bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal field effect on bulk moduli of transition metal oxides
Authors: Ohnishi Shuhei , Mizutani Hitoshi
Published: February 2008

The effects of crystal field stabilization of Fe2+, Co2+, and Ni2+ on isothermal bulk moduli of divalent monoxides are investigated on a simple crystal field theory. The calculated bulk moduli of high‐spin FeO, CoO, and NiO agree well with experimental data. The molar volumes and bulk moduli of low‐spin MnO, FeO, and NiO are also calculated. The ionic radii of low‐spin Mn2+, Fe2+, and Ni2+, calculated from the molar volumes, agree with Shannon and Prewitt's ( 1969) prediction. The bulk modulus of low‐spin FeO is calculated to be 2.23–2.83 Mbar at zero pressure. The spin‐pairing transition gives a lower bulk sound velocity/density ratio than polymorphic transitions of oxides and silicates. This indicates that the spinpairing transition in the lower mantle may favor a mean atomic weight of the lower mantle that is smaller than previous estimates where the effect of spin‐pairing transitions were not taken into account.
Investigation of the Manicouagan Impact Crater, Quebec: An introduction
Authors: Phinney W. C. , Dence M. R. , Grieve R. A. F.
Published: February 2008

A heavily cratered surface is a feature common to the early evolution of all the terrestrial planetary bodies. However, because the multiple impact history and inaccessibility of other bodies provide such poor geologic control, it is considered that the integrated effects of these impacts are best detailed and constrained through the intensive study of terrestrial impact structures. A survey of terrestrial structures indicates that the 65‐km‐diameter Manicouagan ring structure in Quebec, Canada, has by virtue of its size, exposure, and relatively well known geologic and geophysical characteristics the potential to provide details of impact processes, in particular those concerned with impact melting. This paper serves as an introduction to several detailed companion papers that resulted from a recent multi‐disciplinary study of Manicouagan. The major conclusions are as follows: The structure is 214 m.y. old. The melt rocks are texturally inhomogeneous but chemically homogeneous, can be modeled as a mixture of target lithologies, and have a 87Sr/86Sr ratio compatible with the melting of crustal rocks. Grain size and inclusion content of melt rocks vary inversely, grain size increasing upward. The melt had a two‐stage cooling history with clast‐melt interactions indicative of mixing superheated silicate liquid with cold clasts. Extensive digestion and reaction of clasts in the melt during the first stage of cooling resulted in a more refractory clast population than would be provided directly from the target rocks. However, a stronger bias toward refractory clasts in the interior zones of the sheet suggests a higher equilibration temperature and a lower initial content of clasts in these zones. Thermal equilibration of clasts and melt during the first stage of cooling required tens of seconds to a few minutes, whereas completion of crystallization during the second stage required about 1600 more years. Geophysical data suggest the presence of uplifted mafic rocks in the center of the structure and are compatible with an initial transient cavity with a radius of 15 to 22 km and a modeled depth of 6 to 9 km.
Petrogenesis of melt rocks, Manicouagan Impact Structure, Quebec
Authors: Simonds Charles H. , Floran Robert J. , McGee Patricia E. , Phinney William C. , Warner Jeffrey L.
Published: February 2008

Abstract. The 65‐km‐diameter Manicouagan impact structure has an eroded 230‐m‐thick sheet of clast‐laden, impact melt rock with an estimated preerosional volume of >270 km3. All samples are characterized by mineral and lithic clasts or their incompletely digested remnants. Drawing upon previous theoretical studies of shock waves, we suggest that the Manicouagan melt formed in 1 or 2 s in a 5‐km‐radius hemisphere near the point of impact. The melt accelerated to a few kilometers per second, and the melt and the less shocked debris surrounding it flowed downward and outward for a few minutes until the melt formed a lining of a 5‐ to 8‐km‐deep, 15‐ to 22‐km‐radius cavity. Extremely turbulent flow thoroughly homogenized the melt and promoted the incorporation and progressive digestion of slower moving, less shocked, cooler debris surrounding the melt. This debris had been finely fragmented, but not melted, to grain sizes of less than 1 mm by the passage of the shock waves. Because of the fine grain size, the melt and fragmented debris equilibrated thermally in about 100 s. During thermal equilibration, virtually all clastic debris (i.e. alkali feldspar, biotite, hornblende, garnet, and scapolite), other than highly refractory quartz and plagioclase as well as many of the centimeter size lithic clasts other than anorthosite, were digested. The preservation of quartz and plagioclase mineral clasts implies that the clasts and melt equilibrated to temperatures near but not above the liquidus. Plagioclase nucleation was initiated by the drop in temperature and possibly by direct nucleation on undigested debris. The initiation of crystallization vastly increased the melt viscosity, preventing settling of 10‐mm clasts of basement. Flow of melt through basement fractures is evidence that readjustment of the crater floor took place during the period of clast‐melt thermal equilibration.
Rb‐Sr isochron age of the Manicouagan Melt Sheet, Quebec, Canada
Authors: Jahn Bor‐Ming , Floran R. J. , Simonds C. H.
Published: February 2008

