Abstract
The upper part of the crystalline basement of the Northern Apennines is made up of a complex pile of tectonic wedges derived from Alpine low-grade metamorphic units (T= temperature = 300°–450°C; P= pressure = 3–4 Kbar), Paleozoic (?Late Cambrian-Permian) to Tertiary in age. The Paleozoic rocks also show evidence of pre-Alpine deformation (e.g. Hercynian schistosity relics and folding).
In the subsurface of the Larderello geothermal region (southern Tuscany) this tectonic wedge complex (Monticiano-Roccastrada Unit) also includes Hercynian low-/medium-grade metamorphic rocks (Micaschist Complex) which are strongly imprinted by the Alpine tectono-metamorphism. At a depth of about 2000 m (below the ground level), an Alpine mylonite horizon divides the Micaschist Complex from an underlying highly recrystallized succession (Gneiss Complex) which shows a well preserved Hercynian framework modified only by weak Alpine crenulations. The Hercynian near-isothermal decompressional tectono-metamorphic evolution of the Gneiss Complex is similar to that of the Micaschist Complex and matches those of several areas of the Variscan basement in SW Europe with a barrovian-type intermediate P event (T= 550°–650°C; P≅ 6–7Kbar) to a syntectonic high T-low P event (T= 500°–600°C; P= 2.8–3.5 Kbar).
The tectonic superposition of the Alpine units (including the Micaschist Complex) on the Hercynian-preserved Gneiss Complex could be the result of their being overthrust onto the undeformed basement of the Apenninic foreland or onto “rigid” Hercynian section belonging to the Northern Apennines pile.
Geophysical studies reveal probable deeper crustal shear-zones in the inner part of the mountain chain (3–8 Km deep seismic “K” horizon; 10–14 Km deep doubling of the crystalline basement in the magnetic and gravimetric profiles). Collectively, these data point to a strong Alpine reactivation of the Hercynian basement of the Northern Apennines at relatively deep crustal levels.
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Elter, F.M., Pandeli, E. (1996). Structural Setting of the Paleozoic Crystalline Basement of the Northern Apennines (Italy). In: Oncken, O., Janssen, C. (eds) Basement Tectonics 11 Europe and Other Regions. Proceedings of the International Conferences on Basement Tectonics, vol 5. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1598-5_6
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