[1] The anelastic structure of a subduction zone can place first-order constraints on variations in temperature and volatile content. We investigate seismic attenuation across the western Pacific Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment. This 11-month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc, island arc, and back-arc spreading center. We compute amplitude spectra for P and S arrivals from local earthquakes and invert for the path-averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake. Additionally, we investigate earthquake source parameter assumptions and frequencydependent exponents (a) ranging from 0 to 0.6. Tomographic inversion of nearly 3000 t* estimates (at a = 0.27) for 2-D Q P À1 and Q P /Q S structure shows a $75 km wide columnar-shaped high-attenuation anomaly with Q P $ 43-60 beneath the spreading center that extends from the uppermost mantle to $100 km depth. A weaker high-attenuation region (Q P $ 56-70) occurs at depths of 50-100 km beneath the volcanic arc, and the high-attenuation regions are connected at depths of 75-125 km. The subducting Pacific plate is characterized by low attenuation at depths greater than 100 km, but high attenuation is found in the plate between 50 and 100 km depth. The fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc. Q S structure is less well resolved than Q P because of a smaller data set, but Q P /Q S ratios are significantly less than 2 throughout the study region. As temperatures estimated from Q S À1 are unusually high, we interpret the arc and wedge core anomalies as regions of high temperature with enhanced Q À1 due to hydration and/or melt, the slab and fore-arc anomalies as indicative of slab-derived fluids and/or large-scale serpentinization, and the columnar-shaped high Q P À1 anomaly directly beneath the back-arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis.

Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, slab dehydration, arc volcanism, and backarc spreading. The mantle wedge is characterized by a low attenuation forearc, an inclined zone of low velocity and high attenuation underlying the volcanic front, and a broad region of low velocity and high attenuation beneath the backarc spreading center when present. Seismic velocities, bathymetry, and basalt chemistry suggest mantle temperature variations of ∼100°C between different backarc regions. Rock physics experiments and geodynamic modeling are essential for interpreting seismic observations. Seismic anisotropy indicates a complex pattern of mantle flow that can be modeled with along-strike flow in a low viscosity channel beneath the arc and backarc. Comparison of geodynamic models with seismic tomographic results using experimentally derived relations between velocity, attenuation, and temperature suggests the existence of small melt fractions in the mantle at depths of 30–150 km.

[1] Although the Pacific and Nazca plates share the East Pacific Rise (EPR) as a boundary, they exhibit many differing characteristics. The Pacific plate subsides more slowly and has more seamounts than the Nazca plate. Both the seismic and magnetotelluric components of the Mantle ELectromagnetic and Tomography Experiment (MELT) found pronounced asymmetry in mantle structure across the spreading axis near 17°S. The Pacific (west) side has lower S-wave velocities, exhibits greater shear wave splitting, and is more electrically conductive than the Nazca (east) side. These results suggest asymmetric mantle flow and melt distribution beneath the EPR. To better understand the causes for these asymmetric properties, we construct numerical models of melting and mantle flow beneath a midocean ridge migrating to the west over a fixed mantle. Although the ridge is migrating to the west, the migration has little effect on the upwelling rates, requiring a separate mechanism to create the asymmetry. Models that produce asymmetric melting with a temperature anomaly require large (>100°C) excess temperatures and may not be consistent with the observed subsidence and crustal thickness. A possible mechanism for creating asymmetry without a temperature anomaly is across-axis asthenospheric flow, possibly driven by pressures created by upwelling beneath the Pacific Superswell to the west. Pressure-driven asthenospheric flow follows the base of the lithosphere, extending the upwelling region to the west as it follows the thinning lithosphere toward the axis, and shutting off melting as it crosses the axis and encounters an increasingly thick lithosphere to the east.

The MELT Experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise [1][2][3][4][5][6]. Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick [7]. Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric [2,3,6]; at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity [1][2][3][4]6]. Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies.Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.-2 -

S U M M A R YShear wave splitting measurements provide significant information about subduction zone mantle flow, which is closely tied to plate motions, lithospheric deformation, arc volcanism, and backarc spreading processes. We analyse the shear wave splitting of local S waves recorded by a large 2003-2004 deployment consisting of 58 ocean-bottom seismographs (OBSs) and 20 land stations and by nine OBSs from a smaller 2001-2002 deployment. We employ several methods and data processing schemes, including spatial averaging methods, to obtain stable and consistent shear wave splitting patterns throughout the arc-backarc system. Observed fast orientation solutions are dependent on event location and depth, suggesting that anisotropic fabric in the mantle wedge is highly heterogeneous. Shear waves sampling beneath the northern island arc (latitudes 17.5 • -19 • N) and between the arc and backarc spreading centre show arcparallel fast orientations for events shallower than 250 km depth; whereas, fast orientations at the same stations are somewhat different for deeper events. Waves sampling beneath the central island arc stations (latitudes 15.5 • -17.5 • ) show fast orientations subparallel to both the arc and absolute plate motion (APM) for events <250 km depth and APM-parallel for deeper events. Ray paths sampling west of the spreading centre show fast orientations ranging from arc-perpendicular to APM-parallel. Arc-parallel fast orientations characterize the southern part of the arc with variable orientations surrounding Guam. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than ∼250 km and at large distances from the trench. We conclude that the arc-parallel fast orientations are likely the result of physical arc-parallel mantle flow and are not due to recently proposed alternative lattice preferred orientation mechanisms and fabrics. This flow pattern may result from along-strike pressure gradients in the mantle wedge, possibly due to changes in slab dip and/or convergence angles.

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Mantle dynamics can strongly affect melting processes beneath spreading centers and volcanic arcs. A 2‐D numerical model of the Tonga subduction zone, with the slab viscously coupled to the mantle beneath the brittle‐ductile transition but faulted above, shows that induced corner flow may cause asymmetric melting at the Lau back‐arc spreading center, 400 km away. The down‐going slab also entrains the high‐viscosity base of the overlying lithosphere, drawing hot, low‐viscosity asthenosphere upwards into the gap, triggering decompression melting in the wedge. Because the slab is decoupled from the brittle overlying plate, a cold upper corner develops, inhibiting melting where the slab is shallow. The cold corner is consistent with seismic attenuation and heat flow at arcs. Decompression melting may be a substantial fraction of magma production at some arcs, but less at others. Possibly more important, the shallow decompression melting structure may govern the pathways of melt extraction beneath volcanic arcs.

[1] The Tonga arc and associated Lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production. We create seismic tomograms of the Tonga-Lau region by jointly inverting for Vp and Vp/Vs structure using data from the LABATTS ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge. Jointly using P and S data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and/ or volatiles. Because high attenuation in the wedge limits the S wave data set, we focus on 2-D inversions beneath the linear OBS array where resolution is best and also parameterize the solution in terms of the Vp/ Vs ratio. As expected, the subducting slab has fast Vp and Vs and a low Vp/Vs ratio, consistent with the cold downgoing plate. The Central Lau Spreading Center (CLSC) exhibits stronger anomalies in Vp/Vs than in Vp, with the anomalies larger than would be predicted purely by temperature variations. The CLSC anomaly extends >100 km to the west of the axis, suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms. The anomaly is asymmetric about the axis, suggesting that slab-induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc. There is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc, likely outlining a hydrated region of melt production that feeds the volcanic front. Hydration possibly continues throughout the wedge to at least 400 km depth. The Lau ridge exhibits a thicker lithosphere relative to the rest of the Basin, while the Fiji platform likely has a thinner lithosphere than the Lau Ridge from more recent extension. There is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform.