Tectonometamorphic evolution of the Moyale region, southern Ethiopia

Abstract

The Precambrian tectonometamorphic evolution of the Moyale-Agal region, southernmost Ethiopia is described. This area forms part of the Pan-African, Mozambique Belt, forming the eastern margin of the Tanzanian Craton. It comprises polydeformed and metamorphosed mafic and ultramafic rocks, granodiorite, and subordinate amounts of metasedimentary rock. Four phases of deformation and two metamorphic events are recognised. The most pervasive phase, D1, is interpreted to have occurred during the Baragoian (c. 620 Ma) orogenic event. Peak metamorphism (M1) is synchronous with the development of the steep, N-S- or NW-SE-trending, S1 regional schistosity. In the Moyale sub-area amphibolite facies conditions prevailed, whereas in the Agal sub-area peak conditions did not exceed epidote-amphibolite facies. Plagioclase-hornblende thermometry gives estimates for peak metamorphic temperatures of 490°–650°C for the Moyale sub-area and <490°C for the Agal sub-area. The Moyale Granodiorite was intruded syn- to late D1 and is interpreted as one of the G3 granitoids described by Key et al. (1989). Post-D1 structures are considered to have formed during the end Pan-African, Barsaloian event (c. 580−570 Ma). D2 and D3 produced variably oriented, moderate or shallow-plunging, open fold structures and local cleavage development, whereas D4 was of major regional significance. It gave rise to NW-SE- and N-S-oriented, strike-slip ductile shear zones and brittle faults. Both dextral and sinistral shear-sense is recorded although dextral movement appears dominant. An M2 retrogressive event was syn-D4 and shows that late Pan-African orogen-parallel shearing occurred under greenschist facies conditions, with the shear zones acting as important fluid conduits.