The continent-continent collision of Europe and Africa has led to the subduction of continental lithosphere. In the Alpine nappes, only slices of the uppermost continental lithosphere have been preserved, leaving huge volumes of continental lithosphere unaccounted for. Support for this conclusion has come from high-pressure/low-temperature metamorphism in units of continetal crust that can only be explained if the crust had been subducted and then later exhumed (Dal Piaz et al., 1972). On a large scale, the existence of a high-velocity 'block' within the upper mantle under the Alps has been deduced from the dispersion of Rayleigh waves (Panza & Mueller, 1979). This lithospheric 'block' of higher velocity could correspond to two slabs of lower lithosphere subducted to the south and to the north, respectively, during the plate collision process or subfluence that formed the Alps. This model of the crust-mantle system in the Alps (Mueller & Panza, 1986) agrees with geo-tectonic models previously suggested. Ever since Eocene/Oligocene times (i.e. for the last 40 Ma), the continuing collision of the African plate with the Eurasian plate has led to considerable shortening of the lithosphere (Mueller, 1989). An anomalous mantle zone under the Alps has been postulated for a long time. Figure 1(a) illustrates a schematic model of a "double orogen" that has lithospheric material 'flowing down into the mantle' from both sides (Mueller, 1989). In the foreland, extensional structures associated with rifting and volcanism may exist. A consequence of such deep-reaching lithosphere would be the peeling off and piling up of slivers of the upper crust, creating the complex nappe structure shown in Figure 1(b) (Laubscher, 1974). Figure 2 shows a complimentary interpretation based on gravity anomalies.
(a) Schematic model of the Alps as a "double orogen" with lithospheric material flowing downwards into the mantle. (b) Schematic cross section through the crust-mantle system of the Alps (after Laubscher, 1974). Caused by the plate collision, the subducting northern and southern lithospheres seem to form a nearly vertical "bivergent zone of subfluence" which penetrates deep into the asthenosphere.
from Mueller, 1989
Geodynamic model cross section across the Alps, Po Plain, and northern Apennines (after Werner, 1985). (a) Model structure with topographic 'load', varying crustal thickness, and 'lithospheric root'. (b) Residual gravity (RG) effect due to the mantle anomaly. Dotted curve (BG - Bouguer gravity anomalies) is the combined theoretical gravity effect caused by the mantle anomaly and density jump at crust-Moho boundary as compared to observed gravity anomalies (solid line BG) after density disturbances have been eliminated (after Schwendener & Mueller, 1985).
from Mueller, 1989
Mass balancing of at a lithospheric scale of the Western Alps reveals that a large amount of continental crust is missing and was probably subducted (Fig. 3). The tomographic data suggest that slab-detachment has occurred under the Western Alps. Most probably, this event took place in the late Eocene and triggered the emplacement of the Oligocene peri-Adriatic plutons (Marchant, 1993) shown in Figure 4.
Mass balancing of the NRP20 Western traverse at a lithospheric scale. Two (a maximum and minimum estimate) early Cretaceous palinspastic reconstructions are shown with a present-day cross section for comparison. Even with the minimum estimate, quite a large amount of continental crust is missing in the present state. It was probably subducted and now forms an eclogitic root below the Po plain (Marchant, 1993).
from Marchant, 1993
3D reconstruction of the Western Alps in the late Oligocene, showing a westward migration of the slab-detachment process (Marchant, 1993).
from Marchant, 1993
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