The influence of initial fold geometry on type 1 and type 2 interference patterns: an experimental approach

Abstract

Experiments on superposed folding were performed in a plane strain-pure shear rig using paraffin wax as an analogue for rocks. A series of pre-formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction, the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis (Type 1 interference geometry). Roundness, tightness and amplitude were varied to investigate the influence of first-fold geometry on the interference patterns. Experimental folding of an initially planar layer oriented parallel to the first-fold envelope provides a reference geometry for comparison. The results suggest that hinge roundness has less influence on the interference pattern than other factors. Close initial folds refold into Type 2 interference patterns, which become even more pronounced as the tightness increases; open folds produce Type 1 interference patterns. Folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude: folds with relatively large amplitude are refolded into Type 2 patterns, whereas folds with small amplitude give clear dome and basin structures. In a second set of experiments, the compression direction also acted parallel to the first-fold hinge, but the orientations of the intermediate and extension axes were interchanged. The interference patterns obtained are very similar to those in Type 1 interference geometry, suggesting that the major factor in determining whether Type 1 or Type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation.