A major ductile extensional structure throwing down several kilometres to the ESE probably marks the southern portion of the NW Caledonian orogenic margin in Scotland. This structure, the Sound of Iona Fault, is probably related to the intensely mylonitized zones that cut the Lewisian and adjacent Iona Group (?Torridonian) metasedimentary cover on Iona. The fault-related mylonites predated the 414±3 Ma Ross of Mull Granite and are inferred to have postdated the c.425 Ma Moine Thrust. It is concluded that the Sound of Iona Fault is one of the many late Caledonian faults in Scotland, but is exposed at a deeper structural level than normal. Its position at the orogenic margin of the Caledonides is similar to that of the Loch Gruinart Fault on Islay, hinting that (a) major steep fault(s) may be a significant feature of the largely concealed Caledonian margin south of Skye
- Received June 16, 1995.
- Revision received July 24, 1995.
- © Geological Society 1995