Fission-track reconstruction of the front of the northern Apennine thrust wedge and overlying Ligurian unit.

In the Northern Apennines foreland, the Marnoso-arenacea Formation (MA Fin) records post-depositional burial temperatures overlapping with those of the zone of partial annealing of apatite fission tracks. Because the stratigraphy, sedimentology, petrography and structural evolution of this turbidite succession has been intensively studied over the last 40 years, the MA Fin provides an ideal case to apply the apatite fission-track method. The data show a general decrease of the maximum paleotemperature undergone by the MA samples toward the foreland area. The maximum burial, calculated using a geothermal gradient of 20degreesC/km, spans from more than 5 km to less than 2.5 km and indicates that the reconstructed total thickness of the MA succession is not enough to justify the determined burial values. Stratigraphic data indicates that the missing section consisted of a lower component (foredeep successions) and an upper allocthonous component composed of disrupted and chaotic oceanic sediments and minor ophiolites (Ligurian unit). The Ligurian unit overrode the foredeep succession during deposition. Its advancement toward the foreland area was associated with subsidence and only locally contrasted by thrust growth. The reconstructed wedge shape of the Ligurian limit, in its final and pre-erosional configuration at time of the maximum burial of the MA succession, shows a flat upper surface, which corresponds to a paleosurface of Early Pliocene age. This surface marks at 4 to 5 Ma the onset of the exhumation phase, which occurred at a mean rate of 1.2 mm/yr.