Mantle Wedge Flow Research Articles

Slab steepening and rapid mantle wedge replacement during back-arc rifting in the New Hebrides

The effects of the composition and angle of the subducting slab and mantle wedge flow on tectonic and magmatic processes in island arcs and associated back-arcs are poorly understood. Here we analyse the ages and compositions of submarine lavas from the flanks and the floor of the back-arc Futuna Trough some 50 km east of Tanna Island in the New Hebrides arc front. Whereas >2.5 Ma-old back-arc lavas formed from an enriched mantle source strongly metasomatized by a slab component, the younger lavas show less slab input into a depleted mantle wedge. The input of the slab component decreased over the past 2.5 million years while the enriched mantle was replaced by depleted peridotite. The change of Futuna Trough lava compositions indicates rapid (10 s of km/million years) replacement of the mantle wedge by corner flow and slab steepening due to rollback, causing extensional stress and back-arc rifting in the past 2.5 million years.

Anisotropic structure in the mantle wedge beneath southeastern Mexico from shear-wave splitting tomography

Cocos intraslab earthquakes were used to make a 3-D tomographic inversion to define a crystallographic orientation model for the mantle wedge beneath southeastern Mexico. This model provided insights regarding the pattern of the mantle wedge flow and its relationship to the geometry of the subducting slab. The mantle wedge was parametrized as a 3-D block model of crystallographic orientations assuming the elastic constants of olivine and orthopyroxene with orthorhombic symmetry (hexagonal symmetry was also tested). A linearized, damped, and iterative least-squares approach was used to account for the nonlinear behavior of the shear-wave splitting, numerically recalculating partial derivatives after each iteration. The best-fitting model is consistent with two main flow regimes: (1) 2-D corner flow in a mantle wedge core made up of A-type olivine fabric northwest of the Tehuantepec Ridge extension, and (2) 3-D trench-parallel mantle flow in a mantle wedge core made up of A-, C-, or E-type olivine fabric southeast of this geological feature. Around the Tehuantepec Ridge extension, a partially serpentinized mantle wedge tip is inferred since olivine a-axis orientations are trench-parallel regardless of whether a 2-D corner flow or a 3-D trench-parallel flow prevails. Right above the Tehuantepec Ridge extension (beyond the 100 km isodepth contour of the subducting slab), a change of well-resolved olivine a-axis orientations from trench-normal to trench-parallel while going from northwest to southeast is observed. It signals an abrupt change in the mantle flow pattern possibly through a vertical tear in the Cocos slab. 3-D toroidal flow could be driving subslab mantle material into the mantle wedge around the deepest slab segment. Lastly, approximately trench-normal olivine a-axis orientations are observed in the mantle wedge tip near the Mexico and Guatemala border region, and they could be explained by assuming the presence of B-type olivine fabric.

On unusual conditions for the exhumation of subducted oceanic crustal rocks: How to make rocks hotter than models

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

Shear‐Wave Splitting in the Mantle Wedge: Role of Elastic Tensor Symmetry of Olivine Aggregates

AbstractUsing a 3‐D mantle wedge flow field for a generic oblique subduction system, we calculate elastic tensors of mineral aggregates in the mantle wedge for A‐, B‐, C‐, and E‐type olivine crystal preferred orientations (CPO) and apply the calculated elastic tensor in the forward calculation of shear‐wave splitting (SWS) through the mantle wedge. We find that the hexagonal approximation of the full tensor does not affect the SWS parameters (the fast direction and the delay time) significantly for all CPO types except that the delay time for C‐type CPO becomes shorter. Additionally, we find that despite the 3‐D mantle flow field that results from oblique subduction, the fast direction is margin‐normal for A‐, C‐ and E‐type CPOs and margin‐parallel for B‐type CPO.

