Slab

About: Slab is a research topic. Over the lifetime, 31617 publications have been published within this topic receiving 318693 citations.

Stress and Flow beneath Island Arcs

Abstract: Summary. The flow pattern, stress distribution, topography, and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the Aleutian arc. The results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench. In models with simple distributions of rheological parameters, the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc. Material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab. Models with the resistance to flow thus reduced had outer rises, deep trenches, horizontal tension seaward of the trench, horizontal compression under the trench, and downdip tension in the slab. Free air gravity anomalies, which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow, were not fit excellently by any of the models, in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc. An alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models. Shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench. This is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism.

Insulated concrete building panels and method of making the same

Abstract: A three-layer insulated concrete panel includes as the middle layer an insulating slab having grooves which provide a form for casting of concrete supporting ribs integral with a layer of concrete cast over the grooved face. A layer of material, such as particle board, is bonded to the ungrooved face of the slab. In preparing the panel, the slab is placed on a flat surface with the particle board face down. Forms are then placed in spaced-apart relation to panel edges, and concrete is cast into the forms and grooves and over the grooved panel face. The insulating slab provides a form for casting of supporting ribs and is permanently retained in the panel, giving it a high insulating value.

Modeling anisotropy and plate‐driven flow in the Tonga subduction zone back arc

Abstract: The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5–1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (∼4% for shear strains of 1.5, with a maximum value of ∼6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

Modeling anisotropy and plate-driven flow in the Tonga

Abstract: The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orien- tation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5-1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (-4% for shear strains of 1.5, with a maximum value of-6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

Dynamic models of subduction: geophysical and geological evidence in the Tyrrhenian Sea

Abstract: SUMMARY Predictions based on a 2-D finite-element model for subduction underneath the Calabrian Arc in southern Italy are compared with a variety of geophysical and geological data, such as the present-day stress pattern within the slab, uplift from the elevation of marine terraces in Calabria and subsidence in the Tyrrhenian Marsili Basin from ODP Leg 107. We model the behaviour of the slab driven by slab pull, in agreement with the present tectonic style in this part of the Mediterranean as suggested by several investigators. The model accounts for the crustal, lithospheric and mantle structures in a vertical cross-section perpendicular to the Calabrian subduction zone. The shape of the slab is constrained on the basis of new tomographic images in the southern Tyrrhenian Sea, which were obtained from the regional seismic stations of the Istituto Nazionale di Geofisica, while the rheological properties of the mantle are taken from global dynamic models. Density contrasts between the subducted slab and the surrounding mantle, based on petrological models, drive the flow in our viscoelastic model; stress values, displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached. Our estimates are appropriate for a time window of 100 kyr; the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day. Two families of models, with unlocked and locked subduction faults, are considered. The unlocked models allow for roll-back of the trench of about 20 mm yr-', in agreement with some geological estimates; the same family of models predicts uplift of the Calabrian Arc of about 1 mm yr-l and subsidence in the Marsili Basin of 1-2 mm yr-', in agreement with geological surveys. The deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern Tyrrhenian Sea, with minor concentration within the lithospheric wedge. Locked models fail to reproduce these geophysical and geological observations. Predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern Tyrrhenian Sea. Deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate: dynamic topography and horizontal motions are reduced, when compared with the continuous plate, with deviatoric stresses concentrated within the relict slab. Our results indicate that subduction is a major tectonic process in the southern Tyrrhenian Sea.