DIRECT FIELD EVIDENCE FOR SINISTRAL DISPLACEMENTS ALONG THE GREAT GLEN FAULT ZONE - LATE CALEDONIAN REACTIVATION OF A REGIONAL BASEMENT STRUCTURE

The Great Glen Fault Zone is a major, subvertical, reactivated fault w ithin the Scottish Caledonides. Post-Caledonian dextral movements of a few tens of kilometres have been demonstrated previously from the dis placement of geological markers. Late Caledonian sinistral displacemen t has been generally inferred, but only by comparison with apparently related structures in the Central and Northern Highlands. Critical out crops of low-middle greenschist-facies phyllonites, mylonites and cata clasites, particularly at Torcastle near Fort William, provide new evi dence for the nature and sense of early displacements along the Great Glen Fault Zone. Shallow to moderately plunging mineral lineations and shear-sense indicators suggest sinistral displacements that are thoug ht to be late Caledonian (c. 425-390 Ma) based on regional considerati ons. Although the Great Glen Fault Zone may at present separate contra sting basement blocks, there is no evidence that the Caledonian displa cements were associated with the reactivation of an older Proterozoic structure.