Neogene extension in the Northern Apennines (Italy): insights from the southern part of the Mt. Amiata geothermal area

Abstract

Fieldwork data, boreholes stratigraphy and seismic interpretations revealed that stretched upper crust typifies the southern part of the Mt. Amiata geothermal area. Crustal stretching took place during Miocene extensional tectonics which gave rise to widespread tectonic delamination within the tectonic pile of the inner Northern Apennines, stacked from the Cretaceous to the Early Miocene. Extensional horses (boudins) composed of the Tuscan Nappe, the deepest non-metamorphic tectonic unit of the Northern Apennines, were generated from subhorizontal detachments linked to east-dipping ramps. Minor detachments and related ramps produced minor Tuscan Nappe boudins composed of only part of the Tuscan Nappe succession. West and south of the Mt. Amiata geothermal area, three main Tuscan Nappe boudins have been discovered and geometrically investigated by boreholes, fieldwork and seismic data. These geological bodies partially crop out in the Sasso d'Ombrone, Roccalbegna-Torrente Rigo and Castell'Azzara areas. Seismic images and boreholes have allowed better control of the geometry of the Roccalbegna-Torrente Rigo boudin and reconstruction of its possible developmental mechanism, characterised by superposed normal faulting. The Tuscan Nappe boudinage produced structural depressions which were mainly infilled by continental sediments. The Cinigiano-Baccinello basin lies between the Roccalbegna and Sasso d'Ombrone Tuscan Nappe boudins and was mainly infilled by Late Miocene continental deposits. Seismic profiles imaging the geometrical relationships between the Tuscan Nappe boudins and the Late Miocene deposits infilling the Cinigiano-Baccinello basin in its eastern margin, indicate that this basin was a boudinage-related basin.