bluuebunny/crossref_metadata_2025 huggingface dataset:
The subglacial birth of Olympus Mons and its aureoles
Authors: Ann Hodges Carroll , Moore Henry J.
Published: February 2008

The vast volcanic plains and shields of Mars, together with the thermal, spectrographic, and morphological evidence for water ice at the poles, for several percent water in Viking soil samples, for ground ice or permafrost over much of the planet, and for the existence of surface water at some time in the past, suggest that magma and water or ice may have interacted during evolution of the planet's landscape. Relatively small mesas and buttes, with and without summit craters, are remarkably similar to the table mountains of Iceland that formed by subglacial eruption during the late Quaternary period. Table mountains typically comprise foundations of pillow lava and palagonitized tuff breccias (móberg), overlain by subaerial lava flows that commonly culminated in a typical shield volcano; some table mountains, however, failed to reach the subaerial stage and thus lack the cap rock. Subglacial fissure eruptions produced ridges composed of pillow lava and móberg. Conical knobs on steep‐sided Martian plateaus are reminiscent of the small Icelandic shield volcanoes atop móberg pedestals. Candidate table mountains on Mars are especially numerous in the region between latitude 40°N and the margin of the north polar cap, and interaction of lava with a formerly more extensive ice cap may have occurred. More significant is the possibility that Olympus Mons and the broad lobate aureole deposits around its base may have had similar subglacial beginnings. This hypothesis requires an ice cap several kilometers thick in the vicinity of this enormous shield during its initial stages of eruption. The amounts of water ice required do not appear excessive, given the limits to present knowledge of the water budget throughout the planet's history. A mechanism for localizing such ice, however, is required.
Yardangs on Mars: Evidence of recent wind erosion
Authors: Ward A. Wesley
Published: February 2008

Yardangs are streamlined erosional wind forms, similar in form to inverted boat hulls, that in terrestrial deserts range from meters to kilometers in length. On Mars the best examples are seen in the equatorial region. In the Amazonis region, hundreds of ridges and sawtooth‐edged mesas have been wind sculptured in layered rocks. Individual ridges are tens of kilometers long with intervening valleys nearly 1 km wide. The wind‐stripped surface seems to be relatively young and therefore must be easily erodible. Possible lithologies include ignimbrites, mudflows, or lithified regolith. Other wind‐sculpted features occur in the Aeolis region, in Ares Valles, and in the Iapygia region. White Rock, a light‐colored plateau inside a crater, is interpreted to be a yardang cluster eroded in a deposit inside the host crater. White Rock may be a jointed, wind‐eroded pyroclastic deposit. Yardangs on Mars, especially when they are sculpted in young geologic units, demonstrate that much of the observed eolian erosion is recent. Yardang azimuths often are not parallel with wind streak directions, indicating that the yardangs were formed by different (older or weaker) winds from those that formed the streaks.
Mars surface composition from reflectance spectroscopy: A summary
Authors: Singer Robert B. , McCord Thomas B. , Clark Roger N. , Adams John B. , Huguenin Robert L.
Published: February 2008

Visible and near‐infrared (0.3–2.6 μm) reflectance spectra of the martian surface have been obtained primarily from earth‐based telescopic observations, and multispectral images have been obtained both from spacecraft and earth‐based observations. Observations in this wavelength region have confirmed the bimodal albedo distribution of surface materials, first observed visually. All spectra of Mars are characterized by strong Fe3+ absorptions from the near‐UV to about 0.75 μm. Darker regions show this effect to a lesser degree, and are interpreted to be less oxidized materials. In addition, dark areas have Fe2+ absorptions near 1.0 μm, attributed primarily to olivines and pyroxenes. There is evidence at infrared wave‐lengths for highly dessicated mineral hydrates and for H2O‐ice and/or adsorbed H2O. Observations of the north polar cap show a strong H2O‐ice spectral signature but no spectral evidence for CO2‐ice, while only CO2‐ice has been identified in spectra of the south polar cap. While the brightest materials on Mars are widespread and correlate with aeolian dust, darker materials show greater mineralogic variability and are thought to be closer in petrology and physical location to their parent rock. At present the best model for the dark materials is somewhat oxidized basaltic or ultramafic rock, regionally variable in composition and details of oxidation. The bright materials appear to be finer‐grained assemblages of primarily highly oxygen‐sharing dessicated mineral hydrate, some ferric oxides, and other less major constituents, including a small amount of relatively unaltered mafic material. The bright materials seem likely to be primary and/or secondary alteration products of the basaltic or ultramafic dark materials.
Oxygen diffusion in forsterite
Authors: Reddy K. P. R. , Oh S. M. , Major L. D. , Cooper A. R.
Published: February 2008

Oxygen self‐diffusion coefficients for forsterite single crystals were determined in different orientations by the single‐spectrum proton activation technique. In the temperature range 1275°–1625°C the diffusion coefficients in b direction can be represented by Db = 3.5 × 10−3 exp [(−89 ± 3 kcal/mol)/RT] cm2/s.
Reversal chronology from a 7.9‐ to 11.5‐M.Y.‐old volcanic sequence in central Arizona: Comparison with ocean floor polarity record
Authors: McKee Edwin H. , Elston Donald P.
Published: February 2008

A magnetic reversal chronology has been obtained from a section over 700 m thick of radiometrically dated lava flows (7.9–11.5 m.y.) from the Hackberry Mountain area in central Arizona. There are an adequate number of polarity reversals in the Hackberry Mountain volcanic sequence to allow an apparent correlation with the sea floor record assigned to this time interval. A correlation using a few radiometric ages widely spaced in the section for temporal control proves to be incorrect because it does not assess the length of hiatuses in the terrestrial sequence and assumes that the rate of volcanism and sedimentation is uniform. When a large number of stratigraphically controlled radiometric ages are used, temporal compression, extension, and shifting of parts of the Hackberry Mountain reversal pattern produce a different correlation with the ocean floor reversal pattern. In spite of such adjustments, not all of the stratigraphically controlled K‐Ar ages and associated polarity reversals are in accord with ages estimated for polarity boundaries in the sea floor record. The Hackberry Mountain pattern can be less well fitted to the marine sediment polarity record than to the sea floor anomaly record, implying that there are unrecognized hiatuses in the marine sediment record. We conclude that a lack of apparent similarity in the adjusted 8‐ to 11‐m.y.‐old Hackberry Mountain pattern and the currently accepted reversal time scale derived from marine magnetic anomalies reflects (1) loss or absence of the geologic record in the terrestrial volcanic sequence, (2) perhaps brief reversals not recognized in the marine record, and (3) a possible lack of strict linearity in the assignment of estimated ages to the sea floor pattern, which is based on the assumption of uniform spreading rates. A reversal pattern obtained from a terrestrial volcanic sequence older than about 5 m.y. thus is of no value, in itself, for establishing a chronology.
Ultrasonic P and S wave attenuation in dry and saturated rocks under pressure
Authors: Johnston David H. , Toksöz M. Nafi
Published: February 2008

Ultrasonic P and S wave Q values as functions of hydrostatic pressure are presented for the Berea and Navajo sandstones, Bedford limestone, and Colorado oil shale. In most cases the attenuation was obtained for both dry and water‐saturated samples. Q is determined by measuring the attenuation of the rock sample relative to a high Q standard of identical geometry using Fourier spectral ratios. The data show that Q increases with pressure, the rate of increase being dependent on rock type and crack density and distribution. Qs is about equal to or larger than Qp for dry rocks and less than Qp for saturated rocks. In all cases, Q for saturated rocks are lower than for dry samples. The attenuation in the oil shale shows extreme anisotropy.
A harmonic analysis of lunar gravity
Authors: Bills Bruce G. , Ferrari Alfred J.
Published: February 2008

An improved model of lunar global gravity has been obtained by fitting a sixteenth‐degree harmonic series to a combination of Doppler tracking data from Apollo missions 8, 12, 15, and 16, and Lunar Orbiters 1, 2, 3, 4, and 5, and laser ranging data to the lunar surface. To compensate for the irregular selenographic distribution of these data, the solution algorithm has also incorporated a semi‐empirical a priori covariance function. Maps of the free‐air gravity disturbance and its formal error are presented, as are free‐air anomaly and Bouguer anomaly maps. The lunar gravitational variance spectrum has the form V(G; n) = O(n−4), as do the corresponding terrestrial and martian spectra. The variance spectra of the Bouguer corrections (topography converted to equivalent gravity) for these bodies have the same basic form as the observed gravity (V(ΔG; n) = O(n−4)), and, in fact, the spectral ratios σn (ΔG )/σn(G) are nearly constant throughout the observed spectral range for each body. Despite this spectral compatibility, the correlation between gravity and topography is generally quite poor on a global scale.
Seismicity and fault plane solutions of intermediate depth earthquakes in the Pamir‐Hindu Kush Region
Authors: Roecker S. W. , Soboleva O. V. , Nersesov I. L. , Lukk A. A. , Hatzfeld D. , Chatelain J. L. , Molnar P.
Published: February 2008

Relocations of earthquakes, recorded by a local network of stations in Afghanistan and Tadjikistan in 1966 and 1967, indicate a narrow seismic zone (width ≲30 km) dipping steeply into the mantle to a depth of 300 km beneath the Pamir and Hindu Kush ranges. Very low seismicity was observed at depths less than about 70 km, the approximate depth of the Moho. Clear gaps in activity exist also within the zone of intermediate depth seismicity. One gap, about 50 km wide near 37°N and at depths greater than 100 km, separates a steeply northward dipping zone to the southwest from a steeply southeastward dipping zone to the northeast. This gap probably marks either a tear in the downgoing slab or a gap between two oppositely dipping slabs. Fault plane solutions, determined by Soboleva for events between 1960 and 1967, generally show steeply plunging T axes approximately within the planar seismic zone. They therefore are grossly similar to those at island arcs where no deep earthquakes occur and presumably result from gravitational body forces acting on a relatively dense slab of lithosphere. At the same time there is a very large variation in the fault plane solutions, much larger than is common at island arcs.

