Title: Structual mapping of Olkaria Domes Geothermal Field using geochemical soil gas surveys, remote sensing and GIS

Abstract

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.