Phyllosilicate injection along extensional carbonate-hosted faults and implications for co-seismic slip propagation. Case studies from the central Apennines, Italy

We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate hosted extensional faults from the seismically-active domain of the central Apennines, Italy. The shallow portion of this domain is characterized by a sedimentary succession consisting of ~5-6 km thick massive carbonate deposits overlain by ~2 km thick phyllosilicate-rich deposits (marls and siliciclastic sandstones). We show that the phyllosilicates observed within the studied carbonate-hosted faults derived from the overlying phyllosilicate-rich sedimentary deposits and were involved in the faulting processes. We infer that, during fault zone evolution, the phyllosilicates downward injected into pull-aparts (i.e., dilational jogs) that were generated along staircase extensional faults. With further displacement accumulation, the clayey material was smeared and concentrated into localized layers along the carbonate-hosted fault surfaces. These layers are usually thin (a few centimeters to decimeters thick), but can reach also a few meters in thickness. We suggest that, even in tectonic settings dominated by high frictional strength rocks (e.g., carbonates), localized layers enriched in weak phyllosilicates can occur along shallow fault surfaces thus reducing the expected fault strength during earthquakes, possibly promoting co-seismic slip propagation up to the Earth's surface.