Tectonics, climate, process and scale: Landscape evolution in the eastern Southern Alps, New Zealand

The Ben Ohau, Neumann and Young Ranges are parallel, glaciated mountain ranges on the eastern side of the Southern Alps, New Zealand. All three ranges were bounded by substantial glaciers during the last glaciation. The ranges all trend obliquely to the Alpine Fault, so there are along-strike gradients in the amount and rate of uplift (both greater towards the north), precipitation and, therefore, equilibrium line altitude (ELA - lower towards the wetter north). Furthermore, the Neumann Range is substantially narrower than the Ben Ohau and Young Ranges, so the Neumann Range drainage basins are significantly smaller. We analysed digital topographic data, using swath and longitudinal profiles, and hypsometry, to investigate the impact of these gradients in uplift and ELA (taken from published work), and the effects of drainage basin size, on landscape evolution. As in previous studies elsewhere, glaciers in larger drainage basins have efficiently carved characteristic U-shaped cross-sections and shallow downvalley gradients, and contributed to divide migration. However, mean and maximum elevations rise towards the north, in concert with greater uplift rates (up to 2 mm/yr). This contradicts the "glacial buzzsaw" hypothesis, which proposes that topography parallels trends in ELA. Hypsometry suggests that drainage basins in the neighbouring Ben Ohau and Neumann Ranges have similar morphologies, but in detail glaciers in the Neumann Range have done less to modify previous fluvial profiles and reduce downvalley gradients. Glacial landforms are scarce in the smallest drainage basins, where debris flow processes appear to dominate landscape evolution. Fluvial and debris flow erosion have substantially reworked the glacial deposits at the mouth of each drainage basin. These results suggest that the response of small mountain ranges to glaciation is complex, and the glacial buzzsaw hypothesis is not necessarily an accurate summary in tectonically-active regions.