Upper mantle and continental crust anisotropy in southeastern Mexico determined from shear-wave splitting measurements using local intraslab earthquakes

Cocos intraslab earthquakes were used to make shear-wave splitting measurements to explore the factors that control seismic anisotropy and to study the mantle wedge flow patterns in southeastern Mexico, where the Cocos plate subducts beneath the North American plate. Cocos intraslab earthquakes reach depths of 250 km, making it possible to sample the mantle wedge. The Silver and Chan (1991) covariance method was used to measure the splitting parameters: the fast polarization direction (φ) and the delay time (δt). The measurements can be divided into three regions: (1) northwest and (2) southeast of the Tehuantepec Ridge extension (i.e., the subducted Tehuantepec Ridge within the Cocos slab) and (3) the region above the subhorizontal Cocos slab. (1) In the first region, northeast of the 100 km isodepth contour of the Cocos slab, the fast axes are trench-perpendicular. This can be explained assuming the development of A-type olivine fabric and the existence of 2-D corner flow driven by the downdip motion of the Cocos slab. Southwest of the 100 km isodepth contour, measurements show trench-parallel fast polarization directions that are also consistent with corner flow, albeit in a serpentinized mantle wedge. Right above the Tehuantepec Ridge extension (northeast of the 100 km isodepth contour of the subducting slab), a change in the fast polarization directions from trench-normal to trench-parallel while going from northwest to southeast is observed and signals a change in the mantle flow pattern possibly through a vertical tear in the Cocos slab. 3-D toroidal flow could drive subslab mantle material around this slab edge and into the mantle wedge. (2) In the second region, the measured fast polarization directions show a trench-parallel orientation that is interpreted to result from southeastward trench-parallel flow through a serpentinized mantle wedge tip and also through a mantle wedge core made up of A- or C-type olivine fabrics with their fast axes oriented parallel to the flow direction. Trench-perpendicular fast polarization directions are observed beneath the fore-arc region of the Central America Volcanic Arc, near Tacaná Volcano, and trench-parallel polarizations are observed beneath the arc. These orientations could be explained by assuming the presence of B-type olivine fabric in the mantle wedge tip, A- or C-type olivine fabric in the mantle wedge core, and trench-parallel flow, so that the orientations of the fast axes become perpendicular to the mantle flow direction beneath the fore-arc and parallel to it beneath the arc. Lastly, (3) in the third region, over the flat slab, the observed delay times (0.04–0.42 s) are consistent with crustal anisotropy magnitudes, and the fast polarization directions seem to be controlled by the orientations of fault systems and alignments in foliations. Therefore, crustal faults and folds seem to be the dominant factors controlling the observed anisotropy.

Mantle Wedge Seismic Anisotropy and Shear Wave Splitting: Effects of Oblique Subduction

AbstractWe investigate the evolution of olivine crystal preferred orientation (CPO) and its effect on local shear wave splitting (SWS) in the mantle wedge of oblique subduction zones. Based on model‐predicted 3‐D mantle wedge flow fields, we compute the A‐type and E‐type olivine CPO distribution for a range of subduction obliquity. The results show that the seismically fast axis does not necessarily align with the flow direction. To model the local SWS parameter distribution for oblique subduction zones, we apply a full range of initial polarization to multilayer models that approximate the model‐predicted CPO distributions. These models result in a bimodal SWS parameter distribution, which relaxes as subduction obliquity increases. Unlike non‐oblique subduction models, these models indicate considerable variations in the SWS parameters with subduction obliquity and initial polarization and also among the forearc, arc, and backarc regions. Because of this variability, a single SWS measurement cannot constrain the CPO distribution, and shear waves with a range of initial polarization are required to interpret the SWS parameters in oblique subduction zones. Our results indicate that 3‐D mantle wedge flow due to oblique subduction cannot explain commonly observed margin‐parallel fast direction in the forearc region but can explain margin‐normal fast directions that are observed in the arc and backarc regions of oblique subduction zones.

