Showing papers on "Slab published in 1998"

The dynamics of cenozoic and mesozoic plate motions

Abstract: Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present-day plate motions A fuller understanding, how- ever, can be derived from consideration of the history of plate motions Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate-driving forces We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions The present-day field is not equipartitioned in poloidal and toroidal compo- nents; toroidal motions account for only one third of the total The toroidal/poloidal ratio has changed substan- tially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone We develop a self- consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) cal- culating the induced plate motions for each stage of the Cenozoic The "slab" heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core-mantle boundary The slab model pre- dicts the observed geoid extremely well, although com- parison between predicted and observed dynamic topog- raphy is ambiguous The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present The plate motion model itself successfully predicts Cenozoic plate motions (glob- al correlations of 07-09) for mantle viscosity structures that are consistent with a variety of geophysical studies We conclude that the main plate-driving forces come from subducted slabs (! 90%), with forces due to litho- spheric effects (eg, oceanic plate thickening) providing a very minor component (" 10%) For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs Unfortunately, we cannot re- produce the toroidal/poloidal partitioning ratios ob- served for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawai- ian-Emperor seamount chain Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian-Emperor bend However, we find that this collision has no effect on the motion of the Pacific plate, implying that impor- tant plate boundary effects are missing in our models Future progress in understanding global plate motions requires (1) more complete plate reconstruction infor- mation, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc man- tle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity con- straints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change

Mineralogy and dynamics of a pyrolite lower mantle

Abstract: There is a growing consensus that the Earth's lower mantle possesses a bulk composition broadly similar to that of the upper mantle (known as pyrolite)1,2,3. But little is known about lower-mantle mineralogy and phase chemistry4,5, especially at depth. Here we report diamond-anvil cell experiments at pressures of 70 and 135 GPa (equivalent to depths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silicate perovskite (MgPv), calcium-silicate perovskite (CaPv) and magnesiowustite (Mw). Contrary to recent speculation6,7, no additional phases or disproportionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultra-high pressures where thermal expansivities are low, buoyancy forces inherent in subducted slabs because of their lithology will be of similar magnitude to those required for thermally driven upwelling. So slabs would need to be about 850 °C cooler than their surroundings if they are to sink to the base of the mantle. Furthermore, initiation of plume-like upwellings from the core–mantle boundary, long attributed to superheating, may be triggered by lithologically induced buoyancy well before thermal equilibration is attained. We estimate that ascent would commence within ∼0.5 Gyr of the slab reaching the core–mantle boundary, in which case the lowermost mantle should not be interpreted as a long-term repository for ancient slabs.

Subduction of ophicarbonates and recycling of CO2 and H2O

Abstract: Because subducted serpentinites may release significant quantities of volatiles, high-pressure phase equilibria were computed for two end-member hydrothermally altered mantle harzburgite protoliths (ophicarbonates): calcite + antigorite + brucite and calcite + antigorite + talc. For both bulk compositions, most of the H 2 O released by metamorphic dehydration occurs at subarc depths; thus dehydration of serpentinites could be a major source for H 2 O in arc magmas. In contrast, for both model compositions a significant fraction of the original carbonate is retained to depths exceeding 200 km. Consequently, deep subduction of ophicarbonate rocks of the oceanic lithosphere and/or downward drag of mantle wedge ophicarbonates provide a mechanism for carbonating the mantle and thus a potentially significant CO 2 source for deep mantle melts. The probable CO 2 sources for arc magmas are metamorphic decarbonation of marine sediments and/or carbonated mafic volcanics in the subducted slab.

Slab in the wrong place : lower lithospheric mantle delamination in the last stage of the eastern carpathian subduction retreat

Abstract: A model with a new type of delamination of the lower lithospheric mantle is proposed to explain the Pliocene to recent tectonic evolution of the Eastern Carpathians. We suggest that, after the continental collision in middle Miocene time, break-off of the west-dipping subducting slab occurred at a depth of 70 km. Slab break-off propagated horizontally toward the east, inducing lithospheric delamination and movement of the Vrancea slab into its present position. Delamination was followed by rapid asthenospheric rise, resulting in magma generation and the explosive alkaline basaltic magmatism of the Persani Mountains. Contamination of the former subduction-related magmatic reservoirs with the alkalic basaltic material and differentiation processes produced the Harghita calc-alkaline and shoshonitic rocks. The asthenospheric rise induced crustal uplift, which is the triggering mechanism for extension and formation of the Brasov-Gheorghieni hinterland basins. The extension was accommodated by shortening and folding in the foreland. The vertical Vrancea slab appears in our model as a segment of delaminated lower lithospheric mantle that is seismically active due to the ongoing pull of the eclogitized oceanic crust. The model explains the displaced and shallow position of the slab relative to the Eastern Carpathian suture zone.

