HR: 10:50h
AN: T22B-03    [Abstracts]
TI: The nature of the Lehmann discontinuity in subcontinental mantle
AU: Woodhouse, J
EM: john.woodhouse@earth.ox.ac.uk
AF: Dept. Earth Sciences, Oxford University, Oxford, OX1 3PR United Kingdom
AU: * Deuss, A
EM: deuss@esc.cam.ac.uk
AF: Bullard labs, University of Cambridge, Cambridge, CB3 0EZ United Kingdom
AB: The nature of the Lehmann discontinuity is of major importance for our understanding of upper mantle composition and flow. While the existence of the Lehmann discontinuity at 220 km in Earth's mantle has been known for over 40 years from seismology, it is still debated what causes this discontinuity and if it is a global feature or not. Some studies using seismic SS precursor data have suggested that it primarily exists beneath the continents (Gu et al. 2001), while others have found it in both continental and oceanic regions (Deuss & Woodhouse, 2002). Here we study the details of this discontinuity using a global data set SS precursors, which has been extended to also include PP precursors. We find clear reflections of the Lehmann discontinuity in many different continental areas, in particular below North America where is was first discovered. Using SS-precursors, we have measured seismological Clapeyron slopes for the Lehmann discontinuity, which are key to explaining the seismic discontinuity as either a manisfestation of phase transitions or of other mineral physical processes. The Clapeyron slopes are measured by correlating discontinuity depths with local velocity perturbations from a tomographic model, assuming that the velocity perturbations are solely due to temperature variations. We find that in most continental regions the Lehmann discontinuity is characterised by a regionally varying negative seismological Clapeyron slope. Known phase transitions in the upper mantle above 400 km depth all have positive Clapeyron slopes. In the case of the Lehmann discontinuity the only remaining hypothesis for a negative Clapeyron slope is that it represents the transition in deformation mechanism from dislocation to diffusion creep (Karato, 1992). This corresponds to a change from anisotropic structure above, to isotropic structure below the discontinuity and thus is independent evidence for the maximum depth extend of upper mantle anisotropy. This transition is predicted to appear at 340 km depth for a dry mantle and 240 km depth for a wet mantle rheology. Thus our observations are also an indicator of the existence of a significant amount of water in the Earth's upper mantle.
DE: 3902 Creep and deformation
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005