The Rb‐Sr isotopic data for mineral separates from a rock of the Manicouagan impact melt sheet define an isochron of age T = 214 ± 5 (2σ) m.y. and an initial Sr87/sr86 ratio I = 0.70991 ± 0.00007 (2σ). This age is in excellent agreement with the K‐Ar age of 210 ± 8 (2σ) m.y. of Wolfe [1971] and represents the time of crystallization of the melt sheet. Four whole rock samples yield age‐corrected I values ranging from 0.70965 to 0.71006. These high I values are consistent with geological and geochemical arguments that the melt sheet was derived by total melting of Grenville basement rocks during the impact event 214 m.y. ago.
In situ stress determination by hydrofracturing: A fracture mechanics approach
Authors: Abou‐Sayed A. S. , Brechtel C. E. , Clifton R. J.
Published: February 2008

The in situ stress field was determined at a depth of 837 m (2745 feet) in Devonian Shale (‘gray’ shale) within the Rome Basin in West Virginia. Logging data and laboratory observations of core samples reveal vertical cracks oriented at N50°–60°E. Because of these cracks and their preferred orientation a new approach based on fracture mechanics concepts is used to evaluate the in situ stresses from the field and laboratory data. The resulting prediction of the maximum horizontal stress (σHmax) is compared to the maximum horizontal stress predicted by Haimson and Fairhurst's (1967) method; the latter method appears to overestimate the value of this stress component because the effect of loading the faces of any preexisting crack is neglected.
A case study comparison of microwave radiometer measurements over bare and vegetated surfaces
Authors: Barton I. J.
Published: February 2008

Airborne microwave measurements with a nadir‐viewing X band radiometer operating at a wavelength of 2.65 cm are described. The measurements over adjacent bare and vegetated surfaces are compared with ground truth samples of soil moisture content (SMC). For the bare surface the emissivity is highly correlated (r = −0.97) with the SMC of the top 0.5 cm, with an antenna temperature dependence of −2.1°K/% SMC. In contrast, the correlation over vegetated surfaces is very poor (−0.25). Thus it cannot be expected that the technique at this wavelength would be generally useful as a measure of SMC, and this is borne out by comparison with the Nimbus 5 electrically scanned microwave radiometer data. If aircraft or satellite radiometers are to measure SMC under vegetated conditions, it will be necessary to increase their wavelengths well beyond the X band.
The tectonic and volcanic history of mercury as inferred from studies of scarps, ridges, troughs, and other lineaments
Authors: Dzurisin Daniel
Published: February 2008

This paper consists of separate descriptive and interpretive sections which address the issues of tectonic and volcanic modification of the Mercurian surface. In section 1 a nongenetic classification system based on planimetric form is outlined and applied to scarps, ridges, troughs, and other lineaments on Mercury. Nine landform classes are defined, and type examples of each are presented and discussed. The resulting set of observational constraints provides the basis for the following evolutionary chronology for Mercury developed in section 2. After formation of a solid lithosphere but before the end of heavy meteoritic bombardment, tidal spindown and planetary contraction owing to cooling combined to produce a global pattern of linear fractures and arcuate thrust faults. Throughout this period, intercrater plains formed by extrusion from linear vents or by local lithospheric melting. Near the end of heavy meteoritic bombardment the Caloris basin‐forming event drastically altered the morphology of much of the Mercurian surface. Soon after basin excavation, fluid material covered its floor to form smooth plains. Planimetrically irregular ridges formed inside Caloris in response to compressive stresses induced by regional subsidence. Gravitational readjustment to emplacement of plains material outside Caloris was responsible for scarp and ridge formation there. Uplift of plains inside Caloris produced extensional fractures which transect the previously established pattern of ridges. At roughly the same time, intense modification of preexisting landforms occurred in a region roughly antipodal to Caloris Basin to produce a distinctive terrain unit characterized by scattered hills and linear, open‐ended troughs. By this time, volcanic plains emplacement was largely restricted inside large, fresh craters. Irregular scarps and ridges developed in these plains in response to local stresses, perhaps induced by magma redistribution at depth. The period following the most recent smooth plains formation on Mercury has been characterized by relatively light cratering and only mild landform modification.
The physical state of the oceanic crust: Results of downhole geophysical logging in the Mid‐Atlantic Ridge at 23°N
Authors: Kirkpatrick R. James
Published: February 2008

