Showing papers on "Slab published in 2013"

Rheology of the Upper Mantle and the Mantle Wedge: A View from the Experimentalists

Abstract: In this manuscript we review experimental constraints for the viscosity of the upper mantle. We first analyze experimental data to provide a critical review of flow law parameters for olivine aggregates and single crystals deformed in the diffusion creep and dislocation creep regimes under both wet and dry conditions. Using reasonable values for the physical state of the upper mantle, the viscosities predicted by extrapolation of the experimental flow laws compare well with independent estimates for the viscosity of the oceanic mantle, which is approximately 10 19 Pa s at a depth of ∼100 km. The viscosity of the mantle wedge of subduction zones could be even lower if the flux of water through it can result in olivine water contents greater than those estimated for the oceanic asthenosphere and promote the onset of melting. Calculations of the partitioning of water between hydrous melt and mantle peridotite suggest that the water content of the residue of arc melting is similar to that estimated for the asthenosphere. Thus, transport of water from the slab into the mantle wedge can continually replenish the water content of the upper mantle and facilitate the existence of a low viscosity asthenosphere.

Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity

Abstract: A new P wave tomographic model of the mantle was constructed using more than 10 million travel times The finite-frequency effect of seismic rays was taken into account by calculating banana-donut kernels at 2 Hz for all first arrival time data, and at 01 Hz for broadband differential travel time data Based on this model, a systematic survey for subducted slab images was developed for the circum-Pacific; including the Kurile, Honshu, Izu-Bonin, Mariana, Java, Tonga-Kermadec, southern and northern South America, and Central America, arcs This survey revealed a progressive lateral variation of the configuration of slabs along arc(s), which we interpret as an indication for successive stages of slab subduction through the Bullen's transition region with the 660 km discontinuity at the middle We identified the four distinct stages: I - slab stagnant above the 660 km discontinuity; II - slab penetrating the 660 km discontinuity; III - slab trapped in the uppermost lower mantle (at a depth of 660–1000 km); and IV - slab descending well into the deep lower mantle The majority of slab images are found to be either at Stage I or III, suggesting that Stages I and III are relatively stable or neutral and II and IV are relatively unstable or transient There is a remarkable distinction for the deepest hypocentral distribution between slabs at Stage I and slabs at Stages II or III

Tracers of the Slab

Abstract: A new global compilation of high precision trace element and isotopic analyses from mafic island arc lavas is explored to highlight the key geochemical features of volcanic front lavas Two distinct components from the slab can be identified in island arc lavas One component dominates the budgets of most incompatible trace elements and another affects a smaller range of elements, most notably Ba, Pb and Sr These contrasting slab components require different ultimate sources and mechanisms of transport to the mantle wedge The first component is argued to be a melt of the down-going sediment, while the second is likely an aqueous fluid derived from the altered mafic oceanic crust U-series nuclides constrain the timing of slab component additions The sediment component is close to 238 U 230 Th equilibrium implying >350ky since the last major fractionation of U and Th, plausibly the time since sediment melting Lavas dominated by the 'fluid' component can have extreme 226 Ra- 230 Th disequilibrium together with elevated ratios of the stable Ba/Th analogue In its simplest interpretation, this observation implies only a few thousand years elapse between 'fluid' release from the slab, and eruption of this component in arc lavas In most cases the two subduction components appear to be added to a recently melt-depleted mantle This suggests the mantle wedge is fed with material processed through a back-arc melting regime These first order geochemical observations and inferences place important constraints on physical models of the subduction zone

Constraints on subduction geodynamics from seismic anisotropy

Abstract: [1] Much progress has been made over the past several decades in delineating the structure of subducting slabs, but several key aspects of their dynamics remain poorly constrained. Major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology, slab hydration and dehydration, mechanical coupling between slabs and the ambient mantle, the geometry of mantle flow above and beneath slabs, and the interactions between slabs and deep discontinuities such as the core-mantle boundary. Observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy: when mantle rocks are deformed, a preferred orientation of individual mineral crystals or materials such as partial melt often develops, resulting in the directional dependence of seismic wave speeds. Measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems. Here I review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge, slab, and sub-slab anisotropy. I also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations. I evaluate different models in light of constraints from seismology, geodynamics, and mineral physics. Finally, I discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress, particularly in frontier areas such as understanding slab dynamics in the deep mantle.

Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

Abstract: Received 23 July 2013; revised 14 October 2013; accepted 17 October 2013 [1] We present a new whole mantle P wave tomographic model GAP_P4 We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation Finite frequency kernels were calculated at the corresponding frequency bands for both long- and short-period data With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor This new model is different from a model in which the full data set is interpreted with classical ray theory BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments Citation: Obayashi, M, J Yoshimitsu, G Nolet, Y Fukao, H Shiobara, H Sugioka, H Miyamachi, and Y Gao (2013), Finite frequency whole mantle P wave tomography: Improvement of subducted slab images, Geophys Res Lett, 40, doi:101002/ 2013GL057401

Barbados-based estimate of ice volume at Last Glacial Maximum affected by subducted plate

Abstract: Coral records from the Barbados have been used to infer that, due to ice expansion, sea level was 120 m lower than today during the Last Glacial Maximum. A 3D simulation of the mantle in this region suggests these estimates were biased by the presence of a subducted slab, and indicates the sea-level difference was closer to 130 m.

A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate

Abstract: We present numerical subduction models to investigate overriding plate deformation at subduction zones. All models show forearc shortening, resulting predominantly from shear stresses at the subduction zone interface and opposite-sense mantle shear stresses at the base of the forearc lithosphere. Models dominated by backarc extension show that it results from trench-normal positive velocity gradients in the mantle below the overriding plate. Such gradients result from toroidal mantle flow induced by slab rollback, with velocities below the leading part of the backarc faster than the overriding plate velocity. The velocity gradients induce basal shear stresses that increase trenchward and cause trenchward overriding plate motion at a velocity (v OP⊥) whose spatial average is below the trench retreat velocity (vT⊥). The combination of basal shear stresses and average vOP⊥ vT⊥. This results in trench-normal deviatoric compression and shortening in the leading part of the overriding plate as it collides with the subduction hinge. Ultimately, the geodynamic models demonstrate that backarc extension is favored for narrow slabs and near lateral slab edges and is driven by rollback induced toroidal mantle flow, while backarc shortening is favored for the center of wide slabs and is driven by poloidal mantle flow resulting from downdip slab motion. Key Points Slab rollback driven toroidal mantle flow causes trench-normal velocity gradient The velocity gradient induces basal shear stresses that drive backarc extension Backarc shortening results from overriding plate collision with subduction hinge

Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

Abstract: Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure–temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

Temperature distributions in pavement and bridge slabs heated by using vertical ground-source heat pump systems

Abstract: Temperature distribution which occurs in pavement and bridge slabs heated for de-icing and snow melting during cold periods is determined by using vertical ground-source heat pump (GSHP) systems with U-tube ground heat exchanger (GHE). The bridge and pavement models (slabs) for de-icing and snow melting were constructed. A three-dimensional finite element model (FEM) was developed to simulate temperature distribution of bridge slab (BS) and pavement slab (PS). The temperature distribution simulations of PS and BS were conducted numerically by computational fluid dynamics (CFD) program named ‘Fluent’. Congruence between the simulations and experimental data was determined.

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.

Mountain building and mantle dynamics

Abstract: [1] Mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large-scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen. A general, dynamic model for this process is still lacking. Here we propose that mountain belts can be classified between two end-members. First, those of “slab pull” type, where subduction is mainly confined to the upper mantle, and rollback trench motion lead to moderately thick crustal stacks, such as in the Mediterranean. Second, those of “slab suction” type, where whole-mantle convection cells (“conveyor belts”) lead to the more extreme expressions of orogeny, such as the largely thickened crust and high plateaus of present-day Tibet and the Altiplano. For the slab suction type, deep mantle convection produces the unique conditions to drag plates toward each other, irrespective of their nature and other boundary conditions. We support this hypothesis by analyzing the orogenic, volcanic, and convective history associated with the Tertiary formation of the Andes after ~40 Ma and Himalayas after collision at ~55 Ma. Based on mantle circulation modeling and tectonic reconstructions, we surmise that the forces necessary to sustain slab-suction mountain building in those orogens derive, after transient slab ponding, from the mantle drag induced upon slab penetration into the lower mantle, and from an associated surge of mantle upwelling beneath Africa. This process started at ~65–55 Ma for Tibet-Himalaya, when the Tethyan slab penetrated into the lower mantle, and ~10 Myr later in the Andes, when the Nazca slab did. This surge of mantle convection drags plates against each other, generating the necessary compressional forces to create and sustain these two orogenic belts. If our model is correct, the available geological records of orogeny can be used to decipher time-dependent mantle convection, with implications for the supercontinental cycle.

Mantle flow in subduction systems: The mantle wedge flow field and implications for wedge processes

Abstract: [1] The mantle wedge above subducting slabs is associated with many important processes, including the transport of melt and volatiles. Our understanding of mantle wedge dynamics is incomplete, as the mantle flow field above subducting slabs remains poorly understood. Because seismic anisotropy is a consequence of deformation, measurements of shear wave splitting can constrain the geometry of mantle flow. In order to identify processes that make first-order contributions to the pattern of wedge flow, we have compiled a data set of local S splitting measurements from mantle wedges worldwide. There is a large amount of variability in splitting parameters, with average delay times ranging from ~0.1 to 0.3 s up to ~1.0–1.5 s and large variations in fast directions. We tested for relationships between splitting parameters and a variety of parameters related to subduction processes. We also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge. We find that no simple model can explain all of the trends observed in the global data set. Mantle wedge flow is likely controlled by a combination of downdip motion of the slab, trench migration, ambient mantle flow, small-scale convection, proximity to slab edges, and slab morphology, with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology. There is also a likely contribution from B-type olivine and/or serpentinite fabric in many subduction zones, governed by the local thermal structure and volatile distribution.

Energy Release of the 2013 Mw 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity

Abstract: Earth’s deepest earthquakes occur in subducting oceanic lithosphere, where temperatures are lower than in ambient mantle. On 24 May 2013, a magnitude 8.3 earthquake ruptured a 180-kilometer-long fault within the subducting Pacific plate about 609 kilometers below the Sea of Okhotsk. Global seismic P wave recordings indicate a radiated seismic energy of ~1.5 × 10^(17) joules. A rupture velocity of ~4.0 to 4.5 kilometers/second is determined by back-projection of short-period P waves, and the fault width is constrained to give static stress drop estimates (~12 to 15 megapascals) compatible with theoretical radiation efficiency for crack models. A nearby aftershock had a stress drop one to two orders of magnitude higher, indicating large stress heterogeneity in the deep slab, and plausibly within the rupture process of the great event.

Tibetan and Indian lithospheres in the upper mantle beneath Tibet: Evidence from broadband surface‐wave dispersion

Abstract: Broadband seismic experiments over the last two decades have produced dense data coverage across Tibet. Yet, the mechanism of the India-Asia lithospheric convergence beneath it remains a puzzle, with even its basic features debated and with very different end-member models advocated today. We measured highly accurate Rayleigh- and Love-wave phase-velocity curves in broad period ranges (up to 5–200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of interstation measurements made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by extensive series of nonlinear inversions of the data, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy. Temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships. Our results reveal strong heterogeneity in the upper mantle beneath Tibet. Very large high-velocity anomalies in the upper mantle are consistent with the presence of underthrust (beneath southwestern Tibet) and subducted (beneath central and eastern Tibet) Indian lithosphere. The corresponding thermal anomalies match those estimated for subducted Indian lithosphere. In contrast to the Indian lithosphere, Tibetan lithosphere and asthenosphere display low-to-normal shear speeds; Tibetan lithosphere is thus warm and thin. Radial anisotropy in the upper mantle is weak in central and strong in northeastern Tibet, possibly reflecting asthenospheric flow above the subducting Indian lithospheric slab.