bluuebunny/crossref_metadata_2025 huggingface dataset:
Incidence of plane waves upon a fracture
Authors: Gu Boliang , Suárez‐Rivera R. , Nihei Kurt T. , Myer Larry R.
Published: February 2004

This paper investigates the details of reflection, transmission, and conversion of plane waves incident upon a fracture at arbitrary angles. The elastic compliance of fractures that is produced by the presence of a planar collection of void spaces and asperities of contact is modeled as a displacement‐discontinuity boundary condition between two elastic half‐spaces. Closed‐form expressions for the transmission and reflection coefficients on a fracture are derived by replacing the boundary conditions for a welded interface by those for a fracture into the standard procedure for plane wave analysis. The closed‐form expressions reveal that a single fracture can produce a variety of potentially diagnostic waves such as transmitted waves, reflected waves, converted waves, head waves, and P interface waves and introduce a finite group time delay to all these waves with respect to the incident wave. The amplitude and group time delay of the fracture‐induced waves are controlled by the fracture stiffness, wave frequency, and the Poisson's ratio of the medium. The head wave and inhomogeneous P interface waves are generated when an SV wave is incident upon a fracture, at and beyond a critical angle, respectively, which is determined by Snell's law. For some combinations of the fracture stiffness and the Poisson's ratio of the half‐spaces, no reflection or transmission of a P wave or an SV wave occurs.
Deep structure of the southern Kerguelen Plateau (southern Indian Ocean) from ocean bottom seismometer wide‐angle seismic data
Authors: Operto Stéphane , Charvis Philippe
Published: February 2004

Wide‐angle seismic data collected during the Kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of the origin of the plateau. Velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22‐km‐thick crust composed of ∼1.6 km of sedimentary cover, ∼5.3 km of upper crust, ∼11.0 km of lower crust, and a 4‐ to 6‐km‐thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8–4.5 km/s at top to 6.0–6.5 km/s at bottom) are consistent with the basaltic nature of this layer, the top of which was sampled during the Ocean Drilling Program. Velocities in the lower crust increase continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km depth. The reflective zone at the base of the crust identified by wide‐angle reflections is observed only along the NNW‐SSE direction. It consists of alternating high‐ and low‐velocity layers with an average velocity of 6.70 km/s in the NNW‐SSE direction and ≥6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW‐SSE and E‐W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large‐volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated from the Antarctic margin during the early opening of the Indian Ocean.
Normal‐mode constraints on the structure of the Earth
Authors: He Xiong , Tromp Jeroen
Published: February 2004

We have analyzed the splitting characteristics of 75 spheroidal free oscillations excited by the Great 1994 Bolivia and Kuril Islands earthquakes. These spheroidal modes may be roughly subdivided in terms of 8 radial modes, 40 mantle modes, and 27 core‐sensitive modes. The splitting of each mode is corrected for the effects of rotation and hydrostatic ellipticity. The remaining signal is due to lateral variations in the mantle and core and may be expressed in terms of so‐called splitting functions, which represent a local radial average of the Earth's even three‐dimensional heterogeneity. In the surface‐wave limit, splitting functions are the equivalent of an even‐degree phase velocity map. Each mode is uniquely sensitive to the Earth's structure. Some modes are predominantly sensitive to compressional velocities in the upper mantle, others to shear velocity variations in the lowermost mantle, and some modes “see” the inner core. As part of our analysis, we determine the center frequency and quality factor of each individual mode; these observations constrain the terrestrial monopole. Collectively, the normal‐mode splitting observations presented in this paper put constraints on the large‐scale, even structure of the entire Earth. We compare the observed splitting functions with predictions from three recent Harvard models: SH12WM13, SKS12WM12, and PS12WM13. These models are constrained by traveltime and waveform data but contain no normal‐mode information. We demonstrate that large‐scale, even structure is quite accurately represented in current Earth models, but that the splitting of some predominantly compressional modes is not satisfactorily explained. Although a distinct mantle signal is observed in the splitting functions of core‐sensitive modes, a characteristic zonal degree 2 pattern is missing. This missing signal is believed to be the result of inner core anisotropy.
Centerline temperature of mantle plumes in various geometries: Incompressible flow
Authors: Leitch A. M. , Steinbach V. , Yuen D. A.
Published: February 2004

We investigate the temperature at the center of the vertically rising and sinking limbs of convection cells as a function of geometry. We carried out finite element numerical simulations of steady state convection in three different two‐dimensional domains, a cartesian box, an axisymmetric cylinder, and an axisymmetric spherical shell, and looked at the centerline temperature as a function of height for the sheets and/or axial plumes at cell boundaries. We found that there is a significant change in centerline temperature near the base of the sheets due to conduction of heat in the horizontal direction. This drop does not occur for the axial plumes, so that generally temperature loss from plumes is significantly less. Numerical results were compared with the predictions of simplified mathematical models, and key parameters controlling temperature loss were identified in terms of boundary layer velocities, Rayleigh number Ra, and cell shape. Centerline temperature loss decreases strongly as the aspect ratio (length/height of the cell) increases for both sheets and plumes, but while the loss is nearly independent of Ra for sheets, for plumes it decreases approximately as Ra−1/3. For a bottom heated cell of aspect ratio 1, centerline temperature loss is about 60–80% for sheets and about 8–10% for plumes at Ra = 106. We also find that with the same catchment area, plumes are about 2–3 times wider than sheets. These results may be applied to two‐dimensional simulations of steady convection and can address questions regarding the extent that temperature is preserved in mantle plumes in their upward passage.
Thermal conductivity of U.S. coals
Authors: Herrin James M. , Deming David
Published: February 2004

Coal samples in the form of randomly oriented aggregates were obtained from the Pennsylvania State University Coal Bank for the purpose of thermal conductivity measurements. Samples represented 55 locations from throughout the United States and included 6 lignites, 10 subbituminous coals, 36 bituminous coals, and 3 anthracite samples. Matrix thermal conductivities measured at 22°C in the laboratory ranged from 0.22 to 0.55 W/m°K, with an arithmetic mean of 0.33 W/m°K and a standard deviation of 0.07 W/m°K. The thermal conductivity of lignites, subbituminous, and bituminous coals is controlled by composition and can be predicted by a three‐component (moisture, ash, and carbon + volatiles) geometric mean model with an rms residual of 6.1%. The thermal conductivity of bituminous and anthracite samples was found to be positively correlated with matrix density. With the exception of three anthracite samples, rank was not correlated with thermal conductivity, nor was the ratio of carbon to volatiles. The relatively high thermal conductivity of three anthracite samples (mean of 0.49 W/m°K) may have been related to graphitization.
Seismic velocities for hydrate‐bearing sediments using weighted equation
Authors: Lee M. W. , Hutchinson D. R. , Collett T. S. , Dillon W. P.
Published: February 2004

A weighted equation based on the three‐phase time‐average and Wood equations is applied to derive a relationship between the compressional wave (P wave) velocity and the amount of hydrates filling the pore space. The proposed theory predicts accurate P wave velocities of marine sediments in the porosity range of 40–80% and provides a practical means of estimating the amount of in situ hydrate using seismic velocity. The shear (S) wave velocity is derived under the assumption that the P to S wave velocity ratio of the hydrated sediments is proportional to the weighted average of the P to S wave velocity ratios of the constituent components of the sediment. In the case that all constituent components are known, a weighted equation using multiphase time‐average and Wood equations is possible. However, this study showed that a three‐phase equation with modified matrix velocity, compensated for the clay content, is sufficient to accurately predict the compressional wave velocities for the marine sediments. This theory was applied to the laboratory measurements of the P and S wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment. The results are comparable to the results obtained by repeatedly applying the two‐phase wave scattering theory. The theory predicts that the Poisson's ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases. In consequence, the amplitude versus offset (AVO) data for the bottom‐simulating reflections may reveal positive, negative, or no AVO anomalies depending on the concentration of hydrates in the sediments.
A geothermal climate change observatory: First year results from Emigrant Pass in northwest Utah
Authors: Putnam Scott N. , Chapman David S.
Published: February 2004

Temperature‐depth profiles, measured in boreholes, contain a temporal record of past changes in surface ground temperature and provide valuable constraints on climatic variations over the last few centuries. However, the linkage between ground temperature and meteorological variables including air temperature is imperfectly known. To understand that linkage better and to document in detail how the surface ground temperature changes propagate into the subsurface where they are later measured in temperature‐depth logs in boreholes, we have designed and installed a geothermal climate change observatory. Installed in arid northwest Utah, our Emigrant Pass Observatory (EPO) consists of an array of thermistor strings in the subsurface and a meteorological station at the borehole collar. Results from our first complete annual cycle, November 1, 1993, though October 31, 1994, are presented. Ground and air temperatures generally track each other but with important time‐varying offsets. The mean surface ground temperatures for the period are 11.3°C on the granite outcrop and 9.5°C for the partially shaded regolith site; mean air temperature at a 2‐m mast height above the ground is 8.8°C. The ground‐air temperature differences are variable on timescales from days to seasons, largely governed by level of absorbed solar radiation. Marginal precipitation (8.6 mm) and ephemeral snow cover did not significantly disturb the ground‐air temperature difference during the year monitored. Instrumental measurement of air temperature has nonrandom sampling biases that present problems for observing long term changes. The calculation of average annual air temperature at EPO decreases by 0.34 K as the sampling rate of air temperature is decreased from 60 s to every 12 hours. The attenuation and phase lag of thermal waves with depth confirm that heat conduction theory adequately describes the transient temperature field at this site, and yield in situ estimates of thermal diffusivity, a quantity needed to reconstruct surface ground temperature histories. Thermal diffusivity for the granite and regolith is 0.88 × 10−6 m2 s−1 and 0.45 × 10−6 m2 s−1, respectively. Energy flux calculations for the Emigrant Pass Observatory site suggest that a geothermal climate change observatory has the capability of detecting a century scale energy perturbation that is one part in a million of the instantaneous flux.
Ratio of relative S to P velocity heterogeneity in the lower mantle
Authors: Robertson G. S. , Woodhouse J. H.
Published: February 2004

The parameter v = ∂lnνs/∂lnνp, the ratio of relative S to P velocity heterogeneity, can be measured seismically and is important in determining the mineral physics of the deep mantle. However, seismic results are inconclusive because they have generally been based on P and S velocity models which are similar, but not proportional, and which are derived from seismic data sets in different frequency ranges and sample the mantle in different ways. Hence current values of v are subject to inconsistencies which arise from comparing models of differing resolution. Furthermore, heterogeneity within the Earth may be frequency dependent, and this characteristic may also bias v. We overcome these difficulties by computing P and S models from data sets with the same period and similar data coverages and by deriving the ratio in its most rigorous form, whereby we assume proportionality of P and S models and calculate v as a proportionality factor. We proceed by jointly inverting P and S arrival time data from the International Seismological Centre between the years 1964 and 1993 for ratios and compressional models expanded to at most degree 8 laterally in terms of spherical harmonics and up to degree 6 in depth in terms of Legendre polynomials. We also quantify the sensitivity of our results to data weighting and three‐dimensional model parameterization and determine to what depth we are justified in assuming proportionality of P and S heterogeneity. We parameterize v in terms of Legendre polynomials and find that there is an insignificant improvement in variance reduction beyond degree 1. Hence we find that v increases linearly from 1.7 to 2.6 between 700 and 2000 km depth, which is the greatest depth to which we believe our results are reliable. We also compare our results with those from theoretical mineral physics and find that seismically derived values are significantly larger and increasing at a greater rate. Reconciliation of these results requires at least modifications to the physical mechanisms currently postulated.
Amplitudes of regional seismic phases in relation to crustal structure of the Sierra Nevada, California
Authors: Paul A. , Jongmans D. , Campillo M. , Malin P. , Baumont D.
Published: February 2004

We have recorded and modeled the amplitudes of regional seismic waves produced by the Nevada Test Site Non Proliferation Experiment explosion along a 400‐km profile across the southern Sierra Nevada of California. Along this profile the amplitudes of the crustal phases Pg and Lg decrease monotonically with distance because of the cumulative effects of geometrical spreading and a frequency‐dependent crustal attenuation given by Q ≃ 138 f0.76 (1 ≤ f ≤ 8 Hz). This observation implies that the crust of the Sierras is not more attenuative to P and S waves than the crusts of the adjacent Great Valley and Basin and Range. The only amplitude anomaly related to the crustal structure is a strong attenuation of Lg on the vertical component in the Great Valley due to the refraction of rays in the low‐velocity sediments of the basin. In contrast to Pg and Lg, Pn amplitude increases across the Sierra Nevada and the eastern edge of the Great Valley. Then, some 30 km west of the exposed Sierran batholith, the amplitude of Pn suddenly decreases by a factor of 10. On the basis of a new refraction model produced by the Southern Sierra Nevada Continental Dynamics study and on numerical simulations of the Pn/Pg energy ratio, these changes in Pn amplitude are shown to be due to a local increase in crustal thickness, from roughly 35 to 45 km, centered under the western Sierra. The size and location of the amplitude anomaly can be explained with a gently sloping two‐layer crust‐mantle model with P velocities of 6.0 km/s and 8.0 km/s. The small offset crustal root focuses Pn waves toward the westernmost Sierra Nevada and the easternmost Great Valley, significantly increasing the Pn energy in these regions. The focusing effects diminish rapidly toward the west as the Moho is again horizontal.
Isothermal flow of an anisotropic ice sheet in the vicinity of an ice divide
Authors: Mangeney Anne , Califano Francesco , Castelnau Olivier
Published: February 2004

Simulations of glacier flow are commonly based on the assumption that ice has an isotropic viscosity. Here we examine the plane flow of ice in the special region of an ice divide using a constitutive relation for an anisotropic, incompressible viscous body that is orthotropic and transversally isotropic. Ice is assumed to be isotropic at the ice sheet surface, with the continuous development of a vertical single maximum c axis fabric with increasing depth. We consider the theoretical case of an isothermal ice sheet over a horizontal bedrock, with no slip at the ice‐bedrock interface. The ice sheet surface elevation is imposed, and the flow corresponding to the steady state is calculated, using a two‐dimensional finite difference model based on the resolution of a pressure‐Poisson equation. In this model, all components of the stress and strain rate tensor are calculated. The main conclusion is that for a fixed surface elevation, the general flow pattern accelerates when the anisotropic behavior of the ice is taken into account due to the greater fluidity with respect to shear stress. The downward motion of the ice is faster, despite a higher resistance to vertical deformation. As a result, the dominance of shear strain rate in the flow of polar ice is stronger in the anisotropic case than in the isotropic case. The shear stresses are slightly relaxed, while the longitudinal stresses are significantly increased in the anisotropic case.
Absolute paleointensity and magnetomineralogical changes
Authors: Valet Jean‐Pierre , Brassart Jacques , Le Meur Isabelle , Soler Vicente , Quidelleur Xavier , Tric Emmanuel , Gillot Pierre‐Yves
Published: February 2004

Determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings. Such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization (PTRM) are reflected by negative pTRM checks, a deviation between the original pTRM and a repeated measurement after heating at a higher temperature. Because the deviations indicated by the PTRM checks can provide a direct estimate of the effects to the pTRMs, we suggest that they can be used to correct pTRMs. The correction method we present requires additional measurements, in particular pTRM checks performed after each pTRM and specific diagrams to detect acquisition of chemical remanence. Performing multiple successive pTRM checks can also provide information on the blocking temperatures of the alteration product. These experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step. The corrections have been tested on recent and historic lava flows from several localities. We have also compared results on samples from the same lava flows that were heated in air and in vacuum. The mean paleointensity values were found to lie within less than 10% from the expected field intensity at each site. The corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65% additional data for specimens heated in air. Successive heatings at the same temperature in air are often accompanied by a time‐dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field, a problem that is not related to the corrections. We conclude that corrections can be used with fairly high confidence to the pTRMs performed in vacuum and in air.
Detailed observations of California foreshock sequences: Implications for the earthquake initiation process
Authors: Dodge Douglas A. , Beroza Gregory C. , Ellsworth W. L.
Published: February 2004

We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML = 6.4), the 1986 Stone Canyon (ML = 4.7), the 1990 Upland (ML = 5.2), the 1992 Joshua Tree (MW = 6.1), and the 1992 Landers (MW = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small‐scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.
S wave attenuation and site effects in the region of Friuli, Italy
Authors: Castro Raúl R. , Pacor Francesca , Sala Alfio , Petrungaro Carmine
Published: February 2004

