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Showing papers on "Slab published in 2001"


Journal ArticleDOI
TL;DR: A region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models, was performed in this paper.
Abstract: We made a region-by-region examination of subducted slab images along the circum-Pacific for some of the recent global mantle tomographic models, specifically for two high-resolution P velocity models and two long-wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660-km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu-Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga-Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre-Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

630 citations


Journal ArticleDOI
TL;DR: In this paper, the evolution of the Central Mediterranean subduction zone is reconstructed using geophysical and geophysical constraints, and the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross-sections is derived.
Abstract: SUMMARY Geological and geophysical constraints to reconstruct the evolution of the Central Mediterranean subduction zone are presented. Geological observations such as upper plate stratigraphy, HP–LT metamorphic assemblages, foredeep/trench stratigraphy, arc volcanism and the back-arc extension process are used to define the infant stage of the subduction zone and its latest, back-arc phase. Based on this data set, the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross-sections from the northern Apennines and from Calabria to the Gulf of Lyon can be derived. Further, the reconstruction is used to unravel the main evolutionary trends of the subduction process. Results of this analysis indicate that (1) subduction in the Central Mediterranean is as old as 80 Myr, (2) the slab descended slowly into the mantle during the first 20–30 Myr (subduction speeds were probably less than 1 cm year x1 ), (3) subduction accelerated afterwards, producing arc volcanism and back-arc extension and (4) the slab reached the 660 km transition zone after 60–70 Myr. This time-dependent scenario, where a slow initiation is followed by a roughly exponential increase in the subduction speed, can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull. Finally, the third stage is controlled by the interaction between the slab and the 660 km transition zone. In the southern region, this results in an important re-shaping of the slab and intermittent pulses of back-arc extension. In the northern region, the decrease in the trench retreat can be explained by the entrance of light continental material at the trench.

618 citations


Journal ArticleDOI
TL;DR: For the last 30 years, few in the field have seriously believed that the subducting slab is the source of arc basalts as mentioned in this paper, and the accepted hypothesis involves melting of the mantle wedge above the slab via hydrous fluids produced during the transition from amphibolite to eclogite.
Abstract: Most recent geology textbooks state that subduction-related volcanism is due to the melting of the down-going lithosphere. However, for the last 30 years, few in the field have seriously believed that the subducting slab is the source of arc basalts. The accepted hypothesis involves melting of the mantle wedge above the slab via hydrous fluids produced during the transition of the subducting basalt from amphibolite to eclogite. The parental basalts differentiate primarily through crystal fractionation, magma mixing, and differentiation at the Mohorovicic discontinuity into andesites and dacites as they ascend; the basalts are too dense to rise through the lower continental crust. This explains the relative abundance of differentiated rocks in arcs.

303 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone based on mineral physics observations incorporating grain-size, stress, temperature and pressure dependence of rheology.

300 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography, gravity and geoid.

250 citations


Journal ArticleDOI
TL;DR: In this paper, the authors quantitatively investigate the breakoff process and the subsequent thermal evolution of a plate boundary region involving continental collision after a prolonged period of oceanic lithosphere subduction.
Abstract: Slab detachment (or breakoff) has been proposed as a cause of temperature-related processes associated with subduction, such as postcollisional magmatism, mineralization, and metamorphism In this study, we quantitatively investigate the breakoff process and the subsequent thermal evolution of a plate boundary region involving continental collision after a prolonged period of oceanic lithosphere subduction Our two-dimensional time-dependent thermomechanical modeling shows that the dense, oceanic part of the slab can become detached at depths as shallow as 35 km The detached part of the slab sinks into the mantle, creating a gap in the lithospheric system which is filled with upwelling hot asthenosphere The resulting temperature increase in the overlying material can be more than 500°C It allows for partial melting of the asthenosphere and the overriding metasomatized lithosphere for a timespan of a few millions of years Crustal anatexis and related magmatism and mineralization can proceed over a considerably longer period The quantification of the conditions required for shallow slab detachment will contribute to warranted assessments concerning the role of slab detachment (relative to other proposed heat sources, such as tectonically accreted radioactive material) in the geodynamical evolution of former convergent plate boundaries

194 citations


Journal ArticleDOI
TL;DR: In this article, the influence of subducting slabs on overriding plates is examined, and the results provide information about the structure of the lithosphere in this complex region and about the vertical tectonics.

191 citations


Patent
29 Nov 2001
TL;DR: A flat-panel camera comprises a tapered transparent slab 1, a prismatic sheet 3 for introducing light into one face of the slab at near the critical angle so that it is reflected along the slab towards the thick end, and a miniature camera 2 receiving light emerging from the thin end at an angle depending on where the light entered the slab.
Abstract: A flat-panel camera comprises a tapered transparent slab 1, a prismatic sheet 3 for introducing light into one face of the slab at near the critical angle so that it is reflected along the slab towards the thick end, and a miniature camera 2 receiving light emerging from the thick end of the slab at an angle depending on where the light entered the slab. An image processor 7 eliminates distortion in the image such as gaps caused by the light missing the camera.

189 citations


Journal ArticleDOI
TL;DR: In this paper, the trend of decreasing δ11B with slab depth shown by cross-arc magmatic suites in the Izu and Kurile arcs of the western Pacific are extended to shallow depths (∼25 km).

173 citations


Journal ArticleDOI
TL;DR: In this paper, a regional pattern of shear-wave birefringence near the junction of Aleutian Island and Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific.
Abstract: The junction of the Aleutian Island and the Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific. The regional pattern of shear-wave birefringence near the junction indicates that trench-parallel strain follows the seismogenic Benioff zone, but rotates to trench-normal beyond the slab edge. Asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge, and may influence the shallowing dip of the Benioff zone at the Aleutian junction.

170 citations


Journal ArticleDOI
TL;DR: In this article, the authors made density measurements of model MORB and picritic melts up to about 15 GPa using the floating diamond method and fitted the compression behavior of the melts by the Vinet equation of state and extrapolated to higher pressure.

Journal ArticleDOI
TL;DR: In this article, a soft coupling is assumed that is strong enough to enable elongation of the slab, but weak enough to inhibit "quasi-static" stress transfer to the overlying crust.
Abstract: Slab break-off is a plate-tectonic process which does not only return lithospheric material into the deeper mantle, but also has severe effects on surface movements and seismic hazard: slab-pull induced seismicity is reduced when the subducted slab decouples from the overlying crust. In the Vrancea region (SE Carpathians), strong earthquakes frequently occur at intermediate depths (70‐180 km) in a laterally small volume, while the crust shows low but distributed seismicity. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust. Both features indicate a decoupling between slab and overlying crust, either by slab break-off or delamination. However, the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically. Thus, a ‘soft’ coupling is assumed that is strong enough to enable elongation of the slab, but weak enough to inhibit ‘quasi-static’ stress transfer to the overlying crust.

Journal ArticleDOI
TL;DR: In this article, the authors employed the modified weighted slab technique along with recent values of the relevant cross sections to compute primary to secondary ratios including B/C and sub-Fe/Fe for different Galactic propagation models.
Abstract: In an attempt to understand the source and propagation of Galactic cosmic rays, we have employed the modified weighted slab technique along with recent values of the relevant cross sections to compute primary to secondary ratios including B/C and sub-Fe/Fe for different Galactic propagation models. The models that we have considered are the disk-halo diffusion model, the dynamical halo wind model, the turbulent diffusion model, and a model with minimal reacceleration. The modified weighted slab technique will be briefly discussed and a more detailed description of the models will be given. We will also discuss the impact that the various models have on the problem of anisotropy at high energy and discuss what properties of a particular model bear on this issue.

Journal ArticleDOI
TL;DR: Yang et al. as discussed by the authors used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern Luzon and propose a model for describing the geometry of the subducted slab of the Eurasian plate beneath the northern segment of the Manila Trench.

Journal ArticleDOI
TL;DR: In this paper, the authors tried to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and/or mantle and found that the double seismic zone can be explained by its dehydration.
Abstract: The seismicity in the subducting Philippine Sea slab (PHS) beneath southwest Japan shows a variety of modes of occurrence. We try to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehydration embrittlement in the hydrated subducting oceanic crust only, implying the lack of hydrated slab mantle. The PHS beneath Kanto, however, shows a double seismic zone (Hori, 1997) in the mantle part. Here the serpentinized mantle wedge of the Izu-Bonin fore-arc is subducting, and the double zone can be explained by its dehydration. Beneath Kii Peninsula and Kyushu, seismic events within the slab mantle have also been detected. This indicates that the PHS mantle beneath these areas is also hydrated, which may have resulted from subduction of the serpentine stable in the Izu-Bonin back-arc area. Aqueous fluids released from the serpentinized mantle beneath Kii Peninsula may have initiated partial melting in the mantle wedge, as indicated by the presence of high 3He/4He ratios in the natural gasses and the shallow seismic swarms in this region (Wakita et al., 1987).

Journal ArticleDOI
TL;DR: The edge-pumped slab design permits symmetric conduction cooling and efficient pump absorption and accepts large-numerical-aperture pump sources.
Abstract: Experimental demonstrations of two edge-pumped zigzag slab lasers are presented. The Nd:YAG slab laser generated 127 W of multimode output power with 300W of pump power. Preliminary results with a Yb:YAG slab produced 46 W of output power with 315W of pump power. The edge-pumped slab design permits symmetric conduction cooling and efficient pump absorption and accepts large-numerical-aperture pump sources.

Journal ArticleDOI
TL;DR: In this paper, a suite of fresh alpine-type peridotites from the Horoman ultramafic complex, Hokkaido, Japan, were extracted and compared to the corresponding mid-oceanic ridge basalts.

Journal ArticleDOI
29 Jun 2001-Science
TL;DR: It is shown that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere.
Abstract: We combine spatial variations of P- and S-wave speeds, 1000 fault plane solutions, and 6600 well-determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the Tonga back-arc. We show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere. The detached slab may be from a previous episode of subduction along the fossil Vitiaz trench about 5 to 8 million years ago. The juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle. Such a configuration, if common in the past, would allow the preservation of a primordial component in the lower mantle.

Journal ArticleDOI
TL;DR: In this article, the authors used the finite element method for nonlinear transient analysis of reinforced concrete (RC) two-way slabs subjected to blast loading and analyzed both as-built and retrofitted slabs with carbon fiber reinforced polymer composite strips.
Abstract: Computational models using the finite element method for nonlinear transient analysis of reinforced concrete (RC) two-way slabs subjected to blast loading are presented. Both as-built and retrofitted slabs with carbon fiber reinforced polymer (CFRP) composite strips are analyzed. The models are used to investigate different parameters including (a) loading duration, and (b) effect of CFRP retrofit on damage accumulation. In this study, damage is globally quantified by the amount of reduction of the first two vibrational frequencies of the slabs. Local representation of damage in terms of reinforcing steel strains is also discussed. The computational models for both the as-built and the retrofitted slabs are verified using experimental results. In these experiments, a slowly increasing uniform pressure is applied to the bottom surface of large-scale RC slab specimens using high-pressure water bag. Experimental results showed that an increase up to 200% in the load carrying capacity is achieved when using the CFRP composite retrofit system. Transient nonlinear analysis results proved the efficiency of the CFRP composite retrofit in improving the slab behavior under blast loading for different loading durations, i.e. for small, medium, and large charge weights at the same applied maximum pressure. In particular, less than 50% reduction of the fundamental frequency due to concrete damage is obtained for the retrofitted slab compared to more than 85% reduction for the as-built slab. Moreover, the maximum displacement is reduced by 40–70% with the CFRP retrofit compared to the as-built slab. As for reinforcing steel strains, the application of CFRP retrofit significantly limited the spread of yielding in time and space. The improved slab behavior with CFRP is best when retrofitting is applied to both sides of the slab.

Journal ArticleDOI
Dapeng Zhao1
TL;DR: In this article, a review of the structure, magmatism, and dynamics of subduction zones is presented, which suggests that arc magmatic systems are not limited to the near surface areas, but are related to the deep processes, such as the convective circulation in the mantle wedge and dehydration reactions in the subducted slab.

Journal ArticleDOI
TL;DR: In this paper, 10 years of avalanche occurrence data from the Swiss Alps have been analyzed to find characteristics of human-triggered dry snow slab avalanches, and the analysis of the 90 profiles available did support most of the mainly unstructured knowledge used in stability evaluation based on snow profiles and support the simple model of skier triggering.

Journal ArticleDOI
TL;DR: In this paper, phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs, both under equilibrium and disequilibrium (metastable) conditions.

Journal ArticleDOI
TL;DR: In this article, gas data were collected from geothermal production fields, fumaroles, and hot springs in Central America in order to investigate the relation between volatile output and spatial distribution of volcanic systems.
Abstract: [1] Gas data were collected from geothermal production fields, fumaroles, and hot springs in Central America in order to investigate the relation between volatile output and spatial distribution of volcanic systems. The 3He/4He ratios are 6.5 ± 0.7 Ra throughout the region, indicating that helium is predominantly of mantle origin and is largely independent of variations in the characteristics of the arc. Lower ratios produced by radiogenic production within the Chortis Block are restricted to the Berlin geothermal field and the region behind the volcanic front. Ratios of CO2/3He are inversely related to the distance between the volcanic system and the trench. In the southwestern portion of the arc, where the arc-trench gap is short and the subduction angle is shallow (Miravalles, Costa Rica), decarbonation is enhanced relative to the mantle helium flux resulting in higher CO2/3He. In the northwest, where the gap is greater and the subduction angle steeper (Ahuachapan, El Salvador), decarbonation decreases relative to the helium flux. While variations in the carbon isotopic signature have traditionally been linked to the composition of the subducted sediments, the Central American data provide evidence that other factors within the convergent plate boundary such as arc-trench gap, crustal thickness, and subduction angle play an important role in controlling the flux of CO2 from the subducting slab. The Central American Volcanic Arc gases show no apparent contribution of carbon dioxide derived from subducted organic sediments. Shallow crustal processes, including partitioning and isotopic fractionation, account for the minor deviations from direct mixing of mantle and carbonate-derived end-members. Given that the Central American arc system is not unique in terms of the composition of the subducted sediments or the volcanic output, previous interpretations of global volcanic flux in terms of carbonate and sediment output should be reconsidered. Carbon-helium relationships in Central America require that only 0.3–3.3% of the subducted carbon is released in volcanic eruptions, while the rest is presumably reintroduced into the deeper mantle. This is generally an order of magnitude lower than global averages and is limited by the availability flux of mineral-bound water and the temperature of release. The δ13C and CO2/3He ratios suggest that even though the amount of carbon that is released from the slab and subducted sediments is relatively low in Central America, it still makes up 86–98% of the total carbon released from arc volcanics.

Journal ArticleDOI
TL;DR: In this paper, a model for analyzing the shear-lag effect in composite beams with flexible shear connection is proposed, which is based on the virtual work theorem for three-dimensional bodies.
Abstract: Taking into account the long-term behavior of the concrete, this paper proposes a model for analyzing the shear-lag effect in composite beams with flexible shear connection. By assuming the slab loss of planarity described by a fixed warping function, the linear kinematics of the composite beam is expressed by means of four unknown functions: the vertical displacement of the whole cross section, the axial displacements of the concrete slab and of the steel beam, and the intensity of the warping (shear-lag function). A variational balance condition is imposed by the virtual work theorem for three-dimensional bodies, from which the local formulation of the problem, which involves four equilibrium equations with the relevant boundary conditions, is achieved. The assumptions of linear elastic behavior for the steel beam and the shear connection and of linear viscoelastic behavior for the concrete slab lead to an integral-differential type system, which is numerically integrated. The numerical procedure, based on the step-by-step general method and the finite-difference method, is illustrated and applied to a composite beam to get information on the complex time-dependent behavior, including shear-lag and connection deformability effects.

Journal ArticleDOI
TL;DR: In this paper, a constitutive model is developed for material non-linear analysis of steel fiber reinforced concrete slabs supported on soil, where the energy absorption capacity provided by fibre reinforcement is taken into account in the material constitutive relationship.

Journal ArticleDOI
Scott D. King1
TL;DR: In this paper, a review of subduction and geodynamic models is organized around three central questions: (1) Why is subduction asymmetric? (2) Are subducted slabs strong or weak? (3) How do subducting slabs interact with phase transformations, changes in mantle rheology, and possibly chemical boundaries in the mantle?

