Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy) STEVEN A. F. SMITH, 1 ANDREA BILLI, 2 GIULIO DI TORO, 1,3 and RICHARD SPIESS 3 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 tempera- tures are 100°C. 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. Key words: Slip zones, limestone, localization, clast-cortex grains, earthquakes. 1. Introduction A range of geological and geophysical observa- tions from both active and exhumed fault zones suggest that the bulk of co-seismic displacement during individual earthquake ruptures is accommo- dated within highly localized slip zones less than a few centimetres thick, with some evidence of extreme localization within zones millimetres or less in thickness (CHESTER and CHESTER, 1998; CHESTER et al., 1993;POWER and TULLIS, 1989;SIBSON, 2003; WIBBERLEY and SHIMAMOTO, 2003). The slip zones are normally found within a surrounding fault core tens of centimetres to metres thick containing tabular zones of well-developed fault rock materials often associated with syn-tectonic veining and mineralisa- tion (AGOSTA and KIRSCHNER, 2003;BILLI et al., 2003; CAINE et al., 1996). Surrounding the fault core is a damage zone of fractured host rock that may contain secondary faults but in general does not accommo- date high shear strains (CAINE et al., 1996;CHESTER and CHESTER, 1998;CHESTER et al., 2004). Although this picture of fault zone structure is primarily based on observations made in surface exposures, mines, and boreholes, useful constraints on the broad struc- ture of fault zones can be gained using a range of geophysical imaging techniques (BAKUN et al., 2005, COCHRAN et al., 2009,LI et al., 1998,UNSWORTH and BEDROSIAN, 2004). Geophysical methods, however, 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV), 605 Via di Vigna Murata, 00178 Rome, Italy. E-mail: steven.smith@ingv.it 2 Istituto di Geologia Ambientale e Geoingegneria, CNR, Via Salaria km 29.3, 00015 Monterotondo (Rome), Italy. 3 Dipartimento di Geoscienze, Universita ` degli Studi di Padova, 1 Via Giotto, 35137 Padova, Italy. Pure Appl. Geophys. 168 (2011), 2365–2393 Ó 2011 Springer Basel AG DOI 10.1007/s00024-011-0267-5 Pure and Applied Geophysics