Phyllosilicate microlayers and injections along the Mattinata fault: implications for weakening processes in carbonate-hosted faults

Micrometer-thick phyllosilicate films localized along fault slip surfaces can substantially reduce friction, although their occurrence is unexpected in clay-poor carbonate platform rocks. We investigate how a major carbonate-hosted, seismogenic strike-slip fault can develop mechanically weak slip interfaces and experience dynamic weakening despite the overall strength of its carbonate host rock. We combine structural mapping and drone-based virtual outcrop analysis with microstructural, mineralogical, and geochemical analyses, including optical microscopy, SEM/FESEM-EDS, EPMA, and XRD, along the Mattinata Fault (Gargano Promontory, southern Italy). We identify illite- and kaolinite-rich microlayers, locally up to ∼15 µm thick, developed along selected slip surfaces. These layers occur together with ultracataclasites and phyllosilicate- bearing injection veins that branch from the slip surfaces into otherwise nearly pure calcite host rocks. The observed microtextures, vein geometries, and mineral assemblages suggest that clay minerals were externally sourced and emplaced during transient episodes of fluid pressurization associated with coseismic slip, pointing to episodic open-system conditions within the fault zone. Since high-velocity friction experiments demonstrate that even very small amounts of phyllosilicates, when organized into thin films, can promote dynamic weakening, our results provide field evidence for rapid fault weakening in carbonate-hosted foreland settings. More broadly, our observations suggest that carbonate-hosted seismogenic faults may locally develop cryptic clay-rich slip interfaces through transient fluid-assisted processes, with potential implications for fault weakening and rupture propagation in clay-poor carbonate fault zones.