Journal ArticleDOI
01 Nov 2001
TL;DR: In this paper, the results of laboratory tests on fifteen reinforced concrete slab strips typical of a bridge deck slab and compares them to predicted strengths using the current codes and compressive membrane action theory.
Abstract: The corrosion of reinforcement in bridge deck slabs has been the cause of major deterioration and high costs in repair and maintenance. This problem could be overcome by reducing the amount of reinforcement and/or altering the location. This is possible because, in addition to the strength provided by the reinforcement, bridge deck slabs have an inherent strength due to the in-plane arching forces set up as a result of restraint provided by the slab boundary conditions. This is known as arching action or Compressive Membrane Action (CMA). It has been recognised for some time that laterally restrained slabs exhibit strengths far in excess of those predicted by most design codes but the phenomenon has not been recognised by the majority of bridge design engineers. This paper presents the results of laboratory tests on fifteen reinforced concrete slab strips typical of a bridge deck slab and compares them to predicted strengths using the current codes and CMA theory. The tests showed that the strength of lat...

Journal ArticleDOI
TL;DR: The assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab, at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern Japan.
Abstract: The assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab, at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern Japan. The combination of evidence from velocity tomography in the mantle wedge above the slab and mapping of earthquake size distribution within it strongly points to a source of fluids at the top of the slab at 140 to 150 km depth, from where material rises along an inclined path to the volcanoes.

Journal ArticleDOI
TL;DR: In this article, the deformation-induced macrosegregation in continuous casting of steel has been simulated using a finite-volume scheme, where the mass conservation and Darcy equations were solved in a Eulerian reference frame.
Abstract: The deformation-induced macrosegregation in continuous casting of steel has been simulated using a finite-volume scheme. For that purpose, a two-dimensional heat-flow computation was first performed in a Eulerian reference frame attached to the mold, assuming a unique solidification path, i.e., a unique relationship between temperature and enthalpy. This gave the stationary enthalpy field in the longitudinal section of the slab. On the other hand, bulging of the slab between two rolls was calculated in the same section, assuming plane-strain deformation and using the ABAQUS code. The Lagrangian reference frame was attached to the slab, and the rolls were moved at the surface until a stationary, bulging deformation profile was reached. The bulging of the surface was then used as an input condition for the calculation of the velocity and pressure fields in the interdendritic liquid. Using a fairly simple hypothesis for the deformation of the solid skeleton, the mass conservation and Darcy equations were solved in a Eulerian reference frame. This calculation was performed in an iterative loop, within which the solute conservation equation was also solved. At convergence and using the enthalpy field, this calculation allowed to obtain the temperature, the volume fraction of solid, and the average concentration fields, in addition to the fluid velocity and pressure. It is shown that the positive centerline segregation of carbon in the slab is well reproduced with this model. The effects of shrinkage and soft reduction were also investigated.

Journal ArticleDOI
TL;DR: In this article, a method of seismic tomography was used to invert 28,230 P wave arrival times from 2666 local earthquakes that occurred in and around Vancouver Island from 1970 to 1990.
Abstract: At the Cascadia margin the Juan de Fuca plate is subducting beneath the North America plate, causing active seismicity within both plates. Earthquakes occur down to a maximum depth of 80 km within the descending oceanic plate and to about 30 km in the overriding continental plate. We use a method of seismic tomography to invert 28,230 P wave arrival times from 2666 local earthquakes that occurred in and around Vancouver Island from 1970 to 1990. The tomography model uses about 30 km horizontal and 12–19 km vertical grid spacing and assumes that the seismic velocity perturbations vary continuously between grid points. Velocity structures can be obtained to a depth of 65 km. The obtained tomographic image shows an extensive low velocity zone above the subducted slab at about 45 km depth and patches of low velocities at shallower depths just seaward of the volcanic front. The deeper extensive low velocity zone may indicate the presence of partially hydrated mantle, most likely serpentinite, as a result of slab dehydration associated with the transformation of metabasalt to eclogite. One of the shallow low velocity patches coincides with an abrupt increase in surface heat flow and may reflect the presence of partial melts or water in the crust.