Supporting Information for Weakening processes in carbonate-hosted faults: phyllosilicate microlayers and injections along the Mattinata fault (Southern Italy)

Micrometric phyllosilicate films along fault slip surfaces can drastically reduce friction, yet they are unexpected in clay-poor carbonate platforms. We enquire how a major carbonate-hosted, seismogenic strike-slip fault can develop weak slip interfaces and undergo dynamic weakening despite an intrinsically strong protolith. Along the Mattinata Fault (Gargano Promontory, southern Italy), we integrate structural mapping and drone-based virtual outcrops with microstructural, mineralogical, and geochemical analyses (optical microscopy, SEM/FESEM-EDS, EPMA, and XRD). We document micrometric illite- and kaolinite-rich layers (locally up to ~15 µm thick) localized on some slip surfaces, together with ultracataclasites and phyllosilicate-filled injection veins that emanate from these surfaces into otherwise nearly pure calcite host rocks, where clay minerals are not detected even at trace levels. Microtextures, vein geometries, and mineral assemblages suggest that clays were derived from outside the carbonate host rock and introduced by transient fluid pressurization during coseismic slip, implying transient open-system behavior of the fault zone. Because high-velocity experiments show that even minimal amounts of phyllosilicates concentrated in thin films can promote strong dynamic weakening, our observations provide a field-based mechanism for rapid weakening and slip localization in carbonate-hosted foreland faults. More broadly, we show that carbonate seismogenic faults may be (unexpectedly and cryptically) clay-bearing, with important consequences for rupture propagation and for laboratory and numerical models that commonly assume closed, clay-free systems.