Tectonic regime, granite emplacement and crustal structure in the inner zone of the Northern Apennines (Tuscany, Italy): A new hypothesis

Geological and geophysical data on the inner part of the Northern Apennines and the deep (4 km) geothermal wells contained therein have been reviewed in order to better characterize the tectonic regime of the region. The area is affected by Neogene-Quaternary igneous activity, reduced crustal thickness, high heat flow and active geothermal systems. The available information leads to the following conclusions: (1) at present, part of the chain, including its inner regions, are undergoing extensional activity, a tectonic situation in southern Tuscany dating back to the Late Tortonian; however, the simple model of solely extensional tectonics cannot exhaustively account for the phenomena observed in the area, as there is ample evidence of compressive events of Pliocene and Pleistocene ages. (2) The geophysical data indicate reduced Lithosphere thickness and the uprising of the asthenosphere to shallow crust levels. The upper crust has been intruded by many granite bodies and the brittle-ductile transition is probably very shallow. An integrated geophysical model which accounts for heat-flow, seismic, gravimetric and magnetic data, indicates that a regional strong reflector, rather than coinciding with the beginning of the ductile crust, more likely indicates the presence of deep brines. Rather than a gneiss core complex and a generalized post-collision extensional model (Cameli et al., 1993) we support a more complex geodynamic framework, including compressive phases of the internal zone and re-activation of the thrust of the Apennines as a consequence of crustal thrust emplacements from the Late Tortonian in the frame of a subducting delaminated lithosphere.