Tectonic interpretation of orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 82

filled by clastic material, especially
greywackes, and volcanics derived
from the volcanic arc on the upper
plate; this region merges into:
5. the continental slope of the upper
plate, beyond the subduction zone,
which is also the site of thick clastic
deposits.

The accretionary prism
This consists of an accumulation of
clastic sediments and volcanic debris
occupying the trench and continental
slope of an active continental margin,
which is piled up in a series of folded
and thrust slices (Figure 10.2). The
accretionary prism is deformed pro-
gressively as subduction of the oceanic
slab proceeds. It is underlain by a basal
thrust, or detachment, above which
the strata are scraped off and com-
pressed. With continued convergence,
the deformation front moves outwards

towards the ocean; here thrusts and
overfolds are directed oceanwards;
the zone of maximum compression,
where the sedimentary wedge is thick-
est, may be uplifted and the structures
on the landward side directed towards
the continent. At deep levels of the
prism, temperatures and pressures are
higher and structures more ductile.
Higher and older thrust sheets may thus
expose rocks that had been taken down
to some depth on the subducting slab
and now exhibit slate-grade metamor-
phism and more ductile deformation.

The fold-thrust belt is a conse-
quence of the convergence of the two
opposing plates. Because the sub-
ducting slab dips beneath the upper
plate, the leading edge of this plate
acts like a snow-plough, scraping
material off the top of the descend-
ing slab and thrusting it upwards and
towards the approaching lower-plate
continent (Figure 10.2). Thus the ori-
ginal lateral sequence of depositional
environments as shown in Figure 10.1B
has been telescoped into a compara-
tively narrow belt (Figure 10.1A).
As explained in Chapter 5, thrusts
usually propagate forwards, away
from the centre of the orogen, so that,
in this case, successive thrust sheets
carry material from progressively
further away from the original site of
the subduction zone. Thus the lowest
and youngest thrust sheets contain
platform sediments similar to those
on the undeformed foreland; higher
thrust sheets contain sediments depos-
ited on the continental slope; above
these are sheets containing material
from the deep-ocean basin, and even
higher sheets carry deposits from the
trench and continental slope of the
upper plate, which together make
up what is known as the accretion-
ary prism or accretionary wedge .

sea level

trench
sediments

upper
continental
crust

underthrust

oceanic crust

1

zone of
compression

2

uplift of zone of
maximum compression

deformation moves
oceanward

deformation
front

basal underthrust ductile

deformation

3

Figure 10.2 Structure of the accretionary prism. The thick sedimentary wedge (the accretionary prism)
occupying the trench and continental slope of an active continental margin is deformed progressively
as the subduction of the oceanic slab proceeds. The prism is underlain by a basal thrust, or
detachment, above which the strata are compressed. With continued convergence, the deformation
front moves outwards towards the ocean; here thrusts and overfolds are directed oceanwards; the
zone of maximum compression, where the sedimentary wedge is thickest, may be uplifted and
the structures on the landward side directed towards the continent. At deep levels of the prism,
temperatures and pressures are higher and structures more ductile. Based on Westbrook (1982).