Bassett William A.
Published: February 2008

It is nearly impossible to undertake a research project in high‐pressure/temperature mineralogy without drawing on the ingenious and imaginative work of John C. Jamieson. Inventor and builder of several devices for making in situ X ray diffraction studies of samples at high pressure and temperature, John laid the ground work for some of the most exciting and productive years of research in experimental mineralogy. Using his devices, he found and described numerous high‐pressure phases in important materials. In addition, he conducted experiments in shock‐produced high‐pressure phases and made ultrasonic measurements on the elasticity of geophysically important minerals. In recent years he recognized the great potential of synchrotron radiation and contributed very significantly to its development as a tool in mineral physics.
Some associations of J. C. Jamieson with the shock wave community
Authors: McQueen Robert G.
Published: February 2008

A brief review is presented of some of the things John Jamieson did while working with people involved with shock wave physics and equation of state at high pressure. This also includes work done on geophysical problems and equation‐of‐state problems that might not be included in the review articles published in this issue. The subjects covered are some recovery work, studies on rutile and rutile structures, gold standard, flash X ray crystal studies of shocked metals, volume effects on the Gruneisen parameter, equation of state of oxygen, and measurement of Poisson's ratio at high pressure. Although most of these topics are not all directly related to shock wave studies, they all were things that the people working in shock wave physics were also investigating or were directly involved with John in his studies.
Paleomagnetism of the Late Proterozoic Tsezotene Formation of northwestern Canada
Authors: Park John K. , Aitken James D.
Published: February 2008

Paleomagnetic evidence from 37 sites of the partly red‐pigmented siliciclastic Tsezotene Formation supports a recently proposed apparent polar wander path for the Hadrynian Mackenzie Mountains supergroup (MMs). The probable primary remanence has a direction at D°, I° = 271, +24 (k = 15; α95° = 8) with an associated pole TA (12°N, 214°W; N = 23 specimens; δp°, δm° = 5, 9). TA becomes the oldest pole from the MMS. It helps bridge the gap between the apparently youngest poles of the Grenville Loop (about 0.88 Ga) and the suggested younger poles from the MMs. A secondary pole TB (23°N, 198°W; N = 18 sites; δp°, δm° = 3, 5), derived from a magnetic direction in hematite pigment (D°, I° = 263, +48; k = 73; α95° = 4), supports a magnetization found in the overlying “Copper cycle” and in younger units of the MMs as a pervasive overprint. Another secondary pole Tc (63°N, 141°W; N = 29 sites; δp°, δm° = 6, 6), derived from a magnetization (D°, I° = 317, +87; k = 89; α95° = 3) partly residing in another phase of hematite pigment, is of postfolding age (post‐Paleocene). This study demonstrates the importance of using several treatment methods, singly and in combination, when analyzing complex magnetizations.
A new regional electrical model for the southern section of the Rio Grande rift and the adjacent basin and range and great plains
Authors: Keshet Y. , Hermance J. F.
Published: February 2008

One of the earliest geophysical studies in the Rio Grande rift involved a geomagnetic induction profile across the southern rift by Schmucker (1964, 1970). The interpretation of these data, and later studies by Reitzel, Gough, and Porath, clearly established the presence of an electrical conductivity anomaly along the axis of the rift extending from Colorado to west Texas. This anomaly was originally thought to lie well within the mantle. The earliest magnetotelluric (MT) studies in this region generally corroborated the magnetic variation models, although the conducting mantle appeared to be somewhat shallower than previous geomagnetic studies had indicated. On the other hand, more recent MT interpretations differ substantially from the earlier work in that they suggest that the major anomalous features are likely to be in the crust rather than in the upper mantle. Our purpose here is to reconcile the interpretation of the magnetic variation and MT data bases in order to obtain a new preliminary regional model along a latitude of 32°N that fits both data sets. Applying constraints from MT interpretations, we argue for the presence of an intracrustal layer at a depth of about 20 km, having a conductance of about 2000 S. Allowing for a small portion of the Z field to be of normal and external origin, the magnetic variation data suggest that the conductor disappears or is greatly attenuated to either side of the rift, but the location of its boundaries are not well resolved by the present data. Neither data set could clarify whether the conductor is underlain by normal mantle. On the other hand, neither the MT data nor the magnetic variation data strictly requires the presence of abnormal mantle material beneath the southeastern Basin and Range, the southwestern Great Plains, and the southern Rio Grande rift.
Time variations in the Earth's gravity field detectable with geopotential research mission intersatellite tracking
Authors: Wagner Carl A. , McAdoo David C.
Published: February 2008