Testing hypotheses on plate-driving mechanisms with global lithosphere models including topography, thermal structure, and faults

Abstract: A popular concept of plate tectonics is that the most important density anomalies are in oceanic plates, which descend from rise to trench while subducting slabs act as velocity regulators and the drag on the base of the lithosphere is resistive. This hypothesis has been shown to be consistent with velocities of rigid plates and to approximately predict stress directions in a global elastic shell. Here I test whether the hypothesis leads to correct plate velocities and stress directions in models of laterally heterogeneous plates of nonlinear rheology separated by faults with low friction. All models have bottom boundary conditions based on simple shear in an olivine asthenosphere extending to the transition zone, where various patterns of lower mantle flow are assumed. These models show that topography alone is not sufficient to drive plate motion at actual rates over a static or sluggish transition zone. If the forward component of velocity of each subducting slab is also imposed, then velocities are improved, but errors in stress direction become unacceptable. Better models are found by assuming that at least some parts of the transition zone have velocities greater than surface velocities, leading to active or forward basal drag. In the most plausible model, this forward drag acts only on continents, while oceanic lithosphere experiences negligible basal shear tractions. Probably the dense descending slabs of oceanic lithosphere not only pull the oceanic plates, but also stir the more viscous lower mantle, and this in turn helps to drive the slower drift of continents.

Refinements to the stability index for skier-triggered dry-slab avalanches

Abstract: Stability indices for skier-triggering of slab avalanches are discussed in terms of an adjustment to the skier-induced stress to allow for ski penetration and an adjustment to shear strength (measured with a shear frame) to allow for normal load due to the slab overlying the weak layer. The proposed adjustment to shear strength depends on the microstructure of the weak layer. These adjustments are incorporated into a refinement of the previously established Swiss stability index for skier-triggering. The percentage of correct predictions for the Swiss and refined indices are evaluated using data from 115 weak layers on skier-tested avalanche slopes, 83 of which were classified as persistent weak layers of surface hoar, faceted crystals or depth hoar and 32 as non-persistent weak layers. The refined index reduces the number of incorrectly predicted slab avalanches for the persistent weak layers.

A new three-dimensional finite-difference bidomain formulation for inhomogeneous anisotropic cardiac tissues

Abstract: Bidomain modeling of cardiac tissues provides important information about various complex cardiac activities. The cardiac tissue consists of interconnected cells which form fiber-like structures. The fibers are arranged in different orientations within discrete layers or sheets in the tissue, i.e., the fibers within the tissue are rotated. From a mathematical point of view, this rotation corresponds to a general anisotropy in the tissue's conductivity tensors. Since the rotation angle is different at each point, the anisotropic conductivities also vary spatially. Thus, the cardiac tissue should be viewed as an inhomogeneous anisotropic structure. In most of the previous bidomain studies, the fiber rotation has not been considered, i.e., the tissue has been modeled as a homogeneous orthotropic medium. Here, the authors describe a new finite-difference bidomain formulation which accounts for the fiber rotation in the cardiac tissue and hence allows a more realistic modeling of the cardiac tissue. The formulation has been implemented on the data-parallel CM-5 which provides the computational power and the memory required for solving large bidomain problems. Details of the numerical formulation are presented together with its validation by comparing numerical and analytical results. Some computational performance results are also shown. In addition, an application of this new formulation to provide activation patterns within a tissue slab with a realistic fiber rotation is demonstrated.

Detailed look at final stage of plate break‐off is target of study in Romania

Abstract: Geophysical experiments next year in Romania may provide insight into a common but short-lived seismic process that can be observed and understood at only one spot on Earth at present. About 150 stations will be set up in the Vrancea area in the southeast Carpathian Mountains to, in effect, record the terminal phase of the detachment of a subducting slab of oceanic lithosphere. This is a major regional tomographic study using a large number of broadband seismometers, which will operate for 6 months. Images will be used for hazard assessment as well as for a delineation of detachment history. Active subduction of oceanic lithosphere at convergent plate boundaries involves earthquakes, magmatism, metamorphism, and deformation—some of the most vivid manifestations of any plate tectonic process. The initiation and termination of subduction, however, remains relatively poorly understood. When convergence of lithospheric plates ceases and the suction force of the subducting plate becomes negligible, the subducting slab moves into an almost vertical position. If subduction occurs in an arcuate geometry, the slab is likely to be segmented.

