- Category: Geology Of Ethiopia
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Current Status of Geoscience Map Coverage
The Government of Ethiopia has approved a five year (2010/2011-2014/2015) Growth and Transformation Plan (GTP). In line with this strategic framework four Geoscience Mapping projects are planned with goals to
• complete the 1:250,000 scale geological map coverage of the country
• cover the country by low-to-medium resolution airborne geophysical survey
• compile 1:500,000 scale gravity map of the country
• increase the coverage of the 1:250,000 scale gravity map of the country to 49%
• increase the coverage of the 1:250,000 scale geochemical map of the country to 49%
• compile 1:2,000,000 scale geochemical atlas and metallogenic map of the country
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 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 Arab_Nubian Shield to the north.Stern (1994) coined the term East African Orogen (EAO) to encompass both the MB and ANS. EAO represent 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 Gondwana (Vail, 1985; Berhe, 1990; Abdelsalam and Stern, 1996;).
The Precambrian rocks 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.
The Precambrian rocks of southern and southwestern Ethiopia were traditionally divided into Lower, Middle and Upper Complex based on the variation in grade of metamorphism (Kazmin, 1972; Kazmin et al., 1978; see table below). The metamorphic grade was thought to decrease from the Lower to the Upper Complex. These authors also noted that the Lower Complex is Archean whereas the Upper Complex is Neoproterozoic in age. Kazmin (1972) conclude that the high-grade gneisses and granulites of the Lower Complex may correlate with the high-grade granulites of Uganda. In southwestern Ethiopia, de Wit and Chewaka (1981) recognized that the lithological boundaries and structural trends are transected by the granulite isograd. They conclude that the granulite facies metamorphism represented a younger event in the metamorphic evolution of the region, and so challenged Kazmin (1972) suggestion that the high-grade gneiss and granulites are old Archean lithosphere similar to those found in Uganda. Moreover, geochronological data (Amenti et al., 1992; Hailu, 1996; Teklay et al., 1998; Yibas et al., 2002; Tadesse, 2003) favored the conclusion that the high-grade gneisses and granulites are not necessarily older than the low-grade volcano-sedimentary rocks.
The Precambrian of south and southwestern Ethiopia comprises both high-grade gneissic terrain of the Mozambique Belt and low-grade metavolcano-sedimentary sequences of the Arabian-Nubian Shield. Various models have been proposed to explain the relationship between the Mozambique Belt and the Arabian-Nubian Shield. Kazmin et al. 1978 proposed that the high-grade rocks of the Mozambique Belt extend beneath the low-grade rocks of the Arabian-Nubian Shield forming a basement-cover relationship. Others such as de Wit and Chewaka, 1981; Vail, 1985; Berhe, 1990; Abdelsalam and Stern, 1996 and Stern, 1994 have suggested subduction-accretion processes between arc terranes of the ANS and predominantly gneissic terranes of the MB, which resulted in collisional amalgamation of lithotectonic terranes across sutures. Furthermore, Burke and Sengor (1986), Bonavia and Chorowicz (1992) and Stern (1994) invoked escape tectonics and proposed that the N-trending structures in southern Ethiopia are the roots of northward expulsion of the Arabian-Nubian Shield from the Mozambique Belt, following a Tibetan-type continent-continent collision between east and west Gondwana along the Mozambique Belt after the consumption of the Mozambique Ocean. However, this model has been refuted by Warden and Horkel (1984), Ghebreab (1992) and Worku and Yifa (1992) all of whom argued that no subduction has occurred in the past and the mafic-ultramafic rocks are intrusions into the high-grade gneissic rocks through brittle-ductile shear zones, suggesting an ensialic or intracratonic rift basin model, which did not develop into a passive plate margin de Wit and Chewaka, 1981; Berhe, 1990; Woldehaimanot, 1995; Hailu, 1996; Bedru, 1999 and Yibas, 2000 all advocated a Wilson cycle orogenic process to explain the evolution of the Precambrian rocks of southern Ethiopia. Age data indicate that magmatism and possibly metamorphism and deformation in southern Ethiopia occurred between about 900 and 500 Ma. However, detrital zircon ages of 1657 Ma from a meta-rhyolite (Teklay et al., 1998) and ages between 1300 and 2050 Ma from a diorite gneiss (Yibas, 2000) are evidence for the existence of Archean to Mesoproterozoic continental lithospheric component in southern Ethiopia.
The Precambrian of western Ethiopia that extends northwards from 60N for about 650 kilometer is the largest Precambrian block in the country. It forms the western and wider branch of the low-grade volcano-sedimentary terrain of the Arabian-Nubian shield (ANS) bounded both to the east and to the west by the gneissic terrain of the Mozambique Belt (MB).
The Geological Survey of Ethiopia (GSE) studied the Precambrian geology of western Ethiopia at different scales (UNDP, 1972; de Wit, 1977b; Kazmin, 1978; Kazmin et al., 1979; Davidson, 1983; Amenti, 1989; Mengesha, 1987; Mengesha and Berhe, 1987; Ayalew and Moore, 1989, Tadesse and Tsegaye, 2000; Solomon and Mulugeta, 2000; Getahun, 2002). Accordingly, the Precambrian of western Ethiopia consists of: - (i) high grade gneiss and migmatites, (ii) low-grade metavolcano-sedimentary rocks and associated intrusive rocks and, (iii) metavolcano-sediments and associated mafic-ultramafic rocks of probable ophiolitic origin.
