Control of rock rheology on deformation style and slip-rate along the active Pernicana Fault, Mt. Etna, Italy

The already known part of the active left-lateral strike-slip Pernicana Fault, on the eastern flank of Mt. Etna Volcano, has a Holocene high slip-rate of 1.5-2.7 cm/yr, but its surface trace abruptly disappears eastward approaching the lower slope of the volcano. Using geological, geomorphological and structural surveys we found that the Pernicana deformation tow extends 8 km further to the east, beyond this point, through a different mechanism. As the thickness of the volcanic pile decreases, the single E-W- to ESE-striking main fault plane gives way to a set of faults, arranged in a splay structure, which partitions the fault motion. Here, slip-rates are 0.4 to 1.4 cm/yr on each fault. Further east, where the lava succession is very thin and unfaulted, we suggest that lavas are decoupled from plastic-deforming sub-Etnean clay deposits which absorb the strain, or that lavas have a too low mass to slide. On the very lowest slope of the volcano (i.e. further east), lavas are once again thicker and the Pernicana deformation zone behaves at the surface once again in a brittle manner locally known as the Fiumefreddo Fault, with a low slip-rate of <0.1 to similar to 0.1 cm/yr. We suggest that here lavas partly are decoupled and partly react in a brittle manner, by faulting, to the strain in the underlying clays or to the seaward sliding of the lava cover. This interpretation of decoupled lavas can be applied to other volcanic areas with similar characteristics, such as Concepcion and Maderas in Nicaragua, and Merbabu, Tjareme, and Old Lawu in Indonesia. (C) 1999 Elsevier Science B.V. All rights reserved.