Various theoretical and numerical models have been proposed in order to
explain joint formation and spacing in layered rock series. However,
most of these models assume that the interfaces between the rock layers
are perfectly welded, i.e. no slip occurs, and that all the layers are
subjected to the same remote strain due to various processes (e.g.
tectonic processes). Other factors may also induce extensional strain in
rocks, e.g. phase transformations. However, such processes may induce
different amounts of strain on the layers in a rock series leading to a
strain mismatch between these layers. In this paper, we present a 1-D
finite difference linear elastic model which allows joint formation
within the middle layer in a three-layer rock series and is induced by a
strain mismatch between the fractured, central layer and the surrounding
matrix. Furthermore, the central layer in our model is not necessarily
welded to the matrix layers and is allowed to slip along the interfaces
between these layers if the shear strength of the material at the
interface is reached. We find that the final fracture spacing to layer
thickness ratio (S/T(f)) in such layered systems is directly
proportional to the ratio of the tensile and shear strength of the
material. Changes in the material properties such as the shear modulus
or Young's modulus do not affect these results. A natural analog of
joint formation driven by phase transformations is found in the
orthopyroxenite dykes of the Leka Ophiolite Complex (LOC), Norway. joint
formation in orthopyroxenite dykes results from serpentinization-driven
expansion of the surrounding dunite matrix Detailed field studies and
measurements (583 sample points) yield S/T(f) ratios between 0.1 and 1.0
with a mean value of 0.45 +/- 0.20. We demonstrate that the strain
mismatch-driven joint formation associated with interfacial slip
explains the low S/T(f) ratios obtained from field measurements and may
also help us constrain rock strength. (C) 2009 Elsevier B.V. All rights
reserved.