Unity University Faculty of Engineering

Department of Mining Engineering

GENERAL GEOLOGY (Geol 2081)

Chapter 7:

GEOLOGY OF ETHIOPIA

Tadesse Alemu Director Basic Geoscience Mapping Directorate Geological Survey of Ethiopia ([email protected])

December 2012 Addis Ababa

1 CONTENTS

SUMMARY OF THE GEOLOGY OF ETHIOPIA ...... 1 Precambrian rocks...... 1 Palaeozoic – Mesozoic sedimentary rocks ...... 2 Cenozoic volcanic rocks and associated sediments ...... 2 SUMMARY OF OF ETHIOPIA ...... 5 PRECAMBRIAN TECTONICS...... 5 PHANEROZOIC TECTONICS ...... 7

SUMMARY OF THE GEOLOGY OF ETHIOPIA

The geology of the country underlies by rock types range in age from Precambrian to Recent. These rocks are categorized into the following geological formations:- 1. Precambrian rocks 2. Paleozoic – Mesozoic sedimentary rocks 3. Cenozoic volcanic rocks and associated sediments

Precambrian rocks

Precambrian metamorphic and associated intrusive igneous rocks make up 25% of the country’s landmass. They are exposed in the northern, western, southern and eastern parts of the country and have a fundamentally important tectonic position in that they occupy the interface between the Mozambique Belt in the south and the Arabian-Nubian Shield to the north. They are dominantly north-trending linear belts of low-grade volcano-sedimentary rocks and mafic-ultramafic rocks, sandwiched between medium- to high-grade gneisses and migmatites. The high-grade gneisses and migmatites are referred to as Lower Complex is part of the Mozambique Orogenic Belt and generally consist of amphibolites facies (locally granulite facies) orthogneisses, parag-neisses, migmatites, and amphibolite with bands of marble. The low- grade volcano-sedimentary rocks with associated mafic to felsic intrusives, which is referred to as Upper Complex, on the other hand, belongs to the Pan-African Arabian-Nubian Shield. The linear belts of mafic and ultra-mafic rocks are commonly confined to major shear zones and often mark the contacts between the high-grade gneisses and migmatites and the low-grade volcano-sedimentary rocks. Most ages obtain from these rocks range between 900 and 500 Ma with exception of older Archean and Mesoproterozoic ages obtained from some of the rock units.

The Precambrian rocks have received attention in the current exploration activity for base and precious metals. The belts of mafic-ultramafic rocks and major shear zones bounding the two contrasting stratigraphic complexes are potential targets for gold, base metals, nickel, platinum and other mineralization.

1 Palaeozoic – Mesozoic sedimentary rocks

A thick succession of Palaeo-Mesozoic sediments covers 25% of the country’s landmass are represented by three distinct sedimentary basins; namely: the (350,000 km2), the Abay (Blue ) Basin (63,000 Km2), and the Mekele Basin (8,000 km2). The development of the basins can be thought of as relating to the polyphase break-up of Gondwanaland, which was accomplished, in general by reactivation along the preexisting Precambrian structures. Rifting associated with the break up of Gondwanaland was dominant from the late carboniferous onwards, and basins developed as a result of this extension both at the margins and within the continental . The Ogaden Basin contained at places over 10,000 meters thick sediments consist of non-marine to deep marine clastics, very thick, shallow-to-deep marine carbonates and evaporites. The Abay Basin consists of Paleozoic and Mesozoic sedimentary succession exceeding 2000 m, which are represented by pre-Adigrat clastics, equivalent to the Karoo system in the Ogaden, the Antalo , and the Amba Aradom Formation (the Upper ). The sedimentary succession of the Mekele Basin comprises 2000 m thick sediments ranging from fluvio-lacustrine to shallow and deep marine types.

The Paleozoic-Mesozoic sediments have high potential for hydrocarbon exploration and development. Discoveries in the Ogaden Basin indicated the gas-condensate fields of Calub and Hilal, with estimated reserves of 2.7 Tcf and 1.3 Tcf respectively. An indication of oil seep from Were-Ilu locality in the northeastern margin of the basin makes the Abay Basin one of the promising areas to undertake petroleum exploration. Moreover, the presence of significant volume of pre-Adigrat source rocks towards the southwest and eastern part of the Abay basin is speculated from geophysical studies may suggest that the Abay basin could be one of the promising areas to undertake petroleum exploration.

Cenozoic volcanic rocks and associated sediments

Cenozoic volcanics and sediments cover 50% of the country’s landmass, and range in age from the late up to historical times. Volcanism started during the Eocene-late with the eruption of flood basalts that have generally been related to either one or two mantle plumes impinging the base of the lithosphere under Afar or Afar-Northern Kenya . The flood basalts with intercalated silicic volcanics built a subaerial volcanic pile typically 500-1500 m thick and locally attaining 3000 m. The total area covered by these volcanics has been estimated as presently 600,000 km2, and not less than 750,000 km2 before erosion. These flood basalts contribute to an estimated volume of 300,000 km3. The earliest flood basalts forming the Ethiopian Plateau apparently erupted in a rather short time interval (<5 Ma) with the greatest eruption rates occurring from 31 to 28 Ma. This strong eruption was concomitant with the onset of continental rifting in the Red Sea-Gulf of Aden systems by 29 Ma, but predates the main rifting phases associated to the development of the Main Ethiopian (MER). However, limited volumes of basalts as old as 45 Ma have been described in southern Ethiopia, in the Broadly Rifted Zone separating the MER from the Kenya rift. Immediately after the peak of volcanic activity related to the flood basalt emplacement, a number of large shield volcanoes developed on the surface of the volcanic plateau. This subsequent less voluminous volcanic activity formed some of the highest relief of the plateau (such as Mts. Ras Dashen, Guna and Choke), rising 1000-2000 m above the top of the surrounding flood volcanics and being characterized by a basal diameter of 50-100 km. A second episode of flood basalt volcanism has been described in the southern Ethiopia at 18-11 Ma and in the MER-Afar transition zone at about 10-11 Ma. Time correlative basaltic units are widespread both in the western (Wollega and Lake Tana basalts, 11-9 Ma) and in the southeastern Ethiopia Plateau at about 10.5-9 Ma. After these episodes of widespread flood basalts and subordinate silicic volcanics, volcanism is closely associated with the tectonic development of the MER. Rifting in the various MER sectors was characterized by volcanism with fundamentally bimodal character. Widespread late -Pliocene rhyolitic ignimbrites (~7-3 Ma) with intercalated minor mafic lavas occur throughout the Northern and Central MER. In the (<1.6-

2 1.8 Ma), bimodal volcanic rocks (lava, pyroclastics and volcanoclastic strata) were generally closely associated with Wonji Fault Belt affecting the rift floor.

Cenozoic volcanics contain various semi-precious stones and industrial minerals including potash, bentonite, opal, diatomite and sulphur. Even though much of the Cenozoic volcanic terrain is not explored, there are good indications of gemstone and coal occurrences within intra-volcanic sedimentary rocks. Moreover, the Rift Valley has tremendous potential for geothermal energy.

3 Simplified geological map of Ethiopia

4 SUMMARY OF TECTONICS OF ETHIOPIA

PRECAMBRIAN TECTONICS

Tectonic development of the Precambrian of Ethiopia involves by subduction-accretion processes between arc terranes of the Arabian-Nubian Shield and predominantly gneissic terranes of the Mozambique Belt, which resulted in collisional amalgamation of lithotectonic terranes across sutures. It represents a plate tectonic cycle spanning a time-period of 350 Ma, beginning by about 900 Ma with rifting and continental break-up and ending by about 550 Ma subsequent to a continent-to- continent convergence between East and West . In Precambrian of Ethiopia, lithotectonic terranes that comprise high-grade gneisses of the Mozambique Belt and low-grade metavolcano-sedimentary sequences of the Arabian-Nubian Shield are juxtaposed along north-south sheared thrust contacts, which are marked by arc-like ophiolite sequences, which are overprinted by post accretionary structures including north-trending shortening zones and major northwest-trending sinistral and minor northeast-trending dextral strike-slip faults. These lithotectonic terranes are mainly represented by Moyale, Adola, Bul Bul (southern Ethiopia), and Tulu Dimtu (western Ethiopia) belts.

The Moyale Belt

The Moyale Belt is formed part of the Mozambique Orogenic Belt, which is characterized by generally north-trending upright, doubly plunging tight to isoclinal folds (Barsaloian event; 580 Ma). Later events produced a large scale fold with a northeast-southwest axial trace in the central-western region, and north- south to northwest-southeast shear orientations. It is interpreted that northwest-southeast shears activated thrusting and that the region can be interpreted as a flower structure in section.

