bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and evolution of the Hengill‐Grensdalur Volcanic Complex, Iceland: Geology, geophysics, and seismic tomography
Authors: Foulger G. R. , Toomey D. R.
Published: February 2008

Recent geological and geophysical research indicates the presence of three volcanic systems in the Hengill‐Grensdalur area and progressive westerly migration of the accretionary plate boundary. The Hengill system comprises the active Hengill central volcano and a NNE trending fissure swarm that is the present locus of crustal accretion in the area. Extinct volcanic systems are identified with the mountain Hromundartindur and the Grensdalur area and associated NNE trending zones. The Grensdalur system was formerly the locus of accretion in the area. A widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems. A tomographic study of the upper 5 km of crust, using local earthquakes, imaged three bodies with velocities up to 15% higher than the average background velocities and volumes of several tens of cubic kilometers. One of these underlies the Grensdalur volcano, and a second underlies the Olkelduhals area within the Hromundartindur system. These are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them. The third high‐velocity body underlies the extinct basalt shield Husmuli, which is not associated with geothermal resources. That body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site. A low‐velocity body with a volume of a few cubic kilometers was imaged in the depth range 2–4 km beneath the northern part of the presently active Hengill central volcano. This volume may contain partial melt and represent the heat source, or part of the heat source, fueling the Hengill field. Heat balance calculations show that subsurface magmatism continued in the extinct Grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the Hengill system.
Mantle dynamics with induced plate tectonics
Authors: Ricard Yanick , Vigny Christophe
Published: February 2008

Mantle circulation and plate tectonics can be described by means of two scalar fields. The poloidal field is related to horizontal divergence, i.e., to the vertical mass flux. The toroidal field expresses the shear deformation or the rotations within spherical shells. For the Earth, the surface kinetic energy is known to be equally distributed between the two fields. In recent years the lateral mantle density variations revealed by seismic tomography have been introduced in dynamic mantle models which have predicted the major features of both the geoid and the surface divergence. These models assumed a spherical symmetry of the rheological properties of the Earth. However, with this assumption, the Navier‐Stokes equation implies that mass heterogeneities cannot drive any toroidal motion. This prompted us to develop a new model which takes into account the existence of rigid and independent plates. These plates break the spherical symmetry assumed in all earlier models. They modify the mantle circulation and hence the predicted surface observables such as displacement and gravity. In this paper, we first explain by means of a very simple twoplate model, how the poloidal flow caused by mantle heterogeneities can be converted to surface toroidal motion. We then apply the model to the real Earth with its major plates and compare our predictions to those deduced from a more classical Earth model having a uniform free slip condition at the surface. Taking the seismic tomography pattern as a picture of the mantle mass heterogeneities, we achieve a highly satisfactory fit to the observed plate velocities. This new theoretical framework also improves the geoid prediction.
Steady state, near‐source models of the Parkfield, Imperial Valley, and Mexicali Valley Earthquakes
Authors: Mendez A. J. , Luco J. E.
Published: February 2008

Some of the gross characteristics of the rupture processes for the 1966 Parkfield, 1979 Imperial Valley, and 1980 Mexicali Valley earthquakes are determined by waveform inversion of near‐source data employing a steady state dislocation model in a layered half‐space. The forward model involves a piecewise‐linear rupture front moving with a constant horizontal rupture velocity on a fault of infinite length and finite width. The inferred shapes for the rupture front and for the distribution of slip as a function of depth are consistent with previous results obtained by use of more general models. The results obtained show that a strong velocity pulse observed in the nearsource region can be modeled as the passage of the rupture front phase and that the supershear propagation of the rupture front in the sedimentary layers of the medium provides a mechanism for the generation of the observed large amplitudes.
Variations in fault slip and strain accumulation at Parkfield, California: Initial results using two‐color geodimeter measurements, 1984–1988
Authors: Langbein John O. , Burford Robert O. , Slater Larry E.
Published: February 2008

Repeated length measurements of several geodetic baselines near Parkfield, California, have revealed significant variations in the local rates of shallow fault slip and strain. This network of baselines, surveyed several times each week, straddles the San Andreas fault in the transition zone between the creeping section to the northwest and the locked section to the southeast. The length measurements, characterized by a precision approaching 0.1 ppm, reveal large fluctuations in the rates of baseline extension. Principal mode analysis of the length change data indicates that the two largest components of the signal are (1) secular extensions and contractions consistent with surface slip on the main strand of the San Andreas fault, and (2) a large seasonal oscillation with no obvious spatial coherence. On most of these baselines, the second component appears to be in phase with seasonal rainfall. When data from the baselines with the largest amplitude of the seasonal signal are excluded, the remaining data can be modeled in terms of both spatial and temporal variations in surface slip, variations in the components of the changes in uniform strain, and the possible displacement of the central monument in this radial network. In parameterizing this model, the spatial variation of slip beneath the near surface is reflected by changes in shear strain. Although the computed secular shear is highly dependent upon the specified parameterization of surface slip, the data are consistent with the hypothesis that slip at intermediate depths lags behind the surface slip rate. However, the range in models that fit the data does not necessarily imply that there is a deficit in slip at depth relative to the surface. Comparison of the inferred values of surface slip from the model with the observed fault slip measured by very short baseline creep meters indicates close agreement in secular rates, but the short‐term variations observed with the creep meters are either highly attenuated or nonexistent in the modeled slip since the modeled slip is a spatial average which smooths out possible short‐wavelength variations in the surface slip for which the creep instruments are most sensitive. An interesting conclusion from the two‐color data is that surface slip on the San Andreas fault appears to be spread over a 2‐km‐wide zone on the south flank of Middle Mountain but is confined to a very narrow zone to the south as the fault passes through the center of the network. This conclusion is dependent upon the assumption that a few critical monuments are stable and track tectonic displacements in the long term. Finally, the largest observed strain change is an extensional strain coincident with the Kettleman Hills earthquake M5.5 in August 1985.
Numerical modeling of water waves recorded by a subbottom seismometer in the southwest Pacific
Authors: Sereno Thomas J.
Published: February 2008

The amplitudes of water waves generated by shallow explosive sources and recorded by a subbottom seismometer provide constraints on velocity and attenuation parameters of oceanic sediments and upper oceanic crust. Analysis of data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific Ocean illustrates this method. The data were recorded by a triaxial geophone emplaced 54 m into the basaltic basement. Synthetic seismograms are computed using wave number integration for several crustal models to determine the sensitivity of water wave amplitudes to specific parameters of the sediments and oceanic crust. The synthetic seismograms fit the recorded data to better than a factor of 2 for frequencies as high as 15 Hz and ranges out to 30 km. The amplitude of the direct water wave beyond critical incidence (“direct wave root”) indicates that the sediment P wave velocity is near 1.8 km/s. Modeling of the attenuation of shear wave reverberations in the sediments indicates that sediment Qβ is between 30 and 50. Modulations in water wave amplitude as a function of receiver offset are related to the velocity of the upper oceanic crust. The distances at which amplitude maxima occur indicate that the shear wave velocity of the upper crust at the Ngendei site is 2.4±0.1 km/s. This is an important result since reliable estimates of the shear wave velocity of upper oceanic crust are rare. The distances at which amplitude minima occur are used to constrain the compressional wave velocity of the uppermost crust. However, the modulations do not resolve the compressional wave velocity of the crust as well as the shear wave velocity. The relative amplitudes of successive water waves indicate that the reflection coefficient at the sediment‐basement interface is 0.66. This further constrains the compressional wave velocity of the upper crust. Using estimates of the sediment properties and crustal shear wave velocity, the compressional wave velocity of the upper crust at the Ngendei site is estimated from this reflection coefficient to be 4.3–4.4 km/s. A method for estimating the velocity gradient in the upper kilometer of the crust from the interference of water waves with upgoing crustal refractions is presented. Application of the method to these data indicate that the shear wave velocity gradient is about 1.0 s−1, which is indistinguishable from the previous determination by Shearer and Orcutt (1986) using a quite different methodology. The velocity of the upper crust at the Ngendei site is consistent with velocities reported for other regions of similar crustal age (>100 m.y.) and is much greater than velocities typically found for young oceanic crust (<5 m.y.). This lends support to current theories regarding the evolution of the uppermost oceanic crust that attribute higher velocities in older crust to the closing of cracks and pores.
Degassing of argon from microclines within the thermal aureole of the Obsidian Dome Conduit, Long Valley Caldera, California: Constraints on emplacement history
Authors: Ryerson F. J. , Harrison T. Mark
Published: February 2008

Age spectra for microclines separated from samples obtained within the thermal aureole of the conduit to Obsidian Dome are compared with those from a reference sample collected in an area removed from the thermal effects of intrusion. The age spectra are characterized by a zero‐age plateau at low fractional release and by an ∼80 Ma plateau at higher fractional releases. These plateaus correspond to subparallel arrays in the Arrhenius data. These arrays are attributed to two discrete diffusion domain size fractions characterized by identical activation energies but by discrete and different frequency grain size fractions (Lovera et al., 1989). The relative losses of 40Ar* do not show a systematic pattern when compared on the basis of total 40Ar* loss. The 40Ar* losses first increase and then fall with distance from the contact. However, when the data composing the high‐temperature array of the Arrhenius data are considered (the large diffusion domain size fraction), the pattern is consistent with that predicted by conductive heating of the aureole. This agreement requires the aureole to have been uniformly preheated to a temperature of ∼250°C prior to intrusion of the rhyolite dike and surrounding breccia funnel that make up the vent structure. Further, the data require that the material forming the breccia funnel must have been heated to a temperature approaching that of the magma prior to emplacement.
A two‐ and three‐dimensional analysis of gravity anomalies associated with the East Pacific Rise at 9°N and 13°N
Authors: Madsen John A. , Detrick Robert S. , Mutter John C. , Buhl Peter , Orcutt John A.
Published: February 2008

