Showing papers on "Slab published in 1990"

Derivation of some modern arc magmas by melting of young subducted lithosphere

Abstract: MOST volcanic rocks in modern island and continental arcs are probably derived from melting of the mantle wedge, induced by hydrous fluids released during dehydration reactions in the subducted lithosphere1. Arc tholeiitic and calc-alkaline basaltic magmas are produced by partial melting of the mantle, and then evolve by crystal fractionation (with or without assimilation and magma mixing) to more silicic magmas2—basalt, andesite, dacite and rhyolite suites. Although most arc magmas are generated by these petrogenetic processes, rocks with the geochemical characteristics of melts derived directly from the subducted lithosphere are present in some modern arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites and sodic rhyolites (dacites seem to be the most common products) or their intrusive equivalents (tonalites and trondhjemites) are usually not associated with parental basaltic magmas3. Here we show that the trace-element geochemistry of these magmas (termed 'adakites') is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.

The subducted component in island arc lavas: constraints from Be isotopes and B–Be systematics

Abstract: Although high concentrations of beryllium-10 and boron are taken as unequivocal indicators of the contribution of subducting plates, controversy persists about the processes by which material is transferred from slabs to the sources of arc magmas. Data on (10)Be/Be and B/Be ratios from four arcs suggest that the contribution from the slab is compositionally homogeneous in each arc and that subducted boron is not stored in the sub-arc mantle. The link between subduction and magma-tism at convergent margins seems to be well regulated.

The Critical Contact Process Dies Out

Abstract: By producing a finite-box criterion for survival in a slab, we show that the critical contact process dies out and that, in two and more dimensions, the critical parameter value coincides with the limit of slab thresholds. Using the techniques developed in this paper, one may obtain the complete convergence and shape theorems for contact processes in all dimensions and for all infection rates. Our results apply also to the discrete-time analogue of the contact process, viz., oriented percolation.

A New Slab Model Approach for Electronic Structure Calculation of Polar Semiconductor Surface

Abstract: A new slab model approach is examined for the electronic structure calculation of a polar semiconductor surface. Our proposed slab model contains fractionally charged H atoms which completely terminate a noninteresting surface of the slab. We have studied in detail the electronic structure of the polar GaAs(001) surface in order to examine the reliability of our slab model. Calculation is performed by the ab initio pseudopotential method. We have found that our slab model can accurately describe a polar semiconductor surface. In addition, this model can greatly reduce the computational time.

When does the diffusion approximation fail to describe photon transport in random media

Abstract: The transport of photons through a slab of random medium is shown to deviate from the diffusion approximation when z/${\mathit{l}}_{\mathit{t}}$ is small, where z is the thickness of the slab and ${\mathit{l}}_{\mathit{t}}$ is the transport mean free path. When z/${\mathit{l}}_{\mathit{t}}$=10 and z=10 mm, the average time of arrival is about 0.9 times that predicted by diffusion theory. Photons are found to arrive earlier than that predicted by the diffusion theory as z/${\mathit{l}}_{\mathit{t}}$ becomes smaller or the anisotropic scattering increases.

Incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the Central American arc

Abstract: The Sr and Nd isotopic ratios of Central American volcanics can be described by the mixing of four components, marine sediment from DSDP Site 495, MORB-source mantle (DM), EMORB-source mantle (EM), and continental crust. Most of the isotopic data define a trend between EM and a modified mantle (MM) formed as a mixture of DM and less than 0.5% marine sediment, or fluid derived there from. The MM to EM trend is equally apparent in the incompatible-element data and is most clearly seen in a Ba/La versus La/Yb plot. A hyperbolic trend connects high Ba/La and low La/Yb at the MM end of the trend to low Ba/La and high La/Yb at the EM end. Smooth regional variations in incompatible-element and isotopic ratios correlate with the dip of the subducted slab beneath the volcanic front and the volume of lava erupted during the last 100,000 years (volcanic flux). Steep dip and low flux characterize the MM end-member and shallow dip and high flux characterize the EM end-member. The simplest model to explain the linked tectonic and geochemical data involves melting in the wedge by two distinct mechanisms, followed by mixing between the two magmas. In one case, EM magma is generated by decompression of EM plus DM asthenosphere, which is drawn in and up toward the wedge corner. EM mantle is preferentially melted to small degrees because of the presence of low melting components. The second melt is formed by release of fluid from the subducted slab beneath the volcanic front to form MM magma. Mixing between EM and MM magmas is controlled by subduction angle, which facilitates delivery of EM magma to the volcanic front at low-dip angles and impedes it at steep-dip angles.

