Abstract

Two-dimensional finite element models of coupled sediment compaction, variable-density ground-water flow, and conductive/convective heat transfer are used in this study to quantify basin hydrodynamics during the initial and flexural stages of continental rifting. The analysis also incorporates a two-stage stretching/cooling geodynamic model of the thermomechanical evolution of the lithosphere underlying the rift in order to specify geologically relevant boundary conditions for basin subsidence and basal heat flow. A sensitivity study is made using the model to explore the controls of both permeability and water table configuration in determining the dominant fluid flow drive (compaction, density, or topography) during basin evolution. The sensitivity analysis incorporates hydrologic conditions and rock properties representative of many extensional terrains.

Assuming that rift basin subsidence and basal heat flow can be represented by the geodynamic model, two distinct ground-water flow systems evolve within continental rifts during basin evolution. During the initial (stretching) phase of rifting, subsidence is accommodated by fault block motion, and a topography-driven ground-water flow system develops within the permeable alluvial-fan deposits. Within the less permeable lacustrine facies located in the center of the basin, compaction-driven ground-water flow dominates. Here, the compacting lacustrine sediments focus pore fluids laterally from the basin center into the alluvial-fan deposits due to the relatively large permeability contrast between the two depositional environments. Thermal anomalies resulting from convective heat transfer are restricted to alluvial-fan facies near the basin-framing fault. During the thermal cooling (flexural) stage of basin development, laterally extensive onlap facies are deposited, and density-driven ground-water flow dominates in the permeable alluvial-fan deposits, while compaction-drivenflow continues within the lacustrine and onlap facies. The presence of a permeable aquifer within the onlap facies resulted in long-range fluid transport to the edge of the basin. During both stages of basin evolution, ground-water velocities varied from 10-5 to 10-1 m/yr between the lacustrine and alluvial-fan deposits, respectively. The observed presence of ore mineralization within alluvial-fan deposits of some continental-rift systems, such as the Cretaceous Rift Basin of Angola, supports the findings of this study.

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