Tectonic interpretation of orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 80

Tectonic interpretation of orogenic belts
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10

Tectonic interpretation of orogenic belts

Most of the more interesting geologi-
cal structures discussed in the previ-
ous chapters are found in orogenic
belts, and the majority of structural
studies have been carried out there.
Such studies are important in under-
standing how orogenic belts can be
explained by plate tectonic models and
in linking structures seen on a small
scale to their Earth-scale origins.

This central core is flanked by regions
of folded and thrust strata incorporat-
ing material deposited in elongate
marginal basins, together with sedi-
mentary sequences originally laid down
in the continental platform regions
or forelands bordering the orogenic
belt (Figure 10.1A). This arrangement
may be symmetric, as in the Alpine-
Himalayan system with forelands on
both sides, or asymmetric, as in the
American Cordilleran belt, where the
foreland is on one side only, the other
side being oceanic plate. Asymmetric
systems are the result of the subduc-
tion of oceanic plate along one side of
the orogen, whereas symmetric systems
result from continent-continent col-
lision but retain features derived from
their previous subduction history.
The geometry of an orogenic belt
is shaped by its plate tectonic history:
for example, the marginal regions will
represent either an active or a passive
plate margin. In the former case, they
will display evidence of igneous activ-
ity consistent with a subduction zone
(e.g. see Figure 3.10B); in the latter case,
igneous activity will generally be absent.
Orogenic belts situated on an active
continental margin are thus typically
asymmetric with many of the important
planar structures, such as major thrusts,
inclined towards the continent; this
sense of asymmetry is referred to as the
'polarity' of the belt. All orogenic belts
must commence as subduction zones,
although these may be subsequently

obscured by continental collision, as
in Figure 3.10C. However, the sub-
duction history is preserved within
the orogenic belt and can be recon-
structed by examining its structure.

The marginal zones
Foreland and foreland basin
A passive continental margin border-
ing a symmetric orogenic belt such as
that shown in Figure 10.1A will display a
characteristic sequence of sedimentary
zones, as illustrated in Figure 10.1B. A
traverse from the interior of the conti-
nent (the foreland ) of the lower (sub-
ducting) plate towards the original site
of the subduction zone will encounter
first a sequence of shallow marine sedi-
ments, often dominated by carbonates,
lying on continental basement: this is
the continental platform . This zone is
followed by the foreland basin (or fore-
deep) which results from the depres-
sion of the continental crust caused by
the loading effect of the rising orogenic
belt (Figure 10.1C) and contains a thick
sequence of predominantly clastic
sediments derived from the erosion
of the main mountain range. This
depositional basin is followed in turn
by the foreland fold-thrust belt, which
is characterised by a set of thrusts and
related folds with a polarity correspond-
ing to that of the subduction zone, that
is, the thrusts dip away from the plat-
form towards the core of the orogen.

Principal tectonic units of the
orogenic belt
The word 'orogenic' means 'mountain-
forming', and in a geological context
has come to be applied specifically
to large-scale linear surface features
exhibiting both lateral contraction and
vertical uplift. The present-day oro-
genic belt system, described in Chapter
2, consists of two main strands: the
circum-Pacific and Alpine-Himalayan,
both of which vary widely along their
lengths, and comprise both continental
mountain ranges such as the American
Cordillera, the Alps and the Himalayas,
and partly submerged oceanic island
arcs such as those in the north and
west Pacific, Indonesia and the Carib-
bean ( see Figure 2.2). In terms of the
plate tectonic model, orogenic belts
are explained as the sites of either plate
subduction or continent-continent
collision ( see Chapter 3, Figure 3.10).
All orogenic belts contain a central
crystalline core consisting of uplifted,
highly deformed, metamorphic rocks
and igneous (largely granitic) plutons.