Mapping the Pacific Slab Edge and Toroidal Mantle Flow Beneath Kamchatka

AbstractTo clarify the upper mantle structure and dynamics of the Kamchatka subduction zone, we conduct Rayleigh‐wave phase‐velocity azimuthal anisotropy tomography beneath Kamchatka using amplitude and phase data of teleseismic fundamental mode Rayleigh‐waves at periods of 25–150 s. With the obtained anisotropic phase‐velocity model, a 3‐D azimuthal anisotropic shear‐wave velocity (Vs) model is determined down to ∼300 km depth. Beneath Kamchatka, the subducting Pacific slab is clearly imaged as a dipping high‐Vs zone on the south of the Shiveluch volcano at ∼57°N latitude, whereas this high‐Vs zone is absent on the north of the volcano, suggesting that a slab edge exists in the region. Visible low‐Vs anomalies are revealed around the slab edge. The subducting Pacific slab beneath Kamchatka generally exhibits a fast‐velocity direction (FVD) of ∼NE‐SW in the area south of ∼54°N latitude, whereas the dominant FVD in the slab rotates to ∼N‐S or ∼NNW‐SSE in the area of ∼54°–57°N latitude. The FVDs in the subducting Pacific slab are generally parallel to the Kamchatka trench, which may result from the shape‐preferred orientation of stratified oceanic lithosphere. At shallow depths (<∼150 km), the low‐Vs mantle wedge generally exhibits ∼ NW‐SE FVDs, whereas ∼ E‐W FVDs appear on the north of the slab edge, which form semi‐toroidal FVDs around the slab edge. The semi‐toroidal FVDs may reflect a combination of corner flow in the mantle wedge and toroidal mantle flow around the slab edge beneath Kamchatka, which may deform and heat or even melt the slab near its edge.

A simple picture of mantle wedge flow patterns and temperature variation

The solid-state mantle flow is an important factor that controls the mass and heat transfer in the solid Earth. This study aims to provide a simple picture of three-dimensional (3-D) mantle flow patterns in the sub-arc region of subduction zones based on the results of 3-D steady-state numerical models with varying subduction parameters. Here, the mantle wedge flow pattern is evaluated based on the azimuthal directions of the mantle inflow from the back-arc and the down-dip outflow. The outflow direction generally parallels the subduction direction, but the inflow direction relative to the outflow direction depends on the local subduction obliquity – the angle between the subduction direction and the strike-normal axis of the subducting slab. A change in the strike of the slab leads to a change in the obliquity and thus the inflow direction. Such change is common along curved margins as the strike of the slab tends to follow that of the margin, or vice versa. Along convex-arc-ward margins, the mantle inflow is deflected towards the region of lowest obliquity but with reduced vigor due to lower dynamic pressure gradients that partly drive the flow, resulting in a cooler mantle wedge. Along concave-arc-ward margins, the mantle inflow is deflected away from the region of lowest obliquity but with increased vigor, resulting in a hotter mantle wedge. These effects increase with decreasing radius of curvature. Along-margin change in the dip of the subducting slab also affects the inflow direction through its impact on the strike of the slab, but its effect is relatively small. We express the azimuthal inner angle between the inflow and outflow directions as a function of obliquity and apply the function to predict sub-arc mantle inflow directions in the circum-Pacific and neighboring regions. Within and among these margins, the inner angle varies over its full range of 0–180°. Most of the margins that are 1000s of kilometers in length are either straight or curved concave-arc-ward with large radii of curvature, for which small or gradual along-margin changes in the mantle inflow direction and the mantle wedge temperature are predicted. A large drop in the mantle wedge temperature by up to a couple of hundred degrees is predicted at short convex-arc-ward segments, such as at the Kuril-Japan and Bonin-Mariana junctions. The fringes of flat slab segments are curved with small radii of curvature, likely resulting in sharp lateral changes in the inflow direction and the mantle wedge temperature.

