bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical resistivity and phase transformation of hematite under shock compression
Authors: Kondo Ken‐ichi , Mashimo Tsutomu , Sawaoka Akira
Published: February 2008

Electrical resistivity measurements of hematite (α ‐ Fe2O3) were performed under shock compression in the range of 240–760 kbar. The specimens used were coarsely polycrystalline natural material from Brazil. The electrical resistivity decreases gradually with increasing shock pressure to 440 kbar and decreases discontinuously to less than 1 Ω cm from several tens of ohm centimeters at pressures of 440–520 kbar. The phase transformation pressure, implied in Hugoniot equation‐of‐state experiments, is made clear. The transition behavior into the low‐resistivity state depends on the crystallographic axis of shock propagation.
Early acquisition of characteristic magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona
Authors: Purucker Michael E. , Elston Donald P. , Shoemaker Eugene M.
Published: February 2008

Four discrete magnetizations associated with events occurring during deposition, early diagenesis, recent weathering, and cloud to ground lightning have been identified in rocks of the Moenkopi Formation at Gray Mountain, Arizona. A paleomagnetic study of more than 800 samples tied closely to the physical stratigraphy of the Moenkopi indicates that most of the magnetization remaining after partial alternating field and thermal demagnetization was acquired during and shortly after deposition. Directions of this remaining (characteristic) magnetization, in both normally and reversely magnetized rock, are closely similar to directions reported for the Moenkopi elsewhere on the Colorado Plateau. Sampling across the Moenkopi Formation at Gray Mountain has revealed two reversed magnetozones separating three normal magnetozones. Generally sharp boundaries of the magnetozones, their close concordance with physical stratigraphic units, and the rarity of anomalous directions of magnetization within the magnetozones indicate that most of the secondary components of the characteristic magnetization were acquired within 104–105 years after deposition. A conglomerate test based on siltstone clasts in sandstone shows highly scattered but not completely random directions of magnetization following partial thermal demagnetization. A secondary Triassic normal magnetization appears to be superimposed on some of the clasts, as suggested by the approximate coincidence of the resultant magnetic vector from the clasts with a Triassic normal direction. A fold test based on samples from highly deformed silty clay stone and sandstone in an intraformational landslide shows that these rocks acquired most of their stable magnetization before folding. A component of Triassic normal magnetization acquired after deformation also occurs in some of these rocks.
Effects of the gravitational field on liquid distribution in partial melts within the upper mantle
Authors: Waff Harve S.
Published: February 2008

The gravitational field has a major effect on the geometrical distribution of liquid in partial melts occurring within the earth, provided that local conditions and kinetic factors favor stabilization of the melt in contiguous association with individual crystalline grains as opposed to vertical melt segregation. As a result of gravity and the density contrast between the liquid and the crystalline phases, the differential pressure ΔP existing between the two will increase with increasing elevation above the base of a partially molten zone whose fluid phase is connected vertically. This differential pressure must be balanced by the effect of surface tension acting along the curved liquid‐crystal interfaces. The changes in ΔP imposed by gravity can be mechanically compensated for only by increased positive curvature of the interfaces with increased elevation. This forces closure of liquid pathways when ΔP exceeds a critical value which is dependent on vertical distance over the base of the zone. Above the elevation at which the fluid becomes unconnected, the pressure difference, due to buoyancy, can no longer be transmitted from below, and melt will once more be interconnected upward through another limited column until excess fluid pressure again forces closure of connected channels. The result is vertical stratification of liquid phase connectivity into horizontal layers of partial melt with high connectivity separated by regions in which the melt occurs as isolated pockets. Stratification heights are estimated for the cases of (1) homogeneously distributed crystalline phases and (2) a rhythmically zoned occurrence of a minor, nonwetting crystalline phase. They are a few centimeters to a few tens of centimeters and a few hundred meters to a few kilometers, respectively, for the two cases. If, however, the melt fraction exceeds a critical value within the range of 5–22%, the liquid phase must interconnect vertically, and some fraction or all of the liquid may collect in sills or migrate vertically out of the system. Possible implications for the seismic and electrical properties of the mantle are also considered.
Seismicity of the Coso Range, California
Authors: Walter Allan W. , Weaver Craig S.
Published: February 2008

A 16‐station seismographic network, approximately 40 km north‐south by 30 km east‐west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of the range. During the first 2 years of network operations, 4216 local earthquakes (0.5 ≤ m ≤ 3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north‐south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north‐south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. The b value calculated for the entire range is 0.99±0.08. The rhyolite field has a significantly higher b value of 1.26±0.16; if only the shallow events (depth <5 km) are used in the calculation, the b value for this area becomes even higher, 1.34±0.24. The higher b values were interpreted as reflecting the existence of short average fault lengths (<5 km) within the rhyolite field. The seismic data and other data suggest that the fault system lying between the rhyolite field and the adjacent Coso Basin is an important tectonic boundary. Present information is insufficient to determine the geothermal production capability of this fault system, but it does suggest that the system is a good target for further exploration.
Three‐dimensional Q−1 model of the Coso Hot Springs Known Geothermal Resource Area
Authors: Young Chi‐Yuh , Ward Ronald W.
Published: February 2008

Observations of teleseismic P waves above geothermal systems exhibit travel time delays and anomalously high seismic attenuation, which is extremely useful in estimating the thermal regime and the potential of the system. A regional telemetered network of sixteen stations was operated by the U.S. Geological Survey in the Coso Hot Springs Known Geothermal Resources Area (KGRA) for such studies from September 1975 to October 1976. Subsequently, they deployed a portable Centipede array of 26 three‐component stations near the center of the anomaly. The seismograms of 44 events recorded by the telemetered array and nine events by the Centipede array were analyzed using the reduced spectral ratio technique to determine the differential attenuation factor δt* for the events recorded with the highest signal‐to‐noise ratio. The δt* variation observed across the Coso Hot Springs KGRA were small (<0.2 s). A three‐dimensional generalized linear inversion of the δt* observations was performed using a three‐layer model. A shallow zone of high attenuation exists within the upper 5 km in a region bounded by Coso Hot Springs, Devils Kitchen, and Sugarloaf Mountain probably corresponding to a shallow vapor liquid mixture or ‘lossy’ near surface lithology. No zones of significantly high attenuation occur between 5‐ and 12‐ km depth. Between the depth of 12–20 km a thick zone of high attenuation (Q <50) exists, offset toward the east from the surface anomaly.
Deep crustal electrical conductivity in the Adirondacks
Authors: Connerney J. E. P. , Nekut A. , Kuckes A. F.
Published: February 2008

Controlled and natural source electromagnetic induction experiments in the Adirondack Precambrian shield region of northern New York State indicate that the electrical conductivity of the crust is consistent with laboratory studies of moist igneous rock and in serious discord with such measurements on dry rock. A highly conductive layer is found in the lower crust at a depth of ∼22 km. Four three‐axis flux gate magnetometers were deployed at distances of 20 to 62 km from a central source location in Newcomb, New York. A single turn horizontal loop of wire 4.5 km in diameter carrying 65 rms amperes of square wave current generated an oscillating magnetic dipole source field at frequencies of .05 and .1 Hz. The magnetic field data were computer processed to determine the amplitude and phase of the vertical and horizontal magnetic field components relative to the source. These data, together with previous data in the frequency range 0.4 to 400 Hz, indicate a conductivity of 1×10−4mhos/m extending to 10 km, underlain by a conductivity of 1×10−3mhos/m to 22 km. At 22‐km depth the conductivity increases to between ∼4×10−2 and ∼8×10−2 mhos/m. Geomagnetic fluctuations in the period range 10 to 200 s have been analyzed using the gradient method of Kuckes and Schmucker. These data indicate that the conductive feature in the lower crust is of finite extent with a conductance of ∼400 mhos. Comparison of the Adirondack results with the results of controlled and natural source experiments in South Africa and North America indicate similar crustal conductivities among these stable shield regions.
Use of soil‐gas helium concentrations for earthquake prediction: Limitations imposed by diurnal variation
Authors: Reimer G. M.
Published: February 2008

The diurnal variations in soil‐gas helium concentration have been monitored at depths of 0.5–2.0 m in three localities in Colorado, Wyoming, and California. Barometric pressure, air temperature, wind speed, soil temperature and moisture, relative humidity, and precipitation were also measured. The helium variation below a 1‐m sampling depth usually did not exceed the analytical sensitivity of 10 ppm He. The meteorologic parameters that have the greatest effect on helium variation are wind speed and precipitation; another factor is the atmospheric pumping created by air temperature changes and its associated effect on near‐surface soil moisture content. The absolute helium variation rarely exceeds 1% of the background helium concentration in air and can easily be compensated for because it follows a regular daily pattern. Similar diurnal changes in soil‐gas helium concentration would not impose any severe limitations on the use of these data for earthquake prediction purposes.
Heat flow from the Liberian Precambrian Shield
Authors: Sass J. H. , Behrendt J. C.
Published: February 2008

Uncorrected heat flow in iron formation rocks from three areas within the Liberian part of the West African Shield ranges from 50 to more than 80 mW m−2. When corrections are applied for topography and refraction, the range of heat flow is narrowed to between 38 and 42 mW m−2. In comparison with heat flows from other parts of the West African Craton, these values are consistent with preliminary results from Ghana (42±8 mW m−2) and Nigeria (38±2 mW m−2) but are somewhat higher than values from Niger (20 mW m−2) and neighboring Sierra Leone (26 mW m−2). The Liberian values are significantly lower than the heat flow offshore in the equatorial Atlantic Ocean (58±8 m W m−2), suggesting large lateral temperature gradients within the lithosphere near the coast. Values of heat production from outcrops of crystalline basement rocks near the holes are between 2 and 2.3 /μW m−3. A heat‐flow/heat‐production relation cannot be established because of the small range of values; however, assuming a ‘characteristic depth’ of 8 km (similar to the North American Craton) the reduced heat flow of from 20 to 25 mW m−2 is consistent with that from other Precambrian shields.
Paleomagnetic results from some southern African kimberlites, and their tectonic significance
Authors: Hargraves R. B. , Onstott Tullis C.
Published: February 2008

