Modelling fluid convection stability in continental faulted rifts with applications to the Taupo Volcanic Zone, New Zealand

Extreme heat flow of 750 mW/m2 recorded in the central Taupo Volcanic Zone (TVZ) is manifest mostly by convective upwelling of meteoric water in a very hot, thin rift. Despite the very high local heat fluxes and episodic normal faulting, the position and intensity of upflow zones appear to be steady over 10s or 100s of thousand years. Using values for heat flow, fluid supply and rock properties derived from physical measurements in the TVZ, and numerical simulations which can simulate effects of both deformation and heat flow on fluid transfer, we have attempted to constrain the importance of different processes in controlling the fluid flow localization. Vigorous and highly unstable convection is predicted in our numerical models if the upper crust is treated as an un-deforming, homogeneous, highly permeable material (ignimbrite). If the permeability of the crust below 4 km is reduced to two orders of magnitude lower than overlying ignimbrites (to simulate greywacke), then more cell stability is attained. Even greater stability is attained if convective heat loss is permitted from the top, rather than imposing a fixed temperature boundary. However, models that also incorporate deformation on basement faults catastrophically disrupt stable fluid convection in the upper crust, although convection cells return, in the same position if sited above basement faults, once deformation ceases. Upper crustal faulting in the TVZ does not appear to have affected the location of the upwelling cells because the permeability of the host rocks is too high, and individual faulting events may be too transient.