A new gravity survey of the Lake District and three-dimensional model of the granite batholith

Abstract

A new regional gravity survey of the Lake District has defined the principal anomalies in considerably more detail than previous surveys and has formed the basis for a three-dimensional interpretation of the sub-surface shape of the Lower Palaeozoic 'granite' batholith. The gravitational effects of the surrounding low density Permo-Triassic, Carboniferous and Silurian sedimentary successions have been calculated and removed from the observed field. The remaining residual gravity field due to the batholith has been interpreted on a 1 km square grid in terms of a generalized two-density model extending to a depth of 9 km. The interpretation shows that the Skiddaw Granite is a steep-sided, relatively flat-topped intrusion. The roof zone encompasses the Grainsgill, Caldew and Sinen Gill outcrops and corresponds well with the mapped thermal aureole. The southern part of the roof region of the Shap Granite coincides with the metamorphic aureole but a broad shoulder extends at about 1 km depth to the northwest beneath the Haweswater Igneous Complex. The Eskdale Granite is linked at shallow depth beneath the Borrowdale Volcanic Group to the Wasdale Granite. The central part of the Lake District is underlain by granite at relatively shallow depth (less than 2 km) which appears to be a subsurface extension of the outcrops at Eskdale and Wasdale.