Geology Reference
In-Depth Information
dip toward the base of the sequence, where dips
are sub-horizontal (Nicolas
1989
).
Except for plate boundaries, only COBs in-
terrupt the considerable petrologic and mechan-
ical regularity of the oceanic crust. These zones,
which mark the transition to the continental do-
main, can be narrow or broad, and character-
ized by the presence of ultra-thinned blocks of
“transitional” continental crust embedded into an
oceanic “matrix”, exhumed mantle not capped by
oceanic crust, or thickened “transitional” oceanic
crust. A modern and more rigorous term (with
respect to the acronym COB) to indicate the
interface region between oceanic and continental
crust is
ocean-continent transition zone
,orsim-
ply OCT. We shall conclude this section dedi-
cated to the oceanic crust with a short discussion
about the principal characteristics of these inter-
esting regions.
The formation of an oceanic basin is always
preceded by a phase of continental rifting that
involves extensional faulting and thinning of the
continental crust. A modern example of this pro-
cess can be observed along the West African Rift
zone, which extends from the region of Afar in
Ethiopia to South Africa. The onset of sea floor
spreading is not necessarily synchronous along
the rift axis, but more generally occurs along
discrete axial cells that form and grow indepen-
dently for some million years until they join into
linear spreading segments, as observed in the Red
Sea (Bonatti
1985
). At the end of this stage, the
geologic setting of the region at the interface
between the thinned continental margin and the
truly oceanic domain depends from many factors,
the most important being: (a) the velocity of rift-
ing, (b) the presence of thermal anomalies, (c) the
fertility of the asthenosphere beneath the rift area,
and (d) the presence of small-scale convective
currents in the asthenosphere (Ligi et al.
2011
,
2012
). Two end-members can be used to illustrate
the possible range of situations. In the case of
volcanic passive margins
, the onset of sea floor
spreading is associated with intense volcanism,
eventually related to the presence of a
mantle
plume
. This can be viewed as an anomalously hot
or fertile region of the asthenosphere or, more in
general, as an upper-mantle area characterized by
Fig. 1.6
Layering of the oceanic crust.
H
is the thickness
in km,
v
P
is the average
P
-waves velocity in km s
1
.
Sym
is the symbol of the corresponding layer
4.7-4.9 km s
1
marks the top of the seismic layer
2B, which can be considered as a transition zone
characterized by the first appearance of dikes.
The underlying low-porosity sheeted dike com-
plex, layer 2C, is
1.5 km thick and has
P
-wave
velocities between 6.6 and 6.7 km s
1
. This layer
provides the most direct evidence of sea floor
spreading, because it is formed by dikes that have
intruded older dikes in so far as the divergent mo-
tion of two tectonic plates has created new space
to be filled. Therefore, these dikes are generally
arranged in a regular temporal sequence, with the
youngest ones located close to the ridge and the
oldest placed along the continental margins. The
presence of a well-developed 2C layer is also evi-
dence of approximate balance between spreading
rate and magma supply (Robinson et al.
2008
).
The sequence of rocks forming the oceanic crust
is completed by the pair of intrusive gabbroid
layers 3A and 3B, having thickness 2-5 km and
P
-wave velocity
v
P
7.2 km s
1
. The isotropic
gabbros that can be found just beneath the sheeted
dike complex grade into gabbros having a weakly
developed near-vertical layering. Such layering
becomes more developed and acquires shallower
