Origin of the ‘Berwick Monocline’: geometrical and geophysical considerations

Abstract

Synopsis 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 easterly vergence of the southern sector of the Berwick monoclinal disturbance or ‘Berwick Monocline’, a discrete NNW-SSE structure on the western shore of the North Sea. The most intense deformation within the Berwick structure is found in its central sector, where displacement on the steep east-directed Boundary reverse fault is about 0.5 km; the footwall is, however, almost undeformed, and extends offshore as the Marshall Meadows flat zone. The Berwick disturbance as a whole is arcuate in plan, the concave side facing east. The association of a weak gravity anomaly low with higher amplitude aeromagnetic anomalies over the Marshall Meadows flat zone is explained by postulating a concealed anomalous basement massif, the Marshall Meadows block, in which a low density, 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 consequent positive buoyancy of the underlying Marshall Meadows block as compared with adjacent basement. A few tens of kilometres south of the Berwick ‘Monocline’, between Holburn and Lemmington, the Carboniferous cover rocks are affected by a pair of monoclinal structures. Their combined geometrical relationship (location, trend, extent, vergence and arcuate form) t o the adjacent granite-cored Cheviot block is comparable with the relationship of the Berwick structure to the postulated Marshall Meadows block. It is suggested that during regional east-west shortening the Berwick and Holburn/Lemmington structures formed in response to contrasts in the Caledonian basement between the stronger, less dense igneous rocks of the Cheviot and postulated Marshall Meadows blocks, and the adjacent weaker, denser Silurian shale and greywacke host rocks.