The Adola Belt

The Adola Belt is a late-Precambrian, north-south trending fold and thrust belt of volcanic-sedimentary and ophiolite-like units overlying ‘basement rocks’ (gneisses and granitic gneisses). Structural analysis reveals that the Adola Belt has undergone three major deformational events. The earliest deformational event produced low-angle thrusts, recumbent folds and associated flat-lying foliation. The second deformational event gave rise to upright folds, north-south trending and generally west dipping regional schistosity and oblique to reverse sense shear zones. The third deformation reactivated the earlier structures and produced subvertical, discrete strike-slip shear zones along previous discontinuities.

The Adola Belt is found to comprise a group of known and potential primary gold deposits in and along the tectonic contacts of metavolcano-sedimentary sequences stretching for more than 100 km in a north- south direction and in the surrounding gneissic rocks mainly being controlled by structures. Between 1979 and 1982, the Adola Belt is systematically prospected by Adola Gold Exploration Project and many placer gold deposits and the Laga Dembi primary gold mine have been discovered and developed. More recently, exploration focused on the investigation of additional primary gold deposits and on the study of genetic aspects of the existing deposits.

The Bul Bul Belt

The Bul Bul Belt is ~ 100 km long, which is characterized by north-south fold and thrust tectonics, overprinted by a north-northeast-south-southwest shear zones. A three stage tectonic model is proposed to explain the evolution of the Bul Bul Belt. The early stage in the evolution of the belt is manifested in the

5 form of east-west trending gneissic and migmatitic layering and folds, and probably high temperature metamorphism. The second deformational event is resulting in the development of the north-south trending regional planar fabric in the form of gneissic and migmatitic layering, which was later folded about a north-northeast trending major antiform. This event seems to be the result of a regional compressive tectonic event, which was associated with collision between the high-grade gneiss and migmatites and the low-grade metavolcano-sedimentary rocks along a suture that has been subsequently overprinted by the north-northeast trending shear zones (Bul Bul and Wadera shear zones). The timing of suturing is not well constrained, however, based on available geochronological data it can be suggested that the age of the suturing event can be bracketed between 820 and 580 Ma. The final stage in the evolution of the Bul Bul Belt is dominated by an oblique normal-slip shearing with considerable strike-slip component. Southeast-plunging stretching lineation coupled with dextral strike-slip component as well as numerous kinematic indicators suggest top-to-the-southeast tectonic transport direction over dominantly east-dipping planes. It can be interpreted that the oblique normal-slip shearing to be related to regional gravitational tectonic collapse.

The Tulu Dimtu Belt

The Tulu Dimtu Belt is a north-northeast-trending litho-tectonic unit, which forms the southwestern and wider branch of the predominantly low-grade volcano-sedimentary terrane of the Pan-African Arabian- Nubian Shield (ANS). The belt can be traced for the entire length of the Precambrian basement of western Ethiopian and appears to continue to the northern Ethiopia, and the proposed Barka ophiolitic suture in Eritrea and the Sudan. It is characterized by a variety of lithological units, including gneisses, metamorphosed volcanic, volcaniclastic and sedimentary succession with associated mafic-ultramafic rocks of probable ophiolitic origin and granitoid intrusives. The belt represents a zone of shortening and is one of the best examples of a polydeformed northwest-verging fold- and-thrust belt in the ANS. Early Deformation is a progressive shortening, which resulted in the development of northwest-verging thrusts and associated recumbent, tight to isoclinal folds with subhorizontal axes and shallowly SE-dipping and north-northeast-trending axial planar foliations. The asymmetry of structures indicates an oblique (top to the northwest) sense of movement. This phase of deformation might be related the closure of the oceanic basin which resulted in the northwestward obduction of the Tulu Dimtu ophiolites. The second deformation resulted in steepening of early structures into upright folds. The third deformational phase represents extensive shortening, which culminated in the formation of major northwest-trending sinistral strike-slip faults and shear zones (Didesa and Surma shear zones), and minor north- and north-northeast- trending dextral strike-slip faults and shear zones that are superimposed at high angle to the earlier structures. The relationship between these deformational phases is consistent with development of the belt during a period of oblique collision in response to a northwest-southeast compressional stress that induced sinistral transpression.

The northern Ethiopian Precambrian rocks are characterized by the occurrence of low-grade volcanic, volcano-sedimentary, mafic and ultramafic rocks of ophiolitic character, and plutonic rocks of typical Arabian-Nubian Shield assemblage. The tectonics of the Precambrian of northern Ethiopia is characterized by thrust and fold belt type tectonics with a predominant northwest directed sense of displacement. Integration of geochemical data with field, lithological and structural studies demonstrate the occurrence of east to west accreted intra-oceanic arc sequences within the Precambrian of northern Ethiopia, which are probably formed in a supra subduction zone tectonic setting, and were conflated by accretion and superimposed strike-slip deformation.

6 PHANEROZOIC TECTONICS

The tectonic evolution of the country during Phanerozoic can be thought of as relating to the polyphase break-up of Gondwanaland, which was accomplished, in general by reactivation along the Precambrian structures. The Phanerozoic tectonics can broadly be divided into; (i) Late Paleozoic to , and (ii) Cenozoic tectonics.

Late Paleozoic to Cretaceous tectonics

Late Paleozoic to Cretaceous tectonics are related to early rifting of Africa from East Gondwana, which led to formation of multiple rift basins and deposition of terrestitial and marine sediments in central and eastern Africa. The Ogaden, Abay and the Mekele Basins are presumed to be intracontinental rift basins formed as a result of extensional stresses induced by the break-up of Gondwanaland in Late Paleozoic. Rifting began in earnest during the Early- to Middle-, which led to formation of the northeastern passive margin of the and conjugate India margin. The Ogaden Basin, located in the area extending from the east to southeast part of Ethiopia, is constituted of triaxially rifted troughs trending northeast-southwest (Mandera-Bodle Rift), northwest-southeast (Blue Nile Rift), and east-northeast-west- southwest (North Shilabo Rift). The Abay Basin is formed the northwest-trending branch of the Blue Nile Rift.

Further rifting and subsidence during Middle to Late Jurassic, led to flooding of the region (including Saudi Arabia and Yemeni areas), and consequently and carbonates deposited over a large area, extending up to the western and northern regions of Ethiopia. This event is probably related with the tectonics of separation of Madagascar from the Africa coast and the opening of the Indian Ocean. Marine sediments varying from shelf to deep basin types deposited over a large area of the east African region and in wide areas of Yemen and Saudi Arabia had been controlled by sub-basins formed by recurrent faulting and tilting of fault blocks during the Jurassic. This was followed in Early Cretaceous times by an uplift event and deposition of discontinuous fluviatile sediments over the Afro-Arabian region.

Cenozoic tectonics

Cenozoic tectonism in Ethiopia is related to the final stage of Gondwanaland rifting that took place between Late Eocene and the Early Miocene, and is generally associated with continued rifting at the margins and within the African-Arabian plate. The Cenozoic tectonics is represented by: (i) The Afar Depression, (ii) The Main Ethiopian Rift, and (iii) The Ethiopian Plateau.

THE AFAR DEPRESSION

Afro-Arabian Rift system, which comprises the southern , the Gulf of Aden Rift and the , the northern most part of which is the Main Ethiopian Rift. These rifts intersect forming a 400 km wide triple-junction zone marked by the Afar Depression. The Afar Depression is bordered on the west and southeast by the Ethiopian plateau, and on the northeast by the Danakil horst. There is a significant drop in elevation from the plateaus that stand well above 3500 m to the lowest point in northern Afar at -146 m. This tectonic depression, which is due to the breakup of the Afro-Arabian plateau in the Oligocene-Miocene, is still active. The ongoing volcanism, recent faulting, and shallow earthquakes are evidence for ongoing extensional deformation in the region. The Afar Depression is probably the only active rift in the world where both the early continental breakup history and the present-day ongoing extensional tectonism can be observed on the surface.

7 The western margins of the Afar Depression are bounded by a seismically active, right-stepping, en echelon system of discontinuous marginal grabens (Borkena, Guf Guf and others), that extend for ~500 km. These marginal grabens are interpreted to have been initiated during early phase of Afar Rift tectonism. The southern limit of the marginal grabens, near Kara Kore coincides with the abrupt change in the direction of the tectonic trend from north-northwest (Afar area) to north-northeast (Main Ethiopian Rift). Between the latitudes of 11N and 10N, the northwest-striking western Afar margin changes direction by 50 to northeast-striking Main Ethiopian Rift. This area is an intensely faulted zone consists of north-trending faults, which are oblique to both fault trends that make up the western Afar (N25W) and Main Ethiopian Rift margins (N25E), which is interpreted as arcuate accommodation zone (AAZ). The AAZ marks the early continental breakup boundary of the Afro-Arabian plateau in Ethiopia.

THE MAIN ETHIOPIAN RIFT

The Main Ethiopian Rift (MER) is a roughly NE-trending sector of the East African Rift system that includes a series of rift segments extending from the Afar Depression at the Red Sea-Gulf of Aden intersection to the Kenya Rift. The MER is characterized by active extensional tectonics accommodating the ~6-7 mm/yr relative movement between the African and Somalian plates. Generalized regional synthesis indicates that the MER has evolved from the development of half grabens with opposing polarity in the early rifting phase (Oligocene to early Miocene) to full symmetrical grabens in later stages. The rift basins, which are contiguous but separated by transfer or accommodation zones, have variable strike lengths (ranging between 50 and 100 km) and widths over the area of the rift sector. Two distinct fault systems are recognizable in the MER. The older includes Miocene, north-northeast to northeast-trending, high-angle border faults, and the younger Quaternary north-northeast-trending faults and aligned volcanic centers affecting the rift floor. The latter is referred to as the Wonji Fault Belt.

The MER has been traditionally differentiated into three main segments: (1) the Northern (NMER), (2) the Central (CMER), and (3) the Southern (SMER), representing different stages of the extension process, from early rifting in the Southern MER to more evolutes stages in the Central and Northern MER preceding the incipient seafloor spreading in Afar.

The Northern MER is considered to extend from the Afar Depression up to the Lake Koka region following the Middle course of the Awash River Valley. The boundary between the Afar Depression and the Northern MER is not readily recognizable within the rift floor. On larger scale, however, the broadening of the Northern MER from a width of ~100 km to >300 km in the vicinity of latitude 10N, is interpreted to define a geomorphic boundary between the two tectonic domains. The main boundary faults in this segment show an average N50 trend and formed since about 10-11 Ma. The Central MER encompasses most of the Lakes Region, up to the Lake Awasa area. Here, the main boundary faults trend roughly N30-35 and the age of faulting onset is estimated to be around 8.3-9.7 Ma. The Southern MER extends south of Lake Awasa into the ~ 300 km-wide systems of basins and ranges, which referred to as broadly rifted zone. Faults in the Southern MER show a dominant N-S to N20 trend and were well established after ~18 Ma. The Southern MER characterizes the overlapping area between the MER and the Kenya Rift. The N-S striking Omo and Chew Bahir basins, which are presumed to be the northward continuations of the Oligocene rift system of north Kenya are located within the Southern MER. Geophysical (gravity and seismic) investigations suggest more than 3.5 km of sedimentary fill within the basins. This may warrants exploring for oil and gas within the basins.

8 THE ETHIOPIAN PLATEAU

The Ethiopian Plateau is separated into western and eastern shoulder by the Afro-Arabian Rift System. The western shoulder (s l. the Ethiopian plateau), has a north-south decrease in crustal thickness allowing division into Northwestern Ethiopian and Southwestern Ethiopian Plateau at the YTVL. The eastern shoulder is referred to as southeastern Ethiopian plateau (s.l. the Somali Plateau). Both the western and eastern plateaus are characterized by east-west trending transverse lineaments constituting shield volcanoes. These lineaments from north to south represented by Adigrat-Adwa Lineament (AAL), Yerer- Tullu Wellel Volcano-Tectonic Lineament (YTVL), and Goba-Bonga Lineament (GBL).

The northwestern Ethiopian Plateau contains the Tana basin, a faulted depression located between the erosional Tana escarpment to the west and, to the east, the tectonic Western Afar Margin overlooking the Afar Depression. The basin occupies an area of 16,500 km2, of which the Lake Tana covers 3156 km2. Data from digital elevation modeling and satellite imagery analysis confirm the basin’s location at the junction of three grabens: (i) the Dengel Ber graben, which is framed by northeast-southwest to north- northeast-south-southwest-striking faults, (ii) the Gondar graben, being framed by north-northwest-south- southeast to north-south-trending faults, and (iii) the Debre Tabor graben, which is characterized by west- northwest-east-southeast-striking faults.

9 Simplified tectonic map of Ethiopia.

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