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Magnetostratigraphy of the Moenkopi Formation at Bears Ears, Utah
Authors: Lienert Barry R. , Helsley C. E.
Published: February 2008

We have performed a paleomagnetic study on over 200 oriented samples collected from the Lower Triassic Moenkopi Formation at Bears Ears in southeastern Utah. Seven intervals of normal and reversed magnetic polarities are present at the Bears Ears site. We have correlated these magnetozones to a similar set established in the same formation 130 km to the northeast. At Bears Ears, magnetozone boundaries occur in different lithologic members of the Moenkopi Formation than corresponding magnetozone boundaries present in the section to the northeast. We conclude that contemporaneous lateral variations of sediment type and environement are a characteristic feature of the Moenkopi Formation.
Aeromagnetic and gravity surveys in the Coso Range, California
Authors: Plouff Donald , Isherwood William F.
Published: February 2008

The effect of an underlying magma reservoir cannot be identified within the complex gravity pattern in the Coso Range, California. Rather, linear gravity contours, which suggest a regional tectonic origin, enclose the location of most of the volcanic activity of the Coso Range. Faults along the edges of northwest trending, magnetic blocks probably provided paths of minimum resistance to the ascending viscous magma that was extruded as rhyolite domes. Dense, magnetic rocks associated with a complex mafic pluton 9 km in diameter form a relatively impermeable north border of the Pleistocene volcanic field. A heat flow high nearly coincides with the west half of a 6‐km‐diameter magnetic low. A 2‐km‐diameter outcrop of a pre‐Cenozoic silicic pluton, which has low magnetization compared to the surrounding metamorphic rocks, presumably typifies the rocks that underlie the magnetic low and heat flow high. Hydrothermal fluids may have destroyed some magnetite in the more magnetic wall rock, further reducing the magnetic intensity.
Speed of longitudinal sound in clathrate hydrates
Authors: Whalley E.
Published: February 2008

The relative speed of longitudinal sound in clathrate hydrates and in ice has been estimated to an accuracy of about ½%. It is 0.939 and 0.945 for the full structure I and structure II clathrates, respectively, and 1.011 for the empty clathrate of both kinds. The difference of the speed of sound in a porous rock when filled with ice and with clathrate is about 240ϕ(ur/ui)3m s−1, where ϕ is the volume fraction of the ice or clathrate and ur and ui are the speeds of sound in rock containing ice or clathrate, and in ice, respectively.
Hydrothermal alteration enthalpy and heat flow in the Roosevelt Hot Springs Thermal Area, Utah
Authors: Parry W. T. , Ballantyne J. M. , Bryant N. L.
Published: February 2008

Mass abundances of hydrothermal alteration product minerals at the Roosevelt Hot Springs thermal area have been calculated using whole rock chemistry and mineral analyses. These computations together with appropriate formulation of the alteration chemical reactions have been used to estimate total enthalpy produced per unit volume of altered rock. Enthalpy of alteration varies widely depending on the model assumed for sulfate production and the intensity of alteration. Maximum enthalpy production for the most altered rock ranges from 2450 to 7070 J/cm3, depending on whether none or all of the sulfate required to produce alunite is derived from H2S oxidation. The mean enthalpies in the lower 30 m of two drill holes 60 and 66 m deep are 345 and 105 J/cm3. Approximate age and depth‐of‐alteration constraints suggest heat flow due to alteration may be as low as 4 mW/m2 or as great as 852 mW/m2 depending on alteration intensity and sulfur oxidation. Even though chemical reactions could account for a substantial fraction of measured heat flow, comparison of measured temperatures with temperature calculations indicates an insignificant contribution of alteration reactions to present‐day heat flow. The present thermal gradient is explained in terms of convective circulation with a mass flux of water of 1.3×10−6 kg/m2 s estimated from alteration mineral abundance.
Joint linear, extremal inversion of seismic kinematic data
Authors: Orcutt John A.
Published: February 2008

A method for treating, in a joint manner, both distance and delay time data, including uncertainties, is presented. Full advantage is taken of the linear relationship between kinematic data and velocity structure. The inversion is computed using linear programing, a technique developed to treat economic optimization problems. Because of the linearity of the inverse problem and the ability to constrain solutions to be physically realizable the method will accurately predict achievable velocities within the earth. In general, after choosing a convenient basis in which to expand the velocity‐depth function, the technique will provide the tightest possible constraints consistent with a kinematic data set composed of upper and lower bounds on τ(p) and X(p). The effects of two nonlinear constraints caused by a possible lack of knowledge of velocities within a low‐velocity zone and/or the surface velocity are examined.
Averaging on the surface of a sphere
Authors: Gaposchkin E. M.
Published: February 2008

Area means of terrestrial gravity and geoid height are often formed from data and used to compute spherical harmonic potential coefficients. One commonly used area shape is the trapezoid, formed with sides corresponding to meridians of latitude and parallels of longitude. Data averaging modifies the spectrum of spherical harmonic coefficients. Explicit closed formulas are given for the relation between the surface harmonic of an unsmoothed and that of a smoothed function, illustrating the smoothing and aliasing due to averaging. The unaveraged coefficients can be calculated, but the aliasing remains. Also shown is that no arbitrary relation exists between the block size of an area and the degree of the corresponding spherical‐harmonic representation.
Foreword to the Norman Watkins Memorial
Authors: Fuller Michael , Harrison Christopher
Published: February 2008

Norman D. Watkins' career in geophysics spanned some of the most exciting years hi the history of the field. He was born in England at Sunbury‐on‐Thames on February 15, 1934. He took a B.Sc. from the University of London hi geology with mathematics and physics. His postgraduate work started in 1957, and by 1964 he had completed an M.Sc. in exploration geophysics at Birmingham, an M.Sc. in geophysics at Alberta, and a Ph.D. at London, also in geophysics.
Paleomagnetic evidence for the existence of the geomagnetic field 3.5 Ga ago
Authors: McElhinny M. W. , Senanayake W. E.
Published: February 2008

A paleomagnetic study has been made of the red dacite and pillow basalt of the Duffer Formation of the Pilbara craton, Australia. The dacite is currently the oldest dated rock unit in Australia, having a zircon U‐Pb age of 3452±16 Ma. Vector analysis of alternating field and thermal demagnetization results yield three components of magnetization. The high‐temperature, high‐coercivity hematite (H) component in the red dacite appears to be postfolding in age, as does a low‐temperature, low‐coercivity (L) component observed in the pillow basalt. However, an intermediate‐temperature, intermediate‐coercivity magnetite (M) component observed in both the pillow basalt (five sites) and the red dacite (six sites) is clearly prefolding in age and thus older than 3.0 Ga. This M component appears to be the oldest magnetization yet recorded in paleomagnetic studies and yields a paleomagnetic pole, probably original, at 43.9°N, 86.3°E (A95 = 7.2°). A tentative Archean apparent polar wander path for Australia suggests that the L and H components are also ancient magnetizations, probably 3.0 and 2.6 Ga old, respectively. The primary M component establishes the existence of the geomagnetic field at least 3.5 Ga ago.
Late Tertiary secular variation of the geomagnetic field in the North Atlantic
Authors: Dodson R. E.
Published: February 2008

The distributions of virtual geomagnetic pole (VGP) vectors of the paleomagnetic data from eastern Iceland, western Iceland, and the Canary Islands have been analyzed. For all these data sets, the null hypothesis that each data set represents a random sampling of Fisher's distribution can be rejected with 95% confidence. Similarly, the Dimroth‐Watson distribution can be rejected for the Canary Island data, but it cannot be rejected (with 90% confidence) for either of the Icelandic data sets. The orientation of the VGP moment ellipsoids and the increase in angular dispersion with increasing latitude is closely duplicated by a simple stochastic vector model in which the geocentric dipole axis wobbles in a Fisherian manner and the nondipole field anomalies are represented by radial dipoles located on the core‐mantle interface and strongly biased to the polar regions. Values of the angular dispersion of the dipole wobble within the interval 8°≤SD≤12° result in vector density functions which resemble the observed densities, whereas values outside this range result in very different densities.
K‐Ar dating, geological and paleomagnetic study of a 5‐km lava succession in northern Iceland
Authors: Saemundsson Kristjan , Kristjansson Leo , McDougall Ian , Watkins N. D.
Published: February 2008

A 5‐km section of basalt lavas exposed at Tröllaskagi on the western flank of the Eyjafjördur structural dome in northern Iceland has been subjected to K‐Ar age and paleomagnetic analysis from which detailed results are tabulated (34 dated lavas, 455 paleomagnetic directions). The upper 4 km of the sequence was erupted between about 11 and 9 m.y. ago. The rate of growth of the lava pile changed about 9.5 m.y. ago. Growth rates in the lower part were of the order of 1 km/m.y., whereas growth rates in the upper part may have been as high as 4 km/m.y. A 1.8 km‐thick normal polarity interval in the middle part of the section is correlated with marine magnetic anomaly 5. A good match is observed between the polarity sequence of the Trðllaskagi section and the marine magnetic polarity pattern above and below anomaly 5. Regression analysis of the K‐Ar data gives an age of 10.5 m.y. for the older boundary of anomaly 5 but a questionable 9.3‐m.y. age for its younger limit. Structural characteristics of the lava pile indicate that eruption of the sequence was associated with a now extinct rift axis in the western part of northern Iceland. A mean paleomagnetic field direction from 292 lavas is slightly far sided and right handed. Low‐latitude virtual poles are more prevalent in this survey than in two comparable recent surveys on younger Icelandic lavas and much more common than may be expected from a Fisherian distribution. There is no evidence to confirm that such transitional poles are predominantly due to nondipole fields. No significant difference was found in overall strength or stability between normal and reverse geomagnetic fields, as recorded in over 1000 lava flows in Iceland.

