Indentation pits: A product of incipient slip on joints with a mesotopography

The mechanism for structural damage during incipient slip on joints within the Melechov Granite, Czech Republic, changes with the misalignment of the joint's mesotopography, largely a plumose surface morphology. Prior to slip, the joint surfaces are well mated so that contact area is organized on a microscopic scale. During the first phase of slip, diffusion-mass transfer is the active deformation mechanism between the sliding surfaces of the joints, as indicated by the extensive growth of crystal-fibre lineations characteristic of slickenside surfaces. After slip of the order of 1 cm or more, the mesotopography becomes mismatched and the contact area is reorganized to form indentation pits aligned on the ridges of hackle plumes. Indentation pits, that are testimony to a brittle process, are generated by the excavation of Hertzian ring cracks that propagate under contact loading of a brittle substrate. The depth of the indentation pits increases with contact width, suggesting that indentation creep is active. Following indentation along Hertzian ring cracks the slip mechanism transforms to a frictional abrasion. The distribution of indentation track lengths is consistent with laboratory wear grooves generated during earthquake-like stick-slip sliding. The elliptical shape of the indentation pits indicates a gradual decrease in contact area, a process that is consistent with a slip-weakening mechanism during a stick-slip cycle.