bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetism of the Jacobsville Formation and the apparent polar path for the interval −1100 to −670 m.y. for North America
Authors: Roy J. L. , Robertson W. A.
Published: February 2008

Chemical, thermal, and alternating field (af) treatments performed on 257 specimens (37 sites) of the Jacobsville Formation show that the magnetization was acquired gradually over a very long period of time. Three phases of magnetization can be recognized and sequentially separated chemically. Thermal (up to 675°C) and af (up to 290 mT) treatments are unable to distinguish and separate the first two phases. The results are consistent with a three‐phase magnetization model previously proposed and again demonstrate the necessity of performing chemical leaching experiments in red bed studies. The early phase remanence isolated chemically indicates that the Jacobsville is of upper Keweenawan age (≃− 1100 m.y.). Poles obtained from three areas, Sault Ste. Marie (SSM) (175°E, 12°N), Marquette (182°E, 03°S), and the Keweenaw Peninsula (KP) (184°E, 10°S), indicate that the sediments of SSM are older than those of KP. A detailed study (nine sites: mean pole at 190°E, 16°S) of a 5.3‐m stratigraphic section at Jacobsville shows significant apparent polar motion and the occurrence of at least one field reversal. The trend of the pole movement relative to North America was then southward. Vector analyses show that during the acquisition of the intermediate and late phase remanences the apparent pole started to move northward; by the end of the magnetization process it had reached high latitude (307°E, 72°N). Explanation of the Jacobsville results and other high‐latitude poles requires a major revision of the apparent polar path during the Hadrynian. The proposed path executes a loop into the southern hemisphere (−1100 to −950 m.y.) and then roughly follows a great circle path (Hadrynian Track) from 160°E near the equator up to the present north geographic pole and down to the equator at 340°E (−670 m.y.). The controversial Grenville poles could easily be incorporated into that loop, supporting the hypothesis that the Grenville Province has always been an integral part of Laurentia. The Hadrynian Track, with its large pole displacement (180° in 300 m.y.), is an extremely useful stratigraphic tool that can be used to great advantage for global correlations of late Precambrian rock units.
Possibility of ferroelectric‐like phenomena in the mantle of the Earth
Authors: Litov Elie , Anderson Orson L.
Published: February 2008

It is suggested that MgSiO3 the major component of the earth's mantle, may under‐go a ferroelectric‐like phase transformation as it transforms into the orthorhomic perovskite phase deep in the mantle. The anomalous electric conductivity and acoustic discontinuity observed in the same region of the mantle are interpreted, respectively, in terms of the anomalous dieelectric and soft mode phenomenon known to accompany ferroelectric‐like transformation.
Compression wave studies on calcite rock
Authors: Grady D. E. , Hollenbach R. E. , Schuler K. W.
Published: February 2008

Dynamic compression wave studies have been conducted on three calcite rocks, Solenhofen limestone, Oakhall limestone, and Vermont marble, in the stress range of 0–5 GPa. Plate impact techniques provided transient stress wave input, and diffuse laser interferometry was used to study the evolution of these pulses through various thicknesses of rock. Both loading response and release wave response were determined from the time‐resolved particle velocity profiles. The calcite I–II and II–III phase transitions dramatically influence wave propagation in this stress region. The I–II transition begins between 0.6 and 1.2 GPa depending on rock type and is found to be consistent with a reversible, displacive polymorphic phase change. A constitutive model based on stress heterogeneity describes most of the features associated with the I–II transition. The calcite II–III transition proceeds above 2.4 GPa, and peak stress values traverse a metastable Hugoniot. This wave feature and others are more characteristic of a slower, reconstructive phase change. We conclude that stress wave features reveal striking differences in the nature of the shock‐induced calcite transitions and that these transitions strongly influence the dynamic constitutive behavior of calcite rock.
Sonobuoy array measurements of active faulting on the Gorda Ridge
Authors: Jones P. R. , Johnson S. H.
Published: February 2008

The observed mismatch between the topography and the epicenter pattern associated with the Gorda Ridge spreading center was studied with two small arrays of four sonobuoys each deployed over the rift valley at 42°37′N and 43°N. Observed microearthquake activity originates from the crestal region and supports earlier suggestions of systematic mislocation of teleseismically determined epicenters on the Gorda Ridge. The observed seismic activity, which includes swarm events, averaged 3.5 events per hour over a total array recording time of 19.3 hours. Located microearthquakes originated from the median valley floor, valley walls, and crestal mountains. Other events, whose location could not be computed, appeared to originate from the surrounding crestal mountains with a predominance of events from west of the intersection of the Gorda Ridge and the Blanco fracture zone. Of 69 events detected, 5 were suitable for the calculation of focal depth. Focal depths at the intersection of the Gorda Ridge and Blanco fracture zone are 6.5–10 km below a 3.5‐km datum, while those farther to the south at a linear portion of the ridge range in depth from 2.5 to 6.5 km below datum. This may imply more rapid cooling near the fracture zone. A composite fault plane solution for three events on the eastern valley wall indicates movement on a high angle, with the inner wall moving upward with respect to the crestal mountains. This is the first direct evidence for uplift of median valley walls, a process which must occur if median valleys are steady state features of slowly spreading ridges.
The effect of stress cycling and inelastic volumetric strain on remanent magnetization
Authors: Martin R. J. , Habermann R. E. , Wyss M.
Published: February 2008

The relation between remanent magnetization (natural and thermal), volumetric strain, and stress was determined for five rock types. Twenty different samples (fine‐grained intrusive, gabbro, basalt, tuff, and andesite) were repeatedly cycled to 95% of their fracture stresses at confining pressures of up to 1250 bars. At the completion of the first cycle there was always a permanent demagnetization of the rock. Subsequent stress cycles show a progressively smaller demagnetization, and the changes of remanence with loading become nearly reproducible by the fourth cycle. The onset of dilatancy produced changes in the rate of magnetization change. For two rock types the magnitude of the remanence increased up to the onset of dilatancy and then decreased as the stress was augmented. For these two rock types the remanent vector first rotated toward the axis of applied stress up to the onset of dilatancy; continued loading caused a rotation away from the applied stress axis. Typically, the changes in remanent intensity were of the order of 5%, which is about 3 times smaller than the changes in the individual components parallel and perpendicular to the axis of the greatest compressive stress. In fact, it was not uncommon for one component to increase while the other decreased. As a consequence, large rotations, up to 10°, of the remanent moment were observed, and at differential stresses in excess of half the compressive strength the remanent vector always rotated away from the axis of greatest compression. In addition, time dependent changes in remanent magnetization of several percent accompanied time dependent increases in crack porosity during constant stress creep tests at high differential stresses.

Table 2 is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 2006. Document J78‐003; $1.00. Payment must accompany order.
X ray diffraction compression studies of hematite under hydrostatic, isothermal conditions
Authors: Wilburn D. R. , Bassett W. A. , Sato Y. , Akimoto S.
Published: February 2008

Compression data for α‐Fe2O3 were collected using two independent X ray diffraction techniques in which a sample was compressed under hydrostatic conditions. Samples of varying quality were used. In each case, a value of 1.99 ± 0.06 Mbar was calculated for K0, assuming a value of 4.0 for K0′. These data are in good agreement with the value obtained by Lieberman and Schreiber (1968) using ultrasonic methods. The bulk modulus was found to be lower, however, than an earlier, nonhydrostatic value of 2.28 Mbar reported by Bassett and Takahashi (1974). The sample of highest purity produced data which deviate from the other observed data in the region of 30 kbar. This discrepancy may be due to the Morin transition.
Late Cenozoic tectonics of the Tibetan Plateau
Authors: Ni James , York James E.
Published: February 2008

Normal faulting interpreted from Landsat imagery and fault plane solutions of earthquakes suggest late Cenozoic east‐west extension in the Tibetan Plateau. Volcanism and earthquake swarms could be additional evidence for extension, although they are not unique to extensional environments. Normal faulting contrasts with major thrusting along the southern boundary and major strike slip faulting near the other boundaries of Tibet. The extension may be related to western Tibet being compressed between thrusting on the south and strike slip faulting on the northwest and consequently spreading along an east‐west trend.
Preliminary sonobuoy study of rapidly accumulating shelf sediments
Authors: Houtz R. E.
Published: February 2008

Sonobuoy records with smoothly curved travel time data are found most commonly near rapidly outbuilding shelves and near the mouths of major rivers. It is judged that the smooth increase of velocity with depth is most likely where sediments are young and have accumulated rapidly. These sonobuoy records are amenable to analytical techniques for determining velocity gradients in shelf sediments. These methods are simpler and more accurate than those used in deep water (if the source and receiver are at the surface). Also, sea floor sound velocity can be determined by measuring the critical angle of reflection from sea floor multiples, which form an energetic and clearly defined wedge of closely spaced multiples slightly later than the simple sea floor reflection. Velocity inversion techniques applied to the travel time data provide accurate velocity‐depth information. These techniques show that shelves with sections of rapidly accumulating sediments are ideal for the measurement of detailed velocity characteristics. A method is proposed for measuring in situ attenuation on shelves where the velocity increases smoothly with depth. The method takes advantage of the apparent lack of prominent reflectors in rapidly accumulating sediment bodies and uses the decay in amplitude observed between first‐arrival refractions and their simple multiples. In this proposed technique, the first‐arrival refractions are considered to be the ‘source.’ Simple ray theory shows that maximum focusing occurs in the shallow curved path arrivals at a velocity cusp. The velocity cusps can form in shallow water (less than about 100 m) only when the velocity contrast at the sea floor is nonexistent or slightly positive.
Petrology of the spilitic rocks from the Michigan Basin Deep Drill Hole
Authors: McCallister Robert H. , Boctor Nabil Z. , Hinze William J.
Published: February 2008

Two igneous bodies were encountered during the drilling of the Michigan Basin deep drill hole. The upper body (unit 1) extended from 4970 to 4998 m, while the lower body (unit 2) was observed from 5252 m to the bottom of the hole at 5324 m. Samples used in this investigation were taken at 1.5‐m intervals in the core section from 5306 to 5322 m. In addition, one sample from the lower contact of unit 1 was examined. The assemblage observed in unit 2 consists of clinopyroxene‐chlorite‐albite‐epidote‐actinolitesphene, while the one sample from unit 1 contains albite‐chlorite‐carbonate‐epidote. Both of these assemblages can be characterized as spilitic and are representative of conditions which prevail during greenschist facies metamorphism. Comparison of the compositions of the various phases coexisting in the two assemblages with published experimental studies suggests conditions of equilibration of approximately 450°C and a fluid pressure of 1–2 kbar.
Velocity anisotropy in semi‐indurated calcareous deep sea sediments
Authors: Carlson R. L. , Christensen N. I.
Published: February 2008

Compressional‐wave velocities have been measured for propagation directions parallel and perpendicular to bedding in 11 calcareous deep sea sediments at confining pressures to 1.0 kbar. The samples, recovered from subbottom depths of 0.39–0.75 km in the western South Atlantic on DSDP leg 39, range in wet bulk density from 1.83 to 2.30 g/cm−3 and in porosity from 23 to 48%. The sediments exhibit significant velocity anisotropy at all pressures, with velocities in the bedding plane higher than those measured in the vertical direction. At 0.1 kbar, horizontal velocities (Vh) range from 1.80 to 2.99 km/s, while vertical velocities range from 1.65 to 2.54 km/s. Corresponding ranges of ΔV = (Vh‐Vv) and anisotropy (A = 2(Vh‐Vv)/(Vh+Vv)) are 0.10–0.45 km/s and 5.2–16.3%, respectively. Both ΔV and A increase markedly with depth of burial: ∂ΔV/∂z = 0.62 km/s/km, and ∂A/∂z = 19%/km. The failure of anisotropy to decrease with increasing confining pressure suggests that this phenomenon is not produced by the alignment of cracks. However, the elastic properties of calcite are such that an alignment of C axes perpendicular to bedding would produce the observed velosity distribution. Suggested mechanisms for producing the fabric are: (1) the alignment of certain microfossils such as Discoaster during compaction, (2) epitaxial growth of aligned forms during diagenesis, and (3) recrystallization of calcite.
Thermal infrared properties of the Martian atmosphere: 2. The 15‐μm band measurements
Authors: Martin Terry Z. , Kieffer Hugh H.
Published: February 2008

Viking infrared thermal mapper observations of Mars in the 15‐μm CO2 band reveal global atmospheric thermal behavior at the 0.3‐ to 0.6‐mbar level. Dust entrained by storms produces major modification of diurnal and latitudinal structure in the brightness temperature T15. In the dust‐laden atmosphere of southern spring and summer 1977, T15 was a maximum in late afternoon at a latitude well south of the subsolar latitude. Diurnal amplitude was as great as 30 K, while diurnal mean temperatures exceeded 220 K. Over the northern winter polar cap, T15 increased dramatically following the second global dust storm of 1977; even in regions of polar night the change was up to 80 K. Inversions of similar magnitude resulted, and the change in downward radiance was sufficient to modify substantially the rate of CO2 condensation at the surface.
Satellite altimeter measurements of sea state—An algorithm comparison
Authors: Fedor L. S. , Godbey T. W. , Gower J. F. R. , Guptill R. , Hayne G. S. , Rufenach C. L. , Walsh E. J.
Published: February 2008

A group of Geos 3 sea state investigators conducted a workshop during which their independently developed algorithms for extracting significant wave height from Geos 3 data were intercompared using simulated Geos 3 data, a long Geos 3 data span which included a variety of sea states, and a number of short data segments in the vicinity of data buoys. Individual differences between the algorithms were small in comparison to the general good agreement among them and with the surface truth data. Details of the algorithms are given together with the intercomparison results.
A brief summary of verification results for the spectral ocean wave model (SOWM) by means of wave height measurements obtained by Geos 3
Authors: Pierson Willard J. , Salfi Robert E.
Published: February 2008

A brief summary of some of the verification results for the spectral ocean wave model (SOWM) using Geos 3 wave height measurements is given. A NASA contractor's report by Pierson and Salfi (1978) contains the analyses of 44 Geos 3 orbit segments for 1975 and 1976, from which significant wave heights were obtained as overlapping 20‐s averages every 2 or 3 s from the altimeter data. The SOWM is the presently operational wave forecasting model at the Fleet Numerical Weather Central (FNWC). The verifications were carried out for the specification (or hindcast) mode; that is, past meteorological data and wind fields derived therefrom were used to calculate the waves as opposed to forecasts of the wind fields. The major results were that the SOWM specifications were biased too low and that the larger differences between what was measured by Geos 3 and what the model gave could be attributed to poor specifications of the winds over the northern hemisphere oceans. These particular features of the SOWM had been recognized at FNWC by means of verifications by other means, and the bias has been removed, although there are still problems connected with the lack of adequate wind data over the oceans.
Gravitational spectra from direct measurements
Authors: Wagner C. A. , Colombo O. L.
Published: February 2008

A simple rapid method is described for determining the spectrum of a surface field (in spherical harmonics) from harmonic analysis of direct (in situ) measurements along great circle arcs. The method is shown to give excellent overall trends (smoothed spectra) to very high degree from even a few short arcs of satellite data. Three examples are taken with perfect measurements of satellite tracking over a planet made up of hundreds of point masses using (1) altimetric heights from a low‐orbiting spacecraft, (2) velocity (range rate) residuals between a low and a high satellite in circular orbits, and (3) range rate data between a station at infinity and a satellite in a highly eccentric orbit. In particular, the smoothed spectrum of the earth's gravitational field is determined to about degree 400(50‐km half wavelength) from 1° × 1° gravimetry and the equivalent of 11 revolutions of Geos 3 and Skylab altimetry. This measurement shows there is about 46 cm of geoid height (rms worldwide) remaining in the field beyond degree 180.
Pre‐tertiary velocities of the continents: A lower bound from paleomagnetic data
Authors: Gordon Richard G. , McWilliams Michael O. , Cox Allan
Published: February 2008

A new technique for estimating minimum absolute velocities of plates or continents from their apparent polar wander paths is applied to North America, Eurasia, and Gondwanaland for the past 350 m.y. For brief periods of time (approximately 30 m.y. long) each of these continents has moved at minimum velocities much greater than the present velocities of plates with large continents and comparable to the present velocities of the oceanic plates. We conclude from this that the resistance to plate motion of continental plates is not necessarily greater than that of oceanic plates and that the present low values of continental velocities are an accident of present plate geometry.
Detailed structure of the crust and upper mantle along the Jordan‐Dead Sea Rift
Authors: Ginzburg Avihu , Makris Jannis , Fuchs Karl , Perathoner Bertram , Prodehl Claus
Published: February 2008

Seismic refraction data shot along the Dead Sea‐Gulf of Elat graben were reevaluated. Travel times and amplitudes of first arrivals and later arrivals of both refracted and reflected phases were studied using true amplitude record sections and synthetic seismogram sections. The computed models show that the crust is divided by a discontinuity into a uniform velocity upper crust and a higher velocity lower crust. Along the graben the lower crust is separated from the upper mantle by a 5‐km‐thick transition zone in which the velocity increases rapidly and smoothly. This transition zone is not present away from the rift valley. The crustupper mantle boundary dips to the north along the Gulf of Elat to the Dead Sea. A discontinuity was found at a depth of 55 km within the upper mantle, below which the velocity increases from 8.0 to 8.6 km s−1. The results indicate an intermingling of upper mantle material with lower crust material along the graben. The upwelling increases in its vertical extent as the Red Sea with its active spreading mechanism is approached.
Seismotectonic aspects of the Markansu Valley, Tadjikstan, earthquake of August 11, 1974
Authors: Jackson James , Molnar Peter , Patton Howard , Fitch Thomas
Published: February 2008

The Markansu Valley earthquake of August 11, 1974 (mb = 6.4), occurred in a structurally complex area of the northern Pamir. It was followed by more than 80 teleseismically recorded aftershocks, 13 with mb ≳ 5.0. Fault plane solutions for the three largest after‐shocks differ from one another and from the main shock and indicate both thrust and strike slip faulting. Geological mapping and after‐shock locations suggest that the strike slip faulting occurred on both north and northwest trends. The region is one in which large northwest trending strike slip faults converge. To the north and south these faults are very clear topographic features. As they approach the northeastern Pamir, they become less distinct on the Landsat imagery, although they remain recognizable geologic features on the ground. The strike slip faults become less distinct toward their ends perhaps because the north‐south motion is taken up progressively by east‐west striking thrusts. It is not possible to fit orthogonal nodal planes to all of the observed first motions of the P waves from the main shock. This earthquake is located at the intersection of two inferred fault systems and appears to be a multiple rupture. The discrepant P wave first motions are all nodal in character. If faulting occurred on more than one fault plane during the main shock, then the apparent first motions in the nodal directions of the first rupture may be anomalous and actually belong to a rupture later than the first one. We are aware of one other similar occurrence: the 1967 Mudurnu, Turkey, earthquake, for which the few anomalous P wave first motions were also nodal and for which the fault plane solutions of the main shock and the largest aftershock are very different. The concentration of large earthquakes near the intersection of two systems is also observed in Iran. The seismic moment of the main shock of the Markansu Valley sequence is about 5×1026 dyn cm.
The rotation of Mars
Authors: Reasenberg Robert D. , King Robert W.
Published: February 2008

