LATERAL VARIATIONS OF CONDUCTIVITY STRUCTURE ACROSS SOUTHERN SCOTLAND AND NORTHERN ENGLAND

High-frequency (100-0.01 Hz) magnetotelluric (MT) soundings have been carried out along a 140 km profile across Southern Scotland and Northern England, where previously longer-period soundings had been undertaken. Examples of the results of the processing of data from 15 sites along the profile and of the application of a number of 1D inversion procedures are presented. The latter include the joint inversion of d.c. resistivity and MT data at Rookhope where borehole stratigraphy is also available. Although the data were severely contaminated by near field sources at some sites, and the station spacing was too large in parts of the traverse for good resolution of model parameters, by incorporation of results from the earlier studies it is possible to obtain a preliminary 2D model of the electrical resistivity structure of the upper crust in this region. Significant differences have been found in the 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 neighbourhood of the Southern Uplands Fault, for which the data have been subjected to tenser decomposition. This has revealed two dominant regional azimuths, one of which dominates at the shorter periods and corresponds to the strike of the Southern Uplands Fault and the other, at longer periods, which corresponds to the strike of the well-known Southern Uplands magnetovariational (MV) anomaly. Moreover, in this region, the electrical resistivity models indicate the presence of a highly conducting 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 the north and the south of the MV anomaly axis, resulting in a structure which has many features in common with those along parallel traverses in SE Scotland and in Ireland. In discussing the implications of the Irish model, another researcher proposed either metamorphosed sediments and/or serpentinised island are crust as being possible sources of the high conductivity and other geophysical and geological data. The probability that the arguments in that study are equally applicable to Southern Scotland is currently under consideration.