Geoscience Reference
In-Depth Information
Fig. 11.4
Map of SV velocity at
∼
340 km depth. A
prominent low-velocity anomaly shows up in the central
Pacific (the Polynesian Anomaly). The fast anomalies under
eastern Asia, northern Africa and the South Atlantic may
represent mantle that has been cooled by subduction.
the structure of ridges and hotspots and the
style of mantle convection, one can calculate
residual maps by excluding from the tomogra-
phy the first-order effects of conductive cooling
of oceanic plates, deep cratonic roots, and par-
tial melting or cooling caused by subducted litho-
sphere (Wen and Anderson, 1995). The good cor-
relations between residual tomography in the
transition zone (400--650 km) with 0--30 Ma sub-
duction can be explained by slab accumulation
in this region. The correlations between seismic
velocities in the transition region and the sub-
duction during earlier periods are poor. This may
indicate that slabs reside near the 650 km dis-
continuity for only a certain period of time. This
discontinuity may not be the place where long-
lived-mantle convective stratification takes place,
as often assumed, and it may not be a bound-
ary between chemically distinct regions of the
mantle. Mantle convection may be decorrelated
closer to 900 km, near a recently rediscovered
mantle discontinuity (the Repetti discontinuity).
This may be a chemical boundary. If so, we expect
it to be highly variable in depth.
western North America, the western Pacific and
south of Australia may be sites of subducted or
over-ridden oceanic lithosphere. Prominent slow
anomalies are under the northern East Pacific
Rise and in the northwest Indian Ocean. The cen-
tral Atlantic and the older parts of the Pacific are
fast.
The upper-mantle shear velocities along three
great-circle paths are shown in Figures 11.5 to
11.7. The open circles are slower than average
regions of the mantle. These show the effects of
ridges and cratons.
Quantitative tectonic correlations
Cratonic 'roots' (
archons
), thickening oceanic
plates and subducting slabs are the first-order
contributors to tomography above about 400 km
depth. In order to investigate the fate of slabs,
