THE INFLUENCE OF INITIAL FOLD GEOMETRY ON TYPE-1 AND TYPE-2 INTERFERENCE PATTERNS - AN EXPERIMENTAL APPROACH

Experiments on superposed folding were performed in a plane strain-pur e shear rig using paraffin wax as an analogue for rocks. A series of p re-formed cylindrical folds were refolded with the compression directi on acting parallel to the initial fold hinge direction, the intermedia te 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 ampl itude were varied to investigate the influence of first-fold geometry on the interference patterns. Experimental folding of an initially pla nar layer oriented parallel to the first-fold envelope provides a refe rence geometry for comparison. The results suggest that hinge roundnes s has less influence on the interference pattern than other factors. C lose initial folds refold into Type 2 interference patterns, which bec ome even more pronounced as the tightness increases; open folds produc e Type 1 interference patterns. Folds with the same interlimb angle an d the same roundness produce different interference patterns depending on relative amplitude: folds with relatively large amplitude are refo lded into Type 2 patterns, whereas folds with small amplitude give cle ar dome and basin structures. In a second set of experiments, the comp ression 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 determin ing whether Type 1 or Type 2 interference patterns develop is the init ial fold geometry and not the kinematics of the second deformation.