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Now showing items 1 - 16 of 22

  • Grain-size distribution in the mantle wedge of subduction zones

    Wada, Ikuko   Behn, Mark D.   He, Jiangheng  

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  • Common depth of slab-mantle decoupling: Reconciling diversity and uniformity of subduction zones

    Wada, Ikuko   Wang, Kelin  

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  • Sharp thermal transition in the forearc mantle wedge as a consequence of nonlinear mantle wedge flow

    Wada, Ikuko   Rychert, Catherine A.   Wang, Kelin  

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  • Focusing of upward fluid migration beneath volcanic arcs: Effect of mineral grain size variation in the mantle wedge

    Wada, Ikuko   Behn, Mark D.  

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  • Mantle wedge flow pattern and thermal structure in Northeast Japan: Effects of oblique subduction and 3-D slab geometry

    Wada, Ikuko   He, Jiangheng   Hasegawa, Akira   Nakajima, Junichi  

    Highlights • Mantle wedge flow and the thermal structure in Tohoku are nearly two-dimensional. • Oblique subduction in Hokkaido leads to 3-D mantle flow with northerly inflow. • Convergence of mantle inflow from Hokkaido and Tohoku results in a cold wedge. • The cold wedge in Hokkaido and Tohoku correlates with the arc location. • Mantle inflow is sub-parallel to volcanic cross-chain orientations. Abstract We develop a 3-D thermal model for the Northeast Japan subduction margin, using a realistic slab geometry for the subducting Pacific plate, and investigate the effects of oblique subduction and 3-D slab geometry on the mantle wedge flow pattern and the thermal structure. In the Tohoku region, the mantle wedge flow pattern is nearly two-dimensional resulting in a thermal structure similar to those obtained by a 2-D model, owing to the simple slab geometry and subduction nearly perpendicular to the margin. However, in Hokkaido, oblique subduction leads to 3-D mantle wedge flow with northerly inflow and west–northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in Tohoku due to lower sinking rate of the slab. Between Hokkaido and Tohoku, the slab has a hinge-like shape due to a relatively sharp change in the dip direction. In this hinge zone, northerly mantle inflow from Hokkaido and westerly mantle inflow from Tohoku converge, discouraging inflow from northwest and resulting in a cooler mantle wedge. The model-predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross-chains. The predicted 3-D thermal structure correlates well with the along-arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down-dip extent of interplate earthquakes.
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  • Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

    Wada, Ikuko   Wang, Kelin   He, Jiangheng   Hyndman, Roy D.  

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  • Diagenetic,metamorphic,and hydrogeologic consequences of hydrothermal circulation in subducting crust

    Spinelli, Glenn   Wada, Ikuko   Wang, Kelin   He, Jiangheng   Harris, Robert   Underwood, And Michael  

    The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within similar to 30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest.
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  • Thermal Structure of the Forearc in Subduction Zones:A Comparison of Methodologies

    van Keken, Peter E.   Wada, Ikuko   Sime, Nathan   Abers, Geoffrey A.  

    Molnar and England (1990, ) introduced equations using a semianalytical approach that approximate the thermal structure of the forearc regions in subduction zones. A detailed new comparison with high-resolution finite element models shows that the original equations provide robust predictions and can be improved by a few modifications that follow from the theoretical derivation. The updated approximate equations are shown to be quite accurate for a straight-dipping slab that is warmed by heat flowing from its base and by shear heating at its top. The approximation of radiogenic heating in the crust of the overriding plate is less accurate but the overall effect of this heating mode is small. It is shown that the previous and updated approximate equations become increasingly inaccurate with decreasing thermal parameter and increasing variability of slab dip. It is also shown that the approximate equations cannot be extrapolated accurately past the brittle-ductile transition. Conclusions in a recent paper (Kohn et al., 2018, ) that modest amount of shear heating can explain the thermal conditions of past subduction from the exhumed metamorphic rock record are invalid due to a number of compounding errors in the application of the Molnar and England (1990, ) equations past the brittle-ductile transition. The use of the improved approximate equations is highly recommended provided their limitations are taken into account. For subduction zones with variable dip and/or low thermal parameter finite element modeling is recommended.
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  • Kaiiki kara Mita Rekishi: Indo-yō to Chichūkai wo Musubu Kōryūshi

    Wada, Ikuko  

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  • Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

    Wada, Ikuko   Wang, Kelin   He, Jiangheng   Hyndman, Roy D.  