Downhole physical property measurements obtained using standard oil field logging tools in ocean crust hole 396B (leg 46) of the Deep Sea Drilling Project indicate that for the upper 200 m of oceanic layer 2 near the mid‐Atlantic ridge in situ sonic velocities, densities, and electrical resistivities are significantly lower and the porosities significantly higher than laboratory measurements on the recovered basalts. In situ sonic velocities vary from about 1.5 to about 4.8 km/s with a mean of about 3.1 km/s. Densities vary from about 1.55 to about 2.6 g cm−3. Porosities are unexpectedly high, ranging from about 13% to about 41%. Electrical resistivities vary from about 5 to about 90 ohm‐m. The mean sonic velocity is typical of values obtained for the upper part of oceanic crust of the age at this site (middle Miocene) using marine seismic refraction techniques. Laboratory values for small ocean ridge basalt samples are typically much higher, about 6 km/s. The agreement between refraction and logging velocities, combined with the lack of radial variation in electrical resistivity away from the hole, indicates that formation damage due to drilling is relatively minor and that the values obtained by logging more nearly reflect the true in situ values than laboratory measurement. The major lithologic boundaries can be detected using each of the tools, and combinations of tools can differentiate between rock types. The high in situ porosity and relatively low in situ electrical resistivity imply that this part of the crust contains extensive large scale porosity and is probably quite permeable. Most of the porosity probably occurs between uncemented or weakly cemented pieces of pillow basalt, which makes up most of the upper crust at this site.
Past obliquity oscillations of Mars: The role of the Tharsis Uplift
Authors: Ward William R. , Burns Joseph A. , Toon Owen B.
Published: February 2008

The present gravitational quadrupole moment of Mars contains a 6½% nonhydrostatic contribution from the uncompensated Tharsis construct. Prior to the Tharsis uplift, the rate of spin axis precession was nearly equal to the frequency of a minor term in the expression for the orbital precession of Mars developed by Bretagnon (1974). In such a situation, small changes in the J2 of Mars could have caused the spin axis precession rate to drift repeatedly through the orbital frequency. We show that even minor, low‐amplitude orbital components can have a significant impact on the obliquity oscillations due to secular spin‐orbit resonance. Our approach to this problem is in three stages. First, we examine a number of processes which have the potential to alter the spin axis precession rate, discussing both the expected magnitude and time scales involved. Of these, Tharsis plays a dominant role, but other processes, i.e., differentiation, mantle convection, etc., may also have significantly influenced the planet's quadrupole moment in the past. Second, a theory describing the consequences of passage through a secular spin‐orbit resonance is developed. The problem is solved analytically for the case of uniform orbital precession. It is then shown that the resonance behavior is almost undisturbed by the presence of other orbital terms (i.e., nonuniform precession), even if these terms are appreciably larger than the resonance terms. Third, numerical integrations of the full equations of motion for Mars are presented that demonstrate the resonance and indicate that the obliquity of Mars may have intermittently been as low as ∼9° and as high as 46° (or more) in its early history.
Systematic monitoring of millisecond travel time variations near Palmdale, California
Authors: Leary P. C. , Malin P. E. , Phinney R. A. , Brocher T. , VonColln R.
Published: February 2008

Repeated measurements of P wave travel time changes have been completed near the San Andreas fault, 25 km west of Palmdale, California. Spanning a 2‐year period from June 1974, these observations were made on 0.1‐ to 8.5‐km base lines and have a worst case accuracy of ±4 ms. The observation area was centered around Bouquet Reservoir where the P wave source, a marine air gun, was secured. Our data were taken approximately every 5 days during June to July 1974, daily during November 1974 to January 1975, and every 2 hours during March to June 1976. Although the area has undergone regional strain at the rate of ∼2·10−7 yr−1 during this period, no net P wave travel time change greater than 1 part in 103 accumulated. We observed instead systematic and continuous arrival time fluctuations that lasted from hours to days and ranged from 1 to 10 ms (Δt/t ≃ 10−3‐10−2). Earth tides and temperature variations, which can produce some signal (Δt/t ∼ 10−3) at diurnal and semidiurnal periods, are at most partially responsible for the observations. We interpret the results in terms of P wave velocity variations in a waterfilled cracked medium in which velocity changes are due to the opening of microcracks during incremental strain. The time scale of anomalous velocity behavior is constrained to be less than the diffusion time of water into newly opened cracks. In this regime little prospect exists of detecting long‐term earthquake precursors.
A heat flow survey at anomaly 13 on the Reykjanes Ridge: A critical test of the relation between heat flow and age
Authors: Sclater John G. , Crowe John
Published: February 2008

We obtained 17 successful 12.5 m heat flow measurements on Anomaly 13 east of the Reykjanes Ridge. All temperature gradients were linear below 7.5 m but showed a large increase toward the sediment surface in the upper 5m. From the linear gradients and thermal conductivities measured on the sediment recovered in the cores we computed a mean flow for Anomaly 13 (35 m.y. B.P.) of 1.98±0.24 μcal/cm2 s (82.9±10.1 mW/m2). Thirteen of the stations were taken in a detailed survey of a well‐sedimented area at 56°N, 29°30′W. These measurements have a mean of 2.01±0.25 μcal/cm2 s (84.2±10.5 mW/m2). Almost all of the scatter in the data in this area can be accounted for by a combination of the refractive effect of the high thermal conductivity of the basement rocks and instrumental error. However, it is not possible from the present data set to rule out hydrothermal circulation in a ‘permeable’ basement as an explanation of the variation in the data. The mean value for the survey area is in excellent agreement with the relation between heat flow and the reciprocal of the square root of age predicted for oceanic crust less than 120 m.y. old. The sharp decrease in the temperature in the upper 5 m of sediment is most plausibly accounted for by a yearly periodic temperature change of an amplitude of 0.40°C in the Norwegian Sea overflow water. We suggest that the major temperature change occurs in late spring and reaches the area of our survey by early summer.
Vertical movement in mare basins: Relation to mare emplacement, basin tectonics, and lunar thermal history
Authors: Solomon Sean C. , Head James W.
Published: February 2008

