Quantifying Water Diffusivity and Metamorphic Reaction Rates Within Mountain Belts, and Their Implications for the Rheology of Cratons

Abstract

The distribution of rheologically strong cratons, and their weakening by metamorphic hydration reactions, is of fundamental importance for understanding first-order strength contrasts within the crust, and the resulting controls on the tectonic evolution of the continents. In this study the Douglas Harbour structural window within the Paleoproterozoic Trans-Hudson Orogen of Canada is used to study the hydration of the footwall Archean Superior craton basement by water released from the overlying Paleoproterozoic Cape Smith thrust-fold belt. Phase equilibria modelling is applied to quantify the Archean and Paleoproterozoic metamorphic conditions, and to determine the effect of hydration on basement mineralogy. The amount of structurally-bound water within the basement is calculated and shown to decrease as a function of distance below the basal d\'ecollement of the thrust-fold belt. Applying a reactive fluid transport model to these results, the rate coefficient for fluid-rock reaction is constrained to be 10$^{-19}$~mol$^{-1}$m$^{3}$s$^{-1}$, and the diffusivity of water through the grain boundary network to be 10$^{-9}$~m$^2$s$^{-1}$ at the ambient metamorphic conditions of 570~$^{\circ}$C and 7.5~kbar. This newly-documented rate of water diffusion is three orders of magnitude slower than thermal diffusion, implying that hydration by diffusion may be the rate-limiting factor in the weakening of cratons, and therefore plays an important role in their geological persistence. This conclusion is consistent with field observations that Paleoproterozoic strain in the Douglas Harbour structural window is restricted to hydrated portions of the Archean Superior craton basement.