Effects of fluids on the seismicity of a fault. Analysis of exhumed structures in the Northern Adamello (Southern Alps, Italy)

A comparison between two major cataclastic faults within the northern composite Adamello batholith (Southern Alps) is attempted. The two investigated faults have strike-slip kinematics and are related to the slip along the adjacent Tonale Line, a segment of the Periadriatic Lineament bordering to the north the Adamello. The exhumed fault sections were active 30Ma ago at 9-11km depth and 250-300°C ambient temperature. The Gole Larghe Fault Zone (GLFZ) has an E-W strike and cut across the Avio tonalites. The Passo Cercen Fault Zone (PCFZ) is located, a few kilometres north of GLFZ, within the Presanella pluton. Both faults consist of a swarm of subparallel green indurated cataclasites exploiting precursor sets of joints developed during the early stages of pluton cooling (PENNACCHIONI et alii, 2006); pseudotachylytes (frictional melts formed during coseismic slip on fault planes) are abundant along the PCFZ, but almost absent along the PCFZ. The field work arid the laboratory (geochemical and microstructural) analyses have highlighted the following main differences between the two faults: (i) Orientation: the PCFZ has direction N130°, almost parallel to the direction of the regional σ1 during the faulting; the GLFZ has orientation N105° and forms an angle of about 30° with σ1. (ii) Fault rocks: a) The GLFZ includes abundant pseudotachylytes, which are very rare along the PCFZ; b) Within the PCFZ, cataclasites are associated to diffuse macroscopic veins of epidote + Kfeldspar + quartz, which are rare along the GLFZ. Our hypothesis is that both faults were seismic (small amount of pseudotachylytes are present within the PCFZ), but processes inhibiting frictional melting were active along the PCFZ. The different orientation in the regional stress field determines a different component of normal stress on fault planes affecting the degree of fluid infiltration along the two fault zones. The orientation of the PCFZ to the regional stress field is favourable to exploitation of the precursor joints as hybrid mode of fractures. Hybrid shear-extensional fractures developing at depth of 9-11km imply high pore pressure and low differential stress; both factors act reducing the effective normal stress, and thus the shear strength along the fault planes. In condition of low shear strength, frictional heating is reduced; furthermore, fluid-rich conditions trigger processes such as fluid pressurization that inhibit frictional melting.