Permeability in Geothermal Systems Associated with Continental Rifts

Abstract

Continental rifts form mainly as half grabens with a border fault system down one side. Heat flow is highest in the centre of the half graben, but deep recharge is likely to be mainly associated with the border fault system. A volcanic centre must therefore form on the edge of the half graben or crosscutting permeable structures must exist for a significant geothermal system to form. Alternatively where there is permeability in pre-rift rocks, a geothermal system may form where they intersect the border fault system. Cross cutting structures can form in rifts where there has been a change in tectonic forces. This is the case in the East Africa rift where basement faults have been reactivated as NW strike slip faults which have produced associated tensional features. Where there is permeability in pre-rift rocks, permeable targets at depth are where the pre-rift permeable feature intersects the border fault system. Where there are strike slip faults, the associated tensional features are the targets at depth, otherwise normal faults parallel to the rift are the main target.