bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction to the Special Section on Physical and Biogeochemical Processes Responsible for the Magnetization of Sediments
Authors: Lund Steve P. , Karlin Robert
Published: February 2008

Sediment palcomagnetism has made important contributions to magnetostratigraphy, tectonic reconstructions, and studies of geomagnetic field behavior. Interpretations of the sedimentary palcomagnetic record depend critically on when and how the natural remanent magnetization (NRM) is acquired and what postdepositional modifications have occurred. This special section of the Journal of Geophysical Research is a collection of 16 papers that address recent advances in our understanding of the physical, chemical, and biological processes which affect the acquisition and alteration of the NRM in sediments. The issue is an outgrowth of a special symposium on magnetization processes in sediments that was convened at the 1987 Spring AGU Meeting in Baltimore, Maryland.
Use of anisotropy to determine the origin of characteristic remanence in the Siwalik red beds of northern Pakistan
Authors: Tauxe Lisa , Constable Cathy , Stokking Laura , Badgley Cathy
Published: February 2008

It is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone. However, anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains; this in turn may reflect the development of the magnetic fabric of the sediment over time, so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence. In this work, we make a study of the anisotropy of magnetic susceptibility (AMS) in Siwalik red beds of Miocene age from northern Pakistan. Comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it, a coherent magnetization. Furthermore, it should be possible to use AMS fabric information to quantify the degree of pedogenesis in these Miocene soils. We attempted to determine the anisotropy of isothermal remanence (AIR) but found AMS to be the technique of choice because of apparent changes in coercivity during AIR experiments. We interpret the AIR data as resulting from metastable domains in hematite grains which change domain state during the AIR experiment.
Paleomagnetism of eocene volcanic rocks, Talkeetna Mountains, Alaska
Authors: Panuska Bruce C. , Stone David B. , Turner Donald L.
Published: February 2008

Previous paleomagnetic studies of Eocene rocks deposited in the Talkeetna Mountains on the northern edge of the combined Peninsular, Wrangellia, and Alexander terranes (the southern Alaska superterrane (SAS)) yield a paleolatitude of 76°N, while Late Cretaceous rocks on the southern edge of the SAS record a 32°N paleolatitude. At face value, these drastically different paleolatitudes imply that Wrangellia moved 44° to the north in only 20 m.y., requiring a northward component of velocity of about 24 cm/yr. Alternatively, the discrepancy might be explained by postulating an unrecognized tectonic boundary separating the two localities, allowing different emplacement ages. We have tested the unrecognized tectonic boundary hypothesis (allowing post‐Eocene emplacement of the SAS) by sampling Eocene volcanic rocks deposited near the southern edge of the SAS in the Talkeetna Mountains. Our data set comprises measurements from 97 oriented cores collected from 26 lava flows and two tuffs distributed over a distance of 50 km. A primary remanence is indicated by positive fold tests and penecontemporaneous intrusion tests. Age control is provided by nine new K‐Ar whole rock age determinations ranging from 38.8 to 53.6 Ma. The mean paleomagnetic pole calculated for this study is not significantly different from the pole derived from the Eocene flows to the north and gives a paleolatitude of 78°, thus ruling out the hypothesis of an unrecognized suture accommodating major post‐Eocene displacement. This result has prompted a reevaluation of the data which suggest rapid movement, including the precision of the age control, the adequacy of averaging of secular variation, and the possibility of sedimentary inclination error. The bulk of the evidence suggests northward displacement from lower latitudes; however, the error limits are such that the dramatic velocities previously postulated are not required.
The Magellan Venus Radar Mapping Mission
Authors: Saunders R. S. , Pettengill G. H. , Arvidson R. E. , Sjogren W. L. , Johnson W. T. K. , Pieri L.
Published: February 2008

The NASA Magellan Venus Radar Mapper spacecraft was launched into an interplanetary transfer trajectory to Venus on May 4, 1989, and will be placed into orbit around Venus on August 10, 1990. The orbiter carries a 12‐cm‐wavelength, multimode radar system. In the synthetic aperture mode it is capable of imaging most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode it will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter where the surface relief is not too extreme. In the radiometer mode the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. Tracking of the orbiter's coherent radio telemetry transmitter will permit observations of small accelerations related to gravitational inhomogeneities in the planet's interior. Objectives of the mission include deducing the geological history of the surface and the geophysical state of the interior. Specific attention focuses on the origin, present distribution, and activity of four geological processes that modify the surface: (1) volcanic and tectonic; (2) impact; (3) erosional, depositional, and chemical; and (4) isostatic and convective. Data products resulting from the mission will be made available to members of the scientific community through NASA's Planetary Data System.
Folded Brookian Thrust Faults: Implications of three geologic/geophysical transects in the Western Brooks Range, Alaska
Authors: Karl Susan M. , Long Carl L.
Published: February 2008

