Seismic refraction and electrical conductivity profiles across northern Scotland, together with geological and other geophysical data, are interpreted in terms of the deep structure of this part of the orthotectonic Caledonides. Our preferred interpretation is that the Moine thrust zone extends to the base of the crust and the Caledonian front is steep; this contrasts with recently proposed ‘ultra thin-skinned’ solutions. The results of the recently completed MOIST deep seismic reflection experiment should, inter alia, test our hypothesis.

In order to test the geometrical feasibility of our interpretation, balanced sections have been constructed to illustrate the crustal evolution of the region during the Caledonian orogeny. These sections permit estimates to be made of crustal shortening, thickening and erosion. Our model requires that the N Highlands were once overlain by major exotic nappes, which by analogy with NW Newfoundland may have been ophiolitic. The proposed regional structural pattern comprises a duplex of crustal dimensions. We suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins.

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