bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral mixture modeling: A new analysis of rock and soil types at the Viking Lander 1 Site
Authors: Adams John B. , Smith Milton O. , Johnson Paul E.
Published: February 2008

A Viking Lander 1 image was modeled as mixtures of reflectance spectra of palagonite dust, gray andesitelike rock, and a coarse rocklike soil. The rocks are covered to varying degrees by dust but otherwise appear unweathered. Rocklike soil occurs as lag deposits in deflation zones around stones and on top of a drift and as a layer in a trench dug by the lander. This soil probably is derived from the rocks by wind abrasion and/or spallation. Dust is the major component of the soil and covers most of the surface. The dust is unrelated spectrally to the rock but is equivalent to the global‐scale dust observed telescopically. A new method was developed to model a multispectral image as mixtures of end‐member spectra and to compare image spectra directly with laboratory reference spectra. The method for the first time uses shade and secondary illumination effects as spectral end‐members; thus the effects of topography and illumination on all scales can be isolated or removed. The image was calibrated absolutely from the laboratory spectra, in close agreement with direct calibrations. The method has broad applications to interpreting multispectral images, including satellite images.
Seismotectonics of the Zagros continental collision zone and a comparison with the Himalayas
Authors: Ni James , Barazangi Muawia
Published: February 2008

Accurate focal depths of 16 medium to large Zagros earthquakes were determined from identification of surface‐reflected phases by comparing observed, long‐period P waves with synthetic seismograms. All events are shallow (6–12 km) and show no tendency to deepen toward the Main Zagros Thrust. Geological and geophysical evidence suggests that most of these earthquakes probably occurred within the uppermost Precambrian basement. Because the focal mechanisms of these events indicate high‐angle, reverse faulting, the events do not occur on a single detachment surface but rather reflect horizontal shortening of the basement on numerous, short‐segmented (<100 km) and subparallel reverse faults. This geometry may explain the absence of great earthquakes (M ≅ 8) in Zagros. Synchronous widespread deformation of both the sedimentary cover and basement is a unique feature for the Zagros region unlike other fold and thrust belts. In contrast to Zagros, medium to large, thrust‐type Himalayan earthquakes define a shallow, north dipping detachment that separates the underthrusting Indian plate from the Lesser Himalayan crustal block; the great Himalayan events (M > 8) probably occurred along this detachment. A comparative study of the Zagros with the Himalayas suggests that the active tectonics of the Zagros continental collision zone and the Iranian blocks arc similar to those of the Himalayas and the Tibetan terranes in Oligocene time.
Further analysis of the 1981 Southern California Field Test for leveling refraction
Authors: Craymer M. R. , Vaníček P.
Published: February 2008

Application of least squares spectral analysis and multiple linear regression techniques to the 1981 southern California field test for leveling refraction has revealed that in addition to differential refraction, rod/instrument settlement and an effect attributed to differential rod miscalibration are also detectable. The object of our analysis was the discrepancy between the forward and backward runnings of a section as this quantity properly reflects the direction of running thereby allowing for the detection of direction dependent effects. A multiple regression model using three arguments representing differential refraction, rod/instrument settlement, and differential rod miscalibration reduced the observed variation of the discrepancies by 61% as opposed to 53% when only the National Geodetic Survey (NGS) computed refraction correction is applied. It was found that of the original 23‐mm accumulated discrepancy, 14 mm was attributed to differential refraction, 20 mm to settlement, and −14 mm to differential rod miscalibration. Analyses with the NGS computed refraction corrections applied (based on Kukkamäki's single sight equation with observed temperatures) gave similar results. It is also shown that the settlement effect is always present in any discrepancy and accumulates in the discrepancies between the forward and backward runnings while it cancels and is thus hidden in the accumulation of the NGS‐derived discrepancies between the short and long sight length runnings.
Thermoelastic response of fluid‐saturated porous rock
Authors: McTigue D. F.
Published: February 2008

A linear theory for fluid‐saturated, porous, thermoelastic media is developed. The theory allows for compressibility and thermal expansion of both the fluid and solid constituents. A general solution scheme is presented, in which a diffusion equation with a temperature‐dependent source term governs a combination of the mean total stress and the fluid pore pressure. In certain special cases, this reduces to a diffusion equation for the pressure alone. In addition, when convective heat transfer and thermoelastic coupling can be neglected, the temperature field can be determined independently, and the source term in the pressure equation is known. Drained and undrained limits are identified, in which fluid flow plays no role in the deformation. In the drained case, the medium behaves as a simple thermoelastic body with the properties of the porous skeleton with no fluid present. In the undrained limit, the fluid is trapped in the pores, and the material responds as a thermoelastic body with effective compressibility and thermal expansivity determined in part by the fluid properties. The theory is further specialized to one‐dimensional deformation, and several illustrative problems are solved. In particular, the heating of a half space is explored for constant temperature and constant flux boundary conditions on the thermal field, and for drained (zero pressure) and impermeable (zero flux) conditions on the fluid pressure field. The behavior of these solutions depends critically upon the ratio of the fluid and thermal diffusivities, with very large and very small values of this parameter corresponding to drained and undrained responses, respectively.
Geochemistry of primary and least fractionated lavas from Okmok Volcano, Central Aleutians: Implications for arc magmagenesis
Authors: Nye Christopher J. , Reid Mary R.
Published: February 2008

