bluuebunny/crossref_metadata_2025 huggingface dataset:
Equations of state of iron sulfide and constraints on the sulfur content of the Earth
Authors: Ahrens Thomas J.
Published: February 2008

New shock and release wave data for pyrrhotite (Fe0.9S) obtained over the pressure range of 3–158 GPa (0.03–1.58 Mbar) suggest that this mineral undergoes a major shock‐induced phase change(s) with an onset in the range 2.7–3.8 GPa. Free‐surface velocities of Fe0.9S released from states between 8.9 and 24.7 GPa indicate a maximum postshock density of ∼5.54 g/cm3. A pressure of ∼25 GPa appears to be required to drive the phase transition(s) to completion. A density for the high‐pressure phase (hpp) consistent with present static high‐pressure X ray data, ∼5.34 g/cm3, was used to calculate a zero‐pressure, adiabatic bulk modulus of 126–128 GPa for the hpp. Release adiabat measurements centered at 152 and 158 GPa are consistent with the assumption that the Hugoniot curve in the 25‐ to 158‐GPa range reflects the properties of a denser polymorph, possibly with eight‐fold coordination, which may be similar to the local bonding of sulfur in the liquid core of the earth. Similar, but less well constrained, reductions are presented for pyrite, FeS2, based on the three data points of Simakov et al. (1974) in the pressure range 88–320 GPa. These are inferred to represent the behavior of an unknown hpp (approximate zero‐pressure density of 5.3 g/cm3) and indicate that this phase forms at a pressure above ∼29 GPa. Reduction of these data yields a zero‐pressure bulk modulus for the hpp in the range 205–244 GPa. The raw Hugoniot data for Fe0.9S, FeS2, and Fe when constrained to the seismologically obtained density‐pressure profiles of the outer core of the earth indicate a systematic decrease of apparent sulfur content from 10 to 6.5% with depth. When the shock data are reduced to isotherms, a nearly constant sulfur content in the range 9–12% is inferred. Using these bounds on the sulfur content of the core, and depending on whether an olivine or pyroxene mantle stoichiometry is assumed, the earth can be modeled as being depleted in S by a factor ranging from 1.7 to 3.2 with respect to the abundances of Si and from 2.8 to 7.5 relative to the abundance of Fe, in CCl carbonaceous chondrites. It is concluded that although the shock wave data permit the major light element in the core to be sulfur, the earth can be modeled as being depleted in sulfur, along with other volatile elements. A systematic relation, C0(km/s) = 7.15 ‐ 0.47
, was also discovered upon comparison of the inferred densities and bulk sound speeds (C0) of the hpp's of the iron sulfides with other measurements for 12 sulfides and elemental sulfur. Here
is the volume (in cubic centimeters) per mole of atoms.
Kinematic models of large‐scale flow in the Earth's mantle
Authors: Hager Bradford H. , O'Connell Richard J.
Published: February 2008

It is likely that the motions of the lithospheric plates strongly influence the accompanying large‐scale flow in the earth's mantle. In order to isolate and understand the effects of the plate motions on the large scale flow, we calculate simple kinematic models in which the observed plate motions and geometries are imposed as boundary conditions; thermal buoyancy forces are ignored and the flow is determined by the mass flux imposed by the moving plates. With the assumption of a layered, radially symmetric, Newtonian rheology, the flow accompanying the observed plate motions is calculated analytically. The flow is in general complicated, but for models in which whole‐mantle flow occurs, the dips of subduction zones are predicted remarkably well by the flow modesl. Circulation does not from simple closed cells and flow under slowly moving plates is not closely related to the direction of plate motion. There is little return flow under the Pacific unless the lithosphere is essentially decoupled from the mantle by an extremely weak low viscosity layer. Only viscosity contrasts between the upper and lower mantle of three to four orders of magnitude are sufficient to confine flow to the upper mantle. For a wide range of mantle viscosity structures, significant mixing between the upper and lower mantle is expected over geologic time. On the basis of these models, the source rock for oceanic ridge basalt probably lies below the asthenosphere. A comparison of the magnitude of the deviatoric stress with theoretical flow mechanism calculations and with models of postglacial rebound indicate that the assumption of Newtonian viscosity is plausible. The drag on the lithosphere is compared to that calculated assuming simple shear. On a regional basis, the stresses can differ by up to 90°, e.g., in eastern North America. Small plates have higher apparent drag coefficients than larger ones. The normal stress at the base of the lithosphere is excessive in models in which the flow is confined to the upper 700 km of the mantle unless the lithosphere is decoupled from the mantle by an extremely low viscosity layer.
Detailed aeromagnetic investigation of the Arctic Basin
Authors: Vogt P. R. , Taylor P. T. , Kovacs L. C. , Johnson G. L.
Published: February 2008

Systematic low‐level aeromagnetic surveys conducted during 1974–1975 reveal details of the magnetic fabric in two parts of the Arctic Basin. These profiles extend coverage of the Nansen (Gakkel; Mid‐Arctic) Ridge from 85.3°N, 13°E to 86°N, 50°E, where these new data overlap previous Soviet aeromagnetic coverage. Prominent magnetic lineations can be identified despite spreading half‐rates as low as 0.3 cm/yr about 25–35 m.y. B.P. The separation of Lomonosov Ridge from Eurasia occurred at or before anomaly 24 time (55 m.y. B.P.). Although there is ‘room’ for anomalies 25–27 between 24 and the continental margin, a broad magnetic negative exists in their place. Either anomalies 25–27 were suppressed or erased by thick sediment fill or some other process associated with initial rifting, or the associated crust is subsided continental material. All anomalies, particularly the central anomaly, exhibit dramatic variations of amplitude along their strike. It is the low values of amplitude that are anomalous with respect to the Mid‐Atlantic Ridge to the south. Bathymetric data demonstrate that the high central anomaly amplitudes correlate with shallower rift valley floors (3500–4000 m) and higher rift mountains. We propose that the pillow basalt layer (2a) is thicker in the magnetic high amplitude zones. There is no bathymetric evidence for sediment gaining access to the valley floor in the area examined. A second survey was flown across the northern Canada Basin and Alpha Ridge. Complex, but lineated, anomalies of 1500 to 2500 nT relief parallel the crest of Alpha Ridge. A crestal valley, 2500 m deep and 30 km wide, flanked by ridges with crests 1200–1500 m deep, correlates well with magnetic and gravity anomalies; this suggests that prominent Alpha Ridge magnetic anomalies are caused by basement topography of high magnetization (20–30 A/M) and normal polarity. If the Alpha Ridge is floored by an anomalous type of oceanic crust, it probably originated during the long mid‐Cretaceous period of normal polarity. In the northern Canada Basin, several prominent linear anomalies cross the surveyed swath on strikes of 040°T to 065°T, subparallel to the Alpha Cordillera. If these lineations reflect sea‐floor spreading and geomagnetic reversals (possibly during the lower Cretaceous to upper Jurassic), the spreading axis cannot have paralleled either the Canadian continental margin or the Alpha Ridge as has been proposed, but instead, lay parallel to the Northwind Escarpment.
A seismic study of the crust and upper mantle of the Jordan‐Dead Sea Rift and their transition toward the Mediterranean Sea
Authors: Ginzburg Avihu , Makris Jannis , Fuchs Karl , Prodehl Claus , Kaminski Werner , Amitai Uri
Published: February 2008

Four long seismic refraction profiles were shot in the Jordon‐Dead Sea‐Gulf of Elat graben and adjacent areas. From the arrival times of the Pg, Pn, and PmP phases, models of the crust and upper mantle were computed. The models show a crustal thickness of 30 km along the graben with a considerable thinning to the south along the Gulf of Elat. Between the Dead Sea, Red Sea, and the Mediterranean, the crust thickens, reaching a maximum thickness of 40 km west of the graben; but toward the Mediterranean, the crust thins rapidly to a thickness of 20 km while the overlying sediments thicken considerably. The results indicate an upper mantle upwelling in the graben area, connected to the Red Sea spreading mechanism, and the presence of a thin, possibly oceanic crust covered by a thick wedge of Mesozoic and Tertiary sediments underneath the Northern Negev of Israel and probably extending offshore to the Eastern Mediterranean.
Optical absorption and radiative heat transport in olivine at high temperature
Authors: Shankland T. J. , Nitsan Uzi , Duba A. G.
Published: February 2008

The optical absorption spectra of natural single‐crystal and polycrystalline olivine (Mg1.84Fe0.16SiO4) have been measured in the wavelength range 300–8000 nm at temperatures ranging from 300 to 1700 K and under oxygen fugacity well within the sample stability field. The absorption significantly increases with increasing temperature due to a regular broadening of both crystal field and multiphonon lattice vibrational absorption bands; these are the principal absorption bands in the spectral region involved in radiative heat transfer. In the ‘window’ region between these bands the absorption coefficient increases from near zero at 300 K to about 10–15 cm−1 at 1700 K. Under mantle conditions the radiative thermal conductivity (KR) calculated from the olivine single‐crystal spectra increases from near zero at 300 K to about 5×10−3 cal/cm s K (2 W/m K) at 1700 K and is only weakly dependent on temperature above 800 K. Our determination of KR is 10–20% lower than previous estimates based on absorption measurements under different experimental conditions. In polycrystalline (dunite) samples the transmission decreased substantially with temperature cycling due to scattering at intergranular interfaces created by differential thermal expansion.
Physical properties of sediments containing gas hydrates
Authors: Stoll Robert D. , Bryan George M.
Published: February 2008

In order to better understand the occurrence and distribution of gas hydrates and their effects on acoustic and thermal measurements in ocean sediments, the authors have conducted a program of experimental research to study thermal conductivity and acoustic wave velocity in hydrates and sediments containing hydrate. The most significant result of these studies is that the formation of hydrate tends to cause a decrease in the thermal conductivity of a sediment. This is contrary to what might be expected on the basis of an analogy with the behavior of frozen sediment and implies that some thermal gradients assumed in earlier studies of in situ deposits of hydrate may be too small. Other results of the research based on measurements of acoustic wave velocity confirm that both pure water and water‐bearing sediment are converted to a stiff elastic mass by the formation of a sufficient quantity of hydrate. This clearly establishes the potential for a sharp acoustic impedance contrast at the boundary of a region of sediment containing a significant quantity of hydrate.
Transitional basalts and tholeiites from the East Pacific Rise, 9°N
Authors: Johnson Jeffrey R.
Published: February 2008

