The Italian Alps: a journey across two centuries of Alpine geology Giorgio Vittorio Dal Piaz Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 36, paper 8 In: (Eds.) Marco Beltrando, Angelo Peccerillo, Massimo Mattei, Sandro Conticelli, and Carlo Doglioni, The Geology of Italy: tectonics and life along plate margins, 2010. Download from: http://virtualexplorer.com.au/article/2010/234/a-journey-across-two-centuries-of-alpine- geology Click http://virtualexplorer.com.au/subscribe/ to subscribe to the Journal of the Virtual Explorer. Email [email protected] to contact a member of the Virtual Explorer team. Copyright is shared by The Virtual Explorer Pty Ltd with authors of individual contributions. Individual authors may use a single figure and/or a table and/or a brief paragraph or two of text in a subsequent work, provided this work is of a scientific nature, and intended for use in a learned journal, book or other peer reviewed publication. Copies of this article may be made in unlimited numbers for use in a classroom, to further education and science. The Virtual Explorer Pty Ltd is a scientific publisher and intends that appropriate professional standards be met in any of its publications. Journal of the Virtual Explorer, 2010 Volume 36 Paper 8 http://virtualexplorer.com.au/ The Italian Alps: a journey across two centuries of Alpine geology Giorgio Vittorio Dal Piaz University of Padua, Via Meneghini 10, 35122 Padova, Italy. Email: [email protected] Abstract: 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. Citation: 2010. The Italian Alps: a journey across two centuries of Alpine geology . In: (Eds.) Marco Beltrando, Angelo Peccerillo, Massimo Mattei, Sandro Conticelli, and Carlo Doglioni, Journal of the Virtual Explorer, volume 36, paper 8, doi: 10.3809/jvirtex.2010.00234 Journal of the Virtual Explorer, 2010 Volume 36 Paper 8 http://virtualexplorer.com.au/ Introduction the early 1900s, Alpine geology played a central role in The Alps are the most studied segment of the Alpine- the development of the nappe theory and modern tecton- Himalayan mountain ranges which extend from Gibraltar ics. Mobilist concepts, cleverly foreseen by Eduard Suess to the far Asia, and are considered as the typical example (1875, 1885, 1894), were established by Bertrand, of a continent-continent collisional belt. The Alps have Schardt, Lugeon, Termier, Steinmann, Argand and Staub been generated by the Cretaceous-Present convergence of (historical reviews and refs. in Dal Piaz, 2001a, and the Adriatic leading plate (Argand’s African promontory) Trümpy, 2001). In the 1920s Wegener's theory of conti- and the subducting lower plate, including the Piedmont- nental drift was endorsed by Argand and Staub in the Al- Ligurian branch of the Mesozoic ocean (Western Tethys) pine-Himalayan ranges, in contrast with hostility on the and the European passive continental margin. The com- western side of the Atlantic. Later, gravity and gliding plete closure (Eocene) of the ocean marked the onset of nappes were preferred by some geologists, but this return the Adria/Europe collision. The collisional zone is repre- to neo-fixist views waned, and Argand's and Staub's clas- sented by the Austroalpine-Penninic wedge, a fossil sub- sic tectonic lines dominated until the beginning, in the duction complex and a ductile to brittle “collisional dam- late 1960s, of the plate tectonics age. The Alps did not age zone” showing that even large and coherent frag- play any relevant role in the birth of the new global theo- ments of light continental crust may be deeply subducted ry: as pointed out by Trümpy (2001), this was the unhap- in spite of their natural buoyancy (Dal Piaz et al., 1972). py fate of land geology, since it was the ocean which The Alps extend from the Gulf of Genoa to Vienna, gave birth to the new global theory, favoured by the ex- through the French-Italian arc of the Western Alps and plosive development of marine geophysics and explora- the east-west-trending central and eastern Alps, where tion techniques (e.g., Hallam, 1973; Smith, 1976; Sengör, their connection with the Carpathians is buried below the 1990; Dal Piaz, 1995). The unexpected impact of the Neogene infill of the Pannonian basins (Fig. 1). plate tectonics theory on classic Alpine geology was ini- tially suffered and not welcomed (Dal Piaz, 1995; Trüm- Figure 1. Satellite image of northern Italy. py, 2001). This was not only due to its conceptual novel- ty, but also to the