Faults inside out: 4DμCT on direct shear dehydrating gypsum experiments

The dehydration of rocks, such as gypsum, is a critical process influencing plate tectonics and fault zone dynamics. Gypsum dehydration, serving as a model for serpentine dehydration, involves complex hydraulic, mechanical, and chemical (HMC) interactions that remain poorly understood under shear stress. Our study investigates how dehydration reactions and microstructural developments relate to macro-scale frictional responses, providing new insights into the conditions leading to mechanical instabilities and shear localisation.

To study the couplings between shear stress, strain and the microstructures formed during the dehydration of gypsum, we have conducted a series of direct shear experiments on Volterra Alabaster slabs and 99% pure gypsum powder. We performed the tests in a new direct shear setup of the x-ray transparent Heitt Mjölnir Cell (Freitas et al. 2024) at the ID19 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). This new setup allows fast 4D microtomography (4DμCT) to record the time evolution of the microstructure. In several 4D operando experiments, the samples were loaded with 10 - 25 MPa confining pressure and 2 MPa fluid pressure while allowing initial thermal equilibration of the system at 60 °C. Following equilibration, temperature was increased to 115 - 125 °C to start the dehydration of the gypsum. Simultaneously, a constant axial displacement rate of 0.2 - 0.3 µm/s was applied, which produced shear strain within the sample. Pore pressure oscillations were applied to monitor changes in hydraulic permeability across the samples.

The 4DµCT datasets allow good discretization of the three phases of interest (gypsum, hemihydrate and pore space) on the relevant microscale. Our ongoing analyses of the various 4DµCT datasets focus on 1) digital volume correlation (DVC) to quantify the deformation in the sample on the grain scale, 2) the calculation of reaction rates for dehydration and 3) the quantification of grain-scale permeability during shearing and reaction. Initial analyses show well-resolved shear structures forming throughout the different tests (e.g., boundary shears, compaction bands, Riedel-shears). By quantifying the reactions and the deformation over time, we identify the minor and major processes controlling the development of the microstructure. These processes are then related to changes in friction and transport parameters.  In future experiments, we will focus on different lithologies to further understand the effects of fault gouge composition and grain geometry as well as the analysis of rate-and-state friction (RSF) for the quantification of sliding stability. Our data demonstrate the potential that 4D operando direct shear experiments hold for the study of friction processes in fault zones.