Salt flow by aggrading and prograding overburdens

Abstract

Abstract The mechanisms responsible for segmentation of salt sheets and their emplacement into higher stratigraphic levels are not separable and act simultaneously. Three sets of centrifuge models with strongly planar anisotropic (anisotropy, δ, the ratio between the effective viscosities in pure and simple shear, ranging between 4.8 and 16.4) microlaminate overburdens are used to study the effect of aggradation and progradation on segmentation and emplacement of allochthonous salt sheets. In the first set of models, a tabular buoyant source layer was overlain by tabular anisotropic overburden simulating aggradation. During centrifuging, the underlying ductile source layer was segmented into individual wall-like diapirs by the subsiding blocks formed due to extension and faulting of the overburden. The extensional zone in these models started at the spreading edge (free face) of the model and migrated backwards. In the second set of models, a tabular buoyant source layer was overlain successively by wedges of anisotropic overburdens simulating progradation. During centrifuging, the buoyant layer was displaced from the back of the model, where loading was higher, towards the free face in the front of the model, where overburden units were thinner. Overburden units extended at the back and the middle of the model while contractional structures dominated at the front, where asymmetric diapirs formed overhangs that spread ‘basinward’ to form ‘salt’ sheets. In the third set of models, a wedge-shaped buoyant source layer was overlain successively by wedges of anisotropic overburden simulating progradation. The overburden wedge created a lateral pressure gradient ranging from 144 Pa at the back of the model to 80 Pa at the front when deformed in the centrifuge. In these models, as in the second set of models, the underlying buoyant mass was displaced ‘basinward’ by the subsiding thicker overburden units at the back of the model. Contractional structures dominated the deformation at the leading edge of the wedge. Comparison of model results suggests that progradation (as in the second and third sets of models) loads underlying ‘salt’ differentially, displaces it downdip and segments it. As it segments at the back, the ‘salt’ flows laterally to areas of lower loading by intruding through the thinner overburden units and forming secondary ‘salt’ sheets at the front. On the other hand, aggradation of uniform overburden segments a buoyant sheet into two-dimensional salt walls or stocks, as in the first set of models.