Geodynamics, thermal structure and depth of the brittle and semi-brittle layers in the Northern Apennine (Italy)

The Northern Apennines are the exposed part of an accretionary prism which was formed as a result of the westward subduction of the continental Adria microplate. This prism occurred during two phases: the first between 11.2 and 5.3 Ma ago and the second from 3.6 Ma onwards, due to the stacking of continental upper crust strata, scraped off during the subduction. A 1D physical-mathematical model has been elaborated to compute the effect of tectonic stacking of the mantle and the radioactive components of the heat flow. The heat contribution, which resulted from the friction between the strata, has also been calculated. We find that the cooling effect of the thrusting is almost compensated by the heating by friction, so that the undisturbed surface geothermal gradient 29 � 0.3�C km-1 of the Adriatic lithosphere is reduced to 24.5�C km-1 at the outcropping sector of the thrust. This agrees with the gradient 27�C km-1 � 15% observed in deep oil wells. As a consequence of the thrusting, it has been found that, in particular, the 300�C and 450�C isotherms which in the stable Adriatic lithosphere are located at 13.2 and 24.8 km respectively, and delimit the semi-brittle layer separating the brittle upper layer from the deeper ductile one, are located at different depths in the Apennines. In fact, the upper limit is located at about 11 km depth, while the deep limit is located at about 50 km depth. Then, the seismically more active brittle layer and the semi-brittle are thicker and deeper.