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

A northwest-southeast oriented extension rate from faulting for a time-aver aged period of c. 50 000 yr (10 000-64 000 yr), across the Ngakuru-Waikite depression (modem Taupo Fault Belt, central Taupo Volcanic Zone), has a bes t 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 ve rtical components of fault displacement across known-age surfaces, or as th e vertical component of displacement in stratigraphic units of known age, w ithin 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 t o northwest-southeast extension by estimating a range of possible fault pla ne dips at the surface and at seismogenic depth. Fault displacement at seis mogenic depth in large events is on average 1.6 times larger than at the su rface, and for earthquake magnitudes of M6.8 and smaller, about one-third o f the displacement occurring with the whole Gutenburg & Richter distributio n of earthquakes in the modem Taupo Fault Belt will not rupture to the grou nd surface. Fault dip averages c. 75 degrees in the near surface, but is po orly constrained at seismogenic depth in the Taupo Fault Belt, From a varie ty of local and literature considerations, we propose a dip of c. 60 degree s at seismogenic depth in the Taupo Fault Belt. Our observations suggest on ly 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 dep th and the ground surface may represent the surface extension rate caused b y a combination of opening of extension fractures and penetrative grain-sca le extensional deformation.