bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of confining pressure and stress difference on static fatigue of granite
Authors: Kranz Robert L.
Published: February 2008

Samples of Barre granite were creep tested at room temperature at confining pressures up to 2 kbar. The time to fracture increased with decreasing stress difference at every pressure. The time to fracture increased with increasing pressure, even when the stress difference was normalized to account for the increase of strength with pressure. At a load equal to 87% of the short‐term fracture strength, for example, the time to fracture increased from about 8 minutes at atmospheric pressure to longer than one day at 2 kbar of pressure. The inelastic volumetric strain at the onset of tertiary creep, Δ, was roughly constant, independent of the applied load at any particular pressure, but increased with pressure in a manner analogous to the increase of strength with pressure. The creep strains were best fit by power functions of both stress and time, but the functional dependence on pressure remains uncertain. At the onset of tertiary creep the number of cracks and their average length increased with pressure. The crack angle and crack length spectra were quite similar, however, at each pressure at the onset of tertiary creep. Two theories of static fatigue are shown to explain the data adequately. Both suggest that the activation enthalpy for the stress corrosion process controls the creep rate and increases with pressure. Purely mechanical effects of pressure are not, however, discounted. The fatigue time may be a function of the rate at which corrosive agents reach crack tips and the rate of crack linking. Risking the extrapolation to longer times, the data suggest that creep rupture in the upper crust is possible if deviatoric stresses of several kilobars can be maintained for millions of years. Pore fluids and temperature can counteract the effect of pressure and substantially lower the fracture times and required stress, but not enough data presently exist to evaluate this properly.
Crystallization in a double‐diffusive system
Authors: Chen C. F. , Turner J. S.
Published: February 2008

The experiments reported here are a first attempt to model in a simple laboratory system some of the physical processes occurring during crystallization in magma chambers and to examine possible effects of different molecular diffusion rates on the formation of layered igneous rocks. Our laboratory results suggest that stratification in the melt and ‘double‐diffusive’effects during solidification of the magma should be taken into account when developing theories of the layering. Explicitly, we have extended earlier experiments on double‐diffusive convection, which show how layers may readily form from smooth gradients when two components with different molecular diffusivities have opposing effects on the density gradient. The new experiments included the influence of crystallization and have used an aqueous solution Of Na2CO3 cooled in various configurations. When cooling was from the top, the crystals formed in horizontal layers, whether the initial fluid was homogeneous, was stratified with a constant gradient, or was stratified in several distinct layers. The dominant effect was the formation of a cold but light fluid layer against the cooled upper boundary as the denser crystals grew. The crystals were small and closely packed in the upper layer (which was vigorously converting) and larger and more loosely packed in the lower, more quiescent fluid. In the case where there were preexisting layers there was an abrupt increase in the growth rate of crystals as they crossed an interface. With sidewall cooling and a constant gradient the lighter solution left behind after crystallization streamed upward in a thin boundary layer right to the top of the tank. The outer flow consisted of circulation in nearly horizontal converting layers, each of which was depressed slowly as the light fluid collected at the top. When a constant gradient of Na2CO3 was cooled from below, crystallization produced a ‘finger’ instability, leading to the growth of a mixed layer in the fluid above. An experiment using opposing gradients of Na2CO3 and K2CO3 (with Na2CO3 rich on top) showed that crystals settling to their own density level and then redissolving can reverse the original sense of the gradient and produce small‐scale double‐diffusive layering in the liquid.
Tectonic relations of carbon dioxide discharges and earthquakes
Authors: Irwin William P. , Barnes Ivan
Published: February 2008

CO2‐rich springs occur worldwide along major zones of seismicity. They are mostly in young orogenic belts, but some are in areas of rifting continental platforms. Analyses of 13C content indicate that much of the CO2 is derived from the mantle and that other important sources are the metamorphism of marine carbonate‐bearing sedimentary rocks and the degradation of organic material. The presence of calc‐silicate minerals, such as pumpellyite in metagraywacke, is evidence of former conversion of carbonate‐bearing rocks into calc‐silicate minerals and release of CO2. The CO2 pressure in fractured rocks of a fault zone reduces the effective normal stress and, if it is sufficiently great, allows the fault to slip. If the pressure were maintained at a sufficiently high level, the fault behavior might be characterized by frequent small earthquakes and aseismic creep such as occur along active segments of the San Andreas system. The presence of CO2‐rich springs may indicate a potentially hazardous seismic region. Monitoring of CO2 discharges could be useful in earthquake prediction.
Disturbance of trees by the 1857 Fort Tejon Earthquake, California
Authors: Meisling Kristian E. , Sieh Kerry E.
Published: February 2008

Trees may suffer damage during major earthquakes due to shaking or faulting of their substrate. Damage may result in temporarily asymmetric growth and/or reduction in width of annual growth rings. To determine whether trees contain useful records of prehistoric earthquakes in southern California, we cored eight conifers along the 1857 trace of the San Andreas fault near Wrightwood and Frazier Park, California. Annual ring widths were measured and plotted against the growth year determined by ring counting. We examined significant departures from normal growth trends and interpreted them in light of the 1857 earthquake and other possible environmental factors. Of eight trees sampled, five showed damage or growth anomalies attributable to the 1857 event. One 120‐year‐old tree straddling the fault is undamaged, suggesting no substantial slip since about 1870. To evaluate asymmetry, ratios of correlative rings widths from opposite sides of three seismically damaged trees were calculated and plotted against growth year. Two types of ratio anomalies can be recognized: (1) short‐term, unilateral suppression of growth resulting from damage and (2) long‐term, unilateral enhancement of growth attributable to tilt or changes in environmental factors. Further study of ring ratio anomalies may facilitate recognition of seismically damaged trees. Success in recognizing the 1857 event in five out of eight trees suggests that a larger suite of even older trees may contain a valuable ‘dendroseismological’ record of large prehistoric earthquakes in southern California.
Continental stretching: An explanation of the Post‐Mid‐Cretaceous subsidence of the central North Sea Basin
Authors: Sclater John G. , Christie P. A. F.
Published: February 2008

The North Sea is a major continental basin filled with early Paleozoic to Recent sediments. Though graben formation started in the Triassic, the last major period of extension occurred between the Middle Jurassic and the mid‐Cretaceous. Following the faulting and graben formation associated with this extension, subsidence within the central North Sea was widespread and uniform and has created a saucershaped sedimentary basin. This was filled successively by chalks, sandstones, and finally, during most of the Tertiary, by shales and mudstones. We examined the subsidence of six wells down the middle and two on the flanks of the Central Graben. In the period of widespread steady subsidence the water‐loaded basement depth in the middle increased by 1100–1400 m. On the flanks the basement subsided 600–700 m. We suggest that most of this subsidence results from the thermal relaxation of the lithosphere which was thinned during a Middle Jurassic to mid‐Cretaceous stretching of the crust. Assuming a crustal stretching and associated lithospheric thinning of between 50 and 100% in the middle and decreasing on either side, we obtained a good match to the observed amplitude and rate of subsidence. The Middle Jurassic to mid‐Cretaceous subsidence which is found within the graben proper we relate to the fault‐controlled initial subsidence which occurred during the actual stretching. The measured heat flow is compatible with such a stretching model. Though there is no seismic refraction data across the Central Graben, this model is strongly supported by evidence of a thinner crust under the Viking Graben to the north and the Witchground/Buchan Graben complex to the east. Using the above observations as the basis for a geological interpretation, we examined the thermal maturity and hydrocarbon potential of certain sedimentary horizons in the northern section of the Central Graben. In analyzing the various wells we extended previous work on the compaction correction to handle overpressuring and mixed lithologies in backstripping studies. Further, we expanded these methods to include the variation of thermal conductivity, and calculations of the degree of thermal maturation of the deposited sediments, through time.
Predicting major element mineral/melt equilibria: A statistical approach
Authors: Hostetler Charles J. , Drake Michael J.
Published: February 2008

A data set has been collected from the literature, comprising 230 synthetic mineral/melt pairs for which phase composition and run temperature are known. All phase pairs were equilibrated at 1 atm under anhydrous conditions. Solid phases represented are olivine, low‐Ca pyroxene, high‐Ca pyroxene, and plagioclase. We have developed empirical equations for calculating the mole fractions of NaO0.5, MgO, AlO1.5, SiO2, KO0.5, CaO, TiO2, and FeO in a solid phase of initially unknown identity given only the composition of the coexisting silicate melt. The approach involves a linear multivariate regression analysis in which solid composition is expressed as a Taylor series expansion of the liquid compositions. We obtain an internally consistent precision of ≃0.94; that is, we can correctly predict the nature of the liquidus phase in our input data set for approximately 94% of the entries. The composition of the liquidus phase may be calculated to better than 5 mol % absolute. An important feature of this ‘generalized solid’ model is its reversibility; that is, the dependent and independent variables in the linear multivariate regression may be inverted to permit prediction of the composition of a silicate liquid produced by equilibrium partial melting of a polymineralic source assemblage. We have added 14 points from a silica‐olivine‐anorthite pseudoternary phase diagram to the data set to deal with polymineralic source assemblages. The composition of the first partial melt can be calculated to better than 3 mol % absolute.
Statistics of paleomagnetic inclination data
Authors: Kono Masaru
Published: February 2008

