Examples of modern orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 94

1-3. The foreland . Mesozoic plat-
form sediments rest on the margin of
the European plate; they are unfolded
in zone 1 (the Plateau Jura ) and folded
in zone 2 (the Folded Jura ). Zone 2
represents the outermost part of the
foreland fold-thrust belt but is separated
from the main part of the fold-thrust
belt by zone 3, the foredeep basin ,
containing non-marine (continental)
clastic sediments ( molasse ) derived
from the evolving mountain chain.

with marine clastic sediments thought
to have formed in a trench or back-
arc basin. The Mesozoic sedimentary
sequence is quite different from that of
the foreland and is underlain by crystal-
line basement, which forms the core of
the large St. Bernard nappe . The over-
folds in this zone are in the form of a
fan, directed towards the foreland in the
north-west, like those of the underlying
zones, but directed towards the south-
east at the opposite side of the zone.

belt affecting Mesozoic platform sedi-
ments belonging to the African plate.

13. The Po basin . This zone consists
of non-marine clastic sediments derived
from the rising Alps and deposited in a
foredeep basin on the Apulian plate.

Structures
The structures of the outermost zones,
the folded Jura and the Southern Alps,
exhibit typical fold-thrust geometry
(Figure 11.7B), with outward-directed
thrusts and thrust-generated overfolds
linked to a sole thrust along a weak
stratigraphic horizon, in the same way
as was illustrated in the outer zones
of the Himalayas. The nappes of the
Helvetic zone, having originated at
greater depths, are more ductile, but
still have an overall, foreland-directed,
overthrust sense of movement (Figure
11.7B). The more complex nature of
the folding in the Brian ΁ onnais and
Piémont zones, illustrated in Figure
11.7B, has been attributed to SSE-
directed ' back-thrusting '. This has had
the effect of moving the Piémont zone
upwards and backwards, towards the
Apulian plate, thus raising the meta-
morphosed interior of the orogen to
a higher level and isolating the Dent
Blanche nappe ( see cross-section 6 of
Figure 11.6). This back-thrusting may
be attributable to continued conver-
gence between the opposing conti-
nental plates at a time when further
overthrusting was inhibited by the
already greatly thickened orogen.
Detailed analysis of the ductile struc-
tures of the Piémont zone indicates
that the movement direction changed
through time in an anticlockwise sense,
suggesting a link with the anticlockwise

4. The Pre-Alps . This zone is
a klippe (thrust outlier) of rocks
belonging to the Piémont zone (8).

8. The Piémont zone . This zone con-
tains ophiolites and typical ocean-floor
sediments representing pieces of Tethys
Ocean crust and upper mantle, which
has been thrust over the continental
crystalline basement that forms the core
of the Monte Rosa nappe . The presence
of ophiolites here is taken to indicate the
presence of a suture between this zone
and the overlying Dent Blanche nappe.

5. The Helvetic zone . This consists of
a set of complex fold nappes underlain
by ductile thrusts or shear zones, and
cored by crystalline European base-
ment. These structures are directed
north-westwards, towards the foreland.
The sedimentary cover consists of
Mesozoic platform sediments that can
be correlated with those of the foreland,
and represent the thinned margin of the
European continent. The three main
nappes shown in the cross-section are
the Morcles , Dent Blanche and Wild-
horn nappes. These nappes form many
of the high mountains of the Swiss Alps.

9-10. The Sesia Lanzo zone and
the Dent Blanche nappe . The Dent
Blanche nappe is a klippe of crystal-
line basement of African parentage
belonging to the Apulian microplate.
It is considered to have rooted in
the Sesia Lanzo zone, which forms
the northern margin of Apulia.

Zones 2-5 are referred to as the
' external zones ' of the orogenic belt
and together make up the foreland fold-
thrust belt.

11. The Ivrea zone . This zone con-
sists of crystalline basement similar
to that of the Sesia Lanzo zone, but
separated from it by a major steep
fault, the Insubric fault , along which
strike-slip movement took place during
the latter stages of the orogeny.

6-7. The Sub-Briançonnais and
Briançonnais zones. These zones
consist of material thought to represent
a small continental terrane, known as
the Pennine terrane , situated on Tethys
Ocean crust ( see Figure 11.6) together

12. The Southern Alps . This zone,
which includes the Dolomites range,
consists of a SE-directed fold-thrust