The Italian Alps: a journey across two centuries of Alpine geology

Giorgio Vittorio Dal Piaz

This review is first and mainly an historical journey across two centuries of Alpine geology, from the early fixist views to the mobilist revolutions produced by the nappe theory and, later, by the global theory of plate tectonics, including the important developments of the last decade. This review is addressed to the Italian students and non-alpine geoscientists, and mainly focusses on the hard rock geology of the Austroalpine-Penninic wedge which is closest to my direct experience. The Alps, made popular by the "Voyages" of Horace-Bénédict de Saussure, are the mountain range where the nappe theory was conceived and rapidly consolidated. Mobilist views, cleverly foreseen by Eduard Suess, were developed by Bertrand, Schardt, Lugeon, Termier, Argand and Staub, and between the two world wars the Alps became a model for the evolution of collisional mountain belts. Wegener's theory of continental drift was endorsed by Argand and Staub in the Alpine-Himalayan ranges, and their tectonic views dominated until the unexpected impact of plate tectonics. In a few years the plate tectonic re-interpretation of the Alps was generally accepted and regionally improved by integration of classic groundwork with modern field and laboratory advances, mainly concerning refinements in stratigraphy, sedimentology, structural analysis, petrology of metamorphic units, isotope dating, deep geophysical experiments, paleostructural restoration back to Tethys, the Pangea supercontinent and older creative reconstructions, as well as numerical and analogical models. The existence of a pre-Gosau orogenic event was confirmed, better documented and related to the closure of the Triassic Meliata ocean. At the beginning of the 1970s, the eclogitic metamorphism in the Sesia-Lanzo zone, firmly related to Paleozoic events, yielded Cretaceous isotope ages, and became the signature of an early Alpine deep subduction of the continental crust. From the half of 1990s, a new generation of isotope dating on retentive systems began to spread in the Western Alps and then in the entire belt, providing robust Eocene ages for the closure of the Piedmont ocean and the subduction metamorphism in oceanic and continental units. A growing number of innovative, provocative, and sometimes repetitive papers appeared in the last decade. Based on actualistic models, the concept of ocean-continent-transition was tested in the Central Alps, became popular and rapidly expanded in the western Alps, from the Canavese zone to the ophiolitic Zermatt-Zaas nappe, without trace, however, of continental basement slices as extensional allochthons inside the Combin zone.

In the second part of the paper, the Structural Model of Italy at 1:500,000 scale and its contribution to Alpine geology are reappraised. This was the most relevant product in Italy of the fruitful integration of geological and geophysical working groups promoted by the National Geodynamic Project. The entire Alpine chain is represented by the sheets 1-2 (Bigi et al., 1990) and 3 (Bigi et al., 1993), that were printed without explanatory notes. Based on updated general lines of the Structural Model, the third and last part summarizes the structural features and kinematic evolution of the Alps. The Alpine orogeny developed from the Cretaceous through subduction of Mesozoic oceans and the European passive margin below the Adriatic leading plate, including the pre-Gosau Eastern Austroalpine thrust system and the underlying Western Austroalpine wedge. The latter derived from some extensional allochthons with Adriatic affinity, still connected to the Adriatic margin and/or trapped within the Piedmont-Ligurian ocean that completely closed during the Eocene. The Western Austroalpine and Penninic wedge is the core of the collisional belt, a fossil subduction complex which in deep seismic images floats over the European foreland lower plate. It is marked by a blueschist to eclogite facies, locally ultrahigh-P imprint of Late Cretaceous-Eocene age, followed by a post-nappe thermal re-equilibration developing Barrovian greenschist to amphibolite facies mineral assemblages throughout the nappe stack. Soon after, a post-collisional magmatic cycle with calc-alkaline to ultrapotassic affinity widely developed during the Eocene (Adamello) and mainly the Oligocene (32-30 Ma) along the Periadriatic igneous belt and fault system, from the lower Aosta valley to the Slovenian eastern edge of the Alps, and fed from partial melting of previously enriched mantle sources. Thermal perturbation and igneous activity are generally related to slab break-off of the lower plate after continental collision, and rising of hot asthenospheric bodies. During the Neogene the exhumed collisional wedge was accreted outside and below the Penninic frontal thrust by a stack of Helvetic basement slices and decollement cover nappes, pushed up and backward indented by the Southalpine lithosphere which in turn was deformed as an antithetic fold-and-thrust retro-belt. The Alpine tectonics is still active, as documented by seismicity, GPS measurements and foreland subsidence.