Great Glen Fault

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The Great Glen Fault is a long strike-slip fault or ancient transform fault that runs through its namesake the Great Glen (Glen Albyn) in Scotland. However, the fault is actually much longer and over 400 million years old.

Aligned northeast to southwest, the Great Glen Fault extends further southwest in a straight line through Loch Linnhe and the Firth of Lorne, and then on into northwestern Ireland, directly through Lough Foyle, Donegal Bay and Clew Bay. To the northeast the fault connects to the Walls Boundary Fault and the associated Melby and Nesting Faults, before becoming obscured by the effects of Mesozoic rifting to the north of Shetland. The fault continues on the North American side of the North Atlantic Ocean, but is no longer part of a contiguous fault, as the complete fault was broken when the Mid-Atlantic Ridge formed 200 million years ago. The North American side of the fault runs through the length of northwestern Newfoundland, Canada, and into the Gulf of St. Lawrence.

The Great Glen Fault has a long movement history. It formed towards the end of the Caledonian orogeny (mountain building) associated with the collision between the Laurentia and Baltic tectonic plates at the end of the Silurian period continuing into the Early Devonian (likely age range - 430-390 Ma (million years)). The movement at this time was sinistral (left-lateral), the same as the closely related set of faults sub-parallel to the main part of the Great Glen Fault, these include the Strathconan and Strathglass Faults to the northwest and the Laggan, Tyndrum and Ericht-Laidon Faults to the southeast. The second main phase of movement was during the Carboniferous, this time with a dextral (right-lateral) sense. The exact timing is uncertain but associated folds within the Devonian are cut by members of the Late Carboniferous to Early Permian dyke swarm. The Great Glen Fault had its final phase of movement during the Late Cretaceous to Early Tertiary.