Basalts from the base of a small seamount on ∼1.5‐m.y.‐old crust west of the East Pacific Rise (EPR) at 9°N are intermediate in chemical and isotopic composition between light‐rare‐earth‐element‐depleted tholeiite (normal midocean ridge basalt (MORB)) and alkali basalt. Like oceanic alkali basalt, these rocks contain significantly more Ba, K, P, Sr, Ti, U, and Zr than normal MORB. Since the absolute abundances of these elements are still well below alkali basalt levels, the label transitional is adopted for these basalts. A series of fractionated MORB also occurs in this area, northwest of the Siqueiros Fracture Zone ‐ Transform Fault. The normal tholeiites are either olivine‐plagioclase or plagioclase‐clinopyroxene phyric, while the transitional basalts are spinel‐olivine phyric. Fractional crystallization quantitatively accounts for the chemical variability of the tholeiitic series but not for the transitional basalts. The tholeiitic series probably evolved in a crustal magma chamber ∼4 km below the crest of the East Pacific Rise. 143Nd/144Nd and other chemical data suggest that the large‐ion‐lithophile‐enriched transitional basalts may represent a hybrid of normal MORB and Siqueiros area alkali basalt. Incompatible element plots of K, P, and U indicate possible derivation of the transitional basalts by magma mixing. Magma mixing of unfractionated normal MORB and Siqueiros alkali basalt has been quantified. Derivation of the transitional basalts from a 1:1 mixture is supported by all available chemical data, including Cr, Cu, Nd, Ni, Sm, Sr, U, and V. This magma mixing apparently occurred at ≲30 km depth within a few tens of kilometers from the EPR axis. These Siqueiros area EPR transitional basalts are compared with Mid‐Atlantic Ridge (MAR) transitional basalts from the Iceland and Azores areas. The Siqueiros area basalts reflect a profound chemical and isotopic heterogeneity in the upper mantle, similar to that found along the MAR. Unlike the MAR, the EPR shows no evidence of plumelike bulges and associated large‐scale outpourings of nonnormal MORB resulting from these mantle heterogeneities. Siqueiros alkali basalt and MORB, as well as transitional basalt and MORB, were recovered from single dredge hauls. Such close spatial and temporal proximity of the inferred mantle sources places severe constraints on geometric and physicochemical upper mantle models.
Heat flow in the central Gulf of California
Authors: Lawver Lawrence A. , Williams David L.
Published: February 2008

One hundred and five new heat flow measurements in the Gulf of California support the premise that conductive heat loss is not the only mode by which heat is lost from a sea floor spreading center, even in an area with thick sediment cover. Theoretical estimates suggest that the average heat flow in the Guaymas and Farallon basins should be at least 11 μcal/cm2s (HFU) (325 mW/m2). Outside a 30‐km‐wide zone centered on the central troughs, the heat flow values measured are reasonably uniform but average only 4.3±0.2 HFU (180±10 mW/m2). Although the high sedimentation rate may depress the measured heat flow, the effect probably does not exceed 15%. Some heat, particularly in the smaller basins, may be lost to the adjacent cooler continental blocks. The discrepancy between the measured and predicted heat losses, which is at least 30%, may be due to the discharge of thermal waters, through the thinner sediment cover in the central troughs or along active faults.
Seismic anisotropy in the oceanic upper mantle: Evidence from the Bay of Islands Ophiolite Complex
Authors: Christensen Nikolas I. , Salisbury Matthew H.
Published: February 2008

Olivine fabrics in 17 field‐oriented ultramafics and mafics from three widely‐spaced traverses in the Bay of Islands Ophiolite Complex, Newfoundland, display a remarkably uniform symmetry in which the olivineacrystallographic axes are aligned subprependicular to the sheeted dikes and thebandcaxes lie within the plane of the sheeted dikes. The ultramafics studied consist entirely of tectonites; any olivine formed at the ridge crest by cumulus processes has since been re‐oriented by translation gliding and/or syntectonic recrystallization. Deformation has extended from the ultramafics into the overlying gabbro, which suggests that in many oceanic regions the deepest levels of layer 3 consist of gabbroic tectonites. Compressional wave velocities computed from these petrofabrics display 5–6% anisotropy in the plane of the Mohorovičić discontinuity, withVpfast parallel to the direction of spreading inferred from dike orientations. Since this pattern is identical to that observed for the oceanic upper mantle, it is concluded that the Bay of Islands Complex is a segment of oceanic crust and upper mantle. Shear wave velocity contours calculated from the same fabrics indicate that the upper mantle is nearly isotropic in terms of the maximum shear wave velocity,, but that the difference in velocity, ΔVs, between shear waves of orthogonal polarization traveling in the same direction may be sufficiently large parallel to the intersection of the Mohorovičić discontinuity and the sheeted dikes to allow detection of two distinct shear wave arrivals.
Compressional and shear waves in saturated rock during water‐steam transition
Authors: Ito Hisao , DeVilbiss John , Nur Amos
Published: February 2008

Compressional and shear wave velocities were measured in water‐filled Berea sandstone as a function of pore pressure, with a constant confining pressure of 300 bars. The measurements were made at room temperature, 145°C, and 198°C. At 145°C, compressional velocity increased from vapor‐saturated (low pore pressure) to liquid‐saturated (high pore pressure) conditions, whereas shear wave velocity decreased. For Compressional waves there was a velocity minimum and increased attenuation near the liquid‐vapor transition. The results at 198°C show decreases of both compressional and shear velocities and a small velocity minimum for compressional velocity without marked attenuation. At both temperatures, Vp/Vs and Poisson's ratios increased from steam‐ to water‐saturated rock. The results are compatible with the mechanical effects of mixing steam and water in the pore space near the phase transition and may be applicable to in situ geothermal field evaluation.
Constant Q‐wave propagation and attenuation
Authors: Kjartansson Einar
Published: February 2008

A linear model for attenuation of waves is presented, with Q, or the portion of energy lost during each cycle or wavelength, exactly independent of frequency. The wave propagation is completely specified by two parameters, e.g., Q and c0, a phase velocity at an arbitrary reference frequency ω0. A simple exact derivation leads to an expression for the phase velocity c as a function of frequency: c/c0 = (ω/ω0)γ, where γ = (1/π) tan−1 (1/Q). Scaling relationships for pulse propagation are derived and it is shown that for a material with a given value of Q, the risetime or the width of the pulse is exactly proportional to travel time. The travel time for a pulse resulting from a delta function source at x = 0 is proportional to xβ, where β = 1/(1 ‐ γ). On the basis of this relation it is suggested that the velocity dispersion associated with anelasticity may be less ambiguously observed in the time domain than in the frequency domain. A steepest descent approximation derived by Strick gives a good time domain representation for the impulse response. The scaling relations are applied to field observations from the Pierre shale formation in Colorado, published by Ricker, who interpreted his data in terms of a Voigt solid with Q inversely proportional to frequency, and McDonal et al., who interpreted their data in terms of nonlinear friction. The constant Q theory fits both sets of data.
The temperature variation of hardness of olivine and its implication for polycrystalline yield stress
Authors: Evans Brian , Goetze Christopher
Published: February 2008

The variation of hardness with temperature was measured for olivine on a number of crystal faces by the Vickers diamond pyramid technique (up to 800°C) and by a mutual indentation technique (for temperatures up to 1500°C). A comparative review of hardness data and compressive creep measurements obtained under large confining pressures confirms the hypothesis of Rice [1971] that single‐crystal hardness measurements, corrected for elastic effects, can be correlated to the fully ductile yielding of a polycrystal by intragranular dislocation mechanisms, including dislocation climb and glide. The computed differential yield stresses, σ (in gigapascals), which empirically correspond to a strain rate
of 10−5 s−1, were well represented by an equation of the form σ = 9.1(±0.3) ‐ 0.23(±0.01)T2, where T is the absolute temperature (in degrees Kelvin), and the quoted variances are for 1 standard deviation. The olivine data therefore predict a high‐stress polycrystalline flow law that may be expressed as
= 1.3 × 1012exp ‐ [(60×103)/T][1 ‐ (σ/9.1)]2 where
is the strain rate in s−1. A similar functional dependence of strain rate on stress is indicated for Al2O3 for temperatures below 900°C but is contraindicated for MgO and NaCl. Using a semiempirical method of dislocation rosette analysis, the critical resolved shear stress on the {110} [001] slip system was estimated (to 20%) over the temperature range 20°C to 780°C as 1.2 GPa and 0.3 GPa, respectively. These data are useful in providing an upper bound to the yield stress in a region of stress and temperature space not easily accessible by other experimental methods.
Ice sheet internal radio‐echo reflections and associated physical property changes with depth
Authors: Ackley S. F. , Keliher T. E.
Published: February 2008

The use of radio‐echo sounding records to indicate the presence of internal layers within large ice sheets is of interest to glaciologists because it offers a possible means of tracking the internal properties of the ice sheets over large distances. In this paper we use the measured physical properties of core to bedrock taken at Cape Folger, East Antarctica (66°22′S, 111°E, 324‐m depth), to compute a depth‐reflection coefficient profile for comparison with the observed radio‐echo reflections. The measurements available on physical properties are density variations, bubble size and shape changes, and crystal fabric variations. In calculations to differentiate the effects of the physical properties, it appears that density variations account for the primary contributions to the calculated dielectric property changes corresponding to the highest observed reflection coefficients. However, bubble changes alone can also account for reasonable, though lower, reflection coefficients at the depths corresponding to observed reflections. Crystal fabric variations correspond poorly with the reflection locations. The close correspondence between the depths of the bubble shape changes (which are definitely deformational features) and the depths of the density variations, and between both of these and the radio‐echo layers, indicates that deformational events in the ice sheet's history are represented by the variations in physical properties and associated radio‐echo records.
Partitioning of energy in a thrust sheet and implications concerning driving forces
Authors: Wiltschko D. V.
Published: February 2008

A model for the equilibrium of the Pine Mountain block of the southern Appalachians shows that fault‐zone drag is the most important energy sink. However, internal deformation accompanying bending at ramps is significant and makes fault‐zone drag a poor estimate of the total energy dissipated in movement for reasonable model parameters. In none of the models considered can gravity provide the energy necessary to move the Pine Mountain block. A regional compression on the order of a kilobar is also necessary. However, gravity sliding is untenable as a source for this regional compression because (1) there is no evidence for a forward‐dipping sole fault now or at the time of deformation as required by this model and (2) a gravity sliding model is inconsistent with some geological observations. But there is no evidence for or against a forward‐dipping erosional surface, a critical aspect of the so‐called ‘glacier analogy’ model. However, unlike gravity sliding, the existence of a forward‐dipping erosional surface at the time of deformation is not inconsistent with known geology. Any solution to the problem of driving forces for fold and thrust belts must consider not only paleotopography but also processes taking place in the interior of mountain belts.
Equilibrium fluid distribution in an ultramafic partial melt under hydrostatic stress conditions
Authors: Waff H. S. , Bulau J. R.
Published: February 2008

The geometrical distribution of the fluid phase in partially molten mafic material has been studied under conditions providing close approach to textural equilibrium. Mechanical mixtures of natural dunite and basalt were held at temperatures slightly above solidus and at 10‐kbar confining pressure for time durations exceeding 160 hours. For melt fractions of 1–2% fluid phase observed as glass in quenched specimens occurs as a completely interconnected network of channels along intergranular edge intersections. The intergranular facial contacts appear dry, and the grains appear to be in mechanical contact across these faces within the 200‐Å resolution of a scanning electron microscope. Grain‐grain intersection angles near 120° occur between the solid phases, suggesting that crystalline anisotropy plays a minor role in defining surface energies. These results confirm that surface energy plays a dominant role in determining fluid distribution in equilibrium partial melts of silicate composition.
Long‐period gravity tides at the South Pole
Authors: Slichter L. B. , Zürn W. , Syrstad E. , Knopoff L. , Smythe W. D. , Uffelman H.
Published: February 2008

Gravity observations obtained in 1970 at Amundsen‐Scott station at the geographic South Pole by an electrostatically nulled LaCoste‐Romberg gravimeter have been analyzed for the long‐period tides. Two records of lengths 50 and 57 days, separated by a large gap, were analyzed using a least squares method. Instrumental drift and a strong correlation with barometric pressure are identified to be the major sources of noise. Our results for the gravimetric factor and the phase lead of long‐period tides are δ = 1.159 ± 0.010 and α = 0.0073 ± 0.0194 days. The largest constituent responsible for these values is the fortnightly tide Mf Within conservative errors these numbers are consistent with seismological results and at most a small correction for ocean loading.
Paleomagnetic studies of the early carboniferous St. Joe Limestone, Arkansas
Authors: Scott G. R.
Published: February 2008

The Kinderhookian (early Tournaisian) St. Joe Limestone was sampled at two quarries in the Ozark Plateau. A total of 110 oriented core samples were collected from brick red to gray crinoidal limestone and mudstone at approximately 20‐cm stratigraphic intervals. All samples were reversely magnetized. A selection procedure based upon directional stability of demagnetized vectors restricts the stable data population to 39 and 43 samples at each site with paleomagnetic pole positions of 46.2°W longitude, 37.0°S latitude, α95 = 1.67, κ = 189; and 50.7°W longitude, 40.6°S latitude, α95 = 1.48, κ = 218, respectively. These results from ‘cratonic’ North America are the first from early Carboniferous rocks and delimit a period of active apparent polar wander to the latest Devonian and earliest Carboniferous. The relative motion involved a clockwise rotation of the plate. This was immediately followed by a quasi‐static period during the Carboniferous. However, beginning in the latest Carboniferous the North American plate rotated counterclockwise with a large northerly component of motion about a tectonic rotational point at 62°N, 150°E.
Topography and tectonics of Mid‐Oceanic Ridge axes
Authors: Sleep Norman H. , Rosendahl Bruce R.
Published: February 2008

Numerical fluid dynamic models of mid‐oceanic ridge axes were constructed using distributions of material properties constrained by seismic studies and thermal calculations. The calculations indicate that spreading is passive except for forces caused by density differences due to thermal expansion and partial melt. Except for geometric differences due to temperature distribution, one set of mechanical properties can adequately explain central rifts of slow ridges and central peaks of fast ridges. Viscous head loss in the upwelling material dominates at low spreading rates where material ascends through a narrow conduit. Thermal expansion and partial melting dominate at high spreading rates where a wide low viscosity crustal magma chamber is present. The preferred rheology is 1020 poise for the upwelling lithosphere; less than 5×1017 for the crustal magma chamber and axial intrusion zone at fast ridges, and a yield stress of 200 bars for the lithosphere. The calculation correctly predicts the existence of central peaks at ‘hot‐spot’ ridges, where seismic evidence indicates a large magma chamber.

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Compressional wave velocities in rocks at high temperatures and pressures, critical thermal gradients, and crustal low‐velocity zones
Authors: Christensen Nikolas I.
Published: February 2008

The velocities of compressional waves have been determined for several igneous and metamorphic rocks to temperatures of 500°C at elevated confining pressures. At 2 kbar and between 25° and 300°C, temperature coefficients (∂Vp/∂T)p for many of the rocks range between −0.5×10−3 and −0.6×10−3 km s−1°C−1. At higher temperatures and 2 kbar the temperature coefficients show extreme variability, which is related to the opening of grain boundary cracks caused by anisotropic thermal expansion of the mineral components. Critical thermal gradients (dT/dZ)c for a low‐velocity layer in the continental crust at pressures of 5–8 kbar are between 10° and 14°C/km. These values are probably high because of porosity; however, they are still lower than estimated crustal temperature gradients in normal and high heat flow provinces. Thus it is concluded that crustal velocity inversions produced by high temperature are likely to be common within the crust. It is significant, however, that the velocity decreases reported for some crustal low‐velocity layers are much greater than the decrease which can be accounted for by temperature alone. At higher pressures (10–30 kbar), (dT/dZ)c = 6.3°C/km for dunite and eclogite, in excellent agreement with estimates from single‐crystal data. The velocities in dunite as a function of temperature further support the conclusion that the observed increase in upper oceanic mantle velocity with age is a consequence of decreasing temperature.
Photogrammetric portrayal of Mars topography
Authors: Wu Sherman S. C.
Published: February 2008

Special photogrammetric techniques have been developed to portray Mars topography, using Mariner and Viking imaging and nonimaging topographic information and earth‐based radar data. Topography is represented by the compilation of maps at three scales: global, intermediate, and very large scale. The global map is a synthesis of topographic information obtained from Mariner 9 and earth‐based radar, compiled at a scale of 1:25,000,000 with a contour interval of 1 km; it gives a broad quantitative view of the planet. At intermediate scales, Viking Orbiter photographs of various resolutions are used to compile detailed contour maps of a broad spectrum of prominent geologic features; a contour interval as small as 20 m has been obtained from very high resolution orbital photography. Imagery from the Viking lander facsimile cameras permits construction of detailed, very large scale (1:10) topographic maps of the terrain surrounding the two landers; these maps have a contour interval of 1 cm. This paper presents several new detailed topographic maps of Mars.
Erosional processes in channelized water flows on Mars
Authors: Baker Victor R.
Published: February 2008

The large‐scale water flows implied by the morphology of the Martian outflow channels could have produced the observed assemblage of erosional bed forms by a combination of ice processes, macroturbulence, streamlining, and cavitation. Although pure liquid water would not be stable under present Martian environmental conditions, evaporative cooling would probably lead to a variety of ice processes, including erosive ice drives and ice jamming at flow constrictions, sharp bends, and islands. Macroturbulence in the deep, high‐velocity flood flows might consist of kolks, rollers in zones of flow separation, and longitudinal arrays of vortices. These processes would contribute to the development of the scabland‐type bed forms on the channel floors, including longitudinal grooves, inner channels, and cataracts. Streamlined uplands are preferentially developed and preserved in this turbulent regime because they minimize flow separation. The Martian streamlined uplands are somewhat more elongate than those in the Channeled Scabland, perhaps indicating formation by flows of higher Reynold's number. The large variety of possible thermodynamic states, flow dynamics, and local pressure environments for the Martian flood flows appears to have been ideal for cavitation inception and bubble collapse. The combination of relatively low gravity and low atmospheric pressure would facilitate Martian fluvial cavitation at much lower velocities than are required for cavitation inception in terrestrial rivers. Cavitation erosion was probably localized on the stream bed by flow constriction‐expansion effects and by macroturbulent flow structure.
Moderate resolution thermal mapping of Mars: The channel terrain around the Chryse Basin
Authors: Christensen Philip R. , Kieffer Hugh H.
Published: February 2008

Moderate resolution (∼30 km) thermal inertia estimates have been made for several regions in the northern hemisphere of Mars. Examples of these maps are presented here for the region 0°–45°N, 0°–90°W. The thermal inertia of Kasei Vallis is found to be significantly higher than that of the surrounding terrain. The assumption of a uniform grain size surface gives maximum diameters of 1.0 mm inside and 0.05 mm outside Kasei Vallis for the surface materials. High inertia regions are well correlated with low albedo (A ∼ 0.14) regions. Three large channels in the Oxia Palus quandrangle also have high inertia floors. There is some indication that the thermal inertia increases toward the mouth of one of these channels. The Chryse and Acidalia basins have uniform high inertia surfaces with no decrease in inertia as distance increases from the major channels. There are numerous craters in the region which have high inertia‐low albedo features on the crater floor. This correlation has been observed for many other craters on Mars, both from Mariner 9 and Viking data. A possible explanation is the accumulation of coarsegrained, windblown material within the craters. Average grain sizes of these materials range from 0.5 to 1.1 mm, corresponding to medium to coarse sand. The Viking 1 landing site is located in the lowest inertia region within the area studied which met the latitude and elevation capabilities of that vehicle.
On the opacity of Martian dust storms derived by Viking IRTM spectral measurements
Authors: Hunt Garry E.
Published: February 2008

In this paper we develop a procedure to estimate the opacity of Martian dust storms from the spectral measurements made by the Viking infrared thermal mapping instrument. Methods are also produced to estimate the surface temperature and the effective temperature of the dust cloud from these observations, in conjunction with the lander opacity observations. The derived opacities, for various locations on the planet, show the basic trends identified in the lander measurements. At the height of the second dust storm we derive 9‐μm opacities of ∼3, which correspond to values of ∼5 at visible wavelengths. The analysis of zonally averaged data for the southern hemisphere at the time of the second major dust storm indicates dust opacities of ∼2–3 remaining at 1800–2400 LT, with the maximum opacity occurring in the late afternoon period.
Mars in the solar wind
Authors: Intriligator D. S. , Smith E. J.
Published: February 2008

