Towards deciphering the tectono-magmatic dynamics of the Auckland Volcanic Field and Hauraki Rift

New Zealand’s largest city Auckland, 400 km into the overriding Australian plate from the Hikurangi subduction margin, sits on top of the active intraplate Auckland Volcanic Field. Low recurrence interval faults are mapped to the south of the city and ~30 km to the east within the active Hauraki Rift which is opening oblique to the plate boundary trend. Faulting within the urban area is obscured by the distributed <200,000 year old volcanics, Quaternary sedimentation and landscape modification. The potential structural control on magma ascent unclear or variable. While Auckland urbanisation provides the riskscape to motivate seismo-volcano-tectonic characterisation, the setting also provides challenges, and some advantages, to investigation. We discuss the ongoing programme of potential field and borehole studies that characterise crustal structure and geodesy, seismology, geomorphology, field studies that are also indicative of regional deformation.

The NNW-SSE trending Hauraki Rift parallels regional basement fabric characterised by the trend of the Mesozoic, ophiolite bearing Dun Mountain-Maitai basement terrane-sourced Junction Magnetic Anomaly. Geodesy has resolved a rifting rate of ~1 mm/year. Dominant NNW-NW and NE fault trends within/beneath Auckland are resolved from Lidar analysis, field mapping and reconstruction of a regional marker horizon, the “Waitemata Group Erosion Surface”, using extensive urban and urban-fringe borehole datasets. Few boreholes penetrate the deeper areas of Mesozoic basement so modelling of gravity data has proved useful to define the topography of the basement surface and interpret significant basement offsets. Although the historic 1891 ~Mw 6.2 Waikato Heads earthquake ~65 km SE of the Auckland CBD demonstrated the seismic potential in the region, rates of microseismicity are low and have been concentrated in South Auckland and the Hauraki Gulf. New seismometer deployments enhance potential resolution of spatial patterns of seismicity, with use of artificial intelligence in catalogue building investigated. These will also provide the potential for increased resolution crustal, crustal thickness, and mantle characterisation. The establishment of new campaign geodetic sites will also increase confidence in potential dislocations across proposed structures. Geomorphic and field studies attempt to characterise the paleoseismology of exposed faults.