Geology Reference
In-Depth Information
Fig. 1.2
Global crustal
thicknesses in the model
SECT (After Bagherbandi
2012
)
The continental crust is characterized by
lateral and vertical chemical heterogeneity, as
well as by variable thickness. These rocks have a
bulk low-density (¡
2,700 kg m
3
) “andesitic”
(intermediate) composition. This implies that a
description of the average chemical composition
of the continental crust in terms of oxide compo-
nents coincides with that of a typical andesite, an
extrusive rock that is characteristic of subduction
zones and is made principally by plagioclase,
mafic (that is, having high Mg and Fe content)
minerals (hornblende, clinopyroxene, and or-
thopyroxene), and possibly quartz and biotite (see
Tab le
1.1
). The thickness varies from a few kilo-
meters along the ultra-thinned continent-ocean
boundaries (COBs) to more than 80 km in regions
of continental collision. Seismology studies
indicate that the greatest crustal thicknesses can
be found in the Himalayas (
80 km) and along
the western Cordillera of the Andes (70-74 km).
Figure
1.2
shows the recent 1
ı
resolution global
model proposed by Bagherbandi (
2012
), which
furnishes a good representation of the crustal
geometry especially in the continental areas.
In general, at least three layers with different
petrologic and physical properties are necessary
to describe accurately the continental crust. The
upper continental crust is formed by sedimen-
tary and granitic rocks, with a bulk granodioritic
composition. In terms of components, it includes
66.6 % SiO
2
, 15.4 % Al
2
O
3
,5.0%FeO
T
(total Fe
computed as FeO), 3.6 % CaO, 3.3 % Na
2
O, and
2.8 % K
2
O (Rudnick and Gao
2003
). The deep
continental crust can be generally divided into
middle
(between 10 and 20 km depth) and
lower
crust
(below 20-25 km). The former is dominated
by amphibolites (metamorphic rocks which have
experienced temperature in excess of 500
ı
Cand
pressure between 0.3 and 0.8 GPa) and has an
overall trace-element pattern that is very simi-
lar to the upper crust, which indicates that also
this layer is formed by products of the origi-
nal phase of intra-crustal differentiation. Its bulk
composition is intermediate and includes 63.5 %
SiO
2
, 15.0 % Al
2
O
3
,6.0%FeO
T
,5.3%CaO,
3.6 % MgO, 3.4 % Na
2
O, and 2.3 % K
2
O (Rud-
nick and Gao
2003
). Therefore, with respect
to the upper crust this layer has lower SiO
2
and K
2
O concentrations and higher FeO, MgO,
and CaO concentrations. The lower continen-
tal crust is believed to be mainly formed by
granulites, metamorphic rocks that are associated
with
HT
(
T
> 650
ı
C) and low (
LP
) to mod-
erate pressure conditions (0.3<
P
< 1.2 GPa).
The
P
-waves seismic velocity is subject to rel-
evant lateral variations, but is generally high
(
v
P
7kms
1
), suggesting a dominance of mafic
granulite and/or amphibolite lithologies. How-
ever, continental arcs and some Archean cra-
tons are characterized by slower seismic velocity,
which is indicative of more evolved composition
(that is, higher SiO
2
content). The lower crust
bulk composition includes 53.4 % SiO
2
, 16.9 %
Al
2
O
3
,8.6%FeO
T
,9.6%CaO,7.2%MgO,
2.7 % Na
2
O, and 0.8 % TiO
2
(Rudnick and
Gao
2003
).
