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Published April 1984 | Published
Journal Article Open

Mapping convection in the mantle


Long-period (100-250 sec.) Love and Rayleigh waves are used to map heterogeneity and azimuthal anisotropy in the upper mantle. Spherical harmonic descriptions of anisotropy up to ℓ = m = 3 and 2θ are derived. Azimuthal anisotropy obtains values as high as 1½%. There is good correlation of fast Rayleigh wave directions with upper mantle return flow directions derived from kinematic considerations. This is consistent with the a-axis of olivine being aligned in the flow direction. The main differences between the flow models and the Rayleigh wave azimuthal variation maps occur in the vicinity of hotspots. Hawaii, for example, appears to perturb the return flow. There is strong correlation of the geoid and surface wave velocity at ℓ = 4 and 5. Slow regions at this scale are associated with geoid highs and high heat flow, consistent with upwelling convective flow or with isostatically compensated regions of low density. The correlation of azimuthal anisotropy with upper mantle return flow directions, rather than with plate directions, suggests that part of the return flow is in the upper mantle and this, in turn, implies a low viscosity channel.

Additional Information

© 1984 American Geophysical Union. Received January 6, 1984; revised March 5, 1984; accepted March 6, 1984. Paper number 4L0373. We thank Ichiro Nakaniski who kindly provides us with data. Digital data from the SRO and IDA networks made this study possible. We thank Brad Hager for many stimulating discussions and for reviewing the manuscript. This research was supported by NSF grant No. EAR811-5236 and NASA grant No. NSG-7610. Contribution number 4007, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

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