We used strong motion records from the 1976 Friuli earthquake (M 6.4) and 10 of the biggest aftershocks recorded by the National Accelerograph Network of the Electrical Power Company of Italy to estimate the quality factor Q of S waves in this region. The wide distance range of the recordings (10 < r < 190 km) permits us to analyze the spectral amplitude decay of the records using a nonparametric approach [e.g., Anderson and Quaas, 1988; Castro et al., 1990; Anderson, 1991]. We obtained attenuation functions for a set of 18 frequencies ranging between 0.4 and 25.0 Hz. The values of Q retrieved from the attenuation functions obtained follow the frequency‐dependent relation Q = 20.4f. A test of the method was made using a second data set consisting of digital seismograms from the Friuli‐Venezia Giulia Seismograph Network. In spite of the different size of the volume sampled by these data (10 < r < 131 km), the frequency dependence of Q obtained (Q = 16.1f0.92) is similar to that obtained with the strong motion data set. The near‐surface attenuation was also estimated using the model proposed by Anderson and Hough [1984] and Anderson [1991]. We found that κ0 is smaller for the strong motion stations located on rock compared to stations located on either shallow or soft sediments. To estimate the site response of the strong motion stations, we corrected the spectral records for the attenuation effect and then inverted the corrected records to separate source and site effects using the inversion scheme proposed by Andrews [1986]. To verify the site amplification estimates obtained, we also calculated the transfer function of each site using Nakamura's [1989] method for S wave [e.g., Lermo and Chavez‐García, 1993]. In general, the shapes of the site functions obtained with the inversion are consistent with the transfer functions obtained calculating the horizontal to vertical component ratio.
Body waves in poroelastic media saturated by two immiscible fluids
Authors: Tuncay Kagan , Yavuz Corapcioglu M.
Published: February 2004

A study of body waves in elastic porous media saturated by two immiscible Newtonian fluids is presented. We analytically show the existence of three compressional waves and one rotational wave in an infinite porous medium. The first and second compressional waves are analogous to the fast and slow compressional waves in Biot's theory. The third compressional wave is associated with the pressure difference between the fluid phases and dependent on the slope of capillary pressure‐saturation relation. Effect of a second fluid phase on the fast and slow waves is numerically investigated for Massillon sandstone saturated by air and water phases. A peak in the attenuation of the first and second compressional waves is observed at high water saturations. Both the first and second compressional waves exhibit a drop in the phase velocity in the presence of air. The results are compared with the experimental data available in the literature. Although the phase velocity of the first compressional and rotational waves are well predicted by the theory, there is a discrepancy between the experimental and theoretical values of attenuation coefficients. The causes of discrepancy are explained based on experimental observations of other researchers.
Melting relations of model lherzolite in the system CaO‐MgO‐Al2O3‐SiO2 at 2.4–3.4 GPa and the generation of komatiites
Authors: Gudfinnsson Gudmundur H. , Presnall Dean C.
Published: February 2004

Isobarically invariant phase relations in the CaO‐MgO‐Al2O3‐SiO2 system (CMAS) involving the lherzolite phase assemblage in equilibrium with liquid have been determined at 2.4–3.4 GPa. These phase relations form the solidus of model lherzolite in the CMAS system. Our data, which include determinations of all phase compositions, are in excellent agreement with the 3.0 and 4.0 GPa points of Milholland and Presnall [1991] and Davis and Schairer [1965], respectively. The invariant transition on the P‐T solidus curve from spinel‐ to garnet‐lherzolite at 3.0 GPa, 1575°C [Milholland and Presnall, 1991], is confirmed, but we observe that the theoretically required temperature depression on the solidus curve at this point is not experimentally detectable. Composition trends along the solidus take a sharp turn at the transition. In the spinel‐lherzolite stability field, melt compositions become increasingly Fo‐normative and less En‐normative with increasing pressure, but become less Fo‐normative and more pyroxenitic as pressure increases in the garnet‐lherzolite stability field. Calculated melting reactions indicate that forsterite is in reaction relationship with the melt up to 3.0 GPa. Orthopyroxene is also in reaction relationship at pressures higher than just over 2.8 GPa and is the only phase in reaction relationship with the melt in the garnet‐lherzolite stability field. Comparison of the normative compositions and the CaO/Al2O3 values of the komatiites of Gorgona Island and of the Reliance Formation in Zimbabwe with the compositions of liquids along the solidus of model lherzolite in the CMAS system indicates that the former komatiites were generated at pressures close to 3.7 GPa and the latter at close to 4.5 GPa, assuming that the melt generation occurred in the presence of the complete garnet‐lherzolite assemblage.
Lateral flow of hot plume material ponded at sublithospheric depths
Authors: Sleep Norman H.
Published: February 2004

