Tectonic interpretation of orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 84

deep-seated crystalline rocks that
have been raised up and unroofed by
erosion to expose a complex region of
deformed high-grade metamorphic
rocks and igneous plutons. Faults in
these deep-seated rocks are replaced
by shear zones, and folds display
the effects of ductile deformation.
Where the upper plate consists of
continental crust (as in Figure 10.1A),
the rocks lying immediately above the
suture, and which have been thrust up
over the units of the foreland thrust belt,
may represent the lowermost part of
the continental crust; in that case, they
will display the effects of high-pressure
metamorphism, including granulite
facies and, especially, eclogite facies .
The latter are particularly important,
since they signify a high-pressure,
moderate-temperature regime cor-
responding to that of the relatively
cool, but very deep, regions above the
subducting slab. These metamorphic
rocks are predominantly gneisses of
both meta-sedimentary and meta-
igneous origin, the latter being mostly
of broadly granitic composition.
Granites, granodiorites and dior-
ites are the characteristic products of
subduction-related igneous activity,
and represent the deep-seated coun-
terparts of volcanic arcs. Such rocks
are usually deformed by the effects of
plate collision, but are intruded by a
later set of plutons (typically granitic),
often in the form of large batholiths.
These have resulted from the melting
of deep-crustal material as a result of
the rise in temperature brought about
by crustal thickening, and are known
as post-tectonic plutons in contrast
to the pre-tectonic plutons that are
generated by the subduction process.

Structures of the core zone
Structures in the central part of the
orogenic belt may be very complex and
difficult to interpret. The oldest will
represent deformation that occurred
during the formation of the conti-
nental basement and will throw no
light on the orogenic process itself.
However, structures affecting the pre-
tectonic plutons and any sedimentary
or volcanic rocks deposited on the
older basement will provide a guide
as to how the orogen developed.
The first structures to form are gener-
ally large recumbent or inclined isocli-
nal folds and related shear zones that
reflect the asymmetry of the fold-thrust
belt of the foreland, but involve higher-
temperature schists and gneisses. The
ductile nature of these structures is
due to the higher temperature within
the central part of the orogen, brought
about partly by the heating effect of the
igneous intrusions and partly by the
crustal thickening. These early folds are
typically refolded by more upright folds
that result from the sub-horizontal com-
pression caused by the convergence of
the opposing plates. In some cases, in
favourable lithologies, several genera-
tions of younger folds with their accom-
panying fabrics can be identified, and
these generally reflect a continuation of
lateral compression within the orogen.
The deformation just described
results in both shortening and thick-
ening of the crust in the core zone to
the extent of doubling or even tripling
the original crustal thickness. Seismic
studies of some orogenic belts have
revealed Moho depths of up to 80 km.

Gravitational spreading and channel
flow
Because of the increased ductility of
the warm lower crust of a thickened
orogen, this region is more susceptible
to gravity-induced flow, or gravitational
spreading , as described in the previous
chapter (see Figure 9.3). Under these
conditions, the continental lithosphere
of the orogen acts like a sandwich with
two stronger layers - the upper crust
and the lower crust/uppermost mantle
- separating a ductile layer which is
subject to lateral flow and is squeezed
towards the sides of the orogenic belt
(Figure 10.4). This process has been
termed channel flow and is a possible
explanation for many of the ductile
structures seen in orogenic belts where
thrust sheets containing medium- to
high-grade gneisses are bounded by
shear zones with opposite shear senses,
and are both underlain and overlain
by stronger, less ductile material.

cooled
crystalline
sheet

orogen

foreland

upper
crust

middle crust

ductile
channel

Figure 10.4 Channel flow. In a developing
orogen, the middle crust becomes heated and
partially melted by the injection of igneous
plutons and by depression to warmer depths
due to crustal thickening, and forms a ductile
channel. The gravitational effect of the thickened
crust squeezes this more ductile middle-crustal
material, enabling it to flow laterally and escape
to the surface as it cools; there it forms a
crystalline sheet bounded by upper and lower
shear zones with opposite senses of shear.