3D structural model and slip-dilation tendency analysis of the Chiweta Zone: Geothermal system implications and fault reactivation potential

Abstract

A 3D structural model and slip-dilation tendency analysis are used to assess the reactivation potential of shear and dilatational structures to investigate structurally controlled geothermal systems in the Chiweta Zone. The Chiweta Zone is in the northern part of the Malawi Rifted Zone, where an elevated heat flow anomaly and one of the hottest hot springs occur. Structurally controlled geothermal systems are poorly understood in Malawi and the entire Western Branch of the East African Rift System. The objectives of this study were to (1) construct a three-dimensional structural geometry of the faults identified with remote sensing and aeromagnetic data, (2) perform a slip-dilation tendency analysis to assess the fault reactivation potential under a stress state condition at depths between ∼700 and 1000 m.b.g.l. of the estimated reservoir, (3) identify, based on the results, which faults are critically stressed and serve as fluid conduits or barriers of the geothermal system, and (4) combine geological, geochemical, and geophysical data with our results to improve the conceptual model of the Chiweta Geothermal System (CGS). Our findings indicate that ∼ NW- and ∼NNE-striking faults southwest- and northwest dipping, respectively, show segments with the highest dilation values of Td ∼0.6 to Td ∼0.8 and probably behave as active fluid pathways. This is supported by the Chiweta Fault, a NW-striking and southwest-dipping fault that controls the only thermal manifestations in the Chiweta Zone. NW- and NE-striking faults dipping to the northeast and southeast, respectively, show lower dilation values of Td ∼0.1 to Td ∼0.5 and probably act as fluid barriers. These results allowed us to define, for the first time, possible structural boundaries of the Chiweta geothermal reservoir and to improve its geothermal conceptual model. The results of this study may help identify favorable areas to target the first exploratory wells and where further subsurface studies (e.g., MT surveys and seismic reflection) should be conducted in the Chiweta Zone. Finally, our methodology is expected to be applied to other geothermal prospects in the Western Branch of the East African Rift System and help advance geothermal energy production in this region.