bluuebunny/crossref_metadata_2025 huggingface dataset:
Elemental analysis of a comet nucleus by passive gamma ray spectrometry from a penetrator
Authors: Evans Larry G. , Trombka Jacob I. , Boynton William V.
Published: February 2008

Comets are believed to contain some of the most primitive material in the solar system. One of the objectives of a Comet Rendezvous Asteroid Flyby mission is to study the composition of a cometary nucleus. Remote sensing gamma ray spectroscopy can be used to obtain some compositional information of surface materials but is limited in sensitivity by signal‐to‐noise problems. In addition, the composition of the materials below the surface may be more characteristic of the pristine cometary material. Both of these considerations have led to the design of a penetrator system that will deliver a passive gamma ray spectrometer to the comet and determine the composition of the subsurface materials. Results of calculations for a model comet (50% CI carbonaceous chondrite, 50% ices) provide data on the expected gamma ray fluxes, the minimum detectable limits, and the detection uncertainties for 35 isotopes. These results show that this technique can determine the composition of the rocky component, the composition of the icy component, and the rock‐to‐ice ratio of the nucleus. The data obtained from this proposed experiment should be sufficient to address some of the important questions on the early stages in the formation of the solar system.
Double seismic zones: Kinematics
Authors: Kawakatsu Hitoshi
Published: February 2008

The kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied. A simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10−15 s−1 and large enough to account for the seismic strain rate in this depth range. The geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be, therefore, considered essentially equivalent phenomenon. We model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere. The thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab. A preliminary result for the double seismic zone beneath Tohoku, Japan, indicates a possibility that the currently available flow law of “dry” olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties (e.g., thermal structure, rheology) of oceanic lithospheres.
Crustal structure of the southern Rio Grande Rift determined from seismic refraction profiling
Authors: Sinno Y. A. , Daggett P. H. , Keller G. R. , Morgan P. , Harder S. H.
Published: February 2008

As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south ‐ central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area and consist of three interlocking regional profiles: a reversed E‐W line across the rift, an unreversed N‐S axial line, and an unreversed SW‐SE line. The reversed E‐W line shows no significant dip along the Moho (′32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N‐S axial line and the NW‐SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4–6 km with respect to the northern rift and the adjacent Basin and Range province and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.
Direct measurement of the mantle attenuation operator from broadband P and S Waveforms
Authors: Choy George L. , Cormier Vernon F.
Published: February 2008

Displacement and velocity records of P and SH body waves with broadband frequency content from 0.01 to 5.0 Hz are obtained either directly or through multichannel deconvolution of earthquake waveforms from digitally recording stations of the Global Digital Seismographic Network, the Regional Seismic Test Network, and the Graefenberg array. Source parameters of a deep focus earthquake are determined by modeling P waveforms with a dynamically realistic source model and a model of Qα that is allowed to be frequency‐dependent. Because the phase behavior predicted by intrinsic attenuation often contrasts sharply with that predicted by realistic rupture models, there are strong constraints on source and attenuation operators. Using the rupture model derived from the P waves and neglecting the effects of scattering, a search is made for the minimum phase operators required to obtain matches to the observed S waves. The variation in the inferred minimum phase operator between S and ScS phases and its behavior with increasing distance and frequency are used to estimate a depth‐ and frequency‐dependent model of intrinsic Qβ. Strong constraints on the frequency dependence of Qβ are given by fine details in the shape and dispersion of the broadband displacements of the S waves. Rise times are most sensitive to the higher end of the frequency band (0.2–1.0 Hz), while the pulse width and tail are sensitive to the lower end of the frequency band (0.01–0.2 Hz). Although the frequency dependence of the derived tβ* is quite simple, having smooth decreases from 0.01 to 5.0 Hz, a relatively complex parameterization of the frequency and depth dependence of Q is required to match this frequency dependence. Either the power n in a frequency‐dependent Q = Cfn varies with depth, the high‐frequency cutoff in the relaxation spectrum varies with depth, or both. The midmantle between depths of 400 and 1600 km contributes primarily to the attenuation of low frequencies (0.01–0.1 Hz) of body waves. Below the mantle depth of 2000 km the P and S waveforms for the paths investigated suggest that little or no attenuation exists in the band of 0.01–5 Hz.
Observations and modeling of seafloor microseisms
Authors: Webb S. C. , Cox C. S.
Published: February 2008

The seismic noise level on the deep seafloor has been essentially unknown at periods longer than 10 s and poorly known at shorter periods. We present data obtained with two new types of seafloor instrumentation: a differential pressure gauge and an antenna which measures a horizontal component of the electric field. The electric field is closely related to the horizontal ground motion. The observed spectra can be divided into three frequency bands. At periods longer than 40 s surface gravity waves produce velocity and pressure fluctuations which are felt at the deep seabed and dominate other sources. This signal is expected to be uniform throughout the ocean basins and will make detecting small seismic waves at periods longer than 40 s difficult. At periods between 10 and 40 s the spectrum is much quieter and may approach the background observed at quiet land stations. The pressure signal in this band may be at least partially caused by very low frequency acoustic waves in the atmosphere. The electric field spectrum is much noisier, suggesting that horizontal motions may be larger than vertical motions. At periods slightly shorter than 10 s the pressure spectrum rises sharply 45 dB into the microseism peak. Many studies of microseisms have established a clear relationship between the ocean surface gravity wave field and microseisms. We have found close agreement between the theory of microseism generation and observations obtained while a small storm moved over a seafloor instrument. The microseism spectrum evolved in concert with the changing wind wave field. We found that plausible estimates of the directionality of the wind wave field as a function of frequency could be derived from the microseism observations. The structure of the microseism peak in frequency is also related to the properties of surface waves trapped in the upper layers of sediment. Our calculations of the forcing of microseisms by a distributed pressure field at the surface elucidate this relationship and allow a partial determination of sediment structure at an instrument site from the microseism observations.
Phase transitions, Grüneisen parameter, and elasticity for shocked iron between 77 GPa and 400 GPa
Authors: Brown J. Michael , McQueen Robert G.
Published: February 2008

Sound velocities determined in iron, shock compressed to pressures between 77 GPa and 400 GPa, indicate that two phase transitions exist on the Hugoniot. A discontinuity in sound velocities at 200 ± 2 GPa may mark the transition of ε iron to γ iron. A second discontinuity at 243 ± 2 GPa is believed to indicate the onset of melting. The calculated temperature at melting lies between 5000 K and 5700 K. When extrapolated from the Hugoniot melting point, the Lindemann criterion yields an estimate of 5800 ± 500 K for the melting of pure iron at the inner core boundary pressure of 330 GPa. The product of density times the thermodynamic Grüneisen parameter in liquid iron, calculated from the present data, is 19.6 ± 0.8 Mg m−3. A temperature profile ranging from 3800 K at the core‐mantle boundary to 5000 K at the earth's center is calculated using the present data. Sound velocities for ε iron provide a better match to seismic velocities for the earth's inner core than do those of γ iron. A comparison between liquid iron velocities and the velocity profile through the outer core provides further evidence for alloying of iron with “lighter elements” in the core.
Exogenic and endogenic albedo and color patterns on Europa
Authors: McEwen Alfred S.
Published: February 2008

New global and high‐resolution multispectral mosaics of Europa have been produced from the Voyager imaging data. Photometric normalizations are based on multiple‐image techniques that explicitly account for intrinsic albedo variations through pixel‐by‐pixel solutions. The exogenic color and albedo pattern on Europa is described by a second‐order function of the cosine of the angular distance from the apex of orbital motion. On the basis of this second‐order function and of color trends that are different on the leading and trailing hemispheres, the exogenic pattern is interpreted as being due to equilibrium between two dominant processes: (1) impact gardening and (2) magnetospheric interactions, including sulfur‐ion implantation and sputtering redistribution. Removal of the model exogenic pattern in the mosaics reveals the endogenic variations, consisting of only two major units: darker (redder) and bright materials. Therefore Europa's visual spectral reflectivity is simple, having one continuous exogenic pattern and two discrete endogenic units.
Iron‐water reaction under high pressure and its implication in the evolution of the Earth
Authors: Fukai Yuh , Suzuki Toshihiro
Published: February 2008

We have established by experiments and thermodynamical calculations that a large amount of hydrogen can be dissolved in iron under high hydrogen pressure, causing appreciable reduction of density and melting point. More recent experiments have shown that the reaction of Fe with hydrous silicates under high pressure and temperatures (≲5 GPa, ≲ 1200°C, 20 min) yields iron hydride (FeHx), olivine and pyroxene, and that coagulation of iron particles is greatly facilitated in the partially molten state of the silicates. On the basis of these experimental findings on Fe‐H and Fe‐H2O systems, we propose that the Fe‐H2O reaction should have played a crucial role in the evolution of the earth, including in particular the core‐mantle separation and dissolution of hydrogen in the earth's core. A tentative calculation on the mass flow in the evolution process was made by assuming a 10:90 mixture of low‐temperature (LT) and high‐temperature (HT) components for the primordial material and Fe/(Fe + Mg) ≃ 0.11 for the whole mantle. The composition of the core thus deduced, together with compressibility data at high pressures, is found to give reasonable values for the density deficit of the outer core. If, conversely, we adopt the density reduction as input, the composition of the outer core is estimated to be FeH0.25C0.05O0.13S0.03. Finally, some implications of the present picture on other terrestrial planets are examined briefly.
Connectivity of melt phase in a partially molten peridotite
Authors: Toramaru Atsushi , Fujii Naoyuki
Published: February 2008

The morphological stability criteria of melts in edge and corner regions in a partially molten system containing several solid phases (three) under textural equilibrium are proposed in terms of dihedral angles. Owing to the variety of edge and corner regions due to combination of the crystalline phases in the multi‐solid phase system, melts are morphologically stable in some types of edges and corners, and unstable in others. In order to apply the stability criteria to the partially molten regions in the upper mantle we conducted a partial melting experiment with a peridotite mainly composed of olivine (OL), orthopyroxene (OPX), and clinopyroxene (CPX) crystals at 1300°C, 1 GPa and for 300 hours. Various types of dihedral angles were measured on the run product which contained about 7% melt. The melt versus OL/OL dihedral angle was significantly smaller than other types of melt versus solid/solid dihedral angles. Applying the stability criteria to these experimental results, we predict that in the partially molten peridotite the melts are morphologically stable only in OL‐OL‐OL edge regions and in OL‐OL‐OL‐OL and OL‐OL‐OL‐OPX corner regions. The connectivity of the melt phase is determined by the melt distribution in the stable edge and corner regions at melt fractions less than 29%. The melt distribution can be modeled by the bond distribution in a lattice, that is, bond percolation. The connectivity of the melt phase is estimated as a function of the modal composition and grain size distribution of the matrix and is graphically presented as a connectivity diagram (OL‐OPX‐CPX modal composition diagram). The trajectory of modal composition of the solid matrix in the connectivity diagram determines the connectivity history of a rock during progressive partial melting in the upper mantle. Three different cases of connectivity history are discerned depending upon the modal composition before melting and are characterized by the critical melt fractions ϕmc, at which the melt phase suddenly becomes connecting, such as ϕmc = 0, 0 < ϕmc < 0.29, and ϕmc ≒ 0.29. In a peridotite the modal portion of olivine but also the relative grain size are the most important determinant of the connectivity behavior in the upper mantle.
Ultrasonic velocity and attenuation measurements on basalt melts to 1500°C: Role of composition and structure in the viscoelastic properties
Authors: Manghnani Murli H. , Sato Hiroki , Rai Chandra S.
Published: February 2008

Viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity Vp and quality factor Qp measurements in the frequency range of 3.4–22 MHz and the temperature range of 1150°–1500°C. The basalt samples are grouped into two types: low magnesia (group A) containing 5–9 wt % MgO and high magnesia (group B) containing 18–23 wt % MgO. The Vp values of the melts determined at 1400°C are in the range 2.53–2.64 km/s for group A basalts and 2.79–2.86 km/s for group B basalts. The relaxed (low frequency) velocity Vr is a principally linear function of the melt density ρ. Least squares fits of the data give velocity‐density relations Vr (km/s) = 0.691 + 0.712ρ (g/cm3) for group A basalts and Vr = 0.328 + 0.907ρ for group B basalts at 1400°C. The mean molecular weights M are 65.0 ± 0.3 and 58.2 ± 0.5 g/mol for groups A and B basalts, respectively, suggesting that M is also a principal factor determining the melt velocity. The relaxed modulus Mr of the basalt melts range from 16.5 to 22.8 GPa and is also a linear function of the melt density. The values of longitudinal ηL and volume ηv viscosities are calculated from the measured Vp and Qp and the available shear viscosity ηs data. The viscosities and related activation energies decrease almost linearly as NBO/T (nonbridging oxygens per tetrahedrally coordinated cation) increases.
Evidence for postseismic viscoelastic relaxation following the 1959 M = 7.5 Hebgen Lake, Montana, Earthquake
Authors: Reilinger Robert
Published: February 2008

A 1983 releveling survey conducted by the National Geodetic Survey, combined with previous surveys in 1923, 1960, 1967, and 1975, provides evidence for ongoing vertical deformation of a broad region (diameter of ∼150 km) surrounding the site of the 1959 Hebgen Lake earthquake. Deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault. Maximum observed elevation change during the postseismic period (1960–1983) exceeds 30 cm. Assuming that the entire uplift (i.e., 1923–1983) occurred following the 1959 earthquake (i.e., uplift measured between 1923 and 1960 was entirely postseismic), the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years. The spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer (thickness of ∼30–40 km) overlying a viscoelastic half‐space (viscosity of ∼1019 Pa s). This model may also account for the anomalously large horizontal strain around the Hebgen Lake region measured by repeated trilateration surveys during the period 1973–1984. The releveling measurements in the Hebgen Lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the United States.
Ultramafic xenoliths: Clues to Earth's late accretionary history
Authors: Morgan John W.
Published: February 2008

Fourteen spinel lherzolites, for which extensive trace element data are available, may be divided into three groups depending upon the percentage loss of basaltic partial melt; averages are slightly depleted, −7% melt (Ca/Sa > 0.09); moderately depleted, −13% (0.09 > Ca/Si > 0.06); and strongly depleted, −20% (0.06 > Ca/Si). Rhenium abundances of the groups are correlated with percent depletion, and the intercept of 0.0071 × Cl chondrite corresponding to undepleted mantle is identical to that independently derived from 187Os/186Os in osmiridium of known age. The distribution of Re in mafic and ultramafic rocks is apparently closely related to S and Se abundances. Lherzolite data suggest that the abundances of highly siderophile elements (Os, Re, Ir, Pd, Au) and chalcogenic elements (S, Se, Te) in a primary basaltic melt are significantly higher than those of average oceanic ridge basalts but may be similar to those of trace element‐rich Indian Ocean ridge basalts. The estimated S content (∼1000 ppm) of a primary basaltic melt is compatible with experimentally measured S solubilities at high temperature and pressure. On the basis of lherzolite depletion and using published trace element data for spinel lherzolites and ocean ridge basalts, estimated abundances in ppb (or × 103/Cl chondrite abundances) in pristine upper mantle are Os, 3.1 (5.9); Re, 0.26 (7.1); Ir, 3.4 (7.1); Pd, 4.5 (8.1); Au, 1.01 (8.4); S, 200,000 (3.2); Se, 57 (3.1). The trace element pattern closely resembles that of the CM2 chondrites, but not of the CO3, CV3 or H chondrites. The pattern is a reasonable match, except for Au, with Apollo 17 lunar breccias that contain a “group 2” ancient meteoritic component, suggesting that rather similar objects bombarded earth and moon during the first 600 m.y. of their history.
Two‐dimensional seismic reflection modeling of the inferred fossil oceanic crust/mantle transition in the Bay of Islands Ophiolite
Authors: Collins John A. , Brocher Thomas M. , Karson Jeffrey A.
Published: February 2008

We investigate the origin and character of oceanic Moho reflections by computing two‐dimensional synthetic seismogram profiles of the inferred fossil oceanic crust/mantle transition observed in the Bay of Islands Ophiolite. To simulate a seismic reflection experiment, we calculated near‐vertical‐incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length. In the synthetic profiles the Moho reflection event varies from a single phase to two or more phases of up to 1‐s (two‐way travel time) total duration. Individual phases show lateral variation in amplitude, and their two‐way travel times vary by as much as 0.25 s over horizontal distances as short as 10 km. Lateral discontinuity of phases results in abrupt variations in the travel times of first‐arriving, high‐amplitude Moho phases. The geological structures generating the highest‐amplitude Moho reflections vary from high‐ and low‐velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the Moho Transition Zone. Reflection amplitudes from the residual upper mantle are insignificant, and our modeling suggests that using the first‐arriving, high‐amplitude Moho phase to estimate thickness of magmatic material might result in errors of up to 1‐s two‐way travel time (∼3–4 km). Multichannel seismic data from both the western Pacific and western North Atlantic show Moho travel time variations similar to those observed in the synthetic profiles. The western North Atlantic data also show multiphase Moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data, suggesting that the structures observed in the inferred fossil crust/mantle transition of the ophiolite are characteristic of oceanic lithosphere.
Most recent eruption of the Mono Craters, eastern central California
Authors: Sieh Kerry , Bursik Marcus
Published: February 2008

The most recent eruption at the Mono Craters occurred in the fourteenth century A.D. Evidence for this event includes 0.2 km3 of pyroclastic fall, flow, and surge deposits and 0.4 km3 of lava domes and flows. These rhyolitic deposits emanated from aligned vents at the northern end of the volcanic chain. Hence we have named this volcanic episode the North Mono eruption. Initial explosions were Plinian to sub‐Plinian events whose products form overlapping blankets of air fall tephra. Pyroclastic flow and surge deposits lie upon these undisturbed fall beds within several kilometers of the source vents. Extrusion of five domes and coulees, including Northern Coulee and Panum Dome, completed the North Mono eruption. Radiocarbon dates and dendrochronological considerations constrain the eruption to a period between A.D. 1325 and 1365. The lack of lacustrine laminae or aeolian and fluvial beds between individual pyroclastic beds suggests that the explosive phases of the eruption took place over a period of not more than several months. Within the resolution of the available radiocarbon and dendrochronologic dates, the North Mono eruption is contemporaneous with the latest eruption of the Inyo volcanic chain, about 20 km to the south. However, the Inyo tephra blanket clearly overlies, and thus postdates, all North Mono tephra. Minor disturbance of the North Mono tephra prior to deposition of the Inyo tephra indicates that the period of time between the North Mono and Inyo eruptions was probably no more than a year or two. This near contemporaneity of the two eruptions suggests a genetic relationship. Liquefaction of North Mono sands on the floor of Mono Lake occurred twice during the waning stages of the North Mono eruption and 3 times immediately before and after pulses of the Inyo eruption. This is evidence that five earthquakes of ML ≳ 5.5 occurred during the North Mono and Inyo eruptions. The chemical and textural similarity of the erupted products and their nearly simultaneous evacuation from aligned vents indicates that the North Mono eruption resulted from intrusion of a dike beneath the northern 6 km of the volcanic chain. Several observations suggest that dike intrusion beneath the Mono Craters has replaced normal faulting as the mechanism for elastic rebound and permanent extension of the crust at this latitude. However, dike widths compatible with relief of purely tectonic strains (≲3 m) are probably too narrow to have allowed the North Mono magma to erupt. Overpressurization of the Mono Craters magma reservoir by another mechanism, perhaps magma mixing, appears necessary as well. The gradually diminishing explosiveness of the North Mono eruption probably resulted from a decrease of water content downward in the dike. The pulsating nature of the early, explosive phase of the eruption may represent the repeated rapid drawdown of slowly rising vesiculated magma to about the saturation depth of the water within it.
Earthquake processes in the Rainbow Mountain‐Fairview Peak‐Dixie Valley, Nevada, region 1954–1959
Authors: Doser Diane I.
Published: February 2008

The 1954 Rainbow Mountain‐Fairview Peak‐Dixie Valley, Nevada, sequence produced the most extensive pattern of surface faults in the intermountain region in historic time. Five earthquakes of M>6.0 occurred during the first 6 months of the sequence, including the December 16, 1954, Fairview Peak (M = 7.1) and Dixie Valley (M = 6.8) earthquakes. Three 5.5≤M≤6.5 earthquakes occurred in the region in 1959, but none exhibited surface faulting. The results of the modeling suggest that the M>6.5 earthquakes of this sequence are complex events best fit by multiple source‐time functions. Although the observed surface displacements for the July and August 1954 events showed only dip‐slip motion, the fault plane solutions and waveform modeling suggest the earthquakes had significant components of right‐lateral strike‐slip motion (rakes of −135° to −145°). All of the earthquakes occurred along high‐angle faults with dips of 40° to 70°. Seismic moments for individual subevents of the sequence range from 8.0 × 1017 to 2.5 × 1019 N m. Stress drops for the subevents, including the Fairview Peak subevents, were between 0.7 and 6.0 MPa.
Earthquakes, quaternary faults, and seismic hazard in California
Authors: Wesnousky Steven G.
Published: February 2008

Data describing the locations, slip rates, and lengths of Quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in California. The expected seismic moment M0e and the strength of ground shaking resulting from the entire rupture of each mapped fault (or fault segment) are estimated using empirical relations between seismic moment M0, rupture length, source to site distance, and strong ground motions. Assuming a fault model whereby the repeat time T of earthquakes on each fault equals M0e/M0g (where the moment rate M0g is proportional to fault slip rate), it is observed that the moment‐frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150‐year historical record. The agreement is consistent with the argument that the geologic record of Quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in California. The estimates of T for each fault are the foundation for constructing maps that depict the average return period of ≳ 0.1g peak horizontal ground accelerations, and the horizontal components of peak acceleration, peak velocity, and the pseudovelocity response (at 1‐period and 5% damping) expected to occur at the level of 0.1 probability during a 50‐year period of time. A map is also formulated to show the probability that ≳ 0.1g horizontal ground accelerations will occur during the next 50 years. The maps serve to illustrate the potential value of Quaternary fault studies for assessing seismic hazard. Interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the San Andreas, San Jacinto, Calaveras, Hayward, and Ventura Basin fault zones. Other regions of similarly high hazard may yet remain unrecognized. This inadequacy results primarily from an incomplete data set. Numerous faults, for example, are mapped along the coastal region of northern California and within the Modoc Plateau, but relatively few studies relating to fault slip rate are reported. A similar problem exists for other stretches of coastal California where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate. Geological and geophysical field studies can work to remove these deficiencies. A concerted effort to locate and define rates of activity on all faults in California is the most promising means to further quantify present levels of seismic hazard in California.
Terrain types and local‐scale stratigraphy of grooved terrain on Ganymede
Authors: Murchie Scott L. , Head James W. , Helfenstein Paul , Plescia Jeffrey B.
Published: February 2008

Grooved terrain is subdivided on the basis of pervasive morphology into (1) groove lanes ‐ elongate parallel groove bands, (2) grooved polygons ‐ polygonal domains of parallel grooves, (3) reticulate terrain ‐ polygonal domains of orthogonal grooves, and (4) complex grooved terrain ‐ polygons with several complexly cross‐cutting groove sets. Detailed geologic mapping of select areas, employing previously established conventions for determining relative age relations, reveals a general three‐stage sequence of grooved terrain emplacement: first, dissection of the lithosphere by throughgoing grooves, and pervasive deformation of intervening blocks; second, extensive flooding and continued deformation of the intervening blocks; third, repeated superposition of groove lanes concentrated at sites of initial throughgoing grooves. This sequence is corroborated by crater‐density measurements. Dominant orientations of groove sets are parallel to relict zones of weakness that probably were reactivated during grooved terrain formation. Groove lane morphology and development consistent with that predicted for passive rifts suggests a major role of global expansion in grooved terrain formation.
Paleomagnetism of the Tertiary Clarno Formation of central Oregon and its significance for the tectonic history of the Pacific Northwest
Authors: Gromme C. Sherman , Beck Myrl E. , Wells Ray E. , Engebretson David C.
Published: February 2008

