A REASSESSMENT OF FISSION-TRACK AGES AND THE THERMAL EVOLUTION OF THE CALABRIAN ARC AFTER THE INITIATION OF SUBDUCTION, SOUTHERN ITALY

Previously published apatite and zircon fission-track ages from the Stilo Unit, the upper plate of the Calabrian subduction zone, have been interpreted to indicate extensional exhumation between 35-15 Ma. However, there is some disagreement as to whether upper-plate normal faults existed between 35-15 Ma.

In southern Calabria, the Stilo-Capo d’Orlando Formation was deposited in a thrust-top basin, which may have been affected by normal faulting as early as the Oligocene. In contrast, in northern Calabria, the earliest evidence for sedimentation in normal-fault controlled basins comes from Serravallian (13.8-11.6 Ma) fossils from the Amantea Basin. This discrepancy suggests that normal faulting may not be sufficient to explain widespread Oligo-Miocene fission-track dates in Calabria.

Instead, we propose that the initiation of subduction of Ionian oceanic lithosphere beneath Calabria is responsible for the cooling of the upper plate and the 35-15 Ma fission-track ages. Near the subduction zone, cooling occurs rapidly, while far from the subduction zone, cooling takes place with a significant time lag.

In order to investigate this possibility, a two-dimensional thermal-kinematic model of subduction initiation was constructed with GALE, a pre-built finite-element modeling program. In this model, subduction initiation occurred along a passive margin as oceanic lithosphere began subducting beneath continental lithosphere. Several parameters were varied, including the angle of subduction, rate of convergence, amount of radiogenic heating, and the geothermal gradient.

Preliminary results indicate that at distances of ~100 km from the subduction trench deepening of the isotherms which correspond to the apatite and zircon closure temperatures occurs after a significant time lag. Although the timing of subduction initiation beneath Calabria is controversial, with estimates varying between Cretaceous and Eocene, the thermal time lag may be long enough to explain the pattern of fission track ages in Calabria.