bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal diffusivity of igneous rocks at elevated pressure and temperature
Authors: Durham W. B. , Mirkovich V. V. , Heard H. C.
Published: February 2008

Thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400°C and pressures of 200 MPa. The measuring method was based on the concept of cylindrical symmetry and periodic heat pulses. The seven rocks measured were Westerly (Rhode Island) granite, Climax Stock (Nevada) quartz monzonite, Pomona (Washington) basalt, Atikokan (Ontario, Canada) granite, Creighton (Ontario, Canada) gabbro, East Bull Lake (Ontario, Canada) gabbro, and Stripa (Sweden) granite. The diffusivity of all the rocks showed a positive linear dependence on inverse temperature and, excluding the East Bull Lake gabbro, showed a linear dependence on quartz content. (Quartz content varied from 0 to 31% by volume.) Diffusivity in all cases rose or remained steady with increasing confining pressure. The pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 MPa, depending on rock type. The pressure effect (measured as a percentage change in diffusivity) is stronger in the four rocks of granitic composition than in the three of basaltic composition. Our results agree well with existing thermal diffusivity measurements at atmospheric pressure.
Earth anisotropic reflection and the orbit of LAGEOS
Authors: Rubincam David Parry , Knocke Philip , Taylor V. Ray , Blackwell Sue
Published: February 2008

Radiation pressure due to sunlight anisotropically reflected from the oceans apparently cannot explain the fluctuations in the anomalous along‐track deceleration of the LAGEOS satellite. It fails by about a factor of 2 to account for the major peaks in the acceleration. This result is based on an extreme model: a cloudless earth whose northern hemisphere consists of continent and whose southern hemisphere consists of ocean. The continent is assumed to reflect sunlight according to Lambert's law, while the ocean reflects anisotropically according to a simple analytical law which mimics Nimbus 7 observations. The inclusion of clouds into the model would reduce the acceleration to perhaps an order of magnitude below those observed. Some other explanation for the fluctuations, which have magnitude ∼2 × 10−12 m s−2, must be sought.
A monochromatic earthquake suggesting deep‐seated magmatic activity beneath the Izu‐Ooshima Volcano, Japan
Authors: Ukawa Motoo , Ohtake Masakazu
Published: February 2008

After a quiescence of 12 years, the Izu‐Ooshima volcano, located about 30 km east of the Izu‐Peninsula in central Japan, erupted on November 15, 1986. On August 27, 1985, about 1 year before the eruption, a peculiar earthquake of magnitude 2.7 took place near the Moho boundary just beneath the Izu‐Ooshima volcano, where seismic activity is otherwise absent. Analysis of digital data obtained by the telemetered seismic network of the National Research Center for Disaster Prevention reveals the following distinctive features of the seismic waves: (1) The main part of the wave train is composed of S waves, and a small P phase precedes it. (2) The spectral density has a sharp peak at 1.0 Hz, which is independent of the epicentral distance, azimuth, and time, indicating a monochromatic spectrum at the source. (3) Ground motion for the initial part of the S wave is polarized in the N‐S direction at almost all the stations. The amplitude ratios of S to P waves and the polarization pattern of S waves suggest that a traction force caused by magma flow is a more probable source than a double‐couple mechanism, free oscillation of the magma reservoir or opening of a tensile crack. We propose a traction‐force model for this earthquake, in which magma flow through a conduit produces unidirectional viscous force on the conduit walls. The observations imply a N‐S force direction and a maximum force of about 2.8 × 1010 N. The pressure difference between the ends of the conduit is estimated as 6.1 × 107 Pa (610 bar), under the assumption of conduit dimensions of 2 km × 1 km. The monochromatic nature of the source spectrum is difficult to account for by the free oscillation of a magma reservoir. We tentatively suggest the intermittent opening of a barrier in the conduit as a plausible mechanism of the periodic oscillation.
A new reduction algorithm for marine heat flow measurements
Authors: Villinger Heiner , Davis Earl E.
Published: February 2008

Multiple penetration heat flow surveys that employ “violin bow” multithermistor array instruments generate large quantities of high‐quality data. To cope with the problems associated with the reduction of these data, an algorithm has been developed which can be used to reduce the data automatically on a microcomputer. The algorithm is designed for use with a pulsed line source method of conductivity measurement, although it could be modified easily for use with the continuous line source method. The multiply iterative algorithm deals, on a sensor by sensor basis, with errors associated with nonideal probe construction and sediment entry, factors that affect the determination of both in situ temperatures and thermal conductivities. Numerical tests of the algorithm show that it is accurate and stable and fast enough for post‐real‐time reduction of data at sea. Practical tests on high‐quality data from the western Pacific show that accuracies are limited by the instrumental resolution; in these cases, the reduction algorithm provided determinations of undisturbed in situ temperatures to better than 1 mK and of in situ conductivities to within 1%.
Tiltmeter measurements in Long Valley Caldera, California
Authors: Mortensen Carl E. , Hopkins Douglas G.
Published: February 2008