A new model of the rotation of Mars is presented. It is designed to support the analysis of radio metric data from the Viking landers and possible future Mars‐lander missions. The precession and nutation are represented by series that include all terms causing a maximum Mars surface displacement of at least 1 cm after 10 years. The precession rate is estimated from Mars physical constants to be (7.58″±0.03″) yr−1 based on Reasenberg's (1977) model of the mass distribution of Mars. We also discuss some geophysical effects that may cause Mars surface displacements larger than the 1‐cm criterion.
Silt‐clay aggregates on Mars
Authors: Greeley Ronald
Published: February 2008

Viking observations suggest abundant silt and clay particles on Mars. It is proposed that some of these particles agglomerate to form sand size aggregates that are redeposited as sandlike features such as drifts and dunes. Although the binding for the aggregates could include salt cementation or other mechanisms, electrostatic bonding is considered to be a primary force holding the aggregates together. Various laboratory experiments conducted since the 19th century, and as reported here for simulated Martian conditions, show that both the magnitude and sign of electrical charges on windblown particles are functions of particle velocity, shape and composition, atmospheric pressure, atmospheric composition, and other factors. Electrical charges have been measured for saltating particles in the wind tunnel and in the field, on the surfaces of sand dunes, and within dust clouds on earth. Similar, and perhaps even greater, charges are proposed to occur on Mars, which could form aggregates of silt and clay size particles. Electrification is proposed to occur within Martian dust clouds, generating siltclay aggregates which would settle to the surface where they may be deposited in the form of sandlike structures. By analog, siltclay dunes are known in many parts of the earth where siltclay aggregates were transported by saltation and deposited as ‘sand.’ In these structures the binding forces were later destroyed, and the particles reassumed the physical properties of silt and clay, but the sandlike bedding structure within the ‘dunes’ was preserved. The bedding observed in drifts at the Viking landing site is suggested to result from a similar process involving siltclay aggregates on Mars.
Development and analysis of a twelfth degree and order gravity model for Mars
Authors: Christensen Edward J. , Balmino Georges
Published: February 2008

Satellite geodesy techniques previously applied to artificial earth satellites have been extended to obtain a high‐resolution gravity field for Mars. Two‐way Doppler data collected by 10 Deep Space Network (DSN) stations during Mariner 9 and Viking 1 and 2 missions have been processed to obtain a twelfth degree and order spherical harmonic model for the martian gravitational potential. The quality of this model was evaluated by examining the rms residuals within the fit and the ability of the model to predict the spacecraft state beyond the fit. Both indicators show that more data and higher degree and order harmonics will be required to further refine our knowledge of the martian gravity field. The model presented shows much promise, since it resolves local gravity features which correlate highly with the martian topography. An isostatic analysis based on this model, as well as an error analysis, shows rather complete compensation on a global (long wavelength) scale. Though further model refinements are necessary to be certain, local (short wavelength) features such as the shield volcanos in Tharsis appear to be uncompensated. These are interpreted to place some bounds on the internal structure of Mars.
Impact crater and basin control of igneous processes on Mars
Authors: Schultz Peter H. , Glicken Harry
Published: February 2008

Numerous martian impact craters have been heavily modified by processes restricted to the crater interior. A common expression of this modification is the presence of extensive fractures arranged in a concentric plan and typically forming moats that engulf the old crater wall. Although similar styles of modification occur on the moon, martian floor‐fractured craters display a greater diversity in morphology. Such craters are closely associated with major regional features and provinces, such as the Vallis Marineris system, the fretted terrains, and the martian plains. The well‐preserved record of this style of crater modification and its proximity to similarly preserved regional features imply that floor‐fractured craters represent crater‐controlled sites of a late‐stage and widespread pulse of igneous activity. Heat from such activity may locally thaw ground ice, resulting in the observed diversity in style of crater modification. Theoretical calculations show that heat released by a mafic sill beneath the brecciated zone of an impact crater may thaw trapped water‐ice at depth over periods on the order of 104–105 years. Thawed materials may gradually escape through peripheral fractures surrounding the crater floor. Alternatively, a metastable state of potential liquification can occur if the material is confined and the rate of thawing exceeds the rate of escape. This establishes conditions for catastrophic release of a warmed slurry which may produce the chaotic terrain and outflow channels as suggested by other investigators. Identified multiring basins associated with Margaritifer Sinus and the fretted terrains are proposed to represent broader scale control of igneous processes by old impact structures, in direct analogy with floor‐fractured craters. Moreover, the arcuate pattern of Noctis Labyrinthus and a concentric arrangement of massifs identify a proposed Tharsis impact basin centered on Syria Planum.
Pseudocraters on Mars
Authors: Frey Herbert , Lowry Barbara L. , Chase Scott A.
Published: February 2008

