Giorgio V. Dal Piaz

Tectonic significance of Alpine eclogites

Abstract A review of P-T peaks and paths of eo- and meso-Alpine eclogite fades rocks occurring along the axial part of the Alpine chain shows that rocks re-equilibrated under high- and low- T (group-B and -C eclogites), are, respectively, hosted within a lower and an upper tectonic level of the Penninic nappe system. If P-T estimates for eclogites are considered peak conditions the two crustal portions, otherwise undistinguishable, were sutured during the collision of the European and Adriatic continental plates, which corresponds to the latest tectonic mechanism of eclogitization. Before collision, formation and preservation of eclogitic rocks up to shallow levels was assisted by subduction of the cold oceanic crust. The two lithospheric processes of oceanic subduction and continental collision, though separated in time, contribute to continuous generation of eclogites under thermal conditions that evolve from higher to lower P-T ratios from the end of ocean consumption. Exhumation trajectories are characterized by low- or high thermal regimes in the same structural domain in different parts of the chain (Western and Eastern Austroalpine), in the same part of the chain (Penninic and ophiolites in Western, Central and Eastern Alps), or even within the same nappe (Dora-Maira, Gran Paradiso and Adula). Late orogenic collapse or slab breakoff processes may have caused late heating at very low pressure (0.3 GPa) during exhumation in some units of the Pennine nappes and ophiolites Mechanisms of nappe emplacement are demonstrably multiphase and inferences on palaeogeographic derivation of eclogitic units can be drawn from interpretation of P-T trajectories.

Late Jurassic blueschist facies pebbles from the Western Carpathian orogenic wedge and paleostructural implications for Western Tethys evolution

In spite of the absence of ophiolitic slices at the surface, some traces of the lost Tethys ocean are recorded along the Pieniny Klippen Belt (PKB), a narrow decollement thrust system sutured at the transpressive boundary between the Outer and Inner Carpathians. The enigmatic precollisional evolution of Western Carpathians can be deciphered from some late Albian to Campanian flysch conglomerates which display chrome spinel grains, ophiolitic detritus and pebbles of blueschist facies tholeiitic metabasalts yielding a 40Ar/39Ar plateau age of 155.4±0.6 Ma. Other detrital components are represented by extrabasinal pebbles of limestones, arc volcanics, and igneous to metamorphic basement rocks from southern sources. Our results suggest a markedly northward extension of the sublongitudinal Triassic Vardar (Meliata) Ocean and its subduction since the late Middle Jurassic, supposedly balanced westward by coeval spreading in the Ligurian-Piedmont basin of the Apennine-Western Alpine Tethys. A lateral kinematic connection between these diachronous and roughly parallel Tethys branches was provided on the north by a left-lateral east-west trending shear zone running from the Swiss-Austrian Penninic domain to the Northern Carpathians. This reconstruction replaces the classic model of two paired North Penninic and South Penninic oceanic basins and eastern homologues with the Brianconnais-Hochstegen and Czorstin microcontinents in between. The Late Jurassic-Early Cretaceous evolution of the Carpathian active margin was characterized by subduction metamorphism and accretion of a wide orogenic wedge; in this time, the shallowing to deeply subsiding basins inferred from facies analyses on the sedimentary units of the PKB were likely floored by individual sections of the growing wedge. Later, some exhuming blueschist ophiolitic units of the wedge were uplifted to the surface and functioned in the Albian-Campanian as an “exotic ridge” supplying clasts to the forearc basin. Finally, the colliding wedge became a cryptic paleostructure when, since the latest Cretaceous, it disappeared beneath the Inner Carpathian orogenic lid and was incorporated within the eastward moving infrastructure of the Carpathian orocline.

History of tectonic interpretations of the Alps

Two centuries of tectonic interpretations of the Alps are briefly reviewed, from the early fixistic tenets emphasizing vertical movements to the impact of the new global tectonics, through the epos of the nappe theory. From 1884 to the early 1900s, Alpine geology played a central role in the development of the nappe theory and modern tectonics. Mobilistic concepts, cleverly foreseen by Suess, were established by Bertrand, Schardt, Lugeon, Termier and Argand, and the Alps became a model for the evolution of mountain belts. 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 other side of the Atlantic. Later, some geologists favoured gravity and gliding nappes, minimizing the role of crustal shortening. Nevertheless, this return to neo-fixistic views waned, and Argand’s and Staub’s classic tectonic lines dominated until the beginning of the plate tectonics age. The Alps did not play any part in the birth of this new global theory, and Alpine geology only benefited later.

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

We present a 1:30,000 geological map resulting from detailed geostructural surveys carried out along the Italian segment of the design corridor for the Brenner Pass railway base tunnel (BBT), extending from Fortezza (Italy) to Innsbruck (Austria). The map covers the southern part of the Austroalpine-Penninic collisional wedge, the Periadriatic Fault System, associated Oligocene igneous bodies (Periadriatic magmatism) and part of the Southalpine basement. The Penninic Zone in the western Tauern Window is represented by the double domal structure of the Europe-derived Tux and Venediger-Zillertal basement and cover nappe system, capped by the ophiolitic Glockner nappe. The overlying Austroalpine nappe system is here represented by the polymetamorphic Merano-Mules basement and minor cover sheets. The Southern Alps domain includes the Bressanone Granite and pre-granite quartz-phyllites. Four Alpine ductile deformation phases have been recognized, followed by ductile-brittle shear zones, and finally brittle deformations along faults with extensional and strike-slip kinematics. The Quaternary is characterized by glacial deposits, large gravitational mass movements and landslides.

Brittle tectonics in the northwestern Alps: remote sensing applications

In the North Western sector of the Alps an intense brittle tectonics overprints the nappe stack and related ductile deformations. On a regional scale, different brittle structural sets can be identified. The main purpose of this work was to test the reliability of optical and SAR data to identify new lineaments and to test their mutual interference in a collisional orogenic belt. ERS-1 Geocoded Terrain Correct data, provided by the Italian Processing and Archiving Facility (I-PAF) were analyzed and compared with optical data. These products were verified to be essentially complementary by geological interpretation. As an integrate result, the lineament length and orientation fit well the kinematic model of the study area, obtained through field analysis and apatite fission track datations (AFT) on differential uplift. Moreover it was also identified a close relationship between lineament sets and deep-seated gravitational slope deformations.