HR: 16:45h
AN: T34A-04    [Abstracts]
TI: Seismic attenuation and velocity constraints on the formation of oceanic lithosphere and the origin of the low-velocity zone
AU: * Yang, Y
EM: YingjiE_yang@brown.edu
AF: Department of Geological Sciences Brown University, 324 Brook street, Providence, RI 02912 United States
AU: Forsyth, D W
EM: donald_forsyth@brown.edu
AF: Department of Geological Sciences Brown University, 324 Brook street, Providence, RI 02912 United States
AU: Weeraratne, D S
EM: weeraratne@dtm.ciw.edu
AF: Department of Terrestrial Magnetism Carnegie Institution of Washington , 5241 Broad Branch Road, NW, Washington, DC 20015 United States
AB: Seismic attenuation has long been neglected in traditional seismic tomography since attenuation effects on amplitude are too hard to separate from other effects such as multi-pathing, focusing/defocusing and scattering. However, attenuation is an important property of Earth­_s materials and can provide us additional knowledge other than elastic velocities about temperature, fluid content, phase changes, and density of solid-state defects in the crust and mantle. Fundamental mode surface wave studies at different periods allow us to constrain regional and vertical variation of the quality factor Q. We use 2-D sensitivity kernels for surface waves based on single-scattering (Born) approximation to account for the effects of scattering on amplitude in regional surface wave studies. We invert phase and amplitude data of Rayleigh waves for shear wave velocities and attenuation (Qu-1) in very young (less than 10 Ma) Pacific regions using teleseismic sources recorded by ocean-bottom seismometers (OBSs). There is a high seismic velocity lid underlain by a low velocity zone. Qu shows a similar variation pattern: large Qu at depths shallower than 50 km and much smaller Qu at depths greater than 60 km. Models that attribute the origin of the low velocity zone beneath old seafloors solely to temperature and pressure effects predict Qu values an order of magnitude too low beneath young seafloor. An alternative model considering the variation of water content in the upper mantle can explain partially the variation pattern of velocities and attenuation. Partial melting in the shallow upper mantle at mid-oceanic ridges during the production of the oceanic crust effectively removes the water in the MORB source region and leads to a ­(°)dry­ñ depleted peridotite layer underlain by a ­(°)wet­ñ fertile peridotite. The presence of water in the asthenosphere lowers Qu and seismic velocity beneath old seafloor, so that the effects attributed to thermal variations are much smaller and thus attenuation in the hot, young lithosphere is less pronounced. In addition to seismic reduction owing to attenuation, about 1% partial melting is required to explain the minimum value of shear wave velocities in the low velocity zone beneath young seafloor. The ~1% melt is produced in the depth range between the ­(°)wet­ñ solidus and the ­(°)dry­ñ solidus due to the presence of relatively small amount of water. Our seismic observations provide strong constraints on the argument that the structure of oceanic plates is controlled by compositional as well as thermal parameters.
DE: 8100 TECTONOPHYSICS
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8180 Tomography (6982, 7270)
SC: Tectonophysics [T]
MN: Fall Meeting 2005