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which seismic velocity increases rapidly with depth (gradients of 1-2 km s
−
1
per kilometre of depth). In very young crust, the P-wave velocity in the top few
hundred metres of layer 2 may be less than 2.5 km s
−
1
. Drilling into the top of layer
2 has shown that it is made up of sediments, extrusive basaltic pillow lavas and
lava debris in varying degrees of alteration and metamorphism. Deeper drilling
has found more consolidated basalts. Towards the base of this basaltic layer are
sheeted dykes, which cause the P-wave velocity to increase further. Together,
this variability in composition and gradation from sediments to basalt to dykes
appears to account for the rapid and variable increase in seismic velocity with
depth (Fig. 9.5(b)).
Layer 3
, sometimes called the
oceanic layer
,isthicker and much more uniform
than layer 2. Typical P-wave velocities are 6.5-7.2 km s
−
1
, with gradients of about
0.1-0.2 km s
−
1
per kilometre depth. Layer 3 is generally presumed to be gabbroic
in composition. Some seismic experiments have shown that the basal part of this
layer (sometimes termed layer 3B) has a higher P-wave velocity (7.2-7.7 km s
−
1
),
indicating perhaps a change to more cumulate-rich gabbros at the base of the crust.
Afew experimenters have suggested that there is a low-velocity P-wave zone at
the base of layer 3, but this is not generally thought to be a universal feature of
the oceanic layer. In 1962, Hess proposed that layer 3 is partially serpentinized
peridotite formed as a result of hydration of the upper mantle. His proposal
was largely discounted in the 1970s, but it is possible that, in regions where the
oceanic crust is significantly thinner than normal (beneath fracture zones; crust
formed at very-slow-spreading ridges), significant serpentinization of the upper
mantle may well occur. Hole 735B penetrated 1.5 km into 11-Ma-old layer 3 on
Atlantis Bank (Southwest Indian Ridge). The rocks are strongly heterogeneous
olivine gabbros - no cumulates were intersected. This is consistent with a crust
constructed from many small intrusions, each crystallizing
in situ
, and there being
no steady-state magma chamber.
Crust formed in back-arc basins is very similar to normal oceanic crust. In the
Lau back-arc basin layer 2, and hence the crust, is 1-1.5 km thicker than normal.
This difference is presumably due to a combination of the physical and chemical
influences of the adjacent Tonga-Kermadec volcanc arc.
In the same way as that in which P-waves are used to model the seismic
structure of the crust, we can use S-waves to make an S-wave velocity model.
Refraction data of sufficient quality for making detailed S-wave models are very
rare, so, unfortunately, crustal S-wave velocity models are uncommon. Figure 9.6
shows a detailed model of the P- and S-wave velocity structures of the oceanic
crust east of Guadalupe Island off the west coast of Baja California. The seafloor
at this location is about 15 Ma old and is part of what is left of the Farallon
Plate (see Section 3.3.4). Of particular interest here are the low velocities in the
shear-wave model for layer 3. The evidence for this low-velocity zone is very
good. Synthetic S-wave seismograms, computed for the best P-wave velocity
model assuming that Poisson's ratio s is 0.28 throughout the crust, do not match
