Showing papers on "Slab published in 2011"

Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps

Abstract: .Carbon should therefore be retained in the slab and transportedto great depths in the mantle, rather than supply the arcvolcanoes. Here we identify diamonds in ultrahigh-pressurerocks from the Italian western Alps that have an oceanicorigin. We assess the geochemistry of diamond-bearing fluidinclusions and find that they contain bicarbonate, carbonateand sulphate ions, silica monomers, and crystals of carbonateand silicate. This fluid geochemistry indicates that carbon wasreleased from the slab at relatively shallow depths throughdissolution, not decarbonation. We conclude that dissolution,driven by fluids released from the subducted slab, is animportant mechanism for the transfer of carbon into themantle and ultimately back into the atmosphere, helping tobalance the carbon flux.

Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation

Abstract: The mantle transition zone beneath China is anomalously hydrated. Geochemical analyses of basalts erupted above the mantle transition zone in northeast China indicate that water may have been added to this zone during the dehydration of subducted slabs, on two separate occasions over the past one billion years.

Slab melting versus slab dehydration in subduction-zone magmatism

Abstract: The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.

Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry

Abstract: The Northern Cordilleran slab window formed beneath western Canada concurrently with the opening of the Californian slab window beneath the southwestern United States, beginning in Late Oligocene–Miocene time. A database of 3530 analyses from Miocene– Holocene volcanoes along a 3500-km-long transect, from the northern Cascade Arc to the Aleutian Arc, was used to investigate mantle conditions in the Northern Cordilleran slab window. Using geochemical ratios sensitive to tectonic affi nity, such as Nb/Zr, we show that typical volcanic arc compositions in the Cascade and Aleutian systems (derived from subduction-hydrated mantle) are separated by an extensive volcanic fi eld with intraplate compositions (derived from relatively anhydrous mantle). This chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the Northern Cordilleran slab window. We suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge, which had developed during extensive early Cenozoic arc and backarc volcanism in western Canada. High heat fl ow throughout the western Canadian Cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window-induced mantle upwelling.

Topography of the Calabria subduction zone (southern Italy): Clues for the origin of Mt. Etna

Abstract: [1] Calabria represents an ideal site to analyze the topography of a subduction zone as it is located on top of a narrow active Wadati-Benioff zone and shows evidence of rapid uplift. We analyzed a pattern of surface deformation using elevation data with different filters and showed the existence of a long wavelength (>100 km) relatively positive topographic signal at the slab edges. The elevation of MIS 5.5 stage marine terraces supports this pattern, although the record is incomplete and partly masked by the variable denudation rate. We performed structural analyses along the major active or recently reactivated normal faults showing that the extensional direction varies along the Calabrian Arc and laterally switches from arc-normal, within the active portion of the slab, to arc-oblique or even arc-parallel, along the northern and southern slab edges. This surface deformation pattern was compared with a recent high resolution P wave tomographic model showing that the high seismic velocity anomaly is continuous only within the active Wadati-Benioff zone, whereas the northern and southwestern sides are marked by low velocity anomalies, suggesting that large-scale topographic bulges, volcanism, and uplift could have been produced by mantle upwelling. We present numerical simulations to visualize the three-dimensional mantle circulation around a narrow retreating slab, ideally similar to the one presently subducting beneath Calabria. We emphasize that mantle upwelling and surface deformation are expected at the edges of the slab, where return flows may eventually drive decompression melting and the Mount Etna volcanism.

Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: new high-resolution results

Abstract: SUMMARY The Pampean flat slab of central Chile and Argentina (30°–32°S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basement-cored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ∼100 km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to ‘normal’ subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70 km in the main Cordillera and ∼50 km in the western Sierras Pampeanas, that shallow to ∼35 km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernandez Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ∼13–19 km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region.

