Growth of South Rough Ridge, Central Otago, New Zealand: Using in situ cosmogenic isotopes and geomorphology to study an active, blind reverse fault

In situ cosmogenic 10 Be and 26 Al measurements on sedimentary quartzites, together with analysis of abandoned river terraces, can be used to show how drainage responded to the lateral propagation of a late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. A close link between the progression of cosmogenic ages and the tectonic geomorphology allows us to confirm that uplift and propagation rates on the anticline are in the ranges 0.08–0.12 mm yr −1 and 0.8–1.5 mm yr −1 , respectively, over the last 550,000 years. The agreement between the isotopic and geomorphological evidence in turn requires that minimum 10 Be exposure ages as great as 660 ka are not in steady state with respect to erosion. This is a geochemical result of significance, as it opens the possibility of a more regional analysis of landscape evolution in this region, using the same rocks. On an adjacent anticline, thought to be even older on geomorphological arguments, 10 Be concentrations give minimum exposure ages in the range 750–1400 ka. These extremely old minimum ages are rare worldwide, as they are usually limited to smaller values by erosion. They are attributable to the very resistant nature of the quartz‐rich boulders in which the measurements were made, which in places are shown to have erosion rates lower than 0.5 mm kyr −1 .