Showing papers on "Slab published in 2012"

Arc magmas sourced from melange diapirs in subduction zones

Abstract: At subduction zones, crustal material enters the mantle. Some of this material, however, is returned to the overriding plate through volcanic and plutonic activity. Magmas erupted above subduction zones show a characteristic range of compositions that reflect mixing in the magma source region between three components: hydrous fluids derived from the subducted oceanic crust, components of the thin veneer of subducted sediments and peridotite mantle rocks. The mechanism for mixing and transport of these components has been enigmatic. A combination of results from the fields of petrology, numerical modelling, geophysics and geochemistry suggests a two-step process. First, intensely mixed metamorphic rock formations—melanges—form along the interface between the subducted slab and the mantle. As the melange contains the characteristic three-component geochemical pattern of subduction-zone magmas, we suggest that melange formation provides the physical mixing process. Then, blobs of low-density melange material—diapirs—rise buoyantly from the surface of the subducting slab and transport the well-mixed melange material into the mantle beneath the volcanoes. Magma erupted at subduction-zone volcanoes contains mantle rocks and a mixture of fluids and sediments derived from the subducted slab. A synthesis of work over past years provides an integrated physico-chemical framework for subduction zones with mixing at the slab–mantle interface and transport towards the surface volcanoes by buoyant diapirs.

Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity

Abstract: [1] A new model of the subducted Juan de Fuca plate beneath western North America allows first-order correlations between the occurrence of Wadati-Benioff zone earthquakes and slab geometry, temperature, and hydration state. The geo-referenced 3D model, constructed from weighted control points, integrates depth information from earthquake locations and regional seismic velocity studies. We use the model to separate earthquakes that occur in the Cascadia forearc from those that occur within the underlying Juan de Fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes. Seismicity within the slab is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic dehydration processes, promotes earthquake occurrence within the subducted oceanic plate. Earthquake patterns beneath western Vancouver Island are consistent with slab dehydration processes. Conversely, the lack of slab earthquakes beneath western Oregon is consistent with an anhydrous slab. Double-differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 M7.1 Mendocino earthquake. We infer that this earthquake ruptured a surface within the Cascadia accretionary margin above the Juan de Fuca plate. We further speculate that this earthquake is associated with a detached fragment of former Farallon plate. Other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes.

Anatomy of the Andean subduction zone: three-dimensional density model upgraded and compared against global-scale models

Abstract: SUMMARY We present an upgraded version of a previously published 3-D density model of the Andean subduction zone between 18°S and 45°S. This model consists of 3-D bodies of constant density, which geometry is constrained by independent seismic data and is triangulated from vertical cross-sections. These bodies define the first-order morphology and internal structure of the subducted Nazca slab and South American Plate. The new version of the density model results after forward modelling the Bouguer anomaly as computed from the most recent version of the Earth Gravitational Model (EGM2008). The 3-D density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental Moho (CMH) and has a trench-parallel resolution doubling the resolution of the previous model. As an example of the potential utility of our model, we compare the geometry of the subducted slab and CMH against the corresponding global models Slab1.0 and Crust2.0, respectively. This exercise demonstrates that, although global models provide a good first-order representation of the slab and upper-plate crustal geometries, they show large discrepancies (up to ±40 km) with our upgraded model for some well-constrained areas. The geometries of the slab, lithosphere–asthenosphere boundary below the continent, CMH and intracrustal density discontinuity that we present here as Supporting Information can be used to study Andean geodynamic processes from a wide range of quantitative approaches.

Martinique: a Clear Case for Sediment Melting and Slab Dehydration as a Function of Distance to the Trench

Abstract: In subduction zones, melting and dehydration of the subducted slab introduce material into the mantle wedge and modify its chemical and isotopic composition. As a consequence, island arc lavas differ significantly from mid-ocean ridge basalts and ocean island basalts. In some arcs, the composition of lavas is strongly influenced by the sedimentary material introduced with the slab; in others, magma composition is mainly affected by aqueous fluids released by the slab. The Lesser Antilles arc is known for its extreme continental-crust-like signature but for some Lesser Antilles lavas subducted sediments are barely involved and enrichment in fluid-mobile elements (Ba, U, Sr, Pb, etc.) is the dominant feature. Here we evaluate whether La/Sm is a quantitative proxy of sediment involvement in volcanic arcs, and we relate dehydration and melting processes to the temperature and pressure conditions of the slab. We use Martinique as a case study because in this island both dehydration and sediment melting fingerprints coexist. We measured major and trace elements for about 130 age-constrained samples, carefully chosen to cover all volcanic phases of Martinique (25 Ma to present). Using these results we demonstrate that: (1) weathering does not modify the La/Sm ratio; (2) fractional crystallization of amphibole and/or garnet does not increase La/Sm by more than 20%; (3) rare earth element transfer from wall-rock to magma during fractionation is not significant; (4) melting of the mantle source increases La/Sm by only about 20%. As a consequence, we show that the proportion of slab sediment incorporated in the mantle wedge controls the La/Sm ratio of the source. The observed correlations between La/Sm and Nd and Hf isotopic compositions indicate that the effect of sediment addition is the overwhelming factor: La/Sm is a good proxy for slab sediment proportion in Martinique. We observe a geographical gradient between slab dehydration and sediment melting on the island...

Scattering device on an arrayed waveguide grating

Abstract: An optical device may include a substrate; an arrayed waveguide grating provided on the substrate and having first and second slabs; multiple first waveguides extending from the first slab, the multiple first waveguides may supply respective first optical signals to the first slab; multiple second waveguides extending from the second slab, the multiple second waveguides may supply respective second optical signals to the second slab; a third waveguide extending from the second slab, the third waveguide outputting a third optical signal from the second slab, the third optical signal including the first optical signals; a fourth waveguide extending from the first slab, the fourth waveguide may output a fourth optical signal from the first slab, the fourth optical signal including the second optical signals; and a first scattering device optically coupled to a portion of an edge of the first slab between the multiple first waveguides and the fourth waveguide.

Chilean flat slab subduction controlled by overriding plate thickness and trench rollback

Abstract: How flat slab geometries are generated has been long debated. It has been suggested that trenchward motion of thick cratons in some areas of South America and Cenozoic North America progressively closed the asthenospheric wedge and induced flat subduction. Here we develop time-dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction. We find that as the craton approaches the trench and the wedge closes, two opposite phenomena control slab geometry: the suction between ocean and continent increases, favoring slab flattening, while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it. When the slab retreats, as in the Peru and Chile flat slabs, the wedge closure rate and dynamic push are small and suction forces generate, in some cases, flat subduction. We model the past 30 m.y. of subduction in the Chilean flat slab area and demonstrate that trenchward motion of thick lithosphere, 200–300 km, currently ∼700–800 km away from the Peru-Chile Trench, reproduces a slab geometry that fits the stress pattern, seismicity distribution, and temporal and spatial evolution of deformation and volcanism in the region. We also suggest that varying trench kinematics may explain some differing slab geometries along South America. When the trench is stationary or advances, the mantle flow within the closing wedge strongly pushes the slab backward and steepens it, possibly explaining the absence of flat subduction in the Bolivian orocline.

Fluid flow during slab unbending and dehydration: Implications for intermediate‐depth seismicity, slab weakening and deep water recycling

Abstract: Subducting oceanic plates carry a considerable amount of water from the surface down to mantle depths and contribute significantly to the global water cycle. A part of these volatiles stored in the slab is expelled at intermediate depths (70-300 km) where dehydration reactions occur. However, despite the fact that water considerably affects many physical properties of rocks, not much is known about the fluid flow path and the interaction with the rocks through which volatiles flow in the slab interior during its dehydration. We performed thermomechanical models (coupled with a petrological database and with incompressible aqueous fluid flow) of a dynamically subducting and dehydrating oceanic plate. Results show that, during slab dehydration, unbending stresses drive part of the released fluids into the cold core of the plate toward a level of strong tectonic under-pressure and neutral (slab-normal) pressure gradients. Fluids progressively accumulate and percolate updip along such a layer forming, together with the upper hydrated layer near the top of the slab, a Double Hydrated Zone (DHZ) where intermediate-depth seismicity could be triggered. The location and predicted mechanics of the DHZ would be consistent with seismological observations regarding Double Seismic Zones (DSZs) found in most subduction zones and suggests that hydrofracturing could be the trigger mechanism for observed intermediate-depth seismicity. In the light of our results, the lower plane of the DSZ is more likely to reflect a layer of upward percolating fluid than a level of mantle dehydration. In our models, a 20-30 km thick DSZ forms in relatively old oceanic plates without requiring an extremely deep slab hydration prior to subduction. The redistribution of fluids into the slab interior during slab unbending also has important implications for slab weakening and the deep water cycle. We estimate that, over the whole of Earth's history, a volume of water equivalent to around one to two oceans can be stored in nominally anhydrous minerals of the oceanic lithosphere and transported to the transition zone by this mechanism, suggesting that mantle regassing could have been efficient even without invoking the formation of high pressure hydrous minerals. Copyright 2012 by the American Geophysical Union.

Global variations in H2O/Ce: 1. Slab surface temperatures beneath volcanic arcs

Abstract: [1] We have calculated slab fluid temperatures for 51 volcanoes in 10 subduction zones using the newly developed H2O/Ce thermometer. The slab fluid compositions were calculated from arc eruptives, using melt inclusion-based H2O contents, and were corrected for background mantle contributions. The temperatures, adjusted to h, the vertical depth to the slab beneath the volcanic arc, range from ∼730 to 900°C and agree well (within 30°C on average for each arc) with sub-arc slab surface temperatures predicted by recent thermal models. The coherence between slab model and surface observation implies predominantly vertical transport of fluids within the mantle wedge. Slab surface temperatures are well reconciled with the thermal parameter (the product of slab age and vertical descent rate) andh. Arcs with shallow h (∼80 to 100 km) yield a larger range in slab surface temperature (up to ∼200°C between volcanoes) and more variable magma compositions than arcs with greater h (∼120 to 180 km). This diversity is consistent with coupling of the subducting slab and mantle wedge, and subsequent rapid slab heating, at ∼80 km. Slab surface temperatures at or warmer than the H2O-saturated solidus suggest that melting at the slab surface is common beneath volcanic arcs. Our results imply that hydrous melts or solute-rich supercritical fluids, and not H2O-rich aqueous fluids, are thus the agents of mass transport to the mantle wedge.

Experimental Investigation on Punching Strength and Deformation Capacity of Shear-Reinforced Slabs

Abstract: This paper presents the results of an extensive experimental campaign on 16 flat-slab specimens with and without punching shear reinforcement. The tests aimed to investigate the influence of a set of mechanical and geometrical parameters on the punching shear strength and deformation capacity of flat slabs supported by interior columns. All specimens had the same plan dimensions of 3.0 x 3.0 m (9.84 x 9.84 ft). The investigated parameters were the column size (ranging between 130 and 520 mm [approximately 5 and 20 in.]), the slab thickness (ranging between 250 and 400 mm [approximately 10 and 16 in.]), the shear reinforcement system (studs and stirrups), and the amount of punching shear reinforcement. Systematic measurements (such as the load, the rotations of the slab, the vertical displacements, the change in slab thickness, concrete strains, and strains in the shear reinforcement) allow for an understanding of the behavior of the slab specimens, the activation of the shear reinforcement, and the strains developed in the shear-critical region at failure. Finally, the test results were investigated and compared with reference to design codes (ACI 318-08 and EC2) and the mechanical model of the critical shear crack theory (CSCT), obtaining a number of conclusions on their suitability.

On the orientation of fibres in structural members fabricated with self compacting fibre reinforced concrete

Abstract: The incorporation of fibres into concrete produces important benefits, mainly on the residual load-bearing capacity. These improvements depend on the type, content and orientation of the fibres, being a strong relationship between the number of fibres in the fracture surfaces and the post peak parameters. Although the fibres could be homogeneously distributed after mixing, the casting and compaction processes can significantly affect the fibre distribution and orientation, and consequently the mechanical performance of the material. In the case of Fibre Reinforced Self Compacting Concrete (FR-SCC) the existence of significant flow and wall effects may influence fibre orientation. This paper analyzes the fibre orientation in thin structural elements cast with FR-SCC and its effects on the residual mechanical properties. A slab of 0.90 × 1.80 × 0.09 m, a wall of 0.50 × 2.00 × 0.08 m, and a beam of 0.15 × 0.15 × 2.50 m were selected as representative elements where different concrete flow conditions take place. A strong heterogeneity in the orientation of the fibres was found. The fibre orientation varied with the flow rate and with the wall effect; the thickness of the elements or the proximity to the bottom of the moulds appeared as important variables. It was demonstrated that in thin elements the residual mechanical properties can be quite different when diverse zones and/or directions of the structural elements are considered.

The role of rheology and slab shape on rapid mantle flow: Three‐dimensional numerical models of the Alaska slab edge

Abstract: [1] Away from subduction zones, the surface motion of oceanic plates is well correlated with mantle flow direction, as inferred from seismic anisotropy. However, this correlation breaks down near subduction zones, where shear wave splitting studies suggest the mantle flow direction is spatially variable and commonly non-parallel to plate motions. This implies local decoupling of mantle flow from surface plate motions, yet the magnitude of this decoupling is poorly constrained. We use 3D numerical models of the eastern Alaska subduction-transform plate boundary system to further explore this decoupling, in terms of both direction and magnitude. Specifically, we investigate the role of the slab geometry and rheology on the mantle flow velocity at a slab edge. The subducting plate geometry is based on Wadati-Benioff zone seismicity and tomography, and the 3D thermal structure for both the subducting and overriding plates, is constrained by geologic and geophysical observations. In models using the composite viscosity, a laterally variable mantle viscosity emerges as a consequence of the lateral variations in the mantle flow and strain rate. Spatially variable mantle velocity magnitudes are predicted, with localized fast velocities (greater than 80 cm/yr) close to the slab where the negative buoyancy of the slab drives the flow. The same models produce surface plate motions of less than 10 cm/yr, comparable to observed plate motions. These results show a power law rheology, i.e., one that includes the effects of the dislocation creep deformation mechanism, can explain both observations of seismic anisotropy and decoupling of mantle flow from surface motion.

Development of mantle seismic anisotropy during subduction-induced 3-D flow

Abstract: [1] The dynamics of subduction can be indirectly constrained by studying the induced mantle flow. However, inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive. We calculated the strain-induced lattice preferred orientation (LPO) developed in 3-D models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench-perpendicular, and becomes trench-parallel deeper, where the toroidal flow accommodates slab retreat. Synthetic SKS splitting shows that in the fore-arc slab rollback favors trench-parallel polarization of the fast shear wave component, while plate advance enhances trench-perpendicular seismic anisotropy. The interference between these two competing mechanisms yields subslab delay times of 0.5 to 1.3 sec, comparable with those observed at most natural subduction zones. The magnitude of the subslab trench-parallel splitting is independent of the rate at which the slab migrates, instead it is proportional to the amount of retreat.

Global variations in H2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes

Abstract: [1] We compiled a data set of 100 primitive arc magma compositions from melt inclusion and whole rock analyses to compare volatile contents, slab tracers, and calculated subduction component compositions between 18 subduction zone segments spanning the global range in slab thermal structure. The average primitive magma H2O content in our data set is 3.3 ± 1.2 wt.% (1 s.d.) for melts erupted within 50 km of the volcanic front. While there is a wide range of volatile contents in magmas within individual arcs, the highest values occur in magmas erupted from vents along the volcanic front, where the subducting slab is located 104 ± 29 km (avg ± 1 s.d.) beneath the surface. This observation, coupled with positive correlations between H2O, Cl, S, and B contents and predictions from geodynamic models, provides strong evidence for the active supply of volatile-rich slab-derived components from the subducting oceanic plate beneath volcanic arcs. We also show that temperature-sensitive ratios (e.g., H2O/Ce) for both primitive arc magmas and calculated subduction components are similar and display monotonic behavior with slab thermal parameter. Furthermore, calculated subduction component compositions have higher trace element to H2O ratios in arcs with hotter slabs (lower thermal parameter), suggesting that hydrous melts of differing compositions are added beneath different arcs. Finally, we present new volatile outflux estimates for Central Cascades magmatism and then compare these to estimates for the Central American and Kamchatka-Kurile arcs to create a combined data set spanning a large range in slab thermal parameter.

Surface expression of eastern Mediterranean slab dynamics: Neogene topographic and structural evolution of the southwest margin of the Central Anatolian Plateau, Turkey

Abstract: [1] The southwest margin of the Central Anatolian Plateau has experienced multiple phases of topographic growth, including the formation of localized highs prior to the Late Miocene that were later affected by wholesale uplift of the plateau margin. Our new biostratigraphic data limit the age of uplifted marine sediments at the southwest plateau margin at 1.5 km elevation to <7.17 Ma, and regional lithostratigraphic correlations imply that the age is <6.7 Ma. Single-grain CA-TIMS U-Pb zircon analyses from a reworked ash within the marine sediments yield dates as young as 10.6 Ma, indicating a maximum age that is consistent with the biostratigraphy. Our structural measurements within the uplifted region and fault inversion modeling agree with previous findings in surrounding regions, with early contraction followed by strike-slip and extensional deformation during uplift. Focal mechanisms from shallow earthquakes show that the extensional phase has continued to the present. Broad similarities in the change in the tectonic stress regime (after 8 Ma) and the onset of surface uplift (after 7 Ma) imply that deep-seated process(es) caused post-7 Ma uplift. The geometry of lithospheric slabs beneath the plateau margin, Pliocene to recent alkaline volcanism, and the uplift pattern with accompanying normal faulting point toward slab tearing and localized heating at the base of the lithosphere as a probable mechanism for post-7 Ma uplift of the southwest margin. Considering previous work in the region, there appears to be an important link between slab dynamics and surface uplift throughout the Anatolian Plateau's southern margin.

Delamination in collisional orogens: Thermomechanical modeling

Abstract: [1] Modes of mantle delamination in collision zones are discussed in the light of 2D numerical modeling. Freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust. Distinct modes are identified, in which orogens undergo mantle delamination concurrently or after collision. Delamination propagates along the Moho of the subducted plate together with the retreating trench, provided that slab pull is sufficient and that the meta-stability of the crust-lithospheric mantle interface is overcome. Topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle (delamination front). Early exhumation of high-pressure rocks is followed by exhumation of high-temperature, partially molten rocks. Convective stabilization of delamination outlasts slab break-offs and impedes renewed build-up of mechanically strong lithospheric mantle by static cooling. Parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate, the collision rate, the mantle rheology, and explore the effects of melting and sedimentation. Mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi-permeable 670 km spinel-perovskite transition, demonstrating that phase-changes are essential factors. Delamination may account for the formation of complex boundary zones between continents, such as between Africa and Europe in the Mediterranean, where transient compression and extension, the lack of mantle lithosphere, and the exhumation of high-grade metamorphic core complexes are features also obtained in our modeling results.

Magnetic superlens-enhanced inductive coupling for wireless power transfer

Abstract: We investigate numerically the use of a negative-permeability “perfect lens” for enhancing wireless power transfer between two current carrying coils. The negative permeability slab serves to focus the flux generated in the source coil to the receiver coil, thereby increasing the mutual inductive coupling between the coils. The numerical model is compared with an analytical theory that treats the coils as point dipoles separated by an infinite planar layer of magnetic material [Urzhumov et al., Phys. Rev. B 19, 8312 (2011)]. In the limit of vanishingly small radius of the coils, and large width of the metamaterial slab, the numerical simulations are in excellent agreement with the analytical model. Both the idealized analytical and realistic numerical models predict similar trends with respect to metamaterial loss and anisotropy. Applying the numerical models, we further analyze the impact of finite coil size and finite width of the slab. We find that, even for these less idealized geometries, the presence of the magnetic slab greatly enhances the coupling between the two coils, including cases where significant loss is present in the slab. We therefore conclude that the integration of a metamaterial slab into a wireless power transfer system holds promise for increasing the overall system performance.

