Geotectonic imaging of the north-western European continent and shelf

Abstract

Gridding and merging of onshore gravity and topography information from the British National Gravity and Topography Data Bank with offshore free-air gravity anomalies derived from SEASAT altimeter data and bathymetry from the National Geophysical Data Center (USA) has allowed, for the first time, the production of digital images for the north-western European continent and shelf (48°–62° lat., 348°-010° long.). Further, using image processing techniques, the images have been selectively filtered (using artificial illumination) to enhance both Caledonian and Variscan trends. As a large amount of terrestrially-acquired offshore gravity data already exists within the North Sea region, it has been possible to test the accuracy of the SEASAT derived free-air gravity anomaly data. Hence its use in geological and geophysical interpretations, by comparing the two. We conclude that while the derived (SEASAT) gravity trends are essentially correct, significant error may exist in the amplitude determination, especially where anomaly wavelengths are short and oceanographic components in the altimeter data are significant. The potential value of ‘geotectonic images’ is as an aid in recognising trends and relationships between geological and/or tectonic domains that are not generally apparent in the more conventional representations of these data (e.g. profile and contour maps). We offer interpretations for both the local free-air/Bouguer gravity anomalies in terms of the known geology of the British Isles and offshore regions, and the regional gravity anomalies in terms of the thermal and mechanical response of the north-western European lithosphere during Variscan compression (continental collision and thrust-sheet migration) and Palaeozoic, Mesozoic, and Cenozoic rifting events (sedimentary basin formation).