bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial distribution and mechanisms of earthquakes in the southern New Hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations
Authors: Coudert E. , Isacks B. L. , Barazangi M. , Louat R. , Cardwell R. , Chen A. , Dubois J. , Latham G. , Pontoise B.
Published: February 2008

In 1977 a wide‐aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southern New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subduction zone. Surprisingly, small differences (less than about 10 km) were found between the locations of shallow‐ and intermediate‐depth earthquakes as located by the temporary local network and by the International Seismological Centre using worldwide stations. Analysis of the P wave travel time data from the OBS station closest to the trench shows limited evidence for a high‐velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by the spatial distribution of small size events located in this study. Shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blocks that are differentially uplifted along mainly NW‐SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the volcanic arc. Well‐determined depths of these events are between 11 and 22 km. The well‐located intermediate‐depth events define a 20‐km‐thick Benioff zone that has a dip of about 70°. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short‐term sample of small events determined by the local network and the long‐term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate‐depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern New Hebrides arc where the trench begins to curve around to the east.
Theory of time‐dependent rupture in the Earth
Authors: Das S. , Scholz C. H.
Published: February 2008

Using the concepts of fracture mechanics, we develop a theory of the earthquake mechanism which includes the phenomenon of subcritical crack growth. The theory specifically predicts the following phenomena: slow earthquakes, multiple events, delayed multiple events (doublets), postseismic rupture growth and afterslip, foreshocks, and aftershocks. The theory also predicts that there must be a nucleation stage prior to an earthquake and suggests a physical mechanism by which one earthquake may ‘trigger’ another. These predictions are obtained by combining two fundamental concepts. The first is that
, and the second that
, where k is the stress intensity factor, Δτ is stress drop, X is rupture length,
is rupture velocity, C is a geometrical factor, and K0, V0, and n are material constants. The first is a fundamental result of fracture mechanics; the second describes stress corrosion cracking, a well‐established physical process that results in subcritical crack growth. We investigate in detail two phenomena of special interest and which are not predicted by ordinary fracture mechanics: nucleation and delayed multiple events. In the first case we find that all earthquakes must be preceded by quasi‐static slip over a portion of their rupture surfaces, but it may be difficult to detect in practice. In the second case we studied two pairs of delayed multiple events that were separated by the same ‘barrier’ in order to calculate n. We find that the stress corrosion index n is ∼24.
Basement drilling in the western Atlantic Ocean: 1. Magma fractionation and its relation to eruptive chronology
Authors: Flower Martin F. J. , Robinson Paul T.
Published: February 2008

Petrologic/geochemical studies of basalts drilled in Cretaceous oceanic crust (DSDP legs 51–53 in the western Atlantic Ocean) are synthesized in quantitative magma fractionation models. From least squares analysis of whole‐rock, glass, and phenocryst phase compositions it is concluded that fractionation by crystallization of olivine, plagioclase, and clinopyroxene involved gravitative separation of mafic phases from plagioclase. Flotation of plagioclase together with disequilibrium reaction morphologies suggest that crystallization began shortly after the release of primitive liquids from the mantle. Polybaric fractionation and mixing of derivative magma batches are indicated by evidence for solid‐liquid reaction and low‐pressure reequilibration. Distinct eruptive series bounded by magnetic and other stratigraphic discontinuities correspond to chemically defined magma fractionation series. The latter are probably differentiated from one another through small differences in degree of partial melting. Their lithophile element character shows evidence for homogeneous, ‘depleted’ source material persisting during crustal construction at the Atlantic ridge axis. The whole‐rock and glass selvedge compositional averages for eruptive units reflect the importance of posteruptive phenocryst redistribution and syn‐eruptive changes in liquid fraction compositions.
Basement drilling in the western Atlantic Ocean: 2. A synthesis of construction processes at the Cretaceous ridge axis
Authors: Flower Martin F. J. , Robinson Paul T.
Published: February 2008

Petrologic, geochemical, and geophysical studies of cores from DSDP sites 417 and 418 show that the Cretaceous eruptive crust was formed at a stable slow‐spreading axis similar to the present Mid‐Atlantic Ridge, probably not close to major transform fractures. Interpretations of whole‐rock and glass chemical variation suggest that magma fractionation occurred in temporally and spatially transient storage reservoirs at various depths between the zone of magma segregation and the ridge crest. We suggest that melt generation, uprise, and eruption are part of a related episode of activity involving rapid upward propagation of fractures. Eruptive episodes are recorded by stratigraphic sequences of contiguous, mostly cogenetic, lavas, separated by breccias, sediment, and/or extreme alteration zones suggestive of lengthly quiescent intervals. The section drilled at 417D comprises three such episodes (in 365 m of basement), whereas seven are recorded in 418A (in 540 m). Patterns of downhole changes in magnetic inclinations and polarities strongly support the suggestion that iterative burial of successive lava episodes occurs with crustal spreading from the axis. This model predicts that the eruptive layer is imbricated and that it shows increasing complexity of lithology, chemical compositions, and magnetic character with depth. Studies of crack distribution and laboratory measurements of core physical properties indicate preferential zones of water circulation which decrease with depth. Permeability of the lava pile is largely controlled by the imbricate eruptive structure.
The use of height data in gravity field approximation by collocation
Authors: Forsberg René , Tscherning C. C.
Published: February 2008

The accuracy of a gravity field model depends on the amount of available data and on the variation of the gravity field. When topographic height data are available, for example, in the form of a digital terrain model, it is possible to smooth the gravity field on a local scale by removing the gravitational effects calculated from models of the topographic masses. In this way, significant improvements of the prediction results are obtained in mountainous areas. In this paper we describe methods for the calculation of such gravitational terrain effects, applicable in collocation approximation of the gravity field. The terrain effects on gravity field quantities such as gravity anomalies, deflections of the vertical, and geoid undulations are calculated using a system of rectangular prisms, representing either a quasi‐traditional model of the topography and the isostatic compensation or a residual terrain model, where only the deviation of the topography from a mean elevation surface is considered. To test the terrain reduction methods, numerical prediction experiments have been conducted in the mountainous White Sands area, New Mexico. From gravity anomalies spaced approximately 6 arc min apart, other known gravity anomalies and deflections of the vertical were predicted using collocation. When using terrain effects calculated on the basis of 0.5 × 0.5 arc min point heights, the rms errors decreased by a factor of nearly 3 to 1 arc sec for the deflections and 3–4 mGal for the gravity anomalies, quite insensitive to the actual type of terrain reduction used. The feasibility of using topographic reductions in collocation is thus effectively demonstrated.
Isostasy of fracture zones in the Atlantic Ocean
Authors: Kogan M. G. , Kostoglodov V. V.
Published: February 2008

The cross‐spectral method of studying the isostasy was applied to profiles of gravity and topography which run across fracture zones in the Atlantic Ocean between 40°N and 40°S; 30 sections in total were used, each section 400 km long. In wavelengths 25 to 80 km the coherence of the free‐air gravity anomaly and topography attains 0.8, while the phase is close to zero, which indicates that the disturbances in gravity are controlled by the topography and its isostatic compensation. The experimental isostatic response function (admittance) deduced in this study is compared with previous results which relied on different spatial distribution of data. A single mechanism of isostasy can explain well the gravity anomalies across fracture zones and across the median valley of the Mid‐Atlantic Ridge. A basic feature of this mechanism is the elastic lithosphere 6 to 9 km in thickness. We show that the topography and gravity anomalies over the Mid‐Atlantic Ridge creast are essentially three‐dimensional. Nevertheless, the admittance is reliably computed from a set of profiles by using the method of D. McKenzie and C. Bowin. This possibility is provided by the favorable pattern of the topography spectrum in the area under consideration.
Gravity‐induced stresses near topography of small slope
Authors: McTigue David F. , Mei Chiang C.
Published: February 2008

Topographic modification of gravity‐induced near‐surface stresses results in significant departures from a lithostatic state. A perturbation scheme provides approximate analytical solutions for plane strain of an elastic half‐space with an irregularly shaped free surface of small characteristic slope, ε. The leading order effect of the topography is equivalent to that of a distributed normal load on a plane boundary, and the correction at order ε is due to a distributed shear traction on a plane boundary. In the near‐surface region, these two effects contribute at the same order to departures from the lithostatic stress field. The contribution of the shearing has been neglected in most previous analyses. We present explicit solutions for several particular geometries: a symmetric ridge or valley, an adjacent ridge and valley, a shelf, and a sharp‐crested ridge. Notable features include (1) for a sufficiently steep ridge, the horizontal normal stress is compressive at the crest and decreases, possibly becoming tensile, with depth; (2) large horizontal tension is induced in a valley bottom. At greater depth, these effects vanish, and the stresses approach lithostatic. Similar analysis is applied to a half‐space under a far‐field tectonic compression or tension. The result shows that the leading order effect of topography is equivalent to a distributed shear traction on a plane surface. Regional horizontal compression can be significantly reduced, or even change to tension, in the neighborhood of a topographic high.
Influence of strain rate on dilatancy and strength of Oshima granite under uniaxial compression
Authors: Sano Osam , Ito Ichiro , Terada Makoto
Published: February 2008

Uniaxial compression tests have been conducted on Oshima granite under various constant axial strain rates ranging from 10−8 to 10−4. The results showed that the strength and the acoustic emission rate increased exponentially with increasing strain rate. The inelastic volumetric strain rate defined by the differentiation with respect to the stress increased with decreasing strain rate. The redistribution of microcracks due to subcritical crack growth was considered theoretically, and the equations derived from the theory were compared with the experimental results. The agreement between the theoretical and experimental results shows that stress corrosion plays not only a major role in the brittle creep under constant load but also dominates the strain rate effects on strength and dilatancy observed in the constant strain rate loadings.
Numerical methods for establishing solutions to the inverse problem of electromagnetic induction
Authors: Parker Robert L. , Whaler Kathryn A.
Published: February 2008

A previous paper (Parker, 1980) sets out a theory for deciding whether solutions exist to the inverse problem of electromagnetic induction and outlines methods for constructing conductivity profiles when their existence has been demonstrated. The present paper provides practical algorithms to perform the necessary calculations stably and efficiently, concentrating exclusively on the case of imprecise observations. The matter of existence is treated by finding the best fitting solution in a least squares sense; then the size of the misfit is tested statistically to determine the probability that the value would be met or exceeded by chance. We obtain the optimal solution by solving a constrained least squares problem linear in the spectral function of the electric field differential equation. The spectral function is converted into a conductivity profile by transforming its partial fraction representation into a continued fraction, using a stable algorithm due to Rutishauser. In addition to optimal models, which always consist of delta functions, two other types of model are examined. One is composed of a finite stack of uniform layers, constructed so that the product of conductivity and thickness squared is the same in each layer. The numerical techniques developed for the optimal model serve with only minor alteration to find solutions in this class. Models of the second kind are smooth. A special form of the response is chosen so that the kernel functions of the Gel'fand‐Levitan integral equation are degenerate, thus allowing very stable and numerically efficient solution. Unlike previously published methods for finding conductivity models, these algorithms can provide solutions with misfits arbitrarily close to the smallest one possible. The methods are applied to magnetotelluric observations made by Larsen in Hawaii.
Caledonian plutonism in Britain: A summary
Authors: Brown G. C. , Cassidy J. , Locke C. A. , Plant J. A. , Simpson P. R.
Published: February 2008

The Caledonian granites of Britain are a wide‐ranging suite of pretectonic and posttectonic tonalite‐granodiorite‐granite intrusions which were emplaced in the time interval 650–380 Ma ago. The volume of magmatic products and the frequency of intrusions increased with time as closure of the Iapetus ocean occurred across the Caledonide belt in the British Isles. Early pretectonic and posttectonic intrusions (older than about 410 Ma) are distinguished from a later (post‐410 Ma) group using geophysical (gravity, aeromagnetic, and heat production) and geochemical data. The evidence is interpreted in terms of a trend with time from relatively local thermal and crust‐dominated melting events to a final widespread melting event at deeper levels. The latter event produced abundant intrusions with characteristics that resemble the calc‐alkaline batholiths of modern destructive margins. Finally, we propose that crustal growth during the Caledonian period took place by northward lateral accretion of the European continental foreland and by vertical accretion of continental crust on both margins by underplating and by later intrusive magmatism.
Petrogenesis of garnet two‐mica granites in the Ruby Mountains, Nevada
Authors: Kistler R. W. , Ghent E. D. , O'Neil J. R.
Published: February 2008

Garnet‐two‐mica granites of Jurassic (160 ± 3 m.y.) and Cretaceous (83 ± 1.3 m.y.) ages intrude amphibolite facies metasedimentary rocks of Precambrian age and lower Paleozoic sedimentary rocks in the northern part of the Ruby Mountains, Nevada. High initial 87Sr/86Sr and high values of δ18O for minerals suggest a dominantly sedimentary source for the magmas. Total rare‐earth contents are low and less fractionated (CeN/YbN = 10) than granitic rocks of similar composition in the Sierra Nevada and show a negative Eu anomaly (Eu/Eu* = 0.57). These data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon, sphene, and apatite. Minor euhedral garnets are manganese‐rich and magnesium‐poor (al60sp34py3.6gr2.8) compared to garnet (al79sp6py13gr2) in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns. Fractionation of magnesium‐iron between garnet and biotite, and of sodium‐potassium between K‐feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures, in the range of 365° to 505°C, whereas oxygen isotope equilibration temperatures for two granites are higher at 480° and 570°C. The compositions of the granites plot near minimum melting compositions in the water‐saturated quartz‐albite‐orthoclase system. Comparison of muscovite plus quartz stability, the water‐saturated granite solidus, and experimental garnet‐melt equilibria suggests pressures of crystallization no lower than about 3.5 kbar. This pressure is compatible with that estimated from garnet‐plagioclase‐sillimanite‐quartz equilibria for the intruded metasedimentary rocks.
The Late Archaean Qôrqut Granite Complex of southern west Greenland
Authors: Brown Michael , Friend C. R. L. , McGregor V. R. , Perkins W. T.
Published: February 2008