In the Cydonia region of southern Acidalium Planitia are small, low‐relief, apparently volcanic domelike structures, whose size, morphology, and general occurrence suggest they are martian analogs of terrestrial pseudocraters, a type of phreatic eruption. Average base diameters are about 800 m, which is somewhat larger than typical Icelandic examples. All the domes have summit pits; elongate domes generally have elongate summit pits or, in extreme cases, double pits. The greatest concentration of these domes is in a region of subdued fractured plains which may be old volcanic flows. Pseudocraters on the earth are produced when lava flows over water‐logged ground. On Mars surface or subsurface ice was the likely medium that produced the steam eruptions resulting in cratered domelike structures on the lava surfaces.
Some final conclusions and supporting experiments related to the search for organic compounds on the surface of Mars
Authors: Biemann K. , Lavoie John M.
Published: February 2008

The Viking molecular analysis experiment has demonstrated the absence (within the detection limits which range from levels of parts per million to below parts per billion) of organic substances in the Martian surface soil at the two Viking landing sites. Laboratory experiments with sterile and nonsterile antarctic samples further demonstrate the capability and reliability of the instrument. The circumstances under which organic components could have escaped detection, such as inaccessibility or extreme thermal stability of organic polymers, are discussed but are found to be unlikely. The inability of the instrument to detect free oxygen evolved from soil samples is pointed out.
Faulting patterns in north‐central Nevada and strength of the crust
Authors: Zoback Mary Lou , Zoback Mark D.
Published: February 2008

NNE normal fault trends characterize much of the northern Basin and Range province. These faults make sharp bends to NNW and ENE trends in north‐central Nevada in the vicinity of a mid‐Miocene rift characterized by a zone of diabase dike swarms, graben‐filling flows, and a coinciding aeromagnetic anomaly. Despite a roughly 45° change in the least principal stress direction since mid‐Miocene time, pre‐existing NNW‐ and ENE‐trending faults in the vicinity of the rift accommodated the extension whereas regionally, major crustal blocks were faulted along a NNE trend, approximately perpendicular to the modern least principal stress direction. An assumed uniform regional stress field (derived from geologic and geophysical indicators of the modern principal stress field) and the observed oblique slip on the pre‐existing faults were combined in an analysis utilizing an empirically derived frictional sliding law and the Coulomb failure criterion. This analysis constrained the ratio of the least principal stress to the greatest principal stress (S3/S1) as well as the inherent shear strength of intact crustal rocks, τc. While both parameters, S3/S1 and τC, are functions of unknowns including pore pressure and the cohesion (frictional strength) of the pre‐existing faults, reasonable assumptions about these parameters lead to τc estimates that agree well with values obtained from laboratory experiments simulating crustal conditions. At a depth of 10 km, the analysis indicates that the minimum inherent shear strength of intact crustal rocks must range between 150–450 bars for zero pore pressure and 150–350 bars for hydrostatic pore pressure, whereas the corresponding maximum shear stresses at 10‐km depth are 970–1200 bars for zero pore pressure and 640–770 bars for hydrostatic pore pressure.
Multiple ScS travel times in the western Pacific: Implications for mantle heterogeneity
Authors: Sipkin Stuart A. , Jordan Thomas H.
Published: February 2008

Multiple ScS travel times have been obtained by wave form cross correlation from seismograms digitally recorded by the High Gain Long Period (HGLP) and Seismic Research Observatory (SRO) networks. The surface projections of the paths corresponding to these data cross the western Pacific on oceanic crust greater than 100 m.y. old or traverse continental regions. The difference between the median ScSn‐ScSn−1 residuals for all western Pacific paths and all continental paths is +5.2 s, in agreement with our World Wide Standardized Seismograph Network (WWSSN) data (Sipkin and Jordan, 1976). These results support the hypothesis that the average mantle shear velocity of old ocean basins is significantly less than that of old continental nuclei. The medians of both the oceanic and continental residuals for the HGLP and SRO data are more positive than those for the higher‐frequency WWSSN data by amounts consistent with attenuative dispersion, which we take to be direct evidence for such dispersion. The residuals for paths crossing China have a median 2 s greater than the median for all continental paths, supporting the inference from dispersion studies that the upper mantle beneath China is characterized by anomalously low shear velocities. The residuals for western Pacific paths show lateral variations of 5 s or more not correlated in any systematic way with crustal ages along the paths. An analysis of these variations suggests that for horizontal scale lengths of the order of 103 km the amplitude of lateral variability is greater along a SW‐NE axis than along a SE‐NW axis. Mesoscale heterogeneity in the western Pacific may thus consist of predominantly NW trending structures.
Upper mantle velocity heterogeneities beneath the Colorado Rocky Mountains determined from P residuals
Authors: Casaceli R. , Wyss M.
Published: February 2008

The teleseismic P wave residuals of three underground nuclear explosions (Rulison, Rio Blanco, Gasbuggy) and three Denver earthquakes were examined as a function of azimuth. The location and origin times of these six sources are known to a high accuracy, and they are all located in or near the Colorado Rocky Mountains. The data covered the azimuth range from approximately 50° west of north to 160° east of north. The P residuals were corrected for the average receiver residuals. It was found that the travel times in the sector from N40°W to N are clearly delayed by about l s compared to the sector from N to N40°E. This observation can be interpreted to be due to a termination of the pronounced western U.S. low‐velocity layer, which coincides approximately with the eastern edge of the Rocky Mountains.
Interpretation of chemical analyses of waters collected from two geothermal wells at Coso, California
Authors: Fournier R. O. , Thompson J. M. , Austin C. F.
Published: February 2008

