Alberto Castellarin

From rifting to drifting: tectonic evolution of the South-Alpine upper crust from the Triassic to the Early Cretaceous

Abstract The tectonic evolution of the South-Alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles. Extension, related to the movements between Adria and Europe, began in the Norian after Variscan orogeny, Late Carboniferous to Middle Permian orogenic collapse and continental-scale wrenching. From the Late Triassic to the Early Liassic stretching was mostly limited to the Lombardian basin. During this time, extension was mainly controlled by four major listric faults, symmetrically centred around the Late Carboniferous—Early Permian Collio grabens. Smaller faults which also started in the Norian, were progressively de-activated during the Late Triassic. After the Middle Liassic, faulting in the Lombardian basin gradually ceased and the site of extension shifted westwards, i.e. towards the future site of crustal separation. Extension was then controlled by a set of west-dipping normal faults. Oceanic crust was formed not later than 157 Ma. The overall extension along the profile (which had a final length of 290 km) was 52 km which corresponds to a stretching factor of 1.22 for the whole length of the preserved margin. The strain rate is ca. 1 × 10 −16 / s . The sedimentation history suggests that the extension of the South-Alpine margin was a continuous process from the Norian to the Middle Jurassic. The changes in tectonic pattern are related to progressive hardening of the lithospheric segment undergoing slow extension.

Neo-Alpine evolution of the Southern Eastern Alps

Abstract The Eastern Southern Alps, located to the east of the N Giudicarie Line, have been originated by polyphase compressional evolution of Tertiary age. The oldest structural system corresponds to the Mesoalpine (Eocene) and early Neoalpine (Oligomiocene) compressional events, which originated the Dinaric system (NW–SE trending) present in the NE side of the Southern Alps. The subsequent tectonic belt is the Valsugana structural system, ENE–WSW trending, Serravallian– Tortonian in age. The intense activity of this compressional event is documented both by stratigraphic and structural data and by fission track studies which indicate uplifting of some 4 km in the hangingwall of the Valsugana overthrust between 12 and 8 Ma B.P. The more external structures NE–SW trending are located in the Montello–Friuli zone which were generated by the Messinian–Pliocene compressions (whose principal stress axis is SE–NW oriented). Structural and kinematic links between the Montello–Friuli belt and the Giudicarie zone have been recognized by new analytical data documenting that the Schio–Vicenza fault system transferred compressive deformations to the Giudicarie belt, where, the previous structures (belonging to the Valsugana system) were strongly enhanced by the subsequent compressions. Since the Montello–Friuli belt and the Schio–Vicenza system developed during the Messinian–Pliocene Adriatic compressional event, the out-of-sequence tectonic reactivations in the Giudicarie belt have to be considered mostly Messinian–Pliocene in age rather than Tortonian, as previously proposed. This compressional Adriatic event produced wide deformations in the Alps, both internally, like in the Montello–Friuli and Giudicarie, and, very likely, also externally, as in the Jura and the frontal external French Western Alps. The three structural systems here described are clearly represented in a N–S trending balanced profile crossing the Central Dolomites, close to the Italian trace of the Central European Profile (CEP) where a seismic reflexion acquisition campaign is presently in progress according to the German, Austrian and Italian joint program of the TRANSALP Project. The crustal and lithospheric interpretation using available data (revision of previous refraction profiles) assumes strong indentation of the Adriatic lithosphere to the N coupled by southward delamination of the Adriatic crust, coherent with the setting of the upper crust shown in the balanced cross section. Similar indentation processes were recognized up to date only in the Western and Central Alps (CROP–ECORS and NFP20 Profiles).

The Middle Triassic magmatic-tectonic arc development in the Southern Alps

Abstract The geodynamic meaning of the Middle Triassic magmatic and tectonic events in the Southern Alps is still uncertain. The main problem is the significance of the compressional tectonic structures recognized in the central Dolomites. In fact these structures are incompatible with the extensional geodynamic interpretation (crustal rifting) assumed by several authors. Therefore the compressional tectonic elements of the Dolomites have been mostly ignored or not seriously taken into account, or considered as local phenomena due to the volcanic-tectonic interplay. Other more modern interpretations are founded on regional transcurrent and/or transpressive tectonics. Recent discovery of a Middle Triassic thrust system located in the Piave River area (some tens of kilometres east of the central Dolomites) lends more credence to the compressional Middle Triassic event. The magmatic series of volcanic and plutonic Middle Triassic rocks display an “orogenic” character, with typical calc-alkaline trends recognized across the whole Southern Alps as well as in the Dinaric and Hellenic domains. Such a character is rather uncommon in the crustal evolution by rifting but is more in accord with a compressional geodynamic setting. Furthermore the time-space distribution of the South Alpine magmatic rocks, coupled with their petrochemical affinity, are close to the modern volcanic arcs. Nevertheless the general geodynamic context of the Southern Alps, owing to their pronounced ensialic character, does not fit the oceanic model.

European orogenic processes research transects the eastern Alps

The Alps—the youngest and most elevated mountain range in Europe—have inspired ideas about orogenic evolution for a long time. During the late 1980s, the western Alps were the site of intensive research using seismic profiling methods by Swiss, Italian, and French national programs [Rome et al., 1990; Pfiffner et al., 1997] .These investigations, some of which formed part of the European Traverse [Blundell et al., 1992], provided a great wealth of new data relevant to the Alpine orogeny. This orogeny is generally viewed in the context of the collision of the European and the Adriatic/African continental plates after the closure and subduction of the Penninic Ocean since about 40–50 Ma.

Basic stratigraphy and tectonics of the Southern Alps around the Giudicarie Lineament (Southern Alps, Italy)

One of the most fascinating problems of the Southern Alps is the meaning of the strong structural indentation between the Sothern Alps and the Austroalpine units along N-Giudicarie fault, where the Europe-verging nappe stack of the Alps (Austroalpine and Penninic nappes) and the Africa-verging thrust belt of the Southern Alps face each other and appear sinistrally displaced more than 50 km. Starting from the beginning of ‘900 century, several generations of geologists considered the sinistral slip on the fault responsible of the indentation of the Alps along the Giudicarie lineament, occurred during N-S Neogene compressions. Old and new data, on the contrary, indicate that most of the extensive sinistral displacement affecting the N Giudicarie Lineament originated during late Cretaceous-early Eocene times. The Pre-Adamello structural belt is present only in the internal Lombardy zone, located W of the Adamello massif. This belt is unknown in the Dolomites and surrounding areas located to the E of the Giudicarie lineament. Upper Cretaceous-early Eocene thick Flysch deposits of Lombardy and Giudicarie are well preserved along the Southern and Eastern border of the Pre-Adamello belt (S-verging Alpine orogen). Towards the E, in the Dolomites and in the Carnic Alps and external Dinarides, only incomplete remnants of Flysch deposits, Aptian-Albian and Turonian-Maastrichtian in age, are present. They can be considered as equivalent to those of Lombardy and Giudicarie formerly in connection along the N-Giudicarie corridor. All these deposits may correspond to the foredeep syntectonic sedimentary records of the S border of the new Cretaceous-Eocene orogenic belt of the Alps. Differences between the eastern and western blocks (the Dolomites versus the pre-Adamello belt) can be related to the Cretaceous-Eocene N Giudicarie transfer zone, which produced the basic structural setting discussed here. Due to these data and interpretations the Giudicarie bending of the Alps appears to be a late Mesozoic-early Tertiary tectonic inheritance rather than the result of the Neogene compressional evolution. During the late Eocene to early Oligocene the transfer zone was utilized for the ascent and emplacement of the Paleogene Adamello batholith. Oligocene to Neogene compressional evolution inverted the N-Giudicarie fault into a back-thrust of the Austroalpine units over the South-Alpine chain rearranging most of the former structural setting.