The Musso fault and its role in the evolution of the northern lake Como basement.

The structural analysis of the MML and the surrounding basement lead to observe that: 1) The MML is completely bounded by main fragile fault planes and ductile structures. 2) The main foliation of the MML is only partly aligned to the regional E-W trending S2 foliation. 3) The MML has different N-S trending mylonitic foliation and fracture cleavage compared to S/SW dipping mylonitic foliation of the southern basement. 4) The apparent absence of garnet in the MML metabasites suggest a different metamorphic imprint in respect to the surrounding basements. 5) There are not sufficient petrographic data to determine the belonging of the MML to the different structural units recognized in the Southern Alpine basement. 6) The MML reached the Permo-Triassic thermal peak similar to that of surrounding basement. Within the MML, the marble-metabasite contact has locally characteristics that we interpreted as not correlated to the metamorphic evolution but resembling relicts of primary magmatic intrusion: i) lobes, ii) compositional layering parallel to the contact of the lobes, iii) diffuse sulphide mineralization at the metabasite-marble contact, iv) small interfingering of metabasite protruding within the marble resembling a fluidification of the hostrock, v) crosscutting of the dykes, vi) parallel tabular structures with geometry not related to any metamorphic event thus resembling dykes, vii) contact band characterized by mixing of phases suggesting a widespread mixing at the protoliths interface. The seven described structures, all found in the MML, resemble those of a basalt intrusion within a carbonatic sediment only partially lithified. Major, trace and REE elements of Musso and Lugano-Val Grande Fault Zone (LGFZ) metabasites, analyzed in this study, are compared to other metabasite and volcanics of the Southern Alps available in the literature: I) metabasites with and without cummingtonite from the Piona Peninsula (DOZ), II) metabasites from the Strona Ceneri Border Zone (SCBZ) and III) Triassic basaltic volcanics of the central Southern Alps. The LGFZ sample have very similar geochemical characteristics to SCBZ that allow to suppose a common origin. Musso samples show an in-between composition: they have similar Ba, Na, Al content to the SCBZ, LGFZ, and Triassic volcanics but have TiO2, Zr, Nb, MgO and SiO2 content similar to Anisian (a group belonging to the Triassic set) and part of the DOZ. This could be due to a partial metamorphic imprint with elements depletion, or a different volcano-tectonic origin, more similar to that of the Anisian basalts. Genetic diagrams, from literature, confirm the division between the DOZ and the SCBZ. SCBZ and LVGZ appear as a compact group with a MORB to tholeiitic composition. The Musso samples belong to intraplate products with alkali-basalt to tholeiitic composition. These data are only partially sufficient to suppose a particular role of the Musso marble lenses and the included metabasites but definitely testify the existence of this tectonic structure since at least Variscan age (Devonian-Early Carboniferous). In Chapter 3, the ductile and brittle structures mapped along the Musso Fault Zone (MFZ) are described and a hypothesis for the Musso fault evolution is presented, touching the following main stages: 1) Marble lenses (Musso marble) are boudinaged along the MFZ and represent the oldest rocks characterizing it. Their age is still not determined, but the included metabasites have unique geochemical characteristics that do not allow to attribute them nor to DOZ nor the Strona Ceneri Border Zone amphibolite suites. We speculate a Devonian perhaps to Early Carboniferous age for the marble and the included metabasites. 2) Geochronological and thermal data from literature shows that the Northern Lake Como region experienced a Triassic thermal peak and development of intra-basement shear zones, thinning the Adriatic crust. In this context the oldest structures directly and incontrovertibly connected to the MFZ are the Fe-carbonate veins, of Early Triassic age, which are preserved also boudinaged within the Musso mylonites. The same can be said also for the LGFZ, as demonstrated by extensive siderite mineralization presence in the Gaeta mine and Val Cavargna. 3) Fe-mineralized veins are found in similar extensional context in all the Southern Alps, along faults bordering Permo-Triassic continental basins. Therefore it is possible that also the MFZ was bordering a small basin to the N, nonetheless justifying the presence of the sedimentary Monte Pozzuolo and Sasso Pelo. In the Lake Como area continental extension started in the Norian and continued to Liassic. 4) The mylonites of the MFZ possibly developed after the Early Triassic thermal peak when temperature dropped to around 250-350°C allowing crystallization of quartz-albite-chlorite-sericite associations, before the emplacement of the 220Ma old Piona’s pegmatites which do not show mylonitization even near the MFZ. 5) From the Liassic to the onset of the Alpine compressive stage there is a long inactivity time span, with no records for the MFZ. 6) Alpine N-S contraction brought into a steep south dipping position the Paleozoic to Mesozoic rocks and lineaments and caused upper and middle crust to detach from their substratum along the MFZ. This contraction was responsible of the ductile-brittle deformation observed along the MFZ with development of widespread and abundant pseudotachylytes. Similar structures dated along the Orobic and Porcile thrusts yielded two sets of ages: an older Late Cretaceous and a younger Eocene ages. 7) Widespread distensive cataclastic bands along the Musso and contiguous faults, possibly indicating a distensive to transpressive stage, by correlation with nearby data of Val Morobbia, may belong to Oligocene. 8) Finally, right strike-slip dextral brittle deformation took place, visible along the largest fault planes, most probably driven by the Insubric Line.