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Three books on salt tectonics
TECTONOPHYSICS ELSEVIER
Tectonophysics 269(1997) 343-349
Book Reviews
Three b o o k s on salt tectonics
The last few years have seen a rennaissance in the field of salt tectonics, for reasons both scientific and economic. Rock salt is an unusual material. It is ductile, under conditions where most other rocks are brittle. It eases the formation of thin-skinned structures, by providing detachments. It is light. It fills the cores of anticlines, rises beneath the hangingwalls of reverse faults or the footwalls of normal faults and forms buoyant domes, some of which pierce their overburdens. Salt-related structures are amazingly diverse and complex. Some would say they are beautiful. The challenge is to find the physical mechanisms responsible for their formation. Salt is also economically important. It provides an efficient and long-lasting seal for oil and gas traps and it is a potential host for waste disposal. Luckily, the tops of salt layers are relatively easy to locate at depth, because they strongly reflect seismic waves. Large numbers of seismic images have therefore been produced, mainly by the petroleum industry. However, little seismic energy is able to penetrate salt and there are therefore many problems in interpreting sub-salt structures. Over the years, a number of hypothetical mechanisms have been suggested to account for the development of salt-related structures. To test these hypotheses, new techniques have been developed for modelling salt structures or for retracting stages in their development. Paramount amongst these techniques have been analogue modelling, numerical modelling and restoration. Analogue modelling, in particular, has helped convince many sceptics that mechanisms such as downbuilding or reactive diapirism are indeed physically viable.
Given the flurry of recent activity, it is hardly surprising that several new books on salt tectonics should have appeared, in rapid succession, over the last few years. The following special issues of journals are concerned with structural styles and physical mechanisms of salt tectonics. Book 1. Cobbold, P.R. (Editor), 1993. New Insights into Salt Tectonics. Tectonophysics, 228 (3/4), 448 pp. Book 2. M.P.A. Jackson, D.G. Roberts and S. Snelson (Editors), 1995. Salt Tectonics: A Global Perspective. Am. Assoc. Petrol. Geol. Mem., 65, 454 PP. Book 3. Alsop, G.I., Blundell, D.J. and Davison, I. (Editors), 1996. Salt Tectonics. Geol. Soc. London, Spec. Publ., 100, 310 pp. To some extent these books have a common history. In 1992, we tried to organize an international meeting on salt tectonics at Rennes, but were handicapped by a lack of support from the petroleum industry, in general, and from the American Association of Petroleum Geologists, in particular. I therefore decided to edit a collection of papers, which appeared as Book 1 in December 1993. In the meantime, the Americans sponsored their own Hedberg International Research Conference, held at Bath in September 1993. Although I felt a little upstaged, I have to admit that this was the best scientific meeting I have ever attended in my life. The proceedings, carefully edited, lavishly illustrated and very nicely printed, found their way, more than two years later, into Book 2. This did not reach my bookshelf until May 1996, but it came carrying a 1995 copyright, to the applause of all contributing authors (myself included). Not to be outdone, the Geological Society of
344
Book reuiews
London entered the race with its own meeting on salt tectonics (which, I am sorry to say, I missed), held in September 1994. It then published the proceedings in truly record time, as Book 3. Physically, this reached my bookshelf in March 1996 - - two months before Book 2! - - but it came carrying a 1996 copyright, which somehow seemed quite appropriate. Now that the race is over and the dust has settled on my bookshelf, the three books can be appreciated for what they are: complementary views on a rapidly evolving subject. When Book 1 appeared, it drew attention to the power of analogue and numerical modelling and, I believe, managed to provide new insights into the mechanisms of salt tectonics, as claimed by the title. However, this book contains relatively few seismic data from the petroleum industry. I therefore suspect that most of its readers, intially at least, have been from academia. Book 2 provides many more examples of seismic data, from areas worldwide, but especially from the American homewaters of the Gulf of Mexico. The images from 3D seismic surveys are particularly striking. They make me wonder how anyone is going to do valid research into salt tectonics in the future without massive support from industry. Other attractive features are