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.