Summary The response of lithosphere to an applied tectonic tensile force and the resulting stress distribution with depth has been investigated using a mathematical model incorporating the elastic, plastic and brittle behaviour of lithospheric material. Lithospheric strength is shown to be primarily controlled by lithospheric rheology and as a consequence is critically dependent on geothermal gradient and lithospheric composition. The rheologies of the upper crust, lower crust and mantle are assumed to be controlled by dislocation creep in quartz, plagioclase and olivine respectively. The critical level of tensional tectonic force required to generate geologically significant strains has been calculated as a function of surface heat flow, and the predicted lithosphere strength compared with available levels of tensile tectonic force arising from subduction plate boundaries and isostatically compensated plateau uplift loads. The model predicts significant extensional deformation in regions with surface heat flow >65 mWm −2 subjected to a tensile tectonic force of 3 × 10 12 N m −1 and is in good agreement with observed examples of intraplate extension. Lithosphere strength is critically controlled by the crustal thickness since the quartzofeldspathic rheology of the crust is weaker than the olivine rheology of the mantle. A decrease in crustal thickness thus increases the strength of the lithosphere. However, lithospheric extension also increases the geothermal gradient serving to weaken the lithosphere. The rate of extension is critical in determining which of these processes predominates. Fast strain rates (> 5 × 10 −15 sec −1 ) produce a weakening of the lithosphere (i.e. strain softening) while slower strain rates lead to strengthening of the lithosphere (strain hardening). Extensional style is consequently controlled by the lithospheric extension rate; fast extension producing, through strain softening, intense localized lithospheric extension with high (potentially infinite) β values, and slow extension, through strain hardening, giving broader regions of lithosphere extension with finite β values of the order of 1.5. For intermediate geothermal gradients ( q = 55−70 mWm −2 ) the model predicts a low stress-low strength region at the base of the crust due to the contrast between plagioclase and olivine rheology at the Moho. Other low-strength regions are predicted within the crust at major compositional (and rheological) boundaries. These low-strength zones are expected to control the location of detachment horizons by which crustal extension occurs particularly at slower strain rates. High geothermal gradients favour the shallower detachment horizons at the expense of the deeper horizons. The reverse is true for low geothermal gradients.

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Summary The main structural, lithological and geochemical features of the Lewisian complex are summarized in relation to available geochronology and an attempt is made to integrate this information into an evolutionary model for the Lewisian from the Archaean through the Proterozoic. The phase of extensive crustal generation which formed the early Lewisian (Scourian) complex occurred at about 2900 Ma and was marked by the production of vast volumes of tonalite which, together with supracrustal components, were affected by strong horizontal thrusting and deformation. The deeper parts of the Lewisian crust were subject to (Badcallian) granulite facies metamorphism culminating at c. 2700 Ma. Major NW-SE shear zones developed shortly after this and resulted in the segmentation of the Lewisian crust and juxtaposition of different crustal levels. This marked a period of quite extensive retrogression of the high-grade gneisses, which continued (with further local shearing) during and after the emplacement of the Scourie dyke suite at c. 2400 Ma. Extension of the Lewisian crust occurred in the southern (Gairloch) area around 2000 Ma and extensive outpourings of mafic volcanics, with associated exhalative mineralization, took place accompanied by sedimentation in the developing basin to form the Loch Maree Group. Igneous activity related to the South Harris igneous complex may have occurred at this period on the Outer Hebrides. Laxfordian deformation and metamorphism began before 1900 Ma and resulted in major reactivation of the earlier shear zones between and within the juxtaposed crustal blocks. More pervasive deformation accompanied the closure of the Loch Maree volcano-sedimentary basin, with major overthrusting and isoclinal folding. Culmination of Laxfordian metamorphic activity occurred at c. 1900 Ma with migmatization and granite injection at Laxford and on Harris, and locally in the southern area. Shear deformation, increasingly more brittle and localized, continued to affect the Lewisian complex until 1400 Ma and probably until 1000 Ma. Interestingly, the main shear zones established at the end of the Archaean crust-forming episode at 2600 Ma continued to be the focus of tectonic, metamorphic and magmatic activity over the following 1500 million years.

Summary A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition. For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m −2 or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m −2 . Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading. The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow q = 50–55 mW m −2 ) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling). Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.

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.