bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen isotopic compositions of enstatite chondrites and aubrites
Authors: Clayton Robert N. , Mayeda Toshiko K. , Rubin Alan E.
Published: February 2008

Oxygen isotopic compositions have been measured for ten EH chondrites, eight EL chondrites, and nine aubrites. The mean δ18O and δ17O values are: 5.31 and 2.74, 5.57 and 2.96, 5.24 and 2.70, respectively. Within the EH group, more equilibrated meteorites are fractionated toward higher heavy isotope abundances than the less equilibrated meteorites. Small departures of individual meteorites from a common mass fractionation occur for some of the EH samples. The aubrites may be genetically related to either the EH or EL group or to both. Almost all of the measured isotopic compositions are indistinguishable from the terrestrial oxygen isotope fractionation line, suggesting a genetic relationship among the earth, the moon, and the enstatite the meteorites.
Crystal chemistry of meteoritic hibonites
Authors: Burns Roger G. , Burns Virginia Mee
Published: February 2008

Structural features influencing relative enrichments, cation stabilities, and colors of vanadium‐, titanium‐, and iron‐bearing hibonite (CaAl12O19) in meteorites are examined. These transition elements may substitute for Al3+ ions, which occur in five different coordination sites in the hibonite crystal structure, including three distinct octahedra [Al(1), Al(3), and Al(4) positions], one tetrahedron [Al(2) position], and an unusual trigonal bipyramid [the Al(5) position] providing fivefold coordination by oxygen ions. Mossbauer spectral measurements of terrestrial and synthetic iron‐bearing hibonites demonstrate that although Fe cations occur in four‐, five‐, and sixfold coordinations, they are relatively enriched in the trigonal bipyramidal Al(5) site, which provides the largest average Al3+‐oxygen distance. Similarities with Mossbauer spectra of blue sapphires indicate that some Fe2+ ions are also located adjacent to Ti4+ cations in hibonite's face‐sharing Al(3) octahedra. Arguments based on ionic radius and crystal field stabilization energy criteria are used to explain the enrichment of Fe2+ ions in the fivefold coordination Al(5) site of hibonite. Similar electronic stabilities apply also to V3+ and Ti3+, but not to Cr3+, providing an explanation for the fractionation of vanadium into meteoritic hibonites. Three mechanisms are proposed for the blue colors of these hibonites, the visible‐region spectra of which show minima at 550 nm (blue) between two absorption bands at about 400 nm and 700 nm. One assignment of these bands is to crystal field transitions within V3+ and Ti3+, respectively, which are located in the symmetry D3h trigonal bipyramidal Al(5) site. A second assignment of the 700 nm band is to an intense Fe2+ → Ti4+ intervalence transition between traces of these cations located in adjacent face‐shared Al(3) octahedra. The orange color and disappearance of the 700 nm band produced by the heat treatment of hibonite at elevated oxygen fugacities may then be the result of oxidation of either Ti3+ to Ti4+ or Fe2+ to Fe3+. A third explanation of the blue‐orange color change involves color centers induced when primordial 26Al decays to 26Mg, or from trapped electrons in the lattice as a result of nonstoichiometry and structural defects in hibonite.
Geologic evolution of Galileo Regio, Ganymede
Authors: Casacchia Ruggero , Strom Robert G.
Published: February 2008

The geology of Galileo Regio is characterized by: (1) two principal morphologic terrain units (rough and smooth terrains); (2) craters in various states of preservation; and (3) a tectonic framework consisting of three furrow systems of different ages. The three furrow systems transect the rough terrain but predate all other terrains as well as the oldest craters larger than about 10‐km diameter. A broadly arcuate NW‐SE furrow system dominates the tectonic pattern and is intermediate in age between widely spaced NE‐SW and N‐S furrow systems. The furrow systems were probably produced by extensional stresses that occurred during crustal solidification but before large craters could be retained. Smooth terrain is concentrated in the southern part of Galileo Regio and probably resulted from fluid extrusions along fractures associated with the dominant furrow system. The distribution of smooth terrain on Galileo Regio suggests that the ancient crust of Ganymede was relatively thin in the equatorial region and thickened poleward in this area. The origin of the furrow systems is very uncertain. The age relationships, morphology, and geometry of the furrow systems do not favor an origin by impact or tidal stressing. A possible, but speculative, origin is crustal uplift caused by a plume‐like convection cell in a fluid mantle underlying a thin crust. Stratigraphic and morphologic relationships among furrows and crater palimpsests suggest that palimpsest morphology is largely the result of impact into a rheologically weak crust rather than viscous relaxation.
The chemistry and origin of the ordinary chondrites: Implications from refractory‐lithophile and siderophile elements
Authors: Fulton Christopher R. , Rhodes J. Michael
Published: February 2008

