A Hertzian Quasi-Oval Fracture Model for Ring-Complexes

A new fracture model is applied to the problematics of structural development of ring-complexes. This model is derived from the Hertzian fracture theory. According to this concept the quasi-oval pattern of isochromatic bands obtained in glass when pressed by an indenter may be regarded as a visual analogy to the fracture system which develops in the crust when pressed (indented) upward by magma, and passes successively into a zone of horizontal tensional stresses and one of vertical tensional stresses. Common occurrences of fractures and dikes with inward, outward and vertical dips in close proximity, as well as related structural features, may be explained by this model as modified by various indentation and erosion histories. Various field cases are cited: the ring fracture zones of the Valles and Lake City are Hertzian fractures that have not been intruded. At Glen Cand Khibina, oblique indentations and fracture preceded the intrusions. It is postulated that saucer-shaped stratified intrusions are basically a consequence of intrusion into fractures prior to a completion of the indentation process. Nigerian, Irish and other ring-complexes demonstrate that at greater depths cone-fractures have lower dips and gradually approach a vertical trend at shallow depths, as expected from the Hertzian quasi-oval fracture model. A distinction is made between primary and secondary fractures.