Mechanism of continental crust subsidence in fold belts: The Urals, Appalachians and Scandinavian Caledonides

Abstract

Formation of deep basins on continental crust is usually explained by lithospheric stretching. Stretching produces the large deformations in the upper, brittle part of the crust that have been found in many rift valleys. The sedimentary cover of deep basins on continental crust is exposed at the surface in fold belts in many places. Some undeformed basins have been covered by detailed seismic profiling and drilling. The sedimentary cover structure in the Urals, Appalachians and in the Scandinavian Caledonides has been analysed. No deformations typical of stretching were found in the greater part of the basins on continental crust. High magnitude subsidence without stretching can be produced by thrust loading or by a large density increase in the crust or mantle. Paleotectonic conditions exclude thrust loading as a cause of formation of the major basins in the above fold belts. Most of them were formed in cool cratonic areas by rapid subsidence in a few m.y. Their initial depth was 1–3 km. Subsidence of this type cannot be produced by thermal relaxation without significant stretching. Gabbro-eclogite transformation in the lower crust is proposed as a possible mechanism. Intense shortening of continental crust is observed only where rapid subsidence of a large magnitude took place before the folding. Not one cratonic block was strongly compressed in the fold belts considered.