bluuebunny/crossref_metadata_2025 huggingface dataset:
Till beneath ice stream B: 2. Structure and continuity
Authors: Rooney S. T. , Blankenship D. D. , Alley R. B. , Bentley C. R.
Published: February 2008

During the 1984–1985 Antarctic field season, 8.3 km of high‐resolution seismic reflection data were collected in order to image a thin till layer beneath ice stream B, West Antarctica. Two parallel seismic reflection lines were oriented transverse to ice flow. These data show that the till layer varies in thickness but is continuous over almost the entire length of the profiles with an average thickness of 6.5 m. The upper surface of the till layer is smooth, but the lower boundary is fluted parallel to flow. These till‐filled flutes are as much as 13 m deep and 1000 m across. Nowhere on the profiles can any feature be discerned to penetrate more than a few meters into the ice from the bed. Reflection events from lithified sediments of unknown type are observed to be truncated by the till in an angular unconformity.
Sediment deformation beneath glaciers: Rheology and geological consequences
Authors: Boulton G. S. , Hindmarsh R. C. A.
Published: February 2008

Experiments beneath Breidamerkurjökull in Iceland have led to development of flow laws for the subglacial till, relating strain rate to shear stress and effective pressure and assuming either Bingham fluid or nonlinearly viscous fluid behavior. Water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment, which inhibits sliding at the ice/sediment interface. Where water pressures are equal or near to ice pressures, infiltration does not occur and sliding may result. A one‐dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation. This predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice/bed interface or unstable states where zero or negative effective pressures are predicted. Time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds. It is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed. It is suggested that unstable deformation at zero or negative effective stress leads to “piping” in subglacial sediments at the glacier terminus and the growth of sediment‐floored, subglacial tunnels. Their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation. Where they discharge large water volumes, subglacial sediments flow laterally toward them producing “tunnel valleys.” This sediment is then removed by water flowing along the axial tunnel. Tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas.
Coexistence curves and equilibrium boundaries for high‐pressure phase transformations
Authors: Jeanloz Raymond
Published: February 2008

A model is presented for analyzing experimental observations of high‐pressure phase equilibria among two crystalline polymorphs. The equilibrium phase boundary is determined in the quasi‐harmonic approximation as a function of three parameters: the zero‐kelvin transition pressure (P0tr), the average vibrational frequency of one of the phases (ω1), and the ratio of the average frequencies of the two phases (ω2/ω1). Thus modeled, the phase boundary can be extrapolated from the moderate to high temperatures at which experiments are carried out to low temperatures, at which quantitative comparisons with quantum mechanical calculations are possible. A kinetic analysis is presented in terms of time‐pressure‐transformation relations. With suitable assumptions, this analysis permits an average activation energy for the transformation (Δg±) to be determined from the hysteresis observed in pressure‐temperature phase equilibrium studies.
Submersible observations along the southern Juan de Fuca Ridge: 1984 Alvin Program
Authors: Normark William R. , Morton Janet L. , Ross Stephanie L.
Published: February 2008

In September 1984, the research submersible Alvin provided direct observations of three major hydrothermal vent areas along the southernmost segment of the Juan de Fuca Ridge (JFR). The submersible operations focused on specific volcanologie, structural, and hydrothermal problems that had been identified during the preceding 4 years of photographic, dredging, acoustic imaging, and geophysical studies along a 12‐km‐long section of the ridge. A continuously maintained (from 1981 to the present) net of seafloor‐anchored acoustic transponders allowed the observations from Alvin to be directly tied to all previous U.S. Geological Survey data sets and National Oceanic and Atmospheric Administration water column surveys from 1984 to the present. The three vent areas studied are the largest of at least six areas identified by previous deep‐towed camera surveys that lie within a deep cleft, which marks the axis of symmetry of the JFR in this region. The cleft appears to be the locus of eruption for this segment of the JFR. The vent areas, at least in part, are localized near what appear to be previous volcanic eruptive centers marked by extensive lava lake collapse features adjacent to the cleft at these sites. Each hydrothermal area has several active discharge sites, and sulfide deposits occur as clusters (15–100 m2) of small chimneys, individual large chimneys, or clusters of large branched chimneys. We review the dive program and present a brief synthesis of the geology of the vent sites together with sample and track line compilations.
Quantitative characterization of microcracks at elevated pressure
Authors: Abdel‐Gawad Monem , Bulau James , Tittmann Bernie
Published: February 2008

A new technique is described that has been used for the first time to measure microcrack densities, size distributions, and aspect ratios in rocks at elevated effective pressures. The distribution of microcracks in Coconino sandstone has been examined quantitatively at effective pressures to 34.5 MPa. The mean crack thickness and aspect ratio decreased monotonically with increasing effective pressure. This was accompanied by a significant reduction in crack porosity. However, overall crack density changed most between 0 and 3.45 MPa and remained nearly constant at higher pressures. The abundance of the relatively thick microcracks (0.5–4 μm) decreased most with increasing pressure, suggesting that changes in seismic quality factor Q and velocity with increasing pressure may be related primarily to the closure of the relatively thick cracks with high aspect ratio, as opposed to the relatively thin cracks with low aspect ratio.
Introduction to Special Section on How Volcanoes Work: Part 2
Authors: Tilling Robert I.
Published: February 2008

