bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical resistivity structure of the crust in the southern extension of the Canadian Shield – Layered Earth Models
Authors: Sternberg Ben K.
Published: February 2008

A combination of electrical sounding methods has been used to study the vertical resistivity structure of the crust on the southern extension of the Canadian Shield in northern Wisconsin. Direct current dipole‐dipole resistivity soundings were made at transmitterreceiver separations of 1 m to 40 km. Electromagnetic transient soundings were made at ranges of 5 to 40 km and with frequency bandwidths of about 0.5 to 10 Hz. The soundings were made in a region where the gross subsurface structure is laterally uniform, so horizontal, plane‐layered models were used to interpret the data. Layered earth models were randomly generated and tested against the observed data to determine the range of models that fit the data. A four‐layer model with the following range for the resistivities and thicknesses fits the data: (1) A surface layer, comprised mainly of glacial till, has a few hundred ohm meters resistivity down to depths of a few tens of meters. (2) A bedrock layer has a resistivity in the range of 3000 to 7000 ohm m down to depths of 4.5 to 11 km. (3) A deeper, high‐resistivity layer has resistivities of greater than 100,000 ohm m down to depths of 14 to 22 km. (4) A lower layer has resistivities of from 50 to 1500 ohm m. The interpreted resistivities down to depths of about 5 to 10 km are consistent with resistivities of wet rocks containing microfractures. Between 5 to 10 km and 15 to 20 km, we interpret much higher resistivities, which may indicate fewer original microfractures or healed microfractures at these depths. The low resistivities interpreted at depths below 15 to 20 km cannot be explained by usual models for the temperature and composition of the lower crust. The low resistivities can be explained by a higher temperature than is usually postulated for the lower crust, or by water of hydration in the rocks.
Viking Orbiter imaging observations of dust in the Martian atmosphere
Authors: Briggs G. A. , Baum W. A. , Barnes J.
Published: February 2008

More than 20 local Martian dust clouds and two global dust storms were observed with the Viking orbiter camera. Sixteen of the local clouds were imaged in two colors or were observed with other instruments confirming their identification as dust clouds. These Viking results are compared with earth‐based observations of Martian dust storms and with Mariner 9 data. Most of the dust activity seen by Viking occurred during southern hemisphere spring and early summer, when Mars was near perihelion and insolation was near maximum. About half the local clouds occurred near the edge of the southern polar cap, where winds are presumably enhanced by a strong regional temperature gradient. The other half occurred mainly in the southern hemisphere near regions where circulation models incorporating topography predict positive vertical velocities. Although dust clouds observed from earth show a similar partial correlation with models, some ambiguity exists concerning interpretation of regions near Hellespontus that have spawned the most spectacular Martian dust storms on record.
Shear fracture precipitated by strain softening as a mechanism of dry slab avalanche release
Authors: McClung David M.
Published: February 2008

Snow stratigraphy studies at the fracture lines of dry slab avalanches often indicate a situation that consists of a harder thick layer overlying a thin weak layer. Prefracture stress conditions and progressive fracture of snow slabs are discussed here from the point of view that slow shear failure in the weak layer results in Griffith‐like critical lengths there, resulting in rapid shear fracture and subsequent avalanche release. The important aspects of the timing of dry slab avalanche release and the relationship between the applied shear stress and the shear strength at the weak layer are discussed in this context. The results show that the main experimental facts on snow and field observations of snow slab avalanches are explainable from this approach, although direct, concrete proof is not presently available.
Topographic and backscatter characteristics of Geos 3 overland data
Authors: Miller L. S.
Published: February 2008

This paper presents results of the analysis of the backscattered signal characteristics at 13.9 GHz., obtained from the Geos 3 radar altimeter. The characteristics discussed include spatial resolution, altimetric accuracy, radar cross section, and signal fluctuation statistics. Spatial resolution is found to be largely a function of the radar cross‐section angular decay process rather than systems response behavior for either pulselength or beamwidth limited operation. The magnitude and angular response characteristics of the radar cross‐section values were found to differ substantially from previously published aircraft measurements.
Equations of state of MgO and ε Fe under static pressure conditions
Authors: Mao H. K. , Bell P. M.
Published: February 2008

The volume equations of state for MgO and ε Fe determined under calibrated static pressure conditions to 1 Mbar are given by V/V0 = [1 + 2.85(3)P (Mbar)]−0.219(3) (for MgO) where V0 = 11.25(2) cm3/mol and V(cm3/mol) = 6.74(3)[1 + 3.4(3)P(Mbar)]−0.185(7) (for ε Fe). The random error of V is 0.3%, and the maximum systematic error of V (caused by the uncertainty of the pressure scale) is 1%. These equations can be employed in geophysical models of the mantle‐core boundary.
Chemical stratification of the mantle
Authors: Anderson Don L.
Published: February 2008

Differentiation of the mantle results in a thick layer of basalt which cannot sink below about 670 km even after it converts to eclogite and garnetite. The mantle is therefore chemically stratified. The upper mantle stratification is possibly garnet peridotite overlying eclogite.
The erosion of planetary and satellite atmospheres by energetic atomic particles
Authors: Haff P. K. , Watson C. C.
Published: February 2008

Analytic expressions are presented which may be used to compute the sputter‐erosion yield from any unimolecular gravitationally bound gas by any atomic charged particle of any energy. A calculation of solar wind proton and alpha particle induced erosion of the CO2 atmosphere of Mars predicts molecular sputtering yields Sp ≈ 0.029 and Sα ≈ 0.21. These numbers are comparable to the results of earlier Monte Carlo simulations. An expression for the emission yield of energetic molecular fragments produced in primary knock‐on events is also given in closed form. Such fragment emission is of secondary importance for mass loss compared to the molecular yield itself. Erosion by radiation belt protons of a hypothetical thin O2 atmosphere associated with the Jovian satellite Ganymede is considered. Molecular sputtering yields for proton energies at 1 keV and 1 MeV are 0.115 and 6.4×10−4, respectively. The yield at the lower energy can have significant effects on atmospheric equilibrium should keV proton fluxes equal or exceed about 108 cm−2 s−1.
Structure, tectonics, and stress field of the Coso Range, Inyo County, California
Authors: Roquemore Glenn
Published: February 2008

The tectonics of the Coso Range has been described as having arcuate and ring faults both suggesting the presence of a circumscribed subsidence bowl or calderalike feature. New information suggests the Coso Range is situated in an area of transition between the stress of the right slip San Andreas fault‐plate interaction and the extensional tectonics of the Basin and Range. Arcuate faults in the Coso Range are interpreted to have been produced by the regional stress field rather than to have been of volcanogenic origin. Focal mechanisms of small‐magnitude earthquakes support the stress directions indicated by local fault patterns. Fumeroles in the area are primarily associated with oblique slip faults rather than with arcuate or ring faults. The geothermal reservior is therefore much different from that of a caldera or subsidence bowl, and the overall geothermal potential is probably less than earlier estimates.
Investigation of the geomagnetic field polarity during the Jurassic
Authors: Steiner Maureen B.
Published: February 2008

A large number of sedimentary rock formations on the Colorado Plateau and in the Wyoming foreland have been investigated paleomagnetically in an attempt to determine the geomagnetic polarity during the Middle and the early Late Jurassic, a period frequently postulated to have been of constant normal polarity. A survey of published paleomagnetic litature shows this postulate to be without grounds. A wide range of sedimentary lithologies, both red and nonred, were investigated. Most formations displayed a complex, multivectorial magnetization, even to very high stability ranges. However, a strikingly large amount of reversed polarity was observed and relatively little normal Jurassic polarity. Because of the widespread geographic data base, the multitude of lithologies, and the amount of time represented, it is tentatively concluded that the data at least partially reflect the character of the geomagnetic field during the Jurassic. Weak intensities in all formations, and the instability, may support the marine magnetic anomaly hypothesis of weak geomagnetic field intensity for this time. These data do not support the hypothesis of a long period of normal polarity during the Jurassic. They in turn, suggest that the Jurassic field might have frequently been of reversed polarity.
The nature of differentiation trends in some volcanic rocks from the Galapagos Spreading Center
Authors: Byerly Gary
Published: February 2008

The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron‐ and titanium‐enriched tholeiites, (3) andesites, and (4) rhyodacites. Interpretation of major element compositions determined for these rocks, associated glassy selvedges, and mineral phases suggests that the entire suite can be produced by fractional crystallization of the observed low‐pressure phenocrysts. Early fractionation to extreme iron and titanium enrichment requires 50–75% crystallization of five parts plagioclase, three parts augite, and one part olivine. In the highly fractionated basalts, pigeonite replaces olivine as a phenocryst phase and is a minor phase in the fractionation. At extreme iron and titanium enrichment, titanomagnetite becomes a phenocryst phase, and fractionation produces a silica enrichment trend to andesite (79% total crystallization) and rhyodacite (87% total crystallization). Apatite occurs as a microphenocryst phase in the silicic rocks. This suite of iron‐ and titanium‐enriched basalt and low‐alumina andesite differs from the calc‐alkaline suite of island arcs and continental margins. It is similar to the suite of fractionated lavas of the Galapagos Islands and Iceland, though these rocks are generally more alkaline than the Galapagos spreading center suite, which is similar to other ocean floor basalt suites though more fractionated. Magma mixing is suggested by the presence of basaltic xenoliths in the silicic rocks; the presence of high‐silica glass inclusions in the glassy selvedges of the basalts, andesite, and rhyodacite; the common occurrence of reversely zoned phenocrysts in the silicic rocks; and the occurrence of some highly fractionated yet aphyric rocks. The unusual extent of fractionation for this region may be related to the size, morphology, and eruption and resupply rates of the magma chambers associated with the Galapagos spreading center. This and the regional distribution of these highly fractionated rocks suggest an important, though indirect, influence of the Galapagos hot spot.
Effect of partial gas/brine saturation on ultrasonic absorption in sandstone
Authors: Frisillo A. L. , Stewart T. J.
Published: February 2008

Ultrasonic compressional wave velocities and absorption have been measured for Berea sandstone as a function of different nitrogen gas and brine saturations at 103.3‐bar net confining pressure. The results of this experiment show larger absorption for partially gas/brine‐saturated sandstone than for either complete gas or complete brine saturations. These results are in agreement with suggestions made by some early experimenters and with recently reported resonance data on Vycor glass and ultrasonic measurements on Berea sandstone. These results suggest the possible use of sonic logging as a means of detecting bypassed gas production in older hydrocarbon producing wells.
Crystal fractionation in a CUmulate Gabbro, Mid‐Atlantic Ridge, 26°N
Authors: Tiezzi Lawrence J. , Scott Robert B.
Published: February 2008

Gabbros dredged from the Mid‐Atlantic Ridge near 26°N have distinct cumulate textures and display phase, rhythmic, and cryptic layering. The gabbros have a tholeiitic iron enrichment trend and a concomitant increase in the incompatible element abundances. The gabbroic phases have large variations in both the Fo content of olivine (64–82) and the An content of the plagioclase (50–80) and the smaller but significant variations in En content of clinopyroxene and orthopyroxene (42–46 and 68–74, respectively). Increasing Fo contents of olivine correlate with increasing iron enrichment and increasing modal abundance of cumulate pyroxene. These chemical variations and observed textures are consistent with a model involving crystal fractionation. Fractionation models linking crystallization of phases in the gabbros to chemical variations in nearby basalts are tested. A model involving clinopyroxene crystallization can, mathematically, produce the chemical variations found in any one dredge site, but none of the basalts contain clinopyroxene, and hence the model is rejected. The observed gabbroic fractionation cannot produce the magmatic source for more evolved basalts; therefore it is likely that basaltic magmas are not greatly influenced by shallow magma chamber processes either by crystal fractionation or by magma mixing with evolved gabbroic magmas during transport to the surface. The chemical variations observed between dredge sites cannot be successfully modeled through fractionation involving any combination of the phases observed in the basalts or the gabbros.
Higher‐mode surface waves and structure of the Great Basin of Nevada and western Utah
Authors: Priestley Keith , Orcutt John A. , Brune James N.
Published: February 2008

Observed seismograms and dispersion data for crust and mantle higher mode surface waves in the Great Basin are compared with theoretical seismograms and dispersion curves computed for the Great Basin model of Priestley and Brune [1978]. This structure was originally derived from fundamental mode surface wave and refraction data. Phases identified as Sa [T∼13 s] are observed to have a phase velocity of 4.50±0.03 km s−1. Crustal second Rayleigh mode (first shear mode) waves have predominant periods varying from about 5 s on some paths to about 8 s on others. The observed excitation and phase velocity of the Sa phase are in agreement with theoretical seismograms and computed phase velocities for a modified Great Basin model. The agreement provides added support, in an unexpected way, for the existence of a mantle lid of velocity about 4.5 km s−1 in the Great Basin. The crustal higher mode group velocity observations, i.e., the predominant periods of the second Rayleigh mode along various paths, reflect variations in crustal thickness within the Great Basin. Crustal thicknesses of approximately 25 km are indicated for some paths in northwestern Nevada and southeastern Oregon, whereas crustal thicknesses of greater than 35 km are indicated for east central Nevada. The crustal thickness of 35 km in the Great Basin model is probably most appropriate for the central part of the Great Basin.
A technique for the inversion of regional data in source parameter studies
Authors: Wallace Terry C. , Helmberger Donald V. , Mellman George R.
Published: February 2008

A least squares waveform inversion technique to determine the source parameters of moderate size earthquakes is presented. The technique involves an iterative scheme which makes use of an error function determined by the cross correlation of a long‐period seismogram and a synthetic. Both the data and the synthetics are normalized to make the solution insensitive to absolute amplitudes. Making use of the three fundamental faults, the error functions can be written as a series of cross correlations multiplied by constants corresponding to source orientation. Once the cross correlations are computed, the source orientation is determined iteratively, and only the constants have to be recalculated. The technique has been applied to long‐period regional seismograms where the beginning portion of the vertical and radial components containing Pn and PL (referred to as Pnl) is inverted. Three western United States earthquakes which have been well studied by other authors were used as test samples. The inversion yields fault plane solutions in good agreement with those in the literature. Moments determined in the inversion are also in excellent agreement with those determined teleseismically.
Seismotectonics of northeastern United States and adjacent Canada
Authors: Yang Jih‐Ping , Aggarwal Yash Pal
Published: February 2008

Data for local earthquakes recorded by a network of stations in northeastern United States and adjacent Canada were analyzed to study the seismicity, the relationship between earthquakes and known faults, the state of stress, and crustal and upper mantle velocity structure. In addition, portable seismographs were deployed in the field to study aftershocks. As a result, accurate locations for about 364 local earthquakes (2 ≤ mb ≤ 5) and 22 focal mechanism solutions were determined. A comparison of the spatial distribution of these events (1970–1979) with historical earthquakes (1534–1959) reveals that seismic activity in the northeast is relatively stationary in space: those areas that have had little or no seismicity historically are relatively aseismic today, whereas the historically active areas are also active today. The instrumental locations, historical seismicity, and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces. (1) The Adirondack‐western Quebec province is a northwesterly trending zone of seismic activity, about 200 km wide and at least 500 km long, extending from the SE Adirondacks into western Quebec, Canada. Thrust faulting on planes striking NNW to NW appears to predominate, and the inferred axis of maximum horizontal compression is largely uniform and trends WSW, nearly parallel to the calculated absolute plate motion of North America. Little or no seismicity is found where anorthosite outcrops at the surface. Correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep NE or SW gradient in Bouguer anomalies. This zone does not appear to extend southeastward to Boston, as proposed by some workers. (2) The Appalachian province is a northeasterly trending zone of seismic activity extending from northern Virginia to New Brunswick, Canada. Highangle reverse or thrust faulting on N to NE trending planes appears to predominate. The western margin of this province, however, appears to be relatively aseismic. We attribute this relative lack of activity to one or more of the following: the presence of igneous activity postdating rifting of Africa from North America, the occurrence of intense metamorphism during the Acadian orogeny which may have annealed preexisting faults, and the predominance of ductile as opposed to brittle deformation in the geologic past. The inferred axis of maximum horizontal compression along the eastern margin of the Appalachians is rather uniform and trends W‐WNW, almost perpendicular to the magnetic lineations offshore. We suggest that this W‐WNW compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers.
Adiabats (∂T/∂P)s and Grüneisen parameter of NaCl up to 50 kilobars and 800°C
Authors: Boehler Reinhard
Published: February 2008

The adiabatic pressure derivative of temperature (∂T/∂P)s of sodium chloride has been measured by adiabatic compression up to 50 kbars and 800°C using an end‐loaded piston cylinder apparatus with solid pressure medium. Our measurements at room temperature are in very good agreement with measurements using hydrostatic fluid cells (Boehler et al., 1977). The results can be represented by a power law relationship between (∂T/∂P)s and volume of the form (∂T/∂P)s,P = (∂T/∂P)s,P=0 (V/V0)n. We estimate powers n of 6.3 and 5.3 at room temperature and 800°C, respectively. The Grüneisen parameter γ can be calculated from the thermodynamic equation γ = Ks/T = (∂T/∂P)s, where Ks is the adiabatic bulk modulus; γ is found to be independent of temperature within our pressure and temperature range.
On tsunami nucleation: 1. A point source
Authors: Ward Steven N.
Published: February 2008

Near‐field tsunami formation is detailed by means of mode summations using point moment tensor sources with a spherically symmetric, self‐gravitating, and elastic earth. Snapshots of vertical sea surface motion calculated within 200 km and 1000 s of an event typically show an initial tsunami pulse slowly grow and then evolve into dispersed wave trains. The general point moment tensor used here produces frequency dependent tsunami radiation patterns of azimuthal degree ≤2. Pure dip and strike slip sources, however, create waves that are symmetric or antisymmetric about the epicenter. With a moment of 1020 N m and a rise time of 25 s, these sources generate initial tsunami pulses 40 km wide when buried 10 km beneath an ocean 4 km deep. Maximum dip slip and strike slip tsunami amplitudes of 209 and 56 cm occur 10 km from the epicenter, 75 s after the start of nucleation. Tsunamis from deeper earthquakes are smaller, broader, and slower to develop because they are depleted of high frequencies. In general, characteristics of tsunamis developing in this dynamic model contradict static concepts which base tsunami formation on permanent deformations of an otherwise rigid seafloor supporting incompressible water. I find that the permanent strain state of the seafloor is not important in tsunami generation because it is defined by frequencies well below the range of interest (100–2000 s)
Coastal submarine hydrothermal activity off northern Baja California: 2. Evolutionary history and isotope geochemistry
Authors: Vidal Victor M. V. , Vidal Francisco V. , Isaacs John D.
Published: February 2008

A geochemical model of the Punta Banda submarine hydrothermal system (PBSHS) and Ensenada quadrangle subaerial hot springs is developed using 18O/16O, D/H, 34S/32S, 3H, water and gas chemistry. The PBSHS water is a primary high temperature, acid, reducing fluid of old seawater origin which has been titrated by cold, alkaline groundwater of meteoric origin. The final exiting solutions represent a 1 : 1 mixture of the two primary mixing components. In contrast, the subaerial hot spring waters are of unmixed meteoric origin. The subaerial hot spring gas is predominantly atmospheric N2, while the PBSHS contains large amounts of CH4 and N2 derived from trapped marine sediments of Cretaceous age; δS34 values of sampled hydrothermal waters are similar to Cretaceous marine sulfate values and suggest that the waters contacted Cretaceous marine sedimentary strata. The presence of the Alisitos and Rosario marine sedimentary formations of Cretaceous age within the Ensenada‐Punta Banda quadrangle renders support to the above hypothesis. The data also demonstrate that pyrite mineralization and deposition in submarine hydrothermal environments result from the complexing of ferrous iron with elemental sulfur and sulfide and that submarine hydrothermal activity acts as a major source of silica, Ca2+, and trace metals and as a major sink for seawater Mg2+ and SO42−.
The mineralogy and chemistry of the anorogenic tertiary silicic volcanics of S.E. Queensland and N.E. New South Wales, Australia
Authors: Ewart A.
Published: February 2008

