Structures in natural serpentinite gouges

Abstract

Serpentinite gouges from cataclastic fault zones in the Voltri Massif (NW Italy) are dominated by few discrete, boundary-parallel ( Y) shears and oblique synthetic Riedel ( R 1) shears separating domains of massive gouge. Unlike natural clay-rich fault gouges, the cohesive microbreccia domains only rarely show foliations. These structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge. The dominant gouge-forming mechanisms were fracturing of olivine and pyroxene grains along (serpentinized) grain boundaries, and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation. Syn-tectonic serpentinization added to the weakening of fragments. Textural similarities with experimentally formed gouges suggest that the Voltri fault zones were dominated by stick-slip sliding, hence that they may represent fossil seismogenic faults. A notable aspect of the gouge texture in the Voltri fault zones is a sigmoidal shape of the Riedel shears which bend into parallelism with the Y shears. It is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on Y or P shears. The average spacing between the R 1 shears increases linearly with 0.47 times the distance between bounding Y shears. This proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes.