Note illustrative della Carta Geologica d’Italia alla scala 1:50.000, foglio 043 Mezzolombardo. Servizio Geologico d’Italia - ISPRA, 256 p., Land Technology & Services, Roma

The mountain landscape of the Sheet 43 – Mezzolombardo – of the 1:50.000 Geologic Map of Italy, is dominated by the valleys of the river Adige and its affluent, the torrent Noce. These valleys were carved most probably during the dramatic Mediterranean base-level fall at the end of the Miocene, also known as Messinian sea-level drop, or salinity crisis. The occurrence of such a deep valley (Adige), whose bedrock lies several hundreds of meters below the sea-level, controlled the subsequent evolution that included alternating fluvial and valley glacier conditions, modulated by the Pleistocene climatic oscillations, but always forced into the same drainage. This evolution brought to strong variation in the main erosional processes, and, most important, in the local base-level, given the growth of valley glacier at elevation of around 1800 m during the climax of the glacial periods. Presently, most of the surficial deposits belong to the Last Glacial Maximum (LGM) and are grouped into a synthem (SGD), that includes the sedimentary cycles related to the deglaciation during the Tardiglacial. These latter form several subsynthem, depending on the local base-level, therefore changing in the different tributaries of the main valley. All the deposits subsequent to the glacial conditions are grouped into the Postglacial Alpine Synthem, 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 pre-LGM cycles are more and more patchy with increasing age, and outcrop as erosional remnants, whereas the deposits are more and more compacted and cemented. These are described with various hyerarchic ranks, from Supersynthem to Formations, depending on the possibility to correlate them with other neighbouring maps and on the facies characters of the units. The main valley cut the bedrock roughly controlled by the main structural grain that is now exposed on the hillslopes and easy to survey. The main structures formed prior or during the main erosional phase, i.e. during the Miocene. In this map, the clearest structure is an anastomosing system of SE-verging thrusts, stretching on the right slope of the Adige around Mezzolombardo up to Cortaccia/Kurtatsch. The Trento – Cles Line is a N- trending fault system running across the whole western side of the map: it is an important transfer zone, reactivating a previous Mesozoic fault system in a transpressional way, mostly left-lateral. The Trodena fault, at the left slope of the Adige valley, is a NE-trending high-angle fault displaying a reverse to left-lateral movement. The present-day seismicity in this area is low, and there are few evidences of fault activity at the surface, if we except a case near Egna/Neumarkt, where a 1800 yr old roman villa was found struck by an earthquake that possibly cut the edifice. We could not find any other evidence for such an important seismogenic surface structure, therefore, more work is needed to fully document a possible high magnitude seismogenic source in the area. The geological history documented by the rocks outcropping in the area of the map is very long and complex, spanning from Permian to the Paleogene across all the events that brought the Adriatic plate from the end of the Variscan orogeny into rifting and subsequent collision with Eurasia. The Lower Permian Athesian Group (Gruppo Atesino) represents a very large (some thousand km2) and very thick (almost 3 km) post-collisional volcano-tectonic basin, developed at the expense of the previous Variscan belt. Various kind of volcanic and volcaniclastic products occur, basically acid ignimbrites. After a period of non-deposition, a new sedimentary cycle was heralded by the Upper Permian continental red sandstones (Val Gardena Sandstone). These deposits encroached the continent toward the west and were followed by transitional to marine sediments (Bellerophon Fm), here representing its westernmost tip, and finally by the shallow-marine Werfen Fm (uppermost Permian to Lower Triassic), characterized by very low platform carbonate productivity, and basically influenced by terrigenous input. The Middle Triassic displays alternating terrigenous to carbonate deposition in a shallow marine to transitional environment (Lower Serla, Voltago Conglomerate, Giovo Fms), more and more influenced by the extensional tectonics. Toward the end of the Anisian, however, and the beginning of Ladinian, a strong pulse in subsidence shut down the terrigenous input and allowed for the deposition of thick platform carbonates (Contrin and Sciliar Fms), interspersed by intraplatform basins (Moena and Buchenstein Fms). The Late Ladinian and Carnian are time of decreased subsidence and rapid infilling of the residual spaces, including a short episode of basic volcanism. The paleoenvironment was then dominated by continental arid conditions and the poor subsidence did not create accommodation, until the end of Carnian, when a new sedimentary cycle begun, again heralded by red continental to transitional deposits (Travenanzes Fm). This is the beginning of a new cycle that eventually brought to the opening of the Jurassic Alpine Tethys, with break-up located to the west of the Southern Alps. The study area was affected by the activity of some rift-related faults, the most important of them is a N-S system, the Trento-Cles line, located to the west of the sheet, that separates a subsiding block (the Brenta Group) with more than 2 km of Norian to Liassic deposits from a more stable area to the east, where, during the same time span, the succession does not reach 400 m, and it is completely lacking the Rhaetian and Lias. Several smaller N- to NW trending faults are visible in the map, especially near the junction of the Noce with the Adige river. The Dolomia Principale represents the most diffuse carbonate platform of Norian age, but it includes some meters of Rhaetian to the east of the Trento-Cles line. It is followed to the west of this line, by the Rhaetian Zu Limestone, and the Calcari Grigi group, more and more thin toward the north and east, pinching against the more stable Adige and western Dolomites area. After the Late Pliensbachian demise of the Calcari Grigi platform, open-shelf crinoid-rich sand waves occurred west of the Trento-Cles line (Encrinite del Peller). The subsequent hiatus, encompassing the upper Aalenian and lower Bajocian, is commonly interpreted as the break-up unconformity. The following unit is a reduced and condensed interval of deep-sea red nodular limestones, rich of ammonoids, the RossoAmmonitico Veronese, encompassing a long time interval: late Bajocian to early Tithonian. The subsequent depositional evolution witnesses the deepening of the whole passive margin: the area east of the Trento-Cles, however, represented a structural high, winnowed by currents. The upper Tithonian to lower Aptian Maiolica, a nannoplancton-rich lime mudstone, is here very reduced in thickness. The sharp lithological change that heralds the overlying unit, the Scaglia Variegata Alpina, represents the local expression of the well-known Oceanic Anoxic Event, a period of alternating productivity of the surface waters of the oceans and deposition of black shales, interbedded with more oxidized white and red (lime) mudstones. The passive margin turned into a retroforeland of the Alps at that time, and the possible early collision of continents toward the north was able to transmit compressional stresses to the study area, with partial inversion of the previous fault systems, such as the Trento-Cles. Its western limb was then uplifted, separating a sector of non-deposition (the Brenta Group), from a perched basin overlying the previous high on the eastern limb. From that event on, the sector as a whole was part of the retroforeland basin, basically forming a ramp succession, still influenced by the previous bathymetry, but smoothing it. The Scaglia Rossa and Ponte Pià Fms represent these units, whereas the deepest portion could receive the distal turbiditic lobes of the Alpine Flysch, the Val d’Agola Fm.