Evaluation of crustal fluid flow in Mesoarchean granite-greenstone terranes via numerical methods

The movement of material, fluids and heat in geological terranes are at the heart of the Earth’s geological history. Through a given tectonic regime the facilitated movement of matter and energy may form ore deposits. A recent thermo-mechanical experiment was conducted to determine whether the gravity driven tectonic process (sagduction), described for Mesoarchean for granite-greenstone terranes, provided the conditions for ore deposit formation. The results of this numerical experiment indicate that the sagduction process is associated with significant lateral temperatures gradients (∼ 26◦ C/km) therefore providing the heat engine required to develop hydrothermal convection cells necessary for ore deposit formation. This dissertation was undertaken to ascertain the conditions required for the generation of convective cells via a detailed fluid flow investigation.

Two sets of fluid flow simulations were undertaken by a one-way coupling of data and processes. Data from Ellipsis, a thermo-mechanical numerical code, such as Geology, Temperature and Strain-rate served as inputs to SHEMAT, a numerical code to solve fluid flow related equations. The first set of simulations serviced as benchmark models mimicking a generalised granite-greenstone terrane. Different permeability conditions and geometries were applied to these models with increasing complexity. Rock alteration index (RAI) was applied to characterise the behaviour of convective cells in the context of the changes in thermal gradient. RAI is constrained further to reflect a conservative second order fluid movement (10−8 to 10−10 m.s−1) in a regional and contact metamorphic setting. The second set of simulations was reproduced from the thermo-mechanical experiment of gravity driven tectonic in a Mesoarchean setting. In this second set of experiment, an evaluation of the dynamic permeability was calculated from strain-rate values generated through the thermomechanical simulations. These strain-induced zones act as fluid pathways and improving the model’s ability for fluid convection.

The result of this study show that early time steps of the sagduction process are associated with thermal regimes capable of developing hydrothermal convection cells heavily influenced by permeability conditions. As the deformation process progressed, the greenstone keel is associated with the development of permeability channels that are funneling fluid flow. These strain-induced permeability pathways were associated with an increase of fluid velocity up to three orders of magnitude. As the deformation process proceed the dynamic permeability homogenise in the upper part of the greenstone keel and fluid flow organised into fluid convection cells in the upper part of the model. In the lower part of the model the fluid flow experience unidirectional advective flow. In the final stage of the sagduction process the distribution of the permeability field reduce the development of hydrothermal convection cells possibly following the thermal equilibration over the crustal profile.

Overall, the high geothermal gradient strongly appear to apply a first order control on the fluid flow regime at the various time-steps of the simulation including a combination of advection and convection patterns. However, the results obtained must be interpreted with care and several important limitations will need to be overcome in order to generate fully coupled simulations. The limitations highlighted in this dissertation include amongst others the need for more realistic characterisation of the dynamic permeability controlling the development of hydrothermal system in a crustal setting.