Seismic heterogeneity and anisotropy of the Honshu arc from the Japan Trench to the Japan Sea

Abstract: SUMMARY We determinedP-andS-wave tomography andP-wave anisotropic structure under the Honshu arcfromtheJapanTrenchtothebackarcareaundertheJapanSeausing310749P-and150563 S-wave arrivals from 4655 local earthquakes recorded by 982 seismograph stations. Arrival timesfrom1451suboceanicearthquakesrelocatedwithsPdepthphasesenableustodetermine the structures under the Pacific Ocean and Japan Sea, which expand the study region from the land area to the whole arc from the Japan Trench to the Japan Sea with a width of more than 500 km. The results show strong heterogeneities above the subducting Pacific slab under the PacificOceanandmostlargethrust-typeearthquakesoccurredinthehigh-velocityareaswhere the Pacific slab and the overriding continental plate may be strongly coupled. Low-velocity (low-V) zones are imaged in the mantle wedge with significant along-arc variations under the volcanic front. The mantle-wedge low-V zone extends westwards under the Japan Sea and it is connected with the subducting Pacific slab at depths of 150–200 km under the backarc. The results indicate that the H2O and fluids brought downwards by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under the Honshu arc. The predominant fast velocity direction (FVD) is E–W in the mantle wedge while it is N–S in the subducting Pacific slab. The anisotropy in the mantle wedgeistheresultofdeformationcausedbythesubductionofthePacificplateandtheinduced mantle-wedge convection, while the FVD pattern in the middle of the mantle wedge argues for the 3-D mantle flow or the specific alignment of the olivine in the partial-melting mantle. The N–S (trench-parallel) FVD in the subducting Pacific slab represents either the original fossil anisotropy when the Pacific plate formed or the trench-parallel crystallographic and shaped preferred orientation in the subducting slab due to the slab bending. The present results shed new light on the structural heterogeneities and seismic anisotropy under the Honshu arc, which may improve our understanding of the dynamic processes of subduction zones.

A Spike and Slab Restricted Boltzmann Machine

Abstract: We introduce the spike and slab Restricted Boltzmann Machine, characterized by having both a real-valued vector, the slab, and a binary variable, the spike, associated with each unit in the hidden layer. The model possesses some practical properties such as being amenable to Block Gibbs sampling as well as being capable of generating similar latent representations of the data to the recently introduced mean and covariance Restricted Boltzmann Machine. We illustrate how the spike and slab Restricted Boltzmann Machine achieves competitive performance on the CIFAR-10 object recognition task.

Migration of dynamic subsidence across the Late Cretaceous United States Western Interior Basin in response to Farallon plate subduction

Abstract: Backstripped cross sections of the Late Cretaceous succession across central Utah, Colorado, and southern Wyoming in the Western Interior Basin, United States, reveal a component of continuously evolving long-wavelength residual subsidence, in addition to subsidence driven by the Sevier thrust belt and associated sediment loads. The loci of maximum rates of this residual subsidence moved eastward from ca. 98 to 74 Ma in phase with the west to east passage of the Farallon slab, as reconstructed from tomography based on quantitative inverse models. These new subsidence data allow testing of existing subduction models and confi rm the dynamic subsidence origin of the Western Interior Basin. Furthermore, regional variations in subsidence rates suggest a possible defi cit of negative buoyancy (mantle loading) inside the slab beneath Colorado, supporting the hypothesis that the thickened slab represents a subducted oceanic plateau. This paper documents how the Cretaceous stratigraphy records the timing, patterns, and position of underlying mantle processes during Farallon slab subduction. The new data also reveal patterns indicative of the commencement of the Laramide orogeny in the western United States.

