bluuebunny/crossref_metadata_2025 huggingface dataset:
Pitch angle diffusion of newborn ions due to intrinsic turbulence in the solar wind
Authors: Ziebell L. F. , Yoon Peter H. , Wu C. S. , Winske D.
Published: February 2008

The objective of the present study is to understand the interaction of the solar wind with newborn ions in far upstream regions of a comet where the level of intrinsic turbulence is moderately low. Based on the assumption that quasi‐linear theory is adequate and applicable, we investigate the pitch angle diffusion process and the time evolution of the newborn ion distribution function. Numerical solutions to the quasi‐linear diffusion equation, including the effect of resonance broadening and that of continuous creation of newborn ions due to ionization process, are obtained under several assumptions and approximations. It is found that theoretical results are consistent with the Giotto observations recently reported by Neugebauer et al. (1989) which enable us to understand the formation of partial shells which were frequently observed in far upstream regions. The pitch angle diffusion process is also examined by the hybrid simulation. The results of the simulation are in qualitative agreement with those obtained from the quasi‐linear analysis.
STARE and EISCAT measurements: Evidence for the limitation of STARE Doppler velocity observations by the ion acoustic velocity
Authors: Kofman W. , Nielsen E.
Published: February 2008

Measurements of the phase velocity of ionospheric plasma waves using the STARE radar system are compared with the electron drift velocity and the ion acoustic velocity calculated from data measured by EISCAT. This comparison shows that for those flow angles that are smaller than the largest flow angle (θs) for which the component of the electron drift velocity along the radar line of sight equals the ion acoustic velocity, the phase velocity observed by STARE tends to be limited by, or lower than, the ion acoustic velocity. Using observations with flow angles larger than θs, one can infer the electron drift from STARE measurements by the well‐known cosine relation.
Field‐independent source localization of Neptune's radio bursts
Authors: Farrell W. M. , Desch M. D. , Kaiser M. L.
Published: February 2008

During the Voyager 2 encounter with Neptune, a narrowbanded bursty radio component was observed between 500 and 1326 kHz by the Planetary Radio Astronomy (PRA) instrument. Based on the emission occurrence pattern, the radio source has been localized without the explicit use of the Neptunian offset‐tilted dipole (OTD) magnetic field model, which is accurate only at distances greater than 4 RN from the planet. We used only assumptions based upon the general nature of radio wave propagation in planetary magnetospheres. A number of different candidate radial positions were sampled. For example, at 1.5 RN, the derived source location was positioned only about 10° from the south magnetic pole. The radiation from this source was beamed into a cone of 77.5°±6.3° half‐angle that was tilted about 10° from the radial direction to the north‐northeast. At other sampled radial positions, similar source locations were obtained. Due to its proximity to the south magnetic pole, the kilometric emission radio source is believed to be associated with an active auroral region, similar in nature to those found at Earth and Saturn.
The control net of Mars: May 1977
Authors: Davies Merton E.
Published: February 2008

In May 1977 a planet‐wide control net of Mars was computed by means of a large single‐block analytical triangulation using 17,224 measurements of 3037 control points on 928 Mariner 9 pictures. Areographic coordinates of the control points are available on microfiche (see Table 2). The direction of the spin axis and the rotation rate as determined by Viking were used. The angle V, measured from Mars' vernal equinox along the equator to the prime meridian (Airy‐0), was found to be V = 148.37° + 350.891986° (J.D. 2,433,282.5).
Mars: The role of the regolith in determining atmospheric pressure and the atmosphere's response to insolation changes
Authors: Fanale F. P. , Cannon W. A.
Published: February 2008

We present a quantitative model for atmosphere‐regolith exchange of CO2 on Mars based on new laboratory measurements of CO2 adsorption on ground rock at temperatures of 158°, 175°, 196°, and 231°K and CO2 pressures from 1.0 to 80 mbar. Our model is consistent with Viking observations, whereas models involving a massive residual CO2 cap and no long‐term atmosphere‐regolith CO2 exchange are not. Our model describes the role of the regolith as a CO2 storehouse, as a long‐term buffer of the atmospheric pressure, and as a major factor in determining the response of the atmosphere to postulated changes in surface insolation. We conclude the following: (1) The atmosphere‐plus‐cap system is buffered on a long‐term basis by several hundred grams per square centimeter of exchangeable CO2 adsorbed in the regolith. (2) If the atmosphere‐plus‐cap system were arbitrarily removed in its entirety, it would eventually be restored to nearly its former state by reequilibration with the regolith. (3) Exchange with the adsorbed phase in the regolith has greatly restricted 18O enrichment in the atmosphere. (4) The layered terrain primarily represents current periodic pressure increases (of several tens of millibars) caused by exchange of CO2 between the regolith and the atmosphere‐plus‐cap system. (5) Pressures of 100–300 mbar might have existed during the early history of the planet.
Repeated creep events on the San Andreas Fault near Parkfield, California, Recorded by a strainmeter array
Authors: Goulty Neil R. , Gilman Ralph
Published: February 2008

Following the 1966 Parkfield‐Cholame (California) earthquakes a creepmeter was installed across the fresh surface break of the San Andreas fault at Carr Ranch (now part of the Jack Ranch), 10 km southeast of Parkfield. It has recorded continuing slip which since the end of 1968 has occurred at about 10 mm yr−1, occurring primarily in discrete events at intervals of a few months. In April 1976 an array of four strainmeters was installed near this creepmeter at distances between 0.2 and 2.2 km from the fault to detect the elastic strain fields associated with creep events on the fault. Four similar sets of signals have since been observed on the strainmeter array, separated by intervals of 4 to 5 months. A week after three of these events, creep began at the Carr Ranch creepmeter, but no signals were detected by the strainmeters while the creepmeter was recording slip. Analysis of the strainmeter signals shows that they can be modeled by a slip zone on the fault 640 m long and extending from 30 to 510 m in depth, with right lateral slip of about 3.5 mm. Propagation of a dislocation from the lower northwest corner of the slip zone to the upper southeast corner is indicated. The phenomenon may be due to an asperity or an area of higher friction on the fault at Carr Ranch, which is repeatedly loaded to failure by steady slip on the fault around it. This is the first report of a well‐defined fault creep event which repeats itself with a high degree of similarity and which has been observed at a distance of over 2 km from the fault.
Synthetic seismograms from model ocean bottoms
Authors: Spudich P. K. P. , Helmberger Donald V.
Published: February 2008

In this work, plausible ocean bottom structures have been represented by stacks of homogeneous, isotropic, nonabsorbing, plane horizontal layers. Generalized ray theory was used to calculate the seismic responses of these bottom models in a frequency band of 1–30 Hz for a receiver near the ocean's surface and located 2–20 km away from a sound source also near the ocean's surface. The models' responses depend most strongly upon the velocity structure of the basement rock and on the presence or absence of a zone of partial lithification between unconsolidated sediments and basement. The thickness and velocity gradients in the unconsolidated sediments have a smaller effect on the models' seismic responses. Joint consideration of both basement head wave and reflection amplitudes constrains basement velocity structure much more than separate consideration of either phase, thus emphasizing the importance of accurate recording and careful analysis of bottom reflections as well as refracted arrivals in marine refraction profiles. The remainder of the step responses and seismograms are available on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J79‐001; $1.00. Payment must accompany order.
Effect of sample quantity on the results of ferromagnetic resonance studies of lunar samples
Authors: Goldberg I. B. , Crowe H. R. , Housley R. M. , Cirlin E. H.
Published: February 2008

Quantitative ferromagnetic resonance (FMR) measurements at 9.5 and 35 GHz were made on lunar fines in order to determine the amount of single domain Fe0 and to ascertain the effect of sample quantity on the results of these experiments. Samples containing more than 10 μg of Fe0 in an X‐band TE104 cavity, or more than 0.11 μg in a Kα‐band TE011 cavity can cause errors greater than 1% in the quantitative determination of Fe0, as well as cause anomalous broadening of the FMR line. Theoretical calculations were derived to show this effect which results from a nonlinear response of the FMR experiment with the sample quantity. Using samples of nominal weights, the amount of Fe0 determined at 35 GHz was 1.2–1.8 times greater than at 9.5 GHz, while the linewidths increased by a factor of 1.2. As the particle size increases, the ratio of Fe0 determined at 35 GHz to that determined at 9.5 GHz increases, suggesting that the additional iron is due to larger spheroidal particles which change from multidomain to single domain at the larger fields.
Evaluation of the geocentric gravitational constant from Viking Doppler and range data
Authors: Esposito Pasquale B.
Published: February 2008

During the launch and near‐earth phase, continuous radio tracking data were acquired from two Viking spacecraft as they receded from earth. Analysis of this data yielded these values for the geocentric gravitational constant GM: 398600.5±0.1 km3 s−2 for Viking 2. These values include the atomosphere of the earth and are consistent with the currently adopted speed of light (299792.458 km s−1).
Preliminary petrological study of the lower metadiabase from the basement of the Michigan Basin
Authors: Honnorez Jose
Published: February 2008

During the drilling of the McClure‐Sparks well, in the Michigan Basin, two metadiabase units were encountered at about 5.3‐km depth, after boring through a sequence of clastics underlying the Cambrian Mt. Simon sandstone. These metadiabases belong to the basement of the Michigan Basin and are probably late Precambrian in age and therefore contemporaneous with the basalts and the gabbros of the Keweenawan Peninsula, Michigan. No evidence exists of the intrusive or extrusive nature of the diabases. The presence of igneous titaniferous clinopyroxene denotes the alkaline affinity of the original diabase. The six studied samples from the lower metadiabse unit display metamorphic mineral parageneses which are characteristic of the lower greenschist facies. The remarkable freshness of the igneous clinopyroxene could not be satisfactorily explained. The chemical analysis of one noncataclastic metadiabase appears to indicate that the rock has been enriched in MgO and depleted in CaO during the metamorphism when compared with the average Keweenawan basalt. The scarcity of sheared zones and deformation features indicates that the pressure during the metamorphic process was essentially hydrostatic. Temperatures not in excess of 400°C and pressures ranging from less than 2 to 7 kbar probably prevailed during the formation of the lower metadiabase. These physical conditions correspond to depth in the continental crust of 6–20 km. The absence of secondary minerals corresponding to the 125°C temperature measured at the bottom of the well and the lack of retrograde alteration imply that the paragenesis of the metadiabase is a metastable assemblage.
Utilization of satellite‐satellite tracking data for determination of the geocentric gravitational constant GM
Authors: Martin C. F. , Oh I. H.
Published: February 2008

Range rate tracking of Geos 3 through the ATS 6 satellite has been used, along with ground tracking of Geos 3, to estimate the geocentric gravitational constant. Using multiple half‐day arcs, a GM of 398,600.36±0.12 km3/s2 was estimated using the GEM 10 gravity model, based on a speed of light of 299,792.458 km/s. Tracking station coordinates were simultaneously adjusted, leaving geopotential model error as the dominant error source. Base lines between the adjusted NASA laser sites show better than 15‐cm agreement with multiple short arc Geos 3 solutions.
Extraction of ocean wave height and dominant wavelength from Geos 3 altimeter data
Authors: Walsh Edward J.
Published: February 2008

The Geos 3 satellite radar altimeter has the capability to provide accurate, low‐noise measurements of the significant wave height of ocean waves through their effect on the shape of the return pulse. When the amplitude and timing biases are removed from the Geos 3 sample and hold (SH) gates, the mean return wave forms can be excellently fitted with a theoretical template which represents the convolution of the radar point target response, the range noise (jitter) in the altimeter tracking loop, the sea surface height distribution, and the antenna pattern as a function of the range to mean sea level. In computing the wave height, tracking loop jitter may be removed by realigning the SH gates to their actual positions with respect to mean sea level before averaging, by using the observed standard deviation on the altitude measurement to remove the integrated effect of the tracking loop jitter, or by using a lookup table to correct for the expected value of range noise. Analysis of skewness in the Geos return wave form demonstrates the potential of a satellite radar altimeter to determine the dominant wavelength of ocean waves. When the jitter in the Geos range tracker is large, a significant reduction in range noise can be achieved through reprocessing the data. However, when the tracker noise is small, only marginal improvement is possible.
soil transport by winds on Mars
Authors: White Bruce R.
Published: February 2008

The eolian transport of surface material on the planet Mars is estimated from results of low‐pressure wind tunnel testing and theoretical considerations. A semiempirical relation is developed that will estimate the total amount of surface material moving in eolian saltation, suspension, and surface traction. The estimated total mass movement of surface material per unit width time on the surface of Mars is q = 2.61ρ(V* ‐ V*t)(V* + V*t)2/g (g/cm s), where ρ is the density of the atmospheric gas, g is the acceleration due to gravity, and V* and V*t are the friction speed and saltation threshold friction speed, respectively. A flat surface composed of particles of nearly uniform size is assumed. A change in the mean particle size changes the threshold friction speed V*t. The path lengths of saltating particles and wavelengths of surface ripples can vary as much as a factor of 2 if the surface temperature varies from 150 to 250 K. The angles between particle paths and the horizontal surface are calculated to be lower on Mars than on earth, and particles travel much faster on Mars than on earth. The ratio of final particle velocity to threshold friction speed, VF/V*t, is found to be several times that of saltation on earth.
Noble gases in Allende minerals: Reply to Manuel's Critique
Authors: Lewis Roy S. , Gros Jacques , Anders Edward
Published: February 2008

Manuel's interpretation of our data is refuted by direct experimental evidence. When phase Q is etched away, the 20Ne/22Ne ratio consistently falls (by up to 4 σ for Allende, more for some other meteorites), showing that Q contains a substantial Ne component of characteristic 20Ne/22Ne ratio (10.3±0.4). A substantial heavy Xe isotope enrichment is also present in Q, as shown by the fall in 136Xe/132Xe on stepped etching: 0.325 ± 0.004 in the first step, 0.310±0.003 in the second step.
Relative plate motions and the formation of marginal basins
Authors: Jurdy Donna M.
Published: February 2008

The location and timing of activity in marginal basins are considered in the context of relative plate motions. The motion of the Pacific and Farallon plates are reconstructed relative to the overriding Eurasian and South American plates for two models: Antarctica as one and as two plates (converging over the interval from 59 to 36 m.y. B.P.). The two‐plate model (the more consistent) indicates that over the last 80 m.y. the motion of the Pacific plate relative to Eurasia was first westward (80–59 m.y. B.P.), then northward (59–36 m.y. B.P.), and then westward again (36 m.y. B.P. to present). For those Pacific marginal basins where magnetic anomalies have been identified, there is a pattern related to the direction of motion of the Pacific. Those basins active during the time period of northward motion of the Pacific opened in a north‐south direction, and most of those which were active during the recent time period of westward motion of the Pacific opened a east‐west direction. No basins were active during the time of change of direction. This strong correlation between the opening of marginal basins and the relative motions of the converging plates suggests that changes in these relative motions are a factor in the episodicity of marginal basins.
Lithospheric structure of the Walvis Ridge from Rayleigh wave dispersion
Authors: Chave Alan D.
Published: February 2008

Rayleigh wave group velocity dispersion has been used to study the lithospheric structure along the Walvis Ridge and for a nearby South Atlantic path. Group velocity is anomalously low in the period range of 15–50 s for both surface wave paths. Results of a formal inversion for the ridge suggest crustal thickening to 12.5 km and anomalously low mantle shear velocity of 4.25–4.35 km/s to depths of 45 km. Lowering the density in this region during inversion does not raise the shear velocity to the oceanic norm. A nearby off‐ridge path that covers the Cape Basin and part of the western Walvis Ridge shows no sign of thickened crust. No significant differences from normal oceanic lithosphere exist below 50 km, and no signs of thinning of the lithosphere under the Walvis Ridge are apparent. Other geophysical data rule out a thermal cause for the low mantle shear velocity, and it is likely that unusual mantle composition is responsible.
The influence of ozone on Martian atmospheric temperature
Authors: Kuhn W. R. , Atreya S. K. , Postawko S. E.
Published: February 2008

Radiative equilibrium temperature calculations indicate that ozone, although a minor constituent in the Martian atmosphere, may play a significant role in determining the deposition of carbon dioxide.
Color changes at the Viking landing sites over the course of a Mars year
Authors: Guinness E. A. , Arvidson R. E. , Gehret D. C. , Bolef L. K.
Published: February 2008

There have been significant color changes at both Viking landing sites over the past Mars year. Both sites exhibited an increase in the red to blue ratio of the soil, in addition to a loss of contrast between soil units and a general brightening. These changes are due to the accumulation of a thin layer of bright red dust from the two global dust storms that were observed over the past Mars year. The dust layer appears to be thicker and more continuous at the Viking Lander 2 site, perhaps because condensation of CO2 and H2O around soil nuclei significantly increased the probability of depositing soil particles onto the surface. Such an interpretation is consistent with the reddish tint exhibited by the condensates seen at Viking Lander 2. Generation of bright areas on Mars appears to occur by blanketing with a thin layer of bright red dust. Formation of dark areas by stripping of bright dust layers would appear to be controlled in part by the distribution of roughness elements capable of disturbing the atmospheric flow field. Such an interpretation is supported by the lander observations before the dust storm activity, which show a number of isolated bright red drifts that may be regions where material deposited during previous storms was protected from wind erosion. Finally, both aeolian erosion and deposition of material not directly connected with the global dust storms have been observed over the past Mars year. Thus aeolian processes are operative at times other than during global storms.
Viking Magnetic Properties Experiment: Extended mission results
Authors: Hargraves R. B. , Collinson D. W. , Arvidson R. E. , Cates P. M.
Published: February 2008

During the Viking extended mission the following were performed: (1) The backhoe magnets on Viking Lander (VL) 2 were successfully cleaned, followed by a test involving successive insertions of the cleaned backhoe into the surface. Rapid saturation of the magnets confirmed evidence from primary mission results that the magnetic mineral in the Martian surface is widely distributed, most probably in the form of composite particles of magnetic and nonmagnetic minerals. (2) An image of the VL 2 backhoe taken via the ×4 magnifying mirror demonstrates the fine‐grained nature of the attracted magnetic material. (3) The reference test chart magnet on VL 1, and by inference also on VL 2, continued to attract magnetic particles from the Martian atmosphere. The new results generally are in agreement with previous estimates of 1–7% of magnetic mineral, probably maghemite, in the surface material. The presence of maghemite and its occurrence as a pigment in, or a thin coating on, all mineral particles or as discrete, finely divided and widely distributed crystallites, are consistent with data from the inorganic analysis experiments and with laboratory simulations of results of the biology experiments on Mars.
Paleomagnetism of ash‐flow tuffs: Microanalytical recognition of TRM components
Authors: Geissman John Wm.
Published: February 2008

A microanalytical technique, employing a cryogenic magnetometer and progressive alternating field demagnetization, is described and used to investigate the natural remanent magnetization of individual components (groundmass, crystals, and lithic and pumice fragments) in seven ash‐flow tuff lithologies and to determine if subblocking temperature flow‐or compaction‐related movements took place in the units on scales ranging from the volume of the individual microspecimen (1–2 mm in diameter) to that of a standard paleomagnetic core specimen. Closely grouped directions from microspecimens of vitrophyre (α95′s = 4.9° ‐ 12.4° ) and close correspondence of these directions with their site mean directions determined from standard samples suggest that the magnetic components of these units have not been disturbed by movements at low temperatures. Microspecimens from devitrified portions of the cooling units are of significantly lower coercivity than vitrophyre material and yield directions either more scattered about their site mean (α95 greater than 15° ) or uniformly displaced in one direction (up to 35°) away from the mean. Devitrification may be as significant a contributor to the scatter of microspecimen directions as are postblocking temperature randomizing or systematic microtectonic movements. Vitrophyres near the base and extremities of ash‐flow tuff cooling units are probably the most accurate indicators of the paleofield in each cooling unit.
Effects of nondipole components on auroral zone configurations during weak dipole field epochs
Authors: Siscoe George L. , Sibeck David G.
Published: February 2008

