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Ethiopia’s Tectonics

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Precambrian tectonics
Three Precambrian are present, Lower, Middle and Upper. They are clearly separable in structural style. The rocks of the Lower Complex from large blocks separated and surrounded by folded belts of the Upper Complex.

The Middle complex is minor in extent, filling some troughs in the Lower Complex. The Upper Complex appears to consist of two north-south trending belts which join together in northern Ethiopia.

Mesozoic-Cenozoic tectonics
The major tectonic elements are
1. The Ethiopian plateau
2. The Somali plateau
3. The Danakil uplift
4. The Aisha uplift
5. The Rift System

The Ethiopian plateau is an uplifted block limited on the east by the Rift System and declining on the west to the Sudan plain. The eastern margin of the plateau is elevated to between 3,000m and 4,000m while, towards the west, the plateau surface descends to between 1,200 and 1,000 metres.

The Somali plateau bounded on the west by the Rift System, is at elevations of 3,000m to 3,500m on the west, and declines gradually eastward to the Indian Ocean.

The Danakil uplift appears to be a northwesterly trending horst block with a core of Precambrian rocks and Mesozoic sedimentary rocks on the limbs. The Danakil uplift continues to the northwest as the Buri peninsula. It may continue buried under Quaternary volcanics to the Gulf of Zula.

The Aisha uplift in its northern part has a crudely anticlinal structure with Jurassic limestone in the core and Cretaceous sandstones on the limbs.

Numerous north-south faults subdivide the uplift into several horst-graben blocks. In its southern part the Aisha uplift consists of a second uplifted block bounded to the southwest by a large sinistral fault.

The Rift System consists of the Red Sea rift, Afar, the Main Ethiopian rift and Lake Stefanie and Lake Rudolf rifts. The Red Sea rift is bordered on the west by the escarpment of the Ethiopian plateau.

The margin of the Red Sea rift is interpreted as a series of stepfaulted blocks, the coastal plain forming one such step.

The Afar rift is a triangle bordered in the west and south by the escarpments of the Ethiopian and Somali plateaus, and in the northeast and east by the Danakil and Aisha uplifts.

The Danakil depression, the Erta Ale, Alayta and Tat Ali volcanic ranges, and the Gawwah, Dobi and Tendaho graben form prominent features in northern Afar. Southern Afar consists of two marginal depressions parallel to the margins of the Ethiopian and Somali plateaus, and a central relatively uplifted part.

The Main Ethiopian Rift extends for 500km from 90 30’N to 4045’N. The Rift floor varies in elevation from 1,000m to 2,000m. The Northern segment of the Main Rift is 70km to 80km wide, within the north a median trough extending from southern Afar.

The central segment of the Main Rift, 280km long and 70km to 80km wide, extends from the Yerer fault to Lake Abaya. Lakes Langeno, Shala and Abijata, to mention the major ones, fill the central depression in this segment.

The southern segment of the Main Rift consists of the Abaya-Chamo Rift, the Amaro Horst, the Galana graben and several other features.

The Lake Stefanie Rift is a narrow, north-south trending graben 100km long by 25km to 30km wide. It is bounded on both sides by steep tectonic escarpments consisting of two to three step-faulted blocks. This graben stretches to the Kenya border.

The Lake Rudolf rift is a tectonic depression which includes the Turkana basin in Kenya and the Lower Omo valley in Ethiopia. The latter widens southward between north-southtrending escarpments, and is filled with Pliocene-Quaternary lacustrine and alluvial sediments.

(Source: National Atlas of Ethiopia)

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