Back-rotation during crenulation cleavage development: implications for structural facing and cleavage-forming processes

Structural facing can be a useful tool for understanding macroscale structural geometries, particularly where poor outcrop inhibits the mapping of fold closures. However, in some situations facing must be determined with considerable care. In graded metaturbidites, where bedding and a near-parallel foliation have been overprinted by a crenulation cleavage, the earlier foliation in the metapelitic layers can be substantially `back-rotatedʹ in the hinges of the overprinting crenulation cleavage. Thus, the rotation in the crenulation hinges is opposite to the rotation in the crenulation limbs. When viewed in the metapelitic layers, relative to a bedding surface, back-rotation can cause an apparent reversal in the structural facing (and vergence) on the rotated foliation. To avoid such misinterpretation, structural facing on the earlier foliation should be determined in the metapsammitic layers, where the effects of the overprinting crenulation cleavage are minimal. Because foliations that intersect bedding at a low angle are commonly hard to identify in metapsammitic outcrops, microstructural analysis may be required. The back-rotation process provides important constraints on mechanisms and kinematics of crenulation cleavage development, and may also have important implications for porphyroblast rotation, folding mechanisms and issues of strain compatibility in compositionally interlayered rocks.