BASEMENT CONTROL ON THE LOCATION OF STRIKE-SLIP SHEAR IN THE SOUTHERNUPLANDS OF SCOTLAND

In the SW part of the Southern Uplands terrane the Moniaive Shear Zone has been superimposed above a geophysically defined basement disconti nuity. Deformation was a protracted event, commencing in the early Wen lock and ending with granite intrusion at about 392 Ma. At surface the shear zone separates contrasting suites of granitic plutons. These ha ve age differences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal disconti nuity deduced from geophysical modelling. It is proposed that sinistra l 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 d eveloping Southern Uplands terrane by the Wenlock.