The spatial and temporal relationships of linear rilles and mare ridges in the Serenitatis basin region of the moon are explained by a combination of lithospheric flexure in response to basin loading by basalt fill and a time‐dependent global stress due to the thermal evolution of the lunar interior. The pertinent tectonic observations are the radial distance of basin concentric rilles or graben from the mare center; the location and orientation of mare ridges, interpreted as compressive features; and the restriction of graben formation to times older than 3.6±0.2 b.y. ago, while ridge formation continued after emplacement of the youngest mare basalt unit (∼3 b.y. ago). The locations of the graben are consistent with the geometry of the mare basalt load expected from the dimensions of multiring basins for values of the thickness of the elastic lithosphere beneath Serenitatis in the range 25–50 km at 3.6–3.8 b.y. ago. The locations and orientations of mare ridges are consistent with the load inferred from surface mapping and subsurface radar reflections for values of the elastic lithosphere thickness near 100 km at 3.0–3.4 b.y. ago. The thickening of the lithosphere beneath a major basin during the evolution of mare volcanism is thus clearly evident in the tectonics. The cessation of rille formation and the prolonged period of ridge formation are attributed to a change in the global horizontal thermal stress from extension to compression as the moon shifted from net expansion to overall cooling and contraction. Restricting the time of peak lunar volume to 3.6 b.y. or earlier, together with the constraint that the accumulated thermal stress since the end of heavy bombardment did not reach levels capable of causing global‐scale lithospheric failure, greatly limits the range of possible lunar thermal histories. The zone of horizontal extensional stresses peripheral to mare loads favors the edge of mare basins as the preferred sites for mare basalt magma eruption in the later stages of mare fill, although subsidence may lead to accumulation of such young lavas in basin centers.
The stability of a rapid mode II shear crack with finite cohesive traction
Authors: Burridge R. , Conn G. , Freund L. B.
Published: February 2008

It has been found in numerical simulations by Andrews [1976] that a growing mode II shear crack may at first rapidly accelerate toward the Rayleigh wave speed vR and then after a short period of readjustment start running at a speed slightly greater than (2)½vS, where vS is the shear wave speed. In order to throw some light on this phenomenon we study in detail the steady motion of a semi‐infinite crack which is driven by a following point load which remains at constant distance d from the crack tip. A cohesive traction which is a function of relative displacement acts in the Dugdale type tip region [Dugdale, 1960] which is small in comparison with d. We consider crack speeds vC in the range vC < vP, the P wave speed, and find that for vC < vR and for vS < vC < (2)½vS the load is a decreasing function of vC, while for (2)½vS < vC < vP the load increases with vC. The former situation is interpreted as instability, and the latter as stability. The range vR < vC < vS appears to be forbidden. This is in agreement with, and partially explains, Andrews's results. It also agrees with an earlier analytic solution obtained by Craggs [1960] for vC < vR. We find that for vC < vR the displacement in the tip region has a shape independent of vC but suffers a contraction analogous to a Lorentz contraction so that the length goes to zero as vC ↑ vR.
Observations relevant to seismic driving stress, stress drop, and efficiency
Authors: McGarr A. , Spottiswoode S. M. , Gay N. C. , Ortlepp W. D.
Published: February 2008

Results of previous studies of seismicity and source parameters of tremors in a deep mine in South Africa have been compared with underground observations of the effects of tremors and laboratory analyses of material properties. These comparisons indicate that the ruptures are driven by shear stresses averaging about 400–700 bars in the source region; the driving stress apparently increases with depth. Comparison of these high driving stresses to seismic stress drops, that had previously been found to range from 5 to 50 bars, implies very low seismic efficiencies. Most of the released energy is consumed in creating the fault zone, with less than 1% being radiated seismically. Underground observations of displacement across fault zones associated with mine tremors show that the faulting mechanism is very inhomogeneous. Measured displacements show considerable variation, and typical values are more than a factor of 10 greater than those anticipated on the basis of Brune's seismic source model. A series of measured shear displacements averaging 6 cm in the well‐explored fracture systems argue strongly for high, localized stress drops, perhaps in the neighborhood of 700 bars or greater. Because the fracture system interacts with the much larger mine excavation during its formation, it apparently absorbs strain energy from a region considerably broader than the fracture itself. The source dimensions determined from the seismic radiation probably indicate the sizes of the broad regions from which strain energy is drawn rather than the dimensions of contiguous fracture systems.
Global seasonal variation of water vapor on Mars and the implications for permafrost
Authors: Farmer C. B. , Doms P. E.
Published: February 2008