Paleomagnetic studies indicate that the present and historical nondipolar fields are typical in some respects to the nondipolar component of the paleofield generally. We apply this principle to the investigation of the global distribution of the auroral zones during epochs of reduced dipole field strength. The poleward borders of the auroral zones map out along field lines to a distance where the dipolar term dominates the nondipolar terms even for reductions to 10% of the present dipole strength. Thus the dipole scaling relations that describe the interaction with the solar wind can be used to map out the contour of the polar borders that occur when the dipole strength is reduced while the nondipolar terms are held fixed. The results show that the nondipolar terms produce elongated auroral zones that extend to appreciably lower latitudes than are reached when the nondipolar terms are ignored.
Characterization of a penny‐shaped reservoir in a hot dry rock
Authors: Sekine H. , Mura T.
Published: February 2008

The mechanical stability of a penny‐shaped reservoir is characterized by fracture mechanics including thermoelastic effects in connection with research into the extraction of geothermal energy from hot dry rocks. The condition for stability of a reservoir, which is not changing radius by propagating or closing, requires 0 < k < kc, where k and kc are the stress intensity factor and the fracture toughness of the rock. From this condition the upper and lower critical flow rates m* and m*, necessary for reservoir stability, are obtained in terms of the outlet fluid pressure on the earth surface, the fluid temperatures at the inlet and outlet, the geometry of the reservoir, and the physical constants of the fluid and the rock. The behavior of a reservoir when the inlet flow is changed also is studied. According to the value of the outlet flow rate m0, the following three cases are considered: case 1; m0 = m*; case 2; m* > m0 > m*; and case 3; m0 = m*.
Viscous flow and crystallization behavior of tektite glasses
Authors: Klein L. C. , Yinnon H. , Uhlmann D. R.
Published: February 2008

The variation of viscosity with temperature has been determined over a wide range of temperature for a Muong Nong tektite material. The viscosity at the liquidus temperature of 1320°C is about 2×104 P. Crystallization heat treatments in the range of temperature between 900° and 1300°C does not produce significant crystallization in the natural sample except when the sample is heated in contact with a synthetic tektite composition. In this case, cristobalite crystals grow from the contact surface into the natural sample. For comparison, two synthetic microtektite compositions with lower SiO2 contents than the Muong Nong material were also examined. These compositions could not be formed free of crystals in sizes as large as the Muong Nong sample. Heat flow calculations were performed for 2.5 cm and 10 cm spheres of tektite material cooling by radiation to their surroundings. The combined results of the crystallization study and the heat flow calculations indicate that the Muong Nong material is an excellent glass former and can be formed as a glass in bodies even larger than the natural samples. The results also indicate that the microtektite compositions require faster cooling rates to be formed as glasses than the Muong Nong material and that nucleation behavior is very likely important in determining the size of bodies found as tektites.
High‐pressure stability of a dense hydrous magnesian silicate Mg23Si8O42H6 and some geophysical implications
Authors: Akaogi Masaki , Akimoto Syun‐Iti
Published: February 2008

Phase equilibria in the system MgO‐SiO2‐H2O have been studied in the pressure range 85–160 kbar in the temperature range 750°–1200°C. In runs for a gel with composition of 64.1 wt % MgO, 31.9 wt % SiO2, 4.0 wt % H2O (atomic ratio Mg/Si = 3.0) a hydrous magnesian silicate, denoted phase B by Ringwood and Major, was found to exist from about 100 kbar to at least 160 kbar at temperatures 750°–1200°C. At pressures be1ow about 100 kbar the assemblage of phase A(Mg7Si2O14H6) + forsterite + periclase was stable at relatively low temperatures, while the assemblage of chondrodite (Mg5Si2O10H2) + periclase + forsterite was stable at relatively high temperatures. X ray diffraction shows that phase B is monoclinic (space group, P21/c) with cell constants a = 10.600(2) Å, b = 14.098(2) Å, c = 10.092(2) Å, and β = 104.05(2)°. The chemical formula of phase B was deduced to be Mg23Si8O42H6. The observed density, 3.32±0.04 g/cm3, is the highest of the hydrous silicate phases reported in the system MgO‐SiO2‐H2O and is about 3% higher than that of forsterite. While this study indicates the stability of phase B for bulk compositions which are richer in MgO than accepted mantle models, it is regarded as a potential hydrous silicate phase in the upper mantle and transition zone in the depth range of 300 km to at least 460 km.
Phase boundary and transition rate of orthorhombic‐cubic transformation in PbO2
Authors: Yagi Takehiko , Akimoto Syun‐Iti
Published: February 2008

The orthorhombic‐cubic phase transformation in PbO2 was investigated employing cubic‐anvil‐type high‐pressure apparatus combined with an energy dispersive‐type X ray diffraction system in the pressure‐temperature region to 84 kbar and 600°C. In situ X ray observations confirmed the stability of both the orthorhombic columbite‐type and the cubic fluorite‐type polymorphs of PbO2. Considering the rate of transformation, the equilibrium phase boundary between PbO2II (orthorhombic structure) and PbO2III (cubic fluorite structure) was determined as P (kbar) = 69 ‐ 0.019T(°C). A negative slope was established.
Mineral paragenesis in Franciscan metagraywackes of the Nacimiento Block, a subduction complex of the Southern California Coast Ranges
Authors: Ernst W. G.
Published: February 2008

Geologic‐petrographic reconnaissance of Franciscan metagraywackes from the Nacimiento block of the southern California Coast Ranges has demonstrated a perceptible eastward increase in metamorphic grade. Most feebly recrystallized zone I metaclastic rocks bordering the Pacific Ocean are rich in calcite veins and patches and contain no new Ca‐Al hydrous silicates; abundant unstable detritus includes lithic fragments, biotite flakes, and epidote granules. Metasedimentary rocks of medial zone II are characterized by fine‐grained intergrowths of pumpellyite ±5 minor regenerated epidote‐clinozoisite. Easternmost zone III metasandstones contain lawsonite, rarely aragonite and/or jadeitic pyroxene. In general, proportions of calcite, rock fragments, epidote, and biotite decrease eastward, and white mica, chlorite, and stilpnomelane increase. On the basis of experimental phase equilibrium data, temperatures of 150±50°C are estimated to have attended recrystallization, with lithostatic pressures increasing eastward from about 2–3 kbar on the west to approximately 5 kbar, and locally approaching 8 kbar, on the east. Absence of zeolites in the lowest grade zone I metagraywackes may indicate dilution of the aqueous fluid phase by CO2 during the metamorphism. In combination with the northwest tectonic ‘grain’ of the Nacimiento block and its inferred imbricate structure, the assemblage of rock types, the general north‐eastward dip of bedding, and predominantly right‐side‐up nature of the section, this continental margin deposit is thought to have been recrystallized during a period of relatively rapid (and perhaps brief) Late Cretaceous plate convergence. The lack of eclogite and amphibolite as tectonic blocks may indicate less profound depths of recovered subduction compared to Franciscan basement lying northeast of the San Andreas fault. Metamorphosed Nacimiento lithic belts evidently were assembled over a brief period of subduction offscraping at the western edge of the American lithospheric plate and seem to mirror events which unfolded over a longer time interval in Franciscan rocks of the northern California Coast Ranges.
Geodesy by radio interferometry: Intercontinental distance determinations with subdecimeter precision
Authors: Herring T. A. , Corey B. E. , Counselman C. C. , Shapiro I. I. , Rönnäng B. O. , Rydbeck O. E. H. , Clark T. A. , Coates R. J. , Ma C. , Ryan J. W. , Vandenberg N. R. , Hinteregger H. F. , Knight C. A. , Rogers A. E. E. , Whitney A. R. , Robertson D. S. , Schupler B. R.
Published: February 2008

Analysis of very‐long‐baseline interferometer (VLBI) observations yielded estimates of the distances between three radio telescopes in the United States and one in Sweden, with formal standard errors of a few centimeters: Westford, Massachusetts‐Onsala, Sweden: 5,599,714.66±0.03m; Green Bank, West Virginia‐Onsala, Sweden: 6,319,317.75±0.03m; and Owens Valley, California‐Onsala, Sweden: 7,914,131.19±0.04m, where the earth‐fixed reference points are defined in each case with respect to the axes of the telescopes. The actual standard errors are difficult to estimate reliably but are probably not greater than twice the formal errors.
Gravity profiles across the Samail Ophiolite, Oman
Authors: Manghnani Murli H. , Coleman Robert G.
Published: February 2008

Four hundred and sixty gravity stations have been established over the Samail ophiolite and adjacent units exposed in the Oman Mountains. The Bouguer anomaly pattern follows more or less parallel to ophiolite exposures. Along the coastal areas the Bouguer anomaly values range from +20 to +140 mGal and increase northward. Four gravity profiles across the ophiolite are correlated with known geology and structure. The main gravity profile extending 325 km northward from Ibra to Muscat and into the Gulf of Oman shows two positive Bouguer anomalies (+85 over the southern folded ophiolite nappe and +110 mGal near the shoreline at Muscat) that are associated with two nappes of tabular form. The nappes are nearly continuous, and there is no evidence that they are rooted in the mantle. The gravity high across the southern nappe is centered over the axis of the synform (maximum nappe thickness); its position over the gabbro‐peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite. At the southern end of the profile, in the Ibra Dome area, the negative Bouguer anomaly (−10 to −15 mGal) is interpreted as an Infracambrian salt piercement similar to those in the Zagros fold belt. Geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units. The nature of the gravity anomaly across the continental margin indicates a transition from ∼42‐km‐thick continental crust to a thinner (16–18 km) oceanic crust. The two gravity profiles in the north Oman region show a doublet of positive residual Bouguer anomalies (+50 to +90 mGal). The anomalies correlate well with imbricated ophiolite exposures, a result of low‐angle thrusting caused by strong compressive forces. In the absence of imbricate thrusting, the gravity profile in the central region has only one large positive residual Bouguer anomaly (+130 mGal). The anomalies of all the four profiles are further analyzed to elucidate the nappe‐transport distances.
Lunar seismology: The internal structure of the Moon
Authors: Goins N. R. , Dainty A. M. , Toksöz M. N.
Published: February 2008

Tables A1–A4 and Figures A1–A3 are available with entire articleon microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐003;$1.00. Payment must accompany order.

The arrival times of direct P and S waves, measured on seismograms recorded from natural lunar seismic events, have been analyzed using linearized inversion and parameter search methods to simultaneously determine event locations, origin times, and structural parameters (seismic velocities). Polarization‐filtered record section plots are correlated with theoretical travel time curves to identify later phases and obtain additional structural and velocity information. Shear wave amplitudes plotted as a function of distance provide data on the existence and magnitude of seismic velocity gradients in the interior. These studies are used to delineate the structure of the moon below the crust to a depth of about 1100 km. This structure has been divided into an upper and a lower mantle. The upper mantle, from 60‐ to 400‐km depth, has average seismic wave velocities of Vp = 7.70 ±0.15 km/s and Vs = 4.45 ± 0.05 km/s with corresponding Q values (taken from the literature) of Qp ∼ 5000 and Qs ∼ 3000. The shear wave velocity decreases with a negative gradient of at most −6 × 10−4 km/s/km, implying a Vs variation of from 4.57 km/s at 60‐km depth to 4.37 km/s at 400‐km depth; this decrease can be accounted for by increasing temperature, and so no major compositional gradient is required in the upper mantle. A small negative P wave velocity gradient may also be present. Between 400‐ and 480‐km depth there is a transition zone with a sharply decreasing shear wave velocity and possibly an accompanying small decrease in Vp. The dominant shear velocity decrease may occur at a 480‐km interface and may represent a compositional change, although the effects of increased temperature cannot be totally ruled out. The lower mantle extends from 480‐km depth to at least 1100‐km depth; few seismic waves recorded by the lunar network penetrate below 1100 km. The average velocities are Vp = 7.6 ± 0.6 km/s and Vs = 4.2 ± 0.1 km/s. Q values (taken from the literature) are Qp ∼ 1500 and Qs ∼ 1000. Below 1100 km there is some indication that the attenuation may increase still further for shear waves, with Qs dropping to a few hundreds or less. This seismic model of the moon is well constrained, with uncertainties on the above values given explicitly by the analysis methods, and so it serves as a strong control on the present‐day lunar compositional and thermal structure.
Modifying Stokes' function to reduce the error of geoid undulation computations
Authors: Jekeli Christopher
Published: February 2008

A comparison is made of the errors in determining the geoid undulation according to the conventional Molodenskii truncation theory and its modifications as suggested by Molodenskii (1958) and Meissl (1971). The undulation is assumed to be computed from gravity anomalies in a spherical cap and a finite set of harmonic potential coefficients (e.g., GEM 9 or GEM 10B). Only the effects of errors in the coefficients, as well as the neglect of higher degree coefficients, are considered. It is found that the error estimates (rms values) are significantly lower for any of the modified methods than for the conventional method. For example, errorless and continuous gravity data in a cap with 10° radius and the GEM 9 coefficients together yield undulations with an average error of 109 cm if the unmodified truncation theory is applied and 41 cm and 33 cm with Meissl's method and a Molodenskii type of modification, respectively.
Numerical experiments on 18O depletion in igneous intrusions cooling by groundwater convection
Authors: Parmentier E. M.
Published: February 2008

The intrusive and host rocks of epizonal igneous intrusions cooled by groundwater convection are depleted in 18O by oxygen exchange with low‐18O meteoric waters. This study describes numerical experiments on the hydrothermal convective cooling and 18O depletion of a family of simple two‐dimensional model intrusions. Cooling and depletion histories are calculated on the basis of conservation laws for mass, thermal energy, and 18O atoms along with Darcy's law to describe flow in permeable rock and a kinetic description of water‐rock oxygen exchange. Exchange is described on the basis of the measured oxygen diffusivity in feldspar which is large compared to that in other rock‐forming minerals. The resulting equations are solved using finite difference approximations. The numerical experiments examine the effect of intrusion size, intrusive and host rock permeabilities, and host rock thermal environment on the 18O depletion that develops as an intrusion cools and show that the depletion patterns in a cooled intrusion are sensitive to parameters describing the intrusion and its environment. The dependence of several general characteristics of these patterns on the model parameters is described. Depletions obtained from the sampling of natural intrusions are discussed in the context of this simple family of models.
Constraints on water transport and alteration in the oceanic crust from the isotopic composition of pore water
Authors: Lawrence J. R. , Gieskes J. M.
Published: February 2008

The oxygen isotopic compositions of pore fluids from sediments of older oceanic crust commonly decrease with depth. Increases in calcium ion concentrations always accompany the 18O/16O decreases.These changes are attributed principally to the alteration of basalts or alteration of volcanic ash in the sediments. Oxygen isotope material balance calculations suggest that 5–20% of the upper kilometer of basalt is altered at the time of subduction of the oceanic crust. This corresponds to a flux of 18O into the oceanic crust of 2.3 × 1015 moles per million years. Evaluation of the oxygen and hydrogen isotope data from the pore fluids also suggests that the principal process of mass transfer in the sediments of older oceanic crust is diffusion. Convection of water in the basalts of the oceanic crust appears to continue until subduction.
Interpretation of changes in water level accompanying fault creep and implications for earthquake prediction
Authors: Wesson Robert L.
Published: February 2008

Quantitative calculations for the effect of a fault creep event on observations of changes in water level in wells provide an approach to the tectonic interpretation of these phenomena. For the pore pressure field associated with an idealized creep event having an exponential displacement versus time curve, an analytic expression has been obtained in terms of exponential‐integral functions. The pore pressure versus time curves for observation points near the fault are pulselike; a sharp pressure increase (or decrease, depending on the direction of propagation) is followed by more gradual decay to the normal level after the creep event. The time function of the water level change may be obtained by applying the filter—derived by A. G. Johnson and others to determine the influence of atmospheric pressure on water level—to the analytic pore pressure versus time curves. The resulting water level curves show a fairly rapid increase (or decrease) and then a very gradual return to normal. The results of this analytic model do not reproduce the steplike changes in water level observed by Johnson and others. If the procedure used to obtain the water level from the pore pressure is correct, these results suggest that steplike changes in water level are not produced by smoothly propagating creep events but by creep events that propagate discontinously, by changes in the bulk properties of the region around the well, or by some other mechanism. In addition, simplistic calculations show that significant pressure field variations and water level changes near the surface may be expected to accompany a propagating creep event on a buried fault. Water level changes of as much as several meters may be expected at the surface for a creep event having a dislocation amplitude of l m on a semi‐infinite dislocation surface extending downward from a depth of 10 km and propagating horizontally at a rate of 10 km/day. The maximum near‐surface effect should be observed at a horizontal distance from the fault about equal to the depth of the top of the dislocation surface. These results are consistent with the observations made in China of large water level changes preceding large earthquakes, if some sort of aseismic creep event at depth precedes these earthquakes.
Gravity and thermal models for the Twin Peaks Silicic Volcanic Center, southwestern Utah
Authors: Carrier Daniel L. , Chapman David S.
Published: February 2008

Gravity, heat flow, and surface geology observations have been used as constraints for a thermal model of a late Tertiary silicic volcanic center at Twin Peaks, Utah. Silicic volcanism began in the area with the extrusion of the Coyote Hills rhyolite 2.74 ± 0.1 m.y. ago, followed by the Cudahy Mine obsidian, felsite, and volcanoclastics, and finally by a complex sequence of domes and flows that lasted until 2.3 ± 0.1 m.y. ago. Basalt sequences span the time 2.5 to 0.9 m.y. Terrain‐corrected Bouguer gravity anomalies at Twin Peaks are shaped by three features of varying characteristic dimensions: (1) a major north‐northeast trending −30 mGal gravity trough roughly 40 km wide caused by a thick sequence of Cenozoic sediments in the Black Rock Desert Valley, (2) a local roughly circular −7 mGal gravity low, 26 km across, probably related to an intrusive body in the basement, and (3) a series of narrow positive anomalies up to +10 mGal produced by the major Twin Peaks volcanic domes. The intrusive bodies have been modeled as three‐dimensional vertical cylinders; the total volume of intrusive material is estimated to be about 500 km3. Simple thermal models, assuming conductive heat transfer and using geometrical constraints from the gravity results, predict that a negligible thermal anomaly should exist 1 m.y. after emplacement of the intrusion. This prediction is consistent with an average heat flow of 96 mW m−2 for the area, not significantly different from eastern Basin and Range values elsewhere. Magmatic longevity of this system, 2.7 to 2.3 m.y. for silicic volcanism or 2.5 to 0.9 m.y. for basaltic volcanism, does not seem to prolong the cooling of this system substantially beyond that predicted by conductive cooling.
Rotation of the inner core
Authors: Gubbins D.
Published: February 2008

We cannot observe the magnetic field inside the earth's core directly, but there is likely to be a large toroidal part of 10–100 Gauss which, together with the dipole component, could produce a magnetic torque on the inner core that tends to rotate it. Estimates based on dynamo calculations give torques of 1019 N m which is large enough to accelerate the inner core to the westward drift velocity of 0.2° per year within a few days. Presumably some equilibrium has been reached in which the inner core rotates with constant angular velocity and experiences no net torque. This rotation should have significant consequences for dynamo calculations because it is a very effective method of stretching field lines, and its helps to drive differential rotation in the liquid outer core. The core is modeled by two solid spheres permeated by a uniform field of 5 Gauss representing the dipole. The inner sphere is free to rotate relative to the outer sphere. When a torque of 1019 N m is applied to the inner sphere it reaches equilibrium with a steady angular velocity corresponding to a rotation period of 2300 years, which is similar to the westward drift speed, and a toroidal field of about 100 Gauss is induced. The inner sphere can also undergo toroidal oscillations with a period of about 10 years which may be related to the observed secular variation.