bluuebunny/crossref_metadata_2025 huggingface dataset:
Small‐scale features in the Earth's magnetic field observed by Magsat
Authors: Cain Joseph C. , Schmitz Dave Ray , Muth Lorant
Published: February 2008

A spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of Magsat data. Global maps representing the contribution due to terms of the expansion above n = 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar, even in polar regions. An expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high‐order field structure at a constant altitude than do component maps derived independently.

The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐008; $2.50. Payment must accompany order.
Postglacial rebound observed by lageos and the effective viscosity of the lower mantle
Authors: Rubincam David Parry
Published: February 2008

Postglacial rebound appears to have been observed gravitationally by the Lageos satellite. Sixty‐four observations of the orbital node made over a 5‐year time interval reveal an acceleration of ( −8.1±1.8)×10−8 arc sec d−2 due to a source which is not presently modeled in the GEODYN orbit determination computer program. This acceleration cannot be explained by the ocean tide with an 18.6‐year period, assuming it to be an equilibrium tide. Instead it seems to be due to postglacial rebound, which changes the J2 coefficient in the spherical harmonic expansion of the earth's gravitational field at the rate of ( −8.2±1.8)×10−19 s−1 this, in turn, accelerates the node. This rate does not agree with the −32×10−19 s−1 predicted by Wu and Peltier's L2 model, which has upper and lower mantle effective viscosities of 1021 and 1022 Pa s, respectively. It does agree well with their L1 model, which gives about −10×10−19 s−1. Since the effective viscosity is 1021 Pa s throughout the entire mantle in the L1 model, the results support the contentions that (1) the effective viscosity is near 1021 Pa s everywhere in the mantle, and (2) this relatively low value for the effective viscosity may have permitted several degrees of polar wander due to glaciation during the Quaternary Ice Age.
Crustal structure in the Orozco Transform Zone
Authors: Tréhu A. M. , Purdy G. M.
Published: February 2008

Seismic refraction data from lines of small shots and from individual large shots recorded during project ROSE are used to determine the crust and upper mantle velocity structure in the Orozco transform zone. The objective of this study is to provide constraints on the origin of a deep north‐south trending trough flanked by two shallow ridges. These topographic features are incompatible with the present plate tectonic framework as defined by the regional spreading direction (approximately east‐west) and did not display any seismic activity during project ROSE. Travel times and amplitudes from reversed profile along one of the ridges reveal an anomalous crustal structure: a velocity increase approximating a gradient of 0.5 s−1 extending through most of the crust and decreasing to 0.1 s−1 at the base and a total crustal thickness of about 5.4 km. In contrast, three profiles from topographically normal oceanic crust indicate a crustal structure typical of oceanic crust throughout the oceans: a 1.5‐km‐thick layer in which velocity increases from about 3 km s−1 to 6.7 km s−1 overlying a 4.5‐km‐thick layer with a velocity of about 6.8 km s−1. The upper mantle velocity beneath both structures, determined from travel times from large shots at ranges of 30–100 km, is about 8.1 km s−1. These results are interpreted to indicate that the trough and flanking ridges represent an extinct spreading center that developed an axial valley as the spreading center progressively shifted to the present axis of the East Pacific Rise.
Recurrence models and Parkfield, California, earthquakes
Authors: Bakun William H. , McEvilly Thomas V.
Published: February 2008

Main shocks of the earthquake sequences that occurred on the Parkfield section of the San Andreas fault in central California in 1922, 1934, and 1966 are characterized by southeast rupture expansion over the same 20‐ to 30‐km‐long section of the fault. Whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10%, the seismic moment for 1966 is 20% greater than for the earlier events to within a precision of 20%. The Parkfield area seismicity, in general, seems well described by a recurring moderate size characteristic earthquake, repeating the same epicenter, magnitude, seismic moment, rupture area, and southeast direction of rupture expansion. An unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the Parkfield earthquakes in 1857, 1881, 1901, 1922, 1934, and 1966 represent a strictly periodic process. Assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period, the next characteristic Parkfield earthquake should occur between 1983 and 1993.
High‐frequeney acoustic backscattering and seismic attenuation
Authors: Dainty Anton M.
Published: February 2008

An expression for the backscattered intensity of acoustic waves singly scattered from a region containing fluctuations of the acoustic velocity has been derived for high frequencies by expanding the autocorrelation of the slowness fluctuations in a Taylor series about zero lag. The resulting expression indicates that the backscattered intensity is independent of frequency and directly proportional to the first derivative of the autocorrelation at zero lag; the next higher term is proportional to the reciprocal of the square of the frequency and directly proportional to the third derivative of the autocorrelation at zero lag. Contributions from terms of the Taylor series involving even numbered derivatives of the autocorrelation are zero. Since for the autocorrelation of a smooth function only the even numbered derivatives are nonzero at zero lag, this result demonstrates that backscattering at high frequencies can only occur from discontinuities of velocity or its derivatives as opposed to fluctuations in which the velocities are smooth. If the backscattered intensity is independent of frequency, the contribution of backscattering to the attenuation parameter Q is proportional to frequency. Such behavior may have been observed for seismic waves of frequencies greater than 1 Hz, suggesting that scattering from discontinuities is an important part of the attenuation of such waves.
Statistical tools for the analysis of geomagnetic reversal sequences
Authors: McFadden P. L.
Published: February 2008

