Silica gel in a fault slip surface: Field evidence for palaeo-earthquakes?

High-velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces. The identification of frictional silica gel products in palaeo-seismic faults is, however, problematic, because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation. The importance of gel formation on natural faults is therefore unknown. Here, we report a structurally distinct and semi-continuous, 0.5–10 mm thick layer of microcrystalline quartz along a major carbonate-hosted fault, the Olive fault, in the Naukluft Nappe Complex, Namibia. The quartz layer is distinguished by flow banding-like textures and unusual cathodoluminescence characteristics. The layer consists of ∼2–20 μm hexagonal quartz crystals, which include distinct, crystalline, pore-bearing micro-to nano-spheres, separated by pore geometries indicative of volumetric contraction, and with grain boundaries enriched in aluminium. We interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel. Because it is found in a carbonate-hosted fault and crosscuts lithological layering, the silica source is not from comminution of local wall rocks. Rather, the gel likely formed from reshear of a quartz-coated fault surface, or incremental shear slip associated with precipitation of silica driven by co-seismic pressure drops. This example of fault-related silica gel may have formed by a different mechanism than the gels produced in high-velocity friction experiments, but once formed, may have comparable rheological effects.