PRINCIPAL SLIP ZONES IN LIMESTONE: NATURAL AND EXPERIMENTAL EXAMPLES AND IMPLICATIONS FOR THE SEISMIC CYCLE

Many earthquakes in central Italy (e.g L’Aquila M_w 6.3, 6 April 2009), and in other areas worldwide, nucleate within and rupture through limestones. During individual earthquakes a majority of fault displacement is accommodated by thin principal slip zones (PSZs). We present microstructural observations of the PSZs of seismically active normal faults that cut limestones in central Italy, and compare these PSZs to experiments on layers of simulated calcite gouge using a recently-installed high velocity rotary shear apparatus at INGV, Rome. Geological constraints indicate that the natural PSZs are exhumed from <3km depth, where ambient temperatures are <100°C. SEM and XRD observations suggest that the PSZs are composed of c. 100% calcite. The PSZs contain a 2-10 mm thick, texturally-distinct ultracataclasite layer immediately adjacent to the slip surface. The ultracataclasite is itself internally zoned; 200-300μm-thick ultracataclastic sub-layers record extreme localization of slip, and some of the sub-layers contain microstructural evidence for fluidization. Peculiar rounded grains up to 3mm in diameter consisting of a central (often angular) clast wrapped by a laminated outer cortex of ultra-fine grained calcite are found in the ultracataclasite. These clast-cortex grains closely resemble: 1) accretionary lapilli in pyroclastic deposits; 2) ‘armoured’ grains found in the basal detachments of mega-landslides, and; 3) ‘clay-clast aggregates’ produced in saturated high-velocity friction experiments and found in some natural clay-bearing faults. We suggest that localized fluidization of ultracataclasite layers and formation of clast-cortex grains occurred at high strain rates during propagation of seismic ruptures. Preliminary experiments and SEM observations indicate that clast-cortex grains are formed in layers of calcite gouge (initial grain size <250µm) when they are sheared under wet conditions at a slip velocity of 6.5 m/s and a normal stress of 2MPa. Future experiments will cover a wide range of slip velocities (from 10 µm/s to 6.5m/s) and normal stresses (1-10 MPa) to investigate whether clast-cortex grains are formed only in a particular velocity/stress range and to address the question of whether the clast-cortex grains may be a diagnostic microstructure of seismic slip in carbonates.