Tubular folds and sheath folds: definitions and conceptual models for their development, with examples from the Grapesvare area, northern Sweden

Abstract

Two parameters are used in the classification of highly non-cylindrical folds: the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross-section of the structure at the position where the hinge angle was measured. If in a non-cylindrical fold a cross-section can be selected so that the hinge angle w measured where the cross-section cuts the cone is less than 90°, and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross-section, the fold is a sheath fold. A tight sheath fold, with w < 20° and x: y > 1, can be termed a tubular fold. A tubular fold is thus an extreme type of sheath fold. While sheath folds with w > 20° but < 90° and x: y > 0.25 but < 1 can be developed by superimposition of layer-parallel simple shear on almost any initial non-cylindrical irregularity, recognizable tubular folds can only be developed from initial non-cylindrical progenitors whose long axes are parallel or almost parallel to the later shearing direction. Such progenitors of tubular folds can be formed by local shortening subperpendicular to the later shearing direction, or they can form in ductile strike-slip zones subparallel to the shearing direction of the later layer-parallel simple shear. Tubular folds from the Grapesvare area comply to the definition given above, and they occur in a structural setting that is consistent with the models described.