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Historical Analysis of Orogenic Belts |
Department of Geology, Sedgwick Museum, Downing Street, Cambridge
This paper discusses some aspects of the dating of specified orogenic events, in terms of a stratigraphical scale, by various methods of correlation.
A standard stratigraphical scale is the common reference to which various data (structural, palaeontological, isotopic, etc.) and events interpreted from them are correlated. Lists of tectonic phases express age estimates of particular tectonic events and cannot usefully substitute for a standard scale.
Some datable elements in orogeny are defined. The direct relation between an orogenic event and a stratigraphical scale, as determined in different ways, yields an age which is limited in precision by distinct components of indeterminacy and uncertainty.
Correlation depends partly on diastrophic gradient which distinguishes the effects of tectonic events that are confined to the immediate neighbourhood from those at greater distance; few events have global effects (e.g., eustatic change). Correlation is generally effected, however, by relatively independent events, (e.g., biological, evolutionary, nuclear decay, climatic, magnetic reversals, etc.).
The primary purpose of dating orogenic events is to enquire into the distribution of earth movements in space and time. The results can be given in a number of ways (e.g., tectonograms, palaeotectonic and palinspastic maps, kinematic tectonic realms). Because the Earth’s surface is a closed system a total synoptic stratigraphical study of many aspects of the time, direction and magnitude of movement becomes possible and is aided by new procedures of co-ordinated research, compilation, and automatic processing of data.
