Variation in the form and distribution of dykes in the Mull swarm, Scotland

Abstract

The dykes of the Mull swarm, part of the British Tertiary Igneous Province, have been examined in a series of well exposed coastal sections. Traverses through the dyke swarm show systematic changes in the thickness and spacing of dykes, both of which are typically log-normally distributed. Both the geometric mean thickness and spacing of dykes increase with distance from the Mull centre, concordant with a decrease in crustal extension. Variations in the thickness and spacing result from changes in character of the magma flow. It is inferred that the magma flux was fairly evenly distributed near the Mull centre, but farther away from the source it becomes focused into larger dykes. The form, orientation and opening direction are also observed to vary with distance away from the volcanic centre on Mull. The dykes intrude a range of pre-existing fractures, the range of fractures used decreases with distance from the centre. This, together with other features of the form of the dykes have been interpreted in terms of the varying magma pressure and regional stress, which is expressed as the R-ratio. Analysis of the variable strikes of fracture-filled dykes is consistent with decreasing magma pressure with distance from the Mull centre, assuming that regional stresses were relatively invariant across the same distance.