Microcracks in crystalline rock from 5.3‐km depth in the Michigan Basin

Laboratory velocity and differential strain analysis measurements indicate that the gabbroic core from 5310- and 5315-m depth beneath the center of the Michigan Basin contains few open microcracks. Approximate agreement between the in situ velocities and the intrinsic laboratory velocities suggests as one hypothesis that these rocks may be crack free in situ. Stress relief, which occurred when the core was brought from depth to the surface, may have created the open microcracks. The majority of the cracks have a closure pressure of 1450 bars, approximately equal to the in situ vertical stress. Open cleavage cracks are observed in chlorite by scanning electron microscopy. The apparent tensile nature of these cracks supports the stress relief hypothesis. Optical microscopy shows that numerous formerly open microcracks are now completely healed or sealed.