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

Abstract

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% confi dence 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 ~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 Pacifi c-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.

National Seismic Hazard Model for New Zealand: 2010 Update

“…Slip rates are well constrained, and decrease northward from 31 mm=yr off central Fiordland (Barnes, 2009), to 23 mm=yr in southern Westland (Sutherland, Berryman, and Norris, 2006), and to 14 mm=yr at the southern end of the northern segment .…”

Deriving a long paleoseismic record from a shallow-water Holocene basin next to the Alpine fault, New Zealand

“…North of Haast, the slip rate is slightly higher (27 ± 5 mm/yr), there is a minor dip-slip component, and the fault dips to the southeast Cooper, 2001, 2007). The offshore fault section, south of Milford Sound, also has a higher slip rate at 27-31 mm/yr (Barnes, 2009). Trench exposures along the Alpine fault have been used to obtain a record of three surfacerupturing earthquakes since A.D. 750 ± 50 (Berryman et al, 2012a;Yetton and Wells, 2010).…”

“…1A; Norris and Cooper, 2001;Sutherland et al, 2006). With a strike-slip rate of 23-31 mm/yr (Barnes, 2009;Norris and Cooper, 2007;Sutherland et al, 2006), it is one of the fastest-slipping faults in the world and is considered to rupture in large to great earthquakes (Sutherland et al, 2007). The Alpine fault has a remarkably straight 400 km onshore trace at the western edge of the Southern Alps ( Fig.…”

A post-glacial relative sea-level curve from Fiordland, New Zealand

“…Fiordland lies in a complex tectonic terrane of primarily igneous and metamorphic rocks lying just southeast of the convergent PacificAustralian plate boundary (King et al, 2008;Barnes, 2009;Turnbull et al, 2010). As a result, Fiordland is uplifting, which poses a challenge for sea-level reconstruction and requires an uplift correction to be applied (House et al, 2002(House et al, , 2005.…”

“…2). During the LGM, glaciers were grounded at, or near, entrance sills at the fjord mouths (Barnes, 2009) when sea level was at least 100 m below modern levels (Pickrill et al, 1992). Entrance sill depths range from 30 to 120 m (Stanton and Pickard, 1981;Wing et al, 2003;Knudson et al, 2011).…”