| Title | GEOLOGICAL STRUCTURES OF THE OKAUIA LOW-T GEOTHERMAL SYSTEM (HAURAKI RIFT ZONE, NEW ZEALAND) INVESTIGATED USING GRAVITY DATA AND DIGITAL TOPOGRAPHIC MODEL |
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| Authors | S. Soengkono and R. Reeves |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Low-temperature geothermal system, 3D gravity modeling, alluvium sediments, normal faults, digital elevation model (DEM), permeable path |
| Abstract | Okauia Low-T geothermal system is located about 7 km northeast of Matamata in the Waikato Region, North Island of New Zealand. It is situated over the eastern edge of the southern (inland) part of the Hauraki Rift Zone (HRZ). Prior to this study, detailed gravity, ground magnetic, TDEM and DC-resistivity investigations have been carried out over this low-T geothermal system. In this study we investigate further the detailed geological structures across the Okauia Low-T system using the 3-D density model from the previous gravity investigation and a high resolution digital elevation model (DEM) of topography available from the Land Information New Zealand (LINZ). Natural surface thermal features at Okauia consist of seeps and springs with temperatures up to 40ºC along a half kilometre stretch of the Waihou River. The Okauia Low-T geothermal system is hosted by upper Pliocene alluvium sediments (the Hinuera Formation) and older Tertiary volcanic rocks (the Waiteariki Ignimbrite) which are exposed about 1 km south east of the Okauia warm springs. Horizontal gradients were calculated from horizontal slices of the 3-D density model and from the high resolution LINZ DEM data. A set of normal faults mainly oriented in the NW/W direction (the HRZ direction) can be recognised from the analyses. Two of these faults form the SW/W boundary of the sub-surface Waiteariki Ignimbrite, but there is another fault that cut through the ignimbrite formation. It appears that these faults and the sub-surface boundary of the Waiteariki Ignimbrite provide permeable paths for ascending warm fluids from deep circulation of ground water facilitated by regional faulting in the HRZ to reach the surface. |