Nanotopographic characterization of microfractures in rocks by Atomic Force Microscopy

Abstract The study of microfractures is one of the keys to understand a variety of geological issues as diverse as seismic rupturing mechanisms, process zone characterization and evolution toward large-scale fracturing, characterization of reservoirs of geological fluids, or identification of microhabitats outside Earth. In earlier works, microcrack initiation and propagation in rocks has been observed at microscale. We test Atomic Force Microscopy (AFM) as a tool to retrieve nanoscale digital elevation models (nDTM) of rock surfaces. Basalt samples from two different geological environments are studied. In a basalt from Siberia, the topography of the granular material infilling of the crack can be accurately characterised, and the pristine crack shape retrieved, opening access to an estimate of the volume of the infilling material. In a basalt from Ethiopia, mixed-mode fracturing (opening-shear) is identified from nanofracture topography, as well as a textbook example of along-strike variations of fracture displacement. These observations demonstrate that fractures already initiate and propagate at nanoscale, and AFM is a powerful tool to conduct quantitative structural geology analyses with a resolution of ∼1 nm.