The initiation and growth of en échelon veins

Abstract

Numerical models are used to understand the evolution of mode I (opening) fractures from spatially random distributions in a brittle elastic material. En échelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry. This provides a new mechanism for en échelon vein formation in rock which is distinctly different than previously proposed mechanisms. It is suggested that some macroscopic en échelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro-cracking and subsequent shear rupture of rocks loaded under compression. Sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path. Numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation.