Exploring fault rocks at the nanoscale

This paper aims to demonstrate the potential role of micro/nanoscale investigations in the study of fault rocks, given that a multiscale and multidisciplinary approach linking structural geology, petrology and mineralogy from the outcrop to the unit-cell scale is essential to the comprehensive characterization of geological materials. To explore fault rocks at the sub-micrometer scale, a fundamental contribution is provided by the Transmission Electron Microscope (TEM), which offers a real-time combination of highresolution images, diffraction patterns, and chemical data for extremely small volumes, providing accurate mineralogical and nanostructural determinations. This paper focuses on a few selected examples, that are particularly appropriate for TEM investigation, in particular: 1) grain-size determinations on ultrafine-grained fault rocks; 2) detection and characterization of poorly crystalline minerals (e.g., clays) and amorphous phases within fault rocks; 3) identification of deformation-induced mineral reactions occurring within fault zones (e.g., dolomite decarbonation and serpentine dehydration); and 4) observations of recurrent deformation nanotextures in phyllosilicates. TEM investigation can provide an unexpected amount of data, much of which cannot be obtained by conventional techniques, and it can therefore significantly contribute to an understanding of the physico-chemical conditions for faulting and fault mechanics.