Fibrous clay mineral authigenesis induced by fluid-rock interaction in the Galera fault zone (Betic Cordillera, SE Spain) and its influence on fault gouge frictional properties

This study presents a mineralogical and mechanical analysis of the clay-rich materials and structures identified in the Galera Fault Zone, southern Spain that formed as a consequence of active deformation processes affecting this seismically active region. Significant differences in clay mineral assemblages and chemical composition were identified in rocks from the Galera Fault Zone, through a series of analytical techniques including XRD, SEM, TEM and XRF. Three distinct mineral assemblages were identified: 1) wall-rock assemblages including white marls and dark lutite layers, with the latter also found in injection features. Their assemblage includes dolomite, gypsum, quartz, calcite and phyllosilicates 2) smectite- and palygorskite-rich fault gouges formed on materials from the upper part of the stratigraphic sequence at the NE area of the fault (Galera Village), and 3) sepiolite-rich gouges in areas of the lower part of the stratigraphic sequence at the central SW segment of the fault (Rambla de los Pilares). Fibrous clay-rich gouges were formed by hydrothermal alteration during periods of fluid-rock interaction that was concentrated in fault planes and fractures. Their mineralogy is dominated by authigenic Mg-rich fibrous clay minerals; sepiolite, precipitated directly from an Mg-rich fluid; and palygorskite as the product of the interaction of the fluid with the Al-rich host rock. Experimental data from frictional sliding experiments on these clay-rich fault gouges reveal strong differences in their mechanical properties. Towards the north-eastern areas of the fault, the smectite- and palygorskite-rich gouge has a low friction coefficient (0.17 wet and 0.59 under vacuum) and its values of the friction stability parameter (a–b) are always positive indicating a stable regime that could be related with creeping and stable-sliding processes. In the central-south-western segment, however, the gouge is rich in authigenic sepiolite and presents complete absence of smectite. The higher friction coefficient (0.47 wet and 0.68 under vacuum) and negative values of a–b for this gouge suggest a more neutral to unstable regime that could favour earthquake nucleation.