Note Illustrative della carta Geologica d'Italia in scala 1:50.000, foglio 013 "Merano"

The new Merano sheet (scale 1:50,000) depicts one of the most complex portions of the central-eastern Alps, extending across the Insubric line. The mapped area includes the Southern Alps, exposed SE of the fault system, and a large area belonging to the Austroalpine domain NW of it. Several tectono-metamorphic units have been distinguished in the Austroalpine domain, based on the definition of their structural and metamorphic evolution and on the occurrence of brittle to ductile shear zones along their boundaries. In the SW part of the sheet four units have been distinguished: the Ultimo and Tonale units belonging to the Tonale nappe, the Marlengo tectonic slice and the Pejo unit, the last one related to the Ortler-Campo nappe. The Ultimo unit extends in a small area between the N-Giudicarie and the S. Elena lines and consists of two-micas staurolite and kyanite paragneiss (TUN), and orthogneiss (TUO). The Tonale unit crops out along the lower segment of the Ulten Valley, forming the northern end of the unit. It is separated from the Ultimo unit by the mylonites of the S. Elena line and from the Marlengo tectonic slice by brittle-ductile shear zones. In the northern part, the Tonale unit is directly juxtaposed to the Southalpine basement along the N-Giudicarie line. The unit mainly consists of sillimanite gneiss (TTP) with minor amphibolites (TTA), often with garnet (TTAa). Pegmatite dykes (OGA) crosscutting the S2 foliation occur close to the Insubric line. The structural setting of this unit is very complex, due to the occurrence of pervasive cataclastic shear zones possibly coeval to the activity of the Periadriatic fault system. The Marlengo slice forms a 5 km wide, sinuous, NNE-SSW trending strip extending between Rifiano and the southern boundary of the sheet. The unit is bounded by the left-lateral cataclastic shear zones of the Forst fault, by the Pavicolo line, the N-Giudicarie line, and finally by the Passeier line N of Merano separating its NE part from the Scena and Punta Cervina units. The unit forms a complex wedge between the Ortler-Campo and the Tonale nappes to the south, whereas to the north it crops out between the Texel unit and the Southalpine basement. The Marlengo tectonic slice has been defined due to its peculiar composition. In fact, no pegmatite and marble layers, common in the Ortler-Campo nappe, occur in this unit. In addition, the regional trend of the S2 foliation is almost different from the one shown by the surrounding units, suggesting a sharp structural separation. The unit includes quartzite gneiss (OUP), banded gneiss (OUPa), garnet,staurolite and kyanite mica schists (OUM), with orthogneiss (OOR), leucocratic orthogneiss (OUO), quartzite (OUQ) and amphibolite (OUA). The Pejo unit forms the entire southern slope of the Vinschgau Valley between Naturno and Forst, extending from the Adige river up to the watershed and also beyond it. The Forst fault separates this unit from the Marlengo tectonic slice, whereas to the north it is separated from the Texel unit by the Thurnstein mylonites, which show a normal to left-lateral kinematics. The Pejo unit displays a medium-grade metamorphism, probably Variscan in age, followed by an Alpine greenschist facies re-equilibration. Two micas gneiss (OOP) with a pervasive mylonitic foliation dominate along the Vinschgau Valley below 1700 m a.s.l. They are often associated with migmatitic-like layers and lenses of foliated Permian pegmatite dykes (OGA), and orthogneiss (OOG). Amphibolite (OAF) and marble layers (OMA) also occur. In the upper part of the unit, garnet and staurolite mica schists (OMI) are exposed, with minor amounts of amphibolite (OAF), Marble (OMA), and quartzite (OQU). Metadiorites (OGD) and metagabbros (OGDa), possibly Permian in age, intrude the unit. The Texel (Tessa) unit, including most of the Tessa massif, crops out west of the Passirio river extending to the Monteneve unit to the north. The Passirio fault to the east and the Thurnstein mylonites to the south also bound the unit. Two main features characterize the Texel unit: the Parcines orthogneiss (OTH), which are