The Clarno Formation, a mostly Eocene and partly early Oligocene sequence of andesitic lavas and volcaniclastic rocks, is the oldest Tertiary formation exposed in north central Oregon. Remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84°N, 278°E with a 95% confidence cone of 7°. Comparison of this pole with the North American reference pole for Eocene time indicates that the Clarno Formation has rotated 16° clockwise with an uncertainty of 10° but has undergone no significant latitudinal displacement. Further comparison with paleomagnetic data from the Miocene Columbia River Basalt Group and Steens Basalt shows that large‐scale block rotations in the eastern Columbia Embayment ceased sometime between 38 m.y. and 15 m.y. A paleogeographic reconstruction for 38 m.y. is offered which is consistent with the observed rotation of the Clarno and with the other paleomagnetic data from the Pacific Northwest, which are briefly reviewed. This reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre‐Miocene Tertiary rocks of the Pacific Northwest by an extensional tectonic model. As a consequence, only part of the rotations in the pre‐Tertiary rocks of the eastern Columbia Embayment need to be the result of accretionary tectonics. The initiation of magmatism in the eastern Columbia Embayment that is represented by the Clarno Formation itself may have been contemporaneous with the beginning of crustal extension in the Pacific Northwest.
Acoustic and resistivity measurements on rock samples containing tetrahydrofuran hydrates: Laboratory analogues to natural gas hydrate deposits
Authors: Pearson C. , Murphy J. , Hermes R.
Published: February 2008

In this paper we report laboratory acoustic velocity and electrical resistivity measurements on Berea Sandstone and Austin Chalk samples saturated with a stoichiometric mixture of tetrahydrofuran (THF) and water. THF and water is an excellent experimental analogue to natural gas hydrates because THF solutions form hydrates similar to natural gas hydrates readily at atmospheric pressures. Hydrate formation in both the chalk and sandstone samples increased the acoustic P wave velocities by more than 80% when hydrates formed in the pore spaces; however, the velocities soon plateaued and further lowering the temperature did not appreciably increase the velocity. In contrast, the electrical resistivity increased nearly 2 orders of magnitude upon hydrate formation and continued to increase slowly as the temperature was decreased. In all cases resistivities were nearly frequency independent to 30 kHz, and the loss tangents were high, always greater than 5. The dielectric loss showed a linear decrease with frequency suggesting that ionic conduction through a brine phase dominates at all frequencies, even when the pores are nearly filled with hydrates. We find that resistivities were strongly a function of the dissolved salt content of the pore water. Pore water salinity also influenced the sonic velocity, but this effect is much smaller and only important near the hydrate formation temperature.
Fracture instabilities accompanying dike intrusion
Authors: Sammis Charles G. , Julian Bruce R.
Published: February 2008

The recent suggestion that earthquakes at Long Valley caldera, California, are caused by rapid tensile failure under high fluid pressure contradicts the assumption that the speed of propagation of a fluid‐driven tensile crack is limited by the speed of motion of the fluid within the crack, so that such cracks cannot radiate seismic waves. In this paper we analyze fluid‐driven cracks of various geometries and find examples of both stable and catastrophic propagation. For an isolated crack in a homogeneous medium with remotely applied compressive stresses the stress intensity decreases as the crack tip extends beyond the driving fluid (assumed stationary), so that crack propagation is stable and limited by the rate of fluid flow. On the other hand, for two cracks approaching each other, initially stable behavior gives way to unstable propagation, and the cracks join catastrophically even if the fluid is stationary. A crack approaching a free surface also exhibits a transition from stable to unstable propagation, with the final episode occurring catastrophically. A crack emanating from a pressurized reservoir of cylindrical or spherical shape can also undergo an episode of catastrophic propagation. Propagation begins when the reservoir pressure rises to a critical value, which depends on the regional stress field, the fracture toughness of the rock, and the size of the largest flaw at the surface of the reservoir. This flaw propagates catastrophically until it reaches a certain size and thereafter propagates stably. Microearthquakes caused by hydraulic fracturing have dimensions of at most a few tens of borehole radii and therefore probably are difficult to detect. Volcanic earthquakes can have source dimensions comparable to the radius of the associated magma chamber, so earthquakes as large as those at Long Valley caldera (source dimensions ≅ 10 km) could be caused by tensile failure.
Age of tilted reefs, Hawaii
Authors: Moore James G. , Campbell J. Frisbee
Published: February 2008

Submerged carbonate reefs are preserved as a series of submarine terraces between Molokai and Hawaii along a 200‐km span of the southeastern Hawaiian Ridge. Limestones from two of the terraces have been sampled from submersibles and dated radiometrically at 13 and 120 ka. Recognition that the terraces are tilted permits assignment of about a dozen terraces from 150 to 1300 m depth to eight general reef platforms. These reefs were drowned by the combined effects of island subsidence and sea level rise at the end of successive glacial stages from 13 to 647 ka. The platforms are tilted 5 m/km southeast toward the locus of volcanic loading centered on the island of Hawaii. The end of the active period of volcanic shield building is represented on each nonactive volcano by a submerged change in slope. Dating of this slope change by reference to the dated reefs provides independent evidence that shield building ended at about the same time that the volcano changed from eruption of tholeiitic to alkalic basalt. The variation in depth of the dated tilted reefs along the ridge indicates that the end of subsidence follows the end of shield building by about one half million years. The present northwest margin of major subsidence probably crosses southeast Maui. During the last half million years the rate of southeast migration of the beginning of both subsidence and of tholeiitic volcanism does not appear to have increased. However, the rate of southeast migration of the end of both subsidence and of tholeiitic volcanism has apparently increased. This shortening of the life history of the younger volcanoes implies an increase in eruption rates.
Petrology and precursors of lithic clasts from feldspathic fragmental breccia 67975
Authors: McGee James J.
Published: February 2008

Fourteen lithic clasts were extracted from feldspathic fragmental breccia 67975 and analyzed by instrumental neutron activation analysis (INAA) by Lindstrom (1984), who classified the clasts into three granulite groups based upon color and grain size. The INAA data demonstrated that there are significant differences in the chemistry among these three groups, with both magnesian and ferroan rocks present in the clast suite. Evaluation of the textures and mineral chemistries of five of these lithic clasts indicates that several rock types are present, including feldspathic microporphyritic melt breccia, metamorphosed mafic‐rich melt breccia with zoned and mantled clasts, “typical” reequilibrated lunar feldspathic granulites with granoblastic texture and homogeneous mineral chemistry, hornfels, and a recrystallized mafic‐rich granulite with relict igneous texture and the composition of a ferroan anorthositic norite. These five lithic clasts were derived from both ferroan‐anorthosite suite and Mg‐suite precursors, with most of them containing more than one precursor. The ferroan anorthositic norite clast represents an unusual but important precursor that is a possible component of many of these clasts. The two light gray clasts (Lindstrom, 1984) examined in this study differ greatly; one is a plagioclase‐rich, possibly monomict rock with ferroan anorthositic precursor(s), and the other represents a mixture of feldspathic granulite and ferroan anorthositic norite lithologies. Lindstrom's dark gray group is a suite of feldspathic microporphyritic melt breccias and the light green group consists of clasts of metamorphosed polymict mafic melt breccias that have distinctive texture, mineralogy, and REE contents.
Uranium, thorium, and potassium in the Venusian rocks at the landing sites of Vega 1 and 2
Authors: Surkov Yu. A. , Kirnozov F. F. , Glazov V. N. , Dunchenko A. G. , Tatsy L. P. , Sobornov O. P.
Published: February 2008

Concentrations of natural radioactive elements in Venusian rocks at the landing sites of Vega 1 and Vega 2 were determined by gamma ray spectrometers on the landers as follows: Vega 1‐K = (0.45 ± 0.22)%, U = (0.64 ± 0.47) × 10−4%, Th = (1.5 ± 1.2) × 10−4%; Vega 2‐K = (0.40 ± 0.20)%, U = (0.68 ± 0.39) × 10−4%, Th = (2.0 ± 1.0) × 10−4%. The abundances of natural radioactive elements in the investigated rocks are similar to that of basic rocks (tholeiitic basalts and gabbros) of the earth's crust. The same conclusion about the type of the rocks at the landing site of Vega 2 was made from the XRF experiment.
Sequence, rheological properties, and effusion rates of volcanic flows at Alba Patera, Mars
Authors: Cattermole Peter
Published: February 2008

Volcanic flows with very large areas characterize the Martian volcano Alba Patera. Photogeologic analysis indicates a change of eruptive style with time, from early flood lavas to relatively short, narrow, leveed summit flows during the dying phase of activity. Estimates of mass effusion rate and rheological properties derived from dimensional parameters confirm the view that the magmas involved were mafic, and indicate extremely high effusion rates (5–1610 × 103m3 sec−1) for the earlier flows. The data substantiates the view that during the active life of Alba volcano mass eruption rates in the Tharsis region of Mars were significantly higher than in most analogous terrestrial provinces, but probably not as high as those for lunar mare basalts.
Some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central Chile
Authors: Irwin James J. , Sharp Warren D. , Spangler Robert R. , Drake Robert E.
Published: February 2008

Paleomagnetic samples collected from 19 sites in radioisotopically dated Jurassic mafic dykes and plutons in the coastal cordillera of Chile between latitude 32°15′ and 33°30′ S yield Paleomagnetic poles (calculated by averaging the virtual geomagnetic poles from approximately coeval sites) at latitude 85.9°S, longitude 36.7°E (N = 8, a95 = 11.9°) for sites dated at close to 170 Ma and at latitude 80.3°S, longitude 207.8°E (N = 10, a95 = 9.5°) for sites dated at close to 145 Ma. These paleomagnetic poles from the coast of Chile coincide with poles from coeval sequences within the South American craton. Some of the igneous rocks that constitute these sampling sites crosscut a high‐grade metamorphic complex that is distinct from other segments of the coastal cordillera in age, structural style, and metamorphic grade. This complex cannot be correlated with metamorphic rocks anywhere else in central western South America and could be considered a potentially “exotic” allochthonous terrane. If this terrane has been added to the western South American margin, then accretion occurred prior to 170 Ma. Paleomagnetic results presented here indicate that no major rotation or large‐scale latitudinal translation of crustal blocks has taken place in this part of central Chile since Middle Jurassic time.
Comparing the jerk with other global models of the geomagnetic field from 1960 to 1978
Authors: Backus G. E. , Estes R. H. , Chinn D. , Langel R. A.
Published: February 2008

About 3300 satellite values of geomagnetic intensity and about 7000 observatory values of annual mean magnetic vector components from 1960 to 1978 were fitted by three global models of the geomagnetic field B Each model includes a spatially constant external field whose time dependence is a constant plus another constant times the Dst index, and each model accepts a time‐independent station correction at each observatory. The time dependence of the internal Gauss coefficients is either cubic, quintic, or biquadratic (two independent quadratics, one before and one after January 1, 1970); and g01 also has an induced term proportional to the Dst index. The rms residual of the data fit is the same for the cubic and biquadratic models and insignificantly smaller for the quintic model. The quintic and biquadratic models have 1164 adjustable parameters, and the cubic has 1038. At a high level of significance the parameters of the best fitting biquadratic rule out a physical model for the magnetic impulse of 1969 in which the level surfaces of electrical conductivity in the lower mantle are approximately spherical, and also the radial magnetic field at the core‐mantle boundary goes from one quadratic time dependence to another in a year or less.
Determination of sensible heat flux in volcanic areas from ground temperature measurements along vertical profiles: The case study of Mount Etna (Sicily, Italy)
Authors: Tabbagh Alain , Trezeguet D.
Published: February 2008

Measurements of ground temperature at shallow depth continuously transmitted and stored in a data base can be of value to follow volcanic events and to give information about heat flux at the surface of the volcano. We propose models to calculate heat flux from temperature measurements for the following cases: conductive transfer, convective transfer, and conductive and convective transfer. Then we examine the results obtained in the summit area of Mount Etna. In convective areas it is difficult to estimate the flux in the absence of complementary information about the liquid content of the outgoing fluid, but the steady flux is more important than the unsteady one. At the Torre Del Filosofo station the transfer is conductive, it can be used as a reference station, and geothermal heat flux can be approached.
In situ cosmogenic 3H, 14C, and 10Be for determining the net accumulation and ablation rates of ice sheets
Authors: Lal D. , Nishiizumi K. , Arnold J. R.
Published: February 2008

We explore the usefulness of the in situ cosmogenic 3H, 14C, and 10Be produced by spallation of oxygen nuclei in ice, as tracers to determine net accumulation/ablation rates of ice sheets. The application of the in situ 3H and 10Be is severely constrained because at deposition, ice contains appreciable amounts of these isotopes from the atmosphere. The case is much more favorable for 14C, which is not carried with wet precipitations; atmospheric 14C gets mechanically trapped in the ice during deposition. We point out that cosmogenic 14C would probably exist as 14CO in ice. This seems to be supported by the published results of Fireman and Norris. Considering their inherent amounts in the ice and the expected in situ production rates, we discuss conditions under which these isotopes can be used to study net accumulation and ablation rates. We also discuss available data on 14C and 10Be on polar ice from accumulation and ablation zones. We conclude that 3H and 14C should find wide applications in studying ice dynamics and 10Be in very special circumstances.
Omori's Power Law aftershock sequences of microfracturing in rock fracture experiment
Authors: Hirata Takayuki
Published: February 2008

A time series of acoustic emissions (AE) in basalt was measured under constant uniaxial compression. Some bursts of AE events considered to be the main shock and aftershock sequences were observed. Two models describing the aftershock sequences (the exponential decay model, n(t) = K exp (‐pt), and the Omori's power law model, n(t) = K/(c + t)p, where n(t) denotes the occurrence rate of aftershocks at time t) were examined. With the progress of the fracturing process, the bursts changed from the exponential decay type to Omori's power law type, and the p value of Omori's power law aftershock sequences decreased. These results indicate that microfracturing gains a longer tail with the evolution of the fracture process. The Reproduction of Omori's power law aftershock sequences of AE events suggests that the self‐similarity of rock fracturing holds good over a range from microfracturing to large earthquakes.