Evidence From Intermediate-Depth Earthquakes of Slab-Derived Fluids Beneath the Taupō Volcanic Zone

Ahi Tupua, the central section of the Taupō Volcanic Zone in the central North Island of New Zealand, encompasses Taupō and Ōkataina calderas and has been the most frequently active and productive silicic magma system worldwide during the Quaternary. The entire Taupō Volcanic Zone is underlain by the Hikurangi Subduction Zone where a Large Igneous Province, the Hikurangi Plateau, is being subducted, but Ahi Tupua exhibits much higher rates of magma output than either the northern or the southern sections. Trace element signatures of Ahi Tupua eruptive products suggest that both decompression melting and flux melting play a role in this rifted arc setting, with fluid flux signatures being more prevalent in regions of active caldera volcanism. Intermediate-depth (50–300 km) earthquakes provide a means of studying faulting and fluid flow processes within and around the subducting slab beneath Ahi Tupua. Using data from the national seismic network and a 13-station temporary network (“ECLIPSE”), we repick and relocate 397 intermediate-depth earthquakes of magnitudes M2.5+ that occurred in a 29-month interval, and compute focal mechanisms for a subset of 47 earthquakes. We observe some seismicity that may be within the mantle wedge but most of the relocated earthquakes occur within the crust and uppermost mantle of the subducted slab and exhibit a weak transition from predominantly normal-faulting in the slab crust to predominantly reverse-faulting in the slab mantle. No double seismic zone is observed beneath Ahi Tupua. These observations are consistent with dehydration of the slab and flow of slab-derived fluids along existing faults into the mantle wedge, that drives flux melting and accounts for the distinctive geochemical signatures and voluminous output of Ahi Tupua calderas.