Discussion

Abstract

The President (Sir Peter Kent) commented that a full scale case of the effect of varying thickness of an incompetent layer was thought to be provided by the Zagros orogenic belt. In the southeast, where abundant salt extrusions over a long period indicated a thick mobile layer at depth, folds were short and irregular. In the middle stretches (the ‘Fars Platform’) the characteristic feature was of very long relatively simple cylindrical folds. The latter were clearly related to abscherung, but the rarity of salt extrusions and their pattern was compatible with a halite layer of limited thickness, sufficient to facilitate simple folding without complex flowage. Dr. R. Cave remarked that the laboratory models of gravity tectonics described closely simulate the concept in Davies & Cave (1967) that much of the folding and related structure in the Lower Palaeozoic rocks of mid-Wales may be related to sliding under gravity. In a recent survey of part of mid-Wales by the Institute of Geological Sciences it was concluded that the pile of Lower Palaeozoic sediment was still wet and largely non-lithified when deformation commenced. This deformation was not confined to an early Devonian orogenic spasm and the concept relates it to mass movement of sediment down a westward declivity, from the shelf margin in east Wales. Structures in the extreme west, for instance near Aberystwyth in the Aberystwyth Grits and in underlying formations further north, are thought to have resulted from collapse of a pile of sediment which had accumulated on, or at