Hydrodynamic mechanism for the Laramide orogeny

Abstract: The widespread presumption that the Farallon plate subducted along the base of North American lithosphere under most of the western United States and ∼1000 km inboard from the trench has dominated tectonic studies of this region, but a number of variations of this concept exist due to differences in interpretation of some aspects of this orogeny. We contend that five main characteristics are central to the Laramide orogeny and must be explained by any successful hypothesis: thick-skinned tectonism, shutdown and/or landward migration of arc magmatism, localized deep foreland subsidence, deformation landward of the relatively undeformed Colorado Plateau, and spatially limited syntectonic magmatism. We detail how the first two elements can be well explained by a broad flat slab, the others less so. We introduce an alternative hypothesis composed of five particular processes: (1) a more limited segment of shallowly subducting slab is created by viscous coupling between the slab and the Archean continental keel of the Wyoming craton, leaving some asthenosphere above most of the slab; (2) dynamic pressures from this coupling localize subsidence at the edge of the Archean Wyoming craton; (3) foreland shortening occurs after the subsidence of the region decreases gravitational potential energy, increasing deviatoric stresses in lithosphere beneath the basin with no change to boundary stresses near the subduction zone or changes to basal shear stress; (4) shear between the slab and overriding continent induces a secondary convective system aligned parallel to relative plate motion, producing the Colorado Mineral Belt above upwelling aligned along the convection cell; (5) the development of this convective system interrupts the flow of fresh asthenosphere into the arc region farther west, cutting off magmatism even in segments of the arc not over the shallowly dipping slab.

Analysis of Dynamic Behavior for Slab Track of High-Speed Railway Based on Vehicle and Track Elements

Abstract: Slab tracks are used worldwide in high-speed railways. The slab may be cast in situ, resulting in a continuous length of concrete, or it may be constructed in discrete precast sections laid end to end. According to structural characteristics of the China Railway Track System (CRTSII) slab track system, a model for dynamic analysis of vehicle-track-subgrade coupling system has been developed. Based on the model, a new type of slab track element is presented, and the associated stiffness matrix, mass matrix and damping matrix for the element are deduced. This element includes rail, rail fastening and pad, prefab slab, cement-asphalt mortar, hydraulically bonded layer, and subgrade. By means of the Lagrange equation, a numerical method for coupling the moving wheel and the rail with explicit formula is presented and the associated finite-element equation is formulated. As application examples, parameter studies on the track vibration of the slab track structure, such as stiffnesses and dampings resulting from the rail pad, CA mortar (cement-asphalt mortar) and subgrade, are investigated. To understand dynamic behavior of track transition from conventional ballast track to slab track, the effects of train speed and track stiffness on track vibration in the transition are evaluated. The obtained results show (1) reasonably chosen parameter values of the slab track structure and suitably specified stiffness of the transition will significantly improve the behavior of the track performance; (2) changing track stiffness in transition has an influence which increases with an increase of railspeed, on the vertical rail acceleration and the wheel/rail contact force; (3) smoothing of track stiffness in transition can reduce track vibration and improve the operational quality of the train. Countermeasures include long ties, additional rails, hot mix asphalt (HMA) underlayment, slab track approach, stone columns and piles to strengthen weak subgrade, rail seat pads, rubber tie mats, and other countermeasures.

An ultrathin twist-structure polarization transformer based on fish-scale metallic wires

Abstract: This study theoretically and experimentally investigates the transmission properties of a metamaterial slab comprised of two layers of metallic fish-scale structure arrays and a sandwiched dielectric layer. Calculations show that the asymmetric transmission can be tuned by varying the slab thickness, due to near-field interlayer coupling. The spatial evolution of the electric field indicates that the twist structure functions as a perfect polarization transformer at certain frequencies. Measured transmission spectra are in good agreement with calculated results when material dissipation is considered.

An ultra-thin waveguide twist constructed using fish-scale metallic wires

Abstract: This study theoretically and experimentally investigates the transmission properties of a metamaterial slab comprised of two layers of metallic fish-scale structure arrays and a sandwiched dielectric layer. Calculations show that the asymmetric transmission can be tuned by varying the slab thickness, due to evanescent interlayer coupling. The spatial evolution of the local field inside the structure indicates that the slab functions as a perfect polarization transformer at certain frequencies in the manner of a waveguide twist. Measured transmission spectra are in good agreement with calculated results when material dissipation is considered.

Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

Abstract: To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10–11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15–0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30–0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.

