GRENVILLIAN EXTENSIONAL TECTONICS IN NORTHWEST SCOTLAND

New structural data indicate that eclogite-bearing gneisses in the Gle nelg-Attadale inlier of the Caledonian Moine thrust nappe, northwest S cotland, were involved in a major pre-Caledonian system of extensional top-to-the-east ductile shear zones. These shear zones coalesce upwar d to constitute a major extensional detachment below metasedimentary r ocks of the Moine Supergroup. Both the eclogite-bearing gneisses and t he Moinian metasedimentary rocks display evidence of extensional defor mation, but the latter contain structures indicative of a lower intens ity of bulk finite shear strain and may have experienced only the late r of two discrete phases of extension that we recognize in the inlier. Coplanar Caledonian brittle-ductile, top-to-the-west thrusting has re sulted in only localized and limited reactivation, Timing of the exten sional deformation is bracketed by a published Sm/Nd age of 1.08 Ga fo r eclogite equilibration and by a minimum age of deposition for the Mo ine Supergroup of 840 Ma, We propose that the eclogite-bearing lower c rust was exhumed as a result of collapse of the Grenville Orogen, and that the evolution of the Moinian basin was controlled by this extensi onal event.