bluuebunny/crossref_metadata_2025 huggingface dataset:
Observational hints for a plume‐fed, suboceanic asthenosphere and its role in mantle convection
Authors: Phipps Morgan Jason , Morgan W. Jason , Zhang Yu‐Shen , Smith Walter H. F.
Published: February 2004

An asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection. In this study we discuss geochemical, seismic, and geoid/depth evidence for lateral flow within this type of asthenosphere. In particular, we suggest that there are large‐scale layered, horizontal flow structures that connect upward plume input beneath hotspots to near‐ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere. Lateral asthenosphere flow is also shaped by oceanic subduction zones, with a partial return flow from trenches, and by deep continental roots that are migrating barriers to asthenosphere flow. This alternative paradigm offers relatively simple explanations for several puzzles about mantle convection, for example, the low mantle heat flow beneath continents. It also offers an explanation for why mid‐ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large‐scale mantle flow, while in contrast, trenches appear to be strongly coupled to mantle‐thick regions of fast (colder) seismic velocity anomalies. We also discuss several implications of this paradigm that should be testable in future studies, such as the prediction of cogenetic off‐axis seamount volcanism that is created between an off‐axis hotspot and its neighboring ridge axis.
Assessment of the accuracy of daily UT1 determinations by very long baseline interferometry
Authors: Ray J. R. , Carter W. E. , Robertson D. S.
Published: February 2004

By comparing UT1 determinations from the International Radio Interferometric Surveying intensive program of quasi‐daily, single‐baseline, 1‐hour very long baseline interferometry (VLBI) measurement sessions with coincident, multibaseline 24‐hour VLBI determinations, we estimate the overall accuracy of the intensive UT1 results, averaged over 5.4 years of data, to be ∼52 μs, depending somewhat on the amount of error attributed to the multibaseline results. The largest error contributions are due to interpolated values for polar motion X and y coordinates (<38 μs), short‐term variability in atmospheric propagation (∼48 μs during summers but much less at other times), and extended brightness structures in the radio sources (∼22 μs). Because the atmospheric effect is most pronounced during summers, the UT1 accuracy of the intensives is ∼47 μs most of the year but degrades to ∼67 μs during the third quarter of each year.
Long‐period regional wave moment tensor inversion for earthquakes in the western United States
Authors: Ritsema Jeroen , Lay Thorne
Published: February 2004

Source parameters of moderate to large size (Mw > 4.5) earthquakes in the western United States from 1992 to 1994 are determined by point source moment tensor inversion of complete long‐period (T > 35–50 s) ground motions recorded at regional distances (1°–12°). Stable long‐period signals are obtained by low‐pass filtering records from the very broadband seismometers recently deployed in several networks in the western United States. These signals are dominated by fundamental mode Rayleigh and Love waves, which have very simple waveforms due to the limited dispersion on the short paths to regional stations. Since long‐period motions are relatively insensitive to the attenuation model and crustal structure used in the inversion, they provide robust constraints on the seismic moment and faulting geometry as long as adequate azimuthal coverage is available. Comparisons of solutions for 21 events with results of other regional and teleseismic wave inversions are made to assess the model dependence and uncertainties of our regional centroid moment tensor (RCMT) solutions. RCMT inversion has limited source depth resolution for shallow crustal events, but the focal mechanism and seismic moment determinations prove quite stable over a range of source depths in the crust, as well as over a range of crustal propagation models. Simultaneous waveform inversion of shorter‐period body wave signals can improve the source depth resolution. By applying path corrections for heterogeneous crustal structure, shorter‐period surface wave energy can also be inverted, allowing the methodology to be extended to lower‐magnitude regional events as well. The RCMT procedure requires minimal signal processing, only a sparse broadband network, and a simple laterally homogeneous propagation model; thus it can readily be automated and applied in near real time to events in the magnitude range from 4.5 to 7.5 distributed over an area as large as the western United States.
Study of the dynamical mechanism of seasonal variation of Earth's rotational velocity
Authors: Zhao Jinping , Qu Weizheng
Published: February 2004

In this study, the seasonal variation of Earth's rotational velocity is studied from a dynamical point of view. Many studies have been conducted in recent years by methods of correlation analysis, dynamical calculation, and numerical simulation. In this paper, we use an analytical method that has not been attempted in this study area to establish a model in which the angular momentum conservation, short‐wave and long‐wave radiation, the radiation transmitting process, and the anomaly of atmospheric circulation are considered. On the basis of the angular momentum conservation law of the Earth‐atmosphere system, we obtain an analytical solution for variation of Earth's angular velocity. Our model shows that the semiannual variation is mainly caused by solar short‐wave radiation and that the annual change is generated by short‐wave radiation from the Sun and long‐wave radiation from the atmospheric underlying surface. Our model also shows that long‐wave radiation from the atmosphere affects the nonuniformity of Earth's rotation by reducing amplitude and changing phase. The results suggest that the action of ocean for seasonal variation is unimportant, that the nonlinear action of zonal averaged field can be neglected, and that only a secondary part of atmospheric circulation anomaly is decisive for the seasonal variation. Our analytical solution depends on general astronomical parameters and therefore can be applied to other planets.
Geodetic determination of the kinematics of central Greece with respect to Europe: Implications for eastern Mediterranean tectonics
Authors: Le Pichon Xavier , Chamot‐Rooke Nicolas , Lallemant Siegfried , Noomen Ron , Veis George
Published: February 2004

