Finite strain effects in experimental mullions

Abstract

Mullions are cylindrical structures developed at the interface between rocks of different competence shortened along the layering. Natural examples include phyllosilicate-rich meta-igneous sheets and massive sulfides in silicate country rocks. In axial profiles they consist of rounded lobes convex into the less competent material which alternate with sharp cusps pointing toward the more competent material. Mullions have been produced experimentally in incompetent single layers of silicone putty (viscosity n L = 2.9 × 10 4 Pa s and power law exponent n = 1) embedded in a strain-rate softening host of plastilina + silicone putty mixture ( n h = 5.5 × 10 6 Pa s and n = 7.1). The initiation, development and changes in geometry of mullions in the two interfering interfaces of a single incompetent layer shortened along its length by pure shear, are reported. Up to ∼10% bulk shortening (% BS) gentle flexures develop first in one interface, then, by ∼17% BS, in both. These rounded irregularities become cuspate at ∼25% BS. Where a lobe develops on one contact, a cusp is slightly preferred on the opposite side. Such antisymmetric mullions contrast with the symmetrical (pinch-and-swell) mullions predicted by Smith, but are like natural examples and those figured in the literature. The amplitudes of the cusps increase while their wavelength range decreases with finite shortening. It has been found that mature cusps distort but do not transgress the original contact even if the viscosity contrast is ∼-1/1000. Beyond 40% BS, cusps either mature to flames or combine to horns, and after 60% BS even the last surviving mullions cease to amplify dynamically and the layer only thickens uniformly. Most stages seen in the models can be matched by natural field examples.