Fault rocks analysis as a key for the kinematic reconstruction in brittle tectonics: an example from Southern Alps

Fault rocks are highly deformed levels, that develop near the principal surface of faults and thrusts. Fault rocks related to brittle deformation and characterized by penetrative development of tectonic discontinuities, such as pressure solution cleavage and shear planes, are here mainly considered. Fault rocks analysis could be utilized as a kinematic indicator for tectonic lineaments. The structure distribution in fact indicates the direction and sense of fault or thrust movement (slip vector). When fault rocks are related to polyphase tectonic lineaments, a system of structures develops for each different phase of movement and each phase may reactivate and rotate the structures of the previous systems; so, it is very difficult to reconstruct the slip vectors from discontinuities distribution. The analysis of striation on shear planes surfaces may be utilized in this situation. Analyses were carried out in the southernmost part or Giudicarie belt (Central Southern Alps), westwards of Garda Lake (Brescia region, Northern Italy). Striation data are very coherent both in a single outcrop and in many different outcrops related to the same system of lineaments. The striation analysis also indicate the relative chronologies of different slip phases. Slip vector data derived from fault rocks analysis were tested by other kinematic indicators such as major drag folds, mesoscale drag folds, pressure solution cleavage and principal directions of stress reconstructed by “diedre droutes” (ANGELIER and MECHLER, 1977) and Carey (CAREY, 1976) methods. The various analysis show very close results and indicate three different types of movement for the considered tectonic lineament: slip vectors from NE to SW (earliest), from N to S (intermediate) and from E to W (latest) have been recognized, that could be related to three deformative events having the same directions of the maximum principal stress.