Large amounts of water carried down in subduction zones are drivenupward into the overlying forearc upper mantle and crust as increasingtemperatures and pressure dehydrate the subducting crust. Through seismictomography velocities we show (a) the overlying forearc mantle in northernCascadia is hydrated to serpentinite, and (b) there is low Poisson's ratioat the base of the forearc lower crust that may represent silica depositedfrom the rising fluids. From the velocities observed in the forearc mantle,the volume of serpentinite estimated is ∼30 %. This mechanicallyweak hydrated forearc region has important consequences in limits to greatearthquakes and to collision tectonics. An approximately 10 km thick lowercrustal layer of low Poisson's ratio (σ = 0.22) in the forearc isestimated to represent a maximum addition of ∼14 % by volume ofquartz (σ = 0.09). If this quartz is removed from risingsilica-saturated fluids over long times, it represents a significant additionof silica to the continental crust and an important contributor to itsaverage composition.
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