Thirty‐eight ordinary chondrites (17 H, 20 L, and 1 LL) have been analyzed for major and selected trace elements. These data indicate that the lithophile elements Mg, Ca, Al, Cr, and V normalized to Si are in higher abundance in the H than in the L chondrites. The siderophile elements Ni, Co, and Fe show very good correlation within, as well as between, the two major ordinary chondrite groups. Twenty‐four of our analyses are of Antarctic finds, while ten are samples of falls. Comparing the Antarctic data with the fall data reveals no evidence that any of the elements studied here have been mobilized by terrestrial weathering processes. Within the H and L chondrite groups there is little chemical variation, indicating that the source of these samples is remarkably homogeneous. Equilibrium condensate fractionation from a nebula of CI composition can result in the observed ordinary chondrite compositions. The fractionation of metal at about 1440°K (and 10−3 atm) into high and low iron groups, followed by a gas‐solid fractionation at about 1380°K with the H group losing more solids than the L, will produce the observed H and L compositions and intragroup trends.
Morphology of the outermost shells of the Tunguska black magnetic spherules
Authors: Żbik Marek
Published: February 2008

A sample population of 100 spherules from the area of the Tunguska event of 1908 was examined by SEM techniques. According to their morphological features, the spherules are subdivided into several types: compact, rough, regolith‐like, mosaic, and ‘astrakhan‐coated.’ Any hypothetical link between the Tunguska body and spherule morphology requires the body to be capable of converting its evaporated debris into spherules resembling ablation products of iron meteorites.
Interpretation of non‐double‐couple earthquake mechanisms derived from moment tensor inversion
Authors: Sipkin Stuart A.
Published: February 2008

Some of the proposed physical explanations for non‐double‐couple moment tensor inversion results for shallow‐focus earthquakes include source multiplicity, rupture on nonplanar fault surfaces, and tensile failure under high fluid pressure. Experiments using synthetic seismograms demonstrate that the first two effects can result in non‐double‐couple mechanisms due to the inadequacy of the source model used in the inversion; the latter explanation, however, implies the existence of non‐double‐couple mechanisms that are intrinsic to the source process. A waveform inversion algorithm for estimating seismic source parameters, in which the elements of the deviatoric moment tensor are allowed to have indepedent time histories, is applied to four approximately equal sized (M ∼ 6) shallow‐focus earthquakes, all of which had large non‐double‐couple components in their moment tensor solutions when the solutions were constrained to be point sources with step function time histories. The Coalinga, California, event of May 2, 1983, is shown to have had a complex source with a significant change in geometry, and the Yemen earthquake of December 13, 1982, is shown to have been a multiple event with little or no change in source orientation. Of the two largest events in the May 1980 Mammoth Lakes, California, sequence, the larger had a complex time history, but the smaller showed no evidence of source complexity. For these two events, tensile failure under high fluid pressure is the most likely explanation for the large non‐double‐couple components. For the larger event the complex time history could be explained by the existence of barriers to fluid flow which are broken during the rupture process. Thus all three of the previously mentioned explanations, either alone or in combination, may result in non‐double‐couple solutions being found.
Water dissolved in olivine: A single‐crystal infrared study
Authors: Freund Friedemann , Oberheuser Gert
Published: February 2008

Polarized infrared spectra of the hydrogen impurities in an olivine single crystal, approximately (Mg0.9Fe0.1)2SiO4, from the Zargabad Island, Red Sea, are reported in the range 4200–3000 cm−1. In the O‐H stretching frequency region, two groups of IR absorption bands occur which show the same pleochroism and the same temperature‐induced shift between room temperature and 80 K. Each of these IR groups is composed of three equidistant bands at 3614, 3591, 3567 m−1 and at 3576, 3530, 3485 cm−1 (80 K values). These distinct multiplets are assigned to the OH−‐compensated octahedral and tetrahedral vacancy defects [OH·V″oct]′ and [OH·V′tetr]*, respectively. It is suggested that the reduced effective charge of the tetrahedral, i.e., Si‐type, vacancy is due to Si entering a nearby nonideal site for partial charge compensation. The multiplet splitting, 23 and 45 cm−1, is due to the isomorphous substitution of Mg and Fe on the adjacent M1 and M2 positions to which the proton‐bearing oxygen is bound. This leads to the combinations (Mg Mg Si), (Mg Fe Si), (Fe Fe Si) and (Mg Mg Mg), (Mg Mg Fe), (Mg Fe Fe) at the octahedral M2 and at the tetrahedral vacancy sites, respectively. An IR signal which would correspond to the fully OH−‐compensated divalent cation vacancy defect [OH·V″octHO·]* appears to be absent. Instead a weak absorption band is observed in the range of the H‐H stretching frequency of molecular H2. By analogy to molecular H2 in MgO and CaO single crystals and in fused silica it is concluded that OH− pairs, associated with this type of defects, undergo a charge transfer conversion into (H2 + O22−). The H2 are loosely bound to the peroxy entities which probably form peroxy bridges between SiO4 tetrahedra, e.g., Si‐O‐O‐Si. Outgassing of H2 would leave excess oxygen behind, shifting the stoichiometry of the olivine in a direction which has never been discussed before.
Stratigraphic relations and lithologic variations in the Jemez Volcanic Field, New Mexico
Authors: Gardner Jamie N. , Goff Fraser , Garcia Sammy , Hagan Roland C.
Published: February 2008

Over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the Jemez volcanic field. Volcanism had begun in the area by about 16.5 Ma with episodic eruptions of alkaline basalts. By 13 Ma, alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite. High‐silica rhyolite, derived from melts of lower crust, also was erupting by about 13 Ma. Basalt and high‐silica rhyolite continued to be erupted until about 7 and 6 Ma, respectively, but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 Ma volumetrically overshadowed all other eruptive products between 10 and 7 Ma. From 7 to 3 Ma the dominant erupted lithology was dacite, which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high‐silica rhyolites. Less than 4–3 Ma volcanism was dominated by eruption of rhyolitic tuffs. Field relations, geochemistry, and dates specifically indicate the following with regards to stratigraphie relations: (1) distinctions among basalt of Chamisa Mesa, Paliza Canyon Formation basalts, and Lobato Basalt for other than geographic reasons are artificial; basaltic volcanism was continuous in volcanic field from >13 to 7 Ma, (2) Canovas Canyon and Bearhead rhyolites form a continuum of high‐silica rhyolite volcanism from >13 to 6 Ma, (3) hypabyssal and volcanic rocks of the Cochiti mining district probably represent the exhumed interior of a Keres Group volcano(s), (4) temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations, and (5) the Tewa Group formation Cerro Rubio Quartz Latite may more appropriately be considered part of the Tschicoma Formation of the Polvadera Group. Preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field's history.
Low permeability rocks: Laboratory measurements and three‐dimensional microstructural analysis
Authors: Lin C. , Pirie G. , Trimmer Donald A.
Published: February 2008

Traditional techniques for measuring permeability and traditional models relating permeability to porosity are inadequate for low‐permeability (≤10−18m2) geologic materials. Special techniques have been developed for measuring fluid flow through such rock samples and for quantifying their threedimensional pore microstructure. Permeability is measured by a modified transient pulse technique, pore structure is quantified using a computerized microgeometric modeling system, and portable threedimensional display is produced by an optical imaging system. Results of applying this processing to a low‐porosity “tight gas sandstone” from the Cotton Valley Formation agree with the well‐documented diagenetic history of these samples. We find that the porosity distribution is very inhomogeneous and skewed. The pore system is slightly anisotropic, which is consistent with the differences in permeability measured in two orthogonal directions. We indicate how such data could be used to develop more geologically and geometrically realistic models relating porosity and permeability in low‐permeability gas formations.
Slab penetration into the lower mantle beneath the Mariana and other island arcs of the northwest Pacific
Authors: Creager Kenneth C. , Jordan Thomas H.
Published: February 2008

Residual sphere methods of imaging near‐source structure are applied to earthquakes in the Mariana and other subduction zones of the Northwest Pacific. Between 17° and 20°N, the nearly vertical Mariana seismic zone provides a particularly good geometry for constraining the velocity structure and penetration depth of the subducted lithospheric slab. Path‐corrected and smoothed residual spheres constructed from International Seismological Centre P wave travel times are dominated by negative residuals along the strike of the slab, even for the deepest focus events. Theoretical residual spheres are computed by tracing rays through three‐dimensional thermal models of the slab for four representative earthquakes, whose epicenters are separated by less than 60 km and whose focal depths range from 149 to 624 km. The velocity structure derived from the thermal models is calibrated by inverting the observed travel times from two intermediate‐focus earthquakes for the temperature coefficient of velocity. Ignoring the perturbations of phase transitions within the slab, we obtain ∂vp/∂T = −0.5 m/s/°K; including these perturbations, we obtain −0.4 m/s/°K. These values are consistent with laboratory measurements and observations of large‐scale lower mantle structure, and they imply that ∂vp/∂T is a weak function of depth in the mantle. Thermal models which fit the observations are characterized by slab penetration into the lower mantle at least several hundred kilometers below the seismicity cutoff; decreasing the total penetration depth to less than about 1000 km significantly degrades the fit to the deep‐focus events. The travel time data are insensitive to, but consistent with, penetration below 1000 km. Similar conclusions are obtained from the residual sphere analysis of deep‐focus earthquakes in the Kuril‐Kamchatka, Japan and Izu‐Bonin seismic zones. We infer that subduction along the entire northwest margin of the Pacific plate involves the flow of upper mantle material into the lower mantle. These results have substantial implications for models of mantle convection and hypotheses regarding the nature of the 650‐km discontinuity. In particular, our data are not consistent with stratified convection models which restrict the return flow associated with plate motions to be above 650 km nor do they favor the hypothesis that the seismic velocity jump at this level is caused by a chemical change.
Tectonic and gravitational stress in long symmetric ridges and valleys
Authors: Savage William Z. , Swolfs Henri S.
Published: February 2008

We present an exact elastic solution for the effects of topography on the near‐surface stresses caused by a uniform, uniaxial tectonic compression or tension acting normal to the axial planes of isolated symmetric ridges and valleys of realistic shape. The solution, obtained by the Kolosov‐Muskhelishvili method of complex potentials, gives stresses in the vicinity of these topographic features that, with the exception of the vertical component, are of the order of the regional tectonic stress. The effect of topography is to reduce a regional tectonic compression near the crest of a ridge and, if the ridge is sufficiently steep, to cause a stress reversal resulting in a small tension. Valleys, on the other hand, concentrate the far‐field tectonic stress. When a previously derived exact solution for the topographic effect on the gravity‐induced stress field is combined with the present solution for the topographic modification of a regional compressive tectonic stress, we find a slight increase in the lateral components of the gravity‐induced compressive stress at the ridge crest and, under the valley bottoms, a decrease in the gravity‐induced tensile stresses. The opposite effects occur when a far‐field tension is superposed on the gravity‐induced stress field.
Effective pressure law for permeability of westerly granite under cyclic loading
Authors: Morrow C. A. , Zhang Bo‐Chong , Byerlee J. D.
Published: February 2008

The permeability of Westerly granite was measured under cyclic loading conditions for confining pressures Pc to 96 MPa and pore pressures Pp to 86 MPa in order to determine the coefficient α of the effective pressure equation Pe = Pc ‐ αPp, where Pe, Pc, and Pp are effective, confining, and pore pressures, respectively. Two methods of determining α are described, based on the cubic root law, k1/3 = A + B In (Pe), which relates permeability k to effective pressure in fractures. A and B of this equation are functions of the geometric parameters of the cracks; α was close to 1 under all conditions of pressure and loading history. Pressure cycling had significant effects on the permeability of the granite, as demonstrated by the large variations in the coefficients A and B of the cubic root law.
Pristine lunar glasses: Criteria, data, and implications
Authors: Delano John W.
Published: February 2008

Major‐element analyses of several thousand glasses from all of the Apollo landing sites have resulted in the identification of 25 groups of pristine (i.e., volcanic) glass. The nickel in these pristine glasses is shown to be indigenous, not meteoritic contamination, and to be correlated with Mg. The chemical data indicate that these glasses are consistently better candidates for primary magmas than the majority of crystalline mare basalts. The pristine glasses support the view that assimilative processes [Ringwood and Kesson, 1976] involving two cumulate systems in the differentiated mantle operated during mare petrogenesis. The reality of those assimilative interactions is evident by the occurrence of two linear arrays among the chemistries of the glasses. Data suggest that these cumulate components in the differentiated mantle persist for lateral distances of at least 1000 km and therefore appear to be products of a vast magma ocean that existed early in lunar history. Alternative hypotheses claiming that the highlands crust and mare source regions were produced by serial magmatism without a magma ocean have not yet considered all of the constraints derived from the pristine glasses.
Styles of tectonic deformations on Venus: Analysis of Venera 15 and 16 data
Authors: Basilevsky A. T. , Pronin A. A. , Ronca L. B. , Kryuchkov V. P. , Sukhanov A. L. , Markov M. S.
Published: February 2008

Analysis of the radar images obtained by Veneras 15 and 16 leads to the conclusion that the ridge‐and‐groove structures on the surface of Venus are the result of tectonic deformation. Although the mechanism of such deformation cannot yet be unequivocally deduced, several styles of deformation can be described. Areal deformation occurs where horizontal stresses have operated over large areas. Shear deformation appears in bands showing differential longitudinal deformation. Transversal stresses operating over long and relatively narrow areas have produced belt deformation. Circular deformation is related to a specific locus of the stresses. The absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment. The origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity‐induced spreading of surface material over upwellings. A general conclusion is that in the surveyed area of Venus neither terrestrial plate tectonics nor lunar‐highland‐type terrain exists.
Cooling rate calculations for silicate glasses
Authors: Birnie Dunbar P. , Dyar M. Darby
Published: February 2008

Series solution calculations of cooling rates are applied to a variety of samples with different thermal properties, including an analog of an Apollo 15 green glass and a hypothetical silicate melt. Cooling rates for the well‐studied green glass arid a generalized silicate melt (density = 2.5 g/cm3, heat capacity = 0.3 cal/g•°C, and thermal conductivity = 0.005 cal/cm•s•°C) are tabulated for different sample sizes (0.01–1000‐cm half thicknesses), equilibration temperatures (2000°, 1400°, and 620°C), and quench media (vacuum, air, liquid nitrogen, water, silice‐ne oil, and liquid tin). Results suggest that cooling rates are heavily dependent on sample size and quench medium and are less dependent on values of physical properties. Thus cooling histories for glasses from planetary surfaces can be estimated on the basis of size distributions alone. In addition, the variation of cooling rate with sample size and quench medium can be used to control quench rate.
Variation of elastic constants of cubic PbF2 with volume
Authors: Jamieson John C. , Manghnani Murli H. , Matsui Teruyuki , Ming Li Chung
Published: February 2008

The single‐crystal elastic constants Cij of β‐cubic PbF2 (fluorite type) have been measured to ∼0.3 GPa at ambient temperature and to 573 K at ambient pressure. The measured Cij are (in gigapascals (GPa)): C11 = 96.075 ± 0.094, CS = (C11 – C12)/2 = 24.475 ± 0.018, and C44 = 20.835 ± 0.012. The results from the least squares linear fit of the pressure data to ∼0.3 GPa yield dC11/dP = 7.79 ± 0.08, dCs/dP = 0.21 ± 0.08, and dC44/dP = 0.82 ± 0.02, and the temperature data in the range of 298–373 K give (in GPa per degree × 102): dC11/dT = −5.63 ± 0.05, dCS/dP = −1.26 ± 0.03, and dC44/dT = −1.11 ± 0.07. The pressure derivatives do not give any indication of the monotropic transition at ≤0.4 GPa; likewise, the temperature derivatives do not indicate the onset of superionicity at high temperatures. Relative stability of the α, β, and β′ phases of PbF2 is discussed in light of thermodynamic parameters.
Phase transitions among the CaGeO3 polymorphs (wollastonite, garnet, and perovskite structures): Studies by high‐pressure synthesis, high‐temperature calorimetry, and vibrational spectroscopy and calculation
Authors: Ross Nancy L. , Akaogi Masaki , Navrotsky Alexandra , Susaki Jun‐ichi , McMillan Paul
Published: February 2008

