bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional seismic velocity structure of the Earth's mantle using body wave travel times from intra‐plate and deep‐focus earthquakes
Authors: Sengupta Mrinal K. , Hassell Ralph E. , Ward Ronald W.
Published: February 2008

This paper combines the recent measurements of P and S wave travel times from intra‐plate earthquakes with travel times from deep‐focus earthquakes reported earlier to infer the velocity structure of the mantle. It is reasonable that the source bias for intra‐plate earthquakes is less pronounced than that for other shallow‐focus earthquakes such as those occurring in island arcs. The inclusion of intra‐plate earthquakes compensates for the lack of uniform areal distribution exhibited by the deep‐focus earthquake data and augments the number of high quality travel time observations. The features observed in the variation of teleseismic travel times from intra‐plate earthquakes are consistent with the travel time variation determined from deep‐focus earthquakes (especially the P wave travel time variation). The combined intra‐plate and deep‐focus earthquake travel time data were converted to determine an average (spherically symmetric) compressional and shear velocity model and, subsequently, a three‐dimensional compressional velocity model for the earth's mantle. Lateral variation in the mantle accounted for most of the scatter in the observed P wave travel times. The average compressional velocity model suggests two rapid velocity increases at 500 and 670 km depths and a small velocity gradient in the depth interval of 100–450 km. The 3‐D model determined from the augmented travel time data set corroborated the earlier findings (1) that the lateral heterogeneities are more pronounced near the upper‐mantle and near the core‐mantle boundary and (2) that the surface tectonic features are correlated with compressional velocity anomalies near the upper mantle, but not with velocity anomalies below the upper mantle.
Steady propagation of delamination events
Authors: Bird Peter , Baumgardner John
Published: February 2008

Delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation, accelerating as the sinking slab of detached lithosphere grows longer. This propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings. The form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material. Without coupling, the tip of the intrusion cools and stiffens to form a wedge dividing the crust (cold mode). With coupling, the intrusion is forced to convect and remains ductile (hot mode). The cold mode can propagate at all velocities; the hot mode has a lower limiting velocity of 1–2 cm/year but offers less resistance at higher speeds. Resistance to delamination includes a constant term from the buoyant crustal downwarp, plus a velocity‐proportional term representing viscous deformation. However, the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities. Matching this resistance to loading lines of 100‐ to 800‐km slabs sinking in a mantle of 1022 P, velocities of 0.3–8.0 cm/year are obtained. Changes in viscosity affect this rate, but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab. The surface expression of delamination is a leading ‘outer rise’ followed by a submarine trough with a large negative free‐air anomaly, which finally evolves into a 1‐km plateau. If crustal viscosity and velocity are both low, however, there is a montonic crustal uplift with no trough. Thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm/year driven by slabs up to 400 km in length.
Shallow velocity structure of the Rio Grande Rift north of Socorro, New Mexico: A reinterpretation
Authors: Brocher Thomas M.
Published: February 2008

Shallow (upper 1.2 km) compressional wave velocity models from line 2A of the Socorro, New Mexico, Consortium for Continental Reflection Profiling (COCORP) survey of the Rio Grande rift supplement a previous refraction study of Socorro COCORP lines 1 and 1A. First‐arrival refraction analysis and slant stacks of the refraction profiles (derived from the reflection data) are used to determine velocity models of Socorro COCORP line 2A. Exposure of Paleozoic and Mesozoic sedimentary formations in the vicinity of Socorro line 2A allow direct correlation of a unit's velocity to its geological era. Weathered Mesozoic and Paleozoic formations within the rift are inferred to have significantly lower velocities than previously believed. Reinterpretation of the Rio Grande rift structure in the survey area results in a model that is consistent with available gravity data and provenance studies. The 13 km of Cenozoic spreading determined from the model is at least 5 km higher than that found 80 km to the north. There is evidence of minor Paleozoic deformation or nondeposition.
Amplitudes of diffracted long‐period P and S waves and the velocities and Q structure at the base of the mantle
Authors: Mula Abdel Hafiz Gadel
Published: February 2008