Paleomagnetic data are reported from kimberlite occurrences of three distinct age groups: (1) National (one pipe), D = 221°, I = −51°, k = 41, α95 = 7.2°, age ∼1140 m.y., (2) Swartruggens (three fissures), mean D = 293°, I = −46°, k = 22, α95 = 26,9°, age = ∼150 m.y., and (3) six bodies in and adjacent to Lesotho, D = 350°, I = −69°, K = 66, α95 = 8.3° with age spread of ∼90 + 5 m.y. Analysis of these and previously published data suggests that most individual kimberlite bodies give acceptable paleomagnetic (as distinct from virtual geomagnetic) poles. This leads to a revised apparent polar wander curve for Africa in upper Mesozoic time and suggests that the time of kimberlite emplacement does coincide with intervals of rapid plate motion, although this cannot be the primary cause of their intrusion.
Investigations of tilt measurements using shallow borehole tiltmeters
Authors: Wyatt Frank , Berger Jon
Published: February 2008

An array of shallow borehole tiltmeters has been operated at Piñon Flat Observatory since early in 1977. The data from this array are examined for coherence between the individual instruments and compared with the corresponding data from three 732‐m laser strainmeters. In general, there is no significant coherence between the signals from the tiltmeters outside the microseismic and tidal bands even though they are spaced as closely as 10 m apart. Comparisons with the strain records show that the observed tilt noise power exceeds the strain noise power by 25–40 dB over the band from 10−6 to 1 Hz. Analysis of the coherence estimates establishes the necessity for an unacceptably large (≃1000) array of instruments in order to determine the common tectonic signals. The observed secular tilt rates are from 40 to 450 times the observed secular strain rates for the same period. The theoretical noise power limit of the tilt transducer renders the instrument incapable of recording background noise at a quiet site over the band from 10−4 to 10−1 Hz.
Deep structure of the Rio Grande Rift from seismic reflection profiling
Authors: Brown L. D. , Chapin C. E. , Sanford A. R. , Kaufman S. , Oliver J.
Published: February 2008

Seismic reflection surveys carried out by the Consortium for Continental Reflection Profiling (COCORP) in the Rio Grande Rift near Socorro, New Mexico, have successfully mapped large‐scale structural variations down to the base of the crust. A total of 155 km of 24‐fold common reflection point coverage was obtained parallel and transverse to major rift structure, including a continuous east‐west profile which spans the central rift from the Sierra Lucero on the west (at the edge of the Colorado Plateau) to the Manzano Mountains on the east. Stacked seismic reflection sections derived from these surveys indicate coherent reflected energy from depths as great as 35 km, with substantial evidence of structural complexity extending throughout the crust. Among the major features apparent on these sections are (1) large‐scale relief on the floor of the Albuquerque Basin, including a major intragraben horst standing over 3 km above the surrounding deep basin floors, and an extensive, shallow structural bench beneath the southeast portion of the basin, (2) well‐developed, antithetic Tertiary normal faulting on the east side of the rift, (3) marked lateral and vertical variation in intrabasement reflection character, including relatively long correlatable events, distinct bands of short, discontinuous reflections, and seismically transparent zones characterized by few, if any, reflections, (4) a complex band of reflections from depths appropriate for the Moho, and (5) unusually strong, coherent reflections from a depth of approximately 20 km, corresponding closely with the top of an active magma body previously postulated to exist beneath the rift in this area on the basis of microearthquake studies. The magma body reflections are among the most prominent deep events yet recorded by COCORP; their geometry and relationship to surrounding structural elements provide important constraints on the depth of brittle faulting and new insight on magma migration and accumulation. Extensional faulting appears to be primarily near vertical near the surface with no clear evidence of large scale listric faulting at depth. The eastern rift boundary at Abo Pass is marked by a high‐angle planar zone devoid of coherent reflections, possibly representing intense structural disruption extending through the crust. In contrast, the seismic section across the western rift boundary is defined by a gently dipping, relatively simple reflection of uncertain origin, with little evidence of a throughgoing fault in the lower crust. The seismic characteristics of many of the intrabasement features suggest particular igneous and/or metamorphic origins and support models which emphasize lateral and vertical heterogeneity at depth.
Velocity and attenuation in partially molten rocks
Authors: Mavko Gerald M.
Published: February 2008

Interpretation of seismic velocity and attenuation in partially molten rocks has been limited, with few exceptions, to models that assume the melt to be distributed either as spheres or as thin films. However, other melt phase geometries, such as interconnected tubes along grain edges, might equally well account for seismic observations if there is a much larger fraction of melt. Seismic velocity and attenuation are estimated in rocks in which the melt phase has the tube geometry, and the results are compared with results expected for the more familiar film model under similar conditions. For a given melt fraction, tubes are found to give moduli intermediate between moduli for rigid spherical inclusions and compliant films. For example, in polycrystalline olivine at 20 kbar the model predicts a decrease in Vs of 10% and a decrease in Vp of 5% at 0.05 melt fraction, without considering inelastic relaxation. Shear attenuation appears to be dominated by viscous flow of melt between the tubes and/or films. For olivine the tube model predicts the increment of relaxation due to melt, Δµ/µ, to be 0.01 at 0.05 melt fraction. Relaxation of the bulk modulus is dominated by flow between melt pockets of different shape, heat flow, and solid‐melt phase change. If melt is present, considerable bulk attenuation is expected, although the relaxation may be observable only at long periods, outside the seismic body wave band.
Mariana Arc structure inferred from gravity and seismic data
Authors: Sager W. W.
Published: February 2008

A two‐dimensional gravity model of the lithosphere was constructed along a seismic refraction line near 18°N latitude. Included in the model are crustal layers constrained by seismic refraction results, an estimate of the gravity anomaly caused by the subducting slab, and a model of the low‐density mantle beneath the Mariana Trough. With a reasonable anomaly assumed for the slab it is shown that the gravity anomaly caused by the low‐density mantle is greatest over the axial bathymetric high and tapers off to the sides. With the bottom of the low‐density mantle set at 200 km the density contrast is −0.033 g/cm3. Other depths and densities are tried as well. Several notable anomalies are found on the crustal layers. East of the trench, the crust has been thinned slightly to account for an outer gravity high. Behind the landward wall of the trench, a small, low‐density body is modeled to explain a slight offset of the minimum of the free air anomaly from the trench axis. A 50‐mGal jump on the observed gravity over the volcanic line is explained by an unusual configuration of the frontal arc Moho.
Evidence of a deep mantle history in terrestrial osmium‐iridium‐ruthenium alloys
Authors: Bird John M. , Bassett William A.
Published: February 2008

Terrestrial alloys of osmium‐iridium‐ruthenium occur in placers from depleted mantle of obducted ophiolites. Some specimens consist of intergrown metal phases having textures and compositions consistent with formation by crystallization from a melt at temperatures greater than various proposed geotherms of the earth's mantle. It is proposed that these specimens were subjected to melting in the lower mantle when the earth had temperatures higher than at present or that they melted at depths greater than 2900 km before that region was occupied by the core or that they once resided within the core. The alloys were probably transported in convecting mantle material and were subsequently incorporated into depleted mantle of lithosphere, now exposed as obducted ophiolites. It is further proposed that the alloys were originally primitive material that has remained essentially intact since accretion of the earth from the solar nebula.
Shear heating and the state of stress on faults
Authors: Scholz Christopher H.
Published: February 2008

Evidence for shear heating associated with crustal faulting has been described for a number of fault zones in a wide variety of tectonic environments. The evidence is that a thermal anomaly associated with the fault has resulted in the metamorphism of adjacent rocks to higher grades than can be accounted for by considering other heat sources. A number of such cases are recapitulated here to conclude that if relative displacements across these faults occurred at plate tectonic rates or slower, then shear stresses of a kilobar or so must have existed on these faults in order to produce the observed shear heating. Alternatively, these faults moved at several tens of centimeters per year, much higher than observed past and present plate tectonic rates. These conditions must be maintained over time approaching the thermal time constant, ca. 107 years. Although the case for shear heating in many individual cases is arguable, the evidence in toto strongly suggests that shear stresses of a kilobar must have existed on some crustal faults, and these are minimum estimates. Transient shear heating during dynamic rupture of faults also can produce melting within fault zones. Two interesting cases of large landslides that produced melting suggested that (1) the dynamic coefficient of friction does not vanish after melting but remains at a significant level and (2) the efficiency for melting rock by frictional heating in these cases was probably less than 10%, implying that the amount of frictional heating necessary to produce melting during an earthquake may be an order of magnitude higher than previously thought. As a side issue, a new model for the origin of paired metamorphic belts is proposed.
Deformation‐induced microstructures, paleopiezometers, and differential stresses in deeply eroded fault zones
Authors: Kohlstedt D. L. , Weathers Maura S.
Published: February 2008

