A late quaternary extension rate in the Taupo Volcanic Zone, New Zealand, derived from fault slip data

Abstract

A northwest-southeast oriented extension rate from faulting for a time-averaged period of c. 50 000 yr (10 000-64 000 yr), across the Ngakuru-Waikite depression (modern Taupo Fault Belt, central Taupo Volcanic Zone), has a best estimate of 1.9 mm/yr (in a range of 1.2-2.8 mm/ yr) in the near surface, but increases to a best estimate of 6.4 mm/yr (in a range of 3.6-10.2 mm/yr) at seismogenic depths of 6-10 km. We obtain this result by summing the vertical components of fault displacement across known-age surfaces, or as the vertical component of displacement in stratigraphic units of known age, within the 14 km wide zone of active normal faulting. We convert the summed vertical slip rate of 7.2 ± 0.4 mm/yr to dip-slip displacement rate and to northwest-southeast extension by estimating a range of possible fault plane dips at the surface and at seismogenic depth. Fault displacement at seismogenic depth in large events is on average 1.6 times larger than at the surface, and for earthquake magnitudes of M6.8 and smaller, about one-third of the displacement occuring with the whole Gutenburg & Richter distribution of earthquakes in the modern Taupo Fault Belt will not rupture to the ground surface. Fault dip averages c. 75° in the near surface, but is poorly constrained at seismogenic depth in the Taupo Fault Belt. From a variety of local and literature considerations, we propose a dip of c. 60° at seismogenic depth in the Taupo Fault Belt. Our observations suggest only a minor component of extension at the surface (c. 5%) is contributed by small scale faulting below our observation threshold of 0.1-0.5 m of fault slip. The c. 4.5 mm/yr difference in extension rate between seismogenic depth and the ground surface may represent the surface extension rate caused by a combination of opening of extension fractures and penetrative grain-scale extensional deformation.