PRELIMINARY ON-FAULT EVIDENCE FOR THE MOST RECENT ALPINE FAULT SURFACE RUPTURE, GAUNT CREEK, NEW ZEALAND

The dextral-reverse Alpine fault forms the boundary structure between the Australian and Pacific plates, and is one of the largest sources of seismic hazard in New Zealand. The central section (~200 km) of the fault has no previously documented on-fault evidence of the most recent surface rupture (MRSR) and is a challenging setting to obtain paleoseismic data due to thick rainforest and high relief along the rangefront of the Southern Alps. Using a combination of light detection and ranging (LIDAR), field geologic and geomorphic mapping, we located an unmapped recent scarp of the central Alpine fault next to Gaunt Creek in Westland. There we excavated an exposure along the edge of a fluvial terrace riser where it intersects this scarp and exposed a 4 m high thrust scarp partially buried by post-earthquake onlapping alluvium. We logged this trench exposure and obtained radiocarbon samples from faulted sediments to assist in dating paleoearthquakes along the Alpine Fault. Here we report this fault-zone as a clay gouge zone that thrusts hanging wall Pacific-plate mylonites and cataclasites over unconsolidated late-Holocene footwall alluvium. Radiocarbon dates from the scarp coupled with the tectonic geomorphology suggests that this scarp was generated during the most recent earthquake along the Alpine Fault, which most likely occurred around 1717 AD. This exposure at Gaunt Creek represents the first exposure of the MRSR in bedrock along the Alpine Fault and represents an important site to characterize recent dynamics of the fault. Further analysis at the site may yield slip-rate data and further sampling may provide insight into earthquakes along the Alpine fault. Because the Alpine fault has not ruptured in historic times, it is approaching the end of its seismic cycle and is a significant seismic hazard to New Zealand.