A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe, SW Highlands, Scotland

• Unique 3-D analysis of shear band morphology in greenschist facies Dalradian rocks.
• Strain partition results in microlithon fold-pair domains enveloped by D2 cleavage seams.
• Layer-parallel slip and back-rotation of D1 spaced cleavage generate D2 microlithons.
• Analogous structures include fault and shear zone lozenges and flexural-slip duplexes.
• Discussion of kinematic interpretation of minor fold asymmetry and vergence.

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D1 spaced cleavage was responsible for the formation of the D2 spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D2 microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D1 microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D2 flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.

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