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

The transformation of water-rich smectite clay minerals into relatively anhydrous illite is a common reaction in sedimentary basins. It is commonly thought to be driven by temperature increase with increasing burial depth. This mineral transformation is also observed in the gouge of large faults, and because it releases bound water from smectite, it is thought to be a key control in hydrologic and mechanical processes at subduction complexes. In this work, the distribution of smectite and illite within a large fault zone of the Southern Apennines of Italy is analysed at scales varying from outcrop to sub millimetre. X-ray diffraction analyses indicate a direct link between the darkening in colour of the sheared mudstone and the illitization of smectite. The heterogeneous distribution of illite and smectite within this shear zone was studied by integrating outcrop-scale observations with a peeling technique that allows microscope and SEM observations to be made over areas in excess of 60 cm2. The observed patterns of smectite illitization suggest that temperature is not the primary cause of the mineral transformation inside the fault. The distribution of illite along planes of the P foliation and in the infill of shears indicates that shear stress facilitates the mineral transformation.