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Caledonides |
Department of Geology, City of London Polytechnic, Walburgh House, Bigland Street, London E1
The geometric and stratigraphic properties of thrust sheets and fold nappes are controlled by the mechanisms by which they were initiated and emplaced. Thrust sheets that have developed by gravity gliding have their internal stratigraphies truncated by the basal thrusts which cut up-section at the trailing edges. In contrast thrust sheets that have developed by gravitational spreading have their internal stratigraphies truncated by the overlying thrust sheets, and their basal thrusts cut up-section at the leading edges. Fold-nappes have complicated geometries and correlation of stratigraphy between successive nappes is difficult. The basic geometric and stratigraphic properties of thrust sheets and fold-nappes developed by different mechanisms may be used as criteria for testing the applicability of these mechanisms to particular areas of orogenic belts. The Valley and Ridge Province of the Appalachians has geometric and stratigraphic properties consistent with thrust sheet development by gravitational spreading. The western nappes of the Central Scandinavian Caledonides are more complex but have properties suggesting development by buoyant upwelling of the orogen infrastructure. There is, however, a lack of detailed cross-sections of orogenic belts based on geophysical and/or borehole data in addition to the geological data. The interpretation of geological data in isolation is often too dependent on subjective information coloured by the mechanism preferred by the individual concerned.
