Examples of modern orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 95

change in the plate convergence direc-
tion noted above. A similar pattern
has been observed in the outer nappes
of the Helvetic zone, with the outer,
younger thrusts showing a more north-
westerly movement direction. Gravity
gliding is thought to be responsible for
the isolated position of the Pre-Alps
and for certain nappes in the Helvetic
zone (e.g. see Chapter 9 and Figure 9.2).
The ophiolites of the Sesia
Lanzo zone exhibit very high-
pressure metamorphism, indicat-
ing that they had been subducted
to considerable depths before being
upthrust to their present level.
Attempts have been made to esti-
mate the total amount of shortening
across the Alpine belt by restoring the
movements along the various thrusts.
Accurate measurement is impossi-
ble because of the complexity of the
structures, especially in the interior
zones, but it is likely that the shorten-
ing in the continental crust has been
in excess of 250 km across the belt,
which is now ~300 km in width. This
compares with a crustal thicken-
ing in the orogen to over 50 km, the
present depth of the Moho there.

commenced during the Cretaceous in
the form of thrusting onto the margin
of the Apulian micro-plate; however,
collision between that plate and the
main European plate probably did not
occur until the late Eocene or early Oli-
gocene . Crustal shortening and orogen
creation then continued through the
Oligocene, reaching its climax in the
late Oligocene, about 25 Ma ago. The
late back-thrusting phase has been
attributed to the Miocene period.
However, some convergent move-
ment and uplift still continue today.

terranes ( see Chapter 10), which have
accreted to the North American plate
during the Mesozoic. The main col-
lision event took place during the
Cretaceous period. The terranes
represent a mixture of crystalline
metamorphic complexes, volcanic
arc material, arc-trench deposits
of various ages, and ophiolite units
containing oceanic sediments and
ultramafic rocks (Figure 11.8A, B).
These in some cases can be shown to
have originated far from their present
position and are believed to have
been transported across the ancestral
Pacific Ocean on the Farallon plate
and scraped off the subducting slab.
Several of the terranes, including the
Wrangellia and Alexander terranes,
accreted together first during the Juras-
sic, before colliding with the continent
in the Cretaceous. This collision was
responsible for much of the deforma-
tion in the orogen. The later history
of the orogen was marked by lateral
movements along major strike-slip
faults, parallel to the transform fault
that forms the present Pacific-Ameri-
can plate boundary. These movements
slid the various terranes northwards
relative to the eastern part of the
orogen. Late-orogenic collapse due to
gravitational spreading has resulted in
extensional faulting that has reversed
the movement on some of the earlier
thrust faults. The north-western part
of the orogen consists of a Cenozoic
accretionary prism, and parts of the
coastal belt are still tectonically active.

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94 95

The Canadian sector of the North
American Cordilleran belt
The Canadian Cordillera extends from
the Yukon Territory in the north to the
northern border of the USA in the south,
and includes the coastal region of south-
ern Alaska (Figure 11.8A). It is approxi-
mately 2,000 km long and 1,000 km
wide. The eastern part of the mountain
range consists of a fold-thrust belt
directed towards the eastern foreland of
the North American plate (Figure 11.8B).
The sedimentary cover in this belt con-
sists of Palaeozoic to Mesozoic platform
and continental slope deposits resting
on the Precambrian basement of the
Laurentian shield . These strata record
a passive margin of the North American
continent which existed through the
Palaeozoic until, in the Mesozoic, sub-
duction of the Farallon plate ( see Figure
3.6B) commenced at its western margin.
The main part of the orogen is
composed of a number of displaced

Timing of the orogeny
The complex shape of both plate
margins, together with the change
in the convergence direction, have
resulted in regional variation in
the commencement of the Alpine
orogeny. In the sector described above,
crustal deformation seems to have