The initiation and early tectonic significance of the Outer Hebrides Fault Zone, Scotland

Abstract

The Outer Hebrides Fault Zone is a major ESE-dipping detachment exposed within basement gneisses of the Archaean–Palaeoproterozoic Lewisian Complex, northwest Scotland. The fault zone exhibits a long-lived displacement history and was active during Proterozoic, end-Silurian, Carboniferous and Mesozoic times. The earliest deformation event preserved onshore was associated with top-to-the-NW ductile thrusting. A previous study proposed that thrust-related protomylonitic and mylonitic fabrics are cross-cut by amphibolites (‘Younger Basics’) and Laxfordian granite and pegmatite sheets. This evidence was used to suggest that ductile thrusting occurred during the Palaeoproterozoic Inverian event at c. 2500 Ma. Our observations demonstrate, however, that mylonitic fabrics within the ductile thrust zone are superimposed on all components of the gneiss complex including amphibolites and Laxfordian intrusions. It therefore follows that the Outer Hebrides Fault Zone cannot be older than c. 1685 Ma, the age of recently dated Laxfordian granites in the Outer Hebrides. Geochronological studies have shown that the basement blocks of the northern Outer Hebrides and Scottish mainland have different geological histories and were amalgamated during Proterozoic times at or after c. 1700 Ma. We propose that early ductile thrusting along the Outer Hebrides Fault Zone formed part of this amalgamation process leading to burial and reheating of the footwall gneisses in Lewis and north Harris. This would account for the c. 1100 Ma thermal event recorded by previous workers and implies that ductile thrusting along the Outer Hebrides Fault Zone is of Grenvillian age.