The Geopotential Research Mission (GRM) is a proposed system of two artificial satellites in tandem, drag‐free polar orbits of low altitude (∼160 km) designed for a detailed global mapping of the earth's gravitational and magnetic fields. The mission should provide a large amount of new information about the gravity field, particularly over continents, resolving spatial variations in the gravity field as short as 100 km (half wavelength). However, GRM should also perform a much more difficult task, the detection of time variations in the gravity field. Among the effects which produce changes in the field that are large enough to be detected by the intersatellite millimeter wavelength (∼ 1 μ/s precision) GRM sensor are the following: (1) rebound from ancient deglaciation, (2) very large dip‐slip earthquakes, (3) detailed ocean tides, and (4) the seasonal growth and decline of continental glacial fields. In particular, many nonzonal as well as zonal components affected by postglacial rebound should be detected in a 6‐month GRM mission. Repeat passes over the fault zones of earthquakes similar to Alaska (1964), Chile (1960), or Sumbawa (1977) should reveal signal changes due to their earth movements. The effect of ocean tides on the GRM signal should be readily detectable over a broad band of frequencies even if all but 10% of the tides are correctly modeled. Continued geophysical interest in these now largely unobserved phenomena warrants extension of the GRM mission to somewhat higher altitudes.
Electrical conductivity of olivine during high‐temperature creep
Authors: Hirsch L. M. , Wang C.‐Y.
Published: February 2008

The electrical conductance of single crystals of olivine has been measured during creep in the temperature range 1610–1725 K, under controlled oxygen fugacity and under uniaxial stresses of 60–140 MPa. During deformation at a constant uniaxial load, the measured conductance increases continuously with time. During high‐temperature annealing in the absence of load, the conductance of deformed crystals remains constant with time. The total change of conductance resulting from deformation is found to be strongly dependent on the total strain. Comparison between this conductance change and that expected from measured geometrical shape changes of the specimens indicates that the latter is the dominant contributor to the total change in conductance. The results presented indicate that the generation and movement of dislocations and other defects associated with plastic deformation do not significantly contribute to the electrical conductivity of olivine. In addition, the results suggest that the electrical conductivity of the upper mantle will not be affected by the lateral variations in tectonic stresses.
On transient rheology and glacial isostasy
Authors: Yuen David A. , Sabadini Roberto C. A. , Gasperini Paolo , Boschi Enzo
Published: February 2008

Theoretical predictions from analytical, self‐gravitating, layered earth models with Maxwell, Burgers' body, and standard linear solid rheologies are employed to explore the extent to which transient creep may have influenced the inference of long‐term mantle viscosity from analysis of the various signatures derived from the process of glacial isostatic adjustment. The compatibility of the various models is checked by their ability to fit simultaneously the relative sea level, free air gravity, and the rotational data. We conclude that transient creep cannot operate throughout the entire mantle. On account of the extreme sensitivity of the uplift and gravity data to any significant departures of the steady upper mantle viscosity from 1021 Pa s, transient rheology would most likely be unimportant in the upper mantle. Restricting transient rheology to the lower mantle, we find that models with an adiabatic density jump at 670 km depth are not satisfactory because of their inability to produce sufficiently large gravity anomalies and polar wander speeds. Our calculations show that the preferred models with transient creep in the lower mantle are chemically stratified with a nonadiabatic density change of around 10%. The long‐term viscosities in the lower mantle, associated with compatible transient rheological models, lie between 5 × 1022 and 1023 Pa s. These values are consistent with viscosity estimates derived from correlation of long‐wavelength geoid anomalies and the density perturbations in the deep mantle.
Paleomagnetism of the Powder Mill Group, Michigan and Wisconsin: A reassessment of the Logan Loop
Authors: Palmer H. C. , Halls H. C.
Published: February 2008

The apex and the western arm of the 1.1‐Ga Logan Loop, a prominent feature of the North American Precambrian apparent polar wander path, are defined by a large body of data from Keweenawan rocks of the Lake Superior Basin. Until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the Powder Mill Group of Michigan. A reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the Logan Loop. Data from the Mellen Complex in Wisconsin, together with our data from the Powder Mill Group, suggest that for steeply dipping volcanic units, the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike. Reversely magnetized sites from localities where the Powder Mill volcanics have shallow dips yield a pole (146°W, 46°N, A95 = 9.2°) on the apex of the loop. A well‐defined, stratigraphically continuous, normal to reversed polarity change near the base of the Powder Mill Group is not confirmed. Normally magnetized flows have been found at two localities, only one of which is at the base of the lava sequence. The higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the Powder Mill Group are secondary and should not be used in definition of the eastern arm of the Logan Loop.
Crystal structure refinement of a lunar anorthite, An94
Authors: Smyth Joseph R.
Published: February 2008

The crystal structure of an anorthite (An94) from a slowly cooled, coarse‐grained lunar troctolite (sample 76535) was refined in space group from 9393 symmetry‐independent X ray intensities of which 5145 were greater than three standard deviations above background. Long‐exposure X ray precession photographs indicate that all classes of diffracted intensities are equally sharp. The final weighted R‐factor for all reflections is 0.042. The results indicate that Na is ordered into the A(000) and A(0i0), and that there may be up to 10% vacancy in these two sites. The vacancy is consistent with a significant deviation from stoichiometry in the electron microprobe analyses. Two of the four A sites are modeled using split atom positions indicating substantial positional disorder in the primitive structure. Observed cation ordering is consistent with slow cooling above the
to
transition, and there is no conclusive evidence of cation ordering in violation of body‐centering.
Forecast model for large and great earthquakes in southern California
Authors: Stuart William D.
Published: February 2008

An earthquake instability model is used to compute theoretical fault slip and ground surface deformation before possible future earthquakes on the San Andreas fault in southern California. The quasistatic model represents the earth's crust by an elastic half‐space and the San Andreas, Imperial, Cerro Prieto, and San Jacinto faults by flat rectangular surfaces whose strikes approximate the actual local strikes. The seismogenic zone of the San Andreas fault is represented by a brittle, strain‐softening patch extending from the ground surface to 11 km depth and from Parkfield to the Salton Sea (500 km). The strength of the patch varies along strike, and unstable failures of parts of the patch are earthquake analogs; failed areas heal immediately after instability. For the model to reproduce the lengths, slips, and times of earthquakes since 1080 A.D. reported by K. Sieh and others, the patch must be divided into five sections along strike. The sections have alternately low and high strength, with the two strongest sections being roughly coincident with the major bends of the San Andreas fault. Ten to twenty years before unstable slippage of patch areas, portions of the patch bottom begin to fail stably (aseismically). The stable failure causes accelerating fault slip at depth and at the fault trace, as well as anomalous changes in lengthening rates of trilateration lines. In the model simulation the next two sections to fail unstably in the future are just north of the Salton Sea and just south of Parkfield. Rupture lengths are about 100 km and 40 km. At favorable locations, precursory anomalies of fault slip and trilateration line length become large enough to be detected in field data a few years before each earthquake.
Nature of the magma chamber underlying the Mono Craters Area, eastern California, as determined from teleseismic travel time residuals
Authors: Achauer Ulrich , Greene Lizbeth , Evans John R. , Iyer H. M.
Published: February 2008