Previous investigations of geomagnetic reversal sequences have shown that a nonstationary gamma distribution provides a good description of the intervals between reversals. The general problems of parameter estimation and testing for differences between the statistical properties of reverse and normal polarity sequences are considered. A likelihood method is suggested to overcome the problems of nonstationarity and the consequent dependence of polarity interval lengths while avoiding the problems associated with the use of sliding windows. Previously, the good fit of a gamma distribution has been interpreted as implying that geomagnetic reversals are a gamma process. An alternative possibility is presented here by showing that incomplete data from a Poisson process leads to a distribution of interval lengths essentially indistinguishable from a gamma distribution.
A note on the rate sensitivity of frictional sliding of westerly granite
Authors: Solberg P. , Byerlee J. D.
Published: February 2008

The frictional sliding of simulated faults containing crushed Westerly granite was studied in a series of constant stepped velocity experiments. Confining pressures were varied from 0.00733 to 7.33 μm/s. The strain rate sensitivity B, that is, the slope of the frictional stress change Δμ with the logarithm of the change in velocity, (log10 (V2/V1)), was found to be low and pressure dependent, increasing with increased confining pressure to a maximum of approximately 0.01 at 300 MPa confining pressure.

Creep curves are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐011; $2.50. Payment must accompany order.
Effects of compression direction on the plasticity and rheology of hydrolytically weakened synthetic quartz crystals at atmospheric pressure
Authors: Linker Mark F. , Kirby Stephen H. , Ord Alison , Christie John M.
Published: February 2008

A hydrothermally grown synthetic quartz crystal with 370±60 ppm hydroxyl impurity was cut into right rectangular prisms in eight crystallographic orientations. We compressed the prisms under constant axial force corresponding to a uniaxial stress of 140.0±0.5 MPa, and temperatures of 510° and 750°C. All but one of the samples sustained permanent axial strains of 2–3%. We established the operating slip systems from specimen shape change, slip bands and dislocation etch pits on polished surfaces, crystallographic orientation changes, stress optical features in thin sections, and transmission electron microscopy. The observed creep behavior and plasticity divided the samples into three groups: (1) Crystals compressed at 45° to [0001] and [
] and those compressed ⊥(
) and ⊥(
) deformed principally by slip parallel to [0001]. Creep rates were relatively high and were not strongly sensitive to test temperature. Dislocation arrays approximately parallel to (
) are common. Dislocation loops are elongate parallel to [0001], indicating that the edge segments were more mobile than the screw segments. (2) The second group of samples were loaded normal to [0001] in three orientations: ⊥(
), ⊥(
), and at 45° to (
). These samples deformed primarily by {
} 〈a〉 slip with some evidence for secondary slip on the other systems. They were more creep resistant than the first group and displayed a much higher sensitivity of creep rate to test temperature. Dislocation loops are very elongate parallel to [0001], indicating that the screw dislocation segments were much more mobile than the edge segments. (3) A sample compressed parallel to [0001] at 750°C crept at a barely detectable rate (∼10−9 s−1) and no optical scale slip features were observed. These results confirm our earlier work on one orientation each from groups 1 and 2, which indicated a strong creep anisotropy for this same crystal. This creep anisotropy parallels a remarkably similar anisotropy in the diffusivity of impurities in quartz, suggesting a causal relationship between impurity diffusion and creep associated with hydrolytic weakening.

Appendix Tables A1‐A5 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐004; $2.50. Payment must accompany order.
“Pressure solution” creep: Some causes and mechanisms
Authors: Green H. W.
Published: February 2008

Despite the widespread evidence of stress‐controlled dissolution and precipitation in diagenetically altered and low‐grade metamorphic rocks, a great deal of controversy remains concerning the driving forces and transport mechanisms involved. To clarify the various driving forces, the free enthalpy equation is expanded here to allow identification of different terms contributing to the overall phenomenon. It is argued that under diagenetic conditions, stress concentrations at grain‐to‐grain contacts will be the largest source of chemical potential gradients and that upon burial and cementation these inhomogeneities decline and the orientation dependence of normal stress in a quasi‐homogeneous stress field becomes important as well. These mechanisms operate efficiently enough under these relatively cold, H2O‐rich conditions that stresses can remain below the threshold for crystal plastic deformation. Water on grain boundaries provides at the very least a high diffusivity path and in cases of large volume losses must also contribute directly through fluid flow. Most experimental work on this phenomenon has not distinguished carefully between stress‐enhanced solubility and solubility enhancement due to plastic deformation or microcracking. A new thermodynamic analysis of the results of some experiments by Sprunt and Nur suggests that in at least some of their experiments, true pressure solution creep has been activated. A related phenomenon, volume transfer creep during phase transformations which involve significant volume change, displays many of the characteristics of pressure solution.
The earthquake deformation cycle, recurrence, and the time‐predictable model
Authors: Thatcher Wayne
Published: February 2008

