Seismological constraints on structure and flow patterns within the mantle wedge

Abstract

The mantle wedge of a subduction zone is characterized by low seismic velocities and high attenuation, indicative of temperatures approaching the solidus and the possible presence of melt and volatiles. Tomographic images show a low velocity region above the slab extending from 150 km depth up to the volcanic front. The low velocities result at least partially from volatiles fluxed off the slab, which lower the solidus and thus raise the homologous temperature of the wedge material. Subduction zones with active back-arc spreading centers also show large low velocity and high attenuation seismic anomalies beneath the backarc basin, indicating that a broad zone of magma production feeds the backarc spreading center. The magnitude of the velocity anomaly is consistent with the presence of approximately 1% partial melt at depths of 30-90 km. The best-imaged arc-backarc system, the Tonga-Lau region, suggests that the zone of backarc magma production is separated from the island arc source region within the depth range of primary magma production. However, the anomalies merge at depths greater than 100 km, suggesting that small slab components of backarc magmas may originate through interactions at these depths. Slow velocities extend to 400 km depth beneath backarc basins, and these deep anomalies may result from the release of volatiles transported to the top of the transition zone by hydrous minerals in the slab. Observations of seismic anisotropy can provide direct evidence for the flow pattern in the mantle wedge. Slab parallel fast directions suggesting along-arc flow within the mantle wedge are found in most, but not all, subduction zones. The Tonga-Lau region shows a complex pattern of fast directions, with along-strike fast directions beneath the Tonga island arc and convergence-parallel fast directions to the west of the Lau backarc spreading center. The pattern of flow in the Lau backarc is consistent with southward mantle flow inferred from geochemical data. Geodynamic modeling suggests several possible mechanisms of flow within the mantle wedge, which may help explain the diverse observations. Viscous coupling between the backarc flow and the downgoing plate should produce induced flow within the backarc, with flow directions parallel to the convergence direction. In contrast, subduction zone roll-back may produce along-arc flow. This latter model matches the observations in most subduction zones suggesting that viscous coupling does not exert a strong control on the flow pattern in the mantle wedge.