A total of 94 teleseismic events of good quality were recorded by a dense mobile array of seismographs located in the Mono Craters volcanic area, eastern California, one of the youngest apparently active rhyolitic volcanic centers in North America. An inversion of travel time residuals from these events reveals a small anomalous volume, 200–600 km3, directly beneath the Mono Craters with at least 7% low velocity and a top approximately 8–10 km deep. It reasonably may be interpreted as a magma chamber of molten or partially molten rock, although smaller, shallower, and differently placed than previously thought. The magma chamber probably is too small and young to produce a caldera‐forming eruption within the foreseeable future.
Azimuthal variations of teleseismic P wave residuals for northern California
Authors: Simila G. W.
Published: February 2008

Teleseismic P wave residuals from 70 events (1971–1979) for seven stations in central and northern California are investigated as a function of azimuth to model upper mantle velocity structure. Relative residuals are determined for each station using Berkeley as the reference station. Maximum relative residual variation is +0.9 s in the northern Coast Ranges to −1.7 s in the Klamath Mountains. Interpretations of the residual variations represent lateral variations of the P wave velocity in the upper mantle. The modeling technique utilizes a simple ray‐tracing technique to calculate time‐residual equivalent path lengths in the anomalous velocity regions. A model of the paleosubduction zone, Farallon plate, beneath northern California represents a region (40‐ to 150‐km depth) of high‐velocity (8.3–8.5 km/s), which explains the negative residual pattern for the Klamath Mountains, Cascades, and Modoc Plateau region. A low‐velocity zone (7.2–7.4 km/s) beneath the Gorda plate (25‐ to 155‐km depth) produces the observed positive residual pattern of the northern Coast Ranges. The low‐velocity region is related to proposed models of the Mendocino triple junction migration and associated upwelling of asthenospheric material.
Pristine Moon rocks: A “large” felsite and a metal‐rich ferroan anorthosite
Authors: Warren Paul H. , Jerde Eric A. , Kallemeyn Gregory W.
Published: February 2008

We report new chemical analyses and (in most cases) descriptions for 19 lunar rock samples, including 5 mare basalts from Apollo 15 and 5 uncommonly feldspathic rocklets from Apollo 17. Two of the samples studied are most extraordinary: a “large” (≈1 g) felsite from Apollo 12, and a pristine ferroan anorthosite from Apollo 15 that is unusually rich in Fe‐metal and trace siderophile elements. The felsite, 12033,507, is mainly a graphic intergrowth of K‐feldspar and a silica phase, with about 6% plagioclase and 1% each of ferroaugite, ilmenite, and fayalitic olivine. This crystalline assemblage is crisscrossed, and apparently intruded by thin veins of brownish, felsic (average SiO2 = 57 wt %) glass. The glass veins are far from uniform in composition; evidently after the glass was intruded it reacted, to varying extents, with the crystalline felsite. Dark globules scattered at apparent nodes in the network of glass veins are interpreted as the least altered of the intruded glasses. These globule glasses are relatively SiO2‐poor (average = 48 wt %) but extremely FeO‐rich (average = 36 wt %). They may have formed as liquid immiscibility caused the same melt that produced the crystalline felsite to split into Fe‐rich melt (the globule glasses) plus Fe‐poor, Si‐rich melt. According to this model, deformation, most likely caused by an impact, then squeezed the Fe‐rich melt into the still nearby, and still hot, crystalline felsite. The Fe‐metal content of ferroan anorthosite 15363 is 1.2 wt % in the thin section we studied, but must have been, based on mass balance for Co and Ni, considerably lower in the chip we used for bulk‐rock analysis. Even so, the measured bulk‐rock concentrations for several siderophile elements (Re, Os, and Ir) are far higher than previously observed among pristine lunar anorthosites. These results underscore the huge uncertainty associated with any attempt to estimate the overall siderophile element contents of the moon's crust. Among pristine rocks from the Apollo 17 site, located on the rim of the Serenitatis Basin, ferroan anorthosites continue to be far outnumbered by Mg‐rich rocks.
A geologic evaluation of thermal properties for the elysium and aeolis quadrangles of Mars
Authors: Zimbelman James R. , Leshin Laurie A.
Published: February 2008

Thermal inertias with spatial resolutions as small as 2 by 5 km were combined in a regional study of the Elysium and Aeolis quadrangles of Mars (30°N to 30°S, 180°W to 225°W). The range of thermal inertias obtained was 1–14 (× 10−3 cal cm−2 s−½ K−1 = 41.84 j m−2 s−½ K−1), with the lowest values corresponding to the Elysium Rise and the highest values correlating with dark patches of aeolian material in the southern highlands. The thermal properties obtained from the high spatial resolution measurements are essentially identical to the results obtained from data with much poorer spatial resolution, indicating a regional homogeneity of surface properties at scales greater than 5 km. Aeolian features, both dark patches and bright materials associated with topographic obstacles such as craters, have higher thermal inertias than do their surroundings. Other surface features do not display distinctive thermal properties, even when clearly resolved from their surroundings. Comparison of the thermal inertias with global data sets shows a pronounced inverse correlation with albedo (consistent with globally observed trends) and brightnesses at red, green, and violet wavelengths but no prominent correlation with elevation. Thermal inertias for individual geologic units within the two quadrangles appear to be more strongly controlled by the location of the terrain in either the northern plains or the southern highlands than by properties intrinsic to the unit.
The natural thermoluminescence of meteorites: I. Twenty‐three Antarctic meteorites of known 26Al content
Authors: Hasan Fouad A. , Haq Munir , Sears Derek W. G.
Published: February 2008

The level of thermoluminescence (TL) naturally present in 23 Antarctic chondrites of known 26Al content has been measured as a means of exploring their thermal and radiation history. Antarctic meteorites tend to have lower natural TL than non‐Antarctic meteorites, presumably because of their generally longer terrestrial ages. Seventeen of the meteorites lie on a band of increasing natural TL and 26Al activity; meteorites with 26Al of 45–60 dpm/kg have natural TL values (normalized to the high temperature TL) of 2–4, whereas those with 26Al values of 30–45 dpm/kg have normalized natural TL values of 1–2, and ALHA 76008, with an 26Al content of 11 dpm/kg, has a TL value of 0.87 ± 0.01. This suggests that the “half‐life” for TL decay is comparable to that of 26Al. The six meteorites not lying on the TL‐26Al trend have much lower TL than others of comparable 26Al. For two of them (RKPA 79001 and RKPA 80202, which are tentatively paired) the low natural TL probably reflects a recent severe shock, while another (ALHA 77294) has previously been measured in this lab and had TL consistent with the TL‐26Al trend. The remaining three, ALHA 78006, ALHA 77296, and ALHA 77297 (of which the last two are probably paired), have suffered a recent reheating such as would be expected from orbits of unusually small perihelia.
Concepts for collection of aerosols in Titan's atmosphere
Authors: Oberbeck Verne R. , O'Hara Dean , Carle Glenn C.
Published: February 2008

In this paper we consider design of instruments for collection of aerosols during entry in Titan's atmosphere. Major constraints on designs are small sample collection time, low aerosol column density, and the need to collect 1–10 μg of aerosol for gas chromatographic analysis. Thus it is important to maximize aerosol collection through collector design, which includes consideration of various types of collectors and maximizing the collection efficiency of a given type of collector. Sampling systems discussed include inertial impactors, filters, electrostatic devices, and multistage instruments. Aerosol sampling is reviewed in the context of high‐altitude (200–70 km) and low‐altitude (60–30 km) regions of Titan's atmosphere.
A single‐force model for the 1975 Kalapana, Hawaii, Earthquake
Authors: Eissler Holly K. , Kanamori Hiroo
Published: February 2008