Geodetic and geologic observations from a handful of well‐studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation. In ideal circumstances the time‐predictable model requires knowledge of only the seismic slip (and time) of the last earthquake and the rate of relative plate motion or fault slip. Practically, indirect measurements must usually suffice, local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate. Permanent, nonrecoverable deformation, often roughly half the coseismic strain offset near great thrust earthquakes, complicates the inference of interplate seismic slip. Owing to short‐ and long‐term postseismic transients, commonly 20–40% of the coseismic strain drop, deformation rates are variable and not simply related to the plate motion rate. At strike‐slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly. When these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete, recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle. Systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are most severe.
Geoid anomalies in a dynamic Earth
Authors: Richards Mark A. , Hager Bradford H.
Published: February 2008

In order to obtain a dynamically consistent relationship between the geoid and the earth's response to internal buoyancy forces, we have calculated potential and surface deformation Love numbers for internal loading. These quantities depend on the depth and harmonic degree of loading. They can be integrated as Green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts. Spherically symmetric, self‐gravitating flow models are constructed for a variety of radial Newtonian viscosity variations and flow configurations including both whole mantle and layered convection. We demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound, much less than the time scale for significant change in the convective flow pattern, by calculating relaxation times for a series of spherically symmetric viscous earth models. For uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads, resulting in net negative geoid anomalies for positive density contrasts. Geoid anomalies from intermediate‐wavelength density contrasts are amplified by up to an order of magnitude. Geoid anomalies are primarily the result of density contrasts in the interior of convecting layers; density contrasts near layer boundaries are almost completely compensated. Layered mantle convection results in smaller geoid anomalies than mantle‐wide flow for a given density contrast. Viscosity stratification leads to more complicated spectral signatures. Because of the sensitivity of the dynamic response functions to model parameters, forward models for the geoid can be used to combine several sources of geophysical data (e.g., subducted slab locations, seismic velocity anomalies, surface topography) to constrain better the structure and viscosity of the mantle.
Moment‐magnitude relations in theory and practice
Authors: Hanks Thomas C. , Boore David M.
Published: February 2008

The observation that motivates this study is the difference in c values in moment‐magnitude relations of the form log MO = cML + d between central and southern California. This difference is not at all related to geographical area; rather, it results from positive curvature in the log MO ‐ ML plane and the relatively large number of ML<5 earthquakes in the central California data set. With the prescription that the far‐field shear waves from which ML is taken be finite‐duration, band‐limited, white Gaussian noise in acceleration, we can estimate ML as a function of MO alone, by fixing the arms stress drop at 100 bars and ƒmax at 15 Hz. These model calculations fit available California moment‐magnitude data for 0≲ML≲7,1017≲MO≲1028 dyne cm with striking accuracy. This range in source strength is entire: earthquakes with MO≥1028 dyne cm are unlikely to occur in California, and earthquakes with ML<0 cannot be recorded in California, at least under ordinary conditions of recording earthquakes at ordinary hypocentral depths. More fundamentally, the remarkably good fit of model to data implies that the arms stress drop of 100 bars (to a factor of 2 or so) is a stable and pervasive feature of all (ML≳2 1/2) California earthquakes whose spectral corner frequency lies in the “visible” bandwidth, ƒ0≤ƒmax.
Estimates of sulfur and chlorine yield to the atmosphere from volcanic eruptions and potential climatic effects
Authors: Devine J. D. , Sigurdsson H. , Davis A. N. , Self S.
Published: February 2008

