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

M. P. Coward and R. G. Park
Geological Society, London, Special Publications, 27, 127-138, 1 January 1987, https://doi.org/10.1144/GSL.SP.1987.027.01.11

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 (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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