In the SW part of the Southern Uplands terrane the Moniaive Shear Zone has been superimposed above a geophysically defined basement discontinuity. Deformation was a protracted event, commencing in the early Wenlock and ending with granite intrusion at about 392 Ma. At surface the shear zone separates contrasting suites of granitic plutons. These have age diVerences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal discontinuity deduced from geophysical modelling. It is proposed that sinistral reactivation of the basement structure caused the development of the shear zone within the overlying allochthonous thrust belt. This would require the basement structure to have been established beneath the developing Southern Uplands terrane by the Wenlock.