Numerical simulations of brine migration by topographically driven recharge

Abstract

The migration of abnormally warm, saline water through the Appalachian basin and North American midcontinent in Paleozoic time has been inferred from fluid inclusion studies, remagnetizations, and widespread potassic alteration. A time‐dependent numerical model of fluid, heat and solute transport is used to evaluate the viability of topographically driven recharge as a mechanism for brine migration. The model represents a wedge‐shaped sedimentary basin 400 km long by 6 km deep (maximum) with a basal aquifer 500 to 750 m thick overlain by a homogeneous aquitard. Temperature predicted by model simulations is found to be inconsistent with constraints inferred from fluid inclusion studies, unless average heat flow values greater than about 100 mW/m2 are used. Model simulations also lead to predictions of low heat flow and subsurface temperature in recharge zones that are generally not observed in modern orogenic zones. The initial solute content of pore waters in the model basin is flushed out by fresh water entering in the recharge zone before fluid velocities high enough to produce significant warming of the discharge zone can develop. Model simulations with source terms reveal that basin sediments can provide enough solute to maintain hot, hypersaline brine migration for about 1 m.y., at most. High fluid velocity in the basal aquifer is required to carry heat to the basin margins, but the higher the fluid velocity, the more quickly the basin's supply of solute is exhausted. Consideration of these constraints implies that topographically driven recharge may be an effective mechanism to explain regional brine migration only if flow is focused from regional scale recharge zones into more spatially restricted discharge zones.