Local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere. This processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources. Variations in the depth to the base of the lithosphere significantly affect lateral flow. The flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots, such as Hawaii. The effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell. Variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes, effectively forming an upside‐down drainage pattern. Plume material flows upslope toward ridge axes from off‐axis plumes and along ridge axes from on‐axis plumes at sufficiently low spreading rates. Thinner lithosphere exists above plume material than above normal asthenosphere. The edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material. Pressure release melt is associated with the upslope flow of plume material. It is expected to cause volcanism away from the plume and to sometimes supply melt‐depleted plume material to ridge axes that are upslope from the region of voluminous melting.
Effect of a retreating subduction zone on deformation in simple regions of plate convergence
Authors: Waschbusch Paula , Beaumont Christopher
Published: February 2004

Plane strain, two‐dimensional, finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat (the process by which space is created between the subducting (“pro”) and overriding (“retro”) lithospheres either by motion of one or both) is active. The model uses Coulomb plastic and thermally activated power law viscous rheologies. Overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model. The effect of incorporating retreat in the subduction setting diffuses deformation, as the moving (retreating) subduction zone deforms a larger cross‐sectional area of material for a shorter period of time than equivalent models without subduction zone retreat. Material is deformed while within the subduction zone, but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed. Thus, relative to a convergent setting without subduction zone retreat, the retreating convergent zone has an areally extensive zone of simple deformation. A deformation factor, γ, is derived which characterizes the deformation (shortening) the material experienced within the subduction zone. Variable γ is the ratio of convergence rate (rate at which material enters the subduction zone) to subduction rate (rate at which material is subducted). Variables γ and f, the fraction of the layer that is subducted, together determine the dominant style of deformation in the orogenic zone. Subduction zone retreat is shown to have significant and predictable effects on the model morphology (e.g., basin or mountain formation) and on the model geology (e.g., its presence suggests lower topography, lower metamorphic grade rocks at the surface, and simpler deformational structures), which agree with observations.
Heat flow and heat production in the Arkoma Basin and Oklahoma Platform, southeastern Oklahoma
Authors: Lee Youngmin , Deming David , Chen Kevin F.
Published: February 2004

Subsurface temperature and thermal gradients along a north‐south cross section through the Arkoma Basin and the Oklahoma Platform in southeastern Oklahoma were estimated from 345 bottom hole temperatures from 199 oil and gas wells. The average geothermal gradient in the southern part of the basin near the Ouachita Front is 20°C/km, exceeds 30°C/km in the middle part of the basin, and is 24°C/km on the Oklahoma Platform to the north. Drill cuttings obtained from 11 oil and gas wells were used for 843 thermal conductivity measurements. Thermal conductivity data, corrected to in situ conditions, were used to estimate heat flow. Estimated heat flow (±20%) in the deep part of the Arkoma Basin near the Ouachita Front is 35–40 mW/m2 and increases systematically northward to 60–65 mW/m2 on the Oklahoma Platform. Average heat production, estimated from gamma ray logs, is 2.3 ± 0.2 μW/m3 for basement rocks underlying the Arkoma Basin and 2.8 ± 0.1 μW/m3 for basement rocks in the Oklahoma Platform area. Numerical models show that heat refraction from the less conductive sedimentary rocks (∼1.6 W/m°K) of the Arkoma Basin to the more conductive crystalline rocks (∼3.0 W/m°K at 25°C) of the Oklahoma Platform and the Ouachita Mountains accounts for about 5–10 mW/m2 of the observed 20–30 mW/m2 decrease in heat flow from north to south. Changes in crustal heat production related to compositional changes and crustal thinning account for another 5–15 mW/m2 of the observed heat flow change. If the remaining 0–20 mW/m2 difference in heat flow is attributed to heat transport by topographically driven groundwater flow, the average basin‐scale permeability of the Arkoma Basin and the Oklahoma Platform can be no greater than 10−15 m2. Results of this study are not generally supportive of theories which invoke topographically driven regional groundwater flow from the Arkoma Basin in Late Pennsylvanian–Early Permian time (∼290 Ma) to explain the genesis of Mississippi Valley‐type lead‐zinc deposits, paleothermal anomalies, and regional diagenesis in the North American midcontinent.
The distribution and mixing of Pacific water masses in the Indonesian Seas
Authors: Hautala Susan L. , Reid Joseph L. , Bray Nan
Published: February 2004

