Micromechanical control of rheological anisotropy in quartz mylonite

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 o f a late recumbent fold and the quartz crystallographic preferred orientati on and optical microstructure were investigated. The pattern of preferred o rientation was found to be related to the local folded orientation of the f oliation and stretching lineation for the overturned limb and to the inferr ed imposed deformation for the normal limb. The mechanical development of t he fold was modelled as a similar fold forming in a homogeneous mechanicall y anisotropic material with linear viscous rheology. A very high mechanical anisotropy was required for active amplification of small initial perturba tions under simple shear applied at 10 degrees to the initial enveloping su rface, The high anisotropy also constrained the local kinematics to approxi mate simple shear parallel to the plane of easy shear, which contributes to the maintenance of the preferred orientation patterns relative to the loca l macroscopic fabric elements. The degree of mechanical anisotropy calculat ed from the preferred orientation assuming the operation of the common quar tz slip systems is lower than that required for mechanical amplification of the folds. Consequently, it is suggested that the mechanical anisotropy de rives from sliding on preferentially aligned grain boundaries. (C) 2000 Els evier Science Ltd. All rights reserved.