Lowermost mantle anisotropy beneath the Pacific: Imaging the source of theHawaiian plume

We utilized recordings of seismic shear phases provided by several North Am erican broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central Pacific Ocean. Using a new analysis method that reduces measurement errors and enables the analysis o f a larger number of available waveforms, we examined relative travel times of teleseismic S and Sdiff that sample a large area of lowermost mantle st ructure. The results of this study provide evidence for small-scale lateral and depth variations in shear wave anisotropy for a broad region of the lo wermost mantle beneath the Pacific Ocean. In particular, we image a localiz ed zone of anomalously strong anisotropy whose strength increases toward th e top of D " beneath Hawaii. Our results, combined with a previous study of V-P/V-SH ratios, indicate that ancient subducted slab material may be resp onsible for observations beneath the northern Pacific, while tenses or laye rs of core-mantle boundary reaction products or partial melt, oriented by h orizontal inflow of mantle material to the Hawaiian plume source, can expla in observations beneath the central Pacific. (C) 2001 Elsevier Science B.V. All rights reserved.