Geophysical investigations of the deep geology of the East Midlands

Late Precambrian meta-sediments and Caledonian granitic intrusions are exposed in Leicestershire. Similar rocks are found at shallow depth in boreholes from Leicester to East Anglia. Two seismic refraction experiments, each comprising two 10-30 km. profiles, have been carried out over potential field anomalies in the East Midlands. The Melton Mowbray experiment, located to the east of the Charnian Inlier, defined the top surface of the Melton Mowbray granitic intrusion at less than 0.5 km. depth, and the southern margin of the Carboniferous Widmerpool Gulf. The Melton intrusion was found to have a similar p-wave velocity (c. 5.7 km.s-1 ) to the surrounding Precambrian basement. The refraction interpretations, together with some seismic reflection data, are used to constrain 3-dimensional modelling of the aeromagnetic anomalies in this area. The Peterborough experiment was located over a negative Bouguer gravity anomaly to the north of Peterborough. Refractor velocities typical of late Precambrian basement were detected at shallow depth (0.5 km.) across the gravity anomaly. The gravity data is modelled as a intra-basement intrusion. Measurements have been made of physical properties of samples of basement rocks from outcrop and from boreholes within the area. These are used in the interpretation of the potential field data. A pre-Carboniferous palaeogeological map has been compiled from borehole and geophysical data. A compilation of nonconfidential borehole data is included as an appendix. The potential field and seismic studies show that two groups of granitic intrusions can be recognised, one group is more basic than the other.