Spacing and strain during multiphase boudinage in 3D

Most models of brittle boudinage predict a dependency of fracture spacing on thickness of the boudinaged layer. This basic relationship can be distorted in the case of multiphase boudinage, where structural inheritance, possibly combined with time evolution of rheology affects boudin geometries, but is not recognized in 2D outcrops. Here we present analyses from a metre-scale, serially sectioned sample of marble from Naxos, Greece, containing a layer of amphibolite, which is known to have experienced two phases of ductile drawn boudinage followed by five generations of brittle boudinage. We show that brittle boudinage depends on the ratio of grain size and layer thickness of the amphibolite. In very thin layers (few grains across), strain is diffuse throughout the entire layer, leading to macroscopically homogeneous stretching. Strain localisation starts when layer thickness exceeds 5 grains: narrow tensile necks and shear zones (35–45° dip) develop where the layer is