regional seismic lines and sections on large fold-outs, a set of balanced cross-sections in colour - - large enough to paper a wall! - - and a 28-page review of salt tectonics since 1856, which no specialist or interested reader can afford to miss. Generally speaking, the editorial standards are of the highest, the figures are of exceptional quality and yet the price is reasonable. By comparison, Book 3 gives the impression of having struggled somewhat to stay in the race. On the editorial side, if one turns a blind eye to such "anomilies" (sic) as the running title on pp. 277289, the standards are fairly high, as one has come to expect from a special publication of the Geological Society. The page sizes being rather small, however, some of the figures and photographs are hard to read. Even on foldouts - - and there are only two of them seismic images and diagrams cannot compete with those of Book 2. In contrast to its American counterpart, Book 3 has a strongly European flavour; most of the regional case studies and most of the authors are from Europe -
-
or the Middle East; next to nothing is said about the Gulf of Mexico; only one paper refers to Brazil and then almost in passing; there is a strong emphasis on small-scale studies; and ductile behaviour of the sedimentary overburden is openly advocated. The first paper in Book 3 (Davison et al.) is partly a preface. However, it also emphasizes some of the novelties in the book and discusses how the papers contribute to the following themes: deformation mechanisms, flow patterns inside the source layer, effective viscosity of salt, overburden anisotropy, strain rates, active versus passive diapirism, overburden deformation, salt welds and dissolution, salt triggering mechanisms, physical modelling of salt diapirism and future research. The paper is in no way an objective review of salt tectonics. The reference list is too short for that and some of the mechanical statements are rather subjective. The next seven papers describe salt structures from mines in Poland (Burliga), at outcrop in Yemen (Davison et al.), Israel (Frumkin), Spain (Hoyos et al., Sand et al.) or Iran (Talbot and Alavi) and from boreholes in England (Smith). The emphasis is on small-scale structures and on deformation mechanisms (neglected in Books 1 and 2). There follow five regional studies of salt tectonism in the Persian Gulf Basin (Edgell), the Angola Basin (Spathopoulos), the North Sea (Buchanan et al., Stewart et al.) and Germany (Zirngast). The Persian Gulf (neglected in Books 1 and 2) is of special interest, because of its enormous petroleum reserves. The study of the Angola Basin is nicely illustrated and covers a large area, just to the south of Gabon (which is discussed in Book 2). As for the North Sea, there is some overlap geographically between the two papers in Book 3, four papers in Book 2 and two papers in Book 1; however, the interpretations differ, sometimes quite sharply, providing no little excitement for the reader. Finally, three papers are dedicated to physical modelling and three to numerical modelling. Given the enormous amount of work done in physical modelling of salt-cored structures, properly scaled for gravity, with or without a centrifuge, I find quaint, if not outmoded, a study based on forceful indentation by a piston (Alsop). Centrifuged plasticine models of aggrading and prograding overburdens (Koyi) make a change from the usual sand-
Book reviews
boxes, but I have a problem with the model ratio of stress in these experiments and I feel that the plasticine is too strong at the surface. Finally, a single experiment is perhaps not enough to explore the mechanisms for generating large counter-regional faults (Szatmari et al.), even if it succeeds in reproducing quite well the geometry of a remarkable structure in the Santos Basin of Brazil. As for the numerical modelling of stratal architecture (Cohen and Hardy), it demonstrates the gross effects of salt withdrawal for different rates of sediment supply - without any faulting, by definition. Mathematical models of heat flow around salt domes (Petersen and Lerche) show that heat focusses into the salt, because of its high conductivity, causing local thermal anomalies. Finally, a numerical study of salt diapirism under an overburden capable of either brittle faulting or viscous flow (Poliakov et al.) shows that pressure-solution creep should perhaps be considered as an overburden mechanism that competes with faulting. Despite being strongly outdistanced by Book 2, I think that Book 3 will provide useful information and understanding for those interested in salt tectonics. It should also make a reasonable addition to the excellent set of Special Issues published so far by the Geological Society. Let us hope that the commercial success of this series will not lead to any decline in scientific and editorial standards. P.R. Cobbold (Rennes) PH S0040-1951(96)00152-7
