HR: 14:40h
AN: DI43A-05    [Abstracts]
TI: The role of water in connecting past and future episodes of subduction
AU: * van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Northwestern University, 1850 Campus Dr., Evanston, IL 60208
AU: Regenauer-Lieb, K
EM: klaus.regenauer-lieb@csiro.au
AF: CSIRO, PO Box 1130, Bentley, WA 6102
AU: Yuen, D A
EM: davey@msi.umn.edu
AF: Minnesota Supercomputer Inst, University of Minnesota, 310 Pillsbury Dr., Minneapolis, MN 55455
AB: S-velocity models for the North American upper mantle show a giant "dike" of S-velocities about 0.1 km/s slower than the average upper mantle. Of temperature or relative content of iron, silicon, carbon, and hydrogen, we deem the latter the most likely explanation for the unusual S-velocity anomaly. Resolution tests, based on a recent set of over a thousand wave trains of continent-wide S and surface waves, show that the anomaly could be blurred but is not entirely an artifact. However, S-velocities in the transition zone are underestimated. The low-velocity dike connects to a weak low-velocity zone at the top of the lower mantle in Grand's model (2002). The giant dike is east of and roughly parallel to the Appalachians and to the strike of the Farallon slab in the top of the lower mantle (Grand, 2002). The dike most likely represents an upwelling. Past dehydration of subducted Iapetus lithosphere and/or more recent dehydration of the subducted Farallon plate are potential sources for the inferred wet rock in the low-velocity dike. Geodynamic modeling shows that large-scale lithospheric faulting associated with subduction zones, can develop when the lithosphere is wet. Dry lithosphere such as that of the Atlantic Ocean thus needs to be hydrated to allow a new subduction zone to form on the US East coast. However, numerical modeling shows that a supra-lithospheric ocean cannot hydrate the lithosphere to the required depths and hence the source of hydration must come from below. The hydrous dike-shaped upwelling discussed above is the perfect candidate for such hydration and subsequent initiation of subduction. Under the sediment load off the US east coast and horizontal stress the hydrated lithosphere will yield and can form a shear zone that cuts across the entire thickness of lithosphere. A new subduction zone is so initiated, up to half a billion years after a former subduction process provided the necessary ingredient, H₂O.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8104 Continental margins: convergent
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005