Simple models of drainage reorganisation on a tectonically active ridge system

We have studied the reorganisation of drainage patterns in response to the tectonically driven emergence of a small ridge (South Rough Ridge) along the flank of a larger ridge (Rough Ridge) as described by previous workers in the Central Otago region of New Zealand. We compare the results of a numerical model of landscape evolution (CASCADE) in which stream incision is linearly related to local slope, discharge (hence catchment size), and sediment load, to present-day drainage patterns in Central Otago in order to infer possible scenarios for the evolution of the stream network. We conclude that the observed drainage patterns along the flanks of Rough Ridge are most likely to have resulted from the lateral propagation of South Rough Ridge. Our conclusion is also supported by simple mathematical derivations which imply that the periodic patterns observed in the modern fluvial network—the regular spacing of wind gaps and stream valleys—may have been created by a constant tectonic driving force. A dimensionless number is found that best describes stream evolution as a function of tectonic uplift, erosion, and slope. We also demonstrate how the shape of a landform may be used to constrain the sequence of events that have led to its formation.