2003 Volume 54 Issue 3-4 Pages 117-129
Characterization of geothermal systems was investigated in two regional scale volcanotectonic depressions, the Kuju-Beppu Graben (KBG), in Japan, and the young-Taupo Volcanic Zone (TVZ) in New Zealand. The distribution patterns of geothermal fields are different. In the KBG they are restricted to areas on and behind only the youngest volcanic front, but are evenly spaced throughout the whole of the young-TVZ. The youngest volcanism (<0.3 Ma) in the KBG comprises andesitic lava domes and stratovolcanoes, but is dominated by caldera-forming or dome-building rhyolite eruptions in the TVZ. The heat sources of the active geothermal fields are assumed to be andesitic magma chambers and consolidated magma plutons in the KBG, and are associated with high-level rhyolitic magma chambers in the TVZ. The different heat sources and heat flow regimes result in different geothermal field characteristics. In the TVZ shallow (500-1500 m depth), exploitable reservoirs contain boiling or near-boiling fluids (250-300oC) stored in Quaternary permeable layered aquifers of volcano-sedimentary origin lying above a poorly permeable Mesozoic basement. In the KBG the reservoirs are deeper (1000-2000 m depth) and lower temperature (ca. 220oC) because the Quaternary andesitic magma-ambient zone is believed to be deeper than that of the TVZ, and deep-penetrating groundwater extracts heat from fractured volcanics above a thicker conductive zone with lower geothermal gradient. Thus it is concluded that the heat discharged by geothermal fluids in the two volcano-tectonic depressions is greatly controlled both by the depth of the magma-ambient zones, and by the depth to which the cool groundwater convection cells can penetrate.
