Slab

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

Deformation of subducted oceanic lithosphere

Abstract: SUMMARY The deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in Benioff zones and by the increasingly well-resolved images of the upper mantle obtained by seismic P-wave tomography. By using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab, we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation. The simplified model assumes that the lithosphere has uniform physical properties, it is denser and much more viscous than the upper mantle, and its penetration below 670 km depth is resisted by a step-like density increase at that level. The primary determinant of the style of deformation is the buoyancy number, F, a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate U,/L, defined in terms of subduction rate U, and slab thickness L. For a small buoyancy number, deformation of the slab is dominated by viscous flexure. For large F, down-dip extension of the slab appears to dominate, and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level. The transition value of F at which this change in behaviour is observed is about 0.05 for constant viscosity, and about 0.2 for stressdependent viscosity with a stress versus strain-rate exponent of n = 3. With n = 3, a boudinage-type instability of the slab is observed for large F. Penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle. Resisted only by the density difference, the slab may initially penetrate several hundred kilometres below the 670 km level (for plausible density parameters) before a buckling instability causes this part of the slab to rotate and ascend. Comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the Tonga subduction zone suggests that the effective buoyancy number F for this slab is approximately equal to the transition value that we determined experimentally. We use this constraint to estimate average rheological parameters for the Tonga slab which, when compared with published creep deformation laws for olivine, are consistent with the average temperature of the slab being about 0.4 to 0.45 times the melting temperature, based on constitutive laws for olivine. At shallow depths (< 100 km) these temperatures correspond to around 590°C for wet olivine or around 700°C for dry olivine.

Subwavelength imaging by metallic slab lens with nanoslits

Abstract: A metallic slab lens featured with specially designed nano slits is presented to realize imaging for arbitrary object and image distances. Based on the particular propagation properties of surface plasmon polaritons in nanostructures, slits perforated in silver slab are designed with variant widths to produce desired optical phase retardations. Numerical simulation of an illustrative lens is performed through finite-difference time-domain method and shows that subwavelength imaging is realized at the designed position. (c) 2007 American Institute of Physics.

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.

Geophysical Constraints on Slab Subduction and Arc Magmatism

Abstract: Seismic tomography studies have revealed highly heterogeneous structures for the mantle wedge beneath several volcanic arcs. These structures are inclined seismic low-velocity and high-attenuation zones at depths shallower than ∼150 km in the mantle wedge sub-parallel to the slab, which probably correspond to the upwelling-flow portion of subduction-induced convection. Seismic studies for NE Japan suggest that temperatures are higher than the wet solidus of peridotite and that melt inclusions with volume fractions of 0.1-1% exist within this upwelling flow. Aqueous fluids supplied from the underlying slab meet this hot upwelling flow at depths of 100-150 km and perhaps cause partial melting. This inclined low-velocity zone crosses the Moho at the volcanic front, suggesting that the location of the volcanic front is determined by the position of this hot upwelling flow. Observations of heat flow and seismic anisotropy also support the existence of the upwelling flow. Seismic tomography study of the mantle wedge of NE Japan has further revealed an along-arc variation of the inclined low-velocity zone: very low velocity regions periodically occur about every 80 km along the strike of the arc. Clustering of Quaternary volcanoes and topographic highs at the surface are located immediately above these very low-velocity areas in the mantle wedge, and low-frequency microearthquakes, perhaps caused by rapid movements of fluids in the lower crust, occur right above them also. These observations show the value of 3D modeling of arc magmatism.