Structural mapping of Olkaria Domes geothermal field using geochemical soil gas survey, remote sensing and GIS

Abstract

Olkaria geothermal field is a high temperature geothermal system located on the eastern arm of the East African Rift System. It is placed within the Central Kenya Rift Valley and is dominated by late Quaternary rhyolitic volcanism. Basalts, tuffs and trachytes occupy the subsurface with the latter forming the reservoir rock. Normal faulting mapped during this study is prominent in the Domes field with dips ranging from 20° to 80°to the west and the east. Fault strikes vary from north-south, northwest-southeast, northeast-southwest and east-west. The oldest faults strike northwest and coincide with the initiation of the Kenyan rift system while the youngest are the North striking and are thought to have formed during a distinct period of dyking that took place along the Ol Njorowa Gorge. East-west striking faults were discovered to the east of Domes field and are inferred to have been formed during a period of transform faulting. Thrust faulting is rare throughout the field but portrays brief periods of crustal compression. Geothermal manifestations were observed as key indicators of permeable zones that gave indication of faults and fractures which may have been obscured by recent pyroclastic deposits. Soil gas surveys indicate an E-W orientation of anomalies along the south of Domes field.

Digital elevation models show step normal faulting to be dominant forming steep shoulder faults along the Central Kenyan rift. Regional faults exhibit near vertical dips to the east and west, forming a graben which is approximately 45km wide. Transform faulting is evidenced along the Central Kenyan rift and may have been responsible for sudden changes in the strike of the rift faults. A cauldron with four arcs was observed south of Lake Naivasha where Olkaria and Longonot calderas occur on its periphery while Suswa caldera is located at its centre. It forms a structure similar to a knee joint along the rift where all shoulder rift faults are arcuate. Magnetic data shows existence of a shallow anomaly which may signify presence of a shallow magma reservoir at a depth of about 15 km.