bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixing in numerical models of mantle convection incorporating plate kinematics
Authors: Gurnis Michael , Davies Geoffrey F.
Published: February 2008

The process by which subducted lithosphere is mixed by mantle convection is investigated in numerical calculations. The results show that the observed isotopic heterogeneity of mantle sources and their ancient (1–2 b.y.) apparent ages are consistent with convective mixing. Passive tracers, which are introduced below “trenches,” are efficiently dispersed, but nonetheless, heterogeneities in tracer density with a large range of length scales are observed to persist for 40 or more transit times (one transit time is the time to travel the fluid depth with the boundary velocity). In particular, there is a strong tendency to form high‐density folds of the tracer strings, which persist much longer than simple shearing indicates. The folds persist because there is a strong tendency for material that enters the flow at the margins of cells to be transferred to adjacent cells, where it is “unmixed.” When the simulations are scaled to the whole mantle, the tight clumps (folds) of tracers are shown to persist for up to 1–2 b.y. There is also a tendency for large‐scale convection cells to remain isolated from recycled material for 1–2 b.y. These results are consistent with the significant chemical heterogeneity of the mantle as revealed by isotopic studies of oceanic basalts. Despite the spatial heterogeneity in tracer density, the average time tracers remain in the box from subduction at trenches to sampling at ridges (i.e., the residence time) is well constrained and within 20% of the mean residence time expected from an analytic model in which tracers are assumed to be sampled randomly. Model ages of the mantle that explicitly incorporate increased convection rates in the past and assume random sampling of heterogeneities bracket the ∼ 2 b.y. apparent Pb‐Pb and Rb‐Sr isochrons of midocean ridge basalts and oceanic island basalts. The conclusion of persistent spatial heterogeneity is different from the conclusions drawn from other studies. The different conclusions result, primarily, from our emphasis on the details of spatial variations as opposed to some average of the mixing, from a difference in flow unsteadiness, and from the different ways tracers have been introduced into the flow.
A forward approach to the problem of nonlinear viscoelasticity and the thickness of the mechanical lithosphere
Authors: De Rito Robert F. , Cozzarelli Francis A. , Hodge Dennis S.
Published: February 2008

The physical significance of the mechanical lithosphere is clarified using a nonlinear Maxwell flexure model. This model reconciles high‐temperature creep data for rock with the observed long‐term strength of the mechanical lithosphere. An approximate correspondence is established between the nonisothermal Maxwell model and an elastic flexure model to define an effective lithosphere thickness he. The model calculations are essentially forward, since he does not depend in a significant way upon the nominal or prescribed value of beam thickness, depending rather upon such variables as temperature and creep activation energy. The effective thickness of the Maxwell model, though a function of time, is stable enough over appropriate time scales to explain the long‐term integrated strength of the lithosphere. The strength index Σ is defined as the ratio of axial stress to the axial stress that would be present in a purely elastic fiber suffering the same strain. The base of the effective lithosphere is marked by a critical isopleth of strength, Σc ≅ 0.3. This critical level (though not its position) is independent of time since loading. The strength index also maps out zones of predominant rheology. The narrowness of the transition zone between the elastic lithosphere and the viscous asthenosphere explains the success of the elastic plate in modeling many examples of lithosphere flexure. The elastic plate approximation may apply best where the lithosphere is relatively young and hot. Isolated, weak zones in the continental crust may or may not weaken the lithosphere as a whole, depending upon the distribution of these zones and the manner in which they deform. In general, such weakening is likely to be most effective when the base of the crust occurs at midlithospheric depths.
Absolute gravity: A reconnaissance tool for studying vertical crustal motions
Authors: Niebauer T. M. , Hoskins J. K. , Faller J. E.
Published: February 2008

A major effort is underway to develop highly portable absolute gravimeters having an ultimate accuracy of 3–5 μGal, an accuracy which translates into a height sensitivity of several centimeters. We are just finishing the construction of six such units. Measurements at the Joint Institute for Laboratory Astrophysics with one of these new instruments agree well with the earlier measurements made in 1981 and 1982 with a previous generation instrument. Recent measurements at the International Bureau of Weights and Measures in Sevres, France, as a part of an international intercomparison of absolute gravimeters, also show good agreement with the other instruments.
Vertical movement and gravity change near the La Grande‐2 Reservoir, Quebec
Authors: Lambert A. , Liard J. O. , Mainville A.
Published: February 2008

Leveling and precise gravity observations were made along a 50‐km profile before and after the filling of the La Grande‐2 reservoir (53.7°N, 77°W) in 1979. Anomalous relative vertical uplift of 4 cm and relative changes in gravity of 14 μGal (140 nm/s2) were observed along the profile. The observed changes are anomalous with respect to theoretical results for loading of a layered elastic earth. Homogeneous, isotropic, porous, elastic models with reasonable hydraulic properties are also unable to explain the observations. In an effort to account for the discrepancies the accuracies of the leveling and the gravity data were investigated. Corrections were made to the leveling data for refraction errors, rod calibrations, magnetic effects, and deflection of the vertical by the water mass. Although the corrections for systematic errors were significant, the discrepancy with respect to homogeneous, isotropic loading models remains. The anomalous increase in gravity is thought to be the result of lateral transport of water in a high‐permeability formerly undersaturated zone, hydraulically coupled to the reservoir. The anomalous uplift is thought to be the result of elastic expansion of cracks and fractures of low aspect ratio in the high‐permeability zone or the result of an unexpectedly high rate of regional tilting in the LaGrande‐2 area.
Hydrostatic levels in precision geodesy and crustal deformation measurement
Authors: Hurst Kenneth , Bilham Roger
Published: February 2008

Previous attempts to apply the principles of hydrostatic leveling to precision geodesy have been limited by the uniformity of the fluid density attainable in field environments. This is largely due to the effects of temperature variations in the fluid tube. We have overcome this difficulty by using water maintained near its maximum density at 3.98°C inside a counterflow heat exchanger to limit the variations in density to less than 1 ppm. We have demonstrated the feasibility of this method with a 14‐m prototype level and have used a computer model of the system to demonstrate the theoretical performance of instruments up to 1 km long. It appears possible, for example, to construct a fluid tube 200 m long, 12.5 cm in diameter, and weighing 3.8 kg/m that would provide uniform density to <1 ppm in field environments between −40° and 50°C. In our attempt to use the 14‐m prototype pressure‐transfer level we were unable to exploit the density stability that we had achieved due to inadequacies in available pressure gauges.
Geoelectrical structure of the northern part of the Senegal Basin from joint interpretation of magnetotelluric and geomagnetic data
Authors: Ritz Michel , Vassal Jacques
Published: February 2008

Magnetotelluric (MT) and differential geomagnetic sounding (DGS) surveys were made in the Senegal basin, West Africa, to investigate the conductivity structure of the crust and upper mantle. Magnetic and electric variations were recorded at 11 sites along a east‐west profile 220 km in length. The period range of the measurements was 10–10,000 s. For the western stations, the MT response displays low skew with moderate anisotropy. The DGS data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area. A telluric current concentration in the thick, conductive sediments of the deep basin might cause this anomaly. Both MT and DGS data are then compared with two‐dimensional model results to explain the observed features along the profile. Results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone (2 ohm m) at depths in the range 4–23 km. Within the deep basin the results of the two‐dimensional modeling are not conclusive; acceptable fits between observed and calculated data have been obtained with alternate models. A continuous conductive zone (0.7–2 ohm m) may exist beneath the uppermost layers at a depth of approximatively 1 km, but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover. This ambiguity is inherent in the two‐dimensional models themselves. Both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast. It is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and/or by variation of the sedimentary thickness. The models also indicate the existence of two decreased resistivity layers in the eastern part of the profile, one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m. No such layers were detected at these depths below the deep basin. At some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile. This may indicate the presence of a small degree of partial melt.
An inherently linear electrostatic feedback method for gravity meters
Authors: Valliant H. D. , Gagnon C. , Halpenny J. F.
Published: February 2008

A pulse‐width‐modulated signal provides inherently linear electrostatic force feedback for application in gravity meters. Previous methods were linearized through the use of dual‐force rebalance plates and careful adjustment. The suggested system works equally well with single‐ and dual‐plate systems and requires no adjustment. Although dial‐setting errors and noise limit individual observations to a few microGals, a calibration accuracy of 0.25% was achieved. A careful analysis of residuals after regression analysis revealed no evidence of nonlinearity.
Upper mantle anisotropy: A preliminary model
Authors: Estey Louis H. , Douglas Bruce J.
Published: February 2008

Seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion. The physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral. The dominant glide systems are assumed to be [100](0l0) for olivine and [001](100) for the pyroxenes. These yield a predominantly orthorhombic fabric with the olivine [100] and the pyroxene [001] axes aligned in the upper mantle flow direction and the olivine [010] and the pyroxene [100] axes aligned normal to the upper mantle flow plane. These glide systems have a threshold temperature of enhanced mobility of 1100–1200 K, which yields a solid‐state, thermally defined lithosphere‐asthenosphere boundary in an olivine‐pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere. Mantle anisotropy due to mineral alignment is then actively maintained below this boundary (asthenosphere and mesosphere) and is a fossil state above this boundary (lithosphere). We use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models (pyrolite and piclogite) for the upper mantle by an extrapolation of single‐crystal, anisotropic mineral elastic properties to a depth of 400 km. The real‐earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy (isotropy) from random mineral alignment. The seismic anisotropy of the upper 220 km is best represented by the pyrolite model, which reduces to quasi‐hexagonal symmetry with the unique axis in the direction of mantle flow. The Lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy. The piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite. The piclogite anisotropy model can appear quasi‐isotropic, when measured by transverse isotropy parameterization, if the mantle flow is mainly horizontal with a horizontal shear plane (that is, the shear flow gradient duH /dR dominates).
Short‐period strain (0.1–105 s): Near‐source strain field for an earthquake (ML 3.2) near San Juan Bautista, California
Authors: Johnston M. J. S. , Borcherdt R. D. , Linde A. T.
Published: February 2008

Measurements of dilational earth strain in the frequency band 25–10−5 Hz have been made on a deep borehole strainmeter installed near the San Andreas fault. These data are used to determine seismic radiation fields during nuclear explosions, teleseisms, local earthquakes, and ground noise during seismically quiet times. Strains of less than 10−10 on these instruments can be clearly resolved at short periods (< 10 s) and are recorded with wide dynamic range digital recorders. This permits measurement of the static and dynamic strain variations in the near field of local earthquakes. Noise spectra for earth strain referenced to 1 (strain)2/Hz show that strain resolution decreases at about 10 dB per decade of frequency from −150 dB at 10−4 Hz to −223 dB at 10 Hz. Exact expressions are derived to relate the volumetric strain and displacement field for a homogeneous P wave in a general viscoelastic solid as observed on colocated dilatometers and seismometers. A rare near‐field recording of strain and seismic velocity was obtained on May 26, 1984, from an earthquake (ML 3.2) at a hypocentral distance of 3.2 km near the San Andreas fault at San Juan Bautista, California. While the data indicate no precursory strain release at the 5 × 10−11 strain level, a coseismic strain release of 1.86 nanostrain was observed. This change in strain is consistent with that calculated from a simple dislocation model of the event. Ground displacement spectra, determined from the downhole strain data and instrument‐corrected surface seismic data, suggest that source parameters estimated from surface recordings may be contaminated by amplification effects in near‐surface low‐velocity materials.
Influence of welded boundaries in anelastic media on energy flow, and characteristics of P, S‐I, and S‐II waves: Observational evidence for inhomogeneous body waves in low‐loss solids
Authors: Borcherdt Roger D. , Glassmoyer Gary , Wennerberg Leif
Published: February 2008

A general computer code, developed to calculate anelastic reflection‐refraction coefficients, energy flow, and the physical characteristics for general P, S‐I, and S‐II waves, quantitatively describes physical characteristics for wave fields in anelastic media that do not exist in elastic media. Consideration of wave fields incident on boundaries between anelastic media shows that scattered wave fields experience reductions in phase and energy speeds, increases in maximum attenuation and Q−1, and directions of maximum energy flow distinct from phase propagation. Each of these changes in physical characteristics are shown to vary with angle of incidence. Finite relaxation times for anelastic media result in energy flow due to interaction of superimposed radiation fields and contribute to energy flow across anelastic boundaries for all angles of incidence. Agreement of theoretical and numerical results with laboratory measurements argues for the validity of the theoretical and numerical formulations incorporating inhomogeneous wave fields. The agreement attests to the applicability of the model and helps confirm the existence of inhomogeneous body waves and their associated set of distinct physical characteristics in the earth. The existence of such body waves in layered, low‐loss anelastic solids implies the need to reformulate some seismological models of the earth. The exact anelastic formulation for a liquid‐solid interface with no low‐loss approximations predicts the existence of a range of angles of incidence or an anelastic Rayleigh window, through which significant amounts of energy are transmitted across the boundary. The window accounts for the discrepancy apparent between measured reflection data presented in early textbooks and predictions based on classical elasticity theory. Characteristics of the anelastic Rayleigh window are expected to be evident in certain sets of wide‐angle, ocean‐bottom reflection data and to be useful in estimating Q−1 for some ocean bottom reflectors.
3He/4He ratio anomalies associated with the 1984 Western Nagano Earthquake: Possibly induced by a diapiric magma
Authors: Sano Yuji , Nakamura Yuji , Wakita Hiroshi , Notsu Kenji , Kobayashi Yoji
Published: February 2008

A magnitude 6.8 earthquake occurred near Ontake Volcano in the western Nagano Prefecture of central Honshu, Japan, on September 14, 1984. After the event, 3He/4He ratios of bubbling gases in hot springs located less than 10 km from the newly formed fault were found to have increased significantly, by up to 15%. Although the number of data is small, considering that 3He can only be of magmatic origin, we propose a model that the earthquake was triggered by an upward migration of 3He‐containing fluids associated with magma intrusion beneath the source region. The proposed model is supported by the uplifting observed in the region since 1967, the continuation of seismic swarm activities in the area since 1976, an unprecedented eruption of Ontake Volcano in October 1978, and the existence of a small melted body in the lower crust beneath the present fault region.
Modeling gravity and trilateration data in Long Valley, California, 1983–1984
Authors: Rundle John B. , Whitcomb James H.
Published: February 2008

During the period July 1982 through July 1984 a series of repeated, high‐precision gravity measurements have been undertaken in and around Long Valley, California. Data were collected at about 40 sites using three Lacoste‐Romberg model G gravity meters. At the same time the U.S. Geological Survey was conducting a program of high‐precision line length measurements throughout the same area. Significant changes were observed with both types of data during the intervals 1982–1983 and 1983–1984. Previous work by us has shown that deformation data during the interval 1975–1983 are consistent with a model for the inflation of Long Valley caldera which includes magma injection beneath the south central caldera at a depth of only 5 km. Simultaneous inversion of the trilateration and gravity data for the period 1983–1984 shows that most of the previous model is still reasonably consistent with the data and implies injected volumes of between 0.01 and 0.02 km3. Additionally, uplift during this period occurred along the Inyo Domes chain of vents and flows, indicating perhaps a new source of inflation in the northwest part of the caldera. This uplift amounts to a few centimeters and implies a volume increase of perhaps 0.004 km3 at reasonably shallow depth (3 km?). The total amount of uplift in the northwest is small in comparison to the total uplift centered on the central resurgent dome (total vertical displacement well in excess of half a meter). For purposes of comparison we examine several models, one of which involves injection at the base of the crust. This model implies much higher injected volumes, as much as 0.15–0.2 km3.
Extensive‐dilatancy anisotropy beneath Mount Hood, Oregon and the effect of aspect ratio on seismic velocities through aligned cracks
Authors: Crampin Stuart , McGonigle Robert , Ando Masataka
Published: February 2008

The azimuthal variations of P wave velocity measured in U.S. Geological Survey (USGS) seismic refraction experiments around Mount Hood, Oregon, are modeled by propagation through the effective anisotropy of parallel vertical cracks. The three refractors at 1, 3.7, and 8.5 km depth show different behavior. The shallow refractor at 1 km displays P wave velocity variations typical of the parallel water‐filled vertical microcracks found throughout much of the upper continental crust. The arrivals from the refractor at 3.7 km display velocity variations consistent with either dry cracks or, more probably, with cracks filled with supercritical water, as one interpretation of the heat flow around Mount Hood suggests that temperatures at 3.7 km would be above the critical temperature for water at in situ pressures. The velocity variations at the deepest refractor at 8.5 km depth are poorly constrained but again suggest liquid‐filled cracks. The possibility of high temperatures suggests that, at this depth, the cracks could be thin skins of melt along grain boundaries. The vertical cracks in all three refractors strike in the northwest to southeast quadrants, which is consistent with the compressional quadrants in the mechanisms of local earthquakes. Shear‐wave splitting reported elsewhere indicates that there are stress‐aligned water‐filled microcracks (extensive‐dilatancy anisotropy or EDA) throughout at least the brittle upper 10 or 20 km of the continental crust. The high heat flow in the High Cascades may confine the range of crack behavior, usually spread over the whole thickness of the crust, to the top 10 km beneath Mount Hood. Thus the P wave velocity variations beneath Mount Hood may indicate that EDA cracks occur throughout the whole of the continental crust. The large number of shot‐to‐recorder travel times measured by the USGS allows extensive‐dilatancy anisotropy to be recognized from the analysis of P wave velocity variations for the first time.
Chemical zoning and homogenization of olivines in ordinary chondrites and implications for thermal histories of chondrules
Authors: Miyamoto Masamichi , McKay David S. , McKay Gordon A. , Duke Michael B.
Published: February 2008

We studied the extent and degree of homogenization of chemical zoning of olivines in type 3 ordinary chondrites in order to obtain some constraints on cooling histories of chondrites. Based on Mg‐Fe and CaO zoning, olivines in type 3 chondrites are classified into four types. A single chondrule usually contains olivines with the same type of zoning. Microporphyritic olivines show all four zoning types. Barred olivines usually show almost homogenized chemical zoning. We have calculated cooling rates or burial depths needed to homogenize the chemical zoning by solving the diffusion equation, using the zoning profiles as an initial condition. Mg‐Fe zoning of olivine may be altered during initial cooling, whereas CaO zoning is hardly changed. Barred olivines may be homogenized during initial cooling because their size is relatively small. To simulated microporphyritic olivine chondrules, cooling from just below the liquidus at moderately high rates is preferable to cooling from above the liquidus at low rates. For postaccumulation metamorphism of type 3 chondrites in order to keep Mg‐Fe zoning unaltered, the maximum metamorphic temperature must be less than about 400°C if we assume cooling rates based on Fe‐Ni data. Calculated cooling rates for both Fa and CaO homogenization are consistent with those by Fe‐Ni data for type 4 chondrites. A hot ejecta blanket several tens of meters thick on the surface of a parent body is sufficient to homogenize Mg‐Fe zoning if the temperature of the blanket is 600–700°C. Burial depths for petrologic types of ordinary chondrites in a parent body heated by 26Al are broadly consistent with those previously proposed.
Linear volcanic features at Alba Patera, Mars ‐ PRobable spatter ridges
Authors: Cattermole Peter
Published: February 2008

A number of extensive linear constructional features occur on the north and east flanks of Alba Patera. They are found in a 150‐km‐wide zone and comprise aligned ridge elements that are locally associated with short volcanic flows. They are among the youngest volcanic features in the region and are believed to be Martian analogs of terrestrial spatter ridges. On the basis of measurements made on Viking images, it appears that active volcanic fissures on Mars were larger than their terrestrial counterparts.
Seismic amplitude modeling and the shallow crustal structure of the East Pacific Rise at 12°N
Authors: Fischer Karen M. , Purdy G. M.
Published: February 2008

Velocity structures of the upper 2 km of the oceanic crust were used to model seismic refraction data collected on the eastern flank of the East Pacific Rise at 12°N during the Rivera Ocean Seismic Experiment (ROSE). Data, generated with a Bolt 1500C air gun source, show multiple narrow (<1 km in range) peaks in the amplitude of the first refracted arrival. Record sections separated by as much as 150 km indicate similar amplitude versus range patterns, suggesting that the amplitude peaks are caused by consistent structural features and not by random crustal inhomogeneities. These amplitude highs do not consistently correlate with seafloor bathymetry and hence cannot be explained solely by topographic focussing of seismic energy. Through travel time analysis and ray tracing we find a crustal velocity model with two vertical zones of concave gradients which focus rays at 6 and 8 km range. Synthetic seismograms and power versus range patterns computed for this model by the reflectivity algorithm are consistent with the two peak amplitude pattern observed on nearly all air gun record sections. These results suggest that the vertical gradient zone structure of the uppermost 2–3 km of ocean crust is continuous to first order over the ROSE area.
Estimation of marine sediment bulk physical properties at depth from seafloor geophysical measurements
Authors: Nobes D. C. , Villinger H. , Davis E. E. , Law L. K.
Published: February 2008

A combination of geophysical techniques can yield estimates of several physical properties versus depth. An example is presented for a thick sequence of sediments in Middle Valley near the present spreading axis of the Juan de Fuca Ridge. We have computed the porosity, temperature, and thermal conductivity versus depth by combining the results of seafloor electrical and heat flow surveys. The electrical resistivity of the sediments is empirically related to the porosity of the medium and to the pore fluid electrical conductivity, which is in turn sensitive to the temperature. The temperature profile is determined from the heat flow and the thermal conductivity. The latter is determined from calculated porosity and estimates of the constituent mineral and pore fluid thermal conductivities. The measured surface heat flow is assumed to be, and the resistivity has been determined to be, approximately constant with depth. The computation of the variation of temperature, porosity, and thermal conductivity with depth yields physically realistic and reasonable results. Other physical properties (void ratio, bulk density, water content, compressional wave velocity) are estimated from the porosity. The shear wave velocity is calculated empirically from the compressional wave velocity, and Poisson's ratio is then computed from the compressional and shear wave velocities. The results are in good agreement with values derived for similar rapidly deposited sediment sequences elsewhere, but the model calculations indicate that Deep Sea Drilling Project measurements may be biased toward higher‐porosity samples.
Global petrologic variations on the Moon: A ternary‐diagram approach
Authors: Davis Philip A. , Spudis Paul D.
Published: February 2008

A ternary‐diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. This new approach uses a ternary diagram that is subdivided into equally spaced segments along each of its three sides and has a triangular area in its center. The segments are assigned distinct colors that cover the range of the visible spectrum. The apexes are assigned the three primary colors, and the center triangle, which represents equal proportions of the three apexes, is assigned the color gray. The apexes are assigned the average Fe and (Th/Ti)c (ratio normalized to chondrites) compositions of KREEP/Mg‐suite rocks, mare basalts, and ferroan anorthosites. The composition of each picture element (pixel) within these Apollo orbital gamma ray databases is used to determine its position within the ternary diagram. The color corresponding to this ternary position is then placed on a classification map at that pixel's position within the orbital databases. Error databases produced for the Fe and (Th/Ti)c data were used to exclude pixels having high errors. The resultant classification map shows spatial transitions among petrologic units, allows direct determination of the relative proportions of each end‐member composition in a pixel, and increases the geologic interpretability of these data over that of the individual elemental databases. The classification units correspond remarkably well to observed geologic units, when we consider the spatial resolution (100 km) of the gamma ray spectrometer. Our results are as follows: (1) The highlands contain large areas of relatively pure ferroan anorthosite. (2) The average composition of the upper lunar crust is represented by an “anorthositic gabbro” composition, which supports the “magma‐ocean” hypothesis for lunar crustal genesis. (3) KREEP/Mg‐suite rocks are a minor fraction of the upper lunar crust. (4) Within the farside highlands, areas of KREEP/Mg‐suite rocks coincide mostly with areas of crustal thinning, which are probably areas of KREEP basalt extrusions or localized excavations of Mg‐suite rocks or KREEP‐rich rocks. (5) Portions of the east limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark‐halo craters; the presence of this component supports the hypothesis that mare volcanism had occurred within this highland region before the end of final heavy bombardment.
Platinum metals and microstructure in magnetic deep sea cosmic spherules
Authors: Bonté Ph. , Jéhanno C. , Maurette M. , Brownlee D. E.
Published: February 2008

Instrumental neutron activation analysis was used to determine the concentrations of Ir, Fe, Ni, Co, Cr, and Sc in 16 “iron” and 8 “chondritic” magnetic deep sea spherules with sizes of about 200 μm. With the exception of two iron spheres, high indium concentrations were found in each sphere, thus confirming their extraterrestrial origin. Moreover, the “depth profile” of the residual mass of iridium was monitored in individual spheres by comparing the variations of the intensities of the γ ray lines of iridium, cobalt, and iron during a very gradual sequential polishing. Each sharp but partial discontinuity, observed only in the Ir profiles, reflects the intersection of a polished section with a tiny Ir‐rich nugget. These profiles show that such nuggets are much more common in both the iron and the chondritic spheres than previously thought. We found new types of disequilibrated clusters of micrometer‐sized nuggets of platinum metals. All types of nuggets, which were characterized with a scanning electron microscope and an energy dispersive X ray spectrometer, seem to be associated with magnetite. The compositions of the nuggets, as well as the fractionation patterns for Ir, Fe, Ni, Co, and Sc in the spheres, suggest that most of the Ir‐rich iron spheres originate from iron‐rich bodies.
IMPLICATIONS OF MEASURED THERMAL GRADIENTS FOR WATER MOVEMENT THROUGH THE NORTHEAST JAPAN ACCRETIONARY PRISM
Authors: Reck Brian H.
Published: February 2008

Thermal gradients of 24°–36°C/km have been found in the slope cover sediments of the accretionary prism off northeast Japan (Deep Sea Drilling Project (DSDP) sites 438A, 439, and 440). These gradients lead to the prediction of temperatures in excess of 400°C at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity. Such temperatures conflict with the predictions of temperatures of 150°C at even greater depths from conductive thermal models and petrologic studies of low‐temperature blueschists at similar ancient margins. One‐dimensional, steady state conduction and advection via Darcian water flow, of heat, up through the prism was simulated numerically. The measured thermal gradients can be reconciled with temperatures less than 125°C at the prism base by water flowing upward at rates of the order of 10−10 m/s. Such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast Japan prism would flow up through a <10 km width of the prism (perpendicular to the trench) and would require the DSDP sites to have been located fortuitously over the localized zone(s) of water flow. Smaller velocities will reproduce the measured gradients, and thus greater widths through which water flow occurs are predicted, if the temperature at the prism base is higher and/or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate. Where water flow is significant, as likely wherever abundant, water‐rich sediments are subducted, thermal gradients below a few kilometers in accretionary prisms may be much lower than near‐surface values. This has profound implications for the thermal structure of the prism off northeast Japan and elsewhere.
Deformation along the San Andreas Fault 1982–1986 as indicated by frequent geodolite measurements
Authors: Savage J. C. , Prescott W. H. , Lisowski M.
Published: February 2008

Frequent (4–12 times per year) measurements of six 3‐line (lengths 10–40 km) strain networks located along the locked sections of the San Andreas fault in southern and central California provide a time history of deformation along the fault in the interval 1982–1986. The standard error in measurement ranges from 0.4 ppm in line length for the shortest lines to 0.2 ppm for the longest lines. Except for one example of coseismic offset and postseismic relaxation the observations are at least marginally consistent with uniform‐in‐time deformation. The records of deformation at the networks closest to the epicenters of two recent magnitude 6 California earthquakes (1984 Morgan Hill and 1986 North Palm Springs) were particularly stable in the year preceding the earthquakes. Thus it is unlikely that these earthquakes were triggered by regional strain events. However, marginally significant anomalies were observed 12–18 months before both earthquakes.
On the statistical properties of estimated rotations
Authors: Chang Ted
Published: February 2008

This paper discusses theorems from the mathematical statistics literature on confidence regions for an estimated rotation. The theorems can be used to analyze pairs of homologous intersections of fracture zones with magnetic anomaly lineations on opposing plates. Two classes of probability models on the data are considered: The “fixed u” and the “random u” models, corresponding to, in the geophysical context, whether error is attributed to the measurements on one or on both plates. For each model, large sample and concentrated error approximate confidence regions are given. It is found that the geometric features of the confidence regions are in rough agreement with the analysis of partial uncertainty rotations by Stock and Molnar. The paper concludes with some rudimentary small sample, diffuse error distribution theory. For illustrative purposes, a data set from the Gulf of Aden is repeatedly analyzed using a variety of probabilistic models.
Evaluation of alterant geophysical tomography in welded tuff
Authors: Ramirez A. L. , Daily W. D.
Published: February 2008

Electromagnetic measurements have been performed at 300MHz to evaluate the applicability of alterant geophysical tomography to delineate in situ flow paths in a welded tuff rock mass. The evaluation has involved a field experiment, which was supplemented by a test bed experiment and a computer simulation study. The field measurements were made before, during, and after a water‐based dye tracer flowed through the rock mass. Alterant geophysical tomographs are compared with independent evidence: maps of fractures intersected by the measurement boreholes and dyed rock samples. Anomalies present in the tomograph match fractures mapped with a borescope. The locations of tracer‐stained fractures coincide with the locations of most image anomalies. The core data show that some of the image anomalies represent flow through single fractures. Other geophysical anomalies required further evaluation because they existed where tracer‐stained fractures were not observed. The field images were also evaluated using evidence from a computer simulation study and from a test bed experiment where the technique was evaluated under controlled conditions. The results from these supplementary tests suggest that the field test image anomalies without corresponding dyed fractures are unlikely to be artifacts and thus probably represent flow paths of the tracer through the rock.
Reflections from midcrustal rocks within the Mesozoic Subduction Complex near the Eastern Aleutian Trench
Authors: Fisher Michael A. , von Huene Roland , Smith Gregory L.
Published: February 2008

Seismic reflection data collected in 1973 by Western Geophysical Company show that highly reflective rocks make up the midcrust of the convergent margin adjacent to the eastern Aleutian Trench. These rocks form an arch that strikes obliquely across the strongly expressed northeast‐southwest structural grain of exposed Mesozoic rocks. In an earlier report we proposed that the deep events mark the location of either the presently subducting plate or the top of underplated rocks. However, the short radius of curvature of this arch precludes the possibility that the deep events indicate the igneous oceanic crust. Instead, the deep reflections could be from underplated rocks that have been arched by the imbrication or underplating of strata below the reflective rocks. High‐amplitude water layer and interbed multiples prevent precise connection of shallow and deep geology. For example, the Border Ranges fault, a suture between major tectonostratigraphic terranes, is not revealed on any of our seismic sections. We speculate, however, that one band of reflections that rises toward but does not reach the surface is from the Eagle River thrust fault, which separates Late Cretaceous melange from deformed turbidite sequences of the same age.
Magnetization of the oceanic crust: Thermoremanent magnetization of chemical remanent magnetization?
Authors: Raymond C. A. , LaBrecque J. L.
Published: February 2008

We propose a model in which chemical remanent magnetization (CRM) acquired within the first 20 m.y. of crustal evolution may account for 80% of the bulk natural remanent magnetization of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low‐temperature alteration. The CRM intensity and direction are controlled by the posternplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate‐wavelength satellite field and the short‐wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the Cretaceous Quiet Zones, while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further, our study indicates varying rates of TRM decay in very young crust which depicts the advance of low‐temperature alteration through the magnetized layer.
Fast glacier flow: Ice streams, surging, and tidewater glaciers
Authors: Clarke Garry K. C.
Published: February 2008

Fast flowing glaciers are unusual but important. Jakobshavns Glacier in western Greenland illustrates this point (Figure 1). Considered to be the world's fastest glacier, its maximum observed flow rate (measured in midsummer near the floating terminus) is 8360 m yr−1 [Lingle et al., 1981]. Except for a tidal influence near the terminus, the flow is believed to be fairly steady [Bindschadler, 1984].
Recent glacial history and rapid ice stream retreat in the Amundsen Sea
Authors: Kellogg Thomas B. , Kellogg Davida E.
Published: February 2008

Microfossil analyses of core top samples from 20 sites in the eastern and southern Amundsen Sea delimit three biotic provinces, which are related in part to the activity of a large ice stream and modern sea ice distribution. In Recent sediments on the Outer Shelf, planktonic and calcareous benthic foraminifera are abundant, but diatoms are rare. Here, low diatom abundances in sediments, despite high abundances in the overlying sea ice, are attributed to nondeposition caused by ocean currents. Pine Island Bay sediments are almost barren of microfossils, even though diatoms are abundant in overlying surface waters. We attribute this discrepancy to deposition of exclusively non‐organic sediments beneath a former extension of the floating terminus of Pine Island Glacier, which probably retreated from this area during the last century. The polynya which occupies this area during present‐day summers is probably so recent a feature that little or no biogenic sediment has accumulated. Recent sediments in the Eastern Margin Province, between Pine Island Bay and the Outer Shelf, contain abundant diatoms and arenaceous foraminifera. High diatom abundances here probably record the presence of the Amundsen Sea polynya during the last few millennia. The presence of a compact diamicton beneath Recent sediments on the Outer Shelf and in the Eastern Margin Province suggests that grounded ice formerly occupied the Amundsen Sea shelf with ice streams in deep, elongated troughs. These ice streams may have retreated up to 200 km during the Holocene.
Investigation of Martian H2O and Co2 via orbital gamma ray spectroscopy
Authors: Evans Larry G. , Squyres Steven W.
Published: February 2008

A number of presently unanswered questions concerning H2O and CO2 on Mars may be addressed using data from an orbital gamma ray spectrometer such as the one planned for the Mars Observer mission. Problems that could be addressed include the ice/dust ratio of the polar layered deposits; the dust content and thickness of polar perennial ice; the water content of hydrated silicates; the depth, distribution, and concentration of ground ice; the possible existence of liquid water “oases”; and the thickness, variability, and H2O content of the seasonal CO2 frost cap. We investigate the capability of an orbital gamma ray spectrometer to address these problems by calculating the gamma ray signal produced by the Martian atmosphere and by several simple models of Martian surface materials. The Martian surface is modeled as one or two layers composed of various arrangements of “Viking soil,” H2O, and CO2. We calculate (1) the production of neutrons in the atmosphere and in the subsurface material by cosmic ray interactions, (2) the scattering of neutrons and the resultant neutron energy spectrum and spatial distributions, (3) the production of gamma rays by neutron prompt capture and nonelastic scatter reactions, (4) the production of gamma rays by natural radionuclides, (5) the attenuation of the gamma ray signal by passage through surface materials and the Martian atmosphere, (6) the production of the gamma ray continuum background, and (7) the uncertainty in gamma ray line strengths that results from the combined signal and background observed by the detector. Two types of data give us information about the concentration and distribution of hydrogen: the strength of the 2.223‐MeV line of H and the ratio of gamma rays from inelastic scatter to gamma rays due to prompt capture for an element such as Si or Fe. The latter provides information about H by giving an indirect indication of the nature of the neutron energy spectrum. The two techniques are sensitive to the presence of H at different depths. Using both techniques then, we in many cases will be able to determine uniquely both the thicknesses and the H2O content of layers on the Martian surface from the gamma ray signal that will be observed. For investigation of CO2 layers, we can use both the 4.945‐MeV prompt capture line of carbon and, in some cases, the attenuation produced by passage of gamma rays from natural radionuclides through a CO2 layer to determine layer composition and thickness. Our results show that orbital gamma ray data can be used to extract a considerable amount of information about the quantity and vertical distribution of volatiles near the Martian surface. The addition of the capability to measure independently the neutron energy spectrum will further enhance the usefulness of this experiment.
Properties of the lower crust from modeling refraction and reflection data
Authors: Wenzel Friedemann , Sandmeier Karl‐Josef , Wälde Walter
Published: February 2008

Data of a refraction and a reflection seismic survey in the Black Forest of Southwest Germany are used for extensive one‐ and two‐dimensional modeling. The data are available along approximately the same line, and therefore the same piece of crust is probed by two seismic methods. We utilize this favorable circumstance for detailed model calculations concerning both data sets. In this paper we focus on the lower crust. Its properties vary on the scale of a wavelength, and thus full solutions of the elastic equations are required. We use the reflectivity method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response. The lamellae of the lower crust appear as reflecting elements of several hundred meters length randomly distributed in space. Rocks with extremely low velocities (5.6 km/s) must be present, indicating that anisotropy and/or fluids play an important role in the lower crust. Ideas concerning lower crustal formation, evolution, and extinction of lamellaetion are developed.