Relations between postglacial fluvial incision rates and uplift rates in the North Island, New Zealand

Abstract

Fluvial incision rates are increasingly being used as a proxy for rock uplift rates for inland settings. We test the validity of this approach by comparing incision rates and rock uplift rates along the Hikurangi Margin, New Zealand. Postglacial fluvial incision rates were calculated by measuring the difference in altitude between the Last Glacial Maximum (LGM) terrace tread (18 ± 2 calendar ka) and the present river bed for 10 major rivers (catchment areas ∼1400–5900 km2). Incision rates have thus been measured over a half glacial cycle. They also represent average rates over the last 18 kyr, during which time variations are likely due to processes such as knickpoint retreat. Rock uplift rates have been estimated from (1) altitude differences between the treads of pairs of fluvial fill terraces (∼55 and ∼18 calendar ka) assumed to have formed at the same altitude (i.e., rock uplift relative to the geoid), (2) present‐day altitudes of Late Neogene paleoshoreline marker horizons (1.6–5.7 Ma) (i.e., rock uplift relative to sea level), and (3) present‐day altitudes of Neogene mudstones for which maximum depth of burial has been calculated from porosity (i.e., rock uplift relative to sea level). These rock uplift rates overestimate tectonic uplift rates because they do not take into account isostatic compensation due to erosion. Mean incision rates range up to 11.7 mm yr−1, and mean rock uplift rates range up to 4, 1, and 0.9 mm yr−1 for the three methods, respectively. We have made some qualitative and semiquantitative observations about nontectonic controls on postglacial incision rates and find that localized influence only is exerted by rainfall, base level, and bedrock lithology, while negative or no correlations are shown with LGM fluvial fill thickness and stream power. Incision rates and uplift rates display a rough correlation, with incision rates typically 1.5–5 times uplift rates. This relation suggests that tectonic uplift is likely to exert an important control on fluvial incision in the Hikurangi Margin.