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

In southern Tuscany (inner zone of the Northern Apennines) the lithosphere has been affected by extensional deformation since the Middle Miocene and i s characterized by high heat flow (similar to 120 mW m(-2)). Both local (sh allow) and regional (deep) seismic reflection profiles clearly show the occ urrence of a sharp reflector (K horizon) within the upper crust, which is p robably located at the top of a shear zone acting as the decollement horizo n for the latest (Pliocene-Quaternary) normal faults. Its depth varies betw een 3 and 6 km in geothermal areas and 9 and 12 km elsewhere. Borehole cont rol in the geothermal areas shows that it occurs at a temperature of approx imately 450 degrees C. The hypothesis that the It horizon represents the to p of the brittle/ductile transition agrees with two lines of evidence: (a) it approximately follows the same isotherm over the whole region; and (b) i ts depth coincides with rheological predictions of the brittle/ductile tran sition for the range of high pore fluid pressures and strain rates estimate d for southern Tuscany. It appears, therefore, that at least under specific conditions (very high pore fluid pressures, strain concentration) the brit tle/ductile transition may have a seismic signature. (C) 1999 Elsevier Scie nce B.V. All rights reserved.