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Mechanics of discontinuous faults
Authors: Segall P. , Pollard D. D.
Published: February 2008

Fault traces consist of numerous discrete segments, commonly arranged as echelon arrays. In some cases, discontinuities influence the distribution of slip and seismicity along faults. To analyze fault segments, we derive a two‐dimensional solution for any number of nonintersecting cracks arbitrarily located in a homogeneous elastic material. The solution includes the elastic interaction between cracks. Crack surfaces are assumed to stick or slip according to a linear friction law. For an array of echelon cracks the ratio of maximum slip to array length significantly underestimates the difference between the driving stress and frictional resistance. The ratio of maximum slip to crack length slightly overestimates this difference. Stress distributions near right‐ and left‐stepping echelon discontinuities differ in two important ways. For right lateral shear and left‐stepping cracks, normal tractions on the overlapped crack ends increase and inhibit frictional sliding, whereas for right‐stepping cracks, normal tractions decrease and facilitate sliding. The mean compressive stress between right‐stepping cracks also decreases and promotes the formation of secondary fractures, which tend to link the cracks and allow slip to be transferred through the discontinuity. For left‐stepping cracks the mean stress increases; secondary fracturing is more restricted and tends not to link the cracks. Earthquake swarms and aftershocks cluster near right steps along right lateral faults. Our results suggest that left steps store elastic strain energy and may be sites of large earthquakes. Opposite behavior results if the sense of shear is left lateral.
Crustal structure of the Shatsky Rise from seismic refraction measurements
Authors: Gettrust J. F. , Furukawa K. , Kroenke L. W.
Published: February 2008

Data from a 130 km long reversed seismic refraction profile on the Shatsky Rise are interpreted.P velocities in the upper crust beneath the rise are essentially the same as those found for the upper crust in ocean basins. No refractor with a P velocity appropriate for the mantle is observed, implying that if an M discontinuity exists, its depth is greater than 26 km. Gravity variations over the Shatsky Rise of 30 mGal can be accounted for by the surficial topography. If this large thickness of crustlike material composing the rise were overlying normal mantle material, the result would be a large negative free air anomaly, which is not observed. This suggests that an anomalously dense mantle underlies the central portion of Shatsky Rise.
Crust‐mantle whispering gallery phases: A deterministic model of teleseismic Pn wave propagation
Authors: Menke William H. , Richards Paul G.
Published: February 2008

Teleseismic Pn waves are modeled as a sum of whispering galley waves, which propagate in a waveguide composed of a simple high velocity mantle lid underlain by a low velocity zone. This model is able to account for those Pn wave propagation properties that are not dominated by scattering, i.e., their apparent velocity and lack of long period energy. Pn apparent velocity data constrain the P wave velocity gradient in the upper 100 km of mantle to be low; dvp/dz<0.001 s−1. The whispering gallery rays are shown to have significant amplitude (when compared to the direct P wave) and to have spectra that rapidly fall off at long periods. Yet this model cannot account for certain details of the observed Pn amplitude spectra. Most important among these is the spectral ratio of Pn to P, which the model underestimates by a factor of 10. Nevertheless, the model presents a useful framework for understanding some characteristics of Pn wave propagation and provides an estimate of the distribution of energy with depth in the Pn waves.
Tectonic stresses in France: In situ measurements with a flat jack
Authors: Froidevaux Claude , Paquin Christian , Souriau Marc
Published: February 2008

In situ stress measurements in thick limestone layers of the Parisian Basin and in neighboring regions yield a remarkably homogeneous stress field. The flat jack method has been used. Comparison with over‐coring gave good agreement for the azimuth of the principal axes. In the Jurassic of Burgundy and Poitou as well as in a Carboniferous outcrop near the English Channel, maximum compression is found along the NNW‐SSE direction.
Theoretical study of structures, stabilities, and phase transitions in some metal dihalide and dioxide polymorphs
Authors: Tossell J. A.
Published: February 2008

A purely theoretical version of the ionic model is applied to the prediction of the bond distances and cohesive energies in Be, Mg, and Ca difluorides and dichlorides and in Si and Ti dioxides. The average error in predicted metal‐anion nearest‐neighbor distance is about 0.05 Å. Preferred structure type is correctly predicted for the fluorides and oxides but not for the chlorides. The most significant discrepancies between calculated and experimental structures are the exaggerations of M‐M distances across shared polyhedral edges in the calculated structures. Such discrepancies are probably the result of attractive interactions between the metal ions, which are ignored in our ionic model calculations. The rutile → fluorite phase transition pressure predicted for MgF2 is 200–420 kbar, in qualitative agreement with the experimental value of 330 kbar at 0K, while that predicted for SiO2 is about 3.9×103 kbar, indicating that SiO2 will probably not undergo such a phase transition within the mantle of the earth.
Propagation of shock waves in one‐dimensional crystal lattices
Authors: Hill T. G. , Knopoff L.
Published: February 2008

Numerical experiments on strong shocks in one‐dimensional Toda lattices initially at rest show that the shock is led by a soliton and that the shock versus particle velocity relationship is inconsistent with the Rankine‐Hugoniot equations. The shock is at all times unstable in the sense that the shock structure is a function of the distance of penetration of the shock into the lattice. The wake of strong shocks on the Toda lattice shows an almost monochromatic vibration spectrum, which we take to be the Einstein frequency of the lattice. A conjecture is offered for the reduction of such idealized shock wave data to isothermal equations of state, which bypasses the conventional two‐stage procedure of application of the Rankine‐Hugoniot equations plus the Grüneisen equation.
Convective fractionation: A mechanism to provide cryptic zoning (macrosegregation), layering, crescumulates, banded tuffs and explosive volcanism in igneous processes
Authors: Rice Alan
Published: February 2008

