Abstract

President's anniversary address 1984

From the end of the Caledonian orogeny until the present, northern Scotland has acted as a structural high separating the North Sea and Atlantic basins or their forerunners. The geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins, with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre-existing basement structure.

From Devonian to early Palaeocene times, the massif was generally low-lying and its behaviour with respect to vertical movements was essentially neutral. The land surface carved in the crystalline basement during the Devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins. Tectonic events of major importance in the basins, such as the formation of the North Sea and Hebridean rift systems, produced little change of conditions in the intervening massif. It is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching.

The uneventful history of the massif was interrupted by two phases of mafic igneous activity—in late Carboniferous and Palaeocene times—which record responses to regional plate motions. The formation of the Palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub-Devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the North Sea. The style of these events and the anomalous character of the continental margin off NW Britain may have been determined by the refractory nature of the Precambrian basement which inhibited stretching prior to sea floor spreading.

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