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Introductory Remarks |
Department of Earth Sciences, The University, Leeds 2
The purpose of this symposium is to discuss the problems which will be encountered in compiling data for orogenic studies. The proposal that the Geological Society should compile such data was made by Mr W. B. Harland. It was felt that while displacements of the ocean floors are being measured rather precisely by geophysical methods, those within the continental crust—which have to be evaluated, mainly by geological methods, from very complex structures—are still very imprecisely known. There is clearly a need to assemble and assess the available data, especially quantitative data, and it was decided that the Society should do this.
We are not here to discuss hypotheses. These are supposed to flow from analysis of the data. That, however, is not how science usually works. We have to be aware of the alternative hypotheses in order to know what data we want. The most important displacements to be measured are those which are associated with orogeny. The orogenic belts are characterized by intense deformation of the rocks. The strata are conspicuously folded and since folds can be produced experimentally by tangential compression it has for a long time been supposed that the orogenic belts represent parts of the crust that have been compressed when more rigid blocks (cratons) moved towards one another. This is the crustal-shortening hypothesis. Let me emphasize at the start that it is still only a hypothesis: there are many very experienced structural geologists who do not accept it (Bemmelen 1966; Belousov 1962).
There is now an overwhelming
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This 250-word extract was created in the absence of an abstract.
