Gravity Modelling of Reporoa Basin, Eastern Taupo Volcanic Zone (TVZ), New Zealand

Abstract

Gravity data over the Reporoa Basin are interpreted using a new approach of three-dimensional (3D) modelling currently being developed by the author. With the help of upward continuation technique, the gravity anomalies are separated into a series of spatial components assumed to be associated with different “depth slices”. Each set of spatial components is used to interpret the lateral density variation of its associated depth slice. The modelling allows semiautomatic determination of subsurface density distribution in 3D. The modelling output indicates that below 2km depth the surface of TVZ greywacke basement is difficult to resolve using gravity data because of the small (much less than 0.06 g/cc), but also variable density contrast between the greywacke and the volcanic infill. However, it also appears that deeper density variation within the TVZ basement may be detected using this modelling approach. The gravity interpretation shows that the Reporoa geothermal field is located close to the northwestern edge of a sub-circular low-density structure of the Reporoa Caldera. The caldera structure extends down to about 500m depth. A similar shallow circular low density structure occurs about 12 km SW, immediately west of the Ohaaki geothermal field. Below 500 m depth the two merge into a single NE-SW elongated low density structure marking the Reporoa Basin down to about 1500 m depth. The gravity modelling output also indicate that the Paeroa Range west of Reporoa Basin is associated with dense volcanic rocks extending down to about 800 m depth. This block of dense rocks follows the western/north-western edge of the Reporoa Basin. The gravity interpretation approach used in this study can be applied to other studies geothermal areas in complex volcanic setting and will help to reduce the bias often needed to be introduced during a more conventional forward modelling.