Slab

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

Relations among subduction parameters

Abstract: Clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones. Based on a critical appraisal of the geophysical and geological literature, 26 parameters are estimated for each of 39 modern subduction zones. To isolate causal relationships among these parameters, multivariate analysis is applied to this data set. This analysis yields empirical quantitative relations that predict strain regime and strike-slip faulting in the overriding plate, maximum earthquake magnitude, Benioff zone length, slab dip, arc-trench gap, and maximum trench depth. Excellent correlation is found between length of the Benioff zone and the product of convergence rate and age of the downgoing slab. This relationship is consistent with the conductive heating model of Molnar et al. (1979), if the model is modified in one respect. The rate of heating of the slab is not constant; it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate. The structural style in the overriding plate is determined by its stress state. Though the stress state of overriding plates cannot be quantified, one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional (class 1, back-arc spreading) to strongly compressional (class 7, active folding and thrusting). This analysis indicates that strain class is probably determined by a linear combination of convergence rate, slab age, and shallow slab dip. Interplate coupling, controlled by convergence rate and slab age, is an important control on strain regime and the primary control on earthquake magnitude. Arc-parallel strike-slip faulting is a common feature of convergent margins, forming a forearc sliver between the strike-slip fault and trench. Optimum conditions for the development of forearc slivers are oblique convergence, a compressional environment, and a continental overriding plate. The primary factor controlling presence of strike-slip faulting is coupling; strongly oblique convergence is not required. The rate of strike-slip faulting is affected by both convergence obliquity and convergence rate. Maximum trench depth is a response to flexure of the underthrusting plate. The amount of flexural deflection at the trench depends on the vertical component of slab pull force, which is very sensitive to slab age and shallow slab dip. Shallow slab dip conforms to the cross-sectional shape of the overriding plate, which is controlled by width of the accretionary prism and duration of subduction. Deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow. Much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike-slip faulting and strain regime. Empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones.

Mantle and Slab Contributions in ARC Magmas

Abstract: The current consensus is that most arc rocks crystallized from parental magmas generated in the mantle wedge, and that melting took place in the presence of water so that the temperature at the solidus was less than that beneath mid-ocean ridges and oceanic islands. The inference is that the water was released from the subducted oceanic crust, and that other more mobile elements were transported in the hydrous fluids, but the size and nature of the contribution from the subducted crust, and how it may be recognized, remain contentious. This review considers ways in which contributions from the subducted slab and the mantle wedge may be recognized, and examines estimated fluxes from subducted material. The emphasis is on the isotope and trace element geochemistry of the rocks themselves, which forms the basis for assessing the size and nature of the mantle and slab-derived components. -from Authors

Seismicity and shape of the subducted Nazca Plate

Abstract: An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study of the geometry of the Wadati-Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with ∼30° dip at latitude 33°S. In contrast, the transition from 30° slab dip beneath Bolivia to a nearly horizontal dip in the region between 28°S–32°S is more gradual, occurring over several hundred kilometers of along-strike distance between 20°S and 32°S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27°S–28°S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the South American plate. We hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin. Focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses. Results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths.

P wave tomography of the mantle under the Alpine-Mediterranean area

Abstract: [1] We study the upper mantle P wave velocity structure below the Euro-Mediterranean area, down to 1000 km depth, by seismic travel time tomography. We invert summary residuals constructed with both regional and teleseismic first arrival data reported by the International Seismological Centre (ISC) (1964–1995), introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure. Our high-resolution model PM0.5 is parameterized with three-dimensional (3-D) linear splines on a grid of nodes with 0.5° spacing in both horizontal directions and 50 km vertical spacing. We obtain about 26% root-mean-square (RMS) reduction of residuals by inversion in addition to roughly 31% reduction after summary rays formation and selection. Sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales. The distribution of large-scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions. The mantle dynamics of western central Europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone. Contrarily, in the eastern Mediterranean, fast velocity material sinks into the lower mantle, suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity. On a smaller scale, the existence of tears in the subducted slab (lithospheric detachment) all along both margins of the Adriatic plate, as proposed by some authors, is not supported by our tomographic images.

Migration of fluid phases and genesis of basalt magmas in subduction zones

Abstract: Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plate boundaries. The subduction of oceanic lithosphere, occurring with increasing pressures and rising temperatures, causes liberation of H2O from the slab. Amphibole, which can be stable to the highest PT conditions among hydrous phases in the slab, decomposes at around 90 km depth. It follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab-derived H2O is not a direct trigger for the production of arc magmas. Instead, the H2O released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals (serpentine, talc, amphibole, chlorite, and phlogopite). This hydrated peridotite is dragged downward on the slab toward higher PT regions and releases H2O to shallower potential magma source regions in the mantle wedge. Combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge, it is concluded that those minerals decompose beneath the forearc region. On the other hand, high PT experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front. Phlogopite in the peridotite decomposes to release H2O at a deeper level (about 200 km). H2O liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites. Along with the migration of H2O through the above processes, subduction components, especially large ion lithophile elements, can be overprinted on the magma source region, which governs the geochemical characteristics of arc magmas.