Sedimentary thicknesses in orogenic belts

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Abstract
Summary: The evidence provided by certain examples of thick sedimentary sequences, both deformed and undeformed, suggests that at least some of the geotectonic processes can be isolated whereby they are accommodated. These are ( a ) isostatic depression of oceanic crust by sedimentary loading, ( b ) down-dragging and compressional warping due to tangential crustal and sub-crustal displacements and ( c ) crustal foundering resulting from vertically acting changes in the mantle and/or lower crust. All three of these factors must be involved to some extent during the evolution of an orogenic belt but some features, such as the subsidence of a miogeosyncline under tension, are still difficult to explain. Within many Alpine orogenic zones, large thicknesses of sediment are found in miogeosynclines, eugeosynclines, flysch troughs and molasse basins but these four environments are not recognizable in all orogenic belts, particularly the older ones. Comparisons between different belts are not always meaningful in the framework of existing geosynclinal concepts and terminology, but it is expected that a new and comprehensive terminology, based on geotectonic considerations, may shortly become possible.