We use a new satellite laser ranging/Global Positioning System (SLR/GPS) solution at seven sites in Anatolia and Aegea to obtain a better definition of the extrusion motion of the Anatolian‐Aegean block with respect to Europe. We show that this motion can be described in a first approximation by a counterclockwise rotation which transfers most of the motion of Arabia to Anatolia. We combine 78 displacement vectors obtained at common points of two triangulation nets measured in central Greece in 1895 and 1975 with the SLR/GPS measurements to compute the velocity field over Greece with respect to Europe. These measurements indicate that central Greece is a zone of extension between the Anatolian‐Aegean counterclockwise rotation to the south and the northern Greece clockwise rotation to the north. This extension is principally localized within the Gulf of Corinth to the east but is distributed to the west. We then extrapolate this velocity field to the whole Aegea and western Anatolia using recently published GPS results as well as the SLR results. The narrow dextral North Anatolian fault, which limits the velocity field to the north, progressively gives way to a much wider boundary zone where extension becomes dominant. We show that the collision between the Mediterranean Ridge and Africa began 3–6 Ma, and we describe the modifications that this collision has produced on the kinematic pattern both in Aegea and on the Mediterranean Ridge.
Spatial migration and compositional changes of Miocene‐Quaternary magmatism in the Western Grand Canyon
Authors: Wenrich Karen J. , Billingsley George H. , Blackerby Bruce A.
Published: February 2004

Lithospheric extension appears to be responsible for erosional and tectonic evolution of the western margin of the Colorado Plateau. Late Cenozoic (20 Ma to 10,000 years) basaltic flows, scoria cones, necks, dikes, and sills exposed in the Western Grand Canyon region offer a unique opportunity to place age constraints on the landscape evolution. New K‐Ar ages show that these basaltic rocks become progressively younger to the northeast from the Hualapai and Shivwits Plateaus onto the Uinkaret Plateau, a distance of about 100 km. Magmatism appears to have migrated at a rate of about 0.45 cm/yr until sometime between 13.5 and 9.2 Ma, when the rate changed to about 1.15 cm/yr. Recent regional geologic mapping indicates that extensional faulting, with ubiquitous downdrop to the west, accompanied this magmatism; the Hurricane Fault offsets lavas as young as 0.29 Ma. Young fault scarps in sedimentary rocks, and historic seismic activity along these faults, indicate that the region is still undergoing faulting. The basalts form three relatively distinct groups based on their age, chemistry, and geographic distribution: (1) 20–11 Ma, (2) 11–4.5 Ma, and (3) ≤4.5 Ma. The older basalts (>11 Ma) are petrographically and compositionally distinct from younger basalts; their olivine and augite are more Mg rich with lower TiO2 and higher Ni, Cr, and incompatible element contents. Group 1 basalts have more radiogenic Sr and less radiogenic Nd isotopic compositions than younger group 2 and 3 basalts. Overall, present‐day Sr isotopic compositions range from 0.70344 to 0.70505, 143Nd/144Nd from 0.51291 to 0.51227 (εNd = +5.4 to −7.1), and 206Pb/204Pb from 17.951 to 19.091. Group 1 basalts were probably derived from asthenospheric mantle melts that underwent crustal contamination. These basalts appear to have been generated originally from more primitive melts than groups 2 and 3, which were probably also derived from asthenospheric mantle‐derived magmas but underwent only minor crustal contaminations. Group 2 basalts appear to be transitional between groups 1 and 3. The intervals 13–9 Ma and 4.6–4.3 Ma temporally separate the three chemically distinct groups of basalts in the Western Grand Canyon. These periods correspond closely with major Miocene and Pliocene plate reorganizations in the western United States at about 12.5–10 Ma and 3.9–3.4 Ma. These changes in plate motion were reflected in changes in the rate of magmatism migration in the Western Grand Canyon. Recent movements on the Hurricane and Toroweap Faults appear directly related to the <4.5 Ma volcanism and are probably a result of recent plate reorganization.
Parkfield seismicity: Fluid‐driven?
Authors: Johnson P. A. , McEvilly T. V.
Published: February 2004

Characteristics of microearthquake occurrence along the locked‐to‐creeping transition of the San Andreas fault at Parkfield in central California are reviewed for evidence that fault zone fluids play a critical role in slip dynamics there. Previous studies at Parkfield have defined a low‐velocity, anisotropic, attenuating fault zone and a high Vp/Vs ratio in the nucleation zone of the repeating M 6 earthquakes. Most of the ongoing seismicity is organized into a temporally‐evolving checkerboard pattern of alternating high and low seismic slip regions divided on the fault zone at 3–6 km spacings. Much of the seismicity is further confined to a few hundred small (20–30 m radius) cells of densely clustered microearthquakes that exhibit periodic recurrence. Space‐time development of small (tens to hundreds of meters), transient (minutes to days) earthquake sequences reveals diffusivelike outward spreading along the fault zone. There is some evidence for anomalous source mechanisms in sequence‐initiating events, possibly indicative of hydraulic fracturing. The data are consistent with a model in which some microearthquake dusters and confined sequences occur by the cyclic pressurization of fluid within localized patches of the fault zone. The consequent modulation of the effective normal stress leads to fluid‐driven slip manifested both as the highly periodic earthquake clusters and as localized earthquake sequences observed at Parkfield.
Electrical conductivity and carbon in metamorphic rocks of the Yukon‐Tanana Terrane, Alaska
Authors: Mathez E. A. , Duba A. G. , Peach C. L. , Léger A. , Shankland T. J. , Plafker G.
Published: February 2004

Electrical conductivity of a water‐saturated quartz‐mica‐garnet‐schist, collected from a surface outcrop near the Denali Fault Zone in the Yukon‐Tanana terrane of east central Alaska, increases slightly with pressure to about 200 MPa. This behavior is unlike that exhibited by other Yukon‐Tanana samples or by most rocks from other locations. Detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 μm thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation. The stringer is probably responsible for the anomalous conductivity change with pressure, making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature. The carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast. The deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism. We interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer, possibly during the main phase of metamorphism and deformation, and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts. Together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth (i.e., >10 km). Brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer, explaining in part why the rock does not exhibit anomalously high conductivity at 0.1 MPa (1 atm) pressure. The brittle deformation is interpreted to have been caused by unloading due to uplift. The observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic, throughgoing fractures.
Coseismic and postseismic subsurface displacements and strains for a dip‐slip normal fault in a three‐layer elastic‐gravitational medium
Authors: Ma X. Q. , Kusznir N. J.
Published: February 2004

A three layer elastic‐gravitational fault displacement model has been developed using elastic dislocation and employed to examine the effects of rigidity layering, gravity, and stress relaxation on surface and subsurface displacements fields for a dip‐slip normal fault. Within our three layer model, layer 1 represents the upper crust, layer 2 the lower crust, and layer 3 the upper mantle. The fault is embedded in the upper crust. Horizontal as well as vertical displacement components have been determined. Displacement field changes due to postseismic stress relaxation have been calculated using both the correspondence principle and relaxed rigidity moduli, and results are in close agreement. The relaxed rigidity method provides an accurate and computationally efficient method of examining postseismic relaxation. Postseismic relaxation within the lower two layers only, gives surface uplift, increasing footwall uplift and decreasing hangingwall subsidence, and also increases tie wavelength of surface vertical displacement. Postseismic stress relaxation within all three layers (i.e., uniform half‐space) produces a shorter wavelength of surface vertical deformation with respect to the coseismic response. Moho topography created during coseismic deformation is initially amplified during postseismic relaxation. The relaxed Moho topography is dependent on the strength of the upper crust as well as the strength of the lower crust and mantle. Gravity has a significant influence on displacements only at the postseismic stage when the effective rigidity of the lower layers is small. Coseismic and postseismic normal strains associated with dip‐slip normal faulting have been examined. For a large normal basement fault intersecting the free surface, the coseismic horizontal surface strain, perpendicular to fault strike, is compressive adjacent to the fault and in the footwall, and tensile in the hangingwall away from the fault. Coseismic stress redistribution may generate significant tensile brittle failure of the upper crust adjacent to large basement faults.
Diffusion of hydrogen in diopside: Results of dehydration experiments
Authors: Ingrin Jannick , Hercule Sarah , Charton Thomas
Published: February 2004

The kinetics of hydrogen diffusion in diopside single crystals have been investigated through dehydration experiments performed in air between 700° and 1000°C. Samples oriented along [001], [100]*, and [010] crystallographic directions were cut in thin slices of less than 1 mm thickness and about 4 × 4 mm2 across. The decrease, with time of heating, of the OH− concentration was measured from the integral of the OH− absorption bands of infrared spectra taken on a few millimeter per millimeter area at the center of the sample slices. The data show no significant effect of crystallographic orientation on the rates of dehydration, suggesting that hydrogen diffusion in diopside is isotropic. The simultaneous fit of diffusion data measured in the three directions to an Arrhenius law follows: ln D = ln D0 ‐ exp[−136±27 kJ/mol/RT] with ln D0 = −14.5±2.8. The rates of dehydration of diopside are slower than the rates of dehydration of olivine. However, if olivine undergoes extensive dehydration during ascent within magma, dehydration of diopside may also be significant if we take into account the smaller grain size of this mineral, as observed in nodules.
U‐Pb geochronology of detrital zircon from Upper Jurassic synorogenic turbidites, Galice Formation, and related rocks, western Klamath Mountains: Correlation and Klamath Mountains provenance
Authors: Miller M. Meghan , Saleeby Jason B.
Published: February 2004

Synorogenic turbidites of the Upper Jurassic Galice Formation overlie a variety of basement terranes within the western Klamath Mountains along the Oregon‐California border, including the early Late Jurassic ophiolite assemblages, ensimatic arc deposits, and sedimentary terranes. U‐Pb analyses of 68 multiple grain fractions from 11 samples of detrital zircon support the correlation of Galice Formation on these various basement terranes, although some new complexities in provenance are revealed. With one exception, upper intercept ages range from 1509−3+3 to 1675−8+8 Ma. Least squares regression of all fractions yields an upper intercept age of 1583±1 Ma, indicating the importance of an ultimately continental, recycled, and generally well‐mixed sedimentary source. Early Mesozoic lower intercept ages range between 183−2+2 and 263−3+4 and average 215±1 Ma. Results from Galice cover on sedimentary basement show significantly older 2.1 Ga Precambrian component, however, that may be locally derived from pre‐Late Jurassic basement rocks that are rich in recycled sedimentary debris. Existing isotopic data from older, zircon‐bearing Klamath units further indicate that Galice detritus was derived from immediate source terranes within the Klamath Mountains. Reworking of fragile limestone clasts from the biogeographically distinctive eastern Klamath terrane (McCloud Limestone) into Galice Formation substrate also supports early paleogeographic ties between terranes. Thus the tectonic setting of the Late Jurassic Nevadan orogeny in the Klamath Mountains is tightly constrained by original paleogeographic ties between subterranes of the western belt and by provenance ties to terranes to the east. Ultimately continent‐derived clastic debris and other distinctive tracers were recycled within this long‐lived ensimatic convergent margin system.
Paleomagnetic and structural evidence for Neogene block rotations in the Central Apennines, Italy
Authors: Mattei M. , Funiciello R. , Kissel C.
Published: February 2004

Magnetic fabric and paleomagnetic analyses on sedimentary sequences in several structural units in the Central Apennines ranging from upper Cretaceous pelagic carbonates to upper Miocene turbidites provide evidence for at least two distinct phases of rotation. The first counterclockwise (CCW) rotation is well recorded in Eocene‐Oligocene sediments and does not affect the lower‐middle Miocene sequences. The second phase is clearly recorded in lower‐middle and upper Miocene sediments and shows both CCW and clockwise (CW) rotations in concordance with trends of the different thrust structures. We detected three domains characterized by different rotation patterns. The westernmost is the southern Sabina area, an arcuate thrust belt which underwent a CW rotation after the early Miocene. The second domain is the Latium‐Abruzzi carbonate platform, where a CCW rotation occurred after the late Miocene. The third is the Marsica area that underwent a CW rotation after the late Miocene. The magnetic fabric analyses confirm the existence of these different structural provinces and show a strong correlation between the present‐day strike of the anisotropy of magnetic susceptibility magnetic lineation and the rotations recorded in Miocene sediments. We attribute present‐day variations in orientation of the compressional structures in the Central Apennines to differential rotations about vertical axes due to strike‐slip faults and out of sequence thrust activity, rather than to a change in the stress field orientation. We also suggest that this deformation mechanism could be common in other collisional chains around the world.
Rate laws for water‐assisted compaction and stress‐induced water‐rock interaction in sandstones
Authors: Dewers Thomas , Hajash Andrew
Published: February 2004

Mineral‐water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones, phase relations in metamorphic rocks, mass redistribution and replacement reactions during diagenesis, and loss of porosity in deep sedimentary basins. As a step toward understanding the fundamental geochemical processes involved, using naturally rounded St. Peter sand, we have investigated the kinetics of pore volume loss and quartz‐water reactions under nonhydrostatic, hydrothermal conditions in flow‐through reactors. Rate laws for creep and mineral‐water reaction are derived from the time rate of change of pore volume, sand‐water dissolution kinetics, and (flow rate independent) steady state silica concentrations, and reveal functional dependencies of rates on grain size, volume strain, temperature, effective pressure (confining minus pore pressure), and specific surface areas. Together the mechanical and chemical rate laws form a self‐consistent model for coupled deformation and water‐rock interaction of porous sands under nonhydrostatic conditions. Microstructural evidence shows a progressive widening of nominally circular and nominally flat grain‐grain contacts with increasing strain or, equivalently, porosity loss, and small quartz overgrowths occurring at grain contact peripheries. The mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts (termed pressure solution or solution transfer), with a lesser component of time‐dependent crack growth and healing. The magnitude of a stress‐dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy‐induced supersaturations, favoring instead the normal stress dependence of molar Gibbs free energy associated with grain‐grain interfaces.
Anisotropic seismic scattering in old Atlantic crust
Authors: Dougherty M. E. , Vincent R. J. , Swift S. A. , Stephen R. A.
Published: February 2004

An oblique seismic experiment (OSE) was carried out at Ocean Drilling Program (ODP) Hole 418A in the western North Atlantic to investigate vertical, lateral, and azimuthal structural variations in the upper portions of relatively old ocean crust. Original analysis of the data for travel time anisotropy did not show systematic azimuthal trends in seismic structure except at shallow depths immediately adjacent to the borehole. Subsequent analysis of the OSE data, presented here, reveals a correlation between scattering strengths versus azimuth and spreading direction of the crust. Analyses are performed on body and coda waves and include coda energy and normalized coda energy calculations, spectral analysis, coda dropoff rates, and cross‐correlation analysis. All of the analyses show trends indicating an increase in seismic scattering along directions parallel to the paleospreading direction. These results suggest that evidence of crustal formation processes is still present and seismically detectable after 108 m.y. of crustal evolution. Scattering is likely caused by crustal cracks and/or aligned heterogeneities indicating that either large crustal cracks are still present in the crust or that homogenization of the crust through hydrothermal deposition of secondary minerals is not complete at this site.
Dynamical influences of high viscosity in the lower mantle induced by the steep melting curve of perovskite: Effects of curvature and time dependence
Authors: van Keken Peter E. , Yuen David A.
Published: February 2004

Recent experiments on the melting temperatures of perovskite have indicated a high melting temperature in the lower mantle. This suggests that a creep law with an activation enthalpy, that increases strongly with depth, should be employed for the lower mantle rheology. We have examined the dynamical consequences of employing an Arrhenius type of dependence in a Newtonian flow law under the Weertman assumption relating activation enthalpy to the variation of the melting temperature with pressure. We have employed finite element techniques to model both steady state and time‐dependent flows for such type of rheology in an axisymmetric spherical‐ shell model. An outstanding dynamical feature from these models is the presence of a local viscosity maximum with a magnitude of around 1023Pa · s for Earthlike surface Nusselt numbers between 10 and 15. This viscosity maximum is found in the middle of the lower mantle and is a site of high deviatoric stresses between 10 and 100 MPa. For the surface Rayleigh numbers examined (104 to 105) the flows developed are not strongly time‐dependent and often tend to a steady state. In spite of the presence of internal heating with chondritic strength, a few large, relatively stationary plumes are found in the lower mantle, while the rest of the lower mantle circulation is being driven by the more vigorous upper mantle flow. There are no small‐scale instabilities developed in the D” layer of these models, thus suggesting that any small‐scale lateral heterogeneities existing there may have chemical origins.
Sodium partitioning between clinopyroxene and silicate melts
Authors: Blundy J. D. , Falloon T. J. , Wood B. J. , Dalton J. A.
Published: February 2004

We present a quantitative model for Na partitioning between clinopyroxene and silicate melt, applicable in the range 0.1 MPa to 6.0 GPa, 1000–2000°C over a wide span of composition. Our model is derived from the Jadeite melting equilibrium, via the relationship ΔGƒ(P,T) = −RTlnKƒ = RTlnD*Na where ΔGƒ(P,T) is the Gibbs free energy of fusion of Jadeite at the pressure (P) and temperature (T) of interest, T is in kelvins, R is the gas constant, Kƒ is the equilibrium constant for the the melting reaction and D*Na is a molar partition coefficient, defined here as the molar ratio of Na in crystal to Na in melt on a six‐oxygen basis. An expression for ΔGƒ(P,T) is obtained from published experimental data on the Jadeite melting curve from 2.8 to 16.5 GPa, combined with available (or estimated) thermochemical data for jadeite crystal and Jadeite melt. The model is tested against: (1) new experimental data in the system diopside‐albite from 0.1 MPa to 6.0 GPa; (2) new analyses of clinopyroxene and glass from the mid‐ocean ridge basalt‐pyrolite sandwich experiments of Falloon and Green (1988); and (3) published experimental data from natural and synthetic systems. The model is in good agreement with the experimental data over the entire P–T range investigated. The only systematic deviations occur in oxidizing systems (ƒO2 > FMQ + 1), due to stabilization of the NaFe3 + Si2O6 (acmite) component in clinopyroxene, and on the diopside‐albite join at 0.1 MPa. The latter can be quantitatively ascribed to nonideality of melts on the diopside‐silica join in the system CaMgSi2O6‐NaAlSi2O6‐Si3O6. The activity‐composition relationships adopted for crystal and melt do not require prior knowledge of the aluminum content of either phase. As D*Na is typically within 3% relative of the weight partition coefficient (DNa) our thermodynamic model provides a quantitative description of Na partitioning between clinopyroxene and melt in the upper mantle.
Tomographic images of the Iberian subcrustal lithosphère and asthenosphere
Authors: Corchete V. , Badal J. , Serón F. J. , Soria A.
Published: February 2004

The S wave velocity structure of the lithopheric mantle and asthenosphere beneath Iberia is displayed by means of tomographic images obtained from dispersion data of Rayleigh waves propagating across the Iberian region. We have used long‐period data recorded at the broadband stations of the Network of Autonomously Recording Seismographs (NARS) installed in the Iberian Peninsula on the occasion of the Iberian Lithosphere Heterogeneity and Anisotropy (ILIHA) project. A total of 64 teleseismic events provided by the ILIHA array and 143 seismic paths have been studied. Surface wave dispersion analysis is carried out using various methods: from methods for a correct acquisition of data and subsequent two‐station surface wave velocity measurements to inversion methods for velocity structure and methods for verifying the reliability of the inversion results. The phase and group velocity dispersion curves of the fundamental mode Rayleigh waves are the basic information from which we have obtained several Earth models represented by shear wave velocity distributions with depth. Using these inversion results, we display the most conspicuous features of the velocity stnicture of the Iberian lithosphere‐asthenosphere system and propose a new regional model, the Iberian Lithosphere‐Asthenosphere (ILA) model, for the deep structure of the Iberian Peninsula down to a depth of 200 km. We use a representation technique based on an iterative Laplacian interpolation method for obtaining tomographic images of the subcrustal lithosphere and asthenosphere of Iberia. We find significant lateral velocity variations in the peninsula, though these differences vary with depth interval. A low‐velocity channel in the lithosphere (41–51 km) appears with nonuniform velocity structure. In contrast, at the greatest lithospheric depths (51–81 km), almost the entire peninsula shows a rather uniform velocity structure. The asthenosphere (81–181 km) is clearly a nonhomogeneous gross layer both laterally and with depth. The relatively higher velocities span the shallower depths within the asthenosphere, whereas the lower ones span the deeper part.
Late Triassic‐earliest Jurassic geomagnetic polarity sequence and paleolatitudes from drill cores in the Newark rift basin, eastern North America
Authors: Kent D. V. , Olsen P. E. , Witte W. K.
Published: February 2004

Paleomagnetic study of about 2400 samples from nearly 7 km of core recovered at seven drill sites in the Newark continental rift basin of eastern North America provides a detailed history of geomagnetic reversals and paleolatitudinal motion for about 30 m.y. of the Late Triassic and earliest Jurassic (Carnian to Hettangian). Northward drift of only about 7° is recorded in the continental sediments and minor interbedded basaltic lavas in the basin, from 2.5° to 6.5° north paleolatitude in the Carnian and from 6.5° to 9.5° north paleolatitude over the Norian‐“Rhaetian” and the early Hettangian. A total of 59 polarity intervals, ranging from about 4 m to over 300 m in thickness, have been delineated in a composite stratigraphic section of 4660 m. The lateral continuity and consistent relationship of lithological lake level cycles and magnetozones in the stratigraphically overlapping sections of the drill cores demonstrate their validity as time markers. A geomagnetic polarity timescale was constructed by scaling the composite section assuming that lithostratigraphic members in the predominant lacustrine facies represent the 413‐kyr orbital periodicity of Milankovitch climate change and by extrapolating a sedimentation rate for the fluvial facies in the lower part of the section; a 202 Ma age for the palynological Triassic/Jurassic boundary was used to anchor the chronology based on published concordant radiometric dates linked to the earliest Jurassic igneous extrusive zone. Geomagnetic polarity intervals range from about 0.03 to 2 m.y., have a mean duration of about 0.5 m.y., and show no significant polarity bias. The cyclostratigraphically calibrated record provides a reference section for the history of Late Triassic‐earliest Jurassic geomagnetic reversals. Correlations are attempted with available magnetostratigraphies from nonmarine sediments from the Chinle Group of the southwestern United States and marine limestones from Turkey.
Climate friction: A possible cause for secular drift of Earth's obliquity
Authors: Ito Takashi , Masuda Kooiti , Hamano Yozo , Matsui Takafumi
Published: February 2004

We investigated one of the mechanisms of secular drift of Earth's obliquity, called “climate friction,” due to the feedback from the periodic variation of solar insolation and the glacial cycles. We estimated the change of the moment of inertia by utilizing Maxwell‐type viscous Earth models and assumed linearity between insolation variation and ice sheet variation. We reproduced the feedback loop of insolation variation → ice sheet variation → change of the moment of inertia → change of the motion of the rotational axis of Earth, including the standard theoretical results of the quasi‐periodic planetary perturbation. Our simulation resulted in a secular increasing drift of Earth's obliquity, which is induced by the time lag of ice sheet formation with respect to the solar insolation variation, and time lag of viscous deformation of the solid Earth with respect to the formation of ice sheets. We have estimated a secular increase of the obliquity 0.4°/10 m.y., which depends on the viscosity of the lower mantle and the degree of ice sheet development.
Global vector and scalar Magsat magnetic anomaly maps
Authors: Ravat D. , Langel R. A. , Purucker M. , Arkani‐Hamed J. , Alsdorf D. E.
Published: February 2004

Empirical and analytical techniques for modeling ionospheric fields in Magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies. This task has been accomplished for equatorial ΔX, ΔZ, and ΔB component and polar ΔZ and ΔB component measurements. The techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data‐processing techniques developed during the last decade. Data‐processing techniques include retention of common signal in a correlation analysis of adjacent passes; analysis of field differences between dawn and dusk data at points where their orbits cross; and retention of common signal in a covariant spherical harmonic analysis procedure. Results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere (vector components as well as scalar).
Holocene slip rate of the Hayward Fault at Union City, California
Authors: Lienkaemper James J. , Borchardt Glenn
Published: February 2004

