APPLICABILITY OF THE DALE-PUMPELLY PRINCIPLE IN THE LIGHT OF FINDS OF TONGUE FOLDS

Abstract

In the Vochelamba test area (Kola Peninsula) the author has found unusual small tongue-shaped folds in Archaean leucocratic plagiogneisses. The folds were carefully removed from the rock, making it possible to examine them three-dimensionally and in all cross sections, i.e., to consider the folds as three-dimensional geologic bodies. The tongue shape of these folds results from strong primary bends of their hinges in the axial plane. The fact that fold hinges of the same generation may not only be nonparallel, but may have any orientation in the ab kinematic plane, is empirically established. On this basis, we may limit the range of applicability of the Dale-Pumpelly principle and assert that it is valid only for planar deformation. There are two methodological consequences. First, if the orientation of fold hinges is used as a correlation feature in developing structural age scales, then additional evidence of their primary parallelism is required. Second, without such evidence, certain procedures of geometric structural analysis (e.g., the formal distinction of different stages of deformation solely from differences in the orientation of the hinges on stereographic diagrams) are incorrect.