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The role of mid-crustal shear zones in the Early Proterozoic evolution of the Lewisian

Department of Geology, Imperial College, London, SW7 2BP, UK
Department of Geology, Keele University, Newcastle, Staffordshire, ST5 5BG, UK

The major shear zones in the Lewisian complex are either steep NW-SE zones, with a generally dextral strike-slip component, or were subhorizontal before subsequent deformation. They appear to share the same NW-SE movement direction. The Outer Hebrides lie mostly within a major mid-crustal ‘flat’ which on the mainland, at a higher structural level, is seen only north of Loch Laxford and south of Loch Torridon. Inclined NW-SE-striking shear zones at Diabaig, Carnmore and Loch Laxford are interpreted as ramps by which this zone descends below the central region.

Movements on these major zones record a long period of probably intermittent activity from c. 2600 Ma to c. 1400 Ma. Early movements (Inverian) were widespread on the mainland, and indicate a generally overthrust regime with a small dextral component. A major change in kinematic regime occurred during dyke emplacement and in the early Laxfordian (D₁–D₂) where relative movements appear to have been dominantly strike-slip and extensional (transtensional). In the later Laxfordian (D₃) major upright folds and steep dextral shear zones indicate a return to a dextral transpressional regime.

This sequence, together with evidence from Greenland, can be interpreted in the context of a reconstructed N. Atlantic, as the result of relative movements between two Archaean ‘plates’ to N and S of the combined Nagssugtoqidian-Lewisian belt.

The Inverian-Nag I structure indicates a dominantly N-S convergence. A change to a NW-SE convergence direction in the Early Laxfordian explains a dextral strike-slip regime in the Lewisian and convergence in W Greenland. A further change to NNW-SSE convergence in the later Laxfordian would explain the D₃ transpressional regime in Scotland.

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				References Geological Society, London, Memoirs, 2002; 26: 73 - 76. [PDF]

				D. B. Snyder, S. B. Lucas, and J. H. McBride Crustal and mantle reflectors from Palaeoproterozoic orogens and their relation to arc-continent collisions Geological Society, London, Special Publications, 1996; 112: 1 - 23. [Abstract] [PDF]

				I. C. Starmer Accretion, rifting, rotation and collision in the North Atlantic supercontinent, 1700-950 Ma Geological Society, London, Special Publications, 1996; 112: 219 - 248. [Abstract] [PDF]

				R. G. Park Palaeoproterozoic Laurentia-Baltica relationships: a view from the Lewisian Geological Society, London, Special Publications, 1995; 95: 211 - 224. [Abstract] [PDF]

				T. J. Wynn Deformation in the mid to lower continental crust: analogues from Proterozoic shear zones in NW Scotland Geological Society, London, Special Publications, 1995; 95: 225 - 241. [Abstract] [PDF]

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