The Late Oligocene‐Early Miocene volcanism of this region is chemically strongly bimodal; the mafic lavas (volumetrically dominant) comprise basalts, hawaiites, and tholeiitic andesites, while the silicic eruptives are mainly comendites, potassic trachytes, and potassic, high‐silica rhyolites. The comendites and rhyolites have distinctive trace element abundance patterns, notably the extreme depletions of Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable enrichment of such elements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytes exhibit these characteristics to lesser degrees. The comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations (e.g., LREE, Nb, Y, and especially Zr) and Zn. The phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases: sodic plagioclase (albite‐andesine), calcic anorthoclase, sanidine, quartz, ferroaugite‐ferrohedenbergite, ferrohypersthene, fayalitic olivine, ilmenite, titanomagnetite, and rarely biotite (near annite) and Fe‐hastingsitic amphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite‐bearing rhyolites only), and allanite (amphibole and botite rhyolites only). The comendites generally contain Ca‐poor anorthoclase‐sanidine, quartz, fluorarfvedsonite, aegirine and aegirine‐augite (Zr‐bearing), aenigmatite, and ± ilmenite. Coexisting Fe‐Ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes. Where present, they indicate equilibration temperatures of 885°–980°C and fo2 between QFM and WM buffers. The magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration, which is also consistent with the general paucity of pyroclastics, the rarity of hydrous mineral phases, and the extreme Fe‐enriched ferromagnesian phenocryst compositions. The chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas, and the comendites, trachytes, and certain trachyte‐rhyolite series are considered to have evolved from a mafic parentage. The available oxygen, Sr, and Pb isotopic data, however, point to some modification of the magnas through crustal equilibration processes. The remaining high‐silica rhyolites are considered to most likely represent crustal partial melts but are again modified by extensive fractionation.
The redskin granite: Evidence for thermogravitational diffusion in a Precambrian granite batholith
Authors: Ludington Steve
Published: February 2008

The 1000‐Ma‐old composite Pikes Peak batholith consists largely of medium‐ to coarse‐grained biotite granite but is intruded by several smaller granite plutons. Two of these, the Tarryall Mountains batholith and the Redskin Granite (Hawley, 1969; Hawley and Wobus, 1977), are here interpreted as cupolas atop the main mass of the Pikes Peak batholith.

Chemical analysis for 38 elements in rocks from the Redskin Granite, the Tarryall Mountains batholith, and the biotite granite of the Pikes Peak batholith support the contention that the compositions of the smaller plutons are unlikely to have evolved from that of the main mass of the batholith by crystal fractionation. It is believed that these compositions arose through liquid‐state fractionation processes, possibly the convection‐aided thermogravitational model of Shaw et al. (1976).

It is important to recognize that the chemical signature of this fractionation process can be discerned in holocrystalline rocks as well as in their glassy equivalents because the process may be instrumental in the formation of some kinds of lithophile ore deposits.
Stability of goethite (α‐FeOOH) and lepidocrocite (γ‐FeOOH) to dehydration by UV radiation: Implications for their occurrence on the Martian surface
Authors: Morris Richard V. , Lauer Howard V.
Published: February 2008

The stability of submicron powders of goethite (α‐FeOOH) and lepidocrocite (γ‐FeOOH) to dehydration by ultraviolet (UV) radiation was investigated by laboratory experiments. Dehydration of these minerals by solar UV radiation is potentially an important process on the surface of Mars. When the spectral irradiance of the incident radiation was about 100 to 300 times that of the solar spectral irradiance at 1.5 AU (the orbit of Mars), radiant heating was so intense that both FeOOH polymorphs were thermally dehydrated. When the above incident radiation was opitcally filtered to remove a large portion of infrared and visible components, but not the intense near‐UV component, no perceptible dehydration, and therefore no UV photodehydration, was observed after irradiation times as long as 420 hours. After scaling the laboratory irradiance to that at the Martian surface, we calculate that goethite, lepidocrocite, and presumably all FeOOH polymorphs would not perceptibly UV photodehydrate there for at least 10 to 100 years. That is, there is as yet no basis in laboratory experiments for inferring that UV photodehydration of FeOOH polymorphs will occur naturally on the surface of Mars on a time scale of 10 to 100 years, if at all. Thus, the observation that goethite may contribute to the spectral reflectance data of Mars (e.g., Singer et al., 1979) is consonant with our results. The formation of maghemite (γ‐Fe2O3) by UV photodehydration of lepidocrocite does not seem a viable mechanism for producing the former to account for the magnetic component apparently present in Martian surficial material (Fuller and Hargraves, 1978; Hargraves et al., 1979). Finally, ferrimagnetic δ‐FeOOH remains a viable candidate (Burns, 1980) for the magnetic material with respect to FeOOH stability to UV photodehydration.
The Marquesas‐Line Swell
Authors: Crough S. Thomas , Jarrard Richard D.
Published: February 2008

Mapping of depth anomalies in the central Pacific reveals a broad, shallow region herein termed the Marquesas‐Line Swell. It has the morphology and geoid signature typical of a midplate hotspot swell and probably marks the Tertiary trace of the Marquesas hotspot. The swell is elongate in the NW‐SE direction, about 800 m high, and about 1500 km wide. It appears to extend continuously between the Marquesas Islands and the Line Cross Trend. A positive geoid height anomaly over the swell can be separated from the large regional trend. The transfer function between depth anomaly and geoid height is not well defined but is consistent with local isostatic compensation with an average root depth about 30 km (±40 km, 95% confidence interval) below the swell's surface. Direct spatial comparisons between geoid height and depth anomaly suggest that this root depth is constant along the length of the swell and has a value of about 40 km (±10 km, 95% confidence interval). The crest of the swell appears to subside as normal seafloor which is only 25 m.y.old, the prediction of the lithospheric thinning hypothesis. A permanent density decrease within the lithosphere can also explain the subsidence and geoid height anomaly; roll‐cell dynamic uplift can explain the subsidence but is difficult to reconcile with the geoid data.
Opening of a fault and resulting slip due to injection of fluid for the extraction of geothermal heat
Authors: Hayashi K. , Abé H.
Published: February 2008

The behavior of a fault with a fluid‐filled region is studied for the extraction of heat from hot, dry rock masses. A fault is viewed as a prefractured plane in the earth's crust and is modeled by an interface across which two elastic bodies contact with each other, where frictional forces against the relative movement between two surfaces of the interface are taken into account. Once fluid is injected between the two surfaces, the fault will be opened up over a region where the pressure acting on the fault plane before the injection is locally smaller than that on the neighboring regions. The size of the fluid‐filled region is compared with that of a crack opened by a hydraulic pressure in a homogeneous medium to show that the fluid‐filled region could be used as a reservoir for the extraction of geothermal heat. The resulting slip caused by the reduction of frictional forces due to the injection of fluid is analyzed by the use of two models, i.e., an interface with frictional resistance only and an interface with both frictional resistance and fracture toughness. It is shown that the two models do not exhibit any significant difference in the size of the slipped region when the fluid‐filled region is of dimensions of practical interest.
The emplacement of ophiolites by collision
Authors: Ben‐Avraham Zvi , Nur Amos , Jones David
Published: February 2008

Ophiolites, recognized in most of the world's orogenic belts, are generally interpreted to be oceanic crust and upper mantle (lithosphere) fragments that have been incorporated into continental margins at consuming plate boundaries. We suggest that the mechanism for ophiolite emplacement is the same in both the Alpine and Andean‐type orogenes. In both geological settings, obduction of oceanic lithosphere onto the continental lithosphere is caused by the convergence of light, buoyant bodies such as oceanic plateaus, continental slivers, island arcs, or old hot spot traces. For example, the Troodos ophiolite complex, previously interpreted by some workers as resulting from continental collision, may have been emplaced by the collision of Cyprus with the Eratosthenes Plateau embedded in the oceanic eastern Mediterranean crust. On the other hand, the Upper Jurassic Coast Range Ophiolites of California, previously interpreted as resulting from typical oceanic subduction, may be the result of a continuous injection of thick nonsubductable packages of light, continentally derived sedimentary rocks, seamounts, and plateaus into the subduction zones. Many other ophiolite complexes may be similarly related to accreted terranes.
A geophysical study of the Taupo Volcanic Zone New Zealand
Authors: Rogan Mary
Published: February 2008

The Taupo Volcanic Zone is a region of intense volcanic activity, from which over 16,000 km3 of lavas and pyroclastics have been erupted since the Late Pliocene. Analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers, each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly. These centers are interpreted as collapse features resulting from large‐scale withdrawal of magma from the upper crust. Four of the depressions occur beneath recognized volcanic centers or calderas; the fifth (Kapenga) is inferred to underlie an extinct hidden volcanic center. Three‐dimensional models of the thicknesses of low density and magnetized rocks in the Taupo Volcanic Zone have been determined by inversion of gravity and magnetic anomalies. Beneath three of the centers, the thicknesses of low density and magnetized rocks are approximately equal (2–3 km); that is, the collapse structures are infilled by magnetized volcanics. However, there is a significant discrepancy between the thicknesses of the gravity (5 km) and magnetic models (1.5 km) in the region of the Okataina Volcanic Centre. It is therefore suggested that a significant volume of low density, nonmagnetic rock, possibly acidic magma, underlies Okataina.
Source parameters of the 1980 Mammoth Lakes, California, earthquake sequence
Authors: Archuleta Ralph J. , Cranswick Edward , Mueller Charles , Spudich Paul
Published: February 2008

From the more than 1500 Mammoth Lakes earthquakes recorded on three‐component digital seismographs (Spudich et al., 1981), 150 were used in an analysis of the locations, mechanism, and source parameters. A composite fault plane solution of nine earthquakes 3.9 ≤ M ≤ 5.1 defines a right‐lateral strike slip mechanism on a steeply dipping nearly east‐west plane striking S75°E or left‐lateral strike slip on a nearly north‐south plane striking N10°E. Vertical cross sections of well‐located aftershocks indicate possibly three east‐west planes that coincide with the locations of the four largest earthquakes with ML ≥ 6.0. Using the spectral analysis of S waves (Brune, 1970), source parameters for 67 earthquakes were determined. Forty‐eight had magnitudes greater than or equal to 3.0. Seismic moments ranged from 9.20×1018 dyn cm to 2.33×1024 dyn cm. Earthquakes with seismic moment greater than about 1.0×1021 dyn cm had nearly constant stress drops (≃ 50 bars); earthquakes with seismic moment less than about 1.0×1021 dyn cm had stress drops that apparently decrease as seismic moment decreases.
An equivalent layer magnetization model for the United States derived from satellite altitude magnetic anomalies
Authors: Mayhew M. A.
Published: February 2008

Long wavelength (>500 km) magnetic anomalies measured by the Pogo series satellites at altitudes 400–700 km over the United States and adjacent areas are inverted to an equivalent layer magnetization model based on an equal area dipole source array at the earth's surface. Minimum source spacing giving a stable solution and a physically meaningful magnetization distribution is 300 km, but a scheme is presented for effectively sampling the distribution on a grid twice as fine. The model in effect expresses lateral variation in the vertical integral of magnetization and is a starting point for models of lateral variation in the form of the magnetization‐depth curve in the magnetic crust. The magnetization model contours correlate with large‐scale tectonic features and in the western part of the country probably reflect Curie isotherm undulations.
A multilayer model of time dependent deformation following an earthquake on a strike slip fault
Authors: Cohen Steven C.
Published: February 2008

A multilayer half‐space model of the earth is used in finite element calculations of time dependent deformation and stress following an earthquake on an infinitely long strike slip fault. The model involves shear properties of an elastic upper lithosphere, a standard viscoelastic linear solid lower lithosphere, a Maxwell viscoelastic asthenosphere, and an elastic mesosphere. Time dependent displacements, strains, and stresses are computed both at the surface of the earth and at depth. The analysis includes both systematic variations of fault and layer depths and comparisons with simpler elastic lithosphere over viscoelastic asthenosphere calculations. For conditions which may be appropriate for the earth, the creep of both the lower lithosphere and the asthenosphere can contribute to the postseismic deformation. The magnitude of the deformation is enhanced by a short distance between the bottom of the fault (slip zone)and the top of the creep layer but is less sensitive to the thickness of the creeping layer. Furthermore postseismic restressing is increased as the lower lithosphere becomes more viscoelastic, but the tendency for the width of the restressed zone to grow with time is retarded.
Electron microprobe studies of Iceland Research Drilling Project high‐temperature hydrothermal mineral geochemistry
Authors: Exley R. A.
Published: February 2008

The geochemistry of epidote‐ and chlorite‐dominated hydrothermal assemblages in the Iceland Research Drilling Project drill core has been studied by electron microprobe. The association of epidote, andradite, and Cu‐Fe sulphides suggests maximum basalt alteration temperatures of 350°–300°C. Near‐pure andradite implies a fluid XCO2 <0.05. The sequence of successively lower temperature minerals (epidote, prehnite, laumontite) in veins and vesicles suggests precipitation during rapid cooling from >300°C to <200°C and together with zonation in garnet and epidote implies fluid depletion in Fe3+ with time. The eradication of probable early zeolite facies assemblages and this cooling sequence suggest that temperatures rose to >300°C during burial of the lower part of the IRDP sequence before vigorous fluid circulation, producing rapid cooling and intense alteration, was established. Transition metals (Fe, Mn, Cu) were released from primary assemblages but were incorporated in epidote, chlorite, and sulphide. Epidote contains average SrO = 0.23%, but intersample variations in trace element contents of secondary minerals are complex, and there are no correlations with depth. It is concluded that the effects on whole rock chemistry (including Sr) and secondary mineral chemistry were largely controlled by kinetic factors such as (1) differing responses to alteration by primary assemblages and (2) permeability variations.
Modeling of microseismic surface wave sources
Authors: Szelwis Rüdiger
Published: February 2008

In a recent paper by Szelwis (1980) microseismic surface waves were inverted with respect to modal contribution and direction of approach. Array cross spectra of two observational runs were analyzed. The microseismic energy peaking in a frequency range of about 0.13 Hz to0.17 Hz was found to be essentially transported in the two lowest Rayleigh modes and the fundamental Love mode, approaching from one directional interval. In the present paper, the direction mode structure is related to the sources of microseisms. The Rayleigh waves are attributed to ocean wave interactions at the Norwegian coast. This is verified on the basis of Hasselmann's (1963) theoretical concept of the generation of microseisms, by estimating the corresponding source spectrum in two different ways. On the one hand, it is computed from the mode structure observed and a model of the wave‐carrying medium including refraction and attenuation. On the other hand, it is computed from a model of the ocean wave field subject to specular reflection from a planar coast. The ocean wave model is taken from sea wave generation research, the input parameters—wind velocity and fetch—are read from the weather charts. The two source spectrum estimates agree within reasonable limits of the model parameters, thus supporting the assumed Rayleigh wave generation mechanism. The Love waves appear to be coupled to the Rayleigh waves, as (1) both wave types approach from the same directions, and (2) their modal spectra relative to the subsurface transfer functions show a high frequency‐by‐frequency correlation, implying corresponding peak frequencies. A common origin, examined in terms of the spectra of one source, is not compatible with the underlying models.
The negative gravity field over the 85°E ridge
Authors: Liu Char‐Shine , Sandwell David T. , Curray Joseph R.
Published: February 2008

An isopach map made from seismic reflection and refraction data in the Bay of Bengal shows two prominent N‐S trending features in the basement topography. One is the northernmost portion of the Ninetyeast Ridge which is totally buried by sediments north of 10°N. The other buried ridge trends roughly N‐S for 1400 km at 85°E to the latitude of Sri Lanka and then curves toward the west. It has basement relief up to 6 km. Two free‐air gravity anomaly profiles across the region show a strong gravity low (∼−60 mGal) over the 85°E Ridge, while the Ninetyeast Ridge shows a gravity high. To understand the negative free‐air gravity anomaly over the 85°E Ridge, we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load. A plausible formation history for a buried ridge consists of at least two major episodes. The first is the formation of the ridge on a lithosphere with a flexural rigidity of D1. At some later time the ridge is buried by an influx of sediments, the lithosphere is cooler, and the flexural rigidity has increased to D2. The character of the gravity field depends primarily upon the initial and final values of flexural rigidity. These D1 and D2 values are varied to obtain good agreement between the model and observed gravity anomalies. Best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial. An approximate relationship between flexural rigidity and crustal age shows that the 85°E Ridge was formed on relatively young lithosphere, 5–15 m.y. old and that it was buried when the lithosphere was 40–80 m.y. old.
Thermal convection in non‐Newtonian fluids: Volumetric heating and boundary layer scaling
Authors: Parmentier E. M. , Morgan J.
Published: February 2008

Thermal convection in a volumetrically heated fluid layer, in which strain rate is proportional to stress with a power law exponent n, is studied by using finite difference approximations. As in earlier studies of convection in a layer heated from below, convection cell structure is found to be relatively independent of n for n ≤ 3. The rate of heat transfer, expressed in terms of an effective Rayleigh number based on a dissipation rate averaged viscosity, is also relatively independent of n. A relationship between effective Rayleigh number and the Rayleigh number based on the viscosity at a reference strain rate is derived on the basis of boundary layer scaling. This relationship agrees with finite difference solutions for both a volumetrically heated layer and a layer heated from below. Applied to planetary interiors, the power law exponent n is shown to influence the time scale of thermal evolution.
Paleomagnetic evidence bearing on the structural development of the Latir Volcanic Field near Questa, New Mexico
Authors: Hagstrum J. T. , Lipman P. W. , Elston D. P.
Published: February 2008

The mid‐Tertiary Latir volcanic field in northern New Mexico comprises intermediate‐composition volcanics overlain by a regional ash flow sheet and associated lavas of rhyolitic composition that are all cut by silicic‐alkalic granitic intrusives. Deeply exposed along the eastern flank of the Rio Grande rift, the silicic extrusive and intrusive rocks all yield radiometric ages of about 23 m.y., within analytical uncertainties. Flow banding and eutaxitic structures within the extrusive units indicate intense structural deformation, which for the most part increases toward the Questa caldera. Paleomagnetic study was undertaken to obtain information on the orientations of the eruptive and intrusive units and to employ the magnetic directions for an evaluation of the tectonic and igneous history of the caldera and enclosing volcanic field. Radiometric ages and the paleomagnetic data indicate that an episode of pronounced extension coincided with a major pulse of igneous activity in the region 23 m.y. ago. Eastward tilting of the volcanic units appears to have been closely followed by caldera collapse and then by resurgent doming of the caldera. Welded tuff units within the caldera were turned on end and together with older volcanic units were incorporated in a collapse megabreccia. The megabreccia and other nearby units were partially or entirely overprinted by a thermochemical aureole associated with the resurgent doming and attendant hydrothermal circulation. The stocks of the resurgent dome appear to have undergone no significant later tilting.