Thermochemical convection in and beneath intracratonic basins: Onset and effects

Abstract

Observations of episodic subsidence and high organic maturity in the intracratonic Michigan basin cannot be explained by simple conductive models. Moreover, the sylvinite deposits within the basin indicate an evaporite evolution other than a simple evapoconcentration of standard seawater. By means of numerical simulations we have investigated whether these deviations can be attributed to periods of free thermochemical convection (TCC) of aqueous fluids within a fractured igneous rift body underlying the sediments. Since crustal rocks are known to be heterogeneous and anisotropic, we have first investigated the onset of TCC in an idealized domain, for temperature and salinity linearly increasing with depth. The sensitivity study considers several heterogeneous permeability fields, by varying the stochastic correlation lengths of the medium and by introducing anisotropy on the subgrid scale. Due to destabilization of the fluid in pockets of relatively high porosity, the onset of convection in the heterogeneous media occurs at ratios of chemical to thermal buoyancy 1 order higher than in homogeneous media. Next we have studied the thermal, chemical, and subsidence effects of the convective flow in and beneath the Michigan basin, for the situations before and after closure from the open sea. TCC explains the elevated level of organic maturity in the sediments and up to a quarter of the observed fast subsidence rates. Finally, we argue that basement involvement could have changed the seawater composition, and so leads to the direct deposition of sylvinite.