Inferring the magmatic roots of volcano-geothermal systems in the Rotorua Caldera and Okataina Volcanic Centre from magnetotelluric models

Abstract

To investigate the underlying magmatic systems that drive high-temperature geothermal fields and volcanism in the north-central Taupo Volcanic Zone (TVZ), we use a combination of land and lake-bottom magnetotelluric (MT) measurements. In total, MT data from 399 sites (including 34 lake-bottom locations) across 2800 km2 covering the Rotorua Caldera and Okataina Volcanic Centre are inverted to image the 3-D resistivity structure of the crust to 20 km depth. Beneath this region, a singular conductive zone in the mid-crust is shown, inferred to represent a complex of relatively mafic magma, consistent with petrologic and other geophysical data. Discrete low-resistivity (conductive) plumes rise from the margins of this magmatic complex and connect 1-to-1 with the locations of high-temperature geothermal fields at the surface. These plumes are inferred to represent pathways of heat transfer through the crust via magmatic intrusion and exsolved saline fluids, and inform connections with other geothermal areas located nearby to the major fields. An additional conductive plume rises beneath the area where the most voluminous recent volcanism has occurred in the Okataina Volcanic Centre; the summit region (e.g. Makatiti dome) in the Haroharo Volcanic Complex that has erupted 39 km3 (magma equivalent) in the past 9 ka. While hot springs locate around this conductive plume at the base of the rhyolite domes, the plume itself is arrested at 1–2 km depth beneath the resistive (inferred impermeable) young (5 ka old) rhyolite lava domes, and is considered to represent a deep ‘blind’ geothermal system in the TVZ.