Segmentation of the Farallon slab

Abstract: Article history:Accepted 19 September 2011Available online 5 October 2011Editor: Y. RicardKeywords:Farallon subductionslab segmentationseismic tomographyviscosityB&R extensiontoroidal mantle flow Recent tomography images reveal a complex 3D mantle structure beneath western United States, with fea-ture morphology varying rapidly with depth. By assimilating plate motion history, paleo-age of sea floor,and paleo-geography of plate boundaries in a 3-D numerical model, we simulate the Farallon–Juan de Fucasubduction during the past 40 Ma. We find that the highly segmented upper mantle structure of westernU.S. is a direct result of the Farallon subduction. We show that the tilted ‘horseshoe’-shaped fast seismicanomaly beneath Nevada and Utah at 300–600 km depth range is in fact a segment of curled slab subductedsince 15 Ma, and the shallower linear slab beneath the Cascades is younger than 5 Ma. The distinct morphol-ogy between these two parts of the subduction system indicates the strong influence of the fast trench roll-back since 20 Ma, the northward migrating JF–PA–NA triple-junction, and the toroidal flow around slabedges. The observed mantle structures are used to constrain the rheology of the upper mantle throughmatching the shape, depth, and location of modeled subducted slab segments. The inferred viscosity forthe asthenosphere is 5×10

Seismic design and shake table tests of a steel post‐tensioned self‐centering moment frame with a slab accommodating frame expansion

Abstract: SUMMARY Post-tensioned (PT) self-centering moment frames were developed as an alternative to welded momentresisting frames (MRFs). Lateral deformation of a PT frame opens gaps between beams and columns. The use of a composite slab in welded MRFs limits the opening of gaps at the beam-to-column interfaces but cannot be adopted in PT self-centering frames. In this study, a sliding slab is used to minimize restraints to the expansion of the PT frame. A composite slab is rigidly connected to the beams in a single bay of the PT frame. A sliding device is installed between the floor beams and the beams in other bays, wherever the slab is allowed to slide. Many shaking table tests were conducted on a reduced-scale, two-by-two bay one-story specimen, which comprised one PT frame and two gravitational frames (GFs). The PT frame and GFs were self-centering throughout the tests, responding in phase with only minor differences in peak drifts that were caused by the expansion of the PT frame. When the specimen was excited by the 1999 Chi-Chi earthquake with a peak ground acceleration of 1.87g, the maximum interstory drift was 7.2% and the maximum lateral force was 270 kN, equal to 2.2 times the yield force of the specimen. Buckling of the beam bottom flange was observed near the column face, and the initial post-tensioning force in the columns and beams decreased by 50 and 22%, respectively. However, the specimen remained self-centering and its residual drift was 0.01%. Copyright 2011 John Wiley & Sons, Ltd.

Fluid Flow-Related Transport Phenomena in Steel Slab Continuous Casting Strands under Electromagnetic Brake

Abstract: In the current study, a three-dimensinal (3D) numerical model is built to investigate the effect of a local-type electromagnetic brake (EMBr) on the fluid flow, heat transfer, and inclusion motion in slab continuous casting strands. The results indicate that the magnetic force affects the jet characteristics, including jet angle, turbulent kinetic energy, and its dissipation rate. To reduce the top surface velocity and stabilize the top surface, the magnetic flux intensity should be larger than a critical value. With a 0.39 T magnetic flux intensity, the top surface velocity and its fluctuation can be well controlled, and less slag is entrained. The motion of argon bubbles is also studied. More bubbles, especially >2.0-mm bubbles, escape from the top surface between the mold submerged entry nozzle (SEN) and \( \frac{1}{4} \) width for the case with a 0.39 T EMBr. This may push the top slag away and create an open “eye” on the top slag. Small bubbles (≤1 mm) tend to escape from one side of wide face no matter with or without EMBr, which is induced by the swirl flow from the SEN outport. EMBr has a little effect on the overall removal fraction of inclusions; however, it affects the local distribution of inclusion in the slab. With EMBr, more inclusions accumulate the region just below the surface, thus a worse subsurface quality, whereas the inner quality of the slab is better than that without EMBr. For heat transfer in the mold, the heat flux on the narrow face and the area of possible break-out zones can be reduced by using EMBr. Prevention of bias flow and/or asymmetrical flow in mold by EMBr is also concluded.