Crustal Modelling from Gravity data in the Ethiopian Rift
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Date
1983-06
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Addis Ababa University
Abstract
A detailed gravity survey of the Main Ethiopian Rift Valley
between latitudes 70 N and SON, has shown that there is a broad
relative positive Bouguer anomaly over the whole of the Rift floor,
and this anomaly is superimposed on the regional, broad negative
anomaly of the Ethiopian and Somali Plateau. The broad relative
positive anomaly over the rift floor is between 60 and 100 km wide
and has an amplitude of 30 - 60 mga!. On the broad relative positive
anomaly over the rift floor are superimposed other short-wave length
relative positive anomalies which seem to be situated along the
displacement lines of the Wonji Fault Belt. There are also much
narrower relative positive anomalies along the margins of the rift
other than those associated with the \'Ionji Fault Belt.
In their interpretation of the gravity minimum near the center
of profile C (page 47), Searle and Gouin (1972) suggested that this
gravity minimum is due to low density lavas of the Aluto volcano and
they further a.8sociated the volcano with a small negative anomaly.
On the contrary the present survey shows that the volcano is
associated with a relative positive anomaly of magnitude
comparable to the short-wave length rela.tive positive anomalies
along the displacement lines of the Vionji Fault Belt.
Due to both, denser spacings of gravity stations and quality
of data, it is believed that the present gravity survey of the Main
Ethiopian Rift between latitudes 70N and SON defines more
accurately the location of the gravity anomalies (both the negative
and the positive anomalies). The priliminary interpretations of
the gravity anomalies made in this work are corroborated by bore
hole data Wigs. 10, 11 ) and surface geology (F.ig. 2) of the study
area. Furthermore, inferred density measurements on surface rocks
and Cores from production bores in the Olkaria geothermal field of
Kenya (Table 2) have been utilized
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Keywords
Ethiopian Rift Valley