Observations of the global distribution and seasonal variation of the Martian atmospheric water vapor have been made from the Viking orbiters for a continuous period covering a complete Martian year. The seasonal dependence of the latitude distribution of the column abundance of vapor is consistent with a model in which the vapor is in equilibrium with the regolith at polar and mid‐latitudes. The results are consistent with there being a permanent reservoir of water ice buried at a depth of 10 cm to 1 m at all latitudes poleward of 40°. The behavior of the vapor in the summer hemisphere suggests an annual net transport of the vapor phase from the southern to the northern hemisphere, with deposition of ice of thickness of the order of a few milligrams per square centimeter in the northern polar latitudes. The hemispheric asymmetry is the result of the propagation of the global dust storms originating in the south.
Geodetic evidence for aseismic subduction of the Juan de Fuca Plate
Authors: Ando Masataka , Balazs Emery I.
Published: February 2008

A down‐to‐the‐east crustal tilt rate across western Washington disclosed by precise leveling over a 70‐year period suggests that the Juan de Fuca plate is aseismically underthrusting the North American plate. According to this hypothesis, the frequent occurrence of large thrust earthquakes that ordinarily accompany plate convergence need not be expected along the Juan de Fuca subduction zone. This conclusion is consistent with the fact that there have been no great earthquakes in western Washington in historical time (the past 140 years).
Deformation across the Salton Trough, California, 1973–1977
Authors: Savage J. C. , Prescott W. H. , Lisowski M. , King N.
Published: February 2008

A trilateration network extending across the San Andreas, San Jacinto, and Elsinore faults in the vicinity of the Salton Sea, California, has been surveyed to very high precision several times in the 5‐year interval 1973–1977. The average strain across the entire network is essentially a uniaxial north‐south contraction at the rate of about 0.3 μstrain/a. There is no substantial strain perpendicular to the Salton trough, indicating no tendency to either open or close that rift. The observed uniaxial north‐south contraction differs from a pure shear parallel to the major faults by a uniform dilatation of about −0.25 μstrain/a that is only partly explained. The shear strain across the network reaches a maximum near the San Jacinto fault and could be explained by right lateral slip at depth on that fault. The displacement pattern also suggests right lateral slip at depth on the San Jacinto fault with total right lateral relative movement near 50 mm/a across the 120‐km breadth of the network. The deformation appears to be uniform in time in the 1973–1977 interval. A dislocation model of the deformation suggests about 50±15 mm/a of relative right lateral slip at depth distributed between the San Andreas, San Jacinto, and Elsinore faults and demonstrates that a significant average dilatation can be generated by pure strike slip on several subparallel faults.
Thermal subsidence and petroleum generation in the southwestern block of the Los Angeles Basin, California
Authors: Turcotte D. L. , McAdoo D. C.
Published: February 2008

When heated continental crust cools, its density increases, and it may subside according to the condition of isostasy. If the subsiding basement is covered with sediments a sedimentary basin is formed. In this paper the isostatic subsidence model is applied to the southwestern block of the Los Angeles basin with particular reference to the Wilmington oil field, an oil giant. The subsidence record of the sediments within this oil field agrees with the theory for thermal isostatic subsidence. The predicted gravity field and near surface thermal gradient are also in good agreement with observations. When the predicted thermal history of the sediments is combined with laboratory derived equations for the conversion of kerogens to petroleum we conclude that near ideal conditions for kerogen conversion have existed near the base of the Upper Miocene sediments in this oil field. Geochemical studies confirm that this is the origin of the petroleum. A discussion of the subsidence history of the remainder of the Los Angeles basin is given.
A vertical groundwater movement correction for heat flow
Authors: Mansure Arthur J. , Reiter Marshall
Published: February 2008

In the presence of forced convection, there exists an energy flux cρVT, because of mass‐transported heat energy. By plotting temperature gradient versus temperature the significance of convective heat transfer can be determined. Such a plot is useful even when the flow system is not one‐dimensional or vertical. As an example of convective heat transfer, a temperature log from the Rio Puerco area of New Mexico shows that downward groundwater movement of 4.7×10−7 cm/s over a 77‐m interval can reduce the observed geothermal gradient and heat flow by a factor of 2 from 4.1 to 2.0 HFU (1 HFU = 1 μcal cm−2 s−1). Such strong perturbations of heat flow by groundwater indicate the need for deep heat flow measurements to obtain a geothermal heat flux undisturbed by near‐surface groundwater. By considering the convective flux one can use the total energy balance at the boundaries of aquifers and a semiconfining medium to obtain information about groundwater seepage within the semiconfining layer.
A new model for analyzing the effect of fractures on compressibility
Authors: Walsh J. B. , Grosenbaugh M. A.
Published: February 2008

Microcracks and joints in rock are assumed to be rough surfaces in contact. Compressibility is found to depend on the parameter Ah/V, where h is the standard deviation of the asperity heights and A/V is the total crack area per unit volume. Theoretical results agree well with experimental studies of the deformation of an in situ joint under normal stress and the change of compressibility with confining pressure for several rocks.
Paleomagnetism of the Devonian Onondaga limestone revisited
Authors: Kent Dennis V.
Published: February 2008

