Hostname: page-component-7479d7b7d-q6k6v Total loading time: 0 Render date: 2024-07-12T04:39:05.632Z Has data issue: false hasContentIssue false
  • English
  • Français

Kinematic and thermal constraints on the reactivation of the Outer Hebrides Fault Zone, NW Scotland

Published online by Cambridge University Press:  22 July 2008

A. G. SZULC ,
G. I. ALSOP  and
G. J. H. OLIVER
A. G. SZULC
Affiliation:
Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, St Andrews, Fife, Scotland KY16 9AL, UK
G. I. ALSOP*
Affiliation:
Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, St Andrews, Fife, Scotland KY16 9AL, UK
G. J. H. OLIVER
Affiliation:
Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, St Andrews, Fife, Scotland KY16 9AL, UK
*
*Author for correspondence: gia@st-andrews.ac.uk
Get access Rights & Permissions [Opens in a new window]

Abstract

The Outer Hebrides Fault Zone is a major easterly dipping reactivated shear zone which displaces Lewisian gneiss of the Laurentian craton, NW Scotland. Despite a number of detailed field studies, the fault zone remains poorly understood with regard to both its age of inception and precise conditions of reactivation. The island of Scalpay in the northern portion of the Outer Hebrides Fault Zone provides exceptional exposures through a variety of reactivated fault rock types and therefore represents an ideal location to investigate fault zone evolution via fluid inclusion studies of syn-tectonic quartz veins. This fluid inclusion study constrains reactivation temperatures more precisely than hitherto possible with top-to-the-NW ductile thrusting occurring at 500 ± 30°C. Subsequent phyllonitization is associated with oblique sinistral top-to-the-NE strike-slip at 230 ± 20°C, followed by a discrete system of top-to-the-NE/SE extensional detachments at 150 ± 20°C. Other recent fluid inclusion studies in the southern portion of the Outer Hebrides Fault Zone constrain phyllonitization associated with top-to-the-E displacement to 370 ± 20°C, with subsequent top-to-the-NE extensional detachments operating at 150–210°C. Thus, late-stage extensional detachment systems record consistent conditions of reactivation along the strike length of the Outer Hebrides Fault Zone. However, our results also clearly emphasize that conditions of earlier fault zone reactivation and phyllonitization were highly heterogeneous between the northern and southern portions, thus suggesting a spatial and temporal variation in the deformation and/or fluid flux system.

Keywords

deformationfaultfluid inclusionsfluidsScotland
Type
Original Article
Information
Geological Magazine , Volume 145 , Issue 5 , September 2008 , pp. 623 - 636
Copyright
Copyright © Cambridge University Press 2008

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Alsop, G. I. & Holdsworth, R. E. 2004 a. The geometry and topology of natural sheath folds: a new tool for structural analysis. Journal of Structural Geology 26, 1561–89.CrossRefGoogle Scholar
Alsop, G. I. & Holdsworth, R. E. 2004 b. Shear zone folds: records of flow perturbation or structural inheritance? In Flow Processes in faults and shear zones (eds Alsop, G. I., Holdsworth, R. E., McCaffrey, K. J. W. & Hand, M.), pp. 177–99. Geological Society of London, Special Publication no. 224.Google Scholar
Borisenko, A. S. 1977. Study of the salt composition of solutions in gas–liquid inclusions in minerals by the cryometric method. Soviet Geology & Geophysics 18, 11–19.Google Scholar
Brewer, J. A., Matthews, D. H., Warner, M. R., Hall, J., Smythe, D. K. & Whittington, R. J. 1983. BIRPS deep seismic reflection studies of the British Caledonides. Nature 305, 206–10.CrossRefGoogle Scholar
Brown, P. E. 1989. FLINCOR: a microcomputer program for the reduction and investigation of fluid inclusion data. American Mineralogist 74, 1390–3.Google Scholar
Butler, C. A., Holdsworth, R. E. & Strachan, R. A. 1995. Evidence for Caledonian sinistral strike-slip motion and associated fault weakening, Outer Hebrides Fault Zone, NW Scotland. Journal of the Geological Society, London 152, 743–6.Google Scholar
Cheadle, M. J., McGeary, S., Warner, M. R. & Matthews, D. H. 1987. Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling. In Continental extensional tectonics (eds Coward, M. P., Dewey, J. F. & Hancock, P. L.), pp. 455–65. Geological Society of London, Special Publication no. 28.Google Scholar
Cliff, R. A. & Rex, D. C. 1989. Evidence for a ‘Grenville’ event in the Lewisian of the northern Hebrides. Journal of the Geological Society, London 146, 921–4.CrossRefGoogle Scholar
Cliff, R. A., Rex, D. C. & Guise, P. G. 1998. Geochronological studies of Proterozoic crustal evolution in the northern Outer Hebrides. Precambrian Research 91, 401–18.CrossRefGoogle Scholar
Clynne, M. A. & Potter, R. W. II. 1977. Freezing point depression of synthetic brines. Geological Society of America Abstracts with Programs 9, 930.Google Scholar
Coward, M. P. & Park, R. G. 1987. The role of mid-crustal shear zones in the Early Proterozoic evolution of the Lewisian. In Evolution of the Lewisian and comparable Precambrian High Grade Terrains (eds Park, R. G. & Tarney, J.), pp. 127–38. Geological Society of London, Special Publication no. 27.Google Scholar
Dearnley, R. 1962. An outline of the Lewisian complex of the Outer Hebrides in relation to that of the Scottish mainland. Quarterly Journal of the Geological Society of London 118, 143–76.Google Scholar
Fettes, D. J. & Mendum, J. R. 1987. The evolution of the Lewisian Complex in the Outer Hebrides. In Evolution of the Lewisian and comparable Precambrian High Grade Terrains (eds Park, R. G. & Tarney, J.), pp. 27–44. Geological Society of London, Special Publication no. 27.Google Scholar
Fettes, D. J., Mendum, J. R., Smith, D. I. & Watson, J. V. 1992. Geology of the Outer Hebrides. Memoir of the British Geological Survey. London: HMSO, 198 pp.Google Scholar
Francis, P. W. & Sibson, R. H. 1973. The Outer Hebrides Thrust. In The early Precambrian evolution of Scotland and related rocks of Greenland (eds Park, R. G. & Tarney, J.), pp. 95–104. University of Keele.Google Scholar
Friend, C. R. L. & Kinney, P. D. 2001. A reappraisal of the Lewisian Gneiss Complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic. Contributions to Mineralogy and Petrology 142, 198–218.Google Scholar
Ghent, E. D., Stout, M. Z. & Parrish, R. R. 1988. Determination of metamorphic pressure–temperature–time (P–T–t) paths. In Heat, metamorphism and tectonic (eds Nisbet, E. G. & Fowler, C. M. R.). Mineralogical Association of Canada, Short Course no. 14.Google Scholar
Graham, R. H. 1980. The role of shear belts in the structural evolution of the South Harris igneous complex. Journal of Structural Geology 2, 29–37.Google Scholar
Heaman, L. M. & Tarney, J. 1989. U–Pb baddelyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events. Nature 340, 705–8.CrossRefGoogle Scholar
Imber, J., Holdsworth, R. E., Butler, C. A. & Lloyd, G. E. 1997. Fault-zone weakening processes along the reactivated Outer Hebrides fault Zone, Scotland. Journal of the Geological Society, London 154, 105–9.CrossRefGoogle Scholar
Imber, J., Holdsworth, R. E., Butler, C. A. & Strachan, R. A. 2001. A reappraisal of the Sibson–Scholz fault zone model: The nature of the frictional to viscous (“brittle-ductile”) transition along a long-lived, crustal scale fault, Outer Hebrides, Scotland. Tectonics 20, 601–24.Google Scholar
Imber, J., Strachan, R. A., Holdsworth, R. E. & Butler, C. A. 2002. The initiation and early tectonic significance of the Outer Hebrides Fault Zone, Scotland. Geological Magazine 139, 609–19.CrossRefGoogle Scholar
Jefferies, S. P., Holdsworth, R. E., Wibberley, C. A. J., Shimamoto, T., Spiers, C. J., Niemeijer, A. R. & Lloyd, G. E. 2006. The nature and importance of phyllonite development in crustal-scale fault cores: an example from the Median Tectonic Line, Japan. Journal of Structural Geology 28, 220–35.CrossRefGoogle Scholar
Jehu, T. J. & Craig, R. M. 1923. Geology of the Outer Hebrides. Part I – Barra Isles. Transactions of the Royal Society of Edinburgh 53, 419–41.CrossRefGoogle Scholar
Kelley, S. P., Reddy, S. M. & Maddock, R. 1994. Laser-probe 40Ar/39Ar investigation of a pseudotachylyte and its host rock from the Outer Isles thrust, Scotland. Geology 22, 443–6.Google Scholar
Lailey, M., Stein, A. M. & Reston, T. J. 1989. The Outer Hebrides Fault: a major Proterozoic structure in NW Britain. Journal of the Geological Society, London 146, 253–29.CrossRefGoogle Scholar
Lambert, R., Myers, J. S. & Watson, J. V. 1970. An apparent age for a member of the Scourie dyke suite in Lewis, Outer Hebrides. Scottish Journal of Geology 6, 214–20.CrossRefGoogle Scholar
MacInnes, E. A., Alsop, G. I. & Oliver, G. J. H. 2000. Contrasting modes of reactivation in the Outer Hebrides Fault Zone, northern Barra, Scotland. Journal of the Geological Society, London 157, 1009–17.Google Scholar
Miyashiro, A. 1994. Metamorphic Petrology. London: University College London Press, 404 pp.Google Scholar
Osinski, G. R., Alsop, G. I. & Oliver, G. J. H. 2001. Extensional tectonics of the Outer Hebrides Fault Zone, South Uist, northwest Scotland. Geological Magazine 138, 325–44.Google Scholar
Park, R. G., Cliff, R. A., Fettes, D. J. & Stewart, A. D. 1994. Precambrian rocks in NW Scotland west of the Moine Thrust: the Lewisian Complex and the Torridonian. In A revised correlation of Precambrian rocks in the British Isles (eds Gibbons, W. & Harris, A. L.), pp. 175–83. Geological Society of London, Special Report no. 22.Google Scholar
Roberts, A. M. & Holdsworth, R. E. 1999. Linking onshore and offshore structures: Mesozoic extension in the Scottish Highlands. Journal of the Geological Society, London 156, 1061–4.Google Scholar
Roedder, E. 1981. Origin of fluid inclusions and changes that occur after trapping. In Short course in fluid inclusions. Application to petrology (eds Hollister, L. S. & Crawford, M. L.), pp. 101–37. Calgary: Mineralogical Association of Canada.Google Scholar
Roedder, E. 1984. Fluid Inclusions. Reviews in Mineralogy, vol. 12. Mineralogical Society of America, 644 pp.Google Scholar
Shepherd, T. J., Rankin, A. H. & Alderton, D. H. M. 1985. A practical guide to fluid inclusion studies. Glasgow: Blackie, 239 pp.Google Scholar
Sibson, R. H. 1977. Fault rocks and fault mechanisms. Journal of the Geological Society, London 133, 191–213.CrossRefGoogle Scholar
White, J. C. 1996. Transient discontinuities revisited: pseudotachylyte, plastic instability and the influence of low pore fluid pressure on deformation processes in the mid-crust. Journal of Structural Geology 18, 1471–86.Google Scholar
White, S. H. & Glasser, J. 1987. The Outer Hebrides Fault Zone: evidence for normal movements. In Evolution of the Lewisian and comparable Precambrian High Grade Terrains (eds Park, R. G. & Tarney, J.), pp. 175–84. Geological Society of London, Special Publication no. 27.Google Scholar
Whitehouse, M. 1990. Isotopic evolution of the southern outer Hebridean Lewisian gneiss complex – constraints on Late Archean source regions and the generation of transposed Pb–Pb palaeoisochrons. Chemical Geology 86, 1–20.Google Scholar
Yardley, B. W. D. 1989. An introduction to metamorphic petrology. Harlow: Longman Scientific & Technical, 248 pp.Google Scholar