bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the elemental composition of rocks on Venus by Venera 13 and Venera 14 (preliminary results)
Authors: Surkov Yu. A. , Moskalyeva L. P. , Shcheglov O. P. , Kharyukova V. P. , Manvelyan O. S. , Kirichenko V. S. , Dudin A. D.
Published: February 2008

Venera 13 and Venera 14 have determined for the first time ever the elemental composition of the Venusian rocks at the probes' landing sites. To study Venusian rocks, rolling upland and flat lowland have been chosen as the most typical of the entire planetary surface. Venera 13 landed on the rolling upland, while Venera 14 landed in the region with lower topography. The chemical composition of the rock at the Venera 13 landing site proved to be similar to the composition of potassium alkaline basalts of the earth's crust, rocks which can be found on oceanic islands and in rift zones. The chemical composition of the rock at the Venera 14 landing site is similar to that of tholeiitic basalts of the oceanic crust of the earth. The comparison of new data on the composition of Venusian rocks with the composition of the rocks of similar provinces on the earth and the moon shows that the evolution of the surface and crust of Venus was not similar to that of the earth and especially not that of the moon.
Geochemical studies of feldspathic fragmental breccias and the nature of North Ray Crater ejecta
Authors: Lindstrom Marilyn M. , Salpas Peter A.
Published: February 2008

Major and trace element analyses of clasts and matrix from feldspathic fragmental breccias show that these breccias are not all alike but vary widely in their bulk compositions and clast assemblages. These studies, together with analyses of other North Ray Crater large rocks, reveal the major components to be anorthosites, ferroan anorthositic norites, magnesian granulites, feldspathic melt rocks, and VHA impact melts. The ferroan anorthositic norites are important lunar crustal rocks identified here for the first time. The KREEPy VHA melts are found to be much more important in North Ray Crater ejecta than was previously assumed. Sodic ferrogabbro is an important minor component. The compositional characteristics of bulk breccias and feldspathic melt rocks are modeled quantitatively as mixtures of two to five components: anorthosites, ferroan anorthositic norites, magnesian granulites, sodic ferrogabbros, and VHA melt rocks. Breccia 67455 is unique in consisting wholly of ferroan, KREEP‐free components.
A model for the accumulation of solar wind radiation damage effects in lunar dust grains, based on recent results concerning implantation and erosion effects
Authors: Borg J. , Bibring J. P. , Cowsik G. , Langevin Y. , Maurette M.
Published: February 2008

In this paper we present our most recent results on ion implantation and erosion effects, intended to reproduce the superficial amorphous layers of radiation damage observed with a high voltage electron microscope on μm‐sized grains extracted from the lunar regolith and which result from the exposure of the grains to the solar wind. We next outline theoretical computations which yield the thickness distribution of such amorphous layers as a function of the exposure time of the grains at the surface of the moon, the He/H ratio, and the speed distribution in the solar wind. From this model, the position of the peak in the solar wind speed distribution is the major parameter controlling the thickness of the amorphous layer.
Charged dust in Saturn's magnetosphere
Authors: Mendis D. A. , Hill Jay Roderick , Houpis Harry L. F.
Published: February 2008

Gravito‐electrodynamic theory of charged dust grains is used to explain a variety of phenomena in those portions of the Saturnian ring system that are known to be dominated by fine (micron and submicron‐sized) dust, and in which collisional forces and Coulomb drag can be neglected. Among the phenomena discussed are the formation and evolution of the rotating near‐radial spokes in the B‐ring, the formation of waves in the F‐ring, the cause of eccentricities of certain isolated ringlets, and the origin and morphology of the broad diffuse E‐ring. Several novel processes predicted by the gravito‐electrodynamic theory, including ‘magneto‐gravitational capture’ of exogenic dust by the magnetosphere, ‘1:1 magneto‐gravitational orbital resonances’ of charged dust with nearby satellites, and ‘gyro‐orbital resonances,’ are used to explain individual observations. The effect of a ring current associated with this charged dust is also evaluated. Finally, the cosmogonic implications of the magneto‐gravitational theory are briefly discussed. While several (although not all) of these processes have been discussed by one or more of the present authors elsewhere, the purpose of this paper is to synthesize all these processes within the framework of gravito‐electrodynamics, and also to show its range of applicability within Saturn's ring system.
Evidence for a thick oceanic crust adjacent to the Norwegian Margin
Authors: Mutter John C. , Talwani Manik , Stoffa Paul L.
Published: February 2008

The oceanic crust created during this first few million years of accretion in the Norwegian‐Greenland Sea lies at an unusually shallow depth for its age, has a smooth upper surface, and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate, seaward‐dipping reflectors. These have been attributed to lava flows generated during a brief period of subaerial seafloor spreading. We describe the results of inversions of digitally recorded sonobuoy measurements and two‐ship expanded spread profiles collected over the oceanic crust adjacent to the Norwegian passive margin. We find that the crust of the deep Lofoten Basin is indistinguishable from normal oceanic crust in thickness and structure. Closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2, while the layer 3 region retains approximately the same thickness. The area over which the seaward‐dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in “Layer 2” thickness. In the very oldest crust immediately adjacent to an escarpment that probably marks the continent‐ocean boundary, we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke‐lava interface in the thick crust. Comparing the structure of the thick crust to that of eastern Iceland, we find a strong resemblance, especially in the expansion in thickness of material with layer 2 velocities. These results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous, building a thick pile of lavas erupted from a subaerial spreading center.
A pore space model for rock permeability and bulk modulus
Authors: Seeburger Donald A. , Nur Amos
Published: February 2008

A simple network model of pore space in rocks has been developed with which permeability and bulk modulus as a function of confining pressure can be calculated. Pores are modeled as straight conduits with circular, elliptic, or tapered cross sections. The interconnection of these conduits is modeled by emplacing them in regular two‐dimensional hexagonal, square, or triangular networks. Flow through each conduit is modeled using Poiseuille's law. Flow through the network is calculated based upon the analogy of fluid flow in Darcy's law to current flow in Ohm's law. An estimate of the effective bulk modulus is obtained by summing the contributions of the individual pores. The effect of confining pressure on permeability and bulk modulus is determined by the solid properties and the shape of the pores. For appropriate aspect ratio distributions, the permeability and bulk modulus characteristics of the network model are similar to those found for laboratory rock samples. For rocks of moderate porosity, like sandstone, the model predicts that the bulk modulus is most affected by small, low aspect ratio pores. In contrast, for rocks like granite and tight sandstones (permeability less than 0.5 mdarcy) in which there are relatively few round pores, the bulk modulus and the permeability are both controlled by easily deformed pores. These types of responses are observed in experimental data. The success of the model in predicting these general bulk modulus and permeability responses indicates that while the models presented are too simple to represent a rock completely, the network theory approach is a promising method for modeling porous media.
A comparison of satellite doppler and gravimetric geoid undulations considering terrain‐corrected gravity data
Authors: Rapp Richard H. , Wichiencharoen Chugiat
Published: February 2008

Geoid undulations from satellite Doppler stations have been compared to geoid undulations computed using three procedures. The first procedure combines potential coefficients with uncorrected 1°×1° free air anomalies in a 10° cap surrounding the computation point. The second procedure uses the Helmert second condensation procedure for anomaly reduction where terrain corrections and topographic indirect effects are considered. The third procedure uses a set of potential coefficients to degree 180. After correcting the original Doppler undulation for a −0.4‐ppm scale change, and a 4‐mZaxis bias, comparisons were made at 10 sites located in mountainous areas of the western United States and 10 sites in the eastern United States where the topography was relatively flat. The mean difference (Doppler minus gravimetric) for the 10 western stations was 1.65 when no corrections were applied to the gravity data, while the difference was reduced to 0.10 m when the terrain corrections and indirect effects were included. For the 10 eastern stations the systematic difference (excluding the negligible terrain effects ) was 0.42 m. For western sites the standard deviation of the differences was ±0.96 m (with no gravity corrections) and ±0.93 m with the correction applied. In the eastern sites the standard deviation was ±0.55 m. Similar results were found with the potential coefficients to degree 180. Our major conclusion is that the neglect of terrain corrections, and to a lesser extent the indirect effect, can cause systematic errors of the order of 1.5 m (for a cap size of 10°) in geoid undulations for stations in mountainous areas.
Regional deformation near Palmdale, California, 1973–1983
Authors: King N. E. , Savage J. C.
Published: February 2008

The Tehachapi trilateration network spans the intersection of the San Andreas and Garlock faults in southern California in the “Big Bend” region of the San Andreas fault. Analysis of data from 1973–1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults. The Palmdale network, spanning the San Andreas fault entirely within the Tehachapi network, showed increases of about 1 μstrain in both east‐west and north‐south extension in late 1979. The Tehachapi strains also jumped at this time, but the magnitude of the increase was only about one third that of Palmdale. The principal strain rates for Tehachapi over the time interval 1973–1983 are
μstrain/yr and
μstrain/yr, with the 1 axis directed N76°E. Strains were also computed for two subregions. The principal strain rates for the southeast Tehachapi region are
μstrain/yr and
μStrain/yr, with the 1 axis directed N73°E. This result differs significantly from the principal strain rates at Palmdale (
μstrain/yr,
μstrain/yr, with the 1 axis directed N71°E). For the northwest Tehachapi region,
μstrain/yr and
μstrain/yr, with the 1 axis directed N93°E.
Slab penetration into the lower mantle
Authors: Creager Kenneth C. , Jordan Thomas H.
Published: February 2008

To investigate whether or not lithospheric slabs descending along subducting margins penetrate below the 670‐km seismic discontinuity, we have analyzed 4040 P and PKIKP travel times from 14 intermediate‐ and deep‐focus earthquakes in the northwestern Pacific and obtained estimates of the near‐source anomaly as a function of position on the lower focal hemisphere. Station and ellipticity corrections are applied, and any component of the anomaly explicable by hypocenter mislocation is removed by orthogonalization of the residual vector with respect to the location parameters; the resulting residual sphere is smoothed and interpolated by a stochastic filtering scheme to average out observational errors and the effects of small‐scale heterogeneities far from the source. The smoothed residual spheres for six Sea of Okhotsk earthquakes deeper than 500 km are dominated by NE‐SW trending troughs of negative anomalies having strikes and dips similar to the seismic zone at these depths; the troughs are bounded on the NW and SE by parallel ridges of positive anomalies, with peak‐to‐trough amplitudes averaging about 1.5 s. As the focal depth decreases, the pattern translates to the NW, so that for hypocenters near 200 km the negative trough has been replaced by a positive ridge. The axis of the anomaly pattern for two Sea of Japan deep‐focus earthquakes is rotated counterclockwise from Okhotsk, consistent with the nearly N‐S strike of the Japan seismic zone. These correlations, and the fact that one nearby earthquake not within a subduction zone shows very little anomaly, suggest that the residual sphere anomalies are caused primarily by slab heterogeneity. Forward modeling experiments corroborate this conclusion. The thermal disturbance of the mantle is calculated for an assumed flow field by a finite difference algorithm, and from it a model of P velocity heterogeneity is constructed; theoretical travel time residuals are computed by tracing rays through this three‐dimensional structure, and the event is relocated and the residual sphere smoothed using the same station set and by the same algorithm applied to the observations. To obtain a good fit to the deep‐focus Okhotsk data requires penetration by the Kuril‐Kamchatka slab to depths of at least 900–1000 km. Penetration to much greater depths is consistent with the data. This conclusion agrees with the S wave results of Jordan (1977); it implies the circulation of at least some upper mantle material into the deep mantle and argues against a rigorously stratified upper mantle/lower mantle convective system.
The earthquake deformation cycle at the Nankai Trough, southwest Japan
Authors: Thatcher Wayne
Published: February 2008

Geodetic measurements made landward of the Nankai Trough, site of great subduction zone thrust earthquakes in 1854 and 1946, provide a uniquely detailed picture of the strain buildup process, supply constraints on the mechanism of strain accumulation, and allow for improved estimates of earthquake recurrence. Provided the two most recent movement cycles are similar, the observations, dating from about 1890, may be used to reconstruct a single complete deformation cycle (coseismic strain release, postseismic transients, interseismic strain accumulation). Very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary, persists for at least 30 years, and shows a clear tendency to become longer wavelength with increasing time. The transient movements have two timescales. The first, of about a year or less, corresponds to deformation, largely uplift, concentrated close to the coseismic fault, and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane. The second, longer, timescale is associated with a diffusion‐like spread of the deformation further landward, an effect qualitatively similar to that first predicted by Elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere. Cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces, although average post‐1890 tilt rates exceed late Quaternary and Holocene averages by at least a factor of three. Because of the nonlinearity of strain buildup and the significant permanent deformation, simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3. Strict application of the time‐predictable model, assumed correct, overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated.
Phase equilibrium controls on the tholeiitic versus calc‐alkaline differentiation trends
Authors: Grove Timothy L. , Baker Michael B.
Published: February 2008

This paper discusses some of the factors that allow tholeiitic and calc‐alkaline differentiation trends to be generated from a common basaltic parent melt, and calculates liquid lines of descent, for the two divergent trends. Phase proportions and compositions measured in 1‐atm experiments on natural basalts are used as input to calculate the tholeiitic trend, and high‐pressure phase proportions, inferred from phase equilibrium studies on natural compositions, are used to calculate the calc‐alkaline trend. An important control on the development of the contrasting tholeiitic versus calc‐alkaline trends is the proportions of olivine, plagioclase, and pyroxene that crystallize from the basaltic parent melt. The tholeiitic trend is produced by fractional crystallization of a basalt magma at low pressures. The crystallization sequence is olivine, followed by plagioclase, followed by augite. Plagioclase dominates the assemblage, and the three‐phase crystallization continues to a reaction point where olivine+liquid react to augite, plagioclase, and pigeonite. Total iron in the liquid increases throughout this crystallization process, and a dramatic increase in iron and a mild decrease in SiO2 occur at the reaction point. The calc‐alkaline trend develops when olivine, calcic plagioclase, and augite crystallize in nearly equal mass proportions. This phase assemblage precipitates under conditions of moderate pressure and water undersaturation in the middle to upper crust and produces a gentle increase in total iron, an increase in SiO2 and a decrease in MgO in derivative liquids. Liquids derived by such a process can evolve to andesites and dacites by continued crystallization involving plagioclase, orthopyroxene, pigeonite, augite, and magnetite. The assimilation of a crustal component by a fractionating basaltic melt and the mixing of basaltic liquids with siliceous residual liquids produced by either fractionation or fractionation aided by assimilation are additional processes that assist in the production of the calc‐alkaline trend.
Effects of chemical environments on slow crack growth in glasses and ceramics
Authors: Freiman S. W.
Published: February 2008

This paper presents a review of our current understanding of environmentally induced slow crack growth in glasses, single crystals, and polycrystalline ceramics. It is shown that the rate of crack growth is controlled by the chemical activity of the active species in the environment as well as by the stress intensity at the crack tip. A recently developed molecular model of stress‐induced chemical reaction between vitreous silica and water is described. The implications of this model for the effects of other chemical species on crack growth are discussed. Finally, the complications introduced by the presence of grain boundaries in polycrystalline ceramics are pointed out.
Crack growth during unroofing of crustal rocks: Effects on thermoelastic behavior and near‐surface stresses
Authors: Bruner William M.
Published: February 2008

Overcoring and hydrofracturing measurements show that the horizontal normal stresses at or near the earth's surface are nearly always compressive, with magnitudes which occasionally reach tens of megapascals. This is surprising because simple theoretical arguments suggest that the crustal extension and cooling which occur during unroofing (exposure of rocks by uplift and erosion) should frequently produce large tensile stresses near the surface. One likely cause of this discrepancy is the growth of microcracks, driven by the grain scale internal stresses produced by cooling and depressurization during unroofing. The unroofing process has been simulated for a simple two‐dimensional model of a rock in which internal stresses arise only from cooling. These calculations show that crack growth leads to compressive changes in macroscopic stress by increasing the elastic compliance of the rock and by decreasing its linear thermal expansion coefficient. The growth and opening of cracks also produces an expansion which must be canceled by increased horizontal compression in order to satisfy the lateral constraints imposed by the presence of adjacent rocks. The resulting stress changes (relative to the stresses expected in a crack‐free rock) could easily reach tens of megapascals at the surface in some rocks, especially granites and marbles. This effect tends to make the near‐surface stresses relatively insensitive to the mean geothermal gradient, in marked contrast to the behavior of uncracked rocks. Crack growth driven by internal stresses is also likely to be triggered by drilling, tunneling, or quarrying in rocks subjected to substantial in situ compressive stresses and could lead to macroscopic deformation quite different from that expected on the basis of a simple elastic analysis. The importance of such effects in a given rock depends upon whether the internal stresses are sufficient to produce significant crack growth, which in turn depends upon the relative values of two key parameters that are poorly known at present, these quantities are the temperature at which ductile deformation effectively ceases during unroofing and the minimum stress‐intensity factor capable of producing significant crack growth during unroofing. Better estimates of these parameters would permit more quantitative conclusions regarding the effects of crack growth in particular rock types or geologic settings and would facilitate the development of constitutive laws describing the macroscopic deformation resulting from crack growth.
The role of aqueous fluids in the internal friction of rock
Authors: Bulau J. R. , Tittmann B. R. , Abdel‐Gawad M. , Salvadó C.
Published: February 2008

The effects of water on elastic energy dissipation in rock under upper crustal conditions are considered briefly in this paper. Experimental evidence indicates that in dry rock the energy dissipation process involves water adsorbed on the surfaces of very thin cracks. In one mechanism the adsorbed water is regarded as a viscoelastic film. Portions of this layer are forced to relax as crack thickness changes with the passing elastic wave. A second possible mechanism involves the relaxation of electrostatic repulsion stresses between oriented polar water molecules on opposing crack faces. When water is present as a bulk fluid, attenuation can be explained by the flow of a viscous fluid into and out of very thin cracks in response to dynamic stresses associated with a passing elastic wave.
The Gravsat signal over tectonic features
Authors: Wagner C. A. , Sandwell D. T.
Published: February 2008

The range rate between two close gravitational satellites (GRAVSAT) in low earth orbit has been evaluated over model tectonic features such as mountains and ranges, fracture zones, and trenches. Models are locally compensated and consist of both point mass dipoles and sheet mass dipoles. Masses and depths of compensation are chosen to approximate known gravity signatures. The results show that for two satellites at 160 km altitude with 3° separation, significant signal power (>1 μm/s) remains for most extended features at wavelengths less than 200 km. Furthermore, there is strong sensitivity in the signal from these features to lateral and vertical changes of the order of 1 km and less. In addition, the signal of hidden geologic structures such as dikes, salt domes, and ore bodies may also stand above 1 μm/s for this low orbiting pair. Thus, it may prove to be efficient to model the high‐frequency GRAVSAT signal directly in terms of the parameters of tectonic‐topographic features and their compensation.
Research on the geometry of shear fracture zones
Authors: Deng Qidong , Zhang Peizhen
Published: February 2008

The geometry of brittle shear fracture zones has been studied by the examination of some strike‐slip faults and shear joints in China. Pinnate and en echelon deformational patterns are two fundamental arrangements of fractures within brittle shear fracture zones. The pinnate angle is usually less than 25° and the en echelon is more than 25°. The deformation in the “rock bridge,” the area between two secondary fractures, depends on the arrangements of secondary fractures and the sense of shear. Some tensile ruptures are formed at the terminations of both primary shear fracture zones and secondary fractures. The direction of a tensile rupture is commonly parallel to the orientation of the principal compression. Along the direction of propagation of the shear fracture, the horizontal displacement is large, and the attenuation of displacements is slow. The maximum horizontal displacement of the shear zone may be located in the region of the initial fracture. Under certain conditions the shear fracture zones may possess pivotal movement. This type of movement may be expressed by the behavior of the fault or by the geomorphology of the fault zone.
A seismotectonic analysis of the Anza Seismic Gap, San Jacinto Fault Zone, southern California
Authors: Sanders Chris O. , Kanamori Hiroo
Published: February 2008

Small earthquake epicenters near the Anza seismic gap define a 20‐km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity. Recent moderate earthquakes on and near the San Jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism. The locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local, active, discontinuous faults and the oblique orientation of the regional maximum compressive stress. Strain is not being relieved by aseismic fault creep. A swarm of small earthquakes in the crustal block 13 km southwest of the Anza gap beneath the Cahuilla Valley recently released stress in an area which was previously highly active before the 1918 (M 6.8) and 1937 (ML 6.0) earthquakes. The occurrence of these periods of increased seismicity near Cahuilla in the years immediately before the nearby (closer than 35 km) large earthquakes and the recent swarm suggest that the ground beneath Cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes. The length of the quiescent fault segment suggests potential for about an M 6.5 earthquake if the entire segment ruptures at once.
On gravity from SST, geoid from Seasat, and plate age and fracture zones in the Pacific
Authors: Marsh B. D. , Marsh J. G. , Williamson R. G.
Published: February 2008

Our earlier measurements of the high degree and order (n, m>12) gravity in the central Pacific using the method of satellite‐to‐satellite tracking (SST) have been extended with an additional 50 passes of data. The SST method utilizes line of sight Doppler tracking between the very high geosynchronous ATS 6 spacecraft and the much lower (840 km) orbiting GEOS 3 spacecraft. The observed changes in velocity with time are reduced in relation to the well‐determined low degree and order (n, m≤12) Goddard Earth Model (GEM) field model, and accelerations are found by differentiation of the range rates. Because this new map is essentially identical to the earlier results, we have produced a composite map by combining all 90 passes of SST data. The resolution of the map is at worst about 5° and is much better in most places. A comparison of this map with conventional GEM models shows very good agreement. A reduction of the Seasat altimeter data has also been carried out for an additional comparison. Although the Seasat geoid contains much more high‐frequency information, it agrees very well with both the SST and the GEM fields. The maps are dominated (especially in the east) by a pattern of roughly east‐west anomalies with a transverse wavelength of about 2000 km. A further comparison with regional bathymetric data shows a good correlation of gravity and geoid with plate age. Each anomaly band is framed by major fracture zones whose regular spacing (≃10°) seems to account for the fabric in these fields. There are other anomalies that are accounted for by hot spots, and altogether, the immediate source of at least part of these Pacific anomalies is in the lithosphere itself. It therefore seems that most of the anomalies in the east half of the Pacific could be partly caused simply by regional differences in plate age. The amplitude of these geoid or gravity anomalies caused by age differences should decrease with absolute plate age, and large anomalies (∼3 m) over old, smooth seafloor may indicate a further, deeper source within or perhaps below the lithosphere. However, because of the significant effect of plate structure on the geoid and gravity fields, especially over young (<80 Ma) seafloor, lithospheric effects must be eliminated before these anomalies can be systematically ascribed to deeper sources such as small‐scale convection.
Block rotation by strike‐slip faulting: Structural and paleomagnetic evidence
Authors: Ron H. , Freund R. , Garfunkel Z. , Nur A.
Published: February 2008