New method for reservoir mapping

Abstract: The sequential indicator simulation (SIS) algorithm allows building alternative, equiprobable, numerical models of reservoir heterogeneities that reflect spatial-connectivity patterns of extreme values (e.g., permeability) and honor data values at their locations. This paper presents a case study of a sampled slab of Berea sandstone.

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

Abstract: We have observed slablike high P and S velocity anomalies around the Wadati-Benioff zone under island arcs of the northwest Pacific through travel time tomographic inversions. Nineteen years of International Seismological Centre travel time residuals for events and stations in this large region are used. Analyses of resolution and noise show that the images are generally well resolved. The images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth. Near the bottom of the upper mantle, fingering of the slabs, including segmenting and spreading, is indicated. The fast anomalies associated with the Japan, Izu-Bonin, and Mariana subduction zones tend to flatten to subhorizontal at depth, while downward spreading may occur under parts of the Mariana and Kurile arcs. The fast anomalies below 700 km are not in the shape of a single coherent sheet. The principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high-velocity slab, even when it bends to subhorizontal at depth. The depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip. Slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the “670-km” discontinuity.

Geometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico

Abstract: THE western coast of Guerrero in southern Mexico has been identified as a seismic gap on the Middle American Trench in which no large earthquakes have occurred at least since 1908. It has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake. A permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region. The seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast. The resulting geometry of the subduction zone shows that the Cocos plate dips at a shallow angle beneath the North American plate to a depth of ∼40 km; from there the subducted slab is bent upward, following a subhorizontal trajectory extending inland at a depth of ∼50 km. This plate geometry is reminiscent of that found in Peru and central Argentina, two other regions where a young oceanic plate is being subducted. In Mexico, however, the slab underplates an overriding plate which is only half as thick as that observed in South America. A possible oceanic origin of the allochtonous terranes comprising southern Mexico may explain the presence of the anomalously thin lithosphere in this region.

The relationship between plate velocity and trench viscosity in Newtonian and power‐law subduction calculations

Abstract: The relationship between oceanic trench viscosity and oceanic plate velocity is studied using a Newtonian rheology by varying the viscosity at the trench. The plate velocity is a function of the trench viscosity for fixed Rayleigh number and plate/slab viscosity. Slab velocities for non-Newtonian rheology calculations are significantly different from slab velocities from Newtonian rheology calculations at the same effective Rayleigh number. Both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate. Non-Newtonian rheology eliminates the need for imposed weak zones and provides a self-consistent fluid dynamical mechanism for subduction in numerical convection models.

Punching of flat plates : a question of concrete properties in biaxial compression and size effect

Abstract: The paper presents a design method to predict the punching strength and deflection of flat plates at interior columns. Failure is assumed to occur when the compression zone of the slab in the vicin ...

Rod laser with optical pumping from a source having a narrow emitting area

Abstract: An Nd/YAG slab laser (1) is pumped by laser diodes (2) placed at a distance from the laser slab. Reflectors (7) convey the light from the diodes to the slab. The distance is such that the density of such light is made uniform. A cooling fluid flows along a compartment (5) between the diodes and the slab. The invention is applicable to producing infrared coherent light.

Casting of gradient gels

Abstract: A method and apparatus are presented for the casting of gradient gels, without voids, in confined support structures such as thin slab-type configurations. Furthermore, by using a differential activation scheme for the casting, the method makes it possible to produce such gels in slabs of capillary cross-sectional dimension. According to a preferred method, a gradient slab gel is cast by filling a countable number of grooves (95) in a first plate (89) with a first gelling liquid. A gel casting region for a slab gel is formed by spacing the first plate (89) apart from a second plate (87) so as to provide a flow path between the first and second plates which has directional component orthogonal to the grooves (95). A second gelling liquid is injected into the gel casting region between the plates, such that the first liquid mixes with the second liquid as the second encounters each groove, thereby forming a gelling mixture with a composition gradient. The gelling mixture is then caused to gel in the gel casting region, thereby forming a gradient slab gel.

Membrane action in reinforced concrete slabs

Abstract: The formation and influence of compressive membrane action in reinforced concrete slabs is discussed. An experimental program is described, in which two large-scale slab specimens were tested under...

Modeling of residual spheres for subduction zone earthquakes: 1. Apparent slab penetration signatures in the NW Pacific caused by deep diffuse mantle anomalies

Abstract: We have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the Pacific using regional and global mantle velocity models from tomographic inversions. The mantle models explain much of the observed residual sphere data and, to a certain extent, suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns. For most deep events considered, the fast slablike residual sphere anomalies are caused by diffuse heterogeneities, mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab. The region immediately below the deepest earthquakes, depths of 650–1500 km, has an effect usually smaller than or comparable to the effect of other regions of the mantle. Without a proper account of the teleseismic effect, attributing the long-wavelength anomalies of the residual sphere to near-source slab effects alone, or even primarily, is not valid. The fast bands in many observed residual spheres agree with seismicity trends. Once the deep mantle and receiver mantle effects are removed, these may give the approximate orientation, but not the depth extent, of near-source fast velocities. For most deep earthquakes under Japan the predominant fast band is subhorizontal rather than near vertical. This type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data.

