Palaeomagnetism of the middle-upper Devonian Esha Ness ignimbrite, W. Shetland

Abstract

The ignimbrite division of the Esha Ness volcanics of W. Shetland has been studied palaeomagnetically. The major blocking temperature spectrum ranges between 650 and 680°C, implying that haematite is the principal remanence carrier. The magnetization build-up is simple: a minor low stability magnetization aligned along the direction of the present Earth's field is superimposed on a well-defined and highly stable palaeomagnetic component with Dec. = 220 and Inc. = + 3. The Esha Ness volcanic sequence is folded into a gentle syncline trending NNE. The rock collection concerned covers 8 sites: 7 from the western flank and 1 from the eastern flank of the syncline. The 7 sites on the west flank are consistently reversely magnetized while the site on the east flank has a normally directed magnetization. After structural unfolding the normal and reverse directions are almost perfectly antiparallel, i.e., the stable magnetization most likely predates the tilting which is probably of Devonian age. Esha Ness is located some 12 km to the west of the Walls Boundary Fault (WBF) which is considered as the northward continuation of the Great Glen Fault (GGF). The shallow palaeomagnetic inclination found in Old Red Sandstone rocks to the west of GGF/WBF corresponds to that encountered in most other European Old Red formations, discounting therefore megascale (> 1000 km) late-post Devonian movements along this dislocation.