Numerous polymetamorphic slices of basement crop out in the Alps. They are involved in the Alpine belt and partially overprinted by Alpine metamorphism which tends to delete the pre-Alpine history. Identification of pre-Alpine vs. Alpine events must be performed to avoid misunderstanding of the geological history. Pre-Alpine structures can be associated to two different geodynamic events: (1) Variscan structures related to Carboniferous collision, (2) Permo- Triassic rifting that overprints the Variscan history. Evidence for this Permo-Triassic event is poorly reported in the literature either because it was spatially localised or is particularly hard to recognize. We investigate a portion of basement characterized by the absence of Alpine overprint: the basement of the southern Alps, which represent the former northern part of the Adriatic margin. Our goal is to clearly separate orogenic (Variscan) from rift-related (Permo-Triassic) structures. We target the Dervio-Olgiasca Zone (DOZ) and the Monte Muggio Zone (MMZ) that crop out along the northern part of Como lake. This basement records a Variscan history (Spalla et al., 2000) and it is cross cut by the Lugano-Val Grande normal fault (186-220 Ma; Bertotti et al., 1999). Micaschists contain pseudomorphs composed of prismatic sillimanite and biotite after staurolite, as well as biotite and fibrolitic sillimanite aggregates after garnet, and muscovite after kyanite. Reconstructing the evolution of these rocks is fundamental to determine the pre-Alpine pressure and temperature conditions of the basement. Datable accessory mineral monazite and zircon are texturally correlated with the tectono-metamorphic evolution of the schist. Preliminary U-Th-Pb analyses on monazite provide evidence for a Late Carboniferous orogenic event followed by a possible Middle Triassic rift-related HT event. The Dervio-Olgiasca-Zone (DOZ) and Monte Muggio Zone (MMZ) represent an example of well-preserved Variscan basement partially overprinted by a Triassic rift-related event. Across this study, we wish to highlight, that the Permo-Triassic metamorphism is well-developed only at the deepest structural level, it could easily be mistaken as Variscan and is only identified by careful micro-structural analysis. Bertotti, G., Seward, D., Wijbrans, J., ter Voorde, M., Hurford, A.J. (1999): Crustal thermal regime prior to, during, and after rifting: A geochronological and modeling study of the Mesozoic South Alpine rifted margin. Tectonics, 18, 185-200. Spalla, M.I., Siletto, G.B., di Paola, S., Gosso, G. (2000): The role of structural and metamorphic memory in the distinction of tectono-metamorphic units: the basement of the Como lake in the Southern Alps. J. Geodynam., 30, 191-204.
The complex polymetamorphic history of the Southalpine realm: evolution of continental crustal slices from Variscan to Alpine time
Real C.;Carosi R.;Ferrando S.;Groppo C.
2017-01-01
Abstract
Numerous polymetamorphic slices of basement crop out in the Alps. They are involved in the Alpine belt and partially overprinted by Alpine metamorphism which tends to delete the pre-Alpine history. Identification of pre-Alpine vs. Alpine events must be performed to avoid misunderstanding of the geological history. Pre-Alpine structures can be associated to two different geodynamic events: (1) Variscan structures related to Carboniferous collision, (2) Permo- Triassic rifting that overprints the Variscan history. Evidence for this Permo-Triassic event is poorly reported in the literature either because it was spatially localised or is particularly hard to recognize. We investigate a portion of basement characterized by the absence of Alpine overprint: the basement of the southern Alps, which represent the former northern part of the Adriatic margin. Our goal is to clearly separate orogenic (Variscan) from rift-related (Permo-Triassic) structures. We target the Dervio-Olgiasca Zone (DOZ) and the Monte Muggio Zone (MMZ) that crop out along the northern part of Como lake. This basement records a Variscan history (Spalla et al., 2000) and it is cross cut by the Lugano-Val Grande normal fault (186-220 Ma; Bertotti et al., 1999). Micaschists contain pseudomorphs composed of prismatic sillimanite and biotite after staurolite, as well as biotite and fibrolitic sillimanite aggregates after garnet, and muscovite after kyanite. Reconstructing the evolution of these rocks is fundamental to determine the pre-Alpine pressure and temperature conditions of the basement. Datable accessory mineral monazite and zircon are texturally correlated with the tectono-metamorphic evolution of the schist. Preliminary U-Th-Pb analyses on monazite provide evidence for a Late Carboniferous orogenic event followed by a possible Middle Triassic rift-related HT event. The Dervio-Olgiasca-Zone (DOZ) and Monte Muggio Zone (MMZ) represent an example of well-preserved Variscan basement partially overprinted by a Triassic rift-related event. Across this study, we wish to highlight, that the Permo-Triassic metamorphism is well-developed only at the deepest structural level, it could easily be mistaken as Variscan and is only identified by careful micro-structural analysis. Bertotti, G., Seward, D., Wijbrans, J., ter Voorde, M., Hurford, A.J. (1999): Crustal thermal regime prior to, during, and after rifting: A geochronological and modeling study of the Mesozoic South Alpine rifted margin. Tectonics, 18, 185-200. Spalla, M.I., Siletto, G.B., di Paola, S., Gosso, G. (2000): The role of structural and metamorphic memory in the distinction of tectono-metamorphic units: the basement of the Como lake in the Southern Alps. J. Geodynam., 30, 191-204.
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