CROSS-COMPARISON OF THE APATITE FISSION-TRACK AND (U-TH)/HE THERMOCHRONOLOGIC SYSTEMS IN POST-DEPOSITIONALLY REHEATED SEDIMENTARY ROCKS

The age-temperature sensitivity of the apatite fission-track (AFT) and (U-Th)/He (AHe) dating methods are similar enough to allow cross-comparison. For the AFT system the rate of apatite track length and hence age reduction over geological time becomes significant at temperatures > ca. 60°C. Above about 100-120°C the rate of track length reduction is so fast that the age is effectively reduced to zero. For the AHe system a sample held isothermally stable for 100 Ma begins to lose significant helium at about 35°C. Rates of helium loss become sufficient to reduce He ages to essentially zero above about 85°C. Although these figures are now generally accepted for the individual systems themselves, little work has been done to cross-compare and calibrate thermochronologic models of these two systems, despite their similar age-temperature sensitivities.

Such cross-comparison can be achieved in a natural geological environment by studying the response of both systems in sedimentary rocks that have undergone post-depositional reburial to suitable temperatures between about 60°C and 85°C. A detailed fission-track study of the Marnoso-arenacea Formation (MA Fm) of the northern Apennines [1] shows that this well-studied Miocene turbiditic foredeep succession is ideal for such cross-comparison. Maximum post-depositional burial temperatures determined from AFT thermal modelling generally decrease toward the foreland from temperatures >120°C to samples unaffected by post-depositional heating (<60°C). Initial AHe ages agree well with the AFT data. External samples with depositional AFT ages yield apparent pre-depositional AHe ages between 16 and 19 Ma. More internal samples with partially reset AFT ages indicating heating to temperatures of ca. 80°C yield reset AHe ages of ca. 6 Ma. These AHe ages likely record the onset of post-burial cooling at ca. 6 Ma implied in a nearby vertical AFT age-elevation profile. A more internal sample, with a totally reset AFT age of 6 Ma (palaeotemperature >120°C), yields an AHe age of 1.7±0.1 Ma. The AFT and AHe ages of this sample indicate a time-averaged post-burial cooling rate of ca. 8.5°C/Myr. A more complete intermethod comparison will be achieved with the acquisition of substantial new AHe data from the Ma Fm.

[1] Zattin et al. (2002). American Journal of Science, v. 302, p.346