The Outer Hebrides fault: a major Proterozoic structure in NW Britain

Mylonites in the Outer Hebrides fault zone on Scalpay, Harris, are cut by metabasite sheets which are probably of Scourie dyke age (2400-2200Ma) and by pegmatites of late Laxfordian age (c.1750Ma). The mylonites contain relicts of a gneissose banding which probably formed in the Archaean c.2700Ma ago. These age constraints suggest that the fault formed in the Inverian event c.2500Ma ago, not in the Caledonian (Silurian-Devonian) as widely reported. The fault is a long-lived structure which was reactivated as an extensional fault in the Laxfordian, late Proterozoic (Torridonian) and the Carboniferous/Early Mesozoic, and as a thrust in the Caledonian. In a regional context this interpretation has important implications for the structural evolution of the Lewisian complex, for the nature of the Caledonian event in the Laurentian craton (Caledonian foreland) and for the age and geometry of the sedimentary basins in the hanging wall of the Outer Hebrides fault. -Authors