Mixed-Mode Strain Localization between Interacting Fault Segments

Abstract

Analyzing how fracture networks develop from preexisting isolated fault segments may provide knowledge of the preparation process leading to fault growth and dynamic rupture. We reproduce this process experimentally in granite core samples that contain preexisting notches. We characterize the microphysical processes of damage and fault growth in two cylindrical Westerly granite samples (10 mm diameter, 20 mm height) that contain two parallel notches oriented at 30o with respect to the axis of the cylinder, that separate an intact rock bridge. We performed triaxial compression experiments at room temperature with a constant confining pressure of 20 MPa. We image microfracture development between the notches using dynamic in situ X-ray tomography at the European Synchrotron Radiation Facility. During compressive loading, we acquired a series of tomograms of the samples, from which we extracted and quantified the statistical properties of the fractures. We analyzed the development of rock damage and the evolution of fractures oriented at 0o-17o (extensile) and 17o-32o (shear) as the rock approaches failure. We also calculated digital volume correlation to elucidate the strain localization process in the deforming rock. Our results indicate that immediately preceding failure, shear fractures dominate the fracture networks. The rock bridge between the notches becomes damaged with a damage rate and a fracture rate diverging as the rock approaches macroscopic failure. DVC analysis showed that the deformation process is mixed-mode, accommodated by dilation and shear strain. The shear strains are more localized than the dilation. Furthermore, the data reveal that regions with high dilative strain host both extensile and shear fractures.

Keywords: Dynamic X-ray tomography, Fault growth, Westerly granite, Rock damage