Imaging of the Shallow Alpine Fault Zone (New Zealand) Using 2D and Pseudo 3D Seismic Reflection Data

The Alpine Fault Zone in New Zealand's South Island is part of the boundary between the Australian and Pacific tectonic plates. We use ultra-high-resolution 2D and high-resolution pseudo-3D seismic reflection imaging to investigate the structure of the fault and adjacent subsurface in a northern part of the Island. Employing acquisition schemes that minimise field effort while yielding high-resolution data together with fine-tuned processing strategies, we derive high-quality images of the shallow subsurface down to about 200 m depth. Gravels with a maximum thickness of ~50 m are seen on the two sides of the fault. On the hangwall side, these gravels overlie up to ~90 m of glaciolacustrine sediments.The basement underneath is interpreted to be an erosional surface that has been folded and faulted. The principal strand of the Alpine Fault Zone is shown to have a dip of 75° - 80° from the surface to the basement at ~60 m depth. The dip at greater depths is not well constrained, but is likely to range from 50° - 80°. The ~25 m apparent vertical offset of the basement yields an average dip-slip rate of 2.0 ± 1 mm/yr.