A single force mechanism is investigated as the source of long‐period seismic radiation from the 1975 Kalapana, Hawaii, earthquake (MS = 7.1). The observed Love wave radiation pattern determined from the spectra of World‐Wide Standard Seismograph Network and High Gain Long Period records at 100 s is two‐lobed with azimuth, consistent with a near‐horizontal single force acting opposite (strike ∼330°) to the observed displacement direction of the earthquake; this pattern is inconsistent with the expected double‐couple pattern. Assuming a form of the force time history of a one‐cycle sinusoid, the total duration of the event estimated from Rayleigh waves at two International Deployment of Accelerometers stations is approximately 180 s. The peak amplitude fo of the time function is 1 × 1015 N from amplitudes of Love and Rayleigh waves. The interpretation is that the bulk of the seismic radiation was produced by large‐scale slumping of a large area of the south flank of Kilauea volcano. The single force is a crude representation of the effect on the earth of the motion of a partially decoupled large slide mass. Using the mass estimated from the tsunami generation area (∼ 1015–1016 kg), the peak acceleration of the slide block (0.1–1 m s−2) inferred from the seismic force is comparable with the acceleration due to gravity on a gently inclined plane. The slump model for the Kalapana earthquake is also more qualitatively consistent with the large horizontal deformation (8 m on land) and tsunami associated with the earthquake, which are difficult to explain with the conventional double‐couple source model. The single‐force source has been used previously to model the long‐period seismic waves from the landslide accompanying the eruption of Mount St. Helens volcano, and may explain other anomalous seismic events as being due to massive slumping of sediments or unconsolidated material and not to elastic dislocation.
Subglacial drainage for an ice sheet resting upon a layered deformable bed
Authors: Shoemaker E. M. , Leung H. K. N.
Published: February 2008

The subglacial hydrology is considered for a temperate ice sheet resting upon a deformable aquitard (till) with a hydraulic conductivity several orders of magnitude lower than that of an underlying aquifer. The evacuation of basal meltwater by the aquifer‐channel‐bed hydraulic system is studied, particularly as it affects aquifer pressurization, till deformation, and channel failure. Sensitive parameters are aquifer hydraulic conductivity, ice sheet length and elevation, and basal meltwater rate. A criterion is developed to determine channel length; channels need not extend all the way to the divide. The model predicts that, in general, there is a region adjoining the terminus where the bed pore water pressure is equal to the overburden pressure and drainage along the bed is possible. Outside this region the bed pore water pressure is below the overburden pressure, and ice penetrates the soil to prevent drainage along the bed. An important result is that deformable substrates beneath temperate‐based ice sheets may be unstable, particularly under conditions of a diminishing profile.
Geophysical investigations in the Strath Halladale‐Altnabreac District (northeast Scotland) in relation to radioactive waste disposal
Authors: Lee M. K.
Published: February 2008

A series of geophysical studies have been carried out over the Strath Halladale Granite (around Altnabreac, northeast Scotland) as part of a multidisciplinary reasearch programme into the suitability of granitic rocks for radioactive waste disposal. Regional gravity and aeromagnetic data have been interpreted to investigate the structural setting of the Strath Halladale Granite. Ground magnetic and VLF surveys have been carried out to test their effectiveness in defining lithlogical variations, faults, and fracture zones beneath the ubiquitous cover of peat and moraine. Interpretations of the gravity and magnetic data suggest that the granite is a composite body approximately 3–4 km thick, comprising a dominant moderately magnetic variety with a subordinate (more evolved) nonmagnetic variety on the southeast margin. The intrusion apparently extends at shallow depth beneath Moine metasedimentary rocks east of the mapped contact at Altnabreac. The ground magnetic data indicate that the main body of (moderately magnetic) granite is interspersed with zones of less magnetic rock (Moine metasedimentary xenoliths and/or the more evolved variety of granite) and basic intrusions. Prominent lineaments in the magnetic field suggest dipping sheets of magnetically contrasting material, or faulted contacts. The VLF‐EM results indicate a number of anomalies which probably relate to fracture zones or lithological contacts.