PALEOMAGNETIC DATING OF CLASTIC DIKES IN PROTEROZOIC BASEMENT, NORTHWEST SCOTLAND: EVIDENCE FOR SYNDEPOSITIONAL FAULTING DURING DEPOSITION OF THE TORRIDONIAN

Paleomagnetic results from Torridonian sands that occur in clastic-filled fractures in Lewisian (early Proterozoic) basement, northwest Scotland, provide evidence for syn-depositional faulting. Clastic-filled fractures in basement rocks below sedimentary cover sequences can be used to date faulting if the fault displacements can be inferred to have occurred during infilling or prior to lithfication, and can be used to reconstruct the paleostress regime during rifting. The Torridonian is divided into two Groups, the Stoer (~1200 Ma) and the Torridon (~900Ma), and are composed of rift-fill sedimentary rocks. To test if we could date the dikes using paleomagnetism, we sampled one dike set (Clachtoll fault zone), and associated Stoer Group sandstones, where the timing of dike emplacement is known (Stoer-age) and another set (near Gairloch) where the dikes are of uncertain age. The Clachtoll dike samples contain a magnetization residing in hematite with a declination of 330°and an inclination of 27° and a pole position at 41° N, 214°E. This magnetization is similar to that found in the associated Stoer sandstones and to other results from the Stoer Group. These results are consistent with previous studies which suggest that the faulting and infilling are syndepositional. The dikes near Gairloch contain a magnetization residing in hematite with a declination of 96° and an inclination of 61°. The pole position, 32° S and 236° E, is similar to poles from the Torridon Group. The magnetizations in both dike sets are inferred to have been acquired during or soon after infilling of the dikes. The results from Gairloch suggest that syn-depositional faulting may also have occurred during deposition of the Torridon Group, although the fabrics in the dikes are not as clearly syndeformational as at Clachtoll and could represent infilling of older fractures.