Transformation-induced jointing as a gauge for interfacial slip and rock strength

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/Tf) 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 Youngʹs 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/Tf 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/Tf ratios obtained from field measurements and may also help us constrain rock strength.