The Superposition of Caledonoid Folds on an Older Fold-System in the Dalradians of Malin Head, Co. Donegal

Abstract

AbstractIn the Malin Head promontory an Older fold-system, with thrusting and folding about N.W.-trending axes, was followed by Caledonoid folding about N.E.-trending axes. The limbs of the Older folds, both major and minor, now lie in the sheet- dip of the Caledonoid folds, the dips of the Caledonoid limbs being equivalent to the plunges of the axes of the Older folds. Most of the S-planes are shared in common by the two fold-systems. The only exceptions are those that outline the closures and rising parts of the Older fold-system, and these form zones in which the S-planes strike at right angles to the Caledonoid trend. By reference to the outcrop-forms in the area described, and with the aid of plasticine models, it is demonstrated that the outcrop-forms of folded folds are readily recognjzable, and that they provide visible evidence of the approximate axial directions and relative ages of the two fold-systems. By comparative study of outcrop-forms on the models and structures seen in sections cut at right angles to each of the fold-axes, it is found that the structures differ radically from those constructed by projecting the outcrop-forms on planes normal to the axes. The “down-the-plunge” method of viewing a geological map thus provides no clue to the actual three-dimensional structure of folded folds.