The statistical method of Briden and Ward is reconsidered to derive an easy but exact method to calculate true mean inclination and precision parameter when the data consist of only inclinations. Expectations of sin I, sin2 I, etc. are obtained for samples following the Fisher distribution. The best estimate of the mean inclination and precision parameter can easily be calculated as solutions of two non‐linear simultaneous equations. This method applies equally well to whichever of inclinations or virtual geomagnetic latitudes having the Fisher distribution.
Progressive deformation of a Tertiary Trench Slope, Kodiak Islands, Alaska
Authors: Moore J. Casey , Allwardt Alan
Published: February 2008

The Eocene Sitkalidak Formation comprises the youngest and seaward‐most deep‐sea sequence exposed in the Kodiak Islands, which border the eastern Aleutian Trench. Fold orientation, structural style, and stereographic projections of poles to bedding define two structural units in the Sitkalidak Formation, which are geometrically distinct from overlying non‐marine and marine shelf rocks of Oligocene and Miocene age. Landward vergence, intense deformation with stratal disruption, a relatively seaward structural position and sedimentologic considerations all argue that the more deformed portion of the Sitkalidak Formation constitutes an obductively offscraped, trenchfilling fan sequence. Conversely, regular folding, stratal continuity, and relatively landward structural position suggest that the less deformed portion of the Sitkalidak Formation accumulated and was deformed as a slope basin or slope apron deposit. In the more deformed portion of the Sitkalidak Formation, zones of stratal disruption resemble melange and are demonstrably of tectonic origin as indicated by elongated, slickensided boudins with microscopic shear and solution zones. The Sitkalidak Formation was rapidly deformed, uplifted, and unconformably overlain by the Oligocene non‐marine Sitkinak Formation. Zeolite facies metamorphism of the Sitkalidak Formation followed, as a consequence of burial during Oligocene and Miocene time. Vitrinite reflectance and porosity data respectively define a peak temperature of 100–125°C and a maximum burial of 2.4–3.9 km, suggesting a paleogeothermal gradient of about 30°C/km during Miocene time.
Possible mechanism for seismic attenuation in rocks containing small amounts of volatiles
Authors: Tittmann B. R. , Clark V. A. , Richardson J. M. , Spencer T. W.
Published: February 2008

This report presents measurements of the specific dissipation factor Q−1s of rocks containing small amounts of volatiles. Q−1s was measured for shear waves as a function of relative partial pressure P/P0 for benzene, hexane, ethanol, methanol, and water. The measurements were carried out at about 10 kHz with the vibrating bar technique in a chamber in which the relative partial volatile pressure P/P0 was varied between almost zero and about 0.9. The results revealed that in the regime of one‐ or two‐monolayer coverage of absorbed volatiles, Q−1s increased dramatically with exposure to the alcohols and water but only negligibly with exposure to the hexane and benzene. The slopes of Q−1 versus monolayer coverage appeared to correlate with the dipole moment per unit volume of the volatiles. These data are presented in the context of previous measurements, which showed that the influence of volatiles on Q−1 persist to high levels of outgassing (1×10−10 torr vacuum), to elevated hydrostatic confining pressures (at least 0.5 kbar), and for a variety of crystalline rocks including terrestrial analogs of lunar basalt. Both direct (ellipsometry data) and indirect evidence (absorption isotherm data) are presented for the presence of thin films of adsorbed volatiles at low partial vapor pressures. Finally, the measurements are discussed and interpreted in terms of a physical model relating Q−1 to the relative mass of the adsorbed volatiles, the surface area of the rock, the heat of desorption of the volatile, and the rock temperature.
The inverse problem of the shear modulus profile of a layered Earth
Authors: Coen Shimon
Published: February 2008

This paper shows that the shear modulus profile of a layered earth is uniquely determined by the static torsional stress and displacement on its surface. Two analytical examples are given in which the shear modulus profile is deduced from analytic data.
Oceanic intraplate earthquakes: Implications for local and regional intraplate stress
Authors: Bergman Eric A. , Solomon Sean C.
Published: February 2008

