Structural Controls and Slip Tendency in the Chiweta Geothermal Zone: Northern Part of the Malawi Rift, Africa

摘要

The Chiweta hot spring is one of the hottest hot spring with reservoir temperatures estimated at ～200 °C by cation geothermometers. The Chiweta hot spring is located in the northwest of the Malawi Rift, where volcanic activity is not present. However, it is suggested that the Chiweta's geothermal system is strongly controlled by faults. We used Shuttle Radar Topography Mission Digital Elevation Model to map brittle structures on the surface and characterize the favorable structures that may control the storage of the Chiweta geothermal system. Our results show that structures NW-SE, NE-SW and N-S are predominant in the Chiweta area. We suggest that the accommodation zone between the Livingstone and Usysia faults controls the Chiweta's geothermal system. Additionally, we performed the Mohr circle and slip tendency analysis to identify the main orientations of potential active segments along faults capable to transport the geothermal fluids. Our results show that ～NW-SE and ～N-S pre-existing structures of Quaternary ages with ～60° dip have high slip tendency. These structures are likely the main pathways of the hot fluids to the surface in the Chiweta geothermal zone. Whereas the ～NE-SW pre-existing structures of Paleo-Mesozoic age have moderate slip tendency and are possible confining structures of the geothermal system. The first stress state shows a preferential failure plane oriented ～332°/56°, the second is oriented ～167°/53°, and the last one is oriented ～176°/63°. The pore fluid pressure increases the slipping tendency and it is probably higher in areas where fluids are present. We concluded that all these intersections and overlapping structures are assisting permeability in the Chiweta zone. Our results and future studies could help to identify the most promising drilling targets for exploration wells to continue geothermal energy development in this country.