Slow Long-Term Erosion Rates of Banks Peninsula, New Zealand

Banks Peninsula, located south of Christchurch, New Zealand, is composed of a multi-aged complex of volcanic centers. The oldest, Lyttelton Volcano is 12 to 10 Ma, and 350 km3. The largest volcano, Akaroa Volcano, is 9 to 8 Ma and 1200 km3. Both of these volcanoes have large embayments (Lyttelton Harbour and Akaroa Harbour) that connect the central volcano (the location of the former volcanic summits) to the ocean. The other eruptive centers, Mt. Herbert ( 9.5 to 8 Ma) and Diamond Harbor (7 to 5.8 Ma), have not eroded to sea level. We used inferred original surfaces and present day topography to calculate the volume of rock eroded from river valleys draining the flanks of Lyttelton (n=11) and Akaroa (n=26) volcanoes and from the large embayments that penetrate the eroding Lyttelton (n=8) and Akaroa (n=25) volcanoes. We used the youngest age of the eruptions as the start of erosion (Lyttelton = 10 Ma and Akaroa = 8 Ma) to determine erosion rates. Preliminary data suggest average erosion rates of 8.2 ± 2.4 m/My (averaged over 10 Ma) on the flanks of Lyttelton Volcano and 12 ± 5.1 m/My (averaged over 8 Ma) on the flanks of Akaroa Volcano. Dating control and formation processes of Lyttelton Harbour and Akaroa Harbour are poorly constrained. The youngest lava flows, Diamond Harbor, are 5.7 Ma and flow into the Lyttelton Harbour embayment. Using endmembers of embayment age for Lyttelton Harbour (10 Ma to 5.7 Ma) the erosion rates range between 18 ± 5.8 m/My and 31 ± 10 m/My. Similarly, the hillslopes of Akaroa Harbour have slow erosion rates (based on endmember ages of 8 Ma and 5.7 Ma) and range between 22 ± 18 and 31 ± 25 m/My. Even the fastest erosion rates on Banks Peninsula are an order of magnitude slower than the erosion rates of other basalt volcanoes in the world's oceans. Using a similar methodology, Tahiti is eroding between 1200 and 2700 m/Ma (Hildenbrand et al., 2008). Other erosion rates, based on sediment yields and water chemistry for La Reunion (400 to 3000 m/Ma; Louvat and Allegre, 1997), Guadeloupe (400 to 1700 m/Ma; Ricci et al., 2014), and Martinique (800 m/Ma; Germa et al., 2010) are also significantly faster than erosion rates of Banks Peninsula.