Deep Sedimentary Basin below Northern Skye and the Little Minch

THE existence of a negative gravity anomaly of amplitude about 20 mgal below northern Skye was first pointed out by Tuson1, who interpreted it as being due to a sedimentary basin bounded at its NE margin by a large NW-SE fault. Our recent geophysical work confirms the existence of this basin, defines its margins more accurately, and provides evidence for the age of the infilling. Fig. 1 shows the Bouguer anomaly map of the region, compiled from marine and land surveys by the Institute of Geological Sciences and detailed traverses on the Trotternish peninsula by the University of Glasgow. The gravity reaches a minimum of about 6 mgal on Trotternish, in the area of the Glasgow seismic profiles 1 and 2, whereas the extremely steep gradient off the coast of Harris continues south parallel to the landward coastline of the Outer Isles about as far as South Uist. Thus the basin seems to be deepest below northern Skye, and to be bounded on its NW side by a large normal fault, which we shall call the Minch Fault, without implying agreement with Dearnley's hypothesis 2 of transcurrent movement. Northwest of the Minch Fault there is a contribution to the gravity peak from Lewisian rocks of abnormally high density. On Harris, gravity falls off slowly to the NW owing to increasing granitization of the basement. Marine refraction surveys in the Minch have been carried out by IGS and Glasgow. The two IGS lines xx' and yy' shot over the west of the basin give the following mean structure: 200 m of material of P-wave velocity 3.0 km S-1, over-lying 1 km of rock of velocity 3.9 km s-1, overlying rock of velocity 4.8 km S-1. The first two layers are presumed to be of Mesozoic age, overlying the third of Torridonian. Two NW-SE refraction lines between Trotternish and Harris shot by Glasgow in 1971 were mostly unsuccessful, because of energy transfer problems caused by the intrusive igneous rocks which 850 km of sparker and magnetic traversing show to outcrop over more than half the area of the sea bed north