Map of the Great Glen Fault and other late Caledonian strike-slip faults in Scotland and northwestern
Ireland
The Great Glen Fault is a
strike-slip fault that runs through the
Great Glen in
Scotland. Occasional moderate tremors have been recorded over the past 150 years.
Location
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 Swilly,
Donegal Bay and
Clew Bay as the
Leannan Fault. To the northeast the fault connects to the Walls Boundary Fault and the associated
Melby Fault and
Nestings Fault, before becoming obscured by the effects of Mesozoic rifting to the north of
Shetland.[1]
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, as the
Cabot Fault (Long Range Fault) and on into the
Gulf of St. Lawrence.[2] It is at least 300 miles (480 kilometres) long.
The Great Glen Fault has a long movement history. It formed towards the end of the
Caledonian orogeny associated with the collision between the
Laurentia and
Baltic tectonic plates at the end of the
Silurian continuing into the Early
Devonian (likely age range 430–390 Ma (million years)). The movement at that 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, which include the Strathconon Fault and Strathglass Faults to the northwest and the Laggan Fault, Tyndrum Fault, and Ericht-Laidon Fault to the southeast.[1] The second main phase of movement was during the
Carboniferous, this time with a
dextral sense.
The exact timing is uncertain, but associated folds within the Devonian are cut by members of the Late Carboniferous to
Early Permiandyke swarm. The Great Glen Fault had its final phase of movement during the Late
Cretaceous to Early
Tertiary. The displacement is estimated to be 64 miles (104 km).[3]
There is not agreement about whether the Great Glen Fault is "active" — accumulating
seismic slip.[4] Some parts of the fault are moving in opposite directions, but the extent of displacement is not agreed on. The displacement could be at least the full length of the exposed fault on mainland Scotland.[5]
Most researchers consider the fault active or a "reactivated strike-slip fault within the continental crust" that is accumulating tectonic strain.[4][6][7] Some researchers say the observed seismic activity of the last 300 years does not support a theory of re-activation. According to
Roger Musson the fault does not show any signs of present activity. Musson places the
1901 Inverness earthquake on a
secondary fault of the Great Glen Fault.[4][8][9]
Occasional moderate tremors have been recorded over the past 150 years which has meant that
seismic buffers are built into the
Kessock Bridge carrying the
A9 road out of
Inverness. In the 19th century, a boat canal known as the
Caledonian Canal was dug through the Great Glen; the canal is still used today.
^Stewart, M.; Strachan, R. A. (1999). "Structure and early kinemtic history of the Great Glen Fault Zone, Scotland". Tectonics. 18 (2): 326–42.
Bibcode:
1999Tecto..18..326S.
doi:
10.1029/1998TC900033.
Rogers, D. A.; Marshall, J. E. A.; Astin, T. R. (1989). "Devonian and later movements on the Great Glen fault system, Scotland". Journal of the Geological Society. 146 (3): 369–372.
doi:
10.1144/gsjgs.146.3.0369.
S2CID128498128.
Stewart, M.; Strachan, R. A.; Martin, M. W.; Holdsworth, R. E. (2001). "Constraints on early sinistral displacements along the Great Glen Fault Zone, Scotland: structural setting, U-Pb geochronology and emplacement of the syn-tectonic Clunes tonalite". Journal of the Geological Society. 158 (5): 821–830.
Bibcode:
2001JGSoc.158..821S.
doi:
10.1144/jgs.158.5.821.
S2CID129801878.
Stephenson, D.; Gould, D. (1995). British Regional Geology. The Grampian Highlands (4 ed.). Her Majesty's Stationery Office, London: British Geological Survey.
Trewin, N. H., ed. (2002). The Geology of Scotland. The Geological Society, London.