THE ROLE OF FLUVIAL AND GLACIAL EROSION IN LANDSCAPE EVOLUTION - THE BEN-OHAU RANGE, NEW-ZEALAND

A morphometric comparison of valleys has been made for the Ben Ohau Ra nge in the central Southern Alps of New Zealand. The range is undergoi ng rapid tectonic transport and uplift. The humid north of the range i s a glacial trough-and-arete landscape, with a temperate glacial clima te. The dry south has rounded divides and plateau remnants dissected b y fluvial valleys. Assuming that space-time substitution allows today' s spatial valley-form transition to represent evolutionary stages in v alley development, the tectonic history allows time constraints to be placed on the rate of transition to an alpine glacial landscape. Morph ometric change has been quantified using hypsometric curves, and dista nce-elevation plots of cirque and valley-floor altitudes. Ancestral fl uvial valleys have less concave long profiles but are stepped at altit ude owing to the presence of high-level cirques and remnant plateau su rfaces, and possess a low proportion of land area at low elevation. In creasing glacial influence is manifest as smoother, more deeply concav e long profiles and U-shaped cross-profiles associated with a higher p roportion of the land area at lower elevation. The full morphological transition has involved up to 2.4 km of vertical denudation over the 4 Ma lifetime of the mountain range, of which 80 per cent would have oc curred by preglacial fluvial erosion. Combining the trajectory of tect onic transport with reconstructed glaciation limits and climatic histo ry, it is indicated that about 200 ka of temperate glacial erosion pro duces recognizable trough-and-arete topography. Mean and modal relief increase where glacial activity is confined to cirques, but decrease w hen trough incision by ice becomes established as a dominant process i n the landscape. (C) 1997 by John Wiley & Sons, Ltd.