Focal mechanisms of intraplate earthquakes provide the only means at present by which to characterize the long‐wavelength tectonic stress field in oceanic lithosphere. Stress orientations inferred from focal mechanisms may not accurately reflect the state of stress in the epicentral area, however, or the measured stresses may be dominated by local rather than regional sources. To establish a data set with which to study these possibilities, a comprehensive catalog of 159 oceanic intraplate earthquakes has been compiled for events since 1963 with mb 4.7 or larger. Focal mechanisms are available for approximately one quarter of the events, and several new mechanisms are presented here. For a representative subset of this catalog (83 events), the bathymetry and tectonic history of the epicentral areas have been assembled, and the earthquakes have been rated according to their association with (1) a preexisting fault zone, which might decouple the P axis of the focal mechanism from the true orientation of maximum compressive stress, and (2) large bathymetric relief, which might be a source of large local stresses. Oceanic intraplate earthquakes are commonly found in association with zones of previous weakness (usually fracture zones), but they do not show any particular association with large bathymetric features. In the central Indian Ocean there are enough focal mechanisms available to establish a well‐defined NW‐SE orientation for P axes and presumably for the direction of greatest compressive stress. The consistency of the P axes of these widely varying mechanisms in the presence of the Ninetyeast Ridge, a site of major intraplate deformation and large bathymetric relief, is remarkable. A possible explanation is that in the presence of a large number of preexisting faults with a range of orientations, slip occurs on those faults which have large resolved shear stresses from the regional stress field. In such an instance the P axis of focal mechanisms will tend to show a consistent alignment with the true direction of maximum stress.
Magnitude of the continental lithospheric stresses inferred from rheomorphic petrology
Authors: Mercier Jean‐Claude C.
Published: February 2008

Lithospheric stress magnitudes may be estimated through the application of geological thermobarometers [Mercier, 1980] and grain size piezometers [Ross et al., 1980]. Discontinuities in grain size profiles based on series of peridotite xenoliths from some representative localities can hardly correspond to abrupt changes in stress conditions at depth, inasmuch as no geochemical difference is observed across the textural discontinuities, and no phase changes are possible. These discontinuities would thus result from reaching a critical temperature or stress beyond which the dominant recrystallization mechanism drastically changes. Indeed, the grain size piezometer is very sensitive to the recrystallization mechanism [Poirier and Guillope, 1979], and whereas recrystallization through subgrain rotation has been recognized in some natural peridotites [e.g., Nicolas et al., 1971], the experimental data available so far apply to grain boundary migration recrystallization. On the basis of two significantly different grain size profile discontinuities, an empirical relationship (σ = 74.5/D) is inferred for the grain size resulting from subgrain rotation. Applying this new relationship when relevant, the stress magnitudes estimated from deep samples remain virtually unchanged relative to previous results [Ave Lallemant et al., 1980], as do their implications regarding large‐scale convection. However, revised magnitudes for the uppermost mantle may be significantly increased to a value as high as 45 MPa (450 bar) for the Basin and Range Province, with stresses in the lower crust possibly reaching up to 80 MPa.
Comparison of mechanical models of the oceanic lithosphere
Authors: Forsyth Donald W.
Published: February 2008

A realistic mechanical model of the oceanic lithosphere must be able to explain more than just the topography of a typical trench and outer rise system. A successful model should also be consistent with our knowledge of rock mechanics from laboratory studies, with the distribution of earthquakes associated with bending of the plate before subduction, with the amount of strain within the plates as indicated by the seismic moment of the earthquakes, and with the topography associated with other loads, such as seamounts and trenches where plate convergence is very slow. These observational tests are applied to six mechanical models of the lithosphere: models in which the regional, horizontal compressive stresses are of the order of several kilobars are the least successful.
Elastic moduli of rock glasses under pressure to 8 kilobars and geophysical implications
Authors: Meister R. , Robertson E. C. , Werre R. W. , Raspet R.
Published: February 2008

Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more‐normal behavior occurs with decrease in SiO2 content. The pressure derivatives of rigidity and bulk modulus increase linearly, from −3.39 to −0.26 and from −5.91 to +2.09, respectively, with decrease in SiO2 content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consistent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500°C, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low‐velocity zone; at 1300°C and below, basalt would be in the glassy state, especially if more felsic than tholeiite. At a temperature of 1370°C and at 30 kbar, reasonable values for the upper mantle at 100 km depth, the basalt glass of this study would have a viscosity of about 1013 P. On the basis of the theory of viscoelasticity the glass would support shear wave propagation at frequencies above 0.1 Hz. Under such conditions of PT, 10 to 30% basalt glass in a matrix either of eclogite or olivine would reduce the seismic velocities by 3 to 9% and could also account for the values observed in the low‐velocity zone.
Stratigraphy of Oceanus Procellarum basalts: Sources and styles of emplacement
Authors: Whitford‐Stark James L. , Head James W.
Published: February 2008

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D. . 20009. Document J80‐005; $1.00. Payment must accompany order.

