Clast-cortex aggregates in experimental and natural calcite-bearing fault zones

We investigated the formation mechanisms of rounded clast-cortex aggregates, a composite grain found in the slipping zones of faults hosted in calcite- and clay-rich rocks. The natural aggregates contain a central clast commonly made of host-rock fragments or reworked cataclasite from the slipping zone. The central clasts are surrounded by an outer cortex of calcite or clay grains a few mu m or less in size. In laboratory experiments on calcite gouges using two rotary-shear apparatus we investigated the dependence of clast-cortex aggregate formation on the applied slip rate, normal stress, total displacement and ambient humidity. Clast-cortex aggregates formed at all investigated slip rates (100 mu m/s to 1 m/s) but only at relatively low normal stresses (<= 5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that clast-cortex aggregates in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces.