bluuebunny/crossref_metadata_2025 huggingface dataset:
On the orbital evolution of the Martian satellites
Authors: Lambeck Kurt
Published: February 2008

The tidal evolution of the Martian satellites has been investigated, and it is found that tides raised by the planet on the satellite can have significant consequences if the rigidity of the satellites is much less than the average rigidity of Mars. This would be the case if the satellites have a carbonaceous chondritic composition. Frequency dependent Q values of the planet or satellites can further enhance the evolution. Phobos, in particular, could have evolved from a parabolic orbit into a circular orbit within the age of the solar system, but the time required to shift Deimos remains inordinately long. If the present evolutions are representative of the past, the two orbits will have, at some time in the past, intersected, but the present orbit of Deimos does not favor a strong gravitational interaction with Phobos. Possibly, Deimos was part of a larger object, perhaps the core of a primordial body containing a large ice mantle. If it were sufficiently massive, such a body would have remained outside of Phobos. The backward extrapolated orbits of both satellites lead to a quite similar origin. The orbital inclinations have undergone little evolution, and this remains a severe constraint on the capture hypothesis, unless the captured bodies were initially in orbits inclined to the ecliptic. This cannot be excluded, as numerous asteroid families lie in orbits inclined by 20° or more to the ecliptic, in particular as these objects lie preferentially toward the outer limits of the asteroid belt, contain a higher percentage of carbonaceous chondritic material, and are more likely to be perturbed by Jupiter into Mars‐crossing orbits.
Terrestrial heat flow in eastern Arizona: A first report
Authors: Reiter Marshall , Shearer Charles
Published: February 2008

Fifteen heat‐flow measurements are presented for eastern Arizona and neighboring areas. In the Gila Mountains of southeastern Arizona a heat flow of 1.9 HFU (the approximate average for the Basin and Range Province) is estimated from the best measurements. In the eastern part of the Saffobrd Valley, just south of the Gila Mountains, subsurface temperature gradients were measured at one site to 1050 m. Heat‐flow data from this site, considered with the reported hydrogeology and hot spring activity of the valley fill deposits, suggest complex hydrothermal phenomena in the valley. Several hundred km to the north, high heat flows (≥2.5 HFU) are observed within the Mogollon Slope of the Colorado Plateau. These high heat flows, about 100 km from the southern boundary of the Colorado Plateau, are believed to result from the sources of extensive Quaternary volcanics in the region. Still farther into the interior of the Colorado Plateau the Black Mesa Basin has an estimated heat flow of 1.5–1.8 HFU and a uniform geothermal character (1 HFU = 41.8 mW/m2).
The origin of fine scale acoustic stratigraphy in deep‐sea carbonates
Authors: Mayer Larry
Published: February 2008

In this paper we investigate the origin and geologic significance of the closely spaced high‐frequency subbottom acoustic reflectors characteristic of pelagic carbonates. A detailed survey was conducted of a small area in the equatorial Pacific with the Marine Physical Laboratory's Deep‐Tow instrument package, providing high‐resolution 4‐kHz profiles and precise positioning of core samples. The cores were sampled at closely spaced intervals for sound velocity and saturated bulk density. Acoustic impedances were calculated, and a reflection coefficient log determined for the upper 10 m of the sediment column. The reflection coefficient log revealed no interfaces with large reflection coefficients that correlated with the reflectors seen on the Deep‐Tow 4‐kHz seismic profile. The calculated reflection coefficients were very low (typically 10−3–10−5) and varied about a wavelength that was on the order of the wavelength of the 4‐kHz pulse, implying that interference plays a role in the composition of the seismic record. Convolving the outgoing 4‐kHz pulse with the reflection coefficient log generated a synthetic seismogram that very closely resembled the 4‐kHz reflection profile. Varying the frequency of the outgoing pulse changed the amplitude and position of the reflectors seen on the synthetic seismograms. Thus we conclude that the reflectors seen on the 4‐kHz seismic profile were not caused by discrete geologic horizons but rather are the result of the interference of many small layers.
On the luminous efficiency of meteoritic fireballs
Authors: ReVelle D. O. , Rajan R. S.
Published: February 2008

Luminous efficiency values have been calculated empirically as a function of velocity and altitude for the Lost City, Innisfree, and Pribram meteorites. We have used a combined approach which utilizes the observed light data from the meteorite entries and the appropriate ablation rates via the entry model of ReVelle, which is generally consistent with the results of cosmic ray track analyses. Although our method reduces the absolute value of computed luminous efficiency via a total kinetic power balance equation, our results still exceed the modified Trailblazer results of Ceplecha and McCrosky for chondrites by about a factor of 5 for entry velocities of ≲20 km/s. Our values of luminous efficiency (deduced with respect to a detector peaked in the visible) are ∼0.2–0.4% in the vicinity of peak ablation for Lost City and Innisfree, respectively. Results for Pribram are less certain but are ∼0.04% at peak ablation. In terms of numerical values and velocity dependencies our results agree well with appropriately modified laboratory simulation data of Givens and Page (1971). We conclude that the deduced photometric mass scale of fireballs should be revised downward by a factor of 5–10 so that a more accurate mass influx determination at the earth's orbit can be made.
Tectonic evolution of Mars
Authors: Wise Donald U. , Golombek Matthew P. , McGill George E.
Published: February 2008

Any model for the tectonic evolution of Mars must account for two major crustal elements: the Tharsis bulge and the topographically low and lightly cratered northern third of the planet. Ages determined by crater density indicate that both of these elements came into existence very early in Martian history, a conclusion that holds no matter which of the current crater density versus age curves is used. The size of these two major crustal elements and their sequential development suggest that both may be related to a global‐scale internal process. It is proposed that the resurfacing of the northern third of Mars is related to subcrustal erosion and isostatic foundering during the life of a first‐order convection cell. With the demise of the cell, denser segregations of metallic materials began to coalesce as a gravitatively unstable layer which finally overturned to form the core. In the overturn, lighter crustal material was shifted laterally and underplated beneath Tharsis to cause rapid and permanent isostatic rise. This was followed by a long‐lived thermal phase produced by the hot underplate and by the gravitative energy of core formation slowly making its way to the surface to produce the Tharsis volcanics.
The relative humidity of Mars' atmosphere
Authors: Davies Donald W.
Published: February 2008

As a general rule, Mars' atmosphere contains as much water vapor as it can hold on a daily basis; it reaches saturation at night, and the vapor appears to be distributed throughout the lowest several kilometers. Interesting exceptions to this occur when there are temperature inversions in the arctic springtime and during dust‐storm activity. As contrasted to the northern hemisphere there appears to be no local source of water in the southern temperate and arctic areas.
Grooves on Phobos: Their distribution, morphology and possible origin
Authors: Thomas P. , Veverka J. , Bloom A. , Duxbury T.
Published: February 2008

A remarkable characteristic of the surface of Phobos is that it is densely covered by grooves—long linear depressions formed in a regolith. Typically, grooves are 100–200 m wide, 10–20 m deep, and have smooth concave‐up cross profiles. Some can be traced for up to 30 km. Groove paths follow the intersection of planes with the surface of Phobos. Groove planes are grouped in four sets of parallel members. All sets are nearly parallel to the intermediate axis of Phobos: one is parallel to the equatorial plane, another perpendicular to the long axis of Phobos, the other two sets make symmetric angles (∼25°) with the equatorial plane. Grooves are widest and deepest near Stickney, the largest crater on Phobos, and are absent from the trailing region of Phobos, roughly opposite to Stickney. Groove formation occurred during a restricted time interval, which, on the basis of superposed craters, closely followed the formation of Stickney. The impact that formed Stickney probably opened fractures along preexisting planes of weakness and caused subsequent formation of the grooves either by drainage of regolith into open fractures or by ejection of regolith overlying the fractures or a combination of both processes. Heating due to the impact may have produced enough steam from the satellite's carbonaceous chondrite material to expell regolith along the fractures. Other proposed mechanisms of groove formation, such as tidal stressing, internal heating, capture fragmentation, etc., cannot account adequately for the pattern, age, and morphology of the grooves. The lack of grooves on Deimos could be explained by the absence on its surface of a crater large enough to have severely fractured the outer satellite.
The planet Mars as seen at the end of the Viking Mission
Authors: Snyder Conway W.
Published: February 2008

In the last 15 years, spacecraft missions to Mars, especially Mariner 9 and Viking 1 and 2, have removed from discussion many of the traditional Martian topics, such as canals, waves of darkening, and blue clearings. We now have a plethora of hard data about the large variety of geologic features on the planet, about the composition of the surface, the atmosphere, and the polar caps, and about many aspects of Martian meteorology, including temperatures, pressures, tides, dust storms, and the abundance and transport of water vapor. Perhaps the only areas of Martian planetology that have not been spectacularly advanced are those relating to the interior structure, the solar wind interaction, and the existence of living organisms. This paper attempts to summarize most of our new knowledge about the planet.
Detailed heat flow measurements over the Juan de Fuca Ridge System
Authors: Davis E. E. , Lister C. R. B. , Wade U. S. , Hyndman R. D.
Published: February 2008

Eleven detailed profiles of heat flow measurements have been completed over young oceanic crust of the Juan de Fuca ridge system. Individual measurements were spaced typically 0.4–1 km apart along multiple‐penetration lines from 3 to 20 km in total length, and each measurement was located with respect to structural and sedimentary features by simultaneous seismic reflection profiling. In all cases, the average heat flow is well below that predicted by simple conductively cooled spreading models, even when the sediment cover is thick and the nearest basement outcrop is 15 km away. This disparity is attributed to ventilated convective circulation of water in the crust. Large heat flow variability is common along all profiles. Variations are present at two scales. Small‐scale variations (from 1 km to a few kilometers between significant heat flow maxima), present in the younger profiles, probably reflect the influence of local venting and permeability variations on permeable layer cellular convection. Large‐scale variations (10–20 km between significant heat flow maxima), present in all profiles, may reflect the influence of regional circulation driven by cold water recharge at isolated basement outcrops. Laboratory experimental data indicate that normal cellular convection can coexist with larger‐scale bilateral flows, so that there is no simple way to extract the permeable layer thickness from the surface heat flow data. There is a considerable reduction in the amplitude of small‐scale heat flow variability over the range of crustal age studied (0.1–12.5 m.y.). This is probably caused by the thermal filtering effects of the sediment cover which increases from about 50 m near the ridge crests to over 700 m on the flanks.

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D.C. 20009. Document J79‐007; $1.00. Payment must accompany order.
Note on measured values for the state of stress in the Earth's crust
Authors: Jamison Dennis B. , Cook Neville G. W.
Published: February 2008

Although no totally satisfactory methods for measuring the complete state of stress have been devised yet, many efforts have been made to determine the state of stress at different locations. From a compilation of many in situ stress measurements, 50 which yielded the complete state of stress and in which one of the principal stresses is vertical have been selected for a statistical analysis in an endeavor to define the nature of the state of stress in the crust. These data have been analyzed as a whole and divided into three groups depending upon whether the vertical stress is the maximum, minimum, or intermediate principal stress. Linear regression analysis of the values of half the maximum stress difference as a function of half the sum of the maximum and minimum principal stresses have been made. The correlation coefficients for these fits are 0.786 for the data as a whole and 0.848, 0.790, and 0.383 for each of the groups. The slopes of these lines interpreted as a Coulomb friction, range from 0.625 (for those measurements where the vertical stress is the maximum principal stress) through 0.427 (for those cases where the vertical stress is the minimum principal stress), to 0.220 (for those cases where vertical stress is the intermediate principal stress). The 98% confidence limits for these values lie within +19.4% to −16.6%. The 98% confidence limits include the origin, except for the case where the vertical stress is the minimum principal stress, where the mean value of the intercept is 1.3 MPa. This suggests that the value of the cohesion is virtually negligible. Also, the bound to the maximum value of the ratio of the average of the horizontal stresses to the vertical stress suggests that there is a maximum stress difference which the crust can resist near surface which is at least 37.5 MPa, given a normal stress sufficient to generate such a stress difference by friction.
Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California
Authors: Duffield Wendell A. , Bacon Charles R. , Dalrymple G. Brent
Published: February 2008

The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre‐Cenozoic plutonic and metamorphic rocks is partly buried by ∼35 km3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) ≤1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being ≤0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north‐trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east‐west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot‐water geothermal system. This system apparently is heated by a reservoir of silicic magma at ≥8‐km depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle‐derived basaltic magma that intrudes the crust in response to lithospheric extension.
Teleseismic evidence for a low‐velocity body under the Coso Geothermal Area
Authors: Reasenberg P. , Ellsworth W. , Walter A.
Published: February 2008

Teleseismic P wave arrivals were recorded by a dense array of seismograph stations located in the Coso geothermal area, California. The resulting pattern of relative residuals reveals an area showing approximately 0.2‐s excess travel time that migrates with changing source azimuth, suggesting that the area is the ‘delay shadow’ produced by a deep, low‐velocity body. Inversion of the relative residual data for three‐dimensional velocity structure determines the lateral variations in velocity to a depth of 22.5 km beneath the array. An intense low‐velocity body, which coincides with the surface expressions of late Pleistocene rhyolitic volcanism, high heat flow, and hydrothermal activity, is resolved between 5‐ and 20‐ km depth. It has maximum velocity contrast of over 8% between 10 and 17.5 km. The shallowest part of this body is centered below the region of highest heat flow; at depth it is elongate in approximately the N‐S direction. The hypothesis that this low‐velocity body is caused by the presence of partial melt in the middle crust is consistent with the local seismic, geologic, and thermal data.
Volcanic explosion clouds: Density, temperature, and particle content estimates from cloud motion
Authors: Wilson Lionel , Self Stephen
Published: February 2008

Photographic records of 10 vulcanian eruption clouds produced during the 1978 eruption of Fuego Volcano in Guatemala have been analyzed to determine cloud velocity and acceleration at successive stages of expansion. Cloud motion is controlled by air drag (dominant during early, high‐speed motion) and buoyancy (dominant during late motion when the cloud is convecting slowly). Cloud densities in the range 0.6 to 1.2 times that of the surrounding atmosphere were obtained by fitting equations of motion for two common cloud shapes (spheres and vertical cylinders) to the observed motions. Analysis of the heat budget of a cloud permits an estimate of cloud temperature and particle weight fraction to be made from the density. Model results suggest that clouds generally reached temperatures within 10 K of that of the surrounding air within 10 seconds of formation and that dense particle weight fractions were less than 2% by this time. The maximum sizes of dense particles supported by motion in the conecting clouds range from 140 to 1700 μm.
Magnetic blocking temperatures of single‐domain grains during slow cooling
Authors: Dodson Martin H. , McClelland‐Brown Elizabeth
Published: February 2008

Acquisition of TRM by a cooling assemblage of identical monodomain grains involves a transition from superparamagnetic to blocked behavior over a narrow but indefinite range of temperature. Blocking temperature may, nevertheless, be precisely defined by comparing the blocked magnetization, after cooling, with the curve of equilibrium magnetization against temperature. At the blocking temperature TB the relaxation time τ of the magnetization is of the same order as the cooling time constant θ, which is the time required for τ to increase by a factor e. An approximate analytic solution of the differential equation for magnetization in a cooling system gives for TB:ε (TB)/kT = 1n(1.78Cθ) where ε is the activation energy for rotation of magnetic moments, k is Boltzmann's constant, and C is the frequency factor (∼1010 Hz). Because ε varies with temperature, the cooling time constant θ is given by 1/θ = (1/kTB)(dε/dT ‐ ε/TB)dT/dt. The 95% blocking interval is about 7.2θ. For steady cooling it is reasonable to assume θ to be constant through this interval; small deviations from constancy have a negligible effect upon TB. In small external fields the variable grain geometry to be found within a real assemblage can be incorporated in a parameter ε0, which is the activation energy at O°C. Curves of ε0 against TB for magnetite and haematite, at cooling rates from 6°C per minute to 3°C per million years, show that (1) a change in ε0 by a factor of 2 corresponds to 12 orders of magnitude change in cooling rate and (2) for a broad distribution of grain sizes a large proportion of the size range will acquire their magnetization within 100°C–200°C of the Curie point. Using these curves, blocking temperature in cooling assemblages can be related to laboratory demagnetization temperatures; the differences are greater at slower cooling rates and lower temperatures and tend to zero as the Curie temperature is approached. Intensity of magnetization should increase by a few percent for each order of magnitude decrease in cooling rate. Magnetization acquired at temperatures near the Curie point should dominate in a chemically homogenous monodomain assemblage with a broad spread of grain sizes. Possible tests of the theory and its application to the determination of cooling history are discussed.
Shock effects in olivine and implications for Hugoniot data
Authors: Jeanloz Raymond
Published: February 2008

Deformation produced in single‐crystal olivine samples by shock wave stresses ranging between 16 and 75 GPa are documented by microscopy and spectroscopy. No evidence was found for phase transformation throughout the ‘mixed‐phase’ region (about 25 to 70 GPa) contrary to expectations based on the current interpretations of Hugoniot data. Samples retrieved from peak pressures above 20 GPa exhibit extensive plastic strain; a distinctive microundulatory extinction (mosaicism) is associated with elevated dislocation densities. Shock effects in olivine are relatively minor compared to those typical of other silicates: only traces of diaplectic glass are found at pressures above about 50 GPa, and there is no evidence of melting, recrystallization, or recovery. Although civilian (reconstructive) phase transformation has been previously inferred from the Hugoniot data for olivine, this textural evidence is inconsistent with such a process. Calculations of transformation rates for olivine similarly indicate that reconstructive phase transformation is very unlikely under experimental shock conditions and that the Hugoniot data represent highly nonequilibrium states. A new, crystallographically based model is presented for the behavior of olivine under shock: it successfully reproduces the Hugoniot data and is in accord with all available observations. According to this model, olivine is compressed into a slighlty randomized, olivinelike structure at shock pressures above about 20–30 GPa and does not undergo a polymorphic transformation or an increase in cation coordination number. Some evidence indicates that many other silicates may behave similarly and remain untransformed in shock experiments. The results of this study suggest that the current interpretations of shock wave data for silicates and their geophysical applications may need reappraisal.
Iron and titanium distribution on the moon from orbital gamma ray spectrometry with implications for crustal evolutionary models
Authors: Davis Philip A.
Published: February 2008

