STRUCTURES IN NATURAL SERPENTINITE GOUGES

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 mass ive gouge. Unlike natural clay-rich fault gouges, the cohesive microbr eccia domains only rarely show foliations. These structures dominated by discrete shears indicate a significant degree of localization of th e deformation at the scale of the gouge. The dominant gouge-forming me chanisms were fracturing of olivine and pyroxene grains along (serpent inized) 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. Tex tural similarities with experimentally formed gouges suggest that the Voltri fault zones were dominated by stick-slip sliding, hence that th ey may represent fossil seismogenic faults. A notable aspect of the go uge texture in the Voltri fault zones is a sigmoidal shape of the Ried el shears which bend into parallelism with the Y shears. It is argued that this geometry was largely controlled by local perturbations of th e stress field near heterogeneities on Y or P shears. The average spac ing between the R(1) shears increases linearly with 0.47 times the dis tance between bounding Y shears. This proportionality may reflect a ba lance between the cohesive strength of the gouge material and the shea r stresses acting on the bounding slip planes.