bluuebunny/crossref_metadata_2025 huggingface dataset:
Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source
Authors: Sarda Philippe , Moreira Manuel , Staudacher Thomas , Schilling Jean‐Guy , Allègre Claude J.
Published: September 2002

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid‐ocean ridge basalt glasses from the Mid‐Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid‐Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal‐related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along‐strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal‐type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.
Statistical features of Earth's continuous free oscillations
Authors: Nishida Kiwamu , Kobayashi Naoki
Published: September 2002

Ensemble averages of spectra on seismically quiet days exhibit fundamental spheroidal modes of Earth's free oscillations from 3 to 7 mHz at 14 stations. We obtain acceleration amplitudes for these modes of ∼0.4 ngal (1 ngal = 10−11 ms−2) by fitting a spectral model to the ensemble averages of the spectra. The following three statistical features are characteristic to these spectra: (1) standard deviations of the spectra show that the amplitude of each mode fluctuates with time, (2) total modal signal power of the spectra shows that the oscillations are excited continuously, and (3) cross‐correlation coefficients between the modal amplitudes show that the modes do not correlate even with adjacent modes. These features suggest a random excitation mechanism. Assuming random excitation, we estimate the correlation length of the source to be shorter than 600 km, the source area to extend over the whole Earth's surface, and the repeat time of excitation to be shorter than the damping time of mode. Thus the sources must be incessant random disturbances on the whole surface of the Earth. On the basis of these results, four possible mechanisms are discussed. Atmospheric turbulent motions are efficient in exciting the oscillations and explains the statistical features, whereas the efficiencies of processes in the ocean remain questionable. The cumulative effects of earthquakes with seismic moments smaller than 1017 Nm cannot explain the observed amplitudes because the observed amplitudes at 3 mHz would require a moment release rate of 1019 Nm d−1. We also argue that slow earthquakes cannot explain the stochastic properties of the source.
El Niño impact on atmospheric polar motion excitation
Authors: Kolaczek B. , Nuzhdina M. , Nastula J. , Kosek W.
Published: September 2002

We study the influences of El Niño phenomena on correlations between short‐period variations of atmospheric and geodetic excitation functions of polar motion during 1980–1998. The correlation between these two excitation functions varies with time, with correlation coefficients computed for 1 year spans reaching maxima of 0.7–0.8. Series of these correlation coefficients themselves correlate with El Niño and Southern Oscillation Index (SOI) data, indicating the influence of El Niño and SOI on atmospheric and geodetic excitation functions of polar motion and on polar motion itself. The maximum correlation coefficients for Niño 4 sea surface temperature data is equal to 0.8 in the case of El Niño events in 1983 and 1987. Impacts of El Niño on polar motion have an impulsive character causing irregular variations of polar motion during the El Niño epochs.
Models for convection in thawing porous media in support for the subsea permafrost equations
Authors: Hutter Kolumban , Straughan Brian
Published: September 2002

When permafrost becomes submerged because of shore line erosion, the covering ocean acts as a thermal insulator, and the submerged permafrost starts to melt. The thawed layer is bounded above by the ocean bed through which salt may intrude and by the phase boundary which for a fixed offshore position is known to progress with the square root of time. This situation gives rise to nonsteady double‐diffusion coupling Bénard convection and liquefaction which can be described by the Darcy‐Oberbeck‐Boussinesq equations. The boundary value problem is formulated, and scalings are introduced which orient themselves on the relative magnitudes of phase boundary and convective bulk velocities of the salt convective regime identified by Harrison [1982]. The multiscale perturbation analysis that is introduced not only verifies the observed thaw rates with a parabolic‐in‐time phase boundary retreat, it equally automatically generates the equations for the corresponding perturbative equations such as the double‐diffusion Bénard problem, the associated eigenvalue problem, its corrections and possible convective flows induced by the various possible currents induced by the ocean circulation overlying the thawed permafrost layer. The demonstration of this systematic approach is the main purpose of this paper.
Applicability of higher‐order ray theory for S wave propagation in inhomogeneous weakly anisotropic elastic media
Authors: Vavryčuk Václav
Published: September 2002