A large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and/or solidifying melts; e.g., macroscopic segregation (cryptic variation, zoning in magma chambers), mineral layering (in mafic intrusives), crescumulates (fingering) in the vertical and horizontal, banding (in pyroclastics), ‘rollover’ with attendant flashing of volatiles (explosive volcanism), etc. Some quantitative and qualitative aspects of convection in solidifying and/or stratified melts (e.g., mineral layer widths such as are observed in the Skaergaards) are examined to show consistency with field evidence. Convective fractionation does not possess the physical implausibilities of gravitational segregation (crystal settling). Neither is the field evidence as ambiguous if interpreted in terms of convective fractionation (which can explain amongst other things heavier material overlying lighter). Convective fractionation may operate on larger scales in the interior of planets.
The intersection of a pressurized crack with a joint
Authors: Keer L. M. , Chen S. H.
Published: February 2008

A model is described for considering the partial slip that occurs when a pressurized crack intersects a joint to which it is perpendicular. The crack is assumed not to penetrate into the undamaged material; rather, opening of the crack occurs at the juncture through interfacial slip. Within the interfacial slip region the resistance to slip is accounted for by prescribing the shear stress there. Results are obtained as the hydraulic loading in the crack is increased, and the subsequent crack growth is investigated. It is found that for a prescribed critical value of stress intensity factor the crack length remains fixed until a certain value of pressure is attained. Above this value the crack length can increase with increased pressure.
Dynamic tensile strength of lunar rock types
Authors: Cohn Stephen N. , Ahrens Thomas J.
Published: February 2008

The dynamic tensile strengths of four rocks have been determined. A flat plate impact experiment is used to generate ∼1‐μs‐duration tensile stress pulses in rock samples by superposing rarefaction waves to induce fracture. A gabbroic anorthosite and a basalt were selected because they are the same rock types as occur on the lunar highlands and mare, respectively. Although these have dynamic tensile strengths which lie within the ranges 153–174 MPa and 157–179 –MPa, whereas Arkansas novaculite and Westerly granite exhibit dynamic tensile strengths of 67–88 MPa and 95–116 MPa, respectively, the effect of chemical weathering and other factors, which may affect application of the present results to the moon, have not been explicitly studied. The reported tensile strengths are based on a series of experiments on each rock where determination of incipient spallation is made by terminal microscopic examination. These data are generally consistent with previous determinations, at least one of which was for a significantly chemically altered (hydroxylated) but physically coherent rock. The tensile failure data do not bear a simple relation to compressive results and imply that any modeling involving rock fracture consider the tensile strength of igneous rocks under impulse loads distinct from the values for static tensile strength. Generally, the dynamic tensile strengths of nonporous igneous rocks range from ∼ 100 to 180 MPa, with the more basic, and even amphibole‐bearing samples, yielding the higher values.
Paleomagnetism and cross‐folding in a key area of the Asturian Arc (Spain)
Authors: Bonhommet N. , Cobbold P. R. , Perroud H. , Richardson A.
Published: February 2008

In the northwest corner of Spain, Paleozoic and older rocks outcrop in an arcuate pattern (Asturian Arc), concave toward the east. The arc may be primary (before sedimentation) or secondary (after sedimentation). Supporting a secondary origin are many structural features, including two sets of folds, one radial and one tangential to the arc. The Villasecino area was selected for a pilot study of the relationships between paleomagnetism and folding. Detailed structural mapping has shown that radial folds postdate tangential folds. Interference of the two sets has produced the doubly plunging Villasecino anticline. Forty two oriented paleomagnetic cores were drilled at six sites distributed around the anticline, all within red limestones (griotte) of the Alba formation ( lower Carboniferous). The typical sample contains 95% by volume of fine‐grained calcite; each grain being rimmed by very fine‐ grained goethite and other iron‐bearing minerals. The natural remanent magnetism (NRM) is of moderate intensity (about 3 × 10−3 A m−1) readily measured and very homogeneous for each site. Within‐site scatter of vectors is low, but the site mean varies significantly from site to site. The angle between bedding and magnetization vector is nearly constant. These data suggest that magnetization predated folding. Thirteen samples from six sites were subjected to alternating field demagnetization up to 50 mT, and this showed the NRM to be very stable. Thermal demagnetization indicated a unique stable component with blocking temperature in the range 300°–600°C. Chemical demagnetization was attempted using sodium sulphite. Results suggest that the carriers of magnetization are not iron hydroxides but probably hematite. Even after demagnetization, a simple bedding correction does not yield a good grouping of magnetization vectors. We have therefore tested five other, more complex models of unfolding, based on structural observations. The statistically best grouping is produced by a model involving two successive fold phases, where first folds have horizontal E‐W axes and second folds have vertical axes. This is compatible with structural evidence. After unfolding, we obtain D = 101.8°, I = 13.1° (α95° = 1.5°) for the orientation of the earth's magnetic field in the area. If we compare with the direction for the Carboniferous of ‘stable Europe’ we get a difference in declination of 50° to 65° according to the model used for closing the Bay of Biscay, possibly in association with tectonic evolution of the Asturian Arc.
Stochastic synthesis of earthquake catalogs
Authors: Kagan Y. Y. , Knopoff L.
Published: February 2008

A model of earthquake occurrence is proposed that is based on results of statistical studies of earthquake catalogs. We assume that each earthquake generates additional a probability that depends on time as t−(1+θ). This assumption together with one regarding the independence of branching events on adjacent branches of the event ‘tree,’ is sufficient to permit the generation of complete catalogs of earthquakes that have the same time‐magnitude statistical properties as real earthquake catalogs. If θ is about 0.5, the process generates sequences that have statistical properties similar to those for shallow earthquakes: many well‐known relations are reproduced including the magnitude‐frequency law, Omori's law of the rate of aftershock and foreshock occurrence, the duration of a recorded seismic event versus its magnitude, the self‐similarity or lack of scale of rate of earthquake occurrence in different magnitude ranges, etc. A value of θ closer to 0.8 or 0.9 seems to simulate the statistical properties of intermediate and deep shocks. A formula for seismic risk prediction is proposed,and the implications of the model for risk evaluation are outlined. The possibilities of the determination of long‐term risk from real or synthetic catalogs that have the property of self‐similarity are dim.
Effects of simulated fault gouge on the sliding behavior of Tennessee Sandstone: Nonclay gouges
Authors: Shimamoto Toshihiko , Logan John M.
Published: February 2008

Stick‐slip has been considered to be a mechanism of shallow focus earthquakes along pre‐existing faults, and thus laboratory studies relevant to stick‐slip may lead to a better understanding of the mechanisms of the earthquakes and eventually to an effective prediction and/or control of them. This paper deals with the effects of various simulated gouges on the sliding behavior of rocks with special emphasis on how fault gouge alters potentially unstable behavior of a brittle rock. About 150 specimens of Tennessee sandstone with various simulated gouges along a 35° precut are deformed dry in a triaxial apparatus at room temperature, with shortening rate of about 2 × 10−4cm/s and at confining pressures to 300 MPa. Tennessee sandstone is a brittle and strong rock (quartz‐cemented quartzose sandstone) which consistently shows stick‐slip without simulated gouge at confining pressures above 30–40 MPa. Sliding behavior with a gouge layer depends markedly on the material used to simulate the gouge. Experimental results from monomineralic gouges show that Mohs' hardness of a mineral is a useful parameter to predict the general sliding behavior. The most unstable sliding is associated with gouge composed of minerals with intermediate hardness (dolomite, anhydrite, and calcite); these gouges undergo the transition from stable sliding to stick‐slip at pressures of 50–80 MPa. Gouge composed of hard mineral such as quartz or feldspar behave more stably, but stick‐slip occurs at pressures above 250 MPa. Only stable sliding is observed for gouge composed of soft minerals like halite. Although anhydrite gouge shows violent stick‐slip at 100 MPa pressure, whereas only stable sliding occurs for quartz at the same pressure, when they are mixed, the behavior changes systematically with composition. The inclusion of only 10% anhydrite permits stick‐slip, and hence even a minor constituent mineral of fault gouge may alter the sliding mode. For gouge composed of two distinct layers, the more stabilizing member of the two dominates the behavior, and the behavior is considerably different from that of homogeneously mixed gouge with the same bulk composition. This suggests the significance of internal structures of natural fault zone in understanding the effects of fault‐zone materials on the fault motion. The gouge composed of calcite or a mineral harder than calcite all show high and approximately the same ultimate frictional strength, with an exception of fluorite. The frictional strength is intermediate for fluorite, galena and barite gouges, and it is low for halite gouge.
Magnetic effects of maghemitization of oceanic crust
Authors: Prévot Michel , Lecaille Alain , Mankinen Edward A.
Published: February 2008

Both theoretical considerations and available experimental results indicate that magnetic effects of maghemitization are strongly dependent on the grain size of the originally unoxidized titanomagnetite. Maghemitization of single‐domain titanomagnetite results in a decrease in coercivity, an increase in susceptibility, and a large decrease in Q ratio. Maghemitization of multidomain titanomagnetite results in an increase in coercivity, a decrease in susceptibility, and no large changes in Q ratio. Single‐domain titanomagnetite is probably resistant to the development of a chemical remanent magnetization (CRM), whereas multidomain titanomagnetite can acquire a CRM during maghemitization. The behavior of pseudo‐single‐domain titanomagnetite, which is the main carrier of remanence in submarine extrusive rocks, is investigated by comparing the magnetic properties of the French‐American Mid‐Ocean Undersea Study (FAMOUS) (less than 0.1 m.y. old) and the Leg 37 (3.5 m.y. old) pillow basalts recovered from the Mid‐Atlantic Ridge near 37°N. Combining electron microprobe analyses, Curie temperature measurements, and cell edge determinations, we find that the FAMOUS rocks are already oxidized (z = 0.38), possibly as a result of some high‐temperature maghemitization during cooling of the magma. Comparison with the more highly oxidized (z = 0.7) Leg 37 pillow basalts indicates that low‐temperature maghemitization of such rocks does not result in appreciable changes of coercivity and susceptibility, although the Q ratio does decrease and CRM seems to be acquired. Such a CRM could account for the anomalously low magnetic inclinations observed at most of the Leg 37 sites.
A simple global model of plate dynamics and mantle convection
Authors: Hager Bradford H. , O'Connell Richard J.
Published: February 2008

Cooling and thickening of lithospheric plates with age and subduction result in large‐scale horizontal density contrasts tending to drive plate motions and mantle flow. We quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions. First, two‐dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions. We are unable to obtain platelike behavior in viscous models with traction‐free boundary conditions. The piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes. If the lithosphere has a Newtonian viscous rheology, the net force on any plate is a strong function of the effective grid spacing used, leading to ambiguities in interpretation. Incorporating a rigid‐plastic lithosphere, which fails at a critical yield stress, into the otherwise viscous model removes these ambiguities. The model is extended to the actual three‐dimensional (spherical) plate geometry. The observed velocities of rigid‐plastic plates are matched to the solution of the viscous Stokes equation at the lithosphere‐asthenosphere boundary. Body forces from the seismically observed slabs, from the thickening of the lithosphere obtained from the actual lithospheric ages, and from the differences in structure between continents and oceans are included. Interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included. The driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three‐dimensional global return flow and resistance to deformation at converging boundaries; the rms residual torque is ∼30% of the driving torque. The density contrasts within the plates themselves can reasonably account for plate motions. Body forces from convection in the interior may provide only a small net force on the plates. At converging boundaries the lithosphere has a yield stress of ∼100 bars; drag at the base of the plates is ∼5 bars and resists plate motion. The net driving forces from subducting slabs and collisional resistance are localized and approximately balance. Driving forces from lithospheric thickening are distributed over the areas of the plates, as is viscous drag. The approximate balance of these two forces predicts plate velocities uncorrelated with plate area, as observed. The model represents a specific case of boundary layer convection; the dynamical results are consistent with either upper mantle or mantle‐wide convection.
Mantle convection with spherical effects
Authors: Olson Peter
Published: February 2008

Results of a similarity theory for spherical mantle convection are presented. The single‐mode mean field equations are analyzed for convection which is so vigorous that temperature disturbances become localized in thin thermal boundary layers. Our purpose is to study effects of spherical geometry, density interfaces, heat source distribution, and cell size. Steady state solutions are found for isoviscous spherical shells in which the field of motion is spatially periodic in a single spherical harmonic degree. Calculations are carried out over the range 2 ≤ l ≤ 40 and for various fractions of internal versus base heating. Three configurations are examined: (1) convection in a single layer of cells extending through the whole mantle, (2) convection in two layers, separated by a density interface at 670‐km depth, and (3) convection in a single layer terminating at 670 km. Results of these calculations are used to give estimates of surface horizontal velocities in terms of the heat loss, viscosity stratification, amount of internal heating, and depth of circulation. The surface velocity is most strongly affected by the thickness of the convecting shell. Deep mantle convection can achieve surface velocities which agree with observed plate speeds, while convection restricted to the upper mantle does not, at least on the scale of the major plates. The temperature distribution is strongly affected by the spherical geometry and by the presence of density interfaces. The principal difference between convection in one and two layers is that the latter produces a ‘hot’ lower mantle, while the former produces a ‘warm’ one.
Analysis of the multicomponent magnetization of the Little Dal Group, Mackenzie Mountains, Northwest Territories, Canada
Authors: Park J. K.
Published: February 2008

A paleomagnetic study on 14 red limestone sites of the Helikian Little Dal Group, ‘basinal sequence’ (Mackenzie Mountains, Northwest Territories, Canada) resolved five magnetizations using thermal and alternating field treatment with vector analysis. One magnetization (C) with low unblocking temperatures (<350°C) is probably a goethite weathering component of Cretaceous age. It gives a direction at 291°, +75° (12 sites, k = 73, α95 = 5°) and a pole at 60°N, 170°E. Two other closely related magnetizations (A, detrital?) probably carried by magnetite (AM) and hematite (AH) respectively, yield directions of 265°,‐29° (k = 87, α95 = 4°) and 264°,‐26° (k = 53, α95 = 6°) with a combined pole at 16°S, 141°E (14 sites, K = 141, A95 = 3°). The two remaining magnetizations (B, red hematite pigment?)—a normal (BN) and a reverse (BR) component recognized in most specimens—have a combined direction of 273°,‐09° (k = 17, α95 = 10°) and a pole at 3°S, 138°E (13 sites, K = 26, A95 = 8°). C and A are prefolding (before Paleocene or pre‐late Cretaceous) with A suggested to be primary. A and B lie close to a recently proposed polar track for the late Helikian and Hadrynian and evidence suggests a magnetization age for A of 900 to 950 Ma.