bluuebunny/crossref_metadata_2025 huggingface dataset:
The distribution of nitrogen oxides off the East Coast of North America
Authors: Misanchuk Bryan A. , Hastie Donald R. , Schiff Harold I.
Published: April 2008

Measurements of NO and NOx' (as measured by a chemiluminescent analyzer equipped with a FeSO4 converter) were made from an aircraft off the east coast of North America in January and February 1986. These measurements show the mixing ratios of the nitrogen oxides to be higher than encountered in remote continental or marine environments. Both the free troposphere and boundary layer mixing ratio distributions are skewed to ward high mixing ratios and are best described by a gamma probability distribution. The mean mixing ratios observed were 2.9 parts per billion by volume (ppbv) in the boundary layer and 0.55 ppbv in the free troposphere for NOx', and 0.56 ppbv in the boundary layer and 0.18 in the free troposphere for NO.
Magma‐driven propagation of cracks
Authors: Spence D. A. , Turcotte D. L.
Published: April 2008

A similarity solution is derived for the two‐dimensional propagation of a liquid‐filled crack. This solution includes both the flow problem within the crack and the fracture problem through a stress intensity factor. The results are applied to the emplacement of dikes and sills and to the migration of magma through the lithosphere. We show that for these applications the propagation of the fracture is limited by the viscosity of the magma; the fracture resistance of the elastic medium can be neglected. Limitations on propagation due to magma solidification during emplacement are also considered. As a typical example for the emplacement of a dike or sill, we conclude that a magma with a viscosity of 102 Pa s can be injected into a crack with a length of 2 km and a width of 0.5 m in a period of 15 min; the velocity of fracture propagation is about 0.5 m s−1. We also conclude that a propagating, liquid‐filled crack is a viable mechanism for the migration of magma through the lithosphere.
A Bayesian approach to nonlinear inversion
Authors: Jackson David D. , Matsu'ura Mitsuhiro
Published: April 2008

Iterated linear inversion theory can often solve nonlinear inverse problems. Also, linear inversion theory provides convenient error estimates and other interpretive measures. But are these interpretive measures valid for nonlinear problems? We address this question in terms of the joint probability density function (pdf) of the estimated parameters. Briefly, linear inversion theory will be valid if the observations are linear functions of the parameters within a reasonable (say, 95%) confidence region about the optimal estimate, if the optimal estimate is unique. We use Bayes' rule to show how prior information can improve the uniqueness of the optimal estimate, while stabilizing the iterative search for this estimate. We also develop quantitative criteria for the relative importance of prior and observational data and for the effects of nonlinearity. Our method can handle any form of pdf for observational data and prior information. The calculations are much easier (about the same as required for the Marquardt method) when both observational and prior data are Gaussian. We present calculations for some simple one and two‐parameter nonlinear inverse problems. These examples show that the asymptotic statistics (those based on the linear theory) may in some cases be grossly erroneous. In other cases, accurate observations, prior information, or a combination of the two may effectively linearize an otherwise nonlinear problem.
Forecast model for moderate earthquakes near Parkfield, California
Authors: Stuart William D. , Archuleta Ralph J. , Lindh Allan G.
Published: April 2008

Earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation. In the forecast procedure proposed here, repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters. The early part of the model simulation corresponds to the available field data, and the subsequent part constitutes an estimate of future faulting and ground deformation. In particular, the time, location, and size of unstable faulting are estimates of the pending earthquake parameters. The forecast accuracy depends on the model realism and parameter resolution. The forecast procedure is applied to fault creep and trilateration data measured near Parkfield, California, where at least five magnitude 5.5 to 6 earthquakes have occurred regularly since 1881, the last in 1966. The quasi‐static model consists of a flat vertical plane embedded in an elastic half space. Spacially variable fault slip of strike‐slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle, strain‐softening fault. The field data are consistent with these approximate values of patch parameters: radius of 3 km, patch center 5 km deep and 8 km southeast of the 1966 epicenter, and maximum brittle strength of 26 bars. Fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21±8 year recurrence interval. However, the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake, as predicted by the model.
Geometric effects of global lateral heterogeneity on long‐period surface wave propagation
Authors: Lay Thorne , Kanamori Hiroo
Published: April 2008

Long‐period Rayleigh waves from Iranian earthquakes have large amplitude asymmetries between minor arc and major arc arrivals (e.g., R2 and R3) at digital stations in the azimuth range N20°W to N60°E. These asymmetries are as large as a factor of 2 at a period of 256 s and persist to periods greater than 300 s. In some cases the entire Rayleigh wave group arrival spanning periods from 100 to 300 s is either uniformly enhanced in amplitude or diminished to such a degree that the group arrival appears to be missing. The amplitude anomalies are generally not accompanied by significant phase anomalies. The irregular azimuthal distribution of the amplitude asymmetries and their occurrence for events with different focal mechanisms and epicentral separations of several hundred kilometers preclude an explanation of these observations by source complexity. Events in the Mediterranean and Nepal do not produce similar amplitude asymmetries at the same stations. The anomalies are thus most likely due to focusing and defocusing propagation effects. As a preliminary investigation of the effects of lateral heterogeneity of upper mantle velocity structure on long‐period surface wave amplitudes, surface wave ray‐tracing calculations are performed using recently proposed global phase velocity distributions. Dramatic deviations from great circle paths are predicted for long propagation paths (e.g., R3). The particular spatial distribution of lateral velocity gradients around a given source location determines whether substantial amplitude asymmetries will be observed between minor arc and major arc arrivals and whether these will persist for sequential great circle orbits. The 200‐s period amplitude asymmetry observed at KIP for the Iranian source region (R2,4 ≪ R3) is well predicted by the ray‐tracing results. The absence of this anomaly for the other source regions is also predicted. Other observed anomalies are not all well predicted, but it is clear that geometric effects can contribute significantly to the observed variations of Rayleigh and Love wave amplitudes. This is the probable explanation for the instability of Q estimates made from surface waves. Other source regions producing large surface wave amplitude anomalies include Japan and southeastern Alaska.
Structure of the Benioff Zone beneath the Shumagin Islands, Alaska: Relocation of local earthquakes using three‐dimensional ray tracing
Authors: Hauksson Egill
Published: April 2008

Seismic rays are traced through a prescribed three‐dimensional inhomogeneity that simulates the subducted slab below the Shumagin Islands region to calculate local station delays for a given hypocenter and a slab model. Hypocenters determined by the Shumagin seismic network are then relocated using the station delays, a flat‐layered velocity structure and a standard earthquake location computer program. Station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data. A set of path corrections that is calculated for each grid point‐station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km. Application of this method to data from 1982 for the digitally recording Shumagin seismic network shows the following results: (1) a previously observed apparent increase in dip of the subducted slab at depths of ≈ 100 km disappears, (2) the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45° toward north‐northwest at depths between 80 and 250–300 km and has a 7% higher velocity than the surrounding mantle, (3) hypocenters determined from Shumagin network data are located only 10–20 km south of high‐quality hypocenters determined from teleseismic data alone, (4) qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high‐frequency coda, possible converted phases, and low amplitudes of first P arrivals for rays that travel mostly along the slab. Conversely, for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first P arrivals that are followed by an insignificant coda and low‐amplitude S waves.
Reinterpretation of morphology and crustal structure in the Central Arctic Ocean Basin
Authors: Weber J. R. , Sweeney J. F.
Published: April 2008

Ice station bathymetric profiles and submarine echograms reveal a steep escarpment at least 200 km long with up to 1700 m of relief subparallel to the Lomonosov Ridge. This feature, formerly called the Marvin Spur, is here interpreted as the Makarov‐facing flank of the Alpha Ridge. East of the 160°W meridian the intervening Makarov Basin is a narrow grabenlike trough between 25 and 60 km wide filled with horizontally stratified sediments that were deposited largely after the Marvin Seamounts were constructed along the basin axis. The association of basement relief with geomagnetic field variations within the trough plus evidence of normal faulting on the Lomonosov Ridge suggest that this segment of the Makarov Basin is an extensional feature produced by other than typical seafloor‐spreading processes. Gravity models indicate that the crust below the Makarov flank of the Alpha Ridge ranges in thickness between 16 and 20 km, considerably less than the 26 km known from seismic refraction work to exist below the Lomonosov Ridge. It is argued, on the basis of gravity data, that adjacent to the Lomonosov Ridge the Fram Basin crust, which is probably truly oceanic, is 3–4 km thinner than that of the Makarov Basin.
Three‐dimensional flow beneath a slow spreading ridge axis: A dynamic contribution to the deepening of the Median Valley toward fracture zones
Authors: Parmentier E. M. , Forsyth D. W.
Published: April 2008

The axial valley along a slow spreading mid‐ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis. Along a ridge segment, the axial valley floor deepens by as much as 2–3 km over distances of several tens of kilometers approaching a ridge‐transform intersection. This deepening may be explained, in part, by a pressure gradient associated with horizontal flow in the ridge axis conduit. Vanishing vertical velocity on a conduit end wall at a ridge‐transform intersection results in reduced vertical flow within about one conduit width of the intersection. Horizontal flow along the conduit, toward the intersection, must occur to form lithosphere at a uniform rate along the ridge axis. A simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge‐transform intersections is considered. The model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls. This model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection. For reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection.
Heat flow in the Balearic and Tyrrhenian Basins, western Mediterranean
Authors: Hutchison I. , Von Herzen R. P. , Louden K. E. , Sclater J. G. , Jemsek J.
Published: April 2008

We present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the Balearic and Tyrrhenian basins in the western Mediterranean. Analysis of 12 measurements within a 10‐km radius from 40°01′, 4°55′E in the Balearic abyssal plain shows a mean heat flow of 92±10 mW m−2. After correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late Oligocene age proposed for the basin. A similar survey with 18 measurements around 40°16′N, 11°19′E in the Western Tyrrhenian gives a flux of 134±8 mW m−2, while the third survey of 26 measurements in the southern Tyrrhenian abyssal plain at 37°17′N, 12°58′E yields a slightly higher heat flow of 151±10 mW m−2. These values are within the range predicted by simple plate cooling models for the late Miocene ages of the deep Tyrrhenian basin. Thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins, the thermal signature is similar. Also, the trend of increasing heat flow from west to east through the Balearic and Tyrrhenian basins is in agreement with models of the formation of the western Mediterranean behind an eastwardly migrating trench system. In all three areas the measured flux shows significant local variability. In the two westernmost surveys this can be attributed to the presence of buried, high‐conductivity salt structures, but in the third area (SE Tyrrhenian) the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone. Instead, evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area.
The Heber Geothermal Field, California: Natural state and exploitation modeling studies
Authors: Lippmann Marcelo J. , Bodvarsson Gudmundur S.
Published: April 2008

Using numerical simulation techniques and an axisymmetric model of the Heber geothermal field, the natural (preexploitation) state of the system and its response to fluid production are analyzed. The results of the study indicate that the Heber geothermal anomaly is sustained by the upflow of hot water through a central zone of relatively high permeability. The best model suggests that in its natural state the system is recharged at depth by a 15‐MWt (megawatts thermal) (reference temperature 0°C) convective heat source. The existence of an axisymmetric convection pattern, whose axis coincides with the center of the Heber anomaly, is also suggested. At the lower part of the ascending hot water plume the deep recharge water mixes with colder water moving laterally toward the axis of the system. In the upper part the rising plume spreads radially outward after reaching the bottom of the cap rock, at about 550 m depth. The model results suggest that the so‐called cap rock is quite permeable (5×10−15 m2) with Convection controlling its temperature distribution. The results also show reduced permeability (10×10−15 m2) of the upper zones in the outer region of the system that may be due to mineral precipitation. In modeling the exploitation of the field the generation rate is allowed to build up over a period of 10 years; after that, 30 years of constant power production is assumed. Full (100%) injection of the spent brines is considered; the fluids being injected 2250 m (“near injection”) or 4250 m (“far injection”) from the center of the system. The study shows that a maximum of 6000 kg/s (equivalent to approximately 300 MWe (megawatts electric)) of fluids may be produced for the near injection case but only 3000 kg/s (equivalent to approximately 150 MWe) for the far injection case. The results indicate that the possible extraction rates (generating capacity) are generally limited by the pressure drop in the reservoir. The average temperature of the produced fluids will decline 10°–18°C over the 40‐year period.
Venus topography: A harmonic analysis
Authors: Bills Bruce G. , Kobrick Michael
Published: April 2008

A model of Venusian global topography has been obtained by fitting an eighteenth‐degree harmonic series to Pioneer Venus orbiter radar altimeter data. The mean radius is (6051.45±0.04) km. The corresponding mean density is (5244.8±0.5) kg m−3. The center of figure is displaced from the center of mass by (0.339±0.088) km towards (6.6±10.1)°N, (148.8±7.7)°E. The figure of Venus is distinctly triaxial, but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes. However, the higher‐degree harmonics of topography and gravity are significantly correlated. The topographic variance spectrum of Venus is very similar in form to those of the moon, Mars, and especially Earth. We suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes. Venus and Earth are particularly similar (and differ from the moon and Mars) in that the larger bodies both exhibit a significant low degree‐deficit (relative to the extrapolated trend of the higher harmonics).
A simple model of whole‐mantle convection
Authors: Loper D. E.
Published: April 2008

Isotopic evidence of the chemical variability of basalts implies the existence of distinct, long‐lived chemical reservoirs in the mantle. This has commonly been interpreted as evidence that mantle convection is layered, since known models of whole‐mantle convection have too much shearing to allow preservation of these reservoirs. However, the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties, casting serious doubt on the viability of these models. This difficulty has been resolved in the present paper by the development of a simple model of whole‐mantle convection which allows the preservation of chemically distinct reservoirs. In this model, heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning. Hence the lower mantle is much less sheared than in the usual two‐dimensional model. Mantle convection is driven by the penetration of cold lithospheric slabs from above. It is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state. These ideas are quantified in a simple one‐dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous. The resulting convection cycles material through the upper mantle more rapidly than through the lower mantle. This provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle.
Recent movement on the Garlock Fault as suggested by water level fluctuations in a well in Fremont Valley, California
Authors: Lippincott Diane K. , Bredehoeft John D. , Moyle W. R.
Published: April 2008

Water levels have been continuously recorded since March 1978 in a well in Fremont Valley, where several strands of the adjacent Garlock fault zone have exhibited both left‐lateral displacement and components of normal displacement. Differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well. During the 4‐year recording period, six sharp fluctuations, or “spikes,” were noted. These fluctuations, occurring over 2‐ to 4‐day periods, have amplitudes of 15–30 cm. They appear to be the result of creep events on a nearby fault. Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up relative to Fremont Valley or (2) left‐lateral slip on the same fault. Because of the nature of the fluctuations we favor the latter interpretation. Dislocation models utilizing exponential, arc tangent, and skewed cosine functions were used to analyze the water level fluctuations, associated pressure distribution, and fault displacements. The results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events. Estimates of cumulative creep for the period 1978–1982 range from 20 to 50 mm, depending on the particular model employed.
Paleomagnetism of rocks from Burin Peninsula, Newfoundland: Hypothesis of Late Paleozoic displacement of Acadia criticized
Authors: Irving E. , Strong D. F.
Published: April 2008

The posttectonic St. Lawrence Granite (360±5 Ma) and associated subvertical dikes have two magnetizations, N and H. N points north with an intermediate upward inclination (declination, 005°; inclination, −50°; alpha (95), 10°; paleopole 12°N, 120°E). We consider N to have been acquired close to the time of initial cooling. The H magnetization points south with low inclination. H is also the dominant magnetization in the nearby Permo‐Carboniferous Spanish Room Formation. H is considered to be an overprint acquired chemically during the Kiaman Reversed Superchron (Late Carboniferous and Permian). The N magnetization corresponds to a paleolatitude of 31±7°S, very different from the near‐equatorial values expected from the commonly accepted Late Devonian and earliest Carboniferous (Tournaisian) reference paleofields for cratonic North America. This is generally consistent with the hypothesis of a southerly displaced Acadia proposed by Kent and Opdyke, although the displacement is much greater (30° rather than 15°). However, we show that all inclinations observed from Middle to Upper Devonian and Tournaisian rocks from the craton are not significantly different either from the Kiaman paleofield, or from the H overprints observed from Newfoundland. Some of these cratonic studies are not very detailed, but it is noteworthy that the declination observed in the most comprehensive of them (Catskill red beds) also agrees with the Kiaman paleofield and with the H overprints from Newfoundland. Declinations in some other cratonic studies show small departures of only a few degrees (with one exception based on limited sampling and two from the Colorado Plateau which may have been rotated) which could be caused by experimental inaccuracies. Moreover the directions obtained from cratonic studies are 97% reversed, as they are in rocks with Kiaman magnetizations. We suggest, therefore, that the commonly accepted Late Devonian to earliest Carboniferous reference paleofield for North America is either entirely Kiaman in age or has been seriously affected by Kiaman overprinting. Consequently, the 30° of southerly displacement which the N magnetization of the St. Lawrence Granite apears to indicate, and the 15° invoked by Kent and Opdyke, we consider to be illusory. Three Late Precambrian rock units (623 to 606 Ma) have scattered declinations but similar inclinations yielding paleolatitudes of about 35°. The scattered declinations presumable were caused by local pre‐Carboniferous relative rotations. There is little or no evidence of Kiaman overprinting in Precambrian rocks, indicating that overprinting is comparatively superficial and is probably related to Late Paleozoic groundwater circulation.
Spectral analysis of groove spacing on Ganymede
Authors: Grimm Robert E. , Squyres Steven W.
Published: April 2008

Grooves on Ganymede are interpreted as extensional tectonic features whose regular spacing is an indicator of the thickness of a brittle surface layer, or “lithosphere,” at the time of deformation. We have performed a statistical analysis of groove spacing on Ganymede, taking Fourier transforms of 157 photometric profiles across sets of grooves and examining the resultant power spectra for peaks representing the dominant topographic periodicities. The distribution of groove spacing on Ganymede is approximately lognormal, with a minimum of ∼3.5 km, a maximum of ∼17 km, and a mean of 8.4 km. There is no correlation of groove spacing with latitude or longitude. The most striking feature of the geographic distribution is that while groove spacing tends to be quite constant within a given groove set, it can vary substantially from one groove set to another in a geographic region. An exception is the Uruk Sulcus region, with a mean groove spacing of 6.5 km and very little variability. We examine several models that relate the observed groove spacing to lithospheric thicknesses. The most important conclusion is that lithospheric thickness varied significantly from one groove set to the next while the grooves were being formed. We examine four possible causes for the heterogeneous distribution: variation in time of formation during a period in which the global mean heat flow was changing, variation in strain rate from one groove set to the next, local excursions in geothermal gradient caused by variations in the vigor of underlying convective upwellings, and local excursions in geothermal gradient caused by resurfacing events. The first three appear viable, while the fourth requires very rapid groove formation due to the short cooling times involved.
Penecontemporaneous metamorphism, fragmentation, and reassembly of ordinary chondrite parent bodies
Authors: Grimm Robert E.
Published: April 2008

The occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three (H, L, LL) parent asteroids of ∼100‐km radius (e.g., Miyamoto et al., 1982). This conventional (“onion‐shell”) model predicts an inverse relation between petrologic type and metallographic cooling rate, but none has been observed. Scott and Rajan (1981) devised a “metamorphosed‐planetesimal” model to explain this discrepancy, whereby metamorphism occurs in planetesimals a few kilometers in radius, which then accrete to form r ∼ 100 km parent bodies. Metallographic cooling rates are then controlled by burial depth. Thermal and collisional constraints on the metamorphosed‐planetesimal model are examined here, and the model is found to be applicable only to highly insulating, 26Al‐rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface. An alternative model is presented here, in which onion‐shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled. If reassembly times are short (∼days), then metallographic cooling rates would be determined by burial depth in the reaccreted parent body. This model, unlike previous ones, can explain both coherent and incoherent cooling rates of breccia clasts, by collisions during or after metamorphism, respectively.
Geomagnetic perturbations at low latitudes observed by Magsat
Authors: Maeda H. , Kamei T. , Iyemori T. , Araki T.
Published: April 2008

Geomagnetic perturbations (ΔH, ΔD, ΔZ, and ΔF) at low latitudes are studied by using Magsat data during the mission lifetime from November 1979 through May 1980, and the following characteristics are found: (1) ΔH generally shows negative values larger at dusk than at dawn, and it is in good correlation with the Dst index on both dawnside and duskside. The difference between dawnside and duskside shows good correlation with the AE index, (2) ΔD shows a distinct variation near the dip equator only on the duskside, with a small shift of the base line, (3) ΔZ shows a general tendency of being positive in the northern hemisphere and negative in the southern hemisphere, and (4) ΔF generally shows negative values larger at dusk than at dawn, and it changes sinusoidally with latitude. These characteristics are discussed in relation to ionospheric and magnetospheric currents.
Mean ionospheric field correction for Magsat data
Authors: Yanagisawa Masahisa , Kono Masaru
Published: April 2008

To obtain a meaningful anomaly map showing magnetic fields of crustal origin from satellite observations, it is necessary to account adequately for and subtract the fields generated by the currents flowing in the ionosphere. Conventionally, corrections are made separately for each path by empirical methods of fitting and subtracting linear, quadratic, or harmonic functions from the original data. This procedure, however, lacks a sound physical basis. We examined the field components after the main field and the magnetospheric field were subtracted from Magsat data and found certain structures in the residual field. The residual fields at 0600 and 1800 local times were separately averaged over longitude to obtain the effective contribution of ionospheric current system at these local times, which is called the mean ionospheric field (MIF). To obtain the crustal anomalies, the MIF was subtracted from the residual fields. To account for the day‐to‐day variation in the ionospheric current system and for the difference in satellite altitude, anomaly maps are constructed on 5° × 5° grid averages. The dawn and dusk anomaly maps after the correction agree well with each other, showing that the effect of the ionospheric field is well taken care of by the MIF correction. The amplitude of anomalies is large even at comparatively long wavelength, and the maps successfully represent these crustal anomalies which have been greatly reduced in the previous maps.
On the identification of Magsat anomaly charts as crustal part of the internal field
Authors: Meyer J. , Hufen J.‐H. , Siebert M. , Hahn A.
Published: April 2008

Satellite magnetic anomaly charts are derived from the measurements by subtracting a global field model regarded as representative of the core field, after removal of the external field effects. The field model comprises all spherical harmonic terms of the internal field up to a certain degree n, say n = 13. Hence the large‐scale constituents of the crustal field are also removed from the anomaly charts. The calculated magnetic field of a global magnetization model of the earth's crust gives an estimate of the lacking lower‐order crustal terms. It turns out that the omitted crustal field part is of the same order of magnitude as the rest of the field. The geographic distribution of the omitted model field part on a continental scale is closely correlated with the depth of the Moho. By analogy, the same conclusion is drawn with respect to the actual crustal field. This result accounts for the fact that in the satellite charts the continental margins are not very well reflected. Considerable corrections are therefore required if the anomaly charts are intended to display the whole crustal field.
Seafloor spreading anomalies in the Magsat field of the North Atlantic
Authors: LaBrecque J. L. , Raymond C. A.
Published: April 2008

We show that a simple magnetization model for the crust of the North Atlantic Basin will explain the major anomalies of the Magsat intermediate‐wavelength magnetic field over the basin. Our seafloor spreading model incorporates anomalous skewness and enhanced magnetization near the spreading axis and at the Cretaceous Quiet Zones. The anomalies over the young oceanic crust indicate a decrease in susceptibility away from the spreading axis. The enhanced magnetization over the Cretaceous Quiet Zone could be due to an increased main field intensity or a change in the spreading center processes during the Cretaceous. Though similar observations are not readily apparent for other basins, the North Atlantic correlation indicates that an investigation of the remanent magnetization of the ocean crust could provide a standard for the evaluation of the intermediate‐wavelength magnetic field. The models presented in this paper demonstrate that a sizable percentage of the intermediate‐wavelength energy in the Magsat field is derived from sources within the earth's crust. It also demonstrates that remanent magnetization is an important source of the Magsat field over oceanic areas. We seek to demonstrate in this paper that although components of the main field are likely to contribute to the intermediate‐wavelength scalar field, we can correlate the anomaly field over the North Atlantic to the well‐studied thermal remanent magnetization of the oceanic crust with a minor component of induced magnetization associated with younger oceanic crust.
Magsat scalar magnetic anomalies at northern high latitudes
Authors: Coles R. L.
Published: April 2008

A new scalar magnetic anomaly map derived from Magsat data at high northern latitudes shows important correlations with major geologic and tectonic features. Positive magnetic anomalies over some high‐grade metamorphic Precambrian terranes contrast with negative anomalies over lower‐grade Precambrian terranes. A positive anomaly is associated with the Aleutian arc‐trench system, a region of active plate convergence. Negative anomaly fields are associated with the Nansen‐Gakkel Ridge, a region of plate accretion in the Arctic Basin, with its extension into the seismically active Cherskiy Mountains in eastern Siberia, and with the Labrador Sea, an extinct spreading center. However, a positive anomaly occurs over Iceland, on the mid‐Atlantic spreading ridge system, and an intense positive anomaly occurs over the Alpha Ridge in the Arctic Basin. The relations among the Magsat orbit, the rotation of the earth, and the movements of the auroral oval have major effects on the spatial distribution of acceptable data and, therefore, on map resolution.
Spherical cap harmonic analysis
Authors: Haines G. V.
Published: April 2008

The solution of Laplace's equation, in spherical coordinates, is developed for the boundary value problem appropriate to fitting the geomagnetic field over a spherical cap. The solution involves associated Legendre functions of integral order but nonintegral degree. The basis functions comprise two infinite sets, within each of which the functions are mutually orthogonal. The series for the expansion of the potential can by design be differentiated term by term to yield uniformly convergent series for the field components. The method is demonstrated by modeling the International Geomagnetic Reference Field 1980 at the earth's surface and upward continuing it to 300 and 600 km. The rate of convergence of the series is rapid, and standard errors of fit as low as the order of a nanotesla can be obtained with a reasonable number of coefficients. Upward continuation suffers from not considering data outside the cap, the deterioration being confined to the boundary at low continuation altitudes but spreading inward over the cap with increasing altitudes. At 600 km the standard error of upward continuation is about 5 times the standard error of fit. Both the fit and the upward continuation can be greatly improved at a given truncation level by subtraction of a known spherical harmonic potential determined from data from the whole earth.
Extraction of magnetic anomalies of crustal origin from Magsat data over the area of the Japanese Islands
Authors: Nakagawa Ichiro , Yukutake Takesi , Fukushima Naoshi
Published: April 2008

Three component data along the Magsat paths in the area of 110°E–170°E and 8°N–68°N were fitted by polynomial series of the coordinates of the satellite position along the path. The coefficients decrease sharply up to the sixth degree and become almost constant. The fields represented by the sixth degree polynomials were subtracted from the data to remove the core and the external fields. For the altitude correction a rectangular harmonic analysis was applied to the residuals thus obtained by employing local Cartesian coordinates. The residual fields were expanded into double Fourier series of the maximum order of n = m = 3 over areas of 4000×4000 km and 2000×2000 km, and anomaly fields were computed for the satellite altitude. It was revealed that (1) an intense positive anomaly of the vertical component ran along the Kuril trench, (2) the Japan Sea was covered with a negative vertical anomaly in the northeastern part, and with a positive in the southwestern part, (3) there was a pair of positive and negative vertical anomalies covering the area from northern Korea to the East China Sea, suggesting the existence of an extremely localized magnetization like a dipole near the surface in the southwestern tip of the Korean Peninsula.
Crustal structure of the Churchill‐Superior boundary zone between 80° and 98°W longitude from Magsat anomaly maps and stacked passes
Authors: Hall D. H. , Noble I. A. , Millar T. W.
Published: April 2008

A modeling technique using spherical shell elements and equivalent dipole sources has been applied to Magsat signatures at the Churchill‐Superior boundary in Manitoba, Ontario, and Ungava. A large satellite magnetic anomaly (12 nT amplitude) on POGO and Magsat maps near the Churchill‐Superior boundary was found to be related to the Richmond Gulf aulacogen. The averaged crustal magnetization in the source region is 5.2 A m−1. Stacking of the magnetic traces from Magsat passes reveals a magnetic signature (10 nT amplitude) at the Churchill‐Superior boundary in an area studied between 80°W and 98°W. Modeling suggests a steplike thickening of the crust on the Churchill side of the boundary in a layer with a magnetization of 5 A m−1. Signatures on aeromagnetic maps are also found in the source areas for both of these satellite anomalies.
The May 18, 1980, eruption of Mount St. Helens: 1. Melt composition and experimental phase equilibria
Authors: Rutherford M. J. , Sigurdsson Haraldur , Carey Steven , Davis Andrew
Published: April 2008

The Mount St. Helens, May 18 pumice is a dacite containing 60% glass by weight and phenocrysts of plagioclase, orthopyroxene, amphibole, titaniferous magnetite, and ilmenite. The glass is uniform in composition, a rhyodacite with 73 wt % SiO2; the phenocrysts are also uniform in composition except for the plagioclase, which has cores averaging An57 and rims averaging An49. Analyses of seven pairs of coexisting Fe‐Ti oxides in a representative sample of the light pumice were recast using various mineral calculation procedures; they yielded temperatures ranging from 920° to 940°C and a ‐log ƒO2 of 10.3–10.1. Electron microprobe analyses of 57 glass inclusions trapped in plagioclase phenocrysts in the light pumice showed little deviation from an average rhyodacitic composition (69.90±0.87 wt % SiO2) when special care was taken to account for Na loss during the analysis. The difference between the average total of these glass inclusion analyses and 100% is 4.6±1 wt %, which is interpreted to be volatiles dissolved in the glass. On an anhydrous basis the average glass inclusion composition is identical to the matrix glass, indicating that neither underwent significant fractionation after melt was trapped by the plagioclase. Experimentally determined phase relations for the representative dacite sample place limits on conditions in the May 18 Mount St. Helens magma chamber, assuming that the dissolved volatiles were 4.6±1 wt % and the temperature was 920°–940°C. Hydrothermal experiments over a range of P, T, and ƒO2 indicate that at no pressure is the observed phase assemblage and residual melt chemistry produced when PH2O = PTotal. Experiments using CO2‐H2O fluids to achieve PH2's less than PFluid did reproduce the observed residual melt chemistry and an An50 plagioclase at a specific set of conditions, i.e., atƒO2's between the NNO and MNO buffers, at a PFluid of 220 MPa (2.2 kb), and at a PH2 = 110 MPa (all at 920°–940°C). Amphibole was not stable under these conditions but possibly would be if the PH2 / PFluid ratio was raised to 0.7 or if fluorine were added to the experimental system. It is concluded that just prior to eruption, the upper part of the Mount St. Helens magma chamber was at a pressure of 220±30 MPa corresponding to a depth of 7.2±1 km, PH2 was 0.5 to 0.7 PTotal, and the temperature was 930°±10°C.
The May 18, 1980 eruption of Mount St. Helens: 2. Modeling of dynamics of the Plinian Phase
Authors: Carey Steven , Sigurdsson Haraldur
Published: April 2008

The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7–10 km depth at a rate of 1.94×107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920°–940° C), and confining pressure (190–250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95‐m‐diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model‐derived vent diameters based on 0.1‐MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.
A Middle/Late Silurian paleomagnetic pole from limestone reefs of the Wabash Formation, Indiana, U.S.A
Authors: McCabe Chad , Van der Voo Rob , Wilkinson Bruce H. , Devaney Kathleen
Published: April 2008

A Middle/Late Silurian paleomagnetic pole has been obtained from reefal limestones in north central Indiana. Eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization. One additional site gave the same characteristic direction after great circle analysis. These 12 site‐level directions were used to calculate the mean paleomagnetic pole. Reef flank beds dip steeply (about 38°) away from reef centers. Field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds. Present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12°. A 12° tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement, indicating that the magnetization was acquired early. The presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence. The mean of the site paleomagnetic poles (with sites from steeply inclined flank beds corrected for 12° of tilt) is 16.8°N, 124.9°E (K = 74, A95 = 5.1°), in good agreement with the Middle Silurian Rose Hill pole for North America.
Prefolding and overprint magnetic signatures in Precambrian (∼2.9–2.7 Ga) Igneous rocks from the Pilbara Craton and Hamersley Basin, NW Australia
Authors: Schmidt P. W. , Embleton B. J. J.
Published: April 2008

A total of 446 oriented samples have been collected from an Archaean intrusion and the Fortescue Group volcanics and intrusions in the Pilbara and Hamersley regions of northwestern Australia. The paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field (af) and thermal demagnetization. The most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization. Evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy. Application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions. A prefolding pole position from the “Millindinna Complex,” is latitude = 11.9°S, longitude = 161.3°E (dp = 6.8°, dm = 8.4°) and is correlated with a radiometric age of 2.86±0.02 Ga. Folding in the “Millindinna Complex” occurred prior to the deposition of the Fortescue Group. New geochronological data indicate that the extrusion of the flood basalts commenced about 2.8 Ga. This provides a minimum age for the magnetization of the “Millindinna Complex.” Deformation responsible for the formation of the mobile belt separating the Pilbara and Yilgarn blocks during the Ophthalmian Orogeny has affected the Fortescue Group and produced a moderate degree of folding in the southern part of the Hamersley Basin. In the region of “Millindinna Complex” outcrop, the Fortescue Group is generally flat lying. Prefolding pole positions from the Fortescue Group are for the Mount Roe Basalt, latitude = 52.4°S, longitude = 178.0°E (dp = 6.4°, dm = 9.1°) and for the Mount Jope Volcanics, latitude = 40.5°S, longitude = 128.7°E (dp = 19.9°, dm = 20.8°). The overprint magnetization in the Mount Jope Volcanics yields a pole position, latitude = 35.0°S, longitude = 211.5°E (dp = 2.8°, dm = 5.5°) and is interpreted as having been acquired during folding relating to the formation of the Ophthalmian Fold Belt.
Thermal evolution models for the Earth
Authors: Christensen Ulrich R.
Published: April 2008

Thermal evolution models for the earth which are based on a parameterization of the convective heat transport are critically reexamined. Traditionally, it has been assumed that the temperature dependence of the mantle viscosity implies that internal temperature and convective heat loss are strongly coupled. Recent numerical work on the heat transport by variable viscosity convection demonstrates that the dependence of the heat flow on the mantle temperature may in fact be much weaker than expected. I compare thermal evolution models with strong and weak dependence of the heat loss on the temperature. With the weaker dependence, plate velocities and heat flow in the Archean were not more than 50% higher than today, while with the strong dependence, much larger differences are predicted. In the former case the Archaean mantle temperatures are somewhat higher, and the present‐day ratio of radioactive heat production over heat loss (Urey ratio) is 50% or slightly less. The Urey ratios in the traditional parameterized evolution models are >70%. The predictions of both kinds of models are compared with the independent geological, geochemical, and palaeomagnetic evidence. Although this evidence is subject to some uncertainties, it favors in every case the evolution models based on a weak coupling of heat loss to the interior temperature.