Phase boundaries for the reactions CaGeO3 (wollastonite → garnet) and CaGeO3 (garnet → perovskite) were determined at high pressure and temperature. The enthalpies of these transitions were measured by high temperature calorimetry. These studies indicate a positive dP/dT for the wollastonitegarnet transition and a negative dP/dT for the garnet‐perovskite transition. These PT slopes are further supported by calculations of lattice vibrational entropies based on Kieffer's model and the infrared and Raman spectra of the three polymorphs.
Rapid determination of Fe3O4 phase diagram by synchrotron radiation
Authors: Huang E. , Bassett W. A.
Published: February 2008

High‐pressure/temperature phase relationships in Fe3O4 have been determined up to 34 GPa and 600°C using an energy dispersive X ray diffraction method on the Cornell High Energy Synchrotron Source. This experiment has shown that the slope (dT/dP) of the transformation from magnetite to its high‐pressure phase is −45°C/GPa with increasing pressure. The average slope of the reversion boundary from high‐pressure phase to magnetite is 170°C/GPa. Neither boundary is considered to represent the true Clapeyron boundary. The Clapeyron boundary for the transition from magnetite to high‐pressure magnetite is tentatively determined as −68°C/GPa when relevant kinetic effects are considered. These results are then used to calculate some of the phase relations in the FeO phase diagram. These preliminary studies show that the rapid data acquisition with synchrotron radiation will make it possible in future investigations to measure rates of reaction and to correct for the effects of sluggishness.
Shock loading of single‐crystal olivine in the 100–200 GPa range
Authors: Furnish Michael D. , Brown J. Michael
Published: February 2008

We report the results of a series of shock transit time measurements in single‐crystal olivine (San Carlos peridot) designed to assess any orientation dependences over the pressure range of 100–200 GPa. Each of the four shots included a specimen oriented normal to (100), one normal to (010), and one normal to the bisector of these two directions (approximately (120)), with time intervals measured by pin detectors (three per specimen). This third, oblique, orientation was chosen to maximize shearing along the closest‐packed (100) planes. By analogy with the restacking model for the olivine‐spinel transition, shear stresses promoting a pervasive (100)[010] system of stacking faults might facilitate the breakdown of the olivine structure, aiding the transition to the high‐pressure phase and increasing the final shocked density. The Hugoniots of the three orientations were found to be coincident within experimental error, a conclusion at variance with earlier results obtained with synthetic forsterite which suggest that the compressibility along the b axis is significantly greater than along the a or c axes. Possible explanations for this discrepancy include experimental errors and a dependence of the high‐pressure state on the orientation of the shock and the composition of the olivine. A softening of the Hugoniot beginning at approximately 150 GPa was found and may reflect melting or a change of the decomposed mixture of solid phases.
Convection in the oceanic crust: Simulation of observations from Deep Sea Drilling Project Hole 504B, Costa Rica Rift
Authors: Williams Colin F. , Narasimhan T. N. , Anderson Roger N. , Zoback Mark D. , Becker Keir
Published: February 2008

Three‐dimensional modeling of convection in the oceanic crust at Deep Sea Drilling Project site 504B using an integral finite difference model reasonably duplicates underpressures, surface heat flow, downhole temperature profiles, and fluid drawdown rates observed by in situ measurements in the borehole. The major constraint to produce such good fits is the permeability versus depth function, a quantity which was actually measured in the borehole. Pronounced “underpressuring” (fluid pressures less than hydrostatic) occurs throughout the convection cell if and only if a tight, nearly impermeable cap rock (chert) exists over the cell. The computed flow rates of water from the ocean through the borehole into the basalt agree closely with the measurements carried out 65, 720, and 1280 days after drilling. The model predicts an inflow rate of 50–60 L/h during the expected next occupation of the site by mid‐1986. Numerical modeling has confirmed that an active hydrothermal convection cell exists in the region of hole 504B and that a very low permeability cap rock is necessary for the existence of the convection cell. Convective hydrothermal cells in the oceanic crust can explain observed periodic heat flow variations in suboceanic crust.
Topographic mapping from interferometric synthetic aperture radar observations
Authors: Zebker Howard A. , Goldstein Richard M.
Published: February 2008

