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 |
