Remote image analysis of structures in the Precambrian basement exposed in small remote islands N of Scotland and possible implications for Lewisian terrane models

Several well exposed islands previously assigned to the Lewisian Complex, give interesting insights into the Precambrian basement geology north and west of the Scottish mainland. Most were last visited by geologists in the 1930s, but by using high quality aerial and ship-based images it is possible to remotely analyse them. Geological analysis of these islands was carried out by lineament analysis of aerial images together with the production of 3D photogrammetric models of the islands. Models were then structurally analysed using the ©Virtual Reality Geological Studio to extract measurements of structural features. A strong NW-SE foliation is seen on North Rona and Sule Skerry, which is not apparent in the islands further to the south and west (Sula Sgeir, Flannan Islands). It is suggested that this foliation may be a manifestation of a regional-scale shear zone. Recent geological and radiometric analyses of offshore basement core samples taken west of Shetland, suggests that the basement rocks here are uniformly Neoarchaean in age (ca 2-7-2.85 Ga) and they lack the widespread Proterozoic Laxfordian event (ca. 1.6-1.75 Ga) overprint seen in mainland Scotland and Outer Hebrides. This requires the presence of a ‘Northern Laxford Front’ north of the Scottish mainland and we suggest that the NW-SE shear zone identified during this study may be that structure. This proposed shear zone links to the terrane boundary that separates the Nagsuggtoqidian belt from the Central Greenland Craton. This proposal should be tested by revisiting these islands to collect field data and samples for geochronological analysis.