Foliation and strain development in ice-mica models

Abstract

The behaviour during experimental deformation of composite foliated rock models composed of ice and ice-mica is described in order to replicate deformation in quartz-mica rocks. The deformation involves pure shear and a component of simple shear oblique to the shortening axis. A foliation is developed which is defined by an alignment of micas and a preferred orientation of airbubbles that closely parallels the principal plane of the local finite strain ellipsoid. The degree of mica re-alignment is not purely dependent on a parameter such as strain, but is also controlled by the nature of the initial mica fabric and the orientation of layering relative to the shortening direction. Where there is some obliquity of a primary layering to the bulk shortening direction, the deformation is markedly inhomogeneous with non-uniform extension parallel to the flow direction over short distances. This results in varying local strain and mica distributions. Values of strain obtained from such mica orientation distributions are always less than the observed finite strain and are therefore incompatible with a March model. However, deformation of an initial layering parallel to the shortening direction, that contains a uniform mica distribution, results in a homogeneous finite strain distribution and the mica distribution strengthens parallel to the principal plane of the local strain ellipsoid. This mica distribution produces values of strain that correspond to the predictions of the March model. In contrast, deformation of initial bimodal fabrics or fabrics asymmetric to the shortening direction will develop strong preferred orientations quicker than an initial uniform distribution, and characteristics of the initial fabric pattern always persist within the deformed mica preferred-orientation. Shortening perpendicular to a strong initial preferred orientation only strengthens the mica fabric. Intracrystalline slip in the ductile ice matrix is the main means of accommodating the deformation and also produces a strong c-axis preferred-orientation. Grain-boundary mobility in the ice is retarded by the dispersed mica phase. However, grain growth during and post-dating deformation does not effect mica orientation distributions. These features resemble and are compared with identical structures observed in many quartz-mica rocks.