Postglacial fault movement and palaeoseismicity in western Scotland: a reappraisal of the Kinloch Hourn fault, Kintail

The Kinloch Hourn fault is the most prominent of a number of suspected postglacial faults in the western Scottish Highlands. These faults are interpreted to have been reactivated by repeated large (M > 6) palaeoseismic events following deglaciation 10,000–13,000 years ago. Based on inferred deflections of drainage courses, previous studies of the fault have estimated 160 ± 40 m cumulative left-lateral displacement along a 14 km long active segment during postglacial times. Reported soft-sediment deformation phenomena imply that activity on the Kinloch Hourn fault has persisted into the late Holocene, with the most recent movement having been associated with a magnitude 5.5–6.0 surface-faulting event between 3500 and 2400 years ago. The marked contrast between such palaeoseismic activity and the present-day seismic quiescence of western Scotland has stimulated this critical reappraisal of the Kinloch Hourn fault. This paper reassesses the key lines of evidence for postglacial fault activity and palaeoseismicty on the Kinloch Hourn fault, combining the analysis of 1:15,000-scale air photos, field-based geomorphic mapping and palaeoenvironmental investigations. Our reappraisal of inferred drainage deflections across the fault contends that previous reports of significant (10^2 m) left-lateral slip on the fault during the Holocene are spurious. Instead, incidences of Holocene channel abandonment along the fault line are non-synchronous and probably reflect non-tectonic drainage changes. The timing of soft-sediment deformation in the vicinity of the fault is revised to an early Holocene date (8990–8580 calendar years BP), which is in accord with both the palaeoenvironmental history of the site and consistent with published ages of earthquake-induced liquefaction phenomena documented elsewhere in western Scotland. An alleged recent (post-2400 radiocarbon years BP) ground rupture on the fault is questioned in the light of uncertainty about both the nature of the faulted soil deposit and the late Holocene age attributed to it. The study concludes that there is no convincing evidence for postglacial surface rupture on the Kinloch Hourn fault and speculates that the case for significant (10^1–10^2 m) postglacial movement on other faults in western Scotland may be similarly `unproven'.