Paleoseismicity of the Rotoitipakau Fault Zone, a complex normal fault in the Taupo Volcanic Zone, New Zealand

Trenching of 6 of the 10 fault strands that comprise the 5 km long, 1 km wi de Rotoitipakau Fault Zone, Taupo Volcanic Zone, New Zealand, shows that su rface rapture has occurred at least eight times in the zone during the past 8500 yr. Single-event displacements on each strand vary from a few decimet res to perhaps more than 2.5 m, and there may have been as much as 4.5-5.0 m of cumulative slip on several strands in a single event. Five airfall tep hra whose ages span the past 8500 yr provide time lines within which cumula tive slip rate has varied by approximately 10 times: from 1-2.5 mm/yr in th e period from the AD 1886 Tarawera Tephra to the present, and in the c. 400 0 yr period from the 4.8 ka Whakatane Tephra to the 0.65 ka Kaharoa Tephra; to I I mm/yr in the c. 500 yr period from 0.65 ka to AD 1886. All but one of the fault strands are downthrown to the southeast, suggesting that the f ault zone is part of the western margin of the Whakatane Graben. This short fault zone may therefore be a splay of the Braemar Fault Zone, and the ver y large displacement:length ratios characteristic of fault rupture in this fault zone may be misleading if the fault connects to the southwest with th e Braemar Fault. Alternatively, faults in this volcano-tectonic province ma y exhibit different surface faulting characteristics than normal faults in nonvolcanic regions where most of the fault parameter scaling relationships have been developed. Large displacement:length ratios are a characteristic of fracturing above dike intrusions. Such fracturing is accompanied by wel l-defined grabens and single-episode (monogenetic) formation. In the Rotoit ipakau Fault Zone, the lack of graben development, and polygenetic movement history, indicates the fault zone is a primary tectonic feature.