Clay-bearing Gouge Research Articles

Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy)

Earthquakes in central Italy, and in other areas worldwide, often nucleate within and rupture through carbonates in the upper crust. During individual earthquake ruptures, most fault displacement is thought to be accommodated by thin principal slip zones. This study presents detailed microstructural observations of the slip zones of the seismically active Tre Monti normal fault zone. All of the slip zones cut limestone, and geological constraints indicate exhumation from \2 km depth, where ambient tempera- tures are � 100C. Scanning electron microscope observations suggest that the slip zones are composed of 100% calcite. The slip zones of secondary faults in the damage zone contain protocata- clastic and cataclastic fabrics that are cross-cut by systematic fracture networks and stylolite dissolution surfaces. The slip zone of the principal fault has much more microstructural complexity, and contains a 2-10 mm thick ultracataclasite that lies immediately beneath the principal slip surface. The ultracataclasite itself is internally zoned; 200-300 lm-thick ultracataclastic sub-layers record extreme localization of slip. Syn-tectonic calcite vein net- works spatially associated with the sub-layers suggest fluid involvement in faulting. The ultracataclastic sub-layers preserve compelling microstructural evidence of fluidization, and also con- tain peculiar rounded grains consisting of a central (often angular) clast wrapped by a laminated outer cortex of ultra-fine-grained calcite. These ''clast-cortex grains'' closely resemble those pro- duced during layer fluidization in other settings, including the basal detachments of catastrophic landslides and saturated high-velocity friction experiments on clay-bearing gouges. An overprinting foliation is present in the slip zone of the principal fault, and electron backscatter diffraction analyses indicate the presence of a weak calcite crystallographic preferred orientation (CPO) in the fine-grained matrix. The calcite c-axes are systematically inclined in the direction of shear. We suggest that fluidization of ultra- cataclastic sub-layers and formation of clast-cortex grains within the principal slip zone occurred at high strain rates during propa- gation of seismic ruptures whereas development of an overprinting CPO occurred by intergranular pressure solution during post-seis- mic creep. Further work is required to document the range of microstructures in localized slip zones that cross-cut different lithologies, and to compare natural slip zone microstructures with those produced in controlled deformation experiments.

Effect of phyllosilicates on fluid-assisted healing of gouge-bearing faults

On the basis of both experimental and field studies, it is widely accepted that solution transfer processes are important in controlling fault strength recovery (healing) in the hydrothermal reaches of the seismogenic zone. Fluid-assisted healing of fault gouge may be strongly influenced by phyllosilicate content and spatial arrangement, but little is known about such effects. To study these effects, we have performed slide–hold–slide experiments using simulated faults containing gouges of halite+kaolinite saturated with brine. In this system, solution transfer processes are extremely fast. The results show rapid healing in the case of pure halite gouge, whereas almost no such effects were observed in foliated, clay-bearing gouge under otherwise identical conditions. The degree of healing increased with increasing shear strain in monomineralic gouge, whereas the opposite was observed in clay-bearing gouge. The above effects are interpreted to be due to phyllosilicates inhibiting halite–halite grain contact healing processes, an effect which becomes more prominent as on-going shear deformation leads to foliation development. The results raise questions regarding recent application of laboratory-derived healing data for monomineralic gouge to natural fault rocks, since these generally contain significant amounts of phyllosilicates and are often foliated.