Introduction
The Alps are the most studied segment of the Alpine-Himalayan mountain ranges which extend from Gibraltar to the far Asia, and are considered as the typical example of a continent-continent collisional belt. The Alps have been generated by the Cretaceous-Present convergence of the Adriatic leading plate (Argand’s African promontory) and the subducting lower plate, including the Piedmont-Ligurian branch of the Mesozoic ocean (Western Tethys) and the European passive continental margin. The complete closure (Eocene) of the ocean marked the onset of the Adria/Europe collision. The collisional zone is represented by the Austroalpine-Penninic wedge, a fossil subduction complex and a ductile to brittle “collisional damage zone” showing that even large and coherent fragments of light continental crust may be deeply subducted in spite of their natural buoyancy (Dal Piaz et al., 1972).
The Alps extend from the Gulf of Genoa to Vienna, through the French-Italian arc of the Western Alps and the east-west-trending central and eastern Alps, where their connection with the Carpathians is buried below the Neogene infill of the Pannonian basins (Fig. 1).
Figure 1. Satellite image of northern Italy.
Alps, nortern Apennines and Po plain (Padane-Adriatic) foreland from satellite image; geology in Sheets 1, 2 and 3 of the Structural Model of Italy at 1:500,000 scale (Bigi et al., 1990, 1993).
The Alps, made popular by the "Voyages" of de Saussure (1779-1796) and mountaineering exploration, rapidly became the most attractive natural laboratory in Europe for interpreting the anatomy and development of mountain ranges, the classic model of collisional belts (Argand, 1916, 1924; Staub, 1924). Indeed, from 1884 to the early 1900s, Alpine geology played a central role in the development of the nappe theory and modern tectonics. Mobilist concepts, cleverly foreseen by Eduard Suess (1875, 1885, 1894), were established by Bertrand, Schardt, Lugeon, Termier, Steinmann, Argand and Staub (historical reviews and refs. in Dal Piaz, 2001a, and Trümpy, 2001). In the 1920s Wegener's theory of continental drift was endorsed by Argand and Staub in the Alpine-Himalayan ranges, in contrast with hostility on the western side of the Atlantic. Later, gravity and gliding nappes were preferred by some geologists, but this return to neo-fixist views waned, and Argand's and Staub's classic tectonic lines dominated until the beginning, in the late 1960s, of the plate tectonics age. The Alps did not play any relevant role in the birth of the new global theory: as pointed out by Trümpy (2001), this was the unhappy fate of land geology, since it was the ocean which gave birth to the new global theory, favoured by the explosive development of marine geophysics and exploration techniques (e.g., Hallam, 1973; Smith, 1976; Sengör, 1990; Dal Piaz, 1995). The unexpected impact of the plate tectonics theory on classic Alpine geology was initially suffered and not welcomed (Dal Piaz, 1995; Trümpy, 2001). This was not only due to its conceptual novelty, but also to the geophysical supporters of the new global views neglecting reference to some fundamental concepts provided by a century of geological studies in the Alps. In a few years, however, the plate tectonic re-interpretation was generally accepted and regionally perfected by integration of classic stratigraphic and tectonic works with modern advances in structural geology, petrology, geochronology and geophysics (Dal Piaz & Gosso, 1984). After half a century, the Alps are continuing to play as one of the most attractive chains in the world, even if there are clues that the classic field work is becoming less appreciated than in the past as essential milestone for laboratory analysis and numerical modeling.
The paper is deliberately addressed to Italian students and non-alpine geoscientists. In the framework of this volume, this review deals mainly with the Italian part of the collisional belt, without forgetting, however, the essential features of other structural domains.
I am aware of having overdeveloped self-referencing of my work and personal comments, but I was unable to prevent at all this last chance. This contribution was warmly requested by Carlo Doglioni and Marco Beltrando, even long after I had retired. I thank them very much, for their kindness and tolerance. Particular thanks are addressed to Marco Beltrando for his very accurate and fruitful review of a first version of the paper. Obviously, any error is mine, and any comment and criticism, especially the most severe, will be welcomed, being a sign that I am somehow still alive in the Alpine geological community.