The relationship between evolution of fluid chemistry and the style of brittle deformation: examples from the Northern Apennines (Italy)

Abstract

Vein systems related to faults and thrusts are often filled by carbonate phases that carry a chemical composition reflecting the fluids once permeating the rocks. Evidence of important fluid circulation is documented at the northwestern and southern edges of the Northern Apennines in the Monferrato and Sabina regions, respectively. In veins linked to transpressive faults, transtensional faults and thrusts, the compositional ranges are the following: in Monferrato −11.2<δ13C<−0.1;−7.8<δ18O<−0.4;0.708017<87Sr/86Sr<0.71018; 0.14<Sr/Ca×103<16.4; in Sabina −7.3<δ13C<3.6;−6.0<δ18O<−0.2;0.70736<87Sr/86Sr<0.709615; 0.30<Sr/Ca×103<2.44. The geochemical data indicate that in Monferrato the fluids circulation generally evolved towards open system conditions characterized by the presence of fluids of superficial origin geochemically similar to present spring waters; the exception is found in some confined systems corresponding to the main compressive structures. In Sabina region the host rock strongly influences the composition of vein fillings under confined system condition, common to both transpressive faults and thrusts. Open system conditions develop only under a clearly extensional tectonic regime in the more recent fracture systems. The geochemical evolution of tectonically controlled hydrologic systems seems thus to be related not only to the composition of the fluids and rocks, but also to the deformation style.