Showing papers on "Slab published in 1995"

Shape of the subducted Rivera and Cocos plates in southern Mexico: Seismic and tectonic implications

Abstract: The geometry of the subducted Rivera and Cocos plates beneath the North American plate in southern Mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes. The hypocenters of the teleseisms were relocated, and the focal depths of 21 events were constrained using a body wave inversion scheme. The suduction in southern Mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the Cocos plate beneath the Caribbean plate to the east and of the Rivera plate beneath North America to the west. The dip of the interplate contact geometry is constant to a depth of 30 km, and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate. On the basis of the seismicity, the focal mechanisms, and the geometry of the downgoing slab, southern Mexico may be segmented into four regions : (1) the Jalisco region to the west, where the Rivera plate subducts at a steep angle that resembles the geometry of the Cocos plate beneath the Caribbean plate in Central America ; (2) the Michoacan region, where the dip angle of the Cocos plate decreases gradually toward the southeast, (3) the Guerrero-Oaxaca region, bounded approximately by the onshore projection of the Orozco and O'Gorman fracture zones, where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km, and (4) the southern Oaxaca and Chiapas region, in southeastern Mexico, where the dip of the subduction gradually increases to a steeper subduction in Central America. These drastic changes in dip do not appear to take place on tear faults, suggesting that smooth contortions accommodate these changes in geometry. The inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the Trans-Mexican Volcanic Belt, suggesting it is directly related to the subduction. Thus the observed nonparallelism with the Middle American Trench is apparently due to the changing geometry of the Rivera and Cocos plates beneath the North American plate in southern Mexico, and not to zones of weakness in the crust of the North American plate as some authors have suggested.

Post-Laramide removal of the Farallon slab, western United States

Abstract: I propose that post-Laramide removal of the subhorizontally subducting Farallon slab occurred by buckling downward along an approximately east-northeast–trending axis. This process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the United States and propagation of the separated edges toward the central axis of downwelling, accompanied by aesthenosphic upwelling behind the trailing edges. Initial buckling probably began near 50 Ma, and slab removal was complete by 20 Ma. This model is based primarily on the space-time evolution of the “ignimbrite flare-up” (a major mid-Tertiary igneous event of mantle origin), which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central Nevada from the north and southeast. Post-Laramide uplift, extension, establishment of the Cascadia subduction zone, and active magmatism may be consequences of lithosphere-scale modifications caused by the Laramide removal of the slab and the resulting asthenospheric upwelling.

Mantle convection with plates and mobile, faulted plate margins.

Abstract: A finite-element formulation of faults has been incorporated into time-dependent models of mantle convection with realistic rheology, continents, and phase changes. Realistic tectonic plates naturally form with self-consistent coupling between plate and mantle dynamics. After the initiation of subduction, trenches rapidly roll back with subducted slabs temporarily laid out along the base of the transition zone. After the slabs have penetrated into the lower mantle, the velocity of trench migration decreases markedly. The inhibition of slab penetration into the lower mantle by the 670-kilometer phase change is greatly reduced in these models as compared to models without tectonic plates.

On the mechanism of seismic decoupling and back arc spreading at subduction zones

Abstract: To address the problem of the great variability of the mechanical state of subduction zones, we investigate the mechanics of back arc spreading and seismic decoupling. Back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench-normal tension reaches a critical value. Seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition. Two forces are important in this problem. The first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench. The second is a sea anchor force exerted on the slab that resists its lateral motion, assumed to occur at the upper plate velocity. Both forces contribute to the coupling problem: only the sea anchor force is responsible for back arc spreading. The unbalanced slab pull force is determined from a force balance for subduction, the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid. The model predicts three regimes: seismically coupled compressional arcs with advancing upper plates; seismically decoupled extensional arcs with retreating upper plates, and strongly extensional arcs which also have back arc spreading. This model is applied in detail to the Izu-Bonin-Marianas system. It shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1×1013 N m−1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed. It shows why the plate interface near Guam is seismically coupled, while the plate boundary everywhere farther north is decoupled. When applied globally, it successfully predicts the state of seismic coupling and back arc spreading in more than 80% of the world's subduction zones. Of the remaining, half can be seen to contain additional complications not included in the model. About 10% of cases remain unexplained, but some of these may have incorrectly determined seismic coupling coefficients.

Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench

Abstract: AT the Tonga trench, old Pacific sea floor subducts at a rapid rate below the Indo-Australia plate, generating most of the world's deep earthquakes (focal depth >300 km)1,2 and producing a deep slab of former oceanic lithosphere. The seismogenic part of the slab has been mapped in detail3,4, but its fate has remained enigmatic. Here I present evidence from seismic tomography that the Pacific plate descends deep into the Earth's mantle along a trajectory that is more complex than previously thought. In the north, the slab deflects in the transition zone (between about 400 and 700 km depth) before continuing into the lower mantle (below 700 km). Further south, penetration into the lower mantle occurs without a kink. The slab morphology can be explained in terms of the recent tectonic evolution of the subduction system, and reconciles pre-existing evidence from this region for both local horizontal flow in the transition zone2–8 and slab penetration into the lower mantle9–12.

Cross-Arc Geochemical Variations in the Kurile Arc as a Function of Slab Depth

Abstract: Lavas from transects across the Kurile Islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab. Enrichment factors for boron, cesium, arsenic, and antimony, all elements with strong affinities for water, decreased across the arc. This decrease is presumably related to losses of water-rich fluids during the dehydration of the subducting plate. Enrichments of potassium, barium, beryllium-10, and the light rare earth elements remained constant; these species may move in silica-rich fluids liberated from the slab at greater depths.

Electromagnetic field quantization in absorbing dielectrics

Abstract: The electromagnetic field is quantized for normal transmission of incident waves through a parallel-sided dielectric slab. The dielectric material is dispersive and it acts as a linear amplifier over limited ranges of the frequency and as a linear attenuator at the remaining frequencies. The field operators derived for the three spatial regions within and on either side of the slab are shown to satisfy the canonical commutation relations. The noise fluxes emitted by the slab are evaluated and shown to satisfy the general requirements for the minimum noise associated with linear amplifiers and attenuators. The behavior of the amplifier gain profile on the approach to the lasing threshold of the slab is determined, but the results are restricted to the below-threshold state of the system. The spectra of the electric-field fluctuations are evaluated for the three spatial regions and for amplifying and attenuating frequencies.

Hypothesis for Cretaceous rifting of east Gondwana caused by subducted slab capture

Abstract: In the process of subducted slab capture, a spreading ridge approaches subparallel to a subduction zone following the trailing edge of a downgoing plate. Eventually the downgoing plate is too young and small to subduct, and spreading stops. The spreading ridge stalls many tens of kilometres outboard of the subduction zone. The subducted plate welds to the outboard plate across the dormant spreading center and is captured by it. The captured plate then acquires the motion of the plate it welded to. In the southwest Pacific the Pacific-Phoenix ridge approached the east Gondwana margin as the Phoenix plate subducted beneath New Zealand, the Chatham Rise and Campbell Plateau, the Lord Howe Rise (collectively, Zealandia), and Marie Byrd Land in Cretaceous time. Spreading and subduction shut down here between 110 and 105 Ma, and some sections of the Phoenix plate became welded to (captured by) the Pacific plate. Pacific plate northward motion began in Aptian time, pulling the captured subducted microplates with it. This movement exerted a basal traction on the overlying east Gondwana margin and resulted in extension of Zealandia and Marie Byrd Land. Continued Pacific northward motion rifted Zealandia from Marie Byrd Land at about 85 Ma.

Three-dimensional numerical modeling of detachment of subducted lithosphere

Abstract: Recent seismological studies suggest that slab detachment has occurred in the Mediterranean and the New Hebrides subduction zones. Subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km, presumably caused by the lateral migration of the tear along the strike of the slab. To investigate the physical mechanism of the slab detachment and in particular its migration, we constructed a viscoelastic three-dimensional finite element model and introduced a small initial tear from one side of the slab. We investigated spatio-temporal variations in the state of stress within the slab, as a function of tear length, rheology, and a variety of force distribution. Our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab, which is probably sufficient to cause further lateral migration of the tear. The stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension. From our modeling, we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate, forming in-plate tensional stress at intermediate depths. Such a condition is indeed observed in the Dinarides/Hellenic and the New Hebrides subduction zone.

Imaging of subducted lithosphere beneath South America

Abstract: Tomographic images are produced for the deep structure of the Andean subduction zone beneath western South America. The data used in the imaging are the delay times of P, pP and pwP phases from relocated teleseismic earthquakes in the region. Regionally, structural features larger than about 150 km are resolved by the data. Presentations of layer anomaly maps and cross sections reveal: (1) The Nazca slab is probably continuous laterally and at depth over most regions; (2) The offset between the north and south deep earthquake zones, containing the 1994 deep Bolivia main shock and its aftershocks, can be modelled by a northwest striking and steeply northeast dipping slab structure; and (3) The Nazca slab clearly penetrates the lower mantle beneath central South America, but is partly deflected in the southern deep zone.

Sectional nog structure for fastening a covering element to a foamed plastic slab and construction element incorporating said structure

Abstract: A sectional nog structure for fastening a covering element to a foamed plastic slab (5, 6, 35, 39, 49, 54) comprises at least one insert (2, 3), flush-insertable in the slab (5, 6, 35, 39, 49, 54) and associated with a tierod (4), having a base (10) accessible from the outside of the slab and constituting respective means for fastening to the slab (5, 6, 35, 39, 49, 54) the chosen covering element. Advantageously, this sectional nog structure allows to manufacture composite and modular construction elements, such as for example lost formworks and panels incorporating at least one foamed plastic slab and being able to support any covering element.

40-W cw, TEM 00 -mode, diode-laser-pumped, Nd:YAG miniature-slab laser

Abstract: We have built a diode-laser-pumped Nd:YAG slab laser that emits 40 W of cw power in a TEM00 mode and 72 W of power in multimode operation when pumped with 235 W. The slope efficiencies are 22% for TEM00-mode operation and 36% for multimode operation. The laser uses a zigzag slab geometry to reduce thermally induced distortions and operates at less than one wave of distortion at the full pump power. A significant advantage of our design over those of previous slab lasers is a new Teflon AF protective coating on the slab total-internal-reflection surfaces, which greatly simplifies the mounting and cooling of the slab laser medium.

A Scattered-Wave Image of Subduction Beneath the Transverse Ranges

Abstract: Over 5600 short-period recordings of teleseismic events were used to create deterministic maps of P-wave scatterers in the upper mantle beneath Southern California. Between depths of 50 and 200 kilometers, the southern flank of the slab subducting beneath the Transverse Ranges is marked by strong scattering. The marked scattering indicates that the edge of the slab is a sharp thermal boundary. Such a boundary could be produced by slab shearing or small-scale convection in the surrounding mantle. The northern limb of the slab is not a strong scatterer, consistent with thicker lithosphere north of the Transverse Ranges.

Time-Dependent Behavior of Continuous Composite Beams at Service Loads

Abstract: An analytical model of the behavior of continuous composite beams under sustained service loads is described. Each beam consists of a rolled or fabricated steel I-section acting compositely with a reinforced concrete slab attached to the top flange. The analysis accounts for cracking of the concrete slab in the negative moment regions at each interior support, as well as the time-dependent deformation caused by creep and shrinkage in the concrete slab. Measurements from a series of full-scale continuous composite beams, which were tested by the writers in a separate study for a period of 340 days under sustained uniformly distributed loads, are used to validate the theoretical model. Close agreement is observed between analytical and experimental results. Finally, the analytical model is used to investigate the effects of material nonlinearity on structural behavior.

Analysis and Verification of Thermal-Gradient Effects on Concrete Pavement

Abstract: An experimental and analytical study was conducted to develop an effective method for determining realistic thermal-load induced stresses in concrete pavements. Temperatures throughout the concrete slabs were measured over an extended time period. Resulting critical stresses were then analytically derived following the procedure developed in this study. To verify these analytical stresses, load-induced strains and deflections were recorded at numerous locations on the test slab at various time periods, using the falling weight deflectometer as a loading device. The findings confirmed the importance and the need to account for the thermal gradient in the design and analysis of concrete pavements. The temperature data indicated that the temperature distributions were mostly nonlinear and can be represented fairly well by a quadratic equation. In addition, theoretical analysis of the induced stresses suggests the consideration of the total temperature distribution throughout the concrete slab depth rather th...

Leaky wave radiation from a periodically photoexcited semiconductor slab waveguide

Abstract: This paper presents investigations on leaky wave antennas that are modeled by periodical illumination of light on a grounded semiconductor slab waveguide using an asymptotic method of singular perturbation procedure based on multiple scales. Analytical results clearly show that the periodical illumination strongly affects the radiation characteristics such as efficiency and the radiation angle. The dominant effects are studied quantitatively and are outlined in the performance diagrams as a function of optically induced plasma density and the grating period. Initial experimental results at Q band using silicon slab guide under an array of 820-nm LED CW excitation are also reported and are in relatively good agreement with the theory. >

Equivalent Beam Model for Flat-Slab Buildings--Part II: Exterior Connections

Abstract: In analyzing flat-slab buildings for gravity and lateral loading, the same effective slab whidth is used at both interior and exterior slab-column connections. Tests of slab-column connections have clearly shown the moment-transfer mechanism at interior connections to be distinctly different than the one at exterior connections. The effective slab width and stiffness of the exterior connections is therefore significantly different from those of the interior connections. Recognition of this fact is important in accurately predicting the lateral drift and unbalanced moments at connections in flat-slab buildings. The equivalent beam model for slabs at exterior slab-column connections is presented. Based on test results of 41 exterior connections, the ultimate moment-transfer capacity is found to be a combination of the torsional capacity of the slab edge and flexural capacity of the slab portion framing into the front face od the column. The test results also show the actual torsional capacity of the spandrel beam or slab adge at exterior connections to be considerably larger than the theoretical capacity calculated as an isolated beam. An equivalent beam model is proposed for exterior connections that gives a better prediction of the unbalanced moment at connections and lateral drift of flat-slab buildings

Far‐field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent

Abstract: During a brief period of 10 to 15 million years in the Middle-Ordovician, the Michigan Basin departed from its bull's-eye subsidence pattern and tilted toward the east, opening to the Appalachian basin. This tilting is observed in maps of tectonic subsidence estimated for the Black River and Trenton Formations and extends over 300 km across the Michigan Basin and into eastern Wisconsin. Contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the Laurentian-Iapetus convergent margin. The distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km. Three alternative models for the tilting are tested, two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab. In the first elastic model we assume the edge of the elastic plate is simply loaded from above (by a fold and thrust sheet, volcanic pile, etc.). This model, however, cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate, even for lithospheres which have a rigidity of 10^28 Nm. In the second elastic model, the Laurentian continental margin descends into a trench of an eastward dipping slab, that is, a slab descending under the Iapetus ocean. This process cannot generate any significant far field displacements, even for extremely rigid plates, and must be rejected. For the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere. Such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin. The magnitude and distribution of far-field displacements depend on the age, length, and dip angle of the slab. In contrast to the elastic models, penetration of a west-dipping slab beneath the continent can reproduce both the extent, magnitude, and rate of tectonic subsidence observed in the Trenton and Black River Formations. The observed data are best fit by an old slab (140 Ma), which initially descended at a moderate dip (20°–30°) for 10–15 m.y., which then steepened as the slab penetrated deeper into the mantle. At the position of the previous Ordovician plate margin, there are narrow, block faulted basins which underwent rapid subsidence ∼10 m.y. before the Michigan Basin tilted toward the east. We propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting Cambro-Ordovician passive margin. The time lag of ∼10 m.y. may be due to the time it takes the slab to penetrate the upper mantle. This result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction.

Partial melting of subducted continental lower crust in the Pyrenees

Abstract: A magnetotelluric profile through the Central Pyrenees indicates the presence of very high conductive zones at lower crustal and upper mantle depths High conductivity at upper mantle depths is interpreted as partial melts within subducted lower crust, whereas high conductivity at lower crustal depths is interpreted as rising magmas derived from the melting crustal slab Such melts suggest that the continental lower crust, together with its lithospheric mantle may have been subducted into the mantle during the Pyrenean continental collision Thus, magma generation can be related to thermal reequilibration of a subducted lower crust, a scenario that may serve as a model for understanding the late evolution of other collisional orogens

Dynamic response analysis of slab-type bridges

Abstract: The response of a bridge under a moving vehicle is a complex phenomenon because of the interaction between the bridge and the vehicle. This complexity makes it difficult to identify the significant parameters that govern the response; yet a knowledge of such parameters is important for developing rational procedures for design. An earlier study by the writers used a beam model to identify the controlling parameters. In the present study, the bridge is idealized by a rectangular isotropic or orthotropic plate and the vehicle is represented by a single sprung mass moving along the deck. The plate is discretized by finite elements and the nonlinear equations of motion are solved in their incremental form by a process of numerical integration. The study shows that the governing parameters for an isotropic plate models are the aspect ratio, the speed parameter, the frequency ratio, and the mass ratio, while those for an orthotropic model are χ and θ, characteristics that depend on the geometry and the natural ...

Single-mode waveguide structure for optoelectronic integrated circuits and method of making same

Abstract: An optical waveguide for use in an optoelectronic integrated circuit and the associated method of manufacturing such a waveguide. The waveguide is formed by the successive layering of varied waveguide materials on a III-V semiconductor substrate, thereby producing a slab waveguide. The various layers within the slab waveguide are relatively thick, thereby producing a slab waveguide that is sized to be more compatible with an optical fiber and therefore more readily coupled to an optical fiber. The larger slab waveguide structure broadens the fundamental mode of the optical signal. However, multiple modes are also produced in the thicker slab waveguide that include higher order modes. A ridge structure is etched into the slab waveguide, wherein the width and height of the ridge structure are selected to impose lateral confinements on the higher order slab modes. The result is a waveguide that has a broadened fundamental mode yet is essentially single-mode. The resulting broadened fundamental mode is closer in symmetry to the mode of standard optical fibers, thereby enabling the waveguide to be more efficiently coupled to an optical fiber.

Three-dimensional finite element model for analysis of concrete pavement support

Abstract: A three-dimensional (3-D) finite element model for concrete pavements called 3DPAVE was developed to analyze the many complex and interacting factors that influence the support provided to a concrete pavement, including foundation support (subgrade k value); base thickness, stiffness, and interface bond and friction; slab curling and warping due to temperature and moisture gradients; dowel and aggregate interlock load transfer action at joints; and improved support with a widened lane, widened base, or tied concrete shoulder The ABAQUS general-purpose finite element software was used to develop a powerful and versatile 3-D model for analysis of concrete pavements The 3DPAVE model easily overcomes many of the inherent limitations of two-dimensional (2-D) finite element models that reduce the accuracy of the results obtained from 2-D models The 3-D model was validated by comparison with deflections and strains measured under traffic loadings and temperature variations at the AASHO Road Test, the Arlington Road Test, and the Portland Cement Association's slab experiments In every comparison with measured field data, 3DPAVE's calculated responses were found to be in good agreement with the measured responses and significantly closer to the measured responses than those calculated by 2-D programs The development and validation of the 3DPAVE model are described

Flow dynamics in thin slab caster moulds

Abstract: This study investigated the dynamic fluid flow phenomena that occurred in water models of a thin slab caster mould. The flow of fluid caused significant surface waves and was observed to be strongly oscillatory. The magnitude and the period of oscillation of these waves represent potential quality constraints in thin slab casting. The oscillation mechanism is explained in terms of the dynamic balance/imbalance between the pressure gradient across the jet emerging from the SEN and the opposing momentum of the recirculating, entrained flow.

Intermediate‐depth seismicity in northern Colombia and western Venezuela and its relationship to Caribbean plate subduction

Abstract: The recent intermediate-depth seismicity in northern Colombia and western Venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern South America. The area included in this study is located to the north and east of the Bucaramanga nest, which is a particular region beneath Bucaramanga, northern Colombia, that presents a high concentration of intermediate-depth earthquakes. To the north of the nest, the seismicity of the area is sparse, and most of the events are of low magnitude (mb≤5.l). Thus only 23 earthquakes were large enough to be investigated using teleseismic data. The focal parameters of the two largest events (mb≥5.4) were obtained from the formal inversion of long-period body waves recorded at teleseismic distances. The focal mechanisms of 10 more events were determined from first-motion data. In total, the focal mechanisms of 12 events were determined from both the inversion of P and S H waveforms and the polarities of first arrivals. For the smaller earthquakes, the focal depths were calculated by measuring the observed pP-P interval, time and comparing it to the theoretical travel time tables. The isodepth curves reflect a slab striking in a NNE–SSW direction and dipping approximately at 25°–32° to the southeast. This observation is corroborated by the direction and plunge of the T axes of the focal mechanisms, which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters. These results indicate that the intermediate-depth earthquakes in western Venezuela and northern Colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of Colombia. The two largest earthquakes, located at a significant distance from the Bucaramanga nest, present similar fault plane solutions. Moreover, they also agree with those of the two largest earthquakes reported inside the nest. This similarity suggests that the Bucaramanga nest lies on the same subducted slab where the other earthquakes occur. There is not enough shallow seismicity to define the location where the Caribbean lithosphere subducted beneath the South American plate. However, the extension of the slab toward the surface, inferred from the intermediate-depth seismicity, suggests that the subducted lithosphere may still be attached to the Caribbean plate.

Evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones

Abstract: A new model, incorporating shear deformation within a subducting slab, is proposed to explain slip partitioning for oblique plate motion at subduction zones. On the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is largely correlated with the calculated slab pull force. Such correlation suggests that other than the upper plate deformation, the slab pull force plays an important role in controlling oblique subduction. Our model proposes that the force balance between the slab pull force, the interplate coupling resistance, and the viscous mantle drag (the latter two are passive forces to the former one) produces a lateral shear within the slab, which causes the slab to deform and change its motion direction gradually toward trench normal as it subducts. The amount of direction change, which would be observed as the slip partitioning during earthquakes, therefore is closely related to the major plate driving force at the subduction zone, which is the slab pull force in our model.