bluuebunny/crossref_metadata_2025 huggingface dataset:
Orbital radar evidence for lunar subsurface layering in Maria Serenitatis and Crisium
Authors: Peeples Wayne J. , Sill William R. , May Thomas W. , Ward Stanley H. , Phillips Roger J. , Jordan Rolando L. , Abbott Elsa A. , Killpack Terry J.
Published: February 2008

Data from the lunar‐orbiting Apollo 17 radar sounding experiment (60‐m wavelength) have been examined in both digital and holographic formats. Surface backscatter (clutter) which masks possible radar returns originating from subsurface changes in lunar electrical properties was reduced by simultaneously comparing radar data from two orbits. Radar returns that correlate from orbit to orbit form two distinct alignments in Mare Serenitatis and one in Mare Crisium. It is proposed that these alignments represent subsurface reflecting horizons. The hypothesis is tested by showing that (1) most of the radar returns fall outside the ambiguity region of the correlation technique, (2) the results are consistent between optically and digitally processed data, (3) the signal levels of the proposed subsurface features are well above the noise floor, (4) the inferred loss tangents appear to be consistent with returned sample measurements, and (5) the discontinuous nature of the reflections most likely arises from interference effects. It is concluded that there are two subsurface radar reflectors with mean apparent depths of 0.9 km and 1.6 km below the surface in Mare Serenitatis and one reflector at a mean depth of 1.4 km below the surface in Mare Crisium. These reflectors represent basin‐wide subsurface interfaces.
Mapping of integrated radon emanation for detection of long‐distance migration of gases within the Earth: Techniques and principles
Authors: Fleischer Robert L. , Mogro‐Campero Antonio
Published: February 2008

Integrated measurements of radon concentrations within the ground but close to the surface are being used extensively in searches for premonitory signals from earthquakes and in exploration for uranium. We present a description of the different components of the radon signals that may be present and describe methods for separating them. We also show quantitatively what signals the different radon‐integrating devices are sensitive to, how readings in a given geometry can be related to actual concentrations in the soil gas under conditions of diffusional equilibrium, and what some of the effects of water in soil are. This knowledge allows the magnitude of the local contributions to be measured, so that distant signals can be recognized as departures from these values. The importance of recognizing where upward transport of soil gas occurs is emphasized.
Necking of the lithosphere and the mechanics of slowly accreting plate boundaries
Authors: Tapponnier Paul , Francheteau Jean
Published: February 2008

Data on ridges with slow spreading rates (1–3 cm/yr), obtained through detailed studies with Deep‐Tow instruments and manned submersibles (French‐American Mid‐Ocean Undersea Study), or where the active structures are not submarine (Afar triangle) yield a precise picture of the axial region. There is evidence for a thinned lithosphere to be present at the axis with a thickness of 4–5 km. The thermal structure and composition of this solid layer can be estimated from seismic data and thermal and petrologic models. Extensional tectonics prevails in a belt some 10 km wide and is expressed at the surface by normal faults and fissures. In most previous mechanical models the existence of the lithosphere at the axis is neglected, and the rise of viscous asthenospheric material in a narrow vertical cleft beneath the axis is considered the main cause for the steady state presence of an axial valley and the development of normal faults. In contrast with these models we suggest here that these features depend on the rheological behavior of the lithosphere at the axis: the lithosphere is continually thinned by tectonic strain but also thickened by cooling at the base and by volcanism at the surface. In the steady state this process can be viewed as a succession of tectonic ‘neckings’ in the central active part of the axial valley (10 km) followed by doming of the lithosphere over the whole width of the axial valley (30 km), in response to isostatic disequilibrium. Creep is likely to control the process at depth and because of the high temperature and large strains will be of the steady state type. The application of experimental flow laws for material constituting layer 3 and the uppermost mantle to this problem, where both temperature profile and strain rate can be estimated, allows an order of magnitude of the ‘strength’ of the lithosphere at a given strain rate to be calculated. With this strength, isostatic recovery in response to vertical shear stresses will occur at a distance of about 8–15 km from the axis. The resulting ‘simple shear’ strain progressively ‘levels off’ the mean topographic slope until it becomes horizontal in the rift mountains. Whereas for rifted ridges or slowly accreting plate boundaries the behavior of the lithosphere controls the mechanics of the axial region, where only small discontinuous transient magma chambers exist, we suggest that in the case of nonrifted ridges the behavior of the asthenosphere is more important, with axial crust in isostatic equilibrium over a large continuous permanent magma chamber.
The effect of nonelliptical cracks on the compressibility of rocks
Authors: Mavko Gerald M. , Nur Amos
Published: February 2008

