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East African Rifts
Tectonophysics, 0 Elsevier
Book
23 (1974)
Scientific
209-210 Publishing Company,
Amsterdam
-
Printed
in The Netherlands
Reviews
East African Rifts. R.W. Girdler (Editor). Elsevier, Amsterdam, 1972, 186 pp., 72 figs., 12 tables, Dfl. 27.50. Developments in Geotectonics 7, Upper Mantle Scientific Report No. 40, reprinted from Tectonophysics Volume 15(1/2). The book contains nineteen unconnected papers on the petrology and geophysics of the Rift system from Tanganyika northwards. The papers were presented in a symposium arranged by Dr. Girdler at the IUGG meeting in Moscow in August 1971. Of the nineteen, two are only abstracts and three others are brief accounts of already published papers. There are no general review papers. Because a meeting on the Red Sea and Gulf of Aden had been held in London at the Royal Society during 1969, Dr. Girdler began the symposium with six papers devoted to the Afar Triangle. Here there is dissention: Paul Mohr interprets the fault pattern in terms of a triple junction whereas Barberi, Tazieff and Varet interpret the magma types of Afar in terms of an ‘inflexion’ between the Red Sea and Gulf of Aden trends, against which the continental rift impinges. Makris and others present an important Bouguer gravity map which must have involved a lot of work in the hottest place on earth. The map might be taken to support Tazieff but, unfortunately, it is not complete in the vital area inland of the Gulf of Tadjura. The aeromagnetic map might tell us much but unfortunately Girdler’s paper about it is just an abstract with no illustration. French refraction results suggest a crust like Iceland, and Searle and Gouin point out that the absence of S, arrivals at Addis Ababa indicates shallow asthenosphere under Afar. It is not possible to mention all of the remaining thirteen papers. All of them are consistent with the idea which McCall puts succinctly, that the Rift from Ethiopia to 2”s is an abhorted ocean, of long existence but never significantly opened up. Fairhead and Girdler recapitulate their studies of seismicity and it is striking how the epicentres in Afar and the Ethiopian rift support the triple-junction hypothesis, whereas farther south the spread of active faults clearly increases. A Soviet study of micro-earthquakes reports that these are not related to visible geological features and that the crust in the southern part of the Gregory Rift, near 4% is normal continental. At the other end of the rift in Kenya, near 2”N, Griffiths reports refraction results showing 6.4 over 7.5 km/set at a depth of 20 km. Seismological results reported by Long from the equator and by Knopoff from farther north suggest an upper-mantle structure beneath the rift similar to that under the axis of the mid-ocean ridge. Gravity work by Girdler and his students has done much to make this view accepted and in an important paper they trace the southern end of the regional Bouguer low at 5”f and of the superimposed
210
axial Bouguer high at 2%. Nevertheless, the rift valley in Kenya is quite different from the mid-ocean ridge: Williams points out that the ratio of basaltic extrusives to silicic in Ethiopia is 6/l but in Kenya it falls to 1.3/l and McCall and Hornung use the differentiation at Silali, a quaternary volcano at 1”N in Kenya, to argue that the rift here is underlain by a chain of cupolas containing mantle-derived basaltic parental material which differentiates under the control of the concentration of volatiles. The papers have been reprinted without change in pagination from the Elsevier journal Tectonophysics Vol. 15 (l/2), published in 1972. The only change is in the list of contents which contains no reference to an important paper on dating results by Logatchev, Beloussov and Milanovsky and to a list of Upper Mantle Project Reports, both of which appear, both in the book and in the journal. Although this was evidently a good symposium and merited publication, the papers are of not less than ordinarily ephemeral interest and it would have been quite sufficient to have made a few extra copies of the journal available. I can see no reason at all for anyone with access to Tectonophysics wishing to spend $5 on this hardback book; D.H. MATTHEWS
(Cambridge)
Tectonics. P. Schmidt-Thorn& Enke, Stuttgart, 1972, 579 pp., 299 figs., DM 118.-. Textbook of General Geology, 2. R. Brinkmann (Editor). This review has appeared as the last of three volumes. It presents Hypotheses, Theories and Ideas of about a century of Geoscience. Many facts from all over the world are described or demonstrated in an excellent collection of figures, many of them being especially prepared for this book. It seems that it was the author’s main concept to present. as objectively as possible in the last phase of the Upper Mantle Project and for the beginning of a period of international geodynamic research, a wide spectrum of thoughts of leading scientists. So it is very typically that the book starts with a reproduction of a drawing (1834) of one of the mountain groups in Switzerland by Escher von der Linth from whence the nappe-theory started, and one of the last figures shows the drift of Australia (Irving, Green, Runcorn) deduced from paleomagnetism. The main chapters are: Main types of crustal deformation; mechanical properties and deformation of rocks; development of crustal movements in time and space; structural types of the earth’s crust; structure of the earth’s crust and the interior of the earth, geotectonics. The book is written with the experience of a university teacher and the ability of a wise scientist who is able to express a maximum of information with a minimum of words. The manuscript has been completed in 1970, The geophysicist who is able to read german can use this book as an excellent source of information. H. CLOSS