Modeling of S waves propagating in inhomogeneous, weakly anisotropic, elastic media creates complications for the zeroth‐order ray theory. These complications are caused mainly by the so‐called S wave coupling. If the S wave coupling is significant in the wave field, then the zeroth‐order ray theory is inapplicable. In this case, modifications of the zeroth‐order ray theory such as the coupling ray theory or the quasi‐isotropic ray theory have so far been used to reproduce the S waves correctly. We show that the failure of the zeroth‐order ray theory can also be overcome by higher‐order ray theory. If we consider not only the zeroth‐order term of the ray series but also higher‐order terms, we can obtain correct results. On a simple example of a plane S wave propagating in a weak transversely isotropic medium with a rotating axis of symmetry, we study how the accuracy of higher‐order ray theory depends on the number of higher‐order ray approximations considered in the solution.
Moho depth variation in southern California from teleseismic receiver functions
Authors: Zhu Lupei , Kanamori Hiroo
Published: September 2002

The number of broadband three‐component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the crustal thicknesses and Vp/Vs ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P‐to‐S converted phase trades off strongly with the crustal Vp/Vs ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H‐Vp/Vs domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and Vp/Vs ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average Vp/Vs ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.
Basal mechanics of Ice Stream B, west Antarctica: 1. Till mechanics
Authors: Tulaczyk Slawek , Kamb W. Barclay , Engelhardt Hermann F.
Published: September 2002

Data from laboratory geotechnical tests on till recovered from beneath Ice Stream B, West Antarctica, at the Upstream B camp (hereinafter the UpB till) show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate. These data support use of a Coulomb‐plastic rheology in modeling of ice stream behavior and subglacial till deformation. Our testing program combined triaxial, ring shear, and confined uniaxial tests to investigate till strength and compressibility. Results show that the UpB till follows closely Coulomb's equation in which shear strength is a linear function of normal effective stress (apparent cohesion near zero and internal friction angle ϕ equal to 24°). Till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress. In general, the behavior of the UpB till is consistent with other experimental evidence regarding mechanical behavior of granular materials. Based on our laboratory results we formulate the Compressible‐Coulomb‐Plastic till model in which there are three interrelated, primary state variables: shear strength, void ratio, and normal effective stress. This model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings. These simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics: (1) occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records, and (2) distributed till deformation to depths of 0.1–1.0 m beneath the ice base. Our laboratory and modeling results substantiate application of the Compressible‐Coulomb‐Plastic model in simulations of the motion of Ice Stream B over its weak till bed.
Magnetism of oriented single crystals of hemoilmenite with self‐reversed thermoremanent magnetization
Authors: Goguitchaichvili Avto , Prévot Michel
Published: September 2002

Ten large, single crystals of hemoilmenite (yFeTiO3 (1‐y)Fe2O3) with y close to 0.54 were extracted from the self‐reversed Pinatubo dacitic pumice erupted in 1991 and oriented with respect to the c axis of the hexagonal structure by means of an X‐ray diffractometer. Hysteresis measurements show that c is the hardest magnetization axis while the softest axes lie in the basal plane. The hysteresis curves observed at room temperature in this plane suggest that in addition to the multidomain (MD) regions commonly seen under the microscope using the Bitter technique, there are also crystal regions which behave like single‐domain (SD) or pseudosingle‐domain (PSD) particles. As could be expected from the hysteresis measurements, the directions of natural rémanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM), and thermoremanent magnetization (TRM) all lie in the basal plane, regardless of the direction of the applied field. Thus the antiferromagnetically coupled spins of hemoilmenite of intermediate composition lie in the basal plane. The partial TRMs (pTRMs) are approximately reversed; that is, their direction is always more than 90° away from the applied field direction and most often close to 180°. In fact, pTRM directions are discretely distributed along three directions in the basal plane which are 60° away from each other. This distribution indicates that the magnetic anisotropy is of magnetocrystalline origin. Both NRM and TRM are exceptionally resistant to alternating field (AF) with no decrease in remanence intensity up to 30 mT at least. This behavior, together with the fact that the Lowrie‐Fuller test is SD‐type, indicates that natural and laboratory TRMs are carried by non‐interacting single‐domain regions. We propose a self‐reversal model based on negative exchange interactions in which the self‐reversed TRM is carried by cation‐ordered (ferrimagnetic) SD‐like regions dispersed within a cation‐disordered (antiferromagnetic with weak ferromagnetism) MD matrix carrying a weak, normal TRM.
Precipitation of quartz during high‐temperature, fracture‐controlled hydrothermal upflow at ocean ridges: Equilibrium versus linear kinetics
Authors: Martin Jeffrey T. , Lowell Robert P.
Published: September 2002

We have addressed quartz precipitation resulting from cooling of silica‐saturated, high‐temperature hydrothermal solutions ascending through crack‐controlled permeable regions beneath an ocean ridge crest. The results of this work indicate that for systems with high initial permeabilities, such as black‐smoker‐like systems, fluid channels can exist for several decades without serious sealing as a result of quartz precipitation, provided the kinetics of silica precipitation are considered. In lower‐permeability systems, even though kinetics of precipitation remain important, reduction of permeability as a result of quartz precipitation can occur on a time‐scale of a decade, leading to a decline in heat output and temperature of fluid as it encounters lower‐temperature regions. In addition, we show that kinetics considerations result in the preferential sealing of thin cracks. This result is consistent with seismic velocity data, which suggest that the increase in seismic velocity with depth and age in the oceanic crust may result from the preferential closing of thinner cracks. These results also suggest that precipitation of quartz should occur near the dike/lava transition zone or in mineralized zones where large temperature gradients may exist as a result of mixing between cool seawater and hot hydrothermal fluids. The results of this model are thus consistent with the observed increase in quartz veins in such regions beneath the TAG mound and in ophiolites.
Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite
Authors: Farley K. A.
Published: September 2002

High‐precision stepped‐heating experiments were performed to better characterize helium diffusion from apatite using Durango fluorapatite as a model system. At temperatures below 265°C, helium diffusion from this apatite is a simple, thermally activated process that is independent of the cumulative fraction of helium released and also of the heating schedule used. Across a factor of ∼4 in grain size, helium diffusivity scales with the inverse square of grain radius, implying that the physical grain is the diffusion domain. Measurements on crystallographically oriented thick sections indicate that helium diffusivity in Durango apatite is nearly isotropic. The best estimate of the activation energy for He diffusion from this apatite is Ea = 33±0.5 kcal/mol, with log(D0) = 1.5±0.6 cm2/s. The implied He closure temperature for a grain of 100 μm radius is 68°C assuming a 10°C/Myr cooling rate; this figure varies by ±5°C for grains ranging from 50 to 150 μm radius. When this apatite is heated to temperatures from 265 to 400°C, a progressive and irreversible change in He diffusion behavior occurs: Both the activation energy and frequency factor are reduced. This transition in behavior coincides closely with progressive annealing of radiation damage in Durango apatite, suggesting that defects and defect annealing play a role in the diffusivity of helium through apatite.
Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus
Authors: McClusky S. , Balassanian S. , Barka A. , Demir C. , Ergintav S. , Georgiev I. , Gurkan O. , Hamburger M. , Hurst K. , Kahle H. , Kastens K. , Kekelidze G. , King R. , Kotzev V. , Lenk O. , Mahmoud S. , Mishin A. , Nadariya M. , Ouzounis A. , Paradissis D. , Peter Y. , Prilepin M. , Reilinger R. , Sanli I. , Seeger H. , Tealeb A. , Toksöz M. N. , Veis G.
Published: September 2002

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1997 at 189 sites extending east‐west from the Caucasus mountains to the Adriatic Sea and north‐south from the southern edge of the Eurasian plate to the northern edge of the African plate. Sites on the northern Arabian platform move 18±2 mm/yr at N25°±5°W relative to Eurasia, less than the NUVEL‐1A circuit closure rate (25±1 mm/yr at N21°±7°W). Preliminary motion estimates (1994–1997) for stations located in Egypt on the northeastern part of Africa show northward motion at 5–6±2 mm/yr, also slower than NUVEL‐IA estimates (10±1 mm/yr at N2°±4°E). Eastern Turkey is characterized by distributed deformation, while central Turkey is characterized by coherent plate motion (internal deformation of <2 mm/yr) involving westward displacement and counterclockwise rotation of the Anatolian plate. The Anatolian plate is de‐coupled from Eurasia along the right‐lateral, strike‐slip North Anatolian fault (NAF). We derive a best fitting Euler vector for Anatolia‐Eurasia motion of 30.7°± 0.8°N, 32.6°± 0.4°E, 1.2°±0.1°/Myr. The Euler vector gives an upper bound for NAF slip rate of 24±1 mm/yr. We determine a preliminary GPS Arabia‐Anatolia Euler vector of 32.9°±1.2°N, 40.3°±1.1°E, 0.8°±0.2°/Myr and an upper bound on left‐lateral slip on the East Anatolian fault (EAF) of 9±1 mm/yr. The central and southern Aegean is characterized by coherent motion (internal deformation of <2 mm/yr) toward the SW at 30±1 mm/yr relative to Eurasia. Stations in the SE Aegean deviate significantly from the overall motion of the southern Aegean, showing increasing velocities toward the trench and reaching 10±1 mm/yr relative to the southern Aegean as a whole.
Distinguishing ultramafic‐from basalt‐hosted submarine hydrothermal systems by comparing calculated vent fluid compositions
Authors: Wetzel Laura Reiser , Shock Everett L.
Published: September 2002

Submarine hydrothermal vent fluid compositions may be controlled by peridotite‐seawater or basalt‐seawater reactions. Previous studies of slow‐spreading ridges indicate that in addition to basalts, peridotites often play a prominent role in the construction of upper oceanic crust. Therefore the surface outcrop at a submarine hydrothermal vent field may not reveal the composition of the rock that hosts the reaction zone. We can, however, predict the compositional differences between ultramafic‐ and basalt‐hosted vent fluids by using theoretical reaction path calculations. These calculations determine equilibrium fluid compositions and mineral assemblages, yielding synthetic hydrothermal vent fluid compositions that can be compared to analytical measurements. Synthetic vent fluid compositions created from basalt and seawater reactants at 350° or 400°C and 500 bars are in close agreement with analytical measurements of end‐member vent fluids from mid‐ocean ridges. Twenty simulations at a 1:1 water to rock ratio using basalt compositions spanning the range of geochemical variability observed amongst mid‐ocean ridge basalts yield vent fluid compositions with <20% variation in major element concentrations. We also performed 15 ultramafic‐seawater simulations using dunite, lherzolite, and harzburgite compositions found in oceanic crust. All produced aqueous SiO2, K, and H2 concentrations that are distinct from the basalt‐seawater calculations. These differing concentrations can be used to attribute analytical measurements of vent fluid compositions to ultramafic or basaltic reaction zones.
Analysis of deformation data at Parkfield, California: Detection of a long‐term strain transient
Authors: Gao Stephen S. , Silver Paul G. , Linde Alan T.
Published: September 2002

Analysis of more than a decade of high‐quality data, particularly those from the two‐color electronic distance meter (EDM), in the Parkfield, California, area reveals a significant transient in slip rate along the San Andreas Fault. This transient consists of an increase in fault slip rate of 3.3±0.9 mm/yr during 1993.0 to 1998.0. The most reliable fault creep instruments show a comparable increase in slip rate, suggesting that the deformation is localized to the fault which breaks the surface. There was also an increase in precipitation around 1993. It is unlikely, however, that this anomaly is due directly to hydrology, as its spatial distribution is what would be expected for increased slip on the San Andreas Fault. The increase in slip rate corresponds temporally to a dramatic increase in seismicity, including the four largest earthquakes in the period 1984–1999 that occurred along a 6‐km segment of the fault just to the north of the EDM network. There was also a previously reported anomaly in borehole shear strain [Gwyther et al., 1996] that closely corresponds temporally to the transient in EDM data. Solely on the basis of EDM data the transient can be modeled as a slip event on a 10‐km‐long segment of the fault. The calculated shear strains from this model, however, are not consistent with the observed ones. A compatible model can be found if there is increased aseismic slip to the northwest in conjunction with the four earthquakes. Support for this northwestern slip is provided by a recent study of slip rate based on microearthquake activity. We speculate that this northwestern event served to load the fault to the southeast, with the stress being partially released by the observed slip.
Mantle variation within the Canadian Shield: Travel times from the portable broadband Archean‐Proterozoic Transect 1989
Authors: Bokelmann Götz H. R. , Silver Paul G.
Published: September 2002

We report travel times from the Archean‐Proterozoic Transect 1989. This type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the Wyoming Craton and the Superior Province of the Canadian Shield. With station separations of 50 to 100 km and frequencies up to 5 Hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle, surface geology, tectonic features and age provinces. Travel times of direct and later phases are obtained from waveform matching. The values are corrected for crustal contribution and inverted for the vertical path upper mantle delay δtUM under each station as well as the azimuthal dependence of this quantity. The prominent feature in the upper mantle delays δtUM is the variation by at least 1.5 s for S, much of which occurs over a narrow zone of just a few hundred kilometers width. This suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the Canadian Shield but is located within the shield. This same transition is also observed in shear wave splitting delay times. Surprisingly, however, the P delays do not exhibit a corresponding variation. We address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere. A simplified thermal model of the lithospheric transition zone, in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy, predicts this phenomenon.
Spatial variations in present‐day deformation, Kenai Peninsula, Alaska, and their implications
Authors: Freymueller Jeffrey T. , Cohen Steven C. , Fletcher Hilary J.
Published: September 2002

From four years of Global Positioning System (GPS) measurements, we find significant spatial variations in present‐day deformation between the eastern and western Kenai Peninsula, Alaska. Sites in the eastern Kenai Peninsula and Prince William Sound move to the NNW relative to North America, in the direction of Pacific‐North America relative plate motion. Velocities decrease in magnitude from nearly the full plate rate in southern Prince William Sound to about 30 mm/yr at Seward and to about 5 mm/yr near Anchorage. In contrast, sites in the western Kenai Peninsula move to the SW, in a nearly trenchward direction, with a velocity of about 20 mm/yr. The data are consistent with the shallow plate interface offshore and beneath the eastern Kenai and Prince William Sound being completely locked or nearly so, with elastic strain accumulation resulting in rapid motion in the direction of relative plate motion of sites in the overriding plate. The velocities of sites in the western Kenai, along strike to the southwest, are opposite in sign with those predicted from elastic strain accumulation. These data are incompatible with a significant locked region in this segment of the plate boundary. Trenchward velocities are found also for some sites in the Anchorage area. We interpret the trenchward velocities as being caused by a continuing postseismic transient from the 1964 great Alaska earthquake. There may be significant along‐strike differences in the long‐term behavior of the plate interface between the western and eastern Kenai, based on roughly coincident boundaries in the coseismic slip distribution, cumulative postseismic uplift, present‐day plate coupling, and stress field. The present postseismic response appears to generate purely trenchward motion, suggesting a creep process that is purely dip slip. Our observations suggest that postseismic processes after the largest earthquakes can influence patterns of deformation for decades after the event.
Contemporary crustal deformation in east Asia constrained by Global Positioning System measurements
Authors: Shen Zheng‐Kang , Zhao Chengkun , Yin An , Li Yanxing , Jackson David D. , Fang Peng , Dong Danan
Published: September 2002

Global Positioning System (GPS) measurements collected since the early 90's allow us to derive geodetic velocities at 16 permanent stations in east Asia and 68 campaign mode sites in north China. The resulting velocity field shows the following: (1) Contrary to the early inferences that the Shanxi Rift has accommodated significant right‐slip motion, our results suggest that the rift system, at least in its northern part in north China, is under ESE‐WNW extension at a rate of 4±2 mm/yr. The velocity field also suggests that an ESE‐WNW trending left‐lateral shear zone deforming at a rate of 2±1 mm/yr may exist along the north rim of north China at the latitude of ∼40°N, separating actively extending north China in the south from relatively stable Mongolia in the north. (2) Central and east China move at a rate of 8–11 mm/yr east‐southeast with respect to Siberia, implying that the overall east‐southeastward motion is the dominant mode of deformation in east China. (3) The India plate moves at a rate of 6±1 mm/yr slower than the NUVEL‐1A model prediction relative to the Eurasia plate. (4) Significant eastward motion (20±2 mm/yr) with respect to Siberia occurs in southeastern Tibet. About half of this eastward motion (∼11 mm/yr) is absorbed by structures along the eastern boundary of the Tibetan Plateau.
Anomalous seafloor spreading of the Southeast Indian Ridge near the Amsterdam‐St. Paul Plateau
Authors: Scheirer Daniel S. , Forsyth Donald W. , Conder James A. , Eberle Michael A. , Hung Shu‐Huei , Johnson Kevin T. M. , Graham David W.
Published: September 2002

The Amsterdam‐St. Paul Plateau is bisected by the intermediate‐rate spreading Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam‐St. Paul hotspot and the spreading center. The plate boundary geometry on the hotspot platform evolves rapidly (on timescales <1 Myr), off‐axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fracture zone extensions. Away from the hotspot platform the ridge‐transform geometry is typical of mid‐ocean ridges globally. In contrast, the Amsterdam‐St. Paul Plateau spreading segments are shorter, they often overlap each other significantly, and the intervening discontinuities are smaller, more ephemeral, and more migratory. Abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is ∼10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is ∼1.6 km, less than half that of adjacent spreading segments. Away from the platform some transform faults contain intratransform spreading centers; on the platform the two transform faults have valleys which may be depocenters for abundant axial or off‐axis volcanism and mass wasting. Although not wellconstrained by magnetics coverage, the Amsterdam‐St. Paul hotspot appears to have been “captured” by the Southeast Indian Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prior to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present‐day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a consequent effect of weakening the lithosphere.
A crustal model of the ultrahigh‐pressure Dabie Shan orogenic belt, China, derived from deep seismic refraction profiling
Authors: Wang Chun‐Yong , Zeng Rong‐Sheng , Mooney W. D. , Hacker B. R.
Published: September 2002

We present a new crustal cross section through the east‐west trending ultrahigh‐pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400‐km‐long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35‐km‐thick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0±0.2 km/s, 6.5±0.1 km/s, and 6.8±0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the northern margin of the orogen, and thus the present‐day root beneath the orogen is only 6.5 km thick. The upper mantle velocity is 8.0±0.1 km/s. Modeling of shear wave data indicate that Poisson's ratio increases from 0.24±0.02 in the upper crust to 0.27±0.03 in the lower crust. This result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant component in the middle or lower crust. Much of the Triassic structure associated with the formation of the UHP rocks of the Dabie Shan has been obscured by post‐Triassic igneous activity, extension and large‐offset strike‐slip faulting. Nevertheless, we can identify a high‐velocity (6.3 km/s) zone in the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh‐pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike‐slip fault.
Recent seismicity and tomographic modeling of the Mount Etna plumbing system
Authors: Chiarabba C. , Amato A. , Boschi E. , Barberi F.
Published: September 2002

The monitoring of seismic activity in eastern Sicily (southern Italy) has been recently improved, in the framework of the Poseidon Project, to investigate both tectonics and volcanic processes of Mount Etna. This effort has produced a homogeneous and complete data set which we use to image the deep structure of the volcano and to define the space and time distribution of the recent seismicity, encompassing the 1995 eruption, diffuse eruptions between 1995 and 1997, a further magma intrusion started in 1997, and an increase of volcanic activity in July 1998. We inverted P wave arrival times from 307 selected local earthquakes to obtain a three‐dimensional velocity model of the volcano, with a simultaneous inversion for hypocenters and velocity parameters. The new tomographic images permit us to define the structure beneath the volcano from the surface down to 18 km depth. The main structural feature revealed by our inversion is a high‐velocity body located beneath the central craters whose lateral extent increases from ∼6 km between 18 and 9 km depth to ∼12 km between 9 and 3 km depth. Near the surface the fast anomaly branches in two separate high‐Vp regions, which are located below the summit craters and the eastern flank (Valle del Bove), respectively. The high‐velocity features are interpreted as high‐density cumulates of solidified magma that intruded the shallow crust. We hypothesize that magma ascends the crust within the relatively small high‐velocity conduit (below 9 km depth) and is stored at depth shallower than 9 km within the broad high‐velocity region, as also suggested by petrological data. A sharp increase of seismicity in 1997, with earthquakes occurring at the border of the high‐velocity body, suggests that Mount Etna sustained recent episodes of intrusions, which possibly herald future eruptions. No large low‐velocity anomaly interprétable as a melted magma chamber is imaged in the upper 18 km of the crust, while a broad low‐velocity anomaly in the uppermost mantle, revealed by regional seismic data, possibly indicates the magma source region at depth. Earthquake occurrence from the deep source to the shallow intrusive area helps to trace the magma migration and the feeding of the volcanic system.
Siluro‐Devonian paleomagnetic results from the Tuva Terrane (southern Siberia, Russia): implications for the paleogeography of Siberia
Authors: Bachtadse V. , Pavlov V. E. , Kazansky A. Y. , Tait J. A.
Published: September 2002

New paleomagnetic data from seven sections of Siluro‐Devonian age sedimentary sequences in the Tuva Terrane (south Siberia, Russia) reveal the presence of up to three different components of magnetization. Component A is of recent or Late Mesozoic‐Cenozoic origin, and component B, identified unambiguously in 25 samples from one section, yields a sample mean direction (Declination/Inclination) of 300.6/−54.7 (k = 42.2, α95 = 4.5°) in geographic and 283.9/−63.5 (k = 45.4, α95 = 4.3°) in stratigraphic coordinates. However, since the results of the fold test are inconclusive, no further attempt has been made to assign an age to this magnetization. Component C, with unblocking temperatures of 500°–680°C, is identified in all seven sections and with dual polarity. After bedding correction, the section mean directions all plot along a common small circle in stereographic projection with a mean inclination of 42.1° (N = 7 sites, k = 184.2, α95 = 2.9°). It passes the inclination‐only fold test, the within‐site fold test (both on the 95% significance level) and the reversal test and is thus interpreted as being primary in origin and Siluro‐Devonian in age. Adopting the normal polarity option for component C yields a paleolatitude of 24°N (±2°) for the Tuva Terrane. On the basis of geological evidence, the Tuva Terrane was clearly accreted to Siberia by early Silurian times. The results obtained in this study, therefore, indicate lower paleolatitudes for Siberia than those predicted from previously published Apparent Polar Wander Paths for the Siberian Platform and demonstrate that Siberia was positioned between the equator and 30°N during latest Silurian/earliest Devonian times. These new results are also supported by paleoecological and paleoclimatological evidence. The dispersion of declinations along a small circle is indicative of clockwise block rotations within the Tuva Terrane of up to 72° with respect to Siberia.
Temporal changes in coda Q−1 and b value due to the static stress change associated with the 1995 Hyogo‐ken Nanbu earthquake
Authors: Hiramatsu Yoshihiro , Hayashi Nobuhiko , Furumoto Muneyoshi , Katao Hiroshi
Published: September 2002

The seismicity in the Tamba region, northeast of the Hyogo‐ken Nanbu earthquake in Japan (January 17, 1995; MJMA 7.2), increased significantly following this earthquake. This increase suggests that the static stress change due to a large earthquake causes a change in the crustal condition or dynamics. In order to reveal the changes quantitatively, we investigate the temporal variation in coda Q−l and b value in the Tamba region. We analyze the waveform data of many shallow microearthquakes (M 1.5–3.0) in the region recorded in a period from 1987 to 1996. Coda Q−1 is estimated in 10 frequency bands in a range of 1.5–24 Hz based on the single isotropic scattering model. At frequencies between 1.5 and 4.0 Hz the temporal variation in coda Q−1 shows significant correlation with the occurrence of the Hyogo‐ken Nanbu earthquake; coda Q−l increases after the event. A variation in b value whose sign is opposite to that of coda Q−1 is recognized. The fracture dimensions of microearthquakes that contribute to the variation in b value are estimated to be 400 m. This scale length is consistent with the characteristic scale length of scatterer, 300–600 m, which contributes effectively to a temporal variation in coda Q−1. The crustal activity in the Tamba region is possibly controlled by the heterogeneity with dominant scale of 102 m. The stress sensitivity of the coda Q−1 change is estimated to be 10 (MPa)−1. This value is an order of magnitude larger than the stress sensitivity of seismic velocity reported before.
Empirical mode skeletonization of deep crustal seismic data: Theory and applications
Authors: Vasudevan Kris , Cook Frederick A.
Published: September 2002

Seismic skeletonization is a pattern recognition technique used to decompose reflection seismic data into events or reflectors and their seismic attributes, and to store the results in an “event file” for an analysis toward seismic interpretation. The extraction of instantaneous frequency from the event file for any statistical analysis of the seismic attribute analysis is currently not possible, thus precluding the complete parameterization of the reflected seismic wave field in terms of its decomposition products. Empirical mode decomposition of reflection seismic data has uniquely addressed the question of computing the instantaneous frequency from its decomposition products using the Hubert transform. Although the decomposition products of the seismic skeletonization and the empirical mode decomposition are conceptually different, they share a common link to the primitive features of the seismic waveforms. In this paper, we introduce a new decomposition technique, empirical mode skeletonization, that combines the features of both seismic skeletonization and empirical mode decomposition. By this process, it is now possible to subject the reflection seismic data to an analysis that could include a parameterization of the reflected wave field. We apply the new technique to a segment of seismic line 1 of the Lithoprobe Slave Northern Cordillera Lithosphere Evolution (SNORCLE) transect to extract event‐oriented instantaneous frequency attributes and also to analyze the decomposition products for any scaling behavior of the reflected wave field.
Numerical models of a dense layer at the base of the mantle and implications for the geodynamics of D″
Authors: Montague Nancy L. , Kellogg Louise H.
Published: September 2002

To investigate the dynamics of the mantle's D″ layer, we explore numerical models of mantle convection which feature a dense basal boundary layer. We use a double‐diffusive finite element convection scheme and vary thermal and chemical Rayleigh numbers and properties including viscosity, thermal conductivity, and internal heating. For isoviscous models with heating only from below, the thermal Rayleigh number (Ra) is set at either 106 or 107. The negative chemical buoyancy of a dense layer produces a stable boundary layer when the ratio of chemical to thermal buoyancy (the buoyancy number B) is around 1. For B = 0.5 the layer is unstable, while B = 0.6 may produce a layer which remains stable for long periods, depending on other factors (such as Rayleigh number, layer thickness, and thermal diffusivity of the layer). At high enough Ra, small‐scale convection can occur within the layer. Using Ra = 107, we look at changes in layer thickness from 100 to 300 km at two values of B(0.6 and 1). Convection within the layer takes place most easily when the layer has a greater initial thickness. For B = 0.6, the layer is not always continuous along the lower boundary, so convection within the layer does not occur unless the initial thickness is fairly high (300 km). Increasing the thermal diffusivity of the layer (to simulate enrichment in metals) enhances heat conduction across the layer and can suppress internal convection within D″. Enhanced conduction also leads to higher plume temperatures. Including internal heating mainly increases the overall heat flow through the mantle, leading to higher surface heat flux. Finally, we examine models with temperature‐dependent viscosity and pressure‐dependent viscosity using Rayleigh numbers in the range of 107. Convection within the dense layer is enhanced by the relative reduction in viscosity. Our models exhibit only a modest decrease in viscosity (an order of magnitude) but illustrate how a dense, low‐viscosity layer interacts with cold, viscous downwellings.
Regional sensible and radiative heat flux estimates for the winter Arctic during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment
Authors: Overland James E. , McNutt S. Lyn , Groves Joanne , Salo Sigrid , Andreas Edgar L. , Persson P. Ola G.
Published: September 2002

We estimate the influence of the mosaic distribution of surface temperatures of sea ice on regional surface sensible and radiative heat fluxes on the basis of advanced very high resolution radiometer (AVHRR) temperatures. The AVHRR data were used to derive ∼1 km2 surface temperature values for 100×100 km2 regions. Regional flux estimates are compared to direct flux measurements taken during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the Beaufort Sea. We describe 48 cases of clear‐sky conditions between December 1997 and February 1998. The distribution of surface temperatures within each region is skewed: most temperatures fall near the value observed at the ice camp, but the distribution has a warm tail corresponding to subregions with thinner ice. Sensible heat fluxes at the ice camp were downward, from the air to the ice. Although the camp was representative of the majority of the region, the upward flux from thin ice generally balanced the downward flux to thicker ice, suggesting a regional equilibrium of near‐zero sensible heat flux. This was true whether the net downward flux at the camp was large or small or whether the ice was less compact, as in January, or more compact, as in December and February. The only exception to this generalization occurred when the ice was under strong compression. The net regional radiative loss from the ice surface was ∼22% greater than that measured on the floe at the SHEBA camp. This increase is due to the fourth‐power temperature dependence of the radiative flux acting on the skewed distribution of surface temperatures. The implications for modeling are that sea ice tends toward small net surface sensible heat flux on an aggregate scale, an emergent property of the system, and that parameterizing the regional mosaic of surface temperatures to resolve correctly the radiative balance in winter is important.
Toward the modeling of enhanced basal melting in ridge keels
Authors: Schramm Julie L. , Flato Gregory M. , Curry Judith A.
Published: September 2002

Observations from sonar data have suggested enhanced melting of thick, ridged ice relative to level ice. There are several mechanisms that may account for this intensified melting. In this paper, we examine the effects of two‐dimensional (2‐D) heat conduction and enlarged basal surface area due to the sloping sides of the keel on heat conduction and melt rates. The cross section of the 2‐D ridge is taken to be an isosceles triangle with a rounded crest. This is roughly the shape observed and allows a convenient numerical representation in polar coordinates. For comparison, ridges of similar shape are also represented as a collection of 1‐D columns of varying thickness, similar to what is implicit in typical ice thickness distribution models. The results show that 2‐D ridges inhibit the heat conduction compared to 1‐D ridges owing to the dominating effect of weaker temperature gradients. The slope of the keel is the dominant factor in determining the temperature gradient. A size distribution of 2‐D ridges reduces heat transfer to the atmosphere by 3 W m−2 compared to a similar distribution of 1‐D ridges. Over an annual cycle, basal ablation along the keel is insignificant for 2‐D ridges with small slopes, whereas ridges with large slopes show ablation rates determined by the ice‐ocean heat flux. These melt rates imply a transition from a triangular to a more rounded shape. The 1‐D ridge geometry is not adequate to simulate the net melting at the keel base over an annual cycle. Melt rates are calculated along the ridge keel and for level ice over a 40 day period for comparison with observations. For 1‐D ridges, all ice thicker than 5 m melts more slowly than the corresponding level ice. The inclusion of 2‐D heat conduction increases the amount of ablation in the thicker ice relative to the 5 m ice, especially for ridges with larger slopes. However, this increase explains only a small fraction of the enhanced basal melting seen in the observations. These results suggest that other mechanisms are important in determining the mass loss from thicker ice.