Geophysical investigations in the English Lake District

Abstract

Lake District which define the principal anomalies in considerably more detail than previous widely-spaced observations. The physical properties of the Shap and Skiddaw granites have been analysed from geophysical logs recorded in 300 m deep heat-flow boreholes. New density determinations have been made on outcrop samples from over 350 localities in the western and central Lake District. Samples have been classified in terms of their lithology and lithostratigraphy, and representative in-situ densities have been calculated for the principal formations. The gravity and aeromagnetic data have been interpreted, using a combination of modelling and image processing, in order to study the form and evolution of the Lake District granite batholith and structures within the Skiddaw and Borrowdale Volcanic groups. The modelling studies indicate that Lake District batholith may comprise up to nine separate deep-seated components, and there may be a further five high level intrusions. The Eskdale/Wasdale Granite forms a major component in the western Lake District, and the Shap and Skiddaw granites form separate, steep-sided intrusions on the south-eastern and north-eastern margins of the batholith respectively. Prominent residual gravity anomalies, which coincide approximately with the Scafell, Haweswater and Ulpha synclines of the Borrowdale Volcanic Group, are also tentatively interpreted in terms of separate batholith components, but alternative interpretations in terms of thickened BVG sequences are possible. Further separate components are postulated along the northern side of the batholith and beneath the Haweswater Complex. The Ennerdale Granophyre and Threlkeld Microgranite are modelled as high-level intrusions, in line with previous interpretations, and it is posssible that the Eskdale Granodiorite is also in this category. There is good evidence for a high-level granitic intrusion beneath the Crummock aureole and some evidence for a similar intrusion near Coniston. On a broader scale, the modelling indicates that long wavelength magnetic anomalies are best interpreted in terms of a 'magnetic basement' which represent either a thick layer of pre-Skiddaw Group (magnetic) sedimentary rocks, magnetic crystalline basement, or a combination of both. This 'basement' reaches nearest to the surface in the southern Lake District, deepens northwards beneath the batholith, and approaches nearer to the surface again along the northern margin. The image processing of the potential field data has revealed three important ENE-trending geophysical lineaments across the Lake District (the Crummock, Ullswater and Southern Borrowdales lineaments). Several prominent, but less extensive, NE-trending lineaments are also visible across the central and western parts of the area. The ENE-trending set appear to divide distinctive tracts within the Skiddaw Group and it seems likely that at least some of the lineaments represent fundamental fractures within the underlying basement which were initiated prior to the Borrowdale volcanism and which influenced the subsequent structural development of the Borrowdale Volcanic Group and the intrusive form of the batholith. It is possible that vertical movement influenced by the pre-existing NE- and ENE-trending lineaments may have initiated the Scafell, Haweswater and Ulpha synclines, and associated anticlines, in the Ordovician, leading to a thicker accumulation of BVG in the synclines and/or the subsequent emplacement of late Ordovician (or early Silurian?) components of the batholith beneath them. Alternatively, it is possible that each was initiated as a volcano-tectonic sag over a separate component of an evolving Ordovician batholith, the position of the batholith components themselves being influenced by earlier structural trends. The geothermal characteristics of Caledonian-age granites in the Lake District and Eastern Highlands of Scotland have been studied (in collaboration with other workers). The study has led to a re-examination of the relationship between heat flow (q0) and heat production (A0) for granites and basement rocks in the UK. The data form four separate clusters on the q0-A0 plot; three corresponding to granite batholiths in SW England, northern England and the Eastern Highlands of Scotland, and a fourth to the basement rocks of central England and Wales. A single linear correlation between q0 and A0 is no longer tenable, and an explanation of the data is proposed in terms of the crustal structure and thermo-tectonic setting of each area. In the case of the granite batholiths the data reflect the contrasting depth extent and radioelement - depth functions of the intrusions. These parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced.