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Now showing items 1 - 16 of 107216

  • Fluid Migration in a Subducting Viscoelastic Slab

    Morishige, M.   van Keken, P. E.  

    Metamorphic dehydration reactions in a subducting slab release fluids that trigger arc volcanism and are thought to be responsible for intermediate-depth seismicity. The fluid flow from the source is controlled by buoyancy and compaction pressure which is modified by viscous and elastic effects. In this paper, we investigate how fluid migrates in viscoelastic slab by using 2-D and 3-D numerical models based on a theory of two-phase flow. When bulk viscosity is sufficiently low, viscosity plays a dominant role and fluid goes up almost vertically soon after its release producing porosity waves. When a higher bulk viscosity is assumed, a large amount of fluid is trapped in a high porosity region produced by the fluid source and migrates along the source except for a case where the ratio of permeability (K) to fluid viscosity () is relatively low. We also find that porosity increases in the deeper part of the fluid source in cases with intermediate and low values of K/. In 3-D, fluid focusing occurs where the slab bends away from the trench causing a local increase in porosity and compaction pressure. These findings may help us explain several types of observations in subduction zones including slow earthquakes at the plate interface, low seismic wave velocities in the oceanic crust, double seismic zones in the slab, and shallow subduction angle at the bend of the slab.
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  • The cold and relatively dry nature of mantle forearcs in subduction zones

    Abers, G. A.   van Keken, P. E.   Hacker, B. R.  

    Some of Earth's coldest mantle is found in subduction zones at the tip of the mantle wedge that lies between the subducting and overriding plates. This forearc mantle is isolated from the flow of hot material beneath the volcanic arc, and so is inferred to reach temperatures no more than 600 to 800 degrees C - conditions at which hydrous mantle minerals should be stable. The forearc mantle could therefore constitute a significant reservoir for water if sufficient water is released from the subducting slab into the mantle wedge. Such a reservoir could hydrate the plate interface and has been invoked to aid the genesis of megathrust earthquakes and slow slip events. Our synthesis of results from thermal models that simulate the conditions for subduction zones globally, however, indicates that dehydration of subducting plates is too slow over the life span of a typical subduction zone to hydrate the forearc mantle. Hot subduction zones, where slabs dehydrate rapidly, are an exception. The hottest, most buoyant forearcs are most likely to survive plate collisions and be exhumed to the surface, so probably dominate the metamorphic rock record. Analysis of global seismic data confirms the generally dry nature of mantle forearcs. We conclude that many subduction zones probably liberate insufficient water to hydrate the shallower plate boundary where great earthquakes and slow slip events nucleate. Thus, we suggest that it is solid-state processes and not hydration that leads to weakening of the plate interface in cold subduction zones.
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  • Deep storage of oceanic crust in a vigorously convecting mantle

    Brandenburg, J. P.   van Keken, P. E.  

    [1] Fractionated isotopic ratios in some oceanic basalts indicates the presence of recycled oceanic crust in the mantle. This crust must have escaped complete remixing for a significant period of time. Gravitational settling into a dense layer at the base of the mantle may facilitate this preservation. Christensen and Hofmann ( 1994) first demonstrated the dynamics of this process by developing scaling laws for extrapolating low convective vigor models to conditions estimated for the mantle. Here this sequestration is studied in models with more Earth-like convective vigor. Scaling laws for geodynamic parameters are updated and the isotopic evolution of the U-Pb and Sm-Nd systems examined. Significant accumulation is still found at high Rayleigh number, but only when the excess density of oceanic crust in the lower mantle is larger than currently suggested from laboratory experiments. These accumulations are found to maintain the fractionated isotopic signature of ancient crust for models with moderate to moderately high convective vigor relative to mantle estimates. At the highest convective vigor tested, the accumulations are not isotopically distinct.
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  • Methods for thermochemical convection in earth's mantle with force-balanced plates

    Brandenburg, J. P.   van Keken, P. E.  

    [1] Models of convection in the mantle can be used to study the effects of differentiation and remixing on the geochemical evolution of the Earth. Implementation of melting and degassing at mid-ocean ridges and subduction zones requires an adequate approximation of plate tectonics as well as temperature-dependent rheology. We have developed a new two-dimensional cylindrical model that combines a force-balance method for energetically consistent stiff plates with tracer-discretized chemical buoyancy. Basaltic crust is extracted at distinct spreading centers and is subducted into the lower mantle. We find that the unmodified implementation of the force-balance equations in a full cylinder causes occasional spurious rotations by amplification of numerical discretization errors. The method is stable if a single internal symmetry boundary condition is used, but this causes artificial pooling of dense crust near the boundary where it is easily disrupted. This results in artificially enhanced remixing of dense crust. We modify the force-balance equations to damp net lateral plate movement. The energetic consistency of this modification is then demonstrated by comparison to a one-plate, single convection cell calculation. With the removal of the symmetry boundary condition a more continuous rate of crustal pooling is observed. This suggests that models with symmetry boundary conditions may overpredict the rate of pooling and remixing of ancient crust.
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  • Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

    van Keken, P. E.   Kita, S.   Nakajima, J.  

