Slab

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

Wave propagation in a magnetically structured atmosphere: II: Waves in a magnetic slab

Abstract: Magnetic fields may introduce structure (inhomogeneity) into an otherwise uniform medium and thus change the nature of wave propagation in that medium. As an example of such structuring, wave propagation in an isolated magnetic slab is considered. It is supposed that disturbances outside the slab are laterally non-propagating. The effect of gravity is ignored.The field can support the propagation of both body and surface waves. The existence and nature of these waves depends upon the relative magnitudes of the sound speed co and Alfvén speed ςA inside the slab, and the sound speed ce in the field-free environment.In general terms the slow mode can always propagate, and does so both as a surface wave and as a body wave. On the other hand, the fast mode may propagate only in slabs that are not hotter than their surroundings (ce ≥ c0), and then it is a body wave or a surface wave accordingly as ce is greater than or less than ςA. For example, if ce > co > ςA then a fast body wave propagates with phase-speed between ce and co, a slow body wave between ςA and cT = coςA/(co2 + ςA2)1/2, and a slow surface wave with phase-speed below cT. There are no modes between co and ςA. As a second illustration, if ςA > ce > co, then in addition to the slow body and slow surface waves, as before, there is a fast surface wave with phase-speed between co and ce. There is no fast body wave.The special case of a slender field is also investigated and it is shown how the slender flux tube approximation relates to the more general results described above. In particular, the tube wave with phase-speed cT studied by Defouw (1976) and Roberts and Webb (1978) is shown to be a slow surface wave (sausage mode). Finally, we discuss briefly the generation of resonant modes in a slender slab.

Toroidal mantle flow through the western U.S. slab window

Abstract: The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western US cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada We suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range

Dubious case for slab melting in the Northern volcanic zone of the Andes

Abstract: It was first suggested by R.W. Kay that adakites may represent melts of subducted slab: since then, the term adakite has become synonymous with slab melts based on their unusual geochemical signature. This contribution, using the Northern volcanic zone of the Andes as an example, aims (1) to expose the weakness in simply associating a geochemical signature with a genetic mechanism and (2) to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting. We conclude that although slab melting probably occurs in some arcs, regional geochemical trends in the Northern volcanic zone and their relationship to the subduction-zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge-derived basaltic magmas.

Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity

Abstract: [1] We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1-D velocity model. Then, traveltime tomography is carried out to estimate three-dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS-PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust-type) earthquakes on the PAC slab is deepened by ∼30 km locally under the slab contact zone. This deepening is probably caused by a lower-temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low-velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS-PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.