Slip zone structure and processes in seismogenic carbonate faults

High velocity rotary shear experiments performed at seismic slip velocities (>1 m/s) have shown that experimental faults are weak; with increasing displacement, friction coefficient values decrease from Byerlee's values (μ = 0.6-0.85) to values of ~0.1. In carbonate rocks, experimental studies have shown that fault lubrication is due to the operation of multiple dynamic weakening mechanisms (e.g., flash heating, thermal pressurization, nanoparticle lubrication), which are thermally activated due to the frictional heat generated along localized slip surfaces during rapid slip. This study has set out to investigate whether evidence for the operation of these weakening mechanisms can be found in naturally occurring carbonate fault zones. Field studies were carried out on the active Gubbio fault zone (1984, Mw = 5.6) in the northern Apennines of Italy. Jurassic-Oligocene carbonates in the footwall are heavily deformed within a fault core of ~15 m thickness, which contains a number of very well exposed, highly localized principal slip surfaces (PSSs). Fault rocks are predominantly breccias and foliated cataclasites. Microstructural analyses of the PSSs reveal that slip is localized within very narrow principal slip zones (PSZs), ranging from 10-85 μm in thickness, with sub-millimetre scale asperities. PSZs are composed of very fine-grained, orange-brown ultracataclasite gouge containing a high proportion of nano-sized particles. The ultracataclasite commonly displays a foliated texture and sub-micron scale zones of extreme shear localization. A broader slip zone, up to 1.5 mm wide and containing multiple slip surfaces, is associated with the most evolved PSSs; it is located on the opposite side of the PSS to the PSZ. Here, the host rock material is heavily fractured, abraded and altered, sometimes with an ultracataclasite matrix. The surrounding wall rock often appears to have a porous texture, and calcite crystals within the slip zone have altered rims with lobate textures, both of which may be indicative of thermally activated chemical reactions. Occasionally, mantled clasts are observed; these consist of a central, sub-rounded monomineralic clast of calcite, or a polymineralic clast of both calcite and clay particles, enclosed by a cortex of ultracataclasite. These are features which are thought to be a product of thermal pressurization processes operating in the slip zone. These microstructures are compared to those in experimentally deformed dolomite gouges, and the slip zone features are found to be strikingly similar. It is clear that as slip accumulates along PSSs, well-developed PSZs are formed with well-defined foliations and R- and Y-shears, indicating progressive localization of deformation. The similarities between the two sets of samples implies that the dynamic weakening mechanisms known to occur in experimental carbonate slip zones are indeed likely to be in operation in their naturally occurring counterparts. Specifically, slip localization in the fault core may be associated with frictional heating; slip zone roughness may be associated with flash heating; mantled clasts may be attributed to thermal pressurization; and nanoparticles may be associated with nanoparticle lubrication.