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Geological Society, London, Special Publications; 1987; v. 27; p. 139-151;
DOI: 10.1144/GSL.SP.1987.027.01.12
© 1987 Geological Society of London

Early Proterozoic structure and kinematic evolution of the southern mainland Lewisian

R. G. Park, A. Crane & M. Niamatullah

Department of Geology, University of Keele, Keele, Staffs ST5 5BG, UK

The southern mainland region of the Lewisian is a belt of intense Early Proterozoic deformation with a minimum width of 55 km, in which all the main structures trend NW-SE. Archaean structures are dominant in the central block to the northeast, and in a small enclave (the Ruadh Mheallan block) between Gairloch and Loch Torridon. Major structures within the belt include upright NW-SE folds (the Carnmore antiform, the Letterewe synform, and the Tollie and Torridon antiforms) and the Gairloch and Diabaig shear zones. The structural history is divided into six phases: (1) Inverian (2600–2400 Ma)—the whole belt represents a major steep Inverian shear zone. Structures in areas little affected by subsequent deformation indicate a N-up overthrust movement with a small dextral component (dextral transpressional), (2) Emplacement of dykes and Loch Maree Group (2400–1900 Ma)—evidence of dextral shear during dyke emplacement indicates a change to a dextral transtensional regime, (3) Laxfordian D1 and, (4) D2 deformation (1900–1800 Ma)—oblique dip-slip shears and asymmetric shear folds suggest a continuation of the previous dextral transtensional movements on inclined lateral ramps, connected with major gently-inclined shear zone ‘flats’, (5) Laxfordian D3 deformation (c. 1600 Ma)—upright folding of previously sub-horizontal fabrics and a reversal of movement to N-up on the Gairloch shear zone indicate a change to a dextral transpressional regime, which is accompanied by retrogressive metamorphism, (6) Laxfordian D4 deformation (c. 1400 or c. 1000 Ma)—steeply plunging large and small-scale asymmetric folds and crush belts indicate a change to a sinistral strike-slip regime. We suggest that these phases of movement represent major changes in relative plate movement direction probably separated by relatively long periods of comparative stability.





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References
Geological Society, London, Memoirs, 2002; 26: 73 - 76.
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L. E. Beacom, R. E. Holdsworth, K. J. W. McCaffrey, and T. B. Anderson
A quantitative study of the influence of pre-existing compositional and fabric heterogeneities upon fracture-zone development during basement reactivation
Geological Society, London, Special Publications, 2001; 186: 195 - 211.
[Abstract] [PDF]