Temporal constraints on glacial valley cross-profile evolution: Two Thumb Range, central Southern Alps, New Zealand

A key problem in understanding valley evolution in alpine landscapes has been the lack of an independent timescale against which to measure morphological change. Here, we outline that by using space as a proxy for time, the role of glaciation in producing 'classic' U-shaped valleys can be quantified. The Two Thumb Range in New Zealand's central Southern Alps is undergoing rapid tectonic transport and uplift as it is advected towards the Alpine Fault. Repeated cycles of glacial erosion during the Quaternary have fashioned an alpine landscape in the north of the range, close to the Main Divide and Alpine Fault, while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross-profiles. Using known rates of tectonic transport and uplift of the Southern Alps, coupled with an offshore oxygen isotope (δ¹⁸O) record from the Chatham Rise and glacio-geological reconstructions, time constraints can be applied to the erosional development of valleys. It is possible to estimate the duration of glacial occupancy of valleys in the Two Thumb Range, and the pattern is one of very limited glaciation of the southern valleys, with glacial occupancy increasing northwards. The key trend identifiable is that valley cross-profiles become U-shaped with c. 400-600 kyr of glacial occupancy. Other models have shown a "flattening" of valley long-profiles with increased glaciation, implying glaciers erode larger volumes of rock from mountains than rivers. From a cross-profile perspective, the simple change in geometry from V- to U-shape concurs with these models. Indeed, the transition from V- to U-shape with increasing relief and glaciation indicates glaciers may erode greater volumes of rock from mountain belts than rivers.