Joints and faults are two types of fractures generally recognized in rock. A popular model for fracture development presented in many college and graduate level textbooks on structural geology and tectonics is based on the Mohr-Coulomb failure criterion. The model suggests that the failure criterion for rock is continuous and parabolic in the tensile regime, describes the formation of both joints and faults, and predicts that hybrid fractures (i.e., fractures with characteristics intermediate to joints and faults) form under mixed compressive and tensile stress states. However, many questions remain concerning the mode of failure and mechanical behavior across the joint to fault transition because few experiments have been conducted under mixed stress states. As such, a fundamental question regarding fracture of rock is whether joints and faults represent end-members in a continuum of fracture types. Preliminary experiments using dog-bone shaped samples of Carrara marble extended under confining pressure in the rock deformation laboratory at Texas A&M University provide strong evidence for the existence of hybrid fractures. Fractured samples appear to display a continuous transition in macroscopic fracture orientation and surface morphology from classic joints to Coulomb faults. Fracture strength, however, does not appear consistent with a parabolic Mohr-Coulomb failure criterion.
The joint to fault transition in Carrara marble and Berea sandstone is being investigated through experimental rock deformation and petrofabric study of brittle failure under mixed tensile and compressive stress states. The experiments are being used to determine the exact form of the failure envelope from jointing to faulting. Acoustic emissions are monitored to determine relative timing and number of microfracture events leading up to macroscopic failure, and sequential strain experiments provide samples for petrofabric analysis of different stages of fracture development. Detailed petrofabric analysis using optical and scanning electron microscopy, and surface profilometry are used to characterize the morphology of the macroscopic fracture surface and determine the microscopic structure and mechanisms of deformation. The relative contribution of intragranular fracture, grain boundary fracture, cleavage, and twinning along the macroscopic fracture is being quantified. Specific goals of the project include testing the stepped-crack model for formation of hybrid fractures, developing a micromechanical model of fracture development consistent with experiment observations, and deriving a failure criterion for hybrid fracture.
