Using a tephrostratigraphic framework to determine the past 40,000 yrs of fault rupture and paleohydrothermal activity on the east strand of the Whirinaki Fault, Ngakuru Graben, central Taupo Volcanic Zone

The Ngakuru Graben in the central Taupo Volcanic Zone, North Island, New Zealand, is largely thermally inactive, but several fossil geothermal systems have been mapped throughout the area parallel to the Whirinaki Fault (WF). A fossil geothermal system was identified on the northeast section of the east strand of the WF at a location on Hossack Road hereafter referred to as the Meade-Hossack site. The aim of this study was to establish a history of fault rupture and sinter development at the Meade-Hossack site, and to relate the observed faulting and paleogeothermal activity to that at other locations on the WF. The objectives were to (i) describe the stratigraphy exposed in a trench and outcrop, (ii) identify (correlate) tephras using stratigraphic principles, ferromagnesian mineralogical assemblages, and electron microprobe-determined major-element compositions of glass shards, and (iii) restore tectonic deformation at the site using tephrochronology to date the timing of fault movement and the development of sinter formation. A paleoseismic trench was excavated across the northeast section of the east strand of the WF. The stratigraphy and deformation in the trench were logged. Samples of tephras were taken for mineral and glass shard analysis by microscope petrography and electron microprobe analysis, respectively. Eight tephras were identified, namely the Kaharoa (AD 1314 ± 12 or 636 ± 12 cal. yr. BP) (95% probability age range), Taupo (AD 232 ± 10 or 1718 ± 10 cal. yr. BP), Rotoma (9423 ± 120 cal. yr. BP), Rotorua (15635 ± 412 cal. yr. BP), Okareka (21858 ± 290 cal. yr. BP), Kawakawa (25358 ± 162 cal. yr. BP), Poihipi (28446 ± 670 cal. yr. BP), and Tahuna (c. 40 ka), as well as the Ohakuri Ignimbrite (c. 240 ka). Five fault rupture events were identified in the trench (MH1–5). The trench logs were used to reconstruct earlier stages of the stratigraphic succession at the site prior to deformation by each rupture events. The tephra ages were used to constrain the timing of fault movement. The slip rates and the vertical displacement measurements were also calculated using tephrochronology, and correlated with trends (fast-slow-fast) for events of similar ages observed in other trenches on the WF. Rupture event MH1 occurred between the Taupo and Kaharoa eruption episodes at a slip rate of 0.87 ± 0.37 mm/yr, MH2 between the Rotoma and Taupo eruptions at a rate of 0.11 ± 0.20 mm/yr, MH3 between the Rotorua and Rotoma eruptions at a rate of 0.20 ± 0.22 mm/yr, MH4 between the Okareka and Rotorua eruptions at a rate of 0.13 ± 0.27 mm/yr, and MH5 between the Ohakuri and Okareka eruptions at a rate of 0.53 ± 0.24 mm/yr. The recurrence interval during the last 21.8 kyrs was calculated to be 1.8–5.4 kyrs, which fits within recurrence intervals calculated for other trenches on the WF. The rupture events at the Meade-Hossack site show evidence of stress transfer from the southeast section of the east strand as well as the northwest section of the west strand of the WF, indicating that the strands are connected at depth. Sinter deposition mirrors that on the west strand from ~40–10 ka, indicating that the paleogeothermal fields were fed by the same source at depth.Sinter deposition mirrors that on the west strand from ~40–10 ka, indicating that the paleogeothermal fields were fed by the same source at depth. Fault rupture MH3 at the Meade-Hossack site altered the local fluid flow regime, leading to the cessation of sinter development on the east strand by 9423 ± 120 cal. yr. BP