bluuebunny/crossref_metadata_2025 huggingface dataset:
The low‐high quartz and quartz‐coesite transition to 40 kbar between 600° and 1600°C and some reconnaissance data on the effect of NaAlO2 component on the low quartz‐coesite transition
Authors: Mirwald Peter W. , Massonne H.‐J.
Published: February 2008

The phase relations of low quartz, high quartz, and coesite were reinvestigated using extremely pure SiO2. The study was conducted with a piston‐cylinder apparatus employing low‐friction cells. While the low quartz‐coesite transition between 600° and 1100°C was determined by conventional quench techniques, the greater portion of this study relied on in situ methods (differential thermal analysis and a volumetric method referred to as differential pressure analysis). The agreement with previous results is good; however, the precision of present data is considerably higher. A comparison of our salt‐cell data with the raw data of previous studies shows a pressure loss due to friction of about 10% for talc cells. A discussion of thermochemical aspects of the low quartz‐coesite transition indicates improved agreement between thermodynamic and experimental data. Reconnaissance data on the influence of the NaAlO2 component show above 600°C in the absence of H2O a considerable shift of the quartz‐coesite boundary toward higher pressure (4 kbar at 1100°C). In the presence of H2O a partial liquid is formed, and the shift is reversed; the transition boundary is reduced 3 kbar and lies between 680° and 1000°C at even lower pressure than that of the pure SiO2 modifications.
Electromagnetic heating of Io
Authors: Colburn D. S.
Published: February 2008

Electrical‐energy deposition within Io is a possible mechanism for heating the interior, the source of the electrical current system being the relative motion of Io with respect to Jupiter's magnetic field. The transverse‐electric mode currents circulate entirely within Io, with the frequency of Jupiter's rotation rate as observed in the Io frame. Maximum heating for Io is obtained if the electrical conductivity is 0.02 mho m−1, a value spanned by estimates of rock conductivity for hot interiors. Under present conditions, the maximum heating available from the transverse‐electric mode is some 3 orders of magnitude less than would be necessary to maintain Io's estimated heat flow. Confinement of Io's field by the external plasma would raise this estimate only moderately. The transverse‐magnetic mode is effectively blocked by the cold outer surface layers, the current being bypassed by an alternative current conduction path through the ionosphere, leaving an insignificant amount in the interior. Although the transverse‐magnetic heat source can be concentrated at local surface hot spots, the temperature rise is not significant on a time scale appropriate to localized heat sources. It is possible that significant heating was generated in the past by a magnetic field that was larger and had a higher frequency. Heating could have been significant also if Io's ionosphere had not been present.
An assessment of the accuracy of the geodetic measurements that define the Southern California Uplift
Authors: Mark Robert K. , Tinsley John C. , Newman Evelyn B. , Gilmore Thomas D. , Castle Robert O.
Published: February 2008

Examination of the charge that the geodetic measurements which define the southern California uplift are seriously flawed by height‐dependent systematic errors indicates that this charge is unfounded. Our review of those factors designed to minimize the effects of rod error shows that systematic errors attributable to rod miscalibration are trivial and would tend to cancel during the particularly pertinent period 1955–1965. Comparisons between the results of measurements in which the correlation between topography and signal is generally poor reveal large aseismic tilts in a number of places within and around the margins of the uplift. Especially significant in this context are the results of preuplift and postuplift levelings over routes characterized by diverse length, topography, and atmospheric conditions that produce closely matching, temporally equivalent heights for a representative bench mark within the uplift. Statistical analyses of the short‐wavelength components of uplift signal and elevation along a frequently repeated survey line indicate variable correlations between the two. However, the spatial and temporal patterns of these correlations are inconsistent with their attribution to height‐dependent systematic errors. Statistically significant short‐wavelength correlations along this survey line are reasonably explained in part as due to real movement associated with differentially subsiding bench marks, and there is a strong likelihood that this movement tends to dominate many of the correlations.
Magnetic signals from the core of the Earth and secular variation
Authors: Alldredge Leroy R.
Published: February 2008

An oscillating, radial magnetic dipole source was assumed to exist in the core of the earth, 100 km beneath the core‐mantle boundary. As an approximation, electromagnetic propagation was assumed in the core in lieu of hydromagnetic propagation, which could not be used because of unknown internal fields.Using Debye potentials, the radial and horizontal components of the surface fields were calculated using various assumed conductivity parameters in the core and in the mantle. It is concluded that most spherical harmonic models of the earth's magnetic field do not include enough terms to describe properly the field of core sources with periods of tens of years. These short‐period variations are especially important in describing the secular variation. Because of this, a proper description of the secular variation requires more spherical harmonic terms than are required for the field itself. Inadequate representation of short‐period variations in spherical harmonic models may contribute to the rapid deterioration of predictive models. Alternatives to spherical harmonic analysis for secular variation should be investigated; regardless of the method used, a much greater spatial distribution of high quality secular variation data is needed.
Kinematic evolution of the Northern Cocos Plate
Authors: Schilt F. Steve , Karig Daniel E. , Truchan Marek
Published: February 2008

