Geoscience Reference
In-Depth Information
Fig. 11.5
Shear wave velocity from 50 to 550 km depth
along the great-circle path shown. Cross-sections are shown
with two vertical exaggerations. Note the low-velocity
regions in the shallow mantle below Mexico, the Afar and
south of Australia and the asymmetry of the North Atlantic.
Velocity variations are much more extreme at depths less
than 250 km than at greater depths. The circles on the map
represent hotspots.
integrated slab locations since Pangea breakup
and fast velocities in the depth range 220--1022
km. Wen and Anderson (1995) quantified the
slab flux by estimating the subducted volume
in the hotspot reference frame and correlated it
with seismic tomography throughout the man-
tle. They found significant correlations in the
depth interval 900--1100 km and attributed these
to the accumulation of subducted lithosphere in
this region. Correlations were found in the upper
mantle and transition zone for more recent sub-
duction.
Some slabs may be stopped at the 650 km
discontinuity for a period of time, while some
slabs may penetrate to 1000 km. The residual
tomography shows high velocity beneath the
Kurile, Japan, Izu-Bonin, Mariana, New Hebrides
and Philippine trenches. This implies that the
subducted slabs may accumulate beneath these
trenches at the 650 km discontinuity. The good
correlations between the 0--30 Ma subduction
and residual tomography can be explained by the
accumulation of slabs beneath the Kurile, Japan,
There is
no clear global, statisti-
cally significant,correlation between
surface hotspots or swells and mantle
structure
below some 400 km.
Slabs and tomography
The relationship between
subduction and
seismic tomography
has been studied widely.
Scrivner and Anderson (1992) correlated subduc-
tion positions since the breakup of Pangea, with
seismic tomography depth by depth throughout
the whole mantle. They found the best corre-
lations in the transition zone region. Ray and
Anderson (1994) found good correlations between
