Composition of the Crust and the Mantle - Quantitative Plate Tectonics - page 13

Fig. 1.8 Typical cross-section of a non-volcanic OCT.
The continental margin is formed by a sequence of blocks
that have been tilted and rotated by listric faults. The
growth of new mantle lithosphere not capped by oceanic
crust occurs towards the spreading center ( brown area ).

The olivine in mantle peridotite is subject to hydration
at the contact with the oceanic seawater, which induces a
metamorphic transformation that converts this mineral to
serpentine (a hydrous Mg-silicate), magnetite, and other
minerals

magma prevents the formation of oceanic crust at
the ridge axis, thereby the new oceanic seaway
will be floored directly by mantle peridotites.
Figure 1.8 illustrates the main features of this
kind of OCTs. Well-known examples are the
western margin of Iberia and its conjugate (the
Newfoundland margin), the northernmost seg-
ments of the central Atlantic passive margins,
the conjugate margins of Labrador and western
Greenland, and the paleo-margins of the western
Tethys. These OCTs, which can be hundreds
of kilometers wide, generally have a seaward
termination represented by a peridotite ridge ,a
basement high partly buried by post-rift sediment
(Boillot and Froitzheim 2001 ). Although both
the peridotite ridge and the serpentinized mantle
exhumed along the OCT may have a strong mag-
netic signal, the truly oceanic domain, character-
ized by MORB accretion and well-developed sea-
floor spreading magnetic anomalies (Chap. 5 ) ,
starts farther seaward.

lithosphere , which also includes the continental
and oceanic crust. In most treatises on plate
tectonics, this composite lithospheric layer
is considered as the true protagonist of the
game, because the usual definition of tectonic
plates refers to a subdivision of the entire
lithosphere (including the crustal and mantle
components) into a set of blocks that move
independently each other and behave, to a first
approximation, like elastic laminae. Therefore,
it is generally supposed that the lithosphere
has coherent mechanical behaviour despite
the lateral and vertical variability of chemical
composition. In reality, both direct observations
of seismicity and laboratory experiments indicate
that only the upper portion of a lithospheric
plate, the elastic - ductile lithosphere , can resist
static shear deformation through an elastic or
plastic response, whereas the lower part will
flow just like the underlying asthenosphere,
although with much higher viscosity. For this
reason, such lower lithospheric layer cannot be
considered as a permanent part of a tectonic plate.
Experimentally, it is found that the boundary
between the elastic-ductile lithosphere and the
lower fluid layer approximately coincides with
the 650 ı C isotherm and is marked by a sharp
cutoff of seismicity (e.g., Bodine et al. 1981 ;
Anderson 1995 ).

1.4 Lithospheric Mantle

The lithospheric mantle composes the strong
outermost part of the Earth's mantle. However,
it is often described as the lower portion

of a more heterogeneous external layer, the