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Douglas Wiens

Washington University in St. Louis
The Structure of Subduction Zones: Implications for Subduction Dynamics and Volatile Recycling
Mapping the mantle seismological structure of island arcs is important for understanding the processes of magma production, transport, and overall flow patterns in the mantle wedge. The mantle wedge, extending from the volcanic front into the backarc region, is characterized by extremely low seismic velocities and high attenuation, as evidenced by tomographic studies of many island subduction zones. The lowest seismic velocities generally represent an inclined region extending above the slab from about 150 km depth up to the volcanic front, and probably represents the source region for island arc magmas. It is unclear from the magnitude and P/S anomaly ratios of these anomalies whether they represent actual in situ detection of magma or simply the relatively hot source regions. The inclined low velocity region may result from the hydration of the mantle wedge by fluids expelled from the slab by dehydration reactions in the upper 150 km. Island arcs with active backarc spreading centers offer the further opportunity of studying the interaction between volcanic arc and backarc spreading center magma systems. Tomographic imaging suggests that the melt production zones of the island arc and backarc spreading center at the shallowest depths, but merge at depths greater than 100 km. This suggests that any direct geochemical interchange between the arc and backarc spreading center occurs at depths of greater than 100 km. The low velocity region beneath the backarc spreading center extends nearly the entire width of the backarc basin, suggesting a broad region of magma production at depths of 30-90 km. Lower amplitude slow velocity anomalies extend to depths of 400 km, perhaps providing evidence for loss of volatiles from the slab at these depths beneath the backarc basin. The mantle flow pattern in arcs probably influences many processes, such as the path of melt from the source region as well as the distribution of geochemical anomalies. Observations of seismic anisotropy can provide direct evidence for the pattern of solid flow in the mantle, as the olivine fast axes should be oriented in the direction of maximum extensional strain, which is approximately the flow direction. Slab parallel fast anisotropy directions, indicating arc parallel flow within the mantle wedge, are found in most arc settings. In several prominent cases, the pattern of melt flow determined from seismic anisotropy is in agreement with geochemical observations, suggesting that the mantle flow pattern can be accurately inferred.
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