Magnetotelluric studies in and adjacent to the Northumberland Basin, Northern England

Details Magnetotelluric studies in and adjacent to the Northumberland Basin, Northern England Magnetotelluric studies in and adjacent to the Northumberland Basin, Northern England

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993pepi...81...43p&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 81, Issue 1-4, p. 43-66.

Physics

4

Scientific paper

During the past decade broadband magnetotelluric (MT) soundings, with d.c. resistivity soundings at some sites, have been undertaken in three separate field studies in and around the Northumberland Basin, a region of great interest to earth scientists on account of the proposed location there of the Iapetus Suture. As a result of an increase in cultural noise during this period, the data from the last two studies have been processed using a new robust constrained impedance tensor estimation 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 both on land and offshore, a detailed basement topography map has been compiled for the region. The deep eletrical 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 basement rock is found to have resistivities which range from about 100 μ m in the Northumberland Basin to more than 1000 μ m in the Alston Block and probably of the same order in the Southern Uplands. A mid-crustal conductor exists along the whole traverse, which is well resolved 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 with 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 response estimates reveals both comparable and disparate features. The features common to the models presented in this paper and at least one of the inversions of Livelybrooks et al. include differences in the resistivity and thickness of the upper crust in each of the Alston Block, the Northumberland Basin and the Southern Uplands, and a conducting zone 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 μ m whereas the latter is about 400-500 μ m, a value which is unexpectedly low for a crust of known granitic composition.

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