Magnetic superlens-enhanced inductive coupling for wireless power transfer

Abstract: We investigate numerically the use of a negative-permeability "perfect lens" for enhancing wireless power transfer between two current carrying coils. The negative permeability slab serves to focus the flux generated in the source coil to the receiver coil, thereby increasing the mutual inductive coupling between the coils. The numerical model is compared with an analytical theory that treats the coils as point dipoles separated by an infinite planar layer of magnetic material [Urzhumov et al., Phys. Rev. B, 19, 8312 (2011)]. In the limit of vanishingly small radius of the coils, and large width of the metamaterial slab, the numerical simulations are in excellent agreement with the analytical model. Both the idealized analytical and realistic numerical models predict similar trends with respect to metamaterial loss and anisotropy. Applying the numerical models, we further analyze the impact of finite coil size and finite width of the slab. We find that, even for these less idealized geometries, the presence of the magnetic slab greatly enhances the coupling between the two coils, including cases where significant loss is present in the slab. We therefore conclude that the integration of a metamaterial slab into a wireless power transfer system holds promise for increasing the overall system performance.

Mantle subducting slab structure in the region of the 2010 M8.8 Maule earthquake (30–40°S), Chile

Abstract: We present a new tomographic model of the mantle in the area of the 2010 M88 Maule earthquake and surrounding regions Increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle, from the region of flat slab subduction north of the Maule rupture to the area of overlapping rupture between the 1960 M95 and the 2010 M88 events to the south We have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region, which we use to conduct nested regional–global tomography The new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the Maule region South of the Maule region, a steeply dipping relic slab is imaged from ∼200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths We interpret the images as revealing both horizontal and vertical tearing of the slab at ∼38°S to explain the imaged pattern of slab anomalies in the southern portion of the model In contrast, the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend We speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

Dynamics of slab detachment

Abstract: [1] We investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach. The study combines results of two-dimensional (2D) state-of-the-art thermomechanical numerical simulations and a 1D analytical solution for viscous necking under gravity. We show that the dominant deformation mechanism during slab detachment is viscous necking, independent of the depth of slab detachment. When the slab dip is moderate (35–70°), slab detachment is partly affected by localized simple shearing in the colder parts of the slab. Brittle fracturing (breaking) plays a minor role during slab detachment. Our 2D thermomechanical models indicate that the duration of slab detachment, quantified from the onset of slab thinning until the actual detachment (i.e. vanishing of slab-pull force), is relatively short ( 250 km) can occur within a short time interval (<1 Ma) which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting, exhumation or surface uplift. The thinning of the slab during detachment, observed in 2D simulations, agrees well with predictions from a 1D analytical solution indicating that the 1D solution captures the first-order features of the detachment process. We also evaluate the impact of shear heating on the duration of slab detachment. The predictions of a simple semi-analytical solution, based on dimensionless parameters, agree well with our and previously published results.

Thermomechanical modeling of slab eduction

Abstract: [1] Plate eduction is a geodynamic process characterized by normal-sense coherent motion of previously subducted continental plate. This mechanism may occur after slab detachment has separated the negatively buoyant oceanic plate from the positively buoyant orogenic root. Eduction may therefore be partly responsible for exhumation of high pressure rocks and late orogenic extension. We used two-dimensional thermomechanical modeling to investigate the main features of the plate eduction model. The results show that eduction can lead to the quasi adiabatic decompression of the subducted crust (≈2 GPa) in a timespan of 5 My, large localized extensional strain in the former subduction channel, flattening of the slab, and a topographic uplift associated with extension of the orogen. In order to further investigate the forces involved in the eduction process, we ran systematic parametric simulations and compared them to analytic plate velocity estimations. These experiments showed that eduction is a plausible mechanism as long as the viscosity of the asthenospheric mantle is lower than 1022 Pa.s while subduction channel viscosity does not exceed 1021 Pa.s. We suggest that eduction can be a viable geodynamic mechanism and discuss its potential role during the orogenic evolution of the Norwegian Caledonides.

The role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction

Abstract: [1] Slab dip varies significantly, both between different, and along single subduction zones. Provided that old subducting plates are colder and denser than young plates, variations in the slab dip should correlate with slab age. However, recent statistical analyses do not show this expected correlation. We present the results of non-Newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition. We systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges. We find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction, as a result of the increased suction force in the mantle wedge. The influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere. Modeling results are qualitatively compared to the large dip variability of the Cocos slab including a flat-slab segment. We suggest that this variability is likely related to the change of the thermal state of the overriding plates, with flat subduction occurring under cold lithosphere in southwestern Mexico and steep subduction under the warmer lithosphere of the northwestern Caribbean plate.

Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California

Abstract: [1] Subduction of the Farallon plate beneath northwestern Mexico stalled by ∼12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (∼24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ∼40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ∼8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

The Pliocene‐Quaternary wedge‐top basins of southern Italy: an expression of propagating lateral slab tear beneath the Apennines

Abstract: In recent years, contrasting seismic tomographic images have given rise to an extensive debate about the occurrence and implications of migrating slab detachment beneath southern Italy. One of the most pertinent aspects of this process is the concentration of the slab pull force, and particularly its surface expression in terms of vertical motions and related basin subsidence/uplift. In this study we focused on shallow-water to continental, Pliocene-Quaternary basins that formed on top of the Apennine allochthonous wedge after its emplacement onto a large foreland carbonate platform domain (Apulian Platform). Due to the thick-skinned style of deformation controlling the Pliocene-Pleistocene stages of continental shortening, a high degree of coupling with the downgoing plate appears to characterize the late tectonic evolution of the southern Apennines. Therefore, the wedgetop basins analysed in this study, although occurring on the deformed edge of the overriding plate, are capable of recording deep geodynamic processes affecting the slab. Detailed stratigraphic work on these wedge-top basins points to a progressive SE-ward migration of basin subsidence from c. 4 to c. 2.8 Ma over a distance of about 140 km along the strike of the Apennine belt. Such a migration is consistent with a redistribution of slab-pull forces associated with the progressive lateral migration at a mean rate in the range of 12–14 cm y–1 of a slab tear within the down-going Adriatic lithosphere. These results yield fundamental information on the rates of first-order geodynamic processes affecting the slab, and on related surface response.

Multi‐scale dynamics and rheology of mantle flow with plates

Abstract: Fundamental issues in our understanding of plate and mantle dynamics remain unresolved, including the rheology and state of stress of plates and slabs; the coupling between plates, slabs and mantle; and the flow around slabs. To address these questions, models of global mantle flow with plates are computed using adaptive finite elements, and compared to a variety of observational constraints. The dynamically consistent instantaneous models include a composite rheology with yielding, and incorporate details of the thermal buoyancy field. Around plate boundaries, the local resolution is 1 km, which allows us to study highly detailed features in a globally consistent framework. Models that best fit plateness criteria and plate motion data have strong slabs with high stresses. We find a strong dependence of global plate motions, trench rollback, net rotation, plateness, and strain rate on the stress exponent in the nonlinear viscosity; the yield stress is found to be important only if it is smaller than the ambient convective stress. Due to strong coupling between plates, slabs, and the surrounding mantle, the presence of lower mantle anomalies affect plate motions. The flow in and around slabs, microplate motion, and trench rollback are intimately linked to the amount of yielding in the subducting slab hinge, slab morphology, and the presence of high viscosity structures in the lower mantle beneath the slab.

Seismic anisotropy and heterogeneity in the Alaska subduction zone

Abstract: SUMMARY We determined P- and S-wave tomography and P-wave anisotropic structure of the Alaska subduction zone using 259 283 P- and 73 817 S-wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to 100–150 km depth under the backarc. The results indicate that H2O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (<90 km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge.