VARIATION IN THE FORM AND DISTRIBUTION OF DYKES IN THE MULL SWARM, SCOTLAND

The dykes of the Mull swarm, part of the British Tertiary Igneous Prov ince, have been examined in a series of well exposed coastal sections. Traverses through the dyke swarm show systematic changes in the thick ness and spacing of dykes, both of which are typically log-normally di stributed. Both the geometric mean thickness and spacing of dykes incr ease with distance from the Mull centre, concordant with a decrease in crustal extension. Variations in the thickness and spacing result fro m changes in character of the magma flow. It is inferred that the magm a 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 dista nce away from the volcanic centre on Mull. The dykes intrude a range o f pre-existing fractures, the range of fractures used decreases with d istance from the centre. This, together with other features of the for m of the dykes have been interpreted in terms of the varying magma pre ssure and regional stress, which is expressed as the R-ratio. Analysis of the variable strikes of fracture-filled dykes is consistent with d ecreasing magma pressure with distance from the Mull centre, assuming that regional stresses were relatively invariant across the same dista nce.