Plate tectonics and the evolution of the British Isles: Thirty-fifth William Smith Lecture

Abstract

This paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics, with illustrations particularly from the geological history of the British Isles. There is special emphasis on the mechanics of continental lithosphere in extension, when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts. An upper lithosphere stress-concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler, viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid. The very nature of plate tectonics as relative motion among a multi-plate mosaic of plates provides a rational semi-quantitative framework for large-scale geological complexity in space and time. The Caledonian and Hercynian history of the British Isles may be explained semi-quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions. The rationalism of plate tectonics is exhibited on a smaller, more direct field-scale by many of the complexities of ophiolite complexes, such as Ballantrae (Scotland), that can be explained as the effects of the evolution of ridge/ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the Southern Uplands. Deformational and sedimentary patterns in NW Europe from Triassic times to the present day resulted from a complex interaction of extensional events related to diachronous Atlantic opening and compressional events related to sequential Alpine shortening phases. Those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences, because huge areas, much larger than the British Isles, must be analysed and synthesized before much sense can be extracted. Further difficulties arise from the palinspastic and preservation problems of convergent boundaries; orogenic shortening telescopes and partially subducts facies belts. Most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened, elongated and straightened to an apparent but deceiving simplicity. Therefore, while plate tectonics provides a rational geological framework against which, for example, British basin evolution can be tested and explained, we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively.