The effects of fault orientation and fluid infiltration on fault rock assemblages at seismogenic depths

The factors controlling the development of different types of fault rock assemblages and, more specifically, the formation of friction melts are still not fully understood. In this study we compared two exhumed strike-slip faults in the Adamello batholith (Southern Alps): the Gole Larghe and the Passo Cercen fault zones, active at 9-11 km depth and temperatures of 250-300 °C. Each fault zone consists of hundreds of sub-parallel strands exploiting pre-existing joints. The Gole Larghe fault strikes N105 ± 5° and is dextral; the fault rocks are cataclasites and widespread, centimetre-thick pseudotachylytes. The Passo Cercen fault strikes on average N130° and is formed by multiple fault horizons: fault segments striking N105°-N130° are mainly dextral, whereas faults striking N135°-N140° are mainly sinistral. Microstructural, mineralogical and geochemical investigations show that the fault rocks are cataclasites associated with thick epidote + K-feldspar + quartz veins and rare, millimetre-thick pseudotachylytes. Field evidence suggests that in both fault zones, the direction of the maximum horizontal stress σ1 was N135°. The Gole Larghe fault strikes at about 30° to σ1 and is favourably oriented for reactivation. By contrast, the Passo Cercen fault strikes at low angles to σ1 and is unfavourably oriented for reactivation, therefore requiring the development of high pore pressures, as suggested by the occurrence of extensive epidote veining and hydraulic breccias. It is proposed that frictional melting in the Passo Cercen fault zone was inhibited by the development of high pore pressures and low effective normal stresses.