Interferometric synthetic aperture radar observations provide a means for obtaining high‐resolution topographic terrain maps from data acquired simultaneously at two slightly displaced antennas. Calculation of the three‐dimensional coordinates of all the points in a radar image can be made from the combination of along‐track, slant range, and interferometer fringe measurements. Thus the result compensates for the layover exhibited by conventional radar maps and removes the false targets generated by multiple signal paths to a given object in the scene. We have derived a topographic map of a portion of the San Francisco Bay Area utilizing data that were recorded by a radar system mounted on a NASA CV990 aircraft and processed by a general purpose digital computer. This map displays the height above sea level of a region approximately 11 km by 10 km in size, sampled on an 11‐m pixel grid. Uncertainties in the estimated height result from imprecise knowledge of the observational geometry, radar bandwidth limitations, and finite signal‐to‐noise ratios. For our system, each factor contributed approximately 3–4 m to a relative rms error of about 6 m. Additional global errors of the order of 10 m can result from inaccurate monitoring of aircraft attitude. The statistical variation of the height measurements from the ocean portion of the map, which we can presume to be quite flat, is 2–10 m rms, which is consistent with the theoretical estimate. Comparison of our results with U.S. Geological Survey contour maps indicates a high degree of correlation between the two sets of altitude data.
Determination of large‐scale velocity structure of the crust and upper mantle in the vicinity of Pavlof Volcano, Alaska
Authors: McNutt S. R. , Jacob K. H.
Published: February 2008

Pavlof Volcano is a 2715‐m‐high stratovolcano located at latitude 55.4°N and longitude 161.9°W, near the western end of the Alaska Peninsula. In this study the large‐scale lateral perturbations to a vertical reference velocity structure in the vicinity of Pavlof Volcano are determined using modified versions of standard three‐dimensional inversion techniques. We use as the primary data set P wave arrival times and residuals from shallow (<40 km deep) regional earthquakes located by the Shumagin Islands seismic network. Relative residuals mainly from refracted (e.g., Pn) arrivals allow us to constrain models for a laterally varying crustal velocity structure. Several strong ray path dependent patterns are seen in the residuals; the most pronounced is that rays passing through the crust beneath the volcano are delayed (positive residuals) by up to 1.0 s, while those rays traveling subparallel to and behind the volcanic axis on the retroarc (Bering Sea) side arrive as much as 0.5 s early (negative residuals). The corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with Emmons Caldera (10 km SW of Pavlof). Computed velocity values are about 12–14% lower than in the surrounding crust and are comparable to values found at other volcanic areas. The exact shape and position of the low‐velocity body and its suitability for geothermal power development have not yet been sufficiently determined; for such an assessment a much denser station distribution is required.
Multiple differentiation processes in early‐rift calc‐alkaline volcanics, northern Rio Grande Rift, New Mexico
Authors: Thompson Ren A. , Dungan Michael A. , Lipman Peter W.
Published: February 2008

Structurally elevated segments of a north trending intrarift horst in northern New Mexico expose Late Oligocene/Early Miocene volcanic rocks erupted during the inception of the Rio Grande rift. Postcaldera rocks of the Latir volcanic field on the intrarift horst (Timber Mountain and Brushy Mountain) include two distinct calc‐alkaline sequences differing in age by up to 3 m.y. The lower sequence (25 Ma) is dominated by dacite (plag + cpx + opx + Fe‐Ti oxides ± hbld) but includes basaltic andesite (ol + cpx + plag) and low‐silica rhyolite (plag + sanidine + qtz + biotite + Fe‐Ti oxides). Dacites contain quenched andesitic micropillows, high‐silica rhyolite glass, and sanidine and oligoclase xenocrysts. Incompatible element concentrations increase with silica content throughout the sequence and are accompanied by concomitant decreases in Sr, Sc, and Cr. The upper sequence (22 Ma) includes basaltic andesite (ol + cpx + plag), dacite lava flows (plag + hbld + cpx + Fe‐Ti oxides + opx + sanidine + sphene + biotite), and a basal lava dome of high‐silica rhyolite (qtz + sanidine ± biotite ± Fe‐Ti oxides ± sphene + hornblende) overlying remnants of the outflow sheet of the 26‐Ma Amalia Tuff. Upper sequence dacites are distinguished from lower sequence dacites by their hydrous mineralogy and lower relative K2O, TiO2, Zr, Y, Nb, Y/Zr, and total rare earth element concentrations for any given silica content. Major and trace element models, to predict fractionation paths from an andesite parent, indicate that suppressed K, Sr, and elevated Mg, Y, and Lu concentrations in the lower sequence dacites resulted from fractionation of phenocryst phases, plus mixing in proportions approaching one to one, with high‐silica rhyolite similar in composition to the Amalia Tuff. Low‐silica rhyolites may have been derived through fractionation of dacites plus mixing with the high‐silica rhyolite. Positive correlations of 87/86Sr0 with SiO2 are interpreted to reflect the increasing proportions of crustally contaminated high‐silica rhyolite in lower sequence, intermediate‐composition melts. These differentiation models support a petrogenic link between the mildly peralkaline Amalia Tuff of the caldera‐forming phase of the Questa magmatic system and early postcaldera lavas. Subsequent eruption of upper sequence volcanics marks the regeneration of more typical metaluminous magmas characteristic of both precaldera volcanics and postcaldera intrusions.
Nature of the eastern boundary of the Rio Grande rift from COCORP surveys in the Albuquerque basin, New Mexico
Authors: de Voogd Beatrice , Brown Larry D. , Merey Carlene
Published: February 2008

