Interpretation of geophysical data from the eastern margin of the Okataina Volcanic Centre

Lithgow R, Massiot C, Bertrand EA, Heise W, Miller CA, Bennie SL, Brakenrig T, Coup L, Macdonald N. 2022. Interpretation of geophysical data from the eastern margin of the Okataina Volcanic Centre. Lower Hutt (NZ): GNS Science. 25 p. (GNS Science report; 2022/12). doi:10.21420/FPEK-CG53.

Abstract
The Okataina Volcanic Centre (OVC) is an active caldera complex located in the Taupō Volcanic Zone that has experienced at least four probable caldera collapses since 550 ka. Imaging the shape of the caldera and its margins is important to reconstruct the timing and mechanisms of the successive caldera collapses, and provide the context to interpret monitoring signals of the volcano’s activity. However, topographic margins observable on the surface do not necessarily reflect the major structures where the collapse is focused at depth. The structural caldera margins are covered by deposits from more recent eruptions and require geophysical surveys to image the subsurface. For example, existing gravity data cover the entire OVC and identify the eastern caldera margin across the Puhipuhi Basin as the steepest gravity gradient in the region, indicating a distinct boundary despite a lack of topographical relief. While these data identify the caldera margin, additional high-resolution geophysical surveys, sensitive to other physical properties (e.g. resistivity), are required to evaluate the depth of the basement and geometry of the caldera margin.
This study presents the interpretation of two geophysical surveys that both measure subsurface electrical properties but at different resolutions and depth extents: magnetotelluric (MT) and electrical resistivity tomography (ERT) along a 2 km section of Tarawera Road and with an extension to the west. These MT and ERT datasets were inverted independently and then compared together with the gravity data.
The inverted MT data imaged to 2 km depth below surface while the ERT survey imaged to a shallower depth of approximately 200 to 300 m but with higher resolution. The MT model shows a clear lateral contrast in resistivity at the location of the strong gravity gradient inferred to mark the caldera margin; conductive within and resistive outside the caldera. The ERT survey agrees well with the shallow portion of the MT model to ~200 m depth and shows a horizontal highly resistive layer at 150 m above sea level with another resistive body below at 50 to -150 m above sea level.
The combination of techniques (MT, ERT and gravity) suggests that the caldera margin forms a fault zone over a 1 km-wide zone dipping to the west. The depth to the top of the greywacke basement is inferred at ca. 300 to 500 m below ground, though uncertainties remain. The combined techniques clarified the position of the structural margin. This study will help improve our understanding of the mechanism of caldera collapse at the OVC. (The authors)