An olivine and spinel phyric picrite from Okmok has Fo92.9 olivine cores, Mg/(Mg+Fe2+) (Mg#) = 0.79 after correction for olivine accumulation, and a calculated liquidus temperature of 1409°C. A basalt which is also olivine and spinel phyric is different from the picrite in most aspects of trace element and isotope geochemistry and has Mg# = 0.72 and a liquidus temperature of 1286°C. Five basalts are fractionated but have trace element and isotope chemistries in between those of the two inferred primary magmas. All the lavas have Pb, Nd, Sr, and O isotopic ratios, and interelement high field strength element (HFSE) ratios similar to enriched oceanic basalts (EMORB) which are transitional between depleted mid‐ocean ridge basalts (NMORB) and ocean island basalts (OIB), although elemental concentrations of HFSE are lower in the primary Okmok magmas than in primary EMORB. Ratios of heavy rare earth element (HREE), and light REE (LREE), large ion lithophile element (LILE) to HFSE are progressively higher than EMORB ratios. Those samples most removed from the EMORB signature have the lowest 206Pb/204Pb. These features suggest progressive enrichment of an EMORB‐like mantle with an incompatible element‐rich component. In the final mantle source, 80–90% of the LILE and 30–40% of the LREE have been added. Given this extent of metasomatism and the isotopic ratios of the lavas, we see little evidence for significant sediment involvement and suggest that the metasomatic fluid is derived from a lower portion of the slab which has undergone little isotopic exchange with seawater. The high liquidus temperatures of the primary liquids, HFSE with concentrations similar to NMORB and relative abundances similar to EMORB, low Ti/V, and high TiO2/Ni in the primary magmas all require high temperatures and high degrees of partial melting of the mantle source. The temperatures required are sufficiently high that induced counterflow and upward migration of hot (1500°–1600°C) mantle is required. Any melting mechanism involving static mantle, or vertically moving diapirs, is specifically prohibited under Okmok. We propose that decompression of the upward moving limb of the induced counterflow is the dominant melting mechanism and that the melting zone is localized by impingement on the subforearc mantle wedge, that is, the melting zone (and thus the location of the volcanic front) is localized for physical rather than physiochemical reasons. Variable mixing between the static and counterflowing mantle and a variety of metasomatic fluids yields a range of primary magma compositions under Okmok and presumably under arcs in general.
Implications of magma chamber dynamics for Soret‐related fractionation
Authors: Carrigan Charles R. , Cygan Randall T.
Published: February 2008

Convection of silicate melts in magma chambers is considered as a possible mechanism for producing significant or, at least, detectable chemical fractionation by the Soret effect. Thermal boundary layer analyses show that Soret fractionation would be, at best, an extremely weak process in evolving magmatic systems. For very large amplitude thermally driven convection at horizontal chamber margins, both the magnitude of the temperature gradient and the time scale for residence of magma in the unstable thermal boundary layer are entirely inappropriate for chemical fractionation of the magma at levels comparable to those obtained in laboratory experiments (5–60%). For a typical convecting body a relative concentration enhancement of 0.04% is obtained as an estimate of the upper limit of Soret fractionation. Even if exceedingly large temperature gradients (of the order of 104 °C/m) in a transient thermal regime are attained by compositionally driven convection, the magma residence time in the thermal boundary layer is so brief (100 s) that much less chemical fractionation results (0.002%). For convection near vertical margins a kinematic model of a countercurrent flow regime provides estimates of chemical separation produced by the thermogravitational fractionation mechanism. Incorporating a range of physical and chemical parameters that characterize magma chamber convection, steady state values of concentration enhancement are even smaller than for fractionation near horizontal boundaries.
Calibrations of phase abundance, composition, and particle size distribution for olivine‐orthopyroxene mixtures from reflectance spectra
Authors: Cloutis Edward A. , Gaffey Michael J. , Jackowski Timothy L. , Reed Kevin L.
Published: February 2008

Spectral reflectance measurements of characterized (phase abundance, particle size) mixtures of olivine and orthopyroxene were utilized to define the correlations between spectral and albedo parameters of such assemblages and their mineralogical or textural properties. Thirty‐three different spectral parameters falling into three general classes (relative or ratioed, absolute or albedo, and wavelength) were investigated for empirical sensitivity to one or more of the mixture properties. Theoretical considerations and previous experimental observations were utilized to understand their functional relationships. The ratio of areas for the 1‐ and 2‐μm absorption bands is shown to be a sensitive indicator of the olivine‐orthopyroxene abundance and is very nearly independent of particle size and mineral composition. In conjunction with an abundance determination, the wavelength position of the 1‐μm absorption feature can be utilized to determine the molar iron contents of the olivine and orthopyroxene phases. This calibration is insensitive to particle size but will produce systematic deviations if the phases have significantly different iron contents or if more than a few percent of a clinopyroxene component is present. The spectral albedo in the 0.6‐ to 0.7‐μm region is relatively insensitive to phase abundance and can be used to constrain particle size if phase composition has been determined.
Exsolution of H2O, CO2, and S during eruptive episodes at Kilauea Volcano, Hawaii
Authors: Gerlach Terrence M.
Published: February 2008

A model for the exsolution of H2O, CO2, and S from Kilauea magma as it rises from a shallow crustal reservoir predicts that vigorous exsolution occurs only after magma has ascended to shallow depths (≲ 150 m lithostatic) where pressures are <3 MPa (30 bars). Exsolved vapor at saturation (30–100 MPa) is CO2‐rich (70 vol %, maximum), but H2O increases and ultimately predominates in the vapor as pressure drops. The model indicates that most H2O remains dissolved, however, down to pressures as low as 2–3 MPa (20–30 bars) rather than being exsolved abundantly below 8 MPa (80 bars) as is commonly assumed. Most CO2 is exsolved above 10 MPa, whereas most S, like H2O, is exsolved below 2–3 MPa (<100–150 m lithostatic). The critical condition for major outgassing and disruption of magma into spray is reached at 1.0–0.6 MPa (∼40–25 m lithostatic). Gas exsolution apparently continues after disruption. The total quantity of exsolved volatiles as predicted by the exsolution model agrees with that predicted by models of fire fountain dynamics. At 0.3–0.4 MPa (≲20 m lithostatic), the exsolved gases resemble volcanic gases of east rift zone eruptions and coexist with melts containing volatiles at concentrations similar to those observed in fountain spatter. Exsolution from coalesced spatter of previously disrupted melt produces residual gases that are H2O‐rich and sharply depleted in CO2. The low pressures and shallow depths required for extensive exsolution imply that magma must be transported nearly to the surface before disruption, outgassing, and fountaining are possible. Therefore eruption forecasting techniques based on detection of volatiles outgassed from ascending magma may not provide useful advance warning at Kilauea and other tholeiite basalt systems.
Gas anomalies at three mineral springs and a fumarole before an inland earthquake, central Japan
Authors: Sugisaki Ryuichi , Sugiura Tutomu
Published: February 2008

Conspicuous anomalies in gas compositions were observed at a fumarole and three mineral springs about 1–3 months prior to an inland earthquake of 6.8 magnitude in central Japan, on September 14, 1984. The epicentral distances are 9 km for the fumarole and 50, 71, and 95 km for the springs. The anomalies in He/Ar, N2/Ar, and CH4/Ar ratios of bubble gases from the springs can be attributed to fluctuations in the emission rate of some deep‐seated gas due to change in pore pressure, which is subjected to crustal stress, resulting in the earthquake. The anomaly at the fumarole showed an increase in discharge rate of deep volcanic gas under a compressional stress near the focal region. Since the behavior of subsurface fluids represented by He/Ar and other ratios is presumably controlled by the change of pore pressure, temporal variations in the fluids can be used as a strain gauge for the crust. On the other hand, the precursory emissions of H2, which were observed after changes in He/Ar and other ratios had occurred, may have resulted from a reaction between groundwater and the surface of newly formed cracks. The timing and amplitude of the geochemical anomalies are compatible with the result predicted by a dilatancy model without fluid diffusion, although H2emission may be related to groundwater permeation into newly formed cracks. Monitoring of several gases with automatic gas chromatographs at several stations may be useful for unraveling earthquake mechanisms and for predicting earthquakes.
Monitoring of hydrogen along the San Andreas and Calaveras faults in central California in 1980–1984
Authors: Sato Motoaki , Sutton A. J. , McGee K. A. , Russell‐Robinson Susan
Published: February 2008

Hydrogen (H2) has been monitored continuously at 1.5‐m depth at nine sites along the San Andreas and Calaveras faults in central California since December 1980. Site characteristic small noninstrumental diurnal variations were recorded during quiescent periods at most sites. Abrupt H2 changes were observed concurrently at two sites on the Calaveras fault; some of these were correlated with oscillatory fault slips. Large (1000–4000 ppm) H2 increases were recorded at some sites on the San Andreas fault between July 1982 and November 1983, which may be correlated with eleven M ≥ 5 earthquakes that occurred near Coalinga during this period. We attribute both the H2 increases and the triggering of the earthquakes to a large‐scale compressive stress field within the ductile mafic crust near the plate boundary. The stress perhaps caused bulging of the base of the brittle upper crust and thus caused dilation of the San Andreas fault zone, allowing the escape of pent‐up H2 generated by hydration reaction of the mafic crust. At the same time, mobile serpentinites may have squeezed into the seismogenic fault beneath the Coalinga area triggering the earthquakes.
Rupture characteristics and tomographic source imaging ofML∼ 3 earthquakes near Anza, southern California
Authors: Frankel Arthur , Fletcher Jon , Vernon Frank , Haar Linda , Berger Jon , Hanks Thomas , Brune James
Published: February 2008

The rupture characteristics of severalML∼ 3 earthquakes near Anza, California, are determined from time domain analysis of path‐corrected, displacement pulses. These pulses are corrected for propagation effects (apparent attenuation, scattering, site resonance) using the waveforms of adjacent small events (ML≤ 2.1) as empirical Green's functions. The earthquakes studied occurred during two swarm sequences near Cahuilla, in an area characterized by swarm activity at shallow depth (< 5 km). The data consist of seismograms recorded by the Anza seismic network, a high dynamic range local array with digital telemetry. The waveforms of adjacent small events (ML≤ 2.1) are deconvolved from those of theML∼ 3 events to yield the source time functions of the larger earthquakes. TheML∼ 3 earthquakes display a variety of rupture modes, similar to those reported for large shocks but over much shorter time scales and distances. Events with single and multiple ruptures are observed. Relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms. Some events (as small asML2.4) display unilateral rupture propagation, as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses. A tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault, using a one‐dimensional fault model. The tomographic inversion for one of the events (ML2.4) reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity (Vs) and fault length of 300 m. A similar inversion for anML3.7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0.8Vs. These rupture velocities are similar to those reported for large earthquakes, indicating that rupture velocity is independent of seismic moment down to 3.1 × 1019dyn cm, the smallest event studied. There are clear variations in pulse width for neighboring events with similar seismic moments, implying significant differences in static stress drops. Factor of five variations in stress drop (11–61 bars) are calculated for events within 200 m of each other. Thus the stress and/or strength along this fault zone varies considerably over distances of hundreds of meters.
Solubility and diffusional uptake of hydrogen in quartz at high water pressures: Implications for hydrolytic weakening
Authors: Kronenberg A. K. , Kirby S. H. , Aines R. D. , Rossman G. R.
Published: February 2008

Attempts to introduce molecular water into dry, natural quartz crystals by diffusive transport and thus weaken them hydrolytically at T = 700°–900°C and PH2O = 400–1550 MPa have failed. Infrared spectroscopy of hydrothermally annealed single crystals of natural quartz reveals the diffusive uptake of interstitial hydrogen (resulting in hydroxyl groups) at rates similar to those previously proposed for intracrystalline water at high water pressures. The solubility of interstitial hydrogen at these conditions is independent of temperature and pressure; instead, it depends upon the initial aluminum concentration by the local charge neutrality condition [Hi·] = [AlSi′]. The rate of interstitial hydrogen diffusion parallel to c is given by an Arrhenius relation with D0 = 1.4 × 10−1 m2/s and Q = 200 ± 20 kJ/mol, in close agreement with H diffusivities reported for much lower pressures (PH2O = 2.5 MPa). Deformation experiments following hydrothermal annealing show no mechanical weakening, and the lack of any detectable broadband absorption associated with molecular water shows that the diffusion rates of structural water are much lower than those of hydrogen. These results are consistent with the available oxygen diffusion data for quartz and with the failure to observe weakening in previous studies of quartz deformation at pressures of 300–500 MPa; they call into question the rapid rates of diffusion originally suggested for the hydrolytic weakening defect. It is suggested that the observed weakening in many previous experiments was complicated by microcracking processes in response to nonhydrostatic stresses and low effective confining pressures. Extensive microcracking may provide a mechanism for molecular water to enter quartz and allow local plastic deformation to occur. It does not appear that molecular water can diffuse far enough into uncracked quartz to allow hydrolytic weakening over annealing times that are feasible in the laboratory.
An analysis of morphologic variation in simple lunar craters
Authors: Ravine M. A. , Grieve R. A. F.
Published: February 2008

Previous photogeologic classification of lunar craters have differentiated them into various morphologic types. In an attempt to identify the physical factors responsible for these morphologic variations, this study examined 44 fresh, simple lunar craters between 1.1 and 14 km in diameter on the maria. Photogeologic analysis was performed using Apollo stereo panoramic photography, while a quantitative analysis of cross‐sectional shape was done using Fourier analysis. These analyses showed that the sample could not be split into two distinct types (flat‐ and round‐floored) as had been done previously. The two crater types at best represent end‐members of a distribution of morphologies. Some correlation was found between the degree of relative degradation and floor roundness, implying that flat floors are primary and round floors are degradational features. No difference was found between the average depth‐to‐diameter ratio of flat‐ and round‐floored craters, indicating that the factor of two variation in depth‐to‐diameter ratio in this sample of fresh craters is a primary morphologic feature and that minor degradation of fresh craters has no significant effect on average crater depth.
The role of spurious correlation in the development of a komatiite alteration model
Authors: Butler John C.
Published: February 2008

Current procedures for assessing the degree of alteration in komatiites stress the construction of variation diagrams in which ratios of molecular proportions of the oxides are the axes of reference. For example, it has been argued that unaltered komatiites related to each other by olivine fractionation will display a linear variation with a slope of 0.5 in the space defined by [SiO2/TiO2] and [(MgO+FeO)/TiO2]. Extensive metasomatism is expected to destroy such a consistent pattern. Previous workers have tended to make use of ratios that have a common denominator. It has been known for a long time that ratios formed from uncorrelated variables will be correlated (a so‐called spurious correlation) if both ratios have a common denominator. The magnitude of this spurious correlation is a function of the coefficients of variation of the measured amounts of the variables. If the denominator component has a coefficient of variation that is larger than those of the numerator components, the spurious correlation will be close to unity; that is, there will be nearly a straight‐line relationship. As a demonstration, a fictitious data set has been simulated so that the means and variances of SiO2, TiO2, and (MgO + FeO) match those of an observed data set but the components themselves are uncorrelated. A plot of (SiO2/TiO2) versus [(MgO + FeO)/TiO2] of these simulated data produces a distribution of points that appears every bit as convincing an illustration of the lack of significant metasomatism as does the plot of the observed data. The assessment of the strength of linear association is a test of the observed correlation against an expected value (the null value) of zero. When a spurious correlation arises as a result of the formulation of ratios with a common denominator, zero is clearly an inappropriate choice as the null. It can be argued that the spurious correlation is, in fact, a more suitable null value. An analysis of komatiites from Gorgona Island and the Barberton suite reveals that the strong linear association could have been produced by forming ratios from uncorrelated starting chemical components. Ratios without parts in common are to be preferred in the construction of petrogenetic models.
Dynamics of a fluid‐driven crack in three dimensions by the finite difference method
Authors: Chouet Bernard
Published: February 2008

The finite difference method is applied to the study of the dynamics of a three‐dimensional fluid‐filled crack excited into resonance by the sudden failure of a small barrier of area ΔS on the crack surface. The impulse response of the crack is examined for various ratios of crack width to crack length and for several values of the crack stiffness C = (b/μ)(L/d), where b is the bulk modulus of the fluid, μ is the rigidity of the solid, and L and d are the crack length and crack thickness, respectively. The motion of the crack is characterized by distinct time scales representing the duration of brittle failure and the periods of acoustic resonance in the lateral and longitudinal dimensions of the source. The rupture has a duration proportional to the area of crack expansion and is the trigger responsible for the excitation of the crack into resonance; the resonant periods are proportional to the crack stiffness and to the width and length of the crack. The crack wave sustaining the resonance is analogous to the tube wave propagating in a fluid‐filled borehole. It is dispersive, showing a phase velocity that decreases with increasing wavelength. Its wave speed is always lower than the acoustic velocity of the fluid and shows a strong dependence on the crack stiffness, decreasing as the stiffness increases. The initial motion of the crack surface is an opening, and the radiated far‐field compressional wave starts with a strong but brief compression which has a duration proportional to the crack stiffness and size of the rupture area; the amplitude of this pulse increases with the area of rupture but decreases with increasing stiffness. Flow into the newly created cavity triggers a pressure drop in the fluid, which produces a partial collapse of the wall propagated over the crack surface at the speed of the crack wave. The collapse of the crack surface generates a weak long‐period component of dilatation following the compressional first motion in the far‐field P wave train; the dilatational component is clearer in the signal from stiffer cracks when seen in the direction of the rupture. The energy loss by radiation is stronger for high frequencies, resulting in a progressive enrichment of the crack response in lower frequencies over the duration of resonance. These source characteristics translate into a far‐field signature that is marked by a high‐frequency content near its onset and dominated by a longer‐period component in its coda. The source duration shows a strong dependence on the fluid viscosity and associated viscous damping at the crack wall.
Use of focal mechanisms to determine stress: A control study
Authors: Michael Andrew Jay
Published: February 2008

To allow focal mechanisms to be inverted for the stress field requires a different inversion algorithm than for slickenside data because focal mechanisms do not represent fault slip data unless one can decide which nodal plane is the fault plane. If one can decide which nodal plane is the fault plane, then the focal mechanisms can be inverted with the slickenside inversion algorithm. This decision cannot always be made, so algorithms for inverting focal mechanisms for the stress field are studied. These algorithms either use both of the possible fault planes or attempt to choose the correct fault plane while determining the stress tensor. Simulated focal mechanisms are made from slickenside data and used to provide a control study for the focal mechanism inversion algorithms. The results of this control study show that focal mechanisms can be inverted to find the best stress tensor, but the resolution is decreased unless the fault planes can be picked a priori. The resolution can also be increased by including constraints on the magnitude of the tangential traction on the fault plane. Therefore, using focal mechanisms to study small variations in the stress field requires that other data (e.g., studies of the hypocenters, surface faulting, or structural information concerning the region) be introduced to pick which of the nodal planes is the fault plane. This study also introduces the method of bootstrap resampling to the statistics of this problem. The non‐Gaussian nature of the data makes the nonparametric formulation of the bootstrap approach ideal for this problem.
Interaction between the Chile Ridge and Chile Trench: Geophysical and geothermal evidence
Authors: Cande S. C. , Leslie R. B. , Parra J. C. , Hobart M.
Published: February 2008

Geophysical and geothermal data are examined from the three southernmost sections of the Chile Ridge, starting at 44°S and continuing south to the triple junction of the Nazca, Antarctic, and South America plates at 47°S. These sections represent three progressively younger stages in a ridge‐trench collision event, corresponding to 3 m.y. before the collision, 1 m.y. before the collision, and culminating in an ongoing collision at the triple junction. Magnetic and bathymetric data across the Chile Rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench. In the collision zone a “normal” looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope. There is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the Pacific‐Farallon spreading center collided with North America. In contrast, the overriding plate is greatly affected by the ridge collision. The landward trench slope steepens and narrows as the collision zone is approached. At the southern end of the collision zone a ridge associated with the Taitao Fracture Zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope. Geothermal measurements were made along three transects of the margin, corresponding to the time 3 m.y. before the collision, during the collision, and 6 m.y. after the collision. The heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge: Values as high as 345 mW/m2 were recorded on the lower trench slope. A prominent bottom‐simulating reflector (BSR) observed over a wide area of the landward trench slope north of the triple junction, and, more locally, south of the triple junction, is used to expand the grid of heat flow observations. Excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the BSR. The heat flow measurements compare favorably with a theoretical model assuming conductive heat flow. We estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion. Periodic ridge collisions in the past may account for the apparent truncation of the Andean forearc region.
On the robust estimation of power spectra, coherences, and transfer functions
Authors: Chave Alan D. , Thomson David J. , Ander Mark E.
Published: February 2008

Robust estimation of power spectra, coherences, and transfer functions is investigated in the context of geophysical data processing. The methods described are frequency‐domain extensions of current techniques from the statistical literature and are applicable in cases where section‐averaging methods would be used with data that are contaminated by local nonstationarity or isolated outliers. The paper begins with a review of robust estimation theory, emphasizing statistical principles and the maximum likelihood or M‐estimators. These are combined with section‐averaging spectral techniques to obtain robust estimates of power spectra, coherences, and transfer functions in an automatic, data‐adaptive fashion. Because robust methods implicitly identify abnormal data, methods for monitoring the statistical behavior of the estimation process using quantile‐quantile plots are also discussed. The results are illustrated using a variety of examples from electromagnetic geophysics.
The effective pressure law for permeability during pore pressure and confining pressure cycling of several crystalline rocks
Authors: Bernabe Yves
Published: February 2008

The permeability of samples of Pottsville sandstone, Pigeon Cove granite, and Westerly granite was measured while simultaneously cycling Pc and Pp. From these data, “tangent” values of α, the coefficient of the effective pressure law, were calculated at intervals during the cycles. Tangent α's were strongly affected by the order in which the increments in Pc and Pp were applied to the samples. When the pore pressure was changed before the confining pressure, α took on values near 1.0, whereas it was significantly lower in the other case. These results confirm observations previously made on two other granites and published elsewhere. However, this dependency on stress path decreased rapidly with the number of cycles. After a few cycles, α approached 1.0, which apparently favors the use of the simple difference, Pc‐Pp, for evaluating the effective pressure.
Fluid flow through rock joints: The effect of surface roughness
Authors: Brown Stephen R.
Published: February 2008

Fluid flow through rock joints is commonly described by the parallel plate model where the volume flow rate varies as the cube of the joint aperture. However, deviations from this model are expected because real joint surfaces are rough and contact each other at discrete points. To examine this problem further, a computer simulation of flow between rough surfaces was done. Realistic rough surfaces were generated numerically using a fractal model of surface topography. Pairs of these surfaces were placed together to form a “joint” with a random aperture distribution. Reynolds equation, which describes laminar flow between slightly nonplanar and nonparallel surfaces, was solved on the two‐dimensional aperture mesh by the finite‐difference method. The solution is the local volume flow rate through the joint. This solution was used directly in the cubic law to get the so‐called “hydraulic aperture.” For various surface roughnesses (fractal dimensions) the hydraulic aperture was compared to the mean separation of the surfaces. At large separations the surface topography has little effect. At small separations the flow is tortuous, tending to be channeled through high‐aperture regions. The parameter most affecting fluid flow through rough joints is the ratio of the mean separation between the surfaces to the root‐mean‐square surface height. This parameter describes the distance the surface asperities protrude into the fluid and accounts for most of the disagreement with the parallel plate model. Variations in the fractal dimension produce only a second‐order effect on the fluid flow. For the range of joint closures expected during elastic deformation these results show that the actual flow rate between rough surfaces is about 70–90% of that predicted by the parallel plate model.
"Centroid depths and mechanisms of mid‐ocean ridge earthquakes in the Indian Ocean. Gulf of Aden, and Red Sea""
Authors: Huang Paul Y. , Solomon Sean C.
Published: February 2008

We have determined the centroid depths and source mechanisms of 14 earthquakes along spreading centers in the Indian Ocean, Gulf of Aden, and Red Sea. The earthquakes are all characterized by normal faulting on planes dipping at about 45° and generally striking parallel to the local trend of the spreading center. For the ridge axis earthquakes in the Indian Ocean, Gulf of Aden, and central Red Sea, seismic moments range from 3 to 9 × 1024 dyn cm, source time functions are all of simple form, and centroid depths of events with adequate azimuthal coverage are all very shallow, between 1 and 4 km beneath the seafloor. If the centroid depth typically marks the mean depth of fault slip, then earthquake faulting extended to 2–8 km beneath the seafloor during these earthquakes. The centroid depths for two earth‐quakes near the mouth of the Gulf of Suez support the view that seismic faulting accompanying continental rifting generally extends to greater depth than does seismic faulting along the axes of oceanic spreading centers. When these findings are combined with our earlier results from the Mid‐Atlantic Ridge, the greatest centroid depths of earthquakes with well‐constrained source mechanisms tend to shoal with increasing spreading rate. This result supports a general decrease with spreading rate in the maximum thickness of the zone of brittle behavior for slow spreading ridges.
Miocene paleomagnetism and tectonic setting of the Baja California Peninsula, Mexico
Authors: Hagstrum J. T. , Sawlan M. G. , Hausback B. P. , Smith J. G. , Grommé C. S.
Published: February 2008

A paleomagnetic pole calculated from 64 virtual geomagnetic poles for Miocene volcanic rocks of the Baja California Peninsula (86.1°N, 142.6°E, α95= 3.9°) is similar to a reference pole for the North American craton (87.4°N, 129.7°E, α95= 3.0°), consistent with the Baja Peninsula having been adjacent to the Mexican mainland in its pre‐Gulf of California position from at least early Miocene time to the Pliocene opening of the Gulf. This result is in agreement with marine geophysical and geologic evidence from the Gulf of California and Baja Peninsula and does not support a long‐period nondipole geomagnetic field in the region during Miocene time, or 10° of northward transport of the peninsula since then, inferred from smaller data sets (included in this work) by previous workers. This study also indicates that a pre‐Gulf of California northward translation (9° of latitude) and clockwise rotation (24°) of the Baja Peninsula with respect to North America, determined from paleomagnetic studies of Cretaceous rocks within peninsular California occurred prior to Miocene time. Paleomagnetic results from Late Cretaceous to early Tertiary basement rocks in western Sinaloa, Mexico are similar to those results from Cretaceous rocks of the Baja Peninsula, supporting the early Tertiary northward translation of the peninsula between 45 and 34 Ma and suggesting that the tectonic boundary separating peninsular California from North America at that time was east of the present‐day Gulf of California.
Chemical composition and origin of Apollo 15 impact melts
Authors: Ryder Graham , Spudis Paul
Published: February 2008

To better understand the Apennine Front, we have analyzed 14 Apollo 15 impact melt rocks, including hand samples, rake samples, and coarse fines, for major (fused bead microprobe) and trace element (INAA) abundances. The melt rocks show a remarkable diversity of chemical composition, although all but one have typical KREEP incompatible‐element patterns. They appear to form five chemical groups, all but the most KREEP‐rich of which may represent single impacts. Three groups are of low‐K Fra Mauro composition; none correspond to very high alumina (VHA) compositions. The compositions are unlike nearly all other impact melts and regoliths from highland sites, indicating a distinct provenance for the Apollo 15 samples. Serenitatis basin impact melt compositions (Apollo 17 poikilitic boulders) are not duplicated, although one Apollo 17 aphanitic melt sample (73215) is similar to one of the Apollo 15 groups. The Apollo 15 group consisting only of 15445 and 15455 remains the best candidate for Imbrium basin impact melt, while the most KREEP‐rich group probably consists of melts produced in varied post‐Imbrium impacts into the Apennine Bench Formation (Apollo 15 KREEP basalt flows). The most populated group was previously unrecognized and could be either pre‐ or post‐Imbrium basin; it is possible that it, not 15445/15455, represents Imbrium melt. One unique sample has a non‐KREEP incompatible element pattern with low abundances and a positive Eu anomaly. Its target was rather primitive and would not appear to be of regional extent or present in the area in post‐Imbrium times; it is either a pre‐Imbrian melt or exotic. The Apennine Front does not consist of a monolithic LKFM composition but shows a much wider diversity of compositions than was previously recognized.
The nature of the meteoritic components of Apollo 16 soil, as inferred from correlations of iron, cobalt, iridium, and gold with nickel
Authors: Korotev Randy L.
Published: February 2008

Concentrations of Co, Ni, Ir, and Au in small samples of submature and mature soil from Apollo 16 are highly variable. Correlations of Fe, Co, Ir, and Au with Ni indicate that the variation in siderophile element concentrations results from variation in the concentration of Fe‐Ni metal. From these correlations, the mean composition of the metal is inferred to be 5.6% Ni and 0.36% Co for a Ni/Co ratio of 15.5 ± 0.7, i.e., dissimilar to the chondritic ratio of 20–21. The Fe‐Ni metal is also characterized by low Ir/Ni and Ir/Au ratios compared to metal in ordinary chondrites. Metal with a similar composition is also found in metal‐rich, noritic impact melt breccias that occur as discrete rocks as well as small clasts in ancient regolith breccias at Apollo 16. Mass‐balance models using lithophile element concentrations indicate that about 35% of the soil is noritic impact melt. The Fe‐Ni metal associated with this component is sufficient to supply virtually all the ancient Fe‐Ni metal found in the typical Apollo 16 soil (0.4–0.5%), i.e., no other source is required. The soil also contains 1–2% carbonaceous chondrite. The concentration of this component appears to be relatively constant from sample to sample and does not contribute significantly to the variation in siderophile element abundances. These two meteoritic components contribute nearly equal concentrations of Ni to the typical Apollo 16 soil.
Core formation in the shergottite parent body and comparison with the Earth
Authors: Treiman Allan H. , Jones John H. , Drake Michael J.
Published: February 2008

The mantle of the shergottite parent body (SPB) is depleted relative to the bulk SPB in siderophile and chalcophile elements; these elements are inferred to reside in the SPB's core. Our chemical model of these depletions rests on a physically plausible process of segregation of partially molten metal from partially molten silicates as the SPB grows and is heated above silicate and metallic solidi during accretion. Metallic and silicate phases equilibrate at low pressures as new material is accreted to the SPB surface. Later movement of the metallic phases to the planet's center is so rapid that high‐pressure equilibration is insignificant. Partitioning of siderophile and chalcophile elements among solid and liquid metal and silicate determines their abundances in the SPB mantle. Using partition coefficients and the SPB mantle composition determined in earlier studies, we model the abundances of Ag, Au, Co, Ga, Mo, Ni, P, Re, S, and W with free parameters being oxygen fugacity, proportion of solid metal formed, proportion of metallic liquid formed, and proportion of silicate that is molten. Abundances of all elements except Au, Mo, and Re may be reproduced adequately with oxygen fugacity near one log‐unit below the iron‐wüstite buffer, proportion of molten silicate near 0.3, and a core composed of subequal parts of solid and liquid metal constituting approximately 30% of the mass of the SPB. The abundances of Au and Re may be reproduced using different (but still permissible) partition coefficients by invoking a small “chondritic veneer” mixed into the mantle after core formation or by assuming that small fractions of solid and liquid metal are trapped in the mantle during core formation. Molybdenum cannot be modeled adequately; problems may include inaccurate partition coefficients, inaccurate estimation of the abundance of Mo in the SPB mantle and the possibility that the mantle source of the SNC meteorites is not representative of the whole SPB mantle. In contrast to the SPB, there is at present no hypothesis capable of quantitatively reproducing elemental abundances in the earth's mantle. The contrast suggests that these two terrestrial planets may have accreted or differentiated differently, but the cause of such differences is unclear.
A test of the longevity of impact‐induced faults as preferred sites for later tectonic activity
Authors: Solomon Sean C. , Duxbury Elizabeth D.
Published: February 2008

The hypothesis that impact‐induced faults have been preferred sites for later deformation in response to lithospheric stresses has been suggested for several planets and satellites. We investigate this hypothesis on earth by examining whether terrestrial impact structures show higher rates of nearby earthquake activity than do surrounding intraplate regions. For 28 of 30 probable impact structures having an original crater 20 km or more in diameter, the rates of nearby seismicity have been no higher than the regional background rates. For two large probable impact structures, Vredefort and Charlevoix, with higher than normal rates of nearby seismicity, factors other than slip on impact‐induced faults appear to control the occurrence of earthquakes. We conclude that impact‐induced faults, at least on earth, do not persist as lithospheric “weak zones” for periods in excess of several million years after the impact event.
layering of alpine‐Type peridotite and the segregation of partial melt in the upper mantle
Authors: Obata Masaaki , Nagahara Nobuhiko
Published: February 2008

For better understanding of the partial melting and melt segregation processes in the upper mantle, we have studied the layered structure of a peridotite mass in Hokkaido, Japan. The layering is characterized by wavy and oscillatory patterns both in terms of bulk rock and mineral chemistry. From the observed linear trends in bulk rock chemistry, we have concluded that such layering was caused by segregation of partial melt in the mantle. Considering the geological and geochemical data, a physical model of melt segregation in the gravity field is put forward. Because permeable flow is strongly coupled with solid creep and compaction, magma waves may be generated by a fluid dynamic oscillation in a partially molten, ascending mantle. A vertical chemical stratification may thus be obtained, starting from a homogeneous mantle. According to the hypothesis, the most depleted part, dunite, is a residue of partial melting, and the least depleted part, plagioclase Iherzolite, represents a melt accumulation region. The partial melt involved was probably picritic in composition.