The basalts within Oceanus Procellarum have been named the Oceanus Procellarum Group and subdivided into four major geologic units on the basis of morphology, spectral reflectance, and other remote sensing information in order to develop a framework of regional stratigraphy for the largest lunar mare. The oldest mare basalt unit identified, the Repsold Formation (blue color, low albedo, high crater density, very weak 3.8‐cm radar backscatter, and a weak 1‐μm reflectance) crops out in NW Procellarum and may be correlative with Apollo 11 and 17 high‐titanium basalts. The next youngest unit, the Telemann Formation (red, higher albedo, high crater density, high DL values, and an average to strong 1‐μm reflectance) extends over wide areas of Procellarum and crops out as extensive surface units and as local patches surrounding earlier, topographically higher units. Numerous large sinuous rules in the Aristarchus Plateau were a major source for some of these deposits. Pyroclastic deposits forming the dark red mantle were also emplaced at the Aristarchus Plateau during these eruptions. The Hermann Formation (intermediate color, low albedo, intermediate crater density, and average to strong 1‐μm reflectance) occurs as extensive plains developed from less voluminous eruptions from sources at the Marius Hills complex, the Rümker Hills, and in Nubium, Cognitum, and near Delisle. The youngest unit, the Sharp Formation (blue color, very low albedo, low crater density, and a strong 1‐μm reflectance) is volumetrically small and is derived primarily from vents near the highland/mare boundary. Many Sharp Formation members have associated sinuous rules. Basalts of the Hermann Formation were sampled by Apollo 12; those of the Telemann Formation may be similar to the Luna 24 VLT basalts, while those of the Repsold Formation may be comparable with the Apollo 11, 17 and Luna 16 samples. The Sharp Formation, the youngest basalts, remains unsampled. The total volume of Procellarum basalts is estimated at 8.7×105 km3 (about 10% of the total mare volume). The relative volumes of the individual formations are not reflected by the surface area covered because the youngest flows tend to be thin and widespread. For example, the Sharp Formation has a surface area of 72,000 km2 but an estimated volume of only 1.8×104 km3, while the Repsold Formation has a present surface area of only 22,000 km2 but an estimated volume of 2.1×105 km3. The vast majority of lavas in Procellarum were extruded early in the phase of lava flooding, prior to about 3.5 b.y.
The origin of kimberlite
Authors: Wyllie Peter J.
Published: February 2008

A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotite‐CO2‐H2O melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C‐H‐O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80‐km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing CO2‐under‐saturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite‐CO2‐H2O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite‐dolomite‐peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than its precursory cause.
Effect of pressure and stress on water transport in intact and fractured gabbro and granite
Authors: Trimmer D. , Bonner B. , Heard H. C. , Duba A.
Published: February 2008

New laboratory data are reported on the effect of confining pressure (to 60 MPa), pore‐water pressure (to 30 MPa), and stress difference (to 0.88 of the fracture stress) on permeability of intact and fractured White Lake gneissic granite, Westerly granite, and Creighton gabbro. Permeabilities as low as 10−24 m2 (10−12 darcy) have been measured using a transient technique. Fracture displacement, electrical conductance, compressional velocity, and pulse amplitude are determined simultaneously. The loads applied to the 0.15‐m‐diameter by 0.28‐m‐length test sample are controlled automatically, and most data are taken by microprocessor. Tests on the intact gneissic granite indicated permeabilities of 10−22 to 10−24 m2 that appeared to be unaffected either by effective pressure or by stress. The granite yielded permeabilities of 4×10−20 m2 that decreased by a factor of 2 as effective pressure increased to 25 MPa and varied by a factor of 2 as stress was increased to 0.5 of the fracture stress. Permeability of the gabbro linearly decreased from 2×10−22 to 8×10−24 m2 with effective pressures to 25 MPa. Loading of the gabbro up to 0.88 of the fracture stress increased permeability by a factor of 7. The introduction of a throughgoing fracture increased the apparent permeability by 106 to 109 over the intact values in both granite and gabbro. When compared to the initial value, compressional velocities increased by 5% with pressure to 30 MPa in the gneissic granite. For granite, pressurization from 2 to 25 MPa increased the velocity and pulse amplitude by 5 and 30%, respectively, and decreased the conductance by 50%. Velocity, amplitude, and conductance were weakly dependent on pressure in gabbro. The addition of stress decreased velocity and amplitude while increasing conductance markedly in both granite and gabbro. All data on both intact and fractured rock are consistent with crack closure and dilatancy with pressure and stress. Conductance and amplitude exhibit the best potential for monitoring changes in permeability and joint behavior in situ.
Microcrack closure in rocks under stress: Direct observation
Authors: Batzle Michael L. , Simmons Gene , Siegfried Robert W.
Published: February 2008

Microcrack closure in rocks under increasing stress was observed directly with a scanning electron microscope. Uniaxial stresses to 300 bars were applied with a small hydraulic press to specimens of Westerly (RI) granite, both unheated and previously heat cycled to 500°C, and of Frederick (MD) diabase, heat cycled to 700°C. Closure characteristics (rate, final closure pressure, etc.) depend on crack orientation, shape, surface roughness, and on the nature of fracture intersections and interactions. Cracks perpendicular to the applied stress closed while those parallel to the stress tended to open. Long, narrow cracks (low aspect ratio) closed at relatively low pressures. At some intersections, one fracture would open while another simultaneously closed, depending upon their orientations. Many fractures closed uniformly even though offset by other fractures. Local stress concentrations often caused new fracturing at low applied stress. Some fractures were propped open until material lodged inside was crushed. Significant irreversible damage occurred during the first stress cycle. Closure characteristics varied significantly among the samples. The unheated granite has cracks with rough, pitted, and mismatched walls. Only partial closure occurred under stress with many sections remaining open. Crack porosity is reduced but continues to be interconnected. Fractures in the preheated granite and diabase are also irregular, but the walls are well‐matched and closure is nearly complete. The cracks in the heated granite closed at lower stresses than in the diabase. As the maximum stress was approached for the heated granite, new transgranular cracks formed and preexisting cracks were enlarged. The variations in closure rate and character were also observed in strain measurements.
Geodetic measurement of horizontal deformation across the Rio Grande Rift Near Socorro, New Mexico
Authors: Savage J. C. , Lisowski M. , Prescott W. H. , Sanford A. R.
Published: February 2008

Trilateration surveys of a geodetic network across the Rio Grande rift near Socorro, New Mexico, in 1972, 1973, 1976, and 1979 have failed to detect any significant strain accumulation. The surveys place an upper bound (95% confidence limit) of 1 mm/a (a = years) on east‐west spreading cross the rift in 1972–1979. There is marginal evidence from triangulation for an episode of east‐west spreading across the rift within the interval 1954–1972. The trilateration network lies on the south flank of an uplift caused by magma intrusion into a midcrustal sill during this century according to Reilinger and Oliver. The horizontal deformation induced by sill inflation is sufficiently small that continued uplift during 1972–1979 cannot be excluded by the observed absence of significant horizontal deformation.
On the scaling of crater dimensions: 1. Explosive processes
Authors: Holsapple K. A. , Schmidt R. M.
Published: February 2008

A theoretical examination of explosive crater scaling rules based upon dimensional analysis is presented. The distinction between the scaling of similar experiments as opposed to the more common practice of scaling nonsimilar experiments is emphasized. Three different sets of dimensionless π groups commonly used in the literature (denoted here as the mass set, the energy set, and the gravity set) are compared, suggesting alternate ways to preserve similarity among experiments. Specific scaling rules using an assumed power law functional relationship among π groups are derived for nonsimilar experiments. These scaling rules result from assumptions that cratering is independent of certain variables. The role of explosive energy density is examined and shown to be an essential variable which must be included in the analysis. None of the derived results lead exclusively to the requirement of cube root or quarter root scaling except under very special circumstances, which are shown to be common to all three formulations. In the usual series of experiments conducted in the same soil type with an identical type of explosive the scaling rules are shown to be bounded by cube root and quarter root rules. Other scaling exponent restrictions are presented.
Magnetic anisotropy and fabric of some progressively deformed ophiolitic gabbros
Authors: Wagner J.‐J. , Hedley I. G. , Steen D. , Tinkler C. , Vuagnat M.
Published: February 2008

The ophiolites, considered remnants of oceanic seafloor, provide a fruitful source of information on the physical and chemical behavior of the deeper uncored lithosphere. A study of the gabbros in a rather well‐preserved ophiolite from the western Alps (Montgenèvre, France) shows strong evidence for intraoceanic plate deformation. To characterize the different steps of deformation observed in the outcrops, a magnetic fabric study was conducted using the anisotropy of initial magnetic susceptibility (Aims) method. The main conclusions are (1) the Aims is controlled by the paramagnetic ferromagnesian minerals, such as hornblende, resulting from the ocean floor metamorphism close to the spreading ridge (partial amphibolitization) and (2) the magnetic lineation and foliation of the gabbros show a deformation path with increasing strain. A good correspondence between petrofabric and magnetic anisotropies is observed; both suggest that the deformation mechanism was simple shear.
Strain accumulation across the Denali Fault in the Delta River Canyon, Alaska
Authors: Savage J. C. , Lisowski M. , Prescott W. H.
Published: February 2008

Deformation along the Denali fault in the Delta River canyon was determined from geodetic surveys in 1941/1942, 1970, 1975, and 1979. The data were best for the 1975–1979 interval; in that period the average strain accumulation was essentially pure right lateral shear at a rate of 0.6 ± 0.1 μrad/a (a is years) (engineering shear) across a vertical plane striking N87°E. The plane of maximum shear is rotated about 30° counterclockwise from the local strike of the Denali fault but closely coincides with the strike of a major linear segment of the fault that begins 50 km farther west. The deformation between 1941/1942 and 1970 is consistent with a similar rate of strain accumulation if one removes the coseismic strain step contributed by the 1964 Alaska earthquake. The 1970–1975 deformation is poorly defined owing to uncertainties in the 1970 survey, but the strain accumulation during that period is certainly much less than during the 1975–1979 interval. The 1975–1979 strain accumulation is interpreted by means of a dislocation model which suggests that the Denali fault in the vicinity of the Delta River Canyon behaves as a leaky transform fault (i.e., a source of spreading as well as lateral slip). The block south of the Denali fault appears to be moving westward (parallel to the strike of the Denali fault west of the 147th meridian) at the rate of about 20 mm/a relative to the North American plate. Because the linear segment of the Denali fault east of the 147th meridian (along which the geodetic network is located) strikes N63°W, accommodation of the westward motion of the southern block requires some spreading within the fault zone as well as right lateral slip on the fault.
Contour maps of lunar remanent magnetic fields
Authors: Hood L. L. , Russell C. T. , Coleman P. J.
Published: February 2008

The 2605 usable orbits of Apollo 15 and 16 subsatellite magnetometer data have been reexamined for intervals suitable for analysis of crustal magnetic anomalies. To minimize plasma‐related disturbances, segments from 274 of these orbits were selected from times when the moon was either in a lobe of the geomagnetic tail or in the solar wind with the subsatellites in the lunar wake. External field contributions which remained in the selected intervals were minimized by (1) quadratic detrending of individual orbit segments with lengths much greater than anomaly wavelengths and (2) two‐dimensional filtering with minimum passed wavelengths less than or equal to anomaly wavelengths. Improvements in coverage, accuracy, and resolution of previously published anomaly maps produced from these data are obtained. In addition to improved maps of the Reiner Gamma and Van de Graaff‐Aitken anomalies studied previously, a third region of relatively high‐amplitude anomalies centered near the crater Gerasimovich on the southeastern far side has been mapped. Both the Van de Graaff‐Aitken region and the Gerasimovich region are marked by the general occurrence of extensive groups of Reiner Gamma‐type swirls.
Theoretical body wave interactions with upper mantle structure
Authors: Cormier Vernon F. , Choy George L.
Published: February 2008

Long‐period seismograms are generated by a spectral method for body waves interacting with the upper mantle discontinuities that define the low‐velocity and transition zones of the spherically symmetric but radially inhomogeneous earth models, PEM‐C and T7. A complex velocity profile incorporates the SL8 Q model. Models PEM‐C and T7 predict distinct differences in displacement waveforms for P waves that interact with the 400‐ and 670‐km discontinuities. The waveforms demonstrate that later arrivals such as interference head waves can be important constraints upon earth models. Theoretical seismograms were computed by a spectral method that modeled the velocity profile with arbitrarily thick inhomogeneous layers (full wave theory). The seismograms agreed well with those generated by methods that approximate the velocity profile with thin homogeneous layers (the Cagniard and reflectivity methods) provided the displacements were convolved with source‐time functions and a long‐period instrument response. Minor differences among the results of the methods do not affect the interpretation of observed waveform data but do help define the limitations of the methods. For example, the thin‐layer methods suffer from numerical limitations on layer thickness and the number of layers needed to approximate the earth model. The full wave theory may occasionally also require the use of thin layers when the zero‐order asymptotic theory it uses fails to describe wave propagation properly in layers having rapid velocity variation with depth.
A technique for incorporating geothermal gradients and nonlinear creep into lithospheric flexure models
Authors: McMullen R. J. , Hodge D. S. , Cozzarelli F. A.
Published: February 2008

A beam flexure model for the lithosphere, which employs a nonlinear viscous creep constitutive relation, has been developed for time‐dependent flexure caused by surficial loads. The lithosphere is buoyantly supported by the underlying asthenosphere, which is modeled as an elastic (Winkler) foundation. Creep is highly dependent on temperature and melting point, which in turn vary with depth; these factors are incorporated into the beam lithospheric model by using Weertman's constitutive relation for creep of rock at elevated temperature. A typical value of the creep powernin the Weertman equation is 3 for rock, but the value ofnmay be varied in the model. The governing nonlinear partial differential equations for moment and deflection are solved by an analytical procedure for the spatial coordinate and by a numerical marching procedure in the time coordinate. Stresses in the lithosphere are also evaluated, with special attention given to an iterative technique for locating the neutral axis, which will not be at the centroid because of the nonlinearity and the geothermal gradient.
Tectonic setting for ophiolite obduction in Oman
Authors: Coleman Robert G.
Published: February 2008