Three geologic/geophysical transects in the Baird Mountains quadrangle in the western Brooks Range, Alaska, provide important insights on the nature of major structures in the area. Independently constructed geologic and geoelectric cross sections show strong correspondence between modeled audiomagnetotelluric resistivity data and mapped surface and inferred subsurface geologic structures. Major features reinforced by this correspondence include (1) the great thickness of the Maiyumerak mafic igneous rocks, (2) the north‐northwestward vergence of the duplex involving sedimentary rocks of the Baird, Endicott, and Lisburne groups, (3) folding and steep normal fault truncation of the thrust faults in this duplex, (4) the westward dipping structural contact between mixed lower Paleozoic metasedimentary rocks exposed along Tukpahlearik Creek and the more homogeneous underlying quartz‐mica schists of the schist belt, (5) a structural detachment beneath Proterozoic metasedimentary and meta‐igneous basement rocks exposed in the northeastern part of the quadrangle, and (6) prominent west dipping structures that represent lateral ramps associated with northward transport of thrust sheets in the western Brooks Range during regional north directed Brookian deformation and west dipping structures that developed during a post‐Brookian east‐west compressional event which folded the imbricated thrust sheets as a package. Audiomagnetotelluric (AMT) data provide a useful and effective mapping tool in areas of limited to poor exposure and in areas affected by permafrost such as northwestern Alaska. Modeled AMT data supplemented geologic data and constrained geologic interpretations.
A possible explanation for foreland thrust propagation
Authors: Panian John , Pilant Walter
Published: February 2008

A common feature of thin‐skinned fold and thrust belts is the sequential nature of foreland directed thrust systems. As a rule, younger thrusts develop in the footwalls of older thrusts, the whole sequence propagating towards the foreland in the transport direction. As each new younger thrust develops, the entire sequence is thickened; particularly in the frontal region. The compressive toe region can be likened to an advancing wave; as the mountainous thrust belt advanced the down‐surface slope stresses drive thrusts ahead of it much like a surfboard rider. In an attempt to investigate the stresses in the frontal regions of thrustsheets, a numerical method has been devised from the algorithm given by McTigue and Mei [1981]. The algorithm yields a quickly computed approximate solution of the gravity‐ and tectonic‐induced stresses of a two‐dimensional homogeneous elastic half‐space with an arbitrarily shaped free surface of small slope. A comparison of the numerical method with analytical examples shows excellent agreement. The numerical method was devised because it greatly facilitates the stress calculations and frees one from using the restrictive, simple topographic profiles necessary to obtain an analytical solution. The numerical version of the McTigue and Mei algorithm shows that there is a region of increased maximum resolved shear stress, τ, directly beneath the toe of the overthrust sheet. Utilizing the Mohr‐Coulomb failure criterion, predicted fault lines are computed. It is shown that they flatten and become horizontal in some portions of this zone of increased τ. Thrust sheets are known to advance upon weak decollement zones. If there is a coincidence of increased τ, a weak rock layer, and a potential fault line parallel to this weak layer, we have in place all the elements necessary to initiate a new thrusting event. That is, this combination acts as a nucleating center to initiate a new thrusting event. Therefore, thrusts develop in sequence towards the foreland as a consequence of the stress concentrating abilities of the toe of the thrust sheet. The gravity‐ and tectonic‐induced stresses due to the surface topography (usually ignored in previous analyses) of an advancing thrust sheet play a key role in the nature of shallow foreland thrust propagation.
Compositions and sources of fluids in sediments of the Peru Subduction Zone
Authors: Elderfield Henry , Kastner Miriam , Martin Jonathan B.
Published: February 2008

Sediments of the middle‐to‐lower slope regions of the Peru continental margin drilled during Ocean Drilling Program (ODP) leg 112 contain interstitial waters with chloride concentrations as low as 454 mM Cl−, equivalent to ∼20% dilution of seawater with fresh water. Strontium isotope data suggest that two separate fluid regimes are present: a northerly one in which fluids contain more radiogenic Sr than seawater (87Sr/86Sr up to 0.70996), possibly originating from continental crust, and a southerly regime where fluids contain Sr less radiogenic than seawater (87Sr/86Sr down to 0.70722), and which are also enriched in Ca2+ and depleted in Mg2+ and δ18O, that appear to have interacted with oceanic basement. O, C, and He isotopic data support these interpretations.
Introduction to Special Section on Logging and Downhole Measurements in Deep Sea Drilling Project/Ocean Drilling Program Deep Crustal Holes
Authors: Becker Keir
Published: February 2008

One of the most important achievements during the first two years of the Ocean Drilling Program (ODP) was the concerted effort to conduct major programs of logging and downhole measurements upon revisiting three of the deepest holes drilled into the oceanic crust by the Deep Sea Drilling Project (DSDP). This effort established holes 395A, 418A, and 504B (Table 1) as our best reference sites to “ground truth” the structure and physical properties of the upper oceanic crust. The results of logging and downhole measurements at these and other ODP holes and their relationship to regional geophysical studies were the subject of a poster symposium at the AGU 1988 Fall Meeting, from which this special section developed.

Although it was not the intent to restrict this special section to results from holes 395A, 418A, and 504B, all of the papers here deal with at least one of these holes. This emphasizes their importance as reference sites, which arises not only because of the extensive downhole measurements but simply because they are among the four deepest DSDP/ODP penetrations into genuine upper oceanic crust (i.e., crust formed at a mid‐ocean ridge; Figure 1). Holes 395A and 418A were drilled in North Atlantic crust formed at a slow spreading rate, and 504B was drilled in eastern Pacific crust formed at a moderate spreading rate. Thus the set of reference sites lacks deep holes in crust formed at fast spreading rates, which is a serious shortcoming that will hopefully be addressed by upcoming ODP drilling in the Pacific Ocean. (The set of deep reference holes has already been complemented by the drilling and logging success in 500 m of tectonically exposed gabbros in hole 735B during leg 118 to the southwest Indian Ocean in 1987 [Robinson et al., 1989]; however, interpretation of the logging results from hole 735B had not progressed enough to be included here and will appear first in the work by Robinson et al. [1990].)
The effect of a changing aspect ratio of aligned cracks on shear wave vertical seismic profiles: A theoretical study
Authors: Douma Jan , Crampin Stuart
Published: February 2008

Rocks containing parallel cracks or preferentially oriented pores behave at low frequencies as transversely isotropic solids. A change in the aspect ratio of these inclusions affects the resultant anisotropy. Curves showing the velocity dependence of the two shear waves on the direction of wave propagation in such rocks intersect in a direction of propagation which depends on the aspect ratio. The angle this direction makes with the symmetry axis of the anisotropic rock decreases for increasing aspect ratio. Characteristic of wave propagation around this angle is a change in the polarization of the leading shear wave. To demonstrate how this affects vertical seismic profile (VSP) measurements, multioffset VSPs at different azimuths are modeled with synthetic seismograms for rocks containing inclusions with a range of aspect ratios. For inclusions with small aspect ratios, the polarization of the initial shear wave may change abruptly as the direction of propagation changes, whereas for large aspect ratios there is no change or the change may be more gradual and spread out over a wider range of directions. This feature may be of interest in assessing the physical character of fluid reservoirs. The observations of changes of polarization and variation of delays between the split shear waves may also be used to monitor changes in stresses in the crust and in monitoring changes during hydrocarbon extraction.
Global changes in the 0–70 km thermal structure of the Mars atmosphere derived from 1975 to 1989 microwave CO spectra
Authors: Clancy R. Todd , Muhleman Duane O. , Berge Glenn L.
Published: February 2008

Microwave spectra of Mars atmospheric CO were obtained in May (LS = 197°) and November (LS = 305°) of 1988 and January (LS = 341°) of 1989. The November observations, which include both 12CO and 13CO spectra, are analyzed to obtain the CO mixing ratio and temperature profile of the atmosphere of Mars for the 0–70 km altitude region. The resulting volume mixing ratio for CO (6±1.5×10−4) compares well with the 1967 infrared reflectance determination (8±2×10−4) by Kaplan et al. (1969). Based upon these observations and other ground‐based observations of O2, we suggest that large variations (>50%) in the global abundances of CO and O2 are not characteristic of the present Mars atmosphere. Assuming a constant CO mixing ratio, we derive atmospheric temperature profiles from the May 1988 and January 1989 12CO spectra which are quite similar to the temperature profile found for the November 1988 period. All of the derived temperature profiles in 1988 and 1989 agree within 10 K with the Viking descent measurements (Seiff and Kirk, 1977), even though the LS values for these observations are distinctly different from those for the Viking measurements (LS = 100°–120°).The same analysis for 12CO spectra observed in November 1975 (Kakar et al., 1977, LS, = 340°), March–April 1980 (Good and Schloerb, 1981, LS = 86°), and January 1982 (Clancy et al., 1983, LS = 77°) yields Mars atmospheric temperature profiles which are distinctly different from the Viking northern summer profiles. The March–April 1980 and January 1982 profiles, in particular, are 20–40 K cooler than the Viking profile for altitudes above 10 km. We conclude that all of the periods corresponding to the microwave observations are characterized by dust‐free, radiative‐convective equilibrium conditions for the Mars atmosphere. We conjecture that, during such cold northern spring‐summer seasons; condensation of H2O2 above 20–30 km altitude may affect the altitude distribution of O3; the base altitude of H2O clouds may decrease to as low as 5–10 km; and CO2 condensation at low latitudes may occur locally in the 30–40 km altitude region. Finally, we propose 25 K solar longitudinal (LS) variations in low‐to‐mid‐latitude temperatures for both dusty and clear periods of the Mars atmosphere.
A numerical simulation of climate changes during the obliquity cycle on Mars
Authors: François L. M. , Walker J. C. G. , Kuhn W. R.
Published: February 2008

