The Great Glen Fault

Abstract

Summary The powerful dislocation which intersects Scotland along the line of the Great Glen has, in the past, been regarded by most geologists as a normal or dip-slip fault with a predominant vertical downthrow to the south-east. A reconsideration of the entire problem now suggests that this view is no longer tenable and that the dislocation is, in reality, a lateral-slip or wrench fault with a horizontal displacement of approximately 65 miles. Such an interpretation is supported by several independent lines of evidence, as follows :— (1) The dislocation possesses physical characters unlike those of most normal faults but similar to the great strike-slip shears of the California Coast Range. (2) It belongs to the same system as the Strathconon, Ericht-Laidon and Loch Tay faults, all of which have proved lateral displacements of up to 5 miles. (3) It displaces the great belt of regional injection which affects the Moine Schists of the northern and Grampian Highlands, the nature and amount of the displacement being consistent with lateral shift but not with vertical downthrow. (4) It similarly displaces the metamorphic zones of the Highlands in an equally significant manner. (5) It truncates the Strontian Granite, the southern portion of which, according to the detailed structural evidence, is missing. The missing portion, moreover, can be identified in the Foyers mass which outcrops on the other side of the fault-line some 65 miles to the north-east and is similarly truncated by the fault. These two major Caledonian intrusions consist of identical rock types and are structurally homologous. (6) Finally, the occurrence of Lewisian and Torridonian rocks in Islay and Colonsay and the presence of the Moine Thrust-plane in the former island are more readily explained on the assumption of a lateral rather than a vertical displacement along the fault. Although the dislocation is still active, the available evidence indicates that the main lateral movement was accomplished prior to the deposition of the Upper Carboniferous sediments of Lochaline and subsequent to the intrusion of the Strontian and Foyers (Lower Old Red Sandstone) granites. Middle Old Red Sandstone strata along the Great Glen have, moreover, suffered intense crushing and deformation during the faulting, which must, therefore, be referred partly if not wholly to a post-Middle Old Red Sandstone epoch. The sinistral nature of the displacement, i.e. towards the south-west on the northwest side of the fracture and towards the north-east on its south-east side, implies the operation of a stress system involving regional compression acting in a general north-and-south direction accompanied by an east-and-west relief of pressure. This is regarded as evidence of the fact that the Hercynian forces, to which the formation of the Great Glen Fault is ascribed, were already in operation during Upper Old Red Sandstone or Lower Carboniferous times.