Central subsidence in the layered Hypersthene-gabbro of Centre II, Ardnamurchan, Argyllshire

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Abstract
Summary: The layering in the Hypersthene-gabbro of Centre II, Ardnamurchan has been mapped in as much detail as possible. The dips are confocal and steepen inwards, up to more than 50°, towards a central area occupied by later intrusions. The pattern of confocal dips is incomplete, partly because the eastern half of the gabbro has been cut out by Centre III intrusions and partly because the gabbro is not rhythmically layered throughout. There is convincing evidence that the layering was produced by bottom accumulation on successive floors which were more or less horizontal, so that the inward steepening must be due to later deformation. It is considered that the deformation concerned represents a downbending produced by movement on a ring fault, which is now obliterated. This process of cauldron subsidence dropped the central part of the gabbro to a lower level and permitted the emplacement of the next intrusion in the Centre II sequence, the granophyric quartz-dolerite. From a survey of the literature it would appear that there are other gabbros for which the attitude of the layering shows a pattern which we think may have been produced by cauldron subsidence.