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to the LAB as seen in long period surface and body
wave studies (Priestley & McKenzie, 2006; Yuan
& Romanowicz, 2010).
Using SS precursors (Gu et al ., 2001; Deuss &
Woodhouse, 2002), global maps of the Lehmann
discontinuity have been made (Figure 10.8b).
Alaska (Figure 10.5a), Asia (Figure 10.5c) and the
Indian Ocean (Figure 10.5d) all show observations
of the Lehmann discontinuity in SS precursors; it
is not that easily seen in PP precursors. Gu et al .
(2001) suggested that the Lehmann discontinuity
is only seen in continental regions, but the
observation in the Indian Ocean (Figure 10.5d)
shows that it can also been seen in oceanic
regions. Using Pds receiver functions, Shen et al .
(1998) also found the Lehmann discontinuity
in an oceanic region near the Southern East
Pacific Rise. So, it might preferentially be seen
under continents, but not exclusively as there
are also several oceanic observations. As oceanic
observations of discontinuities are generally
more dfficult to make, it is quite likely that many
more oceanic regions exist with a Lehmann
discontinuity. The Lehmann discontinuity is
only occasionally seen in PP precursors (Rost
& Weber, 2001) and P P precursors (Adams,
1971; Tkal ci c et al ., 2006). Pds receiver functions
should be used with caution as observations from
the Lehmann discontinuity may be masked by
crustal multiples (Li et al ., 2002).
In order to find out what mineralogical
mechanism might cause the Lehmann dis-
continuity, it is important to determine its
'seismological Clapeyron slope' for comparison
with mineralogical Clapeyron slopes. Using
Equation (10.1), dT can be taken from local
tomographic velocity variations and dP from the
discontinuity depth. A recent study determined
that the Lehmann discontinuity has a negative
seismic Clapeyron slope (Deuss & Woodhouse,
2004). Candidate mineral phase transitions are
coesite to stishovite (Akaogi et al ., 1995) or
orthoenstatite to high clino-enstatite (Angel
et al ., 1992; Woodland, 1998), which both
have positive Clapeyron slopes. Comparing the
seismic Clapeyron slopes with mineral physical
Clapeyron slopes, it was found that none of the
candidate mineral physical phase transitions had
the correct Clapeyron slope. Thybo and Perchuc
(1997) suggest that the Lehmann discontinuity
might be the base of a partially molten zone, but
that would not explain the discontinuity in the
range of regions where it has been observed.
As an alternative, Karato (1992) suggested
that the Lehmann discontinuity could be due
to a change in deformation mechanism from
dislocation creep above, to diffusion creep below.
This would agree with observed anisotropy in the
top few hundred kilometers of the Earth being
due to dislocation creep, and is in agreement
with work on the LAB by Yuan and Romanowicz
(2010). Gaherty and Jordan (1995) also suggested
that the Lehmann discontinuity could be the
base of an anisotropic layer. The depth of the
change in deformation mechanism depends both
on water content and on temperature (Karato,
1992). In particular, the depth will be deeper
in colder regions, which would agree with a
negative Clapeyron slope (Deuss & Woodhouse,
2004). Also, a wet upper mantle is required to
get the deformation change at shallow enough
pressure to explain reflections from around
220 km depth. Thus, the most likely explanation
for the Lehmann discontinuity is a change
in deformation mechanism from dislocation
to diffusion creep. Especially S waves will be
sensitive to a change in deformation mechanism,
potentially explaining why the Lehmann dis-
continuity is most often seen in SS precursor.
Alternatively, Gung et al . (2003) argued that the
Lehmann discontinuity is caused by a change
in anisotropy from weaker anisotropy above, to
stronger anisotropy in the deeper mantle.
10.4.3 X-discontinuity
The name 'X-discontinuity' is loosely used for
any discontinuity observation in the depth range
of 250 to 350 km. It was first mentioned by
Revenaugh and Jordan (1991b) in their work
using ScS reverberations, and it is also seen
in SS precursors (Deuss & Woodhouse, 2002,
2004) and Pds receiver functions. Revenaugh and
Williams (2000) combined different published
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