Preliminary Results From Combined Geomorphic LiDAR Mapping, Radiocarbon Dating, and Slip-rates Along The Central Alpine Fault, New Zealand
Abstract
In central South Island of New Zealand, the dextral-reverse Alpine Fault Zone (AFZ) forms the major plate boundary structure between the Pacific and Australian plates. The AFZ is thought to fail in large earthquakes (~ Mw 7-8+) approximately every 200 to 400 years, to have last ruptured around 1717 AD, and has a geologic slip rate of approximately 2.7 cm/yr. Climate plays a large role in both the surface expression of the AFZ and contributes to our limited knowledge of this important plate boundary. Very high precipitation rates (9-15 m of rain/year) and associated rainforest and steep hillslopes mask the tectonic geomorphology along the central section of the fault and limits the value of airphoto and satellite mapping. However, recently acquired LIDAR data allows accurate mapping along the central section of the fault. We used a 2 meter LIDAR-derived digital elevation model (DEM) to map geological and geomorphological features along the AFZ. A new paleoseismic site was selected along on a previously unmapped scarp based on LIDAR mapping near Gaunt Creek in Westland. The site was excavated and exposed via trenching and the scarp was found to be the surface expression of the Alpine fault with mylonites and cataclasites thrust over young unconsolidated alluvium. Radiocarbon dates from this site suggest that the most recent surface rupture along this section of the fault occurred around 1717 AD - making this the first on-fault record of this event along this ~200 km section of the fault. Radiocarbon dates from select geomorphic surfaces coupled with stratigraphic evidence from creek exposures give us range of sediment ages from Late-Pleistocene to recent around Gaunt Creek. In addition to the paleoseismic site, based on LIDAR mapping, fluvial erosion and deposition clearly dominate the landscape, with large alluvial fans at the Southern Alps range-front that are often faulted. Landslides are also common and possibly will yield information on slip-rates where they cross the Alpine Fault. One landslide crosses the fault and is massive, clast-supported, a minimum of 30 m thick, with schist, mylonite and cataclasite debris ranging from house-sized boulders to cobbles with a sandy matrix. The landslide does not appear to be faulted, however recent (<20 year) fluvial erosion at the toe of the landslide complicates timing. Non-mature rainforest is growing on the landslide and this supports a relatively young age of this deposit. The landslide is large (>225 m2), and was likely deposited either coseismically or shortly after the most recent event (~1717 AD). Through an analysis of the LIDAR data along this section of the fault, we are able to better constrain slip-rates from offsets of young geomorphic features such as streams, landslides, alluvial fans and terrace deposits - giving better insight into fault dynamics at different temporal scales. These results help us to better understand previous recent surface ruptures along the Alpine fault and by dating these geomorphic surfaces, we are able to obtain new slip-rates along this important plate boundary which will help us better constrain seismic hazards for people and infrastructure of the South Island of New Zealand.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T44A..04D
- Keywords:
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- 7221 SEISMOLOGY / Paleoseismology;
- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8111 TECTONOPHYSICS / Continental tectonics: strike-slip and transform;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution