SALT DIAPIRS AND PENETRATIVE STRAIN: HIGH VERTICAL UPLIFT INHIBITS ACCOMMODATION OF PENETRATIVE STRAIN IN COVER ROCKS

In sandbox analog models consisting of a brittle cover over silicon (simulating salt), the structures developed and the amount of internal deformation (penetrative strain) that is accommodated within the cover appear to depend heavily on the thickness of the basal silicon/salt layer. This study consists of a series of models where the thickness of the basal layer was varied between 1 cm, 1.5 cm and 2 cm whilst the cover sequence thickness was held constant at 1.5 cm. Each thickness of the basal layer was shortened to 5, 10 and 15% total shortening, resulting in a series of 9 models in all. Results were compared to a previously published reference set of models where the basal silicon layer was only 0.5 cm thick and the cover was still 1.5 cm thick, run by the same lab and shortened to the same amounts.

In the reference models, the internal deformation (penetrative strain) reached 10% at the maximum amount of bulk shortening considered in this study. By comparison, the penetrative strain in the models with thicker silicon layers reached approximately 3% at the maximum amount of shortening for all thicknesses of silicon. A further observation is the development of box folds in the reference models, in contrast to the development of large amplitude and long wavelength silicon/salt diapirs in the thick silicon models of this study. We speculate that the thickened silicon is leading to increased vertical strain and less horizontal strain as the structures develop, and that the high amplitude diapirs are creating a barrier to the accommodation of horizontal compression. Thus, the cover is deforming by vertical uplift and folding over the edges of the diapirs, and not by internal accommodation of layer parallel strain. Conversely, the thinnest basal silicon in the reference models is acting as a detachment and a sliding surface for the development of detachment folds rather than high amplitude salt diapirs.

These results have implications for the formation mechanisms and internal deformation of salt-pillow-cored ‘buckle folds’ such as those in the North Sea and deserve to be explored in greater detail by further analog and numerical modeling.