Fault controls on the geometry and location of the Okataina Caldera, Taupo Volcanic Zone, New Zealand

Abstract

Okataina Caldera is located within the Taupo Rift and formed due to collapse following eruptions at 325 and 61 ka. Gravity, seismic reflection, topographic and geological data indicate that active rift faults pass into the caldera and have influenced its location and geometry. The caldera has a minimum gravity anomaly of − 50 mGal, is elongate north–south with an inferred minimum depth to caldera floor of 3 ± 0.5 km at the rift axis, and occupies a 10 km hard-linked left step in the rift. The principal rift faults (55–75° dip) define the location and geometry of the northwest and southeast caldera margins and locally accommodate piecemeal collapse. Segments of the east and west margins of the caldera margin are near vertical (70–90° dip), trend north–south, and are inferred to be faults formed by the reactivation of a pervasive Mesozoic basement fabric (i.e. faults and/or lithological contacts). The fault sets which define the caldera geometry predate it, while the step in the rift across the Okataina Volcanic Centre (OVC) is at least as old as the caldera. Within the OVC displacement on rift faults induced by gravitational caldera collapse at 61 ka exceeds tectonic displacement since this time by at least a factor of two. Collapse along pre-existing rift faults and, in particular, Mesozoic basement fabric are important for caldera formation elsewhere in the Taupo Volcanic Zone.