Wellhead and downhole water samples were collected and analyzed from a 114.3‐m well at Coso Hot Springs (Coso No. 1) and a 1477‐m well (CGEH No. 1) 3.2 km to the west. The same chloride concentration is present in hot waters entering both wells (about 2350 mg/kg), indicating that a hot‐water‐dominated geothermal system is present. The maximum measured temperatures are 142°C in the Coso No. 1 well and 195°C in the CGEH No. 1 well. Cation and sulfate isotope geothermometers indicate that the reservoir feeding water to the Coso Hot Spring well has a temperature of about 240°–250°C, and the reservoir feeding the CGEH well has a temperature of about 205°C. The variation in the chemical composition of water from the two wells suggests a model in which water‐rock chemical equilibrium is maintained as a convecting solution cools from about 245°C to 205°C by conductive heat loss.
The effect of pressure on the rate of dislocation recovery in olivine
Authors: Kohlstedt D. L. , Nichols H. P. K. , Hornack Paul
Published: February 2008

Samples cut from single crystals of olivine which had been deformed in the laboratory were annealed for 1 hour at either 0.1‐MPa or 500‐MPa gas pressure to study the effect of pressure on the kinetics of dislocation recovery in olivine. After an anneal the dislocation structure was examined by transmission electron microscopy and compared with the dislocation structure in an unannealed ‘reference’ sample from the same deformed single crystal. For the range of annealing temperatures used, 1525–1675 K, the density of free dislocations decreased by dislocation‐dislocation annihilation. The activation volume and activation energy for this dislocation recovery process were obtained from an analysis of the decrease in dislocation density as a function of temperature, for the two hydrostatic pressures, in terms of second‐order kinetics. This analysis yields V = 11 ± 1 cm3/mol and Q = 300 ± 15 kJ/mol.
Heat flow in the Bering Sea
Authors: Langseth Marcus G. , Hobart Michael A. , Horai Ki‐iti
Published: February 2008

The basic pattern of heat flow in the Bering Sea is revealed by 43 measurements obtained in the Aleutian, Kamchatka, and Bowers basins. Averages of the observed values are 55 mW m−2 in the Aleutian Basin, 120 mW m−2 in the Kamchatka Basin, and 80 mW m−2 for two measurements in the Bowers Basin near the Bowers Ridge. Correcting these averages for the effects of sedimentation gives values of 69 mW m−2, 138 mW m−2, and 95 mW m−2, respectively. A useful nomogram for computing the thermal effect of an extended period of sedimentation is presented. Several methods of averaging heat flow values in marginal basins are compared using the data from the Aleutian Basin. If the Aleutian Basin was formed by the sea floor spreading process, then the heat flow suggests an age of about 44 m.y., which is considerably younger than magnetic data suggest. An alternative explanation is that the high heat flow is a residuum of thermal processes associated with the now extinct Kula Spreading Center and the Kula Plate, which underthrust the Bering Sea in the early Tertiary and thermally rejuvenated the trapped Mesozoic sea floor underlying the Aleutian Basin. The high average heat flow in the Kamchatka Basin suggests a considerably younger age for this basin, and the very high values in the southwestern part indicate either thin lithosphere or recent magmatic activity.
Stress measurements at depth in the vicinity of the San Andreas Fault: Implications for the magnitude of shear stress at depth
Authors: Zoback Mark D. , Tsukahara Hiroaki , Hickman Stephen
Published: February 2008

Using the hydraulic fracturing technique, we have made a systematic series of in situ stress measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the fault in relatively shallow (∼230 m) wells and the variation of stress with depth in a ∼1‐km‐deep well located 4 km from the fault. The shallow wells are located along profiles roughly perpendicular to the fault in the western Mojave desert near Palmdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45° from the local trend of the San Andreas. The two stress profiles show very similar results: (1) shear stress (on planes parallel to the San Andreas) increases with distance from the fault, more markedly in the western Mojave, (2) the far‐field shear stress at ∼200 m depth is ∼50 bars, and (3) the horizontal principal stresses as well as shear stress increase with depth more rapidly in the wells farthest from the fault. The ∼1‐km‐deep well, also located in the western Mojave desert, shows increases of both horizontal principal stresses and shear stress with depth. Shear stress increases from about 25 bars at 150–300 m to about 80 bars at 750–850 m. Although this rapid increase of shear stress with depth suggests that the mean shear stress on the fault at seismogenic depths exceeds several hundred bars, the principal stresses increase with depth in a steplike manner. As this may be a near‐surface effect, extrapolation of the measurements to much greater depths may not be warranted.
The P, T phase and reaction diagram for elemental carbon, 1979
Authors: Bundy Francis P.
Published: February 2008

A brief history of the many investigations of the pressure/temperature phase diagrams of elemental carbon is given. The strengths and weaknesses of the various proposed diagrams are discussed, and the areas where additional or better experimental data are needed are pointed out. Many questions still remain about linear solid carbons; about the low‐pressure/high‐temperature solid/liquid/vapor triple point; about the melting line of diamond; about the existence of a high‐pressure metallic phase; and about the existence of two liquid phases, one insulator and the other metallic.
Temperature and pressure dependence of intrinsic anharmonic and quantum corrections to the equation of state
Authors: Hardy Robert J.
Published: February 2008

Results of detailed calculations of the anharmonic and the quantum corrections to the equation of state and to Grüneisen's parameter γ are given for pressures P from zero to 2B0 (where B0 is the bulk modulus at P = 0 and T = 0) and for temperatures T from zero to (0.6) Tε (where Tε = ∈/k,∈ is the depth of the potential well and k is Boltzmann's constant). Interionic potentials of the form A/r+ϕR with ϕR proportional to r−12, r−8, and e−αr were used. It was found, for example, that at T = (0.2) T∈ the anharmonic corrections to P for the potentials in the order just given were approximately 7, 6, and 4% at P = (0.1)B0, while they were approximately 1, 1, and ½% at P = (0.5)B0. Quantum corrections to P were found to depend very weakly on P and to be about 1% or less for (0.2)B0 < P < 2B0 for temperatures above the zero‐pressure Debye temperature. Anharmonic corrections decreased the values of γ and of P, while quantum corrections decreased the value of γ but increased the value of P.
The Gulf of Aden: Structure and evolution of a young ocean basin and continental margin
Authors: Cochran James R.
Published: February 2008

New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies on Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45°E and to anomaly 2′ (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45°E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphere of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid‐ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 100 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading.
Electrical resistivity changes in tuffs due to stress
Authors: Morrow C. , Brace W. F.
Published: February 2008

Certain tuffs, sampled near active faults in northern California, exhibited the same large electrical resistance changes with stress reported by Yamazaki for tuffs from the Izu Peninsula. These effects, studied in the laboratory, were most pronounced at stresses below 6 MPa, for partial saturation, and for rocks with porosity less than 20%. Relative changes of resistance with strain reached 105 for several of the tuffs near a strain of 10−4. A curious and unexplained aspect of the electrical behavior of the tuffs was the recoverability between stress cycles; this was not observed for other, equally porous rocks like sandstone and for partially saturated granite. Electrically sensitive tuffs near active faults might be used in earthquake prediction studies provided field sites in partially saturated tuffs can be found.
A study of regional temperature and thermohydrologic effects of an underground repository for nuclear wastes in hard rock
Authors: Wang J. S. Y. , Tsang C. F. , Cook N. G. W. , Witherspoon P. A.
Published: February 2008

Heat released by the radioactive decay of nuclear wastes in an underground repository causes a long‐term thermal disturbance in the rock mass. The nature of this disturbance for a planar repository 3000 m in diameter at a depth of 500 m below surface is investigated. Loaded initially with a power density of 10 W/m2 of spent fuel assemblies 10 years after discharge from a reactor, the maximum increase in temperature of the repository in granite is about 50°C and the epicentral thermal gradient about 70°C/km. Different waste forms and periods before burial have significant effects on the thermal disturbance. The effects of temperature changes on the groundwater flow are evaluated with simple models of a vertical fracture connected to a horizontal fracture in the rock mass. The buoyancy groundwater flow through the vertical fracture is a function of both the vertical and the horizontal fracture transmissivities, as well as the changes in density and viscosity of groundwater caused by the temperature changes. Finite hydraulic recharge from the surrounding rock mass affects the thermohydrologic disturbance.
Role of membrane stresses in the support of planetary topography
Authors: Turcotte D. L. , Willemann R. J. , Haxby W. F. , Norberry John
Published: February 2008

The principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres. It is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter τ = Ed/ΔρgR2 where d is the thickness of the elastic lithosphere, Δρ is the density difference between the mantle and crust, and R is the radius of the planetary body. When this parameter is large membrane stresses can fully support topographic loads without flexure, and when it is small the influence of membrane stresses can be neglected. Solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics. Spherical harmonic expansions of the measured gravity and topography for Mars and the moon are compared with the theory. It is found that for Mars the support of topography is primarily due to membrane stresses for n < 8 and for the moon for n < 17. For Mars the data for 4 ≤ n ≤ 7 give τ ≃ 0.5. For the moon the data have considerable scatter that is attributed to the mascons but generally correlate with τ = 0.5. If bending stresses are neglected, the governing equation for the deflection of the spherical shell is Legendre's equation. A general solution is obtained for an axisymmetric load. This solution is applied to the Tharsis region on Mars. The 60–65% compensation of this region requires that τ = 0.6. The well defined fracture pattern surrounding the Tharsis region is attributed to tensional membrane stresses.