Frequency and time domain analysis of amplitudes of long‐period (∼20 s) P and S waves diffracted around the earth's core and registered at the WWNSS and CSN stations show that the velocities at the base of the mantle have slightly positive gradients. This result substantiates the previous ray parameter study of Mula and Müller (1980). Such velocity gradients at the base of the mantle imply that there is no significant departure from homogeneity and/or adiabaticity of the base of the mantle from the rest of the lower mantle as suggested by, for example, Bolt (1972), Jones (1977), Jeanloz and Richter (1979), and Doornbos and Mondt (1979b). In addition to this the Qα factor for the base of the mantle is found not to be less than 250.
On some consequences and possible causes of layered mantle convection
Authors: Richter Frank M. , McKenzie Dan P.
Published: February 2008

Laboratory and numerical experiments are used to study convection confined to discrete layers. The laboratory experiments, which use the water content of a predominantly glycerine solution as a way of establishing ‘chemical’ density variations, show that convection will remain confined to superimposed layers when the chemical density contrast between layers is greater than the density change associated with the greatest temperature difference within the convecting system. The numerical experiments show a double thermal boundary layer at the interface between layers, a feature that would result in an average temperature increase of about 500°C in the mantle near 700‐km depth if the lower mantle is isolated as suggested by recent geochemical models. The possible causes of a layered mantle convection system include major element chemistry variations, the combined effect of major element chemistry and a phase change, or a phase change alone if it has a sufficiently negative Clapeyron slope.
High‐temperature creep of forsterite single crystals
Authors: Darot Michel , Gueguen Yves
Published: February 2008

Creep of forsterite single crystals has been studied with respect to the orientation of the differential stress. Three orientations have been investigated: [110]c, [101]c, and [011]c. Specimens were deformed at high temperature (T ≥ 1400°C) and moderate stresses (15 < σ < 110 MPa) in a dead load creep apparatus at room pressure and under controlled atmosphere. Assuming that the creep law has the general form
= Aσn exp −(Q/RT), two different tests were performed. Stress step and temperature step experiments gave the stress exponent n and the activation energy Q, respectively. A thorough analysis of the dislocation microstructures was carried out in the potential glide planes using the optical microscope (decorated thin section) and transmission electron microscope. Comparison of both mechanical and microstructural data allowed the determination of the different flow laws for the three orientations studied. Forsterite and olivine data are consistent; both flow laws and dislocation microstructures are similar. In light of these results, extrapolation of flow laws to the stress conditions of the mantle is not straightforward. Very high activation energies at low stresses (σ < 15 MPa) may suggest a change of flow mechanisms. Accordingly, further investigations in the low‐stress domain are required.
Detailed aeromagnetic investigation of the Arctic Basin: 2
Authors: Taylor P. T. , Kovacs L. C. , Vogt P. R. , Johnson G. L.
Published: February 2008

Over 150,000 line‐kilometers of low‐level (152 m) aeromagnetic data were recorded in the western Arctic Basin by the U.S. Navy during four field seasons (1975–1978); data from the first 2 years were presented by Vogt et al. (1979a), while the data from the last two years are described in this paper. These data (1977–1978) cover a swath from the north slope of Alaska to the north geographic pole. Flight lines were spaced between 10 and 24 km. The east‐west oriented aeromagnetic profiles across the Canada Basin and the Beaufort Sea suggest that Alaska was moving away from the Queen Elizabeth Islands of the Canadian Arctic from 153 m.y. B.P. (anomaly M‐25 time) to 127 m.y. B.P. (M‐12), at an opening rate of 2.6 cm/yr. An extinct spreading center is defined by a positive free‐air gravity anomaly, with the relic spreading axis generally paralleling the 150°W meridian. We are unable to recognize a coherent pattern of lineated anomalies over the Alpha Ridge; therefore its origin remains uncertain. A series of seafloor spreading type anomalies have been tentatively identified in the Fletcher (Makarov) Basin. Spreading began in the Upper Cretaceous (anomaly 34; 80 m.y. B.P.) and continued until mid‐Eocene (anomaly 21; 53 m.y. B.P.); total opening rate was about 1.7 cm/yr. During the opening of the Fletcher Basin, rifting began in Baffin Bay and the Norwegian and Labrador seas and the Eurasian Basin. Our results suggest a tectonic coupling between these areas at this time, with the Nares Strait acting as a transform fault and serving as a connector with the Baffin Bay/Labrador Sea spreading centers.
Seamount loading and stress in the ocean lithosphere: 2. Viscoelastic and elastic‐viscoelastic models
Authors: Lambeck Kurt , Nakiboglu S. M.
Published: February 2008

Analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts. The models investigated include layered elastic plates, homogeneous viscoelastic plates, and elastic over viscoelastic models. In all cases the composite models overlie an inviscid half space. The principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences. A number of geophysical observations point to stress relaxation when the lithosphere is loaded on geological time scales, and this has been modeled with viscoelastic and elastic‐viscoelastic models. The viscoelastic models explain many of these observations, but the stresses remain unrepresentative of those in the real lithosphere. The elastic‐viscoelastic models, in which the parameters defining each layer are considered as effective moduli or viscosities, represent better the rheological zonation of the earth with the crust being modeled by one or several elastic layers, the subcrustal lithosphere by a viscoelastic layer, and the mantle by an inviscid fluid. Observations of gravity or geoid height over the seamounts cannot readily discern between the viscoelastic and elastic‐viscoelastic models unless detailed observations over the moat and arch are available. A summary of some estimates of effective flexural rigidity indicates that the stress relaxation is an important factor in determining the response of the lithosphere to loading.
The effect of thermal cycling on the seismic quality factor Q of some sedimentary rocks
Authors: Clark V. A. , Spencer T. W. , Tittmann B. R.
Published: February 2008

Changes in Q in rocks in response to thermal cycling can result from the removal of physisorbed water, an increase in the crack density and therefore the internal surface area, at temperatures greater than 400°C, the removal of chemisorbed OH groups which are the adsorption sites for physisorbed water, and the dehydration of clays. When rocks are heated to temperatures of 100°–200°C, the change in Q is reversible and is due to the removal of physisorbed water. Sioux quartzite and Berea sandstone were thermally cycled to 675°C and 385°C, respectively. The Q of both rocks increased when measured over a broad range of relative humidities, but decreased at low humidities (<10–25%). Measurements of adsorption isotherms for the Berea sandstone showed no loss of adsorptive capacity of the clays. Although the surface area of thermally cracked rocks increased, the number of sites where adsorbed water molecules contribute to attenuation decreased.
Tidal dissipation in the Moon
Authors: Cappallo R. J. , Counselman C. C. , King R. W. , Shapiro I. I.
Published: February 2008

Euler's equations of motion, modified to include elasticity and solid friction, were used to study the rotation of the moon. Two heuristic models for the anelasticity were considered: Q independent of frequency and Q inversely proportional to the frequency of the strain oscillation. Parameters in each model were estimated by weighted least squares from 9 years of lunar laser range observations. The root mean square of the postfit range residuals was 19 cm in each case. For a strain period of 1 month, the estimates of Q obtained with the two models were similar and surprisingly low: 23 ± 6. The range of uncertainty, ±6, represents our estimate of the true standard deviation of the estimate of Q, as affected by systematic errors; it is 3 times larger than the formal, statistical, standard error.
The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks
Authors: Pearson Chris
Published: February 2008

Several hundred microearthquakes with local magnitudes ranging between −4 and −2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site, Fenton Hill, New Mexico. These events are probably caused by shear failure induced by high pore fluid pressures. Since the event locations seem to cluster in a narrow band near the hydraulic fracture, we were able to use micro‐seismic techniques to locate the hydraulic fracture and monitor its growth. We calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures. By comparing a pore pressure distribution, calculated using a one‐dimensional diffusion model with the distribution of event foci, we demonstrate that high pore pressures are a probable cause for these microearthquakes. Stress drops for these microearthquakes, calculated from spectral parameters, increased by a factor of 3 during the course of the experiment. The stress drop increase correlates with a migration of seismic activity away from the injection well bore, which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations.
Near‐infrared spectral reflectance of mineral mixtures: Systematic combinations of pyroxenes, olivine, and iron oxides
Authors: Singer Robert B.
Published: February 2008

Near‐infrared spectral reflectance data are presented for systematic variations in weight percent of two component mixtures of ferromagnesian and iron oxide minerals. Mixtures were chosen for application to the study of the dark materials on Mars, but the results are equally applicable to mafic and ultramafic assemblages elsewhere in the solar system. Olivine spectral features are greatly reduced in contrast by admixture of other phases but remain distinctive even for low olivine contents. Clinopyroxene and orthopyroxene mixtures show resolved pyroxene absorptions near 2 μm. The addition of limonite greatly modifies pyroxene and olivine reflectance but does not fully eliminate distinctive spectral characteristics. Using only spectral data in the 1‐μm region, it is difficult to differentiate orthopyroxene and limonite (goethite) in a mixture. Additional spectral coverage or other evidence may be required for a unique interpretation. All composite mineral absorptions observed in this study are either weaker than or intermediate in strength to the end‐member absorptions and have bandwidths greater than or equal to those for the end‐members. In general, spectral properties in an intimate mixture combine in a complex, nonadditive manner, with features demonstrating a regular but usually nonlinear variation as a function of end‐member phase proportions.
The uncertainties of finite rotations in plate tectonics
Authors: Hellinger S. J.
Published: February 2008

Incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates. A reconstruction may be represented by the finite rotation that describes one configuration relative to another. In this paper a method of reconstruction is provided that reflects the uncertainties in the data. The method minimizes a weighted least squares measure of fit as a function of the rotation parameters. For a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points (representing corresponding former plate margin segments) from a common plate margin. The common margin is estimated from the locations of both fixed and rotated data points. The estimated margin consists of a set of great‐circle arcs. It is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation. These techniques are then used to study the uncertainties of reconstructions in the South Pacific for the times of anomalies 13 and 18. The best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work. However, the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators.
Minor and trace element geochemistry of volcanic rocks dredged from the Galapagos Spreading Center: Role of crystal fractionation and mantle heterogeneity
Authors: Clague David A. , Frey Frederick A. , Thompson Geoff , Rindge Susan
Published: February 2008

A wide range of rock types (abyssal tholeiite, Fe‐Ti‐rich basalt, andesite, and rhyodacite) were dredged from near 95°W and 85°W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, the P2O5/K2O ratio averages 0.83 at 95°W and 1.66 at 85°W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with [La/Sm]EF = 0.67 at 95°W and 0.46 at 85°W. The sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95°W. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and volatile?) elements, although the source region beneath 95°W is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe‐Ti‐rich basalt from 85°W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85°W can be related to a common parental magma that contained approximately 9 wt % FeO*, 1 wt % TiO2, and had an Mg number (Mg# = 100 Mg/(Mg + Fe2+)) of about 65. Although the samples from 95°W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silica enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonite, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.
Pleistocene high‐silica rhyolites of the Coso Volcanic Field, Inyo County, California
Authors: Bacon Charles R. , Macdonald Ray , Smith Robert L. , Baedecker Philip A.
Published: February 2008

The high‐silica rhyolite domes and lava flows of the bimodal Pleistocene part of the Coso volcanic field provide an example of the early stages of evolution of a silicic magmatic system of substantial size and longevity. The rhyolites are sparsely porphyritic to virtually aphyric, containing qz + pl + san + bi + hb + mt ± allanite ± opx ± cpx ± fa ± il ± ap ± zircon phenocrysts. Major and trace element compositions of all 38 rhyolite extrusions are consistent with derivation from somewhat less silicic parental material by liquid state differentiation processes in compositionally and thermally zoned magmatic systems. Seven chemically homogeneous eruptive groups emplaced approximately 1.0, 0.6, 0.24, 0.17, 0.16(?), 0.09, and 0.06 m.y. ago can be distinguished on the basis of trace element and K‐Ar data. The oldest two groups are volumetrically minor and geochemically distinct from the younger groups, all five of which appear to have evolved from the same magmatic system. Erupted volume‐time relations suggest that small amounts of magma were bled from the top of a silicic reservoir at a nearly constant long‐term rate over the last 0.24 m.y. The interval of repose between eruptions appears to be proportional to the volume of the preceding eruptive group. This relationship suggests that eruptions take place when some parameter which increases at a constant rate reaches a critical value; this parameter may be extensional strain accumulated in roof rocks. Extension of the lithosphere favors intrusion of basalt into the crust, attendant partial melting, and maintenance of a long‐lived silicic magmatic system. Consideration of the mass of magma that must have been intruded into the crust in order to explain the anomalously high heat flow near the center of the rhyolite field and comparison of age, volume, mineralogic, and compositional characteristics of the rhyolites with those of caldera‐forming systems suggest that the Coso silicic system may contain a few hundred cubic kilometers of magma. The Coso magmatic system may eventually have the potential for producing voluminous pyroclastic eruptions if the safety valve provided by rapid crustal extension becomes inadequate to (1) defuse the system through episodic removal of volatile‐rich magma from its top and (2) prohibit migration of the reservoir to a shallow crustal level.
Hybrid granodiorites intruding the accretionary prism, Kodiak, Shumagin, and Sanak Islands, southwest Alaska
Authors: Hill Malcolm , Morris Julie , Whelan Joseph
Published: February 2008

A narrow belt of tonalite‐granodiorite‐granite plutons and batholiths intruded the accretionary prism in southwestern Alaska about 60 m.y. ago, simultaneously with plutonism over 100 km north along the main arc axis. The presence of metasedimentary xenoliths, kyanite, and garnet within the intrusions exposed on the Kodiak, Shumagin and Sanak islands establishes the presence of a crustal component. Extremely high values of δ18O, from +10.9 to +13.2 o/oo, require a crustal origin for much of the oxygen in the intrusions. Open‐system alteration has disrupted whole‐rock Rb‐Sr systematics in some samples from the Sanak pluton and Shumagin batholith. Mineral isochrons using unaltered minerals yield an age of 58.7 ± 1.2 m.y. and 87Sr/86Sri = 0.70534 ± 10 for the Shumagin batholith, and ages of 62.7 ± 1.2 m.y. and 87Sr/86Sri = 0.70523 ± 14 for the Sanak pluton. Comparison of the isotopic data for the intrusions with those of sedimentary rocks in the Kodiak and Shumagin formations requires that a low‐δ18O, low‐87Sr/86Sr component be present as well. Mixing models combining 87Sr/87Sr and Sr contents of various kinds of mafic magmas with partial melts of metasedimentary wall rocks suggest that three types of mafic magmas can satisfactorily duplicate the oxygen isotope signature of the intrusions: (1) a mid ocean ridge (MORB)‐like magma, (2) a magma derived by partial fusion of altered MORB, or (3) an arc basalt. The MORB model predicts abundances of Rb, Sr, Ba, and rare earth elements which are similar to those in the intrusions, while the altered MORB model does not. The arc magma model provides a better trace element match than the remelted MORB model, but is not as successful as the MORB model. These intrusions were preceded by a pulse of mafic to intermediate volcanism near or within the accretionary prism, recorded in the Paleocene Ghost Rocks Formation. This volcanism is probably the heat and mass source of the mafic component within the intrusions, and may be related to activity of the Kula‐Farallon ridge approximately 60 m.y. ago.
Occurrence and orientation of shatter cones in Pretoria group quartzites in the collar of the Vredefort ‘dome’: Impact origin precluded
Authors: Simpson Carol
Published: February 2008

Shatter cones have been found cutting completely indurated fault breccia associated with the down‐faulting of previously overturned Pretoria Group quartzites around the Vredefort structure. The evidence indicates that the shatter cones formed later than the overturning. Hence they cannot be considered to have been formed by an impact‐generated shock wave which penetrated the strata just before overturning.
A plausible two‐dimensional vertical model of the East Mesa Geothermal Field, California
Authors: Goyal K. P. , Kassoy D. R.
Published: February 2008

A two‐dimensional conceptual model of the East Mesa Geothermal system is developed on the basis of existing geological, geophysical, geochemical, heat flux, and borehole logging data. Hot water rising in a set of faults is assumed to charge the reservoir, which is overlaid by a clay‐rich cap. The temperature‐depth distribution observed at the site implies that the liquid is converting at a high Rayleigh number. In this approximation, liquid rises up the fault and spreads isothermally into the nearby sections of the reservoir. The cooling effect of the surface on the flow in the reservoir is confined to a thin layer adjacent to the cap‐reservoir interface near the fault. This layer grows with the distance from the fault. Eventually, the entire depth of the reservoir is cooled by the surface. The mathematical model is based on the flow of liquid water in a saturated porous medium. Results are obtained for the velocities, pressures, and temperatures in the entire system consisting of fault zone, aquifer, and clay cap. Finally we compare the predicted surface heat flux to that measured at the site in shallow wells. It is concluded that the model represents a plausible description of fault zone controlled systems like that at East Mesa.
Subduction of the Eastern Panama Basin and seismotectonics of northwestern South America
Authors: Pennington Wayne D.
Published: February 2008

Spreading along the Cocos‐Nazca plate boundary since the breakup of the Farallon plate in the Miocene has resulted in the formation of the Panama basin and a complex interaction of plates in and near northwestern South America. Current plate boundaries have been defined, and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes, considering only well‐located events, and through focal mechanism determinations. The existence of relict plate boundaries, bathymetric features, and the Panamanian isthmus has affected the subduction process of the Nazca plate beneath South America and determined the present‐day configuration of the subducting lithospheric plate. There is no single triple junction separating the Caribbean, South American, and Nazca plates. Instead, the Panamanian isthmus and surrounding areas are accommodating east‐west compression (and a lesser degree of north‐south compression) along a series of thrust faults striking NW to NE, and the Andean ranges of Ecuador, Colombia, and Venezeula are moving as a block NNE relative to the rest of the South American plates, along a system of faults following the front of the Eastern Cordillera. The subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study. From north to south, they are (1) a ‘Bucaramanga’ segment continuous with the Caribbean seafloor northwest of Colombia, (2) a ‘Cauca’ segment continuous with oceanic crust (Nazca plate) currently being subducted beneath South America at the Colombia‐Ecuador trench, and (3) an ‘Ecuador’ segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern Peru. The segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted.
Oceans and continents: Similarities and differences in the mechanisms of heat loss
Authors: Sclater John G. , Parsons Barry , Jaupart Claude
Published: February 2008

The principal objective of this paper is to present a simple and self‐consistent review of the basic physical processes controlling heat loss from the earth. To accomplish this objective, we give a short summary of the oceanic and continental data and compare and contrast the respective mechanisms of heat loss. In the oceans we concentrate on the effect of hydrothermal circulation, and on the continents we consider in some detail a model relating surface heat flow to varying depth scales for the distribution of potassium, thorium, and uranium. From this comparison we conclude that the range in possible geotherms at depths below 100 to 150 km under continents and oceans overlaps and that the thermal structure beneath an old stable continent is indistinguishable from that beneath an ocean were it at equilibrium. Oceans and continents are part of the same thermal system. Both have an upper rigid mechanical layer where heat loss is by conduction and a lower thermal boundary layer where convection is dominant. The simple conductive definition of the plate thickness is an oversimplification. The observed distribution of area versus age in the ocean allows us to investigate the dominant mechanism of heat loss which is plate creation. This distribution and an understanding of the heat flow through oceans and continents can be used to calculate the heat loss of the earth. This heat loss is 1013 cal/s (4.2 × 1013W) of which more than 60% results from the creation of oceanic plate. The relation between area and age of the oceans is coupled to the ridge and subducting slab forces that contribute to the driving mechanism for plate motions. These forces are self‐regulating and maintain the rate of plate generation required to achieve a balance between heat loss and heat generation.
Upper crustal structure of the Mount Hood, Oregon, region as revealed by time term analysis
Authors: Kohler W. M. , Healy J. H. , Wegener S. S.
Published: February 2008

Seismic refraction data with a dense areal distribution were collected to study the seismic structure of Mount Hood and the surrounding region. This area is typical of Cascade volcanoes and is geologically quite complex. The prime goals of this project were to search for velocity variations in the upper crustal rocks and to determine if the velocity of these rocks is anisotropic. A new system, including 100 remote recording units, was developed to facilitate the collection of data in this type of survey. The data collected in this study reveal a large variation in velocity and thickness of the uppermost crustal rocks that is probably typical of the High Cascade province. A regional structural pattern surrounding Mount Hood, where there is a marked thinning of low‐velocity near‐surface rocks, suggests that the present edifice of Mount Hood lies on top of a much larger structure, possibly the roof of a large batholith that was emplaced prior to the eruption of the volcanic rocks that form the modern mountain. A straightforward time term analysis of the data reveals this structure. When the time terms are used to correct the travel time observations for the variation in structure under each station, the remaining set of residuals indicates a variation of travel time with azimuth. While a systematic error in the time term reduction cannot be ruled out, this variation is probably the result of anisotropic velocity structure in the underlying refractor. The alignment of the direction of maximum velocity with the direction of maximum principal stress and the decrease in anisotropy with depth suggest that the anisotropic velocity structure is related to the opening and closing of fractures in the present stress field. The direction of maximum velocity is about N25°W, and the degree of anisotropy appears to vary with depth from about 3% near the surface to zero at depths below 8 km.
Paleomagnetism of the Moenkopi Formation, Colorado Plateau: Basis for long‐term model of acquisition of chemical remanent magnetism in red beds
Authors: Larson E. E. , Walker T. R. , Patterson P. E. , Hoblitt R. P. , Rosenbaum J. G.
Published: February 2008

Paleomagnetic studies of Moenkopi samples from several sites on the Colorado Plateau, including a complete 427‐m section at Sinbad Valley, western Colorado, reveal the following: (1) the presence of both normal and reversed Triassic components in many of the same samples at many localities, (2) the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the Sinbad Valley section, (3) a streaked pattern of the remanence directions in any one member in the Sinbad Valley section similar to that expected from polar wander, (4) a lack of progressive change from the base upward, and (5) the common intermediate remanence directions of weak intensity (suggesting the presence of two or more nearly balanced anti‐parallel components) throughout the formation. The results of our investigation, combined with previously reported petrographic and paleomagnetic studies of the Moenkopi Formation, have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism (CRM) acquired diagenetically over a geologically long time interval. From the available data we have formulated the following model to explain how the CRM was acquired through natural processes acting intermittently at geologically reasonable rates. When initially deposited, the Moenkopi sediments consisted primarily of first‐cycle detritus containing a few percent of iron‐bearing accessory minerals such as biotite, hornblende, magnetite, and ilmenite. At some time after deposition, under favorable subsurface conditions, intrastratal alteration began, involving breakdown of iron‐bearing silicate and oxide minerals, partial replacement of some iron‐rich silicate grains by hematite, martitization of magnetite, growth of hematite pods and blebs in ilmenite and biotite, and precipitation of crystalline hematite in both primary and secondary voids. The authigenesis spanned at least two polarity events and probably continued over several tens of millions of years. Stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies. Because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events, any given lithologic unit tended to be dominated by one polarity. The model given above seems to be capable of explaining not only those magnetic and petrographic features found in the Moenkopi Formation but also those of most other red beds.
Aureole deposits of the Martian volcano Olympus Mons
Authors: Morris Elliot C.
Published: February 2008

The aureole of grooved terrain that surrounds the large shield volcano Olympus Mons consists of several overlapping roughly circular sheets, each 0.5 to 1.5 km thick. The surfaces of these materials bear abundant curvilinear ridges and troughs 10 to 100 km long and 1 to 5 km wide, which form anastomosing patterns that vary in length and width over the aureole. The aureole is asymmetric to Olympus Mons, extending almost 1000 km northwest from the center of the volcano but only 600 km southeast. The lobate form of the deposits, the pressure ridges on their surfaces, and the deflection of aureole material around barriers indicate that the deposits were emplaced as viscous flows. High‐resolution Viking orbiter pictures show the aureole material to be unstratified, and it is inferred to contain randomly distributed large dark blocks whose weathering products form dark streaks in the talus on the flanks of ridges. A pyroclastic origin is proposed for the aureole. The following sequence is suggested. At least six great pyroclastic eruptions occurred prior to the construction Olympus Mons, they formed very fluid ash flows that became viscous during late stages of emplacement, and pressure ridges formed on the surfaces of the deposits before they finally came to rest. The oldest and most extensive aureole deposit (finely grooved terrain) was considerably modified by erosion prior to being partly covered by later eruptions. A positive gravity anomaly over the Olympus Mons area may define the site of a near‐surface magma chamber that was the source of the pyroclastic eruptions.
Thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate
Authors: Furlong Kevin P. , Chapman David S. , Alfeld Peter W.
Published: February 2008

The path which subducting lithosphere follows can be broken into three zones: zone I, a subhorizontal region of nearly constant dip near the trench; zone II, a region of rapidly increasing dip; and zone III, a region generally deeper than 125 km, again of nearly constant dip. We have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines, a numerical technique well suited to the solution of time dependent partial differential equations. By use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere, slab velocities, and paths followed by the slab. Thermal modeling results indicate that the geometry of the slab in region II can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity. It follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities (>15 cm yr−1) and old (>120 m.y.) lithosphere. We further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and/or age variations in the subducting lithosphere lead to migration of volcanic arcs.
Noise and target strength degradation accompanying shallow‐buried explosions
Authors: Gaffney Edward S. , Melosh H. Jay
Published: February 2008

Ground shock from shallow‐buried explosions is commonly accompanied by acoustic ‘noise.’ Analysis of records from a series of 10‐ton chemical detonations in Gulf Coast sediments provides a basis for the first quantitative description of such noise during a cratering event. The test series included shots at depths of 3–12 m leading to craters about 20 m in radius. Most of the records studied were taken within the eventual crater; they include stress, velocity, and acceleration. Oscillations arising from reflections off the free surface or subsurface layers are evident in stress records. High‐frequency oscillations, attributed to reflections or scattering from smaller scale inhomogeneities, are particularly striking in the acceleration records, where peak accelerations were 10–100 km/s2. Oscillations with frequencies of 5–25 kHz are obvious. They exhibit exponential decay with the dominant frequency decreasing with time. Approximate values of Q estimated from these accelerograms are 30±20, agreeing with Q for similar materials derived from seismic techniques. Since seismic Q's include both internal dissipation and scattering losses, whereas explosive Q's include internal dissipation and radiative losses, the magnitude of scattering and radiative losses must be comparable. The latter alone correspond to an effective Q of about 300, so that internal dissipation dominates Q in these poorly consolidated sediments. Acoustic fluidization may have occurred for a couple of milliseconds in the top 1 or 2 m of soil within the eventual crater (by this, we mean that the material was liable to flow if the threshold shear stress were exceeded). This result is inferred from ground shock stress duration, acoustic amplitude decay, and initial overburden stress. Fluidization of such limited duration and extent would have no effect on crater growth, just as would be expected for such a low yield event.
A polar wandering curve for North America during the Mesozoic and Cenozoic
Authors: Harrison C. G. A. , Lindh T.
Published: February 2008

If paleomagnetic poles from individual studies are compared with mean paleomagnetic poles, it is found that there are significant numbers of deviations which are greater than would be expected when the formal statistical error associated with the position of the individual pole is considered. This suggests that in many cases, individual poles have not completely sampled the earth's magnetic field and its variation to give a good estimate of the mean field. This further suggests that uniform weighting of individual studies to obtain a mean pole is incorrect. We develop a method of determining mean poles in which the information content of individual studies is used as a weighting factor. No assumption need be made that the individual studies represent a mean pole for the time period under consideration. Studies in which it is known that the mean pole has not been adequately determined may then be used in the analysis by giving such studies a low weighting factor in the analysis. We also introduce a method of weighting dependent on the age overlap between the individual study and the age window for which a mean pole is to be calculated. The most important determinant of the polar wandering path is, however, the data set used.
The Yellowstone‐Snake River Plain Seismic Profiling Experiment: Crustal structure of the Eastern Snake River Plain
Authors: Braile L. W. , Smith R. B. , Ansorge J. , Baker M. R. , Sparlin M. A. , Prodehl C. , Schilly M. M. , Healy J. H. , Mueller St. , Olsen K. H.
Published: February 2008

Seismic refraction profiles recorded along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 Yellowstone‐Snake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q‐1) structure of the ESRP. Travel‐time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an upper‐crustal layer (compressional velocity ≅6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediate‐velocity (≅6.5 km/s) layer extends from ≅10 to ≅20 km depth. A thick (≅22 km) lower crust of compressional velocity 6.8 km/s, a total crustal thickness of ≅42 km, and a Pn velocity of ≅7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to low‐Q values in the volcanic rocks (Qp = 20 to 200) and throughout the crust (Qp = 160 to 300). Based on these characteristics of the crustal structure and volcanic‐age progression data, it is suggested that the ESRP has resulted from an intensive period of intrusion of mantle‐derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate layer. Crustal subsidence and periodic basaltic volcanism as represented by the ESRP complete the sequence of crustal evolution.
Crustal structure of the Eastern Snake River Plain determined from ray trace modeling of seismic refraction data
Authors: Sparlin M. A. , Braile L. W. , Smith R. B.
Published: February 2008

Ray trace travel time modeling of the seismic refraction record sections for a profile from near Soda Springs, Idaho, to near McKay, Idaho, was used to derive a crustal model across the eastern Snake River Plain (ESRP). The derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the ESRP. The interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the ESRP when compared with the upper crust beneath the adjoining Northern Rocky Mountain and Basin and Range provinces. The most prominent features of the crustal structure inferred by the ray trace modeling are as follows: (1) The northwest margin of the ESRP was modeled as a fault structure, downthrown on the SRP side (SE) with an offset of greater than 4 km. The southeast margin, conversely, appears to be downwarped with possible minor faulting. Paleozoic sedimentary rocks are present beneath the Cenozoic volcanics of the ESRP possibly to and beyond 40 km from the southeastern margin of the ESRP near Blackfoot, Idaho. (2) The modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the ESRP. (3) An intermediate 6.5 km/s layer occurring beneath the ESRP, interpreted from a refraction profile located along the axis of the ESRP, was found to be localized within the ESRP margins. This layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region. (4) A density model of the crust across the ESRP was prepared with the densities selected using the interpreted seismic velocities as a constraint. The gravity field calculated from this model resulted in a good match to observed gravity data over the eastern Snake River Plain.