Reprocessing and synthetic seismic modeling of COCORP profiles across the eastern margin of the Rio Grande rift at Abo Pass, New Mexico, suggest that synthetic and antithetic Cenozoic normal faults sole into or are truncated by a northwest dipping listric master fault which bounds the eastern side of the basin and reaches a depth of at least 10 km beneath the southern Albuquerque basin. This fault appears to project toward the surface position of the Los Pinos Cenozoic normal fault, which lies basinward of and trends parallel to a mapped Laramide duplex thrust zone. The Los Pinos fault may sole into the older thrust, reactivating this preexisting compressional structure at depth. A lateral change in reflection amplitude and frequency is conspicuous across the eastern rift margin. Analysis demonstrates that source coupling is the main cause of a much deteriorated signal below the rift basin, although complex ray paths and attenuation by the graben fill may contribute.
The 1977 Sumba earthquake series: Evidence for Slab pull force acting at a subduction zone
Authors: Spence William
Published: February 2008

The great 1977 Sumba earthquake occurred at the eastern Sunda trench, just west of the collision of Australian continental lithosphere with the island arc. The length of the aftershock zone of this normal‐faulting earthquake is about 200 km. Aftershocks are concentrated 65–115 km east of the main shock epicenter, with very few aftershocks in a 50‐km‐long segment that spans the main shock epicenter. Relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere. There is no evidence for thrust faulting of the deeper aftershocks. These data imply that the neutral bending surface must be at least 35–40 km deep. A second aftershock zone, about 180 km northwest of the main shock, became active immediately following the main shock, but events were concentrated during days 50–52. This zone is a 70‐km‐long lineation that trends toward the main shock epicenter and reflects right‐lateral, strike‐slip faulting within the subducted oceanic plate. Seismicity exists to a depth of about 650 km in the very old plate beneath the Sunda‐Banda arc, and that plate's negative buoyancy causes very large slab pull forces. Great interface thrust earthquakes are absent at the Sumba region, and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate. This decoupling permits slab pull stresses to be guided updip to the region of the Sumba main shock. Such shallow‐acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire Sumba earthquake series. In this model, slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake. Postseismically, the released oceanic plate undergoes a pulse of downdip strain, returning the plate to a less extended state. The moment of this downdip plate motion could exceed the seismic moment of the main shock.
Coupled free oscillations of an aspherical, dissipative, rotating Earth: Galerkin theory
Authors: Park Jeffrey , Gilbert Freeman
Published: February 2008

Variational theory based on self‐adjoint equations of motion cannot fully represent the interaction of the earth's seismic free oscillations in the presence of lateral structure, attenuation, and rotation. The more general Galerkin procedure can model correctly the frequencies and attenuation rates of hybrid oscillations. Implementation of either algorithm leads to a generalized matrix eigenvalue problem in which the potential and kinetic energy interactions are separated into distinct matrices. The interaction of the earth's seismic free oscillations due to aspherical structure, attenuation, and rotation is best treated as a matrix eigenvalue problem. The presence of attenuation causes the matrices to be non‐Hermitian and requires the use of a general Galerkin procedure. Physical dispersion, represented as a logarithmic function in frequency, must be represented by a truncated Taylor series about a fiducial frequency in order to be incorporated in the Galerkin formalism in a numerically tractable manner. The earth's rotation introduces an interaction matrix distinct from the potential and kinetic energy matrices, leading to a quadratic eigenvalue problem. A simple approximation leads to an eigenvalue problem linear in squared frequency. Tests show that this approximation is accurate for calculations using modes of frequencies f ≳ 1 mHz, unless interaction across a wide frequency band is modeled. Hybrid oscillation particle motions are represented by matrix eigenvectors that can be significantly nonorthogonal. The degrees of freedom in the low‐frequency seismic system remain distinct, since source excitation is calculated by using dual eigenvectors. Synthetic seismograms that are constructed from Galerkin coupling calculations without reference to this eigenvector nonorthogonality can be disastrously noncausal.
Paleomagnetism of the structurally deformed Latir volcanic field, northern New Mexico: Relations to formation of the Questa caldera and development of the Rio Grande rift
Authors: Hagstrum Jonathan T. , Lipman Peter W.
Published: February 2008

The Oligocene‐Miocene Questa caldera and related rocks of the Latir volcanic field are exposed along the eastern flank of the Rio Grande rift in northern New Mexico. These rocks bracket in age the beginning of extensional tectonics in the region approximately 25–26 m.y. ago. The early phase of extension is characterized by currently low‐angle normal faults that caused steep tilting of volcanogenic units, particularly within the Questa caldera. Paleomagnetic measurements on intrusive and extrusive rocks from the Latir field show that tilting began abruptly after the eruption of the Amalia Tuff and subsequent formation of the Questa caldera 26 m.y. ago. Specifically, most of the tilting occurred after the intracaldera welded tuff and uppermost portion of the subcaldera magma chamber had cooled and become magnetized, but before the subsequent resurgent plutons had been emplaced and cooled sufficiently to also become magnetized about 1 m.y. later. Differential uplift of the region exposes similarly aged subvolcanic plutonic rocks to the south that also have untilted magnetizations. Tilting had ceased altogether by the time the late‐stage plutons were magnetized, at least within the caldera structure and to the south. Younger rift‐related sedimentary rocks and interbedded basalt flows (15 m.y. old) show that tilting continued to the north of the caldera past this time. However, paleomagnetic evidence indicates that a nearby 7‐ to 11‐m.y.‐old rhyolite dome is probably untilted, suggesting that significant tilting in the region had ended by this time. The latest phase of extension is characterized by high‐angle deeply penetrating normal faults, which are responsible for the high structural relief presently observed in the region.
Shock wave equation of state of enstatite
Authors: Watt J. Peter , Ahrens Thomas J.
Published: February 2008

Shock compression data are reported for hot‐pressed Bamble bronzite (En86) loaded to pressures between 104 and 161 GPa. When compared to earlier shock wave data on En90 at lower pressures and to static compression data, our data require the presence of a phase change. In P‐ρ space the data yield two distinct trajectories, which cannot be explained by experimental error. The higher‐density data, corrected for porosity and a small amount of metallic iron impurity, agree with a theoretical En86 high‐pressure phase Hugoniot calculated from static compression equation of state data for perovskite (pv) structure silicates when experimental errors and uncertainties in the equation of state parameters are considered. All the En86 data can be described by a calculated Hugoniot if the first pressure derivative of the MgSiO3 (pv) bulk modulus is taken as 4.5 ± 1.0. Combining the present preferred data with recent shock wave data for single‐crystal forsterite, we find that En86 is slightly more dense than Fo86 at pressures above 110 GPa. Comparison of the forsterite and enstatite data with the Preliminary Reference Earth Model (PREM) lower mantle densities, with corrections applied for the higher shock temperatures relative to lower mantle temperatures, shows that PREM densities are satisfied by olivine or pyroxene stoichiometries with Mg mole fractions from 0.82 to 0.90. These values are lower than estimates of 0.90 to 0.95 developed from extrapolating static compression data to lower mantle conditions.
Stretching instabilities and lithospheric boudinage
Authors: Ricard Yanick , Froidevaux Claude
Published: February 2008

A competent layer with a nonlinear rheology can, under extension, exhibit pinch‐and‐swell instabilities. Such instabilities can explain small‐scale regular deformations of rock. Recently they have also been invoked in relation to the distribution of basins and ranges (Fletcher and Hallet, 1983) and the undulation of the Bouguer map (Froidevaux, 1986) in the western part of North America. For the case of a simple stratified structure, different authors have expressed mathematical solutions describing these instabilities. In this paper we elucidate the general physical meaning of these nonhomogeneous deformations. In the simplest case where the competent layer is fully embedded in a softer material, a fundamental mode develops with a preferred wavelength equal to 4 times the thickness of a competent layer. The novelty is the occurrence of overtones with alternate symmetrical and antisymmetrical deflections of the interfaces between layers. These modes have a large amplitude when the rheology tends to perfect plasticity. The presence of a free surface like the earth's surface for the upper crust is shown to double the preferred wavelength of the fundamental mode and to forbid one overtone out of two. This new effect is caused by gravity which inhibits the growth of surface topography. Another shift is of opposite sign to that caused by a free surface. Numerical solutions for multilayer structures can elucidate the nature of couplings between various layers. The case of a three‐layer lithosphere having a soft lower crust and overlying a mantle asthenosphere has been analyzed in some detail. It shows that, in the range of wavelengths larger than 30 km, up to five preferred modes may be superimposed. This should encourage further analysis of the western U.S. topography and gravity pattern which is known to contain at least two wavelengths: about 40 km for the individual basins and ranges and about 200 km for the Bouguer troughs in Nevada and their associated broad topographies.
Saugus‐Palmdale, California, field test for refraction error in historical leveling surveys
Authors: Stein Ross S. , Whalen Charles T. , Holdahl Sanford R. , Strange William E. , Thatcher Wayne
Published: February 2008

A field test designed to measure atmospheric refraction error in historical and modern leveling was conducted in May–June 1981 on a 50‐km‐long grade from Saugus to Palmdale, California. During 1955–1971, the length of sights made between the level instrument and rods systematically decreased from 60 m to 26 m. The difference in height near Palmdale measured by single‐run long‐sight (42‐m) and short‐sight (22‐m) leveling during the test was 6 times larger than expected random error. Correction for refraction by using either the observed or modeled vertical temperature gradient in Kükkamaki's balanced sight equation reduced the height difference to the level of random error uncertainty. The observed temperature gradient obeyed a power law relation, T = a + bzc, where z is the height and a, b, and c are constants that depended on atmospheric conditions and the ground surface beneath the line of sight. The refraction‐corrected leveling satisfies the specifications and meets all standards of first‐order control surveys. The six historical surveys of the Saugus‐Palmdale grade were corrected for refraction error using the results of the experiment and for rod scale errors and nontectonic subsidence considered in previous investigations. The corrected uplift near Palmdale reached 56 ± 16 mm with respect to Saugus during the period 1955–1965. This amount of uplift is about one third that obtained before removal of refraction error. The corrected displacement profiles also reveal previously unrecognized deformation in the epicentral region of the 1971 San Fernando ML = 6.4 earthquake during the decade before the main shock.