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

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Abstract
Summary: 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 (D 1 –D 2 ) where relative movements appear to have been dominantly strike-slip and extensional (transtensional). In the later Laxfordian (D 3 ) 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 D 3 transpressional regime in Scotland.