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A seismic reflection survey of the Clyde Plateau Lavas in North Ayrshire and Renfrewshire

Synopsis Studies of surface geology and of the results of recent gravity and magnetic surveys have yielded limited information about the development of the Clyde Plateau Lavas south-west of Glasgow. Seismic reflection velocity surveys linked by continuous reflection profiles were carried out to determine the depth of the base of the lavas, the recognition of which is based on the probability of a velocity reversal below it and on correlation with borehole and surface geological evidence. Operational methods, results and interpretation of the surveys are described. The survey was designed to discriminate between two models of the lava pile: post-extrusion folding of a fairly uniform pile and an original maximal development of the lavas along their NW.–SE. trending outcrop. The results of the survey support the latter model, with compaction dips of later sediments banked against the pile and the whole modestly accentuated by later folding. More rapid, but smaller-amplitude variations of thickness occur across NE.–SW. trending faults with long histories of repeated movement. The survey provides further evidence of the gradual diminution of activity of NE.–SW. faults, allied to the emergence of increasingly significant NW.–SE. trending, extensional structures. The inferred change of stress during Carboniferous times is thought to be related to the initiation of Atlantic rifting.