Geoscience Reference
In-Depth Information
The observed topography on the discontinu-
ities does not seem to be explicable by thermal
effects to the extent expected. The observations
are consistent with the decorrelation of seismic
anomalies between the upper and lower man-
tles observed both in tomographic images and
revealed by matched filtering using plate/slab
reconstructions. A few anomalies appear to
extend from the surface through the TZ and
into the deep mantle; it would be interesting to
calculate if they are more numerous than would
be expected by chance.
Detailed study of some specific regions have
yielded surprises. The thinnest TZ region, 181 km
thick, is found in Sumatra, where a thick accu-
mulation of cold slabs is thought to exist, which
would thicken the TZ. In western USA, the thick-
ness of the TZ varies from 220 to 270 km, with
20 to 30 km relief on each discontinuity, and no
correlation with surface geology, topography or
between the discontinuities [
mantleplumes
].
There is evidence from scattering of seis-
mic waves and plate-tectonic--tomographic corre-
lations that there may be a chemical boundary
near 1000-km depth. Much of modern mantle
geochemistry is based on the conjecture that the
650-km phase change is also a major chemical
change, and that this is the boundary between
the upper and lower mantles. Geodynamic
models assume that below 650 km depth, the
mantle radioactivity is high. Mantle geodynamics
is also based on the assumption that if slabs can
penetrate the phase-change region they will sink
to the core--mantle boundary. The
transition
zone of the upper mantle
therefore contin-
ues to be a critical region for investigation.
The Repetti discontinuity
A layered convection model with a chemical
interface near 900 km at the base of Bullen's
Region C explains the geoid and dynamic topog-
raphy. The evidence for stratification includes
the mismatch between tomographic patterns and
spectra between various depth regions, and evi-
dence for slab flattening. There is a good correla-
tion between subducted slabs and seismic tomog-
raphy in the 900--1100 km depth range. The man-
tle does not become radially homogenous and
adiabatic until about 800 km depth. A variety
of evidence suggests that
there might be an
important geodynamic boundary, possi-
bly a barrier to convection,and a ther-
mal boundary at a depth of about 900--
1000 km
, the bottom of the transition region.
Chemical boundaries, in contrast to most
phase-change boundaries, will not be flat, as
assumed in some layered convection models, and
will have little impedance contrast. The latter
inference is based on plausible compositional
differences between various lower-mantle assem-
blages. Complications between 650 and 1300 km
depth in the mantle are perhaps related to slab
trapping or thermal coupling and undulations in
the Repetti discontinuity, the top of Region D.
