INVESTIGATING NEOGENE EXHUMATION OF THE NORTHERN APENNINES BY COMBINING FISSION-TRACK AND (U-TH)/HE DATING IN AREAS OF HIGH RELIEF

Variations in cooling and exhumation rates of the northern Apennines following their emergence and development since the late Miocene have been investigated using both fission-track (AFT) analysis and (U-Th)/He (AHe) dating on apatite. Samples were collected from uplifted Cenozoic turbiditic foredeep deposits collected from three vertical transects in areas of high relief close to the present day topographic divide (Monte Falterona, Monte Cimone, and Val d’Arno).

All profiles show a similar pattern of age versus elevation for both thermochronometers. AFT results indicate exhumation rates of ca. 0.4-0.8 km/Myr between ca. 8 and 5 Ma, while AHe ages give younger, faster exhumation rates with values >1 km/Myr at ca. 4 Ma in the Val d’Arno area and at ca. 3 to 2 Ma for the Mt. Falterona and Mt. Cimone transects. The older values obtained from the Val d’Arno area may relate to its more southerly location. The difference in AFT and AHe age in common samples decrease towards lower elevations in all transects. This can be shown to imply an apparent increase in cooling rate at around 6 to 5 Ma. Reconstructed "psuedo vertical transects", whereby the elevation of AFT ages are increased by an amount proportional to the difference in closure temperature of the AHe system assuming a geotherm to convert temperature to depth, also indicate an increase in exhumation rate at ca. 6-5 Ma. Ongoing investigations will examine whether this apparent increase in cooling rate at ca. 6 to 5 Ma is a real event, or an artefact related to the effects of an increasing transient geotherm, a variation of the geothermal gradient with depth, or possible topographic perturbations of near surface isotherms that particularly influence the lower temperature AHe system.