A set of Fe and Ti maps and regional values are obtained from the Apollo 15 and 16 orbital gamma ray data by energy band analysis. High‐Ti basalts predominate the early and late stages of mare volcanism with high‐Fe basaltic volcanism in the interim. The first evidence of a high‐Ti‐KREEP basalt association is found in the Aristarchus region. A N‐S asymmetry for Fe and Ti in the east limb and farside highlands complicates the E‐W asymmetry for Th but substantiates crustal inhomogeneity. The observed crustal inhomogeneity adds an additional objection to the primitive source model for crustal evolution. The high‐Ti‐KREEP basalt association and the general trend of decreasing mare basalt Ti with time lend support to the cumulate source model; however, this model cannot account for young, high‐Ti maria and the absence of KREEP in high‐Ti maria. The dynamic assimilation model better accounts for chemical variations observed on the moon.
Secular variation and excursions of the Earth's magnetic field
Authors: Harrison C. G. A.
Published: February 2008

Several different models for the secular variation of the earth's magnetic field have been proposed. None of the proposed models fits well the observational evidence about the secular variation, which calls for an increase in the nondipole field contribution toward virtual geomagnetic pole (VPG) scatter of a factor of 2 on going from equator to pole, a symmetric distribution of VGP's at all latitudes (and hence a nonsymmetric distribution of directions), and a drifting portion of the non‐dipole field significantly greater than the standing portion. A new model for secular variation is introduced which fits the observed data better than the others. The secular variation data from over 1000 lava flows from Iceland are discussed, and it is shown that the low‐latitude VGP's do not fit in with a Fisher distribution. Instead, the whole distribution of VGP's is well fit by a combination of about 0.9 of a Fisher distribution and about 0.1 of a random distribution. The low‐latitude poles also come from lavas which have a lower than normal intensity of magnetization. These low‐latitude poles are caused either by excursions of the earth's magnetic field or during times when the earth's magnetic field is undergoing a reversal. By looking at the statistics of when these low latitude poles occur in the section, it is possible to infer that either there are many undetected reversals or there is a high chance (about 40%) that when the dipole field decreases in intensity (before a possible reversal), it can increase in intensity with the same polarity that it originally had.
Magnetism of the oceanic crust: Evidence from ophiolite complexes
Authors: Banerjee Subir K.
Published: February 2008

The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomalies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.
Paleomagnetism of Late Mesozoic pelagic limestones from the southern Alps
Authors: VandenBerg J. , Wonders A. A. H.
Published: February 2008

Paleomagnetic data are presented from the Southern Alps (Belluno‐Bassano del Grappa). These paleomagnetic data specify the megatectonic movements of the Southern Alps for the Late Mesozoic. The detailed paleomagnetic data of the upper Albian, Cenomanian, and Turonian show upper Albian reversals and the possible presence of a short mixed interval in the Cenomanian.
The effect of cooling rate on the intensity of weak‐field trm in single‐domain magnetite
Authors: Halgedahl S. L. , Day R. , Fuller M.
Published: February 2008

The effect of cooling rate on the intensity of low‐field thermoremanent magnetization (TRM) in uniaxial, single‐domain magnetite is calculated, using both numerical and analytical techniques. These calculations indicate that when a single‐domain assemblage is cooled much more slowly than in a typical laboratory experiment, it can remain magnetically unblocked and in thermal equilibrium with an external field at temperatures significantly lower than the ‘laboratory’ blocking temperature. Owing to the rapid rise of spontaneous magnetization just below the Curie temperature the TRM can consequently be elevated by several tens of percent over that acquired in the laboratory. The paleointensity obtained from rocks that have cooled in nature over geologic time may therefore be erroneously high, if single‐domain particles are the primary carriers of NRM. These calculations indicate that when the ratio of ‘natural’ to laboratory cooling rates approximately equals 10−11, as in the case of orogenic bodies that have cooled over several million years, the paleointensity obtained by the Thellier method can be more than 50% too high, unless the role of cooling rate is taken into account.
Apparent velocity measurements for the lower mantle from a wide aperture array
Authors: Burdick L. J. , Powell Christine
Published: February 2008

The California Institute of Technology (CIT) operates a dense network of short‐period vertical seismometers which we have used as a large seismic array to measure the azimuth of approach ζ and ray parameter dT/dΔ of teleseismic P waves. Analysis of over 145 globally distributed events indicates that the ζ and dT/dΔ values measured by the array are in close agreement with the values predicted by the U.S. Geological Survey event locations. Most ζ anomalies (measured minus predicted values) do not exceed 1°. The small magnitude of the anomalies suggests that the measured dT/dΔ values can be used to construct a radial velocity model for the lower mantle. The curve of dT/dΔ versus Δ is in agreement with the curve determined for the Tonto Forest Seismological Observatory (TFSO) by Johnson (1969). The two curves differ in that there is no compelling evidence in the CIT data for first‐ or second‐order discontinuities in the velocity gradient. It is shown that discontinuities in the dT/dΔ data proposed by Johnson near distances of 40.5, 49.5, 59.5, and 70.5° are due to a strong dependence upon azimuth to source. The same least squares straight line can be fit through the CIT and TFSO dT/dΔ values. Residuals from this straight line display the same azimuthal dependence at both arrays. A velocity‐depth curve has been constructed by Wiechert‐Herglotz inversion of the CIT dT/dΔ data. A current upper mantle model for western North America was used as the top of the velocity profile. It is also shown that if fine structure does exist in the lower mantle velocity profile, the best way to find it may be through a combined amplitude dT/dΔ study.
The interrelation between variations in magnetic anomaly amplitudes and basalt magnetization and chemistry along the Southeast Indian Ridge
Authors: Anderson Roger N. , Spariosu Dann J. , Weissel Jeffrey K. , Hayes Dennis E.
Published: February 2008

Magnetic model studies, rock magnetization, and major element chemical analyses show that unusually large amplitude magnetic anomalies found along the easternmost segment of the Southeast Indian Ridge [zone A] are caused by the high intensities of magnetization of basalts intruded at the ridge axis. In turn, these high magnetizations are caused by a threefold increase in weight percent of titanomagnetites from normal midocean ridge basalts intruded in the west [zones B and C] to ferrobasalts intruded in the high magnetic anomaly zone to the east. The Fe‐Ti increase in the opaques reflects a major elemental enrichment in Fe and Ti caused by extensive shallow fractionation of plagioclase, clinopyroxene, and olivine. This shallow fractionation appears related to unusually cold magma crystallization temperatures, perhaps caused by proximity to a postulated downwelling zone in the asthenosphere south of Australia. However, examination of these correlations in greater detail points to several ambiguities: although the entire region is deep, and therefore the lithosphere is integrally colder than normal plates, zone A is shallower than zones B and C, but geothermometrically, its axial magmas are colder. We do not know how cold magmas directly relate to ferrobasalt generation. Also, plumelike geochemical signatures are also seen in some of the analyses. It appears that plume mantle sources are mixing with MORB sources and then undergoing extensive shallow fractional crystallization in zone A.
Late high‐titanium basalts of the Western Maria: Geology of the Flamsteed REgion of Oceanus Procellarum
Authors: Pieters Carle M. , Head James W. , Adams John B. , McCord Thomas B. , Zisk Stanley H. , Whitford‐Stark James L.
Published: February 2008

The Flamsteed region of Oceanus Procellarum is representative of an unsampled volcanic complex on the lunar nearside. The evolution of this region, as portrayed in the current surface, is investigated in detail. A synthesis of remote sensing data, including multispectral images (digital vidicon images and color composite photographs), spectral reflectance measurements, radar topographic maps, and orbital and earth‐based photography, is presented. Additional information is derived from crater degradation studies, radar backscatter, gravity measurements and orbital γ ray data. Seven spectrally distinct basaltic units, which range in age from 2.5±0.5 b.y. to 3.65±0.15 b.y., have been identified. Three have been formally named and related to the Procellarum lithostratigraphic nomenclature scheme. The earliest units within the mapped area are composed of highlands material and include partially flooded impact craters. Another early nonmare unit is spectrally distinct material that occurs as plains and domes and which, in part, is of probable extrusive origin. Small amounts of dark mantle material suggest the presence of an early Ti‐rich basalt (Repsold Formation) beneath the presently exposed mare basalts. The oldest surface exposed mare basalts are very low Ti basalts of the Telemann Formation. These are overlain by low to intermediate Ti basalts of the Hermann Formation. The youngest exposed basalts are the moderately Ti‐rich members of the Sharp Formation. The unsampled Flamsteed basalt, a young Ti‐rich unit defined here, is comparable to Apollo 11 basalts in many measurable parameters except the strength of an absorption feature at 1 μm. This distinction implies a fundamental difference in mineralogy of the two titanium‐rich units; the Flamsteed basalts may be more iron rich than the other lunar basalts. The total mare fill in the Flamsteed region is probably of the order of 400‐m average thickness. Ejecta from young craters such as Kepler and Copernicus overlie some basalts. The major deformation of these units, evidenced by tectonic rules and mare ridges, terminated prior to the emplacement of the youngest mare unit during the emplacement of the Hermann Formation. This complexity of emplacement history and wide variety of observed basalt types, largely unsampled by Apollo and Luna missions, provide additional details of basaltic volcanism on the moon and require models of lunar basalt petrogenesis to be reexamined.
The Indian Ocean Triple Junction
Authors: Tapscott Christopher R. , Patriat Philippe , Fisher Robert L. , Sclater John G. , Hoskins Hartley , Parsons Barry
Published: February 2008

