New structural data indicate that eclogite-bearing gneisses in the Glenelg-Attadale inlier of the Caledonian Moine thrust nappe, northwest Scotland, were involved in a major pre-Caledonian system of extensional top-to-the-east ductile shear zones. These shear zones coalesce upward to constitute a major extensional detachment below metasedimentary rocks of the Moine Supergroup. Both the eclogite-bearing gneisses and the Moinian metasedimentary rocks display evidence of extensional deformation, but the latter contain structures indicative of a lower intensity of bulk finite shear strain and may have experienced only the later of two discrete phases of extension that we recognize in the inlier. Coplanar Caledonian brittle-ductile, top-to-the-west thrusting has resulted in only localized and limited reactivation. Timing of the extensional deformation is bracketed by a published Sm/Nd age of 1.08 Ga for eclogite equilibration and by a minimum age of deposition for the Moine Supergroup of 840 Ma. We propose that the eclogite-bearing lower crust was exhumed as a result of collapse of the Grenville Orogen, and that the evolution of the Moinian basin was controlled by this extensional event.

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