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East African rifts
138
East African Rifts. R. W. Girdler (Editor). Elsevier, Amsterdam (Developmeplts iJl Geotectonics, 7), 1972, 186 pp., Dfl. 27.50. This book - Upper Mantle Scientific Report, no. 40, reprinted from Tectonophysics, 15(1/2) - contains 18 articles describing geophysical, petrological and structurai studies of the East African Rift System mainly between the Afar triangle and latitude 5°S. The papers were originally presented at a special symposium in Moscow in August 1971 during the XVth General Assembly of the I.U.G.G. and to a large extent were original contributions at that time. Six of the papers are concerned with the Afar triangle. The majority of the authors of these papers believe that the more nearly oceanic part of Afar lies in the northern Danakil depression and to the west of the Gulf of Tadjoura. In these areas gravity shows the crust to be attenuated, S n waves are not propagated and the petrology of volcanic rocks is akin to that of oceanic basalts. Seismic refraction results around the Gulf of Tadjoura indicate a crustal structure quite similar to that beneath Iceland. Mohr is of the opinion that crustal extension is not localized in Afar but is spread over the whole region. He postulates intermittent activity over the whole of Cenozoic times. The remaining papers deal with the Ethiopian rift and the various branches of the rift system in Kenya and northern Tanzania. The consensus of geophysical results (mainly seismic and gravity) shows that here there is a relatively narrow zone of dense intrusions in the crust within a broader region of low-density mantle. The Neogene volcanics appear to have a mantle origin and have not resulted from assimilation or remelting of crustal material. First-motion studies of earthquakes indicate ESE tensional forces across the rift in accord with plate tectonics, however, the amount of separation, as estimated from the total width of the intrusions beneath the rift floor, is no more than 50 kin. There are other results, however, conflicting with the above views. Wohlenburg's magnetic surveys across the rift located NW trending long-wavelength anomalies seemingly not caused by bodies within the upper crust. Rykunov obtained, from micro-earthquake studies, a crustal structure at variance with that from a seismic refraction experiment carried out in a different part of the rift and reported by Griffiths. Logatchev et al. believe that the geological evidence does not favour rifting but instead there has been a more complex process of increasing the crustal density by intrusions. This book is a useful compendium of state-of-the-art papers on the East African Rift. For those unfamiliar with the geography of the area it would have been of assistance to have had one good map, somewhere in the volume, of the whole Afar-East African Rift system. The paper by Logatchev et al. is omitted from the contents list.
R. B. WHITMARSH(Wormley)
East African Rifts. R. W. Girdler (Editor). Elsevier, Amsterdam (Developmeplts iJl Geotectonics, 7), 1972, 186 pp., Dfl. 27.50. This book - Upper Mantle Scientific Report, no. 40, reprinted from Tectonophysics, 15(1/2) - contains 18 articles describing geophysical, petrological and structurai studies of the East African Rift System mainly between the Afar triangle and latitude 5°S. The papers were originally presented at a special symposium in Moscow in August 1971 during the XVth General Assembly of the I.U.G.G. and to a large extent were original contributions at that time. Six of the papers are concerned with the Afar triangle. The majority of the authors of these papers believe that the more nearly oceanic part of Afar lies in the northern Danakil depression and to the west of the Gulf of Tadjoura. In these areas gravity shows the crust to be attenuated, S n waves are not propagated and the petrology of volcanic rocks is akin to that of oceanic basalts. Seismic refraction results around the Gulf of Tadjoura indicate a crustal structure quite similar to that beneath Iceland. Mohr is of the opinion that crustal extension is not localized in Afar but is spread over the whole region. He postulates intermittent activity over the whole of Cenozoic times. The remaining papers deal with the Ethiopian rift and the various branches of the rift system in Kenya and northern Tanzania. The consensus of geophysical results (mainly seismic and gravity) shows that here there is a relatively narrow zone of dense intrusions in the crust within a broader region of low-density mantle. The Neogene volcanics appear to have a mantle origin and have not resulted from assimilation or remelting of crustal material. First-motion studies of earthquakes indicate ESE tensional forces across the rift in accord with plate tectonics, however, the amount of separation, as estimated from the total width of the intrusions beneath the rift floor, is no more than 50 kin. There are other results, however, conflicting with the above views. Wohlenburg's magnetic surveys across the rift located NW trending long-wavelength anomalies seemingly not caused by bodies within the upper crust. Rykunov obtained, from micro-earthquake studies, a crustal structure at variance with that from a seismic refraction experiment carried out in a different part of the rift and reported by Griffiths. Logatchev et al. believe that the geological evidence does not favour rifting but instead there has been a more complex process of increasing the crustal density by intrusions. This book is a useful compendium of state-of-the-art papers on the East African Rift. For those unfamiliar with the geography of the area it would have been of assistance to have had one good map, somewhere in the volume, of the whole Afar-East African Rift system. The paper by Logatchev et al. is omitted from the contents list.
R. B. WHITMARSH(Wormley)