Concrete building panel with intermeshed interior insulating slab and method of preparing the same

Abstract: An insulated concrete building panel having an internal slab of insulating material, layers of concrete on each of the slab faces, and a concrete frame around its edges. One face of the slab has a pattern of grooves into which concrete is cast to provide ribs that are cast integral with the outer layer on that face. The other face has ridges, preferably of trapezoidal cross section that, when placed upon a bottom layer of cast but uncured concrete, become intermeshed with the bottom layer. The panels are prepared by first casting a layer of concrete on a horizontal surface, placing the slab, grooved face up, on the cast but uncured layer, and casting concrete over the grooved face, into the grooves and around the slab edges. Panels having this structure are useful for constructing buildings such as warehouses where long panel lengths of concrete inner face and internal insulation are needed.

Observations on earthquake stress axes and seismic morphology of deep slabs

Abstract: SUMMARY Focal mechanism solutions from centroid-moment tensor (CMT) and other studies in the NW Pacific and Tonga-Kermadec regions are analysed in association with velocity images from tomographic inversions. Deep compression axes and fast velocity slab anomalies are usually in consistent alignment, even when the slab is contorted or flattened. The down-dip direction for each event is given based on slab geometry interpreted from velocity anomalies and seismicity. The well-known pattern of tension to compression down-dip stress by Isacks & Molnar is seen and fitted with a tangent curve, which gives the location of the neutral down-dip stress zone (NDSZ). An abnormal pattern of NDSZ exists beneath major junctions of the arcs. The NDSZ is generally in the depth range of 100-250 km in the central parts of the arcs, while it is deeper in the Mariana region. The depth seems to depend on the dip and topology of the slab, but the NDSZ probably starts at more or less the same subducted length along the slab.

Chloride Diffusion in a Porous Concrete Slab

Abstract: Chloride ion diffusion in porous concrete is one of the two major factors that cause corrosion to the reinforcing steel bars. A physical model is given in the present paper for describing ...

The effect of confinement on the isotropic-nematic transition

Abstract: The Lebwohl-Lasher model of a liquid crystal is examined in a slab geometry, with free top and bottom surfaces, using standard molecular-field and a new Bethe-type approximation that includes some effects of correlations. The results are compared with computer simulations for a range of slab widths L. We find that the approximate treatments, while predicting a nematic-isotropic transition that is too strongly first-order, are in semiquantitative agreement with the simulation results. The results of the approximate treatments show that the Kelvin equation for the shift in transition temperature due to confinement is accurate for L ≥ 64 layers. For large L a wetting film of the disordered, isotropic phase intrudes between each surface and the nematic phase. The film thickness, evaluated at the transition in the slab, increases as ln L in accord with the theory of complete wetting for systems with short-ranged forces.

Seismic Response of Connections in Two-Bay Reinforced Concrete Frame subassemblies with a floor slab

Abstract: The effect of the presence of a floor slab on the behavior of beam-column connections was studied by testing two-bay frame subassemblies. Each subassembly consisted of two exterior connections and an interior connection. Individual interior and exterior connections were also tested to study the effect of continuity on the response of connections. The tests have shown that a larger slab width contributed to the flexural capacity of beams in continuous subassemblies compared to that observed in tests on sigle connections. Both the continuity of the test subassembly, as well as the presence of the floor slab, resulted in increased shear in the joints. The energy-dissipation capacity in continuous subassemblies was not affected by the presence of the slab; however, the lateral-load resistance increased by as much as 30 % lateral drift, and the degradation of stiffness was considerably delayed. Based on the results of this study, suggestions are made to include the effect of a floor slab in the procedure for designing beam-column connections.

Simplified model for estimating the punching resistance of reinforced corete slabs

Abstract: A method for predicting the punching strength of symmetric slab—column connections in flat slab structures is presented. The rational simplified model used was derived from a more complete one previously proposed by the author. This model leads to a single equation which is all that is required for design, agrees with the observed slab behaviour and correlates well with test results.

Beam-propagation analysis of bent step-index slab waveguides

Abstract: The electric field distribution and loss in a single-mode bent slab waveguide are analysed by the beam propagation method. The steady-state results obtained at large axial propagation distances are compared with other theoretical results. Reduction in bending loss is demonstrated by introducing a trench section which is placed on the outer side of the bend.