Fault-related carbonate rocks and earthquake indicators: recent advances and future trends

A major problem in seismology and structural geology is the lack, at least apparently, of clear records of seismic fault slip. Some geologists even suggested that, except for pseudotachylytes, earthquakes do not leave detectable geological indicators along faults. Discovering new geological indicators of seismic fault slip is therefore a challenging but also a very important target to advance the knowledge of how, where, when, and why earthquakes occur along faults. High-velocity weakening of faults may drive fault motion during large earthquakes. Laboratory experiments on simulated faults in Carrara marble performed at seismic slip rates (about 1 m/s) showed that thermal decomposition of calcite due to frictional heating induces pronounced fault weakening with very low friction coefficients (about 0.1 instead of a typical rock friction coefficient of 0.7). The ultra-low friction appears to be associated with the flash heating on the ultrafine products of calcite decomposition. It follows that thermal decomposition may be an important process for the dynamic weakening of carbonate-bearing faults and the products of calcite thermal decomposition (i.e. ultrafine particles and decomposition-related minerals) may be clear indicators of seismic slip in exhumed faults. Energy budget calculations predict seismic slip to be localized in ultra-thin slipping zones, compared to crystalline rocks. This research field is among the most promising ones in seismology and structural geology and recent results from laboratory simulations require proper validation in natural examples. Other likely seismic slip indicators are fault zone-related pulverized rocks, consisting of mechanically-pulverized rocks lacking in significant shear and preserving most of their original fabric. Pulverized rocks such as those recently found in the damage zone of active faults (i.e., the Hartebeestfontein mine in South Africa and the San Andreas Fault in California) are still poorly documented and studied. Further field and experimental studies are, therefore, required to establish the diffusion of these fault zone rocks, the mechanism of production, and whether they can be commonly considered as indicators of ancient earthquakes. Carbonate rocks usually occur in the upper portion of the crust and are often known as hypocenters of shallow earthquakes. For this reason, exposures of exhumed faults in carbonate rocks are a promising target to study earthquake rupture dynamics in the shallow crust. Faults exposed in carbonate rocks from three sites of central Italy (i.e. Mattinata, Sperlonga, and Venere sites) and one site in northwestern England (i.e. the Dent Fault) are here indicated as potentially suitable to research earthquake indicators similar to those mentioned above (i.e. mineralogical thermal decomposition and pulverized rocks).