CONICAL FOLDS PRODUCED BY DOME AND BASIN FOLD INTERFERENCE AND THEIR APPLICATION TO DETERMINING STRAIN - EXAMPLES FROM NORTH CANTERBURY, NEW-ZEALAND

Conical folds associated with dome and basin interference are analysed using equal-area plots, which provide data on the geometry of the fold surface and estimates of the finite shortening. To describe individual folds, they are subdivided into subareas defining the domains of each interfering fold set. The resulting plots demonstrate that numerous partial cones of distinct geometry are required to adequately describe the folded surface. To help identify the presence and accurately determine the geometries of such folds, the expected variations in conical fold geometry for interfering folds with varying hinge curvature, tightness and limb dips are analysed and illustrated with examples from North Canterbury. The resulting cone geometries are complex and often difficult to detect, which may in part explain the apparent absence of conical folds in some areas of dome and basin interference. Where parallel folds interfere, the divergence of poles to the conical fold surface and cone apical angle provide estimates of the finite shortening associated with folding. These shortening estimates are consistently larger than those determined by constructing profiles parallel to shortening along the hinge line of the second fold set.