Illite-smectite patterns in sheared Pleistocene mudstones of the Southern Apennines and their implications regarding the process of illitization: A multiscale analysis

► Deformed mudstones in the shear zone are darker than the wallrock mudstones. ► Smectite disappearance and replacement with illite is associated to darkening. ► Illitization occurs along planes of P-foliation and along shears. ► Temperature or fluid chemistry is not the primary cause of the transformation. ► We propose smectite illitization is driven by shear stress concentration.