    The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two-dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation-creep rheology for the mantle wedge, to investigate the effect of this interface "decoupling.'' Decoupling occurs when the temperature-dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (< 0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H(2)O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H(2)O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70 - 80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.
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  • Fluidmigration in themantle wedge:Influence of mineral grain size andmantle compaction

    Cerpa, Nestor G.   Wada, Ikuko   Wilson, Cian R.  

    Mineral grain size in the mantle affects fluid migration by controlling mantle permeability; the smaller the grain size, the less permeable the mantle is. Mantle shear viscosity also affects fluid migration by controlling compaction pressure; high mantle shear viscosity can act as a barrier to fluid flow. Here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2-D numerical fluid migration model. Our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged downdip by the mantle flow due to small grain size (<1 mm) and high mantle shear viscosity that develop along the base of the mantle wedge. Increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at subarc depths. Fluids introduced into the mantle wedge at postarc depths migrate upward due to relatively large grain size in the deep mantle wedge, forming secondary fluid pathways behind the arc. Fluids that reach the shallow part of the mantle wedge spread trench-ward due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55-65 km depths. These results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc.
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  • Sharp thermal transition in the forearc mantle wedge as a consequence of nonlinear mantle wedge flow

    Wada, Ikuko   Rychert, Catherine A.   Wang, Kelin  

    In the forearc mantle wedge, the thermal field depends strongly on slab-driven mantle wedge flow. The flow is in turn affected by the thermal field via the temperature dependence of mantle rheology. Using thermal modeling, we show that the nonlinear feedback between the thermal and flow fields always leads to complete stagnation of the mantle wedge over a shallow, weakened part of the slab-mantle interface and an abrupt onset of mantle flow further down-dip. The abrupt increase in flow velocity leads to a sharp thermal transition from a cold stagnant to a hot flowing part of the wedge. This sharp thermal transition is inherent to all subduction zones, explaining a commonly observed sharp arc-ward increase in seismic attenuation. Citation: Wada, I., C. A. Rychert, and K. Wang (2011), Sharp thermal transition in the forearc mantle wedge as a consequence of nonlinear mantle wedge flow, Geophys. Res. Lett., 38, L13308, doi: 10.1029/2011GL047705.
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  • Case report: A long-term survivor of jejunal leiomyosarcoma with liver metastasis: Effective transcathetel arterial embolization for hepatic metastatic foci

    Hara, Toshiya   Wada, Ikuko   Kajihara, Susumu   Mizuta, Toshihiko   Yamamoto, Kyosuke   Sakai, Takahiro  

    We report a case of jejunal leiomyosarcoma with liver metastases in a 52-year-old Japanese male. An echogram demonstrated multiple cystic liver masses in April 1991. The diagnosis of metastatic leiomyosarcoma was made on the basis of characteristic hepatic angiography images and liver biopsy findings. The jejunal leiomyosarcoma was resected and unresectable liver metastatic foci were treated repeatedly with transcatheter arterial embolization. Transcatheter arterial embolization was considered to be effective in this case as the patient survived 4 years and 9 months after the first treatment.
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  • Intraslab Stresses in the Cascadia Subduction Zone from Inversion of Earthquake Focal Mechanisms

    Wada, Ikuko   Mazzotti, Stephane   Wang, Kelin  

    At the Cascadia subduction zone, intraslab earthquakes occur mostly in the northern part of the margin and its southern end, the Mendocino triple junction (MTJ). We determine intraslab stress orientations by inverting earthquake focal mechanisms and develop working hypotheses to explain the inferred intraslab stresses and observed seismicity. Our inversion results show that the subducting Juan de Fuca (JDF) slab in northern Cascadia is primarily under compression normal to the slab surface and tension in the downdip direction, most likely controlled by the net slab pull. An exception is a northernmost shallow region near the Nootka fault zone where the state of stress is dominated by nearly east-west tension. We hypothesize that the shear force on the Nootka fault zone and margin-parallel mantle resistance to slab motion induce the east-west tension in this region. Near the MTJ, stresses in the JDF plate are dominated by north-south compression down to about 20 km depth, consistent with a strong push by the Pacific plate from south of the Mendocino transform fault, but the deeper part of the slab shows downdip-tension, similar to northern Cascadia. Deviatoric stresses in the JDF slab appear to be very low, resulting in very low intraslab seismicity. In comparison with northern Cascadia, the stresses in most of southern Cascadia are even lower, resulting in nearly no intraslab seismicity.
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  • Wada Receives 2013 Jason Morgan Early Career Award: Response

    Wada, Ikuko  

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  • Mafic High-Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

    van Keken, Peter E.   Wada, Ikuko   Abers, Geoffrey A.   Hacker, Bradley R.   Wang, Kelin  

    The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure-temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 degrees C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.
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