Ikuko Wada

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

Processes in subduction zones such as slab and mantle-wedge metamorphism, intraslab earthquakes, and arc volcanism vary systematically with the age-dependent thermal state of the subducting slab. In contrast, the configuration of subduction zones is rather uniform in that the arc is typically situated where the slab is ∼100 km deep. Toward reconciling the diversity and uniformity, we developed numerical thermal models with a nonlinear mantle rheology for seventeen subduction zones, spanning a large range of slab age, descent rate, and geometry. Where there are adequate observations, such as in Cascadia, northeast Japan, and Kamchatka, we find that surface heat flows can be explained if the interface between the slab and the mantle wedge is decoupled to a depth of 70–80 km. Models with this common decoupling depth predict that the region of high mantle temperatures and optimal fluid supply from the dehydrating slab, both required for melt generation for arc volcanism, occurs where the slab is ∼100 km deep. These models also reproduce the variations of the metamorphic, seismic, and volcanic processes with the thermal state of the slab. The shallow decoupling results in a stagnant fore arc whose thermal regime is controlled mainly by the subducting slab. The deeper coupling leads to a sudden onset of mantle wedge flow that brings heat from greater depths and the back arc, and its thermal effect overshadows that of the slab in the arc region. Our results serve to recast the research of subduction zone geodynamics into a quest for understanding what controls the common depth of decoupling.

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

Supervisory Committee Dr. Kelin Wang Supervisor Dr. George Spence Co-Supervisor Dr. Roy D. Hyndman Departmental Member Dr. John F. Cassidy Departmental Member Dr. Henning Struchtrup Outside Member The thermal structure of subduction zones depends on the age-controlled thermal state of the subducting slab and mantle wedge flow. Observations indicate that the shallow part of the forearc mantle wedge is stagnant and the slab-mantle interface is weakened. In this dissertation, the role of the interface strength in controlling mantle wedge flow, thermal structure, and a wide range of subduction zone processes is investigated through twodimensional finite-element modelling and a global synthesis of geological and geophysical observations. The model reveals that the strong temperature-dependence of the mantle strength always results in full slab-mantle decoupling along the weakened part of the interface and hence complete stagnation of the overlying mantle. The interface immediately downdip of the zone of decoupling is fully coupled, and the overlying mantle is driven to flow at a rate compatible with the subduction rate. The sharpness of the transition from decoupling to coupling depends on the rheology assumed and increases with the nonlinearity of the flow system. This bimodal behaviour of the wedge flow gives rise to a strong thermal contrast between the cold stagnant and hot flowing

Intraslab Stresses in the Cascadia Subduction Zone from Inversion of Earthquake Focal Mechanisms

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 me- chanisms 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 trans- form 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.

Dynamics of Subducting Slabs: Numerical Modeling and Constraints from Seismology, Geoid, Topography, Geochemistry, and Petrology

Slab dynamics poses a critical hurdle for understanding mantle convection and plate tectonics on Earth in general and subduction zone processes, such as intraslab earthquakes, mantle wedge flow, and volatile recycling, in particular. A significant effort has been made for the development of a series of numerical and analog experiments and the synthesis of observational constraints on slab dynamics. This chapter is dedicated to guiding the reader through the current state of our knowledge and some of the long-standing and recently raised questions that resulted from these studies.

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

We use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc. Previous coupled thermal-grain size evolution (T-GSE) models predict small grain size (< 1 mm) in the corner flow of the mantle wedge, a down-dip grain size increase by ∼two orders of magnitude along the base of the mantle wedge, and finer grain size in the mantle wedge for colder-slab subduction zones. We integrate these T-GSE modeling results with a fluid migration model, in which permeability depends on grain size, and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow. Our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down-dip by corner flow due to the small grain size and low permeability immediately above the slab. As grain size increases with depth, permeability increases, resulting in upward fluid migration. Fluids released beneath the arc and the backarc are also initially dragged down-dip, but typically are not transported as far laterally before they begin to travel upward. As the fluids rise through the backarc mantle wedge, they become deflected towards the trench due to the effect of mantle inflow. The combination of down-dip migration in the forearc and trench-ward migration in the backarc results in pathways that focus fluids to the arc. This article is protected by copyright. All rights reserved.

