Spacing and strain during multiphase boudinage in 3D

Abstract

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 < 10–20 grains thick. Thicker layers (<30–40 grains) show extensional (mode 1) fractures while shear fractures form in even thicker layers. The absence of shear fractures in thinner layers may be explained by a geometry-related compressive stress decrease in the pinches. This results in localised shear evolving only in thicker layers. Complete delocalisation where layer thickness approaches the grain scale is expected as the typical shear bandwidth in granular media (10–20 grains) exceeds layer thickness. Successive fracture reactivation causes geometrical complexity in the form of splays and branches, and in addition the thickness-dependence of strain localisation governs fracture distribution in the layer. A second, temporal trend is recorded in the progressive embrittlement of the rocks as they cooled during exhumation, evidenced by increasing fracture density and a switch from shear to extensional fracturing. In the final stages, the marble was brittle enough to allow fracture propagation from the amphibolite across the material interface into the marble and, finally, the formation of brittle fractures affecting both the amphibolite and marble.