Abstract
O’a is the largest of the Quaternary caldera volcanoes that punctuate the axis of the Ethiopian rift valley. The known volcanic history of O’a is brief: eruptions of restricted ash-flow tuffs and «tufolavas» were followed by extensive pumice deposition with intervening paleosols, lacustrine sediments, and flows of occasional welded tuffs and rare basalts. Ensuing caldera collapse at c. 0.24 m.y. ago was accompanied by emplacement of two massive ignimbrite flow units comprising a single cooling unit: the first was much more severely welded than the second which shows lahar characteristics. Post-caldera volcanism at O’a has been sparse compared with most other Ethiopian rift centres.
O’a volcano exemplifies the common rift association of a caldera set tightly between two offset segments of the Wonji fault belt. The Wonji fault belt marks the youngest tectonism of the rift floor, and in the vicinity of O’a has been active in a major way since caldera subsidence. This faulting is clearly younger than the massive rift margin faulting, which to the northeast of O’a occurred during a tectonic climax dated at c. 1.0 m.y. ago.
Radiometric analysis suggests a rather regular level of initial40Ar in O’a basalt lavas sampled near to their original vents. If this level also applies to near-vent basalts dated from other parts of the Ethiopian rift, a regional rift paroxysm of crustal extension and related silicic and basaltic volcanism is evident at c. 0.30–0.20 m.y. ago. Episodic dilatation and associated volcano-tectonism separated by long periods of quiescence appears to be a general feature of continental rift valleys.
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References
Baker, B.H. andMitchell, J.G., 1976,Volcanic Stratigraphy and Geochronology of the Kedong-Olorgesaile Area and the Evolution of the South Kenya Rift Valley. J. Geol. Soc. London,132, p. 467–484.
Baumann, A., Förstner, U. andRohde, R., 1975,Lake Shala: Water Chemistry of Sediments in an Ethiopian Rift Lake. Geol. Rundschau,64, p. 593–609.
Bott, M.H.P., 1976,Formation of Sedimentary Basins of Graben Type by Extension of the Continental Crust. Tectonophysics,36, pp. 77–86.
Brotzu, P., Morbidelli, L., Piccirillo, E.M. andTraversa, G., 1974,Petrological Features of the Boseti Mountains, a Complex Volcanic System in the Axial Portion of the Main Ethiopian Rift. Bull. Volcanol.,38, p. 206–234.
Catlin, D., Largen, M.J., Monod, T. andMorton, W.H., 1973,The caves of Ethiopia. Cave Res. Group Gt. Brit.,15, p. 107–168.
Christiansen, T.B., Schaefer, H.-U. andSchönfeld, M., 1975,Geology of Southern and Central Afar, Ethiopia. In:Afar Depression of Ethiopia, A. Pilger and A. Rosler, eds., Schweitzerbart, Stuttgart, p. 259–277.
Cole, J.M., 1969,Gariboldi Volcanic Complex, Ethiopia. Bull. Volcanol.,33, p. 566–578.
Dakin, F. andGibson, I.L., 1971,A Preliminary Account of Alutu, a Pantelleritic Volcano in the Main Ethiopian Rift. Bull. Geophys. Obs. Addis Ababa,13, p. 110–114.
Dalrymple, G. B., 1969,40 Ar/36 Ar Analyses of Historic Lava Flows. Earth Planet. Sci. Letters,6, p. 47–55.
Di Paola, G. M., 1972,Geology of the Corbetti Caldera Area (Main Ethiopian Rift Valley). Bull. Volcanol.,35, p. 497–506.
Di Paola, 1973,The Ethiopian Rift Valley (Between 7°00′ and 8°40′ lat. North). Bull. Volcanol.,36, p. 517–560.
-----, 1977,Geological Map of the Tullu Mojé Volcanic Area. Consiglio Naz. Ricerche, Firenze. (Map only, scale 1:75,000).
Fairhead, J.D., Mitchell, J. G. andWilliams L.A.J., 1972,New K-Ar determinations on Rift Volcanics of S. Kenya and Their Bearing on the Age of Rift Faulting. Nature Phys. Sci.,238, p. 66–69.
Fiske, R.S., 1963,Subaqueous Pyroclastic Flows in the Ohanapecosh Formation, Washington. Bull. Geol. Soc. Amer.,74, p. 391–406.
Gibson, I.L., 1967,Preliminary Account of the Volcanic Geology of Fantale, Shoa. Bull. Geophys. Obs. Addis Ababa,10, p. 59–68.
—————, 1970,A Pantelleritic Welded Ashflow Tuff from the Ethiopian Rift Valley. Contr. Min. Petrol.,28, p. 89–111.
—————, 1972,The Chemistry and Petrogenesis of a Suite of Pantellerites from the Ethiopian Rift. J. Petrol.,13, p. 31–44.
Grove, A.T., Street, F.A. andGoudie, A.S., 1975,Former Lake Levels and Climatic Change in the Rift Valley of Southern Ethiopra. Geogr. J.,141, p. 177–202.
Levitte, D., Columba, J. andMohr, P., 1974,Reconnaissance Geology of the Amaro Horst, Southern Ethiopian Rift. Geol. Soc. Amer. Bull.,85, p. 417–422.
Lloyd, E.F., 1977,Geological Factors Influencing Geothermal Exploration in the Langano Region, Ethiopia. Privately circulated report, 73 pp.
Meyer, W., Pilger, A., Rosler, A. andStets, J., 1975.Tectonic Evolution of the Northern Part of the Main Ethiopian Rift in Southern Ethiopia. In:Afar Depression of Ethiopia, editors A. Pilger and A. Rosler, p. 352–362, Schweizerbart, Stuttgart.
Milanovski, E. E. andKoronovski, N. V., 1965,«Tuffolavas» and Related Formations of Central Caucasus. Bull. Volanol.,28, p. 205–213.
Mohr, P.A., 1960,Report on a Geological Excursion through Southern Ethiopia. Bull. Geophys. Obs. Addis Ababa,3, p. 9–20.
—————, 1962,The Ethiopian Rift System. Bull. Geophys. Obs. Addis Ababa,5, p. 33–62.
Mohr, P.A., 1966a.Chabbi Volcano (Ethiopia). Bull. Volcanol.,29, p. 797–816.
—————, 1966b.Geological Report on the Lake Langano and Adjacent Plateau Regions. Bull. Geophys. Obs. Addis Ababa,9, p. 59–75.
-----, 1977,1974 Ethiopian Rift Geodimeter Survey. Smithson. Astrophys. Obs., Spec. Rep. 376, 111 pp.
—————, andPotter, E.C., 1976,The Sagatu Dike Swarm, Ethiopian Rift Margin. J. Volc. Geoth. Res.,1, p. 55–71.
—————, andWood, C.A., 1976,Volcano Spacings and Lithospheric Attenuation in the Eastern Rift of Africa. Earth Planet. Sci. Lett.,33, p. 126–144.
Moore, J.M. andDavidson, A., 1978,Rift Structure in Southern Ethiopia. Tectonophysics,46, p. 159–173.
Morandini, G., 1940,Le caratteristiche di alcuni laghi dell’Africa Orientale Italiana. Riv. Geogr. Ital.,47, p. 67–76.
Morton, W.H., Rex, D.C., Mitchell, J.G., andMohr, P., 1979,Riftward Younging of Volcanic Units in the Addis Ababa Region, Ethiopian Rift Valley. Nature,280, p. 284–288.
Schmincke, H.-U., 1970,Ignimbrite Sequence on Gran Canaria. Bull. Volcanol.,33, p. 1199–1219.
—————, andSwanson, D.A., 1967,Laminar Viscous Flowage Structures in the Ash-flow Tuffs from Gran Canaria, Canary Islands. J. Geol.,75, p. 641–664.
Thrall, R., 1975,Gadamsa Caldera, Ethiopia. Bull. Geophys. Obs. Addis Ababa,15, p. 71–81.
United Nations, 1973,Geology, Geochemistry and Hydrology of Hot Springs of the East African Rift System Within Ethiopia. Tech. Rep. DP/SF/UN/116 United Nations, New York, 300 pp.
Vogt, P.R., 1979,Global Magmatic Episodes: New Evidence and Implications for the Steady-state Mid-oceanic Ridge. Geology,7, p. 93–98.
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Mohr, P., Mitchell, J.G. & Raynolds, R.G.H. Quaternary volcanism and faulting at O’A caldera, central ethiopian rift. Bull Volcanol 43, 173–189 (1980). https://doi.org/10.1007/BF02597619
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DOI: https://doi.org/10.1007/BF02597619