A NEW EXPLANATION FOR SHEETING JOINTS BASED ON FRACTURE MECHANICS

Sheeting joints have attracted attention for centuries owing to their broad distribution, spectacular form, and importance in quarrying, slope stability, and hydrology. Sheeting joints are discernibly curved fractures that open at shallow depths and that form subparallel to the topographic surface. They share many geometric, textural, and kinematic features with other joints. Where sheeting joints are geologically young, the surface-parallel compressive stresses near the topographic surface characteristically are large, typically several MPa or greater. Sheeting joints typically are best developed beneath domes, ridges, and saddles, but they also are reported beneath valleys or bowls. Several mechanisms that account for these associations have been explored. The most popular explanation, erosion of overburden, predicts sheeting joints should form where they don’t, and fails to predict well where they do form. Principles of fracture mechanics, together with the mechanical effects of a curved topographic surface experiencing a surface-parallel compression, provide a theoretical framework that accounts for the key characteristics of sheeting joints. A compression along a convex topographic surface induces a tension perpendicular to the surface at shallow depths and can cause fractures to form parallel to the surface. In some places, this combination alone could overcome the weight of overburden to open sheeting joints. Plausible distributions of water pressure in sheeting joints also would help open and drive sheeting joints beneath valleys, saddles, and bowls. Thermal stresses help sheeting joints develop, but only very near the ground surface. Sheeting joints apparently reflect an intricate fracture process primarily involving the shape, slope, and scale of the topography; the regional horizontal stresses; the effect of gravity; and groundwater pressure. A better understanding of how these effects operate in three dimensions through time would illuminate how sheeting joints grow and would provide insight into the effects of sheeting joints on landscapes, slope stability, and fluid flow.