Heat Sources for Geothermal Systems in East African Rift System

Peter Omenda Classification by Temperature Volcano hosted Geothermal Systems

• Typical model for volcano hosted geothermal system Volcano hosted Geothermal Systems-Menengai • Associated with main rift volcanoes Menengai Geothermal field

• Temperatures of > 400oC measured • Shallow magma bodies ~2.3km Geophysical Model across Menengai

• Seismics show high velocity under volcano • Common feature of all volcanoes in EARS rift axis MEQ Studies

T o B MN15 a Marigat MN13 r in g Coffee o Farm MN12 Bahati ru Majani Mingi ru hu ya 9985000 N To MN14 • Studies revealed that

9980000 Kabarak

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g g g brittle ductile transition

MN10

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i CALDERA Settlement

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o MN11

N N N MN07

To

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G G G re zone under Menengai is 9975000 t

MN06 Residential Area MN04 at 6km depth T o Nairobi 9970000 NAKURU MN05 MN03URBAN AREA Lake Rongai Nakuru Lake Nakuru Farms National Park MN02 MN01 165000 170000 175000 180000 Grid Eastings (m) NorthWest Ol Rongai Men. Caldera SouthEast 0

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-8 165000 167500 170000 172500 175000 177500 180000 Distance (M) Menengai Stratigraphy

• Stratigraphy dominated by trachytes and pyroclastics • Magma body at ~2.3km • Syenitic cap present • Tmax 400oC Menengai stratigraphic Model

Tuff layers

Trachyte

Syenite

Magma Menengai Model

200 isotherm

250 isotherm

350 isotherm

Magma Olkaria Volcanic Complex

• The heat source at Olkaria is associated with – discrete magma chambers that underlie the volcanic centres Main Ethiopian Rift Recent volcano tectonic activity is mainly within the axis. Central volcanoes often with calderas, e.g. Aluto Langano, Corbetti

• Aluto-Langano caldera, Ethiopia • Temperatures >300oC Corbetti Volcano

• Post caldera obsidian lavas present on caldera floor • Magma is expected at similar depth to Menengai (2.5km) Asal, Djibouti

• Asal rift zone • Extensional fault zone • Dikes associated with NW rift faults • Shallow magma chambers Magmatic Model

• Active magmatism, shallow magma chamber, Intrusive bodies • Major extrusive features may be absent • E.g. Tendaho, Ethiopia Karthala Volcano, Comoros

• Karthala is youngest and most active volcano (2361masl). • Nested caldera structure at Karthala • Active volcano

Karthala Eruption Karthala Volcano prospect

• Do active volcanoes form viable geothermal prospects? Yes Fault Model

• Deep circulation along the fault plane • Manifestations occur along downthrow side of faults • No centralized heat sources • Common on rift shoulders and between volcanic centres High heat flow due to Crustal thinning Extension Domain: Lakes Baringo - Bogoria

• No recent volcanism • Fracture controlled • Geothermometry: <200oC • Dikes and crustal heat Buranga, Uganda

• Fault controlled deep circulation along border fault • Other Heat sources? Gisenyi – Karisimbi Geothermal area

- Would the volcanoes be viable heat sources? Basement Play Type Zambia Geothermal areas Heat sources: - Crustal heat and fault circulation - Radiogenic heat? Kapisya

Heat source Model • Normal Thermal gradient and deep circulation • Radiogenic heat from granites • Rift related heating

Kapisya Geothermal Model

Model • Fracture/fault system • Deep circulation

Heat sources • Heat generating granites • Thinned Crust • Dikes Intra-Cratonic Basins Thanks