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

Abstract

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 temperatures 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 protocataclastic 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 networks spatially associated with the sub-layers suggest fluid involvement in faulting. The ultracataclastic sub-layers preserve compelling microstructural evidence of fluidization, and also contain 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 produced 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 ultracataclastic sub-layers and formation of clast-cortex grains within the principal slip zone occurred at high strain rates during propagation of seismic ruptures whereas development of an overprinting CPO occurred by intergranular pressure solution during post-seismic 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

Hydrogeological properties of fault zones in a karstified carbonate aquifer (Northern Calcareous Alps, Austria)

“…These show grain-size reduction by comminution (producing high volumes of fine-grained matrix), shear fracturing and chipping, resembling the "mature" cataclasite fabric described by Billi (2010). Cataclasites type 2 are always very cohesive rocks, with complete cementation or eventually recrystallization (Smith et al 2011) of the finegrained matrix. Cataclasites type 1 show only localized portions of fine-grained matrix, indicating that these rocks are formed by (intra-granular) extensional fracturing and in-situ grain-size reduction by crushing of entire intermediate components with increasing deformation (Schröckenfuchs et al 2015).…”

Episodic Dissolution, Precipitation, and Slip along the Heart Mountain Detachment, Wyoming

“…Banded grains, also referred to as clast-cortex grains, have been described along rooted faults, including the carbonate-hosted Tre Monti fault in Italy (Smith et al 2011). This texture was proposed to be a potential indicator of rapid slip, in part because of the similarity to "armoured carbonate grains found within the basal detachment horizons of catastrophic landslides," with the citations referring solely to studies of the Heart Mountain basal layer (Smith et al 2011(Smith et al , p. 2386.…”

“…This texture was proposed to be a potential indicator of rapid slip, in part because of the similarity to "armoured carbonate grains found within the basal detachment horizons of catastrophic landslides," with the citations referring solely to studies of the Heart Mountain basal layer (Smith et al 2011(Smith et al , p. 2386. These authors wanted experimental validation before using this texture as evidence of rapid slip.…”

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

“…Boullier et al (2009) suppose that the presence of CCAs in TCDP samples indicate that the gouge was fluidized as a result of frictional heating and thermal pressurization. Smith et al (2011) assume a syntectonic origin of the CCAs by localized fluidization at seismic slip velocities within the principal slip zone. On the other hand, rotary shear experiments from Han and Hirose (2012) produced CCAs at slip rates, which are considerably slower than seismic slip rates.…”