The Effects of Layer Thickness on Brittle Boudinage in 3D

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Bob Bamberg, Christoph von Hagke , Simon Virgo , Janos L. Urai


We analyse the effects of thickness on brittle boudinage in a metre-scale sample of marble containing a layer of amphibolite recording two phases of ductile pinch-and-swell followed by five generations of brittle boudinage. The amphibolite geometry was reconstructed in 3D, employing a method we call ‘outcrop-scale tomography’. Our data suggests that strain localisation depends on the ration of grain size and layer thickness of amphibolite. In very thin layers (few grains across), strain is diffuse throughout the entire layer, leading to macroscopically homogeneous stretching. Strain localisation increases when layer thickness is more than 10 grains, first through narrow tensile necks and shear zones (<10-20x average grain size), then through extension fractures, and finally shear fractures emerge. The disappearance of shear fractures in thinner layers can be explained by a geometry-related compressive stress decrease in the pinches and expected shear band width exceeding layer thickness. This results in localized shear evolving only in thicker layers. Successive reactivation between fracture generations, geometrical complexity, in the form of splays and branches, and the thickness-dependence of localised strain govern fracture distribution in the layer. We infer a second, temporal trend that records the progressive embrittlement of the rocks as they cool during exhumation, evidenced by a switch from shear to extensional fracturing. In the final stages, the marble is brittle enough to allow fracture propagation from the amphibolite across the material interface and the formation of throughgoing brittle faults.



Earth Sciences, Physical Sciences and Mathematics, Tectonics and Structure


brittle, 3D, boudinage, tomography, fracture spacing, brittle, 3D, strain analysis, fracture spacing


Published: 2021-10-01 05:49


CC BY Attribution 4.0 International

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