Granites and granite pegmatites composing the ∼ 2550‐Ma Qôrqut granite complex occur in a SSW‐NNE trending linear belt >150 km long extending through the Buksefjorden‐Ameralik‐Godthåbsfjord region of southern West Greenland. The main body of the complex crops out over a distance of 50 km from Ameralik to Kapisigdlit kangerdluat and reaches a maximum outcrop width of 18 km between Storø and Qôrqut. Around Qôrqut the complex comprises three main groups of granites: early leucocratic granites, various grey biotite granites, and late aplogranite‐granite pegmatites. Within the 1500‐m vertical section available in this area the complex has a tripartite structure comprising a lower zone dominantly of polyphase granite, an intermediate zone where country rock occurs as rafts in polyphase granite with a complex sheeted structure, and an upper zone dominantly of country rock sheeted by granite. Fifty‐two specimens of granite have been analyzed for major, minor, and some trace elements. Geochemical variation within the complex is consistent with either fractional crystallization or partial melting, but in both cases, feldspar + biotite must have been involved either as fractionating phases or as residual phases during melting to account for the trace element chemistry. Two possible models for the generation of the complex are either anatexis of granulite facies rocks in the lower crust following an influx of volatiles and heat from the mantle or melting at intermediate depths of amphibolite facies rocks with volatiles supplied by breakdown of hydrous phases.
Moon: Near‐infrared spectral reflectance, A first good look
Authors: McCord Thomas B. , Clark Roger N. , Hawke B. Ray , McFadden Lucy A. , Owensby P. D. , Pieters Carle M. , Adams John B.
Published: February 2008

A positive identification of the minerals olivine, plagioclase, and several types of pyroxenes were made at several locations on the lunar surface by using remote measurements. For example, the crater Aristarchus is found to have an average pyroxene composition of augite, and plagioclase is obviously present. A dark mantle deposit in the crater J. Herschel is at least partly composed of a mixture of 70% olivine and 30% pyroxene. These determinations were possible because the reflectance spectra for 10–20 km diameter lunar areas have been measured for the first time in the IR spectral region (0.65–2.5 μm) with sufficient spectral resolution and photometric precision to define mineral electronic absorption bands. The reflectance for all lunar regions observed (over 100 to present) continues to increase toward longer wavelengths to at least 2.5 μm, and several mineral absorption bands appear. The telescopic spectra are of similar quality and contain features similar to laboratory spectra of lunar samples. The absorption features in several spectra have been quantitatively analyzed using newly developed computer processing techniques, including thermal flux removal and absorption band fitting, to produce these mineral identifications. Detection and quantitative analysis of these absorptions provide a much improved basis for remotely determining and mapping surface mineralogy quantitatively from the ground or using airborne or spacecraft platforms.
Seismic refraction shooting on the continental margin west of the Outer Hebrides, northwest Scotland
Authors: Jones E. J. W.
Published: February 2008

Seventeen sonobuoy refraction profiles have been shot to determine the nature of the basement and the broad pattern of sedimentation on the continental margin west of the Outer Hebrides, NW Scotland. Under much of the shelf, crystalline rocks (Vp > 5.1 km/s) lie within 100 m of the seafloor, the basement being largely an extension of the Precambrian (Lewisian) metamorphic complex of western Scotland. Vp/Vs gives Poisson's ratios (σ) of 0.26–0.30 for the Lewisian, values which are significantly higher than σ in the deep crust under northern Britain, implying important compositional differences. Comparisons with ultrasonic velocities in rocks from the Scourian (∼ 2700 Ma) and Laxfordian (∼ 2200–1500 Ma) belts of the Scottish mainland suggest that the Lewisian on the inner continental shelf is predominantly Laxfordian (Vp ∼ 5.5 km/s). Higher‐velocity rocks, probably Scourian with only a moderate degree of Laxfordian reworking (Vp ∼ 5.9 km/s), and Cenozoic intrusions occur locally. Two seismic profiles indicate that the outer continental shelf may be underlain by a zone of dense Scourian/early Laxfordian granulites, whose presence possibly influenced the siting of the continental slope.The sediments covering the basement are generally thin.Thicknesses exceeding 1 km are restricted to a fault‐bounded trough off the Isle of Lewis and to the outer shelf and continental slope. The deposits can be divided into Cenozoic (1.7–1.9 km/s) and Mesozoic (3.0–4.4 km/s)units, velocity variations in the latter probably reflecting the abundance of early Cenozoic basic intrusions. The distribution of the Mesozoic is partly controlled by faults which appear to be related to early Precambrian shear zones in the basement. These highly foliated belts seem to have facilitated stress relief by normal faulting during Permo‐Triassic rifting activity. The general lack of subsidence of the Outer Hebridean block is attributed to the buoyancy of granitic material incorporated at an early stage of its history.
Petrogenesis of Uivfaq Iron, Disko Island, Greenland
Authors: Bird John M. , Goodrich Cyrena Anne , Weathers Maura S.
Published: February 2008

