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In-Depth Information
Fig. 2.13 The simple
shear model of rifting of
Werni cke ( 1985 )
continental extension is represented by the so-
called core complexes, in which high-grade meta-
morphic rocks from the middle to lower crust are
exposed at the surface, surrounded and overlain
by low-grade metamorphic rocks. This rifting
mode requires high extension rates over nar-
row zones and an extremely weak, low-viscosity
lower crust. In this instance, while extension pro-
ceeds, a lower crust inflow from the surrounding
region keeps the topography uniform.
Orogenic belts represent the last kind of con-
tinental plate boundaries. These are collisional
structures that should be distinguished from
other kinds of mountain belts, for example from
orogens associated with accretionary wedges,
like the northern Apennine chain of Italy (Treves
1984 ), or from Andean-type orogens associated
with subduction magmatism and accretion of
exotic terranes. Orogenic plate boundaries are
the product of continental collisions, which
follow the closure of intervening oceans in the
context of the Wilson cycle (see Sect. 1.3 ) . The
tectonic style of these compressive structures is
sometimes called the Alpine style of orogeny
(e.g., Frisch et al. 2011 ), essentially because the
spectacular Alpine-Himalayan belt, extending
from western Europe to China, is the unique
example of active orogenic boundary in the
modern Earth. The formation of this mountain
belt started after the collision of three continental
masses,
( 50 Ma). This event followed the closure of the
neo-Tethys ocean, a wide oceanic domain that
existed between Gondwana and Eurasia since
the early Mesozoic (e.g., Schettino and Turco
2011 ). The collisional structures of this orogenic
belt are still active. This is confirmed both by
the diffuse seismicity (Fig. 2.14 ) and by space
geodetic observations across the mountain ranges
(Kreemer et al. 2003 ). Figure 2.14 shows the
chain of convergent and transpressive boundaries
composing the Alpine-Hymalaian belt. In the
next section we shall learn how the set plate
boundaries that are active at any given time
can be linked together to form a plate tectonic
configuration.
2.5
Triple Junctions
Both the direct observation of modern plates and
plate reconstructions show that plate boundaries
are joined together in groups of three, at loca-
tions that are called triple junctions . The lack of
higher order junctions is not casual but depends
from their instability. For example, it is easy to
show that a four-order junction always splits into
two triple junctions (e.g., Cox and Hart 1986 ).
McKenzie and Morgan ( 1969 ) showed that there
are 16 possibilities to form triple junctions by
linking three plate boundaries at a point. If we
designate by R , T ,and F , respectively a mid-
ocean ridge, a trench, and a strike-slip fault, then
Africa,
Arabia,
and
India,
with
the
southern
Eurasian
margin during the
Eocene
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