The boundaries of three major plates (Africa, India, and Antarctica) meet in a triple junction in the Indian Ocean near 25°S, 70°E. Using observed bathymetry and magnetic anomalies, we locate the junction to within 5 km and show that it is a ridge‐ridge‐ridge type. Relative plate motion is N60°E at 50 mm/yr (full rate) across the Central Indian Ridge, N47°E at 60 mm/yr across the Southeast Indian Ridge, and N3°W at 15 mm/yr across the Southwest Indian Ridge; the observed velocity triangle is closed. Poles of instantaneous relative plate motion are determined for all plate pairs. The data in the South Atlantic and Indian oceans are consistent with a rigid African plate without significant internal deformation. Two of the ridges at the triple junction are normal midocean spreading centers with well‐defined median valleys. The Southwest Indian Ridge, however, has a peculiar morphology near the triple junction, that of an elongate triangular deep, with the triple junction at its apex. The floor of the deep represents crust formed at the Southwest Indian Ridge, and the morphology is a consequence of the evolution of the triple junction and is similar to that at the Galapagos Triple Junction. Though one cannot determine with precision the stability conditions at the triple junction, the development of the junction over the last 10 m.y. can be mapped, and the topographic expressions of the triple junction traces may be detected on the three plates.
Seafloor and near‐surface sound velocities from Barents Sea sonobuoy data
Authors: Houtz Robert E.
Published: February 2008

The uppermost identifiable head waves from the vicinity of the seafloor andgroup velocities developed from multiples of critical reflections are utilized to contour sound velocities from near the floor of the Barents Sea, an area with divergent velocity structures. Where the low‐velocity sediments are 30 to 80 m thick and cap relatively high‐velocity materials, the seafloor sound velocities determined from critical reflections are strongly dependent on frequency. Multiple reflections from thick‐layered sediments, without major velocity discontinuities, are not affected by passbands that vary from 15–40 up to 100–200 Hz. Certain sonobuoy records are compared with wave equation solutions at fluid/solid and solid/solid interfaces modeled from the Barents Sea data. Results indicate that the presence of quite thin sediments (∼30 m), not otherwise detectable, may be required to account for the behavior of p waves from apparently high‐velocity seafloors. Under the fairly restrictive conditions of a fluid/solid interface, shear wave velocities can be estimated from the pattern of reflected p wave energy.
Determination of the geopotential from satellite‐to‐satellite tracking data
Authors: Douglas Bruce C. , Goad Clyde C. , Morrison F. Foster
Published: February 2008

Ground‐based observation of the deviation of artificial satellite trajectories from a reference path is the classical means for determining the parameters of the global geopotential. However, for the short wave‐lengths (<10°), tracking coverage from ground stations of sufficient density is impossible to obtain. But one or more satellites can observe another satellite and obtain the needed global data coverage. In this paper we present the results of error analyses of possible satellite‐to‐satellite tracking missions to determine the geopotential at a resolution of 1°×1°. To achieve an accuracy of a few milligals (1 mGal = 10−3 cm/s2) at this resolution requires a satellite altitude at or near 150 km and measurement of intersatellite speed to 10−6 cm/s.
Microwave properties of solid CO2
Authors: Simpson R. A. , Fair B. C. , Howard H. T.
Published: February 2008

Laboratory measurements on the electrical properties of solid carbon dioxide (dry ice) were made over the frequency range 2.2 to 12 GHz. These give a dielectric constant which varies with density according to the Rayleigh mixing formula and is independent of frequency; at 1 g/cm3 the dielectric constant is 1.7. The loss tangent was below the sensitivity threshold of our measurement technique at all frequencies and densities used; we set an upper limit of 0.005.
Flow stress from microstructures of mylonites: Example and current assessment
Authors: Christie John M. , Ord Alison
Published: February 2008

Several recent studies have attempted to determine the flow stresses during geological deformations from the microstructures of rocks, utilizing quantitative estimates of such structural parameters as grain size, subgrain size, and dislocation density. The basis for these stress determinations is the apparent correlation between flow stress and microstructures in experimental deformation studies of several crystalline materials. Estimates of flow stresses are presented for a group of quartz‐bearing mylonitic rocks from the Coyote Mountain mylonite zone, near Borrego Springs, California, using the grain sizes and dislocation densities of quartz grains. Neglecting complexities in the geological history of the rocks, flow stresses inferred from grain sizes range from 16 to 74 MPa (160 to 740 bars), whereas those derived from dislocation densities range from 131 to 50 Mpa, respectively, for the same rocks. Stress estimates by the two methods are not well correlated and may differ by an order of magnitude for the same rock. The major causes of the inconsistencies are considered to be (1) inadequacy of the currently available experimental data and (2) complexity of the thermomechanical history of the rocks. These problems are discussed, and the potential value of the methods is assessed. It appears likely that the rocks were subjected to flow stresses (σ1–σ3) of 100 MPa (1 kbar) or more during mylonitization.
Tectonic stresses in the lithosphere: Constraints provided by the experimental deformation of rocks
Authors: Kirby Stephen H.
Published: February 2008

The strengths of rocks clearly place an upper limit on the stress that can be sustained by the upper half of the lithosphere. Laboratory data on rock rheology are generally lacking at intermediate temperatures and pressures on the important rock types expected in the lithosphere, so a definitive accounting of the strength distribution with depth in the upper lithosphere is presently unattainable. Analogies are drawn between the fragmentary strength data on silicates at intermediate temperature and the more extensive experimental data on marble and limestone, and several tentative conclusions are drawn: First, brittle processes, such as faulting and cataclasis, are expected to control rock strength at low pressures and temperatures. The strengths associated with these brittle mechanisms increase rapidly with increasing effective pressure and are relatively insensitive to temperature and strain rate. Second, the transitions between brittle and ductile processes occur at critical values of the least principal stress σ3. I suggest that the concept of the deformation mechanism map of Ashby (1972) be extended to brittle‐ductile transitions by normalizing the applied differential stress σ by σ3, i.e., the transitions occur at critical values of σ/σ3. Third, the high temperature flow law of olivine, widely applied to problems involving flow in the asthenosphere, is inappropriate to the conditions of temperature found in the lithosphere, because different dislocation flow mechanisms dominate at low to intermediate temperatures. The fragmentary rheological data suggest the following rheological structure of the lithosphere where it is inelastically deforming: A rapid pressure‐driven increase in rock strength with depth culminates with a shear strength maximum of up to 8 kbar at depths that depend on the state of stress and on the temperature distribution. A review of the mechanisms of weakening associated with water suggests that water weakening effects are probably not important in the oceanic lithosphere but are likely to be controlling in the continental crust.
Implications of regional gravity for state of stress in the Earth's crust and upper mantle
Authors: McNutt Marcia
Published: February 2008

Topography is maintained by stress differences within the earth. Depending on the distribution of the stress we classify the support as either local or regional compensation. In general, the stresses implied in a regional compensation scheme are an order of magnitude larger than those corresponding to local isostasy. Gravity anomalies, a measure of the earth's departure from hydrostatic equilibrium, can be used to distinguish between the two compensation mechanisms and thus to estimate the magnitude of deviatoric stress in the crust and upper mantle. Topography created at an oceanic ridge crest or in a major continental orogenic zone appears to be locally compensated. Such features were formed on weak crust incapable of maintaining stress differences much greater than the stress from the applied load. Oceanic volcanoes formed on an already cooled, thickened lithosphere are regionally supported with elastic stresses. Similarly, the broad topographic rise seaward of subduction zones is elastically supported as the lithosphere is bent near the plate margin. Although the implied stress is to some degree dependent on the rheological model assumed, the gravity anomalies and surface deformation produced by these features demonstrate that the upper 30–40 km of the oceanic lithosphere is capable of regionally supporting stress differences in the 100‐MPa range. Given certain conditions of load emplacement, continental crust can also support loads regionally over 100‐m.y. time scales, but the effects of erosion only allow an estimate of a lower bound on stress. Data from space probes indicate that the upper layers of other terrestrial planets also support topographic‐induced stress differences in excess of 100 MPa.
Interpretation of African gravity and its implication for the breakup of the continents
Authors: Brown C. , Girdler R. W.
Published: February 2008

To obtain a more quantitative interpretation of the ‘great negative Bouguer anomaly’ over Africa, a study is made of all the gravity data for the whole of the African continent. Histograms from 1°×1° means are presented for heights, free air, and Bouguer anomalies. The modal values are found to be 400 m, +70±5.1 gu (gravity unit; equal to 0.1 mGal or 10−6m/s2) and −480 gu, respectively. These are considered to be the best estimates for Africa away from the regions of rifting and are used to construct a ‘standard African lithosphere’ with an assumed thickness of 100 km. The large‐scale Bouguer anomalies are then interpreted as being due to variations in the thickness of this assumed standard lithosphere rather than to the traditional compensation at the base of the crust. On this assumption, the relative density contrast at the lithosphere‐asthenosphere boundary is found to be −0.05. After corrections for the effects of light volcanics and sediments, a massive replacement of the lithosphere is required beneath much of the east African rift system. The interpretation requires: (1) a wide variation in lithospheric structure beneath different parts of the rift system, (2) that the lithosphere be thinnest beneath the eastern rift, and (3) that the thinning increases from south to north. The ‘lesser negative Bouguer anomaly’ associated with the Adrar‐Hoggar‐Tibesti‐Jebel Marra chain of volcanoes is treated in a similar way, and the replacement of the lithosphere is shown to be correspondingly less. Possible implications for the mechanism of rifting and break up of the continents are given.