Multisciplinary constraints to the apatite annealing temperatures: a case history from the southern Apennines

A precise determination of the annealing temperatures requires the use of indipendent techniques. In this work, the thermal evolution of sedimentary successions from a sector of the southern Apennines has been studied by using different methodologies: (i) vitrinite reflectance, (ii) fluid inclusions, (iii) illite/smectite (I/S) mixed layers in clayey sediments, and (iv) apatite fission-tracks. The studied area is around Monte Alpi, where the reservoir for the major oil fields in southern Italy are exposed at surface. Samples analyzed with fission-track analysis have been collected from Messinian rocks transgressive on the Monte Alpi carbonates, and from a melange tectonically overlying the carbonates.All the samples but one yield two single grain age populations. Most of the grains belong to the youngest peak whose age is of about 2.4 Ma. Some grains are much older than stratigraphic age. These data indicate that the maximum temperature during burial was very close to the total annealing temperature. Constraints given by modelling of others thermal indicators suggest a maximum temperature of about 145°C. This temperature is compatible with known annealing models given the short heating time (about 1 M.yr.). The meaning of oldest grain ages is not clear but possible influences given by variations of apatite composition has been investigated by Raman spectroscopy. This methodology allows a fast and reliable distinction among apatites with different chemical compositions.