HR: 14:25h
AN: T13D-04 INVITED     [Abstracts]
TI: Seismological Applications of Laboratory Measurements of Dispersion and Attenuation in Upper-Mantle Materials
AU: * Jackson, I
EM: Ian.Jackson@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Faul, U
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Fitz Gerald, J
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: Progress in the use of seismic-frequency experimental techniques for the measurement of dispersion and attenuation in melt-free and melt-bearing olivine polycrystals will be reviewed. The generally mild frequency and grain-size sensitivities of the observed attenuation are suggestive of grain-boundary relaxation - particularly grain-boundary sliding for which there is not yet an adequate microphysical theory. Under these circumstances, our approach is to model the high-temperature viscoelastic rheology with an empirically successful creep function. We have recently identified an optimal creep function of the generalised Burgers type that simultaneously represents our shear modulus and dissipation data obtained at oscillation periods of 1-1000 s and temperatures of 1000-1300 C for a suite of four genuinely melt-free olivine polycrystals ranging in mean grain size from 3 to 165 micron. Application of this laboratory-based seismic-frequency model to the conditions of teleseismic wave propagation in the upper mantle requires only modest extrapolation in grain size, temperature and pressure - the latter through an activation volume. The calculated shear wave speeds and attenuation for melt-free olivine for plausible upper-mantle grain sizes and geotherms reproduce many of the first order features of the seismic structure of the oceanic upper mantle. A zone of low wave speed and high attenuation is predicted that becomes progressively less pronounced and deeper with increasing lithospheric age in accord with seismological observations. Similar calculations suggest that distinctive upper-mantle seismic structures for contrasting tectonic provinces within continental regions are attributable mainly to systematic differences in the depth at which the conductive geotherm intersects a common adiabat. The continental geotherms are calculated with literature values for surface heat flow and heat production; comparison with seismological models confirms observations from Canada and South Africa (Jaupart and Mareschal, Lithos, 1999) that surface heat-flow variations are due to crustal heat production with low and near constant heat-flow across the Moho.
DE: 5144 Wave attenuation
DE: 7218 Lithosphere and upper mantle
DE: 3909 Elasticity and anelasticity
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting