ORIGIN OF THE BERWICK MONOCLINE - GEOMETRICAL AND GEOPHYSICAL CONSIDERATIONS

Analysis of minor fold vergence and thrust directions in Carboniferous strata at Sea House, north Northumberland, has revealed the synclinal member of a major monoclinal fold pair and thus established the easte rly vergence of the southern sector of the Berwick monoclinal disturba nce or 'Berwick Monocline', a discrete NNW-SSE structure on the wester n shore of the North Sea. The most intense deformation within the Berw ick structure is found in its central sector, where displacement on th e steep east-directed Boundary reverse fault is about 0.5 km; the foot wall is, however, almost undeformed, and extends offshore as the Marsh all Meadows flat zone. The Berwick disturbance as a whole is arcuate i n plan, the concave side facing east. The association of a weak gravit y anomaly low with higher amplitude aeromagnetic anomalies over the Ma rshall Meadows flat zone is explained by postulating a concealed anoma lous basement massif, the Marshall Meadows block, in which a low densi ty, possibly granitoid core is associated with a locally more strongly magnetized margin. The marked attenuation of Dinantian strata in the Marshall Meadows flat zone is consistent with lower density and conseq uent positive buoyancy of the underlying Marshall Meadows block as com pared with adjacent basement. A few tens of kilometres south of the Be rwick 'Monocline', between Holburn and Lemmington, the Carboniferous c over rocks are affected by a pair of monoclinal structures. Their comb ined geometrical relationship (location, trend, extent, vergence and a rcuate form) to the adjacent granite-cored Cheviot block is comparable with the relationship of the Berwick structure to the postulated Mars hall Meadows block. It is suggested that during regional east-west sho rtening the Berwick and Holburn/Lemmington structures formed in respon se to contrasts in the Caledonian basement between the stronger, less dense igneous rocks of the Cheviot and postulated Marshall Meadows blo cks, and the adjacent weaker, denser Silurian shale and greywacke host rocks.