Postglacial (after 20 ka) dextral slip rate of the offshore Alpine fault, New Zealand

Geological displacement rates for major plate boundary strike-slip faults have seldom been determined with precision from the marine environment. In this paper I present high-quality slip rates derived from the southern Alpine fault using multibeam bathymetric data and dated samples from the Fiordland continental margin. These rates are derived from dextral displacements of relict but well-preserved glacial geomorphology (moraines and outwash fans), interpreted to 17 (+2/-1, calendar) ka. The weighted mean slip rate is 27.2 (-3.0/+1.8) mm/yr on the shelf between Milford and George Sounds, increasing southward to 31.4 (-3.5/+2.1) mm/yr between Caswell and Doubtful Sounds, with uncertainties at the 95% confidence level. These offshore rates are higher than those from an adjacent 80-km-length section of the fault on land in south Westland, and are in good agreement with global positioning system data. The southern slip rates represent some of the highest strike-slip rates on Earth, and similar to 90% of the total plate motion in southern New Zealand. The southward increase in strike-slip rate on the Alpine fault occurs despite a southward reduction in Pacific-Australian relative plate motion rate and despite the prediction of southward-decreasing plate-parallel motion and southward-increasing normal convergence. The Fiordland region is thus an excellent example of geological strain being highly partitioned within an obliquely convergent plate boundary zone.