Evaporites bearing faults as a tool to understand the deformation processes into the seismogenic zone of the Northern Apennines [Zone di faglia in rocce evaporitiche ed interpretazione dei processi defoimativi nella zona sismogenetica dell'Appennino Settentrionale]

In the Northern Apennines the integration of commercial seismic reflection profiles with the location of the mainshocks of the recent seismic sequences (the 1997 Mw = 6.0 Colfiorito and the 1998 Mw = 5.1 Gualdo Tadino) constrains the nucleation of the major events at ∼6 km of depth within the Triassic Evaporites, TE, formation. In order to investigate the deformation processes responsible for earthquake nucleation we have studied ancient and exhumed Evaporites bearing normal faults cropping out in Tuscany. Within the TE formation, that is a 2.5 km thick sequence, composed of decimetric-to-decameter scale interbeds of foliated gypsumanhydrites and brecciated dolostones, we have studied fault zone architecture and deformation processes of both small (<10 m) and large (>100 m) displacement faults. Small and large scale faults display a relatively well structured internal architecture with a sharp reduction in grain-size between the damage zone (coarse grained protocatclasite) and the fault core (fine/very fine grained cataclasite). Extreme localization of slip to discrete and very thin (Y and B shear up to 10-100 mm) sliding surfaces is observed within the fine-grained dolomitic bearing cataclasite layers within the fault core. Fluid assisted processes have been inferred by fault fracture mesh development with crack and seal textures within dolomite-dominated damage fault zone. The deformation processes observed in the field are consistent with elastic friction behaviour, recorded by random fabric fault rocks developed within localised zones of high shear strain. Coseismic slip along very narrow slip zones (< 1 mm) can produce very high temperature rise (T> 700 °C); it is currently under investigation whether such temperature increase which produces thermal decomposition of dolomite and phase transition in anhydrite rocks is able to cause dramatic coseismic slip weakening within the fault zone.