APPARENT EXTENSIONAL STRUCTURES DUE TO VOLUME LOSS
Paul D. Bons · Proceedings of the Estonian Academy of Sciences Geology · 1999
Extensional structures, such as boudins, extensional faults, and shear bands, are often used to infer a general tectonic extension.However, observations on stromatic migmatites and theoretical considerations, both presented here, show that volume loss can also cause apparent extensional structures, even when the overall deformation is shortening.Extensional structures form in a layer when adjacent layers extend faster, or contract slower than the layer itself.The latter can be due to volume loss in that layer.The relative rates of layer-parallel shortening or stretching determine what structures form, not only the overall applied tectonic deformation.Stromatic migmatites often show layer-parallel extensional structures, which can be explained by the volume loss due to melt extraction.Volume loss due to diagenetic compaction and pressure solution can give rise to apparent extensional structures in sedimentary basins, while it is speculated that on an even larger scale, volume loss in the lower crust due to melt extraction can cause the formation of crustal-scale apparent extensional structures.