Palaeozoic reconfigurations of north Scotland based on palaeomagnetic results from Orkney dykes

Abstract

Abstract The camptonite and monchiquite dykes of the Orkney Islands, intruding Middle Devonian sedimentary strata, constitute two significantly different axes of magnetization. The inferred older of these remanences has an inclination corresponding to an Upper Devonian to early Carboniferous age while the inclination of the secondary magnetization is in line with the Kiaman overprint repeatedly recorded in Orcadian Basin rocks. However, the unique palaeomagnetic property of these dykes is that both magnetizations have mean declinations of ∼ 170°, i.e., the palaeomagnetic axes are rotated ∼ 20° anticlockwise with respect to the corresponding NW European reference fields. The data represent vital new information for the discussion of transcurrent motions along the Great Glen Fault. It is concluded that the Orkney dykes originated at a late stage of an Acadian age sinistral megashear (∼ 600 km) at a time when the Northern Highlands block was positioned to the south of the Central Highlands. Partial remagnetization of the dykes took place at an early stage of Hercynian fault reactivation. The present declination discrepancy of both primary and overprinted magnetizations was brought about by a Hercynian dextral displacement on the Great Glen Fault (∼ 300 km) leading essentially to the present juxtaposition of the Northern and Central Highlands.