Measured offsets of well‐dated alluvial fan deposits near the Masonic Home in Union City constrain Holocene slip rate of the Hayward fault between 7 and 9 mm/yr. Our best minimum geologic slip rate over the past 8.27 ± 0.08 kyr (i.e., 8270 years) is 8.0 ± 0.7 mm/yr. A steep stream (its channel cut into bedrock) flows southwest out of the East Bay Hills, crosses the fault, and deposits its load on an alluvial fan. We cut two 5‐m‐deep, fault‐parallel trenches 20–30 m southwest of the main fault through the crest of the fan. Walls of the trenches reveal a series of nested distributary channel fills. These channels had cut into old surfaces that are indicated by paleosols developed on flood silts. We distinguished many channel fills by their shape, clast size, flow direction, elevation, and relation to paleosols, enabling us to correlate them between both trenches. Two distinct episodes of fan deposition occurred during the Holocene. Reconstructing the apex positions of these fan units indicates that about 42 ± 6 m and 66 ± 6 m of fault slip has occurred since their inceptions at about 4.58 ± 0.05 ka, and 8.27 ± 0.05 ka, respectively. We lowered the age and age uncertainty of the younger unit from earlier reports based on new multiple radiocarbon dates. The 4.58 ka slip rate of 9.2 ± 1.3 mm/yr is not significantly different at 95% confidence from the 8.27 ka slip rate of 8.0 ± 0.7 mm/yr. Because current regional strain rates are fully consistent with Neogene plate tectonic rates (Lisowski et al., 1991) and the historic surface rate of creep in Union City is only 4.7 ± 0.1 mm/yr (Galehouse, 1994), the larger, ≥8 mm/yr, Holocene slip rate implies that strain is now accumulating on a locked zone at depth. The 8 mm/yr rate is probably minimal because earlier trenching evidence nearby implies that some unknown additional amount of fault deformation occurs outside of the narrow fault zone assumed in measuring slip. Lienkaemper et al. (1991) suggest that the fast creep rate of 9 mm/yr, measured near the southern end of the Hayward fault, may underestimate the deep slip rate, because 1868 surface slip occurred there in addition to the continuing fast creep. If the historic deep slip rate equals the long‐term rate, then the 9 mm/yr creep rate reflects the minimum seismic loading rate of the Hayward fault better than the ≥8 mm/yr Holocene rates do.
Synthetic seismicity models of multiple interacting faults
Authors: Robinson Russell , Benites Rafael
Published: February 2004

A synthetic seismicity model for multiple, interacting faults, of any strike and dip in a three‐dimensional elastic half‐space has been developed. The ultimate purpose is to determine how hazard is modified by elastic interactions of events in complex fault systems and to examine the synthetic seismicity for unusual behavior before large events that might be observable in the real world. Each fault is subdivided into some number of equal‐sized patches, and a coefficient of static friction, random within a specified range, is assigned to each. The patch static strength is then the product of friction and differential normal stress. Stress (both shear and normal) accumulates due to some predefined tectonic driving force until one or more patches fail and slip instantaneously. The slip can be in any direction within the fault plane and its magnitude is determined by a specified fractional stress drop. This initial slip induces shear stress and strength changes on all other patches and faults and perhaps results in other patches failing. Patches can slip more than once; once having failed, their strength is reduced, resulting in an overshoot that approximates the effect of dynamic stress enhancement. An event is over when all patches are stable. Although present computational constraints do not allow models with more than about 1500 patches, or a large number of faults, experiments with simple models of two parallel strike‐slip faults (25 km length, 15 km depth, 694 m × 714 m patch size), driven at different rates and separated by 10 or 15 km, give some indicative results. A range of friction between 0.1 and 0.2, with a stress drop of 10% (about 3 MPa or 30 bars), produces a b value (500,000 events) close to 1. The models generate a distinct class of large “characteristic” events that rupture the full fault plane. The distribution of interevent times for the background seismicity is very similar to that for a Poisson process, while the characteristic events are quasiperiodic. Nearly half the larger events directly induce some activity on the other fault. The effect of a characteristic event on the occurrence of another characteristic event on the opposite fault is small and depends on the separation: at 10 km distance other events are retarded while at 15 km distance they are advanced. At 10‐km separation the probability of a pair of characteristic events within 1 year is reduced to 0.3% from 1.7% for the case with no interaction. Consideration of interactions between slip on the faults and the driving mechanism, as well as higher stress drops and more overshoot, would increase the magnitude of the interaction effects.
Petrography and mineral compositions of eclogites from the Koidu Kimberlite Complex, Sierra Leone
Authors: Fung Agnes T. , Haggerty Stephen E.
Published: February 2004

The origins of eclogite (clinopyroxene + garnet) and the relative proportions of eclogite in the upper mantle are issues of considerable uncertainty and debate that bear upon the chemical and physical dynamics of petrogenesis, recycling, and remote sensing interpretation. Forty‐one upper mantle eclogites from the Koidu Kimberlite Complex, Sierra Leone, were selected for detailed petrographic and chemical examination to bolster an earlier database with a view to the identification of protoliths and possible source regions of eclogite origin. On the basis of MgO contents in garnets, eclogites are divided into a high‐MgO suite and a low‐MgO suite. High‐MgO eclogites contain pyrope (16.5–20.2 wt % MgO), with an average garnet composition of Pyr65Alm20Gross15. Almandine and grossular (5.3–13.2 wt % MgO) are present in low‐MgO eclogites and vary from Pyr20Alm60Gross20 to Pyr35Alm30Gross35. Pyroxenes in high‐MgO eclogites are diopsidic (Jd11Di89‐Jd26Di74); those in low‐MgO eclogites range from jadeitic‐diopside to omphacite (Jd20Di80‐Jd48Di52). Oriented apatite crystals in garnet and clinopyroxene are interpreted to be products of exsolution and, coupled with coexisting rutile, imply that garnet is a major repository for P, Cl, F, OH, and Ti in the upper mantle. Reconstructed bulk compositions of high‐(15.0–18.9 wt %) and low‐MgO (7.1–12.2 wt %) eclogites are distinct, and major elements in these xenoliths broadly resemble basalts, picrites, and komatiites. Most high‐MgO eclogites equilibrated at 1080°C at 4.7 GPa to 1130°C at 5.2 GPa, whereas most low‐MgO eclogites cluster at 880°C at 3.3 GPa to 930°C at 3.8 GPa. Estimated PT and depths of origin of the Koidu eclogites imply that high‐MgO eclogites are asthenospheric, low‐MgO eclogites are lithospheric, and both are likely products of plume activity. Diamondiferous eclogites, worldwide, have characteristic Na, K, Ti, and IVAl (Si) in garnet‐clinopyroxene pairs that point to distinctive source regions and petrogenesis; these chemical variables may be applied to diamond exploration and evaluation.
Strain accumulation across the central Nevada seismic zone, 1973–1994
Authors: Savage J. C. , Lisowski M. , Svarc J. L. , Gross W. K.
Published: February 2004

Five trilateration networks extending for 280 km along the central Nevada seismic zone (1915 Pleasant Valley, M = 7.3; 1954 Dixie Valley, M = 6.8; 1954 Stillwater, M = 6.8; 1954 Rainbow Mountain, M = 6.6; 1954 Fairview Peak, M = 7.1; and 1932 Cedar Mountain, M = 7.2) have been surveyed 6 times since 1973 to determine deformation along the zone. Within the precision of measurement the deformation appears uniform along the zone and is described by the principal strain rates 0.036±0.008 μstrain/yr N60°W±3° and −0.031±0.008 μstrain/yr N30°E±3°, extension reckoned positive. The observed strain rates are consistent with simple, right‐lateral, tensor shear at the rate of 0.033 μstrain/yr across a shear zone striking N15°W. This central Nevada shear zone appears to be the northward continuation of the eastern California shear zone. The orientation of the strike‐slip and normal‐slip ruptures within the central Nevada seismic zone are consistent with principal stress axes parallel to the measured principal strain rate axes. Space‐based geodetic measurements (very long baseline interferometry) indicate that the relative motion accommodated across the Basin and Range province west of Ely, Nevada, is about 9.1±1.5 mm/yr N16°W±8° (Dixon et al., 1995.) Notice that the right‐lateral shear zone postulated to explain deformation in the central Nevada seismic zone is properly oriented to accommodate that relative motion. However, a 135‐km effective width of the shear zone would be required to accommodate all of the 9.1 mm/yr relative motion at the strain rates observed in the Nevada seismic zone; only about 3 mm/yr of that relative motion is accommodated within the span of the trilateration networks.
A preliminary thermal budget for lava tubes on the Earth and planets
Authors: Keszthelyi Laszlo
Published: February 2004

Lava tubes are a very common and important feature in mafic lava flows. The insulation provided by lava tubes allows molten lava to travel large distances from the vent with little cooling. This paper presents the first attempt to quantify the processes that control this cooling. The resulting thermal budget balances heat loss by (1) conduction, (2) convection of air in the wall rocks, (3) vaporization of rainwater, and (4) radiation out of skylights against (1) viscous dissipation, (2) latent heat released during crystallization, and (3) the cooling of the lava. When applied to the Waha'ula tube on Kilauea Volcano, Hawaii, this thermal budget reproduces the observed ∼1°C/km cooling of the lava inside the active tube. The thermal budget is also used to compare the insulating ability of hypothetical lava tubes in a continental flood basalt setting, on the ocean floor, Venus, the Moon, and Mars. This analysis demonstrates the large effect of rainfall and atmospheric convection, the importance of the volumetric flux of lava through the tube, and the overwhelming importance of compositional (i.e., rheological) differences. This work suggests that basaltic tube‐fed flows several hundred kilometers long can be produced by eruptions with effusion rates of only a few tens of cubic meters per second. Thus even the longest lava flows observed in our solar system could have been produced by low to moderate effusion rate eruptions, if they were tube‐fed.