LATERAL VARIATIONS OF CONDUCTIVITY STRUCTURE ACROSS SOUTHERN SCOTLANDAND NORTHERN ENGLAND

High-frequency (100-0.01 Hz) magnetotelluric (MT) soundings have been carried out along a 140 km profile across Southern Scotland and Northe rn England, where previously longer-period soundings had been undertak en. Examples of the results of the processing of data from 15 sites al ong the profile and of the application of a number of 1D inversion pro cedures are presented. The latter include the joint inversion of d.c. resistivity and MT data at Rookhope where borehole stratigraphy is als o available. Although the data were severely contaminated by near fiel d sources at some sites, and the station spacing was too large in part s of the traverse for good resolution of model parameters, by incorpor ation of results from the earlier studies it is possible to obtain a p reliminary 2D model of the electrical resistivity structure of the upp er crust in this region. Significant differences have been found in th e three main geological zones crossed by the survey-the Alston Block, the Northumberland Basin and the Southern Uplands of Scotland. In the discussion, special attention has been paid to four sites in the neigh bourhood of the Southern Uplands Fault, for which the data have been s ubjected to tenser decomposition. This has revealed two dominant regio nal azimuths, one of which dominates at the shorter periods and corres ponds to the strike of the Southern Uplands Fault and the other, at lo nger periods, which corresponds to the strike of the well-known Southe rn Uplands magnetovariational (MV) anomaly. Moreover, in this region, the electrical resistivity models indicate the presence of a highly co nducting zone beginning at a depth of about 4 km at Station CAP, which is located on the axis of the MV anomaly, and extending to a depth of at least 25 km. The depth to this conducting zone increases both to t he north and the south of the MV anomaly axis, resulting in a structur e which has many features in common with those along parallel traverse s in SE Scotland and in Ireland. In discussing the implications of the Irish model, another researcher proposed either metamorphosed sedimen ts and/or serpentinised island are crust as being possible sources of the high conductivity and other geophysical and geological data. The p robability that the arguments in that study are equally applicable to Southern Scotland is currently under consideration.