Modelling geothermal circulation in a 'TVZ-like' setting

Abstract

Heat is transported through the brittle crust of the Taupo Volcanic Zone (TVZ) by up-flow of hot fluid through more than 20 distinct convective plumes, the locations of which depend upon prevailing permeability and temperature conditions. Complete knowledge of such conditions would greatly assist modelling studies investigating deep circulation or field evolution over geological time; processes that are typically impenetrable to direct surface observation. In practice, data are available only for the near surface, and for depths greater than 2 km estimates of temperature and permeability are largely inferred or reasoned. In this paper we detail a 3-D numerical model for geothermal circulation in a rifted setting, in which heat is transported through a 50 x 80 km region by numerous convection cells. The model describes statistical properties of the TVZ, matching average values or distributions of geophysical observables such as heat output across the rift and the average dimensions and properties of geothermal fields. This approach permits permeability properties and temperature boundary conditions to be specified stochastically, i.e., by an average value about which some degree of fluctuation and spatial correlation is permitted. The resulting model reproduces the average surface heat output of 0.6-0.8 W m-2 and an across strike bimodal heat distribution consistent with the TVZ. Fluid flow paths indicate that recharge is sourced from an area 5-10 km distant from the field boundary, in which fluids slowly percolate to depths of 3-7 km before rapidly ascending to the surface. Return times vary widely both within and between fields but average 10-30 kyr, of which the final 25% represents buoyant upwelling of the fluid. Over a period of 300 kyr field boundaries, as demarcated by contours of heat flow, are observed to migrate several km. Assuming distributions of hydrothermal alteration, and thus resistivity modification, correlate to regions of past and present upflow, we are able to juxtapose the current distribution of geothermal activity upon a historical context.