We report analytical methods based on electron microprobe analysis of volcanic tephra which allow estimation of minimum masses of sulfur and chlorine released to the atmosphere by volcanic eruptions. Differences between sulfur and chlorine contents of glassy melt inclusions (trapped by tephra phenocrysts) and matrix glasses, respectively, scaled to estimated eruption volumes, provide estimates of minimum sulfur and chlorine yield to the atmosphere. Declines in mean northern hemisphere surface temperatures associated with several Recent eruptions (Laki 1783, Tambora 1815, Krakatau 1883, Agung 1963, Mount St. Helens 1980) are positively correlated with our estimates of the rnimimum mass of sulfur released to the atmosphere, reinforcing the idea that sulfate aerosols have a greater climatic impact than volcanic “dust.” Injection of volcanic emissions into the stratosphere increases the optical thickness of the atmosphere which may result in cooling of the earth's surface. Our results indicate that decreases in mean surface temperatures associated with the eruptions listed above appear to vary linearly with the cube root of the estimated minimum mass of sulfur released by the eruptions. This apparent geometric relationship suggests that increases in optical thickness may be completely related to increases in the mass of particulates in the stratospheric aerosol layer‐Correlations between observed mean surface temperature decreases and other eruption‐related parameters (estimated chlorine yield, estimated total sulfur and chlorine yield, estimated tephra volume dense rock equivalent) are poor compared to the correlation with our estimates of sulfur yield. Application of our methods to the 1982 eruption of El Chichon volcano results in underestimation of the minimum mass of sulfur released to the atmosphere, judging from independent estimates based on airborne sampling and satellite observations of the eruption plume. The presence of anhydrite phenocrysts in El Chichon tephra suggests that decomposition of anhydrite may have contributed to the sulfur yield of this eruption. Potential contributions of volatile species to the atmosphere from eruptions of basaltic magma are generally about an order of magnitude higher in sulfur than eruptions of equal volumes of silicic magma, in spite of the fact that silicic magmas inject more fine ash and total volatiles (H2O, CO2) into the upper atmosphere. In view of the significant climatic impact of the Laki 1783 fissure eruption of basaltic magma, we suggest that flood or fissure basalt eruptions should be taken into account along with explosive silicic volcanism in paleoclimatic reconstructions.
Spectral properties of ice‐particulate mixtures and implications for remote sensing: 1. Intimate mixtures
Authors: Clark Roger N. , Lucey Paul G.
Published: February 2008

The spectral properties of water ice‐particulate mixtures are studied for the purpose of deriving the ice and particulate abundances from remotely obtained spectra (particulates referring to nonicy materials in the form of grains). Reflectance levels and ice absorption band depths are a complex function of the single scattering albedo of the particulates embedded in the ice. The ice absorption band depths are related to the mean optical path length of photons in ice through Beers law, Fresnel reflection from the ice‐crystal faces on the surface, and ice absorption coefficient as a function of wavelength. Laboratory spectra of many ice‐particulate mixtures are studied with high‐, medium‐, and low‐albedo particulates. From the laboratory spectra the band depth, the continuum reflectance, and the ratio of band depth to continuum reflectance are derived for each particulate albedo as a function of the logarithm of the particulate weight fraction in the sample. Ice band depths are dependent on the particulate albedo and increase with smaller weight fractions of particulates until the bands saturate and their depths decrease. The continuum reflectance is a complex function of the particulate albedo and wavelength of light, while the band depth to continuum reflectance ratio appear independent of particulate albedo such that, for a given grain size of particulate, a calibration curve to abundance of ice and particulates is derived. The derived abundance calibrations are accurate if the source of scattering is dominated by particulates and not ice‐vacuum (air) interfaces. If ice‐vacuum interfaces are dominant, then the derived particulate abundance is an upper limit. Theoretically produced frost spectra show similar curves of growth of band depth, continuum reflectance, and corresponding ratio versus the photon mean optical path length similar to that for intimate mixtures of ice and high‐albedo particulates. In all scattering cases the ice absorption coefficients determine the path length needed to produce a given band depth; thus different absorptions are sensitive to different amounts of particulates, with the 3‐μm fundamental being the most sensitive to the presence of ice (very low ice abundance) and the 1.04‐μm overtone being the most sensitive to the presence of particulates (very high ice abundance).
Normal modes of the Coupled Earth and Ocean System
Authors: Wahr John M.
Published: February 2008

When two physical systems are dynamically coupled, the eigenmodes for the combined system will differ, in general, from the modes for the two original, uncoupled systems. This paper examines certain normal modes of the coupled earth and ocean system and discusses differences from corresponding results for the two individual systems. Particular attention is paid to those modes which involve wobble of the solid earth. It has been suggested that the coupled system may possess a long—period wobble mode, not part of the spectrum for the solid earth alone, which might be identified as the Markowitz wobble. The results presented below do not support that conclusion.
Leveling precision and bench mark motions, Piñon Flat Observatory, California
Authors: Sylvester Arthur G.
Published: February 2008

