ENERGY-TRANSPORT PROCESSES IN A BRITTLE-DUCTILE INTRUSIVE MODEL OF THE TAUPO VOLCANIC ZONE, NEW-ZEALAND

The implications of the findings of recent GPS and micro-seismic studi es in the Taupo Volcanic Zone (TVZ), New Zealand, on models of process es transporting mass, heat and chemicals are discussed. It is argued t hat in addition to the well established process of groundwater convect ion extracting heat and chemicals by interacting with magmatic intrusi ves under the TVZ, that two other processes may be important. Firstly, the existence of a ductile layer with very low permeability between a bout 8 to 15 km depth will produce a region of 'enhanced conduction' i n which very high conductive fluxes of energy arise from a temperature distribution which varies exponentially with depth. Secondly, water m ay transport up through the ductile layer, as a result of extensional processes in the ductile region. If extension is occurring at about 8 mm/yr, then geothermal heat transfer in the TVZ of about 4200 MW is ma de up from about 1200 MW from the cooling of intrusives in the brittle region in the upper 8 km; of about an additional 1900 MW of conducted heat entering the brittle region from the ductile region; and about a n additional 1100 MW from water transport through the ductile region. Provided this water flow has a chloride concentration similar to that emitted from nearby volcanoes, then the total chloride transport from the TVZ is about 3.5 kg/s, as suggested by average enthalpy to chlorid e ratios in the TVZ of about 1.2 MJ/g. The present high heat and mass transport processes in the TVZ are assumed to result from the passive filling of volume created from extensional processes under the TVZ, pl us conductive and/or convective heating processes below 15 km depth. ( C) 1998 Elsevier Science B.V. All rights reserved.