Probing active deformation - Fault healing through the lens of 4D operando imaging

Direct observations of geological processes are often limited by available observation time, the spatial resolution of imaging techniques or the accessibility of active sites. These limitations also apply to fault healing, whereby faults progressively regain strength throughout the interseismic phase of the earthquake cycle. Here, conventional approaches either capture a static snapshot of the final microstructure in exhumed natural fault rocks or focus on the bulk mechanical behaviour through slide–hold–slide or direct shear experiments. However, these approaches generally fail to resolve the dynamic evolution of the microstructural record, and the associated chemo-mechanical feedback that controls a rock’s hydraulic properties. To overcome these limitations and constrain the spatiotemporal coupling between mechanical, chemical, and hydraulic processes in healing fault gouges, we conducted a series of direct shear experiments on analogue fault gouges composed of a quartz–hemihydrate mixture. We then monitored their microstructural evolution using operando 4D synchrotron-based X-ray CT imaging.

Our experiments, performed at constant shear rates of 0.3–1 µm/s, were designed to mimic gouge-rich faults in the uppermost continental crust during the interseismic phase. In the presence of a reactive pore fluid, we simulated chemical fault healing through dissolution-reprecipitation and cementation, which are associated with the hydration reaction of CaSO₄ hemihydrate to gypsum. In our deforming samples, these time-dependent healing processes compete with mechanical weakening processes, such as frictional granular flow.

Our novel approach combines an innovative experimental setup [1, 2] with high-resolution 4D imaging and advanced image analysis techniques, including digital volume correlation (DVC). In this contribution we discuss the benefits of integrating micromechanical data with high-resolution 4D imaging by linking active deformation mechanisms to the evolving mechanical and hydraulic response of the simulated fault gouge. Further, we demonstrate a shear-rate-dependent competition between time-dependent healing processes and mechanical weakening.   

 

[1] Freitas, D. et al. (2024): Heitt Mjölnir: a heated miniature triaxial apparatus for 4D synchrotron microtomography. Journal of Synchrotron Radiation 31, 150-161. doi.org/10.1107/S1600577523009876 

[2] Jayawickrama, E. et al. (2026): Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619.