Formation of mullions in two and three dimensions: Results from analogue modelling

Scale-model experiments, involving pure shear shortening of a single incompetent layer embedded in a competent matrix, were carried out to study the influence of the bulk strain geometry and an inclined layer on the growth rate and shape of mullions. The viscosity ratio between non-linear viscous layer and matrix varied from 1/4-1/35. If layer parallel shortening (eZ) is the same, the growth rate of mullions is highest under bulk constriction, moderate under bulk plane strain, and lowest under bulk flattening. Because of their low growth rate in a flattening strain field, formation of mullions is hardly possible. Initial layer inclination under bulk constriction led to antisymmetric mullions. Increasing the viscosity ratio between layer and matrix results in (1) faster rotation of the layer towards the main stretching axis, X, (2) earlier attainment of a stable mullion wavelength, and (3) earlier formation of flames. The style of deformation (coeval folding and boudinage vs. shearing) of a thin competent sheet, inserted into the incompetent layer, is controlled not only by the viscosity ratio, but also by the bonding between competent and incompetent material. The new results shed light on the deformation behavior of Sederholm-type dikes and sills at deep crustal levels.