Discussion on structures found in thrust belts

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Abstract
Drs D. Elliott and M. R. W. Johnson write as follows—In companion papers on the structures found in thrust belts, Price (1977) and Blay et al. 1977 discuss gravity gliding and the development of folds by buckling illustrated by model studies. We wish to point out some alternative interpretations of thrusting, and of the folding encountered in thrust sheets. Price ( op. cit. ) and Blay et al. ( op. cit. ) adopt the classical gravity gliding interpretation in which the basal décollement dips towards the foreland. However, in the Jura it has been known for about fifty years that the basal décollement dipped towards the hinterland during deformation and this is also true for the central and southern Appalachians, the Canadian Rockies and the Idaho-Wyoming thrust belt. The classical gravity theory also requires the décollement eventually to cut up section at the back leaving a region of tectonic denudation rather like the scar produced by a gigantic landslide. Yet no case can be made for the existence of this type of tectonic denudation in the thrust belts mentioned above. We would like to draw attention to some simple model experiments performed by Bucher (1956) which were the starting point for what we believe is the more realistic theory of gravity spreading. In this theory the décollement dips towards the hinterland, there is no tectonic denudation and the rock mass moves in the direction of the surface slope (Price & Mountjoy 1970, Elliott 1976 a , b ). Blay et al . ( op. cit. ) attribute the major

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