Paleostress, deformation mechanisms and finite strain related to chocolate-tablet boudinage and folding of a Permian rock salt/anhydrite sequence (Morsleben site, Germany)

Abstract

Polyphase chocolate-tablet boudinage of competent anhydrite layers in incompetent Permian rock salt of the Morsleben repository (Germany) developed during Triassic/Jurassic graben formation and late Cretaceous graben inversion. The competence contrast between layers and matrix is related to the deformation mechanisms of anhydrite and halite, which have been constrained in the present study using deformation microfabrics and geochemical data. Boudinage implies a finite longitudinal strain of ca. 100% in NNW-SSE direction. There is a clear impact of the layer thickness on both the deformation mechanisms and the shape of anhydrite boudins. Pervasive solution-precipitation creep is the dominant deformation mechanism in thin anhydrite layers (thickness < 15 cm), which are dominated by drawn boudins with aspect ratios ranging from 1.5 to 5.4. Deformation of thick anhydrite layers (thickness > 15 cm) was accommodated by dislocation creep succeeded by localized solution-precipitation, kinking, twinning and fracturing resulting in torn boudins with aspect ratios between 0.5 and 2.0. Halite of the incompetent rock salt matrix deformed viscously under low differential stress (2.0–3.1 MPa). The deformation mechanisms were dislocation creep in form of subgrain formation and strain-induced grain boundary migration. Moreover, close to anhydrite boudins, solution-precipitation creep of halite is indicated by decreased bromide content in halite of necks between brittle torn boudins, which should have precipitated from a fluid phase. Halite in necks of drawn boudins, on the other hand, displays a continuous decrease of bromide content, pointing to predominantly crystal plastic inflow of halite.