Palaeomagnetic properties of a Precambrian metamorphic terrane: The Lewisian complex of the Outer Hebrides, NW Scotland

Abstract

The palaeomagnetism of a large area of Precambrian metamorphic basement comprising the Outer Hebrides archipelago in NW Scotland is described. A stable magnetite-held imprint characterises much of the complex. Dispersed easterly steep (‘A1’) and NE intermediate (‘C’) components are present in the northern sector but are subordinate to NW positive to NW negative (‘A2’ → ‘A5’, ‘B’) components. NW shallow (‘A4’) components predominate in the south of the outcrop area. ‘A1’, ‘A2’ and ‘C’ components are confined largely to areas which have escaped tectonism since Scourian (> 2650 Ma) times and are probably a relict record. The ‘A4’ components are most prominent in areas profoundly affected by the Laxfordian episode and record the peak of a later magnetising event. By correlation with APW from the Laurentian and Fennoscandian Shields, relict components are interpreted as a possible acquisition at 1820-1750 Ma close to the peak of Laxfordian tectonism. The younger components are linked to retrogressive amphibolite facies metamorphism and rapid crustal uplift at 1700-1630 Ma. Anisotropy of susceptibility is variable in magnitude and predominantly foliated. It appears to be controlled mainly by Laxfordian strain although the effect of the earlier Scourian episode is also recorded. A younger lamprophyre dyke suite yields a palaeomagnetic pole from nine sites (349° E, 47° S) dated as mid-Permian by reference to the Eurasian APW path. The spatial distribution of magnetisations in the Outer Hebrides basement mirrors the distribution observed in the Mainland outcrop comprising the NW foreland to the Caledonian orogen but is displaced laterally. The two distributions can be matched by adjustment for ca. 95 km of sinistral strike slip on the site of the Minches Basin between the two outcrops. This fault zone is the NW member of a suite of NE-SW trending transcurrent faults active in late Caledonian times and possesses the largest strike-slip offset.