The structural evolution of active fault and fold systems in central Otago, New Zealand: Evidence revealed by drainage patterns

Central Otago in New Zealand is an area of active continent al shortening in which a peneplain surface cut into schist has been deformed by folds, which are developed above buried reverse faults. We use the drainage patterns in this region to demonstrate various processes in fold (and fault) growth and interaction that would be difficult to identify by other means. In particular we show: (1) how simple asymmetric folds can develop into box folds; (2) how apparently continuous ridges were formed by the coalescing of quite separate propagating fold (and fault) segments; (3) evidence for the relative ages (or relative uplift rates) of adjacent structures; and (4) evidence for the propagation direction of folds (or faults) as they grow. The few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10-50 m per earthquake on faults about 20 km long. These estimates are very uncertain, but are similar in magnitude to an estimate made in Nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models. They raise the question of whether fault growth, earthquake recurrence rates and climate change can interact to produce semi-regular discrete features in an active landscape.