In 1983, the U.S. Geological Survey instrumented Long Valley caldera, near Mammoth Lakes, California, with an array of borehole tiltmeters to detect the rapid deformation that might accompany incipient volcanic activity, such as the intrusion of a dike at shallow depth. The array consists of seven sites, within and surrounding the zone of the 1983 earthquake swarm. Five of these sites have instruments installed at a depth of 6 m and consist of two closely spaced installations, one containing a Rockwell/Kinemetrics instrument and the other an instrument designed by J. Westphal of the California Institute of Technology. The sixth site consists of two of Westphal's instruments and one Rockwell/Kinemetrics instrument, and the seventh site has two of the Rockwell/Kinemetrics instruments only, installed at a depth of 2 m. Since July 1985, all but one site have been equipped with a telemetry system that uses the Geostationary Operational Environmental Satellite (GOES). Most sites are also equipped with a telemetry system based on telephone lines and radio links. This tiltmeter array affords a unique opportunity to compare the relative merits of different instrument types and installation techniques under similar settings. In general, the results are similar for the two different instruments and installations, but local geologic conditions substantially affect the performance at each site. The records from the 6‐m‐deep boreholes have better stability and lower noise than those from the 2‐m‐deep boreholes. At one site, the tilt record appears to be dominated by perturbations related to local geothermal production. Performance at most other sites indicates that sudden deformation, with time spans shorter than about 1 week and amplitudes of about 5–10 μrad or greater, should easily be detected by the instruments within the array.
Microseismicity and focal mechanisms of the intermediate‐depth Bucaramanga Nest, Colombia
Authors: Schneider John F. , Pennington Wayne D. , Meyer Robert P.
Published: February 2008

The Bucaramanga nest of intermediate‐depth seismicity in northern Colombia was investigated from microearthquakes (mb ≤ 4.3) recorded from June 20 to July 6, 1979, by a local array of 12 digital three‐component seismographs. Of 161 events studied, 142 are from the nest and 19 define a thin Wadati‐Benioff (W‐B) zone, mainly from 110 to 190 km depth. The W‐B zone, part of the Bucaramanga segment, strikes N10°E with a dip increasing with depth from 30°E to near‐vertical beneath the nest. The 89 best located events from within the nest define a volume of about 8 km (NW) by 4 km (NE) by 4 km (depth) centered at 161 km depth, with relative location accuracies of ±0.5 to ±1.0 km. Double‐couple fault plane solutions of 59 events were determined from an objective procedure using P wave polarities and S/P amplitude ratios. There is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data. The subduction and assimilation of a linear feature (such as an island arc undergoing a phase transformation) may explain the existence of the Bucaramanga nest. However, the extraordinarily high rate of seismicity suggests that the nest may be moving at a rate several orders of magnitude faster than the rate of relative plate motion alone. The intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids, perhaps brine or magma, that will ultimately result in the initiation of volcanic activity over this recently emplaced W‐B zone.
Mantle rheology and satellite signatures from present‐day glacial forcings
Authors: Sabadini Roberto , Yuen David A. , Gasperini Paolo
Published: February 2008

We study the changes of the long‐wavelength portion of the Earth's gravity field as a result of present‐day glacial discharges and the possible growth of the Antarctic ice sheet. We employ both Maxwell and Burgers' body rheologies and find that there are significant differences in the responses between the two rheologies for time spans less than a century. Both present‐day glacial forcing and the ice buildup on Antarctica can cause non‐negligible perturbations in j2 of about one‐third the observed amount. The contributions from the two mechanisms have opposite signs in j2. Only for l = 3 and 6 do these cryospheric contributors act in concert with one another in disturbing the gravity field. All other degree harmonics, up to l = 9, have opposite signs from these two excitations. For a steady‐state lower‐mantle viscosity of 1022 Pa s, j3 can attain a maximum value of around 1.5×10−11 yr−1 and j6 a value of −1.3×10−11 yr−1 for current forcings. These have signs opposite to those predicted for Pleistocene deglaciation. All harmonics, up to l = 9, are sensitive to variations of the steady‐state viscosity in the lower mantle, even for time scales shorter than 100 years. It is proposed that detailed satellite monitoring of present‐day ice movements in conjunction with geodetic satellite missions will eventually provide a viable alternative for estimating deep mantle viscosity, useful in mantle convection models.
Origin of reflections from the Brevard Fault Zone
Authors: Christensen Nikolas I. , Szymanski Daniel L.
Published: February 2008

The Brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from Alabama into Virginia. Seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth. The origin of these reflections has been investigated by measuring compressional wave velocities to 800 MPa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m. Densities are reported as well as chemical analyses at selected depths. Anisotropy at elevated pressures ranges within the fault zone from less than 5% to over 26%. Similarly, the fault zone rocks show significant chemical variability, with SiO2 ranging from 58.0 to 70.4% by weight. Synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the Brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain. Essential to this study has been the continuous coring of a major section of the Brevard fault zone, which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies.
Body wave attenuation in the Tonga Subduction Zone
Authors: Bowman J. Roger
Published: February 2008

Attenuation P and S waves of in the Tonga subduction zone is estimated from locally recorded data using a spectral ratio method. The data consist of three‐component, digital seismograms of 77 earthquakes recorded at two stations in Fiji and one in Tonga. The stations in the Fiji Islands lie above the deep earthquakes in the subduction zone while the Tonga station is close to the top of the subducted slab. Variations in the high‐frequency spectral decay rate define four attenuation provinces. Paths from earthquakes deeper than 350 km to stations on Viti Levu (Fiji) and the Lau ridge traverse mantle with normal attenuation (Qp=570, Qs=200–400). The subducting Tonga slab has very low attenuation (Qp>2000, Qs≥1800) relative to the surrounding mantle. The wedge of mantle above the slab at depths shallower than 350 km and east of the Lau ridge is extremely attenuating for P waves (Qp=240). Several paths from main thrust zone events to the station in Tonga show intermediate attenuation (Qp=300–500) in the forearc region. The observed spectra do not require Q to be frequency dependent in the 1.0 Hz to 7.6 Hz band in this region.
Compensation of swells and plateaus in the north Pacific: No direct evidence for mantle convection
Authors: Sandwell David T. , Renkin Miriam L.
Published: February 2008

At intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation. A low geoid/topography ratio (<2 m/km) signifies shallow Airy compensation. A higher ratio (∼6 m/km) signifies thermal isostasy and/or dynamic uplift from a mantle plume. A very high geoid/topography ratio (>8 m/km) in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection. After subtracting a reference geoid from the observed geoid, previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000–4000 km. Since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid/topography ratios (10–20 m/km), they were believed to reflect the planform of mantle convection. We show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10. Since the geoid spectrum is “red,” the residual geoid is dominated by degree 11. When the harmonics of the reference geoid are rolled off gradually, the regular pattern of geoid highs and lows disappears. In the Northeast Pacific, the new residual geoid reflects the lithosphere age offsets across the major fracture zones. In the Northwest Pacific, the residual geoid corresponds to isostatically compensated swells and plateaus. We have calculated the geoid/topography ratio for 10 swells and plateaus and have found a range of compensation depths. The highest geoidAopography ratio of 5.5 m/km occurs on the flanks of the Hawaiian Swell. Intermediate ratios occur in four areas, including the Midway Swell. These intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of Airy‐compensated seamounts. Low geoid/topography ratios occur over the remaining five areas (e.g., Emperor Seamounts), reflecting the absence of a thermal swell. Our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid. We see only indirect evidence of thermal plumes reheating the lower lithosphere.
Eruption styles of basalt in oceanic spreading ridges and seamounts: Effect of magma temperature and viscosity
Authors: Bonatti Enrico , Harrison C. G. A.
Published: February 2008

Basalts erupted along oceanic zones of spreading and on intraplate seamounts, at depths where hydroexplosions are not likely, follow various styles of eruption. “Quiet” eruptions produce a variety of forms, including lava lakes and sheet, pillow, and pavingstone lavas. “Reactive” eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites. The stresses induced in a rapidly cooling lava flow are important in determining the different lava forms, with lava lakes and sheet flows resulting from low stresses, pillows from higher stresses, and hyaloclastites from even higher stresses which cause rupture. Calculations show that if we consider basaltic lavas of the same composition, the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption. The viscosity is strongly dependent on the crystal/liquid ratio of the magma at eruption. The temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below; viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts. Within a given ridge segment, temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary. Thus the model predicts that going from fast to slow spreading ridges (i.e., from East Pacific Rise to Mid‐Atlantic Ridge), the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases. Moreover, within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached. Sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts, pillows and hyaloclastites in later stages. The probability of forming hyaloclastites should be low in spreading ridges, higher in intraplate seamounts. Recent high‐resolution observations on the East Pacific Rise, the Galapagos Rift, the Mid‐Atlantic Ridge, and the Red Sea are consistent with the model, because they show that (1) the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges, (2) within a given ridge segment it decreases approaching a transform, and (3) hyaloclastites are important on seamounts only.
Shear wave splitting in the Anza Seismic Gap, southern california: Temporal Variations as possible precursors
Authors: Peacock Sheila , Crampin Stuart , Booth David C. , Fletcher Jon B.
Published: February 2008

