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about 3.5--4.0 km/s. Shear velocities can be deter-
mined from both body waves and the dispersion
of short-period surface waves. The top of the man-
tle under continents usually has velocities in the
range 8.0--8.2 km/s for compressional waves and
4.3--4.7 km/s for shear waves.
The compressional velocity near the base of
the oceanic crust usually falls in the range
6.5--6.9 km/s. In some areas a thin layer at the
base of the crust with velocities as high as
7.5 km/s has been identified. The oceanic upper
mantle has a P-velocity (Pn) that varies from
about 7.9 to 8.6 km/s. The velocity increases with
oceanic age, because of cooling, and varies with
azimuth, due to crystal orientation or to dikes
and sills. The fast direction is generally close
to the inferred spreading direction. The average
velocity is close to 8.2 km/s, but young ocean has
velocities as low as 7.6 km/s. Tectonic regions also
have low velocities. The shear velocity increases
from about 3.6--3.9 to 4.4--4.7 km/s from the base
of the crust to the top of the mantle.
Ophiolite sections found at some continental
margins are thought to represent upthrust or
obducted slices of the oceanic crust and upper
mantle. These sections grade downward from
pillow lavas to sheeted dike swarms, intrusives,
pyroxene and olivine gabbro, layered gabbro and
peridotite and, finally, harzburgite and dunite
(Figure 8.3). Laboratory velocities in these rocks
are given in Table 8.7. There is good agree-
ment between these velocities and those actually
observed in the oceanic crust and upper man-
tle. The sequence of extrusives, intrusives and
cumulates is consistent with what is expected at
a midocean-ridge magma chamber.
The velocity contrast between the lower crust
and upper mantle is commonly smaller beneath
young orogenic areas (0.5--1.5 km/s) than beneath
cratons and shields (1--2 km/s). Continental rift
systems have thin crust (less than 30 km) and
low Pn velocities (less than 7.8 km/s). Thinning of
thecrustintheseregionsappearstotakeplace
by thinning of the lower crust. In island arcs
thecrustalthicknessrangesfromabout5kmto
35 km. In areas of very thick crust such as in the
Andes (70 km) and the Himalayas (80 km), the
thickening occurs primarily in the lower crustal
layers. Oceanic crust also seems to thicken by
Table 8.7 Density, compressional velocity
and shear velocity in rock types found in
ophiolite sections
V p
V s
Poisson's
ρ
Rock Type
(g/cm 3 ) (km/s) (km/s)
Ratio
Metabasalt
2.87
6.20
3.28
0.31
Metadolerite
2.93
6.73
3.78
0.27
Metagabbro
2.95
6.56
3.64
0.28
Gabbro
2.86
6.94
3.69
0.30
Pyroxenite
3.23
7.64
4.43
0.25
Olivine gabbro
3.30
7.30
3.85
0.32
Harzburgite
3.30
8.40
4.90
0.24
Durite
3.30
8.45
4.90
0.25
Salisbury and Christensen (1978),
Christensen and Smewing (1981).
Approx.
Depth
(km)
Rock type
SHEAR VELOCITY (P
=
0)
Vs
density
(g/cc)
3
4
5
6
km/s
3.62
3.68
3.71
3.68
3.57
3.79
3.73
3.72
3.83
3.69
3.28
3.84
3.78
3.64
3.80
4.03
4.30
4.05
2.62
2.68
2.74
2.77
2.79
2.79
2.80
2.80
2.81
2.86
2.87
2.87
2.93
2.95
2.99
2.98
3.07
3.10
granite
granodiorite
gneiss restite
anorthosite
gneiss
diorite
anorthosite
diorite
serpentinite
gabbro
metabasalt
gabbro
dolerite
gabbro
diabase
gneiss restite
amphibolite
granulite-mafic
13
CRUST
20
3
4
5
6
3.20
3.23
3.80
4.43
amphibole
pyroxenite
3.24
4.28
eclogite
60
3.29
4.68
Avg.ultramafic rock
bronzite
cpx
dunite
PHN1569
continental
moho
3.29
3.30
3.30
3.31
4.59
4.60
4.90
4.87
80
Fig. 8.3
Crustal minerals.
increasing the thickness of the lower layer. Pale-
ozoic orogenic areas have about the same range
of crustal thicknesses and velocities as platform
areas.
The seismic lithosphere or LID
The top of the mantle is characterized, in most
places, by a thin high-velocity layer, a seismic lid.
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