Interpretation of magnetic anomalies over the Blackstones Bank Igneous Centre, West Scottish Shelf

Abstract

A 32 × 32 km2 region of the B. G. S. one inch aeromagnetic anomaly map over the Blackstones Bank igneous centre was digitized at 1 km intervals and transformed into pseudogravimetric anomalies. A three dimensional model was obtained for the subsurface structure of the Blackstones Bank igneous centre by interpreting the pseudogravimetric anomaly. The main body of the model extends to a depth of 15 km with a reverse magnetization of 0.93 A m−1. A shallow normally magnetized body occurs within this to depths of 1.0–5.0 km with a magnetization in the range 0–2.47 A m−1. Bullerwell (1963) recognized the similarity between aeromagnetic anomalies over the Blackstones Bank situated about 50 km SW of Mull and over the known Tertiary igneous centres such as Mull, Skye and Rhum. This similarity led him to propose the existence of a Tertiary igneous centre related to the Islay and Jura dyke swarms at the Blackstones Bank. Geophysical and geological investigations have been carried out by several workers on the Blackstones Bank and their results have confirmed Bullerwell's suggestion (McQuillin and Binns 1975). This note presents an interpretation of aeromagnetic anomalies over the Blackstones Bank igneous centre (Fig. 1). The almost circular nature of the anomaly is distorted by its elongation along the NW–SE axis. The magnetic anomaly over the Blackstones Bank is dominantly negative with a relatively narrow positive anomaly superimposed. This indicates the presence of a reversely magnetized body possibly with a normally magnetized core. The complicated nature . . .