Recent mapping and radiocarbon dating of three giant landslides in northern Fiordland, New Zealand

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Hancox, G.T.; Langridge, R.M.; Perrin, N.D.; Vandergoes, M.J.; Archibald, G.C. 2013 Recent mapping and radiocarbon dating of three giant landslides in northern Fiordland, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/45 52 p.

Abstract: This study of three giant landslides in northern Fiordland has involved the geological and geomorphic mapping, description, and radiocarbon dating of the Lake Adelaide rock-slide avalanche (~750 Mm3), the John O’Groats rock avalanche (~1000 Mm3), and the Lake Gunn rock block-slide avalanche (~300 Mm3). These three landslides are part of a cluster of at least 58 very large (>/=1 Mm3) and giant (>/=100 Mm3) postglacial age landslides which have occurred within ~50 km of the southern section of the Alpine Fault and the underlying Puysegur (Fiordland) subduction zone. Both of these active tectonic structures are potential sources of large (Mw7) and great (Mw8) earthquakes which, based on historical evidence of coseismic landsliding in New Zealand, are considered most likely to have triggered the numerous very large bedrock collapses in Fiordland and south Westland. The calibrated 14C ages of organic material within or deposited on top of the landside deposits are thought to provide minimum ages (±2[sigma]) for the three landslides dated in our study, namely: John O’Groats Landslide: 5785 ±125 years BP; Lake Adelaide Landslide: 6144 ±143 years BP; and Lake Gunn Landslide 7630 ±52 years BP. These ages are generally consistent with the age of the beech forest on the landslide deposits, multiple displacements of the landslide deposits in John O’Groats valley by the Alpine Fault over the last ~6000 years, and only moderate to minor erosional modification of the landslide head scarps, source areas, and slide debris deposits. The ages of these landslides have been tentatively correlated against the paleoseismic record of earthquakes on the Alpine Fault over the last 8000 years at Hokuri Creek. Based on that record, the John O’Groats Landslide is correlated with the Hokuri Creek event Hk15 (3951–3761 years BCE, or 5902–5711 years BP); the Lake Adelaide Landslide with Hk16 (4237–4055 years BCE, or 6187–6005 years BP); and the Lake Gunn Landslide with Hk21 (5752–5422 years BCE, or 7702–7372 years BP). However, because the dates for the Lake Adelaide and John O’Groats landslides may be minimum ages, they could have occurred during earlier large earthquakes. Because there is no similar record of past earthquakes on the Puysegur subduction zone, that potential earthquake source cannot be confirmed or ruled out, although its existence is believed to increase the likelihood that the three landslides studied, and others like them in the region were earthquake-triggered. Given the 14C dating and historical precedent evidence available at present, we therefore conclude that the Lake Adelaide, John O’Groats, and Lake Gunn landslides were most probably triggered by large or great earthquakes on the Alpine Fault or the subduction zone between 5500 and 7500 cal. years BP. Although other faults in the region are capable of producing large earthquakes, the Alpine Fault a nd the Puysegur subduction zone are believed to be the most likely sources of >/=MM9 shaking at the landslide sites. This initial study has made significant progress in the dating of giant landslides in Fiordland by establishing the probable age of the John O’Groats, Lake Adelaide, and Lake Gunn landslides, and linking them to the Alpine Fault earthquakes that are likely to have triggered landslides of that size. However, more field work and landslide dating, especially of samples that can provide maximum landslide ages, is required before a clear understanding of postglacial large landslides triggered by Alpine Fault earthquakes, or earthquakes on the Puysegur subduction zone can be established with more certainty. (auth)