Terrestrial Heat Flow and Geothermal Energy in Asia, by M.L. Gupta and M. Yamano (Editors). Balkema, Rotterdam, 1995, hardcover, XII + 516 pp., Price: Dr. 206,70. ISBN 90-5410-258-6. The heat flux from the Earth's interior as measured at the surface has long been an important boundary condition for models of lithospheric temperatures, heat transfer processes and geodynamics, on global, regional and local scales. Although the global average of the heat flux on the continents is 65 mW m -2, and in the ocean basins 101 mW m -2, the regional variation within each setting ranges over
345
three orders of magnitude. Thus knowledge of the regional variation, and of the relationship of the variation to tectonic settings, is an important goal of geothermal studies, and is of practical value in assessing the utility of the Earth's heat as an energy resource. The International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior has been an energetic promoter of regional geothermal investigations, and equally vigorous in its encouragement of the publication of continental scale syntheses of such research in special volumes. In this context appeared Terrestrial Heat Flow in Europe edited by V. Cermak and L. Rybach and published by the Springer Verlag in 1979, and followed in 1992 by the Geothermal Atlas of Europe, edited by E. Hurtig, V. Cermak, R. Haenel, and V. Zui, and published by the H. Haack Verlagsgesellschaft. Now we have another volume describing the thermal regime over the broad expanses of Asia. Of the 13,000 or so measurements of terrestrial heat flow on continents, some 5000 have been made in Asia, so the topics discussed in this volume derive from and speak to a very significant block of measurements. This book comprises papers written by leading researchers in the Asian region. It was no small editorial task to bring these papers to completion and to publication in a well-produced volume, particularly when one recognizes that for almost all of the authors and editors English is not a mother tongue. The book comprises two sections: the first seventeen papers address various aspects of the heat flow distribution and its implications for the crustal thermal regime in Asia; the final eight papers describe the geothermal energy resources in tectonically, and in particular, volcanically active areas. Contributions derive from investigations in Israel, Turkey, Azerbaijan, India, Thailand, Indonesia, Vietnam, China (6), Japan (3), Korea, Russia (3), Mongolia and from some of the adjacent marine areas. Inevitably there is some geographic insularity imposed on the discussions, because of the reality of research efforts constrained by national boundaries. Nevertheless, there are some fine overviews and summaries of the research carried on for more than three decades in the principal geothermal laboratories of Asia. Of particular note are the comprehensive papers addressing: (1)
Tectonophysics 269(1997) 343-349
Book Reviews
Three b o o k s on salt tectonics
The last few years have seen a rennaissance in the field of salt tectonics, for reasons both scientific and economic. Rock salt is an unusual material. It is ductile, under conditions where most other rocks are brittle. It eases the formation of thin-skinned structures, by providing detachments. It is light. It fills the cores of anticlines, rises beneath the hangingwalls of reverse faults or the footwalls of normal faults and forms buoyant domes, some of which pierce their overburdens. Salt-related structures are amazingly diverse and complex. Some would say they are beautiful. The challenge is to find the physical mechanisms responsible for their formation. Salt is also economically important. It provides an efficient and long-lasting seal for oil and gas traps and it is a potential host for waste disposal. Luckily, the tops of salt layers are relatively easy to locate at depth, because they strongly reflect seismic waves. Large numbers of seismic images have therefore been produced, mainly by the petroleum industry. However, little seismic energy is able to penetrate salt and there are therefore many problems in interpreting sub-salt structures. Over the years, a number of hypothetical mechanisms have been suggested to account for the development of salt-related structures. To test these hypotheses, new techniques have been developed for modelling salt structures or for retracting stages in their development. Paramount amongst these techniques have been analogue modelling, numerical modelling and restoration. Analogue modelling, in particular, has helped convince many sceptics that mechanisms such as downbuilding or reactive diapirism are indeed physically viable.