As the East Pacific Rise approached and impinged upon the western margin of North America, the Farallon plate began to break into multiple smaller plates, whose relative movements changed strongly from those before breakup. Movements of the northern Cocos plate, inferred from magnetic lineations, show rapidly changing rates and pole positions which may be characteristic of other small remnants of the Farallon plate. Finite difference poles and rates for Cocos‐Pacific relative motion during the time intervals 2–5, 5–7, 7–10, and 10–14 m.y. have been determined from the geometry of magnetic isochrons. Prior to about 2 m.y. the Cocos‐Pacific pole was 10° to 20° south of its present location, and angular rates were 2–4 times the present rate. From 10–7 m.y., we deduce a large degree of oblique convergence between the Cocos and North American plates, which may be an important constraint on the evolution of the structure of the continental margin of southwestern Mexico. The Tehuantepec Ridge originated as the ridge‐ridge transform which generated the Clipperton fracture zone. Spreading rate estimates for the southern part of the Cocos plate suggest the possibility that the Tehuantepec Ridge was reactivated as a ridge‐trench transform between the two Cocos plate fragments prior to about 7 m.y.
The deep lunar electrical conductivity profile: Structural and thermal inferences
Authors: Hood L. L. , Herbert F. , Sonett C. P.
Published: February 2008

Simultaneous Apollo 12 surface and Explorer 35 orbital magnetometer records have been reexamined for intervals suitable for measuring the lunar inductive response in the solar wind and terrestrial magnetosheath. Improvements in the selected time series are obtained by (1) editing of intervals containing extraneous surface field fluctuations caused by plasiha interaction with the local crustal magnetic field, and (2) minimization of magnetometer data errors in the Apollo 12 record. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the frequency range 10−5 to 10−3 Hz using methods introduced earlier by Wiskerchen and Sonett (1977). Maximum consistency of estimates from different time intervals is found when the analysis is limited to the tangential direction of maximum incident power as measured at Explorer 35 for each interval, i.e., the direction in which the ratio of signal to background noise is greatest. A simplified version of spherically symmetric plasma (SSP) confinement theory is used to interpret the transfer function data via forward model calculations under the assumption that the electrical conductivity increases continuously with depth. The inferred bounds on the internal electrical conductivity profile are narrowest in the depth range 450–1350 km and are more restrictive at these depths than those obtained previously from plasma‐confined response data. An upper limit of ≤360 km for the radius of a metallic core with assumed conductivity ≥102 mhos/m is estimated from the one standard deviation error limits of the measurements. Using the bulk conductivity versus temperature model proposed by Huebner et al. (1979), rough limits on the selenotherm are calculated which appear to be in agreement with independent geophysical constraints at depths less than about 800 km.
Analysis of long‐period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens—A terrestrial monopole?
Authors: Kanamori Hiroo , Given Jeffrey W.
Published: February 2008

Long‐period (100 to 260 s) Love and Rayleigh waves excited by the eruption of Mount St. Helens on May 18, 1980, and recorded by IDA, SRO, and ASRO stations were analyzed to determine the mechanism of the eruption. The amplitude radiation patterns of both Rayleigh and Love waves are two lobed with a nodal direction in E5°S for Rayleigh waves and in N5°E for Love waves. These radiation patterns preclude any double‐couple mechanism. The radiation pattern, the initial phase, the relatively large amplitude ratio of Love to Rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher‐mode Rayleigh waves suggest that the source is represented by an almost horizontal (less than 15° from the horizontal) single force pointed toward S5°W. The surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process. Although the details of the source time history could not be determined, a smooth bell‐shaped time function: fos(t) = (1/2)fo(
) for 0 ≤ t ≤ 2τ and fos(t) = 0 for t ≥ 2τ, with τ = 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum. The peak value of the force f0 is about 1018 dynes. The tailing end of the source time function could not be resolved, and some overshoot may be added. The magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption. Two distinct events about 110 s apart can be identified on body wave and short‐period surface wave records. The first event may correspond to the earthquake which triggered the landslide and the lateral blast. The second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake.
Constraints on the structure of the East Pacific Rise from seismic refraction data
Authors: Lewis B. T. R. , Garmany J. D.
Published: February 2008

University of Washington seismic data taken during ROSE phase 1 have been used to study the structure of the East Pacific Rise in the vicinity of 12°N. Off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2.5 km, a zone of low but uniform velocity gradient from 2.5 to about 5.5 km and a transitional zone about 1 km thick to mantle material. There is also evidence for substantial anisotropy in mantle velocity gradients, with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise. Under the rise axis the data allow at most a very narrow zone of low velocity (less than about 2 km wide) in the crust.
Evidence for and consequences of thermal rejuvenation
Authors: Menard H. W. , McNutt Marcia
Published: February 2008

Anomalously shallow regions in the form of bathymetric swells are a common occurrence in ocean basins. The elevation of these swells could originate from pressure forces at the base of the lithosphere caused by mantle convection, compositional changes leading to increased buoyancy of the lithosphere, or thermal expansion caused by reheating of the lithosphere. The third explanation, thermal rejuvenation, must be an important factor in the formation and evolution of bathymetric swells because in every test we applied, we found midplate swells to behave like oceanic crust reset to a younger thermal age. From the subsidence of atolls and guyots on lithsopheric swells, it appears that they sink along the thermal cooling curve for normal oceanic crust of the same depth but younger age as defined by magnetic isochrons. The distribution of volcanoes in the oceans suggests that older crust acts as a thermal barrier to midplate vulcanism. If swells are thermally rejuvenated, therefore, they should be more likely spots for vulcanism. As predicted, swells commonly show overprinting of several distinct hot spot traces and remain extremely favorable sites for midplate vulcanism even when the direction of plate motion changes. Flexural rigidity values obtained from islands and seamounts superimposed on swells place constraints on the form of the thermal rejuvenation process. For the Hawaiian swell, the fact that the effective elastic‐plate thickness is much larger than would be expected from younger lithosphere at the same depth requires that the heating be confined to the lower lithosphere only. In the case of Great Meteor Guyot, the initial swell depth and elastic‐plate thickness are both approprite for lithosphere with an equivalent thermal age of 20 m.y. At least in this case, reheating of the entire lithospheric column remains a possibility. Lithosphere subjected to several reheating events, such as in the CookAustral and Society Islands area, is particularly likely to display a significant reduction in the elastic‐plate thickness.
Remagnetization and tectonic rotation of Upper Precambrian and Lower Paleozoic strata from the Desert Range, southern Nevada
Authors: Gillett Stephen L. , Van Alstine David R.
Published: February 2008

In the Desert Range of southern Nevada, miogeoclinal sedimentary rocks, mostly shallow‐water limestones of latest Precambrian through Early Ordovician age, yield three components of magnetization having different blocking temperature spectra: (1) a low blocking temperature component near the direction of the present axial‐dipole field; (2) an intermediate blocking temperature component with northerly declination and inclination ∼+60°; and (3) a characteristic component with southeasterly declination and inclination ∼−20°. Combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite. The intermediate magnetization probably reflects a viscous partial thermoremanent magnetization (VPTRM) imposed between the Late Cretaceous and mid‐Cenozoic; it was probably acquired when the strata were dipping slightly eastward. The characteristic magnetization is probably a VPTRM imposed during regional uplift in the Late Permian. The degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the Ordovician rocks; additionally, the characteristic magnetization in those rocks is younger than bedding disruption caused by major, late stylolitization. Red‐purple mudstones from the middle member of the Wood Canyon Formation, in which a directionally similar characteristic magnetization resides in fine‐grained hematite, also appear to have been remagnetized; in these rocks, the remagnetization probably reflects partial recrystallization, as the blocking temperatures are too high to have been reset by burial heating. The sampled sections have undergone relative tectonic rotation about a vertical axis, consistent with late Tertiary oroflexural bending that had been proposed on independent geologic evidence. The characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations, as the regional geology suggests the Desert Range strata were essentially horizontal throughout the Paleozoic. The total observed relative rotation is 44°±5°, representing absolute counterclockwise rotation (27°±4°) of the northern part of the Desert Range, and absolute clockwise rotation (17°±5°( of the southern part. The only unit in the Desert Range sequence that may retain a primary magnetization is the late Precambrian Rainstorm Member of the Johnnie Formation. The characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite; after correction for 18° of counterclockwise rotation, the resulting pole (5°N, 151°E) is near other late Hadrynian poles from North America.
Pore water chemistry of the Mounds Hydrothermal Field, Galapagos Spreading Center: Results from Glomar Challenger Piston Coring
Authors: Bender Michael L.
Published: February 2008

On DSDP Leg 70, Glomar Challenger piston cored hydrothermal MnO2‐encrusted nontronite mounds and adjacent pelagic sediments through to basement. Pore waters were collected by centrifuging, squeezing, and in situ sampling; analyses are presented here for Ca, Mg, Si, NH3, Mn, and Fe. Our results confirm Maris and Bender's (1982) conclusions that hydrothermal solutions enriched in Ca by 1–2 mM and depleted in Mg by ∼2 mM are upwelling through the mounds and the surrounding pelagic sediments. Si, NH3, and Mn2+ concentrations generally increase upcore, reflecting addition of products of metabolic reactions to upwelling hydrothermal solutions. Pore water iron concentrations decrease upcore, probably as a result of oxidation and precipitation of upwelling hydrothermal iron. The formation of nontronite (Fe(III)4Si8O20(OH)4) involves oxidation of dissolved Fe2+. Several models, constrained by the electron balance, are proposed to explain the process of nontronite formation. The stratigraphy of the mounds (thick nontronite covered by a thin MnO2 crust) may be explained by postulating Fe2+ oxidation by MnO2 and replacement of MnO2 by nontronite at the base of the MnO2 crust, followed by upward migration of Mn2+ and precipitation of MnO2 at the sediment water interface.
Global multispectral mosaics of the icy Galilean satellites
Authors: Johnson T. V. , Soderblom L. A. , Mosher J. A. , Danielson G. E. , Cook A. F. , Kupferman P.
Published: February 2008

Multispectral mosaics of the three icy Galilean satellites have been produced from low‐ to moderate‐resolution Voyager images. The digital image data have been calibrated using preflight and in‐flight calibration data and reduced to normal albedos for four‐color band passes, centered at 0.35, 0.41, 0.48, and 0.59 μm. The individual images have been geometrically corrected, mosaiked, and presented in a normal cylindrical map projection at a uniform scale of 8 km per pixel. Preliminary analysis of the normal albedo data shows that (1) the darkest terrains on the satellites are the dark regions on Callisto, (2) the brightest regions are bright plains regions on Europa and bright, fresh craters on Ganymede, (3), bright craters on Ganymede and its dark, cratered regions are both significantly brighter than comparable terrains on Callisto, and (4) Europa exhibits several distinct spectral units, with regions on the trailing hemisphere having lower relative ultraviolet spectral reflectance than areas on the leading hemisphere. This is consistent with proposals that Io‐related plasma impact on Europa's trailing side has altered its optical properties.
Possible seismic reflections from the downgoing Pacific Plate, 275 kilometers arcward from the Eastern Aleutian Trench
Authors: Fisher Michael A , von Huene Roland , Smith Gregory L. , Bruns Terry R.
Published: February 2008

Multichannel seismic reflection data collected near the southern Kenai Peninsula, Alaska, show deep events that could stem either from a linear diffractor or from a curved reflector. Whichever is the cause, it must be at a minimum depth of 12 km. If a linear diffractor causes the most obvious events, it strikes at a 60° angle across the surface trend of Mesozoic rocks. Thus the diffractor could be associated with a transverse tectonic boundary that, on the basis of other evidence, is thought to strike through the area of this study. Alternatively, these deep events could be reflections from rocks associated either with a detached piece of oceanic crust or with the downgoing Pacific plate. These associated rocks could be porous and overpressured sedimentary rocks that are squeezed between indurated or metamorphosed sedimentary rocks in the overthrust plate and igneous rocks in the downgoing oceanic crust.
A review of lunar paleointensity data and implications for the origin of lunar magnetism
Authors: Cisowski S. M. , Collinson D. W. , Runcorn S. K. , Stephenson A. , Fuller M.
Published: February 2008

The saturation remanence normalization method of paleointensity estimation has been applied to more than 50 lunar samples, and has been calibrated in terms of standard intensity determinations done on 14 samples. These results suggest that a lunar surface field, comparable in intensity to the earth's magnetic field, existed from about 3.6 to 3.8 AE. Absolute paleointensity estimates and lunar surface field strength determinations are consistent with this observation. The data presented here differ from previous descriptions of the variation in the lunar field with time in that (A) the oldest samples (>3.9 AE) are among the most weakly magnetized; and (B) an order of magnitude drop in paleointensity is suggested between the time of extrusion of the Apollo 11 low and high potassium basalts. The limited range in age of the strongly magnetized lunar samples and the absence of shocked samples in this set argues against a solar source for the lunar paleofield or for fields arising from transient phenomena such as impacts. The coincidence of this high field era with the termination of basin forming impacts and the beginning of mare basalt extrusion suggests a common cause. A close approach of the moon to the earth at about 3.8 AE might account for all three phenomena. The confined duration of the strong lunar field may allow for the assignment of relative ages for certain lunar samples and surface features.
Rapid discrimination of granitic rock compositions by low‐resolution near‐infrared reflectance
Authors: Baird Alex K.
Published: February 2008

The slopes of near‐infrared spectra between ∼1 and ∼2 μm from quartz‐bearing plutonic rocks are strongly correlated with rock chemistry determined by X ray spectrometry. The empirically derived predictive equations provide compositional data of adequate precision and resolution to discern patterns of regional geochemical variation in granitic batholithic rocks of southern California. As an analytical method, infrared spectrometry is rapid and inexpensive, and the method has potential in applications to direct field measurements and to data from aircraft and spacecraft scanner systems of relatively low spectral and spatial resolution, provided vegetative cover and surface alteration are not prohibitively masking.
Hugoniot data for pyrrhotite and the Earth's core
Authors: Brown J. Michael , Ahrens Thomas J. , Shampine Dennis L.
Published: February 2008

New shock wave Hugoniot data for pyrrhotite (Fe0.9S) now describe the equation of state to nearly twofold compression at a maximum pressure of 274 GPa. A minor discontinuity on the Hugoniot between 100 and 150 GPa is interpreted as the melting transition. While not tightly constrained, the inferred melting point lies below lower‐bound temperature estimates based on the Lindemann criterion. The highest‐pressure Hugoniot data (representing the melted phase) are used to model the equation of state for liquid iron sulfide. A density for liquid pyrrhotite of 7.80±0.20 Mg/m3 under core‐mantle boundary conditions (P = 135 GPa, T = 4000 K) is calculated. Assuming that sulfur is the primary alloying element in a predominately iron core, the present data are consistent with a homogeneous outer core containing 10±4 wt % sulfur.
Venus banded terrain: Tectonic models for band formation and their relationship to lithospheric thermal structure
Authors: Solomon Sean C. , Head James W.
Published: February 2008

Recent radar images of the mountains of Ishtar Terra, Venus, at approximately 3‐km radar resolution show a series of linear bands of alternatively greater and lesser backscatter; the bands are generally aligned with the topographic strike of the mountain ranges. We test the hypothesis that these band features are tectonic in origin, the product either of folding during lithospheric compression or of block faulting during lithospheric extension. Tested models for folding include uniform elastic and viscous plates overlying in viscid substrates, layered plates, and a viscous half space in which viscosity decreases exponentially with depth. The uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on Venus, about 15 to 20 km, if the elastic or high‐viscosity layer is, at most, a few kilometers in thickness, though the required compressive stresses are several kilobars in magnitude. For the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness, and similar results hold. Layered elastic or viscous plates, however, can fold at the required wavelength with subkilobar stresses and, therefore, are favored over uniform plate models. Tested extensional models include graben and horst formation in a surface elastic‐brittle layer, imbricate normal faulting, and necking of a surficial layer that is everywhere in a state of extensional failure. The spacing between adjacent extensional features can match the spacing between bands on Venus if the surficial brittle layer is no more than a few kilometers in thickness. Both compressional and extensional models for band formation are therefore consistent with radar images of Venus mountain ranges at the image resolution presently available. Additional evidence, however, including the linearity and continuity of individual bands, the relationship between band trends and topographic contours in some regions near the ends of mountain ranges, and the very different manifestations of lithospheric extension in other highland areas of Venus, leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of Ishtar Terra as a result of horizontal compression of the Venus lithosphere. Independent arguments on the thermal structure of the Venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of Venus is approximately 1–10 km thick. We therefore suggest that the surficial layer of elastic‐brittle or high‐viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the Venus highlands at the time of band formation.
Lunar dark‐haloed impact craters: Origin and implications for Early Mare volcanism
Authors: Bell Jeffrey F. , Hawke B. Ray
Published: February 2008

Remote sensing and photogeologic data were used to determine the composition and origin of lunar dark‐haloed craters. Near‐infrared reflectance spectra were obtained for numerous dark‐haloed impact craters located on either the ejecta blankets of large impact craters or Imbrian and Nectarian age light plains deposits. Spectral, thermal, radar, and photogeologic data conclusively demonstrate that Copernicus H and other dark‐haloed craters on the ejecta blanket of Copernicus excavated mare basalts from beneath lighter surface deposits rich in highlands material. Analyses of reflectance spectra of dark‐haloed craters on light plains indicate that in every instance these craters exposed mare basalt which had previously been covered by varying thicknesses of highlands debris. In the Schiller‐Schickard region a relatively thick highlands unit was emplaced as a result of the Orientale impact event. On the interior of Balmer basin, marelike basalt has been excavated from beneath much thinner highlands deposits emplaced by nearby craters. Mafic orbital geochemical anomalies are commonly associated with regions with abundant light plains deposits which exhibit dark‐haloed impact craters. Apparently, some mare material has been incorporated into the regolith either by local mixing during the emplacement of the highlands debris or by subsequent vertical mixing. The results of recent remote‐sensing, photogeologic, and lunar sample studies strongly indicate that mare volcanism was a significant process during much of the pre‐Imbrian and may have been initiated as early as 4.2–4.3 Ga. These very early volcanic episodes contributed materials to the lunar surface which were incorporated into the upper portion of the highlands crust by subsequent impact mixing. Models concerning the extent and duration of mare volcanism as well as those involving the composition and thermal evolution of the lunar interior will have to be revised.
An analysis of the orientations of large‐scale crustal structures: A statistical approach based on areal distributions of pointlike features
Authors: Lutz Timothy M.
Published: February 2008