A one‐dimensional seasonal energy balance climate model has been developed for the Martian surface and coupled to a model of CO2 distribution between atmosphere, regolith, and polar caps. This model takes into account the greenhouse warming of carbon dioxide, the meridional transport of heat, the CO2 condensation and sublimation cycle, and its adsorption in the regolith. The model takes into consideration the diurnal variation of solar irradiation, since it is shown that disregard of this effect yields temperatures too high by several degrees. The yearly‐averaged temperatures calculated from this climate model at different obliquities are used to estimate the importance of CO2 exchanges between the regolith and atmosphere‐cap systems during the obliquity cycle. For this purpose, the equation of thermal diffusion into the ground is solved for each latitude belt. The results differ substantially from those of previous studies, due in part to the consideration of the diurnal and seasonal variations of the solar irradiance. The model shows the importance of taking these short‐period variations into account instead of using yearly‐averaged quantities, due to the strong nonlinearity of the climate system on Mars. The roles of meridional heat transport and greenhouse warming are analyzed and shown to be important. For example, a permanent polar cap of carbon dioxide is destroyed by heat transport when the obliquity is high, while at low obliquity, high‐pressure systems without permanent cap can exist if enough exchangeable carbon dioxide is available. Further, the results show the possible existence of hysteresis cycles in the formation and sublimation of permanent deposits during the course of the obliquity cycle.
A nonsteady one‐dimensional theoretical model of Mars' neutral atmospheric composition between 30 and 200 km
Authors: Rodrigo R. , García‐Álvarez E. , López‐González M. J. , López‐Moreno J. J.
Published: February 2008

There has been a big advance in the knowledge of the composition of the atmosphere of the planet Mars since its exploration by different missions in the 1970s, and this will be deeply increased in the following years as the upcoming programs to Mars develop. In this context, we have elaborated a model of the Mars' neutral atmosphere including the following compounds: O(3P), O(1D), O2, O3, H, H2, OH, H2O, HO2, H2O2, CO, and CO2, between 30 and 200 km of altitude. The model is carried out for middle latitudes in equinox conditions and with moderate solar activity and provides the day‐to‐night evolution of the atmosphere. The scarcity of observations corresponding to the nightside of the planet has made it necessary to calculate the atmospheric temperature profile based on the available observations and on theoretical estimations. The model includes a detailed treatment of both the photochemical and the dynamical processes. In this sense, the most recent values of the reaction rates and photodissociation cross sections have been used, and a new height profile of the eddy diffusion coefficient has been computed which is able to explain the vertical distribution of carbon monoxide. The concentration profiles obtained show, in general, a very good agreement with the available experimental measurements.
Three‐dimensional P and S velocity structure in the Coalinga Region, California
Authors: Eberhart‐Phillips Donna
Published: February 2008

The Coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three‐dimensional velocity and hypocenter inversion in an area of complex three‐dimensional structure dominated by folding and blind thrusts. Additionally, since other varied geological and geophysical studies have been completed in this area, the three‐dimensional inversion solution could be compared to other interpretations. Inversion of 7696 P and 1511 S first arrivals from earthquakes and 696 P first arrivals from refraction shots produced a three‐dimensional velocity model with grid spacing of 1–2 km in the hypocentral area. The overall shape and location of velocity features correspond well to the mapped surface geology. The three‐dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower. The amounts of structural relief inferred for the local folds are similar to values inferred from geologic data. The three‐dimensional velocity solution has several distinctive features. There is a linear high‐velocity body (6.1–6.5 km/s), about 25 km long, from 6 to 8 km depth, that may represent a fragment of Coast Range ophiolite. A shallow low‐velocity zone (LVZ), which extends for 20 km along the fold axis at about 6 km depth and correlates with LVZs observed in both the refraction and the reflection data, may indicate high pore pressure caused by lateral compressive strain. Deeper LVZs occur within inferred Franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness, 4–8 km wide and 5–10 km long. These LVZs may represent multiple thrust faults. Their locations and geometry are consistent with thrust faults inferred with seismic reflection data. The three‐dimensional velocity solution compares well to prior two‐dimensional seismic reflection and refraction models and observed gravity. The shape of the inferred sedimentary section agrees well with the reflections from the Cenozoic strata. Compared to the refraction model, the three‐dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data. The gravity computed from the three‐dimensional velocities is similar to the observed gravity in both shape and amplitude. Both a simple one‐dimensional initial model and a complex initial model derived from the refraction interpretation were tried. A simple starting model gave the best results. The S velocity solution has different resolution than the P velocity solution because it uses a different set of stations, and it has lower resolution because it uses fewer arrival times. While the general patterns of velocity variation are similar for both Vp and Vs, the Vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features.
Ferric iron in silicate melts in the system Na2O‐Fe2O3‐SiO2 at high pressure
Authors: Brearly Mark
Published: February 2008

To assess the effect of pressure on the coordination state of Fe3+, experiments have been conducted on silicate melts in the Na2O‐Fe2O3‐SiO2 system in the presence of O2 to pressures of 4 GPa and temperatures of 1700°C. Quenched melts (glasses) have been analyzed using 57Fe Mössbauer spectroscopy at 298K. Fe2+ was not detected in any glasses and the absorption in the Mössbauer spectra, consisting of two peaks, is entirely from Fe3+. Two peaks in the spectrum are symmetric at low pressure and are attributed to Fe3+(IV). The isomer shift of the Fe3+ doublet increases ∼0.10 mm/s from 1 bar to 4 GPa. Above 1.5 GPa in iron‐rich compositions, the spectra become asymmetric with the low‐velocity peak becoming less intense and broader suggesting the presence of a second component. This component can be either a result of increasing distortion of Fe3+(IV) or a result of the gradual coordination change of Fe3+ from IV to VI. This coordination change for Fe3+ has been previously observed in glasses in the Na2O‐SiO2‐Fe‐O at high pressure and lower values of fO2 as the Fe3+ is partially reduced to Fe2+. The effects of pressure on the coordination state of Fe3+ and the Fe3+/∑Fe affect the compressibility of a magma at depth. It is anticipated that the presence of Fe3+(IV) at high pressure will favor a higher compressibility than if Fe3+ transforms to Fe3+(VI) or if the Fe3+ is reduced to Fe2+.
Melt structure in the system nepheline‐diopside
Authors: Sykes Dan , Scarfe Christopher M.
Published: February 2008

The Raman spectra of nine 1‐atm glass samples in the system nepheline‐diopside are presented. Nepheline glass consists of a fully polymerized network structure with largely six‐membered rings of Q4 tetrahedra. The structure of diopside glass contains Q0, Q1 Q2, and Q3 units. Intermediate compositions with up to 18 mol% nepheline have Q0, Q1, Q2, Q3, and Q4 structural units. Nepheline‐rich compositions contain two discrete molecular structures with different polymerization modes (Q2 and Q4 units). This tendency to form molecular clusters is facilitated by the inability of Ca and Mg to compete with Na on nonframework sites in structural units having only bridged oxygens.
Terrain and geology near the WTVD tower in North Carolina: Implications for Non‐Newtonian gravity
Authors: Bartlett D. F. , Tew W. L.
Published: February 2008

Eckhardt and coworkers have taken gravity measurements on and around a TV tower in North Carolina. They have interpreted these as evidence for a non‐Newtonian force. As originally reported, the discrepancy between prediction and measurement reaches about 500 μGal at the top of the 550‐m‐high tower. We have already given preliminary results suggesting that most of this discrepancy could have come from inadequate modeling of the local terrain. Here we present our final result. We show that about 3/4 of the effect comes from the terrain within 20 km of the tower. Left is somewhat more than 100 μGal for the contribution of the far terrain, for measurement error, or for a “fifth” force.
Magmatic inclusions in rhyolites of the Spor Mountain Formation, western Utah: Limitations on compositional inferences from inclusions in granitic rocks
Authors: Christiansen Eric H. , Venchiarutti Daniel A.
Published: February 2008

Inclusions of quenched mafic magma occur in a 21 Ma rhyolite lava and precursory non‐welded tuff that form the Spor Mountain Formation in west‐central Utah. The mafic inclusions are not lithic inclusions; no comparable volcanic unit was present at the surface when the Spor Mountain Formation erupted. Mineral and bulk compositions preclude liquid immiscibility. The mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite. Globular inclusions from both units are vesicular, phenocryst‐poor, plagioclase‐sanidine‐clinopyroxene‐orthopyroxene‐magnetite‐ilmenite latites and trachytes with quench temperatures of about 1000°C. Although overlapping in SiO2, TiO2, Zr, and Hf concentrations, inclusions from the underlying tuff lack negative Eu anomalies and are enriched in P2O5, K2O, Al2O3, Sr, Pb, Cr, and Ni whereas those hosted in the overlying lava have small negative Eu anomalies and two‐fold enrichments in Fe2O3, MnO, HREE, Y, Ta, Th, Rb, and Cs. The most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption. Limited mechanical mixing occurred after the inclusions solidified and became chemically modified. The textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor‐phase transport occurred in this low‐pressure volcanic environment. If such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling, it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions, mineralogies, or information about their ultimate sources. Using the compositions of such chemically modified inclusions as end‐members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks.
Three‐dimensional morphologies of metasedimentary and mafic enclaves From Ascutney Mountain, Vermont
Authors: Srogi Leeann , Lutz Timothy M.
Published: February 2008

Two types of enclaves are found in the main syenite unit of the Cretaceous igneous complex at Ascutney Mountain, Vermont. Metasedimentary xenoliths consist of fine‐grained hornfels with bands and lenses of coarser‐grained quartz. Mafic enclaves of igneous origin contain phenocrysts in an aphanitic groundmass. We have reconstructed the three‐dimensional shapes of a metasedimentary xenolith and 15 mafic enclaves from serial sections created by sawing blocks of syenite containing enclaves into parallel slabs. Enclave outlines on each side of the slabs were digitized and computer software was used to reconstruct three‐dimensional images. Results are presented for three typical mafic enclaves and for both the hornfels and quartz portions of the metasedimentary enclave. Stereographic drawings show that the metasedimentary and the mafic enclaves have complex shapes in three dimensions, with branches, lobes, and concavities. Syenite from the host magma penetrated the xenolith along hornfels‐quartz boundaries, producing complementary concave‐convex surfaces. Adjacent mafic enclaves commonly have complementary surfaces and are not observed to impinge on each other. Syenite also penetrated both the metasedimentary and mafic enclaves along finger‐like projections. Syenite infiltration into the metasedimentary xenolith apparently occurred by a combination of mechanical and chemical processes, controlled by the presence of compositionally favorable boundaries. A similar model is suggested for the mafic enclaves, in which syenite infiltration occurred preferentially along quartz‐feldspar grain boundaries or involved larger‐scale compositional heterogeneities within the mafic enclaves. Three‐dimensional views of enclaves provide information complementary to chemical and petrologic studies of assimilation.
formation of Crater Palimpsests on Ganymede
Authors: Thomas Paul J. , Squyres Steven W.
Published: February 2008

Crater palimpsests on Ganymede are circular features of high albedo apparently formed by impacts very early in the satellite's history. Some palimpsests have distinct outward‐facing slopes at their outer margins, suggesting that they are distinct deposits of significant thickness. This view is supported by observations of superimposed craters that excavate through palimpsest deposits into underlying materials of contrasting albedo. We suggest that crater palimpsests may result from volcanic extrusions triggered by large impact events early in Ganymede's history. Impact excavation that occurred to a sufficient depth would penetrate the satellite's thin, primordial lithosphere, allowing mobile, buoyant material from a vigorously convecting mantle to rise to the surface. Upon reaching the surface, the material would spread radially to produce a circular deposit of relatively high albedo. As the lithosphere cooled and thickened, palimpsest formation would have ceased, consistent with the observed concentration of palimpsest formation in the earliest part of Ganymede's recorded geologic history. Constraints on the nature of the extruded material may be derived from a simple model of emplacement. We use an analytic model for axisymmetric viscous flow to constrain the flow viscosity in terms of the aspect (height/radius) ratio γ of the palimpsest. A measured value of γ = 0.02 would imply a viscosity marginally consistent with glacial ice viscosities. Smaller values of γ, which are likely, would imply still lower viscosities, more appropriate to partially liquid flow. If such flows were described by a Bingham rheology, their yield strengths were of the order of 1 kPa. This is lower than is typical for even very mobile silicate lavas, but may be reasonable for a water/ice slush. Ice extrusion could have been a slow process taking thousands of years, while slush extension would have been essentially contemporaneous with the impact event. For the case of either solid ice and slush extrusion, our conclusions for the thermal structure of Ganymede at the time of palimpsest formation are similar: There was a thin (∼10 km) conducting lithosphere underlain by a vigorously convecting mantle that was very close to the solidus temperature.
Volcanism, isostatic residual gravity, and regional tectonic setting of the Cascade Volcanic Province
Authors: Blakely Richard J. , Jachens Robert C.
Published: February 2008

A technique to locate automatically boundaries between crustal blocks of disparate densities was applied to upward continued isostatic residual gravity data. The boundary analysis delineates a narrow gravitational trough that extends the length of the Pliocene and Quaternary volcanic arc from Mount Baker in northern Washington to Lassen Peak in California. Gravitational highs interrupt the trough at two localities: a northwest trending high in southern Washington and a northeast trending high between Mount Shasta and Lassen Peak. The latter anomaly is one of a set of northeast trending anomalies that, within the Quaternary arc, appear related to volcanic segmentation proposed previously on the basis of spatial and compositional distributions of volcanoes. These northeast trending anomalies extend hundreds of kilometers northeast of the arc, are caused by sources in the upper crust, and in some cases are related to exposed pre‐Tertiary rocks. Segmentation models invoke geometric characteristics of the subducting plate as the primary factor controlling location and chemistry of volcanism, and these northeast trending gravity sources also may be a product of disturbance of the upper crust by the subduction process. More likely, the gravity sources may reflect upper crustal structures older than the High Cascades, possibly relicts from earlier accretionary events or more recent crustal deformation, that have actively influenced the spatial location of more recent volcanism. Much of the Pliocene and Quaternary volcanism of the Cascade arc has concentrated on or near contacts between crustal blocks of disparate density. These contacts may promote the ascension of magma to the Earth's surface.
Miocene calc‐alkaline magmatism, calderas, and crustal extension in the Kofa and Castle Dome Mountains, southwestern Arizona
Authors: Grubensky Michael J. , Bagby William C.
Published: February 2008

