Combining geomorphic observations with in situ cosmogenic isotope measurements to study anticline growth and fault propagation in Central Otago, New Zealand

Measurements of Be-10 concentrations in quartz-rich boulders exposed by the uplift of anticlinal ranges in Central Otago, New Zealand, are used to investigate the fault propagation styles and rates for the underlying blind reverse faults. Be-10 ages along Little Rough Ridge reveal the propagation rate for this fault to be between zero (i.e., not propagating at all, if the ages reflect emergence of a fault of fixed length through a sloping surface) and a maximum of 8.1 mm/yr, if the ages reflect true lateral growth. Regardless of the propagation rate, Little Rough Ridge has established its length rapidly compared with the rate at which it has accumulated displacement. On Raggedy Range, the Be-10 ages, together with structural data, imply that the ridge reached its current length by the merging of several small fault segments. This study emphasises the importance of combining Be-10 measurements with geomorphic observations. Without understanding the geomorphological context, Be-10 exposure ages are difficult to interpret at all; but even with geomorphic data, the interpretation of these ages can still be ambiguous.