Seismic Velocity Structure of the Shallow Part of the Alpine Fault and Gravity Study of the Basement Features in the Whataroa River Flood Plain, Central Westland, South Island

<p>The deep and middle sections of the Alpine fault have extensively been studied, however, the shallow part has had relatively minor geophysical attention. This study focuses on the basement geometry and the determination of the upper-crustal velocity structure of the Alpine fault in the vicinity of the Whataroa River flood plain in Central Westland, South Island. Data from a temporary gravity survey collected in November 2006, the GNS gravity database and four of the westernmost shot gathers from the SIGHT96's transect 1 were used for this project. A ray-tracing software was used to establish the velocity structure of the shallow part of the Alpine fault. Seismic velocities decrease to 3.8 km/s immediately southeast of the mylonite strip, which is adjacent to the Alpine fault's ramp heading towards the fault's surface trace from the southeast or from depth. Velocities of 5 km/s reach 2 km depth to the southeast of the Alpine fault's ramp. Results of the gravity and seismic models coincide in the positions and the dimensions of two northwest-orientated glacial overdeepings. The strike of their alignment is offset to the northeast by 3.5 km and is sub-parallel to the mouth of the Whataroa River. We propose that these kettle holes, thought to have been carved successively during the Waimea and Otira glaciations, are the beheaded river mouth of the Whataroa river. By supposing that the furthest kettle hole was carved during the Waimea glaciation, the 3.5 km offset thus corresponds to 140 Ka of dextral slip on the Alpine fault, we could approximate the mean displacement rate over the time interval of 140-18 Ka of 25 mm/yr.</p>