    The cause of intermediate-depth (> 40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths > 80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.
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  • Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

    van Keken, P. E.   Kita, S.   Nakajima, J.  

    The cause of intermediate-depth (> 40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths > 80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.
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  • A comparison of methods for the modeling of thermochemical convection

    van Keken, P. E.   King, S. D.   Schmeling, H.   Christensen, U. R.   Neumeister, D.   Doin, M.-P.  

    Compares several methods of studying thermochemical convection in a Boussinesq fluid at infinite Prandtl number. For the representation of chemical heterogeneity tracer, marker chain, and field methods are employed. In the case of an isothermal Rayleigh-Taylor instability, good agreement is found for the initial rise of the unstable lower layer; however, the timing and location of the later smaller-scale instabilities may differ between methods. For a simulation of entrainment by thermal convection of a dense layer at the bottom of the mantle the authors found good agreement for a few overturn times. After this, differences between the results can be large. The authors propose intrinsic differences between the methods and possibly chaotic mixing effects may be the cause of the lack of detailed agreement. The comparison shows that high resolution is necessary for a reasonable thermochemical study. This will pose severe restrictions on the applicability of these methods to three-dimensional situations
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  • Rheological control of oceanic crust separation in the transition zone

    van Keken, P. E.   Karato, S.   Yuen, D. A.  

    Mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle. We have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle. Confirming previous results, the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling. However, a soft layer may exist between the strong garnet crust and cold slab interior. Models employing a simplified Newtonian sandwich model show that this thin, weak layer can effectively decouple the crust and slab. Once entrained into the lower mantle, the then lighter crust can rise sufficiently fast as a Rayleigh-Taylor instability to avoid further entrainment. These results suggest that the crustal component of slabs may be trapped at 670 km depth, leading to a garnet enriched transition zone.
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  • Dynamics of a laminar plume in a cavity: The influence of boundaries on the steady state stem structure

    van Keken, P. E.   Davaille, A.   Vatteville, J.  

    The steady state structure of thermal plumes rising from a small heater is studied in high Prandtl number fluids. We show good agreement between laboratory experiments and numerical simulations. We study the effect of the boundaries on the plume development by numerically simulating the plume rise in very large geometries. The thermal structure of the plume axis is similar for all box sizes considered, but the velocity structure changes strongly as box sizes are increased. We show that the effect of the side boundaries becomes unimportant for large aspect ratio, but that the free-slip top boundary has a strong influence on the velocity structure under all conditions. We show that the use of an outflow boundary condition significantly reduces the influence of the top boundary. Under these conditions we recover to good precision the theoretical predictions for plumes rising in an semi-infinite half-space. The strong influence of the boundaries in high Prandtl number fluids is important in the interpretation of laboratory experiments and numerical simulation for the dynamics of the Earth's mantle.
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  • Implications for Mantle Dynamics from the High Melting Temperature of Perovskite

    van Keken, P. E.   Yuen, D. A.   van den Berg, A. P.  

    Recent studies have implied that (Mg,Fe)SiO3-perovskite, a likely dominant mineral phase in the lower mantle, may have a high melting temperature. The implications of these findings for the dynamics of the lower mantle were investigated with the use of numerical convection models. The results showed that low homologous temperatures (0.3 to 0.5) would prevail in the modeled lower mantle, regardless of the effective Rayleigh number and internal heating rates. High-temperature ductile creep is possible under relatively cold conditions. In models with low rates of internal heating, local maxima of viscosity developed in the mid-lower mantle that were similar to those obtained from inversion of geoid, topography, and plate velocities.
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  • Kinetic Models for Healing of the Subduction Interface Based on Observations of Ancient Accretionary Complexes

    Fisher, D. M.   Smye, A. J.   Marone, C.   van Keken, P. E.   Yamaguchi, A.  

    Sand-shale melanges from the Kodiak accretionary complex and Shimanto belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350 degrees C. We use observations from these melanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid-filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: (1) a transient drop in fluid pressure P-f, and (2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than P-f, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which is loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab-top geotherms. The model provides (1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and (2) demonstration that kinetically driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment-dominated convergent margins. Plain Language Summary Geophysical monitoring of active subduction zones has revealed plate boundary slip behaviors such as creep, slow slip events, and earthquakes that vary spatially and temporally for different plate boundaries and downdip along a given boundary. Fault rocks exposed on land from paleosubduction plate boundaries provide a record of the deformation processes that likely occur during slip along active boundaries, so we review the characteristics of these ancient rocks to develop insight into slip behavior in subduction zones. We find that plate boundary deformation in these cases occurs within a wide fault zone through processes that involve redistribution of silica from shearing mudstones to cracking sandstone blocks. We use a geochemical model to calculate how long it would take to seal a crack by this process and conclude that cracks seal at rates that could influence the earthquake cycle, with rates of crack healing dependent on the temperature structure and the depth where slip occurs. Our results suggest that processes of frictional failure and geochemical healing in downgoing sediments may influence the slip stability along the subduction interface.
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  • The interaction of a plume with a rheological boundary: a comparison between two- and three-dimensional models

    van Keken, P. E.   Gable, C. W.  

    The discrete nature of volcanic hot spot chains, whose origin is believed to be related to mantle plumes, is suggestive that the plumes feeding them may be pulsating. A possible origin of the pulsations is the interaction of mantle plumes with a rheological interface in the transition zone. The authors have studied the time-dependent, three-dimensional interaction of an upwelling mantle plume with a rheological interface that separates a Newtonian lower mantle from a Newtonian upper mantle with a lower viscosity. It was found that the disappearance of pulsating behavior in three dimensions indicates a case where conclusions about plume dynamics in two dimensions do not apply to 3-D situations.
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  • Dynamical models of mantle volatile evolution and the role of phase transitions and temperature-dependent rheology

    van Keken, P. E.   Ballentine, C. J.  

    Helium isotopic variations demand the preservation of a primitive volatile source for ocean island basalts (OIB) in conjunction with a well-mixed and more radiogenic source for mid-oceanic ridge basalts (MORB). Dynamical models of the Earth's evolution should be able to predict this basic geochemical observation. The authors have developed a number of increasingly more realistic models of mantle convection that satisfy present-day heat loss and plate velocities to study the effects of convective mixing, radiogenic ingrowth, and degassing on the mantle 3He/ 4He evolution. The authors have included mechanisms that have been proposed to enhance the isolation of individual reservoirs in the mantle such as high lower mantle viscosity, strongly temperature- and pressure-dependent rheology, and an endothermic phase transition at 670 km depth. Although the combination of these mechanisms can produce regions of lower mixing efficiency, the models cannot satisfactorily explain the existence of two distinct OIB and MORB 3He/ 4He sources. If further improvements to the model, such as simulated plates and continents still fail to explain the geochemical constraints, it may be prudent to consider sources of primitive helium beyond the current paradigm requiring them to be stored within the terrestrial mantle since early in the Earth's history
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  • Variable spatio-temporal clustering of microseismicity in the Hellenic Subduction Zone as possible indicator for fluid migration

    Ruscic, M.   Bocchini, G. M.   Becker, D.   Meier, T.   van Keken, P. E.  

    This study investigates the variability of spatio-temporal microseismicity clustering and the occurrence of mutual triggering of events along the subduction interface in south-eastern Aegean as indication for fluid flow along and above the plate interface. We quantify spatial, temporal and spatio-temporal microseismicity clustering from the outer to the inner forearc and at intermediate depths. Waveform similarity indicates a decreasing of spatially clustered events from the outer and central towards the inner forearc and at intermediate depths. Highly similar events (cross-correlation >0.9), used as proxy for spatial clustering, decrease from the outer (30.2%) and central forearc (34.9%), towards the inner forearc (205%) and at intermediate depth (6.9%). Such highly similar events show increasing median inter-event times from the outer and central towards the inner forearc and at intermediate depth: 0.35, 0.34, 16.45, and 70 days, respectively. The Epidemic-Type-Aftershock-Sequences (ETAS) model, employed to investigate microseismicity temporal clustering, indicates an increase of the percentage of independent events from the outer (32%) and central (46%) forearc, to the inner forearc (93%) and at intermediate depth (93%). Hence, ETAS results suggest that mutual triggering of events is significant in the outer and central forearc, and it is almost absent in the inner forearc and at intermediate depths. Autocorrelation analysis, investigating spatio-temporal clustering, shows the tendency of earthquakes to occur close in space and time in the outer and central forearc, while in the inner forearc, and especially at intermediate depth, earthquakes are more homogeneously and randomly distributed. Combining the results from spatial, temporal and spatiotemporal analysis, we suggest that the different spatio-temporal patterns hint at systematic variations in the presence of migrating fluids on active faults close to failure. Triggering of seismicity is significant in the outer and central forearc, indicating fluid flow from the subduction interface, and it is diminishing towards the inner forearc. At intermediate depths, the nearly complete absence of mutual triggering of earthquakes indicates that there is little evidence for migration of fluids on active faults close to failure. Because intermediate depth seismicity in the Hellenic subduction zone occurs at P-T conditions where dehydration reactions are expected, fluids released by dehydration reactions within the slab are very likely migrating directly into the overlying mantle without triggering earthquakes. (C) 2019 Elsevier B.V. All rights reserved.
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  • Geskiedenis werken en streven van S. P. E. Trichardt

    Ploeger, Jan  

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  • Geskiedenis werken en streven van S. P. E. Trichardt

    Ploeger, Jan  

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