Massive blocks of native iron and iron carbide, weighing up to 22 tons, have been found at Uivfaq, Disko Island, western Greenland. The iron blocks are associated with Tertiary basalts that erupted during rifting between Greenland and Baffin Island, early in the opening of the Davis Strait. The ‘iron’ is an asemblage of iron and iron carbide (cohenite) and various silicate, sulphide, and oxide inclusions containing highly reduced and iron‐rich phases, including fayalite, wustite, troilite, ulvospinel, ilmenite, ferriferous pigeonite, and FeO‐rich glass. The chemistry and petrography of the inclusions are used to derive a paragenetic history for the Uivfaq iron assemblage. Petrographic study of a massive Uivfaq iron block shows that the block formed through accumulation of partly molten spherules of iron and cohenite,∼0.5 to 1.0 mm in diameter. Basaltic liquid was trapped by the accumulating metal spherules and reacted with the metal to form various highly reduced and iron‐enriched immiscible liquids. These liquids solidified as inclusions in the iron mass. Various models are considered for the origin of the iron‐cohenite spherules; at present, a carbon‐reduction mechanism is preferred. The association of graphite‐rich plagioclase‐spinel xenoliths (thought to be derived from sediments) with the iron masses suggests a genetic relation between the two. We suggest that graphite was assimilated by mantle‐derived basaltic magma. The carbon reduced iron oxide components of the basalt at a depth of 3 km or less and formed small immiscible droplets of high‐carbon iron‐carbon liquid. These droplets were erupted with the magma. The droplets settled and accumulated near or at the base of a flow or sill to form massive iron blocks.
Investigation of induced magnetization variations caused by triaxial stresses
Authors: Zlotnicki J. , Pozzi J. P. , Cornet F. H.
Published: February 2008

Laboratory experiments have been conducted to investigate the change in induced magnetization caused by triaxial stress variations on andesite specimens. Induced axial magnetization was found to decrease with the increment of axial load (as is the case for uniaxial stress conditions) for a constant confining pressure, while induced radial magnetization increased with axial load increments. This relationship between stress changes and induced magnetization variation was found to depend on the confining pressure magnitude and the maximum axial stress previously reached: the larger the confining pressure, the larger the effect on induced axial magnetization, but the smaller the effect on induced radial magnetization; the larger the maximum differential stress, the smaller the effect of stress variation on both axial and radial induced magnetization except for uniaxial stress condition. For small maximum differential stresses or maximum axial stress conditions close to the maximum load bearing capacity of the material, this relationship was found to be linear so that a tensorial formulation could be proposed for small magnetic fields. This led to the definition of a fourth‐order tensor, which was called the piezomagnetic tensor.The important geophysical implications of these results is that the influence of deviatoric stress variations on induced magnetization increases with depth, provided the maximum differential stress supported in the past by the material is not close to that necessary for failure to occur; for this last instance the influence of a pure deviatoric stress variation is independent of the spherical component of the stress state. Accordingly, pore pressure variations could generate induced magnetization variations for those rocks which have not been loaded close to failure.
Interpretation of seismic data from hydraulic fracturing experiments at the Fenton Hill, New Mexico, hot dry rock geothermal site
Authors: Aki Keiiti , Fehler Michael , Aamodt R. L. , Albright J. N. , Potter R. M. , Pearson C. M. , Tester J. W.
Published: February 2008

An attempt was made to synthesize the results of active seismic experiments carried out by the Los Alamos National Laboratory's Hot Dry Rock Project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot, low‐permeability rock. Interpretation of data from several reflection, transmission, and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 104 Hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small‐scale cracks. We found an interesting coincidence between the mean square fluctuation of P arrival time arid Q−1 of coda waves. Both are increased by nearly the same amount (35∼40%) when fluid pressure in the reservoir is increased. Another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of P arrival time. These results suggest that the seismic attenuation in the fractured region is caused primarily by scattering.
Thermal isostasy: Response of a moving lithosphere to a distributed heat source
Authors: Sandwell David T.
Published: February 2008

Spreading ridges and hot spot swells are identified by their high surface heat flow, shallow seafloor, and high geopotential. To understand these and other thermotectonic features, the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three‐dimensional, time‐independent heat source. The heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes. The Fourier transforms of three Green's functions (response functions), which relate the three observable fields to their common heat source, are determined analytically. Each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source. However, the ratio of the gravity response function to the topography response function (i.e., gravity/topography transfer function) is nearly isotropic and has a maximum lying between the flexural wavelength and 2π times the thickness of the thermal boundary layer. The response functions are most useful for determining the surface heat flow, seafloor topography, and geopotential for complex lithospheric thermal structures. In practice, these three observables are calculated by multiplying the Fourier transform of the heat source by the appropriate response function and inverse transforming the products. Almost any time‐independent thermotectonic feature can be modeled using this technique. Included in this report are examples of spreading ridges and thermal swells, although more complex geometries such as ridges offset by transform faults and RRR‐type triple junctions can also be modeled. Because forward modeling is both linear and computationally simple, the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields.
Geomagnetic local and regional harmonic analyses
Authors: Alldredge Leroy R.
Published: February 2008

Procedures are developed for using rectangular and cylindrical harmonic analyses in local and regional areas. Both the linear least squares analysis, applicable when component data are available, and the nonlinear least squares analysis, applicable when only total field data are available, are treated. When component data are available, it is advantageous to work with residual fields obtained by subtracting components derived from a harmonic potential from the observed components. When only total field intensity data are available, they must be used directly. Residual values cannot be used. Cylindrical harmonic analyses are indicated when fields tend toward cylindrical symmetry; otherwise, rectangular harmonic analyses will be more advantageous. Examples illustrating each type of analysis are given.
Extremal travel time inversion of explosion seismology data from the Eastern Snake River Plain, Idaho
Authors: Priestley Keith , Orcutt John
Published: February 2008

We have inverted travel time data from seismic refraction profiles within the Snake River Plain, a volcanic‐tectonic depression in southern Idaho, for crustal and uppermost mantle compressional velocity structure. The data in the vicinity of the youngest, northeastern volcanics in Yellowstone require the presence of a significant low velocity zone in the lower crust at depths between 20 and 40 km in order to explain the presence of a caustic at short ranges resulting from the crust‐mantle transition. Data to the west are compatible with a model in which the velocity increases monotonically with depth. These observations are consistent with other data including the progressively lower crustal resistivity toward the northeast, the dated progression of volcanics from the southwest to the northeast at a rate of approximately 3.5 cm yr−1 and surface wave dispersion studies. The velocity reversals required by the data presented here and surface wave studies in the area are almost certainly related to elevated temperatures in the lower crust which approach or exceed the solidus of the deep crustal composition. The entire crustal section in the eastern Snake River Plain is characterized by very high velocities and can be represented by models with no first‐order discontinuities with the possible exception of the Moho or crust‐mantle transition.
The seasonal and global behavior of water vapor in the Mars atmosphere: Complete global results of the Viking Atmospheric Water Detector Experiment
Authors: Jakosky Bruce M. , Farmer Crofton B.
Published: February 2008

The water vapor content of the Mars atmosphere was measured from the Viking Orbiter Mars Atmospheric Water Detectors (MAWD) for a period of more than 1 Martian year, from June 1976 through April 1979. Results are presented in the form of global maps of column abundance for 24 periods throughout each Mars year. The data reduction incorporates spatial and seasonal variations in surface pressure and supplements earlier published versions of less complete data. Column abundances vary between 0 and about 100 precipitable microns (pr μm), depending on location and season, while the entire global abundance varies seasonally between an equivalent of about 1 and 2 km3 of ice. The first appearance of vapor at nonpolar latitudes as northern summer approaches and the drop in abundance at mid‐latitudes as summer ends both strongly imply the existence of a seasonal reservoir for water within the regolith. There appear to be no net annual sources away from the poles that contribute significant amounts of water. However, the strong annual gradient of vapor from north to south implies a net annual flow of vapor toward the south. This southward flow may be balanced by a northward flow during the global dust storms, by transport in the form of clouds or adsorbed onto dust grains, or during other years. The perenially cold nature of the south polar residual cap, along with the relatively large summertime vapor abundances over the cap, implies a net annual condensation of vapor onto the cap. Comparison with earlier (Earth‐based) observations of vapor in the south during the local summer indicates that all of the seasonal CO2 cap may sublime away in some years to reveal the water ice cap which must lie underneath, with corresponding large southern summer vapor abundances. The global distribution of the annual average abundance of vapor correlates well with Martian topography, as might be expected for a uniform constant atmospheric mixing ratio. If this topographic effect is divided out, the resulting residual map correlates well with maps of surface albedo and thermal inertia; this correlation may be related to the control exerted by the surface and subsurface temperatures on the adsorption/desorption process and on the atmospheric temperature profile and, hence, the vapor holding capacity of the atmosphere.
Barbados: Architecture and implications for accretion
Authors: Speed R. C. , Larue D. K.
Published: February 2008

The island of Barbados exposes the crestal zone of the remarkably broad accretionary prism of the Lesser Antilles foreacrc. The architecture of Barbados is three‐tiered: an upper arched cap of Pleistocene reefs that record rapid and differential uplift of the island, an intermediate zone of nappes of mainly abyssal or deep bathyal pelagic rocks, and a basal complex whose lithotypes extend to substantial depth and may be representative of the bulk of the western or inner accretionary prism. The exposed basal complex consists of generally steeply dipping ENE to NE‐striking fault‐bounded packets which contain rocks of one of three lithic suites: terrigenous (quartzose turbidite and mudstone), debris flow, and hemipelagic (chiefly radiolarite). Present but imcomplete rock dating indicates that the terrigenous and hemipelagic suites and the pelagic rocks of the intermediate zone are age overlapping in Early and Middle Eocene time. Deformation within packets of the basal complex is systematic, pre‐ or synfault, and indicative of shortening that is generally normal to packet boundaries. A unit of terrigenous materials that probably underwent local resedimentation in the Miocene is recognized in wells, but its relationship to exposed rocks is uncertain. The packet‐bounding faults of the basal complex are interpreted to have been primary accretionary surfaces which may have been reactivated by later intraprism movements. Exposed sedimentary rocks of Barbados can be successfully assigned to contemporaneous depositional sites associated with an accretionary prism: terrigenous beds to a trench wedge that was connected to South American sediment sources, debris flow to trench floor or slope basin accumulations of material derived from the lower slope, hemipelagic to Atlantic plain strata, and pelagic rocks of the intermediate zone to deep outer forearc basin sites. The decollement at the base of the intermediate zone is probably due to uplift and arcward motion of the crestal zone of the accretionary prism with respect to the forearc basin during progressive prism growth. Principal uplift of the prism seems to have started, apparently abruptly, in the Miocene. Quaternary uplift of Barbados may be due partly to local diapirism. Paleogene subduction that created the arcward region of the prism probably occurred in a differently configured zone from the present one.
Attenuation of short‐period P, PcP, ScP, and pP waves in the Earth's mantle
Authors: Bock G. , Clements J. R.
Published: February 2008

The parameter t* (ratio of body wave travel time to the average quality factor Q) was estimated under various assumptions of the nature of the earthquake sources for short‐period P, PcP, and ScP phases originating from earthquakes in the Fiji‐Tonga region and recorded at the Warramunga Seismic Array at Tennant Creek (Northern Territory, Australia). Spectral ratios were calculated for the amplitudes of PcP to P and of pP to P. The data reveal a laterally varying Q structure in the Fiji‐Tonga region. The high‐Q lithosphere descending beneath the Tonga Island arc is overlain above 350 km depth by a wedgelike zone of high attenuation with an average Qα between 120 and 200 at short periods. The upper mantle farther to the west of the Tonga island arc is less attenuating, with Qα between 370 and 560. Qα is about 500 in the upper mantle on the oceanic side of the subduction zone. The t* estimates of this study are much smaller than estimates from the free oscillation model SL8. This can be partly explained by regional variations of Q in the upper mantle. If no lateral Q variations occur in the lower mantle, a frequency‐dependent Q can make the PcP and SeP observations consistent with model SL8. Adopting the absorption band model to describe the frequency dependence of Q, the parameter τ2, the cut‐off period at the high‐frequency end of the absorption band, was determined. For different source models with finite corner frequencies, the average τ2 for the mantle is between 0.01 and 0.10 s (corresponding to frequencies between 16 and 1.6 Hz) as derived from the PcP data, and between 0.06 and 0.12 s (2.7 and 1.3 Hz), as derived from the ScP data.
Delineation and interpretation of seismotectonic domains in western North America
Authors: Sbar Marc L.
Published: February 2008

Indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western North America. The San Andreas, East‐Central California (ECC), Mendocino Triple Junction (MTJ), and Pacific Northwest (PNW) domains are found to have similar stress fields, although the style and rate of deformation in each is different. We propose that all of these domains are generated by shear between the Pacific and North American plates. In the MTJ and PNW domains this shear may be evidence for the initiation of a transform fault connection between the San Andreas and the Queen Charlotte Island fault systems. The ECC, a region including much of the Sierra Nevada, appears to result from the relatively stiff nature of the San Andreas fault system. The Big Bend and region of relatively new faulting in northern California act to make the San Andreas fault system appear stiff on a regional scale. Two other domains, the Northern Basin and Range (NBR) and the Intermountain Seismic Belt (ISB), show evidence of being controlled by more local forces. The NBR has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the Juan de Fuca‐North America plate boundary. The ISB can be explained by readjustments between the Colorado Plateau and the Basin and Range Province, which may be caused by the lowering of the Basin and Range due to extension.
Long‐period lunar tides at the South Pole
Authors: Rydelek P. A. , Knopoff L.
Published: February 2008

Vertical gravity data from 6.3 years of observations at the geographic South Pole have been analyzed to determine the gravimetric factor and phase angle of the long‐period tides. Annual and semiannual tides are obscured by the combination of signals arising from instrumental drift and the large annual component of barometric pressure changes at Pole. Gravimetric factors for the three major long‐period tides are δ(Mƒ) = 1.1589 ± 0.0017, δ(Mm) = 1.1406 ± 0.0085, and δ(Mtm) = 1.1692 ± 0.0068. The only statistically significant phase shift is for the fortnightly component λ = 0.0151 ± 0.0032 days, a phase lead. Our data analysis for the gravitational factor for Mƒ at the South Pole indicates that the calculation of the influence of the long‐period ocean tides on the earth tides at the South Pole should be reconsidered.
The evolution of impact basins: Viscous relaxation of topographic relief
Authors: Solomon Sean C. , Comer Robert P. , Head James W.
Published: February 2008

We evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon. We adopt a representative topographic profile of the Orientale basin, the youngest large impact basin on the moon, as an estimate of the initial topography of older basins of similar horizontal dimensions, and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography. At wavelengths greater than the thickness of a high‐viscosity lithosphere, both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength‐dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins. The effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths, while the rate of relaxation decreases substantially for the fraction of long‐wavelength topography which is isostatically compensated as an initial condition. These models are applied to two pre‐Nectarian basins on the moon, the Tranquillitatis basin on the lunar nearside and the much larger and older South Pole‐Aitken basin on the farside. The topographic profile of Tranquillitatis, after correcting for the effect of mare basalt fill, is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit. The large topographic relief of the larger farside basin, on the contrary, is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as Tranquillitatis probably occurred. A difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference; such a nearside‐farside asymmetry in near‐surface temperature may date from the time of crust‐mantle differentiation and may have persisted because of the different histories of late‐stage impact basin formation and mare volcanism on the two hemispheres.
Thermochemical plumes and mantle phase transitions
Authors: Olson Peter , Yuen David A.
Published: February 2008

Integral relations based on boundary layer theory are derived to study the motion of an isolated, two‐dimensional thermal plume through a viscous mantle containing polymorphic phase changes. Analytical results are obtained which show that phase transitions alter average mantle convective velocities by less than 50%. In particular we find that the olivine‐spinel transition, approximated as univariant, can enhance the circulation velocity of upper mantle convection by 30–40%, while it can enhance the overall amplitude of whole mantle convection by a few percent only. Our calculations demonstrate that a possible endothermic phase change located at 650 km will not prevent deep mantle convection unless the Clapeyron slope defining the transition exceeds −0.3 kbar/°K. This large value is more than one order of magnitude greater than what has been proposed for the 650‐km discontinuity. We then extend the method to include compositional buoyancy and effects of the divariant nature of the olivine‐spinel transition. Analysis of the motion of a compositionally buoyant plume (one having an anomalous Mg/Fe ratio relative to the ambient mantle) reveals that the chemical plume locally distorts the transition in a way which contributes buoyancy and enhances convective amplitudes by 10% or less. Finally, we combine thermal and compositional buoyancy to investigate the interaction between a thermochemical plume and a compositionally induced density interface. The results are used to determine the stability criterion for two‐layered mantle convection. We find that a compositionally produced density increase of 4% at 650 km is needed to prevent mixing between the upper and lower mantle. Seismological observations indicate that density jump at the 650‐km discontinuity is between 5% and 9%. If all the density increase observed there is due to change in composition, then two‐layered mantle convection is stable. However, if phase transitions account for part of the density increase, so that the chemically produced component is less than 4%, then two‐layered mantle convection may be unstable.