Did the Dalradian slides originate as extensional faults?

Rocks of the Dalradian Supergroup^1,2 of Scotland and Ireland were deposited in late Proterozoic-early Cambrian times on what became the southern margin of the Laurentian plate, and underwent metamorphism and deformation when this margin was affected by the Grampian orogeny in the late Cambrian^3,4. The resulting complex structure is still not fully understood, and a continuing problem is presented by the origin and significance of the `slides', a series of ductile faults which lie sub-parallel to bedding⁵. Although numerous slides have been recognized within the Dalradian terrain⁶ the term has seldom been used outside the British Caledonides. It is generally assumed that these slides are synmetamorphic and were formed during Grampian deformation. However, a consideration of their geometry, stratigraphic relationships and context now suggests that they may have been initiated as syndepositional extensional faults which were only modified, and not formed, during the Grampian orogeny.