Shear wave splitting above small earthquakes in the Anza seismic gap on the San Jacinto fault, southern California, displays the effective anisotropy of vertical fluid‐filled microcracks. The delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence. These changes can be modeled by increasing the aspect ratio (“bowing”) of stressoriented microcracks, which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid‐filled microcracks. This is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake.
Differential rotation of the eastern part of southwest Japan inferred from paleomagnetism of Cretaceous and Neogene rocks
Authors: Itoh Yasuto
Published: February 2008

Paleomagnetic directions have been determined on Late Cretaceous and Miocene rocks in the Nohi area, in the eastern part of southwest Japan, in an effort to clarify within‐block deformation of the rotated part of southwest Japan. Twenty‐two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods. Tiltcorrected paleomagnetic data in the Nohi area are divided into three stages based on geologic age: Late Cretaceous, early Miocene (23–15 Ma), and middle Miocene (12–10 Ma). The formation‐mean direction of 10 sites obtained from Late Cretaceous Nohi rhyolite is D=17.0°, I=49.8°, and α95=6.1°. Neogene rock units underlain by Nohi rhyolite show two distinct formation‐mean directions: D=13.0°, I=52.7°, and α95=7.2° for eight sites of early Miocene, and D=−2.5°, I=55.6°, and α95=6.6° for four sites of middle Miocene. Differential rotation is proposed between the eastern part of southwest Japan including the Nohi area and the central part of southwest Japan, which is known to have rotated clockwise through about 50° around 15 Ma. The spatial distribution of contemporaneous paleomagnetic data in southwest Japan suggests that two differentially rotated parts are connected in the western neighborhood of the Nohi area. It is concluded that the Nohi area had rapidly rotated counterclockwise through 48° relative to the central part of southwest Japan between 15 and 12 Ma. Together with previous paleomagnetic data from early Miocene rocks in the Kanto area, which is situated to the east of the Nohi area, the present results suggest that differential rotation of the eastern part of southwest Japan is attributed to the formation of northward cuspate shapes of pre‐Neogene terranes which constitute the framework of the Japan arc. One probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the Philippine Sea plate against the Japan arc in the middle Miocene.
Effect of a shallow low‐viscosity zone on small‐scale instabilities under the cooling oceanic plates
Authors: Robinson Elizabeth M. , Parsons Barry
Published: February 2008

We have studied the role of a low‐viscosity zone in the uppermost mantle on convective instabilities in the cooling oceanic plates. The upper mantle is represented by three layers in a finite element model: a conducting lid overlies a low‐viscosity zone, which in turn overlies a layer of constant viscosity extending to the base of the upper mantle (at 675 km in depth). The viscosity contrast, the Rayleigh number, the relative fluid layer thicknesses, and the conducting lid thickness are varied. If the Rayleigh number of the low‐viscosity zone exceeds a critical value, then convection will be confined to the low‐viscosity zone for a period which depends on the viscosity contrast and the Rayleigh number. However, the scale of the convection will eventually grow and extend throughout the upper mantle. We compare our model to the small‐scale geoid and topography anomalies observed in the central Pacific, which are 50–80 cm in geoid and ∼ 250 m in topography 10–40 m.y. after plate formation and whose wavelength (150–500 km) increases with age. The magnitude, early onset time, and lifetime of the anomalies suggest a viscosity contrast of greater than 2 orders of magnitude. The increase in wavelength with time also suggests a high Rayleigh number (≥ 106), and the initial 150–250 km wavelength indicates a low‐viscosity zone thickness of 75–125 km. Small‐scale convection does not alter the slope of the depth‐age curve but does elevate the depth‐age curve by ≥ 250 m. It is not until the onset of long‐wavelength convection that the depth‐age curve departs from (age)½ predicted by a cooling half‐space. These results agree with the depth‐age behavior observed in the Pacific, where the depth‐age curve is elevated in the region of small‐scale anomalies and departs from (age)½ at 70 m.y. The models also predict heat flow values on old ocean floor which are higher than the plate model (50–55 mW/m2) but which are in agreement with heat flow measurements in the Atlantic and Pacific.
Basement heating by a cooling lava: Paleomagiietic constraints
Authors: Audunsson Haraldur , Levi Shaul
Published: February 2008

Paleomagnetism can provide quantitative, empirical data on basement heating by cooling lava flows, thereby constraining theoretical models of heat transport processes. We used paleomagnetism to study heating in a Frenchman Springs (FS) lava by the overlying cooling Roza flow. The flows belong to the Miocene Columbia River Basalt (CRB) group, and they recorded very different primary paleomagnetic directions: FS has normal polarity, and Roza is transitional. The solid base of Roza typically rests on a few meters of the brecciated and vesicular top of FS, which is otherwise solid. Thermal demagnetization of FS specimens was usually successful at separating the overprinting due to Roza from the primary FS direction and for establishing limits on the unblocking temperatures of the overprinting. The unblocking temperatures agree well between sites separated by up to 70 km, irrespective of Roza thickness at the sites, which vary from 35–62 m. In specimens from 0.3 to l m below the contact, the overprinting was unblocked in the laboratory between 600 and 300°C and close to 200°C at 4‐m depth. No overprinting was apparent below 6 m. The influence of longer heating times in the field on the unblocking temperatures was estimated by viscous remanence acquisition at elevated temperatures. The results suggest typical reductions of the unblocking temperatures by a few tens of degrees. Our observations imply much less heating of the basement than predicted by simple conductive thermal models. Accounting for the low conductivity breccia in the contact zone results in a better agreement with the experimental temperature profile, but unrealistically low conductivities are needed to sufficiently reduce the absolute temperature. The observed heating is effectively explained by postulating a wet basement at the time of Roza extrusion, as well as groundwater, to maintain a low‐temperature isotherm (∼100°C) a few meters below the contact. The presence of water during the time of Roza extrusion and some other CRB flows has been suggested by field observations.
The Giant Submarine Alika Debris Slide, Mauna Loa, Hawaii
Authors: Lipman Peter W. , Normark William R. , Moore James G. , Wilson John B. , Gutmacher Christina E.
Published: February 2008

A 4000‐km2 area of submarine slump and slide deposits along the west flank of Mauna Loa volcano has been mapped with GLORIA side‐scan sonar images, seismic reflection profiles, and new bathymetry. The youngest deposits are two debris avalanche lobes that travelled from their breakaway area near the present shoreline as much as 100 km into the Hawaiian Deep at water depths of 4800 m. The two lobes partly overlap and together are designated the Alika slide. They were derived from the same source area and probably formed in rapid succession. Distinction hummocky topography, marginal levees, and other features on lower slopes (0.3°–0.6°) of these deposits resemble subaerial volcanic debris avalanche deposits such as 1980 Mount St. Helens and suggest high emplacement velocities. The breakaway area for the Alika slide (10°–15° slopes) is characterized by large block slumps, bounded by normal faults, that probably represent multiple subsidence events before, during, and after the debris avalanches. Lower slopes of the slide contain distinctive lobate‐terraced deposits that are interpreted as having been emplaced more slowly, prior to the debris avalanches. Estimated thicknesses of 50–200 m suggest volumes of 200–600 km3 for the two lobes. The combined volume of the entire slide and slump terrane is probably 1500–2000 km3. The slide deposits predate a 13‐ka coral reef and probably postdate the block‐faulted Ninole Basalt, roughly dated as a few hundred thousand years old. The Alika slide, or a similar deposit recognized on GLORIA images further north along the Hawaiian Ridge, probably triggered a giant wave that washed 325 m high on Lanai at about 100 ka. Slumping on Mauna Loa has been most intense adjacent to the large arcuate bend in its southwest rift zone, as the rift zone migrated westward away from the growing Kilauea volcano. Slumping events were probably triggered by seismic activity accompanying dike injection along the rift zone. Such massive slumps, landslides, and distal submarine turbidity flows appear to be widespread on the flanks of Hawaiian volcanoes.
Chemical controls on the composition of vent fluids at 13°–11°N and 21°N, East Pacific Rise
Authors: Bowers Teresa Suter , Campbell Andrew C. , Measures Chris I. , Spivack Arthur J. , Khadem Mitra , Edmond John M.
Published: February 2008