(Hanover)
Book
23 (1974)
Scientific
209-210 Publishing Company,
Amsterdam
-
Printed
in The Netherlands
Reviews
East African Rifts. R.W. Girdler (Editor). Elsevier, Amsterdam, 1972, 186 pp., 72 figs., 12 tables, Dfl. 27.50. Developments in Geotectonics 7, Upper Mantle Scientific Report No. 40, reprinted from Tectonophysics Volume 15(1/2). The book contains nineteen unconnected papers on the petrology and geophysics of the Rift system from Tanganyika northwards. The papers were presented in a symposium arranged by Dr. Girdler at the IUGG meeting in Moscow in August 1971. Of the nineteen, two are only abstracts and three others are brief accounts of already published papers. There are no general review papers. Because a meeting on the Red Sea and Gulf of Aden had been held in London at the Royal Society during 1969, Dr. Girdler began the symposium with six papers devoted to the Afar Triangle. Here there is dissention: Paul Mohr interprets the fault pattern in terms of a triple junction whereas Barberi, Tazieff and Varet interpret the magma types of Afar in terms of an ‘inflexion’ between the Red Sea and Gulf of Aden trends, against which the continental rift impinges. Makris and others present an important Bouguer gravity map which must have involved a lot of work in the hottest place on earth. The map might be taken to support Tazieff but, unfortunately, it is not complete in the vital area inland of the Gulf of Tadjura. The aeromagnetic map might tell us much but unfortunately Girdler’s paper about it is just an abstract with no illustration. French refraction results suggest a crust like Iceland, and Searle and Gouin point out that the absence of S, arrivals at Addis Ababa indicates shallow asthenosphere under Afar. It is not possible to mention all of the remaining thirteen papers. All of them are consistent with the idea which McCall puts succinctly, that the Rift from Ethiopia to 2”s is an abhorted ocean, of long existence but never significantly opened up. Fairhead and Girdler recapitulate their studies of seismicity and it is striking how the epicentres in Afar and the Ethiopian rift support the triple-junction hypothesis, whereas farther south the spread of active faults clearly increases. A Soviet study of micro-earthquakes reports that these are not related to visible geological features and that the crust in the southern part of the Gregory Rift, near 4% is normal continental. At the other end of the rift in Kenya, near 2”N, Griffiths reports refraction results showing 6.4 over 7.5 km/set at a depth of 20 km. Seismological results reported by Long from the equator and by Knopoff from farther north suggest an upper-mantle structure beneath the rift similar to that under the axis of the mid-ocean ridge. Gravity work by Girdler and his students has done much to make this view accepted and in an important paper they trace the southern end of the regional Bouguer low at 5”f and of the superimposed
210
axial Bouguer high at 2%. Nevertheless, the rift valley in Kenya is quite different from the mid-ocean ridge: Williams points out that the ratio of basaltic extrusives to silicic in Ethiopia is 6/l but in Kenya it falls to 1.3/l and McCall and Hornung use the differentiation at Silali, a quaternary volcano at 1”N in Kenya, to argue that the rift here is underlain by a chain of cupolas containing mantle-derived basaltic parental material which differentiates under the control of the concentration of volatiles. The papers have been reprinted without change in pagination from the Elsevier journal Tectonophysics Vol. 15 (l/2), published in 1972. The only change is in the list of contents which contains no reference to an important paper on dating results by Logatchev, Beloussov and Milanovsky and to a list of Upper Mantle Project Reports, both of which appear, both in the book and in the journal. Although this was evidently a good symposium and merited publication, the papers are of not less than ordinarily ephemeral interest and it would have been quite sufficient to have made a few extra copies of the journal available. I can see no reason at all for anyone with access to Tectonophysics wishing to spend $5 on this hardback book; D.H. MATTHEWS
(Cambridge)
Tectonics. P. Schmidt-Thorn& Enke, Stuttgart, 1972, 579 pp., 299 figs., DM 118.-. Textbook of General Geology, 2. R. Brinkmann (Editor). This review has appeared as the last of three volumes. It presents Hypotheses, Theories and Ideas of about a century of Geoscience. Many facts from all over the world are described or demonstrated in an excellent collection of figures, many of them being especially prepared for this book. It seems that it was the author’s main concept to present. as objectively as possible in the last phase of the Upper Mantle Project and for the beginning of a period of international geodynamic research, a wide spectrum of thoughts of leading scientists. So it is very typically that the book starts with a reproduction of a drawing (1834) of one of the mountain groups in Switzerland by Escher von der Linth from whence the nappe-theory started, and one of the last figures shows the drift of Australia (Irving, Green, Runcorn) deduced from paleomagnetism. The main chapters are: Main types of crustal deformation; mechanical properties and deformation of rocks; development of crustal movements in time and space; structural types of the earth’s crust; structure of the earth’s crust and the interior of the earth, geotectonics. The book is written with the experience of a university teacher and the ability of a wise scientist who is able to express a maximum of information with a minimum of words. The manuscript has been completed in 1970, The geophysicist who is able to read german can use this book as an excellent source of information. H. CLOSS
(Hanover)