Geoscience Reference
In-Depth Information
motion on the left-lateral East Anatolian fault is
1.5 cm yr
−
1
. The compression
occurring to the east in the Caucasus is also a consequence of the collision between
Arabia and Eurasia. The strain there is much greater than can be accounted for
by recorded earthquakes, indicating that considerable aseismic deformation is
occurring.
An interpretation of the stages of the evolution of the Alps is shown in
Fig. 10.21. During the Mesozoic extensional rift systems were operating in the
initial crust and mantle of Pangea as Gondwana and Laurasia separated and the
Tethys and central Atlantic Oceans formed (Fig. 3.30). By the end of the Jurassic
major extension had taken place: the so-called Neo-Tethys or Piemont Ocean had
formed and separated the rifted and thinned European and Adriatic continental
margins (Fig. 10.21(a)). Estimates of the width of this ocean are 100-500 km.
Some of this oceanic material may now be represented in the ophiolite sequences
that occur along the length of the Alpine chain. The initial formation of the Alps
resulted from northeast-southwest convergence between Europe and Africa. By
the mid-Cretaceous (Fig. 10.21(b)) the Piemont oceanic lithosphere had been
subducted and the European continental margin was being thrust beneath the
Adriatic margin. The main episode of continental collision took place in the
Te r tiary with north-south convergence. This resulted in the major deformation,
uplift and subsequent erosion which formed the Alps as we observe them today.
Figure 10.21(c) shows the situation during the Oligocene (30 Ma) with the thinned
European continental margin being delaminated - the European upper crust was
peeled off and thrust northwards. At the same time, on the Adriatic margin
the upper crustal layers were also being removed; they were thrust southwards.
During the late Eocene a foreland basin developed to the north of the Alps. The
exact method of loading and deformation and the peeling off and stacking of
slices of crust from the colliding plates remain matters of research.
Figure 10.22 shows a series of sections across the Swiss Alps. Negative
Bouguer gravity anomalies characterize the Alps, which is consistent with major
crustal thickening. The details of the geology of the Alpine chain are very
complex, but overall the geology is straightforward. The northern ranges are
molasse (sediments) from the foreland sedimentary basin. The rocks then pro-
gressively age southwards, until finally the highly metamorphosed crystalline
core is reached: these crystalline rocks were at deep levels in the crust until
thrusting and erosion brought them to the surface. (See Section 7.8.4 on meta-
morphic belts.) The southern Alps, south of the Insubric Line, originated on the
continental shelf of the Adriatic promontory of the African continent. It is esti-
mated that some 100 km of shortening has occurred (by folding and thrusting)
across the Alps during the last 40 Ma. In the western Alps, there is a large positive
gravity anomaly caused by the
Ivrea body
,aslice of lower-crustal-upper-mantle
material that was obducted from the southern (Adriatic) plate and thrust to a
shallow level, in some places outcropping at the surface.
∼