Deformation‐induced dislocation densities, subgrain sizes, and grain sizes in rocks from deeply eroded fault zones provide estimates of the differential stresses that existed when the faults were active. Laboratory experiments have demonstrated that the dislocation density, subgrain size, and grain size generated during steady state deformation depend primarily on the magnitude of the applied differential stress; temperature has only a minor influence on these three quantities through materials parameters such as the shear modulus. Theoretical arguments support these experimental results. Differential stresses in fault zones, determined from the empirical stress‐microstructure relations, lie in the range 40–200 MPa. In many fault zones, such as the Moine thrust zone in Scotland, the progressive development of the deformation‐induced microstructure can be observed, particularly in quartzites. Far from the fault surface, quartz grains are nearly equant but show undulatory extinction. As the central portion of the fault is approached, extinction remains undulatory as the grains become increasingly elongated (aspect ratios of 100∶1 are common) and recrystallized. Quartzites are often 100% recrystallized near the centers of fault zones. This progressive development of the microstructure indicates that recovery at low or zero stress has not erased information on the last tectonic event in the plastic deformation regime.
Observations of flexure and the state of stress in the oceanic lithosphere
Authors: Watts A. B. , Bodine J. H. , Steckler M. S.
Published: February 2008

Observations of flexure indicate the effective elastic thickness of the oceanic lithosphere is 2 to 3 times smaller than the seismic or thermal thickness of oceanic lithosphere. The effective elastic thickness is a function of temperature and hence age of the lithosphere at the time of loading. Recent results of experimental rock mechanics indicate that the strength of rocks is a strong function of temperature and that the oceanic lithosphere responds to loading by thinning rapidly from its seismic thickness to its rheologic thickness. We have used a yield stress envelope based on experimental rock mechanics to estimate the maximum bending stresses associated with the load of the Hawaiian Islands near Oahu. These results indicate that the oceanic lithosphere is capable of supporting stresses of at least 1 kbar for long periods of geological time (>50 m.y.).
Dedication to Special Issue Honoring George Kennedy
Authors: Boettcher Art , Newton Robert C.
Published: February 2008

This volume of the Journal of Geophysical Research was intended to be dedicated to Professor George C. Kennedy of the Institute of Geophysics and Planetary Physics, University of California, Los Angeles (UCLA), to commemorate his 60th birthday. However, George died of cancer on March 18, 1980, and this volume must now commemorate his career.

On September 19 and 20, 1979, the Institute hosted a conference at UCLA to celebrate George's birthday, and most of the papers presented there, together with others by former coworkers, appear in this issue. All of the authors appreciate this opportunity to honor a man who made so many significant contributions to geochemistry and geophysics.
Conditions of origin of natural diamonds of peridotite affinity
Authors: Boyd F. R. , Finnerty A. A.
Published: February 2008

Studies of mineral inclusions in natural diamonds and rare diamondiferous xenoliths from kimberlites show that most diamonds are associated with a dunite or harzburgite paragenesis. The diamondiferous peridotites and dunites have predominantly coarse or tabular textures that suggest low‐temperature (<1100°C) equilibration. Application of the KD Fe/Mg(Ga/Ol) geothermometer of O'Neill and Wood to analytical data for the minerals in these rocks shows that most have equilibrated below 1100°C. Application of this thermometer to pairs of olivine and garnet crystals included in individual diamonds indicates that the diamonds have crystallized in the range 900°–1300°C, with a majority of estimated equilibration temperatures falling in the range below 1150°C. Comparison of these estimates of equilibration temperature with the zone of invariant vapor composition solidus for kimberlite and garnet lherzolite determined by Eggler and Wendlandt suggests that many diamonds may have formed in subsolidus events.
Studies in synthetic carbonatite systems: Solidus relationships for CaO‐MgO‐CO2‐H2O to 40 kbar and CaO‐MgO‐SiO2‐CO2‐H2O to 10 kbar
Authors: Boettcher A. L. , Robertson J. K. , Wyllie P. J.
Published: February 2008

The system CaO‐MgO‐SiO2‐CO2‐H2O is an important model for many igneous and metamorphic processes, including the generation and differentiation of carbonatite and associated silicate magmas. We have experimentally established the vapor‐saturated solidus for the system CaO‐MgO‐CO2‐H2O from 595°C/1 kbar to <500°C/40 kbar, about 25°C lower than the corresponding temperatures for the CaO‐CO2‐H2O system. Brucite replaces periclase on the vapor‐saturated solidus at about 750 bars pressure, remaining as the stable phase to pressures of at least 40 kbar; no dolomite or magnesite was encountered. At a pressure between 35 and 40 kbar, the assemblage portlandite + brucite + aragonite + vapor changes to one containing phase W, a previously unreported Ca‐Mg carbonate. At 20 kbar, the vapor‐saturated liquid contains at least 24 wt % H2O. The vapor‐saturated solidus for the system CaO‐MgO‐SiO2‐CO2‐H2O ranges from 613°C/1 kbar to 565°C/10 kbar, experimentally indistinguishable from that for CaO‐MgO‐CO2‐H2O, about 10°C lower than that for CaO‐CO2‐H2O, and about 25°C lower than that for CaO‐SiO2‐CO2‐H2O. In the quinary system, monticellite is replaced by dellaite and an unidentified silicate on the vapor‐saturated solidus above 4.1 kbar. MgO‐poor liquids are similar in composition to the magma from which the Magnet Cove calcite carbonatite crystallized. More magnesian magmas would produce dolomite at moderate depths, such as at Alnö Island.
Shock compression of KFeS2 and the question of potassium in the core
Authors: Somerville Malcolm , Ahrens Thomas J.
Published: February 2008

