Geology Reference
In-Depth Information
ern oceanic sea floor (Müller et al.
2008
) suggests
that a complete recycling of the oceanic crust may
take more than 200 Myrs. Recent estimates sug-
gest
3 Ga as a probable starting time of this pro-
cess (Shirey and Richardson
2011
; Dhuime et al.
2012
), although other authors claim that plate
tectonics did not initiate during the Archean (e.g.,
Hamilton
1998
). Therefore, the starting time of
the Wilson cycle could be as young as 2.5 Ga.
New oceanic crust is generated by crystal-
lization of a mid-ocean ridge basaltic (MORB)
magma in the empty space that is continuously
created at a spreading center from the horizontal
displacement of two oceanic plates that are mov-
ing apart. Several lines of evidence suggest that
mid-ocean ridges have deep “roots” in the mantle
asthenosphere. Although this layer is made by
solid-state peridotite rocks having considerable
density (between 3,400 and 3,500 kg m
3
), we
shall prove in Chap.
13
that it has a distinct fluid
behaviour, with relatively low viscosity of the
order of 10
20
Pa s. The lateral divergent motion
of two oceanic plates induces passive upwelling
of asthenosphere material, because the separating
plates exert viscous drag on the underlying hot
mantle, determining the continuous formation
of void that must be filled by a vertical flow.
The asthenospheric upwelling beneath mid-ocean
ridges has a lower bound velocity of the order
v
10 mm yr
1
and can be traced downwards to
a depth of 250-300 km on the basis of seismic
1.3
Oceanic Crust
We now consider the
oceanic crust
. With respect
to the continental counterpart, this is a thin layer
(Fig.
1.1
) with different and much more homo-
geneous composition, and different mechanical
properties (in terms of density and seismic ve-
locities). It formed much later at the expenses
of the originary undifferentiated basaltic layer
mentioned above and can be considered as the
most evident product of the global plate tectonics
process, which is historically known as
Wilson
cycle
(Dewey and Burke
1974
). This process
governs the formation of new continents by ag-
gregation of continental masses after episodes of
collision (orogenic phases). It also governs their
splitting and subsequent dispersal, with forma-
tion of new oceanic seaways, as a consequence
of extensional forces. Figure
1.3
shows schemati-
cally the four constitutive elements of the Wilson
cycle. A quantitative description of these pro-
cesses is the ultimate objective of this topic. The
Wilson cycle implies that oceanic crust is con-
tinuously accreted at
spreading centers
that are
localized along the
mid-ocean ridges
, and con-
tinuously destroyed at
subduction zones
,where
it bends downwards and sinks passively into the
asthenosphere. The morphological expression of
these bending lines is represented by the
oceanic
trenches
. The global age distribution of the mod-
Fig. 1.3
The Wilson cycle paradigm. Oceanic crust and
underlying mantle lithosphere are shown in
orange
. Con-
tinental crust and underlying mantle lithosphere are shown
in
green
. The asthenosphere and transition zone are shown
in
light brown
and
blue
respectively. Four distinct inter-
connected processes contribute to the Wilson cycle:
R
Rifting
, where continents are split apart by extensional
force fields,
S Spreading
, where new oceanic crust is
created by cooling of basaltic magma associated with as-
thenosphere upwelling;
T Subduction
, where old oceanic
crust and mantle lithosphere sink into the asthenosphere as
a consequence of gravitational instability; and
C Collision
,
where continental masses join together and form mountain
belts after complete closure of the intervening oceans
