MAGNETOTELLURIC STUDIES IN AND ADJACENT TO THE NORTHUMBERLAND BASIN, NORTHERN ENGLAND

During the past decade broadband magnetotelluric (MT) soundings, with d.c. resistivity soundings at some sites, have been undertaken in thre e separate field studies in and around the Northumberland Basin, a reg ion of great interest to earth scientists on account of the proposed l ocation there of the Iapetus Suture. As a result of an increase in cul tural noise during this period, the data from the last two studies hav e been processed using a new robust constrained impedance tenser estim ation program. The resulting apparent resistivity and phase data from these studies, together with those from the first broadband study and some earlier MT responses from the region, have now all been modelled using an interpretative modelling procedure. New information has been provided by the MT models on basement depths and, by integrating these new estimates with those from gravity modelling and seismic studies b oth on land and offshore, a detailed basement topography map has been compiled for the region. The deep electrical resistivity structure has been modelled along a NW-SE traverse from the Weardale Granite of the Alston Block across the Northumberland Basin to the Southern Uplands of Scotland. Underlying the more conductive sedimentary rocks, the bas ement rock is found to have resistivities which range from about 100 O mega m in the Northumberland Basin to more than 1000 Omega m in the Al ston Block and probably of the same order in the Southern Uplands. A m id-crustal conductor exists along the whole traverse, which is well re solved and has a southward dip beneath the Weardale Granite. Under the Northumberland Basin, the conductor is less well resolved and thus an apparent northward dip can only be regarded as tentative. Comparison of the pseudo-2D and full 2D models resulting from this study and from earlier MT and magnetovariational (MV) studies in Southern Scotland w ith new MT and joint MT and MV inversions of Livelybrooks et al. (Phys . Earth Planet, Inter., 81: 67-84 (1993)) for a subset of the same res ponse estimates reveals both comparable and disparate features. The fe atures common to the models presented in this paper and at least one o f the inversions of Livelybrooks et al. include differences in the res istivity and thickness of the upper crust in each of the Alston Block, the Northumberland Basin and the Southern Uplands, and a conducting z one at mid-crustal depths which vary along the traverse from about 10 km near the southern edge of the Northumberland Basin to more than 30 km under the Alston Block and about 30 km in the southern part of the Southern Uplands. The main difference between the pseudo-2D and the 2D forward models and the 2D inversions of Livelybrooks et al, is in the resistivity of the Alston Block, the former being more than 1000 Omeg a m whereas the latter is about 400-500 Omega m, a value which is unex pectedly low for a crust of known granitic composition.