The basic interaction between Mars and the solar wind is reexamined by using recent spacecraft observations and model calculations. It is found that the particle pressure is not large enough to stand off the solar wind unless the electron temperature is 4 times the ion temperature in the ionosphere. However, the additional pressure is presumably magnetic provided by a planetary magnetic field, although an induced field cannot be excluded. A planetary field corresponding to a surface field of ∼20 gammas and a dipole moment of ∼8×1021 gauss cm3 (0.01% of the earth’s dipole moment) are implied. On the basis of our calculation, 60–70% of the pressure is supplied by the magnetic field, and the remaining 30–40% by the ionosphere. This suggests that the interaction of Mars in the solar wind is unique in that it is both atmospheric (i.e., Venuslike) and magnetospheric (i.e., similar to that at earth and Jupiter). Which mode of interaction is dominant is likely to depend on the external solar wind, the magnetic field interaction and the atmospheric interaction being characteristic of quiet and disturbed conditions, respectively. The implications of an earthlike interaction are reexamined by using magnetospheric scaling laws, and it is found that plasma convection may play a major role in the Martian magnetosphere and ionosphere and that the auroral oval may extend to low latitudes. The implications of a Venuslike interaction are also examined, in particular the heating and removal of atmospheric ions as a result of the direct interaction between the solar wind plasma and the ionosphere. The existence of a planetary field also implies that Mars has a liquid rather than a frozen core.
The stopping of a rising, liquid‐filled crack in the Earth's crust by a freely slipping horizontal joint
Authors: Weertman J.
Published: February 2008

The stress intensity factor K at the upper tip of a vertical, liquid‐filled crack in the earth's crust is large for relatively small cracks (K>3 MPa m½when the crack length is larger than about 40 m). Consequently, the large water‐filled cracks considered for use in the extraction of geothermal heat might be expected to pop to the earth' surface. In this paper it is shown that a freely slipping horizontal joint (as well as a not so freely slipping joint) can stop the upward motion of a large liquid‐filled crack. If the joint is tightly fitting, there is no limit to the length of a stopped crack. If slip on a joint results in the joint being propped open, the maximum crack length of a stopped crack is given by the crevasse expression: total crack length = πT/2(ρ −ρ ′)g, where T is the tectonic stress, g is the gravitational acceleration, and ρ −ρ′ is the difference between the rock and liquid density. The results can also be applied to the problem of magma transport in the earth's crust. In particular, laccolith emplacement is favored when the nonhydrostatic tectonic stress component is compressive in character.
Petrology and porosity of an oceanic crustal site: Results from wave form modeling of seismic refraction data
Authors: Spudich Paul , Orcutt John
Published: February 2008

P and converted S waves observed in refraction stations FF1, FF2, and FF4 of the 1959 Fanfare cruise of the Scripps Institution of Oceanography are analyzed by synthetic seismogram modeling of the data using the reflectivity algorithm and by inversion of the P and S wave travel time data to obtain extremal bounds on the P and S velocity (vp and vs) profiles. While the FF1 data are inadequate for detailed analysis, the FF2 and FF4 data yield vp profiles displaying rapidly increasing velocity with depth in layer 2, a small velocity discontinuity between layers 2 and 3, gently increasing velocity with depth in layer 3, and a 1‐km‐thick Moho transition. The vs profiles for FF2 and FF4 show rapidly increasing velocity with depth in layer 2, fairly uniform velocities in the top of layer 3, a slight low‐velocity zone extending through most of layer 3, and a 1‐km‐thick Moho transition. Using theories of seismic wave velocities in cracked media and a laboratory velocity measurement made on a basalt sample from this site, a porosity of 18% is inferred for the top of the igneous crust at this site. A further reduction of porosity to 2% can explain the observed velocity gradients only to a depth of 0.6 km into the igneous crust. In the 0.8‐ to 1.5‐km depth interval, Poisson's ratio appears to drop below 0.27 to a minimum of 0.24, which may indicate a zone of trondhjemites or other quartz‐rich rocks at this depth or which may be related to state of fluid saturation of the rocks. Within layer 3, observed vp and vs agree well with laboratory velocity measurements of ophiolite samples from the western U.S. and from the Bay of Islands, Newfoundland. The observed velocities suggest the disappearance of hornblende and the appearance of augite and olivine with increasing depth in layer 3. There is no evidence for more than 30% serpentine anywhere within the crust or upper mantle at this site, except possibly within unresolvably thin zones or pods. Evidence is also given which suggests that velocities and velocity gradients in the shallow crust may be partly controlled by differential pressure (externally applied pressure minus pore fluid pressure) and its spatial gradients and that laboratory velocity measurements made on water‐saturated basalt samples at zero differential pressure are more representative of in situ velocities in the shallow crust than lab measurements made at which are usually employed as in situ conditions, namely, elevated externally applied pressure and zero pore fluid pressure. The factors affecting the efficiency of shear wave conversion at the sea floor are investigated, and the important role of basement vp and especially vs are shown. Since basement vs is very sensitive to fracture geometry, the high lateral variability of shear wave conversion may be related to variability in the extent and character of basement porosity. A useful explosive source function for marine synthetic modeling is presented, and a nomenclature for marine seismic phases is suggested.
Electrical conductivity measurement of fayalite under shock compression up to 56 GPa
Authors: Mashimo Tsutomu , Kondo Ken‐Ichi , Sawaoka Akira , Syono Yasuhiko , Takei Humihiko , Ahrens Thomas J.
Published: February 2008