Ninety‐six oriented samples were collected from 13 sites in the Onondaga limestone of New York for paleomagnetic study, including rock magnetism and magnetic susceptibility anisotropy. The sites were located in a small penecontemporaneous fold and a larger fold of imprecisely known origin, as well as flatlying beds. The natural remanent magnetizations (NRM) give directions (declination 195.1°, inclination 84.4°, and α95 = 6.9° for N = 13 sites) that are similar to those originally measured for the Onondaga limestone by Graham (1956). Alternating field and thermal demagnetization analyses show that the NRM is a composite of a large viscous component along the present geomagnetic field and a smaller, more stable component of magnetization with a shallow and southerly direction. The site mean directions after 300 Oe of magnetic cleaning, but without correction for bedding tilt, give a formation mean of declination 168.7°, inclination 13.6°, and α95 = 8.3° for N = 9 sites (the magnetizations at four sites could not be adequately resolved and are omitted). Although the stable magnetization is shown to be of postfolding origin, essentially the same direction is obtained at sites with flat‐lying bedding as at the sites with folded bedding prior to tilt correction. Rock magnetic investigations indicate that some form of magnetite is the important magnetic mineral in the limestone. The similarity in magnetic fabric between a small penecontemporaneous fold and a larger fold suggests acquisition of magnetic properties while the sediment was still unconsolidated. The postfolding origin of stable remanence is therefore interpreted as a form of postdepositional detrital remanent magnetism. The paleomagnetic pole position derived from the formation mean direction is located at latitude 39.7°N, longitude 120.7°E (dp = 4.3°, dm = 8.5°) and agrees reasonably well with other Devonian poles from cratonic North America, although a Permian age for the magnetizations of the Onondaga cannot be entirely discounted.
Can the ratios of single‐wavelength EDM data improve the resolution of small changes in line length?: A comparison with multiwavelength EDM data
Authors: Slater L. E.
Published: February 2008

A variety of electromagnetic techniques are now in use for high‐precision measurement of distance through the earth's atmosphere. One of the major obstacles to improving the precision of single‐wavelength electromagnetic distance‐measuring (EDM) measurements is the uncertainty in the value of the index of refraction along the path being measured. If the index of refraction were slowly varying and the relative change of the index nearly uniform, the problem would be considerably simplified; deformations, except uniform compression and extension, could be determined to high precision by analyzing the ratios of various uncorrected line lengths in an array. The development of a field‐worthy multiwavelength EDM (multiwavelength distance measuring (MWDM)) instrument has allowed a direct comparison of this ratio technique with high‐precision multiwavelength data. Three widely spaced lines in the Hollister, California, MWDM array were selected for this study. The objective was to determine whether a single observer, without access to expensive or sophisticated means of sampling the atmosphere, could collect data capable of resolving changes of less than 1 part per million (ppm) in line length. No corrections were made to the single‐wavelength data (i.e., the optical path lengths) before the ratio was formed. Results indicate that the requirement of near constancy during a particular survey is more realizable than that of near uniformity. It appears difficult to achieve a precision of better than 0.5 ppm routinely within an arbitrary radial array of substantial size. The technique does offer promise in properly designed arrays where all lines can be restricted to a reasonably small solid angle (e.g., the monitoring of some man‐made structures such as dams) or where they are between isolated mountain peaks of comparable elevation. Within such arrays a precision of 0.2 ppm should be possible.
Rates and processes of crystal growth in the system anorthite‐albite
Authors: Kirkpatrick R. James , Klein Lisa , Uhlmann D. R. , Hays James Fred
Published: February 2008

Rates of plagioclase crystal growth at constant temperature have been measured for a series of compositions in the system CaAl2Si2O8(An)—NaAlSi3O8 (Ab). A microscope heating stage was used at temperatures near the liquidus to observe and photograph the growing crystals, and a resistance furnace quenching technique was used at larger degrees of undercooling with observations and measurements made on quenched samples of heat‐treated glasses. Results from the two techniques agree within the temperature range of overlap. Growth rates are independent of time and increase (at either constant temperature or constant undercooling) with increasing An content. The crystal morphologies for all compositions are faceted near the liquidus and become progressively skeletal, dendritic, and fibrillar with increasing undercooling. Even the spherulitic or fibrillar crystals may be faceted at the growing interface. Maximum growth rates vary from about 5×10−1cm/min at an undercooling of 150°C (T = 1400°C) for pure CaAl2Si2O8 (An100) to about 2 × 10−5 cm/min at an undercooling of 380°C (T = 960°C) for An20.
The electrical resistivity of oceanic basalts
Authors: Drury M. J. , Hyndman R. D.
Published: February 2008

The electrical resistivity of a large number of ocean crust basalt samples obtained by the Deep Sea Drilling Project has been studied, including the effect of temperature and pressure, the correlation with porosity, and the changes with degree of dehydration. The data are compared to those of samples from several oceanic islands and a few gabbros that might be representative of the lower oceanic crust. The geometric mean resistivity of 153 ocean crust basalts, seawater saturated, at 22°C and atmospheric pressure is 190 ohm m. There is no significant change with the low‐temperature weathering or halmyrolysis characteristic of the upper part of old oceanic crust. Higher‐temperature hydrothermal alteration such as that experienced by subaqueous Bermuda lavas appears to reduce the porosity and thus increase the resistivity. In contrast, subaerial basalts such as those from the Azores have high porosity and thus low resistivity, but Azores subaqueous samples of similar composition have the same resistivity as deep‐sea basalts. The gabbros have low porosity and resistivity an order of magnitude higher than the basalts. The basalt resistivities correlate directly with porosity according to Archie's law, but with a very high slope, which suggests that pore fluid conduction is dominant but that it is affected by the presence of clay minerals. The resistivity of all saturated samples increases with increasing pressure, as is expected for pore fluid conduction. The resistivity of saturated samples decreases markedly with increasing temperature. For some of the most resistive samples, temperature dependence parallels that of seawater, but for most samples, resistivity decreases more rapidly with increasing temperature, again suggesting pore fluid conduction modified by the presence of clay minerals. Samples dried at low temperature and those dried at high temperature so that the clays are dehydrated both exhibit thermally activated conduction. In the completely dehydrated samples, conduction may be through the metallic oxide grains. The low basalt resistivities under simulated crustal conditions are in agreement with electromagnetic induction data from oceanic islands and the deep‐sea floor.
Spacing of water‐free crevasses
Authors: Nemat‐Nasser S. , Oranratnachai A. , Keer L. M.
Published: February 2008

On the basis of the analysis of the stability of an interacting system of straight edge cracks it is shown that when the fracture strength of the ice is taken to be nonzero, there exists a minimum spacing for water‐free crevasses (about 6–8 m). This spacing is, in general, much larger than the minimum crack spacing (about ⅓ m) needed in order to be able to attain the critical value of the stress intensity factor; and therefore, in general, the minimum spacing of the water‐free crevasses is dictated by the stability consideration, rather than by the consideration of the stresses alone. In terms of dimensionless parameters a master chart is developed which separates stable and unstable states. It is shown that the minimum spacing decreases with decreasing fracture strength of the ice, and when the fracture strength of the ice is taken to be zero, the spacing can be very small. At this limit the penetration depth is approximately given by the value found by Nye, which is then independent of the corresponding spacing.
Elastic expansion of the lithosphere caused by groundwater depletion
Authors: Holzer Thomas L.
Published: February 2008

Several centimeters of man‐induced elastic expansion of the lithosphere is inferred to have been caused by the depletion and removal from the area of large masses of groundwater in parts of Arizona, California, and Texas. The water was removed principally by evapotranspiration by crops of groundwater pumped from aquifers. Arguments for elastic expansion are based on both theoretical estimates and comparison with the elastic deformations observed at Lake Mead and Lake Kariba, where comparable masses of water were loaded on the earth's surface. In one area, south central Arizona, 6 cm of crustal uplift observed from 1948 to 1967 may be related to the removal of 4.35×1013 kg of groundwater. First, if the observed uplift is assumed to be due to man‐induced unloading, a Young's modulus of the lithosphere of approximately 0.68 Mbar is implied, which is comparable to values reported elsewhere. Second, the uplift compares favorably with the depression caused by the formation of Lake Mead, 430 km northwest of the study area, when allowance is made for the different magnitudes, sense, and areas of surface loading.
Interpretation of borehole magnetometer data
Authors: Parker Robert L. , Daniell Geoffrey J.
Published: February 2008

Samples recovered from Deep‐Sea Drilling Project (DSDP) holes into the basaltic layer strongly suggest that a good model of the magnetization would be a spatially random one. To interpret future magnetometer observations made in the holes, we develop a theory relating the observed fields to the magnetization autocorrelation tensor, a function completely describing the second‐order statistics of the medium. We examine special models with long‐ and short‐range order and conclude that certain properties of the medium are highly desirable in making interpretation possible. Among these are that the autocorrelation tensor be independent of direction in a horizontal plane and that the direction of magnetization be uniform. Furthermore, useful interpretation is feasible only if three components of the magnetic field are measured, and it is preferable (but not essential) that the magnetometer is absolutely orientated about a vertical axis. We show that only in special circumstances (e.g., horizontal layering of the medium) does the magnetic field correlate directly with the average magnetization in a region surrounding the magnetometer. We analyze the natural remanent magnetization data from leg 37 of the DSDP to obtain partial information on the spatial statistics. There is no significant order vertically on scales longer than 10 m, but in two of the five holes there is correlation in the range 0.3–2 m. In three holes there is evidence of fairly good uniformity of direction, a highly desirable property for interpretational purposes. Unfortunately, drill hole samples give us no information about the behavior of the autocorrelation tensor as a function of horizontal distance.
A thermodynamically based equation of state for the lower mantle
Authors: Brennan Barry J. , Stacey Frank D.
Published: February 2008

An observation by O. L. Anderson that for close‐packed minerals, (∂KT/∂T)V = 0, approximately, implies without further assumption the Birch relationship (αKT)T = const, to a similar approximation. At sufficiently high temperatures (and in materials that are not too near to phase transitions) we may also make the classical assumption CV = const, and then we have the thermal Grüneisen parameter γ inversely proportional to density ρ. The fact that the free volume formulation for γ is also believed to be a good approximation for close‐packed materials leads immediately to an equation of state. In integrated form this is P = (Ko/2γo)(ρ/ρo)4/3{exp[2γo(1 ‐ ρo/ρ)] ‐ 1}. Earth model data for the lower mantle, extra‐polated to zero temperature, give only modest agreement with the new equation and indicate that in the lower mantle, γ is less dependent upon ρ than the equation implies, although doubt about the model data is such that this is not a secure conclusion. Nevertheless, the agreement is sufficient to give reasonable zero‐pressure parameters for the lower mantle, ρo = 4180 ± 30 kg m−3, Ko = (2.42±0.05) × 1011 Pa, and γo = 1.17 ± 0.05, and to indicate a mean temperature gradient (1.3±0.3) times adiabatic with a value extrapolated to the coremantle boundary (3200±900) K. However, the model data are consistent with a much steeper gradient in the lowest 200 km of the mantle, and our preferred fit to the data omits this range. The implied interatomic potential function has a Coulomb attractive term and a repulsion term in the form of an incomplete gamma function.
Isostasy and flexure of the lithosphere under the Hawaiian Islands
Authors: Suyenaga Wayne
Published: February 2008

Previous flexural studies on Hawaii were done under the assumption that the amount of deflection caused by the loading of the islands is equivalent to the change in Moho depth. Crustal profiles based on seismic refraction suggest an alternative measurement of deflection as the change in depth of the boundary between the basement and oceanic layers. This alternative implies the existence of an unflexed portion of the crust that represents the difference between the basement‐oceanic and Moho displacements and that effectively lessens the load due to its buoyancy. This material represents either a phase change of mantle to crustal material or intrusion into the lower crust. From the basic equations of flexure and Airy isostasy a more general form of equilibrium equation is developed and can be interpreted as an addition of an elastic flexure term to the isostatic equations or an addition of a term accounting for changes in the crustal column to the presently used flexure equations. The study of simple models indicates that the most reliable parameter in estimating flexural rigidity is the wavelength of the flexure. When elastic flexure and buoyancy of added crust are considered in relation to the Hawaiian Ridge at Oahu, a plausible case is made for the presence of both. A flexural rigidity of about 1.2×1023 N m for an unfractured lithosphere is derived for the Hawaiian Islands; this value agrees favorably with results of other studies.
Intraplate earthquakes in the Svalbard Archipelago
Authors: Mitchell Brian J. , Zollweg James E. , Kohsmann James J. , Cheng Chiung‐Chuan , Haug Eric J.
Published: February 2008

A small, seismically active region, designated the Heerland seismic zone, has been delineated in the Svalbard archipelago. Over 500 earthquakes in this region, of which nearly 50 were reliably located, were recorded during a 9‐week field season in 1977. This zone provides an example of intraplate earthquake activity in continental‐type crust not far from an active plate boundary. A large proportion of the located earthquakes suggests a trend with a dominant east‐west direction which is 25 km or less in length. The orientation of this grouping of epicenters is nearly transverse to all of the major mapped faults and structural trends in that region. A fault plane solution was obtained for one relatively large teleseismically recorded earthquake which was located near the Heerland zone. One of the derived nodal planes approximately corresponds with the trend of locally obtained epicenters. First‐motion data from the larger locally recorded earthquakes are consistent with the solution obtained for the teleseismically recorded earthquake. These data, if combined with the observed seismicity pattern, indicate sinistral strike‐slip motion along a vertical fault trending in an approximate eastsoutheast west‐northwest direction. Presentday seismicity along the major mapped north‐south faults of Spitsbergen is insignificant in comparison to that of the Heerland zone. Since movement along those faults has been documented at least into the Paleocene, the paucity of recent earthquakes along them and the unexpected pattern of seismicity in the Heerland zone suggest that the stress field acting in the crust of that region is quite different from that acting during the early Tertiary.
Microseisms at Palisades: 2. Rayleigh wave and Love wave characteristics and the geologic control of propagation
Authors: Rind D. , Down W. L.
Published: February 2008

The wave type of the microseisms recorded at Palisades, New York, is investigated to determine the Love and/or Rayleigh wave content. It is found that microseisms arriving from the southeast and northwest consist almost entirely of Rayleigh waves, while microseisms from the northeast and southwest have significant Love wave energy. The Love waves appear to be generated in the same source region as the Rayleigh waves. To account for the variation in Love wave energy as a function of source direction, the geologic structure along the various propagation paths is reviewed, and it is found that Love waves appear to be inhibited when propagating across the discontinuity surface between ocean and continental crust and when propagating across continental shield regions, both obstacles to a continuous surface layer. Observations from around the world are discussed, and we show that these two generalities appear to account for many of the reported discrepancies in microseism composition.