The special section on “How Volcanoes Work” is a collection of some of the nearly 300 papers presented at the Hawaii Symposium on How Volcanoes Work, held in Hilo in January 1987 [Tilling, 1987a]. The response of the symposium participants to the invitation to submit their papers for consideration in the special section exceeded expectations. In all, more than 70 manuscripts were received, reviewed, and handled following regular JGR editorial procedures.

Part 1 of the special section, composed of the papers that were processed and accepted in time to meet the production schedule, was published in the December 1987 issue of the Journal of Geophysical Research‐Solid Earth and Planets [Tilling, 1987a]. Even though part 1 contained only eight papers, it nonetheless provided a representative sampling of the multidisciplinary nature of the investigations undertaken to improve our understanding of volcanic systems. The papers published in part 1 included ground deformation studies on caldera systems [Dzurisin and Yamashita, 1987; Mortensen and Hopkins, 1987], geochemical and observational studies of the lava fountains and of volcanic fume during the 1983–1984 activity of Pu'u 'O'o in Kilauea's east rift zone [Crowe et al., 1987; Head and Wilson, 1987], magmatic crystal stratigraphy and theoretical petrology [Pearce et al., 1987; Russell, 1987], dynamics of active lava lakes [Tilling, 1987b], and seismic evidence for a subcrustal intrusive complex beneath the Island of Oahu, Hawaii [ten Brink and Brocher, 1987].
A temporal change in coda wave attenuation observed during an eruption of Mount St. Helens
Authors: Fehler Michael , Roberts Peter , Fairbanks Tom
Published: February 2008

During the past few years there have been numerous reports of changes in coda wave attenuation occurring before major earthquakes. These observations are important because they may provide insight into stress‐related structural changes taking place in the focal region prior to the occurrence of large earthquakes. The results of these studies led us to suspect that temporal changes in coda wave attenuation might also accompany volcanic eruptions. By measuring power decay envelopes for earthquakes at Mount St. Helens recorded before, during, and after an eruption that took place during September 3–6, 1981, we found that coda Q−1 for frequencies between 6 and 30 Hz was 20–30% higher before the eruption than after. The change is attributed to an increase in the density of open microcracks in the rock associated with inflation of the volcano prior to the eruption. Q−1 was found to be only weakly dependent on frequency and displayed a slight peak near 10 Hz. The weak frequency dependence is attributed to the dominance of intrinsic attenuation over scattering attenuation, since it is generally accepted that intrinsic attenuation is constant with frequency, whereas scattering attenuation decreases strongly at higher frequencies. The weak frequency dependence of Q−1 at Mount St. Helens contrasts with results reported for studies in non volcanic regions. The peak in Q−1 near 10 Hz at Mount St. Helens is attributed to the scale length of heterogeneity responsible for generating backscattered waves. Results for non volcanic regions have shown this peak to occur near 0.5 Hz. Thus a smaller scale length of heterogeneity is required to explain the 10‐Hz peak at Mount St. Helens.
The El Cajete Series, Valles Caldera, New Mexico
Authors: Self S. , Kircher D. E. , Wolff J. A.
Published: February 2008

Three rhyolite eruptive units totaling 3 km3 dense rock equivalent volume are the youngest products from the Valles caldera, New Mexico. These pyroclastic and effusive units, herein called the El Cajete Series, were erupted over an appreciable time period with Plinian and ignimbrite‐forming activity preceding an effusive phase by a lengthy but indeterminable interval. New U‐Th disequilibrium and published fission track dates support an age for the event between 170 and 130 ka. The explosive phases formed the El Cajete Plinian deposit (about 1.3 km3), minor dry pyroclastic surges, and the Battleship Rock tuff (about 1.0 km3), a valley‐confined welded ignimbrite. The Plinian eruption column is estimated to have been 28 km high during deposition of the most widely dispersed pumice fall unit. Slow effusion of the Banco Bonito obsidian lava flow (<1.0 km3) onto a dissected surface cut into the El Cajete and Battleship Rock pyroclastic deposits, accompanied by minor explosive activity, terminated the event. Twenty‐four bulk‐rock samples from all three eruptive stages exhibit little significant compositional variation, and the three units are petrographically identical apart from differences arising from contrasting eruption styles. Very few phenocrysts appear to have been in equilibrium with their enclosing high‐silica rhyolitic liquid. Plagioclase grains are typically strongly resorbed, while biotite and hornblende frequently occur in aggregate grains in which textures characteristic of plutonic rocks can be discerned. These features result from partial melting of preexisting crustal igneous rock, probably of dioritic to granodioritic composition, and subsequent eruption of the resulting liquid plus restite crystals. Rapid generation and eruption of rhyolitic magma during this most recent phase of activity in the Jemez Mountains may imply that the Valles magma system is presently in a state where small magma bodies are transient phenomena.
Crystallization remanent magnetization during the transformation of maghemite to hematite
Authors: Özdemir Özden , Dunlop David J.
Published: February 2008

We have studied crystallization remanent magnetization (CRM) in the γFe2O3 → αFe2O3 transformation, starting from synthetic, equidimensional maghemite crystals of single‐domain size (median diameter of 245 Å). Inversion of maghemite to hematite on heating was indicated by an exothermic peak in differential thermal analysis (DTA) at 555°C. In CRM experiments, samples of dispersed maghemite were heated in zero field to temperatures between 300°C and 604°C, held for 3 hours in a 50‐μT field HCRM, and cooled in zero field to 20°C. The intensity of the resulting CRM MCRM increased with increasing conversion to hematite, reaching a maximum of 784 A m−1 (0.784 emu cm−3) after the 556°C run, then decreased to 1.3 A m−1 (0.0013 emu cm−3) after the 604°C run. Maximum CRM intensity and the most rapid inversion indicated by DTA both occurred around 555°C. MCRM partially inverted samples was always parallel to HCRM. Hematite cannot explain the high intensity and moderate coercivity of the CRM. The bulk of the CRM seems to be carried by maghemite and to be acquired viscously rather than by grain growth. In a second set of experiments, CRM was produced in the presence of a preexisting anhysteretic remanent magnetization (ARM) MARM perpendicular to HCRM. The resulting remanence MT was a composite of surviving anhysteretic remanent magnetization ARM plus CRM directed parallel to HCRM, rather than a single vector of intermediate direction. MT can be synthesized by orthogonally combining CRMs measured in the first set of experiments with ARMs heated in zero field. Phase coupling of maghemite and hematite is not necessary to explain the main features of CRM, with or without an m initial remanence.
Lateral heterogeneity in the upper oceanic crust at Deep Sea Drilling Project Site 504
Authors: Stephen Ralph A.
Published: February 2008

The lateral variability of the upper oceanic crust can be studied by mapping the amplitude (or power) content of arrivals from a dense array of identical sources to a single receiver. In this paper we present the results from a borehole seismic experiment at Deep Sea Drilling Project site 504 in the eastern equatorial Pacific. Seismic amplitude anomalies of 20 dB and more are present with a horizontal correlation length of 1–3 km. Finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry, basement relief, and lateral variations within basement. A three‐dimensional model of the velocity structure around the borehole can be constructed. The resultant model has horizontal gradients greater than 2.0 s−1 in the upper crust. These gradients are comparable to the vertical gradients normally associated with oceanic crust. Assuming that velocity anomalies in the upper crust reflect changes in porosity, the resultant structure provides constraints on the hydrothermal circulation at the site. In addition, the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver.
Friction of ice
Authors: Beeman M. , Durham W. B. , Kirby S. H.
Published: February 2008

The frictional strength of ice seems to be well below that for all other rocks. Triaxial testing of cylinders of pure water ice containing a 45° inclined sawcut, at temperatures of 77 ≤ T ≤ 115 K and confining pressures 0.1 ≤ P ≤ 250 MPa, reveals the frictional laws τ = 0.20 σn + 8.3 MPa for P ≥ 10 MPa and τ = 0.55 σn + 1.0 MPa for P ≤ 5 MPa, where τ and σn are the shear and normal stresses on the sawcut surface. Friction is independent of T and, over the one order of magnitude tested, of average sliding velocity. The sliding behavior is invariably stick slip. Examination of sliding surfaces shows only minor gouge development. A curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength. These results apply to models of near‐surface tectonic activity on Jupiter's three large icy moons, Europa, Callisto, and Ganymede. They indicate, for example, that a global expansion on Ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7±3 km.
Seismic imaging of the Puritan Batholith, Wisconsin, using split array cross‐correlator processing
Authors: Doll William E. , Clay C. S.
Published: February 2008

A seismic data set was collected in northern Wisconsin in the summers of 1980–1981. The study area overlies a less heterogeneous portion of the Precambrian Puritan batholith, a body which has been intruded, and otherwise deformed since its emplacement. High‐velocity bedrock lies beneath a thin glacial till layer. Other than a shallow penetration first arrival, there was no evidence of refractors or reflectors in the upper few kilometers. The data contain an abundance of energy, and our goal was to determine the source of that energy. A processing technique, based on the split array cross correlator (Jacobsen, 1957; Doll, 1983), was developed for locating diffractors with a sparse data set. The technique is similar to those used in underwater acoustics and radio astronomy for locating point sources. It assumes an acoustic medium with known velocity structure, in which inhomogeneities may be treated as point diffractors. The output is the windowed cross correlation of two stacks of delayed traces. The delays are determined by ray tracing for each source‐to‐grid point and grid point‐to‐receiver path. The cross correlations are calculated over a two‐dimensional array of grid points. The method is useful when data are sparse and conventional linear methods fail to image subsurface structure. Our images show reflections and diffractions from a dipping interface at about 6–8 km depth, as well as other features associated with the Puritan batholith.
Tectonic evolution of the Explorer‐Northern Juan de Fuca Region from 8 Ma to the present
Authors: Botros Mona , Johnson H. Paul
Published: February 2008

New magnetic anomaly data, in conjunction with Sea Beam and SeaMARC II data, has been used to schematically model the tectonic development of the Explorer‐Northern Juan de Fuca Region with a series of five ridge‐offset propagators during the last 8 m.y. The Sovanco Fracture Zone played a major role in the evolution of this area since its origin as a spreading center offset between the Explorer and Juan de Fuca Ridges approximately 7.4 Ma. Between this time and 2.5 Ma, the transform migrated southwards due to southward propagation of Explorer Ridge, and lengthened by the addition of three propagating ridge offsets as well as by asymmetric spreading to the east on Explorer Ridge. Since the independence of Explorer Plate at 4 Ma, there has been clockwise rotation of the Explorer Spreading Center relative to the Juan de Fuca Ridge. In response to forces acting at the convergent margin, the Sovanco Fracture Zone also underwent clockwise reorientation, and in the process developed into a broad zone of shear. The tectonic deformation associated with this rotation distorted the previously existing N‐S magnetic anomaly pattern. Explorer Seamount, a major geological feature in the region, is modeled as a presently extinct southern spreading segment of the Explorer Ridge system which was isolated by the northward migration of the west end of the Sovanco Fracture Zone. The recent (<1 Ma) tectonic evolution of Explorer Ridge involved both northward and southward propagation of Southern Explorer Ridge, as well as an eastward ridge jump during the Brunhes. Ih the northern portion of the study area, the axis of spreading jumped 40 km to the NW around 0.3 Ma, from the eastern Explorer Deep to the western Explorer Rift.
Rikitake two‐disk dynamo system: Statistical properties and growth of instability
Authors: Hoshi Masayuki , Kono Masaru
Published: February 2008

The apparent randomness in the observed behaviors of the geomagnetic field (polarity reversals, intensity fluctuations, etc.) can be simulated either by simplified physical models (e.g., disk dynamo models) or by stochastic models (e.g., the Cox model). The Rikitake two‐disk dynamo is particularly interesting as a simulator, since it displays aperiodic changes of current direction resembling the geomagnetic reversals. The behaviors of the Rikitake system are investigated in some detail, with various values for parameters (μ, k). The current intensity fluctuates around one of the two stationary states, and the amplitude of oscillation grows monotonically with time, following a growth curve common to models with the same k. When the amplitude becomes so big that the peak of oscillation exceeds some threshold value, the system flips to the opposite polarity and the oscillation resumes around the new stationary state. The growth of the instability and the eventual flip of the polarity occur as deterministic process as dictated by the differential equation governing the system, and yet the system behavior appears almost random for a wide range of parameters (μ, k). The randomness enters as the growth rate of the amplitude is exceptionally large just before the polarity reversal, and the system behavior in the next polarity interval depends critically on the largest amplitude gained in the previous interval. The two statistical distributions, those of polarity intervals and field intensity, can be classified into three types which are distinctly different from each other. The transition between these types occur almost in parallel in the two distributions in the (μ, k) space.
Laboratory Studies of the Acoustic Properties of Samples From Salt on Sea Scientific Drilling Project and Their Relation to Microstructure and Field Measurements
Authors: Tarif P. A. , Wilkens R. H. , Cheng C. H. , Paillet F. L.
Published: February 2008

Compressional and shear wave velocities were measured at confining pressures up to 200 MPa for 12 core samples from the depth interval of 600–2600 m in the California State 2–14 borehole. Samples were selected to represent the various lithologies, including clean, heavily cemented sandstones, altered, impermeable claystones, and several intermediate siltstones. Velocities measured at ultrasonic frequencies in the laboratory correspond closely with velocities determined from acoustic waveform logs and vertical seismic profiles. The samples exhibit P wave velocities around 3.5 km/s at depths above 1250 m but increase to nearly 5.0 km/s at 1300 m in depth. Further increases with depth result in compressional wave velocity increasing to nearly 6.0 km/s. These increases in velocities are related to systematic variations in lithology, microstructure, and hydrothermal alteration of originally clay‐rich sediments. Scanning electron microscope observations of core samples confirm that local core velocities are determined by the combined effects of pore size distributions and the proportion of clays and alteration minerals such as epidote present in the form of pore fillings and veins.
Chemistry and Geothermometry of Brine Produced From the Salton Sea Scientific Drill Hole, Imperial Valley, California
Authors: Thompson J. M. , Fournier R. O.
Published: February 2008

The December 29–30, 1985, flow test of the State 2–14 well, also known as the Salton Sea Scientific drill hole, produced fluid from a depth of 1865–1877 m at a reservoir temperature of 305° ± 5°C. Another flow test at a depth of 3170 m produced brine contaminated by drilling fluid and diesel oil. Therefore we focus on the first flow test. Samples were collected at five different flashing pressures. The brines are Na‐Ca‐K‐Cl‐type waters with very high metal and low SO4 and HCO3 contents. Compositions of the flashed brines were normalized relative to the 25°C densities of the solutions, and an ionic charge balance was achieved by adjusting the Na concentration. The composition of the preflashed reservoir fluid was calculated using enthalpy‐chloride relations applied to the normalized and charge‐balanced brines. The calculated total dissolved solids in the preflashed reservoir fluid ranges from about 24.8 wt %, assuming insignificant thermal losses from the erupting fluid before sampling, to 26.0 wt %, assuming a 10% enthalpy loss by conduction of thermal energy through casing and surface piping. The preferred total dissolved solids of the reservoir fluid is 25.05 wt %. The calculated specific density of the preflashed reservoir fluid at 305°C and 1870 m depth ranges from 0.9980 (no thermal loss prior to sampling) to 1.0107 ± 0.0023 g cm−3 (10% thermal loss). Of the various cation geothermometers that are now in common use, the Na‐K‐Ca method gives a temperature (310°C) closest to the measured temperature (305°C) in the production horizon. Calculated Na/K geothermometer temperatures, using equations suggested by different investigators, range from 326° to 364°C. The Mg/K2 method gives a temperature of about 350°C, Mg/Li2 about 282°, and Na/Li 395°–418°C.
Magnetic Fabric and Its Significance in the 1400 Ma Mealy Diabase Dykes of Labrador, Canada
Authors: Park John K. , Tanczyk Elizabeth I. , Desbarats Alexandre
Published: February 2008

An anistropy of magnetic susceptibility (AMS) study of the relatively fresh Mealy diabase dyke swarm indicates a correlation between the AMS and the deformational effects in the rocks. Several AMS components were revealed during experimental work on 211 samples that had been heated to 640°C during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields. Seventeen sites with slight or no deformation have a magnetic fabric composed of two major components. The stable component, attributed to a primary shape anisotropy, has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike (≈N 60°E), respectively. This configuration is consistent with an origin due to a post‐emplacement stress field, whereby horizontal extension acted on a vertical sheet of partially crystallized magma. The orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement. The unstable component, removed by fields up to 250 mT, is attributed to induced (domain) anisotropy produced by mild tectonic stress during the Grenville Orogeny. The approximate orientation of its principal components, as found by the method of substitutional ellipsoids, suggests an east‐west‐directed maximum stress. Four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed, but which may be largely composed of domain and stress‐induced anisotropies developed during the Grenville Orogeny.
Petrochemical variations of island arc andesites (Kurile Island Arc)
Authors: Zharinov Sergei E.
Published: February 2008

The problem of comparison of island arc andesites in terms of petrochemical data is examined. It has been proved that in the case of the Kurile Arc, two main factors exist governing the petrochemical variety of andesites. We have made use of original multivariate statistical techniques and computer software to extract these factors. The following interpretation is suggested: the first factor (“mantle” factor) is the depth to the Benioff seismic zone, and the second (“crustal” factor) is supposedly the position of the peripheral magma chamber. Trends in the variations of the initial chemical components depending on changes in the controling factors have been determined. It has also been established that the chemical composition of volcanics depends on the type and structure of the crust.
Continuation of a deep borehole stress measurement profile near the San Andreas Fault: 2. Hydraulic fracturing stress measurements at Black Butte, Mojave Desert, California
Authors: Stock J. M. , Healy J. H.
Published: February 2008

Hydraulic fracturing stress measurements were obtained in the Black Butte drill hole, 18 km northeast of the San Andreas fault in the Mojave Desert, at depths from 251 to 635 m. In all tests the least and greatest horizontal principal stresses (Sh and SH, respectively) exceeded the vertical stress (Sν), indicating a thrust faulting stress regime. A single good‐quality hydraulic fracture impression from 309 m depth indicates an SH direction of N41°E ± 10°. This SH direction should be interpreted with caution because it is based on only one observation. This orientation is fairly compatible with nearby surface stress measurements but is incompatible with most of the hydraulic fracturing stress orientations reported from comparable depths in the Mojave Desert and is not favorable for right‐lateral slip on either the San Andreas fault or NW striking faults present farther to the east. The stress regime measured in the Black Butte hole is comparable to that measured at nearby shallow depths but differs from the strike‐slip or transitional (strike‐slip to thrust faulting) stress regime present at similar depths in two nearby holes: Crystallaire, 4 km northeast of fhe San Andreas fault, and Hi Vista, 32 km northeast of the San Andreas fault. The SH direction measured in these holes is approximately 60° counterclockwise of that observed in the Black Butte hole. The differences in stress magnitudes and orientation among these holes substantiate previous indications of local variations in stress in the upper kilometer of the crust in this area and cast doubt on the validity of linear elastic models in which the effects of the San Andreas fault dominate the stress field in the western Mojave Desert.
Gravity study of the boundary between the western Transverse Ranges and Santa Maria Basin, California
Authors: Up De Graff Jaye Ellen , Luyendyk Bruce P.
Published: February 2008

Three gravity profiles were completed across the inferred trace of the Santa Ynez River fault in southern California. This fault is believed to be the structural boundary between the Santa Maria Basin and the western Transverse Ranges to the south. A large, steep gravity gradient is associated with the inferred east‐west trace of this fault. Gravity to the north of the fault is approximately 60 mGal lower. The gravity anomalies were analyzed to answer two questions: What density contrasts (and therefore rock types) are juxtaposed across the fault at depth? What is the geometry of the fault at depth? Forward and inverse modeling was performed on the anomalies using structural constraints from geologic cross sections and a seismic refraction study plus density constraints from well logs. The Bouguer gravity was modeled as a combination of three sources: a deep long‐wavelength contribution from the slight northward deepening of the Moho; an upper crust contribution from the density contrast between the low‐density Miocene Monterey and Sisquoc formations (present in the Santa Maria Basin) and the denser shallow basement beneath the western Transverse Ranges to the south; and a high‐density midcrust contribution, extending from 3 to 11 km depth below the Transverse Ranges. The origin of the midcrust contribution is unknown, but we suggest that it may arise from a layer of oceanic crust beneath the western Transverse Ranges south of the Santa Ynez River fault.
Seismic velocities in mantle minerals and the mineralogy of the upper mantle
Authors: Duffy Thomas S. , Anderson Don L.
Published: February 2008

Comparison of seismic velocities in mantle minerals, under mantle conditions, with seismic data is a first step toward constraining mantle chemistry. The calculation, however, is uncertain due to lack of data on certain physical properties. “Global” systematics have not proved very useful in estimating these properties, particularly for the shear parameters. A new approach to elasticity estimation is used in this study to produce estimates of unknown quantities, primarily pressure and temperature derivatives of elastic moduli, from the structural and chemical trends evident in the large amount of elasticity data now available. These trends suggest that the derivatives of unmeasured high‐pressure phases can be estimated from “analogous” low‐pressure phases. Using these predictions and the best available measurements, seismic velocities are computed along high‐temperature adiabats for a set of mantle minerals using third‐order finite strain theory. The calculation of density and moduli at high temperature, to initiate the adiabat, must be done with care since parameters such as thermal expansion are not independent of temperature. Both compressional and shear seismic profiles are well‐matched by a mineralogy dominated by clinopyroxene and garnet and with an olivine content of approximately 40% by volume. Between 670 and 1000 km, perovskite alone provides a good fit to the seismic velocities. Combining seismic velocities with recent phase equilibria data for a hypothetical pure olivine mantle suggests that a mineralogy with a maximum of 35% olivine (shear profile) or 40–53% olivine (compressional profile) by volume can satisfy the constraint imposed by the 400‐km discontinuity. Other features of the upper mantle can then be matched by appropriate combinations of pyroxenes, garnets, and their high‐pressure equivalents. While mantle models with a substantially larger fraction of olivine cannot be ruled out, they are acceptable only if the derivatives of the spinel phases are substantially different from olivine and deviate from trends in the larger data set.
Deformation of the so‐called Gorda Plate
Authors: Wilson Douglas S.
Published: February 2008

A new compilation of magnetic anomaly identifications, recent side scan sonar data, and other data provide constraints on the mechanism of deformation of the area commonly known as the Gorda plate. A sharp boundary between the rigid Juan de Fuca plate and deformed material trends southeast from the northern part of the Gorda ridge. In material older than 2–3 Ma, the boundary is gradational and trends more to the east. Previous observations that the anomalies east of the ridge are shorter than their counterparts on the Pacific plate are confirmed. There is no evidence for subduction or obduction along the Mendocino transform. Adjacent to the ridge, the deformation is interpreted to be distributed NW‐SE right‐lateral simple shear, possibly controlled by ductile deformation of the lower crust. Deformation of this sense is capable of producing the observed shortening the anomalies. Away from the ridge, apparently in crust of age 2–3 Ma, there must be a transition to deformation on observed NE‐SW left‐lateral faults parallel to the original spreading fabric.
Analysis of earth rotation solution from Starlette
Authors: Schutz B. E. , Cheng M. K. , Shum C. K. , Eanes R. J. , Tapley B. D.
Published: February 2008

