bluuebunny/crossref_metadata_2025 huggingface dataset:
Passive, off‐axis convection through the southern flank of the Costa Rica Rift
Authors: Fisher A. T. , Becker K. , Narasimhan I. T. N. , Langseth M. G. , Mottl M. J.
Published: February 2008

Pore fluids are passively convecting through young oceanic sediments and crust around Deep Sea Drilling Project (DSDP) site 504 on the southern flank of the Costa Rica Rift, as inferred from a variety of geological, geochemical, and geothermal observations. The presence of a fluid circulation system is supported by new data collected on Ocean Drilling Program (ODP) leg 111 and a predrilling survey cruise over the heavily sedimented, 5.9 Ma site; during the latter, elongated heat flow anomalies were mapped subparallel to structural strike, with individual measurements of twice the regional mean value, and strong lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores. Also, there is a strong correlation between heat flow, bathymetry, sediment thickness, and inferred fluid velocities up through the sediments. On an earlier DSDP leg, an 8‐bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover. Although the forces which drive passive circulation are not well understood, it has generally been thought that the length scale of heat flow variations provides a good indication of the depth of hydrothermal circulation within the oceanic crust. This assumption was based on analytical and numerical analyses of relatively simple porous media models. Deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4–7 km but appears to be precluded by low crustal permeability, as measured in DSDP hole 504B. The widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines (1) basement relief, (2) irregular sediment drape, (3) largely conductive heat transfer through the sediments overlying the crust, and (4) thermal and geochemical homogenization of pore fluids at the sediment/basement interface, which results from (5) topographically induced, passive hydrothermal circulation with large aspect ratio, convection cells. This convection involves mainly the permeable, upper 200–300 m of crust; the deeper crust is not involved. This convection is induced through a combination of buoyancy fluxes, due to heating from below, and topographic variations on the seafloor and at the basement‐sediment interface. This model was designed to incorporate data from both near the sediment surface and deep within boreholes; it is successful in duplicating numerous field observations.
Tectonic and volcanic evolution of dark terrain and its implications for the internal structure and evolution of Ganymede
Authors: Murchie Scott L. , Head James W. , Plescia Jeffrey B.
Published: February 2008

Furrows in ancient dark terrain on Ganymede form three systems that are each hemispheric or greater in scale. The oldest of the systems, designated system III, is dominated by approximately concentric troughs centered on about 60°N, 50°W. System I, in the anti‐Jovian hemisphere, contains concentric and subradial furrows arrayed around a large, degraded palimpsest centered at 15°S, 165°W. Furrows in each system formed on and locally are buried by dark volcanic materials that embay and infill preexisting topographic features; they crosscut extremely few well preserved older craters; and they occur on surfaces having significantly different relative crater ages. System II, also in the anti‐Jovian hemisphere, contains widely spaced, radially arrayed furrows commonly 500–2000 km in length, which are organized around a large area of dark smooth resurfacing material, intense dark terrain fracturing, and some of the globally oldest light material. The total thickness of dark terrain resurfacing, estimated using the stratigraphy of different dark material deposits and the rim heights of the largest crater size class whose density was depleted by each deposit, is probably in the range of 3–8 km. Multiple models of the origin of each furrow system were tested using observed geologic features and patterns. Systems I and III were found to be most consistent with reactivation of impact‐generated, multiringed structures by endogenic global extension, during a period of widespread dark material volcanism that obliterated a dense, ancient crater population. System II was found to be most consistent with fracturing of a single, circular, isostatic uplift covering an entire hemisphere. On the basis of geologic observations and interpretations and theoretical models of convection in spheres, it is hypothesized that the uplift developed by long‐term warming of the upwelling current of a single axisymmetric convection cell in an initially cooler, undifferentiated interior. Such wanning would also have created global expansion and supplied the tensional stress inferred to have formed systems I and III. This hypothesis is supported by the concentration around the center of system II of intense fracturing and relatively young dark volcanic deposits, suggestive of high lithospheric heat flow, lithospheric thinning, and stress concentration. An observed long‐term decrease in the width of extensional tectonic features interpreted to be of endogenic origin is also consistent with lithospheric thinning due to warming of a cool interior.
The upper mantle degree 2: Constraints and inferences from global mantle wave attenuation measurements
Authors: Romanowicz Barbara
Published: February 2008

We present the results of an analysis of global lateral variations in anelasticity of the upper mantle, as measured from very long period Rayleigh waves observed on the GEOSCOPE network. Four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account, in a linear manner, focussing effects due to lateral variations in elastic parameters. Local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length, as an alternative to spherical harmonics expansion. Comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high‐Q and high‐velocity regions (respectively low Q and low velocity) both for great circle and minor arc data, at periods around 200 s. The detailed analysis of the degree 2 pattern, well resolved in the attenuation maps around this period, shows that it originates in the depth range 250–500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin, possibly related to the topmost part of the large‐scale convective system. Corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid. This study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods, before performing any comparison with short‐period data or other geophysical parameters.
Temporal changes in shear wave splitting during an earthquake swarm in Arkansas
Authors: Booth David C. , Crampin Stuart , Lovell John H. , Chiu Jer‐Ming
Published: February 2008

Shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three‐component digital seismometers during the 1982 Enola, Arkansas, earthquake swarm. Polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field. The time delay between split shear waves appears to increase before, and to decrease at or after the time of each main shock. This behavior is similar to observations before and after a M = 6.0 event near the Anza seismic gap, southern California, and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes. There are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction.
An integrated study of seismic anisotropy and the natural fracture system at the Conoco Borehole Test Facility, Kay County, Oklahoma
Authors: Queen John H. , Rizer William D.
Published: February 2008

A significant body of published work has developed establishing fracture‐related seismic anisotropy as an observable effect. To further the understanding of seismic birefringence techniques in characterizing natural fracture systems at depth, an integrated program of seismic and geologic measurements has been conducted at Conoco's Borehole Test Facility in Kay County, Oklahoma. Birefringence parameters inferred from the seismic data are consistent with a vertical fracture model of density 0.04 striking east‐northeast. That direction is subparallel to a fracture set mapped both on the surface and from subsurface data, to the in situ maximum horizontal stress, and to the inferred microfabric.
Simultaneous iterative reconstruction technique: Physical interpretation based on the generalized least squares solution
Authors: Trampert Jeannot , Leveque Jean‐Jacques
Published: February 2008

The Simultaneous Iterative Reconstruction Technique is a very suitable technique for inverting large sparse linear systems, since it is iterative and does not need the whole matrix to be stored in the internal computer memory. It was designed for medical tomography, but is nowadays commonly used in seismic tomography. We propose to analyze this inversion technique, which has nevertheless a few drawbacks, such as inconsistency and the introduction of nonphysical, a priori information into the solution resulting from an implicit rescaling of the problem. The introduction of a damped version of the original algorithm and the study of its relationship with the generalized least squares algorithm [Tarantola and Valette, 1982] enables us to explain the physical behavior of the method. The damped algorithm appears to be more efficient than the original one because it allows us to control the inversion. There are two main options: (1) a fast convergence with, unfortunately, nonphysical, a priori information in the solution and (2) complete control of the a priori information, but at the expense of the convergence speed. Finally, we show how to compute the resolution matrix and an approximation of the estimated model covariance, without passing through memory‐consuming matrix inversions.
Origin of the Martian global dichotomy by crustal thinning in the Late Noachian or Early Hesperian
Authors: McGill George E. , Dimitriou Andrew M.
Published: February 2008

The marked dichotomy in topography, surface age, and crustal thickness between the northern lowland and southern upland of Mars has been explained as due to an initially inhomogeneous crust, a single mega‐impact event, several overlapping large basin impacts, and first‐order convective overturn of the martian mantle. All of the published hypotheses propose that the dichotomy was formed early in martian history; before the end of the primordial heavy bombardment. A primordial origin is inherent in the initial crustal inhomogeneity hypothesis, and required for both impact hypotheses. Endogenic hypotheses are not so constrained. Geological data indicate episodes of fracturing and faulting in the late Noachian and the early Hesperian. This fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland. Igneous activity also peaked in the late Noachian and early Hesperian. These data suggest a tectonic event near the Noachian/Hesperian boundary characterized by enhanced heat loss and extensive fracturing, including formation of the faults that define much of the highland/lowland boundary. We argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume.
High spectral resolution reflectance spectroscopy of minerals
Authors: Clark Roger N. , King Trude V. V. , Klejwa Matthew , Swayze Gregg A. , Vergo Norma
Published: February 2008

The reflectance spectra of minerals are studied as a function of spectral resolution in the range from 0.2 to 3.0 μm. Selected absorption bands were studied at resolving powers (λ/Δλ) as high as 2240. At resolving powers of approximately 1000, many OH‐bearing minerals show diagnostic sharp absorptions at the resolution limit. At low resolution, some minerals may not be distinguishable, but as the resolution is increased, most can be easily identified. As the resolution is increased, many minerals show fine structure, particularly in the OH‐stretching overtone region near 1.4 μm. The fine structure can enhance the ability to discriminate between minerals, and in some cases the fine structure can be used to determine elemental composition. For example, in amphiboles and talcs, four absorption bands are observed in the samples analyzed in this study that are due to hydroxyl linked to Mg3, Mg2Fe, MgFe2, and Fe3 sites. The band intensities have been shown by other investigators to give the Fe:Fe+Mg ratio from transmission spectra. This study shows that the same equations can be used to obtain the ratio from reflectance spectra of unprepared samples. High‐resolution reflectance Spectroscopy of minerals may prove to be a very important tool in the laboratory, in the field using field‐portable spectrometers, from aircraft, and from satellites looking at Earth or other planetary surfaces.
Geochemical evolution of the Northern Plains of Mars: Early hydrosphere, carbonate development, and present morphology
Authors: Schaefer Martha W.
Published: February 2008

It is likely that early in Mars' history, abundant liquid water was available. Under a thick (several bars) carbon dioxide atmosphere, this water could have formed an ocean, located primarily in the lowlands of the northern hemisphere. An equilibrium geochemical model of this ocean and its interactions with the atmosphere and regolith of Mars was developed, and the results of this model were used to discuss the evolution of the volatile budget of Mars, including the deposition of large carbonate beds on the northern plains. Differential solutional weathering of these carbonate beds may have caused the formation of some of the enigmatic features seen on the northern plains of Mars, such as the thumbprint terrain and enclosed depressions.
Possible Martian brines: Radar observations and models
Authors: Zent Aaron P. , Fanale Fraser P. , Roth Ladislav E.
Published: February 2008

The 1971 and 1973 Goldstone 12.6‐cm radar observations of Mars are separate data sets which include reflectivity as a function of latitude, longitude, and season. It has been argued that secular reflectivity variations of Mars' surface are indicated by the data and that shallow subsurface melting is the causal mechanism most compatible with the observations; however, the melting hypothesis conflicts with accepted notions of the state and distribution of water on Mars. We examine the data to identify temporal and spatial domains within which statistically significant changes in measured reflectivity are clustered. A few reflectivity changes may be genuine; others may be due to ephemeris errors or binning during data reduction. Brines which might satisfy the best supported reflectivity variations are out of equilibrium with the chemical megaenvironment. It is unclear whether such a brine, if emplaced in the Martian regolith at a depth shallow enough to affect the radar reflectivity, could survive even a single freeze‐thaw cycle. We suggest that some combination of unique scattering properties or some as yet unidentified process other than melting is responsible for any genuine reflectivity variations.
Attenuation of high‐frequency shear waves in the crust: Measurements from New York State, South Africa, and southern California
Authors: Frankel Arthur , McGarr Art , Bicknell John , Mori Jim , Seeber Leonardo , Cranswick Edward
Published: February 2008

We compare the attenuation of high‐frequency (3–30 Hz) shear waves for crustal paths in New York State, South Africa, and southern California over source‐receiver distances of about 10–400 km. The data consist of digital recordings of S waves (Δ = 5–100 km) and Lg waves (Δ = 100–400 km) produced by earthquakes. We use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the S and Lg waves. Over the entire distance range studied (10–400 km), the amplitude decay of 3‐Hz shear wave energy is considerably less for the tectonicaily stable areas of New York and South Africa than for the tectonicaily active region of southern California. High‐frequency (30 Hz) S wave attenuation is significantly less for New York and South Africa than for southern California, for distances between 15 and 90 km. We parameterize the decay with distance (R) of coda‐normalized shear wave amplitudes with a frequency‐independent Q and geometrical spreading exponent γ, where geometrical spreading is proportional to R−γ. For New York State the S wave amplitude decay (3–30 Hz) is well described by a frequency‐independent Q of 2100−330+490 and γ of 1.3±0.1. The decay of Lg wave amplitudes from 3 to 15 Hz in the New York State region is fit with a frequency‐independent Q of 1600−280+330 γ of 0.70±0.2. The S wave amplitudes (3–30Hz) in South Africa yield a Q of 1500−190+380 and γ of 1.3±0.1. Fixing the geometrical spreading at R−0.5 produces an Lg wave Q estimate at 3 Hz in South Africa of 360−50+80. This Lg wave Q is low considering that South Africa is a cratonic, tectonicaily stable area. The S wave amplitudes from southern California are described with a frequency‐independent Q of 800−150+240 and a large geometrical spreading exponent of γ of 1.9±0.2. We find an Lg wave Q at 3 Hz of 260±30 for southern California, after constraining the geometrical spreading at R−0.5.
Some additional observations on inclusions in the granitic rocks of the Sierra Nevada
Authors: Dodge F. C. W. , Kistler R. W.
Published: February 2008

Microgranular quartz diorite and diorite inclusions are widespread in central Sierra Nevada granitoid rocks and are almost exclusively restricted to hornblende‐bearing rocks, most commonly felsic tonalites and mafic granodiorites. Strontium 87/strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages. Neodymium 143/neodymium 144 values of five inclusions ranged from 0.51229 to 0.51248 and yield εNd values that correlate closely with values of their respective hosts. The Nd‐Sm and Rb‐Sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation. Silica contents of inclusions and granitoids are contiguous, but inclusions generally contain less than, and granitoids more than, 60% SiO2. Linear continuity of the femic oxide and H2O+ trends relative to SiO2 suggests many inclusions formed as concentrations of hydrous mafic minerals. Variation of other major element oxides and trace elements support this inference. The aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle‐derived basalt. However, many initial Sierra Nevada magmas were produced by mixing of mafic, mantle‐derived melts and felsic crustal materials, resulting in isotopically varied tonalitic and granodioritic magmas. Most inclusions thus represent fragmented crystal accumulations of early‐formed, near‐liquidus minerals generated from these previously mixed magmas.
Hydrothermal circulation in an anisotropic sedimentary basin: Application to the Okinawa Back Arc Basin
Authors: Genthon Pierre , Rabinowicz Michel , Foucher Jean Paul , Sibuet Jean Claude
Published: February 2008

We explore the pattern of two‐dimensional convection in an highly anisotropical porous medium. This physical situation is relevant to passive margin sedimentary basins consisting of interbedded coarse‐grained pervious and shale or limestone semi pervious layers, in particular to deltaic‐type basins with sandstone lenses in a shale matrix. We show that permeability anisotropies of the order of 102–104 allow for long convective cells, of aspect ratio greater than 10, but that a combination of this parameter with a slight slope of the order of a few percent of the sedimentary layers is required to stabilize these long cells. As an example, we present the Okinawa basin, an active submarine back arc basin, with a sedimentary thickness of about 2 km and a heat flow profile across this basin, varying from 32 to 232 mWm−2 over a distance of 30 km. It is shown that this heat flow variation is difficult to explain with conductive mechanisms only but is well reproduced by different convective models relying on permeability anisotropy plus slope. Although the insuficient thermal and structural constraints did not allow us to build a unique model, the whole set of possible fits to the heat flow data may restrict the mean hydraulic parameters of the basin. A vertical permeabilty of a few tens of millidarcy and an anisotropy greater than 100 are required to produce the expected stable and active large‐scale circulation. It is suggested in conclusion that this type of circulation might be active in oil‐or oreforming element migration.
Basal slip and mechanical anisotropy of biotite
Authors: Kronenberg Andreas K. , Kirby Stephen H. , Pinkston John
Published: February 2008

The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45° to (001) exhibit gentle external rotations associated with dislocation glide. High‐angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample corners. Samples shortened perpendicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical response of biotite shortened at 45° to (001) is nearly perfectly elastic‐plastic; stress‐strain curves are characterized by a steep elastic slope, a sharply defined yield point, and continued deformation at low (mostly < 100 MPa), relatively constant stresses at strains >1%. Stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain rate and temperature. Assuming an exponential relationship between differential stress σd and strain rate
of the form
, the data collected over strain rates and temperatures of 10−7 to 10−4 s−1 and 20° to 400°C, respectively, are best fit by an exponential constant α of 0.41±0.08 MPa‐1 and an activation energy Q of 82±13 kJ/mol. A power law fits the data equally well with n = 18±4 and Q = 51±9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110] are measurably weaker than those shortened at 45° to [010] and [310], consistent with the reported Burgers vectors 〈100〉, 1/2 〈110〉, and 1/2 〈110〉. The anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands. The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limit locally the stresses that can be supported.
Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology
Authors: Blackwell David D. , Steele John L. , Frohme Michael K. , Murphey Charles F. , Priest George R. , Black Gerald L.
Published: February 2008

New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass‐Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two‐dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high‐frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.
Block rotation in western Turkey and elsewhere: 2. Theoretical models
Authors: Westaway Rob
Published: February 2008

Rotation of domains within the upper crustal brittle layer around vertical axes has been documented in many actively deforming regions. Some domains in western Turkey and elsewhere comprise sets of blocks that are bounded by subparallel faults, are elongated and angular in plan view, coupled to the underlying plastic deformation, and rotate in the same sense at a uniform rate that may equal approximately half the vertical vorticity in the underlying plastic deformation. However, other domains in western Turkey that appear approximately circular in plan view and comprise a single block with little internal deformation appear to rotate in the opposite sense to the regional counterclockwise vertical vorticity, and are thus apparently decoupled from the underlying plastic deformation. Rotational behavior of any block in the brittle layer may be affected by torques acting both on its base and on its margins. The torque acting on the base of a circular block with radius R caused by viscous coupling with the underlying plastic deformation is likely to be proportional to R4, as well as proportional to the difference between the angular velocity of the block and that expected given the vertical vorticity in the underlying plastic deformation. The frictional torque acting on the margins of such a circular block is likely to be proportional to R2. Thus, provided a circular block has sufficiently small radius, the frictional torque acting its margins will exceed the viscous torque acting on its base. Frictional torques generated in this manner may cause circular blocks to rotate in the opposite sense to the regional vertical vorticity beneath the brittle layer. The circular domains identified in western Turkey comprise single blocks, are within the size range for frictional torques to dominate viscous torques, and their geometrical relationship to adjacent domains and to the regional deformation pattern allows them to rotate in the opposite sense to their surroundings. This suggests that frictional torques may cause their observed clockwise rotations.
The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes
Authors: Foster David A. , Harrison T. Mark , Miller Calvin F. , Howard Keith A.
Published: February 2008

Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of 40Ar/39Ar thermochronology to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The 40Ar/39Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The 40Ar/39Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1°–5°C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman‐Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500°C which reset the K–Ar systems of minerals from Proterozoic rocks. High‐grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman‐Piute batholith at 73±1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were ∼100°C/m.y. between 73 and 70 Ma and 5°–10°C/m.y. from 70 to ∼30 Ma. Between 65 and 25 Ma the entire eastern Mojave Desert underwent a period of cooling at a rate of 2°–10°C/m.y. By 30 Ma, rocks exposed in the Old Woman‐Piute, Marble, Ship, Clipper, and Turtle Mountains were below ∼100°C. The 40Ar/39Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23±1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at ∼180°C and ∼350°C, respectively. This suggests that the exposed part of the Chemehuevi detachment fault initiated at a dip of 5°–30° or as a series of higher‐angle faults that cut to a depth of 10–12 km and were later rotated to their present dip. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10°–50°C/m.y. between 22 and 15 Ma.
Rhyolitic calderas of the Yukon‐Tanana Terrane, east central Alaska: Volcanic remnants of a Mid‐Cretaceous magmatic arc
Authors: Bacon Charles R. , Foster Helen L. , Smith James G.
Published: February 2008

Four large but poorly exposed rhyolitic calderas are present in the Yukon‐Tanana terrane (YTT) in east central Alaska. At least two are mid‐Cretaceous in age (∼93 Ma). Similar volcanic rocks, the South Fork Volcanics, occur northeast of the Tintina fault in Yukon Territory. Evidence for the calderas consists of thick deposits of de vitrified crystal‐ and lithic‐rich densely welded tuff, interpreted as caldera fill, associated with lava domes or shallow intrusive rocks. Coeval outflow sheets have been largely stripped by erosion. The calderas are preserved within a northeast trending depression extending across the axis of the elongate mid‐Cretaceous plutonic province. Trace element abundances in andesites and rhyolites associated with the caldera structures are similar to those of volcanic and plutonic rocks of subduction‐related magmatic arcs developed on continental crust and thus are suggestive of formation in such an environment. Late Cretaceous and early Tertiary igneous rocks in the YTT near the calderas are interpreted to have been emplaced in a more extensional setting when the subduction‐related magmatic front was farther ocean ward.
Laboratory study of fluid pressure diffusion in rock using acoustic emissions
Authors: Kranz R. L. , Satoh T. , Nishizawa O. , Kusunose K. , Takahashi M. , Masuda K. , Hirata A.
Published: February 2008

In many geological situations, there is a need to monitor fluid migration and associated phenomena. Acoustic emissions (AE) that result from fluid pressure diffusion from a borehole can potentially provide useful information about migration paths, local stress states and local strength distributions. However, AE may also result from regional differential stress as well as mechanical stress concentrations associated with the injection borehole. To attempt to differentiate between these possible AE generating situations, we performed several experiments in unjointed granite rock samples containing a borehole. Fluid diffusion into and out of the borehole was achieved, under hydrostatic to high deviatoric far‐field stress states, while concurrently recording digitized waveforms of AE at 20 transducer locations and measuring acoustic velocity changes. Over 2000 AE events were located with a precision better than 2 mm, and focal mechanisms were obtained for the majority of them. Most acoustic emission events could be modeled by a shear displacement source mechanism giving rise to a quadrature polarity pattern usually associated with a double couple. A significant fraction of the events, however, were not so simply modeled and may indicate a combination of extension and shear displacement at the source. Very few pure dilatational or compressional events occurred. Results indicate that diffusing fluids, at pressures less than the local tensile strength, produce useful accompanying AE only in the presence of a large differential stress state surrounding the borehole. It is difficult, however, to distinguish events produced solely as a result of fluid pressure diffusion from those resulting from regionally applied shear stresses. A high‐pressure fluid leak at a metal‐rock interface was easily detected using AE, but mapping of isobaric pore pressure fronts in the presence of background seismicity was less successful. A model is developed to explain the difficulty. The spatial patterns of the acoustic emission locations appeared to be fractal and might be used to infer information about the magnitude and spatial distributions of local strength in the rock.
Characterizing windows for shock wave radiation studies
Authors: McQueen R. G. , Isaak D. G.
Published: February 2008

Sapphire and lithium fluoride have been used as windows in shock wave experiments to transmit reflected light from lasers to perform velocity measurements and to measure thermal radiation for temperature measurements, while maintaining substantial pressure at the interface. In the latter case there is a fundamental problem in that the radiation coming through the window is not that of the shocked sample but is the radiation from the interface, which in the ideal case is at some temperature between the temperature of the sample and the relatively cold windows. The interface temperature can be used to calculate the temperature of the sample, but values for the thermal diffusivities of both the window and the sample must be used in the heat flow calculation. Experiments are described that may resolve that difficulty. Two other problems thought to exist in these measurements are due to interface imperfections, such as gaps and the transparency of the windows under shock loading. Preliminary experiments are reported, which showed that in the pressure regime studied, the sapphire remained transparent. These experiments also showed that imperfections at the interface are a major problem, but it is a recognizable one and can be minimized or eliminated so that meaningful temperature measurements can be made.
Quantitative analysis of long‐period events recorded during hydrofracture experiments at Fenton Hill, New Mexico
Authors: Ferrazzini Valerie , Chouet Bernard , Fehler Mike , Aki Keiiti
Published: February 2008

A three‐dimensional fluid‐filled crack model recently developed by Chouet is used to reproduce and explain the spectral characteristics of different classes of long‐period events recorded during a hydrofracture experiment conducted at Fenton Hill, New Mexico. We study the dependence on the model parameters of the far‐field P‐wave radiation due to the vibration of the fluid‐filled crack. Those parameters are given by the properties of the fluid and solid, the crack dimensions, the area and location of the crack surface over which the excess pressure is applied, the time history of this excess pressure, and the station location. In this model, the resonance of the crack is sustained by a very slow and dispersive wave called “crack wave”. The phase velocity of the crack wave depends critically on the impedance contrast between fluid and solid and on the crack dimensions. We are able to fit the dominant features of the Fenton Hill data in the time and frequency domains and draw inferences on the impedance contrast between the fluid and solid. The various classes of events observed can be modeled by a single crack over which the geometry of the applied excess pressure changes. The length, width, and thickness of the crack are estimated to be on the order of 3 m, 1 m, and 3 mm, respectively. The observed spectral roll‐off can be explained by a ramp function time dependence of the pressure transient. The rise time necessary to simulate the observed data varies between 2 and 4 ms, depending on the event considered. Assuming the source‐receiver distance of 700 m, the amplitude of displacement of the data agree with the model's prediction if the excess pressure applied on the crack is on the order of 20 bars.
Radon series disequilibrium in southern California coastal air
Authors: Shapiro Mark H. , Kosowski Rebecca , Jones Daniel A.
Published: February 2008

High‐resolution gamma ray spectroscopy has been used to obtain 214Bi/214Pb specific activity ratios from an extended series of high‐volume air samples taken at a height of 20 m. Afternoon samples exhibited 214Bi/214Pb activity ratios that ranged from 0 to 2.5 with a mean value of 0.84 ± 0.04. Morning samples exhibited ratios that ranged from 0 to 1.6 with a mean value of 0.87 ± 0.03. Afternoon samples with ratios greater than 1.0 were associated with southwest winds and 214Pb specific activities less than 130 pCi/m3. Morning samples with ratios greater than 1.0 were associated in most cases with southwest winds and with 214Pb specific activities less than 220 pCi/m3. Afternoon 214Pb specific activity values ranged from 5 to 388 pCi/m3 with a mean of 91.7 ± 7.1 pCi/m3. Morning 214Pb specific activities ranged from 23 to 664 pCi/m3 with a mean of 224.2 ± 20.3 pCi/m3. An improved sampling system used for these measurements reduced the likelihood that observed abnormal activity ratios were artifacts of the sampling procedure.
Modulation of coherent microwave backscatter by shoaling waves
Authors: Plant W. J. , Keller W. C. , Wright J. W.
Published: February 2008

The orbital speed of long shoaling waves and the modulation of centimetric wind‐generated waves by the shoaling waves were obtained from the frequency and amplitude modulations, respectively, of the backscattered signal of a coherent CW 9.375‐GHz radar operated from the end of a pier on the Outer Banks of North Carolina. Modulations anomalously large in comparison with those predicted by the relaxation time model (Keller and Wright, 1975) were observed in the range of wind speeds 4.5–6.5 m s−1, which was perhaps coincidentally near the shallow water wave speed. Outside this range of wind speeds the model qualitatively accounted for the magnitude and wind speed dependence of the measured modulations. The phase of the modulation leads to the inference that the maximum amplitude of the short waves occurs near, and generally leeward of, the crests of long waves for wind speeds of up to 8 m s−1, the highest wind speed encountered in the experiment.
On the existence of temperature waves at a wavy air‐water interface
Authors: Miller A. Woodruff , Street Robert L.
Published: February 2008

The amplitude and phase of temperature waves associated with plane progressive waves were determined in a laboratory experiment under a range of conditions, including wind speeds between 1 and 10 m/s and water‐air temperature differences between 2.5°C and 20°C. Temperature waves were found, and at low wind speeds the results are in rough agreement with Writing's (1972) theory. However, the character of the temperature waves is strongly dependent on wind speed. Hence the amplitude and phase of the temperature waves change as the energy transfer across the air‐water interface comes to be dominated by latent and sensible heat transfer at high wind speeds. Radiation is the major mechanism of heat transfer at low wind speeds.
Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation
Authors: Deardorff J. W.
Published: February 2008

An efficient time‐dependent equation for predicting ground surface temperature devised by Bhumralkar (1975) and Blackadar (1976) is tested against a 12‐layer soil model and compared with five other approximate methods in current use. It is found to be generally superior if diurnal forcing is present and very much superior to the use of the insulated surface assumption. An analogous method of predicting ground surface moisture content is presented which allows the surface to become moist quickly during rainfall or to become drier than the bulk soil while evaporation occurs. These improved methods are not of much relevance unless the main influences of a vegetation layer are included. An efficient one‐layer foliage parameterization is therefore developed that extends continuously from the case of no shielding of the ground by vegetation to complete shielding. It includes influences of both ground and foliage albedos and emissivities, net leaf area index, stomatal resistance, retained water on the foliage, and several other considerations. When it is tested against data for wheat measured by Penman and Long (1960), it appears quite adequate despite the many simplifying assumptions. The parameterization predicts that errors of up to a factor of 2 in evapotranspiration can be incurred by ignoring the presence of a vegetation layer.
On the statistical distribution of wave heights in a storm
Authors: Forristall G. Z.
Published: February 2008

There has been recent controversy over how well the Rayleigh distribution matches the observed distribution of wave heights. Most of this controversy stems from comparisons based on different definitions of the significant wave height. Once consistent definitions are used, all available data support the conclusion that the Rayleigh distribution overpredicts the heights of the higher waves in a record. Analysis of 116 hours of hurricane‐generated waves in the Gulf of Mexico permitted the empirical fitting of the data to a Weibull distribution. Statistics developed from the empirical distribution include the prediction that the highest wave in 1000 is only 0.907 times the height predicted by the Rayleigh distribution.
The dependence of bulk evaporation coefficients on air‐water interfacial conditions as determined by the isotopic method
Authors: Merlivat Liliane
Published: February 2008

The analysis of the natural distribution of deuterium and oxygen 18 in moisture inside the turbulent boundary layer developed above a water surface makes possible the investigation of the mechanism of evaporation. The distribution of isotopes in water vapor allows the calculation of the relative contributions of molecular and turbulent transfer to the total mass transport (Merlivat and Coantic, 1975). We have applied this method to assess the influence of the conditions at the liquid surface on the evaporation flux. In particular, the influence of wind waves and superposed artificially generated waves was studied. Experiments have been carried out at the Institut de Mécanique Statistique de la Turbulence air‐water tunnel, specially designed for the simulation of ocean‐atmosphere energy exchanges. Here the wind velocities could be varied from 0.7 to 6 m/s. Waves were generated by a motor‐driven paddle. The wave frequency was 1 Hz, and the maximum height 12 cm, while the surface roughness Reynolds numberResvaried from 0.02 to 2. Mean wind velocity profiles and isotopic profiles were measured. Experimental drag coefficients are described quite well by Charnock's relationZ0=u*2/ag., wherea= 81.1. No specific difference is observed when waves are artificially generated. Agreement between experiment and theoretical isotopic distribution is checked for six models proposed to describe evaporation processes. Quite good agreement is found with Brutsaert's model (Brutsaert, 1975a,b) for a smooth surface ifRes< 1 and for a rough surface ifRes> 1. Again, no specific effect due to the presence of artificially generated waves is observed. The above observations allow, then, the calculation of the drag and bulk evaporation coefficients,CDandCq, as a function of the surface roughness Reynolds number: these coefficients, as well as their ratio, when calculated for a height ofz= 10 m, vary from 0.99 × 10−3to 1.24 × 10−3and from 1.31 to 0.84, respectively, whenResincreases from 0.02 to 10, corresponding to a mean wind speed range extending from 2 to 13 m/s. These evaporation bulk transfer coefficients are given for near‐neutral stability conditions of the atmospheric layer. In the case of moderate instability, frequently observed over the sea, we would expect, and we observe, that the transfer coefficients are generally higher.
Directional VLF sferics from the Union City, Oklahoma, tornadic storm
Authors: Brown Rodger A. , Hughes Herbert G.
Published: February 2008

The Naval Electronics Laboratory Center (now Naval Ocean Systems Center) made directional measurements of very low frequency sferics in the Union City, Oklahoma, tornadic storm on May 24, 1973. These data verify the interesting findings of two earlier directional sferics studies. Whereas sferics activity in tornado‐producing storms is known to peak near tornado time, all these studies show conclusively that sferics do not originate in the tornado itself but rather throughout the parent thunderstorm.
Atmospheric 210Pb fluxes determined from soil profiles
Authors: Nozaki Y. , DeMaster D. J. , Lewis D. M. , Turekian K. K.
Published: February 2008

Estimates of the atmospheric 210Pb flux can be made from old (∼ 100 years) soil profiles because of the strong retention of this nuclide in the organic rich surface layers. Correction for radon loss can be made by modeling the observed 210Pb and 226Ra profiles. This is shown to have only a small effect for most of the soils studied. Comparison of soil‐based 210Pb fluxes with other estimates indicates good agreement except in the forested montane region of eastern New Mexico; the high 210Pb content of soils there remains to be explained.