The Hugoniot equation of state of KFeS2 (initial density 2.663 g/cm3) has been determined for pressures up to 110 GPa. The Hugoniot data demonstrate a transformation at 13±1 GPa to a phase with an apparent zero‐pressure density of 3.7±0.2 g/cm3. A comparison of the inferred isentrope of KFeS2 (high‐pressure phase) with those of Fe, FeS0.9, and FeS2 indicates that the atomic volume of potassium in KFeS2 is approximately twice that of iron at 75 GPa. In the temperature and pressure range of the experiments, potassium fails to meet the empirical Hume‐Rothery and Raynor (HRR) criterion for solubility of an element in iron, namely, that the molar volume of the element should not exceed that of iron by a factor greater than ∼1.4. However, both the applicability of the HRR solubility criterion and the inferred isentrope of KFeS2 at high pressure are uncertain. Thermochemical calculations of the partitioning of K between a sulfide and silicate phase (e.g., KFeS2 and KAlSiO4 or KAlSi3O8 (hollandite)) indicate that pressure does not have a pronounced effect on the relative stability of solid KFeS2 and potassium aluminosilicate high‐pressure phases. The calculations suggest that the high‐pressure phase of KFeS2 would not be stable in relation to KAlSiO4 (kalsilite) in the upper mantle, or in relation to KAlSi3O8 (hollandite) in the lower mantle. However, the calculations do not bear directly on the question of partitioning of K into an iron sulfide melt from lower mantle aluminosilicate phases. Although the present results cannot absolutely rule out the hypothesis that a large fraction of the terrestrial potassium budget has dissolved into a molten iron sulfide‐bearing core, the present analysis of the pressure‐volume relation for potassium, iron, iron sulfides, potassium aluminosilicate, and potassium iron sulfide yields no evidence in support of this hypothesis.
Paleomagnetic record of plate‐margin tectonic processes along the western edge of North America
Authors: Beck Myrl E.
Published: February 2008

The paleomagnetic record for the western edge of North America shows consistent, systematic discordance. Virtually none of roughly three dozen high‐quality paleomagnetic studies of pre‐Pliocene rock units located within several hundred kilometers of the continental margin are lying anywhere near their appropriate reference poles (constructed from high‐quality paleomagnetic data from the craton). Nor do these discordant paleomagnetic studies show simple random scatter, as might be expected for rock bodies whose postmagnetization histories include reheating, complex tectonic deformation, or chemical change. Instead, their paleomagnetic poles are clearly displaced systematically away from the reference curve into the general area of the Atlantic Ocean. To accomplish this tectonically requires that the magnetized rock body move northward, rotate clockwise, or both, in relation to North America. The consistency of paleomagnetic record argues that most of the western edge of North America has undergone such block movement and therefore is allochthonous, at a scale at least as large as the area of a typical paleomagnetic study. It further argues that northward transport and clockwise rotation have been prime elements in shaping the Cordillera. Differences between paleomagnetic records for separate parts of the Cordillera probably reflect differences in specific platetectonic histories. For instance, although northward transport of allochthonous blocks is important south of Cape Mendocino and north of southern Vancouver Island, in the area between (the Pacific Northwest), only clockwise rotations are found. This is consistent with steady under‐thrusting of the Pacific northwest coastline by the Farallon plate for most of the Tertiary, which is in contrast to the steady or intermittent transform activity to the north and south. Batholith belts, including the Peninsular Range batholith of Southern California and the Coast Plutonic Complex of British Columbia, also seem to have been involved in the general northward transport and clockwise rotation, although the possibility of undetected tectonic tilts clouds the record. However, consideration of the thermal behavior of large, tilted plutonic blocks suggests that post‐magnetization tilts probably are small. If so, then most western Cordilleran Cretaceous batholiths (not including the Sierra Nevada) also are allochthonous. Northward transport and clockwise rotation could be accomplished by a variety of mechanisms, involving recognized plate tectonic processes. Some of these mechanisms are discussed. However, dextral shear between North America and plates to the west is central to each mechanism. It seems likely that shear also has disrupted and internally modified older accreted terranes, producing internal block rotations. Such older, disrupted allochthonous terranes appear to be drawn out into highly attenuated tectonic laminae plastered on the western edge of the continent.
Thermal neutron interferences in the 40Ar/39Ar dating technique
Authors: Tetley Neil , McDougall Ian , Heydegger H. R.
Published: February 2008

Dating geological samples by the 40Ar/39Ar method involves an irradiation in a reactor during which interfering nuclear reactions also occur that require corrections to be made to the observed data. The (39Ar/37Ar)Ca and (36Ar/37Ar)Ca correction factors for HIFAR reactor agree with those reported for other reactors, whereas the (40Ar/39Ar)K correction factor (about 0.3) is more than an order of magnitude greater than that found for most other reactors. A positive correlation found between reported thermal/ fast neutron flux ratios and the (40Ar/39Ar)K correction factors for different reactors leads to the inference that significant 40Ar can be produced by the 40K(n, p)40Ar reaction through interaction with thermal neutrons. This effect of thermal neutrons also causes substantial variation in (40ArX39Ar)K as a function of position in the reactor. Cadmium shielding has been used to modulate these effects over about 2 orders of magnitude and thereby to reproduce artificially the entire range of (40Ar/39Ar)K values reported for other reactors. Elimination of the thermal neutron interference (via Cd shielding ≥0.5 mm) can increase the range of useful neutron fluencies by about an order of magnitude and also reduce corrections in young samples, where it can be relatively large.
Surface deformation, crack formation, and acoustic velocity changes in pyrophyllite under polyaxial loading
Authors: Spetzler Hartmut A. , Sobolev Gennady A. , Sondergeld Carl H. , Salov Boris G. , Getting Ivan C. , Koltsov Anotoli
Published: February 2008

Jacketed cubes of pyrophyllite (31.6 mm on an edge) with variable water content were stressed monotonically to failure under polyaxial compression (σ1 > σ2 = 2σ3). The maximum and minimum principal stresses, σ1, and σ3, were applied with pistons, and the intermediate stress σ2 with a transparent fluid. An optical window in the pressure vessel allowed in situ measurements of the σ2 face deformation by optical holography. In addition, the more conventional techniques of stress, strain, and elastic wave velocity measurements as well as optical and electron microscopy were used to study the formation and propagation of fractures. The strength ( σ1 − σ3)f of the samples increased by about 50% as σ2 was increased from 5 MPa to 100 MPa. Air dry samples were stronger than water‐soaked samples by about ∼20%. Increasing the strain rate from 2 × 10−8 to 10−6 s−1 at σ2 = 5 MPa and 30 MPa increased the strength by about 10%. At σ2 = 100 MPa this trend was reversed, and samples that were deformed at the low strain rate were stronger by approximately 5%. The crack morphology of recovered samples was studied by optical and scanning electron microscopy. Subparallel sets of fractures and en échelon fracture patterns ahead of the macrofracture were easily visible with the unaided eye when σ2 was 5 MPa or 30 MPa. However, at σ2 = 100 MPa these fracture patterns were only visible under the microscope, and the cracks appeared much thinner. The holographic observations of the σ2 face revealed the following: as σ1 was increased, broad bulges formed in a crosslike pattern along the lines of the maximum shear stress. Macrofracture initiation, which occurred in a corner, was preceded by concentrated surface deformation. As the macrofracture propagated across the sample, it deviated from the direction of the maximum shear stress. Ahead of the tip of the macrofracture and migrating with it was a pronounced bulge. In response to monotonically increasing σ1, the displacement in the σ2 direction of a point adjacent to the eventual crack plane went through a local maximum that was followed by a local minimum as the crack passed. The results of elastic wave velocity measurements in the σ2 direction were very sensitive to the spatial relationship of the macrofracture and the elastic wave travel path. However, in general, above ∼50% of (σ1 − σ3)f the velocities decreased. As the migrating bulge approached an elastic wave travel path, the velocity decrease became more pronounced. The velocity increased again as the bulge passed.
Variable rates of Late Quaternary strike slip on the San Jacinto Fault Zone, southern California
Authors: Sharp Robert V.
Published: February 2008

Three strike slip displacements of strata with known approximate ages have been measured at two locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6 km in no more than 0.73 m.y. northeast of Anza indicates 8–12‐mm/yr average slip rate since late Pleistocene time. Two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the Coyote Creek fault in western Imperial Valley. Horizontal slip of 1.7 m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 275–510 yr B.P. The corresponding slip rate is 2.8–5.0 mm/yr. Right lateral offset of 10.9 m measured on a buried stream channel older than 5060 yr B.P. but younger than 6820 yr B.P. yields average slip rates for the intermediate time periods, 400± to 6000± yr B.P., of 1–2 mm/yr. The average rates of slip for these three time intervals suggest a relatively quiescent period for the San Jacinto fault zone from about 4000 B.C. to about 1600 A.D. To the extent that long‐term variations in seismic activity of major strike slip faults elsewhere are known, fluctuations in slip rate for the San Jacinto fault would not appear to be abnormal. If the San Jacinto and adjacent segments of the San Andreas fault alternately assume dominant roles in absorbing motion between the Pacific and American plates, perhaps even more recently than 400 yr ago, the San Andreas fault south of the Transverse Ranges expressed most of the motion but has since become relatively inactive.
Mixing characteristics and compositional differences in mantle‐derived melts beneath spreading axes: Evidence from cyclically layered rocks in the ophiolite of North Oman
Authors: Smewing John D.
Published: February 2008

Within the ophiolite of north Oman, interlayered gabbros and peridotites form a unit up to 3.5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites. Spatial and chemical relationships within this sequence require the existence of a large reservoir (20 km wide, 3.5 km high) of tholeiitic magma that was periodically replenished by mantle‐derived picritic to tholeiitic melts during the spreading process. Cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks. Within these cyclic units, basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases. This abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs. By virtue of its greater density than tholeiitic magma (Sparks et al., 1979) any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid‐liquid interface. Cooling of this basal layer by contact with the cooler, overlying magma would result in the precipitation of olivine + chrome spinel ± pyroxene from the picritic liquid. This process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir. The precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle. Variable thicknesses of the cyclic units, different liquid lines of descent, and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume, primary composition, and the extent and nature of prefractionation in the mantle. Liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm. The existence of a large magma reservoir supports earlier interpretations based on age (Tilton et al., 1981) and geochemical data (Pearce, 1980) that the Oman ophiolite is an analogue for fast spreading ridges.
Tillamook Volcanic Series: Further evidence for tectonic rotation of the Oregon Coast Range
Authors: Magill James , Cox Allan , Duncan Robert
Published: February 2008

Paleomagnetic directions from the Eocene Tillamook Volcanic Series of the Oregon Coast Range point 46° clockwise from the expected Eocene field direction. Potassium argon dating of six dikes and flows from this formation yields a mean age of 44.3 ± 0.6 m.y. These results establish that the Oregon coastal block of Simpson and Cox (1977) extends north to the Oregon‐Washington border and that this block has rotated clockwise 46° ± 13° during the past 44 m.y. The block has undergone no detectable north‐south translation. A two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the Pacific northwest region. Phase I of the rotation, active between 50 and 42 m.y. B.P., results from fragmentation of the Farallon plate with rotation of a fragment during its accretion to North America. Phase II of the rotation, active between 20 m.y. b.p. and the present, occurs in association with extension in the Basin and Range province.
Migration of radionuclides in fissured rock: The influence of micropore diffusion and longitudinal dispersion
Authors: Rasmuson Anders , Neretnieks Ivars
Published: February 2008

The migration of radionuclides in the fissures in the bedrock surrounding a repository is discussed. A one‐dimensional transport model is presented. It includes diffusion of the nuclides into the microfissures of the rock, and linear sorption, and longitudinal dispersion in the bedrock. An analytical solution to the model is given in terms of an infinite integral. The integrand is a sometimes highly oscillatory function of the system parameters. A special integration method is developed to evaluate the infinite integral. The method utilizes the oscillatory behavior of the integrand. The assessment of input parameters is discussed in some detail. Dimensionless breakthrough curves are given for the approximate range of variation of the input parameters. Calculations are made for a repository of spent fuel surrounded by fissured but fairly good rock (Kp = 10−9 m/s and fissure spacing S = 50 m). Longitudinal dispersion may significantly affect the amount of radioactive material reaching the biosphere. Radionuclides, which would decay completely without longitudinal dispersion, may arrive in nonnegligible concentrations. Dispersion effects of the magnitude considered in this study can significantly diminish the retardation effects of matrix diffusion.
Continental crust beneath the Agulhas Plateau, southwest Indian Ocean
Authors: Tucholke Brian E. , Houtz Robert E. , Barrett Douglas M.
Published: February 2008

The Agulhas Plateau lies 500 km off the Cape of Good Hope in the southwestern Indian Ocean. Acoustic basement beneath the northern one third of this large, aseismic structural high has rugged morphology, but basement in the south is anomalously smooth, excepting a 30‐ to 90‐km‐wide zone with irregular relief that trends south‐southwest through the center of the plateau. Seismic refraction profiles across the southern plateau indicate that the zone of irregular acoustic basement overlies thickened oceanic crust and that continental crust, locally thinned and intruded by basalts, underlies several regions of smooth acoustic basement. Recovery of quartzo‐feldspathic gneisses in dredge hauls confirms the presence of continental crust. The smoothness of acoustic basement probably results from erosion (perhaps initially subaerial) of topographic highs with redeposition and cementation of debris in ponds to form high‐velocity beds. Basalt flows and sills also may contribute locally to form smooth basement. The rugged basement of the northern plateau appears to be of oceanic origin. A plate reconstruction to the time of initial opening of the South Atlantic places the continental part of the southern plateau adjacent to the southern edge of the Falkland Plateau, and both abut the western Mozambique Ridge. Both the Agulhas and Falkland plateaus were displaced westward during initial rifting in the Early Cretaceous. Formation of an RRR triple junction at the northern edge of the Agulhas continental fragment during middle Cretaceous time may explain the origin of the rugged, thickened oceanic crust beneath the northern plateau as well as the apparent extension of the continental crust and intrusion of basaltic magmas beneath the southern plateau.