Shear wave anisotropy in the Mariana Subduction Zone

Abstract: To determine the location, strength, and orientation of seismic anisotropy in the Mariana subduction zone beneath Guam, we evaluated shear wave splitting in local S, regional S and ScS, and teleseismic core phases such as SKS recorded at station GUMO. Fast directions from the local S phases have an average azimuth of −45°, and splitting times range from 0.1 s to 0.4 s. For local S phases from events within the southeastern half of the subducting slab, splitting parameters manifest minimal frequency dependence in both fast direction and splitting time. However, for the remaining local S phases in the data set, fast directions vary with frequency content. No well-constrained splitting parameters were obtained for the regional and teleseismic phases, but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of ∼−45°. Anisotropy due solely to olivine oriented by slab-entrained flow in the mantle wedge would produce local S fast directions at ∼−66°, and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at −20° to −30°. Neither of these predictions is consistent with the observed fast directions. However, the observed splitting, including the frequency-dependent fast directions, can be explained by models containing anisotropy in both the slab and wedge, and possibly (although not necessarily) anisotropy due to recent extension in the overriding Philippine sea plate.

Topography of the 660-km seismic discontinuity beneath Izu-Bonin : Implications for tectonic history and slab deformation

Abstract: We analyze the P wave codas of 65 paths from deep northwestern Pacific earthquakes recorded by arrays of stations in Germany, the western United States, India, and Turkmenistan. We identify a phase resulting from a near-source S-to-P conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km, which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition. We migrate these data along with 39 more from Wicks and Richards [1993], accounting for three-dimensional ray bending by the sloping discontinuity, to produce a high-resolution topography map of the 660-km discontinuity in the Izu-Bonin region. Assuming an equilibrium phase transition, we interpret the discontinuity depth in terms of local temperatures. The slab, if defined by a thermal anomaly greater than −400°K, is only about 100 km thick near 28°N suggesting the slab is penetrating into the lower mantle with little or no advective thickening. Farther to the north, however, cold material appears spread out over a wide region, consistent with the slab having been laid down flat on the 660-km discontinuity as the trench retreated 2000 km eastward. Both the narrow slab to the south and the flat-lying slab to the north are consistent with recent high-resolution tomographic images. The depression to 745 km along the arc is consistent with a maximum thermal anomaly of about 1100°K. Along the entire arc, the depression occurs directly beneath the deepest earthquakes, even where seismicity is dipping at 45° and stops at 450 km depth, suggesting that the slab steepens to a vertical dip at the deepest seismicity. This change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease.

Modeling the Tonga slab: Can travel time data resolve a metastable olivine wedge?

Abstract: We present the results of detailed modeling of the Tonga slab with the goals of determining whether high-resolution travel time data (1) can be fit by simple thermal and petrological slab models and (2) can resolve a metastable olivine wedge at depths greater than the equilibrium olivine-spinel phase boundary. We model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers (OBS) and island broadband seismic stations extending from the Lau backarc basin, across the Tonga trench and onto the Pacific plate. The data consist of 388 local, P wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3-month OBS deployment in late 1994. We locate the events using both local and teleseismic arrival times, and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference Earth model. The modeling consists of grid searches using a three-dimensional finite difference algorithm to compute local, first arriving travel times for equilibrium and metastable P wave velocity models constructed from thermal, mineralogical, and morphological constraints. The travel time anomalies are well fit by standard slab thermal models and P velocity temperature derivatives of −0.4 to −0.3 ms−1°C−1. Forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low-velocity region. Wedge velocity models provide a slightly better fit to the data than equilibrium models, but F tests indicate the improvement is not significant at the 95% level. Our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary.

System for transferring loads between cast-in-place slabs

Abstract: A load plate (900) for transferring loads between a first cast-in-place slab (102-1) and a second cast-in-place slab (102-2) separated by a joint (104). The load plate (900) having a substantially tapered end having substantially planar upper and lower surfaces adapted to protrude into and engage the first slab (102-1), and the load plate being adapted to transfer between the first and second slabs (102) a load directed substantially perpendicular to the intended upper surface of the first slab (102-1). A blockout sheath (1600) embedded within the first slab (102-1) could also be included. The blockout sheath (1600) could have a substantially planar top surface and a substantially planar bottom surface substantially parallel to the upper surface of the first slab (102-1).

Heat transfer controlled pyrolysis kinetics of a biomass slab, rod or sphere

Abstract: A theory for deriving the pyrolysis rate of a single infinite slab, infinite cylinder or sphere in a constant temperature furnace is suggested. In analogy with the shrinking-core model a pyrolysis propagation front velocity is defined. The velocity is thereafter used in a compartment-model approach for deriving a set of ordinary differential equations for solving the burn-off over time. A comparison with experimental and published data is also made.

Post-punching resistance of connections between flat slabs and interior columns

Abstract: The paper reports tests designed to study the improvement in the post-punching resistance of slab–column connections by the provision of bottom bars passing through the columns and anchored in the slab. A series of model tests on slabs was used to developan expression for the strength limit governed by the slab concrete, while large-scale tests on abbreviated specimens were used to formulate the criterion dependent on the failure of the reinforcement. It is demonstrated that bottom reinforcement can be of significant benefit and that the resistance given by it can be assessed with reasonable accuracy.

Extracting convergent surface formation energies from slab calculations

Abstract: It is known that surface formation energies obtained as differences between slab energies and an independently determined bulk energy will diverge linearly with the slab thickness. A recent paper by Fiorentini and Methfessel presented `a solution to this problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies'. Although their work is correct, the solution they propose is not new. In this comment, we provide some of the historical background neglected by Fiorentini and Methfessel.

Propagation in bianisotropic media - reflection and transmission

Abstract: In this paper a systematic analysis that solves the wave propagation problem in a general bianisotropic, stratified media is presented. The method utilizes the concept of propagators, and the representation of these operators is simpli- fied by introducing the Cayley-Hamilton theorem. The propagators propagate the total tangential electric and magnetic fields in the slab and only outside the slab the up/down-going parts of the fields need to be identified. This procedure makes the physical interpretation of the theory intuitive. The re- flection and the transmission dyadics for a general bianisotropic medium with an isotropic (vacuum) half space on both sides of the slab are presented in a coordinate-independent dyadic notation, as well as the reflection dyadic for a bianisotropic slab with perfectly electric backing (PEC). In the latter case the current on the metal backing is also given. Some numerical computations that illustrate the algorithm are presented. (Less)

Transverse-pumped sLAB laser/amplifier

Abstract: A light amplification device comprises a laser slab and optics for establishing an optical beam at the lasing wavelength. The beam travels inside the slab in a zig-zag path between two reflecting faces. The slab is also cooled through the reflecting faces. The slab is pumped through one or more pumping faces, each pumping face being distinct from the reflecting faces. Light from the pump is incident upon the slab in a direction that is transverse to the plane of propagation of the lasing wavelength beam. This arrangement separates the cooling interfaces from the pumping interfaces. Distortions in the beam due to thermal effects are largely averaged out by the zig-zag path. The device may function as either a laser or an optical amplifier.

Transmission of Interior Concrete Column Loads through Floors

Abstract: Clause 10.15.3 of the 1995 ACI code (ACI 318-95) defines the effective strength of a high-strength concrete interior column with an intervening normal strength concrete slab. This provision is based on laboratory tests in which no load was applied to the slab. Such tests may overestimate the strength of the column because they do not properly model the confinement conditions of the column-slab joint. This paper reports experimental results of tests with properly modeled confinement conditions. The test results show that the intensity of slab load, the aspect ratio of the joint, and the rectangularity of the column affect the strength of an interior column-slab joint. A new design equation replacing the provision in Clause 10.15.3 is proposed.

Moisture transfer analysis during drying of slab woods

Abstract: This article presents an analytical technique for determining the moisture diffusivities and moisture transfer coefficients for slab shaped woods subjects to drying process. The analysis of transient moisture diffusion is carried out on the basis of two important practical criteria: 0.1 100. The drying coefficients and lag factors are defined for wood-drying applications and incorporated into the models. In order to verify the present models, the model results are compared with experimental measurements taken from the literature and good agreement was found. Results show that the technique presented here is capable of determining the moisture diffusivities and moisture transfer coefficients for slab woods in a simple and accurate manner for practical applications and will be beneficial to the relevant wood␣drying industries. This approach can be extended to␣different wood products of regular and irregular shapes.

Rolling method for thin flat products and relative rolling line

Abstract: Rolling method for the production of flat products, such as metal strip, which provides a production step of a thin slab with a thickness comprised between 30 and 90 mm, preferably between 35 and 70 mm, a temperature maintenance and/or possible heating step of the slab to be sent for rolling, and a rolling step with multiple passes performed in a reversing rolling mill (14) comprising at least a reversing rolling stand ( 15), in which at least a winding reel furnace (16a) is also present upstream of the stand (15) and a winding reel furnace (16b) downstream of the stand (15), a winding step also being provided downstream of the rolling in at least a coiler (19a, 19b) to form a relative coil of strip of a defined weight, comprised between 20 and 30 tons, and a cooling step of the strip being provided between the reversing rolling mill (14) and said at least one coiler (19a, 19b). The method provides that the thin slab is disposed, for example, sheared to size, upstream of the maintenance and/or possible heating step in order to form a segment of slab having a length equivalent, in weight, greater than that of a coil, and that, already after the first rolling pass in the reversing rolling mill (14), the thickness of the segment of slab is reduced to a value in the range of 20-25 mm so as to render it windable on the reel furnace (16b) downstream of the stand (15).