Crustal shortening and duplication of the Moho in the northern Apennines: A view from seismic refraction data

A reappraisal of the DSS refraction seismic campaigns of 1978 and 1974 in the Northern Apennines and the northern Tyrrhenian Sea, after digitization of the original analog data, and a new interpretation of crustal structures are presented in this paper. The layering of the Adria crust consists of a low-velocity lower crust topped by a 6.7 km/s horizon and an upper crust again formed by a low-velocity layer capped by a faster one. In the Tuscany sector the crust-mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the Adriatic counterpart. These differences are due to the extensional tectonics, restricted to Tuscany and occurred in a high heat flow regime related to an uplifting astenosphere. It has been confirmed that a deep thrust is responsible for Moho doubling in correspondence with the zero-Bouguer anomaly line: the Tuscan Moho overlaps the Umbro-Marchean one for a length of about 30 km. It is hypothesized that another shallower thrust involves upper crust in the Mid-Tuscany Range, Mt. Cetona and Perugia Massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks. No conclusive proof of the existence of a European Moho below a Tuscan one in the area underneath Elba has yet been found. Revision of DSS data excludes the presence of a refractor-reflector 60 km deep; the same data, however, are compatible with the existence of a crust-mantle boundary at 30-35 km depth.