Geometry of quartz boudins in marble (abstract only)

Geometry of quartz boudins in marble (abstract only)

Brunt, R.

Molengraaff Fonds

2002

The aim of this project is to find out what caused the differences in geometry of quartz boudins in marbles. The project is focussed on the influence of the layer thickness the elastic modulus of the layer and the competence ratio (layer to matnx) on the geometry of the boudins. Samples of the Ugab and Goantagab Valley in Namibia were analysed. A discrete element code within the model Elle was used to model the full dynamics of fracturing of brittle layers within a less-competent matrix. Simulations showed that fractures start to grow at the rim of the brittle layer, where tensile stress is highest. An initial fracture set is first developed. Successive fracturing is possible with continuing strain, even if the boudins have compressive stresses in the center. Fractures are not a continuous function of strain. The boudins were divided into four types: pinch-and-swell structures, rectangular-, granular- and shear fracture boudins. A linear relation is found between layer thickness and spacing for each boudin type. The spacing increases with increasing layer thickness and decreases with increasing strain. The aspect ratio decreases non-Iinearly with increasing layer thickness and strain. The minimum spacing is independent of strain and layer thickness, but differs per boudin type. The minimum aspect ratio depends on the strain and the layer thickness and is different per boudin Further, the aspect ratio decreases non-linearly with increasing thickness ratio (layer to matrix) and decreases non-linearly with increasing elastic modulus of the layer (with constant competence ratio between layer and matrix). The well-rounded pinch-and-swell structures and the shear fracture boudins have both smaller competence ratios compared to the rectangular boudins. An ill-constraint correlation is found between the absolute elastic modulus and the shape of the boudins. Rectangular boudins have probably a larger elastic modulus than the pinch and- swell structures. More natural samples need to be analysed to confirm this relation.

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Utrecht University

Geoscience Reports

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(c) Rianne Brunt