Glacial valley cross-profile development: the influence of in situ rock stress and rock mass strength, with examples from the Southern Alps, New Zealand

The evolution of the glacial valley cross-profile form is commonly attributed primarily to the glaciological variables that control the erosion of the channel. However, studies in the New Zealand Southern Alps suggest that the rock mass strength (RMS) of the eroded rock mass is a major control on slope stability, and hence on the final form of the trough. RMS of the slope rock will alter with time and erosional excavation and oversteepening of the valley slopes. The in situ rock stress field induced by: (1) the extreme topography in the axial ranges of the New Zealand Southern Alps, and (2) tectonically by collision of the Australian and Pacific crustal plates, may play a role in valley slope development by controlling the location of rock failure and reducing RMS. This provides weakened rock that may provide sites for selective glacial erosion of the rock mass. Hence, valley form evolution models should also take into account the RMS and the in situ stress field acting in the eroded rock mass. This study has implications for the development and modification of alpine glacial troughs in similar tectonic settings elsewhere.