Late quaternary hydrothermal explosion breccias at Kawerau Geothermal Field,New Zealand

Two large (10⁶–10⁷ m³ erupted volume) hydrothermal explosions occurred from craters on the eastern margin of Kawerau Geothermal Field at c. 14,500 and 9,000 yrs B.P. Explosion products are interbedded within C14 dated pyroclastic fall deposits and contain clasts of hydrothermally altered ignimbrite, rhyolite and tuff, in a silty hydrothermal clay matrix. No magmatic ejecta are found. Some ejected blocks record earlier pre-eruption episodes of shallow hydraulic fracturing and silica cementation. Drillhole stratigraphy indicates that explosion extended to about 190 m below present ground level. The explosion is analysed as a rock/water interaction with eruptive energy provided by flashing of about half the available water. Although surface heat flow and shallow temperatures are now low at eastern Kawerau, the hydrothermal explosions demonstrate the previous existence of a high temperature shallow geothermal system, probably related to a major fault feeding water up through the basement.