The reliability of relative vertical motions detected by levelings of bench marks installed at Pinyon Flat is highly dependent on the kinds of bench marks installed for monitoring purposes. The most stable marks are the end monuments for two 535‐m‐long fluid tiltmeters. Nine direct measurements of average slope between tiltmeter end monuments agree to within a standard deviation about the mean of 0.2 mm (0.4μrads), comparable to the slope measured by the tiltmeters themselves. Motions of class B steel rod marks are random with standard deviations ranging from 0.5 to 3.2 mm, averaging 1.0 mm. The motions of nearly all rod marks decreased greatly in the fourth year, evidently because of a time‐dependent establishment of stability in the decomposed granite fundament. Other kinds of bench marks are stable between 0.5 and 0.8 mm. Tectonic tilt is not revealed in 15 levelings of the array in 4 years, consistent with the tiltmeters, but the random bench mark motions dominate the errors in the estimated tilt.
Gravity empirical covariance values for the continental United States
Authors: Goad Clyde C. , Tscherning C. Christian , Chin M. Miranda
Published: February 2008

Variances and correlation distances have been determined for terrain corrected Bouguer anomalies in all 30×30 arc min blocks covering the continental United States. Also linear trend values were subtracted from the data. In order to make the determination independent of the frequently irregular local data distribution, gravity values predicted in a regular grid were used. The prediction was made using least squares collocation, constructing local surfaces representing the gravity anomalies for each 30×30 arc min block. For this purpose 544,000 terrain‐corrected Bouguer anomalies were available in the U.S. National Geodetic Survey gravity data base. Therefore in order to limit the computational effort, a sequence of surfaces was determined by selecting in each block an increasing number of data points until a limit of the absolute value of the error equal to 4 mGal was reached. This procedure reduced the number of gravity observations used per block to a very few (in areas of little gravity variation) to maximally 188. The correlation distance was found to be smallest in areas of high topography. Large variances are associated with the midcontinental gravity high and tectonic features on the Pacific coast.
Welded tuffs deformed into megarheomorphic folds during collapse of the McDermitt Caldera, Nevada‐Oregon
Authors: Hargrove Howard R. , Petro‐Lewis Denver , Sheridan Michael F.
Published: February 2008

The Montana Mountains contain part of a resurgent dome complex within the southwestern sector of the McDermitt caldera. Exposed intracaldera units consist of welded tuffs, lavas, domes and epiclastic deposits. The majority of volcanic units have a comenditic composition, but a few units are nonperalkaline and biotite bearing. A deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m. thick. The geometry of vitrophyres, flow bands, foliations, and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes. However, the contacts between units are conformable, the vitrophyres are continuous from one fold to the next, and the units display pumice, shards, and other features common in welded tuffs. The deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted (but not folded) and beneath relatively undeformed tuffs and lake deposits. This relationship suggests that their large‐scale folds in the deformed sequence developed shortly after caldera collapse
Seismic source influence in pulse attenuation studies
Authors: Blair D. P. , Spathis A. T.
Published: February 2008

Analysis of pulse rise time data produced from repetitive piezoelectric and magnetostrictive seismic sources shows that the change in rise time as a function of distance is source dependent. Thus a linear rise time law of the form τ = τ0 + CT/Q, with C source independent, cannot be used to assess the attenuation of rock in situ. In fact, application of such a law to the data results in a significant overestimate in the value of Q. A numerical example is also presented to show that in terms of filter theory a linear rise time law is ill founded. An alternative, source independent method of assessing the attenuation is employed for both constant‐Q and non‐constant‐Q models, and it is found that either model may be made to give a reasonable fit to the data. However, it is shown that if a non‐constant‐Q mechanism is operating, then there is probably less than a 5% change in Q over the frequency range 0.80–70.OkHz.
Effects of dilatant hardening on the development of concentrated shear deformation in fissured rock masses
Authors: Rudnicki J. W.
Published: February 2008

This study examines the effects of dilatant hardening on the development of concentrated shear deformation. Specifically, the analysis considers the shear of an inelastic ally deforming rock mass containing a weakened layer of thicknessh. The presence of the weakened layer causes localization instability, characterized by an unbounded ratio of a strain increment in the weakened layer to that in the far field, to occur earlier than it would be predicted from the response of the material surrounding the embedded layer. The development of instability in time depends on the ratio of the rate of imposed shear strainto that for fluid mass exchange between the layers,c/h2, wherecis an effective diffusivity. In the limit, the pressure in the weakened layer is equal to that in the surrounding material and localization instability occurs at the peak of the drained stress‐strain curve. For finite, instability is delayed until the material in the weakened layer has been driven to the peak of its undrained, dilatantly hardened stress‐strain curve. The time delay between final instability and the time at which the weakened layer passes the peak of its drained stress‐strain curve is called the precursor timetprbecause rapid straining of the weakened layer occurs during this period. For small, as appropriate for tectonic applications and most laboratory experiments, a nonlinear asymptotic analysis predicts that, where λ is the half width of the peak of the stress‐strain curve, Δ is the difference in the peak stresses of the rock mass and the weak layer divided by λ times the elastic shear modulus, and α is a nondimensional measure of the strength of dilatant hardening. For a wide range of numerical values the precursor times are very short: less than a few hours for tectonic strain rates and less than a few tens of seconds for typical laboratory strain rates.
Global plate motions relative to the hot spots 64 to 56 Ma
Authors: Jurdy Donna M. , Gordon Richard G.
Published: February 2008

We examined the early Tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early Tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole. In our reconstruction, the velocities of the Pacific and African plates were inferred directly from their hot spot tracks, and the velocities of other plates with respect to the hot spots were calculated from the motion of the Pacific or African plate and by relative motions estimated from seafloor spreading data. The relative motion between assumed‐separate North Pacific and South Pacific (Chatham Rise) plates was estimated by assuming the hot spots in the Atlantic Ocean basin have been fixed with respect to those in the Pacific Ocean basin, thereby avoiding the use of a poorly defined relative motion circuit. We found that the characteristics of early Tertiary plate motions with respect to the hot spots resemble those at present. The root‐mean‐square velocity of every major continental plate with respect to the hot spots during the early Tertiary exceeds its present velocity but (with the exception of the Indian plate) is less than the root‐mean‐square velocity of every early Tertiary oceanic plate. Equatorial lithosphere moved faster than polar lithosphere during the early Tertiary, but the difference is less than at present. Over the interval 64 to 56 Ma, the no net‐torque absolute velocities differ from the assumed fixed‐hot spot velocities by only 0.04°/Ma, which is insignificant. Thus two dissimilar approaches, fixed hot spots and no net torque of the lithosphere, when applied to identical early Tertiary relative plate velocities, yield similar absolute plate velocities. We attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here.
Evidence for a hot spot origin of the Caroline Islands
Authors: Keating B. H. , Mattey D. P. , Helsley C. E. , Naughton J. J. , Epp D. , Lazarewicz A. , Schwank D.
Published: February 2008

The results of paleomagnetic, petrographic, and radiometric studies of the Eastern Caroline Islands in the western Pacific indicate that the islands were formed by a hot spot located near the paleoequator between 1 and 11 Ma. The islands show a linear progression of mean ages from 1 Ma in the east (Kusaie) to 11 Ma in the west (Truk). The results of volumetric measurements and geochemical studies suggest that the hot spot source is waning and perhaps was slowly dying during the time Truk, Ponape, and Kusaie were being formed. The dominant shield‐building magmas in the Caroline Islands are part of a differentiated alkalic series. The posterosional lavas are highly silica undersaturated and trace element enriched nephelinites. The latter were erupted subsequent to the cessation of the main shield phase of volcanism. The petrography and geochemical evolution of Truk are strongly reminiscent of that of the Hawaiian chain; however, the shield‐building lavas are compositionally similar to the alkalic lavas that typically form only thin, late‐stage caps on many Hawaiian volcanos. No tholeiitic rocks were found despite sampling deep within the eroded volcanic structure of the islands. This absence of tholeiitic lavas and dominance of alkalic lavas stand in contrast with Hawaii, where tholeiitic volcanism dominates and alkalic lavas form only a minor component of the exposed lavas. The absence of tholeiitic lavas in the main shield‐building phase of construction, however, is not unique to the Caroline Islands. Dominant alkalic volcanism appears characteristic of other seamounts in the Pacific, including the Samoan, Austral‐Cook, and Line Islands.
Implications of volcano and swell heights for thinning of the lithosphere by hotspots
Authors: Epp David
Published: February 2008

The heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere. The depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot. There is a square‐root relationship between volcano height and lithospheric age. Relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities. If the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere, then a relationship between the thickness of the lithosphere before and after reheating can be derived. Combining the upper limit on the reheated thickness/age relationship with a theoretical expression, derived by S. T. Crough, which relates swell height to lithospheric age, reset thickness, and physical constants, yields a swell‐height/age relationship that is in good agreement with empirical swell‐height data. This agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere, a result that, in turn, supports the thermal origin of hotspot swells.
Intraplate seismicity of the southern part of the Pacific Plate
Authors: Okal Emile
Published: February 2008

We present a compilation of all the known seismicity of the part of the Pacific plate located south of the equator. This includes historical earthquakes dating back to 1937. In addition to the Gilbert Islands swarm of 1981–1983 we identify 60 events at 32 epicenters in the magnitude range 4.3–6.9. Our results indicate that the level of seismicity of the southern Pacific plate is comparable to that of other plates, including continent‐bearing ones. Most of the seismic activity is concentrated in the youngest parts of the plate, but a few large events are present in Cretaceous lithosphere. With the exception of events from the Gilbert Islands swarm, earthquakes located in lithosphere 35 Ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push; in younger areas the situation is much less clear‐cut. The final geometry of the focal mechanisms is controlled by local tectonics; in particular, due to the Miocene reorientation of the spreading at the East Pacific Rise, the bulk of the Pacific plate is particularly vulnerable to ridge push, and this explains the numerous strike‐slip events and the absence of magnitude 6 events in most of the plate. Correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal. Finally, a number of areas with high seismic release are identified as potential targets for future exploration.
On the sensitivity of parameterized convection to the rate of decay of internal heat sources
Authors: Jackson Michael J. , Pollack Henry N.
Published: February 2008

Thermal histories for the earth based on parameterized mantle convection models have previously indicated that the earth departs significantly from a steady thermal state. The non‐steady state is manifest by a present‐day excess of heat loss over heat production. We investigate the dependence of the heat production/heat loss ratio and mantle viscosity on heat source decay rate by varying the relative and absolute concentrations of the heat‐producing isotopes of K, U, and Th in the mantle. We examine three models with progressively increasing effective half‐lives, respectively characterized by K/U ratios of 6×104 (“chondritic”), 1×104 (“terrestrial”), and 0.25×104 (“low Ka/U”). Each of these models can be constrained to yield a common present‐day heat flow, mantle temperature, and mantle viscosity, thus demonstrating the inability of those present‐day constraints to differentiate between widely varying blends of the earth's nuclear fuel. The present‐day heat production/heat loss ratio ranges from about 73% for the chondritic model to about 86% for the low K/U model. Other possible criteria diagnostic of the earth's radioisotope mix are the present‐day isotopic abundances. The low K/U model requires U and Th greater by a factor of 2.5 over the chondritic model and K less by a factor of 10. However, measurements of concentrations in candidate mantle rocks reveal a range that far exceeds the relatively small variations that distinguish the different compositional models. Paleothermal conditions also may be indicative of the earth's radioisotope blend. The thermal state in the Archean calculated for the low K/U earth model is characterized by a mean mantle temperature about 100°C hotter and heat flow about 2 times greater than the present‐day, whereas a chondritic earth model yields a mantle temperature 300°C hotter and heat flow 4 times greater. However, current estimates of paleo‐heat flow and mantle melting temperatures are burdened with uncertainties sufficiently large to preclude their use as criteria diagnostic of the earth's radioisotope endowment.
The geometry and high‐temperature brittle deformation of the Skaergaard Intrusion
Authors: Norton Denis , Taylor H. P. , Bird Dennis K.
Published: February 2008

The Skaergaard magma chamber formed approximately 55 m.y. ago along the embryonic rift between North America and Europe as tholeiitic basalt magma flowed upward along fractures in basement gneiss and then infiltrated the stratigraphic unconformity at the base of a 7‐ to 9‐km‐thick section of continental basalts. The magma deflected and faulted the overburden as it formed a 4.5‐km‐thick, 189 km3, laccolithlike chamber with elliptical form (a= 6 km,b= 4 km) in map view. As the chamber grew, its feeder pipes were eroded into conical depressions; blocks of gneiss were rafted to the chamber top, and some blocks were fused and entrained in the main magma as “immiscible” fluids. Crystallization and cooling produced at least four distinct fracture events: (1) At 1050°–1000°C, residual magma accumulated in fractures in the Layered Series, forming gabbro pegmatites, (2) at 1050°–700°C, near‐vertical fractures were formed, providing channels for the main pulse of meteoric‐hydrothermal activity; these fractures developed near the margin of the magma chamber, then expanded outward into the permeable basalts and inward, following the gabbro‐magma interface. Ground waters derived from joints in the surrounding basalts flowed into the gabbro, were heated, lowered the18O/16O ratio of the intrusion, and filled its fractures with hornblende, clinopyroxene, biotite, and magnetite‐ilmenite, (3) at 800°–750°C, volatile‐rich granophyric melts derived from sloped blocks of basement gneiss expanded and crystallized as both sill‐like and dike‐like bodies in the gabbro, and (4) below 700°C, fractures continued to form and hydrothermal activity continued to cool the intrusion. The relative age, abundance, continuity, and mineralogy of the veins are consistent with parameters used in previous studies of this intrusion that predict the occurrence of a high‐temperature hydrothermal system and a time‐ and volume‐averaged permeability of 10−13cm2. Our new data indicate that the permeability of the layered gabbro decreased with time because the flow channels were sealed by high‐temperature mineral deposition. We thus conclude the following: (1) layered gabbros fracture in response to local stress at conditions just below their solidus temperature if the confining pressures are typical of the upper crust. This observation contravenes the conceptual viewpoint that the style of deformation at such elevated temperatures is only by plastic flow, and (2) because an extensive fracture network develops at these near‐solidus temperatures in layered gabbros, the bulk of the hydrothermal alteration of such bodies takes place at extremely high temperatures. This helps clarify the apparent paradox that extreme18O depletions are found in “fresh” layered gabbros.
Rayleigh wave scattering at a basin type heterogeneity
Authors: Szelwis Rüdiger
Published: February 2008

