A. Nicolas – författare

1.1. HISTORICAL DEVELOPMENT OF THE OPHIOLITE CONCEPT. Ophiolite, Greek for 'the snake stone', appears to have received its first written definition by Brongniart (1813) as a serpentine matrix containing various minerals. Later in 1821 and 1827, Brongniart determined that volcanic and gabbroic rocks were also present, associated with cherts, and he ascribed an igneous origin to the ophiolite. Amstutz (1980) gives an excellent exegesis of these early contributions and traces the further use of the term and concept of ophiolite. This concept had been forged in the western Alps and Apennines where, thanks to talented Italian geologists, in particular A. Sismonda, B. Gastaldi, V. Novarese and S. Franchi, the study on metamorphic ophiolites (the 'pietre verdi') has rapidly progressed. At the tum of the century the association of radiolarite, diabase, gabbro (euphotide), and serpentinite-peridotite was clearly identified, even through their metamorphic transformations.In 1902, Franchi developed the hypothesis introduced earlier by Lotti (1886), of a submarine outflow to explain the 'pietre verdi' association, on the basis of the attribution of the variolites and metamorphic prasinites to an hypabyssal volcanism, also responsible for the formation of radiolarites. Thus, before the popular work of Steinmann in 1927, the various components constituting an ophiolite had been identified and its hypabyssal origin proposed. As recalled by Amstutz (1980), the so-called 'Steinmann trinity', which consists of the association of radiolarites, diabases and serpentinites, was more completely and better defined in these earlier works.

Ophiolites are key sources of information regarding the genesis and evolution of oceanic lithosphere. Over the past decades, the geological ~tudy of ophio- lites has provided a wealth of insight into lithospheric processes and has proved to be an indispensible prerequisite to interpreting geophysical and other investigations of the crust underlying recent oceans. The Oman Ophiol- ite offers the most complete and structurally undisturbed sections of the oceanic crust in vast, clean exposures. It is, therefore, most fortunate for the scientific community that Mhd. Kassim, Director General of Minerals and Dr. Hilal Al Azri, Director of the Geological Survey, took upon themselves the task of organizing in Oman an international meeting on ophiolites. Having planned for an attendance of only 100 to 150 persons, the logistics of the organizing committee were put to a severe test by th~ 300 participants who eventually arrived from 27 countries. The 14 field trips, most of which were conducted twice, provided the participants with an excellent introduc- tion to the geology of Oman, the ophIolite sequence, and re~ated phenomena.

This volume follows a Specialized Symposium on "Mantle denudation in slow spreading ridges and in ophiolites", held at the XII EUG Meeting in Strasbourg, spring 1993. During the meeting it was felt that the contribu­ tions to the Symposium justified a volume presenting its main scientific achievements. The present title of the volume shows that the center of inter­ est has slightly shifted with respect to the initial objective: in order to under­ stand the processes involved in accretion taking place at oceanic ridges, it is crucial to study the interaction between uppermost mantle and lower crust. The approach favored here is that of petrological and structural analysis of oceanic rocks in present-day oceanic ridges combined with similar studies in ophiolites. Rock specimen collected by submersibles or dredge hauls in oceanic ridge environments provide a "ground truth". However, except for areas such as the MARK (Mid-Atlantic Ridge ne ar Kane fracture zone) where, thanks to multiple submersible dives, the local geology is known with aprecision even better than in many onshore ophiolites, mutual rela­ tionships between uppermost mantle and lower crust are poorly known. In contrast, onshore ophiolites provide a necessary large-scale picture built up over many years of structural and petrological mapping.

Physicists attempt to reduce natural phenomena to their essential dimensions by means of simplification and approximation and to account for them by defining natural laws. Paradoxically, whilst there is a critical need in geology to reduce the overwhelming field information to its essentials, it often re­ mains in an over-descriptive state. This prudent attitude of geologists is dictated by the nature of the subjects being consi­ dered, as it is often difficult to derive the significant parame­ ters from the raw data. It also follows from the way that geolo­ gical work is carried out. Geologists proceed, as in a police investigation, by trying to reconstruct past conditions and events from an analysis of the features preserved in rocks. In physics all knowledge is based on experiment but in the Earth Sciences experimental evidence is of very limited scope and is difficult to interpret. The geologist's cautious approach in accepting evidence gained by modelling and quantification is sometimes questionable when it is taken too far. It shuts out potentially fruitful lines of advance; for instance when refu­ sing order of magnitude calculations, it risks being drowned in anthropomorphic speculation. Happily nowadays, many more studies tend to separate and order the significant facts and are carried out with numerical constraints, which although they are approxi­ mate in nature, limit the range of hypotheses and thus give rise to new models.

Physicists attempt to reduce natural phenomena to their essential dimensions by means of simplification and approximation and to account for them by defining natural laws. Paradoxically, whilst there is a critical need in geology to reduce the overwhelming field information to its essentials, it often re­ mains in an over-descriptive state. This prudent attitude of geologists is dictated by the nature of the subjects being consi­ dered, as it is often difficult to derive the significant parame­ ters from the raw data. It also follows from the way that geolo­ gical work is carried out. Geologists proceed, as in a police investigation, by trying to reconstruct past conditions and events from an analysis of the features preserved in rocks. In physics all knowledge is based on experiment but in the Earth Sciences experimental evidence is of very limited scope and is difficult to interpret. The geologist's cautious approach in accepting evidence gained by modelling and quantification is sometimes questionable when it is taken too far. It shuts out potentially fruitful lines of advance; for instance when refu­ sing order of magnitude calculations, it risks being drowned in anthropomorphic speculation. Happily nowadays, many more studies tend to separate and order the significant facts and are carried out with numerical constraints, which although they are approxi­ mate in nature, limit the range of hypotheses and thus give rise to new models.

This volume follows a Specialized Symposium on "Mantle denudation in slow spreading ridges and in ophiolites", held at the XII EUG Meeting in Strasbourg, spring 1993. During the meeting it was felt that the contribu­ tions to the Symposium justified a volume presenting its main scientific achievements. The present title of the volume shows that the center of inter­ est has slightly shifted with respect to the initial objective: in order to under­ stand the processes involved in accretion taking place at oceanic ridges, it is crucial to study the interaction between uppermost mantle and lower crust. The approach favored here is that of petrological and structural analysis of oceanic rocks in present-day oceanic ridges combined with similar studies in ophiolites. Rock specimen collected by submersibles or dredge hauls in oceanic ridge environments provide a "ground truth". However, except for areas such as the MARK (Mid-Atlantic Ridge ne ar Kane fracture zone) where, thanks to multiple submersible dives, the local geology is known with aprecision even better than in many onshore ophiolites, mutual rela­ tionships between uppermost mantle and lower crust are poorly known. In contrast, onshore ophiolites provide a necessary large-scale picture built up over many years of structural and petrological mapping.

Physicists attempt to reduce natural phenomena to their essential dimensions by means of simplification and approximation and to account for them by defining natural laws. Paradoxically, whilst there is a critical need in geology to reduce the overwhelming field information to its essentials, it often re­ mains in an over-descriptive state. This prudent attitude of geologists is dictated by the nature of the subjects being consi­ dered, as it is often difficult to derive the significant parame­ ters from the raw data. It also follows from the way that geolo­ gical work is carried out. Geologists proceed, as in a police investigation, by trying to reconstruct past conditions and events from an analysis of the features preserved in rocks. In physics all knowledge is based on experiment but in the Earth Sciences experimental evidence is of very limited scope and is difficult to interpret. The geologist''s cautious approach in accepting evidence gained by modelling and quantification is sometimes questionable when it is taken too far. It shuts out potentially fruitful lines of advance; for instance when refu­ sing order of magnitude calculations, it risks being drowned in anthropomorphic speculation. Happily nowadays, many more studies tend to separate and order the significant facts and are carried out with numerical constraints, which although they are approxi­ mate in nature, limit the range of hypotheses and thus give rise to new models.

Ophiolites are key sources of information regarding the genesis and evolution of oceanic lithosphere. Over the past decades, the geological ~tudy of ophio- lites has provided a wealth of insight into lithospheric processes and has proved to be an indispensible prerequisite to interpreting geophysical and other investigations of the crust underlying recent oceans. The Oman Ophiol- ite offers the most complete and structurally undisturbed sections of the oceanic crust in vast, clean exposures. It is, therefore, most fortunate for the scientific community that Mhd. Kassim, Director General of Minerals and Dr. Hilal Al Azri, Director of the Geological Survey, took upon themselves the task of organizing in Oman an international meeting on ophiolites. Having planned for an attendance of only 100 to 150 persons, the logistics of the organizing committee were put to a severe test by th~ 300 participants who eventually arrived from 27 countries. The 14 field trips, most of which were conducted twice, provided the participants with an excellent introduc- tion to the geology of Oman, the ophIolite sequence, and re~ated phenomena.

1.1. HISTORICAL DEVELOPMENT OF THE OPHIOLITE CONCEPT. Ophiolite, Greek for 'the snake stone', appears to have received its first written definition by Brongniart (1813) as a serpentine matrix containing various minerals. Later in 1821 and 1827, Brongniart determined that volcanic and gabbroic rocks were also present, associated with cherts, and he ascribed an igneous origin to the ophiolite. Amstutz (1980) gives an excellent exegesis of these early contributions and traces the further use of the term and concept of ophiolite. This concept had been forged in the western Alps and Apennines where, thanks to talented Italian geologists, in particular A. Sismonda, B. Gastaldi, V. Novarese and S. Franchi, the study on metamorphic ophiolites (the 'pietre verdi') has rapidly progressed. At the tum of the century the association of radiolarite, diabase, gabbro (euphotide), and serpentinite-peridotite was clearly identified, even through their metamorphic transformations.In 1902, Franchi developed the hypothesis introduced earlier by Lotti (1886), of a submarine outflow to explain the 'pietre verdi' association, on the basis of the attribution of the variolites and metamorphic prasinites to an hypabyssal volcanism, also responsible for the formation of radiolarites. Thus, before the popular work of Steinmann in 1927, the various components constituting an ophiolite had been identified and its hypabyssal origin proposed. As recalled by Amstutz (1980), the so-called 'Steinmann trinity', which consists of the association of radiolarites, diabases and serpentinites, was more completely and better defined in these earlier works.