How great is the Great Glen Fault?

A popular conceptual tectonic model envisages the Great Glen Fault to be part of a sinistral strike-slip system active during the mid-Silurian to early Devonian with c. 700 km of displacement. Here we use sedimentological, geochemical and detrital zircon age data to show that restoring 250–300 km of displacement suffices to fulfil key geological constraints and reveal three new pre-strike-slip relationships: (1) Paleoproterozoic Makkovik–Ketilidian crust becomes placed proximal to numerous immature sandstone units in the Grampian and Northern Highlands of Scotland and County Mayo, Ireland, that are marked by single-mode peaks of 1.8–1.6 Ga detrital zircons sourced from that crust; (2) the two most concentrated occurrences of appinite and metadolerite/gabbro in the Scottish–Irish Caledonides become matched; (3) the Donegal and Argyll granite suites can be paired. That amount of displacement provides, at least in part, the separation required between the Northern and Grampian Highlands to account for Scandian-age (Silurian) deformation in the former and its absence in the latter.