P Wave Azimuthal Anisotropic Tomography in Northern Chile: Insight Into Deformation in the Subduction Zone

AbstractBased on a large data set of local body wave travel times, we determined the first 3‐D model of azimuthal P wave anisotropic tomography of northern Chile in order to study the deformation in the subduction zone. Our results indicate different deformation patterns in the overriding lithosphere, mantle wedge, and the subducting Nazca slab as well as significant along‐arc variations. Radiating fast velocity directions (FVDs) around the rupture zone of the 2014 Iquique earthquake (Mw 8.2) are notable in the crust, which may reflect the specific crustal extension induced by a point‐like stress source in the form of a fully locked asperity at the plate interface. In most of the study region, FVDs in the mantle wedge are trench normal, which we interpret to have resulted from mantle wedge flow driven by the oceanic plate subduction. However, trench‐parallel FVDs are found beneath the northern segment, which may imply the occurrence of B‐type olivine fabrics in the cold forearc mantle. Strong along‐arc variations are also observed in the subducting slab, which may reflect intraslab heterogeneity and change in the slab geometry. In the northern segment, slab bending induces generally trench‐normal extension and leads to trench‐normal FVDs in the upper part of the slab. In contrast, in the southern segment, flat slab subduction and thus slab unbending dominate, producing trench‐normal compression and trench‐parallel FVDs instead. Along the subducting slab interface, FVDs change from trench‐parallel to trench‐normal in the downdip direction, which may indicate the first‐order transition from compression in the locking area to dominant shear in the decoupling area along the slab interface.

P-wave attenuation structure of the Lau back-arc basin and implications for mantle wedge processes

We image 3-D seismic P-wave attenuation structure of the Tonga subduction zone and the Lau back-arc basin using local earthquake waveforms recorded by land and ocean bottom seismographs of the 2009–2010 Lau Spreading Center Imaging project. The amplitude spectra of P waves from local earthquakes are inverted for the path-average attenuation operator (t⁎), with a variety of frequency-dependence exponent values (α). Analysis shows that the data are best fit by the assumption of a weak frequency dependence (α≈0.3), consistent with laboratory results modeled as the effects of grain boundary sliding. The t⁎ measurements are inverted for a 3-D tomographic QP−1 model. The results show high P-wave attenuation within the upper 100 km of the back-arc mantle, with a sharp boundary at the Tofua arc between high attenuation sub-arc mantle and the low attenuation fore-arc. Perhaps the highest P-wave attenuation (QP−1≈0.036 or QP≈28) known in the mantle occurs at 20–60 km depth beneath the back-arc spreading centers. High attenuation anomalies form an inclined zone dipping from the spreading centers to the west away from the slab, indicating abnormally high temperature and the existence of hydrous partial melt. These observations suggest that hot materials supplied from the Australian mantle upwell along with the mantle wedge flow pattern, triggering extensive decompression melting near the back-arc spreading centers. The along-strike variation of P-wave attenuation is different from that of SV-velocity, possibly implying the different effects of partial melt on seismic attenuation and velocity.

The effect of obliquity on temperature in subduction zones: insights from 3-D numerical modeling

Abstract. The geotherm in subduction zones is thought to vary as a function of the subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction may strongly vary due to changes in the angle between the trench and the plate convergence vector, i.e., the subduction obliquity, due to trench curvature. We currently observe such curvature in, e.g., the Marianas, Chile and Aleutian trenches. Recently, strong along-strike variations in subduction obliquity were proposed to have caused a major temperature contrast between Cretaceous geological records of western and central Turkey. We test here whether first-order temperature variation in a subduction zone may be caused by variation in the trench geometry using simple thermo-kinematic finite-element 3-D numerical models. We prescribe the trench geometry by means of a simple mathematical function and compute the mantle flow in the mantle wedge by solving the equation of mass and momentum conservation. We then solve the energy conservation equation until steady state is reached. We analyze the results (i) in terms of mantle wedge flow with emphasis on the trench-parallel component and (ii) in terms of temperature along the plate interface by means of maps and the depth–temperature path at the interface. In our experiments, the effect of the trench curvature on the geotherm is substantial. A small obliquity yields a small but not negligible trench-parallel mantle flow, leading to differences of 30 °C along-strike of the model. Advected heat causes such temperature variations (linked to the magnitude of the trench-parallel component of velocity). With increasing obliquity, the trench-parallel component of the velocity consequently increases and the temperature variation reaches 200 °C along-strike. Finally, we discuss the implication of our simulations for the ubiquitous oblique systems that are observed on Earth and the limitations of our modeling approach. Lateral variations in plate sinking rate associated with curvature will further enhance this temperature contrast. We conclude that the synchronous metamorphic temperature contrast between central and western Turkey may well have resulted from reconstructed major variations in subduction obliquity.

Transient rheology of the Sumatran mantle wedge revealed by a decade of great earthquakes

Understanding the rheological properties of the upper mantle is essential to develop a consistent model of mantle dynamics and plate tectonics. However, the spatial distribution and temporal evolution of these properties remain unclear. Here, we infer the rheological properties of the asthenosphere across multiple great megathrust earthquakes between 2004 and 2014 along the Sumatran subduction zone, taking advantage of decade-long continuous GPS and tide-gauge measurements. We observe transient mantle wedge flow following these earthquakes, and infer the temporal evolution of the effective viscosity. We show that the evolution of stress and strain rate following these earthquakes is better matched by a bi-viscous than by a power-law rheology model, and we estimate laterally heterogeneous transient and background viscosities on the order of ~1017 and ~1019 Pa s, respectively. Our results constitute a preliminary rheological model to explain stress evolution within earthquake cycles and the development of seismic hazard in the region.

Heterogeneous subduction structure within the Pacific plate beneath the Izu-Bonin arc

The Izu-Bonin subduction zone is a subduction system formed in early Eocene. The structure of the subduction zone becomes complicated with the evolution of the surrounding plate motion, and many aspects are still unkown or ambiguous. The geodynamic implications are further investigated in related to published seismic observations and geochemical characters of the Izu-Bonin subduction zone.As indicated by seismic tomography and epicentral distributions, the dip angle of the plate beneath the segment to the south of 29°–30°N (the southern Izu-Bonin) is much steeper than the northern one (the northern Izu-Bonin). Deep focus events in the southern segment extend to the depth of ∼600 km, whereas in the northern section deep events just terminate at 420–450 km. Particularly, tomographic images show an obvious boundary between the northern and southern Izu-Bonin at depths of 150–600 km neglected in the previous studies. The northern and southern segments are even separated by a wide range of low-velocity anomaly in P and S wave tomography at 380 km and 450 km depths. In this depth range, three events near 30°N are characterized by strike-slip mechanisms with slab parallel σ1 and horizontally north-south trending σ3, which differ with the typical down-dip compression mechanisms for neighboring events. These events could be attributed to an abrupt change of the morphology and movement of the slab in the transition segment between the northern and southern Izu-Bonin. Indicated by the focal mechanisms, the northern and southern Izu-Bonin exhibits an inhomogeneous stress field, which is closely related to age differences of the downgoing slab. Because of the reheating process, the thermal age of the Pacific plate entering the Izu-Bonin trench in the past 10 Ma, is only 60–90 ± 20 Ma, along with the younger plate subducting in the northern segment. The seismic anisotropy implies that mantle wedge flow orientation is between the motion direction of the Pacific plate and trench strike, which may be caused by the viscous coupling to the subducting plate and along-trench N-S shearing. The NE splitting direction oblique or perpendicular to the NW-NNW movement of the Pacific plate beneath the transition segment results from the “tearing” of the slab, which is also confirmed by the slab-related velocity anomalies, a sharp change in the dip angle of the Wadati-Benioff zone, the tectonic stress characteristics and along-arc variations of Sr-Nd-Pb isotope ratios in the transition segment.

Thermal structure of the Kanto region, Japan

AbstractUsing 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 toward 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 a 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 pattern and distributions of interplate seismicity and arc volcanoes in Kanto.

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.