The spatial distributions of agmatic complexes and other features thought to be associated with major crustal structures may be important sources of information about large‐scale structural patterns. However, attempts to incorporate these features into quantitative analyses of linear features have used arbitrary or inaccurate criteria to judge hypothetical geological relationships. In this paper, features of limited spatial extent are considered pointlike, and the concept of a probabilistic lattice point distribution is used to formulate a statistical method that leads to a quantitative and reproducible analysis of directional patterns based solely on the locations of the points. Thus this analysis is independent of linear patterns and provides a measure of the directional information intrinsic to point patterns. The procedure determines the most likely trends of structural anisotropies; Monte Carlo simulations of random point patterns provide a reference distribution from which confidence levels can be determined. Applications to published data for magmatic complexes, magnetic contour closures, and structural change points are used as examples. The results suggest that there has been a tendency to overestimate the amount of information available from point patterns.
Teleseismic estimates of the energy radiated by shallow earthquakes
Authors: Boatwright John , Choy George L.
Published: February 2008

The energy flux contained in the P‐wave groups (P + pP + sP) or the S‐wave groups (S + pS + sS) radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently. By defining generalized radiation patterns which incorporate this neutral interference, the wave groups are analyzed as though they were comprised of a single phase. Measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body‐wave attenuation and the frequency band of the recording. The corrected measurements are then used to estimate the seismic energy radiated by the earthquake. This analysis is applied to digital recordings of the teleseismic wave groups radiated by the May 2, 1983, Coalinga, California, earthquake and the October 28, 1983, Borah Peak, Idaho, earthquake. For the Coalinga earthquake, an estimate of Es = 1.6 ± 0.4×1021 dyn cm was determined from six P‐wave groups, while the SH wave group recorded at station COL returned an estimate of Es = 1.2×1021 dyn cm. For the Borah Peak earthquake, an estimate ofEs = 3.2 ± 0.5×1021 dyn cm was determined from seven P‐wave groups. The distribution of the isoseismals for the Borah Peak earthquake indicate that the energy radiated by this event was focussed to the northwest, in the direction of rupture propagation. Correcting the teleseismic estimate for this focussing gives Es = 3.6 ± 0.5×1021 dyn cm. Combining these estimates of the radiated energy with broad‐band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the Coalinga and the Borah Peak earthquakes, respectively.
An electron microscope study of the stability field and degree of nonstoichiometry in olivine
Authors: Boland J. N. , Duba A. G.
Published: February 2008

The stability of San Carlos olivine (Fo 90.5) relative to changes in oxygen fugacity at 1300°C has been determined by studying the microstructural changes in single crystals using transmission and analytical electron microscopy. The first detectable metallic phase, heterogeneously nucleated along dislocations within (100) and (001) subboundaries, had a composition Ni65Fe35 equilibrated with a CO/CO2 gas mixture having an oxygen fugacity fO2 of 10−5.6 Pa. Although the (100) subboundaries contained a high density of dislocations with Burgers vector [100], the nucleation sites were restricted to [010] and [001] dislocations: a result consistent with the theory of heterogeneous nucleation along dislocations. Under the reducing conditions of an fO2 of 10−12 Pa, a forsteritic olivine is produced at the gas‐mineral interface with a high density of metallic precipitates (Fe96Ni4) homogeneously nucleated in the olivine. Non‐stoichiometry of the olivine was inferred from the absence of any additional silica‐rich phases. The nonstoichiometry was analyzed in terms of possible crystalline defect structures, especially vacancies, assuming no evaporative losses from the surface except an oxygen exchange reaction. On the basis of the model proposed, the highly reduced olivine would contain an excess of SiO2 (xSiO2 = 0.355 compared with a stoichiometric value of 0.333) from the surface to a depth of several micrometers.
An inversion for slip distribution and fault shape from geodetic observations of the 1983, Borah Peak, Idaho, Earthquake
Authors: Ward Steven N. , Barrientos Sergio E.
Published: February 2008

This paper develops an inversion technique for static surface displacements associated with shallow faulting and applies the method to geodetic observations of the Borah Peak, Idaho, earthquake (M8 = 7.3) of October 28, 1983. This technique improves upon classical, uniform slip planar (USP) models by admitting earthquake faults which are curved and contain variable slip. By summing elemental point sources, employing a gradient strategy with positivity constraints, and combining both geodetic and surface scarp height data, we can now resolve fault orientation, dimension, shape, and slip distribution. Large formal misfits to the geodetic data and disagreements in observed surface scarp locations found with the best USP fault are eliminated in variable slip planar (VSP) models. The best VSP fault reaches to 18 km depth in the southeast but only to 3 km depth in the northwest. Surface slip and slip at depth are poorly correlated in the VSP model. In fact, toward the southeast, slip at depth extends 13 km beyond the terminus of the fault's surface expression. Three or four knots of concentrated slip are found in VSP models. These knots are 3–6 km in dimension and may represent the breaking of asperities. To test for listric‐like behavior of the Borah Peak fault, we considered all possible variable slip listric (VSL) faults of parabolic shape. The best VSL model has a slight downward curvature; however, it does not fit the data significantly better than VSP models. The geodetic measurements do not support significant listric curvature at depths shallower than 20 km. Our techniques permit the calculation of sensitivity kernels for surface displacement. These functions explicitly reveal fault locations where a certain geodetic measurement is most sensitive. For faults like Borah Peak, a surface measurement at point r samples slip to a depth roughly equal to the distance from r to the fault trace. Consequently, near fault bench marks and surface scarp height measurements are very weak constraints on slip at depth. We feel that variable slip fault models will allow significant new interpretations to be drawn from the geodetic data pool concerning the physics of earthquakes and their potential hazard.
Visible and near‐infrared spectra of carbonate rocks: Reflectance variations related to petrographic texture and impurities
Authors: Crowley James K.
Published: February 2008

Grain size statistics derived from thin section point measurements were used to classify carbonate rock textures and to identify related spectral characteristics in the visible and near‐infrared (0.4–2.5 μm) wavelength range. Although the statistics did not permit all the carbonate textural varieties generally recognized by petrologists to be distinguished, three broad textural classes of pure carbonate material were found that produced distinctive spectra. Further refinement of the statistical technique may permit other textural classes to be identified. However, internal scattering in carbonate rocks is a complex phenomenon involving the grain size distribution, as well as the nature and abundance of inclusions and internal grain defects. To yield an optimum textural classification for spectroscopic studies, all such scattering contributors would have to be quantified. The spectral effects of iron oxide, clay, silica, and organic matter impurities were examined by comparing texturally similar pure and impure carbonate rock samples. Of these impurities, organic matter had the greatest spectral influence, and amounts as low as 0.01 wt % commonly caused marked reductions in sample brightness and absorption band intensity. In contrast, the spectral effects of rock texture were subtle and could only be identified in samples that were nearly free of organic matter impurities. The pronounced organic matter spectral effects imply that the organic matter in most carbonates is very finely disseminated. Samples collected from different regions usually showed different degrees of organic spectral quenching. However, no correlations were evident between sample reflectance and contained kerogen H/C ratios or kerogen amounts. Additional analytical work is needed to examine organic matter compositional variation and associated spectral quenching effects in detail. From the present study, it appears likely that contained organic matter will be a dominant factor in the visible and near‐infrared spectral expression for most carbonate rocks, and perhaps for other sedimentary lithologies. Spectral quenching by organic matter will likely complicate remote sensing and laboratory spectral determinations of carbonate composition. On the other hand, variations in quenching may prove to be useful for distinguishing rocks that have experienced unusual diagenetic, thermal, or other conditions affecting the organic matter contents.
Continuous measurement of nontidal variations of gravity
Authors: Goodkind John M.
Published: February 2008

Records from seven superconducting gravimeters operated at five different locations in California and one in Boulder, Colorado, are examined after removal of tides and the gravitational attraction of the atmosphere. Fluctuations over periods between a few days and several months were observed at all sites with peak amplitudes of order 10 μGal. By contrast a 640‐day record obtained with one of the instruments in Germany showed peak fluctuations of only 2 μGal. In most of the records the causes of these aperiodic variations were not determined so that they serve to set limits on the vertical motion or displacement of mass at the respective locations. However, at The Geysers geothermal field, much of the gravity variation is correlated with seismic activity, reinjection rate, and rainfall. Measurements of this type were not possible prior to the development of the superconducting device. Consequently, these results provide the first evidence for the existence of gravity variations on the time scale and of the magnitude described here. Vertical crustal motion, motion related to seismic events, and hydrological phenomena can lead to variations on this scale. Unambiguous identification of causal relationships will require either special circumstances such as found at The Geysers or operation of the instruments in pairs.
Melting and phase relations in the system Mg2SiO4‐MgSiO3 at 20 GPa under hydrous conditions
Authors: Kato Takumi , Kumazawa Mineo
Published: February 2008

An experimental study was performed on the melting and subsolidus phase relations in a synthetic magnesium silicate system at a pressure of about 20 GPa and over the temperature range of 1700∼2500°C under hydrous conditions. The modified spinel, Mg2SiO4, is stable up to 2250°C, above which it melts incongruently into phase B, Mg23Si8O42H6, and liquid. At higher temperatures, phase B melts again incongruently into periclase and liquid. The liquid coexisting with periclase has an atomic ratio Mg/Si∼1.5 at 2360°C and ∼1.7 at 2500°C. For MgSiO3 composition the ilmenite is stable at lower temperatures, and it transforms to a garnet with increasing temperatures to 2000°C. At above 2200°C, clinopyroxene replaces the garnet and then melts congruently at 2250°C. The triple point of clinopyroxene, garnet, and liquid is closely located at 20 GPa and 2250°C. The binary system of Mg2SiO4‐MgSiO3 shows a eutectic melting between modified spinel and garnet with a eutectic point located at 2150°C and Mg/Si∼1.3.
Elysium Region, Mars: Tests of lithospheric loading models for the formation of tectonic features
Authors: Hall J. Lynn , Solomon Sean C. , Head James W.
Published: February 2008

The second largest volcanic province on Mars lies in the Elysium region. Like the larger Tharsis province, Elysium is marked by a topographic rise and a broad free air gravity anomaly and also exhibits a complex assortment of tectonic and volcanic features. We test the hypothesis that the tectonic features in the Elysium region are the product of stresses produced by loading of the Martian lithosphere. We consider loading at three different scales: local loading by individual volcanoes, regional loading of the lithosphere from above or below, and quasi‐global loading by Tharsis. A comparison of flexural stresses with lithospheric strength and with the inferred maximum depth of faulting confirms that concentric graben around Elysium Mons can be explained as resulting from local flexure of an elastic lithosphere about 50 km thick in response to the volcano load. Volcanic loading on a regional scale, however, leads to predicted stresses inconsistent with all observed tectonic features, suggesting that loading by widespread emplacement of thick plains deposits was not an important factor in the tectonic evolution of the Elysium region. A number of linear extensional features oriented generally NW‐SE may have been the result of flexural uplift of the lithosphere on the scale of the Elysium rise. The global stress field associated with the support of the Tharsis rise appears to have influenced the development of many of the tectonic features in the Elysium region, including Cerberus Rupes and the systems of ridges in eastern and western Elysium. The comparisons of stress models for Elysium with the preserved tectonic features support a succession of stress fields operating at different times in the region. While the order in which those stress fields operated cannot be determined from present geological observations, thermal and mechanical arguments favor the hypothesis that any flexural uplift of the lithosphere by a mantle thermal anomaly preceded or occurred contemporaneously with emplacement of the largest volcanic loads.
Early evolution of the Earth: Accretion, atmosphere formation, and thermal history
Authors: Abe Yutaka , Matsui Takafumi
Published: February 2008

Atmospheric and thermal evolution of the earth growing by planetesimal impacts was modeled by taking into account the blanketing effect of an impact‐induced H2O atmosphere and the temperature dependence of H2O degassing. When the water content of planetesimals is larger than 0.1% by weight and the accretion time of the earth is less than 5 × 107 years, the surface of the accreting earth melts and thus a “magma ocean” forms and covers the surface. The formation of a “magma ocean” will result in the initiation of core‐mantle separation and mantle differentiation during accretion. Once a magma ocean is formed, the surface temperature, the degree of melting in the magma ocean, and the mass of the H2O atmosphere are nearly constant as the protoplanet grows further. The final mass of the H2O atmosphere is about 1021 kg, a value which is insensitive to variations in the model parameter values such as the accretion time and the water content of planetesimals. That the final mass of the H2O atmosphere is close to the mass of the present oceans suggests an impact origin for the earth's hydrosphere. On the other hand, most of the H2O retained in planetesimals will be deposited in the solid earth. Free water within the proto‐earth may affect differentiation of the proto‐mantle, in particular, the mantle FeO abundance and the incorporation of a light element in the outer core.
Structure of Zabargad Island and early rifting of the Red Sea
Authors: Nicolas Adolphe , Boudier Françoise , Montigny Raymond
Published: February 2008

A structural and kinematic analysis of the peridotites and associated gneisses on Zabargad Island in the Red Sea indicates a rift setting and not a transform setting, because the foliations in the peridotite trend NW‐SE and are vertical, and the stretching lineations plunge 50°NW. A clear distinction can be made between a higher‐T structure developed mainly in feldspathic lherzolites and characterized by dextral shear flow and a lower‐T to mylonitlc structure developed at hydrous conditions (syntectonic amphiboles) and characterized by sinistral shear flow. The first structure is ascribed to a diapiric mantle upwelling. The dextral shear sense suggests that the island originated on the western limb of the diapir. Flow lines are not vertical, but moderately inclined; this seems to be a consistent feature of asthenosphere flow beneath rifts. The low‐T deformation records the final emplacement of the peridotites into the crust, represented by mafic gneisses as also these gneisses are affected by it. The metamorphism of the gneisses dated as lower Miocene (K/Ar amphibole in amphibolite) is, at least partly, the result of their juxtaposition with the hotter peridotites. Thus, the intrusion of the mantle diapir coincides with the early rifting of the Red Sea. The sinistral shear flow in low‐T peridotites and gneisses could be related to the sinistral component of lithospheric stretching during the Red Sea rifting.