In this study, a consider covariance analysis of error sources limiting the accuracy of the Earth rotation parameters (ERP), which include the polar motion coordinates x and y and the length of the day (LOD), determined from satellite laser ranging (SLR) data to near‐Earth geodetic satellites, has been performed. The solution for the ERP has been obtained from the analysis of the SLR data to Starlette collected primarily during the 14‐month MERIT Campaign that began September 1, 1983. A preliminary Starlette ERP solution based on a Starlette‐tailored Earth gravity model, PGS 1331, was compared with the solutions derived from SLR to LAGEOS, and the weighted rms about the mean of the difference between these solutions was 13 milliarcseconds (mas), 11 mas, and 0.6 milliseconds (ms) for Δx, Δy, and ΔLOD, respectively. A covariance analysis indicated that the accuracy of the Starlette ERP solution was limited primarily by errors in the Earth's gravity field model. In particular, this analysis showed that the first‐order geopotential coefficients produced a systematic perturbation in the solutions for x and y with a maximum value up to 29 mas and a dominant period of 74 days. The LOD determination by Starlette is limited primarily by errors in the Earth's zonal harmonics and long‐period tides. With the improvement of the Starlette force model by simultaneous estimation of selected geopotential coefficients and ocean tide parameters, the weighted rms of the Starlette solution with respect to LAGEOS was reduced to 9 mas and 6 mas for Δx and Δy, respectively. However, this technique was not able to improve the accuracy of the LOD solution. Using an improved gravity field recently developed at the University of Texas Center for Space Research, the weighted rms differences between Starlette and LAGEOS ERP solutions have been further reduced to 4.4 mas and 3.6 mas for Δx and Δy, respectively. These results show that a significant improvement of the ERP solution from Starlette can be achieved through further refinements in the Earth's gravity field and tide model and that a precision of <5 mas in x and y coordinates of the pole is feasible from Starlette.
Qp‐melting temperature relation in peridotite at high pressure and temperature: Attenuation mechanism and implications for the mechanical properties of the upper mantle
Authors: Sato Hiroki , Sacks I. Selwyn , Murase Tsutomu , Muncill Gregory , Fukuyama Hiroyuki
Published: February 2008

The anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures (to 1280°C) and pressures (to 0.73 GPa) to warrant comparison with seismic measurements of the Earth. A substantial decrease of Qp is observed at temperatures well below the onset of partial melting. Qp systematically increases with increasing pressure over the entire temperature range. Of major significance is the finding that Qp is dependent on the ratio of the temperatures to the melting (solidus) temperature; i.e., Qp depends on the homologous temperature. The pressure dependence of Qp appears through the pressure dependence of the solidus of the peridotite. Within the uncertainties of measurement of both Qp and the phase diagram, it appears that melting and high‐temperature anelastic properties have a common origin in peridotite. The homologous temperature dependence of Qp suggests that we may estimate the temperature and pressure dependence of Qp for peridotites of different compositions and possibly even for hydrous peridotites, if solidus temperatures are known as a function of pressure (a far easier measurement than elastic and anelastic properties). The activation volume of Qp is greatly reduced at high pressure, since the slope of solidus versus pressure rapidly decreases with increasing pressure. Pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature. The Qp‐homologous temperature relation suggests a connection between Qp and the properties of the grain boundaries; that is, the major loss of seismic energy occurs at the grain boundaries. Grain boundary relaxation or high‐temperature background attenuation is a possible mechanism for the grain boundary damping. No frequency dependence of Qp is resolved (0<α<0.2 in Qp ∝ fα) over the pressure, temperature and frequency ranges of the measurement. The present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic Q that is applicable to the Earth. Experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle. Implications of the results are as follows: (1) Seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90–100% of the solidus temperature. (2) Partial melting may not produce any abrupt change of seismic velocity and Q; rather, elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus. (3) There may be no sharp mechanical boundary between the lithosphere and the asthenosphere.
Tests of crustal divergence models for Aphrodite Terra, Venus
Authors: Grimm Robert E. , Solomon Sean C.
Published: February 2008

Aphrodite Terra, the largest highland region of Venus, is a likely site of mantle upwelling, active volcanism, and extensional tectonics. We examine three alternative kinematic models for the interaction of mantle convection and the surface: one in which little horizontal surface displacement results from mantle flow (“vertical tectonics”), one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation (“plate divergence”), and one in which strain from large horizontal motions is broadly accommodated (“distributed deformation”). In support of a divergent plate model for Aphrodite, Head and Crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones; small‐scale bilateral symmetry of topographic elements, hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus; and subsidence of topography proportional to the square root of distance, consistent with that expected for a divergent thermal boundary layer. We undertake quantitative tests of each of these assertions, and we compare the results with those from similar tests of the Mid‐Atlantic Ridge. We find that apart from the long‐wavelength symmetry of broadly elevated regions, there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet. The fit of the topography of Aphrodite to that of a thermal boundary layer is in general much poorer than for the Earth, and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography. The broad saddle‐shaped region between Thetis and Atla Regiones shows the best fit to the root‐distance relation, yielding apparent spreading half rates of a few centimeters per year, but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed. Calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two‐plate model, regardless of past pole motions. A multiple‐plate geometry is therefore called for, and one or more of the lineaments must act as a plate boundary along nearly its full length, if the plate divergence hypothesis is correct. Such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming Magellan radar images. In the absence of such evidence, as long as the existence and distribution of the lineaments are verified, then broad disruption of a thin lithosphere is favored. In such a model, lineaments may be surface manifestations of mantle convective flow. A model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by Magellan.
Two‐dimensional magnetotelluric inversion of the EMSLAB Lincoln Line
Authors: Jiracek George R. , Curtis Jeffrey H. , Ramirez Jorge , Martinez Mario , Romo Jose
Published: February 2008

Two‐dimensional, Backus‐Gilbert inversion of the EMSLAB land magnetotelluric (MT) data along the 200‐km‐long Lincoln Line has yielded optimally smooth geoelectric sections. Inversions were performed on the apparent resistivity and impedance phase data approximating the transverse magnetic (TM) mode. The land portion of the Lincoln Line traverses the edge of the North American plate that is being underthrust by the Juan de Fuca plate system. The inversion reveals three centralized conductive zones in the depth range of 20–40 km. A slightly conducting (<100 S) zone is centered at 30–35 km depth under the Oregon Coast Range; this feature may be the top of the subducting Juan de Fuca plate since there is complementary evidence here from Consortium for Continental Reflection Profiling seismic data. A prominent conduction zone of several hundred Siemens (S) is also detected at 30–35 km depth under the very resistive (>1000 ohm m) Western Cascades. Here the depth is too shallow for the zone to be the subducting plate. There is also evidence for a highly conducting (>1000 S) lower crust east of the High Cascades on the east end of the Lincoln Line. Two vertical conductive regions are also exposed in the inversion model. One occurs at 70–80 km from the coast under the Willamette Valley where a postulated Eocene trench may have left a suture zone. The second region is coincident with surface hydrothermal activity along the Western‐High Cascades boundary. There are ample sources of water in the crust, e.g., in subducted sediments, from dehydration reactions along the upper plate boundary, and in volcanic arc magmas, to lead us to believe that hot, saline water is the major source of the conductive occurrences along the Lincoln Line. However, the various zones appear to be distinct, and the water may be trapped by different mechanisms.
A magnetotelluric study of the High Cascades graben in central Oregon
Authors: Livelybrooks Dean W. , Clingman William W. , Rygh John T. , Urquhart Scott A. , Waff Harve S.
Published: February 2008

We have analyzed wideband magnetotelluric data from three east‐west transects across the Central Oregon High Cascades. The data were modelled using preliminary one‐dimensional inversions of rotationally invariant parameters followed by conventional two‐dimensional analysis. The resulting electrical profiles show offsets at the boundaries of the High Cascades that are consistent with downfaulting beneath the High Cascades Graben. Conductive and resistive layers in the upper crust are interpreted in terms of the known and inferred volcanotectonic history of the range. A pervasive conductor is present in the lower crust. It deepens eastward across the Western Cascades, High Cascades, and adjoining Deschutes Plateau and may indicate concomitant eastward crustal thickening. Regional heat flow and gravity studies have been interpreted as evidence of a transition from thinner crust on the west to thicker crust on the east. This transition may mark a “suture” between pre‐Tertiary continental crust and an accreted oceanic terrane that underlies western Oregon, or it may indicate the westward limit of the anomalous lithosphere that is thought to underlie the Basin and Range province.
Introduction to special section on the Yukon‐Koyukuk Basin and Its Borderlands, Western Alaska
Authors: Patton William W. , Box Stephen E.
Published: February 2008

This special section of the Journal of Geophysical Research is devoted to a study of the origin and evolution of the Yukon‐Koyukuk basin and its borderlands in western Alaska. The Yukon‐Koyukuk basin, which is variously referred to as the “Yukon‐Koyukuk geosyncline,” the “Koyukuk basin,” or the “Yukon‐Koyukuk province,” is a large, wedge‐shaped tract of Jurassic and Cretaceous sedimentary and igneous rocks that extends over much of western Alaska. The basin is bordered on three sides by an angular “orogenic loop” composed of Proterozoic and Paleozoic continental sedimentary rocks metamorphosed under greenschist and blueschist facies conditions in the late Mesozoic. Investigators have long recognized that the basin and the orogenic borderlands that wrap around it not only hold the key to the tectonic history of western Alaska but also are of global tectonic significance because of their location at the juncture of the North American and Eurasian continents.