Correlation between seismic reflectivity and rheology in extended lithosphere: southern Tuscany, inner Northern Apennines, Italy

Abstract

In southern Tuscany (inner zone of the Northern Apennines) the lithosphere has been affected by extensional deformation since the Middle Miocene and is characterized by high heat flow (∼120 mW m −2). Both local (shallow) and regional (deep) seismic reflection profiles clearly show the occurrence of a sharp reflector (K horizon) within the upper crust, which is probably located at the top of a shear zone acting as the décollement horizon for the latest (Pliocene–Quaternary) normal faults. Its depth varies between 3 and 6 km in geothermal areas and 9 and 12 km elsewhere. Borehole control in the geothermal areas shows that it occurs at a temperature of approximately 450°C. The hypothesis that the K horizon represents the top of the brittle/ductile transition agrees with two lines of evidence: (a) it approximately follows the same isotherm over the whole region; and (b) its depth coincides with rheological predictions of the brittle/ductile transition for the range of high pore fluid pressures and strain rates estimated for southern Tuscany. It appears, therefore, that at least under specific conditions (very high pore fluid pressures, strain concentration) the brittle/ductile transition may have a seismic signature.