A Study of Echelon Fracture Sets in the Killarney Igneous Complex, Killarney, Ontario

Abstract

<p> The igneous rocks of the Killarney Complex, Killarney, Ontario exhibit numerous echelon fracture sets near their contact with mylonites of the Grenville Front. The echelon fracture sets are younger than the Grenville Front mylonites and are therefore part of the late brittle history of the Grenville Province. These fracture sets are confined to a small (5 km2) area but display a wide range of orientations and morphologies. They can be divided into four peak orientations and these can be compared with inferred stress states for several periods in the tectonic history of the Grenville Province from 1200 Ma to the present. Based on these orientations and other observations it is suggested that the echelon fracture sets formed over a period of time spanning the end of the Grenvillian Orogeny 1000 Ma) to the beginning of Ottawa-Nippissing rifting (post 575 Ma). </p> <p> Measurement of the geometrical characteristics of the echelon fracture sets and comparison with those quoted in the literature has led to the conclusion that the geometry of an echelon fracture set does not unequivocally indicate its origin or mode of formation. </p> <p> Current models using fracture-zone angle and overlap ratio for the classification of echelon fracture sets seem to be unapplicable to the echelon fracture sets examined in this study and must be used with caution. The echelon fracture sets studied here are interpreted to have formed in "shear zones" and to consist of dominantly tensile fractures. Some of the fractures have originated as shear fractures or have undergone shearing at some point in their history. </p>