HR: 0800h
AN: T51B-0548    [Abstracts]
TI: Where Does Subducted Water Go? Estimates from Seismic Tomography of Serpentinite in the Forearc Mantle and Quartz in the Deep Crust
AU: * HYNDMAN, R D
EM: rhyndman@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre9860 W Saanich Road PO Box 6000, Saanichton, BC V8L4B2, Canada
AU: RAMACHANDRAN, K
EM: kumar-ramachandran@utulsa.edu
AF: University of Tulsa, Department of Geosciences University of Tulsa, Tulsa, OK 74104-3189, United States
AB: Much of the large amounts of water are carried down in subduction zones is driven upward into the overlying upper mantle and crust. The world oceans are consumed in approximately a billion years. Although this water is concluded to be responsible for the volatile-rich volcanic arc magmas, and reduced upper mantle viscosity and shallow convection in the backarc, the largest amount of water expulsion is estimated to be beneath the forearc, at least for subduction of young hot oceanic lithosphere such as Cascadia and SW Japan. The upward expelled fluids appear to be responsible for low-velocity serpentinite in the forearc mantle and in a concentration of silica deposited at the base of the forearc lower crust. High-resolution seismic tomography P- and S-wave velocities in northern Cascadia are now of adequate resolution to quantitatively estimate the total fluid incorporated into serpentinite hydration, and the rising silica-rich fluid required to deposit the silica-rich layer. The average serpentinization is ~30% from both P- and S-wave velocities (Vp=7.6, Vs=4.2). Just seaward of the arc where the forearc mantle wedge is thickest, this hydration approximately represents the total amount of water estimated to be released from the downgoing plate under this region in the most recent ~43 Ma subduction phase. Little fluid may reach the crust and surface. Similar results are found for a number other subduction zones. The ~10 km thick lower crustal layer of remarkable low Vp/Vs of 1.65 (Poisson's Ratio, σ, of 0.22) is concentrated mainly just seaward of the mantle corner. It may be explained by a ~20% volume quartz content removed from rising silica-saturated fluids. This represents a significant addition of silica to the continental crust, and to the average composition of some crustal accretion. The required total water flux again is similar to that estimated to be released from the underlying subducting slab. In this mid-forearc region, much of the fluid may reach the upper crust and surface. We conclude that within the forearc, arc, and backarc, we are now able to account for most of the total water carried into the earth by the subducted oceanic plates.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting