A microstructural study of the extension-to-shear fracture transition in Carrara Marble

Triaxial extension experiments on Carrara Marble demonstrate that there is a continuous transition from extension to shear fracture on the basis of mechanical behavior, macroscopic fracture orientation and fracture morphology where hybrid fractures with extension and shear fracture characteristics are formed at the intermediate stress conditions. Extension fracture surfaces display discrete, highly reflective cleavage planes and shear fracture surfaces are covered with calcite gouge and display grooves and striations that are aligned parallel to slip. This study uses the fractured samples that were formed under triaxial extension experiments to characterize 1) microscopic surface features using scanning electron microscopy, 2) fracture surface morphology using laser profilometry, and 3) off-fracture damage using optical microscopy. These data are used to test the step-crack model of fracture development for the formation of hybrid fractures. Spectral analysis of the profiles demonstrates that microscopic roughness decreases gradually across the extension-to-shear fracture transition in both the orientations parallel and perpendicular to slip. However, macroscopic roughness gradually increases then decreases across the transition in the direction parallel to slip. The greatest macroscopic roughness occurs at the transition from extension fractures to tensile-hybrid fractures and is attributed to the presence of macroscopic steps in hybrid fractures surfaces. The treads of the steps in the hybrid fracture surfaces have characteristics of extension fracture surfaces and the risers have characteristics of shear fracture surfaces. The treads have a right-stepping left lateral geometry that is consistent with the step-crack model. Thin sections of hybrid fractures display systematically spaced, pinnate, microfractures that emanate from both sides of the macroscopic fracture surface. The pinnate fractures on both sides correlate across the macroscopic fracture, suggesting that they are precursory to the formation of the macroscopic fracture surface. The spacing to length ratio of the pinnate fractures and the macroscopic orientation of the fracture surface are also consistent the relationship dictated by the step-crack model of fault formation.