The Gravity Filed and Crustal Structure of the Southern end of the Ethiopian Rift System
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Date
1993-07
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Addis Ababa University
Abstract
The Bouguer anomaly map of the rift zone south of the southern
end of the Main Ethiopian rift between Lat. 4° OO'N .;; 7° OO'N
and Long. 36° 00 I E - 39° 45 ' E is presented. The computed Bouguer
anomaly map shows a complex pattern of positive and negative
Bouguer gravity anomalies. A positive Bouguer gravity anomaly
of -165 mGal occurs over the floor of the Abaya-Chamo rift and
is flanked by negative anomalies that reside on its shoulders.
The anomaly pattern associated with the Chew Bahir rift is
completely different from the other regions. The floor has a
negative gravity anomaly of magnitude -135 mGal and is flanked
by positive gravity anomalies residing on its shoulders. This
may be attributed to the swamp and volcano-Iacostrine nature of
the rift floor. Further south in Kenya, the Bouguer anomaly map
shows that there is a relative increase in magnitude of the
regional gravity field with a complex pattern of alternating
positive and negative anomalies which are generally oriented a
NNW-SSE and E-W direction.
Two profiles that run through the major tectonic systems (the
Abaya-chamo rift and the Chew Bahir rift) of interest in the
area were extracted from the Bouguer anomaly map. 2%-0 gravity
models were constructed along these profiles. The results of
the model calculations are that:
Along Profile AA, the crust beneath the plateaus is 35 to 40 km
thick. within the axis of the Abaya-Chamo rift the crust thins
to 29 km. Along profile BB, the crust beneath the high lands in
the vicinity of the Amaro horst is 35 to 40 km thick. Beneath
the rift zone, i.e., the floor of the Chew Bahir rift and the
flanking uplifted regions the crust thins to 27 km. The
observed crustal thinning beneath_the rift zones along both
profiles is thought to be caused by an upward progression of
low density mantle material (anomalous mantle) that intrudes
itself into the lower crust
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Keywords
Rift Structure in Southern Ethiopia