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Department. of Geology, Imperial College, Prince Consort Road, London SW7 2BP, UK
GeoScience Ltd, Falmouth Business Park, Bickland Water Road, Falmouth, Cornwall TR11 4SZ, UK
A suite of 1.8–1.7 Ga (Laxfordian), E-W striking, amphibolite-facies shears in the Lewisian of NW Scotland have been mapped across a pre-existing rheological boundary in the Laxford-Scourie area. The rheological contrast is provided by the southern boundary of the NW striking Laxford shear zone (LSZ), which displays complex Proterozoic folding and shearing of relict Archaean granulites. The more competent Archaean gneisses lie to the SW of the LSZ which has been a focus of shearing, amphibolitization and metasomatism from 2.6 to 1.4 Ga. The shears of interest display a second-order, extensional shear band relationship to the LSZ and formed during a phase of sinistral transtensional shearing and granite intrusion at 1.8–1.7 Ga. The second-order shears show displacement variations consistent with their propagation from the strongly foliated, less competent gneisses in the LSZ into the Archaean granulites to the SW.
A map of the Laxford-Scourie area can be regarded as a section through the mid to lower crust as observed on some deep seismic lines such as the BIRPS DRUM line from the north of Scotland. In the Laxford area, the change in geometry of the second order shears from SW to NE, as they curve into the LSZ, is comparable to the curvature of extensional shears in the mid to lower continental crust. Displacement variations indicate that the shear zones may nucleate at the base of the upper/mid crust and propagate up and down dip. Alternatively the shears may nucleate in the mid/lower crust and propagate up dip, becoming steeper as they do so.