Given the flurry of recent activity, it is hardly surprising that several new books on salt tectonics should have appeared, in rapid succession, over the last few years. The following special issues of journals are concerned with structural styles and physical mechanisms of salt tectonics. Book 1. Cobbold, P.R. (Editor), 1993. New Insights into Salt Tectonics. Tectonophysics, 228 (3/4), 448 pp. Book 2. M.P.A. Jackson, D.G. Roberts and S. Snelson (Editors), 1995. Salt Tectonics: A Global Perspective. Am. Assoc. Petrol. Geol. Mem., 65, 454 PP. Book 3. Alsop, G.I., Blundell, D.J. and Davison, I. (Editors), 1996. Salt Tectonics. Geol. Soc. London, Spec. Publ., 100, 310 pp. To some extent these books have a common history. In 1992, we tried to organize an international meeting on salt tectonics at Rennes, but were handicapped by a lack of support from the petroleum industry, in general, and from the American Association of Petroleum Geologists, in particular. I therefore decided to edit a collection of papers, which appeared as Book 1 in December 1993. In the meantime, the Americans sponsored their own Hedberg International Research Conference, held at Bath in September 1993. Although I felt a little upstaged, I have to admit that this was the best scientific meeting I have ever attended in my life. The proceedings, carefully edited, lavishly illustrated and very nicely printed, found their way, more than two years later, into Book 2. This did not reach my bookshelf until May 1996, but it came carrying a 1995 copyright, to the applause of all contributing authors (myself included). Not to be outdone, the Geological Society of
344
Book reuiews
London entered the race with its own meeting on salt tectonics (which, I am sorry to say, I missed), held in September 1994. It then published the proceedings in truly record time, as Book 3. Physically, this reached my bookshelf in March 1996 - - two months before Book 2! - - but it came carrying a 1996 copyright, which somehow seemed quite appropriate. Now that the race is over and the dust has settled on my bookshelf, the three books can be appreciated for what they are: complementary views on a rapidly evolving subject. When Book 1 appeared, it drew attention to the power of analogue and numerical modelling and, I believe, managed to provide new insights into the mechanisms of salt tectonics, as claimed by the title. However, this book contains relatively few seismic data from the petroleum industry. I therefore suspect that most of its readers, intially at least, have been from academia. Book 2 provides many more examples of seismic data, from areas worldwide, but especially from the American homewaters of the Gulf of Mexico. The images from 3D seismic surveys are particularly striking. They make me wonder how anyone is going to do valid research into salt tectonics in the future without massive support from industry. Other attractive features are regional seismic lines and sections on large fold-outs, a set of balanced cross-sections in colour - - large enough to paper a wall! - - and a 28-page review of salt tectonics since 1856, which no specialist or interested reader can afford to miss. Generally speaking, the editorial standards are of the highest, the figures are of exceptional quality and yet the price is reasonable. By comparison, Book 3 gives the impression of having struggled somewhat to stay in the race. On the editorial side, if one turns a blind eye to such "anomilies" (sic) as the running title on pp. 277289, the standards are fairly high, as one has come to expect from a special publication of the Geological Society. The page sizes being rather small, however, some of the figures and photographs are hard to read. Even on foldouts - - and there are only two of them seismic images and diagrams cannot compete with those of Book 2. In contrast to its American counterpart, Book 3 has a strongly European flavour; most of the regional case studies and most of the authors are from Europe -
-
or the Middle East; next to nothing is said about the Gulf of Mexico; only one paper refers to Brazil and then almost in passing; there is a strong emphasis on small-scale studies; and ductile behaviour of the sedimentary overburden is openly advocated. The first paper in Book 3 (Davison et al.) is partly a preface. However, it also emphasizes some of the novelties in the book and discusses how the papers contribute to the following themes: deformation mechanisms, flow patterns inside the source layer, effective viscosity of salt, overburden anisotropy, strain rates, active versus passive diapirism, overburden deformation, salt welds and dissolution, salt triggering mechanisms, physical modelling of salt diapirism and future research. The paper is in no way an objective review of salt tectonics. The reference list is too short for that and some of the mechanical statements are rather subjective. The next seven papers describe salt structures from mines in Poland (Burliga), at outcrop in Yemen (Davison et al.), Israel (Frumkin), Spain (Hoyos et al., Sand et al.) or Iran (Talbot and Alavi) and from boreholes in England (Smith). The emphasis is on small-scale structures and on deformation mechanisms (neglected in Books 1 and 2). There follow five regional studies of salt tectonism in the Persian Gulf Basin (Edgell), the Angola Basin (Spathopoulos), the North Sea (Buchanan et al., Stewart et al.) and Germany (Zirngast). The Persian Gulf (neglected in Books 1 and 2) is of special interest, because of its enormous petroleum reserves. The study of the Angola Basin is nicely illustrated and covers a large area, just to the south of Gabon (which is discussed in Book 2). As for the North Sea, there is some overlap geographically between the two papers in Book 3, four papers in Book 2 and two papers in Book 1; however, the interpretations differ, sometimes quite sharply, providing no little excitement for the reader. Finally, three papers are dedicated to physical modelling and three to numerical modelling. Given the enormous amount of work done in physical modelling of salt-cored structures, properly scaled for gravity, with or without a centrifuge, I find quaint, if not outmoded, a study based on forceful indentation by a piston (Alsop). Centrifuged plasticine models of aggrading and prograding overburdens (Koyi) make a change from the usual sand-
Book reviews
boxes, but I have a problem with the model ratio of stress in these experiments and I feel that the plasticine is too strong at the surface. Finally, a single experiment is perhaps not enough to explore the mechanisms for generating large counter-regional faults (Szatmari et al.), even if it succeeds in reproducing quite well the geometry of a remarkable structure in the Santos Basin of Brazil. As for the numerical modelling of stratal architecture (Cohen and Hardy), it demonstrates the gross effects of salt withdrawal for different rates of sediment supply - without any faulting, by definition. Mathematical models of heat flow around salt domes (Petersen and Lerche) show that heat focusses into the salt, because of its high conductivity, causing local thermal anomalies. Finally, a numerical study of salt diapirism under an overburden capable of either brittle faulting or viscous flow (Poliakov et al.) shows that pressure-solution creep should perhaps be considered as an overburden mechanism that competes with faulting. Despite being strongly outdistanced by Book 2, I think that Book 3 will provide useful information and understanding for those interested in salt tectonics. It should also make a reasonable addition to the excellent set of Special Issues published so far by the Geological Society. Let us hope that the commercial success of this series will not lead to any decline in scientific and editorial standards. P.R. Cobbold (Rennes) PH S0040-1951(96)00152-7
Terrestrial Heat Flow and Geothermal Energy in Asia, by M.L. Gupta and M. Yamano (Editors). Balkema, Rotterdam, 1995, hardcover, XII + 516 pp., Price: Dr. 206,70. ISBN 90-5410-258-6. The heat flux from the Earth's interior as measured at the surface has long been an important boundary condition for models of lithospheric temperatures, heat transfer processes and geodynamics, on global, regional and local scales. Although the global average of the heat flux on the continents is 65 mW m -2, and in the ocean basins 101 mW m -2, the regional variation within each setting ranges over
345
three orders of magnitude. Thus knowledge of the regional variation, and of the relationship of the variation to tectonic settings, is an important goal of geothermal studies, and is of practical value in assessing the utility of the Earth's heat as an energy resource. The International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior has been an energetic promoter of regional geothermal investigations, and equally vigorous in its encouragement of the publication of continental scale syntheses of such research in special volumes. In this context appeared Terrestrial Heat Flow in Europe edited by V. Cermak and L. Rybach and published by the Springer Verlag in 1979, and followed in 1992 by the Geothermal Atlas of Europe, edited by E. Hurtig, V. Cermak, R. Haenel, and V. Zui, and published by the H. Haack Verlagsgesellschaft. Now we have another volume describing the thermal regime over the broad expanses of Asia. Of the 13,000 or so measurements of terrestrial heat flow on continents, some 5000 have been made in Asia, so the topics discussed in this volume derive from and speak to a very significant block of measurements. This book comprises papers written by leading researchers in the Asian region. It was no small editorial task to bring these papers to completion and to publication in a well-produced volume, particularly when one recognizes that for almost all of the authors and editors English is not a mother tongue. The book comprises two sections: the first seventeen papers address various aspects of the heat flow distribution and its implications for the crustal thermal regime in Asia; the final eight papers describe the geothermal energy resources in tectonically, and in particular, volcanically active areas. Contributions derive from investigations in Israel, Turkey, Azerbaijan, India, Thailand, Indonesia, Vietnam, China (6), Japan (3), Korea, Russia (3), Mongolia and from some of the adjacent marine areas. Inevitably there is some geographic insularity imposed on the discussions, because of the reality of research efforts constrained by national boundaries. Nevertheless, there are some fine overviews and summaries of the research carried on for more than three decades in the principal geothermal laboratories of Asia. Of particular note are the comprehensive papers addressing: (1)