The electrical conductivity measurements of single‐crystal fayalite are performed under shock wave compression in the pressure range 19.5–56 GPa. The electrical conductivity under shock compression increases by a factor of more than 1010 from 0 to 40 GPa. Over the range 19.5–42 GPa the specific conductivity σ can be closely described by log10σ (S/m) = −4.65 + 0.15P (GPa). The conductivity versus pressure relation at room temperature is calculated by using measured values of the activation energy to correct the high‐temperature shock data to room temperature. After the effect of temperature is removed, the increase in conductivity with pressure approximately agrees with that measured under static pressure using a diamond anvil cell by Mao and Bell. Above pressures of ∼42 GPa the conductivity remains at a relatively high level of ∼100 S/m from 40 to 55 GPa.
Automated radon monitoring at a hard‐rock site in the southern California transverse ranges
Authors: Shapiro M. H. , Melvin J. D. , Tombrello T. A. , Whitcomb J. H.
Published: February 2008

Data are presented from 20 months of near‐real‐time (three samples per day) radon monitoring at a hard‐rock site in the Transverse Ranges of southern California. An annual cycle is evident in the data which is attributed to thermoelastic strains in the vicinity of the borehole site. Between April 1, 1977, and October 31, 1978, there were 11 earthquakes with magnitudes ≥2.0 within 25 km of the monitoring site. Three of these events appeared to be preceded by precursory signals, four were preceded by ‘possible’ precursory signals, and four were not preceded by any apparent precursors. Before the 4.6 M Malibu earthquake of January 1, 1979, a ‘possible’ precursory signal sequence of 40–50 days' duration was observed.
Postseismic viscoelastic deformation and stress: 2. Stress theory and computation; dependence of displacement, strain, and stress on fault parameters
Authors: Cohen Steven C.
Published: February 2008

This paper reports on a viscoelastic model for deformation and stress associated with earthquakes. The model consists of a rectangular dislocation (strike slip fault) in a viscoelastic layer (lithosphere) lying over a viscoelastic half space (asthenosphere). The first part of the paper contains an analysis of the time‐dependent surface stresses. The model predicts that near the fault a significant fraction of the stress that was reduced during the earthquake may be recovered by viscoelastic softening of the lithosphere. By contrast, the strain shows very little change near the fault. The model also predicts that the stress changes associated with asthenospheric flow extend over a broader region than those associated with lithospheric relaxation, even though the peak value is less. The second part of the paper studies the dependence of the displacements, strains, and stresses on fault parameters. Peak values of strain and stress drop increase with increasing fault height and decrease with fault depth. Under many circumstances, postseismic strains and stresses show an increase with decreasing depth to the lithosphere‐asthenosphere boundary. Values of the strain and stress at distant points from the fault increase with fault area but are relatively insensitive to fault depth.
Acquisitional chronology of remanent magnetization along the ‘Grenville Polar Path’: Evidence from 40Ar/39Ar ages of hornblende and biotite from the Whitestone Diorite, Ontario
Authors: Dallmeyer R. D. , Sutter J. F.
Published: February 2008

The Whitestone diorite is a prekinematic intrusion located in the southwestern portion of the Grenville province of the Canadian Shield. Hornblende and biotite from the diorite display undisturbed 40Ar/39Ar release spectra which define average ages of 980 m.y. and 945 m.y., respectively. These dates are interpreted to record times of cooling through temperatures required for retention of radiogenic argon following Grenville metamorphism of the pluton at approximately 1150 m.y. Ueno et al. (1975) reported that the diorite carries a natural remanent magnetization which loses directional coherence at 580°–640°C. They concluded that these temperatures may be close to the geologic temperatures at which the magnetization was actually acquired. If so, this magnetization was likely acquired at a time and a temperature closely represented by argon retention in hornblende. The virtual geomagnetic pole (VGP) defined by the Whitestone diorite magnetization overlaps, within analytical uncertainty, the location of other VGP's which define the southwestern portion of the ‘Grenville polar path,’ and therefore all may have been acquired at approximately the same time. 40Ar/39Ar hornblende dates previously reported by Dallmeyer (1978a) for some of the rocks whose VGP's define the northeastern portion of the Grenville polar path are younger and suggest later cooling through argon retention (and magnetic blocking) temperatures between 880 and 900 m.y. The suggested time span for the Grenville polar path is 980‐1000 m.y. (southwest) to 880–900 m.y. (northeast) and indicates that Grenvillian poles can be considered to form a late Proterozoic part of the North American polar path. Separate Grenvillian and Interior Laurentian polar paths do not seem reasonable in view of the available geochronologic and geologic controls.