Note Illustrative della Carta Geologica d'Italia alla scala 1:50:000 Foglio 009 Anterselva

The n. 009 Anterselva Sheet at 1:50.000 scale of the Italian Geological Map covers ~400 km2 of a mountainous area in the Eastern Alps (Italy). It takes its name from the Anterselva Valley, the most important valley of this area. The 400 km2 belong to the Bolzano Province, while the rest of the sheet is in Austrian territory. The highest peaks of this area (many over 3200 m) are located inside the Vedrette di Ries Group, delimited by the Riva Valley and the Anterselva Valley: Collalto (3436 m, the highest peak), M. Nevoso (3358 m), Collaspro (3273 m), M. Magro (3273 m), Pizzo delle Vedrette (3249 m). Along the crest separating Austria from Italy many other peaks are higher than 3000 m. From the hydrographic/orographic point of view three are the main valleys, whose NE-SW trend follows some tectonic lineaments: the Riva Valley, the Anterselva and the Casies Valley. The geological setting of the Anterselva area has been strongly influenced by the alpine orogeny due to the collision of the African (Adria micro-plate) and European plates after the closure of the Mesozoic Tethys Ocean. In this part of the Alpine chain, the austroalpine and pennidic metamorphic basements of the Eastern Alps and minor Mesozoic cover sequences occur. The pennidic basement represents the European continental crust (passive margin) and the pennidic ocean lithosphere, which were subducted during alpine collision underneath the austroalpine nappe stack, which in its turn represents the African continental crust (passive margin). As a consequense of the post-collisional distension and erosion the deepest structural units of the chain, the pennidic units, now crop out inside the Hohe Tauern tectonic window. The southernmost part of this tectonic window extends E-W in the northern portion of the Sheet, where only the Glockner nappe, composed by oceanic metasediments and ophiolites, crops out. The deeper Gran Veneziano and Picco dei Tre Signori/Greiner nappes crop out to the north, in the Vetta d’Italia Sheet. The Glockner nappe shows a HP eo-alpine metamorphism (in two stages: eclogitic + blue schists facies) concealed by an earlier Oligocene-Miocene green schist facies overprint (peak at T ~500°C and P ~0.7-0.8 GPa: the so called “Tauern metamorphism”). This pennidic unit is separated from the austroalpine ones by a narrow E-W trending melange zone called “Matrei Zone”, which represents the subduction trench composed by both pennidic and austroalpine lithotypes (only the first ones are present in this area). The austroalpine basement extends over most of the Sheet’s area between the Matrei Zone and the Periadriatic Line (here called “Pusteria Line”), which is just south of the Sheet’s borders. The basement is composed of crustal units of African origin decoupled from their lithospheric roots, forming the alpine nappe stack. This last was accreted in front of the African margin as a part of the subductionrelated alpine orogenic wedge. Here the austroalpine basement is represented by two tectono-metamorphic units: the Anterselva unit and the Tures unit, which show different evolutions and are separated by an important tectonic lineament: the DAV (Defereggen-Anterselva-Valles) Line. The Anterselva unit, extending in the southern part of the area to the south of the DAV, is mainly composed by a monotonous sequence of garnet-staurolite (± kyanite and sillimanite) banded paragneisses and by the Anterselva and Casies granitic/tonalitic ortogneiss bodies; it suffered Variscan amphibolite facies deformation and metamorphism (metamorphic peak at T ~650°C and P ~0,7 GPa), and probably a previous Ordovician metamorphism, which produced the acid intrusions, now metamorphosed into Anterselva and Casies ortogneiss by the Variscan event (Zircon U/Pb geochronology gives ages of intrusion in the range ~430-470 Ma). This unit completely lacks Alpine metamorphic overprint as testified by cooling ages obtained on micas (Rb/Sr method) which are constantly around 300 Ma: i.e. these rocks never experienced T over 300°C after Variscan time. On the other hand, to the North of the DAV line the same ages inside the Tures unit are always around 30 Ma (20-60 Ma). In fact this basement was affected by both the Variscan amphibolite facies metamorphism (metamorphic peak at T ~650-680°C and P ~0,7-0,8 GPa) and the Alpine multistage green schists facies one (metamorphic peak at T ~450-550°C and P ~0,7-0,8 GPa). The Tures basement comprehends two sectors: a northern one characterized by a phillonitic sequence (philladic micaschists or Fillade di Cima Dura Auct.), in contact with the Matrei Zone, and by amphibolites, marbles, quartzites, fine-grained paragneisses, augen gneisses (age of intrusion of the magmatic protolith ~445 Ma). In the southern sector amphibolites, marbles, quartzites, and, above all, two mica bearing, sometimes migmatitic, paragneisses occur. In the Tures unit we have also discordant layer of metapegmatites of Permian age. These rocks of the Tures unit were intruded close to the DAV line, during Oligocene (~30 Ma), by the Vedrette di Ries and Cima di Vila granodioritic/tonalitic plutons. 147 The permo-triassic sedimentary cover of the austroalpine basement is only present in a small area between Forcella Ciarnil and Bad Kalkstein (Austria) (south-eastern area of the Sheet), where a clastic-carbonate succession crops out with an erosive boundary directly on the Anterselva basement. The deformation during the alpine collision was only brittle in the Anterselva unit, while in the Tures unit it was mainly ductile (mylonitic) and not homogeneous: from micro to mega scale, lithons occur, which escaped this deformation, preserving Variscan paragenesis. The DAV line is the most important tectonic lineament of the Sheet and playes a fundamental role in this part of the alpine chain for 2 reasons: • as already mentioned, it is the ENE-WSW trending green schist facies (~350-400°C) mylonitic Eo-Alpine fault zone which separates the Tures from the Anterselva unit, as proved by the huge difference of alpine metamorphic grade. This is in line with the model proposed by Hoinkes et alii (1999) and redefined afterwards by Schuster (2003), Schmid et alii (2004) e Froitzheim et alii (2008) who call this prominent fault zone the SAM (Southern Limit of Alpine Metamorphism) which divides the tectonic units of the lower crust that suffered alpine greeschist metamorphism – the Tures Unit – from the ones who do not show any or only a very light Alpine overprint – the Anterselva unit; • The DAV in the younger literature was described as the path of ascension and emplacement of the tonalitic melts of the Vedrette di Ries and Cima di Vila pluton. As the mapping, fieldwork and detailed analyses of this mapsheet shows, it would be more appropriate to call the large sinistral strike slip shear zone at the northern and southern rim of the oligocene plutons between the eastern end of the Vedrette di Ries to the western end of the Rensen pluton the “Oligocene Shear Belt (OSB)”. The reasons for doing this are the following: only the easternmost part of the Rieserferner Pluton touches the DAV line, here the sinistral shear zone of the Ologocene Shear Belt superimposes the structures of the Eo-Alpine steep normal fault of the DAV. Everywhere else the broad shear zone with many, also alternating, mylonites and cataclasites in the basement but even in the plutonic rocks lies within the Tures unit. Deformation style and age are very different to the DAV line sensu first authors. Before the magma emplacement the kinematics of the DAV was left-lateral transpressive, but just after the intrusion (~30 Ma) there was a big change into a mainly brittle, right-lateral transpression, as along the Pusteria Line. This sudden change was determined by the σ1 rotation from NNE-SSW to NNW-SSE during the alpine collision. The Quaternary deposits cover the 40% of Anterselva Sheet area and consists of continental deposits from Upper Pleistocene to Holocene in age. Part of the surficial deposits belong to the Last Glacial Maximum (LGM) and they 148 are included into the Garda Synthem (SGD). Indeed, during the Last Glacial Maximum glaciers covered the whole region: in the Pusteria valley and its tributaries, a huge glacial plateau expanded between Brunico and San Candido slowly moving toward East, West and South. Pre-LGM sediments are only preserved in the Rio Liccio catchment at the base of the “Piramidi of Plata” and consist of debris flow deposits belonging to the Perca synthem. During the phase of glacial decay we have documentation of the resumption of local glacialism; in the Rio di Riva Valley and in the smaller ones, flowing from the south into the Rio Aurino Valley, glacigenic deposits correlated with a phase of maximum advance of a local glacier have been distinguished and mapped. The single units, referred to as the sub-synthesis of Riva di Tures and the sub-synthesis of Casere, also include all the deposits of the following minor phases of advance and retreat. Moreover, even if the two subsyntheses can be framed in a generic “Late Glacial” no assumption can be made with respect to their precise chronological location, let alone whether they are equivalent in time.All quaternary sediments subsequent to the glacial conditions, form the Postglacial Alpine Synthem (PTG) that includes deposits associated to all the different processes acting on the landscape. Its base is strongly diachronous, following the retreat of the glaciers up to upper reaches of the valleys. The well-preserved and diffused morphologies and structures related to the LIA (Little Ice Age) are incorporated in the Amola Subsynthem.