BASEMENT TECTONICS OF THE NEGASH & SAMRE AREAS AND THE ECONOMIC POTENTIAL OF THE LIMESTONE UNIT IN THE AREAS, TIGRAY, NORTHERN ETHIOPIA

ABSTRACT

The geology and structure of the Negash & Samre areas is described. The rocks of the areas consist of mainly metavolcanics, metasediments and post-tectonic granitoid intrusives. The metavolcanic unit is composed of fine-grained intermidite volcanic rock (andesitic) as well as metabreccia of pyroclastic origin. The metasedimentary unit comprises rocks of turbiditic and shallow marine origin including metaconglomerates, graphitic phyllite (or black slate), dolomites and black limestone, variegated slate and pebbly slate. The metasedimentary rocks show only very slight recrystallization whilst the metavolcanic rocks appear to have attained greenschist facies metamorphism.

At least four phases of deformation are recognized in the Negash and Samre areas. The first phase (D1) produced pervasive regional foliation and, perhaps, the composite fabric in the metasedimentary rocks. It was followed by local D2 semi-ductile shear deformation. The formation of folds (as D3) and development of brittle structures (D4) such as dextral strike-slip faults is also characteristic feature of the region.

The structural and lithological style in the Negash and Samre areas is not identical. In Negash area there is a structural and metamorphic contrast between the metavolcanic and metasedimentary units. Their boundary is interpreted as an activated unconformity.

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