Geometric analysis shows that motion of sets of strike‐slip faults should cause block rotation; otherwise the distorted fault domain would not fit with its surrounding. The sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults, all of which can be obtained from structural data. The predictions of the geometric model can then be independently tested by paleomagnetic measurements. The model was tested in northern Israel. Structural data reveal several domains of contemporaneous right‐ and left‐lateral strike‐slip faults. Paleomagnetic measurements show that in a domain of NNW trending left‐lateral faults, blocks rotated 23.3°±8.2° clockwise, and in a domain of right‐lateral faults, blocks rotated 22.4°±9.0° anticlockwise. These results are in agreement with the prediction of the geometric analysis of the structure. The combined results of the heterogenous deformation of the area resembles a pure shear which allows N‐S extension by about 1.3. In other domains, rotations of 34.6°±9.1° and 53.1°±11.0° were found. In their original orientation the left‐ and right‐lateral faults intersected at angles of 60°–70°, enclosing the principal axis of shortening, in accordance with theories of brittle failure. As a result of block rotations, this angle is now larger and may reach 110°. The results demonstrate large block rotations, in different senses, in domains of strike‐slip faults. The independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model. Northern Israel is considered to be a good model for this efficient mechanism of intraplate deformation, which is expected to occur in similar tectonic settings elsewhere.
A method for the display and analysis of transitional paleomagnetic data
Authors: Hoffman Kenneth A.
Published: February 2008

Application of a simple coordinate transformation to data from polarity transition and excursion records renders a form of display having strong similarities, yet distinct advantages, over the use of virtual geomagnetic poles. Specifically, the procedure consists of a single rotation about the east‐west line so as to align the +z axis of the local Cartesian system with the normal polarity axial dipole field direction associated with the recording site at the time of the event. The method makes possible the redefinition of concepts relevant to transitional field studies in terms of vector directions, a more meaningful approach than the use of virtual poles when nondipole field components are significant. Visually, polar stereographic plots in (D′,I′) space present paleodirectional movement as if one were literally peering down the dipole direction. Analysis of individual transition records in this rotated directional space makes easy the determination of (1) the angular distance of a given paleodirection from that of the axial dipole field, (2) the number of strictly transitional directions contained within a given record, and (3) the degree of near‐sidedness or far‐sidedness of each paleodirection. In addition, the comparison of records from distant sites for the purpose of harmonic analysis can be accomplished without the need for additional normalization. Such comparison plots in (D′,I′) space display transitional data in a form strongly reminiscent of virtual geomagnetic poles plotted with respect to a common site longitude. Records of polarity transitions and excursions are presented which illustrate the utility of the proposed approach.
Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications
Authors: Clark Roger N. , Roush Ted L.
Published: February 2008

Several methods for the analysis of remotely sensed reflectance data are compared, including empirical methods and scattering theories, both of which are important for solving remote sensing problems. The concept of the photon mean optical path length and the implications for use in modeling reflectance spectra are presented. It is shown that the mean optical path length in a particulate surface is in rough inverse proportion to the square root of the absorption coefficient. Thus, the stronger absorber a material is, the less photons will penetrate into the surface. The concept of apparent absorbance (‐In reflectance) is presented, and it is shown that absorption bands, which are Gaussian in shape when plotted as absorption coefficient (true absorbance) versus photon energy, are also Gaussians in apparent absorbance. However, the Gaussians in apparent absorbance have a smaller intensity and a width which is a factor of √2 larger. An apparent continuum in a reflectance spectrum is modeled as a mathematical function used to isolate a particular absorption feature for analysis. It is shown that a continuum should be removed by dividing it into the reflectance spectrum or subtracting it from the apparent absorbance and that the fitting of Gaussians to absorption features should be done using apparent absorbance versus photon energy. Kubelka‐Munk theory is only valid for materials with small total absorption and for bihemispherical reflectance, which are rarely encountered in geologic remote sensing. It is shown that the recently advocated bidirectional reflectance theories have the potential for use in deriving mineral abundance from a reflectance spectrum.
Geologic thermobarometry of retrograded metamorphic rocks: An indication of the uplift trajectory of a portion of the northern Scandinavian caledonides
Authors: Hodges K. V. , Royden L.
Published: February 2008

