Geology Reference
In-Depth Information
Because sedimentation continues as
subduction and convergence proceed,
younger sediments are less deformed
than older, as shown in Figure 10.2.3.
platform sediments
Figure 10.3 Inversion.
Faults formed during the
extensional period when
the passive continental
margin was created are
re-used during orogenic
compression by a reversal
of the movement sense on
them.
10
extension
82
83
Inversion
Some of the thrusts affecting the
continental margin may represent
re-activated normal faults formed as
a result of the extensional event that
originally created the passive conti-
nental margin (Figure 10.3). This re-
activation is termed inversion , and
occurs widely where compression of
a piece of crust takes advantage of
zones of weakness already created
by a previous extensional episode.
uplifted fault blocks
foreland
basin
foreland basin
compression
by a unit consisting of sheet-like mafic
and ultramafic intrusions, followed
in turn by a sheeted dyke complex,
basalt pillow lavas and ocean-floor
sediments. The origin of this sequence
is due to the way that ocean crust is
created, and is discussed in Chapter
3 (see Figure 3.9A). The ocean-floor
assemblage contains the characteristic
red cherts, which are thinly-bedded
silicic deposits containing fossil radio-
larian - radiolarian cherts ; these are
diagnostic of the deep ocean, since
they are deposited beneath the level
where carbonate can be preserved.
Complete sequences should be
around 6 km thick at least, and do occur
in some orogenic belts, but fragmentary
sequences are quite common and occur
in accretionary prisms where they have
been scraped off the top of the down-
going slab. Some complete sequences
occur in large thrust slices that are
continuous for many hundreds of kilo-
metres and may mark the boundary
between two plates, as shown in Figure
10.1A, since they are the first slice to be
scraped from the subducting slab, and
therefore form the highest thrust unit
beneath the upper plate. The boundary
between the plates is known as a suture
or suture zone and is discussed below.
Eu-geosynclines and mio-geosynclines
Before the plate tectonic theory was
established, it was recognised from
studies of many orogenic belts that
there were two contrasting types of
depositional basin: the eu-geosyncline
and the mio-geosyncline , both contain-
ing thick sequences of predominantly
clastic deposits of equivalent age to the
much thinner platform sequences. The
mio-geosyncline corresponds to the
continental slope environment and the
eu-geosyncline to the deep-ocean basin,
ocean trench and the marginal deposi-
tional environment of the upper plate. A
diagnostic characteristic of the eu-geo-
syncline was held to be the presence of
volcanics and, importantly, ophiolites.
Outer zones of the opposing plate
Fold-thrust belts and foreland basins are
also typically present on the opposite
plate, as shown on the right-hand side
of Figure 10.1A, but may have a dif-
ferent origin. The sense of asymmetry
exhibited by that part of the orogen
closest to the subduction zone (i.e. the
fold-thrust belt just described) does
not appear to be transmitted through
the orogen to the far side. The conver-
gence between the two plates, and the
consequent uplift of the central core
complex, have the effect of impos-
ing the opposite sense of asymmetry
to the marginal zones on the other
plate. Thus it seems that all orogenic
belts are bordered by thrust belts that
combine to squeeze the orogen out-
wards towards the bordering plates.
Ophiolites
The term ophiolite refers to a sequence
of rock types that are interpreted as
pieces of oceanic crust. A 'complete'
ophiolite sequence would commence
with ultramafic material assumed to be
from the uppermost mantle, succeeded
The central crystalline core
complex
The central core of the orogenic
belt consists essentially of originally
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