Assessing the Pliocene–Recent erosion history of New Zealand's eastern Southern Alps using cosmogenic radionuclides, tracer techniques and grain size analyses

The Southern Alps / Kā Tiritiri o te Moana in Aotearoa New Zealand have attracted scientists to study the interactions between climate and tectonics for decades. It has long been argued that tectonic uplift of this orogen is approximately balanced by erosion. The prevailing westerly airflow at the latitudes of the Southern Alps has created a strong orographic effect with precipitation decreasing sharply across the orogen’s main divide. The signature of this orographic effect is apparent in erosion rates that decrease from west to east, and from the dominant types of erosional processes that operate on either side of the orogen’s main divide. Most studies quantifying erosion over geologic timescales have focussed on the wetter—but areally significantly smaller—side of the orogen. Here, we seek to quantify the Pliocene–Recent erosion history of the Southern Alps’ much larger and drier eastern side using cosmogenic radionuclides (¹⁰Be and ²⁶Al), tracer techniques (U–Pb) and a grain size analysis on fluvial deposits in the Canterbury region that record concomitant erosion of this mountain range. Cosmogenic radionuclides provide a powerful tool to constrain catchment-scale erosion rates on timescales of 100–100,000 years, which is the temporal range at which tectonic and climatic forcings overlap and meso-scale stratigraphic architecture is created, thereby offering critical insights into the dynamics between tectonics, climate, and surface processes. Detrital grain U–Pb analysis of the fluvial deposits will be used to establish the sediment’s provenance, while a grain size analysis of the river sediments will provide insights into associated past stream dynamics. With this multi-method study, we seek to constrain both spatial patterns and catchment-scale rates of erosion of the eastern Southern Alps, as well as their changes through time and see if erosion has been affected by major climatic shifts during the Pliocene and Pleistocene epochs. Finally, this research will provide a benchmark for assessments of anthropogenically influenced erosion of the eastern Southern Alps. Preliminary results from ¹⁰Be and ²⁶Al analyses and dating of fluvial terraces will be presented.