Kyanite grade pelitic schists from the Efjord area (68°15′N, 16°45′E), northern Norway, show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the Scandinavian Caledonides. Graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure‐temperature conditions. Three element‐partitioning geothermometers and geobarometers yield consistent PT estimates in individual samples, which together define a trend in PT space. The sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates, whereas samples exhibiting abundant reequilibration textures give lower PT estimates. We suggest that the trend indicated by these samples reflects a portion of the PT trajectory assumed by the Efjord area during uplift. Thus the judicious use of element‐partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains.
A comparison of the complete and truncated versions of the polar wander equations
Authors: Sabadini R. , Yuen D. A. , Boschi E.
Published: February 2008

We give a theoretical development of a uniformly valid solution for describing the polar motions of a layered viscoelastic earth. Such solutions require the usage of two classes of eigenspectra to describe the processes of rotational deformation and viscous relaxation. One set consisting of real eigenvalue describes the isostatic relaxation of the mantle by viscous creep. The second one governs the readjustment of the rotational axis of the viscoelastic earth, and the eigenspectrum is complex valued. These solutions are capable of describing rotational phenomena ranging from Chandler wobble excitation to long‐term polar drift. A comparative study is conducted between the complete solution for polar wander and one in which a number of the rotational modes has been truncated. For a four‐layer model consisting of an elastic lithosphere, a two‐layer, adiabatically stratified viscoelastic mantle, and an inviscid core, such a comparison shows that at most a 30% difference exists in the viscosity solutions of the lower mantle, which are obtained by fitting the theoretical predictions to the observed polar wander data. Although the polar speeds from the various models can vary by as much as a factor of 2, the differences in the inverted viscosity solutions are sharply reduced upon fitting the calculations with the actual data for the time window, between 5 and 7×103 years, which characterizes the hiatus between the termination time of the deglaciation period and today. The averaged polar drift from typical glacial cycles in the Pleistocene shows an initial magnitude of 0(1°/m.y., which after a few million years decays to a steady state value of a few tenths of a degree per million years.
On the thermal structure of subduction complexes: A preliminary study
Authors: Wang Chi‐Yuen , Shi Yao‐Lin
Published: February 2008

The common occurrence of high‐pressure tectonic terranes along the Pacific coast and other convergent origins suggests that the evolution of subduction complexes may be an important phase in the development of continents. In this study we model the steady state thermal structure in an active subduction complex. Following Cloos, we treat the movements of sediments in deep subduction complexes as laminar flow in a viscous fluid. Driven by the descending plate and blocked by the hanging wall of the nonsubducted plate, a part of the sedimentary pile is forced to convect. Temperatures in subduction complexes are determined by the velocity of flow, radiogenic heating, and heat flow across the boundaries of these complexes. Heating becomes important when the rate of flow is small, of the order of 10 mm yr−1. Under such conditions, temperatures in subduction complexes are suitable for the formation of the characteristic metamorphic facies and may lead to the progression in metamorphic age and grade observed in many high‐pressure tectonic terranes. Fast convection cools the accretionary prisms; convective cooling can be so effective that temperatures even in the deepest interiors of subduction complexes may become too low for the formation of the metamorphic mineral assemblages that are currently used as criteria to identify sediments that experienced deep burial in the past. The results of this study suggest that thermal modeling and prediction of the distribution of metamorphic grades and ages of sediments in subduction complexes can provide powerful tests on hypotheses on the evolution of these complexes.
Near‐surface in situ stress: 4. Residual stress in the Tully Limestone Appalachian Plateau, New York
Authors: Engelder Terry , Geiser Peter
Published: February 2008

The direction of maximum expansion during strain relaxation of the Tully Limestone at Ludlowville, New York, is oriented about NO4°E and thus is within a couple of degrees of the strike of both set Ia cross‐fold joints and the compression direction indicated by the teeth of tectonic stylolites. These joints and stylolites are members of a suite of structures accommodating approximately 9% layer‐parallel shortening during the main phrase of the Alleghanian Orogeny within the Appalachian Mountains. Because of the parallelism of the maximum expansion with the main phase Alleghanian compression and because our measurements were made in joint‐bounded blocks, we suggest that the expansion represents the relief of a residual stress locked into the Tully Limestone during the Alleghanian Orogeny. The magnitude of the strain relaxation indicates that the residual differential stress was 14 MPa. Using a flow law for pressure solution, we infer that the Tully Limestone deformed at a strain rate of about 3×10−15s−1, and thus the layer‐parallel shortening observed in the Tully Limestone may have required an aggregate deformation interval of only 1 m.y.
The empirical age‐depth relation and depth anomalies in the Pacific Ocean Basin
Authors: Schroeder William
Published: February 2008

This paper presents a new age‐depth relation and a comprehensive depth anomaly map for the Pacific Ocean. Age, depth, and sediment thickness were averaged for each 1°×1° area. Bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water. Seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep. Hot spots were treated as thermal anomalies, and data within 800 km of them were eliminated from further analysis. An age‐depth equation was determined by fitting a least squares line to the remaining data. The resulting depth versus age distribution confirms the linear dependence of depth on √age up to 80 Ma. All older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted. This “flattening” of the age‐depth distribution requires that a separate function be fit to data older than 80 Ma. As a result, data representing the deepest seafloor (older than 80 Ma) were selected for curve fitting, and the age‐depth distribution was approximated by an exponential function. It is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age‐depth data from the Pacific Ocean. A depth anomaly map prepared by comparing observed depth with the depth predicted by the new age‐depth formula shows several newly recognized features, including swells over the Louisville Ridge in the South Pacific and in the Gulf of Alaska Seamounts.
Seasat altimetry, the North Atlantic geoid, and evaluation by shipborne subsatellite profiles
Authors: Vogt P. R. , Zondek B. , Fell P. W. , Cherkis N. Z. , Perry R. K.
Published: February 2008

A geological evaluation of geoid anomalies over the North Atlantic is presented with emphasis on short (40–300 km) wavelength undulations mapped by Seasat radar altimetry. Long (≥3000 km) and intermediate (300–3000 km) wavelength undulations constrain subcrustal processes and are briefly reviewed. Portions of three Seasat revolutions (revs) totaling 9300 km were followed by research ship, which measured gravity, magnetics, topography, and for a 800‐km segment also seismic reflection data, to form a ground truth data set for comparison to the altimetry. The intermediate‐wavelength geoid, the residual upon removal of a (14,14) long‐wavelength surface, correlates with plate age (slope ∼10 cm/m.y.) over Cenozoic crust, and major fracture zones appear as geoid steps. A 3.5‐ to 5‐m high over the Bermuda Rise may reflect thermal rejuvenation created within the plate 40–50 Ma. Short‐wavelength geoid anomalies, typically ±10–50 cm in the open ocean, were analyzed by high‐pass (200‐ or 400‐km cutoff) along‐track filtering and smoothing. The vertical deflection computed from the Seasat geoid compares well with one based on seismic reflection. Shipborne free air gravity, except for a higher‐frequency cutoff, correlates closely with geoid anomalies. Except near seamounts, the principal contribution to geoid anomalies is fracture zone topography. All major and many minor fracture zones (FZ's) appear as geoid lineations, particularly between the 15°20′and Kane FZ, from the ridge axis to anomaly 34. The WSW trend of FZ's between anomaly 21 and 34 is clearly expressed in the altimetry, as is the smooth geoid over mid‐Cretaceous smooth basement formed by fast spreading. Crustal ribbons, 50–150 km wide, of higher or lower geoid, parallel FZ trends. Some geoid lineations are oblique to both isochron and FZ trends; east of the Puerto Rico Trench such anomalies may reflect compressional deformation within Atlantic lithosphere.
Propagation and linkage of oceanic ridge segments
Authors: Pollard David D. , Aydin Atilla
Published: February 2008

We have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates. The ridge‐propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other. The ridge‐propagation force increases as ridge ends approach but then declines sharply as the ends pass, so ridge segments may overlap somewhat. The sign of the path factor changes as ridge ends approach and pass, so the overlapping ridge ends may diverge and then converge following a hook‐shaped path. The magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting. For different loading conditions simulating ridge push, plate pull, and ridge suction, we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated. For all conditions, the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap; thus crust in this zone may fracture and weaken in preparation for the formation of a through‐going transform fault. The calculated shear planes rotate toward an orientation about 90° from the strike of ridges as the ends pass, thus favoring the orthogonal arrangement of ridges and transforms. The magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated. The mean stress is tensile in the interaction zone, so basins may form there, except in the case of ridge push loading. The planes across which the maximum tension acts are oblique to ridges, thus favoring obliquely oriented normal faults bounding the transform valley.