considered Lower Palaeozoic (Caledonian) meta-intrusives, and the occurrence of a penetrative Alpine metamorphism. The Texel unit displays a polyphase metamorphic evolution with a dominant syn-D2 amphibolite facies imprint of Alpine age, followed by a pervasive greenschist facies retrogression during a D3 event. Eclogites (D1) are locally preserved as small boudins within garnet amphibolites. Garnet amphibolites with plagioclase or amphibole+plagioclase symplectites are also widespread. An HP syn-D1 metamorphic event is also recorded in the metapelites of the unit. Radiometric data constrain the eclogitic event at 85 Ma. The Texel unit mainly consists of banded paragneiss (OTP), garnet- and staurolite-mica schists (OTM), with amphibolite (OTF) with eclogitic relics (OTE), quartzite (OTQ), thick marble layers (OTB), and calc-schists (OTC). The upper part of the unit, rich in marble and calc-schists layers forming the transition between the Texel and the Monteneve unit, was previously related to the so called “Laas Serie” attributed by several authors to the Monteneve unit. Nice folds with superposed interference patters resulting from the D1+2 and D3 deformational events are exposed in the high peaks of the Tessa massif. Steeply plunging folds characterize the last important stage of ductile deformation (D3) especially in the Pfossen Valley close to the boundary with the Monteneve unit. The Parcines orthogneiss (OTH) largely extends in the central-southern part of the unit forming a large WSW-ENE trending body including also well preserved meta-intrusive (OTL) with relics of intrusive textures. The Monteneve unit is exposed in a small area along the high mountain ridge separating Italy from Austria in the NE part of the sheet. The contact with the underlying Texel unit is often marked by discontinuous mylonitic shear zones. The Monteneve unit here includes mica schists with large garnets (OSF), amphibolite (OSA), often rich in garnet (OSAa), thick garnet-rich quartzite layers (OSQ), marbles (OSB), and calc-schists (OSC). This peculiar lithological association has been traditionally considered as a monometamorphic unit, that experienced only Alpine metamorphism with peak metamorphic conditions close to the eclogiteamphibolite facies transition. We suggest that at least 3 different deformational events affected the unit, the last one (D3) being in common with the evolution of the Texel unit. The Monteneve unit is deeply involved in the D3 folds, also affecting the tectonic boundary with the Texel unit. East of the Passirio River, four tectono-metamorphics units are exposed: the Scena, Punta Cervina, Pennes and S. Leonardo units. Common features are a general pre-Alpine radiometric age of minerals, and the occurrence of important NNW-dipping shear zones marking their tectonic boundaries, which are sharply cut by the late Tertiary left-lateral Passirio fault. The Scena unit extends in the southernmost part of the area between the Insubric and Rio Masul lines. It mainly consists of garnet-staurolite-sillimanite gneisses (OVP) and mica schists (OVM) with minor amphibolites (OVA), and amphibolic gneisses. Mylonitic marble layers (OVB), and quartzite (OVQ) occur close to the Insubric line. The occurrence of sillimanite replacing garnet and, locally, of K-feldspar within garnet-biotite gneisses, indicates higher temperatures with respect to the nearby Punta Cervina and S. Leonardo units during the Variscan evolution. A large pegmatite dyke (OGA) crops out at the old mine of Rio Masul. The Punta Cervina unit is directly located north of the Scena unit forming a NW-SE trending belt developed in the hanging wall of the Rio Masul shear zone. The Punta Cervina unit is bounded to the north by the mylonites of the Pennes unit. The Punta Cervina unit consists of garnet-kyanite-staurolite gneiss (OCP and OCPa), with large kyanite crystals (up to 10 cm). Minor amounts of amphibolite (OCA), quartzite (OCQ), and thick layers of granitic gneisses (OCO) also occur. The km-thick high-angle NW-dipping shear zone of Rio Masul, forms the southern part of the unit; it consists of mylonitic gneisses (OCX) including other lithologies characterized by a pervasive mylonitic fabric with superimposed discrete cataclastic fault zones often injected by pseudotachylyte veins. Fine-grained ultramylonites show greenschist facies conditions with stable biotite. The kinematics of the shear zone strongly recalls the Merano-Mules line, showing an almost along-dip stretching lineation and a top-to-the SE shear sense. The Pennes unit forms a few km large stripe between the S. Leonardo unit to the north and the Punta Cervina unit to the south. This poorly known unit represents an important intra-Austroalpine shear zone extending from the Passirio Valley to Vipiteno out of the mapped area. It includes mylonitic orthogneiss (OPG) and deformed but poorly metamorphic metasediments. Metaconglomerates with metasandstones (OPC), and carbonates (OPD), belonging to the Permian to Triassic Austroalpine sedimentary cover, are preserved along the Sarentino-Passiria watershed. The shear zone shows a complex polyphase evolution recording a progressive exhumation from ductile to brittle condition. A mylonitic fabric with white mica, chlorite and quartz ribbons occurs both in metasandstones and in the deformed orthogneiss. Stretching lineations mainly suggest a motion of the top to the S/SW. The mylonitic shear zone is overprinted by right-lateral brittle-ductile shear zones. The S. Leonardo unit crops out in the NE part of the sheet, north of the Pennes unit. It extends up to the normal left-lateral Giovo fault out of the Merano sheet. It mainly consists of retrogressed garnet-staurolite gneisses (OJP), with orthogneiss derived from a granitic protholith (OJO), showing large K-feldspar porhyroclasts (OJF). Mica schists (OJM), quartzite (OJQ) and amphibolite boudins (OJA) have been also mapped. The Southern Alps domain is localized in the SE sector of the Sheet, SE of the Insubric Line, and is characterized by the absence of any Alpine metamorphic overprint. This domain includes a Variscan metamorphic basement, intruded by lower Permian acid plutons (Ivigna and Monte Croce bodies) and several related dykes. The basement is covered by: a) a thick Lower Permian sub-aerial volcanic sequence (Athesian Volcanic Group); b) an Upper Permian-Lower Triassic sedimentary cover of continental and shallow water marine sediments (Valgardena Sandstones and Werfen Fm). The metamorphic basement mainly consists of garnet bearing phyllites (+ biotite, muscovite, plagioclase, chlorite) and acid meta-volcanites (“porfiroidi” Auct.) with minor muscovite quartzites and metabasites. The metamorphism is Variscan (350-320 Ma) and the metamorphic grade reached the greenschist facies (garnet sub-facies) with a metamorphic peak around T ∼ 460-520°C (for P ∼ 0.3-0.5 GPa). The S2 schistosity is cut by many rhyodacitic dykes and by two huge intrusion bodies: the Ivigna Granodiorite and the Monte Croce Granodiorite; both of them form NE-SW elongated plutons emplaced ∼ 285 Ma very close to the Insubric Line and are composed by granodiorite - monzogranite calcalkaline rocks. Their emplacement was related to an extensional-transtensional regime in the Lower Permian, which produced voluminous basic to acid volcanic and plutonic rocks in the Southern Alps. The volcanic activity, during a span of 10 Ma (285-274 Ma), gave rise to the Athesian Volcanic Group and to the formation of large calderas: a thick succession (up to 2000 m outside this Sheet) of lavas and pyroclastic flow deposits (lapilli tuff - ignimbrites) with minor pyroclasic surge. The composition ranges from andesitic to rhyolitic with a general evolution from basic-intermediate to acid. Continental silico-clastic sedimentary deposits are present at the base (Conglomerato Basale) and inside the volcanic sequence. The basal conglomerate is clearly erosive on the metamorphic basement and starts the volcanic cycle, which is covered by the Upper Permian Valgardena sandstones, a continental silicoclastic formation, erosive on the upper volcanic formations (IGG or ORA). The sedimentary cover ends in this area with the Werfen Fm (Upper Permian p.p. - Olenekian p.p.): an alternation of carbonate-silico-clastic deposits of shallow water marine sediments, representing a general marine transgression. In the Merano sheet area, the Austroalpine units are