Six vent fields sampled at 13°–11°N, East Pacific Rise (EPR) in May 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21°N. Measured pH at 25°C are acidic, ranging from 3.1 to 3.7. Sodium and chloride vary from 40% lower to 30% higher than seawater. Iron concentrations range from 2 to 10 mmol/kg, compared with 0.7–2.5 mmol/kg at 21°N. Other sulfide‐forming metals (Cu, Zn, Cd, and Pb) are generally lower at 11°–13°N than at 21°N. Reliable temperature measurements were obtained at only two of the six vents and both were 350° ± 5°C. The vent fields at 21°N, EPR were resampled in August 1985, thus extending to almost 6 years the time period over which they have been monitored (previous expeditions were made in November 1979 and November 1981). Campbell et al. (this issue) have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation. Equilibrium calculations for the fluids from the fields at 13°N and 21°N, using a greatly improved thermodynamic data base, are described in this report. They indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist‐type mineral assemblage at depth. Calculated high‐temperature pH of the fluids range from 4.1 to 4.7 with those from 13°–11°N at the more acidic end of the range. Calculated affinities show that the fluids are close to, or at, saturation with respect to quartz, albite, muscovite, smectite/chlorite, epidote, and pyrrhotite. The computations imply that lower‐temperature vents such as National Geographic Smoker (NGS) (273°C) may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz. A comparison of fluid chemistry between NGS and vent 5 at 11°N suggests that the latter vent may also have a temperature < 300°C. While the fluid chemistry of a given field is compositionally stable, the compositions themselves are unique for a particular field and cover a wide range between fields. The processes controlling this variability probably include the depth of reaction, i.e., pressure, which is difficult to treat thermodynamically at present. In addition, increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water‐rock ratio. It may be difficult to separate these two effects. If the age differences between the fields (at present unknown) are of the order of decades, then their temporal histories may have an important influence on their particular chemistries.
A seismic refraction study of the crustal structure in the active seismic zone east of Taiwan
Authors: Hagen Rick A. , Duennebier Frederick K. , Hsu Vindell
Published: February 2008

An array of 10 Hawaii Institute of Geophysics (HIG) ocean bottom seismometers (OBSs) was emplaced off the east coast of Taiwan for a cooperative study conducted by scientists from HIG and institutions in Taiwan. As part of this experiment, three overlapping seismic refraction profiles were shot across the array parallel to the coast of Taiwan. The results of the ray trace modeling of these data indicate that the crust off southern and central Taiwan is about 8 km thick in this area and can be modeled by several layers with velocities and thicknesses that lie within the range associated with “normal” oceanic crust. Near 23.5°N a downwarping and thickening of the crustal layers occurs in the model. This downwarped trough of low‐velocity materials may represent the sediment‐filled axis of the Ryukyu Trench. If this is true, it indicates a more southerly trench position in this area than that previously published by other investigators. To the north of this downwarping, the bottom shoals rapidly, and the velocity structure undergoes a transition that may indicate a change to arc or continental type crust.
Geochemical and experimental study of the genesis of magmas of Reunion Island, Indian Ocean
Authors: Fisk M. R. , Upton B. G. J. , Ford C. E. , White W. M.
Published: February 2008

Major element, trace element, and Sr and Nd isotopic compositions were determined on a suite of lavas and intrusives from Piton des Neiges and Piton de la Fournaise volcanoes of Reunion Island. The temperatures of appearance of mineral phases in six basalt samples were determined by melting and recrystallization experiments at pressures from 1‐atm to 2000 MPa. The compositions of minerals, both in the starting basalts and in the experimental run products, were analyzed by electron microprobe and compared to determine the temperature and depth of their formation in the basalts. The Sr and Nd isotopes and ratios of highly incompatible trace elements of basalt samples from both volcanoes are virtually identical, indicating that the mantle source for the magmas is chemically homogeneous, both in time and space. The lavas of the active volcano (Piton de la Fournaise) and the late stage Differentiated Series of Piton des Neiges are slightly more alkali‐rich than the primitive shield lavas of Piton des Neiges. The alkali enrichment could be the result of lower degrees of partial melting of the mantle for the young eruptions, but Zr and the rare earths do not fit this model. An alternative explanation is that the alkali enrichment resulted from contamination of a Piton des Neiges‐type magma with the low‐melting component of the lava pile. The melting experiments and least squares modeling indicate that augite was an important fractionating phase and that the fractionation of olivine plus augite occurred at depths of at least 5 km. The temperature of the magma chamber at this depth was between 1200° and 1300°C. The parent magma composition for Reunion had between 15 and 17% MgO, and all Piton des Neiges magmas can be derived from it by fractional crystallization at 5 to 15 km depth. Piton de la Fournaise magmas can be derived from this parent magma if they are also enriched with alkali elements by an independent process.
Conductive heat flux in VC‐1 and the thermal regime of Valles Caldera, Jemez Mountains, New Mexico
Authors: Sass J. H. , Morgan Paul
Published: February 2008

Over 5% of heat in the western United States is lost through Quaternary silicic volcanic centers, including the Valles caldera in north central New Mexico. These centers are the sites of major hydrothermal activity and upper crustal metamorphism, metasomatism, and mineralization, producing associated geothermal resources. We present new heat flow data from Valles caldera core hole 1 (VC‐1), drilled in the southwestern margin of the Valles caldera. Thermal conductivities were measured on 55 segments of core from VC‐1, waxed and wrapped to preserve fluids. These values were combined with temperature gradient data to calculate heat flow. Above 335 m, which is probably unsaturated, heat flow is 247±16 mW m−2. The only deep temperature information available is from an uncalibrated commercial log made 19 months after drilling. Gradients, derived from uncalibrated temperature logs, and conductivities are inversely correlated between 335 and 737 m, indicating a conductive thermal regime, and component heat fluxes over three depth intervals (335–539 m, 549–628 m, and 628–737 m) are in excellent agreement with each other with an average of 504±15 mW m−2. Temperature logs to 518 m depth with well‐calibrated temperature sensors result in a revised heat flow of 463±15 mW m. We use shallow thermal gradient data from 75 other sites in and around the caldera to interpret the thermal regime at the VC‐1 site. A critical review of published thermal conductivity data from the Valles caldera yields an average thermal conductivity of ≥1 W m−1 K−1 for the near‐surface tuffaceous material, and we assume that shallow gradient values (°C km−1) are approximately numerically equal to heat flow (mW m−2). Heat loss from the caldera is asymmetrically distributed, with higher values (400 mW m−2 or higher) concentrated in the west‐southwestern quadrant of the caldera. This quadrant also contains the main drainage from the caldera and the youngest volcanism associated with the caldera. We interpret the shallow thermal gradient data and the thermal regime at VC‐1 to indicate a long‐lived hydrothermal (and magmatic) system in the southwestern Valles caldera that has been maintained through the generation of shallow magma bodies during the long postcollapse history of the caldera. High heat flow at the VC‐1 site is interpreted to result from hot water circulating below the base of the core hole, and we attribute the lower heat flow in the unsaturated zone to hydrologic recharge.
Shallow seismic study of a fault scarp near Borah Peak, Idaho
Authors: Treadway Jeffrey A. , Steeples Don W. , Miller Richard D.
Published: February 2008

Shallow seismic‐reflection surveys conducted near Borah Peak, Idaho, define subsurface structure across a segment of the Lost River fault that was reactivated in 1983. Four different seismic lines using different sets of recording parameters produced reflections from different depth ranges and at different resolutions. The combined interpretation shows the Lost River fault zone in the upper 50 m to be 180 m wide and to be composed of high‐angle faults of various ages. The northeast edge of the Lost River fault is clearly defined at a depth of approximately 43±2 m. The dip of the fault plane in the upper 43±2 m is approximately 55° southwest with an uncertainty of 5°. The interpretation presented correlates with surface structure as well as with deeper geophysical data.
Along‐axis variations within the plate boundary zone of the southern segment of the Endeavour Ridge
Authors: Barone Angela M. , Ryan William B. F.
Published: February 2008

The width of and along‐axis variations within the plate boundary zone of the southern segment of the Endeavour Ridge of the Juan de Fuca Ridge System have been quantified using Sea MARC I side‐looking sonar imagery. The sonar imagery was calibrated with video and photographic surveying to provide visual corroboration of the activity, spacing, and offset of faults and fissures. This ridge segment contains an axial high bisected along its length by a summit depression whose width, relief of its rims, depth of its floor, and spacing of faults and fissures vary systematically away from the mid‐point of the ridge segment toward its tips. The axial high and its summit depression are likened to an elongated shield volcano that is being disrupted by collapse of its crest into a widening linear caldera. The observed distribution of faults and fissures could be produced by mechanisms which stretch young ocean crust which is characterized by rheological properties which vary with distance from the center of the elongated volcano.
Magnetotellurics in southwestern Sweden: Evidence for electrical anisotropy in the lower crust?
Authors: Rasmussen T. M.
Published: February 2008

Magnetotelluric data from measurements conducted on the southwestern part of the Baltic Shield are analyzed with respect to crustal and upper mantle structures. The interpretations have been done using ordinary two‐dimensional and transverse anisotropic layered models in which distortions of the impedance tensors due to near‐surface electrical structures are included in the interpretation procedures. The anisotropic model is able to explain the gross features of the data, whereas the two‐dimensional model only displays features observed at long periods. The two main results from the study with anisotropic models are the interpretation of a possible transverse anisotropic lower crust and a depth to the uppermost conductive layer within the mantle of more than 200 km. The minimum depth to the conductive mantle layer is found from a most squares analysis.
Horizontal extent of anomalously thin crust near the Vema Fracture Zone from the three‐dimensional analysis of gravity anomalies
Authors: Prince Roger A. , Forsyth Donald W.
Published: February 2008

Recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust, with total crustal thickness less than half that of normal oceanic crust. The horizontal extent of this anomalous crust is not well known. To map the lateral distribution of thin crust in a major fracture zone, we have constructed free‐air anomaly, bathymetry, and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the Vema fracture zone with the Mid‐Allantic Ridge. We isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water‐crust interface, the sediment layer, and the crust‐mantle interface, assuming constant crustal thickness and density. These calculations are performed using a fully three‐dimensional model. The remaining mantle Bouguer anomaly is dominated by the effects of the cooling lithosphere. Variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three‐dimensional conduction of heat. The attraction of this three‐dimensional density structure is subtracted from the mantle Bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density. The residual gravity anomaly map indicates that there is thin crust along much of the length of the Vema fracture zone. Thin crust is concentrated primarily beneath the walls of the fracture zone, rather than beneath the center of the fracture zone valley, an interpretation confirmed by seismic refraction data. The degree of thinning along the length of the fracture zone is variable, ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km. The excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust. Other major topographic features, such as the median valley and the elevated transverse ridge along the south wall of the fracture zone, are also not locally compensated by changes in crustal thickness.
Accretional heating of the satellites of Saturn and Uranus
Authors: Squyres Steven W. , Reynolds Ray T. , Summers Audrey L. , Shung Felix
Published: February 2008

We consider the heating of the larger satellites of Saturn and Uranus (with the exception of Titan) by deposition of impactor kinetic energy during satellite accretion. Satellite growth takes place from cisplanetary and transplanetary planetesimal swarms, proceeding until the planetesimal reservoirs are exhausted. Impact velocities are controlled by the initial orbital eccentricities of the planetesimals, and by gravitational focusing by the planet and the growing satellite. Heat is deposited below the satellite's surface by dissipation of impact‐generated shocks. An effective thermal diffusivity produced by impact mixing provides most of the energy transport within the satellite during accretion. A number of parameters that go into the calculation are poorly constrained. The ones that can have an important influence on the results are the Safronov parameter describing the mean orbital eccentricity of bodies in the cisplanetary impactor swarm, the exponent in the power law size distribution of impacting bodies, and the fraction of the impactor kinetic energy partitioned into subsurface heat. Typical post accretion thermal profiles show temperature maxima several tens of kilometers beneath the surface. The maximum temperature reached is in all cases insufficient to melt H2O ice. However, if NH3 or CH4 is present within the satellites, accretional heating can produce a zone of relatively warm, mobile, and buoyant material tens of kilometers below the surface. Cooling of the outer regions of the satellite immediately following accretion will produce thermal stresses sufficient to fracture surface materials, perhaps producing conduits that will allow the mobilized material to ascend to the surface. Accretional heating therefore may have been responsible for some of the resurfacing and tectonism that has taken place on many of the Saturnian and Uranian satellites.
Microearthquakes beneath Median Valley of Mid‐Atlantic Ridge near 23°N: Tomography and tectonics
Authors: Toomey Douglas R. , Solomon Sean C. , Purdy G. M.
Published: February 2008

Data from a microearthquake experiment in the median valley of the Mid‐Atlantic Ridge near 23°N in 1982 are used to measure earthquake source parameters, to determine the laterally heterogeneous seismic velocity structure across the inner floor, and to develop a kinematic tectonic model for this portion of the median valley. Fifty‐three microearthquakes occurred over a 10‐day period beneath the median valley inner floor and eastern rift mountains. Twenty of 23 well‐located inner floor epicenters define a line of activity, about 17 km long, having a strike of N25°E and located near an along‐axis depression some 300–400 m deeper than surrounding regions. Earthquakes with well‐resolved hypocenters generally have focal depths of 4–8 km beneath the seafloor of both the inner floor and the rift mountains; the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure. Composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45°. Normal faulting mechanisms, although poorly constrained, are also indicated for the rift mountain microearthquakes. Seismic moments, approximate fault dimensions, and average stress drops for the largest events recorded are 1019–1020 dyn cm, 200–400 m, and 1–70 bars, respectively. A twodimensional tomographic inversion of P wave travel time residuals from microearthquakes and local shots indicates a well‐resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor. P wave velocities at 1–5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions. The most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column. By this view, the seismic velocity structure at the ridge axis evolves, probably by the sealing of cracks and pores, within the first few hundred thousand years of crustal accretion. Consideration of the detailed Sea Beam bathymetry in this region of the inner floor, the characteristics of large earthquakes that the region has experienced during the past 25 years, and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years.