Progress in the Project for Development of GPS/Acoustic Technique Over the Last 4 Years

GPS/Acoustic (GPS/A) survey is the most promising way to detect crustal deformation in the ocean far from the coast, where a dense onshore GPS network is not available. Monitoring seafloor deformation is crucial to understand the tectonic state in regions of geophysical significance such as subduction zones. We, Tohoku University, together with Nagoya University and Japan Coast Guard have been dedicated to GPS/A survey around the Japanese Islands and developing its instruments for more than a decade. Especially in 2010, a new project for the development of the GPS/A technique commenced, and since 2012 following the Tohoku earthquake, further acceleration of the project has been taken place. Tohoku and Nagoya Universities have been working on this project for 4 years. In the project, Tohoku University worked on several topics, such as realtime/continuous monitoring of crustal deformation using a moored buoy, automatic survey using an Autonomous Surface Vehicle (ASV), which makes the survey as efficient as possible, and constructing a new GPS/acoustic survey network along the Japan Trench and their intensive survey using a chartered ship. In this paper, we summarize the achievements in each of the topics above.

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid-oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA-type basalts (H2O/Ce = 215 ± 30, δDSMOW = -45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = -30 to -40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = -57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (-68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = -94 ± 3 ‰). A multi-stage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration and re-equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of <1% carbonated eclogite- ± sediment-derived C-O-H-Cl fluids to depleted mantle at 180 to 280 km (EM) or within the transition zone (PREMA).

Clustering of arc volcanoes caused by temperature perturbations in the back-arc mantle

Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated. The sub-arc mantle is brought in from the back-arc largely by slab-driven mantle wedge flow. Dynamic processes in the back-arc, such as small-scale mantle convection, are likely to cause lateral variations in the back-arc mantle temperature. Here we use a simple three-dimensional numerical model to quantify the effects of back-arc temperature perturbations on the mantle wedge flow pattern and sub-arc mantle temperature. Our model calculations show that relatively small temperature perturbations in the back-arc result in vigorous inflow of hotter mantle and subdued inflow of colder mantle beneath the arc due to the temperature dependence of the mantle viscosity. This causes a three-dimensional mantle flow pattern that amplifies the along-arc variations in the sub-arc mantle temperature, providing a simple mechanism for volcano clustering.

Modeled temperatures and fluid source distributions for the Mexican subduction zone: Effects of hydrothermal circulation and implications for plate boundary seismic processes

In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the sliding behavior of the plate boundary fault. The pressure-temperature paths for subducting material control the distributions of dehydration reactions, a primary control on the pore fluid pressure distribution. Thus, constraining subduction zone temperatures are required to understand the seismic processes along the plate interface. We present thermal models for three margin-perpendicular transects in the Mexican subduction zone. We examine the potential thermal effects of vigorous fluid circulation in a high-permeability aquifer within the basaltic basement of the oceanic crust and compare the results with models that invoke extremely high pore fluid pressures to reduce frictional heating along the megathrust. We combine thermal model results with petrological models to determine the spatial distribution of fluid release from the subducting slab and compare dewatering locations with the locations of seismicity, nonvolcanic tremor, slow-slip events, and low-frequency earthquakes. Simulations including hydrothermal circulation are most consistent with surface heat flux measurements. Hydrothermal circulation has a maximum cooling effect of 180°C. Hydrothermally cooled crust carries water deeper into the subduction zone; fluid release distributions in these models are most consistent with existing geophysical data. Our models predict focused fluid release, which could generate overpressures, coincident with an observed ultraslow layer (USL) and a region of nonvolcanic tremor. Landward of USLs, a downdip decrease in fluid source magnitude could result in the dissipation in overpressure in the oceanic crust without requiring a downdip increase in fault zone permeability, as posited in previous studies.

Thermal structure of the Kanto region, Japan

Using a 3-D numerical thermal model, we investigate the thermal structure of the Kanto region of Japan where two oceanic plates subduct. In a typical subduction setting with one subducting slab, the motion of the slab drives solid-state mantle flow in the overlying mantle wedge, bringing in hot mantle from the back-arc towards the forearc. Beneath Kanto, however, the presence of the subducting Philippine Sea plate between the overlying North American plate and the subducting Pacific plate prevents the typical mantle wedge flow pattern, resulting in a cooler condition. Further, frictional heating and the along-margin variation in the maximum depth of slab-mantle decoupling along the Pacific slab surface affect the thermal structure significantly. The model provides quantitative estimates of spatial variations in the temperature condition that are consistent with the observed surface heat flow patterns and distributions of interplate seismicity and arc volcanoes in Kanto.