An investigation of the crustal structure of north-west England using quarry blasts as seismic sources

Abstract

SUMMARY Seismic refraction profiles have been established between quarries at Outscar and Oddendale (on the north-eastern margin of the Lake District), Holmepark (between Lancaster and Kendal), and Arcow (Ribblesdale). The profile between Outscar and Holmepark traverses the Shap granite intrusion and outcrops of Silurian meta-sedimentary rocks. The eastern line crosses the Ingleton Coal Measures basin and continues onto the Askrigg Block, where the basement is formed of Lower Ordovician (Ingletonian) metamorphic rocks. The P wave velocity, S wave velocity, and Poisson’s ratio of the Carboniferous Limestone were estimated to be 5.250 km s −1 , 2.854 km s −1 , and 0.290 respectively. The corrresponding values for Silurian rocks were 5.635 km s −1 , 3.144 km s −1 ,0.274, and for Lower Ordovician metamorphic rocks on the Askrigg block were 5.984 km s −1 , 3.326 km s −1 , and 0.276. In addition, at the longer ranges, arrivals with a P velocity of 6.25 ± 0.15 km s −1 were detected on both the profiles. Seismic models derived by ray tracing are compatible with the view that the pre-Silurian basement between the Lake District and the Askrigg block is of Lower Ordovician age, equivalent to the Ingletonian rocks on the block, and that it extends westwards to the point at which it is intruded by the granite batholith that underlies the Lake District.