Cirque development in a steadily uplifting range: rates of erosion and long‐term morphometric change in alpine cirques in the Ben Ohau Range, New Zealand

Abstract

The intractable obstacle to understanding cirque evolution has been the lack of an independent timescale against which to measure morphometric change. By using known rates of transport and uplift of the Southern Alps, combined with the oxygen isotope record contained in marine sediments, it is possible to estimate the duration of glacial occupancy of cirques in the Ben Ohau Range. The pattern is one of very limited glaciation of the southern range, with glacial occupancy increasing northwards. Map‐based measurement of 18 morphometric properties in each of 92 cirques reveals systematic northward trends in cirque size, form and altitude, which are explained as reflections of increasing cirque ‘age’ (i.e. length of glacial occupancy). Thus, the validity of space–time substitution is demonstrated rather than assumed, and morphometric trends define evolutionary stages of cirque development. For cirques with up to c. 750 ka occupancy, rates of downcutting average 0·29 mma−1 and headwall retreat 0·44 mm a−1. There is no indication of attainment of an equilibrium form within this time: both cirque size and form continue to evolve throughout, with few parameters showing a convergence of value with cirque ‘age’. Some morphometric parameters perform much better than others: the k coefficient of the long‐profile descriptor is the most powerful predictor of cirque ‘age’, and simple measures correlate better than derived measures (e.g. ratios of dimensions). The trends identified concur with some previously published ideas, but for the first time provide empirically quantified rates of change at an appropriate timescale. Copyright © 2006 John Wiley & Sons, Ltd.

Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems

“…Higher values of channel profile concavity and lower channel gradients in headwater basins that are characterized by erosion of local cirque glaciers imply lower transport capacities of the postglacial geomorphic processes in formerly glaciated headwaters. Furthermore, the shaping of glacial troughs, which are characterized by steep rock walls and a pronounced transition to flat cascading valley bottoms (Brocklehurst and Whipple, 2004;Brook et al, 2006), results in a distinctive pattern of postglacial sediments. This in turn i) disconnects many lateral sediment flow paths within mountain headwater basins and ii) affects their effective contributing area (Brardinoni et al, 2009;Cavalli et al, 2013;Heckmann and Schwanghart, 2013;Messenzehl et al, 2014) and…”

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

“…The temporal extent of glaciation increases with distance northwards along ranges in this zone of the Southern Alps (e.g., Kirkbride and Matthews, 1997;Brook et al, 2006), hence by plotting morphological and size descriptors against distance along the range, spatial difference in valley cross-profile form, influenced by increased glacierization, can be identified. …”

Small valley glaciers and the effectiveness of the glacial buzzsaw in the northern Basin and Range, USA

“…Freeze-thaw activity and frost cracking are important mechanisms for weathering and liberating material from the steep exposed rockwalls above the heads of glaciers (e.g., Matsuoka, 1990Matsuoka, , 2008Hales and Roering, 2007), and quarrying beneath a glacier may be promoted by elevated sub-glacial water-pressure fluctuations at the bergschrund, increasing erosion at the lower headwall (Hooke, 1991). Surveys of the temporal evolution of cirque form from the Ben Ohau Range, New Zealand (Brook et al, 2006) and the mountains of Wales, UK (Evans, 2006), demonstrate that lateral enlargement outpaces vertical incision at the cirque floor, and morphometric analyses from the Kyrgyz Range, central Asia (Oskin and Burbank, 2005) and the Bitterroot Range, Montana, USA (Naylor and Gabet, 2006) suggest that cirques can erode laterally at rates 2-4 times greater than vertical incision. Other studies have emphasised the role that lateral erosion of the cirque headwall plays in alpine landscape evolution.…”