Evaluating a Crustal-Scale Rheological Equation Using Stress Measurements and Mechanical Property Estimations from Discrete Fracture Network Models: A Case Study of Fennoscandia

Crystalline rocks can be highly fractured at shallow depths due to tectonic stresses and topographic effects. This fracturing largely controls the rheological properties of the rock in a way that depends on the properties of the fracture networks, including the distribution of fracture sizes and orientations, the distribution of sealing minerals in the fracture network, and the normal and shear stiffnesses. Better knowledge of these properties at the site scale would improve both mechanical and hydrological models, essential for risk assessment or resource management. To this end, we combine models of mechanical properties based on Discrete Fracture Network (DFN) properties (Davy et al., 2018), stress measurements, and strain deduced from large-scale lithosphere models. The consistency of this rheological equation provides a basis for discussing the hypothesis used to infer the mechanical properties of the rock mass.

We apply this methodology to the Forsmark site in Sweden, which is being studied as the future location for a deep nuclear waste repository, supported by a comprehensive database of fractures. Fracture network models have been developed based on core and outcrop observations. Fracture density decreases with depth, showing a significant reduction down to 300-400 meters, followed by a more gradual decline. Fracture stiffnesses and matrix elasticity have been extensively measured in the laboratory. As a rule for upscaling, we assume that openness and stiffness are influenced by fracture size and normal stress. The complete 3D compliance matrix of the effective properties is calculated, facilitating the identification of the primary anisotropy planes of the rock mass.

Our findings indicate that, under specific conditions, the closure of the crustal-scale rheological equation can be guaranteed, i.e. the combination of DFN-inferred mechanical properties and measured stresses gives reasonable deformations, compatible with most lithospheric models. Using additional glaciation models, we then infer paleostresses and paleostrains during the Last Glacial Maximum, 20,000-10,000 years ago.

 

References:
Davy, P., Darcel, C., Le Goc, R., Mas Ivars, D., 2018. Elastic Properties of Fractured Rock Masses With Frictional Properties and Power Law Fracture Size Distributions. JGR Solid Earth 123, 6521–6539. https://doi.org/10.1029/2017JB015329