The salinity and dissolved oxygen of water masses in the Indonesian Seas, from historical hydrographic data, are examined on isopycnal surfaces. We focus primarily on the Banda Sea, from which the bulk of the throughflow transport flows into the Indian Ocean. Dissolved oxygen proves to be a problematic conservative tracer in this region due to biological consumption in the upwelling regime of the Arafura Sea and the subsequent spreading of relatively low oxygen water over a broad area. The remaining analysis is thus restricted to salinity. We first consider a hypothesis of simple isopycnal advection and mixing between North and South Pacific low‐latitude western boundary current sources. Three regimes are apparent. The surface and upper thermocline layers, down to 25.8σθ, are too fresh to fit the hypothesis. Vertical mixing of surface precipitation and runoff excesses down into the water column must be invoked. Vertical mixing is also apparent on deeper isopycnals, below 27.0σθ, where a contribution from the deep Indian Ocean can be discerned. In between, in the lower thermocline, the θ‐S data are consistent with the hypothesis of purely isopycnal spreading, given a simple variation in the ratio of sources toward increasing South Pacific contribution with depth. In this regime the juxtaposition of sources leads to relatively strong gradients in water mass properties across the Banda Sea. This gradient translates into a difference in outflow characteristics between Timor and Ombai Straits, which appear to draw their waters, respectively, from the eastern and western Banda Sea. We then consider how the presence of vertical mixing modifies our inferred water mass ratio under a variety of boundary conditions. Vertical mixing effects are particularly important in the upper thermocline. In the Banda Sea, on all isopycnals down to σθ = 26.5, an increase in the degree of vertical mixing tends to decrease the relative importance of the North Pacific source. The sensitivity of the water mass ratio to degree of vertical mixing decreases with depth, and the tendency is reversed below 26.5σθ. In the Banda Sea, for plausible values of throughflow residence time and vertical diffusivity (λ = (κt)1/2 < 100 m), the North Pacific contributes 80–90% of the water mass at 26σθ, 50–60% at 26.5σθ, and only 10–30% at 27σθ. This large South Pacific contribution in the lower thermocline is supported by other studies.
Carbon budget of sea‐ice algae in spring: Evidence of a significant transfer to zooplankton grazers
Authors: Michel C. , Legendre L. , Ingram R. G. , Gosselin M. , Levasseur M.
Published: February 2004

The fate of ice‐bottom algae, before and after release from the first‐year sea ice into the water column, was assessed during the period of ice‐algal growth and decline in Resolute Passage (Canadian Arctic). During spring 1992 (from April to June), algae in the bottom ice layer and those suspended and sinking in the upper water column (top 15 m) were sampled approximately every 4 days. Ice‐bottom chlorophyll a reached a maximum concentration of 160 mg m−2 in mid‐May, after which it decreased to lower values. In the water column, chlorophyll a concentrations were low until the period of ice‐algal decline (∼0.1 mg m−3), with most biomass in the <5‐μm fraction. In both the suspended and sinking material, large increases of algal biomass occurred at the beginning of June, following the release of ice‐algae into the water column. The input of ice‐algal derived carbon to the upper water column and the proportions exported through sinking or remaining in suspension were assessed using a carbon budget for the two periods of ice‐algal growth and decline. For each period the output terms closely balanced the input. The carbon budget showed that most of the biomass introduced into the upper water column remained suspended (>65% of total export) and that ice‐algae were ingested by under‐ice grazers after release from the ice. These results stress the importance of ice algae for pelagic consumers during the early stages of ice melt and show that the transfer of ice algae to higher trophic levels extends beyond the period of maximum algal production in the ice bottom.
Formation and evolution of the surface mixed layer and halocline of the Arctic Ocean
Authors: Rudels B. , Anderson L. G. , Jones E. P.
Published: February 2004

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen‐Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen‐Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen‐Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low‐salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double‐diffusive convection, is trapped below the mixed layer.
The temperature dependence of frost flower growth on laboratory sea ice and the effect of the flowers on infrared observations of the surface
Authors: Martin Seelye , Yu Yanling , Drucker Robert
Published: February 2004

