POST-LGM RIVER INSICION IN THE HIKURANGI SUBDUCTION ZONE, NEW ZEALAND

The Last Glacial Maximum (LGM) was a time of regional river aggradation within the mid-latitude, sub-humid continent of New Zealand. Aggradation in the non-glaciated catchments of the eastern North Island occurred in response to reduced vegetation cover, for which the modern day, forest-clearance induced, aggradation in two catchments provides an analogue. The terrace deposits are generally well preserved along the trunk rivers of the nine largest catchments (catchment areas 1730 to 5870 km²) and thus provide a datum to measure river incision rates in the actively uplifting Hikurangi Subduction Margin.

The LGM terrace has been mapped at 1:50 000 scale using a combination of aerial photo analysis and field mapping. The terrace is characterised by a paired, flat, surface, underlain by gravel, sand and silt fluvial deposits averaging 5-6 m thickness, with little to no loess cover, but in the north, tephra cover, sourced from the Taupo Volcanic Zone. Terrace dating has utilised a combination of tephrostratigraphy, loess stratigraphy, radiocarbon dating and luminescence dating, and is still in progress. The c. 26 500 cal. yrs BP Kawakawa Tephra is a key marker horizon found both within the terrace deposits and overlying older surfaces. Longitudinal profiles have been measured using RTK GPS, supplemented by Local Government survey data, and 1:50 000 scale topographic contour data for the uppermost reaches. In the reaches where the LGM terrace is preserved, terrace and river elevations are accurate to ±2 m.

River incision rates have been calculated using a preliminary terrace abandonment age of 18 ± 2 ka. Average rates are generally low (<3 mm/yr) in the south, and are more variable in the north, with rates as high as 11 mm/yr in the Mohaka Catchment. Potential factors contributing to the variability in incision rates, such as changes in climate, lithology, catchment area and slope, are presented for discussion. Further application will be to separate tectonic from climatic controls, and early interpretations are that variability in incision rates are primarily controlled by tectonics.