Rayleigh waves are investigated numerically on a subsurface model composed of a layer over a substratum, and a basin embedded in the layer. The basin elastic parameters are varied to obtain three laterally heterogeneous models of different velocity contrast between the basin and the layer. The wave incident upon the basin is defined to be a pure fundamental mode having wavelengths comparable to the basin dimensions. Analysis of the wave transmitted reveals mode conversion, i.e., higher mode excitation which is increasingly important with increased velocity contrast. In terms of (rate of transport of) energy, the contribution of the first higher mode (being the only higher mode in the analysis interval) is relatively small, which differs from earlier results for the case of Love waves. To enable direct comparison, a fundamental Love mode having spectral properties analogous to the Rayleigh wave, is propagated across the subsurface models of the present paper. Hence, the discrepancy between the two waves basically arises from the structure of the subsurface transfer functions. The fraction of modal energy transmitted across the basin proves a sensitive indicator of the velocity contrast, showing similarity in amount and frequency dependence for Rayleigh and Love waves. In terms of modal amplitudes, conventionally considered in practice, the first higher mode contribution (due to mode conversion) is roughly comparable to the fundamental mode contribution, for both transmitted waves. It implies that the amplitude spectrum of the transmitted surface wave is modulated in the horizontal direction. The effect adds to the spectral distortion due to reflection at the basin. Hence, standard interpretation based upon a laterally homogeneous model, of measurements in corresponding regions, may be significantly biased.
Alkali gabbronorite, ultra‐KREEPy melt rock and the diverse suite of clasts in North Ray Crater feldspathic fragmental breccia 67975
Authors: Lindstrom Marilyn M.
Published: February 2008

Lunar breccia 67975 is a feldspathic fragmental breccia from North Ray crater, Apollo 16. It contains clasts of alkali gabbronorite and ultra‐KREEPy mafic fragment‐laden melt breccias, which are unique among Apollo 16 samples. Both are alkali‐ and iron‐rich rocks with moderate to high REE concentrations. They more strongly resemble Apollo 14 gabbronorites and alkali anorthosites and KREEP‐rich rocks than they do other Apollo 16 samples. The other clasts in 67975 are the ferroan anorthosites, feldspathic melt rocks, and magnesian granulites, which are typical of other feldspathic fragmental breccias. Examination of bulk and mineral compositions of other breccias and melt rocks suggests that alkali gabbronorite may be a minor component in other North Ray crater breccias and feldspathic melt rocks. This implies that alkali gabbronorite was a fairly early (4.0 b.y.) crustal component in the North Ray crater region.
Cosmic ray exposure histories and 235U‐136Xe dating of Apollo 11, Apollo 12, and Apollo 17 mare basalts
Authors: Eugster O. , Eberhardt P. , Geiss J. , Grögler N. , Schwaller H.
Published: February 2008

The history of exposure to cosmic rays was studied for nine basaltic rocks based on the cosmic‐ray‐produced noble gas isotopes. The results for the five basalts 10003, 10047, 12004, 12051, and 77135 are consistent with a single‐stage exposure. Four basalts, 10057, 10069, 12040, and 12053, experienced a multistage exposure. Using the 235U‐136Xe dating method, the time of their early exposure to cosmic rays on the lunar surface was dated to have begun 3–4 aeons ago, immediately after rock formation as defined by their crystallization ages. The durations and shielding depths for the proposed two‐stage exposure models of the four basalts were calculated. From the shielding at the early stage we estimate a lower limit of 50 cm for the thickness of the lava flows from which the investigated basalts originate.
Composition of orientale basin deposits and implications for the lunar basin‐forming process
Authors: Spudis Paul D. , Hawke B. Ray , Lucey Paul
Published: February 2008

The Orientale basin, on the lunar western limb, has served as the prototype lunar multiring basin for many years. In order to constrain possible models of the formation and excavation of basin ejecta, we have studied the composition of Orientale basin deposits, using both earth‐based spectral reflectance and Apollo orbital geochemical data. Results indicate that Orientale basin ejecta are dominantly anorthositic with compositions ranging from pure anorthosite to noritic anorthosite. Although minor quantities of mare basalt and low‐K Fra Mauro basalt are present, ultramafic and KREEP components appear to be absent. Portions of the inner rings of the basin may be composed of anorthosite whose shock levels exceed 20 Gpa (200 kbar). In conjunction with photogeologic evidence for the preservation of prebasin topography and structure within the Cordillera basin ring, we suggest that the diameter of the excavation crater of the Orientale basin is on the order of 500 to 600 km. Effective depths of excavation are probably less than about half the local crustal thickness, which in this region is on the order of 100 km. We find that the Orientale impact excavated dominantly upper crustal rocks with a depth of excavation consistent with a proportional‐growth, basin‐forming model.