Gas emissions and sub-surface architecture of fault-controlled
geothermal systems: a case study of the North Abaya geothermal area
East Africa hosts significant reserves of untapped geothermal energy.
Most exploration has focused on geologically young (<1 Ma)
silicic caldera volcanoes, yet there are many sites of geothermal
potential where there is no clear link to an active volcano. The origin
and architecture of these systems is poorly understood. Here, we combine
remote sensing and field observations to investigate a fault-controlled
geothermal play located north of lake Abaya in the Main Ethiopian Rift.
Soil gas CO2 and temperature surveys were used to examine permeable
pathways and showed elevated values along a ~110 m high
fault which marks the western edge of the Abaya graben. Ground
temperatures are particularly elevated where multiple intersecting
faults form a wedged horst structure. This illustrates that both deep
penetrating graben bounding faults and near-surface fault intersections
control the ascent of hydrothermal fluids and gases. Total CO2 emissions
along the graben fault are ~300 t d–1; a value
comparable to the total CO2 emission from silicic caldera volcanoes.
Fumarole gases show δ13C of –6.4 to –3.8 ‰ and air-corrected 3He/4He
values of 3.84–4.11 RA, indicating a magmatic source originating from
an admixture of upper mantle and crustal helium. Although our model of
the North Abaya geothermal system requires a deep intrusive heat source,
we find no ground deformation evidence for volcanic unrest nor recent
volcanism. This represents a key advantage over the active silicic
calderas that typically host these resources and suggests that
fault-controlled geothermal systems offer viable prospects for further
exploration and development.