Micromechanical control of rheological anisotropy in quartz mylonite

Abstract

The Moine nappe mylonites of the area of Loch Eriboll are inferred to have deformed predominantly by simple shear. Samples were taken from each limb of a late recumbent fold and the quartz crystallographic preferred orientation and optical microstructure were investigated. The pattern of preferred orientation was found to be related to the local folded orientation of the foliation and stretching lineation for the overturned limb and to the inferred imposed deformation for the normal limb. The mechanical development of the fold was modelled as a similar fold forming in a homogeneous mechanically anisotropic material with linear viscous rheology. A very high mechanical anisotropy was required for active amplification of small initial perturbations under simple shear applied at 10 ° to the initial enveloping surface. The high anisotropy also constrained the local kinematics to approximate simple shear parallel to the plane of easy shear, which contributes to the maintenance of the preferred orientation patterns relative to the local macroscopic fabric elements. The degree of mechanical anisotropy calculated from the preferred orientation assuming the operation of the common quartz slip systems is lower than that required for mechanical amplification of the folds. Consequently, it is suggested that the mechanical anisotropy derives from sliding on preferentially aligned grain boundaries.