Kinematics of Rifting at the Transition from Continental to Seafloor Spreading in the Northern Taupo Volcanic Zone and Bay of Plenty

Abstract:

The transition between magmatic rifting and back-arc seafloor spreading occurs in the Whakatane rift zone beneath the Bay of Plenty, New Zealand. The time-space patterns of faulting and magmatism within the northern Taupo Volcanic Zone (TVZ) onshore and its offshore continuation in the Whakatane graben remain poorly understood. We use seismicity data from the 7-station Back-Arc Rifting in New Zealand (BARNZ) temporary seismic array and permanent GEONET stations to unravel the kinematics of linkage between the TVZ, the North Island Dextral Fault Zone, and the Whakatane graben. The array captured a Mw4.1 earthquake and aftershock sequence in 2018 near Kawerau, and in 2019 an extended sequence of earthquakes at the Whakatane graben and Whakaari (White Island) volcano. We use the catalogue earthquakes recorded between October 2017 and October 2019 to determine first motion focal mechanisms. The solutions are primarily NE-SW striking normal faults or dextral strike-slip, with one nodal plane subparallel to the N-S striking North Island Dextral Fault Zone. We use a grid search algorithm to invert for principal stresses within different tectonic domains of this complex region. For events within the 2018 Kawerau swarm that initiated with a Mw4.1 earthquake, and the May-June 2019 Whakatane graben swarm, we use waveform cross-correlation and double difference relocation to map the time space patterns of faulting, and to evaluate links between faulting and magmatism.