Fore-arc structure, plate coupling and isostasy in the Central Andes: Insight from gravity data modelling

The central segment of the Peru-Chile subduction zone has not seen a major earthquake of similar scale to the megathrust Iquique event in 1877 (Magnitude ∼8.8). The plate interface between the subducting and overriding plates in the central segment of the subduction zone is highly coupled and is accumulating elastic energy. Here, we assessed the locking mechanism and isostatic state of the Central Andes based on gravity models of the crust and upper mantle structure. The density models are based on satellite gravity data and are constrained by velocity models and earthquake hypocenters. The gravity models indicate a high density batholithic structure in the fore-arc, overlying the subducting Nazca plate. This high density crustal structure is pressing downward into the slab and locking the plate interface. Thus, plate coupling in the Central Andes may result from pressure exerted by high density fore-arc structures and buoyancy force on the subducting Nazca plate. The increased compressive stress closer to the trench, due to the increased contact between the subducting and overriding plates, may increase the intraplate coupling in the Central Andes.To assess the isostatic state of the Central Andes, we determined the residual topography of the region (difference between observed and isostatic topography). There is a residual topography of ∼800m in the western part of the Central Andes that cannot be explained by the observed crustal thicknesses. The residual topography may be attributed to mantle wedge flow and subduction of the Nazca plate. Thus, part of the observed topography in the western part of the Central Andes may be dynamically supported by mantle wedge flow below the overriding plate.

Backarc spreading and mantle wedge flow beneath the Japan Sea: insight from Rayleigh-wave anisotropic tomography

We present the first high-resolution Rayleigh-wave phase-velocity azimuthal anisotropy tomography of the Japan subduction zone at periods of 20–150s, which is determined using a large number of high-quality amplitude and phase data of teleseismic fundamental-mode Rayleigh waves. The obtained 2-D anisotropic phase-velocity models are then inverted for a 3-D shear-wave velocity azimuthal anisotropy tomography down to a depth of ∼300km beneath Japan. The subducting Pacific slab is imaged as a dipping high-velocity zone with trench-parallel fast-velocity directions (FVDs) which may indicate the anisotropy arising from the normal faults produced at the outer-rise area near the Japan trench axis, overprinting the slab fossil fabric, whereas the mantle wedge generally exhibits lower velocities with trench-normal FVDs which reflect subduction-driven corner flow and anisotropy. Depth variations of azimuthal anisotropy are revealed in the big mantle wedge beneath the Japan Sea, which may reflect past deformations in the Eurasian lithosphere related to backarc spreading during 21 to 15 Ma and complex current convection in the asthenosphere induced by active subductions of both the Pacific and Philippine Sea plates.

The mantle wedge's transient 3‐D flow regime and thermal structure

AbstractArc volcanism, volatile cycling, mineralization, and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner‐flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small‐scale‐convection (SSC). Here, we systematically explore mantle‐wedge dynamics in 3‐D simulations. We find that longitudinal “Richter‐rolls” of SSC (with trench‐perpendicular axes) commonly occur if wedge hydration reduces viscosities to Pa s, although transient transverse rolls (with trench‐parallel axes) can dominate at viscosities of Pa s. Rolls below the arc and back arc differ. Subarc rolls have similar trench‐parallel and trench‐perpendicular dimensions of 100–150 km and evolve on a 1–5 Myr time‐scale. Subback‐arc instabilities, on the other hand, coalesce into elongated sheets, usually with a preferential trench‐perpendicular alignment, display a wavelength of 150–400 km and vary on a 5–10 Myr time scale. The modulating influence of subback‐arc ridges on the subarc system increases with stronger wedge hydration, higher subduction velocity, and thicker upper plates. We find that trench‐parallel averages of wedge velocities and temperature are consistent with those predicted in 2‐D models. However, lithospheric thinning through SSC is somewhat enhanced in 3‐D, thus expanding hydrous melting regions and shifting dehydration boundaries. Subarc Richter‐rolls generate time‐dependent trench‐parallel temperature variations of up to K, which exceed the transient 50–100 K variations predicted in 2‐D and may contribute to arc‐volcano spacing and the variable seismic velocity structures imaged beneath some arcs.

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

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.

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.