This paper describes a laboratory study of frost flower growth on young sea ice at different temperatures and the effect of these flowers on the surface temperature observed with an infrared radiometer. The flowers grew on sea ice which formed in a salt water tank at room temperatures of −20, −24, and −30°C, with an additional experiment at −16°C, where no flowers appeared. The growth habit and height of the observed crystals depended on the existence of a region of supersaturated vapor adjacent to the surface and on the range of temperatures in the surface boundary layer. The source of the surface brine from which the flowers grew was probably brine transport within the ice toward the cold upper surface driven by the thermomolecular pressure gradient. The evaporation of vapor from this liquid into the atmospheric boundary layer provided the supersaturated region adjacent to the ice surface. Two kinds of flowers were observed; at −20 and −24°C, dendritic crystals grew approximately between the −12 and −16°C isotherms, and, at −30°C, rod‐like flowers appeared between −16 and −25°C. These limits correspond to earlier work on crystal growth from the vapor. In each case, the maximum flower height approximately equaled the height of the isotherm corresponding to the colder temperature limit for each crystal type, −16°C for the dendrites and −25°C for the rods. The effect of the flowers on the radiometer surface temperature was as follows: because the flowers protrude 10–20 mm above the surface into the boundary layer, the infrared temperature of the flower‐covered ice was about 4–6°C colder than that of the same ice cleared of flowers. We also found that the insulating effect of the flowers caused the ice surface temperature beneath the flowers to be 1–2°C warmer than the surrounding bare ice. The importance of the flower growth is that infrared satellite observations of thin ice in winter will be colder than the actual surface temperature, which may account for the absence of warm young ice in infrared satellite images.
Energy‐containing scales of turbulence in the ocean thermocline
Authors: Moum J. N.
Published: February 2004

From measurements of the energy‐containing scales of turbulence in the ocean thermocline, two new formulations are examined: (1) an inviscid estimate for the viscous dissipation rate of turbulent kinetic energy and (2) a mixing length estimate for the turbulent heat flux. These formulations are tested using coincident measurements of the relevant properties of both energy‐containing and dissipation scales of stratified turbulence in the ocean's main thermocline obtained from a vertical microstructure profiler. It is found that energy‐containing scale estimates of both dissipation rate and heat flux compare favorably with dissipation scale estimates. Since the energy‐containing scales are many times greater than the dissipation scales, the measurement constraints on these new estimates are considerably less strict than for dissipation scale estimates of the same quantities. These observations also suggest that the timescale for viscous decay of turbulent motions is greater than that for diffusive smoothing of scalar fluctuations. It is argued that this is consistent with current estimates of mixing efficiencies.
Efficiency of mixing in the main thermocline
Authors: Moum J. N.
Published: February 2004

Estimates of heat flux from direct measurements of vertical velocity‐temperature fluctuation correlations have been obtained from vertical profiles through turbulent patches in the main thermocline. These have been compared to more indirect flux estimates derived from dissipation rates of turbulent kinetic energy and temperature variance. Because record lengths are limited by the thickness of observed turbulent patches, uncertainties are larger than would be expected from relatively longer horizontal records. The best estimate of dissipation flux coefficient from these data is about 0.15–0.2, but it is characterized by a large range of sample values. This implies mixing efficiencies (flux Richardson numbers) are about 0.13–0.17. This is within the range of laboratory estimates but is different from measurements in turbulent tidal fronts.
Study of the generation and propagation of internal waves in the Strait of Gibraltar using a numerical model and synthetic aperture radar images of the European ERS 1 satellite
Authors: Brandt Peter , Alpers Werner , Backhaus Jan O.
Published: February 2004

A weakly nonhydrostatic, two‐layer numerical model based on the Boussinesq equations is presented which is capable of describing, among others, the generation and propagation of nonlinear weakly dispersive internal waves in the Strait of Gibraltar. The model depends on one space coordinate only, but it retains several features of a fully three‐dimensional model by including a realistic bottom profile, a variable channel width, and a trapezoidal channel cross section. The nonlinear primitive Boussinesq equations include horizontal diffusion, bottom friction, and friction between the two water layers. The model is driven by a height difference of the mean interface depth between the Atlantic and the Mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport. The model presented in this paper describes (1) the mean and tidal flow in the Strait of Gibraltar, (2) the variation of the depth of the interface during a tidal cycle, (3) the generation of strong depressions of the interface at the western sides of the Spartel Sill and the Camarinal Sill, (4) the generation of strong eastward propagating internal bores, and (5) their disintegration into trains of internal solitary waves. The surface convergence patterns associated with depressions of the interface at the Camarinal Sill, internal bores, and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar (SAR) images of the first European Remote Sensing Satellite ERS 1. In total, 155 ERS 1 SAR scenes from 94 satellite overflights over the Strait of Gibraltar, which were acquired in the period from January 1992 to March 1995, have been analyzed. It is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the Strait of Gibraltar as well as the observed east‐west asymmetry of the internal wave field.