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 shorten ing in which a peneplain surface cut into schist has been deformed by folds, which are developed above buried reverse faults. We use the dra inage patterns in this region to demonstrate various processes in fold (and fault) growth and interaction that would be difficult to identif y by other means. In particular we show: (1) how simple asymmetric fol ds can develop into box folds; (2) how apparently continuous ridges we re formed by the coalescing of quite separate propagating fold (and fa ult) segments; (3) evidence for the relative ages (or relative uplift rates) of adjacent structures; and (4) evidence for the propagation di rection of folds (or faults) as they grow. The few quantitative estima tes we obtain for fault propagation rates suggest an increase in lengt h of 10-50 m per earthquake on faults about 20 km long. These estimate s 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 que stion of whether fault growth, earthquake recurrence rates and climate change can interact to produce semi-regular discrete features in an a ctive landscape.