Two widespread lower Miocene rhyolite ash flow tuffs in the Kofa and Castle Dome mountains of southwestern Arizona are products of caldera‐forming eruptions. These closely erupted tuffs, the tuff of Yaqui Tanks and the tuff of Ten Ewe Mountain, are approximately 22 Ma in age and their eruptions culminate a 1‐ to 2‐m.y.‐long burst of calc‐alkaline volcanic activity centered on the northern Castle Dome Mountains. Exotic blocks of Proterozoic and Mesozoic crystalline rocks up to 20 m across are present in exposures of the tuff of Yaqui Tanks exposed in the central Castle Dome Mountains and the southern Kofa Mountains. A single, thick cooling unit of the tuff of Ten Ewe Mountain that includes thick lenses of mesobreccia marks the location of the younger caldera that extends from Palm Canyon in the western Kofa Mountains eastward more than 7 km along strike to the central part of the range. The tuffs show rapid thinning away from their inferred sources. They were probably associated with high‐volume (100 kms) eruptions. Large residual Bouguer gravity anomalies, one beneath each inferred caldera, are interpreted as batholithic rocks or low‐density caldera fill. Caldera‐related volcanism in the Kofa region occurred during a transition in extensional tectonic regimes: from a regime of east–west trending uplifts and basins to a regime manifest primarily by northwest striking normal faults. A narrow corridor of folding and strike‐slip faulting formed during volcanism in the southern Kofa Mountains. Upper Oligocene or lower Miocene coarse sedimentary rocks along the southern flank of the Chocolate Mountains anticlinorium in the southern Castie Dome Mountains mark the periphery of a basin similar to other early and middle Tertiary basins exposed in southern California. The volcanic section of the Kofa region was dissected by high‐angle normal faults related to northeast‐southwest oriented crustal extension typical of the southern Basin and Range province.
Orbit determination and gravitational field accuracy for a mercury transponder satellite
Authors: Vincent Mark A. , Bender Peter L.
Published: February 2008

Covariance studies were performed to investigate the orbit determination problem for a small transponder satellite in a nearly circular polar orbit with 4‐hour period around Mercury. With X band and Ka band Doppler and range measurements, the analysis indicates that the gravitational field through degree and order 10 can be solved for from as few as 40 separate 8‐hour arcs of tracking data. In addition, the Earth‐Mercury distance can be determined during each ranging period with about 6‐cm accuracy. The expected geoid accuracy is 10 cm up through degree 5, and 1 m through degree 8. The main error sources were the geocentric range measurement error, the uncertainties in higher degree gravity field terms, which were not solved for, and the solar radiation pressure uncertainty.
Thermal variations of domain wall thickness and number of domains in magnetic rectangular grains
Authors: Xu Song , Merrill Ronald T.
Published: February 2008

Equilibrium domain wall thickness and number of domains in rectangular magnetic grains are determined by using a modified Amar model. It is shown that domain structure, particularly domain wall thickness, in a magnetized grain depends strongly on grain shape and orientation. These dependencies are attributed to the existence of two competing self‐magnetostatic interactions, one from the ends of the grain and the other from the sides. One of the consequences of this is that the thermal variation of domain wall thickness in an elongated grain is greater (smaller) than predicted by classical theory when the grain is magnetized along the shortest (longest) dimension. For magnetite, classical theory provides a good approximation in predicting both domain wall thickness and number of domains in equal‐dimensional grains larger than about 4 μm.
Elasticity, shear‐mode softening and high‐pressure polymorphism of wüstite (Fe1‐xO)
Authors: Jackson Ian , Khanna S. K. , Revcolevschi A. , Berthon J.
Published: February 2008

Elastic wave travel times have been determined as functions of hydrostatic pressure to 3 GPa for five modes of propagation in synthetic single‐crystal wüstite Feo.943O by ultrasonic phase comparison. The measured travel times, corrected for transducer‐bond phase shifts, constrain very accurately the zero‐pressure elastic moduli (GPa) and, for the first time, their first pressure derivatives (dimensionless) as follows: C11∶218.4, dC11/dP∶9.65, C12∶123.0, dC12/dP∶2.77, C44∶45.5, dC44/dP∶−1.03. The zero‐pressure moduli are in good agreement with the results of previous determinations by ultrasonic wave propagation but not with all of the moduli determined by resonance techniques. The variation of bulk modulus with pressure calculated from the Cij (P) is extrapolated to much higher pressures via third‐order Eulerian isotherms and isentropes based on K0S = 154.9 GPa and (dKs/dP)0T = 4.90. The resulting isothermal and shock compression curves satisfactorily reproduce the experimental data to ∼70 GPa, thereby providing a unified description of essentially all data bearing on the compressibility of wüstite. At higher pressures, published shock compression studies provide clear evidence for the existence of a different phase of much greater density and incompressibility. Metallic values of electrical conductivity have been reported for pressures >70 GPa under conditions of shock and high‐temperature static loading. Polyhedral face‐sharing in either the B8(NiAs) or B2(CsCl) (or derivative) structures would result in shorter Fe‐Fe distances, allowing greater 3d orbital overlap conducive to metallic conductivity. However, none of these possibilities satisfactorily accounts for the large inferred increase (14–20%) in zero‐pressure density unless the Fe‐O distance is also reduced by 3–5% by electron delocalization or spin‐pairing. The marked violation of the Cauchy condition associated with the very low value of C44 and its unusual temperature and pressure derivatives are attributable mainly to exchange coupling between nearest and next‐nearest neighbor spins.
Preliminary comparison of satellite BUV and surface‐based Umkehr observations of the vertical distribution of ozone in the upper stratosphere
Authors: DeLuisi John J. , Nimira Jerome
Published: February 2008

Data from surface‐based Umkehr and satellite ultraviolet backscatter observations of the vertical distribution of ozone in the upper stratosphere are compared. The satellite data are limited to four single‐pass observations of the vertical‐meridional distribution of ozone at different seasons of the year during 1970. The Umkehr data are climatological means over several years of observations. Although most of the observations made with the surface and satellite systems do not coincide in time, they nevertheless show a general agreement in seasonal variations that clearly appear in the more complete set of Umkehr data. Average values of ozone concentrations for satellite and Umkehr observations and a standard ozone profile are intercompared and show some significant differences.
The surface temperature structure associated with the Keweenaw Current in Lake Superior
Authors: Green T. , Terrell R. E.
Published: February 2008

A number of closely spaced airborne radiometer flights over the entire Keweenaw Current in Lake Superior suggest that bathymetry plays a major role in determining the overall features of the current. The crosscurrent surface temperature variations are correlated over long‐current distances of kilometers only in a loose sense, and the crosscurrent fine structure is essentially uncorrelated over the same distance. The crosscurrent temperature variations are often dominated by very sharp fronts, which seem to become unstable and spawn meanders along their outer edge.
The Iwex spectrum
Authors: Müller P. , Olbers D. J. , Willebrand J.
Published: February 2008

We describe the analysis of current and temperature cross spectra from the trimoored internal wave experiment Iwex and present a spectral model for the fluctuations in the internal wave range. The model consists of internal waves contaminated by temperature and current fine structure and by current noise. Except for the inertial and tidal frequencies the distribution of wave energy is vertically symmetric and horizontally isotropic. The wave number spectrum is characterized by a bandwidth of about 20–10 equivalent modes (decreasing with frequency), by a power law at high wave numbers, and by a sharp peak at low wave numbers. The general features of this spectrum agree fairly well with the one proposed by Garrett and Munk (1975). The contaminations increase with frequency. Contamination by temperature fine structure is confirmed by independent measurements. The Iwex spectral model provides a consistent description of all observed cross spectra.
Observations of the intrusion of a narrow warm tongue into the Sargasso Sea using satellite and in situ data
Authors: Vukovich Fred M. , Crissman Bobby W.
Published: February 2008

An intense narrow intrusion of warm Gulf Stream water into the Sargasso Sea was detected by the NOAA 4 satellite in late April 1975. Analysis of satellite‐derived sea surface temperature distribution and in situ data obtained within the intrusion indicated that the intrusion was a shallow zone (extending from the surface to about 200 m) of warm, less saline (in relation to the Sargasso Sea) Gulf Stream water about 95 km wide and extending 210 km in the north‐south direction. The intrusion turned eastward in a cyclonic pattern, the southern portions extending another 150 km to the southeast. Southward flow was detected at the surface in the north‐south portion of the intrusion with an average speed of approximately 1 m/s. The geostrophic computations suggested that the maximum depth of the zone of southward flow was about 200 m. Existing evidence suggested that the intrusion developed about April 9, 1975, following an intense coastal storm.
Energy exchange between surface waves and motion of sediment
Authors: Rosenthal W.
Published: February 2008

The energy transfer from surface waves to the sediment is calculated from the measurement of bottom movement in the North Sea. The contribution of this process to surface wave attenuation is compared with measurements from the Joint North Sea Wave Project experiment.
The formation of banded salt finger structure
Authors: Linden P. F.
Published: February 2008

Stepped fine structure of temperature and salinity observed in the Tyrrhenian Sea at a depth of between 600 and 1400 m is an established feature which has been studied over a number of years. At these depths the temperature and the salinity both decrease with depth, so it is tempting to relate this fine structure to salt finger convection. Recent observations with an optical device have shown that the fingers do not exist continuously across the interfaces between the steps but are found only in certain regions inside the interface. This discontinuous finger structure had not been observed previously in the laboratory, and this paper describes some experiments in which the breakup of salt fingers in an interface is documented. The conditions under which the fingers break up are investigated, and some implications of these new observations to the interpretation of oceanic fine structure are discussed.