| Abstract |
Geothermal resources within the Taupo Volcanic Zone (TVZ) currently supply 13% of the energy used in New Zealand, but could make up to one third of the total electricity used in this country if fully developed. Improvement in targeting of wells to intercept high-flux fluid pathways is a key component in geothermal development, and depends in part on a good understanding of the geometry of geological structures at depth. Here we present new geological and geophysical (gravity) data to develop an improved tectonic and structural model of the Whakatane Graben (WG), which hosts the Kawerau Geothermal Field. Interpretations of new gravity and published seismic data (Lamarche et al, 2006) indicate different evolutionary stages in rifting within the WG, including an eastward shift of its western shoulder. Sediment rates within the WG have increased significantly in the last 350 thousand years due to 1) a flare-up in ignimbrite eruptions and consequent supply of volcaniclastic sediment and 2) a constriction of the depocentre for the Whakatane, Rangitaiki and Tarawera Rivers as the WG itself became narrower. Results from field work in the Awatarariki stream behind the township of Matata and from Lamarche’s (2006) offshore seismic reflection interpretation show that the accumulation of sediment and the subsequent thickness is largely fault controlled. As a consequence, applying average sedimentation rates across the whole graben (and to the full depth to basement rock) conceals a more intricate development of the WG with respect to the growth of faults and the creation and movement of discrete tectonic blocks over time. Here, we present a new picture for the evolution of the WG, and in particular, the structure and geometry of tectonic blocks with implications for what might lie beneath the Kawerau Geothermal Field. |