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and formed with little melting in the mantle. Such non-volcanic margins have
very thin stretched continental crust, or even lack any crust, in the continent-
ocean transition zone. Along the Iberian margin, the ocean-continent transition
is a region of exposed mantle (drilled at basement highs) that has been serpen-
tinized (Fig. 10.43). The width of this transition zone varies considerably along
the margin, at its greatest being over 150 km. The stretching/extension along
this margin must have been so slow that the unroofed continental mantle was
cooled by conduction rather than undergoing any decompression melting. Since
exposed mantle peridotite would become serpentinized through reaction with sea
water, its seismic velocity and density would be appropriate for crust rather than
mantle.
Whether the rifting process itself focusses mantle thermal/plume activity or
whether the thermal activity initiates the rifting is much debated - chicken-or-
egg-wise. Rifting of continents is a slow process - the time interval between
the start of flood volcanism and the formation of oceanic crust can be tens
of millions of years. Following flood volcanism and with the onset of exten-
sion, the magmatism changes from silicic to felsic and in style and volume.
Thick extrusive layers (subaerial and submarine volcanic and sedimentary rocks)
form at high levels and are imaged as the sequences of 'seaward-dipping reflec-
tors'. Additionally, the thick zones of igneous material are underplated to the
lower crust in the transitional region between the stretched continental crust
and the normal oceanic crust. The thickness of these high-velocity zones can
be well in excess of the thickness of normal oceanic crust, reaching 10-15
km (Fig. 10.42).
Figure 10.44 outlines the situations that occur when the lithosphere is uni-
formly stretched by a factor of
. The initial subaqueous subsidence
S
i
can be
determined by assuming the
before
and
after
columns to be in isostatic equilib-
rium (see Section 5.5.2). In this case, we can write
β
h
l
−
−
S
i
h
c
β
ρ
c
+
h
l
−
h
c
β
h
l
β
h
c
ρ
c
+
(
h
l
−
h
c
)
ρ
l
=
ρ
l
+
ρ
a
+
S
i
ρ
w
(10.8)
where
ρ
c
is the average density of the crust,
ρ
l
the average density of the subcrustal
lithosphere,
ρ
w
the density of water,
h
c
the
thickness of the crust,
h
l
the thickness of the lithosphere and
ρ
a
the density of the asthenosphere,
the stretching
factor. Rearranging Eq. (10.8)gives an expression for the initial subsidence:
β
1
−
h
l
(
ρ
a
−
ρ
l
)
+
h
c
(
ρ
l
−
ρ
c
)
ρ
a
−
ρ
w
1
β
S
i
=
(10.9)
If the temperature gradient in the lithosphere is assumed to be linear and the
temperature of the asthenosphere
T
a
is assumed to be constant, the densities
ρ
c
,