We have used a unique data set collected during the 1985 East Pacific Rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the East Pacific Rise (EPR). The close spacing of track lines within the two main survey areas (8°45′N‐9°55′N and 12°20′‐13°30′N) allowed us to perform three‐dimensional analyses of high‐quality gravity data (+/−1 mGal uncertainity) obtained with the Bell Aerospace BGM‐3 gravity meter. our gravity modeling was enhanced by the availability of high‐resolution Sea Beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data. To model the crustal and upper mantle density structure at the ridge axis, we first calculated the gravity anomalies due to the density contrasts at the water/crust and crust/mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free‐air anomaly. The residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the EPR. Our results show that over 90% of the power in the observed free‐air anomaly can be modeled by these predictable components of the gravity signal. The amplitude and wavelength of the small residual anomaly can be modeled by a broad region (∼20 km wide), centered on the rise axis, of slightly lower‐than‐normal crustal and/or upper mantle densities (−0.03 Mg m−3). This region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high. The anomalies are consistent with, but do not require, the presence of a low‐density, crustal magma chamber at the rise axis. If a largely molten magma chamber exists, then the gravity data require that it be a narrow, volumetrieally small body. There are first‐order differences in the distribution of anomalous mass between the two survey areas with the 9°N region generally being characterized by lower densities in the axial region. Along‐axis variations in anomalous mass within each of the survey areas correlate with first‐order changes in the depth and cross‐sectional shape of the EPR axial high. We attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along‐strike changes in the size of the crustal magma chamber, the width of the surrounding zone of cooling crustal rocks, and densities in the uppermost mantle beneath the rise axis.
Borehole determination of formation thermal conductivity using a thermal pulse from injected fluid
Authors: Silliman S. E. , Neuzil C. E.
Published: February 2008

Thermal conductivity (λ) of a formation can be determined in situ by pouring or injecting liquid at an arbitrary temperature into a thermally equilibrated borehole and analyzing dissipation of the resulting thermal pulse. We used this technique in a shale by pouring diesel fuel into a borehole and obtained values for λ within a few percent of the mean value obtained from cores. The test showed that the technique is relatively simple to apply and can readily be used to test multiple horizons simultaneously. A priori estimation of formation specific thermal capacity was necessary to obtain good results, apparently because the form of the thermal pulse was not well characterized in our test. This resulted from adding liquid to the borehole in a poorly controlled manner; injecting liquid using a tremie pipe would provide a better characterized pulse and is probably advisable when implementing this technique.
Long‐period perturbations in starlette orbit and tide solution
Authors: Cheng M. K. , Shum C. K. , Eanes R. J. , Schutz B. E. , Tapley B. D.
Published: February 2008

Continuous and dynamically consistent orbits have been computed using satellite laser ranging (SLR) data to Starlette for a one‐year period during 1976 and 1977 and the three‐year period from 1983 through 1985. A low degree and order ocean tide solution, consisting of a total of 66 tidal parameters from 14 tidal constituents, was obtained in a combined solution using both the one‐ and three‐year Starlette orbits. An enhanced model of the Schwiderski [1980] tide solution was used both as the a priori and the background tide model. The model contains 368 tidal coefficients from 21 tidal constituents. Yearly values for the second‐degree annual tides, Sa, which produce seasonal variations on the Starlette node, were obtained also. These variations were found to have year‐to‐year fluctuations of more than 25% about the mean value and produced significant long‐period perturbations in the Starlette orbit. The Starlette tide solution is in good agreement with the Schwiderski tide solutions and with other satellite solutions, including multi‐satellite tide solutions. The average uncertainties are approximately 0.1 cm for the estimated semidiurnal and diurnal tides and 0.4 cm for the long‐period tides. The discrepancies of effective k2 inferred from the Starlette tide solution and other tide solutions are less than 0.01. The tide solutions were evaluated using orbit fits of a three‐year LAGEOS and a one‐year Starlette long arc. The results indicate that the Starlette tide solution produces significantly improved orbit fits for both LAGEOS and Starlette. The uncertainties for the estimated tide model coefficients were used to predict tidal errors for the LAGEOS orbit and were found to be consistent with the apparent tidal errors present in this orbit. The computed secular change in the Moon's mean motion due to tidal dissipation is −24.8±0.8 arc second/century2, which is in good agreement with the results obtained from other investigations.
Interrelationship of fluid venting and structural evolution: Alvin observations from the frontal accretionary prism, Oregon
Authors: Moore J. Casey , Orange Dan , Kulm Lavern D.
Published: February 2008

Seismic reflection and Sea Beam bathymetric data plus submarine geological measurements define a ramp anticline at the deformation front of the central Oregon subduction zone. At its northern termination the ramp anticline is deeply incised by a large 500‐m‐deep submarine canyon and cut by a probable backthrust. To the south along the strike of the fold, a smaller submarine canyon shallowly erodes the anticline, and backthrusting is not apparent in the submersible observations. Two Alvin dives along a transect through the southern canyon show active fluid vents demarked by biological communities at the frontal thrust and at the breached crest of the anticline. Along a northern transect, encompassing the large submarine canyon, 10 Alvin dives indicated no venting on the frontal thrust, limited venting in the canyon, but numerous biological communities along a scarp interpreted as the surface trace of the backthrust. These observations suggest a scenario of vent and structural‐geomorphic development consisting of (1) frontal thrust faulting and associated venting, facilitated by high fluid pressure; (2) erosion of the oversteepened seaward flank of the ramp anticline assisted by seepage forces and leading to fluid flow out of stratigraphically controlled conduits in the limbs of the overthrust deposits; (3) locking of the frontal thrust due to dewatering or a local decrease in wedge taper associated with development of the large canyon, leading to failure along the backthrust; and (4) redirection of fluid flow by the backthrust. Thus, within ≤0.3 m.y., deformation of the relatively permeable sediments of the Oregon margin results in stratigraphically controlled flow being partially captured by faults.
Evidence for an ancient impact basin in Daedalia Planum, Mars
Authors: Craddock Robert A. , Greeley Ronald , Christensen Philip R.
Published: February 2008

Geologic mapping has provided observations that some features on Mars are morphologically similar to lunar Imbrium Sculpture materials. These grooved features occur in clustered sets of a dozen or more, are spaced 1 to 10 km apart, and are unlike other linear Martian features such as yardangs, faults, ancient valley networks, and mass‐wasting features. We propose that they represent impact materials from an ancient basin. The orientations of these features were plotted as lines on a Schmidt net, and the density of the resulting intersections suggests a likely source area at approximately −26.0°, 125.0°, or roughly the center of Daedalia Planum. Numerous features, geologic materials, and Earth‐based radar data indicate that a basin with a diameter of approximately 4500 km with an early Noachian age is centered at this location. Four interior rings with diameters of 1100, 1500, 2200, and 3200 km and an exterior ring with a diameter of approximately 6400 km were found. The rings are spaced with a ratio of
, which is similar to the basin ring spacing on the other terrestrial planets. Both concentric and, infrequently, radial basin‐related faults were recognized, including Claritas Fossae, Gordii Dorsum, and the Thaumasia graben. Faults or zones of weakness related to the basin rings may have influenced the position of the Mangala Valles source area and Arsia Mons. Based on the location and morphology of early Hesperian to early Amazonian volcanics composing Daedalia Planum, the Daedalia basin may also have extensive, deep‐seated interior faults capable of extruding mare materials to the surface.
Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System
Authors: Hutchinson D. R. , White R. S. , Cannon W. F. , Schulz K. J.
Published: February 2008

The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end‐member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.
Memorial to Christopher Martin Scarfe
Authors: Dingwell D. B.
Published: February 2008

I first spoke with Chris Scarfe in the summer of 1980. I'd been flown out of the Newfoundland bush for a 4‐day break from chopper camp, and I had to get moving on a graduate school decision for September. I contacted Scarfe by telephone and mentioned to him some of my vague proposals for graduate work. He told me about his research, and I responded by saying that I certainly didn't know very much about silicate melts. He say that was o.k., we could learn together. All I could think was what a decent guy he must be to say something like that to a novice like myself. I've met a large number of people in the field of experimental and igneous petrology since that time. I don't think any of them ever had a better attitude toward the subject than Chris.
Introduction to C. M. Scarfe Memorial: Special Section on Silicate Melts and Mantle Petrogenesis
Authors: Dingwell Donald B.
Published: February 2008

In the summer of 1988 it was decided to commemorate Chris's career with a special session at an international meeting and a publication containing contributions volunteered by his former friends and associates. The 1989 Spring Meeting of the American Geophysical Union was chosen for the memorial session and the Journal of Geophysical Research was chosen for the publication of contributed scientific papers. This section would not have been possible without the active encouragement, advice, and direction provided by Bob Liebermann. I also extend my thanks to the participants and to the reviewers.
Origin of mantle peridotite: Constraints from melting experiments to 16.5 GPa
Authors: Herzberg Claude , Gasparik Tibor , Sawamoto Hiroshi
Published: February 2008

Experimental data are reported for the melting of komatiite, peridotite, and chondrite compositions in the pressure range 5–16.5 GPa. All experiments were run using the multiple‐anvil apparatus facilities at Nagoya and Stony Brook. Equilibrium between coexisting crystals and liquid is demonstrated to occur in less than 3 min in the 2100°C range. The anhydrous solidus in CaO‐MgO‐Al2O3‐SiO2 has been calibrated and is shown to be about 100° higher than that for naturally occurring peridotite (KLB1). All melting curves have positive dT/dP. The effect of pressure is to expand the crystallization field of garnet at the expense of all other phases, resulting in a change in the liquidus phase from olivine to garnet at high pressures. The melting of rocks which contain the four crystalline phases olivine, orthopyroxene, clinopyroxene, and garnet is restricted to enstatite‐rich compositions such as chondrite. For these it is demonstrated that melting is peritectic, rather than eutectic, and takes the form L+Opx = Ol+Cpx+Gt. Partial melting yields liquids with the following properties: 5 GPa for komatiite; and 10–15 GPa for liquid peridotite with about 40% MgO, but one that is unlike mantle peridotite in that it is distinctly enriched in silica. These results provide a test and refutation of the model that upper mantle peridotite originated by direct initial melting of a chondritic mantle (Herzberg and O'Hara, 1985). Unlike chondrite, partial melting of peridotite does not usually involve orthopyroxene. Instead, it occurs by the generation of ultrabasic liquids along a cotectic involving L+Ol+Cpx+Gt. Although the thermal and compositional characteristics of this cotectic have not been fully calibrated, it is very likely that it will degenerate into a thermal minimum (L+Ol+Cpx+Gt), compositionally similar to komatiite at 5 GPa and mantle peridotite at 10–15 GPa. Peridotite liquids that occupy a thermal minimum can be derived from those formed from the melting of chondrite by removal of orthopyroxene, followed by fractional crystallization of olivine, clinopyroxene, and garnet. The possibility exists that the thermal minimum is compositionally identical to mantle peridotite in the 10–15 GPa range. If this can be confirmed by experiment, the upper mantle can be understood as having originated by the fractional crystallization of peridotite liquids in a large‐scale differentiation event, consistent with magma ocean models for an early Earth.
Origin of leucite‐rich and sanidine‐rich flow layers in the Leucite Hills Volcanic Field, Wyoming
Authors: Gunter W. D. , Hoinkes Georg , Ogden Palmer , Pajari G. E.
Published: February 2008

Two types of orendite (sanidine‐phlogopite lamproite) and wyomingite (leucite‐phlogopite lamproite) intraflow layering are present in the ultrapotassic Leucite Hills Volcanic Field, Wyoming. In large‐scale layering, wyomingites are confined to the base of the flow, while in centimeter‐scale layering, orendite and wyomingite alternate throughout the flow. The mineralogy of the orendites and wyomingites are the same; only the relative amount of each mineral vary substantially. The chemical compositions of adjacent layers of wyomingite and orendite are almost identical except for water. The centimeter‐scale flow layering probably represents fossil streamlines of the lava and therefore defines the path of circulation of the viscous melt. Toward the front of the flow, the layers are commonly folded. Structures present which are indicative that the flows may have possessed a yield strength are limb shears, boudinage, and slumping. Phlogopite phenocrysts are poorly aligned in the orendite layers, while they are often in subparallel alignment in the wyomingite layers; and they are used as a measure of shearing intensity during emplacement of the flow. Vesicle volumes are concentrated in the orendite layers. In the large‐scale layering, a discontinuous base rubble zone of autobreccia is overlain by a thin platy zone followed by a massive zone which composes more than the upper 75% of the flow. Consequently, we feel that the origin of the layering may be related to shearing. Two extremes in the geometry of shearing are proposed: closely spaced, thin, densely sheared layers separated by discrete intervals throughout a lava flow as in the centimeter‐scale layering and classical plug flow where all the shearing is confined to the base as in the large‐scale layering. A mechanism is proposed which causes thixotropic behavior and localizes shearing: the driving force is the breakdown of molecular water to form T‐OH bonds which establishes a chemical potential gradient for water in the melt. The higher activity of water in the nonsheared regions allows sandine to crystallize, whereas the lower activity of water in the areas of active shearing causes leucite to crystallize.
Mafic enclaves in the Wilson Ridge Pluton, northwestern Arizona: Implications for the generation of a calc‐alkaline intermediate pluton in an extensional environment
Authors: Larsen Lance L. , Smith Eugene I.
Published: February 2008

The Wilson Ridge pluton is an epizonal calc‐alkaline pluton that formed about 13.5 Ma during a period of mid‐Miocene extension. Faulting and erosional dissection provide a cross section of the pluton. The apex of the pluton, in the Boulder Wash area, Nevada, is composed of hypabyssal quartz monzonite and dacite. The base of the pluton is 20 km to the south where quartz monzodiorite, monzodiorite, and diorite are in low‐angle intrusive contact with Precambrian basement. The pluton was separated from cogenetic volcanic rocks in the River Mountains by movement along the Saddle Island detachment fault at about 13.4 Ma. The River Mountains now lie 20 km to the west of the pluton. The Wilson Ridge pluton is composed of the Teakettle Pass suite consisting of foliated monzodiorite and quartz monzodiorite and unfoliated quartz monzonite and the older Horsethief Canyon diorite. Rocks of the pluton contain 2–4 modal percent sphene. Intermediate rocks of the Teakettle Pass suite contain abundant basaltic and diorite enclaves. Basaltic enclaves are lensoidal and pillow‐like and commonly have crenulate and fine‐grained margins. Enclaves are chemically similar to mafic dikes of the Wilson Ridge pluton and to cogenetic alkali basalt flows in the River Mountains. They probably represent blobs of mafic liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton. Basaltic enclaves commonly occur as inclusion‐rich zones that represent synplutonic mafic dikes that were injected into a quartz monzonite host. Mafic magma was entrained and mechanically broken down by magmatic flow shear. A continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin, tabular mafic selvages and schlieren and ultimately to the mafic component in foliated quartz monzodiorite and monzodiorite. Diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths. Field evidence and major and trace element models suggest that the intermediate rocks of the pluton were produced by the commingling of a large volume of mafic magma with a smaller volume of felsic magma (a mafic‐felsic ratio of about 70∶30) as well as fractional crystallization. Similar open system processes may be responsible for the production of calc‐alkaline intermediate rocks in other parts of the Great Basin.
Olivine analcimite in the Cascade Range of Oregon
Authors: Conrey Richard M.
Published: February 2008

A 10‐m‐thick lava flow of olivine analcimite occurs in drill core near latitude 45°N in the Cascade Range in Oregon. The analcimite contains phenocrysts of calcic clinopyroxene (∼23 wt % CaO) and olivine (Fo88–84 with Cr–Al spinel inclusions) in a fine‐grained groundmass containing analcime, but no nepheline. Chemically and mineralogically similar rocks are usually found on ocean islands and in intracratonic rifts, both within‐plate tectonic settings. The high Ti, Nb, and Ta contents of the analcimite are typical of within‐plate and some back arc magmas. The anakimite is unlike many other arc rocks described as alkaline but which are enriched in K and depleted in Ti, Nb, and Ta. The analcimite is K‐poor (K2O = 0.68 wt %) even for a within‐plate magma. Pb and Sr isotope ratios (208pb/204Pb = 38.915; 207Pb/204Pb = 15.6; 206Pb/204Pb = 19.543; 87Sr/86Sr = 0.702848) for the analcimite are similar to ratios found in within‐plate alkaline rocks from throughout the Pacific Northwest and NE Pacific Ocean. This uniformity is consistent with the suggestion that similar mantle underlies the accreted terranes of Oregon, Washington, and British Columbia. The analcimite Pb is much more radiogenic than Pb in other rocks from the Cascade Range. The analcimite is approximately 4 Ma and was erupted at about the time of intraarc graben formation in the northern portion of the Cascade Range. It is suggested that the analcimite was generated by a very small degree of partial melting in a shallow (∼45–50 km) subarc metasomatized mantle containing amphibole and apatite. Similar small‐degree partial melts are presumably present in many arcs at the margins of the large‐degree partial melt zone that generates arc basalts. In the absence of structural disruption, however, small‐degree melts fail to reach the surface. The metasomatism which led to the growth of amphibole and apatite in the analcimite mantle source is probably not related to subduction but rather to earlier melting events that generated oceanic lithosphere before its accretion to North America.
Ironworkers Convention I: Workshop on Physics of Iron
Authors: Shankland Thomas J. , Anderson Orson L. , Young David
Published: February 2008

This special section reports on a Workshop on the Physics of Iron that was held June 14–16, 1989, at Los Alamos National Laboratory, Los Alamos, New Mexico. Sponsored by the Institute of Geophysics and Planetary Physics (IGPP) of the University of California, the workshop was hosted by the Los Alamos branch. The IGPP's Center for Earth and Planetary Interiors (CEPI), of which the three convenors are members, provided coordination during the publication stages. Topics of study included the physical properties of iron, its phase transitions and phase diagram to high temperatures and pressures, its alloying properties, and theoretical explanations of these features.
Comparison of ground motion from tremors and explosions in deep gold mines
Authors: McGarr A. , Bicknell J. , Churcher J. , Spottiswoode S.
Published: February 2008

Seismic body waves, from tamped chemical explosions, two with yields of 50 and one of 150 kg, were compared with corresponding data from three mining‐induced tremors with a view to testing methods of discriminating between the two types of events. Detonated at depths of about 2 km, all three explosions generated P waves for which the low‐frequency spectral asymptotes agree well with corresponding results scaled down from nuclear shots at the Nevada Test Site. For the two smaller explosions, recorded underground at hypocentral distances ranging from 234 to 871 m, the P wave corner frequencies, defined by the intersections of the low‐ and high‐frequency asymptotes, are also in fine agreement with scaled‐down results from the Nevada Test Site; for the 150‐kg explosion, which was recorded only at the surface, it appears that 2.39 km of upward propagation caused the corner frequency to be at least a factor of 4 lower than anticipated due to attenuation. All three explosions generated S waves that appear to be a consequence of deviatoric stress release in the immediate environs of the explosions. The three tremors analyzed here generated P and S waves whose spectra and source parameters agree well with standard earthquake source models. With regard to discrimination based on P wave spectra, we conclude that for events of fixed low‐frequency spectral asymptotes, the explosions typically have higher corner frequencies than tremors or earthquakes, although counterexamples certainly exist. Interestingly, the 150‐kg explosion was identified as such on the basis of P and S wave polarities that are incompatible with the normally expected double‐couple source model; instead, these initial motions are consistent with an explosion in conjunction with normal faulting. The body wave spectra of this explosion and those of a nearby tremor, however, were indistinguishable.
Large cyclonic rings from the northeast Sargasso Sea
Authors: McCartney M. S. , Worthington L. V. , Schmitz W. J.
Published: February 2008

Expendable bathythermograph observations have revealed large cold core cyclonic current rings to the east of 60°W in a region that mechanical bathythermograph observations (Parker, 1971) indicated to be devoid of rings. As a class these rings are larger than typical Gulf Stream rings that form and drift west of 60°W. The typical diameter (15°C at 500 m) there is around 100 km, while the eastern Sargasso rings are 200 km and more in diameter. Several of these eastern rings were observed on each of four cruises in the northern Sargasso Sea in 1974 and 1975. The overall picture of the region east of 60°W obtained was a very noisy one, dominated by large‐diameter, large‐amplitude eddies. One of the eastern rings was seen in all four cruises and was observed to drift westward for over 730 km at an average speed of 4.4 km/d, starting at 56°30′W and 34°40′N and passing north of Bermuda. The character of the dissolved oxygen anomalies in the cores of the eastern rings suggests a possible formation region at the eastern end of the Sargasso Sea gyre, around 40°W. Hence the eastern rings may have already been a year old when first observed in November 1974. A single deep hydrographic section showed the center of the deep circulation to lie considerably further southwest than the near‐surface circulation center, although this could be a distortion due to a large seamount. Moored current meter data suggest a level of no motion within eastern rings at about 2000 m, giving a weak anticyclonic circulation of 4 × 106 m3/s below that level, compared with the 45 × 106 m3/s cyclonic circulation above 2000 m. On several occasions, smaller‐scale upward displacements of the thermal structure were seen at the sides of eastern rings. It is not known whether these represented interactions with smaller rings or some breakdown of the circular symmetry.
Spatial and temporal variation of the wind stress off the Somali Coast
Authors: Bruce J. G.
Published: February 2008

Wind observations from ships along the sea‐lane off the Somali coast are used for estimating the seasonal increase in wind stress during the southwest monsoon. The stress has a relatively large maximum (∼4–6 dyn cm−2) in the region between 8°N and 12°N during June, July, and August. Maximum values of the zonal wind stress curl occur about 6°N–8°N. During this monsoon period a large‐scale eddy (∼600 km in diameter) or eddies occur in the Somali basin. Wind stress averages (1922–1972) for the north‐western Indian Ocean are also given. Yearly southwest monsoon averages for the region subjected to the strong monsoon winds show that stress components may vary from year to year by nearly a factor of 2.
Turbulent interface layers
Authors: Csanady G. T.
Published: February 2008

The ‘law of the wall,’ familiar from turbulent boundary layer theory, has been extended to turbulent flow along a sharp density interface. The effective gravity of such an interface is an additional independent variable, so that in nondimensional form the ‘law of the interface’ becomes a one‐parameter family of curves. A convenient label for each curve is its ‘Keulegan parameter,’ a nondimensional combination of inner variables, which also plays a role in the theory of interfacial stability. One important difference between the law of the wall and the law of the interface is that the velocity gradient at an interface is typically much less than the stress divided by the molecular viscosity. Even in the nonturbulent core of an interface the effective viscosity is appreciably higher than the molecular viscosity. This finding is explained by the presence of viscous wavelets on the interface which contribute to momentum transfer through a combination of ‘sheltering’ and the generation of an interfacial viscous boundary layer. A simple approximate theory based on these ideas reproduces the observed functional form of effective viscosity versus Keulegan parameter (a square root relationship for miscible fluids). At interfaces with free surface energy, for low relative velocities, capillarity plays an important role. Capillary‐gravity waves of minimum celerity become key agents of momentum transfer, so that effective interface viscosity comes to depend on surface tension. The second major effect of variations of effective gravity is changing hydrodynamic roughness. The extent of this influence is governed by the buoyancy flux, and it also occurs over solid walls, although this has so far been ignored in the literature. Evidence from a variety of sources now suggests that hydrodynamic ‘roughness’ is determined by the energy balance of turbulence very close to the wall. A simple approximate theory based on this postulate reproduces qualitatively the observed interface roughness changes. Bulk relationships (drag, shear, and mass transfer coefficients) for interfaces may be determined by matching inner and outer laws in the conventional manner. The most practical independent variables for the presentation of such laws appear to be a gravity to viscous force ratio (rather than Reynolds number) and a Keulegan parameter (rather than Froude number), both based on bulk external variables.
Stratospheric dust measurements, 1970–1977
Authors: Clemesha B. R. , Simonich D. M.
Published: February 2008

Stratospheric aerosol measurements have been made by laser radar at São José dos Campos (23°S, 46°W) since July 1970. From 1970 to 1974 the measured scattering ratios for the 20‐km layer were similar to those reported for other locations. A large increase was observed in April 1975, 6 months after the eruption of Volcán de Fuego in Guatemala. This delay in the transport of the Fuego dust to our latitude is interpreted as being due to the inhibition of southward eddy transport during the southern summer by the mean meridional circulation. A maximum monthly average scattering ratio of 1.28 measured at a wavelength of 5890 Å was observed at 20 km in August 1975. Since early 1976 the 20‐km scattering ratio has oscillated around a value of about 1.15, but the integrated columnar backscattering coefficient above 17 km, indicative of the total stratospheric dust loading, has increased by about 40%. Scattering from the upper stratosphere, at heights above 30 km, was observed in the southern springs of 1971, 1972, and 1973. There appears to be some evidence that the dust responsible for this scattering was of extraterrestrial origin.
Thunderstorm on July 16, 1975, over Langmuir laboratory: A case study
Authors: Winn William P. , Moore C. B. , Holmes C. R. , Byerley L. G.
Published: February 2008

The electric field along the path of an instrumented balloon was closely coupled to the wind profile and to the radar echo structure of a weak thunderstorm over Langmuir Laboratory on July 16, 1975. The balloon ascended at 3.5 m/s into the southern part of the storm, where a stable layer had stopped the cloud's vertical convection at 6.4 km above sea level. At lower altitudes, near the cloud base, the balloon rose past two nearby oppositely charged regions which were associated with a precipitation echo and with an outflow of air from the storm. When the balloon ascended into clear air through the top of the lower cloud at 6.4‐km altitude, its motion indicated a sharp change in wind direction, and its electric field meter showed an abrupt decrease in field intensity, probably from a screening layer at the cloud boundary. Above 7.5 km the balloon encountered a slanted and charged downdraft just before entering the northern part of the storm under its anvil cloud. This downdraft, which had a velocity of about 6 m/s, was a prominent and persistent feature of the cloud's circulation. It held the balloon at a nearly constant altitude of 7.7 km for 10 min while carrying it 3 km toward the center of the storm. When the electric field meter descended, after release from the balloon, it encountered the downdraft a second time, 24 min after its first encounter. Electric field measurements suggest that the downdraft was carrying a negative charge. Our measurements on this storm also contain evidence for vertical transport of horizontal momentum and for a net positive charge in the upper part of the storm.
On the use of zirconium 95 data from chinese atmospheric thermonuclear explosions to study stratospheric transport in a one‐dimensional parameterization
Authors: Bauer Ernest , Oliver Robert C. , Wasylkiwskyj Wasyl
Published: February 2008

We analyze data (Telegadas, 1974, 1976) on the transport of 95Zr from five Chinese 3‐Mt thermonuclear explosions which deposited their debris clouds at approximately 18‐km altitude and location (40°N, 90°E) in terms of one‐dimensional diffusive transport into the troposphere. The motivation for the work is that the dynamics of oxides of nitrogen and other materials injected into the lower stratosphere by supersonic transport aircraft in the general region of 15‐ to 18‐km altitude, 40°–60°N latitude, is not well known, and different parameterizations of the transport by different authors vary substantially. A technique is developed here that allows data from pulsed sources at different seasons to be used to parameterize stratospheric motions in terms of a mean stratospheric eddy diffusivity
and a mean tropopause height z0 (which characterizes an effective height of injection above the local tropopause) and thus to estimate the atmospheric residence time (equal to burden divided by flux) and also the ‘injection coefficient’ of McElroy et al. (1974) for continuous sources. A correction is made for the sedimentation of the particulate tracers used.