The high-grade gneiss and migmatites referred to as Lower Complex (UNDP, 1972; Kazmin, 1972) are considered as the northern continuation of the Pan-African Mozambique belt. These rocks were regarded as Archean in age mainly on the basis of correlation with similar rocks in east Africa (Kazmin, 1972; Kazmin et al., 1978). However, geochronological investigations indicate that the granitoids from the high-grade gneiss and migmatites fall within the time range of 550 to 810 Ma (Ayalew et al., 1990; Tesfaye et al., 2000), and some of the granitoids contain inherited zircon as old as 1571 ± 9 Ma (Tesfaye et al., 2000). This might suggest that the gneiss and migmatites are not juvenile Pan-African terrane but consists of Mesoproterozoic crust that was reworked in the East African Orogen.
The low-grade metavolcano-sedimentary rocks and associated intrusives outcrop is remarkably persistent and can be traced for the entire length of the Precambrian of western Ethiopia. The assemblage varies in width along strike that is wider in the north and narrower in the south. Further south the assemblage pinches out and truncates by NW-trending Surma shear zone. Their contact with the high-grade gneiss and migmatites is not exposed, nor has it been mapped in detail. Usually the contact is tectonic represented by tightly folded and strike-slip shear zones. Part of the contact is also marked by SE-dipping and NW-verging thrust faults and folds (Tadesse and Tsegaye, 2000, 2002).
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. Felsic plutonism of syntectonic, late- and post-tectonic granitoid range in age from 800 to 600 Ma is common in Precambrian of northern Ethiopia (Tadesse, 1998; Tarekegn et al., 1999; Asfawossen et al., 2001).Beyth, 1972; Kazmin, 1972 and Mengesha et al., 1996 have grouped the Precambrian rocks of northern Ethiopia into: - (i) Tsaliet Group, (ii) Tambien Group, (iii) Didikama Formation, and (iv) Shiraro Formation (see table). However, geological mapping and regional compilation of the Axum sheet by the GSE (Tarekegn, 1997) showed that part of what was considered as the Tsaliet Group, which consists of widespread low-grade metavolcano-sedimentary rocks are divided into four tectono-stratigraphic blocks (Adi Hageray, Adi Nebrid, Chila, and Adwa). These tectono-stratigraphic blocks are separated by mafic-ultramafic belts and characterized by the occurrence of lithological units having different structural, metamorphic and magmatic history. 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 (Tarekegn, et al., 1999).
The Precambrian geology of eastern Ethiopia is little known compared to the other Precambrian exposures of the country. Berhe (1985) proposed a three-fold division for the Precambrian rocks of eastern Ethiopia. These are: - (i) Lower Complex, comprised of high-grade gneiss and migmatites, (ii) the Boye Group (Middle Complex of Kazmin, 1972), comprising meta-arkose, metapelite, quartzite and marble, and (iii) the Soka Group (Upper Complex of Kazmin, 1972), consisting phyllite, chlorite schist, metavolcanics and lenses of ultramafic rocks. Tadesse and Kibre (2010) in their review of the geology of the northern Ogaden Basin suggested a three-fold subdivision to the Precambrian rocks of eastern Ethiopia similar to that of Berhe (1985) except that they rename the Boye and Soka group as domain and the Lower Complex as Eastern Gneissic domain.Pb-Pb zircon geochronological investigations from the Precambrian basement of eastern Ethiopia between Hirna and Harar (Teklay et al., 1998) indicated older (Archean and Paleoproterozoic) ages. This suggests that the Precambrian basement of eastern Ethiopia is much older than the other Precambrian exposures inthe country. According to the Neodymium isotopic data summarized for basement rocks of the EAO, the Precambrian rocks of Eastern Ethiopia are correlatable with basement rocks of Somalia and Yemen, and Afif terrane of Saudi Arabia, characterized by older (Paleoproterozoic and Archean) ages (Stern, 2002). The Nabitah Orogenic Belt separates the Paleoproterozoic and Archean crust in the east from the mainly Neoproterozoic crust to the west. However, further south in Ethiopia it is difficult to trace the belt because of extensive cover of Tertiary volcanics and Mesozoic sediments.
Field relationships have shown that the Precambrian rocks of eastern Ethiopia are affected by three phases of deformation (D1 to D3), which are related in time and space to continuous progressive deformation (Tadesse and Hailemariam, 2006; Tadesse and Kibre, 2010). The first two (D1 and D2) deformations are related to compressional deformation, which apparently indicating an overall progressive shortening starts with folding that ultimately leads to thrusting and strike-slip shearing. The D3 deformation, which is characterized by brittle-ductile low-angle shear zone formed under conditions of non-coaxial shear directed northeast to east-northeast related to extensional deformation.
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