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian‐African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian‐African plate drifted northward about 10°. These events combined with the opposite rotation of Eurasia and Africa initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin combined with the compressional forces of closure from the Eurasian plate initiated obduction of the Tethyan oceanic crust along preexisting transform faults, and still hot oceanic crust was detached along oblique northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large‐scale island arc volcanism did not commence until after the Tethyan sea began to close and after the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics, and detached fragments of the basal amphibolites which in turn rest on autochthonous shelf carbonates of the Arabian platform. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.
Structure of the sheeted dike complex of the Samail Ophiolite near Ibra, Oman
Authors: Pallister John S.
Published: February 2008

The sheeted dike complex is a regionally mappable geologic unit with a consistent position in the Samail ophiolite stratigraphy. Discontinuous exposures of the complex in the Ibra area of the southeast Oman Mountains provide structural data that allow reconstruction of a paleo‐spreading ridge axis of 347° for the region and suggests a maximum spreading width of 275 km for the ophiolite. Contact relations within the well‐exposed Al Ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo‐ridge axis. Split dikes are abundant; facing directions indicate a spreading ridge southwest of the present outcrop area, in conflict with other spreading axis direction indicators.
The biostratigraphy of sediments in the volcanic unit of the Samail Ophiolite
Authors: Tippit P. R. , Pessagno E. A. , Smewing J. D.
Published: February 2008

Sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the Oman ophiolite contain datable radiolarian faunas that indicate (1) that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early Cenomanian to the early Turonian and (2) that the age progression within this volcanic sequence matches its development from a ridge axis to an off‐axis setting. Emplacement of the ophiolite has been dated as late Campanian‐early Maestrichtian [Glennie et al., 1974]; thus the time gap between formation and emplacement was very narrow.
Uranium‐lead isotopic ages of the Samail Ophiolite, Oman, with applications to Tethyan ocean ridge tectonics
Authors: Tilton G. R. , Hopson C. A. , Wright J. E.
Published: February 2008

Plagiogranites are a minor but widespread component of the Samail ophiolite plutonic member. They crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the Tethyan spreading ocean ridge, and their ages are thought to mark the time of ocean crust formation. Isotopic U‐Pb ages of zircons from 13 plagiogranites collected along a 270‐km segment of the Samail ophiolite subparallel to the regional trend of the sheeted dike complex (the former spreading ridge axis direction) define a narrow time interval of 93.5–97.9 m.y., with a pronounced clustering about 95 m.y. The zircon ages of the plagiogranites agree remarkably well with the early Cenomanian to early Turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them (Tippit et al., 1981). The agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite U‐Pb ages closely date the time span of ocean crust formation. No step changes in age patterns are observed along the ridge axis (sheeted dike) direction, suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse. One possible exception occurs at the southeastern end of the sampled interval (Ibra area), where a 3 m.y. discontinuity might be caused by an unmapped fault. Assuming that the regional trend of the sheeted dikes (N10°–25°W) marks the direction of the former spreading ridge axis, the present array of sample localities spans a distance of 130 to 195 km normal to that axis (i.e., in the spreading direction). The data as a whole do not define a clear‐cut age trend normal to the spreading axis, but by eliminating samples that may be aberrant due to faulting, the data array suggests a pattern of increasing ages from east to west. This would indicate that the Samail ophiolite is derived from the western (southwestern) side of the Tethyan spreading system—a conclusion reached independently from geological evidence. Also, the gentle age gradient in the spreading direction (i.e., between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m.y.) suggests a very fast spreading rate. This conclusion also is supported by independent geologic evidence.
K‐Ar ages of metamorphic rocks at the base of the Samail Ophiolite, Oman
Authors: Lanphere Marvin A.
Published: February 2008

Hornblendes from amphibolites in the sheet of metamorphic rocks beneath the peridotite member of the Samail ophiolite and phyllites farther from the peridotite contact have weighted mean 40Ar/39Ar total fusion ages of 90.0 ± 3.0 m.y. and 79.5 ± 3.0 m.y., respectively. The amphibolites represent the first tectonic slice welded to the base of the Samail ophiolite after it was detached from the Tethyan oceanic crust. Formation of the amphibolites occurred no more than 3 to 7 m.y. after crystallization of plagiogranite in the ophiolite. The phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the Tethyan spreading axis. The K‐Ar ages suggest, assuming a half‐spreading rate of 2 to 5 cm/yr, that detachment of the Samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from the spreading center. Using the same spreading rate, one can calculate a minimum half width of 300 to 750 km for the Tethyan Ocean during the Late Cretaceous.