Modelling the Flow of Hydrothermal Fluids in the Crust Above an Evolving Continental Rift

Abstract

We use a thermo-mechanical code (SULEC) in combination with a supercritical version of TOUGH2 to model the evolution and deformation of a continental rift over millions of years. SULEC can sequentially couple Darcy fluid flow, heat flow and rock mechanics. Rock deformation is a combination of elasticity, thermally-activated dislocation creep, and pressure-dependent brittle yield. We use SULEC to predict the change in crustal properties and the evolution in the brittle-ductile transition. A simple melt function is used model partial melt in mantle and crust, although advective transport of heat by melt migration is not included. For each SULEC output time, the basal heat-flow, material properties, degree of melting, brittle and ductile material evolution and rift geometry and an inferred permeability distribution for the upper 30 km of the crust are passed to TOUGH2, and the evolution of fluid flow is followed for 100-200 kyr. This one-way coupling allows us to examine the changing fluid flow regime predicted by TOUGH2 for the crust as the rift evolves.

We find that the spatially evolving brittle-ductile transition predicted by SULEC has a strong influence on the fluid flow regime predicted by TOUGH2. Since the brittle-ductile transition depends on temperature, pressure, ambient strain-rates, and fluid pressure, we can demonstrate the feedback between deformation and crustal fluid flow for an evolving rift such as the Taupo Volcanic Zone. In addition, the development of partial melt predicted by SULEC changes the flux of heat and fluid into the TOUGH model in time.