bluuebunny/crossref_metadata_2025 huggingface dataset:
Spacelab 2 electron beam wave stimulation: Studies of important parameters
Authors: Reeves G. D. , Banks P. M. , Neubert T. , Harker K. J. , Gurnett D. A. , Raitt W. J.
Published: February 2008

The Spacelab 2 space shuttle mission included experiments on the production of waves in the ionospheric plasma by the injection of pulsed electron beams. The Spacelab 2 mission took place during July and August of 1985 and provided an extensive data set which continues to yield new results. The experimental results reported here were obtained with the combined use of the University of Iowa plasma diagnostics package (PDP) and the Stanford/Utah State vehicle charging and potential experiment (VCAP). To date most of the analysis has focused on the sequences performed during the release of the PDP as a free‐flying satellite. However, over 300 beam sequences were conducted with the PDP mounted in the orbiter's payload bay. The results of these experiments provide important new information which helps answer some of the outstanding questions which remain. The wave environments which exist in the orbiter payload bay and at several hundred meters away are compared both during ambient conditions and during electron beam injections. The dependence of beam‐generated wave characteristics on the duty cycle and the frequency of the pulsed beam are investigated. The duty cycle of the beam is found to be one of the most important parameters affecting the amplitude and the harmonic structure of the waves. Maximum amplitudes are produced by 50% duty cycles and amplitudes are proportional to instantaneous, rather than average, beam current. Harmonic structure also provides insight into the spatial integrity of the propagating beam. The dependence on pulsing frequency is found to show good agreement with theory for whistler mode waves produced through the Cherenkov resonance confirming previous results. Theoretical predictions for the dependence of wave amplitudes on parameters which could not be experimentally investigated are also provided. The results of both the experimental and theoretical studies have important practical consequences for experiments attempting to produce and detect propagating radiation using artificial electron beams.
Models of inertial range spectra of interplanetary magnetohydrodynamic turbulence
Authors: Zhou Ye , Matthaeus William H.
Published: February 2008

Recently developed transport theories for small‐scale solar wind fluctuations explicitly treat convection, expansion, and other interactions with large‐scale gradients, while separating nonlinear effects associated with triple correlations of the fluctuations. To close these models, approximations for the nonlinear terms are needed, since exact treatment of the triple correlations is tantamount to a full solution of the turbulence problem, which is unavailable even for the case of homogeneous turbulence. In this paper we present a framework, based on turbulence theory, to develop approximations for the local turbulence effects that are required in transport models. Two approaches are given, based on, first, Kolmogoroff‐style dimensional analysis and, second, a wave number diffusion picture. Further extensions are given, including a unified approach to the Kolmogoroff and Kraichnan spectral theories, the inclusion of cross helicity, and a discussion of nonlinearities associated with the energy difference spectrum.
The solar cycle dependence of the location and shape of the Venus bow shock
Authors: Zhang T.‐L. , Luhmann J. G. , Russell C. T.
Published: February 2008

From initial Pioneer Venus observations during the maximum of solar cycle 21 it was evident that the position of the Venus bow shock varies with solar activity. The bow shock radius in the terminator plane changed from 2.4 Rv to 2.1 Rv as solar activity went from maximum to minimum and, as activity has increased in cycle 22, it has increased again. Our recent studies of the subsolar region show that the altitude of the nose of the bow shock varies from 1600 km at solar minimum to 2200 km at intermediate solar activity in concert with the terminator altitude so that the shape remains constant and only the size varies during the solar cycle. Using a gas dynamic model and the observed bow shock location, we infer the variation in the size of the effective obstacle during the solar cycle. At solar maximum, the effective obstacle is larger than the ionopause as if a “magnetic barrier” exists in the inner magnetosheath. This magnetic barrier acts as the effective obstacle deflecting the magnetosheath plasma about 500 km above the surface of Venus. However, at solar minimum the effective obstacle is well below the subsolar ionopause, and some absorption of the solar wind plasma by the Venus neutral atmosphere is suggested by these observations. The dependence of the solar cycle variation of the shock position on the orientation of the interplanetary magnetic field reinforces the idea that planetary ion pickup is important in the interaction of the solar wind with Venus.
Linear prediction filter analysis of relativistic electron properties at 6.6 RE
Authors: Baker D. N. , McPherron R. L. , Cayton T. E. , Klebesadel R. W.
Published: February 2008