Dislocation theory is used to study the deformation of nonelliptical thin cracks in a loaded elastic material. The cracks considered are two‐dimensional with nonblunted, tapered ends such that opposite faces are tangent to each other at points of contact. Under compression the cracks shorten by closing near the crack tips, the proportion of crack surface area in contact becoming gradually larger. Some cracks make contact between the crack tips, becoming multiple cracks. Normal stresses on the crack surface vary rapidly over the closed portions from zero near the open surfaces to a peak value at the original crack tip. Stresses remain finite everywhere. At a given load the effective rock compressibility due to arbitrarily shaped, tapered cracks depends only on crack length, giving results identical to a distribution of elliptical cracks of the same lengths. However, at different loads the varying length causes the modulus to vary. As a result, interpretation of features like porosity and modulus under varying applied stress will depend on the specific crack model chosen. In particular, a single aspect ratio of a simply tapered crack yields the same nonlinear effect as a flat distribution of elliptical cracks. Consequently, estimates of crack spectra from nonlinear strain data are totally nonunique.
Extension behind island arcs and motions relative to hot spots
Authors: Chase Clement G.
Published: February 2008

The spatial distribution and episodic nature of behind‐arc spreading is as yet unexplained. The present sites of rapid marginal basin opening well‐developed enough to be producing unmistakable oceanic crust are the Tonga‐Kermadec, Mariana, and Scotia arcs and the Andaman Sea. In relation to a hot spot reference frame the major plate lying behind the trench in all these spreading arc systems moves away from the subduction zone (retreats). Taking into account the behind‐arc spreading, almost all of the oceanic subduction zones advance in the hot spot frame, i.e., move toward the side from which the subducted slab approaches. This leads to the suggestion that marginal basin opening may be related to a component of ‘trench suction’ in the driving forces of plate tectonics.
Egress of hot water from midocean ridge hydrothermal systems: Some thermal constraints
Authors: Sleep Norman H. , Wolery Thomas J.
Published: February 2008

The flow rates necessary for hot water to vent directly into the ocean at midocean ridge hydrothermal systems are obtained by balancing heat lost by conduction through the walls of the hydrothermal passages with the heat brought up in the water. Widespread porous flow is unlikely to vent hot water even at the ridge axis. More spatially restricted flow was modeled by planar and tubular crack zones. The planar zones can vent water only through young (age <1 m.y. B.P.) crust and then must be spaced on the order of kilometers. Tubular zones vent hot water on moderately old crust (1 m.y. B.P. <age <11 m.y. B.P.). A factor of 2 or 3 change in flow rate can change the temperature in a crack zone from slow cooling as the fluid rises to venting of hot water. Clogging will eliminate vents where extensive cooling or mixing occurs in the uppermost regions. The flow volumes predicted for cracks are large enough that minor and trace constituents may clog crack zones.

Appendix is available with entire article on microfiche. Order fromAmerican Geophysical Union, 1909 K Street, N. W., Washington, D. C. 20006. Document J78‐008; $1.00. Payment must accompanyorder.
A numerical treatment of melt/solid segregation: Size of the eucrite parent body and stability of the terrestrial low‐velocity zone
Authors: Walker David , Stolper Edward M. , Hays James F.
Published: February 2008

Crystal sinking to form cumulates and melt percolation toward segregation in magma pools can be treated with modifications of Stokes' and Darcy's laws, respectively. The velocity of crystals and melt depends, among other things, on the force of gravity (g) driving the separations and the cooling time of the environment. The increase of g promotes more efficient differentiation, whereas the increase of cooling rate limits the extent to which crystals and liquid can separate. The rate at which separation occurs is strongly dependent on the proportion of liquid that is present. As a result, cumulate formation is a process with a negative feedback; the more densely aggregated the crystals become, the slower the process can proceed. In contrast, melt accumulation is a process with a positive feedback; partial accumulation of melt leads to more rapid accumulation of subsequent melt. This positive feedback can cause melt accumulation to run rapidly to completion once a critical stability limit is passed. The observation of cumulates and segregated melts among the eucrite meteorites is used as a basis for calculating the g (and planet size) required to perform these differentiations. The eucrite parent body was probably at least 10–100 km in radius. The earth's low velocity zone (LVZ) is shown to be unstable with respect to draining itself of excess melt if the melt forms an interconnecting network. A geologically persistent LVZ with a homogeneous distribution of melt can be maintained with melt fractions only on the order of 0.1% or less.
Back‐arc opening and the mode of subduction
Authors: Uyeda Seiya , Kanamori Hiroo
Published: February 2008

Trench‐arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back‐arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back‐arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench‐arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back‐arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back‐arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back‐arcs.
Thermal conductivity of crystalline rocks associated with energy extraction from hot dry rock geothermal systems
Authors: Sibbitt W. L. , Dodson J. G. , Tester J. W.
Published: February 2008

Because the lifetime of and heat extraction rate from a hot dry rock (HDR) geothermal reservoir can be substantially controlled by the in situ rock thermal conductivity, information concerning the dependence of thermal conductivity on moisture content and temperature is important for proper design and management of the reservoir. Results of thermal conductivity measurements are given for 14 drill core rock samples taken from two exploratory HDR geothermal wellbores (maximum depth of 2929 m (9608 ft)) drilled into Precambrian granitic rock in the Jemez Mountains of northern New Mexico. These samples have been petrographically characterized and in general represent fresh competent Precambrian material of deep origin. Thermal conductivities, modal analyses, and densities are given for all core samples studied under dry and wet (‘water‐saturated’) conditions. A cut bar thermal conductivity comparator and a transient needle probe were used for the determinations with fused quartz and Pyroceram 9606 as the standards. The maximum temperature range of the measurements was from the ice point to 250°C. The measurements on wet rock were limited to the temperature range below room temperature. Conductivity values of the dense core rock samples were generally within the range from 2 to 2.9 W/mK at 200°C. Excellent agreement was achieved between these laboratory measurements of thermal conductivity and those obtained by in situ measurements run in the HDR wellbores. By using samples of sufficient thickness to provide a statistically representative heat flow path, no difference between conductivity values and their temperature coefficients for orthogonal directions (heat flow parallel or perpendicular to core axis) was observed. This isotropic behavior was even found for highly foliated gneissic specimens. Estimates of thermal conductivity based on a dispersion and/or mixture analysis utilizing pure minerallic phase conductivities and detailed modal analyses usually agreed to within 9% of the experimental values.
b values and ω−γ seismic source models: Implications for tectonic stress variations along active crustal fault zones and the estimation of high‐frequency strong ground motion
Authors: Hanks Thomas C.
Published: February 2008

In this study the tectonic stress along active crustal fault zones is taken to be of the form
, where
is the average tectonic stress at depth y and Δσp(x, y) is a seismologically observable, essentially random function of both fault plane coordinates; the stress differences arising in the course of crustal faulting are derived from Δσp(x, y). Empirically known frequency of occurrence statistics, moment‐magnitude relationships, and the constancy of earthquake stress drops may be used to infer that the number of earthquakes N of dimension ≥r is of the form N ∼ 1/r2 and that the spectral composition of Δσp(x, y) is of the form
, where
is the two‐dimensional Fourier transform of Δσp(x, y) expressed in radial wave number k. The γ = 2 model of the far‐field shear wave displacement spectrum is consistent with the spectral composition
, provided that the number of contributions to the spectral representation of the radiated field at frequency ƒ goes as (k/k0)2, consistent with the quasi‐static frequency of occurrence relation N ∼ 1/r2; k0 is a reference wave number associated with the reciprocal source dimension. Separately, a variety of seismologic observations suggests that the γ = 2 model is the one generally, although certainly not always, applicable to the high‐frequency spectral decay of the far‐field radiation of earthquakes. In this framework, then, b values near 1, the general validity of the γ = 2 model, and the constancy of earthquake stress drops independent of size are all related to the average spectral composition of
. Should one of these change as a result of premonitory effects leading to failure, as has been specifically proposed for b values, it seems likely that one or all of the other characteristics will change as well from their normative values. Irrespective of these associations, the far‐field, high‐frequency shear radiation for the γ = 2 model in the presence of anelastic attenuation may be interpreted as band‐limited, finite duration white noise in acceleration. Its rms value, arms, is given by the expression arms = 0.85[21/2(2π)2/106] (Δσ/ρR)(ƒmax/ƒ0)1/2, where Δσ is the earthquake stress drop, ρ is density, R is hypocentral distance, ƒ0 is the spectral corner frequency, and ƒmax is determined by R and specific attenuation 1/Q. For several reasons, one of which is that it may be estimated in the absence of empirically defined ground motion correlations, arms holds considerable promise as a measure of high‐frequency strong ground motion for engineering purposes.
The vertical distribution of Mars water vapor
Authors: Davies Donald W.
Published: February 2008

Analysis of observations made from the Viking 1 Orbiter indicates that the water vapor over the Viking 1 landing site is uniformly mixed with the atmosphere and not concentrated near the surface. The analysis incorporates the effects of atmospheric scattering and explains why previous earth‐based observations showed a strong diurnal variation in water content. It also explains the lack of an early morning fog and removes the necessity of daily exchange of large amounts of water between the surface and the atmosphere. A water vapor volume mixing ratio of 1.5×10−4 is inferred for the Viking 1 site in late summer.
Formation of Martian flood features by release of water from confined aquifers
Authors: Carr Michael H.
Published: February 2008

Many large Martian channels arise full born from discrete areas of chaotic terrain. Estimates of peak discharges based on channel dimensions range from 106 to 108 m3/s. It is here proposed that the large channels were eroded by water released rapidly, under great pressure, from deeply buried aquifers. Early in the planet's history the old cratered terrain was probably highly permeable to depths of several kilometers as a result of its volcanic origin and intense brecciation by meteorite impact. Extensive dissection of the old cratered terrain by fine channels suggests that fluvial action was widespread at that time and that warmer climatic conditions prevailed. Much of the water which cut the fine channels was probably removed from surface circulation and entered the groundwater system. Subsequent global cooling trapped the groundwater under a thick permafrost layer and formed a system of confined aquifers. Thickening of the permafrost and warping of the surface created high pore pressures within the aquifers, particularly in low areas. Episodic breakout of water from the aquifers could have been triggered either by impact or by the pore pressure reaching the lithostatic pressure. The rate of outflow would have depended on the aquifer thickness and permeability, its depth of burial, and the diameter of the region over which water had access to the surface. Plausible values give discharges that range from 105 to 107 m3/s. Outflow from the aquifer probably caused undermining of the adjacent areas and collapse of the surface to form chaos. Flow ceased when the aquifer was depleted, or when the hydraulic gradient around the chaos, and thus the flow, was so reduced that the flow could freeze. The process could be repeated if the aquifer were recharged.
Rifting of the plate boundary in north Iceland 1975–1978
Authors: Björnsson Axel , Johnsen Gunnar , Sigurdsson Sven , Thorbergsson Gunnar , Tryggvason Eysteinn
Published: February 2008

A rifting episode started in 1975 on the accreting plate boundary in North Iceland after 100 years of quiescence. Horizontal extension of some 3 m has been observed in the Krafla caldera and the associated 80 km long fissure swarm. The rifting occurs periodically in short active pulses at a few months intervals. Between these active pulses, continuous inflation of 7–10 mm/day of the caldera is caused by 5 m3/s inflow of magma into a magma chamber at 3 km depth. The active pulses are caused by a sudden east‐west expansion of the fissure swarm and a contraction of zones outside the fissure swarm. Rapid flow of magma out of the magma chamber and into the fissures toward north or south is indicated. These pulses are accompanied by earthquake swarms and vertical ground movements of up to 2 m and sometimes also volcanic eruptions and formation of new fumaroles. The magma chamber below the Krafla caldera thus acts as a trigger for the plate movement along the constructive plate boundary in North Iceland.
Dielectric relaxations of coal and conductive impurities
Authors: Alvarez Román , de Paiva José Airton C.
Published: February 2008

Bituminous coal mixed with pyrite and water impurities yields dielectric relaxations of the Davidson‐Cole type. Measurements were made on mixtures ranging from 100% coal to 50% coal‐50% pyrite in the frequency range of 30 Hz to 100 kHz. Wet samples consisted of one of the above mixtures (20 g) plus 1 g of water. Mixtures with water and 40–50% pyrite suggest relaxations of the Debye type. Interconnections between pyrite grains dispersed on coal give rise to conducting needles which are in part responsible for the observed dispersions. The mixtures analyzed herein varied in a dc conductivity range of 6 decades, from 10−11to 10−5(ohm m)−1, suggesting that various types of coal combined with small amounts of impurities can yield the full, 12‐decade range of conductivity variation determined for coal elsewhere.
Active faulting and cenozoic tectonics of the Tien Shan, Mongolia, and Baykal Regions
Authors: Tapponnier Paul , Molnar Peter
Published: February 2008

We present a study of the active tectonics of central Asia based on an interpretation of Landsat imagery and supplemented with published field observations and seismic data. Reverse faulting dominates the tectonics of the Tien Shan but is associated with prominent northwest trending right lateral strike slip fault systems. Both types of faulting imply approximately north‐south maximum compressive stress. The active tectonics of the Altai and of southern Mongolia are controlled by largescale conjugate strike slip faulting; left lateral on east‐west planes and right lateral on north‐northwest planes. This implies that the maximum compressive stress is oriented approximately northeast‐southwest. Farther north, strike slip faulting gives way to predominantly normal faulting in the Baykal rift system. We interpret all of the active faulting to be a consequence of lateral displacements of the crust caused by the penetration of the Indian subcontinent of Eurasia. We also interpret the gradual change from thrust faulting and high altitudes in the south and west to normal faulting and lower mean elevations in the north and east to reflect a smooth change in the average state of stress. This suggests that the details of the complex intracontinental deformation in Asia are better described by the deformation of a continuum than by the relative motion of a small number of rigid blocks. Intracontinental rifting in the northeast, in particular, may result from a state of stress analogous to the secondary tension that commonly arises within bounded plastic materials indented by a rigid die.
Estimation of surface wind speeds using satellite‐borne radar measurements at normal incidence
Authors: Brown Gary S.
Published: February 2008

The results of a study to determine the correlation of the surface scattering cross section per unit area at normal incidence σ°(0°) and 13.9 GHz with surface wind speed are presented. Comparing σ°(0°) measurements with ship reports of wind speed, it was found that an algorithm could be devised which is capable of producing wind speed estimates from σ°(0°) measurements to a precision of 2.6 m/s for winds in the range of 1–21 m/s. Correlations with buoy measurements in the range of 1–10 m/s show an rms precision of 2.1 m/s with the possibility of a slight bias in the ship report based algorithm. A study of σ°(0°)‐inferred wind speeds in relation to hindcast estimates shows a distinct tendency in the hindcast results to be biased toward the low side in the absence of sufficient ship reports of wind and pressure. The results of this study indicate that a short‐pulse radar altimeter can provide reasonably accurate estimates of surface wind speeds over a rather large range of wind speeds.
Paleomagnetism of Upper Precambrian sedimentary rocks from the Desert Range, Nevada
Authors: Van Alstine David R. , Gillett Stephen L.
Published: February 2008

About 300 oriented samples were obtained from upper Precambrian and possibly lowest Cambrian strata in a homoclinal sequence conformable with Lower Paleozoic strata in the Desert Range, Nevada. These samples are from miogeoclinal sedimentary rocks of the uppermost Johnnie Formation (upper Precambrian) and overlying Stirling Quartzite (uppermost Precambrian and Lower(?) Cambrian). For about 100 of the samples, thermal demagnetization revealed a characteristic magnetization with a low inclination. The characteristic magnetization of the uppermost Johnnie has two polarities, which allows its direction to be established despite incomplete removal of a secondary component of recent origin. Petrographic and stratigraphic evidence suggest that this characteristic magnetization was acquired penecontemporaneously with deposition. The age of the magnetization probably lies in the range 675–575 m.y. B.P., based on estimates of sedimentation rates in the Desert Range and on proposed lithologic correlations and isotopic age control elsewhere in the Cordilleran Geosyncline. A fine‐grained, red‐purple unit in the middle Stirling Quartzite exhibits a single‐polarity (reversed) characteristic magnetization with a direction roughly like the directions of the uppermost Johnnie or the Lower Cambrian Wood Canyon Formation, which conformably overlies the Stirling. The paleomagnetic pole from the uppermost Johnnie is about 47° from the pole for the Wood Canyon. It seems probable therefore that at least 45° of apparent polar wandering occurred with respect to North America in latest Precambrian time.
The double seismic zone in downgoing slabs and the viscosity of the mesosphere
Authors: Sleep Norman H.
Published: February 2008

The seismic zone beneath several island arcs between about 100 and 200 km depth consists of an upper zone having down‐dip compression and a lower zone having down‐dip tension. Several numerical models of the Aleutian arc were computed to test the hypothesis that these double seismic zones are due to sagging of the slab under its own weight. This sagging occurs because the asthenosphere (between about 100 and 200 km) provides little support or resistance to the slab, which is supported from below by the more viscous mesosphere and from above by the lithosphere. The viscosity of the mesosphere was constrained to the interval between 0.25×1022 and 0.5×1022 P by noting that the slab would have mainly down‐dip compression at higher viscosities and mainly down‐dip tension at lower viscosities. The deviatoric stress in the slab and the fault plane between the slab and the island arc is about 200–300 bars (expressed as shear stress). The models were calibrated to the observed depth and gravity anomalies in the trench.
The Galapagos Rift at 86°W: 1. Regional morphological and structural analysis
Authors: Allmendinger Richard W. , Riis Fridtjof
Published: February 2008

This study presents an analysis of a detailed topographic survey with a multi‐beam bathymetric survey system, of the crest and upper flank of the Galapagos Rift between 85°20′ and 86° 17′W. The morphology of this region is distinctly lineated due to elongate ridges and troughs parallel to the ridge axis (first order relief). These ridges and troughs are called lineaments, and occur in pairs of strikingly similar appearance on both sides of the axis. Each lineament exhibits a second order relief consisting of highs and lows that alternate along the strike of the lineament. A high in one of a pair of lineaments is opposed to a low in the other. Finally, a third order relief of small, rounded hills is a result of volcanic construction in constrast to the first and second order relief which are of structural origin. The evolution of the rise is primarily due to processes operating within 5 km to either side of the axis. In this zone, lineaments form by progressive faulting over a long period of time. Two paired lineaments start to form at slightly different times. This is related to the instantaneous position of the primary zone of extrusion, which migrates continuously across the rift valley floor. The process of formation of the lineaments is completed in the crestal ranges; beyond, the pattern is frozen and the lineaments are transferred downslope with only subsidence and relief reduction by faulting. The constant width of the lineaments probably reflects the depth near the axis of a transition from brittle to ductile crust at approximately 1 km. The extreme symmetry of the lineaments across the axis remains unexplained. The antisymmetric high‐low second order relief is attributed to shallow intrusions. It is not clear why the pattern is so regular. Comparison with the Famous area at the Mid‐Atlantic ridge suggests a relation between the height of the rift walls, the depth of the rift valley floor, the relative amount of structural extension and the ease with which the magma can reach the surface from the source. The magma source in the Famous area may be relatively deep so that the only surface access is by localized, diapric upwelling. This is reflected in the short ridge segments and short, closely spaced transform faults of that region. In contrast, the magma source in the Galapagos area seems to be much shallower so that magma is available everywhere along the rise axis. Here, ridge segments are long, and only major transforms occur.
Dilatancy anisotropy and the response of rock to large cyclic loads
Authors: Scholz C. H. , Koczynski T. A.
Published: February 2008

An experimental study of cyclic fatigue of granite and diabase in triaxial compression was conducted. It was found that dilatancy progressively developed during cyclic loading of these rocks with a form in time very similar to a creep curve. Dilatancy was in all cases highly anisotropic. This anisotropy was controlled by the crack microstructure in the granite and by the residual stress in the diabase. The strike of the fracture plane was found typically to form normal to the axis of maximum dilatancy. Increasing dilatancy during cyclic loading was found to be caused by progressive dilatant creep plus additional damage produced by the cycling itself. The former leads to a pronounced loading rate effect on fatigue, the latter to a complex effect of cycle amplitude on fatigue. No stabilization of fatigue by pressure was found for confining pressures up to 300 MPa. The total dilatant strain at fracture was found to increase strongly with peak stress and decrease with increasing loading rate. Three types of cracking are shown to result in dilatancy: type 1, stress‐induced cracking; type 2, stress corrosion cracking; and type 3, fatigue cracking. Rock fracture is sensitive to which type is prevalent.
Dynamic recrystallization during creep of single‐crystalline halite: An experimental study
Authors: Guillope M. , Poirier J. P.
Published: February 2008

Single crystals of pure and impure halite have been dynamically recrystallized during compression creep at temperatures between 250° and 790°C and stresses between 1.5 and 120 bars. Recrystallization was found to occur by two different mechanisms: at lower temperatures and stresses the new grains result from the rotation of subgrains without grain boundary migration (rotation recrystallization), and at higher temperatures and stresses the final texture results from the migration of the high‐angle grain boundaries of the rotated subgrains. Migration recrystallization was shown to occur for critical stress and temperature conditions, allowing rapid grain boundary migration. A curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur. In each recrystallization regime the recrystallized grain size is uniquely related to the applied stress, thus yielding two different geopiezometers, which should not be applied indiscriminately to natural tectonites to determine lithospheric or mantle deviatoric stresses. The experimental results are interpreted by the Lücke et Stüwe theory for impurity‐controlled grain boundary migration.
Ice in the lunar polar regions
Authors: Arnold James R.
Published: February 2008

The idea that ice and other trapped volatiles exist in permanently shadowed regions near the lunar poles was proposed by Watson, Murray, and Brown [1961]. It is reexamined in the present paper, in the light of the vast increase of our lunar knowledge. The stability of the traps and the trapping mechanism are verified. Four potential sources of lunar H2O, (1) solar wind reduction of Fe in the regolith, (2) H2O‐containing meteoroids, (3) cometary impact, and (4) (the least certain) degassing of the interior, can supply amounts of trapped H2O estimated in the range of 1016–1017 g. Two important destructive mechanisms have been identified: photodissociation of H2O molecules adsorbed on the sunlit surface and sputtering or decomposition of trapped H2O by solar wind particles. The effect of impact gardening is mainly protective. The question of the presence of H2O in the traps remains open; it can be settled by experiment.
Determination of aquifer parameters from well tides
Authors: Rhoads George H. , Robinson Edwin S.
Published: February 2008

An equation for the specific storage of an inland artesian aquifer of infinite extent is developed from a consideration of the tidal dilatation of an idealized well‐aquifer system. It was found that for a particular tidal frequency the specific storage Ss is related to tidal water level and barometric pressure fluctuations according to the relationship Ss = −0.5 W2 cos Φe/{ag [dhw cos Φw + B dhb cos Φb]}, where W2, dhw, dhb are the amplitudes of the periodic tidegenerating potential, the water level fluctuations in the well, and the barometric pressure fluctuations; Φe, Φw, and Φb are the phase angles between the tide‐generating potential and the earth tide dilatation, the water level fluctuation in the well, and the barometric pressure fluctuations; B is the barometric efficiency; a is the radius of the earth; and g is the acceleration of gravity. The earth tide dilatation phase angle Φe ≈ 0.0 can be assumed. Two arbitrary numerical methods are devised for estimating barometric efficiency from aperiodic water level and barometric pressure fluctuations. If the specific storage and barometric efficiency are known, in situ values of porosity n and the matrix bulk modulus Em can be calculated by the following expressions: n = {1 ± [1 ‐ (4 Ss Ew [B‐B2]/ρg)]½}/2(1‐B) and Em = ρg {1 ± [1 ‐ (4 Ss Ew [B‐B2]/ρg)]½}/2 Ss (1‐B) where Ew is the bulk modulus of water and ρ is the density of water. Tidal water level fluctuations were recorded in three observation wells in Montgomery County, Virginia. Physically reasonable values of specific storage, barometric efficiency, porosity, and matrix bulk modulus were calculated, but the accuracy of these values is limited by the imprecise measurements of water level and barometric pressure and the possibility of leakage from imperfectly confined aquifers.
Characterization of barriers on an earthquake fault
Authors: Aki Keiiti
Published: February 2008

Recently, Bouchon (1979a) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1966 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 109 erg cm−2. Using the formulas of Ida (1973a), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high‐velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.
Heat flow at the spreading centers of the Guaymas Basin, Gulf of California
Authors: Williams David L. , Becker Keir , Lawver Lawrence A. , Von Herzen Richard P.
Published: February 2008

Fifty‐four new heat flow measurements in the central troughs of the Guaymas basin support the hypothesis that they are sites of active intrusion. In the northern trough a distinct pattern of hydrothermal cooling is revealed, with venting along the western boundary fault of the trough. In the southern trough an analogous pattern is apparently superimposed upon a conductive cooling anomaly associated with a recent central intrusion. The discharge of thermal waters occurs along the boundary faults and through other faults associated with a possible horst block located in the north central floor of the southern trough. The heat flow patterns suggest that the intrusions are episodic and do not occur simultaneously along the length (15–40 km) of a spreading segment. A review of all available heat flow measurements for the Guaymas basin suggests that most of the recharge for a pervasive regional hydrothermal system is limited to the central depressions, with perhaps some contribution from pore water. The discharge of thermal waters occurs predominantly in the central depressions and possibly along the boundary transform faults and fracture zones. The regions of the basin more than a few kilometers in distance from the spreading axis, although presumably underlain by a hydrothermal system, are probably not the location of numerous vents or recharge zones.
A model for the evolution of the Indian Ocean and the breakup of Gondwanaland
Authors: Norton I. O. , Sclater J. G.
Published: February 2008

Magnetic anomaly and fracture zone information is used to develop a self‐consistent tectonic history of the Indian and South Atlantic oceans. Working backward in time we have made three reasonably well constrained (39, 53, and 65 Ma) and two speculative (80 and 115 Ma) reconstructions of the positions of the Gondwana continents (Ma is m.y.B.P.). Our final fit, which is constrained by the recognition of Mesozoic anomalies off Antarctica and in the Mozambique Basin, places Dronning Maud Land against southern Mozambique and Madagascar in the northern position against Kenya. We suggest that after the initial rifting, Antarctica moved away from Africa in a southerly direction relative to present‐day Africa. This started the formation of the Southwest Indian Ridge. Most of the present length and geometry of the ridge result from migration of triple junctions so do not reflect predrift continental outlines. India and Madagascar moved with Antarctica until India separated from first Antarctica then Madagascar, when it started moving north toward Asia. In our reconstructions we find no necessity for significant relative motion between the Antarctic Peninsula and South America from the early Cretaceous to the Oligocene. From the breakup of Gondwanaland to the present we identify seven significant events. These are (1) first break in the late Triassic/early Jurassic between East and West Gondwanaland with initial motion along long transform faults parallel to the present African east coast, (2) early Cretaceous separation of Africa and South America and possibly simultaneous separation between India and Australia‐Antarctica, (3) cessation of motion between Africa and Madagascar, (4) break between India and Madagascar in the late Cretaceous, (5) Paleocene reorganization in the northwest Indian Ocean when the Seychelles left India, (6) Eocene separation between Australia and Antarctica with Australia joining the Indian plate, and (7) India's collision with Asia and subsequent commencement of spreading on the Central Indian Ridge, and later opening of Drake Passage.
Pitted and fluted rocks in the western desert of Egypt: Viking comparisons
Authors: McCauley J. F. , Breed C. S. , El‐Baz Farouk , Whitney M. I. , Grolier M. J. , Ward A. W.
Published: February 2008

The Western Desert of Egypt is one of the most arid regions on Earth and is probably our closest terrestrial analog to the surface of Mars. An expedition to the area in 1978 revealed an abundance of quartzite and basalt rocks that have been pitted and fluted by wind erosion and deflation of the desert surface. These pitted rocks are internally homogeneous, show no internal holes or vesicles, and are considered an important but neglected type of ventifact. They bear a striking resemblance to the pitted and fluted rocks seen by the Viking Landers, rocks that have generally been interpreted as vesicular basalts only slightly modified by wind erosion. Wind tunnel studies of the air flow over and around nonstreamlined hand specimens from the Western Desert show that windward abrasion coupled with negative flow, secondary flow, and vorticity in a unidirectional wind can explain the complex arrays of pits and flutes. These field and laboratory observations suggest that the pitted rocks at the Viking Lander sites are also ventifacts, and thus the Martian surface may be far more wind eroded than previously thought.
The effects of nonideal surfaces on the derived thermal properties of Mars
Authors: Jakosky Bruce M.
Published: February 2008

The thermal inertia of the surface of Mars varies spatially by a factor of 8. This is attributable to changes in the average particle size of the fine material, the surface elevation, the atmospheric opacity due to dust, and the fraction of the surface covered by rocks and fine material. The effects of these nonideal properties on the surface temperatures and derived thermal inertias are modeled, along with the effects of slopes, CO2 condensed onto the surface, and layering of fine material upon solid rock. The nonideal models are capable of producing thermal behavior similar to that observed by the Viking infrared thermal mapper, including a morning delay in the postdawn temperature rise and an enhanced cooling in the afternoon relative to any ideal, homogeneous model. The enhanced afternoon cooling observed at the Viking 1 landing site is reproduced by the nonideal models while that atop Arsia Mons volcano is not, but may be attributed to the observing geometry. A histogram of surface thermal inertia versus elevation shows at least two distinct classes: a single region near Amazonis Planitia has low inertias at low elevations; many of the remaining data show an anticorrelation between inertia and elevation, expected because of the change in thermal inertia produced by changes in the atmospheric pressure and dust opacity with elevation.
Thermal cracking and amplitude dependent attenuation
Authors: Johnston David H. , Toksöz M. Nafi
Published: February 2008

The role of crack and grain boundary contacts in determining seismic wave attenuation in rock is investigated by examining Q as a function of thermal cycling (cracking) and wave strain amplitude. Q values are obtained using a longitudinal resonant bar technique in the 10‐ to 20‐kHz range for maximum strain amplitudes varying from roughly 10−8 to 10−5. The samples studied include the Berea and Navajo sandstones, Plexiglas, Westerly granite, Solenhofen limestone, and Frederick diabase, the latter two relatively crack free in their virgin state. Measurements were made at room temperature and pressure in air. Q values for both sandstones are constant at low strains (<10−) but decrease rapidly with amplitude at higher strains. There is no hysteresis of Q with amplitude. Q values for Plexiglas show no indication of amplitude dependent behavior. The granite, limestone, and diabase are thermally cycled at both fast and slow heating rates in order to induce cracking. Samples slowly cycled at 400°C show a marked increase in Q that cannot be entirely explained by outgassing of volatiles. Cycling may also widen thin cracks and grain boundaries, reducing contact areas. Samples heated beyond 400°C, or rapidly heated, result in generally decreasing Q values. The amplitude dependence of Q is found to be coupled to the effects of thermal cycling. For rocks slowly cycled to 400°C or less, the transition from low‐amplitude contant Q to high‐amplitude variable Q behavior decreases to lower amplitudes as a function of maximum temperature. Above 400°C, and possibly in the rapidly heated samples also, the transition moves to higher amplitudes.
On the variation of continental heat flow with age and the thermal evolution of continents
Authors: Vitorello Icaro , Pollack Henry N.
Published: February 2008

The observed decrease of continental heat flow with tectonic age is interpreted in a three‐component model. The first component is radiogenic heat from the zone of crustal isotopic enrichment, which yields about 40% of the observed heat flow in terrains of all tectonic ages. Because the surface heat flow varies with age between about 90 and 45 mW m−2, this radiogenic component also varies with age between about 36 and 18 mW m−2, a net decrease of some 18 mW m−2. The absolute decrease with time of this component is achieved by erosional removal of radioisotopes from the surface; the erosion, exponentially decreasing with age, has a characteristic time of some 300–400 Ma (m.y. B.P.). The second component of surface heat flow is residual heat from a transient thermal perturbation associated with tectogenesis. This transient yields approximately 30% of the heat flow (27 mW m−2) in Cenozoic tectonic zones, diminishing effectively to zero in late Precambrian to mid‐Precambrian terrains. The decay of this component has been fitted with temperature perturbation models that show a maximum perturbation to a cool shield geotherm of 750°–800°C at a depth of 80–100 km. From the analysis of this transient component we conclude that for residual heat to be observable up to and beyond 500 Ma, conductive cooling likely extends at least to depths as great as 300–350 km, implying that the continental crust and upper mantle beneath 500‐Ma and older terrains has remained as a single conductive unit to such depths for at least the age of the terrain. For younger terrains the residual heat is yet sufficient to weaken them and make them vulnerable to recurrent tectonism. The third component of surface heat flow is a background flux of 27 mW m−2. Some 15 mW m−2 arises within the thick continental conductive root and is likely to be radiogenic, although the isotopic concentrations required imply some modest posttectonic metasomatic enrichment. The remaining 12 mW m−2 of the background comes from deeper within the earth, perhaps from the core.