Seismic reflection imaging of the Haraharo Caldera boundary beneath Lake Tarawera, Okataina Volcanic Centre, New Zealand

Abstract

The Okataina Volcanic Centre forms one of the most active volcanic centres of the Taupo Volcanic Zone, New Zealand, having erupted c. 400 km3 of magma over the last 380 000 yr. Lake Tarawera lies within the Haroharo collapse caldera formed by two major Okataina ignimbrite eruptions at 0.28 Ma and 65 ka. Marine seismic surveying has imaged the Mamaku Ignimbrite basement horizon dipping east beneath Lake Tarawera. This horizon terminates mid‐lake at a fault with a throw >300 m, interpreted as the caldera collapse boundary associated with the 65 ka Rotoiti eruption. The dip of the Mamaku Ignimbrite and overlying sediment are the result of subsidence peripheral to the >60 km3 of collapse deduced within the central Haroharo Caldera. Two prominent, highly reflective, reversed phase sedimentary interfaces are interpreted as gas‐charged pumice layers deposited as late phases of the 5 ka Whakatane and 7.5 ka Hainini eruptions. Island‐like stratigraphy suggests a shallow (<10 m) lake level at the conclusion of the Whakatane eruption. Lake‐floor mounds and gas‐charged disrupted sediments are interpreted to have formed by the passage of water, with entrained volcaniclastic sediment and gas, up faults to the near surface.