Electrons with energies >500 keV in the Earth's outer magnetosphere exhibit very hard energy spectra and highly variable absolute intensities. The fluxes often show a strong 27‐day periodicity which is related to recurrent solar wind stream variations at 1 AU. We have used available solar wind speed data as well as continuous geomagnetic indices such as Kp and AE in order to characterize the relationship of relativistic electrons to these geophysical parameters. The present analysis emphasizes data taken in 1982–1985 and employs electron measurements from 3 to 40 MeV at geostationary orbit along with geomagnetic data from the National Geophysical Data Center CD ROM. The method of linear prediction filter (LPF) analysis is used to characterize and predict the general relationship between solar wind or geomagnetic indices as input time series and electron properties as the output time series. Filters are found that generally decrease strongly at zero lag time and then peak strongly at lags of 2–3 days. Cross‐covariance analyses show strong correlative peaks between electron fluxes and geomagnetic parameters at multiples of 13 and 27 days. The present analysis allows enhanced understanding of the relativistic electron behavior on both short and long time scales and permits improved prediction of both high‐altitude spacecraft operational environments and magnetosphere‐atmosphere coupling relationships.
Nonlinear evolution of high‐latitude ionospheric interchange instabilities with scale‐size‐dependent magnetospheric coupling
Authors: Keskinen M. J. , Huba J. D.
Published: February 2008

The Naval Research Laboratory high‐latitude ionosphere‐magnetosphere mesoscale coupling model has been generalized to include scale‐size‐dependent magnetospheric coupling. With this model, the nonlinear evolution of plasma interchangelike instabilities and processes in the high‐latitude ionosphere has been studied using computer simulation techniques. We find that the principal effect in the high‐latitude ionosphere of magnetospheric coupling, i.e., increased inertial effects, is most pronounced for long‐wavelength interchange‐driven ionospheric fluctuations of several tens of kilometers and greater. For scale sizes of a few kilometers to tens of kilometers the phenomenology and time scales of the nonlinear evolution of ionospheric interchange modes can be characterized as neither purely inertial (magnetospheric control) nor purely collisional (ionospheric control). For scale sizes less than a few kilometers the nonlinear interchange evolution can be described as a purely collisional process. Density and electric field fluctuation power spectra have been computed from the simulations and found to be in agreement with experimental observations. With scale‐size‐dependent magnetospheric coupling, spectral breaks at scale sizes of the order of a few kilometers are observed in the density and electric field spectra. Applications to mesoscale dynamics, structure, and the stability of convecting inhomogeneities and large‐scale plasma density structures in the high‐latitude ionosphere have been made.
An observational study of the nightside ionospheres of Mars and Venus with radio occultation methods
Authors: Zhang M. H. G. , Luhmann J. G. , Kliore A. J.
Published: February 2008

An analysis of Mars and Venus nightside electron density profiles obtained with radio occultation methods shows how the nightside ionospheres of both planets vary with solar zenith angle. From previous studies it is known that the dayside peak electron densities at Mars and Venus show a basic similarity in that they both exhibit Chapman layer‐like behavior. In contrast, the peak altitudes at Mars behave like an ideal Chapman layer on the dayside (peak altitudes increase with solar zenith angle), whereas the altitude of the peak at Venus is fairly constant up to the terminator. The effect of major dust storms can also be seen in the peak altitudes at Mars. All Venus nightside electron density profiles show a distinct main peak for both solar minimum and maximum, whereas many profiles from the nightside of Mars do not show any peak at all. This suggests that the electron density in the Mars nightside ionosphere is frequently too low to be detected by radio occultation. On the Pioneer Venus orbiter, “disappearing ionospheres” were observed near solar maximum in the in‐situ data when the solar wind dynamic pressure was exceptionally high. This condition occurs because the high solar wind dynamic pressure decreases the altitude of the ionopause near the terminator below ∼250 km, thus reducing the "normal" nightward transport of dayside ionospheric plasma. While the “disappearing ionosphere” condition is usually considered important for the upper ionosphere only, the peaks in the deep Venus nightside also show some dependence on the solar wind dynamic pressure. However, an additional persistent source (e.g., particle precipitation) makes a major contribution to these peaks. The observations suggest that this other source may in fact increase with solar wind pressure. This is different from the nightside ionosphere of Mars where even the peak cannot be detected on occasion. We do not know whether this more complete disappearance of the nightside ionosphere at Mars is, like the Venus “disappearing ionosphere,” a response to solar wind conditions because of the lack of solar wind data at Mars. On the basis of the Venus observations, one might predict that if a positive correlation of nightside peak density with dynamic pressure was found, it could mean that transport from the dayside is the only significant source for the nightside ionosphere of Mars. The lack of a correlation would imply that the precipitation source at Mars is quite variable.
Temperature gradients at mesopause heights inferred from OH nightglow data
Authors: Schubert G. , Walterscheid R. L. , Hecht J. H. , Sivjee G. G.
Published: February 2008

A theoretical method is proposed to obtain thermal structural information about the mesopause using multiple rotational lines from ground‐based observations of a single vibration‐rotation nightglow Meinel band of the electronic ground state of OH. The method determines the mean temperature of the emission region, the temperature difference across the region, and the magnitude of the temperature gradient for physically realistic assumptions about emission region thickness. It is applied to the OH(6, 2) Meinel band spectra of the nightglow obtained during periods of low geomagnetic activity during January 1986 at Longyearbyen, Spitzbergen. The results of our analyses indicate that during the periods of observation the emission region of the nightglow OH Meinel bands is significantly nonisothermal, with temperature gradient magnitudes of about 10 K km−1 (for an emission region thickness of 8 km). Such large temperature gradients are consistent with other independent measurements of a highly structured mesopause reported in the literature. Large‐amplitude gravity waves propagating through the mesopause are probably responsible for the large temperature gradients.
An observation of ionospheric convection and auroral arc motion
Authors: Doolittle J. H. , Mende S. B. , Robinson R. M. , Swenson G. R. , Valladares C. E.
Published: February 2008

An all‐sky imager operated at Sondre Stromfjord, Greenland has been used to make auroral measurements in correlation with data from the incoherent scatter radar. The auroral images were recorded on video tape for later display as time lapse sequences. Reviewing the images as movie sequences with an apparent time compression ratio of 60 allows the eye to discern features which are not apparent in the individual frames. The faint 630 nm airglow background lying poleward of the bright auroral oval arcs appears to be structured in the time lapse observations and drifting at a uniform rate. Analysis of the motion of the airglow structures shows excellent agreement with Doppler radar measurement of the ion drift velocities. Electron density profiles derived from the radar data corroborate the existence of high altitude F‐region ionization enhancements. The electron temperature in the patches was about 1000 K and no higher than outside of the patch signifying that the patches were not produced by local soft electron precipitation. According to the radar Doppler data there was a small component of the drift velocity parallel to the arc at either side of the auroral boundary. The sense of those small components was consistent with drifts driven by a convergent electric field which would also produce an up going field aligned current. The larger component of drift velocity perpendicular to the arc appeared to be continuous on either side of the arc. This drift component shows the existence of an electric field parallel to the arc, showing that the arc was not on an equipotential. During the observations the arc moved slowly in the opposite sense to the ion drift, signifying that the auroral arc motion is not driven by the electric field impressed on the atmosphere by the magnetosphere. Calculations of the expected intensity of the 630 nm emissions are in good agreement with the measured intensities of the moving patches. The observation of the motion of these airglow structures could provide a routine measurement of the ionospheric convection speeds in regions where the radar is not available.
Solar wind stream interaction: Electron temperature and heat flux rise in the low‐speed stream
Authors: Alexander P. , Duhau S.
Published: February 2008

The strong compression produced in two‐stream interaction regions in the solar wind is a local source of heating. The study of the distribution of that energy between heat and internal energy provides valuable information about transport processes. In the present work, the electron heat flux and temperature rise in the compression produced within the low‐speed portion of the interaction region is predicted using a new heat conduction law valid for collisionless plasmas with isotropic electron temperature, introduced recently by one of the authors. Equations are found for the electron heat flux and temperature rise as functions of two parameters, one related to the strength of the compression and the other one to the heat flux at the boundaries of the region under study. These equations lead to agreement between theory and experiment.
Ascending flow between the descending lithosphere and the overlying asthenosphere
Authors: Ito Keisuke
Published: February 2008

A model is proposed to explain the high heat flow and active volcanism above the descending plate. The slip zone between the descending plate and the overlying asthenosphere is assumed to be a low‐viscosity and low‐density fluid layer enriched with liquid. The flow rate and the layer thickness are estimated as functions of viscosity from simple model calculations of fluid motion and thermal and force balances. The flow tends to be upward when the viscosity is low. The amount of heat transported by the ascending flow is greater than that generated by viscous dissipation. Basalts and andesites in arc volcanism are considered to be derived from the ascending liquid fraction and remelting of the subducted oceanic crust, respectively. Seismic evidence supporting the existence of a low‐viscosity layer between the sinking plate and the overlying mantle is discussed. The region over the descending lithosphere is associated with high heat flow and active volcanism. This is generally explained by the following hypotheses: (1) frictional heating in the slip zone and (2) upwelling of high‐temperature mantle material induced by the descending plate [Turcotte and Oxburgh, 1968; McKenzie and Sclater, 1968; Oxburgh and Turcotte, 1970; Andrews and Sleep, 1974]. Either hypothesis has some difficulties in explaining all geophysical and petrological observations. In this paper I present an alternative model to explain the high heat flow and volcanic activity above the descending lithosphere.
The magnetic properties of some DSDP basalts from the North Pacific and inferences for Pacific plate tectonics
Authors: Marshall Monte
Published: February 2008

One hundred seven basalt samples from seven DSDP sites in the North Pacific studied for magnetic properties were found to have a mean magnetization of 0.005 emu/cm3, a mean susceptibility of 0.001, and a mean Koenigsberger ratio of 10. The mean remanence coercivity as measured by the mean demagnetizing field is 90 Oe. The titanomagnetite in most of these old (13–115 m.y.) and highly fractured basalts has been largely oxidized to titanomaghemite during sea floor weathering. The titanomaghemitization is manifest in an increase in Curie temperature from about 160°C to 300°–350°C and a threefold to fourfold decrease in the saturation magnetization. Although a 50% decrease in remanence intensity and susceptibility can be attributed to oxidation at one site, the relation between degree of oxidation and these two properties is obscured at most sites by magnetic grain size differences. A decrease in remanence intensity no doubt occurs but was not observed because the most highly oxidized basalt, that in thin flows and in the margins of thicker flows, is also the finest grained and therefore had a higher initial remanence intensity. Remanence coercivity, while mainly dependent on magnetic grain size, increases about 50% with oxidation. The remanence direction does not appear to be affected by oxidation. Although the remanance inclinations differ considerably from the present geomagnetic field inclination at most sites, the expected range of secular variation is such that the remanence, with the exception ofthat in Meiji Seamount, could have been acquired by the basalts at the present site latitudes. But the site paleolatitudes calculated from the remanence inclinations correspond more closely to the paleolatitudes predicted by assuming that the Pacific plate has moved northward with the motions deduced by Morgan and others from linear volcanic chains. In particular, the coincidence of the magnetic paleolatitude, 19°, measured in Meiji Seamount at the northern end of the Emperor Seamount chain, with the latitude of the island of Hawaii, supports the fundamental assumption of the Morgan model of Pacific plate motion, wherein the volcanic locus or ‘hot spot’ forming the Emperor‐Hawaiian chain has remained essentially stationary with respect to the earth's axis.
Properties of single‐domain, pseudo‐single‐domain, and multidomain magnetite
Authors: Levi Shaul , Merrill Ronald T.
Published: February 2008

Despite the influence that paleomagnetism has exerted on the earth sciences, our understanding of thermal remanent magnetization (TRM), which is one of the primary sources of paleomagnetic information, is at best fragmentary and incomplete. In this paper we report on experiments studying TRM properties of magnetites whose particle sizes vary from single domain (SD) to pseudo single domain (PSD) to multidomain (MD). TRM stability has been measured by using alternating field (af) demagnetization, hot af demagnetization, low‐temperature treatments, thermal demagnetization, and storage tests. Low‐temperature treatments are a great aid in helping to determine whether a sample's TRM is carried primarily by SD, PSD, or MD magnetite particles. Thermal demagnetization is relatively insensitive for predicting particle sizes, while af demagnetization remains the best method for reducing the MD contribution to the remanence. Theoretical considerations indicate that the thickness of a domain wall should initially increase with temperature until a critical value is reached, beyond which the thickness decreases. This result is useful in the interpretation of the hot af demagnetization experiments of MD particles whose median destructive alternating field increases with temperature up to some maximum value before decreasing, while the median destructive field of SD and PSD particles decreases monotonically with temperature. Characteristics of the Lowrie‐Fuller test (Lowrie and Fuller, 1971) for equidimensional 0.2‐μm‐sized particles indicate that a critical TRM‐inducing field hc exists, where 0.10 < hc < 0.49 Oe; SD‐like behavior is exhibited when the inducing field is greater than hc, and MD behavior is exhibited for fields less than hc. This suggests that the critical sizes for transitions of domain structure depend on the intensity of the inducing field. It is argued that the presence of an external field and, similarly, blocking at high temperatures increase the effective size region of SD/PSD remanence and that remanence might be blocked in grains with a nonequilibrium domain configuration. Estimates of critical sizes for transitions between SD, PSD, and MD behavior depend on several factors including grain shape, temperature, type of remanence given to the sample, mineralogy, and intensity of the field used to induce the remanence.
Systematic behavior of compressional velocity in Franciscan rocks at high pressure and temperature
Authors: Stewart Roger , Peselnick Louis
Published: February 2008

Compressional velocity was measured to 300°C and 8 kbar in 14 dry rocks ranging from zeolite facies sandstone to jadeitized metagraywacke from the Franciscan complex of central and northern California. At pressures above 2.5 kbar, velocity in any one of these rocks can be expressed as a linear function of pressure and temperature with an error of less than 1%. For all the rocks taken together, velocity may be expressed as a linear function of temperature, pressure, density, and porosity or as a linear function of pressure, temperature, and bulk density; in either case, root‐mean‐square velocity residuals are about 1.5%. Critical thermal gradients for individual rocks average about 15°C/km. Small increases in bulk density due to elimination of porosity or crystallization of dense minerals can maintain constant velocities in the face of supercritical geothermal gradients. The laboratory data are consistent with available seismic refraction and rock density measurements in the Franciscan terrane of central California.
Tidal stresses in the Moon
Authors: Cheng Chuen Hon , Toksöz M. Nafi
Published: February 2008

The tidal stresses in a radially heterogeneous moon are calculated numerically as a function of time and location by using the latest moon model obtained from seismic data. Theoretical results are compared with the observed features of deep moonquakes and in particular with those of the most active A1 hypocenter. The results show that (1) the magnitudes of tidal stresses are small, of the order of 1 bar and less, (2) they reach maxima in the seismically active depth range of 600–1200 km, (3) the geographic distribution of moonquake epicenters does not appear to be controlled solely by the highest tidal stresses, and the presence of heterogeneities and weak zones may be important in determining moonquake locations, (4) the occurrence times of moonquakes correlate with one or more of the tidal stress components, and (5) the presence of an ambient tectonic stress of about the same order of magnitude as the tidal stress is necessary to explain the observed reversal in polarity of the A1 moonquakes.
Characterization of oriented cracks with differential strain analysis
Authors: Siegfried Robert , Simmons Gene
Published: February 2008

Linear strain of a rock sample as a function of hydrostatic pressure can be measured with a precision of 2 × 10−6. Such high‐precision data for three orthogonal directions allow calculation of the distribution function for the porosity due to cracks closing completely at a given pressure. Such data for at least six independent directions yield the zero‐pressure strain tensor due to cracks closing completely at a given pressure. The principal values and axes of this tensor distribution function provide information about the orientation of cracks as a function of closure pressure. In this manuscript we first develop the mathematical basis for the technique and then illustrate it with differential strain data for two samples, the Westerly (Rhode Island) granite and the Twin Sisters (Washington) dunite. Strain tensor calculations reveal that each of these samples has a different type of anisotropic crack distribution.
Carbonate formation in Marslike environments
Authors: Booth Michael C. , Kieffer Hugh H.
Published: February 2008

Carbonate growth was examined within rock powders subjected to Marslike environmental simulations. Rates of growth under experimental conditions were 1012–1013 molecules cm−3 s−1 with or without an aqueous phase of H2O present and were found to be proportional to CO2 pressure, H2O abundance, and particle surface area. Direct ultraviolet illumination of powders was found not to affect carbonate growth significantly, but photochemistry of absorbed H2O was thought to play an important role in chemical alteration activity. Transitory damp periods induced in powders of some experiments by the melting of surface frosts were found to provide only slight assistance to the rate of growth in comparison with experiments in which sublimation of frosts occurred. Liquid water did, however, have a noticeable effect upon the distribution of carbonate material within experimental samples. On the basis of experimental results, carbonate formation is shown to be fully compatible with the low‐pressure arid conditions characteristic of the present Mars surface environment. It is further concluded that carbonate formation is likely to be an important aspect of Martian chemical weathering, that carbonate material is likely to represent a major chemical reservoir for the outgassed CO2 inventory of Mars, and that in the absence of an aqueous phase of H2O in the environment this carbonate should form as submicron coatings on regolith particle grains.
On the theoretical evaluation of the Grüneisen function in the harmonic approximation
Authors: Mulargia Francesco
Published: February 2008

A theoretical expression is derived for the harmonic (in the Leibfried and Ludwig [1961] sense) Grüneisen γ function by means of a Legendre polynomial expansion of the frequency spectrum (Thirring series). At high temperatures the harmonic Grüneisen function is found to be rigorously independent of temperature; the postulate definition made by Born is also given a mathematical proof, and it is proven that the acoustical branches of the spectrum give an irrelevant contribution to γ, as was first suggested by Knopoff and Shapiro [1969], The explicit expression of the harmonic high temperature γ function, its volume derivatives, and its asymptotic behavior are then derived also for three of the most used kinds of intermolecular pair potentials: the Lennard‐Jones potential, the Morse potential, and the Rydberg potential. A practical application to sodium, copper, and iron is also presented. The expression found for the Grüneisen function is simple, compact, and consistent, however, a major problem arises from the theory: an evaluation of this function requires the knowledge of the intermolecular potential function up to the fourth derivative; since the pair potentials commonly used generally agree with the ‘true’ function at most on the second derivative, the accuracy which can be achieved is poor. This is clearly shown by the inconsistent results found in the practical applications presented. The same objection is obviously valid for any other theoretical expression of γ involving high‐order derivatives of the intermolecular potential.
New high‐pressure modifications of GeO2 and SiO2
Authors: Liu Lin‐Gun , Bassett William A. , Sharry John
Published: February 2008

Glass samples of GeO2 and SiO2 were subjected to pressures in excess of 250 kbar in a diamond anvil press and heated to temperatures over 1000°C by YAG laser. X ray diffraction study of the quenched and unloaded samples yielded powder patterns consisting of the rutile phase plus a new modification which can be indexed as being hexagonal. The lattice parameters of the new phase at room temperature and atmospheric pressure are a0 = 2.729 ± 0.002 and c0 = 4.312 ± 0.004 Å for GeO2 and a0 = 2.561 ± 0.004 and c0 = 4.112 ± 0.008 Å for SiO2. Calculated intensities based on hexagonally close‐packed oxygens with cations randomly filling one half of the octahedral sites are in agreement with the observed intensities. This structure (Fe2N) can best be visualized as the niccolite structure with half the cations randomly removed.
Xenon in CO2 well gas revisited
Authors: Phinney D. , Tennyson J. , Frick U.
Published: February 2008

We have analyzed the rare gases in a CO2 well gas from Harding County, New Mexico, by mass spectrometry. We have reconfirmed the monoisotopic excess of 129Xe measured previously. The excesses of fissiogenic Xe isotopes have relative proportions which coincide with the spontaneous fission Xe spectrum of 238U. No 244Pu‐derived, fissiogenic Xe was detected. The uncertainties in the fission Xe component place an upper limit of 20% on the 244Pu contribution to fissiogenic 136Xe. Using this upper limit and the excess of 129Xe, we calculate an upper limit on the formation interval of the earth of 76 m.y. The trapped Ne, Ar, Kr, and Xe are dominated by a component which is similar to that found in mantle‐derived terrestrial samples. We also report for the first time an excess of 3He, which is evidence for primordial 3He in the source region of the well gas.
Manicouagan Impact Melt, Quebec 2. Chemical interrelations with basement and formational processes
Authors: Grieve R. A. F. , Floran R. J.
Published: February 2008

The basement rocks of Manicouagan exhibit a wide compositional range (40–72% SiO2), and factor analysis techniques have been employed to determine the average composition of chemically distinct lithologies. Least squares mixing calculations using some of these lithologies as melt components indicate that the average composition of the Manicouagan melt sheet can be modeled for 9 major and 11 trace elements by a mixture of 4.5% anorthosite, 55.5% mafic gneiss, and 40.0% tan gneiss with a granitic‐granodioritic composition. These lithologies have a large areal extent close to the center of the structure, and the mixing model is considered compatible with the genesis of the melt rocks during a hypervelocity impact event. The underrepresentation of anorthosite relative to its present distribution is in keeping with evidence that the bulk of the presently exposed anorthosite is a horst which was uplifted to form a central peak after melt formation. The homogeneous composition of the melt (57.75 ± 1.21% SiO2) relative to that of the postulated target is explained in a model outlining the dynamic conditions existing during the formation of the melt and its accompanying movement into the excavated cavity. Gross differences in target composition may be reflected in a minor variation of less than 1% Al2O3 between the average melt in the northern and southern areas of the structure. A relatively rare mafic composition occurs sporadically within the melt sheet and may have resulted from the incomplete homogenization of local concentrations of anorthosite with a high proportion of mafic bands.
The thermal history of the Manicouagan Impact Melt Sheet, Quebec
Authors: Onorato P. I. K. , Uhlmann D. R. , Simonds C. H.
Published: February 2008

Calculations for the heat transfer between superheated silicate melt and evenly dispersed 1‐mm cold clasts (a model for impact melt rocks) indicate that (1) most of the thermal gradients are smoothed out in 100 s, (2) the extent to which cold debris is melted is slightly more than proportional to the fractional difference of the equilibration temperature between the solidus and the liquidus of the melt, (3) smaller clasts are preferentially dissolved, (4) the melting of clasts depends on the instantaneous local temperature of the melt in the region of the clasts, and hence clasts can melt during cooling whose liquid uses are higher than the equilibration temperature, and (5) the rate of equilibration is sufficiently high that clasts whose dissolution in the melt is slow may be preserved. Calculations for cooling of the 200‐m‐thick melt sheet at Manicouagan suggest that complete crystallization takes 35 years at 10 m from the edge and 1600 years at the center. Failure to consider the latent heat of fusion results in estimates of these times which are too short by about a factor of 2. The time to cool to a given temperature is proportional to the square of the distance from the boundary of the melt sheet only when the finite nature of the sheet is unimportant.
The isostatic state of the lunar apennines and regional surroundings
Authors: Ferrari Alfred J. , Nelson David L. , Sjogren William L. , Phillips Roger J.
Published: February 2008

High‐resolution gravity and topography data taken over the Apennine Mountains have been used to compute their isostatic state. Results show that the Apennines are uncompensated; thus this state implies that the lunar crust and upper mantle have been strong enough over 3.9 b.y. to support the load exerted by this topographic excess. The Apennines produce a maximum shear stress of 60 bars at a depth of 60 km. A lower bound on the lunar crustal viscosity of 1 × 1027 P is calculated on the basis of the assumption of a 10% relaxation over 3.9 b.y. Studies of a broad negative regional anomaly located between Maria Serenitatis and Imbrium necessitate a locally thicker crust to satisfy the observed data. This anomaly may have been produced by a lateral transport of crustal material from beneath the giant impact basins as a result of rebound at the crust‐mantle interface.
Plate kinematic implications of Atlantic equatorial fracture zone trends
Authors: Sibuet Jean‐Claude , Mascle Jean
Published: February 2008

We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Ceara and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present‐day positions with respect to South America and Africa.
The effect of shock on the magnetism of terrestrial rocks
Authors: Cisowski S. M. , Fuller M.
Published: February 2008

Experimental projectile impacts into solid intrusive rocks confirm that shock pressures on the order of tens of kilobars are sufficient to affect the natural remanent magnetism (NRM) of terrestrial materials. The principal effect is to demagnetize the preexisting remanence. A secondary effect is the acquisition of an additional component of remanence whose direction is related to the ambient field direction at the time of impact. Although longer and more intense shock pulses may affect the higher‐coercivity fractions, the low‐coercivity fraction is the most susceptible to magnetic resetting by shock. Thus igneous rocks with abundant low‐coercivity carriers (low remanent coercive force, HRC) are more disposed toward magnetic recording of low‐pressure shock events. Evidence is presented that these low‐HRC rocks are also apparently capable of recording the remanent field of a magnetized projectile at the time of impact. Analysis of the NRM of samples from a road cut in the Palisades Sill, New Jersey, demonstrated that the explosions associated with road cutting are sufficient to reset partially the magnetization in the direction of the present field. This result illustrates possible hazards in the interpretation of paleomagnetism for such samples. The variability of remanence in the Deccan basalts from the Lonar Crater, India, can be explained in terms of their observed differing response to shock, as a function of HRC. However, the lack of oriented samples precludes a conclusive test that the secondary component observed in nature is a shock remanent magnetization. The magnetism of Lonar microbreccias seems to be related to the temperatures they reached during the impact event. Despite widespread remagnetization of the Kaibab formation since the impact event, certain oriented dolomite samples from Meteor Crater, Arizona, retain evidence of partial magnetic resetting at the time of the crater‐forming event, which may well be a shock remanence. The Coconino sandstone from Meteor Crater shows changes in remanence and hysteresis properties with increasing degrees of shock metamorphism. An important application of the effects of shock upon magnetization lies in the interpretation of lunar magnetism, since all lunar samples were collected from an impact‐generated regolith. The ‘hard’ NRM curves of many mare basalts may in part be due to preferential shock demagnetization of ‘soft’ NRM. In the soil breccias the observed NRM is likely to be directly associated with the lithification process. The mechanism of shock‐associated magnetization is not well understood. In the low‐shock range of a few kilobars it appears to be similar to static‐stress‐related magnetization. In extreme shock of hundreds of kilobars the transient and residual temperatures are high enough so that the acquired remanence is transitional to thermoremanent magnetization.
Tectonomagnetic event detection using empirical transfer functions
Authors: Poehls Kenneth A. , Jackson David D.
Published: February 2008

We have devised a method of analyzing geomagnetic array data which effectively filters out variations caused by atmospheric disturbances, their associated ground currents, and local variations in magnetic susceptibility. This method has application to the detection of tectonomagnetic variations. We use vector field data at one station, from which scalar field data at several stations are predicted. Prediction error statistics are then carefully studied, and suspected tectonomagnetic events are described in terms of the probability of their being random fluctuations in normal data. We have applied this method to data from the U.S. Geological Survey magnetometer array in central California, using vector data from Castle Rock Observatory. Prediction errors appear to be Gaussian. The lower limit to the variance appears to be the sensitivity of the vector data at Castle Rock (the hourly means used are reported to the nearest gamma). No believable tectonomagnetic events occur from January 1, 1975, to February 28, 1975.
The viscoelastic stiffness model of seismicity
Authors: Cohen Steven C.
Published: February 2008

A viscoelastic stiffness model of seismicity is developed by introducing a viscoelastic element into the stiffness model for fault dynamics. The introduction of this element permits modeling of transient anelastic deformations in response to stress loading and relaxation and provides a mechanism for partial stress recovery following an earthquake. As a consequence, several phenomena not present in elastic stiffness theory emerge. These include postseismic creep, foreshocks, and aftershocks. Numerical simulations of fault motion also reveal episodes of stable sliding, tertiary creep preceeding earthquakes, and long‐term aseismic creep.
Fission xenon isotope dating
Authors: Teitsma A. , Clarke W. B.
Published: February 2008

The fission xenon isotope dating method has been evaluated by dating four zircon samples, one sphene sample, and one carbonatite sample. The zircon and sphene samples had been dated in other laboratories by the conventional U‐Th‐Pb and Rb‐Sr methods. Samples were irradiated with thermal neutrons in order to introduce xenon from neutron fission of 235U. After a short cooling period (before decay of 133Xe), stepwise heating of the sample and mass spectrometric analysis were used to determine the isotopic composition in the gas released at each temperature step. Fission xenon ages were calculated from the ratio of spontaneous fission xenon from 238U, and neutron fission xenon ages from 235U. The method appears to have several advantages over the conventional U‐Xe technique: (1) Isotope ratio measurements are inherently more precise than measurements of absolute quantities. (2) If a geological disturbance has caused loss of fission xenon, a valid date can still be obtained using the low‐mobility xenon released at high temperatures. (3) In some samples, dates of disturbance and initial formation can be measured.
Heat flow in the eastern Caribbean
Authors: Clark Tony F. , Korgen Ben J. , Best David M.
Published: February 2008

A west‐to‐east traverse of 24 heat flow measurements was made across the Caribbean Sea floor from the eastern Venezuelan basin to 80 km east of the Lesser Antilles arc crest. The resulting heat flow profile includes an increase from basin values in the eastern Venezuelan basin and a zone of high values extending the length of the transect. Superimposed on this zone are two heat flow peaks containing maximum values, one at the Aves swell and the other at the Lesser Antilles arc. Based on 21 acceptable values, average heat flows were 1.87 heat flow units (1 HFU = 1 μcal cm−2 s−1) (8 values) for basin regions, 3.63 HFU (9 values) for the Aves swell peak, and 4.16 HFU (4 values) for the Lesser Antilles arc peak. High heat flow is expected for the Lesser Antilles arc, the present site of seismic and volcanic activity associated with subduction of the North American plate at the West Indies trench. The heat flow peak at the Aves swell agrees with an island arc formation for this presently inactive feature, with its high heat flow anomaly attributed to radiogenic heat sources concentrated during subduction in the past. Both the Aves swell and the Lesser Antilles arc are believed to be heated by radiogenic sources accumulated during past subduction, but only the Lesser Antilles arc is now receiving new heat from mechanical processes directly coupled to active underthrusting. This suggests that differences in the two heat flow peaks at the Aves swell and Lesser Antilles arc reflect the relative importance of different heat‐producing mechanisms at island arcs.
Viscoelastic crustal deformation by finite quasi‐static sources
Authors: Rundle John B.
Published: February 2008

A method is formulated for the exact calculation of Green functions for finite quasi‐static sources in an elastic layer over a linear viscoelastic half space. The displacements due to the same sources in an elastic layer over an elastic half space are first computed, and then the well‐known correspondence principle of linear viscoelasticity is applied. The most difficult part of the problem, the inversion into the time domain, is accomplished by means of a novel technique. As a first example of the method, the growth of the error in an approximate Green function for a strike slip source is evaluated. Second, the viscoelastic uplifts due to an expanding sphere are calculated for two half‐space rheologies. Third, the time dependent uplifts due to point dip slip sources are computed. Finally, the dip slip Green functions are integrated over a finite rectangular fault surface, and the coseismic and postseismic uplifts are compared.
Stress pattern near the San Andreas Fault, Palmdale, California, from near‐surface in situ measurements
Authors: Sbar Marc L. , Engelder Terry , Plumb Richard , Marshak Stephen
Published: February 2008

We made 29 in situ doorstopper strain relaxation measurements distributed among eight sites spaced on a 35‐km transect running from the foothills of the San Gabriel Mountains, across the San Andreas fault, into the western Mojave desert southeast of Palmdale, California. This was a pilot study to test if such measurements can detect the regional stress field and any modification of it in a tectonically complex area. Strain was measured by overcoring strain gauge rosettes which had been bonded to the flattened bottom of a shallow borehole. Measurements of stress were repeatable at each site. We found NNE trending maximum compressive stress (σ1) at our sites farthest from the fault. This orientation is parallel to the σ1 inferred from fault plane solutions of major southern California earthquakes and compares favorably with deep hydrofracture stress measurements made near our sites. Nearer the San Andreas fault, the orientation of σ1 was approximately east‐west north of the fault, and northwest‐southeast to northsouth south of the fault. The repeatability of measurements at a site and the favorable comparison of our measurements with Tullis' [1977] near‐surface stress measurements suggest that we reliably determined the stress field orientation present at a site. It was not possible, using the available data, to distinguish the stress caused by residual or topographic effects from tectonically applied stress or to account for modification of the stress field by decoupling across fractures.
Geometry of triple junctions related to San Andreas Transform
Authors: Dickinson William R. , Snyder Walter S.
Published: February 2008

Understanding the evolution of the San Andreas transform is a key not only to broad aspects of regional geology but also to the development of specific structural provinces. Though stable in the regional sense, the paired triple junctions at the ends of the transform have had transient unstable configurations wherever the trends of the prior trench and the newly developing transform were locally not collinear. The resulting instabilities were mainly of kinds inferred to induce extensional tectonics within a nearby region. Passage of the Mendocino fault‐fault‐trench triple junction northward along the central California coast coincided well with pulses of initial subsidence in Neogene sedimentary basins near the continental margin and with eruptions at local volcanic centers in the Coast Ranges. Passage of the Rivera ridge‐trench‐fault triple junction southward was associated with the rifting events that formed the California Continental Borderland and the Gulf of California.
An in situ velocity study: The Stone Canyon Well
Authors: Stierman Donald J. , Kovach Robert L.
Published: February 2008

Velocities of seismic p‐waves are unusually low in the vicinity of a 600‐meter deep well drilled into quartz diorite located 1.2 km from the San Andreas fault in central California. These low velocities are attributed to the presence of extensive fracturing of rocks along the fault. Now here in the well does the P‐velocity exceed 70% of the velocity measured in samples of core from the well. Analyses of electrical and gravity logs suggest that saturated macrocracks, while producing only 5–10% porosity, are responsible for the 30–50% reduction of in situ seismic wave velocities. These macrocracks fail to close in response to confining stress. We attribute this to dilatancy induced by deviatoric stresses associated with the San Andreas fault, although the possibilities of high pore fluid pressures in the rocks or of an increase in fracturing and gouge formation with depth cannot be ruled out. Serious questions are raised concerning the use of laboratory data to model fault zones. In situ measurements of stress, seismic velocity and fluid pressures are necessary before we can hope to completely understand the distribution and mechanism of shallow focus earthquakes.