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Comment

Don L. Anderson

dla@gps.caltech.edu

Tan & Helmberger (2007) developed a pure-path shear velocity model PAC06 for the mantle between Tonga-Fiji and California. The model contains a fast lid (Vsh = 4.78 km s−1, Vsv = 4.58 km s−1) ~60 km thick. The underlying LVZ is prominent with the lowest velocities Vsh = 4.34 km s−1, and Vsv = 4.22 km s−1, inconsistent with a subsoliduus mantle. The anisotropy (Vsv < Vsh) extends to a depth of ~300 km. Besides the 406 km and 651 km discontinuities, PAC06 also has a small (~1%) velocity jump at ~516 km. PAC06 explains a large data set from various events. Therefore it is ideally suited for comparing with mineralogical models. Although the study refers to a relatively homogeneous part of the Pacific, the paths cross over what has been called a superplume in the lower mantle. Clear difference between the paths close to Hawaii and those away from Hawaii are not observed.

If anything, the average Pacific model is slower than the near-Hawaii model in the LVZ and LID. There is no systematic lateral variation along the path, i.e., no continuous velocity decrease or increase.  The thickness of the TZ is remarkably constant, indicating thermal and chemical homogeneity,  and a simple history. If  the lower mantle is hot under the south central Pacific, this heat is not communicated across the TZ or LVZ. 

Citation: Tan, Y., and D. V. Helmberger, Trans-Pacific upper mantle shear velocity structure, J. Geophys. Res., 112, B08301, doi:10.1029/2006JB004853, 2007.

last updated 10th September, 2007

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