juxtaposed to the South Alpine domain along the Insubric Line (Periadriatic fault system), which includes two different branches, both representing a restraining bend of the fault. They are: the NNE-SSW trending N-Giudicarie line south of S of Merano, and the NE-SW trending Merano-Mules line east of the town, both dipping at high angle respectively to WNW and NW, and showing an important N-side-up component. Oligocene magmatism is recorded along the line by the occurrence of thin foliated tonalite bodies (TLG). Radiometric ages on mylonites and pseudotachylytes along the North Giudicarie segment of the fault also suggest deformation around 30 Ma, possibly continuing up to 22 Ma (Müller et alii, 2001). The N-Giudicarie line forms the northern termination of the Giudicarie fault system. The hanging wall consists of paragneiss of the Tonale unit intruded by poorly deformed pegmatite dykes. Low grade S-C mylonites show a superimposed cataclastic fabric mainly indicating a left-lateral motion of the fault. The footwall comprises the low grade phyllite of the Bressanone unit showing contact metamorphism with the Permian Mt. Croce granodiorite. The Merano-Mules is a NW-SE trending high-angle reverse fault. Quartz-rich mylonites, calc-mylonites and phyllonitic layers showing an almost along-dip lineation occur along the fault, suggesting a top to SE motion of the Austroalpine domain. Strongly sheared pseudotachylytes injecting mylonitic gneiss belonging to the hanging wall of the fault near Merano. A late cataclastic imprint is given by discrete fault zones consistent with the ductile structures. The Merano-Mules fault is clearly displaced by a splay of the Passirio fault, showing a left-lateral offset of about 1 km across the Passirio River just N of Merano. As regards the Quaternary deposits, four main unconformity-bounded stratigraphic units (UBSU) have been recognized. The Monte Spinale supersyntheme (Upper and Middle Pleistocene) is set up: it includes a part which is subdivided into synthemes (Caldaro syntheme and Sonvigo syntheme) and another part which is undifferentiated. All deposits are formed before the LGM. The Caldaro syntheme is composed by alluvial and mixed (fluvial + debris flow + avalanche) deposits, which crop out in the Passiria Valley and at Castel Tirolo, with a thickness locally up to 200 m. These deposits testify a refilling phase of the Val Passiria, with a valley floor ~ 50-80 m higher then today, which is joined by the stream fans coming from both the valley mountainsides. Four C-14 ages allow attributing these deposits to the Upper Pleistocene, inside the Marine Isotope Stage (MIS) 3. Inside the Sonvigo syntheme (Upper and Middle (?) Pleistocene) are only present lodgement tills. The top of the unit is marked by a buried soil, over which there are tills attributed to the LGM. Carbons present inside this soil have given an age of 56000 yrs BP. This age allow attributing these deposits to a glacial event which can be set inside the lower part of the Upper Pleistocene or inside the Middle Pleistocene. The Garda syntheme (Upper Pleistocene) is made up of deposits formed during the phases of increase, maximum expansion and retirement of the local valley glaciers during LGM. The main unit is prevalently formed by till deposits up to 100 m thick. This unit is composed by two subsynthemes linked to the retirement phase: the San Pancrazio subsyntheme in the Ultimo Valley, characterized by glacigenic deposits associated to the first Late-glacial phases, and the Malga Fontana Bianca subsyntheme with glacigenic and rock-glacier deposits probably related to the last glacial re-advance phases. The Postglacial Alpine syntheme (PTG) (Upper Pleistocene p.p. - Holocene) formed after the local LGM expansion. It includes different kinds of deposits. In the Adige Valley there are mainly alluvial and debris-flow/torrential fan deposits; C-14 ages allow calculatating average sedimentation rates of 2-2,7 m/1000 years (in places with mainly coarse-grained sedimentation). Outside the Adige Valley gravitational and debris-flow deposits prevail. Inside the PTG there is also the Amola subsyntheme which includes deposits from the Little Ice Age to the present.